JPH0639614B2 - Ultra-low S, ultra-low A (1) steel manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention relates to a method for producing ultra-low S and ultra-low Al steel.

[Conventional technology]

For example, as seen in JP-A-61-79745,
In order to obtain a steel with good weld joint toughness in which a fine oxide is used as a transformation nucleus of ferrite, Al ≦ 0.005%, preferably Al ≦ 0.
It must be 003% low Al steel.

Furthermore, in order to obtain steel with excellent lamella tear resistance, HIC resistance, and toughness, MnS drawn by rolling is used.
Is harmful, and it is generally well known that S in steel is preferably low.

That is, S ≦ 0.003%, Al ≦ 0.005%, preferably Al ≦ 0.003% as a steel that has good lame resistance, HIC resistance, and toughness, and at the same time satisfies steel with good weld joint toughness using fine oxides as ferrite nuclei. Manufacturing is required.

[Problems to be Solved by the Invention]

As seen in the Iron and Steel Handbook (Edited by the Iron and Steel Institute),
Generally, there are a method of removing S with hot metal and a method of removing S with molten steel, but the method for producing S ≤ 0.003% with a product is
Since it is difficult to do this alone, it is usually processed by combining molten steel de-S and molten steel de-S.

In order to reduce S in steel by removing S from molten steel, it is necessary to reduce the oxygen potential in steel and the oxygen potential in slag as much as possible. In general, ultra-low S steel is produced by using Al as killed steel.

If Al in the molten steel is lowered to obtain an extremely low Al steel, oxygen in the steel becomes high. Therefore, if the de-S treatment is performed in this state, the de-S rate is poor and it becomes difficult to achieve S ≦ 0.003%. On the other hand, de-S
In order to improve the temperature, if oxygen in the steel is reduced by a method other than Al deoxidation such as vacuum treatment, even if Al is not used, according to the equilibrium reaction between the slag, metal, and refractory,
Slag Al ₂ O ₃ and refractories Al ₂ portion of O ₃ is reduced in the steel Al
Therefore, in the conventional method, S ≦ 0.003%, Al ≦ 0.00
It has been considered very difficult to produce an extremely low S and extremely low Al steel with 5% desirably Al ≦ 0.003%.

[Means for Solving the Problems]

The present invention has been made to advantageously solve the above-mentioned problems of the prior art, the gist of which is to perform deoxidation of Al while adding a component adjusting alloy to molten steel in a ladle that has been tapped from a converter. Al in the steel to 0.02 to 0.10%, and then CaO or Ca alloy is added to make S in the steel 0.002% or less, and then oxygen is supplied to the molten steel without mixing and stirring between the slag and the molten steel, Blowing until oxygen in the steel reaches 60 to 150 ppm, Al in the steel
Of 0.002% or less, and then deoxidizing with one or more deoxidizing agents selected from Ti, Zr, Ca, REM, Mg, and Nb. .

The present invention will be described in detail below.

After refining the molten iron deoxidized to S ≤ 0.006% in the converter, the molten steel in the ladle that is tapped from the converter is S within the target steel composition range.
Along with the addition of i and Mn, Al 0.02 to 0.10% for promoting molten steel de-S.
Al deoxidation is performed, and molten steel is deoxidized by performing S reaction in which the slag having a high deoxidizing ability such as CaO powder or Ca-based alloy injection is formed in a state where the oxygen in the steel is lowered to cause S ≦ S ≦
You get 0.002% steel. After that, the RH method of vacuum gas treatment and a kind of simple ladle refining method are used to dip the lower part of the cylinder into molten steel to remove slag, and to perform mixing and stirring between de-S slag and retaining steel by the CAS method for refining. Oxygen is blown so that oxygen is supplied only to the molten steel in the RH tank or the CAS dip pipe in a state where it does not occur as much as possible, and it is blown until the oxygen in the steel reaches 60 to 150 ppm to oxidize Al in the steel and Al ≦ 0.002 Get% Molten Steel. Then Ti etc. Al
Deoxidizing agents other than S are used to perform the necessary deoxidation, S ≦ 0.003%, Al ≦ 0.
005% It is a method for producing ultra-low S and ultra-low Al steel, preferably Al ≦ 0.003%.

Here, the amount of Al in the Al deoxidizing is set to 0.02 to 0.10% because the Al in the steel is oxidized and lost due to the stirring and mixing between the slag and the metal and the air oxidation during the subsequent de-S treatment, but the oxygen in the steel at that time is lost. Of Al> 0 even after oxidation loss so that S rises and S removal does not deteriorate.
To secure 01%, the lower limit must be 0.02% or more.

On the other hand, the upper limit is that the amount of Al ₂ O ₃ produced by the oxidation of Al during oxygen blowing after de-S is so large that Al ₂ O ₃ % in the slag becomes too high, and during the subsequent deoxidation by Ti etc. From the equilibrium reaction between the slag and metal to the reverse reaction until it is cast
In order to avoid that Al ₂ O ₃ is reduced and Al in the steel rises to Al> 0.003%, the molten steel temperature was set to 0.10% so that the heat generated by the oxidation heat of Al would not cause the molten steel temperature to become too high.

On the other hand, the S-removing step is performed under a sealed atmosphere, in an atmosphere of an inert gas such as Ar gas, or under vacuum.
After injecting or adding a desulfurizing agent such as a system alloy, the mixture is agitated and blown with Ar to perform S removal treatment up to S ≦ 0.002%.

After that, oxygen in the steel is increased to blow Al and oxidize Al to Al ≤ 0.002%, but if slag / metal mixing occurs at that time, the S produced during the S removal process according to the S removal equilibrium reaction between the slag and the metal In order to prevent re-S from high slag in the ladle, CaO, CaCO ₃ etc. are added or blown after the S removal to solidify or increase the viscosity of the S slag into an inactive state, and oxygen blowing and then It is necessary to perform deoxidation by the RH method or CAS method so that the mixing reaction between ladle slag and molten steel does not occur as much as possible until after oxygen blowing and subsequent deoxidation by Ti and the like.

That is, oxygen gas is blown into the RH tank that does not come into contact with the slag in the ladle or into the dipping pipe of the CAS equipment to oxidize Al in steel to 0.002% or less, and then a deoxidizing agent such as Ti is placed in the RH tank or the CAS dipping pipe. Is added to deoxidize the molten steel. Most of Al ₂ O ₃ produced during oxygen blowing is floated and separated under the slag outside the RH or CAS immersion pipe.

It is not possible to completely prevent S from recovering S by the above-mentioned method so that S ≦ 0.002% is removed during the S removal process. Therefore, ladle slag that occurs after the subsequent oxygen blowing is not possible. This is because the allowable amount of return S from S is considered up to S = 0.001%.

Here, in order to reliably oxidize Al in steel to ≤ 0.002%, molten steel free oxygen was measured by an oxygen sensor of the commercially available oxygen concentration cell method, and the lower limit of the free oxygen value was 60 pp.
Let m. Equilibrium reaction in steel When free oxygen is 60 ppm, in steel
Al is 0.001% or less, but was set to 60 ppm in consideration of the representativeness of sampling in a ladle and the accuracy of oxygen analysis.

If the free oxygen is too high, the yield of the deoxidizing agent such as Ti thereafter deteriorates, and the deoxidizing product grows, grows, aggregates, and grows, and becomes a harmful inclusion, so the upper limit was made 150 ppm.

In addition, deoxidation of Al in steel after oxidation is removed by using a deoxidizer other than Al.
i, Zr, Ca, REM, Si, Mn, Mg, Nb, etc., and one or more of their alloys are used. Here, in order not to raise Al in the steel, a deoxidizing agent or a deoxidizing agent that makes the total amount of Al mixed in the molten steel from the alloy 10 g or less per molten steel Ton is used.

Also, after deoxidation, the oxygen in the steel falls again, and between the slag and metal,
Due to equilibrium reaction between refractory metals in slag or refractory
To prevent Al ₂ O _{3 from} being reduced and Al in steel rising, the Al ₂ O ₃ content in the slag in the ladle before deoxidation, in the ladle refractory, in the refractory of the secondary refining equipment, and in the TD refractory was adjusted. It is desirable that each be 10% or less.

〔Example〕

Table 1 shows the manufacturing conditions of the steel manufactured according to the present invention and the compositional changes with time of the obtained steel, and Table 2 shows the compositional changes of the steel obtained by the conventional manufacturing method.

Nos. 1 to 5 of the examples of the present invention shown in Table 1 have the same manufacturing process. That is, the hot metal pretreated with S <0.006% of hot metal by the CaO-based flux injection method in the topeed was smelted in the converter, and then tapped, and Fe-Si within the range of the target composition of the steel at tapping, Fe-Mn alloy and Al were added so as to contain 0.02 to 0.10%. After that, in a ladle sealed with an Ar atmosphere, 5-8 kg of CaO (+ CaF ₂ ) per molten steel Ton was subjected to S deoxidation treatment to S ≦ 0.002% by a powder injection method. After that, add ₃ to 5 kg / T of CaCO ₃ on the slag in the ladle to cool and deactivate the de-S slag, and then perform the RH degassing process.
Oxygen gas is blown into the RH tank until the free oxygen in the steel reaches 0.006 to 0.015%, and the Al in the steel is oxidized.
One or two kinds of Fe-Ti or Fe-Zr and Ca-Si alloy were added and stirred for deoxidation. In both cases, S ≦ 0.003%, Al ≦ 0.
003% ultra low S ultra low Al steel was obtained.

On the other hand, in Comparative Examples Nos. 1 to 5 of the conventional method shown in Table 2, the hot metal de-S-treated to S <0.006% by the hot metal pretreatment was scoured in the converter. No. 1 among them is S by CaO powder injection after deoxidizing Al at the time of tapping from the converter to the ladle.
= 0.001% deoxidation treatment followed by Ti deoxidation gives an extremely low S steel, but Al in the steel is high.

In the case of No.2, the CaO powder injection de-S treatment was performed without deoxidizing Al at the time of tapping, but since oxygen in the steel was too high, it was hardly de-S and Ti and Zr were used during RH treatment. After deoxidation, the Al content in the steel was low, but low S steel with S ≦ 0.003% or less was not obtained.

The example of No.3 is 6kg / T during de-S treatment without deoxidizing Al during tapping
Injecting 1kg / T Ca-Si powder with CaO powder (+ CaF ₂ ) and deoxidizing with Ca,
Although S = 0.003% progressed de S for oxygen in the steel is low is obtained, Al ₂ O ₃ contained in the Al ₂ O ₃ and the slag in the ladle refractory is reduced in the steel Al Becomes higher and Al <0.005% is not obtained.

[Effect of the Invention] As described above, according to the present invention, there is a remarkable effect that an extremely low S and extremely low Al steel having 0.003% or less S in steel and 0.003% or less Al can be easily manufactured.

Claims (1)

[Claims] 1. An Al content in the steel is adjusted to 0.02 by adding an alloy for adjusting the composition to molten steel in a ladle discharged from a converter.
To 0.10%, and then CaO or Ca alloy is added to reduce S in the steel to 0.002% or less, and then oxygen is supplied to the molten steel without mixing and stirring between the slag and the molten steel. Blow to 150ppm to reduce the Al content in the steel to 0.002% or less, and then select Ti, Zr, Ca, REM, Mg, Nb 1
A method for producing ultra-low S, ultra-low Al steel, characterized by deoxidizing with one or more deoxidizing agents.