JP5776959B2 - Die steel with excellent hot workability - Google Patents

Description

  The present invention relates to a mold for molding plastic or rubber, for example, and relates to a mold steel used for mold parts such as a nest that constitutes a cavity portion or a holder that holds the nest.

  2. Description of the Related Art Conventionally, in a mold for molding plastic or the like, excellent machinability is required for mold steel constituting the mold. In the whole of these molds, a large “nesting hole” is formed in the holder that forms the outer frame for tightly holding the insert serving as the mold body. Therefore, more excellent machinability is required among steels for molds, especially for steels for holders. Moreover, since this holder is provided with water cooling holes for shortening the molding cycle, it is also required to have excellent rust resistance against water and the like.

  Medium C—Mn—Cr—Mo—Fe steel, which is a low alloy such as JIS-SCM440, has been used for the holder steel. In recent years, in response to strong demands such as shortening of the production delivery time and reduction of processing costs, steel for molds with improved machinability by adding S, which is a free-cutting property imparting element, is generally used ( Patent Document 1). Moreover, after adding S, the tool steel made into low C has been proposed for the purpose of further improving rust resistance (Patent Documents 2 and 3).

Japanese Patent Laid-Open No. 03-097829 JP 05-279800 A Special table 2008-505786 gazette

  The mold steel of Patent Document 1 is excellent in machinability. However, in mass% (hereinafter referred to as%), C: 0.20% or more and Cr: 14% or more of a medium C-high Cr component composition, so when coarse carbides precipitate, hot working There is a concern that it will be difficult to manufacture stable products.

  On the other hand, the steel for molds of Patent Document 2 is such that C is lowered to a level of 0.1% to suppress a decrease in machinability due to carbides, and the hot workability is also good. However, since the decrease in hardness due to low C is compensated by precipitation of NiAl intermetallic compound, 0.5% or more of Al should be added. Therefore, when hard alumina-based nonmetallic inclusions are formed, there is a concern that machinability and toughness are reduced. In particular, alumina-based non-metallic inclusions in the base for plastic mold steels are factors that should be excluded because they cause pinhole formation and deteriorate the mirror finish.

  Since the steel for molds of Patent Document 3 has extremely reduced C to 0.09% or less, it is excellent in hot workability and rust resistance, but when it has a high hardness of 30 HRC or more, for example, It is difficult to obtain it stably.

  Accordingly, the present invention provides individual characteristics that conventional steel for molds cannot satisfy all, that is, strength and toughness as steel for molds, and in particular, the work required for steel for holders of plastic molds. In addition, the steel for molds is provided with excellent hot workability for facilitating the production of the steel material while maintaining the properties and rust resistance.

  The inventor has reviewed a technique by which mold steel can achieve excellent hot workability without impairing conventional machinability and rust resistance. As a result, the main factor for reducing the hot workability is coarse carbides and MnS inclusions, and it has been found that it is effective to refine these. In order to refine the carbide, there is an optimum narrow range of C content and Cr content, and even if S is added, the conventional MnS inclusion has been refined. It was found that such an added amount is not acceptable. And according to these findings, the decrease in hardness accompanying the reduction of the C and Cr content can be compensated by adjusting the content of other Si, Mo, V, Cu, and sufficient machinability In addition, it has reached the steel for molds which has rust resistance and excellent hot workability.

That is, the present invention, in mass%, C: less than 0.1~0.2%, Si: 0.1 ~1.2% , Mn: 1.2~2.0%, S: 0.01~0 0.1%, Ni: 0.5 to 1.5%, Cr: 12.0 to 13.0%, Mo: 0.3 to 0.8%, V: 0.07 to 0.3%, Cu: 0.1 to 1.0%, Al: 0.05% or less, N: 0.03 to 0.1%, excellent in hot workability, characterized by comprising the balance Fe and inevitable impurities Steel for molds. Preferably, Si is 0.1 to 0.8%, S is 0.05 to 0.1%, N is 0.04 to 0.1%, and O which is an inevitable impurity is 0.02% or less. To do. Alternatively, the hardness of the mold steel is 30 to 45 HRC.

  In the present invention, in addition to strength, toughness, machinability, and rust resistance, the individual properties that conventional steel for molds could not satisfy all, such as hot workability related to the manufacture of steel materials. This also contributes to a significant reduction in the number of processing steps during mold production and an improvement in the characteristics of the mold. Therefore, this is an effective technique for improving the mold technology.

In the rust-proof test of Example 1, it is a figure which shows the condition of the rust produced on the mirror surface finishing surface of this invention steel 1-4 and comparative steel 1-6. In Example 2, it is a figure explaining the tempering hardness of this invention steel 3 and the comparative steel 3. FIG.

  The mold steel of the present invention contains a low C—Mn—Ni—Cr—Fe alloy as a basic component. Among them, by setting C to less than 0.1 to 0.2%, for example, high hardness of 30 to 45 HRC is achieved by high-temperature tempering at 550 ° C. or higher. Further, since Cr has a high content of 12.0 to 13.0%, a large amount of Cr is dissolved in the matrix, and a dense oxide film is formed to give excellent rust resistance. The excellent rust resistance prevents rusting of the water-cooled holes when used as a mold, so that the cooling effect lasts long and contributes to the improvement of the mold life.

  And the steel for metal mold | die of this invention excellent in machinability can be supplied in the said 30-45HRC prehardened state, for example. In other words, a predetermined part shape is used after being machined such as cutting in its hardness state and then subjected to a polishing finish to prepare the shape. Therefore, since the heat treatment after the processing can be omitted, it is possible to deal with mold parts that require strict dimensional accuracy.

Hereinafter, the reason for limiting the components of the steel of the present invention will be described.
C is a basic additive element necessary for maintaining the mold strength, and preferably maintaining the quenching and tempering hardness of about 30 to 45 HRC. And in order to suppress the processing distortion which generate | occur | produces at the time of cutting etc., it is desirable to reduce the residual stress in steel, For this purpose, it is necessary that said tempering temperature can be made high. Therefore, in the steel of the present invention, it is important to add a sufficient amount of C that can stably achieve a hardness of 30 HRC or higher even when tempering at 550 ° C. or higher, for example. However, if the amount is too large, huge residual carbides are formed, and machinability and hot workability are lowered. Therefore, it is made less than 0.1-0.2%.

  Si is an element added as a deoxidizer during the production of steel ingots. And in this invention steel, it is an element which has an effect in a machinability improvement by containing. If the amount is too large, deterioration of mechanical properties such as toughness is caused. Preferably they are 0.1% or more and / or 0.8% or less.

  Mn is an important element that is added to increase the hardenability of the steel of the present invention and to suppress the formation of ferrite to give an appropriate quenching and tempering hardness. However, if it is too much, it will increase the viscosity of the base and reduce the machinability. Therefore, it is set to 1.2 to 2.0%. Preferably it is 1.7% or less.

  When S is present in the structure as a nonmetallic inclusion MnS, it 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. Moreover, coarse MnS also inhibits hot workability in the manufacturing process of steel materials. Therefore, in the steel of the present invention, S is set to 0.01 to 0.1%. In order to improve the machinability, the content is preferably 0.05% or more.

  Ni is an important element for increasing the base viscosity and improving toughness. However, if it is too much, it becomes excessively viscous and the balance between toughness and machinability deteriorates. Therefore, 0.5 to 1.5% is set. Preferably it is 0.8% or more and / or 1.3% or less.

  Cr is a very important element that dissolves in the matrix and improves the rust resistance of the steel of the present invention. In addition, in the tempering treatment, Cr-based carbides are generated and added to form the strength of the steel of the present invention. If it is too much, hot workability and machinability will deteriorate. Therefore, it is 12.0 to 13.0%.

  Mo precipitates and aggregates fine carbides during tempering, and improves the strength and temper softening resistance of the steel of the present invention. Further, a part of Mo is 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. On the other hand, however, if the amount is too large, it causes a decrease in machinability. Therefore, it was 0.3 to 0.8%. Preferably, the content is 0.4% or more and / or 0.6% or less.

  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 0.07% or more is required, but if it is too much, the machinability is lowered, so the content is made 0.3% or less. Preferably it is 0.25% or less.

  Cu partly dissolves in the base, and in combination with the effect of Cr, the rust resistance of the steel of the present invention is further improved. Moreover, by depositing part from the base, the ductility of the base is moderately lowered and excellent machinability is imparted. However, if it is too much, in addition to reducing hot workability, machinability is also reduced. Therefore, it is set to 0.1 to 1.0%. Preferably they are 0.2% or more and / or 0.6% or less.

Al is usually used as a deoxidizing element during melting, but as a result, a large amount of alumina-based nonmetallic inclusions (Al ₂ O ₃ ) that remain in the steel can be mirror-finished or heat-treated. Reduces inter-workability. Therefore, in this invention, it is necessary to regulate to 0.05% or less. Preferably it is 0.04% or less.

  N (nitrogen) has the effect of increasing rust resistance by its addition. Moreover, it is an element which forms fine nitride in steel, and has the effect of increasing the hardness of the steel of the present invention. Further, in the cutting process, when fine nitrides are formed in the steel, these increase the notch effect at the time of chip formation, so that the effect of improving the machinability is also exhibited. However, when nitrogen is added excessively, coarse nitrides are excessively formed, and this is a factor that significantly deteriorates the toughness, machinability and polishability of the mold. Therefore, N of the steel of the present invention is 0.03 to 0.1%. Preferably, it is 0.04% or more.

O (oxygen), which is an inevitable impurity, is an element that forms an oxide in steel, and this causes a significant deterioration in cold plastic workability and polishability. Then, especially in the present invention, since it is important to suppress the formation of the Al ₂ O _3, O is preferably managed low. Therefore, the preferable upper limit is 0.02%, and further 0.01% or less and 0.005% or less are the preferable order. At the same time, for the present invention which also manages the reduction of Al, the effects of the present invention are exhibited even if the O amount is about 0.001%, considering the ease of manufacture.

  Table 1 shows the chemical compositions of the inventive steel and the comparative steel. The comparative steels 1 and 2 are the mold steel of Patent Document 1 described above, the comparative steel 3 is the mold steel of Patent Document 2, and the comparative steel 4 is the mold steel of Patent Document 3. Each corresponds.

  These samples were tempered from 1030 ° C., and then tempered with a target of 40 HRC hardness, and tempered to a predetermined hardness. Then, the tempered steel of the present invention and the comparative steel were subjected to the evaluation of their mechanical properties and the following test to be cut and rust resistance test. Mechanical properties were determined by measuring the impact value by a 2 mm U notch Charpy impact test and the tensile strength, elongation and drawing by a tensile test at 1100 ° C. A tensile test evaluates the hot workability which concerns on the steel materials manufacture.

  In the test to be cut, drilling of φ1 mm was performed on the sample after the tempering, and the number of holes at the drill life was evaluated by an index with the comparative steel 1 being 100 (the larger the index, the more the workpiece to be cut) Excellent). The rust resistance test was performed by spraying a 35% 5% NaCl aqueous solution for 3 hours on a 20 mm × 40 mm mirror-finished surface of the above sample, and evaluating the rust area by an index with Comparative Steel 1 being 100. (The smaller the index, the better the rust resistance). The rusting situation of the steel of the present invention and the comparative steel at that time is as shown in FIG. The results are summarized in Table 2.

  From Table 2, the present invention steels 1 to 4 have the same excellent machinability as that of the comparative steel 1, but the hot workability is improved and the toughness is tempered to a hardness exceeding 40 HRC. Inventive steels 3 and 4 are sufficiently excellent. In particular, the inventive steels 1 to 3 having a high N amount are remarkably excellent in rust resistance.

  On the other hand, since the comparative steel 2 is higher C and higher Cr than the steel of the present invention, the amount of residual carbides is increased and the hot workability is reduced. Further, since the amount of N is also small, the notch effect at the time of chip formation is low, and the machinability is reduced.

  The comparative steel 3 shows sufficient characteristics in any of toughness, hot workability, machinability, and rust resistance. However, as in Example 2 to be described later, there are limitations on the heat treatment conditions for obtaining a hardness of 30 HRC or higher.

The comparative steel 4 is excellent in rust resistance by optimizing the C amount. However, in combination with the highly controlled Cr content and the low Cu and N content, since the S content itself is low, the machinability is poor. And for excessive amounts of Ni and Al, since Comparative Example 4 is a mold steel to which hardness is imparted by precipitating a fine Ni ₃ Al intermetallic compound by mutual addition of Ni and Al, This is a factor that drastically reduced toughness.

The comparative steels 5 and 6 have optimum compositions in the main components of the mold steel. However, since the comparative steel 5 has an excessive amount of Al and N, hard non-metallic inclusions such as Al ₂ O ₃ and coarse AlN are formed, and the toughness, hot workability, and machinability decrease. ing. And since the comparative steel 6 has little N amount, the notch effect at the time of the chip formation in cutting processing falls, and machinability has fallen.

  Invention steel 3 and comparative steel 3 in Table 1 were quenched from 1030 ° C. And the change of the hardness by the tempering temperature at this time was measured by tempering these at each temperature. The resulting tempering curve is shown in FIG. The steel 3 of the present invention to which a C amount of 0.1% or more is added can stably achieve a hardness of 30 HRC or more even at a tempering temperature of 550 ° C. or more. On the other hand, it can be seen that the comparative steel 3 having a C amount of less than 0.1% cannot stably achieve a hardness of 30 HRC at the same tempering temperature.

  The mold steel of the present invention has excellent mechanical properties and machinability, and therefore can be applied to other products that require precision machining.

Claims (6)

By mass%, C: less than 0.1~0.2%, Si: 0.1 ~1.2% , Mn: 1.2~2.0%, S: 0.01~0.1%, Ni : 0.5 to 1.5%, Cr: 12.0 to 13.0%, Mo: 0.3 to 0.8%, V: 0.07 to 0.3%, Cu: 0.1 to 1 0.0%, Al: 0.05% or less, N: 0.03-0.1%, comprising the balance Fe and unavoidable impurities, and having excellent hot workability, for mold steel.   The steel for molds having excellent hot workability according to claim 1, wherein the mass ratio is Si: 0.1 to 0.8%.   The steel for molds having excellent hot workability according to claim 1, wherein S is 0.05 to 0.1% by mass%.   The steel for molds excellent in hot workability according to any one of claims 1 to 3, wherein N is 0.04 to 0.1% by mass%.   The steel for molds having excellent hot workability according to any one of claims 1 to 4, wherein O, which is an inevitable impurity, is 0.02% or less in mass%.   Hardness is 30-45HRC, Steel for metal molds excellent in hot workability in any one of Claim 1 thru | or 5 characterized by the above-mentioned.