TWI808760B - Method of electric furnace steelmaking - Google Patents

Abstract

The present invention relates to a method of an electric furnace steelmaking, including a step of distributing materials, a step of slagging and a step of dephosphorizing. In the step of distributing the materials, waste steel with an initial amount, additive, the waste steel with a remaining amount, and molten iron are sequentially provided, and the initial amount and an amount of the additive are controlled to early produce alkaline slags, thereby enhancing dephosphorizing effect of the electric furnace steelmaking.

Description

electric furnace steelmaking method

The present invention relates to a method of electric furnace steelmaking, and in particular to a method of electric furnace steelmaking in which additives are distributed among scrap steel.

Electric furnace steelmaking includes dephosphorization and desulfurization to obtain molten steel with low phosphorus and low sulfur content, which is conducive to the subsequent production of high-quality steel. The main raw materials used in electric furnace steelmaking include steel scrap and molten iron. Hot metal can provide heat energy for steelmaking and a source of iron, but compared with other main raw materials (such as scrap steel with a phosphorus content of about 0.02 weight percent), the phosphorus content of molten iron is higher (0.12 to 0.15 weight percent phosphorus), so it increases the difficulty of dephosphorization. Therefore, dephosphorization must use additives containing calcium oxide (such as lime and dolomite), so that calcium oxide and the oxides of the main raw materials and oxides oxidized by molten iron (such as silicon dioxide, iron oxide and aluminum oxide) undergo slagging reactions, and then dephosphorization is carried out through the generated slag.

The conventional electric furnace steelmaking method is to put scrap steel into the electric furnace first, mix in molten iron from the runner, then put lime in from the silo, blow oxygen into the electric furnace, and then heat the electric furnace to start the subsequent slagging and dephosphorization. In the case of this cloth, the molten iron will flow to the bottom of the electric furnace through the gap formed by the accumulation of scrap steel, so the scrap steel is located between the lime above and the molten iron below, hindering the contact between the two. Therefore, it is necessary to wait until at least a part of the steel scrap is melted before the lime can contact the molten iron. At this time, the calcium oxide contained in the lime can produce alkaline slag with the molten steel formed by the molten steel scrap and the molten iron. In other words, the time point when slag starts to form is in the middle and late stages of steelmaking, so the time for dephosphorization by basic slag is shortened.

Furthermore, at the aforementioned time when basic slag can be produced, the temperature of the electric furnace is too high (for example: higher than 1500° C.), which makes the exothermic dephosphorization reaction more difficult to proceed, thus reducing the dephosphorization effect. Therefore, more additives need to be invested to compensate for the reduced dephosphorization effect. However, in addition to increasing the cost of additives, additional additives will also lead to excessive slag, which is mixed with a lot of molten steel after smelting, and the molten steel will be removed along with the slag during the process of raking the slag, thus reducing the output of molten steel after smelting.

In view of this, there is an urgent need to develop a new electric furnace steelmaking method to improve the above-mentioned shortcomings of the conventional electric furnace steelmaking method.

In view of the above problems, one aspect of the present invention is to provide a method for electric furnace steelmaking. This method is to generate basic slag early by controlling the initial amount of steel scrap and the amount of additives through a specific distribution sequence, thereby improving the dephosphorization effect of electric furnace steelmaking.

According to an aspect of the present invention, a method for electric furnace steelmaking is proposed. The method includes feeding oxygen and natural gas into the electric furnace to carry out the step of distributing; and after the step of distributing, performing a slagging step and a dephosphorization step in the contact area of scrap steel, additives and molten iron to obtain slag and molten steel. The distributing step includes sequentially supplying the initial amount of steel scrap, additives, remaining amount of steel scrap and molten iron into the electric furnace, wherein the initial amount (x1) satisfies the following formula (I), and the amount of additives (c1) satisfies the following formula (II): In formula (I), x2 represents the density of steel scrap, x3 represents the bulk density of steel scrap, y1 represents the amount of molten iron, and y2 represents the density of molten iron; In formula (II), x4 represents the remaining amount of steel scrap, x5 represents the silicon content of steel scrap, and y3 represents the silicon content of molten iron.

According to one embodiment of the present invention, the additive comprises calcium oxide.

According to another embodiment of the present invention, based on 100% by weight of the additive, the content of calcium oxide is greater than 87% by weight.

According to yet another embodiment of the present invention, the method excludes the use of steel slag and/or iron slag.

According to another embodiment of the present invention, after the distributing step, the liquid level of the molten iron is substantially in contact with the additive.

According to yet another embodiment of the present invention, during the dephosphorization step, the basicity of the slag is 1.0 to 3.0.

According to another embodiment of the present invention, the ratio of the phosphorus content of the slag to the phosphorus content of the molten steel is greater than 100.

According to another embodiment of the present invention, the final phosphorus content of the molten steel is less than 0.020% by weight.

The electric furnace steelmaking method of the present invention is applied, wherein the distributing step is to provide the initial amount of scrap steel, additives, remaining amount of scrap steel and molten iron in order, and control the initial amount of scrap steel and the amount of additives to generate alkaline slag early, thereby improving the dephosphorization effect of electric furnace steelmaking.

The making and using of embodiments of the invention are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are illustrative only and do not limit the scope of the invention.

The electric furnace steelmaking method of the present invention uses a specific sequence of material distribution steps and a specific initial amount of steel scrap to generate basic slag in advance, thereby improving the dephosphorization effect of the electric furnace steelmaking. The aforementioned specific sequence is to add the initial amount of steel scrap, additives, remaining amount of steel scrap and molten iron in the electric furnace in sequence, and the specific initial amount of steel scrap must satisfy the formula (I) described later, so that the liquid level of the added molten iron can substantially contact with the additive located between the steel scrap. After the electric furnace starts heating, the generated slag can be exposed to additives immediately, so that the calcium oxide contained in it can be chemically diffused into the slag at the early stage of smelting, so that the basic slag that is beneficial to dephosphorization is formed early. Therefore, the dephosphorization reaction can be carried out at a favorable low temperature (such as 1300°C to 1500°C), thereby prolonging the effective dephosphorization time and improving the dephosphorization effect of electric furnace steelmaking. In addition, since the additive is buried between the steel scrap, the heat energy absorbed by the scrap steel from the electric furnace can be transferred to the additive immediately to melt the additive quickly, thereby increasing its melting rate, thus reducing the amount of the additive and correspondingly reducing the amount of slag generated. In addition, the electric furnace steelmaking method of the present invention also uses a specific amount of additives to help the formation of basic slag, so as to control the basicity of the slag in a range that is conducive to dephosphorization (basicity is 1.0 to 3.0), thereby further improving the dephosphorization effect of electric furnace steelmaking.

The steel scrap referred to in the present invention refers to factory steel scrap (for example: iron and steel scrap), general household steel scrap (for example: steel parts of discarded furniture) or other scrap steel, wherein based on the weight of the scrap steel as 100%, the iron content of the scrap steel can be 80% to 95%. In some embodiments, the density of steel scrap can be, but not limited to, 7 metric tons/m ³ to 8 metric tons/m ³ , and the phosphorus content of steel scrap can be, but not limited to, 0.01 to 0.05 weight percent.

The molten iron referred to in the present invention refers to iron that melts into a liquid state, and its temperature can be 1250°C to 1500°C. In some embodiments ^, the density of the molten iron may be, but not limited to, 6.8 to 7.5 metric tons/m ³ , and the phosphorus content of the molten iron may be, but not limited to, 0.120 to 0.150 weight percent.

The basicity of the slag referred to in the present invention refers to the ratio of the weight of calcium oxide in the slag divided by the weight of silicon dioxide in the slag. When the basicity of the slag is greater than 1.0, the slag is called basic slag. Alkaline slag is beneficial to dephosphorization, but too high calcium oxide will increase the melting point of slag and reduce the fluidity of slag, so it will reduce the probability of contact between molten steel and slag, thereby reducing the dephosphorization effect. Therefore, slag with a basicity of 1.0 to 3.0 has a better dephosphorization effect.

Please refer to FIG. 1 , the electric furnace steelmaking method 100 is to first feed oxygen and natural gas into the electric furnace for the step of distributing materials, as shown in operation 110 . In the distributing step, the initial amount of scrap steel, additives, remaining amount of scrap steel, and molten iron are supplied to the electric furnace in sequence. The above-mentioned order of distributing can make the additive buried in the steel scrap, so that the heat absorbed by the scrap steel from the electric furnace can be transferred to the additive immediately, thereby increasing its melting rate. The more molten additives are beneficial to the alkalinization of the slag and become basic slag, so the dephosphorization effect is improved.

Conversely, if the fabric order is not in the aforementioned order, when additives are added first, since the amount of additives is significantly less than other raw materials (scrap and molten iron), the additives will be pressed under the scraps, and the liquid level of the molten iron will be much higher than where the additives are located. At this time, the contact area of scrap steel, additives and molten iron is under the molten iron surface, not at the liquid surface (or close to the liquid surface), so oxygen is insufficient, which makes it difficult for scrap steel and molten iron to undergo oxidation reaction, thus making it difficult to generate alkaline slag.

In the case of post-addition of additives (that is, scrap steel is added in one stage, and additives are added after adding scrap steel), this is the conventional method of electric furnace steelmaking. As mentioned in the previous art, the scrap steel will be located between the upper additive and the lower molten iron, which hinders the contact between the two, so the slagging can only be carried out after at least a part of the scrap steel is melted.

In order to achieve that the liquid level of molten iron falls substantially at the position where the additives are located, the initial amount of steel scrap must be controlled to satisfy the following formula (I): In formula (I), x1 represents the initial amount of scrap, x2 represents the density of scrap, x3 represents the bulk density of scrap, y1 represents the amount of molten iron, and y2 represents the density of molten iron.

If the initial amount of steel scrap does not satisfy the above formula (I), the liquid level of the molten iron cannot substantially fall on the position of the additive, so it is not conducive to the formation of alkaline slag, and the dephosphorization effect will be reduced. In addition, in some embodiments, the bulk density of steel scrap can be, but not limited to, 0.2 ton/m ³ to 0.8 ton/m ³ , and is preferably 0.5 ton/m ³ .

In addition, in order to be able to control the basicity of the slag to be in the range that is beneficial to dephosphorization (the basicity is 1.0 to 3.0), the amount of the additive must satisfy the following formula (II): In formula (II), c1 represents the amount of additives used, x4 represents the remaining amount of steel scrap, x5 represents the silicon content of steel scrap, and y3 represents the silicon content of molten iron.

If the amount of additives does not satisfy the above formula (II), the basicity of the slag will be less than 1.0 or greater than 3.0, which is not conducive to dephosphorization. In addition, in some embodiments, the silicon content of scrap steel may be but not limited to 0.10 to 0.50 weight percent, and the silicon content of molten iron may be but not limited to 0.10 to 0.20 weight percent.

In some embodiments, additives may include, but are not limited to, lime, magnesia, cement, marble, serpentine, dolomite, feldspar, mica, and/or talc. In addition, the additive may contain calcium oxide, and optionally magnesium oxide.

In some specific examples, based on the weight of the additive as 100% by weight, the content of calcium oxide can be greater than 87% by weight, and preferably greater than 90% by weight, so that the dephosphorization effect can still be maintained when the amount of the additive is reduced, thereby reducing the amount of slag generated.

In some embodiments, the electric furnace steelmaking method 100 of the present invention can exclude the use of steel slag and/or iron slag. Since both steel slag and iron slag are residues after steelmaking, the silicon content varies greatly, and it is difficult to control the basicity of slag by the aforementioned formula (II), so the method 100 for electric furnace steelmaking can exclude the use of steel slag and/or iron slag.

During the distributing step, the electric furnace steelmaking method 100 is to feed oxygen and natural gas into the electric furnace, so that the gaps in the scrap steel are filled with oxygen and natural gas, so as to facilitate the oxidation of scrap steel and molten iron in the subsequent slagging step, thereby facilitating the formation of slag. In some embodiments, the volume ratio of oxygen to natural gas may be 1.5 to 2.5.

Please refer to FIG. 1 , after operation 110 , the electric furnace steelmaking method 100 performs a slagging step and a dephosphorization step in the contact area of steel scrap, additives and molten iron to obtain slag and molten steel, as shown in operation 130 . This slagging step is a chemical diffusion reaction between the oxidized oxides of scrap steel and/or molten iron and the additive calcium oxide and/or magnesium oxide to form oxides that do not melt at high temperatures in the electric furnace. The oxides that do not melt are slag. The composition of slag can include basic oxides, acidic oxides and amphoteric oxides, wherein basic oxides such as calcium oxide can increase the basicity of slag, and acidic oxides such as silicon dioxide can reduce the basicity of slag.

In some embodiments, during the slagging step, the volume ratio of oxygen and natural gas is increased to greater than 2.5 and less than or equal to 10, so as to facilitate oxidation of scrap steel and molten iron, thereby facilitating slag formation.

In addition to the basic slag, in the electric furnace steelmaking method 100 of the present invention, the temperature of the electric furnace will gradually increase with the smelting time. However, the dephosphorization reaction is an exothermic reaction, so when the temperature of the electric furnace is too high, the dephosphorization effect will be reduced. The temperature for effective dephosphorization is 1300°C to 1500°C. Therefore, the early formation of basic slag can also enable the dephosphorization reaction to proceed at a lower temperature, so that the dephosphorization effect can be further improved.

Accordingly, in some embodiments, the ratio of the phosphorus content of the slag to the phosphorus content of the molten steel (also referred to as phosphorus distribution ratio) may be greater than 100, and preferably greater than 140. In addition, in some specific examples, after the molten steel is tapped, the phosphorus content at the end point of the molten steel is measured, and the phosphorus content can be less than 0.020% by weight. Therefore, the electric furnace steelmaking method 100 of the present invention can achieve excellent dephosphorization effect.

The following examples are used to illustrate the application of the present invention, but they are not intended to limit the present invention. Anyone skilled in this art can make various changes and modifications without departing from the spirit and scope of the present invention.

Preparation of molten steel

Example 1

The preparation of molten steel in Example 1 is to first add 6.73 tons of steel scrap, 5.2 tons of lime and 40 to 50 tons of steel scrap into the electric furnace (abbreviated as "two-stage addition method"), wherein the purity of lime (that is, the content of calcium oxide) is greater than 90% by weight, the density of steel scrap is 7.8 tons/m ³ , the bulk density of steel scrap is 0.5 tons/m ³ , and the density of molten iron is 7.14 tons/m ³ , and the silicon content of steel scrap and molten iron is 0.10% by weight to 0.20% by weight respectively. Feed oxygen and methane with a volume ratio of 2.0 into the electric furnace, and add 90 tons of molten iron at a temperature of 1250°C to 1400°C into the electric furnace at a rate of 2 to 5 tons per minute. Next, increase the volume ratio of oxygen to methane to 3 to 10 and heat the electric furnace. The changes in the basicity of the slag were recorded over time. Obtain the molten steel and slag of embodiment 1 after smelting finishes. Then, it was evaluated in the evaluation manner described later.

Comparative example 1

The preparation system of the molten steel of comparative example 1 is carried out with the method similar to embodiment 1. The difference is that in Comparative Example 1, the addition method of steel scrap is changed and the smelting time is increased, that is, a one-stage addition method is used, in which all steel scrap is added first, and then lime is added into the electric furnace. The specific conditions and evaluation results of the aforementioned Example 1 and Comparative Example 1 are shown in Table 1 and Figures 2 to 4 below.

Evaluation method

1. Test of basicity of slag

The basicity test of the slag is to record the change of the basicity of the slag with the time of smelting. At the time point of the test, the slag is taken out from the electric furnace, and the content of calcium oxide and silicon dioxide in the slag is detected by an elemental analyzer, and then the basicity (the ratio of the content of calcium oxide divided by the content of silicon dioxide) is obtained, and the detection conditions are the conditions commonly used by those with ordinary knowledge.

2. Lime melting rate test

The test of lime melting rate is to rake out unmelted lime before tapping, and measure the weight of unmelted lime, then calculate the ratio of unmelted lime weight based on the weight of added lime, and calculate the ratio of melted lime from the ratio of unmelted lime, which is the lime melting rate.

3. Test of phosphorus distribution ratio The test of the phosphorus distribution ratio is to use an optical emission spectrometer (OES) to detect the phosphorus content of the slag and the phosphorus content of the molten steel before tapping, and use the ratio of the two to express the phosphorus distribution ratio. When the ratio is larger, more phosphorus is moved from molten steel to slag, so the dephosphorization effect is better. The opposite is the opposite.

4. Test of end-point phosphorus content

The end-point phosphorus content test is to use an optical emission spectrometer (OES) to detect the phosphorus content of the prepared molten steel before tapping, and use it to represent the end-point phosphorus content of the molten steel. When the end-point phosphorus content is lower than 0.020 weight percent, the molten steel meets the requirements of the end-point phosphorus content specification.

Table 1 Example comparative example 1 1 steelmaking method Cloth method Two-stage addition method One-stage addition method Scrap usage (metric tons) The first stage is 6.73 and the second stage is 40~50 46.73~56.73 Lime consumption (metric tons) 5.2 6.7 Hot metal (metric tons) 90 90 Smelting time (minutes) 80 80 Evaluation method Lime melting rate (weight percent) 95.3 66.9 Phosphorus distribution ratio 149.4 90.3 End point phosphorus content (weight percent) 0.0117 0.0133

Please refer to Table 1 and Figure 2, Embodiment 1 uses a two-stage addition method. At the initial stage of smelting (25 minutes), the middle stage of smelting (the smelting temperature is 1530°C, 50 minutes), the later stage of smelting (the smelting temperature is 1600°C, 60 minutes) and before tapping (75 minutes), the basicity of the slag is 1.3, 1.9, 2.5 and 2.8 respectively.

After distributing, the liquid level of the molten iron falls to the position where the lime is, so the lime touches the molten iron, and the lime is buried between the steel scraps, so that the calcium oxide of the lime can chemically diffuse into the slag at the initial stage of smelting, and the basic slag that is beneficial to dephosphorization is formed in advance, so the dephosphorization effect is improved, and the effective dephosphorization time is increased (about 50 minutes). Therefore, embodiment 1 can shorten the time of smelting.

However, please refer to Table 1 and Figure 3. Comparative Example 1 uses a one-stage addition method. At the initial stage of smelting (25 minutes), the middle stage of smelting (1600°C, 35 minutes), the late stage of smelting (1530°C, 60 minutes), and before tapping (80 minutes), the basicity of the slag is 0.7, 0.8, 2.9 and 3.5, so the effective dephosphorization time is 25 Minutes, so it is necessary to increase the smelting time to obtain molten steel that meets the phosphorus content specification at the end point.

According to Table 1, compared with Comparative Example 1, Example 1 uses less lime, so the two-stage addition method can reduce the amount of lime, and because it uses less lime, it can reduce the amount of slag generated.

Please refer to Table 1 and Figure 4. Compared with Comparative Example 1, Example 1 uses a two-stage addition method to bury the lime between the scrap steel, so the heat energy absorbed by the scrap steel from the electric furnace can be immediately transferred to the lime to increase its melting rate and chemically diffuse the phosphorus contained in the molten steel into the alkaline slag, thus improving the dephosphorization effect.

Furthermore, according to the test results of the phosphorus distribution ratio in Table 1, Example 1 can improve the dephosphorization effect through the early generation of basic slag and high lime melting rate, thereby reducing the final phosphorus content of the prepared molten steel and increasing the phosphorus distribution ratio.

To sum up, the electric furnace steelmaking method of the present invention utilizes a specific sequence of distributing steps, a specific initial amount of steel scrap, and a specific amount of additives to generate alkaline slag early, thereby improving the dephosphorization effect of electric furnace steelmaking. The specific sequence is to add the initial amount of scrap steel, additives, and the remaining amount of scrap steel and molten iron in the electric furnace. The initial amount of steel scrap and the amount of additives must satisfy formulas (I) and (II) respectively. Substantial contact with additives to achieve the aforementioned dephosphorization effect.

Although the present invention has been disclosed above in terms of implementation, it is not intended to limit the present invention. Anyone with ordinary knowledge in the technical field of the present invention can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be defined by the scope of the appended patent application.

100: method 110,130: operation

In order to have a more complete understanding of the embodiments of the present invention and their advantages, please refer to the following descriptions together with the corresponding drawings. It must be emphasized that the various features are not drawn to scale and are for illustration purposes only. The contents of relevant diagrams are explained as follows: FIG. 1 is a flowchart illustrating an electric furnace steelmaking method according to an embodiment of the present invention. Fig. 2 is a graph showing the variation of the basicity of slag with the smelting stage according to Example 1 of the present invention. Fig. 3 is a diagram showing the variation of the basicity of the slag with the smelting stage in Comparative Example 1 according to the present invention. FIG. 4 is a graph showing the results of lime melting rate according to Example 1 and Comparative Example 1 of the present invention.

100: method

110,130: operation

Claims (8)

A method of electric furnace steelmaking, comprising: feeding oxygen and natural gas into an electric furnace to perform a distributing step, wherein the distributing step comprises: sequentially providing an initial amount of steel scrap, an additive, a remaining amount of the scrap and molten iron into the electric furnace, wherein the initial amount (x1) satisfies the following formula (I), and an amount of the additive (c1) satisfies the following formula (II):

In the formula (I), x2 represents a density of the steel scrap, x3 represents a bulk density of the steel scrap, y1 represents a consumption of the molten iron, and y2 represents a density of the molten iron;

In the formula (II), x4 represents the remaining amount of the steel scrap, x5 represents the silicon content of the steel scrap, and y3 represents the silicon content of the molten iron; and after the distributing step, a slagging step and a dephosphorization step are carried out in the contact area of the steel scrap, the additive and the molten iron to obtain a slag and a molten steel. The method for electric furnace steelmaking as claimed in claim 1, wherein the additive contains calcium oxide. The method for electric furnace steelmaking as described in claim 2, wherein When the weight of the additive is 100% by weight, the content of the calcium oxide is greater than 87% by weight. The method for electric furnace steelmaking as described in Claim 1, wherein the method excludes the use of steel slag and/or iron slag. The method for electric furnace steelmaking according to claim 1, wherein after the distributing step, a liquid level of the molten iron is in substantial contact with the additive. The method for electric furnace steelmaking as claimed in claim 1, wherein during the dephosphorization step, the slag has a basicity of 1.0 to 3.0. The method for electric furnace steelmaking according to claim 1, wherein the ratio of the phosphorus content of the slag to the phosphorus content of the molten steel is greater than 100. The method for electric furnace steelmaking according to claim 1, wherein the phosphorus content of the molten steel at an end point is less than 0.020% by weight.

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