JPH03180424A - Method for controlling end point carbon concentration for vacuum refining - Google Patents

Abstract

PURPOSE:To control the carbon concn. at the time when decarburization ends with high accuracy by determining the decarburization end point timing from the relation between the CO concn. and the decarburization time at the time of smelting a low- carbon steel by vacuum refining. CONSTITUTION:The correlations between the CO concn. and the decarburization time in exhaust gases are determined by each decarburization speed to determine the timing for the end point of the decarburization by simultaneously holding equations I, II, III, IV at the time of smelting the low-carbon steel of <=50ppm carbon concn. by vacuum refining. The end point carbon concn. corresponding to the fluctuation in the decarburization speed is controlled in accordance with this timing. In the equations I to IV, [C] denotes the carbon concn. (ppm) in steel; (t) denotes the decarburization time (min), Kc denotes a coefft. (1/min)) [C]0 denotes the initial carbon concn. in the steel; [C] denotes the decarburization quantity per minute; (x) denotes the amt. of the molten steel to be treated (ton); (CO) denotes the amt. of the CO generated (kg/hr); %CO denotes the CO concn. in the exhaust gases; Y denotes the amt. of the gas exclusive of the CO to be introduced into a vacuum system (kg/hr). The excessive decarburization treatment is averted in this way.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a method for controlling the carbon concentration at the end of decarburization when melting low carbon steel by vacuum refining, and in particular to a method for controlling the carbon concentration at the end of decarburization when the carbon concentration is 50 ppm or less. The present invention relates to an end point carbon concentration control method that can accurately control the carbon concentration at the end of decarburization when producing a certain low carbon steel.

[Conventional technology]

-i, when producing ultra-low carbon steel with a carbon concentration of 50 ppm or less, first perform a rough decarburization treatment in an atmospheric refining furnace, and then process RH, DH, LFV, VOD, etc. Decarburize to a specified carbon concentration in a vacuum smelting furnace.

In each vacuum smelting furnace, the following methods are used to speed up the decarburization process in vacuum smelting. R
) In I, the reflux flow rate of molten steel is increased by increasing the reflux Ar gas or increasing the immersion pipe diameter. DI speeds up the suction cycle. In addition, in LFV and VOD, the flow of molten steel is improved by increasing the amount of bottom-blown stirring gas.

In this way, even if high-speed decarburization processing is performed in vacuum refining, if the end point of decarburization processing (decarburization end point timing) cannot be accurately determined, excessive decarburization processing will be necessary. This is a hindrance to promoting rationalization. In addition, when trying to precisely control the deep drawability or strength of the product by precisely controlling the carbon concentration,
It is not enough to simply improve decarburization ability; highly accurate control of decarburization is required.

Under these circumstances, various control methods have been proposed as methods for controlling the carbon concentration of molten steel in vacuum refining. A typical control method will be briefly explained below.

The method disclosed in JP-A-1-222018 (hereinafter referred to as prior method 1) estimates the amount of carbon in molten steel based on the correlation between the CO gas concentration in the vacuum degassing tank and the amount of carbon in molten steel. This is a method to control the decarburization reaction. Furthermore, the method disclosed in JP-A No. 62-263916 (hereinafter referred to as prior method 2) quantifies the molten steel carbon content during the vacuum oxygen decarburization period and the vacuum degassing period as a function of operating conditions. The molten steel carbon content in the vacuum smelting furnace is accurately controlled by determining the appropriate end point of vacuum oxygen decarburization and vacuum degassing using the relationship. Also, JP-A-6
1-195913 (hereinafter referred to as prior method 3), the molten steel carbon content in each period of the vacuum oxygen decarburization period and the vacuum decarburization period of the vacuum smelting furnace is determined in advance as a function of operating conditions. This is quantified, and the appropriate end point of vacuum oxygen decarburization and vacuum decarburization is determined using this quantified relationship. Furthermore, the method disclosed in JP-A-59-185720 (hereinafter referred to as prior method 4) dynamically calculates the amount of carbon in molten steel based on information on exhaust gas in the exhaust duct during operation of the vacuum degassing equipment. The amount of carbon in the molten steel at a certain time is calculated by subtracting the cumulative amount of carbon transferred into the exhaust gas from the amount of carbon in the molten steel before the start of vacuum treatment. In addition, the method disclosed in JP-A No. 49-61013 (hereinafter referred to as prior method 5) is related to the production of standard stainless steel, and is based on the measured values of the degree of vacuum and exhaust gas components during blowing, and the overall decarburization oxygen efficiency. Find the correlation in advance with
The end point carbon content is estimated based on this correlation. Furthermore, the method disclosed in JP-A No. 49-61014 (hereinafter referred to as "prior method 6") is related to the production of ultra-low carbon stainless steel. The carbon concentration at the boundary with the low carbon region where the diffusion rate is rate-determining is defined as the critical carbon concentration, and this critical carbon concentration is determined by the molten steel conditions before blowing or the CO□ in the exhaust gas.
The oxygen injection time is calculated empirically from the concentration and the pressure inside the vacuum container, and from a formula created assuming that the subsequent decarburization rate is rate-limited by carbon diffusion.

[Problem to be solved by the invention]

Each of the methods described above has the following problems.

Prior method 1 is not effective in actual operation because the correlation between the CO gas concentration and the amount of carbon in molten steel changes if the operating conditions change. The previous method 2゜3 is a static control that does not incorporate changing conditions such as the CO gas concentration per hour during vacuum refining into the control conditions, and does not take into account the variations in the decarburization reaction in actual operation. Therefore, satisfactory end point control cannot be performed. In addition, in the preceding method 4, the end point carbon value can be controlled with relatively high accuracy due to dynamic control, but since the amount of carbon in molten steel is determined using an integral type, the accumulation of measurement errors is unavoidable. and lacks accuracy. Prior method 5 cannot be realized in processes where oxygen is not blown, and is limited to stainless steel. In the prior method 6, in actual refining, there is no sudden transition from the rate-limiting state of oxygen supply to the rate-limiting state of carbon diffusion, and it is not effective in actual operations.

As described above, there are various problems with conventional control methods, and no method has yet been proposed for controlling the carbon concentration at the end of decarburization so as to obtain satisfactory results during actual operation.

The present invention has been made in view of the above circumstances, and it is possible to control with high precision the carbon concentration at the end of decarburization when low carbon steel with a carbon concentration of 50 ppm or less is melted by vacuum refining. An object of the present invention is to provide a method for controlling end point carbon concentration in vacuum refining, which can avoid excessive decarburization and promote rationalization.

[Means to solve the problem]

The end point carbon concentration control method in vacuum refining according to the present invention is a method for controlling the carbon concentration at the end of decarburization when melting low carbon steel with a carbon concentration of 50 ppm or less by vacuum refining, By combining the following four equations, calculate the relationship between the degree of CO'a (XCO) and time t for each carbon concentration reached when the coefficient Kc fluctuates, and determine the decarburization end point timing based on this calculated relationship. The end point carbon concentration is controlled based on the determined timing.

[C] - [CF6 exp (-KcHt) However, [C]: Carbon concentration in m (ppm).

t: decarburization time (min), Kc: coefficient (1/min) [C].

: Initial carbon concentration in steel (ppm).

Δ[C]: Decarburization amount per valve (ppm/min).

X: Amount of molten steel processed (tons).

(Co): Amount of CO generated in steel (kg/hour).

(If Co is generated, convert it to the equivalent number of moles of CO and correct it.)

(XCO): CO concentration (%) in exhaust gas.

(Co□ is corrected by converting it to CO) Y: Amount of gas other than CO group introduced into the vacuum system into steel (kg
/hour) [Operation] In the end point carbon concentration control method of the present invention, in low carbon steel where the carbon concentration is 50 ppm or less, the basic equation of the decarburization reaction, which is assumed to be the following reaction, and the mass balance equation of CO are At the same time, the relationship between the CO concentration in the exhaust gas and the decarburization time is determined for each decarburization rate, and the end point carbon concentration is controlled based on the relationship between the two in response to fluctuations in the decarburization rate.

〔principle〕 The principle of the control method of the present invention will be explained below.

The decarburization reaction in the low carbon region (carbon concentration of about 400 ppm or less) apparently follows the -order reaction formula, so the following (1
) is admonished.

however, [C]: Carbon concentration in steel (ppm).

t: decarburization time (min), t: coefficient (1/min).

Furthermore, in vacuum refining, the inflow and outflow of gas is strictly controlled, and accurate information on exhaust gas can be obtained.

OverviewIf the amount of coal is the same, the total amount of CO generated (COz
If both occur at the same time, the generated Co□ is the current molar C
Convert to O> is also the same. When the decarburization rate is high, a large amount of CO is generated initially, and the amount of CO generated rapidly decreases as the carbon concentration decreases. On the other hand, when the decarburization rate is low, the initial ρCO generation decreases, but
Since the total amount of CO generated is the same, the amount of CO generated after a certain period of time is rather larger.

As described above, since the relationship between time and the amount of CO generated changes depending on the decarburization rate, this relationship is utilized in the present invention to estimate the carbon concentration in molten steel.

Solving the differential equation (11) above, [C at t=Q
Set the initial value of I to [CI. Then, the following equation (2) is obtained.

[C.I. : Initial carbon concentration in steel (ppm) where (11
, (Since the variables in the formula 2+ are Kc, t, and [CI, the relationship between B and [CI is calculated by setting . Since the amount Δ[CI is also calculated for each set Kc,
According to the following formula (3), the amount of CO generated per unit time, [
C] o is required.

FIL, Δ[CI: Amount of decarburization per needle (ppm/min) X: Amount of molten steel processed (tons) (Co): Amount of CO generated in steel (kg/hour).

(If Co2 is generated, it is corrected by converting it into an equimolar number of CO.) Therefore, for each set Kc, the CO concentration (XCO) corresponding to time t is also calculated in (4) 2 below.

However, (XCO): CO concentration (%) in exhaust gas (COX is C
(corrected by converting into O) ゛t: Amount of COO40 gas introduced into the vacuum system into steel (k
g/hour) Here, initial (I!\[C1o = 300 (ppm), amount of processed molten steel X = 275 ()n), COO40 gas amount Y =
1580 (kg/hour) is substituted as the actual value of R1t processing. And Kc=0.1.0.2.0.3 (
Fig. 1 shows the relationship between the decarburization time and the carbon content [CI] and the CO concentration (XCO) in the exhaust gas when the decarburization time is 1/min). Figure 1(a) shows the decarburization time t (min) and the carbon content [CI
(ppm), and FIG. 1(b) shows the relationship between decarburization time L (min) and co? in exhaust gas. fi degree (XCO)
(%).

When the decarburization rate (coefficient Kc) is high, the exhaust gas CO concentration becomes small after 6 minutes from the start of the decarburization process, as can be seen from the graph in FIG. 1(b).

Carbon M at 6 minutes from the start of decarburization [the lowest value of CI is 5
0 ppm (see Fig. 1 f8)), the carbon concentration of the low carbon steel targeted in the present invention is 50 ppm or less.

In Fig. 1(a), [C] for each Kc = 20 ppm
Find the time t to reach , and calculate the co? in the exhaust gas for each Kc corresponding to this time t. Find the fi degree. Then, in Figure 1 (bl), after 6 minutes, the larger the coefficient, that is, the faster EC] reaches a low value, the faster (XCO
) is small, so after 6 minutes from the start of decarburization, K
, [C'l ≦20 ppm is always held below the dashed-dotted line connecting the [CI = 20 ppm reaching points]. Therefore, after 6 minutes from the start of decarburization, that is, [CI
In the range ≦50ppm, see Figure 1 (bl or Figure 1f).
− point chain 1 in FIG. 1(C) which is a partially enlarged view of b)
When reaching the region below mA, [CI-20pp
It may be determined that m has been reached.

As described above, in the region of [C'2 ≦50 ppm, the time point at which C'2 in the exhaust gas reaches
It can be accurately determined based on the O concentration (XCO) and the decarburization time t.

〔Example〕

Examples of the present invention will be described below.

FIG. 2 is a schematic diagram showing the implementation state of the control method of the present invention in the RH method, and in the figure, 1 indicates a ladle in which molten steel 11 is accommodated. Above the ladle 1 are two wooden dip tubes 2a, 2.
An RH vacuum tank 2 is provided, and the tips of two immersion tubes 2a and 2b are immersed in the molten liquid in the ladle 1. Further, a gas supply pipe 3 for introducing reflux ^r gas into the pipe is provided in one of the immersion pipes 2a. The R1+ vacuum tank 2 is connected to a vacuum evacuation device 5 via an exhaust pipe 4, and the inside of the tank is maintained in a vacuum state. In the middle of the exhaust pipe 4, CO+ COz 'ly in the exhaust gas is
i degrees and other gases (Hz, 0□+ NZ+
An infrared spectrometer (or mass spectrometer) 6 is installed to measure the concentration of Ar, etc.). Further, a timer (not shown) is provided to measure the decarburization time from the start of circulation of the molten n.

Then, ^r gas is blown into the chaste pipe 2a to circulate the molten steel 11 in the ladle 1 as shown by the arrow using the principle of a gas slit pump. 11 is brought into contact with vacuum to perform vacuum refining. In this example, the processing conditions are: processing melt lff1 = 250 to 270 tons.

The diameter of the immersion tubes 2a and 2b was 750 m, the reflux Ar gas was 2000 to 250 ON#/min, and the control value of the carbon concentration at the end of decarburization was 15 ppm.

The relationship between time t and the carbon concentration [C] in the steel and the decarburization amount Δ "C" per unit time are calculated for each coefficient KC using the formulas (11 and (21) above, and the calculated Δ[C] Using equation (3) above, CO generation per unit time l, [C
]o is obtained, and then using the obtained [C]o'c-, CO? in the exhaust gas is determined by the above equation (4). Calculate W degrees (ZCO).

Similarly to the case explained in "Principle", t is calculated for each coefficient Kc.
Find the relationship between and [C] and t and (XCO), [C]
= 15 ppm is determined in advance as shown in FIG. 1(b) (or (C)).

Then, vacuum refining is performed on the molten steel 11 while calculating the < end.

The distribution of carbon concentration [C] at the end point (decarburization end point) in the example of the present invention, which was conducted in this way, is shown in FIG. Third
In the figure, the horizontal axis shows the carbon concentration (ppm) after the decarburization process, and the vertical axis shows the frequency of molten steel (%), and the area surrounded by the large frame is the example of the present invention.

Note that FIG. 2 also shows the results of a comparative example in which the refining conditions were the same and the carbon concentration at the end point was similarly controlled to 15 ppm, and the patched portion is the comparative example.

Setting of the expected time to reach the end point in this comparative example will be explained. When the above equation (2) is solved for time t, the following equation (5) is obtained.

t= 12n([C ko/ [C
] ) −(5)c Under the refining conditions described above, the average value of the coefficient Kc is 0.22 (1/min), so K is added to the above equation (5).
c = 0.22. By substituting [C] = 15 ppm, the following equation (6) is obtained.

Various initial values [C] based on the above formula (6). When the expected arrival time t is calculated for [C]. (ppm) =
200, 250.300.350.400.450, t (minutes) = 11.8. 12.8. 1
3.6.14.3.14.9. It becomes 15.5. Therefore, in this comparative example, the initial value is [C]. The expected time to reach the end point was set according to the range of , as shown in Table 1 below.

Table 1 Comparing the inventive example and the comparative example, it is found that in the comparative example, control was possible only within the range of +13 to -5 ppm due to the variation in the coefficient Kc, but in the inventive example, control was possible only in the range of +13 to -5 ppm. Since the end point is determined accordingly, it is possible to control the target value of 15 ppm to a range of +3 to -5 ppm.

In this embodiment, vacuum refining using the R11 method has been described, but it goes without saying that the control method of the present invention can also be applied to vacuum refining using other methods such as DH, LFV, and VOD.

〔Effect of the invention〕

As detailed above, in the control method of the present invention, when producing low carbon steel with a carbon content of 50 ppm or less using vacuum refining, the carbon concentration after the completion of decarburization can be adjusted with extremely high precision. As a result, excessive decarburization can be avoided and rationalization can be promoted.

[Brief explanation of drawings]

FIG. 1 is a graph for explaining the principle of the control method according to the present invention, and FIG. 2 is a graph for explaining the principle of the control method according to the present invention.
FIG. 3, which is a schematic diagram of the H vacuum refining equipment, is a graph showing the carbon concentration distribution after the RH treatment in the example of the present invention and the conventional example.

Claims (1)

[Claims] 1. A method for controlling the carbon concentration at the end of decarburization when melting low carbon steel with a carbon concentration of 50 ppm or less by vacuum refining, the method comprising: Then, calculate the relationship between the CO concentration (CO) and time t for each carbon concentration reached when the coefficient K_c fluctuates, determine the decarburization end point timing based on this calculated relationship, and use this determined timing to determine the decarburization end point timing. A method for controlling end point carbon concentration in vacuum refining, the method comprising controlling end point carbon concentration. -d[C]/dt=K_c・[C] [C]=[C]_oexp(-K_c・t)(CO)=
Δ[C]×(1/1000)×(28/12)×60×
x(%CO)={(CO)/[(CO)+Y]}×10
0 However, [C]: Carbon concentration in steel (ppm), t: Decarburization time (minutes), K_c: Coefficient (1/minute), [C]
_o: Initial carbon concentration in steel (ppm), Δ[C]: Amount of decarburization per minute (ppm/min), x: Amount of molten steel processed (tons), (CO): Amount of CO generated per hour (kg/hour), (
If CO_2 is generated, it is corrected by converting it into an equimolar number of CO), (%CO): CO concentration in exhaust gas (%), (CO_2
is corrected by converting it to CO) Y: Amount of gas other than CO introduced into the vacuum system per hour (kg/hour)