JP2019035107A - Steel plate and method of producing steel plate - Google Patents

Abstract

To provide a steel plate having excellent toughness in an HAZ (Heat-Affected Zone) and having excellent mechanical characteristics in a base metal when high heat input welding is performed on the steel plate.SOLUTION: The steel plate is provided which has a predetermined chemical composition and in which an effective crystal particle diameter at 1/4 position in a plate thickness direction is in a specific range, a ferrite fraction, a bainite fraction and a pearlite fraction are in specific ranges, B, carbon equivalent Ceq. and arrest index which are obtained from specific expressions are in specific ranges, the total of a content ratio of a value of an Al oxide expressed in terms of mass and a content ratio of values of a Zr oxide and a Ti oxide in terms of mass satisfy a specific ratio, the steel plate contains an oxide having a number density having a specific equivalent circle diameter of a specific value or more, 10% or more of the number ratio of the oxide has an oxide A having a specific composition A and an oxide B having a specific composition B (smaller Zr amount and larger Al amount than those of oxide A), a length in contact with the oxide A and the oxide B/outer periphery of oxide A is 30% or more, and a ratio of a length in contact with the oxide B and ferrite/outer periphery of the oxide B is 30% or more.SELECTED DRAWING: None

Description

  The present invention relates to a steel plate and a method for manufacturing a steel plate.

  Examples of the use of the steel sheet include ships, high-rise buildings, other buildings, bridges, offshore structures, LNG storage tanks, other large tanks, line pipes, and the like. In recent years, the size of welded structures has been increasing in order to increase the height of building structures and increase the loading weight of container ships. In connection with this, thickening and high intensity | strength of the steel plate thickness are calculated | required. Further, with respect to the welded portion, further safety and reliability must be ensured, and the toughness of the welding heat affected zone (hereinafter sometimes referred to as “HAZ”) (hereinafter referred to as “the weld heat affected zone”). "Toughness" may be referred to as "HAZ toughness"). Furthermore, even if a brittle crack is generated at a welded joint location, the steel sheet is required to have a capability of stopping the brittle crack at the base material (hereinafter sometimes referred to as “arrestability”).

  Conventionally, it has been known that the austenite (γ) crystal grain size, transformation structure, HAZ hardness, coarse hard phase, etc. have a great influence on the HAZ toughness of high-tensile steel sheets, and various countermeasures have been proposed. ing. Among these, the refinement of the HAZ structure is the most effective for improving the HAZ toughness, and many methods of utilizing inclusions have been proposed.

  The refinement of the HAZ structure utilizing inclusions includes, for example, a pinning effect that suppresses the growth of crystal grains and an intragranular transformation that newly generates ferrite. Intragranular transformation is a method of refining the structure by generating ferrite with inclusions as nuclei in austenite grains coarsened by the heat effect during welding. So far, in addition to nitrides such as TiN and sulfides such as MnS, techniques that use oxides that are chemically stable even at high temperatures as ferrite nuclei have been proposed (for example, Patent Documents 1 to 5). reference).

  The technique disclosed in Patent Document 1 is a composite oxide of Ti and Zr that becomes a nucleus of intragranular transformation (hereinafter sometimes referred to as “IGF nucleus”) in a steel sheet that does not substantially contain Al. This proposes a method of refining the structure of the weld heat-affected zone by finely dispersing. In the method disclosed in Patent Document 1, in order to produce a composite oxide of Ti and Zr that works effectively as an IGF nucleus, Ti and Zr are simultaneously added, and the balance of Ti, Zr and O is optimized. It has become.

  The technology disclosed in Patent Document 2 proposes a method of improving HAZ toughness by inclusions containing REM and Zr by adding REM, Zr and Ti to a steel sheet which does not substantially contain Al. Is.

  The technique disclosed in Patent Document 3 proposes a method for improving HAZ toughness by inclusions containing Zr, REM and Ca by adding REM, Ca and Zr to a steel sheet containing Al. It is.

  The technique disclosed in Patent Document 4 is based on the HAZ toughness by suppressing the austenite grain coarsening by adding Ti, Al, Ca in the order of Ti, Al, Ca and dispersing fine oxides. It proposes a method to improve the above.

  The technique disclosed in Patent Document 5 refines HAZ by the pinning effect by TiN and intragranular transformation by BN, and improves the toughness by suppressing the softening of HAZ by utilizing the hardenability improvement by B. A method is proposed.

JP-A-01-159356 JP 2008-291347 A JP 2014-001432 A JP 2001-342537 A JP 2007-177327 A

When the present inventors examined the techniques disclosed in Patent Documents 1 to 5, the following knowledge was obtained.
As a result of examining the technique disclosed in Patent Document 1, in order to produce a composite oxide of Ti and Zr, Ti and Zr are simultaneously added, and the balance of Ti amount, Zr amount and O amount is optimized. It has been found that further improvement in the HAZ toughness is insufficient.

  As a result of examining the technique disclosed in Patent Document 2, it was found that REM is stronger deoxidation than Zr and inhibits the formation of oxides of Zr and Ti.

  As a result of examining the technique disclosed in Patent Document 3, it has been found that both REM and Ca are stronger deoxidation than Zr and inhibit the formation of Zr oxide.

  As a result of examining the technique disclosed in Patent Document 4, in the high heat input welding, the portion adjacent to the weld metal is exposed to high temperature for a long time, so even if the oxide is finely dispersed, the austenite grains are coarsened. Since it cannot suppress, it turned out that deterioration of HAZ toughness is not suppressed.

  As a result of examining the technique disclosed in Patent Document 5, in high heat input welding, a portion adjacent to the weld metal is exposed to a high temperature for a long time, so that TiN using the pinning effect disappears in a solid solution, and HAZ is lost. It was found that toughness degradation was not suppressed.

  By the way, the welding construction cost which occupies the whole construction cost of a welded structure is large, and in order to reduce this expense, it is calculated | required to perform highly efficient welding. Specifically, it is effective to perform welding with high heat input and reduce the number of welding passes. However, when high heat input welding is performed, the HAZ structure of the steel sheet becomes coarse and deterioration of toughness is inevitable.

  Conventionally, dispersed particles such as steel plate inclusions have been used to improve HAZ toughness. However, it has been difficult to stably improve the HAZ toughness of a steel sheet when performing high heat input welding with a heat input exceeding 40 kJ / mm in order to increase the efficiency of welding. As this cause, for example, inclusions such as oxides tend to aggregate in the molten steel, it is difficult to uniformly disperse in the steel sheet, and inclusions are altered by being exposed to high temperature for a long time during high heat input welding, It is considered that it is difficult to control so as to easily act as a nucleus of intragranular transformation.

As described above, the actual situation is that a technique for improving the HAZ toughness has not been established during high heat input welding.
The present invention has been made in view of such circumstances, and has excellent toughness in HAZ when large heat input welding is performed, and in a base material that is a part other than HAZ and a weld metal part. An object of the present invention is to provide a steel sheet having excellent mechanical properties.

  The present invention is a result of intensive studies focusing on oxides and solute B as intragranular transformation nuclei as intragranular ferrite nuclei capable of refining the HAZ structure in steel sheets containing Al. The inventors have found that the above problems can be solved, and have completed the present invention.

  The gist of the present invention is as follows.

(1)
% By mass
C: 0.01% to 0.20%,
Si: 0.02% to 0.50%,
Mn: 0.30% to 2.50%,
Ti: 0.003% to 0.024%,
B: 0.0005% to 0.0050%,
N: 0.0010% to 0.0090%,
O: 0.0010% to 0.0050%,
Zr: 0.0005% to 0.0100%,
Sol. Zr: 0% to 0.0020%,
Cu: 0.1% to 1.5%,
Ni: 0.1% to 3.0%,
Al: 0.0055% to 0.0550%,
P: 0.050% or less,
S: 0.0080% or less,
Nb: 0% to 0.035%
Cr: 0% to 1.0%
Mo: 0% to 1.00%,
V: 0% to 0.10%
Ca + REM: 0% to 0.0005% or less, and the balance: a chemical composition consisting of Fe and impurities,
_BF represented by the following formula (1) is 0.0005% to 0.0030%,
Carbon equivalent represented by the following formula (3) Ceq. Is 0.35% to 0.50%,
In crystal orientation analysis using electron beam backscatter diffraction (EBSD) at 1/4 position in the thickness direction of the cross section perpendicular to the rolling direction, the effective crystal grain size is 30 μm or less,
The microstructure at the 1/4 position in the plate thickness direction is the area ratio, the ferrite fraction is 20% to 70%, the bainite fraction is 30% to 75%, and the pearlite fraction is 0% to 5%.
The arrest index represented by the following formula (4) is -10 or less,
As the average composition, the Ti, Zr, and Zr when assumed to be a single oxide of Ti, Zr and Al elements, obtained from the measured values of O amount, Ti amount, Zr amount, and Al amount of the whole oxide The content ratio of the mass converted value of the Al oxide is 5% to 70% or less, and the content ratio of the mass converted value of the Zr oxide is 5% to 70% with respect to the total mass converted value of the oxide of each element of Al. Below, the total content of Ti oxides in terms of mass is within the range of 5% to 70%, and the number density with the equivalent circle diameter of 0.5 μm to 10 μm is 10 / mm ² or more. Containing
Of the oxides, 10% or more of the total number of the oxides includes the oxide A having the following composition A and the oxide B having the following composition B, and the oxide A and the oxidation Object B is in contact with each other,
Composition A: The content ratio of the mass-converted value of the Al oxide is 0% to 60% or less, and the content ratio of the mass-converted value of the Zr oxide is 15% to 95 with respect to the sum of the mass-converted values of the oxides of the respective elements. % Or less, and the content ratio of the mass converted value of the Ti oxide is 15% to 95% or less Composition B: The content ratio of the mass converted value of the Al oxide to the total of the mass converted values of the oxides of the respective elements is 15 % To 95% or less, the content ratio of the mass conversion value of the Zr oxide is 0% to 60% or less, and the content ratio of the mass conversion value of the Ti oxide is 15% to 95% or less. Zr content of the oxide A Is Zr (A), the mass ratio is Zr (A) / Zr (B)> 1 when the Zr content of the oxide B is Zr (B), and the Al content of the oxide A is Al (A ), And the mass ratio when the Al content of the oxide B is Al (B) is Al (A) / Al B) satisfies the <1,
The ratio of the length of contact between the oxide A and the oxide B to the outer periphery of the oxide A (the length of contact between the oxide A and the oxide B / the outer periphery of the oxide A) is 30%. ~ 100%
The ratio of the length at which the oxide B and the base iron are in contact with the outer periphery of the oxide B (the length at which the oxide B and the base iron are in contact / the outer periphery of the oxide B) is 30% to 100%. A certain steel plate.

(However, in the formula (1), B _asBN is represented by the formula (2). Further, B is the content (mass%) of the B element contained in the steel plate, and 0 ≦ B _F ≦ B. Fulfill.)

However, in the formula (2), the relationship 0 ≦ _BasBN ≦ B is satisfied, and N, Ti, O, and Al are the contents (mass of each of the elements N, Ti, O, and Al contained in the steel plate). %), Insol. Zr represents the content (% by mass) of acid-insoluble Zr.

  Ceq. = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (3)

  However, C, Mn, Cu, Ni, Cr, Mo, and V in Formula (3) represent content (mass%) of each element contained in a steel plate.

  Arrest index = 0.34 × t + 0.40 × vTrs (table) + 0.12 × NDTT (t / 2) (4)

  However, in the formula (4), t is the plate thickness [mm], vTrs (table) is the brittle ductile transition temperature [° C.] in the Charpy impact test in the 5 mm position below the table and in the direction parallel to the rolling direction, and NDTT ( t / 2) represents a Nil-Ductility Transition (non-ductile transition) temperature in the Naval Research Laboratory drop weight test at a position 1/2 of the sheet thickness direction from the steel sheet surface.

(2)
The temperature at which the plate thickness is 55 mm or more, the yield stress of the base material, which is a portion other than the weld heat affected zone and the weld metal portion, is 460 MPa or more, and the arrest toughness value Kca is 6000 N / mm ^1.5 is − The steel plate as described in (1) which is 10 degrees C or less.

(3)
Absorbed energy of Charpy impact test in which the weld heat affected zone generated when large heat input welding is performed at a heat input of 40 kJ / mm to 60 kJ / mm when the plate thickness is 55 mm to 80 mm at a test temperature of −40 ° C. Is 100 J or more at all positions on the front side of the plate thickness, the position of the plate thickness center (t / 2), and the back side of the plate thickness in the plate thickness direction, and other than the weld heat affected zone and the weld metal portion The steel sheet according to (1) or (2), wherein the brittle ductile transition temperature of the base material, which is a part, is −40 ° C. or lower.

(4)
A method for producing the steel sheet according to any one of (1) to (3),
In secondary refining in a reduced-pressure atmosphere, at least one of Al and C is added to the molten steel, and the dissolved oxygen content in the molten steel is adjusted to 0.0005% to 0.0100% by mass%, and the dissolved oxygen content is adjusted. After adding Ti, Al, and Zr in the order of Ti, Al, and Zr to the molten steel after casting, casting the molten steel after addition of Ti, Al, and Zr to obtain a slab, and
A heating step of heating the steel slab after the casting step in a temperature range of 1000 ° C to 1150 ° C;
The steel slab after the heating step is rolled in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction is 50% or more, and the temperature after 1 sec from the completion of finish rolling is the rolling start temperature −80 ° C. to the rolling start temperature. A rolling process for rolling to + 80 ° C .;
Water cooling is started when the steel sheet after the rolling process is in a temperature range of 650 ° C. to 850 ° C., and the cooling rate at a position of 5 mm from the steel plate surface and a 1/4 position in the plate thickness direction is 3 ° C./sec to 30 ° C. / Sec and a cooling step for stopping water cooling in a temperature range where the surface temperature is 500 ° C. or lower,
The manufacturing method of the steel plate which has.

(5)
A method for producing the steel sheet according to any one of (1) to (3),
In secondary refining in a reduced-pressure atmosphere, after adjusting the amount of dissolved oxygen in the molten steel to 0.0005% to 0.0100% in mass% without adding Al and C to the molten steel A casting step of adding Ti, Al, and Zr to the molten steel in the order of Ti, Al, and Zr, and then casting the molten steel after addition of Ti, Al, and Zr to obtain a slab;
A heating step of heating the steel slab after the casting step in a temperature range of 1000 ° C to 1150 ° C;
The steel slab after the heating step is rolled in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction is 50% or more, and the temperature after 1 sec from the completion of finish rolling is the rolling start temperature −80 ° C. to the rolling start temperature. A rolling process for rolling to + 80 ° C .;
Water cooling is started when the steel sheet after the rolling process is in a temperature range of 650 ° C. to 850 ° C., and the cooling rate at a position of 5 mm from the steel plate surface and a 1/4 position in the plate thickness direction is 3 ° C./sec to 30 ° C. / Sec and a cooling step for stopping water cooling in a temperature range where the surface temperature is 500 ° C. or lower,
The manufacturing method of the steel plate which has.

(6)
Furthermore, the manufacturing method of the steel plate as described in (4) or (5) which has the heat processing process which reheats the steel plate after the said cooling process to the temperature of 300 to 600 degreeC.

  According to the present embodiment, a steel sheet having excellent toughness in HAZ when performing high heat input welding and excellent mechanical properties in a base material that is a portion other than HAZ and a weld metal part is provided. it can.

It is the photograph showing an example which image | photographed the steel plate of this embodiment with the scanning electron microscope. It is a graph showing the relationship between the arrest index in the steel plate of this embodiment, and the temperature which the arrest toughness value Kca becomes 6000 N / mm ^1.5 .

Hereinafter, an example of a preferred embodiment of the present invention will be described in detail.
In addition, in this specification, the numerical range represented using "to" means the range which includes the numerical value described before and behind "to" as a lower limit and an upper limit.

Conventionally, when Ti oxide and B nitride are dispersed in weld metal and HAZ, it is known that intragranular ferrite is generated and the structure is refined. Conventionally, it is known that the solid solution B segregated at the prior austenite grain boundaries of the steel sheet suppresses the formation of coarse grain boundary ferrite during welding and improves the HAZ toughness.
However, Zr is not an element that is generally added to steel sheets, and research conducted in the past has been very limited as steel sheets to which Zr is added. So far, when an oxide containing Zr (especially an oxide containing Zr and Ti) is dispersed in a steel sheet, the effect of solid solution B on improving HAZ toughness has never been studied.

In the steel sheet containing Al, the present inventors have intensively studied paying attention to oxides, solute B, and B nitrides that form intragranular ferrite formation nuclei that can refine the HAZ structure. . As a result, new findings were obtained mainly on the following (A) oxide composition and number density, (B) solute Zr, (C) solute B, (D) deoxidation method, and (E) microstructure. It was.
Hereinafter, these new findings will be described.

(A): Oxide composition and number density The inventors of the present invention have investigated in detail for each oxide, which is the core of intragranular ferrite, and investigated the effect on the improvement of HAZ toughness. went.
As a result, it was found that the HAZ toughness was improved by containing the following oxides.
The oxide has a specific range with respect to the total mass converted value of Ti oxide, Zr oxide, and Al oxide as an average composition, and has a specific equivalent circle diameter (the diameter of the circle when assumed to be a circle). Corresponding oxide). And among this oxide, it has oxide A which has Zr and Ti as a main component, and oxide B with much content of Zr oxide and less content of Al oxide than oxide A. . In the oxide A, at least a part (30% or more) around the oxide A is in contact with the oxide B, and at least a part (30% or more) around the oxide B Is an oxide that comes into contact with the surface.

Specifically, the average composition includes an oxide having the following composition with respect to the sum of mass converted values of Ti oxide, Zr oxide, and Al oxide. In addition, the sum total of the mass conversion value of Ti oxide, Zr oxide, and Al oxide is 100%.
Content ratio of mass converted value of Al oxide: 5% to 70%
Content ratio of mass converted value of Zr oxide: 5% to 70%
Content ratio of mass converted value of Ti oxide: 5% to 70%
(The lower limit of each oxide is preferably 10% or more, more preferably 15% or more, still more preferably 20% or more. The lower limit of each oxide is preferably 65% or less, more preferably 60% or less, still more preferably. Is 55% or less.)

The oxide having the above average composition has an equivalent circle diameter of 0.5 μm to 10 μm and a number density of 10 / mm ² or more.
When the equivalent circle diameter is smaller than 0.5 μm, the function as the intranuclear ferrite formation nucleus (IGF nucleus) is lowered. On the other hand, if the equivalent circle diameter is larger than 10 μm, the possibility that the coarse oxide itself acts as a starting point of destruction increases. The number density of the oxide having the above composition having an equivalent circle diameter of 0.5 μm to 10 μm is 10 / mm ² or more (preferably 20 / mm ² or more, more preferably 30 / mm ² or more. In the case of more preferably 50 pieces / mm ² or more, most preferably 60 pieces / mm ² or more, it is clear that HAZ toughness is improved by refining the HAZ structure as compared with a steel sheet not containing Zr. It became. In addition, the upper limit of the number density of the first oxide is not particularly limited, and examples thereof include 200 pieces / mm ² or less.

Further, among the oxides having the above average composition, 10% or more in terms of the number ratio includes the oxide A and the oxide B in contact with each other. Further, the oxide A has the following composition A, and the oxide B has the following composition B. In addition, in the following composition A and the following composition B, the total of the mass conversion value of Zr oxide, Ti oxide, and Al oxide is 100%.
Further, when the Zr content of the oxide A is Zr (A) and the Zr content of the oxide B is Zr (B), the mass ratio is Zr (A) / Zr (B)> 1, The mass ratio when the Al content is Al (A) and the Al content of the oxide B is Al (B) satisfies Al (A) / Al (B) <1.

Composition A: Each of Ti, Zr, and Al when assumed to be a single oxide of elements of Ti, Zr, and Al, obtained from measured values of O amount, Ti amount, Zr amount, and Al amount of the whole oxide For the total mass converted value of elemental oxides,
Content ratio of mass converted value of Zr oxide: 15% to 95%
Content ratio of mass converted value of Ti oxide: 15% to 95%
Content ratio of mass converted value of Al oxide: 0% to 60%

Composition B: Each of Ti, Zr, and Al assuming that it is a single oxide of elements of Ti, Zr, and Al, obtained from measured values of O amount, Ti amount, Zr amount, and Al amount of the whole oxide For the total mass converted value of elemental oxides,
Content ratio of mass converted value of Zr oxide: 0% to 60%
Content ratio of mass converted value of Ti oxide: 15% to 95%
Content ratio of mass converted value of Al oxide: 15% to 95%

  Further, the ratio of the length of contact between the oxide A and the oxide B to the outer periphery of the oxide A (the length of contact between the oxide A and the oxide B / the outer periphery of the oxide A) is 30% to 100%. Satisfied. Furthermore, the ratio of the length of contact between the oxide B and the base iron to the outer periphery of the oxide B (the ratio of the length of contact between the oxide B and the base iron / the outer periphery of the oxide B) is 30% to 100%. Satisfied.

Among the oxides having the above average composition, 10% or more of oxides as a number ratio have oxide A and oxide B satisfying the above conditions, so that intranuclear ferrite nuclei (IGF) It has been found that it can function as a nucleus).
From the viewpoint of further improving the HAZ toughness, the number ratio of the oxide A and the oxide B satisfying the above conditions is preferably 20% or more, and more preferably 30% or more.
In addition, the number ratio is the oxide A and oxide B that satisfy the above conditions with respect to the number of oxides (referred to as oxide 1) that satisfy the average composition, the equivalent circle diameter, and the number density that are analyzed. It is expressed as a percentage of the number of [specific oxides] [(number of oxides 1 / number of specific oxides)].

  Here, when oxides (including oxide A and oxide B) deviated from the range of the above conditions, they did not form the nuclei of intragranular ferrite. When Zr was not contained, the number of intranuclear ferrite formation nuclei (IGF nuclei) decreased, and HAZ toughness deteriorated. Further, when Zr was excessive, Zr clusters were generated, IGF nuclei were similarly reduced, and HAZ toughness could not be ensured.

  The equivalent circle diameter, number density, and composition of the oxide containing Al, Ti, and Zr contained in the steel sheet according to the present embodiment, and the number ratio in which oxide A and oxide B are in contact with each other, oxide A The ratio of the length in contact with the oxide B, the ratio of the length in which the oxide B contacts the ground iron, and the composition of the oxide A and the oxide B are determined by analysis using a scanning electron microscope (SEM). To do.

Specifically, first, a heat cycle test piece having a thickness direction of 12 mm, a width direction of 12 mm, and a rolling direction of 70 mm is collected at the position of t / 4 in the thickness direction of the steel sheet. Then, after the test piece was heated and held at 1400 ° C. for 25 seconds, the cross section in the direction perpendicular to the rolling direction of the steel sheet cooled at a cooling rate of 1 ° C./sec was mirror-polished to obtain a SEM / EDX (scanning electron microscope). / Energy dispersive X-ray spectroscopy) is observed by analysis, and the inclusions observed in the observation field are quantitatively analyzed and measured. In the SEM / EDX analysis, for example, the acceleration voltage is 15 kV, the current is 89 μA to 91 μA, and the observation area is 25 mm ² (5 mm × 5 mm) or more (preferably, the observation area is 100 mm ² (10 mm × 10 mm)). As an example, FIG. 1 shows a photograph by SEM. 11 represents a ground iron, and 12 represents an inclusion. As shown in the photograph in FIG. 1, quantitative analysis of the average composition of the inclusions is included for each of the inclusions 12 that appear granular due to the contrast (contrast) of the color tone with respect to the ground iron 11 (background). To do.

  The size of inclusions to be analyzed is an equivalent circle diameter (diameter) of 0.5 μm to 10 μm, and at least 500 or more analysis items are analyzed.

The elements to be analyzed are O, Ti, Zr, and Al, and the relationship between the X-ray intensity and the element concentration of each element is obtained in advance as a calibration curve using a known substance. Then, the element concentration contained in the inclusions to be analyzed is quantified from the X-ray intensity obtained from the inclusions to be analyzed and the calibration curve. Of the inclusions, those judged to be oxides are those in which an oxygen peak is clearly recognized, and the lower limit depends on the measurement conditions and the measurement apparatus.
For example, a case where SEM / EDX analysis is measured at an acceleration voltage of 15 kV and a current of 89 μA to 91 μA will be described. The total of the O content, the Ti content, the Zr content, and the mass% of the Al content is obtained. When the O content is 1.0 mass% or more with respect to the total, this inclusion is oxidized. It is a thing. And about this oxide, using the following formula (5)-following formula (7), from the mass% of each element, the mass conversion value of the oxide of each element when it is assumed that it is a single oxide by these elements Is calculated.
Ti ₂ O ₃ = Ti × 3.003 (5)
ZrO ₂ = Zr × 1.351 (6)
Al ₂ O ₃ = Al × 3.779 (7)

However, in Formula (5)-Formula (7), Ti, Zr, and Al are content (mass%) of each element measured by SEM / EDX analysis. The total content of each element measured by the SEM / EDX analysis is 100% by mass.
The total of the mass conversion values of Ti ₂ O ₃ , ZrO ₂ , and Al ₂ O ₃ obtained from the formulas (5) to (7) is obtained, and the ratio of the oxide of each element to the total is included in the oxide The content ratio (%) of the oxide of each element.

The content ratio of Ti ₂ O ₃ , ZrO ₂ , and Al ₂ O ₃ is represented by the following formula (8) to the following formula (10).
Ti ₂ O ₃ content ratio (%) = Ti ₂ O ₃ / (Ti ₂ O ₃ + ZrO ₂ + Al ₂ O ₃ ) (8)
The content of _{ZrO 2 (%) = ZrO 2} / (Ti 2 O 3 + ZrO 2 + Al 2 O 3) ··· (9)
Al ₂ O ₃ content ratio (%) = Al ₂ O ₃ / (Ti ₂ O ₃ + ZrO ₂ + Al ₂ O ₃ ) (10)

Next, among the oxides measured for number density, a region corresponding to the oxide A (in the SEM composition image, a region that exhibits a relatively bright (eg, white) cotton tone in the oxide) And a region corresponding to the oxide B in contact with at least a part of the oxide A (in the SEM composition image, a region of the oxide that is darker (for example, black) than the ground iron and presents a cotton trust) Ti _2. The O ₃ , ZrO ₂ , and Al ₂ O ₃ content ratio and the number ratio are measured. The analysis number of oxide A and oxide B in contact with oxide A is at least 20 of the oxides measured for the number density. And the total of O content, Ti content, Zr content, and the mass% of Al content is calculated | required, Al oxide, Zr oxide, and said Formula (5)-Formula (10) The amount of Ti oxide is determined.

  Further, from the structure image of the SEM, the length of the region corresponding to the oxide A and the region corresponding to the oxide B is measured, the outer peripheral length of the region corresponding to the oxide A is measured, and the oxide A and A value obtained by dividing the length of contact with the oxide B by the outer periphery of the oxide A (the length of contact between the oxide A and the oxide B / the outer periphery of the oxide A) is calculated. And the length which the area | region applicable to the oxide B and the ground iron contact is measured, the outer periphery length of the area | region applicable to the oxide B is measured, and the area | region which corresponds to the oxide B and the ground iron contact length Is divided by the outer peripheral length of the region corresponding to the oxide B (the ratio of the length of contact between the oxide B and the ground iron / the outer periphery of the oxide B).

(B): Solid solution Zr (Sol. Zr)
As a condition of the oxide containing Zr that contributes to the refinement of the HAZ structure, it is necessary that Zr is contained in a certain amount or more in the oxide. On the other hand, Zr (solid solution Zr (solid solution Zr is expressed as “Sol.Zr”) remaining in the steel sheet without forming an oxide significantly deteriorates not only the HAZ but also the toughness of the steel sheet itself. It is necessary to reduce the Sol.Zr.The smaller the Sol.Zr, the more the toughness tends to improve, and in order to obtain a steel sheet with excellent HAZ toughness, the Sol.Zr may be limited to 0.0020% by mass or less. In order to further improve the HAZ toughness, it is preferable to limit to 0.0010% by mass or less (more preferably 0.0005% by mass or less), where Sol.Zr is an acid-soluble Zr. And corresponds to Zr solid-dissolved in the steel plate, which can be measured by electrolytic extraction residue analysis method, etc. The acid-insoluble Zr is Insol.Zr (Insol.Zr in Formula (2)), steel Zr amount in is the total amount of acid-soluble Zr and acid-insoluble Zr.

(C): Solid solution B (B _F )
Solid solution B segregating at the prior austenite grain boundaries of the steel sheet suppresses the formation of coarse grain boundary ferrite during welding and improves the HAZ toughness. It has been found that in a steel sheet in which an oxide containing Zr is dispersed in an oxide, the solid solution B increases compared to a steel sheet in which an oxide not containing Zr is dispersed. The mass% (B _F ) of the solid solution B in the steel sheet containing the Zr oxide in the oxide can be obtained by subtracting the mass% of B that becomes B nitride from the content of B contained in the steel sheet. That is, _BF is represented by the following formula (1). When this value is 0.0005% or more (preferably 0.0010% or more), the effect of improving the HAZ toughness by the solid solution B is obtained. When _BF becomes excessive, there is a concern that the HAZ toughness deteriorates. Therefore, the upper limit of _BF is 0.0030% or less. Preferably it is 0.0025% or less, More preferably, it is 0.0020% or less.

However, B in Formula (1) is content (mass%) of B contained in a steel plate, B _asBN is mass% of B used as B nitride. Also, _{B F} satisfies the relationship of 0 ≦ _{B F} ≦ _B.
Further, when B _F <0, B _F = 0, and when B _F > B, B _F = B. That is, _when the value of B _asBN is a negative value, B _F = B, and when the value of B _asBN is larger than B, B _F = 0.

In the steel plate, Ti acts as a nitride forming element in addition to B. However, Ti also forms an oxide. Therefore, in order to obtain B _asBN , it is necessary to take into consideration the formation of inclusions including oxides and nitrides.

  Here, since the oxide is formed from an element having a strong deoxidizing power, first, Zr having a higher deoxidizing power than Al is preferentially oxidized in the molten steel to form a Zr oxide. Next, it is considered that surplus oxygen and Al are combined to form an Al oxide, and further surplus oxygen is combined with Ti to form a Ti oxide. Then, it is considered that the remaining Ti without forming an oxide combines with nitrogen to form Ti nitride, and the remaining nitrogen combines with B to form B nitride.

It is considered that Zr forms ZrO ₂ , Al forms Al ₂ O ₃ , Ti forms Ti ₂ O ₃ , TiN, and B forms BN. For this reason, the mass% (B _asBN ) of B to be B nitride is obtained using the following formula (2) based on these atomic weights or molecular weights.

However, N, Ti, O, and Al in Formula (2) are contents (mass%) of each element of N, Ti, O, and Al contained in a steel plate, Insol. Zr is the content (% by mass) of acid-insoluble Zr. B _asBN satisfies the relationship of B in the formula (1) and 0 ≦ B _asBN ≦ B.
Further, when B _asBN <0, B _asBN = 0, and when B _F > B, B _asBN = B. That _is, when the value of _{B AsBN} is a negative _value, and _{B asBN} = _0, when the value of _{B AsBN} is greater than B in the formula (1) _is a _{B asBN} = _B.
In addition, Sol. Zr is acid-soluble Zr, and is the Zr content (% by mass) dissolved in the steel sheet measured by electrolytic extraction residue analysis or the like. Insol. Zr is the content (mass%) of acid-insoluble Zr. The Zr content is subtracted. In addition, 0 ≦ Insol. It satisfies Zr ≦ Zr.
In the steel sheet according to the present embodiment, B _{asBN preferably} has a relationship of B _asBN ≦ 0 in that the added B is used as the solid solution B to suppress the grain boundary ferrite. In other words, B _asBN represented by the above formula (2) is _preferably 0% or less.

(D): Deoxidation method Oxide particles are generated when the molten steel is deoxidized. This is referred to as a primary oxide. Furthermore, during casting and solidification, an oxide containing Al, Zr, and Ti is generated as the molten steel temperature decreases. This is called a secondary oxide. In the present embodiment, either a primary oxide or a secondary oxide may be used. However, it is preferable to use a secondary oxide because the oxide produced with the drop in the molten steel temperature during casting and solidification produces finer particles than the primary oxide produced when the molten steel temperature is high. .

Furthermore, the manufacturing conditions of such a slab were examined in detail.
Production process of slab: In the process of converter → ladle → secondary refining → continuous casting, oxide inclusions remaining in the slab are especially dissolved oxygen in the molten steel before deoxidation start in secondary refining The amount is controlled to 0.0005% to 0.0100% by mass% (preferably upper limit is 0.0080% or less, more preferably 0.0050% or less) and Ti, Al, and Zr are deoxidizing elements. It has been found that the average particle size of the oxide is remarkably refined and the number density of the oxide is increased by adding in this order.

Through this step, the oxide finally remaining in the steel sheet has an average composition of 5% to 70% of the content of Al oxide in terms of mass, and 5% of the content of Zr oxide in terms of mass. -70%, and the total content of mass converted values of Ti oxide satisfies the range of 5% to 70%, and the equivalent circle diameter (diameter) of these Al oxide, Zr oxide, and Ti oxide is It was found that the number density of the oxide having a thickness of 0.5 μm to 10 μm is an oxide having a density of 10 / mm ² or more.
Further, it has been found that 10% or more of the oxides satisfying the following conditions as a percentage of the total number of oxides.
It has oxide A containing Zr and Ti and oxide B containing Al and Ti, and oxide A and oxide B are in contact with each other.
The mass ratio of the Zr content Zr (A) of the oxide A to the Zr content Zr (B) of the oxide B is Zr (A) / Zr (B)> 1, and the Al content of the oxide A The mass ratio of Al (A) to the Al content Al (B) of the oxide B satisfies Al (A) / Al (B) <1.
The ratio of the length of contact between the oxide A and the oxide B / the outer periphery of the oxide A is 30% to 100%, and the ratio of the length of contact between the oxide B and the ground iron / of the oxide B The outer circumference) is 30% to 100%.

  Here, secondary refining shows the process performed by the vacuum refining apparatus or the refining apparatus in an inert gas after converter refining. Ti, Al, and Zr may be added in the form of a single metal or an alloy.

(E): Microstructure The matrix structure is a mixed structure of ferrite, bainite and pearlite, or a mixed structure of ferrite and bainite. However, in a structure in which ferrite and bainite are mixed, the basic structure unit is objectively defined and the size is measured only by the structure observation with a normal optical microscope (hereinafter sometimes referred to as “light microscope observation”). It is very difficult to do. Therefore, in addition to light microscopic observation, the present inventors performed crystal orientation analysis using an electron back scatter diffraction pattern (EBSD) and analyzed the microstructure.
In addition, when calling it a base material in this specification, a base material shows parts other than HAZ and a weld metal part.

More specifically, a sample for observing the structure is taken from the position of the sheet thickness ¼ in the sheet thickness direction from the steel sheet surface (hereinafter sometimes referred to as “t / 4 part”), and the sample is observed in the main rolling direction. The cross section in the vertical direction (width direction) is mirror-polished. The sample of t / 4 parts was subjected to Nital corrosion, photographed with 4 optical fields at 500 times using an optical microscope, measured the pearlite fraction in each field of view, and the average value was calculated as the pearlite content of t / 4 parts. Rate.
Further, for each part of t / 4 part, a 500 μm × 500 μm region was measured at a 1 μm pitch by EBSD method with respect to a cross section perpendicular to the main rolling direction (width direction). The boundary where the crystal orientation difference with the adjacent grain is 15 ° or more is defined as the grain boundary, and the weighted average of the equivalent circle diameter (diameter) of the region surrounded by the grain boundary is defined as the effective grain size of each part. did.

The ferrite was a part having a KAM (Kernel Average Misoration) value of 1 ° or less when the measurement points measured by the previous EBSD method are close to each other. The area fraction of this ferrite was calculated | required for every site | part of t / 4 part.
The t / 4 part bainite fraction was the remainder other than the t / 4 part pearlite fraction and the t / 4 part ferrite fraction. That is, the sum of the bainite fraction at t / 4 part, the pearlite fraction at t / 4 part, and the ferrite fraction at t / 4 part is 100% in area ratio.
The weighted average was obtained by the following method. And there are N crystal grains in a single field, 1 area of each crystal grain _{A, A 2, A 3,} ··· A i, there is · · · _{A N,} equivalent diameter (diameter circle of each particle ) is _{D 1, D 2, D 3} , ··· D i, and a · · · _{D N.} In that case, the effective crystal grain size (Deff) is obtained by the following formula (11).

As a result of investigating the relationship between the toughness of the base metal at t / 4 part and the microstructure, the brittle ductile transition temperature of the base material (hereinafter referred to as “vTrs”) as the effective crystal grain size at t / 4 part becomes finer. There is a low temperature. It has been found that when the effective crystal grain size is 30 μm or less (preferably 20 μm or less, more preferably 25 μm or less, further preferably 20 μm or less, most preferably 15 μm or less), vTrs is −40 ° C. or less. When at least one of the effective crystal grain size was more than 30 μm and the pearlite fraction at t / 4 part was more than 5%, vTrs exceeded −40 ° C., and the base material toughness could not be secured. In order to ensure the base material toughness, the pearlite fraction is preferably low, and the fraction may be 0%.
In this embodiment, the effective crystal grain size of 1 μm or more was measured as the effective crystal grain size of t / 4 part. The effective crystal grain size of t / 4 part is preferably as small as possible, and the lower limit is not particularly limited, but examples include 5 μm or more, and further 1 μm or more.

  As a result of investigating the relationship between the base metal strength and the microstructure, the ferrite fraction at t / 4 part decreased and the base metal strength at t / 4 part improved as the bainite fraction at t / 4 part increased. . When the area percentage is 70% or less (preferably 65% or less, more preferably 60% or less, more preferably 55% or less, most preferably 50% or less), the yield stress of the base metal is 460 MPa. It became clear that it became the above. When the ferrite fraction exceeds 70%, the base material strength cannot be secured. It was found that the ferrite fraction was 20% or more (preferably 25% or more, more preferably 30% or more) in order to set the brittle ductile transition temperature (vTrs) of the base material to −40 ° C. or less. In order to secure the base material strength, the bainite fraction needs to be 30% or more (preferably 35% or more, more preferably 40% or more, and further preferably 45% or more), and the brittle ductile transition of the base material. In order to set the temperature (vTrs) to −40 ° C. or less, the bainite fraction was found to be 75% or less (preferably 70% or less, more preferably 65% or less, and even more preferably 60% or less). .

FIG. 2 shows the results of investigating the relationship between arrest properties, plate thickness, vTrs (table), and arrest index calculated from NDTT (t / 2). In FIG. 2, the horizontal axis is the arrest index calculated by the following equation (4), and the vertical axis is the temperature at which the arrest toughness value Kca is 6000 N / mm ^1.5 (hereinafter referred to as “T _Kca6000 ”). May be referred to). In FIG. 2, the result of approximating the arrest index and _TKca6000 with a linear function is indicated by a dotted line. As shown in FIG. 2, it was found that there is a strong correlation between the arrest index and _TKca6000 . From this first-order approximation curve, it was found that the Arrest index needs to be −10 or less in order to make T _Kca6000 −10 ° C. or lower (for example, −50 ° C. to −10 ° C.). In consideration of measurement variations, the arrest index is preferably −15 or less, more preferably −20 or less, still more preferably −25 or less, and most preferably −30 or less. The lower limit value of the arrest index is not particularly limited, but considering the production load such as an increase in rolling load at the time of rolling to improve arrestability, a decrease in productivity, etc., for example, −60 or more (preferably −50 or more, More preferably, it is −40 or more.
Arrest index = 0.34 × t + 0.40 × vTrs (table) + 0.12 × NDTT (t / 2) (4)

Here, in the formula (4), “t”, “vTrs (table)”, and “NDTT (t / 2)” are represented as follows.
t: Plate thickness (mm)
vTrs (Table): Brittle ductile transition temperature [° C.] in a Charpy impact test parallel to the rolling direction of the steel sheet surface 5 mm part
NDTT (t / 2): NDT (Nil-Ductility Transition) temperature [° C.] in an NRL (Naval Research Laboratory) drop weight test parallel to the rolling direction at a position of 1/2 the thickness of the steel sheet

  The NDT temperature (hereinafter sometimes referred to as “NDTT”) is determined by a method based on the NRL drop weight test defined in ASTM E208.

It has been clarified that a steel sheet satisfying these conditions improves the HAZ toughness through refinement of the HAZ structure in a high heat input welded joint and is excellent in the mechanical properties of the base material. Specifically, a steel plate having a yield stress of 460 MPa or more (for example, 460 MPa to 600 MPa) and _{TKca 6000} of −10 ° C. or less (for example, −50 ° C. to −10 ° C.) is obtained. In addition, when the plate thickness is 55 mm to 80 mm, the Charpy impact test performed at a test temperature of −40 ° C. with respect to the weld heat affected zone that occurs when large heat input welding is performed at a heat input of 40 kJ / mm to 60 kJ / mm. Absorbed energy is 100 J or more at all positions on the front side of the plate thickness, the position of the plate thickness center (t / 2), and the back side of the plate thickness in the plate thickness direction, and the weld heat affected zone and weld metal zone It was found that the brittle ductile transition temperature of the base material, which is a part other than the above, becomes −40 ° C. or lower.

Furthermore, the reason for limitation of the chemical composition of the steel plate of this embodiment is described.
In the following description, “%” in the description of each element means “mass%”.

(C: 0.01% to 0.20%)
C is an element necessary for ensuring strength. If the C content is less than 0.01%, the required strength cannot be ensured. However, if the amount of C exceeds 0.20%, it becomes difficult to secure toughness for both the base material and the HAZ. The minimum with the preferable amount of C is 0.03% or more, More preferably, it is 0.05% or more. The upper limit with preferable C amount is 0.15% or less, More preferably, it is 0.10% or less.

(Si: 0.02% to 0.50%)
Si increases the hardenability of the steel sheet and contributes to an increase in the strength of the steel sheet. In order to acquire this effect, it is necessary to contain 0.02% or more of Si. Preferably, the Si amount is 0.05% or more. On the other hand, since Si has a high reactivity with oxygen and a deoxidizing action, it affects the formation of a composite oxide containing Zr and Ti. When Si is contained in an amount exceeding 0.50%, the composition of the oxide changes, the HAZ structure is not refined, and the HAZ toughness is reduced. A more preferable upper limit of the Si amount is 0.40% or less, and a still more preferable upper limit is 0.30% or less.

(Mn: 0.30% to 2.50%)
Mn has an effect of enhancing the hardenability of the steel sheet and is an effective component for securing strength and toughness. If the amount of Mn is less than 0.30%, strength and toughness cannot be obtained due to insufficient hardenability. However, if Mn is contained exceeding 2.50%, the toughness of the center segregation part is lowered due to Mn segregation during solidification, and the hardenability is excessively increased, leading to an increase in hardness of both the base material and HAZ. to degrade. The preferable lower limit of the amount of Mn is 0.60% or more, and the preferable upper limit is 2.00% or less.

(Ti: 0.003% to 0.024%)
Ti forms not only a single oxide of Ti but also a complex oxide with Zr. In particular, this composite oxide functions as an intragranular ferrite formation nucleus in the HAZ and contributes to refinement of the HAZ structure. In order to obtain this effect, it is necessary to contain 0.003% or more of Ti. On the other hand, Ti produces nitride, but if Ti nitride is produced in a large amount, the amount of B nitride produced is suppressed, and the desired effect in this embodiment cannot be obtained. Further, excess Ti forms TiC and degrades the toughness of the base material and HAZ. Therefore, the upper limit of the Ti amount needs to be 0.024% or less. The preferable lower limit of the Ti amount is 0.005% or more, and the preferable upper limit is 0.020% or less.

(B: 0.0005% to 0.0050%)
B combines with nitrogen in the steel sheet to form a film-like B nitride around the composite oxide containing Zr and Ti. By making the amount of B 0.0005% or more, the intragranular ferrite-forming ability in HAZ is enhanced and contributes to the improvement of toughness through refinement of the structure. Further, the solid solution B segregates at the austenite grain boundaries, thereby suppressing the formation of coarse grain boundary ferrite. In order to further improve the HAZ toughness, the B content is preferably 0.0010% or more. On the other hand, when the amount of B is excessive, the effect of increasing the strength is saturated, and the tendency of deterioration of toughness becomes remarkable in both the base material and HAZ. Therefore, the B amount is set to 0.0050% or less. The upper limit with preferable B amount is 0.0030% or less, More preferably, it is 0.0025% or less, More preferably, it is 0.0020% or less.

(N: 0.0010% to 0.0090%)
N is an element necessary for bonding with B in the steel sheet to form B nitride, and for this purpose, it is necessary to contain 0.0010% or more of N. On the other hand, when the amount of N is excessive, the toughness of the base material and the HAZ is deteriorated, so the upper limit is made 0.0090% or less. The preferable lower limit of the N amount is 0.0020% or more, and the preferable upper limit is 0.0060% or less.

(O: 0.0010% to 0.0050%)
O (oxygen) is an element indispensable for producing a composite oxide containing Zr and Ti, and it is necessary to contain 0.0010% or more of O. However, when the amount of O is excessive, oxides are generated excessively, deteriorating the cleanliness of the steel sheet and adversely affecting the base material toughness and ductility such as stretch drawing. For this reason, the upper limit of the amount of O is made 0.0050% or less. The preferable lower limit of the amount of O is 0.0015% or more, and the preferable upper limit is 0.0040% or less.

(Zr: 0.0005% to 0.0100%)
Zr is an element indispensable for fine dispersion of oxides and increase of solid solution B, and it is necessary to contain it in an amount of 0.0005% or more. Zr oxide, a composite oxide of Zr and Ti functions as an intragranular ferrite formation nucleus in HAZ and contributes to refinement of the HAZ structure. In order to acquire this effect, it is necessary to make Zr 0.0005% or more. Preferably it is 0.0010% or more, More preferably, it is 0.0015% or more. On the other hand, if Zr is excessive, nozzle clogging may occur during casting, so the upper limit is made 0.0100% or less. A preferable upper limit is 0.0050% or less, More preferably, it is 0.0040% or less.

(Sol. Zr: 0% to 0.0020%)
Sol. Zr stands for acid-soluble Zr and represents Zr that is solid-solved in the steel sheet. Sol. When the Zr content increases, the HAZ toughness is remarkably deteriorated, so the upper limit must be limited to 0.0020% or less. Sol. The upper limit with preferable Zr is 0.0010 mass% or less, and a more preferable upper limit is 0.0005 mass% or less. Sol. Since Zr is preferably as small as possible, the lower limit is not particularly defined and may be 0%. Sol. Zr can be measured by electrolytic extraction residue analysis. The electrolytic extraction residue analysis method is a method in which a matrix is dissolved in a steel plate by electrolysis in a non-aqueous solvent, and residues (precipitates and inclusions) are extracted with a filter having a pore size of 0.1 μm to 0.2 μm and separated. is there. After separation, the amount of Zr contained in the solution was determined as Sol. This is the amount of Zr. Insol. Zr is acid-insoluble Zr. Zr amount and Sol. The amount obtained by adding the amount of Zr is the amount of Zr.

(Cu: 0.1% to 1.5%)
Since Cu has an effect of improving strength and corrosion resistance, Cu is contained by 0.1% or more. More preferably, the amount of Cu is 0.2% or more. On the other hand, even if Cu is contained in excess of 1.5%, the performance improvement commensurate with the increase in the alloy cost is not seen, which may cause the steel sheet surface crack. Preferably, the upper limit of the Cu amount is 1.0% or less, more preferably 0.5% or less.

(Ni: 0.1% to 3.0%)
Ni has the effect of increasing the toughness of the steel matrix (dough) in the solid solution state. In order to acquire the effect containing Ni, 0.1% or more of Ni is contained. On the other hand, even if Ni is contained exceeding 3.0%, improvement in characteristics commensurate with an increase in alloy cost cannot be obtained. Preferably, the upper limit of the Ni amount is 2.0% or less, more preferably 1.0% or less.

(Al: 0.0055% to 0.0550%)
Al is an important deoxidizing element, and the lower limit is made 0.0055% or more. The minimum with preferable Al amount is 0.0060% or more, More preferably, it is 0.0065% or more, More preferably, it is 0.0070% or more. On the other hand, when the amount of Al is excessive, formation of coarse cluster-like alumina (Al ₂ O ₃ ) -based inclusions is promoted, and the toughness of the base material and the HAZ is deteriorated. Therefore, the upper limit value of the Al amount is 0.0550% or less. The upper limit with preferable Al amount is 0.0500% or less, More preferably, it is 0.0400% or less, More preferably, it is 0.0300% or less.

(P: 0.050% or less)
P is unavoidably present in the steel sheet as an impurity. However, if the amount of P exceeds 0.050%, it not only segregates at the austenite grain boundaries and lowers the toughness, but also causes hot cracking during welding. The upper limit with the preferable amount of P is 0.030% or less, More preferably, it is 0.010% or less. Since the lower the amount of P is, the lower limit is not particularly specified. However, from the viewpoint of manufacturing cost, it may be 0.001% or more.

(S: 0.0080% or less)
S is unavoidably present in the steel sheet as an impurity. If the amount of S is too large, a large amount of MnS stretched in the center segregation part is generated, and the toughness and ductility of the base material and HAZ deteriorate. For this reason, the upper limit of the amount of S is made 0.0080% or less. The upper limit with preferable S amount is 0.0050% or less, More preferably, it is 0.0040% or less, More preferably, it is 0.0030% or less. Since the lower the amount of S, the better. The lower limit is not particularly specified, but may be 0.0001% or more from the viewpoint of manufacturing cost.

  The steel plate according to this embodiment may contain one or more of the following elements in place of part of Fe.

(Cr: 0% to 1.0%)
Cr increases corrosion resistance and is useful for improving strength by improving hardenability, so it may be contained in the steel sheet as necessary. In order to effectively obtain the effect of containing Cr, it is preferable to contain 0.1% or more of Cr. On the other hand, even if Cr is contained exceeding 1.0%, the effect of improving the corrosion resistance is saturated, and the HAZ is hardened and the toughness may be deteriorated. Preferably, the upper limit of Cr content is 0.5% or less.

(Mo: 0% to 1.00%)
Mo has an effect of improving the strength and toughness of the base material, and may be contained in the steel sheet as necessary. In order to effectively obtain the effect of containing Mo, it is preferable to contain 0.01% or more of Mo. On the other hand, when Mo is contained exceeding 1.00%, the hardness of HAZ is particularly increased and the toughness may be deteriorated. Preferably, the upper limit of the Mo amount is 0.50% or less, more preferably 0.30% or less.

(Nb: 0% to 0.035%)
Since Nb improves the strength and toughness of the base material by refining and carbide precipitation, it may be contained in the steel sheet as necessary. In order to effectively obtain the effect of containing Nb, it is preferable to contain 0.005% or more of Nb. On the other hand, when Nb is contained exceeding 0.035%, the effect is saturated and the toughness of the HAZ may be impaired. More preferably, the upper limit of the Nb amount is 0.025% or less, and further preferably 0.015% or less.

(V: 0% to 0.10%)
V has an effect of improving the strength of the base material mainly by precipitation of carbonitride during tempering, and may be contained in the steel sheet as necessary. In order to effectively obtain the effect of containing V, it is preferable to contain 0.01% or more of V. On the other hand, when V is contained exceeding 0.10%, the effect is saturated, the hardness is increased, and the toughness may be deteriorated. Preferably, the upper limit of the V amount is 0.05% or less.

(Ca + REM [total of Ca and REM]: 0% to 0.0005%)
Ca and REM are elements that easily react with oxygen more preferentially than Al. In order to form a composite oxide containing desired Zr and Ti, the total content of Ca and REM (Ca + REM) is limited to 0.0005% or less. More preferably, Ca is less than 0.0003% and REM is less than 0.0003%, and the total content is 0.0005% or less. Since Ca and REM are preferably as small as possible, the lower limit is not particularly defined and may be 0%.
In addition, since Ca and REM act as strong deoxidation elements in the steel sheet and inhibit the formation of oxides of Zr and Ti, it is necessary to reduce them as much as possible without intentionally containing them.
Here, “REM” is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to the total content of one or more elements of REM.

(Carbon equivalent Ceq .: 0.35% to 0.50%)
The steel plate according to the present embodiment has a carbon equivalent Ceq. Obtained by the following formula (3). Is 0.35% to 0.50%.
Ceq. = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (3)
Here, each component is the mass% of each component contained in the steel sheet. When there is an element having a content of 0% by mass, calculation is performed by substituting 0% by mass as the content of the corresponding element in the formula (1).

  When the carbon equivalent is less than 0.35%, the strength required for the high-strength steel sheet cannot be satisfied. If the carbon equivalent exceeds 0.50%, the hardenability becomes excessive and the joint toughness cannot be satisfied. The lower limit of the carbon equivalent is preferably 0.37%, more preferably 0.39%. The upper limit of the carbon equivalent is preferably 0.48%, more preferably 0.46%, and still more preferably 0.44%.

  The steel plate excellent in the weld heat affected zone toughness of the present embodiment contains each of the above elements, and the balance is made of Fe and impurities. An impurity means the component mixed by raw materials and other factors, such as an ore and a scrap, when manufacturing a steel plate industrially.

  In addition, in the actual manufacturing process, the added elements are not necessarily contained in 100% molten steel. Therefore, extra elements may be added in consideration of the yield so that each element has a desired content. . The addition method is not particularly limited. Any method may be used as long as the chemical composition can be contained in the steel sheet so as to satisfy the above conditions.

  Although it does not specifically limit as plate | board thickness of a steel plate, For example, it is 55 mm or more. Moreover, 55 mm-80 mm are mentioned.

  Next, the preferable manufacturing method for obtaining the steel plate which concerns on this embodiment is demonstrated.

A preferred method for producing a steel sheet according to this embodiment is as follows:
In secondary refining in a reduced-pressure atmosphere, the amount of dissolved oxygen in molten steel is adjusted to 0.0005% to 0.0100% in mass%, and the molten steel after adjusting the amount of dissolved oxygen contains Ti, Al, and Zr. After adding in the order of Ti, Al, and Zr, casting the molten steel after addition of Ti, Al, and Zr to obtain a slab,
A heating step of heating the steel slab after the casting step in a temperature range of 1000 ° C to 1150 ° C;
The steel slab after the heating step is rolled in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction is 50% or more, and the temperature after 1 sec from the completion of finish rolling is the rolling start temperature −80 ° C. to the rolling start temperature. A rolling process for rolling to + 80 ° C .;
Water cooling is started when the steel sheet after the rolling process is in a temperature range of 650 ° C. to 850 ° C., and the cooling rate at a position of 5 mm from the steel plate surface and a 1/4 position in the plate thickness direction is 3 ° C./sec to 30 ° C. / Sec and a cooling step for stopping water cooling in a temperature range where the surface temperature is 500 ° C. or lower,
Have In addition, a slab (steel piece) has the above-mentioned chemical composition.
Hereinafter, each step will be described.

(Casting process)
In order to obtain the steel sheet according to the present embodiment, the amount of dissolved oxygen before the start of deoxidation is controlled as described above.
Specifically, in secondary refining in a reduced-pressure atmosphere, molten steel after adding Ti, Al, and Zr in this order to molten steel whose dissolved oxygen content was adjusted to 0.0005% to 0.0100% in mass%. Cast. The amount of dissolved oxygen in the molten steel in secondary refining in a reduced-pressure atmosphere may be adjusted after adding at least one of Al and C, or may be adjusted without adding Al and C. Ti, Al, and Zr may be added in any form of a single metal or an alloy.
In addition, the method of performing secondary refining is not particularly limited, but for example, a method by RH (Ruhrstahl-Heraeus) can be mentioned.

  Here, it is important to add Ti, Al, and Zr in this order to the molten steel in which the dissolved oxygen content is adjusted to 0.0005% to 0.0100% by mass%. By adding in this order, first, an oxide A mainly containing Zr and Ti is produced during casting, and at least a part of the periphery of the oxide A (using the oxide A mainly containing Ti and Zr as a nucleus) ( 30% or more), an oxide B mainly containing Al and Ti is produced.

Examples of the method for obtaining a slab (steel piece) include the following method.
For example, in secondary refining in a reduced pressure atmosphere performed by a vacuum refining apparatus or a refining apparatus in an inert gas after converter refining, the dissolved oxygen content of molten steel is 0.0005% to 0.0100% in mass%. Adjust to the range. Thereafter, Ti, Al, and Zr are added in this order, and the molten steel is adjusted to have the above-described chemical composition. And a slab (steel piece) is obtained by continuous casting or the like.
As described above, in secondary refining, at least one of Al and C may be added to the molten steel before adjusting the dissolved oxygen content of the molten steel, and Al and C may not be added.
Moreover, about the addition method of each above-mentioned element, if it can be contained in a steel plate so that a chemical composition may satisfy the said conditions, it will not specifically limit.

  Then, the suitable conditions of each process for manufacturing the steel plate which concerns on this embodiment are demonstrated.

(Heating process)
First, a steel slab having the predetermined chemical composition described above is heated at 1000 ° C. to 1150 ° C. and held at the heating temperature for a certain period of time. The holding time is not particularly specified as long as the trace alloy element (for example, Nb in the case of containing Nb) is uniformly dissolved, but it is preferably performed for 30 minutes to 500 minutes, for example. The holding time is a time from when the temperature reaches 20 ° C. lower than the set furnace temperature until extraction. The heating temperature is defined as the average temperature during that time.

(Rolling process)
Next, the steel slab after the heating process is rolled. First, in order to effectively refine the γ grains (austenite grains) generated by heating by recrystallization, the steel slab is subjected to rough rolling. Rough rolling is preferably performed in a temperature range of 900 ° C. or higher.

  After rough rolling, finish rolling is subsequently performed on the steel sheet. This process is an extremely important process that determines the effective crystal grain size, ferrite fraction, and hardness distribution that contribute to arrestability in combination with the cooling process described later.

  In finish rolling, rolling is started at a temperature range of 650 ° C. to 850 ° C. of the surface temperature of the steel sheet 1 sec before the finish rolling (hereinafter sometimes referred to as “rolling start temperature”). In this temperature range, the rolling reduction is 50% or more, and the temperature after 1 second from the completion of rolling (hereinafter sometimes referred to as “finishing temperature”) is the rolling start temperature −80 ° C. to the rolling start temperature + 80 ° C. Finish rolling.

  The lower limit of the rolling start temperature is preferably 680 ° C, more preferably 700 ° C. The upper limit of the rolling start temperature is preferably 830 ° C, more preferably 800 ° C. When the rolling start temperature is 650 ° C. or higher, the strength is easily secured, and when it is 850 ° C. or lower, the arrestability and the base material toughness are easily secured.

  A preferable range of the finishing temperature is a range of rolling start temperature −50 ° C. to rolling start temperature + 50 ° C., and a more preferable range is a range of rolling start temperature −40 ° C. to rolling start temperature + 40 ° C.

The lower limit of the cumulative rolling reduction is preferably 55% or more, more preferably 57% or more, and further preferably 60% or more. The upper limit is not particularly limited, but the cumulative rolling reduction is preferably 80% or less in order to prevent the cooling start temperature from becoming too low.
The rolling reduction in the rolling process represents the cumulative rolling reduction in finish rolling. Cumulative rolling reduction is expressed by (input side plate thickness of the first pass−outside plate thickness of the last pass) / input side plate thickness of the first pass) × 100 (%) in a plurality of passes in a predetermined temperature range. The

(Cooling process)
After finishing rolling is completed, water cooling is started from a temperature of the plate surface of 650 ° C. to 850 ° C., the cooling rate at the 5 mm position and the 1/4 position from the steel plate surface is 3 ° C./sec to 30 ° C./sec, and the surface Water cooling is stopped at a temperature range of 500 ° C. or lower.
When the cooling start temperature is 650 ° C. or higher, the strength of the base material is easily secured. When the temperature at which finish rolling is performed (finish rolling temperature) is 850 ° C. or less, the finish rolling temperature does not become too high, and it becomes easy to ensure the toughness of the base material.
Further, when the cooling rate at the 5 mm position and the 1/4 position from the steel sheet surface is 3 ° C./sec or more, it is easy to secure the base material strength. Moreover, since hardness distribution can be controlled as it is 30 degrees C / sec or less, the hardness of 5 mm position and 1/4 position from a steel plate surface does not become excess, and it becomes easy to ensure arrestability.
In addition, the cooling rate of 5 mm position and 1/4 position from the steel plate surface can be controlled, for example, by adjusting the amount of cooling water and the plate passing speed. The above cooling rate can be calculated by calculation from the cooling water amount, the sheet passing speed, the thermal conductivity of the steel plate, and the like.

Furthermore, when the cooling stop temperature is 500 ° C. or lower, the strength is easily secured, and the effective crystal grain size is easily refined. Or it becomes easy to ensure arrestability because pearlite produces | generates in 5% or less of range.
The steel plate according to the present embodiment is obtained by the above manufacturing method.

(Heat treatment process)
The preferable manufacturing method of the steel plate which concerns on this embodiment may have further the heat processing process which reheats to the steel plate after a cooling process at the temperature of 300 to 600 degreeC.
A heat treatment process is a process of performing reheating (tempering heat treatment) with respect to the steel plate which passed through the cooling process in order to adjust the strength and toughness of the steel plate. When the reheating temperature is 300 ° C. or higher, ductility and toughness are easily improved, and when it is 600 ° C. or lower, a decrease in arrestability can be suppressed. The lower limit of the heat treatment temperature may be 400 ° C.

  In addition, the manufacturing method of the steel plate which concerns on this embodiment is not limited to the above-mentioned manufacturing method. Even if the manufacturing method of a steel plate is a manufacturing method other than the above, if the steel plate is within a specified range, the steel plate is considered to be included in the range of the steel plate according to the present embodiment.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be modified to be applied within a range that can be adapted to the above or the gist described below. These are all possible and are within the scope of the present invention.

Tables 1 and 2 show the chemical components of the steel sheet. Here, Sol. Zr represents acid-soluble Zr. Sol. Zr is separated by extracting the residue (precipitates and inclusions) with a 0.1 μm pore size filter by dissolving the parent phase by electrolysis in a non-aqueous solvent by electrolytic extraction residue analysis. The amount of Zr contained in the solution is measured. In Tables 1 and 2, Sol. The location where Zr is represented by “−” is determined by Sol. Indicates that Zr was not measured. And Insol. Zr represents acid-insoluble Zr. Insol. The amount of Zr is calculated from the amount of Zr by Sol. It can be obtained by subtracting the amount of Zr. B _AsBN is calculated by Equation (2), _{B F} is calculated by the equation (1), Ceq. Was determined by equation (3).

  Tables 3 and 4 show the amount of dissolved oxygen before Ti deoxidation in the RH vacuum refining equipment, the order of addition of Ti, Al, and Zr, and the presence or absence of prior addition of Al and C. Moreover, heating conditions, rolling conditions, cooling conditions, and heat treatment conditions (tempering temperature) are shown. However, in the “dissolved oxygen amount before Ti deoxidation” column in Table 4, steel 56 indicates the dissolved oxygen amount before Zr deoxidation in the RH vacuum refining equipment.

Tables 5 and 6 show the plate thickness, effective crystal grain size, ferrite fraction, pearlite fraction, and bainite fraction.
Moreover, the content ratio of the mass conversion value of Al oxide is 5% to 70% or less, the content ratio of the mass conversion value of Zr oxide is 5% to 70% or less, and the total content ratio of the mass conversion value of Ti oxide Represents the number density of oxides satisfying 5% to 70% and an equivalent circle diameter of 0.5 μm to 10 μm.
Furthermore, the number ratio of the specific oxide to the whole oxide is shown.
Note that, in the specific oxide, the oxide A and the oxide B are in contact with each other, the Zr oxide content of the oxide A is larger than the Zr oxide content of the oxide B, and the oxide A The Al oxide content is less than the Al oxide content of oxide B, the length of contact between oxide A and oxide B / the outer periphery of oxide A is 30% to 100%, The ratio of the length in contact with the base iron / the oxide satisfying that the outer periphery of the oxide B is 30% to 100%.
And base material toughness, base material strength, welding conditions (heat input), arrest index, and HAZ toughness are shown.
In Tables 5 and 6, “α fraction” represents an area fraction of ferrite, “B fraction” represents an area fraction of bainite, and “P fraction” represents an area fraction of pearlite.
“Table 5” represents the 5 mm portion of the steel sheet surface, and “t / 4” represents the t / 4 portion in the thickness direction.

  Steel strips obtained by melting so that the chemical components of the steel plates have the values shown in Tables 1 and 2 were subjected to the conditions shown in Tables 3 and 4 as follows, and the plate thickness was 55 mm to 80 mm. Each steel plate was manufactured.

Steel 1 to 27 are examples, and steel 28 to steel 56 are comparative examples.
Steel is melted in a 400-ton converter and deoxidized during vacuum degassing in secondary refining by RH (Ruhrstahl-Heraeus). Adjust the dissolved oxygen before adding Ti, Al, and Zr to the values shown in Tables 3 and 4, and then add Ti, Al, and Zr in the order of addition shown in Tables 3 and 4. The deoxidation was performed. Then, after casting into a 280 mm-360 mm thickness slab by continuous casting, it manufactured as a steel plate with a plate thickness of 55 mm-80 mm through each process of heating, rolling, and cooling on the conditions shown in Tables 3 and 4. Thereafter, heat treatment was performed as necessary to adjust the material. The temperature of the temper during the heat treatment was 440 ° C. to 570 ° C. Some steels added at least one of Al and C to the molten steel before adjusting the dissolved oxygen.
The obtained steel plate was welded and used for each test. The heat input under welding conditions is 40 kJ / mm to 60 kJ / mm.

The effective grain size, pearlite fraction, ferrite fraction, and bainite fraction were measured by the following procedure.
First, a method for measuring the effective crystal grain size will be described. Test specimens were sampled from the center of the width of the steel sheet, 5 mm part of the steel sheet surface and 1/4 position in the sheet thickness direction, and the surface perpendicular to the rolling direction was mirror-polished, and the surface was 500 μm × 500 μm in 1 μm area by EBSD method Measured with pitch. A boundary where the crystal orientation difference between adjacent grains is 15 ° or more is defined as a crystal grain boundary, and a weighted average of equivalent circle diameters (diameters) in a region surrounded by the crystal grain boundary is defined as an effective crystal grain size. The weighted average was obtained by the above-described equation (11).
The pearlite fraction is obtained by taking a test piece from the center of the width of the steel sheet and a 1/4 position in the thickness direction, mirror-polishing the surface perpendicular to the rolling direction, performing nital corrosion, and using an optical microscope, a magnification of 500 times 4 fields of view were taken, the perlite fraction of each field of view was determined, and the average value was taken as the perlite fraction. The size of one visual field is 200 μm × 200 μm. Further, pearlite was obtained by performing image analysis on the assumption that it looks like a blocky black when it corroded at night.
The ferrite has a KAM (Kernel Average Misoration) value of 1 ° or less when the measurement points measured by the EBSD method are close to each other in the first proximity, and the area fraction of this ferrite is t / 4 parts. Asked.
The bainite fraction was the remainder of the pearlite fraction and the ferrite fraction.

Inclusion investigation was measured by the following procedure. First, a thermal cycle test piece having a plate thickness direction of 12 mm, a plate width direction of 12 mm, and a rolling direction of 70 mm was collected from the width center of the steel plate and a 1/4 position in the plate thickness direction. And after heat-maintaining at 1400 degreeC for 25 second, the cross section of the direction perpendicular | vertical to the rolling direction of the steel plate cooled on the conditions of the cold speed of 1 degree-C / sec was grind | polished. The surface of the thermal cycle test piece as mirror-polished was measured by SEM / EDX (scanning electron microscope / energy dispersive X-ray spectroscopy) analysis using “JXA-8530F” manufactured by JEOL. The observation conditions were an acceleration voltage of 15 kV, a current of 89 μA to 91 μA, an observation visual field area of 90 mm ^{2 to} 100 mm ² , and an analysis number of 500 or more. The analysis target elements were O, Zr, Ti, and Al.

FIG. 1 shows an example of the observation result. In FIG. 1, 11 is a ground iron, and 12 is an observed inclusion. Table 7 shows the mass% for each target element when the inclusion shown in FIG. 1 is analyzed (see the entire column in Table 7). Note that the total mass% of O, Ti, Zr, and Al is 100%. Here, an inclusion having an O mass% of 1.0 mass% or more was defined as an oxide. Then, from a single oxides of these _{elements, Ti 2 O 3, ZrO 2} , and _Al 2 _{O 3} the following formula weight conversion value of oxide of each element, assuming the (5) to formula (7) calculate.
Ti ₂ O ₃ = Ti × 3.003 (5)
ZrO ₂ = Zr × 1.351 (6)
Al ₂ O ₃ = Al × 3.779 (7)
Table 8 shows the mass-converted values of the oxides of each element (see the entire column in Table 8).

With respect to the total of the mass conversion values of these oxides, the content ratio of the mass conversion value of the Al oxide is 5% to 70% or less and the content ratio of the mass conversion value of the Zr oxide is 5% to 70 as an average composition. %, And the total content of Ti oxides in terms of mass, satisfying 5% to 70%, the number of oxides whose equivalent circle diameter is not less than 0.5 μm and not more than 10 μm The density was determined.
Ti ₂ O ₃ content ratio (%) = Ti ₂ O ₃ / (Ti ₂ O ₃ + ZrO ₂ + Al ₂ O ₃ ) (8)
The content of _{ZrO 2 (%) = ZrO 2} / (Ti 2 O 3 + ZrO 2 + Al 2 O 3) ··· (9)
Al ₂ O ₃ content ratio (%) = Al ₂ O ₃ / (Ti ₂ O ₃ + ZrO ₂ + Al ₂ O ₃ ) (10)
The calculation results of the average composition of this oxide are shown in Table 9 (see the entire column in Table 9).

Next, as an average composition, the content ratio (%) of Al ₂ O ₃ (Al oxide) is 5% to 70%, the content ratio of ZrO ₂ (Zr oxide) is 5% to 70% or less, and Ti _2. The composition analysis of the surface of the oxide which satisfies 5 to 70% or less of the total content (%) of O ₃ (Ti oxide) was performed.

Using the sample as it was mirror-polished, the number density of the oxide was determined by SEM / EDX (scanning electron microscope / energy dispersive X-ray spectroscopy) analysis using “JXA-8530F” manufactured by JEOL. The observation conditions were an acceleration voltage of 15 kV, a current of 89 μA to 91 μA, an observation visual field area of 90 mm ^{2 to} 100 mm ² , and an analysis number of 20. The analysis target elements were O, Ti, Zr, and Al.

  FIG. 1 shows an example of the observation result. In FIG. 1, A is a region A in the inclusion 12, and B is a region B in the inclusion 12. A shows a relatively bright cotton trust, and B shows a darker cotton trust. This means that the composition of inclusions varies from place to place. Therefore, composition analysis was performed on regions having different contrasts.

Table 7 shows the mass% for each target element in the region A and the region B when the inclusion shown in FIG. 1 is analyzed (see the region A column and the region B column in Table 7). Table 8 shows mass-converted values of oxides of the respective elements in the regions A and B, and Table 9 shows the content ratio (%) of Al ₂ O ₃ , ZrO ₂ , and Ti ₂ O ₃ in the regions A and B. The result calculated | required using Formula (5)-Formula (10) is shown (refer the area | region A column and the area | region B column of Table 8 and Table 9).
As shown in Table 9, in the oxide shown in FIG. 1, the Zr oxide content in the oxide constituting the region A is smaller than the Zr oxide content in the oxide constituting the region B. It exceeds 1 time. Further, the content of the Al oxide in the oxide constituting the region A is less than 1 times the content of the Al oxide in the oxide constituting the region B. The Al content in the oxide constituting the region A is less than the Al content in the oxide constituting the region B.
And the ratio of the length which the oxide which comprises the area | region A and the oxide which comprises the area | region B contact | abut with respect to the outer periphery of the oxide which comprises the area | region A satisfied the range of 30%-100%. Moreover, the ratio of the length which the oxide which comprises the area | region B and the ground iron contact | connect with respect to the outer periphery of the oxide which comprises the area | region B was satisfying the range of 30%-100%.

  A similar analysis was performed on the remaining 19 inclusions. Further, of the 30 oxides analyzed, the ratio of the length of the contact between the oxide constituting the region A and the oxide constituting the region B to the outer periphery of the oxide constituting the region A is 30% to 100 % Of oxides satisfying the range of 30% to 100% in which the ratio of the length of contact between the oxide constituting the region B and the ground iron to the outer periphery of the oxide constituting the region B is satisfied. Was measured, and the number ratio was determined.

The base material toughness was in accordance with JIS Z 2242 (2005), and 2 mm V-notch Charpy impact test specimens were taken from the direction parallel to the rolling direction at the 5 mm position and 1/4 position below the sheet thickness direction. The test piece was tested three times in the range of 0 ° C. to −140 ° C. to determine the brittle ductile transition temperature (vTrs). A material having vTrs of −40 ° C. or lower is considered to have excellent base material toughness.
The base material strength was based on JIS Z 2241 (2011), and tensile test specimens were collected from the direction perpendicular to the rolling direction at 1/4 position in the thickness direction. Two each of the tensile test pieces were measured by test and the average value was obtained. The tensile test piece was a No. 4 test piece of JIS Z 2241 (2011).
The NRL drop weight test was performed according to ASTM E208. A test piece was taken from a direction parallel to the rolling direction so that an embrittled bead can be imparted to a half position (t / 2) in the sheet thickness direction, and a Nil-Ductility Transition (non-ductile transition) temperature (hereinafter, Which may be referred to as “NDTT”).
By substituting the brittle ductile transition temperature (vTrs) at the lower 5 mm position obtained by the above test, the NDTT at the 1/2 position in the sheet thickness direction, and the sheet thickness into the above formula (4), The index was calculated.

The HAZ toughness conforms to the NK class steel ship rule M-part welding (2015), and the welding direction is parallel to the width direction (so that it is perpendicular to the rolling direction). Gas arc welding was performed. Welding is performed while using SB-60VT (manufactured by Nippon Steel & Sumikin Welding Co., Ltd.) as the backing material under the condition that the groove angle of the groove shape is 20 ° and the interval between the groove-shaped tips is 8 mm. As the wire, EG-47T (manufactured by Nippon Steel & Sumikin Welding Co., Ltd.) was used. The amount of heat input during welding is 40 kJ / mm to 60 kJ / mm.
And according to NK class steel ship rule K knitting material (2015), the position of U4 test piece from the direction perpendicular to the welding line direction from the front side of the plate thickness to the plate thickness center direction 6mm (bottom), Three pieces are sampled centering on the position (t / 2) at the center of the plate thickness and the position 6mm from the back side of the plate thickness in the direction of the center of the plate thickness (under the back), and a 2mm V notch is processed in the fusion line (boundary part). Created. The test was performed three times under the condition of a test temperature of −20 ° C., and from this average value, the absorbed energy (vE-20) of HAZ was determined for each position under the table, t / 2, and under the back. The absorption energy (vE-20) of HAZ at each position below the table, t / 2, and under the back was evaluated as having excellent HAZ toughness at 100 J or more.

In order to evaluate arrestability, a full thickness test piece (size: t (plate thickness) x 500 mm x 500 mm) was used based on the Japan Welding Association standard WES 2815 (2014) "Brittle crack arrest toughness test method". Then, a temperature gradient type ESSO test was conducted. The temperature at which the arrest toughness value Kca becomes 6000 N / mm ^1.5 , that is, T _Kca6000 was determined. And it evaluated that _TKca6000 is -10 degrees C or less excellent in _{arrestability} .

As is clear from Tables 1 to 6, Steel 1 to Steel 27 all had excellent HAZ toughness when high heat input welding was performed. Moreover, it had the outstanding mechanical characteristic in the base material which is parts other than HAZ and a weld metal part.
On the other hand, since the steel 28-steel 56 are outside the range prescribed | regulated with the steel plate which concerns on this embodiment, when large heat-input welding was performed, the outstanding HAZ toughness was inferior. Moreover, even if it was excellent in HAZ toughness, the mechanical characteristics in the base material which is a part other than HAZ and the weld metal part were inferior.

  The steel plate according to the present embodiment is a steel plate having excellent toughness in the weld heat affected zone when performing high heat input welding and having excellent mechanical properties in the base material. Therefore, according to the steel plate concerning this embodiment, while improving safety, highly efficient welding is possible and it becomes possible to reduce the construction cost of a welded structure dramatically.

11 Railways, 12 Inclusions

Claims (6)

% By mass
C: 0.01% to 0.20%,
Si: 0.02% to 0.50%,
Mn: 0.30% to 2.50%,
Ti: 0.003% to 0.024%,
B: 0.0005% to 0.0050%,
N: 0.0010% to 0.0090%,
O: 0.0010% to 0.0050%,
Zr: 0.0005% to 0.0100%,
Sol. Zr: 0% to 0.0020%,
Cu: 0.1% to 1.5%,
Ni: 0.1% to 3.0%,
Al: 0.0055% to 0.0550%,
P: 0.050% or less,
S: 0.0080% or less,
Nb: 0% to 0.035%
Cr: 0% to 1.0%
Mo: 0% to 1.00%,
V: 0% to 0.10%
Ca + REM: 0% to 0.0005% or less, and the balance: a chemical composition consisting of Fe and impurities,
_BF represented by the following formula (1) is 0.0005% to 0.0030%,
Carbon equivalent represented by the following formula (3) Ceq. Is 0.35% to 0.50%,
In crystal orientation analysis using electron beam backscatter diffraction (EBSD) at 1/4 position in the thickness direction of the cross section perpendicular to the rolling direction, the effective crystal grain size is 30 μm or less,
The microstructure at the 1/4 position in the plate thickness direction is the area ratio, the ferrite fraction is 20% to 70%, the bainite fraction is 30% to 75%, and the pearlite fraction is 0% to 5%.
The arrest index represented by the following formula (4) is -10 or less,
As the average composition, the Ti, Zr, and Zr when assumed to be a single oxide of Ti, Zr and Al elements, obtained from the measured values of O amount, Ti amount, Zr amount, and Al amount of the whole oxide The content ratio of the mass converted value of the Al oxide is 5% to 70% or less, and the content ratio of the mass converted value of the Zr oxide is 5% to 70% with respect to the total mass converted value of the oxide of each element of Al. Below, the total content of Ti oxides in terms of mass is within the range of 5% to 70%, and the number density with the equivalent circle diameter of 0.5 μm to 10 μm is 10 / mm ² or more. Containing
Of the oxides, 10% or more of the total number of the oxides includes the oxide A having the following composition A and the oxide B having the following composition B, and the oxide A and the oxidation Object B is in contact with each other,
Composition A: The content ratio of the mass-converted value of the Al oxide is 0% to 60% or less, and the content ratio of the mass-converted value of the Zr oxide is 15% to 95 with respect to the sum of the mass-converted values of the oxides of the respective elements. % Or less, and the content ratio of the mass converted value of the Ti oxide is 15% to 95% or less Composition B: The content ratio of the mass converted value of the Al oxide to the total of the mass converted values of the oxides of the respective elements is 15 % To 95% or less, the content ratio of the mass conversion value of the Zr oxide is 0% to 60% or less, and the content ratio of the mass conversion value of the Ti oxide is 15% to 95% or less. Zr content of the oxide A Is Zr (A), the mass ratio is Zr (A) / Zr (B)> 1 when the Zr content of the oxide B is Zr (B), and the Al content of the oxide A is Al (A ), And the mass ratio when the Al content of the oxide B is Al (B) is Al (A) / Al B) satisfies the <1,
The ratio of the length of contact between the oxide A and the oxide B to the outer periphery of the oxide A (the length of contact between the oxide A and the oxide B / the outer periphery of the oxide A) is 30%. ~ 100%
The ratio of the length at which the oxide B and the base iron are in contact with the outer periphery of the oxide B (the length at which the oxide B and the base iron are in contact / the outer periphery of the oxide B) is 30% to 100%. A certain steel plate.

(However, in the formula (1), B _asBN is represented by the formula (2). Further, B is the content (mass%) of the B element contained in the steel plate, and 0 ≦ B _F ≦ B. Fulfill.)

(However, in the formula (2), the relationship of 0 ≦ B _asBN ≦ B is satisfied, and N, Ti, O, and Al are the contents of each element of N, Ti, O, and Al contained in the steel sheet ( Insol.Zr represents the content (mass%) of acid-insoluble Zr.)
Ceq. = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (3)
(However, C, Mn, Cu, Ni, Cr, Mo, and V in Formula (3) represent the content (mass%) of each element contained in the steel sheet.)
Arrest index = 0.34 × t + 0.40 × vTrs (table) + 0.12 × NDTT (t / 2) (4)
(In the formula (4), t is the plate thickness [mm], vTrs (table) is the brittle ductile transition temperature [° C.] in the Charpy impact test in the 5 mm position below the table and in the direction parallel to the rolling direction, and NDTT (T / 2) represents the Nil-Ductility Transition (non-ductile transition) temperature in the Naval Research Laboratory drop weight test at a position 1/2 of the sheet thickness direction from the steel sheet surface. The temperature at which the plate thickness is 55 mm or more, the yield stress of the base material, which is a portion other than the weld heat affected zone and the weld metal portion, is 460 MPa or more, and the arrest toughness value Kca is 6000 N / mm ^1.5 is − The steel sheet according to claim 1, which is 10 ° C or lower.   Absorbed energy of Charpy impact test in which the weld heat affected zone generated when large heat input welding is performed at a heat input of 40 kJ / mm to 60 kJ / mm when the plate thickness is 55 mm to 80 mm at a test temperature of −40 ° C. Is 100 J or more at all positions on the front side of the plate thickness, the position of the plate thickness center (t / 2), and the back side of the plate thickness in the plate thickness direction, and other than the weld heat affected zone and the weld metal portion The steel plate according to claim 1 or 2, wherein the brittle ductile transition temperature of the base material, which is a part, is -40 ° C or lower. A method for producing the steel sheet according to any one of claims 1 to 3,
In secondary refining in a reduced-pressure atmosphere, at least one of Al and C is added to the molten steel, and the dissolved oxygen content in the molten steel is adjusted to 0.0005% to 0.0100% by mass%, and the dissolved oxygen content is adjusted. A casting process in which Ti, Al, and Zr are added to the molten steel after being added in the order of Ti, Al, and Zr, and then the molten steel after addition of Ti, Al, and Zr is cast to obtain a slab;
A heating step of heating the steel slab after the casting step in a temperature range of 1000 ° C to 1150 ° C;
The steel slab after the heating step is rolled in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction is 50% or more, and the temperature after 1 sec from the completion of finish rolling is the rolling start temperature −80 ° C. to the rolling start temperature. A rolling process for rolling to + 80 ° C .;
Water cooling is started when the steel sheet after the rolling process is in a temperature range of 650 ° C. to 850 ° C., and the cooling rate at a position of 5 mm from the steel plate surface and a 1/4 position in the plate thickness direction is 3 ° C./sec to 30 ° C. / Sec and a cooling step for stopping water cooling in a temperature range where the surface temperature is 500 ° C. or lower,
The manufacturing method of the steel plate which has. A method for producing the steel sheet according to any one of claims 1 to 3,
In secondary refining in a reduced-pressure atmosphere, after adjusting the amount of dissolved oxygen in the molten steel to 0.0005% to 0.0100% in mass% without adding Al and C to the molten steel A casting step of adding Ti, Al, and Zr to the molten steel in the order of Ti, Al, and Zr, and then casting the molten steel after addition of Ti, Al, and Zr to obtain a slab;
A heating step of heating the steel slab after the casting step in a temperature range of 1000 ° C to 1150 ° C;
The steel slab after the heating step is rolled in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction is 50% or more, and the temperature after 1 sec from the completion of finish rolling is the rolling start temperature −80 ° C. to the rolling start temperature. A rolling process for rolling to + 80 ° C .;
Water cooling is started when the steel sheet after the rolling process is in a temperature range of 650 ° C. to 850 ° C., and the cooling rate at a position of 5 mm from the steel plate surface and a 1/4 position in the plate thickness direction is 3 ° C./sec to 30 ° C. / Sec and a cooling step for stopping water cooling in a temperature range where the surface temperature is 500 ° C. or lower,
The manufacturing method of the steel plate which has.   Furthermore, the manufacturing method of the steel plate of Claim 4 or Claim 5 which has a heat processing process which reheats the steel plate after the said cooling process to the temperature of 300 to 600 degreeC.