JP3305313B2 - Decarburization method using RH degasser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for melting ultra-low carbon steel using an RH degassing apparatus.

<Prior Art> Various methods for melting ultra-low carbon steels in which the C concentration is reduced as much as possible with the improvement in the quality of cold-rolled steel sheets and the continuation of the annealing process have been disclosed. A common method for producing ultra-low carbon steel is a converter to RH degassing process.

In this method, after the steel is turned out of the converter at 200 to 500 ppm of C and 300 to 700 ppm of O, decarbonization is performed by an RH degassing device capable of preferential decarburization up to a low carbon concentration region by lowering Pco.
It is a method of quickly decarburizing to ppm, and thereafter, deoxidizing the molten steel by adding Al or the like as a reducing agent into a vacuum chamber.

However, a method for rapidly obtaining molten steel having an extremely low carbon concentration is not easy, and various attempts have been made in the past. For example, Inoue et al .: CAMP-ISIJ, vol. 2 (1989), P. 1226-112.
No. 27 discloses a technique for injecting Ar gas from molten tuyere into molten steel in an RH vacuum tank to accelerate the decarburization rate in an extremely low carbon concentration region. Further, as disclosed in JP-A-63-190113, O ≧ 400 ppm from the start of RH treatment to C = 30 ppm,
After that, decarburization was continued with O = 50 to 200 ppm.
There is a method of performing a complete deoxidation treatment.

<Problems to be Solved by the Invention> In the technology disclosed in the former document, Ar gas must be injected even during RH degassing other than ultra-low carbon steel processing, which increases the cost of unnecessary processing. The cost is high.

In the method disclosed in JP-A-63-190113, O is changed from 400 ppm or more to 50 to 200 ppm before and after C = 30 ppm, but the decarburization rate is changed according to the C concentration. Cannot be significantly improved (just a one-step change). Since the reducing agent was added for the first time in the ultra-low carbon region of C = 30 ppm, there was a problem that the time for deoxidizing treatment (killing treatment) after the decarburization was completed could not be significantly reduced.

The present invention has been made to provide a technique for rapidly processing ultra-low carbon steel that solves the above problems.

<Means for Solving the Problems> According to the present invention, when performing decarburization treatment of molten steel using an RH vacuum degassing apparatus, based on measured or predicted values of C and O in the molten steel, O ≦ In the range of 1.3C + 35, oxygen for decarburization is supplied to the molten steel, and in the range of 1.3C + 200 ≦ O, a reducing agent is supplied, and C ≦ 20pp while satisfying the following formula (1).
1.3C + 35 ≦ O ≦ 1.3C + 200 (1) where: O: oxygen concentration in molten steel (ppm), C: in molten steel Carbon concentration (ppm).

In addition, the carbon concentration in the molten steel before the decarburization treatment is 200 ppm or more and 50
The decarburization method using the RH degassing device according to the preceding paragraph, wherein the oxygen concentration is 0 ppm or less and the oxygen concentration is 300 ppm or more and 700 ppm or less.

<Operation> The present invention will be specifically described.

FIG. 4 shows an example of the transition of carbon during the treatment of ultra-low carbon steel by the normal RH method. In this example, after reducing the free oxygen in the steel by adding a reducing agent (Al) after the limd treatment (decarburization treatment) for 20 minutes, a killing treatment is performed for 10 minutes to reduce the total oxygen concentration in the steel to 15 to Reduce to about 30ppm. In this method, if the killing treatment time is short, the total oxygen concentration in the steel increases, and blistering defects and the like occur frequently in the steel sheet. Therefore, a certain time is required after the introduction of the reducing agent.

FIG. 5 is a characteristic diagram showing the relationship between C and O in molten steel during normal RH degassing. Although the oxygen concentration slightly decreases with the progress of decarburization, there is no decrease as much as the stoichiometric relation line of C + O → CO shown in FIG.

As a result of investigating the transition of CO in FIG. 5 in detail, O in the molten steel causes decarburization by reaction with C, and at the same time, acts as a surface active element in the steel. It has been found that when excess O is present, in addition to decarburization by O, O is adsorbed on the interface with the molten steel due to the surface activation action of O, thereby reducing the decarburization rate.

Therefore, the present inventors have invented a method of controlling the O concentration in accordance with the C concentration so that oxygen for decarburization is required and supplied sufficiently and the decarburization is promptly caused in the entire C concentration range. is there.

According to the present invention, O in the molten steel is controlled as a function of C in a region represented by the following equation, and is processed to a predetermined extremely low carbon concentration (see FIG. 1).

 1.3C + 35 ≦ O ≦ 1.3C + 200 (1) Here, O: oxygen concentration in molten steel (ppm), C: carbon concentration in molten steel (ppm).

By such a control, advantages such as rapid progress of decarburization from the initial stage to the extremely low carbon concentration region and reduction of the killing time can be obtained.

The method of controlling O is as follows: In the region of O ≦ 1.3C + 35, supply gas containing oxygen to molten steel in the vacuum chamber,
In a region where the decarburization reaction rate becomes excessive oxygen of ≤O, the excess oxygen is adjusted by adding a reducing agent to the molten steel in the vacuum chamber.

For this purpose, the supply rate and time of the oxygen gas are adjusted, and the input amount and the input pattern of the reducing agent are adjusted based on the measured or predicted values of C and O obtained by continuous measurement of the molten steel.

At the end of decarburization, oxygen in steel is excessive compared to carbon in steel (fifth
In the present invention, the reducing agent is continuously or intermittently charged so as to satisfy the range of 1.3C + 200 ≧ O. At this time, since the total oxygen in the steel is also removed (usually only by killing), the killing time after completely removing free oxygen can be shortened.

In addition, the present invention relates to
= Oxygen from 400 ppm or more to 50 to 200 ppm based on 30 ppm
This method is completely different from the method of changing oxygen in that oxygen is continuously controlled.

<Examples> Examples and comparative examples are shown below.

In each of the examples and comparative examples, the molten steel processing amount was 290 to 3
00t, the chemical composition before treatment is C = 0.020-0.05% by weight (hereinafter abbreviated as%), Mn = 0.05-0.15%, P = 0.01-0.02%, S
= 0.004 to 0.006%, N = 0.0015 to 0.002%, O = 0.03 to
0.07%.

Molten steel temperature is 1590 ~ 1610 ℃ before treatment, 1590 ~ 1600 ℃ after treatment
And Note that “after processing” means after killing processing. The reflux gas flow rate is constant at 2000 N / min, and the degree of vacuum during the killing process is 0.4
~ 0.5 torr.

Note that C was measured by Cantback analysis, and O was measured using a solid electrolyte.

The C transition and the CO relationship of the example and the comparative example are shown in the second.
FIG. 3 and FIG. From FIG. 2, the rimmed treatment time of Examples 1 and 2 is 15 minutes at C = 16 ppm, and the killed treatment time is 5 minutes and the total oxygen is 15 to 17 ppm, whereas the comparative example is 20 minutes. Despite the rimmed processing time, C becomes only 25 ppm, and the killed processing 10
The total oxygen in a minute was 23 ppm.

As can be seen from FIG. 3, both Examples 1 and 2 have C-
The relationship of O is in the shaded area except for the processing start time. The symbols ３ and ● in FIG. 3 indicate the chemical compositions before the RH treatment and are not in the shaded region.

Usually, when the carbon concentration at the time of converter tapping is 200 to 500 ppm,
The oxygen concentration is between 300 and 700 ppm. When tapping the converter, C <200
When O> 700 ppm in ppm, the degree of oxidation of the slag becomes excessive, and the oxygen concentration after the RH treatment increases. In addition, the load on the converter increases, which is not a good idea. On the other hand, C> 500ppm, O
If it is <300 ppm, it takes time to raise the oxygen to the target level, and the load of RH increases because the amount of decarburization increases. Therefore, C, before proper decarburization treatment in the present invention,
O level is 200ppm ≦ C ≦ 500ppm, 300ppm ≦ O ≦
700 ppm.

Table 1 shows the amount of oxygen blown into the tank from the top blowing lance and the amount of Al charged in the examples and comparative examples.

The oxygen blowing flow rate was in the range of 50 to 60 Nm ³ / min, and the Al charging rate was in the range of 5 to 20 kg / min. Table 2 shows the results of other examples and comparative examples.

Thus, according to the method of the present invention, rapid decarburization and deoxidation of ultra-low carbon steel became possible.

<Effects of the Invention> According to the present invention, rapid and economical melting of ultra-low carbon steel has become possible.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a region of CO in the present invention,
FIG. 2 is a characteristic diagram showing the transition of C in the example and the comparative example, FIG. 3 is a characteristic diagram showing the relationship of CO in the example and the comparative example, and FIG. FIG. 5 is a characteristic diagram showing the transition of C, and FIG. 5 is a characteristic diagram showing the relationship with CO during normal RH processing.

Continuing from the front page (72) Inventor: Santo Nakato 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corporation Research and Development Headquarters (72) Inventor Tetsuya Fujii 1-Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corporation Research headquarters

Claims (2)

(57) [Claims] When performing decarburization processing of molten steel using an RH vacuum degassing apparatus, the molten steel is desorbed in the region of O ≦ 1.3C + 35 based on the measured or predicted value of C and O in the molten steel. Supply oxygen for decarburization, and supply a reducing agent in the region of 1.3C + 200 ≦ O, and satisfy C ≦ 20ppm while satisfying the following formula (1).
A decarburization method using an RH degassing device, characterized in that decarburization is performed up to a maximum. 1.3C + 35 ≦ O ≦ 1.3C + 200 (1) Where, O: Oxygen concentration in molten steel (ppm) C: Carbon concentration in molten steel (ppm) 2. The decarburization method according to claim 1, wherein the carbon concentration in the molten steel before the decarburization treatment is 200 ppm or more and 500 ppm or less, and the oxygen concentration is 300 ppm or more and 700 ppm or less.