TWI902198B - Method for manufacturing molten steel - Google Patents

Abstract

This invention provides a method for manufacturing molten steel, which, when using a vacuum degassing device for deoxidation and refining, can control the Al concentration of the molten steel to a target concentration of 0.01% by mass or higher without significantly increasing processing costs. The method of manufacturing molten steel in this invention involves adding Al to the molten steel more than twice within a vacuum degassing device to produce molten steel with an Al concentration of 0.01% by mass or higher. The Al yield is predicted based on the temperature change of the slag before and after the first Al addition, and the predicted Al yield is used to determine the amount of Al added for subsequent additions.

Description

Methods for manufacturing molten steel

This invention relates to a method for manufacturing molten steel, which involves manufacturing molten steel deoxidized using a vacuum degassing device.

When degassing molten steel that is not fully deoxidized or partially deoxidized using vacuum degassing equipment such as RH (Ruhrstahl-Heraus) or DH (Dortmund-Horder), deoxidation and refining are typically achieved by adding metallic Al to the molten steel in a vacuum environment during the process. However, the yield of metallic Al during deoxidation varies considerably depending on the operating conditions, and this variation can contribute to the removal of certain components or increase the cost of Al alloys. Therefore, several techniques have been reported in the industry to reduce the variation in Al yield.

For example, Patent 1 discloses the following technique: adding deoxidizing materials to the slag on the molten steel in the ladle after tapping from the converter to inhibit Al oxidation caused by the slag, thereby controlling the Al concentration in the molten steel. Patent 2 discloses the following technique: determining the degree of oxidation of the slag by measuring the concentrations of T, Fe, and MnO in the slag using an oxygen probe before vacuum refining, and determining the amount of Al added based on the measured value of the slag's oxidation degree. [Prior Art Documents] [Patent Documents]

Patent Document 1: Japanese Patent Application Publication No. 63-183117 Patent Document 2: Japanese Patent Application Publication No. 2006-283089 [Non-Patent Document]

Non-Patent Document 1: Thermodynamic data for steelmaking, edited by Mitsutaka Hi no and Kimihisa Ito Tohoku, University Press, 2010

(The problem that the invention aims to solve)

However, in the method disclosed in Patent 1, although deoxidizing materials are added to the slag, the oxides in the slag are not completely reduced by adding these materials. Therefore, some Al reacts with the slag, resulting in a larger deviation in the Al concentration of the molten steel. When, in the case of ultra-low carbon steel, after tapping without deoxidation, oxygen is introduced for decarburization using vacuum degassing, FeO is generated by adding deoxidizing materials to the slag and then introducing oxygen. The FeO reacts with Al, further increasing the deviation in the Al concentration of the molten steel.

In patent document 2, an oxygen probe is used to determine the concentrations of T, Fe, and MnO in slag. However, in order to quickly determine the oxidation degree of slag during the steelmaking process, an oxygen probe for slag is required. The unit price of oxygen probes for slag is high, thus significantly increasing the processing cost.

This invention was made in view of the above-mentioned circumstances, and its purpose is to provide a method for manufacturing molten steel that, when using a vacuum degassing device for deoxidation and refining, can control the Al concentration of the molten steel to a target concentration of 0.01% by mass or higher without significantly increasing processing costs. (Technical means to solve the problem)

The means for solving the above problems are as follows. [1] A method for manufacturing molten steel in which Al is added to molten steel more than twice in a vacuum degassing apparatus to produce molten steel with an Al concentration of 0.01% by mass or more; wherein, the Al yield is predicted based on the temperature change of the slag before and after the first Al addition, and the predicted Al yield is used to determine the amount of Al added after the second addition. [2] The method for manufacturing molten steel as described in [1] wherein the evaporation ratio of Al is determined based on the heat change of the molten steel before and after the first Al addition, and the Al yield is predicted based on the temperature change of the slag and the evaporation ratio. [3] The method for manufacturing molten steel as described in [2] uses the following formula (1) to predict the Al yield and uses the following formula (2) to determine the amount of Al added after the second addition. [Number 1] [Number 2] In equations (1) and (2) above, η is the Al yield (-), ΔT is the temperature change of the slag (°C), d is the thickness of the slag (mm), α is the correction coefficient (-), β is the evaporation ratio (-), _a0 is the oxygen concentration (mass%) of the molten steel before the first Al addition, _f0 is the oxygen activity coefficient of the molten steel (-), _Wsteel is the weight of the molten steel (kg), _{WAl_1st} is the amount of Al added in the first addition (kg), _{WAl_2nd} is the amount of Al added after the second addition (kg), [Al] _Target is the target Al concentration (mass%) of the molten steel, and [Al] ₀ is the Al concentration (mass%) of the molten steel before the first Al addition. [4] The method for manufacturing molten steel as described in [2] or [3], wherein the above evaporation ratio is determined using the following equations (3) to (6). [3] [Number 4] [Number 5] [Number 6] In equations (3) to (6) above, ΔQ is the change in heat of the molten steel caused by the first addition of Al (kcal/t-steel), ΔQ _exo is the change in heat of reaction of the molten steel caused by the first addition of Al (kcal/t-steel), ΔQ _sens is the change in sensible heat of the molten steel caused by the first addition of Al (kcal/t-steel), a ₀ is the oxygen concentration (mass%) of the molten steel before the first addition of Al, f ₀ is the oxygen activity coefficient of the molten steel (-), β is the evaporation ratio (-), and X and Y are constants. (Effects compared to prior art)

According to the present invention, it is possible to manufacture molten steel that controls the Al concentration of molten steel to a target concentration of more than 0.01% by mass without significantly increasing processing costs.

The embodiments of the present invention will now be described with reference to the accompanying drawings. Vacuum degassing equipment suitable for implementing the steel-melting manufacturing method of the present invention includes RH vacuum degassing devices, DH vacuum degassing devices, and revolutionary degassing accelerator (REDA) vacuum degassing devices. The embodiments of the steel-melting manufacturing method of the present invention will now be described using the most representative of these, the RH vacuum degassing device.

Figure 1 is a cross-sectional schematic diagram of the RH vacuum degassing device 10. In Figure 1, symbol 12 is a steel container, symbol 14 is molten steel, and symbol 16 is slag. Symbol 18 is a vacuum tank, which is composed of an upper tank 20 and a lower tank 22. Symbol 24 is an ascending side impregnation pipe (ascending pipe), and symbol 26 is a descending side impregnation pipe (descending pipe). Symbol 28 is a gas injection pipe for dry distillation, symbol 30 is a guide pipe, symbol 32 is a raw material inlet, and symbol 34 is a top-blown spray gun. The top-blown spray gun 34 is a device that sprays oxygen or solvent into the molten steel in the vacuum tank. It is located above the vacuum tank 18 and can move up and down inside the vacuum tank 18. Symbol 36 is a thermal imaging camera, which is used to measure the temperature of slag.

In the RH vacuum degassing apparatus 10, a steel tank 12 containing molten steel 14 that has undergone dephosphorization or decarburization treatment is raised using a lifting device (not shown), which immerses the rising-side impregnation pipe 24 and the descending-side impregnation pipe 26 in the molten steel 14 within the steel tank 12. Then, the interior of the vacuum tank 18 is depressurized using an exhaust device (not shown) connected to the guide pipe 30, and circulating gas is blown from the dry distillation gas inlet pipe 28 into the interior of the rising-side impregnation pipe 24. When the interior of the vacuum tank 18 is depressurized, the molten steel 14 in the steel ladle 12 rises proportionally to the pressure difference between atmospheric pressure and the vacuum tank (vacuum degree), and flows into the vacuum tank 18. Simultaneously, due to the air-lift effect generated by the circulating gas blown in from the dry distillation gas inlet pipe 28, the molten steel 14 and the circulating gas rise together in the rising side impregnation pipe 24 and flow into the vacuum tank 18. Subsequently, the flow returning to the steel ladle 12 via the descending side impregnation pipe 26 forms a so-called circulation, thus performing vacuum degassing and refining. The molten steel 14 is exposed to a depressurized environment in the vacuum tank 18, and the gaseous components in the molten steel 14 move into the environment in the vacuum tank 18, thereby carrying out the degassing reaction of the molten steel 14.

In vacuum degassing refining, when deoxidizing undeoxidized or semi-deoxidized molten steel, an alloy that reacts with oxygen to form oxides is added as a deoxidizing material to the molten steel 14 in the vacuum tank 18 through the raw material inlet 32. Regarding the deoxidizing material, depending on the strength of its deoxidizing force, metallic Al or Al-containing alloys are typically used. In the molten steel manufacturing method of this embodiment, metallic Al and Al-containing alloys are also used as deoxidizing materials; these metallic Al and Al-containing alloys are collectively referred to as Al.

When Al, added as a deoxidizing material, reacts with the oxides of slag 16, slag 16 is heated by the heat of reaction of Al. The inventors assumed that the slag temperature increased according to the amount of Al reacted and used a thermal imaging camera to measure the slag temperature before and after the addition of Al during vacuum degassing. The results confirmed a good correlation between the temperature change of slag 16 and the amount of Al reacted. Therefore, the inventors have discovered that when Al is added to molten steel 14 more than twice, the Al yield is first predicted based on the temperature change of the slag 16 before and after the first Al addition. The Al yield takes into account the amount of Al consumed by the slag 16. The Al yield is then used to determine the amount of Al added to molten steel 14 after the second addition. In this way, the Al concentration of molten steel 14 after deoxidation treatment can be controlled with high precision.

Therefore, in the molten steel manufacturing method of this embodiment, during the deoxidation of molten steel in the RH vacuum degassing device 10, Al, which is a deoxidizing material, is added more than twice to produce molten steel with an Al concentration of 0.01% by mass or more. First, based on past operating results and referring to the oxygen concentration of molten steel 14, the approximate Al yield is estimated. The amount of Al added for the first time is determined and added to molten steel 14 so that the oxygen concentration of molten steel 14 after Al addition is less than 0.0002% by mass. At this time, a thermal imaging camera 36 is pre-installed at a position where the slag 16 in the steel-holding ladle 12 can be measured from above, and the slag temperature is continuously measured using the thermal imaging camera 36. An example of a thermal imaging camera 36 series radiation thermometer.

When Al is added for the first time, Al reacts with oxides such as FeO in the slag 16, and the slag temperature rises. At this time, if the slag temperature after the first Al addition is subtracted from the slag temperature before the Al addition and the resulting value is set as the slag temperature change ΔT, the yield of the first Al addition can be predicted using the following formula (1). The slag temperature is the average temperature of a predetermined area measured by the thermal imaging camera 36. The slag temperature after Al addition is the slag temperature after the rise in slag temperature caused by the addition of Al stops.

[Number 7] In equation (1) above, η is the Al yield (-). ΔT is the temperature change of slag 16 (°C). d is the thickness of slag 16 (mm). α is the correction coefficient (-). β is the evaporation ratio of the first added Al (-). _a0 is the oxygen concentration (mass %) of the molten steel before the first Al addition. _f0 is the oxygen activity coefficient in the molten steel 14 (-). _Wsteel is the weight of the molten steel 14 (kg). _{WAl_1st} is the amount of Al added in the first time (kg). The unit (-) refers to dimensionless. The correction coefficient is determined in the following way: the Al yield obtained from the actual Al addition amount and Al analysis value (average of the past 3 months to 1 year) is consistent with the Al yield obtained from equation (1). The activity coefficient of oxygen in molten steel 14 is obtained using the following formula (7).

[Number 8] In equation (7) above, f _₀ is the activity coefficient of oxygen in molten steel 14 (-). e ^_i ₀ is the interaction coefficient (-) of component i in molten steel 14 on oxygen. (%i) is the concentration (mass %) of component i in molten steel 14. The interaction coefficient e_ⁱ ₀ is the value recorded in Non-Patent Document 1.

The slag thickness in equation (1) above is determined by measuring the height of the molten steel surface using a molten steel level gauge; or by immersing a metal rod in the molten steel 14 in the steel container 12 and measuring the length of the portion melted by the slag 16. The oxygen concentration of the molten steel 14 can be the oxygen concentration of the molten steel 14 measured before deoxidation and after dephosphorization and decarburization. The Al yield obtained from equation (1) above and equation (2) below are used to determine the amount of Al added after the second addition. When using an Al-containing alloy as Al, the amount of Al-containing alloy added is determined based on the amount of metallic Al added, which is obtained by multiplying the amount of Al-containing alloy added by the purity of Al in the alloy.

[Number 9] In equation (2) above, _{WAl_2nd} represents the amount of Al added after the second addition (kg). [Al] _Target represents the Al concentration (mass %) of the target molten steel 14. [Al] ₀ represents the Al concentration (mass %) of the molten steel 14 before the first Al addition. _Wsteel represents the weight of the molten steel 14 (kg). η represents the Al yield (-). _{WAl_1st} represents the amount of Al added in the first addition (kg).

In this manner, the determined amount of Al is added after the second addition. The term "after the second addition" can mean adding the entire determined amount of Al in the second addition, or it can be divided into two or more additions, for example, dividing the determined amount of Al into two or three additions to the molten steel 14.

By determining the amount of Al added after the second addition in this way, the amount of Al added after the second addition can be determined based on the Al yield, which takes into account the amount of Al consumed by the slag 16, without using an oxygen probe for the slag. By adding the determined amount of Al to the molten steel 14, the deviation in the amount of Al caused by the slag 16 can be reduced, and molten steel with an Al concentration controlled at a target concentration of 0.01% by mass or higher can be produced with high precision without significantly increasing processing costs.

In equation (1) above, β represents the evaporation ratio of the first added Al. The evaporation ratio of the first added Al can be set as a predetermined amount based on past operating results, but it is preferable to determine the evaporation ratio of Al based on the change in heat of the molten steel 14 before and after the first addition of Al. Specifically, it is preferable to use equations (3) to (5) below to determine the change in heat ΔQ of the molten steel 14 before and after the first addition of Al, and use ΔQ and equation (6) below to determine the evaporation ratio of the first added Al.

[Number 10]

[Number 11]

[Number 12]

[Number 13] In equations (3) to (6) above, ΔQ is the change in heat of molten steel 14 caused by the first addition of Al (kcal/t-steel). ΔQ _exo is the change in heat of reaction of molten steel 14 caused by the first addition of Al (kcal/t-steel). ΔQ _sens is the change in sensible heat of molten steel 14 caused by the first addition of Al (kcal/t-steel). _{a 0} is the oxygen concentration (mass %) of the molten steel before the first addition of Al. f ₀ is the oxygen activity coefficient of the molten steel (-). W _{Al_1st} is the amount of Al added in the first addition (kg). _{W steel} is the weight of molten steel 14 (kg). β is the evaporation ratio of the first addition of Al (-). X and Y are constants. _{f 0} is obtained using equation (7) above. The constants X and Y can be obtained from laboratory experimental results, or they can be obtained by fitting the actual evaporation rate Al in actual operation with the calculated value.

Thus, by determining the Al evaporation ratio based on the change in heat of the molten steel 14 before and after the first Al addition, the Al evaporation ratio can be calculated with high precision. Using this evaporation ratio and the above equations (1) and (2), the amount of Al added after the second addition can be determined. By adding the determined amount of Al to the molten steel 14, molten steel with an Al concentration controlled to a target concentration of 0.01% by mass or higher can be produced with even higher precision. [Example]

Next, an example of the following test will be explained. In this test, 300 tons of molten steel, produced by decarburizing and refining molten iron in a converter, was tapped from the converter into a steel ladle. Ten pairs of RH vacuum degassing devices (as shown in Figure 1) were used to vacuum degas and refine the molten steel in the ladle. The target steel was an extremely low-carbon steel with an upper limit of 25 ppm carbon concentration and a target Al concentration of 0.04% by mass. The composition of the molten steel used in the test was: C: 0.04–0.06% by mass, Si: 0.15–0.25% by mass, Mn: 1.2–1.4% by mass, P: less than 0.02% by mass, S: less than 0.003% by mass. The temperature of the molten steel before deoxidation is 1580–1630℃, and the oxygen concentration of the molten steel before deoxidation is 200–600 ppm. The vacuum degree of the RH vacuum degassing device is set to 2 torr, and the circulating gas flow rate is set to 2500 NL/min. Metallic Al is used as the deoxidizing material.

After thorough decarburization, the oxygen concentration a _₀ of the molten steel was measured using an oxygen probe. Based on the value of a _₀ and the amount of molten steel (within the range of 200–600 kg), the amount of Al added for the first time was determined and added accordingly. The experiment was conducted in three feed groups, A, B, and C, with each group consisting of 100 feeds. The amount of Al added for the second time was determined for each feed group in the following manner.

In feed group A (comparative example), the oxygen concentration of the molten steel is measured using an oxygen probe, and the Al concentration of the molten steel is estimated based on the Al-O balance to determine the amount of Al added in the second addition. In feed group B (invention example 1), the amount of Al added in the second addition is determined using equations (1) and (2) above. In feed group B, a constant derived from the average value of past operations is used as the evaporation ratio of the Al added in the first addition. In feed group C (invention example 2), the amount of Al added in the second addition is determined using equations (1) and (2) above. In feed group C, the value derived from equations (3) to (6) above is used as the evaporation ratio of the Al added in the first addition.

Metal samples were collected from each feed group after the RH vacuum degassing process was completed, and the Sol.Al concentration of the molten steel was measured.

Figure 2 is a graph showing the relationship between the Sol.Al concentration and the number of feeds after RH vacuum degassing. In Figure 2, the horizontal axis represents the Sol.Al concentration (mass %), and the vertical axis represents the number of feeds (ch). As shown in Figure 2, compared with feed group A, which belongs to the comparative examples, the deviation of the Sol.Al concentration in feed groups B and C, which belong to the invention examples, is smaller relative to the target concentration of 0.04% by mass. The standard deviation of the Sol.Al concentration of the molten steel after RH vacuum degassing in each feed group is shown in Table 1 below.

[Table 1] Category Standard deviationσ(%) Invention Example 1 Feeding Group B 0.0027 Invention Example 2 Feeding Group C 0.0009 Comparative example 1 Feeding Group A 0.0114

As shown in Table 1, the standard deviation of the Sol.Al concentration in the molten steel after RH vacuum degassing treatment in Examples 1 and 2 is smaller than that in Comparative Example 1. Based on this result, it was confirmed that by predicting the Al yield based on the temperature change of the slag before and after the first Al addition, and using the predicted Al yield to determine the amount of Al added in the second addition, the Al concentration in the molten steel can be controlled with high precision at the target concentration (0.04% by mass) of 0.01% by mass or higher.

The standard deviation of the Sol.Al concentration of the molten steel after the RH vacuum degassing treatment in Invention Example 2 is smaller than that in Invention Example 1. Based on this result, it was confirmed that the evaporation ratio of the first added Al in the above formula (1) can be determined by the change in heat of the molten steel before and after the first Al addition, thereby enabling the Al concentration of the molten steel to be controlled at the target concentration (0.04 mass%) above 0.01 mass% with higher precision.

10: RH Vacuum Degassing Device 12: Steel Tank 14: Molten Steel 16: Slag 18: Vacuum Tank 20: Upper Tank 22: Lower Tank 24: Rising Side Impregnation Pipe 26: Falling Side Impregnation Pipe 28: Dry Distillation Gas Inlet Pipe 30: Guide Pipe 32: Raw Material Inlet 34: Top-Blow Spray Gun 36: Thermal Imaging Camera

Figure 1 is a schematic cross-sectional view of the RH vacuum degassing device 10. Figure 2 is a graph showing the relationship between the Sol.Al concentration after RH vacuum degassing and the number of feed charges.

Claims (2)

A method for manufacturing molten steel, wherein Al is added to molten steel more than twice in a vacuum degassing device to produce molten steel with an Al concentration of more than 0.01% by mass; wherein the Al yield is predicted by using the temperature change of the slag before and after the first Al addition and the following formula (1), and the predicted Al yield is used to determine the amount of Al added after the second addition and the following formula (2); [Equation 2] WAl_2nd = 0.01 × ([ Al ] _Target - _{[ Al} ] ₀ ) _{× Wsteel} - η × WAl_1st ‧‧‧(2) In the above equations (1) and (2), η is the above _{Al yield} (-), ΔT is the above slag temperature change (°C), d is the above slag thickness (mm), α is the correction coefficient (-), β is the above evaporation ratio (-), _a0 is the oxygen concentration (mass%) of the molten steel before the first Al addition, _f0 is the oxygen activity coefficient of the above molten steel (-), _Wsteel is the weight of the above molten steel (kg), _{WAl_1st} is the above first Al addition amount (kg), _{WAl_2nd} is the above Al addition amount after the second addition (kg), [Al] _Target is the target Al concentration (mass %) of the molten steel, and [Al] ₀ is the Al concentration (mass %) of the molten steel before the first Al addition. For example, in the method of manufacturing molten steel according to claim 1, the heat change of the molten steel before and after the first addition of Al and the following formulas (3) to (6) are used to determine the evaporation ratio of Al, and the Al yield is predicted based on the temperature change of the slag and the evaporation ratio; [3] ΔQ = ΔQ _exo + ΔQ _sens ‧‧‧(3) [Equation 6] β = X × ln ΔQ + Y ‧‧‧(6) In the above equations (3)~(6), ΔQ is the change in heat of the molten steel caused by the first addition of Al (kcal/t-steel), ΔQ _exo is the change in the heat of reaction of the molten steel caused by the first addition of Al (kcal/t-steel), ΔQ _sens is the change in the sensible heat of the molten steel caused by the first addition of Al (kcal/t-steel), a ₀ is the oxygen concentration (mass%) of the molten steel before the first addition of Al, f ₀ is the oxygen activity coefficient (-) of the molten steel, W _{Al_1st} is the amount of Al added in the first addition (kg), W _steel is the weight of the molten steel (kg), β is the evaporation ratio (-), and X and Y are constants.