JP2961332B2 - Manufacturing method of unmaintained continuous cast slab of Ti-containing 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 which can be made free of slab of Ti-containing steel.

[0002]

2. Description of the Related Art Ti-containing steel (particularly Ti-containing stainless steel)
Contains Ti, which has a strong affinity for O and N, so that the surface of the hot-rolled steel strip (hot coil) or cold-rolled steel strip (cold-rolled coil) has TiN or TiO ₂ called Ti streak.
It is well known that band-shaped barbed flaws are easily generated due to the above. Ti streak flaws not only significantly lower the coil yield, but must be turned into scraps if the flaws cannot be removed. Therefore, many investigations and countermeasures have been taken for these flaws. For example, powders have been developed so that their characteristics do not change even if TiO ₂ absorption occurs in the mold powder during continuous casting. However, it is currently extremely difficult to reduce Ti streak flaws. .

[0003] Further, when hot rolling is performed without grinding the surface layer of a Ti-containing stainless steel continuous cast slab without grinding and removing, the presence of inclusions on the surface of the obtained hot coil is not recognized, but the spindle-like shape is obtained. Scalping flaws may occur.
Regarding this defect, no abnormalities were confirmed by visual inspection of the slab, and there were almost no reports in other reports that Ti-containing stainless steel was hot-rolled without care. Not in. For these reasons, there is a problem in eliminating the need for a Ti-containing stainless steel continuous cast slab. Therefore, the slab surface is ground before being subjected to hot rolling.

[0004]

Accordingly, the present invention provides
In order to achieve a maintenance-free Ti-containing stainless steel continuous cast slab, a continuous Ti-containing steel capable of producing a hot coil having no spindle-shaped stubble flaws without generating Ti streak flaws due to TiN or TiO _2. The task is to establish a casting method.

[0005]

According to the present invention, in a continuous casting of Ti-containing steel, a Ti-containing molten steel in a tundish is continuously injected into a mold while a sealing gas is supplied into the tundish. 0.25% by weight of Ti content in Ti-containing steel
And the N content is restricted to 0.009% by weight or less, and the flow rate of the seal gas supplied to the tundish is set to 1.
1 Nm ³ /T.Steel and negative strip time t _N determined by casting speed, mold amplitude and frequency 0.31 s
By controlling the slab to less than ec, we will manufacture continuous cast slabs that can be maintained without using Ti-containing steel. Here, t _N = 1 / f−t _P t _P = cos ⁻¹ (−v / 2πaf) / πf where t _N : negative strip time (sec) t _P : positive strip time (sec) V: casting speed (m / min) a: amplitude (mm) f: mold frequency (cycle / min) In the present invention, the Ti-containing steel includes a Ti-containing stainless steel that requires particularly beautiful surface.

DETAILED DESCRIPTION OF THE INVENTION As described above, Ti-containing steel is
Since it contains Ti, which has a strong affinity with O and O, it is not necessary to make the continuous cast slab maintenance-free. Had to be forced. However, [Ti] amount in steel and [N]
By controlling the amount to a certain level or less, the occurrence of Ti streak flaws mainly composed of TiN was eliminated, and it was confirmed that the slab could be kept free. However, when a Ti-containing steel continuous cast slab at a level where Ti streak does not occur was rolled without care, a new spindle-shaped flaw other than a Ti streak flaw was generated. No inclusions were present in this flaw. Therefore, it was presumed that the cause was due to something other than inclusions at the time of the slab. A detailed investigation of the slab and a follow-up investigation of the coil revealed that the cause was bubble defects.

More specifically, the relationship between the levels of Ti and N, which are the main components of Ti streak flaws, in steel and the Ti streak flaws was investigated in detail.
It has been found that it occurs when the i and N contents exceed a certain level. FIG. 1 shows the results. As a base steel, C: 0.06% or less, Si: 0.75% or less, Mn;
0.70% or less, Cr; 10.50 to 12.00%, Ti; 0.15 to 0.5%,
N: Continuous casting of stainless steel of 0.006 to 0.012%, and after slab surface treatment and hot rolling to produce hot coils, the presence or absence of Ti streak flaws on the surface of the hot coils was examined. It is organized by medium Ti content and N content. As is clear from FIG. 1, if [Ti] is 0.25% or less and [N] is 0.009% or less, no Ti streak flaw occurs.

However, when a hot coil is manufactured by hot rolling a slab having a Ti, N content that should not cause Ti streak flaws without care, spindle-shaped barbed flaws may be observed. . Then, a sample was cut out from the unmaintained continuous cast slab, and after thunder buffing, a thorough microscopic examination was performed. As a result, a bubble-like defect was confirmed in the surface layer of the slab. No inclusions were present in the bubble flaws, and no inclusions were found in the spindle-shaped barbed flaws themselves. Therefore,
The correlation between the bubble flaw and the spindle-shaped barb flaw was estimated. A follow-up study was conducted to make a hot coil by drilling the same hole as the bubble flaw in the slab, and a flaw of exactly the same shape as the spindle-shaped barbed flaw occurred on the surface of the hot coil. From these facts, it is presumed that the occurrence of spindle-shaped barbed flaws can be prevented by reducing the bubble flaws observed in the slab. In order to realize this, the relationship between the bubble flaws and the continuous casting conditions was investigated in detail. .

As a result, it has been found that two factors in the continuous casting conditions influence the generation of bubble defects. One is the amount of the sealing gas supplied to the tundish, and the other is the negative strip time of the mold.

FIG. 2 is a graph showing the argon gas supplied to the tundish in the casting operation performed on the same type of steel as in FIG. 1 in terms of the ton of molten steel per ton of molten steel. Flaw + spindle-shaped scab flaw). From FIG. 2, it is clear that the flow rate (Nm ³ /T.Steel) of the sealing gas sent to the tundish per ton of molten steel affects the generation of bubble defects. That is, irrespective of the size of the tundish or the casting scale, the larger the unit flow rate of the sealing gas in contact with the molten steel, the larger the amount of bubble defects generated on the surface of the hot coil. The results in FIG. 2 indicate that the gas flow rate for the tundish seal needs to be less than 1.1 Nm ³ / TSteel to prevent bubble defects. However, if the flow rate of the seal gas is reduced too much, there is a problem in the sealing performance with the atmosphere. Therefore, it is necessary to supply the seal gas, and it is necessary to supply the seal gas at 0.5 Nm ³ / T Steel or more, preferably 0.7 Nm ³ / T Steel. The above is good to send air. FIG. 2 shows a collection of operation examples when the negative strip time of the mold is set to 0.31 sec or less.

FIG. 3 shows the negative strip time determined by the casting speed, mold stroke (amplitude) and frequency in the casting operation performed on the same type of steel as in FIG. Shows the relationship,
Gas flow for tundish seal is almost 1.0Nm ³ / T Steel
It is when it is. FIG. 3 shows a clear correlation between the negative stripping time and the number of the flaws. As the negative stripping time becomes shorter, the number of bubble flaws decreases and the occurrence of spindle-shaped barbed flaws also disappears. The results in FIG. 3 indicate that it is important to set the negative strip time to 0.31 sec or less in order to prevent the generation of the flaw.

From the above findings, in order to suppress the generation of Ti streak flaws appearing in hot coils of Ti-containing steel and spindle-shaped barbed flaws which appear even after slab care, the contents of Ti and N must be 0.25% by weight. % And N content 0.009%
Weight percent or less (this makes slabs maintenance-free), and the flow rate of seal gas sent to the tundish is less than 1.1 Nm ^{3 /} T. Steel and the negative strip time of the mold is 0.31 sec. It is clear that the regulation should be made as follows. If these requirements according to the present invention are satisfied, the slab can be maintained without any care even with the Ti-containing steel, and a sound hot coil without spindle-shaped barbed flaws can be manufactured.

As described above, according to the present invention, not only Ti streak flaws but also spindle-shaped barbed flaws can be suppressed. The reason is considered as follows. In order to suppress the generation of these flaws, it is important to perform casting under casting conditions in which bubbles are not captured in the slab surface layer. For this reason, sealing gas carried into the molten steel in the tundish, for example, A
It is better to reduce the amount of blowing such as r to the extent that there is no problem in the sealing property, specifically, it is necessary to make it less than 1.1 Nm ³ / TSteel. Furthermore, the solidification rate should be reduced as far as the shell strength allows, so that the gas (bubbles) that cannot be lifted up in the tundish and is brought into the mold is not trapped by the solidification shell in the mold. As vibration conditions for slowing down the solidification rate, a stroke of 4 to 9 mm and a vibration frequency of 70 are generally used.
The negative strip time may be reduced within the range of ~ 120 cycle / min and the casting speed of 0.4 ~ 1.0m / min.
It is good to be less than sec. In addition, it is considered that the reason why a large number of bubble defects are observed in the Ti-containing steel type is that the viscosity of the molten steel-slab interface is increased due to the inclusion of Ti, so that bubbles are difficult to escape.

[0014]

EXAMPLES Table 1 shows that C ≒ 0.2%, Si ≒ 0.55%, Mn ≒ 0.2
%, Cr ≒ 11.0% stainless steel to which 0.30% or less of Ti has been added and N has been reduced to 0.015% or less, for negative stripping time, the flow rate of the seal gas (argon gas) sent to the tundish Continuous casting was performed under the indicated casting conditions, and the resulting slabs were hot-rolled without any maintenance to produce hot coils. It is shown.
As can be seen from the results in Table 1, no generation of Ti streak flaws and spindle-shaped barbed flaws are observed in the hot coil manufactured under the conditions according to the present invention.

[0015]

[Table 1]

[0016]

According to the present invention, generation of Ti streak flaws can be prevented in both Ti-containing steel and slab-free care rolling.
In addition, since spindle-shaped barbed flaws that are generated even with a care slab can also be prevented, Ti-free steel slabs need not be maintained.
High quality hot coils can be manufactured, which can greatly contribute to quality improvement in addition to the economic effect of slab maintenance.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the Ti content and N content in steel and the occurrence of Ti streak in a hot coil.

FIG. 2 is a diagram showing the relationship between the flow rate of gas for sealing a tundish and the presence or absence of bubble defects and spindle-shaped barbed defects in a hot coil.

FIG. 3 is a view showing the relationship between the negative strip time of a mold and the presence or absence of bubble defects and spindle-shaped barbed defects in a hot coil.

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Claims (2)

(57) [Claims] In a continuous casting of a Ti-containing steel in which a Ti-containing molten steel in a tundish is continuously injected into a mold while a sealing gas is supplied into the tundish, the Ti content in the Ti-containing steel is reduced to 0.25. Wt% or less and the content of N is regulated to 0.009 wt% or less, and the flow rate of the seal gas sent to the tundish is set to less than 1.1 Nm ³ /T.Steel, and is determined by the casting speed, the amplitude and the frequency of the mold. A method for producing a cast-free slab of Ti-containing steel, characterized in that the negative strip time t _N is regulated to 0.31 sec or less, where t _N = 1 / f−t _P t _P = cos ⁻¹ (−v / 2πaf) / πf where t _N : negative stripping time (sec) t _P : positive stripping time (sec) V: casting speed (m / min) a: amplitude (mm) f: mold frequency (cycle / min) ). 2. The continuous casting method according to claim 1, wherein the Ti-containing steel is stainless steel.