KR100862030B1 - Atmosphere corrosion resisting steel producting method in mini mill process - Google Patents

Abstract

The present invention relates to a method for manufacturing high weather resistance molten steel in a mini mill process,

In manufacturing high weather resistance molten steel in the mini-mill process, HMS, which is a scrap containing a large amount of residual elements, is used at 70 to 80% of the total loading, and cold or molten iron is used at the remaining loading of 20 to 30%, but the Cu concentration in the steel is used. In order to control the amount of waste motor, 0.13 to 0.33% of the total amount of waste motor is charged with the HMS, and the quicklime input into the electric furnace to control the concentration of phosphorus (P) is limited to 0.33 to 1% of the total amount of main raw material. After controlling the end point oxygen to 500 ~ 700ppm during tapping, vacuum degassing is performed in the secondary refining VTD without removing the oxygen of molten steel during tapping,

By reducing the cost of expensive ferroalloy in the steel and improving molten steel cleanness by reducing deoxidation cost and reducing deoxidation product, it is possible to reduce cost, improve quality and stable production when producing high weather resistant steel in mini mill process. Provides the effect of planning.

Mini Mill, Electric Furnace, High Weatherability Molten Steel, Main Raw Materials, Secondary Raw Materials

Description

Manufacturing method of high weatherability molten steel in mini mill process {ATMOSPHERE CORROSION RESISTING STEEL PRODUCTING METHOD IN MINI MILL PROCESS}

1 is a graph showing the weather resistance effect index according to the amount of elements added to the steel,

2 is a schematic diagram of a typical electric arc furnace (Electric Arc Furnace),

3 is a schematic diagram of a general tapping operation after the completion of the electric furnace work,

4 is a secondary refinement arrival [Cu] component graph according to HMS (heavy metal scrap) weight%;

5 is a graph showing the amount of increase in the copper (Cu) component according to the input amount of the waste motor,

Figure 6 is a graph showing the phosphorus (P) component in the molten steel at the time of tapping by the present invention and the conventional method,

7 is a flow chart for manufacturing high weather resistance steel in a mini mill.

<Description of Symbols for Main Parts of Drawings>

1: Graphite electrode (cathode) 2: Molten steel in electric furnace

3: electric furnace slag 4: billet (anode, lower electrode)

5: sleeve tail and 6: stamp material

7: exit hole 8: oxygen lance                 

9: tapping flow 10: ferroalloy injection hopper in ladle

11: ladle alloy iron 12: ladle

13: molten steel in ladle

The present invention relates to a method for manufacturing high weather resistance molten steel in a mini mill process, and to a method for manufacturing a high corrosion resistant steel, which is mainly used in a corrosive environment such as a container, an unpainted steel bridge, and a building exterior plate material, in a mini mill process. In particular, by using scrap iron and waste motor containing a large amount of tramp elements, and adjusting the input of secondary raw materials and ferroalloy during tapping, the composition of molten steel is controlled and the end point oxygen is controlled by electric tapping. The present invention relates to a method for manufacturing high weather resistance molten steel to reduce expensive ferroalloy and reduce processing load in secondary refining by transferring the vacuum to secondary refining without removing oxygen from molten steel.

1 is a graph showing the weather resistance effect index according to the amount of elements added to the steel, Figure 2 is a schematic diagram of a typical electric arc furnace, Figure 3 is a schematic diagram of a general tapping work after the completion of the furnace work to be.

In general, high weathering steel is more than 2 ~ 4 times more corrosion resistant than corrosive steel. It is used for containers, unpainted (steel) bridges, street lamps, building panels, trains, trains, boilers, etc. It is done.                         

Such weatherability is determined by the properties of the elements to be added. Generally, the special elements added in the manufacture of high weathering steel are phosphorus (P), copper (Cu), silicon (Si), chromium (Cr), and nickel (Ni). to be. These additional elements are to ensure the weather resistance (temperature resistant properties of the steel) of the steel, the effect increases with the wt% increase as shown in FIG.

In general, phosphorus (P) in the steel is a harmful element, an element that strives for the ultimate delinquency in steelmaking or steelmaking operations. This is because phosphorus (P) in steel is an element with high segregation, in particular, it is dissolved in ferrite in steel to form Fe ₃ P, thereby making the steel vulnerable, such as reducing impact resistance and causing brittleness at room temperature. In particular, this brittleness is more pronounced with higher carbon (C).

However, due to these characteristics, it is effective to improve the machinability of the steel, so that a certain amount of phosphorus (P) is required to improve the free machinability of the steel. In addition, brittleness by phosphorus (P) is brittle at room temperature, and when the temperature is high, it has toughness, and thus, forging or rolling at high temperature is different from sulfur (S).

However, as shown in FIG. 1, phosphorus (P) is the most effective element for improving the weather resistance of steel, and has the property of increasing the ductility without increasing the toughness, thereby improving the workability. to be.

On the other hand, copper (Cu) in steel is more difficult to oxidize than iron (Fe), and it is difficult to remove by refining, and the steel surface precipitates under the oxide film during oxidation and penetrates into the Fe grain boundary, thereby forming a film and causing red brittleness. However, this property disappears in the presence of 0.4-0.6% manganese (Mn) in the steel. In addition, the same effect is obtained when nickel (Ni) in steel is added in the same amount.

Such copper is added in the production of high weather resistance steel due to precipitation in steel, giving aging hardening, improving tensile strength and hardness, and having high corrosion resistance in the air.

In addition, elements such as silicon (Si), chromium (Cr), and nickel (Ni) are added to the steel in order to improve the strength, toughness, and corrosion resistance of the high weather resistance steel.

In general, steel grades are designed taking into account the operating conditions and the characteristics required by the product. A method of manufacturing the steel thus designed in an electric furnace is shown in FIG. 2.

Scrap metal charged in the electric furnace is dissolved using an arc generated in the graphite electrode rod 1, oxygen is injected into the molten steel using an oxygen lance 8, and carbon is dissolved in the molten steel 2 in the form of carbon monoxide or carbon dioxide. Remove (decarburization).

In addition, the slag (3) is prepared by adding an auxiliary material into the furnace during the melting operation, the furnace slag (3) prepared in this way to minimize the protection of the furnace and heat emission from the arc, and to minimize the reaction with the molten steel (2) Phosphorus (P) in the molten steel (2) is removed by the slag (3) through the (tallin reaction).

3CaO + 2P + 5O = 3CaOP ₂ O ₅

When the molten steel 2 manufactured through such a process reaches an appropriate temperature and oxygen according to the characteristics of the steel, it is poured into the ladle 12 through the tap hole 7 as shown in FIG. 3. At this time, the ferroalloys (11) added in consideration of the design characteristics of the steel is weighed in advance, and then stored in the hopper (Hopper) 10, the molten steel (2) and during the tapping operation from the electric furnace to the ladle (12) Into the ladle 12 together, it is to control the components in the steel.

However, the above conventional method has the following problems.

First, the mini mill is designed to be made of low carbon steel, unlike conventional blast furnace mills, due to the process characteristics. Minimills feature thin slab casting, so the casting speed is very fast, the slabs are thin, and they are very sensitive to cracks. Due to this, in mini-mill, carbon% is set to 0.02 to 0.06% to avoid the area of 0.07 to 0.13% of carbon content in steel, which is the area that causes breakout, which is a performance accident. High weather resistance steel is designed.

However, in order to satisfy the characteristics of the high weather resistance steel, a large amount of ferroalloy is added to the ladle during the tapping, and the carbon concentration in the steel increases due to the carbon contained in the ferroalloy. This increases the workload for carbon pick-up during secondary refining and increases the risk of breakout during casting.

Second, considering the pick-up of ferrous alloys and carbon refining during secondary refining, the end point oxygen of high weathering steel is higher than that of general steel. This is because the oxygen potential of the molten steel dissolved in the electric furnace is sufficiently high, so that the carbon component in the steel is sufficiently low to correct the carbon% picked up.

However, by increasing the oxygen potential of molten steel (including slag), the amount of deoxidizer added during tapping increases, thereby raising the cost and causing the problem of deterioration of the molten steel cleanness due to the increase in the deoxidation product. In addition, an increase in the amount of oxygen in molten steel (including slag) increases the melting loss of the furnace refractory and acts as a factor of reducing the furnace refractory life.

Third, ferrous alloys (P, Cu, Si, Ni, Cr) that are introduced during tapping are expensive, and the increase in the amount of these ferroalloys increases the manufacturing cost. In addition, the introduction of a large amount of ferroalloy leads to excessive drop of molten steel temperature during tapping, resulting in a load of secondary refining operation. In order to compensate for such a drop in temperature, the tapping temperature in the electric furnace must be operated about 10 to 20 ° C higher than that of general steel grades, which directly leads to problems such as an increase in electric power unit and an increase in refractory loss.

The present invention is to solve the conventional problems as described above, by performing the vacuum treatment in the secondary refining through the control of the steel components through the appropriate use of the main / secondary raw materials, ferroalloy and the management of the end point oxygen furnace, It is an object of the present invention to provide a method for manufacturing high weather resistance molten steel in a mini mill process, which facilitates production of high weather resistance steel in a mini mill process and reduces manufacturing costs.

In order to achieve the above object, the present invention, in the manufacture of high weather resistance molten steel in the mini-mill process, 70% to 80% of the total amount of HMS (Heavy Melting Scrap) of scrap iron containing a large amount of residual elements (Tramp Element) And use the cold or molten iron at a residual charge of 20 to 30%, but adjust the Cu concentration in the steel to 0.25 to 0.35%, and the waste motor of 0.13 to 0.33% relative to the total charge of the HMS and the cold or molten iron. Charge with HMS and limit the quicklime added to the furnace to control the concentration of phosphorus (P) in the steel at 0.075 ~ 0.085% to 0.33 ~ 1% compared to the total charge of HMS and the cold or molten iron. After controlling the end point oxygen to 500 ~ 700ppm, vacuum degassing is carried out in the secondary refining (Vacuum Tank Degasser) without removing the oxygen of molten steel during tapping.

Hereinafter, the present invention will be described in detail.

4 is a graph showing the secondary refinement arrival [Cu] component according to HMS (Heavy Metal Scrap) weight%, FIG. 5 is a graph showing an increase amount of copper (Cu) component according to the amount of waste motor input, and FIG. Graph showing phosphorus (P) component in molten steel at the time of tapping by the method, FIG. 7 is a flowchart of manufacturing high weather resistance steel in a mini mill.

In the present invention, by using a scrap containing a large amount of residual elements that are generally difficult to remove in the mini mill as a main raw material, a method of reducing the special alloy iron costs rather developed. Scrap metal used in the present invention is commonly called HMS (Heavy Melting Scrap), and is a scrap generated during disassembly of construction and machinery (ships, automobiles, etc.), and is composed of beams, rebars, pipes, motor blocks, and the like. .

In more detail, the Institute of Scrap Recycling Industries, Inc. (ISRI) stipulates various scrap specifications. For # 1 HMS, the ISRI codes 200 to 202 are defined. 0.25%, Mn 0.3%, Si 0.01%, P 0.02%, S 0.04%, Cu 0.25%, Cr 0.10%, Ni 0.09%, Sn 0.025%, Mo 0.03%) There is.

Since such scrap iron contains a large amount of residual elements (particularly, Cu components), it is common to limit the amount of use to 10% or less of the total amount of charge when producing general steel. However, in the present invention, HMS was used for 70 to 80% of the total loading in order to actively utilize the Cu component contained in the scrap metal.

In this case, when the HMS used is less than 70%, the Cu pick-up of the molten steel by Cu included in the HMS is less than 0.25 wt%, which is the lower limit of the Cu component among the high weatherability steel component standards shown in Table 1, After the melting is completed, the added Cu alloy is increased, resulting in an increase in cost.

In addition, when the HMS exceeds 80%, the volume of the charged scrap compared to the volume of the furnace is increased, resulting in scrap pushing, thereby increasing the workload. In addition, in order to secure slag forming and heat source during the operation of the electric furnace, it is necessary to secure the minimum amount of carbon in the main raw materials. Therefore, cold or molten iron of 20-30% of the total charge should be used.

Therefore, in order to prevent scrap pushing and to secure the minimum charged carbon, it is preferable to use HMS of about 70 to 80% of the total charged amount. Table 1 below shows the target value, the upper limit and the lower limit of each composition component, and the unit is wt%.                     

division C Mn Si P S S.Al Ni Cr Cu N goal 0.04 0.40 0.50 0.080 - 0.015 0.30 0.45 0.30 0.0130 Lower limit 0.02 0.35 0.45 0.075 - 0.005 0.25 0.35 0.25 0.0080 maximum 0.05 0.50 0.55 0.085 0.008 0.030 0.35 0.55 0.35 0.0180

In addition, as shown in Figure 4, when using the 70% to 80% HMS Cu in the molten steel is about 0.20 ~ 0.26wt%, 0.04 ~ 0.10wt% of the target Cu wt% is insufficient. In order to make up for the shortage of Cu components in the molten steel, in the present invention, 0.13 to 0.33% of the total amount of the main raw material is added to the scrap metal when the scrap motor is a by-product sold outside after scrap metal sorting.

About 30% of Cu is contained in the injected waste motor, and when the waste motor is put in the amount of 200 ~ 500kg into molten steel with the total amount of 150 ton in the electric furnace, Cu in the molten steel picked up % Is shown in FIG. 5.

As shown in FIG. 5, when the waste motor is used at less than 200 kg, Cu% to be picked up is less, resulting in an increase in the amount of Cu to be added after tapping, resulting in an increase in ferroalloy cost. In addition, if more than 500kg is input, the amount of pick-up in the molten steel is excessively high, exceeding the design standard value. The amount of waste motor used is 0.13 to 0.33% of the total amount of main raw materials (200 to 500kg for 150 ton of main raw materials). Is preferred.

Phosphorus (P), the most effective element to improve the high weather resistance of steel, is an element with high segregation in the steel. In particular, it is dissolved in ferrite in the steel to form Fe ₃ P. It makes it vulnerable, and in general steel it is extremely delinquent.

This dephosphorization usually takes place in the furnace as an oxidation reaction (2P + 5O = P ₂ O ₅ ). However, the resultant P ₂ O ₅ is therefore very unstable and exhibited a strong acid in a steel-making temperature, the reverse reaction (bokrin) also for effective Tallinn due to occur easily secure the P ₂ O ₅ with a basic material in the slag in a stable form for activity Should be lowered.

The basic substance used in the steelmaking process is usually quicklime (CaO), and by adjusting the amount of the quicklime, the delining reaction can be controlled. Therefore, in the present invention, the end point phosphorus (P) component was controlled by limiting the amount of quicklime introduced into the electric furnace to 0.33 to 1% (500 to 1500 kg based on 150 tons of the total main raw material) compared to the total main raw material loading.

Due to the nature of the furnace operation in which the main material is dissolved and heated by the arc generated from the electrode, slag forming is generally performed to inflate the slag in the furnace in order to protect the furnace equipment from heat and minimize heat emission to the atmosphere. (Slag Foaming) Conduct the operation.

If the amount of quicklime injected into the furnace is less than 0.33% (500kg), the minimum slag for the forming operation is not made and the slag forming is not performed, causing problems such as equipment trouble and heat loss.

In addition, when the amount of quicklime in excess of 1% (1500kg) is added, the dephosphorylation reaction by the caO in the slag (fixing P ₂ O ₅ in a stable form) is suppressed, and the abdominal chlorine into molten steel is suppressed. Due to this, a problem arises in that the input amount of ferroalloy increases (cost increases) to secure a target phosphorus (P) component in the designed steel.

Therefore, the quicklime injected into the furnace is preferably 0.33 to 1% (500 to 1500kg for 150 ton of the main raw material) compared to the total amount of the main raw material. The change of the phosphorus (P) component in molten steel controlled by this method is shown in FIG. As shown in FIG. 6, it can be seen that the phosphorus (P) component increased by about 2.3 times during the manufacture of the steel according to the present invention, thereby suppressing Tallinn.

When the molten steel manufactured as described above reaches an appropriate temperature and oxygen (commonly called tapping temperature and end point oxygen) according to the characteristics of the designed steel, the ladle 12 is provided through the tapping hole 7 as shown in FIG. 3. Will be poured into. In the present invention, after controlling the end point oxygen to 500 ~ 700ppm in this electric furnace, the deoxidizer is not added during tapping to remove the oxygen of molten steel, vacuum degassing in the VTD (Vacuum Tank Degasser) equipment as a secondary refining The method was adopted. This process is illustrated in FIG. 7, where LF represents Ladle Furnace and CC represents Continuous Cast.

Typically, the oxygen arriving in the VTD process should be between 200ppm and 400ppm. This is because the amount of oxygen blows extra oxygen for the degassing (decarburization) process in the VTD process, or degassing due to natural decarburization without the problem of increased deoxidizer use due to excessive molten oxygen after deoxidation. This is because oxygen is within the range where deoxidation can proceed.

When the end point oxygen is less than 500ppm in the electric furnace, the oxygen content of the VTD is less than 200ppm, which causes problems of oxygen blowing in the VTD for smooth decarburization.

In addition, when the terminal oxygen exceeds 700ppm, the amount of oxygen in the molten steel becomes excessive after the decarburization operation, thereby increasing the molten steel manufacturing cost due to an increase in the use of a deoxidizer for deoxidation. In addition, due to the increase in the deoxidation product (Al ₂ O ₃ ), the cleanliness of the molten steel is lowered, the possibility of operation accidents such as clogging the nozzle increases.

Therefore, it is preferable that the end point oxygen in the electric furnace is controlled to 500 to 700 ppm when manufacturing high weatherability steel.

Such a method for producing high weather resistance molten steel in the mini mill process of the present invention utilizes the residual elements (particularly Cu components) contained in scrap iron and waste motors and suppresses the dephosphorization reaction in the furnace, thereby costing expensive ferroalloys in steel. Reduction of deoxidation cost through end point oxygen control and vacuum treatment in secondary refining and improvement of molten steel cleanness by reduction of deoxidation product, resulting in cost reduction and quality improvement in production of high weathering steel in mini mill process. It provides the effect of promoting stable production.

Claims (1)

In manufacturing high weatherability molten steel in mini mill process, Use HMS, scrap metal containing a large amount of residual elements, at 70 to 80% of the total loading amount, and use cold or molten iron at the remaining charge of 20 to 30%, In order to adjust the Cu concentration in the steel to 0.25 to 0.35%, additionally charge 0.13 to 0.33% of the waste motor with the HMS, and adjust the phosphorus (P) concentration in the steel to 0.075 to 0.085%. To control the quicklime injected into the electric furnace is limited to 0.33 ~ 1% of the total charge amount of the HMS and the cold or molten iron, while controlling the end oxygen of the furnace at 500 ~ 700ppm, after removing the oxygen of the molten steel during tapping The high-temperature-resistant molten steel manufacturing method in the mini-mill process characterized by performing vacuum degassing in the secondary refining VTD instead of.

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