JPS5922765B2 - Manufacturing method for low-oxygen, low-sulfur steel that controls sulfide formation - Google Patents

Description

[Detailed description of the invention] The present invention controls the form of sulfides in molten steel to ensure toughness,
This paper relates to improvements in deoxidizing and desulfurizing methods aimed at improving the quality of steel, such as preventing hydrogen-induced cracking.

It has been known that Ca has a high affinity for oxygen and sulfur, is effective in reducing total oxygen and total sulfur in molten steel, and has an important effect on improving the toughness of steel pipes.

And industrially, it is used as a Ca additive. 1 Ca alloy alloyed with elements that dissolve in iron.

For example, CaSi, CaA1, Ca5iBa, CaSiM
nAl etc 2 CaO-containing flux.

For example, it can be broadly classified into Cab and CaO+CaF2.

However, no systematic or theoretical study has yet been conducted on the deoxidation, desulfurization, or sulfide morphology control effects of these molten steel additives.

Previously, the effects of Ca or Ca compounds were viewed comprehensively, and it was thought that deoxidation, desulfurization, and sulfide morphology control could all be performed uniformly by adding additives (e.g., CaSi or Ca0) to molten steel. .

However, in reality, such a simple operation does not uniformly satisfy all of the above objectives, and the main objective is to control the sulfide morphology, and the Ca addition treatment is performed with the expectation of deoxidation and desulfurization at the same time. However, the deoxidation was not always sufficient and the cleanliness of the molten steel was not improved.

For example, when CaSi, which is most commonly used industrially, is added to molten steel with the main purpose of controlling sulfide morphology and desulfurization and expecting deoxidation, the deoxidation rate is lower than that of CaO-containing flux, which is the highest. The total oxygen level reached is at most around 40 ppm.

Therefore, the current situation is that it is insufficient to meet the strict requirements of steel quality in recent years, such as ensuring the toughness of line pipes for extremely cold regions or preventing hydrogen-induced cracking of line pipes for transporting natural gas containing sour gas. .

There has long been a desire for a method of manufacturing steel that satisfies both excellent cleanliness with an oxide inclusion level of 20 to 30 ppm and control of sulfide morphology.

In view of the above-mentioned circumstances, the present invention has been made to meet these demands.The present invention has been made by intensively studying the conventional Ca addition method and reaching a solution to the problem.

That is, the effects of deoxidation, desulfurization, and sulfide form control were tested and studied for Ca alloys and CaO-containing fluxes.

As a result, in order to satisfy all of the above objectives, 1. Preliminary deoxidation treatment step during tapping 2. Deoxidation and desulfurization step by injecting powder into the ladle molten steel 3. By injecting powder into the ladle molten steel It is necessary to separate each step of the sulfide morphology control process, and after preliminary deoxidation treatment of molten steel in a ladle during tapping, CaO-containing flux and Ca alloy are injected into the molten steel. We found that the timing and selection and order of additives have important implications.

That is, in the present invention, CaO is added to molten steel that has been deoxidized in advance in a ladle.
Blow the contained flux with carrier gas, deoxidize,
This invention relates to a method for producing low-oxygen, low-sulfur copper with controlled sulfide morphology, which is characterized by injecting Ca alloy into the molten steel after desulfurization to control the sulfide morphology in the molten steel.

The present invention will be explained in detail below. (1) Regarding deoxidation, there is clearly a difference between Ca alloy and CaO-containing flux.

(Part 1) The deoxidation rate equation for both sides can be expressed as follows: 0f10i = η + (1-η) exp (-kt ) Of
, Ol ; Total oxygen before and after treatment η = Q oo /Q 1 Qoo ; Achievement limit total oxygen (Figure 2) K; Rate constant t: Treatment time From the experimental results, for Ca alloy = 0.20 For CaO-containing flux: = 0.35, and it was possible to quantitatively derive the difference in deoxidation rate.

That is, when the purpose is deoxidation, a CaO-containing flux is more effective than a Ca alloy.

In addition, in the case of a mixture of Ca alloy and CaO-containing flux, Ca
The results showed that deoxidation by the alloy determined the reaction rate, and the deoxidation rate constant was the same as that of the Ca alloy alone.

This is thought to be because, although Ca alloy has a stronger affinity for oxygen than CaO-containing flux, the oxide formed by Ca alloy is worse in terms of the buoyancy of the generated oxide.

It is also clear that there is a limit total oxygen value and that it depends on the composition of the slag.

(Fig. 2) 2 Regarding desulfurization The mechanism of desulfurization refining when powder additives are injected is (a) reaction while additive particles rise in the steel bath (
(transitory reaction) (b) Reaction by slag on a steel bath (permanent rea
There are two possible options.

-Ca alloy and CaO-containing flux The contribution of reaction (b) is very large.

That is, the reason for this is that there is no clear correlation between the desulfurization rate and the blowing material consumption rate, and that the desulfurization behavior is greatly influenced by the slag composition and slag formation (Figures 3 and 4). From the figure, m annes m anSl is used as an index of slag composition.
ag Index (Cao/Sio2: A1
It was found that the desulfurization rate was dependent on 203).

When comparing Ca alloys and CaO-containing fluxes, CaO-containing fluxes tend to have better slag formation and better desulfurization efficiency than Ca alloys, but there is no fundamental difference.

3 Regarding sulfide morphology control The sulfide mentioned here refers to MnS, and is generally C
It is well known that a suppresses the formation of MnS and produces spherical inclusions of calcium, aluminate, and sulfide, and these spherical inclusions have the property of not deforming in rolled steel materials.

However, according to experiments conducted by the present inventors, Ca alloy has a function of controlling the morphology of sulfides, whereas CaO-containing flux does not have such a function.

The above 1.2.3 is a fact that became clear from the results of numerous experiments conducted by the present inventors.

Therefore, when producing low-oxygen, low-sulfur copper with controlled sulfide morphology, it is necessary to perform the following series of steps in sequential combination based on the above knowledge.

That is, when first tapping the molten steel, for example, AI
, Al-8i, AI-8i-Mn, etc. are deoxidized in advance, and then a CaO-containing flux is blown into the molten steel. After the flux is blown, a carrier gas is successively blown into the bath. It is preferable to perform stirring.

Finally, a series of steps are carried out in sequence, including adding Ca alloy by blowing it into the steel bath to destroy the form of the sulfide.

However, when a CaO-containing flux and a Ca alloy are added at the same time, almost the same results are obtained in terms of sulfide morphology control and desulfurization as when they are added sequentially, but the resulting steel is very poor in cleanliness.

After addition of Ca alloy, the Ca content in the steel can be adjusted to the desired value by blowing with inert gas. Also, the composition of the slag is extremely important for deoxidation and desulfurization.
If a large amount of , Mn01 is contained in the slag, the deoxidizing and desulfurizing effects will both deteriorate, so it is necessary to consider this adjustment by preventing the slag from flowing out during tapping.

The CaO-containing flux Ca alloy used in the present invention is both pulverized and introduced into a steel bath from a blowing lance with a carrier gas (inert gas).

Examples will be described below.

The method of the present invention was applied to a thick plate 50 that had been deoxidized with AI, Fe-8i, and Fe-Mn alloys at the time of tapping, using killed steel as the target steel type.

For comparison, a conventional method was also conducted.

In the blowing shown in the attached table, a lance coated with a refractory material was immersed in 2.50 m of molten steel, and Ar gas was used as a carrier gas.

In the examples in which Ca alloy was injected (Comparative Example 2.3.4, the present invention), it was shown that there was an effect of controlling the form of the imaged object, and the result was spherical composite inclusions of CaO.Al2O3.S.

Looking at the total oxygen level, it is lower in the CaO-containing flux injection example (Comparative Example 1, the present invention), but in Comparative Example 4, Ca
Similar to the alloy injection example, simultaneous injection of CaO-containing flux and Ca alloy with a high concentration of around 46 ppm shows that there is no effect of reducing total oxygen.

Furthermore, it can be seen from Comparative Examples 2.3.4 and the present invention that the calcium content is affected by after-blowing.

As described above, by the method of the present invention, it was possible to produce low oxygen (22 ppm) and low sulfur (6 ppm) steel with controlled sulfide morphology.

[Brief explanation of drawings]

Figure 1 shows the change in total oxygen before and after treatment with respect to treatment time, and shows that deoxidation is smaller with Ca alloy (or simultaneous injection of Ca alloy + flux) than with CaO-containing flux. Figure 2 shows the reaching limit oxygen of the slag after treatment (
Figure 3 shows the relationship between desulfurization rate and slag Cab/5in2.
;A1203(mannesman Slag Ind.
Figure 4 shows the dependence on S after processing.
Distribution ratio (S) f/(S, If and (Fe in the slag after treatment)
FIG. 5 is a diagram showing the correlation between the sum of O)% and (MnO)%, and FIG. 5 is a diagram showing the influence of afterblowing using only a carrier gas on the Ca yield.

Claims (1)

[Claims] 1. After deoxidizing and desulfurizing the molten steel that has been previously deoxidized in the ladle by blowing a CaO-containing flux into the molten steel using a carrier gas, a Ca alloy is subsequently blown into the molten steel to control the sulfide form in the molten steel. A method for producing low-oxygen, low-value brass that controls the form of sulfides.