Steel sheet, method for producing same, and steel pipe

Abstract

Provided is a steel sheet having excellent HIC resistance. This steel sheet has a prescribed component composition, and contains inclusions having an average composition containing 10-40% by mass of MgO and 10.0% by mass or less of MnS, the average aspect ratio of MnS being 15 or less, in regions from both surfaces of the steel sheet to the inside of a depth of 1 / 4 of the thickness of the steel sheet in the sheet thickness direction, the inclusions having an average composition containing 10-40% by mass of MgO and 10.0% by mass or less of MnS. The average value of upper 10% of the circle-equivalent diameter of the inclusions is 3.5 [mu] m or less, and the area ratio CAR of cracks after the HIC test of the region is 5.0% or less.

Description

Technical Field

[0001] The present invention relates to a steel plate excellent in hydrogen-induced cracking resistance (HIC (Hydrogen Induced Cracking) resistance) suitable for use in pipeline pipes for transporting crude oil or natural gas, and a method for manufacturing the same. Further, the present invention relates to a steel pipe using the above steel plate. Background Art

[0002] Generally, pipeline pipes can be manufactured by forming a steel plate manufactured by a heavy plate mill or a hot rolling mill into a steel pipe by UOE forming, press bending forming, roll forming, or the like.

[0003] In addition to strength, toughness, weldability, etc., pipeline pipes used in the transportation of crude oil or natural gas containing hydrogen sulfide also require so-called acid resistance such as HIC resistance and sulfide stress corrosion cracking resistance (SSCC (Sulfide Stress Corrosion Cracking) resistance). Among them, HIC means that after hydrogen ions generated by a corrosion reaction are adsorbed on the steel surface, they penetrate into the steel interior in an atomic state, diffuse and aggregate around non-metallic inclusions such as MnS in the steel or hard second-phase structures, and become molecular hydrogen, and cracks are generated due to the internal pressure thereof.

[0004] As non-metallic inclusions in steel that are likely to be the starting points of HIC, MnS (manganese sulfide) stretched by hot rolling can be cited. In this regard, the following technique is known: by adding calcium (Ca) to molten steel, Ca reacts with sulfur (S) in the steel to generate a more stable sulfide CaS (calcium sulfide) than MnS, suppressing the generation of MnS and improving HIC resistance. The added Ca also reacts with oxygen (O) in the steel and the deoxidation product Al2O3 (aluminum oxide) to generate a CaO-Al2O3-based non-metallic inclusion.

[0005] Therefore, when Ca in the molten steel is insufficient, the amount of Ca reacting with S in the steel is insufficient, a large amount of MnS is generated, and the HIC resistance of the center segregation part deteriorates. Further, when Ca is excessive, a CaO-Al2O3-based non-metallic inclusion cluster with a high CaO concentration is generated, and the HIC resistance deteriorates.

[0006] Here, in recent years, by increasing the content of manganese (Mn) and reducing the content of other alloy components, the development of steel plates aiming at cost reduction has been promoted. In the case of this steel plate, MnS that deteriorates HIC resistance is generated from enriched Mn and enriched S in the center segregation part. Therefore, there is a problem that even if the S content of the entire steel plate is reduced, the generation frequency of MnS in the center segregation part increases.

[0007] As a countermeasure, if the operation of further increasing the amount of Ca added is carried out to suppress the formation of MnS, it is easy to generate the CaO-Al2O3-based non-metallic inclusion clusters with a high CaO concentration as described above, which becomes the cause of the deterioration of the HIC resistance.

[0008] Therefore, in order to improve the HIC resistance of steel plates with a high Mn content, it is necessary to suppress the formation of MnS and the formation of inclusion clusters by methods other than increasing the amount of Ca added.

[0009] As a means to solve the above problems, a method of using an element with a strong affinity for S and a high sulfide-forming ability in combination with Ca is considered. For example, it is known that magnesium (Mg), like Ca, is an element that can form a sulfide more stable than MnS. When comparing the stability of sulfides thermodynamically, the order is CaS > MgS > MnS, so Mg can be expected to have the same effect as Ca in suppressing the formation of MnS.

[0010] Here, Patent Document 1 and Patent Document 2 disclose acid-resistant steel containing not only Ca but also Mg. Patent Document 3 discloses acid-resistant steel containing Mg as an element effective for improving the HIC resistance.

[0011] As a method of adding Mg to molten steel, Patent Document 4 discloses the following method: by containing 2 to 10% by mass of MgO in the slag, supplying MgO to non-metallic inclusions to control the composition of non-metallic inclusions.

[0012] As other Mg addition methods, Patent Document 5 discloses a method of adjusting the composition in a refining vessel using a refractory material containing MgO for a part or all of the lining refractory material, and Patent Document 6 discloses a method of adding a Ti-Mg alloy.

[0013] As a method of adding Ca and Mg, Patent Document 7 discloses a method of injecting an inert gas and addition materials such as Ca-Si and Mg-Si into the molten steel flow injected from the ladle into the tundish in a continuous casting facility.

[0014] It should be noted that regarding the shape of inclusions, it is known that the greater the aspect ratio (major axis / minor axis) of the inclusions, the greater the stress concentration caused by the inclusions, and it is also known that the inclusions are deformed by rolling the cast slab. That is, if the aspect ratio of the inclusions becomes larger by rolling, the stress concentration caused by the inclusions becomes larger, and HIC is likely to occur. Therefore, it is difficult to manufacture steel plates with excellent HIC resistance by conventional rolling methods.

[0015] Prior art documents

[0016] Patent documents

[0017] Patent Document 1: Japanese Patent Application Laid-Open No. 2017-172010

[0018] Patent Document 2: Japanese Patent Application Laid-Open No. 2017-110249

[0019] Patent Document 3: Japanese Patent Application Laid-Open No. 2014-208891

[0020] Patent Document 4: Japanese Patent Application Laid-Open No. 2012-188696

[0021] Patent Document 5: Japanese Patent Application Laid-Open No. 2004-043838

[0022] Patent Document 6: Japanese Patent Application Laid-Open No. 2002-266019

[0023] Patent Document 7: Japanese Patent Application Laid-Open No. 2017-087229 SUMMARY OF THE INVENTION

[0024] However, Patent Documents 1 and 2 disclose that Mg exists in the form of fine Mg-based oxides, and these fine Mg-based oxides function as precipitation nuclei for TiN, which is effective in improving the toughness of the heat-affected zone (HAZ) of welding. That is, in the technologies described in Patent Documents 1 and 2, since the surface of the Mg-based oxides is covered by precipitated TiN, Mg cannot contribute to the formation of MgS.

[0025] In the method described in Patent Document 4, the MgO-based non-metallic inclusions generated by the reduction of the slag involved in the molten steel are coarse, and there is a concern that they may become the starting point of HIC.

[0026] Regarding the technologies described in Patent Documents 5 and 6, considering the repair cost of refractory materials and the cost of alloys containing expensive Ti (titanium), they are uneconomical.

[0027] The method described in Patent Document 7 has the following problems: there is a concern that the blown inert gas may be involved in the molten steel flow, resulting in deterioration of the quality of the cast sheet, and it is difficult to balance the control of the morphology of non-metallic inclusions and the suppression of bubble defects by adding alloys.

[0028] Thus, although acid-resistant steel containing Ca and Mg has been proposed in the above prior technologies, they all have the above problems. In addition, the steel sheet containing Ca and Mg has the following problems.

[0029] That is, for the addition operations of Ca and Mg not described in Patent Documents 1 to 3 and Patent Document 7 at all, there are the following problems. Oxides and sulfides of Ca are thermodynamically more stable than oxides and sulfides of Mg. Therefore, when non-metallic inclusions (oxides, sulfides) containing Ca have already formed in molten steel, the influence of subsequently added Mg on the morphology of the non-metallic inclusions is small.

[0030] Since oxides and sulfides of Ca are more stable than oxides and sulfides of Mg, S and O (oxygen) in molten steel easily react with Ca, and it is difficult for Mg to react with S and O (oxygen) in molten steel. Therefore, it is difficult to expect desulfurization brought about by the formation of MgS and deoxidation brought about by the formation of MgO and the control of the morphology of non-metallic inclusions accompanying the addition of Mg.

[0031] In addition, since CaO formed in steel cannot be effectively reduced by Mg, the proportion of Ca contributing to desulfurization cannot be increased either. Therefore, it is difficult to reduce the amount of Ca added to suppress the formation of MnS, and the HIC resistance of the steel plate cannot be sufficiently ensured.

[0032] Therefore, in view of the above problems, an object of the present invention is to provide a steel plate having excellent HIC resistance and an advantageous manufacturing method thereof. In addition, an object of the present invention is to provide a steel pipe using the above steel plate having excellent HIC resistance.

[0033] The inventors of the present invention repeatedly conducted a large number of experiments and studies on the composition and manufacturing conditions of the steel plate in order to improve the HIC resistance, and obtained the following findings.

[0034] That is, for the process of adding a Mg-containing substance and a Ca-containing substance to molten steel, the temperature of the molten steel and the supply rates of Mg and Ca are appropriately controlled, and then the order of starting to add the Ca-containing substance after starting to add the Mg-containing substance or starting to add the Mg-containing substance and the Ca-containing substance simultaneously is adopted. Thereby, dissolved Mg can be appropriately present in the molten steel simultaneously with dissolved Ca. As a result, due to the influence of dissolved Mg present in the molten steel, the activities of oxygen and sulfur in the molten steel decrease, and the equilibrium of the reaction of non-metallic inclusions shifts toward the formation of CaS. Thus, by promoting the formation of CaS, the improvement of the desulfurization effect is effectively achieved. In addition, as a result of adopting the above-described addition order, the reaction of Mg with S in the molten steel and the reaction of Mg with O (oxygen) in the molten steel can suppress the formation of MnS without forming coarse inclusion clusters. It should be noted that "dissolved Ca" and "dissolved Mg" refer to Ca and Mg dissolved in the molten steel in atomic states, respectively.

[0035] Furthermore, in addition to the optimization of the above addition process, the time from the end of the addition of the Ca-containing substance to the start of casting is optimized as a casting condition. As a result, in the regions on both sides of the steel plate, respectively, deeper than 1 / 4 of the plate thickness in the plate thickness direction (hereinafter, also referred to as the "inner region" in this specification), there are inclusions having an average composition containing 10 to 40% by mass of MgO and 10.0% by mass or less of MnS, and the average value of the aspect ratio of MnS is 15 or less. Moreover, the average value of the upper 10% of the equivalent circle diameter of these inclusions can be made 3.5 μm or less. As a result, a steel plate excellent in HIC resistance with a crack area ratio CAR of 5.0% or less after the HIC test in the inner region can be obtained.

[0036] The gist configuration of the present invention completed based on the above findings is as follows.

[0037] [1] A steel plate, characterized by having the following component composition, containing C: 0.030 to 0.080%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.080%, Nb: 0.080% or less, N: 0.0080% or less, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and O: 0.0003 to 0.0030% by mass, the ACR calculated by the following formula (1) is 1.00 or more, and the balance is composed of Fe and inevitable impurities.

[0038] In the regions on both sides of the steel plate, respectively, deeper than 1 / 4 of the plate thickness in the plate thickness direction, there are inclusions having an average composition containing 10 to 40% by mass of MgO and 10.0% by mass of MnS, and the average value of the aspect ratio of MnS is 15 or less.

[0039] The average value of the upper 10% of the equivalent circle diameter of the above inclusions is 3.5 μm or less.

[0040] The crack area ratio CAR after the HIC test in the above region is 5.0% or less.

[0041] ACR = {[Ca] - (1.23[O] - 0.000365)} / (1.25[S]) ··· (1)

[0042] It should be noted that [X] refers to the content (mass%) of element X in the steel.

[0043] [2]The steel sheet according to [1] above, wherein the above composition further contains, by mass%, one or more selected from Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.100% or less, Ti: 0.100% or less, Zr: 0.0200% or less, and REM: 0.0200% or less.

[0044] [3]A method for manufacturing a steel sheet, characterized in that the addition step of adding a Mg-containing substance and a Ca-containing substance to the molten steel after the end of refining is carried out under the following conditions:

[0045] (A) The temperature of the above molten steel at the start of the addition step is in the range of 1580 to 1620 °C,

[0046] (B) The above Ca-containing substance is added after starting to add the above Mg-containing substance or the above Mg-containing substance and the above Ca-containing substance are added simultaneously,

[0047] (C) The supply rates of Mg and Ca are 15 to 30 kg / min respectively;

[0048] Then, the molten steel is cast under the condition that the time from the end of the addition of the above Ca-containing substance to the start of casting is within 90 minutes to obtain a steel sheet,

[0049] Then, the above steel sheet is hot-rolled to manufacture the steel sheet according to the above [1] or [2].

[0050] [4]According to the method for manufacturing a steel sheet according to [3] above, wherein the above hot rolling is carried out under the conditions that the heating temperature of the above steel sheet is 1000 to 1250 °C, the cross rolling ratio is 20 or less, and the average value of the rolling shape ratio in the rolling passes of rough rolling is 0.7 or less,

[0051] Then, the controlled cooling of the above steel sheet is carried out under the condition that the surface temperature of the steel sheet at the start of cooling is above the Ar3 point calculated by the following formula (2).

[0052] Ar3 point (°C) = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni] - 80[Mo] ··· (2)

[0053] It should be noted that [X] refers to the content (mass%) of element X in the steel.

[0054] [5]A steel pipe using the steel sheet according to the above [1] or [2].

[0055] The steel sheet and steel pipe of the present invention have excellent HIC resistance. In addition, according to the method for manufacturing a steel sheet of the present invention, a steel sheet with excellent HIC resistance can be manufactured. Detailed implementation mode

[0056] (Steel plate)

[0057] The steel plate of an embodiment of the present invention has a specified composition, has specified inclusions in the regions (inner regions) that are more internal than 1 / 4 of the plate thickness from both sides of the steel plate in the plate thickness direction, and has excellent HIC resistance in the inner regions. It should be noted that in this specification, the positions at 1 / 4 of the plate thickness from both sides of the steel plate in the plate thickness direction are simply referred to as "1 / 4 plate thickness positions". In addition, the inner region is a region including the plate thickness center sandwiched between a pair of 1 / 4 plate thickness positions.

[0058] [Composition]

[0059] First, the composition of the steel plate and the reasons for its limitations will be described. In the following description, the unit represented by "%" is "mass%" unless otherwise specified.

[0060] C: 0.030 - 0.080%

[0061] C effectively contributes to the improvement of the strength of the steel plate. However, when the C content is less than 0.030%, sufficient strength cannot be ensured. Therefore, the C content is 0.030% or more, preferably 0.035% or more. On the other hand, if the C content exceeds 0.080%, the hardness of the surface layer and the center segregation part increases during accelerated cooling, so the HIC resistance deteriorates. Therefore, the C content is 0.080% or less, preferably 0.070% or less.

[0062] Si: 0.01 - 0.50%

[0063] Si is added for deoxidation. However, when the Si content is less than 0.01%, the deoxidation effect is insufficient. Therefore, the Si content is 0.01% or more, preferably 0.05% or more. On the other hand, if the Si content exceeds 0.50%, the low-temperature toughness deteriorates or the surface properties deteriorate. Therefore, the Si content is 0.50% or less, preferably 0.45% or less.

[0064] Mn: 0.80 - 1.80%

[0065] Mn inhibits the formation of ferrite during cooling. However, when the Mn content is less than 0.80%, its effect cannot be fully demonstrated. Therefore, the Mn content is 0.80% or more, preferably 1.00% or more. On the other hand, if the Mn content exceeds 1.80%, it promotes center segregation and the HIC resistance deteriorates. Therefore, the Mn content is 1.80% or less, preferably 1.70% or less.

[0066] P: 0.015% or less

[0067] P is an inevitable impurity element, which increases the hardness of the surface layer and the center segregation part, thereby deteriorating the HIC resistance. If the P content exceeds 0.015%, this trend becomes significant. Therefore, the P content is 0.015% or less, preferably 0.008% or less. It should be noted that the lower the P content, the better, but from the perspective of refining cost, the P content is preferably 0.001% or more.

[0068] S: 0.0015% or less

[0069] S is an inevitable impurity element, which becomes MnS inclusions in the steel and deteriorates the HIC resistance. Therefore, the S content is 0.0015% or less, preferably 0.0010% or less. It should be noted that the lower the S content, the better, but from the perspective of refining cost, the S content is preferably 0.0002% or more.

[0070] Al: 0.010 - 0.080%

[0071] Al is added as a deoxidizer, but when the Al content is less than 0.010%, its effect cannot be fully demonstrated. Therefore, the Al content is 0.010% or more, preferably 0.015% or more. On the other hand, if the Al content exceeds 0.080%, problems such as alumina blockage of the immersion nozzle during continuous casting will occur. Therefore, the Al content is 0.080% or less, preferably 0.070% or less.

[0072] Nb: 0.080% or less

[0073] If Nb exists in the form of solid solution Nb, it expands the unrecrystallized temperature range during controlled rolling and helps to improve the low-temperature toughness. From the perspective of fully obtaining this effect, the Nb content is preferably 0.005% or more, more preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.080%, coarse carbides will precipitate during solidification, so the HIC resistance deteriorates. Therefore, the Nb content is 0.080% or less, preferably 0.060% or less.

[0074] N: 0.0080% or less

[0075] N brings about the effect of microstructure refinement by forming nitrides. From the perspective of fully obtaining this effect, the N content is preferably 0.0010% or more, more preferably 0.0015% or more. On the other hand, if the N content exceeds 0.0080%, the HIC resistance and low-temperature toughness deteriorate due to the formation of coarse nitrides. Therefore, the N content is 0.0080% or less, preferably 0.0070% or less.

[0076] Ca: 0.0005 - 0.0050%

[0077] Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide-based inclusions. However, when the Ca content is less than 0.0005%, the addition effect is insufficient. Therefore, the Ca content is 0.0005% or more, preferably 0.0008% or more. On the other hand, when the Ca content exceeds 0.0050%, not only does the above effect saturate, but the cleanliness of the steel decreases, resulting in deterioration of HIC resistance. Therefore, the Ca content is 0.0050% or less, preferably 0.0045% or less.

[0078] Mg: 0.0005 - 0.0050%

[0079] Mg is an element with deoxidation and desulfurization effects, and has the effect of improving the formation efficiency of CaS and the desulfurization effect of non-metallic inclusions through interaction. In addition, Mg is an element that refines inclusions, so it is effective in suppressing the formation of inclusion clusters and improving HIC resistance. In order to stably exert this effect, the Mg content is 0.0005% or more. On the other hand, if the Mg content increases, the effect of Mg saturates, so the Mg content is 0.0050% or less.

[0080] O: 0.0003 - 0.0030%

[0081] O (oxygen) is an inevitable impurity element. By increasing the formation amounts of Al2O3, CaO, and CaS, it not only deteriorates the surface layer but also deteriorates the HIC resistance in the internal region. Therefore, the O content is 0.0030% or less. It should be noted that the lower the O content, the better, but from the perspective of steelmaking cost, the O content is 0.0003% or more.

[0082] The basic components in the composition of the steel plate of this embodiment have been described above. However, in this composition, in order to further improve the strength and toughness of the steel plate, one or more selected from Cu, Ni, Cr, and Mo can be arbitrarily contained within the following ranges.

[0083] Cu: 0.30% or less

[0084] Cu is an element effective in improving low-temperature toughness and increasing strength. In order to obtain this effect, the Cu content is preferably 0.01% or more. On the other hand, Cu accumulates in the center segregation part. Therefore, if the Cu content exceeds 0.30%, the HIC resistance deteriorates. Therefore, when Cu is contained, the Cu content is 0.30% or less, preferably 0.25% or less.

[0085] Ni: 0.30% or less

[0086] Ni is an element effective in improving low-temperature toughness and increasing strength. To achieve this effect, the Ni content is preferably 0.01% or more. On the other hand, Ni accumulates in the center segregation part. Therefore, if the Ni content exceeds 0.30%, the HIC resistance deteriorates. Thus, when Ni is contained, the Ni content is 0.30% or less, preferably 0.25% or less.

[0087] Cr: 0.50% or less

[0088] Like Mn, Cr is an element effective in obtaining sufficient strength even at a low C content. To achieve this effect, the Cr content is preferably 0.01% or more. On the other hand, if the Cr content is excessive, it promotes center segregation and the HIC resistance deteriorates. Thus, when Cr is contained, the Cr content is 0.50% or less, preferably 0.45% or less.

[0089] Mo: 0.50% or less

[0090] Mo is an element effective in improving low-temperature toughness and increasing strength. To achieve this effect, the Mo content is preferably 0.01% or more, more preferably 0.10% or more. On the other hand, if the Mo content is excessive, the weldability deteriorates. Thus, when Mo is contained, the Mo content is 0.50% or less, preferably 0.40% or less.

[0091] In the chemical composition of the steel plate of this embodiment, one or more selected from V, Ti, Zr, and REM can be arbitrarily contained within the following ranges.

[0092] V: 0.100% or less, Ti: 0.100% or less

[0093] Both V and Ti are elements that can be arbitrarily contained to increase the strength and low-temperature toughness of the steel plate. To achieve this effect, the content of each element is preferably 0.005% or more. On the other hand, if the content of each element exceeds 0.100%, the toughness of the welded part deteriorates. Thus, when these elements are contained, the content of each element is 0.100% or less.

[0094] Zr: 0.0200% or less, REM: 0.0200% or less

[0095] Zr and REM are elements that can be arbitrarily contained to increase low-temperature toughness by grain refinement or to improve crack resistance by controlling the inclusion morphology. To achieve this effect, the content of each element is preferably 0.0005% or more. On the other hand, if the content of each element exceeds 0.0200%, the effect saturates. Thus, when these elements are contained, the content of each element is 0.0200% or less.

[0096] It should be noted that the above Cu, Ni, Cr, Mo, V, Ti, Zr and REM are arbitrary elements, so the content of each element can of course be 0% or exceed 0%.

[0097] It should be noted that the remaining part other than the above elements is composed of Fe and inevitable impurities. In addition, as long as the effects of the present invention are not impaired, it does not prevent the inclusion of other trace elements. B (boron) is allowed in the present embodiment as long as its content is 0.0010% or less, preferably 0.0005% or less.

[0098] ACR: 1.00 or more

[0099] In the compositional composition of the steel plate of the present embodiment, the ACR calculated by the following formula (1) is 1.00 or more. ACR is an index for quantifying the morphological control of Ca on MnS. If the ACR is 1.00 or more, the generation of MnS in the center segregation part is suppressed, and the HIC resistance of the internal region is improved. It should be noted that the upper limit of ACR is not particularly limited, but considering the contents of Ca, O and S, the ACR can be 3.00 or less.

[0100] ACR = {[Ca] - (1.23[O] - 0.000365)} / (1.25[S]) ··· (1)

[0101] It should be noted that [X] refers to the content (mass%) of element X in the steel.

[0102] [Inclusions]

[0103] In the steel plate of the present embodiment, it is important that in the region more internal (internal region) than 1 / 4 of the plate thickness from both sides of the steel plate along the plate thickness direction, there are inclusions having an average composition containing 10 - 40 mass% of MgO and 10.0 mass% or less of MnS, and the average value of the aspect ratio of MnS is 15 or less, and the average value of the upper 10% of the equivalent circle diameter of the inclusions is 3.5 μm or less.

[0104] [MgO: 10 - 40 mass%]

[0105] In the average composition of the inclusions present in the internal region, the content of MgO is 10 mass% or more. This is because when the content of MgO is less than 10 mass%, the effect of suppressing the formation of inclusion clusters cannot be obtained, and the HIC resistance deteriorates. In addition, in the average composition of the inclusions present in the internal region, the content of MgO is 40 mass% or less. This is because when the content of MgO exceeds 40 mass%, the effect of suppressing the formation of inclusion clusters decreases, and the HIC resistance deteriorates.

[0106] The content (mass %) of MgO in the average composition of the inclusions present in the internal region is determined by the following method. Taking the cross-section perpendicular to the rolling direction of the steel plate as the observation surface, a rectangular evaluation region is set with a longitudinal dimension from one 1 / 4 position of the steel plate to the other 1 / 4 position and a transverse dimension of 10 mm in the width direction. The inclusions in this evaluation region are observed using a scanning electron microscope (SEM) with particle analysis function. For each of all the inclusions in the evaluation region, the Mg content (mass %), Al content (mass %), S content (mass %), Ca content (mass %), and Mn content (mass %) are measured, and MgO (mass %) is calculated using the following formula (3).

[0107] MgO (mass %) = (MgO[g] × 100) / (MgO[g] + Al2O3[g] + CaO[g] + CaS[g] + MnS[g]) ··· (3)

[0108] It should be noted that in formula (3), MgO[g], Al2O3[g], CaO[g], CaS[g], and MnS[g] represent the contents of MgO, Al2O3, CaO, CaS, and MnS in the inclusions calculated from the Mg content (mass %), Al content (mass %), S content (mass %), Ca content (mass %), and Mn content (mass %) contained in the inclusions, respectively. At this time, it is assumed that all Mg and Al form MgO and Al2O3 inclusions respectively, all Ca forms CaO - CaS inclusions, and all Mn forms MnS.

[0109] It should be noted that inclusions such as carbon particles in the grinding debris that do not contain MgO, Al2O3, CaO, CaS, and MnS are excluded from the measurement objects. In addition, inclusions with an equivalent circle diameter less than 0.4 μm are also excluded from the measurement objects because they do not affect the effects of the present invention.

[0110] The average value obtained by dividing the total value of MgO (mass %) of all the inclusions to be measured by the number of inclusions to be measured is taken as the content (mass %) of MgO in the average composition.

[0111] 〔MnS: 10.0 mass % or less〕

[0112] In the average composition of the inclusions present in the internal region, the content of MnS is 10.0 mass % or less. This is because when the content of MnS exceeds 10.0%, MnS with a large aspect ratio that is stretched by hot rolling is generated in the internal region, and the HIC resistance of the internal region deteriorates. The lower limit of the content of MnS is not particularly limited, but in order to suppress the increase in manufacturing cost, the content of MnS can be 1.0 mass % or more.

[0113] The content (mass %) of MnS in the average composition of the inclusions present in the internal region can be determined in the same manner as the method for determining the content (mass %) of the above-mentioned MgO. That is, by substituting "MgO [g]" in the denominator of formula (3) with "MnS [g]", the MnS (mass %) is obtained. Then, the average value obtained by dividing the total value of MnS (mass %) of all the inclusions to be measured by the number of inclusions to be measured is taken as the content (mass %) of MnS in the average composition.

[0114] 〔Average aspect ratio of MnS: 15 or less〕

[0115] In the present embodiment, the average aspect ratio of MnS in the inclusions present in the internal region is 15 or less. This is because if the average aspect ratio of MnS exceeds 15, stress is likely to concentrate at the ends of MnS, and thus the CAR in the internal region exceeds 5.0%. The lower limit of the average aspect ratio of MnS is not particularly limited, but in actual manufacturing, the average aspect ratio of MnS can be 3 or more.

[0116] The average aspect ratio of MnS in the inclusions present in the internal region is determined by the following method. The cross-section perpendicular to the rolling direction of the steel plate is used as the observation surface, and a randomly selected part (area: 200 mm 2 ) in the internal region is set as the evaluation region, and the inclusions in the evaluation region are observed using a scanning electron microscope (SEM) with a particle analysis function. MnS appears dark in the backscattered electron image in the SEM, so MnS can be identified within the inclusions. Inclusions with a major axis of 1 μm or more in size are taken as the measurement objects. For all MnS in the inclusions with a major axis of 1 μm or more present in the evaluation region, the aspect ratio (major axis / minor axis) is calculated, and the average value thereof is taken as the "average aspect ratio of MnS in the inclusions present in the internal region". It should be noted that the major axis refers to the maximum Feret diameter, and the minor axis refers to the minimum Feret diameter.

[0117] The number density of the inclusions in the internal region is preferably 1.0 piece / mm 2 or more, preferably 50.0 pieces / mm 2The following is the case because the desired HIC resistance can be obtained. The number density of inclusions in the internal region is determined by the following method. Take the cross-section perpendicular to the rolling direction of the steel plate as the observation surface, and set the longitudinal direction: from one 1 / 4 position of the steel plate to the other 1 / 4 position, and the transverse direction: a rectangular evaluation region of 10 mm in the width direction. Observe the inclusions in this evaluation region using a scanning electron microscope (SEM) with particle analysis function. Count the number of inclusions with an equivalent circle diameter of 0.4 μm or more existing in the evaluation region, and find the value obtained by dividing the number by the total area of the evaluation region, and adopt it as the "number density of inclusions in the internal region".

[0118] [Average value of the upper 10% of the equivalent circle diameter of inclusions: 3.5 μm or less]

[0119] In this embodiment, it is important that the average value of the upper 10% of the equivalent circle diameter of inclusions existing in the internal region is 3.5 μm or less. Here, "upper 10%" means 10% in terms of the number of existences starting from the side with a larger equivalent circle diameter in the number distribution of the equivalent circle diameter of inclusions. If the average value of the equivalent circle diameter of these 10% of inclusions is specified, it is possible to evaluate the coarse inclusions that may be the starting point of HIC among the inclusions within the measurement range, so it becomes an index for improving HIC resistance. In this embodiment, if the average value of the upper 10% of the equivalent circle diameter of inclusions existing in the internal region exceeds 3.5 μm, the HIC resistance of the internal region deteriorates, and thus the crack area ratio CAR after the HIC test of the internal region exceeds 5.0%. In this embodiment, the lower limit of the average value of the upper 10% of the equivalent circle diameter of inclusions existing in the internal region is not particularly limited, but in order to suppress the increase in steelmaking cost, the average value of the upper 10% of the equivalent circle diameter of inclusions is preferably 1.5 μm or more.

[0120] The average value of the upper 10% of the equivalent circle diameter of inclusions existing in the internal region is determined by the following method. Take the cross-section perpendicular to the rolling direction of the steel plate as the observation surface, and set the longitudinal direction: from one 1 / 4 position of the steel plate to the other 1 / 4 position, and the transverse direction: a rectangular evaluation region of 10 mm in the width direction. Observe the inclusions in this evaluation region using a scanning electron microscope (SEM) with particle analysis function. Measure the equivalent circle diameter of inclusions with an equivalent circle diameter of 0.4 μm or more existing in the evaluation region, and for the upper 10% of the inclusions in terms of equivalent circle diameter among them, find the average value of their equivalent circle diameter, and adopt it as the "average value of the upper 10% of the equivalent circle diameter of inclusions existing in the internal region".

[0121] [HIC resistance]

[0122] In this embodiment, the crack area ratio CAR after the HIC test in the internal region is 5.0% or less. Thus, excellent HIC resistance can be achieved. In this embodiment, the crack area ratio CAR after the HIC test in the internal region can be 0.0% or more.

[0123] The crack area ratio CAR after the HIC test in the internal region is obtained by the following method. A sample with a total thickness × 20 mm width × 100 mm length is taken from the center position in the rolling direction and width direction of the steel plate. An HIC test is carried out by immersing the sample in a solution of NACE standard TM0177 Solution A for 96 hours at a hydrogen sulfide partial pressure of 1 bar. The crack area ratio (CAR) is measured after the HIC test in the internal region of the steel plate. Specifically, using an ultrasonic flaw detector and a water immersion type probe (frequency: 10 MHz, diameter: 0.375 inches, focal depth: 3 inches), an ultrasonic flaw detection test is carried out at a pitch of 0.1 mm on the regions more internal than 1 / 4 of the plate thickness from both sides of the steel plate in the plate thickness direction, and the magnification is determined so that the echo of the bottom surface wave is 100%. The area ratio of the part with a defect echo height of 25% or more in the internal region of the steel plate is taken as the crack area ratio (CAR).

[0124] [Thickness]

[0125] The plate thickness of the steel plate of this embodiment is not particularly limited, and a thickness of 12 to 39 mm is preferably provided.

[0126] (Manufacturing method of steel plate)

[0127] Hereinafter, a manufacturing method of a steel plate according to an embodiment of the present invention for manufacturing the steel plate of this embodiment will be described.

[0128] A manufacturing method of a steel plate according to an embodiment of the present invention has: an addition step of adding a Mg-containing substance and a Ca-containing substance to the molten steel after refining; a step of then casting the molten steel to obtain a steel sheet; a step of then performing hot rolling on the steel sheet to obtain a steel plate; and a step of then performing controlled cooling of the steel plate.

[0129] The refining process of molten steel can be carried out by the methods known in the past. For example, after the pretreatment of hot metal, primary refining is carried out using a converter, and then secondary refining is carried out, whereby the cleanliness of the molten steel in the ladle can be improved. In the above secondary refining, the molten steel is transferred from the converter to the ladle, and desulfurization treatment is carried out in a ladle refining furnace, and further vacuum degassing treatment is carried out in an RH vacuum degassing device. It should be noted that in the manufacturing method of acid-resistant steel plates, the ladle refining furnace is used to desulfurize the S content in the molten steel to less than 0.0010%. In addition, the RH vacuum degassing device is used to remove hydrogen (H) in the molten steel and separate and remove oxide-based non-metallic inclusions such as Al2O3 in the molten steel.

[0130] [Process of adding Mg-containing substance and Ca-containing substance]

[0131] The addition time of the Mg-containing substance and the Ca-containing substance to the molten steel in the ladle is from the end of the above refining process to the start of casting. This is to prevent Mg and Ca, which are strong deoxidizers and strong desulfurizers, from reacting with dissolved oxygen or reducing oxide-based non-metallic inclusions and being consumed, so that Mg and Ca can contribute to the morphology control of non-metallic inclusions.

[0132] In the present embodiment, it is important to carry out the process of adding the Mg-containing substance and the Ca-containing substance to the molten steel after refining under the following conditions: (A) the temperature of the above molten steel at the start of this addition process is in the range of 1580 - 1620 °C, (B) the addition of the above Ca-containing substance starts after the addition of the above Mg-containing substance or the addition of the above Mg-containing substance and the above Ca-containing substance starts simultaneously, (C) the supply rates of Mg and Ca are 15 - 30 kg / min respectively.

[0133] Both Mg and Ca are elements with high vapor pressures and evaporate at normal molten steel temperatures. In addition, Mg and Ca not only have high reactivity with molten steel but also have high reactivity with air. Therefore, in order to suppress the evaporation of Mg and Ca and improve the yield of Mg and Ca, pure metals of Mg and Ca are not added, but Mg-containing substances and Ca-containing substances are added. The Mg-containing substance is preferably an Mg alloy obtained by alloying Mg with one or more selected from silicon, aluminum, etc. The Ca-containing substance is preferably a Ca alloy obtained by alloying Ca with one or more selected from silicon, aluminum, etc. From the viewpoints of treatment and reactivity with molten steel, the pure Mg of the Mg alloy and the pure Ca of the Ca alloy are preferably 5 - 30 mass% respectively. In addition, the simultaneous addition of the Mg-containing substance and the Ca-containing substance includes the addition of a substance containing both Mg and Ca.

[0134] The Mg-containing material can be, for example, a powder or granule of a Mg alloy, or an iron-coated Mg wire formed by coating a powder or granule of a Mg alloy with a thin steel plate. The Ca-containing material can be, for example, a powder or granule of a Ca alloy, or an iron-coated Ca wire formed by coating a powder or granule of a Ca alloy with a thin steel plate. In addition, when adding the Mg-containing material and the Ca-containing material simultaneously, the iron-coated Mg wire and the iron-coated Ca wire can be added separately, but as a material containing both Mg and Ca, an iron-coated Mg-Ca wire formed by coating a powder or granule of a Mg alloy and a powder or granule of a Ca alloy with one thin steel plate can also be added.

[0135] The method of adding the Mg-containing material and the Ca-containing material can be a method of adding the above-mentioned metal wire to the molten steel, or a method of blowing a powder or granule of a Mg alloy and a powder or granule of a Ca alloy from an injection gun immersed in the molten steel.

[0136] 〔Temperature of the molten steel at the start of the addition process: 1580 - 1620 °C〕

[0137] If the temperature of the molten steel at the start of the addition process is too high, the evaporation amounts of Mg and Ca into the atmosphere increase, and the yield rates (dissolved amounts in the molten steel) of Mg and Ca decrease. On the other hand, if the temperature of the molten steel at the start of the addition process is too low, the stirring of the molten steel caused by evaporation becomes insufficient, and the yield rate also decreases in this case. In addition, if the temperature of the molten steel at the start of the addition process is too high or too low, the MnS content in the average composition of the inclusions becomes too large, and the HIC resistance of the internal region deteriorates. Therefore, in order to stabilize the yield rates of Mg and Ca and ensure the HIC resistance, the temperature of the molten steel at the start of the addition process is in the range of 1580 - 1620 °C.

[0138] 〔Addition order of the Mg-containing material and the Ca-containing material〕

[0139] In the present embodiment, the first method of starting to add the Ca-containing material after starting to add the Mg-containing material or the second method of starting to add the Mg-containing material and the Ca-containing material simultaneously is adopted. Thereby, an appropriate amount of dissolved Ca and dissolved Mg can exist in the molten steel. Due to the influence of an appropriate amount of dissolved Mg in the molten steel, the activities of oxygen and sulfur in the molten steel decrease, and the equilibrium of the reaction of non-metallic inclusions shifts in the direction of CaS formation. The formation of CaS is promoted, and the improvement of the desulfurization effect is achieved. In addition, the reaction of Mg with S in the molten steel and the reaction of Mg with O (oxygen) in the molten steel also proceed. As a result, the average value of the upper 10% of the equivalent circle diameters of the inclusions existing in the internal region can be 3.5 μm or less.

[0140] In contrast, in the addition sequence where the Mg-containing material is added after starting to add the Ca-containing material, the effects of desulfurization due to the formation of MgS accompanying the addition of the Mg-containing material, deoxidation due to the formation of MgO, and the control of the morphology of non-metallic inclusions cannot be obtained. This is because the oxides and sulfides of Ca are thermodynamically more stable than those of Mg. That is, the average value of the upper 10% of the equivalent circle diameters of the inclusions present in the inner region exceeds 3.5 μm.

[0141] It should be noted that in the first method where the Ca-containing material is added after starting to add the Mg-containing material, there can be a period during which both the Mg-containing material and the Ca-containing material are added (i.e., the overlapping period between the addition period of the Mg-containing material and the addition period of the Ca-containing material), or the Ca-containing material can be added after the addition of the Mg-containing material is completed. That is, there can also be no overlapping period. However, when the Ca-containing material is added after the addition of the Mg-containing material is completed, from the viewpoints of productivity and the effects of the present invention, the shorter the interval, the better, and it is preferably 5 minutes or less.

[0142] In addition, in the case of the second method where the Mg-containing material and the Ca-containing material are added simultaneously, it is also preferable that the addition of the Mg-containing material and the Ca-containing material be completed simultaneously. As a method that is easy to implement this operation, a method of adding an Fe-coated Mg-Ca wire to the molten steel can be cited. However, even in the second method, it is of course possible to separately and simultaneously start adding an Fe-coated Mg wire and an Fe-coated Ca wire. In this case, "simultaneously start adding" does not mean that the addition starts strictly in units of seconds and is consistent, and of course, the deviation in the start of addition that is inevitable in the operation of the input machine (for example, within 30 seconds) is allowed.

[0143] [Supply rates of Mg and Ca: 15 - 30 kg / min respectively]

[0144] If the supply rates of Mg and Ca (the addition rates of the Mg-containing material and the Ca-containing material) are too fast, the content of MgO in the average composition of the inclusions becomes too large, and the HIC resistance deteriorates. On the other hand, if the supply rates of Mg and Ca (the addition rates of the Mg-containing material and the Ca-containing material) are too slow, the content of MgO in the average composition of the inclusions becomes too small, and the HIC resistance still deteriorates. Therefore, in the present embodiment, the Mg-containing material and the Ca-containing material are added in such a way that the supply rates of pure Mg and pure Ca are 15 - 30 kg / min respectively. It should be noted that the supply rates of pure Mg and pure Ca respectively refer to the supply amounts of Mg and Ca per minute during the period of adding the Mg-containing material and the Ca-containing material.

[0145] [Casting process]

[0146] In this embodiment, it is important to cast molten steel to obtain steel sheets such as slabs and bloom segments under the condition that the time from the end of adding the Ca-containing substance to the start of casting is within 90 minutes. Other conditions in the casting process can be in accordance with conventional methods. In the casting process, continuous casting can be carried out using a continuous casting apparatus.

[0147] [Time from the end of adding the Ca-containing substance to the start of casting: within 90 minutes]

[0148] In the ladle, inclusions float and separate from the molten steel over time. The floating and separation of inclusions contribute to improving the cleanliness of the molten steel. Therefore, conventionally, the time from the end of adding the Ca-containing substance to the start of casting was usually 100 minutes or more. In contrast, in this embodiment, inclusions generated by adding the Mg-containing substance and the Ca-containing substance are necessary for improving the HIC resistance. Therefore, if too many inclusions float and separate from the molten steel, the HIC resistance deteriorates. Therefore, the time from the end of adding the Ca-containing substance to the start of casting is within 90 minutes. This time is preferably within 80 minutes. The lower limit of this time is not particularly limited, but considering cleanliness, this time is preferably 40 minutes or more. It should be noted that in this embodiment, by satisfying the above-mentioned addition amount of Ca, the cleanliness of the molten steel reaches a level where there is no problem even for the above-mentioned time.

[0149] [Hot rolling process]

[0150] Next, the steel sheet is hot-rolled to obtain a steel plate. The hot rolling is preferably carried out under the conditions that the heating temperature of the steel sheet is 1000 - 1250°C, the cross rolling ratio is 20 or less, and the average value of the rolling shape ratio in the rolling passes of rough rolling is 0.7 or less. Other conditions in the hot rolling process can be in accordance with conventional methods.

[0151] [Heating temperature of the steel sheet: 1000 - 1250°C]

[0152] When the heating temperature of the steel sheet during hot rolling is less than 1000°C, controlled cooling cannot be started above the Ar3 point, and HIC resistance cannot be obtained. Therefore, the heating temperature of the steel sheet is 1000°C or more, preferably 1030°C or more. On the other hand, if the heating temperature of the steel sheet exceeds 1250°C, the energy consumption increases. Therefore, the heating temperature of the steel sheet is 1250°C or less, preferably 1200°C or less. It should be noted that this temperature is the temperature inside the heating furnace, and the steel sheet is heated to this temperature up to the center part.

[0153] [Cross rolling ratio: 20 or less]

[0154] In this embodiment, the cross rolling ratio defined by the following formula (4) is preferably 20 or less. If the cross rolling ratio is 20 or less, elongation of MnS in inclusions can be effectively prevented, and the HIC resistance is further improved. The lower limit of the cross rolling ratio is not particularly limited, but from the viewpoint of restrictions on rolling equipment, the cross rolling ratio can be 6 or more.

[0155] Cross rolling ratio = rolling ratio in the rolling direction / rolling ratio in the direction perpendicular to rolling ··· (4)

[0156] It should be noted that the "rolling ratio in the rolling direction" refers to the rolling ratio in the rolling direction relative to the total rolling during cross rolling in hot rolling. In addition, the "rolling ratio in the direction perpendicular to rolling" refers to the rolling ratio in the direction perpendicular to the total rolling during cross rolling in hot rolling.

[0157] [Average value of rolling shape ratio in rough rolling passes: 0.7 or less]

[0158] In this embodiment, it is preferable that the average value of the rolling shape ratio in the rough rolling passes is 0.7 or less. Thereby, the reduction in the thickness center part becomes smaller, elongation of MnS caused by rolling in the internal region can be prevented, and the CAR in the internal region can be effectively made 5.0% or less. On the other hand, when the average value of the rolling shape ratio exceeds 0.7, although the recrystallized grain generation effect in the thickness center part can be obtained, elongation of MnS is caused, and thus the HIC resistance in the internal region deteriorates. The lower limit of the average value of the rolling shape ratio is not particularly limited, but from the viewpoint of restrictions on rolling equipment, the average value of the rolling shape ratio can be 0.3 or more. It should be noted that in this embodiment, a grain refinement effect in the thickness center part of the steel plate can be obtained through finish rolling after rough rolling. Here, the average value of the rolling shape ratio refers to the value of Ld / hm, that is, {length of contact between the roll and the steel plate (roll contact arc length: Ld)} / {average thickness of the plate thickness on the roll inlet side and the plate thickness on the roll outlet side: hm}, and is obtained by the following formula (5).

[0159] Ld / hm = {R(hi - ho)} 1 / 2 / {(hi + 2ho) / 3} ··· (5)

[0160] It should be noted that R: roll radius of each rolling pass

[0161] hi: plate thickness on the inlet side of each rolling pass

[0162] ho: plate thickness on the outlet side of each rolling pass.

[0163] [Controlled cooling process]

[0164] Next, a controlled cooling process of the steel plate is carried out. This process is preferably carried out under the condition that the surface temperature of the steel plate at the start of cooling is above the Ar3 point calculated by the following formula (2). When the surface temperature of the steel plate at the start of cooling is lower than the Ar3 point, there is a possibility of ferrite formation before cooling, resulting in deterioration of the HIC resistance. Therefore, it is preferable that the surface temperature of the steel plate at the start of cooling is above the Ar3 point. Here, the Ar3 point refers to the start temperature of the ferrite phase transformation during cooling and can be calculated by the following formula (2) according to the composition of the steel, for example. It should be noted that the surface temperature of the steel plate can be measured using a radiation thermometer or the like.

[0165] Ar3 point (°C) = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni] - 80[Mo] ··· (2)

[0166] It should be noted that [X] represents the content (mass%) of element X in the steel, and the content of elements not contained can be substituted with 0. Other conditions in the controlled cooling process can be in accordance with conventional methods.

[0167] (Steel pipe)

[0168] The steel plate of one embodiment of the present invention can be formed into a tubular shape by press bending, roll forming, UOE forming, etc., and then the butt joint is welded to manufacture the steel pipe of one embodiment of the present invention (UOE steel pipe, electric resistance welded steel pipe, spiral steel pipe, etc.). The steel pipe of this embodiment is suitable for the transportation of crude oil or natural gas.

[0169] For example, in the case of a UOE steel pipe, the end of the steel plate is beveled, formed into a steel pipe shape by C stamping, U stamping, and O stamping, and then the butt joint is seam welded using internal surface welding and external surface welding, and further manufactured via an expanding process as required. In addition, as long as the welding method can obtain sufficient joint strength and joint toughness, any method can be used. From the viewpoints of excellent welding quality and manufacturing efficiency, submerged arc welding is preferably used. In addition, after forming the steel plate into a tubular shape by press bending, an expanding process can also be performed on the steel pipe with the butt joint seam welded.

[0170] Examples

[0171] In a converter, hot metal is decarburized and refined to melt molten steel, and the molten steel is tapped into a ladle. After tapping from the converter, metallic Al is added to the ladle to deoxidize the molten steel.

[0172] Then, the molten steel contained in the ladle is desulfurized in a ladle refining furnace. In this desulfurization process, a CaO - Al2O3 - SiO2 - based flux is used as the desulfurizing agent. Then, the temperature of the molten steel is raised using the arc heat from a graphite electrode, and at the same time, the above-mentioned desulfurizing agent is slagged, and 100 - 150 Nm is applied from an immersion lance immersed in the molten steel. 3Argon gas is blown in at a speed of / h as the stirring gas to stir and mix the molten steel and the desulfurizing agent, and desulfurization treatment is carried out.

[0173] Furthermore, vacuum degassing refining is carried out in the RH vacuum degassing device. Specifically, degassing treatment, adjustment of the molten steel composition, and floating and separation of non-metallic inclusions caused by stirring are carried out. The treatment time of the above vacuum degassing refining is 20 minutes.

[0174] Next, during the period from the end of refining in the RH vacuum degassing device to the start of continuous casting in the continuous casting equipment, an Fe-coated Mg wire as a Mg-containing substance and an Fe-coated Ca wire as a Ca-containing substance are added to the molten steel in the ladle to obtain a steel (steel grades A to U) having the composition shown in Table 1. The molten steel temperature at the start of the addition process under each condition, the addition order of the Mg-containing substance and the Ca-containing substance, and the supply rates of Mg and Ca are shown in Table 2. It should be noted that in the "Mg, Ca addition order" column in Table 2, "Mg→Ca" means starting to add the Fe-coated Mg wire and immediately starting to add the Fe-coated Ca wire after the end, "Ca→Mg" means starting to add the Fe-coated Ca wire and immediately starting to add the Fe-coated Mg wire after the end, and "Mg, Ca simultaneously" means adding an Fe-coated Mg-Ca wire.

[0175] Next, the molten steel is cast by continuous casting to form a slab. The time from the end of the addition of the Ca-containing substance to the start of casting under each condition is shown in Table 2.

[0176] Next, the slab is heated to the slab heating temperature shown in Table 2, and hot rolling is carried out on the slab under the conditions of the cross rolling ratio shown in Table 2 and the average value of the rolling shape ratio in the number of rolling passes of rough rolling to form a steel plate with the final plate thickness shown in Table 2. Then, controlled cooling of the steel plate is carried out at the steel plate surface temperature at the start of cooling shown in Table 2.

[0177] Then, beveling is carried out on the ends of the steel plate, and it is formed into a steel pipe shape by C stamping, U stamping, and O stamping, and then the butt joint is seam welded from the inner surface side and the outer surface side by submerged arc welding, and a steel pipe is made through an expanding process. It should be noted that the composition of the steel plates and steel pipes manufactured from the above steel grades A to U is the same as that of the steel grades A to U that become the steel billet materials within the industrial error range. It should be noted that in Table 1, for the elements with a hyphen (-), their contents are regarded as zero in the calculation of Ar3 (°C).

[0178] [Evaluation of Inclusions]

[0179] Table 2 shows the "contents of MgO and MnS in the average composition of inclusions present in the inner region", the "average value of the upper 10% of the equivalent circle diameters of inclusions present in the inner region", the "number density of inclusions in the inner region", and the "average value of the aspect ratios of MnS in inclusions present in the inner region" obtained by the above method.

[0180] [Evaluation of HIC resistance]

[0181] Table 2 shows the "crack area ratio CAR after the HIC test in the inner region" obtained by the above method.

[0182]

[0183]

[0184] As shown in Table 2, Nos. 1 to 10 are inventive examples in which the component composition and manufacturing conditions satisfy the appropriate ranges of the present invention. In any example, inclusions having an average composition containing 10 to 40% by mass of MgO and 10.0% by mass or less of MnS and an average aspect ratio of MnS of 15 or less are present in the inner region, the average value of the upper 10% of the equivalent circle diameters of the inclusions is 3.5 μm or less, and the crack area ratio CAR after the HIC test in the inner region is 5.0% or less.

[0185] In contrast, the component compositions of the steel plates of Nos. 11 to 13 are outside the inventive range, so center segregation is promoted and the CAR does not reach the desired value.

[0186] The S content of the steel plate of No. 14 is more than the inventive range, so excessive MnS is generated and the CAR does not reach the desired value.

[0187] The Nb content of the steel plate of No. 15 is more than the inventive range, so coarse carbides precipitate and the CAR does not reach the desired value.

[0188] The N content of the steel plate of No. 16 is more than the inventive range, so coarse nitrides are generated and the CAR does not reach the desired value.

[0189] The temperature of the molten steel at the start of the addition process of No. 17 is too high, so the yield is reduced. As a result, the Ca content is excessively reduced and the CAR does not reach the desired value. It should be noted that there is a deviation in the yield, and an example in which only the Ca content is reduced is shown in this No. 17.

[0190] The Ca content of the steel plate of No. 18 is more than the inventive range, so the cleanliness of the steel is reduced and the CAR does not reach the desired value.

[0191] The temperature of the molten steel at the start of the addition process for No.19 was too low, so the yield rate decreased. As a result, the Mg content decreased excessively and the CAR did not reach the desired value. It should be noted that there are differences in the yield rate, and an example of only a decrease in the Mg content is shown in this No.19.

[0192] The O content of the steel plate of No.20 is more than the invention range, so the CAR did not reach the desired value.

[0193] The ACR of No.21 is less than 1.00, so coarse MnS is generated and the CAR did not reach the desired value.

[0194] The cross rolling of No.22 is insufficient, so the average value of the aspect ratio of MnS in the inclusions becomes larger and the CAR did not reach the desired value.

[0195] No.23 increases the average value of the rolling shape ratio, and as a result, the average value of the aspect ratio of MnS in the inclusions becomes too large and the CAR did not reach the desired value.

[0196] No.24 adds Mg after adding Ca, so the average value of the upper 10% of the equivalent circle diameter of the inclusions becomes too large and the CAR did not reach the desired value.

[0197] The addition speed of the wire of No.25 is too slow, and as a result, the MgO content of the inclusions becomes too little and the CAR did not reach the desired value.

[0198] The time from the end of Ca addition to the start of casting for No.26 is long, so the MnS content of the inclusions becomes too large and the average value of the upper 10% of the equivalent circle diameter of the inclusions becomes too large, and the CAR did not reach the desired value.

[0199] The addition speed of the wire of No.27 is too fast, and as a result, the MgO content and MnS content of the inclusions become too much and the CAR did not reach the desired value.

[0200] Industrial availability

[0201] According to the present invention, steel plates and steel pipes with excellent HIC resistance can be provided.

Claims

1. A steel plate, characterized in that, It has the following composition, containing C: 0.030 - 0.080% by mass, Si: 0.01 - 0.50% by mass, Mn: 0.80 - 1.80% by mass, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 - 0.080% by mass, Nb: 0.080% or less, N: 0.0080% or less, Ca: 0.0005 - 0.0050% by mass, Mg: 0.0005 - 0.0050% by mass, and O: 0.0003 - 0.0030% by mass. The ACR calculated by the following formula (1) is 1.00 or more, and the balance is composed of Fe and inevitable impurities. Moreover, in the regions on both sides of the steel plate, respectively, deeper than 1 / 4 of the plate thickness in the thickness direction, there are inclusions having an average composition containing 10 - 40% by mass of MgO and 10.0% by mass or less of MnS, and the average value of the aspect ratio of MnS is 15 or less. The average value of the upper 10% of the equivalent circle diameter of the inclusions is 3.5 μm or less. The crack area ratio CAR after the HIC test in the said region is 5.0% or less. ACR = {[Ca] - (1.23[O] - 0.000365)} / (1.25[S]) ··· (1) It should be noted that [X] refers to the content of element X in the steel by mass%.

2. The steel plate according to claim 1, wherein, The said composition further contains one or more selected from Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.100% or less, Ti: 0.100% or less, Zr: 0.0200% or less, and REM: 0.0200% or less by mass%.

3. A method for manufacturing a steel plate, characterized in that, The addition process of adding a Mg-containing substance and a Ca-containing substance to the molten steel after the end of refining is carried out under the following conditions: (A) The temperature of the molten steel at the start of this addition process is in the range of 1580 - 1620 °C. (B) In such a way that the addition of the Ca-containing substance starts after the addition of the Mg-containing substance or the addition of the Mg-containing substance and the Ca-containing substance starts simultaneously. (C) The supply rates of Mg and Ca are 15 - 30 kg / min respectively. Then, the molten steel is cast to obtain a steel sheet under the condition that the time from the end of the addition of the Ca-containing substance to the start of casting is within 90 minutes. Then, the steel sheet is hot-rolled to manufacture the steel plate described in claim 1 or 2.

4. The method for manufacturing a steel plate according to claim 3, wherein, The said hot rolling is carried out under the conditions that the heating temperature of the steel sheet is 1000 - 1250 °C, the cross rolling ratio is 20 or less, and the average value of the rolling shape ratio in the rolling passes of rough rolling is 0.7 or less. Then, the controlled cooling of the steel plate is carried out under the condition that the surface temperature of the steel plate at the start of cooling is above the Ar3 point calculated by the following formula (2). Ar3 point = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni] - 80[Mo] ··· (2) It should be noted that [X] refers to the content of element X in the steel by mass%, and the unit of the Ar3 point is °C.

5. A steel pipe using the steel plate according to claim 1 or 2.

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