JP2016125137A - Steel sheet and steel pipe for line pipe excellent in hydrogen-induced crack resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a steel sheet or a steel pipe excellent in hydrogen-induced crack resistance, and further to realize a steel sheet or a steel pipe allowing evaluation of hydrogen-induced crack resistance from an internal quality of a cast slab without conducting a hydrogen-induced crack test after rolling.SOLUTION: A steel sheet excellent in hydrogen-induced crack resistance satisfies regular C, Si, Mn, P, S, Al, Ca, N and O, and further, includes one or more elements selected from a group consisting of regular REM and Zr, and the balance consists of iron and inevitable impurities. The ratio of the Ca and the S (Ca/S) is 2.0 or higher. The Ca, the S and the O satisfy (Ca-1.25S)/O≤1.80. A Ca drop amount obtained by subtracting a Ca concentration of a slab from a Ca concentration of molten steel within a tundish is a threshold value Caor lower. The threshold value Cais the maximum Ca drop amount capable of avoiding a hydrogen-induced crack in a steel sheet obtained by rolling the slab.SELECTED DRAWING: Figure 6

Description

  The present invention is suitable for natural gas / crude oil transportation line pipes and storage tanks, and has excellent hydrogen-induced crack resistance and line pipe excellent in hydrogen-induced crack resistance obtained using the steel sheet. It relates to steel pipes.

  In line with the development of inferior resources containing hydrogen sulfide, so-called sour resistance, such as resistance to hydrogen-induced cracking and stress corrosion cracking, is required mainly for oil and gas transportation line pipes and storage tanks. The Hereinafter, the steel plate having sour resistance may be referred to as “sour-resistant steel plate”. Hydrogen-induced cracking (hereinafter sometimes referred to as “HIC”) is a phenomenon in which hydrogen that has penetrated into a steel material due to the corrosion reaction caused by hydrogen sulfide or the like starts with MnS or Nb (C, N). It is known that it is a crack that accumulates in non-metallic inclusions and is caused by gasification.

  It is known that HIC is likely to be generated starting from a segregated portion including center segregation of the slab, internal cracks, etc., particularly inclusions such as MnS. Thus, several techniques for improving the HIC resistance have been proposed. For example, Patent Document 1 discloses a steel material having improved HIC resistance by suppressing the segregation degree of Mn, Nb, and Ti at the center of the plate thickness. Patent Document 2 discloses a method of suppressing HIC starting from MnS or Ca-based oxysulfide by a parameter formula including Ca, O, and S contents.

  Although many HICs are suppressed by these methods, a large number of fine HICs may be generated locally.

  On the other hand, the steel sheet is obtained through melting, casting, and hot rolling, and then subjected to an HIC test before shipping as a product. However, the HIC test takes several weeks before the results are known. In addition, when HIC occurs in the HIC test, the steel sheet cannot be shipped as a product excellent in hydrogen-induced cracking resistance, and the HIC test is performed again on the product obtained by remanufacturing, that is, remelting. There is a need to do. If it does so, a manufacturing period will become long and it will cause a delay in delivery.

  Therefore, if the HIC resistance can be evaluated at the stage of the cast slab after the casting instead of performing the HIC test after the hot rolling, it is considered that the manufacturing period can be greatly shortened. As described above, since HIC occurs starting from the segregation part (center segregation, internal crack) and inclusions such as MnS, if these can be evaluated at the stage of the slab, the HIC resistance can be improved based on the evaluation result. It can be evaluated.

For example, in the conventional method of performing the HIC test after rolling, the following long process A-1 is performed from casting to shipment. On the other hand, if the HIC resistance can be evaluated at the stage of the slab, it is possible to omit “sample adjustment (for HIC test) → HIC test” when performing the HIC test as shown in the following step B-1. Can be shipped early.
Process A-1: Casting → Rolling → Sample preparation (for HIC test) → HIC test → Shipping process B-1: Casting → HIC resistance evaluation → Rolling → Shipping

Further, when the result of the HIC test is NG, in the conventional method, the following process A-2 is long from casting to remelting. On the other hand, if HIC resistance can be evaluated at the slab stage as shown in the following process B-2, even if this evaluation is NG, “rolling → (for HIC test) sample in the following process A-2” “Adjustment → HIC test” can be omitted, and remelting can be started at an early stage.
Step A-2: Casting → Rolling → Sample preparation (for HIC test) → HIC test → Remelting Step B-2: Casting → Evaluation of HIC resistance → Remelting

  As such a method, Patent Document 3 discloses a method of evaluating internal cracks at the stage of a slab. In this method, whether or not HCR (Hot Charge Rolling) operation is possible is determined from the evaluation result of the internal crack.

  Moreover, although it does not evaluate CaO inclusions, Patent Documents 4 to 8 disclose methods for evaluating the quality of a slab before rolling. For example, in Patent Documents 4 to 7, 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. Further, in Patent Document 8, the quality of the slab is evaluated from the analysis result of the molten steel in the tundish (primary determination), and the quality of the slab is determined from the analysis result of the slab sample if this determination accuracy does not satisfy the predetermined accuracy. (Secondary judgment).

JP 2010-209461 A Japanese Patent Laid-Open No. 06-136440 JP 2006-198649 A JP-A-62-277539 JP 2002-214222 A JP-A-10-122854 Japanese Patent Laid-Open No. 10-249505 JP 2000-292418 A

  Patent Documents 3 to 8 do not evaluate CaO inclusions as described above. However, as an evaluation method for CaO inclusions, the amounts of inclusions and elemental amounts of cast slabs and tundish molten steel as in Patent Documents 3 to 8 are used. It is possible to evaluate from the above.

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

  It is also conceivable to evaluate CaO inclusions from the amount of inclusions and element amounts of molten steel in the tundish. However, CaO inclusions aggregate and accumulate after injection into the mold. Therefore, 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.

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is to realize a steel plate and a steel pipe excellent in hydrogen-induced cracking resistance, and further, without carrying out a HIC test, It is to realize a steel plate and a steel pipe that can evaluate the HIC resistance from the internal quality of the piece.

A steel sheet excellent in hydrogen-induced crack resistance of the present invention that has solved the above problems is
% By mass
C: 0.02 to 0.15%,
Si: 0.02 to 0.50%,
Mn: 0.6 to 2.0%,
P: more than 0% and 0.030% or less,
S: more than 0% and 0.003% or less,
Al: 0.010 to 0.08%,
Ca: 0.0003 to 0.0060%,
N: 0.001 to 0.01%, and O: more than 0% and 0.0045% or less,
Including one or more elements selected from the group consisting of REM: more than 0% and 0.02% or less, and Zr: more than 0% and 0.010%, the balance consisting of iron and inevitable impurities,
The ratio of Ca to S (Ca / S) is 2.0 or more, and the Ca, S and O satisfy (Ca−1.25S) /O≦1.80,
Furthermore, the amount of Ca reduction obtained by subtracting the Ca concentration of the slab from the Ca concentration of the molten steel in the tundish is not _more than the threshold value Ca _dropθ , and the threshold value Ca _dropθ does not cause hydrogen-induced cracking in the steel sheet obtained by rolling the slab. It is characterized by the maximum amount of Ca decrease.

The threshold value Ca _dropθ may be a value obtained in advance by the following methods (i) to (iii).
(I) The Ca concentration of the molten steel in the tundish and the Ca concentration of the slab are measured, and the amount of Ca decrease is calculated by subtracting the Ca concentration of the slab from the Ca concentration of the molten steel in the tundish.
(Ii) A hydrogen-induced cracking test is performed on a steel plate obtained by rolling a slab cast under the same casting conditions as the slab.
(Iii) From the Ca decrease amount measured in (i) above and the hydrogen induced crack test result of (ii) above, the maximum Ca decrease amount at which no hydrogen induced crack occurs is obtained.

  The slab cast under the same casting conditions as the slab may be a slab obtained by measuring the Ca decrease amount.

  The Ca concentration of the slab may be the minimum Ca concentration of the two or more Ca concentrations obtained by examining the Ca concentration at two or more positions different in the thickness direction in the slab.

The threshold value Ca _dropθ may be 4 ppm (mass ppm).

The said steel plate may contain any one or more of following (A) and (B) as another element.
(A) By mass%, B: more than 0% to 0.005% or less, V: more than 0% to 0.1% or less, Cu: more than 0% to 1.5% or less, Ni: more than 0% to 1.5% or less Cr: more than 0% and 1.5% or less, Mo: more than 0% and 1.5% or less, and Nb: more than 0% and 0.06% or less. One or more elements selected from the group consisting of Ti: more than 0% and 0.03% or less and Mg: more than 0% and 0.01% or less

  The steel sheet is suitable for line pipes and pressure vessels. The present invention also includes a steel pipe for line pipe formed of the steel plate.

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate and steel pipe which were surely excellent in hydrogen-induced crack resistance can be provided. Furthermore, the steel plate and steel pipe which can evaluate HIC resistance from the internal quality of a slab can be provided, without performing a HIC test. These are suitably used for pressure vessels such as natural gas / crude oil transportation line pipes and storage tanks.

FIG. 1 is a schematic diagram illustrating the flow of CaO inclusions. FIG. 2 is a diagram showing Ca concentration distributions of various slabs. FIG. 3A is a cross-sectional view of a slab, and FIG. 3B is a cross-sectional view of a product. FIG. 4 is a cross-sectional view of the slab. FIG. 5 is a diagram for explaining the investigation surface of the slab. FIG. 6 shows the threshold value determination results of the first embodiment in the examples, and is a diagram showing the relationship between the Ca concentration Ca _{TD1 of the} tundish molten steel and the Ca concentration Ca _{S1 of the} slab, and the HIC test results. FIG. 7 shows the threshold value determination result of the second embodiment in the example, and shows the relationship between the Ca concentration Ca _{TD1 of the} molten steel in tundish and the minimum value Ca _min1 of the Ca concentration of the slab and the HIC test result. It is.

  The inventors of the present invention have made extensive studies to solve the above problems. First, the inventors focused on the fact that HIC is likely to be generated starting from MnS inclusions. As a result, it was conceived that the rare earth element or Zr, which is an element having a desulfurization action, can be contained in the steel material to suppress the generation of MnS and enhance the resistance to hydrogen-induced cracking. Furthermore, in order to effectively exhibit the desulfurization action, an appropriate content described later has been found.

  Next, the present inventors paid attention to the fact that HIC is likely to be generated starting from a CaO accumulation portion that is generated when a slab is manufactured. As a result, paying attention to the “Ca reduction amount obtained by subtracting the Ca concentration of the slab from the Ca concentration of the molten steel in the tundish” that can evaluate the presence or absence of the CaO accumulation portion, this Ca reduction amount is made a predetermined threshold value or less at the slab stage It was found that a steel plate with high resistance to hydrogen-induced cracking can be obtained if it is contained, and that the product can be shipped early. This point will be described in detail later.

  First, the component composition will be described. In the following, “%” means “mass%” and “ppm” means “mass ppm” for the components.

  In order to ensure excellent HIC resistance, it is necessary to control the composition of the steel material. Furthermore, as another characteristic required as a steel material for line pipes, for example, in order to ensure high strength and excellent weldability, the component composition of the steel sheet needs to be as follows. Hereinafter, the reasons for defining each component including the above-described rare earth element and Zr will be described.

(Component composition)
[C: 0.02 to 0.15%]
C is an indispensable element for securing the strength of the base material and the welded portion, and needs to be contained by 0.02% or more. The amount of C is preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the amount of C is too large, the HAZ toughness and weldability deteriorate. On the other hand, if the amount of C is excessive, NbC and island-shaped martensite that become the starting point of HIC and the fracture propagation path are likely to be generated. Therefore, the C amount needs to be 0.15% or less. Preferably it is 0.12% or less, More preferably, it is 0.10% or less.

[Si: 0.02 to 0.50%]
Si is an element that has a deoxidizing action and is effective in improving the strength of the base material and the welded portion. In order to obtain these effects, the Si content is set to 0.02% or more. The amount of Si is preferably 0.05% or more, and more preferably 0.15% or more. However, if the amount of Si is too large, weldability and toughness deteriorate. If the amount of Si is excessive, island martensite is generated and HIC is generated and progresses. Therefore, the amount of Si needs to be suppressed to 0.50% or less. The amount of Si is preferably 0.45% or less, more preferably 0.35% or less.

[Mn: 0.6 to 2.0%]
Mn is an element effective for improving the strength of the base material and the welded portion, and is contained in an amount of 0.6% or more in the present invention. The amount of Mn is preferably 0.8% or more, and more preferably 1.0% or more. However, if the amount of Mn is too large, not only MnS is produced and the hydrogen-induced cracking resistance deteriorates, but also the HAZ toughness and weldability deteriorate. Therefore, the upper limit of the amount of Mn is set to 2.0%. The amount of Mn is preferably 1.8% or less, more preferably 1.5% or less, and still more preferably 1.2% or less.

[P: more than 0% and 0.030% or less]
P is an element inevitably contained in the steel material. If the amount of P exceeds 0.030%, the toughness of the base material and the HAZ part is significantly deteriorated, and the resistance to hydrogen-induced cracking is also deteriorated. Therefore, in the present invention, the amount of P is suppressed to 0.030% or less. The amount of P is preferably 0.020% or less, more preferably 0.010% or less.

[S: more than 0% and 0.003% or less]
If S is too much, it is an element that produces a large amount of MnS and significantly deteriorates the resistance to hydrogen-induced cracking. Therefore, in the present invention, the upper limit of the amount of S is set to 0.003%. The amount of S is preferably 0.002% or less, more preferably 0.0015% or less, and still more preferably 0.0010% or less. Thus, the smaller one is desirable from the viewpoint of improving hydrogen-induced crack resistance.

[Al: 0.010 to 0.08%]
Al is a strong deoxidizing element. When the amount of Al is small, the Ca concentration in the oxide increases, that is, Ca inclusions are easily formed in the surface layer portion of the steel sheet and fine HIC is generated. Therefore, in the present invention, Al needs to be 0.010% or more. The amount of Al is preferably 0.020% or more, more preferably 0.030% or more. On the other hand, when there is too much Al content, the oxide of Al will produce | generate in cluster shape and will become the starting point of a hydrogen induced crack. Therefore, the Al amount needs to be 0.08% or less. The amount of Al is preferably 0.06% or less, and more preferably 0.05% or less.

[Ca: 0.0003 to 0.0060%]
Ca has the effect | action which controls the form of sulfide, and there exists an effect which suppresses formation of MnS by forming CaS. In order to obtain this effect, the Ca content needs to be 0.0003% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when the Ca content exceeds 0.0060%, a large amount of HIC is generated starting from Ca-based inclusions. Therefore, in the present invention, the upper limit of the Ca amount is set to 0.0060%. The Ca content is preferably 0.0045% or less, more preferably 0.0035% or less, and still more preferably 0.0025% or less.

[N: 0.001 to 0.01%]
N is an element that precipitates as TiN in the steel structure, suppresses coarsening of the austenite grains in the HAZ part, further promotes ferrite transformation, and improves the toughness of the HAZ part. In order to acquire this effect, it is necessary to contain N 0.001% or more. The N amount is preferably 0.003% or more, and more preferably 0.0040% or more. However, if the amount of N is too large, the HAZ toughness deteriorates due to the presence of solute N, so the amount of N needs to be 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.0060% or less.

[O: more than 0% and 0.0045% or less]
O (oxygen) is preferably low from the viewpoint of improving cleanliness. When a large amount of O is contained, in addition to deterioration of toughness, HIC is generated starting from oxide, and resistance to hydrogen-induced cracking is deteriorated. . From this viewpoint, the amount of O needs to be 0.0045% or less, preferably 0.0030% or less, more preferably 0.0020% or less.

[Ca / S (mass ratio): 2.0 or more]
As described above, S forms MnS as sulfide inclusions, and HIC is generated starting from the MnS. For this reason, Ca is added to control the form of the sulfide inclusions in the steel as CaS, thereby detoxifying S against HIC resistance. In order to fully exhibit this effect, Ca / S needs to be 2.0 or more. Ca / S is preferably 2.5 or more, more preferably 3.0 or more. Incidentally, the upper limit of Ca / S is about 17 from the Ca amount and S amount specified in the present invention.

[(Ca-1.25S) /O≦1.80]
In order to suppress the generation of HIC due to Ca-based oxysulfides, it is effective to suppress CaO, which easily forms aggregated coal, among Ca-based inclusions. For this purpose, the Ca amount (Ca-1.25S) obtained by subtracting the Ca component existing as sulfide (CaS) from the total Ca amount in the steel must not be excessive with respect to the O amount. When the amount of Ca (Ca-1.25S) is excessive with respect to the amount of O, CaO is likely to be formed as oxide inclusions, and the aggregated coalescence (coarse Ca-based inclusions) of the CaO is the steel sheet surface layer portion. It becomes easy to be formed in large quantities. Since these coarse Ca-based inclusions are the origin of HIC, it is necessary to set (Ca-1.25S) / O to 1.80 or less in order to obtain excellent HIC resistance. (Ca-1.25S) / O is preferably 1.40 or less, more preferably 1.30 or less, still more preferably 1.20 or less, and particularly preferably 1.00 or less. In addition, the lower limit of (Ca-1.25S) / O is about 0.1 from the viewpoint of suppressing Al ₂ O ₃ that easily forms an aggregated coal as in CaO.

[REM: more than 0% and 0.02% or less]
As described above, REM (Rare Earth Metal) is an element effective for suppressing the generation of MnS by the desulfurization action and enhancing the resistance to hydrogen-induced cracking. In order to exhibit such an effect, it is preferable to contain REM 0.0002% or more. The amount of REM is more preferably 0.0005% or more, and further preferably 0.0010% or more. On the other hand, the effect is saturated even if a large amount of REM is contained. Therefore, the upper limit of the REM amount needs to be 0.02%. From the viewpoint of increasing productivity by suppressing the clogging of the immersion nozzle during casting, the REM content is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.0050% or less. is there. In the present invention, the REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y.

[Zr: more than 0% and 0.010% or less]
Zr is an element that contributes to the improvement of HAZ toughness by improving HIC resistance by desulfurization and forming oxides and finely dispersing them. In order to exert these effects, the Zr content is preferably 0.0003% or more. The amount of Zr is more preferably 0.0005% or more, still more preferably 0.0010% or more, and still more preferably 0.0015% or more. On the other hand, when Zr is added excessively, coarse inclusions are formed and the hydrogen-induced crack resistance and the base metal toughness are deteriorated. Therefore, the amount of Zr needs to be 0.010% or less. The amount of Zr is preferably 0.0070% or less, more preferably 0.0050% or less, and still more preferably 0.0030% or less.

The components of the steel material (steel plate, steel pipe) of the present invention are as described above, and the balance consists of iron and inevitable impurities. In addition to the above elements,
(A) By containing one or more elements selected from the group consisting of the following amounts of B, V, Cu, Ni, Cr, Mo, and Nb, the strength and toughness can be further increased,
(B) By containing one or more elements selected from the group consisting of Ti and Mg in the following amounts, the HAZ toughness can be improved and desulfurization can be promoted to further improve the HIC resistance. Hereinafter, these elements will be described in detail.

[B: more than 0% and 0.005% or less]
B enhances hardenability, enhances the strength of the base metal and the welded part, and bonds with N during the process of cooling the heated HAZ part during welding, thereby precipitating BN and causing ferrite transformation from within the austenite grains. In order to promote, HAZ toughness is improved. In order to acquire this effect, it is preferable to contain B amount 0.0002% or more. The amount of B is more preferably 0.0005% or more, and further preferably 0.0010% or more. However, if the B content is excessive, the toughness between the base material and the HAZ part deteriorates or weldability deteriorates, so the B content is preferably 0.005% or less. The amount of B is more preferably 0.004% or less, and still more preferably 0.0030% or less.

[V: more than 0% and 0.1% or less]
V is an element effective for improving the strength. To obtain this effect, V is preferably contained in an amount of 0.003% or more. More preferably, it is 0.010% or more. On the other hand, if the V content exceeds 0.1%, weldability and base metal toughness deteriorate. Therefore, the V amount is preferably 0.1% or less, and more preferably 0.08% or less.

[Cu: more than 0% and 1.5% or less]
Cu is an element effective for improving the hardenability and increasing the strength. In order to acquire this effect, it is preferable to contain 0.01% or more of Cu. The amount of Cu is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the Cu content exceeds 1.5%, the toughness deteriorates, so it is preferable to set it to 1.5% or less. The amount of Cu is more preferably 1.0% or less, still more preferably 0.50% or less.

[Ni: more than 0% and 1.5% or less]
Ni is an element effective for improving the strength and toughness of the base material and the welded portion. In order to obtain this effect, the Ni content is preferably 0.01% or more. The amount of Ni is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if Ni is contained in a large amount, it becomes extremely expensive as a structural steel material. Therefore, the Ni content is preferably 1.5% or less from an economical viewpoint. The amount of Ni is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Cr: more than 0% and 1.5% or less]
Cr is an element effective for improving the strength, and in order to obtain this effect, it is preferable to contain 0.01% or more. The amount of Cr is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, if the Cr content exceeds 1.5%, the HAZ toughness deteriorates. Therefore, the Cr content is preferably 1.5% or less. The amount of Cr is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Mo: more than 0% and 1.5% or less]
Mo is an element effective for improving the strength and toughness of the base material. In order to obtain this effect, the Mo amount is preferably 0.01% or more. The amount of Mo is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the Mo amount exceeds 1.5%, the HAZ toughness and weldability deteriorate. Therefore, the Mo amount is preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.50% or less.

[Nb: more than 0% and 0.06% or less]
Nb is an element effective for increasing strength and base metal toughness without degrading weldability. In order to obtain this effect, the Nb content is preferably 0.002% or more. The Nb amount is more preferably 0.010% or more, and still more preferably 0.020% or more. However, if the Nb content exceeds 0.06%, the toughness of the base material and the HAZ deteriorates. Therefore, in the present invention, the upper limit of the Nb amount is preferably 0.06%. The Nb amount is more preferably 0.050% or less, still more preferably 0.040% or less, and still more preferably 0.030% or less.

[Ti: more than 0% and 0.03% or less]
Ti is an element effective for improving the toughness of the HAZ part because it precipitates as TiN in the steel to prevent coarsening of austenite grains in the HAZ part during welding and promote ferrite transformation. . Further, Ti is an element effective for improving the HIC resistance since it exhibits a desulfurization action. In order to obtain these effects, it is preferable to contain 0.003% or more of Ti. The amount of Ti is more preferably 0.005% or more, and still more preferably 0.010% or more. On the other hand, if the Ti content is excessive, the toughness of the base material and the HAZ part deteriorates due to an increase in solid solution Ti or an increase in TiC precipitation, so 0.03% or less is preferable. The amount of Ti is more preferably 0.02% or less.

[Mg: more than 0% and 0.01% or less]
Mg is an element effective for improving toughness through refinement of crystal grains, and is an element effective for improving HIC resistance since it exhibits a desulfurization action. In order to acquire these effects, it is preferable to contain 0.0003% or more of Mg. The amount of Mg is more preferably 0.001% or more. On the other hand, since the effect is saturated even if Mg is contained excessively, the upper limit of the amount of Mg is preferably 0.01%. The amount of Mg is more preferably 0.005% or less.

The steel plate of the present invention is a steel plate having a high hydrogen-induced cracking resistance with a Ca reduction amount obtained by subtracting the Ca concentration of the slab from the Ca concentration of the molten steel in the tundish _being equal to or less than the threshold value Ca _dropθ . Here, the threshold value Ca _dropθ means the maximum amount of Ca decrease that is obtained in advance and does not cause hydrogen-induced cracking in the steel sheet obtained by rolling the slab.

[Ca reduction amount]
As described above, by setting the Ca decrease amount obtained by subtracting the Ca concentration of the slab from the Ca concentration of the molten steel in the tundish to a predetermined threshold value or less, it is possible to obtain a steel plate with high resistance to hydrogen-induced cracking. Explain what can be shipped. Below, it demonstrates from the reason which made the said Ca fall amount the evaluation index first.

  The present inventors paid attention to MnS inclusions and advanced research on adding Ca to molten steel by secondary refining in order to suppress the formation of MnS.

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 amount of Ca added to the molten steel is not appropriate, for example, when excessive addition exceeding the required amount required for the suppression of MnS formation and the modification of Al ₂ O ₃ is performed, the molten steel contains CaO—Al. _In addition to ₂ O ₃ inclusions, pure CaO inclusions are also produced. Pure CaO inclusions tend to agglomerate in molten steel because of poor wettability with molten steel. Aggregated and coalesced CaO becomes coarse inclusions and induces HIC.

  Since the coarsened CaO inclusion has a density lower than that of the molten steel, most of the CaO inclusion floats and separates. However, as shown in FIG. 1, a part of the molten steel flows into the mold and gets into the deep part of the slab, receiving buoyancy and trapped by the solidified shell to form a CaO accumulation zone. The CaO accumulation zone is the starting point of HIC.

  Therefore, if an appropriate amount of Ca added to the molten steel can be determined in advance, generation of HIC due to CaO inclusions can be suppressed. For that purpose, it is necessary to grasp | ascertain correctly the amount of inclusions in the molten steel before Ca addition, its composition, and sulfur concentration. However, since it is impossible to grasp these in advance in actual operation, the Ca addition amount is set to a sufficient amount for suppressing MnS generation. As a result, the Ca addition amount tends to be excessive, and a CaO accumulation zone is likely to be formed.

  If the CaO accumulation band 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 CaO accumulation whether a CaO accumulation band is generated in the slab.

  However, as described above, the position where the CaO accumulation band is generated varies in the thickness direction of the slab depending on the casting conditions (such as the casting speed and the angle of the discharge hole of the immersion nozzle). For example, as shown in FIG. 2, 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 accumulation band is generated (ac). 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 for 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. In consideration of this, the relationship between “Ca concentration at an arbitrary position in the thickness direction of the slab” and “Ca concentration of molten steel in tundish” when a CaO accumulation zone occurs was examined. As a result, since “Ca concentration of slab” is relatively low at a position where no CaO accumulation zone is generated, “a value obtained by subtracting“ Ca concentration of slab ”from“ Ca concentration of molten steel in tundish ””, that is, It turned out that "the amount of decrease in Ca concentration from tundish to slab" increases.

  Then, when the “Ca concentration decrease amount from the tundish to the slab” is large, it is considered that an accumulation band is not generated at that position, but a CaO accumulation band is generated at another position. Can be evaluated as it occurs. On the other hand, when the “Ca concentration drop from the tundish to the slab” is small, there is almost no difference between the Ca concentration of the tundish and the Ca concentration of the slab, that is, it is assumed that there is no high Ca concentration position in the slab. it can. 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.

  In the present invention, a value obtained by subtracting “Ca concentration of slab” from “Ca concentration of molten steel in tundish” related to presence / absence of CaO accumulation zone (hereinafter referred to as “Ca reduction amount”) is used. It was decided to evaluate the HIC property.

[Determining the threshold value of Ca decrease amount]
Next, the threshold value Ca _dropθ of the Ca reduction amount for judging whether the obtained steel plate exhibits excellent HIC resistance, that is, the maximum Ca reduction that does not generate HIC in the steel plate obtained by rolling the slab. Explain how to find the quantity.

The threshold value Ca _dropθ is obtained in advance, but the method is not particularly limited. As a method for obtaining the threshold value Ca _dropθ , the following methods (i) to (iii) may be used in advance.
(I) The Ca concentration of the molten steel in the tundish and the Ca concentration of the slab are measured, and the amount of Ca decrease is calculated by subtracting the Ca concentration of the slab from the Ca concentration of the molten steel in the tundish.
(Ii) A hydrogen-induced cracking test is performed on a steel plate obtained by rolling a slab cast under the same casting conditions as the slab.
(Iii) From the Ca decrease amount measured in (i) above and the hydrogen induced crack test result of (ii) above, the maximum Ca decrease amount at which no hydrogen induced crack occurs is obtained.

A specific method for obtaining the threshold value Ca _dropθ will be described in detail below with reference to the following first and second embodiments.
[First Embodiment]
(Investigation of Ca concentration in molten steel in tundish)
Tundish molten steel is collected and its Ca concentration (Ca _TD1 ) is analyzed. 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.

(Investigation of slab Ca concentration)
Next, the Ca concentration (Ca _S1 ) of the slab is investigated. As shown in FIG. 3A, a sample is taken from a region R4 in the D / 2 range in the thickness direction from the reference side surface of the slab (hereinafter referred to as “reference side region R4”), and the Ca concentration Ca _S1 is determined. analyse. As shown in FIG. 3A, the “reference side region R4” is a range from D / 2 to D in the thickness direction of the slab from the non-reference side surface.

  As described above, since the density of CaO inclusions is smaller than the density of molten steel, CaO inclusions in the molten steel are levitated due to buoyancy caused by the density difference from the molten steel. As shown in FIG. 1, in a continuous casting machine in which a bent portion and a horizontal portion are formed, when CaO inclusions float, they are captured by the solidified shell on the anti-reference side, so the CaO accumulation zone is generated on the anti-reference side of the slab. Does not occur on the reference side.

Therefore, in the present invention, as shown in FIG. 3 (a), the “D / 2 in the thickness direction from the reference side surface (reference side region R4)” where no CaO accumulation band is generated, that is, in the embodiment described later, The Ca concentration Ca _S1 is investigated in the range from the center of the thickness D to −0.50 D toward the reference side surface. Since the “Ca reduction amount” at a position where no CaO accumulation band is generated can be calculated from the Ca concentration Ca _S1 of the reference side region R4, the presence or absence of the CaO accumulation band can be accurately evaluated.

Then, the “Ca reduction amount Ca _drop1 ” is calculated by subtracting the “Ca concentration Ca _{S1 of the} slab” from the “Ca concentration Ca _{TD1 in the tundish} ”. Ca _drop1 is represented by the following equation.
Ca _drop1 = Ca _TD1 -Ca _S1

(rolling)
A slab cast under the same casting conditions as the slab in which the Ca concentration Ca _S1 is measured is hot-rolled to produce a steel plate for threshold measurement. For example, rolling is performed under the following conditions. That is, after heating the slab to 1050 to 1250 ° C., the steel sheet surface temperature is 900 ° C. or higher, the steel sheet average temperature obtained by calculation as described below is 1000 ° C. or higher, and the cumulative rolling reduction is 40% or higher. Hot rolling is performed so that the pass with a rolling reduction per pass of 10% or more becomes 2 passes or more. Thereafter, hot rolling is performed so that the cumulative rolling reduction at 700 ° C. or more and less than 900 ° C. becomes 20% or more, so that the rolling end temperature becomes 700 ° C. or more and less than 900 ° C. Thereafter, water cooling is started from a temperature of 650 ° C. or higher, stopped at a temperature of 350 to 600 ° C., and then air cooled to room temperature. The steel sheet average temperature is determined as follows. That is, based on data such as a rolling pass schedule during rolling and a cooling method (water cooling or air cooling) between passes, the temperature at an arbitrary position in the plate thickness direction is calculated using a method suitable for calculation such as a difference method, Let the average value of the temperature from the surface of the obtained steel piece to the back surface be steel plate average temperature.

(HIC test)
Then, an HIC test is performed on the steel sheet to examine whether or not HIC is generated. The HIC test may be performed by a method defined in NACE (National Association of Corrosion and Engineer) standard TM0284-2003, as shown in the examples described later.

  As shown in FIG. 3B, the target region of the HIC test is a region R41 excluding the vicinity of the thickness center portion in the product region R40 corresponding to the non-reference side region. As shown in FIG. 1, the coarse CaO integrated band is easily formed on the anti-reference side of the slab, and the 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 R41 excluding the vicinity of the thickness center portion.

(Determination of threshold)
Subsequently, a threshold value Ca _dropθ of a Ca drop amount at which no HIC is generated is determined from the “Ca drop amount Ca _drop1 ” and the “HIC test result”. By comparing a plurality of Ca reduction amounts Ca _drop1 and the HIC test results, the maximum Ca reduction amount when no HIC is generated is defined as “threshold Ca _{drop θ} ”. In particular, by using a plurality of slab measurement results and test results, a more accurate threshold value can be obtained, and erroneous determination of whether or not HIC has occurred can be reduced.

[Second Embodiment]
Next, a second embodiment in which the Ca decrease amount calculation method is different from the first embodiment will be described with reference to FIG. The description of the same configuration as that of the first embodiment will be simplified. Also in FIG. 4, the same reference numerals are used for the same components as those in the first embodiment described above, and the description thereof is omitted as appropriate.

(Investigation of Ca concentration in molten steel in tundish)
Investigate Ca concentration (Ca _TD1 ) of molten steel in tundish.

(Investigation of slab Ca concentration)
Next, in the slab cast with the same charge, as shown in FIG. 4, samples are taken at two or more investigation positions different in the thickness direction, and the Ca concentration of each sample is analyzed. The minimum Ca concentration (Ca _min1 ) is selected from the obtained two or more Ca concentrations (Ca _S1 , Ca _S2 ...).

Then, “Ca decrease amount Ca _drop11 ” is calculated using a value obtained by subtracting “minimum Ca concentration Ca _{min1 of} slab” from “Ca concentration Ca _{TD1 in tundish} ”. Ca _drop11 is expressed 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, and it is considered that HIC can be generated due to this.

  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 CaO accumulation band exists at a specific position in the thickness direction determined by casting conditions, a position where no CaO 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 ...) Include the Ca concentration in the accumulation band and the Ca concentration at the position where the accumulation band is not generated. By selecting (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 generation mechanism of the CaO accumulation band is the same for CaO inclusions and Al ₂ O ₃ inclusions, and the thickness of the accumulation band of Al ₂ O ₃ inclusions is reported to be 10 mm (reference: ISIJ). International, Vol. 43 (2003), No. 10, pp. 1548-1555). From this report, the thickness of the accumulation zone of CaO inclusions can be estimated to be 10 mm. Then, as shown in FIG. 4, when 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, no accumulation band is generated at the other investigation positions. Position. For this reason, it is preferable that the two or more survey positions are separated by more than 10 mm in the thickness direction. Note that FIG. 4 shows a case where there are two survey positions and the thickness direction distance l between the two survey positions exceeds 10 mm (thickness direction distance l between the two survey positions> 10 mm).

  Further, as shown in FIG. 1, since CaO inclusions are captured in a wide range in the vicinity of the bent portion of the casting path, in the regions R1 and R2 of D / 2 from both ends in the width direction of the slab shown in FIG. It occurs in a wide range in the thickness direction. Therefore, in the regions R1 and R2, there is a possibility that even if the Ca concentration investigation position is changed in the thickness direction, a position where no accumulation band is generated cannot be investigated. Therefore, it is preferable to set the Ca concentration investigation position to a region R3 having a width WD excluding D / 2 from both ends in the width direction, which is mainly cooled only from the wide surface side.

(rolling)
A slab cast under the same casting conditions as the slab in which the Ca concentration Ca _{S1 and the} like are measured is hot-rolled to produce a steel plate for threshold measurement.

(HIC test)
Then, an HIC test is performed on the steel sheet, and the presence or absence of HIC occurrence in the “region R41 corresponding to the vicinity of the anti-reference side surface” is examined. The HIC test can be performed by a method defined in NACE standard TM0284-2003, as shown in Examples described later.

(Determination of threshold)
Subsequently, a threshold value Ca _dropθ of the Ca reduction amount at which HIC does not occur is determined from “Ca reduction amount Ca _drop11 ” and “HIC test result”. In the present embodiment, the maximum Ca decrease amount when no HIC occurs is defined as “threshold Ca _{drop θ} ”.

[Measurement of Ca decrease in judgment target charge]
The Ca concentration Ca _{TD11 of the} molten steel in the tundish of the determination target charge is investigated. For example, as in the second embodiment, in a slab cast with the same charge, the Ca concentration is investigated at two or more different locations in the thickness direction, and the minimum is determined from two or more Ca concentrations (Ca _S11 , Ca _S12 ...). The Ca concentration (Ca _min11 ) is selected. It is preferable that two or more investigation positions are spaced apart from each other by more than 10 mm in the thickness direction.

Then, subtracted "minimum Ca concentration Ca _Min11 slab" from "Ca concentration Ca _TD11 in the tundish", it calculates the "Ca decrease Ca _drop" to be determined. Ca _drop is represented by the following equation.
_{Ca drop} = Ca _TD11 -Ca min11

[Evaluation of Ca decrease in judgment target charge]
When the determination target Ca _drop is compared with the threshold value Ca _{drop θ,} and the Ca _drop is _{equal to} or less than the threshold value Ca _{drop θ} , it is determined that the obtained steel sheet has excellent HIC resistance, and the Ca _drop exceeds the threshold value Ca _{drop θ.} If it is, the obtained steel sheet is judged to be inferior in HIC resistance.

  The slab investigation position (survey surface) is preferably a stationary part, but may be an unsteady part. The “unsteady portion” is a portion cast when a casting condition is changed, and includes a portion cast at an early stage of casting such as when the casting speed is increased, or a portion cast at the end of casting such as 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 part shows the same HIC resistance as the HIC test result, more accurate evaluation can be performed.

Steel sheet of the present invention, at the stage of the slab that is a state before rolling, by subtracting the Ca concentration of the slab from the Ca concentration in the tundish and calculating the "Ca decrease Ca _drop", the "Ca decrease Ca _{drop Is} a steel sheet satisfying Ca _drop ≦ threshold Ca _{drop θ} . The steel plate of the present invention satisfies the above Ca _drop ≦ threshold Ca _{drop θ,} and it is considered that no CaO accumulation band is generated in the slab, so that no HIC is generated.

  As described above, in this embodiment, “Ca concentration decrease from tundish to slab” is used for the evaluation of HIC resistance. Since the internal quality of the slab (accumulation degree of CaO inclusions) can be accurately evaluated from this, the HIC resistance can be evaluated at the stage of the slab based on this evaluation result. Thereby, since the HIC test which requires several weeks can be omitted, the period from manufacture to shipment can be greatly shortened.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

(1) Casting Tables 1-1 to 4 and FIGS. 6 and 7 show experimental conditions and experimental results for determining the threshold. By continuous casting, a slab having a slab thickness D of 280 mm and a slab width W of 2100 mm was obtained. The casting conditions of the first embodiment are shown in Table 1-1 and Table 1-2, and the casting conditions of the second embodiment are shown in Table 2-1 and Table 2-2, respectively. In this example, 25 charges were manufactured to obtain API (The American Petroleum Institute) X65 grade steel plate and APIX70 grade steel plate.

Here, the conditions shown in Table 1-1, Table 1-2, Table 2-1, and Table 2-2 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, a 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 caster [l / min.]) / (Cast slab weight per unit time [kg / min.])
・ Casting speed Pulling speed of slab [m / min. It was calculated from the diameter (peripheral length) of the roll (major roll) in contact with the slab and the rotation speed (number of rotations per unit time).

(2) Investigation of Ca decrease amount When the total length of the slab reached 10 m, the molten steel in the tundish was collected, and the Ca concentration Ca _{TD1 of the} molten steel in the tundish was investigated. After casting, the Ca concentration Ca _s1 or Ca _min1 of the _slab was investigated. Table 1-1 and Table 1-2 show the survey position and the Ca concentration Ca _s1 when the Ca concentration is investigated in the reference side region R4 of the slab. In Table 3-1, Table 3-2, and Table 4, 2-10 places (total number of N shown in Table 3-1, Table 3-2, and Table = 2-10) differing in the thickness direction of the slab. The survey position when the Ca concentration is investigated and the Ca concentration at each location are shown. Among Table 3-1, Table 3-2 and Table 4, Test Nos. 51 to 57 and 69 to 100 were measured at two locations. Test No. 58-64 are 3-8 locations, test no. 65-68 investigated 10 places. And the minimum Ca density _| concentration _Camin1 is shown among several Ca density _| concentrations. The 2 to 10 positions are positions separated by more than 10 mm in the thickness direction.

(3) Rolling Then, after heating the slab to 1050 to 1250 ° C., the surface temperature of the steel plate is 900 ° C. or higher, the average steel plate temperature calculated by calculation is 1000 ° C. or higher, and the cumulative rolling reduction is 40% or higher. In addition, hot rolling is performed so that a pass having a reduction rate of 10% or more per pass becomes 2 passes or more, and then, hot rolling is performed so that a cumulative reduction rate of 700 ° C. or more and less than 900 ° C. becomes 20% or more. The surface temperature at the end of rolling was set to 850 ° C. Thereafter, cooling is started at a cooling start surface temperature: 950 ° C. at an average cooling rate: 10 ° C./s, stopped at a temperature of 350 to 600 ° C., and then air-cooled to room temperature to obtain various component compositions A steel plate having a size of 9 to 50 mm plate thickness × 2000 to 3500 mm width × 12000 to 35000 mm length was obtained.

(4) HIC test In order to determine the threshold value tθ, in this example, an HIC test was performed after rolling.
(A) A sample was cut out from each steel sheet after rolling, and an HIC test was performed. The HIC test was performed according to the method specified in NACE standard TM0284-2003.
(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 HIC. The observation region was defined as a region R41 excluding a range within ± 5.3% of the plate thickness from the product thickness center in the “product region R40 corresponding to the anti-reference side region” shown in FIG.

(5) Determination of Ca Decrease Amount Threshold FIG. 6 shows the threshold determination result of the first embodiment. “Ca concentration Ca _{TD1 of} molten steel in tundish” and Table 1-1 investigated in (2) above. And the relationship between the “IC concentration Ca _{S1 of the} slab” in Table 1-2 and the HIC test results is shown. FIG. 7 shows the threshold value determination result of the second embodiment. “Ca concentration Ca _{TD1 of} molten steel in tundish” investigated in the above (2), and Table 3-1, Table 3-2 and Table 4 The relationship between the minimum Ca concentration Ca _min1 of the slab and the HIC test results is shown.

From the said FIG. 6, in the determination method of 1st Embodiment, when Ca fall amount was 4 ppm or less, HIC did not generate | occur | produce. On the other hand, when Ca fall amount exceeded 4 ppm, the case where HIC generate | occur | produced and the case where it did not occur were mixed. From this result, 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 for the Ca decrease amount is 4 ppm, that is, Ca _dropθ = 4 ppm.

Further, from FIG. 7, even in the determination method of the second embodiment, no HIC was generated when the Ca decrease amount was 4 ppm or less. On the other hand, when Ca fall amount exceeded 4 ppm, the case where HIC generate | occur | produced and the case where it did not occur were mixed. From this result, it was found that the occurrence of HIC can be reliably suppressed when the amount of Ca decrease is ≦ 4 ppm. Therefore, also in the example of the second embodiment, the threshold value of the Ca decrease amount is set to 4 ppm, that is, 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.

(6) Evaluation of determination target slab Using the above threshold, the HIC resistance of the determination target slab having the component composition shown in Table 5 was evaluated.

  Steels having the composition shown in Table 5 were melted, and a slab having a slab thickness D of 280 mm and a slab width W of 2100 mm was obtained by continuous casting.

With investigating determination target charge tundish molten steel Ca concentration Ca _TD11, determining the minimum of the Ca concentration of the slab to be determined _(Ca min11), the Ca decrease Ca _drop of the slab to be determined and calculated as described above . And, using the threshold value Ca _dropθ = 4 ppm obtained from the first and second embodiments of (5) above, no HIC due to CaO occurs when the Ca _drop amount Ca _drop of the judgment target slab is 4 ppm or less, The HIC resistance evaluation was determined to be OK, and when the Ca drop amount Ca _drop was more than 4 ppm, HIC due to CaO was generated, that is, the HIC resistance evaluation was determined to be NG. The results are shown in Table 6.

  Thereafter, the slab was heated to 1050 to 1250 ° C., and then “TMCP” or “QT” in the column of “Hot rolling / cooling method” in Table 6, two patterns of hot rolling / cooling. By the method, various steel compositions (9 to 90 mm plate thickness x 2000 to 3500 mm width x 12000 to 35000 mm length) were obtained. The “TMCP” is a pass in which the cumulative rolling reduction when the steel plate surface temperature is 900 ° C. or higher, the average steel plate temperature calculated by calculation is 1000 ° C. or higher is 40% or higher, and the rolling reduction per pass is 10% or higher. After hot rolling so that it becomes 2 passes or more, after that, hot rolling is performed so that the cumulative rolling reduction of 700 ° C. or more and less than 900 ° C. becomes 20% or more, and the rolling finish surface temperature becomes 850 ° C. The cooling start surface temperature is 950 ° C., the cooling is started at an average cooling rate of 10 ° C./s, stopped at a temperature of 350 to 600 ° C., and then cooled to room temperature. The “QT” is a method in which after hot rolling, air-cooled to room temperature, reheated to a temperature of 850 ° C. or higher and 950 ° C. or lower, quenched, and then tempered at 600 to 700 ° C.

  Using the steel plate, an HIC test was performed according to the method specified in NACE standard TM0284-2003, and the presence or absence of cracks in the HIC resistance test was confirmed. The results are shown in Table 6.

  Table 5 and Table 6 show the following. Steel type no. Nos. 1 to 7 and 10 to 16 are steel plates of the present invention that satisfy the prescribed component composition and have a reduced amount of Ca in the slab that is suppressed to a threshold value or less and that have excellent HIC resistance.

  On the other hand, steel grade No. In Nos. 9 and 18, since the amount of Ca decrease in the slab exceeded the threshold value, the HIC resistance evaluation of the slab was NG. Further, in the HIC test performed after rolling, it was confirmed that the steel plate was cracked and inferior in HIC resistance. Steel grade No. 9 and 18 are examples in which the chemical composition of the steel sheet deviates from the definition of the present invention. That is, the steel type No. Steel plate No. 9 was inferior in HIC resistance because REM and Zr were 0%, and the value of (Ca-1.25S) / O was out of specification. Steel type no. No. 18 was inferior in HIC resistance because the value of (Ca-1.25S) / O was out of specification. Steel type no. 8 and 17 are examples in which the chemical component composition of the steel sheet deviates from the definition of the present invention, although the amount of Ca decrease in the slab is suppressed to be smaller than the threshold value. That is, the steel type No. No. 8 was inferior in HIC resistance because REM and Zr were 0% and the value of (Ca / S) was out of specification. Steel grade No. No. 17 was inferior in HIC resistance because the value of (Ca / S) was not specified.

  In the example in which the HIC resistance evaluation on the slab was OK, the period (casting → rolling → shipment) from the start of casting to the shipment of the product steel plate, that is, the sour-resistant steel plate, was 19 days. On the other hand, when the HIC test is performed using the steel sheet obtained after rolling and the HIC resistance is evaluated, the period from casting start to shipping (casting → rolling → HIC test → shipping) is as long as 28 days. It took a period. In this example, since the HIC test after rolling could be omitted, the period from the start of casting to shipment could be greatly shortened from 28 days to 19 days.

  Further, in the case where the HIC resistance evaluation on the slab was NG, when remelting was started at the slab stage, the period from the start of casting to the shipment of the product steel plate, that is, the sour steel plate (casting → Remelting → rolling → shipping) was 54 days. On the other hand, as a result of performing the HIC test using the steel sheet obtained after rolling and evaluating the HIC resistance of the product, if the evaluation is NG, remelting was started after the HIC test was performed. The period from the start of casting to the shipment of the steel sheet as a product (casting → rolling → HIC test → remelting → rolling → HIC test → shipping) required 72 days. In this example, since the HIC test after rolling could be omitted, even when remelting was necessary, the period from the start of casting to shipment could be greatly shortened from 72 days to 54 days.

  As described above, according to the present invention, since the HIC resistance could be evaluated at the stage of the slab, which is a slab, without performing the HIC test after rolling, the manufacturing lead time could be greatly shortened. In this example, the determination method of the present invention is highly accurate because the HIC test for determining the threshold for evaluating the HIC resistance of the slab and the HIC test for confirmation are the same.

Claims (10)

% By mass
C: 0.02 to 0.15%,
Si: 0.02 to 0.50%,
Mn: 0.6 to 2.0%,
P: more than 0% and 0.030% or less,
S: more than 0% and 0.003% or less,
Al: 0.010 to 0.08%,
Ca: 0.0003 to 0.0060%,
N: 0.001 to 0.01%, and O: more than 0% and 0.0045% or less,
Including one or more elements selected from the group consisting of REM: more than 0% and 0.02% or less, and Zr: more than 0% and 0.010%, the balance consisting of iron and inevitable impurities,
The ratio of Ca to S (Ca / S) is 2.0 or more, and the Ca, S and O satisfy (Ca−1.25S) /O≦1.80,
Furthermore, the amount of Ca reduction obtained by subtracting the Ca concentration of the slab from the Ca concentration of the molten steel in the tundish is not _more than the threshold value Ca _dropθ , and the threshold value Ca _dropθ does not cause hydrogen-induced cracking in the steel sheet obtained by rolling the slab. A steel sheet excellent in hydrogen-induced crack resistance, characterized by being the maximum amount of Ca decrease. The steel sheet according to claim 1, wherein the threshold value Ca _dropθ is a value _obtained in advance by the following methods (i) to (iii).
(I) The Ca concentration of the molten steel in the tundish and the Ca concentration of the slab are measured, and the amount of Ca decrease is calculated by subtracting the Ca concentration of the slab from the Ca concentration of the molten steel in the tundish.
(Ii) A hydrogen-induced cracking test is performed on a steel plate obtained by rolling a slab cast under the same casting conditions as the slab.
(Iii) From the Ca decrease amount measured in (i) above and the hydrogen induced crack test result of (ii) above, the maximum Ca decrease amount at which no hydrogen induced crack occurs is obtained.   The steel plate according to claim 2, wherein the slab cast under the same casting conditions as the slab is a slab obtained by measuring the Ca decrease amount.   The Ca concentration of the slab is a minimum Ca concentration of the two or more Ca concentrations obtained by investigating the Ca concentration at two or more positions different in the thickness direction in the slab. The steel plate in any one of. The steel sheet according to any one of claims 1 to 4, wherein the threshold value Ca _dropθ is 4 ppm. Furthermore, as other elements,
B: more than 0% and 0.005% or less,
V: more than 0% and 0.1% or less,
Cu: more than 0% and 1.5% or less,
Ni: more than 0% and 1.5% or less,
Cr: more than 0% and 1.5% or less,
The steel plate according to any one of claims 1 to 5, comprising one or more elements selected from the group consisting of Mo: more than 0% and 1.5% or less, and Nb: more than 0% and 0.06% or less. Furthermore, as other elements,
The steel plate according to any one of claims 1 to 6, comprising one or more elements selected from the group consisting of Ti: more than 0% and 0.03% or less, and Mg: more than 0% and 0.01% or less.   The steel plate according to any one of claims 1 to 7, which is for a line pipe.   It is an object for pressure vessels, The steel plate in any one of Claims 1-7.   The steel pipe for line pipes formed with the steel plate in any one of Claims 1-8.