JP3374761B2 - Continuous cast slab, continuous casting method thereof, and method of manufacturing thick steel plate - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a continuously cast slab of steel, a continuous casting method for the slab, and a method for producing a thick steel plate by hot rolling the slab.

[0002]

2. Description of the Related Art Center segregation and V segregation are likely to occur near the center of the thickness of continuously cast slabs of steel. The center segregation appears when the segregated components such as C, S, P, and Mn are concentrated in the final solidified portion of the cast slab, and the V segregation is a V-shaped segregated component near the final solidified portion of the cast slab. It is segregated into a shape.

It is known that such segregation causes, for example, reduction in toughness of thick steel plates and hydrogen-induced cracking of large-diameter steel pipe manufactured by welding after bending from thick steel plates.

The generation mechanism of such segregation is considered as follows. As the solidification progresses, the segregation component is concentrated in the dendrite tree, which is one of the solidification structures. This concentrated molten steel flows out from between the dendrite trees due to shrinkage of the slab during solidification or swelling of the slab called bulging. The concentrated molten steel that has flowed out locally accumulates in places, or flows toward the solidification completion point of the final solidification portion, and solidifies as it is as a thickened zone. Therefore, these segregations occur.

As a countermeasure for preventing such segregation, it is effective to prevent the concentrated molten steel from moving between the dendrite trees and to prevent the localized accumulation of the concentrated molten steel. Proposed.

[0006] As one of them, there is a light reduction method using a reduction roll group, but under a light reduction slightly exceeding the amount of solidification shrinkage,
There is a limit to the improvement of segregation.

In addition, in order to effectively improve these segregation, there is a method of applying a large reduction to a cast piece with a reduction roll. However, when a large reduction is applied, there is a concern that internal slab cracking may occur due to the reduction.

For iron and steel, 60 (1974), P875, when a slab having a square cross section with a side of 120 mm was rolled over a rolling reduction of more than 30%, internal cracking occurred at 30% or less. It is disclosed that the center segregation is improved by preventing the generation of the negative segregation part in the thickness center part. However, in this method, since both ends of the completely solidified slab will be rolled down, when applied to a large continuously cast slab having a cross-sectional shape such as a slab, the equipment for rolling down is enlarged. There must be.

In Japanese Unexamined Patent Publication No. 9-57410, a slab containing an unsolidified portion is bulged by about 20 to 100 mm,
A method is disclosed in which at least a pair of pressing rolls is used to reduce the amount corresponding to the bulging amount before the solidification completion position. In this method, since both ends of the completely solidified slab are not pressed, it is not necessary to use excessive equipment.
However, there is no mention of the timing of starting the reduction, and if the timing of starting the reduction is not appropriate, the effect of improving segregation may not be obtained.

The above-mentioned iron and steel, 60 (1974),
In the method disclosed in P875, when a slab containing a negative segregation portion is used as a raw material and hot-rolled into, for example, a thick steel plate, in the vicinity of the boundary between the negative segregation portion and the surrounding solidification structure portion, the surrounding solidification structure Internal cracks may occur in the thick steel plate of the product corresponding to the positions of the parts due to hot rolling, and the internal cracks may remain in the thick steel plate. If the thick steel sheet has internal cracks, the mechanical properties deteriorate, and in a large-diameter steel pipe made of this thick steel sheet, the mechanical properties deteriorate and hydrogen-induced cracking occurs.

[0011]

DISCLOSURE OF THE INVENTION The present invention is a slab having an equiaxed crystal and a negative segregation in the central portion, and even if the slab is hot-rolled to a thick steel plate, the thick steel plate of the product is An object of the present invention is to provide a continuously cast slab of steel in which cracking does not occur, a continuous casting method for the slab, and a method for producing a thick steel plate by hot rolling the slab.

[0012]

The gist of the present invention is to provide a continuous cast slab as shown in (1) below, a continuous casting method as shown in (2) below, and a method for manufacturing a thick steel plate as shown in (3) below. It is in.

(1) The center of the cross section of the cast slab is equiaxed crystal, and the periphery thereof is provided with branched columnar crystals and a cast structure composed of columnar crystals or columnar crystals. its periphery Ri negative segregation area der, between its Fuhen析領zone and billet surface
Is a continuously cast slab of steel containing 0.02 to 0.6% by weight of carbon in which equiaxed crystals are not present .

(2) After the slab having an unsolidified portion is bulged by 5 to 25% of the thickness of the slab at the start of bulging, a bulging-corresponding amount of thickness is paired before the start of equiaxed crystal formation. The method for continuous casting of a slab as described in (1), wherein the slab is rolled down by the roll.

(3) A method of manufacturing a thick steel plate, which comprises hot rolling the slab according to (1) above.

Negative segregation present in the continuously cast slab of the present invention is the ratio of the content (% by weight) C of the component in the thickness center of the slab divided by the C analysis value C ₀ of the ladle. C / C
A value of ₀ is less than 1. That is, it means that the content of C, Si, Mn, etc. in the negative segregation portion of the cast slab is smaller than the Ladle analysis value.

When the present inventors manufacture a thick steel sheet by hot rolling a continuously cast slab having a negative segregation portion at the center,
It was found that internal cracking occurs in thick steel plates for the following reasons. That is, since there is a difference in hardness between the negative segregation portion of the cast slab and the solidification structure portion around it, stress tends to concentrate at the boundary between the negative segregation portion and the solidification structure around it during hot rolling. At this time, if semi-macro segregation such as granular segregation is present in the surrounding solidified structure, these segregation become the starting points of internal cracks during hot rolling. This crack during rolling remains as an internal crack in the thick steel plate.

The presence of granular segregation in the solidification structure around the negative segregation portion is when the surrounding solidification structure is an equiaxed crystal. In the case of a columnar crystal or a branched columnar crystal, these particles are solidified during solidification. No granular segregation occurs between these crystals because the crystals grow densely.

In the cast slab of the present invention, since the solidification structure between the negative segregation portion and the cast slab surface is a columnar crystal or a branched columnar crystal, there is no granular segregation, and therefore a thick steel sheet is produced by hot rolling. When doing, internal cracks are less likely to occur. It should be noted that equiaxed crystals may exist inside the negative segregation portion. This is because granular segregation does not occur between equiaxed crystals in the negative segregation part. That is, when the negative segregation portion is generated, a large rolling force is working, and at this time, segregation is not generated.

The slab of the present invention has a carbon content of 0.02.
For steels up to 0.6% by weight. The solidification structure of the continuously cast slab of this steel is from the surface of the slab toward the center,
Solidified in the order of columnar crystals, branched columnar crystals, equiaxed crystals or columnar crystals, equiaxed crystals. Therefore, when continuously casting this steel, if negative segregation is generated by the reduction of the slab,
If the pressure reduction is performed before the formation of equiaxed crystals, formation of equiaxed crystals does not occur around the negative segregation portion.

The thick steel plate produced by hot rolling the cast product of the present invention has mechanical properties such as tensile strength equivalent to those of ordinary products. Generally, in a thick steel plate having a negative segregation portion, the reduction of the mechanical properties, particularly in the Z direction (the thickness direction of the plate), is reduced. However, in the cast slab of the present invention, since the solidification structure of the negative segregation portion is dense due to the effect of reduction, the drawing in the Z direction does not decrease.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The solidification structure of the cross-section of the slab of the present invention is such that the central part of the slab is equiaxed and the periphery of the equiaxed crystal,
That is, from the equiaxed crystal at the center of the slab toward the surface,
It has branched columnar crystals and columnar crystals or only columnar crystals.

The thickness of this equiaxed crystal varies depending on the target steel, the amount of secondary cooling water of the slab, the degree of superheated molten steel in the tundish, etc., but the C content is 0.02 to 0.6% by weight.
In the case of steel, the thickness of the slab is usually about 20 to 3
0% is the upper limit. Also in the case of the present invention, it is preferable that this level or less.

The negative segregation portion of the cast slab is the entire area of the equiaxed crystal portion or the area of the solidification structure of the equiaxed crystal portion and its peripheral portion. The fact that the region of the negative segregation portion is a region including the equiaxed crystal portion or the peripheral portion thereof means that there is no equiaxed crystal between the negative segregation portion and the surface of the cast piece.

The thickness of the negative segregation portion, including the peripheral portion of the equiaxed crystal, is preferably about 30% or less of the thickness of the cast slab.

FIG. 1 is a schematic diagram for explaining a continuous casting method for obtaining the above-mentioned cast piece of the present invention. Molten steel is injected into the mold 1 from the immersion nozzle 8. The solidified shell 2a formed in the mold is pulled out from the mold 1 and then cooled by spray water to increase its thickness. Further, the slab 2 is pulled out by a pinch roll 5 via a guide roll 3 and a pressing roll 4. In order to cause the slab to bulge, the interval in the thickness direction of the slab of the guide roll 3 is arranged so as to increase stepwise in the drawing direction from the mold outlet side to immediately before the reduction roll 4. After that, a pair of reduction rolls 4 reduces the amount of bulging.

The bulging amount is 5 to 25% of the thickness of the slab at the start of bulging. If bulging exceeds 25%, excessive equipment is required for rolling down the slab, so the upper limit was made 25%. Further, in order to stably generate the negative segregation portion inside the slab, it is necessary to bulge at 5% or more. Therefore, the bulging amount is set to 5 to 25% of the thickness of the slab at the start of bulging.

The reduction of the cast slab is performed before the start of formation of equiaxed crystals. This is because equiaxed crystals do not exist between the negative segregation portion generated by rolling and the surface of the cast slab. The generation start time of this equiaxed crystal is the carbon content of steel, the amount of secondary cooling water of the slab,
Determined by the degree of superheat of molten steel in the tundish. If the amount of secondary cooling water in the cast slab is large or if the degree of superheat of the molten steel is high, columnar crystals develop and the formation start time of equiaxed crystals is delayed. The carbon content of steel, the amount of secondary cooling water, the degree of superheat of molten steel, etc. are changed, the thickness of the equiaxed crystal of the slab is confirmed, and the generation start time of the equiaxed crystal is determined for each casting condition. The time to start the reduction may be decided based on the time.

Furthermore, in order to prevent equiaxed crystals from existing between the negative segregation portion and the surface of the cast slab, it is necessary to control the thickness of the negative segregation portion. The present inventors have found that the thickness of this negative segregation portion is
At the time of starting the reduction, it was found that the interval between the solidified interfaces of the thickness of the slab having a solid fraction of 0.8, that is, the thickness of the unsolidified portion between the solidified interfaces having a solid fraction of 0.8 was obtained. It was From this fact, the thickness of the negative segregation portion of the cast slab after the reduction can be controlled by setting the timing of the reduction based on the thickness of the unsolidified portion between the solidification interfaces having the solid phase ratio of 0.8. However, the value of the solid phase ratio of 0.8 is not necessarily general, and may change if the thickness of the slab or the continuous casting machine is changed. At this time, the time of rolling down may be changed, the thickness of the negative segregation portion may be examined, and the solid fraction to be used as a reference may be experimentally determined.

The solidification interface corresponding to the solid fraction, such as the solid fraction of 0.8 as described above, is the casting speed, the secondary cooling of the slab,
It can be calculated by the one-dimensional unsteady heat transfer analysis method in the thickness direction of the slab considering the physical properties of steel.

The amount of reduction is defined as the amount corresponding to bulging because the amount of reduction does not reduce the short side portion of the slab that has been solidified, and that no excessive equipment is required. This is because the negative segregation portion is not stably formed at the center of the cast piece under the pressure of. In addition, the reason why the rolling rolls are a pair of rolls is that when there are two or more pairs, the rolling amount of each pair is small and the negative segregation portion is difficult to form.

In the case of the method of the present invention, the unsolidified portion may be stirred by electromagnetic stirring before the pressing. The electromagnetic stirrer 6 shown in FIG. 1 is a device for stirring the unsolidified portion 2b to make the solidified structure equiaxed. When a large amount of equiaxed crystals are generated, there is a problem that the area of the negative segregation part must be increased. However, it may be effective to perform this electromagnetic stirring.

Depending on the type of steel, internal cracking tends to occur in the negative segregation portion of the cast during rolling. In such cases, equiaxed crystals are less likely to cause internal cracking than columnar or branched columnar crystals, so that electromagnetic stirring creates equiaxed crystals in the center of the slab to prevent internal cracking. It is desirable to do. However, although the negative segregation portion is generated by pressing down the slab after electromagnetic stirring, it is desirable that the thickness of the equiaxed crystal is within the range of the thickness of the negative segregation portion. It is also desirable to limit the steel that is applied.

When an equiaxed crystal is generated in the central portion of a slab by electromagnetic stirring, the thickness of this equiaxed crystal is the thickness of the unsolidified portion between solidification interfaces having a solid fraction of 0.4 at the electromagnetic stirring position. Almost matches. A solid phase ratio of 0.4 is a state in which both solid phase and liquid phase can flow simultaneously. However, the value of the solid phase ratio of 0.4 is not necessarily general, and may be different if the thickness of the slab or the continuous casting machine is changed. At this time, the time of rolling may be changed and the thickness of the equiaxed crystal may be investigated.

As described above, the thickness of the negative segregation portion is determined by the thickness between solidification interfaces having a solid fraction of 0.8 at the time of rolling down.
The thickness of the equiaxed crystal generated by electromagnetic stirring is determined by the thickness between solidification interfaces with a solid fraction of 0.4 at the stirring position. The stirring timing may be selected.

When the slab of the present invention is hot-rolled to manufacture a thick steel plate, the heating temperature of the slab, in-furnace time, rolling temperature, etc. are the same as those of the ordinary slab. I do not care.

[0037]

EXAMPLE Molten steel was cast using a slab continuous casting apparatus having the apparatus configuration shown in FIG. The slab size is 220m thick
m, width 1800 mm, the used steel has a C content of 0.15
It is a steel for steel plates for weight percent. The secondary cooling specific water amount was 2 liter / kg-steel, and the bulging amount was 10 to 40 mm. Using a pair of upper and lower rolling rolls having a diameter of 210 mm, the bulging equivalent amount was rolled down with a maximum rolling force of 200 ton / roll.

The casting speed was changed during casting at about 1 m / min to change the thickness of the unsolidified portion between the solidified interfaces with the solid fraction of 0.8 at the time of rolling. Further, the thickness of the non-solidified portion between solidified interfaces having a solid phase ratio of 0.4 at the time of performing electromagnetic stirring was also changed. The magnetic field strength of the electromagnetic stirrer was a magnetic flux density of 300 gauss, and the maximum stirring speed was 300 rpm.

Further, the degree of superheat of molten steel in the tundish was changed to control the solidification structure. Table 1 shows the casting conditions.

[0040]

[Table 1]

A sample having a length of 1 m in the casting direction was sampled from the obtained slab and the macrostructure of its cross section was observed to confirm the solidification structure of the slab cross section. The thickness of the equiaxed crystal was measured.

Next, a slab having a length of 5 m was sampled, heated in an ordinary heating furnace, and then hot rolled into a thick steel plate having a thickness of 30 mm.

From the center of the width of the thick steel plate and the position of 1/4 width, J
The No. 14 tensile test piece specified in IS Z 2201 was sampled and the tensile strength was investigated. Further, in order to investigate the influence of the granular segregation around the negative segregation portion of the cast slab on the internal cracking of the thick steel sheet, the drawing in the Z-direction tensile test of the thick steel sheet was measured. The Z-direction tensile test was carried out as follows. That is, five samples each having a total plate thickness of 10 mm in diameter and 30 mm in length in the plate thickness direction were taken from each of the width center and the quarter width position of the thick steel plate for each test. Cylindrical round steel having the same diameter and a length of 50 mm was welded to both end faces of these samples to prepare tensile test samples. As the round steel, a homogenized steel having a C content of 0.2% by weight was used. Drawing was determined from the tensile test results of these samples at room temperature, and the average value was used as the drawing for each test. Further, the fracture surface of the sample after the tensile test was observed, and the results are also shown in Table 1.

In each test of the examples of the present invention, the negative segregation region is within the range defined by the present invention, that is, the thickness of the negative segregation portion is thicker than the thickness of the equiaxed crystal. The reduction was 73 to 76%, and the breakage position was also good at the center of the sample. This means that the thick steel sheet has no internal cracks. Also,
The normal tensile strength of the thick steel plate was also 520 to 530 MPa, which was a value comparable to that of the thick steel plate using the target ordinary slab as a material, and was good.

Test No. of the present invention example In No. 1, the thickness of the equiaxed crystal was 8 mm and the thickness of the negative segregation portion was 10 mm. Test No. In No. 2, since the degree of superheat of molten steel was increased to 80 ° C., the thickness of the equiaxed crystal was as thin as 6 mm. The negative segregation portion had a thickness of 20 mm, and the entire periphery of the negative segregation portion was columnar crystals. Test No. of the present invention example 3 and No. In No. 4, electromagnetic stirring was performed to increase the thickness of the equiaxed crystal to 23 to 28 mm.
The thickness of the negative segregation portion is 30 m by controlling the thickness of the non-solidification portion between solidification interfaces having a solid fraction of 0.8 at the time of reduction.
m, and the thickness of the equiaxed crystal was made thinner than the thickness of the negative segregation portion within the range specified in the present invention.

Test No. of the comparative example. 5 to No. In No. 8, the tensile strength of the thick steel plate was 520 to 530 MPa, which was similar to that of a normal thick steel plate, but the drawing in the Z direction tensile test was low at 55 to 65%, and the fracture position of the sample was the negative segregation region. It was the border of. This breaking position is the position of the thick steel plate corresponding to the granular segregation portion around the negative segregation portion of the cast slab. This is because in each of the tests, equiaxed crystals exist up to the outside of the negative segregation portion, and granular segregation is generated around the negative segregation portion. Test No. 5 and No. In No. 6, electromagnetic stirring was not performed, and the molten steel superheat degree in the tundish was lowered to 12 to 13 ° C. for casting. Since the degree of superheat of molten steel was low, the thickness of the equiaxed crystal was as thick as 20 to 22 mm, whereas the thickness of the negative segregation portion was only 10 to 20 mm. Test No. 7
And No. In No. 8, since magnetic stirring was performed in all cases,
The thickness of the equiaxed crystal was as thick as 23 to 28 mm, and the thickness of the negative segregation portion was 20 mm.

Test No. of the comparative example. In No. 9, a conventional light reduction method without bulging and a test without electromagnetic stirring were performed. No. 10 was also tested by the conventional light reduction method and with magnetic stirring. In the light reduction method at this time, reduction is started when the thickness of the unsolidified portion between the solidification interfaces having a solid fraction of 0.8 is 20 mm, and a total reduction of 10 mm is performed during a length of 4 m in the casting direction. I went.

Test No. Center segregation remains in the slab of No. 9, and in the tensile test result of the thick steel plate obtained by hot rolling this slab, the drawing is low at 55%, and the fracture position of the tensile test sample is the center of the sample. It was a center segregation part, and the adverse effect of center segregation was clearly recognized.

Test No. In the tensile test results of the thick steel plate obtained by hot rolling 10 cast pieces, the reduction was as low as 58%, and the fracture position of the tensile test sample was at these segregated portions in the center of the sample. This is because V segregation and granular segregation remained in the center of the slab.

[0050]

The slab of the present invention is a slab in which the central portion is equiaxed and at least that region is a negative segregation portion. By hot rolling this slab as a raw material, a thick steel plate with good quality without internal cracks can be obtained. According to the continuous casting method of the present invention and the method of manufacturing a thick steel plate of the present invention, the above cast piece and thick steel plate can be easily manufactured.

[Brief description of drawings]

FIG. 1 is a schematic diagram for explaining a continuous casting method of the present invention.

[Explanation of symbols]

1: mold 2: slab 2a: solidified shell 2b: unsolidified portion 3: Guide roll 4: Roll down 5: Pinch roll 6: Electromagnetic stirrer 7: Molten steel 8: Immersion nozzle 9: Casting direction

Claims (3)

(57) [Claims] 1. A cast slab is provided with a equiaxed crystal at the center of its cross-section and a cast structure composed of branched columnar crystals and columnar crystals or columnar crystals in the periphery thereof. periphery Ri negative segregation area der, said between its Fuhen析領zone and billet surface
0.02 to 0. characterized by the absence of equiaxed crystals.
Continuous cast slab of steel containing 6% by weight of carbon. 2. A slab having an unsolidified portion is bulged in an amount of 5 to 25% of the thickness of the slab at the start of bulging, and before the start of formation of equiaxed crystals, a bulging-corresponding amount of The continuous casting method for a cast slab according to claim 1, wherein the rolling is performed by rolling. 3. A method for producing a thick steel plate, which comprises hot rolling the slab according to claim 1.