JP2000094101A - Continuous cast slab, continuous casting method thereof, and method of manufacturing thick steel plate - Google Patents

Abstract

(57) [Summary] [Problem] To provide a continuous cast slab of steel which has little center segregation and V segregation and does not generate internal cracks even when hot-rolled to a thick steel plate, a continuous casting method of the slab, and a slab thereof. Provision of a method for manufacturing a hot-rolled thick steel plate. The slab has a cast structure in which the center of the cross section is composed of equiaxed crystals and the periphery thereof is composed of branched columnar crystals and columnar crystals or columnar crystals. And its periphery are negatively segregated regions, containing 0.02 to 0.6% by weight of carbon. At the time of casting, after bulging 5 to 25% of the thickness of the slab at the start of bulging, the thickness equivalent to bulging is reduced by a pair of rolls before the start of the generation of equiaxed crystals. A thick steel plate is manufactured using the above slab under normal conditions.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuously cast slab of steel, a method of continuously casting the slab, and a method of manufacturing a thick steel plate by hot rolling the slab.

[0002]

2. Description of the Related Art Central segregation and V segregation are likely to occur near the center of the thickness of a continuously cast slab of steel. Central segregation is a phenomenon in which segregation components such as C, S, P, and Mn appear in the final solidified portion of the slab, and V segregation is a V-shaped segregation component near the final solidified portion of the slab. It segregates in a shape.

It is known that such segregation causes, for example, a decrease in toughness of a thick steel plate and hydrogen-induced cracking of a large-diameter steel pipe manufactured by bending a thick steel plate and then welding.

The mechanism of such segregation is considered as follows. Solidification proceeds, and segregated components are concentrated between dendrite trees, which are one of solidified structures. The 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 thickened molten steel that has flowed out locally accumulates locally, or flows toward the solidification completion point of the final solidification portion, and solidifies as a thick thickened zone as it is. Therefore, these segregations occur.

[0005] As measures to prevent these segregations, it is effective to prevent the movement of the concentrated molten steel remaining between the dendrite trees and to prevent local accumulation of the concentrated molten steel. Proposed.

One of the methods is a light reduction method using a reduction roll group.
There is a limit to the improvement of segregation.

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

In iron and steel, 60 (1974), and P875, when a slab having a square cross section of 120 mm on a side was rolled down at a rolling reduction rate of more than 30%, internal cracks generated at a rolling reduction of 30% or less were obtained. It is disclosed that the segregation does not occur and a negative segregation part is generated at the center of the thickness, thereby improving the center segregation. However, in this method, both ends of a completely solidified slab are reduced, so when applying to a continuous cast slab having a large cross-sectional shape such as a slab, equipment for the reduction is increased in size. There must be.

Japanese Patent Application Laid-Open No. 9-57410 discloses that a slab including an unsolidified portion is bulged by about 20 to 100 mm.
A method is disclosed in which the bulging amount is reduced by at least a pair of reduction rolls before the solidification completion position. In this method, since both ends of the slab that has been completely solidified are not reduced, excessive equipment does not have to be used.
However, the timing of starting the rolling is not mentioned, and if the timing of starting the rolling is not appropriate, the effect of improving segregation may not be obtained.

In addition, the above-mentioned iron and steel, 60 (1974),
In the method disclosed in P875, when a cast slab including a negatively segregated portion is used as a raw material, for example, when hot rolling is performed on a thick steel plate, the solidified structure around the boundary between the negatively segregated portion and the surrounding solidified structure portion is formed. In some cases, the steel plate of the product corresponding to the position of the part has an internal crack due to hot rolling, and the internal crack may remain in the steel plate. If there is an internal crack in the thick steel plate, the mechanical properties deteriorate, and in a large-diameter steel pipe made from this thick steel plate, the mechanical properties deteriorate and hydrogen-induced cracks occur.

[0011]

SUMMARY OF THE INVENTION The present invention relates to a slab having an equiaxed crystal and a negatively segregated portion in the center thereof, and even if the slab is hot-rolled into a thick steel plate, the slab is formed in the steel plate of the product. It is an object of the present invention to provide a continuous cast slab of steel that does not cause cracking, a continuous casting method of the slab, and a method of manufacturing a thick steel plate by hot rolling the slab.

[0012]

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

(1) The cast slab has a cast structure in which the center of the cross section is composed of equiaxed crystals and the periphery thereof is composed of branched columnar crystals and columnar crystals or columnar crystals. A continuous cast slab of steel containing 0.02 to 0.6% by weight of carbon, the periphery of which is a negative segregation region.

(2) The slab having an unsolidified portion is bulged by 5 to 25% of the thickness of the slab at the start of bulging. The continuous casting method for cast slabs according to the above (1), wherein the slabs are rolled down.

(3) A method for producing a thick steel plate by hot rolling the slab according to (1).

Negative segregation present in the continuous cast slab of the present invention is defined as a ratio obtained by dividing the content of the component in the center of the slab thickness, for example, C content (% by weight) C by the ladle C analysis value C _0. C / C
The value of ₀ is less than 1. That is, it means that the content of C, Si, Mn, and the like in the negative segregation portion of the slab is smaller than the ladle analysis value.

[0017] The present inventors, when hot rolling a continuous cast slab having a negative segregated portion in the center to produce a thick steel plate,
We have found that internal cracks occur in thick steel plates for the following reasons. That is, since there is a difference in hardness between the negatively segregated portion of the slab and the solidified structure around it, stress tends to concentrate on the boundary between the negatively segregated portion and the solidified 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 segregations become the starting points of internal cracking during hot rolling. The cracks at the time of rolling remain as internal cracks in the thick steel plate.

Further, the granular segregation in the solidification structure around the negative segregation part is when the surrounding solidification structure is an equiaxed crystal, and when the solidification structure is a columnar crystal or a branched columnar crystal, these solidification structures are formed during solidification. Since the crystals grow densely, no granular segregation occurs between these crystals.

In the slab of the present invention, since the solidification structure between the negatively segregated portion and the slab surface is a columnar crystal or a branched columnar crystal, there is no granular segregation. Therefore, a thick steel plate is manufactured by hot rolling. In doing so, internal cracks are less likely to occur. In addition, an equiaxed crystal may be present inside the negative segregation part. This is because no granular segregation is generated between equiaxed crystals in the negative segregation portion. That is, when the negative segregation portion is generated, a large rolling force is acting, and at this time, no segregation is generated.

The slab of the present invention has a carbon content of 0.02.
For steel that is ~ 0.6% by weight. The solidification structure of the continuous cast slab of this steel, from the surface of the slab toward the center,
Solidify in the order of columnar crystals, branched columnar crystals, equiaxed crystals or columnar crystals, and equiaxed crystals. Therefore, when continuously casting this steel, when the negative segregation part is generated by the reduction of the slab,
If the pressure is reduced before the generation of the equiaxed crystal, the equiaxed crystal is not generated around the negatively segregated portion.

[0021] In a steel plate manufactured by hot rolling the slab of the present invention, mechanical properties such as tensile strength are equivalent to those of a normal product. Generally, in the case of a thick steel plate having a negative segregation portion, the reduction of the drawing in the Z direction (the thickness direction of the plate) is reduced among the mechanical properties. However, in the cast slab of the present invention, the reduction in the Z direction does not decrease because the solidification structure of the negatively segregated portion is dense due to the effect of the reduction.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The solidified structure of the cross section of a slab of the present invention is such that the center of the slab is equiaxed,
That is, from the equiaxed crystal at the center of the slab toward the surface,
There are branched columnar crystals and columnar crystals or only columnar crystals.

The thickness of the equiaxed crystal varies depending on the target steel, the amount of secondary cooling water for the slab, the degree of superheat of molten steel in the tundish, and the like.
In the case of steel having a thickness of about 20 to 3 mm,
0% is the upper limit. Also in the case of the present invention, this level or less is preferable.

The negative segregation portion of the slab is the entire region of the equiaxed crystal portion or the region of the solidified 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 its peripheral portion means that no equiaxed crystal exists between the negative segregation portion and the surface of the slab.

It is preferable that the thickness of the negatively segregated portion is about 30% or less of the thickness of the slab including the peripheral portion of the equiaxed crystal.

FIG. 1 is a schematic diagram for explaining a continuous casting method for obtaining the above-mentioned cast slab of the present invention. Molten steel is injected into the mold 1 from the immersion nozzle 8. After the solidified shell 2a generated in the mold is pulled out of the mold 1, it is cooled by spray water and its thickness increases. 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 bulging of the slab, the interval between the guide rolls 3 in the slab thickness direction is arranged so as to increase stepwise in the drawing direction from the mold discharge side to immediately before the pressing roll 4. Thereafter, the bulging amount is reduced by a pair of reduction rolls 4.

The amount of bulging is 5 to 25% of the thickness of the slab at the start of bulging. If the bulging exceeds 25%, excessive equipment is required to reduce the slab, so the upper limit was set to 25%. In addition, in order to stably generate a negative segregation portion in the slab, it is necessary to perform bulging by 5% or more. Therefore, the bulging amount was 5 to 25% of the thickness of the slab at the start of bulging.

The rolling of the slab is performed before the start of the formation of equiaxed crystals. This is because no equiaxed crystal is present between the negatively segregated portion generated by the rolling and the surface of the slab. The start time of the formation of this equiaxed crystal depends on the carbon content of the steel,
It is determined by the degree of superheat of the molten steel in the tundish. If the amount of secondary cooling water in the slab is large or the degree of superheat of the molten steel is high, columnar crystals develop, and the start of the generation of equiaxed crystals is delayed. Change the carbon content of the steel, the amount of secondary cooling water, the degree of superheat of the molten steel, etc., check the thickness of the equiaxed crystal of the slab, and find the start time of the production of the equiaxed crystal for each casting condition. What is necessary is just to determine the time to start the reduction based on the time.

Further, it is necessary to control the thickness of the negatively segregated portion so that no equiaxed crystal exists between the negatively segregated portion and the surface of the slab. The present inventors, the thickness of the negative segregation portion,
Obtained the knowledge that the interval between the solidification interfaces of the slab thickness with a solid fraction of 0.8 at the time of starting rolling, that is, the thickness of the unsolidified portion between the solidification interfaces with a solid fraction of 0.8 Was. From this, if the time for rolling down is set based on the thickness of the unsolidified portion between the solidification interfaces having a solid fraction of 0.8, the thickness of the negatively segregated portion of the slab after rolling can be controlled. However, the value of the solid fraction of 0.8 is not necessarily general, and may be different if the thickness of the slab or the continuous casting machine changes. At this time, the timing of the reduction may be changed, the thickness of the negatively segregated portion may be checked, and the solid fraction to be used as a reference may be experimentally determined.

The solidification interface corresponding to the solid phase ratio, such as the solid phase ratio of 0.8 described above, has a casting speed, secondary cooling of the slab,
It can be obtained by calculation using a one-dimensional unsteady heat transfer analysis method in the thickness direction of the slab in consideration of the physical properties of steel.

The reduction amount is equivalent to the bulging amount. This reduction amount does not reduce the short side of the solidified slab, so that excessive equipment is not required. This is because a negative segregation portion is not stably generated at the center of the slab under the pressure of (1). In addition, the reason why the reduction roll is a pair of rolls is that if two or more pairs are used, the reduction amount of each pair of rolls decreases, and it is difficult to generate a negative segregation portion.

In the case of the method of the present invention, the unsolidified portion may be stirred by electromagnetic stirring before the reduction. The electromagnetic stirring device 6 shown in FIG. 1 is a device for imparting stirring to the unsolidified portion 2b to make the solidified structure an equiaxed crystal. When a large number of equiaxed crystals are generated, there is a problem that the area of the negatively segregated portion must be increased. However, it may be effective to perform this electromagnetic stirring.

In some steels, internal cracks are likely to occur in the negatively segregated portion of the slab during rolling. In such a case, equiaxed crystals are less likely to cause internal cracks than columnar or branched columnar crystals, so electromagnetic stirring creates an equiaxed crystal at the center of the slab to prevent internal cracks. It is desirable to do. However, after the electromagnetic stirring, the slab is reduced to form a negative segregation part, and it is desirable that the thickness of the equiaxed crystal be within the range of the thickness of the negative segregation part. Also, it is desirable to limit the applied steel.

When an equiaxed crystal is formed at the center of a slab by electromagnetic stirring, the thickness of the equiaxed crystal is determined by the thickness of the unsolidified portion between solidification interfaces having a solid fraction of 0.4 at the electromagnetic stirring position. And almost match. The solid phase ratio of 0.4 indicates that both the solid phase and the 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 thickness of the equiaxed crystal may be investigated by changing the rolling timing.

As described above, the thickness of the negatively segregated portion is determined by the thickness between solidification interfaces at a solid phase ratio of 0.8 at the time of reduction.
The thickness of the equiaxed crystal generated by electromagnetic stirring is determined by the thickness between the solidification interfaces at a solid phase ratio of 0.4 at the position of stirring. What is necessary is just to select the stirring time.

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

[0037]

EXAMPLE A molten steel was cast using a continuous slab casting apparatus having the apparatus configuration shown in FIG. The slab size is 220m thick
m, width 1800 mm, and the steel used had a C content of 0.15.
It is a steel for steel plate of weight%. 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 reduced with a maximum rolling force of 200 ton / roll.

The casting speed was changed during casting at around 1 m / min, and the thickness of the unsolidified portion between the solidification interfaces with a solid fraction of 0.8 at the time of reduction was changed. In addition, the thickness of the unsolidified portion between solidification interfaces having a solid fraction of 0.4 at the time of performing electromagnetic stirring was also changed. The intensity of the magnetic field of the electromagnetic stirring device was 300 gauss of magnetic flux density, and the maximum stirring speed was 300 rpm.

Further, the solidification structure was controlled by changing the degree of superheating of the molten steel in the tundish. Table 1 shows the casting conditions.

[0040]

[Table 1]

From the obtained slab, a sample having a length of 1 m in the casting direction was sampled, the macrostructure of the cross section was observed, and the solidification structure of the slab cross section was confirmed. 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 and 1/4 width of the thick steel plate, J
A No. 14 tensile test piece specified in IS Z 2201 was collected and examined for tensile strength. Further, in order to investigate the effect of the granular segregation around the negative segregated portion of the slab on the internal cracks of the thick steel plate, the drawing in the tensile test in the Z direction of the thick steel plate was measured. The tensile test in the Z direction was performed as follows. That is, five samples each having a diameter of 10 mm and a length of 30 mm and a total thickness of 30 mm were taken from the center of the width and the quarter width of the thick steel plate for each test. A cylindrical round steel having the same diameter and a length of 50 mm was welded to both end surfaces of these samples to prepare a sample for a tensile test. The round steel used was a steel having a C content of 0.2% by weight and homogenized. The aperture was determined from the results of the tensile test at room temperature of these samples, and the average value was used as the aperture for each test. In addition, the fracture surface of the sample after the tensile test was observed, and the results are also shown in Table 1.

In each of the tests of the present invention, since the negative segregation region is in the slab within the range specified by the present invention, that is, the thickness of the negative segregation portion is larger than the thickness of the equiaxed crystal, the tensile test in the Z direction is performed. The drawing was 73 to 76%, and the breaking position was good at the center of the sample. This means that no internal crack has occurred in the thick steel plate. Also,
The normal tensile strength of the thick steel plate was also 520 to 530 MPa, which was a value similar to that of a thick steel plate using a target ordinary cast 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 negatively segregated portion was 10 mm. Test No. In No. 2, since the degree of superheat of the molten steel was increased to 80 ° C., the thickness of the equiaxed crystal was as thin as 6 mm. The thickness of the negative segregation part was 20 mm, and the periphery of the negative segregation part was all columnar crystals. Test No. of the present invention example. 3 and No. 3 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 part is 30 m by controlling the thickness of the unsolidified part between solidification interfaces having a solid fraction of 0.8 at the time of reduction.
m, and the thickness of the equiaxed crystal was smaller than the thickness of the negatively segregated portion within the range specified in the present invention.

Test No. of Comparative Example 5-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 tensile test in the Z direction was as low as 55 to 65%, and the fracture position of the sample was in the negative segregation region. Was the border. This fracture position is a position of the thick steel plate corresponding to the granular segregation part around the negative segregation part of the slab. In each of the tests, equiaxed crystals existed outside the negative segregation part, and granular segregation was generated around the negative segregation part. Test No. 5 and No. 5 In No. 6, casting was performed without reducing the superheating degree of molten steel in the tundish to 12 to 13 ° C. without performing electromagnetic stirring. Due to the low degree of superheat of the molten steel, the thickness of the equiaxed crystal was as large 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 8, the magnetic stirring was performed in each case,
The thickness of the equiaxed crystal was as large as 23 to 28 mm, and the thickness of the negatively segregated portion was 20 mm.

Test No. of Comparative Example In No. 9, the test was performed by the conventional light pressure method without bulging and without performing electromagnetic stirring. In the test No. 10, the test was carried out by the same conventional method under light pressure and with electromagnetic stirring. In the light reduction method at this time, the reduction starts when the thickness of the unsolidified portion between the solidification interfaces having a solid phase ratio of 0.8 is 20 mm, and a total reduction of 10 mm is performed over a length of 4 m in the casting direction. Was done.

Test No. In the slab No. 9, the center segregation remained. According to the results of the tensile test of the hot-rolled thick steel plate, the drawing was as low as 55%, and the breaking position of the tensile test sample was at the center of the sample. It was the center segregation part of the part, and the adverse effect of the center segregation was clearly recognized.

Test No. Tensile test results of a thick steel plate obtained by hot rolling 10 slabs showed that the drawing 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 the region is a negatively segregated portion. By hot rolling this slab as a raw material, it is possible to obtain a thick steel plate of good quality without internal cracks. 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 slab and thick steel plate can be easily manufactured.

[Brief description of the drawings]

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

[Explanation of symbols]

 1: mold 2: cast slab 2a: solidified shell 2b: unsolidified portion 3: guide roll 4: rolling roll 5: pinch roll 6: electromagnetic stirring device 7: molten steel 8: immersion nozzle 9: casting direction

Claims (3)

[Claims] 1. A cast slab having a cast structure in which a central portion of a transverse section is composed of an equiaxed crystal and a periphery thereof is composed of a branched columnar crystal and a columnar crystal or a columnar crystal. 0.02 to 0.6% by weight, characterized in that the periphery is a negative segregation region
Continuous cast slab of steel containing carbon. 2. A slab having an unsolidified portion is bulged by 5 to 25% of the thickness of the slab at the start of bulging. 2. The method for continuous casting of slabs according to claim 1, wherein the reduction is performed by a roll. 3. A method for producing a thick steel plate, comprising hot rolling the slab according to claim 1.