JP6043217B2 - How to change the destination by judging the quality of the sour steel slab - Google Patents

Description

  The present invention relates to a method for evaluating HIC properties from the internal quality of a slab.

  Since petroleum sulfide and natural gas contain hydrogen sulfide, the steel materials (hereinafter referred to as sour-resistant steel) used for transportation of these oils are always in a hydrogen sulfide atmosphere. In such an atmosphere, hydrogen enters and diffuses into the steel material and accumulates and gasifies into inclusions in the steel material. As a result, when an internal pressure is applied to the steel material, fine cracks are generated around the inclusions, which propagate along the hardened phase, thereby generating hydrogen-induced cracking (hereinafter referred to as HIC). Therefore, the sour-resistant steel is required to have excellent HIC resistance.

  It is known that HIC is generated starting from the center segregation and internal cracks generated by inclusions and a hardened phase generated during casting. Therefore, conventionally, a technique (casting method) for suppressing the occurrence of center segregation and internal cracks has been proposed (see Patent Documents 1 to 6).

  However, even if the above method is carried out, if an operation abnormality occurs, for example, if the gap between rolls is clogged, the immersion nozzle is clogged, the molten steel injected from the immersion nozzle is drifted, the casting speed is changed, etc. The above effects cannot be obtained, and the center segregation and internal cracks may be caused in the slab. Since HIC occurs when these defects remain in the product, the presence or absence of HIC is usually confirmed by performing an HIC test on the product (steel material) before shipment.

  However, the HIC test requires several weeks before the results are known. In addition, when HIC occurs, the product cannot be shipped as sour-resistant steel, so it is necessary to perform a new HIC test on the product that has been remelted and manufactured. If it does so, a manufacturing period will become long and it will become a cause, such as a delivery-date delay.

  Therefore, if the HIC property can be evaluated at the slab stage without performing the HIC test, the process of rolling → sample adjustment → HIC test can be omitted when HIC occurs and remelting is required. Moreover, when remelting is not necessary, since the sample adjustment after casting → HIC test can be omitted, the period from the start of production to shipment can be greatly shortened. As described above, HIC occurs starting from center segregation and internal cracks. Therefore, if center segregation and internal cracks can be evaluated at the stage of the slab, it is considered that HIC properties can be evaluated based on the evaluation results.

  As such a method, Patent Document 7 discloses a method of evaluating internal cracks at the stage of a slab. In Patent Document 7, whether or not the HCR operation is possible is determined from the evaluation result of the internal crack.

Patent No. 4508087 Japanese Patent No. 3297703 Japanese Patent No. 3524750 Japanese Patent No. 3114671 Japanese Unexamined Patent Publication No. 63-317237 Japanese Patent Laid-Open No. 60-6257 Japanese Patent No. 4397825

  By the way, the internal cracks that are a problem in sour-resistant steel are very small fine cracks. However, in Patent Document 7, an internal crack that is a problem in HCR operation (a large crack having a crack length of 10 mm or more). I am evaluating. Therefore, in the method of Patent Document 7, a fine internal crack that causes a problem in sour-resistant steel may be missed, and the HIC property caused by the internal crack cannot be accurately evaluated at the stage of the slab.

  Further, since Patent Document 7 does not describe a method for evaluating the center segregation, it is considered that the HIC property caused by the center segregation cannot be evaluated at the slab stage.

  Then, the objective of this invention provides the method of evaluating HIC property from the internal quality of a slab, without performing a HIC test.

The present invention is a quality determination method for determining whether a slab of thickness D cast by a continuous casting machine can be applied to sour-resistant steel,
In a plurality of slabs cast under different casting conditions, a first opening thickness measurement step for measuring the maximum opening thickness of an internal crack existing in a first range of width D / 2 from both ends in the width direction of the slab;
A plurality of slabs cast by different casting conditions, when separated in the second 1 range in the width direction or a plurality of predetermined section width W-D from both ends in the width direction except for D / 2 of the slab, In each of the predetermined sections, a first segregated grain measuring step of measuring the number density of segregated grains having a maximum diameter of the segregated grains and a predetermined diameter or more at a thickness center portion;
For the steel material produced by rolling a plurality of slabs cast under the same conditions as the slabs in the first hole thickness measurement step and the first segregated grain measurement step, the first range and the second range A test process for performing an HIC test in each of the areas corresponding to one or more predetermined sections of the range;
Based on the plurality of maximum opening thicknesses obtained in the first opening thickness measurement step and the plurality of test results in the region corresponding to the first range in the test step, the threshold value of the maximum opening thickness at which no HIC is generated. A threshold value determining step for determining
Wherein the first polarized析粒measuring a plurality of number density of the plurality of maximum diameter of the polarized析粒and polarization析粒on the predetermined diameter or less obtained in step, one or more predetermined section of the second range in the test step and a plurality of test results of the region corresponding to the HIC generation range determination step of determining the maximum diameter and the scope of the number density of polarized析粒on the predetermined size or less polarized析粒that HIC is generated,
A second hole thickness measurement step for measuring the maximum hole thickness of an internal crack existing in the range of width D / 2 from both ends in the width direction of the slab to be determined;
In the range of width WD excluding D / 2 from both ends in the width direction of the slab to be determined, when this range is divided into one or more predetermined sections in the width direction, the thickness in each of the predetermined sections a second polarization析粒measuring step of measuring the maximum diameter and the number density of polarized析粒on the predetermined size or less polarized析粒in the center,
The maximum aperture thickness obtained in the second aperture thickness measurement step is compared with the threshold value of the maximum aperture thickness determined in the threshold determination step, and the maximum of the segregated particles obtained in the second segregation particle measurement step. diameter and then compared with the maximum diameter and the scope of the number density of polarized析粒on the predetermined size or less polarized析粒that HIC was determined by the number density and the HIC generation range determining step of polarization析粒occurs on the predetermined size or less The segregated particles obtained when the maximum pore thickness obtained in the second pore thickness measurement step exceeds the threshold value of the maximum pore thickness determined in the threshold value determination step and in the second segregation particle measurement step. maximum diameter and number density of polarized析粒on said predetermined diameter or less is in the maximum diameter and the scope of the number density of polarized析粒on the predetermined size or less polarized析粒that HIC was determined by the HIC generation range determining step occurs in If you meet at least one of the times And a changing step of changing towards destination elephant slab to products other than sour steel.

  As described above, in the present invention, “opening thickness of internal cracks” and “maximum diameter and number density of segregated grains” are used for evaluation of HIC properties. By these, since the internal quality (segregation degree of an internal crack and center segregation) of a slab can be evaluated correctly, HIC property can be evaluated in the stage of a slab, without performing a HIC test. And the use of a slab can be determined based on the evaluation result. Thereby, since the HIC test which requires several weeks can be omitted, the period from the start of production to shipment can be greatly shortened.

  According to the present invention, the HIC property can be evaluated from the internal quality of the slab without performing the HIC test. And since the use of a steel material can be determined in the stage of a slab from this evaluation result, the period from a manufacture start to shipment can be shortened significantly.

It is a schematic diagram which shows the structure of a continuous casting machine. It is a flowchart of the determination method of this invention. (A) is a sectional view of a slab, (b) is an enlarged view of the section r _1. It is a figure explaining an internal crack. It is a figure which shows sectional drawing of a slab, and sectional drawing of a product. It is a figure which shows the result (opening thickness and HIC property) which investigated the multiple cross section. It is a figure which shows the result (maximum segregation particle size and number density, and HIC property) which investigated the multiple cross section. It is a figure explaining the investigation surface of a slab. It is a figure which shows the result (opening thickness and HIC property) of an Example. It is a figure (maximum segregation particle diameter and number density, and HIC property) which shows the result of an Example.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

  Here, a determination method according to an embodiment of the present invention will be described with reference to FIGS. In FIGS. 3 and 5, hatching indicating a cross section is omitted.

[Continuous casting machine]
As shown in FIG. 1, the continuous casting machine 100 is a vertical bending type continuous casting machine, in which a tundish 1, an immersion nozzle 2 attached to the bottom of the tundish 1, and a lower part of the immersion nozzle 2 are arranged. The casting mold 3, a plurality of rolls 4 provided along the casting path Q from directly below the casting mold 3, and a cooling nozzle 5 disposed between the rolls 4 adjacent to each other in the casting direction. The tundish 1 is supplied with a molten steel 6 whose components have been adjusted by secondary refining from a ladle (not shown). The mold 3 is formed with a substantially rectangular opening in plan view, and a slab (thickness D × width W cast piece) can be cast.

  The casting path Q has a vertical portion extending in the vertical direction, a bent portion gently curved from the vertical portion, and a horizontal portion extending in the horizontal direction from the bent portion.

  When casting is performed, the molten steel 6 in the tundish 1 is injected into the mold 3 through the immersion nozzle 2. The molten steel 6 is cooled in the mold 3, drawn downward while forming a solidified shell, and solidified to the inside, whereby a slab (slab) is cast. Thereafter, the slab is subjected to a rolling process or the like to become a steel plate, which is applied to sour-resistant steel (hydrogen-resistant induced cracking steel) and other products.

  As described above, the sour steel is required to have excellent HIC resistance (hydrogen induced cracking resistance). HIC is generated starting from a position (segregation part) where inclusions such as MnS, Ti (C, N), and Nb (C, N) are present in the steel, and propagates along the hardening phase. It is known that segregation is present in the internal crack portion and the center segregation portion of the slab, and that the higher the degree of segregation, the more likely HIC is generated (see Japanese Patent Application Laid-Open No. 2007-136396).

  Therefore, in the present invention, the degree of segregation of internal cracks and center segregation is investigated, and HIC properties are evaluated at the slab stage based on these.

  In addition, although segregation exists also between secondary dendrite trees (micro segregation), since the secondary dendrite trees are very small, inclusions and hardened structures that cause HIC are not generated. Therefore, it is considered that HIC does not occur starting from the segregation.

Further, the internal cracks include “horizontal cracks” and “other internal cracks”, which are caused by bulging between rolls, unbalance of cooling water, and deformation during correction. As shown in FIG. 4, the “horizontal crack” is a crack that exists in the range of D / 2 from the width end in the width direction of the slab, and is a crack that has propagated in the slab width direction and the casting direction. On the other hand, the “other internal cracks” are cracks existing in the entire width of the slab, and are cracks propagated in the slab thickness direction and the slab width direction, or in the slab thickness direction and the slab casting direction. Therefore, “horizontal cracks” are extended by rolling, while “other internal cracks” are reduced by rolling. Therefore, when HIC occurs, HIC easily propagates and extends in “horizontal cracking”, but HIC does not propagate and extend in “other internal cracks”, so there is no problem in quality.
Further, when the HIC test was performed, HIC sometimes occurred in the “horizontal crack” occurrence portion, but no HIC occurred in the “other internal crack” occurrence portion. Therefore, in the present invention, only “horizontal cracks” among the internal cracks are examined.

  Below, the determination method of this invention is demonstrated in detail along the flowchart of FIG.

[Judgment method]
The slab after casting is cut in a direction perpendicular to the casting direction (see FIG. 3), and the internal quality (the degree of segregation of horizontal cracks and center segregation) is examined on the cut surface.

  The position where the horizontal crack occurs is more likely to vary in the slab width direction and the thickness direction than in the casting direction. In addition, the level of center segregation (segregation grain size, the number of segregation grains having a predetermined grain size or more) may vary in the width direction of the slab and deteriorate at a specific portion in the width direction. Therefore, by using a cut surface perpendicular to the casting direction as an object of investigation, it is possible to investigate a portion where horizontal cracking and center segregation are most deteriorated.

  The slab is made of the same steel type (component) as the sour-resistant steel. This is because when the steel type is different, the concentration of the additive element changes, so that the amount of inclusions in the segregation part and the hardening phase change.

Next, on the slab cut surface (see FIG. 3), the maximum opening thicknesses t ₁ and t ₂ of horizontal cracks existing in the regions R ₁ and R ₂ in the range of the width D / 2 from both ends in the width direction are measured (FIG. 3). 2 S1, first hole thickness measuring step). Here, the maximum aperture thickness t ₁ is the maximum aperture thickness in the region R ₁ , and the maximum aperture thickness t ₂ is the maximum aperture thickness in the region R ₂ .

Horizontal cracking occurs in the process of solidification progressing from both ends (narrow surfaces) in the width direction of the slab toward the center in the width direction. In the regions R ₁ and R ₂ (first range), solidification proceeds toward the center in the width direction under the influence of cooling on the narrow surface side (short side). On the other hand, in the region R ₃ (second range) having a width WD excluding D / 2 from both ends in the width direction, the cooling on the narrow surface side (short side) has little influence, so that the solidification hardly occurs in the width direction. Does not progress. For this reason, it is considered that horizontal cracks occur in the regions R ₁ and R ₂ . Therefore, in step S2, horizontal cracks are examined in the regions R ₁ and R ₂ . Here, each region R _1, if two or more horizontal cracking R ₂ in is present, the maximum aperture thickness of each region R _1, R ₂ and maximum opening thickness t _1, t _2. For example, if there are two or more horizontal cracks in the region R ₁ measures the opening thickness of the most aperture to which part of the horizontal cracks (thickest portion opening thickness) having the largest openings The opening thickness is defined as a maximum opening thickness t ₁ .

  In the present invention, the degree of segregation of “horizontal cracking” is evaluated from “opening thickness (maximum opening thickness)”. A “horizontal crack” is a crack that occurs at the solid-liquid interface during solidification, and is accompanied by a segregation line that is formed when the concentrated molten steel enters between dendritic trees. When the degree is bad, holes are formed along the segregation line. There is a correlation between “degree of segregation” and “opening thickness (opening width)”, and the larger the “opening thickness”, the higher the “segregation degree”. It was found that HIC is more likely to occur as the “degree of segregation” is higher, so that the HIC is more likely to occur as the “opening thickness” is larger, and the presence or absence of HIC can be determined by the opening thickness. Therefore, in the present invention, the HIC property is evaluated based on the “opening thickness”.

  A fine horizontal crack having an “opening thickness” of about several tens [μm] is pressed during rolling and does not become a UT defect at the product stage, but causes HIC. Then, it is considered that HIC is generated not because of the opening, but because of the high degree of segregation.

Next, (see Fig. 3) in the slab cut surface segregation ratio of center segregation in the region R ₃ of the width W-D, except for the D / 2 from both ends in the width direction ( "up polarized析粒diameter (maximum diameter of the polarized析粒) And “number density of segregated grains having a predetermined diameter or more”) (S2 in FIG. 2, first segregated grain measuring step). In the following, “number density of segregated grains having a predetermined diameter or more” may be simply referred to as “number density of segregated grains” or “number density”.

Center segregation is a defect generated in the final solidified part. As shown in FIG. 3, the regions R ₁ and R ₂ are cooled from the wide surface side and the narrow surface side, but the region R ₃ is mainly cooled only from the wide surface side. Therefore, in the region R _3, solidification proceeds toward the thickness center from broad surface, the thickness center portion is the final solidified portion. Thus, the central segregation occurs in the thickness center portion near the region R _3. Therefore, in step S2, to investigate the segregation ratio of the center segregation in the thickness center portion near the region R _3.

  In the present invention, the segregation degree of the center segregation is evaluated from “maximum segregation particle diameter” and “number density of segregation grains having a predetermined diameter or more”. There are various methods for investigating the degree of segregation, but there is a correlation between “segregation particle size” and “segregation degree”. The larger the “segregation particle size”, the higher the “segregation degree” (reference) Document: Nippon Steel Pipe Technical Report No. 121 (1988)). As the “segregation degree” is higher, HIC is more likely to occur. Therefore, it can be said that the larger the “segregation particle diameter” is, the easier it is to generate HIC.

  In addition, there is a correlation between "number density of segregated grains having a predetermined diameter or more" and "maximum segregated particle diameter", and "maximum segregated particle diameter" tends to increase as the "number density" increases (references: CAMP-ISIJ Vol.2 (1989) p, 1150). As described above, since the “segregation degree” tends to be higher as the “segregation particle size” is larger, it can be said that as the “number density” is larger, the “segregation degree” is higher, that is, HIC is more likely to occur.

  From the above, it is considered that the “degree of segregation” can be evaluated by the “maximum segregation particle diameter” and the “number density of segregation grains”. In addition, since the “segregated particle diameter” and “number density” can be measured visually, the degree of segregation can be investigated easily and in a short time. Therefore, in the present invention, the HIC property is evaluated based on the “maximum segregated particle diameter” and the “number density of segregated grains”.

  Here, an example of a method for measuring “maximum segregated particle diameter” and “number density of segregated grains” will be described with reference to FIG.

As shown in FIG. 3A, the region R ₃ is divided into n predetermined sections r ₁ , r ₂ , r ₃ ... In the width direction.
Divide into r _n (n is a natural number of 1 or more), and measure “maximum segregation particle size” and “number density” at the center of thickness in each section. Here, the predetermined interval _{r 1, r 2, r 3} ··· r n are each rectangular region having a width W ₁ × a thickness D _1. (See FIG. 3B). The “number density of segregated grains having a predetermined diameter or more” is calculated from the following equation.
When N segregated grains having a predetermined diameter or more exist in the section r ₁ (see FIG. 3B),
“Number density of segregated grains having a predetermined diameter or more” in the section r ₁ = N / (W ₁ × D ₁ )

  Next, the slab cast under the same casting conditions as the slab investigated in step S1 and step S2 is rolled under rolling conditions for rolling sour steel (rolling start surface temperature, rolling end surface temperature, final product thickness, etc.) (Steel) is manufactured. Then, an HIC test is performed on the product (S3 in FIG. 2), and the presence or absence of HIC generation is examined (evaluation of HIC properties).

  Here, “same casting conditions” means i) that the casting speed is constant, ii) that there is no operational abnormality such as nozzle clogging, iii) that the cooling conditions and roll gap are the same, etc. It is. Since these operating factors have a large effect on horizontal cracking and center segregation, the evaluation of HIC properties differs when these factors change. In the threshold value determination step described later (S4), the segregation degree obtained in step S1 and step S2 and the HIC test result obtained in step S3 are made to correspond to each other. Therefore, if these HIC properties are different, the threshold value cannot be determined. Therefore, the same casting conditions are used. The “slab cast under the same casting conditions” includes the slab investigated in step S1 and step S2.

In addition, the HIC test is performed in the regions (product regions) corresponding to the slab regions R ₁ , R ₂ , and R ₃ (see FIG. 3). When the region R ₃ is divided into _n predetermined sections r ₁ , r ₂ , r ₃ ... R _n in the width direction in step S2, the sections r ₁ , r ₂ , r ₃ . Each HIC test is performed in the area corresponding to _n (product area).

For example, when the slab is rolled in the casting direction (when the rolling direction is the casting direction), as shown in FIG. 5A, the width does not change before and after rolling, so the slab width W = the product width W. It is. In this case, “regions corresponding to (slab) regions R ₁ , R ₂ ” are “regions R ₁₁ , R ₁₂ within a range of width D / 2 from both ends in the width direction of the product”, and “(slab) region R ₃ The “region corresponding to” is “region R _{13 in} the range of width WD excluding D / 2 from both ends in the width direction of the product”.

Also, "(slab) sections _{r 1, r 2, r 3} ··· r n the region corresponding to the region _{R 3"} when separated regions _{R 13} of the product in the width direction into n predetermined section Corresponding to “predetermined sections r ₁₁ , r ₁₂ , r ₁₃ ... R _n1 ”. Here, the predetermined sections r ₁₁ , r ₁₂ , r ₁₃ ... R _n1 are rectangular regions of width W ₁ × thickness D ₁ .

On the other hand, when the slab is rolled in the width direction (when the width direction is included in the rolling direction), as shown in FIG. 5B, the width changes before and after rolling, so the slab width W <the product width. Wa. In this case, the regions R ₂₁ , R ₂₂ , R ₂₃ corresponding to the slab regions R ₁ , R ₂ , R ₃ are determined by the rolling ratio (product width Wa / slab width W). Also, "(slab) sections _{r 1} of region _{R 3, r 2, r 3} ··· region corresponding to _{r n"} is also determined by the rolling ratio Wa / W "interval _{r 21, r 22, r 23} ··・ R _n2 ”.

Next, referring back to the flowchart of FIG. 2, an internal crack is obtained from “the maximum opening thickness t ₁ , t ₂ ” obtained in step S ₁ and “the evaluation result” (HIC property) obtained in step S 3. The “maximum aperture thickness threshold value (t _θ )” relating to the HIC property caused by (no HIC occurs) is determined (S4, threshold determination step).

At this time, the results obtained in the areas corresponding to each other in the slab and the product are made to correspond.
For example, when the slab is rolled in the casting direction (see FIG. 5A),
When Step S3 (HIC test) results, in the product region _{R 11} "HIC generation Yes", it is in the region _{R 12} "HIC generated no",
i) “HIC occurs” at the opening thickness t ₁ (opening thickness of slab region R ₁ ) (result of product region R ₁₁ )
i) “No HIC generation” at the opening thickness t ₂ (opening thickness of the slab region R ₂ ) (result of the product region R ₁₂ )
And

Moreover, when rolling a slab in the width direction (refer FIG.5 (b)),
When the result of step S3 (HIC test) is “HIC generated” in the product region R ₂₁ and “HIC not generated” in the region R ₁₂ ,
i) “HIC occurs” at the opening thickness t ₁ (opening thickness of slab region R ₁ ) (result of product region R ₂₁ )
i) “No HIC generation” at the opening thickness t ₂ (opening thickness of the slab region R ₂ ) (result of the product region R ₂₂ )
And

A plurality of results of the determines the threshold t _theta apertures thickness bounding the HIC occurrence or non-occurrence. In the present embodiment, the maximum opening thickness at which no HIC is generated is defined as “threshold value t _θ ”.

Next, returning to the flowchart of FIG. 2, from the “maximum segregation particle size” and “number density” obtained in step S2 ”and“ evaluation result ”(HIC property) obtained in step S3, the HIC property is obtained. “Determines whether or not HIC occurs” is determined (threshold function f _θ (d, m) of maximum segregation particle size and number density) (S4). Since the degree of segregation of center segregation can be evaluated by “maximum segregation grain size” and “number density of segregation grains”, the value (threshold) that serves as a boundary on whether or not HIC occurs due to center segregation is the maximum segregation grain size. And a function of number density.

Also at this time, the results obtained in the areas corresponding to each other in the slab and the product are made to correspond.
For example, when the slab is rolled in the casting direction (see FIG. 5A),
When Step S3 (HIC test) results, in the product region _{r 11} "HIC generation Yes", in the region _{r 12} "HIC generation Yes", ..., is in the region _{r n1} "HIC generated no",
i) “HIC is present” when the maximum segregation particle diameter d ₁ and number density m ₁ (maximum segregation particle diameter and number density of slab region r ₁ ) (result of product region r ₁₁ )
ii) “HIC is present” when the maximum segregation particle size d ₂ and number density m ₂ (maximum segregation particle size and number density of slab region r ₂ ) (result of product region r ₁₂ )
iii) maximum deviation析粒diameter _{d n,} the number density _{m n} (maximum deviation析粒size of the slab region _{r n,} number density) "HIC generated no" (the result of the product region _{r n1} at)
And

Moreover, when rolling a slab in the width direction (refer FIG.5 (b)),
When the result of step S3 (HIC test) is “HIC occurs” in the product region r ₂₁ , “HIC occurs” in the region r ₂₂ ,..., “HIC does not occur” in the region r _n2 ,
i) “HIC is present” when the maximum segregation particle size d ₁ and number density m ₁ (maximum segregation particle size and number density of slab region r ₁ ) (result of product region r ₂₁ )
ii) “HIC is present” when the maximum segregation particle size d ₂ and number density m ₂ (maximum segregation particle size and number density of slab region r ₂ ) (result of product region r ₂₂ ).
iii) maximum deviation析粒diameter _{d n,} the number density _{m n} (maximum deviation析粒size of the slab region _{r n,} number density) "HIC generated no" (the result of the product region _{r n2} when)
And

Based on the above results, the threshold function f _θ (d, m) of the maximum segregation particle size and number density, which becomes the boundary of whether or not HIC occurs, is determined. Here, the threshold function f _θ (d, m) is a function serving as a threshold for determining whether or not HIC occurs, which is determined from the maximum segregated particle diameter d and the number density m. Then, from the threshold function f _θ (d, m), “maximum segregation particle size and number density range where HIC occurs” (HIC generation range) and “maximum segregation particle size and number density range where HIC does not occur” ( HIC non-occurrence range) is determined (S5 in FIG. 2, HIC occurrence range determination step).

As described above, the “thickness threshold value t _θ ” and the “maximum segregation particle size and number density threshold function f _θ (d, m)” are determined from steps S1 to S4. Further, from the “maximum segregation particle size and number density threshold function f _θ (d, m)”, “maximum segregation particle size and number density range where HIC occurs” (HIC generation range) and “maximum where no HIC occurs” The “range of segregated particle size and number density” (range in which no HIC occurs) is determined (step S5).

  Next, the HIC property of the judgment target slab is evaluated.

A slab to be judged is cut in a direction perpendicular to the casting direction, and a horizontal crack exists in the cut surface (see FIG. 3) in the range of width D / 2 from both ends in the width direction (regions R ₁ and R ₂ ). (S6 shown in FIG. 2). When the horizontal crack exists (S6: YES), the “maximum hole thickness t” of the horizontal crack is measured in the range of the width D / 2 (S7, second hole thickness measuring step). In addition, when two or more horizontal cracks exist in the range of the width D / 2, as in step S1, the most open part of the horizontal crack having the largest opening (the thickness of the hole is the thickest). The opening thickness of the portion) is defined as the maximum opening thickness t.

Then, the “opening thickness t” is compared with the “threshold (t _θ )” determined in step S4. If the opening thickness t> the threshold t _θ (S8: YES), the segregation degree of the horizontal crack portion Therefore, it is determined that HIC caused by horizontal cracking occurs. Since such a slab cannot be applied to sour-resistant steel, the destination of the judgment target slab is changed to a product other than the sour-resistant steel (S9, changing step).

On the other hand, if the opening thickness t ≦ threshold t _theta is (S8: NO), due to the low segregation ratio of horizontal cracking unit, it determines a horizontal crack HIC caused does not occur. Further, returning to step S6, even when there is no horizontal crack in the slab cut surface regions R ₁ and R ₂ (S6: NO), the segregation degree of the horizontal crack portion is low, so that HIC occurs due to the horizontal crack. Judge not to.

However, since there is a possibility that HIC due to center segregation may occur, the process proceeds to step S10, and the range of the width WD excluding D / 2 from both ends in the width direction of the slab cut surface to be determined (region R ₃ ) (See FIG. 3), “maximum segregation particle diameter d” and “number density m of segregation grains having a predetermined diameter or more” are measured at the thickness center (S10, second segregation grain measurement step).

Here, the “predetermined diameter” of the “number density m of segregated grains having a predetermined diameter or more” is the same as the “predetermined diameter” of the “number density of segregated grains having a predetermined diameter or more m ₁ ” in step S 2. Is the diameter. For example, when the “predetermined diameter” in step S2 is 1.2 [mm], the “predetermined diameter” in step S10 is 1.2 [mm].

If the slab region R ₃ is divided into _n predetermined sections r ₁ , r ₂ , r ₃ ... Rn in the width direction in step S2 (see FIGS. 3 and 5), also in step S10 the region R ₃ in the width direction separated into n predetermined interval _{r 1, r 2, r 3} ··· r n, a thickness center portion of each section "maximum polarization析粒diameter d""number density m" Measure. In this case, the “predetermined section” in step S10 is the same section as the “predetermined section” in step S2 (rectangular region of width W ₁ × thickness D ₁ ).

Next, the “maximum segregation particle size d” and “number density m” of the judgment target slab (each section) are compared with the “HIC generation range” determined from the threshold values d _θ and m _θ , When “d” and “number density m” are in the “HIC generation range” (S11: YES), the segregation degree of the center segregation portion is high, and it is determined that HIC is caused by center segregation. When the slab region R ₃ is divided into _n predetermined sections r ₁ , r ₂ , r ₃ ... Rn in the width direction (see FIG. 3), the maximum segregated particle diameter d> threshold value d _θ. When there is at least one section (S11: YES), it is determined that HIC is caused by center segregation, and the destination of the determination target slab is changed to a product other than sour steel (S9).

  On the other hand, when the “maximum segregation particle size d” and “number density m” of the judgment target slab (each section) are not in the “HIC generation range” (S11: NO), “maximum segregation particle size d” and “number density” m ”is determined to be in the“ HIC non-occurrence range ”. In this case, since the segregation degree of the center segregation portion is low, it is determined that HIC due to center segregation does not occur, and the determination target slab is applied to sour-resistant steel (S12).

When the slab region R ₃ is divided into _n predetermined sections r ₁ , r ₂ , r ₃ ... Rn in the width direction (see FIG. 3), the “maximum segregation” is applied to all the n sections. when the particle diameter d "and" number density m "is not in the" HIC generation range "(S11: nO), the entire range of the region R _3" maximum polarization析粒diameter d "and" number density m "is" HIC not Since it is in the “occurrence range”, it is determined that HIC due to center segregation does not occur, and the determination target slab is applied to the sour-resistant steel (S12).

  As described above, in this embodiment, the “opening thickness of the internal crack” and the “maximum segregation particle size and number density” are used for the evaluation of the HIC property. As a result, the internal quality of the slab (the degree of segregation of internal cracks and center segregation) can be accurately evaluated, so that the HIC property can be evaluated at the slab stage without performing the HIC test. And the use of a product can be determined from this evaluation result. As a result, the product can be shipped without an HIC test that takes several weeks, so that the manufacturing period can be greatly shortened.

  In the present embodiment, the degree of segregation of center segregation is evaluated by “maximum segregation particle size and number density”. EPMA and combustion infrared absorption methods are known as methods for evaluating the degree of segregation, but these methods require (1) the introduction of equipment and (2) a long time to measure multiple segregations. (3) It is necessary to secure analysts. On the other hand, in the present invention, since the “maximum segregation particle size and number density” can be measured simply by visually observing the solidified structure of the slab, the degree of segregation can be evaluated easily and in a short time.

  In this embodiment, when it is determined that HIC occurs, the destination of the slab is changed to a product other than the sour-resistant steel, but the slab cast for the sour-resistant steel can be applied to other line pipe materials. Quality. In particular, the level of horizontal cracking and center segregation required for sour-resistant steel is stricter than the level of horizontal cracking and center segregation required for other line pipe materials. Even if it is determined that HIC occurs due to center segregation), it is of good quality that can be applied to other line pipe materials.

  Moreover, it is preferable to use the measurement results and test results of a plurality of slabs for determining the threshold values (S1 to S4). By using the measurement results and test results of a plurality of slabs, it is possible to obtain a more accurate threshold value and reduce the erroneous determination of whether or not HIC has occurred.

[Number of slab cross sections]
The investigation of internal quality and HIC property may be evaluated from one section of a slab or product, or may be evaluated from two or more sections. The results (Examples 1 and 2) of examining a plurality of cross sections in the slab having the same charge will be described below. Here, Example 1 is an example in which two cross sections of the same charge are examined (“Example 15” (see Table 1 described later)), and Example 2 is an example in which three cross sections of the same charge are investigated (described later in “ Example 2 "(see Table 1)).

<Opening thickness and HIC properties>
As shown in FIG. 6, in Example 1, the aperture thickness was 0 [mm] in both cross sections, and no HIC was generated starting from the horizontal crack in the HIC test.
Moreover, in Example 2, the aperture thickness of the three cross sections was 0.72 [mm], 0.73 [mm], and 0.71 [mm], which were substantially the same thickness. Further, HIC occurred from the horizontal cracks in all the cross sections.
As described above, in the same charge, substantially the same result was obtained even if the cross sections were different.
In addition, when only one cross section was examined (50 charges), it was found that there was no misjudgment and accurate evaluation was possible.

<Maximum segregation particle size, number density and HIC properties>
As shown in FIG. 7, in Example 1, the maximum segregation particle diameters of the two cross sections were 1.12 [mm] and 1.15 [mm], and the number density was 0 [pieces / m ² ]. . In the HIC test, no HIC starting from the center segregation portion was generated in any cross section.
In Example 2, the maximum segregation particle size of the three cross sections was 2.45 [mm], 2.42 [mm], 2.48 [mm], and the number density was all 2000 [pieces / m ² ]. It was. In all the cross sections, HIC was generated starting from the central segregation part.
As described above, in the same charge, substantially the same result was obtained even if the cross sections were different.
In addition, when only one cross section was examined (50 charges), it was found that there was no misjudgment and accurate evaluation was possible.

  As described above, in the same charge, substantially the same result was obtained in each cross section. An accurate evaluation was also obtained when a cross section was examined. Therefore, it was found that the number of cross sections to be investigated may be one or two or more.

[Slab survey location]
The investigation position (survey surface) of the slab is preferably a stationary part, but may be an unsteady part. Here, the “unsteady part” is a part cast when the casting conditions are changed, such as a part cast at the beginning of casting (when the casting speed is increased) or at the end of casting (when the casting speed is lowered). It is done. When investigating in the unsteady part, as shown in FIG. 8, it is preferable to use a part adjacent to the site where the HIC test is performed. Since such a portion shows the same HIC property as the HIC test result, more accurate evaluation can be performed.

  Next, examples using the determination method of the present invention will be described. Table 1 and FIGS. 9 and 10 show experimental conditions and experimental results for determining the threshold.

<Casting>
Under the casting conditions shown in Table 1, a slab was cast using a vertical bending continuous casting machine. And the slab was cut | disconnected in the position (stationary part) where the full length of a slab is 10-15 [m]. Here, the stationary part is a part that satisfies the following conditions.
1) The casting speed is constant.
2) No more operation such as clogging of immersion nozzle has occurred.
3) Cooling conditions have not changed.
4) The roll gap has not changed.
Also, 21 charges were cast to determine the threshold. And the horizontal crack was investigated by 21 charges, and center segregation was investigated by 7 charges.

Next, the conditions shown in Table 1 will be described.
(Components of molten steel in tundish)
The concentrations of C, Mn, Nb, P, and Ca were measured by emission spectroscopy. Since the S concentration was low, it was difficult to measure by emission spectroscopic analysis. Therefore, the combustion-infrared absorption method was used for measuring the S concentration.
(Casting conditions)
-Specific water amount Specific water amount = (Total secondary cooling water amount per unit time from directly under the mold to the final roll of the continuous casting machine [l / min.]) / (Cast slab weight per unit time [kg / min.])
-Casting speed It is the drawing speed [m / min.] Of the slab, and was calculated from the diameter (peripheral length) of the roll (major roll) contacting the slab and the rotational speed (number of rotations per unit time).
(Rolling conditions)
-Rolling end surface temperature It is the surface temperature [° C] of the steel sheet at the end of rolling, and was measured with a radiation thermometer.
-Cooling start surface temperature It is the surface temperature [° C] of the steel sheet at the start of cooling after rolling, and was measured with a radiation thermometer.
Cooling rate This is the average surface cooling rate [° C./S] during cooling after rolling.

<Investigation of horizontal cracking>
Work was done in the following order.
(1) The range of D / 2 from the both ends in the width direction of the slab cut surface was polished to # 800.
(2) The polished surface was corroded with picric acid (20 g / L), cupric chloride (5 g / L) and a surfactant (60 ml / L).
(3) The corroded surface was visually confirmed, and a portion where a horizontal crack was present was cut into a size of 40 mm × 70 mm.
(4) The cut sample was buffed and finished to a roughness of 1 μm or less.
(5) Line analysis was performed on the Mn segregation degree of the horizontal crack portion in the sample with a beam diameter of 20 μm using EPMA (C _max (Mn)).
(6) C _max (Mn) / C ₀ (Mn) was calculated from the Mn concentration (C ₀ (Mn)) and C _max (Mn) of the molten steel in the tundish measured during casting.
(7) Horizontal cracks in the portion where EPMA was performed were observed with a microscope (20 to 50 times), and the thickness of the hole was measured.

<Investigation of central segregation>
Work was done in the following order.
(1) The width WD range excluding D / 2 from both ends in the width direction of the slab cut surface was polished to # 800.
(2) The polished surface was corroded with picric acid (20 g / L), cupric chloride (5 g / L) and a surfactant (60 ml / L).
(3) The maximum segregation particle size was calculated by the following method.
(a) The range of width WD excluding D / 2 from both ends in the width direction of the slab is divided into sections of 110 mm in the width direction, and the major axis a of the segregated grains existing in the range of ± 15 mm from the thickness center in each section The minor axis b was measured visually using a straight scale.
(b) The equivalent circle diameter (particle diameter) ds of the segregated grains was calculated from the following formula.
π × a / 2 × b / 2 (elliptical area) = π × (ds / 2) ²
ds = (a × b) ^0.5
(c) The maximum particle size ds _max out of the particle size ds of all sections was defined as the maximum segregated particle size (the maximum diameter of the segregated particles).
(4) The number density of segregated grains having a predetermined diameter or more was calculated by the following method.
(a) A range of width WD excluding D / 2 from both ends in the width direction of the slab is divided into sections of 110 [mm] in the width direction, and each section exists in a range of ± 15 [mm] from the thickness center. The number of segregated grains (segregated grains larger than a circle having a diameter of 1.2 [mm]) was visually counted.
Here, whether or not the segregated grains were larger than a circle having a diameter of 1.2 [mm] was confirmed by superimposing a transparent sheet on which the circle having a diameter of 1.2 [mm] was printed on the segregated grains.
(b) The number density of each section was calculated from the following formula.
Number density = number / area of one section
= Number / (0.11 [m] × 0.03 [m])

<Rolling conditions>
The slab was rolled in the casting direction until the slab thickness 280 [mm] reached 45 [mm]. Note that rolling was not performed in the slab width direction.

<HIC test>
(a) The product after rolling was divided into sections of 110 mm in the width direction, and samples of width 110 [mm] × thickness 20 [mm] × 30 [mm] in the casting direction were cut out from each section.
(b) An HIC test was performed on each sample. The HIC test was performed according to the method specified in NACE standard TM0284-2003.
(c) After the HIC test, the sample was cut at three locations, and each cross section (three cross sections) was observed with a microscope to check for cracks.

These results are shown in Table 1 and FIGS. FIG. 9 shows the relationship between “opening thickness” and “C _max (Mn) / (Co (Mn))” and the HIC property. FIG. 10 shows the relationship between “maximum segregated grains” and “number density of segregated grains having a predetermined diameter or more” and the HIC property.

<Determination of threshold>
From FIG. 9, HIC did not occur when the hole thickness ≦ 0.054 [mm], but HIC sometimes occurred when the hole thickness> 0.054 [mm]. Therefore, the threshold value of the opening thickness was set to 0.054 [mm] (t _θ = 0.054 [mm]). Note that no HIC occurred in horizontal cracks (opening thickness = 0 [mm]) that were not open.
In the slab to be judged from the above threshold,
(a) When the hole thickness ≦ 0.054 [mm], it is determined that no HIC occurs.
(b) When the hole thickness is> 0.054 [mm], it is determined that HIC occurs.

From FIG.
i) When the maximum segregated particle size ≦ 1.30 [mm], no HIC was generated.
ii) In the case of 1.30 [mm] <maximum segregation particle size <1.86 [mm], a case where HIC occurred and a case where HIC did not occur were mixed. In this range, the boundary of whether or not HIC occurs can be expressed as y = −4166.67 × x + 8083.33,
When y ≦ −4166.67 × x + 8083.33, no HIC occurs,
It was found that HIC occurred when y> −4166.67 × x + 8083.33.
Here, x is “maximum segregation particle diameter”, and y is “number density of segregation grains larger than a circle having a diameter of 1.2 mm”.
iii) When the maximum segregation particle size ≧ 1.86 [mm], HIC was generated.

Therefore, the threshold function of the maximum segregated particle diameter and number density (number density of segregated grains larger than a circle having a diameter of 1.2 mm) was set as follows.
・ X = 1.30 [mm]
・ Y = −4166.67 × x + 8083.33 (1.30 [mm] <x <1.86 [mm])
・ X = 1.86 [mm]
And
i) The HIC generation range is the following two ranges,
1.30 [mm] <x <1.86 [mm] and y> −4166.67 × x + 8083.33
X ≧ 1.86 [mm] (y includes all values)
ii) The HIC non-occurrence range was set to the following two ranges.
・ X ≦ 1.30 [mm] (y includes all values)
1.30 [mm] <x <1.86 [mm] and y ≦ −4166.67 × x + 8083.33
From the above, in the slab to be judged,
(a) When x ≦ 1.30 [mm], it is determined that no HIC occurs regardless of the value of y.
(b) For 1.30 [mm] <x <1.86 [mm],
If y ≦ −4166.67 × x + 8083.33, it is determined that no HIC occurs,
When y> −4166.67 × x + 8083.33, it is determined that HIC occurs.
(c) When x ≧ 1.86 [mm], it is determined that HIC occurs regardless of the value of y.

  And the HIC property of the judgment object slab was evaluated using the threshold value, the HIC occurrence range, and the HIC non-occurrence range. The results are shown in Table 2.

Here, the investigation method and the evaluation method in Table 2 will be described.
<Horizontal cracking>
(a) The range of width D / 2 from both ends in the width direction of the slab cut surface to be judged was milled, and a dye penetration test (JIS Z2343) was performed.
(b) When horizontal cracks were not detected (no cracks), it was determined that the aperture thickness was below the detection lower limit (less than about 10 μ [m]). Since this thickness is equal to or less than the threshold value 0.054 [mm], it was determined that no HIC caused by horizontal cracking occurred (tests a and c).
(c) When a horizontal crack was detected (with cracks), the holed portion was buffed and the polished surface was observed with a 20 to 50 times microscope to measure the hole thickness (test b). , d, e).
In test b, since the hole thickness was 0.04 [mm] (<threshold 0.054 [mm]), it was determined that no HIC caused by horizontal cracking occurred.
On the other hand, in tests d and e, the hole thicknesses were 0.11 [mm] and 0.16 [mm] (> threshold value 0.054 [mm]), so it was determined that HIC occurred due to horizontal cracking. .

<Center segregation>
・ Test a
The largest maximum segregation particle diameter in all sections is 1.41 [mm], and the maximum number density (number density of segregation grains larger than a circle having a diameter of 1.2 mm) is 1000 [pieces / m ² ] (<threshold value 2208). .33), which is within the HIC non-occurrence range. Therefore, it was determined that no HIC caused by center segregation occurred.
・ Test b
The largest maximum segregation particle diameter in all sections is 1.36 [mm], and the maximum number density (number density of segregation grains larger than a circle having a diameter of 1.2 mm) is 1333 [pieces / m ² ] (<threshold 2416). .67) and is in the HIC non-occurrence range. Therefore, it was determined that no HIC caused by center segregation occurred.
・ Test c
The largest maximum segregation particle diameter in all sections is 2.52 [mm] (> threshold value 1.86), and the maximum number density (number density of segregated grains larger than a circle having a diameter of 1.2 mm) is 2000 [pieces / piece. m ² ], which is within the HIC generation range. Therefore, it was determined that HIC caused by center segregation occurred.
・ Test d
The largest maximum segregation particle diameter in all sections is 1.15 [mm] (<threshold value 1.30), and the maximum number density (number density of segregated grains larger than a circle having a diameter of 1.2 mm) is 0 [piece / m ² ] and is in the HIC non-occurrence range. Therefore, it was determined that no HIC caused by center segregation occurred.
・ Test e
The largest maximum segregation particle diameter in all sections is 2.75 [mm] (> threshold value 1.86), and the maximum number density (number density of segregation grains larger than a circle having a diameter of 1.2 mm) is 3000 [pieces / m ² ], which is within the HIC generation range. Therefore, it was determined that HIC caused by center segregation occurred.

  From the above results, in Experiment Nos. A and b, it was determined that no HIC was generated, and the product was applied to sour-resistant steel (the destination was not changed). Moreover, when the HIC test was conducted for confirmation, no HIC was generated. Thus, the determination result was the same as the test result.

On the other hand, in Experiment Nos. C to e, it was determined that HIC occurred, and the product destination was changed to X60 for the line pipe.
For confirmation, when the HIC test is performed,
(i) In experiment No. c, HIC occurred in the range of width WD (center segregation investigation range) excluding D / 2 from both ends in the width direction of the slab,
(ii) In Experiment No. d, HIC was generated in the range of width D / 2 (horizontal crack investigation range) from both ends in the width direction of the slab,
(iii) In Experiment No. e, HIC was generated in both the range of slab width D / 2 (horizontal cracking investigation range) and width WD (central segregation investigation range).
Thus, the determination result was the same as the test result. Even if the product destination was changed, there was no problem in the product characteristics.

  In Experiment Nos. A and b, the period from casting start to shipment (casting → rolling → shipment) was 19 days. On the other hand, in the method of evaluating the HIC property by the HIC test, the period from casting start to shipment (casting → rolling → HIC test → shipment) required a long period of 28 days. In this example, since the HIC test could be omitted, the period from the start of casting to shipment could be greatly shortened from 28 days to 19 days.

  In Experiment Nos. C to e, when remelting was started at the slab stage, the period from casting start to shipment of sour-resistant steel (casting → remelting → rolling → shipping) was 54 days. . On the other hand, in the method of evaluating the HIC property of the product by the HIC test, since the remelting was started after the HIC test, the period from the start of casting to the shipment of sour-resistant steel (casting → rolling → HIC test → restarting Melting → rolling → HIC test → shipping) took 72 days. Thus, in this example, since the rolling and HIC tests could be omitted, the period from the start of casting to shipment could be greatly shortened from 72 days to 54 days.

  As described above, when the determination method of the present invention is used, the HIC property can be evaluated at the slab stage without performing the HIC test, so that the manufacturing lead time can be greatly shortened.

  Further, when 50 charges were cast in another test and the HIC property of each charge was evaluated, it was determined that no HIC was generated at 47 charges, and HIC was determined to be generated at the remaining three charge slabs. After that, when the HIC test was performed for confirmation, HIC did not occur in the above 47 charges, and HIC occurred in the remaining 3 charges. Thus, the determination result according to the present invention is the same result as the HIC test result, and it can be said that the determination method of the present invention has high accuracy. Moreover, although the product manufactured with the remaining 3 charges was applied to the line pipe material in which HIC property is not requested | required, the problem on quality did not arise.

  As mentioned above, although embodiment of this invention was described based on drawing, it should be thought that a specific structure is not limited to these embodiment. The scope of the present invention is defined by the terms of the claims, rather than the description above, and includes all modifications within the scope and meaning equivalent to the terms of the claims.

  For example, in the above-described embodiment and example, as shown in FIG. 2, first, the “opening thickness” of the horizontal crack was measured, and then the “maximum segregation particle size” and “number density” were measured (S1 → S2, S6 to S10), the order of these may be changed. For example, after measuring “maximum segregation particle diameter” and “number density”, “opening thickness” of horizontal cracks may be measured (S2 → S1, etc.). Further, measurement of “opening thickness”, “maximum segregation particle size” and “number density” of horizontal cracks may be performed at the same time.

Furthermore, in the above-described embodiment, when evaluating the HIC property of the judgment target slab (in FIG. 2), first, the “opening thickness t” of the horizontal crack is compared with the threshold value t _θ, and then “ The maximum segregation particle size d ”and“ number density m ”” were compared with “HIC generation range” determined from threshold values d _θ and m _θ (S6 → S7 → S8 → S10 → S11). May be changed. For example, after comparing “maximum segregation particle diameter d” and “number density m” with “HIC generation range”, “hole thickness t” may be compared with threshold t _θ (S10 → S11 → S6). → S7 → S8).

Further, in the above-described embodiment, when evaluating the HIC property of the determination target slab (see FIG. 2), the presence / absence of the “opening thickness” of the horizontal crack is confirmed in step S6, but step S6 may be omitted. Good. For example, the opening thickness t = 0 when "opening Thickness" is not, may compare the t = 0 and the threshold t _theta.

  Furthermore, in the above-described embodiments and examples, “number density of segregated grains having a predetermined diameter or more” is set to “number density of segregating grains having a diameter of 12 [mm] or more”, but the predetermined diameter is 12 [mm]. It is not limited to and can be changed.

In addition, in the above-described embodiment, when the “maximum segregation particle size” and the “number density” are measured, in FIGS. 3 and 5, the range of the width WD excluding D / 2 from both ends in the width direction of the slab ( It has been shown a case where region _{R 3)} was separated in the width direction into two or more predetermined interval _{r 1, r 2, r 3} ··· r n, the region _{R 3} may be one section.

  Moreover, the measuring method of "opening thickness", "maximum segregation particle size", and "number density" is not limited to the method described in the above-described embodiment, and other methods may be used.

  Furthermore, in the above-described embodiments and examples, the slab obtained by the vertical bending type continuous casting caster has been described. However, the determination method of the present invention can also be applied to a slab cast by a casting caster having another configuration.

  Since this invention can evaluate HIC property in the stage of a slab, without performing an HIC test, it can utilize for the quality judgment method of the slab for the steel plates used in order to transport oil and natural gas containing hydrogen sulfide. it can.

1 tundish 2 immersion nozzle 3 mold 4 roll 5 cooling nozzles 6 the molten steel 100 caster R _{1, R 2} region (first range)
R ₃ region (second range)

Claims (1)

A quality judgment method for judging whether a slab of thickness D cast by a continuous casting machine can be applied to sour steel,
In a plurality of slabs cast under different casting conditions, a first opening thickness measurement step for measuring the maximum opening thickness of an internal crack existing in a first range of width D / 2 from both ends in the width direction of the slab;
In a plurality of slabs cast under different casting conditions, when the second range of the width WD excluding D / 2 from both ends in the width direction of the slab is divided into one or more predetermined sections in the width direction, In each of the predetermined sections, a first segregated particle measuring step for measuring the maximum diameter of the segregated particles and the number density of the segregated particles having a predetermined diameter or more at the center of the thickness;
For the steel material produced by rolling a plurality of slabs cast under the same conditions as the slabs in the first hole thickness measurement step and the first segregated grain measurement step, the first range and the second range A test process for performing an HIC test in each of the areas corresponding to one or more predetermined sections of the range;
Based on the plurality of maximum opening thicknesses obtained in the first opening thickness measurement step and the plurality of test results in the region corresponding to the first range in the test step, the threshold value of the maximum opening thickness at which no HIC is generated. A threshold value determining step for determining
Wherein the first polarized析粒measuring a plurality of number density of the plurality of maximum diameter of the polarized析粒and polarization析粒on the predetermined diameter or less obtained in step, one or more predetermined section of the second range in the test step and a plurality of test results of the region corresponding to the HIC generation range determination step of determining the maximum diameter and the scope of the number density of polarized析粒on the predetermined size or less polarized析粒that HIC is generated,
A second hole thickness measurement step for measuring the maximum hole thickness of an internal crack existing in the range of width D / 2 from both ends in the width direction of the slab to be determined;
In the range of width WD excluding D / 2 from both ends in the width direction of the slab to be determined, when this range is divided into one or more predetermined sections in the width direction, the thickness in each of the predetermined sections a second polarization析粒measuring step of measuring the maximum diameter and the number density of polarized析粒on the predetermined size or less polarized析粒in the center,
The maximum aperture thickness obtained in the second aperture thickness measurement step is compared with the threshold value of the maximum aperture thickness determined in the threshold determination step, and the maximum of the segregated particles obtained in the second segregation particle measurement step. diameter and then compared with the maximum diameter and the scope of the number density of polarized析粒on the predetermined size or less polarized析粒that HIC was determined by the number density and the HIC generation range determining step of polarization析粒occurs on the predetermined size or less The segregated particles obtained when the maximum pore thickness obtained in the second pore thickness measurement step exceeds the threshold value of the maximum pore thickness determined in the threshold value determination step and in the second segregation particle measurement step. maximum diameter and number density of polarized析粒on said predetermined diameter or less is in the maximum diameter and the scope of the number density of polarized析粒on the predetermined size or less polarized析粒that HIC was determined by the HIC generation range determining step occurs in If you meet at least one of the times Toward destination change process according to the quality determination of the sour steel slab, characterized in that it comprises a changing step of changing the directed destination elephant slab to products other than sour steel.