KR100836699B1 - Die steel - Google Patents

Abstract

This invention is 0.10-0.25 mass% carbon, 1.00% mass or less Si, 2.00 mass% or less Mn, 0.60-1.50 mass% Ni, more than 1.00 mass% 2.50 mass% Cr, Formula (Mo + (W / 2)) at least one of Mo and W satisfying ≤ 1.00 mass%, 0.03 to 0.5 mass% of V (vanadium), 0.50 to 2.00 mass% of Cu, 0.05 mass% or less of S (sulfur), 0.10 A steel for a mold is provided comprising a balance consisting of Al by mass or less, 0.06% by mass or less N (nitrogen), 0.005% by mass or less O (oxygen), and Fe and inevitable impurities. The steel for a mold of this invention has a columnar lower bainite metal structure, and has the hardness of HRC 34-45.

Mold, bainite, forming, bottom

Description

Die Steels {DIE STEEL}

1 is an example of the micrograph which shows the typical metal structure of the steel for metal mold | die of this invention.

2 is an example of a micrograph showing a typical metal structure of a conventional die steel.

3 is an example of a micrograph showing a typical metal structure of a conventional die steel.

3 is an example of a micrograph showing a typical metal structure of a conventional die steel.

The present invention relates to a steel for a mold which has a high hardness and is mainly used as a plastic molding mold and has excellent toughness, machinability, abrasiveness and wear resistance.

Steel for plastic molding dies requires the following characteristics:

(1) the mirror finish workability is good, pinholes and very fine pit should not occur easily;

(2) satinizing workability is good;

(3) good corrosion resistance and rust resistance;

(4) good strength, wear resistance and toughness;

(5) Machinability should be good.

Conventionally, medium C-Mn-Cr-Mo-Fe type, such as JIS SCM440, has been used as the steel for plastic molding dies, but the demand for the above characteristics is increased, and more recently, in order to satisfy the needs of customers, There is a strong need for shortening. In order to meet this demand, to reduce the machining step, for example, a low C-Mn-Cr-Mo-S-Fe-based steel (see JP-A-48-093518) or this steel (ie JP -A-48-093518) modified steel with Ni to improve hardenability (see JP-A-52-065557), and steel containing Cu without adding sulfur to improve machinability (JP- A-58-067850 or JP-A-60-204869) are commonly used.

In addition, low carbon, low sulfur and low Al Mn-Cr-Mo based steels with improved mirror finish machinability, machinability and weldability are known (JP-A-03-115523). In addition, the matrix structure and the precipitates are properly combined, and the hardness is adjusted to about HRC 30 through heat treatment, thereby exhibiting excellent machinability without adding a large amount of cutting elements, and low C-Mn- having excellent rust resistance, abrasion resistance, and abrasiveness. Cr-Mo (W) -V-Cu-Fe alloys or low C-Mn-Ni-Cr-Mo (W) -V-Cu-Fe alloys have been proposed (see JP-A-48-062491).

In recent years, in order to improve strength, abrasion resistance, heat resistance, and the like, engineering plastic materials to which glass fibers, carbon fibers, and the like are added are widely used. Accordingly, the demand for wear resistance, toughness, and machinability for plastic molding die materials has also increased. Accordingly, a low C-Mn-Ni-Cr-Mo-Cu-A1 steel having excellent toughness and machinability is proposed by finely depositing Cu to a hardness level of HRC 40 (see JP-A-02-182860). ). In addition, in order to extend the life of the mold, pre-harden steel for molding low C-Mn-Ni- (Mo, W) -Cu-A1 based plastics adjusted to have a more tough lower bainite structure ( preharden steel) is proposed (see JP-A-07-278737).

The low C-Mn-Cr-Mo-S-Fe-based or low C-Mn-Ni-Cr-Mo-S-Fe-based plastic molding steel can be used to produce large molds having a maximum length of about 2 m. In this case, abrasiveness, abrasion resistance, toughness, and the like are lowered due to segregation of sulfides and the like, so that there is a problem that a satisfactory mold life cannot be obtained. In addition, the low carbon and sulfur-free Mn-Cr-Cu-Fe-based steel disclosed in JP-A-58-067850 has a low softening resistance in quenching and tempering, and thus nitriding at around 550 ° C. Hardness falls when it processes. The low C-Mn-Ni-Cr-Mo (W / 2) -Cu-Fe-based steel disclosed in JP-A-48-093518 has a low carbon content and cannot sufficiently obtain a precipitation strengthening effect. Can't.

In the low carbon, low sulfur, low Al, and low oxygen Mn-Cr-Mo steels disclosed in JP-A-03-115523, the hardness of the base metal is reduced to about HRC 30 to improve machinability, thereby suppressing the formation of nonmetallic inclusions. As a result, mirror finish workability was improved, but hardness was low, and abrasiveness and wear resistance were not satisfactory. Low C-Mn-Cr-Mo (W) -V-Cu-Fe steels, or low C-Mn-Ni-Cr-Mo (W) -V-Cu-Fe steels disclosed in JP-A-07-062491 In the case of manufacturing a large mold, the above requirements for the plastic molding die are satisfied, and sufficient precipitation reinforcement by Cr, Mo (W / 2), Cu, and V has high strength and excellent machinability. . However, it does not satisfy the abrasive and wear resistance of engineering plastic materials widely used.

The low C-Mn-Ni-Cr-Mo-Cu-A1 based steel disclosed in JP-A-02-182860 has a hardness of HRC 40 level by precipitation of intermetallic compounds by A1 and Ni, and has a carbon content. By being adjusted to have a uniform upper bainite structure by lowering it, it has excellent machinability, but there is a problem that sufficient toughness cannot be obtained.

The low C-Mn-Ni- (Mo, W) -Cu-A1 based plastic hardening steel disclosed in JP-A-07-278737 has an upper bainite structure that was considered to be an essential structure for enhancing machinability. Rather, it is fabricated from the lower bainite structure to have both machinability and toughness. In recent years, however, such toughness is not necessarily satisfactory due to the necessity of reducing the use cost of the mold and extending the mold life while maintaining the conventional mold performance.

SUMMARY OF THE INVENTION The present invention has been made in view of the above requirements, and has extremely excellent toughness and machinability, which can extend the life of tools such as molds while maintaining the performance of conventional molds, and have both abrasiveness and wear resistance. To provide a steel for a mold mainly used for plastic molding.

The present inventors have conducted studies on the relationship between chemical composition and metallographic structure, machinability, toughness and abrasiveness, and as a result, by combining appropriately the components, in particular, (l) mutually controlling the content of Ni and Cu, Steel is manufactured to have lower bainite structure rather than upper bainite structure, which was considered essential to improve machinability, and (2) is considered an essential reinforcing mechanism to combine toughness and machinability at HRC 40 hardness. By adopting the precipitation strengthening mechanism that optimally controls Cr, Mo (W / 2), Cu, and V, instead of adopting the precipitation strengthening of the intermetallic compounds of A1 and Ni and the precipitation strengthening of Cu, which are excellent Finding the best steel for molds, which has the toughness, machinability and hardness, and also has excellent abrasive and abrasion resistance. The.

That is, according to the present invention, 0.10 to 0.25% by mass of carbon, 1.00% by mass or less of Si, 2.00% by mass or less of Mn, 0.60 to 1.50% by mass of Ni, and more than 1.00% by mass of 2.50% by mass of Cr, a formula ( At least one of Mo and W satisfying Mo + (W / 2)) ≤1.00% by mass, 0.03 to 0.5% by mass of V (vanadium), 0.50 to 2.00% by mass of Cu, 0.05% by mass or less of S (sulfur ), 0.10 mass% or less Al, 0.06 mass% or less N (nitrogen), 0.005 mass% or less O (oxygen), and the remainder consisting of Fe and an unavoidable impurity, and the primary phase is lower bainite It has a metal structure and is provided with a steel for a mold having a hardness of HRC 34 to 45.

It is preferable that the said steel for metals satisfy | fill the following chemical composition (mass%).

(1)% Ni + 1.2 (% Cu) = 1.30-2.70, and

(2) 60 (% C) +1.5 (% Si) +% Ni + 6 (% Cr) +2 (% Mo + 1/2 (% W)) + 20 (% V) + 0.2% (Cu)

    = 21.00 to 28.70.

In addition, it is preferable that the total amount of at least one of Mo and W is Mo + (W / 2) = 0.10 to 1.00 mass%.

Moreover, it is preferable that the content of the said sulfur is 0.003-0.05 mass%.

The steel for metal mold | die of this invention satisfy | fills one or more of the following.

0.05 mass% or less Al;

More than 0.001 mass% and less than or equal to 0.005 mass% oxygen;

0.60 to 1.20 mass% of Ni;

0.60 to 1.50% by mass of Cu;

0.13-0.20 mass% carbon; And

1.40-2.20 mass% chromium

According to the present invention, by appropriately combining the components, in particular, by adjusting the Ni and Cu content to mutually optimize the structure, and employing a mechanism to enhance the precipitation by optimizing the contents of Cr, Mo (W / 2), Cu, and V By this, even if a large amount of cutting elements such as S (sulfur) is not added, not only excellent toughness, machinability and hardness but also excellent abrasiveness and wear resistance can be imparted. In addition, segregation, which is a problem in the production of a large mold, can be remarkably alleviated by optimizing the content of S (sulfur) (uniform dispersion of sulfide). Since the steel for metal mold | die of this invention has a big temper softening resistance, even if it carries out the nitriding process of the working surface of a metal mold | die, there is little fall of hardness. In addition, since it has sufficient strength and wear resistance, it is particularly effective when applied to large plastic molding molds.

One of the main features of the present invention is the use of a low C-Mn-Ni-Cr-Mo (W) -V-Cu-Fe-based alloy as the steel of the present invention, the constituent elements of the steel, in particular Ni and Cu By optimally adjusting the content, the steel has a metallic structure on the bottom bainite. In addition, by adopting a mechanism that enhances precipitation by optimizing the contents of Cr, Mo (W / 2), Cu, and V, it is possible not only to add a large amount of cutting elements such as sulfur, but also to have excellent toughness, machinability and hardness. It also has excellent abrasive and wear resistance.

As described above, conventional low C-Mn-Cr-Mo (W) -V-Cu-Fe-based alloy steel, C-Mn-Ni-Cr-Mo (W) -V-Cu-Fe-based alloy steel, and The low C-Mn-Ni-Cr-Mo-Cu-A1 alloy steel was adjusted to have an upper bainite structure in order to ensure machinability. However, the upper bainite structure has excellent machinability but low toughness. In order to secure toughness, it was necessary to adjust the hardness to about HRC 30. Therefore, the present inventors adopted a method of adjusting the metal structure to have a lower bainite structure by appropriately combining the compositions, and in particular by mutually controlling the contents of Ni and Cu.

In the case of the conventional die steel, the metal structure was adjusted to become the upper bainite phase, and in order to obtain such a structure, strict control was required in the heat treatment process at the time of manufacture. That is, since the cooling rate had to be finely adjusted, there existed a drawback that the heat processing process is enormously large. However, in the steel of the present invention, by controlling the chemical composition appropriately, the management difficulty in the heat treatment step for achieving the target lower bainite structure is greatly improved. In other words, even if the steel of the present invention is applied to direct quenching after the hot working with a cooling rate equal to or higher than air cooling, the lower bainite structure can be obtained.

In general, the bainite steel structure is one of the transformation products produced when the austenite is cooled, and is a phase produced in the intermediate temperature range between the pearlite formation temperature and the martensite formation temperature. In detail, the phase near the pearlite transformation temperature is in the form of a feather, and the phase near the martensite formation temperature is in the form of a needle. The former is referred to as the upper bainite tissue, the latter as the lower bainite tissue. The lower bainite structure prescribed | regulated by this invention is a structure specifically, for example shown in FIG. And, for comparison, martensite (FIG. 2; low C-Mn-Ni-Cr-Mo (W) -Fe based alloy) of conventional steel, upper bainite structure (FIG. 3; low C-Mn-Ni-Cr -Mo (W) -V-Cu-Fe-based alloy), and upper bainite structure (Fig. 4; low C-Mn-Ni-Cr-Mo-Cu-Al-Fe-based alloy).

In the present invention, the term "main column" refers to the lower bainite structure in consideration of the occurrence of non-uniform structure due to a somewhat non-uniform composition and a slightly non-uniform temperature during the heat treatment (ie, cooling) process for obtaining the structure. Considering some of the upper bainite tissue, martensite tissue, and others that may be included in the lower bainite tissue, in the case of the present invention, in the microscopic field of view according to FIG. If lower bainite structure of more than area% is secured, there is no problem. The amount of lower bainite tissue is preferably at least 75 area%, more preferably at least 80 area%. 1 shows a steel in which the amount of lower bainite tissue is substantially adjusted to 80 area%.

Moreover, in this invention, aiming at uniform dispersion of sulfide is carried out by suppressing sulfur content suitably and adding Cu. The steel of the present invention is quenched to produce a uniform bottom bainite structure. The Fe-Cu solid solution, Cr, Mo (W), and V carbide can be precipitated by applying to high temperature tempering of 550 ° C or higher to obtain hardness of HRC 34 to 45. Further, by tempering, the precipitates are aggregated to improve the strength of the steel, and the steel is appropriately weakened to give extremely good machinability to the steel matrix itself. Therefore, even if the amount of S (sulfur) added in a large amount is usually limited as a means for imparting machinability to steel, extremely excellent machinability can be obtained. The steel of the present invention can be made 0.05% or less even when S is added. Due to segregation of sulfides, it is possible to prevent various problems such as pinhole generation during welding, roughing of the discharge-machined surface, abrasiveness, abrasion resistance and roughing, and use of Cr, Mo, W, Cu, or Ni together. By doing so, excellent corrosion resistance and rust resistance can be obtained.

As mentioned above, the steel of this invention is characterized by reducing the content of sulfur by imparting good machinability to the matrix itself. In the steel of the present invention, Mo and tungsten (W) may be used alone or in combination. In this case, the content of Mo has the same effect as that of W / 2. And Mo and tungsten (W) of the steel of this invention raise the softening resistance at the time of quenching and tempering, it is solid-solution in Fe-Cr oxide film or Cr oxide film of the metal mold | die surface, reinforces a film, and corrosion resistance of a metal mold | die It is also an important element to improve.

The steel of the present invention is supplied in a pre-hardened state (generally, after tempering, tempered at 550 ° C. or higher) having a hardness of HRC 34 to 45, and is directly used for mold forming and finishing polishing. That is, in the tempered state, the steel has good machinability and excellent abrasiveness, so that the heat treatment after the mold forming process is unnecessary. In addition, since the steel of the present invention uses only a small amount of cutting elements such as sulfur, there is no need to worry about remarkable segregation due to the enlargement of the mold made of the steel. Therefore, the steel of the present invention exhibits a great effect not only for a small article mold but also a large mold, for example, a mold having a maximum length of one side of about 2000 mm, and thus can provide a long life without fear of abrasion. have.

Hereinafter, the reason which limited the component of the steel of this invention is described.

Carbon (C) is a basic addition element required for maintaining the quenched structure in the lower bainite structure with good machinability and for tempering to precipitate and strengthen carbides of Cr, Mo (W) and V. If the carbon content is too high, the matrix is transformed into martensite to reduce machinability, and at the same time, excessive carbides are formed to reduce machinability. Therefore, the content of carbon is made 0.25% or less. On the other hand, if the content of carbon is too small, the precipitation of ferrite is caused, so it is made 0.10% or more. Preferred carbon content is 0.13% to 0.20%.

Si is an element which strengthens corrosion resistance in the atmosphere at the time of use of a metal mold | die. An excessively high Si content causes the formation of ferrite, so it is 1.00% or less. In addition, if the content of Si is lowered, the anisotropy of the mechanical properties is reduced, and the stripe segregation can be reduced to obtain excellent mirror surface finish workability. Therefore, the content of Si is made 0.60% or less. On the other hand, the addition amount of silicon (Si) for imparting the corrosion resistance is at least 0.10%, more preferably at least 0.20%.

Mn is an element that enhances the lower bainite quenchability of the steel of the present invention, suppresses the formation of ferrite, and imparts appropriate quenching and tempering hardness. However, when the content of Mn is too high, it is difficult to manage the heat treatment to maintain the lower bainite structure, promote martensite transformation, and also improve the toughness of the matrix, thereby reducing machinability. Therefore, the content of Mn is made 2.00% or less. The amount of Mn added to impart the hardenability is l.00% or more, more preferably 1.20% or more.

Cr is added in order to precipitate and aggregate fine carbide in a tempering process, and to improve the strength of the steel of this invention. In addition, Cr improves the corrosion resistance of the steel of the present invention and suppresses the occurrence of rust during polishing or storage of the mold. In addition, when nitriding is performed, there is an effect of improving the rigidity of the nitride layer. However, if the Cr content is excessively large, the lower bainite structure is refined to promote martensite transformation, and the toughness of the matrix is improved to lower machinability. On the other hand, when Cr content is too small, the effect of the said addition cannot be acquired. Therefore, the content of Cr is more than l.00% and less than 2.50%. The content of Cr is preferably 1.40 to 2.20%, more preferably 1.60 to 2.00%.

As described above, in order to improve the strength of the mold, Cr may be added in a large amount. However, if the Cr content is increased, the machinability is also reduced, and therefore there is a limit to the addition of Cr. Therefore, it is necessary to improve the strength of the mold by a method that does not depend only on the addition of Cr. If the mold is used after nitriding, tempering softening resistance of 550 ° C. or higher should be ensured. In this respect, addition of Cr is not enough. Therefore, in the steel of this invention, in order to solve the said subject, containing of Mo and W becomes important.

Mo and tungsten (W) are added alone or in combination, and precipitate and agglomerate fine carbide during the tempering treatment, thereby improving the strength of the steel of the present invention and increasing the softening resistance in quenching tempering. In addition, since Mo and tungsten (W) are partially dissolved in an oxide film on the surface of the mold, there is also an advantage of improving the corrosion resistance to corrosive gas generated in plastic, for example, during use of the mold. For these applications it is not necessary to add Mo and W in large quantities to the steel. If the content of Mo and W is too large, the machinability is lowered, so that Mo + 1/2 (W) is 1.00% or less. The range of 0.10 to 1.00% is preferable, and the range of 0.10 to 0.70% is more preferable.

Vanadium (V) increases the temper softening resistance, suppresses coarsening of crystal grains, and contributes to improvement of toughness. In addition, vanadium has an effect of finely forming hard carbide and improving wear resistance. For this purpose, the content of vanadium needs to be 0.03% or more, but if it is too large, the machinability deteriorates. Therefore, the content of vanadium is limited to 0.15% or less, preferably 0.05 to 0.12%.

Cu is suitable for depositing and flocculating Fe-Cu solid solution in the tempering treatment of the steel of the present invention. It is to be noted that the structure is adjusted to the lower bainite structure by appropriately adjusting the addition amount of Cu and Ni (to be described later). Due to the precipitation and solidification of these solid solutions and the adjustment of the structure to the lower bainite, the steel of the present invention is endowed with excellent machinability. Moreover, since Cu also has the effect of providing excellent corrosion resistance, it is important to make it 0.50% or more. When the content of Cu is too large, the hot workability is lowered, which also acts on the martensite transformation of the structure, and rather the machinability is reduced. Therefore, the content of Cu is made 2.00% or less, preferably 0.60 to 1.50%.

Ni is an element for improving the lower bainite hardenability of the steel of the present invention and suppressing the formation of ferrite. As described above, by adjusting Ni and Cu appropriately, it is an important element for adjusting the structure to the lower bainite structure, and in order to impart excellent machinability to the steel of the present invention, the content of Ni is made 0.60% or more. An excessively high content of Ni excessively refines the lower bainite structure, promotes martensite transformation, and increases the toughness of the matrix, thereby reducing machinability. Therefore, the content of Ni is made 1.50% or less, and preferably 1.20% or less.

Sulfur (S) exists in the form of MnS which is a nonmetallic inclusion, and has a big effect in improving machinability. However, when a large amount of MnS is present, not only the damage during mold processing, such as pinhole generation during welding, pinhole generation during polishing, roughing of the discharge-machined surface, etc., but also as a starting point for rust, the anisotropy of mechanical properties It is a factor that lowers the performance of the mold itself, such as to encourage it. In particular, in the large mold, the above-mentioned damage due to segregation of MnS becomes remarkable. Therefore, in order to obtain the said effect, it is preferable to make content of S into 0.003% or more, but in order to suppress such a problem, it is necessary to limit to 0.05% or less.

A1 can be generally used as a deoxygenating element during melting, but in the steel state of the present invention, since A1 ₂ O ₃ present in the steel degrades the mirror surface finish workability, the Al content is limited to 0.10% or less. Needs to be. The content of Al is preferably 0.05% or less, more preferably 0.01% or less, and even more preferably 0.002% or less.

Oxygen (O) is an element which forms an oxide in steel, and becomes a factor which remarkably deteriorates cold plastic workability and abrasiveness. In particular, in the present invention, it is important to suppress the formation of the A1 ₂ O _3, and the upper limit of the Al content of 0.005%. Preferably it is 0.003% or less. On the other hand, in order to improve polishing property, it is also preferable to limit it to 0.001% or less, for example. However, in the present invention in which A1 ₂ O ₃ should be reduced, since the content of Al is already controlled low, it is not necessary to particularly strictly control the content of O to a low range. Therefore, it is allowed that the content of Al exceeds 0.001%.

Nitrogen (N) is an element which forms nitride in steel. If the nitride is excessively formed, the toughness, machinability and abrasiveness of the mold are significantly degraded. Therefore, it is preferable to limit nitrogen (N) in the steel to a low level, and in the present invention, the content of nitrogen (N) is limited to 0.06% or less. The content of nitrogen (N) is preferably 0.02% or less, more preferably 0.005% or less.

In order to realize the structure which consists of substantially the lower bainite phase aimed at this invention, and to make both the machinability and toughness of the steel of this invention excellent, the composition range of the said component is more effective, precise, and clear. Should be limited.

More specifically, as a result of repeating studies in the above-described areas of the basic components of the present invention, the present inventors found a precise component range as shown in the following formula.

Equation 1: (% Ni) +1.2 (% Cu) = 1.30 to 2.70, and

Equation 2: 60 (% C) +1.5 (% Si) +% Ni + 6 (% Cr) +2 (% Mo + 1/2 (% W) (alone or in combination)) + 20 (% V) + 0.2% (Cu) = 21.00-28.70.

More specifically, the object of the steel of the present invention is that the structure consists substantially of the lower bainite phase. Equation 1: If the value by (% Ni) +1.2 (% Cu) is less than 1.30, ferrite and upper bainite tissue may be produced, and when the value exceeds 2.70, the lower bainite tissue may be refined and martensite Zites can be generated. In addition, the value according to Equation 1 satisfies 1.3 to 2.70, and Equation 2: 60 (% C) +1.5 (% Si) +% Ni + 6 (% Cr) +2 (% Mo + 1/2 (% W). When the value by + (alone or in combination)) + 20 (% V) + 0.2% (Cu) is less than 21.00, it is difficult to achieve a predetermined hardness or the toughness tends to be lowered, and the value is 28.70 When exceeding, machinability falls and toughness tends to decrease.

In this invention, a toughness improving element and a machinability improving element can be added further within the range which does not impair the said effect. For example, as the toughness improving element, one or more of Nb: 0.5% or less (preferably 0.01 to 0.1%), Ti: 0.15% or less, Zr: 0.15% or less, and Ta: 0.15% or less can be added. As the machinability improving element, Zr: 0.003-0.2%, Ca: 0.0005-0.01%, Pb: 0.03-0.2%, Se: 0.03-0.2%, Te: 0.01-0.15%, Bi: 0.01-0.2%, In: One or more of 0.005 to 0.5% and Ce: 0.01 to 0.1% can be added. In addition, Y, La, Nd, Sm and other REM elements may be contained in 0.0005 to 0.3% of the total.

First, in a 30 kg high frequency vacuum melting furnace, various kinds of steels having a composition composed of the balance of iron and unavoidable impurities shown in Table 1 were dissolved (the values represented by Formulas 1 and 2 of the present invention are shown in Table 2). Subsequently, each of these steels was forged into a bar shape of 50 mm x 100 mm, and then heat-treated, and the following evaluation was performed. The heat treatment was carried out in the following manner to obtain the desired hardness: Each steel was heated to an austenite region at 900 ° C., maintained at this temperature for 1 hour, and the actual ingot was cooled at the assumed cooling rate. The steel was then tempered at an appropriate temperature between 520 ° C. and 590 ° C. for 1 hour and then air cooled.

TABLE 1

Machinability was evaluated through the drill processing test. Specifically, with a φ2 mm drill made of high speed steel, the maximum of the outer peripheral end of the tool after machining 50 holes in the specimen steel at a cutting speed of 15 m / min, a feed rate of 120 mm / min, and a machining hole depth of 20 mm. The wear width was measured.

The toughness was evaluated by Charpy impact test using a 2 mm U-notch specimen (JIS No. 3 specimen), and measured the Charpy impact value at room temperature.

Abrasiveness was evaluated in the following manner: after preparing specimens with an evaluation surface of 50 mm ² , the hardness was adjusted by quenching and tempering under the above heat treatment conditions, and then each specimen was mirrored in a grinder → paper → diamond compound method. Cotton processed. Subsequently, use a magnifying glass (10x) to count the number of minute pits generated, A for specimens with fewer than six pits, B for 6-10 pits, C for 11 to 20 pits, and 20 pitches. When exceeded, it was set to D. The results are shown in Table 2. The tissue of each specimen shown in Table 2, excluding specimens 16 to 18, was substantially a single phase tissue, with the phases shown in Table 2 accounting for at least 90 area percent.

TABLE 2

Invention steel

Specimens 1 to 12, which satisfy the chemical composition of the present invention, are mold steels consisting substantially of a lower bainite single phase, and satisfy the hardness of the present invention because the values of Equations 1 and 2 are within the above range. The machinability of each of these specimens shows the best results with a maximum wear width of 0.16 mm or less at the outer periphery of the tool. In addition, the toughness shows a very good result of about 60 J / cm ² or more, and the polishing property is also good.

Comparative Example  River

Specimen 13, which has less carbon (C) than the component range of the present invention, has a top bainite structure since the value of Equation 1 is less than 1.30, and is low in hardness, so that the machinability and toughness are good, but the polishing property is not sufficient. . Specimens 14 high in Si, specimen 16 low in Ni, and specimen 18 low in Cr satisfy the hardness of the present invention because the values of Equations 1 and 2 are within the scope of the present invention, Since ferrites of at least area% are mixed, the machinability and the abrasiveness are rather low.

Specimen 15 with sulfur exceeding 0.05% is a lower bainite structure that satisfies the hardness of the present invention, and since a large amount of sulfide-based nonmetallic inclusions is contained, the machinability is also the best. However, the toughness is low because the value of Equation 2 exceeds 28.70. In addition, sulfides are much softer than the matrix, and pits are easily generated from sulfides during polishing, resulting in poor polishing. Ni-rich specimen 17 has a texture in which martensite (approximately 60 area%) is mixed with the excessively micronized lower bainite structure because the value of Equation 1 exceeds 2.70. Thus, the specimen has good abrasiveness and good toughness, but somewhat poor machinability.

Specimen 19 containing less Cr and a large amount of N than the component range of the present invention has excessively fine martensite. Specimen 20, which is high in Mo and contains a large amount of oxygen (O), is substantially the lower bainite single phase. However, these specimens have a slightly lower machinability because the value of Equation 2 exceeds 28.70 and the carbides are contained in the tissue. In addition, these specimens have significantly lower abrasive properties because specimen l9 contains excess nitride and specimen 20 contains excess oxide.

Specimens 21-24 are practically a lower bainite single phase. However, Specimen 21, which has less vanadium than the component range of the present invention, has a somewhat low hardness because the value of Equation 2 is less than 21.00, and thus has good machinability but somewhat low abrasive property. On the other hand, Specimen 22, which contains a lot of vanadium (V), does not satisfy the region defined in the present invention by the values of Equations 1 and 2, and contains a lot of carbides, so that the machinability is somewhat low. In addition, the specimen 23 containing a lot of Cu has a value of Equation 1 exceeding 2.70, and the microstructure has maintained an excessively fine lower bainite structure, but the machinability is somewhat inferior. Specimen 24 with high A1 satisfies the region defined in the present invention by the value of Equation 2, but reduced toughness due to precipitation strengthening of the intermetallic compound of A1 and Ni. In addition, there is a tendency to contain a lot of nitride, and the polishing property is also somewhat low.

Conventional steel

Specimens 25 of all conventional steels do not satisfy the component ranges of the invention of Formulas 1 and 2, and have more carbon (C) and less vanadium (V) and Cu than the component ranges of the invention. Although the hardness is satisfied, it is a martensite microstructure, which has good toughness and abrasiveness but low machinability. Specimens 26 with little Cu and specimens 27 with little Ni satisfy the hardness of the present invention and have good abrasiveness, but have poor upper toughness and lower machinability because they have an upper bainite structure.

Specimen 28, which has less Ni and Cu than the component range of the present invention, has an upper bainite structure and has low hardness, and thus has good machinability and toughness. However, the specimens are not abrasive enough. Specimen 29, which contains many Ni and A1 and has little Cr, is an upper bainite structure satisfying the hardness of the present invention. In addition, since the specimen adopts both the precipitation strengthening of the intermetallic compound of A1 and Ni and the precipitation strengthening of Cu, the machinability and abrasiveness are best, but the toughness is remarkably low.

The steel of the present invention, which has excellent toughness not found in conventional plastic-molded pre-hardened steels, is hard to generate cracking due to thermal stress when processing or processing a mold, so that more precise mold processing can be performed. It is particularly suitable for the manufacture of molds.

Claims (7)

0.10-0.25 mass% carbon, 1.00% mass or less Si, 2.00 mass% or less Mn, 0.60-1.50 mass% Ni, more than 1.00 mass% 2.50 mass% Cr or less, Formula (Mo + (W / 2)) At least one of Mo and W satisfying ≤1.00% by mass, 0.03 to 0.5% by mass of V (vanadium), 0.50 to 2.00% by mass of Cu, 0.05% by mass or less of S (sulfur), 0.10% by mass or less Remainder consisting of Al, 0.06 mass% or less N (nitrogen), 0.005 mass% or less O (oxygen), and Fe and inevitable impurities It includes, the primary phase (primary phase) has a lower bainite metal structure, characterized in that the mold for steel having a hardness of 34 to 45 HRC. The method of claim 1, A steel for a mold, characterized by satisfying the following chemical composition (mass%): (1)% Ni + 1.2 (% Cu) = 1.30-2.70, and (2) 60 (% C) +1.5 (% Si) +% Ni + 6 (% Cr) +2 (% Mo + 1/2 (% W)) + 20 (% V) + 0.2% (Cu)     = 21.00 to 28.70. The method of claim 1, Mold steel, characterized in that the total amount of at least one of Mo and W is Mo + (W / 2) = 0.10 to 1.00% by mass. The method of claim 1, The content of the sulfur is 0.003 to 0.05% by mass of steel for a mold. The method of claim 1, Al for 0.05 mass% or less, 0.001 mass% or more 0.005 mass% or less, The steel for metal molds characterized by the above-mentioned. The method of claim 1, 0.60 to 1.20% by mass of Ni, 0.60 to 1.50% by mass of Cu, the die steel. The method of claim 1, 0.13-0.20 mass% carbon and 1.40-2.20 mass% Cr, The steel for metal mold | die characterized by the above-mentioned.

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