JP4508087B2 - Continuous casting method and continuous cast slab - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a continuous casting method for steel plate production slabs excellent in hydrogen-induced cracking resistance and a continuous casting slab for steel plate production cast by the continuous casting method.

  In continuous cast slabs of steel, central segregation, which is a segregation band in which impurity components such as C, S, P, and Mn and alloy components are concentrated in the center of the slab thickness direction, or the above components are between equiaxed crystals Concentrated granular segregation is one of the serious casting defects that cause deterioration of mechanical properties in thick plate products. In particular, in the case of line pipes used for transportation of crude oil and natural gas containing hydrogen sulfide, if such segregation remains, it becomes a site of hydrogen accumulation, often hydrogen induced cracking (hereinafter also referred to as “HIC”). Is the starting point. These defects are caused by shrinkage when the residual molten steel in the unsolidified phase at the end of casting solidifies, resulting in a negative pressure state. Molten steel with fine segregation between the dendritic trees is sucked out from between the dendritic trees. It flows out and accumulates in the closed space where the solidified tissue locally adheres and solidifies, resulting in macroscopic segregation.

  Conventionally, there has been disclosed a method for reducing defects due to macrosegregation by controlling the solidification structure of the slab and applying mechanical reduction in the thickness direction from the surface of the slab at the end of solidification.

  For example, in Patent Document 1, a bulging force is positively applied to a solidified shell between a mold and a liquid phase crater end of a slab to increase the thickness of an unsolidified layer in the slab, and then the liquid phase crater A continuous casting method characterized by reducing the occurrence of central segregation by reducing the slab between the end and the solidus crater end is disclosed. The reason for applying the reduction is to remove the root cause of the macrosegregation described above by applying the reduction from the outside to the solidification shrinkage inside the slab. The unsolidified reduction method of a slab that applies a reduction that compensates for solidification shrinkage inside the slab is called a “light reduction method” and is widely applied in the field of continuous casting.

  For the reduction of macro segregation defects, a considerable effect was obtained by the conventional light reduction method as described above, but considering the recent high demands on product quality, in the conventional light reduction method, It must be said that the effect is insufficient.

  In recent years, there has been a demand for the production of steel sheets having high strength and high toughness that can withstand increasingly severe sour environments and excellent in HIC (hydrogen induced cracking resistance) performance. Conventionally, the main causes of HIC are macro-segregation such as center segregation and semi-macro segregation, and it has been considered that the anti-HIC performance can be improved by reducing these segregation. However, it has become clear that it is necessary to reduce the center porosity generated in the center portion of the slab in order to meet the demand for manufacturing a steel plate having such high-performance HIC resistance as described above. That is, when the degree of center porosity is large, the trace of the center porosity cannot be completely erased even if the slab is rolled, and the trace portion tends to be a hydrogen accumulation site in a sour environment. is there.

  The formation of the hydrogen accumulation site is speculated to be specifically due to the following mechanism. That is, it is assumed that even after rolling, the center porosity is not completely pressed and remains as fine voids in the steel sheet, which becomes a hydrogen accumulation site. Alternatively, during the solidification of the slab, non-metallic inclusion particles are accumulated in a form protruding in the form of dots at the interface of the porosity due to the balance of interfacial tension when the porosity is formed. Since the form remains, it is assumed that the boundary between the accumulation inclusion and the matrix becomes a hydrogen accumulation site.

  The center porosity is formed by solidification shrinkage when the center portion of the slab is completely solidified. However, only the light reduction method that provides only a reduction amount that compensates for the amount of solidification shrinkage has a very small effect on improving the center porosity. Therefore, it is not possible to meet the demand for manufacturing a steel plate with high-performance HIC resistance. There wasn't.

Japanese Patent Publication No. 62-34461 (Claims and column 2, line 23 to column 3, line 4)

  The present invention has been made to solve the above-mentioned problems, and the problem is that a slab for manufacturing a steel sheet having high-performance HIC resistance that simultaneously reduces the center porosity and the occurrence of macro-segregation and semi-macro segregation. It is providing the casting method and the continuous casting slab using the method.

  In order to solve the above-mentioned problems, the present inventors have investigated and studied a continuous casting method and a continuous cast slab of a slab for producing a high-performance HIC-resistant steel sheet based on the conventional problems. The knowledge of a) to (e) was obtained and the present invention was completed.

  (A) By improving the macro segregation by a method of reducing the thickness of the slab in a taper shape with respect to the casting direction, the HIC generation rate of the steel sheet can be reduced to a very low value. It is inevitable that this will occur.

  (B) By reducing the generation degree of the center porosity of the slab to a certain level or less, the slight HIC generation rate described in the above (a) can be clearly reduced.

  (C) The center porosity can be improved by reducing the macro segregation of the slab to some extent and then applying a reduction of several millimeters in the thickness direction immediately before the center of the slab is completely solidified. There is a proper range related to taper reduction for improving macrosegregation of a slab and reduction immediately before complete solidification.

  (D) That is, after bulging 3 to 10 mm in the slab thickness direction until the slab solidified shell thickness reaches 60 mm, the slab is at a position where the central solid phase ratio is less than 0.8. Is reduced in the slab thickness direction by taper roll alignment, and in the slab thickness direction using a single-stage reduction roll pair at a position where the central solid phase ratio is in the range of 0.8 to 0.95. The center porosity of the slab can be reduced by reducing the stepped shape within a range of 5 to 10 mm.

(E) By reducing the porosity volume at the center of the slab thickness to 2 × 10 ⁻⁴ (cm ³ / g) or less by the method of (d) above, the HIC resistance of the steel sheet obtained from the slab Performance is greatly improved.

  The present invention has been completed based on the above findings, and the gist thereof resides in the continuous casting method shown in the following (1) and the slab shown in (2).

(1) At the slab position where the solidified shell thickness on one side of the slab is 60 mm or less, the slab thickness is bulged within a range of 3 to 10 mm at the center of the slab width, and then the bulging state is performed. Thickness of bulging the slab from the slab position where the solid phase is generated in the thickness center to the slab position where the central solid fraction is less than 0.8 in the thickness center while maintaining The slab thickness is reduced in a taper direction with respect to the casting direction of the slab within the range of slab, and subsequently, at the slab position where the central solid fraction in the thickness center is in the range of 0.8 to 0.95. The porosity volume at the center of the thickness is 0.8 × 10 ⁻⁴ to 2 × 10 ⁻⁴ (cm) by using a pair of rolling rolls in a step within a range of 5 to 10 mm in the direction. in ³ / g), and the slab segregation ratio is 1.10 to 1.25 of Mn in the thickness center portion Resistance to hydrogen induced cracking performance superior continuous casting method of manufacturing a steel sheet for insert piece, characterized by forming (hereinafter, referred to as "first invention").

(2) A slab cast by the continuous casting method according to (1), wherein a porosity volume at a thickness central portion of the slab is 0.8 × 10 ⁻⁴ to 2 × 10 ⁻⁴ (cm ^3). / G) and the segregation ratio of Mn at the center of the slab thickness is 1.10 to 1.25, and in accordance with the test method specified in NACE T0284, hydrogen-induced cracking in the steel sheet after rolling A continuous cast slab for producing a steel sheet (hereinafter, also referred to as “second invention”) , wherein the area ratio is 0.5% or less .

  In the present invention, the “center solid phase ratio” means the fraction of the area occupied by the solid phase with respect to the entire area occupied by the solid phase and the liquid phase in the center of the slab.

  “To taper down” means to roll down the slab by a plurality of roll pairs arranged so that the roll gap of the roll pair decreases with a predetermined gradient with respect to the casting direction.

  In addition, “one-stage reduction roll pair” refers to a reduction roll pair having one pair of roll pairs in the casting direction, and “to reduce in a stepped manner” uses the one set of roll pairs described above. This means that the slab thickness is reduced so as to decrease in a single step.

  And "porosity volume" means the maximum value among the volumes of porosity per slab unit mass determined by the method described in detail later.

  According to the continuous casting method of the present invention, it is possible to cast a slab slab with significantly reduced center porosity, macro segregation and semi-macro segregation, and to produce a steel sheet having excellent HIC resistance using the slab as a raw material. Is possible. Moreover, the continuous cast slab of the present invention is most suitable for the production of a steel plate having excellent HIC resistance used for a line pipe or the like.

As described above, in the present invention, the slab thickness of 60 mm or less on one side is bulged within a range of 3 to 10 mm, and the thickness center is maintained while maintaining the state. The slab between the position where the solid phase is generated in the part and the position where the central solid fraction in the thickness center is less than 0.8 is tapered in the casting direction within the range of the bulging thickness, For steel sheet production where the center solid phase ratio is reduced to a step using a pair of reduction rolls within a range of 5 to 10 mm in the slab thickness direction at a position within the range of 0.8 to 0.95. A slab continuous casting method, and a slab cast by the above-described continuous casting method, wherein the porosity volume at the center of the thickness is 2 × 10 ⁻⁴ (cm ³ / g) or less. is there. The reason why the present invention is limited to the above range and the preferable range will be described below.

(1) Bulging application position and bulging amount The reason for applying bulging to the slab is to avoid the reduction of the width direction end of the slab slab, including the unsolidified portion other than the end, as in the above-mentioned Patent Document 1. This is because the entire slab width direction is effectively reduced in a taper shape or in a step shape just before complete solidification.

  When bulging is performed under the condition that the solidified shell thickness of the slab exceeds 60 mm on one side of the slab, fine internal cracks (that is, solidification) occur due to bending strain due to bulging at the solidified shell interface on the short side of the slab. It was found that the bulging needs to be started before the solidified shell thickness reaches 60 mm because the interface cracks once, and molten steel with microsegregation is sucked into the cracked part and a solidified structure is generated. did. It has also been found that the presence of such fine internal cracks causes the generation of HIC in the steel sheet. Therefore, bulging was started before the solidified shell thickness on one side of the slab reached 60 mm.

  The reason why the appropriate range of the bulging amount is 3 to 10 mm is that if the bulging amount is less than 3 mm, the effect of reducing the bulging slab is not sufficient, whereas if the bulging amount exceeds 10 mm, the solidified shell This is because even when the thickness is 60 mm or less, the risk of occurrence of internal cracks at the short-side solidified shell interface of the slab increases.

(2) Cast slab position and amount of taper reduction Taper reduction is a slab position where the center of the slab begins to solidify, that is, a slab where a solid phase is generated at the center, as is generally done. It is preferable to carry out continuously from the position (or time) to the position (or time) of the slab where it completely solidifies with a small taper. Therefore, the appropriate starting position under the taper-shaped reduction is defined as the slab position where a solid phase is generated at the center. Further, when the center solid phase ratio of the slab becomes about 0.02, the solidified shells on the upper side and the lower side of the slab start to start bridging, and the influence of solidification shrinkage becomes remarkable. It is preferable to start the tapered reduction from the position onward. Further, it is even more preferable that the taper-shaped reduction is started from a slab position where bridging is large and the central solid phase ratio is 0.3 or more.

  The end position of the taper-shaped reduction was set to a slab position having a central solid phase ratio of less than 0.8 in consideration of an appropriate range of step-like reduction immediately before complete solidification of the slab described later. The maximum solid phase rate at which the molten steel can flow is considered to be 0.8. At this solid phase rate, the flow of the residual molten steel enriched with segregation components whose solidification shrinkage is the driving force is almost stopped. Therefore, in consideration of securing a step-like reduction region on the downstream side, the end position of the taper-like reduction is defined as a position where the central solid fraction is less than 0.8. Although the segregation can be improved even if the end position of the taper-shaped reduction is further reduced to a position where the central solid phase ratio is low, it is preferable to continue the taper-shaped reduction at least until the slab position where the central solid ratio is about 0.6. .

  The taper amount under the taper pressure is preferably 0.8 to 2.0 mm per 1 m in the casting direction. When the taper amount is less than 0.8 mm per 1 m in the casting direction, compensation of the solidification shrinkage amount becomes insufficient and segregation may be deteriorated. On the other hand, if the taper amount becomes larger than 2.0 mm per 1 m in the casting direction, the reduction amount becomes larger than the solidification shrinkage amount, and the reverse flow of the residual molten steel concentrated in the segregation component may occur, which may cause reverse V segregation. is there.

(3) Casting slab position and amount of step-like reduction Step following the taper-like reduction, using a pair of rolls, at the slab position where the central solid fraction is in the range of 0.8 to 0.95 The reduction is applied within the range of the reduction amount of 5 to 10 mm. The reason is as follows. That is, even if the taper-shaped reduction of (2) is performed, the solidification shrinkage amount of the slab cannot be completely compensated, and as a result, a center porosity is formed as a contraction hole due to solidification shrinkage at the end of solidification. This is because, at this timing, it is extremely effective to give the slab a slightly larger amount of reduction per one squeeze roll and press the shrinkage hole formed inside the slab. That is, at this stage, since solidification latent heat remains in the center part of the slab, the temperature of the center part is maintained at a high temperature near the melting point. Therefore, the temperature difference between the slab surface layer portion and the center portion is large, and the inside is relatively softer than the surface layer portion, so that deformation due to rolling is likely to concentrate on the center portion of the slab.

  However, when the central solid phase ratio is less than 0.8, if the step-like reduction amount is too large, the residual molten steel having fluidity concentrated in the segregation component is caused to flow backward to the upstream side of the reduction roll pair. . In this case, the reverse flow rate due to the discharge of the residual molten steel is larger than the reverse flow rate of the residual molten steel due to excessive taper reduction that causes the occurrence of reverse V segregation, and a lump of several mm or more is formed upstream of the roll pair. Segregation is formed once, and in some cases, it may remain until complete solidification. On the other hand, at the slab position where the central solid phase exceeds 0.95 and the solidification is completed, the formation of the center porosity is completed and the reduction effect is reduced. Therefore, the appropriate position of the slab where the step-like reduction is performed is the slab position having a central solid phase ratio of 0.8 to 0.95.

  When the reduction amount due to the step-like reduction is 5 mm or more, a center porosity reduction effect is obtained, and the center porosity reduction effect increases as the reduction amount increases. However, in the range where the amount of reduction exceeds 10 mm, the reduction effect is almost saturated. Therefore, the appropriate range of the amount of reduction was set to 5 to 10 mm.

  In addition, the center solid phase ratio at the time of slab reduction and the solidified shell thickness at the start of bulging can be obtained by unsteady heat transfer analysis. That this analysis method has sufficient accuracy to be used in the practice of the present invention was confirmed by a commonly used method such as a hammering test into an unsolidified slab and measurement of the slab surface temperature. .

(4) Porosity volume Although the slab obtained as described above has reduced segregation and the center porosity of the slab is also extremely reduced, the center porosity is reduced even if the segregation state is good. It was proved that HIC was generated in a steel plate manufactured using the cast slab as a raw material when it was not good. As a result of quantifying the porosity of the center part of the slab as follows, the porosity volume of the center part is reduced to 2 × 10 ⁻⁴ (cm ³ / g) or less, thereby suppressing the occurrence of HIC in the steel sheet. It was found that it can be maintained at a low level. Therefore, in the second invention, the porosity volume at the center of the slab thickness is defined as 2 × 10 ⁻⁴ (cm ³ / g) or less.

Here, the porosity volume is determined by the following method. The volume of the porosity is Pv (cm ³ / g), the density of a representative sample of a ¼ thickness portion of the same slab (portion at a ¼ thickness direction of the slab) is ρ ₀ , and the center portion of the slab When the sample density of ρ is ρ, the volume of porosity can be calculated by Pv = 1 / ρ−1 / ρ ₀ (cm ³ / g). In the present invention, as described later, the porosity (Pv) of the porosity was obtained by the above method for samples collected from a plurality of locations of the slab, and these maximum values were used as “porosity volume (Pvmax)”.

In this case, the size of the sample is preferably a cube within 30 mm × 30 mm × 30 mm. If the sample size is too large, the sample originally includes almost no porosity, and even a portion off the center of the slab is included in the sample, so that the sensitivity of detecting the porosity at the center becomes dull. The reason why the density ρ ₀ of the representative sample of the ¼ thickness portion of the same slab is selected as a reference is that, for example, according to inspection by a microscope, the porosity is hardly detected in the portion. This is because the density of the pieces can be equal to the original density of the material.

  In order to confirm the effect of the continuous casting method of the present invention, the following continuous casting test was performed, and the obtained slab was rolled into a steel plate to evaluate its hydrogen-induced crack resistance.

(Test method)
1) Casting Method FIG. 1 shows an example of a vertical bending die continuous casting apparatus used for testing the continuous casting method of the present invention. As the mold used for the test, a mold having a casting slab thickness of 310 mm and a slab width of 2300 mm was used. The target steel types are steel components in mass%, C: 0.06 to 0.08%, Si: 0.18 to 0.25%, Mn: 1.5 to 1.55%, P: 0.007-0.009%, S: 0.0004-0.0006%, Ti: 0.015-0.02%, Nb: 0.03-0.04%, Ca: 0.002-0. 0025% HIC steel. The casting speed was variously changed in the range of 0.60 to 0.65 m / min. The secondary cooling specific water amount was 0.8 to 1.5 L / kg-steel.

  Further, the thickness of the solidified shell at the start of bulging and the central solid phase ratio before the reduction were adjusted by changing the casting speed and the secondary cooling specific water amount.

The molten steel 4 injected into the mold 3 from the tundish through the immersion nozzle 1 is cooled by spray water sprayed from the mold 4 and a group of secondary cooling spray nozzles (not shown) below the mold 4 to form a solidified shell 5. Thus, the slab 8 is obtained. The slab is pulled out by the pinch roll 10 through the guide roll group 6 while holding the unsolidified portion 9 inside.
A strong reduction roll pair (hereinafter also referred to as “reduction roll pair” or “reduction roll”) 7 used for stepwise reduction immediately before complete solidification was installed at a position 21 m downstream from the molten steel meniscus 2 in the mold. The diameter of the rolls constituting the above-mentioned rolling roll pair 7 was 450 mm, and the rolling force was 5.88 × 10 ⁶ N at the maximum. Moreover, taper-shaped reduction was implemented using the guide roll group 6 upstream from this reduction roll. In addition, although the continuous casting apparatus used for the test is a vertical bending type continuous casting apparatus, it goes without saying that a curved type continuous casting apparatus may be used.

  In the bulging and taper reduction zone indicated by arrows B1-B2 in FIG. 1, the guide roll group 6 is arranged so that the interval in the thickness direction of the slab can be controlled to a predetermined value. A rolling pass line can be provided.

  The central solid fraction during rolling is mainly determined by the casting speed, the secondary cooling strength (ie, secondary cooling specific water amount) and the thickness of the center of the slab width (ie, the bulging amount of the slab). The central solid fraction was determined by performing heat transfer calculation with various casting speeds for the bulging amount of the piece.

  Moreover, the superheat degree ((DELTA) T) of the molten steel in a tundish was made substantially constant between 40 degreeC-50 degreeC. ΔT is the difference between the molten steel temperature and the liquidus temperature. The total amount of taper reduction is determined by the slab reduction range and taper amount in the casting direction, but in the case of the reduction range and taper amount of the present invention, it is about 10 mm at the maximum.

2) HIC resistance test of steel plate and porosity volume and segregation ratio investigation method An 8 m long slab was collected and heated to 1150 ° C in a heating furnace, then rolled from about 800 ° C to about 500 ° C. The steel sheet was rolled into a steel plate having a thickness of 20 mm and a width of 2400 mm under general rolling conditions for completing the rolling.

A total of 3 positions (hereinafter also referred to as “1/4 width position”) and a position in the center in the width direction from the width direction end of each plate obtained by dividing the longitudinal direction of each steel plate into four equal parts. From the position of the part, a total thickness test piece of length 100 mm × width 100 mm × thickness 20 mm was collected, and 5 mass% NaCl + 0.5 mass% CH ₃ COOH + 1 atm in accordance with the HIC test method defined in NACE T0284 It was immersed for 96 hours in a NACE TM0177 solution at a temperature of 25 ° C. with H ₂ S saturation. The area of cracks due to HIC generated in the test piece after immersion was measured by ultrasonic C-scan to determine the area ratio of cracks due to HIC (hereinafter also referred to as “CAR”) in the total area of the test piece. The average value of 3 places was calculated.

  In addition, in order to investigate the influence of internal cracks during bulging, it is also possible at two locations from the width direction end of each plate to 1/10 of the plate width (hereinafter also referred to as “1/10 width position”). The above CAR was obtained, and the average value of these two locations was calculated.

Furthermore, a sample was taken from the above slab by the following method for examining the porosity volume and the segregation ratio. Samples for investigating the porosity volume (Pvmax) were collected from 15 positions evenly in the width direction of the slab at the center of the thickness of the cross section block in the slab of the continuous part of continuous casting. As for the size of the sample, the plane parallel to the transverse section was 30 mm × 30 mm, and the thickness (casting direction) was 20 mm. Similarly, as a sample for measuring the reference density (ρ ₀ ), a sample of the same size was taken from the position of the 1/4 thickness at the center in the width direction of the slab.

  The density was calculated from the mass and volume of each sample. The volume was calculated from the buoyancy and the density of water by immersing the sample in water and measuring the weight in water to obtain the buoyancy. Using these results, the volume (Pv) of porosity in the slab width direction was determined by the method described in (4) above, and the porosity volume (Pvmax), which was the maximum value, was determined.

The segregation ratio was investigated by the following method. Drill holes with a diameter of 5 mm and a depth of 3 mm were drilled at 22 locations at a pitch of 100 mm in the width direction of the slab from the thickness center of the slab cross-sectional block, and Mn analysis was performed using the obtained chip sample. . The segregation ratio (C / C ₀ ) was obtained by obtaining the arithmetic average value (C) of the Mn analysis value and dividing by the base material concentration (concentration in the pan: C ₀ ).

(Test results)
Tables 1 and 2 show a series of test conditions and test results performed to confirm the effects of the present invention.

In the same table, test numbers H1 to H10 are tests for the inventive examples that satisfy all the conditions defined in the present invention.
In the above example of the present invention, the bulging amount (indicated by “BA” in the same table), the solidified shell thickness during bulging (indicated by “BS” in the same table), and stepped shape at three casting speed conditions Casting was performed by changing the central solid phase ratio (fs) during the reduction and the step-like reduction amount within the range specified in the first invention, and a test was performed in which the obtained slab was further rolled into a steel plate. In addition, the range of the reduction position under taper reduction (the range of the central solid phase ratio (fs)), the reduction taper amount, and the total reduction amount due to the taper reduction are also shown.

According to the results in the table, in the test numbers H1 to H10 that satisfy the conditions specified in the first invention, the porosity volume (Pvmax) at the center of the slab thickness is 2.0 × 10 ⁻⁴ ( cm ³ / g) or less, the Mn segregation ratio (C / C ₀ ) is stable at a low level in the range of 1.25 or less, and the occurrence of internal cracks in the short side of the slab during bulging is also observed. An unsatisfactory slab of good quality was obtained. Furthermore, the area ratio (CAR) of HIC in the steel sheet after rolling was as extremely low as 0.5% or less, and it was confirmed that a steel sheet having high-performance HIC resistance was obtained.

  On the other hand, test numbers C1 to C22 are tests for comparative examples that do not satisfy at least one of the conditions defined in the first invention.

  Test No. C1 is a test for a comparative example in which the same conditions as in Test No. H1 except for the taper reduction and the stepwise reduction at the end of coagulation were performed under the same conditions as in Test No. H1. Both the Mn segregation ratio and the porosity volume at the center are very high, and the slab quality is inferior. As a result, the HIC area ratio of the steel plate was extremely high at 15.3%, and the steel plate was extremely inferior in HIC resistance.

  Test numbers C2, C4, and C6 are tests for a comparative example in which only taper reduction was not performed under the same conditions as test numbers H2, H4, and H8. Although the values of the porosity volume were all low and good, the value of the Mn segregation ratio was high, and the HIC resistance performance of the steel sheet was inferior.

  Test numbers C3, C5, and C7 are tests for a comparative example in which stepwise reduction at the end of coagulation was not performed under the same conditions as test numbers H1, H4, and H8. The value of the Mn segregation ratio was relatively low and good, but the value of the porosity volume was very high. As a result, the HIC resistance of the steel sheet was inferior.

Test numbers C8, C9, and C10 are comparative examples in which the stepwise reduction amount at the end of coagulation is less than 5 mm under the same conditions as test numbers H1, H5, and H9. As a result, the value of the porosity volume was larger than 2.0 × 10 ⁻⁴ (cm ³ / g), and the HIC resistance performance of the steel sheet was somewhat poor.

  Test numbers C11 to C16 are tests for a comparative example in which only the timing of the step-like pressure reduction at the end of solidification (center solid phase ratio) is changed under the same conditions as in test numbers H1, H5, and H9. In test numbers C11, C13, and C15, stepwise reduction was performed at the slab position where the center solid phase ratio of the slab was less than 0.8. As a result, the value of the porosity volume was low and good, but there were some parts with poor segregation status, the value of Mn segregation ratio was high, and the HIC resistance performance of the steel sheet was also poor. This is presumably because the residual molten steel enriched with segregation components was discharged and flowed back upstream, and remained as component segregation.

On the other hand, in test numbers C12, C14, and C16, stepwise reduction was performed at the slab position where the center solid phase ratio of the slab exceeded 0.95. As a result, the value of the porosity volume was larger than 2.0 × 10 ⁻⁴ (cm ³ / g), and the HIC resistance performance of the steel sheet was somewhat poor. As described above, this is because the effect of the step-like reduction is reduced in the situation where the solidification has progressed in this way.

  Test numbers C17 to C22 are tests for comparative examples in which the bulging conditions are changed. In test numbers C17 and C18, under the same conditions as in test number H1, the thickness of the solidified shell and the amount of bulging during bulging were largely changed beyond the range specified in the first invention. Similarly, in test numbers C19 and C20, the thickness of the solidified shell and the amount of bulging during bulging were largely changed beyond the ranges specified in the first invention under the same conditions as in test numbers H6 and H8. As a result, in all cases, internal cracks presumed to have occurred during bulging occurred, and the HIC resistance of the steel sheet was inferior particularly at the 1/10 width end.

In test numbers C21 and C22, casting was performed under the same conditions as in test numbers H1 and H4 with only the bulging amount being less than 3 mm, which is the lower limit of the range defined in the first invention. As a result, the value of the Mn segregation ratio slightly increased, the porosity volume became higher than 2.0 × 10 ⁻⁴ (cm ³ / g), and the HIC resistance performance of the steel sheet also became slightly poor.

  The superiority of the present invention was proved from the comparison of the respective test results of the present invention examples and comparative examples.

  According to the continuous casting method of the present invention, it is possible to cast a slab slab with significantly reduced center porosity, macro segregation and semi-macro segregation, and to produce a steel sheet having excellent HIC resistance using the slab as a raw material. Is possible. Moreover, the continuous cast slab of the present invention is most suitable for the production of a steel plate having excellent HIC resistance used for a line pipe or the like. Therefore, the continuous casting method and continuous cast slab of the present invention can be widely applied in the field of steel sheet manufacturing technology that requires high performance HIC resistance.

It is a figure which shows the example of the continuous casting apparatus for enforcing the continuous casting method of this invention.

Explanation of symbols

1: immersion nozzle, 2: molten steel meniscus, 3: mold, 4: molten steel,
5: Solidified shell, 6: Guide roll, 7: Roll pair for high pressure reduction, 8: Slab,
9: Unsolidified molten steel, 10: Pinch roll,
B1-B2: bulging and tapered reduction zones

Claims (2)

At the slab position where the solidified shell thickness on one side of the slab is 60 mm or less, the slab thickness is bulged within a range of 3 to 10 mm at the center of the slab width,
While maintaining the bulging state, the slab between the slab position where the solid phase is generated in the center of the thickness and the slab position where the central solid fraction in the center of the thickness is less than 0.8 , Squeezed down in a taper direction with respect to the casting direction of the slab within the bulging thickness
Subsequently, at the slab position where the central solid phase ratio in the central portion of the thickness is in the range of 0.8 to 0.95, a single rolling roll pair is used within the range of 5 to 10 mm in the slab thickness direction. By rolling down into steps ,
Casting having a porosity volume of 0.8 × 10 ⁻⁴ to 2 × 10 ⁻⁴ (cm ³ / g) in the thickness center and a Mn segregation ratio of 1.10 to 1.25 in the thickness center. A continuous casting method for a steel plate production slab excellent in hydrogen-induced cracking resistance, characterized by producing a piece. A slab cast by the continuous casting method according to claim 1,
The porosity volume at the center of the slab thickness is 0.8 × 10 ⁻⁴ to 2 × 10 ⁻⁴ (cm ³ / g) , and the segregation ratio of Mn at the center of the slab thickness is 1.10. 1.25,
A continuous cast slab for producing a steel sheet, wherein the area ratio of hydrogen-induced cracking in the steel sheet after rolling is 0.5% or less in accordance with a test method defined in NACE T0284 .

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