CH685865A5 - Continuous casting mould improving cooling and casting performance - Google Patents

Abstract

A mould for continuous casting of steel, in which walls of copper or a copper alloy enclose a hollow mould chamber and which has a cooling water gap of predetermined dimensions for a cooling water circuit between the copper walls and a cooling water surface, is novel in that the copper walls on the cooling water side have a surface roughness Ra of 5-600 mu m at least along one sub-range step. The fig. shows on the vertical axis the temp. in deg C and the heat flow density q in W/cm<2> and on the horizontal the cooling water flow rate. The graphs correspond to a continuous casting mould producing a 200 \* 200 mm<2> prod. having 16 mm thick copper walls with a heat conductivity of 322 W/m(m.K). Curve (a) is for a hydraulically smooth wall, i.e. a surface roughness of 1-2 mu m, (b) for roughness of 20 mu m and (c) for 80 mu m.

Description

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description

The invention relates to a mold for the continuous casting of steel, according to the preamble of claim

1.

In the continuous casting of steel, it is known to manufacture the walls of the mold cavity of molds from copper or from copper alloys and to cool them by means of one or more water circuits.

In order to achieve a good cooling performance of the mold on the one hand and a low temperature of the copper wall during the casting operation on the other hand, it is also known to cool a copper wall that is as thin as possible by means of cooling water and to let the cooling water flow past the copper wall at a relatively high flow rate. The temperature of the cooling water, which usually circulates in a closed circuit, should not exceed 30-40 ° C.

With increasing casting performance, especially with increasing casting speed, the heat output of the strand into the mold increases and at the same time the wall temperature of the mold. In parallel with the increase in the wall temperature, the temperature gradient in the mold wall between the mold cavity and the cooling water side also increases. The mold wall temperature and the temperature gradient are highest in the bath level area. The mold wall temperature on the mold cavity side can reach temperatures of up to 350 ° C in the cooling gap at normal water speeds and up to 150 ° C on the opposite water side.

Heat loads of this magnitude influence the geometry (thermal expansion) and the service life (softening or recrystallization temperature, creep) of such molds and thus the economy of continuous casting and the quality of the strands produced from them.

In order to lower the wall temperature in the area of the bathroom mirror, it has been proposed in DE-OS 3 621 073 to provide the copper wall of the mold on the side of the cooling water gap with grooves running in the direction of the strand between 1-8 mm wide and 0.5-3 mm deep. Groove lengths of 60-600 mm are recommended. Through these grooves, the cooling surface in the water gap of the mold is enlarged similar to that of cooling fins and the heat transfer between the mold wall and the cooling water is thereby improved. The production of such grooves, particularly in the case of tubular molds for continuous sheet casting systems, is associated with relatively high costs.

The invention has for its object to provide a mold, in particular a tubular mold, with improved cooling in a cost-effective manner, which has a reduced wall temperature between the mold cavity side and the water gap side of the copper wall, at least in some areas of the copper wall, in order to thereby also overcome the temperature differences to reduce the length of the mold tube during casting. This measure is intended to increase the casting performance and / or the service life of the mold. The crystallization of the strand crust that is formed should also be controlled or uniformized by locally improved cooling, at least in the area of the bath level.

According to the invention, this object is achieved by the sum of the features of claim 1.

With the mold according to the invention, it is possible to increase the copper wall temperature at a specific cooling water speed by 15-30 ° C. and the heat flow density q in W / cm 2 through the copper wall. Furthermore, it is possible to keep the water-side copper wall temperature at the pressure prevailing in the water gap below the associated boiling point temperature in order to prevent the cooling water from evaporating.

The copper wall temperature can also be reduced and the heat flow density increased further if the cooling water speed is increased to 5 to 15 m / s with a cooling water gap of, for example, 2-10 mm.

In order to influence the heat flow density and the wall temperatures, the mold wall thickness can e.g. can be selected differently in the direction of the strand. However, economical production is achieved when the copper walls have a uniform thickness in the partial area.

The surface roughness is generally chosen to be uniform in the partial area. According to one embodiment, it is additionally proposed to provide the surface roughness as a function of the thermal load on the copper wall by means of a strand crystallizing in the mold, the surface roughness increasing with increasing thermal load. The surface roughness can usually be reduced in the strand running direction.

In addition to the surface roughness Ra or Ks (sand roughness), the roughness density in the strand running direction can also be selected irregularly, in particular in the direction of movement of the water in the water gap.

An optimization of the surface roughness for an optimal heat flow density and an optimal temperature of the copper wall is achieved according to a further exemplary embodiment if the surface roughness can be determined via the pressure loss in the cooling water gap with the proviso that the ratio of the pressure loss between a smooth wall and a wall with surface roughness is approximately Is 1: 2.

In the following, the invention is to be additionally explained using a diagram.

A continuous casting mold for a 200 x 200 mm2 strand format is manufactured as a tubular mold with 16 mm wall thickness made of copper with a thermal conductivity of 322 W / (m • K). For such a mold

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in diagram 1 the temperature in ° C and the heat flow density q in W / cm2 are plotted on the vertical and the cooling water speed on the horizontal. Lower curve groups a), b), c) show the copper wall temperature on the side of the cooling water and middle curve groups a), b), c) the mold wall temperature on the inside of the copper wall in the area of the bath level. Another upper family of curves a), b), c) shows the heat flow density through the copper wall.

Curve a) shows the values for a hydraulically smooth wall, i.e. for a roughness of 1-2 µm, e.g. a drawn copper wall,

curve b) shows the values for a roughness Ra 20 um and curve c) shows the values for a roughness Ra 80 um.

Table 1 shows the curve values along a vertical line at a water speed of 8 m / s.

Table 1

Copper wall roughness Ra on the water side

Copper wall temperature on the water side

Copper wall temperature on the string side

Heat flow density in the area of the bathroom mirror

2 um

131 ° C

312 ° C

356 W / cm2

20 [in

115 ° C

298 ° C

361 W / cm2

80 (im

98 ° C

283 ° C

365 W / cm2

The table values show that with increasing roughness Ra of e.g. 2 µm to 80 µm the copper wall temperature outside from 131 ° C to 98 ° C, the copper wall temperature inside, i.e. on the mold cavity side, drops from 312 ° C to 283 ° C. These temperature drops appear to be relatively small, but are of particular importance in this application because the softening or recrystallization temperature for copper or for the copper alloys in question is 280-400 ° C. The hardness achieved with cold-formed copper molds is reduced when these temperatures are exceeded and the geometry is changed by reducing thermal or mechanical stresses. The heat transfer rate increases when the roughness Ra increases from 2 to 80 firn from 356 W / cm2 to 365 W / cm2.

The roughness can be applied to the copper wall using all technically known means. In the case of arc molds, a cold-forming operation is usually carried out with the mandrel inserted. It is therefore particularly advantageous if, after the cold forming, the mold tube with the mandrel inserted is subjected to a sand or steel gravel blasting process and the desired roughness can be produced at the desired locations by appropriate selection of the blasting agent.

The surface roughness Ra in um can also be specified in other scales for determining the roughness. A specific roughness Ra in (im can be assigned, for example, an equivalent sand roughness Ks.

Claims (5)

Claims 1. Mold for the continuous casting of steel, wherein walls made of copper or a copper alloy enclose a mold cavity and a cooling water gap with predetermined dimensions is provided for a cooling water circuit between the copper walls and a cooling water jacket, characterized in that the copper walls on the cooling water side at least along a partial area Have surface roughness Ra = 5-600 µm. 2. Chill mold according to claim 1, characterized in that the copper walls in the partial area have a surface roughness Ra = 10-200 µm. 3. Chill mold according to claim 1 or 2, characterized in that the cooling water gap between the copper wall and the cooling water jacket is 2-10 µm. 4. Chill mold according to one of claims 1-3, characterized in that the copper walls have a uniform thickness in the partial area mentioned. 5. Chill mold according to one of claims 1-4, characterized in that the surface roughness decreases in the strand running direction. 3rd