Patent Application
20130167688
This invention relates generally to a material and method for reducing the carbon and alloy content in molten steel.
Various steel making technologies have been developed since the Bessemer/Kelly bottom blown furnace and the Siemens open hearth furnace breakthroughs in the mid 19th century. Open hearth steelmaking predominated steel production through the mid 1960's. Electric arc and basic oxygen furnaces have completely replaced Bessemer/Kelly processes and open hearth steelmaking in the USA and most of the world.
Molten steel is normally produced in an Electric Arc Furnace (EAF) using primarily solid ferrous scrap or other solid iron derivatives, or a Basic Oxygen Furnace (BOF) using hot molten iron containing up to 4.0% C and scrap, or other solid iron derivatives. In the EAF, steel is melted using a combination of electrical and chemical energy. Melting of scrap in the BOF process is accomplished by chemical energy alone. In both the EAF and BOF process, the molten metal is refined using a flux to remove some of the sulfur and most of the phosphorous while providing protection to the refractory lining. Oxygen is blown into the molten metal to remove carbon, phosphorous, aluminum, chrome and silicon from the molten bath through an oxidation process. The oxidation process is exothermic which emits heat and assists in taking the molten metal up to the proper tapping temperature.
Once the molten metal is at the proper temperature and chemistry it is tapped from the EAF or BOF into a refractory lined ladle. During tapping operations, steel is poured out of the furnace vessel into a ladle. At some primary steel making shops the steel is tapped with between 50 to 1500 ppm dissolved oxygen. During tapping operations the only addition to the ladle may be high carbon ferro manganese (Mn 75% minimum, Si 4.0% maximum, C 7.0 maximum, S 0.03% maximum and P 0.7% maximum). The ferro manganese is an alloying agent and a deoxidizer. The carbon, silicon and manganese contained in ferro manganese all combine with the dissolved oxygen in the molten steel. Carbon forms gaseous carbon monoxide. Silicon forms silica. Manganese forms manganous oxide. Not all of the carbon forms carbon monoxide. A small amount of the carbon will alloy with the molten steel. Carbon levels may range up to 0.10% in the molten steel in the ladle depending on the carbon level in the EAF just prior to tap and the total amount of ferro manganese added to the ladle.
During tapping in some melt shops, high carbon ferro manganese additions may be limited up to 0.20% of the total molten steel weight in the ladle if an artificial ladle slag is added. Artificial slags may contain calcium aluminate, calcia, doloma and calcium fluoride (spar). If no artificial slag is added the high carbon ferro manganese addition may range up to 0.60%.
In order to further lower the carbon in the ladle just after tap, steelmakers will stir the ladles with argon or nitrogen through a porous plug in the ladle or through a stirring lance to induce oxygen from air into the steel. Stirring produces a roiling action on top of the molten metal bath. Bare molten metal is exposed to atmospheric air. The addition of dissolved oxygen from the air to the molten metal combines with some of the carbon to form carbon monoxide. Sometimes a workman will point a hand oxygen lance at the bare metal roiling on top of the ladle to enhance the formation of carbon monoxide in the molten steel. The gaseous carbon monoxide evolves out of the molten steel thus removing carbon according to the following reactions:
Fe+½O2=>FeO 1.
FeO+C=>Fe+CO 2.
and
C+½O2=>CO 3.
As the dissolved oxygen content in the molten steel increases, the carbon level drops in the molten steel. When the carbon level in the molten steel gets below 0.06%, dissolved oxygen in the steel exponentially increases resulting in the formation of large amount of molten iron oxide in the slag. As the dissolved oxygen content increases in the molten steel, the iron oxide level in the slag can rapidly increase from 10% to 55%. Iron oxide increases in the slag represent a yield loss of iron available in the furnace for further processing. Yield from raw ferrous charge materials can drop from 1% to 5% depending on the slag weight and iron oxide percentage in the slag. Additionally, iron oxide in the slag attacks furnace refractories causing erosion and possible steel and slag leakage from the furnace shell.
Thermodynamically it is possible increase the dissolved oxygen content from atmospheric air to remove carbon but the ladle must be violently stirred with resulting damage due to liquid metal roiling against the ladle refractory lining. Stirring gasses are induced to the ladle at rates up to 2 meters cubed per minute (70 cubic feet per minute). Stirring results in a violent roil in the slag layer on top of the ladle. Severe erosion of the ladle refractory lining occurs in the region of the roiling.
Vacuum degassing may also be used to lower carbon in a ladle containing molten steel. Vacuum degassing is very useful for making steel with <0.025 carbon. Many carbon steel plants do not have vacuum degassers available. Vacuum degassers represent a large capital cost and an additional processing step.
Another process used for steelmaking which produces a low carbon steel is known as the OBM or Q-BOP. This process involves injecting gaseous oxygen and hydrocarbons into the bottom of the refining vessel through tuyeres. A very intense mixing is induced in the molten metal leading to carbon levels as low as 0.015% without excess iron oxide production.
In liquid iron at 1873 K (1600° C.), thermodynamic equilibrium is defined by the following equation:
([ac]=[ao]/Pco)=2.0×10−3
The equation clearly indicates that as the dissolved oxygen increases, the partial pressure of carbon monoxide must increase to maintain equilibrium thus resulting in the removal of carbon from the molten iron. When the sum of the partial pressures of all of the dissolved gasses in the molten steel is greater than 1.0 atm, removal of carbon occurs in the form of CO gas evolving out of the molten steel.
Chemical reheating is regularly used in the steel industry to provide temperature increases to molten steel. Chemical heating is performed by reacting a fuel and an oxidizer. Heat is emitted from the reaction. Dissolved oxygen in the molten metal and in oxides such as MnO and SiO2 serve as oxidizers. The primary fuels are Al or Si. The basic reactions are either:
2Al+3/2O2(dissolved)=>Al2O3
with 31,129 kJ/kg (13,383 BTU/lb) Al heat generated; or
Si+O2(dissolved)=>SiO2(solid)
With 33,857 kJ/kg (14,556 BTU/lb) Si heat generated.
Additional reactions to consider are:
2Al+Fe2O3=>Al2O3+2Fe
with 16,298 kJ/kg (7007 BTU/lb) Al heat generated; or
3Si+2Fe2O3=>3SiO2+4Fe
with 14,798 kJ/kg (6362 BTU/lb) Si heat generated.
Both reactions are exothermic and generate heat in the molten steel. Furthermore, Ca, CaSi, and Mg can be used as fuels to increase temperature in molten steel but at a higher energy cost per kJ as compared to aluminum or silicon.
Prior to the widespread availability of bulk gaseous oxygen, ferrous oxide was added into the slag on top of steel melts to provide for decarburization. Steelmakers would add mill scale, a common source of ferrous oxide, to the top of the steel bath in the final refining stages. The mill scale containing ferrous oxide would melt in the slag. At steelmaking temperatures, the mill scale would transfer oxygen to the molten steel at the slag-metal interface. The oxygen would then combine with the carbon in the molten steel to form CO. The CO would out gas from the molten steel. CO formation from the carbon in the molten steel and the additional oxygen from mill scale would reduce the carbon content in the steel. This method has largely been abandoned due to the fact that reduction of carbon by gaseous oxygen injection provides for faster carbon reduction.
Additionally, it has been found that certain materials may be added to the molten metal during the steel making process for various reasons. One method of introducing these desirable additives is the use of a cored wire injection. Use of cored wire injection in the steel making is known in the art. For example, Sarbendu et al, U.S. Pat. No. 7,682,418 describes a cored wire injection process. It describes a method of injecting cored wire into the liquid steel bath. Cored wire allows for the subsurface release of additives while controlling the zone of release. The addition of additives can be controlled by changing dimensions of the cored wire and the speed of injection depending on the needs of the steel making process. Cored wire commonly has an outer coating, usually a continuous steel tube, which is filled with various additives, including lead, sulfur, selenium, tellurium, and bismuth as filling material. Cored wire containing calcium or mixture of calcium silicon is normally injected to liquefy alumina inclusions and ameliorate ladle and tundish nozzle clogging. A different type of cored wire method for treating molten metal is seen in King et al, U.S. Pat. No. 6,508,857. This is primarily an aluminum sheath forming a composite core with a calcium inner core encased in a steel jacket. The Weiner U.S. Pat. No. 4,773,929 patent deals with a chemical method of reheating molten steel using solid metal as a fuel and an oxidizer contained in a cored wire. Ferrous oxide is used in the mixture to provide a source of oxygen for the chemical reaction. This reaction is more commonly known as the thermite reaction and is very well known among chemists, metallurgists and welders. The Tiekink EP 1,715,065 uses a metal or compound with a vapor pressure higher than the vapor pressure of calcium at steel making temperatures to provide a gas which will stir the molten steel and provide for better distribution of calcium in the molten steel. It's well known that metals such as zinc or magnesium or sodium transform to a gas at molten steel making temperatures. The addition of small amounts of Zn or Mg or Na to a cored wire containing calcium can provide an intense localized stirring action.
The current invention uses a cored wire injection with a filling of iron oxides and a mineral carbonate to provide an improved method and apparatus for increasing and maintaining dissolved oxygen in the steelmaking process while also providing a method for forming carbon dioxide for stirring and carbon oxidation in the molten steel bath. This invention is particularly useful for low carbon steel production. This invention allows for tapping electric arc or basic oxygen furnaces at higher levels of carbon thus lowering the tap oxygen content in the furnace and preventing the high amounts of iron oxide in the slag which in turn leads to iron yield improvements and cost savings. Injecting a cored wire containing a mineral carbonate in the ladle after the furnace melting process provides sources of oxygen and a method of stirring the steel to lower the carbon content. Thus the dangerous and expensive effects of increasing dissolved oxygen in either the electric arc or basic oxygen furnace can be eliminated.
The use of a cored wire injection with the iron oxides and mineral carbonate filling reduces the risk to an operator in the steelmaking process who otherwise would be using a lance to stir the steel in the ladle. Use of the cored wire reduces the risk of damage to the ladle refractory lining. It provides more precise control of the process for the operator while reducing costs. Injecting a cored wire containing ferrous oxide and a mineral carbonate is useful for making low carbon steel grades with 0.015 to 0.06% carbon as opposed to the current methods of blowing oxygen into the furnace or using a vacuum degasser.
1. Contains the filling (200);
2. Keeps the filling (200) dry;
3. Prevents the filling (200) from reacting in the liquid slag layer on top of the ladle; and
4. Provides rigidity for the filling (200) to penetrate into the molten steel.
The cored wire (100) is preferably wound into a coil (400) and placed on a reel. The metal jacket (110) starts as a flat ribbon like construction and is formed into the cylinder that holds the filling (200). The flat ribbon like material is bent into a cylinder with the seam (120) holding the filling (200) in place inside the cored wire (100).
circumference of the cored wire (100). The cored wire (100) will then be wound into a coil (400) with the weight of the coil ranging from 113.4 kg to 2.268 kg (250 to 5000 lb).
The metal jacket (110) forming the outer shell of the cored wire (100) prevents premature melting of the filling (200) so reactions can take place in the molten steel (600) and not in the slag layer. The feeding speed can be varied to allow melting of cored wire (100) at various depths in the ladle (500).
Using the above described cored wire injection process a cored wire (100) is injected into the ladle (500) containing the molten steel. The cored wire includes a metal jacket (110), and a filling (200) that contains oxides of iron and a mineral carbonate. Oxides of iron are used to provide oxygen to the steel in the ladle. Mineral carbonates are used to form CO2 for stirring and further carbon oxidation in the molten steel bath.
Three reactions may occur depending on the type of mineral carbonate used:
CaCO3=>CaO+CO2 1.
MgCO3=>MgO+CO2 2.
CaMg(CO3)2=>CaO+MgO+2CO2. 3.
In a non-deoxided steel such as being treated in this application the CO2 undergoes a further reaction to help decarburize the steel: CO2+C=>2CO.
Additionally two secondary reaction occurs with the CO2 to further help decarburize the steel:
CO2+Fe=>CO+FeO 1.
FeO+C=>Fe+CO. 2.
The formation of CO2 in the ladle provides a mechanism to promote intense localized sub-surface mixing kinetics in the ladle.
In a synergistic effect, the use of CO2 as a stirring agent helps to lower the partial pressure of CO needed for carbon oxidation.
The following table indicates the maximum volume of CO2 gas generated at standard temperature and pressure conditions (STP):
The CO2 production rate is controlled by the injection speed and weight of mineral carbonate contained in a given length of cored wire. The cored wire has various oxides of iron that have varying amounts of oxygen as a by weight percentage. This percentage ordinarily varies between 10% and 30%. The use of the cored wire (100) contains a filling (200) that comprises in part various forms of oxides of iron containing wustite, hematite, and/or magnetite, and allows a precise method and apparatus for increasing and maintaining dissolved oxygen somewhere between 1 and 1800 parts per million in a ladle of molten steel. Ordinarily the amount of oxygen to the ladle of steel may vary from 1 part per million and 1800 parts per million. A simple calculation based on the filling (200) and the cored wire (100) an injection rate allows an operator to control the amount of oxygen added. In industrial applications the actual range added will usually fall between a low of 0.00333 kilograms of oxides of iron per metric ton added to a high of 18 kilograms per metric ton of molten steel of oxides of iron added using the cored wire (100). The amount of mineral carbonates on a weight basis mixed in with the ferrous oxide will vary from 1 to 90%. The final percentages of a ferrous oxide and mineral carbonate mixture used at a particular operation will depend on cored wire injection velocity, cored wire diameter, and the desired outcome. It should be noted that if the percentage of mineral carbonate is too high and the injection speed is too high, violent stirring will take place in the ladle and steel will splash out of the ladle. Thus, a lower percentage of mineral carbonates will be needed for high injection rates. A case may be made for very slow injection speeds which will allow for more reaction times. In this situation a higher percentage of mineral carbonate will be needed in the cored wire to provide adequate CO2 gas generation rates.
Combining ferrous oxide and a mineral carbonate in the same cored wire provides a synergistic effect for removing carbon. As the mineral carbonates degrade into a mineral oxide and gaseous CO2, an intense localized stirring action occurs deep inside the ladle of molten steel. Kinetic action stirs the ferrous oxide into solution and forces faster formation of CO which removes the carbon from the molten steel. With the intense stirring and the lowering of the partial pressure of CO gas needed for carbon removal by CO2 generation from the mineral carbonate, minimum carbon levels of 0.015% can be realized as opposed to 0.025% C achieved with normal oxygen blowing methods. Additionally, some of the CO2 will react further with the molten steel's carbon to help form supplementary CO gas for accelerated carbon removal. The addition of a mineral carbonate greatly assists carbon removal as opposed to injecting only a ferrous oxide material. Injecting a cored wire containing ferrous oxide and mineral carbonates provides a novel and unobvious method for decarburization by providing an oxygen source, stirring the molten metal, and lowering the partial pressure of CO needed for carbon removal. The total pressure of gasses required for carbon removal from molten steel must exceed 1 atm:
PCO+PN2+PAr+PCO2>1 atm. As the partial pressures of Ar, N2 and CO2 are increased, the partial pressure of CO needed to remove C is reduced. The CO2 obtained from the calcium carbonate will produce very small gas bubbles thus increasing the surface area for reactions. The use of nitrogen is discouraged in low carbon steelmaking since it can produce unwanted physical properties when the steel solidifies. High volumes of argon gas injected into a ladle through a lance or porous plug tends to coalesce and form very large bubbles thus reducing the amount of surface area for reactions.
Mineral carbonates have been added to steelmaking slag as a foaming and refining agent. As the mineral carbonate breaks down, carbon dioxide is evolved in gaseous form. Some of the carbon dioxide will further degrade into CO and O2. Additionally, any remaining metallic P, Cr, B, Ti, Si, Al, Ca, and Mg will be oxidized into inclusions of P2O5, Cr2O3, B2O3, TiO2, SiO2, Al2O3, CaO and MgO. The inclusions will float to the top of the bath and be trapped in the slag layer.
The slag rises due to the gas evolution much like foam on a glass of beer. This invention injects the mixture of mineral carbonates below the surface of the molten metal so all of the contained liquid is mixed, not just the slag. This provides momentum to the molten metal and causes intense localized mixing. Rather than relying on additional stirring action from a porous plug or ladle lance, the injected cored wire provides its own agents for stirring the molten metal.
The following method may be used for increasing the dissolved oxygen content in a ladle of steel:
1. The molten metal contained in the melting furnace is at the desired temperature and chemistry. A steelmaker normally tests the molten steel using a thermocouple, oxygen probe and sampler. The sampler sucks up a small quantity of molten metal where it is instantly solidified into a shape suitable for optical emission spectrographic (OES) testing. The oxygen probe provides the level of dissolve oxygen in the molten metal in the bath. If the temperature, oxygen level and chemistry are within specification, the furnace is ready to be drained in an operation known as tapping.
2. The furnace is opened and molten metal is tapped into a ladle. The furnace is primarily a melting vessel. Further refinement of the molten steel takes place in a refractory lined ladle. This step also frees up the furnace to melt the next order of steel. Tapping operations normally take from 3 to 12 minutes. Tapping is a very spectacular operation with hot glowing molten metal flowing through the air from the furnace into the ladle. The flow rate out of the furnace is normally controlled by an operator continuously adjusting the tilt angle.
3. During tapping, ferro-manganese may or may not be added to the ladle. No other ferro alloys are added during tapping operations for this invention. The addition of ferro alloys is restricted to allow for the achievement of high dissolved oxygen levels, >100 ppm, in the ladle post tap. In some operations aluminum, silicon or silico-manganese are added during tap to decrease dissolved oxygen in the molten metal. The intent of this invention is to keep the dissolved oxygen levels at a >100 ppm level so that the removal of carbon is easier.
4. Fluxes such as calcium aluminate, calcia, doloma or calcium fluoride (spar) are added to the ladle during tapping operations. Fluxes are used primarily for the removal of sulfur and protection of the ladle during refinement operations. Fluxes also help to reduce the activity of dissolved oxygen in the molten metal so if an operator wishes a higher dissolved oxygen level for carbon oxidation from the molten steel, the amount of fluxes added during tapping operations will be reduced as compared to normal levels.
5. Cored wire containing various forms of oxides of iron containing wustite, hematite or magnetite and a mineral carbonate are injected into the ladle during tapping operations. Tapping operations alone generate a roil in the ladle to promote mixing. Simultaneously, argon or nitrogen is injected into the ladle either through a lance or porous plug to aid in the agitation of the molten metal. The agitation is further intensified by the evolution of carbon dioxide gas from the cored wire addition. The intense kinetic energy imparted by stirring promotes the mixing of ferrous oxides with the dissolved carbon in the melt. An intense but controlled stir from the tapping operation, argon or nitrogen gas injection and carbon dioxide evolution from the mineral carbonate addition in the cored wire accelerates the reaction time. This is unique.
Alternatively, the ferrous oxide plus mineral carbonate cored wire may be injected after tapping is completed. This step can be conducted behind the melting furnace, at an intermediate station or at a ladle refining operation. The kinetic energy from the molten metal flowing from the furnace into the ladle is lost but in some operations it may be more practical to inject the cored wire later in the process. The ladle would be stirred with nitrogen or argon through a lance or porous plug while addition stirring energy would be imparted from the evolution of carbon dioxide gas splitting off from the mineral carbonates. This is also unique.
6. The dissolved oxygen content in the molten metal is increased by one to 1800 ppm by the injection into the ladle of cored wire containing various forms of oxides of iron containing wustite, hematite and or magnetite and a mineral carbonate. It is desirable that the dissolved oxygen content in the molten metal is increased by one to 1800 ppm by the injection into the ladle of cored wire, with the process conditions dictating the amount of oxygen increase needed for carbon removal. The injection speed will range from 10 feet per minute up to 1500 feet per minute.
Carbon removal is accomplished by increasing the oxygen content in the molten metal. A simple reaction of C+O═CO provides the essential description of carbon removal from molten steel. As the dissolved oxygen content increases, the level of retained carbon decreases. Both ferrous oxide and carbon dioxide provide oxygen to the process for carbon oxidation and removal. Some additional oxygen for carbon reduction may be provided from the air atmosphere directly above the ladle.
In general, the higher the dissolved oxygen content, the lower the level of retained carbon in the molten metal. Optimizing this is unique.
7. P, Cr, B, Ti, Si, Al, Ca and Mg alloyed with the molten steel will be oxidized prior to C affecting their removal from the molten steel bath.
8. After the dissolved oxygen content is increased and the carbon is at the desired level, additions of cored wire rod form or alloy lump of aluminum, silicon, ferro-silicon, silico-manganese, calcium-silicon, calcium metal, or magnesium may be added to the molten metal in the ladle. An addition of any of the above mentioned materials causes an exothermic reaction thus raising the temperature of the molten metal. The dissolved oxygen level is reduced and the molten metal is ready for further refinement.
9. The ladle of molten metal is then taken to the ladle furnace or other metallurgical refinement process, such as a vacuum degasser, for final treatment into a castable, sellable material. Most steel produced now uses a secondary refining station. Use of secondary refining stations has led to productivity and quality improvements.
10. Alternatively, as mentioned in Step 5, after tapping is complete and the ladle is at a secondary refining station, injection of cored wire containing various forms of oxides of iron containing wustite, hematite and or magnetite and a mineral carbonate are injected into the ladle at the start of secondary refining operations prior to the addition of any ferro alloys or carbon. No other alloys other than ferro manganese have been added up to this point.
Due to the physical layout of many steel melt shops, injection of cored wire may be more easily accomplished at the secondary refining unit rather than just after tapping.
11. Alternatively, at the secondary refining station, after the cored wire containing various forms of oxides of iron containing wustite, hematite and or magnetite and a mineral carbonate are injected into a ladle, additional alloying operations may be accomplished after the carbon is at the desired level as detailed in Step 7. Additional fluxes may be added to the ladle of steel for sulfur removal during and after the alloying operations. The addition of ferrous oxides containing mineral carbonates, most ferro alloys and fluxes tend to reduce temperature in the molten metal. At a ladle furnace, the molten metal can be reheated to the desired temperature. The use of a ladle furnace station for carbon removal using a cored wire containing ferrous oxide and mineral carbonates is a desirable alternative to injection behind the furnace or at an intermediate station. This is unique.
Injecting oxygen using a lance into a ladle or stirring the molten metal in the air is a slow process as compared to increasing oxygen using a cored wire containing ferrous oxide. When comparing the oxygen injection rate between a lance running at 2.12 normal cubic meters per minute (75 scfm) and a 13 mm diameter cored wire containing ferrous oxide, the oxygen injection rate using the cored wire is 7 times faster. Operators could tap an EAF at a lower oxygen level which will lead to faster production times, less refractory wear and lower yield losses.
Lowering carbon in a ladle using a cored wire containing ferrous oxides and mineral carbonates provides an inexpensive method for lowering carbon outside of the melting furnace. Current methods for post melting carbon reduction involve using a vacuum degasser at a large capital cost. Any steel melt shop could readily acquire and install a wire feeder needed for this process at 1/10 to 1/20th of the cost of a vacuum degasser and start making low carbon steel in the ladle. Steel could be tapped from the melting furnace at higher carbon levels leading higher productivity and yields from raw materials.
It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims. In addition, the same method could be applied to achieve other stoichiometric goals. By way of example, if the alloy level is too high in a ladle of steel the injection of ferrous oxide will help to readily oxidize excessive alloys such as P, Cr, B, Ti, Si, Al, Ca and Mg and remove the materials in oxide form to the slag layer on top of the ladle. It should also be understood that ranges of values set forth inherently include those values, as well as all increments between. For example, 1-5 includes 1; 1.1; 1.2 and so forth until 5. It should also be understood that, as used herein, “approximately” and the like are +/−5%, and “substantially” means to the extent reasonably possible when considering human and machine variations. Also, it should be understood that these processes can take place in an industrial ladle having a capacity of approximately 10 tons to approximately 400 tons of molten metal.
This application claims benefit and priority from U.S. provisional application No. 61/631,423 accorded a filing date of Jan. 4, 2012.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 61631423 | Jan 2012 | US |
| Child | 13671584 | US |
