JPS58181817A - Manufacture of low-phosphorus al-killed steel - Google Patents

Abstract

PURPOSE:To extremely efficiently manufacture high-grade low-phosphorus Al-killed steel, by smelting pig iron melt together with necessary adjutant raw materials into a state having low C content with upper-blown oxygen and bottom-blown gas, vacuum degasifying the formed steel to deoxidize and decarburize it, adding Al to it, and then continuously casting it. CONSTITUTION:Pig iron melt tgether with necessary adjutant raw materials is bottom- blown with gas during and/or after being refined with upper-blown O2 in the converter furnace which enables the upper blowing of O2 and the bottom blowing of gas to make steel of [C] below 0.07%, and the resulting steel melt is tapped under an undeoxidized condition. The undeoxidized tapped steel melt is then treated in a vacuum degasifier to perform decarburization, deoxidation and the adjustment of necessary components. From the time when [C] comes close to an objective value with the progress of the decarburizing reaction of C+O CO in the vacuum degasifier, Al is added in installments to reduce [O] down to about 0.002-0.005%. Thereafter, the steel melt after being treated in this way is formed into a cast piece of predetermined size by a continuous casting process. The formation of Al2O3 clusters is little in this continuous cast piece, and said cast piece is a low-phosphorus Al-killed one extremely excellent in quality as compared with a conventional Al-killed cast steel piece.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for producing low phosphorus, low carbon A/killed steel, particularly by rationally combining the steps from refining to continuous casting.
The present invention relates to a method for economically producing the above-mentioned high-grade steel.

Ill, which is used as a material for cold-rolled steel sheets for automobiles, is required to have a variety of properties depending on its use, and research and development in various fields such as chemical acid content and rolling conditions are being carried out. On the other hand, this type of steel is a typical mass-produced product, and reducing its manufacturing cost is also a top priority for the industry. The low phosphorus, low carbon A/killed steel that is the object of the present invention has recently been widely used, especially as cold rolled steel sheets for automobiles, but the process currently adopted as its manufacturing method involves There are some drawbacks in terms of quality and manufacturing cost, but the best product is the best! −
It's hard to say.

In general, low-carbon 1-killed steel is produced by a process of melting in an oxygen top-blowing converter (hereinafter referred to as LD converter), followed by deoxidation and composition adjustment, followed by continuous B-casting. i! It is left behind. In this case, in order to lower the product CI, the decarburization reaction must be sufficiently carried out in the converter, and the oxidation loss of Fe, Mn, etc. inevitably increases with this decarburization reaction. Normally, the blow-off point [C] in converter blowing is set lower than the carbon content of the product, taking into account the increase in the amount of coal from Fe, Mn, etc. added later for component adjustment. Therefore, especially when melting low carbon steel for cold rolling using an LD converter, peroxidation of the molten steel,
The increase in FeO in the slag is significant. When producing Ar-killed steel, such peroxidized molten steel is exposed to A after the steel is tapped.
Deoxidation by IK must be performed, but A for decarburization
An increase in the amount of / not only significantly affects the manufacturing cost of steel, but also increases in Ar and O produced as a result of the decarburization reaction, leading to undesirable problems such as nozzle clogging during continuous casting and an increase in product clusters. cause no problems.

On the other hand, from the viewpoint of reducing phosphorus, which is required to improve the impact properties, weldability, etc. of steel, a certain degree of overoxidation of molten steel is required. That is, the equilibrium equation for the dephosphorization reaction in a conventional converter is the Bajiva equation, where K is a constant that changes depending on the temperature: 20.08 at 1550°C, 20.41 at 1585°C, 2 at 1635°C.
0.83 The formula for the H axis 1)l has been proposed, in which the values in parentheses indicate the components in the slag, and those in parentheses indicate the components in the molten steel. %T, Fe) or if(%Fed) is said to be necessary. P by blowing with a conventional LD converter
&0.018% steel, approximately 16% (%T, Fe) is appropriate.

However, increasing (%T, Fe) in this way not only reduces the yield of Fe as described above, but also causes many problems, such as an increase in the amount of 1/ as a thinning agent, which increases manufacturing costs. There is.

Recently, the present inventors have developed and are putting into practical use a so-called composite blowing technique in which gas is also blown from below the surface of the molten steel bath instead of the conventional converter blowing method in which only oxygen is blown from above. This composite blowing method introduces primarily a stirring gas under each surface along with top-blown oxygen, and has many advantages compared to the conventional LD method, such as shortening reaction time and improving yield. It is something. In the process of putting the composite blowing method into practical use, the present inventor recognized that by appropriately combining this method with a vacuum degassing method, low phosphorus, low carbon Ar killed steel can be produced extremely efficiently. The present invention is based on this knowledge and provides a new manufacturing method that solves many of the difficulties in manufacturing low phosphorus, low carbon, Ar killed steel using conventional LD converter filtration.

The method for producing low phosphorus low carbon Ar killed steel of the present invention includes (a)
Hot metal and required auxiliary raw materials are subjected to gas downward blowing during oxygen top blowing refining or at least either after oxygen top blowing refining in a converter capable of oxygen top blowing and gas downward blowing [C] 0. 07%
A process of melting the following steel and tapping it without deoxidizing, (B) The above-mentioned undeoxidizing tapped steel [C) 0.07% or less 1
A step of treating SIII4 in a vacuum degassing device to deoxidize and decarburize, C9 during the above treatment or after the completion of the treatment, A
The method is characterized by comprising the following four steps: a step of adding r or Ar and necessary additives, and a) a step of continuously casting the molten steel that has been treated up to Cf above.

Hereinafter, the method of the present invention will be explained in detail along each of the above steps.

FIG. 1 is a conceptual diagram showing changes in carbon [C] in molten steel and iron (T, Fe) in slag during blowing in the conventional LD method and combined blowing method. The experimental conditions for obtaining this figure are outlined below.

Common items Hot metal ratio: 285-100% Hot metal composition: C4, 50% Si 0.45% Mn 0.30% P 0.120% SO, 01% Converter capacity: 250 tons Top blown acid rate: 2.1 ~2.5Nd 1 minute O ton Combined blowing method only Bottom blowing gas: Ar, N, Co, 0° Nozzle: Bottom blowing double pipe Flow rate: o, o3N-/min・ton Figure 1 As can be seen, (T, Fe)Fi increases as decarburization progresses in all cases, but in the combined blowing method, the curve is much more to the left than in the LD method. As the broken line in the figure shows, [C] is 0.05%
In the case of performing blowing to lower Fi (T, Fe) using the LD method,
) reaches 20%, but in the composite blowing method, (T, Fe)#
-i stays at 15%. In other words, according to the composite blowing method, even if low-carbon blowing is performed to lower the length of longitudinal cables in molten steel, the loss of Fe hardening is small, and this means that not only FiFe but also useful materials such as Mn can be used. The same can be said about ingredients. This is mainly due to the fact that in the combined blowing method, gas is blown below the bath surface, which actively stirs the molten steel and promotes the decarburization reaction of c+o-co.

In addition, after the end of Shunso, 1llll Ar for a very short time,
It is also possible to obtain extremely low [C] molten steel by a so-called rinsing process that only performs downward blowing, such as pipeCOl. It takes shape. That is, in the first step C4) of the method of the present invention, it is necessary to carry out the downward blowing of the gas during the top blowing of the sulfur element or at least once after the top blowing of the sulfur element is completed. It may be done during both periods. Here, "bottom blowing" means blowing gas through a nozzle or Volks brick provided at the bottom of the converter furnace or at a position below the bath surface of the furnace wall. The bottom blowing gas to be used is N* *CO!
I Ot * C@H@ + A r is used, but
Fi N t is used for the melting of low carbon low phosphorus steel which is the object of the present invention.
, COt.

It is desirable to use Ow, Cs, and Hs.

FIG. 2 shows an example of the gas blowing pattern in step (A) in the method of the present invention. 0. Before blowing up, blow N. down, and then blow 0. After starting top blowing, N, -0, is first supplied from the inner tube of the double tube nozzle, and then FiN* is supplied from the outer tube.
COt is injected, and in the low carbon region at the end of blowing, CO, -O, are injected from the inner tube, and CO7 is injected from the outer tube. After the end of the blowing process, blow the N chisel down again.

In the early and middle stages of blowing, [%N] is within the allowable range and is inexpensive! It is preferable to use a mixture of gases, and in the final stage, it is preferable to increase the flow rate to suppress overoxidation and strengthen the stirring by the downward blowing gas.

Figure 3 shows Fe in the slag under the same conditions as in Figure 1.
(T, Fe) and changes in phosphorus [P] in molten steel were investigated. The fact that 1i(P) decreases as (T, Fe) increases is the same tendency as in the equilibrium equation of Healy and Balijin.

However, according to the combined blowing method, the amount of (T, Fe) Vi, LD required to reach the same level is much less than that required for obtaining steel with CP) of 0.015%. The required (T, Fe) tf is about 20% in the LD method, but it is only 15% in the composite blowing method.

In other words, even in the melting of low-phosphorus steel, if the combined blowing method is used, it is possible to minimize the decrease in the Fe yield, and at the same time, it is possible to keep the amount of iron in the molten steel low. A/Difficulties associated with deoxidation will be eliminated. The promotion of the dephosphorization reaction in such a composite blowing method is due to the activation of the slag-metal reaction by increasing the molten steel stirring, that is, the CaO
It is thought that K increases the chance of capturing P, O, and strengthens the stirring, which reduces the temperature difference between the slag and metal, lowers the temperature, and promotes the deP reaction.

In the present invention, the phosphorus content is approximately 0.0% as a low phosphorus steel.
Our goal is to obtain 0.018% or less, so
As seen from FIG. 3, (T, Fe) may be about 12% or more. The amount of [C] corresponding to this amount of (T, Fe) is about 0.07% as seen in FIG. In other words, it is possible to obtain molten steel with a concentration of 0.07% (EC) or less by the combined blowing method.
, Fe)Fi is about 12% or more, [P] 0.018%
The following low phosphorus steels can be manufactured.

Of course, if a low phosphorus steel with [P] of 0.010% or less is required, blowing that increases Vi (T, Fe), for example, by reducing the bottom blowing gas flow rate, Although it is possible to adopt a method such as using a so-called soft plow to make the distance between the surfaces larger than usual, such an operation should be decided based on the balance between increasing the cost of the entire process and improving the quality of the product.

The steel produced as described above is tapped without being deoxidized. Here, undeoxidized steel is melted, and during tapping from the converter to the ladle, the deoxidation treatment is not substantially performed in the ladle after tapping. The deoxidation here must be done in the atmosphere, or in the molten steel (
9) and the presence of slag with high FeO content, the yield of the defrosting agent (A/) is extremely poor. Also A4,0. The production of a large amount of silica leads to undesirable results such as product defects and nozzle formation during continuous casting. Furthermore, if Atl- is added here, (CaO)sP*Os in the slag
is reduced, and so-called rephosphorization occurs, in which [P] in the molten steel increases. However, it is permissible to add a small amount of At411 for the purpose of preventing 7 ohms of slag.

The second step (b) of the method of the present invention is to treat the molten steel that has been de-expanded as described above in a vacuum degassing device to efficiently progress decarburization, deoxidation, and necessary component adjustment. It is a process. The reaction here is that [Cl and (6) in the steel are C+0
→It is discharged from the system due to the reaction of CO, and decarburization and deoxidation proceed simultaneously. The degree of decarburization and deoxidation is determined by the degree of vacuum and processing time, but according to the RH method, which is one of the representative vacuum processing methods, under normal operating conditions (molten steel is heated in a 250-ton pot) When molten steel of -0,09% -0,03% is processed at (C1-0,06
It takes approximately 10 minutes to reach %. Thereafter, the decarburization rate rapidly decreases, and it becomes extremely difficult to reduce Cl to 0.04% or less unless special operations such as external oxygen supply are added.

Long-term processing in vacuum processing equipment not only causes problems such as increased equipment operating costs and erosion of the equipment's refractories.
If you eliminate the inconvenience in casting work caused by a drop in molten steel temperature and increase the converter tapping temperature in anticipation of this drop in molten steel temperature,
This results in shortening the life of the converter.

In order to eliminate the above-mentioned problems, it is necessary to keep the [Cl] as low as possible before treatment. As explained at the beginning, the method of the present invention has the disadvantage of using a certain amount of (T, Fe) to reduce phosphorus.
In order to ensure that the EC) at the end of converter blowing is 0.07
% or less, and by adopting the composite blowing method as described above, even if the ni] is lowered to 0.607% or less, the oxidation loss of Fe, Mn, etc. is still the same as in the conventional LD method. does not increase. That is, the molten steel having an EC of 0.07% or less produced by the first step of the method of the present invention is decarburized to a predetermined low [q slope] in a relatively short time in a vacuum processing apparatus. For example, when 1-0.07% molten steel is treated under the above conditions by the RH method, [
Cl = 0.04%, and simultaneous K(6) is also 0.02%
go down to C
The [C
It is better to keep l as low as possible. In order to produce ultra-low-leg steel for cold rolling, such as C0.01% or less, which has recently been expanded in use, vacuum treatment is required to keep Q at 0.04% or less during converter tapping. This is desirable from the standpoint of efficiency.

Next, to explain the process (c), the decarburization reaction of C+0 → CO progressed in the vacuum processing equipment, and [after cl decreased to the target value, C] became close to the target value. From this point onwards, AI is added in several portions. Since a considerable amount of oxygen in the molten steel has already been removed by the reaction of 10JFiC+0-Co, the amount of At required for deoxidation is smaller than before the vacuum treatment. Furthermore, since the addition is performed in a vacuum processing apparatus, the yield of At addition is significantly improved compared to when the At is added in the atmosphere. In both the RH method and the DH method, since slag does not substantially enter the vacuum vessel, there is almost no oxidation loss of At due to contact between the added At and the slag. However, when producing ultra-low phosphorous copper, it is preferable to separate and remove the slag before vacuum treatment in order to minimize the possibility of rephosphorization.

When processing molten steel with [Cl0.06% and [◇0.04%] by the RH method, the amount of At added is 0.6 to 1.04/)
With molten steel, the K content can be lowered to about 0.002 to 0.005%. The amount of At added (approximately 1.6~
2.0-/ton-molten steel), A/! Approximately 1&F/ton of molten steel can be saved in units of Il.

The molten steel that has been decarburized, decarburized and, if necessary, compositionally adjusted, in the vacuum processing apparatus as described above is made into slabs of a predetermined size by continuous casting in step K). Generally A
/In continuous casting of killed sea bream, when the molten steel temperature decreases, Al
Nozzle clogging accidents may occur due to e's precipitation.

However, the amount of A/killed steel #iA/ produced by the method of the present invention is suppressed to the minimum necessary amount,
A/, 0. Since there is little generation of , there is no need to worry about nozzle clogging as in the case of continuous casting of ordinary A/killed steel.

Continuously cast slabs A/, 0.0, manufactured by the present invention as detailed above. It produces fewer clusters and is extremely superior in quality compared to conventional A/killed steel slabs.

Table 1 shows the processing conditions and casting conditions for Examples (1) and (2) of the method of the present invention and the conventional method.

This is a list of slag components and quality.

[Brief explanation of drawings]

Fig. 1 is a graph showing the relationship between Fi [c] - (T, Fe) for the LD method and the composite blowing method. Fig. 2 is an explanatory diagram showing an example of the gas injection pattern of the method of the present invention. FIG. 3 is a graph showing the relationship between (T, F e ) [P] for the LD method and the composite blow fIL method. Patent applicant: Sumitomo Tsubo Industries Co., Ltd. Agent, Patent attorney: Noboru Kono [0(”/,) 1st circle (T-Fe) ('/,)

Claims (1)

[Claims] 1. (a) Hot metal and necessary auxiliary materials are heated at least either in an oxygen top-blown converter or after the oxygen top-blowing converter in a converter capable of oxygen top-blowing and gas bottom-blowing. At , blow the gas down to (C)0
．． 0.07% or less steel and tapping it without deoxidizing; (b) the above [C] step of vacuum descaling the molten W4 of 0.07% or less to deoxidize and decarburize; e →The process of vacuum degassing the upper part and then adding necessary additives in the vacuum processing equipment, and 2) the process of continuously casting the molten steel after the vacuum treatment of the upper part 1, A method for producing low phosphorus A/guild steel, the method comprising: