JP6126503B2 - Redirecting method based on quality judgment of sour line pipe steel slabs - Google Patents

Description

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

  Steel materials such as line pipes are used for transportation of oil and natural gas, but oil and natural gas may contain a lot of hydrogen sulfide. In a hydrogen sulfide atmosphere, hydrogen enters and diffuses into the steel material, and accumulates and gasifies the inclusions in the steel material. Thereby, hydrogen-induced cracking (hereinafter referred to as HIC) is generated by applying an internal pressure to the steel material. Therefore, when oil and natural gas contain a lot of hydrogen sulfide, the above steel materials are required to have excellent HIC resistance (hereinafter, line pipe steel materials used in an atmosphere containing a lot of hydrogen sulfide are referred to as sour line pipes). Called steel).

  It is known that HIC tends to occur starting from segregation parts (center segregation, internal cracks) of the slab, particularly inclusions such as MnS. Therefore, I) a method for suppressing the generation of MnS (Patent Document 1) and II) a casting method for reducing segregation itself (Patent Documents 2 to 7) have been proposed.

  In I), generation of MnS is suppressed by adding Ca in secondary refining. Such a method is also disclosed in Patent Document 8. However, when the Ca addition amount exceeds the appropriate amount, CaO inclusions are generated. CaO inclusions become a starting point for HIC generation by agglomerating and coarsening in molten steel.

  In particular, a product having a relatively high strength grade (for example, a product corresponding to X65, X70) is likely to generate MnS because of its high Mn content. In order to suppress this, a large amount of Ca is added, but CaO inclusions are easily generated as the amount of Ca added increases. Therefore, HIC due to CaO is likely to occur.

  On the other hand, II) is a method that assumes a steady state, so when an operation abnormality occurs, for example, deviation of roll interval, clogging of immersion nozzle, drift of molten steel injected from immersion nozzle, change of casting speed, etc. When this occurs, the center segregation and internal cracks may be poor in the slab. HIC occurs at such a defective portion.

  Therefore, even if the methods I) and II) are carried out, the aggregated and coarsened CaO inclusions and the poor degree of segregation are generated, so that HIC can be generated. Since such products cannot be shipped as sour line pipe steel, the HIC test is performed before shipping, and only products that do not generate HIC are shipped as sour line pipe steel.

  However, the HIC test takes several weeks before the results are known. In addition, when HIC occurs, the product cannot be shipped as sour line pipe 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, it is considered that the manufacturing period can be greatly shortened. For example, when the HIC test is performed, the following long process is performed from casting to remelting. “Casting → Rolling → Sample adjustment (for HIC test) → HIC test → Remelting”
If HIC properties can be evaluated at the slab stage, “casting → HIC properties → remelting”
Only the above steps are sufficient, and “rolling → sample adjustment → HIC test” in the case of performing the HIC test can be omitted. Therefore, remelting can be started early after casting.

In addition, even if the product can be shipped without remelting, if the HIC test is performed, the following long process is carried out from casting to shipment. “Casting → Rolling → Sample adjustment (for HIC test) → HIC test → shipment"
If HIC properties can be evaluated at the slab stage, “Casting → Evaluation of HIC properties → Rolling → Shipping”
Since only the above-described steps are necessary, and “sample adjustment → HIC test” in the case of performing the HIC test can be omitted, the product can be shipped early.

  As described above, since HIC occurs starting from segregation parts (center segregation, internal cracks) and CaO inclusions, if these can be evaluated at the stage of the slab, HIC properties can be evaluated based on the evaluation results. Conceivable.

  As such a method, Patent Document 9 discloses a method of evaluating internal cracks at the stage of a slab. In this method, whether or not the HCR operation is possible is determined from the evaluation result of the internal crack.

  Moreover, although it does not evaluate CaO inclusions, Patent Documents 10 to 14 disclose a method for evaluating the quality of a slab before rolling. For example, in Patent Documents 10 to 13, the quality of the slab is evaluated from the amount of inclusions, the amount of elements, etc. of the slab and the molten steel in the tundish. In Patent Document 14, the quality of the slab is evaluated from the analysis result of the molten steel in the tundish (primary determination). If the determination accuracy does not satisfy the predetermined accuracy, the quality of the slab is determined from the analysis result of the slab sample. (Secondary judgment). Furthermore, in patent document 15, the quality of steel materials is evaluated from the amount of inclusions in steel materials.

JP 2010-189722 A 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 JP 54-121262 A Japanese Patent No. 4397825 JP-A-62-277539 JP 2002-214222 A JP-A-10-122854 Japanese Patent Laid-Open No. 10-249505 JP 2000-292418 A JP 2006-226951 A

  By the way, the internal cracks which are a problem in the sour line pipe steel are very small fine cracks, but in Patent Document 9, the internal cracks which are a problem in HCR operation (large cracks with a crack length of 10 mm or more). Is evaluated. For this reason, in the above method, a fine internal crack that causes a problem in the sour-line pipe steel may be missed, so that the HIC property caused by the internal crack cannot be accurately evaluated at the slab stage.

  Moreover, although HIC is easy to generate | occur | produce not only from an internal crack but a center segregation, patent document 9 does not describe the evaluation method of a center segregation. Therefore, it is considered that the method of Patent Document 9 cannot evaluate the HIC property due to the segregated portion (center segregation, internal crack) at the stage of the slab.

  On the other hand, as a method for evaluating CaO inclusions, it is conceivable to evaluate from the amounts of inclusions and elements of cast slabs and molten steel in tundish as in Patent Documents 10 to 14.

  In order to evaluate CaO inclusions at the slab stage, it is necessary to analyze the CaO amount (or Ca concentration) at the position where the CaO accumulation zone is generated. However, it is difficult to predict the position at which the accumulation band is generated because the position in the width direction, the thickness direction, and the casting direction of the slab varies. Moreover, even if a predetermined portion of the slab is analyzed, the analysis result is not necessarily the amount of CaO in the accumulation zone. Therefore, CaO inclusions cannot be evaluated from the analysis result of the slab.

  In addition, it is conceivable to evaluate the CaO inclusions from the amount of inclusions and elements in the molten steel in the tundish, but the CaO inclusions aggregate and accumulate even after the mold. For this reason, even if it is evaluated from the CaO amount (or Ca concentration) of the molten steel in the tundish that there is no CaO accumulation zone, there is a possibility that HIC may occur due to aggregation of CaO inclusions thereafter.

  Moreover, in patent document 15, since the amount of inclusions in steel is used, the HIC property cannot be evaluated at the stage of the slab.

  Thus, since the segregation part (center segregation, internal crack) and CaO inclusion cannot be accurately evaluated at the slab stage even if the methods of Patent Documents 9 to 15 are used, the HIC properties caused by these cannot be evaluated. .

  Therefore, an object of the present invention is to provide a method for evaluating the HIC property from the internal quality of a slab without performing an HIC test.

The present invention is mainly divided into the following three steps.
1) Step of determining the threshold value of the segregation part (the degree of segregation of internal cracks and center segregation) 2) Step of determining the threshold value of the amount of Ca decrease 3) Internal quality of the slab to be judged based on the threshold values of 1) and 2) ("Segregation degree of internal crack and center segregation" and "Accumulation degree of CaO inclusions") and determining the HIC property and destination of the product from the evaluation results

In particular,
It is a quality judgment method for judging whether a slab of thickness D cast by a continuous casting machine having a bent portion can be applied to sour line pipe steel,
The step 1) includes a first opening thickness measurement step for measuring the opening thickness of an internal crack existing in the first range of width D / 2 from both ends in the width direction of the slab,
In the second range of the width WD excluding D / 2 from both ends in the width direction of the slab, the second range is divided into one or more predetermined sections in the width direction. In each of them, a first segregated grain measuring step of measuring the maximum density of segregated grains within a range including the thickness center and the number density of segregated grains having a predetermined diameter or more;
For steel materials produced by rolling a slab cast under the same conditions as the slabs in the first hole thickness measurement step and the first segregated grain measurement step, one of the first range and the second range. Or a first test step of performing an HIC test in each of regions corresponding to a plurality of predetermined sections;
A first threshold value for determining an opening thickness at which no HIC is generated is determined from the opening thickness obtained in the first opening thickness measurement step and the test result of the region corresponding to the first range in the first test step. 1 threshold determination step;
Corresponding to the the maximum diameter and the number density of the polarized析粒on the predetermined size or less polarized析粒obtained by the first polarization析粒measurement step, one or more predetermined section of the second range in the first test step from the test results of the region has a 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
The process of 2) is a first investigation process for investigating the Ca concentration of molten steel in the tundish,
A second investigation step of investigating a Ca concentration in a range of D / 2 in the thickness direction from the reference-side surface in a slab of thickness D cast with the same charge as the first investigation step;
A second test step of performing an HIC test on a steel material produced by rolling a slab cast under the same conditions as the slab of the second investigation step;
The amount of decrease in Ca that does not generate HIC from the amount of decrease in Ca, which is a value obtained by subtracting the concentration of Ca obtained in the second investigation step from the concentration of Ca obtained in the first investigation step, and the test result of the second test step. A second threshold value determining step for determining a threshold value,
The step 3) includes a second opening thickness measurement step for measuring the opening thickness of the internal crack existing in the range of width D / 2 from both ends in the width direction of the slab to be judged,
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a range including the center,
A third investigation step for investigating the Ca concentration of the molten steel in the tundish for the charge to be judged;
A fourth investigation step of investigating a Ca concentration in a range of D / 2 in the thickness direction from the reference side surface in a slab of thickness D cast by the charge to be determined;
When the aperture thickness obtained in the second aperture thickness measurement step exceeds the aperture thickness threshold determined in the first threshold determination step, the segregated particles obtained in the second segregation particle measurement step when the maximum diameter and the number density of the 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 And a value obtained by subtracting the Ca concentration obtained in the fourth investigation step from the Ca concentration obtained in the third investigation step exceeds at least one of the Ca decrease amounts determined in the second threshold determination step. A change process for changing the destination of the slab to be judged to a product other than the sour line pipe steel.

  As described above, in the present invention, the “opening thickness of internal cracks”, “maximum diameter and number density of segregated grains”, and “Ca decrease in Ca concentration from tundish to slab” are investigated at the stage of the slab. . From these, the internal quality of the slab (“segregation degree of internal cracks and center segregation” and “accumulation degree of CaO inclusions”) can be accurately evaluated. Based on this result, the HIC property can be evaluated at the stage of the slab. . And the use of slab can be determined based on this evaluation. Thereby, since the HIC test which requires several weeks can be omitted, the period from manufacture 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 steel materials can be determined in the stage of a slab from the evaluation result, the period from manufacture to shipment can be shortened significantly.

It is a schematic diagram which shows the structure of a continuous casting machine. It is a schematic diagram explaining an internal crack, (a) has shown the slab (before rolling), (b) has shown the product (after rolling). It is a schematic diagram explaining the flow of CaO inclusion. It is a figure which shows Ca density | concentration distribution of slab (slab). It is a flowchart of the determination method of 1st Embodiment. (A) is a sectional view of a slab, (b) is an enlarged view of the section r _1. It is a figure which shows sectional drawing of a slab, and sectional drawing of a product. (A) is sectional drawing of a slab, (b) is sectional drawing of a product. It is a flowchart of the determination method of 2nd Embodiment. It is sectional drawing of a slab. 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 (X65). It is a figure which shows the result (opening thickness and HIC property) of an Example (X70). It is a figure which shows the result (the maximum segregation particle size, number density, and HIC property) of an Example (X65). It is a figure which shows the result (the maximum segregation particle diameter and number density, and HIC property) of an Example (X70). It is a figure which shows the result (Ca density | concentration of molten steel in a tundish, and Ca density | concentration of a slab) of an Example (Claim 1). It is a figure which shows the result (The minimum value of Ca density | concentration of molten steel in a tundish, and Ca density | concentration of a slab) of an Example (Claim 2). It is a figure which shows the result (Ca density | concentration of molten steel in a tundish, and Ca density | concentration of a slab) of a comparative example (Claim 1). It is a figure which shows the result (minimum value of Ca density | concentration of molten steel in a tundish, and Ca density | concentration of a slab) of a comparative example (Claim 2).

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

[First Embodiment]
Here, the determination method of the first embodiment of the present invention will be described with reference to FIGS.

  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 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. In the present embodiment, one side of the casting path Q (corresponding to the lower side of the slab) is referred to as “reference side”, and the other side of the casting path Q (corresponding to the upper side of the slab) is referred to as “anti-reference side”. .

  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 rolled into a steel sheet and applied to sour line pipe steel (hydrogen resistant induced cracking steel) and other products.

  As described above, the sour line pipe steel is required to have excellent HIC resistance (hydrogen induced crack resistance). HIC is generated starting from segregated parts and CaO inclusions. Therefore, in the present invention, the segregated portion and the CaO inclusion are evaluated at the stage of the slab, and the HIC property is evaluated based on the evaluation result.

(Segregation part)
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). Further, segregation generates a hardened phase such as MA (island martensite martensite-authentite constituent) or pearlite band. The higher the degree of segregation, the more easily the hardened phase is generated, and the HIC propagates and propagates along the hardened phase. In the present invention, in order to predict these in advance, the degree of segregation of internal cracks and center segregation is investigated, and the HIC property is evaluated at the stage of the slab.

  Segregation also exists between secondary dendrite trees (micro-segregation), but the secondary dendrite trees are very small, and HIC does not propagate or extend, so there is no problem in quality. Therefore, in the present invention, microsegregation is not an object of investigation.

  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. 2A, the “horizontal crack” is a crack existing in the range of D / 2 from the width end portion in the width direction of the slab, and is a crack 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.

When the slab is rolled, as shown in FIG. 2B, the “horizontal crack” extends, but the “other internal cracks” shrink. When HIC occurs starting from the above-mentioned cracks, HIC easily propagates and extends in “horizontal cracks”, 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 investigated.

(CaO)
HIC tends to occur starting from MnS inclusions. Therefore, the production of MnS is suppressed by adding Ca to the molten steel by secondary refining.

When the amount of Ca added to the molten steel is appropriate, CaO—Al ₂ O ₃ inclusions are generated in the molten steel. Since CaO—Al ₂ O ₃ has good wettability with molten steel, it does not agglomerate in the molten steel, remains fine, and does not adversely affect the HIC resistance.

However, when the Ca addition amount is not appropriate, for example, when excessive addition exceeding the required amount required for the suppression of MnS generation and the modification of Al ₂ O ₃ is performed, CaO—Al ₂ O ₃ intervenes in the molten steel. In addition to the product, pure CaO inclusions are produced. Since pure CaO inclusions have poor wettability with molten steel, they tend to aggregate in molten steel. Aggregated and coalesced CaO becomes coarse inclusions and induces HIC.

  Since coarse CaO inclusions have a density lower than that of molten steel, most of them are levitated and separated. However, as shown in FIG. 3, a part of the molten steel flows in the mold 3 and buoyancy is received while sinking deep into the slab, and is trapped by the solidified shell to form a CaO accumulation zone. The CaO accumulation zone is the starting point of HIC.

  Thus, if an appropriate Ca addition amount can be determined in advance, the generation of HIC due to CaO inclusions can be suppressed. To that end, it is necessary to accurately grasp the amount of inclusions in the molten steel before Ca addition, its composition, and the sulfur concentration. However, since it is impossible to grasp these in advance in actual operation, the amount of Ca added is set to an amount sufficient to suppress MnS generation. As a result, the Ca addition amount may be excessive, and as a result, a CaO accumulation zone is formed.

  Here, if the CaO accumulation zone always occurs at the same position, the accumulation degree of CaO inclusions can be grasped by analyzing the Ca concentration at that position. Further, it can be estimated from the degree of integration whether a CaO accumulation band is generated in the slab.

  However, the position where the CaO accumulation band is generated varies in the thickness direction of the slab depending on the casting conditions (the casting speed and the angle of the discharge hole of the immersion nozzle). For example, as shown in FIG. 4, in the three slabs (A to C) having different casting conditions (casting speed and angle of the discharge hole of the immersion nozzle), the position (ac) where the high concentration of Ca occurs where the accumulation band is generated. Are different. Since the position of the CaO accumulation band cannot be predicted in this way, it is difficult to evaluate whether the CaO accumulation band is generated from the degree of accumulation (Ca concentration).

  Therefore, the present inventors changed the viewpoint with respect to the survey position of the Ca concentration and focused on the position where the Ca concentration becomes low. When the CaO accumulation band is generated, it is considered that the Ca concentration is high in the CaO accumulation band, while the Ca concentration is relatively low in the position where the CaO accumulation band is not generated. Taking this into consideration, when the relationship between “Ca concentration at any position in the thickness direction of the slab” and “Ca concentration of molten steel in the tundish” when the CaO accumulation zone occurs, the accumulation zone occurs. Since the “Ca concentration of the slab” is relatively low at the position where it is not, it was found that “the value obtained by subtracting the“ Ca concentration of the slab ”from the“ Ca concentration of the molten steel in the tundish ”” becomes large.

  Then, if the “Ca concentration drop from the tundish to the slab” is large, there is no accumulation band at that position, but it is considered that a CaO accumulation band has occurred at another position. Then you can evaluate. On the other hand, when the “Ca concentration drop from the tundish to the slab” is small, it can be assumed that there is almost no difference between the Ca concentration of the tundish and the Ca concentration of the slab, that is, there is no position of high Ca concentration in the slab. . In this case, since it is considered that no CaO accumulation band has occurred in the slab, it can be evaluated that no HIC occurs.

  From the above, in the present invention, the presence / absence of the CaO accumulation band is determined using “a value obtained by subtracting“ Ca concentration of slab ”from“ Ca concentration of molten steel in tundish ”” (hereinafter referred to as “Ca reduction amount”). The HIC property is evaluated from the evaluation result.

(Evaluation of segregation part and CaO)
In the above description, the segregation part (horizontal cracking and center segregation) and the Ca decrease amount have been described as the evaluation method of the HIC property, but when the strength grade of the product is low, the HIC property is evaluated by either the segregation part or the Ca decrease amount. It is considered possible.

  For example, in a product with a relatively low strength grade (API (American Petroleum Institute) standard X60, etc.), if Ca is added in an appropriate range, HIC due to CaO does not occur. However, as for the segregation part, if an operation abnormality occurs even if the prior art methods (Patent Documents 2 to 7) are performed, the segregation cannot be reduced and HIC occurs. Therefore, it is considered that the product having a relatively low strength grade can evaluate the HIC property only from the evaluation result of the segregation part.

  However, when the strength grade is relatively high (product of API (American Petroleum Institute) standard X65 or higher), since the Mn content is large, MnS and internal cracks are likely to occur in the segregated portion. Then, although the production | generation of MnS is suppressed by increasing Ca addition amount, CaO inclusion tends to produce | generate as Ca addition amount increases. Thereby, it becomes easy to generate HIC resulting from CaO. Moreover, about HIC resulting from segregation, HIC may be generated if an operation abnormality occurs even when the prior art methods (Patent Documents 2 to 7) are performed as in the case where the strength grade is relatively low. Therefore, in a product with a relatively high strength grade, it is necessary to use a segregation part and a Ca decrease amount in order to evaluate the HIC property.

  In the present invention, not only when the strength grade is relatively low, but also when the strength grade is relatively high, the HIC property is evaluated from the segregated portion and the amount of Ca decrease. Examples of products having a relatively high strength grade include the above-described products of X65 or higher.

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

(Judgment method)
In this embodiment, 1) the threshold value of the segregation part (the degree of segregation of internal cracks and center segregation) is determined, 2) the threshold value of the amount of Ca decrease is determined, and then 3) the slab to be determined based on these threshold values. The internal quality of the product is evaluated, and the HIC property and destination of the product are determined from the evaluation result.

[Determination of threshold value of segregation part, S1 to S4 in FIG. 5]
First, the cast slab is cut in the thickness direction (direction perpendicular to the casting direction) (see FIG. 6), and the segregated portion (the degree of segregation of horizontal cracks and center segregation) is investigated.

  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 line pipe steel. This is because the amount of inclusions in the segregation part and the hardening phase change due to the change in concentration of the additive element when the steel type is different.

At the slab cut surface (see FIG. 6), 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 (S1 in FIG. 5). , First aperture 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 in which solidification proceeds from both ends (narrow surfaces) in the width direction of the slab toward the center of the width. In the regions R ₁ and R ₂ (first range), the 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 the width WD excluding D / 2 from both ends in the width direction, the solidification is not affected by the cooling on the narrow surface side (short side side). Hardly progress. Thus, horizontal cracks would be considered to occur in the region _{R 1, R 2,} to investigate the horizontal cracks in the region _{R 1, R 2} in step S1.

Here, the region R _1, if two or more horizontal cracks in each of R ₂ are present, the maximum opening up of the opening thickness of the plurality of apertures of thickness present in each region R _1, R ₂ The thicknesses are t ₁ and t ₂ . For example, if there are three horizontal cracks in the region R _1, to select the horizontal cracks having the largest opening of the three horizontal cracks, most opening and a portion (a hole thickness of the horizontal cracks are most The aperture thickness of the thick portion) is defined as “maximum aperture 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. “Horizontal cracks” are accompanied by segregation lines caused by the intrusion of concentrated molten steel between dendrite trees. When the degree is poor, holes are formed along the segregation lines. There is a correlation between “degree of segregation” and “opening thickness (opening width)”, and the larger the “opening thickness”, the higher the “segregation degree”. Since HIC is more likely to occur as the “segregation degree” is higher, it is considered that HIC is more likely to occur as the “opening thickness” is larger. Therefore, the segregation degree of “horizontal crack” can be evaluated from “opening thickness (maximum opening thickness)”, and it has been found that the HIC property can be judged by “opening thickness”.
From the above, in the present invention, the HIC property is evaluated based on the “opening thickness”.

  It is known that a fine horizontal crack whose “opening thickness” is about several tens [μm] is pressed during rolling and does not become a UT defect at the product stage, but causes HIC. Considering this, it is considered that HIC does not occur due to opening, but occurs due to a high degree of segregation.

Next, (see Fig. 6) 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. 5, 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. 6, 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. Solidification proceeds toward the thickness center from broad surface in the region R _3, the thickness center portion is the final solidified portion. Center segregation, since it occurs in the thickness center portion near the final solidified portion of the region R _3, 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 the “segregation degree” is higher as the “number density” is larger, that is, HIC is likely to occur.

  From the above, it is considered that the “segregation degree” can be evaluated based on the “maximum segregation particle diameter” and the “number density of segregation grains”, and the HIC property can be evaluated based on the evaluation. Further, since “segregated particle diameter” and “number density” can be measured visually, there is an advantage that 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. 6 (a), n predetermined sections r ₁ , r ₂ , r ₃ ... In the width direction in the vicinity of the thickness center of the region R ₃ (for example, ± 15 [mm] from the thickness center). - Separate the r _n (n is a natural number of 1 or more), to measure the "maximum polarization析粒diameter" and "number density" in each section. Here, the predetermined interval _{r 1, r 2, r 3} ··· r n, respectively, is a rectangular region having a width _{W 1} × a thickness of _{D 1.} (See FIG. 6 (b)). Further, “number density” (“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. 6B),
“Number density of segregated grains not less than a predetermined diameter” in section r ₁ = N / (W ₁ × D ₁ )

  Subsequently, the slab cast under the same casting conditions as the slab investigated in step S1 and step S2 is rolled under rolling conditions (rolling start surface temperature, rolling end surface temperature, final product thickness, etc.) for sour line pipe steel. , Manufacture products (steel). Then, an HIC test is performed on the product (S3 in FIG. 5, first test step), and the presence or absence of HIC generation is examined (HIC evaluation).

  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. When determining the threshold value of the segregation degree (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. Cannot be determined. The operating factors of i) to iii) have a great influence on horizontal cracking and center segregation, and as a result, HIC properties. Therefore, the HIC property changes if the operation factor is different. Therefore, a slab cast under the same casting conditions (operating factors) as the slab whose degree of segregation was investigated in step S1 and step S2 is used as the product for which the HIC test is performed in step S3. The “slab cast under the same casting conditions” includes the slab investigated in step S1 and step S2.

In step S3, it is checked whether HIC is generated (whether HIC is generated) in a region (product region) corresponding to the slab regions R ₁ , R ₂ , R ₃ (see FIG. 6). Further, n-number of predetermined section _{r 1} region _{R 3} in the width direction in step S2, _{r 2, r} 3 when divided into · · · _{r n,} section _{r 1, r 2, r 3} ··· r n It is checked whether HIC is generated in the area corresponding to (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. 7A, the width does not change before and after rolling, so the slab width W = the product width W. is there. 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 _1n ”. Here, the predetermined sections r ₁₁ , r ₁₂ , r ₁₃ ... R _1n 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. 7B, the width changes before and after rolling, so the slab width W <the product width Wa. It becomes. 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 _2n ". It is confirmed whether HIC has occurred in each of these regions R ₂₁ , R ₂₂ , R ₂₃ and each section r ₂₁ , r ₂₂ , r ₂₃ .

  And based on the said result, the threshold value regarding the HIC property of a segregation degree is determined (S4 of FIG. 5).

First, from the “maximum aperture thickness t ₁ , t ₂ ” obtained in step S ₁ and the “HIC test result obtained in step S 3”, “threshold value (t _θ ) for maximum aperture thickness” where no HIC occurs. Is determined (S4, first threshold value determining step).

When the threshold value _tθ is determined, the results obtained in the areas corresponding to each other in step S1 and step S3 are associated with each other.
For example, when the slab is rolled in the casting direction (see FIG. 7A),
When in step S3 (HIC test), the product area _{R 11} "HIC generation Yes", it is in the region _{R 12} "HIC generated no",
i) “HIC is present” when the hole thickness is t ₁ (in the slab region R ₁ ) (result of the product region R ₁₁ )
ii) “No HIC generation” when the hole thickness is t ₂ (in the slab region R ₂ ) (result of the product region R ₁₂ )
And

Moreover, when rolling a slab in the width direction (refer FIG.7 (b)),
In step S3 (HIC test), "there HIC generation" products area _{R 21} In, when the region _{R 12} is a "no HIC occurred",
i) “HIC occurs” when the hole thickness is t ₁ (in the slab region R ₁ ) (result of the product region R ₂₁ )
ii) “No HIC generation” when the hole thickness is t ₂ (in 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, from the “maximum segregated particle size” and “number density” obtained in step S2 and the “HIC test result obtained in step S3”, the range of the maximum segregated particle size and number density in which HIC occurs. (S4 in FIG. 5, HIC generation range determination step).

Since the segregation degree of center segregation can be evaluated by “maximum segregation particle size” and “number density of segregation particles”, the boundary (threshold) for determining whether or not HIC occurs due to center segregation is the maximum segregation particle size d. And a function f _θ (d, m) of the number density m. Therefore, in the present embodiment, the “maximum segregation particle size and number density threshold function f _θ (d, m)” is determined, and the HIC generation range is determined based on this.

When determining the threshold value, the results obtained in the regions corresponding to each other in step S2 and step S3 are made to correspond.
For example, when the slab is rolled in the casting direction (see FIG. 7A),
In step S3 (HIC test), the product region _{r 11} "HIC generation Yes", in the region _{r 12} "HIC generation Yes", ..., when a in the region _{r 1n} "HIC generated no",
i) “HIC is present” when the maximum segregation particle diameter d ₁ (in the slab region r ₁ ) and the number density m ₁ (result of the product region r ₁₁ )
ii) “HIC is present” when the maximum segregation particle diameter d ₂ (in the slab region r ₂ ) and the number density m ₂ (result of the product region r ₁₂ ).
iii) ( "HIC generated no" when the slab region of _{r n)} maximum deviation析粒diameter _{d n,} the number density _{m n} (results of product region _{r 1n)}
And

Moreover, when rolling a slab in the width direction (refer FIG.7 (b)),
In step S3 (HIC test), when the product region r ₂₁ is “HIC generated”, the region r ₂₂ is “HIC generated”,..., And the region r _2n is “HIC not generated”.
i) “Has HIC generated” when the maximum segregation particle diameter d ₁ (in the slab region r ₁ ) and the number density m ₁ (result of the product region r ₂₁ )
ii) “HIC is present” when the maximum segregation particle diameter d ₂ (in the slab region r ₂ ) and the number density m ₂ (result of the product region r ₂₂ ).
iii) ( "HIC generated no" when the slab region of _{r n)} maximum deviation析粒diameter _{d n,} the number density _{m n} (results of product region _{r 2n)}
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. Further, 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 (S4 in FIG. 5, 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 “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 segregation where no HIC occurs” “Range of particle size and number density” (range in which HIC does not occur) is determined.

[Determining the threshold value of Ca decrease amount, S5 to S9 in FIG. 5]
The molten steel in the tundish is collected and its Ca concentration (Ca _TD1 ) is analyzed (S5, first investigation step). Since the molten steel in the tundish is constantly supplied from the ladle, the Ca concentration (Ca _TD1 ) is constant regardless of the time of collection.

Next, the Ca concentration (Ca _S1 ) of the slab is investigated (S6, second investigation step). In the slab cast with the same charge as step S3, a sample is taken from a region R _{4 in} the range of D / 2 in the thickness direction from the reference side surface (hereinafter referred to as “reference side region R ₄ ”) (FIG. 8). (See (a)), and analyze the Ca concentration. As shown in FIG. 8A, the “reference side region R ₄ ” is a range of D / 2 or more and D or less in the thickness direction of the slab from the non-reference side surface.

  Since the density of the CaO inclusion is smaller than the density of the molten steel, the CaO inclusion in the molten steel is levitated due to buoyancy caused by the density difference from the molten steel. In the continuous casting machine in which the bent part and the horizontal part as shown in FIG. 1 are formed, the CaO inclusions are trapped by the solidified shell on the anti-reference side when the CaO inclusions float (see FIG. 3). It occurs on the reference side and does not occur on the reference side.

Therefore, in the present embodiment, the Ca concentration is examined in the “D / 2 in the thickness direction from the reference-side surface (reference-side region R ₄ )” in which no CaO accumulation band is generated (see FIG. 8A). Since the “Ca reduction amount” at the position where the accumulation band is not generated can be calculated from the Ca concentration in the reference side region R ₄ , the presence or absence of the CaO accumulation band can be accurately evaluated.

Then, “Ca concentration Ca _{S1 in} slab” obtained in “Step S6” is subtracted from “Ca concentration Ca _{TD1 in} tundish” obtained in “Step S5” to calculate “Ca reduction amount Ca _drop1 ” (S7). ). Ca _drop1 is represented by the following equation.
Ca _drop1 = Ca _TD1 -Ca _S1

  Next, the slab cast under the same casting conditions as the slab investigated in Step S6 is rolled under rolling conditions for sour line pipe steel to produce a product. Then, an HIC test is performed on the product (S8 in FIG. 5, second test process), and it is investigated whether HIC has occurred (evaluation of HIC property). Here, the “same casting condition” is the same condition as the “same casting condition” described in [Determination of threshold value of segregation part].

Further, as shown in FIG. 8B, the investigation range is a region R ₄₁ excluding the vicinity of the thickness center portion in the product region R ₄₀ corresponding to the non-reference side region. This is because the coarse CaO accumulation band is formed on the anti-reference side of the slab (see FIG. 3), so that HIC caused by CaO is likely to occur in the region corresponding to the vicinity of the anti-reference side surface. In addition, since HIC due to segregation is likely to occur at the center of thickness, it cannot be evaluated as HIC due to CaO. Therefore, it is examined whether or not HIC is generated in the region R ₄₁ excluding the vicinity of the thickness center portion.

Subsequently, a “threshold value (Ca _{drop θ} )” of the Ca reduction amount at which HIC does not occur is determined from “Ca reduction amount Ca _drop1 ” obtained in step S7 and “HIC test result obtained in step S8” ( S9, second threshold value determination step). In the present embodiment, the maximum amount of Ca decrease when no HIC occurs is defined as “threshold (Ca _dropθ )”.

[Evaluation of HIC property of judgment target slab, S10 to S20 in FIG. 5]
The slab to be judged is cut in the thickness direction (perpendicular to the casting direction) (see FIG. 6), and horizontally in the range of the width D / 2 (regions R ₁ and R ₂ ) from both ends of the cut surface in the width direction. It is examined whether there is a crack (S10 in FIG. 5). When the horizontal crack exists (S10: YES), the “maximum hole thickness t” of the horizontal crack is measured in the range of the width D / 2 (S11, 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 horizontal crack having the largest opening is selected, and the most open part of the horizontal crack ( The opening thickness of the portion having the thickest opening thickness is defined as the maximum opening thickness t.

Then, the “maximum aperture thickness t” is compared with the “threshold value (t _θ )” determined in step S4. When the maximum aperture thickness t> the threshold value t _θ (S12: YES), segregation of the horizontal crack portion is performed. Since the degree is high, it is determined that HIC due to horizontal cracking occurs. Since such a slab cannot be applied to the sour line pipe steel, the destination of the judgment target slab is changed to a product other than the sour line pipe steel (S13, changing step).

On the other hand, when the maximum opening thickness t ≦ threshold t θ _(S12: NO), due to the low segregation ratio of horizontal cracking unit, determines a horizontal crack HIC caused does not occur. Further, returning to step S10, even when there is no horizontal crack in the slab cut surface regions R ₁ and R ₂ (S10: NO), the segregation degree of the horizontal crack portion is low, so that HIC due to horizontal crack does not occur. Judge.

However, since there is a possibility that HIC due to center segregation may occur, the process proceeds to step S14, 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. 6), “maximum segregation particle diameter d” and “number density m of segregation grains having a predetermined diameter or more” are measured at the thickness center (S14, 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 S14 is 1.2 [mm].

Further, when 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. 6 and 7), also in step S14, 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" and "number density m" taking measurement. In this case, the “predetermined section” in step S14 is the same section as the “predetermined section” in step S2 (a rectangular area having a 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” (S15: YES), it is determined that HIC due to center segregation occurs because the segregation degree of the center segregation portion is high. 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. 6), the maximum segregated particle diameter d> the threshold value d _θ . When there is even one section (S15: YES), it is determined that HIC due to center segregation occurs, and the destination of the determination target slab is changed to a product other than the sour line pipe steel (S13).

  On the other hand, when the “maximum segregation particle diameter d” and “number density m” of the determination target slab (each section) are not in the “HIC generation range” (S15: NO), “maximum segregation particle diameter d” and “number density m”. "Is in the" HIC non-occurrence range ". In this case, since the segregation degree of the center segregation part is considered to be low, it is determined that no HIC due to center segregation occurs.

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 grains” when the diameter d "and" number density m "is not in the" HIC generation range "(S15: nO), the entire range of the region R _3" maximum polarization析粒diameter d "and" number density m "is" HIC nonoccurrence range Therefore, it is determined that no HIC due to center segregation occurs.

  However, since there is a possibility that HIC due to CaO may occur, next, the HIC property due to CaO is examined.

The Ca concentration Ca _{TD11 of the} molten steel in the tundish to be determined is investigated (S16, third investigation step).

Further, the Ca concentration Ca _S11 of the slab cast with the same charge as in Step S16 is investigated (S17, fourth investigation step). Interrogation position as in step S5, in the thickness direction from the reference surface of the slab and reference-side region R ₄ in the range of D / 2 (see FIG. 8 (a)).

Then, “Ca concentration Ca _{S11 in the} slab” obtained in “Step S17” is subtracted from “Ca concentration Ca _TD11 in the tundish” obtained in “Step S16” to calculate “Ca _drop Ca _drop ” (S18). ). Ca _drop is represented by the following equation.
Ca _drop = Ca. _TD11 -Ca _S11

Next, the “Ca drop amount Ca _drop ” is compared with the “threshold value (Ca _{drop θ 2} ) determined in step S 9”. If Ca _drop > threshold value (Ca _{drop θ 2} ) (S 19: YES), the CaO accumulation band is _{added to} the slab. Therefore, it is determined that HIC occurs. Since such a slab cannot be applied to the sour-line pipe steel, the determination target slab is applied to a product for an application other than the HIC steel (S13). On the other hand, if Ca _drop ≦ threshold (Ca _{drop θ} ) (S19: NO), it is considered that no CaO accumulation band has occurred in the slab, so it is determined that no HIC has occurred, and the slab to be determined is identified as a sour resistant line. Appropriate for pipe steel (S20).

  As described above, in this embodiment, the “horizontal crack opening thickness”, “maximum diameter and number density of segregated grains”, and “Ca concentration decrease from tundish to slab” are used for evaluation of HIC properties. . From these, the internal quality of the slab ("Segregation degree of horizontal crack and center segregation" and "Accumulation degree of CaO inclusions") can be accurately evaluated. Based on this evaluation result, the HIC property is evaluated at the stage of the slab. it can. And based on this evaluation, HIC property can be evaluated in the stage of a slab. Thereby, since the HIC test which requires several weeks can be omitted, the period from manufacture to shipment can be greatly shortened.

  Further, in this embodiment, not only the segregation part ("opening thickness of internal cracks" and "maximum diameter and number density of segregated grains") but also the amount of Ca decrease ("from tundish to slab" By using the "Ca concentration reduction amount"), the HIC property can be accurately evaluated at the slab stage even for products with a large amount of added Ca, for example, products with relatively high strength.

  Furthermore, in this embodiment, the segregation degree of the 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 line pipe steel, but the slab cast for the sour line pipe steel is made of other line pipe materials. The quality can be applied to. In particular, the level of horizontal cracking and center segregation required for sour-resistant pipe 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 cracking or center segregation), it is a good quality applicable 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 S9). 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.

[Second Embodiment]
Next, a second embodiment of the present invention will be described with reference to FIGS. The second embodiment is different from the first embodiment in a Ca decrease amount calculation method. In addition, about the structure same as 1st Embodiment mentioned above, the same code | symbol is used and the description is abbreviate | omitted suitably. The same configuration as that of the first embodiment is that among [determination of threshold value of segregation part, S1 to S4 in FIGS. 5 and 9] and [evaluation of HIC property of determination target slab], S10 to S15 in FIGS. It is.

[Determining Ca Threshold Amount, S5 to S9 in FIG. 9]
The Ca concentration (Ca _TD1 ) of the molten steel in the tundish is investigated (S5, first investigation step).

Next, in the slab cast with the same charge as in step S1, samples are taken at two or more places different in the thickness direction (see FIG. 10), and the Ca concentration of each sample is analyzed (S61). A minimum Ca concentration (Ca _min1 ) is selected from the obtained two or more Ca concentrations (Ca _S1 , Ca _{S 2} ...) (S62, second investigation step).

Then, “Ca reduction amount Ca _drop11 ” is calculated using a value obtained by subtracting “minimum slab Ca concentration Ca _min1 ” obtained in step S 62 from “Ca concentration Ca _{TD1 in} tundish” obtained in step S 5. (S4). Ca _drop1 is represented by the following equation.
Ca _drop11 = Ca _TD1 -Ca _min1

  When the survey position of the Ca concentration is one in the entire range in the thickness direction of the slab, a significantly high Ca concentration is detected when the position is an accumulation zone. Since the amount of Ca decrease calculated from the high Ca concentration is small, it is determined that no CaO accumulation band has occurred, and it is evaluated that no HIC has occurred. However, an integrated band is actually generated, which causes HIC. Therefore, when the Ca concentration survey position is one in the entire range of the slab thickness direction, the presence or absence of the CaO accumulation zone cannot be accurately evaluated, and therefore the HIC property cannot be accurately evaluated.

  Therefore, in this embodiment, the Ca concentration is investigated at two or more positions that are different in the thickness direction of the slab. Since the accumulation band exists at a specific position in the thickness direction determined by the casting conditions, a position where no accumulation band is generated can be included in the investigation object by changing the investigation position in the thickness direction.

Further, the two or more Ca concentrations (Ca _S1 , Ca _S2 ...) Obtained in step S61 include the Ca concentration in the accumulation band and the Ca concentration at the position where the accumulation band is not generated. By selecting the minimum Ca concentration (Ca _min1 ), it is possible to select the Ca concentration at a position where no accumulation band is generated. Since the amount of Ca decrease at a position where no accumulation band is generated can be calculated from this concentration, the presence or absence of the CaO accumulation band can be accurately evaluated.

Here, the formation mechanism of the accumulation band is the same between the CaO inclusion and the Al ₂ O ₃ inclusion, and the thickness of the accumulation band of the Al ₂ O ₃ inclusion is reported to be 10 [mm] (reference: ISIJ International, Vol. 43 (2003), No. 10, p.1548-1555). From this report, the thickness of the accumulation zone of CaO inclusions can also be estimated as 10 [mm]. Then, as shown in FIG. 10, if each Ca concentration investigation position is separated from the thickness direction by more than 10 [mm], even if one of the investigation positions is an accumulation band, an accumulation band is generated at the other investigation positions. It is a position that is not. For these reasons, it is preferable that the two or more survey positions are separated by more than 10 [mm] in the thickness direction. FIG. 10 shows a case where there are two survey positions and the distance in the thickness direction 1 between the two survey positions exceeds 10 [mm] (l> 10 [mm]).

Further, in the vicinity of the bent portion of the casting path Q (see FIG. 3), since CaO inclusions are captured in a wide range, the CaO integrated band is thick in the regions R ₁ and R _{2 of} D / 2 from both ends in the width direction of the slab. It occurs in a wide range in the direction (see FIG. 10). Therefore, in the regions R ₁ and R ₂ , even if the survey position of the Ca concentration is changed in the thickness direction, there is a possibility that the position where no accumulation band is generated cannot be investigated. Therefore, the Ca concentration interrogation position is mainly cooled only from broad surface side, it is preferable that the area R ₃ of the width W-D, except for the D / 2 from both ends in the width direction.

Next, the slab cast under the same casting conditions as the slab investigated in Step S61 is rolled under rolling conditions for sour line pipe steel to produce a product. Then, an HIC test is performed on the product (S8 in FIG. 5, second test process), and it is investigated whether HIC is generated in “region R ₄₁ corresponding to the vicinity of the anti-reference side surface” as in the first embodiment. (HIC evaluation, see FIG. 8 (b)). Here, the “same casting conditions” are the same conditions as the “same casting conditions” in the first embodiment.

Subsequently, a “threshold value (Ca _{drop θ} )” of the Ca decrease amount at which HIC does not occur is determined from “Ca decrease amount Ca _drop1 ” obtained in step S63 and “HIC test result obtained in step S8” ( S9, second threshold value determination step). In the present embodiment, the maximum amount of Ca decrease when no HIC occurs is defined as “threshold (Ca _dropθ )”.

[Evaluation of HIC property of judgment target slab, S10 to S20, S71 to S73 in FIG. 9]
If it is determined that no HIC due to horizontal cracking or HIC due to center segregation occurs in the determination target slab (S10 to S15, S15: NO), the HIC property due to CaO is examined.

The Ca concentration Ca _{TD11 of the} molten steel in the tundish of the determination target charge is investigated (S16, third investigation step).

Further, in the slab cast with the same charge as in step S16, the Ca concentration is investigated at two or more places different in the thickness direction (S71), and the minimum Ca is determined from the two or more Ca concentrations (Ca _S11 , Ca _S12 ...). The concentration (Ca _min1 ) is selected (S72, fourth investigation step). It is preferable that the two or more survey positions are separated from each other by more than 10 [mm] in the thickness direction. Further, when determining the threshold value (step S61 in FIG. 9), (see FIG. 10) when investigating Ca concentration in the region _{R 3} of the slab, it is preferable to investigate the Ca concentration in the region _{R 3} even step S71. More accurate evaluation can be performed by comparing the results of the corresponding regions.

Then, subtracted "obtained in step S72" minimum Ca concentration Ca _min1 slab "" from the "" Ca concentration Ca _TD11 in the tundish "obtained in step S16", it calculates the "Ca decrease Ca _drop" ( S73). Ca _drop is represented by the following equation.
Ca _drop = Ca _TD11 -Ca _min1

Next, the “Ca drop amount Ca _drop ” is compared with the “threshold value (Ca _{drop θ} )” determined in step S9. If Ca _drop > threshold value (Ca _{drop θ} ) (S19: YES), the CaO accumulation band is _{added to} the slab. Therefore, it is determined that HIC occurs. Since such a slab cannot be applied to the sour-line pipe steel, the determination target slab is applied to a product for an application other than the HIC steel (S13). On the other hand, if Ca _drop ≦ threshold (Ca _{drop θ} ) (S19: NO), it is considered that no CaO accumulation band has occurred in the slab, so it is determined that no HIC has occurred, and the slab to be determined is identified as a sour resistant line. Appropriate for pipe steel (S20).

  As described above, in the second embodiment, as in the first embodiment, the internal quality of the slab (“the degree of segregation of horizontal cracks and center segregation” and “the degree of accumulation of CaO inclusions”) can be accurately evaluated. Therefore, the HIC property can be evaluated at the stage of the slab based on this evaluation result. Based on this evaluation, the HIC property and the product destination can be determined at the slab stage. In addition, HIC properties can be accurately evaluated at the slab stage even for products with relatively high strength.

[Number of slab cross sections]
The segregation part (“segregation degree of horizontal cracking and center segregation”) and the HIC investigation may be evaluated from one section of the 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" described later (see Table 1)), and Example 2 is an example in which three cross sections of the same charge are investigated ("Examples" described later). 2 ”(see Table 1)), and all were investigated using slabs applicable to X65.
In addition, although the present Example was implemented on the conditions applicable to X65, since not only X65 but X70 or more, center segregation and internal crack formation and variation do not change, the segregation part and the HIC property are X65 and X70 or more. Even the strength grade may be evaluated from one cross section or may be evaluated from two or more cross sections.

<Opening thickness and HIC properties>
As shown in FIG. 11, in Example 1, the opening thickness was 0 [mm] in both of the two cross sections, and no HIC occurred from the horizontal crack in the HIC test.
Moreover, in Example 2, the hole thickness of three cross sections was 0.065 [mm], 0.067 [mm], and 0.066 [mm], and it was the same thickness. In all the cross sections, HIC occurred starting from the horizontal crack.
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. 12, in Example 1, the maximum segregation particle diameters of the two cross sections were 1.12 [mm] and 1.14 [mm], and the number density was 0 [pieces / m ² ]. . Further, in the HIC test, no HIC was generated starting from the central segregation portion in any cross section.
In Example 2, the maximum segregation particle diameters of the three cross sections were 2.23 [mm], 2.25 [mm], and 2.26 [mm], and the number density was all 1667 [pieces / m ² ]. 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.

  Although the slab applicable to X65 has been described above, the variation in segregation does not change in the casting direction even if the strength grade changes, so the number of cross sections to be investigated is not limited even in other strength grades.

[Slab survey location]
The slab investigation position (survey surface) is preferably a stationary part, but may be an unsteady part. The “unsteady part” is a part cast when a casting condition is changed, and includes a part cast at an initial stage of casting (when the casting speed is increased) or a final stage of casting (when the casting speed is decreased). When investigating in the unsteady part, it is preferable to investigate the part adjacent to the site where the HIC test is performed, as shown in FIG. Since such a portion shows the same HIC property as the HIC test result, more accurate evaluation can be performed.

〔Example〕
Next, examples using the determination method of the present invention will be described. Tables 1 and 2 and FIGS. 11 to 18 show experimental conditions and experimental results for determining the threshold. Table 1 shows examples according to the determination method of the first embodiment (Claim 1), and Table 2 shows examples according to the determination method of the second embodiment (Claim 2). In this example, slabs and products applicable to X65 and X70 were manufactured.

(Secondary refining)
The hot metal was dephosphorized and desulfurized in the hot metal pretreatment, and then decarburized in a converter. Thereafter, in secondary refining, the molten steel was desulfurized with an LF device, then degassed with a reflux degasser (RH), and Ca was added. Here, molten steel of 235 to 255 [ton] was processed by one refining. Moreover, the hot metal preliminary treatment method, the decarburization method in the converter, the treatment method in the LF apparatus, the degassing treatment method in RH, and the Ca addition method were carried out by commonly performed methods.

(casting)
Slabs were cast using a vertical bending continuous casting machine under the conditions shown in Tables 1 and 2. Other casting conditions were the same as those of ordinary skill in the art. Further, as the immersion nozzle, a two-hole nozzle having an ejection hole angle of 15 ° to 35 ° was used.

Here, the conditions shown in Tables 1 and 2 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).

(Segregation degree of horizontal crack and center segregation)
The slab was cut at a position having a total length of 10 to 15 [m] (stationary part), and the segregation degree of horizontal cracking and center segregation was investigated. 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.
Further, in order to determine the threshold value, slabs corresponding to X65 and X70 were each charged with 21 charges. The horizontal crack was investigated at 21 charges, and the center segregation was investigated at 7 charges.

<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])

(Ca reduction amount)
When the total length of the slab reached 10 [m], the molten steel in the tundish was collected, and the Ca concentration was investigated (25 charges). After casting, the Ca concentration of the slab was investigated. Table 1 shows the conditions under which the Ca concentration was investigated in the reference side region R of the slab. Table 2 shows the conditions under which the Ca concentration was investigated at 2 to 10 locations (N = 1 to 10 shown in Table 2) that differ in the thickness direction of the slab, and the minimum Ca concentration (Ca minimum value). The 2 to 10 positions are positions separated by more than 10 [mm] in the thickness direction.

(Rolling conditions)
The slab was rolled in the casting direction until the thickness was changed from 280 [mm] to 45 [mm]. Note that rolling was not performed in the slab width direction.
The rolling conditions shown in Tables 1 and 2 are shown below.
-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.

(HIC test)
(a) A sample was cut out from the rolled product and an HIC test was performed. The HIC test was performed according to the method specified in NACE standard TM0284-2003. In the sample to confirm “HIC due to horizontal cracking and center segregation”, the product is divided into sections of 110 mm in the width direction, and each section has a width of 110 [mm] × thickness of 20 [mm] × 20 [mm] in the casting direction. The sample was cut out and obtained.
(b) After the HIC test, the sample was cut at three locations, and each cross section (three cross sections) was observed with a microscope to confirm the presence or absence of cracks (HIC). Here, for “HIC due to horizontal cracking”, check the presence or absence of cracking in “regions R ₁₁ and R _{12 in} the range of D / 2 from both ends in the width direction of the product” (see FIG. 7A).
-Regarding “HIC caused by center segregation”, “± 15 [mm] / (280 [from the thickness center of the product in“ region R _{13 in} the range of width WD excluding D / 2 from both ends in the width direction of the product ”” mm] / 45 [mm] (= reduction ratio)) ≒ ± 2.41 [mm] or less range ”(see Fig. 7 (a))
-For “HIC due to CaO”, “15 mm from the thickness center / (280 [mm] / 45 [mm] (= reduction ratio) in the product area R ₄₀ corresponding to the anti-reference side area”) ) ≈Region excluding the range of 2.41 [mm] (R ₄₁ ) ”” to confirm the presence or absence of cracks (see FIG. 8B).

Note that “regions R ₁₁ and R _{12 in which} “ HIC due to horizontal cracking ”is confirmed” and “region R _{41 in which} “ HIC due to CaO ”is confirmed” partially overlap. In the overlapping region, “HIC due to horizontal crack” and “HIC due to CaO” coexist, but “HIC due to horizontal crack” tends to be large in the rolling direction, whereas “HIC due to CaO” tends to be small.

  Since horizontal cracks (segregation) are elongated by rolling, “HIC due to horizontal cracks” also extends in the rolling direction along the horizontal cracks, but the CaO accumulation zone (coarse CaO inclusions) is rolled. This is because it does not expand. Therefore, it is possible to distinguish which HIC from the size of the HIC.

  In order to confirm the difference between “HIC due to horizontal cracking” and “HIC due to CaO”, a product was manufactured under the conditions shown in Table 3 separately from the experiments in Tables 1 and 2, and around the HIC (HIC part) Surface analysis of Mn by EPMA was performed. When a segregation peak occurred in the vicinity of the HIC, it was considered that there was segregation of MnS, and it was judged as “HIC due to horizontal cracking”. On the other hand, when no segregation peak occurred in the vicinity of the HIC, it was considered that there was almost no MnS, and was determined as “CaO-derived HIC”. As a result, “HIC due to horizontal cracking” has a length of 13 mm or more in the rolling direction, whereas “HIC due to CaO” has a length of 8 mm or less in the rolling direction (Table 3). reference).

  Therefore, in this example, when the length in the rolling direction of the HIC is 13 [mm] or more, it is determined as “HIC due to horizontal cracking”, and when the length in the rolling direction of the HIC is 8 [mm] or less, It was judged that the HIC originated from CaO.

(Determination of threshold)
<Threshold of hole thickness>
14 and 15 show the relationship between ““ horizontal crack opening thickness ”and“ C _max (Mn) / Co (Mn) ”” and “whether or not HIC occurs”. FIG. 14 shows the result of investigating the threshold value at which HIC occurs with the component corresponding to X65 (see Tables 1 and 2), and FIG. 15 examines the threshold value at which HIC occurs with the component equivalent to X70 (see Tables 1 and 2). It is a result.

In the slab applicable to X65, HIC did not occur when the hole thickness ≦ 0.047 [mm] from FIG. 14, but HIC sometimes occurred when the hole thickness> 0.047 [mm]. It was. Therefore, in the slab applicable to X65, the opening thickness threshold is set to 0.047 [mm] (t _θ = 0.047 [mm]).

In the slab that can be applied to X65 from the above threshold,
(a) When the hole thickness ≦ 0.047 [mm], it is determined that no HIC occurs.
(b) When the hole thickness is> 0.047 [mm], it is determined that HIC occurs.

On the other hand, in the slab applicable to X70, HIC did not occur when the hole thickness ≦ 0.042 [mm] from FIG. 15, but HIC occurred when the hole thickness> 0.042 [mm]. was there. Therefore, in the slab applicable to X65, the opening thickness threshold is set to 0.042 [mm] (t _θ = 0.042 [mm]).

In the slab that can be applied to X65 from the above threshold,
(a) When the hole thickness ≦ 0.042 [mm], it is determined that no HIC is generated.
(b) It is determined that HIC is generated when the hole thickness is> 0.042 [mm].

  In FIGS. 14 and 15, no HIC occurred due to horizontal cracks (opening thickness = 0 [mm]) that were not open.

<Threshold of segregation degree of center segregation>
16 and 17 show the relationship between ““ maximum segregation particle size ”and“ number density ”” and “whether or not HIC occurs”. FIG. 16 shows the result of investigating the threshold value at which HIC occurs with the component corresponding to X65 (see Tables 1 and 2), and FIG. 17 shows the HIC with the component corresponding to X70 in Tables 1 and 2 (see Tables 1 and 2). It shows the result of investigating the threshold value at which occurrence occurs.

For slabs applicable to X65, from Figure 16
i) No HIC was generated when the maximum segregated particle size ≦ 1.26 [mm].
ii) When 1.26 [mm] <maximum segregation particle size <1.78 [mm], cases where HIC occurred and cases where HIC did not occur were mixed. In this range, the boundary of whether or not HIC occurs can be expressed as y = −3846 × x + 7178,
When y ≦ −3846 × x + 7178, no HIC occurs,
HIC occurred when y> −3846 × x + 7178.
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) HIC was generated when the maximum segregation particle size ≧ 1.78 [mm].

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 = 1.26 [mm]
Y = −3846 × x + 7178 (1.26 [mm] <x <1.78 [mm])
・ X = 1.78 [mm]

And
i) The HIC generation range is the following two ranges,
1.26 [mm] <x <1.78 [mm] and y> −3846 × x + 7178
・ X ≧ 1.78 [mm] (y includes all values)
ii) The HIC non-occurrence range was set to the following two ranges.
・ X ≦ 1.26 [mm] (y includes all values)
1.26 [mm] <x <1.78 [mm] and y ≦ −3846 × x + 7178

From the above, in the slab to be judged,
(a) When x ≦ 1.26 [mm], it is determined that no HIC occurs regardless of the value of y.
(b) For 1.26 [mm] <x <1.78 [mm],
When y ≦ −3846 × x + 7178, it is determined that no HIC occurs,
When y> −3846 × x + 7178, it is determined that HIC occurs.
(c) When x ≧ 1.78 [mm], it is determined that HIC occurs regardless of the value of y.

On the other hand, in the slab applicable to X70, from FIG.
i) When the maximum segregation particle size ≦ 1.22 [mm], no HIC was generated.
ii) When 1.22 [mm] <maximum segregation particle size <1.72 [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 = −3333 × x + 6067,
HIC does not occur when y ≦ −3333 × x + 6067,
When y> −3333 × x + 6067, HIC occurred.
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) HIC was generated when the maximum segregation particle size ≧ 1.72 [mm].

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 = 1.22 [mm]
Y = -3333 × x + 6067 (1.22 [mm] <x <1.72 [mm])
・ X = 1.72 [mm]

And
i) The HIC generation range is the following two ranges,
1.22 [mm] <x <1.72 [mm] and y> -3333 × x + 6067
・ X ≧ 1.72 [mm] (y includes all values)
ii) The HIC non-occurrence range was set to the following two ranges.
・ X ≦ 1.22 [mm] (y includes all values)
1.22 [mm] <x <1.72 [mm] and y ≦ −3333 × x + 6067

From the above, in the slab to be judged,
(a) When x ≦ 1.22 [mm], it is determined that no HIC occurs regardless of the value of y.
(b) For 1.22 [mm] <x <1.72 [mm],
When y ≦ −3333 × x + 6067, it is determined that no HIC occurs,
When y> −3333 × x + 6067, it is determined that HIC occurs.
(c) When x ≧ 1.72 [mm], it is determined that HIC occurs regardless of the value of y.

<Threshold of Ca decrease amount>
FIG. 18 shows the relationship between “Ca concentration Ca _{TD1 of} molten steel in tundish” and “Ca concentration Ca _{S1 of} slab” in Table 1 (Claim 2). FIG. 19 shows the relationship between “Ca concentration Ca _{TD1 of} molten steel in tundish” and “Minimum value Ca _min1 of Ca concentration of slab” in Table 2 (Claim 2).

From FIG. 18, in the determination method of the first embodiment, when the Ca decrease amount was 4 [ppm] or less (Ca decrease amount ≦ 4 [ppm]), no HIC occurred.
On the other hand, when the Ca reduction amount exceeded 4 [ppm] (Ca reduction amount> 4 [ppm]), the case where HIC occurred and the case where it did not occur were mixed.
From the above, it was found that the occurrence of HIC can be reliably suppressed when the amount of Ca decrease is ≦ 4 [ppm]. Therefore, in the example of the first embodiment, the threshold value of the Ca decrease amount is set to 4 [ppm] (Ca _{drop θ} = 4 [ppm]).

Further, from FIG. 19, even in the determination method of the second embodiment, when the Ca decrease amount is 4 [ppm] or less (Ca decrease amount ≦ 4 [ppm]), no HIC occurred.
On the other hand, when the Ca reduction amount exceeded 4 [ppm] (Ca reduction amount> 4 [ppm]), the case where HIC occurred and the case where it did not occur were mixed.
From the above, it was found that the occurrence of HIC can be reliably suppressed when the amount of Ca decrease is ≦ 4 [ppm]. Therefore, in the example of the second embodiment, the threshold value of the Ca decrease amount is also set to 4 [ppm] (Ca _dropθ = 4 [ppm]).

  The “Ca reduction threshold value” is determined for all products regardless of the strength grade. This is because the ease of occurrence of HIC due to coarse CaO is not related to the strength grade of the product.

  On the other hand, since horizontal cracks and segregation are affected by the Mn content, the Mn content increases as the strength grade increases. Along with this, the segregation degree (opening thickness and central segregation segregation degree) is different, and the ease of occurrence of HIC changes. Therefore, the "opening thickness threshold value" and the "center segregation segregation degree threshold value" The strength grades were determined.

(Evaluation of judgment target slab)
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 slab to be determined was a slab applicable to X65 or X70. The results are shown in Tables 4 and 5. Table 4 shows the results of the determination method of the first embodiment (Claim 1), and Table 5 shows the results of the determination method of the second embodiment (Claim 2). In the experiment of Table 5, “Ca concentration of slab” was investigated at 2 to 6 places different in the thickness direction. These investigation points are separated by more than 10 [mm] in the thickness direction.

Here, the investigation method and the evaluation method in Tables 4 and 5 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 a horizontal crack was not detected (no crack: ◯), it was determined that the hole thickness was below the detection lower limit (less than about 10 μ [m]). Since this thickness is equal to or less than the threshold (0.047 [mm] or less for X65, 0.042 [mm] or less for X70), it was determined that no HIC due to horizontal cracking occurred.
(c) When horizontal cracking was detected (with cracking: x), the portion where the hole was opened was buffed and the polished surface was observed with a 20 to 50 times microscope to measure the hole thickness.
(C-1) In the slab applicable to X65, it is judged that HIC due to horizontal cracking does not occur when the hole thickness ≦ threshold 0.047 [mm] (determination: ◯), and the hole thickness> threshold 0. When 047 [mm], it was determined that HIC caused by horizontal cracking occurred (determination: x).
(C-2) In the slab applicable to X70, it is judged that HIC due to horizontal cracking does not occur when the hole thickness ≦ threshold 0.042 [mm] (determination: ◯), and the hole thickness> threshold 0. At 042 [mm], it was determined that HIC due to horizontal cracking occurred (determination: x).

<Center segregation>
Whether the maximum segregation particle size and number density are within the HIC non-occurrence range or the HIC generation range was examined from the threshold values of each product grade, and based on this, it was determined whether HIC due to center segregation occurred.

<Ca reduction amount>
The amount of Ca decrease Ca _drop of the slab to be determined was calculated, and it was determined from the threshold whether or not HIC caused by CaO occurred.

  From the above three evaluations, the slab that was judged not to generate any HIC was applied to the sour line pipe steel (HIC judgment in the slab: ○). For confirmation, no HIC occurred when the HIC test was performed.

  On the other hand, if it is determined that any one of the HICs is generated, the product destination is changed to X65 or X70 for a general line pipe that does not require anti-HIC characteristics (HIC judgment in slab: x). For confirmation, HIC occurred when the HIC test was performed. There was no problem in product characteristics even when the product destination was changed.

  Moreover, in the example in which the HIC judgment on the slab was “◯” and the product was applied to the sour line pipe steel, the period from casting start to shipment (casting → rolling → shipping) was 19 days. On the other hand, when the HIC property was evaluated by the HIC test, the period from casting start to shipment (casting → rolling → HIC test → shipping) required 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.

  Furthermore, in the case where the HIC judgment on the slab is “x”, when remelting is started at the slab stage, the period from the start of casting to the shipment of sour line pipe steel (casting → remelting → rolling → shipping) ) Was 54 days. On the other hand, when the HIC property of the product was evaluated by the HIC test, since remelting was started after the HIC test, the period from the start of casting to the shipment of sour line pipe steel (casting → rolling → HIC test → Remelting → 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. Moreover, since the determination by the slab and the HIC test result for confirmation were the same, it can be said that the determination method of the present invention has high accuracy.

[Comparative Example 1]
In the example (Table 1), the Ca concentration of the slab was investigated in the reference side region R in order to determine the threshold value for the Ca decrease amount. In Comparative Example 1, the region other than the reference side region R (thickness from the surface opposite to the reference side) The range was less than D / 2 in the vertical direction) (see FIG. 8). Table 6 and FIG. 20 show experimental conditions and experimental results. From FIG. 20, the “Ca reduction amount” and the “HIC test result” were not regular, and the threshold value for the Ca reduction amount could not be determined.

[Comparative Example 2]
In the example (Table 2), in order to determine the threshold value for the Ca decrease amount, the Ca concentration of the slab was investigated at two or more locations in the entire range in the thickness direction. Table 7 and FIG. 21 show experimental conditions and experimental results. From FIG. 21, the “Ca reduction amount” and the “HIC test result” were not regular, and the threshold value for the Ca reduction amount could not be determined.

  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 first embodiment and the second embodiment, “slab used for determination of“ threshold of segregation part ”” and “slab used for determination of“ Ca threshold value ”” may be the same slab or different slabs. Good. Moreover, these slabs may be cast under the same casting conditions, or may be cast under different casting conditions.

  Further, in the first embodiment and the second embodiment, the “Ca threshold reduction amount” is determined after the “Segregation portion threshold” is determined (for example, “S1 to S4” in FIG. 5 → “S5 to S9”). ”),“ Ca decrease threshold value ”may be determined first (“ S5 to S9 ”→“ S1 to S4 ”). Further, the “threshold value of the segregation part” and the “threshold value of the Ca decrease amount” may be determined at the same time or may be mixed. For example, the “opening thickness” of horizontal cracks is measured (S1), the “maximum segregation particle size” and the “number density” are measured (S2), and the amount of Ca decrease is calculated (S5 to 7). A test may be performed (S3, S8), and a threshold value may be determined (S4, S9).

  In addition, in the first embodiment and the second embodiment, when evaluating the HIC property of the determination target slab, after evaluating the “HIC property due to the segregation part” (S10 to S12, S14, 15), Although “the HIC property due to CaO” was evaluated (S17 to S19), the “HIC property due to CaO” may be evaluated first (S17 to S19 → S10 to S12, S14, 15). Moreover, these may be performed simultaneously and may be mixed. For example, “HIC property due to horizontal cracking” is evaluated (S10 to S12), “HIC property due to CaO” is evaluated (S17 to S19), and finally “HIC property due to center segregation” is evaluated. Good (S14, 15). Even if the order is changed, if it is estimated that at least one HIC is generated, the destination of the slab is changed to a product other than the sour line pipe steel.

  Further, in the first embodiment and the second embodiment, when determining the threshold value of the segregation part, the “maximum segregation particle size” and the “number density” are measured after measuring the “opening thickness” of the horizontal crack. (S1 → S2 in FIGS. 5 and 9), “maximum segregation particle size” and “number density” may be measured first (S2 → S1). In addition, measurement of “opening thickness”, “maximum segregation particle size”, and “number density” may be performed at the same time.

  Furthermore, in 1st Embodiment and 2nd Embodiment, when evaluating HIC property resulting from a segregation part in the judgment object slab, after confirming "HIC property resulting from a horizontal crack" (S10 of FIG. 5, 9). S12) Although “HIC property due to center segregation” was confirmed (S14, S15), “HIC property due to center segregation” may be confirmed first (S14, S15 → S10 to S12). These may be performed at the same time.

In addition, the first embodiment and the second embodiment include a step of confirming the presence or absence of horizontal cracks when evaluating the HIC property due to horizontal cracks in the judgment target slab (S10 in FIGS. 5 and 9). This step may be omitted. For example, the opening thickness t = 0 when there is no horizontal cracking (S11) may be compared with t = 0 and the threshold t θ _(S12).

  In the first embodiment and the second embodiment, the “number density of segregated grains having a predetermined diameter or more” is set to “number density of segregated grains having a diameter of 1.2 [mm] or more”, but the predetermined diameter is 1 It is not limited to 2 [mm] but can be changed.

Furthermore, in the first embodiment and the second embodiment, when measuring the “maximum segregation particle size” and the “number density”, the width WD excluding D / 2 from both ends in the width direction of the slab in FIGS. range have been shown the case where separated in (region _{R 3)} 2 or more predetermined interval _{r 1} a in the width _{direction, r 2, r 3 ··· r} n, the region _{R 3} may be one section.

  In addition, the measurement method of “open hole 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.

  Further, in the first embodiment, when calculating the Ca decrease amount, the “Ca concentration of the molten steel in the tundish” is investigated and then the “Ca concentration of the reference side region of the slab” is investigated (in FIGS. 5 and 9). S5 → S6, S16 → S17), “Ca concentration in the reference side region of the slab” may be investigated first (S6 → S5, S17 → S16). These surveys may be conducted at the same time.

  Furthermore, in 1st Embodiment and the Example of Table 1, although the Ca density | concentration investigation position of slab was made into one place, it is good also as two or more places. In this case, any Ca concentration may be used to calculate the Ca decrease amount.

  In addition, in the second embodiment, when calculating the Ca decrease amount, the “Ca concentration of the molten steel in the tundish” is investigated, then the “Ca concentration of the slab” is investigated, and the “minimum Ca concentration” is selected. (S5 → S61 → S62, S16 → S71 → S72 in FIG. 9), the order of these may be changed. For example, “Ca concentration of slab” may be investigated, and “Minimum Ca concentration” may be selected and then “Ca concentration of molten steel in tundish” may be investigated (S61 → S62 → S5, S71 → S72 → S16). . Further, the investigation of “Ca concentration of slab” and “Ca concentration of molten steel in tundish” may be performed at the same time.

  Moreover, in 2nd Embodiment, when investigating Ca density | concentration of a slab, although 2 or more investigation positions are all made into the same width direction position (refer FIG. 10), you may change these width direction positions.

  Furthermore, the investigation position of the Ca concentration of the slab in the second embodiment is not limited to 2 to 10 places, and may be 11 or more places.

  In addition, in the above-described embodiment, the spark discharge emission spectrometry is used for the analysis of the Ca concentration, but other methods such as an atomic absorption analysis may be used.

  Moreover, in 1st Embodiment and 2nd Embodiment, although the continuous casting machine which has a vertical part in FIG. 1 was illustrated, you may use the continuous casting machine which does not have a vertical part.

  Furthermore, in the above-described embodiments and examples, slabs and products applicable to X65 and X70 have been illustrated, but the determination method of the present invention can also be applied to slabs and products of strength grades of X60 and X80 or higher. .

  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)
R ₄ region

Claims (1)

It is a quality judgment method for judging whether a slab of thickness D cast by a continuous casting machine having a bent portion can be applied to sour line pipe steel,
A first hole thickness measuring step for measuring an 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 the second range of the width WD excluding D / 2 from both ends in the width direction of the slab, the second range is divided into one or more predetermined sections in the width direction. In each of them, a first segregated grain measuring step of measuring the maximum density of segregated grains within a range including the thickness center and the number density of segregated grains having a predetermined diameter or more;
For steel materials produced by rolling a slab cast under the same conditions as the slabs in the first hole thickness measurement step and the first segregated grain measurement step, one of the first range and the second range. Or a first test step of performing an HIC test in each of regions corresponding to a plurality of predetermined sections;
A first threshold value for determining an opening thickness at which no HIC is generated is determined from the opening thickness obtained in the first opening thickness measurement step and the test result of the region corresponding to the first range in the first test step. 1 threshold determination step;
Corresponds to the maximum diameter of the segregated grains obtained in the first segregated grain measuring step and the number density of segregated grains greater than or equal to the predetermined diameter, and one or more predetermined sections in the second range in the first test step. HIC generation range determination step for determining the range of the maximum density of the segregated grains in which HIC is generated and the number density of segregated grains having a predetermined diameter or more from the test results of the area to be
A first investigation step for investigating the Ca concentration of the molten steel in the tundish;
A second investigation step of investigating a Ca concentration in a range of D / 2 in the thickness direction from the reference-side surface in a slab of thickness D cast with the same charge as the first investigation step;
A second test step of performing an HIC test on a steel material produced by rolling a slab cast under the same conditions as the slab of the second investigation step;
The amount of decrease in Ca that does not generate HIC from the amount of decrease in Ca, which is a value obtained by subtracting the concentration of Ca obtained in the second investigation step from the concentration of Ca obtained in the first investigation step, and the test result of the second test step. A second threshold value determining step for determining a threshold value;
A second aperture thickness measurement step for measuring the aperture thickness of the internal crack existing in the range of the 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 segregated grain measuring step for measuring the maximum diameter of the segregated grains within a range including the center and the number density of segregated grains having the predetermined diameter or more;
A third investigation step for investigating the Ca concentration of the molten steel in the tundish for the charge to be judged;
A fourth investigation step of investigating a Ca concentration in a range of D / 2 in the thickness direction from the reference side surface in a slab of thickness D cast by the charge to be determined;
When the aperture thickness obtained in the second aperture thickness measurement step exceeds the aperture thickness threshold determined in the first threshold determination step, the segregated particles obtained in the second segregation particle measurement step When the number density of segregated grains having the maximum diameter and the predetermined diameter or more is within the range of the maximum diameter of segregated grains generated by the HIC determined in the HIC generation range determining step and the number density of segregated grains having the predetermined diameter or more. And a value obtained by subtracting the Ca concentration obtained in the fourth investigation step from the Ca concentration obtained in the third investigation step exceeds at least one of the Ca decrease amounts determined in the second threshold determination step. Change method for changing the destination of the slab to be judged to a product other than the sour line pipe steel, and changing the destination by quality judgment of the sour line pipe steel slab .