JP2001105101A - Melting method of steel plate for thin sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a melting method of molten low carbon steel for steel thin sheet blank, capable of surely preventing the surface flaw and the nozzle clogging by executing deoxidization mainly with Ti so as not to develop alumina base intervening substance. SOLUTION: This melting method of the low carbon steel thin sheet is that the component adjustment of this molten steel is executed after decarburizing to <=0.01% carbon content with a vacuum degassing treatment and the total Ti adding amount is divided into >=two parts and added into the molten steel and thereafter, Ca is added and the substance is floated up and separated by stirring the molten steel with a circulating type vacuum degassing apparatus or an Ar gas bubbling. Then it is desirable to control the dissolved oxygen amount after adding the first Ti to be 10 ppm-100 ppm. Further, it is desirable to secure the substance floating-up time of >=30 sec till adding Ti at the next time after the first Ti, and to secure the stirring time of >=30 sec after adding Ca.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low carbon steel sheet having excellent workability and formability.

[0002]

2. Description of the Related Art Molten steel refined in a converter or a vacuum processing vessel contains a large amount of dissolved oxygen. This excess oxygen is deoxidized by Al which is a strong deoxidizing element having a strong affinity for oxygen. It is generally done. However, Al generates alumina-based inclusions by deoxidation, and these aggregate and coalesce to form coarse alumina clusters. The alumina clusters cause surface flaws during the production of the steel sheet, and greatly deteriorate the quality of the thin steel sheet. Further, the alumina-based inclusions in the molten steel easily adhere to a continuous casting nozzle such as a tundish nozzle, and when the nozzle is closed, smooth casting becomes difficult. In particular, in low-carbon molten steel, which is a material for thin steel sheets with a low carbon concentration and a high dissolved oxygen concentration after refining, the amount of alumina clusters is very large, surface flaws and nozzle blockage are extremely likely to occur, and alumina inclusions Reduction measures have become a major issue.

On the other hand, in the prior art, Japanese Patent Laid-Open No. 5-1042
No. 19, a method for removing alumina-based inclusions by adding a flux for adsorption of inclusions to the surface of molten steel;
A method of adding CaO flux to molten steel by using an injection flow disclosed in Japanese Patent Application Laid-Open No. 3-149057 and thereby adsorbing and removing alumina-based inclusions has been proposed and implemented. On the other hand, as a method of not forming alumina inclusions instead of removing them, as described in Japanese Patent Laid-Open No. 5-302112, molten steel is deoxidized with Mg, and almost no deoxidation is performed with Al. A method is also disclosed.

[0004]

However, in the above-described method of removing alumina-based inclusions, it is necessary to reduce the amount of alumina-based inclusions generated in low-carbon molten steel to a level that does not cause surface flaws or nozzle blockage. Is very difficult.
Further, in the Mg deoxidation that does not produce any alumina-based inclusions, the vapor pressure of Mg is high and the yield to molten steel is very low.
A large amount of Mg is required to deoxidize only with g, and it is not a practical process in view of the production cost.

[0005] In view of these problems, the present invention provides a thin steel sheet which can reliably prevent surface flaws and nozzle blockage by performing deoxidation mainly using Ti so as not to form alumina inclusions. An object of the present invention is to present a method for smelting low carbon molten steel as a material.

[0006]

Means for Solving the Problems In order to solve the above problems, the present invention has the following features. That is, (1) the carbon content is reduced to 0.01% in the low carbon thin steel sheet smelting method.
After decarburizing to the following, Ti is added to the molten steel in two or more portions, and Ca is further added. Then, the molten steel is refluxed by a reflux vacuum degassing apparatus, or bubbling is performed by blowing Ar gas to remove inclusions. This is a method for smelting a low carbon steel sheet, which is characterized by floating separation. (2) In the method for producing a low-carbon thin steel sheet, after decarburizing the carbon content to 0.01% or less by vacuum degassing, the composition of the molten steel is adjusted.
The total amount of Ti is divided into two or more parts and added to molten steel.
a, and refluxed by a reflux type vacuum degassing device, or Ar
This is a method for smelting a low-carbon thin steel sheet, in which inclusions are floated and separated by bubbling by blowing gas. (3) The method according to any one of claims 1 and 2, wherein the dissolved oxygen amount is from 10 ppm to 100 p after the first Ti addition.
pm, which is a method for melting low carbon thin steel sheets. (4) In claim 1 to claim 3, after the first Ti addition, by the next Ti addition.
This is a method for melting a low carbon thin steel sheet, which secures an inclusion floating time of 30 seconds or more. (5) The low-carbon thin steel sheet according to any one of (1) to (4), wherein a recirculation time of the recirculation type degassing device after addition of Ca or a bubbling time of Ar gas injection is secured for 30 seconds or more. This is a smelting method.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
In the smelting method of the present invention, molten steel that has been refined in a steelmaking furnace such as a converter or an electric furnace and then degassed by vacuum degassing to a carbon content of 0.01% or less, which is necessary as a material for thin steel sheets, Is added in two or more portions to perform deoxidation and component adjustment.
Is added and then stirred to promote floating separation of inclusions. The basic idea of this smelting method is to increase the cleanliness of molten steel by lowering the melting point of inclusions and promoting flotation by the first addition of Ti, and then to reduce a small amount of inclusions generated by the second and subsequent additions of Ti. In order to obtain ultra-clean steel.

[0008] The first Ti added in a state where the dissolved oxygen concentration in the molten steel is high reacts rapidly with the dissolved oxygen in the molten steel,
It becomes a complex inclusion of iron oxide and titania. The inclusions are in the liquid phase in the molten steel and are almost separated by flotation by coarsening due to agglomeration / coalescence. Therefore, the concentration of the inclusions greatly decreases as the dissolved oxygen in the molten steel decreases. The main purpose of the first addition of Ti is to deoxidize it, but as described above, it controls the composition of inclusions with a low melting point that are relatively easy to aggregate and coalesce, and also has the role of minimizing the amount of inclusions by flotation. ing. Therefore, the dissolved oxygen in the molten steel is reduced as much as possible,
In addition, it is important to control the inclusions in the molten steel to composite oxides of iron oxide and titania that are easily floated and separated, and in an experimental study, the dissolved oxygen in the molten steel was reduced from 10 ppm to 100 ppm.
It is effective to add Ti for the first time so as to leave a certain amount. The reason is that if the dissolved oxygen in the molten steel after the first addition of Ti is deoxidized to less than 10 ppm, solid-phase titania-based inclusions are formed and it is difficult to float and separate, and the dissolved oxygen is reduced to 100 ppm after the first addition of Ti. This is because a large amount of solid-phase titania-based inclusions, which are difficult to float and separate by adding Ti from the second time onward, will be generated if they remain excessively. Further, it is preferable to provide a floating separation time for inclusions before the second addition of Ti, and the floating separation time was experimentally evaluated with this inclusion composition, and it was found that 30 seconds or more should be secured.

The Ti added separately from the second time onwards reacts with the remaining dissolved oxygen to form solid titania-based inclusions. Because this high melting point inclusion is difficult to float and separate,
Further, the cleanliness of the molten steel cannot be improved. For this reason, after the addition of Ti, Ca is added to modify the high-melting titania-based inclusions to low-melting titania-calcia-based inclusions, and to promote agglomeration and coalescence of the inclusions by adding stirring. And achieve rapid flotation. If Ti is added at once, all dissolved oxygen after the degassing process becomes titania-based inclusions and does not float and separate, so a large amount of Ca must be added and long-time stirring is required to reform this. Become. As a method of stirring, it is effective to circulate the molten steel by a circulating vacuum degassing apparatus or to stir by blowing Ar gas into the molten steel, and a stirring time of 30 seconds or more is secured in any method. It was confirmed experimentally that this was sufficient. As a result, the inclusions in the molten steel can be efficiently reduced without generating alumina inclusions, so that it is possible to smelt a low-carbon molten steel as a material for a thin steel sheet that can reliably prevent surface flaws and nozzle blockage. .

The total amount of Ti is an amount necessary for deoxidizing dissolved oxygen in the molten steel and fixing carbon and nitrogen in the molten steel in order to secure the quality of the material. Although it depends on the amount of dissolved oxygen and the concentrations of carbon and nitrogen, 0.0
It is advisable to add it in consideration of the yield so as to be at least 05%, preferably at least 0.01%. The Ti to be added is not limited to high-purity Ti as in the case of sponge-like Ti, and the above effects are not impaired even when added as an alloy such as Fe-Ti.

[0011] The amount of Ca added is the amount of T
The amount of titania-based inclusions generated by the reaction between i and dissolved oxygen is greater than or equal to the amount required to make the low-melting titania-calcia-based composite oxide, and Ca reacts with the refractory or mold powder. It is below the amount that does not contaminate the molten steel. That is, the appropriate range is about 1 to 100 ppm in the Ca concentration in the molten steel. The addition of Ca may be performed after the addition of Ti, and may be added in any of a ladle, a tundish, a continuous casting mold, or the like by an injection method, a wire addition method, or the like. Further, Ca addition can be performed with pure Ca, but since the vapor pressure of Ca is high, C such as Ca-Si
You may add as an a alloy.

It is preferable not to add Al in the molten steel, but if necessary, it may be added at 0.01% or less.
This is because if the Al concentration is attained, the addition of Ca also lowers the melting point of alumina-based inclusions.

In ultra-low carbon steel sheets and the like which are severely processed for automobile outer panels, it is necessary to make C as low as possible in order to impart workability, and the C concentration is 0.01% or less, preferably 0.005%. It is better to do the following.

[0014]

The present invention will be described below with reference to examples and comparative examples. Example: 200 ton of molten steel refined in a converter was decarburized to a carbon concentration of 30 ppm by a reflux vacuum degassing apparatus, and then 150 kg of Ti was added to the molten steel in the ladle to obtain a dissolved oxygen concentration of 20 ppm.
ppm, and then the molten steel in the ladle was refluxed for 1 minute with a reflux vacuum degassing apparatus. After adjusting the composition by adding 50 kg of Ti to the molten steel, Ca was further added to the molten steel in the ladle by a wire addition method to obtain a Ti concentration of 0.02% and a Ca concentration of 0.02%.
Molten steel having a concentration of 5 ppm was smelted, and this was further refluxed for 30 seconds by a reflux vacuum degassing apparatus. This molten steel was cast by a continuous casting method to cast a slab having a thickness of 250 mm and a width of 1800 mm. The nozzle opening during continuous casting was constant, and no nozzle blockage occurred. The cast slab is 8
It was cut to a length of 500 mm to make one coil unit. The slab thus obtained was hot-rolled and cold-rolled by a conventional method to finally obtain a cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1800 mm. Regarding the slab quality, visual observation was performed on an inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, no surface defects occurred.

Comparative Example: 200 ton of molten steel refined in a converter was decarburized to a carbon concentration of 30 ppm by a reflux vacuum degassing apparatus, and then 200 kg of Al was added to molten steel in a ladle to deoxidize the steel, and then 45 kg. And an Al concentration of 0.0
The composition was adjusted to 4% molten steel having a Ti concentration of 0.02%. This molten steel is cast by a continuous casting method and has a thickness of 250 mm and a width of 180 mm.
A 0 mm slab was cast. The nozzle opening during continuous casting gradually opened from the start of casting, and was fully opened at the end of casting, causing nozzle blockage. The cast slab is 850
It was cut to a length of 0 mm to make one coil unit. The slab thus obtained was hot-rolled and cold-rolled by a conventional method to finally obtain a cold-rolled steel sheet having a thickness of 0.7 mm and a width of 1800 mm. Regarding the slab quality, visual observation was performed on an inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, as many as 20 surface defects were generated per coil.

[0016]

As described above, according to the present invention, the inclusions in the molten steel can be reduced and the inclusions can be further refined without producing alumina-based inclusions. It becomes possible to smelt a low carbon molten steel for a thin steel sheet having excellent workability and formability capable of reliably preventing surface flaws and nozzle blockage.

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Claims (5)

[Claims] In a method for producing a low carbon thin steel sheet, after decarburizing the carbon content to 0.01% or less, Ti is added to the molten steel in two or more parts, and Ca is further added. A method for melting a low carbon thin steel sheet, comprising refluxing by a reflux vacuum degassing apparatus or bubbling by blowing in Ar gas to float and separate inclusions. 2. In a method for producing a low carbon thin steel sheet, after decarburizing the carbon content to 0.01% or less by vacuum degassing, the composition of the molten steel is adjusted, and the total amount of Ti added is divided into two or more. Melting steel, then adding Ca, and buoyant inclusions by bubbling with reflux or Ar gas injection in a reflux type vacuum degassing apparatus to separate inclusions by flotation. Method. 3. The method according to claim 1, wherein the amount of dissolved oxygen is from 10 ppm to 100 ppm after the first addition of Ti.
A method for melting low carbon thin steel sheets, characterized in that the control is performed so that 4. The low carbon thin steel sheet according to claim 1, wherein an inclusion floating time of 30 seconds or more is secured after the first Ti addition and before the next Ti addition. Melting method. 5. The low carbon material according to claim 1, wherein a recirculation time by a recirculation type degassing device after Ca addition or a bubbling time by bubbling Ar gas is maintained for 30 seconds or more. Melting method of thin steel sheet.