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, contains inclusions having an average composition containing 10-40% by mass of MgO and having an average aspect ratio of 2.5 or less in a pair of regions extending from both surfaces of the steel sheet to a depth of 1 / 4 of the sheet thickness in the sheet thickness direction, and has an average value of 3.5 [mu] m or less for upper 10% of the equivalent circle diameter of the inclusions. The crack area ratio (CAR) of the region after an HIC test 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. In addition, 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 accumulate around non-metallic inclusions such as MnS or hard second-phase structures in the steel, become molecular hydrogen, and cracks are generated due to its internal pressure.

[0004] As non-metallic inclusions in steel that are likely to be the starting points of HIC, MnS (manganese sulfide) obtained by hot rolling and stretching 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) and deoxidation product Al 2 O 3 (aluminum oxide) to generate CaO - Al 2 O 3 series non-metallic inclusions.

[0005] Therefore, when Ca in the molten steel is insufficient, there is insufficient Ca to react with S in the steel, a large amount of MnS is generated, and the HIC resistance of the center segregation part deteriorates. In addition, when Ca is excessive, a CaO - Al with a high CaO concentration is generated 2 O 3 series non-metallic inclusion clusters, and the HIC resistance of the steel plate surface layer deteriorates.

[0006] Here, in recent years, the development of low-cost steel plates has been advanced by increasing the content of manganese (Mn) and decreasing the content of other alloying elements. In the case of such steel plates, MnS that deteriorates the 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 in the entire steel plate is decreased, the generation frequency of MnS in the center segregation part increases.

[0007] As a countermeasure, if the operation of further increasing the Ca addition amount is carried out to suppress the generation of MnS, it is easy to generate a CaO–Al 2 O 3 system non-metallic inclusion cluster as described above. Moreover, this inclusion cluster is trapped in the surface layer of the ingot during continuous casting, and thus becomes the starting point of HIC in the surface layer of the steel plate, causing 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 generation of MnS and the generation of inclusion clusters by methods other than increasing the Ca addition amount.

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

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

[0011] As a method of adding Mg to molten steel, Patent Document 4 discloses the following method: by containing 2 to 10 mass% of MgO in the slag, supplying MgO to non-metallic inclusions, and controlling 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 inner 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 poured from the ladle into the tundish in a continuous casting apparatus.

[0014] It should be noted that regarding the shape of inclusions, it is known that the larger the aspect ratio (major axis / minor axis) of the inclusions, the greater the stress concentration caused by the inclusions. Furthermore, it is 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 a steel sheet with excellent HIC resistance by the conventional rolling method.

[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 Document 1 and Patent Document 2 disclose that Mg exists in the form of fine Mg-based oxides, and these fine Mg-based oxides function as precipitation nuclei of TiN and are effective in improving the toughness of the heat affected zone (HAZ) of welding. That is, in the technologies described in Patent Document 1 and Patent Document 2, since the surface of the Mg-based oxides is covered with 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 Document 5 and Patent Document 6, considering the repair cost of the refractory material and the cost of the alloy 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-in inert gas is 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 blister defects by adding alloys.

[0028] Thus, although acid-resistant steel containing Ca and Mg has been proposed in the above prior arts, 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 the molten steel, the influence of the 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 the molten steel easily react with Ca, and it is difficult for Mg to react with S and O (oxygen) in the 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 the steel cannot be effectively reduced by Mg, the proportion of Ca that helps desulfurization cannot be increased either. Therefore, it is difficult to reduce the Ca addition amount to suppress the formation of MnS, and inclusion clusters are trapped on the surface layer of the cast sheet, whereby the HIC resistance of the steel sheet surface layer cannot be sufficiently ensured.

[0032] Therefore, in view of the above problems, an object of the present invention is to provide a steel sheet 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 sheet 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 sheet in order to improve the HIC resistance, and obtained the following findings.

[0034] That is, for the process of adding Mg-containing substances and Ca-containing substances 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. In this way, 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 are carried out, and the formation of MnS can be suppressed without forming coarse inclusion clusters. It should be noted that "dissolved Ca" and "dissolved Mg" respectively refer to Ca and Mg dissolved in the molten steel in atomic states.

[0035] Furthermore, in addition to the optimization of the above-described addition process, the time from the end of the addition of the Ca-containing substance to the start of casting and the average flow rate of the molten steel in the mold during casting are optimized as casting conditions. Thereby, in a pair of regions (hereinafter, also referred to as "surface regions" in this specification) from both sides of the steel plate along the plate thickness direction to a depth of 1 / 4 of the plate thickness, there are inclusions having an average composition containing 10 to 40% by mass of MgO and an average aspect ratio of 2.5 or less. Moreover, the average value of the upper 10% of the equivalent circle diameters of these inclusions can be 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 surface region can be obtained.

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

[0037] [1] A steel plate, characterized in that it has the following composition, containing C: 0.030 to 0.080% by mass, 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%, and Mg: 0.0005 to 0.0050% by mass, and the balance is composed of Fe and inevitable impurities.

[0038] In a pair of regions from both sides of the steel plate along the plate thickness direction to a depth of 1 / 4 of the plate thickness, there are inclusions having an average composition containing 10 to 40% by mass of MgO and an average aspect ratio of 2.5 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] [2] The steel sheet according to the above [1], 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.

[0042] [3] A method for manufacturing a steel sheet, characterized in that an addition step of adding a Mg-containing substance and a Ca-containing substance to molten steel after refining is carried out under the following conditions:

[0043] (A) The temperature of the above molten steel at the start of the addition step is in the range of 1580 - 1620 °C.

[0044] (B) The above Ca-containing substance is added after the start of adding the above Mg-containing substance or the above Mg-containing substance and the above Ca-containing substance are added simultaneously.

[0045] (C) The supply rates of Mg and Ca are 15 - 30 kg / min respectively.

[0046] Then, the above molten steel is cast under the conditions that (i) the time from the end of adding the above Ca-containing substance to the start of casting is within 90 minutes and (ii) the average flow velocity of the molten steel in the mold during casting is 0.10 m / s or more to obtain a steel sheet.

[0047] Then, the above steel sheet is hot-rolled to manufacture the steel sheet described in the above [1] or [2].

[0048] [4] The method for manufacturing a steel sheet according to the above [3], wherein the above hot rolling is carried out under the conditions that the heating temperature of the above steel sheet is 1000 - 1250 °C and the cross rolling ratio is 20 or less.

[0049] 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 Ar 3 point obtained by the following formula (1).

[0050] Ar 3 point (°C) = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni] - 80[Mo] ··· (1)

[0051] It should be noted that [X] refers to the content (mass%) of element X in steel.

[0052] [5] A steel pipe using the steel plate described in [1] or [2] above.

[0053] The steel plate and steel pipe of the present invention have excellent HIC resistance. In addition, according to the manufacturing method of the steel plate of the present invention, a steel plate with excellent HIC resistance can be manufactured. Detailed Embodiments

[0054] (Steel Plate)

[0055] The steel plate according to an embodiment of the present invention has a specified composition, has specified inclusions in a pair of first regions (surface regions) from both sides of the steel plate along the plate thickness direction to a depth of 1 / 4 of the plate thickness, and has excellent HIC resistance in the surface region. It should be noted that in this specification, the position at a depth of 1 / 4 of the plate thickness from both sides of the steel plate along the plate thickness direction is simply referred to as the "1 / 4 plate thickness position".

[0056] [Composition]

[0057] 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.

[0058] C: 0.030 - 0.080%

[0059] 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, and thus the HIC resistance deteriorates. Therefore, the C content is 0.080% or less, preferably 0.070% or less.

[0060] Si: 0.01 - 0.50%

[0061] 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.

[0062] Mn: 0.80 - 1.80%

[0063] Mn inhibits the formation of ferrite during cooling. However, when the Mn content is less than 0.80%, its effect cannot be fully manifested. 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.

[0064] P: 0.015% or less

[0065] P is an inevitable impurity element that 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.

[0066] S: 0.0015% or less

[0067] S is an inevitable impurity element that deteriorates the HIC resistance by becoming MnS inclusions in the steel. 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.

[0068] Al: 0.010 - 0.080%

[0069] Al is added as a deoxidizer. However, when the Al content is less than 0.010%, its effect cannot be fully manifested. 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.

[0070] Nb: 0.080% or less

[0071] If Nb exists in the form of solid solution Nb, it expands the non-recrystallization temperature range during controlled rolling and helps 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.

[0072] N: 0.0080% or less

[0073] N brings about the effect of refining the structure by generating nitrides. From the viewpoint 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 resistance to HIC 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.

[0074] Ca: 0.0005 - 0.0050%

[0075] Ca is an element effective for improving the resistance to HIC 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 the resistance to HIC. Therefore, the Ca content is 0.0050% or less, preferably 0.0045% or less.

[0076] Mg: 0.0005 - 0.0050%

[0077] Mg is an element having a deoxidizing effect and a desulfurizing effect, and has the effect of improving the formation efficiency of CaS and the desulfurizing effect of non-metallic inclusions through interaction. In addition, Mg is an element that refines inclusions, so it is effective for suppressing the formation of clusters of inclusions in the surface layer of the steel plate and improving the resistance to HIC. In order to stably exhibit 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.

[0078] The basic components in the component composition of the steel plate of the present embodiment have been described above. However, in this component 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.

[0079] Cu: 0.30% or less

[0080] Cu is an element effective for 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 resistance to HIC deteriorates. Therefore, when Cu is contained, the Cu content is 0.30% or less, preferably 0.25% or less.

[0081] Ni: 0.30% or less

[0082] 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.

[0083] Cr: 0.50% or less

[0084] 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.

[0085] Mo: 0.50% or less

[0086] 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.

[0087] In the chemical composition of the steel plate of this embodiment, one or more selected from V, Ti, Zr, and REM may be optionally contained within the following ranges.

[0088] V: 0.100% or less, Ti: 0.100% or less

[0089] Both V and Ti are elements that can be optionally 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.

[0090] Zr: 0.0200% or less, REM: 0.0200% or less

[0091] Zr and REM are elements that can be optionally 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.

[0092] It should be noted that the above Cu, Ni, Cr, Mo, V, Ti, Zr, and REM are arbitrary elements. Therefore, the content of each element can of course be 0% or exceed 0%.

[0093] It should be noted that the remaining part other than the above elements is composed of Fe and inevitable impurities. For example, O (oxygen) is an inevitable impurity element, but as long as the O content is 0.0030% or less, it is allowed in this embodiment. 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 this embodiment as long as its content is 0.0010% or less, preferably 0.0005% or less.

[0094] [Inclusions]

[0095] In the steel plate of this embodiment, it is important that in a pair of regions (surface regions) from both sides of the steel plate along the plate thickness direction to a depth of 1 / 4 of the plate thickness, there are inclusions having an average composition containing 10 to 40% by mass of MgO and an average aspect ratio of 2.5 or less, and the average value of the upper 10% of the equivalent circle diameter of the inclusions is 3.5 μm or less.

[0096] [MgO: 10 to 40% by mass]

[0097] In the average composition of the inclusions present in the surface region, the content of MgO is 10% by mass or more. This is because when the content of MgO is less than 10% by 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 surface region, the content of MgO is 40% by mass or less. This is because when the content of MgO exceeds 40% by mass, the effect of suppressing the formation of inclusion clusters decreases and the HIC resistance deteriorates.

[0098] The content (mass%) of MgO in the average composition of the inclusions present in the surface region is obtained by the following method. Take the cross-section perpendicular to the rolling direction of the steel plate as the observation surface, and set the vertical: from one surface of the steel plate to the 1 / 4 position on the one surface side, horizontal: a rectangular evaluation region of 10 mm in the width direction, and vertical: from the other surface of the steel plate to the 1 / 4 position on the other surface side, horizontal: a rectangular evaluation region of 10 mm in the width direction. Observe the inclusions in these two evaluation regions using a scanning electron microscope (SEM) with a particle analysis function. For each of all the inclusions in the two evaluation regions, measure the Mg amount (mass%), Al amount (mass%), S amount (mass%), Ca amount (mass%), and Mn amount (mass%), and calculate MgO (mass%) using the following formula (2).

[0099] MgO (mass%) = (MgO[g] × 100) / (MgO[g] + Al2 O 3 [g] + CaO[g] + CaS[g] + MnS[g]) ··· (2)

[0100] It should be noted that in formula (2), MgO[g], Al 2 O 3 [g], CaO[g], CaS[g] and MnS[g] respectively represent the contents of MgO, Al 2 O 3 in the inclusions calculated from the amounts of Mg (mass %), Al (mass %), S (mass %), Ca (mass %) and Mn (mass %) contained in the inclusions. At this time, it is assumed that Mg and Al respectively completely form MgO and Al 2 O 3 inclusions, Ca completely forms CaO - CaS inclusions, and Mn completely forms MnS.

[0101] It should be noted that inclusions such as carbon particles in the grinding chips that do not contain MgO, Al 2 O 3 , CaO, CaS and MnS are excluded from the measurement objects. In addition, inclusions with an equivalent circle diameter of less than 0.4 μm are also excluded from the measurement objects because they do not affect the effects of the present invention.

[0102] The average value obtained by dividing the total value of MgO (mass %) of all inclusions to be measured by the number of inclusions to be measured is used as the content (mass %) of MgO in the average composition.

[0103] The number density of inclusions in the surface layer region is preferably 1.0 piece / mm 2 or more, and preferably 50.0 pieces / mm 2 or less. This is because the desired HIC resistance can be obtained. The number density of inclusions in the surface layer region is obtained by the following method. The cross-section perpendicular to the rolling direction of the steel plate is used as the observation surface, and a rectangular evaluation region with a length: from one surface of the steel plate to the 1 / 4 position on the side of that one surface, a width: 10 mm in the width direction, and a length: from the other surface of the steel plate to the 1 / 4 position on the side of that other surface, a width: 10 mm in the width direction is set. The inclusions in these two evaluation regions are observed using a scanning electron microscope (SEM) with a particle analysis function. The number of inclusions with an equivalent circle diameter of 0.4 μm or more existing in the two evaluation regions is counted, and the value obtained by dividing the number by the total area of the two evaluation regions is adopted as the "number density of inclusions in the surface layer region".

[0104] [Average aspect ratio of inclusions: 2.5 or less]

[0105] In this embodiment, the average aspect ratio of the inclusions present in the surface layer region is 2.5 or less. This is because if the average aspect ratio exceeds 2.5, the stress concentration caused by the inclusions becomes large, and thus the HIC resistance deteriorates. The lower limit of the average aspect ratio is not particularly limited, but since the aspect ratio of the inclusions increases during rolling, the average aspect ratio can be 1.1 or more.

[0106] The average aspect ratio of the inclusions present in the surface layer region is obtained by the following method. The cross-section perpendicular to the rolling direction of the steel plate is used as the observation surface, and evaluation regions are set as follows: longitudinal: from one surface of the steel plate to the 1 / 4 position on the side of that one surface, transverse: a rectangle with a width of 10 mm in the width direction, and longitudinal: from the other surface of the steel plate to the 1 / 4 position on the side of that other surface, transverse: a rectangle with a width of 10 mm in the width direction. The inclusions in these two evaluation regions are observed using a scanning electron microscope (SEM) with a particle analysis function. For the inclusions with an equivalent circle diameter of 0.4 μm or more present in the two evaluation regions, the aspect ratio (major axis / minor axis) is obtained, and its average value is used as the "average aspect ratio of the inclusions present in the surface layer 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.

[0107] 〔Average value of the upper 10% of the equivalent circle diameter of inclusions: 3.5 μm or less〕

[0108] In this embodiment, it is important that the average value of the upper 10% of the equivalent circle diameter of the inclusions present in the surface layer region is 3.5 μm or less. Here, "upper 10%" means 10% in terms of the number of existing particles starting from the side with a larger equivalent circle diameter in the number distribution of the equivalent circle diameter of the inclusions. If the average value of the equivalent circle diameter of these 10% of the inclusions is specified, it is possible to evaluate the coarse inclusions that may be the starting points of HIC among the inclusions within the measurement range, and thus it becomes an index for improving the HIC resistance. In this embodiment, if the average value of the upper 10% of the equivalent circle diameter of the inclusions present in the surface layer region exceeds 3.5 μm, the HIC resistance of the surface layer region deteriorates, and thus the crack area ratio CAR after the HIC test of the surface layer region exceeds 5.0%. In this embodiment, the lower limit of the average value of the upper 10% of the equivalent circle diameter of the inclusions present in the surface layer region is not particularly limited, but in order to suppress the increase in steelmaking costs, the average value of the upper 10% of the equivalent circle diameter of the inclusions is preferably 1.5 μm or more.

[0109] The average value of the upper 10% of the equivalent circle diameters of the inclusions present in the surface layer 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 vertical: from one surface of the steel plate to the 1 / 4 position on the side of that one surface, horizontal: 10 mm in the width direction, and vertical: from the other surface of the steel plate to the 1 / 4 position on the side of that other surface, horizontal: 10 mm in the width direction. The inclusions in these two evaluation regions are observed using a scanning electron microscope (SEM) with particle analysis function. The equivalent circle diameters of the inclusions with an equivalent circle diameter of 0.4 μm or more present in the two evaluation regions are measured, and for the inclusions in the upper 10% of the equivalent circle diameters among them, the average value of their equivalent circle diameters is determined and adopted as the "average value of the upper 10% of the equivalent circle diameters of the inclusions present in the surface layer region".

[0110] [HIC resistance]

[0111] In this embodiment, the crack area ratio CAR after the HIC test in the surface layer 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 surface layer region can be 0.0% or more.

[0112] The crack area ratio CAR after the HIC test in the surface layer region is determined 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 using the NACE standard TM0177 Solution A solution, immersing the sample in the solution 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 surface layer 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 on a pair of regions from both sides of the steel plate along the plate thickness direction to a depth of 1 / 4 of the plate thickness at a pitch of 0.1 mm, determining the magnification so that the echo of the bottom wave is 100%, and taking the area ratio of the part with a defect echo height of 25% or more in the surface layer region of the steel plate as the crack area ratio (CAR).

[0113] [Thickness]

[0114] The plate thickness of the steel plate in this embodiment is not particularly limited, and preferably has a thickness of 12 to 39 mm.

[0115] (Manufacturing method of steel plate)

[0116] 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.

[0117] The manufacturing method of a steel plate according to an embodiment of the present invention includes: an adding step of adding a Mg-containing substance and a Ca-containing substance to molten steel after the end of refining; then a step of casting the molten steel to obtain a steel sheet; then a step of performing hot rolling on the steel sheet to obtain a steel plate; and then a step of performing controlled cooling on the steel plate.

[0118] The refining step of the molten steel can be carried out by a conventionally well-known method. For example, after hot metal pretreatment, primary refining is carried out using a converter, and then secondary refining is carried out, thereby improving the cleanliness of the molten steel in the ladle. In the above secondary refining, the molten steel is tapped from the converter into the ladle, 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 an acid-resistant steel plate, the ladle refining furnace is used to desulfurize the S content in the molten steel to 0.0010% or less. 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 2 O 3 aluminum oxide.

[0119] [Adding Step of Mg-Containing Substance and Ca-Containing Substance]

[0120] The addition timing 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 step to the start of casting. This is to suppress the consumption of Mg and Ca, which are strong deoxidizers and strong desulfurizing agents, by reacting with dissolved oxygen or reducing oxide-based non-metallic inclusions, and to make Mg and Ca contribute to the morphology control of non-metallic inclusions.

[0121] In the present embodiment, it is important to carry out the adding step of adding the Mg-containing substance and the Ca-containing substance to the molten steel after the end of refining under the following conditions: (A) the temperature of the molten steel at the start of this adding step is in the range of 1580 - 1620 °C, (B) in such a manner 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, and (C) the supply rates of Mg and Ca are 15 - 30 kg / minute respectively.

[0122] Both Mg and Ca are elements with high vapor pressures and evaporate at the temperature of molten steel under normal conditions. Additionally, Mg and Ca not only have high reactivity with molten steel but also with air. Therefore, in order to suppress the evaporation of Mg and Ca and improve the yield rate of Mg and Ca, instead of adding pure metals of Mg and Ca, substances containing Mg and substances containing Ca are added. The substance containing Mg is preferably an Mg alloy obtained by alloying Mg with one or more selected from silicon, aluminum, etc. The substance containing Ca 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 in the Mg alloy and the pure Ca in the Ca alloy are each preferably 5 to 30 mass%. Additionally, adding the substance containing Mg and the substance containing Ca simultaneously includes adding a substance containing both Mg and Ca.

[0123] The substance containing Mg can be, for example, the powder or granule of an Mg alloy, or an iron-coated Mg wire formed by coating the powder or granule of an Mg alloy with a thin steel plate. The substance containing Ca can be, for example, the powder or granule of a Ca alloy, or an iron-coated Ca wire formed by coating the powder or granule of a Ca alloy with a thin steel plate. Additionally, when adding the substance containing Mg and the substance containing Ca simultaneously, the iron-coated Mg wire and the iron-coated Ca wire can be added separately, but as a substance containing both Mg and Ca, an iron-coated Mg-Ca wire formed by coating the powder or granule of an Mg alloy and the powder or granule of a Ca alloy with one thin steel plate can also be added.

[0124] The addition method of the substance containing Mg and the substance containing Ca can be a method of adding the above-mentioned metal wire into molten steel, or a method of blowing the powder or granule of an Mg alloy and the powder or granule of a Ca alloy from an injection lance immersed in molten steel.

[0125] 〔Temperature of molten steel at the start of the addition process: 1580 - 1620 °C〕

[0126] If the temperature of molten steel at the start of the addition process is too high, the evaporation amount of Mg and Ca into the atmosphere increases, and the yield rate (dissolved amount in molten steel) of Mg and Ca decreases. On the other hand, if the temperature of molten steel at the start of the addition process is too low, the stirring of molten steel caused by evaporation becomes insufficient, and the yield rate also decreases in this case. Therefore, in order to stabilize the yield rate of Mg and Ca, the temperature of molten steel at the start of the addition process is in the range of 1580 - 1620 °C.

[0127] 〔Addition sequence of the substance containing Mg and the substance containing Ca〕

[0128] In the present embodiment, a first method of starting the addition of the Ca-containing substance after starting the addition of the Mg-containing substance or a second method of starting the addition of the Mg-containing substance and the Ca-containing substance simultaneously is adopted. Thereby, an appropriate amount of dissolved Ca and dissolved Mg can be present 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 toward the formation of CaS. The formation of CaS is promoted, and an improvement in 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 present in the surface region can be 3.5 μm or less.

[0129] In contrast, in the addition order of starting the addition of the Mg-containing substance after starting the addition of the Ca-containing substance, the desulfurization effect brought about by the formation of MgS accompanying the addition of the Mg-containing substance, the deoxidation effect brought about by the formation of MgO, and the effect of controlling 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 surface region exceeds 3.5 μm.

[0130] It should be noted that in the first method of starting the addition of the Ca-containing substance after starting the addition of the Mg-containing substance, there may be a period during which both the Mg-containing substance and the Ca-containing substance are added (i.e., the overlapping period between the addition period of the Mg-containing substance and the addition period of the Ca-containing substance), or the addition of the Ca-containing substance may be started after the addition of the Mg-containing substance is completed. That is, there may be no overlapping period. However, when the addition of the Ca-containing substance is started after the addition of the Mg-containing substance is completed, from the viewpoints of productivity and the effect of the present invention, the shorter the interval, the better, and it is preferably 5 minutes or less.

[0131] In addition, in the case of the second method of starting the addition of the Mg-containing substance and the Ca-containing substance simultaneously, it is also preferable that the addition of the Mg-containing substance and the Ca-containing substance 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, "starting the addition simultaneously" does not mean that the start of the addition is strictly consistent in seconds, and of course, a deviation in the start of the addition that is inevitable in the operation of the input machine (for example, within 30 seconds) is allowed.

[0132] 〔Supply rates of Mg and Ca: 15 to 30 kg / min respectively〕

[0133] If the supply rates of Mg and Ca (the addition rates of the Mg-containing substance and the Ca-containing substance) are too high, the MgO content 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 substance and the Ca-containing substance) are too low, the MgO content in the average composition of the inclusions becomes too small, and the HIC resistance still deteriorates. Therefore, in the present embodiment, the Mg-containing substance and the Ca-containing substance are added such that the supply rates of pure Mg and pure Ca are 15 to 30 kg / minute, 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 substance and the Ca-containing substance.

[0134] [Casting process]

[0135] In the present embodiment, it is important to cast molten steel to obtain steel sheets such as slabs and billets under the conditions that (i) the time from the end of adding the Ca-containing substance to the start of casting is within 90 minutes and (ii) the average flow rate of the molten steel in the mold during casting is 0.10 m / s or more. 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 device.

[0136] [Time from the end of adding the Ca-containing substance to the start of casting: within 90 minutes]

[0137] 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 is usually 100 minutes or more. In contrast, in the present embodiment, the 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 the present 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.

[0138] [Average flow rate of the molten steel in the mold during casting: 0.10 m / s or more]

[0139] In the mold, the flow of molten steel is controlled by electromagnetic stirring, thereby suppressing the coarsening of inclusions due to their agglomeration at the interface between the solidified shell and the molten steel. Thus, in order to sufficiently suppress the coarsening of inclusions and improve the HIC resistance of the surface layer region, the average flow velocity of the molten steel in the mold is 0.10 m / s or more. The average flow velocity of the molten steel in the mold is preferably 0.20 m / s or more. On the other hand, when the average flow velocity of the molten steel in the mold exceeds 0.40 m / s, the effect of suppressing the coarsening of inclusions saturates. Therefore, the average flow velocity of the molten steel in the mold is preferably 0.40 m / s or less.

[0140] The average flow velocity of the molten steel in the mold can be calculated using the following formula (3) by measuring the slope (dendrite inclination angle) of the primary dendrite with respect to the direction perpendicular to the surface of the cast sheet. The flow velocity of the molten steel is measured at a total of 5 positions (10 positions in total for both sides) at the 1 / 6 position, 1 / 3 position, 1 / 2 position, 2 / 3 position, and 5 / 6 position in the width direction of each of the front and back surfaces of the cast sheet, and the average value thereof is taken as the "average flow velocity of the molten steel in the mold".

[0141] θ = {(0.35 × C 0 2 ) / (C 0 2 + 0.0005) + 0.65} × 11.5 × V F －0.177 × log{(5.38 × 10 －1 × V F 2.08 ) / V} ··· (3)

[0142] It should be noted that θ (degree) represents the dendrite inclination angle, C 0 (mass%) represents the carbon concentration, V F (m / s) represents the flow velocity of the molten steel, and V (m / s) represents the solidification velocity. The dendrite inclination angle θ is obtained by the following method. That is, a sample having a cross-section perpendicular to the rolling direction is taken from the cast sheet after casting, ground, etched with a saturated aqueous solution of picric acid, and observed with an optical microscope. In the surface layer portion within a depth of 5 mm from the surface of the cast sheet in the cross-section, the slope of the primary dendrite with respect to the direction perpendicular to the surface of the cast sheet is measured with a protractor, and the dendrite inclination angle θ is thus obtained. The carbon concentration C 0 is analyzed using a sample taken from the tundish before casting with a solid emission spectroscopic analyzer. The solidification velocity V is obtained from the time change of the solidified shell thickness.

[0143] [Hot rolling process]

[0144] 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 and the cross rolling ratio is 20 or less. Other conditions in the hot rolling process can be in accordance with conventional methods.

[0145] [Heating temperature of steel sheet: 1000 - 1250 °C]

[0146] When the heating temperature of the steel sheet during hot rolling is less than 1000 °C, controlled cooling cannot be started above the Ar 3 point, and HIC resistance cannot be obtained. Therefore, the heating temperature of the steel sheet is 1000 °C or higher, preferably 1030 °C or higher. 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 lower, preferably 1200 °C or lower. It should be noted that this temperature is the temperature inside the heating furnace, and the steel sheet is heated to this temperature until the center part.

[0147] [Cross rolling ratio: 20 or less]

[0148] 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 inclusions can be effectively prevented, and 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.

[0149] Cross rolling ratio = Rolling ratio in the rolling direction / Rolling ratio in the direction perpendicular to the rolling direction ··· (4)

[0150] 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 the rolling direction" refers to the rolling ratio in the direction perpendicular to the rolling direction relative to the total rolling during cross rolling in hot rolling.

[0151] [Controlled cooling process]

[0152] 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 Ar 3 point obtained by the following formula (1). When the surface temperature of the steel plate at the start of cooling is less than the Ar 3 point, ferrite may be generated before cooling, and HIC resistance deteriorates. Therefore, it is preferable that the surface temperature of the steel plate at the start of cooling is above the Ar 3 point. Here, the Ar 3 point refers to the starting temperature of ferrite phase transformation during cooling, and can be obtained, for example, by the following formula (1) according to the composition of the steel. It should be noted that the surface temperature of the steel plate can be measured using a radiation thermometer or the like.

[0153] Ar 3 point (°C) = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni] - 80[Mo] ··· (1)

[0154] It should be noted that, [X] represents the content of element X in the steel (mass %), and the content of the element not contained is substituted with 0. Other conditions in the controlled cooling process can be in accordance with conventional methods.

[0155] (Steel pipe)

[0156] The steel plate of an 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, thereby manufacturing the steel pipe (UOE steel pipe, electric resistance welded steel pipe, spiral steel pipe, etc.) of an embodiment of the present invention. The steel pipe of this embodiment is suitable for the transportation of crude oil or natural gas.

[0157] For example, the UOE steel pipe is manufactured by beveling the ends of the steel plate, forming it into a steel pipe shape by C stamping, U stamping, and O stamping, and then seam welding the butt joint by internal surface welding and external surface welding, and further through an expanding process as needed. In addition, as long as the welding method can obtain sufficient joint strength and joint toughness, it can be any method. From the viewpoints of excellent welding quality and manufacturing efficiency, submerged arc welding is preferably used. In addition, after the steel plate is formed into a tubular shape by press bending, the steel pipe formed by seam welding the butt joint can also be expanded.

[0158] Examples

[0159] 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 into the ladle to deoxidize the molten steel.

[0160] Then, the molten steel contained in the ladle is desulfurized in a ladle furnace. In this desulfurization process, a CaO-Al 2 O 3 -SiO 2 -based flux is used as a desulfurizing agent. Then, the molten steel is heated by the arc heat from the graphite electrode, and at the same time, the above-mentioned desulfurizing agent is slagged. Argon is blown in as a stirring gas from the immersion lance immersed in the molten steel at a speed of 100~150 Nm 3 / h, and the molten steel and the desulfurizing agent are stirred and mixed to carry out the desulfurization process.

[0161] Furthermore, vacuum degassing refining is carried out in an 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-mentioned vacuum degassing refining is 20 minutes.

[0162] Next, during the period from the end of refining in the RH vacuum degassing apparatus 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 V) 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 addition is completed, "Ca→Mg" means starting to add the Fe-coated Ca wire and immediately starting to add the Fe-coated Mg wire after the addition is completed, and "Mg, Ca simultaneously" means adding an Fe-coated Mg-Ca wire.

[0163] Next, the molten steel is cast by continuous casting to produce a slab. The time from the end of the addition of the Ca-containing substance to the start of casting and the average flow rate of the molten steel in the mold during casting under each condition are shown in Table 2. It should be noted that the average flow rate of the molten steel in the mold is obtained by the above method.

[0164] Next, the slab is heated to the slab heating temperature shown in Table 2, and hot-rolled under the conditions of the cross-rolling ratio shown in Table 2 to produce a steel plate having the final plate thickness shown in Table 2. Next, controlled cooling of the steel plate is carried out at the surface temperature of the steel plate at the start of cooling shown in Table 2.

[0165] Then, the end of the steel plate is beveled, formed into a steel pipe shape by C stamping, U stamping, and O stamping, and 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 produced 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 V is the same as that of the steel grades A to V that become the steel billet material within the industrial error range. It should be noted that in Table 1, for elements with a hyphen (-), their content is regarded as zero in the calculation of Ar 3 (℃).

[0166] [Evaluation of inclusions]

[0167] Table 2 shows the "content of MgO in the average composition of inclusions existing in the surface layer region", the "number density of inclusions in the surface layer region", the "average value of the upper 10% of the equivalent circle diameters of inclusions existing in the surface layer region", and the "average value of the aspect ratios of inclusions existing in the surface layer region" obtained by the above method.

[0168] [Evaluation of HIC resistance]

[0169] Table 2 shows the "crack area ratio CAR after the HIC test in the surface layer region" obtained by the above method.

[0170]

[0171]

[0172] As shown in Table 2, Nos. 1 to 10 are inventive examples where the component composition and manufacturing conditions satisfy the appropriate range of the present invention. In any example, inclusions having an average composition containing 10 to 40% by mass of MgO and an average aspect ratio of 2.5 or less are present in the surface layer region, the average value of the upper 10% of the equivalent circle diameter of the inclusions is 3.5 μm or less, and the crack area ratio CAR after the HIC test in the surface layer region is 5.0% or less.

[0173] In contrast, the component compositions of the steel plates of Nos. 11 to 13 and 20 to 22 are outside the inventive range, thus promoting center segregation and the CAR not reaching the desired value.

[0174] 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.

[0175] The Nb content of the steel plate of No. 15 is more than the inventive range, so coarse carbides are precipitated and the CAR does not reach the desired value.

[0176] 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.

[0177] 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 where only the Ca content is reduced is shown in this No. 17.

[0178] 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.

[0179] The temperature of the molten steel at the start of the addition process of No. 19 is too low, so the yield is reduced. As a result, the Mg 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 where only the Mg content is reduced is shown in this No. 19.

[0180] The cross rolling of No. 23 is insufficient, so the average aspect ratio of the inclusions becomes larger and the CAR does not reach the desired value.

[0181] Mg is added after Ca addition in No. 24, so the average value of the upper 10% of the equivalent circle diameter of the inclusions becomes too large and the CAR does not reach the desired value.

[0182] The addition rate of the wire No.25 was too slow, and as a result, the MgO content of the inclusions became too low, and the CAR did not reach the desired value.

[0183] The addition rate of the wire No.26 was too fast, and as a result, the MgO content of the inclusions became too high, and the CAR did not reach the desired value.

[0184] The time from the end of Ca addition to the start of casting for No.27 was long, and therefore the average value of the upper 10% of the equivalent circle diameter of the inclusions became too large, and the CAR did not reach the desired value.

[0185] The average flow rate of the molten steel in the mold for No.28 was slow, and therefore the average value of the upper 10% of the equivalent circle diameter of the inclusions became too large, and the CAR did not reach the desired value.

[0186] Industrial availability

[0187] According to the present invention, it is possible to provide steel plates and steel pipes having excellent HIC resistance.

Claims

1. A steel plate, characterized in that it has the following composition, containing by mass% C: 0.030 - 0.080%, Si: 0.01 - 0.50%, Mn: 0.80 - 1.80%, P: below 0.015%, S: below 0.0015%, Al: 0.010 - 0.080%, Nb: below 0.080%, N: below 0.0080%, Ca: 0.0005 - 0.0050% and Mg: 0.0005 - 0.0050%, and the balance consists of Fe and inevitable impurities. In a pair of regions from both sides of the steel plate along the plate thickness direction to a depth of 1 / 4 of the plate thickness, there are inclusions with an average composition containing 10 - 40 mass% of MgO and an average aspect ratio of 2.5 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 of the region is 5.0% or less.

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

3. A method for manufacturing a steel plate, characterized in that the adding step of adding a Mg-containing substance and a Ca-containing substance to the molten steel after refining is carried out under the following conditions: (A) The temperature of the molten steel at the start of this adding step is in the range of 1580 - 1620 °C. (B) In a manner that the Ca-containing substance is added after starting to add the Mg-containing substance or the Mg-containing substance and the Ca-containing substance are added simultaneously. (C) The supply rates of Mg and Ca are respectively 15 - 30 kg / min. Then, the molten steel is cast under the conditions that (i) the time from the end of adding the Ca-containing substance to the start of casting is within 90 minutes and (ii) the average flow rate of the molten steel in the mold during casting is 0.10 m / s or more to obtain a steel sheet. Then, the steel sheet is hot-rolled to manufacture the steel plate according to claim 1 or 2.

4. The method for manufacturing a steel plate according to claim 3, wherein the hot rolling is carried out under the conditions that the heating temperature of the steel sheet is 1000 - 1250 °C and the cross rolling ratio is 20 or less. Next, 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 Ar point obtained by the following formula (1). 3 ​ Ar 3 Point = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni] - 80[Mo] ···(1) It should be noted that, [X] represents the content of element X in steel in mass%, and Ar 3 The unit of the point is °C.

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

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