Steel sheet and method for producing the same

Abstract

To provide a steel sheet having high strength and excellent toughness, and a method for producing the same.SOLUTION: The steel sheet has the chemical composition described in the specification, where Ceq is 0.25-0.60%, and BF' is more than 0%. In a C cross section, the metal structure at a position of 1 / 4t includes bainite in an amount of 70% or more in terms of area%, and an average grain size of bainitic ferrite constituting the bainite is 10 μm or less. Among the bainitic ferrite grains, a proportion of bainitic ferrite grains having an intragranular orientation difference of less than 5° is 1% or more and less than 30% in terms of area%. An average grain size of parent phase austenite obtained by inverse analysis from crystal orientation information of a matrix structure is 80 μm or less. A number density of B-containing precipitates having a grain size of 1 μm or more present on crystal grain boundaries of the parent phase austenite is 30 particles / mm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a method for manufacturing the same. [Background technology]

[0002] Applications of steel plates include welded structures such as ships, high-rise buildings, other buildings, bridges, marine structures, LNG storage tanks and other large tanks, and line pipes (see, for example, Patent Documents 1 to 5). For example, in the field of ships, welded structures have been increasing in size in recent years due to factors such as an increase in the load weight of container ships. Accordingly, steel plates are required to have increased thickness and higher strength. In addition, in the above-mentioned welded structures, further improvement in toughness is an issue from the viewpoint of further safety and reliability. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-023322 [Patent Document 2] Japanese Patent Application Publication No. 2019-023323 [Patent Document 3] Japanese Patent Application Publication No. 2019-023324 [Patent Document 4] Japanese Patent Application Publication No. 2019-035107 [Patent Document 5] International Publication No. 2019 / 069771 Summary of the Invention [Problem to be solved by the invention]

[0004] However, since there is generally a so-called trade-off relationship between strength and toughness, it has not been easy to achieve both high strength and improved toughness at the same time.

[0005] The present invention aims to solve the above problems and to provide a steel plate having high strength and excellent toughness, and a method for manufacturing the same. [Means for solving the problem]

[0006] As a result of detailed investigations into the above problems, the present inventors have come to the following findings.

[0007] As described above, there is a so-called trade-off relationship between strength and toughness. Therefore, the inventors investigated methods for achieving both high strength and improved toughness. As a result, they found that high strength can be achieved by making the metal structure mainly bainite, and that a decrease in toughness can be suppressed by refining the bainite structure and generating bainitic ferrite with a specific intragranular misorientation.

[0008] Furthermore, they found that by performing finish rolling at a predetermined reduction rate in the recrystallization temperature range to recrystallize the austenite and then cooling to the non-recrystallization temperature range before the grain growth associated with recrystallization occurs, or by continuing to perform rolling at a predetermined reduction rate in the non-recrystallization temperature range and controlling the finish rolling end temperature to a high level, it is possible to partially recrystallize the austenite, thereby producing bainitic ferrite with a specific intragranular misorientation and achieving a refined bainite structure.

[0009] On the other hand, if the heating temperature is too high, the austenite grain size coarsens, and the resulting bainite also coarsens, resulting in a deterioration in toughness. Also, if the finish rolling start temperature and / or finish rolling temperature are too high, the recrystallized austenite grains grow, causing the austenite grain size to coarsen, which similarly reduces toughness.

[0010] In addition, in this invention, we have investigated a manufacturing method that utilizes boron (B) to achieve a specified strength with less alloying. Even with trace amounts of B, it segregates at prior γ grain boundaries, suppressing ferrite formation, making it possible to ensure a bainite structure with lower alloying. However, B is prone to form nitrides and carbonitrides, and if these B compounds precipitate during thermomechanical treatment, the hardenability-improving effect of B is lost, and ferrite transformation is accelerated at high temperatures, which may result in a significant decrease in strength.

[0011] Therefore, we investigated a manufacturing method that would allow us to obtain the hardenability-improving effect of B while also achieving the formation of bainitic ferrite with the specific intragranular misorientation and the refinement of the bainite structure. As a result, we found that the recrystallization start temperature T rex It was found that it is important to control the reduction ratio, rolling time, and the time from rolling to the start of accelerated cooling using these as control parameters.

[0012] In addition, when a combined analysis of the state of B in steel using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and crystal orientation analysis using electron backscatter diffraction (EBSD) was performed, it was discovered that when the metal structure has a characteristic state of B, B has the effect of improving hardenability.

[0013] The present invention has been made based on the above findings, and is summarized as follows: a steel sheet and a method for producing the same.

[0014] (1) The chemical composition of the steel plate is, in mass%, C: 0.040~0.180%, Si: 0.01 to 0.50%, Mn: 0.70 to 2.50% P: 0.030% or less, S: 0.020% or less, Al: 0.001 to 0.100%, N: 0.0010~0.0080%, Nb: 0.003 to 0.050%, B: 0.0001 to 0.0050%, Ti: 0.003 to 0.050%, Mo: 0.010% or less Ce: 0.010% or less, O: 0.0040% or less, The balance is Fe and impurities. Ceq defined by the following formula (i) is 0.25 to 0.60%, B calculated using the following formula (ii) F ´ is greater than 0%, In a cross section perpendicular to the rolling direction of the steel plate, when the thickness of the steel plate is t, the metal structure at a position of 1 / 4t from the surface of the steel plate is Contains 70% or more bainite by area%, and The average grain size of bainitic ferrite constituting the bainite is 10 μm or less, The proportion of bainitic ferrite having an intragranular misorientation of less than 5° in the bainitic ferrite is 1% or more and less than 30% by area, The average grain size of parent phase austenite obtained by reverse analysis from the crystal orientation of the matrix structure constituting the steel sheet is 80 μm or less, the number density of B-containing precipitates with a grain size of 1 μm or more present on the grain boundaries of the parent austenite phase is 30 precipitates / mm or less; steel plate. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 ···(i) B F ´=B-(N-Ti×(14 / 47.867))×(10.811 / 14) ···(ii) However, the element symbols in the above formula represent the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained.

[0015] (2) The chemical composition contains, in mass %, one or more elements selected from the following groups A, B, C, and D, in place of a portion of the Fe: The steel sheet according to (1) above. [Group A] Cu: 1.50% or less, Ni: 2.50% or less, Cr: 1.00% or less, V: 0.15% or less, One or more selected from the group consisting of [Group B] Mg: 0.0100% or less, Ca: 0.0100% or less, REM: 0.0100% or less, One or more selected from the group consisting of [Group C] Zr: 0.0100% or less, Te: 0.0100% or less, One or two selected from the group consisting of [Group D] W: 1.00% or less, Sn: 0.50% or less, One or two selected from the group consisting of

[0016] (3) The chemical composition contains one or more elements selected from Group A. The steel sheet according to (2) above.

[0017] (4) The chemical composition contains one or more elements selected from Group B. The steel sheet according to (2) above.

[0018] (5) The chemical composition contains one or more elements selected from Group C. The steel sheet according to (2) above.

[0019] (6) The chemical composition contains one or more elements selected from Group D. The steel sheet according to (2) above.

[0020] (7) A method for producing a steel sheet according to any one of (1) to (6) above, A method for producing a steel plate, comprising sequentially subjecting a steel slab having a chemical composition according to any one of (1) to (6) above to a heating step, a hot rolling step, an accelerated cooling step, and a tempering step, In the heating step, the steel slab is heated to a temperature of 950 to 1200°C, The hot rolling step includes rough rolling and finish rolling, The rough rolling is performed by rolling the steel billet until the surface temperature of the steel billet reaches T rex Carry out at temperatures above +30°C, The cumulative reduction rate in the rough rolling is 10 to 75%, and the interpass time is 15 seconds or less, The finish rolling is performed by rolling the steel billet until the surface temperature of the billet reaches T rex ~T rex Rolling begins at a temperature in the range of +50°C, and T rex The cumulative rolling reduction in the above temperature range is more than 35% and less than 80%, T rex The cumulative reduction rate is less than 55% and the interpass time is 15 seconds or less. The finish rolling is completed at a temperature of Ar3+30°C or higher, The time from the completion of the finish rolling to the start of cooling in the accelerated cooling step is 100 seconds or less, In the accelerated cooling step, the material is water-cooled from a cooling start temperature of Ar3 or higher to a cooling stop temperature of 550°C or lower under conditions in which the average cooling rate from the start to the end of cooling is 5 to 50°C / second, In the tempering step, heating is performed in a temperature range of 350 to 650 ° C. Steel plate manufacturing method. However, Ar3 is calculated by the following formula (iii), and T rex is calculated using the following formula (iv). The element symbols in the following formulas represent the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained. Ar3=910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo...(iii) T rex =-91900×[Nb*] 2 +9400×[Nb*]+770 (iv) However, when the amount of dissolved Nb (% by mass) determined by the following formula (v) is defined as sol.Nb, when Nb ≥ sol.Nb, [Nb*] = sol.Nb when Nb < sol.Nb, [Nb*] = Nb shall be satisfied. sol.Nb = (10 (-6770 / (T+273)+2.26) ) / (C + 12 / 14 × N) ··· (v) In the above formula, T represents the heating temperature (°C) of the steel slab in the heating process.

Advantages of the Invention

[0021] According to the present invention, it becomes possible to obtain a steel sheet having high strength and excellent toughness.

Embodiments for Carrying Out the Invention

[0022] Each requirement of the present invention will be described in detail below.

[0023] (A) Chemical Composition The reasons for limiting each element are as follows. In the following description, “%” means “% by mass”.

[0024] C: 0.040 to 0.180% C is contained at 0.040% or more in order to ensure the strength of the steel sheet. On the other hand, when the C content exceeds 0.180%, it becomes difficult to ensure good toughness. Therefore, the C content is 0.180% or less. Thus, the C content is 0.040% or more, preferably 0.050% or more, more preferably 0.060% or more. Also, the C content is 0.180% or less, preferably 0.160% or less, more preferably 0.140% or less, and still more preferably 0.120% or less.

[0025] Si: 0.01 to 0.50% Since Si is effective as a deoxidizing and strengthening element, it is contained in an amount of 0.01% or more. On the other hand, if the Si content exceeds 0.50%, toughness is significantly reduced, so the Si content is set to 0.50% or less. Therefore, the Si content is 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. Furthermore, the Si content is 0.50% or less, preferably 0.40% or less, more preferably 0.35% or less, and even more preferably 0.30% or less.

[0026] Mn: 0.70 to 2.50% Mn is contained in an amount of 0.70% or more to ensure the strength of the steel sheet. On the other hand, if the Mn content exceeds 2.50%, center segregation becomes significant and the toughness of the area where center segregation occurs deteriorates, so the Mn content is set to 2.50% or less. Therefore, the Mn content is 0.70% or more, preferably 0.90% or more, and more preferably 1.10% or more. In addition, the Mn content is 2.50% or less, preferably 2.20% or less, more preferably 2.00% or less, and even more preferably 1.80% or less.

[0027] P:0.030% or less P is an element present in steel as an impurity. To stably ensure toughness, the P content is set to 0.030% or less, preferably 0.020% or less, and more preferably 0.015% or less. The lower limit is 0%, but in consideration of the cost of reducing the P content, the P content may be set to 0.0001% or more.

[0028] S: 0.020% or less S is an element present in steel as an impurity. If the S content exceeds 0.020%, a large amount of elongated MnS is formed in the center segregation region, resulting in deterioration of toughness and ductility. For this reason, the S content is set to 0.020% or less, and preferably 0.010% or less. Since the lower the S content, the better, and no lower limit is particularly specified, but from the viewpoint of production costs, the S content may be 0.0001% or more.

[0029] Al: 0.001 to 0.100% Al is generally an element that is intentionally added as a deoxidizing element. It also forms Al nitrides, which have the effect of suppressing the growth of austenite grain size during heating of a slab. Therefore, the Al content is set to 0.001% or more. However, excessive Al content promotes the formation of coarse, cluster-like alumina (Al2O3)-based inclusions, resulting in a deterioration of toughness. Therefore, the Al content is set to 0.100% or less, preferably 0.050% or less.

[0030] N: 0.0010~0.0080% N forms aluminum nitrides and has the effect of suppressing the growth of austenite grain size during heating of the slab, so it must be contained in an amount of 0.0010% or more. However, if the N content exceeds 0.0080%, not only does the steel sheet become embrittled, but it also precipitates as a compound with B (BN), which inhibits the hardenability improvement effect of B. Therefore, the N content is set to 0.0080% or less. Therefore, the N content is set to 0.0010% or more, preferably 0.0015% or more, and more preferably 0.0020% or more. Furthermore, the N content is set to 0.0080% or less, preferably 0.0070% or less, and more preferably 0.0060% or less.

[0031] Nb: 0.003 to 0.050% Nb can improve the strength and toughness of steel sheets. Furthermore, Nb is an element effective in expanding the non-recrystallization temperature range toward higher temperatures and contributes to increasing the rolling temperature. Therefore, Nb is essential for rolling in the non-recrystallized austenite range and for increasing the rolling finish temperature to obtain a desired microstructure. To achieve this effect, a Nb content of 0.003% or more is required. However, since a Nb content exceeding 0.050% reduces toughness and weldability, the Nb content is set to 0.050% or less. Therefore, the Nb content is set to 0.003% or more, preferably 0.005% or more, and more preferably 0.008% or more. Furthermore, the Nb content is set to 0.050% or less, preferably 0.035% or less, and more preferably 0.025% or less.

[0032] B: 0.0001 to 0.0050% B is an element that contributes to improving the strength of steel sheets by improving hardenability and generating a structure mainly composed of bainite. To achieve this effect, a B content of 0.0001% or more is necessary. However, an excessive B content reduces toughness, so the B content is set to 0.0050% or less. Therefore, the B content is 0.0001% or more, preferably 0.0005% or more, and more preferably 0.0008% or more. The B content is also 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0033] Ti: 0.003 to 0.050% In addition to improving the strength and toughness of steel sheets, Ti forms a compound (TiN) with N, reducing the amount of solute N in steel and inhibiting the formation of a compound (BN) with B. This makes Ti an essential element for achieving the hardenability-improving effect of B. Therefore, Ti must be contained in an amount of 0.003% or more. However, excessive Ti content hardens welds and significantly reduces toughness, so the Ti content is set to 0.050% or less. Therefore, the Ti content is set to 0.003% or more, preferably 0.006% or more, and more preferably 0.010% or more. The Ti content is set to 0.050% or less, preferably 0.035% or less, and more preferably 0.020% or less.

[0034] Mo: 0.010% or less Mo is an element that not only impairs economic efficiency but also causes an excessive increase in strength when contained in combination with B, even in trace amounts. Therefore, the Mo content is limited to 0.010% or less, and may be 0%. The Mo content is preferably 0.008% or less, and more preferably 0.005% or less.

[0035] Ce: 0.010% or less Ce, like Mo, is an element that not only impairs economic efficiency but also increases strength more than necessary when contained in combination with B, even in trace amounts. Therefore, the Ce content is limited to 0.010% or less, and may be 0%. The Ce content is preferably 0.008% or less, and more preferably 0.005% or less.

[0036] O: 0.0040% or less O is an element contained in steel, and exists either dissolved or in the form of an oxide. It is difficult to clearly separate the two. Therefore, the O content is defined as the total oxygen content, which includes both. If the O content exceeds 0.0040%, coarse oxides are formed, and stress concentration occurs in the oxides, resulting in a deterioration of toughness. Therefore, the O content is set to 0.0040% or less. Since a lower O content is preferable, no lower limit is particularly specified, but from the viewpoint of production costs, the O content may be 0.0001% or more.

[0037] In addition to the above elements, the chemical composition of the steel sheet of the present invention may further contain the following elements of Group A for the purpose of improving strength. The reasons for limiting each element will be explained below. [Group A] One or more selected from the group consisting of Cu: 1.50% or less, Ni: 2.50% or less, Cr: 1.00% or less, and V: 0.15% or less

[0038] Cu:1.50% or less Cu has the effect of improving the strength and toughness of steel sheets, so it may be added as needed. However, if excessive Cu is added, the performance improvement commensurate with the increase in alloy cost is not achieved, and rather, toughness deteriorates due to Cu precipitation. Therefore, the Cu content is 1.50% or less, preferably 1.20% or less, and more preferably 1.00% or less. To ensure the above effects, the Cu content is preferably 0.10% or more, and more preferably 0.20% or more.

[0039] Ni: 2.50% or less Ni is an element that has the effect of improving the strength of steel sheet, and may be added as needed. Furthermore, Ni is an element that has the effect of increasing the toughness of the steel matrix (base) in a solid solution state. However, excessive Ni content deteriorates toughness and weldability. Therefore, the Ni content is 2.50% or less, preferably 1.00% or less, more preferably 0.50% or less, and even more preferably 0.30% or less. To ensure the above effects, the Ni content is preferably 0.10% or more, more preferably 0.20% or more.

[0040] Cr:1.00% or less Cr is an element that has the effect of improving the strength of steel sheets, and may be added as needed. However, excessive Cr content deteriorates toughness and weldability. Therefore, the Cr content is 1.00% or less, preferably 0.80% or less, more preferably 0.50% or less, and even more preferably 0.30% or less. To ensure the above effects, the Cr content is preferably 0.10% or more, more preferably 0.20% or more.

[0041] V: 0.15% or less V is an element that has the effect of improving the strength of steel sheet, and may be contained as needed. However, excessive V content deteriorates toughness and weldability. Therefore, the V content is 0.15% or less, preferably 0.10% or less, more preferably 0.07% or less, and even more preferably 0.05% or less. To ensure the above effects, the V content is preferably 0.01% or more, more preferably 0.02% or more.

[0042] In addition to the above elements, the chemical composition of the steel sheet of the present invention may further contain the following elements of Group B for the purpose of controlling inclusions. The reasons for limiting each element will be explained below. [Group B] One or more selected from the group consisting of Mg: 0.0100% or less, Ca: 0.0100% or less, and REM: 0.0100% or less

[0043] Mg: 0.0100% or less Mg is a deoxidizing element that suppresses the formation of coarse inclusions by forming sulfides and also suppresses the formation of harmful inclusions by forming fine oxides. Therefore, Mg may be added as needed. However, excessive Mg content tends to form coarse oxides, sulfides, and oxysulfides, resulting in a decrease in toughness. Therefore, the Mg content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. To ensure the above effects, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0044] Ca:0.0100% or less Ca is a deoxidizing element that suppresses the formation of coarse inclusions by forming sulfides and also suppresses the formation of harmful inclusions by forming fine oxides. Therefore, it may be added as needed. However, excessive Ca content tends to form coarse oxides, sulfides, and oxysulfides, resulting in a decrease in toughness. Therefore, the Ca content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. To ensure the above effects, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0045] REM: 0.0100% or less REM is a deoxidizing element that suppresses the formation of coarse inclusions by forming sulfides and also suppresses the formation of harmful inclusions by forming fine oxides. Therefore, it may be added as needed. However, excessive REM content tends to lead to the formation of coarse oxides, sulfides, and oxysulfides, resulting in a decrease in toughness. Therefore, the REM content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. To ensure the above effects, the REM content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0046] In the present invention, REM refers to 16 elements excluding Ce from the total of 17 elements, Sc, Y, and lanthanoids, and the content of REM refers to the total content of these elements. Note that lanthanoids are industrially added in the form of misch metal.

[0047] In addition to the above elements, the chemical composition of the steel sheet of the present invention may further contain the following elements of Group C for the purpose of refining the metal structure. The reasons for limiting each element will be explained below. [Group C] One or two selected from the group consisting of Zr: 0.0100% or less and Te: 0.0100% or less

[0048] Zr: 0.0100% or less Zr is an element that contributes to improving toughness by refining the structure of the steel sheet. Zr also functions as a deoxidizing element. Therefore, Zr may be added as needed. However, excessive Zr content reduces toughness. Therefore, the Zr content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. To ensure the above effects, the Zr content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0049] Te: 0.0100% or less Te is an element that contributes to improving toughness by refining the structure of the steel sheet, and therefore may be added as needed. However, if Te is added in excess, toughness decreases. Therefore, the Te content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. To more reliably obtain the above effects, the Te content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0050] In addition to the above elements, the chemical composition of the steel sheet of the present invention may further contain the following elements of Group D for the purpose of improving corrosion resistance. The reasons for limiting each element will be explained below. [Group D] One or two selected from the group consisting of W: 1.00% or less and Sn: 0.50% or less

[0051] W: 1.00% or less W dissolves and forms oxygen acid ions WO4 - W is an element that adsorbs to rust in the form of , inhibits the permeation of chloride ions through the rust layer, and improves corrosion resistance, so it may be contained as needed. However, if excessive W is contained, not only will the above effect saturate, but toughness may also decrease. Therefore, the W content is 1.00% or less, preferably 0.75% or less. To ensure the above effect, the W content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0052] Sn: 0.50% or less Sn is Sn 2+ Sn dissolves as a cation and acts as an inhibitor in acidic chloride solutions to inhibit corrosion. Sn also inhibits the anodic dissolution reaction of steel, improving corrosion resistance. Therefore, Sn may be added as needed. However, excessive Sn content not only saturates the above-mentioned effect, but also deteriorates toughness. Therefore, the Sn content is set to 0.50% or less, preferably 0.30% or less. To ensure the above-mentioned effects, the Sn content is preferably set to 0.03% or more, more preferably 0.05% or more.

[0053] In addition, in the chemical composition of the steel sheet according to the present invention, Ceq, defined by the following formula (i), must be 0.25 to 0.60%. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 ···(i) However, the element symbols in the above formula represent the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained.

[0054] By setting the Ceq value to 0.25% or more, the strength required for the steel sheet can be ensured. Furthermore, by setting the Ceq value to 0.60% or less, excellent toughness can be ensured. Ceq is 0.25% or more, preferably 0.28% or more, more preferably 0.31% or more, and even more preferably 0.34% or more. Furthermore, Ceq is 0.60% or less, preferably 0.57% or less, more preferably 0.54% or less, and even more preferably 0.51% or less.

[0055] Furthermore, in the chemical composition of the steel sheet according to the present invention, B calculated by the following formula (ii) F ´ must be greater than 0%. B F ´=B-(N-Ti×(14 / 47.867))×(10.811 / 14) ···(ii) However, the element symbols in the above formula represent the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained.

[0056] B F ' is a value that serves as an index of the amount of solute B. It is difficult to measure the amount of solute B during thermomechanical treatment. Therefore, in the present invention, it is calculated using the above formula. The above formula determines whether or not solute B remains in the steel by estimating the total amount of B-containing precipitates in the steel, since after solute N is consumed with the precipitation of TiN during thermomechanical treatment, the remaining solute N forms B and BN in the steel. The presence of solute B in the steel causes it to segregate at prior γ grain boundaries and suppresses the formation of ferrite, thereby contributing to improving strength. The lower limit of the amount of solute B is 0%, and the upper limit is the B content in the steel sheet. From this perspective, in the present specification, the B calculated using the above formula (ii) is F If ´ is less than 0%, B F The value of ´ is 0%, and B F If ´ exceeds the B content in the steel plate, B F The value of ´ is the same as the B content in the steel plate.

[0057] The balance of the chemical composition of the steel sheet of the present invention is Fe and impurities. Here, "impurities" refers to components that are mixed in during industrial production of steel sheet due to various factors in raw materials such as ores and scraps, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0058] (B) Metal structure of steel plate The metal structure of the steel sheet of the present invention will be described. In the following description, "%" means "area %." In addition, in the present invention, when the thickness of the steel sheet is t, a position of 1 / 4t from the surface of the steel sheet in a cross section perpendicular to the rolling direction of the steel sheet will be referred to as the "1 / 4t position on the C cross section."

[0059] Bainite: 70% or more In the present invention, the metal structure is mainly composed of bainite. Specifically, by making the area fraction of bainite at the 1 / 4t position on the C cross section 70% or more, it is possible to ensure the strength of the steel plate. The area fraction of bainite is preferably 80% or more. However, there is no need to set an upper limit to the area fraction of bainite; in other words, it may be a single bainite phase.

[0060] The remaining structure may contain ferrite, pearlite, a martensite-retained austenite mixed phase (Martensite-Austenite Constituent; MA phase), and martensite, but this is acceptable as long as the total area ratio of these is 30% or less. The total area ratio is preferably 20% or less. The lower the total area ratio, the better, and there is no particular lower limit. For example, the total area ratio may be 0%. It may also be greater than 0%, or may be 1% or greater.

[0061] In the present invention, the area fractions of the metallographic structure, including bainite, are determined as follows: First, a sample is taken from the steel sheet so that the observation surface is the 1 / 4t position on a cross section (C cross section) perpendicular to the rolling direction. The observation surface is then subjected to nital etching, and after etching, eight fields of view are photographed at 500x magnification using an optical microscope. The obtained microstructure photographs are then subjected to image analysis, and the area fractions of white-appearing ferrite, black-appearing pearlite, and gray-appearing bainite, martensite, or the MA phase are determined.

[0062] Next, the nital-etched portion is re-polished and then subjected to Repera etching. Image analysis is performed on the portions that appear gray after nital etching, and the area percentage of those that appear white is determined as the MA phase. The area percentage of bainite or martensite is calculated by subtracting the area percentage of the MA phase from the area percentage of those that appear gray after nital etching. Of those judged to be bainite or martensite, those with carbides present between blocks and aligned in only one direction are considered to be bainite, and their area percentage is calculated. The area percentage of martensite is calculated by subtracting the area percentage of bainite from the area percentage of bainite or martensite.

[0063] The bainite structure determined by the above procedure can be refined and bainitic ferrite with a specific intragranular misorientation can be generated to achieve both strength and toughness in the steel plate. Specifically, the bainite structure must satisfy the following requirements:

[0064] Average grain size of bainitic ferrite: 10 μm or less At the 1 / 4t position on the C cross section, the average grain size of the bainitic ferrite that constitutes the bainite is set to 10 μm or less. Here, the average grain size is the average value of the circle equivalent diameter. Refining the bainitic ferrite that constitutes the bainite is necessary to ensure toughness. The average grain size of the bainitic ferrite is preferably 8 μm or less.

[0065] Percentage of bainitic ferrite with intragranular misorientation of less than 5°: 1% to less than 30% Of the bainitic ferrite constituting the bainite structure, the proportion of bainitic ferrite with an intragranular misorientation of less than 5° is set to 1% or more and less than 30% in terms of area ratio. The intragranular crystal orientation is thought to correlate with the dislocation density contained in the crystal grain. The higher the dislocation density, the greater the crystal misorientation. Generally, an increase in intragranular dislocation density improves strength, but also makes dislocation movement more difficult, reducing workability and toughness. Therefore, to ensure toughness, 1% or more of bainitic ferrite with an intragranular misorientation of less than 5° is required. The proportion of bainitic ferrite with an intragranular misorientation of less than 5° is preferably 3% or more. If the proportion of bainitic ferrite with an intragranular misorientation of less than 5° is too high, the average grain size of the bainitic ferrite increases, making it difficult to ensure toughness, so it is set to less than 30%. The proportion of bainitic ferrite with an intragranular misorientation of less than 5° is preferably less than 25%.

[0066] The average grain size of bainitic ferrite is measured using the following method. A sample is taken so that the 1 / 4t position in the C-section serves as the observation surface. The crystal orientation of a rectangular region of the observation surface, measuring 500 μm in the rolling direction (RD) and 500 μm in the normal direction (ND) of the rolling surface, is observed at 1 μm intervals using electron backscattering diffraction (EBSD). Crystal orientation information for this region is obtained. Next, a crystal grain is defined as a region surrounded by grain boundaries with a misorientation of 15° or more and with an equivalent circle diameter of 1 μm or more. Furthermore, a map based on local crystal misorientation is created using KAM (kernel average misorientation) analysis, where the average misorientation value between a pixel at the measurement point and its six neighboring pixels is used as the value of the central pixel. The region where this local misorientation exceeds 1.0° is bainitic ferrite, and the circle-equivalent diameter of the crystal grains corresponding to this region is calculated.

[0067] The proportion of all crystal grains with an intragranular misorientation of less than 5° is measured using the following method. First, the observation surface is analyzed using the EBSD method in the same way as above, and crystal orientation information for this region is obtained. Next, using the obtained crystal orientation information, regions surrounded by grain boundaries with an orientation misorientation of 15° or more and with an equivalent circle diameter of 1 μm or more are defined as crystal grains, the intragranular misorientation is calculated, and the proportion of all crystal grains with an intragranular misorientation of less than 5° is determined. The proportion obtained in this way is an area fraction, but is equivalent to a volume fraction.

[0068] "Intra-grain orientation misalignment" refers to "Grain Orientation Spread (GOS)," which is the orientation dispersion within a crystal grain. The intra-grain orientation misalignment is calculated as the average misorientation between a reference crystal orientation and the crystal orientations at all measurement points within the crystal grain. The "reference crystal orientation" is the average of the crystal orientations at all measurement points within the crystal grain. The intra-grain orientation misalignment and KAM analysis described above can be calculated using, for example, the EBSD analysis software "OIM Analysis Version 8.6.101" manufactured by TSL Solutions.

[0069] Average grain size of parent austenite obtained by inverse analysis from crystal orientation information of the matrix structure: 80 μm or less A specific crystal orientation relationship called the Kurdjumov-Sachs (KS) relationship exists between the body-centered cubic (BCC) phase (ferrite, bainite, martensite), which serves as the matrix structure, and the parent austenite phase. By utilizing this relationship and performing inverse analysis of the crystal orientation information of the matrix, it is possible to reconstruct the parent austenite phase. The finer the average grain size of the austenite grains, the finer the bainite in the matrix, resulting in better toughness. On the other hand, if the average grain size of the austenite grains exceeds 80 μm, the matrix structure becomes coarse, the GOS increases, and bainitic ferrite with an intragranular misorientation of less than 5° is not formed, resulting in poor toughness. For this reason, the average grain size of the austenite grains is limited to 80 μm or less. It is preferably 70 μm or less, and more preferably 60 μm or less.

[0070] The average grain size of the parent austenite grains is measured using the following method. First, the observation surface is analyzed using the EBSD method as described above to obtain crystal orientation information for this region. Next, the parent austenite is reconstructed based on the obtained crystal orientation information based on the K-S relationship. Regions surrounded by grain boundaries with a misorientation of 15° or more and with an equivalent circle diameter of 1 μm or more are defined as crystal grains, and the average austenite grain size is calculated. The austenite reconstruction based on the K-S relationship and the average grain size of the austenite grains described above can be calculated using, for example, the EBSD analysis software "OIM Analysis Version 8.6.101" manufactured by TSL Solutions.

[0071] Number density of B-containing precipitates with a diameter of 1 μm or more present on the grain boundaries of the parent austenite phase: 30 precipitates / mm or less During the thermal processing of the base material, solute B segregates at the austenite grain boundaries, ensuring hardenability. On the other hand, the B segregated at the austenite grain boundaries dissolves in Fe. 23When B precipitates such as (C,B)6 are precipitated, the hardenability is reduced, the strength of the base material is reduced, and the coarse B precipitates act as the starting point for brittle fracture, resulting in a deterioration in toughness. Therefore, the number density of B precipitates with a grain size of 1 μm or more present on the grain boundaries of the parent austenite phase is limited to 30 precipitates / mm or less, preferably 25 precipitates / mm or less, and more preferably 20 precipitates / mm or less.

[0072] The number density of B-containing precipitates present on the grain boundaries of the parent austenite phase is measured by the following method. First, the location of B is measured using time-of-flight secondary ion mass spectrometry (TOF-SIMS). A sample is taken so that the 1 / 4t position in the C cross section becomes the observation surface, and the surface is mirror-finished with colloidal silica. A rectangular area 200 μm long in the RD direction and 200 μm long in the ND direction is used as the measurement target. + is the irradiated ion, and BO2 - Observe the location of B from the measurement data. - The dot-like portions having a circle equivalent diameter of 1 μm or more, where the signal intensity is higher than that of the matrix portion, are regarded as B-containing precipitates. For the TOF-SIMS, for example, TOF.SIMS5 manufactured by ION-TOF can be used.

[0073] Next, the EBSD observation described above is performed in the same position and field of view as the TOF-SIMS observation described above, and crystal orientation information for the observation surface is obtained. Next, the parent austenite phase is reconstructed from the EBSD data using the inverse analysis described above. Furthermore, the grain boundary map of the parent austenite phase is compared with the B presence map obtained by the TOF-SIMS observation described above, and coarse B-containing precipitates with a circle equivalent diameter of 1 μm or more present on the austenite grain boundaries are counted. The number of B-containing precipitates is then divided by the total length of the austenite grain boundaries in the field of view, allowing the number density of B-containing precipitates present on the parent austenite grain boundaries to be calculated.

[0074] (C) Mechanical properties of steel plate Although there are no particular limitations on the mechanical properties of the steel sheet according to the present invention, the steel sheet according to the present invention has high strength and excellent toughness. Specifically, it is preferable that the yield stress (YS) is 305 to 750 MPa and the tensile strength (TS) is 440 to 930 MPa. It is also preferable that the fracture appearance transition temperature (vTrs) is -60°C or lower.

[0075] Tensile strength (TS) and yield stress (YS) are measured using No. 1B tensile test specimens taken from the center of the plate thickness in a direction perpendicular to the rolling direction, in accordance with JIS Z 2241:2022. Specifically, yield stress (YS) is the proof stress measured using the permanent elongation method at a permanent elongation of 0.2%. Furthermore, the fracture transition temperature (vTrs) is evaluated in accordance with JIS Z 2242:2018, using V-notch test specimens taken to include the 1 / 4t position of the steel plate.

[0076] (D) Thickness of steel plate There are no particular restrictions on the thickness of the steel plate according to the present invention, but when used as a welded structure, the plate thickness is preferably 6 to 70 mm, more preferably 10 to 60 mm, and even more preferably 20 to 60 mm.

[0077] (E) Steel plate manufacturing method There are no particular restrictions on the manufacturing conditions for the steel plate according to the present invention, but for example, the steel plate can be manufactured by subjecting a steel slab having the above-described chemical composition to a heating step, a hot rolling step, an accelerated cooling step, and a tempering step in this order under the conditions shown below. Each step will now be described.

[0078] (a) Heating process The heating step is a step that contributes to controlling the structure of the austenite phase by heating the slab. In the heating step, the slab is heated to a heating temperature of 950 to 1200°C. The heating step is preferably carried out in a heating furnace. Heating the slab to 950 to 1200°C means heating the slab so that the average temperature through the thickness of the slab when removed from the heating furnace is in the range of 950 to 1200°C. In this specification, this average temperature through the thickness of the slab is referred to as the heating temperature of the slab.

[0079] If the heating temperature is less than 950°C, the solid solution of Nb becomes insufficient, and coarse Nb-containing precipitates remain, resulting in a deterioration in toughness and a failure to obtain a structure mainly composed of bainite. If the heating temperature exceeds 1200°C, the austenite grains become coarse, making it difficult to refine the bainite structure in the final structure. The preferred heating temperature range is 1000 to 1150°C.

[0080] (b) Hot rolling process The hot rolling process includes a rough rolling process and a finish rolling process. rex +30℃ or higher. In other words, the surface temperature of the steel piece is T rex Rough rolling begins when the surface temperature of the slab is +30°C or higher. rex Rough rolling is completed when the temperature is +30°C or higher. rex is the recrystallization temperature, which will be explained in more detail later. For the recrystallization of austenite grains, T rex It is acceptable to roll at a temperature above T rex ~T rex To keep the temperature within the range of +50℃, rough rolling is performed at T rex The rough rolling is carried out at a temperature of +30°C or higher. Note that the surface temperature at the end of rough rolling may be higher than the surface temperature at the start of rough rolling. This is thought to be due to the effect of heat generated by rough rolling and the effect of heat transfer in the thickness direction of the billet due to the internal temperature being higher than the surface temperature. rex If the finish rolling start temperature is less than +30°C, rex Therefore, the conditions for finish rolling described later cannot be satisfied.

[0081] The cumulative reduction in rough rolling is in the range of 10 to 75%. The cumulative reduction in rough rolling is the value obtained by subtracting the plate thickness after rough rolling from the plate thickness at the start of rough rolling, and dividing the result by the plate thickness at the start of rough rolling. If the cumulative reduction in rough rolling is less than 10%, porosity may remain, causing internal cracks and resulting in deterioration of ductility and toughness. If the cumulative reduction exceeds 75%, the number of passes increases, reducing productivity. A preferable cumulative reduction is 30 to 60%. In the following description, the steel slab after rough rolling is referred to as a steel plate.

[0082] The subsequent finishing rolling is carried out when the surface temperature of the steel plate reaches T rex ~T rex Finish rolling is started when the surface temperature of the steel plate is in the range of Ar3 + 50℃, and finish rolling is completed when the surface temperature of the steel plate is Ar3 + 30℃ or higher. rex ~T rex Start at +50°C, T rex By rolling at a predetermined cumulative reduction rate, austenite is recrystallized. rex After rolling at a predetermined cumulative reduction rate below Ar3, finish rolling is completed at Ar3+30°C or higher. rex If rolling is not performed below T rex After rolling at the above temperature, T rex The temperature is lowered to less than Ar3 + 30°C. This allows the creation of a structure in which recrystallized austenite and non-recrystallized austenite are mixed. Bainitic ferrite with an intragranular misorientation of less than 5° is generated from this partially recrystallized austenite. If the finish rolling temperature is less than Ar3 + 30°C, B-containing precipitates precipitate from the austenite grain boundaries at such a low temperature, losing the effect of improving hardenability and preventing the formation of a bainite structure.

[0083] In addition, T in finishing rolling rex The cumulative reduction in the above temperature range is in the range of more than 35% and not more than 80%. rex The cumulative reduction in the above temperature range is the thickness of the steel plate at the start of finish rolling (after rough rolling) until the surface temperature of the steel plate reaches T rexThis is the value obtained by subtracting the thickness after the last rolling pass when the thickness becomes less than 1 / 2, and dividing the result by the thickness at the start of finish rolling. rex By setting the cumulative reduction rate in the above temperature range to be more than 35% and not more than 80%, it becomes possible to recrystallize the austenite grains and refine the grains. rex By lowering the temperature to less than 1000 K, a structure in which recrystallized austenite and unrecrystallized austenite are mixed can be produced.

[0084] T rex If the cumulative reduction rate in the above temperature range becomes too large, the recrystallization rate becomes too high, and the proportion of bainitic ferrite with an intragranular misorientation of less than 5° becomes 30% or more. As a result, the grain size of the bainitic ferrite becomes coarse, and the toughness deteriorates. rex The cumulative rolling reduction in the above temperature range shall be 80% or less. rex If the cumulative reduction rate in the above temperature range is too small, T rex When rolling is performed at temperatures below 1000 K, and when the temperature drops between the end of finish rolling and the start of water cooling, B-containing precipitates are formed, which prevents the hardenability improvement effect of B from being obtained, resulting in a decrease in strength. Furthermore, the B-containing precipitates become fracture initiation points, which also reduces toughness. In addition, toughness is also degraded when bainitic ferrite with a grain orientation misorientation of less than 5° is no longer formed. Therefore, T rex The cumulative rolling reduction rate above is more than 35%.

[0085] Furthermore, T in finish rolling rex The cumulative reduction in the temperature range below T shall be less than 55%. rex The cumulative reduction in the temperature range below T rex The thickness after the last rolling pass when the surface temperature of the steel plate becomes less than T is subtracted from the thickness after the finish rolling. rex This is the value obtained by dividing the thickness by the thickness after the rolling pass immediately before the rolling pass when the thickness becomes less than T rex If the cumulative reduction rate is 55% or more in the temperature range below T rexDuring rolling in a temperature range below T, coarse B-containing precipitates precipitate from the austenite grain boundaries, and the number density of B-containing precipitates with a diameter of 1 μm or more present on the grain boundaries of the parent austenite phase exceeds 30 precipitates / mm. When B-containing precipitates precipitate from the austenite grain boundaries, hardenability decreases and the required bainite fraction cannot be obtained. Therefore, the T in finish rolling is rex The cumulative rolling reduction in the temperature range below T rex While recrystallization progresses in the temperature range above T rex Partial recrystallization can occur even by lowering the temperature below T rex The lower limit of the cumulative rolling reduction in the temperature range below is not particularly limited, and may be 0% or 1% or more, but is preferably 10% or more.

[0086] Furthermore, the inter-pass time for all rolling passes in both rough rolling and finish rolling is set to 15 seconds or less. First, if the inter-pass time exceeds 15 seconds in the rough rolling process, the grain size will become coarse due to grain growth after austenite recrystallization, and the average grain size of the parent austenite phase obtained by inverse analysis from the crystal orientation information of the matrix structure will exceed 80 μm. In addition, the T rex In rolling in the above temperature range, if the interpass time exceeds 15 seconds, the strain imparted by the rolling is recovered, and the proportion of bainitic ferrite with an intragranular misorientation of less than 5° becomes 30% or more, resulting in an increase in the average grain size of bainitic ferrite. rex When rolling in a temperature range below T, if the interpass time exceeds 15 seconds, rex During rolling in a temperature range below 1000 K, coarse B-containing precipitates precipitate from the austenite grain boundaries, and the number density of B-containing precipitates with a diameter of 1 μm or more present on the grain boundaries of the parent austenite phase exceeds 30 precipitates / mm. When B-containing precipitates precipitate from the austenite grain boundaries, hardenability decreases and the required bainite fraction cannot be obtained.

[0087] Since the shorter the interpass time, the better, there is no need to set a lower limit, but from the viewpoint of operability, it is preferable to set it to 3 seconds or more. Finish rolling is generally performed by reverse rolling. The interpass time in finish rolling refers to the time from when the steel sheet is rolled by the rolling rolls while moving forward, when the rear end of the steel sheet leaves the rolling rolls, until the direction of travel of the steel sheet reverses to the rear, and the rear end of the steel sheet is again engaged by the rolling rolls.

[0088] The time from the completion of finish rolling to the start of cooling in the accelerated cooling step described below is set to 100 seconds or less. If the time from the completion of finish rolling to the start of cooling exceeds 100 seconds, B-containing precipitates precipitate from the austenite grain boundaries, and the number density of B-containing precipitates with a grain size of 1 μm or more present on the grain boundaries of the parent austenite phase exceeds 30 precipitates / mm. Note that the time from the completion of finish rolling to the start of cooling refers to the time from when the leading edge of the steel sheet moving forward passes through the reduction rolls in the final pass until water cooling is started.

[0089] In the above explanation, Ar3 means the transformation start temperature at which the metal structure transforms from FCC (Face Center Cubic) (austenite) to BCC (Body Center Cubic) (ferrite, bainite, martensite) during the temperature drop process, and is calculated by the following formula (iii). rex means the recrystallization temperature, which is the lowest temperature at which equiaxed recrystallized grains can be generated and grow, and is calculated by the following formula (iv): In the formula, the element symbols represent the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained.

[0090] Ar3=910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo...(iii) T rex =-91900×[Nb*] 2 +9400×[Nb*]+770 (iv) However, when the amount of dissolved Nb (mass%) calculated by the following formula (v) is taken as sol.Nb, When Nb ≥ sol.Nb, [Nb*] = sol.Nb When Nb < sol.Nb, [Nb*] = Nb Let it be so. sol.Nb = (10 (-6770 / (T+273)+2.26) ) / (C + 12 / 14 × N) ···(v) In addition, T in the above formula represents the heating temperature (°C) of the steel slab in the heating process.

[0091] (c) Accelerated cooling process In the accelerated cooling process, the steel plate after the finish rolling is water-cooled. At this time, water-cooling is performed from the cooling start temperature above Ar3 to the cooling stop temperature of 550°C or lower under the condition that the average cooling rate from the start of cooling to the end of cooling at the plate thickness t / 4 is 5 to 50°C / second. The average cooling rate is calculated based on the temperature at the plate thickness t / 4 obtained by heat transfer calculation from the surface temperatures of the steel plate at the start and end of cooling. By water-cooling at an average cooling rate of 5 to 50°C / second from the cooling start temperature above Ar3 to the cooling stop temperature of 550°C or lower, the final structure can be made mainly bainite. In addition, the average cooling rate and the cooling stop temperature are adjusted according to the value of Ceq in the chemical composition of the steel plate so as not to cause martensite transformation. [[ID=​​​​​​​​​​​​​​Steel plates with thicknesses of 6 to 70 mm were produced using steel billets having the chemical compositions shown in Tables 1 and 2 under the manufacturing conditions shown in Tables 3 and 4. The maximum interpass times in the rough rolling process and the finish rolling process shown in Tables 3 and 4 indicate the longest interpass times for each rolling pass.

[0095] [Table 1]

[0096] [Table 2]

[0097] [Table 3]

[0098] [Table 4]

[0099] The metallographic structure of the obtained steel sheets was observed by the method described above, and the area ratio of each structure, the average grain size of bainitic ferrite, the proportion of crystal grains with an intragranular misorientation of less than 5° to all crystal grains, the average grain size of the parent phase austenite, and the number density of coarse B-containing precipitates on the austenite grain boundaries were measured.

[0100] Furthermore, tensile strength (TS) and yield stress (YS) were measured in accordance with JIS Z 2241:2022. Test specimens were taken from the center of the plate thickness, with the longitudinal direction perpendicular to the rolling direction (width direction), using No. 1B tensile test specimens. Yield stress (YS) was measured as the proof stress of the permanent elongation method at a permanent elongation of 0.2%. V-notch test specimens were then taken to include the 1 / 4t position of the steel plate, and the fracture transition temperature (vTrs) was evaluated in accordance with JIS Z 2242:2018.

[0101] The results of these measurements are shown in Tables 5 and 6. In the tables, the area fraction of ferrite is represented as the "F fraction," the area fraction of pearlite as the "P fraction," the area fraction of bainite as the "B fraction," the B fraction with an intragranular misorientation of less than 5° as the "B' fraction," the area fraction of the MA phase as the "MA fraction," the martensite fraction as the "M fraction," the average grain size of bainitic ferrite as the "BF grain size," the average grain size of parent austenite as the "average γ grain size," and the number density of B-containing precipitates with a grain size of 1 μm or more present on the grain boundaries of parent austenite as the "number density of B-containing precipitates."

[0102] [Table 5]

[0103] [Table 6]

[0104] As can be seen from Table 5, the inventive examples (test numbers A1 to A38) that satisfied the requirements of the present invention had high strength and excellent toughness. In contrast, as shown in Table 6, the comparative examples (test numbers B1 to B47) had poor strength and / or toughness.

[0105] In Comparative Example B1, TS and YS deteriorate due to the low C content. In Comparative Example B2, toughness deteriorates due to the high C content. In Comparative Example B3, toughness deteriorates due to the high Si content. In Comparative Example B4, TS and YS deteriorate due to the low Mn content. In Comparative Example B5, toughness deteriorates due to the high Mn content. In Comparative Example B6, toughness deteriorates due to the high P content. In Comparative Example B7, toughness deteriorates due to the high S content. In Comparative Example B8, toughness deteriorates due to the high Al content. In Comparative Example B9, toughness deteriorates due to the low Nb content. In Comparative Example B10, toughness deteriorates due to the high Nb content. In Comparative Example B11, toughness deteriorates due to the low Ti content. In Comparative Example B12, toughness deteriorates due to the high Ti content. In Comparative Example B13, toughness deteriorates due to the high B content. In Comparative Example B14, toughness deteriorates due to the high N content. In Comparative Example B15, toughness deteriorates due to the high O content.

[0106] In Comparative Example B16, the Cu content is high; in Comparative Example B17, the Cr content is high; in Comparative Example B18, the V content is high; in Comparative Example B19, the Mg content is high; in Comparative Example B20, the Ca content is high; in Comparative Example B21, the REM content is high; in Comparative Example B22, the Zr content is high; in Comparative Example B23, the Te content is high; in Comparative Example B24, the W content is high; and in Comparative Example B25, the Sn content is high. In Comparative Example B26, TS and YS are low because Ceq is low. In Comparative Example B27, TS and YS exceed the limits due to high Ceq, and toughness is low. In Comparative Example B28, B F ´ is low, which causes deterioration of TS and YS.

[0107] In Comparative Example B29, T rex Since the cumulative reduction rate is low and the finish rolling temperature is low, the B' fraction is low and the number density of B-containing precipitates is high, which deteriorates TS, YS and toughness. rex As a result of the long interpass time in the above finish rolling, the B' fraction increases and the BF grain size also increases, resulting in a deterioration in toughness. In Comparative Example B31, the heating temperature before hot rolling is low, resulting in a low B fraction and the presence of coarse Nb-containing precipitates, resulting in a deterioration in TS, YS, and toughness. In Comparative Example B32, the heating temperature before hot rolling is high, resulting in a coarse BF grain size and a deterioration in toughness. In Comparative Example B33, the T is low due to the low rough rolling temperature. rex The cumulative reduction rate is low above T rex Because the cumulative reduction rate is high at less than 1000 kJ / min, B-containing precipitates precipitate, and the hardenability-improving effect of B cannot be obtained. As a result, the B and B' fractions are low, and the number density of B-containing precipitates becomes excessive, resulting in deterioration of TS, YS, and toughness. In Comparative Example B34, the interpass time of rough rolling is long, and in Comparative Example B35, the start temperature of finish rolling is high, resulting in large BF grain sizes and deterioration of toughness.

[0108] In Comparative Example B36, the finish rolling start temperature was low, and in Comparative Example B37, T rex Due to the low cumulative reduction rate above, Trex The cumulative reduction rate above this level becomes low, B-containing precipitates precipitate, and the hardenability improving effect of B is not obtained. Therefore, the B fraction and B' fraction are low, and the number density of B-containing precipitates becomes excessive, resulting in deterioration of TS, YS, and toughness. rex Because the cumulative reduction rate is high at less than 1000 kJ / s, the number density of B-containing precipitates becomes excessive, resulting in deterioration of TS, YS, and toughness. rex In Comparative Example B40, the finish rolling end temperature is low because of the long interpass time of finish rolling in Comparative Example B41, and the time from the end of finish rolling to the start of accelerated cooling is long because of this, resulting in an excessive number density of B-containing precipitates, and TS, YS, and toughness are deteriorated. In Comparative Example B42, the cooling rate during accelerated cooling is low, and in Comparative Example B43, the water cooling end temperature during accelerated cooling is high, resulting in a low B fraction and deteriorated TS and YS. [Industrial Applicability]

[0109] According to the present invention, it is possible to obtain a steel plate having high strength and excellent toughness, and therefore the steel plate according to the present invention can be suitably used as a material for welded structures such as ships, high-rise buildings, other buildings, bridges, marine structures, LNG storage tanks and other large tanks, and line pipes.

Claims

1. The chemical composition of the steel plate is, in mass%, C: 0.040-0.180%, Si: 0.01 to 0.50%, Mn: 0.70-2.50%, P: 0.030% or less, S: 0.020% or less, Al: 0.001-0.100%, N: 0.0010-0.0080%, Nb: 0.003 to 0.050%, B: 0.0001 to 0.0050%, Ti: 0.003 to 0.050%, Mo: 0.010% or less, Ce: 0.010% or less, O: 0.0040% or less, The balance is Fe and impurities. Ceq defined by the following formula (i) is 0.25 to 0.60%, B calculated by the following formula (ii) Ｆ ´ is greater than 0%, In a cross section perpendicular to the rolling direction of the steel plate, when the thickness of the steel plate is t, the metal structure at a position of 1 / 4t from the surface of the steel plate is Contains 70% or more bainite by area%, and the average grain size of bainitic ferrite constituting the bainite is 10 μm or less, The proportion of bainitic ferrite having an intragranular misorientation of less than 5° in the bainitic ferrite is 1% or more and less than 30% by area, the average grain size of parent phase austenite obtained by inverse analysis from the crystal orientation of the matrix structure constituting the steel sheet is 80 μm or less, the number density of B-containing precipitates having a particle size of 1 μm or more present on the grain boundaries of the parent austenite phase is 30 precipitates / mm or less; steel plate. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15...(i) B Ｆ ´=B-(N-Ti×(14 / 47.867))×(10.811 / 14) ・・・(ii) In the above formula, the element symbols represent the content (mass%) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.

2. The chemical composition contains, in mass %, one or more elements selected from the following Group A, Group B, Group C, and Group D in place of a portion of the Fe: The steel sheet according to claim 1. [Group A] Cu: 1.50% or less, Ni: 2.50% or less, Cr: 1.00% or less, V: 0.15% or less, One or more selected from the group consisting of [Group B] Mg: 0.0100% or less, Ca: 0.0100% or less, REM: 0.0100% or less, One or more selected from the group consisting of [Group C] Zr: 0.0100% or less, Te: 0.0100% or less, One or two selected from the group consisting of [Group D] W: 1.00% or less, Sn: 0.50% or less, One or two selected from the group consisting of

3. The chemical composition contains one or more elements selected from Group A. The steel sheet according to claim 2.

4. The chemical composition contains one or more elements selected from Group B. The steel sheet according to claim 2.

5. The chemical composition contains one or more elements selected from Group C. The steel sheet according to claim 2.

6. The chemical composition contains one or more elements selected from the D group. The steel sheet according to claim 2.

7. The method for manufacturing a steel sheet according to any one of claims 1 to 6, A method for producing a steel plate, comprising sequentially subjecting a steel slab having the chemical composition according to any one of claims 1 to 6 to a heating step, a hot rolling step, an accelerated cooling step, and a tempering step, In the heating step, the steel slab is heated to a temperature of 950 to 1200°C, The hot rolling step includes rough rolling and finish rolling, The rough rolling is performed until the surface temperature of the steel billet reaches T ｒｅｘ Carry out at +30°C or higher, The cumulative reduction rate in the rough rolling is 10 to 75%, and the interpass time is 15 seconds or less, The finish rolling is performed such that the surface temperature of the steel billet is T ｒｅｘ ~T ｒｅｘ Rolling begins at a temperature in the range of +50°C, and T ｒｅｘ The cumulative rolling reduction in the above temperature range is more than 35% and less than 80%, T ｒｅｘ The cumulative rolling reduction is less than 55% and the interpass time is 15 seconds or less, The finish rolling is performed using Ar ３ It ends at temperatures above +30°C, The time from the completion of the finish rolling to the start of cooling in the accelerated cooling step is set to 100 seconds or less, In the accelerated cooling step, the average cooling rate from the start of cooling to the end of cooling is 5 to 50° C. / sec. ３ Water cooling from the cooling start temperature to a cooling stop temperature of 550°C or less, In the tempering step, heating is performed in a temperature range of 350 to 650 ° C. Steel plate manufacturing method. However, Ar ３ is calculated by the following formula (iii), and T ｒｅｘ is calculated using the following formula (iv). The element symbols in the following formulas represent the content (mass%) of each element contained in the steel sheet, and 0 is substituted if the element is not contained. 2) ３ $B ‐B-based"5 t5 B                                                                                        not  not notes not notes is no ises __×_______________________________________________________________・・・ (iii) T ｒｅｘ =-91900×[Nb*] ２ +9400×[Nb*]+770 ・・・(iv) However, when the amount of dissolved Nb (mass%) calculated by the following formula (v) is defined as sol. Nb, If Nb ≥ sol. Nb, then [Nb*] = sol. Nb If Nb<sol. Nb, then [Nb*] = Nb Let's say. sol.Nb=(10 （－６７７０／（Ｔ＋２７３）＋２．２６） ) / (C+12 / 14×N) ・・・(v) In the above formula, T represents the heating temperature (°C) of the steel slab in the heating step.

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