Patent Application
20160107226
This invention relates to the continuous slab casting of steel. In the continuous slab casting of steel, molten steel from a steelmaking ladle is poured indirectly through a subentry nozzle into an oscillating casting mold, and the steel is continuously cast in a semi-finished strand to make a slab, bloom, or billet. The semi-finished shape of the strand is determined by the continuous casting mold with a molten inner core and a solidified outer surface as the strand moves downwardly through the mold. The strand is subjected to secondary cooling upon exiting from the mold until the entire strand is solidified. The strand is then cut into slabs, blooms, or billets.
In the continuous caster, the molten steel flows from the tundish into the mold through a submerged entry nozzle (SEN). The SEN discharges the molten metal into the mold to a selected depth below the surface (the “meniscus”) of the melt in the mold. The flow of the molten melt from the tundish is gravitationally fed by the pressure difference between the liquid levels of the tundish and that of the melt in the mold. The melt flow from the tundish may be controlled by a stopper rod which at least partially blocks the exit port to the SEN, or a slide gate that moves across the outlet port of the tundish to the SEN. As the molten metal enters the mold, the steel solidifies at the water cooled mold walls to form a shell, which is continuously withdrawn at the casting speed to produce the steel strand by oscillation of the mold walls.
One of the prime difficulties in such continuous casting of steel is having a uniform and consistent mold heat transfer rate, which affects the final casted steel. The origin of the majority of surface defects in the cast strip is at, or within, a very short distance of the meniscus in the mold. Whether the defects propagate into cracks depends on the heat transfer in the remainder of the mold and events and conditions at and below the mold exit. As such, regulating the heat transfer rate is essential.
The heat transfer rate is can be affected by the amount of dissolved gases, particularly hydrogen, in the molten metal. As such, fluctuation in hydrogen levels in the molten metal may cause defects in the steel product and even breakouts as the steel is casted, which in turn, would increase maintenance costs and decrease productivity.
Excessive hydrogen concentrations may decrease the heat transfer rate through the liquid metal causing various defects and risk possible breakouts in the mold. Due to its high mobility, hydrogen can easily diffuse through the lattice of the steel microstructure. Hydrogen may be picked up by the molten metal through steelmaking additions or processes, several of these could be hydrated lime, wet alloys or excessive furnace slag carry over. Hydrogen may also be picked up by the molten metal from the atmosphere. As shown in FIG. 11 of the paper titled “Hydrogen and Nitrogen in Steel Making at U.S. Steel” published in the November 2009 issue of Iron & Steel Technology, higher humidity conditions characteristic of steelmaking facilities located in northern U.S. resulted in higher average hydrogen levels.
Conversely, abnormally low hydrogen concentrations may increase the heat transfer rate of the liquid metal causing various casting defects, such as surface cracks in the finished product. Decrease in hydrogen levels is aggravated by cold and dry weather conditions. It is known that the extent of hydrogen pickup strongly depends on the partial pressure of water vapor (i.e. humidity) in the atmosphere. Since the amount of moisture in the air depends on the temperature and the relative humidity, cold and dry winter days provide conditions for unusually low hydrogen levels when compared to summer days.
Currently, there are known methods for regulating the heat transfer rate by altering the casting conditions in the mold. One of these methods is the variation of the physical composition of the mold powder at the continuous caster. The physical components of the mold powder affects the heat transfer rate. As the mold powder melts and solidifies in the mold, the mold powder interacts with the hydrogen in the molten steel and the glass state of the solidified mold powder is altered, affecting the heat transfer rate of said powder. However, developing mold powders dependent on hydrogen levels requires having an extensive inventory of mold powders of different compositions. Additionally, it requires supervision by a trained operator to select the correct mold powder from the various mold powder compositions to control the hydrogen levels in the mold with different operational conditions.
Previous methods control hydrogen levels by modifying the refining process or by using a downstream degasser. Degassing the steel has proven effective in reducing the hydrogen levels and altering the method and timing of the alloy additions have also proved effective in reducing the hydrogen levels in the produced steel.
Accordingly, there remains a need for a method for increasing the hydrogen levels in steel composition for consistent heat transfer in continuous casting that is both effective and economical.
Presently disclosed is a method of controlling the amount of hydrogen in steel for consistent heat transfer in continuous casting by adding a hydrocarbon to the molten metal. Disclosed is a method of continuous casting comprising the steps of:
The hydrocarbon may be delivered to the molten steel in the ladle metallurgy furnace in an amount sufficient to provide between 5 and 9 ppm of hydrogen in the molten steel delivered to the caster for continuous casting into a steel product. Alternatively, the hydrocarbon may be delivered to the molten steel in the ladle metallurgy furnace in an amount sufficient to provide between 6 and 8 ppm of hydrogen in the molten steel delivered to the caster for continuous casting into a steel product.
The hydrocarbon may be methane. The hydrocarbon may be delivered to the molten metal in the ladle metallurgy furnace by bottom stirring. In some embodiments, the hydrocarbon may be stirred at a rate of 15 SCFM. In other embodiments, the hydrocarbon may be stirred at a rate of 20 SCFM.
Also disclosed is a method of continuously casting comprising the steps of:
The hydrocarbon may be delivered to the molten steel in the ladle metallurgy furnace in an amount sufficient to provide between 5 and 9 ppm of hydrogen in the molten steel delivered to the caster for continuous casting into a steel product. Alternatively, the hydrocarbon may be delivered to the molten metal in the ladle metallurgy furnace in an amount sufficient to provide between 6 and 8 ppm of hydrogen in the molten steel delivered to the caster for continuous casting into a steel product.
The hydrocarbon may be methane. The hydrocarbon may be delivered to the molten metal in the ladle metallurgy furnace by bottom stirring. In some embodiments, the hydrocarbon may be stirred at a rate of 15 SCFM. In other embodiments, the hydrocarbon may be stirred at a rate of 20 SCFM.
In order that the invention may be more fully explained, illustrative results of experimental work carried out to date will be described with reference to the accompanying drawings in which:
Molten steel for steel making is made mostly from scrap melt in an electric arc furnace. Referring to
Provided is a slide door 17 for charging and a backdoor 18 with a slag apron 19 for discharging the slag from the furnace. The electric arc furnace 10 may have a split shell top portion 21 including a roof 13 capable of being quickly decoupled and removed from a bottom portion 22. This facilitates and reduces downtime due to change out of the top portion 21 of the furnace, and provides for rapid relining of the bottom 11 and side walls 12 in bottom portion 22 of the furnace. A sill line 22A divides the upper portion 21 from the bottom portion 22 of the electric arc furnace.
The sidewalls 12 above the slag line are usually comprised of water-cooled panels 23 supported by a water-cooled cage 23A. The furnace roof 13 is also comprised of water-cooled panels with the center section of roof 13 surrounding the electrode ports 24 (called the roof delta 25), generally a cast section of refractory, which may be also water-cooled. Electrodes 26 extend through the electrode ports 24 into the furnace. The electrodes 26 are supported by electrode holders 27, electrode mast arms 28, and electrode mast 29. Roof 13 of the furnace may be removed and supported by jib structure 30, which may be supported by the operating floor level structure 31. The transformers (not shown), housed in an electrical equipment vault 32, supply the electrical current to the electrodes 26 and the steel melt in the electric arc furnace.
Referring now to
Once the heat is completed in the furnace and discharged through the shroud, the molten metal is tapped through the bottom of the furnace into a ladle 61 and transferred to a ladle metallurgy furnace 60 on a ladle car 62, which is configured to move the ladle from the ladle metallurgy furnace 60 along the factory floor 63 to a caster (not shown). The molten steel is then delivered from the ladle metallurgical furnace after trimming, as discussed below, to the continuous slab caster.
The cast strand 136 leaves the caster mold through a support roller assembly 140 adjacent broad mold faces 133 and 134 in a generally horizontal direction, which directs the cast strand to a cutting point 150 as the strands cools to a solid form. During casting, water (or some other coolant) is circulated through the caster mold 130 to cool and solidify the surfaces of the cast strand 136. Each time the strand 136 is cut at the cutting point 150, a solid slab 160 is formed having a predetermined length 165.
Before the molten metal is delivered to the caster, the molten steel composition is trimmed in the ladle metallurgy furnace to the exact chemistry desired for casting in the continuous caster. Hydrogen levels in the molten steel may vary with the atmospheric humidity at the steel making plant, which varies generally with the season of the year. For example,
As illustrated in
As illustrated by the upper graph in
A hydrocarbon may be added to the molten metal in the ladle to control the hydrogen levels for consistent heat transfer as needed and desired. Hydrocarbon refers to any of a class of organic chemical compounds composed only of the elements carbon (C) and hydrogen (H). The addition of hydrocarbon to the molten metal increases hydrogen levels. The hydrogen and carbon of the hydrocarbon disassociate, increasing hydrogen levels in the molten steel.
The hydrocarbon may be stirred through the bottom of the ladle and into the molten metal. A hydrocarbon, such as methane, may be added to the molten metal in the ladle metallurgy furnace in an amount sufficient to provide between 5 and 9 ppm, or alternatively between 6 and 8 ppm, of hydrogen in the molten metal.
The theoretical and actual recovery rates were also determined. The theoretical recovery is identified with square notations. The actual recovery is identified with diamond notations. As
Furthermore, several tests were performed varying the flow rates at which methane was added and stirred into the molten metal. We found that the slower the flow rate, the better the recovery. For example, a flow rate of 15 SCFM with a methane flow volume of 35 SCF for 1 ppm hydrogen results in 83% recovery rate. A flow rate of 20 SCFM with a methane flow volume of 42 SCF for 1 ppm hydrogen results in 69% recovery rate. Whereas, more than doubling the flow rate to 50 SCFM with a methane flow volume of 47 SCF for 1 ppm hydrogen resulted in 62% recovery rate.
While the invention has been described with reference to certain embodiments it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiments falling within the scope of the appended claims.
This application claims priority to provisional application No. 62/065,319 filed on Oct. 17, 2014.
| Number | Date | Country | |
|---|---|---|---|
| 62065319 | Oct 2014 | US |
