[發明欲解決之課題] [0014] 然而，為了獲得專利文獻1、2中記載之鋼材，於鋼材表層部暫時冷卻後，必須復熱且於前述復熱中施以加工而控制組織。因此，以實際生產規模之控制並不容易，對壓延、冷卻設備之負荷大。 [0015] 又，如前述之先前技術，均係以板厚50mm左右之鋼板為對象者，應用於板厚70mm左右之厚壁材時，是否可獲得必要特性尚不清楚。尤其，關於船體構造所必要之板厚方向之龜裂擴展特性亦不清楚。 [0016] 本發明係有利於解決上述問題者，目的在於提供即使板厚超過50mm時亦具有優異之脆性龜裂擴展停止特性，且可以工業上極簡易之製程製造之高強度厚鋼板。又，本發明之目的係以工業上簡易之製程，可安定地製造前述高強度厚鋼板的高強度厚鋼板之製造方法。 [用以解決課題之手段] [0017] 本發明人等為解決上述課題，針對具有優異脆性龜裂擴展停止特性之高強度鋼板及可安定地獲得該鋼板之製造方法重複積極研究之結果，獲得以下見解。 [0018] （1）於奧氏體區域完成壓延時，壓延時之溫度越為低溫越能獲得高的韌性值與集合組織。然而，如板厚超過50mm之厚鋼板中，若壓延溫度降低至變態點附近，則如圖1所示之鋼板表面與板厚中央部之溫度差變大，故表層部變態為鐵素體組織，該鐵素體經壓延而使表層部之韌性劣化。 [0019] （2）為了抑制表層部之鐵素體生成而有必要提高壓延溫度，但提高壓延溫度時，無法使板厚中心之壓延溫度充分降低。 [0020] （3）板厚中心部之壓延溫度未充分降低時，板厚中心部之結晶粒徑變粗大而有韌性變不充分之情況，或有利於龜裂擴展停止特性之集合組織之積集度不充分之情況。 [0021] 為了解決上述問題而進而重複檢討之結果，想到於壓延中途藉由對鋼板表背面加熱而可減低如圖2所示之板厚方向之溫度差，可於迄今以上之低溫安定地壓延。藉此，於迄今相同程度之條件下進行熱軋時，可獲得更高的脆性龜裂擴展停止特性。又，為了獲得相同程度之脆性龜裂擴展停止特性之必要壓延條件與迄今相比可緩和。 [0022] 因此，發現使用上述製程，控制板厚1/2位置及鋼板表面之{113}＜110＞方位強度，可獲得具有優異母材韌性並且獲得極優異之脆性龜裂擴展停止特性。 [0023] 基於以上之見解進行檢討，因而完成本發明。亦即，本發明之要旨構成如下。 [0024] 1. 一種高強度厚鋼板，其以質量%計，含有 C：0.03~0.20%， Si：0.03~0.5%， Mn：0.5~2.2%， P：0.02%以下， S：0.01%以下， Ti：0.005~0.03%， Al：0.005~0.080%，及 N：0.0050%以下， 其餘部分由Fe及不可避免雜質所成， 且具有以下述（1）式定義之Ceq滿足下述（2）式之條件之成分組成， 具有板厚1/2位置之{113}＜110＞方位強度為4.0以上，鋼板表面之{113}＜110＞方位強度為1.7以上之集合組織，其中，上述（1）式中之括弧表示前述高強度厚鋼板之該括弧內之元素的含量（質量%），未含有該元素時表示為0。 [0025] 2. 如上述1之高強度厚鋼板，其板厚為50~ 100mm， Kca（-10℃）為7000N/mm3/2
以上， 板厚1/4位置之vE（-40℃）為250J以上，且 板厚1/4位置之拉伸強度TS為570MPa以上。 [0026] 3. 如上述1或2之高強度厚鋼板，其中板厚1/2位置之組織所佔之貝氏體（bainite）之面積分率為85%以上。 [0027] 4. 如上述1~3中任一項之高強度厚鋼板，其中前述成分組成進而含有以質量%計，選自下述所成之群中之1或2者以上： Nb：0.005~0.05%， Cu：0.01~0.5%， Ni：0.01~1.5%，及 Cr：0.01~0.5%。 [0028] 5. 如上述1~4中任一項之高強度厚鋼板，其中前述成分組成進而含有以質量%計，選自下述所成之群中之1或2者以上： Mo：0.01~0.5%， V：0.001~0.10%， B：0.0030%以下， Ca：0.0050%以下，及 REM：0.0100%以下。 [0029] 6. 如上述1~4中任一項之高強度厚鋼板，其中距鋼板表面5mm位置與板厚1/2位置之vE（-40℃）均為250J以上。 [0030] 7. 一種高強度厚鋼板之製造方法，其係如上述1~6中任一項之高強度厚鋼板之製造方法，且具有下述步驟： 使具有如上述1、4及5中任一項之成分組成之鋼在1000~1200℃之加熱溫度加熱的加熱步驟；及 使經加熱之前述鋼進行熱軋成為熱軋鋼板之熱軋步驟， 前述熱軋步驟包含： 在板厚1/2位置為奧氏體（austenite）再結晶溫度區域之熱軋，與 在板厚1/2位置為奧氏體（austenite）未再結晶溫度區域之熱軋， 於前述熱軋步驟之間，自表背兩面加熱前述鋼。 [0031] 8. 如上述7之高強度厚鋼板之製造方法，其中進而具有以3℃/s以上之冷卻速度，將前述熱軋鋼板冷卻至500℃以下之冷卻停止溫度之冷卻步驟。 [0032] 9. 如上述8之高強度厚鋼板之製造方法，其中進而具有使於前述冷卻步驟冷卻之熱軋鋼板於Ac1
點以下之回火溫度回火的回火步驟。 [0033] 10. 如上述7~9中任一項之高強度厚鋼板之製造方法，其中於自前述表背兩面之加熱結束的時點之前述鋼的表面與板厚1/2位置之溫度差為30℃以下。 [0034] 11. 如上述7~10中任一項之高強度厚鋼板之製造方法，其中自前述表背兩面之加熱係在比前述板厚1/2位置為奧氏體未再結晶溫度區域之熱軋開始更早進行。 [發明效果] [0035] 依據本發明，由於適當控制板厚1/2位置與鋼板表面兩者之集合組織，故即使於板厚超過50mm之情況下，亦可獲得脆性龜裂擴展停止特性優異之高強度厚鋼板。本發明之高強度厚鋼板藉由應用於例如造船領域中之集裝箱船、散裝船等之強力甲板部構造中之與艙口邊緣圍板接合之甲板構件，而有助於船舶之安全性提高，於產業上極為有用。[Problems to be Solved by the Invention] However, in order to obtain the steel materials described in Patent Documents 1 and 2, after the steel sheet surface portion is temporarily cooled, it is necessary to reheat and apply the processing in the reheating to control the structure. Therefore, it is not easy to control the actual production scale, and the load on the rolling and cooling equipment is large. Further, as in the prior art described above, it is not necessary to obtain a steel sheet having a thickness of about 50 mm, and it is not clear whether or not a necessary thickness is obtained when applied to a thick wall material having a thickness of about 70 mm. In particular, the crack propagation characteristics in the thickness direction necessary for the hull structure are also unclear. [0016] The present invention has been made in an effort to solve the above problems, and an object thereof is to provide a high-strength thick steel plate which has excellent brittle crack propagation stop characteristics even when the thickness exceeds 50 mm, and can be manufactured in an industrially simple process. Further, the object of the present invention is to produce a high-strength thick steel plate of the above-described high-strength steel plate in a stable manner by an industrially simple process. [Means for Solving the Problem] In order to solve the above-described problems, the present inventors have repeatedly conducted active research on a high-strength steel sheet having excellent brittle crack propagation stop characteristics and a method for stably obtaining the steel sheet. The following insights. [0018] (1) The pressure delay is completed in the austenite region, and the higher the temperature of the pressure delay, the higher the toughness value and the aggregate structure can be obtained. However, in a thick steel plate having a thickness of more than 50 mm, if the rolling temperature is lowered to the vicinity of the transformation point, the temperature difference between the surface of the steel sheet and the central portion of the thickness of the sheet becomes large as shown in Fig. 1, so that the surface portion is transformed into ferrite structure. The ferrite is calendered to deteriorate the toughness of the surface portion. [0019] (2) In order to suppress the formation of ferrite in the surface layer portion, it is necessary to increase the rolling temperature. However, when the rolling temperature is increased, the rolling temperature at the center of the sheet thickness cannot be sufficiently lowered. (3) When the rolling temperature at the center portion of the plate thickness is not sufficiently lowered, the crystal grain size at the center portion of the plate thickness becomes coarse and the toughness is insufficient, or the product of the aggregate structure which is favorable for the crack propagation stop characteristic Insufficient concentration. [0021] In order to solve the above problems and repeat the results of the review, it is conceivable that the temperature difference in the thickness direction as shown in FIG. 2 can be reduced by heating the front and back surfaces of the steel sheet in the middle of rolling, and it can be stably rolled at a low temperature so far. . Thereby, higher hot brittle crack propagation stop characteristics can be obtained when hot rolling is performed under the same conditions to date. Moreover, the rolling conditions necessary for obtaining the same degree of brittle fracture propagation stop characteristics can be alleviated as compared with the prior art. [0022] Therefore, it has been found that using the above-described process, controlling the sheet thickness of 1/2 and the {113}<110> azimuth strength of the steel sheet surface, it is possible to obtain an excellent base material toughness and obtain an extremely excellent brittle crack propagation stop characteristic. [0023] Based on the above findings, the present invention has been completed. That is, the gist of the present invention is as follows. [0024] 1. A high-strength thick steel plate containing C: 0.03 to 0.20%, Si: 0.03 to 0.5%, Mn: 0.5 to 2.2%, P: 0.02% or less, and S: 0.01% or less by mass%. , Ti: 0.005 to 0.03%, Al: 0.005 to 0.080%, and N: 0.0050% or less, the remainder is formed of Fe and unavoidable impurities, and Ceq defined by the following formula (1) satisfies the following (2) The composition of the conditional composition, having a {113}<110> azimuth intensity of 1/2 or more at a plate thickness of 4.0 or more, and a {113}<110> azimuth intensity of 1.7 or more on the surface of the steel sheet, In the above formula (1), the brackets indicate the content (% by mass) of the elements in the brackets of the high-strength steel plate, and when the element is not contained, it is represented as 0. [0025] 2. The high-strength thick steel plate according to the above 1 has a plate thickness of 50 to 100 mm, a Kca (-10 ° C) of 7000 N/mm 3/2 or more, and a plate thickness of 1/4 position vE (-40 ° C). It is 250 J or more, and the tensile strength TS of the sheet thickness of 1/4 position is 570 MPa or more. [0026] 3. The high-strength thick steel plate according to the above 1 or 2, wherein the area of the bainite occupied by the structure having a thickness of 1/2 is 85% or more. [0027] 4. The high-strength thick steel plate according to any one of the above 1 to 3, wherein the component composition further contains, in mass%, one or more selected from the group consisting of: Nb: 0.005 ~0.05%, Cu: 0.01~0.5%, Ni: 0.01~1.5%, and Cr: 0.01~0.5%. [0028] 5. The high-strength thick steel plate according to any one of the above 1 to 4, wherein the component composition further contains, in mass%, one or more selected from the group consisting of: Mo: 0.01 ~0.5%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, and REM: 0.0100% or less. [0029] 6. The high-strength thick steel plate according to any one of the above 1 to 4, wherein a vE (-40 ° C) of 5 mm from the surface of the steel sheet and 1/2 of the thickness of the steel sheet is 250 J or more. [0030] A method for producing a high-strength thick steel plate, which is the method for producing a high-strength thick steel plate according to any one of the above 1 to 6, and having the following steps: having the above-mentioned 1, 4, and 5 a heating step of heating the steel of any one of the components at a heating temperature of 1000 to 1200 ° C; and a hot rolling step of hot rolling the heated steel into a hot rolled steel sheet, wherein the hot rolling step comprises: The /2 position is hot rolling in the austenite recrystallization temperature region, and hot rolling in the austenite non-recrystallization temperature region at the sheet thickness of 1/2, between the aforementioned hot rolling steps, The aforementioned steel is heated from both sides of the front and back. 8. The method for producing a high-strength thick steel plate according to the above seventh aspect, further comprising a cooling step of cooling the hot-rolled steel sheet to a cooling stop temperature of 500 ° C or lower at a cooling rate of 3 ° C/s or more. 9. The method for producing a high-strength thick steel plate according to the above aspect, further comprising a tempering step of tempering the hot-rolled steel sheet cooled in the cooling step at a tempering temperature of Ac 1 or less. [0033] 10. The method for producing a high-strength thick steel plate according to any one of the above items 7-9, wherein a temperature difference between a surface of the steel and a thickness of 1/2 at a time point from the end of heating of the front and back sides It is below 30 °C. [0034] 11. The method for producing a high-strength thick steel plate according to any one of the above items 7 to 10, wherein the heating from the front and back sides is austenite non-recrystallization temperature region at a position 1/2 of the thickness of the sheet. The hot rolling began earlier. [Effect of the Invention] According to the present invention, since the collective structure of both the plate thickness 1/2 position and the steel sheet surface is appropriately controlled, even when the plate thickness exceeds 50 mm, the brittle crack propagation stop characteristic can be obtained excellent. High strength thick steel plate. The high-strength thick steel plate of the present invention contributes to the safety improvement of the ship by being applied to a deck member joined to a hatch edge coaming in a strong deck structure such as a container ship or a bulk ship in the shipbuilding field. Very useful in industry.
[0037] 以下具體說明本發明。本發明一實施形態之高強度厚鋼板中，成分組成及集合組織規定如上述。 [0038] [成分組成] 首先，說明將本發明之鋼成分組成限定為如上述之理由。又，成分組成中有關之「%」表示，只要未特別限定，則意指「質量%」。 [0039] C：0.03~0.20% C係提高鋼強度之元素，本發明中，為確保期望強度有必要含有0.03%以上。然而，C含量超過0.20%時，不僅是熔接性劣化，對韌性亦有不良影響。因此，C含量設為0.03~0.20%。又，C含量較好為0.05~0.15%。 [0040] Si：0.03~0.5% Si係作為脫氧元素，且作為鋼的強化元素而有效，但未達0.03%之含量時並無該效果。另一方面，Si含量超過0.5%時，不僅損及鋼的表面性狀，亦使韌性極端劣化。因此，Si含量設為0.03~0.5%。Si含量較好為0.04~0.40%。 [0041] Mn：0.5~2.2% Mn係作為強化元素而含有。Mn含量少於0.5%時，其效果不充分。另一方面，Mn含量超過2.2%時，除了熔接性劣化以外，鋼材成本亦上升。因此，Mn含量設為0.5~2.2%。 [0042] P：0.02%以下 P係鋼中不可避免之雜質，含量多時韌性劣化。因此，為了於如板厚超過50mm之厚鋼板亦保有良好韌性，P含量設為0.02%以下。P含量較好設為0.01%以下，更好0.006%以下。另一方面，下限並未限定，雖亦可為0%，但工業上而言係超過0%。 [0043] S：0.01%以下 S係鋼中不可避免之雜質，含量多時韌性劣化。因此，為了於如板厚超過50mm之厚鋼板亦保有良好韌性，S含量設為0.01%以下。S含量較好設為0.005%以下，更好0.003%以下。另一方面，下限並未限定，雖亦可為0%，但工業上而言係超過0%。 [0044] Ti：0.005~0.03% Ti藉由微量含有，而形成氮化物、碳化物或碳氮化物，具有使結晶粒微細化並提高母材韌性之效果。前述效果若於Ti含量為0.005%以上即可獲得。另一方面，Ti含量超過0.03%時，母材及熔接熱影響部中之韌性降低。因此，Ti含量設為0.005~0.03%。Ti含量較好為0.006~ 0.028%。 [0045] Al：0.005~0.080% Al係作為脫氧材而添加之元素，為了獲得其效果必須添加0.005%以上。另一方面，Al含量超過0.080%時，韌性降低並且熔接時熔接金屬部之韌性降低。因此，Al含量設為0.005~0.080%。又，Al含量較好為0.020~0.040%。 [0046] N：0.0050%以下 N係與鋼中之Al結合，調整壓延加工時之結晶粒徑，強化鋼的元素。然而，N含量超過0.0050%時，韌性劣化，故N含量設為0.0050%以下。另一方面，N含量之下限並未特別限定，但較好為0.0010%以上，更好為0.0015%以上。 [0047] 本發明一實施形態之高強度鋼板之成分組成係由上述元素、其餘部分之Fe及不可避免雜質所成。 [0048] 又，本發明其他實施形態中，為了進而提高特性，上述成分組成可進而任意含有選自Nb、Cu、Ni及Cr所成之群之1或2者以上。 [0049] Nb：0.005~0.05%， Nb係於鐵素體變態時或再加熱時以NbC析出，有助於高強度化。又，Nb具有於奧氏體區域之壓延中擴大未再結晶區域之效果，有助於鐵素體之細粒化，故對於韌性之改善亦有效。其效果藉由含有0.005%以上而得以發揮，但含有超過0.05%時，因析出粗大NbC，反而導致韌性降低。因此，含有Nb時，Nb含量設為0.005~0.05%。 [0050] Cu：0.01~0.5% Cu係提高銅之淬透性的元素，除了提高壓延後之強度以外，亦有助於韌性、高溫強度、耐候性等之功能。該等效果藉由含有0.01%以上而得以發揮，但過度含有時韌性或熔接性反而劣化。因此，Cu含量設為0.01~0.5%。 [0051] Ni：0.01~1.5% Ni係提高鋼的淬透性之元素，除了提高壓延後之強度以外，亦有助於韌性、高溫強度、耐候性等之功能。該等效果藉由含有0.01%以上而得以發揮。另一方面，過度含有時韌性或熔接性反而劣化，此外亦導致合金之成本增加。因此，Ni含量設為0.01~1.5%。 [0052] Cr：0.01~0.5% Cr與Cu同樣係提高鋼的淬透性之元素，除了提高壓延後之強度以外，亦有助於韌性、高溫強度、耐候性等之功能。該等效果藉由含有0.01%以上而得以發揮，但過度含有時韌性或熔接性反而劣化。因此，Cr含量設為0.01~0.5%。 [0053] 又，本發明其他實施形態中，為了進而提高特性，上述成分組成可進而任意含有選自Mo、V、B、Ca及REM所成之群之1或2者以上。 [0054] Mo：0.01~0.5% Mo與Cu及Cr同樣係提高鋼的淬透性之元素，除了提高壓延後之強度以外，亦有助於韌性、高溫強度、耐候性等之功能。該等效果藉由含有0.01%以上而得以發揮，但過度含有時韌性或熔接性反而劣化。因此，Mo含量設為0.01~0.5%。 [0055] V：0.001~0.10% V係藉由作為V（CN）析出之析出強化，而提高鋼強度之元素。該效果藉由含有0.001%以上之V而得以發揮。另一方面，V含有超過0.10%時，反而使韌性降低。因此，含有V時，V含量設為0.001~0.10%。 [0056] B：0.0030%以下 B係微量即具有提高鋼的淬透性效果之元素，可任意含有。然而，B含量超過0.0030%時，熔接部之韌性降低。因此，B含量設為0.0030%以下。又，B含量下限並未特別限定，含有B時，基於獲得良好淬透性之觀點，B含量較好設為0.0006%以上。 [0057] Ca：0.0050%以下 Ca係具有使熔接熱影響部之組織微細化並提高韌性之效果的元素，若適量含有則不會損及本發明效果。因此，可根據需要含有Ca。然而，過度含有Ca時，形成粗大之介隔物並使母材之韌性劣化。因此，含有Ca時，Ca含量設為0.0050%以下。另一方面，Ca含量之下限值並未特別限定，但添加Ca時，為了充分獲得添加效果，較好Ca含量設為0.0001%以上。 [0058] REM：0.0100%以下 REM（稀土類金屬）與Ca同樣，係具有使熔接熱影響部之組織微細化並提高韌性之效果的元素，若適量含有則不會損及本發明效果。因此，可任意含有REM。然而，過度含有REM時，形成粗大之介隔物並使母材之韌性劣化。因此，含有REM時，REM含量設為0.0100%以下。另一方面，REM含量之下限並未特別限定，但添加REM時，為了充分獲得添加效果，較好REM含量設為0.0005%以上。 [0059] [Ceq] 再者，上述成分組成係以下述（1）式定義之碳當量Ceq滿足下述（2）式之條件者。其中，上述（1）式中之括弧表示高強度厚鋼板之該括弧內之元素的含量（質量%），未含有該元素時表示為0。 [0060] 藉由將Ceq設為0.40以上，即使為板厚超過50mm之厚鋼板，亦可確保強度及集合組織強度。另一方面，Ceq之上限並未特別限定，但較好為0.55以下，更好為0.53以下，又更好為0.50以下。 [0061] [集合組織] 本發明中，為了提高對於壓延方向或壓延直角方向等之與板面平行方向擴展之龜裂的龜裂擴展停止特性，而規定板厚1/2位置及鋼板表面之{113}＜110＞方位強度。板厚1/2位置及鋼板表面中，若{113} ＜110＞方位發達，則在龜裂進展之前，發生微觀的龜裂而成為龜裂進展之阻力。又，此處所謂「板厚1/2位置」意指板厚方向之中央位置，所謂「鋼板表面」意指去除垢後之距鋼板表面0.5mm之深度位置。 [0062] 具體而言，成為板厚1/2位置之{113}＜110＞方位強度為4.0以上，鋼板表面之{113}＜110＞方位強度為1.7以上之集合組織。藉由以滿足前述條件之方式控制集合組織，即使為最近於集裝箱船或散裝船等船體外板所用般之板厚超過50mm之厚鋼板，除了確保構造安全性以外，亦可獲得成為目標之Kca（-10℃）≧7000N/mm3/2
之脆性龜裂擴展停止特性。其中，Kca（-10℃）係-10℃下之脆性龜裂擴展停止韌性。又，於要求更優異之龜裂擴展停止特性時，較好板厚1/2位置之{113} ＜110＞方位強度為4.1以上，鋼板表面之{113}＜110＞方位強度為1.9以上。另一方面，板厚1/2位置之{113}＜110＞方位強度之上限並未特別限定，越高越佳，但一般宜為7.0以下。又鋼板表面之{113}＜110＞方位強度之上限亦未特別限定，越高越佳，但一般宜為5.0以下。 [0063] 又，板厚1/2位置之{113}＜110＞方位強度與鋼板表面之{113} ＜110＞方位強度可分別藉由X射線極點圖法，以隨機強度比求得，具體而言，可藉由實施例記載之方法測定。又，前述測定中，容許數%之位置誤差。 [0064] [鋼板內部之組織] 板厚1/2位置之貝氏體之面積分率較好為85%以上。藉由如此控制組織，可提高有利於脆性龜裂擴展停止特性之{113}＜110＞方位。又，前述貝氏體之面積分率更好為90%以上。另一方面，前述貝氏體之面積分率上限並未特別限定，可為100%。又，貝氏體以外之其餘部分並未特別限定，可為任意組織。該等其餘部分組織之面積分率合計較好為15%以下。前述面積分率可藉實施例中記載之方法測定。 [0065] [母材韌性] 藉由如上述控制成分組成與集合組織，可獲得具有優異母材韌性之高強度厚鋼板。具有優異之母材韌性於抑制龜裂進展上具重要性。具體而言，板厚1/4位置之-40℃下之夏比（charpy）吸收能：vE（-40℃）較好為250J以上，更好為280J以上，又更好為300J以上。另一方面，前述vE（-40℃）之上限並未特別限定，但一般可為420J以下，亦可為400J以下。 [0066] 再者，距高強度厚鋼板表面5mm位置（深度）之vE（-40℃）較好為250J以上，更好為280J以上，又更好為300J以上。另一方面，前述vE（-40℃）之上限並未特別限定，但一般可為420J以下，亦可為400J以下。 [0067] 本發明中，如後述，藉由於熱軋步驟之間自表背兩面加熱鋼，可使距鋼板表面5mm位置與板厚1/2位置之vE（-40℃）兩者均為250J以上。 [0068] 又，板厚1/4位置之夏比斷口轉變溫度較好為 -40℃以下。前述夏比斷口轉變溫度之下限並未特別限定，但一般宜為-130℃以上。 [0069] [脆性龜裂擴展停止韌性] 如上述，本發明之高強度厚鋼板中藉由控制集合組織而可實現Kca（-10℃）為7000N/mm3/2
以上之優異脆性龜裂擴展停止特性。Kca（-10℃）較好為7500N/mm3/2
以上，更好為8000N/mm3/2
以上，又更好為9000N/mm3/2
以上。另一方面，由於Kca（-10℃）之值越高越好，故其上限並未特別限定，但一般宜為13000N/mm3/2
以下。又，前述Kca（-10℃）之值可藉由溫度梯度型ESSO試驗測定，具體而言可藉由實施例記載之方法獲得。 [0070] [拉伸強度] 本發明之高強度厚鋼板之拉伸強度（TS）並未特別限定，較好板厚1/4位置之拉伸強度TS為570MPa以上，更好為580MPa以上，又更好為590MPa以上。另一方面，關於TS上限亦未特別限定，但一般板厚1/4位置之拉伸強度TS宜為700MPa以下。 [0071] [板厚] 本發明之高強度厚鋼板之板厚並未特別限定，可為任意值。然而，由於板厚越厚本發明效果越顯著，故板厚較好為50mm以上，更好超過50mm，又更好為60mm以上，再更好為70mm以上。另一方面，板厚上限亦未特別限定，但一般宜為100mm以下。 [0072] [製造方法] 其次，說明本發明一實施形態之高強度厚鋼板之製造方法。 [0073] 本發明之高強度厚鋼板可藉由將具有上述成分組成之鋼在特定條件下熱軋而製造。具體而言，依序進行如下（1）及（2）。 （1）使鋼在1000~1200℃之加熱溫度加熱的加熱步驟。 （2）使經加熱之前述鋼進行熱軋成為熱軋鋼板之熱軋步驟。 而且，前述（2）熱軋步驟中，依序進行如下之（2-1）及（2-2）之步驟。 （2-1）在板厚1/2位置為奧氏體再結晶溫度區域之熱軋（再結晶區域壓延）。 （2-2）在板厚1/2位置為奧氏體未再結晶溫度區域之熱軋（未再結晶區域壓延）。 [0074] 又，前述（2）熱軋步驟之後，亦可任意進行如下之（3）之步驟。 （3）以3℃/s以上之冷卻速度，將前述熱軋鋼板冷卻至500℃以下之冷卻停止溫度之冷卻步驟。 [0075] 再者，前述（3）冷卻步驟之後，亦可任意進行如下之（4）之步驟。 （4）使於前述冷卻步驟冷卻之熱軋鋼板於Ac1
點以下之回火溫度回火的回火步驟。 [0076] 以下針對上述各步驟之條件的限定理由加以說明。 [0077] [加熱步驟] 加熱溫度：1000~1200℃ 於熱軋之前，加熱具有上述成分組成之鋼。此時，加熱溫度未達1000℃時，無法充分確保奧氏體再結晶溫度區域之壓延時間。另一方面，加熱溫度超過1200℃時，奧氏體粒粗大化，反而導致韌性降低，或氧化損失變顯著而使良率降低。因此，加熱溫度設為1000~1200℃。又，基於鋼板之韌性提高之觀點，前述加熱溫度較好為1000~1170℃，更好為1050~1170℃。 [0078] 又，供於前述加熱步驟之鋼並未特別限定，可藉任意方法製造。例如，可使用將具有上述成分組成之熔鋼以轉爐等進行熔製，藉由連續鑄造而得之鋼片（鋼坯）。 [0079] [熱軋] 其次，進行熱軋。熱軋步驟中，首先，進行（2-1）在板厚1/2位置為奧氏體再結晶溫度區域之熱軋（再結晶區域壓延），其次進行（2-2）在板厚1/2位置為奧氏體未再結晶溫度區域之熱軋（未再結晶區域壓延）。前述熱軋步驟中之壓延結束溫度並未特別限定，但較好為Ar3
點以上。 [0080] 上述熱軋之累積壓下率並未特別限定，但（2-1）在板厚1/2位置為奧氏體再結晶溫度區域之熱軋之累積壓下率（再結晶溫度區域累積壓下率）較好為12%以上。前述再結晶溫度區域累積壓下率之上限並未特別限定，但基於壓延負荷之觀點，較好為60%以下。又，（2-2）在板厚1/2位置為奧氏體未再結晶溫度區域之熱軋之累積壓下率（未再結晶溫度區域累積壓下率）較好為45%以上。前述未再結晶溫度區域累積壓下率之上限並未特別限定，但基於壓延負荷之觀點，較好為90%以下。基於集合組織控制之觀點，較好如此控制累積壓下率。 [0081] 本發明中，於前述（2）熱軋步驟期間，自表背兩面加熱前述鋼。藉由前述加熱，可控制板厚方向之溫度分佈，可減小鋼板表面與內部之溫度差，其結果，可獲得上述之板厚中央部與鋼板表面之集合組織。又，藉由進行自前述兩面之加熱，可使距鋼板表面5mm位置與板厚1/2位置之vE（-40℃）均為250J以上。 [0082] 自前述表背兩面之加熱較好以於該加熱結束的時點之前述鋼的表面與板厚1/2位置之溫度差為30℃以下之方式進行。藉此，可使板厚中央於更低溫度下壓延並且可抑制表面生成鐵素體。前述溫度差更好為20℃以下，又更好為10℃以下。另一方面，前述溫度差越小越好，故下限並未特別限定，宜為0℃以上。 [0083] 又，進行上述加熱之時點並未特別限定，只要於熱軋步驟之期間即可。然而，基於集合組織控制之觀點，較好在以加熱結束時更後進行之熱軋的累積壓下率為45%以上之時點進行該加熱，更好在未再結晶區域壓延開始前進行，又更好在再結晶區域壓延結束厚且未再結晶區域壓延開始前進行。於未再結晶區域壓延開始前進行加熱時，基於溫度控制之觀點，較好於該加熱結束後30秒以內開始未再結晶區域壓延。 [0084] 前述熱軋步驟中之加熱並未特別限定，可以感應加熱或爐加熱等之任意方法進行。 [0085] 於未再結晶區域壓延之期間鋼板表面與板厚1/2位置間之溫度差較大時，亦可進而於未再結晶區域壓延之期間，僅對表面加熱。 [0086] [冷卻步驟] 冷卻速度：3℃/s以上 冷卻停止溫度：500℃以下 壓延結束之鋼板，基於保持壓延時發達之集合組織之觀點，較好以3℃/s以上之冷卻速度，冷卻至500℃以下之冷卻停止溫度。冷卻速度之上限並未特別限定，但較好為10℃/s以下。又，冷卻停止溫度之下限並未特別限定，但較好為0℃以上。又，前述冷卻步驟中之冷卻開始溫度較好為Ar3
點以上。 [0087] [回火步驟] 回火溫度：Ac1
點以下 於前述冷卻步驟之後進行回火處理時，較好於Ac1
點以下之回火溫度進行回火。其理由為回火溫度高於Ac1
點時，有失去壓延時發達之集合組織之情況。回火溫度之下限並未特別限定，但較好為400℃以上。 [0088] 又，以上說明中，板厚1/2位置之溫度係藉由自以輻射溫度計測定之鋼板表面溫度之傳熱計算，或基於事先測定之中心溫度計算而求得。又，壓延後之冷卻條件中之溫度係板厚1/2位置之溫度。 [0089] （實施例） 以下針對本發明實施例加以說明。 將具有表1所示各成分組成之熔鋼以轉爐熔製，藉由連續鑄造法作成鋼坯。其次，加熱前述鋼坯厚，以成為板厚：50~100mm之方式進行熱軋。前述加熱與熱軋之條件如表2所示。隨後，以表2所示之條件進行冷卻，隨後，放冷獲得高強度厚鋼板。針對一部分鋼板，於冷卻後以表2所示之溫度進行回火。 [0090] 前述熱軋期間，除一部分比較例以外，自表背兩面加熱鋼。前述加熱係於再結晶區域壓延結束後、未再結晶區域壓延開始前進行。又，此時，未再結晶區域壓延於結束加熱後於30秒以內開始。前述加熱可藉由使用環境爐之爐加熱及高頻之感應加熱而實施。 [0091][0092][0093] 針對所得高強度厚鋼板之各者，藉由以下方法，評價韌性、拉伸強度、集合組織、組織及脆性龜裂擴展停止特性。評價結果示於表3。 [0094] [韌性] 為了評價所得高強度厚鋼板之韌性，進行夏比衝擊試驗，測定各鋼板之（1）距鋼板表面5mm之位置、（2）板厚1/4位置及（3）板厚1/2位置之3部位之-40℃下之夏比吸收能vE（-40℃）。前述夏比衝擊試驗係使用JIS（日本工業規格）規定之4號衝擊試驗片（長55mm，寬10mm，厚10mm），以使該試驗片之長度方向平行於鋼板之壓延方向之方式採取前述試驗片。又，用以測定距前述鋼板表面5mm之位置之vE（-40℃）之試驗片，係去除鋼板表面所形成之垢（黑皮）後，自該鋼板表面採取。試驗片厚度為10mm，故前述試驗片之測定位置為該試驗片厚度方向之中心位置，亦即距鋼板表面於板厚方向5mm之位置。 [0095] [拉伸強度] 自所得高強度厚鋼板之板厚1/4位置，以試驗片之長度方向與壓延方向垂直之方式，採取JIS 4號試驗片。使用前述試驗片，依據JIS Z 2241之規定進行拉伸試驗，求出板厚1/4位置之拉伸強度（TS）。 [0096] [集合組織] 為了評價所得高強度厚鋼板之集合組織，藉以下方法測定（1）板厚1/2位置及（2）鋼板表面之{113}＜110＞方位強度。首先，去除形成於前述鋼板表面之垢後，以使（1）板厚1/2位置及（2）距鋼板表面0.5mm深之位置成為測定位置之方式，採取板厚厚度1mm之樣品。其次，藉由機械研磨‧電解研磨所採取之樣品與板面平行之面，而準備X射線繞射用之試驗片。又，針對板厚表面之樣品，研磨接近鋼板表面之側的面。針對所得試驗片之各者，使用利用Mo線源之X射線繞射裝置實施X射線繞射測定，求出（200）、（110）及（211）正極點圖。自所得正極點圖求出三次元結晶方位密度函數，藉此算出{113}＜110＞方位強度相對於隨機強度之比。 [0097] [組織] 以與壓延方向平行之面作為觀察面之方式，自板厚1/2位置採取試料。鏡面研磨前述試料表面後，拍攝藉由蝕刻而露出之金屬組織之光學顯微鏡照片，藉由圖像解析評價貝氏體之面積分率。 [0098] [脆性龜裂擴展停止特性] 為了評價脆性龜裂擴展停止特性，進行溫度梯度型ESSO試驗，求出前述高強度厚鋼板之-10℃之Kca值（以下亦記為Kca（-10℃））。於前述溫度梯度型ESSO試驗中使用全厚試驗片。 [0099] 如由表3所示結果所了解，滿足本發明條件之高強度厚鋼板，距鋼板表面5mm位置、板厚1/2位置及板厚1/4位置之vE（-40℃）均為250J以上，Kca（-10℃）為7000N/mm3/2
以上，具備優異之脆性龜裂擴展停止特性。另一方面，未滿足本發明條件之比較例之高強度厚鋼板，板厚1/4位置之vE（-40℃）、Kca（-10℃）之至少一者劣化。 [0100] [0037] The present invention is specifically described below. In the high-strength thick steel plate according to the embodiment of the present invention, the component composition and the aggregate structure are as described above. [Component Composition] First, the reason for limiting the steel component composition of the present invention to the above is explained. In addition, the "%" in the component composition means "% by mass" unless otherwise specified. C: 0.03 to 0.20% C is an element which increases the strength of the steel. In the present invention, it is necessary to contain 0.03% or more in order to secure the desired strength. However, when the C content exceeds 0.20%, not only the weldability is deteriorated, but also the toughness is adversely affected. Therefore, the C content is set to 0.03 to 0.20%. Further, the C content is preferably from 0.05 to 0.15%. [0040] Si: 0.03 to 0.5% Si is a deoxidizing element and is effective as a strengthening element of steel, but this effect is not obtained when the content is less than 0.03%. On the other hand, when the Si content exceeds 0.5%, not only the surface properties of the steel but also the toughness are extremely deteriorated. Therefore, the Si content is set to 0.03 to 0.5%. The Si content is preferably from 0.04 to 0.40%. Mn: 0.5 to 2.2% Mn is contained as a strengthening element. When the Mn content is less than 0.5%, the effect is insufficient. On the other hand, when the Mn content exceeds 2.2%, the steel cost increases in addition to the deterioration of the weldability. Therefore, the Mn content is set to 0.5 to 2.2%. P: 0.02% or less The unavoidable impurities in the P-based steel, and the toughness is deteriorated when the content is large. Therefore, in order to maintain good toughness in a thick steel plate having a thickness of more than 50 mm, the P content is set to 0.02% or less. The P content is preferably set to 0.01% or less, more preferably 0.006% or less. On the other hand, the lower limit is not limited, and may be 0%, but industrially it is more than 0%. [0043] S: 0.01% or less of unavoidable impurities in the S-based steel, and the toughness is deteriorated when the content is large. Therefore, in order to maintain good toughness in a thick steel plate having a thickness of more than 50 mm, the S content is set to 0.01% or less. The S content is preferably set to 0.005% or less, more preferably 0.003% or less. On the other hand, the lower limit is not limited, and may be 0%, but industrially it is more than 0%. Ti: 0.005 to 0.03% Ti forms a nitride, a carbide or a carbonitride by being contained in a small amount, and has an effect of making the crystal grains finer and improving the toughness of the base material. The above effect can be obtained if the Ti content is 0.005% or more. On the other hand, when the Ti content exceeds 0.03%, the toughness in the base material and the heat-affected zone is lowered. Therefore, the Ti content is set to 0.005 to 0.03%. The Ti content is preferably from 0.006 to 0.028%. [0045] Al: 0.005 to 0.080% Al is an element added as a deoxidizing material, and it is necessary to add 0.005% or more in order to obtain the effect. On the other hand, when the Al content exceeds 0.080%, the toughness is lowered and the toughness of the welded metal portion at the time of welding is lowered. Therefore, the Al content is set to 0.005 to 0.080%. Further, the Al content is preferably from 0.020 to 0.040%. N: 0.0050% or less N-type is bonded to Al in steel, and the crystal grain size at the time of rolling processing is adjusted to strengthen the element of steel. However, when the N content exceeds 0.0050%, the toughness deteriorates, so the N content is made 0.0050% or less. On the other hand, the lower limit of the N content is not particularly limited, but is preferably 0.0010% or more, more preferably 0.0015% or more. The composition of the high-strength steel sheet according to the embodiment of the present invention is composed of the above-mentioned elements, the remaining Fe, and unavoidable impurities. Further, in another embodiment of the present invention, in order to further improve the characteristics, the component composition may further optionally contain one or more selected from the group consisting of Nb, Cu, Ni, and Cr. [0049] Nb: 0.005 to 0.05%, and Nb precipitates as NbC when ferrite is metamorphosed or reheated, contributing to high strength. Further, since Nb has an effect of expanding the non-recrystallized region in the rolling of the austenite region and contributes to the fine graining of the ferrite, it is also effective for improving the toughness. The effect is exhibited by containing 0.005% or more. However, when it contains more than 0.05%, coarse NbC is precipitated, and the toughness is rather lowered. Therefore, when Nb is contained, the Nb content is set to 0.005 to 0.05%. [0050] Cu: 0.01 to 0.5% Cu is an element which improves the hardenability of copper. In addition to improving the strength after rolling, it also contributes to functions such as toughness, high-temperature strength, and weather resistance. These effects are exhibited by containing 0.01% or more, but the toughness or the weldability deteriorates when excessively contained. Therefore, the Cu content is set to 0.01 to 0.5%. [0051] Ni: 0.01 to 1.5% Ni is an element which improves the hardenability of steel, and in addition to improving the strength after rolling, it also contributes to functions such as toughness, high-temperature strength, and weather resistance. These effects are exhibited by containing 0.01% or more. On the other hand, when excessively contained, the toughness or the weldability deteriorates, and the cost of the alloy increases. Therefore, the Ni content is set to 0.01 to 1.5%. [0052] Cr: 0.01 to 0.5% Cr is an element which improves the hardenability of steel in the same manner as Cu, and in addition to improving the strength after rolling, it also contributes to functions such as toughness, high-temperature strength, and weather resistance. These effects are exhibited by containing 0.01% or more, but the toughness or the weldability deteriorates when excessively contained. Therefore, the Cr content is set to 0.01 to 0.5%. Further, in another embodiment of the present invention, in order to further improve the characteristics, the component composition may further contain one or more selected from the group consisting of Mo, V, B, Ca, and REM. Mo: 0.01 to 0.5% Mo is an element which improves the hardenability of steel in the same manner as Cu and Cr. In addition to improving the strength after rolling, it also contributes to functions such as toughness, high-temperature strength, and weather resistance. These effects are exhibited by containing 0.01% or more, but the toughness or the weldability deteriorates when excessively contained. Therefore, the Mo content is set to 0.01 to 0.5%. V: 0.001 to 0.10% V is an element which increases the strength of steel by precipitation strengthening as V (CN) precipitation. This effect is exhibited by containing V of 0.001% or more. On the other hand, when V is more than 0.10%, the toughness is rather lowered. Therefore, when V is contained, the V content is set to 0.001 to 0.10%. [0056] B: 0.0030% or less B-based trace element, which has an effect of improving the hardenability of steel, can be optionally contained. However, when the B content exceeds 0.0030%, the toughness of the welded portion is lowered. Therefore, the B content is set to 0.0030% or less. Further, the lower limit of the B content is not particularly limited, and when B is contained, the B content is preferably 0.0006% or more from the viewpoint of obtaining good hardenability. Ca: 0.0050% or less Ca is an element having an effect of refining the structure of the heat-affected zone and improving the toughness, and if it is contained in an appropriate amount, the effect of the present invention is not impaired. Therefore, Ca can be contained as needed. However, when Ca is excessively contained, a coarse barrier is formed and the toughness of the base material is deteriorated. Therefore, when Ca is contained, the Ca content is made 0.0050% or less. On the other hand, the lower limit of the Ca content is not particularly limited, but when Ca is added, in order to sufficiently obtain the effect of addition, the Ca content is preferably made 0.0001% or more. REM: 0.0100% or less REM (rare earth metal) has an effect of refining the structure of the heat-affected zone and improving the toughness, similarly to Ca, and does not impair the effects of the present invention if it is contained in an appropriate amount. Therefore, REM can be optionally contained. However, when REM is excessively contained, a coarse barrier is formed and the toughness of the base material is deteriorated. Therefore, when REM is contained, the REM content is set to 0.0100% or less. On the other hand, the lower limit of the REM content is not particularly limited, but in the case of adding REM, in order to sufficiently obtain the effect of addition, the REM content is preferably 0.0005% or more. [Ceq] Further, the component composition is such that the carbon equivalent Ceq defined by the following formula (1) satisfies the condition of the following formula (2). Here, the brackets in the above formula (1) represent the content (% by mass) of the elements in the brackets of the high-strength thick steel sheet, and when the element is not contained, it is represented as 0. [0060] By setting Ceq to 0.40 or more, strength and aggregate structure strength can be ensured even for a thick steel plate having a thickness of more than 50 mm. On the other hand, the upper limit of Ceq is not particularly limited, but is preferably 0.55 or less, more preferably 0.53 or less, still more preferably 0.50 or less. [Collection Organization] In the present invention, in order to improve the crack propagation stop characteristic of the crack in the direction parallel to the plate surface in the rolling direction or the rolling orthogonal direction, the plate thickness is 1/2 position and the surface of the steel sheet is specified. {113}<110> Azimuth strength. When the {113} <110> orientation is developed at the 1/2 position of the plate thickness and the surface of the steel sheet, microscopic cracks occur before the crack progresses, and the crack progresses. Here, the term "plate thickness 1/2 position" means the center position in the thickness direction, and the "steel plate surface" means a depth position of 0.5 mm from the surface of the steel sheet after the scale removal. Specifically, the {113}<110> azimuth intensity at a plate thickness of 1/2 is 4.0 or more, and the {113}<110> azimuth intensity of the steel sheet surface is 1.7 or more. By controlling the assembly organization in such a manner as to satisfy the above conditions, even if it is a thick steel plate having a thickness of more than 50 mm as used in a hull plate such as a container ship or a bulk ship, in addition to ensuring structural safety, a target Kca can be obtained. (-10 ° C) 脆 7000 N / mm 3 / 2 brittle crack propagation stop characteristics. Among them, Kca (-10 ° C) is a brittle crack propagation at -10 ° C to stop the toughness. Further, when a more excellent crack propagation stop characteristic is required, the {113} <110> azimuth intensity at a plate thickness of 1/2 is preferably 4.1 or more, and the {113}<110> azimuth intensity of the steel sheet surface is 1.9 or more. On the other hand, the upper limit of the {113}<110> azimuth intensity at the plate thickness of 1/2 is not particularly limited, and the higher the better, but it is generally preferably 7.0 or less. Further, the upper limit of the {113}<110> azimuth strength of the surface of the steel sheet is not particularly limited, and the higher the ratio, the more preferable, but it is preferably 5.0 or less. [0063] Moreover, the {113}<110> azimuth intensity of the plate thickness 1/2 position and the {113}<110> azimuth intensity of the steel plate surface can be obtained by the X-ray pole pattern method at a random intensity ratio, respectively. The measurement can be carried out by the method described in the examples. Further, in the above measurement, a position error of several % is allowed. [Structure inside the steel sheet] The area fraction of bainite at a plate thickness of 1/2 is preferably 85% or more. By controlling the structure in this way, the {113}<110> orientation which is advantageous for the fragile crack propagation stop characteristic can be improved. Further, the area fraction of the bainite is more preferably 90% or more. On the other hand, the upper limit of the area fraction of the bainite is not particularly limited and may be 100%. Further, the rest of the bainite is not particularly limited and may be any structure. The total area fraction of these other organizations is preferably less than 15%. The aforementioned area fraction can be measured by the method described in the examples. [Base Material Toughness] By controlling the composition of the components and the aggregate structure as described above, a high-strength thick steel plate having excellent base material toughness can be obtained. It is important to have excellent base metal toughness in suppressing the progress of cracks. Specifically, the Charpy absorption energy at -40 ° C at a plate thickness of 1/4 ° is preferably 250 J or more, more preferably 280 J or more, and more preferably 300 J or more. On the other hand, the upper limit of the above vE (-40 ° C) is not particularly limited, but may be generally 420 J or less, or 400 J or less. Further, the vE (-40 ° C) at a position (depth) of 5 mm from the surface of the high-strength steel plate is preferably 250 J or more, more preferably 280 J or more, and more preferably 300 J or more. On the other hand, the upper limit of the above vE (-40 ° C) is not particularly limited, but may be generally 420 J or less, or 400 J or less. [0067] In the present invention, as will be described later, by heating the steel from both sides of the front and back sides during the hot rolling step, both the position 5 mm from the surface of the steel sheet and the vE (-40 ° C) at the plate thickness 1/2 are both 250 J. the above. Further, the summer-to-fracture transition temperature of the sheet thickness of 1/4 is preferably -40 ° C or lower. The lower limit of the aforementioned Charpy fracture transition temperature is not particularly limited, but is generally preferably -130 ° C or higher. [Fragile Cracking Expansion Stop Toughness] As described above, in the high-strength steel plate of the present invention, excellent brittle crack propagation of Kca (-10 ° C) of 7000 N/mm 3/2 or more can be achieved by controlling the aggregate structure. Stop feature. Kca (-10 ℃) preferably 7500N / mm 3/2 or more, more preferably 8000N / mm 3/2 or more, and more preferably 9000N / mm 3/2 or more. On the other hand, since the higher the value of Kca (-10 ° C) is, the upper limit is not particularly limited, but it is generally preferably 13,000 N/mm 3 / 2 or less. Further, the value of Kca (-10 ° C) can be measured by a temperature gradient type ESSO test, and specifically, it can be obtained by the method described in the examples. [Tensile Strength] The tensile strength (TS) of the high-strength thick steel sheet of the present invention is not particularly limited, and the tensile strength TS at the 1/4 position of the sheet thickness is preferably 570 MPa or more, more preferably 580 MPa or more. It is more preferably 590 MPa or more. On the other hand, the upper limit of TS is not particularly limited, but the tensile strength TS at a quarter of the thickness of the sheet is preferably 700 MPa or less. [Sheet Thickness] The thickness of the high-strength thick steel plate of the present invention is not particularly limited and may be any value. However, since the effect of the present invention is more remarkable as the thickness of the plate is thicker, the plate thickness is preferably 50 mm or more, more preferably 50 mm or more, more preferably 60 mm or more, and still more preferably 70 mm or more. On the other hand, the upper limit of the sheet thickness is not particularly limited, but it is generally preferably 100 mm or less. [Manufacturing Method] Next, a method of manufacturing a high-strength thick steel plate according to an embodiment of the present invention will be described. [0073] The high-strength thick steel plate of the present invention can be produced by hot rolling a steel having the above composition under specific conditions. Specifically, the following (1) and (2) are sequentially performed. (1) A heating step of heating the steel at a heating temperature of 1000 to 1200 °C. (2) A hot rolling step of hot rolling the heated steel to a hot rolled steel sheet. Further, in the above (2) hot rolling step, the following steps (2-1) and (2-2) are sequentially performed. (2-1) Hot rolling (recrystallization region rolling) in the austenite recrystallization temperature region at a sheet thickness of 1/2. (2-2) Hot rolling (no recrystallization region rolling) in a region where the austenite is not recrystallized at a sheet thickness of 1/2. Further, after the hot rolling step (2), the following step (3) may be arbitrarily performed. (3) A cooling step of cooling the hot-rolled steel sheet to a cooling stop temperature of 500 ° C or lower at a cooling rate of 3 ° C/s or more. [0075] Further, after the cooling step (3), the following step (4) may be arbitrarily performed. (4) A tempering step of tempering the hot-rolled steel sheet cooled in the cooling step at a tempering temperature of Ac 1 or less. [0076] The reasons for limiting the conditions of the above steps will be described below. [Heating Step] Heating Temperature: 1000 to 1200 ° C Before the hot rolling, the steel having the above composition is heated. At this time, when the heating temperature is less than 1000 ° C, the rolling time in the austenite recrystallization temperature region cannot be sufficiently ensured. On the other hand, when the heating temperature exceeds 1200 ° C, the austenite grains are coarsened, and conversely, the toughness is lowered, or the oxidation loss is remarkable, and the yield is lowered. Therefore, the heating temperature is set to 1000 to 1200 °C. Further, the heating temperature is preferably from 1,000 to 1,170 ° C, more preferably from 1,050 to 1,170 ° C, from the viewpoint of improvement in toughness of the steel sheet. Further, the steel to be supplied to the heating step is not particularly limited and may be produced by any method. For example, a steel sheet (slab) obtained by continuously casting a molten steel having the above-described composition into a melting furnace or the like can be used. [Hot Rolling] Next, hot rolling is performed. In the hot rolling step, first, (2-1) hot rolling (recrystallization region rolling) in the austenite recrystallization temperature region at the plate thickness 1/2 position, and second (2-2) in the plate thickness 1/ The 2 position is hot rolling (no recrystallization region rolling) in the austenite non-recrystallization temperature region. The rolling end temperature in the hot rolling step is not particularly limited, but is preferably at least 3 points of Ar. [0080] The cumulative reduction ratio of the hot rolling is not particularly limited, but (2-1) the cumulative reduction ratio of the hot rolling in the austenite recrystallization temperature region at the plate thickness 1/2 position (recrystallization temperature region) The cumulative reduction ratio is preferably 12% or more. The upper limit of the cumulative reduction ratio in the recrystallization temperature region is not particularly limited, but is preferably 60% or less from the viewpoint of the rolling load. Further, (2-2) the cumulative reduction ratio (the cumulative reduction ratio in the non-recrystallization temperature region) of the hot rolling in the austenite non-recrystallization temperature region at the plate thickness 1/2 is preferably 45% or more. The upper limit of the cumulative reduction ratio in the non-recrystallization temperature region is not particularly limited, but is preferably 90% or less from the viewpoint of the rolling load. Based on the viewpoint of collective organization control, it is better to control the cumulative reduction rate as such. In the present invention, during the hot rolling step (2), the steel is heated from both sides of the front and back sides. By the above heating, the temperature distribution in the thickness direction can be controlled, and the temperature difference between the surface of the steel sheet and the inside can be reduced, and as a result, the aggregate structure of the center portion of the above-mentioned thickness and the surface of the steel sheet can be obtained. Further, by heating from both surfaces, vE (-40 ° C) at a position of 5 mm from the surface of the steel sheet and a position of 1/2 of the thickness of the steel sheet can be 250 J or more. The heating from both sides of the front and back surfaces is preferably performed such that the temperature difference between the surface of the steel and the thickness of the sheet at the time of the end of the heating is 30° C. or less. Thereby, the center of the sheet thickness can be calendered at a lower temperature and the formation of ferrite on the surface can be suppressed. The aforementioned temperature difference is more preferably 20 ° C or less, and still more preferably 10 ° C or less. On the other hand, the smaller the temperature difference, the better, so the lower limit is not particularly limited, and is preferably 0 °C or higher. Further, the timing at which the heating is performed is not particularly limited, and may be performed during the hot rolling step. However, from the viewpoint of collective organization control, it is preferred to carry out the heating at a point where the cumulative reduction ratio of the hot rolling performed at the end of the heating is 45% or more, more preferably before the start of the rolling in the non-recrystallization region, and It is more preferable to carry out the calendering in the recrystallization region until the end of the rolling and the non-recrystallization region is started before the rolling. When heating is performed before the start of rolling in the non-recrystallization region, it is preferable to start the rolling of the non-recrystallization region within 30 seconds after the completion of the heating, from the viewpoint of temperature control. The heating in the hot rolling step is not particularly limited, and may be carried out by any method such as induction heating or furnace heating. When the temperature difference between the surface of the steel sheet and the position of the thickness of the steel sheet is large during the rolling in the non-recrystallization region, the surface may be heated only during the rolling of the non-recrystallization region. [Cooling step] Cooling rate: 3° C./s or more Cooling stop temperature: The steel sheet having a rolling end of 500° C. or less is preferably a cooling rate of 3° C./s or more based on the viewpoint of maintaining a developed structure of the pressure delay. Cool to a cooling stop temperature below 500 °C. The upper limit of the cooling rate is not particularly limited, but is preferably 10 ° C / s or less. Further, the lower limit of the cooling stop temperature is not particularly limited, but is preferably 0 ° C or higher. Further, the cooling start temperature of the cooling step is preferably not less than Ar 3 point. [tempering step] Tempering temperature: When the tempering treatment is performed at a point of Ac 1 or less after the cooling step, it is preferable to temper at a tempering temperature of Ac 1 or less. The reason is that when the tempering temperature is higher than Ac 1 point, there is a case where the aggregate structure in which the pressure delay is developed is lost. The lower limit of the tempering temperature is not particularly limited, but is preferably 400 ° C or higher. Further, in the above description, the temperature at the plate thickness of 1/2 is determined by heat transfer from the surface temperature of the steel sheet measured by a radiation thermometer or based on a central temperature measured in advance. Further, the temperature in the cooling condition after rolling is a temperature at a plate thickness of 1/2. (Embodiment) Hereinafter, an embodiment of the present invention will be described. The molten steel having the composition of each component shown in Table 1 was melted in a converter, and a slab was produced by a continuous casting method. Next, the thickness of the steel slab is heated, and hot rolling is performed so as to have a thickness of 50 to 100 mm. The conditions of the aforementioned heating and hot rolling are shown in Table 2. Subsequently, cooling was carried out under the conditions shown in Table 2, followed by cooling to obtain a high-strength thick steel plate. For a part of the steel sheet, it was tempered at a temperature shown in Table 2 after cooling. [0090] During the aforementioned hot rolling, the steel was heated from both sides of the front and back sides except for a part of the comparative examples. The heating is performed after the end of the rolling in the recrystallization zone and before the rolling of the non-recrystallization zone is started. Further, at this time, the non-recrystallization region was rolled and started within 30 seconds after the completion of the heating. The above heating can be carried out by using furnace heating in an environmental furnace and induction heating at a high frequency. [0091] [0092] For each of the obtained high-strength thick steel sheets, the toughness, the tensile strength, the aggregate structure, the structure, and the brittle crack propagation stop characteristics were evaluated by the following methods. The evaluation results are shown in Table 3. [Toughness] In order to evaluate the toughness of the obtained high-strength thick steel plate, a Charpy impact test was performed to measure (1) the position of each steel plate by 5 mm from the surface of the steel sheet, (2) the thickness of the plate 1/4 position, and (3) the plate. The summer specific absorption energy at a temperature of -40 ° C in the third portion of the thickness 1/2 position is vE (-40 ° C). The aforementioned Charpy impact test was carried out by using the No. 4 impact test piece (length 55 mm, width 10 mm, thickness 10 mm) prescribed by JIS (Japanese Industrial Standards) so that the longitudinal direction of the test piece was parallel to the rolling direction of the steel plate. sheet. Further, a test piece for measuring vE (-40 ° C) at a position of 5 mm from the surface of the steel sheet was taken from the surface of the steel sheet after the scale (black skin) formed on the surface of the steel sheet was removed. The thickness of the test piece was 10 mm, so the measurement position of the test piece was the center position in the thickness direction of the test piece, that is, the position of the steel plate surface at a thickness of 5 mm from the plate thickness direction. [Tensile Strength] A JIS No. 4 test piece was taken from the 1/4 position of the obtained high-strength thick steel plate so that the longitudinal direction of the test piece was perpendicular to the rolling direction. Using the test piece, a tensile test was carried out in accordance with JIS Z 2241, and the tensile strength (TS) at a thickness of 1/4 of the sheet thickness was determined. [Collection Organization] In order to evaluate the aggregate structure of the obtained high-strength thick steel sheets, (1) the sheet thickness 1/2 position and (2) the {113}<110> orientation strength of the steel sheet surface were measured by the following methods. First, after the scale formed on the surface of the steel sheet was removed, a sample having a thickness of 1 mm was used so that the position of (1) the thickness of 1/2 and the position of (2) the depth of 0.5 mm from the surface of the steel sheet were measured. Next, a test piece for X-ray diffraction was prepared by mechanically grinding and polishing the sample taken in parallel with the surface of the plate. Further, the sample on the side of the surface of the steel sheet was polished to the surface of the surface of the steel sheet. For each of the obtained test pieces, X-ray diffraction measurement was performed using an X-ray diffraction apparatus using a Mo line source, and (200), (110), and (211) positive electrode dot patterns were obtained. The cubic element azimuth density function is obtained from the obtained positive point map, thereby calculating the ratio of the {113}<110> azimuth intensity to the random intensity. [Organization] A sample was taken from the plate thickness 1/2 position so that the surface parallel to the rolling direction was used as the observation surface. After the surface of the sample was mirror-polished, an optical microscope photograph of the metal structure exposed by etching was taken, and the area fraction of bainite was evaluated by image analysis. [Fragile crack propagation stop characteristic] In order to evaluate the brittle fracture propagation stop characteristic, a temperature gradient type ESSO test was performed to determine the Kca value of -10 ° C of the high-strength thick steel plate (hereinafter also referred to as Kca (-10). °C)). A full thickness test piece was used in the aforementioned temperature gradient type ESSO test. [0099] As understood from the results shown in Table 3, the high-strength thick steel plate satisfying the conditions of the present invention has a vE (-40 ° C) position of 5 mm from the surface of the steel sheet, 1/2 position of the sheet thickness, and 1/4 position of the sheet thickness. It is 250 J or more, Kca (-10 ° C) is 7000 N/mm 3/2 or more, and has excellent brittle crack propagation stop characteristics. On the other hand, the high-strength thick steel plate of the comparative example which did not satisfy the conditions of the present invention deteriorated at least one of vE (-40 ° C) and Kca (-10 ° C) at a sheet thickness of 1/4. [0100]