GB2046151A

GB2046151A - Method of Reducing Segregation in The Continuous Casting of Steel

Info

Publication number: GB2046151A
Application number: GB8011499A
Authority: GB
Original assignee: Elkem Spigerverket AS
Current assignee: Elkem ASA
Priority date: 1979-04-06
Filing date: 1980-04-08
Publication date: 1980-11-12
Also published as: DE3012398A1; FR2452985A1; FI800869A; IT8020858A0; NO151072C; IT1209319B; GB2046151B; FR2452985B1; NO151072B; NO791157L; SE8002412L; DK101780A; SE443735B

Abstract

The invention relates to a method of reduction of segregation in continuous casting of steel by addition of cold, solid or pulverulent steel directly to the molten material in the tundish or in the mould, whereby the melt is cooled and the superheat removed. The cold, solid steel is added in the form of wire, rods, cuttings, poflider, granules, etc. <IMAGE>

Description

SPECIFICATION Method of Reducing Segregation in the Continuous Casting of Steel In the continuous casting of steel there will as a rule take place a segregation, i.e. an enrichment of carbon, sulphur and other elements in the centre of the casting. The segregation ratio will increase with increasing carbon content, and with carbon content above 0.6%, the segregations will often result in practical problems. The segregated material in the centre of the billet can then have a supereutectoid composition and free cementite will be liberated. This is a brittle phase which can cause breaks in subsequent use such as during wire drawing or in other ways impair the mechanical properties of the products.

Figure 1 a of the accompanying drawings shows a schematic section through a mould and a cast strand. As an example the strand may have a cross-section of 135 mm square, and the length of the meniscus to completely solidified material can be 8000 mm. The strand can also be bent with a radius of 6000 mm.

Figure 1 b of the accompanying drawings shows in principle the cast structure as shown by deep etching of a billet section which is cast at normal superheat, for instance 500C above the liquidus temperature of the steel.

The formation of the central segregations can be explained in the following way: In the solidification of steel there is liberated solid dendritic crystals. Between the dendrites there will be a residual melt. The dendrites have a lower and the residual melt a higher content of carbon, sulphur, etc. than the average analysis. A microscale segregation will therefore arise. With further solidification the dendrites will grow and the residual melt will also gradually solidify. The dendrites will chiefly grow in the direction of their axis. The result is that the steel which solidifies against the wall of the mould will consist of tiny dendrites which are orientated in all directions. When these have grown enough they will strike against each other and be prevented from further growth.Only those dendrites which have a favourable direction away from the surface will have the possibility of growing further, in the most favourable cases just into the centre as shown by Figure 1 b. As shown in Figure 1 a the strand will as a result of normal superheat during the solidification comprise an outer solid shell, indicated by S. Inside this shell there is a zone which consists of dendrites with residual melt in between, indicated by S/L, and on the top an area of molten material, indicated by L. The area indicated by S/L forms a socalled solidification crater.

With solidification there will take place a reduction in volume. This results in hollow spaces, pipe and suction arising in the central portion of the billet. Enriched residual melt will therefore be sucked through that channel system which is formed between the dendrites and will collect in the central part.

The segregations will take place in an irregular way. The central segregations will therefore vary considerably along a length of 100--200 mm.

Figure 2a of the accompanying drawings shows what happens when the casting takes place without superheat or at temperatures which are somewhat below the liquidus temperature. Freely flowing separated dendrites will be found in the melt. These will be carried with the convection flow and will precipitate on the walls and bottom of the solidification boundary. The residual melt will also be locked up by these collections of dendrites in different areas. The reduction in volume by solidification will cause a sliding of dendrites and residual melt within the solidification crater. The collections of residual melt will therefore show as V-shaped segregations across a big area of the cross-section, but as a channel system of dendrites which points against the centre will not be formed, there will only be little segregation in the central part.V-shaped segregations are considerably smaller and far less detrimental than the central segregations.

Figure 2b of the accompanying drawings shows in principle what an etched section looks like when the billet has been cast without superheat.

In practical operation there will be a temperature fall in the course of a casting period so that there is only a small part of one charge which can be cast by temperatures which are sufficiently low to avoid segregation in the central part. Casting at low temperatures is therefore usually not any practical solution of the problem.

Attempts to solve the problems discussed above have included methods which employ electromagnetical stirring of the melt. As shown in Figure 3a of the accompanying drawings an induction coil is provided which causes a vertical or horizontal flow within the melt. This flow will break off dendrites which will then trickle down into the solidification crater. This results in the casting structure which is shown in Figure 3b of the accompanying drawings. This method can be fairly effective, but it cannot remove those segregations which take place by casting at higher superheat.

Accordingly the present invention provides a method of reducing segregation during the continuous casting of steel, comprising the addition of cold, solid steel directly in the tundish or directly into the mould during casting whereby the steel is cooled so that its superheat is removed.

The process according to the invention is based on a real cooling and removal of the superheat in the liquid steel. This is effected by the addition of cold, solid steel preferably in the form of wire, rods, cuttings, powder, granules, etc. into the mould itself and/or into the tundish which receives the steel from the ladle and distributes it to one or more moulds.

In order to promote a fuller understanding of the above and other aspects of the present invention, some examples of the method will now be described, by way of example only, with reference to Figures 4 and 5 of the accompanying drawings which show schematically a method of the invention.

The method can be exemplified by describing the following tests which were carried out by continuous casting of a charge containing 0.36% C. The strand had a cross-section of 135 mm square.

The casting speed was about 2 m/min, or about 280 kg steel per minute per strand. The temperature in the tundish was 1 5550C, which is about 600C above the liquidus temperature of the steel. For the cooling there was employed 5.5 mm rolled wire which was fed into the mould. The wire was in beforehand cleaned of any oxide shell by etching and neutralized in a solution of borax.

The rolled wire was introduced into the mould as quickly as the resistance in the solidification crater admitted, see Figure 4a, in which the rolled wire is marked by R. The speed was in average 1 1.6 m per minute, i.e. 2.1 6 kg per minute or 0.77% of cast quantity. With this addition a shell of steel solidified around the wire about 15 mm in diameter. At a point 2 to 3 meters below the surface the wire was so hot that it broke off. The remainders remained standing in the solidification crater as shown in Figure 4a. Figure 4b shows in principle the casting structure which was obtained in the body.

The structure consisted of elongated dendrites until a distance of 25-30 mm from the surface. Inside this area the structure consisted of small dendrites with equal axes which were oriented in ali directions. There were not many pores and segregations in the centre, but many small V-formed segregations could be observed across a bigger part of the cross-section. The rolled wire with the solidified shell was found again in the etched section.

Comparative samples were taken of a strand which was cast in the usual way. An etched section from this strand looked as shown in Figure 1 b.

In order to determine the segregations, chips were drilled for analysis of carbon, with a 7 mm diameter drill (0.2% of the cross-section of the sample). 7 mm deep holes were drilled at 7-8 mm spacing in the pipe in the longitudinal direction of the billet. Where the pipe was not visible the heat centre of the sample was found by etching of the sample, and the drill was located in this area.

Samples were taken along a length of 1 50-200 mm.

The result is shown in Figure 5. Diagram A shows the degree of segregation in the sample without any additions, the diagram B shows the sample with addition of rolled wire.

The degree of segregation, I, shown in the diagrams, is defined as the ratio between the contents of carbon in the sample taken from the funnel or the heat centre of the sample and the analysis of the charge:

Of greatest importance is the maximum degree of segregation, 1may' which can occur. For this reason the average value, I, and standard deviation, sX, have been calculated. For statistical purposes only 0.13% of the samples shall have a segregation which is bigger than Imax=I+3s, The tests gave the following result: Without With addition addition of rolled of rolled wire: wire: Number of samples N 17 22 Average value 1 1,1229 1,0636 Standard deviation s 0,1152 0,0602 Maximum degree of segregation Imax 1,47 1,24 The result shows that addition of rolled wire has reduced the maximum segregation to about 50% of the original one.

The capacity of the equipment was also considerably increased. By addition of cold steel to the mould there is obtained an extra cooling of the inner portion of the strand which results in an earlier solidification and which makes it possible to increase the casting speed. This results in an increase in capacity which is considerably bigger than the quantity of added material represents.

Claims

1. A method of reducing segregation during the continuous casting of steel, comprising the addition of cold, solid steel directly in the tundish or directly into the mould during casting whereby the steel is cooled so that its superheat is removed.

2. A method as claimed in Claim 1, in which the cold, solid steel is in the form of wire, rods, cuttings, granules, powder, etc.

3. A method of reducing segregation during the continuous casting of steel substantially as herein described with reference to Figures 4 and 5 of the accompanying drawings.