JP6316548B2 - Steel plates and line pipe steel pipes with excellent hydrogen-induced crack resistance and toughness - Google Patents

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 toughness, and excellent hydrogen-induced crack resistance and toughness obtained using the steel sheet. The present invention relates to a steel pipe for line pipe.

  Mainly for oil and gas transportation line pipes and storage tanks, in order to suppress brittle fracture from the viewpoint of safety, and to prevent and suppress the propagation of cracks when fracture occurs, the line pipe High toughness is required for the steel materials constituting these materials. Furthermore, in recent years, with the development of inferior resources containing hydrogen sulfide, so-called sour resistance such as hydrogen-induced crack resistance (HIC resistance) and stress corrosion crack resistance (SSCC resistance) is also required. Hydrogen-induced cracking (hereinafter sometimes referred to as “HIC”) is caused by non-metallic inclusions such as MnS and Nb (C, N) that are formed by hydrogen that has penetrated into the steel material due to the corrosion reaction caused by hydrogen sulfide or the like. It is known that it is a crack generated by gasification. If HIC occurs during use of the line pipe or the like, the toughness of the structure is lowered.

  Conventionally, several techniques for improving the HIC resistance have been proposed. For example, Patent Document 1 discloses a steel material that has improved hydrogen-induced cracking properties 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, as described later, there are cases where many fine HICs that are difficult to observe at the optical microscope level are locally generated, and local after the HIC test (in actual use). Cause toughness reduction. Therefore, high stabilization of toughness after the HIC test (during actual use) is required.

JP 2010-209461 A Japanese Patent Laid-Open No. 06-136440

  The present invention has been made paying attention to the above-described circumstances, and an object thereof is to realize a steel plate and a steel pipe excellent in hydrogen-induced crack resistance and toughness.

The steel sheet excellent in hydrogen-induced cracking resistance and toughness according to the present invention that has solved the above problems is C: 0.02 to 0.15% (% means mass%, the same applies hereinafter), Si: 0.02 -0.50%, Mn: 0.6-2.0%, P: 0.030% or less (not including 0%), S: 0.003% or less (not including 0%), Al: 0 0.010 to 0.08%, Ca: 0.0003 to 0.0060%, N: 0.001 to 0.01%, and O (oxygen): 0.0045% or less (not including 0%) The balance is made of iron and inevitable impurities, the ratio of Ca to S (Ca / S) is 2.0 or more, and the Ca, S and O are (Ca-1.25S) / O ≤ met 1.80, further, Ar gas content in the steel material 0.50MyuL (microliters) / cm ³ in the are where less With a butterfly.
The steel sheet, as another element,
(A) B: 0.005% or less (not including 0%), V: 0.1% or less (not including 0%), Cu: 1.5% or less (not including 0%), Ni: 1.5% or less (not including 0%), Cr: 1.5% or less (not including 0%), Mo: 1.5% or less (not including 0%), and Nb: 0.06% One or more elements selected from the group consisting of:
(B) Ti: 0.03% or less (not including 0%), Mg: 0.01% or less (not including 0%), REM: 0.02% or less (not including 0%), and Zr : One or more elements selected from the group consisting of 0.010% or less (not including 0%) may be included.
The steel sheet is suitable for line pipes. Moreover, the steel pipe for line pipes manufactured using the said steel plate is also contained in this invention.

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate and steel pipe which were reliably excellent in hydrogen-induced crack resistance and toughness can be provided. These are suitably used for natural gas / crude oil transportation line pipes and storage tanks.

  The inventors of the present invention have made extensive studies to solve the above problems. First, the present inventors performed HIC tests specified in NACE (National Association of Corrosion and Engineer) TM0284 for various steel plates, and evaluated HIC resistance. The NACE test is a test for evaluating the generation of HIC after 96 hours by saturating hydrogen sulfide gas in a solution of 5% NaCl solution + 0.5% acetic acid, pH 2.7.

  Next, the inventors have investigated the steel sheet surface layer portion (for example, see CAMP-ISIJ Vol. 24 (2011) -P671), which is known to have a particularly high hydrogen concentration during the HIC test, and the Charpy after the HIC test. Impact properties were performed according to ASTM A370. As a result, it was found that there was variation in the value of Charpy impact absorption energy.

  Investigation of the cause, particularly when the value of Charpy impact absorption energy varies and the sample showing a low value was examined, it was not observed by microscopic observation at 100 times as specified in the NACE test (below the observation limit). ) It was found that fine HIC was generated. Furthermore, as a result of examining the cause of the generation of the fine HIC, it was first found that the cause was Ar gas in the steel.

  Therefore, when the relationship between the Ar gas content in the steel material and the toughness variation of the steel sheet surface layer after the HIC test was investigated, a clear correlation was found between the two. As a result of further detailed investigation, when the Ar gas content is large, the Ar bubbles remaining in the slab are not completely crimped during hot rolling and remain in the steel sheet as fine defects. It was found that Charpy impact absorption energy was reduced because a fine HIC was generated and a separation that caused cracks parallel to the steel sheet surface was generated by the Charpy test.

Next, as evaluated in the examples described later, it was examined how much Ar gas content in the steel material should be suppressed in order to obtain a highly stable toughness of the steel sheet surface layer after the HIC test. As a result, the present inventors have found that the Ar content in the steel material may be 0.50 μL / cm ³ or less. The Ar content is preferably 0.30μL / cm ³ or less, more preferably 0.25 [mu] L / cm ³ or less. However, it is necessary to blow Ar into the molten steel in the manufacturing process, for example, by suppressing clogging of the injection nozzle, refluxing with RH for degassing, stirring in a tundish (TD) for inclusion floating separation, etc. Therefore, it is difficult to suppress the Ar content in the steel material to zero.

  The “Ar gas content in the steel” is determined by the method described in the examples described later.

  In order to ensure excellent HIC resistance and toughness, it is necessary to control the component composition of the steel material together with the control of the Ar content in the steel material. Furthermore, in order to ensure other characteristics (such as high strength and excellent weldability) required as steel materials for line pipes, for example, it is necessary to set the component composition of the steel sheet as follows. Hereinafter, the reasons for defining each component 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 Mn amount 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: 0.030% or less (excluding 0%)]
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: 0.003% or less (excluding 0%)]
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: 0.0045% or less (excluding 0%)]
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. In order to suppress this, the present inventors examined the relationship between (Ca-1.25S) / O and toughness, and in order to obtain excellent toughness, (Ca-1.25S) / O is 1. It was found that it was necessary to make it 80 or less. (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.

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 including one or more elements selected from the group consisting of Ti, Mg, REM, and Zr in the following amounts, the HAZ toughness is improved and desulfurization is promoted to further improve the HIC resistance. be able to. Hereinafter, these elements will be described in detail.

[B: 0.005% or less (excluding 0%)]
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: 0.1% or less (excluding 0%)]
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: 1.5% or less (excluding 0%)]
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: 1.5% or less (excluding 0%)]
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. More preferably, it is 0.05% or more, More preferably, it is 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: 1.5% or less (excluding 0%)]
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: 1.5% or less (excluding 0%)]
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: 0.06% or less (excluding 0%)]
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: 0.03% or less (excluding 0%)]
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 because it exhibits a desulfurization action. In order to obtain these effects, it is preferable to contain 0.003% or more of Ti. More preferably it is 0.005% or more, and still more preferably 0.010% or more. On the other hand, if the Ti content is excessive, solid solution Ti or TiC is precipitated and the toughness of the base material and the HAZ part is deteriorated. Therefore, the content is preferably 0.03% or less. The amount of Ti is more preferably 0.02% or less.

[Mg: 0.01% or less (excluding 0%)]
Mg is an element effective for improving toughness through refinement of crystal grains, and is also 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. More preferably, it is 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.

[REM: 0.02% or less (excluding 0%)]
REM (rare earth element) is an element effective for suppressing the formation 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 is preferably 0.02%. From the viewpoint of increasing productivity by suppressing the clogging of the immersion nozzle during casting, the REM content is more preferably 0.015% or less, still more preferably 0.010% or less, and still more preferably 0.0050%. It is as follows. In the present invention, the REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y.

[Zr: 0.010% or less (excluding 0%)]
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 Zr content is preferably 0.010% or less. The amount of Zr is more preferably 0.0070% or less, still more preferably 0.0050% or less, and still more preferably 0.0030% or less.

  In the above, the steel plate prescribed | regulated by this invention was demonstrated. The method for producing the steel plate of the present invention is not particularly limited as long as the Ar gas content in the steel material is a method for obtaining a steel plate satisfying the above-mentioned rules. The following method is recommended as a method for easily obtaining the steel sheet specified above.

〔Production method〕
In order to achieve the Ar gas content, in the continuous casting process, the number density of inclusions having a major axis of 3 μm or more with a melting point of 1550 ° C. or more is preferably 3 pieces / cm ² or more in the mold. Is recommended.

  Inclusions with a melting point of 1550 ° C. or higher are present as solids in the mold, so that the wettability with molten steel is poor, the inclusions aggregate together, and float due to volume expansion by involving Ar gas in the inclusions. Becomes easy. In addition, relatively coarse inclusions having a major axis of 3 μm or more come into contact with each other in the mold and become coarser, and involve Ar bubbles, thereby promoting the floating separation of Ar bubbles in the mold. As a result, the Ar gas content in the steel material can be reduced. In particular, when Ar gas is used in the process before pouring molten steel into the mold, Ar gas tends to remain in the steel material, so that the floating separation by the inclusions is effective.

Examples of inclusions having a melting point of 1550 ° C. or higher include Al ₂ O ₃ and CaO, and composite inclusions thereof. If the melting point is unknown for composite inclusions, etc., quantitative analysis of the inclusions is performed by energy dispersive X-ray spectroscopy (EDX), etc., artificial inclusions simulating the composition are created, The melting point can be determined by measuring the temperature at which the dissolution of the artificial inclusion starts. More simply, by utilizing the fact that liquid inclusions in a mold are observed in a spherical shape after solidification, inclusions having an aspect ratio of 1.3 or more can be handled as inclusions having a temperature of 1550 ° C. or more. Good.

The number density of the inclusions is more preferably 5 pieces / cm ² or more, and even more preferably 10 pieces / cm ² or more. However, the toughness of the base material and the HAZ part even if the number density of the inclusions is excessive. Therefore, the upper limit of the number density of the inclusions is approximately 100 / cm ² .

As a specific means for achieving the number density of inclusions, for example, the recirculation time in RH in the refining process is 45 minutes or less, and after 15 minutes or more have elapsed from the addition of Ca in the RH,
(A) In a continuous casting machine using a hot reuse tundish, casting is performed using a tundish that has passed 30 minutes or more after the completion of the pre-charge casting; and / or
(B) Metal Al can be added to the molten steel in the hot reuse tundish, for example, 0.04 kg / ton or more (for example, about 0.2 kg / ton. The upper limit is about 0.50 kg / ton or less, preferably 0.40 kg / ton or less) and then casting.

  Another means for reducing the Ar gas content in the steel material is to suppress and stop the use of Ar in the injection nozzle, RH, and tundish. However, in order to suppress the blockage of the injection nozzle and prevent a decrease in yield, it is effective to blow Ar from a position of 50 mm or more from the upper part of the discharge hole of the injection nozzle. Not recommended. In order to achieve the prescribed Ar gas content, the Ar blowing amount (flow rate) in the injection nozzle is preferably 9.0 L (liter) / t (tons) or less (more preferably 6.0 L / t). The following is recommended: Although it is conceivable to change the gas used for blowing in the injection nozzle from Ar gas to nitrogen gas, the nitrogen gas is not preferable because the N amount of the steel sheet cannot be controlled and the toughness tends to deteriorate.

  In this invention, it does not ask | require in particular about the process after casting as mentioned above, According to a conventional method, it can hot-roll and can manufacture a steel plate. Moreover, the steel pipe for line pipes can be manufactured by the method generally performed using this steel plate. The steel pipe for line pipes obtained using the steel sheet of the present invention is also excellent in HIC resistance and toughness.

  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.

Steel having the component composition shown in Table 1 was melted and a steel piece (slab) having a thickness of 280 mm was obtained by continuous casting. The casting was performed so that the number of inclusions having a major axis of 3 μm or more in the mold was 3 / cm ² or more. Specifically, in the present example, the number of inclusions was controlled such that the reflux time in RH was 5 minutes to 45 minutes, Ca was added after RH reflux, and then 15 minutes to 45 minutes were passed. . Thereafter, in a continuous casting machine using a hot reuse tundish, the molten steel is filled with the tundish that has passed 30 minutes or more and 60 minutes or less after the completion of the casting of the precharge, and then 0.04 kg / Casting was performed by adding metallic Al of ton or more (upper limit is about 0.50 kg / ton). The inclusion number density was measured using a sample taken from the mold 10 minutes after casting.

The number of inclusions in the mold was determined by taking a molten steel sample from the mold and observing with a SEM (field size: 400 times, number of fields: 30 fields of view), the major axis was 3 μm or more, and the aspect ratio was 1. The number density was determined by regarding three or more inclusions as inclusions having a melting point of 1550 ° C. or more. Table 2 shows “◯” when the number density of the inclusions is 3 or more / cm ² or more in the mold by the above method, and “x” otherwise.

  Then, after reheating the slab so that the steel slab produced by continuous casting has a temperature of 1050 to 1250 ° C., the steel sheet surface temperature is 900 ° C. or higher, and the cumulative reduction of the steel plate average temperature (1000 ° C. or higher) obtained by calculation. The hot rolling is performed so that the pass having a reduction rate of 40% or more and the reduction rate per pass of 10% or more becomes 2 passes or more, and then the cumulative reduction rate of 700 ° C. or more and less than 900 ° C. is 20% or more. Hot rolling is performed so that the rolling end temperature is 700 ° C. or higher and lower than 900 ° C., then water cooling is started from a temperature of 650 ° C. or higher, and stopped at a temperature of 350 to 600 ° C. To obtain steel plates (9 to 50 mm plate thickness x 2000 to 3500 mm width x 12000 to 35000 mm length) having various component compositions.

  And using each steel plate, as shown below, the Ar gas content in the steel was measured. Moreover, the HIC test was conducted to evaluate the HIC resistance, and the Charpy impact test was conducted to evaluate the toughness.

[Measurement of Ar gas content in steel]
A test piece of product plate thickness × 15 mm × 15 mm cut out from the steel surface was placed in a vacuum chamber and the degree of vacuum was set to 2 × 10 ⁻⁵ [Torr] or less. GSDD3000, diameter D1: 3.0 mm, groove length L3: 32 mm, full length: 71 mm, blade diameter: 3.0 mm) to extract the gas component in the steel by drilling from the steel sheet surface to 5 mm below the surface, Then, the gas component was quantitatively analyzed using the quadrupole mass spectrometer (M-101QA-TDM type) (mass number measurement range: 1 to 100 amu) manufactured by Anerva. And ratio (microliter / cm < ³ >) of Ar amount with respect to the volume of the steel materials drilled by the said drill process was calculated | required. This measurement was performed at 10 positions at arbitrary positions on the steel sheet, and the maximum value among the 10 positions was defined as “Ar gas content in steel”.

[HIC test (NACE test)]
The HIC test was performed and evaluated according to NACE standard TM0284-2003. Specifically, 20 test pieces (size: plate thickness × (width) 100 mm × (rolling direction) 20 mm) were sampled from 1/4 W position and 1/2 W position in the width direction of each steel plate. Then, the test piece was immersed in an aqueous solution of 25 ° C. (0.5% NaCl + 0.5% acetic acid) saturated with 1 atm of hydrogen sulfide for 96 hours and subjected to cross-sectional evaluation (according to NACE standard TM0284-2003 FIGURE 3). (The ratio (%) of the total crack length to the specimen width, the crack length ratio) was measured. And when CLR was 3% or less, it evaluated that it was excellent in HIC resistance ((circle)), and the case where CLR exceeded 3% was evaluated as inferior to HIC resistance (x).

[Charpy impact test]
After the NACE test, 10 Charpy specimens with a thickness of 10 mm and a rolling direction of 10 mm were sampled in a direction perpendicular to the rolling direction in accordance with ASTM A370 centering on 6 mm below the steel sheet surface, and notched in the thickness direction of the steel sheet. . The Charpy impact test was performed according to ASTM A370, the test temperature was 20 ° C., and Charpy absorbed energy and brittle fracture surface ratio were evaluated. For a total of 10 Charpy specimens collected, samples with a brittle fracture surface ratio of 5% or less were extracted, the standard deviation σ of the Charpy absorbed energy value was determined, and those with a standard deviation σ of 20 J or less were tough. (In particular, the variation in toughness is small and high toughness can be reliably achieved).

  These results are shown in Table 2.

  Table 1 and Table 2 show the following. No. Nos. 1 to 13 satisfy the specified component composition, and the Ar gas content in the steel material is suppressed within the specified range, so that the HIC resistance is excellent and the excellent toughness is stably obtained.

  In contrast, no. In Nos. 14 and 15, since the Ar gas content in the steel material is excessive, the variation in toughness became large. No. No. 16 resulted in inferior HIC resistance because the chemical composition (Ca / S) of the steel sheet was not within the scope of the present invention. No. No. 17 had a large value of (Ca-1.25S) / O and a large amount of Ca-based inclusions (particularly CaO), which resulted in inferior HIC resistance and increased toughness variation.

Claims (5)

C: 0.02 to 0.15% (% means mass%, the same shall apply hereinafter)
Si: 0.02 to 0.50%,
Mn: 0.6 to 2.0%,
P: 0.030% or less (excluding 0%),
S: 0.003% or less (excluding 0%),
Al: 0.010 to 0.08%,
Ca: 0.0003 to 0.0060%,
N: 0.001 to 0.01%, and O (oxygen): 0.0045% or less (excluding 0%) is satisfied, and the balance consists 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, a steel sheet excellent in hydrogen-induced crack resistance and toughness, characterized in that the Ar gas content in the steel material is 0.50 μL (microliter) / cm ³ or less. As other elements,
B: 0.005% or less (excluding 0%),
V: 0.1% or less (excluding 0%),
Cu: 1.5% or less (excluding 0%),
Ni: 1.5% or less (excluding 0%),
Cr: 1.5% or less (excluding 0%),
Mo: 1.5% or less (not including 0%), and Nb: 0.06% or less (not including 0%)
The steel plate according to claim 1, comprising at least one element selected from the group consisting of: As other elements,
Ti: 0.03% or less (excluding 0%),
Mg: 0.01% or less (excluding 0%),
REM: 0.02% or less (not including 0%), and Zr: 0.010% or less (not including 0%)
The steel plate according to claim 1 or 2, comprising one or more elements selected from the group consisting of:   It is an object for line pipes, The steel plate in any one of Claims 1-3.   The steel pipe for line pipes manufactured using the steel plate in any one of Claims 1-4.