JP2574582B2 - Adjustment method of slab strand segregation in continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a center segregation unavoidably generated in a slab strand obtained by continuous casting.
The purpose of the present invention is to obtain a continuous cast slab having a stable quality with the entire length kept within an appropriate range.

[0002]

2. Description of the Related Art The central segregation of a continuous cast slab of steel is caused by the concentration of molten steel components such as C, S, and P at the center of the thickness, which is the final solidified portion of a slab strand drawn from a continuous casting mold. It appeared as positive segregation and was one of the quality defects that could not be avoided by the conventional continuous casting method.

[0003] The mechanism of generation of center segregation is not only the solidification shrinkage of the solidification front end of the slab strand obtained by continuous casting, but also the vacuum suction force generated by bulging of the solidification shell and the like, and the solidification front end is concentrated. It is considered that the molten steel is sucked and remains as positive segregation at the center of the thickness of the slab strand. For example, in continuous casting of blooms for hard steel wires having a thickness of 400 mm and a width of 560 mm, significant V segregation and intermittent cavities are generated near the axis of the slab strand. Such V segregation has a width of 100 mm around the axis of the slab strand.
It occurs in the area of about mm, and the negative segregation zone is clearly recognized in the area adjacent to this area, indicating that the solute-concentrated molten steel moves about 100 mm in width around the axis of the strand. ing.

[0004] Measures to reduce center segregation in slabs and blooms obtained by continuous casting are to reduce the degree of superheat of molten steel, or to add a wire rod to molten steel in a mold to form an equiaxed crystal inside a slab strand. There is known a method of forming a solidified structure having a predetermined shape, and a method of obtaining equiaxed crystals by electromagnetically stirring molten steel in a mold or unsolidified molten steel in a slab strand. All of these methods aim at fine dispersion of a solute by obtaining a solidified structure composed of equiaxed crystals, thereby reducing center segregation. However, this method has advantages and disadvantages, and although it is widely used, the effect of improving segregation is not always sufficient.

In addition, as a countermeasure for reducing center segregation,
As seen in JP-A-52-104220 and JP-A-54-107831, there are also known methods of applying a light reduction by a roll.However, in these methods, when the reduction amount by the roll is small, It is not enough to prevent the concentrated molten steel from moving downward (moving to the solidification completion point) caused by bulging or solidification shrinkage. Conversely, if the reduction amount is too large, cracks occur at the solidification interface. There is a fatal drawback that the pressure is generated, and it is very difficult to adjust the rolling reduction.

As a method of reducing the segregation of the center of the slab strand including the solution of the problem relating to this point, JP-A-63-18376 discloses a method.
Reference is made to Publication No. 5. The method proposed here is
Before the solidification point of the slab strand obtained by continuous casting, particularly at the position where the solid fraction becomes 0.5 to 0.9, δ / D ≧
0.5 (where δ: total reduction by forging (mm), D:
It is intended to perform forging processing to satisfy the unsolidified thickness (mm) at the forging position. According to this method, central segregation is effectively reduced without occurrence of internal cracks and significant negative segregation. It was confirmed that it could be done.

[0007]

In the above-mentioned method using forging, the unsolidified molten steel having a high solute concentration remaining at the center of the thickness of the slab strand is directed toward the top side of the slab strand. Forcibly squeezed, the unsolidified molten steel and the molten steel located upstream of it become mixed, and after prolonged operation that repeats this situation, the center of the obtained slab is The solute concentration in the part changed along the longitudinal direction of the slab strand, and in some cases, the quality of the slab could be adversely affected.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and its gist is as follows.

That is, according to the present invention, when a slab strand drawn from a continuous casting mold is drawn out, a forging process is performed in a region near the solidification completion point of the slab strand with an anvil sandwiching the slab strand. Adjusting the casting speed or cooling speed of the slab strand to gradually increase or decrease the solid fraction of the slab strand at the forging point in accordance with the progress of forging. This is a method for adjusting segregation of slab strands in continuous casting.

FIG. 1 shows a continuous casting machine suitable for carrying out the present invention. In the figure, reference numeral 1 denotes a continuous casting mold, 2 denotes a slab strand drawn from the mold 1, 3a, 3b. Are a pair of anvils sandwiched on both sides of the slab strand 2, and the anvils 3a and 3b are disposed in the vicinity of the solidification completion point of the slab strand 2, and the driving means D
The strands 2 are subjected to forging processing by repeating the mutual approach and separation by the drive of (2) (the drive mechanism is not shown).
Reference numeral 4 denotes an electromagnetic stirring device, and reference numeral 5 denotes a pinch roll for pulling out the slab strand 1.

[0011]

In order to reduce the segregation of the center of the slab strand 2 by forging with the anvils 3a and 3b, the solidification rate of the slab strand 2 at the forging point is adjusted by adjusting the casting speed and the cooling speed. It is effective to perform operations that are almost constant. However, in such a forging method, as described above, the unsolidified molten steel having a high solute concentration remaining in the center of the thickness of the slab strand is forcibly squeezed toward the top side of the strand, and is further upstream. This causes a problem that the solute concentration in the central portion of the slab strand becomes non-uniform in the longitudinal direction because of mixing with the sound molten steel.

The present invention further improves the above-described forging process by appropriately changing the solid fraction of the slab strand at the forging point within an allowable range in which center segregation can be reduced. The aim is to make the quality uniform in the region.

It is preferable that the allowable range of the solid fraction at the forging point of the slab strand is 0.5 to 0.95, and a method of gradually increasing or decreasing the solid fraction with the lapse of casting time. Is valid. The amount of unsolidified molten steel squeezed out by forging varies depending on the solid fraction, and for the same forging amount, the squeezed amount is small when the solid fraction is high and large when the solid fraction is low, On the other hand, the solute concentration in the unsolidified molten steel is high when the solid fraction is high, and low when the solid fraction is low. Thus, by controlling the molten mass squeezed by changing the solid phase ratio of the forging point, molten steel having a high solute concentration can be dispersed and solidified in the longitudinal direction of the slab strand.

In order to change the solid fraction of the slab strand at the forging point with the elapse of casting time, it is necessary to change the time until the slab strand drawn from the continuous casting mold reaches the forging point. Is most effective, and more specifically, can be achieved by sequentially changing the casting speed. However, the amount of cooling water in the secondary cooling may be changed.

[0015]

EXAMPLE A continuous caster as shown in FIG. 1 above was applied, and C: 0.64 wt%, Si: 0.23 wt%, Mn: 0.73 wt%, P:
Thickness 270 mm, width 340 mm containing 0.010 wt%, S: 0.010 wt% and Al: 0.025 wt%, with the balance being substantially Fe.
Casting speed: 0.85 to 1.10m / to obtain a bloom for wire rod
min, continuous casting under the condition of the solid portion ratio of the center of the slab strand at the forging point: 0.75 to 0.95 (worked so that the center becomes a complete solid phase by forging) Was evaluated by the central segregation index in the longitudinal direction. The result is shown in FIG. The above-mentioned center segregation index refers to the analysis of C (carbon) at 20 points in the casting direction at intervals of 30 mm using a 5 mmφ drill from the center of the slab in the thickness direction of the slab, performing C (carbon) analysis, The result is divided by the C concentration (C ₀ ) to obtain an average value and indexed.

In FIG. 2, Example 1 shows a case where the solid phase ratio at the processing point of the slab strand at the start of forging is 0.95, and the solid phase ratio is sequentially changed to 0.75 with the lapse of casting time. It was confirmed that C / C ₀ was approximately 0.98 to 1.0 in the longitudinal direction of the slab by changing the solid phase ratio in this way, and that the quality was stable.

In FIG. 2, in Example 2, the solid phase ratio at the processing point of the slab strand at the start of forging was set to 0.75, and thereafter this solid phase ratio was sequentially changed to 0.95 with the lapse of casting time. This is the case where negative segregation was made positively in the center of the slab.By performing such an operation, C / C _{0 in} the longitudinal direction of the slab was approximately 0.92 to 0.93, and a stable value was obtained. ing.

Comparative Example 1 in FIG. 2 is a case where ordinary continuous casting was performed without performing forging processing. Comparative Example 2
This is a case where continuous casting was performed at a casting speed of 0.98 m / min and a solid phase ratio of the slab strand at the forging point of 0.85, which was almost constant. In the case of Comparative Example 1, the center segregation at the center of the slab is very large. In Comparative Example 2, although the center segregation is reduced, the change of the center segregation index in the initial stage of casting is large.

[0019]

As described above, according to the present invention, it is possible to obtain a continuous cast slab with reduced center segregation and stable quality in the longitudinal direction.

[Brief description of the drawings]

FIG. 1 is an explanatory view of a configuration of a continuous casting machine suitable for carrying out the present invention.

FIG. 2 is a graph showing a result of examining a center segregation index in a longitudinal direction of a bloom slab.

[Explanation of symbols]

 DESCRIPTION OF REFERENCE NUMERALS 1 Continuous casting mold 2 Slab strand 3a Anvil 3b Anvil 4 Electromagnetic stirrer 5 Pinch roll for drawing slab strand

Claims (1)

(57) [Claims] When a forging process is performed in a region near the solidification completion point of a slab strand with an anvil sandwiching both sides of the slab strand while the slab strand is being drawn from a continuous casting mold, Continuous casting characterized by gradually increasing or decreasing the solid phase ratio of the slab strand at the forging point by adjusting the casting speed or cooling speed, in accordance with the progress of forging. Of adjusting slab strand segregation in the above.