DE1135618B - Plain for casting molds - Google Patents

Description

Sizing for casting molds The invention relates to linings for Casting molds and especially those linings that contain rare earth salts.

It is common practice to line molds with fabrics that prevent them from sticking of the cast block on the wall, which should prevent the mold. This is usually done Used vaporizable hydrocarbons between the molten iron or steel and the wall of the mold form a layer of vapor.

When the pouring stream hits what is already in the mold liquid metal creates splashes. When these hit the wall of the mold and stick there, the ingot has irregularities. Aluminum oxides and silicates are often found on the surface and cause defects that have a particularly detrimental effect when the ingot is in a rolling mill is rolled into plates, sheets or strips. The inclusions cause streaks in the rolled steel material and lead to the formation of soft spots, which are especially in the further processing of sheet steel in drawing or deformation processes adversely affect.

Mold linings are generally made from hydrocarbon materials produced that are solid at room temperature and largely even at considerably higher temperatures remain in the area to which they were applied, however then, when they come into contact with hot molten steel, evaporate and spray quickly. The materials used for such linings include gilsonite, Tar, petroleum still bottoms, and Mexican graphite are used. Included Use of the latter silicon oxide get onto the surface of the ingot can, is its use with regard to the final use of the relevant Limited metal. A commercially available material known as "Dannold" sold and made from colloidal electrolyte graphite, an antioxidant and a desulfurizing agent in a refractory, non-toxic hydrocarbon carrier is also widely used. Any binder through which no harmful substances are introduced into the melt can be used.

The known lining materials are mostly sprayed onto the inner surface of the casting mold. The liner is chosen to withstand the preheating temperature of the mold, which is generally 260 ° C. The thickness of the liner applied is generally about 1 mm. When the lining material comes into contact with the molten metal, which in the case of steel generally has a temperature between 1540 and 1600 ° C. , it evaporates and thus forms a vapor layer protecting the wall of the casting mold.

A problem that often arises is that the cast metal splashed and so on the surface of the liner that has not yet been cast by the Metal is covered, steel splashes stick. These steel splashes are in one oxidized to a certain extent. Often even greater difficulties arise when Killed steel is used to which aluminum is added to remove oxygen because there was aluminum oxide on the edge and surface of the cast metal accumulates and rises on the sides of the melt. This also makes a kind of coating on the steel particles adhering to the lining of the mold which, if you, lining evaporated, immerse in the steel and so the above described Bring impurities and oxide inclusions into the steel.

The invention is based in particular on the object that is provided by the Steel splash adhering to the liner caused adverse phenomena eliminate and remove oxide inclusions and other impurities that too irregular Lead cast blocks and their usability for Impact rolling of steel sheets.

This object is achieved by the use of a size which consists of a binder that can withstand heat stresses up to about 260 ° C and a complex salt containing rare earths in finely divided form with the general formula NRX4, where N is an alkali metal, preferably sodium or potassium, R at least one rare earth with an atomic number between 57 and 71 and X is a halogen. Fluorine is preferably used as the halogen. This salt is contained in the size preferably in an amount of at least 55 g, advantageously about 115 g, per ton of steel.

It has been found that these rare earth fluorides are deposited in deposits find the mineral "bastnasite". The mineral is treated through a flotation process and divided into concentrates of different enrichment. The analyzes of the salary on rare earths were carried out on these concentrates.

The oxides of the rare earths from which the alkali-rare-earth fluorides are formed have the following compositions in percentages by weight, based on the total weight of all rare earths present. The table gives a typical example of four concentrate compositions. rehearse 1 2 3 4 Cerium ......... 53.75 48.90 55.12 52.46 Lanthanum .... 27.10 34.00 25.12 27.40 Neodymium ..... 11.56 11.20 12.69 14.00 Praseodymium . . 4.16 4.25 4.42 4.84 Samarium ... 0.71 0.75 0.75 0.75 Europium . . . 0.17 0.17 0.16 0.17 Gadolinium . . 0 0.21 0.19 0.23 When using the lining according to the invention for the casting mold, the surface of the cast block is improved by avoiding oxide inclusions and impurities previously in the surface of the cast block, which are caused by drops of liquid steel splashing, solidifying, oxidizing and impurities on the walls of the mold take up and are so included in the surface of the ingot.

When the killed steel is poured into the mold, it has alumina slag on its surface. This slag is advanced from the molten steel, moves to the sides of the mold, and rises up the walls of the mold. When the killed steel is poured into the mold, it has a temperature between about 1540 and 1600 ° C. At this temperature the lining is burned or evaporated. The alkali rare earth fluorides decompose, releasing at least some fluoride ions. In addition, carbon monoxide, carbon dioxide and water are produced. Part of the aluminum slag is converted to aluminum fluoride. The oxides on the surface of the sprayed steel particles are apparently removed by the decomposition products and the surface of the particles is cleaned so that they do not acquire an aluminum coating. As a result, they are again homogeneous from the melt was added, in which they go into hiding when these 5 has reached the level of splashes on the liner. The cast block obtained in this way has virtually no cast skin and is free from deeper oxide inclusions. It has a uniform structure and can be rolled out very easily into sheet steel of uniform quality. Steel sheets made from the ingots cast according to the invention are essentially free of grain boundary defects, the presence of which makes sheets unsuitable for drawing processes.

The complex rare earth salts can be prepared by first preparing a halide which is then e.g. For example, with sodium fluoride is melted in a suitable manner at higher temperatures of about 1300 to about 14001 C.

For example, the complex rare earth fluorides can be made from rare earth chlorides by converting the chlorides to fluorides and then melting the fluoride with sodium fluoride. The chlorides are dissolved in water and filtered to remove insoluble impurities. A sodium fluoride solution is added to the chloride filtrate, a pinkish-white precipitate being formed. This precipitate is filtered off and washed until it is free of chloride. The rare earth fluorides are dried and mixed with solid sodium fluoride in a ratio of about 1: 4, then melted at about 1300 to 1400 ° C. Excess sodium fluoride is removed by repeated washes with boiling water, and the remaining substance is dried and / or melted.

In one embodiment, 200 g of a natural mixture of rare earth fluorides was dissolved in 100 cm3 of water and the solution was filtered to remove insoluble constituents. To facilitate the filtration, 50 ems of about 30% hydrochloric acid were added. A saturated solution of sodium fluoride was then added to the solution of the rare earth fluoride until the fluoride was completely precipitated. The precipitate was removed by filtration, washed with hot distilled water until free of chlorides, dried at 110 ° C. , and crushed into particles about 76 m [t in diameter (200 mesh). In this way, 102.23 g of rare earth fluorides was obtained.

65.34 g of sodium fluoride were thoroughly mixed with the rare earth fluorides in a molar ratio of 1: 3 and this mixture is melted in a platinum cone in a "Globar» oven for 60 minutes. The starting temperature of the furnace was 1250 ° C., the end temperature 12851 ° C. After melting, the substance was cooled to room temperature, crushed and washed with hot distilled water to remove excess sodium fluoride. The solid parts were separated by centrifugation and suction of the clear liquid. The washed substance was dried and ground.

Other methods of preparing the complex salts can also be used the rare earths are used, such. B. direct fluorination and melting.

The following is an example of how a steel casting can be carried out. Example A carbon steel was melted with the composition given in percentages below: Carbon .............. 0.06 to 0.10 Manganese ................ 0.20 to 0.40 Silicon ................ 0.01 - maximum Sulfur ................ 0, Oj - maximum Phosphorus ................ 0.01 - maximum The batch consisted of: Limestone ................... 8 400 kg Iron ore .................... 26 300 kg Scrap ..................... 55 800 kg Hot metal ............... 148,000 kg After melting in the blast furnace, the following were added: Quick lime ............ 3 900 kg Iron ore .................... 10000 kg During the tapping process, the following were added to the ladle: Ferromanganese (85 IM Manganese.. 1360 kg Aluminum ................. 40 kg The casting mold was lined with 9 kg of complex rare earth fluorides, which were made from bastnasite and crushed to particle sizes between 76 and 100 m # t. This finely divided rare earth salt was mixed with 14 kg of a binder, which consisted of colloidal electrolyte graphite, antioxidant and a desulphurizing agent in a high-melting, non-toxic hydrocarbon carrier. The liner was sprayed onto the mold. Then the mold was preheated to about 2600 ° C. The cast steel ingot had the following composition: Carbon .................... 0.08% Manganese ........................ 0.380 / 0 Silicon ....................... 0.0061 / o Phosphorus ...................... 0.007% Sulfur ....................... 0.022% Iron .......................... rest The surface of the ingot cast in the mold lined according to the invention was essentially free of surface defects and oxide inclusions. The bar produced in this way was later rolled into sheet steel in a rolling mill. This sheet steel was particularly pure and uniform in structure. Its drawability was uniformly good over the entire bar, and it also showed no tendency towards greater scaling or local brittleness.

Claims (2)

PATENT CLAIMS: 1. Sizing for casting molds, especially for iron and steel, consisting of a binder that can withstand thermal stresses of up to about 260 ° C, and a complex salt that contains rare earths in finely divided form, with the general formula N RX 4, where N is an alkali metal, preferably sodium or potassium, R is at least one rare earth with an atomic number between 57 and 71 and X is a halogen. 2. Sizing according to claim 1, characterized in that fluorine is used as the halogen. 3. Sizing according to claim 1 or 2, characterized in that it contains per ton of metal to be cast, in particular steel, at least about 55 g, preferably about 115 g of the complex rare earth salt.