RU2267379C1 - Process for electroslag casting of rolling rolls - Google Patents

Abstract

FIELD: processes for casting bimetallic cylindrical articles such as rolling rolls.

SUBSTANCE: process comprises steps of casting melt metal under layer of slag to mold; using melting electrodes connected with DC source at successively alternating polarity of electrode connection. Electrodes are in the form of coaxial helixes with variable winding direction and they are uniformly shifted one relative to other according to azimuth. Number of electrodes is selected in such a way that is multiple to four. Azimuth spacing between adjacent electrodes exceeds spacing between ends of electrodes and locking plate of mold.

EFFECT: enhanced quality of rolling rolls due to averaged properties of melts.

2 dwg

Description

The invention relates to the field of casting, and more particularly to methods of casting bimetallic cylindrical products, such as rolling rolls.

A known method of electroslag surfacing (USSR author's certificate No. 1215251, priority 02.08.84), which consists in surfacing a hard alloy on the surface of the part with simultaneous heating of the bath with several electrodes. Heating is not more than one, but several parallel-connected electrodes are more uniform than one vertical or steeply inclined electrode, since the thermal power is distributed between all local heating zones. But at the same time, the heating of the slag bath with vertical electrodes is less uniform than with one lying or flat, because instead of one ring active spot several smaller active spots are formed under each electrode.

The direct application of this method for electroslag casting is complicated by the fact that with the vertical arrangement of the electrodes there are no spiral thickenings of the transition layer on the inner border of the bandage. In addition, with a vertical arrangement of the electrodes, azimuthal mixing practically does not occur, and the slag and metal baths according to the mixing conditions are divided into several independent zones. Far from each other zones can vary significantly in temperature. In case of differences in the chemical composition of the electrodes, the chemical composition of the zones will also differ. These differences may persist throughout the electroslag casting process.

There is also known a method of horizontal electroslag surfacing (RF patent No. 1619573, priority from 02.27.03), in which a framework pre-sintered from carbide plates is installed in the mold, a flux layer is applied to it, a plate electrode is installed on top, and a variable-thickness flux layer is applied to it . This method is unsuitable for casting cylindrical products. In addition, its use leads to excessive uniformity of heating the slag bath, and does not lead to the formation of a relief on the inner surface of the bandage.

The closest in technical essence is the method of electroslag casting of rolls, which consists in pouring metal under a slag layer heated by a spiral electrode consisting of two halves with the opposite direction of winding, and implementing its device (RF patent No. 2183530, priority from 13.11.2000, cl . 22 D 19/16). Due to the concentration of power in the spot zone and the spiral displacement of this zone during the process, the thickness of the transition layer turns out to be uneven, it has a spiral thickening making it difficult to delaminate the part, and the direction of the spiral in the middle of the part (in height) is reversed, which makes it impossible to unscrew the product.

The disadvantages of this method include the fact that it does not provide sufficiently uniform azimuthal heating of the slag. With a low current, its power is insufficient to conduct the process, the slag bath solidifies and hardens. And this is technologically unacceptable. With a significant current, you have to use a thick electrode. And the thicker it is, the greater its step. Consequently, the angle of inclination also increases, but at large angles of inclination of the electrode, the current drop in the azimuth of the cylinder also increases. Consequently, the concentration of power in the area of the active spot becomes excessive - the minimum of the concentration of power can be an order of magnitude less than the maximum and part of the bath freezes. Too uneven heating of the slag is unacceptable, as it leads to instability of the electroslag process. With a large step of the spiral, the electrode at each particular moment in time is located only in one place of the slag bath. Since the current density under the electrode and far from it is significantly different from each other, most of the bath is cold. The dependence of the thermal power P allocated on the slag bath on the current strength I has the form of an inverted pseudo-parabola. With a decrease in the current strength I in the peripheral parts of the bath (relative to the place where the electrode enters the slag), the mode point shifts to the left unstable part of the dependence P (I). As a result, areas with a similar release of power P continue to cool, and the electroslag process dies out in them. The bath in these areas becomes low-conductivity, and the released power approaches zero. With a large cylinder diameter, this leads to the fact that the supercooled area does not have time to warm up when the active spot moves along the slag ring, the spot is compressed, and its power is insufficient to heat the entire bath.

At a small step of the spiral, the electrode in the slag bath is located almost parallel to the slag – metal interface. At the same time, the entire slag bath is heated. The uneven heating of its individual sections becomes insufficient and the positive effect of selective melting of the cylinder wall disappears. This effect is preserved only with a relatively small diameter of the cylinder and a relatively large pitch of the spiral.

For large product diameters, it is impossible to achieve an acceptable fusion quality in this way.

The objective of the invention is to simultaneously ensure acceptable quality of castings and the stability of the technological process of their manufacture.

For this, it is proposed to use several coaxial spiral electrodes, a multiple of four, uniformly shifted relative to each other in azimuth and connected to a source with sequential alternating polarity, with the azimuthal distance between adjacent electrodes being chosen greater than the distance between the ends of the electrodes and the plate.

This allows you to have several active spots of equal power. The difference in power concentration occurs at a distance less than the circumference of the cylinder, the temperature is partially equalized due to the heat-conducting and convective heat transfer. The residual unevenness of the temperature distribution provides a relief fusion of parts of the product, and the relief of the transition layer repeats the shape of the electrodes and additionally works as a pseudo-thread, which makes the resulting connection more reliable.

To exclude the possibility of unwinding the product, the direction of the spiral at a certain level in height (for example, in the middle of the product) is reversed (as in patent No. 2183530). Moreover, to reduce the volume of the metal and slag baths, metal is poured into the gap between the finished roll axis and the mold. Mixing the bath is provided by free convection. Moreover, the inclined arrangement of hot and cold layers of the bath initiates the azimuthal flow of slag, and the mutual adhesion of slag and metal at the interface is the azimuthal flow of metal. This provides a better averaging of the properties of the melts.

The drawings show a device for implementing this method (figures 1 and 2). It contains a current source (not shown), a mold 1 with a finished product axis 2 installed in it, and several melting spiral electrodes 3 forming a multi-helix changing the curling direction. The electrodes are fixed in holders (not shown) and connected to a current source with sequential alternating polarity. In the lower part of the form on the support plug 4 lies a metal locking plate 5 - a flat ring. The number of electrodes is a multiple of four. The roll axis is a continuous body of revolution, consisting of three coaxial cylinders: one long, large diameter and two shorter and smaller diameters with their smooth mating.

The method is implemented as follows. The electrodes are lowered into the mold. In this case, a gap is left between the lower ends of the electrodes and the locking plate, which is less than the azimuthal distance between adjacent electrodes. After pouring liquid slag, voltage is applied to all electrodes. The chain closes through the melt. Moreover, the main current will flow between the electrodes and the metal bath. After slag guidance and the formation of a slag bath, liquid metal is poured into the gap between the form and the axis of the product. At the metal - slag interface, several active spots arise, the temperature of which is higher than the temperature of the spaces between them. As a result, the transition layer is embossed. As the ends of the electrodes melt, spots and gaps shift azimuthally along a circle — the projection of the electrodes onto a horizontal plane. At the same time, as the metal is poured, spots and slag rise. As a result of the simultaneous rotation and rise of areas of intense melting, the shape of the transition layer is spiral and repeats the shape of the electrodes. This relief of the transition layer resembles a thread and works similarly to it. After the lower spirals are melted, the upper ones curled in the opposite direction are turned on, the direction of rotation of the spots and the gaps between them is reversed, therefore the direction of the transition layer spiral above the junction of the electrode spirals is opposite to the direction of the transition layer spiral in the lower part of the product. The relief of one part of the transition layer acts as a left pseudo-thread, the other as the right, which excludes the possibility of unwinding the product, that is, it cannot be stratified without complete destruction of the transition layer. But since the distance between adjacent electrodes is less than the circumference of the projection of the electrodes on a horizontal plane, the difference between the maximum and minimum temperatures is less than when using one electrode, which reduces the initial internal stresses and makes the distribution of product properties in azimuth more uniform. As a result, the operational properties of the product will be better than the properties of similar products of large diameter cast by known methods.

Claims (1)

The method of electroslag casting of rolling rolls, including the use of melting electrodes made in the form of coaxial spirals with a change in the direction of winding, pouring liquid metal under a slag layer into a mold, characterized in that the number of electrodes that are a multiple of four, uniformly shifted in azimuth, is connected them to a current source with sequential alternating polarity, and the azimuthal distance between adjacent electrodes is chosen to be large distances between the ends of the electrodes and the locking plate that form.

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