EP0163784B1

EP0163784B1 - Two stage deoxidation process in steel-making

Info

Publication number: EP0163784B1
Application number: EP84303555A
Authority: EP
Inventors: Maw-Cheng Shieh; Seng-Jung Chen; Li-Jung Hu; Jin-Luh Su
Original assignee: China Steel Corp
Current assignee: China Steel Corp
Priority date: 1984-05-25
Filing date: 1984-05-25
Publication date: 1989-11-02
Also published as: ZA852015B; AU567212B2; DE3480350D1; EP0163784A1; ATE47727T1; AU4022085A

Abstract

A weak pre-deoxidation process (W.P.D. process) for the production of aluminium- and titanium-killed steel. In this process an adequate amount of a silicium-containing ferro-alloy is added to the molten steel during its tapping. The ladle is subsequently transferred to an aluminium wire feeder or to a ladle-injection treating equipment for performing the final stage deoxidation process.

Description

This invention relates to deoxidation practice in steelmaking.
An object of the present invention is to provide an improved deoxidation process whereby steel products of improved cold heading formability may be obtained. In the production of rimmed steel, the use of very minor amounts of deoxidizer results in a high free oxygen content in the molten rimmed steel, which results in a solid skin layer around the ingot surface of good surface quality and soft characteristics but the inner portion of the rimmed steel ingot is not suitable for high grade applications because of contamination by impurities. Furthermore, rimmed steel with a high free oxygen content cannot be cast in a continuous casting process.
For fully killed steel, the deoxidizer (Al, Si, Ti, Mn) added during the tapping process is oxidized by the free oxygen in molten steel. The reaction is shown below:

M: Deoxidizer, such as Al, Si, Ti, Mn etc.
(O): free oxygen in molten steel
x, y: coefficient

Because of the lower free oxygen after deoxidizing, such fully killed molten steel can be cast using a continuous casting process. Generally speaking, the production yield and internal quality of the continuous cast products are superior to those of ingots. Lower recovery rate and higher addition amount of deoxidizer for fully killed steel causes higher production costs and results in a residue of deoxidizing formations remaining in steel. Such residual deoxidation formations are harmful to processing formability.
An object of the invention is to overcome or alleviate the defects stated above to provide lower cost and higher quality steel products.
In the production of steel for cold working or forming applications by continuous casting, fully killed steel is usually adopted to avoid casting accidents and blow hole formation in steel. AI and/or Ti are the major deoxidizers used in the continuous casting process. Killed steel for cold working or forming applications could be classified into AI-killed and Ti-killed steel according to the deoxidizer adopted. In order to reduce the work hardening effect in (for example) At-kitted steel, Si-containing ferroalloy cannot be added to molten steel during the steelmaking process-only Al is used as the deoxidizer. Owing to the deoxidation reaction of AI (2A[+3[01=AI₂0₃), alumina clusters (AI ₂0₃₁ form in the molten steel and remain in the solid steel as inclusions. These inclusions cannot be elongated during deformation, and thus interfere with the cold heading or working formability.
An object of the invention is to overcome the above shortcoming of deoxidation practice, that is to reduce the work hardening effect.
US 2,705,196 discloses a method for the deoxidation of molten steel produced in a furnace by a steelmaking process, comprising a predeoxidizing step in which a silicon-containing ferroalloy is added to the molten steel, and a subsequent deoxidizing step in which a deoxidizing metal is added to the pre-deoxidized molten steel. The method of the invention starting from this known method provides that said silicon-containing ferroalloy is added in a controlled amount sufficient to reduce the free oxygen content of the melt to a level below 340 ppm but insufficient to result in substantial retention of elemental silicon in the melt; after the pre-deoxidizing step the molten steel is stirred, by gas-bubbling to separate silicon dioxide from the molten steel, and said deoxidizing metal is added in finely divided form by a continuous feeder arrangement so as substantially to prevent retention of silicon in the steel.
The process of the invention increases the recovery rate of deoxidizing metal, decreases the amount of deoxidizer and ferroalloy consumption and saves on production cost. Because of the reduced deoxidizing metal and alloy addition, deoxidized formations can be reduced, resulting in a remarkable improvement in the internal cleanliness of the steel products. Optional features of the invention are set out in Claims 2 to 8 below. After treating by AI and/or Ti with the above process, good shrouding systems should be adopted during continuous casting or ingot teeming processes to protect the molten steel from reoxidizing by the atmosphere. Consequently, cleaner steel could be acquired by this new predeoxidation process, which is referred to below as a WPD (weak predeoxidation) process.
This invention will now be explained by way of example only with reference to Figures 1 to 4 of the accompanying drawings of which:

Figure 1 is a schematic flow diagram showing steel making processes employing the deoxidation process of the invention;
Figure 2 is a plot of the amount of silicon-containing ferroalloy against free oxygen content in the molten steel, prior to deoxidation with aluminum;
Figure 3(a) shows the distribution of silicon content of a number of batches of molten steel deoxidised in accordance with the invention;
Figure 4(b) shows the corresponding distribution when the pre-deoxidation step is omitted; and
Figure 4 shows graphically the degree of aluminum recovery for aluminum killed steel produced by a process in accordance with the invention and a conventional deoxidation process.

In general, in order to prevent the liquid steel from containing residual Si, no Si containing ferroalloy could be permitted to be added to molten steel to adjust the chemical composition in producing AI-killed steel; ferromanganese is usually added. But manganese itself is not a good deoxidizer. Therefore, if weak pre-deoxidation with Si contained ferroalloy is not performed before AI addition in producing AI-killed steel, the residual free oxygen content in the liquid steel will be very high and unstable.
Figure 2 indicates that with appropriate amount of Si containing ferroalloy addition the free oxygen content in the liquid steel before AI deoxidation can evidently be lowered. By using this process, the recovery of deoxidizer can be improved and the oxides retained in the liquid steel after deoxidation can be reduced as well, thus the quality of bloom, slab and ingot can be improved.
Figure 3(a) and 3(b) compare the Si content in the liquid steel between WPD Process and non-WPD Process. Figure 3(a) shows the distribution of Si contents in the final molten steel treated by weak pre-deoxidizing with Si contained ferroalloy. Figure 3(b) shows the distribution of Si contents in the final molten steel without WPD treatment.
Figure 3(a) and 3(b) indicates the percent of the number of melts which contain Si less than 0.02% in the liquid steel by using WPD Process is 96.8%, while that of non-WPD Process is 95.8%. The data obviously shows that the proportion of Si content below 0.02% in the liquid steel of WPD Process is even a little bit higher than that of non-WPD Process. The Si content analyzed by spectro-scope is total Si content (including silica), thus confirms that Si contained ferroalloy will not cause Si to be retained in the liquid steel. (It can also be confirmed by microscope). While Si contained ferroalloy is added into liquid steel, Si will react with free oxygen first and forms silicon dioxide (Si0₂) particles, which distribute in the whole liquid steel. Manganese will then react with the oxygen around Si0₂ and forms silicon-manganese oxides, which can float up almost completely after gas stirring. Therefore, it is a characteristic of the present invention that by adding appropriate amount of Si containing ferroalloy during tapping (or into furnace) the free oxygen content can be reduced effectively before AI and/or Ti addition, without fear of Si being retained.
Figure 4 shows the comparison of the rate of A) recovery between AI-killed steel produced by Weak Pre-Deoxidation Process and conventional deoxidation process. For AI-wire feeder system, the rate of AI recovery was evidently increased by this invention as indicated in Figure 4, that is due to the content of free oxygen in molten steel as remarkably decreased. Because of higher recovery rate of Al, caused less AI addition, deoxidation formations could be effectively reduced. Consequently, the internal cleanliness and surface quality of the steel product was remarkably improved by this new process.
Table 1 (page 4) shows the comparison of free oxygen content between WPD Process and conventional deoxidation process before aluminum and/or titanium addition, when the content of free oxygen according to the invention is 1.
A feature of this invention is to reduce the free oxygen content of molten steel as much as possible before the addition of deoxidizers (aluminum and/or titanium) but without retention of Si in the melt. The data listed in the table obviously show that after WPD Process treatment the free oxygen content can be greatly decreased before the addition of deoxidizers.
The amount of free oxygen content lowered can be controlled directly by adjusting the amount of Si-containing ferroalloy addition. Owing to the decrease of free oxygen content, recovery of aluminum can be improved, cost can be lowered, and the quality of steel products can be improved remarkably.
Table 2 (page 4) shows the comparison of typical chemical compositions between the general cold working AI-killed steel grade and the steel designed according to this invention for the same end use. The main difference is that typical chemical composition designed according to this invention has lower aluminum content than that of conventional AI-killed steel grade. The reason for this composition design is to decrease the inclusion formation of deoxidation to get cleaner molten steel. Because of more deoxidizers are added, more chances to form inclusins would result and the cost is also higher. Therefore, the principle of chemical composition design by this invention is to lower the addition of deoxidizers such as aluminum and/or titanium under the condition of no poor deoxidation and good formabilty. And with the aid of WPD Process, the amount of deoxidizers added can be decreased, cleaner steel and lower production cost will be resulted.
This deoxidation method is also suitable for af other kind of AI-killed steel grade.
Table 3 (page 4) shows the comparison of estimated index of inclusions between different deoxidation processes. In respect of quality, the main purpose of WPD Process is to improve the internal cleanliness, and improve the quality of casted steel. The table obviously shows that under this new process, the estimated index of inclusions is much better than that of conventional process. It can also be sured that the WPD Process has much improvement on internal quality of casted steel.
Table ₄ (page 4) shows the comparison of grinding speed of billets between different deoxidation processes. In respect of quality, the WPD process improves not only the internal cleanliness of the casted steel, but also its surface quality. Data listed in the table represent pieces of billets to be ground within unit time (per hour).
(The worse in surfacial quality, the greater the area and depth of grinding required resulting in fewer billets being treated per unit time in order to get same level of surfacial quality).
This table shows that the grinding speed of billets treated by the WPD Process is faster than that of conventional deoxidation process.
Therefore, the WPD Process can make much improvement on surfacial quality of casted steel, and save much surface conditioning cost.
The invention is applicable both to Basic Oxygen and Electric Arc Furnace Steel making processes. In such processes after blowing end or during tapping of the Basic Oxygen or Electric Arc Furnace, Si-containing ferroalloy is added to the molten steel as weak predeoxidation agent.
After adding the optimal amount of Si-containing ferroalloy, the free oxygen content of molten steel can be lowered efficiently whilst ensuring that silcon will not remain in molten steel. This method not only increases the recovery of aluminum and/or titanium, saves much production cost, improves surfacial and internal quality of steel which is good for formability, but also keeps steelmaking operation in good stability. The weak pre-deoxidation step is suitably executed after blowing end and before the addition of aluminum and/or titanium deoxidizer.

Remark: Estimated index value 0-4, 0 is the best.
Remark: Depth of defects to be ground are more than 1.2 mm

Claims

1. A method for the deoxidation of molten steel produced in a furnace by a steelmaking process, comprising a pre-deoxidizing step in which a silicon-containing ferroalloy is added to the molten steel, and a subsequent deoxidizing step in which a deoxidizing metal is added to the pre-deoxidized molten steel; wherein said silicon-containing ferroalloy is added in a controlled amount sufficient to reduce the free oxygen content of the melt to a level below 340 ppm but insufficient to result in substantial retention of elemental silicon in the melt, after the pre-deoxidizing step the molten steel is stirred by gas-bubbling to separate silicon dioxide from the molten steel, and said deoxidizing metal is added in finely divided form by a continuous feeder arrangement so as substantially to prevent retention of silicon in the steel.

2. A method as claimed in Claim 1, further characterised in that said pre-deoxidizing and deoxidizing steps are carried out at separate stations.

3. A method as claimed in Claim 1 or Claim 2, further characterised in that said deoxidizing metal is injected into the molten steel in the form of wire or shot.

4. A method as claimed in any preceding claim wherein said deoxidizing metal is aluminum or titanium.

5. A method as claimed in any preceding claim further characterised in that the molten steel prior to the pre-deoxidizing step is substantially free of dissolved silicon and the amount of added silicon-containing ferroalloy is controlled such that after the deoxidizing step the molten steel has a total silicon content of less than 0.02%.

6. A method as claimed in any preceding claim wherein said steel additionally contains manganese.

7. A method as claimed in any preceding claim wherein the molten steel is produced in a basic oxygen or an electric arc furnace.

8. A method of producing cast steel products comprising providing molten steel, deoxidizing said molten steel by a method as claimed in any preceding claim, and continuously casting the resulting deoxidized molten steel to form said cast steel products.