JPS5877516A - Converter refining method - Google Patents

Abstract

PURPOSE:To prevent an increase in the content of hydrogen in steel in the stage of refining steel by decarburization in a converter provided with bottom blowing double pipe tuyeres by changing over a refining gas from pure oxygen to a gaseous mixture of oxygen and an inert gas on progression of decarburization reaction. CONSTITUTION:In the case of refining molten steel by decarburization in a bottom or top and bottom blown converter, a gas for refining by decarburization is blown into the molten steel through the inside pipe of the double tuyere pipes under the melt surface and a hydrocarbon gas for cooling and protecting the tuyeres through the outside pipe. Pure oxygen is first blown as a gas for refining by decarburization through the inside pipe to decarburize the C on the melt as CO. The content of C decreases to 0.8-0.3% with progression of the decarburization, and when the partial pressure of CO decreases on account of a decrease in the rate of generation of CO, the partial pressure of H2 produced by the decomposition of a hydrocarbon gas such as propane as a protecting gas increases and the content of H2 in the steel increases, thus degrading the quality. At this time the gas for refining is changed over to a gaseous mixture of O2 and an inert gas such as Ar of >=0.6 ratio, whereby the partial pressure of the H2 is decreased by the inert gas and the entry of the H2 into the molten iron is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention applies to a converter in which a double pipe wall is provided below the molten steel thirst + ni position i1 a, for example, a bottom blowing converter and a top and bottom blowing converter (vine smelting Regarding the method, in particular, a refining gas is blown into the molten steel from the inner α of the double pipe tuyere, and a hydrocarbon-based cooling medium for protection is inserted between the inner pipe of the double pipe tuyere and the tA pipe. This relates to a refining method that uses flowing water.

Recently, bottom blowing converters have been developed in which tuyeres are installed at the bottom of the hearth and the slag is injected into the molten steel through the tuyere, and a combination of blowing from the bottom tuyere and soil blowing from a lance has been developed. Top-bottom blowing converters have come into practical use. In these converters, the tuyeres at the bottom of the furnace are protected from melting damage! In order to cool the tuyeres, the tuyere has a double-tube structure, and the tuyeres are blown into the molten steel from the inside and a, and the cooling medium is flowed into the gap between the inner and outer tubes. There are many. This cooling medium for protecting the tuyere and (2) u (・11) have been used,
Not only the sensible heat but also the absorption Mj at the time of Ij contributes to cooling.
Liquid hydrocarbons are often used to remove the cooling effect of liquid hydrocarbons.

However, the refrigerant is used as a double layer to protect the lining □
When a hydrocarbon-based material is used as the material, the hydrocarbons blown into the molten steel decompose and generate hydrogen, and some of the hydrogen dissolves into the molten steel, resulting in normal pure i. The aqueous concentration in the steel is 1'; There was a problem in which this tendency was particularly strong in the case of melting low carbon steel and ultra-low element steel. If the hydrogen concentration in the steel becomes high as described above, there is a problem that many copper holes are generated during solidification of the molten steel, and surface defects and internal defects in the copper piece lead to a decrease in the quality of the steel.

As a method for solving this problem, there is a method of degassing the steel using an RH degassing device or the like after tapping the steel from the converter, but in this case, there is a problem that the overall external manufacturing cost of the steel increases.

This invention was developed in view of the following circumstances, and when a hydrocarbon-based cooling medium is used as a cooling medium for protecting the siding of a double-pipe tuyere, it is possible to prevent an increase in water-based concentration in steel while The purpose is to prevent the increase in hydrogen concentration in steel during oil production, especially for low carbon steel and ultra-low carbon steel, and to eliminate the need for secondary descaling treatment. .

In order to achieve the above-mentioned objective, the Ministry of the Invention conducted an intensive HB investigation into the cause of the increase in hydrogen concentration in steel during blowing using a hydrocarbon coolant for one nail, and found the following: It was found that this was due to the change in the hydrogen concentration in the steel over time during blowing. It was discovered that the aqueous concentration in the steel suddenly increased during the 1-day period. - It was discovered that the aqueous concentration in the steel suddenly increased.
J' Oxygen, which is normally used as needle waste at tL, is effective in decarburizing the whole grain at Jυ1, which is the maximum decarburization stage, so the decarburization oxygen efficiency is 1), and the injection size is 1). 1 mol of pure oxygen generates 2 mol of CO sludge, but if the carbon dioxide is low, the decarburization oxygen efficiency will be low and the raw CO sludge will decrease. From this, it can be understood that if the flow rate of the hydrocarbon cooling medium is constant, the partial pressure of hydrogen sludge often generated by decomposition of hydrocarbons will rapidly increase at low carbon dioxide levels after the peak decarburization period has passed. From these results, it has been found that the increase in hydrogen concentration in steel is caused by an increase in the partial pressure of decomposed hydrogen gas in the hydrocarbon cooling medium due to a decrease in the amount of CO gas generated in the d1 low coal region.

To explain in more detail the rise in hydrogen partial pressure at low carbon dioxide levels due to the use of a hydrocarbon-based cooling medium in the conventional refining method as described above, for example, propane gas is used as the cooling medium, and propane gas is blown into molten steel from an inner pipe. Assuming that the flow rate ratio of propane gas to the flow rate of pure oxygen is a%, and that propane gas is completely decomposed into carbon and hydrogen,
Since 4 moles of hydrogen are produced from 1 mole of propane gas,
At the peak of decarburization, the hydrogen partial pressure Pl (2 is the total pressure of 1 s
PH2 (atm) as tm, = (seedling) / (2+con),,
,,,, (1) Totoru. Since the propane gas to oxygen flow rate ratio a in normal pure oxygen bottom-blowing converter operation is about 4%, by substituting a-4 (%) into equation (1), PH2=
0.074(5) (aim). On the other hand, after the peak decarburization period has passed and the carbonation becomes low, and the decarburization r night view efficiency is 1 or more], for example, in 05 and Todore, the hydrogen partial pressure P112 is P112 ("") "" (x? +71) / (
2X O, 5+ person) ...... can be expressed as (2).

Therefore, by substituting a-4 (%) in the same way as above, P
H2-, 0138 (at m) ・The hydrogen partial pressure is twice that of the peak decarburization period. Furthermore, the decarburization oxygen efficiency is 0.
If it decreases to 3, then PH2 = 0.211 (atm) at a = 4 (%) as described above.O' As shown above, in the low carbon dioxide at the end of blowing, the hydrogen partial pressure increases significantly, and this increases the hydrogen partial pressure in the steel. This results in an increase in water-based concentrations.

Therefore, in order to prevent an increase in the hydrogen concentration in steel, it is considered that it is sufficient to suppress the increase in hydrogen scum partial pressure at low carbon dioxide levels at the end of blowing. One possible method is to flow an inert gas together with a hydrocarbon-based cooling medium through the gap between the inner tube and the outer tube of the 2<i><i><i> tube siding when the carbon dioxide is low. However, since the cross-sectional area of the gap between the inner tube and the outer tube is significantly smaller than that of the inner tube through which pure oxygen (6) flows, the absolute flow rate of inert gas that can be supplied is small, and therefore the In the coal region, it is difficult to compensate for the decrease in the amount of CO gas generated due to oxygen in the inner tube with the inert gas supplied together with the hydrocarbon cooling medium. It is difficult to prevent.

Therefore, in this invention, in the final stage of blowing, inert gas is blown in together with oxygen from the inner tube, thereby diluting the hydrogen produced by the decomposition of the hydrocarbon cooling medium with inert scum, thereby increasing the hydrogen partial pressure. This is to suppress the hydrogen concentration in the steel and prevent an increase in the hydrogen concentration in the steel.

Therefore, in the refining method of the present invention, the gas injected into the molten steel from the inner pipe of the double-pipe tuyere is pure oxygen gas at the beginning of the blowing, and in the latter half of the refining, when the carbon concentration in the molten steel reaches 03 1 tJ] before the gas force blown from the inner pipe, the inert coupling flowmeter ratio from pure oxygen gas to oxygen was 06
The present invention is characterized in that the gas mixture is switched to the above-mentioned mixed gas of oxygen gas and inert dregs, and the blowing of the mixed gas is continued at the end of blowing 1.

Further explaining the refining method of the present invention in detail 1, in the refining method of the present invention, there is a gap between the inner pipe and the outer pipe of the double pipe tuyere from the start of refining to the end of the refining process. A hydrocarbon-based gas such as gas or a liquid hydrocarbon, that is, a hydrocarbon-based cooling medium, is caused to flow.

On the other hand, pure oxygen gas is injected from the inner pipe at the beginning of the N refining process.In this state, the maximum decarburization period has passed and the carbon concentration in the steel reaches 0.3% in the latter half of the refining process. When the carbon concentration is 0.3% or more, the gas blown from the inner tube is switched from pure oxygen gas to a mixed gas of oxygen scum and inert gas. tli+] If the switch is made at a time when the carbon concentration in the steel is lower than the 0.3 factor, the hydrogen partial pressure due to the decomposition of the hydrocarbon coolant described in -II will increase and the hydrogen concentration in the steel will increase. Therefore, at least the carbon concentration in the steel is 0.
It is necessary to switch before it reaches 3%. On the other hand, if the switching time is too early, decarburization (a> rb) may be delayed and the refining time may become longer, so it is necessary to switch at least in the latter half of the refining process, especially if the carbon concentration in the steel is o8. It is desirable to switch after the time when it reaches the relevant level. In the end, the appropriate time for switching is when the carbon concentration in the steel is between 08 and 03. In addition, when the flow rate ratio of inert gas to oxygen sludge is less than 0.6, the mixed gas after switching will have a lower hydrogen partial pressure due to the decomposition of the hydrocarbon cooling medium in the low coal region, as shown in the examples described later. It becomes difficult to suppress the double rise, and therefore, the mixing ratio (flow rate ratio) of the inert gas to the oxygen gas in the mixed gas needs to be 0.6 or more.

As mentioned above, if the effect of suppressing the rise in hydrogen partial pressure when inert gas is injected together with oxygen gas from the inner pipe in the low coal region is calculated according to the above calculation example, the ratio of inert gas to oxygen is Blow the mixed gas of 06 from the inner pipe,
In addition, when propane gas with an oxygen flow rate ratio of 4% is flowed between the outer pipe and the inner pipe as a hydrocarbon-based cooling medium, under the condition that the decarburization oxygen efficiency is 05, the hydrogen partial pressure PH2 is 0.0909 ( at+n), and under the condition that the decarburization oxygen efficiency is 0.3, the hydrogen partial pressure PH2 is 0118<
aim). These values are inert (
9) It is much smaller than the hydrogen partial pressure when no gas is used (PH2=0..138 at decarburization oxygen efficiency 05, PH=0.211. at decarburization oxygen efficiency O3).

Therefore, inert gas [practical hydrogen partial pressure J-+1
It is clear that the inhibitory effect is large.

An actual example of this invention and a comparative example will be described below.

Actual H1! 1 Example: 5 tons of hot metal was charged into a 5-ton bottom-blowing converter with 4 double-pipe tubes installed at the bottom of the furnace, and was used as a cooling medium between the inner and outer tubes of the tuyere. At the beginning of blowing, propane is flowed, and pure oxygen is blown into the inner pipe at the beginning of blowing, and during the latter half of blowing, the stubs in the inner pipe are replaced with oxygen and argon gas. Finally, the gas was switched to a mixed gas with kl nitrogen gas, and the blowing was completed.The pure oxygen gas flow before changing the gas in the inner tube to the mixed gas was 15 N7717.
Assuming that m1n is constant, the ratio of propane to oxygen flow rate is 4.
Assuming a constant value of

In this example, the composition and temperature of hot metal, the composition and +? +λ river”, inner pipe waste switching condition,
The components and weight ratios at the time of blow-off are shown in Table 1, Test (10) Test Nos. 1 to 6.

Comparative Example 1 Almost the same as the example except that a mixed gas of 05 inert gas (argon gas) and oxygen with a flow rate ratio to oxygen smaller than 06 was used as the mixed gas blown from the inner pipe at the final stage of blowing. It was then blown.

Comparative Example 2 Same as the example except that the timing for switching the gas blown from the inner tube from pure oxygen gas to a mixed gas of oxygen and argon gas or nitrogen gas was set at the time when the C concentration in the steel became lower than 03. It was blown in the same way.

Comparative Example 3 Pure oxygen was blown from the start of blowing to the end of blowing without changing the inner pipe waste at the end of blowing. Other conditions are the same as in the example.

The composition and temperature of the hot metal, the composition and temperature at the time of switching the inner tube gas, the conditions for switching the inner tube scum, and the composition and temperature at the time of blow-off in each of the above comparative examples were determined from test numbers 7 to 11 in Table 1.
Shown below.

(11) Comparing Test Numbers 1 to 4 of the Examples and Comparative Example 1 (Test Number 7), it is found that if the ratio of inert gas to oxygen in the mixed scum is 0.6 or more, the concentration of hydrogen at the end of the flow is lower. - It is clear that the increase in the temperature is suppressed to a minimum amount.

In addition, test numbers 1 to 6 of Examples and comparative example 2 (test number 8,
9), it is clear that if the timing of switching the gas blown from the inner pipe is at a time when the C# degree in the steel is 03% or more, the increase in the concentration of blown hydrogen can be suppressed. Furthermore, comparative example 3
(Experiment No. 10.11), when the gas blown from the inner tube is not switched from pure oxygen gas to a mixed gas of oxygen gas and inert gas at low carbon dioxide, the concentration of blown hydrogen increases significantly. It is clear that

In this way, the examples and comparative examples show that in order to suppress the increase in hydrogen concentration in steel, it is effective to blow inert gas together with oxygen from the inner pipe of the double pipe panel at the final stage of blowing. In that case, the inert gas to oxygen ratio is 0.6 or higher and the timing to start blowing (time to switch from pure oxygen gas)
It has become clear that the effect cannot be obtained unless the carbon concentration in the steel is 03 or below.

As described above, in the method of the present invention, in order to enhance the protective cooling effect of the tuyeres in a bottom-blown converter or a top-bottom-blown converter having double-pipe linings, the gap between the inner and outer tubes of the tuyere is When a hydrocarbon coolant is passed through the steel, it can effectively prevent the hydrogen concentration in the steel from increasing due to the hydrogen generated by the decomposition of the hydrocarbon coolant, and therefore blowholes occur when the molten steel solidifies. In addition to effectively preventing deterioration of the quality of steel slabs, there are also benefits such as lowering the hydrogen concentration in the steel and eliminating the need for descaling treatment after tapping, thereby reducing steel manufacturing costs. 474 will be added.

Applicant Kawasaki Steel Corporation

Claims (2)

[Claims] (1) Using a converter with a double-pipe tuyere installed below the molten steel level, refining gas is blown into the molten steel from the inner pipe of the double-pipe tuyere, and the inner and outer pipes are connected. In a converter refining method in which a hydrocarbon-based cooling medium for lining protection is flowed in between, pure oxygen gas is blown from the inner tube as the refining gas at the beginning of refining, and the molten steel carbon concentration in the latter half of refining is 0.3% or more. At the time of , the gas blown from the inner pipe is switched from pure oxygen gas to a mixture of oxygen gas and inert gas with a mixing ratio of inert gas to oxygen gas of 06 or more, and in this state, the refining is completed. A converter refining method characterized by the following. (2) The scope of the patent claims that the timing at which the gas blown from the inner tube is switched from pure oxygen gas to a mixed gas of oxygen gas and inert gas is set at the time when the molten steel carbon concentration 10゜ is below 08 degrees and above 03 degrees. Converter steel needle method as described in item 1.