Molding powder for continuous casting of steel and method...

Details Molding powder for continuous casting of steel and method... Molding powder for continuous casting of steel and method...

2000-08-07

2002-10-08

King, Roy (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Compositions

Solid treating composition for liquid metal or charge

C148S540000

Reexamination Certificate

active

06461402

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to a mold powder for continuous casting of steel and a method for continuous casting of steel using the mold powder which can greatly suppresses the corrosion of continuous casting equipment, reduces fluorine concentration in the waste water, and which can realize stable casting even with reduced consumption.
RELATED ART
Mold powder is added on the surface of molten steel inside a mold, is melted by heat derived from the molten steel to form a molten slag layer, and progressively flows into the gap between the mold and the solidifying shell, to be consumed. Some of the major roles that the mold powder plays during this time are: (1) lubrication. between the mold and the solidifying shell; (2) dissolution and absorption of the inclusions which come to the surface of the molten steel; (3) prevention of reoxidation, and heat insulation of the molten steel; and (4) control of the speed of heat dissipation from the solidifying shell.
Regarding points (1) and (2), it is important to control the softening point, viscosity, etc. of the mold powder, and to determine the chemical composition. Regarding point (3), important factors are the melting rate and powder characteristics such as the bulk specific gravity and spreadability, which are controlled mainly by carbonaceous materials. Regarding point (4), the crystallization temperature etc., must be controled and the determination of the chemical composition is crucial.
A typical mold powder contains as base materials, Portland cement, synthetic calcium silicate, wollastonite, blast furnace slag, yellow phosphorus slag, dicalcium silicate (2CaO.SiO
2
), etc., and also contains if necessary, siliceous materials for controlling the alkalinity and powder characteristics such as bulk density. Further, it generally contains flux materials such as fluorides including fluorite, cryolite, magnesium fluoride, etc., as moderators for controlling the melting characteristics such as softening point and viscosity, and carbonaceous materials such as carbonate including sodium carbonate, lithium carbonate, strontium carbonate, barium carbonate, etc., as moderators for controlling the speed of slag-form melting. As for the chemical composition, a mold powder contains SiO
2
and CaO as the main components, and Al
2
O
3
, MgO, BaO, SrO, Li
2
O, Na
2
O, F, MnO, B
2
O
3
, etc.
Among the roles of mold powders, the roles of of cuspidine crystals (3CaO.2SiO
2
.CaF
2
) in the slag film are significant role with regards to (4), control of the heat dissipation from the solidifying shell. Thus, fluorine, which is a constituent element of cuspidine, is an essential component for controlling the heat dissipation. Especially in the case of casting steels which tends to cause cast slab fractures, such as hypoperitectic steel, the role played by fluorine in the mold powder is important. To achieve slow cooling and uniform heat dissipation in the mold, the mold powder must have a high crystallization temperature. Accordingly, typical mold powders have compositions including high fluorine content. Fluorine also plays an important role regarding viscosity control and crystallization temperature control.
PROBLEMS TO BE SOLVED BY THE INVENTION
Since almost all of the currently-used mold powders purposely have fluorides, such as CaF
2
, NaF, and NaAlF
6
added thereto, as flux, fluorine is contained therein, and so, they have the following problems. Mold powder melts when it comes into contact with molten steel, and then flows into the gaps between the cast slab and the mold to be consumed as a lubricant; however, since it contains fluorine, there is a problem that when it comes into contact with the secondary cooling water at the bottom of the mold, hydrofuluoric acid (HF) is generated by a reaction between the fluorine and the water, which lowers the pH of the cooling water. Accordingly, corrosion attacks equipment around the continuous casting equipment, which contacts the cooling water, especially the structures made of metal such as molds, rolls, pipes, nozzles, etc. Further, waste cooling water must be be neutralized. Still further, fluorine involves environmental problems, and concentration in the waste water is regulated. Still further, there is also a problem that a mold powder containing much fluorine increases the dissolution loss speed at the powder line of the submerged nozzle.
To solve the above-described problems caused by fluorine, there is disclosed, for example, in Japanese Patent Laid-open No. 50-86423, an additive for continuous casting of steel which is characterized by consisting of 10 to 50% of CaO, 20 to 50% of SiO
2
, 1 to 20% of Al
2
O
3
, 0.1 to 10% of Fe
2
O
3
, 1 to 20% of Na
2
O, 1 to 15% of C, 0.1 to 10% of K
2
O, 0.1 to 5% of MgO, 0.1 to 20% of B
2
O
3
if necessary, and other impurities, and having the form of powder.
In Japanese Patent Laid-open No. 51-132113, there is disclosed an additive for continuous casting of steel which is characterized by consisting of 10 to 50% of CaO, 20 to 50% of SiO
2
, 1 to 20% of Al
2
O
3
, 0.1 to 10% of Fe
2
O
3
, 1 to 20% of Na
2
O, 1 to 15% of C, 0.1 to 10% of K
2
O, 0.1 to 5% of MgO, 0.1 to 10% of F if necessary, 0.1 to 20% of B
2
O
3
if necessary, 0.5 to 10% of inorganic and organic binders, and other impurities in small quantities, and having the form of grain of which the diameter is, 0.1 to 5 mm.
In Japanese Patent Publication No. 56-29733, there is disclosed a refining agent for continuous casting of cast slabs, which do not contain any fluorides, and of which the compositions includes, 20 to 45% of CaO, 20 to 45% of SiO
2
, 0.5 to 5% of B
2
O
3
, 3 to 15% of Na
2
O+K
2
O+Li
2
O, where CaO/SiO
2
is controlled to be in the range of 0.8 to 1.2.
In Japanese Patent Laid-open No. 51-67227, there is disclosed a flux for casting of steel which consists of base material(s), flux(s), and slag formation moderator(s), and of which the chemical composition in the molten state includes the following: 30 to 60 wt % of SiO
2
, 2 to 40 wt % of CaO, 1 to 28 wt % of Al
2
O
3
, 1 to 15 wt % of alkali metal oxide, 7 to 18 wt % of B
2
O
3
, 5 to 15 wt % of MnO, 1 to 5 wt % of FeO, and 0 to 17 wt % of C.
In Japanese Patent Laid-open No. 51-93728, there is disclosed a flux for continuous casting of steel which consists of, 50 to 80 parts by weight of SiO
2
-CaO-Al
2
O
3
ternary system base material, 1 to 15 parts by weight of alkali metal compound, 1 to 15 parts by weight of, at least one of manganese carbonate, manganese monoxide, ferromangenese, ferric oxide, and ilmenite, and less than 5 parts by weight of carbonaceous material as a slag formation moderator, and which do not contain fluoride.
In Japanese Patent Laid-open No. 58-125349, there is disclosed a mold additive for continuous casting, which is characterized by consisting of 30 to 40% of CaO, 30 to 45% of SiO
2
, 3 to 20% of, at least one of Na
2
O, K
2
O, Li
2
O, 3 to 6% of carbon in total, and 2 to 5% of Al
2
O
3
if necessary, wherein compound ratio of CaO and Sio
2
follows the condition of CaO/SiO
2
=0.68 to 1.2.
In Japanese Patent Laid-open No. 3-151146, there is exemplified a composition of a mold powder for the use in continuous casting of Al-killed ultra low carbon steel for deep drawing, which is, 0.5 to 5.0% of carbon in total, 20.0 to 40.0% of SiO
2
, 20.0 to 40.0% of CaO, zero or not more than 8.0% of Al
2
O
3
, zero or not more than 10.0% of Na
2
O, zero or not more than 6.0% of MgO, zero or not more than 10.0% of F, 5.0 to 30.0% of B
2
O
3
, zero or not more than 12.0% of TiO
2
. This exemplification suggests the mold power in which the content of F is zero. According to this publication, however, all of the mold powders used in the examples contain 9.0% of F. Also, it is described that the viscosity of the mold powder at 1,300° C. is 1.0 to 1.3 poise.
In Japanese Patent Laid-open No. 5-208250, there is disclosed a mold additive for continuous casting of steel which is characterized by having chemical composition of, 30 to 45 wt % of CaO, 20 to 35 wt % of SiO
2
, w

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