EP0091897B1 - Strain hardening austenitic manganese steel and process for the manufacture thereof - Google Patents

Description

The invention relates to a strain-hardening austenitic manganese steel with an elongation at break of 10 to 80% and to a method for producing the same.

Cold-hardening austenitic manganese steels have a wide range of applications, whereby both castings and forgings as well as rolled material are used. The wide range of applications is particularly given by the high ductility and the good work hardenability. The area of application ranges from castings for hard comminution to bulletproof objects. The valuable properties of the manganese hard stands lie in the combination of the above-mentioned properties of the work hardening ability and its lactility. The work hardening occurs whenever the manganese steel is mechanically, e.g. B. is claimed by impact or shock. Hiebie is likely to convert the austenite in one surface layer into an epsilon martensite. Measurements of work hardening show an increase in hardness from 200 HB to up to 550 HB. There is thus an increase in hardness during use when the casting, forging or the like is subjected to mechanical stress. However, since such objects are also subject to an abrasive stress, this surface layer is continually removed, but a renewed transformation of the austenite now on the surface occurs due to renewed mechanical stress. The alloy located under the surface layer has a high ductility, so that the manganese steels can also withstand high mechanical impact loads without risk of breakage, even with a smaller wall thickness.

No specific steel can be found in British patent specification 1 187 027, but only limits in percent by weight are given which are to be complied with. The teaching that a certain ratio of carbon to manganese should be adhered to cannot be found at any point in this reference. The teaching of achieving grain refinement by adding zircon and at the same time maintaining an increase in mechanical properties cannot be inferred. The teaching can be found in the Russian copyright certificate 581.165, a steel which among other things Contains 0.9 - 1.5% by weight of C and 9 - 15% by weight of Mn, to be alloyed with molybdenum, vanadium and aluminum. The limits for vanadium are 0.05 and 0.5% by weight. Various processes for the production of general manganese steels can be found in the handbook of special steel customers, first volume, pages 510 - 515.

For all objects to be made from manganese steel, it is a prerequisite that a casting, be it a molded casting or a block casting, is initially manufactured. Due to the properties of this casting, the properties of the objects made from it are then predetermined. If the structure is too coarse in the casting, the ductility of the commodity is then too low. In the case of large castings, as is known per se, this has different grain zones over its cross section. On the outside there is a thin, relatively fine-grained edge zone, which is followed by a zone with coarse stem crystals, which is followed by the center of the casting with a globular structure. Although the steel is essentially austenitic and work hardenable over the entire cross-section, due to the different structure it shows great differences in mechanical properties, in particular in ductility.

In order to achieve a ductility as uniform as possible over the entire cross section, it has already been proposed that the casting temperature be as low as possible, e.g. to keep at 1410 ° C, which increases the number of germs with increasing supercooling and a finer grain should be achieved. However, such low casting temperatures lead to great difficulties in production. Cold welding occurs on the casting, furthermore, the rheological properties of the melt are already such that an exact shape, especially in edges, is no longer given. In addition, the melt solidifies on the pan lining during casting, which leads to pan shells which then have to be removed and refurbished. During the casting itself, the plug can become stuck in the spout, as a result of which the casting must be stopped. As can easily be seen from these explanations, the economic disadvantages which have to be accepted for a non-reproducible grain refinement are so serious that the processes with such a low casting temperature could not prevail.

Another method for grain refinement consists of a targeted heat treatment, in which the casting is annealed at a temperature between 500 and 600 ° C. for 8 to 12 hours, whereby a high proportion of the austenite is to be converted to pearlite. Austenitization annealing is then carried out at a temperature between 970 and 1110 ° C. The two-fold change in structure is said to produce a finer grain, but on the equilibrium side the cause is that the product becomes extremely brittle during the heat treatment and breaks even without deformation under less mechanical stress. Another major disadvantage is that this method requires a large amount of energy.

For the reasons given above, attempts have already been made to achieve grain refinement by adding additional alloying elements. Chromium, titanium, zirconium and nitrogen, for example, were used as alloying elements. The additives were selected so that the manganese high-carbon steel had contents of at least 0.1 or 0.2% by weight of the corresponding alloying elements. These additives really cause grain refinement at low casting temperatures, however there is a significant deterioration in the mechanical properties, in particular the elongation and the impact strength.

Manganese steels usually have a carbon content of 0.7 to 1.7% by weight, with a manganese content of between 5 and 18% by weight to be maintained. essential for the properties of a Manganese steel is also that a ratio of carbon to manganese between 1: 4 and 1:14 is maintained. If the ratio is lower, there is no longer any austenitic steel and it can therefore no longer be work hardened, and at the same time there is less toughness. At a higher ratio, the austenite is too stable, which means that it can no longer be work hardened, so that the desired properties cannot be achieved either.

A phosphorus content of more than 0.1% by weight causes an extreme drop in toughness, so that, as is known per se for phosphorus, a particularly low value is aimed for.

ASTM A 128/64 describes four different types of high manganese steels, the carbon cold varying between 0.7 and 1.45% by weight and the manganese content between 11 and 14.0% by weight. The carbon content is changed to vary the work hardenability, which is also to be influenced by the addition of chromium. The chromium additives then range between 1.5 and 2.5% by weight. Coarse carbide deposits are to be suppressed by adding up to 2.3% molybdenum. The addition of up to 4.0% by weight of nickel is said to stabilize the austenite, which prevents the formation of pearlite in thick-walled castings.

Furthermore, a high manganese steel has already become known which has a manganese content of approximately 5% by weight. With such steels, however, the toughness is already low. However, it has a high wear resistance.

The present invention has set itself the goal of creating a strain-hardening austenitic manganese high-carbon steel which has an elongation at break of 10 to 80%, which has a structure that is as uniform as possible over the entire cross section and has a particularly fine grain, at the same time without any deterioration in the mechanical properties should occur.

• 0,7 bis 1,7 C
• 5,0 bis 18,0 Mn
• 0 bis 3,0 Cr
• 0 bis 4,0 Ni
• 0 bis 2,5 Mo
• 0,1 bis 0,9 Si
• max. 0,1 P

The cold-hardening austenitic high manganese steel with an elongation at break of 10 to 80% measured according to L = 5 d or L = 10 d with a content in% by weight of

• 0.7 to 1.7 C.
• 5.0 to 18.0 Mn
• 0 to 3.0 cr
• 0 to 4.0 Ni
• 0 to 2.5 months
• 0.1 to 0.9 Si
• Max. 0.1 p

and provided that the ratio of carbon to manganese is between 1: 4 and 1:14 has up to 0.05 titanium, 0.05 zirconium and 0.05 vanadium as microalloying elements with the proviso that the sum of titanium and zirconium between 0.002% and 0.05%, optionally also containing vanadium as a microalloying element, the sum of these microalloying elements being up to 0.05% by weight. It was quite surprising that with such a small addition of alloying elements, grain refinement can be achieved while maintaining or increasing the mechanical properties, since additions of 0.1% and above caused the mechanical properties to deteriorate. An exact explanation for this fact has not yet been found. Zircon and vanadium are particularly effective at high casting temperatures.

If the manganese steel has an additional 0.002 to 0.008% by weight boron, an even smaller grain can be achieved.

Grain refinement is particularly good when only 0.01 to 0.025% by weight of titanium is present as the microalloying element.

If the manganese steel has a content of 0.01 to 0.05% by weight of aluminum, the titanium content can be maintained particularly precisely.

In the manufacture of a manganese steel casting according to the invention, an insert being melted in an electric furnace, after which calcareous, slag-forming additives are added to the liquid melt and the desired analysis is set and brought to a tapping temperature of 1450 to 1600 ° C. with an oxygen affinity Element is deoxidized and tapped into the ladle, consists essentially in the fact that the content of the microalloying elements titanium, zirconium and vanadium is set in the ladle, and the melt is poured at a temperature between 1420 and 1520 ° C. and again after the casting has cooled heated to austenitizing temperature from 980 to 1150 ° G and then cooled rapidly.

By adding the microalloying elements in the ladle, a reproducible content of microalloying elements is achieved. A particularly high level of toughness is achieved by the heat treatment of the casting, heating to an austenitizing temperature of 980 to 1150 ° C. and then rapidly cooling.

If the casting is lowered to a temperature of 980 to 1000 ° C after heating to 1030 to 1150 ° C, whereupon it is cooled rapidly after temperature compensation in the casting, the susceptibility to cracking in the casting obtained can be significantly reduced. Manganese steel has a lower thermal conductivity than other steels (only one sixth of that of iron), so special attention must be paid to temperature compensation.

A reliable dissolution of the grain boundary carbides even with larger cross-sections and with lower energy consumption can be achieved at a temperature between 1080 and 1100 ° C during solution annealing, whereupon the temperature is reduced to 980 to 1000 ° C and equalized and then rapidly cooled becomes.

A casting with particularly low internal stresses can be achieved by alternately applying coolants of different thermal conductivity after heating to the austenitizing temperature. Water and air are to be used as particularly suitable coolants.

If the casting is already removed from the mold at a temperature between 800 and 1000 ° C and then placed in a heat treatment furnace in which the temperature of the casting is equalized, whereupon the temperature is immediately raised to the austenitizing temperature, a particularly energy-saving process is provided, with a build-up of high Tension in the casting is prevented, whereby pearlitization can be avoided.

The invention is explained in more detail below with the aid of examples.

example 1

• 1,21 Gew.-% Kohlenstoff; 12,3 Gew.-% Mangan; 0,47 Gew.-% Silizium; 0,023 Gew.-% Phosphor; 0,45 Gew.- % Chrom und Spuren von Nickel und Molybdän. Die Schmelze wurde mit einer Schlacke aus 90 Gew.% Kalkstein und 10 Gew.-% Kalziumflorid abgedeckt, worauf eine Abstichtemperatur von 1520°C eingestellt wurde. Sodann wurde eine Schlußdesoxidation mit metallischen Aluminium durchgeführt. Nach der Desoxidation wurde in die Gießpfanne abgestochen. In diese wurde eine Temperatur von 1460°C gemessen. Die Schmelze wurde in eine basische Sandgußform (Magnesit) vergossen. Das erhaltene Gußstück, ein Turas (englisch: Tumbler) Bruttogewicht 14 t - Nettogewicht 11 t, wies Wandstärken von 60 bis 180 mm auf. Das Gußstück wurde auf Raumtemperatur erkalten gelassen, sodann wurde entformt und langsam auf 1050°C erhitzt. Nach einer Haltezeit von vier Stunden wurde der Turas in Wasser getaucht und rasch abgekühlt. Das so erhaltene Gußstück wies Risse auf, die durch Schweißen mit artgleichem Material wieder geschlossen werden mußten. Die metallographische Untersuchung ergab eine extrem ausgebildete Transkristallitenzone, mit einer anschließenden globulitischen Zone. Proben aus der globulitischen Zone hatten eine Dehnung gemessen mit einer Probe L = 10 d von 8,4 %. Die Zugfestigkeit lag bei 623 N/mm².

15 tons of high manganese steel of the following composition were melted in an electric arc furnace:

• 1.21 wt% carbon; 12.3% by weight of manganese; 0.47 wt% silicon; 0.023 wt% phosphorus; 0.45% by weight of chromium and traces of nickel and molybdenum. The melt was covered with a slag made of 90% by weight limestone and 10% by weight calcium fluoride, whereupon a tapping temperature of 1520 ° C. was set. Final deoxidation was then carried out with metallic aluminum. After the deoxidation, the ladle was tapped. A temperature of 1460 ° C. was measured in this. The melt was poured into a basic sand mold (magnesite). The casting obtained, a Turas (English: Tumbler) gross weight 14 t - net weight 11 t, had wall thicknesses from 60 to 180 mm. The casting was allowed to cool to room temperature, then was demolded and slowly heated to 1050 ° C. After a four hour hold, the turas was immersed in water and cooled rapidly. The casting thus obtained had cracks that had to be closed again by welding with the same type of material. The metallographic examination revealed an extremely well-developed transcrystalline zone, followed by a globulitic zone. Samples from the globulitic zone had an elongation measured with a sample L = 10 d of 8.4%. The tensile strength was 623 N / mm ² .

Example 2

The procedure was analogous to Example 1, with titanium in the form of ferrotitanium being added to the ladle. The ladle was brought to the mold and it was poured at 1460 ° C. After the casting had cooled, it was heated to 1100 ° C., at which temperature it was kept for four hours, whereupon the temperature of the furnace was lowered to 1000 ° C. After an hour, a temperature equalization in the casting was reached, followed by cooling with alternating immersion in a water bath. The Turas thus obtained was free from cracks. With the exception of the marginal zone, which was microcrystalline, the metallographic examination revealed a completely uniform, fine-grained structure. The casting had an average titanium content of 0.02% by weight. The mechanical properties were almost identical for the specimens removed in the center and on the edge, the tensile strength being 820 and 830 N / mm ² and the elongation being 40 and 45%, respectively.

Example 3

For the production of a drop-forged hammer with pin for a Gastein mill, which had a weight of 180 kg, a block was cast analogously to Example 2. This block was then cut up, and these parts were drop-forged into impact hammers at a forging temperature of 1050 ° C. These impact hammers had a perfectly fine structure in the area of the pin attachments, which was retained even after solution annealing and quenching. In the case of a hammer which was produced with an alloy according to Example 1, coarse-grained crystals appeared in the area of the pin attachments, which in some cases caused microcracks.

Example 4

• 1,0 Gew.-% Kohlenstoff; 5,2 Gew -% Mangan; 0,4 Gew.-% Silizium; 1,7 Gew.-% Chrom; 1,0 Gew.-% Molybdän und 0,03 Gew.-% Phosphor. Die Schmelze wurde mit einer Schlacke aus 90 Gew-% Kalkstein und 10

Gew.-% Kalziumflorid abgedeckt, worauf eine Abstichtemperatur von 1490° C eingestellt wurde. Sodann wurde eine Schlußdesoxidation mit metallischem Aluminium durchgeführt. Nach der Desoxidation wurde in die Gießpfanne abgestochen, in welcher eine Temperatur von 1430° C gemessen wurde. Der Schmetze wurde in der Gießpfanne Ferrotitan und eine Zirkon-Vanadin-Legierung zugegeben. Bei dem Gießen von Platten für Kugelmühlen wurde eine Temperatur von ca. 1430°C eingehalten. Die erhaltenen Platten wiesen eine Wandstärke von 80 mm auf. Sie wurden bei einer Temperatur von 850°C entformt und in einem Wärmebehandlungsofen, der auf einer Temparatur von 850°C eingestellt war, zwei Stunden bis auf Temperaturausgleich gelagert, worauf auf 1100°C erwärmt und sodann abgekühlt wurde. Die metallographische Untersuchung ergab mit Ausnahme der Randzone, die mikrokristallin war, ein vollkommen gleichförmiges feinkörniges Gefüge. Der durchschnittliche Gehalt an Titan, Vanadin und Zirkon betrug 0,03 Gew.-%. Die mechanischen Eigenschaften der am Wand bzw. im Zentrum entnommenen Probestücke war fast ident, wobei die Zugfestigkeit bei 850 bzw. 835 Nfmm² und die Dehnung bei 45 bzw. 48 % lag.In an electric arc furnace, 10 tons of high manganese steel of the following composition were melted:

• 1.0 wt% carbon; 5.2% by weight of manganese; 0.4 wt% silicon; 1.7 wt% chromium; 1.0 wt% molybdenum and 0.03 wt% phosphorus. The melt was treated with a slag made of 90% by weight limestone and 10

Wt .-% calcium fluoride covered, whereupon a tapping temperature of 1490 ° C was set. Final deoxidation was then carried out with metallic aluminum. After the deoxidation, the ladle was tapped, in which a temperature of 1430 ° C. was measured. Ferrotitan and a zircon-vanadium alloy were added to the butcher in the ladle. When casting plates for ball mills, a temperature of about 1430 ° C was maintained. The plates obtained had a wall thickness of 80 mm. They were removed from the mold at a temperature of 850 ° C. and stored in a heat treatment oven which was set to a temperature of 850 ° C. for two hours until the temperature had equalized, whereupon the temperature was raised to 1100 ° C. and then cooled. With the exception of the marginal zone, which was microcrystalline, the metallographic examination revealed a completely uniform, fine-grained structure. The average content of titanium, vanadium and zircon was 0.03% by weight. The mechanical properties of the test pieces taken from the wall or in the center were almost identical, the tensile strength being 850 or 835 Nm ² and the elongation being 45 or 48%.

Example 5

The procedure was analogous to Example 2, with boron also being added to the ladle in addition to titanium. The temperature trip was observed as in Example 2. The casting had an average titanium content of 0.02% by weight and an average boron content of 0.005% by weight. The micrographs of samples taken at analogous locations showed that there were 50 grains per millimeter in the samples containing only titanium, whereas the samples containing boron had an average of 60 grains, resulting in a decrease in the average grain diameter of 0.02 mm to 0.017 mm.

Example 6

• 1,35 Gew.-% Kohlenstoff: 17,2 Gew.-% Mangan; Spuren von Nickel und Chrom und 0,02 Gew.-% Phosphor. Die Schmelze wurde mit einer Schlacke aus 90 Gew.-% Kalkstein und 10 Gew.-% Kalziumflorid abgedeckt, worauf eine Abstichtemperatur von 1600°C eingestellt wurde. Die Schlußdesoxidation wurde mit metallischem Aluminium durchgeführt, wonach in die Gießpfanne abgestochen und dieser Titan zugegeben wurde. Bei 1520°C erfolgte dann ein Gießen von Rundstäben mit einem Durchmesser von 110 mm. Die abgekühlten Rundstäbe wurden sodann entformt und auf 1030°C erwärmt und fünf Stunden auf dieser Temperatur gehalten. Danach wurde die Temperatur des Ofens auf 980°C abgesenkt und eineinhalb Stunden auf dieser Temperatur gehalten. Die Gußstücke sind sodann im Wasserbad rasch abgekühlt worden. Die Schmelzen wurden mit unterschiedlichem Titangehalt wiederholt, wobei die in der Tabelle angeführten mechanischen Werte bei den verschiedenen Probestücken, die dem Zentrum bzw. der Randzone entnommen wurden, gemessen werden konnten.
  

500 kg of manganese steel with the following composition were melted in an induction furnace:

• 1.35% carbon: 17.2% manganese; Traces of nickel and chromium and 0.02% by weight phosphorus. The melt was covered with a slag made of 90% by weight limestone and 10% by weight calcium fluoride, whereupon a tapping temperature of 1600 ° C. was set. The final deoxidation was carried out with metallic aluminum, after which the ladle was tapped and this titanium was added. Round bars with a diameter of 110 mm were then cast at 1520 ° C. The cooled round bars were then removed from the mold and heated to 1030 ° C. and kept at this temperature for five hours. The temperature of the furnace was then reduced to 980 ° C and held at this temperature for one and a half hours. The castings were then quickly cooled in a water bath. The melts were repeated with different titanium contents, the mechanical values given in the table being able to be measured for the various specimens which were removed from the center or the edge zone.
  

As can be seen in the table, the addition of one tenth of a% by weight of titanium causes a deterioration in the mechanical properties, at the same time there being a relatively large difference between the edge and center samples. With a titanium content of less than 5 hundredths, the properties of the edge and center samples are almost identical, whereby an increase in the mechanical properties compared to the non-microalloyed manganese steel is shown.

The tensile strength or elongation at break was determined in accordance with DIN 5 D 145/1975.

Claims (7)

1. A strain-hardening austenitic manganese steel with an elongation at fracture of 10% to 80%, measured according to L = 50 d or L = 10 d, with a content in wt.% of

0.7 to 1.7 C

5.0 to 18.0 Mn

0 to 3.0 Cr

0 to 4.0 Ni

0 to 2.5 Mo

0.1 to 0.9 Si

max. 0.1 P

under the condition that the ratio of carbon to manganese is between 1:4 and 1:14, and the content of micro-alloy elements in wt.% amounts to

0.0 to 0.05 Ti

0.0 to 0.05 Zr

under the condition that the sum of Ti + Zr lies between 0.002 wt.% and 0.05 wt.%, and where appropriate vanadium is additionally contained as a micro-alloy element, the sum of these micro-alloy elements amounting to up to 0.05 wt.%, and where appropriats 0.002 to 0.008 wt.% of boron and where appropriate 0.01 to 0.05 wt.% of aluminium are additionally contained, and the remainder is iron and impurities due to the melting process.

2. A strain-hardening austenitic manganese steel according to claim 1, characterized in that it contains only 0.01 to 0.025 wt.% of titanium.

3. A process for the manufacture of a strain-hardening austenitic manganese steel moulded or monobloc casting with a composition according to one of Claims 1 and 2, a charge being melted in an electric furnace, after which calcareous, slag-forming aggregates are added to the liquid melt, the desired analyses are adjusted, the melt is brought to a tapping temperature of 1450 to 1600°C, is deoxidized with an element having an affinity to oxygen, and is run off into the ladle, the content of the micro-alloy elements Ti, Zr and V is set in the ladle, and the melt is cast at between 1420 and 1520°C, and after cooling the casting heating to an austeniting temperature of 980 to 1150°C is again carried out and cooling then takes place rapidly.

4. A process according to claim 3, characterized in that after being heated to 1030 to 1150° C, preferably 1080 to 1100°C, the casting is reduced to a temperature of 980 to 1000°C, after which cooling takes place rapidly after temperatur&-€qua!ization in the casting.

5. A process according to claim 3 or 4, characterized in that after being heated to an austeniting temperature the casting is acted upon alternately with cooling agents of different thermal conductivity.

6. A process according to claim 5, characterized in that water and air are used alternately as the cooling agents.

7. A process according to any one of claims 3 to 6, characterized in that the casting is removed from the mould at a temperature between 800 and 1000°C and is conveyed to a heat-treatment furnace in which temperature equalization of the casting takes place, after which an immediate increase to the austeniting temperature takes place.