JPH0931524A - Treatment of ca in molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel which never cause hydrogen induced cracking by simultaneously adding Ca alloy and Al2 O3 in specific ratio into molten steel in a ladle. SOLUTION: The Ca alloy and the Al2 O3 are simultaneously added into the molten steel in the ladle after deoxidation and degassing treatment. In the above Ca treating method of the molten steel, the addition is executed in the condition satisfying the inequality of the ratio of pure Ca adding quantity and Al2 O3 adding quantity [wherein, [O] is total oxygen concn. in the molten steel (ppm), (Ca) is pure Ca adding quantity per ton of the molten steel (kg/T) and (Al2 O3 ) is adding quantity of Al2 O3 simultaneously added with Ca alloy per ton of the molten steel (kg/T)]. By this method, the steel quality securing the ductility and preventing the hydrogen induced cracking, etc., can be improved by producing an extremely low sulfur steel and controlling the composition and the shape of inclusion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Ca treatment method capable of improving the quality of steel such as ensuring toughness and preventing hydrogen-induced cracking by controlling extremely low sulfurization of steel, composition and morphology of inclusions.

[0002]

2. Description of the Related Art Hydrogen-induced cracking (hereinafter referred to as HIC) that occurs in a line pipe or the like is an Al that is linearly crushed during rolling.
It is well known that it originates from ₂ O ₃ clusters and linearly elongated MnS. Therefore, C has been conventionally used for molten steel.
By adding an a-containing substance, the morphology of Al ₂ O ₃ clusters is controlled into spherical inclusions that do not act as the origin of HIC, and Ca reacts with molten steel [S] to produce CaO-Al ₂ O ₃ -CaS.
There is known a technique for suppressing the production of MnS by desulfurizing as a system inclusion.

Generally, HIC resistant steel such as line pipe is manufactured by the following method.

After deoxidizing the molten steel after the steel is discharged from the converter with Al or the like,
Nitrogen and hydrogen in molten steel are removed using an RH vacuum degassing device. After this, molten steel desulfurization treatment may be performed. Thus, the Ca-containing substance is added to the molten steel after the deoxidizing and degassing treatments.

Generally, the added Ca-containing substance is C
A Ca alloy containing about 30% of Ca such as a-Si and Ca-Al is used. In addition, as a method of adding the Ca-containing substance, the lance immersed in the molten steel and the Ca
Injection method of blowing the contained substance into molten steel, C
A wire feeder method is known in which a wire containing a-containing substance powder is coated with iron to form a wire, which is fed into molten steel.

However, if the Ca treatment method is inappropriate, the spheroidization of Al ₂ O ₃ clusters becomes insufficient, or during continuous casting by CaS clusters produced by the reaction between added Ca and molten steel [S]. The immersion nozzle is clogged.

In order to solve such a problem, several methods have been conventionally proposed.

JP-A-60-9816 discloses that the action of Ca on molten steel can be appropriately controlled by a metal refining agent in which a Ca alloy and a flux containing Al ₂ O ₃ and CaO as main components are closely bonded. It has been shown that this makes it possible to control the inclusions to the CaO-Al ₂ O ₃ system and to achieve desulfurization.

The inventors of the present invention have disclosed in Japanese Unexamined Patent Publication (Kokai) No. 64-75622 that Al ₂ O ₃ and a Ca alloy are simultaneously added or Al is added.
By adding Ca alloy after adding ₂ O ₃ , inclusions
Modifying to CaO-Al ₂ O ₃ -CaS and making it 100% spherical,
A method to effectively desulfurize was proposed.

[0010]

Conventionally, MnS or Al ₂ O ₃ clusters, which are the starting points of HIC by adding Ca, are modified into CaO-Al ₂ O ₃ -CaS-based spherical inclusions which are harmless to HIC resistance. And suppress CaS generated when Ca is added,
It has been considered that practically sufficient HIC resistance of steel can be obtained.

However, in reality, even if Ca is added, the harmful inclusions that are the starting point of HIC are detoxified and modified into spherical CaO-Al ₂ O ₃ -CaS inclusions, and even if CaS inclusions are suppressed, HIC generation is prevented. It cannot be suppressed enough. Therefore, the inventor
The steel material added with a was evaluated by the HIC resistance evaluation test, and H
As a result of observing the morphology of inclusions in the steel material in which IC occurred and analyzing the composition, the following findings were obtained.

All the inclusions present in the slab are spherical and have a composition of Ca-Al-OS. When this slab was rolled and the inclusions in the steel after this rolling were investigated, some of the inclusions were spherical, but most of the inclusions were crushed into linear shapes.

Further, when this rolled material was evaluated by the HIC evaluation test, it was found that HIC was generated from the linearly crushed inclusions as the starting point.

Therefore, after the rolling, the compositions of the linearly crushed inclusions and the inclusions that remained spherical were examined in detail, and the following was revealed.

The composition of the inclusions crushed linearly is CaO>
60%, CaS> 8%, Al ₂ O ₃ <27%, or CaO <
35% and Al ₂ O ₃ > 65%. On the other hand, inclusions that were spherical even after rolling were 35% ≤ CaO ≤ 60%, CaS ≤ 8%
And the balance had a composition of Al ₂ O ₃ . Therefore, if the composition of the inclusions is inappropriate, even if the inclusions are spherical in the molten steel and the slab, they are crushed during rolling and become the starting point of HIC.

Furthermore, [S] concentration in molten steel and CaS in inclusions
When the correlation with the concentration was investigated, when the [S] concentration in the molten steel was 7 ppm or more, the CaS concentration in the inclusions increased during the solidification process, and the CaS concentration in the inclusions eventually became 1
It was found to be 0% or more.

Therefore, in order to sufficiently improve the HIC resistance of the steel material, in addition to suppressing the formation of CaS simple substance inclusions and completely spheroidizing the inclusions, the following conditions must be achieved.

The composition of inclusions is 35% ≤CaO≤60%, Ca
S ≦ 8% and the balance Al ₂ O ₃ .

The [S] concentration in the molten steel is set to 7 ppm or less.

However, both the refining agent of JP-A-60-9816 and the desulfurization method of JP-A-64-75622 aim to desulfurize molten steel to some extent and to spheroidize inclusions. However, since the degree of achievement of suppression of CaS formation and spheroidization of inclusions is low, the above conditions cannot be satisfied, and the HIC resistance of steel cannot be sufficiently improved.

[0021]

The gist of the present invention resides in the following Ca treatment method for molten steel.

A Ca treatment method in which Al ₂ O ₃ is simultaneously added to a molten steel in a ladle together with a Ca alloy, and the ratio of the pure Ca content and the Al ₂ O ₃ content is expressed by the following equation (1). A method for treating Ca in molten steel, characterized in that they are added simultaneously under the conditions that satisfy the above conditions.

[0023]

[Equation 2]

However, [O]: total oxygen concentration in molten steel (p
pm) (Ca): Amount of pure Ca added per ton of molten steel (kg / T) (Al ₂ O ₃ ): Amount of added Al ₂ O ₃ added together with Ca alloy per ton of molten steel (kg / T)

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Since Ca has a high reactivity with Al ₂ O ₃ inclusions and [S] in molten steel, when a Ca alloy is added, the reaction rapidly proceeds near the addition position,
The inclusions are very rich in CaO and CaS. Since this rapid reaction is a local reaction that occurs near the addition position, it is difficult to mitigate it due to general operating factors such as the amount of Ca alloy added and the addition rate, and as a result, the composition of inclusions and It is impossible to precisely control the morphology.

Therefore, in order to precisely control the composition and morphology of the inclusions and improve the HIC resistance, such C
a) Addition of Ca
It is essential to establish a technology for more effectively acting on molten steel.

When a Ca alloy and Al ₂ O ₃ are simultaneously added, Ca
The reaction of the following formula (2) occurs near the addition position.

Ca + Al ₂ O ₃ = mCaO · nAl ₂ O ₃ (2) As a result, the added Ca is Al ₂ O ₃ inclusions in the molten steel,
Simultaneously with the reaction with [S], it is consumed by the reaction with the added Al ₂ O ₃ , so that the rapid reaction of Ca is suppressed. For this reason,
It is possible to precisely control the composition of inclusions. However, mCaO.nAl generated by the above reaction (2) equation
₂ O ₃ is quickly floated and separated from the molten steel and removed.

At this time, by simultaneously adding the Ca alloy and Al ₂ O ₃ under the condition that the following formula (1) is satisfied, the composition of the inclusions can be controlled and, at the same time, the desulfurization can be performed more effectively.

[0030]

(Equation 3)

However, [O]: total oxygen concentration in molten steel (p
pm) (Ca): Addition amount of pure Ca per ton of molten steel (kg / T) (Al ₂ O ₃ ): Addition amount of Al ₂ O ₃ added together with Ca alloy per ton of molten steel (kg / T) Above The basis for obtaining the equation (1) will be described with reference to FIGS. 1 and 2.

FIG. 1 is a diagram showing the relationship between the [O] concentration in molten steel and (Ca) / (Al ₂ O ₃ ) (hereinafter referred to as the compounding ratio) in the above formula (1). The white circles in the figure indicate that the inclusion composition is 35.
% ≤ CaO ≤ 60%, CaS ≤ 8%, balance Al ₂ O ₃ , HIC did not occur, black squares are CaO
HIC that originated from inclusions with a composition of <35%, Al ₂ O ₃ > 65%, black coating triangles are CaO> 60
%, CaS> 8%, Al ₂ O ₃ <27% as the starting point of the occurrence of HIC. Further, the curve A shows the following formula (3), and the curve B shows the following formula (4).

[0033]

(Equation 4)

[0034]

(Equation 5)

The area below the curve A in FIG. 1, that is, Ca
If the added amount of Al ₂ O ₃ is excessive with respect to the added amount, the added Ca
Most of the Al reacts with the added Al ₂ O ₃ to form CaO-Al ₂ O ₃ , which floats and is removed from the molten steel. As a result, the apparent amount of Ca supplied to the molten steel is reduced, and Al ₂ O ₃ inclusions in the molten steel are sufficiently removed.
It could not be reformed to CaO-Al ₂ O ₃ and the inclusions were CaO <3
5%, Al ₂ O ₃ > 65%, crushed during rolling, HI
It is the starting point of C.

On the other hand, in the region above the curve B in FIG. 1, that is, when the amount of Al ₂ O ₃ added is insufficient with respect to the amount of Ca added, C
The sudden reaction of a cannot be suppressed sufficiently, and most of the added Ca
Since Al ₂ O ₃ functions to modify inclusions, Ca in inclusions
Higher concentration, resulting in inclusion composition of CaO> 60
%, CaS> 8%, Al ₂ O ₃ <27%, and they are crushed during rolling and become the starting point of HIC.

As described above, in order to precisely control the composition of inclusions and improve the HIC resistance, it is indispensable to add Ca and Al ₂ O ₃ in the compounding ratio represented by the formula (1).

FIG. 2 shows the amount of Ca added and the amount reached [S].
It is a figure which shows the relationship with density. The white circles in Figure 2
It is the ultimate [S] concentration when Ca alloy and Al ₂ O ₃ are simultaneously added according to the equation (1). The area sandwiched between the broken lines in FIG.
When only Ca-Si is added and after CaO is added Ca-Si
Is the ultimate [S] concentration in the case of adding. The [S] concentration before addition is 30 ppm.

As shown in FIG. 2, when the compounding ratio is controlled according to the equation (1), desulfurization proceeds and the reached [S] concentration decreases. As the desulfurization reaction are shown in the following equation (5), adding Ca and the addition Al ₂ O ₃ mCaO · produced by a reaction of nAl ₂ O ₃
This is because the system oxide and [S] in the molten steel react to form CaO-Al ₂ O ₃ -CaS, which floats and separates from the molten steel.

[0040] _{mCaO · nAl 2 O 3 + S} = (m-1) when _{CaO · CaS · Al 2 O 3} ···· (5) blending ratio (1) exceeds the upper limit value specified by expression contributes to desulfurization Since the CaO concentration in mCaO.nAl ₂ O ₃ is extremely lowered, it cannot be desulfurized sufficiently. On the other hand, when the compounding ratio becomes less than the lower limit value defined by the equation (1), mCaO generated
・ Since the absolute amount of nAl ₂ O ₃ is small, it cannot be desulfurized sufficiently.

The amount of Ca to be added is not particularly limited as long as it satisfies the expression (1), but is preferably 0.05 kg / T or more and 0.4 kg / T or less. When it exceeds 0.4 kg / T,
The residual amount of mCaO.nAl ₂ O ₃ generated by Ca and added Al ₂ O _{3 in} the molten steel increases, and as a result, the cleanliness of the molten steel deteriorates. On the other hand, when the amount of Ca added is less than 0.05 kg / T
Al ₂ O ₃ cluster reforming becomes insufficient and, at the same time, sufficient desulfurization cannot be performed and MnS is produced.

Any Ca alloy may be added, but Ca--Si, Ca--Al, Ca--Al--Si
Is desirable. Al ₂ O ₃ added at the same time has a grain size of 0.
It is preferably about 2 to 2 mm. If the particle size is less than 0.2 mm, the floatation from the molten steel becomes slower, which deteriorates the cleanliness of the molten steel. On the other hand, if the particle size exceeds 2 mm, Al ₂ O ₃
Not only is it difficult to add, but the temperature of the Al ₂ O ₃ grains does not rise rapidly after addition, so the reaction efficiency decreases and the function cannot be fully achieved.

The Ca alloy and Al ₂ O ₃ may be added by premixing and blowing them together with an inert gas by injection, or by filling them in a wire and feeding it into molten steel. Further, while cutting out Ca alloy and Al ₂ O ₃ from separate hoppers and mixing them in a predetermined amount using a load cell,
It may be blown together with an inert gas using an injection lance.

Before adding Ca, the [S] concentration in the molten steel is 30 ppm.
Hereinafter, it is desirable to keep the hydrogen concentration at 2 ppm or less. As a method of desulfurizing before adding Ca, a method of gas stirring molten steel in the presence of slag having excellent desulfurization ability or CaO
Any method such as a method of blowing a desulfurizing agent containing as a main component into molten steel may be used. Alternatively, in an RH-type vacuum degassing device, etc., a method of adding a desulfurizing agent from a hopper in a vacuum tank, a method of blowing a desulfurizing agent from a lance provided in the vacuum tank, a method of soaking in a vacuum tank or desulfurization from an upper blowing tuyere Any method such as a method of blowing an agent may be used. It is desirable to apply a method of performing dehydrogenation using a vacuum degassing device.
Furthermore, it is desirable to use a combination of the above desulfurization and dehydrogenation. As a combination method, desulfurization treatment may be followed by dehydrogenation treatment, or dehydrogenation treatment may be followed by desulfurization treatment.

[0045]

[Example] Molten steel 250 that was tapped from a converter and stored in a ladle
The composition of T 1 was adjusted by deoxidizing T 2.

[0046]

[Table 1]

Ar using a lance immersed in molten steel in a ladle
After stirring the molten steel with gas, the RH vacuum degasser was used to perform reflux treatment, and then Ca alloy and Al ₂ O mixed in a predetermined amount.
₃ was simultaneously added by the injection method using Ar gas. The [S] concentration in the molten steel before the addition of Ca was 25 ppm. Table 2 shows each processing condition.

[0048]

[Table 2]

As the Ca alloy, a Ca-Si alloy of 30 Wt% Ca-70 Wt% Si was used, and as Al ₂ O ₃ , powder having an average particle size of 0.5 mm was used. The addition rate is 0.13 kg / (T
・ Min), Ca addition amount is 0.16 kg / T in terms of pure Ca
And

The treated molten steel was made to a thickness of 235 by a continuous casting machine.
mm slab, which was rolled into a 26.5 mm thick plate. Then, the sample was cut out, the morphology of inclusions was observed by a microscopic method, and the composition was analyzed and subjected to a HIC resistance evaluation test.

The HIC resistance test conditions were NACE conditions. Table 2 also shows the HIC evaluation test results. In the table, ◯ indicates that HIC did not occur at all, and X indicates that HIC occurred.

As shown in Table 2, when the compounding ratio was controlled according to the conditions defined by the method of the present invention, HIC did not occur at all, but when the compounding ratio deviated from the conditions specified by the method of the present invention, HIC occurred.

[0053]

According to the method of the present invention, it is possible to obtain steel in which HIC does not occur at all.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between [O] concentration in molten steel and Ca / Al ₂ O ₃ (mixing ratio).

FIG. 2 is a diagram showing the relationship between the amount of added pure Ca and the reached [S] concentration.

Claims (1)

[Claims] 1. A Ca treatment method of simultaneously adding Al ₂ O ₃ together with a Ca alloy into molten steel in a ladle, wherein the ratio of the amount of pure Ca added and the amount of Al ₂ O ₃ added is represented by the following formula (1). The method for treating Ca in molten steel is characterized in that they are added at the same time under the conditions that satisfy the relation [Equation 1] However, [O]: total oxygen concentration in molten steel (ppm) (Ca): amount of pure Ca added per ton of molten steel (kg / T) (Al ₂ O ₃ ): Al ₂ O ₃ added at the same time as Ca alloy Addition amount per ton of molten steel (kg / T)