JPH02107714A - Production of low-carbon steel by smelting - Google Patents

Abstract

PURPOSE:To efficiently reduce the carbon content to a desired value in a short time by precisely discriminating the time when the carbon content is reduced to the desired value from the initial carbon content and decarburizing rate at the time of producing an extremely low carbon steel from molten steel by vacuum degassing equipment. CONSTITUTION:Molten steel is decarburized by an RH-type vacuum degassing equipment to obtain an extremely low carbon steel contg. <30ppm C. In this case, the C content is analyzed when the raw molten steel is charged into the equipment. The C content in the molten steel sample during decarburization is then analyzed, and the decarburizing rate in the low-carbon region by the equipment is estimated from both results of analysis. The CO in exhaust gas when the molten steel is decarburized to a desired value is obtained by analysis from the decarburizing rate and the measured C content in the exhaust gas. As a result, the molten steel is precisely decarburized to a desired C content in a short time, the O2 content in the decarburized molten steel is adjusted to 150-500ppm, and an extremely low carbon steel appropriate for a continuously annealed steel sheet for deep drawing is stably produced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for stably and inexpensively producing ultra-low carbon steel suitable for continuous annealing steel sheets for deep drawing, electrical steel sheets, and the like.

(Prior Art) Generally, molten steel melted and refined in the atmosphere, such as in a converter or an electric furnace, is contaminated by gas components such as H, N, and O.

As demands for steel quality have increased, various steel-making technologies have been developed and improved.Recently, converters, electric furnaces, and other furnaces are used as melting-only furnaces for special refining processes that require high quality. is a smelting method using another suitable equipment,
The so-called out-of-furnace refining method is the mainstream.

Typical methods for out-of-furnace refining include the R1+ degassing method and the DH skin degassing method.

The principle of the RH degassing method is as follows. That is, ffj
In a vacuum vessel with two legs for suction and discharge, the two wooden legs are immersed in the bay steel in the ladle and the vessel is evacuated, allowing the molten steel to rise inside the vacuum vessel. come. What if we blow Ar gas etc. into the riser pipe here? The apparent specific gravity of the 8 steel becomes smaller, and the 8 steel in the upper tube rises and is sent into the vacuum chamber.

The molten steel is degassed in the vacuum chamber, and the degassed molten steel descends into the ladle. The above is the principle of the RH skin degassing method.

On the other hand, the principle of the DH degassing method is that, unlike the RH skin degassing method, molten steel is sucked up and discharged using one leg.

When the legs are immersed in the /8 wire in the ladle and the pressure inside the vacuum chamber is reduced,
18 tl rises through the vacuum chamber to a height equivalent to atmospheric pressure. When the ladle is then raised or the vacuum chamber is lowered, the surface of the molten steel descends within the vacuum chamber by the same height. By performing this raising and lowering 3 to 4 times per minute, the /8 steel is processed in the vacuum chamber. The above is the principle of the D11 degassing method.

By the way, using this degassing method, the carbon content can be reduced to 3.
Various methods have been attempted to produce steel with a concentration of 0 ppm or less at high speed.For example, (i) the fli1 degassing method has 1. A method of promoting and strengthening the large circulation flow rate by increasing the diameter of the immersion tube, and improving the vacuum pattern in the vacuum chamber by increasing the capacity of the exhaust ejector and booster ("Tetsu to Hagane J 73 (1)
987), 5939 (P2O3)), and (ii) the DH degassing method, a method of speeding up the suction cycle of a vacuum chamber using a vacuum pump has been carried out.

Furthermore, in the VOD method, in which molten steel is decarburized using a 02 blowing lance while blowing Ar gas from the bottom of the ladle in a vacuum vessel, decarburization is achieved by intensifying the stirring of the molten steel by blowing meter gas from the bottom of the ladle. Engineers are finding ways to increase the speed of coal.

These methods have certainly increased the decarburization rate.

(Problem to be solved by the invention) By the way, this decarburization rate Kc (1/m1n) is calculated by the following formula % formula % (]) where [clo': initial [CI 'a degree (p
pm) IC1: [CI 4 degrees (ppm) at time t
L is defined as two hours (min). In general, til + degassing degassing -
It is known that Kc is in the range of 0.1 to 0.35 (1/m1n). On the other hand, as shown in Fig. 8, the exhaust gas balance can be calculated for each value of the decarburization rate Kc, and for each Kc, the CO concentration (%) can be estimated from the CI concentration. Can be done. This estimation result is
Shown in the table.

Table 1 Here, as mentioned above, the value of Kc is 0.1 to 0.35 (
In general, [CI
= around 20 (ppm), Kc = 0.10 (1/m1
n), and a method has been used in which decarburization is completed when the COa degree reaches 4 to 5%.

However, as mentioned above, when decarburization progresses at high speed due to the recent development of steelmaking technology, for example, Kc-0.20 (1
/m1n), decarburization ended when the CO concentration reached 4%, as shown in Figure 9 [CI<30
Even after achieving ppm, decarburization continues for about 10 minutes, and [C
Excessive decarburization will be performed to around I'=IOppm. In this case, although the decarburization reaction became faster,
This does not lead to a reduction in processing time. In other words, even though the decarburization rate was improved, the timing of decarburization completion was not properly determined, so it was not possible to achieve the effect of shortening the processing time by increasing the decarburization rate. be.

The purpose of the present invention is therefore to develop a method for melting low-carbon steel that can maximize the effects of high-speed decarburization and shorten processing time by appropriately determining the end time of decarburization. Our goal is to provide manufacturing methods.

(Means for Solving the Problems) As a result of various studies to solve the above problems, the inventor calculated the decarburization speed in low and medium carbon loading based on the decarburization speed Kc in the high carbon content region. The present invention was completed based on the knowledge that it is possible to accurately determine the end of decarburization from the calculated value of the decarburization rate in this low carbon content region.

The gist of the present invention is that the carbon content is 30
ppm or less in a vacuum processing furnace, (i) Calculate the decarburization rate by measuring the carbon content in the molten steel before and during the decarburization process. , the process of estimating the decarburization rate in the low carbon content range from this calculated value, and (ii) the decarburization rate from the decarburization rate in the low carbon content range obtained in step (i) and the carbon content in the molten steel. The process of determining CO4 degrees in the exhaust gas at the end of the process, and (ij) (i
A method for producing low-carbon steel, comprising: a step of determining the fertilization at the end of decarburization based on the cofR degree determined in step i);

That is, from the results of measuring the carbon content before the start of decarburization and during the decarburization treatment, the decarburization rate during this period, in other words, in the high carbon content range is calculated, and from this calculated value, the decarburization rate in the low carbon content range is calculated. Estimate speed. This is a low carbon steel melting method that accurately determines the end of decarburization based on the decarburization rate in this low carbon content range and the carbon content in molten steel.

(Function) The present invention will be explained in detail together with its function and effects. In this specification, "%" means "volume %" unless otherwise specified.

Figures 1 and 2 show the high carbon N region (30 to 200 ppm
) and low/medium carbon mounting (15-30.20-5 oppH)
This shows the relationship between the decarburization rate and the decarburization rate. As is clear from FIGS. 1 and 2, if decarburization progresses at high speed in the high carbon content region, it was also found that decarburization progresses at high speed in low and medium carbon loading.

In other words, if we find the decarburization rate Kc in the high carbon content region,
It is possible to predict the decarburization rate in low- and medium-carbon installations. In other words, by measuring the [CI value during decarburization, the decarburization rate in the high carbon content region from the start of decarburization to the time of [CI value measurement] can be determined from the relationship with the initial [CI value]. Based on the decarburization rate in the second high carbon content region, it is possible to estimate the decarburization rate for low and medium carbon loading.

Therefore, it is possible to predict the decarburization trajectory of low and medium carbon loading during decarburization in the high carbon content region.

More specifically, the reason why the CI value is measured once before and during the decarburization process is as follows.

In other words, in RH degassing treatment, etc., the decarburization rate Kc value varies, so unless you accurately know the Kc value in the high carbon content range, it is difficult to accurately predict the Kc value in the low carbon content range. Although I am unable to do so.

The rCI value before decarburization is determined by (1) using the fcI analysis value at the bottom of the converter after tapping the steel, and (ii) using the [CI] value when the molten steel arrives at the RH degasser.
Using analytical values (iii) When the molten steel arrives at the IIH degasser, the amount of residual oxygen in the T8 steel is measured with an oxygen sensor [CI 4M
Any one of the three methods of measuring .

Furthermore, the measurement of the CI value during the decarburization process is carried out by (1) collecting a sample and analyzing the CI value. However, it is preferable that the sampling time be between 50% and 70% of the expected decarburization time. This is because if it is less than 50%, there is a risk that the estimation accuracy of [cLa and [CI] are close <Kc in equation (1) will decrease, and if it exceeds 70%, the analysis time will take 3 to 4 minutes. This is because there is a risk that the end point determination timing may be missed.

(11) Depending on the total amount of exhaust gas and changes in CO gas over time,
This is done by integrating the total amount of CO generated and estimating the fcI value during treatment from the CI value before decarburization.

Any of the above methods) and (ii) may be used.

Next, the reason why the end point is estimated based on the [CI value, predicted Kc value, and exhaust gas component value during the decarburization process] will be explained. In the conventional method, the end point of decarburization was determined by fixing the end point when the amount of CO reached 4 to 5%, and therefore, when the decarburization rate was high, overaction, that is, decarburization proceeded excessively. Therefore, in order to determine the behavior in the high carbon content region and make an appropriate end point determination, it is necessary to determine Kc from the CI value during processing and to determine the end point while referring to the exhaust gas component value, which is exhaust gas information.

In other words, since the Kc value in the low carbon content range can be predicted with high accuracy, it is possible to use Table 1 and fix the decarburization end time, such as when the CO content drops from 4% to 5%, as in the conventional method. First, decarburization ends when the amount of CO decreases from 6% to 8% (Kc
= 0.15 to 0.20).

As detailed above, the [C1 value during decarburization is measured, and the trajectory of decarburization below the [CI value] and [C1'r20p
By the melting method according to the present invention that predicts the CO gas concentration around pm, it is possible to accurately determine the end time of decarburization during the melting of low carbon steel.

By the way, in the method for producing low carbon steel according to the present invention, it is more effective to adjust the amount of dissolved oxygen in the molten steel (hereinafter referred to as "a, j") at the end of decarburization to 150 ppm or more and 500 ppm or less. The reason will be explained in detail.

(2) Reason for a, ≧150 ppm Since the decarburization reaction proceeds as follows: [CI + [O] → co (g), if ao is significantly lowered, there is a risk that the decarburization rate will decrease. For example, the amount of refluxed Ar gas is 30
00 N j2/min, and the immersion pipe diameter is 660 mm.
Figure 3 shows the results of changing ao in the 1-11 degasser. As is clear from Figure 3, a. is 15O
In the pp field and below, the decarburization rate decreased significantly from around CI -50 to 7 oppm, resulting in poor decarburization. For this reason a
It is effective to control the amount to 0≧150 ppn+ and determine the end time of decarburization.

■Reason for a0≦500ppm There are two purposes: adjusting i8 M m (hereinafter simply referred to as rsol [ΔQIJ) and reducing the amount of AQ used, and we will separate them from each other.

(i) Since the he Q yield is relatively stable when the three-period adjustment of sol[△Q] is a0≦500 ppm, the AQ for deoxidizing and sol[△Q1 is based on the 80 measurement value of the oxygen sensor. Adjustment can be made by simply adding -degrees, but if ao>500ρpI11, the AQ yield will not be stable and -degrees a. After deoxidizing with a target of ≦500 ppm, a. This is because the time for the RH degassing treatment may be extended by 3 to 4 minutes because it is necessary to measure the amount of I2 and then add 8 I2. Therefore, it is effective that 80≦500 ppm.

To (11), in addition to the reason for Q consumption reduction (1), deoxidation is as follows: 2[8Q] + 3+0] → (AQzo
*) The reaction proceeds as follows. If ao is extremely high, the unit consumption will increase, leading to an increase in processing cost. Figure 4 shows a at the end of decarburization. A graph showing the relationship between Tohe and Q basic unit is shown. Therefore, it was set as a0≦500 ppm.

Next, the present invention will be explained in detail using examples. Note that this is an illustration of the present invention, and the present invention is not unduly limited thereby.

Example 1 a at the end of decarburization. 150 to 500 ppm, same as the conventional method (when decarburization is completed at CO = 4% and when decarburization is started at 7
A sample was taken at ~8 minutes and compared when the method according to the invention was implemented.

That is, using an RH degassing device in which the diameter of the immersion tube is 660 mm and the amount of refluxed Ar gas is 3000 Nf/min, the
~280-410 ppm)? 8 steel [C1<3
Decarburization was achieved to 0 ppm.

In other words, if the target [CI = 20 ppm building, the second
Using the diagram, adopt the lower limit line of KC5° - 2° and Kcs
o, 211 = 0.50XKczoo+*o H・
・It was set as (A).

Initial [C) a - Sampling was performed at 400 ppm for 7 minutes and found that [C1 = 45 ppI11].

Considering this Kc=0.312 as KCZOO+ffO, 8) Kcso from 2, zo=0.50xO, 312=0
．． 156 (1/mtn) (i) was obtained. According to Table 1, [CI = 20ppm, = 0.156, so it is the intermediate value between 0.1 and 0.2, that is, (4,4+8
．． 2) x+A””6.3, GO=6.3% and [C
It was predicted that I would reach 20 ppm, and the timing for stopping decarburization and adding Q to C0 was set at 6%.

On the other hand, in the case of the target [CI = 15 ppm, prediction was made in the same manner as above using FIG. In this case, the formula corresponding to the above formula (A) was as follows.

Kcffo-++s = 0.35XKczoo-+, o
...' (B) Figure 5 shows the distribution and average value of decarburization time.

When decarburizing was performed by the method according to the present invention, the decarburizing treatment time was shortened by an average of 5.1 minutes (16.7 minutes → 11.6 minutes).

Example 2 A sample for CI analysis was taken 8 minutes after the start of decarburization, and the influence of 80 when the decarburization method according to the present invention was performed was divided into the following cases (1) and (2) and compared.

■If a6<150 +)P+ll and ao =
150 to 500 ppm Similar to Example 1, the diameter of the immersion tube was 6 chm, and the reflux A
250 to 270 tons of molten steel was treated using an RH degassing device with r gas of 3000 Nf/min. 6th
The results are shown in Figure (a) and Figure 6 (c).

From Figure 6(a) and the 6th factor), (i) If ao>150 PPT1 [CI! =
The decarburization rate becomes low from around 50 to 70 ppm, and 18
(ii) In the case of ao-150 to soo PfHl, the decarburization was smooth and [CI < 30 ppm was reached in about 10 minutes. (iii) The method according to the present invention ~1
It is clear that decarburization could be completed in 5 minutes.

■When ao > 500 ppm and ao = 150
Comparison with the case of ~500 ppm 250 to 280 tons of molten steel was treated using R1+ similar to (2). The decarburization time was 12.0 min on average (n
= 8ch) ao = 150 to soo ppni: Average time was 11.6 minutes (n = 18ch), which was within the range of operational variation and there was no difference.

On the other hand, the components were adjusted by performing deoxidation and sol [AQI adjustment, and then [Til adjustment].

The distribution and average value of component adjustment times in this case are shown in FIG.

As is clear from FIG. 7, when a0≦500 ppm, deoxidizing 10S. L[8Q] Since 3Pl adjustment can be performed by adding AQ at once, the component adjustment time is reduced by 2°3 minutes (6,8 minutes → 4,5 minutes) compared to the case of >500 ppm, and R
It can contribute to speeding up the H degassing process.

(Effects of the Invention) According to the present invention, it is possible to provide a method for determining the optimal decarburization end time, and it has become possible to reduce the processing time required for decarburization.

Furthermore, by limiting the amount of dissolved oxygen in the molten steel at the end of decarburization to an appropriate range, it has become possible to perform high-speed decarburization even more easily.

The practical significance of the present invention having such effects is extremely significant.

[Brief explanation of the drawing]

Figure 1 shows the high carbon content region ([CI = 30-200 pp
Figure 2 is a graph showing the relationship between the decarburization rate Kc in the high carbon content range ([CI = 30 to 200 ppm) and the low carbon content range ([CI = 15 to 30 ppm)];
Figure 3 is a graph showing the relationship between 80 and [CI] at the end of decarburization. The figure shows 80 at the end of decarburization and (Decarburization 1S., [l'l
A galaf representing the relationship between the basic units of AQ for QI; Figure 5 is
A graph showing the decarburization time between the melting method according to the present invention and the conventional method; FIGS. 6(a) and 6(b) are graphs showing the relationship between changes in decarburization behavior due to changes in ao; The figure is a graph showing the relationship between a0 at the end of decarburization and component adjustment time: Figure 8 shows l? A graph showing the relationship between changes in decarburization behavior due to speeding up of the l+ treatment method; and FIG. 9 is a graph showing the gas balance in the R1+ treatment method.

Claims (2)

[Claims] (1) When melting steel with a carbon content of 30 ppm or less in a vacuum processing furnace, (i) measuring the carbon content in the molten steel before and during decarburization; A process of calculating the decarburization rate and estimating the decarburization rate in the low carbon content range from this calculated value, and (ii) calculating the decarburization rate in the low carbon content range determined in step (i) and the carbon content in the molten steel. (iii) determining the end time of decarburization based on the CO concentration determined in step (ii); Steel melting method. (2) The method for producing low carbon steel according to claim (1), further comprising adjusting the amount of dissolved oxygen in the molten steel at the end of the decarburization treatment to within a range of 150 ppm to 500 ppm. Manufacturing method.