Field of the Invention
The present invention relates to an electromagnetic stirring method which provides a satisfactory solidified structure in continuous casting.
Besides Fe, molten steel contains various alloying elements and impurity elements, and the solidification of molten steel is sometimes attended by the remarkable segregation of segregative elements, such as C, P, and S, into the final solidifying portion of the steel ingot or cast slab and bloom. Products made of a material having such a segregated portion would have inferior product characteristics due to the non-ununiformity of their mechanical properties, and they would have case trouble during welding; thus it is important to decrease segregation. Particularly in continuous casting process, noticeable segregation develops in a direction at right angles with the cast slab and bloom drawing direction; however, past examination of various operating conditions have not been successful in improving the mechanical properties of the cast slab and bloom.
The most promising of the measures heretofore taken is to apply electromagnetic stirring to molten steel during solidification. Although this method has been recognized as having the effect of breaking to some extent the columnar crystals growing during solidification, such a degree of breaking of columnar crystals is insufficient for elimination of marked segregation. Thus, to enhance the stirring effect, an attempt has been made to increase the electromagnetic stirring force so as to provide an increased stirring force capacity, but it has the drawback of producing a white band in the form of negative segregation. The white band portion is not only lower in the percentages of alloying elements than their average values, forming a qualitative defect, but also presents an undesirable outside appearance.
The present invention, made with this serious situation in mind, is intended to establish electromagnetic stirring conditions for enhancing the effect of breaking columnar crystals to reduce negative segregation and avoid formation of white bands. Thus, the method of electromagnetically stirring molten steel in continuous casting according to the present invention is characterized in that an electromagnetic stirrer is installed between drawing positions where the unsolidified thickness is 45% and 15%, preferably 35% and 20%, respectively, of the thickness as viewed in the direction of the thickness of the cast slab and bloom, and in that where the magnetic flux density at the interface between the unsolidified and solidified portions (said interface being hereinafter referred to as the solidification interface) is represented by B (gauss),
the stirring effective length of the electromagnetic stirrer by 1 (mm),
the casting speed by v (m/min.), and
the stirring time by T (min.) =1/v, stirring in the casting direction is applied in such a manner that the product B.times.T is 1,600 gauss-min. or more per m.sup.3 of the total volume of the unsolidified molten steel present in a region extending to the drawing side from the position where the electromagnetic stirrer is located.
The aforesaid conditions have been determined with the flow condition of molten steel during solidification taken into account. The arrangement and functions and effects of the invention will now be described along with the process of development of the invention.
In continuous casting, the cause of segregation taking place in the central portion of the cast slab and bloom is generally considered as follows.
It is known that although the central portion of the cast slab and bloom, when viewed in the casting direction (drawing direction), has very little temperature gradient, the flow of the solid-liquid coexistence layer in this portion can be induced by the so-called suction (a phenomenon of contraction of the solid-liquid coexistence layer taking place in the last stage of solidification of molten steel). However, all the solid-liquid coexistence layer does not flow at the same time, but owing to solidification contraction which proceeds in the lower region (on the drawing side), the region which overlies the same (mold side) flows downward, and as this flowing region solidifies, the region which overlies the same flows downward and solidifies; such stepwise flow is repeated, whereby the periodicity of V segregation is formed. This situation will now be further described. The solid-liquid coexistence condition is established in several regions along the cast slab and bloom drawing direction i.e., the casting direction and these regions flow in block but the flow of these regions takes place successively with some time lag, with the lower side flowing first. Therefore, between adjacent regions, the dendrites separate from each other in accordance with the flow time lag, so that cavities with some periodicity are formed. Such a cavity has a temperature gradient in a direction at right angles with the cast slab and bloom drawing direction and a flow of molten steel is formed between the dendrites, so that the aforesaid suction effect becomes greater toward the center of the cast slab and bloom. Under these influences, the aforesaid cavities assume a V-shape inclined toward the center axis, and it seems that the surrounding segregated liquid present between the dendrites flows into the V-shaped cavities, resulting in V segregation.
On the basis of this analysis, we thought we would attain reduction of segregation in the central portion of the cast slab and bloom by adjusting the electromagnetic stirring force so as to change the aforesaid solidification mechanism.
The region where V segregation takes place is, after all, a region with little temperature gradient. The factors which determine the size of this region are supposed to include the molten steel composition (particularly the carbon concentration) and superheating of molten steel, but a statistical examination of regions where V segregation is formed has revealed that even the maximum value does not exceed 45% of the thickness as viewed in the direction of the thickness of the cast bloom.