JPS63190113A - Production of dead low carbon steel - Google Patents

Abstract

PURPOSE:To reach carbon content in steel to dead low carbon range in a short time and to economically and effectively execute decarbonizing treatment by controlling dissolved oxygen in the steel in the prescribed range at the time of decarbonized-treating the molten steel in a ladle after tapping off from a converter under reducing pressure. CONSTITUTION:The non-deoxidized molten steel, tapped off from the converter is decarbonized-treated under reducing pressure favorably in thermodynamics by RH, DH, etc. Then, the dissolved oxygen in the steel is controlled to >=400ppm during refining from start of the treatment until reaching 30ppm carbon concn. in the steel. Or, in addition, further in the range of <30ppm carbon content, deoxidizing agent is charged in a lump. And, the dissolved oxygen in the steel is made 50-200ppm. Next, by further treating, the deoxidation is completely executed.

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a method for decarburizing molten steel in a ladle after being tapped in a converter under reduced pressure. (Prior Art) In order to improve workability such as deep drawability and stretchability of hot-cold rolled steel sheets for automobiles, it is necessary to reduce the (C) concentration in the steel material as much as possible. In order to meet this need, in the steelmaking process, various treatment devices are used to reduce blow-out (C) in the converter and decarburize under reduced pressure in the secondary refining process (see 3rd Edition Iron and Steel Handbook ■ Pigmaking 111671-685). ) has been dealt with. In particular, in processes that utilize the thermodynamically advantageous decarburization process under reduced pressure such as RH, 011, it has been possible to gradually achieve (C) after treatment by increasing the recirculation flow rate and improving the degree of vacuum. Although it has decreased (C) 20 ppm or less, the normal treatment is 15~
This cannot be achieved easily in about 20 minutes. (Problems to be Solved by the Invention) The present invention provides a method for decarburizing under reduced pressure that can eliminate the above-mentioned limitations of the conventional decarburization process under reduced pressure in secondary refining and can reach an extremely low carbon range in a short time. The purpose is to provide. (Means for solving the problems) The gist of the present invention is as follows. (1) Undeoxidized molten steel tapped from the converter is subjected to RH and DH
When decarburizing under reduced pressure such as
Dissolved oxygen in steel until reaching 0 ppm

[0] f to 40
A method for producing ultra-low carbon steel, characterized by controlling the carbon content to 0 ppm or more. (2) Undeoxidized molten steel tapped from the converter is subjected to RH and DH
When decarburizing under reduced pressure such as
Until reaching 0 ppm, the dissolved oxygen (O)f in the steel was reduced to 40
Control to 0 ppn+ or higher, then (C) 30ppm
An ultra-low carbon steel characterized by weakly deoxidizing the dissolved oxygen (O)f in the steel to 50 to 200 ppn+ by adding a deoxidizing agent all at once in a region of less than manufacturing method. (3) Undeoxidized molten steel tapped from the converter is subjected to RH and DH
When decarburizing under reduced pressure such as
Dissolved oxygen in the network until reaching 0ppm+

[0] f to 4
00 ppm or more, and then (C) 30 ppm
After weakly deoxidizing the dissolved oxygen (O)f in the steel to 50 to 200 ppn+ by adding a deoxidizing agent continuously or intermittently to a predetermined basic unit according to the RH reflux amount in the region below, further processing. A method for producing ultra-low carbon steel, which is characterized by completely deoxidizing the steel. (4) Ti or Al can be used as a weak deoxidizing agent.
3. The method for producing ultra-low carbon steel according to item 2 or 3, characterized in that the method uses the following. (5) The method for producing ultra-low carbon steel according to item 3 or 4, wherein the weak deoxidation is performed by partially deoxidizing with Al and then deoxidizing with Ti. The present invention will be specifically explained below. The present inventors mainly used RH to investigate and study various factors that affect decarburization behavior in order to improve the ability of the decarburization process under reduced pressure. As a result, as shown in Fig. 1, the decarburization transition under reduced pressure can be divided into three regions: the large decarburization rate region, the small decarburization speed region, and the small region. Low decarburization rate caused by! The following factors were found to affect the decarburization rate in the ■ area and ■ area, excluding the area. Although it is well known that the decarburization rate in the 193 region up to about 30 ppm increases due to an increase in the recirculation amount, etc., the present inventors have shown in Figure 2 regarding the decarburization in the II 6i region. Perfect as shown

We found that [0]1 exists. In FIG. 2, C is the decarburization rate constant (1/a+in), which is obtained from equation (1). -d (C) --Kc (c) ...... (1) t In other words, basically more than the (C) equivalent of the decarburization reaction

The amount [0] is necessary, and the experimental results of the present inventors show that if the pre-treatment (C) is in the range of about 150 to 350 ppm, the optimum (0) is 400 ppm or more. Moreover,

As a control method for [0], controlling the converter blow-off (0), oxygen blowing (OB) before or immediately after the start of treatment, or adding oxygen by adding iron oxide, etc. is effective, but approximately Oxygen addition by OB etc. after 5 minutes is optimal [0
], it has become clear that it is either ineffective or even has the opposite effect. In addition, the decarburization rate in the low (C) region of less than 30 ppm + (0) shown in Japanese Patent Application No. 82239/1980 was also found in the inventors' actual experiments. As shown in Figure 3, weak deoxidation with M etc.
It was found that the decarburization rate was the highest when (o)1 was 0 to 200 ppm. Furthermore, as a result of experiments by the present inventors, the (O)f in the
When weakly deoxidized to 200 ppm and controlled, as shown in Figure 4, the

It has become clear that the effect is large when [0] is a high value of 400 ppm or more, and that even if weak deoxidation is performed from less than 400 ppm, the effect is small. Therefore, considering the entire n, mjJl range, the

[0]
, is preferably 400 ppm or more. In addition, when Ti, which has weak deoxidizing power, is used as a weak deoxidizing agent that performs weak deoxidizing,
It has also become clear that it is more effective than weak deoxidizing agents such as M. As a method of weak deoxidation, a method of partially deoxidizing with Al and then deoxidizing with Ti is also effective. In addition, in the I95 area

[0], as a control, O during processing
The reason why B has no effect, or rather has the opposite effect, is that an FjO film is formed on the free surface of molten steel that inhibits the C-O reaction, and
This is thought to be because it takes a certain amount of time for the O film to disappear. ■Ideal for decarburization in the area

[0], as range

The reason why there is an upper limit of [0] is excessive.

This is thought to be because [0] acts as a surface active element and inhibits the decarburization reaction. In addition, in the area

The reason why it is effective in controlling the weak deoxidation of [0] when (0)f before weak deoxidation is as high as 400 ppm or more is because the weak deoxidation product acts as a CO bubble generating color. This is thought to be due to the fact that the more deoxidized products there are, the more effective it is. Furthermore, the reason why Ti is more effective than M etc. as a deoxidizing agent used for weak deoxidation control is that when using M etc. with strong deoxidizing power,

[0], is 50-20
At 0ppa+, no M remains in the steel, but in the case of Ti, the equilibrium value recommended by equations (2) and (3) (1982
According to the 19th Steelmaking Committee of the Japan Society for the Promotion of Science (published in January), it is optimal because it exists at about 30 to 290 ppm.

[0], even within the range of surface active elements.

It is thought that this is because [0] has the effect of preventing enrichment on the free surface of molten steel to some extent, promoting the decarburization reaction. art+50-TisOs(S)...(2)log
K=log a'yiao'--89300/T+30
．． 30...(3) Also, when adding a deoxidizing agent for weak deoxidation all at once into the vacuum chamber, immediately after addition, the molten steel in the vacuum chamber is strongly deoxidized locally and the deoxidizing agent is removed. As the molten steel in the ladle is gradually and uniformly mixed from an undeoxidized state, the whole becomes weakly deoxidized steel, but during this time the decarburization reaction does not progress sufficiently, so the specified unit consumption is adjusted according to the molten steel circulation flow rate. If the deoxidizing agent is added continuously or intermittently, for example in 2 to 10 portions, so that This is effective because it starts immediately after adding the deoxidizing agent. As a formula for expressing the molten steel recirculation flow rate, the following formula is proposed as a result of an experiment at actual temperature in Tetsu to Hagane, 54 (1968) p. 1342.Based on this experimental formula, the molten steel recirculation flow rate can be changed during processing. be able to. W 0.02D'-'α0.33 ・・・・・・(
4) Here, W: Molten steel recirculation flow rate (t/win) D: Riser pipe inner diameter (c+*) G@: Circulation Ar gas flow rate (NZ/m1n) (action) II In the 9M region

[0], low (C) in a short time by control
area is reached, and furthermore, the molten steel free surface of the area

[0],
Control and 7igQ3. Due to the generation of oxides such as AlB12, the decarburization rate decreases in the low (C) range, and as a deoxidation process under reduced pressure, low (C) of 20 ppm or less can be achieved in a short time.
It becomes possible to manufacture molten steel. (Examples) Hereinafter, the present invention will be further explained based on Examples. FIG. 5 shows the test results at RH for the invention examples (■, ■) and the comparative example (■). Both the invention examples (■, ■) and the comparative example ■ are examples in which IT+ control is not performed due to OB etc. at RH (0), and in the case of the comparative example ■,

[0] is less than 400 ppm in the ■ area, and in the ■ area

[0] The result is when f is 300 ppm or more, and the (C) level reached is as high as 20 to 25 ppm after a processing time of 15 to 20 minutes. On the other hand, in the case of the present invention example (■), (O)f in the area (■) is 510 to 5
50 ppm, 10 minutes after starting treatment (C) 22 ppm
When reaching m, Al is added all at once to deoxidize and reduce (O)f to 1
After setting it to 65ppn+, for 6 minutes

[0] 1 to 150 to 16
When treated at 5 ppm, the decarburization rate in the ■ region is large, and the decarburization rate in the IIIfil region is small, and the decarburization rate is large compared to the comparative example ■ (as a result of 16 minutes of treatment).
C) 1'6 ppm was obtained. Furthermore, in the case of the present invention example (■), (O)f in the area (■) is 630~
(C) 19ppm for 10 minutes after starting processing for 680ppp
At that point, Ti was continuously introduced into the RH tank in proportion to the recirculation amount to deoxidize [O] to goppm, and then treated at (O)f of 80 to 100 ppm for 6 minutes. The decarburization rate in the If 81 region is high, similar to Example 2 of the present invention, and the decarburization rate in the region 2 is higher than that of Example 2 of the present invention. In the case of the present invention example ■, processing time is 16 minutes (C) 12p
p+w was obtained. Furthermore, Fig. 6 shows the converter blow-off.

[0] Example of the present invention when f is low and oxygen is added by oxygen blowing (OB) at RH ■
and comparative example ■ are shown. In the case of the present invention example (■), the converter blow-off

[0] is 450 ppm, and before the start of RH treatment

[0] became 380 ppm, but due to OB

[0] This is an example in which f is set to 680 ppp and then RH treatment is started, and (0) z is set to 600 to 68
0 ppm and then (C) M at 28 ppm.
This is an example in which weak deoxidation was carried out to bring (O)f to 120 ppm, followed by treatment for about 8 minutes, and the decarburization rate in the If fil region was high, and (C) 16 ppm was obtained in 16 minutes of treatment. In the case of comparative example ■,

[0] is 420 ppm at the converter stop.
, 350 ppm at the start, OB was performed 5 minutes after R1+ treatment, and (O)f was increased from 330 ppm to 7
Enrich to 20 ppm and continue treatment, then (C) 25
Weakly deoxidized with M at the ppm point, (0]t80p
This is an example of processing for about 7 minutes after setting the temperature to pm, but if 8
Even when OB was performed in the N range and oxygen was added, no improvement in the decarburization rate was observed, and as a result, it took 20 minutes to obtain (C) of 20 ppm or less. (Effects of the Invention) As explained above, the present invention enables economical and efficient decarburization treatment under reduced pressure that can easily reach an extremely low carbon range in a short time.

[Brief explanation of the drawing]

Figure 1 shows the (C) transition during the decarburization process under reduced pressure, and Figure 2 shows the decarburization rate constant (Kc) and dissolved oxygen CO] in the region ■ in the decarburization process under reduced pressure. Figure 3 shows the relationship between the decarburization rate constant (KC) and dissolved oxygen in the region ■ in the decarburization process under reduced pressure.

[0] A diagram showing the relationship between

[0]!
5 and 6 are diagrams showing the (C) and (0) transitions in the present invention example and the comparative example. Fig. 1 Fig. 2 CO] f CPP groan Fig. 3 CO) f (ppm) Fig. 4 I

Claims (5)

[Claims] (1) When decarburizing undeoxidized molten steel tapped from a converter under reduced pressure such as RH or DH, (C) 30p from the start of treatment
Until the pm is reached, the dissolved oxygen (O)_f in the steel is reduced to 400
A method for producing ultra-low carbon steel, characterized by controlling the carbon content to ppm or higher. (2) When decarburizing undeoxidized molten steel tapped from a converter under reduced pressure such as RH or DH, (C) 30p from the start of treatment
Until the pm is reached, the dissolved oxygen (O)_f in the steel is reduced to 400
After weakly deoxidizing the dissolved oxygen (O)_f in the steel to 50 to 200 ppm by controlling the dissolved oxygen (O)_f to 50 to 200 ppm by controlling the dissolved oxygen (O)_f to 50 to 200 ppm by controlling the dissolved oxygen (O)_f to 50 to 200 ppm by controlling the dissolved oxygen (O)_f to 50 to 200 ppm by controlling the dissolved oxygen (O)_f to 50 to 200 ppm by controlling the dissolved oxygen (O)_f to 50 to 200 ppm by controlling the dissolved oxygen (O)_f to 50 to 200 ppm by controlling the dissolved oxygen (O)_f to 50 to 200 ppm.
A method for producing ultra-low carbon steel, characterized by further treatment and subsequent complete deoxidation. (3) When decarburizing undeoxidized molten steel tapped from a converter under reduced pressure such as RH or DH, (C) 30p from the start of treatment
Until the pm is reached, the dissolved oxygen (O)_f in the steel is reduced to 400
ppm or more, and then (C) in the region of less than 30 ppm, a deoxidizing agent is added continuously or intermittently to a predetermined basic unit according to the RH recirculation flow rate to reduce the dissolved oxygen (O)_f in the steel to 50 ppm or more. A method for producing ultra-low carbon steel, which comprises weakly deoxidizing to ~200 ppm, further treatment, and then complete deoxidation. (4) The method for producing ultra-low carbon steel according to claim 2 or 3, characterized in that Ti or Al is used as a deoxidizing agent that performs weak deoxidation. (5) The method for producing ultra-low carbon steel according to claim 3 or 4, wherein the weak deoxidation is performed by partially deoxidizing with Al and then deoxidizing with Ti.