EP4343007A1

EP4343007A1 - Steel material and mold

Info

Publication number: EP4343007A1
Application number: EP23198533.4A
Authority: EP
Inventors: Masamichi Kawano
Original assignee: Daido Steel Co Ltd
Current assignee: Daido Steel Co Ltd
Priority date: 2022-09-22
Filing date: 2023-09-20
Publication date: 2024-03-27
Also published as: US20240102140A1; KR20240041265A; JP2024046069A; CN117737609A

Abstract

The present invention relates to a steel material including: 0.25 mass% ≤ C ≤ 0.37 mass%; 0.08 mass% ≤ V ≤ 0.28 mass%; 6.60 mass% ≤ Mn + Cr ≤ 7.40 mass%; Mn/Cr ≤ 0.150; Mn ≥ 0.60 mass%; Cr ≤ 6.60 mass%; Cu + Ni ≤ 0.84 mass%; 0.40 mass% ≤ Si ≤ 0.90 mass%; 0.60 mass% ≤ Mo ≤ 2.00 mass%; 0.001 mass% ≤ Al ≤ 0.080 mass%; and 0.003 mass% ≤ N ≤ 0.040 mass%, with the balance being Fe and unavoidable impurities.

Description

TECHNICAL FIELD

The present invention relates to a steel material and a mold, and more particularly to a steel material suitable for producing a mold having a large mass and a large size, and a mold using the same.

BACKGROUND ART

Since a stress and heat repeatedly act on a mold during use, a steel material for the mold is required to be excellent in a plurality of properties such as a hardness, impact resistance, heat check resistance, and wear resistance. Therefore, various proposals have been made in the related art regarding a steel material having such properties.
For example, Patent Literature 1 discloses a hot work tool steel containing predetermined amounts of C, Si, Mn, Cr, Mo, and V, with the balance being Fe and unavoidable impurities.
The Patent Literature 1 discloses that (A) in the case where an amount of Si is set to 0.01 mass% or more and less than 0.25 mass%, a hot work tool steel can be obtained that has machinability enough to be industrially processed into a mold shape and has thermal conductivity higher than that of a general-purpose mold steel (for example, JIS SKD61), and (B) in the case where an amount of Mn, an amount of Cr, an amount of Mo, and an amount of V are optimized, a hot work tool steel having high hardenability and a high impact value can be obtained.
A mold production process generally includes (a) a first step of producing a steel material suitable for mold production, and (b) a second step of producing a mold from the obtained steel material.
The first step (a step of producing a steel material for a mold) includes various steps. Main steps thereof include a melting step, a refining step, a casting step, a homogenization heat treatment step, a hot working step, a normalizing step, a tempering step, and a spheroidizing annealing step. Among these, either one or both of the normalizing step and the tempering step may be omitted.
The second step (a step of producing a mold from a steel material) includes an HT step as one step thereof.
The HT step generally includes (a) a step of machining (rough machining) a spheroidized annealed steel material into a rough mold shape, (b) a step of performing quenching (H) and tempering (T) on the roughly machined mold, (c) a step of performing finish machining on the quenched and tempered mold, and (d) if necessary, a step of performing surface modification on the finished mold.
Properties required for the steel material to be subjected to the HT step and the mold produced by the HT step include (1) a spheroidizing annealing (SA) property, (2) machinability, (3) an impact value when a quenching rate is low, (4) heat check resistance, and (5) softening resistance.
In order to obtain a high impact value even in the case where the quenching rate is low, it is necessary to satisfy three factors, that is, (a) a small amount of a coarse foreign matter, (b) fine austenite crystal grains at the time of quenching, and (c) high hardenability.
However, it is not easy to produce a steel material satisfying all of the above five properties. For example, SKD61, which is a general-purpose steel for a die casting mold, is excellent in SA property and machinability, but is inferior in impact value, heat check resistance, and softening resistance.
On the other hand, a steel material obtained by reducing defects (the impact value, the heat check resistance, and the softening resistance) of SKD61 is generally inferior in SA property and machinability. That is, since influences of alloy elements affecting the above five properties are opposite, it is very difficult to improve the five properties at the same time.
Patent Literature 1: JP2011-001572A

SUMMARY OF INVENTION

An object of the present invention is to provide a steel material which is good in all five properties of SA property, machinability, impact value, heat check resistance, and softening resistance.
Another object of the present invention is to provide a steel material which is good in all five properties of SA property, machinability, impact value, heat check resistance, and softening resistance even in the case where the steel material is large in both mass and size.
Further, yet another object of the present invention is to provide a mold produced from such a steel material.
In order to solve the above problems, a steel material according to the present invention includes:

0.25 mass% ≤ C ≤ 0.37 mass%;
0.08 mass% ≤ V ≤ 0.28 mass%;
6.60 mass% ≤ Mn + Cr ≤ 7.40 mass%;
Mn/Cr≤ 0.150;
Mn > 0.60 mass%;
Cr ≤ 6.60 mass%;
Cu + Ni ≤ 0.84 mass%;
0.40 mass% ≤ Si ≤ 0.90 mass%;
0.60 mass% ≤ Mo ≤ 2.00 mass%;
0.001 mass% ≤ Al ≤ 0.080 mass%; and
0.003 mass% ≤ N ≤ 0.040 mass%,
with the balance being Fe and unavoidable impurities.

A mold according to the present invention is produced from the steel material according to the present invention and has a mass of 2000 kg or more.
There are two major features of the steel material according to the present invention. The first feature is that an amount of C and an amount of V are relatively small. Accordingly, it is possible to prevent a decrease in an impact value due to a coarse foreign matter. On the other hand, in the case where the amount of C and the amount of V are small, austenite crystal grains are likely to be coarsened during quenching. However, by decreasing the amount of C and the amount of V, and at the same time, adding appropriate amounts of Al and N and adjusting quenching conditions, it is possible to prevent the decrease in the impact value due to the coarsening of the austenite crystal grains during quenching.
The second major feature is that an amount of Cr and an amount of Mn are individually defined, and at the same time, parameters "Mn + Cr" and "Mn/Cr" are introduced to find optimum ranges of the amount of Mn and the amount of Cr. In the case where the amount of Mn and the amount of Cr are optimized, an SA property is improved, the decrease in the impact value due to a decrease in hardenability is prevented, and softening resistance is improved.
In particular, the "SA property" and the "hardenability", as well as the "hardenability" and the "softening resistance" are properties in which influences of elements are opposite. However, in the case where the amount of Cr and the amount of Mn are optimized, both properties can be achieved.
Further, "machinability" and "heat check resistance" are generally properties in which influences of elements are opposite. On the other hand, in the present invention, in addition to the above two features, an amount of Si is optimized, so that both machinability and heat check resistance can be achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a relationship between an amount of C and an impact value.
FIG. 2 is a diagram showing a relationship between an amount of V and an impact value.
FIG. 3 is a diagram showing ranges of the amount of C and the amount of V
FIG. 4 is a schematic diagram of a CCT diagram.
FIG. 5 is a schematic diagram showing a difference in critical cooling rate X depending on a type of a steel.
FIG. 6 is a diagram showing an influence of an amount of Mn + Cr on the critical cooling rate.
FIG. 7 is a diagram showing an influence of the amount of Mn + Cr on an impact value in the case where a quenching rate is low.
FIG. 8 is a diagram showing an influence of Mn/Cr on a hardness of a test piece after SA.
FIG. 9 is a diagram showing ranges of an amount of Mn and an amount of Cr.
FIG. 10 is a diagram showing an influence of an amount of Si on machinability.
FIG. 11 is a diagram showing an influence of the amount of Si on heat check resistance.
FIG. 12 is a diagram showing an influence of the amount of Mo on fracture toughness.
FIG. 13 is a schematic diagram illustrating a cut-out position of a test piece.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described in detail.

[1. Steel Material]

[1.1. Composition]

[1.1.1. Main Constituent Element]

A steel material according to the present invention includes the following elements, with the balance being Fe and unavoidable impurities. Types of additive elements, component ranges thereof, and reasons for limitation thereof are as follows.

(1) 0.25 mass% ≤ C:S 0.37 mass%:

Fine particles (carbides, carbonitrides) having a diameter of less than 0.5 µm function as "pinning particles" that prevent growth of austenite crystal grains during heating for quenching. In the case where an amount of C is too small, an amount of pinning particles becomes insufficient during heating for quenching. As a result, crystal grains may be coarsened, and steel material properties such as an impact value, a fracture toughness value, and ductility may be deteriorated.
In addition, in the case where the amount of C is too small, a martensite transformation start temperature (a Ms point) becomes excessively high. As a result, hardenability is increased, but the impact value may be decreased.
Further, in the case where the amount of C is too small, it is difficult to obtain a hardness of 45 HRC or more by tempering at 560°C to 600°C. In order to ensure high heat check resistance, the hardness of 45 HRC or more is required.
Therefore, the amount of C needs to be 0.25 mass% or more. The amount of C is preferably 0.26 mass% or more, and more preferably 0.27 mass% or more.
On the other hand, in the case where the amount of C is excessive, coarse carbides or carbonitrides may be crystallized during casting. These crystallized matters become a "foreign matter" that reduces the impact value. It is difficult to dissolve and eliminate the coarse foreign matter by a heat treatment (a homogenization heat treatment, normalizing, and spheroidizing annealing). The coarse foreign matter often remains without being completely dissolved even after quenching and tempering. The coarse foreign matter is dissolved and reduced in size during the homogenization heat treatment, but is still observed with a diameter exceeding 3 µm. The foreign matter that remains without being completely dissolved becomes a starting point of fracture and causes a decrease in the impact value and a fatigue strength.
Further, in the case where an ingot is formed into a block-shaped or bar-shaped steel material by hot working, when a cooling rate after the hot working is low, the impact value may be decreased. In the case where the amount of C is excessive, this phenomenon is clear.
Therefore, the amount of C needs to be 0.37 mass% or less. The amount of C is preferably 0.36 mass% or less, and more preferably 0.35 mass% or less.

(2) 0.08 mass% ≤ V ≤ 0.28 mass%:

V is bonded to C and/or N in a steel to form a carbide, a carbonitride, and/or a nitride. All of these function as the pinning particles. Therefore, in the case where an amount of V is too small, the amount of pinning particles is insufficient during heating for quenching.
In addition, in the case where the amount of V is too small, a degree of secondary hardening during tempering is decreased. As a result, at the time of tempering at 560°C to 600°C, it is difficult to obtain the hardness of 45 HRC or more.
Therefore, the amount of V needs to be 0.08 mass% or more. The amount of V is preferably 0.09 mass% or more, and more preferably 0.10 mass% or more.
On the other hand, in the case where the amount of V is excessive, the coarse foreign matter is increased. In addition, in the case where the cooling rate after the hot working is low, the phenomenon that the impact value is decreased may be clear.
Therefore, the amount of V needs to be 0.28 mass% or less. The amount of V is preferably 0.27 mass% or less, and more preferably 0.26 mass% or less.

(3) 6.60 mass% ≤ Mn + Cr ≤ 7.40 mass%:

Both Mn and Cr affect the hardenability. In the case where an amount of Mn + Cr is too small, the hardenability is insufficient. As a result, in particular, in an inside of a large mold (a region where a quenching rate is low), the impact value may be significantly decreased. Therefore, the amount of Mn + Cr needs to be 6.60 mass% or more. The amount of Mn + Cr is preferably 6.65 mass% or more, and more preferably 6.70 mass% or more.
On the other hand, in the case where the amount of Mn + Cr is excessive, thermal conductivity is significantly decreased. As a result, heat check resistance is deteriorated due to an increase in a thermal stress. Therefore, the amount of Mn + Cr needs to be 7.40 mass% or less. The amount of Mn + Cr is preferably 7.35 mass% or less, and more preferably 7.30 mass% or less.

(4) Mn/Cr ≤ 0.150:

A ratio (Mn/Cr) of a mass of Mn to a mass of Cr contained in the steel affects an SA property. In the case where Mn/Cr is too large, the SA property is deteriorated. Therefore, in order to soften the steel material to 98 HRB or less in SA at a heating temperature exceeding an A_c3 point, the cooling rate needs to be less than 10 °C/H. As a result, an SA step becomes longer, and productivity is decreased. In addition, in the case where the crystal grains are coarse and Mn/Cr is large, a SA failure is likely to occur.
Therefore, Mn/Cr needs to be 0.150 or less. Mn/Cr is preferably 0.148 or less, and more preferably 0.145 or less.

(5) Mn ≥ 0.60 mass%:

Mn affects the hardenability. In the case where an amount of Mn is too small, the hardenability is deteriorated. In order to ensure the hardenability in the case where the amount of Mn is small, it is necessary to increase an amount of Cr. However, in the case where the amount of Cr is excessive, problems to be described later may become apparent.
Therefore, the amount of Mn needs to be 0.60 mass% or more. The amount of Mn is preferably 0.62 mass% or more, and more preferably 0.65 mass% or more.

(6) Cr ≤ 6.60 mass%:

In the case where the amount of Cr is excessive, softening resistance is decreased. That is, a surface of a mold that is in contact with a molten metal during use as a die casting mold is heated to a high temperature, and the surface of the mold heated to the high temperature is likely to be softened. In the case where a high-temperature strength is decreased due to the softening, the heat check resistance is also deteriorated. In addition, softening of a region with a hardness exceeding a maximum hardness is remarkable, and it is difficult to adjust a tempering hardness. This is because the hardness is sensitive to a variation of a furnace temperature.
In addition, in the case where the amount of Cr is excessive, the thermal conductivity is decreased. As a result, a thermal stress is increased and the heat check resistance is deteriorated. Further, in the case where an amount of Si is 0.50 mass% or less, when the amount of Cr is increased, machinability is significantly decreased.
Therefore, the amount of Cr needs to be 6.60 mass% or less. The amount of Cr is preferably 6.55 mass% or less, and more preferably 6.50 mass% or less.

(7) Cu + Ni ≤ 0.84 mass%:

In the present invention, as described above, the SA property, the hardenability, and the softening resistance are ensured by the balance between Cr and Mn (the amount of Cr, the amount of Mn, the amount of Mn + Cr, the ratio of Mn/Cr). On the other hand, Cu and Ni both have an effect of improving the hardenability, but deteriorate the SA property. Further, Cu and Ni do not significantly affect the softening resistance, but rather have a noticeable adverse influence on the SA property. In the case where an amount of Cu is more than an amount of Ni, hot workability is deteriorated. Therefore, a total amount of Cu and Ni is defined, and an upper limit of the total amount is defined within a range in which the influence on the hardenability and the SA property is small.
A "hardenability property value" is used as an index of an effect of an alloy element on improving the hardenability of the steel. The hardenability property value means that the larger the value, the higher the effect of improving the hardenability. The hardenability property value is determined for each type of alloy element and an addition amount thereof. The hardenability of steels having different components is evaluated by an added value of hardenability property values corresponding to the types and amounts of alloy elements.
Here, the hardenability property value is 0.125 in the case where 0.10 mass% of Mn is added. On the other hand, the hardenability property value is 0.062 in the case where 0.42 mass% of Ni is added, and the hardenability property value is also 0.062 in the case where 0.42 mass% of Cu is added. That is, the hardenability property value (the added value) is 0.124 in the case where 0.42 mass% of each of Cu and Ni is added (addition of 0.84 mass% in total). This value is substantially equal to the hardenability property value (= 0.125) when 0.10 mass% of Mn is added. This fact means that in the case where an amount of Cu + Ni is 0.84 mass% or less, an influence on the improvement of the hardenability is small. In the case where the amount of Cu + Ni is about 0.84 mass%, an influence on an increase in the high-temperature strength is also small.
On the other hand, in the case where the amount of Cu + Ni is about 0.84 mass%, various problems become apparent. Specifically, for example, a crack is likely to occur during hot working, the SA property is deteriorated, and a cost is increased. Therefore, the amount of Cu + Ni needs to be 0.84 mass% or less. Since the amount of Mn + Cr for ensuring the hardenability is 6.60 mass% or more, it is clear that the hardenability is not significantly affected in the case where the amount of Cu + Ni is 0.84 mass% or less. The amount of Cu + Ni is preferably 0.78 mass% or less, and more preferably 0.72 mass% or less.

(8) 0.40 mass% ≤ Si ≤ 0.90 mass%:

In the case where the amount of Si is too small, the machinability is decreased, and it is difficult to industrially stably perform machining on a large mold. In particular, since the steel material of the present invention is intended for producing a large mold, an amount to be cut is large, and good machinability is required. Therefore, the amount of Si needs to be 0.40 mass% or more. The amount of Si is preferably 0.45 mass% or more, and more preferably 0.50 mass% or more.
On the other hand, in the case where the amount of C, the amount of V, and an amount of N are large, when the amount of Si is excessive, coarse crystallized substances may be increased. In addition, in the case where the cooling rate after the hot working is low, the phenomenon that the impact value is decreased may be clear. Further, due to a decrease in the thermal conductivity, the thermal stress in the case where the steel material is used as a mold may be increased, and the heat check resistance may be deteriorated. Therefore, the amount of Si needs to be 0.90 mass% or less. The amount of Si is preferably 0.85 mass% or less, and more preferably 0.80 mass% or less.

(9) 0.60 mass% ≤ Mo ≤ 2.00 mass%:

In the case where an amount of Mo is too small, the degree of secondary hardening during tempering is decreased. Therefore, in the case where the amount of Mo is too small, it is difficult to obtain the hardness of 45 HRC or more at the time of tempering at 560°C to 600°C. In addition, the softening resistance and the high-temperature strength may be insufficient, and the heat check resistance may be deteriorated. Therefore, the amount of Mo needs to be 0.60 mass% or more. The amount of Mo is preferably 0.70 mass% or more, and more preferably 0.80 mass% or more.
On the other hand, in the case where the amount of Mo is excessive, the machinability is decreased. In particular, in the case where the amount of Si is small, when the amount of Mo is excessive, the machinability is significantly decreased. In addition, in the case where the amount of Mo is excessive, the fracture toughness may be decreased. This tendency is apparent in the case where the amount of Si is large. Therefore, the amount of Mo needs to be 2.00 mass% or less. The amount of Mo is preferably 1.95 mass% or less, and more preferably 1.90 mass% or less.

(10) 0.001 mass% ≤ Al ≤ 0.080 mass%:

In the steel material according to the present invention, the amount of C and the amount of V are much smaller than those of the existing hot work die steel (SKD61). Therefore, amounts of V-based carbides, carbonitrides, and nitrides serving as the pinning particles during heating for quenching are smaller than those of SKD61. Therefore, in the present invention, AlN particles are also used to prevent the growth of the austenite crystal grains.
In the case where an amount of Al is too small, it is difficult to reduce oxygen during refining, and an amount of oxides is increased, which may decrease the impact value. In addition, in the case where the amount of Al is too small, an amount of AlN serving as the pinning particles is insufficient. As a result, the austenite crystal grains may be coarsened during heating for quenching, and the impact value, the fracture toughness, and/or the ductility may be decreased. Therefore, the amount of Al needs to be 0.001 mass% or more. The amount of Al is preferably 0.002 mass% or more, and more preferably 0.003 mass% or more.
On the other hand, in the case where the amount of Al is excessive, coarse alumina particles may be increased, and the impact value and the fatigue strength may be decreased. In addition, the thermal conductivity may be decreased, and the heat check resistance may be deteriorated. Therefore, the amount of Al needs to be 0.080 mass% or less. The amount of Al is preferably 0.070 mass% or less, and more preferably 0.060 mass% or less.
In the case where Ca is added in order to improve the machinability, the amount of Al is very important for optimizing a form of a compound.

(11) 0.003 mass% ≤ N ≤ 0.040 mass%:

In the present invention, in order to disperse the A1N particles in an austenite phase during heating for quenching, the amount of N is also defined together with the amount of Al. In the case where the amount of N is too small, the amount of AlN serving as the pinning particles is insufficient. As a result, the austenite crystal grains may be coarsened during heating for quenching, and the impact value, a fracture toughness value, and/or the ductility may be decreased. In addition, in the case where the amount of N is too small, the amounts of the V-based carbonitrides and nitrides which are also the pinning particles may be insufficient. Therefore, the amount of N needs to be 0.003 mass% or more. The amount of N is preferably 0.004 mass% or more, and more preferably 0.005 mass% or more.
On the other hand, in order to add N in an amount exceeding an amount that can be adjusted in normal refining, it is necessary to positively add N using dedicated equipment, which increases a material cost. In addition, in the case where the amount of N is excessive, the coarse crystallized substances may be increased. This tendency is apparent in the case where the amount of C, the amount of Si, and the amount of V are large. Further, in the case where the amount of N is excessive, an amount of coarse AlN may be excessively large, and the impact value may be decreased. Therefore, the amount of N needs to be 0.040 mass% or less. The amount of N is preferably 0.038 mass% or less, and more preferably 0.036 mass% or less.

(12) Unavoidable Impurities:

The steel material according to the present invention may include unavoidable impurities. Elements that can be included as impurities in the steel material according to the present invention and contents thereof are as follows. P ≤ 0.03 mass%, S ≤ 0.006 mass%, O ≤ 0.006 mass%, W ≤ 0.30 mass%, Co ≤ 0.30 mass%, B ≤ 0.0002 mass%, Nb ≤ 0.004 mass%, Ta ≤ 0.004 mass%, Ti ≤ 0.004 mass%, Zr ≤ 0.004 mass%, Ca ≤ 0.0005 mass%, Se ≤ 0.03 mass%, Te ≤ 0.005 mass%, Bi ≤ 0.01 mass%, Pb ≤ 0.03 mass%, and Mg ≤ 0.02 mass%.
In the present invention, the "content" refers to an "average amount of elements in the steel material" obtained by dissolving a predetermined mass of a steel material (preferably 1 g or more per analysis of one elemental), including a portion with a high segregation, a portion with a low segregation, and a portion with an average segregation, in an acid and performing derivation using a chemical analysis method.

[1.1.2. Sub-constituent Element]

The steel material according to the present invention may further includes one or two or more elements described below in addition to the main constituent elements and the unavoidable impurities described above. Types of additive elements, component ranges thereof, and reasons for limitation thereof are as follows.

[A. Group A]

(13) 0.30 mass% < W ≤ 2.00 mass%:

In the steel material according to the present invention, the amount of C and the amount of V are smaller than those of the hot work die steel in the related art, and thus a strength may be insufficient depending on an application. In such a case, it is effective to add W for increasing the strength. In order to obtain such an effect, an amount of W preferably exceeds 0.30 mass%. The amount of W is more preferably 0.80 mass% or more.
On the other hand, in the case where the amount of W is excessive, the material cost is increased. In addition, appearance of segregation may cause deterioration of mechanical properties or an increase in anisotropy. Therefore, the amount of W is preferably 2.00 mass% or less. The amount of W is more preferably 1.50 mass% or less.

(14) 0.30 mass% < Co ≤ 1.00 mass%:

As with W, Co has an effect of increasing the strength. Therefore, in the case where the strength is insufficient, it is effective to add Co for increasing the strength. In order to obtain such an effect, an amount of Co preferably exceeds 0.30 mass%. The amount of Co is more preferably 0.50 mass% or more.
On the other hand, in the case where the amount of Co is excessive, the material cost is increased. In addition, appearance of segregation may cause deterioration of mechanical properties or an increase in anisotropy. Therefore, the amount of Co is preferably 1.00 mass% or less. The amount of Co is more preferably 0.90 mass% or less.
The steel material according to the present invention may include either or both of Co and W.

[B. Group B]

(15) 0. 0002 mass% < B ≤ 0.0080 mass%:

In the case where an amount of P in the steel material is relatively large, P segregated in a grain boundary decreases a grain boundary strength, and the impact value is decreased. Addition of B is effective for increasing the grain boundary strength. In order to increase the grain boundary strength, B needs to be present alone (without forming a compound) in the steel. When B forms BN, an effect of the addition of B is lost. Therefore, in the steel material containing N, in the case where B is added for a purpose of increasing the grain boundary strength, N needs to be bonded to an element other than B.
Specifically, N is preferably bonded to a nitride forming element such as Ti, Zr, or Nb, which easily forms a nitride. These elements are effective even in a content at an impurity level, but in the case where the elements are insufficient, it is preferable to add the elements with an amount exceeding the impurity level.
BN has an effect of improving the machinability of the steel material. Therefore, in the case where B is added for a purpose of improving the machinability, it is not necessary to positively add the nitride forming element to the steel material.
In order to obtain the above effect, an amount of B is preferably 0.0002 mass% or more. The amount of B is more preferably 0.0003 mass% or more, and still more preferably 0.0004 mass% or more.
On the other hand, even in the case where B is added more than necessary, there is no difference in the effect, and there is no practical benefit. In the case where the amount of B is excessive, a cost of the steel material is increased. Therefore, the amount of B is preferably 0.0080 mass% or less. The amount of B is more preferably 0.0075 mass% or less, and still more preferably 0.0070 mass% or less.

[C. Group C]

(16) 0.006 mass% < S ≤ 0.180 mass%, (17) 0.0005 mass% < Ca ≤ 0.0500 mass%, (18) 0.03 mass% < Se ≤ 0.50 mass%, (19) 0.005 mass% < Te ≤ 0.100 mass%, (20) 0.01 mass% < Bi ≤ 0.50 mass%, and (21) 0.03 mass% < Pb ≤ 0.50 mass%:
In the steel material according to the present invention, addition of free-cutting elements is effective for improving the machinability. Specific examples of the free-cutting element include S, Ca, Se, Te, Bi, and Pb. The steel material according to the present invention may include any one of these free-cutting elements, or may include two or more thereof.
In order to obtain a sufficient free-cutting property, contents of the free-cutting elements are preferably greater than the above lower limits, respectively.
On the other hand, in the case where the content of the free-cutting element is excessive, a crack is likely to occur during hot working. In addition, in the case where the content of the free-cutting element is excessive, the impact value, the fatigue strength, the heat check resistance, and the like may be decreased. Therefore, the contents of the free-cutting elements are preferably equal to or less than the above upper limits, respectively.

[D. Group D]

(22) 0.004 mass% < Nb ≤ 0.100 mass%, (23) 0.004 mass% < Ta ≤ 0.100 mass%, (24) 0.004 mass% < Ti ≤ 0.100 mass%, and (25) 0.004 mass% < Zr ≤ 0.100 mass%:

In the steel material according to the present invention, a carbonitride forming element other than V and Al may be added to increase amounts of the carbides, the carbonitrides, and/or the nitrides. Specific examples of the carbonitride forming element include Nb, Ta, Ti, and Zr. The steel material according to the present invention may include any one of these carbonitride forming elements, or may include two or more thereof.
In order to prevent excessive grain growth of the austenite crystal grains, contents of the carbonitride forming elements are preferably greater than the above lower limits, respectively.
On the other hand, in the case where the content of the carbonitride forming element is excessive, the carbides, the carbonitrides, and/or the nitrides are crystallized in a coarse state during casting. Coarse crystallized particles remain as a foreign matter without being eliminated even during the homogenization heat treatment, SA, and quenching, which causes the decrease in the impact value and the fatigue strength. Therefore, the contents of the carbonitride forming elements are preferably equal to or less than the above upper limits, respectively.

[1.2. Properties of Steel Material]

[1.2.1. Mass and Size]

As described above, properties required for a steel material to be subjected to an HT step and for a mold produced by the HT step include five properties of the SA property, the machinability, the impact value, the heat check resistance, and the softening resistance. Among these five properties, a problem with a large steel material is a low impact value inside a large mold produced from the large steel material.
The first reason for a decrease in an impact value in the large mold is that a large foreign matter is likely to be crystallized inside the large steel material. This is because a solidification rate during ingot production is low inside the large steel material.
The second reason for the decrease in the impact value in the large mold is that a cooling rate after hot working is low, so that carbides are likely to be precipitated.
The third reason for the decrease in the impact value in the large mold is that a quenching rate is decreased inside the large steel material.
In the steel material according to the present invention, the amount of C and the amount of V are small, and the amount of Mn and the amount of Cr are optimized, and thus an influence of a large foreign matter, a low cooling rate after hot working, or a low quenching rate is small. That is, in the steel material according to the present invention, even in the case where a mass and a size are large, the five properties of the SA property, the machinability, the impact value, the heat check resistance, and the softening resistance can be achieved at a high level.
For example, in the case where the composition and production conditions of the steel material are optimized, a steel material having a mass of 3000 kg or more can be obtained in addition to the fact that all of the five properties reach a practical level. In the case where the composition and the production conditions of the steel material are further optimized, even a steel material having a mass of 4000 kg or more or 5000 kg or more can be produced.
In addition, in the case where the composition and the production conditions of the steel material are optimized, a steel material having the above properties and having a dimension (L_min), which is minimum among a longitudinal dimension (L₁), a lateral dimension (L₂), and a height dimension (L₃), of 300 mm or more can be obtained. In the case where the composition and the production conditions of the steel material are further optimized, even a steel material having L_min of 350 mm or more or 400 mm or more can be produced.
Here, the "longitudinal dimension (L₁)", the "lateral dimension (L₂)", and the "height dimension (L₃)" refer to lengths of three sides of a rectangular parallelepiped body with a minimum volume that circumscribes the steel material, respectively.

[1.2.2. Hardness]

In the present invention, the "hardness of the steel material" refers to a Rockwell B scale hardness obtained by (a) cutting out a test piece from a vicinity of a center (a region where a solidification rate is low) of a cross section of a steel material subjected to SA, and (b) performing measurement at room temperature using the test piece.
FIG. 13 shows a schematic diagram illustrating a cut-out position of the test piece. For example, in the case where the steel material is a block material 10 of a [mm] × b [mm] × c [mm] (a ≤ b ≤ c, a ≥ 300 mm), a first material 12 of a [mm] × b [mm] × d [mm] is cut out from an end surface in a c-axis direction. A value of d is not particularly limited, but is preferably 30 mm to 70 mm.
Next, a second material 14 of e [mm] × f [mm] × d [mm] is cut out from substantially a center of an ab surface of the first material 12. Values of e and f are not particularly limited, but e = 90 mm to 120 mm and f = 130 mm to 160 mm are preferred. Further, a test piece for hardness measurement is cut out from the second material 14, and used to measure the hardness.
When SA is performed on the steel material according to the present invention under appropriate conditions, the hardness of the steel material is appropriately decreased, and machining is possible. In the case where the composition and the production conditions (in particular, SA conditions) of the steel material are optimized, the hardness at room temperature is 98 HRB or less. In the case where the composition and/or the production conditions of the steel material are further optimized, the hardness at room temperature is 97 HRB or less or 96 HRB or less.

[1.2.3. Impact Value]

In the present invention, the "impact value of the steel material" refers to an impact value obtained by (a) cutting out a square bar (12 mm × 12 mm × 55 mm) from the vicinity of the center (the region where the solidification rate is low) of the cross section of the steel material subjected to SA, (b) thermally refining the square bar into 44.5 HRC to 45.5 HRC by a heat treatment, (c) producing an impact test piece from the thermally refined square bar, and (d) performing an impact test at 15°C to 35°C.
A cut-out position of the square bar is the same as the cut-out position of the test piece for the hardness measurement. That is, the square bar is cut out from the second material 14 shown in FIG. 13.
The "heat treatment" for thermally refining the square bar refers to a treatment in which in one cycle (a) the square bar is held at 950°C for 1 hour, then cooled from 950°C to 750°C at 8 °C/min, cooled from 750°C to 500°C at 5 °C/min, cooled from 500°C to 200°C at 0.5 °C/min, and cooled from 200°C to 100°C or lower at any cooling rate, and (b) subsequently, heating to a temperature range of 560°C to 600°C and cooling to 100°C or lower are performed once or more times.
The "impact test piece" refers to a test piece conforming to JIS Z2242:2018 (10 mm × 10 mm × 50 mm, an arc radius of a notch tip: 1 mm, a notch depth: 2 mm, a cross-sectional area at a lower part of a notch bottom of the test piece: 0.8 cm²).
The "impact value (J/cm²)" refers to a value obtained by dividing an absorption energy [J] by the cross-sectional area (0.8 [cm²]) at the lower part of the notch bottom of the test piece.
An "average impact value (J/cm²)" refers to an average value of impact values of 10 or more (preferably 10 to 20) impact test pieces.
A "low impact value ratio (%)" refers to a ratio (= n × 100/n₀) of the number (n) of impact test pieces having an impact value of less than 20 [J/cm²] to the total number (n₀) of impact test pieces subjected to the impact test.
In the steel material according to the present invention, in the case where the composition and the production conditions are optimized, a steel material that exhibits a high impact value in spite of a large size and has a small variation in the impact value can be obtained.
Specifically, when the composition and the production conditions are optimized, a steel material having an average impact value of 25 [J/cm²] or more and a low impact value ratio of 30% or less can be obtained.
When the composition and the production conditions are further optimized, the average impact value is 26 [J/cm²] or more or 27 [J/cm²] or more.
In addition, when the composition and the production conditions are further optimized, the low impact value ratio is 20% or less or 10% or less.

[1.2.4. Prior Austenite Crystal Grain Size]

A "prior austenite crystal grain size" of the steel material refers to a value obtained by (a) producing the impact test piece or a test piece subjected to a heat treatment under the same conditions as the impact test piece, (b) corroding the test piece to reveal a prior austenite crystal grain boundary, or determining a prior austenite crystal grain boundary by crystal orientation analysis, and (c) observing the test piece at a magnification such that 50 or more crystal grains are included in one field of view, and calculating an equivalent circle diameter of each prior austenite crystal grain included in the field of view.
An "average value of the prior austenite crystal grain size" refers to an average value of crystal grain sizes of all prior austenite crystal grains included in one or two or more fields of view having a total area of 0.5 mm² or more.
Since the steel material according to the present invention includes a relatively large amount of pinning particles, the austenite crystal grains are less likely to be coarsened during heating for quenching. When the composition and the production conditions of the steel material are optimized, the average value of the prior austenite crystal grain size is 150 µm or less. When the composition and the production conditions of the steel material are further optimized, the average value of the prior austenite crystal grain size is 135 µm or less or 120 µm or less.

[2. Mold]

A mold according to the present invention is made of the steel material according to the present invention and has the following properties.

[2.1. Mass and Size]

The steel material according to the present invention is excellent in SA property, machinability, impact value, heat check resistance, and softening resistance even in the case where the mass and the size are relatively large. Therefore, when such a steel material is used, a mold excellent in impact value, heat check resistance, and softening resistance can be obtained even in the case where a mass and a size are relatively large.
When a composition and production conditions of the mold are optimized, a mold having a mass of 2000 kg or more can be obtained in addition to the fact that the impact value, the heat check resistance, and the softening resistance reach a practical level. When the composition and the production conditions of the mold are further optimized, even a mold having a mass of 3000 kg or more or 4000 kg or more can be produced.
In addition, in the case where the composition and the production conditions of the mold are optimized, a mold having the above properties and having a dimension (L'_min), which is minimum among a longitudinal dimension (L'₁), a lateral dimension (L'₂), and a height dimension (L'₃), of 250 mm or more can be obtained. When the composition and the production conditions of the mold are further optimized, even a mold having L'_min of 300 mm or more or 350 mm or more can be produced.
Here, the "longitudinal dimension (L'₁)", the "lateral dimension (L'₂)", and the "height dimension (L'₃)" refer to lengths of three sides of a rectangular parallelepiped body with a minimum volume that circumscribes the mold, respectively.

[2.2. Hardness]

In the present invention, the "hardness of the mold" refers to a Rockwell C scale hardness obtained by (a) cutting out a test piece from a surface of the mold, and (b) performing measurement at room temperature using the test piece.
Alternatively, the surface of the mold may be evaluated according to the Rockwell C scale using a portable hardness tester (a hardness meter). In addition, a hardness (for example, a Shore hardness) evaluated by the portable hardness tester (the hardness meter) may be converted into the Rockwell C scale hardness.
The mold according to the present invention can obtain a high hardness even in the case where the mass and the size are relatively large. When the composition and the production conditions of the mold are optimized, the hardness at room temperature is 38 HRC to 48 HRC. When the composition and the production conditions of the mold are further optimized, the hardness at room temperature is 39 HRC to 49 HRC or 40 HRC to 50 HRC.

[2.3. Impact Value]

In the present invention, the "impact value of the mold" refers to an impact value obtained by (a) cutting out an impact test piece conforming to JIS Z2242:2018 from a center of a thickest portion of the mold, and (b) performing an impact test at 15°C to 35°C.
The details of the "impact value", an "average impact value", and a "low impact value ratio" are as described above, and the description thereof is omitted.
In the mold according to the present invention, when the composition and the production conditions are optimized, a mold that exhibits a high impact value in spite of a large size and has a small variation in the impact value can be obtained.
Specifically, when the composition and the production conditions are optimized, a mold having an average impact value of 25 [J/cm²] or more and a low impact value ratio of 30% or less can be obtained.
When the composition and the production conditions are further optimized, the average impact value is 26 [J/cm²] or more or 27 [J/cm²] or more.
In addition, when the composition and the production conditions are further optimized, the low impact value ratio is 20% or less or 10% or less.

[2.4. Prior Austenite Crystal Grain Size]

A "prior austenite crystal grain size" of the mold refers to a value obtained by (a) producing a test piece from the center of the thickest portion of the mold, (b) corroding the test piece to reveal a prior austenite crystal grain boundary, or determining a prior austenite crystal grain boundary by crystal orientation analysis, and (c) observing the test piece at a magnification such that 50 or more crystal grains are included in one field of view, and calculating an equivalent circle diameter of each prior austenite crystal grain included in the field of view.
An "average value of the prior austenite crystal grain size" refers to an average value of crystal grain sizes of all prior austenite crystal grains included in one or two or more fields of view having a total area of 0.5 mm² or more.
Since the mold according to the present invention includes a relatively large amount of pinning particles, austenite crystal grains are less likely to be coarsened during heating for quenching. When the composition and the production conditions of the mold are optimized, the average value of the prior austenite crystal grain size is 150 µm or less. When the composition and the production conditions of the mold are further optimized, the average value of the prior austenite crystal grain size is 135 µm or less or 120 µm or less.

[3. Method for Producing Steel Material]

A method for producing the steel material according to the present invention includes (a) a first step of melting raw materials blended to have a predetermined composition, refining a molten metal, and casting the molten metal into a mold, (b) a second step of performing a homogenization heat treatment on an ingot, (c) a third step of hot working the ingot after the homogenization heat treatment, (d) if necessary, a fourth step of normalizing a rough material after the hot working, (e) if necessary, a fifth step of tempering the rough material, and (f) a sixth step of performing spheroidizing annealing on the rough material.

[3.1. First Step]

First, the raw materials blended to have the predetermined composition are melted, the molten metal is refined, and the molten metal is cast in the mold (the first step). Melting conditions, refining conditions, and casting conditions are not particularly limited, and optimum conditions can be selected according to a purpose.

[3.2. Second Step]

Next, the ingot is subjected to the homogenization heat treatment (the second step). The homogenization heat treatment is performed to homogenize components by diluting a component segregation generated at the time of solidification and dissolving a foreign matter crystallized at the time of the solidification as much as possible. Conditions of the homogenization heat treatment are not particularly limited, and optimum conditions can be selected according to a purpose.

[3.3 Third Step]

Next, the ingot after the homogenization heat treatment is subjected to the hot working (the third step). The hot working is performed to make the ingot into a rough material having a desired shape. Conditions of the hot working are not particularly limited, and optimum conditions can be selected according to a purpose.

[3.4. Fourth Step]

Next, if necessary, the rough material after the hot working is normalized (the fourth step). The normalizing is performed in the case where it is necessary to homogenize and refine a structure of the rough material. Normalizing conditions are not particularly limited, and optimum conditions can be selected according to a purpose. The normalizing step may be omitted.

[3.5. Fifth Step]

Next, if necessary, the rough material is tempered (the fifth step). The tempering is performed in the case where it is necessary to temper martensite or bainite generated in a cooling process after the normalizing, or in the case where it is necessary to precipitate carbides in preparation for the spheroidizing annealing. Tempering conditions are not particularly limited, and optimum conditions can be selected according to a purpose. The tempering step may be omitted.

[3.6. Sixth Step]

Next, the rough material is subjected to the spheroidizing annealing (the sixth step). Thereby, the steel material according to the present invention is obtained.
In the present invention, the "spheroidizing annealing (SA)" refers to a treatment in which (a) a steel material is heated in a furnace at a temperature of [A_c3 point - 10°C] to [A_c3 point + 50°C] (hereinafter, also referred to as an "SA treatment temperature") to form a "structure with dispersed carbides in an austenite phase and very little or no ferrite phase, and (b) a slow cooling method or a constant temperature holding method is applied to the steel material having the above structure.
The "slow cooling method" refers to a method in which (a) controlled cooling is performed from the SA treatment temperature at 5 °C/H to 60 °C/H to transform a mother phase into ferrite and grow carbides, and (b) the controlled cooling is terminated when austenite is removed, and the steel material is taken out from the furnace.
In this case, it is preferable to select optimum conditions for a cooling rate of the controlled cooling according to the components and grain sizes of the steel material. A temperature at which the austenite is removed is about 550°C to 800°C, depending on the components of the steel material and the cooling rate at the time of the controlled cooling.
The "constant temperature holding method" refers to a method in which (a) cooling is performed from the SA treatment temperature at any cooling rate and constant temperature holding is performed in a temperature range of 620°C to 780°C to transform a mother phase into ferrite and grow carbides, and (b) the constant temperature holding is terminated when austenite is removed, and the steel material is taken out from the furnace.
In this case, a holding time during which the austenite is removed is about 1 hour to 24 hours, depending on the components and the grain sizes of the steel material.
The SA treatment temperature is often 830°C to 950°C, depending on the components of the steel material. Regardless of whether the slow cooling method or the constant temperature holding method is used, the hardness of the steel material after SA is about 98 HRB or less (about 240 Hv or less), and is in a soft state that facilitates rough machining (processing into a rough mold shape by machining).

[4. Method for Producing Mold]

A method for producing the mold according to the present invention includes (a) a first step of rough machining a spheroidized annealed steel material, (b) a second step of performing quenching and tempering on the roughly machined mold, (c) a third step of performing finish machining on the quenched and tempered mold, and (d) if necessary, a fourth step of performing surface modification on the finished mold.
A method and conditions of each step are not particularly limited, and optimum methods and conditions can be selected according to a purpose.

[5. Effects]

Properties required for the steel material to be subjected to the HT step and the mold produced by the HT step include (1) a spheroidizing annealing (SA) property, (2) machinability, (3) an impact value when a quenching rate is low, (4) heat check resistance, and (5) softening resistance. Hereinafter, the reason for the necessity of these five properties will be described mainly using die casting as an example.

[5.1. SA Property]

It is preferable to perform SA until the austenite is completely removed, and then take out the steel material from the furnace. In the case where untransformed austenite remains in the steel material when the steel material is taken out from the furnace, the austenite is transformed into bainite or martensite due to cooling after the steel material is taken out from the furnace.
In such a steel material, (a) a hard portion (400 Hv or more) including bainite or martensite and (b) a portion where carbides are dispersed in a mother phase of ferrite, that is, a soft portion (98 HRB or less ≈ 240 Hv or less) including an SA structure are mixed.
A state in which the hard portion and the soft portion are mixed is called an "SA failure", and is likely to occur in a steel material having high hardenability or in the case where austenite crystal grains at the time of SA heating are coarse.
In the case where a mold is produced from a steel material having the SA failure by the HT step, the following three problems occur. (A) At the time of rough machining, the hard portion severely wears a tool for machining (cutting), and a tool life is shortened. (B) The hard portion appears as a pattern on a surface of the roughly machined mold, and a smooth surface cannot be obtained. (C) At the time of heating for quenching, coarse austenite crystal grains are generated in a vicinity of the hard portion, which decreases an impact value of the mold and shortens a life of the mold.
Accordingly, the steel material for the mold is required to be a steel material that has no "hard portion" having a hardness of 400 Hv or more and can be softened to 240 Hv or less by the SA treatment, that is, a "steel material having a good SA property".
However, the steel material having the good SA property generally has poor hardenability. In addition, the steel material having the good SA property generally has high C and low Mn in many cases. In such a steel material, since carbides are likely to be precipitated during cooling at the time of quenching and ferrite transformation is also likely to proceed, it is difficult to obtain a bainite or martensite structure (the hardenability is low). That is, generally, the "good SA property" and the "high hardenability" are opposite to each other.
On the other hand, the steel material according to the present invention has optimized amounts of Cr and Mn. Therefore, the steel material according to the present invention exhibits the good SA property without significantly impairing the hardenability.

[5.2. Machinability]

A steel material to be subjected to machining is required not to cause much wear of a machining tool even when machined at a high speed. In the case where the wear of the tool is severe, a replacement frequency of the tool is increased, and a machining cost is increased. On the other hand, in the case where a machining speed is decreased in order to avoid the wear of the tool, machining efficiency is decreased. For the above reasons, the steel material for the mold is required to be able to be efficiently machined at a low cost, that is, to have the "good machinability".
On the other hand, a steel material having good machinability generally includes Si, P, and/or S in a large amount. A mold made of such a steel material generally has poor heat check resistance. The "poor heat check resistance" means that a heat check is likely to occur and is likely to be developed. Hereinafter, the reason will be described.
A high-Si steel material has low thermal conductivity. When a mold produced from a steel material having low thermal conductivity is used for die casting, a temperature amplitude of a surface of the mold is increased, and thus a generated thermal stress is increased.
A mold produced from a high-P steel material has low toughness. Therefore, when such a mold is used for die casting, a crack is likely to occur and to be developed.
Further, the high-S steel material includes a relatively large amount of sulfide. In a mold produced from such a steel material, the sulfide serves as a starting point or a development path of a crack, and thus the crack is likely to occur and to be developed.
That is, a steel material including Si, P, and/or S in a relatively large amount has good machinability. However, when such a steel material is used as a mold, a high thermal stress acts on a matrix. Therefore, a heat check, which is a thermal fatigue crack, is likely to occur and to be developed. That is, the "good machinability" and "good heat check resistance" are opposite to each other.
On the other hand, the steel material according to the present invention includes substantially no P and no S and has an optimized amount of Si. Therefore, the steel material according to the present invention exhibits the good machinability without significantly impairing the heat check resistance.

[5.3. Impact Value in the case where Quenching Rate is Low]

The mold is used in a state of being thermally refined to a predetermined hardness by quenching and tempering. The mold requires not only the hardness but also a high impact value. The reason for this is that a mold having a high impact value is less likely to be severely cracked.
In order to obtain a high impact value, it is necessary to satisfy the following three items. That is, (a) a small amount of a coarse foreign matter, (b) fine austenite crystal grains during quenching, and (c) high hardenability.

[5.3.1. Small Amount of Coarse Foreign Matter]

The "foreign matter" is a substance having a composition different from that of the matrix, and refers to a carbide, a nitride, a carbonitride, a sulfide, an oxide, or the like.
In the present invention, the "coarse foreign matter" refers to a foreign matter having a size (= equivalent circle diameter) of 3 µm or more.
When a stress acts on the mold, the coarse foreign matter is likely to become a starting point of a crack and become a propagation path of the crack that occurs. Therefore, in order to obtain a high impact value, the smaller the number of coarse foreign matters, the better.
The foreign matter includes a foreign matter containing one metal element and a foreign matter containing two or more metal elements. Since a related-art steel for a die casting mold includes large amounts of C and V, a coarse foreign matter is usually constituted by a carbide or carbonitride containing V. A size and an amount of the V-based carbide or carbonitride are affected not only by chemical components of a steel material, but also by a solidification rate during casting, a temperature and a time of a homogenization heat treatment, and the like.

[5.3.2. Fine Austenite Crystal Grains during Quenching]

When martensite or bainite is generated from austenite, austenite crystal grains are divided into several packets. Each packet is divided into a plurality of strip-shaped blocks. In this case, the shorter the martensite or bainite block, the more difficult it is for the crack propagates. The block cannot exceed an austenite crystal grain boundary.
Therefore, when the austenite crystal grains during quenching are fine, blocks of a transformed structure are also fine, and the impact value is increased. An austenite crystal grain size during quenching is affected not only by chemical components of a steel material but also by a heating temperature for quenching, a holding time, and the like.

[5.3.3. High Hardenability]

As a size of the mold is increased, a cooling rate during quenching is decreased. This tendency is particularly remarkable inside the mold. Therefore, with the recent increase in the size of the mold, the cooling rate inside the mold is decreased, and a decrease in the impact value becomes a problem.

[5.3.4. High Impact Value]

From the above circumstances, there is a strong demand for a steel material that can obtain a high impact value even in the case where a quenching rate is low, that is, a "steel material having good hardenability". In other words, the "good hardenability" means that coarse bainite is not generated even in the case where the quenching rate is low.
On the other hand, the steel material having the good hardenability has a poor SA property. Generally, the steel material having the good hardenability has low C, high Ni, and high Mn. This is because, in such a steel material, since carbides are difficult to be precipitated during slow cooling of SA and ferrite transformation is difficult to proceed, it is difficult to obtain an SA structure (a structure in which carbides are dispersed in a mother phase of ferrite).
On the other hand, the steel material according to the present invention has optimized amounts of Cr and Mn. Therefore, the steel material according to the present invention exhibits the good hardenability without significantly impairing the SA property.

[5.4. Heat Check Resistance]

A surface of a die casting mold is subjected to a cycle of temperature rise due to contact with a molten metal and cooling due to application of a release agent. A thermal stress is generated due to such a temperature amplitude, and a mechanical stress due to mold clamping or injection is also applied, so that a crack (a heat check) due to fatigue is generated on the surface of the mold. When the molten metal enters the crack and is solidified, this solidified portion is transferred to a casting surface, which is a cause of impairing aesthetic appearance.
Therefore, the mold is required to be less likely to cause a heat check, that is, to have "good heat check resistance".
As described above, the "good machinability" and the "good heat check resistance" are opposite to each other. Generally, a steel material having good heat check resistance has small amounts of Si, P, and S. However, such a steel material is likely to adhere to a cutting tool, includes a small amount of an S compound that provides a lubricating action on a cut surface, has high toughness, and is sticky, and thus is inferior in machinability.
On the other hand, the steel material according to the present invention includes substantially no P and no S and has an optimized amount of Si. Therefore, the steel material according to the present invention exhibits the good heat check resistance without significantly impairing the machinability.

[5.5. Softening Resistance]

A temperature of the surface of the die casting mold rises due to the contact with the molten metal. In the case where the number of casting shots is increased, an accumulated time of exposure to a high temperature is also increased, and thus a hardness of the surface of the mold may be decreased. Such softening causes a decrease in a high-temperature strength, and as a result, the heat check resistance is deteriorated.
For the above reasons, the die casting mold is required to be less likely to soften, that is, to have "high softening resistance". However, it is necessary to pay attention to the fact that a steel material whose softening resistance is improved by decreasing the amount of Cr has a low high-temperature strength. This is because a low-Cr steel is poor in solid-solution strengthening at a high temperature. The decrease in the high-temperature strength deteriorates the heat check resistance. That is, "good softening resistance" and the "good heat check resistance" are opposite to each other.
On the other hand, the steel material according to the present invention has optimized amounts of Cr and Mn. Therefore, the steel material according to the present invention exhibits the good softening resistance without significantly impairing the hardenability and the heat check resistance.

EXAMPLES

[1. Verification Test for Suitable Element Amounts]

[1.1. Overview]

Hereinafter, items to be achieved in the present invention are described again.

(1) SA property
(2) Machinability
(3) Impact value in the case where a quenching rate is low
1. (a) Small amount of a coarse foreign matter
2. (b) Fine austenite crystal grains during quenching
3. (c) High hardenability
(4) Heat check resistance
(5) Softening resistance

In the following verification tests, the items other than (3)(a) were targeted. There are three reasons.
The first reason is that (3)(a) can be accurately verified only with a steel material produced from an ingot with an industrial size (a mass of 8 ton or more) having a low solidification rate.
The second reason is that in the case where a large steel material is actually produced using an ingot of 8 ton or more, a cost and an examination time are excessive.
The third reason is that since an influence of (3)(a) is very large, the influence of (3)(a) is to be excluded in order to accurately verify an influence of (3)(b) or (3)(c) on an impact value.
Therefore, a steel material having a small cross section (about diameter: 82 mm × length: 3000 mm) was produced from an ingot having a high solidification rate (a small ingot having a mass of 150 kg). Next, a test piece produced from the steel material was subjected to a heat treatment simulating an industrial production method (that is, a method for producing a large mold steel material and a large mold). In this way, it is considered that properties of the steel material "other than (3)(a)" can be appropriately evaluated when a mold is produced from an ingot having an industrial size.
On the other hand, (3)(a) was evaluated with a steel material actually produced from an ingot (an ingot having a mass of 8 ton or more) having an industrial size and having a low solidification rate.

[1.2. Verification Test of Upper Limit of Amount of C]

[1.2.1. Production of Sample]

[A. Production of Round Bar in SA State]

Hereinafter, a decrease in an impact value when an amount of C exceeded 0.37 mass% was verified.
Components (mass%) of a steel material were set to 0.90 Si-0.06 Cu-0.13 Ni-0.81 Mn-6.23 Cr-1.79 Mo-0.019 Al-0.028 N-0.28 V, and an amount of C was systematically changed. These types of steels were cast into an ingot of 150 kg. After the ingot was produced, a homogenization heat treatment, hot working, normalizing, tempering, and SA were performed. In this verification, the normalizing and the tempering were performed, but these treatments may be omitted. Through the above steps, a steel material (a round bar) in an SA state of about diameter: 82 mm × length: 3000 mm was produced.

[B. Heat Treatment of Square Bar Simulating Hot Working]

Ten square bars of 12 mm × 12 mm × 55 mm were produced from the round bar in the SA state.
The obtained square bar was heated to reproduce an austenite crystal grain size during industrial hot working. That is, the square bar was held in vacuum at 1240°C for 2 hours. In an industrial step, the heat treatment is not necessarily performed in vacuum, but a purpose of the verification was to simulate a temperature history, so that the heat treatment was performed in vacuum.
Next, the square bar was subjected to a heat treatment simulating cooling after hot working the ingot into a steel material having a large size. That is, following the holding at 1240°C for 2 hours, the square bar was cooled to 1000°C at 1 °C/min, and then cooled from 1000°C to 600°C at 0.5 °C/min.
In a case of a type of a steel having a large amount of C, a carbide was precipitated at an austenite crystal grain boundary in this temperature section. The carbide had a bar shape, a V shape, a W shape, or a wave shape, and had a size in a long axis direction of 0.5 µm to 3 µm. The precipitated carbide was smaller in size than a foreign matter crystallized at the time of solidification in a large steel material, but was intermittently distributed at the crystal grain boundary so as to cover the grain boundary. In the case where such a grain boundary carbide is present, fracture is likely to occur at the grain boundary, and an impact value is significantly decreased.
In a temperature range of 600°C or lower, an inert gas was introduced into a vacuum furnace to pressurize to 3 Torr to 4 Torr (0.40 kPa to 0.53 kPa), and the inert gas was further forced convection to rapidly cool the square bar.
A cooling rate at 600°C or lower does not simulate a steel material having a large size which is industrially produced. However, since a purpose of this evaluation is to examine an "influence of a grain boundary carbide precipitated in a high temperature range after hot working", the purpose can be achieved even in the case where a cooling history at 600°C or lower is a history as described above. The grain boundary carbide is not eliminated even by "normalizing-tempering-SA-quenching and tempering" after the hot working, and finally remains in the mold to greatly decrease the impact value.

[C. Normalizing, Tempering, SA, Quenching, and Tempering of Square Bar]

Next, the square bar subjected to the heat treatment simulating the hot working was subjected to normalizing, tempering, and SA in vacuum in accordance with an industrial production method.
Further, the square bar in an SA state was vacuum-quenched. That is, the square bar was held at 950°C for 1 hour in vacuum. Next, the inert gas was introduced into the vacuum furnace to pressurize to 3 Torr to 4 Torr (0.40 kPa to 0.53 kPa), and the inert gas was further forced convection to rapidly cool the square bar to 100°C or lower.
A cooling time from 950°C to 100°C during the quenching was within 60 minutes. That is, the cooling of the square bar is different from cooling of a steel material having a large size which is industrially produced. However, since the purpose of this evaluation is to examine the "influence of a grain boundary carbide precipitated in a high temperature range after hot working", the purpose can be achieved even in the case where the quenching is rapid cooling.
Subsequently, the square bar after the quenching was further tempered. The tempering was performed by holding the square bar at 560°C for 2 hours and then cooling the square bar to 100°C or lower.
Further, tempering was additionally performed. That is, the above square bar was held at 560°C to 600°C for a predetermined time, and then cooled to 100°C or lower. This treatment was performed once or more, and the square bar was thermally refined to 44.5 HRC to 45.5 HRC. A temperature and a time of the holding, and the number of treatments were changed according to a type of a steel (an amount of C). This is because softening resistance varies when an amount of C varies.

[1.2.2. Test Method]

An impact test piece was produced from the square bar thermally refined to 44.5 HRC to 45.5 HRC. The impact test piece had a shape (10 mm × 10 mm × 50 mm, an arc radius of a notch tip: 1 mm, a notch depth: 2 mm, a cross-sectional area at a lower part of a notch bottom of the test piece: 0.8 cm²) conforming to JIS Z2242:2018 . An impact test was performed at 15°C to 35°C using the obtained impact test piece.
An impact value was used for evaluation. The impact value [J/cm²] referred to here is a value obtained by dividing an absorption energy [J] by the cross-sectional area of 0.8 cm² at the lower part of the notch bottom of the test piece, and refers to an average value often test pieces.

[1.2.3. Results]

An impact value required for a die casting mold is 20 J/cm² or more for a mold with a small load and 25 J/cm² or more for a mold with a large load. In a die casting mold having an impact value of 30 J/cm² or more, a risk of fracture is considerably reduced.
In this evaluation, the quenching is rapid cooling, but in the case where quenching is slow cooling in an actual large mold, the impact value is decreased by about 5 J/cm². Therefore, here, a threshold value of the impact value for performing quality determination was set to 25 J/cm².
FIG. 1 shows a relationship between the amount of C and the impact value. Unlike a large mold, in the impact test piece used in the test, the quenching is rapid cooling. Nevertheless, in the case where the amount of C was excessive, a test piece had an impact value of less than 20 J/cm². From this, it can be seen that an influence of a grain boundary carbide precipitated in a high temperature range after hot working is very large. From FIG. 1, it can be seen that a type of a steel having an amount of C of 0.37 mass% or less has an impact value of 25 J/cm² or more.

[1.3. Verification Test of Upper Limit of Amount of V]

[1.3.1. Production of Impact Test Piece and Impact Test]

Hereinafter, a decrease in an impact value in the case where an amount of V exceeded 0.28 mass% was verified.
Components (mass%) of a steel material were set to 0.37 C-0.90 Si-0.04 Cu-0.13 Ni-0.82 Mn-6.21 Cr-1.97 Mo-0. 013 Al-0. 028 N, and an amount of V was systematically changed. These types of steels were cast into an ingot of 150 kg. Thereafter, in the same manner as in the verification test of the amount of C, an impact test piece was produced, and an impact test was performed.

[1.3.2. Results of Impact Test]

FIG. 2 shows a relationship between the amount of V and the impact value. Unlike a large mold, in the impact test piece used in the test, the quenching is rapid cooling. Nevertheless, in the case where the amount of V was excessive, a test piece had an impact value of less than 20 J/cm². From this, it can be seen that an influence of a grain boundary carbide precipitated in a high temperature range after hot working is very large. From FIG. 2, it can be seen that a type of a steel having an amount of V of 0.28 mass% or less has an impact value of 25 J/cm² or more.

[1.3.3. Suitable Ranges of Amount of C and Amount of V]

FIG. 3 shows ranges of the amount of C and the amount of V In the present invention, the amount of C and the amount of V were defined in consideration of a "hardness", an "amount of a coarse foreign matter", an "amount of a grain boundary carbide precipitated after hot working", and an "amount of pinning particles". A hot work die steel in the related art has an amount of C of 0.32 mass% or more and an amount of V of 0.30 mass% or more. On the other hand, the steel material according to the present invention has an amount of C of 0.37 mass% or less and an amount of V of 0.28 mass% or less, so that ranges of the amount of C and the amount of V are different from those of the steel in the related art.
In addition, the present invention is different from the steel in the related art in terms of a quenching temperature. A quenching temperature of the steel in the related art is as high as 1010°C to 1040°C in order to sufficiently dissolve C and V On the other hand, a quenching temperature of the present invention having the small amounts of C and V may be as low as 920°C to 980°C, which is advantageous in that deformation of a quenched mold is small and a crack is less likely to occur in a mold.

[1.4. Verification Test of Lower Limit of Amount of Mn + Cr]

[1.4.1. Overview of Critical Cooling Rate]

Hereinafter, it was verified that hardenability is improved by optimizing an amount of Mn + Cr, and thus a high impact value can be obtained even in the case where a quenching rate is low.
A CCT diagram is often used to evaluate hardenability. A transformation point was determined based on a dimensional change (shrinkage -> expansion -> shrinkage) during a step of cooling a steel material from a quenching temperature at a constant rate.
FIG. 4 shows a schematic diagram of a CCT diagram. In FIG. 4, "•" represents a transformation temperature determined according to the above criteria, and represents data in which constant cooling rates from the quenching temperature are different. Based on •, a line corresponding to the transformation temperature is drawn. A solid line represents a highly accurate line, and a broken line represents an estimated line. In FIG. 4, a cooling rate (a line of an arrow) passing through a boundary between martensite and bainite is a critical cooling rate X.
The critical cooling rate X is greatly affected by Mn, Cr, and the like. FIG. 5 shows a schematic diagram of a difference in the critical cooling rate X depending on a type of a steel. In FIG. 5, a position of bainite of a steel B is on a right side (a long time side) of that of a steel A. This indicates that X of the steel B is smaller than that of the steel A.
That is, the steel B is easily to have a fine structure (martensite, or bainite transformed at a low temperature) even in the case where X is small. In this way, a steel having a small X is regarded as having "good hardenability". A steel having good hardenability is easily quenched even for a "large mold" in which X is small, and is easily to have a fine structure.

[1.4.2. Production of Round Bar in SA state]

Hereinafter, an influence of the amount of Mn + Cr on the hardenability and the impact value was verified.
Components (mass%) of a steel material were set to 0.32 C-0.45 Si-0.08 Cu-0.11 Ni-1.06 Mo-0.18 V-0.028 Al-0. 011 N, and an amount of Mn and an amount of Cr were systematically changed. These types of steels were cast into an ingot of 150 kg. Thereafter, in the same manner as in the verification test of the amount of C, a steel material (a round bar) in an SA state of about diameter: 82 mm × length: 3000 mm was produced.

[1.4.3. Test Method]

[A. Measurement of Critical Cooling Rate]

A test piece having a substantial shape of diameter: 4 mm × length: 10 mm was produced from the round bar in the SA state. The test piece was heated to a quenching temperature of 950°C and cooled to 100°C or lower at any constant rate. A transformation point was determined based on a dimensional change (shrinkage -> expansion -> shrinkage) at that time. Further, a critical cooling rate X was determined based on the obtained transformation point.

[B. Impact Test]

Ten square bars of 12 mm × 12 mm × 55 mm were produced from the round bar in the SA state. The square bar was held in vacuum at 950°C for 1 hour and then quenched. A cooling rate during the quenching was 8 °C/min from 950°C to 750°C, 5 °C/min from 750°C to 500°C, and 0.5 °C/min from 500°C to 200°C, and a cooling rate from 200°C to 100°C or lower was not particularly controlled.
The above quenching step is one of examples assuming an inside having a lowest cooling rate in the case where a large mold of 2000 kg or more is quenched. Since phase transformation was substantially completed when a temperature reached 200°C, the cooling rate from 200°C to 100°C or lower was not particularly controlled.
Subsequently, the square bar was tempered. The tempering was performed by holding the square bar at 560°C for 2 hours and then cooling the square bar to 100°C or lower.
Further, tempering was additionally performed. That is, the above square bar was held at 560°C to 600°C for a predetermined time, and then cooled to 100°C or lower. This treatment was performed once or more, and the square bar was thermally refined to 44.5 HRC to 45.5 HRC. A temperature and a time of the holding, and the number of treatments were changed according to a type of a steel (an amount of Mn and an amount of Cr). This is because softening resistance varies when an amount of Mn and/or an amount of Cr varies.
An impact test piece was produced from the square bar thermally refined to 44.5 HRC to 45.5 HRC. The impact test piece had a shape (10 mm × 10 mm × 50 mm, an arc radius of a notch tip: 1 mm, a notch depth: 2 mm, a cross-sectional area at a lower part of a notch bottom of the test piece: 0.8 cm²) conforming to JIS Z2242:2018 . An impact test was performed at 15°C to 35°C using the obtained impact test piece.
An impact value was used for evaluation. The impact value [J/cm²] referred to here is a value obtained by dividing an absorption energy [J] by the cross-sectional area of 0.8 cm² at the lower part of the notch bottom of the test piece, and refers to an average value of ten test pieces.

[1.4.4. Results]

[A. Critical Cooling Rate]

FIG. 6 shows an influence of the amount of Mn + Cr on the critical cooling rate. From FIG. 6, it can be seen that the critical cooling rate is decreased, that is, the hardenability is increased, as the amount of Mn + Cr is increased.

[B. Impact Test]

From the above, it was found that the hardenability is increased as the amount of Mn + Cr is increased. Hereinafter, an effect of increasing the amount of Mn + Cr was actually verified based on an impact value in the case where a quenching rate was low. This is because even if the hardenability is high (even if X is small), the impact value does not increase in the case where a martensite transformation point is high.
FIG. 7 shows an influence of the amount of Mn + Cr on an impact value in the case where a quenching rate is low. From FIG. 7, it can be seen that the impact value is increased as the amount of Mn + Cr is increased. In FIG. 7, a cooling rate was set to 0.5 °C/min in a temperature range of 500°C to 200°C at which transformation of martensite or bainite occurs.
A type of a steel having an amount of Mn + Cr of 6.60 mass% or more had an impact value of 25 J/cm² or more. In a case of a type of a steel having an amount of Mn + Cr of 6.60 mass%, one of the ten test pieces had an impact value of less than 20 J/cm².
Further, a type of a steel having an amount of Mn + Cr of 6.70 mass% or more had an impact value of 30 J/cm² or more. In a case of a type of a steel having an amount of Mn + Cr of 6.70 mass%, none of the ten test pieces had an impact value of less than 20 J/cm².
No coarse foreign matter was observed on a fracture surface of the impact test piece. There is no guarantee that the foreign matters are not present throughout the steel material. However, in evaluation of the ten impact test pieces, the number of test pieces having an impact value of less than 20 J/cm² was one or less, and thus it is estimated that even if a foreign matter is present, an amount thereof is very small.
A fractured test piece after the impact test was corroded to observe a metal structure. As a result, an average value of a prior austenite crystal grain size was as fine as about 25 µm to 80 µm.
From the above, it was possible to confirm effects of (3)(b) and (3)(c) under conditions that an influence of (3)(a) is very small.

[1.5. Verification Test of Upper Limit of Mn/Cr]

[1.5.1. Production of Test Piece]

Hereinafter, an influence of Mn/Cr on the SA property was verified.
In the same manner as in the verification test of the amount of Mn + Cr, a round bar in an SA state was produced. Next, a test piece (12 mm × 12 mm × 20 mm) for SA property evaluation was produced from the round bar in the SA state. The test piece was subjected to a vacuum heat treatment simulating "hot working-normalizing-tempering-spheroidizing annealing" in a production process of a large mold material, and an SA property was evaluated. In this verification, the normalizing and the tempering were performed, but these treatments may be omitted.
Details of a vacuum heat treatment step are as follows. That is, first, the test piece was held in vacuum at 1240°C for 0.5 hours. This treatment is a treatment for reproducing coarse crystal grains of a hot-worked material. After holding at 1240°C for 0.5 hours, a nitrogen gas was introduced into a furnace, and the nitrogen gas was pressurized to cause forced convection to cool the test piece to 100°C or lower.
Subsequently, the test piece was held in vacuum at 970°C for 1 hour. This treatment simulates the normalizing. After holding at 970°C for 1 hour, the nitrogen gas was introduced into the furnace, and the nitrogen gas was pressurized to cause the forced convection to cool the test piece to 100°C or lower.
Subsequently, the test piece was held in vacuum at 680°C for 6 hours. This treatment simulates the tempering. After holding at 680°C for 6 hours, the nitrogen gas was introduced into the furnace, and the nitrogen gas was pressurized to cause the forced convection to cool the test piece to 100°C or lower.
Finally, SA was performed. That is, the test piece was held in vacuum at 900°C for 1 hour. Thereafter, the test piece was cooled to 600°C at 15 °C/H in vacuum. Thereafter, the nitrogen gas was introduced into the furnace, and the nitrogen gas was pressurized to cause the forced convection to cool the test piece to 100°C or lower.
The above vacuum heat treatment step is not exactly the same as a step of producing a steel material for a large mold, but important conditions for evaluating the SA property, such as a heating temperature and a cooling rate of SA, can simulate industrial steps.

[1.5.2. Evaluation of Hardness]

A hardness of the test piece after SA was evaluated at room temperature. FIG. 8 shows an influence of Mn/Cr on the hardness of the test piece after SA. From FIG. 8, it can be seen that the hardness after SA is 98 HRB or less when Mn/Cr is set to 0.150 or less. In addition, from FIG. 8, it can be seen that the hardness after SA is 97 HRB or less when Mn/Cr is set to 0.145 or less.
Further, in the case where Mn/Cr was 0.145 or less, even if a cooling rate (a cooling rate from 890°C to 600°C) after SA was increased to 20 °C/H, the hardness after SA was softened to 98 HRB or less.

[1.5.3. Suitable Ranges of Amount of Mn and Amount of Cr]

From the above, suitable ranges of the amount of Mn and the amount of Cr were determined. FIG. 9 shows the ranges of the amount of Mn and the amount of Cr. A pentagonal region surrounded by five lines in FIG. 9 is a range of the steel material according to the present invention.
In the present invention, by introducing parameters such as the amount of Mn, the amount of Cr, the amount of Mn + Cr, and Mn/Cr, narrow ranges of the amount of Mn and the amount of Cr were found in which (1) the SA property, (3) the hardenability, and (5) the softening resistance are maintained high. In addition, it was possible to achieve both (1) the SA property and (3) the hardenability in which influences of elements are opposite, and to achieve both (3) the hardenability and (5) the softening resistance in which influences of elements are opposite.

[1.6. Verification Test of Lower Limit and Upper Limit of Amount of Si]

[1.6.1. Production of Round Bar in SA state]

Hereinafter, an influence of an amount of Si on the machinability and the heat check resistance was verified.
Components (mass%) of a steel material were set to 0.31 C-0.81 Mn-0.08 Cu-0.11 Ni-6.19 Cr-1.01 Mo-0.18 V-0. 028 Al-0. 011 N, and an amount of Si was systematically changed. These types of steels were cast into an ingot of 150 kg. Thereafter, in the same manner as in the verification test of the amount of C, a steel material (a round bar) in an SA state of about diameter: 82 mm × length: 3000 mm was produced. All the types of steels were softened to 98 HRB or less.

[1.6.2. Test Method]

[A. Evaluation of Machinability]

A test piece of 53 mm × 53 mm × 200 mm was produced from the round bar in the SA state. The test piece was cut with a cutting tool, and a wear amount of the cutting tool was measured. A cutting distance at which the wear amount of the cutting tool reached 300 µm was defined as a tool life. It is determined that the longer the cutting distance, the better the machinability.

[B. Heat Check Resistance Test]

A cylindrical test piece of diameter: 72 mm × thickness: 50 mm was produced from the round bar in the SA state. Next, the cylindrical test piece was quenched and tempered in the same manner as in the impact test piece (the verification test of the amount of Mn + Cr) used in FIG. 7, and thermally refined to about 45 HRC.
A temperature cycle of heating and cooling was applied to an end surface (a surface having a diameter of 72 mm) on one side of the thermally refined test piece. High-frequency heating to 580°C and cooling to 100°C or lower with jetted water were set as one cycle, and 10,000 cycles were applied.
The test piece after the 10,000 cycles was cut vertically in a vicinity of a center thereof, and a depth of a crack (a heat check) generated in the heated and cooled surface was evaluated. The heat check resistance was evaluated by selecting three cracks in descending order of depth from a plurality of cracks observed, and averaging depths of the three cracks (hereinafter, also referred to as a "crack depth").

[1.6.3. Results]

[A. Machinability Test]

FIG. 10 shows an influence of the amount of Si on the machinability. In the case where the amount of Si was less than 0.40 mass%, the tool life was equivalent to that of the existing steel evaluated as having "poor machinability". In the case where the amount of Si is less than 0.40 mass%, the machinability is poor, and the number of steps of cutting work is enormous. In particular, since a cutting amount is large in a large mold, it is difficult to achieve industrial work if machinability is poor.
From FIG. 10, it can be seen that in the case where the amount of Si is 0.40 mass% or more, the tool life is 15 m or more. In addition, it can be seen that in the case where the amount of Si is 0.80 mass% or more, a tool life equivalent to that of SKD61 evaluated as having "very good machinability" can be obtained.

[B. Heat Check Resistance Test]

FIG. 11 shows an influence of the amount of Si on the heat check resistance. In the case where the amount of Si exceeded 0.9 mass%, the crack depth was equivalent to that of the existing steel (SKD61 or the like) having poor heat check resistance. In a large mold, in addition to the fact that it is not easy to repair a heat check, a load acting on the mold is also large, and thus a large crack is likely to occur starting from the heat check. From FIG. 11, it can be seen that the amount of Si is preferably 0.90 mass% or less.

[1.7. Verification Test of Lower Limit and Upper Limit of Amount of Mo]

[1.7.1. Production of Round Bar in SA state]

Hereinafter, an influence of an amount of Mo on fracture toughness was verified.
Components (mass%) of a steel material were set to 0.28 C-0.48 Si-0.82 Mn-0.07 Cu-0.12 Ni-6.23 Cr-0.18 V-0.026 Al-0. 010 N, and an amount of Mo was systematically changed. These types of steels were cast into an ingot of 150 kg. Thereafter, in the same manner as in the verification test of the amount of C, a steel material (a round bar) in an SA state of about diameter: 82 mm × length: 3000 mm was produced. All the types of steels were softened to 98 HRB or less.

[1.7.2. Measurement of Fracture Toughness]

A plate material of 13 mm × 62 mm × 65 mm was produced from the round bar in the SA state. Next, the plate material was quenched and tempered in the same manner as in the impact test piece (the verification test of the amount of Mn + Cr) used in FIG. 7, and thermally refined to about 45 HRC.
A fracture toughness test piece (a test piece having a notch and two holes) having a substantial shape of 12.5 mm × 61 mm × 64 mm was produced from the plate material. Further, fracture toughness was evaluated at room temperature.

[1.7.3. Results]

FIG. 12 shows the influence of Mo on the fracture toughness. Both in the case where the amount of Mo was too small and in the case where the amount of Mo was too large, a fracture toughness value decreased. In order to prevent rapid development of a crack generated in a mold, the higher the fracture toughness, the better. From FIG. 12, it can be seen that in the case where the amount of Mo is 0.60 mass% or more and 2.00 mass% or less, the fracture toughness value is 30 MPa_·m^0.5 or more.

[2. Verification Test Using Large Ingot]

[2.1. Overview]

In the verification test for the suitable elemental amounts, the small-sized (150 kg) ingot for research was used to produce the steel material having the small cross section, and the test piece produced from the steel material was subjected to the heat treatment simulating the industrial production method (the method for producing a steel material for a large mold and a large mold). Accordingly, it was possible to appropriately evaluate the properties "other than 3(a)" when the steel material was produced by the industrial production method and turned into a mold.
On the other hand, in the following Examples, effects of the present invention were confirmed by actually using an ingot having a mass of 8 ton or more. In this case, a steel material was quenched and tempered, and an internal impact value was verified. That is, the above "3(a)" was verified. This is because verification of the other properties has been completed in the verification test for the suitable elemental amounts.

[2.2. Production of Sample]

[2.2.1. Production of Block Material]

Table 1 shows compositions of steels (Examples 1 to 13 and Comparative Examples 1 to 3) whose properties were verified. Comparative Example 1 corresponds to JIS SKD6 (AISI H11). Comparative Example 2 corresponds to a commercially available steel in which Si-Mn-Cr of SKD6 has been adjusted, and is a steel having better hardenability and heat check resistance than SKD6. Comparative Example 3 is a steel in which an amount of C and an amount of V exceed the upper limits of those of the present invention. Although not shown in Table 1, each steel includes an impurity element such as P in a range not exceeding the above upper limit.

Table 1

	Composition (mass%)
	C	Si	Mn	Cu	Ni	Cr	Mo	V	Al	N	Mn + Cr	Mn/Cr	Cu + Ni	Selective addition
Example 1	0.32	0.50	0.81	0.06	0.13	6.20	1.15	0.15	0.027	0.011	7.01	0.131	0.19
Example 2	0.25	0.61	0.65	0.04	0.07	6.50	0.62	0.22	0.054	0.005	7.15	0.100	0.11
Example 3	0.37	0.42	0.86	0.01	0.02	6.32	1.90	0.28	0.011	0.023	7.18	0.136	0.03
Example 4	0.30	0.80	0.82	0.17	0.16	6.03	0.89	0.08	0.018	0.016	6.85	0.136	0.33
Example 5	0.28	0.71	0.73	0.24	0.26	5.97	1.53	0.10	0.003	0.036	6.70	0.122	0.50
Example 6	0.34	0.90	0.91	0.31	0.39	6.30	0.70	0.26	0.041	0.004	7.21	0.144	0.70
Example 7	0.30	0.50	0.85	0.11	0.51	5.95	0.80	0.12	0.007	0.007	6.80	0.143	0.62	0.0021 of B, 0.025 of Ti
Example 8	0.33	0.66	0.65	0.14	0.43	6.06	0.99	0.18	0.022	0.030	6.71	0.107	0.57	1.49 of W
Example 9	0.35	0.73	0.79	0.35	0.21	5.92	1.85	0.24	0.015	0.020	6.71	0.133	0.56	0.92 of Co
Example 10	0.29	0.75	0.80	0.20	0.30	6.46	1.07	0.22	0.048	0.009	7.26	0.124	0.50	0.023 of Nb
Example 11	0.27	0.58	0.87	0.03	0.34	6.16	1.64	0.14	0.060	0.005	7.03	0.141	0.37	0.046 of Ti
Example 12	0.31	0.80	0.72	0.08	0.58	6.30	1.34	0.26	0.034	0.040	7.02	0.114	0.66	0.024 of S
Example 13	0.28	0.54	0.75	0.27	0.45	6.12	1.44	0.16	0.005	0.023	6.87	0.123	0.72	0.0012 of Ca
Comparative Example 1	0.39	0.93	0.42	0.07	0.11	5.04	1.20	0.44	0.024	0.009	5.46	0.083	0.18
Comparative Example 2	0.36	0.50	0.71	0.06	0.12	5.52	1.24	0.58	0.019	0.019	6.23	0.129	0.18
Comparative Example 3	0.39	0.52	0.79	0.06	0.14	6.21	1.14	0.46	0.027	0.011	7.00	0.127	0.20

These steels were cast into an ingot having a mass of about 21 ton. The ingot of 21 ton has a lower solidification rate than an ingot of about 10 ton, and thus a coarse foreign matter is likely to affect an impact value. Under such adverse conditions, adequacy of an amount of C and an amount of V was verified.
The ingot of 21 ton was subjected to a homogenization heat treatment at a high temperature for a long time, and then formed into a block shape by hot working. This block material was subjected to normalizing, tempering, and spheroidizing annealing to finally obtain a spheroidized annealed material of 740 mm × 1060 mm × 2440 mm (a mass of about 15 ton). A difference in mass (about 6 ton) between the ingot and the block material is a mass of a portion removed due to a quality or shape problem.
Appropriate conditions for the normalizing, the tempering, and the spheroidizing annealing were set according to a type of a steel. For example, a heating temperature of the normalizing and the spheroidizing annealing was set to a temperature of A_c3 point or higher and a temperature at which an undissolved carbide is present. In addition, a tempering temperature was set to be lower than an A_c1 point. A transformation point and an amount of an undissolved carbide vary according to a type of a steel.

[2.2.2. Production of Second Material]

A portion with many coarse foreign matters is in a vicinity of a center where a solidification rate is low. Therefore, as shown in FIG. 13, the first material 12 was cut out from an end of the block material 10 (a = 740 mm, b = 1060 mm, c = 2440 mm, w ≈ 15000 kg) in the c-axis direction. Next, the second material 14 was cut out from substantially the center of the ab surface of the first material 12. In this experiment, d = 35 mm, e = 95 mm, and f = 135 mm were set.

[2.3. Test Method]

[2.3.1. Hardness]

A small piece of 15 mm × 15 mm × 35 mm was cut out from a corner portion of the second material 14 (95 mm × 135 mm × 35 mm). The small piece was ground and polished, and adjusted to have a parallelism and a surface roughness that allow hardness measurement. Using this small piece, a Rockwell B scale hardness was measured at room temperature.

[2.3.2. Impact Test]

Twenty square bars of 12 mm × 12 mm × 55 mm were produced from the second material 14 (95 mm × 135 mm × 35 mm). The obtained square bar was quenched.
In Examples 1 to 13 and Comparative Example 3, the square bar was held in vacuum at 950°C for 1 hour, and then quenched. A cooling rate during the quenching was 8 °C/min from 950°C to 750°C, 5 °C/min from 750°C to 500°C, and 0.5 °C/min from 500°C to 200°C, and a cooling rate from 200°C to 100°C or lower was not particularly controlled.
In Comparative Examples 1 and 2, the square bar was held in vacuum at 1030°C for 1 hour, and then quenched. A cooling rate during the quenching was 8 °C/min from 1030°C to 750°C, and cooling rates at 750°C or lower were the same as in Examples 1 to 13.
The above quenching step is one of examples assuming an inside having a lowest cooling rate in the case where a large mold of 2000 kg or more is quenched. Since phase transformation was substantially completed when a temperature reached 200°C, the cooling rate from 200°C to 100°C or lower was not particularly controlled.
Subsequently, the tempering was performed by holding the square bar at 560°C for 2 hours and then cooling the square bar to 100°C or lower. Further, tempering was additionally performed on the square bar by holding the square bar at 560°C to 600°C and then cooling the square bar to 100°C or lower. This additional tempering was performed once or more, and the square bar was thermally refined to 44.5 HRC to 45.5 HRC. A temperature and a time of the holding, and the number of treatments were changed according to a type of a steel. This is because softening resistance varies in the case where a content of an alloy element varies.
Twenty impact test pieces were produced from the twenty square bars which were thermally refined to about 45 HRC. Further, an impact test was performed at 15°C to 35°C.

[2.3.3. Prior Austenite Crystal Grain Size]

The test piece after the impact test was corroded to expose a prior austenite crystal grain boundary. Further, a metal structure was observed with a microscope to determine a prior austenite crystal grain size.

[2.4. Results]

[2.4.1. Hardness]

In all of Examples 1 to 13 and Comparative Examples 1 to 3, a hardness after SA was 98 HRB or less. It was reconfirmed that when Mn/Cr ≤ 0.150, a steel was sufficiently softened by spheroidizing annealing.

[2.4.2. Impact Test]

Table 2 shows an average impact value, the number of test pieces having the impact value of less than 20 [J/cm²], and a low impact value ratio. In all of Examples 1 to 13, the average impact value was 25 [J/cm²] or more, and the low impact value ratio was 30% or less. In Examples 1 to 13, in addition to high hardenability (a large amount of Mn + Cr), since an amount of C and an amount of V are small, an amount of a coarse foreign matter is small. As a result, an impact value was stable at a high level even when a material cut out from a vicinity of a center of a large-cross-section material having a low solidification rate was subjected to slow quenching. However, a coarse foreign matter derived from C or V was not completely absent, and a foreign matter not including C or V was also present, and thus a test piece having an impact value of less than 20 [J/cm²] was generated at a low probability.
On the other hand, in Comparative Examples 1 to 3, the average impact value was less than 25 [J/cm²], and the low impact value ratio exceeded 30%. In Comparative Examples 1 to 3, in addition to low hardenability (a small amount of Mn + Cr), since an amount of C and an amount of V are large, an amount of a coarse foreign matter is increased. As a result, an impact value was decreased when a material cut out from a vicinity of a center of a large-cross-section material having a low solidification rate was subjected to slow quenching.

Comparative Example 3 is a steel in which the amount of C and the amount of V are increased as compared with Examples 1 to 13. Therefore, in Comparative Example 3, although the hardenability was high, the amount of the coarse foreign matter was increased. As a result, the impact value of Comparative Example 3 was smaller than those of Examples 1 to 13. That is, in a case of a large-cross-section material, it was clear that an impact value cannot be evaluated by hardenability alone, and it was possible to confirm an importance of decreasing an amount of C and an amount of V

Table 2

	Average impact value [J/cm²]	The number less than 20 [J/cm²]	Low impact value ratio [%]
Example 1	30	2	10
Example 2	31	2	10
Example 3	32	1	5
Example 4	29	1	5
Example 5	28	1	5
Example 6	33	3	15
Example 7	29	4	20
Example 8	28	3	15
Example 9	27	2	10
Example 10	28	4	20
Example 11	27	4	20
Example 12	25	6	30
Example 13	25	5	25
Comparative Example 1	16	15	75
Comparative Example 2	18	11	55
Comparative Example 3	21	9	45

[2.4.3. Prior Austenite Crystal Grain Size]

In Examples 1 to 13 and Comparative Example 3, an average value of a prior austenite crystal grain size was 30 µm to 120 µm. On the other hand, in Comparative Examples 1 and 2, an average value of a prior austenite crystal grain size was 25 µm to 75 µm. That is, none of types of steels had a coarse grain structure (a structure having an average grain size exceeding 150 µm) which had an adverse influence on an impact value.
In Examples 1 to 13, since the amount of C and the amount of V are small, crystal grains are likely to be coarsened. However, by optimizing an amount of Al and an amount of N and optimizing a quenching temperature, it was possible to prevent the coarsening of the crystal grains. This point is also considered to contribute to stabilizing the impact values of Examples 1 to 13 at a high level. However, the average value of the prior austenite crystal grain size in Examples 1 to 13 tended to be larger than that of a sample used for evaluation of FIG. 7 (a sample produced from an ingot of 150 kg).

[3. Versatility]

As described above, the verification has been performed assuming a die casting mold, but the present invention can be applied not only to die casting but also to molds and parts used in various types of casting. In addition to casting, the present invention can be applied to molds and parts used for forging, hot stamping, extrusion, injection molding of a resin, blow molding of a resin, molding or processing of a rubber or fiber-reinforced plastic, and the like.
In the above verification, the steel material was quenched from 950°C, tempered at 560°C to 600°C, and thermally refined to about 45 HRC to evaluate the properties, but a steel material adjusted to a wide range of hardness can be applied to molds and parts at a wide range of quenching and tempering temperatures depending on an application.
In the verification of the properties, a molten block material was used, but the steel material according to the present invention may also be used as a powder, a bar material, a wire material, or a plate material.
For example, in the case where the steel material according to the present invention is used as a powder, the present invention can be applied to various sequential forming such as additive manufacturing (a Selective Laser Melting (SLM) method, a Laser Metal Deposition (LMD) method, or the like) and Plasma Powder Welding (PPW).
In the case where the steel material according to the present invention is used as a molten bar material, a mold or a part can be produced from the steel material. Alternatively, in the case where the steel material according to the present invention is used as a molten bar material or wire material, the steel material can be applied to welded additive manufacturing or repair using Tungsten Inert Gas (TIG) welding or laser welding.
In the case where the steel material according to the present invention is used as a plate material, a mold or a part can also be produced by joining the steel materials. Of course, a mold or a part can also be produced by joining members made of the steel material according to the present invention.
As described above, the steel material according to the present invention can be applied to members having various shapes. It is possible to produce or repair a mold or a part using various methods from materials having various shapes made of the steel material according to the present invention.
Although the embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the gist of the present invention.
The present application is based on Japanese Patent Application No. 2022-151231 filed on September 22, 2022 , and the contents thereof are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The steel material according to the present invention can be used as a mold or a part thereof used in various processing such as casting, forging, hot stamping, extrusion, injection molding, and blow molding.

Claims

A steel material comprising:
0.25 mass% ≤ C ≤ 0.37 mass%;

0.08 mass% ≤ V ≤ 0.28 mass%;

6.60 mass% ≤ Mn + Cr ≤ 7.40 mass%;

Mn/Cr≤ 0.150;

Mn > 0.60 mass%;

Cr < 6.60 mass%;

Cu + Ni ≤ 0.84 mass%;

0.40 mass% ≤ Si ≤ 0.90 mass%;

0.60 mass% ≤ Mo ≤ 2.00 mass%;

0.001 mass% ≤ Al ≤ 0.080 mass%; and

0.003 mass% ≤ N ≤ 0.040 mass%,

with the balance being Fe and unavoidable impurities.
The steel material according to claim 1, further comprising:
at least one Group selected from the group consisting of Group A, Group B, Group C, and Group D,

wherein Group A is at least one element selected from the group consisting of 0.30 mass% < W ≤ 2.00 mass%, and 0.30 mass% < Co ≤ 1.00 mass%,

Group B is 0.0002 mass% < B ≤ 0.0080 mass%,

Group C is at least one element selected from the group consisting of 0.006 mass% < S ≤ 0.180 mass%, 0.0005 mass% < Ca ≤ 0.0500 mass%, 0.03 mass% < Se ≤ 0.50 mass%, 0.005 mass% < Te ≤ 0.100 mass%, 0.01 mass% < Bi ≤ 0.50 mass%, and 0.03 mass% < Pb ≤ 0.50 mass%, and

Group D is at least one element selected from the group consisting of 0.004 mass% < Nb ≤ 0.100 mass%, 0.004 mass% < Ta ≤ 0.100 mass%, 0.004 mass% < Ti ≤ 0.100 mass%, and 0.004 mass% < Zr ≤ 0.100 mass%.
The steel material according to claim 1 or 2, having a mass of 3000 kg or more, and having a dimension (L_min), which is minimum among a longitudinal dimension (L₁), a lateral dimension (L₂), and a height dimension (L₃), of 300 mm or more.
A mold being produced from the steel material according to claim 1, 2 or 3 and having a mass of 2000 kg or more.
A method of producing the steel material of one of claims 1 to 3, the method comprising a first step of melting raw materials blended to have the defined composition, refining and casting the molten material; a second step of performing a homogenization treatment on an ingot; a third step of hot working the ingot after the homogenization treatment; and a final step of performing spheroidizing annealing of the rough material.
The method of claim 5, further including a fourth step of normalizing a rough material after the hot working.
The method of claim 5 or 6, further including a fifth step of tempering the rough material.
Use of the steel material according to one of claims 1 to 3, or the steel material obtained according to the method of one of claims 5 to 7, to produce the mold according to claim 4.
Use of the mold according to claim 4, or the mold produced according to claim 8, for casting, forging, hot stamping, extrusion, injection molding, or blow molding.