JP5487870B2 - Low nitrogen steel manufacturing method - Google Patents

Description

  The present invention relates to a method for producing low nitrogen steel by preventing air from entering the furnace at the end of decarburization blowing in a converter.

  In the decarburization blowing of hot metal in a converter, where oxygen gas is blown up or blown down to oxidize and remove the carbon in the hot metal, the concentration of carbon in the molten iron in the furnace is reduced at the end of the oxygen blowing. The amount of exhaust gas generated from the converter is reduced. As a result, the timing when the pressure inside the converter becomes negative with respect to the atmospheric pressure is generated. At that time, the external atmosphere enters the converter and comes into contact with the molten iron in the furnace. The nitrogen concentration of the molten steel absorbed and melted by the molten iron increases. In the decarburization blowing of hot metal in the converter, the hot metal charged into the converter is gradually decarburized and refined to become molten steel, but in the course of decarburization refining, the distinction is made between hot metal and molten steel. Since this is difficult, in the present invention, both are collectively indicated as molten iron.

Many proposals have been made to suppress the increase in the nitrogen concentration of the molten iron at the end of this blowing process. For example, Patent Document 1 proposes a method in which slag in a converter is formed (foamed) at the end of blowing and blown so that the atmosphere does not directly contact the molten iron in the furnace. In Patent Document 1, limestone (CaCO ₃ ) is cited as an example of a gas generating material for forming slag, but iron oxide such as scale reacts with carbon in molten iron when it is dissolved in molten iron. Since it generates CO gas, it is used as a gas generating substance.

  Further, Patent Document 2 proposes a method of preventing the atmosphere from entering the furnace by adding a gas generating substance into the furnace at the end of blowing and maintaining the gas pressure at the converter furnace at a positive pressure. ing. In Patent Document 2, a sedative that is an organic substance mainly composed of paper sludge is cited as a gas generating substance, but it is not limited thereto.

  On the other hand, Patent Document 3 discloses a smelting furnace that is not intended to prevent an increase in the nitrogen concentration of molten iron, but is intended to recover thermally effective exhaust gas even for refining with a small amount of exhaust gas generation. There has been proposed a refining method in which a plastic or a mixture of plastic and iron-made dust or briquette is put in and a gas generated by a refining reaction is recovered together with a gas generated from the plastic. However, in Patent Document 3, since the purpose is gas recovery, it is difficult to recover gas when the amount of thermally effective gas generated at the end of the reaction is small. It has stopped.

Japanese Patent Laid-Open No. 59-208090 Japanese Patent Laid-Open No. 61-157609 Japanese Patent Laid-Open No. 2002-339012

  However, the above prior art has the following problems.

  That is, in Patent Document 1, a gas generating material is added to form slag, and gas is generated by an endothermic reaction regardless of whether the gas generating material is limestone or iron oxide. By adding the gas generating substance, the molten steel temperature at the end of the converter is lowered. Therefore, when the molten iron has no heat margin, the gas generating substance cannot be added, and nitrogen pickup occurs.

In Patent Document 2, paper sludge is used as a gas generating substance. When the paper sludge is heated to a high temperature, CO ₂ gas, H ₂ gas and hydrocarbon gas are generated. When the amount is small, the converter furnace pressure cannot be maintained at a positive pressure, and the atmosphere enters the converter and nitrogen pick-up of molten steel occurs. Patent Document 2 does not disclose how much gas is generated to prevent nitrogen pickup. In addition, paper sludge contains ash like coal and coke and is not optimal as a gas generating substance.

  In Patent Document 3, plastic is used as a gas generating material, and plastic is generally an excellent gas generating material because it does not contain ash and is mostly gasified at high temperatures. In No. 3, the introduction of plastic was stopped at the end of refining, and there was no effect in preventing nitrogen pick-up of molten steel.

  The present invention has been made in view of the above circumstances, and its purpose is to enter the atmosphere in the converter at the end of decarburization blowing in the converter even when the hot metal has no heat margin. It is to provide a method for reliably preventing low temperature, suppressing an increase in nitrogen concentration of molten iron in the converter, and stably producing low nitrogen steel.

The method for producing low nitrogen steel according to the first invention for solving the above-mentioned problem is to remove a hot metal in a converter from a molded body containing 20 to 70% by mass of a plastic and the balance being a metal or a metal oxide. At the end of charcoal blowing, it is put into the converter, and the total amount of the gas generated from the plastic and the gas generated by the decarburization blowing is secured at 400 Nm ³ / hr or more per ton of molten steel in the converter to convert the atmosphere. It is characterized by preventing entry into the furnace.

The method for producing low nitrogen steel according to the second invention is the method for producing low nitrogen steel according to the first invention, wherein the amount of gas generated by decarburization blowing is 600 Nm ³ / hr or less per ton of molten steel in the converter in the first invention and It is characterized in that it is put into the furnace at a time of 400 Nm ³ / hr or more.

According to the present invention, at the end of decarburization and refining of hot metal in a converter, a molded body containing 20 to 70% by mass of plastic and the balance being metal or metal oxide is put into the converter and is generated from the plastic. Since the total amount of gas and gas generated by decarburization blowing is ensured to be 400 Nm ³ / hr or more per ton of molten steel in the converter, the intrusion of air into the converter is hindered. Nitrogen pickup of molten steel in the furnace is prevented, and stable production of low nitrogen steel is realized. Since plastic is a highly exothermic material, there is no problem even if the hot metal has no heat margin.

It is a figure which shows the result of having investigated the nitrogen concentration of the molten iron in a furnace from the start of blowing and the exhaust gas flow rate from a converter from the end of blowing. It is a figure which compares and shows the nitrogen concentration in the molten steel manufactured with the converter by the example of this invention, and a comparative example.

  Hereinafter, the present invention will be described in detail.

In order to prevent the pick-up of nitrogen in molten steel due to the intrusion of air into the converter at the end of decarburization blowing in the converter, the present inventors have completed the blowing from the start of the blowing of the top-blown oxygen gas The nitrogen concentration of the molten iron in the furnace and the exhaust gas flow rate from the converter were investigated. The survey results are shown in FIG. FIG. 1 shows the result of decarburization blowing by supplying oxygen gas from an upper blowing lance and blowing Ar gas as a stirring gas from a bottom blowing nozzle at the furnace bottom in a converter having a furnace capacity of 300 tons. . Incidentally, blowing of Ar gas for agitation, 6.0 nm per ton of the molten steel ³ / hr (hereinafter referred to as "Nm ³ / (hr · t)") is about, compared to the exhaust gas flow rate from the converter It can be seen that the bottom blown Ar gas does not affect the exhaust gas flow rate.

As shown in FIG. 1, the exhaust gas flow rate increases simultaneously with the start of oxygen blowing, but in the initial stage of blowing, the oxidation of silicon in the hot metal has priority, and the exhaust gas flow rate is about 400 to 600 Nm ³ / (hr · t). is there. After the blowing is progressed and the silicon in the hot metal is oxidized and removed, the total amount of oxygen gas supplied is consumed for the oxidation of carbon in the hot metal, and the exhaust gas flow rate is about 800 Nm ³ / (hr · t). To reach. Then, after about 12 minutes have passed since the start of blowing, when the carbon concentration in the molten iron in the furnace decreases due to the decarburization reaction, the decarburization reaction changes from the supply-controlled rate of oxygen gas to the rate of transfer of carbon in the molten iron. As a result, the exhaust gas flow rate decreases. This is because when the decarburization reaction reaches the rate of carbon movement control, the excess oxygen at the reaction interface inevitably reacts with component elements such as iron and manganese that constitute the molten iron, and this causes oxidation of these components. This is because the amount of exhaust gas is reduced. And it falls to about 200 Nm ³ / (hr · t) at the end of decarburization blowing. It should be noted that the time point at which the rate-determining rate of the decarburization reaction changes from the rate-controlled rate of supplying oxygen gas to the rate of carbon transfer in the molten iron is when the carbon concentration in the molten iron is about 0.6% by mass. In terms of expression, it is about 2 minutes to 3 minutes before the end point of decarburization blowing.

  On the other hand, the nitrogen concentration (N concentration) in the molten iron is intensively stirred under the condition that no nitrogen gas is present in the atmospheric gas after the start of blowing. It decreases to about 10 ppm when about 12 minutes have passed. However, after that, it starts to rise and rises to about 20 ppm at the end of blowing.

As a result of detailed analysis of these phenomena, it was found that when the exhaust gas flow rate from the converter becomes less than 400 Nm ³ / (hr · t), nitrogen pickup of molten iron occurs. This is because when the exhaust gas flow rate is less than 400 Nm ³ / (hr · t), the pressure in the furnace may be lower than the atmospheric pressure, and the atmosphere enters the furnace and the nitrogen gas in the atmosphere turns into molten iron. By being absorbed.

Therefore, if the exhaust gas flow rate from the converter at the end of blowing is 400 Nm ³ / (hr · t) or more, preferably 500 Nm ³ / (hr · t) or more, nitrogen pickup by the molten iron atmosphere is prevented. I found out. That is, the gas generating material is introduced into the converter to generate gas, and the total amount of gas generated by the decarburization reaction and gas generated from the gas generating material is 400 Nm ³ / (hr · t) or more, preferably 500 Nm. ^{If it is 3} / (hr · t) or higher, the pressure inside the converter will be maintained higher than the atmospheric pressure, preventing the atmosphere from entering the converter, and preventing the pickup of nitrogen from the molten iron by the atmosphere. I understood.

  Next, the gas generating materials to be charged into the furnace were examined. As a result, it did not contain ash, most gasified, and had a calorific value equal to or greater than the heat of decomposition when burned, indicating that plastic is the most suitable gas generating substance. Moreover, it turned out that plastics, such as a polypropylene and polyethylene which do not contain sulfur, are the most suitable among plastics. When using a plastic containing sulfur, the sulfur concentration in the molten steel increases, which is not preferable.

  That is, the plastic used is not particularly limited as long as it does not contain sulfur, such as polypropylene, polyethylene, and polystyrene, and it has been found that there is no problem even if it is a plastic (waste plastic) recovered from waste. In addition to sulfur, phosphorous, nitrogen and the like are not preferable because they are a contaminated source of molten steel, and silicon, aluminum and the like are also undesirable because they lead to an increase in slag. Accordingly, it is necessary to select a plastic having a small amount of these components as the plastic to be used.

The plastic is mainly composed of [C], [H], and [O], and carbon, CO gas, H ₂ gas, hydrocarbon gas, and the like are generated by the decomposition reaction. Since these decomposed products also function as a fuel and a reducing agent, the plastic charged into the converter not only functions as a gas generating substance but can also be used as a heat source and a reducing agent.

  However, these plastics have a specific gravity of 0.9 to 1.0, and if they are put into the converter as they are, the flow rate of the exhaust gas at the converter furnace is high, and the exhaust gas flows out of the furnace due to the gas flow. There is a fear. Therefore, in the present invention, the plastic is processed into a molded body of metal or metal oxide to increase the apparent specific gravity. That is, the metal and metal oxide which comprise a molded object bear the function for the specific gravity adjustment of a molded object.

  In this case, the plastic content in the molded body is required to be 20 to 70% by mass. If the plastic content in the molded body is less than 20% by mass, the amount of gas generated is small and the amount of the molded body charged increases, which is not preferable. In addition, when the plastic content in the molded body is less than 20% by mass, the amount of heat generated by the plastic is small, the cooling effect by the metal or metal oxide in the remainder of the molded body is increased, and the thermal margin of molten iron is insufficient. In some cases, it cannot be used, which is not preferable. On the other hand, when the content of the plastic in the molded body exceeds 70% by mass, the specific gravity of the molded body is reduced, and stable charging into the furnace is impaired, which is not preferable.

  As the metal composing the compact, magnetically selected ingots collected by magnetically sorting converter slag, steel scraps generated at machine shops, and small iron scraps such as chopper scraps are suitable. Various alloy irons may be used. Moreover, as a metal oxide which comprises a molded object, iron ore, a mill scale, iron-making dust, Mn ore, Cr ore, Ni ore etc. are suitable.

  As a method of molding a molded body from plastic and metal or metal oxide, an extrusion molding method in which these are mixed and extruded from a porous die or a wire mesh with a strong extrusion force, and these are mixed to form a fixed container There is a compression molding method in which the material is strongly compressed and molded, and a melt molding method in which a mixture of these is heated to melt the plastic, and any method can be used. In this case, the extrusion molding method is preferable because it can be molded relatively easily.

The present invention has been made on the basis of these examination results. That is, in the present invention, a molded body containing 20 to 70% by mass of a plastic and the balance being a metal or a metal oxide is removed from the hot metal in a converter. At the end of charcoal blowing, it is put into the converter, and the total amount of gas generated from the plastic and gas generated by the decarburization blowing is ensured to be 400 Nm ³ / hr or more per ton of molten steel in the converter. Prevents air from entering the converter at the end of charcoal blowing.

In the present invention, the charging start timing of the formed body is set to a later stage of blowing than the time when the rate-determining reaction rate of the decarburization reaction is changed from the rate-controlled rate of supplying oxygen gas to the rate of movement of carbon in the molten iron. It is preferable to set the time after the amount of gas generated by smelting is 600 Nm ³ / (hr · t) or less and 400 Nm ³ / (hr · t) or more. Further, in one-on, the exhaust gas flow rate 400Nm ³ / (hr · t) or more, preferably may be charged the input amount of the total amount needed to ensure the 500Nm ³ / (hr · t) more but The plastic decomposition reaction is rapid, and it is effective to continuously generate gas for at least about 1 minute. It is preferable to add continuously until just before the end of smelting. The exhaust gas flow rate from the converter can be directly determined by a flow meter provided in the exhaust gas duct above the converter.

As described above, according to the present invention, at the end of the decarburization and refining of hot metal in the converter, a molded body containing 20 to 70% by mass of plastic and the balance being metal or metal oxide is placed in the converter. The total amount of gas generated from plastic and gas generated by decarburization blowing is ensured to be 400 Nm ³ / hr or more per ton of molten steel in the converter, which prevents air from entering the converter. Further, nitrogen pickup of molten steel in the furnace by nitrogen in the atmosphere is prevented, and stable production of low nitrogen steel is realized.

  The present invention was applied to decarburization blowing of hot metal in a converter having a furnace capacity of 300 tons, in which oxygen gas was supplied from an upper blowing lance and Ar gas was supplied as a stirring gas from a bottom blowing nozzle at the bottom of the furnace.

Waste plastic mainly composed of polypropylene and polyethylene and iron-making dust were mixed at a blending ratio of 6: 4, and this was extruded to form a cylindrical molded body. One molded body had a mass of 30 to 40 g and a specific gravity of about 1.2. During hot metal decarburization, the exhaust gas flow rate from the converter was measured with a flow meter installed in the exhaust duct, and when the exhaust gas flow rate reached 600 Nm ³ / (hr · t), 1.0 kg per ton of molten steel The above molded bodies were all put into a furnace. By introducing this molded body, the exhaust gas flow rate was ensured to be 400 Nm ³ / (hr · t) or more until the end of blowing.

  After completion of decarburization, the molten steel was discharged from the converter into the ladle in an undeoxidized state. Immediately after the extraction, an analytical sample was taken from the molten steel in the ladle and the nitrogen concentration was investigated. The survey results are shown in FIG. In FIG. 2, the nitrogen concentration in molten steel when refining without adding a compact is also shown as a comparative example. Note that the input carbon amount on the horizontal axis in FIG. 2 is the carbon concentration of the hot metal when charged into the converter and the total amount of carbonaceous material (coke, coal) supplied into the converter was dissolved in the molten iron. It is the sum with the carbon concentration calculated | required as.

  As shown in FIG. 2, as an overall trend, the nitrogen concentration in molten steel tends to decrease as the amount of input carbon increases, but in the present invention example, the nitrogen concentration decreases by about 8 ppm relative to the comparative example. Was confirmed. That is, according to the present invention, it was confirmed that the nitrogen pickup of the molten iron in the furnace by the atmosphere was prevented, and the low nitrogen steel could be manufactured stably.

Claims (2)

A molded body containing 20 to 70% by mass of a plastic and the balance being a metal or a metal oxide is put into the converter at the end of decarburization blowing of the hot metal in the converter, and the gas generated from the plastic and the degassing. A method for producing low nitrogen steel, characterized in that a total amount of gas generated by charcoal blowing is secured to 400 Nm ³ / hr or more per ton of molten steel in a converter to prevent air from entering the converter. The molded body, characterized in that introducing the furnace at and 400 Nm ³ / hr or more below the molten steel ton 600 Nm ³ / hr of the amount of generated gas converter furnace caused by decarburization blowing, wherein Item 2. A method for producing low nitrogen steel according to Item 1.

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