JP2740278B2 - Continuous casting method of molten stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a continuous casting method for casting a castable slab by reducing segregation generated in an oscillation mark valley of a continuous cast slab of molten stainless steel. The present invention relates to a continuous casting method using a casting mold.

<Conventional technology> Segregation of oscillation marks (hereinafter abbreviated as OSM) valleys on the surface of a continuous slab causes direct cracking (hot charging) of the slab because ordinary steel causes lateral cracking along the OSM. One of the inhibiting factors is mentioned. Further, since stainless steel, particularly austenitic stainless steel, contains elements that are hardly oxidized such as Cr and Ni, the oxide scale generated on the surface in a heating furnace before rolling is much thinner than that of ordinary steel. Here, when rolling is performed in a state where OSM with segregation remains, a pattern (irregularity) is generated on the product surface. Also,
If the segregation is large, lateral cracks will occur along the OSM as in ordinary steel. Therefore, it is necessary to maintain the slab surface before rolling, and the production capacity is significantly reduced.

Many techniques have been proposed to reduce the segregation of OSM valleys. However, optimization of the mold vibration conditions (frequency, negative strip rate, and vibration waveform) has resulted in OSM valleys.
Techniques for making the depth shallow and thereby reducing segregation are the mainstream (for example, JP-A-61-159255, JP-A-57-1159).
No. 48).

However, in order to solve the above-mentioned problem, it is necessary to carry out a high frequency casting of the mold with a frequency of the mold of 100 to 500 cpm and an amplitude of the mold of 2 to 6 mm. The amount of powder supplied to the space between the mold and the solidified shell is reduced, so that lubricity is significantly deteriorated and breakout is likely to occur.

<Problems to be Solved by the Invention> The present invention can reduce the OSM depth, reduce segregation generated in the OSM valley, and prevent a decrease in the amount of powder flowing into the space between the mold and the solidified shell. The present invention has been made to provide a continuous casting method of molten stainless steel capable of producing a stainless steel slab having excellent surface quality without causing breakout.

<Means for Solving the Problems> In the present invention, when continuously casting molten stainless steel while vibrating the mold in the vertical direction, the slag rim is formed at least above the meniscus portion on the long side of the mold, at least during casting. This is a continuous casting method for molten stainless steel that is cast using a mold having a wider range.

<Procedure and Function of Making the Present Invention> First, the reason why segregation occurs in an OSM valley formed in a stainless steel slab will be described below with reference to FIG. In FIG. 4, OSM8 is the vibration speed of the mold 1 (V _M ).
Is generated at a time faster than the slab drawing speed (V _C ), that is, during the negative strip period. At this time, when the upper end of the solidified shell 2 is pushed inward by the slag rim 3 formed on the inner surface near the meniscus of the mold, the formation of the OSM 8 is further promoted. On the other hand, when the vibration of the mold shifts from the negative strip period to the period at which the vibration speed of the mold is lower than the slab withdrawal speed, that is, the bodily strip period, the solidified shell pressed inwardly is bent outward by the molten steel static pressure. Pressed and bent. At this time, the molten steel side of the solidified shell becomes a tensile stress, and the concentrated molten steel existing on the front surface of the solidified shell exudes to the surface of the solidified shell and segregates in the OSM valley described above. This becomes more pronounced as the deformation of the solidified shell increases. Therefore, it has been found that preventing the contact between the slag rim and the upper end of the solidified shell, which causes the upper end of the solidified shell to bend inward during the negative strip period, can solve the above-described problem, and completed the present invention from this finding. .

That is, the present invention is above the meniscus portion,
This is a continuous casting method in which the distance between the slag rim formed on the inner surface of the mold and the upper end of the solidified shell is increased by increasing the mold at least in the range where the slag rim is formed during casting, thereby solving the problems of the prior art. .

Next, the operation of the present invention will be described with reference to FIG. Since the mold above the meniscus part is wider in the negative strip period, the contact between the slag rim and the upper end of the solidified shell is relaxed, and the inward deformation of the upper end of the solidified shell is reduced. Deformation is also reduced, and thus the OSM depth is reduced. In this case, the tensile stress on the molten steel side of the solidified shell is also small, and the exudation of the concentrated molten steel can be prevented, and segregation of OSM valleys is reduced.

Furthermore, since the distance between the slag rim and the solidified shell upper end in the negative strip period is large, the inflow path of the powder is secured and the lubricity between the mold and the slab is improved,
It has features such as casting without causing breakout in high frequency casting of a mold, which was a drawback of the prior art.

In the present invention, it is considered that the upper wide taper amount of the mold is optimally about 2% / m from the top end of the mold to the meniscus portion.

The reason why the taper amount of 2% / m is optimal is based on the following examination results. That is, the tensile stress acting in the casting direction of the solidified shell when a compressive stress corresponding to the fluctuating distance of the meniscus level is applied to the solidified shell is determined, and the solidified shell is compared with the high-temperature tensile strength of the steel type to be cast. It was found that the amount of taper at which no breakage occurred was 2% / m or less. On the other hand, when the taper amount is significantly smaller than 2% / m, the distance between the slag rim and the upper end of the solidified shell cannot be sufficiently secured, and the effect of the present invention cannot be exhibited.

The purpose can be achieved without using the method of the present invention by using a powder composition that does not easily generate slag rim or a substance that has low strength attached to the mold wall of the slag rim, but these technical ideas are easily analogized from the present invention. it can.

<Example> Using a mold of FIG. 1 according to the present invention and a conventional mold of FIG. 4, a continuous cast slab having a cross section of 200 × 1040 mm was cast from austenitic stainless steel molten steel under the same casting conditions. The OSM depth of one surface was measured and the segregation frequency of OSM valleys (number of occurrences of segregation / total number of OSMs investigated x 100) was investigated.

The results are shown in FIGS. 2 and 3. The OSM depth was shallower in the examples of the present invention than in the comparative examples under any casting conditions, and as a result, the frequency of segregation at OSM valleys was significantly reduced. Such a slab is rolled to a cold-rolled coil while keeping the surface untreated, and the uneven glossiness on the surface ｛(the number of uneven glossy coils /
As a result of examining (the total number of cold-rolled coils) × 100 °, the conventional mold was as high as 86%, while the mold of the present invention was reduced to 5%, indicating a remarkable effect.

<Effects of the Invention> According to the present invention, it is possible to reduce the occurrence of segregation in OSM valleys of a stainless steel slab, which was a drawback of the prior art, and to reduce the slab surface maintenance rate by preventing the reduction of defects caused by segregation. And the surface can be rolled without care, thereby improving the product yield.

[Brief description of the drawings]

FIG. 1 is an explanatory view schematically showing the relationship between the vibration waveform of the mold of the embodiment of the present invention, the solidified shell formation state, and the slag rim, and FIG. 2 shows the relationship between the OSM depth and the frequency. FIG. 3 is a characteristic diagram showing the segregation frequency of the OSM valley in comparison with the method of the present invention and the conventional method, and FIG. 4 is a first graph of the comparative example. It is explanatory drawing which showed the phenomenon similar to a figure. 1 ... mold, 2 ... solidified shell, 3 ... slag rim, 4 ... concentrated molten steel, 5 ... molten powder, 6 ... molten steel, 7 ... segregation, 8 ... oscillation mark (OSM), ... … Negative strip period,… Bositive strip period, …… Bossive strip period.

Continuation of the front page (56) References JP-A-60-158955 (JP, A) JP-A-64-83349 (JP, A) Jpn.

Claims (1)

(57) [Claims] In a continuous casting of molten stainless steel in which a mold is vibrated in a vertical direction, a mold is used which is above a meniscus portion on a long side of the mold and at least widens a range where a slag rim is formed at the time of casting. A continuous casting method of molten stainless steel, characterized by casting.