EP0721995B1 - Use of an iron based alloy for plastic molds - Google Patents

Description

The invention relates to the use of a chromium-containing, martensitic Iron-based alloy for plastic molds.

For the production of corrosion-resistant plastic molds for processing of chemically attacking molding compounds are predominant Iron-based alloys with a chromium content of over 12% are used. Depending on required or desired material hardness, temperable Cr steels come with approx. 13.0% Cr and approx. 0.2 or approx. 0.4% by weight C, for example according to DIN Material numbers 1.2082 and 1.2083, for use. This, essentially Iron and base alloys containing carbon and chromium are for less stressed forms can be used economically, but have the disadvantage that that for highly corrosive molding compounds and plastics with wearing additives insufficient tool life can be achieved.

By increasing the chromium content to approx. 14.5% by weight and increasing it of the carbon content to about 0.48% by weight and an addition of about 0.25 % By weight of molybdenum according to DIN material number 1.2314 can do better corrosion-resistant iron-based alloys for plastics processing be preserved. Such materials are mostly in practical use sufficiently resistant to chemical attack, however, especially when Mineral fibers containing molding compounds, insufficient resistance to Wear.

Improved usage properties of plastic molds Oxidation / corrosion and wear are comparatively high Chromium contents, high carbon contents as well as molybdenum and vanadium contents of the used steel attainable. A typical iron base alloy for The highly stressed plastic tools are made of material no. 1.2361 according to DIN In the manufacture of tools or molds from this alloy However, there may be material distortion or an uneven dimensional change, which of which often expensive reworking or retirement of the processed part required. Such an uneven dimensional change, such as is known to the person skilled in the art, is essentially due to a deformation texture or a line arrangement of the carbides. Now, as suggested was reduced, the carbon content and thus the carbide content in the matrix, so the wear resistance of the material also decreases, whereby the removal of the form increases with high friction stress and the Tool life are reduced. Another disadvantage of a high carbon content consists in a low elasticity and a low toughness of the Steel.

A stainless martensitic steel with nitrogen and reduced carbon contents is known from EP-0638 658 A1, this material from 14 to 17% by weight Chromium and from 0.10 to 0.70 wt .-% vanadium. Another heat resistant corrosion-resistant chrome steel with nitrogen concentrations of 0.20 to 1 wt .-% and a ratio of carbon: nitrogen from 0.4 to 0.8 is disclosed in DE 42 12 966 A1, which alloy contains 0.2 to 1.0% by weight vanadium and the elements tungsten and has nickel.

The object of the invention was to avoid the above disadvantages and to use a chromium-containing martensitic iron-based alloy for thermally tempered plastic molds with High corrosion resistance and improved usage properties to propose which shapes economically small changes in size can be produced.

To achieve this object, the use of a Iron-based alloy with the composition according to claim 1 for the production thermally tempered plastic molds with a hardness of at least 45 HRC, preferably from 50 to 55 HRC, and with high corrosion resistance suggested.

The advantages achieved by the invention are essentially to be seen in that the molded part or the workpiece largely isometric dimensional changes shows a heat treatment. Furthermore, the corrosion resistance of the Improved material and its matrix is more homogeneous.

Both the mechanical properties and, surprisingly, the Wear resistance of the plastic molds from that used according to the invention Alloy are significantly increased. The cause of this property improvement of the molding material is seen in that the iron base alloy is nitrogen contains which element is a strong austenite former on the one hand and on the other hand formation of intermetallic hard phases with nitride-forming elements causes. The concentrations of all essential alloying elements are included synergistically on each other, taking into account the effect of nitrogen the solidification, on the excretions, on the conversion kinetics in one Heat treatment and on the corrosion and cracking behavior of the Iron-based alloy matched so that when used according to the invention of the material for the production of thermally tempered plastic molds have significantly improved performance characteristics. This is especially true for a high gloss polishability of the plastic mold, which often, among other things use of the mold in the electronics industry. All The reasons for this have not yet been fully clarified scientifically, but have been found the following relationships: during solidification and deformation as well as The usual difference in heat treatment is the difference in concentration of chromium in the matrix of the molding material used according to the invention is low and also the carbide content is compared to nitrogen-free martensitic chrome steels low, which is high corrosion resistance and obviously a special one good polishability. Cr contents lower than 14% by weight lead however, to an abruptly increased chemical attack, especially through organic acids. At chrome contents over 25% by weight Embrittlement phenomena of the material when used for Plastic forms were observed, with the best long-term results at Cr concentrations from 16.0 to 19.0% by weight were found. With exception of Contains up to 17% by weight of chromium.

A minimum content of 0.5% by weight of molybdenum is important to support the corrosion resistance or the stabilization of the surface passive layer, but contents higher than 3.0% by weight can have a ferrite-stabilizing effect, making it difficult to harden the alloy . Particularly good results, also with regard to the effect of molybdenum nitride (Mo ₂ N) on the mechanical material properties, but in particular on the wear resistance, were found at contents in the range from 0.3 to 1.5% by weight of Mo.

Vanadium has a very high affinity for both carbon and nitrogen. The fine dispersed monocarbides (VC) or the mononitrides (VN) and the Mixed carbides are advantageously effective in the range from 0.04 to 0.4% by weight of vanadium regarding the material properties of the material in the tempered state, whereby particularly good hardness values and high in the range between 0.05 and 0.2% by weight V Tempering resistance with good dimensional stability of the shape were achieved presumably on the germination effect of the small homogeneously distributed Vanadium compounds is attributable. Levels from 0.1 are excluded % By weight vanadium.

The total effect of carbon and nitrogen in the iron-based alloy is essential in the selected concentration ranges of the alloy metals. At minimum concentrations of either carbon and / or nitrogen of 0.25 or 0.1% by weight, the sum of the contents must be at least 0.5% by weight in order to bring about an advantageous interaction of the alloying elements, as previously mentioned . With a total content in the range from 0.5 to 1.2% by weight of C + N, it has surprisingly been found that the fatigue strength in particular in the case of alternating stresses such as occurs in the case of plastic forms due to the filling cycles is significantly increased. This is probably due to the stabilization of the passive layer in the atomic or micro range caused by nitrogen and thus a avoidance of crack initiation by local material attack. Nitrogen atoms, which will be examined in more detail, could have a beneficial effect on the material's alternating corrosion stress, as was found. Furthermore, with the above minimum total content, the cubic body-centered lattice obviously begins to be destabilized, so that there are no remaining areas with alpha and delta structures in the coating in a simple manner, which eliminates the tendency of the material to crack corrosion. With the same hardness and wear resistance, alloying the chromium-containing martensitic steel with carbon and nitrogen results in a lower carbide content, the matrix having increased strength, which significantly improves the performance properties of a highly stressed plastic mold. Total values of carbon and nitrogen higher than 1.2% by weight bring about an extraordinarily great hardness in the case of complex tempering and deep-freezing treatments of the mold, but they also suddenly increase the risk of breakage.
In a range from 0.61 to 0.95% by weight of the total content of carbon and nitrogen of the iron-based alloy, the longest service lives were achieved with thermally tempered plastic molds with a material hardness of 50 to 55 HRC, especially when processing strongly chemically aggressive ones Pressing compounds and plastics with wearing additives determined. It was surprising that the adhesion of the plastic product or compact in the mold, in particular in the case of high production numbers, was significantly lower than at low nitrogen concentrations in the alloy, which made it easier to eject the compact. The cause of a reduction in sliding friction on the mold wall has not yet been fully clarified.

Tungsten contents up to 3.0% by weight improve hardness and wear resistance, however, higher values affect the high carbon affinity of tungsten because of the workability and the annealing behavior of the material.

Niobium and / or titanium are monocarbide and mononitride formers in higher proportions; these become up to a concentration of 0.18% by weight or 0.2% by weight However, elements mainly stored in mixed carbide improve the mechanical properties of the steel and reduce the risk of overheating essential. Higher levels can be particularly high at carbon levels 0.7% by weight increase the brittleness of the molds.

Low levels of cobalt and nickel improve up to 2.8% by weight and 3.9, respectively % By weight of the material toughness, with nickel, an austenite-forming element, which Hardenability is not preferred because of a concentration value of 1.5% by weight should exceed.

As is known per se, an improvement in the machinability of the material is by alloying sulfur with a concentration between 0.02 and 0.45 % By weight achievable, the most favorable values in a concentration range from 0.2 to 0.3% by weight were found.

To further harden or increase the wear resistance of the surface of the Is plastic molds made of an iron-based alloy used according to the invention, as extensive work has shown, advantageous if especially on the Work surface one, preferably according to a CVD or PVD process manufactured, hard material layer is formed.

The invention is described below for the purpose of further clarification using examples which are summarized in a table. Eight iron-based alloys were used for plastic molds of the same design, which are particularly high, but of the same chemical and wear, whereby the result values of the mold from the DIN material no To be able to clearly display shapes made of different materials. The respective values are rounded total values. The corrosion behavior, the mechanical properties, the fatigue strength, the hard material coating and the wear index are better with higher results, a lower dimensional stability and better polishability of the material are indicated by lower identification numbers.

Claims (3)

1. Use of an iron-based alloy comprising, in % by weight: C 0.25 to 1.0 Si to 1.0 Mn to 1.6 optionally S 0.02 to 0.45 N 0.10 to 0.35 Al to 1.0 Co to 2.8 Cr 14.0 to 25.0, excluding to 17.0 Mo 0.5 to 3.0 Ni to 3.9 V 0.04 to 0.4, excluding from 0.1 W to 3.0 Nb to 0.18 Ti to 0.20 with the proviso that the sum of the concentration of carbon and nitrogen gives a value A of at least 0.5 % and at most 1.2 % by weight, remainder iron and impurities caused by melting, for the production of heat-treated plastics moulds with a hardness of at least 45 HRC, preferably 50 to 55 HRC, and with high corrosion resistance and/or high polishability.
2. Use of an iron-based alloy comprising, in % by weight: C 0.4 to 0.8 Si to 1.0 Mn 0.3 to 0.8 optionally S 0.20 to 0.30 N 0.12 to 0.29 Al 0.002 to 0.8 Co to 2.8 Cr 16.0 to 19.0, excluding to 17.0 Mo 0.8 to 1.5 Ni to 1.5 V 0.05 to 0.2, excluding from 0.1 W to 3.0 Nb to 0.18 Ti to 0.20 with the proviso that the sum of the concentration of carbon and nitrogen gives a value A of at least 0.61 % and at most 0.95 % by weight, remainder iron and impurities caused by melting, for the production of heat-treated plastics moulds with a hardness of at least 45 HRC, preferably 50 to 55 HRC, and with high corrosion resistance and/or high polishability.
3. Use of an iron-based alloy according to either one of claims 1 and 2, with a surface, in particular a working surface, on at least part of which is formed a hard-material layer, preferably of carbide and/or nitride and/or oxide in single or mixed forms, in particular of the elements titanium and/or vanadium and/or aluminium.