SE441420B - CHILL - Google Patents

Description

79090734; in particular occurs in the form of surface porosity and surface cracks. Further one must always count on abrasion of the layers, which leads to one for many products (eg foils) unacceptable contamination of the casting surface.

Finally, it has been found that casting of many aluminum alloys s with concomitant molds is only possible then the initial solidification occurs so steeply that the surface cell size of the casting strip is at 10 - 20 sqm.

However, the usual coating layers lead to softer solidification, which results in the occurrence of surface defects, in particular surface porosity.

Permanent ceramic protective layers have the disadvantage, that they, with respect on the high occupancy costs, shows only a limited life expectancy length. Even with this coating procedure, it is difficult to set a sufficiently steep initial solidification for casting alloys.

In the case of a mechanically knurled mold, the heat transfer through is checked If a melt comes in contact with a molded surface, for example by irradiation with steel ball scrap, then, when the metallostatic pressure is not too high, it the peaks of the irregularities, while in the grooves air cushions are formed, By a suitable dimensioning of the relative contact surface wherein Fi denotes the contact surface on an uneven peak, Fo the entire mold surface, and n denotes the number of unevenness peaks, as well as the groove depth and the average distance between adjacent unevenness peaks, it is possible to control the heat transfer through the guys.

According to the state of the art, there are two mechanical methods for rough roughening of the casting surface of accompanying molds: a) Using chip removal methods (milling, planing) grooves in the surface. This procedure is associated with various disadvantages.

W ... ..... v .. -...._..__..__.._._.......__._ _.4, __... _., __._ _ ___., _, _ _ vs ov eovz-4 Due to the high demands on homogeneity of the heat transfer through the mold surface, the requirement for homogeneous knurling is extremely high. Modern be- working tools meet these requirements only for grooves with grooves distance up to about 1 mm. With finer grooves, it becomes difficult to keep the groove depth and the contact surface constant. Furthermore, the processing the costs of increasing the fineness of the groove to a considerable extent especially when the machined surface of continuous casting plants with running coils are quite coarse (a plant with running, traction torband-like mold strips exhibit, for example, at a casting width of 2 m and a casting length of 3 m a mold surface of a total of about 30 m2).

Rough grooves, ie. groove distance> 0.5 mm, however, when casting wide bands to cracks, then through an excessively deep penetration of the metal in the groove valleys at the staple friction between it solidified goods and the mold become so large that a solidification shrinkage prevented. b) By irradiating the mold surface with hard particles, in particular steel balls, depressions are pressed into the surface. This procedure leads to a homogeneous inhibition of the heat flow, which at a suitable metallo- static pressure also results in the casting of high-alloy metals (eg AlMg 4,5) with concomitant molds, especially then the molds consists of copper. However, practical experience has revealed a disadvantage of this procedure, which is illustrated below.

With longer casting operation, it is unavoidable that in the In the depressions formed, contaminants accumulate, which on heating emits gases. These include organic substances, hydro- xides and various salts, containing crystalline water. If at casting the metal comes into contact with a surface contaminated in this way, then the corresponding gas is split off. Especially at higher casting speeds this gas is trapped between the melt and the mold, then due to the specific groove, proximity to each other in craters, which are separated apart through ridges, the flow of the gas is prevented in parallel with the mold surface to a high degree, as soon as the melt touches the surface. Gas- However, blisters between the mold and the solidifying melt lead to defects in the casting. band, which generally means committee. Furthermore, it has been shown that that a removal of the contaminants by various purification procedures does not solve the problems of the specificity required for production operations certainty. The object of this invention is to provide a mold with 7999073-'4 grooved or uneven surface for casting metals, which surface will the required homogeneous and precisely dosed braking of the heat flow between melt and mold and at the same time errors in the casting surface, which are caused by gas inclusions between the melt and kokilll According to the invention, this object is fulfilled by molds, in particular for casting aluminum or aluminum alloys, the interior of which surfaces show a pattern of roughness formed by grooves with such measure, that the melt only touches the peaks of the irregularities while i the grooves air cushions are formed whereby the molds are characterized by that the grooves mutually cross each other and thus form a fall unevenness peaks whose distance from each other amounts to 0.05 to 1 mm. By the valleys of the grooves are connected to each other on in such a manner, upon contact with the melt with the mold may be released gases escape unhindered into the groove valleys parallel to the mold surface thereby preventing a lifting of the melt from the casting surface. nom an excessive increase.of the gas pressure.

According to an advantageous embodiment of the invention, the mold the surface an uneven pattern consisting of a regular arrangement of pyramid- or truncate-like elevations or irregularities peaks.

If a molten metal flows on this mold surface, it only touches this in the area of the surfaces parallel to the entire mold surface f on the unevenness peaks. Thereby, the heat transfer at the initial solidification is set by establishing the similarity a fp d. = / d2 The distance d between adjacent unevenness peaks is hereby defined as the distance between the centers on corresponding surfaces f.

According to a particularly preferred embodiment of the invention for the values of 0.05 <. ~. << 0.5, preferably 0.1 <° š <0.25, wherein the distance d amounts to 0.05 - 1 mm, preferably 0.2 - 0.5 mm.

It has further proved advantageous to keep the height h too uneven. the hot peaks, i.e. the distance perpendicular to the surface f measured the surface f and that through the deepest points on the ridge valleys imagined planet, within borders 0.1 d <h <d, preferably 0.15 d <h <0.4 d. _ 790907341 Extensive operating tests with different aluminum alloys on a string casting plant with an accompanying, tractor belt-like molding belt has shown that the molded surfaces grooved in this way are avoided formation of the said gas inclusions and thus a per- effective casting band quality is achieved.

The improvement of the casting strip quality when using the molds according to the invention can be explained by the gas which is formed at the first contact between melt and mold surface, may flow freely in the continuous channels between the peaks of unevenness and thus depart thence.

For the production of the unevenness pattern, special methods are suitable, which on the basis of a smooth mold surface do not produce any mechanical deformations on the mold surface. Preferably, the unevenness is etched. the pattern in the mold surface.

For the formation of a precisely defined pattern of unevenness, it has shown it is particularly advantageous to carry out the etching by means of chemical etching methods, which are commonly used for manufacturing of textile pressure rollers or for printed couplings in electronics.

Comparative experiments with different aluminum alloys on a continuous casting plant with an accompanying tractor-belt-like molding belt has shown that especially when using copper molds it is photochemical the etching method is preferred for forming the defined the pattern of unevenness compared to mechanical methods. Mechanically knurled copper surfaces always show a certain surface deformation. This is according to danger unit due to corrosion, hydrogen and oxygen embrittlement. Further creep processes are observed at the mechanically grooved surfaces, which may adversely affect the geometry of accompanying coils. All of these unfavorable phenomena are not observed in photochemical and thus lye deformation-free grooved surfaces.

Furthermore, experiments have shown that photochemically knurled mold surfaces, for reasons similar to casting with accompanying molds, including die casting and in continuous casting with sliding molds leads to a significant improvement of the casting surface, which means a reduction of terbcarbetninqskostnader.

In the following, the concept of invention is undermined by means of schematic drawings. nings, there 'ÜGQÛVS-lr Fig. 1 shows a cross section through a part of an example genome irradiation with steel ball scrap grooved mold surface, and I Fig. 2 in perspective view shows a section of the surface of a mold according to the invention.

In Fig. 1, the melt (2) is in contact with an e.g. with steel ball scrap grooved mold surface (1) and thereby only touches the area around the unevenness peaks (3), while between the melt (2) and the unevenness valleys (4) an air cushion (5) is present¿ By appropriate dimensioning of the relative contact surface, ie. here the ratio of the sum of the areas FT - * F4 to the corresponding total surface Fo, as well as the unevenness depth t and the average distance a between adjacent unevenness peaks, the heat transfer can be controlled between melt and cook.

The section of the surface of a mold according to the invention in Fig. 2 shows a regular pattern of uneven tops (6) similar to truncated ones pyramids. These unevenness peaks are characterized by a height h as well a surface parallel to the entire mold surface f {Adjacent uneven hot peaks are at a distance of d from each other.

From Fig. 2 it is immediately apparent that such a regular unevenness pattern ent provides an accurate and reproducible control of the heat transfer between melt and mold, at the same time the coherent channel the system between the various unevenness peaks guarantees unhindered exhaust gases formed.

Claims (4)

79090734 »Patent claims 1. Kokiller. especially for casting aluminum or aluminum alloys. whose inner surfaces show a pattern of roughness formed by grooves with such dimensions. that the melt only touches the peaks of the irregularities while air cushions form in the grooves. characterized in that the grooves mutually cross each other and thus form a plurality of unevenness peaks (6), the distance (d) of which amounts to 0.05 to 1 mm, Mold according to Claim 1, characterized in that a regular arrangement of pyramid- or truncated-cone-like unevenness peaks (6). Mold according to claim 2, characterized in that the unevenness peaks (G) with respect to their distance d have a height h between 0.1 d and 1.0 d. And that the pyramid or truncated cone surfaces f satisfy the condition 0.05 <f / dz < 0.5. Mold according to Claim 3, characterized in that the unevenness peaks (Q) have a distance d of 0.2 to 0.5 mm and a height h between 0.15 d and 0.4 d and in that the pyramid or the truncated surfaces meet the condition 0.1 <f / 62 <0.25.