JP2017173100A - Determination method of hic resistance performance in slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correctly determine a HIC resistance performance in a slab stage.SOLUTION: A determination method of a HIC resistance performance in a slab 1 creates a transfer image 5 of a solidification structure from a cut surface of the slab 1 which is vertical to a direction of casting upon determination of the HIC resistance performance in the stage of the slab 1 defines the area of the slab 1 toward the inside of a curve along a thickness direction from a position of a center D/2 of the thickness direction which is the area (W-D) of the center side in the width direction in the transfer image 5 and the area of a2 toward the outside of the curve as a determination area 6. The maximum particle size from among segregation particles 7 located inside the determination area 6 is obtained as the maximum segregation particle size d[mm]. In a final solidification position 8 inside the determination area 6, when a columnar crystal ratio r obtained by dividing a total of a length in the width direction of the area in which neither branching columnar crystal 3 nor an equated crystal 4 exists by a (W-D) is 0.95≤r<1 or 0≤r<0.95, the plus 1 is determined to satisfy the HIC resistance performance in the case that the maximum segregation particle size d satisfies the desired relationship.SELECTED DRAWING: Figure 11

Description

  The present invention relates to an HIC resistance determination method for determining HIC resistance at a slab stage for a continuously cast slab. More specifically, the present invention relates to sour steel slab quality judgment that accurately determines HIC resistance, which is one of the qualities, in the slab stage in sour steel used in an atmosphere containing hydrogen sulfide. It is about technology.

  Conventionally, natural gas contains highly corrosive hydrogen sulfide, and a steel material (hereinafter referred to as sour steel) having improved corrosion resistance to a hydrogen sulfide environment has been used for a natural gas transport pipe. Under the hydrogen sulfide atmosphere, hydrogen sulfide hydrogen penetrates into the steel, and hydrogen gas is generated around the inclusions present in the steel (for example, inclusions such as MnS and Nb (C, N)). Hydrogen induced cracking (HIC) occurs due to internal pressure. Therefore, in the sour-resistant steel, it is preferable to suppress the occurrence of central segregation, which is an inclusion generation site, and the use of techniques such as those shown in Patent Documents 1 to 3 prevents the occurrence of hydrogen-induced cracking. .

  For example, Patent Document 1 discloses a quality determination method for determining whether a slab cast by a continuous casting machine having a bent portion can be applied to sour-resistant steel. In the quality judgment method of Patent Document 1, first, the step of measuring the opening thickness of the internal crack existing in the range of the width D / 2 from both ends in the width direction of the slab, and D / 2 were removed from both ends in the width direction of the slab. The step of measuring the maximum diameter of the segregated grains and the number density of segregated grains having a predetermined diameter or more at the thickness center of the region is performed. By performing these steps, the HIC generation range can be determined. Next, in the method of Patent Document 1, the threshold value of the Ca decrease amount at which HIC does not occur is determined from the Ca decrease amount obtained by subtracting the slab Ca concentration from the Ca concentration of the tundish molten steel and the result of the HIC test. When the determination target slab exceeds the threshold value of the Ca reduction amount with respect to the threshold value determined in this way, it is determined that the cast slab cannot be applied to sour steel, and the destination of the slab is determined. Change to a non-sour resistant product.

  Patent Document 2 discloses a continuous casting method in which a low carbon steel having a carbon content of less than 0.1% by mass is continuously cast. A method of rolling down a slab including an unsolidified portion on the downstream side using a rolling roll pair is disclosed. In the method of Patent Document 2, an equiaxed crystal ratio is 6 by placing an electromagnetic stirrer at a position 3 m to 7 m upstream of the most upstream side roll pair and applying electromagnetic force to the unsolidified molten steel. It is described that it is possible to obtain a cast structure suppressed to not more than%. In the method of Patent Document 2, it is also described that 40% or more of the unsolidified portion thickness of the slab including the unsolidified portion is reduced using a reduction roll pair. In the continuous cast slab cast by the continuous casting method of Patent Document 2, the total area ratio of granular or linear semi-macro segregation or macro segregation occurring in a region within 20% of the central portion in the plate thickness direction is 5% or less. It is said that it is possible to produce a steel sheet having excellent hydrogen-induced cracking resistance.

  Furthermore, Patent Document 3 discloses a continuous casting method capable of producing a slab with very little occurrence of center segregation. In the continuous casting method of Patent Document 3, the molten steel in the tundish is heated so that the heating degree is 25 ° C. or more, and the heated molten steel is cast into a continuous casting mold to start solidification of the molten steel. . Then, a static magnetic field with a magnetic field strength of 0.15 Tesla or more is applied within 4 minutes from the start of solidification to the slab at the initial stage of solidification drawn out from the mold, and the slab is applied while applying a static magnetic field to the slab immediately before final solidification. By lightly reducing the slab, it is possible to produce a slab with very little center segregation.

JP, 2014-172074, A JP 2005-305517 A JP-A-6-608

By the way, in the quality judgment method of patent document 1, although the generation | occurrence | production state of a segregated grain is determined by the thickness center part of a sour-resistant steel slab, this thickness center part is only the middle of the thickness direction, and solidification structure | tissue It has nothing to do with it. Therefore, it does not mention the influence of the solidified structure on the generation of segregated grains.
Further, in a solidified structure such as an unsteady portion, a branched columnar crystal or an equiaxed crystal may exist at the center of the slab thickness. In the region where such branched columnar crystals or equiaxed crystals exist, the calculation result of the threshold value is different from the case where only the columnar crystals exist in the center of the thickness, and the HIC resistance is accurately determined at the slab stage. It is difficult to make a judgment.

  Furthermore, the equiaxed crystal ratio in Patent Document 2 is a value obtained from the entire cross section of the slab perpendicular to the casting direction, and the results of the thickness other than the central portion of the slab that are not so much related to the HIC resistance and both ends in the width direction. It is included. In addition, Patent Document 2 does not mention how the position or region where the equiaxed crystal is generated affects the HIC resistance, and the HIC resistance has a concentration of solute components at the end of solidification. Therefore, it is not clear whether the equiaxed crystal ratio can be used as a factor for evaluating the HIC resistance.

Furthermore, the method of Patent Document 2 is to reduce the total area ratio of segregation by reducing the equiaxed crystal ratio. Whether it can be evaluated is unknown.
On the other hand, in the method of Patent Document 3, the solidified structure at the position where the solidification is finally performed (final solidification position) is converted into a columnar crystal. If the solidified structure is a columnar crystal, the resulting HIC resistance is reduced. It only discloses qualitatively that it will improve. Therefore, the HIC resistance when a branched columnar crystal or equiaxed crystal is present in the solidified structure, in other words, the HIC resistance when a branched columnar crystal or equiaxed crystal is present, is not quantitatively evaluated.

  The present invention has been made in view of the above-mentioned problems, and accurately determines HIC resistance at the slab stage for a continuously cast slab when a solidified structure other than columnar crystals exists at the final solidification position. An object of the present invention is to provide a method for determining the HIC resistance of a slab that can be used.

In order to solve the above problems, the method for determining the HIC resistance in the slab of the present invention employs the following technical means.
That is, according to the method of determining the HIC resistance of a slab of the present invention, when determining the HIC resistance of a product manufactured from the slab at the slab stage, the slab is perpendicular to the casting direction of the slab. The solidified tissue transfer image is created from the cut surface, the slab thickness in the created transfer image is D [mm], and the slab width is W [mm]. (WD) region on the center side in the width direction excluding D / 2 from both ends in the width direction of the slab in the image, from the position of the center D / 2 in the thickness direction toward the inside of the curve along the thickness direction a1 [mm, 10 ≦ a1 ≦ D / 2] region and a2 [mm, 10 ≦ a2 ≦ D / 2] region from the center D / 2 position in the thickness direction toward the curved outer side along the thickness direction And the maximum segregation grain size d among the segregated grains located in the judgment area. [mm] and is equiaxed or branched columnar between the columnar dendrites generated from the surface on the long side of the slab toward the final solidification position at the final solidification position existing in the determination region. Measure the total length in the width direction of the region in which there is no Wc [mm], and calculate the columnar crystal ratio r = Wc / (WD) by dividing the measured Wc by (WD), When the calculated columnar crystal ratio r satisfies 0.95 ≦ r <1, the maximum segregated particle diameter d satisfies the relationship of the formula (1), or the calculated columnar crystal ratio r is 0 ≦ r <0.95 When the maximum segregated particle diameter d satisfies the relationship of the formula (2), the slab is determined to satisfy the HIC resistance.

  According to the method for determining the HIC resistance of a slab of the present invention, when a solidified structure other than columnar crystals exists at the final solidification position, the HIC resistance is accurately determined at the slab stage for a continuously cast slab. can do.

It is the figure which showed the state of the heat | fever input and heat extraction in continuous casting. It is the figure which showed the relationship between the segregation worst deterioration frequency and the width direction position. It is the figure which showed the difference of the last solidification position and the thickness direction center part (D / 2). It is the figure which showed the relationship between columnar crystal ratio and the maximum segregation particle size. It is the figure which expanded and showed the A section of FIG. It is the figure which showed the final solidification position in the cross section of a slab perpendicular | vertical to a casting direction. It is the figure which showed the measurement area | region of the segregation particle size in the cross section of a slab perpendicular | vertical to a casting direction. It is the figure which showed the calculation method of the segregation particle size of a segregation grain. It is the figure which showed typically the solidification structure of a columnar crystal, a branched columnar crystal, and an equiaxed crystal. It is the figure which showed the relationship between the inclination of the columnar crystal with respect to the thickness direction, and the segregation degree. It is the figure which showed the calculation method of the columnar crystal ratio. It is the figure which showed the structure of the continuous casting machine. It is the photograph which expanded and showed the solidification structure in which the columnar crystal has occurred. It is the photograph which expanded and showed the solidification structure in which the branched columnar crystal has occurred. It is the photograph which expanded and showed the solidification structure in which the equiaxed crystal has occurred.

[First Embodiment]
Hereinafter, an embodiment of a “method for determining HIC resistance in a slab” according to the present invention will be described in detail with reference to the drawings.
In the determination method of the present invention, when the slab 1 for sour-resistant steel used in an environment where hydrogen sulfide exists (sour environment) is continuously cast, the cast slab 1 is sufficient for hydrogen-induced cracking. It is determined at the stage of the slab 1 whether it has resistance (HIC resistance).

  In other words, steel materials (hereinafter referred to as sour-resistant steel) used in natural gas transport pipes containing highly corrosive hydrogen sulfide are required to have characteristics that can be used even in an environment (a sour environment) in a hydrogen sulfide atmosphere. More specifically, in an atmosphere where hydrogen sulfide is present, hydrogen in hydrogen sulfide penetrates into the steel, and the infiltrated hydrogen is in the vicinity of inclusions (for example, MnS, Nb (C, N), NbN, etc.). And hydrogen-induced cracking (Hydrogen Induced Cracking, hereinafter simply referred to as HIC). For example, in the case of a continuously cast slab 1, there are many inclusions at the center of the slab 1 such as this, and in order to evaluate the HIC resistance of sour-resistant steel, the inclusions are generated. It is known that it is necessary to know the state of occurrence of central segregation, which is a site.

  Conventionally, a method for evaluating HIC resistance from the internal quality of the slab (center segregation state inside the slab) has been proposed, and the standard of center segregation necessary to satisfy the HIC resistance is shown. For example, a method for indicating a standard in terms of Mn segregation using concentration mapping analysis has been proposed. However, when performing concentration mapping analysis, mirror polishing is required, and in order to investigate the degree of segregation of segregated grains satisfying the HIC resistance, it is necessary to perform mapping after mirror polishing. In other words, this method requires time for the mirror polishing and is expensive.

  A method for evaluating HIC resistance from the size and number density of segregated grains has been proposed. With this method, HIC resistance can be evaluated quickly and inexpensively at the slab stage. However, as the survey progresses, it has been found that some slabs have different HIC resistance even if the segregation particle size and density are equivalent. It was. In other words, the segregated grains change in HIC resistance not only by size and number but also by the solidified structure. For example, in the solidified structure inside the slab, dendrite (dendritic metal crystal, which will be referred to in the following specification) grows along the thickness direction of the slab (the direction along the short side of the cross section perpendicular to the casting direction). Branched columnar crystals that grow in a direction that intersects the thickness direction (other than simply columnar crystals) (columnar crystals that grow in a direction different from the columnar crystals that grow along the short side direction described above) In this specification, there is also a branched columnar crystal to distinguish it from a columnar crystal growing along the thickness direction), and there is no difference in the growth of the crystal structure between the three axes. There are also equiaxed crystals.

As a result of the investigation, it was found that many such branched columnar crystals and equiaxed crystals exist in slabs with inferior HIC resistance, and it was found that these slabs and slabs with many columnar crystals have different HIC resistance. did.
In other words, if the state of occurrence of branched columnar crystals and equiaxed crystals is the same solidification structure, the segregation degree is the same, so by determining the threshold value for determining the HIC resistance for each solidification structure of the slab, Although it is possible to appropriately evaluate the HIC resistance between each other, it is necessary to determine a threshold value for each solidified structure (for each crystal structure) when the state of occurrence of branched columnar crystals and equiaxed crystals differs. In addition, the process for determining the threshold value increases, and a large amount of cost and a long time are required for the evaluation.

Therefore, in the determination method of HIC resistance of the present invention, even when the branched columnar crystal 3 or the equiaxed crystal 4 exists in addition to the columnar crystal 2, the HIC resistance is determined from the segregation level (segregation state) at the center of the slab 1. I am trying to evaluate easily. Specifically, the determination method of the HIC resistance of the present invention includes the following operations (A) to (E) when determining the HIC resistance of a product manufactured from the slab 1 at the stage of the slab 1. It is intended to do.
Operation (A):
“The slab 1 is cut in a direction perpendicular to the casting direction of the slab 1 and a transfer image 5 of the solidified tissue is created from the cut surface.”
Operation (B):
“When the thickness of the slab 1 in the transfer image 5 created by the operation of (A) is D [mm] and the width of the slab 1 is W [mm], D from both ends in the width direction of the slab 1 in the transfer image 5 (W−D) region excluding / 2, which is a1 [mm, 10 ≦ a1 ≦ D / from the center D / 2 position in the thickness direction toward the inside of the curve along the thickness direction. 2) and the region a2 [mm, 10 ≦ a2 ≦ D / 2] from the position of the center D / 2 in the thickness direction toward the curved outer side along the thickness direction are determined as the determination region 6. ”
Operation (C):
“The maximum particle size is obtained as the maximum segregated particle size d [mm] among the segregated particles 7 located in the determination region 6 determined by the operation (B).”
Operation (D):
“At the final solidification position 8 existing in the determination region 6 determined by the operation of (B), the equiaxed crystal 4 or between the columnar dendrites generated from the long side surface of the slab 1 toward the final solidification position 8. The total length in the width direction of the region where the branched columnar crystal 3 does not exist is measured as Wc [mm], and the columnar crystal ratio r = Wc / (WD) is calculated by dividing the measured Wc by (WD). To do. "
Operation (E):
“When the maximum segregation particle diameter d satisfies the relationship of formula (1) when the columnar crystal ratio r calculated by the operation of (D) is 0.95 ≦ r <1, or the calculated columnar crystal ratio r When the maximum segregation particle diameter d satisfies the relationship of the formula (2) when 0 ≦ r <0.95, it is determined that the slab 1 satisfies the HIC resistance. ”
In the present specification, in the cross section of the slab 1 perpendicular to the casting direction, the direction along the short side of the slab 1 (the direction from the curved inner periphery to the curved outer periphery) is referred to as the thickness direction. A direction along the side (a direction crossing the slab 1 in a direction perpendicular to the thickness direction) may be referred to as a width direction.

Next, the operations (A) to (E) described above that constitute the HIC resistance determination method of the present invention will be described.
The operation (A) is to cut the slab 1 in a direction perpendicular to the casting direction of the slab 1 and create a transfer image 5 of the solidified tissue from the cut surface.
The reason why the slab 1 is cut in such a manner as to be perpendicular to the casting direction is as follows.

That is, the transfer image 5 of the cut surface is actually created, and the solidified tissue is confirmed from the transfer image 5. Then, the location where the segregation is worse can be confirmed from the transfer image 5 of the solidified tissue. This is because a segregated grain 7 having a large segregated grain size is generated at a place where the segregation is considered to be the worst.
For example, FIG. 1 is a diagram illustrating heat input to the slab 1 and heat removal from the slab 1 as images in a continuous casting facility. As shown in FIG. 1, in the molten steel stored in the mold 9 of the continuous casting equipment, the molten steel flow discharged from the immersion nozzle 10 has variations in the width direction of heat input to the solidified shell and the width direction distribution of secondary cooling. Exists. In addition, there is an influence such as support by the roll of the continuous casting machine (for example, the presence or absence of the bearing box 11), the slab 1 has variations in the width direction of heat removal, and the slab 1 in the continuous casting machine has a uniform heat. Have no distribution. Therefore, in the center of the slab 1 where crystal structures grown from the front surface and the back surface of the slab 1 collide with each other, a difference occurs in the generation state of the segregated grains 7 due to the nonuniformity of the heat distribution. 7 is confirmed in the transfer image 5 as a difference in the solidified tissue located at the center of the slab 1. This is the reason why the solid-state transferred image 5 is created from the cut surface of the slab 1 perpendicular to the casting direction in the operation (A).

  The operation (B) is performed when the thickness of the slab 1 in the transfer image 5 created by the operation (A) described above is D [mm] and the width of the slab 1 is W [mm]. This is a (WD) region on the center side in the width direction excluding D / 2 from both ends in the width direction of the slab 1, and from the position of the center D / 2 in the thickness direction toward the inside of the curve along the thickness direction a1 [ mm, 10≤a1≤D / 2], and a2 [mm, 10≤a2≤D / 2] area from the center D / 2 position in the thickness direction along the thickness direction toward the outside of the curve The region 6 is determined.

The reason why the region on the cut surface of the slab 1 is set as the determination region 6 is as follows.
That is, the segregated grains 7 having a large particle size are formed in the solidified structure due to the non-uniformity of the heat distribution described above, and the segregated grains 7 having a large particle size are segregated. Can be evaluated as worsening.

For example, the actual slab 1 is used to determine the position in the width direction where the segregated grain 7 has the maximum grain size (the maximum segregated grain size d is confirmed) in the transferred image 5 of the solidified structure of the cut surface. The results shown in Fig. 2 are obtained.
As shown in FIG. 2, when the occurrence frequency (number of occurrences) in which the maximum segregated particle diameter d is confirmed in the transfer image 5 of the solidified structure of the cut surface is arranged for each position in the width direction, in the range of WD, It has been found that the maximum segregation particle diameter d can be confirmed at any position, in other words, the center segregation can be most deteriorated. The range of WD is a range obtained by subtracting 1/2 (= 1/2 × D) of the thickness of the slab 1 from both ends in the width direction from the full width W of the slab 1 as shown in FIG. Indicated.

  On the other hand, as described above, dendrites (dendritic columnar crystals) grow from the surface of the slab 1 toward the center along the thickness direction substantially parallel to the thickness direction, and the dendrite grown from the front side of the long side of the slab 1 A dendrite grown from the back side of the long side collides with the center of the slab 1 to form a solidified structure. Here, the final solidification position where the solidification is finally performed is the position farthest from the surface of the slab 1 when the heat distribution is uniform and the cooling is performed uniformly. The thickness center is not necessarily the final solidification position 8 because it is affected by operating conditions such as the heat distribution of the molten steel, secondary cooling, and variations in operation.

For example, as shown in FIG. 9, it is possible that the final solidification position 8 meanders on the cut surface and becomes a position away from the thickness center. However, it is also true that segregated grains 7 are likely to be generated at the final solidification position 8, and it is necessary to always evaluate the final solidification position 8 when evaluating the segregation state.
Here, since the final solidification position 8 can be confirmed by visual observation of the solidified structure or the like, when the difference between the confirmed final solidification position 8 and the thickness center of the slab 1 is measured, a result as shown in FIG. 3 is obtained.

  As shown in FIG. 3, when the solidification state is evaluated with the measurement range of less than ± 10 mm in the thickness direction from the thickness center (thickness direction D / 2) of the slab 1, the final solidification position 8 exists outside the measurement range. There is a possibility that the final solidification position 8 where the solidification state should be evaluated most is not included in the measurement range. Therefore, as shown in gray in FIG. 7, a region of a1 [mm, 10 ≦ a1 ≦ D / 2] from the position of the center D / 2 in the thickness direction toward the inside of the curve along the thickness direction, and the thickness direction If the region of a2 [mm, 10 ≦ a2 ≦ D / 2] is determined as the determination region 6 from the center D / 2 position toward the outside of the curve along the thickness direction, the final coagulation position 8 is always the determination region 6 The segregation state can be accurately evaluated.

This is because the solidification proceeds from the respective surfaces in the thickness direction and the width direction, so that the final solidification position 8 does not become larger than WD.
In the operation (C), the maximum particle size among the segregated particles 7 located in the determination region 6 determined in the operation (B) is obtained as the maximum segregated particle size d [mm]. It is known that when the segregated grains 7 are large, the degree of segregation (the value obtained by dividing the maximum solute concentration by the average solute concentration) increases (see Nippon Steel Pipe Technical Report No. 121 (1988)), and MnS that causes HIC generation And inclusions such as Nb (C, N) are easily generated (see, for example, JP-A-2002-363689), the maximum segregation particle diameter d is necessary for evaluating HIC.

That is, the segregated grains 7 actually observed in the determination region 6 have various dimensions in the width direction and the thickness direction. For example, as shown in FIG. 8, if four segregated grains α, segregated grains β, segregated grains γ, and segregated grains δ are confirmed in the solidified structure, these four segregated grains 7 have a thickness of t _α . Since t _δ and the width are W _{α to} W _δ , a simple comparison cannot be made. Therefore, as shown by the following formula (3), the square root (root mean square) of the product of the major axis and the minor axis of the segregated grain 7 is obtained, and the segregated grain 7 having the largest square root value is called the maximum segregated grain. The maximum segregated grain size indicated by is defined as the maximum segregated grain size d.

  The operation (D) is performed between the columnar dendrites generated from the long side surface of the slab 1 toward the final solidification position 8 at the final solidification position 8 existing in the determination region 6 determined by the operation (B). The total length in the width direction of the region where no branched columnar crystal 3 or equiaxed crystal 4 is present is measured as Wc [mm], and the columnar crystal ratio obtained by dividing the measured Wc by (WD) r = Wc / (WD) is calculated. In other words, the operation (D) indicates the ratio in which only the columnar crystal 2 excluding the branched columnar crystal 3 and the equiaxed crystal 4 exists in the range of (WD).

The center segregation occurs at the final solidification position 8, and HIC due to MnS and Nb (C, N) does not occur at other than the final solidification position 8. This is because the solute concentration at the end of solidification is not promoted even if the branched columnar crystal 3 or the equiaxed crystal 4 is present at a position other than the final solidification position 8.
Here, the branched columnar crystal 3 described above is one in which the main axis of the dendrite is inclined at an inclination angle of 50 ° or more with respect to the thickness direction, as shown in FIG. For example, in the case of a normal columnar crystal 2 that is not a branched columnar crystal 3, the main axis of the dendrite is often along the thickness direction as shown in FIG. 13, and the inclination of the main axis with respect to the thickness direction is less than 50 °. . On the other hand, the branched columnar crystal 3 has a large inclination of the main axis with respect to the thickness direction, has many portions with a high degree of segregation (maximum component concentration / average component concentration), and cracks tend to propagate, so HIC is likely to occur. Become.

  Specifically, as shown in FIG. 10, when the change in the segregation degree is investigated by changing the inclination of the dendrite main axis, the inclination of the main axis is 0 ° or more and less than 15 °, or 15 ° or more and less than 50 °. In comparison, the degree of segregation increases when the inclination of the main axis is 50 ° or more or when there is no directionality of the main axis. From this, when the inclination of the dendrite main axis becomes 50 ° or more, in other words, when the branched columnar crystal 3 is present, the solidification of the slab 1 becomes non-uniform, and the molten steel with concentrated solute components solidifies as it is and the segregation state deteriorates. It is thought to do.

 Further, as shown in FIG. 15, the equiaxed crystal 4 described above has the main axis of the dendrite completely lost directionality, the secondary arm (the short axis of the dendrite perpendicular to the main axis) is unclear, and the appearance is It is a solidified structure close to granularity. That is, the equiaxed crystal 4 is a crystal in which the lengths of three crystal axes orthogonal to each other are substantially equal, and the segregation degree is likely to be deteriorated similarly to the branched columnar crystal 3 described above. This can be determined from the fact that the degree of segregation increases even in the case where there is no directionality of the main axis in FIG.

If it can be determined whether the solidification structure at the final solidification position 8 belongs to the branched columnar crystal 3 and the equiaxed crystal 4 in this way, the length in the width direction where the final solidification position 8 is the columnar crystal 2 is measured. For example, as shown in FIG. 11, when there are a plurality of places where the final solidification position 8 is the columnar crystal 2, the respective widthwise lengths W _c1 , W _c2 , W _c3 are summed according to the equation (5), final solidification position 8 is determined the total value W _c in the width direction length of columnar crystals 2. Then, those final solidification position 8 is the total value W _c in the width direction length of columnar crystals 2, divided by the measurement range W-D is a columnar Akiraritsu r.

When such a columnar crystal ratio r is obtained, the ratio of the columnar crystals 2 at the final solidification position 8, in other words, how many branched columnar crystals 3 and equiaxed crystals 4 that deteriorate the segregation degree are present at the final solidification position 8. It is possible to evaluate the influence of the branched columnar crystal 3 and the equiaxed crystal 4 on the HIC resistance.
The operation of (E) is performed when the maximum segregation particle diameter d satisfies the relationship of formula (1) when the columnar crystal ratio r calculated by the operation of (D) is 0.95 ≦ r <1, or When the columnar crystal ratio r is 0 ≦ r <0.95, when the maximum segregated particle diameter d satisfies the relationship of formula (2), it is determined that the slab 1 satisfies the HIC resistance.

  That is, when either of the two conditions is satisfied, it is determined that the slab 1 satisfies the HIC resistance.

  These formulas (1) and (2) are derived from the results of actual HIC tests. That is, it was evaluated as an HIC test whether or not HIC actually occurs when both the columnar crystal ratio r and the maximum segregation particle diameter d are changed. The results of the HIC test are shown in FIGS. Based on the experimental results, the threshold was drawn with a straight line at the boundary between pass and fail of the HIC test. In other words, as is clear from FIGS. 4 and 5, the “◯” point where the HIC test passes and the “x” point where the HIC test fails are clearly distributed in the drawing. Different. Therefore, if a large number of data is secured to the extent shown in FIGS. 4 and 5, the boundary between the region where “◯” is distributed and the region where “×” is distributed can be clearly determined. The straight line to be shown can be uniquely determined. The boundary lines determined in this way are a straight line inclined at r = 0.95 as shown in FIG. 4 and a horizontal straight line shown in FIG. In other words, since the slope of the HIC test results changes at r = 0.95, the boundary line formulas are obtained separately for 0 ≦ r <0.95 and 0.95 ≦ r <1. The inequalities of the above formulas (1) and (2) are derived from the relationship between whether they are above or below the boundary line thus obtained.

  As shown in FIG. 4 and FIG. 5, when the columnar crystal ratio r is the same value, increasing the maximum segregation particle diameter d results in good HIC resistance when the maximum segregation particle diameter d is small. If the maximum segregation particle diameter d exceeds a certain limit value, the HIC resistance tends to deteriorate. Further, in the range of 0 ≦ r <0.95, the limit value of the maximum segregated particle diameter d that deteriorates the good HIC resistance increases as the columnar crystal ratio r increases. On the other hand, in the range of 0.95 ≦ r <1, the maximum segregated particle diameter d that deteriorates the HIC resistance depending on the columnar crystal ratio did not change (is constant). This is considered to be because when 0.95 ≦ r <1, there are few sites where the degree of segregation is high due to the branched columnar crystal 3 and the equiaxed crystal 4, and cracks are difficult to propagate.

From the above, by dividing the range of 0 ≦ r <0.95 and the range of 0.95 ≦ r <1, the above formula (1) and The relationship of Formula (2) can be obtained.
The present invention is intended for the case where the branched columnar crystal 3 or the equiaxed crystal 4 is present at the final solidification position 8. Only the columnar crystal 2 having a low degree of segregation is not targeted because the HIC resistance is different.

Next, the effects of the method for determining the HIC resistance in the slab 1 according to the present invention will be described in more detail using examples and comparative examples.
In Examples and Comparative Examples, the slab 1 having the composition shown in “a” to “d” in Table 1 is actually continuously cast.

In addition, as shown in FIG. 12, the continuous casting uses a casting mold 9 (230 to 280 × 900 to 2100 mm) having a two-hole immersion nozzle 10 and a casting speed of 1.0 to 1.3 m / min. Continuous casting was performed at a water amount of 0.8 to 1.4 L / kg-steel.
With respect to the slab 1 continuously cast in this manner, the slab 1 was cut perpendicular to the casting direction to create a transfer image 5 of the solidified structure.

  The transfer image 5 was created by polishing the cut surface with a buff or the like until the finish was # 800. Then, the polished surface after polishing is etched with a corrosive solution containing 20 g / L picric acid, 5 g / L cupric chloride and 60 mL / L surfactant, forcing the cut surface. Corroded. At this time, in the segregation part, the corrosion proceeds more than the part where there is no segregation, and a hole (pit) is generated in the cut surface. When etching is completed, the substrate is washed with water and dried, and a mixture of grease and carbon powder is applied to the corroded surface. When the mixture is removed from the corroded surface after application, the mixture preferentially enters a portion where corrosion has progressed (for example, a hole generated by the corrosion). Finally, the adhesive surface of the adhesive tape is affixed to the corroded polished surface, and the adhesive tape is peeled off together with the mixture of holes generated by corrosion adhering to the adhesive surface, whereby a transfer image 5 of the solidified tissue can be obtained ( (See, for example, “Transfer method of segregation of steel” in Japanese Patent No. 2790426). In the transfer image 5 of the solidified structure thus obtained, the segregated grains of the solidified structure were measured, and the maximum segregated particle diameter d was calculated. Since the segregated grains have a microsegregated grain size of about 0.25 mm, a larger width of 0.3 mm and a thickness of 0.3 mm or more were measured as segregated grains.

Moreover, about the continuously cast slab 1, the HIC resistance was evaluated by performing the HIC test.
The HIC test was performed according to the method specified in NACE standard TM0284-2003. Specifically, a specimen taken from a steel plate that has undergone a rolling process in the steel plate manufacturing process was immersed in a saturated aqueous solution (0.5% NaCl + 0.5% acetic acid) at 25 ° C. saturated with 1 atm hydrogen sulfide for 96 hours. . Each test piece after immersion is cut at a 10 mm pitch in the longitudinal direction, the cut surface is polished, and then the entire cross section is observed at a magnification of 100 times using an optical microscope, and the HIC crack length is 1 mm or more. The number of each was measured. Then, those having no cracks of 1 mm or more were evaluated as being excellent in HIC resistance (pass, ○), and those having cracks of 1 mm or more were evaluated as being inferior in HIC resistance (fail, x).

In the rolling process described above, the slab 1 is heated to 1100 ° C. or higher and hot-rolled at a reduction rate of 40% or higher in the recrystallization temperature range, and this is cooled from 780 ° C. to a cooling rate of 10-20 ° C./s. It is to cool at. In this embodiment, tempering after cooling is not performed.
Tables 2 and 3 show the evaluation results of the maximum segregation particle diameter d and HIC resistance described above.
Table 2 corresponds to the range of 0.95 ≦ r <1, and Table 3 corresponds to the range of 0 ≦ r <0.95.

Looking at Table 2 above, the evaluation of HIC resistance is “◯” until the maximum particle diameter d is 1.20, but when the maximum particle diameter d exceeds 1.20, the evaluation of HIC resistance is “×”. " From this, in the range of 0.95 ≦ r <1, it is judged that the HIC resistance is improved when the maximum particle size d ≦ 1.2.
Further, when Table 3 is viewed, Examples 22 to 36 in which the value of “0.95r + 0.3” is equal to or less than the maximum segregation particle diameter d are “◯”, but “0. In Comparative Examples 37 to 61 in which the value of “95r + 0.3” exceeds the maximum segregated particle diameter d, the evaluation of HIC resistance is “x”. From this, in the range of 0 ≦ r <0.95, it is judged that the HIC resistance is improved when the columnar crystal ratio d ≦ 0.95r + 0.3.

Moreover, according to the determination method of HIC resistance in the slab 1 of the present invention, it is possible to accurately determine the HIC resistance in one stage of the slab, so that it is possible to determine whether or not a new slab 1 casting is necessary after the slab 1 casting. Become. This eliminates the need to wait for the HIC test results after rolling, greatly reducing the production period when the HIC resistance is poor.
The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. In particular, in the embodiment disclosed this time, matters that are not explicitly disclosed, for example, operating conditions and operating conditions, various parameters, dimensions, weights, volumes, and the like of a component deviate from a range that a person skilled in the art normally performs. Instead, values that can be easily assumed by those skilled in the art are employed.

DESCRIPTION OF SYMBOLS 1 Slab 2 Columnar crystal 3 Branched columnar crystal 4 Equiaxial crystal 5 Transfer image 6 Judgment area 7 Segregated grain 8 Final solidification position 9 Mold 10 Immersion nozzle 11 Bearing box D Slab thickness W Slab width

Claims (1)

In determining the HIC resistance of a product manufactured from the slab at the slab stage,
While cutting the slab in a direction perpendicular to the casting direction of the slab, creating a transfer image of the solidified tissue from the cut surface,
When the thickness of the slab in the created transfer image is D [mm] and the width of the slab is W [mm], the center in the width direction excluding D / 2 from both ends in the width direction of the slab in the transfer image. A region of (WD), a region of a1 [mm, 10 ≦ a1 ≦ D / 2] from the position of the center D / 2 in the thickness direction toward the inside of the curve along the thickness direction, and in the thickness direction The area of a2 [mm, 10 ≦ a2 ≦ D / 2] from the center D / 2 position toward the outside of the curve along the thickness direction is set as the determination region.
Of the segregated grains located in the determination region, the maximum grain size is determined as the maximum segregated grain size d [mm],
The length in the width direction of the region where no equiaxed crystal or branched columnar crystal exists between the columnar dendrites generated from the long side surface of the slab toward the final solidification position at the final solidification position existing in the determination region. And the columnar crystal ratio r = Wc / (WD) calculated by dividing the measured Wc by (WD).
When the calculated columnar crystal ratio r satisfies 0.95 ≦ r <1, the maximum segregated particle diameter d satisfies the relationship of the formula (1), or
When the calculated columnar crystal ratio r is 0 ≦ r <0.95, when the maximum segregated particle diameter d satisfies the relationship of formula (2), it is determined that the slab satisfies the HIC resistance. A method for determining the HIC resistance of a slab.