JP2006028564A - Steel for plastic molding die having excellent specular finishability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for a plastic molding die having excellent specular finishability. <P>SOLUTION: The steel for a plastic molding die is a precipitation hardening steel having a composition comprising, by mass, 1.0 to 5.0% Ni, 0.5 to 1.5% Al, 0.1 to 2.5% Cu, 0.01 to 0.4% Zr and ≤0.02% S, and in which the content of O is regulated to ≤20 ppm and N to ≤50 ppm. Preferably, the content of Al is ≥0.6%, or further, the content of Ca is regulated to ≤5 ppm and Mg to ≤30 ppm. Regarding these steels for a die, at the time when oxides in each steel are floated up by electron beam button melting, and the floated surfaces of the oxides are observed, the ratio of ZrO<SB>2</SB>occupied in all the oxides is desirably ≥95 area%. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  TECHNICAL FIELD The present invention relates to a mold steel that is optimal for use in injection molding of plastic products and the like that requires a high degree of mirror finish.

  Conventionally, for molding applications of plastic products that require a high degree of mirror finish (for example, a diamond compound abrasive mesh number of 8000 or more) such as home appliances, OA equipment casings, food containers, medical equipment, etc. A high-cleanness JIS-SUS420J2 or similar stainless steel die hardened and tempered to about 50HRC is used, or precipitation-hardened steel added with Ni, Al, Cu typified by AISI-P21 before cutting A mold made of steel that has been conditioned to a working hardness of around 40 HRC in advance has been used. In particular, the latter mold material is excellent in machinability for a high hardness of 40 HRC, and does not require heat treatment after rough machining, so that it is possible to save man-hours for heat treatment on the user side and correction of heat treatment strain, etc. Therefore, it has been used extensively.

As steel having improved cutting performance and mirror finish of AISI-P21, for example, it contains about 0.1% C, about 3% Ni, about 1% Al, about 1 to 2% Cu, and Cr. A steel type not particularly added is most commonly used (Patent Document 1). Furthermore, a steel type containing an appropriate amount of Cr and having improved corrosion resistance is also available on the market (Patent Document 2). In addition, a technique has been proposed for improving machinability and mirror finish by using steel for plastic molds in which the size of non-metallic inclusions such as Al ₂ O ₃ is 50 μm or less (Patent Document 3). ).
JP-A-63-114942 JP-A-11-220891 JP-A-11-335775

  The above-described AISI-P21 modified mirror finish mold steel is usually refined in a vacuum after being melted in an electric furnace, or is also produced in a vacuum induction furnace. Recently, the material melted in an electric furnace is used as an electrode. It is manufactured by the electroslag remelting method and the vacuum remelting method. This is because with the ordinary electric furnace melting method, the added Al forms a hard oxide in the molten metal and easily aggregates during casting, and this aggregated Al oxide (alumina) significantly impairs the mirror finish. This is because the oxide is reduced and the size is reduced by the remelting method. However, the application of the remelting method can be a significant cost increase factor from the viewpoint of manufacturing man-hours and yield.

  In order to omit the above-described remelting step, for example, as seen in Patent Document 3, a method of performing thorough deoxidation in the refining and casting process during melting in an electric furnace has been studied. Such a method is effective for reducing oxides, but it is difficult to completely remove the oxides, and there is a problem that it cannot be guaranteed that the aggregation is completely prevented during casting. According to the inventor's experience, even when the oxygen amount was actually suppressed to the 1 ppm level, the mirror finish of the above-mentioned # 8000 level could not be sufficiently ensured by itself. Alumina is the cause.

  Moreover, although it is important to obtain the effect of precipitation hardening by a method other than the above, there is a concern about the formation of the oxide, especially a high-level mirror finish mold to which high Al of 0.5% or more is added. For steel, there has not been proposed a method for sufficiently ensuring the mirror finish and omitting the remelting process.

  An object of the present invention is to provide a steel for precipitation hardening plastic molding dies added with Ni, Al and Cu, which can be manufactured at a lower cost and with a higher mirror finish.

  As a result of studying the above problem, the present inventor has found that, even if an Al-added steel of 0.5% or more is added, an appropriate amount of Zr is added, and if other conditions that can be easily adjusted are prepared, the oxidation in the steel is achieved. It was found that most of the product could be replaced with an oxide of Zr, in which case the oxide of Zr would not be a hindrance to mirror finish such as pinholes. Zr not only forms oxides, but also forms sulfides and traces of hard carbonitrides, but it has also been found that these do not adversely affect mirror finish under certain conditions. Reached.

  That is, the present invention has a mass ratio of Ni: 1.0 to 5.0%, Al: 0.5 to 1.5%, Cu: 0.1 to 2.5%, and Zr: 0.01 to 0.4% and S is 0.02% or less precipitation hardening type steel, and O is controlled to 20 ppm or less and N is controlled to 50 ppm or less. Steel. Here, S is an element that may be added, but is restricted to 0.02% or less. And preferably, Al: 0.6% or more, or Ca: 5 ppm or less, Mg: 30 ppm or less.

  And about said steel for metal mold | die, the ratio of the Zr oxide to the total oxide when the oxide in steel is levitated by electron beam button melt | dissolution and the floating surface of the oxide is observed is 95. It is desirable that it is area% or more.

  In addition to the above specified elements, the component composition of the steel for plastic molds of the present invention is, by mass ratio, C: 0.4% or less, Si: 2.0% or less, Mn: 2.0% or less P: 0.03% or less, Cr: 18% or less, Mo: 4% or less, which can be composed of the remaining Fe and unavoidable impurities, which is a preferred embodiment. In the steel for plastic molding die of the present invention described above, it is further desirable that C is 0.15% or less, including a preferred embodiment.

  According to the present invention, it becomes possible to supply a pre-hardened mold material having high hardness with excellent mirror finish at a low cost without a remelting step, which is an indispensable technique for manufacturing high-grade plastic products.

  As described above, an important feature of the present invention is that it has a high specular surface without a remelting step in its manufacture, which has been considered difficult for precipitation hardened steel with a high Al content of 0.5% or more. It is possible to impart finishability, and found the necessary condition for designing the alloy steel.

First, the component composition of the present invention will be described. In addition, the description of the mass ratio contained therein is indicated by “%” and “ppm” unless otherwise specified.
Ni: 1.0-5.0%, Al: 0.5-1.5%, Cu: 0.1-2.5%
Ni and Al are constituent elements such as a main precipitation hardened phase Ni ₃ Al, and Cu is an important element providing its precipitation site. In general, the greater the precipitation hardening phase, the better the machinability, but if it is too much, the toughness is lowered, so there is an appropriate range for each other. If Ni is less than 1.0%, the required hardness (about 40 HRC) is difficult to obtain, and if it exceeds 5.0%, the contribution to hardening is not obtained and the machinability is impaired. %. If Al is less than 0.5%, the required hardness (about 40 HRC) is hardly obtained, and if it exceeds 1.5%, the toughness is remarkably lowered, so 0.5 to 1.5%. A preferred range is 0.6 to 1.5%. If Cu is less than 0.1%, it becomes difficult to obtain a uniform hardness, and if it exceeds 2.5%, the toughness and hot workability are greatly reduced, so 0.1 to 2.5%.

・ Zr: 0.01 to 0.4%
Zr is the most important element that can solve the problems of the present invention. That is, alumina that causes pinholes during mirror polishing, and further Mg oxide (magnesia) and the like are reduced to form Zr oxide (zirconia) that does not easily become pinholes. At the same time, since Zr also forms sulfides and carbonitrides, the necessary addition amount is determined in relation to these amounts, but by limiting to the following oxygen and nitrogen, Al and Ni of the present invention In the range of Cu, a certain effect can be obtained at 0.01% or more. However, if it exceeds 0.4%, the toughness is adversely affected. A desirable range is 0.03 to 0.2%, and more desirably 0.05 to 0.15%.

S: 0.02% or less S may be added to improve machinability, but it forms a compound with Mn or Zr to produce orange peel (yuzu skin) during mirror finish. Become. On the other hand, if the amount is too large, the effectiveness of Zr for reducing Al ₂ O ₃ is remarkably reduced. Therefore, it is an element that requires upper limit management for the present invention. Even when it is added, by controlling to 0.02% or less, excellent mirror finish is easily achieved.

O (oxygen): 20 ppm or less O is an indispensable element for solving the problems of the present invention. That is, when O is excessively present, the effect of adding Zr is not sufficient, and a large amount of Al ₂ O ₃ and MgO harmful to mirror finish are produced in the steel. In the balance of Al, Ni, Cu and Zr of the present invention, O must be at most 20 ppm or less. A desirable range is 5 ppm or less.

N (nitrogen): 50 ppm or less As described above, N forms a hard carbonitride, ZrCN, together with Zr and C, which adversely affects mirror finish. Therefore, N in the steel in the present invention needs to be 50 ppm or less. Desirably, it is 20 ppm or less.

The element definition essential for achieving the effect of the present invention has been described above. Preferred conditions for further improving the mirror finish will be described.
-Ca: 5 ppm or less Ca is unavoidably present in steel, or forms S and a compound that is added as necessary, and becomes a pinhole factor during mirror polishing. Therefore, it is desirable to regulate to 5 ppm or less. The smaller the number, the better.

Mg: 30 ppm or less Mg forms a complex compound with other oxides and sulfides as MgO as described above, and can become a pinhole factor during mirror polishing. Therefore, it is desirable to regulate to 30 ppm or less. More desirably, it is 10 ppm or less.

-The ratio of Zr oxide in the total oxide when the oxide in the steel is levitated by electron beam button melting and the floating surface of the oxide is observed: 95 area% or more. Most of the oxide can be made of zirconia, and it is possible to minimize alumina and magnesia which are harmful to mirror finish. This effect is achieved by separating the oxide in the steel by electron beam button melting and observing the floating surface of the oxide (scanning electron microscope, element map analysis). Evaluation is possible by examining the area ratio of occupying zirconia. Preferably, the ratio of zirconia in the total oxide when observed is 95 area% or more, more desirably 99% or more.

  Electron beam button melting used in the evaluation of the present invention means that a sample of about 5 g can be measured using an electron beam in a vacuum chamber, then solidified into a spherical shape, and inclusions can be floated on the uppermost surface. To do.

  Next, a desirable specific component composition suitable for the application of the present invention will be shown for the above precipitation hardening steel. In addition, the desirable range of the element species described in these can be applied singly or in plural in addition to the above-described root or steel for plastic molds satisfying a preferable component composition.

  C has effects such as improvement of age hardness, grain refinement, and improvement of grain boundary strength by appropriate addition, but it is not necessary to add excessively, and if added excessively, hard carbide is formed and machinability is improved. Deterioration of the surface finish and the mirror finish. In the present invention, care must be taken to prevent the formation of carbides and carbonitrides with Zr, and at most 0.4% or less is desirable. In addition, More preferably, it is 0.03 to 0.15%.

  Si has an action of forming an oxide on the tool surface during high-speed cutting of steel to develop a lubrication effect (Belag effect), and is an important element for improving the machinability. There is also an effect of giving aging softening resistance. However, if it is too high, it causes a decrease in toughness and the like, so 2.0% or less is desirable.

  Mn works as a deoxidizer for steel and is important for fixing S, but if it is too high, it causes a decrease in cutting workability, so it is desirable to make it 2.0% or less.

  P lowers the toughness, but if it is contained at 0.03% or less, it is not practically harmful, and is preferably 0.03% or less.

  Cr is an element effective for improving the corrosion resistance, but the necessity and amount of addition should be determined according to the application needs such as the type of molding resin. However, if added over 18%, the toughness decreases, so 18% or less is desirable.

  Mo is one of the elements contributing to age hardening, and can be used when it is desired to further increase the hardness in addition to the precipitation hardening by NiAl of the present invention. However, even if it exceeds 4% in this effect, the effectiveness on the characteristics is low, so it is desirable to make it 4% or less.

  The remainder other than the above may be Fe and other inevitable impurities, for example, W: 1% or less, V: 1% or less, Co: 3% or less, Nb: 0.5% or less, Ta: One or more of 0.5% or less, Ti: 0.5% or less, and B: 0.1% or less may be added as necessary.

  A steel ingot obtained by the electric furnace melting method was hot-worked to prepare a sample composed of the remaining Fe and unavoidable impurities of the chemical components shown in Table 1. Then, these samples were heat-treated (solution treatment and aging treatment) and tempered to a predetermined hardness. And, for each sample after tempering, a survey of the proportion of zirconia in the floating oxide by the electron beam button dissolution test according to the above procedure, and a mirror finish polishing test with # 8000 abrasive grains (existence evaluation of pinhole and orange peel) Was done. The results are shown in Table 2 together with the tempering hardness.

  Although the steel of the present invention has a large amount of Al, most of the oxides can be converted into zirconia by appropriate control of oxygen, nitrogen and sulfur and addition of appropriate amounts of Zr. Met. The zirconia distribution of the invention steel 1 and the comparative steels 6 and 7 is as shown in the scanning electron microscope images of the floating inclusions and the map analysis results of the O, Zr, and Al elements shown in FIGS. In the map images of FIGS. 1 to 3, a portion exhibiting white (light color) is a concentrated portion of each analytical element. In the case of the steel 1 of the present invention (FIG. 1), only one alumina is recognized on the analysis map, but the number is small and it is not agglomerated. Moreover, the individual size of zirconia is small, and the situation where aggregation is difficult is also recognized.

In Comparative Steel 6, because of the insufficient amount of Zr as compared with the amount of Al, most of the oxide became agglomerated alumina, which was crushed and dropped off with a # 8000 mirror finish, generating pinholes. In Comparative Steel 7, although the same amount of Zr as that of the steel of the present invention was added, S was as high as 0.051%, so the action of reducing Al ₂ O ₃ by Zr was reduced, and Zr oxide As a result, pinholes due to alumina were generated. Also, orange peel due to sulfide was generated.

  The comparative steel 8 is a sample to which the amount of the present invention is not added, but in this case, even when the amount of Zr is as low as 0.007%, the entire amount of zirconia of the oxide is achieved. However, since high hardness cannot be obtained due to insufficient precipitation hardening action, a sufficiently smooth polished surface is difficult to obtain, and as a result, an orange peel-like skin was obtained despite the low amount of S. In the comparative steel 9 having a low Al content, zirconia of the oxide is achieved by adding Zr. However, although not as much as comparative steel 8, sufficient hardness was not obtained due to the lack of precipitation hardening elements, and some orange peel was generated.

Scanning electron microscope image and map analysis of O, Zr, and Al elements in which the oxide in the steel for plastic molding die of the present invention was separated by floating by electron beam button melting and the floating surface of the oxide was observed. It is a statue. Scanning electron microscope image and map of O, Zr, and Al elements in which the oxide in the steel for plastic molding die for comparison was floated and separated by electron beam button melting, and the floating surface of the oxide was observed. It is an analysis image. Scanning electron microscope image and map of O, Zr, and Al elements in which the oxide in the steel for plastic molding die for comparison was floated and separated by electron beam button melting, and the floating surface of the oxide was observed. It is an analysis image.

Claims (6)

In terms of mass ratio, Ni: 1.0-5.0%, Al: 0.5-1.5%, Cu: 0.1-2.5%, Zr: 0.01-0.4% S is a precipitation hardening type steel of 0.02% or less, and O is controlled to 20 ppm or less and N is controlled to 50 ppm or less. The mass ratio of Al is 0.6% or more, and the steel for plastic molds with excellent mirror finish according to claim 1. 3. The steel for plastic molds with excellent mirror finish according to claim 1 or 2, wherein the mass ratio is restricted to Ca: 5 ppm or less and Mg: 30 ppm or less. The ratio of the Zr oxide in the total oxide when the oxide in the steel is levitated by electron beam button melting and the floating surface of the oxide is observed is 95 area% or more. The steel for plastic molding dies excellent in mirror finish in any one of 1 to 3. In addition to the element species of any one of claims 1 to 3, by mass ratio, C: 0.4% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.03% or less Cr: 18% or less, Mo: 4% or less, comprising the balance Fe and unavoidable impurities, for a plastic mold excellent in mirror finish according to any one of claims 1 to 4 steel. The steel for plastic molds with excellent mirror finish according to any one of claims 1 to 5, wherein C: 0.15% or less in terms of mass ratio.