JP3217535B2 - Low oxygen steel smelting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a Ti-added low carbon steel having a limited Si content and using almost no Al alloy as a deoxidizing agent, and is mainly used for thick plates or pipes.

[0002]

2. Description of the Related Art In recent years, material properties required for high-grade steels such as marine structures, ships, and line pipes have become increasingly severe. In particular, drastic improvement of low-temperature toughness in a weld is desired. On the other hand, by dispersing fine oxides mainly composed of Ti and mainly 5 μm or less uniformly in the steel material, many fine ferrites are formed in the austenite grains using these as nuclei in the cooling process after welding. Japanese Patent Application Laid-Open No. Sho 60-7015 discloses a method for producing a steel material which can be formed to obtain excellent toughness by effectively refining the crystal grains of the weld heat affected zone.

[0003]

However, when the Si content is higher than 0.10 mass%, island-like martensite tends to be generated during welding. In steel materials used in harsh environments such as under stress and low temperatures in cold regions, this island-like martensite becomes a problem.
It is necessary to make it lower than 0 mass%. In addition, in this steel material, the lower limit of the Ti content and the upper limit of the Al content are determined from the formation of intragranular ferrite, and the upper limit of the Ti content is specified from the viewpoint of suppressing precipitation of hard Ti carbide. : 0.005 to 0.020 mass% and Al:
A component range of 0.003 mass% or less is required.

When the steel material is melted, oxygen tends to increase due to low contents of Al, Si and Ti which are deoxidizing elements. The converter slag contains an oxidizing component (% FeO), and is usually inevitably mixed into a ladle during tapping after converter blowing. In molten steel containing low Al and low Si containing a small amount of deoxidizing elements, the reduction of FeO in the converter slag flowing out is insufficient, the (% FeO) of the ladle slag is high, and the oxygen of the molten steel is high. When the oxygen of the steel material increases, the amount of oxide larger than 10 μm, which is the starting point of crack generation, increases, and the toughness deteriorates. In particular, when the oxygen content exceeds 0.0025 mass%, the deterioration of the material becomes remarkable. Therefore, in these steel materials, it is necessary to reduce the oxygen content.
It needs to be 0025 mass% or less.

An object of the present invention is to provide a low-Al, low-Si, Ti-added steel (Al: 0.003 mass% or less, Si: 0.10%).
mass% or less, Ti: 0.005 to 0.020mas
s%) to suppress re-oxidation from the slag and reduce the oxygen content to 0.0025 mass% or less. In addition,
In the present invention, since the toughness decreases as the C content increases, C is targeted at a steel material of 0.20 mass% or less.

[0006]

The present invention has been made to solve the above-mentioned problems, and has a C: 0.20% by mass or less;
l: 0.003 mass% or less, Si: 0.10 mass
s% or less, Ti: 0.005 to 0.020 mass% in a method for producing a steel material, in which the steel receiving amount exceeds 70% of the target steel receiving amount until the completion of the steel receiving in the converter tapping. 0.15 to 0.50 kg of A per ton of molten steel with lime
1 is added to a slag layer to reduce FeO of the slag on the molten steel in the ladle and reduce the oxygen of the steel material to 0.0025 mass% or less.

[0007]

In the following, low Al, low Si, Ti added steel (Al:
0.003 mass% or less, Si: 0.10 mass%
Hereinafter, reoxidation from slag is suppressed by molten steel of Ti: 0.005 to 0.020 mass%), and the oxygen content is reduced to 0.0
A method for reducing the content to 025 mass% or less will be described.

After blowing the converter, the outflow of converter slag is suppressed as much as possible at the time of tapping, but the slag inevitably flows out, and the slag causes reoxidation. Therefore, it is necessary to reduce the reoxidation of molten steel by controlling the composition of the slag. The composition of the ladle slag is largely controlled by the following Si content of the molten steel. Si + 2FeO → SiO ₂ + 2Fe

That is, as shown in FIG. 1, as the molten steel Si decreases, the reduction of FeO in the slag, which is the oxidizing source, is insufficient and the (% FeO) of the slag increases. Also, (% Fe
Due to the rise of O), [O] in the slab (steel material) increases as shown in FIG. Therefore, it is necessary to reduce the degree of oxidation (% FeO) in the slag.

In the present invention, Al is added to the slag to react with FeO in the slag to reduce FeO, and the generated Al ₂ O ₃ is absorbed in the slag to minimize the penetration into the molten steel. Suppresses the increase in Al concentration in molten steel. In this steel material, the content of Al is 0.003.
mass% or less, and it is necessary to reduce Al in the molten steel. In order to reduce (% FeO) in the slag, it is necessary to suppress the supply of Al to the molten steel as much as possible and supply the Al to the slag to react with the slag.

The outflow of slag at the time of tapping the converter is remarkable in the latter half of tapping, particularly during the period from when the ladle receiving amount exceeds 70% of the target receiving amount to completion. At the time of tapping, a part of the Mn alloy is mainly added to the Si alloy. The deoxidized product reacts with the slag flowing out of the converter, and the mixed slag becomes ladle slag.
The addition of Al is desirably at a time when the outflow of the slag becomes remarkable in order to give priority to the reaction with the slag, that is, during a period from when the steel receiving amount of the ladle exceeds 70% of the target steel receiving amount to completion.

If the addition amount of Al is too low, the reduction of slag is insufficient, and if it is too large, a large amount of alumina-based inclusions is formed in the molten steel, deteriorating the cleanliness of the steel, and further increasing the Al content. As a result, the dissolved oxygen in the molten steel decreases, and the concentration of Ti ₂ O ₃ in minute oxides of 5 μm or less decreases, and it becomes difficult to generate intragranular ferrite. In a series of tests this time, it was found that the addition amount of Al was 0.15 to 0.50 kg / t-molten steel.

The lime absorbs the produced alumina, dilutes the FeO component with an increase in the amount of slag, and converts the slag basicity (=
(% CaO) / (% SiO ₂ )) is added to reduce the degree of oxidation as the activity of FeO decreases with an increase in (Fe). If the added amount of lime is too small, the absorption of the formed alumina will be insufficient, and if the amount of lime increases, the melting temperature of the slag will increase and the absorption capacity will decrease.
T-molten steel has been found to be suitable.

[0014]

Examples are shown below. Fig. 3 shows Al during tapping
The change of FeO in slag with the addition amount and the analysis values of oxygen and Al of the cast slab are shown. The input Al amount is 0.15kg
When it exceeds / t- molten steel, the reduction of FeO in the slag progresses to approximately 2 mass% or less, and accordingly, the oxygen content can be reduced to 25 ppm or less. However, when the amount of Al added exceeds 0.5 kg / t-molten steel, the Al in the steel material becomes 0.0
When the content exceeds 03 mass%, the formation of intragranular ferrite is deteriorated, and the toughness during welding is reduced.

FIG. 4 shows the F of the ladle slag according to the addition time of Al.
Indicates eO. According to the method A of the present invention, if Al is added during the time when the steel receiving amount during the converter tapping exceeds 70% of the target steel receiving amount and the steel receiving is completed, the FeO can be reduced to 2 mass% or less and the oxygen amount 2
A steel material of 5 ppm or less could be produced. On the other hand, method B, which is a comparative method, is a method in which Al is added during the period from the start of converter tapping until the steel receiving amount reaches 70% of the target steel receiving amount. Al is added almost evenly between them,
In any case, FeO is 1 to 5 mass%, and the reduction of FeO in slag is insufficient.
pm or less could not be achieved. It can be seen that the addition time of Al is optimum during the output of the converter when the steel receiving amount exceeds 70% of the target steel receiving amount and until the steel receiving is completed.

[0016]

According to the present invention, between 0.15 and 0.50 kg / t-molten steel is added together with lime until the steel receiving amount during the tapping of the converter exceeds 70% of the target steel receiving amount and the steel receiving is completed. Al is added to the slag layer to reduce FeO in the slag on the ladle molten steel, so that even steel materials having a low content of deoxidizing elements can be stably added.
The oxygen amount could be reduced to 0025 mass% or less.

As an additive alloy, in addition to Al, Ca-A
Although an 1 alloy or a Zr alloy is also conceivable, as can be seen from FIG. 5, Al is the cheapest, and according to the method of the present invention, low-oxygen steel can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 (% F in molten steel Si and ladle slag after tapping from converter
graph showing the relationship of eO)

FIG. 2 is a graph showing the relationship between (% FeO) in ladle slag and oxygen in a slab.

FIG. 3 is a graph showing the relationship between the amount of Al added and (% FeO) in ladle slag, oxygen in a slab, and Al concentration.

FIG. 4 is a graph showing the relationship between the time of Al addition and (% FeO) in ladle slag.

FIG. 5 is a graph comparing the cost of each additive alloy.

Claims (1)

(57) [Claims] 1. C: 0.20 mass% or less, Al:
0.003 mass% or less, Si: 0.10 mass%
Hereinafter, in a method of manufacturing a steel material having a Ti content of 0.005 to 0.020 mass%, the lime is supplied from the time when the steel receiving amount exceeds 70% of the target steel receiving amount to the completion of the steel receiving in the converter tapping. And 0.15 to 0.50 kg of Al per ton of molten steel is added to the slag layer to reduce FeO of the slag on the ladle molten steel and reduce the oxygen of the steel material to 0.0025 mass% or less. Oxygen steel smelting method.