[發明所欲解決之課題] [0008] 此外，就船舶用的鋼材而言，已知有開發作為貨油艙用或壓載艙用之鋼材。然而，煤船及煤・礦石兼用船的船艙的使用環境係如上述，在因腐蝕環境（溫度・濕度・腐蝕性物質等）及內容物而有無引起機械損傷等方面係與貨油艙或壓載艙的使用環境完全不同。因此，煤船及煤・礦石兼用船船艙用的鋼材需要獨特的材料設計或特性評定。 [0009] 就此，專利文獻1所示之鋼材係以改善船舶外板或壓載艙、貨油艙、礦石船貨艙等在共同的使用環境下的耐蝕性為目標，就鋼材的耐蝕性之評定，係考量貨油艙及壓載艙的使用環境。惟，專利文獻1中並未顯示考量到煤船及煤・礦石兼用船的船艙使用環境，亦即乾濕反覆循環且因煤的硫成分所引起之低pH環境的腐蝕試驗結果。 [0010] 又，專利文獻2及3中，亦評定模擬礦石搬運船的船艙的使用環境之鋼材在腐蝕環境下的耐蝕性，惟其仍未顯示考量到煤船及煤・礦石兼用船的船艙使用環境的腐蝕試驗結果。 [0011] 再者，船艙通常焊接底板與漏斗給料板、上甲板背板與縱樑等而構成，於其焊接連接部，會沿板厚方向承受拉伸應力。而且，於此種焊接連接部，近來已闡明有發生層狀撕裂的危險性。於此，所稱層狀撕裂，係指在十字形連接部、T形連接部、方形連接部等沿板厚方向承受拉伸應力的焊接連接部，因拉伸應力而沿與鋼板表面平行的方向，於鋼材內部加深龜裂而產生破裂的現象。 因此，煤船及煤・礦石兼用船的船艙用鋼材，除上述煤船及煤・礦石兼用船的船艙在使用環境下的耐蝕性外，亦要求耐層狀撕裂性優良。 [0012] 然而，專利文獻1～3均完全未考量到在焊接連接部發生層狀撕裂的風險，且未考量到任何關於耐層狀撕裂性者。 [0013] 本發明係有鑑於上述現況而開發者，茲以提供一種煤船及煤・礦石兼用船的船艙在使用環境下的耐蝕性優異，且耐層狀撕裂性亦優良的煤船及煤・礦石兼用船船艙用鋼材為目的。 又，本發明係以提供一種使用上述之煤船及煤・礦石兼用船船艙用鋼材而成的船舶為目的。 [解決課題之手段] [0014] 從而，本案發明人等為解決上述課題而致力反覆研究，獲得以下見解： （1）要提升煤船及煤・礦石兼用船的船艙在使用環境，亦即乾濕反覆循環且因煤的硫成分所引起之低pH環境下的耐蝕性，與Sn共同複合添加選自Cu、Ni、Sb、W、Mo及Nb當中的1種或2種以上係屬有效。 （2）另一方面，基於耐層狀撕裂性觀點，減少鋼中的S量並同時減少Sn係屬有效。 [0015] 如此，基於提升煤船及煤・礦石兼用船的船艙在使用環境下的耐蝕性的觀點，添加Sn係屬有效，但基於耐層狀撕裂性的觀點，減少Sn則屬有效。因此，本案發明人等以上述見解為基礎，為了兼具耐蝕性與耐層狀撕裂性而進一步反覆研究。 [0016] 其結果，獲得以下見解： （3）只要抑制Sn的中心偏析，使Sn朝鋼材全體極力擴散，即使含有既定量的Sn也能獲得優良的耐層狀撕裂性；亦即，只要適當地調整Sn量，同時抑制Sn的中心偏析，使Sn朝鋼材全體擴散，則可兼具煤船及煤・礦石兼用船的船艙在使用環境下的耐蝕性與耐層狀撕裂性。 又，獲得以下見解： （4）依據S量嚴謹地控制Sn量，可進一步提升耐層狀撕裂性。 本發明係基於上述見解，進一步反覆研究而完成者。 [0017] 亦即，本發明之要旨構成如下。 1.一種煤船及煤・礦石兼用船船艙用鋼材，其係具有：含有以質量%計為 C：0.03～0.18%、 Si：0.01～1.50%、 Mn：0.10～2.00%、 P：0.030%以下、 S：0.0070%以下、 Al：0.005～0.100%、 Sn：0.01～0.20%及 N：0.0080%以下 ，同時含有選自 Cu：0.01～0.50%、 Ni：0.01～0.50%、 Sb：0.01～0.30%、 W：0.01～0.50%、 Mo：0.01～0.50%及 Nb：0.0010～0.10% 當中的1種或2種以上，且其餘由Fe及無可避免之雜質所構成的成分組成，而且 Sn偏析度未達18； 於此，Sn偏析度係依下式(1)定義： [Sn偏析度]=[中心偏析部的Sn濃度]/[平均Sn濃度]--- (1)。 [0018] 2.如前述1之煤船及煤・礦石兼用船船艙用鋼材，其中前述成分組成中的S含量與Sn含量係滿足下式(2)之關係： 10000×[%S]×[%Sn]2
≦ 1.40 --- (2) 於此，[%S]及[%Sn]分別為成分組成中S及Sn的含量（質量%）。 [0019] 3.如前述1或2之煤船及煤・礦石兼用船船艙用鋼材，其中前述成分組成進一步含有選自以質量%計為 Cr：0.01～0.50%及 Co：0.01～0.50% 當中的1種或2種。 [0020] 4.如前述1～3中任一項之煤船及煤・礦石兼用船船艙用鋼材，其中前述成分組成進一步含有選自以質量%計為 Ti：0.001～0.100%、 Zr：0.001～0.100%及 V：0.001～0.100% 當中的1種或2種以上。 [0021] 5.如前述1～4中任一項之煤船及煤・礦石兼用船船艙用鋼材，其中前述成分組成進一步含有選自以質量%計為 Ca：0.0001～0.0100%、 Mg：0.0001～0.0200%及 REM：0.0002～0.2000% 當中的1種或2種以上。 [0022] 6.如前述1～5中任一項之煤船及煤・礦石兼用船船艙用鋼材，其中前述成分組成進一步含有以質量%計為 B：0.0001～0.0300%者。 [0023] 7.一種船舶，其係使用如前述1～6中任一項之煤船及煤・礦石兼用船船艙用鋼材而成。 [發明之效果] [0024] 根據本發明，可獲得一種煤船及煤・礦石兼用船的船艙在使用環境下的耐蝕性優異，且耐層狀撕裂性亦優良的煤船及煤・礦石兼用船船艙用鋼材。 而且，藉由將本發明之煤船及煤・礦石兼用船船艙用鋼材應用於船舶的船艙，可確保高安全性，同時可降低船艙的檢查或塗裝所需的費用。[Problems to be Solved by the Invention] [0008] In addition, steel materials for ships have been developed as steel materials for use in cargo oil tanks or ballast tanks. However, the use environment of the ship's cabin of the coal-fired vessel and the coal-and-or-pod ship is as described above, and it is related to the cargo oil tank or pressure in terms of corrosion environment (temperature, humidity, corrosive substances, etc.) and contents. The environment in which the tank is used is completely different. Therefore, the steel used for coal ships and coal/ore combined ship cabins requires unique material design or characterization. [0009] In this regard, the steel material shown in Patent Document 1 is intended to improve the corrosion resistance of steel materials in order to improve the corrosion resistance of ship outer panels or ballast tanks, cargo oil tanks, ore cargo tanks in a common use environment. Consider the use environment of cargo oil tanks and ballast tanks. However, Patent Document 1 does not show the results of the corrosion test of the low-pH environment which is considered to be caused by the wet and dry cycle and the low-pH environment caused by the sulfur content of the coal, considering the use environment of the ship to the coal ship and the coal/ore. [0010] Further, in Patent Documents 2 and 3, the corrosion resistance of the steel in the environment in which the cabin of the simulated ore carrier is used is also evaluated, but it has not been shown to be used in the cabin of the coal ship and the coal/ore combined vessel. Environmental corrosion test results. [0011] Furthermore, the cabin is usually welded with a bottom plate and a funnel feed plate, an upper deck back plate and a longitudinal beam, etc., and the welded joint portion is subjected to tensile stress in the thickness direction. Moreover, in such welded joints, the risk of occurrence of lamellar tears has recently been clarified. Here, the term "layered tear" refers to a welded joint that is subjected to tensile stress in a thickness direction in a cross-shaped joint portion, a T-shaped joint portion, a square joint portion, etc., and is parallel to the steel sheet surface due to tensile stress. In the direction of the steel, the crack is deepened inside the steel to cause cracking. Therefore, in addition to the corrosion resistance of the ship's cabin of the coal ship and the coal/ore ore ship, the steel for the ship and the coal/ore ore ship is also required to have excellent lamellar tear resistance. [0012] However, Patent Documents 1 to 3 all fail to consider the risk of lamellar tearing at the welded joint portion, and do not consider any one relating to the resistance to lamellar tearing. [0013] The present invention has been made in view of the above-mentioned situation, and is intended to provide a coal ship with excellent corrosion resistance in a use environment of a coal ship and a coal/ore combined ship, and excellent in layer tear resistance. Coal and ore are used for steel for ship cabins. Moreover, the present invention is to provide a ship which is obtained by using the above-mentioned coal ship and coal ore and ship steel for ship cabin. [Means for Solving the Problem] In order to solve the above problems, the inventors of the present invention have tried their best to study repeatedly and have obtained the following findings: (1) To improve the use environment of the ship and the coal and ore combined ship, that is, dry It is effective to add one or more selected from the group consisting of Cu, Ni, Sb, W, Mo, and Nb in combination with Sn in the wet-reverse cycle and the corrosion resistance in a low-pH environment due to the sulfur component of the coal. (2) On the other hand, based on the viewpoint of resistance to lamellar tearing, it is effective to reduce the amount of S in the steel while reducing the Sn system. [0015] As described above, it is effective to add the Sn system from the viewpoint of the corrosion resistance of the cabin of the coal-fired vessel and the coal/ore-use vessel in the use environment, but it is effective to reduce Sn from the viewpoint of resistance to layer tearing. Therefore, the inventors of the present invention have further studied in order to have both corrosion resistance and lamellar tear resistance based on the above findings. [0016] As a result, the following findings are obtained: (3) As long as the center segregation of Sn is suppressed and Sn is strongly diffused toward the entire steel material, excellent lamellar tear resistance can be obtained even if a predetermined amount of Sn is contained; that is, as long as When the amount of Sn is appropriately adjusted and the center segregation of Sn is suppressed, and Sn is diffused toward the entire steel material, the corrosion resistance and the lamellar tear resistance of the cabin of the coal ship and the coal/ore ore vessel can be used. Further, the following findings were obtained: (4) The amount of Sn is strictly controlled in accordance with the amount of S, and the layer tear resistance can be further improved. The present invention has been completed based on the above findings and further research. [0017] That is, the gist of the present invention is as follows. 1. A steel material for a coal ship, a coal ore and a ship cabin, comprising: C: 0.03 to 0.18% by mass, Si: 0.01 to 1.50%, Mn: 0.10 to 2.00%, P: 0.030% Hereinafter, S: 0.0070% or less, Al: 0.005 to 0.100%, Sn: 0.01 to 0.20%, and N: 0.0080% or less, and a content selected from the group consisting of Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, and Sb: 0.01 to 0.30%, W: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and Nb: 0.0010 to 0.10%, one or more of them, and the balance of Fe and inevitable impurities, and Sn The segregation degree is less than 18; here, the Sn segregation degree is defined by the following formula (1): [Sn segregation degree] = [Sn concentration of the center segregation part] / [Average Sn concentration] - (1). [0018] 2. The steel material for coal ship and coal/ore combined ship cabin according to the above 1, wherein the S content and the Sn content in the composition of the composition satisfy the relationship of the following formula (2): 10000 × [% S] × [ %Sn] 2 ≦ 1.40 --- (2) Here, [%S] and [%Sn] are the contents (% by mass) of S and Sn in the component composition, respectively. [0019] 3. The steel material for a coal ship and a coal/ore combined ship cabin according to the above 1 or 2, wherein the component composition further contains, by mass%, Cr: 0.01 to 0.50% and Co: 0.01 to 0.50%. One or two. [0020] 4. The steel material for a coal ship and a coal ore combined ship cabin according to any one of the above 1 to 3, wherein the component composition further contains, by mass%, Ti: 0.001 to 0.100%, Zr: 0.001 ～0.100% and V: 0.001 to 0.100% One or two or more of them. (5) The steel material for a coal ship and a coal/ore ore ship cabin according to any one of the above 1 to 4, wherein the component composition further contains, by mass%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 ～0.0200% and REM: 0.0002 to 0.2000% One or two or more. (2) The steel material for a coal ship and a coal ore combined ship cabin of any one of the above-mentioned items 1 to 5, wherein the component composition further contains B: 0.0001 to 0.0300% by mass%. [0023] A ship according to any one of the above 1 to 6, wherein the coal ship and the coal and ore are used for ship cabin steel. [Effects of the Invention] According to the present invention, it is possible to obtain a coal ship, coal, ore which is excellent in corrosion resistance in the use environment of a coal ship and a coal/ore ore ship, and which is excellent in layer tear resistance. Use steel for ship cabins. Further, by applying the steel ship of the present invention to the ship's cabin, it is possible to ensure high safety and reduce the cost of inspection or painting of the ship cabin.
[實施發明之形態] [0025] 以下，具體地說明本發明。首先，針對在本發明中將鋼的成分組成限定於前述範圍的理由加以說明。此外，鋼的成分組成中元素的含量的單位皆為「質量%」，以下，除非特別合先敘明，否則僅以「%」表示。 [0026] C：0.03～0.18% C為用來提高鋼的強度之元素。為確保所要的強度（490～620MPa），C量係取0.03%以上。惟，C量若超過0.18%，則焊接性及焊接熱影響部的韌性會劣化。從而，C量係取0.03～0.18%的範圍。較佳為0.05%以上、0.16%以下。 [0027] Si：0.01～1.50% Si為添加作為脫氧劑之元素。又，Si亦為對提高鋼的強度屬有效的元素，為確保所要之強度，Si量係取0.01%以上。惟，Si量若超過1.50%，則會使鋼的韌性劣化。從而，Si量係取0.01～1.50%的範圍。較佳為0.03%以上、1.00%以下。更佳為0.04%以上、0.50%以下。 [0028] Mn：0.10～2.00% Mn為用來提高鋼的強度之元素。為確保上述之所要的強度，Mn量係取0.10%以上。惟，Mn量若超過2.00%，則鋼的韌性及焊接性會劣化。又，因Mn的中心偏析，使耐層狀撕裂性亦劣化。從而，Mn量係取0.10～2.00%的範圍。較佳為0.60%以上、1.80%以下。更佳為0.80%以上、1.60%以下。 [0029] P：0.030%以下 P會使韌性及焊接性劣化。因此，P量係取0.030%以下。較佳為0.025%以下。更佳為0.015%以下。此外，就其下限不特別限定，較佳取0.003%。 [0030] S：0.0070%以下 S為參與耐層狀撕裂性的重要元素。亦即，S為會形成大體積的MnS，其為非金屬夾雜物，此MnS會成為層狀撕裂的起點。尤其是，S量超過0.0070%的話，會導致耐層狀撕裂性大幅劣化。從而，S量係取0.0070%以下。較佳為0.0060%以下。更佳為0.0050%以下。此外，就其下限不特別限定，較佳取0.0003%。 [0031] Al：0.005～0.100% Al為添加作為脫氧劑之元素，Al量係取0.005%以上。惟，Al量若超過0.100%，則鋼的韌性會劣化。因此，Al量係取0.005～0.100%的範圍。 [0032] Sn：0.01～0.20% Sn係為了提升煤船及煤・礦石兼用船的船艙在使用環境下的耐蝕性所需的元素，同時為參與耐層狀撕裂性的重要元素。具體而言，Sn為可提升耐蝕性，但另一面會使耐層狀撕裂性劣化的元素。 亦即，Sn在煤船及煤・礦石兼用船的船艙之反覆發生乾濕且低pH的腐蝕環境下，於鋼材表面形成難溶性被膜。與此同時，Sn會摻入於鋼材表面的鐵鏽中，而抑制會促進腐蝕的SO4 2-
等陰離子物種之擴散。藉此，可提升耐蝕性。此等效果可藉由將Sn量取0.01%以上而顯現。較佳為0.02%以上。 另一方面，由於Sn易偏析於鋼材中心部，且於此種偏析部，硬度會顯著增大，以致耐層狀撕裂性劣化。尤其是Sn量超過0.20%的話，耐層狀撕裂性會大幅劣化。從而，基於確保耐層狀撕裂性的觀點，Sn量係取0.20%以下。較佳為0.15%以下。更佳為0.10%以下。 [0033] N：0.0080%以下 N為會使韌性劣化的有害元素，故以極力減少為佳。尤其是N量超過0.0080%的話，韌性會大幅劣化。從而，N量係取0.0080%以下。較佳為0.0070%。此外，就其下限不特別限定，較佳取0.0005%。 [0034] 選自Cu：0.01～0.50%、Ni：0.01～0.50%、Sb：0.01～0.30%、W：0.01～0.50%、Mo：0.01～0.50%及Nb：0.0010～0.10%當中的1種或2種以上 Cu、Ni、Sb、W、Mo及Nb為透過與Sn共同複合添加，而能夠提升煤船及煤・礦石兼用船的船艙在使用環境下的耐蝕性之元素。 諸如上述，Sn雖為可有效提升耐蝕性的元素，但基於耐層狀撕裂性觀點卻無法使其大量含有。因此，要兼具煤船及煤・礦石兼用船的船艙在使用環境下的耐蝕性與耐層狀撕裂性，則需使其含有選自Cu：0.01～0.50%、Ni：0.01～0.50%、Sb：0.01～0.30%、W：0.01～0.50%、Mo：0.01～0.50%及Nb：0.0010～0.10%當中的1種或2種以上。 於此，Cu、Ni、Sb及Nb各自會隨著腐蝕的進行，自鋼材表面以Cu2 ＋
、Ni2 ＋
、Sb5 ＋
及Nb4 ＋
游離，使腐蝕生成物緻密化，而抑制SO4 2 －
等腐蝕性陰離子向鋼界面（鐵鏽層與基底鐵之界面）的穿透。又，W及Mo會各自以WO4 2-
及MoO4 2-
游離，摻入至鐵鏽中，對鐵鏽賦予陽離子選擇穿透性，而電性抑制SO4 2-
等腐蝕性陰離子向鋼界面的穿透。 此等效果，在上述Sn之防蝕作用共存時更為顯著，Cu、Ni、Sb、W及Mo量分別為0.01%以上、Nb量為0.0010%以上則可顯現。惟，使任一種元素均大量含有時，則會使焊接性或韌性劣化，基於成本觀點亦屬不利。 從而，Cu量係取0.01～0.50%的範圍，Ni量係取0.01～0.50%的範圍，Sb量係取0.01～0.30%的範圍，W量係取0.01～0.50%的範圍，Mo量係取0.01～0.50%的範圍，Nb量係取0.0010～0.10%的範圍。 較佳的是，Cu量為0.02%以上、0.40%以下，Ni量為0.02%以上、0.40%以下，Sb量為0.02%以上、0.25%以下，W量為0.02%以上、0.40%以下，Mo量為0.02%以上、0.40%以下，Nb量為0.0020%以上、0.08%以下。 [0035] 又，諸如上述，Sn所引起之耐層狀撕裂性的劣化機構係有別於S所引起之耐層狀撕裂性的劣化機構。惟，S與Sn所引起之耐層狀撕裂性的劣化彼此會相乘性地作用。因此，基於進一步提升耐層狀撕裂性的觀點，就S及Sn的含量，宜使其滿足下式(2)之關係： 10000×[%S]×[%Sn]2
≦ 1.40 --- (2) 於此，[%S]及[%Sn]分別為成分組成中S及Sn的含量（質量%）。 [0036] 上述(2)式係意指Sn量對耐層狀撕裂性的影響遠大於S量對其之影響。亦即，其意指在確保耐層狀撕裂性上，特別重要的是嚴謹地控管Sn。 於此，10000×[%S]×[%Sn]2
更佳定為1.20以下。就10000×[%S]×[%Sn]2
的下限而言不特別限定，較佳定為0.001。 此外，欲抑制層狀撕裂時，理當應以將S量與Sn量均限定於上述範圍為前提。 [0037] 以上，已針對基本成分加以說明，惟在本發明之煤船及煤・礦石兼用船船艙用鋼材中，可適宜使其含有以下所述元素。 選自Cr：0.01～0.50%及Co：0.01～0.50%當中的1種或2種 Cr及Co會隨著腐蝕的進行而轉移至鐵鏽層中，藉由阻斷SO4 2 －
等腐蝕性陰離子向鐵鏽層的侵入，可抑制SO4 2 －
等腐蝕性陰離子在鐵鏽層與基底鐵之界面的濃縮，由此有助於耐蝕性的進一步提升。 此種效果在Cr量或Co量未達0.01%時無法充分地獲得。另一方面，Cr量或Co量若超過0.50%，則會使焊接部的韌性劣化。又，就Cr而言為發生水解反應的元素，而會使腐蝕部的pH降低。亦即，過量添加Cr，則有使整體的耐蝕性劣化之虞。 從而，使其含有Cr及Co時，其量均取0.01～0.50%的範圍。較佳為0.02%以上、0.30%以下。更佳為0.03%以上、0.20%以下。 [0038] 選自Ti：0.001～0.100%、Zr：0.001～0.100%及V：0.001～0.100%當中的1種或2種以上 Ti、Zr及V，基於確保所要之強度的觀點，可單獨或複合性地添加。惟，使任一種元素均過量地含有時，則會使韌性及焊接性劣化。因此，使其含有Ti、Zr及V時，其量均取0.001～0.100%的範圍。較佳為0.005%以上、0.050%以下。 [0039] 選自Ca：0.0001～0.0100%、Mg：0.0001～0.0200%及REM：0.0002～0.2000%當中的1種或2種以上 Ca、Mg及REM，基於提升焊接部的韌性的觀點，可單獨或複合性地添加。惟，使任一種元素均過量地含有時，反而會導致焊接部的韌性劣化。又，也會增加成本。從而，使其含有Ca、Mg及REM時，Ca量係取0.0001～0.0100%、Mg量係取0.0001～0.0200%、REM量係取0.0002～0.2000%的範圍。 [0040] B：0.0001～0.0300% B為使鋼材的淬透性提升的元素。又，基於確保所要之強度的觀點，可使其含有B。由此種觀點而言，將B量取0.0001%以上係屬有效。惟，使B過量含有，尤其是B量超過0.0300%的話，則會導致韌性大幅劣化。從而，使其含有B時，其量係取0.0001～0.0300%的範圍。 [0041] 上述以外的成分為Fe及無可避免之雜質。 [0042] 以上，已針對本發明之煤船及煤・礦石兼用船船艙用鋼材的成分組成加以說明，惟就本發明之煤船及煤・礦石兼用船船艙用鋼材，如下控制Sn偏析度係極為重要。 Sn偏析度：未達18 由於Sn的中心偏析，偏析部的硬度會大幅增加。而且，此種偏析部會成為層狀撕裂發生的起點。亦即，要確保含有Sn的成分組成中優良的耐層狀撕裂特性，重要的是抑制Sn的中心偏析而抑制偏析部的硬度增加。由此種觀點而言，Sn偏析度係定為未達18。較佳為未達16。更佳為15以下。就其下限不特別限定，較佳定為2。 [0043] 此外，此處所稱Sn偏析度，係指在與鋼材的軋製方向平行地切割出來的剖面（與鋼材表面垂直的剖面）上，藉由電子束微分析器（以下表示為EPMA）的線分析所得之中心偏析部的Sn濃度對平均Sn濃度的比。 具體而言，設鋼材的厚度為t（mm）、寬度（與鋼材的軋製方向及厚度方向垂直的方向）為W（mm）時，首先，在與鋼材的軋製方向平行地切割出來的剖面（與鋼材表面垂直的剖面）之鋼材的厚度方向：(0.5±0.1)×t、軋製方向：15mm的面區域（即包含鋼材的厚度方向之中心位置的面區域），以束徑：20μm、間距：20μm的條件實施Sn的EPMA面分析。此外，Sn的EPMA面分析係於1/4×W、1/2×W及3/4×W之位置此3個剖面視野實施。 接著，由上述EPMA面分析選出在各剖面視野中Sn濃度最高的位置，在該位置分別沿鋼材的厚度方向以束徑：5μm、間距：5μm的條件實施Sn的EPMA線分析。此外，在實施EPMA線分析之際，係由鋼材的表背面分別排除25μm前的區域。 然後，按每條測定線求出Sn濃度（質量濃度）的最大值，以此等的平均值作為中心偏析部的Sn濃度（質量濃度），並以此中心偏析部的Sn濃度除以測定線之全部測定值的算術平均值，即平均Sn濃度（質量濃度）所得的值作為Sn偏析度。 亦即， [Sn偏析度]＝[中心偏析部的Sn濃度]/[平均Sn濃度]。 [0044] 諸如上述，本發明之煤船及煤・礦石兼用船船艙用鋼材，基於確保優良的耐層狀撕裂特性的觀點，極為重要的是抑制Sn的中心偏析，也就是將表示Sn的中心偏析之程度的Sn偏析度控制成既定值以下。於此，即使成分組成相同，Sn偏析度仍會隨製造條件大幅變化。因此，要抑制Sn的中心偏析，極為重要的是適切地控制鋼材的製造方法。 以下，就本發明之煤船及煤・礦石兼用船船艙用鋼材的較佳製造方法加以說明。 [0045] 亦即，本發明之鋼材可藉由將調整成上述之成分組成的鋼，使用轉爐或電爐、真空除氣等周知之精煉程序進行熔製，並以連續鑄造法或者造塊-分塊壓延法作成鋼素材（鋼胚, slab），接著將此鋼素材視需求再加熱後進行熱軋，作成鋼板或型鋼等來製造。此外，鋼材的厚度不特別限定，較佳為2～100mm。更佳為3～80mm。再更佳為4～60mm。 於此，採連續鑄造時，鑄造速度（拉取速度）較佳取0.3～2.8m/min。鑄造速度未達0.3m/min，作業效率會變差。另一方面，鑄造速度若超過2.8m/min，則會發生表面溫度不均，而且無法充分地向扁胚內部供給熔鋼，而促進Sn的中心偏析。基於抑制Sn的中心偏析的觀點，更佳為0.4m/min以上、2.6m/min以下。再更佳為1.5m/min以下。 此外，較佳進行輕輥軋法，其係將具有未凝固層的凝固末期之扁胚，一邊以相當於凝固收縮量與熱收縮量的和之程度的輥軋總量及輥軋速度，藉由輾壓輥群緩緩地進行輥軋一邊進行鑄造。 [0046] 其次，將上述之鋼素材熱軋成所要尺寸形狀之際，較佳加熱至900℃～1350℃的溫度。加熱溫度未達900℃，變形阻力較大，不易進行熱軋。另一方面，加熱溫度若超過1350℃，則會產生表面痕、或使氧化皮損耗或燃料原單位增加。 又，尤其是加熱溫度愈高則愈可促進中心偏析部之Sn的擴散，因此由確保耐層狀撕裂性觀點而言係屬有利。由此種觀點而言，加熱溫度更佳取1030℃以上。 再者，上述加熱溫度下的保持時間較佳取60min以上。藉此，可充分促進中心偏析部之Sn的擴散。更佳為150min以上。此外，就其上限不特別限定，較佳取1000min。 [0047] 此外，當鋼素材的溫度原本即為1030～1350℃的範圍時，且經保持於此溫度範圍60min以上時，可無需加熱而直接供予熱軋。又，亦可對熱軋後所得的熱軋板實施再加熱處理、酸性、冷軋，而製成既定板厚的冷軋板。 於熱軋中，精軋結束溫度較佳取650℃以上。精軋結束溫度未達650℃，會因變形阻力增大而使軋製負載增加，而不易實施軋製。 [0048] 熱軋後的冷卻可採氣冷、加速冷卻任一種方法，而欲獲得更高的強度時，係以進行加速冷卻為佳。 於此，進行加速冷卻時，較佳將冷卻速度設為2～100℃/s、冷卻停止溫度設為700～400℃。亦即，冷卻速度未達2℃/s、及/或冷卻停止溫度超過700℃時，加速冷卻的效果較小，而無法達到充分的高強度化。另一方面，冷卻速度超過100℃/s、及/或冷卻停止溫度未達400℃時，鋼材的韌性會劣化、或鋼材的形狀會產生變形。惟，於後續步驟中實施熱處理時則不在此限。 [實施例] [0049] 將成分組成為表1所示者的鋼（其餘為Fe及無可避免之雜質）以真空熔解爐或轉爐進行熔製，藉由表2所示條件的連續鑄造作成鋼胚。將此等鋼胚再度加熱至1150℃後，以表2所示條件保持，再實施精軋結束溫度：930℃的熱軋，而得到板厚：30mm的鋼板。此外，熱軋後的冷卻係採用冷卻速度：10℃/s、冷卻停止溫度：550℃的水冷卻（加速冷卻）。 然後，根據上述方法，求取所得鋼板中的Sn偏析度。將結果併記於表2。 [0050] 再者，針對如上述方式所得的鋼板，依以下要領進行模擬煤船及煤・礦石兼用船的船艙之使用環境的腐蝕試驗，並進行煤船及煤・礦石兼用船的船艙在使用環境下的耐蝕性的評定。 （1）耐蝕性的評定 由如上述方式所得之No.1～60之鋼板分別採取5mmt×50mmW×75mmL的試片，對其表面進行噴砂，來去除表面的鏽皮或油分。以此面為試驗面，評定塗膜剝離後之鋼材的耐蝕性。以矽系密封劑塗敷背面與端面後，嵌入於丙烯酸製之治具，於其上鋪滿5g的煤，並藉由恆溫恆濕器賦予環境A（溫度：60℃，相對濕度：95%，20小時)⇔環境B（溫度：30℃，相對濕度：95%，3小時)、各過渡時間：0.5小時的溫度濕度循環84天。於此，記號「 ⇔ 」係以重複之意義使用。此外，煤係使用秤量5g，在常溫下浸漬於100ml的蒸餾水2小時後，進行過濾並稀釋成200ml之煤滲出液的pH經調成3.0者。於此，係藉由在上述之條件下進行試驗，來模擬煤船及煤・礦石兼用船的船艙內底板的腐蝕環境。 試驗後，使用鐵鏽剝離液剝離各試片的鐵鏽並測定各試片在腐蝕試驗前後的質量減少量，以其為腐蝕量。又，使用深度計，測定各試片的最大孔蝕深度。然後，以未添加Sn或Cu、Ni、Sb、W、Mo及Nb的No.53為基底鋼，根據相對於此基底鋼的質量減少量及最大孔蝕深度的比率，按以下基準評定耐蝕性。 ○（合格）：相對於基底鋼的質量減少量及最大孔蝕深度的比率皆未達70% △（不合格）：相對於基底鋼的質量減少量及最大孔蝕深度的比率當中，任一者為70%以上且未達80%，而且另一者未達80% ×（不合格）：相對於基底鋼的質量減少量及最大孔蝕深度的比率當中，至少一者為80%以上 [0051] 進而，依以下要領，進行耐層狀撕裂性的評定。 （2）耐層狀撕裂性的評定 依據ClassNK 鋼船規則・同檢查要領（K篇第2章），對如上述方式所得之No.1～60之鋼板實施鋼板之板厚方向（Z方向）的拉伸試驗，並算出縮面率（RA, Reduction of Area）。然後，基於算出之縮面率（RA），按以下基準評定耐層狀撕裂性。 ◎（合格，特優）：70以上 ○（合格）：35以上且未達70 △（不合格）：25以上且未達35 ×（不合格）：未達25 [0052] 將（1）及（2）之評定結果併記於表2。此外，表2中的綜合評定係將上述（1）及（2）之評定全為「○」或「◎」時評為「合格」，將（1）及（2）之評定中任1個有「△」或「×」時評為「不合格」。 [0053] [0054][0055] 如表2所示，發明例皆兼備優良的耐蝕性與耐層狀撕裂性。 相對於此，就比較例，就耐蝕性及耐層狀撕裂性之至少一者，未能獲得充分之特性。 [0056] 亦即，比較例No.50及52由於S量超過上限，且未含有既定量的Cu、Ni、Sb、W、Mo及Nb，就耐蝕性及耐層狀撕裂性，未能獲得充分之特性。 比較例No.51、55及58由於Sn量超過上限，就耐層狀撕裂性，未能獲得充分之特性。 比較例No.54由於S量及Sn量超過上限，就耐層狀撕裂性，未能獲得充分之特性。 比較例No.56及60由於S量超過上限，就耐層狀撕裂性，未能獲得充分之特性。 比較例No.57由於未含有既定量的Cu、Ni、Sb、W、Mo及Nb，就耐蝕性，未能獲得充分之特性。 比較例No.59由於S量超過上限，且Sn量低於下限，就耐蝕性及耐層狀撕裂性，未能獲得充分之特性。 比較例No.61～64由於Sn偏析度超過上限，就耐層狀撕裂性，未能獲得充分之特性。[Mode for Carrying Out the Invention] [0025] Hereinafter, the present invention will be specifically described. First, the reason why the component composition of steel is limited to the above range in the present invention will be described. In addition, the unit of the content of the element in the composition of the steel is "% by mass", and the following is only expressed by "%" unless otherwise specified. C: 0.03 to 0.18% C is an element for increasing the strength of steel. In order to ensure the required strength (490 to 620 MPa), the amount of C is 0.03% or more. However, if the amount of C exceeds 0.18%, the weldability and the toughness of the welded heat affected zone may deteriorate. Therefore, the amount of C is in the range of 0.03 to 0.18%. It is preferably 0.05% or more and 0.16% or less. [0027] Si: 0.01 to 1.50% Si is an element added as a deoxidizer. Further, Si is also an element effective for increasing the strength of steel. To ensure the required strength, the amount of Si is 0.01% or more. However, if the amount of Si exceeds 1.50%, the toughness of the steel is deteriorated. Therefore, the amount of Si is in the range of 0.01 to 1.50%. It is preferably 0.03% or more and 1.00% or less. More preferably, it is 0.04% or more and 0.50% or less. [0028] Mn: 0.10 to 2.00% Mn is an element for increasing the strength of steel. In order to secure the above-mentioned desired strength, the amount of Mn is 0.10% or more. However, if the amount of Mn exceeds 2.00%, the toughness and weldability of the steel may deteriorate. Further, the layer-like tear property is also deteriorated due to segregation at the center of Mn. Therefore, the amount of Mn is in the range of 0.10% to 2.00%. It is preferably 0.60% or more and 1.80% or less. More preferably, it is 0.80% or more and 1.60% or less. P: 0.030% or less P deteriorates toughness and weldability. Therefore, the amount of P is taken to be 0.030% or less. It is preferably 0.025% or less. More preferably, it is 0.015% or less. Further, the lower limit thereof is not particularly limited, and is preferably 0.003%. [0030] S: 0.0070% or less S is an important element involved in the resistance to lamellar tearing. That is, S is a large volume of MnS which is a non-metallic inclusion which becomes the starting point of the lamellar tear. In particular, when the amount of S exceeds 0.0070%, the layer tear resistance is largely deteriorated. Therefore, the amount of S is 0.0070% or less. It is preferably 0.0060% or less. More preferably, it is 0.0050% or less. Further, the lower limit thereof is not particularly limited, and is preferably 0.0003%. [0031] Al: 0.005 to 0.100% Al is an element added as a deoxidizing agent, and the amount of Al is 0.005% or more. However, if the amount of Al exceeds 0.100%, the toughness of the steel deteriorates. Therefore, the amount of Al is in the range of 0.005 to 0.100%. [0032] Sn: 0.01 to 0.20% Sn is an element required to improve the corrosion resistance of the cabin of a coal ship and a coal/ore ore vessel in use environment, and is an important element involved in the resistance to layer tearing. Specifically, Sn is an element which can improve corrosion resistance, but the other side deteriorates the lamellar tear resistance. In other words, Sn forms a poorly soluble film on the surface of the steel in a corrosive environment in which the ship's cabin of the coal ship and the coal/ore combined ship is dry and wet and has a low pH. At the same time, Sn is incorporated into the rust on the surface of the steel to suppress the diffusion of anionic species such as SO 4 2- which promotes corrosion. Thereby, corrosion resistance can be improved. These effects can be exhibited by taking the amount of Sn by 0.01% or more. It is preferably 0.02% or more. On the other hand, since Sn tends to be segregated in the center portion of the steel material, the hardness is remarkably increased in such a segregation portion, so that the lamellar tear resistance is deteriorated. In particular, when the amount of Sn exceeds 0.20%, the layer tear resistance is greatly deteriorated. Therefore, the amount of Sn is 0.20% or less from the viewpoint of ensuring the resistance to lamellar tearing. It is preferably 0.15% or less. More preferably, it is 0.10% or less. N: 0.0080% or less N is a harmful element which deteriorates toughness, and therefore it is preferable to reduce it as much as possible. In particular, when the amount of N exceeds 0.0080%, the toughness is largely deteriorated. Therefore, the amount of N is taken to be 0.0080% or less. It is preferably 0.0070%. Further, the lower limit thereof is not particularly limited, and is preferably 0.0005%. [0034] One selected from the group consisting of Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Sb: 0.01 to 0.30%, W: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and Nb: 0.0010 to 0.10% Two or more types of Cu, Ni, Sb, W, Mo, and Nb are added together with Sn, and the corrosion resistance of the cabin of the coal ship and the coal/ore combined ship can be improved. For example, although Sn is an element which can effectively improve corrosion resistance, it cannot be contained in a large amount based on the viewpoint of resistance to lamellar tearing. Therefore, in order to have the corrosion resistance and the lamellar tear resistance of the cabin of a coal ship and a coal/ore ore vessel, it is necessary to contain a selected from the group consisting of Cu: 0.01 to 0.50%, and Ni: 0.01 to 0.50%. And Sb: 0.01 to 0.30%, W: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and Nb: 0.0010 to 0.10%, one or more. Here, each of Cu, Ni, Sb, and Nb is freed from Cu 2 + , Ni 2 + , Sb 5 + , and Nb 4 + from the surface of the steel as the corrosion progresses, densifying the corrosion product and suppressing SO 4 . 2 - Penetration of corrosive anions to the steel interface (the interface between the rust layer and the base iron). Moreover, W and Mo are each freed from WO 4 2- and MoO 4 2- , incorporated into rust, imparting cation selective permeability to rust, and electrically inhibiting corrosive anions such as SO 4 2- to the steel interface. penetrate. These effects are more remarkable when the anti-etching action of Sn is present, and the amounts of Cu, Ni, Sb, W, and Mo are each 0.01% or more, and the amount of Nb is 0.0010% or more. However, when any of the elements is contained in a large amount, the weldability or toughness is deteriorated, which is disadvantageous from the viewpoint of cost. Therefore, the amount of Cu is in the range of 0.01 to 0.50%, the amount of Ni is in the range of 0.01 to 0.50%, the amount of Sb is in the range of 0.01 to 0.30%, and the amount of W is in the range of 0.01 to 0.50%, and the amount of Mo is taken. In the range of 0.01 to 0.50%, the amount of Nb is in the range of 0.0010 to 0.10%. Preferably, the amount of Cu is 0.02% or more and 0.40% or less, the amount of Ni is 0.02% or more and 0.40% or less, the amount of Sb is 0.02% or more and 0.25% or less, and the amount of W is 0.02% or more and 0.40% or less. The amount is 0.02% or more and 0.40% or less, and the amount of Nb is 0.0020% or more and 0.08% or less. Further, as described above, the deterioration mechanism for the layer tear resistance caused by Sn is different from the deterioration mechanism for the layer tear resistance caused by S. However, the deterioration of the layer tear resistance caused by S and Sn acts synergistically with each other. Therefore, based on the viewpoint of further improving the resistance to lamellar tearing, the content of S and Sn should be such that it satisfies the relationship of the following formula (2): 10000 × [% S] × [% Sn 2 ] ≦ 1.40 --- (2) Here, [%S] and [%Sn] are the contents (% by mass) of S and Sn in the component composition, respectively. [0036] The above formula (2) means that the influence of the amount of Sn on the lamellar tear resistance is much larger than the influence of the S amount. That is, it means that it is particularly important to strictly control Sn in ensuring the resistance to lamellar tearing. Here, 10000 × [% S] × [% Sn] 2 is more preferably set to 1.20 or less. The lower limit of 10000 × [% S] × [% Sn 2 ] is not particularly limited, and is preferably 0.001. Further, in order to suppress the lamellar tear, it is reasonable to assume that both the amount of S and the amount of Sn are limited to the above range. In the above, the basic components are described. However, in the steel materials for the coal ship and the coal ore combined ship cabin of the present invention, the following elements may be suitably contained. One or two kinds of Cr and Co selected from Cr: 0.01 to 0.50% and Co: 0.01 to 0.50% are transferred to the rust layer as corrosion progresses, by blocking corrosive anions such as SO 4 2 - The intrusion into the rust layer suppresses the concentration of the corrosive anion such as SO 4 2 - at the interface between the rust layer and the base iron, thereby contributing to further improvement in corrosion resistance. Such an effect cannot be sufficiently obtained when the amount of Cr or the amount of Co is less than 0.01%. On the other hand, when the amount of Cr or the amount of Co exceeds 0.50%, the toughness of the welded portion is deteriorated. Further, in the case of Cr, an element which undergoes a hydrolysis reaction lowers the pH of the corrosion portion. That is, when Cr is excessively added, there is a possibility that the overall corrosion resistance is deteriorated. Therefore, when Cr and Co are contained, the amount thereof is in the range of 0.01 to 0.50%. It is preferably 0.02% or more and 0.30% or less. More preferably, it is 0.03% or more and 0.20% or less. One or more of Ti, Zr, and V selected from the group consisting of Ti: 0.001% to 0.100%, Zr: 0.001% to 0.100%, and V: 0.001% to 0.100% can be used alone or in view of ensuring the required strength. Added in combination. However, when any of the elements is excessively contained, the toughness and weldability are deteriorated. Therefore, when Ti, Zr, and V are contained, the amount thereof is in the range of 0.001 to 0.100%. It is preferably 0.005% or more and 0.050% or less. One or more of Ca, Mg, and REM selected from the group consisting of Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0200%, and REM: 0.0002 to 0.2000% may be separately used from the viewpoint of improving the toughness of the welded portion. Or added in combination. However, when any one of the elements is excessively contained, the toughness of the welded portion is deteriorated instead. Also, it will increase costs. Therefore, when Ca, Mg, and REM are contained, the amount of Ca is 0.0001 to 0.0100%, the amount of Mg is 0.0001 to 0.0200%, and the amount of REM is 0.0002 to 0.2000%. [0040] B: 0.0001 to 0.0300% B is an element which improves the hardenability of the steel material. Further, it is possible to contain B based on the viewpoint of ensuring the required strength. From such a viewpoint, it is effective to take B amount of 0.0001% or more. However, if B is excessively contained, especially if the amount of B exceeds 0.0300%, the toughness is largely deteriorated. Therefore, when B is contained, the amount thereof is in the range of 0.0001 to 0.0300%. [0041] The components other than the above are Fe and inevitable impurities. [0042] In the above, the composition of the steel material for the coal ship and the coal and ore used in the ship cabin of the present invention is described. However, the steel segregation system for the coal ship and the coal and ore used for the ship cabin of the present invention is controlled as follows. Extremely important. Sn segregation degree: less than 18 Due to the center segregation of Sn, the hardness of the segregation portion is greatly increased. Moreover, such a segregation portion becomes a starting point for the occurrence of a layered tear. In other words, in order to secure excellent lamellar tear resistance in the composition of Sn, it is important to suppress center segregation of Sn and suppress an increase in hardness of the segregation portion. From this point of view, the degree of Sn segregation is determined to be less than 18. It is preferably less than 16. More preferably 15 or less. The lower limit is not particularly limited, and is preferably set to 2. Further, the term "Sn segregation degree" as used herein refers to a cross section (a cross section perpendicular to the surface of the steel material) which is cut in parallel with the rolling direction of the steel material, and is represented by an electron beam microanalyzer (hereinafter referred to as EPMA). The ratio of the Sn concentration to the average Sn concentration in the center segregation portion obtained by the line analysis. Specifically, when the thickness of the steel material is t (mm) and the width (the direction perpendicular to the rolling direction and the thickness direction of the steel material) is W (mm), first, it is cut in parallel with the rolling direction of the steel material. Thickness direction of the steel material of the cross section (section perpendicular to the steel surface): (0.5 ± 0.1) × t, rolling direction: 15 mm surface area (ie, the surface area including the center position of the steel in the thickness direction), the beam diameter: The EPMA surface analysis of Sn was carried out under conditions of 20 μm and pitch: 20 μm. Further, the EPMA surface analysis of Sn was carried out in the three cross-sectional fields at positions of 1/4×W, 1/2×W, and 3/4×W. Next, the position where the Sn concentration was the highest in the cross-sectional field of view was selected by the EPMA surface analysis, and EPMA line analysis of Sn was performed at the position in the thickness direction of the steel material at a beam diameter of 5 μm and a pitch of 5 μm. In addition, in the EPMA line analysis, the area before the 25 μm was excluded from the front and back sides of the steel. Then, the maximum value of the Sn concentration (mass concentration) is obtained for each measurement line, and the average value is used as the Sn concentration (mass concentration) of the center segregation portion, and the Sn concentration of the center segregation portion is divided by the measurement line. The arithmetic mean value of all the measured values, that is, the value obtained by the average Sn concentration (mass concentration) is taken as the Sn segregation degree. That is, [Sn segregation degree] = [Sn concentration of the center segregation portion] / [Average Sn concentration]. [0044] In the above, the steel material for the coal ship and the coal and ore used in the ship of the present invention is extremely important to suppress the center segregation of Sn, that is, to represent Sn, from the viewpoint of ensuring excellent lamellar tear resistance. The degree of Sn segregation of the degree of center segregation is controlled to be less than or equal to a predetermined value. Here, even if the composition of the components is the same, the degree of Sn segregation greatly changes depending on the manufacturing conditions. Therefore, in order to suppress the center segregation of Sn, it is extremely important to appropriately control the method of manufacturing the steel. Hereinafter, a preferred method of producing the steel material for a coal ship and a coal ore combined ship cabin of the present invention will be described. [0045] That is, the steel material of the present invention can be melted by a known refining process such as a converter or an electric furnace, vacuum degassing, and the like, or by a continuous casting method or agglomerate-- The block rolling method is used to produce a steel material (steel blank, slab), and then the steel material is further heated as required, and then hot rolled to produce a steel sheet or a steel or the like. Further, the thickness of the steel material is not particularly limited, but is preferably 2 to 100 mm. More preferably 3 to 80 mm. More preferably, it is 4 to 60 mm. Here, in the case of continuous casting, the casting speed (pull speed) is preferably from 0.3 to 2.8 m/min. The casting speed is less than 0.3 m/min, and the work efficiency is deteriorated. On the other hand, when the casting speed exceeds 2.8 m/min, surface temperature unevenness occurs, and molten steel cannot be sufficiently supplied to the inside of the flat embryo to promote center segregation of Sn. From the viewpoint of suppressing center segregation of Sn, it is more preferably 0.4 m/min or more and 2.6 m/min or less. More preferably, it is 1.5 m/min or less. Further, it is preferable to carry out a light rolling method in which a flat embryo having an unsolidified layer at the end of solidification is used, and the total amount of rolling and the rolling speed corresponding to the sum of the amount of solidification shrinkage and the amount of heat shrinkage are borrowed. Casting is carried out while the rolling roll group is slowly rolling. [0046] Next, when the steel material described above is hot rolled into a desired size, it is preferably heated to a temperature of from 900 ° C to 1350 ° C. When the heating temperature is less than 900 ° C, the deformation resistance is large and it is difficult to perform hot rolling. On the other hand, if the heating temperature exceeds 1350 ° C, surface marks may be generated or the scale loss or the fuel original unit may be increased. Further, in particular, the higher the heating temperature, the more the diffusion of Sn in the center segregation portion is promoted, and therefore it is advantageous from the viewpoint of ensuring the resistance to lamellar tearing. From such a viewpoint, the heating temperature is more preferably 1030 ° C or more. Further, the holding time at the above heating temperature is preferably 60 minutes or more. Thereby, the diffusion of Sn in the center segregation portion can be sufficiently promoted. More preferably 150min or more. Further, the upper limit thereof is not particularly limited, and it is preferably 1000 minutes. [0047] Further, when the temperature of the steel material is originally in the range of 1030 to 1350 ° C, and is maintained at this temperature range of 60 min or more, hot rolling can be directly supplied without heating. Further, the hot-rolled sheet obtained after the hot rolling may be subjected to reheating treatment, acidification, and cold rolling to obtain a cold-rolled sheet having a predetermined thickness. In hot rolling, the finish rolling temperature is preferably 650 ° C or higher. When the finishing rolling temperature is less than 650 ° C, the rolling load increases due to an increase in deformation resistance, and rolling is not easy. [0048] The cooling after hot rolling may be any method of gas cooling or accelerated cooling, and in order to obtain higher strength, it is preferred to perform accelerated cooling. Here, in the case of performing accelerated cooling, it is preferred to set the cooling rate to 2 to 100 ° C/s and the cooling stop temperature to 700 to 400 ° C. That is, when the cooling rate is less than 2 ° C / s, and / or the cooling stop temperature exceeds 700 ° C, the effect of accelerated cooling is small, and sufficient strength cannot be achieved. On the other hand, when the cooling rate exceeds 100 ° C / s, and / or the cooling stop temperature does not reach 400 ° C, the toughness of the steel material may deteriorate or the shape of the steel material may be deformed. However, this is not the case when heat treatment is carried out in the subsequent steps. [Examples] [0049] The steel having the composition shown in Table 1 (the balance being Fe and the inevitable impurities) was melted in a vacuum melting furnace or a converter, and was continuously cast by the conditions shown in Table 2. Steel embryo. These steel blanks were again heated to 1,150 ° C, and then held under the conditions shown in Table 2, and further subjected to hot rolling at a finish rolling temperature of 930 ° C to obtain a steel sheet having a thickness of 30 mm. Further, the cooling after the hot rolling was performed by cooling with a cooling rate of 10 ° C/s and a cooling stop temperature of 550 ° C (accelerated cooling). Then, according to the above method, the degree of segregation of Sn in the obtained steel sheet was obtained. The results are also recorded in Table 2. [0050] Further, in the steel sheet obtained as described above, the corrosion test of the environment in which the cabin of the coal ship and the coal/ore combined ship is used is carried out in the following manner, and the cabin of the coal ship and the coal/ore ore ship is used. Evaluation of corrosion resistance in the environment. (1) Evaluation of Corrosion Resistance A steel sheet of No. 1 to 60 obtained as described above was subjected to a test piece of 5 mmt × 50 mm W × 75 mmL, and the surface thereof was subjected to sandblasting to remove scale or oil on the surface. Using this surface as a test surface, the corrosion resistance of the steel after the peeling of the coating film was evaluated. After coating the back surface and the end surface with a enamel sealant, it is embedded in a jig made of acrylic, and is covered with 5 g of coal, and is given environment A by a constant temperature and humidity device (temperature: 60 ° C, relative humidity: 95%) , 20 hours) ⇔ environment B (temperature: 30 ° C, relative humidity: 95%, 3 hours), each transition time: 0.5 hours of temperature and humidity cycle for 84 days. Here, the symbol "⇔" is used in a repetitive sense. Further, the coal was weighed to 5 g, and after immersing in 100 ml of distilled water at normal temperature for 2 hours, the pH of the coal exudate which was filtered and diluted to 200 ml was adjusted to 3.0. Here, the test was carried out under the above conditions to simulate the corrosive environment of the inner bottom of the ship's cabin of the coal ship and the coal/ore. After the test, the rust of each test piece was peeled off using the rust stripping liquid, and the amount of mass reduction of each test piece before and after the corrosion test was measured, and this was used as the amount of corrosion. Further, the maximum pitting depth of each test piece was measured using a depth gauge. Then, No. 53 to which Sn or Cu, Ni, Sb, W, Mo, and Nb were not added was used as the base steel, and the corrosion resistance was evaluated according to the ratio of the mass reduction amount to the maximum pitting depth to the base steel. . ○ (Qualified): The ratio of the mass reduction to the base steel and the maximum pitting depth are less than 70% △ (failed): Any of the ratios of the mass reduction to the base steel and the maximum pitting depth 70% or more and less than 80%, and the other is less than 80% × (failed): at least one of the ratio of the mass reduction amount to the base steel and the maximum pitting depth is 80% or more [ 0051] Further, the evaluation of the layer tear resistance was carried out in the following manner. (2) Evaluation of the resistance to the lamellar tearing According to the rules of the ClassNK steel ship and the inspection method (Chapter 2 of the K), the plate thickness direction of the steel plate of the No. 1 to 60 obtained as described above (Z direction) The tensile test was performed, and the reduction ratio (RA) was calculated. Then, based on the calculated shrinkage ratio (RA), the layer tear resistance was evaluated on the basis of the following criteria. ◎ (passed, excellent): 70 or more ○ (qualified): 35 or more and less than 70 △ (failed): 25 or more and less than 35 × (failed): less than 25 [0052] (1) The evaluation results of (2) are also shown in Table 2. In addition, the comprehensive assessment in Table 2 is rated as "qualified" when all the evaluations of (1) and (2) above are "○" or "◎", and one of the assessments of (1) and (2) is "△" or "×" was rated as "failed". [0053] [0054] As shown in Table 2, the inventive examples all have excellent corrosion resistance and lamellar tear resistance. On the other hand, in the comparative example, at least one of corrosion resistance and lamellar tear resistance was not obtained. That is, in Comparative Examples Nos. 50 and 52, since the amount of S exceeds the upper limit and does not contain a predetermined amount of Cu, Ni, Sb, W, Mo, and Nb, corrosion resistance and lamellar tear resistance are not obtained. Get the full range of features. In Comparative Examples Nos. 51, 55, and 58, since the amount of Sn exceeded the upper limit, the layer tear property was prevented, and sufficient characteristics were not obtained. In Comparative Example No. 54, since the amount of S and the amount of Sn exceeded the upper limit, the layer tear property was prevented, and sufficient characteristics were not obtained. In Comparative Examples Nos. 56 and 60, since the amount of S exceeded the upper limit, the layer tear property was prevented, and sufficient characteristics were not obtained. In Comparative Example No. 57, corrosion resistance was not obtained because Cu, Ni, Sb, W, Mo, and Nb were not contained, and sufficient characteristics were not obtained. In Comparative Example No. 59, since the amount of S exceeded the upper limit and the amount of Sn was less than the lower limit, corrosion resistance and lamellar tear resistance were not obtained, and sufficient characteristics were not obtained. In Comparative Examples Nos. 61 to 64, since the Sn segregation degree exceeded the upper limit, the layer tear property was prevented, and sufficient characteristics were not obtained.