WO2011102402A1

WO2011102402A1 - Steel for molds with excellent hole processability and reduced processing deformation, and method for producing same

Info

Publication number: WO2011102402A1
Application number: PCT/JP2011/053327
Authority: WO
Inventors: 隆一朗菅野; 英司中津
Original assignee: 日立金属株式会社
Priority date: 2010-02-18
Filing date: 2011-02-17
Publication date: 2011-08-25
Also published as: JP5668942B2; JPWO2011102402A1; CN102770566A; KR20120106900A; DE112011100602T5

Abstract

Provided is steel for molds that has excellent cuttability, that particularly allows small-diameter holes to be formed therein, and in which deformation due to processing has been reduced. Also provided is a method for producing the steel for molds. The disclosed steel for molds has a composition comprising, in mass%: 0.15-0.30% of C; 1.0% or less of Si; 2.0% or less of Mn; 0.6-1.5% of Ni; greater than 1.0% to 2.0% of Cr; 1.2% or less of Mo or W alone, or Mo+1/2W in combination; 0.03-0.15% of V; 0.1% to less than 0.5% of Cu; 0.05% or less of S; 0.1% or less of Al; 0.06% or less of N; 0.005% or less of O; and Fe and inevitable impurities as the remainder. In said steel, the value found from formula (1), [%Ni]+1.2[%Cu], is from 0.70 to 1.80 in mass%, and the sectional structure includes 50 area% or more of lower bainite. The steel has a hardness of 34 to 45 HRC. The disclosed method for producing the steel for molds involves adjusting said sectional structure and said hardness by quenching and by tempering at 570°C or higher.

Description

Die steel excellent in hole workability and suppression of machining strain and method for producing the same

The present invention relates to a mold for plastic molding, for example, and relates to a mold steel that is most suitable for mold parts such as a nest that constitutes a cavity portion and a holder that holds the nest, and a manufacturing method thereof.

Conventionally, in a mold for molding plastics, etc.,
(1) Good mirror finish and low tendency to generate pinholes and other fine pits.
(2) Good texture processing,
(3) Good corrosion resistance and rust resistance,
(4) Good strength, wear resistance, toughness,
(5) Good machinability,
Etc. are required.

Among these, excellent machinability, that is, improvement of the cutting tool life, is an important improvement characteristic for the recent demand for reduction in mold production cost. Accordingly, the applicant of the present application has adopted a low C—Mn—Ni—Cr—Mo (adopting precipitation strengthening by optimum adjustment of Cr, Mo (1/2 W), Cu, and V for the purpose of improving the machinability. W) -V-Cu-Fe-based mold steel was developed (see Patent Document 1).

Japanese Patent No. 4269293

The steel for molds of Patent Document 1 whose structure is mainly controlled by lower bainite is excellent in machinability. And in this improvement of machinability, the lower bainite of the prehardened structure after tempering (quenching and tempering) by mutual adjustment of Ni and Cu greatly contributes.

By the way, in recent years, with the increase in size of the mold, the steel material used for this is required to have a very large cross-sectional dimension. And the increase in size of the mold steel is one factor that slows the cooling rate during quenching. Therefore, the lowering of the cooling rate hinders the formation of lower bainite, and the steel for molds of Patent Document 1 is excellent in lower bainite hardenability. For example, quenching with a slow cooling rate such as direct quenching after hot working is performed. Even so, a sufficient amount of lower bainite can be obtained.

However, even in the steel for molds of Patent Document 1 which is excellent in lower bainite hardenability, a sufficient amount of lower bainite is not formed uniformly even if the cooling rate is decreased (the formation is unevenly formed). If so, the toughness and machinability of the mold steel will be reduced. Among various types of cutting, drilling with drills, etc., especially when drilling with a small diameter of φ5 mm or less, allows smooth discharge of chips generated during machining in mold steel with non-uniform lower bainite. As a result, the tool life is shortened.

Also, with the increase in size of the mold, the processing area for the mold steel increases, and the mold carving depth also increases. Therefore, in the steel for mold before processing, for example, if the residual stress introduced at the time of quenching is distributed remarkably and non-uniformly, the residual stress is partially released at the time of die engraving. There is a concern that processing distortion may occur in the mold. Therefore, in order to suppress this processing strain, it is effective to reduce the residual stress in the mold steel.

Accordingly, an object of the present invention is to provide excellent machinability, particularly φ5 mm or less, because even a die steel with a large cross-sectional dimension can maintain a uniform lower bainite structure and reduce residual stress. It is intended to provide a die steel that achieves a hole workability that can cope with small-diameter hole machining and a reduction in machining strain.

First, in order to maintain excellent machinability and toughness, the present inventor examined a method capable of compensating a sufficient and uniform lower bainite structure even when the cooling rate during quenching described above is slow. And even if residual stress that may cause processing strain is introduced during the quenching, a method that can efficiently reduce the residual stress later was studied. As a result, it was found that these techniques can be achieved together by improving the component composition based on the component composition of the mold steel of Patent Document 1, and reached the mold steel of the present invention.

That is, the present invention is, in mass%, C: 0.15 to 0.30%, Si: 1.0% or less, Mn: 2.0% or less, Ni: 0.6 to 1.5%, Cr: 1 More than 0.0 to 2.0%, Mo and W alone or in combination (Mo + 1 / 2W): 1.2% or less, V: 0.03 to 0.15%, Cu: 0.1 to 0.5% Less than, S: 0.05% or less, Al is controlled to 0.1% or less, N is controlled to 0.06% or less, O is controlled to 0.005% or less, and the balance is composed of Fe and inevitable impurities. The composition is expressed by mass%. Formula 1: [% Ni] +1.2 [% Cu] has a value of 0.70 to 1.80, and the cross-sectional structure is 50% by area of the lower bainite. As described above, the steel for molds is excellent in hole workability and processing strain suppression, characterized by having a hardness of 34 to 45 HRC.

Furthermore, the mold steel of the present invention has the following composition by mass%: (1) C: 0.17 to 0.25%, Cr: 1.2 to 1.8%, (2) Ni: 0.00. 6 to 1.2%, Cu: 0.1 to 0.45%, (3) Formula 1: 1 of [% Ni] +1.2 [% Cu] is 1.10 to 1.60 It is preferable to satisfy one or more conditions.

Further, the mold steel of the present invention preferably has the above (Mo + 1 / 2W) of 0.2 to 1.0%, or more preferably S of 0.005 to 0.05%. And the composition of the steel is expressed by the formula 2 expressed by mass%: 60 [% C] +1.5 [% Si] + [% Ni] +6 [% Cr] +2 [% Mo + 1/2% W (single or composite)] The value of +20 [% V] +0.2 [% Cu] is preferably 16.20 to 38.10.

The present invention is characterized in that a steel having the above composition is quenched and tempered at 570 ° C. or higher, thereby adjusting the cross-sectional structure to 50 area% or lower of the lower bainite and the hardness to 34 to 45 HRC. This is a method for producing steel for molds excellent in suppressing hole workability and processing strain. In the quenching, the cooling rate in the range from 450 ° C. to 400 ° C. is preferably 3 ° C./min or more. Alternatively, the heating temperature is preferably 850 to 1050 ° C.

According to the present invention, the structure and the tempering properties described later are optimized by an appropriate combination of the component composition, in particular, mutual adjustment of C, Cr, Ni, and Cu, so that it has high hardness and excellent toughness. In addition, it is possible to provide a mold steel that has both machinability and ability to suppress processing strain and can be easily applied to a large mold.

It is a microscope picture which shows an example of the cross-sectional structure of this invention steel. It is a schematic diagram of the micrograph of FIG. It is the figure which showed the drill life with respect to Cu amount of this invention steel and comparative steel which were performed in the Example, and is a figure explaining an example of the effect of this invention.

The feature of the present invention is that for the steel for molds of Patent Document 1 excellent in bainite hardenability, the readjustment of the constituent element species, in particular, the relationship between the amounts of C, Cr, Ni, and Cu has been reviewed. The present inventors have found an optimum component composition that can improve the lower bainite uniformity of the structure and can cope with the reduction of residual stress that the structure can have.

The mold steel of the present invention is assumed to be so-called pre-hardened steel which is supplied with a predetermined tempered hardness and is used after being engraved and polished after being tempered. And the tempered hardness is adjusted by a series of heat treatments by quenching and generally tempering at 550 ° C. or higher, which includes high hardness for obtaining a mirror finish and high toughness and From the viewpoint of achieving both low hardness for obtaining machinability (tool life), it is preferably in the range of 34 to 45 HRC.

And in said heat processing, the structure | tissue of this invention steel is controlled to a lower bainite in the hardening process. That is, in order to achieve both excellent toughness and machinability, the structure mainly composed of lower bainite, which is also adopted in the steel for molds of Patent Document 1, is adopted in the present invention. At this time, the lower bainite occupying the entire structure is 50 area% or more in the cross-sectional structure. Preferably it is 60 area% or more, More preferably, it is 70 area% or more. FIG. 1 is a photomicrograph showing an example of a cross-sectional structure of the steel of the present invention, and FIG. 2 is a schematic diagram thereof. In the case of FIG. 1, the area ratio of the lower bainite in the cross-sectional structure is 85%.

Here, even if it is steel for metal mold | die of patent document 1, the lower bainite hardenability is excellent, for example, even if it is quenching with slow cooling speeds, such as direct quenching by air cooling, sufficient amount of lower bainite. It is possible to get However, as the mold steel size increases, the cooling rate during quenching becomes further slower, and if the lower bainite becomes non-uniform, it leads to a decrease in machinability as described above. It is. Therefore, in the present invention, the lower bainite uniformity of the steel of the present invention could be improved by reviewing the component composition of the steel for molds of Patent Document 1 as described below.

And even if the structure can be adjusted to the lower bainite by the above quenching, if a non-uniform residual stress is introduced into the steel at the same time, it can lead to the generation of processing strain during the die processing. Therefore, in order to suppress this processing strain, it is effective to reduce the residual stress in the steel material, but it does not reduce the residual stress itself introduced at the time of quenching, but after the introduction. We studied a method that can effectively remove residual stress in steel. In other words, this is the use of a tempering process performed after quenching, and if the residual stress can be reduced or removed during heating and holding at this time, it is possible to suppress the occurrence of processing strain accompanying the increase in size of the mold steel. .

However, in order to maintain the working hardness of the mold, the above-mentioned residual stress is removed while the general tempering temperature applied to the mold steel is set from around 550 ° C. The effective heating / holding temperature is 570 ° C. at the lowest, and a high temperature range of 580 ° C. or more and even 600 ° C. is required to exhibit the remarkable removal effect. That is, a simple increase in the tempering temperature for removing the residual stress, on the other hand, leads to softening of the mold steel. Therefore, the present invention has excellent temper softening resistance that can maintain the hardness of 34 to 45 HRC with good reproducibility even when heated at least 570 ° C., preferably 580 ° C. or more, in accordance with excellent lower bainite uniformity ( Aimed at steel for molds that can be tempered at high temperature. And it came to this achievement by reviewing the component composition of the steel for metal mold | die of patent document 1 as follows. Hereinafter, the reason for limiting the components of the steel of the present invention will be described.

・ C: 0.15-0.30%
C is necessary to keep the quenched structure in a lower bainite structure with good machinability (if it is too low, it will lead to precipitation of ferrite), and in tempering, it will be strengthened by the precipitation of Cr, Mo (W), V carbides. Is a basic additive element. And it is an element required only for providing the temper softening resistance which is the characteristic of this invention. Therefore, it is important that C is 0.15% by mass or more. If the amount is too large, the base is martensite-organized and excessive carbides are formed to reduce the machinability, so the amount is 0.30% by mass (hereinafter simply referred to as%) or less. Preferably, the content is 0.17% or more and / or 0.25% or less.

Si: 1.0% or less Si is an element that enhances the corrosion resistance to the atmosphere when using the mold, but if it is too much, it will lead to the formation of ferrite, so 1.0% or less. Further, when Si is reduced, the anisotropy of the mechanical properties is reduced, and stripe segregation is reduced to obtain excellent mirror workability. Therefore, the Si content is preferably 0.6% or less. In addition, in providing said corrosion resistance, it is preferable to add 0.1% or more, further 0.2% or more.

Mn: 2.0% or less Mn is an element that enhances the lower bainite hardenability and suppresses the formation of ferrite to impart moderate quenching and tempering hardness. However, if the amount is too large, the heat treatment management for maintaining the lower bainite structure becomes severe, and martensitic transformation is promoted. Moreover, since the machinability is lowered by increasing the viscosity of the base, the content is made 2.0% or less. In addition, in providing said hardenability, it is preferable to add 1.0% or more, and also 1.2% or more.

・ Ni: 0.6-1.5%
Ni is an element that enhances the lower bainite hardenability of the steel of the present invention and suppresses the formation of ferrite. And it is an important element which controls a structure | tissue to lower bainite by adjustment of the appropriate addition amount with Cu mentioned later, and this provides the machinability excellent in this invention steel. However, if the amount is too large, the lower bainite structure is excessively refined, the martensitic transformation is promoted, the base viscosity is increased, and the machinability is lowered. Therefore, Ni is set to 0.6 to 1.5%. Preferably it is 1.2% or less.

・ Cr: more than 1.0 to 2.0%
Cr is added to precipitate and agglomerate fine carbides during the tempering treatment and form the strength of the steel of the present invention. In addition, the corrosion resistance of the steel of the present invention is enhanced to suppress rusting during polishing or during mold storage. Further, when nitriding is performed, it has the effect of increasing the hardness of the nitrided layer. Therefore, it adds exceeding 1.0%. However, if the amount is too large, the effect of refining the lower bainite structure promotes martensitic transformation, increases the base viscosity, and reduces machinability. And, in order to reduce the temper softening resistance, which is a feature of the present invention, it is important to make it 2.0% or less. Preferably it is 1.2% or more and / or 1.8% or less.

Mo and W are single or composite (Mo + 1 / 2W): 1.2% or less Mo and / or W precipitate and agglomerate fine carbides during the tempering treatment, and improve the strength of the steel of the present invention. In consideration of the reduction of residual stress in the steel material, which is a feature of the present invention, and the case where the mold is subjected to nitriding treatment, it is a useful element for increasing the softening resistance in quenching and tempering. Furthermore, Mo and W are partly dissolved in the oxide film on the mold surface, thereby improving the corrosion resistance against corrosive gas generated from, for example, plastic during use of the mold. There is also. However, if the amount is too large, the machinability is deteriorated. Therefore, (Mo + 1 / 2W) is set to 1.2% or less. And preferably it is 0.2% or more and / or 1.0% or less. More preferably, it is 0.3% or more and / or 0.9% or less.

・ V: 0.03-0.15%
V increases the resistance to temper softening and suppresses the coarsening of crystal grains, thereby contributing to the improvement of toughness. In addition, there is an effect of improving the wear resistance by forming hard carbide finely. For this purpose, at least 0.03% or more is required, but if it is too much, the machinability is reduced, so the content was made 0.15% or less. Preferably they are 0.05% or more and / or 0.12% or less.

Cu: 0.1 to less than 0.5% Cu precipitates or agglomerates the Fe—Cu solid solution in the tempering treatment of the steel of the present invention as in the steel for molds of Patent Document 1. And a structure | tissue is controlled to a lower bainite by adjustment of the appropriate addition amount with Ni mentioned above. Combined with the precipitation or solidification of these solid solutions and the structure control of the lower bainite, the steel of the present invention is imparted with excellent machinability. Cu also has an effect of providing excellent corrosion resistance. However, if the amount is too large, in addition to lowering hot workability, it also works on the martensitic transformation of the structure, which in turn reduces machinability.

And it should be noted that the Cu content is greatly related to the non-uniformity of the lower bainite, which is the subject of the present invention. That is, in the process of bainite transformation by quenching, if the cooling rate at that time becomes slow, transformation to upper bainite occurs at a relatively high temperature, and therefore Cu precipitates at the interface between the transformed region of upper bainite and untransformed austenite. . Thereby, since the transformation to the upper bainite further proceeds around the Cu precipitate, it becomes difficult to obtain a uniform lower bainite. And since precipitation of this Cu is accelerated | stimulated, so that Cu content in steel increases, this promotes the nonuniformity of a lower bainite. Then, when the content of Cu was reviewed based on this knowledge, it was found that the structure could be controlled to lower bainite even if Cu was less than 0.5% by adjusting the relational expression with Ni described later. Therefore, the Cu of the steel of the present invention is 0.1 to less than 0.5%. Preferably it is 0.25% or more and / or 0.45% or less.

S: 0.05% or less S is present in the structure as MnS of non-metallic inclusions, and has a great effect on improving the machinability. However, the presence of a large amount of MnS becomes a factor that degrades the performance of the mold itself, such as promoting mechanical properties, particularly toughness anisotropy. Therefore, S of the steel of the present invention that has excellent toughness, machinability and hardness, and also achieves excellent polishing finish needs to be limited to 0.05% or less. In order to improve machinability, the content is preferably 0.005% or more, more preferably 0.01% or more.

-Al: 0.1% or less Al is usually used as a deoxidizing element at the time of melting, but in the steel of the present invention in a state after tempering, Al2O3 present in the steel has mirror surface workability. Since it decreases, it is necessary to regulate to 0.1% or less. Preferably it is 0.05% or less. And it is more preferably 0.01% or less, and still more preferably 0.005% or less.

N: 0.06% or less N is an element that forms nitrides in steel. If the nitride is excessively formed, the toughness, machinability and polishability of the mold are significantly deteriorated. Therefore, it is preferable to regulate N in steel low, and in the present invention, it is specified to be 0.06% or less. Preferably it is 0.02% or less, More preferably, it is 0.015% or less.

O: 0.005% or less O (oxygen) is an element that forms an oxide in steel, and causes a significant deterioration in cold plastic workability and polishability. In particular, in the present invention, since it is important to suppress the formation of the Al2O3, the upper limit of O is set to 0.005%. Preferably it is 0.003% or less. In order to improve the polishability, it is also a desirable condition that the regulation is controlled to a lower level, for example, 0.001% or less. Therefore, there is no particular requirement for the low amount management of the O amount itself. Therefore, exceeding 0.001% is sufficiently acceptable.

Formula 1: The value of [% Ni] +1.2 [% Cu] is 0.70 to 1.80
In the steel of the present invention, even if the content of Cu is reduced in order to reduce non-uniformity of the lower bainite, the value of the following formula 1: [% Ni] +1.2 [% Cu] by mass% is reduced to 0. If it is secured to .70 to 1.80, a sufficient amount of the lower bainite structure itself can be realized, and the machinability, toughness and hardness can be combined at a high level. That is, if the value of Formula 1 is less than 0.70, ferrite and upper bainite are likely to be generated. If the equivalent value is greater than 1.80, excessively refined lower bainite and martensite are likely to be generated. Preferably it is 1.10 or more and / or 1.60 or less.

Here, in order to more reliably suppress the formation of the upper bainite and to set the area ratio of the lower bainite in the structure to 50% or more, the value according to the above formula 1 is controlled to 0.70 or more, and further The cooling range from 450 ° C. to 400 ° C. during quenching is preferably a cooling rate of 3 ° C./min or more. The mold steel of the present invention has a component composition in which upper bainite is more easily generated than the mold steel of Patent Document 1 by reducing the amount of Cu. On the other hand, the component composition has a quenching characteristic in which a bainite nose is located in a range of 450 to 400 ° C. in an isothermal transformation diagram. Therefore, at the time of quenching the mold steel of the present invention, the above temperature range is allowed to pass at a high cooling rate, so that the formation of upper bainite can be suppressed even in the mold steel having a Cu amount of 0.1%. . And it becomes easier to achieve the amount of lower bainite of 60 area% or more, further 70 area% or more by this quenching. The heating temperature during quenching is preferably 850 to 1050 ° C. at which sufficient austenitization can be achieved.

Formula 2: 60 [% C] +1.5 [% Si] + [% Ni] +6 [% Cr] +2 [% Mo + 1/2% W (single or combined)] + 20 [% V] +0.2 [% The value of Cu] is 16.20 to 38.10.
In the case of the steel of the present invention, the following formula 2: 60 [% C] +1.5 [% Si] + [% Ni] +6 [% Cr] +2 [% Mo + 1/2% W (independent) introduced in Patent Document 1 Alternatively, the range of 16.20 to 38.10 is acceptable for the value of compound)) + 20 [% V] +0.2 [% Cu]. However, in order to achieve high hardness and excellent toughness and machinability, it is preferable to satisfy the range of 21.00 or more and / or 28.70 or less. A range of 23.00 or more and / or 28.00 or less is more preferable.

In the present invention, further toughness improving elements and machinability improving elements can be added as long as the above-described effects are not impaired. For example, as a toughness improving element,
Nb: 0.5% or less (preferably 0.01% or more and / or 0.1% or less),
Ti: 0.15% or less (preferably 0.01% or more),
Zr: 0.15% or less (preferably 0.01% or more),
Any one or more of Ta: 0.15% or less (preferably 0.01% or more) can be added. As a machinability improving element,
Zr: 0.2% or less (preferably 0.003% or more),
Ca: 0.01% or less (preferably 0.0005% or more),
Pb: 0.2% or less (preferably 0.03% or more),
Se: 0.2% or less (preferably 0.03% or more),
Te: 0.15% or less (preferably 0.01% or more),
Bi: 0.2% or less (preferably 0.01% or more),
In: 0.5% or less (preferably 0.005% or more),
Any one or more of Ce: 0.1% or less (preferably 0.01% or more) can be added. Furthermore, Y, La, Nd, Sm, and other REM (rare earth) elements may be contained in total of 0.3% or less (preferably 0.0005% or more).

The mold steel of the present invention is excellent in suppressing processing strain, which can maintain a hardness of 34 to 45 HRC even when the tempering temperature during tempering is 570 ° C. or higher. However, the above tempering temperature is preferably 650 ° C. or lower in order to more reliably maintain a hardness of 34 HRC or higher.

Table 1 shows the chemical composition of the steel of the present invention and the comparative steel. Sample No. which is a comparative steel. Nos. 11 to 15 are steels for metal molds disclosed in Patent Document 1. Sample No. 10 is a sample No. 10 of the steel of the present invention. 3 is a mold steel having the same composition as that of No. 3.

Regarding the preparation of samples by these, first, a steel ingot melted 10 kg at a time in a vacuum induction melting furnace was subjected to homogenization heat treatment at 1250 ° C. for 5 hours, and then hot forged at 1100 ° C. to obtain a thickness of 30 mm × A steel material having a width of 60 mm was produced. Next, after annealing at 860 ° C., quenching was performed at 900 ° C. Quenching is performed by pressurized gas cooling, and the time required for cooling from the quenching temperature (900 ° C.) to the intermediate temperature (450 ° C.) between the quenching temperature and room temperature (20 ° C.) is expressed as “half-cooling time ( When it is defined as “semi-cooled ** minutes”, it was cooled in about half-cooled 150 minutes as a part corresponding to a part where the cooling rate becomes slow like the center part of a large-sized steel material. The cooling rate in the range of 450 to 400 ° C. is the sample No. All were controlled at 3 ° C./min or more except that 10 was set to about 1 ° C./min. The sample temperature was measured by a thermocouple attached in the vicinity of the center of the sample. In tempering, the target hardness was 37 to 41 HRC, and the mixture was heated for 2 hours at an appropriate temperature in the range of 550 ° C. to 650 ° C. and then air-cooled (at this time, the hardness of sample No. 10 was significantly lower than 37 HRC).

And each sample after tempering was subjected to the following drilling test to evaluate their machinability. That is, drilling was performed with a JIS-SKH51 high-speed steel φ1 mm drill under the conditions of cutting speed: 20 m / min, feed rate: 31.83 mm / min, and drilling hole depth: 10 mm. The number of holes until the drill broke was defined as the tool life, and the value was measured.

Thereafter, the hardness of the drill incident surface and the microstructure observation of the sample subjected to the drilling test were performed. Microscopic observation is performed by finishing the drill incident surface to a mirror surface by buffing 1 μm abrasive grains, and then observing each sample surface corroded with a mixed solution of 10% nitric acid and 90% ethyl alcohol with an optical microscope with a magnification of 400 times. Then, the area ratio of the lower bainite occupying the surface was measured by photographing.

Moreover, the evaluation of toughness was carried out by conducting a Charpy impact test using a 2 mm U notch test piece (JIS No. 3 test piece) and measuring the Charpy impact value at room temperature. The results are shown in Table 2.

Specimen No. satisfying the composition of the steel of the present invention. Nos. 1 to 8 maintain a predetermined high hardness even by tempering at 570 ° C. or higher, specifically 580 ° C. or higher. In particular, the sample No. C added with the amount of C exceeding 0.25%. No. 8 is tempered at 620 ° C. and maintains a hardness of 37.0 HRC and is excellent in temper softening resistance. And sample no. Sample Nos. 1 to 7 have excellent toughness and a high C content. No. 8 while ensuring sufficient toughness, their machinability (hole workability) is the same as the sample No. 1 of Patent Document 1. It is improved from 11-15.

Sample No. with a high value of Equation 1 In No. 9, the lower bainite in the structure is refined and excellent in toughness, but the machinability of the present invention is not achieved. And sample no. 10 is a steel for molds which satisfies the component composition of the present invention, but whose structure is mainly upper bainite. Sample No. No. 10 has a lower hardness than other samples, and has a high Charpy impact value. However, the machinability of the present invention is not achieved.

FIG. 3 is a summary of the results of drill life (number of holes) with respect to the amount of Cu in Table 2. As shown in FIG. 3, the steel for molds satisfying the composition of the present invention has an improved drill life. And in FIG. No. 10, because the structure is mainly upper bainite, the drill life is shorter than other samples having a hardness of about 37 HRC despite the low hardness.

The steel of the present invention has excellent machinability that is not found in conventional pre-hardened steel for plastic molding. In addition to low processing strain, it is suitable for products that require precision processing because it reduces the number of man-hours required for processing the product and it is difficult for cracks to occur due to thermal stress associated with processing. Become.

Claims

In mass%, C: 0.15 to 0.30%, Si: 1.0% or less, Mn: 2.0% or less, Ni: 0.6 to 1.5%, Cr: more than 1.0 to 2 0.0%, Mo and W alone or in combination (Mo + 1 / 2W): 1.2% or less, V: 0.03 to 0.15%, Cu: 0.1 to less than 0.5%, S: 0 .05% or less, Al is 0.1% or less, N is 0.06% or less, O is controlled to 0.005% or less, and the balance is a steel composed of Fe and inevitable impurities. The composition is expressed by mass%. Formula 1: The value of [% Ni] +1.2 [% Cu] is 0.70 to 1.80, and the cross-sectional structure is such that the lower bainite is 50 area% or more and the hardness is Is a steel for molds excellent in hole workability and processing strain suppression, characterized in that is 34 to 45 HRC.
2. The gold excellent in hole workability and processing strain suppression according to claim 1, characterized in that C: 0.17 to 0.25% and Cr: 1.2 to 1.8% in mass%. Mold steel.
3. It is excellent in suppressing hole workability and processing strain according to claim 1 or 2, characterized in that Ni: 0.6 to 1.2% and Cu: 0.1 to 0.45% by mass%. Mold steel.
4. The hole workability according to claim 1, wherein the value of the formula 1: [% Ni] +1.2 [% Cu] expressed by mass% is 1.10 to 1.60. Die steel excellent in suppressing processing strain.
The hole workability and work strain according to any one of claims 1 to 4, characterized in that Mo and W are individually or in combination of (Mo + 1 / 2W): 0.2 to 1.0% by mass%. Steel for molds with excellent suppression of corrosion.
The mold steel excellent in suppression of hole workability and processing strain according to any one of claims 1 to 5, wherein S is 0.005 to 0.05% by mass%.
The composition of the steel is expressed by mass%: formula 2: 60 [% C] +1.5 [% Si] + [% Ni] +6 [% Cr] +2 [% Mo + 1/2% W (single or compound)] + 20 The excellent hole workability and processing strain suppression according to any one of claims 1 to 6, wherein the value of [% V] + 0.2 [% Cu] is 16.20 to 38.10. Steel for molds.
The method for manufacturing a steel for a mold according to any one of claims 1 to 7, wherein the steel having the composition is quenched and tempered at 570 ° C or higher so that the cross-sectional structure is lower than 50 area% of lower bainite. A method for producing a steel for molds, which is excellent in suppressing hole workability and processing strain, characterized by adjusting the hardness to 34 to 45 HRC.
The production of mold steel excellent in hole workability and work strain suppression according to claim 8, wherein the quenching is performed at a cooling rate in a range from 450 ° C to 400 ° C of 3 ° C / min or more. Method.
10. The method for producing steel for molds according to claim 8 or 9, wherein the quenching is performed at a heating temperature of 850 to 1050 ° C., and excellent in hole workability and processing strain suppression.