JP2008038219A - Prehardened steel with excellent machinability and toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide prehardened steel having extremely excellent machinability, further combining toughness with hardness and most suitably used for steel for metallic molds for use in plastic molding. <P>SOLUTION: The prehardened steel is characterized in that: it has a composition composed of, by mass, 0.10 to 0.14% C, 0.1 to 0.5% Si, 1.2 to 1.6% Mn, 0.002 to 0.005% S, 2.8 to 3.2% Ni, 0.35 to 0.85% Cr, 0.2 to 0.4% Mo, <0.03% (including 0%) V, 1.3 to 1.8% Cu, 0.8 to 1.2% Al, <0.01% N, <0.002% O and the balance Fe with inevitable impurities; a value of (1.2×Mn%+1.3×Cr%+Cu%) ranges, preferably, from 3.50 to 4.16, further preferably, from 3.58 to 4.08; and hardness is 34 to 45 HRC. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a new pre-hardened type tool steel mainly used for plastic molding dies, which has excellent machinability and has both toughness and hardness.

  Pre-hardened steel used mainly as a mold for plastic molding requires excellent strength and wear resistance as well as machinability, as well as appropriate toughness from the viewpoint of shortening the production period of molds and improving the service life. It is supposed to be. However, these required properties are contradictory properties, and it cannot be said that a steel satisfying all the properties has been obtained.

In response to the above requirements, for example, for a low C—Mn—Ni—Mo (W) —Cu—Al alloy, the size of crystal grains is set in the range of grain size numbers 4 to 6 to cut the workpiece. A proposal has been made to have both strength and toughness (see Patent Document 1). The structure of this steel is adjusted to a uniform upper bainite structure mainly by prescribing C low, and the machinability is ensured by this upper bainite structure. On the other hand, the applicant of the present application mainly uses a low C—Mn—Ni—Mo (W) —high Cu— having both machinability and toughness by optimizing the amount of Mn and adjusting it to a uniform lower bainite structure. An Al-based alloy has been proposed (see Patent Document 2).
Japanese Patent Laid-Open No. 05-070887 JP 07-278737 A

  Each of the above proposals provides a means for contributing to the improvement of the performance of pre-hardened steel for plastic molding. However, although the upper bainite structure of Patent Document 1 is excellent in machinability, it cannot be said that the toughness is sufficient. On the other hand, although the lower bainite structure of Patent Document 2 is excellent in toughness, the machinability is slightly inferior, and it is difficult to say that it is sufficient. As described above, the conventional steel for molds has room for improvement in characteristics in sufficiently satisfying the requirements for shortening the mold production period and improving the service life.

  In view of the above-described requirements, the object of the present invention is to provide a pre-hardened steel that is particularly suitable for use in a plastic molding die because it can achieve both shortening of the mold production period and improvement of the service life. That is.

  The inventor has a composition and structure of a low C—Mn—Ni— (Mo, W) —Cu—Al based alloy and a low C—Mn—Ni— (Mo, W) —high Cu—Al based alloy, When the relationship between machinability and toughness was examined in detail, first, the basic composition was machinability excellent in being a low C—Mn—Ni—Cr—Mo—Cu—Al based alloy, and It has been found that it is an essential requirement for toughness. On the other hand, it has been found that the mixed structure of the upper bainite and the lower bainite can combine machinability superior to the single-phase structure of the upper bainite and toughness equivalent to the single-phase structure of the lower bainite. And the optimal mixed structure according to these findings is relatively low by using an alloy having a limited narrow composition among Cu-Al systems in the low C-Mn-Ni-Cr-Mo- The inventors have found that it can be achieved even under heat treatment conditions that are easy to manage, and have reached the present invention.

  That is, this invention is mass%, C: 0.10-0.14%, Si: 0.1-0.5%, Mn: 1.2-1.6%, S: 0.002-0. 005%, Ni: 2.8 to 3.2%, Cr: 0.35 to 0.85%, Mo: 0.2 to 0.4%, V: less than 0.03% (including 0%), Cu: 1.3-1.8%, Al: 0.8-1.2%, N: less than 0.01%, O: less than 0.002%, balance Fe and inevitable impurities Pre-hardened steel with excellent machinability and toughness.

  Preferably, pre-hardened steel satisfying 3.50 ≦ 1.2 × Mn% + 1.3 × Cr% + Cu% ≦ 4.16 in terms of mass%, more preferably 3.58 ≦ 1 with the above steel as a basic composition. .2 × Mn% + 1.3 × Cr% + Cu% ≦ 4.08 pre-hardened steel. Alternatively, the steel is preferably prehardened steel having a hardness of 34 to 45 HRC.

  The steel of the present invention has excellent machinability and toughness at a high level not found in conventional pre-hardened steel. Therefore, other characteristics are not greatly deteriorated, and, for example, extension of the tool life for plastic molding can be achieved, which is extremely effective. In addition, since the steel of the present invention has high toughness, it is less likely to crack due to thermal stress accompanying processing of a mold or the like, and is particularly suitable for performing more precise mold processing. Furthermore, since the management conditions during the heat treatment are relatively easy, there is also an advantage that time, labor, and cost can be reduced during manufacturing.

  One of the basic features of the present invention is that a low-C—Mn—Ni—Cr—Mo—Cu—Al-based alloy must be adopted as the basic component of the steel. That is, for the problem of combining excellent machinability and toughness, the present invention is important for the structure control, but in this basic composition, particularly in the basic element such as Cu, when the above basic system is deviated, Even if the targeted organization control can be achieved, it is difficult to achieve this characteristic.

And even though it is a low C—Mn—Ni—Cr—Mo—Cu—Al alloy, it has both machinability and toughness, even under heat treatment conditions that are relatively easy to manage. There is an optimum narrow composition region that can be adjusted to a particularly excellent mixed microstructure of upper bainite and lower bainite, that is, another feature of the present invention is that this narrow composition region is also specifically specified. .

  As described above, a conventional low C—Mn—Ni— (Mo, W) —Cu—Al alloy or a low C—Mn—Ni— (Mo, W) —high Cu—Al system is used as the prehardened steel. In some cases, in order to ensure the machinability, it was prepared aiming at a single phase structure of an upper bainite structure or a lower bainite structure. However, although the upper bainite structure is a structure with excellent machinability, it is a structure with low toughness, and the lower bainite structure is a structure with excellent toughness, but the machinability is slightly inferior. It was an organization.

  Therefore, the present inventor has investigated in detail the relationship between the structure and machinability and toughness. As a result, the upper bainite structure excellent in machinability is mixed with 20 to 80% of the lower bainite structure, which is a different structure. It has been found that, by occasionally embrittlement moderately, machinability and toughness superior to a uniform upper bainite structure can be obtained. Furthermore, it was also found that machinability is most excellent when the lower bainite structure is 60 to 70%, and that toughness equivalent to the lower bainite structure is obtained. The steel according to the present invention is a low C—Mn—Ni—Cr—Mo—in-Cu—Al alloy, which is an essential requirement. The present inventors have found that a mixed structure of the upper bainite and the lower bainite, which is a stable target, can be obtained under conditions, and that both the shortening of the mold manufacturing period and the improvement of the service life can be achieved.

  In addition, bainite in a steel structure is one of transformation products generated when austenite is cooled, and refers to that generated in a temperature range intermediate between the pearlite generation temperature and the martensite generation temperature. Those occurring near the pearlite transformation temperature are feather-like (lumpy), and those occurring near the martensite formation temperature are needle-like. The former is called upper bainite and the latter is called lower bainite.

  In addition, a predetermined steel structure, particularly a mixed structure of upper bainite and lower bainite, as in the present invention, is not obtained simply by determining the steel composition, and usually depends greatly on heat treatment conditions such as the cooling rate during quenching. It is what is done. However, in the steel of the present invention, the component composition is adjusted to a sufficiently narrow range by optimizing the element amounts of Mn, Cr, Cu in particular. It is kept as low as possible. And, the degree of influence on the structure by the other heat treatment condition is a condition that is easy to manage in view of the heat treatment condition for achieving the mixed structure of the upper bainite and the lower bainite, which is the aim of the present invention. Thus, the component area is set narrow. That is, for example, a mixed structure of upper bainite and lower bainite can be stably obtained by heating to the austenite region and then performing air cooling (cooling). Furthermore, even if the cooling rate during direct quenching in which quenching is performed immediately after hot working is air cooling (cooling), a mixed structure of upper bainite and lower bainite can be obtained stably.

Hereinafter, the reasons for defining the steel composition defined in the present invention will be described.
C keeps the quenched structure of Cu-Al pre-hardened steel in low C-Mn-Ni-Cr-Mo- in a bainite structure and precipitates Cu-Fe solid solution, Ni-Al intermetallic compound and Mo carbide in tempering Is a basic additive element that provides a substrate for effecting precipitation hardening based on. If the amount is too large, the base is martensite-organized to reduce the machinability, and excessive carbides are formed to reduce the machinability and mirror finish. Therefore, in the present invention, it was specified to be 0.10 to 0.14%.

  Si is an element that enhances the corrosion resistance to the atmosphere during use as a steel product. However, if the amount is too large, ferrite is generated and machinability is lowered. When Si is reduced, the anisotropy of mechanical properties is reduced, stripe segregation is reduced, and excellent mirror surface workability is obtained. Therefore, the Si content is preferably 0.4% or less.

  Mn is one of the most important elements for prehardened steel having a high toughness mixed structure. Mn is an element that basically increases the toughness of the matrix, and has the effect of suppressing the formation of ferrite and imparting appropriate quenching and tempering (aging) hardness. Further, Mn is an element that enhances the bainite hardenability and makes it easier to obtain an upper / lower bainite mixed structure, which is a desirable structural feature of the present invention, by relatively easy heat treatment. However, if there is too much Mn, the toughness becomes too high and excellent machinability cannot be maintained. From the above, Mn is specified to be 1.2 to 1.6%.

  S is very effective for improving machinability by forming non-metallic inclusions MnS. However, when the amount is too large, mirror surface workability is deteriorated. Even if it is added in a small amount, the effect of sufficiently improving machinability can be obtained, so 0.002 to 0.005% is set.

  Ni enhances the bainite hardenability, suppresses the formation of ferrite, further precipitates a Ni-Al intermetallic compound during tempering (aging), obtains the required hardness and moderately reduces ductility, Added to improve machinability. If it is too low, the effect of the addition shown on the left cannot be obtained, so it is 2.8% or more. However, if it is too high, the bainite transformation temperature is lowered, the bainite structure is excessively refined, and further martensitic transformation is performed. Since the machinability is lowered by increasing the viscosity of the steel, it is set to 3.2% or less.

  Cr is one of the most important elements for obtaining a mixed structure of upper bainite and lower bainite. That is, there is an effect of appropriately miniaturizing the bainite structure even in the cooling rate range in which the condition management during the heat treatment process of quenching is relatively easy. It also has the effect of enhancing corrosion resistance, increasing the hardness when nitriding, and suppressing rusting during polishing or product storage. However, if the amount is too large, the bainite structure is excessively refined and further the martensitic transformation is performed to deteriorate the machinability. Therefore, the content is made 0.35 to 0.85%.

  Mo precipitates fine carbides in the tempering (aging) treatment of the steel of the present invention, particularly high temperature tempering (aging) treatment exceeding 500 ° C., thereby causing precipitation (aging) hardening and corrosion resistance to the atmosphere during product use. It is an element that has an enhancing effect. In the case of the present invention, it is not necessary to add a large amount, and if it is too much, the machinability is lowered, so the content is made 0.2 to 0.4%. Although W is known to have the same effect as Mo, W has a lower diffusion rate than Mo, and therefore there is a high possibility that undissolved carbides remain during quenching and hot working. The present invention steel uses only Mo.

  V has the effect of increasing the temper softening resistance and suppressing the coarsening of the crystal grains, but if it is too much, the hard carbide adversely affects the mirror surface workability and machinability, so it is made less than 0.03% ( 0% included). More preferably, it is less than 0.02%.

  Cu, like Cr, is one of the most important elements for obtaining a mixed structure of upper bainite and lower bainite. Again, the cooling rate is relatively easy to manage the conditions during the heat treatment process of quenching. Even if it is within the range, there is an effect of appropriately miniaturizing bainite. Moreover, in the tempering (aging) treatment, precipitation (aging) hardening by fine precipitation of the Fe—Cu solid solution is brought about, and the required machinability and toughness of the steel of the present invention are imparted and the required hardness is obtained. In addition, it provides excellent corrosion resistance. However, if the amount is too large, the hot workability is lowered, the bainite is excessively refined, and further, it works in the martensitic transformation. And

  Al is one of the important elements that brings about precipitation (aging) hardening by fine precipitation of Ni—Al intermetallic compounds in the tempering (aging) treatment and forms the excellent machinability of the present invention. Al is also an additive element for obtaining the required hardness. If nitriding is performed, the effect of increasing the nitriding hardness at that time is brought about. If the amount is too high, the amount of alumina inclusions increases, the mirror surface workability is lowered, the pitting corrosion resistance is lowered, and the ductility is excessively lowered. Is 0.8% or more. Preferably, it is 0.95% or more in order to further improve machinability.

  N is an element that forms nitrides in steel. If the nitride is excessively formed, the toughness, machinability and mirror surface workability of the mold are remarkably deteriorated, so N is restricted to less than 0.01%. Preferably, it is less than 0.008%.

  O is an element that forms an oxide in steel. Oxides significantly reduce cold plastic workability and mirror surface workability, so O is restricted to less than 0.002%. Preferably, it is less than 0.001%.

  In the steel of the present invention satisfying such a component range, it is more desirable to satisfy 3.50 ≦ 1.2 × Mn% + 1.3 × Cr% + Cu% ≦ 4.16. That is, as described above, it is important for the steel of the present invention that the adjustment to the bainite mixed structure is the basis of the action and effect to adjust the contents of Mn, Cr, and Cu. It is desirable to adjust to the optimal component composition in consideration of the difference in the degree of influence on the tissue. As a relational expression for this purpose, the present inventor has found that index management by “1.2 × Mn% + 1.3 × Cr% + Cu%” is effective. If the above formula is less than 3.50, the upper bainite structure is slightly increased and the toughness may be slightly inferior. Conversely, if it exceeds 4.16, the lower bainite structure is slightly increased and the machinability is increased. It may be slightly inferior. More preferably, 3.58 ≦ 1.2 × Mn% + 1.3 × Cr% + Cu% ≦ 4.08 is satisfied.

  The steel of the present invention is supplied in a pre-hardened state with a hardness of 34 to 45 HRC, for example, and processed into a product shape as it is. If it is a die, it is engraved and then subjected to polishing, advanced mirror finishing and embossing, etc. Used. When it is less than 34 HRC, problems such as wear occur during use (during molding as a mold), and when it exceeds 45 HRC, the machinability is adversely affected. Therefore, the desired hardness is set to 34 to 45 HRC. However, the steel according to the present invention achieves further excellent machinability and toughness by setting it to a low hardness of less than 34 HRC. Therefore, if the use with such a low hardness is allowed, naturally It does not exclude it.

(Example 1)
After hot-rolling samples 1 to 5 consisting of the remaining Fe and unavoidable impurities of the chemical components shown in Table 1, the sample is heated to an austenite region of 880 ° C., then air-cooled (cooled), and a temperature range of 500 to 590 ° C. And tempered. And this was used as a test material, and machinability and toughness were evaluated.

  The machinability was evaluated by drilling and face milling with a diameter of 125 mm. Evaluation of drilling was performed after machining 50 holes with a Φ2 mm drill made of high-speed steel under a machining condition of a cutting speed of 15 m / min, a feed rate of 120 mm / min, and a machining depth of 20 mm. The amount of tool wear was measured. The face milling was evaluated using a cermet cutting tip with a cutting speed of 150 m / min, a feed rate of 0.13 mm / tooth, a notch of 2 (depth) x 100 (width) mm, and a number of cutting edges of 1. The amount of tool wear when cutting for 49.7 minutes under the processing conditions of the sheet was measured.

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

  From Table 4, Sample 1 which is the steel of the present invention has a mixed structure of upper bainite and lower bainite (ratio 30:70 on the surface to be examined) even when the cooling rate from the austenite region is relatively easy to manage. Thus, it can be seen that the amount of tool wear is small, a high impact value is obtained, and both machinability and toughness are provided. However, the sample 2 of the comparative steel exhibits an ideal bainite mixed structure as the structure, and the machinability is slightly inferior to that of the steel of the present invention. Less toughness. Samples 3 and 4 were mainly composed of lower bainite due to a large amount of Cr or a large amount of Cu, and were excellent in toughness but inferior in machinability. Sample 5 has a structure in which Cr, Cu, and Ni are low and is controlled by upper bainite. However, since the hardness at the time of tempering is low, machinability and toughness are relatively good. However, in addition to the above, since the amounts of Ni and Al are low, only a maximum hardness of 32.0 HRC can be obtained, and it is difficult to use in a use environment where damage such as wear easily occurs.

(Example 2)
After hot-rolling samples 6 and 7 consisting of the remainder of the chemical components shown in Table 3 and inevitable impurities, direct quenching was performed at a cooling rate of air cooling (cooling), and tempering was performed at a temperature range of 500 to 590 ° C. went. And this was used as a test material, and machinability and toughness were evaluated.

  The machinability was evaluated by performing two types of drilling and face milling of Φ100 mm. Evaluation of drilling was performed with a Φ5 mm drill made of high-speed steel, after machining 100 holes under machining conditions of a cutting speed of 20 m / min, a feed rate of 63.35 mm / min, and a machining depth of 25 mm. The amount of tool wear was measured. Also, using a tabletop drilling machine, a high-speed steel Φ4mm drill, cutting speed of 17.6m / min, feed rate of manual feed, machining depth of 20mm, in the number of machining holes until continuous cutting is impossible Was also evaluated. The face milling was evaluated using a cutting tip made of cermet, the cutting speed was 150 m / min, the feed speed was 0.13 mm / blade, the notch was 2 (depth) x 80 (width) mm, and the number of cutting edges was 1. The amount of tool wear when cutting for 29 minutes under the sheet processing conditions was measured. The toughness was evaluated in the same manner as in Example 1. The results are shown in Table 4.

  From Table 4, when the steel of the present invention is compared with the comparative steel, the amount of tool wear in drilling does not change, but the steel of the present invention is clearly superior in the amount of tool wear in milling and the number of drillable holes. . The impact value is about twice that of the comparative steel. Therefore, it turns out that this invention steel has the outstanding machinability and toughness.

  The pre-hardened steel of the present invention, which excels in machinability and toughness, is optimal for mold steels used for plastic molding, for example, and it requires hardness, such as screws for injection molding machines, and emphasizes machinability. It can also be applied to machine parts and the like.

Claims (4)

In mass%, C: 0.10 to 0.14%, Si: 0.1 to 0.5%, Mn: 1.2 to 1.6%, S: 0.002 to 0.005%, Ni: 2.8 to 3.2%, Cr: 0.35 to 0.85%, Mo: 0.2 to 0.4%, V: less than 0.03% (including 0%), Cu: 1.3 -1.8%, Al: 0.8-1.2%, N: less than 0.01%, O: less than 0.002%, balance of Fe and inevitable impurities, and machinability and Pre-hardened steel with excellent toughness. The pre-hardened steel excellent in machinability and toughness according to claim 1, characterized by satisfying 3.50 ≦ 1.2 × Mn% + 1.3 × Cr% + Cu% ≦ 4.16 in mass%. The pre-hardened steel excellent in machinability and toughness according to claim 1, characterized by satisfying 3.58 ≦ 1.2 × Mn% + 1.3 × Cr% + Cu% ≦ 4.08 in mass%. The pre-hardened steel excellent in machinability and toughness according to any one of claims 1 to 3, wherein the hardness is 34 to 45 HRC.