JP3494136B2 - Continuous cast slab, casting method thereof, and method of manufacturing thick steel plate - Google Patents

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, a method for casting the slab, and a method for producing a thick steel sheet having excellent hydrogen-induced crack resistance by hot rolling the slab.

[0002]

2. Description of the Related Art Line pipes used for mining, refining and transportation of natural gas and crude oil are manufactured by processing a thick steel plate obtained by hot rolling a continuously cast slab. . It is known that accidents such as oil leakage occur in steel materials such as line pipes due to hydrogen induced cracking (HIC). Hydrogen in steel materials
Non-metallic inclusions (MnS, cluster oxides, CaS
Etc.), the hydrogen gas pressure increases, and due to the notch effect based on the shape of the nonmetallic inclusions, hydrogen-induced cracking starts from the nonmetallic inclusions. At that time, if there is an abnormal structure in which Mn, P, etc. are segregated, cracks propagate along the abnormal structure and larger cracks are likely to occur.

Japanese Patent Laid-Open No. 54-38214 discloses C
It has been proposed that the addition of a, REM, etc. suppresses the precipitation of non-metallic inclusions MnS that enhances the hydrogen-induced cracking susceptibility of steel materials and controls the morphology of non-metallic inclusions so that MnS becomes spherical. . However, in this method, since the optimum values of the contents of Ca, REM, etc. are in a narrow range, it is difficult to properly adjust the addition amounts of these. If the added amount of Ca, REM, etc. is less than the optimum value, the effect of controlling the morphology of non-metallic inclusions cannot be exhibited,
Further, if the addition amount is large, the amount of sulfides, oxides, etc. generated by the addition increases, and the susceptibility of the steel material to hydrogen-induced cracking increases.

The abnormal structure of the steel product caused by segregation of Mn, P, etc. means that the center segregation generated in the continuously cast slab remains even in the steel product obtained by hot rolling the slab and the hardness of that part. Is a metallographic structure that has become higher. The center segregation of the slab is the concentration of segregation components between the dendrite trees in the process of solidification, and this concentrated molten steel flows out from between the dendrite trees due to bulging of the slab and the solidification completion point, which is the final solidification part. Occurs to flow toward and solidify as it is to form a concentrated zone of segregated components.

Therefore, as a measure for preventing the center segregation of the cast slab, it is effective to prevent the migration of the molten steel enriched in the segregation component between the dendrite trees and to prevent the local accumulation of the molten steel enriched in the segregation component. In JP-A-9-57410 and JP-A-9-206903, a slab containing a non-solidified portion is bulged, and a bulging amount corresponding to the bulging amount is reduced on the upstream side in the casting direction of the final solidified portion. The method of doing is proposed. However, in these methods, when it is difficult to stably suppress the occurrence of center segregation over the entire length in the casting direction and the entire width of the slab, depending on the rolling reduction conditions such as the central solid fraction during rolling and the rolling amount. There is.

In view of the above situation, there is a demand for a continuously cast slab that stably suppresses hydrogen-induced cracking in steel products such as line pipes, and a method for performing continuous casting to obtain the slab. .

[0007]

DISCLOSURE OF THE INVENTION The present invention provides a continuously cast slab of steel capable of stably suppressing the occurrence of hydrogen-induced cracking in steel products such as line pipes, and a continuous casting method for obtaining the slab. Further, it is an object of the present invention to provide a method for producing a thick steel sheet having excellent hydrogen-induced cracking resistance by hot rolling these slabs.

[0008]

The gist of the present invention resides in the continuous cast slab shown in (1) below, the continuous casting method shown in (2), and the method for producing thick steel plate shown in (3). (1) Mn content is 1.0 to 2.5% by mass% and
S content is 0.0001 to 0.0060% (1 to 60 p
pm) and the Ca content is 10 to 50 ppm.
A slab obtained by casting a carbon steel and a low-alloy steel, in which the final solidified portion of the slab is included in the central portion in the thickness direction.
In a region within 0 mm, and the S content of 10ppm or less, and continuous casting slab you, wherein the Mn content is 2.0 mass% or more of the area ratio is less than 0.2%. (2) In a method in which a slab containing an unsolidified portion is bulged and then reduced by one or more pairs of reduction rolls until the central portion of the thickness is completely solidified, the central solid fraction is 0.2 to
Continuous casting according to (1) above, in which the slab is rolled under the condition that the rolling reduction Lf represented by the following formula (A) satisfies 0.9 to 1.5 at the position of 0.7 slab. Piece casting method.

Lf = D1 / D2 (A) where D1: the reduction amount (mm) in the width center portion of the slab including the unsolidified portion D2: the solid fraction at the start of reduction is 0.8 or less Thickness of unsolidified portion (mm) (3) A method for producing a thick steel plate, which comprises hot rolling the continuously cast slab according to (1) above.

The continuous cast slab targeted by the present invention is a transverse casting.
It is a slab with a rectangular surface shape. Further, the present invention defines
"Thickness center including the final solidified part of the slab in the central part in the thickness direction
The area within 10 mm "means the area of the slab shown below.
To taste. FIG. 1 is a schematic view showing a cross section of a cast piece. Long
The final solidified portion Tc of the rectangular slab 2 has a thickness of about 1/2 of the thickness.
It is a substantially linear portion corresponding to the position. "Final of the slab
10 mm or more of the thickness center part including the solidified part in the center part in the thickness direction
Is a drawing of the almost solid final solidification portion Tc.
In the above, the straight line T at the upper 5 mm position₁ And the lower 5
Straight line T at mm position₂ The area of the slab between and (hatched in the figure
Area). Width of rectangular slab
From both ends, 1/2 x T₀ (T ₀ Is the slab thickness
Up to the distance), the progress of coagulation is fast,
It does not become the final solidification part. In FIG. 1, T₁ Or
T₂ The interval between Tc and Tc is exaggerated.

The S content of 10 ppm or less in the region within 10 mm of the thickness center portion including the final solidified portion of the cast slab in the center portion in the thickness direction means the following. That is,
Except for the vicinity of the above-mentioned both ends of the cross section of the slab, at any position in the area within 10 mm of the thickness center including the final solidified portion in the center in the thickness direction, for example, by a drill blade having a diameter of 3 to 5 mm. It means that the value of S analyzed by the usual high frequency combustion-infrared absorption method etc. was 10 ppm or less.

Further, in a region within 10 mm of the thickness center portion including the final solidified portion of the cast piece in the center portion in the thickness direction, Mn
The area ratio of content of 2.0% by mass or more and less than 0.2% means the following. That is, except for the vicinity of the above-mentioned both ends of the cross section of the slab, at any position in the area within 10 mm of the thickness center portion including the final solidified portion in the center portion in the thickness direction, Wmm of the width direction of the slab is Range, for example, 50μ
Mn beam size X-ray microanalyzer
It means that, when measuring the content rate, the number of measurement points at which the Mn content rate is 2.0% by mass or more is less than 0.2% of the total number of measurement points. For example, when W = 20 mm,
200 points in the thickness direction of the slab, 4 points in the width direction
00 points, all measuring points are 200 × 400 = 80000
It becomes a point. This means that the number of measurement points at which the Mn content is 2.0% by mass or more is less than 0.2% of all measurement points, that is, less than 160 points.

FIG. 2 is a view showing the influence of the S and Mn content in the region within 10 mm near the thickness center of the cast piece on the hydrogen-induced crack area ratio of the thick steel plate. It is a figure which shows the result of having performed the test of the following content.

That is, the area ratio of S content and Mn content of 2.0 mass% or more within 10 mm of the thickness center portion including the final solidified portion of the slab in the center portion in the thickness direction has various values. The slab samples with a thickness of 230 mm were prepared by casting with an actual continuous casting machine, and these slab samples were hot rolled into a steel plate with a thickness of 19 mm using a laboratory scale rolling apparatus. A test piece was taken from the obtained steel plate, and 5
The results of investigating the state of occurrence of hydrogen-induced cracking by immersing for 96 hours in a saturated H ₂ S NACE corrosive solution (pH: about 3.0) containing mass% NaCl and 0.5 mass% CH ₃ COOH are shown. The hydrogen-induced crack area ratio (%) shown in the figure is
Ultrasonic flaw detection (flaw detector; point focus type, 50 MH)
It is the ratio of the area of the portion determined to be cracked by z) divided by the total surface area of the test piece of the steel sheet subjected to ultrasonic flaw detection.

The following can be seen from FIG. That is,
Even if Ca is contained to suppress the precipitation of non-metallic inclusions MnS and the MnS is spheroidized, the final solidified portion of the slab is contained in the central portion in the thickness direction in a region within 10 mm of the thickness center portion. If the S content exceeds 10 ppm, hydrogen-induced cracking will occur even if the area ratio of Mn content of 2.0% by mass or more in these regions is zero.

Further, even if the S content in the region within 10 mm of the thickness center portion including the final solidified portion of the cast piece in the center portion in the thickness direction is 10 ppm or less, if Ca is not contained,
Hydrogen-induced cracking occurs even when the area ratio of Mn content in these regions is 2.0% by mass or more and less than 0.2%.

Further, the S content in the region within 10 mm of the thickness center portion including the final solidified portion of the slab in the thickness direction is set to 10 ppm or less, Ca is contained, and the Mn content ratio in these regions is When the area ratio of 2.0 mass% or more is less than 0.2%, the occurrence of hydrogen-induced cracking can be suppressed. Since the amount of non-metallic inclusions MnS existing in these regions is small and the existing MnS has a stable spherical shape, the hardness of these regions becomes uniform.
Therefore, the occurrence of hydrogen-induced cracking of the steel material can be suppressed.

In the continuous casting method of the present invention, after the slab containing the unsolidified portion is bulged, the slab is pressed down until the solidification of the central portion of the thickness is completed. The neighborhood can be effectively reduced.

Further, under the condition that the rolling reduction Lf represented by the above-mentioned formula (A) satisfies 0.9 to 1.5 at the position of the slab having the central solid fraction of 0.2 to 0.7. Since the slab is pressed down,
The reduction effect effectively extends to the center of the thickness of the cast slab, and the molten steel in the vicinity of the final solidified portion with concentrated segregation components can be discharged at a high speed upstream in the casting direction. Therefore, since the molten steel enriched in the segregation component is not captured at the solidification interfaces on both sides in the thickness direction of the slab to be crimped by pressing, the region having a low S and Mn content is the entire length in the casting direction, Stable formation over the entire width of the slab.

In the above formula (A), the thickness D2 of the unsolidified portion of the cast piece at the start of reduction is defined as the thickness between solidification interfaces having a solid fraction of 0.8 on both sides in the thickness direction inside the cast piece. The reason for this is that the rolling force is not transmitted in such a region having a solid fraction of 0.8 or less, and this region is defined as an unsolidified portion.
The solidification interface having a solid phase ratio of 0.8 can be calculated by using a commonly used solidification heat transfer analysis method if the slab size, the degree of superheated molten steel, the casting speed, the secondary cooling specific water content, etc. are determined.

Further, according to the method for manufacturing a thick steel plate of the present invention,
Hot rolling is performed using the above continuously cast slab of steel.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the Mn content is 1.0 to 2.5% and the S content is 0.0001 to 0.
A 0060% shall be the subject of carbon steel and low alloy steels.
In carbon steels and low alloy steels containing Mn and S in this level, center segregation is likely to occur in continuously cast slabs, and hydrogen-induced cracking is likely to occur in thick steel plates made from the slabs. is there.

Further, the present invention is directed to a steel containing Ca. The Ca content of steel is 10 to 50 p in mass%.
It shall be the pm. If it is less than 10 ppm, it is difficult to exert the effect of controlling the morphology of the non-metallic inclusion MnS by adding Ca, and if it is more than 50 ppm, the amount of sulfides and oxides produced by the addition of Ca becomes too large, and rather hydrogen induction is caused. Increases susceptibility to cracking.

The continuously cast slab of the present invention has an S content of 10 ppm or less and a Mn content in a region within 10 mm of the thickness center portion including the final solidified portion of the slab in the central portion in the thickness direction. The area ratio of which the ratio is 2.0 mass% or more is 0.2%
The slab is less than

The area within 10 mm of the thickness center including the final solidified portion of the slab in the center in the thickness direction is the area of the slab corresponding to the area where the hydrogen-induced cracking of the steel material often occurs. Because it is.

Further, the area ratio of the Mn content of 2.0 mass% or more is defined by the fact that when the Mn content is 2.0 mass% or more, the hardness of the steel material reaches 330 Hv and hydrogen-induced cracking occurs. This is because it tends to occur. At that time, the upper limit of the Mn content is not particularly limited. However, the steel targeted by the present invention has a Mn content of 1.0 to 2.
Since it is 5% by mass of carbon steel or low alloy steel, the Mn content may be about 3.8% by mass when analyzed using the above-mentioned X-ray microanalyzer with a beam size of 50 μm. In any case, the Mn content is 2.
The area ratio of 0% by mass or more shall be obtained.

Further, the reason why the upper limits of the area ratios where the S content and the Mn content are 2.0 mass% or more in the slab areas are defined are the non-metallic inclusions existing in these slab areas. This is because the precipitation amount of the substance MnS decreases. Combined with the effect of controlling the morphology of MnS by containing Ca, the hardness of the regions of these cast pieces becomes uniform, and the occurrence of hydrogen-induced cracking of the steel material can be suppressed.

From the region within 10 mm of the thickness center portion including the final solidified portion of the slab in the center portion in the thickness direction, for example, the diameter 3
When analyzing the S content by taking a cut with a mm-5 mm drill blade, it is preferable to take a cut from at least two places, the center of the slab width and the quarter of the width. Further, when the Mn content is analyzed by using, for example, an X-ray microanalyzer having a beam size of 50 μm, W = W in the width direction of the slab at least at two positions, that is, the central part of the slab width and the ¼ part of the width. It is preferable to measure an area of about 20 mm. FIG. 3 is a schematic diagram showing an example of a continuous casting machine for carrying out the continuous casting method of the present invention.
The molten steel 3 injected into the mold 1 through the immersion nozzle 4 is cooled by the mold and solidified. The formed solidified shell 2a is cooled by cooling water sprayed from a nozzle group (not shown) below the mold, and its thickness increases. The cast slab 2 including the unsolidified portion 2b therein is pulled out by the pinch roll 7 while being supported and guided by the guide roll 5. At that time, the slab containing the unsolidified portion is bulged in the bulging zone 8 by expanding the interval of the guide roll in the slab thickness direction. Further, the bulged slab is rolled down by the rolling roll pair 6 in the rolling down zone 9 until the solidification of the central portion of the thickness is completed. In Figure 3,
An example of rolling down a slab using two rolling roll pairs is shown.
In the continuous casting method of the present invention, after the slab containing the unsolidified portion is bulged, the central solid phase fraction of the slab of 0.2 to 0.7 is obtained until the solidification of the central portion of the thickness is completed. In position, the slab is rolled using one or more pairs of drafting rolls.

Since the slab containing the unsolidified portion is bulged and then the slab is pressed down, it is possible to effectively reduce the vicinity of the thickness center of the slab. When the central solid fraction is less than 0.2, the rolling time is too early, the unsolidified portion is thick, and the rolling ratio Lf is 0.
To achieve 9 or more, it is necessary to make the amount of reduction excessively large, and it is actually difficult to secure such a large amount of reduction. When the central solid fraction exceeds 0.7,
Since the rolling time is too late, the molten steel in the unsolidified part has poor fluidity, so the molten steel with concentrated segregation components cannot be completely discharged to the upstream side in the casting direction, and the molten steel with locally concentrated segregation components remains. To do.

In the continuous casting method of the present invention, the above (A)
The slab is rolled under the condition that the rolling reduction Lf represented by the formula satisfies 0.9 to 1.5.

If the reduction ratio Lf is less than 0.9, the reduction amount is small, so that the amount of molten steel enriched in the segregation component is discharged to the upstream side in the casting direction, and the discharging speed becomes slow. Therefore, the molten steel in which the segregation component is concentrated is likely to be left in the vicinity of the thickness center of the cast slab. Reduction ratio Lf is 1.5
If it exceeds, the reduction amount becomes excessively large, and it is actually difficult to secure such a large reduction amount.

Further, it is desirable that the reduction amount is 50% or more of the bulging amount. When it is 50% or more, the vicinity of the central portion of the thickness of the cast slab can be reduced more effectively.

Assuming that one rolling roll pair is one, rolling is performed by one or more rolling rolls. When a plurality of rolls are rolled down, rolling is performed for each rolling roll pair under the condition that the rolling reduction ratio Lf represented by the formula (A) is 0.9 to 1.5.

Although the vertical type continuous casting machine is shown in the schematic view of FIG. 3, the method of the present invention can be applied to a curved type continuous casting machine and a vertical bending type continuous casting machine.

A thick steel plate having excellent resistance to hydrogen-induced cracking can be produced by hot rolling the continuously cast slab of the present invention. The heating temperature, in-furnace time, rolling temperature, etc. of the slab may be the same as those in the case of hot-rolling an ordinary continuously cast slab of similar chemical composition.

[0036]

[Example] A total of 12 heat casting tests were conducted using a vertical bending type continuous casting machine. Each of the slabs had a thickness of 230 mm and a width of 2300 mm, and was cast by one heat single casting. One heat is about 250 tons. Table 1 shows the chemical composition of the steel used. The steel used has a Mn content of 1.50% by mass, an Nb content of 0.042% by mass, and a C
Steel for thick steel plate having an a content of 0.0025 mass% was used.

[0037]

[Table 1] Superheat of molten steel in the tundish is 25 ~ 35 ℃
And The casting speed is 0.90 to 1.13 m / min, the secondary cooling specific water amount is 1.2 to 1.5 liter / kg-steel, and the casting speed and the secondary cooling specific water amount are changed to The test was performed by changing the central solid fraction at the start of rolling. The central solid fraction was calculated by using a usual solidification heat transfer analysis method.

The bulging amount before reduction is 2 at the center of the slab width.
The diameter was set to 0 mm, and then the slab containing the unsolidified portion was pressed down by one pressing roll pair having a diameter of 400 mm. The reduction roll pair was placed at a position 21 m from the molten steel meniscus. A slab cross-section sample having a length of 100 mm in the casting direction was taken. At the central portion of the thickness of this cross-section sample, cutting was taken from the central portion in the width direction of the slab and ¼ portion of the width with a drill blade having a diameter of 3 mm, S was analyzed, and the average value was obtained. Also, remove the part where the cutting was taken with the drill blade,
2 in the width direction from the slab width direction center and 1/4 of the width
A sample with a total thickness of 0 mm was cut out, and the Mn content was measured with an X-ray microanalyzer at a beam size of 50 μm. At that time, the area ratio was obtained by dividing the number of measurement points at which the Mn content was 2.0% by mass or more by the total number of measurement points.
All measurement points for each measurement sample are 200 × 400 = 8000
It is 0 point.

The obtained slab is used as a raw material and has a thickness of 19
The steel sheet was rolled into a steel plate having a thickness of mm, and the tensile strength of the steel plate was investigated.
Further, a thick steel plate sample having a length of 100 mm and a width of 150 mm was sampled, and 5 mass% NaCl and 0.5 mass% CH ₃ C were used.
It was immersed in a saturated H ₂ S NACE etchant (pH: about 3.0) containing OOH for 96 hours, and the occurrence of hydrogen-induced cracking was investigated. The presence or absence of hydrogen-induced cracking was investigated by ultrasonic flaw detection (sensitivity 8 dB). The ratio of the area of the region where hydrogen-induced cracking occurred to the area of the thick steel plate sample subjected to ultrasonic flaw detection was determined as the hydrogen-induced cracking area ratio. Table 2 shows the test conditions and the test results.

[0040]

[Table 2] Test No. In No. 1, the casting speed was 1.0 m / min, the specific water amount for secondary cooling was 1.4 liter / kg-steel, and the rolling reduction D1 was 2.
It was set to 0 mm. At the start of the rolling reduction, the central solid fraction is 0.5 and the thickness D2 of the unsolidified portion is 20 mm, so the rolling reduction Lf was 1.0. In addition, the test No. In No. 2, the casting speed was 0.95 m / min and the specific water amount in the secondary cooling was 1.3 liters / min.
kg-steel, the reduction amount D1 was 20 mm. At the start of rolling, the central solid fraction is 0.7 and the thickness D2 of the unsolidified portion is 15 m.
Therefore, the rolling reduction Lf was 1.3. These test No. 1 and No. The central solid fraction and the rolling reduction Lf at 2 are values within the range of the conditions specified in the present invention.

Test No. In No. 1, in the region within 10 mm near the thickness center of the slab, the S content is 9 ppm, M
The area ratio with an n content of 2.0% by mass or more was 0.1%, which was a low value. In addition, the test No. In No. 2, the S content was 7 pp in the region within 10 mm near the thickness center of the cast slab.
Area ratio of m, Mn content of 2.0 mass% or more is 0.05
% Was a low value. These test No. 1 and No.
The S content in 2 and the area ratio in which the Mn content is 2.0% by mass or more are within the range defined by the present invention. Since the effect of rolling down effectively reaches the central portion of the thickness, molten steel enriched in the segregation component is not captured at the solidification interfaces on both sides in the thickness direction of the slab, and S and Mn A region with a low content was stably generated. Furthermore, these test No.
1 and No. No hydrogen-induced cracking occurred in the thick steel plate using the slab of No. 2 as a raw material. The tensile strength of these thick steel plates was 590 MPa or 591 MPa. This tensile strength is within the target strength range. The target tensile strength of the thick steel plate is 500 MPa or more because it is the steel having the chemical composition shown in Table 1 above.

Test No. In 3, the casting speed is 1.05m
/ Min, the secondary cooling specific water amount is 1.4 liters / kg-steel,
The reduction amount D1 was set to 20 mm. At the start of the rolling reduction, the central solid fraction is 0.3 and the thickness D2 of the unsolidified portion is 25 mm, so the rolling reduction Lf was 0.8. In addition, the test No. Four
Then, the casting speed was 0.95 m / min, the specific water amount of the secondary cooling was 1.3 liter / kg-steel, and the rolling reduction D1 was 10 mm. At the start of the rolling reduction, the central solid fraction was 0.7 and the thickness D2 of the unsolidified portion was 15 mm, so the rolling reduction Lf was 0.7. These test No. 3 and No. Reduction ratio L at 4
f is a small value outside the conditions specified by the present invention.

Test No. In No. 3, in the region within 10 mm near the center of the thickness of the cast slab, the S content is 13 ppm,
The area ratio of Mn content of 2.0% by mass or more was 0.5%. In addition, the test No. In No. 4, in the region within 10 mm near the thickness center of the slab, the S content is 14 ppm,
The area ratio with the Mn content of 2.0 mass% or more was 0.8%. These test No. 3 and No. The S content in 4 and the area ratio in which the Mn content is 2.0 mass% or more are
This is a large value outside the conditions specified in the present invention. This is because the rolling reduction Lf was a small value and the rolling reduction effect did not reach the central portion of the thickness. Furthermore, these test No. 3 and No. Hydrogen-induced cracking occurred in the thick steel plate made of the slab of No. 4 and the hydrogen-induced cracking area ratio was 7.6% or 9.2%. The tensile strength of these thick steel plates was 589 MPa or 592 MPa.

Test No. In 5, the casting speed is 1.13m
/ Min, the secondary cooling specific water amount is 1.5 liter / kg-steel,
The reduction amount D1 was set to 20 mm. At the start of rolling, the central solid fraction is 0.1 and the thickness D2 of the unsolidified portion is 35 mm, so the rolling reduction Lf was 0.57. In addition, the test No. 6
Then, the casting speed was 0.9 m / min, the secondary cooling specific water amount was 1.2 liter / kg-steel, and the rolling reduction D1 was 20 mm. At the start of the reduction, the central solid fraction was 0.8 and the thickness D2 of the unsolidified portion was 11 mm, so the reduction rate Lf was 1.8. These test No. 5 and No. The central solid fraction at the start of rolling and the rolling reduction Lf at 6 are small or large values outside the conditions specified in the present invention.

Test No. In No. 5, in the area within 10 mm near the thickness center of the cast slab, the S content is 13 ppm,
The area ratio with the Mn content of 2.0% by mass or more was 2.1%. In addition, the test No. In No. 6, in the region within 10 mm near the thickness center of the cast slab, the S content is 16 ppm,
The area ratio with the Mn content of 2.0% by mass or more was 6.3%. These test No. 5 and No. The area ratio of S content in 6 and Mn content of 2.0 mass% or more is
This is a large value outside the conditions specified in the present invention. Test N
o. In No. 5, both the central solid fraction and the rolling reduction Lf were small, and the rolling reduction effect did not reach the central portion of the thickness. In addition, the test No. In No. 6, the central solid fraction and the rolling reduction Lf were both large, and the rolling reduction effect did not sufficiently reach the central portion of the thickness. Furthermore, these test No. 5 and No. Hydrogen-induced cracking occurred in the thick steel plate made of the slab of No. 6 and the hydrogen-induced cracking area ratio was 26.8% or 32.2%. The tensile strength of these thick steel plates is 590 MPa or 592 MPa.
Met.

[0046]

EFFECTS OF THE INVENTION By applying the continuously cast slab of the present invention, the method for casting the slab, and the method for producing a thick steel plate by hot rolling the slab, hydrogen resistance suitable for use in line pipes, oil country tubular goods, etc. It is possible to obtain a thick steel plate having excellent induced cracking property.

[Brief description of drawings]

FIG. 1 is a schematic view showing a cross section of a cast piece.

FIG. 2 shows S and Mn in the region within 10 mm near the thickness center of the slab, which affects the hydrogen-induced crack area ratio of the thick steel plate.
It is a figure which shows the influence of the content rate of.

FIG. 3 is a schematic view showing an example of a continuous casting machine for carrying out the continuous casting method of the present invention.

[Explanation of symbols]

1: Mold 2: Cast piece 2a: Solidified shell 2b: Unsolidified part 3: Molten steel 4: Immersion nozzle 5: Guide roll 6: Rolling roll pair 7: Pinch roll 8: Bulging zone 9: Rolling zone Tc: Almost linear Final solidification part T ₁ : Straight line at a position 5 mm above Tc T ₂ : Straight line at a position 5 mm below Tc T ₀ : Thickness of slab

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ identification code FI B22D 11/16 B22D 11/16 C (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/20 B22D 11 / 00 B22D 11/12 B22D 11/128 B22D 11/128 350 B22D 11/16

Claims (3)

(57) [Claims] 1. A Mn content of 1.0 to 2.5% by mass.
And S content is 0.0001 to 0.0060% (1 to
60 ppm), and the Ca content is 10 to 50 p.
It is a slab made by casting pm carbon steel and low alloy steel.
The S content is 10 pp in the region within 10 mm of the thickness center part including the final solidified part of the slab in the center part in the thickness direction.
m or less, and continuous casting slab you, wherein the Mn content is 2.0 mass% or more of the area ratio is less than 0.2%. 2. A method in which, after bulging a slab containing an unsolidified portion, the slab is reduced by one or more pairs of reduction rolls until the solidification of the central portion of the thickness is completed, and the central solid fraction is 0. The slab is pressed at a position of 2 to 0.7 at a position where the rolling reduction Lf represented by the following formula (A) satisfies 0.9 to 1.5. The method for casting a continuously cast slab as described in. Lf = D1 / D2 (A) Here, D1: Reduction amount (mm) in the width center part of the slab including the unsolidified portion D2: Unsolidified portion having a solid fraction of 0.8 or less at the start of the reduction Thickness (mm) 3. A method for producing a thick steel plate, which comprises hot rolling the continuously cast slab according to claim 1.