HK1048341A - Steel for plastic molds and process for the heat treatment thereof - Google Patents

Description

Steel for plastic forming die and heat treatment method thereof

The invention relates to a maraging steel with improved machinability, good weldability and high corrosion resistance, and to the use thereof. The invention also includes a method for heat treating a maraging steel with improved machinability, which method enables the production of objects that are through-hardened even in large cross-sections.

In products which are required in everyday life and industry, plastics are contained, which plastic parts should generally have a fixed shape and surface properties. The shaping can be carried out, for example, by pressing, injection molding or injection molding the material into a molded article. The surface properties of an article that are important to the aesthetic judgment of the customer are determined primarily by the surface properties of the mold used during the molding process.

Objects or elements, which are usually made of steel, in particular maraging steel, are used as molds for the shaping of plastics and also as clamping elements, such as frames and the like. The refining of steel for the above purposes takes into account, on the one hand, the interests of the person who makes the mould and, on the other hand, the interests of the user of the mould.

In order to economically produce plastic molding dies and accessories in a short time, die makers want to use as large workpieces as possible made of stock of thermally age-hardened bars or plates. This allows, on the one hand, a reduction in production costs by eliminating the heat treatment while generating less waste. On the other hand, when processing from already age-hardened material, a plastic molding tool or element with particularly precise dimensions can be provided to the tool user, since this avoids deformation of the tool as a result of the subsequent heat treatment after the molding process is completed. In particular, good machinability of thermally age-hardened starting materials is required by the mould manufacturers in order to facilitate the processing and to achieve sufficient surface quality of the plastic parts. Furthermore, in order to achieve the same mechanical properties of the plastic molding tool produced over the entire cross-section, it is necessary that the workpiece used is completely through-hardened.

For economic reasons, in addition to good weldability, the corrosion resistance of the mould material is also an aspect of great importance to the mould user for the low-cost production and in particular for the repair of the mould. Only plastic molding tools and tool elements made of steel with sufficient corrosion resistance are suitable for multiple and/or long-term use with an interval of storage time. The surface of the mould is in frequent contact with corrosive chemical agents causing corrosion due to contamination of the injection, in particular of the plastic, for example by acids generated in acid-catalysed polymerisation reactions.

It is well known that steels containing more than 12% by weight of chromium are used to produce plastic forming molds having high corrosion resistance. Such steels are known from alloys with german industry standard material numbers 1.2085, 1.2314 or 1.2316. These types of steels contain at least 14.0% by weight of chromium and 0.33% by weight or more of carbon, the high carbon content of which reduces the chromium concentration in the matrix during carbide formation and thus the corrosion resistance, in particular in the medium surrounding the carbides.

In order to achieve better corrosion resistance, a class of steels with lower carbon content is used, such as the german industry standard material No. 1.4005 steel. In US 6,045633 a low carbon martensitic steel for making plastic moulding tools is proposed. The steel contains, in addition to iron and impurities introduced during the production, 1.0 to 1.6% by weight of manganese, 0.25 to 1.0% by weight of silicon, 0.5 to 1.3% by weight of copper, 12.0 to 14.0% by weight of chromium, 0.06 to 0.3% by weight of sulfur and other elements. For this alloy, the corrosion resistance is increased by copper in addition to the reduced carbon content. However, such a high copper content causes major problems in the thermal deformation of the workpiece, in particular in the formation of fine cracks on the surface of the workpiece to be machined, which in turn are prone to crevice corrosion.

The known steels for producing plastic moulding tools or similar fittings are intended to meet the various requirements of mechanical and chemical corrosion properties set by the tool manufacturers and users. However, such materials are often observed to fail to have machinable properties. Furthermore, it has been shown in the case of thermal age hardening that workpieces made from these alloys are sometimes very difficult to achieve through-hardening, in particular for large bodies with large cross sections, for example forged bars, which are frequently produced in production technology. Furthermore, the moulds produced from these workpieces have inhomogeneous mechanical properties and therefore weak spots, which are manifested primarily by premature material defects.

It is therefore the object of the present invention to provide a maraging steel which simultaneously has good weldability, high corrosion resistance and improved machinability.

It is a further object of the invention to provide a heat treatment method for maraging steels with improved cutting properties, with which objects can be produced that are through-hardened even with large cross-sections.

Finally, the object of the invention is to illustrate the use of the steel according to the invention.

The task of providing a maraging steel with improved machinability, good weldability and high corrosion resistance can be solved by a maraging steel with the following contents of alloying elements (% by weight),

0.02-0.075% carbon

0.1-0.6% of silicon

0.5-0.95% manganese

0.08-0.25% of sulfur

Phosphorus with an upper limit of 0.04%

12.4-15.2% of chromium

0.05-1.0% molybdenum

0.2-1.8% of nickel

An upper limit of 0.15% of vanadium

0.1-0.45% of copper

Aluminum with an upper limit of 0.03%

0.02-0.08% nitrogen

And optionally one or more further alloying elements with an upper limit of 2.0%, the balance being iron and impurities introduced during production, the proportion of ferrite in the structure of the steel being less than 28% by volume.

The advantage of the invention is, in particular, that maraging steels are produced by defined alloy technology measures, which, in conjunction with a certain ferrite proportion in the structure, which can be adjusted as required by the heat treatment, have improved machinability, while having a carbon content of 0.02 to 0.075 wt.% and a chromium content of 12.4 to 15.2 wt.%, ensuring high corrosion resistance and good weldability. The steel according to the invention also contains 0.2 to 1.8 wt.% nickel, which is used on the one hand to adjust the proportion of ferrite in the structure. On the other hand, the nickel and chromium are combined to achieve corrosion resistance against reducing chemical agents, so that the steel according to the invention achieves high chemical corrosion resistance.

In order to form manganese sulfide in the material which is advantageous for machinability, a manganese content of 0.5-0.95 wt.% and a sulphur content of 0.08-0.25 wt.% are important for the invention, where the ratio of manganese to sulphur content is determined such that good corrosion resistance is achieved while excellent machinability is achieved. It was found that when the manganese content is less than 0.5% by weight, there is a tendency to form chromium sulfide which adversely affects machinability. Conversely, manganese sulfides which are free of chromium are generally formed when the manganese content is greater than 0.95% by weight. An upper limit of 0.01 wt% calcium can cause the manganese sulfide envelope to have a more uniform morphology.

The copper content is in the range of 0.1 to 0.45 wt.%, and this copper content range improves the corrosion resistance. The upper limit of the copper content is 0.45% by weight, up to which workpieces, in particular large forgings, made from the steel according to the invention can achieve good heat deformability.

Molybdenum in an amount of 0.05 to 1.0 wt.% leads to the desired deformation kinetics at maraging with increased hardness, and it has been found that the above-mentioned concentration ranges on the one hand contribute to a further suppression of the formation of chromium carbides and on the other hand increase the corrosion resistance. Conversely, molybdenum in amounts greater than 1.0 wt.% reduces its cracking resistance during thermal deformation of the workpiece in some temperature ranges.

Other elements of the steel according to the invention are carbon, silicon, phosphorus, vanadium, aluminium and nitrogen in the weight contents given in claim 1.

Alloy constituents, such as those of sub-groups 5 and 6 in particular, can be present in an upper total content of 2% by weight without observable adverse effects on machinability and possibly also for improving corrosion resistance.

The steel according to the invention in a preferred embodiment contains 0.80-0.90 wt.% manganese and 0.1-0.16 wt.% sulphur. Good corrosion resistance is achieved with particularly improved machinability in the composition range with a manganese/sulphur weight ratio of 5.0 to 9.0, this effect being attributable to the influence of the spherical morphology of the manganese sulphide particles in the above-mentioned solution. It also appears advantageous to form manganese sulphide particles that may contain very small amounts of chromium when the manganese/sulphur stoichiometric ratio is less than 3: 1, which allows the material to achieve superior corrosion performance.

The higher chromium content is mainly used to improve the corrosion resistance of the alloy with little reduction in the workability of the alloy. The steels according to the invention preferably have a chromium content of 13.8 to 15.0%, preferably 14.1 to 14.7%, within which composition range the desired advantageous properties are simultaneously achieved.

For good corrosion resistance and also for precise adjustment of the ferrite fraction and improved machinability, it is important that the nickel content in the steel according to the invention is 0.25 to 1.6 wt.%, preferably 0.35 to 1.1 wt.%, most preferably 0.8 to 1.0 wt.%. Higher nickel concentrations are generally very disadvantageous for stabilizing the austenite at higher temperatures, whereas lower nickel contents have a negative effect on the deformability of the material during thermal age hardening.

A copper content of 0.25 to 0.35 wt.% proves advantageous in the optimization of the material properties. In this copper concentration range, the corrosion resistance of the hot-deformed body is improved to the greatest possible extent by the alloying elements, while at higher copper contents the increased tendency to crevice corrosion is explained by the formation of fine surface cracks.

Good workability of the material according to the invention can be achieved at a ferrite content in the microstructure of up to 15% by volume. For only a negligible small workability, however, the ferrite content may preferably be up to 10% by volume and up to 6% by volume, which may improve the mechanical properties of the die element, in particular transverse to the direction of deformation.

Another object is to provide a method for heat treatment of maraging steel with improved machinability, with which an article can be produced that is through-hardened even at large cross-sections, the object being solved by a steel ingot with the following composition (in wt.%):

0.02-0.075% carbon

0.1-0.6% of silicon

0.5-0.95% manganese

0.08-0.25% of sulfur

Phosphorus with an upper limit of 0.04%

12.4-15.2% of chromium

0.05-1.0% molybdenum

0.2-1.8% of nickel

An upper limit of 0.15% of vanadium

0.1-0.45% of copper

Aluminum with an upper limit of 0.03%

0.02-0.08% nitrogen

And possibly other alloying elements in a total of at most 2.0%, the balance being iron and impurities introduced during production, in a first step the ingot is annealed to form and adjust the ferrite fraction in the structure, in a second step the ingot is hot deformed with a degree of deformation of at least 4 times, in a third step the forging is soft annealed, followed by a thermal age-hardening treatment consisting of at least one hardening treatment and at least one tempering treatment.

The method according to the invention is innovative in that a heat-treated metal object can be produced which also has a completely hardened structure in the case of large cross-sections, so that plastic molding die elements with uniform mechanical properties and high quality can be produced from such a workpiece. The superior hardenability is mainly attributed to the bonding of nickel to other alloying elements.

It is possible with the method according to the invention to adjust the ferrite content of the microstructure over a wide range, which can affect the machinability of the workpiece.

In the case of nickel contents of 0.2 to 1.8 wt.%, the ferrite content in the microstructure can be set in a targeted manner according to the invention to 0 to 70 vol.% by selecting the temperature and time of the annealing treatment. At sufficient annealing time, the ferrite content can be obtained according to the following empirical equation relating nickel content: 0.5% nickel: ferrite content [ volume% ] -0.345 × annealing temperature [ ° c ] -370 (1) 1.0% nickel: ferrite content [ volume% ] -0.355 × annealing temperature [ ° c ] -390 (2) 1.5% nickel: ferrite content [% by volume ] - [ 0.375X annealing temperature [ ° C ] -430 (3)

The content of ferrite in the structure can also be adjusted at a given annealing temperature simply by varying the nickel content in the steel.

As has proven advantageous, when the annealing treatment is carried out between 1080 ℃ and 1350 ℃ for at least 12 hours, preferably at least 24 hours, an exact adjustment of the ferrite content to the advantageous stability required for the further processing of the material is ensured.

In a further advantageous form of the method according to the invention, the ferrite content in the microstructure is adjusted by means of an annealing treatment up to 15% by volume, preferably up to 10% by volume, most preferably up to 6% by volume, so that good strength properties can be achieved at the same time as the desired good machinability of the workpiece.

It has also been found that when the steel contains 13.8-15.0 wt.%, preferably 14.1-14.7 wt.% chromium and/or 0.25-1.6 wt.%, preferably 0.35-1.1 wt.%, most preferably 0.8-1.0 wt.% nickel, the ferrite content of the workpiece produced according to the invention is determined particularly accurately, and workpieces having corrosion resistance properties exceeding those of the prior art can be produced within this range of chromium and nickel contents.

As already mentioned, the corrosion resistance of the material is particularly improved when the copper content in the steel is between 0.2 and 0.35 wt.%. However, a copper content of more than 0.35 wt.% adversely affects the thermal deformation of the material and reduces the surface properties of the component. In addition, a copper content of more than 0.35 wt.% increases the tendency of the material to crevice corrosion.

The use of the steel according to the invention for the frame construction of plastic moulding tools has proven to be particularly advantageous and economical. Due to the improved machinability, high corrosion resistance and good weldability, the steel is suitable for the economical production of mould elements which require high chemical resistance and long service life for the application. The use of forgings made from the alloy according to the invention has proven particularly suitable for the production of moulds or components, in particular because of its extremely high economy.

The following is an embodiment which is intended to be illustrative only and to be understood as an embodiment of the invention.

Figure 1 shows a graph of wear mark width of a die versus life of a workpiece of steel according to the invention (marked 1-3) and a workpiece of reference steel (marked 4-6).

For the examination of the material properties, the workpieces produced according to the invention and the reference workpieces having the chemical compositions listed in table 1 were used. The chemical composition of the workpieces 1 to 3 each relates to a steel according to the invention, while the chemical composition of the workpieces 4 to 6 relates to a reference steel known from the prior art.

Table 1: chemical composition of the inspected workpiece, wt%

(impurities introduced during the preparation are not shown)

Workpiece composed of (wt%)

123456C 0.050.040.060.340.080.05 Mn 0.80.700.861.400.801.35 Si 0.340.430.370.350.450.48S 0.240.10.140.120.180.22P 0.020.010.010.020.020.02 Cr 13.912.514.516.012.812.6 Ni 0.630.40.950.65 Cu 0.150.250.340.95 Mo 0.10.250.920.15V 0.060.030.090.08 Al 0.030.020.020.040.040.04N 0.040.050.050.050.050.04 balance Fe Fe Fe Fe Fe Fe Fe Fe

The workpieces are heat-treated, i.e. annealed in a first step between 1080 ℃ and 1350 ℃ for 15 hours, the workpieces according to the invention being adjusted to the desired ferrite content at a given content of nickel by this temperature. In a second step, the workpieces are each forged at about 1000 ℃ with a 6-fold deformation, and then soft annealed at 590 ℃. Finally, carrying out thermal age hardening, including hardening at 1020 ℃ and tempering within 530 ℃. The hardenability of these workpieces was checked. Machinability and corrosion resistance were also examined.

Five hardness measurements along the horizontal axis of the cross-section of the workpiece quantitatively determine the hardenability across the cross-section of the workpiece. The hardness differences of the workpieces produced according to the invention with respect to the maximum hardness values are within a maximum of + -5%, while the hardness differences of the reference workpieces 4, 5 or 6 are + -10% or more.

For the determination of the machinability, a ring cutter equipped with rolled pieces of hard metal was used, the cutting coefficient being as follows:

cutting speed: 350m min^-1

Inlet/gear milling: 0.3mm

The improvement according to the invention is illustrated in fig. 1, from which fig. 1 it is evident that the service life of the mold produced during the treatment of the steel according to the invention is significantly increased.

Workpieces produced according to the method of the invention with a ferrite content in the microstructure of up to approximately 5% prove to be easier to machine with the same chemical composition.

The corrosion resistance test was carried out according to german industrial standard 50021 by means of a series of salt spray tests, in which the corroded area fraction was determined after a treatment time of 2 hours and 5 hours. The size of the detected workpiece sample is 36cm²。

Table 2: testing the corrosion resistance of a workpiece according to German Industrial Standard 50021

Eroded area fraction [% ] spray time workpiece

[h] 1 2 3 4 5 6

2 28 33 32 35 50 37

5 57 61 63 68 71 69

The welding of the mould elements made of steel according to the invention does not cause problems.

Tests carried out on steels according to the invention which also contain up to about 1.5% by weight of other alloying elements in the steel have shown similar results, the elements added being mainly elements of metals of sub-groups 5 and 6 of the periodic table.

Claims (13)

1. A maraging steel with improved machinability, good weldability and high corrosion resistance, comprising%

0.02-0.075% carbon

0.1-0.6% of silicon

0.5-0.95% manganese

0.08-0.25% of sulfur

Phosphorus with an upper limit of 0.04%

12.4-15.2% of chromium

0.05-1.0% molybdenum

0.2-1.8% of nickel

An upper limit of 0.15% of vanadium

0.1-0.45% of copper

Aluminum with an upper limit of 0.03%

0.02-0.08% nitrogen

And possibly up to 2.0% of one or more other alloying elements, the balance being iron, and impurities introduced during the refining process, the steel having a ferrite content of less than 28% by volume in the microstructure.

2. Steel according to claim 1, containing 0.80-0.90% manganese and 0.10-0.16% sulphur.

3. Steel according to claim 1 or 2, containing 13.8-15.0%, preferably 14.1-14.7% chromium.

4. Steel according to one of claims 1 to 3, containing 0.25-1.6%, preferably 0.35-1.1%, most preferably 0.8-1.0% nickel.

5. Steel according to one of claims 1 to 4, containing 0.25 to 0.35% copper.

6. Steel according to one of claims 1 to 5, having a structure comprising up to 15% ferrite (in vol.%), preferably up to 10%, most preferably up to 6%.

7. A method for heat treating maraging steel with improved machinability, by means of which an object is produced that is through-hardened even at large cross-sections, characterized in that a steel ingot has the following composition (in weight%),

0.02-0.075% carbon

0.1-0.6% of silicon

0.5-0.95% manganese

0.08-0.25% of sulfur

Phosphorus with an upper limit of 0.04%

12.4-15.2% of chromium

0.05-1.0% molybdenum

0.2-1.8% of nickel

An upper limit of 0.15% of vanadium

0.1-0.45% of copper

Aluminum with an upper limit of 0.03%

0.02-0.08% nitrogen

And possibly up to 2.0% of other alloying elements, the balance being iron, and impurities entrained during the refining, the ingot being subjected in a first step to an annealing treatment for forming and adjusting the ferrite content in the structure, in a second step to a hot deformation with a degree of deformation of at least 4 times, and then in a third step to a softening annealing, followed by a thermal age-hardening consisting of at least one hardening treatment and at least one tempering treatment.

8. Method according to claim 7, characterized in that the annealing treatment for forming and adjusting the ferrite content in the microstructure between 1080 ℃ and 1350 ℃ is carried out for at least 12 hours, preferably at least 24 hours.

9. Method according to one of claims 7 or 8, characterized in that the ferrite content (vol.%) is adjusted by means of an annealing treatment to a maximum of 15%, preferably to 10%, most preferably to 6%.

10. Method according to one of claims 7 to 9, characterized in that the steel contains (wt.%) 13.8-15.0%, preferably 14.1-14.7% chromium and/or 0.25-1.6%, preferably 0.35-1.1% and particularly preferably 0.8-1.0% nickel.

11. Method according to one of claims 7 to 10, characterized in that the steel contains 0.25-0.35 wt.% copper.

12. Use of a steel according to one of claims 1 to 6 for the production of a frame construction material for plastic moulding tools.

13. Use of a steel according to one of claims 1 to 6, for a steel having a thickness of at least 0.32m and at least 0.1m²A cross-sectional area of a wrought product heat treated according to the method of any of claims 7 to 9 for making a plastic forming die and/or die element made by a drawing process.