METHOD FOR PRODUCING GRAINED IRON, AND GRAINED IRON

Description

TECHNICAL FIELD

The present invention relates to grained iron with a reduced P concentration, and a method for producing the same.

BACKGROUND ART

In recent years, there has been a growing demand for increased use of cold iron sources (scrap) in the steel industry. To build a recycling society, recycling iron sources is essential. Moreover, increasing the use of scrap is essential from the perspective of preventing global warming and responding to the demand for reducing CO₂ emissions. The amount of scrap used can reduce CO₂ emissions because the production process for the scrap does not require a reduction step unlike with iron ore which is iron oxide (Fe₂O₃). Thus, the amount of cold iron sources used is increasing more and more.

The blast furnace-converter method is a steelmaking process that includes supplying iron ore (Fe₂O₃) as a raw material into a blast furnace together with coke (a carbon source) as a reducing agent, to obtain molten pig iron with a C concentration of approximately 4.5 to 5%, and then supplying the obtained molten pig iron into a converter to remove C, Si, and P, which are impurities, through oxidation. When molten pig iron is produced in a blast furnace, a reduction process for iron ore and the like require approximately 500 kg of carbon source to produce 1 ton of molten pig iron, generating approximately 2 tons of CO₂ gas. Meanwhile, when molten steel is produced using iron scrap as a raw material, no carbon source is required for the reduction process for iron ore. When the energy needed to melt iron scraps is considered, replacing 1 ton of molten pig iron with 1 ton of iron scrap leads to a reduction of CO₂ emissions by about 1.5 tons. This shows that the amount of scraps used should be increased to reduce greenhouse gas emissions while maintaining production activity.

However, the tight supply-demand balance for iron scrap, especially high-grade iron scrap, which is essential for producing high-grade steel, has increased the need for reduced iron to replace scrap. Reduced iron is produced by reducing iron ore, and it is not necessary to set the C concentration in the produced iron to a high level as in the blast furnace-converter method. Thus, since an excessive amount of carbon source is not used, CO₂ emissions can be reduced by approximately 0.2 tons per 1 ton of iron. Further, when a hydrogen gas or a hydrocarbon-based gas, such as a natural gas, is used as a reducing agent instead of a carbon source, CO₂ emissions can be further reduced.

Problems faced when reduced iron is used include phosphorus contained in the reduced iron. Since phosphorus contained in a steel product would cause a decrease in quality, such as hot brittleness, it is necessary to reduce the P concentration to a level corresponding to the required quality. However, when molten iron is produced by melting reduced iron by an electric furnace method, a major part of phosphorus in the reduced iron remains in the molten iron (which is referred to as rephosphorization). Therefore, the currently available reduced iron is produced using high-grade iron ore with a low P concentration (a P concentration of approximately 0.01 mass %), and the P concentration in the reduced iron is approximately 0.02 mass %.

Meanwhile, high-grade iron ore with a low P concentration is predicted to be depleted in the future, and it will be thus required to produce molten steel using, as a raw material, reduced iron produced using low-grade iron ore with a high P concentration. The P concentration in iron ore used for the current blast furnace method is 0.05 to 0.10 mass % (or 0.10 to 0.15 mass % when converted into the P concentration in reduced iron), and the P concentration is predicted to further increase in the future. Such a P concentration is 5 to 10 times or more the P concentration in the above-mentioned reduced iron produced using high-grade iron ore with a low P concentration. To prevent the quality of a steel product from deteriorating due to phosphorus contained therein, it is necessary to remove phosphorus during the production of molten steel by melting reduced iron with a high P concentration, or remove phosphorus during the production of reduced iron from iron ore with a high P concentration. Several phosphorus removal technologies have been proposed.

Patent Literature 1 proposes a dephosphorization refining flux for use in an arc furnace for removing phosphorus in molten steel to achieve a low phosphorus concentration in a relatively short time with the arc furnace alone, the flux containing calcium oxide as the main ingredient, and also containing 5 to 15 mass % of aluminum oxide and 25 to 35 mass % of iron oxide, with the balance being unavoidable impurities.

Patent Literature 2 proposes a method of removing phosphorus by bringing iron ore, titanium-containing iron ore, nickel-containing ore, chromium-containing ore, or a mixture containing such ores, each having a CaO content of 25 mass % or less and a ratio of CaO/(SiO₂+Al₂O₃) of 5 or less, as the main ingredient into contact with a gas selected from the group consisting of Ar, He, N₂, CO, H₂, and hydrocarbon, or a mixture gas thereof at a temperature of 1600° C. or higher.

Patent Literature 3 proposes a method including crushing iron ore with a high P concentration to have a size of 0.5 mm or less; adding water thereto to achieve a pulp concentration of approximately 35 mass %; adding H₂SO₄ or HCl to a solvent to allow a reaction to take place at a pH of 2.0 or less, thereby decomposing and eluting phosphorus minerals; collecting a magnetically attracted substance such as magnetite by magnetic sorting to separate therefrom SiO₂, Al₂O₃, and so on, which are magnetically non-attracted substances, as slime by sedimentation separation; and also adding slaked lime or quicklime to thereby neutralize P eluted into the solution at a pH in the range of 5.0 to 10.0 to separate and recover P as calcium phosphate.

CITATION LIST

Patent Literature

• • Patent Literature 1: JP-H8-120322A
  • Patent Literature 2: JP-S54-83603A
  • Patent Literature 3: JP-S60-261501A

SUMMARY OF INVENTION

Technical Problem

However, each of the above-described conventional technologies has the following problems that should be solved.

That is, the technology disclosed in Patent Literature 1 is based on the premise of using an iron source with a low P concentration, such as scrap. Specifically, 350 g of flux is added to 7000 g of molten steel to reduce the P concentration in the molten steel from 0.020 mass % to 0.005 mass %. Assuming that the P concentration in reduced iron is 0.15 mass %, the amount of flux required to reduce the P concentration to 0.01 mass %, which is approximately the same level in a steel product, is 230 kg per 1 ton of molten steel. This results in a high proportion of the flux volume in the arc furnace, which is problematic in that the amount of molten steel processed would decrease, reducing production efficiency.

In the method disclosed in Patent Literature 2, the treatment temperature is 1600° C. or higher. Further, the specification of Patent Literature 2 describes that “the temperature is preferably 1800° C. or higher to allow more effective dephosphorization, and such a high temperature range is difficult to achieve by an ordinary heating method, but can be achieved by using a plasma arc or high-frequency induced plasma, for example.” Therefore, the method requires more energy, and thus is not suitable for large-scale dephosphorization, which is problematic.

The method disclosed in Patent Literature 3 is problematic in that it involves wet processing using acid, requiring a long time and high cost to dry the collected magnetically attracted substance for use as the main raw material. The method is also problematic as it requires a long time and high cost to crush iron ore into particles of 0.5 mm or less in advance.

The present invention has been made in view of the circumstances described above and aims to provide a technology capable of efficiently producing grained iron with a low P concentration even when reduced iron obtained from low-grade iron ore with a high P concentration is used as a raw material.

Solution to Problem

A method for producing grained iron according to the present invention that advantageously solves the above-described problems includes a first step of melting reduced iron to produce primary molten iron, a second step of separating the primary molten iron from slag, a third step of subjecting the primary molten iron separated from the slag to dephosphorization to produce secondary molten iron, and a fourth step of solidifying the secondary molten iron in a grain state to form grained iron, characterized in that in the third step, the dephosphorization is performed by supplying an oxygen source and a CaO source to the primary molten iron, and a temperature of the secondary molten iron at the end of the dephosphorization is set to a temperature of the primary molten iron at the start of the dephosphorization or lower.

Note that for the method for producing grained iron according to the present invention, it is considered that the following can be more preferable solution means, for example.

• • (a) A temperature T_f of the secondary molten iron at the end of the dephosphorization is set higher than a solidifying temperature T_m of the secondary molten iron at the end of the dephosphorization by 20° C. or more.
  • (b) A composition of the slag at the end of the dephosphorization is set to have a slag basicity in a range of 1.0 to 4.0, wherein the slag basicity is a ratio of a CaO concentration (% CaO) to a SiO₂ concentration (% SiO₂) on a mass basis.
  • (c) The fourth step is performed using a grained iron producing apparatus including a granulation device that forms the secondary molten iron into droplets, a water-flow control vessel that is disposed at a position for receiving the droplets and accommodates cooling water, and at least one cooling water pipe that is connected to the water-flow control vessel and supplies cooling water to the water-flow control vessel. The water-flow control vessel includes an inclined surface that is inclined such that a horizontal cross-sectional area of the water-flow control vessel becomes narrower in a downward direction, and a discharge port is provided below the inclined surface.

Grained iron according to the present invention which advantageously solves the foregoing problems is produced from reduced iron with a P concentration of 0.050 mass % or more as a raw material, characterized in that a P concentration of the grained iron is 0.030 mass % or less, and grain size of the grained iron is in the range of 1 mm to 50 mm inclusive.

Advantageous Effects of Invention

The method for producing grained iron according to the present invention can efficiently produce grained iron with a low P concentration from reduced iron obtained from low-grade iron ore with a high P concentration. Furthermore, the grained iron according to the present invention satisfies the P concentration required for most steel products, i.e., 0.030 mass % or less. Thus, molten iron with a P concentration corresponding to the level in a steel product can be obtained simply by re-melting the grained iron according to the present invention.

Further, since the grained iron is produced by solidifying molten iron into a grained form once after the dephosphorization, it is possible to produce iron and steel not in a large-scale iron-making plant but in such a manner that a plant for producing an iron source is separated from a place where iron source is demanded. For example, such a production method can be considered in which a process up to the production of dephosphorized grained iron is performed in a raw material-producing country, and iron or steel is produced in an iron steel-producing country, using the dephosphorized grained iron as a raw material.

When the production of reduced iron and the production of grained iron according to the present invention are both performed in an iron ore-producing country, it is possible to separate a gangue portion, as slag, contained in iron ore, which is a raw material. Thus, by transporting only grained iron, the amount of transportation per unit of Fe can be reduced, thus reducing the transportation cost to a place where grained iron is demanded, as well as energy consumption. Furthermore, when considering the transport of grained iron to the place where grained iron is demanded, the storage of grained iron therein, the supply of grained iron to a facility in the place where grained iron is demanded, and so on, the degree of freedom of facilities used for the transport, storage, and supply can be increased by setting the grain size of grained iron in the range of 1 to 50 mm. There is another advantage that a risk such as bridging in a feed hopper can be reduced.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be specifically described below. Note that the following embodiment only describes examples of an apparatus (or a device) and a method for embodying the technical idea of the present invention. Thus, the configuration of the present invention is not limited thereto. That is, the technical idea of the present invention can be modified in various ways within the technical scope described in the claims.

The inventors have considered as follows to implement the present invention.

Reduced iron produced using iron ore as a raw material has different properties, such as a metallization rate and composition, depending on the brand of the iron ore used, the type and unit consumption of a raw material composition adjusting agent to be mixed, the type and unit consumption of a reducing agent, a reduction temperature, and a scheme adopted for a facility for producing the reduced iron. Table 1 shows examples of the ingredient compositions of reduced iron. In the examples in Table 1, the P concentration converted to the P concentration in molten iron, which is obtained by dividing the P concentration by the T.Fe (total iron) concentration, is 0.057 to 0.152 mass %. Therefore, if such reduced iron is melted as is, it will be difficult to reduce the P concentration to the level required for a steel product (0.030 mass % or less). In addition, if such reduced iron is simply melted to be subjected to dephosphorization, the amount of slag produced will become huge due to gangue, such as SiO₂, contained in the reduced iron, and the proportion of the slag to the volume of a processing facility will become high, resulting in reduced productivity, and also, the amount of a CaO source required to secure the amount of phosphorus to be removed from the molten iron will increase, resulting in an increase in cost, which are problematic.

In response, the inventors have arrived at a process of producing grained iron by melting reduced iron once to obtain molten iron, and also removing at least a part of the slag derived from gangue, and then supplying an oxygen source and a lime source to the obtained molten iron to effect dephosphorization, and further solidifying the dephosphorized molten iron into a grained form.

An embodiment of the present invention will be specifically described below.

As a first step, reduced iron is heated and melted in an electric furnace to produce primary molten iron. The reduced iron to be used herein may be the one transferred as is at a high temperature from an adjacent plant for producing reduced iron, for example. Of course, reduced iron that has been once cooled to room temperature may also be used. The electric furnace may be an arc furnace, submerged arc furnace, or induction melting furnace. The thermal energy to be supplied in the first step to heat and melt the reduced iron, which is a solid iron source, can be not only electrical energy but also, supplementally, the combustion heat of gaseous fuel such as a natural gas or a propane gas, liquid fuel such as heavy oil, or combustible solid such as coal or metallic Al or Si, for example. Such energy is preferably renewable from the perspective of reducing CO₂ emissions.

As a second step, slag, which is a gangue portion of the reduced iron, and the primary molten iron are separated from each other. For example, the molten metal is tapped into a vessel for transport and then transported to a facility for performing dephosphorization. When dephosphorization is performed in the following step, a CaO source is added to produce slag for dephosphorization. To secure the amount of the slag and adjust the ingredient composition thereof, at least a part of the slag containing a large amount of SiO₂ produced with the melting of the reduced iron may be carried over. The slag may also be removed from a vessel for heating and melting the reduced iron used in the first step, for example, by means of a slag dragger.

As a third step, the molten metal is subjected to dephosphorization to produce secondary molten iron. A dephosphorization reaction requires an oxygen source and a CaO source as represented by the following Expression (1).

2 [ P ] + 5 / 2 · O 2 ( g ) + 3 ⁢ CaO ( s ) = 3 ⁢ CaO · P 2 ⁢ O 5 ( s ) , ( 1 )

• • where [P] represents phosphorus in the molten iron.

A pure oxygen gas is normally used as the oxygen source for dephosphorization. The inventors have come to the conclusion that it is advantageous to perform dephosphorization at a low temperature, since a dephosphorization reaction is an exothermic reaction, and also to reduce the temperature of the molten iron within the range that does not adversely affect dephosphorization, taking into account that the resultant is solidified to form grained iron in the following step.

As a result of examination, the inventors have found that sufficient dephosphorization can be achieved while cooling the molten iron, by supplying air or an iron oxide source such as iron ore or mill scale, as the oxygen source. When air is used, heat removal proceeds as sensible heat of a nitrogen gas contained in the air, achieving a better cooling effect than when a pure oxygen gas is used. Meanwhile, when an iron oxide source is used, an endothermic reaction occurs as the iron oxide source is reduced to form metallic Fe, or heat absorption occurs as a molten slag is formed in the form of iron oxide, achieving a better cooling effect than when a pure oxygen gas is used.

Next, using limestone as the CaO source can cool the molten iron because calcium carbonate contained in limestone absorbs heat as it decomposes into CaO and CO₂. A similar cooling effect is achieved by supplying carbonate, such as raw dolomite. However, if the proportion of CaO in an auxiliary material is low, the amount of the auxiliary material to be added will increase, and the amount of the produced slag will thus increase, and the time required to add the auxiliary material will also increase, which is problematic in operation. Therefore, it is preferable to adjust the type and the amount of the auxiliary material to be added by taking into consideration the required cooling effect and a stable operation.

It is preferable to adjust the supply rate of pure oxygen or air and the height of a top-blowing lance in accordance with the operation condition of dephosphorization, as the behavior of the occurrence of spitting differs depending on the height of a freeboard (the height from the surface of the molten iron to the upper end of a vessel) of a vessel in which dephosphorization is performed and the nozzle shape of the lance. In addition, an inert gas is preferably blown into the molten iron to agitate it. The inert gas is preferably blown into the molten iron via a porous plug disposed at the bottom of the furnace or by immersing an injection lance in the molten iron. Regarding the composition of the slag at the end of the dephosphorization, slag basicity, which is the ratio of the CaO concentration (% CaO) to the SiO₂ concentration (% SiO₂) on a mass basis, is preferably in the range of 1.0 to 4.0. The slag basicity is adjusted based on the amount of slag containing a large amount of SiO₂ that is carried over to the second step, and the type and the amount of the CaO source added. It is also possible to add a SiO₂ source, such as silica stone or ferrosilicon, and a CaO source, such as quicklime, as appropriate.

If the slag basicity is low, the amount of phosphorus to be removed in dephosphorization will be small. Meanwhile, if the slag basicity is high, a part of the slag will solidify and thus become attached to a refractory when the temperature of the molten iron drops. This makes it difficult to remove the slag after dephosphorization and causes problems such that an abnormal reaction may occur the next time molten iron is charged, or the residual slag may be mixed into the produced slag, causing the composition to fall out of range.

Further, since a large amount of an exhaust gas at a high temperature is generated through such dephosphorization that involves the use of air, it is also possible to recover the exhaust heat using a boiler, for example.

As a fourth step, the secondary molten iron after the dephosphorization is solidified into a grained form to obtain grained iron. Examples of a method for producing grained iron include a method of flowing down molten iron subjected to dephosphorization to cause it to collide with a surface plate of a refractory, and a method of causing water to collide with the molten iron, which has flowed out, to obtain molten iron droplets, and then dropping the molten iron droplets into a water-flow control vessel to obtain solidified grained iron. At this time, since the diameter of grained iron varies in accordance with the flowing-down speed of the molten iron, it is preferable to transfer the molten iron subjected to dephosphorization, to a tundish where a falling speed can be kept constant.

The temperature of the molten iron decreases while the molten iron is being transported after the dephosphorization to be supplied to a grained iron producing apparatus. If the temperature of the molten iron after the dephosphorization is too low, part of the molten iron in the vessel will solidify before the molten iron is entirely supplied to the grained iron production apparatus, resulting in reduced production yields. Meanwhile, if the temperature of the molten iron after the dephosphorization is high, the heat load when the molten iron is solidified by the grained iron production apparatus will increase, increasing the amount of cooling water to be used, so that the productivity may decrease due to the cooling rate, or the waiting time until the temperature of the molten iron decreases and grained iron is obtained may become long. As described above, considering forming into grained iron after the dephosphorization, there is a suitable range of the temperature of the molten iron after the dephosphorization. Specifically, the temperature T_f of the molten iron after the dephosphorization is set to the temperature T_i of the molten iron at the start of the dephosphorization or lower, from the viewpoint of increasing productivity. In addition, if the temperature T_f at the end of the dephosphorization is set higher than the solidifying temperature T_m of the secondary molten iron at the end of the dephosphorization by 20° C. or more, the molten iron can be supplied to the grained iron producing apparatus in a high yield, which is preferable.

Note that the solidifying temperature T_m (° C.) may be determined by either of the following methods. First, it may be directly measured as the solidifying temperature of a sample. Alternatively, it can be a temperature read from a liquidus temperature in an Fe—C state diagram, based on the C concentration in the molten iron subjected to dephosphorization that is estimated from past records of operation (the C concentration and the temperature before dephosphorization, and the type and supply conditions of the oxygen source).

The grained iron producing apparatus includes a granulation device which forms the molten iron into droplets, and a water-flow control vessel which is disposed at a position for receiving the droplets and accommodates cooling water. At least one cooling water pipe which supplies cooling water is connected to the water-flow control vessel into which the molten iron is dropped to solidify. As the cooling water is discharged from the cooling water pipe to form a water flow, the formation of a stagnation region of the cooling water within the vessel is suppressed. This can suppress a local temperature rise of the cooling water and efficiently cool grained iron to suppress the fusion of grained iron caused by insufficient cooling of grained iron. In addition, the water-flow control vessel has an inclined surface which is inclined such that the horizontal cross-sectional area of the vessel becomes narrower in a downward direction, and a discharge port is provided below the inclined surface. Setting the inclination angle of the inclined surface to be the angle of repose of grained iron in water or more allows grained iron to be directed to the discharge port without accumulation of grained iron on the inclined surface.

By using the thus-obtained grained iron as a part of an iron source in a blast furnace or a converter, the effect of diluting the P concentration in accordance with the proportion of the grained iron used can be achieved. This can reduce the load in dephosphorization and ease restrictions on the raw materials to be used in the blast furnace and converter.

Note that when the grained iron obtained in this embodiment is used as an iron source in an electric furnace, blast furnace, or converter, there is a range of grain sizes that are convenient to work with. To obtain the desired grain sizes, it is preferable to adjust the flowing-down speed in the tundish. It is also preferable to perform classification as required. Typically, grained iron with a grain size in the range of 1 to 50 mm is convenient to use. When grained iron with a grain size of less than 1 mm is included, there is a higher possibility of clogging a conveyor for transport or bridging in a hopper. Therefore, it is preferable to perform classification so as to obtain grained iron with a grain size of 1 mm or more for use. On the other hand, if grained iron with a grain size of more than 50 mm is used, there is a higher risk of wear damage that may occur to a facility, such as the conveyor for transport or the hopper, when collision due to falling of grained iron occurs, for example. Therefore, it is preferable to reduce the flowing-down speed in the tundish to obtain grained iron with a grain size of 50 mm or less. It is also possible to perform classification as appropriate to remove grained iron with a grain size of more than 50 mm. Herein, the grain size in the range of 1 to 50 mm may include particles on a sieve with an opening of 1 mm to particles that have passed through a sieve with an opening of 50 mm.

EXAMPLES

Example 1

The reduced iron A shown in Table 1 was melted in an electric furnace with a capacity of 250 tons, and, after adjusting the temperature of the resultant, transferred to a ladle. Among the slag produced due to the gangue content in the reduced iron during the melting of the reduced iron in the electric furnace, approximately 10 kg of slag per 1 ton of molten iron was transferred to the ladle together with the molten iron, and the rest of the slag was transferred to a slag vessel. The ladle was transferred to a dephosphorization facility to perform dephosphorization while changing the types and amounts of an oxygen source and a lime source supplied. The dephosphorization facility included a gas top-blowing lance, an auxiliary material feeding hopper, and a bottom-blowing porous plug. The gas top-blowing lance was capable of supplying gas containing pure oxygen or air at a rate of approximately 1 Nm³/minute per 1 ton of molten iron. Three auxiliary material feeding hoppers, each filled with iron ore, quicklime (CaO), and calcium carbonate (CaCO₃), can feed them at a rate of approximately 10 kg/minute. The bottom-blowing porous plug can supply gas. In this example, a pure Ar gas was supplied at a rate of approximately 0.1 Nm³/minute per 1 ton of molten iron.

The melting temperature in the electric furnace was adjusted to allow the temperature of the molten iron before dephosphorization to be approximately 1590° C. “Before dephosphorization” refers to the time before the gas top-blowing lance is lowered, while “after dephosphorization” refers to the time when the gas top-blowing lance has been completely raised after the dephosphorization. At each timing, temperature measurements and sampling were conducted using a sublance. The obtained samples were cut and polished and subjected to an emission spectrochemical analysis to evaluate the C concentration [C] and the P concentration [P] in the molten iron from calibration curves determined in advance. It was possible to measure the solidifying temperature of the molten metal at the timing when the temperature measurement and sampling were performed using the sublance, and the solidifying temperature T_m of the molten iron subjected to the dephosphorization was actually measured.

The start of the dephosphorization was defined as when the gas top-blowing lance started to be lowered. After the top-blowing lance reached a predetermined height, the supply of an oxygen gas source and the addition of auxiliary materials were started. The dephosphorization was terminated when the supply of predetermined amounts of oxygen gas source and auxiliary materials was completed and the top-blowing lance was raised to a standby position. The duration of the period was determined as a processing time t_f (minutes).

After the dephosphorization, the ladle was tilted to remove the slag on the molten iron with a slag dragger. Part of the removed slag was collected and subjected to a chemical analysis. The ladle was lifted and tilted using a crane to transfer the molten iron to the tundish. The molten iron was caused to flow down from the tundish so as to collide with a surface plate of a refractory, and the resulting molten iron droplets were dropped into the water-flow control vessel and solidified to produce grained iron. The grain sizes of the obtained grained iron ranged from 0.1 to 30 mm. The grain size distributions were: +0.1 mm to −1 mm: 17.2 mass %, +1 mm to −10 mm: 31.3 mass %, +10 mm to −20 mm: 38.8 mass %, and +20 mm to −30 mm: 12.7 mass %. Herein, “+N to −M” means particles on a sieve with an opening of N to particles that have passed through a sieve with an opening of M.

Table 2 shows, as Test Nos. 1 to 5, the temperatures T_i and T_f (° C.), the C concentrations [C]_i and [C]_f (mass %), and the P concentrations [P]_i and [P]_f (mass %) of the molten iron before and after the dephosphorization, respectively; the types and the amounts of the oxygen source and the CaO source supplied; the processing time t_f (minutes); and the basicity of the slag after the process ((% CaO)/(% SiO₂), i.e., the ratio of the CaO concentration (% CaO) to the SiO₂ concentration (% SiO₂) on a mass basis; hereinafter referred to as C/S).

As shown in Table 2, in all the invention examples, the temperature T_f of the molten iron after the process was lower than the temperature T_i of the molten iron before the process, and the P concentration [P]_f after the process was sufficiently lowered. In the comparative example, the temperature T_f after dephosphorization increased higher than the temperature T_i before dephosphorization, and consequently the P concentration [P]_f after the process was high, and a waiting time was caused during the grained iron production step, resulting in decreased productivity. In Test No. 4, compared with Test Nos. 1 to 3, the temperature T_f of the molten iron after the process decreased to reduce the P concentration [P]_f sufficiently. However, part of the molten iron solidified in the tundish during the production of grained iron, resulting in a reduced yield. In each of Test Nos. 1 to 3, the temperature T_f of the molten iron after the process was lower than the temperature T_i of the molten iron before the process, and the temperature T_f of the molten iron after the process was higher than the solidifying temperature T_m of the molten iron by 20° C. or more. Also, the P concentration [P]_f after the process was sufficiently low, and the whole molten iron was formed into grained iron in a high yield with no decrease in productivity.

Example 2

Dephosphorization and a production of grained iron were conducted using a method similar to that of Example 1. Table 3 shows the temperatures T_i and T_f (° C.), the C concentrations [C]_i and [C]_f (mass %), and the P concentrations [P]_i and [P]_f (mass %) of the molten iron before and after the dephosphorization, respectively; the types and amounts of the oxygen source and CaO source supplied; the processing time t_f (minutes); and the basicity C/S of the slag after the process, as Test Nos. 6 to 12. As shown in Table 3, in Test No. 11, the basicity C/S of the slag was low compared with Test Nos. 6 to 10, and thus the P 10 concentration after the process was high. Meanwhile, in Test No. 12, the basicity C/S of the slag was high, and the solidification of the slag was confirmed.

Example 3

The reduced iron A shown in Table 1 was melted with anthracite in an electric furnace with a capacity of 250 tons to produce molten iron with a C concentration of approximately 2.0 mass %. After adjusting the temperature of the molten iron, the molten iron was transferred to a ladle, where dephosphorization and a production of grained iron were conducted by a method similar to those of Examples 1 and 2. Table 4 shows the temperatures T_i and T_f (° C.), the C concentrations [C]_i and [C]_f (mass %), and the P concentrations [P]_i and [P]_f (mass %) of the molten iron before and after the dephosphorization, respectively; the types and amounts of the oxygen source and CaO source supplied; the processing time t_f (minutes); and the basicity C/S of the slag after the process, as Test Nos. 13 to 19. As shown in Table 4, the basicity C/S of the slag in Test No. 18 was low compared with Test Nos. 13 to 17, and thus the P concentration [P]_f after the process was high. Meanwhile, in Test No. 19, the basicity C/S of the slag was high, and the solidification of the slag was confirmed.

The grained iron produced in each of Test Nos. 8 to 10, 12, 14 to 17, and 19 were found to have a P concentration of 0.030 mass % or less. When the reduced iron of each process was melted in an electric furnace, the obtained molten iron was found to have a P concentration of 0.030 mass % or less. Such a P concentration has reached a level required of a steel product, thus requiring no additional dephosphorization. After being classified based on a grain size of 1 mm or more, the grained iron obtained in each of Test Nos. 8 to 10, 12, 14 to 17, and 19 could be used in an electric furnace, a blast furnace, or a converter without any problem.

In this specification, the unit “t” of a mass represents 103 kg. In addition, the symbol “N” added to the unit “Nm³” of a volume represents the standard state of gas. In this specification, the standard state of gas corresponds to 1 atm (=101325 Pa) and 0° C. Symbol [M] in a chemical formula represents that an element M is melted in molten iron or reduced iron.

INDUSTRIAL APPLICABILITY

According to the method for producing grained iron and grained iron of the present invention, it is possible to efficiently produce grained iron with a low P concentration even when reduced iron obtained from low-grade iron ore with a high P concentration is used as a raw material. In addition, only remelting the grained iron according to the present invention can obtain molten iron with a P concentration corresponding to the level in a steel product. Thus, the present invention is industrially advantageous.