EP2483014B1 - Method for strip casting steel and system for strip casting - Google Patents

Description

The invention relates to a method for casting a steel having a chromium content of more than 15 percent by weight, in particular more than 33 percent by weight.

High-temperature-resistant and wear-resistant steels with a high chromium content and carbon-containing alloys have hitherto been produced as individual parts in a discontinuous sand casting process. The cast product is then mechanically processed directly without additional thermomechanical treatment.

In the case of horizontal strip casting of steels, the previously known methods for casting a steel always assume an almost final casting in combination with an offline or an inline rolling. The forming or rolling step has both the purpose of reducing the thickness and the structure formation, d. H. the recrystallization. It is a process aimed at the production of hot rolled strip for steel alloys. To achieve the mechanical properties, the transformation of the cast structure is absolutely necessary, especially in conventional steels.

In strip casting, liquid steel is fed by a feed system with a correspondingly formed nozzle onto a circulating transport belt cooled from below with water. The conveyor belt is driven and guided by two pulleys. The melt applied to the conveyor belt solidifies completely in the area of the primary cooling. After solidification, the strip enters inline rolling in rolling stands. After inline rolling and another cooling process, the belt is rewound. Such a casting method for strip casting is from the DE 198 52 275 A1 known.

Also from the WO 02/064288 A1 For example, there are known a method and apparatus for potting and solidifying liquid metal and its division, which is particularly intended for ferroalloys or non-ferrous metals and their fragmentation. In this case, endless strips of predeterminable thickness are produced from the liquid phase by the use of a strip casting plant. A crushing plant divides these into pieces, wherein in the surface of the bands during casting and solidification, a pattern of predetermined breaking points is impressed, which determines the predefined, optimal size of the product pieces when breaking the tape.

On the other hand, selected components, for example bearing sleeves for turbochargers, are cast in a discontinuous casting process, for example by casting in sand casting molds. Since a mechanical reworking of the individual components is always required, the castings are cast with a slight excess and then turned off. If necessary, further manufacturing steps such as drilling are required. However, no hot rolling takes place with the aim of recrystallization or thickness reduction. From the EP 0 060 661 A1 For example, a method for casting steel or an iron alloy in a horizontal strip casting plant is known, wherein the iron alloy may comprise ferrochrome. The summary of the JP 51 103 035 A calls a method for casting ferrochrome.

It is the object of the invention to produce special steel alloys while saving process steps and energy, ie simpler than previously possible.

The inventive method is defined by the features of claim 1.

The use of the strip casting method makes it possible to produce steel strips of very low thickness, for example of 50 mm or less, continuously.

Advantageous developments of the invention will become apparent from the dependent claims.

The invention is particularly suitable for casting a steel which additionally has a carbon content of more than 1 percent by weight.

Likewise, the strip casting method is advantageously used if the steel additionally has a silicon content of more than 2 percent by weight.

It can be provided that the cast steel strip or casting plates produced by casting are cut, milled, drilled or annealed.

The invention also relates to a horizontal strip casting plant for carrying out the method according to the invention.

The invention thus aims at the use of a horizontal Bandgießvorrichtung for the production of building or insert parts from, for example, high-alloy tool steels whose dimensions match the cast strip thickness in horizontal strip casting, so that a hot rolling with the goal of recrystallization or thickness reduction is not necessary. The blanks can be poured continuously, and the creation of single molds as in sand casting is eliminated.

The strip caster may be associated with other equipment for the mechanical aftertreatment of cast by the conveyor belt casting plates. These are, for example, cutting devices for the exact definition of the width and the length of the cast blanks which already have dimensions close to their final dimensions during the casting process. Alternatively, the cast semi-finished product is processed by various mechanical methods to obtain the desired final dimensions.

Advantageously, it can also be provided that a separating or cutting device for severing the cast product is arranged downstream of the conveyor belt in the conveying direction.

The invention will be explained in more detail in exemplary embodiments.

Fig. 1 a, b

eine schematische Seitenansicht und eine Draufsicht einer Anlage zum Bandgießen nach einer ersten Ausführungsform,

Fig. 2a, b

eine seitliche Schnittansicht eines Transportbandes zum Gießen von Gussplatinen und eine Draufsicht auf das Transportband, mit dem Gussplatinen mit verschiedenen Abmessungen nebeneinander gegossen werden, und

Fig. 3a - d

Temperaturverläufe von mit einer Bandgießanlage gegossenen Gussstücken als Funktionen der Zeit.

Show it:

Fig. 1 a, b

a schematic side view and a plan view of a plant for strip casting according to a first embodiment,

Fig. 2a, b

a side sectional view of a conveyor belt for casting of cast iron plates and a plan view of the conveyor belt, with the casting boards of different dimensions are cast side by side, and

Fig. 3a - d

Temperature profiles of castings cast with a strip caster as functions of time.

A strip casting plant 1 ( Fig. 1a, b ) for casting a steel having a chromium content of more than 15 percent by weight comprises a liquid steel supply system comprising a furnace 2 and a tundish or ladle 3 for storing or making up a certain quantity of liquid steel which is fed through an outlet nozzle 4 to an endless, preferably also existing steel conveyor belt 5 is applied with a cooling device 6, wherein the cooling device 6 comprises, for example, a tank 13 with a cooling liquid, through which the conveyor belt 5 is passed on its underside.

The outlet nozzle 4 has a width which corresponds to the width of a casting plate 7 to be cast on the revolving conveyor belt 5. The conveyor belt 5 is driven and guided by two deflection rollers 10, 11 each equipped with its own drive 8, 9.

Preferably, at the two narrow band sides of the conveyor belt 5 with this follower forming segments 12 are provided, which are arranged overlapping each other or close to each other to prevent leakage of the liquid steel. The distance between the segments 12 is either predetermined by the width of the conveyor belt 5 or adjustable according to the desired width of the board to be cast 7. For cooling, the conveyor belt 5 passes through together with the segments 12, the container 13 of the cooling device 6, which may also include other means for cooling the board 7. For example, the two deflection rollers 10, 11 can each flow through a cooling liquid. Also below the running on the top of the strip casting line 1 belt area of the conveyor belt 5, the system 1, for example, a spray cooling device 14, which sprays the conveyor belt 5 from the bottom with a cooling liquid. In this way, a board 7 pour, the length of which corresponds approximately to the length of the system 1 or beyond its length. The casting plate 7 either already has the desired length resulting from a determination of the amount of liquid steel in the tundish, or it is separated by means of a cutting or crushing device 15 after leaving the conveyor belt 5 and fed to a semi-finished stock 16. From there, it is forwarded for further treatment, for example milling, drilling, annealing or further cutting or other machining processes, to the corresponding devices and devices. Since the board 7 already has the desired thickness, it needs no rolling process as a thermomechanical treatment process to go through. However, depending on the desired material properties, a rolling process may also be provided as a treatment step to the casting process, either immediately after this or after the castings have cooled.

According to an alternative of the invention, a conveyor belt 17 (FIG. Fig. 2a, b ) in the strip caster 18 a plurality of recesses 19, whose dimensions are preferably close to the final dimensions of the castings to be cast in them. The recesses 19 are arranged either only one behind the other or depending on the width of the conveyor belt 17 in addition to each other. The dimensions of the adjacent recesses 19 may have different dimensions. Corresponding to the number of rows of recesses 19 mounted side by side on the conveyor belt 17, a plurality of outlet nozzles 20 with a width adapted thereto are also provided. After leaving the conveyor 17, the castings cast in it, as in Fig. 1b shown, fed to a semi-finished stock 16.

Casting can be used to produce castings of various thicknesses; Usually, the casting thickness is between 8 and 25 mm, preferably 15 mm. Typical applications of the high-alloy material are bearing sleeves, for example, the dimensions of 13 mm (thickness) x 120 mm (length) and are made by machining of castings, for example, with the thickness 15 mm and the same width on a lathe, wherein the hollow cylindrical Form is produced by drilling out of the core. All other forms are obtained by machining superficial material of the castings.

In another embodiment, the casting geometry is 15 mm (width) x 125 mm (length); this results from mechanical, in particular Machining, machining a component geometry of 12.7 mm (width) x 120 mm (length).

In addition to the chromium content of the steel, this preferably also contains carbon of more than 1 percent by weight and / or silicon of more than 2 percent by weight. The property profile of the castings to be cast with the strip caster 1 or 18 includes good high temperature resistance combined with good wear resistance. The Rockwell hardness (HRC) is 33 to 38 at a tensile strength of about 1000 MPa.

In the procedure according to the invention, the process-typical rapid cooling has positive effects on the microstructure of the castings. Grain size is reduced and precipitates, such as carbides, are finely dispersed in the matrix due to the impeded diffusion. This results in advantageous mechanical properties.

According to the invention, various possibilities of temperature control for cooling the castings ( Fig. 3a - d ). According to a first approach ( Fig. 3a ), the temperature of the cast products is first held for a predetermined time and then cooled at a defined cooling rate (curve 21). Alternatively, after a prolonged period of time during which the temperature is maintained, a heating phase may also follow (curve 22). In this case, the castings are not cooled until later.

According to a second method, the castings are heated immediately after casting to a defined temperature ( Fig. 3b ) and then cooled for a predetermined period of time at a predetermined cooling rate.

According to another approach, the castings are directly through Quenching rapidly cooled ( Fig. 3c ) and then subjected to a controlled temperature control over time, as far as they have not yet assumed the ambient temperature by the quenching process.

In a further procedure ( Fig. 3d ), the castings with alternating cooling rates are cooled to ambient temperature.

LIST OF REFERENCE NUMBERS

1

strip casting plant

2

oven

3

ladle

4

outlet nozzle

5

conveyor belt

6

cooler

7

cast board

8th

drive

9

drive

10

idler pulley

11

idler pulley

12

segments

13

container

14

Spray cooler

15

Cutting or crushing device

16

Semi-finished products warehouse

17

conveyor belt

18

strip casting plant

19

wells

20

outlet nozzles

21

Curve

22

Curve

Claims (7)

1. Method of casting a steel strip with chromium content, wherein the steel strip is cast in a horizontal strip casting plant (1, 18),
   characterised in that
   the steel has a chromium content of more than 15 weight percent and
   for formation of cast plates (19) within the steel strip the steel is cast on a transport belt (17) with depressions or the steel is cast on a transport belt with transversely extending projections for formation of frangible locations within the steel strip.
2. Method according to claim 1, characterised in that the steel is cast with a carbon component of more than 1 weight percent.
3. Method according to claim 1 or 2, characterised in that the steel is cast with a silicon component of more than 2 weight percent.
4. Method according to any one of claims 1 to 3, characterised in that the cast plates (19) created by casting are cut, milled, drilled or annealed or the cast steel strip is cut, milled, drilled or annealed.
5. Horizontal strip casting plant for carrying out a method according to any one of claims 1 to 4, wherein the plant comprises a smelting furnace (2), a casting ladle (3) and a transport belt (5, 17) for reception and cooling of a liquid steel flowing out of the casting ladle (3),
   characterised in that
   the transport belt (17) is furnished with recesses for formation of cast plates (19) within the steel strip or with transversely extending projections for formation of frangible locations within the steel strip.
6. Strip casting plant according to claim 5, characterised in that machining devices for fixing the width and length of the cast plates (7, 19) are present.
7. Strip casting plant according to claim 5, characterised in that a separating or cutting device (15) for parting the steel strip is arranged downstream of the transport belt (5) in conveying direction.

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