JP4790284B2 - Steel continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting method of steel, and more particularly, to a continuous casting method of steel in which a continuous cast slab of steel, particularly extremely low carbon mild steel, can be subjected to hot rolling in the next step without surface maintenance.

Since ultra-low carbon mild steel, which is mainly used for automotive skins, requires extremely clean surface quality, conventional ultra-low carbon mild steel continuous cast slabs have been slab refining lines (non-patent) (Refer to Document 1), after surface preparation such as grinding or grinder grinding, it is subjected to hot rolling.
Edited by Japan Iron and Steel Institute: 3rd Edition Steel Handbook III (1) Rolled foundations and steel plates p.114,171,350,353, 2nd edition, June 30, 1980, published by Maruzen

  Extremely low carbon mild steel is a mass-produced steel grade, and all of them have been surface-treated in the past. Therefore, there has been a problem that the load on the slab finishing line is large and is an obstacle to cost reduction. However, assuming that ultra-low carbon mild steel slabs are sorted according to the same criteria as other steel types for whether surface maintenance is performed, there is a risk of significant yield reduction. The problem remains unresolved. Regarding steels other than ultra-low carbon mild steel, quality variation is relatively large with untreated materials, while quality is stable with care materials, but tends to be over-serviced. It has been desired to establish a continuous casting method that can stabilize the temperature.

  Accordingly, an object of the present invention is to provide a continuous casting method of steel capable of ensuring a surface quality pass rate comparable to that of a treated material after cold rolling even if steel, particularly extremely low carbon mild steel, is made free of slabs. To do. Here, the ultra-low carbon mild steel refers to steel containing C: 0.001 mass% or less.

The present invention that has achieved the above object is to cast steel under an operating condition of an effective Ar amount defined by the following formula (1): 5 L / ton or less during continuous casting of steel with a casting width of less than 1450 mm. The drift rate defined by the formula (2) is measured, and the slab whose measured value is 6.0% or less is a non-care material. This is a continuous casting method for steel.
Further, in the present invention, when continuously casting steel with a casting width of 1450 mm or more, the nozzle immersion depth is less than 290 mm, the effective Ar amount defined by the following equation (1): 5 L / ton or less, under the operating conditions, A drift rate defined by the following formula (2) is measured during the casting, and a slab having a measured value of 6.0% or less is treated as a non-care material. is there.

Effective Ar amount = Ar gas blow amount per unit time into molten steel in nozzle / throughput (amount of cast molten steel per unit time) (L / ton) (1)
Diffusion rate = | Two-surface heat removal amount−4-surface heat removal amount // 2−4 surface average heat removal amount × 100 (%) (2)
Here, the 2nd surface and the 4th surface indicate either one or the other of both end surfaces in the mold long side direction (both short side surfaces of the mold).

  According to the present invention, the rate of degradation due to surface defects after cold rolling of ultra-low carbon mild steel slabs that are continuously cast is less than that of conventional slabs. Therefore, most of the ultra-low carbon mild steel slab can be made free, and the load reduction and cost reduction of the slab finishing line can be achieved. The same effect can be obtained when applied to steels other than ultra-low carbon mild steel.

In order to achieve the above object, the present inventor has sought for casting conditions for enabling maintenance. And I thought that it was effective to use the following three factors as an index largely related to the actual flow of molten steel in the mold, which was not considered in the conventional condition for omitting care.
[1] Drift rate:
At the time of continuous casting of steel, for example, as shown in FIG. 1, the discharge flow F from one hole of the two-hole immersion nozzle 1 toward one of the two short side surfaces facing the mold 2 (for example, two surfaces) and the other hole There is often a case where a difference occurs in the flow velocity of the molten steel, that is, uneven flow occurs between the discharge flow F ′ toward the other side (for example, four sides). When drift occurs, local meniscus flow velocity deficiency and powder melting inhibition occur, and slab surface defects are promoted. In addition, when drift occurs, the amount of heat removal (the amount of heat removed from the molten steel by the mold cooling water) on the side where the molten steel flow velocity is large (for example, the second surface side) is larger than that on the opposite side (for example, the four surfaces). That is, when a drift occurs, a difference in heat removal occurs between the two short side surfaces facing each other. This difference increases as the degree of drift increases. Therefore, it was considered that the drift rate was defined in relation to the amount of heat removed as in the above equation (2), and if this was used as an index, the degree of drift and thus the degree of slab surface defects could be estimated.

[2] Effective Ar amount:
In the continuous casting of steel, Ar gas is blown into the molten steel in the nozzle in order to prevent nozzle clogging of the immersion nozzle and to promote the floating of inclusions in the molten steel in the mold. At this time, even when the effective Ar amount defined by the equation (1) is too large or too small, as shown in the right half of FIG. 2, the flow when the discharge flow F ′ reaches the short side surface is unstable. If it is too large, the surface defect is promoted by the bubble bursting and the ball ingot is generated, and if it is too small, the inclusion flotation effect is insufficient and the internal defect is promoted. Therefore, in order to suppress surface defects and achieve maintenance, it was considered important to limit the effective Ar amount to a certain upper limit. The lower limit is effective for suppressing internal defects, but is considered to be ineffective for surface defects.

[3] Casting width and nozzle immersion depth:
When the casting width is wide, if the nozzle immersion depth (distance from the meniscus to the center of the nozzle hole when the nozzle is immersed) is deep, at the interface between the solidified shell near the immersion nozzle (long side surface center) and the molten steel The molten steel flow rate is insufficient, and the amount of inclusions trapped in the solidified shell increases, leading to an increase in surface defects (or subsurface latent defects). Therefore, in order to suppress surface defects and eliminate the need for maintenance, it was considered necessary to regulate the nozzle immersion depth to a certain extent when the casting width is larger than a certain extent.

  Based on the above idea, an uncontrolled experiment was conducted on a continuous cast slab of ultra-low carbon mild steel. In this experiment, a plurality of slabs randomly selected from continuous cast slabs of ultra-low carbon mild steel were hot-rolled and then cold-rolled without care, and the obtained cold-rolled coils were subjected to a surface per 100 m length. When even one defect was detected, the slab corresponding to the coil was determined as an NG slab, and the relationship between the NG slab ratio of different heats and the above three factors was analyzed. This NG slab criterion is much stricter than the normal surface quality acceptance criterion in the inspection after cold rolling. The result in the case of wide casting with a casting width of 1450 mm or more is shown in FIG. In FIG. 3, instead of the NG slab ratio, an NG slab ratio index, which is a relative ratio based on the NG slab ratio with respect to all the slabs to be tested, is shown.

  From FIG. 3, the heat having an NG slab ratio index of 0.0 is condition A (nozzle immersion depth: less than 290 mm), condition B (drift rate: 6.0% or less), condition C (effective Ar amount: 5.0 L / ton or less) is satisfied (“satisfy” is indicated by “☆”). Of the remaining (one or more of the conditions A to C are not satisfied), the conditions A and B are satisfied, and the condition C is not satisfied (“not satisfied” is indicated by “★”). The slab ratio index is 0.4. Further, among the remaining conditions, the condition A is satisfied but the condition B is not satisfied, and the condition C is not determined whether it is satisfied or not satisfied (“unidentified” is indicated by “−”). The slab ratio index is 1.1. In the heat excluding these, that is, the heat that does not satisfy the condition A and has not yet been determined for the conditions B and C, the NG slab ratio index is 2.1.

  That is, this result shows that, in the case of wide casting where the casting width of the factor [3] is 1450 mm or more, the nozzle immersion depth of the factor [3] is less than 290 mm (condition A), and the drift rate of the factor [1] is Slab free material from heat satisfying the three conditions of 6.0% or less (Condition B) and the effective Ar amount of the factor [2] is 5.0 L / ton or less (Condition C) However, surface defects do not occur after cold rolling, while surface defects may occur after cold rolling in slab-free materials from heat that do not satisfy any one or more of these three conditions. In addition, as the number of unsatisfied conditions increases to a few, the frequency of occurrence increases. From this, in the case of wide casting, slabs from heat that satisfy the three conditions according to the above three factors are shown. Even if this is not necessary, It was found that the downgrade rate in the surface quality inspection after rolling did not deteriorate (increase).

  Moreover, when the same analysis was performed also in the case of narrow casting with a casting width of less than 1450 mm, in this case, the condition A is canceled, that is, the nozzle immersion depth is less than or equal to 290 mm. Regardless of this, slabs from heat that satisfy the two conditions of a drift rate of 6.0% or less (Condition B) and an effective Ar amount of 5.0 L / ton or less (Condition C) should be used as a maintenance-free material. However, it was found that the downgrade rate in the surface quality inspection after cold rolling did not deteriorate (increase).

  Based on the above knowledge, in the present invention, when continuously casting steel with a casting width of less than 1450 mm, the effective Ar amount defined by the above formula (1) is cast under operating conditions of 5 L / ton or less, and during the casting, The drift rate defined by the above equation (2) is measured, and a slab whose measured value is 6.0% or less is to be made a maintenance-free material, and steel is continuously cast with a casting width of 1450 mm or more. In this case, the nozzle immersion depth is less than 290 mm, the effective Ar amount defined by the formula (1) is cast under the operating condition of 5 L / ton or less, and the drift rate defined by the formula (2) is set during the casting. The slab of which the measured value was 6.0% or less was determined to be a maintenance-free material.

  As for the drift rate, if it can be cast while controlling this, it will not exceed that, but in reality it is extremely difficult to control, so it has to be a monitoring item. The amount of heat removed from equation (2) that gives this drift rate can be determined from the amount of increase in the water temperature per unit water flow rate of the mold cooling water on the opposing two short sides.

  In accordance with the present invention, ultra-low carbon mild steel continuous cast slabs were made free of maintenance. Compared with the results of conventional care materials, the rate of deterioration obtained by performing pass / fail judgment of surface quality as usual on the inspection line after cold rolling for the wide castings carried out so far FIG. 4 shows the disability rate index as a relative ratio. As shown in the figure, when the maintenance was made in accordance with the present invention, the rate of decline was improved (reduced) compared to the conventional maintenance materials.

  This is because the implementation of the present invention strictly controlled the continuous casting operation conditions of ultra-low carbon mild steel, and as a result, the slab surface flaw depth was reduced to such an extent that it could be almost removed by hot rolling descaling. It's thought to be. On the other hand, in the case of conventional slab casting conditions for slabs, slab surface defects or subsurface latent defects with a depth that remains even after hot rolling descaling after care such as cutting There must have been many.

It is sectional drawing which shows the view of a drift rate. It is sectional drawing which shows the view of effective Ar amount. It is a graph which shows the analysis result of a slab careless experiment. It is a graph which shows the downgrade rate about the unpurchased material by this invention compared with the track record of the conventional treated material.

Explanation of symbols

1 Immersion nozzle 2 Mold 3 Molten steel F Discharge flow

Claims (2)

When continuously casting steel with a casting width of less than 1450 mm, the effective Ar amount defined by the following formula (1): cast under operating conditions of 5 L / ton or less, and the drift defined by the following formula (2) during the casting A continuous casting method for steel, characterized in that the rate is measured, and the slab whose measured value is 6.0% or less is used as a maintenance-free material.
Effective Ar amount = Ar gas blow amount per unit time into molten steel in nozzle / throughput (amount of cast molten steel per unit time) (L / ton) (1)
Diffusion rate = | Two-surface heat removal amount−4-surface heat removal amount // 2−4 surface average heat removal amount × 100 (%) (2)
Here, the 2nd surface and the 4th surface indicate either one or the other of both end surfaces in the mold long side direction (both short side surfaces of the mold). In continuous casting of steel with a casting width of 1450 mm or more, the nozzle immersion depth is less than 290 mm, the effective Ar amount defined by the following formula (1) is cast under operating conditions of 5 L / ton or less, and the following ( 2) A continuous casting method for steel characterized in that the drift rate defined by the equation is measured, and a slab whose measured value is 6.0% or less is made a maintenance-free material.
Effective Ar amount = Ar gas blow amount per unit time into molten steel in nozzle / throughput (amount of cast molten steel per unit time) (L / ton) (1)
Diffusion rate = | Two-surface heat removal amount−4-surface heat removal amount // 2−4 surface average heat removal amount × 100 (%) (2)
Here, the 2nd surface and the 4th surface indicate either one or the other of both end surfaces in the mold long side direction (both short side surfaces of the mold).