DE1806155A1 - Propellant for addition to molten steel and process for making unkilled steel - Google Patents

Description

Propellant for addition to molten steel and process for Manufacture of unkilled steel Summary: manufacture of unkilled steel Steel melts, especially unkilled steel melts with a medium and high carbon content, due to the addition of a blowing agent containing iron carbonate to a molten steel bath.

The invention relates to a novel propellant (additive for unquenched steel-rimmig agent), spatial iron carbonate (FeCO3), for the manufacture Contained from unaffected steel masses of medium and high carbon content.

It is used for the production of high quality raw steel the development of large quantities of gas, chiefly carbon monoxide, during the Solidification of the block required. The gas development takes place mainly along the side walls of the block, as the walls are good nucleation sites and since the metal in the vicinity of the side walls cools down first and gives rise to solubility of an ases in liquid steel decreases as the temperature decreases. That walls Metal adjacent to the mold solidifies first and the development of gas in it The area decreases the carbon content near the wall, creating one on the outside lying ferrite zone known as the "rim" is formed, the has a minimum of oxide inclusions.

Blocks with a low rimming action (weak Carbon monoxide) contain many gas bubbles near the surface of the block, those trapped by the solidifying metal interface during solidification will. When a block is subsequently rolled out, many of these gas bubbles, which are referred to as voids or gas inclusions, and those near the surface of the block had been trapped, exposed to the atmosphere, where they oxidize and Cause vein-like and silver-like surface defects (seam d silver type surface defect).

It is well known that medium and high carbon steels, i.e. steels with carbon held by about 0.12% and above, difficult to disrupt or are difficult to @gen to a strong gas development (to rim) due to the in the. Steel available @ insufficient amount of oxygen for union with carbon in the steel to evolve carbon monoxide gas that for good Rimming action is necessary. The product of dissolved carbon and dissolved oxygen in molten steel is for any given temperature a constant. This notation is expressed by the equation [C] [O] = K. The value of the constant K @ im @ t increases with temperature, but with the relationship between the dissolved carbon and the dissolved oxygen remains valid. the end From this relationship it can be seen that as the carbon content of the steel increases, the dissolved oxygen content decreases. Thus, in steel masses with a mean and high carbon content, the oxygen content is low and it is insufficient Amount of oxygen for generating the necessary rimming action available.

The problem of obtaining a good propellant effect in steel compositions with medium and high carbon C; C14ki3 "of f gel lt has gained increasing importance in recent years with the advent of very large molds, which are used for the production of unusually large plates or large weld-free windings or coils, which are now required by steel consumers. The increase in the pouring height of an ingot causes the ferrostatic head in the mold to increase in size, thus increasing the suppression of the evolution of carbon monoxide gas.

Many measures have been suggested and many tools have been used in an attempt to investigate the driving effect in steels with high and medium carbon content to improve. It was suggested to put the steel on one lower Carbon content too fine to increase the oxygen content of the steel and then by adding carbon to the steel when it is drained charcoal the steel in a pan. This is a dangerous practice as this can lead to an unsuitable carbon content. In addition, the addition causes from excessive amounts of carbon to liquid steel, a strong foaming and since the reaction is very endothermic, the temperature is often too high in temperature lost to the molten steel before casting.

The addition of common fluoride-containing promoters for the Propulsion (degassing effectY, for example of sodium fluoride is generally with reference to the promotion of propulsion in steels with medium and high carbon content ineffective. These promoters drive off fluoride gas, which nucleates others Gas developments supported; however, no oxygen is present in the promoter is to support the formation of the necessary carbon monoxide.

Nitrate-containing promoters for promoting propulsion (gas development) result in excessive amounts of oxygen, but with the nitrogen, which also occurs dissolves in the steel, in terms of the physical properties of the final product due to the rapid increase in aging associated with increased nitrogen levels night is ilig.

Iron oxide, usually in the form of mill scale, was added to steel added during pouring as a propellant (degassing agent). However included Additions of this kind enough other impurities to cause excessive undesirable non-metallic inclusions in to guide the steel. aside from that The iron oxides decompose relatively quickly, so that there is no propellant effect has sufficient duration.

Sodium carbonate has also been studied and found to be unsatisfactory as a 'llre-ib or degassing agent. When adding sodium carbonate to a steel bath, the following reaction is obtained: The sodium oxide (Na20) is stable and therefore the oxygen bound to the sodium is not available for the conversion with carbon and for the promotion of propulsion. In addition, the rapid decomposition of sodium carbonate requires the addition of excessive amounts to maintain rubbing. The stable sodium oxide floats on the top of the block and creates a strong slag that can suppress the development of gas.

In addition, deslagging must excessively affect the molten ones Block tops are designed to ensure a suitable capping of the block to enable.

It has now been found, n, that a violent drifting or degassing (rimming action) in numbers of medium and high carbon content by the Addition of iron carbonate (FeCO3) to the molten steel during its casting can be obtained. In addition, the disadvantages listed above are by the use of iron carbonate as a propellant or degassing agent for the unsettled Steel turned off.

Isene carbonate - reacts with steel according to the equation given below: For every mole of iron carbonate added, 3 moles of carbon monoxide are formed. Iron carbonate is thus 50% more effective than sodium carbonate, which only forms 2 moles of carbon monoxide per mole of carbonate. In addition, visene carbonate does not decompose as quickly as sodium carbonate, and it also does not form stable compounds that have to be removed as slag. Finally, bis-carbonate is free of non-metallic components so that there is no increase in the inclusion content of the steel.

Although iron carbonate is very effective by itself, it can also be used in conjunction with other promoters promoting drifting or degassing are used. For example, iron carbonate can be used with a sodium fluoride promoter. The iron carbonate supplies the dissolved oxygen and the fluoride supplies a nucleation gas for better carbon monoxide removal. The iron carbonate should be at least 40% the combined total amount of blowing or degassing agents and in general an iron carbonate addition of 70% should be used.

The amount of iron carbonate to use depends on factors which are generally known to the person skilled in the art, for example slimming or diminishing of carbon, amount and type of ladle alloy additives, ingot casting height or the like. In general, the amount of biscarbonate will vary from 0.23 to 1.40 kg 0.9 metric tons of S'tahl each.

If only a small amount of tightness is necessary, this can be done immediately can be added after cushioning. If more iron carbonate is required this can be added during potting. In general, it is convenient and desirably a larger amount of the bottom portion of the block due to the larger ferrosta'tic head, the one to suppress a signed Gas development tends to clog.

Although the invention is particularly focused on the manufacture of unquieted Steels with medium and. high carbon, there are cases in which iron carbonate is very effective as a blowing agent or degassing agent the manufacture of low carbon steels can no. If e.g. a scrap load applied in excess in a Siemens-Martin furnace is used, or pig iron with a low silicon content and high phosphorus content is used the steel produced often has a very low oxygen content. In an oxygen inflation steelmaking process, the oxygen content is generally less than the oxygen content in the steel for a given carbon content for a comparable carbon content in e'inem according to other working methods produced steel because of the kinetics of the reaction in the bar making process is so rapid after the oxygen inflation process that equilibrium conditions cannot even come close. In these cases, iron carbonate can be successful as a propellant or degassing agent to deliver the necessary amount of oxygen for a suitable propellant or degassing effect even with steels with a low carbon content be applied.

Claims (4)

Claims 1) Propellant for addition to molten oil during pouring in a block form, characterized in that it contains iron carbonate. 2) Process for the production of unkilled steel, characterized in that that liquid steel is progressively poured into an ingot mold during casting Propellant containing iron carbonate added. 3) Method according to claim 2, characterized in that one is a Propellant is added which contains at least 40% carbonate of carbonate. 4) Method according to claim 2, characterized in that 0.23 Adds up to 1.40 kg of iron carbonate per 0.9 metric ton of steel.