BR112012024297B1 - METHOD OF PRODUCTION OF A STEEL SHEET - Google Patents

Abstract

Steel sheet production method. a method of producing a steel plate that is applicable to a large structural steel subjected to high heat input welding, and which is excellent in the toughness of the base metal and the heat affected zone by suppressing the stiffening of the structure of a part of the base metal surface and the central portion of the plate thickness of the zone affected by the chlorine, the method including casting, in which cooling rate during cooling is 0.1 <198> c / s or more over a temperature range. 1100 <198> and 1300 <198> c in the central part of the plate thickness, a strand which is a steel that meets as a chemical composition a predetermined condition such as additions of c, mn, nb, ti, and including tin with a circle equivalent diameter of 0.02 to 0.05 µm of 5.0x10 ^ 4 ^ pieces / mm ^ 2 ^ or more at the central part of the sheet thickness; heat the strand in a predetermined condition; perform finishing lamination in which the cumulative reduction of lamination is 40% or more at an air temperature ~ 3 ~ up to 880 <198> c after roughing lamination; and performing subsequent accelerated cooling in which the average plate thickness cooling rate is 5 <198> c / s or more from air temperature ~ 3 ~ or more to a temperature of 550 <198> c or less.

Description

Descriptive Report of the Invention Patent for METHOD OF PRODUCTION OF A STEEL SHEET.

Technical Field [001] The present invention relates to a method of producing a steel sheet that is excellent in toughness of the base metal and the zone affected by heat (ZTA). The steel sheet produced by the present invention is suitable for welded structures such as ships, buildings, bridges, tanks and offshore structures. In addition, the steel sheet produced by the present invention is not limited to a thick steel sheet, but applies to manufactured products such as steel tubes, columns, etc., so that these are also subjects of the invention.

Fundamentals of the Technique [002] In recent years, in addition to the increase in the size of steel structures and the high reinforcement and high thickness of the steel sheet used, toughness requirements are becoming more stringent in terms of ensuring safety. Since these structures are produced by welding, the toughness of a welded hair is also necessary at the same time. In particular, in the recent past, ensuring the toughness of the ZTA becomes increasingly difficult, because welding with high heat input, such as submerged arc welding, electrogas arc welding, is applied in many cases to improve the welding efficiency of the construction.

[003] Many methods of improving the toughness at ZTA in a welded joint of high heat input welding have been proposed so far. The methods can be divided basically into the following two types by the technical idea. One is a technique to prevent the hardening of the austenite grain (γ) by using a fixing effect that uses particles on a steel. Another is an effective grain refining technique using an intragranular transformation

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2/32 from γ to a ferrite (a).

[004] As a technique related to the first method (the fixation effect), it is mentioned the investigation of a suppression effect of γ grain growth by various types of nitrides, carbides, oxides, sulfides, etc. For example, in steel containing Ti, fine particles of TiN form in steel and are capable of effectively suppressing the growth of γ grains in the ZTA at the welded joint of high heat input welding (for example, Non-Patent Document 1) .

[005] Regarding oxides and sulfides, the technique of improving the toughness in the ZTA for steel is proposed, which includes 0.04 to 0.10% of Al, 0.002 to 0.02% of Ti, and 0.003 to 0 , 05% of rare earth metals (REM) (for example, Patent document 1). This is the method that uses oxides and sulfides to suppress the hardening of the grains in the ZTA during welding with high heat input.

[006] On the other hand, as a technique concerning the Second method (intragranular transformation), steel that includes at least one between Ti oxides and Ti oxide compounds with a particle diameter of 0.1 to 3.0 pm and with a number of particles from 5x10 ³ to 1x10 ⁷ pieces / mm ³ is proposed (for example, Patent document 2). This is the technique that uses Ti oxide particles and Ti oxide compounds and Ti nitrides as nuclei for the intragranular transformation from γ to ferrite (a) in ZTA, so that the toughness is improved by refining the structure of the ZTA.

[007] Furthermore, since BN also acts as a transformation core for α, steel that includes 0.005 to 0.08% Al, 0.0003 to 0.0050% B, and at least one element between Ti , Ca, and REM of 0.03% or less is proposed (for example Patent document 3). This is the technique that forms BN by using oxides and sulfides from an undissolved RFEM and / or Ca, or TiN as a nucleation point in the ZTA during cooling, so that the toughness in the ZTA is

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3/32 improved by the formation of α from BN.

[008] Although the aforementioned technique improves the toughness in the ZTA in the welded joints of high heat input welding, it is also proposed the technique that improves the toughness of the base metal by using inclusions (for example, document Patent 4). This is the technique that conducts a lamination in a predetermined condition by using a shaft that includes oxide particles with a predetermined size and number, so that the tenacity of the base metal is improved by the effective refining of the grain size y.

List of Citations Patent Document

[Patent Document 1] Japanese Unexamined Patent Application, First Publication n ° S60-184663 [Patent Document 2] Japanese Unexamined Patent Application, First Publication n ° S60-245768 [Patent Document 3] Japanese Unexamined Patent Application, First Publication n ° S61-253344 [Patent Document 4] Japanese Unexamined Patent

Application, First Publication n ° 2002-309315

Non-Patent Document [Non-Patent Document 1] Kanazawa, Nakajima, Okamoto, and Kaneya, Improved Toughness of Weld Fusion Zone by Fine TiN Particles and Development of a Steel for Large Heat Input Welding, Tetsu-to-Hagane, Vol.61 (1975), p.2589

Summary of the Invention

Technical Problem [009] The technique described in Non-Patent Document 1 is just the general technique that uses TiN particles, and there is no detailed description about the control of the composition, the particle diameter, and the distribution. So it is difficult for an excellent tenacity at ZTA in

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4/32 case of welding with high heat input, as the target of the present invention, is obtained only by technique.

[0010] The method described in the document Patent 1 is the use of sulfides and oxides of REM. However, in general, it is difficult to disperse sulfides and oxides finely. There is a limit to obtaining small grain size γ in the ZTA at the welded joint with high heat input. If crude sulfides and oxides exist in steel, toughness can deteriorate.

[0011] The technique described in the document Patent 2 is the use of Ti oxides and particles composed of Ti oxides and Ti nitrides as a core for the intragranular transformation from γ to ferrite (a). However, there is a limitation in obtaining the refining of the structure of the ZTA only by intragranular transformation when γ grains become stale by welding with high heat input. In addition, since the size of the particles that are effective as the α transformation core is relatively large, the toughness can deteriorate.

[0012] The technique described in the document Patent 3 is the use of BN which is formed in the oxides and sulfides of REM, those of Ca, or TiN as a transformation nucleus a. However, the effect is limited when γ grains are crude, and oxides and sulfides can become the source of fractures.

[0013] The technique described in the document Patent 4 is the use of the oxide particles that is generally used to improve the toughness of the ZTA, to effectively control the toughness of the base metal. However, when crude oxides exist, toughness can deteriorate.

[0014] The present invention was made in consideration of the problems mentioned above, and an objective of the present invention is to provide a method of producing the steel sheet that is applicable to a large structural steel and is excellent in toughness in the base metal and

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5/32 in the heat affected area.

Solution to the problem [0015] In consideration of the nitrides with Ti included that are formed in the shaft and in the condition of heating of a hot lamination, the present Invention provides the method to control the nitrides with Ti included which contributes to the refining of the base metal and the suppression of grain size bruising in the ZTA, and provides the method of producing the steel sheet which is excellent in toughness of the base metal and the zone affected by heat. For example, steel sheet has an elastic limit of 315 MPa to 580 MPa and a tensile strength of 440 MPa to 720 MPa. The yield strength can be 500 MPa or less, and the tensile strength can be 490 MPa to 620 MPa. For example, the thickness of the sheet is 10 to 100 mm, and its lower limit can be 12 mm, 20 mm or more preferably 30 mm. The upper limit of the plate thickness can be 70 mm or 50 mm. For example, the target of the toughness of the base metal is to obtain -50Ό or less of vTrs or a higher value of absorbed energy Charpy at 40 * C such as 31 J or more, 47 J or more, or 100 J or more. For example, the target of the toughness of the heat affected zone is to obtain 40 * C or less of vTrs or high value of energy absorbed Charpy at 21 * C as 27 J or more, 34 J or more, or 70 J or more in the heat-affected zone of the welded joint where the heat input from the weld is 200 kJ / cm or more. One aspect of the present invention employs the following:

(1) A method of producing a steel sheet, the method including:

[0016] casting, in which the cooling rate during cooling is 0.10 / s or more in a temperature range of 1100 to 1300Ό in the central part of a shaft thickness, a pass which is a steel including as chemical composition, in mass%,

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C: 0.03 to 0.16%,

Si: 0.03 to 0.5%,

Mn: 0.3 to 2.0%,

Nb: 0.005 to 0.030%,

Ti: 0.003 to 0.050%,

Al: 0.002 to 0.10%,

N: 0.0020 to 0.0100%,

P: limited to 0.020% or less,

S: limited to 0.010% or less, and the balance consisting of iron and the unavoidable impurities and satisfying equations (1) and (2) below, where the shaft includes nitrides with Ti included with an equivalent circle diameter of 0, 02 to 0.05 pm of 7.0x10 ⁴ or more per mm ² on a part of the surface and nitrides with Ti included with the equivalent circle diameter of 0.05 to 0.2 pm of 5.0x10 ⁴ or more per mm ² in the central part.

heating the shaft in a condition that satisfies equations (3) and (4) below;

perform roughing lamination in which the cumulative lamination reduction is 30% or more at a temperature of 900Ό or more;

perform the finishing lamination in which the cumulative lamination reduction is 40% or more at a temperature of the Ar ₃ point up to 880Ό; and then perform accelerated cooling in which an average cooling rate of the sheet thickness is 5 ° C / s or more from an Ar ₃ point temperature or more to a temperature of 550Ό or less.

0.32 <[C] + 0.15 [Mn] + 3.8 [Nb] <0.39 - (1)

1.5 <[Ti] / [N] <3.0 - (2)

56000 / (1,2 - 0,18 x log [Nb]) <(T + 273) x {log (t) + 25} <

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91000 / (1,9 - 0,18 χ log [Ti]) - (3) t> 30 - (4) where [X] is the addition of element X in units of mass%, T is the reheat temperature in units of “C, et is the retention time in units of minutes.

(2) The method of producing a steel sheet according to item (1), the method also including:

[0017] Heat treatment at a temperature of 650Ό or less after accelerated cooling.

(3) The method of producing a steel sheet according to item (1) or (2), steel also including, in mass%, at least one element between:

Cu: 1.5% or less,

Cr: 0.5% or less,

Mo: 0.5% or less,

Ni: 2.0% or less,

V: 0.10% or less,

B: 0.0030% or less,

Mg: 0.0050% or less,

Ca: 0.0030% or less, and

REM: 0.010% or less.

Advantageous effects of the invention [0018] According to the present invention, it is possible to provide, by an effective production method, a steel sheet with a tensile strength of 440 MPa or more, a sheet thickness of 10 mm or more, and especially 30 mm or more, which is excellent in toughness with base metal and in toughness from ZTA even if a high heat input welding of 200 to 500 kJ / cm is conducted. Consequently, the present invention has significant industrial applicability.

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Brief description of the drawings [0019] Fig. 1 is a graph that indicates the relationship between the TiN density of 0.02 to 0.05 pm on a part of the surface and the transition temperature of the appearance of fracture of the base metal.

[0020] Fig. 2 is a graph that indicates the relationship between the TiN density of 0.05 to 0.2 pm in a central part and the transition temperature of fracture appearance of an ATZ.

[0021] Fig. 3 is a graph that indicates the relationship between the cooling rate of the casting and the density of TiN.

[0022] Fig. 4 is a graph indicating the heating condition range of the present invention in the case of a steel that includes Nb: 0.02% Ti: 0.01%.

Description of configurations [0023] The toughness of the ZTA deteriorates due to the conduction of high heat input welding, because the γ grain size becomes stubborn in a heat-affected zone (ZTA) at a temperature of 1400Ό or more and a weakening phase that has martensite - austenite as constituents (MA) it forms during cooling in the case of components with high hardening capacity. Therefore, to improve the toughness in the ZTA, it is necessary to fundamentally combine the control of the chemical composition and inclusions for refining the structure of the ZTA.

[0024] Especially in the component with high hardening capacity, the guarantee of toughness becomes much more difficult, because connecting elements, such as Mn, are concentrated in a central part of the plate thickness and the formation of M-A is promoted. In addition, in a total thickness destruction test, once the stress state of the central part of the sheet thickness becomes the most severe, it is necessary to refine the structure of the central part of the sheet thickness as much as possible by using the control of

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9/32 microstructure.

[0025] On the other hand, to improve the tenacity of the base metal, a production method is also important in addition to controlling the chemical composition and inclusions. For example, TMCP (Thermomechanical Control Process) is applied. In the production process of the base metal, since the highest temperature of a heating process is approximately 1250Ό, it is possible to easily refine the structure of the central part of the sheet thickness compared to the ZTA by performing the typical TMCP such as lamination controlled (CR) and accelerated cooling (ACC).

[0026] However, the temperature in a part of the surface of a steel plate easily increases locally in the heating process, and raw γ grains formed in that part remain after recrystallization by a process of rough lamination so that the final structure becomes uneven and the toughness can deteriorate. Therefore, in order to guarantee the toughness of the steel sheet, it is necessary to suppress the formation of crude γ grains on the surface part. Then, the inventors considered whether nitride particles with Ti included, which is used to guarantee the toughness of the ZTA of a welded joint from high heat input welding, can also be used for the refinement of the base metal part or not , and carried out several investigations. In addition, although nitride particles with Ti included include compounds with oxides or sulfides, hereinafter nitride particles with Ti included are referred to as TiN.

[0027] Although the stability of TiN in a high temperature range is less than that of oxides, TiN can be finely dispersed in a steel with comparative ease. In addition, since the size distribution changes with the thermal history, as described below, it was found that the heating process is very important.

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10/32 between a casting process that produces a shaft and processes that produce the steel sheet.

[0028] Initially, in the process of heating the shaft to conduct a hot rolling, the distribution of TiN, which is necessary to suppress the formation of crude γ grains and ensure toughness, was investigated. Then, in a welding process, the distribution of TiN, which is necessary to suppress the growth of γ grains in the ZTA in the central part of the plate thickness and guarantee toughness, was investigated. Tenacity was assessed as the transition temperature of fracture appearance (vTrs) by conducting a Charpy impact test. In addition, based on these results, the distribution of TiN, which is necessary for the surface part and the central part of the plate thickness of the shaft, was investigated in detail. In addition, the size and density of TiN were measured using an electronically transmitted microscope, after preparing replicas of the extraction from samples that are taken from the shaft, the steel plate and the ZTA.

[0029] Based on these research results, the relationship between the size and density of the TiN on the surface part of the shaft and the toughness of the surface part of the base metal and the relationship between the size and density of the TiN in the central part the plate thickness of the shaft and the toughness of the ZTA in the central part of the plate thickness were evaluated. The results are shown in Fig. 1 and Fig. 2.

[0030] As shown in Fig. 1, when the TiN with an equivalent circle diameter of 0.02 to 0.05 pm is 7.0x10 ⁴ pieces or more per mm ² on the surface part of the shaft (for example, a position 1/20 of the shaft thickness from the shaft surface), the toughness of the shaft surface part improves. The reasons why the density of TiN with the equivalent circle diameter of 0.02 to 0.05 pm on the shaft surface is observed are as follows. When TiN is lower

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11/32 than 0.02 pm, TiN is dissolved as a solid solution during heating for hot rolling. When TiN is greater than 0.05 pm, the grain growth suppression effect becomes insufficient because the fixation effect is ineffective. Although it is not necessary to determine the upper limit of the TiN density of the particular part of the shaft surface, when excessively large, there is a possibility that surface defects may occur on the surface part. Therefore, it is preferable that the TiN from 0.02 to 0.05 pm is 4.0 x 10 ⁵ pieces or less per mm ² on the surface part.

[0031] Also, as shown in Fig. 2, when the TiN with an equivalent circle diameter of 0.05 to 0.2 pm is 5.0x10 ⁴ pieces or more per mm ² in the central part of the shaft thickness, the ZTA toughness of the central part of the plate thickness improves. The reasons why the density of TiN with the equivalent circle diameter of 0.05 to 0.2 pm in the central part of the thickness of the shaft plate is observed is as follows. When TiN is less than 0.05 pm, TiN is dissolved as a solid solution by a thermal influence during welding and the grain growth suppression effect becomes insufficient. When TiN is greater than 0.2 pm, the tenacity of the base metal deteriorates. Although it is also not necessary to determine an upper limit of the density of the TiN of the central part of the particular shaft, when very large, there is a possibility that the tenacity of the central part of the plate thickness of the base metal will deteriorate. Therefore, it is preferable that TiN from 0.05 to 0.2 pm is 3.0x10 ⁵ pieces or less per mm ² m at the central part of the plate thickness.

[0032] To obtain the distribution of TiN, it is necessary to control the cooling rate of the casting process. The inventors also investigated, and found that it was necessary that the ratio of the Ti content to the N (Ti / N) content, in mass%, is 1.5 to 3.0 and the cooling rate in the central part of the came in a tempera range

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12/32 tures from 1100 to 1300Ό that is equivalent to a peak of precipitation is 0.1 Ό / s or more. As shown in Fig. 3, when the cooling rate in the central part of the shaft is 0.1 “C / s or more, it is possible that the TiN with the equivalent circle diameter from 0.02 to 0.05 pm is 7 , 0x10 ⁴ pieces / mm ² or more on the surface part of the shaft and the TiN with the equivalent circle diameter from 0.05 to 0.2 pm is 5.0x10 ⁴ pieces / mm ² or more in the central part of the thickness of shaft plate.

[0033] However, the TiN distribution may not necessarily be uniform in the width direction or in the longitudinal direction of the plate (shaft), and can be irregular with density measurement methods. Thus, when the cooling rate is 0.1 “C / s or less, the density of TiN or the part of the surface or the central part of the plate's thickness can satisfy the evaluation criteria. Concrete methods that increase the cooling rate are as follows, increase the pressure and the dosage of water in the cooling zone of a continuous casting machine, reduce the thickness of the mold, reduce the thickness of the plate by laminating a non-solidification layer of the came, etc. Although it is not necessary to determine an upper limit on the cooling rate in particular, it is difficult to obtain a cooling rate greater than ΙΌ / s due to the plate thickness (shaft thickness) and equipment restrictions. Although the thickness of the slab for casting is not particularly designated, most thicknesses of steel slab casting are in the range of 150 mm to 400 mm.

[0034] It is necessary to control the heating temperature and the retention time of the plate, when the hot lamination is conducted by using the shaft with the TiN distribution. For this reason, it is necessary to sufficiently dissolve the Nb as a solid solution which contributes to a greater reinforcement by a function of increasing the non-recrystallization temperature and controlling the heating temperature and the

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13/32 retention time in which TiN on the surface part of the shaft can be obtained. The inventors performed several experiments and thermodynamic calculations concerning the precipitation behavior of Nb and Ti, and derived the following equations (3) and (4) from the results.

56000 / (1.2 - 0.18 x log [Nb]) <P _H 91000 / (1.9 - 0.18 x log [Ti]) - (3) where P _H = (T + 273) χ ( log (t) + 25}.

t> 30 - (4) where [X] is the addition of element X in units of mass%, T is the heating temperature in units of “C, and t is the retention time in units of minute.

[0035] The functional type of P _H refers to a tempering parameter that is used for converting a temperature and a tempering time. The left side of the inequality is a lower limit of a heating condition that changes according to the Nb content, and the right side of the inequality is an upper limit of the heating condition that changes according to the Ti content. Each coefficient was obtained experimentally at from the limiting conditions that are to form crude γ and that are to guarantee the amount of solid Nb solute. The reason why the retention time has been adjusted to 30 minutes or more is to dissolve evenly as a solid solution of binding microelements such as Nb. Here, the time since reaching the temperature that is 20Ό lower than the predicted temperature of the oven for extraction is defined as the holding time, and the average temperature during that time is defined as the heating temperature. If the plate is excessively heated to a high temperature, a very thick scale can form and surface defects can occur on the steel plate. Thus, the heating temperature of the plate can be limited to 1300Ό or less, 1250Ό or less, 1200Ό or less, or 1180X3 or less. Embo ra not be

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14/32 it is necessary to determine an upper limit of the retention time in particular, to avoid the decrease in productivity for the long retention time, the upper limit of the retention time can be 500 minutes, 400 minutes and 300 minutes.

[0036] Fig. 4 shows that a state of Nb as a solid solution, the state of γ hardening, and the thermal condition range in the Nb vessel of 0.02% and Ti of 0.01%. When the plate is satisfactorily heated in the condition range, it is possible to suppress the γ grain stiffness and use maximally solid Nb solute, so that the tenacity of the base metal can be improved without an increase in the production load of subsequent processes such as hot rolling and accelerated cooling.

[0037] The reason for the limitations of processing conditions after heating is explained below. In the present invention, hot rolling performed at 900Ό or more at Wed I recrystallization proceeds easily is defined as crude rolling, and hot rolling performed at 880Ό or less in which recrystallization is suppressed and refining of the structure is promoted. defined as finishing lamination. Thus, rough rolling does not have to be carried out by a roughing mill, and finishing rolling does not have to be carried out by a finishing laminator. For example, all rough and finishing laminations can be performed by a finishing laminator. Hot rolling performed greater than 880Ό and less than 900Ό can be performed, but the influence on the structure or mechanical properties is not noticeable.

[0038] Crude lamination is conducted, in which the cumulative reduction of the lamination is 30% or more at a temperature of 900Ό or more. When the temperature is less than 900Ό or the cumulative lamination reduction is less than 30%, γ recrystallization may not proceed

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15/32 enough, the structure can become duplex grains, and the material properties can become uneven. It is not necessary to determine an upper limit of the rough rolling temperature, and the upper limit is properly determined according to the heating temperature of the shaft and the starting temperature of the finishing lamination. In addition, it is not necessary to determine an upper limit of the cumulative rolling reduction, and the upper limit is properly determined according to the thickness of the shaft, the thickness of the sheet, and the cumulative reduction of the finishing lamination.

[0039] The finishing lamination is conducted, in which the cumulative reduction of lamination is 40% or more at an Air temperature ₃ to 880Ό. When the temperature is lower than Ar ₃ , a deformed ferrite can form and the toughness can deteriorate. When the temperature is greater than 880Ό or the cumulative reduction in lamination is less than 40%, since it is difficult to increase the displacement density, the structure cannot be refined sufficiently and the toughness can deteriorate.

[0040] After finishing the finishing lamination, accelerated cooling is conducted in which an average cooling rate of the sheet thickness is õO / s from the temperature of Ar ₃ or more to a temperature of 550Ό or less. When the cooling rate is less than õO / s or the finishing temperature of the finishing lamina is greater than 550Ό, the strength may be insufficient, the structure may not be sufficiently refined, and the toughness of the base material may deteriorate. In the steel sheet of the present invention, it is not necessary to determine an upper limit on the cooling rate, because the low temperature transformed structure that deteriorates toughness is not formed even if the cooling rate of the accelerated cooling increases. However, it is difficult for the cooling rate

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16/32 to be greater than 10000 / s, due to limitations depending on the thickness of the plate and the capacity of the equipment. The upper limit of the cooling rate can be limited to ϊδΌ / β, õOO / s and õOO / s.

[0041] Furthermore, it is also not necessary to determine the lower limit of the cooling stop temperature in the present invention, and the lower limit is suitably determined according to the required properties of the steel sheet. To suppress grain and precipitate growth and improve productivity, it is preferable that the accelerated cooling end temperature should be 550Ό or less. Since the time for accelerated cooling can become long and productivity can deteriorate when accelerated cooling is stopped at less than 300Ό, it is preferable that the cooling stop temperature can be 200Ό or more. To improve resistance, the lower limit of the cooling stop temperature can be 300 ° C, 400Ό or 450Ό.

[0042] After accelerated cooling, a heat treatment (tempering) can be conducted at a temperature of 650Ό or less, to control strength and toughness. When the temperature is higher than 650Ό, cementite and grain can grind, a brittle fracture can be promoted, and the toughness of the base metal can deteriorate. To improve the toughness of the steel plate, it is preferable that the heat treatment temperature can be 400Ό or more. The heat treatment temperature can be 490Ό or more, to sufficiently improve toughness.

[0043] The steel sheet produced under predetermined conditions by using the shaft with the aforementioned TiN distribution is excellent in the toughness of the surface part. In the steel plate, even when welding with high heat input is performed, the TiN in the ZTA in the central part of the plate thickness is not completely re

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17/32 solved, and the very TiN of 0.05 pm or less that can suppress the growth of γ grain remains effectively. Therefore, the hardening of the structure in the ZTA in the central part of the plate thickness of the welded joint of the welding with high heat input can be suppressed to some extent. On the other hand, since the TiN on the surface part of the base metal is thin, the very TiN is dissolved as a solid solution by the thermal influence of welding with high heat input. However, due to the effect of precipitated compounds of TiN, MnS and the like that are re-precipitated during cooling, the intragranular transformation is accelerated. As a result, in the central part of the plate thickness, the stuttering of the structure of the ZTA is suppressed. The toughness in the ZTA of the welded joint of the welding with high heat input that is sufficient for the practice, is obtained, since the structure of the ZTA of the surface part is refined.

[0044] In the following, the reason for the limitations of the chemical composition in the present invention will be explained. Here the described% of the compositions means% by mass.

[0045] Carbon (C) is an essential element to improve strength, so that 0.03% or more of C is added. On the other hand, since the guarantee of the toughness of the ZTA is difficult when the addition is excessive, the upper limit of the C content is 0.16%. To improve strength, the lower limit of the C content can be 0.05%, 0.06%, or 0.07%. To improve toughness in the ZTA, the upper limit of C can be 0.14%, 0.13%, or 0.12%.

[0046] Silicon (Si) is a cheap and deoxidizing element, and also contributes to the strengthening of the solute, so that 0.03% or more of Si is added. On the other hand, since the weldability and toughness of ZTA deteriorate when the Si content exceeds 0.5%, the upper limit is 0.5%. To deoxidize with certainty, the lower limit of Si can be 0.05%, 0.08% or 0.12%. To improve the cover

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18/32 welding strength and tenacity in HAQZ, the upper limit of Si can be 0.40%, 0.35% or 0.30%.

[0047] Manganese (Mn) is an element that is effective in improving the strength and toughness of the base metal, so that 0.3% or more of Mn is added. To improve the welding capacity, the lower limit of Mn can be 0.5% or 0.7%. Most preferably, the addition can be 0.9% or more or 1.0% or more. On the other hand, since the toughness in the ZTA and the fracture in the weld deteriorate when the addition is excessive, the upper limit is 2.0%. Preferably, the Mn content can be 1.8% or less. Most preferably, the Mn content can be 1.6% or less.

[0048] Phosphorus (P) and sulfur (S) are unavoidable impurities. To improve the toughness of the base metal and the ZTA, the upper limit of P is 0.020%, and the upper limit of S is 0.010%. To also improve toughness, the upper limit of P can be 0.017% or 0.015%, and the upper limit of S can be 0.008%, 0.006%, or 0.004%. While it is preferable that the P content and the S content are as low as possible, the lower limit of P can be 0.001% and the lower limit of S content can be 0.0001%, because significant costs are needed to reduce the contents of P and S industrially.

[0049] Niobium (Nb) is an element that contributes to the refining of the structure, the reinforcement of transformation, and the reinforcement of precipitation even if a small amount of Nb is added. In the present invention, to obtain the strength of the base metal, 0.005% Nb or more is added. To also improve strength, etc., the Nb content can be 0.008% or more or 0.010% or more. On the other hand, since the ZTA hardens and the toughness deteriorates when the addition of Nb is excessive, the upper limit of Nb is 0.030% or less. Preferably, the upper limit of the Nb content can be 0.020%.

[0050] Aluminum (Al) is an important deoxidizing element, of

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19/32 so that 0.002% Al or more is added. To deoxidize with certainty, the addition can be 0.008% or more or 0.012% or more. However, since inclusions, which impair surface quality and are detrimental to toughness, are formed when Al is excessively added, the upper limit of Al is 0.10%. Preferably, the upper limit of Al is 0.10%. Preferably, the upper limit of the Al content can be 0.07% or 0.05%.

[0051] Titanium (Ti) is a very important element for the present invention. Ti is effective in improving structure refinement and reinforcing precipitation by a small amount of addition and improving the strength and toughness of the base metal and the toughness of ZTA by the formation of fine TiN. Thus 0.003% of TI or more is added. Preferably the lower limit of Ti can be 0.005% or more, and more preferably the addition of Ti can be 0.008% or more. On the other hand, since the toughness of the ZTA deteriorates noticeably when the addition of Ti is excessive, the upper limit of Ti is 0.050%. Preferably, the upper limit of Ti can be 0.040%. The upper limit can be 0.030%, 0.025% or 0.020%.

[0052] Nitrogen (N) improves the toughness in the ZTA by the formation of nitride with Ti, so that 0.0020% of N or more are added. Preferably, the lower limit of the N content can be 0.0030% or more, and more preferably it can be 0.0035% or more. On the other hand, since embrittlement occurs by solid N solute when the addition of N is excessive, the addition is limited to 0.0100% or less. To avoid embrittlement, the addition can be 0.0080% or less or 0.0060% or less.

[0053] C, Mn, and Nb are the elements that contribute to the hardening capacity, and the additions must satisfy equation (1) below to guarantee the resistance of the base metal and the toughness of the ZTA.

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0.32 <[C] + 0.15 [Mn] + 3.8 [Nb] <0.39 - (1) [0054] [C], [Mn], and [Nb] in the above equation are the addition of each element (mass%), and the coefficient was obtained experimentally from the contribution to the hardening capacity. When [C] + 0.15 [Mn] + 3.8 [Nb] is less than 0.32, the resistance becomes insufficient. On the other hand, since the suppression of the central segregation of Mn and Nb is especially difficult, the central segregation becomes pronounced when [C] + 0.15 [Mn] + 3.8 [Nb] exceeds 0.39, of so that the toughness in the ZTA of the welded joint with high heat input deteriorates. To improve the toughness of the ZTA, the upper limit can be 0.38 or 0.37. To improve resistance, the lower limit can be 0.33.

[0055] When TiN particles are used to guarantee the toughness of the ZTA, it is necessary to consider the balance of Ti and N in addition to the individual addition. That is, it is necessary to control the ratio of the addition of Ti and N in a range of 1.5 to 3.0 as shown in equation (2) below. When the Ti / N ratio is less than 1.5, the amount of solid N solute is excessive, so that the toughness of the ZTA deteriorates. On the other hand, when Ti / N exceeds 3.0, crude oxides or sulfides are formed by excess Ti or the resistance increases by the precipitation of TiC, so that the toughness of the ZTA deteriorates.

1.5 <[Ti] / [N] <3.0 - (2) [0056] [Ti] and [N] in the above equation are the addition of each element (mass%).

[0057] In addition, to improve strength and toughness, at least one element between Cu, Cr, Mo, Ni, V, B, Mg, Ca, and REM can be added. However, it is preferable to avoid adding these elements to reduce connection costs.

[0058] Copper (Cu), Chromium (Cr), and Molybdenum (Mo) are elements

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21/32 that improve the hardening capacity respectively. 0.05% or more of Cu, Cr, and Mo can be added to increase the strength of the base metal and suppress the softening of the ZTA. On the other hand, since the toughness of ZTA deteriorates when the addition is excessive, the upper limit of Cu can be 1.5% or less, the upper limits of Cr and Mo can be 0.5% or less. To prevent the tenacity in the ZTA from deteriorating, the upper limit of Cu can be limited to 0.5%, 0.35%, or 0.2%, the upper limit of Cr can be limited to 0.3%, 0, 2% or 0.1%, and the upper limit of MO can be limited to 0.2%, 0.1% and 0.08%.

[0059] Nickel (Ni) is effective in improving the resistance, the ability to retain, and the toughness of the ZTA, so that 0.05% or more of Ni can be added. On the other hand, since an increase in Ni content results in an increase in the connection cost, the upper limit may be 2.0%. To suppress the increase in connection costs, the upper Ni limit can be 0.8%, 0.6% or 0.4%.

[0060] Vanadium (V) contributes to the improvement of resistance by reinforcing precipitation, so that 0.005% or more of V can be added. On the other hand, since the toughness of ZTA deteriorates when the addition of V is excessive, it is preferable that the upper limit of the V content is 0.10% or less. Preferably, it can be 0.080% or less, and more preferably it can be 0.05% or less.

[0061] Boron (B) is an element that improves the hardening capacity, so that 0.0002% of B or more can be added to increase the strength of the steel. On the other hand, since the hardening capacity deteriorates when the addition of B is excessive, the upper limit of B can be 0.0030%. It can be 0.0020% or 0.0015%.

[0062] Magnesium (Mg), Calcium (Ca), and REM are elements that

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22/32 contribute to improving the toughness of ZTA by the formation of fine oxides or sulfides, so that 0.0003% or more of Mg can be added, 0.0005% or more of Ca can be added, and 0.0005 % or more of REM can be added. On the other hand, since the toughness deteriorates due to the hardening of inclusions when these elements are added excessively, it is preferable that the upper limit of Mg is 0.0050% or less, the upper limit of Ca is 0.0030% or less, and the upper limit of REM is 0.010% or less. Here, REM are rare earth metals such as La, Ce and the like.

Example [0063] Steel sheets with a sheet thickness from 30 mm to 70 mm were produced by production conditions as shown in Tables 2 and 3, by using plates (shafts) that have chemical compositions as shown in Table 1. Here , “Ceq” in Table 1 is a calculated value of [C] + 0.15 [Mn] + 3.8 [Nb]. In Table 2, the caster cooling rate is a cooling rate in the range 1100 to 1300X3, ML is a calculated value of 56000 / (1.2 0.18 x log [Nb]), P _H is a calculated value of (T + 273) x (log (t) + 25}, and MU is a calculated value of 91000 / (1.9 - 0.18 x log [Ti]). In Tables 2 and 3, Ar ₃ (X3 ) was calculated using the following equation.

Ar ₃ = 910 - 310C - 80Mn - 20Cu - 15Cr - 55Ni - 80Mo + 0.35 + 0.35 (t-8) where t is the plate thickness (mm).

[0064] TiN density in the shaft, strength and toughness of the base metal, and toughness of the ZTA are shown in Tables 4 and 5.

[0065] To measure the density of TiN, the extraction of replica samples from a part of the shaft surface, specifically at a position 1/20 of the shaft surface, and from a central part of the shaft thickness were prepared, the size and the TiN number were me

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23/32 taken by taking microphotographs of 50 to 100 arbitrary visual fields at a magnification of 20,000 to 50,000 times with an electronic transmission microscope (TEM), and the density of TiN was calculated.

[0066] To assess the strength of the base metal, No. 4 tensile test bodies regulated by JIS Z 2201 were sampled from a direction perpendicular to the rolling direction in the central part of the steel sheet thickness, tensile tests that were regulated by JIS 2241 were performed, and the yield strength (YP) and tensile strength (TS) were measured. In order to assess the toughness of the base metal, the test plugs with a 2 mm v-notch, which were regulated by JIOS 2242, were sampled from the rolling direction in a more upper part of the surface and in the central part of the steel plate thickness. . Charpy impact tests were performed at various temperatures, and the transition temperature of fracture appearance (vTrs) was calculated. In addition, the target for the toughness of the base metal (the center part and the surface part) was -50Ό or less of vTrs.

[0067] To assess the toughness in the ZTA by vTrs, an arc gas welding (EGW) was performed in a heat input condition of 200 to 450 kJ / cm, the notched Charpy test pieces were sampled from the ZTA which was at a distance of 1 mm from the cast line of the central part of the plate thickness, and the tests were performed. In addition, the target for the toughness of the ZTA (the central part) was -40Ό or less of the vTrs.

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24/32 ro φ η φ

Ti / N 2.89 1.67 2.91 1.63 2.90 2.63 2.97 2.44 2.55 2.22 2.41 3.18 1.47 2.63 Ceq ' 0.368 0.336 0.387 0.371 0.320 0.326 0.321 0.334 0.342 0.399 0.305 0.333 0.324 0.372 Chemical composition (% by mass) | REM 0.0041 Here 0.0016 0.0012 O) 0.0023 0.0034 0.0011 0.0024 > 0.045 0.066 Mo 0.05 0.45 O 0.26 0.39 z 0.65 1.96 0.75 Ο, ³⁴ O 0.25 1.39 0.30 z 0.0038 0.0048 0.0055 0.0049 0.0062 0.0038 0.0091 0.0041 0.0055 0.0036 0.0029 0.0044 0.0068 0.0076 < 0.027 0.025 0.034 0.022 0.003 0.045 0.039 0.028 0.019 0.028 0.018 0.024 0.037 0.072 P 0.011 0.008 0.016 0.008 0.018 0.010 0.027 0.010 0.014 0.008 0.007 0.014 0.010 0.020 Nb 0.012 0.007 0.013 0.027 0.014 0.009 0.019 0.011 0.014 0.008 0.013 0.016 0.021 0.015 ω 0.003 0.003 0.004 0.002 0.001 0.002 0.002 0.004 0.003 0.002 0.003 0.005 0.006 0.009 0- 0.013 0.006 0.010 0.008 0.005 0.009 0.004 0.006 0.010 0.008 0.003 0.013 0.009 0.010 Μη 1.46 1.35 1.28 1.56 1.45 1.06 1.40 1.62 Ο 1.43 0.87 0.98 ώ 0.22 Ο, ¹⁵ 0.27 0.30 0.45 0.18 0.36 0.05 0.14 0.23 Ο, ³⁴ 0.08 Ο, ²⁷ 0.45 ο 0.151 0.090 0.135 0.076 0.033 0.074 0.090 0.082 0.032 0.126 0.091 0.058 0.114 0.168 Steel < Ο Q LL LL · <5 τ - "3 k: _ι ζ

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25/32

CXI ro φ n

Rough rolling  ά ω tb o 3 c £ ® '8. E S E * 3 to S -3, With POO 00 The m m CO 54 | CO The m hm 45 | 45 | CO m CO ⁴⁵ I O Temp. in End (Ό) 1062 1004 997 966 1040 1061 1059 00 00 σ> 1016 997 1075 1060 967 1006 1038 Temp. start (C) 1089 1060 1026 1096 1062 1105 1081 1020 1084 1034 1128 1099 1001 1043 O 00 o

l

Al

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Note Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Heat treatment CO temperature 675 525 Accelerated cooling | 2 | g 467 510 436 532 397 485 509 278 1 125 276 357 Temp. beginning CC) 801 807 732 804 797 862 810 801 1 CD O 00 760 766 * ® o “ii ^-0 8 E ~ ί- φ · - ' CO LO - CXI O) O O) O 00 O) Finishing lamination g Έ §, Ϊ § íc 50 46 50 44 50 42 47 52 53 45 37 55 <p 783 773 775 797 745 676 769 772 794 791 712 709 Temp. terminating CC) 830 838 751 825 00 o 00 882 827 819 813 820 774 779 Temp. beginning CC) 845 865 770 850 825 905 858 842 830 835 799 810 Rough rolling Cumulative lamination reduction (%) 56 63 50 54 50 57 50 46 35 45 53 27 c Φ i ° - | It's 1 p 1060 1103 965 1000 066 1016 1071 1046 696 1007 1035 1017 Has p. from start CC) 1090 1093 1027 1021 1085 1024 1094 1005 1045 1074 1043 Heating 4.04 4.00 4.09 4.00 4.03 4.17 4.17 4.03 4.03 4.07 4.07 3.82 4.04 3.87 3.67 3.73 3.75 3.91 3.81 3.63 3.71 3.81 3.70 3.62 3.53 3.64 3.78 3.65 3.57 3.71 3.71 3.61 3.61 3.65 3.65 Retention time (min) 125 70 95 104 230 O 00 95 24 175 128 152 63 CC temperature) 1135 1233 1160 1087 1090 1122 1176 1170 1060 1096 1130 1108 Casting £ φξ ® o? 0.09 0.28 0.13 0.24 0.07 0.22 0.25 00 θ ' θ ' 0.08 0.14 0.21 ώ «£ <o <o cq E ^ω ®« ⁷³ o 40 30 50 45 50 45 40 50 70 09 09 50 <i “<o ά <o E HJ ^ ^O ã ° E 180 150 200 175 200 180 150 195 230 200 200 150 Steel < m O Q LU LL 0 0 I I - - The Z CD r- CO O) 20 c \ i 22 23 24 25 26 27

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Note Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Trat. Thermal έ E E _ £ £ 3 g 487 Accelerated cooling F ® d. ° 1 g 384 423 510 525 541 d. O :! g 807 810 σ> o 00 801 799 if-S 8 E ° S O) CO HI HI CO Finishing lamination ε | $ E '8. ^ ® t> g φ ^ φ φ ^ 50 44 53 50 54 <g 756 00 o 00 783 792 789 Temp. CO termination 819 818 836 830 814 Temp. beginning CC) 846 835 867 865 861 Rough rolling Cumulative rolling reduction (%) 50 50 43 47 49 F ® d. ° 1 ’2 f p 994 950 976 1012 1047 Has p. from start CC) 1025 998 1024 1072 1105 Heating 4.00 3.98 4.07 4.03 4.13 3.65 3.64 3.72 3.87 3.93 3.55 3.64 3.68 3.73 3.66 Retention time (min) CO 274 2nd 167 86 CC temperature) 1075 1055 1093 1147 1185 Casting § ω 1 = ω o 0.15 0.19 0.20 0.26 0.22 i ffl i ώ <g £ φ φ φ E ^ω ® « ⁷³ o 50 50 40 40 35 ώ 2 φ φ φ E 200 180 150 150 150 Steel "3 * -1 z The Z 28 29 30 dog 32

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Note Example Example Example Example Example Example Example Example Example Example Example Example Example Example Example Tenacity of ZTA vTrs (the center, the middle part, the nucleus) CO LO 1 -54 -43 -42 -43 -46 1 -44 -47 -46 09- -44 O 1 -45 -42 Welding heat input (kJ / cm) 248 203 308 316 380 297 368 310 286 344 257 305 451 372 363 Base metal toughness vTrs (the center, the middle part, the nucleus) CC) -55 -58 -56 -54 -57 -60 -53 s 1 -63 -60 -55 -55 -52 -58 -53 vTrs (part of the surface) CC) -66 -65 s 1 -59 -62 -66 s 1 -67 -70 -65 -58 -60 -64 -65 -60 base metal TS (central part) (MPa) 516 547 493 542 534 647 618 521 099 625 631 566 564 537 649 Resistance YP (central part) (MPa) 368 388 342 401 394 477 495 373 487 448 490 454 410 412 481 TiN Number Density of Shaft 0.05 to 0.2 pm (central part) (x10 ⁴ / mm ³ ) CO 9.4 5.2 5.5 6.4 7.0 LD CO LD 6.9 6.0 8.5 CO LD 5.2 CD 6.0 0.02 to 0.05 pm (part of the surface) (x10 ⁴ / mm ² ) 6’8 CO θ ' CO l < LO l < O CO CO CO (NI l < LO l < 00 * CD l < (NI O) rl < l < LO l < l < Plate thickness (mm) 40 30 50 50 09 45 09 50 45 55 40 50 70 09 09 < m m O O Q Q LU LL LL 0 0 I I - O Z - (NI CO LO CD r- CO O) O - (NI CO LO

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ΙΌ ro φ φ

Note Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Comp. ZTA toughness vTrs (central part) (“C) - ²⁰ I -38 | O 1 -34 1 -39 | -45 | -44 I -42 I 1 CXI 1 -42 I -45 | -24 I -43 | CN 1 1 LO 1 Welding heat input (kJ / cm) ²⁴⁶ I ²⁰⁵ I ³¹⁵ I 300 | ^{312 284} I ²⁵⁷ I 305 | 450 | ³⁷⁴ | 362 | 316 | 312 | 320 | ²⁵¹ I ²⁵⁶ 1 236 Base metal toughness vTrs (central part) (“C) -57 | - ²⁸ I 00 1 -42 | -56 | -38 | -38 | 1 -33 1 -52 | -36 | -37 | -43 | - ⁵⁴ I O 1 -43 | -33 vTrs (part of the surface) (Ό) -64 | - ³⁴ I - ²⁸ 1 -47 1 -39 | - ⁵⁵ I -42 I - ⁴⁵ 1 -38 | -36 | -44 | -45 | I s 1 -46 | -49 I -38 Base metal resistance TS (central part tral) (MPa) ⁵¹⁸ I ⁵⁵¹ I 553 | 586 | ⁵¹⁹ I 671 | ⁵⁸⁷ I ⁵¹⁷ I 476 | 538 | 653 | 664 | 565 | 435 | 496 | 477 | 509 YP (central part) (MPa) ³⁷⁰ I 390 I ⁴²² 1 ⁴²¹ I ³⁷⁹ I 495 | 471 I 393 | 360 | 414 461 | 470 | 420 | dog 355 374 | 373 TiN Number Density of Shaft 0.05 to 0.2 μτη (central part) (x10 ⁴ / mm ³ ) 3 9.4 I 5.5 | 7.0 | 5.0 | 6.9 | 8.5 | CO LD 5.2 | 6.0 | 6.5 | 6.4 | 6.7 | 6.8 | 7.2 | 7.0 0.02 to 0.05 μτη (part of the surface) (x10 ⁴ / mm ² ) CO θ ' 7.5 | ⁸ > ³ I CDl CO 9.2 | 7.7 3 7.4 | 8.2 | 7.7 O CO CO 8.5 | 8.3 Plate thickness (mm) ⁴⁰ I ³⁰ I ⁵⁰ 1 ⁴⁵ I ⁵⁰ I ⁴⁵ I ⁴⁰ I ⁵⁰ I 70 | I I ^{99 59} I ⁵⁹ I ⁵⁰ 1 40 | 40 | 35 Steel < m Ο O LU LL 0 0 I I - - "3 -1 z The Z CD r- CO O) 20 | cxi 22 | 23 | ²⁴ 1 ²⁵ I 26 | ²⁷ I 28 | 29 | 30 | dog 32

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30/32 [0068] As shown in Table 4, with respect to Examples ^Nos 1 to 15, since the chemical compositions were within the specified ranges and these conditions were produced by specified, all properties were satisfactory so that the the specified density of TiN was satisfied, these had sufficient strength of 440 MPa or more, the tenacity of the base metal was -50Ό or less of the vTrs, and the tenacity of the ZTA of high heat input welding was -40Ό or less of the vTrs. In particular, it can be understood that the paragraphs 5, 7, 13 and 15, which had thickness of 60 mm or more plate and in which welding heat input was large, had excellent toughness. On the other hand, as shown in Table 5, regarding Comparative Examples ^Nos 16 to 32, since at least one of chemical composition and the production conditions were outside the ranges of the present invention, at least one of resistance of the base metal, the tenacity of the base metal and the tenacity of ZTA deteriorated.

[0069] Nos ⁰⁵ 16, 20, and 25 were the comparative examples in which the density of TiN was insufficient and at least one between the tenacity of the base metal and the tenacity of the ZTA deteriorated because the cooling rate of the casting was slow. In no. 16, since the TiN in the central part of the shaft plate thickness was insufficient and the γ grain became hard in the ZTA, the toughness deteriorated. At No. 20, since the TiN on the surface part was insufficient, ο γ crude was formed during the heating of the shaft and the toughness of the base metal surface part and the toughness of the ZTA deteriorated. As mentioned above, when the caster cooling rate is slow, the specified density of both the surface part and the central part of the sheet thickness may not be met, or the specified density or the surface part or the central part of the thickness of the plate may not be satisfied due to irregularityPetition 870180028151, of 04/09/2018, p. 36/47

31/32 des.

[0070] Paragraphs 17, 19 and 23 were comparative examples in which the heating conditions for hot rolling were outside the range of the present invention. At No. 17, once the P _H exceeded the upper limit, ο γ stiffened during heating and the tenacity of the base metal deteriorated. At No. 19, since the P _H was less than the lower limit, the Nb did not dissolve sufficiently as a solid solution, the resistance was insufficient, and the tenacity of the base metal deteriorated.

[0071] In No. 23, since the heating retention time was short, the solubilization of the connecting elements was insufficient and the tenacity of the base metal deteriorated.

[0072] No. 27 was the comparative example in which the rough rolling condition was outside the range of the present invention. Since the cumulative reduction in lamination was small, the structure was not refined and the tenacity of the base metal deteriorated.

[0073] Nos. ^05, 18, 21, and 26 were the comparative examples in which the finishing lamination conditions were outside the ranges of the present invention. At No. 18, since the starting temperature and the finishing temperature of the finishing lamination were below Ar ₃ , deformed α was formed and the tenacity of the base metal, especially the tenacity of the surface part, deteriorated noticeably. At No. 21, since the starting temperature and the finishing temperature of the finishing lamination were higher, the structure, especially the central part of the sheet thickness, became stiff and the toughness deteriorated. No. 26 was the comparative example in which the cumulative reduction in lamination of the finishing lamination was small, so that the structure became brutish and the toughness deteriorated.

[0074] Nos ⁰⁵ 22 and 24 were the comparative examples in which

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32/32 the conditions of accelerated cooling and heat treatment after hot rolling were outside the range of the present invention. No. 24 was the comparative example in which accelerated cooling was not conducted, so that the structure was not refined and the toughness deteriorated. At No. 22, since the temperature of the heat treatment was higher, the cementite and the structure became brutish and the tenacity of the base material deteriorated.

[0075] Paragraphs 28 to 32 were comparative examples in which the chemical compositions were outside the range of the present invention. At No. 28, since the Ceq 'parameter consisting of C, Mn and Nb exceeded the upper limit, the central segregation became pronounced, and especially the tenacity of the ZTA deteriorated. At No. 29, since the Ceq 'parameter was less than the lower limit, the resistance of the base metal deteriorated. At No. 30, since Ti / N was higher, crude oxides of Ti remained and especially the toughness at ZTA deteriorated. At No. 31, since Ti / N was lower, especially the toughness in the ZTA deteriorated due to the solid N solute. At No. 32, since the C content was excessive, the resistance was excessive and especially the toughness of the ZTA deteriorated.

Claims (3)

1. Method of producing a steel sheet, the method characterized by the fact that it comprises: caster, in which the cooling rate during cooling is 0.1 “C / s or more in a temperature range of 1100 to 1300Ό in a central part of the shaft thickness, which is a steel including as chemical composition, in mass%, C: 0.03 to 0.16%, Si: 0.03 to 0.5%, Mn: 0.3 to 2.0%, Nb: 0.005 to 0.030%, Ti: 0.003 to 0.050%, Al: 0.002 to 0.10%, N: 0.0020 to 0.0100%, P: limited to 0.020% or less, S: limited to 0.010% or less, and the balance consisting of iron and the inevitable impurities and satisfying equations (1) and (2), in which the shaft includes nitrides with Ti included with an equivalent circle diameter of 0.02 at 0.05 pm of 7.0x10 ⁴ or more per 1 mm ² on a part of the surface and the nitrides with Ti included with an equivalent circle diameter of 0.05 to 0.2 pm of 5.0x10 ⁴ or more per 1 mm ² in the central part; heating the shaft in a condition that satisfies equations (3) and (4) below; perform roughing lamination in which the cumulative lamination reduction is 30% or more at a temperature of 900Ό or more; perform finishing lamination in which the cumulative lamination reduction is 40% or more at an air temperature of ₃ to 880Ό. and Petition 870180028151, of 04/09/2018, p. 39/47 2/2 perform accelerated cooling after which the average cooling rate of the sheet thickness is 5 ° C / s or more from the temperature of Ar ₃ or more to a temperature of 550Ό or less. 0.32 <[C] + 0.15 [Mn] + 3.8 [Nb] <0.39 - (1) 1.5 <[Ti] / [N] <3.0 - (2) 56000 / (1.2 - 0.18 x log [Nb]) <(T + 273) x {log (t) + 25} <91000 / (1.9 - 0.18 x log [Ti]) - ( 3) t> 30 - (4) where [X] is the addition of element X in units of mass%, T is the reheat temperature in units of “C, and t is the retention time in units of minutes. 2. Steel sheet production method, according to claim 1, the method characterized by the fact that it also comprises heat treatment at a temperature of 650Ό or less after accelerated cooling. 3. Steel plate production method according to claim 1 or 2, characterized by the fact that steel also includes, in mass%, at least one element between Cu: 1.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, Ni: 2.0% or less, V: 0.10% or less, B: 0.0030% or less, Mg: 0.0050% or less, Ca: 0.0030% or less, and REM: 0.010% or less.