CA1121970A

CA1121970A - Slag powders

Info

Publication number: CA1121970A
Application number: CA000319333A
Authority: CA
Inventors: Robin J. Gray; David Holliday
Original assignee: British Steel Corp
Current assignee: British Steel Corp
Priority date: 1979-01-09
Filing date: 1979-01-09
Publication date: 1982-04-20

Abstract

ABSTRACT

A fluxing agent for adding to the surface of molten steel in the mould of a continuous casting machine. The fluxing agent is prepared from a mixture of raw materials including glass, cement clinker, limestone, fluorspar, sodium carbonate and carbon to give a specified chemical analysis and specified viscosity to the molten slag formed when the fluxing agent is melted on contact with the molten steel.

Description

This invention relates to fluxing agents as used in the continuous casting steel. When liquid steel is teemed into the mould of a continuous casting machine it is desirable to cover the surface of the liquid steel in the mould with a fluxing agent which will melt on contact with the steel to fonm a molten slag cover. The purpose of the slag cover is multifold providing for the absorption of non-metallic inclusions, lubrication and control of heat transfer between the casting and th~ mould wall, the pre-vention of oxidation of the steel and a reduction in radiated heat loss from tl~e steel.
Numerous fluxing agents for use in the continuous casting of steel are commercially available anà it is an object of the present invention to provide an improved agent for such use and which will improve the quality of the cast product.
. In accordance with the invPntion there is provided a fluxing agent for adding to the mould in the ~.ontinuous casting of steel - which contains by weight a mixture of raw materials comprising 30~/0 to 65% glass, up to 60% cement clinker, up to 60% limestone, up to 10% fluorspar, up to 25% sodium carbonate and 1% to 5% carbon;
the chemical analysis of the mixture being such that the ratio of CaO to SiO2 ls within the range 0.6 to 1.4, maximum S of 0.2%, maximum Fe203 of 0.5%, maximum C02 of 30%, maximum H20 of 0.7%, maximum B203 of 5-0% and maximum Al203 of 8%, the viscosity of the molten slag formed by melting the agent being in the range of 0.5 to 10 poise at 1300 C and also less than 50 poise at 1200C.

~ 7 0 Also in accordance with the inventlon there is provid~d a method of continuously casting steel wherein molten steel is teemed into an open ended coo1ed mould and wherein a fluxing agent is defined in the preceding paragraph is added in the mould to cover the top surface of the molten steel. Further in accordance with the invention there is provided continuously cast steel whenever produced by the method described in the preceding paragraph.
Conveniently the fluxing agent is in the form of a powder, the powder parti~les having a size such that 90% of the powder is less that 75~ and a 100% less than 250~.
The composition of fluxing powder as recited above has a CaO
- SiO2 base with Na20 and CaF2 to ma~ntain low melting point and viscosity values; carbon to control the melting rate, to reduce heat loss from the meniscus and to maintain reducing conditions in the slag phase and A1203 below 8% in order to give the maximum capacity for absorbing alumina floating out of the ste~lO The raw materials are chosen (1) to be consistent in composition, non-hygroscopic and as free as possible from impurities such as iron and sulphur;

(2) to minimise the amount of hign melting point constituents, and ~3) to be cheap, few in number, readily available and easy to grind and blend.
To prepare fluxing powders in accordance with the invention the glass which is preferably lead free flint gla5s is added to-gether with cement clinker, or the raw materials of cement clinker, to a silica pebble ball mill and ground to a fine powder ~90% less than 75~ . Cement clinker may be omitted from the mixture and substituted with limestone. The other constituellts of the powder are then added to the mill and blended to give a homogenous mix~
Alternatively thP starting materials ntay each be pre-ground and then blended to form the fluxing powder.
Various trial batches of fluxing powder in accordance with the invention wPre made and tested on a continuous casting machine~
the cast steel strand subsequently being rolled to plate. Specific formulations of raw ~aterials as tested are listed on Table I, whilst their chemical an~lyses are listed on Table II.
All of the powders tested gave satisfactory strand and plate surface qualities and the following conclusions concerning the effect of the powders on longitudinal surface cracking in slabs can be drawn from the trials:
(1) the slag viscosity for option surface quality decreases with increasing casting speed;
(2) surface quality tends ~o be poorer with powders having a high sulphur content. Such powders form slags which do not readily de-vitrify during cooling from the molten state;

(3) when casting under conditions where longitudinal surface crack-ing may occur, such as thin slabs in the region of 0~12%
carbony the fluxing powder used should have a low sulphur content.

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Powder 5l2% CaO% ~ Al2O3% j IU
~ t- - I
2~ 34 1.00 2.~4 3.84 4.3 21 4.0 2 30 36 0.63 5.20 4.48 4.1 lS 0.2 3 36 42 2.20 9.49 4.09 2~78.S 0.2

4 36 40 2.04 7.11 4.09 2.78.1 0~2 36 0.63 7~24 4.o3 3.0 19 0.2 6 30 36 0.~3 8.g3 4.48 1.1 20 0.2 . .._ _ ~__ _ ~ _

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A fluxing agent for adding to the mould in the continuous casting of steel which contains by weight a mixture of raw materials comprising 30% to 65% glass, up to 60% cement clinker, up to 60% limestone, up to 10% fluorspar, up to 25% sodium carbonate and 1% to 5% carbon; the chemical analysis of the mixture being such that the ratio of CaO to SiO2 is within the range 0.6 to 1.4 maximum S of 0.2%, maximum Fe2O3 of 0.5%, maximum CO2 of 30%, maximum H2O of 0.7%, maximum B2O3 of 5.0% and maximum Al2O3 of 8%, the viscosity of the molten slag formed by melting the agent being in the range of 0.5 to 10 poise at 1300°C and also less than 50 poise at 1200°C.

2. A fluxing agent as claimed in claim 1 wherein the mixture is in the form of a powder, the powder particles having a size such that 90% of the powder is less than 75µ and 100% of the powder is less than 250µ.

3. A fluxing agent as claimed in either one of claims 1 or 2 wherein the glass is a lead-free glass.

4. A fluxing agent as claimed in either one of claims 1 or 2 wherein the mixture is prepared by grinding the glass together with cement clinker and/or limestone to form a powder, grinding the other raw materials to form a powder and subsequently blending the ground powders together to form the mixture.

5. A fluxing agent as claimed in either one of claims 1 or 2 wherein the mixture is prepared by grinding the raw materials individually to a powder and subsequently blending the ground powders together to form the mixture.

6. A method of continuously casting steel wherein the molten steel is teemed into an open-ended cooled mould and wherein a fluxing agent is added in the mould to cover the top surface of the molten steel, the fluxing agent containing by weight a mixture of raw materials comprising 30% to 65% glass, up to 60% cement clinker, up to 60% limestone, up to 10% fluorspar, up to 25% sodium carbonate and 1% to 5% carbon; the chemical analysis of the mixture being such that the ratio of CaO to SiO2 is within the range 0.6 to 1.4, maximum S of 0.2%, maximum Fe2O3 of 0.5%, maximum CO2 of 30%, maximum H2O of 0.7%, maximum B2O3 of 5.0% and maximum Al2O3 of 8%, the viscosity of the molten slag formed by melting the agent being in the range of 0.5 to 10 poise at 1300°C and also less than 50 poise at 1200°C.