JP3388933B2 - Continuous casting method of titanium-added ultra low carbon steel - Google Patents

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 continuously cast slab for a titanium-added ultra low carbon steel sheet having excellent surface properties.

[0002]

2. Description of the Related Art Conventionally, as a material for steel sheets having good workability mainly for automobiles, as disclosed in Japanese Examined Patent Publication No. 44-18066, "Manufacturing Method of Cold Rolled Steel Sheets with Excellent Press Formability",
An ultra-low carbon steel containing titanium is widely used in which the content of carbon (C) and nitrogen (N) in the steel is reduced as much as possible and titanium (Ti) that forms a compound with C and N is added in an equivalent amount or more. .

The purpose of adding titanium here is to prevent the formation of wrinkle-like defects, which are called stretcher strains, during deep-drawing and strong forming, so that C and N are dissolved from the matrix. This is because it is combined with titanium and completely removed to render it harmless.

As a method for casting such a cast slab for a titanium-added ultra-low carbon steel sheet, a hollow mold having a periphery made of a water-cooled copper plate is used from the viewpoints of reduction of manufacturing cost by mass production and stabilization of high quality of cast slab. A so-called continuous casting method in which molten steel is continuously supplied and solidified to continuously produce a slab slab is generally adopted.

[0005]

However, when a titanium-added ultra-low carbon steel is cast and rolled by using the above-mentioned conventional technique, it is generated when the slab slab is heated before rolling.
However, surface defects caused by the oxide scale on the surface layer frequently occur, and the yield of the product remains low.

[0006]

Therefore, the present inventors have proposed a method for improving the surface properties of a titanium-added ultra-low carbon steel sheet as follows.
By accumulating research and solving the above problems by applying the following means, it was found that a continuous cast slab for a titanium-added ultra-low carbon steel sheet having excellent surface properties can be obtained, and the present invention has been completed. It was

That is, according to the present invention, the carbon content in steel is 0.01% by weight or less, and the titanium content is 0.005 to 0.005%.
When continuously casting ultra-low carbon steel added in the range of 0.150% by weight, a direct current magnetic flux across the entire width in the thickness direction of the slab is applied in the continuous casting mold, and in the width direction of the mold by the direct current magnetic flux. The ultra-low carbon steel is injected below the static magnetic field band to be formed, and the carbon content of the molten steel above the static magnetic field band.
Is a continuous casting method for titanium-added ultra-low carbon steel, wherein the casting is carried out while supplying carbon from the outside so that the content becomes higher than 0.01% by weight . Where the above
Contains carbon when casting while supplying carbon from the part
Casting while supplying carbon from the mold flux
And supply carbon by wire containing carbon
It is also characterized by casting while.

[0008]

The function of the present invention will be described in detail below. In order to solve the above problems in the prior art, the present inventors have
First, it occurs when heating the continuously cast slab slab,
The formation behavior of oxide scale was investigated in detail.

FIG. 1 (a) is a titanium-added ultra-low carbon steel (C:
0.0015% by weight, Ti: 0.05% by weight),
There are few surface defects due to the oxide scale on the surface. Figure 1
(B) Aluminum killed low carbon steel (C: 0.05% by weight)
Is a schematic view showing the state of the surface oxide scale after heating at 1200 ° C. for 120 minutes. The oxygen concentration in the atmosphere in the heating furnace was 3%. In the text below, (a) represents a titanium-added ultra-low carbon steel, and (b) similarly represents an aluminum killed steel.

The aluminum-killed low carbon steel of (b) is characterized in that a decarburized layer in which the C concentration in the surface layer on the base metal side is reduced is formed in addition to the surface scale. On the other hand, in the case of the titanium-added ultra-low carbon steel of (a), the formation of the decarburized layer on the surface side of the base metal is not remarkable, and instead, the grain boundary of the surface layer or a part of the grain is locally oxidized. The feature is that As a result of the compositional analysis of this locally oxidized portion, it was found that these were mainly the titanium in the steel that was oxidized.

FIG. 2 shows the results of measuring the Vickers hardness of the surface layer on the base metal side. (B) is the case of aluminum killed low carbon steel, and the hardness of the surface layer tends to be lower than that of the inside. It is presumed that this is due to the formation of the decarburized surface layer. On the other hand, (a) is the case of a titanium-added ultra-low carbon steel, and on the contrary, the hardness of the surface layer tends to be higher than that of the inside. It is presumed that this is due to local oxidation at the grain boundary of titanium on the surface layer or in a part of grain.

From the above, the titanium-added ultra-low carbon steel of (a), which is likely to cause surface flaws due to the oxide scale generated during heating of the slab before rolling, and the surface flaw due to the same oxide scale are small (b). Of aluminum killed low carbon steel,
It can be seen that there is a large difference in the formation behavior of oxide scales during slab heating, especially so-called subscales on the surface layer of the base metal.

That is, in the titanium-added ultra-low carbon steel of (a), the titanium oxide in the grain boundaries near the surface layer or in some grains remains in the surface layer even after the descaling treatment (the scaling), and therefore is oxidized. It is considered that surface defects due to scale are likely to occur. It is also presumed that the surface layer hardness locally increases due to the formation of these oxides, resulting in a decrease in workability, which also promotes the generation of surface defects due to the oxide scale during rolling. .

Such an oxidation reaction of the element M in steel is generally represented by the following equation.

[0015]

[Equation 1]

Here, m and n are stoichiometric coefficients. Further, the change in free energy of formation ΔG per mol of oxygen at this time is expressed by the following equation.

[0017]

[Equation 2]

Where ΔG ^O is the standard free energy of formation change, R is the gas constant, T is the temperature, a _X is the activity of the X component,
p _O2 is oxygen potential. Also, the larger the negative value of ΔG, the stronger the tendency of oxide formation. From equation (2), it can be seen that the change in free energy of formation of oxide ΔG depends on the activity of the components in the steel and the oxygen potential, in other words, the concentration of the components in the steel and the oxygen partial pressure in the atmosphere.

Table 1 assumes that the oxygen partial pressure in the atmosphere is 0.04 atm (oxygen concentration 4%), which is almost the same as that in the slab heating furnace, and the (a) titanium-added ultra-low carbon steel (b) is used. ) This is the result of trial calculation of change in free energy of oxide formation in aluminum-killed low carbon steel. From this, it can be inferred that in the (b) aluminum-killed low carbon steel, the oxidation tendency of C in the steel is stronger than the oxidation of the base iron, and the decarburization reaction of the surface layer is likely to occur.

On the other hand, in the case of (a) titanium-added ultra-low carbon steel, the oxidation tendency of C in the steel is weaker than the oxidation of base iron, and the decarburization reaction of the surface layer is less likely to occur, and the oxidation of base iron It can be seen that the oxidation tendency of titanium is stronger than that of titanium and the titanium is more likely to be preferentially oxidized. From the same calculation, the limit concentration at which titanium can be preferentially oxidized over base iron is 0.0002% by weight.
It is estimated that the titanium is industrially added, and the titanium is preferentially oxidized in the range of 0.005 to 0.150% by weight, which is added to the ultra-low carbon steel.

[0021]

[Table 1]

FIG. 3 shows the results of trial calculation of the relationship between the C concentration in steel and the change in free energy of formation of CO. Here, the oxygen partial pressure is 0.04 atm and the CO partial pressure is 0.01 a.
It was calculated assuming tm. From this, the C concentration is about 0.0
If it is 1% by weight or more, the deoxidization reaction tends to occur more easily than that of base iron, and if the concentration of C is less than about 0.01% by weight, it tends to oxidize C rather than the oxidation of base iron. It is presumed that the decarburization reaction is weak and that the decarburization reaction is unlikely to occur. This is almost in agreement with the formation tendency of the decarburized layer on the surface layer when various slab slabs having different C concentrations are heated. Thus, it can be explained semi-quantitatively from the results of thermodynamic studies that the oxide scale, especially the subscale formation behavior during slab heating differs depending on the steel type.

Therefore, the present inventors investigated the relationship between the C concentration in steel and the occurrence of scale-based surface flaws after rolling. FIG. 4 shows the results of investigating the occurrence of scale-based surface flaws after rolling by changing the C concentration of steel containing 0.05% by weight of titanium. In addition, the surface flaw generation rate is obtained by sampling a 1 m long sample from the cold rolled coil,
It is the ratio (%) of the number of detected surface defects due to oxide scale during slab heating to the total number of extracted samples. The local oxidation of titanium generated during the heating of the slab before rolling at the grain boundary and a part of the grain in the surface layer on the base steel side becomes slight as the C concentration in the steel increases, and particularly the C concentration is about 0.
It was found that, in the region where the decarburization reaction was remarkable when the amount was more than 01% by weight, the effect on the surface flaw was almost harmless industrially.

Next, the present inventors set the C concentration in the steel of the surface layer of the slab slab to 0.01% by weight or more to promote the decarburization reaction, thereby producing titanium produced during heating of the slab before rolling. The studies were repeated on the method of reducing the surface defects caused by the local oxidation in the grain boundary and some grain in the surface layer of the steel. As a result, a DC magnetic flux across the width of the cast piece in the continuous mold is applied over the entire width, and the ultra-low carbon steel is injected below the static magnetic field band formed in the mold widthwise direction by the DC magnetic flux. It was also found that the above problems can be solved by casting while supplying carbon from the outside so that the carbon content of the molten steel on the upper side of the static magnetic field band is 0.01% by weight or more.

According to this method, first, a DC magnetic flux that traverses the thickness direction of the slab is applied across the entire width in the continuous mold, and the fireproof is applied to the lower side of the static magnetic field band formed by the DC magnetic flux in the mold width direction. By injecting the molten steel with a material-made nozzle or the like, the flow in the mold pool is electromagnetically damped by the static magnetic field zone, and the upper side of the static magnetic field zone has an amount of molten steel that is consumed as a solidification shell. Only the molten steel is supplied from the lower side of the static magnetic field band, and the supply of the molten steel from the upper side to the lower side of the static magnetic field band is suppressed. Next, if C is supplied to the molten steel above the static magnetic field zone from the outside so as to meet the required amount, the C concentration of the solidified shell formed above the static magnetic field zone will be high, and after that A slab having a low C concentration in the solidified shell formed on the side, that is, a so-called multi-layer slab made of ultra-low carbon steel having a high C concentration only on the surface and a low C concentration inside is obtained.

The inventors of the present invention have made extensive studies on the method of supplying C to the upper molten steel in the static magnetic field zone, and in the continuous casting of steel, the lubricant between the slab and the mold, and the heat retention of the molten steel in the mold. By increasing the C concentration of the mold flux used as an agent and supplying it from the upper surface of the molten steel in the mold, or by dipping and melting an alloy-adding wire containing C in the molten steel We found that it can be made stable and inexpensive.

Further, the inventors of the present invention investigated the material properties of the product after rolling, and showed a large difference in the material properties between the multi-layer ultra low carbon steel according to the present invention and the conventional single layer ultra low carbon steel. It was confirmed that there was no. This means that merely increasing the C concentration only in the extreme surface layer of the slab does not have a great influence on the properties of the entire slab. However, if the C concentration in the surface layer is too high, problems such as surface cracking tend to occur. Therefore, it is practically possible to control the surface C concentration to 0.20% by weight or less where surface cracking does not easily occur. desirable.

[0028]

EXAMPLE FIG. 5 (a) schematically shows a continuous casting apparatus in which a static magnetic field generator (DC electromagnet) is installed in a continuous mold. Under the static magnetic field thus formed, a titanium-added ultra-low carbon steel having the chemical composition shown in Table 2 was injected using a refractory immersion nozzle. Further, by supplying a mold flux containing 5% by weight of (A) C to the molten steel above the static magnetic field, or by supplying an alloy-added wire containing (B) C of 10% by weight, the C concentration of the molten steel is reduced to 0. .0
It was made to be 1% by weight or more.

The main casting conditions at this time are: casting size = 250 mm thickness × 1200 mm width, casting speed Vc = 1 m
/ Min, degree of molten steel heating in tundish ΔT = 25 ~
45 ° C. The magnetic flux density of the static magnetic field was 0.5T. The length of the mold is 900 mm, and the position of the static magnetic field band (the position of the iron core of the DC electromagnet) is 250 to 450 mm from the upper end of the mold. As the immersion nozzle, a two-hole type nozzle as shown in FIG. 5A is used, and the upper end position of the discharge hole is 50
It was set to be 0 mm. Note that FIG. 5B shows a cross-sectional view of the cast slab.

[0030]

[Table 2]

The slab after casting has an oxygen concentration of 3 in the atmosphere.
After heating at 1200 ° C. for about 2 hours in a slab heating furnace of ˜4%, hot rolling was performed, and further cold rolling was performed, and then the surface texture was inspected. Table 3 shows the results of the investigation of the occurrence rate of scale-based surface flaws due to the oxide scale during slab heating. In the present invention example, the occurrence rate of the surface flaws was significantly reduced and the product yield was significantly improved. .

[0032]

[Table 3]

[0033]

As described above, according to the present invention, it is possible to industrially manufacture a titanium-added ultra-low carbon steel with few surface defects due to oxide scale during slab heating, which is stable and inexpensive.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing the formation of surface oxide scale during slab heating in (a) titanium-added ultra-low carbon steel and (b) aluminum-killed low-carbon steel.

FIG. 2 is a diagram showing the results of measuring the Vickers hardness of the surface layer on the ground iron side.

FIG. 3 is a diagram showing a result of trial calculation of a relationship between a C concentration in steel and a change in free energy of CO formation.

FIG. 4 is a diagram showing the relationship between the C concentration and the occurrence status of scale-based surface flaws after rolling.

5A and 5B are schematic views showing a continuous casting apparatus in which a static magnetic field generator (DC electromagnet) is installed in a continuous casting mold and an example of the present invention using the same. FIG. 5A is a side view, and FIG. Plan

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI B22D 11/115 B22D 11/115 B (56) References JP-A-4-313447 (JP, A) JP-A-4-313448 ( JP, A) JP 56-68562 (JP, A) JP 3-243245 (JP, A) JP 51-107234 (JP, A) JP 7-178521 (JP, A) JP Sho 63-108947 (JP, A) JP 4-279249 (JP, A) JP 4-314844 (JP, A) JP 3-20295 (JP, B2) (58) Fields investigated (Int .Cl. ⁷ , DB name) B22D 11/00 B22D 11/04 311 B22D 11/108 B22D 11/115

Claims (3)

(57) [Claims] 1. Continuous casting in continuous casting of ultra-low carbon steel having a carbon content of 0.01% by weight or less and titanium added in the range of 0.005 to 0.150% by weight. Direct current magnetic flux across the width in the thickness direction of the slab in the mold is applied over the entire width, the ultra-low carbon steel is injected below the static magnetic field band formed in the mold width direction by the direct current magnetic flux, and, A continuous casting method for a titanium-added ultra-low carbon steel, which comprises casting while supplying carbon from the outside so that the carbon content of the molten steel above the static magnetic field band is higher than 0.01% by weight. 2. Continuous casting in continuous casting of ultra-low carbon steel having a carbon content of 0.01% by weight or less and titanium added in the range of 0.005 to 0.150% by weight. Direct current magnetic flux across the width in the thickness direction of the slab in the mold is applied over the entire width, the ultra-low carbon steel is injected below the static magnetic field band formed in the mold width direction by the direct current magnetic flux, and, The titanium-added electrode according to claim 1, wherein casting is performed while supplying carbon from a mold flux containing carbon so that the carbon content of the molten steel above the static magnetic field band is higher than 0.01% by weight. Continuous casting method for low carbon steel. 3. Continuous casting in continuous casting of an ultra-low carbon steel having a carbon content of 0.01% by weight or less and titanium added in the range of 0.005 to 0.150% by weight. Direct current magnetic flux across the width in the thickness direction of the slab in the mold is applied over the entire width, the ultra-low carbon steel is injected below the static magnetic field band formed in the mold width direction by the direct current magnetic flux, and, The titanium-added extra low according to claim 1, wherein casting is performed while supplying carbon with a wire containing carbon so that the carbon content of the molten steel above the static magnetic field band is higher than 0.01% by weight. Continuous casting method for carbon steel.