JP6828638B2 - Steel plate and steel plate manufacturing method - Google Patents

Description

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

Examples of the use of the steel plate include ships, high-rise buildings, other buildings, bridges, marine 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 the load weight of container ships. Along with this, it is required to increase the thickness and strength of the steel sheet. Further, it is necessary to ensure the safety and reliability of the welded portion as well, and the toughness of the weld heat-affected zone (hereinafter, may be referred to as “HAZ”) (hereinafter, “weld heat-affected zone”). "Toughness" may be referred to as "HAZ toughness"). Further, even if a brittle crack is generated at a welded joint, the steel sheet is required to have the ability to stop the brittle crack at the base metal (hereinafter, may be referred to as "arrestability").

Conventionally, it has been known that the grain size of austenite (γ), the transformed structure, the hardness of HAZ, the coarse hard phase, etc. have a great influence on the HAZ toughness of a high-strength steel plate, and various measures have been proposed. ing. Of these, miniaturization of the HAZ structure is the most effective for improving the HAZ toughness, and many methods utilizing inclusions have been proposed.

The miniaturization 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 produces ferrite. Intragranular transformation is a method in which ferrite is generated in austenite grains, which have been coarsened by the heat effect during welding, with inclusions as nuclei to refine the structure. So far, in addition to nitrides such as TiN and sulfides such as MnS, techniques for utilizing oxides that are chemically stable even at high temperatures as ferrite nucleation 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, which is a nucleus of intragranular transformation (hereinafter, may be referred to as "IGF nucleus") on a steel sheet containing substantially no Al. We propose a method of making the structure of the weld heat-affected zone finer by finely dispersing. In the method disclosed in Patent Document 1, Ti and Zr are added at the same time and the balance of Ti, Zr and O amounts is optimized in order to generate a composite oxide of Ti and Zr that effectively acts as an IGF nucleus. It has become.

The technique disclosed in Patent Document 2 proposes a method for improving HAZ toughness by inclusions containing REM and Zr by adding REM, Zr and Ti to a steel sheet containing substantially no Al. It is a thing.

The technique disclosed in Patent Document 3 proposes a method of improving HAZ toughness by inclusions containing Zr, REM and Ca by adding REM, Ca and Zr to an Al-containing steel sheet. Is.

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

The technique disclosed in Patent Document 5 refines HAZ by pinning effect by TiN and intragranular transformation by BN, suppresses softening of HAZ by utilizing the improvement of hardenability by B, and improves toughness. It proposes a method.

Japanese Unexamined Patent Publication No. 01-159356 Japanese Unexamined Patent Publication No. 2008-291347 Japanese Unexamined Patent Publication No. 2014-001432 Japanese Unexamined Patent Publication No. 2001-342537 JP-A-2007-177327

When the present inventors examined the techniques disclosed in the above-mentioned Patent Documents 1 to 5, the following findings were obtained.
As a result of examining the technique disclosed in Patent Document 1, in order to generate a composite oxide of Ti and Zr, Ti and Zr are added at the same time, and the balance of Ti amount, Zr amount and O amount is optimized. It was found that it was not enough to further improve the HAZ toughness.

As a result of examining the technique disclosed in Patent Document 2, it was found that REM is more strongly deoxidizing than Zr and inhibits oxide formation of Zr and Ti.

As a result of examining the technique disclosed in Patent Document 3, it was found that both REM and Ca are more strongly deoxidized than Zr and inhibit the production of Zr oxide.

As a result of examining the technique disclosed in Patent Document 4, in large heat-affected welding, the portion adjacent to the weld metal is exposed to a high temperature for a long time, so that even if the oxide is finely dispersed, the austenite grains become coarse. It was found that the deterioration of HAZ toughness was not suppressed because it could not be suppressed.

As a result of examining the technique disclosed in Patent Document 5, in the large heat-affected zone, the portion adjacent to the weld metal is exposed to a high temperature for a long time, so that TiN utilizing the pinning effect disappears by solid dissolution, and HAZ It was found that the deterioration of toughness was not suppressed.

By the way, the welding construction cost accounts for a large amount of the total construction cost of the welded structure, and in order to reduce this cost, it is required to perform high-efficiency welding. Specifically, it is effective to perform welding with a large amount of heat and reduce the number of welding passes. However, when welding with large heat input is performed, the HAZ structure of the steel sheet becomes coarse and deterioration of toughness is unavoidable.

Conventionally, dispersed particles such as inclusions in a steel plate have been used to improve HAZ toughness. However, in order to improve the welding efficiency, it has been difficult to stably improve the HAZ toughness of the steel sheet when the large heat input welding exceeding 40 kJ / mm is performed. The causes of this are, for example, that inclusions such as oxides are easily aggregated in the molten steel and are difficult to be uniformly dispersed in the steel sheet, and that the inclusions are deteriorated by being exposed to a high temperature for a long time during high heat input welding. It is considered that it is difficult to control so that it easily acts as a nucleus of intragranular transformation.

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

The present invention is the result of diligent studies focusing on oxides and solid solution B, which are intragranular transformation nuclei, as intragranular ferrite-forming nuclei capable of miniaturizing the structure of HAZ in steel sheets containing Al. , The present invention has been completed by finding that the above problems can be solved.

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: having a chemical composition of Fe and impurities.
_BF represented by the following formula (1) is 0.0005% to 0.0030%.
Carbon equivalent Ceq. Represented by the following formula (3). However, it is 0.35% to 0.50%,
In the crystal orientation analysis using the electron backscatter diffraction method (EBSD) at the 1/4 position in the plate 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 expressed by the following formula (4) is -10 or less,
As an average composition, the Ti, the Zr, and the said Ti, the said Zr, and the said Ti, the said Zr, and the said | said Ti, the said Zr, and the said Ti The content ratio of the mass conversion value of Al oxide is 5% to 70% or less, and the content ratio of the mass conversion value of Zr oxide is 5% to 70% with respect to the total mass conversion value of the oxides of each element of Al. Oxide in which the total content of Ti oxides in terms of mass is 5% to 70% or less, the equivalent circle diameter is 0.5 μm to 10 μm, and the number density is 10 pieces / mm ² or more. Contains things,
Of the oxides, 10% or more of the oxides as a whole has an oxide A having the following composition A and an oxide B having the following composition B, and the oxide A and the oxidation It is in contact with object B and
Composition A: The content ratio of the mass-converted value of Al oxide to the total mass-converted value of the oxide of each element is 0% to 60% or less, and the content ratio of the mass-converted value of Zr oxide is 15% to 95. % Or less, and the content ratio of the mass conversion value of Ti oxide is 15% to 95% or less Composition B: The content ratio of the mass conversion value of Al oxide to the total mass conversion value of the oxides of the above elements is 15. % To 95% or less, the content ratio of the mass-converted value of Zr oxide is 0% to 60% or less, and the content ratio of the mass-converted value of Ti oxide is 15% to 95% or less. Zr content of the oxide A. Is Zr (A), the mass ratio when the Zr content of the oxide B is Zr (B) is Zr (A) / Zr (B)> 1, and the Al content of the oxide A is Al (A). ), The mass ratio when the Al content of the oxide B is Al (B) satisfies Al (A) / Al (B) <1.
The ratio of the length of contact between the oxide A and the oxide B (the length of contact between the oxide A and the oxide B / the outer circumference of the oxide A) with respect to the outer circumference of the oxide A is 30%. ~ 100%,
The ratio of the length of contact between the oxide B and the base iron (the length of contact between the oxide B and the base iron / the outer circumference of the oxide B) with respect to the outer circumference of the oxide B is 30% to 100%. A steel plate.

(However, in the formula (1), _BasBN is represented by the formula (2). B is the content (mass%) of the element B contained in the steel sheet, and has a relationship of 0 ≦ _BF ≦ 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 (mass) of each element of N, Ti, O, and Al contained in the steel sheet. %), And 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 the formula (3) represent the content (mass%) of each element contained in the steel sheet.

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

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

(2)
The temperature at which the plate thickness is 55 mm or more, the yield stress of the base metal, which is the part other than the weld heat affected zone and the weld metal part, is 460 MPa or more, and the arrest toughness value Kca is 6000 N / mm ^1.5 is-. The steel plate according to (1), which has a temperature of 10 ° C. or lower.

(3)
When the plate thickness is 55 mm to 80 mm, the absorbed energy of the Charpy impact test is performed at the test temperature of -40 ° C for 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. However, in the plate thickness direction, it is 100 J or more at all points on the front side of the plate thickness, the position of the center of the plate thickness (t / 2), and the back side of the plate thickness, and other than the weld heat affected zone and the weld metal portion. The steel plate according to (1) or (2), wherein the brittle weldable transition temperature of the base metal, which is a portion, is −40 ° C. or lower.

(4)
The method for manufacturing a steel sheet according to any one of (1) to (3).
In the secondary refining in a reduced pressure atmosphere, at least one of Al and C is added to the molten steel, and the amount of dissolved oxygen in the molten steel is adjusted by mass% to 0.0005% to 0.0100% to adjust the amount of dissolved oxygen. After adding Ti, Al, and Zr to the molten steel in the order of Ti, Al, and Zr, the molten steel after adding Ti, Al, and Zr is cast to obtain a slab.
A heating step of heating the steel pieces after the casting step in a temperature range of 1000 ° C. to 1150 ° C.
Rolling of the steel pieces after the heating step is started in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction ratio is 50% or more, and the temperature 1 sec after the completion of finish rolling is the rolling start temperature -80 ° C to the rolling start temperature. A rolling process that carries out rolling at + 80 ° C and
Water cooling is started when the steel sheet after the rolling step is in the temperature range of 650 ° C. to 850 ° C., and the cooling rate at the position 5 mm from the surface of the steel sheet and the 1/4 position in the plate thickness direction is 3 ° C./sec to 30 ° C. A cooling step that stops water cooling in a temperature range of / sec and a surface temperature of 500 ° C or less,
A method for manufacturing a steel sheet having.

(5)
The method for manufacturing a steel sheet according to any one of (1) to (3).
In the secondary refining in a reduced pressure atmosphere, the amount of dissolved oxygen in the molten steel is adjusted to 0.0005% to 0.0100% by mass% without adding Al and C to the molten steel, and then the amount of dissolved oxygen is adjusted. Ti, Al, and Zr are added to the molten steel 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 pieces after the casting step in a temperature range of 1000 ° C. to 1150 ° C.
Rolling of the steel pieces after the heating step is started in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction ratio is 50% or more, and the temperature 1 sec after the completion of finish rolling is the rolling start temperature -80 ° C to the rolling start temperature. A rolling process that carries out rolling at + 80 ° C and
Water cooling is started when the steel sheet after the rolling step is in the temperature range of 650 ° C. to 850 ° C., and the cooling rate at the position 5 mm from the surface of the steel sheet and the 1/4 position in the plate thickness direction is 3 ° C./sec to 30 ° C. A cooling step that stops water cooling in a temperature range of / sec and a surface temperature of 500 ° C or less,
A method for manufacturing a steel sheet having.

(6)
The method for producing a steel sheet according to (4) or (5), further comprising a heat treatment step of reheating the steel sheet after the cooling step to a temperature of 300 ° C. to 600 ° C.

According to the present embodiment, there is provided a steel sheet having excellent toughness in HAZ when subjected to large heat-affected zone and having excellent mechanical properties in a base material other than HAZ and the weld metal part. it can.

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

Hereinafter, an example of a preferred embodiment of the present invention will be described in detail.
In the present specification, the numerical range represented by using "~" means a range including the numerical values before and after "~" as the lower limit value and the upper limit value.

Conventionally, it is known that when Ti oxide and B nitride are dispersed in a weld metal and HAZ, intragranular ferrite is generated and the structure is refined. Further, conventionally, it is known that the solid solution B segregating at the old austenite grain boundaries of a steel sheet suppresses the formation of coarse grain boundary ferrites during welding and improves the HAZ toughness.
However, Zr is not an element generally added to a steel sheet, and research conducted in the past as a steel sheet to which Zr is added has been very limited. So far, the effect of solid solution B on improving HAZ toughness when a Zr-containing oxide (particularly an oxide containing Zr and Ti) is dispersed in a steel sheet has not been investigated.

In the steel sheet containing Al, the present inventors have focused on oxides, solid solution B, and B nitrides, which are nucleation of intragranular ferrite capable of refining the structure of HAZ. .. As a result, new findings were obtained mainly on the following composition and number density of (A) oxide, (B) solid solution Zr, (C) solid solution B, (D) deoxidation method, and (E) microstructure. It was.
Hereinafter, these new findings will be described.

(A): Oxide composition and number density The present inventors investigated in detail the oxide that is the core of the intragranular ferrite for each oxide, 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 as an average composition with respect to the total mass conversion value of Ti oxide, Zr oxide, and Al oxide, and has a specific circle-equivalent diameter (the diameter of a circle when assumed to be circular). Contains oxides with (equivalent). Among these oxides, there is an oxide A containing Zr and Ti as main components, and an oxide B having a higher Zr oxide content and a lower Al oxide content than the oxide A. .. In this 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 ground iron. It is an oxide that comes into contact with.

Specifically, the average composition includes an oxide having the following composition with respect to the total mass-converted values of Ti oxide, Zr oxide, and Al oxide. The total mass-converted value of Ti oxide, Zr oxide, and Al oxide is 100%.
Content ratio of Al oxide in terms of mass: 5% to 70%
Content of Zr oxide in terms of mass: 5% to 70%
Content ratio of Ti oxide in terms of mass: 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 preferable. Is 55% or less.)

The oxide having the above average composition has a circle-equivalent diameter of 0.5 μm to 10 μm and a number density of 10 pieces / mm ² or more.
If the equivalent circle diameter is smaller than 0.5 μm, the function of the intragranular ferrite as a nucleation nucleation (IGF nuclei) deteriorates. 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 fracture increases. The number density of oxides having the above composition having a circle equivalent diameter of 0.5 μm to 10 μm is 10 pieces / mm ² or more (preferably 20 pieces / mm ² or more, more preferably 30 pieces / mm ² or more). In the case of 50 pieces / mm ² or more, more preferably 60 pieces / mm ² or more), it is clear that the HAZ toughness is improved by miniaturizing the HAZ structure as compared with the steel plate containing no Zr. It became. 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 of the oxides have oxides A and B which are in contact with each other. Further, the oxide A has the following composition A, and the oxide B has the following composition B. In the following composition A and the following composition B, the total 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, and the oxide A When the Al content is Al (A) and the Al content of the oxide B is Al (B), the mass ratio satisfies Al (A) / Al (B) <1.

Composition A: Each of Ti, Zr, and Al when it is assumed that the oxide is a single oxide of the elements of Ti, Zr, and Al, which is obtained from the measured values of the O amount, Ti amount, Zr amount, and Al amount of the entire oxide. For the total mass conversion of elemental oxides
Content of Zr oxide in terms of mass: 15% to 95%
Content ratio of Ti oxide in terms of mass: 15% to 95%
Content ratio of Al oxide in terms of mass: 0% to 60%

Composition B; Each of Ti, Zr, and Al when it is assumed that the oxide is a single oxide of the elements of Ti, Zr, and Al, which is obtained from the measured values of the O amount, Ti amount, Zr amount, and Al amount of the entire oxide. For the total mass conversion of elemental oxides
Content ratio of Zr oxide in terms of mass: 0% to 60%
Content ratio of Ti oxide in terms of mass: 15% to 95%
Content ratio of Al oxide in terms of mass: 15% to 95%

Further, the ratio of the length in which the oxide A and the oxide B are in contact with the outer circumference of the oxide A (the length in which the oxide A and the oxide B are in contact / the outer circumference of the oxide A) is 30% to 100%. I am satisfied. Further, the ratio of the length of contact between the oxide B and the base iron (the ratio of the length of contact between the oxide B and the base iron / the outer circumference of the oxide B) with respect to the outer circumference of the oxide B is 30% to 100%. I am satisfied.

Of the oxides having the above average composition, 10% or more of the oxides have oxides A and B that satisfy the above conditions, so that the nucleation of intragranular ferrite (IGF) It turned out 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.
The number ratio is the number of oxides (referred to as oxide 1) satisfying the above average composition, the equivalent circle diameter, and the number density of the analysis targets, and the oxides A and B (referred to as oxide 1) satisfying the above conditions. It is expressed as a percentage of the number of (referred to as a specific oxide) [(the number of oxides 1 / the number of specific oxides)].

Here, when the oxide (including oxide A and oxide B) is out of the range of the above conditions, it does not become a nucleation of intragranular ferrite. When Zr was not contained, the number of intragranular ferrite nucleating nuclei (IGF nuclei) decreased, and HAZ toughness deteriorated. In addition, when Zr became excessive, Zr clusters were formed, IGF nuclei decreased as well, and HAZ toughness could not be secured.

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

Specifically, first, a thermal cycle test piece having a thickness direction of 12 mm, a plate width direction of 12 mm, and a rolling direction of 70 mm is collected at the center of the width of the steel plate and at a position of t / 4 in the plate thickness direction. Then, after heating and holding the test piece at 1400 ° C. for 25 seconds, the cross section of the steel plate cooled at a cooling speed of 1 ° C./sec in the direction perpendicular to the rolling direction is mirror-polished, and SEM / EDX (scanning electron microscope) is used. / Energy dispersive X-ray spectroscopy) Observe by analysis, and quantitatively analyze and measure inclusions found in the observation field. 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 base iron and 12 represents an inclusion. As shown in the photograph shown in FIG. 1, for inclusions 12 that appear to be granular due to the difference in color tone (contrast) with respect to the base iron 11 (background), the average composition of the entire inclusions is quantitatively analyzed for each of these inclusions. To do.

The size of the inclusions to be analyzed is 0.5 μm to 10 μm in the equivalent circle diameter (diameter), and at least 500 or more 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 X-ray intensity obtained from the inclusions to be analyzed and the element concentration contained in the inclusions to be analyzed are quantified from the calibration curve. Among the inclusions, those judged to be oxides shall have a clearly recognized peak of oxygen, and the lower limit thereof depends on the measurement conditions and the measuring device.
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 mass% of the O content, the Ti content, the Zr content, and the Al content is obtained, and when the O content is 1.0% by mass or more with respect to the total, this inclusion is oxidized. Make it a thing. Then, for this oxide, using the following formulas (5) to (7), from the mass% of each element, the mass conversion value of the oxide of each element assuming that it is a single oxide of 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 the formulas (5) to (7), Ti, Zr, and Al are the contents (mass%) of each element measured by SEM / EDX analysis. The total content of each element measured by these SEM / EDX analyzes is 100% by mass.
The total of the mass-converted values of Ti ₂ O ₃ , ZrO ₂ , and Al ₂ O ₃ obtained from the formulas (5) to (7) was obtained, and the ratio of the oxide of each element to the total was included in the oxide. The content ratio (%) of oxides of each element.

The content ratios of Ti ₂ O ₃ , ZrO ₂ , and Al ₂ O ₃ are represented by the following formulas (8) to (10).
Ti ₂ O ₃ content ratio (%) = Ti ₂ O ₃ / (Ti ₂ O ₃ + ZrO ₂ + Al ₂ O ₃ ) ... (8)
ZrO ₂ content ratio (%) = 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 above oxides whose number density was measured, the region corresponding to the above oxide A (in the composition image of SEM, the region exhibiting a relatively bright (for example, white) cotton trust among the oxides). And Ti _{2 in} the region corresponding to the oxide B in contact with at least a part of the oxide A (in the composition image of SEM, the region of the oxide exhibiting the coton trust darker (for example, black) than the base iron). Measure the O ₃ , ZrO ₂ , and Al ₂ O ₃ content ratios and the number ratio. The number of the oxide A and the oxide B in contact with the oxide A to be analyzed shall be at least 20 of the oxides whose number density has been measured. Then, the sum of the mass% of the O content, the Ti content, the Zr content, and the Al content is obtained, and the above formulas (5) to (10) are used to obtain the Al oxide, the Zr oxide, and the Al oxide, the Zr oxide, and the Al oxide. Determine the amount of Ti oxide.

Further, from the structure image of SEM, the length of contact between the region corresponding to oxide A and the region corresponding to oxide B is measured, the outer peripheral length of the region corresponding to oxide A is measured, and the region corresponding to oxide A is measured. The value obtained by dividing the length in contact with the oxide B by the outer circumference of the oxide A (the length in contact between the oxide A and the oxide B / the outer circumference of the oxide A) is calculated. Then, the length of contact between the region corresponding to oxide B and the base iron is measured, the outer peripheral length of the region corresponding to oxide B is measured, and the length of contact between the region corresponding to oxide B and the base iron. Is divided by the outer peripheral length of the region corresponding to the oxide B (ratio of the length in which the oxide B and the base iron are in contact with each other / the outer circumference of the oxide B) is calculated.

(B): Solid solution Zr (Sol.Zr)
As a condition of the oxide containing Zr that contributes to the miniaturization of the HAZ structure, it is necessary that the oxide contains a certain amount or more of Zr. On the other hand, Zr (solid-dissolved Zr (solid-dissolved Zr is referred to as "Sol.Zr") that does not form an oxide and remains on the steel sheet significantly deteriorates the toughness of not only HAZ but also the steel sheet itself. It is necessary to reduce Sol.Zr. The smaller the Sol.Zr, the better the toughness tends to be. In order to obtain a steel sheet having excellent HAZ toughness, Sol.Zr may be limited to 0.0020% by mass or less. It is important. For further improvement of HAZ toughness, it is preferable to limit it to 0.0010% by mass or less (more preferably 0.0005% by mass or less). Here, Sol.Zr is acid-soluble Zr. It corresponds to Zr dissolved in a steel sheet, which can be measured by an electrolytic extraction residue analysis method or the like. The acid-insoluble Zr is Insol.Zr (Insol.Zr in the formula (2)). The amount of Zr in the steel sheet is the total amount of acid-soluble Zr and acid-insoluble Zr.

(C): Solid solution B ( _BF )
The solid solution B segregating at the old austenite grain boundaries of the steel sheet suppresses the formation of coarse grain boundary ferrites 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, solid melt B is increased as compared with a steel sheet in which an oxide containing no Zr is dispersed. The mass% ( _BF ) of the solid solution B in the steel sheet containing the Zr oxide in the oxide is obtained by subtracting the mass% of B as the B nitride from the content of B contained in the steel sheet. That is, _BF is represented by the following equation (1). When this value is 0.0005% or more (preferably 0.0010% or more), the effect of improving HAZ toughness by the solid solution B can be obtained. If _BF becomes excessive, there is a concern that HAZ toughness deteriorates. Therefore, the upper limit of _BF is set to 0.0030% or less. It is preferably 0.0025% or less, more preferably 0.0020% or less.

However, B in the formula (1) is the content (mass%) of B contained in the steel sheet, and _BasBN is the mass% of B that becomes the B nitride. Further, _BF satisfies the relationship of 0 ≦ _BF ≦ B.
Further, when _BF <0, _BF = 0, and when _BF > B, _BF = B. That is, _when the value of B _asBN is a negative value, _BF = B, and when the value of B _asBN is larger than B, _BF = 0.

In the steel sheet, Ti acts as a nitride forming element in addition to B. However, Ti also forms oxides. Therefore, in order to obtain _BasBN , it is necessary to consider the formation of inclusions including oxides and nitrides.

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

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

However, N, Ti, O, and Al in the formula (2) are the contents (mass%) of each element of N, Ti, O, and Al contained in the steel sheet, and Insol. Zr is the content (% by mass) of acid-insoluble Zr. B _asBN satisfies the relationship of 0 ≦ B _asBN ≦ B with B in the formula (1).
If B _asBN <0, then B _asBN = 0, and if B _F > B, then B _asBN = B. That is, _when the value of B _asBN is a negative value, B _asBN = 0, and when the value of B _asBN is larger than B in the equation (1), B _asBN = B.
In addition, Sol. Zr is an acid-soluble Zr, which is a Zr content (% by mass) dissolved in a steel sheet measured by an electrolytic extraction residue analysis method or the like. Insol. Zr is the content (% by mass) of acid-insoluble Zr, and from the Zr content, Sol. It is obtained by subtracting the Zr content. In addition, 0 ≦ Insol. Satisfy Zr ≦ Zr.
In the steel sheet according to the present embodiment, _BasBN may have a relationship of _BasBN ≤ 0 in that the _added B is _fully utilized as the solid solution B and the grain boundary ferrite is suppressed. That is, it is preferable that the _BasBN represented by the above formula (2) is 0% or less.

(D): Deoxidizing method Oxide particles are generated when deoxidizing molten steel. This is called a primary oxide. Further, during casting and solidification, as the molten steel temperature decreases, oxides containing Al, Zr, and Ti are produced. This is called a secondary oxide. In this embodiment, either a primary oxide or a secondary oxide may be used. However, it is preferable to use a secondary oxide because finer particles can be obtained from the oxide generated as the molten steel temperature decreases during casting and solidification than the primary oxide generated when the molten steel temperature is high. ..

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

The oxides finally remaining in the steel plate by this step have an average composition of 5% to 70% of the mass-converted value of Al oxide and 5% of the mass-converted value of Zr oxide. The total content ratio of ~ 70% and the mass conversion value 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 oxides of 0.5 μm to 10 μm is 10 pieces / mm ² or more.
Furthermore, it was found that 10% or more of these oxides as the total number of oxides are oxides that satisfy the following conditions.
It has an oxide A containing Zr and Ti and an oxide B containing Al and Ti, and the oxide A and the oxide B are in contact with each other.
Further, 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 length in which the oxide A and the oxide B are in contact with each other / the ratio of the outer circumference of the oxide A is 30% to 100%, and the ratio of the length in which the oxide B and the base iron are in contact with each other / the ratio of the oxide B. The outer circumference) is 30% to 100%.

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

(E): Microstructure The base metal 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 coexist, the basic structure unit is objectively defined and its size is measured only by observing the structure with a normal optical microscope (hereinafter, may be referred to as "light microscopic observation"). It's very difficult to do. Therefore, the present inventors performed crystal orientation analysis using an electron backscatter diffraction method (EBSD: Electron Backscatter Diffraction pattern) in addition to photovisual observation, and analyzed the microstructure.
When referred to as a base material in the present specification, the base material indicates a portion other than the HAZ and the weld metal portion.

More specifically, a sample for microstructure observation is taken from a plate thickness 1/4 position (hereinafter, may be referred to as "t / 4 part") in the plate thickness direction from the surface of the steel plate, and the sample is taken in the main rolling direction. The cross section in the vertical direction (width direction) is mirror-polished. Then, nital corrosion was performed on the sample of t / 4 parts, four fields of view were photographed at 500 times using an optical microscope, the pearlite fraction of each field of view was measured, and the average value was the pearlite component of t / 4 parts. It was a rate.
Further, for each part of the t / 4 part, a region of 500 μm × 500 μm was measured at a pitch of 1 μm by the EBSD method with respect to the cross section in the direction (width direction) perpendicular to the main rolling direction. A boundary with a crystal orientation difference of 15 ° or more from adjacent grains is defined as a crystal grain boundary, and the weighted average of the equivalent circle diameter (diameter) of the region surrounded by the crystal grain boundaries is defined as the effective crystal grain size of each site. did.

Ferrite was defined as a portion where the KAM (Kernel Average Measurement) value when the measurement points measured by the above EBSD method were first close to each other was 1 ° or less. The surface integral of this ferrite was determined for each part of t / 4 parts.
The bainite fraction of t / 4 part was the balance other than the pearlite fraction of t / 4 part and the ferrite fraction of t / 4 part. That is, the total of the bainite fraction of t / 4 parts, the pearlite fraction of t / 4 parts, and the ferrite fraction of t / 4 parts is 100% in area ratio.
The weighted average was calculated by the following method. Assuming that there are N crystal grains in one field of view, the area of each crystal grain is A ₁ , A ₂ , A ₃ , ... A _i , ... _AN , and the equivalent circle diameter (diameter) of each grain. ) is _{D 1, D 2, D 3} , ··· D i, and a · · · _{D N.} In that case, the effective crystal grain size (Deff) is calculated by the following formula (11).

As a result of investigating the relationship between the toughness of the base metal in t / 4 parts and the microstructure, the brittle ductility transition temperature of the base material (hereinafter referred to as "vTrs") as the effective crystal grain size of t / 4 parts becomes finer. There is.) Has cooled down. It was revealed 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 the effective crystal grain size was more than 30 μm and the pearlite fraction of t / 4 part was more than 5%, vTrs exceeded −40 ° C., and the toughness of the base metal could not be ensured. In order to secure the toughness of the base metal, it is preferable that the pearlite fraction is low, and the fraction may be 0%.
As for the effective crystal grain size of t / 4 part, in this embodiment, the effective crystal grain size of 1 μm or more was measured. The effective crystal grain size of t / 4 part should be as small as possible, and the lower limit value is not particularly limited, but is, for example, 5 μm or more, and further 1 μm or more.

As a result of investigating the relationship between the strength of the base metal and the microstructure, the strength of the base metal of the t / 4 part improved as the ferrite fraction of the t / 4 part decreased and the bainite fraction of the t / 4 part increased. .. When the area is% and the ferrite fraction is 70% or less (preferably 65% or less, more preferably 60% or less, further 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 was more than 70%, the strength of the base metal could not be secured. It was found that the ferrite fraction was 20% or more (preferably 25% or more, more preferably 30% or more) in order for the brittle ductile transition temperature (vTrs) of the base metal to be −40 ° C. or less. In order to secure the strength of the base material, the bainite fraction needs to be 30% or more (preferably 35% or more, more preferably 40% or more, still more preferably 45% or more), and the brittle ductility transition of the base material is required. It was found that the bainite fraction was 75% or less (preferably 70% or less, more preferably 65% or less, still more preferably 60% or less) in order for the temperature (vTrs) to be −40 ° C. or less. ..

FIG. 2 shows the results of investigating the relationship between the arrest property and the plate thickness, the arrest index calculated from vTrs (table) and the NDTT (t / 2). In FIG. 2, the horizontal axis is the arrest index calculated by the following formula (4), and the vertical axis is the temperature at which the arrest toughness value Kca becomes 6000 N / mm ^1.5 (hereinafter, “ _{TKca 6000} ””. It may be called.). In FIG. 2, the result of approximating the arrest index and _TKca6000 with a linear function is shown 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 for _TKca6000 to be -10 ° C or lower (for example, -50 ° C to -10 ° C). Considering the variation in measurement, 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 of the arrest index is not particularly limited, but in consideration of an increase in rolling load during rolling for improving arrestability, a production load such as a decrease in productivity, etc., for example, -60 or more (preferably -50 or more, More preferably, -40 or more) can be mentioned.
Arrest index = 0.34 x t + 0.40 x vTrs (table) + 0.12 x NDTT (t / 2) ... (4)

Here, in the equation (4), what is represented by "t", "vTrs (table)", and "NDTT (t / 2)" is as follows.
t: Plate thickness (mm)
vTrs (table): Brittle ductility transition temperature [° C] in the Charpy impact test parallel to the rolling direction of the steel sheet surface 5 mm.
NDTT (t / 2): NDT (Nil-Ductility Transition) temperature [° C.] in the NRL (Naval Research Laboratory) drop test parallel to the rolling direction at the position of 1/2 the thickness of the steel sheet.

The NDT temperature (hereinafter, may be referred to as "NDTT") is determined by a method conforming to the NRL weight drop test specified in ASTM E208.

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

Further, the reason for limiting the chemical composition of the steel sheet of the present embodiment will be 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 amount of C is less than 0.01%, the required strength cannot be secured. However, if the amount of C exceeds 0.20%, it becomes difficult to secure toughness for both the base material and HAZ. The preferable lower limit of the amount of C is 0.03% or more, more preferably 0.05% or more. The preferable upper limit of the amount of C is 0.15% or less, more preferably 0.10% or less.

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

(Mn: 0.30% to 2.50%)
Mn has the effect of enhancing the hardenability of the steel sheet, and is an effective component for ensuring 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 in excess of 2.50%, the toughness of the central segregated portion is lowered due to Mn segregation during solidification, and the hardenability is excessively increased, resulting in an increase in hardness of both the base material and HAZ, resulting in increased toughness. 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 a composite oxide together with Zr as well as a single oxide of Ti. In particular, this composite oxide functions as an intragranular ferrite nucleation in HAZ and contributes to the miniaturization 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 nitrides, but when a large amount of Ti nitrides are produced, the amount of B nitrides produced is suppressed, and the desired effect cannot be obtained in the present embodiment. Furthermore, excess Ti forms TiC and deteriorates the toughness of the base metal and HAZ. Therefore, it is necessary to set the upper limit of the Ti amount to 0.024% or less. The preferable lower limit of the amount of Ti 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 setting the amount of B to 0.0005% or more, the ability to generate ferrite in the grain in HAZ is enhanced, and it contributes to the improvement of toughness through the miniaturization of the structure. Further, the solid solution B segregates at the austenite grain boundaries to suppress the formation of coarse grain boundary ferrites. In order to further improve the HAZ toughness, the amount of B 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 toughness deterioration of both the base material and HAZ becomes remarkable. Therefore, the amount of B is set to 0.0050% or less. The preferable upper limit of the amount of B is 0.0030% or less, more preferably 0.0025% or less, still more preferably 0.0020% or less.

(N: 0.0010% to 0.0090%)
N is an element required to bond with B in a steel sheet to form a B nitride, and for this purpose, it is necessary to contain 0.0010% or more of N. On the other hand, if the amount of N is excessive, the toughness of the base metal and HAZ deteriorates, so the upper limit is set to 0.0090% or less. The preferable lower limit of the amount of N 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 the formation of 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 excessively generated, which deteriorates the cleanliness of the steel sheet and adversely affects the toughness of the base material and ductility such as elongation and drawing. Therefore, the upper limit of the amount of O is set to 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 0.0005% or more. The Zr oxide and the composite oxide of Zr and Ti function as an intragranular ferrite-forming nucleus in HAZ and contribute to the miniaturization of the HAZ structure. In order to obtain this effect, Zr needs to be 0.0005% or more. It is preferably 0.0010% or more, more preferably 0.0015% or more. On the other hand, if Zr is excessive, nozzle blockage may occur during casting, so the upper limit is set to 0.0100% or less. The preferred upper limit is 0.0050% or less, more preferably 0.0040% or less.

(Sol.Zr: 0% to 0.0020%)
Sol. Zr stands for acid-soluble Zr and represents Zr dissolved in a steel sheet. Sol. As the Zr content increases, the HAZ toughness deteriorates significantly, so it is necessary to limit the upper limit to 0.0020% or less. Sol. The preferred upper limit of Zr is 0.0010% by mass or less, and the more preferable upper limit is 0.0005% by mass or less. Sol. Since the smaller the amount of Zr, the more preferable it is, the lower limit is not particularly specified 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 steel plate is electrolyzed in a non-aqueous solvent to dissolve the matrix, and the residue (precipitate and inclusions) is extracted by 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 is Sol. The amount of Zr. Insol. Zr is acid-insoluble Zr, and Insol. Zr amount and Sol. The sum of the Zr amount is the Zr amount.

(Cu: 0.1% to 1.5%)
Since Cu has an effect of improving strength and corrosion resistance, it contains 0.1% or more of Cu. More preferably, the amount of Cu is 0.2% or more. On the other hand, even if Cu is contained in an amount of more than 1.5%, the performance is not improved in proportion to the increase in alloy cost, which may cause cracks on the surface of the steel sheet. The upper limit of the amount of Cu is preferably 1.0% or less, and more preferably 0.5% or less.

(Ni: 0.1% to 3.0%)
Ni has the effect of increasing the toughness of the steel matrix (fabric) in the solid solution state. In order to obtain the effect of containing Ni, 0.1% or more of Ni is contained. On the other hand, even if Ni is contained in excess of 3.0%, the property cannot be improved in proportion to the increase in alloy cost. The upper limit of the amount of Ni is preferably 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 0.0055% or more. The preferable lower limit of the amount of Al is 0.0060% or more, more preferably 0.0065% or more, still more preferably 0.0070% or more. On the other hand, when the amount of Al is excessive, the formation of coarse cluster-like alumina (Al ₂ O ₃ ) -based inclusions is promoted, and the toughness of the base metal and HAZ is deteriorated. Therefore, the upper limit of the amount of Al is set to 0.0550% or less. The preferable upper limit of the amount of Al is 0.0500% or less, more preferably 0.0400% or less, and further preferably 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 segregates at the austenite grain boundaries and not only lowers the toughness, but also causes high-temperature cracking during welding. The preferable upper limit of the amount of P is 0.030% or less, more preferably 0.010% or less. Since the smaller the amount of P is, the more preferable it is, the lower limit is not particularly specified, but 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 central segregation portion is generated, so that the toughness and ductility of the base metal and HAZ deteriorate. Therefore, the upper limit of the amount of S is set to 0.0080% or less. The preferable upper limit of the amount of S is 0.0050% or less, more preferably 0.0040% or less, still more preferably 0.0030% or less. Since the smaller the amount of S is, the more preferable it is, the lower limit is not particularly specified, but from the viewpoint of manufacturing cost, it may be 0.0001% or more.

The steel sheet according to the present embodiment may contain one or more of each of the following elements instead of a part of Fe.

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

(Mo: 0% to 1.00%)
Since Mo has the effect of improving the strength and toughness of the base material, it may be contained in the steel sheet if necessary. In order to effectively obtain the effect of containing Mo, it is preferable to contain Mo in 0.01% or more. On the other hand, if Mo is contained in excess of 1.00%, the hardness of HAZ may be particularly increased and the toughness may be deteriorated. The upper limit of the amount of Mo is preferably 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 metal by fine granulation and precipitation of carbides, it may be contained in the steel sheet if necessary. In order to effectively obtain the effect of containing Nb, it is preferable to contain Nb in an amount of 0.005% or more. On the other hand, if Nb is contained in excess of 0.035%, the effect may be saturated and the toughness of HAZ may be impaired. More preferably, the upper limit of the amount of Nb is 0.025% or less, still more preferably 0.015% or less.

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

(Ca + REM [total of Ca and REM]: 0% to 0.0005%)
Ca and REM are elements that are more likely to react with oxygen more preferentially than Al. In order to form a composite oxide containing the 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%, REM is less than 0.0003%, and the total content thereof is 0.0005% or less. Since the smaller the amount of Ca and REM is, the more preferable it is, the lower limit is not particularly specified and may be 0%.
Since Ca and REM act as strong deoxidizing elements in the steel sheet and inhibit the oxide formation of Zr and Ti, it is necessary not to intentionally contain them and to reduce them as much as possible.
Here, "REM" is a general term 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 sheet according to this 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, 0% by mass is substituted as the content of the corresponding element in the formula (1) for calculation.

If 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 carbon equivalent is preferably 0.37%, more preferably 0.39%. The upper limit of carbon equivalent is preferably 0.48%, more preferably 0.46%, still more preferably 0.44%.

The steel sheet having excellent toughness in the weld heat affected zone of the present embodiment contains each of the above elements, and the balance is composed of Fe and impurities. Impurities mean components that are mixed in by raw materials such as ore and scrap and other factors when steel sheets are manufactured industrially.

In the actual manufacturing process, the added element is not 100% contained in the molten steel. Therefore, it is sufficient to add an extra element 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.

The thickness of the steel sheet is not particularly limited, but for example, it may be 55 mm or more. Further, 55 mm to 80 mm can be mentioned.

Next, a preferable manufacturing method for obtaining the steel sheet according to the present embodiment will be described.

A preferred method for producing a steel sheet according to this embodiment is
In the secondary refining in a reduced pressure atmosphere, the amount of dissolved oxygen in the molten steel is adjusted to 0.0005% to 0.0100% by mass%, and Ti, Al, and Zr are added to the molten steel after the amount of dissolved oxygen is adjusted. , Ti, Al, and Zr are added in this order, 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 pieces after the casting step in a temperature range of 1000 ° C. to 1150 ° C.
Rolling of the steel pieces after the heating step is started in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction ratio is 50% or more, and the temperature 1 sec after the completion of finish rolling is the rolling start temperature -80 ° C to the rolling start temperature. A rolling process that carries out rolling at + 80 ° C and
Water cooling is started when the steel sheet after the rolling step is in the temperature range of 650 ° C. to 850 ° C., and the cooling rate at the position 5 mm from the surface of the steel sheet and the 1/4 position in the plate thickness direction is 3 ° C./sec to 30 ° C. A cooling step that stops water cooling in a temperature range of / sec and a surface temperature of 500 ° C or less,
Have. The 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 the secondary refining in a reduced pressure atmosphere, the molten steel after adding Ti, Al, and Zr in this order to the molten steel whose dissolved oxygen amount was adjusted to 0.0005% to 0.0100% by mass% was added. Cast. The amount of dissolved oxygen in the molten steel in the 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 either the form of a single metal or an alloy.
The method for performing the secondary refining is not particularly limited, and examples thereof include a method using RH (Ruhrstahl-Heraeus).

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

Examples of the method for obtaining the slab (steel piece) include the method for obtaining the slab (steel piece) as follows.
For example, in secondary refining under a reduced pressure atmosphere performed by a vacuum refining device or a refining device in an inert gas after converter refining, the amount of dissolved oxygen in the molten steel is 0.0005% to 0.0100% by mass. Adjust to the range of. Then, Ti, Al, and Zr are added in this order to adjust the molten steel so as to have the above-mentioned chemical composition. Then, a slab (steel piece) is obtained by continuous casting or the like.
As described above, in the secondary refining, at least one of Al and C may be added to the molten steel before adjusting the dissolved oxygen amount of the molten steel, and Al and C may not be added.
Further, the method of adding each element described above is not particularly limited as long as the chemical composition can be contained in the steel sheet so as to satisfy the above conditions.

Subsequently, suitable conditions of each step for manufacturing the steel sheet according to the present embodiment will be described.

(Heating process)
First, a steel piece having a predetermined chemical composition described above is heated at 1000 ° C. to 1150 ° C. and held at that heating temperature for a certain period of time. The holding time may be a trace alloy element (for example, Nb when Nb is contained) may be uniformly dissolved in a solid solution, and is not particularly specified, but may be, for example, between 30 minutes and 500 minutes. The holding time is the time from reaching a temperature 20 ° C. lower than the set furnace temperature to extraction. The heating temperature is defined as the average temperature during that period.

(Rolling process)
Next, the steel pieces that have undergone the heating step are rolled. First, in order to effectively miniaturize the γ grains (austenite grains) generated by heating by recrystallization, the steel pieces are roughly rolled. Rough rolling is preferably performed in a temperature range of 900 ° C. or higher.

After rough rolling, the steel sheet is subsequently finish rolled. This step, together with the cooling step described later, is an extremely important step for determining the effective crystal grain size, ferrite fraction, and hardness distribution that contribute to arrestability.

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

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, it becomes easy to secure the strength, and when it is 850 ° C. or lower, it becomes easy to secure the arrest property and the base metal toughness.

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

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

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

Further, when the cooling stop temperature is 500 ° C. or lower, the strength is easily secured and the effective crystal grain size is easily refined. Alternatively, by generating pearlite in the range of 5% or less, it becomes easy to secure the arrest property.
The steel sheet according to the present embodiment can be obtained by the above manufacturing method.

(Heat treatment process)
A preferred method for producing a steel sheet according to the present embodiment may further include a heat treatment step of reheating the steel sheet after the cooling step at a temperature of 300 ° C. to 600 ° C.
The heat treatment step is a step of reheating (tempering heat treatment) the steel sheet that has undergone the cooling step in order to adjust the strength and toughness of the steel sheet. When the reheating temperature is 300 ° C. or higher, ductility and toughness are likely to be improved, and when the reheating temperature is 600 ° C. or lower, the decrease in arrestability can be suppressed. The lower limit of the heat treatment temperature may be 400 ° C.

The method for manufacturing the steel sheet according to the present embodiment is not limited to the above-mentioned manufacturing method. Even if the manufacturing method of the steel sheet is a manufacturing method other than the above, if the steel sheet is within the specified range, the steel sheet is considered to be included in the range of the steel sheet according to the present embodiment.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples do not have properties that limit the present invention, and may be changed to application within a range that can be adapted to the above-mentioned or later gist. It is possible, and they are all within the scope of the invention.

Tables 1 and 2 show the chemical composition of the steel sheet. Here, Sol. Zr represents an acid-soluble Zr. Sol. For Zr, the matrix was dissolved by electrolysis of the steel plate in a non-aqueous solvent by an electrolytic extraction residue analysis method, and the residue (precipitate and inclusions) was separated by filtering with a pore size of 0.1 μm, and after separation. The amount of Zr contained in the solution of is measured. In Tables 1 and 2, Sol. The part where Zr is represented by "-" is Sol. Indicates that Zr was not measured. And Insol. Zr represents an acid-insoluble Zr. Insol. The amount of Zr is determined from the amount of Zr. 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 calculated by equation (3).

Tables 3 and 4 show the amount of dissolved oxygen before Ti deoxidation in the RH vacuum smelting facility, the order of addition of Ti, Al, and Zr, and the presence or absence of prior addition of Al and C. It also shows heating conditions, rolling conditions, cooling conditions, and heat treatment conditions (temper temperature). However, in the “Dissolved oxygen amount before Ti deoxidation” column of Table 4, the steel 56 shows the dissolved oxygen amount before Zr deoxidation in the RH vacuum refining facility.

Tables 5 and 6 show the plate thickness, effective grain size, ferrite fraction, pearlite fraction, and bainite fraction.
Further, 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. Indicates the number density of oxides in which 5% to 70% or less is satisfied and the equivalent circle diameter is 0.5 μm to 10 μm.
Furthermore, the number ratio of the specific oxide to the whole oxide is shown.
In the specific oxide, the oxide A and the oxide B are in contact with each other, and the Zr oxide content of the oxide A is higher than the Zr oxide content of the oxide B. 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 circumference of oxide A is 30% to 100%, and the oxide B and It represents an oxide that satisfies the ratio of the length in contact with the base iron / the outer circumference of the oxide B is 30% to 100%.
Then, the toughness of the base metal, the strength of the base metal, the welding conditions (heat input), the arrest index, and the HAZ toughness are shown.
In Tables 5 and 6, "α fraction" represents the area fraction of ferrite, "B fraction" represents the area fraction of bainite, and "P fraction" represents the area fraction of pearlite.
“5 below the table” represents a 5 mm portion of the steel sheet surface, and “t / 4” represents a t / 4 portion in the plate thickness direction.

The steel pieces obtained by melting the steel pieces so that the chemical composition of the steel sheet has the values shown in Tables 1 and 2 are subjected to the following conditions according to the conditions shown in Tables 3 and 4, and the sheet thickness is 55 mm to 80 mm. Each steel sheet of was manufactured.

Steels 1 to 27 are examples, and steels 28 to 56 are comparative examples.
The steel is melted in a 400-ton converter and deoxidized during the vacuum degassing treatment of the secondary refining by RH (Ruhrstahl-Heraeus). Dissolved oxygen was adjusted before adding Ti, Al, and Zr so as to have the values shown in Tables 3 and 4, and then Ti, Al, and Zr were added in the order of addition shown in Tables 3 and 4. Deoxidized. Then, it was cast into a slab having a thickness of 280 mm to 360 mm by continuous casting, and then subjected to each step of heating, rolling, and cooling under the conditions shown in Tables 3 and 4, to produce a steel sheet having a thickness of 55 mm to 80 mm. Then, heat treatment was performed as necessary to adjust the material. The temper temperature during the heat treatment was 440 ° C to 570 ° C. For some steels, at least one of Al and C was added to the molten steel before adjusting the dissolved oxygen.
The obtained steel sheet was welded and subjected to each test. The heat input under the 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. Specimens are sampled from the center of the width of the steel sheet, 5 mm on the surface of the steel sheet, and 1/4 of the thickness direction, and the surface perpendicular to the rolling direction is mirror-polished. Measured at pitch. A boundary having a crystal orientation difference of 15 ° or more from adjacent grains was defined as a crystal grain boundary, and the weighted average of the equivalent circle diameter (diameter) of the region surrounded by the crystal grain boundaries was defined as the effective crystal grain size. The weighted average was calculated by the above equation (11).
For the pearlite fraction, a test piece is taken from the center of the width of the steel plate and 1/4 of the plate thickness direction, the surface perpendicular to the rolling direction is mirror-polished, nital corroded, and a magnification of 500 times is used using an optical microscope. Four fields of view were photographed with, the pearlite fraction of each field of view was obtained, and the average value was taken as the pearlite fraction. The size of one field of view is 200 μm × 200 μm. In addition, pearlite was determined to appear as a lumpy black color when it was corroded by nital, and was obtained by performing image analysis.
Ferrite is a portion where the KAM (Kernel Average Measurement) value when the measurement points measured by the above EBSD method are first close to each other is 1 ° or less, and the area fraction of this ferrite is divided into t / 4 parts. I asked for it.
The bainite fraction was the balance of the pearlite fraction and the ferrite fraction.

The inclusion survey was measured by the following procedure. First, thermal cycle test pieces having a thickness direction of 12 mm, a plate width direction of 12 mm, and a rolling direction of 70 mm were collected from the center of the width of the steel plate and a quarter position in the plate thickness direction. Then, after heating and holding at 1400 ° C. for 25 seconds, the cross section of the steel sheet cooled at a cooling speed of 1 ° C./sec in the direction perpendicular to the rolling direction was polished. The surface of the heat cycle test piece as it was 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 field area of 90 mm ^{2 to} 100 mm ² , and an number of analyzes of 500 or more. The elements to be analyzed 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% of each target element when the inclusions shown in FIG. 1 were analyzed (see the entire column of Table 7). The total mass% of O, Ti, Zr, and Al is 100%. Here, inclusions having a mass% of O of 1.0% by mass or more were used as oxides. Then, the mass conversion values of the oxides of each element assuming the single oxides of these elements, Ti ₂ O ₃ , ZrO ₂ , and Al ₂ O _3, are calculated from the following formulas (5) to (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 conversion values of the oxides of each element (see the whole column of Table 8).

As an average composition, the content ratio of the mass conversion value of Al oxide is 5% to 70% or less, and the content ratio of the mass conversion value of Zr oxide is 5% to 70% with respect to the total mass conversion value of these oxides. % Or less, and the total number of Ti oxides in terms of mass content is 5% to 70% or less, and the equivalent circle diameter of this oxide is 0.5 μm or more and 10 μm or less. The density was calculated.
Ti ₂ O ₃ content ratio (%) = Ti ₂ O ₃ / (Ti ₂ O ₃ + ZrO ₂ + Al ₂ O ₃ ) ... (8)
ZrO ₂ content ratio (%) = 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 of 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 of the surface of the oxide satisfying the total content ratio (%) of O ₃ (Ti oxide) of 5% to 70% or less was analyzed.

Using a sample as it was mirror-polished for which the number density of oxides was determined, it was measured by SEM / EDX (scanning electron microscope / energy dispersive X-ray spectroscopy) analysis by "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 field area of 90 mm ^{2 to} 100 mm ² , and an number of analyzes of 20. The elements to be analyzed were O, Ti, Zr, and Al.

FIG. 1 shows an example of the observation result. In FIG. 1, A is the region A in the inclusions 12, and B is the region B in the inclusions 12. A has a relatively bright cotton trust, and B has a relatively dark cotton trust than the ground iron. This means that the composition of inclusions varies from place to place. Therefore, the composition of each contrast region was analyzed.

Table 7 shows the mass% of each target element in the regions A and B when the inclusions shown in FIG. 1 were analyzed (see the regions A and B columns of Table 7). Table 8 shows the mass conversion values of the oxides of each element in the regions A and B, and Table 9 shows the content ratios (%) of Al ₂ O ₃ , ZrO ₂ , and Ti ₂ O ₃ in the regions A and B. , The results obtained by using the formulas (5) to (10) are shown (see the area A column and the area B column of Tables 8 and 9).
As shown in Table 9, in the oxides shown in FIG. 1, the content of Zr oxide in the oxide constituting region A is larger than the content of Zr oxide in the oxide constituting region B. It is over 1 times. Further, the content of Al oxide in the oxide constituting region A is less than 1 times the content of Al oxide in the oxide constituting region B. The Al content in the oxide constituting the region A is smaller than the Al content in the oxide constituting the region B.
The ratio of the length of contact between the oxides constituting the region A and the oxides constituting the region B with respect to the outer circumference of the oxides constituting the region A was satisfied in the range of 30% to 100%. In addition, the ratio of the length of contact between the oxide constituting the region B and the base iron with respect to the outer circumference of the oxide constituting the region B was satisfied in the range of 30% to 100%.

A similar analysis was performed on the remaining 19 inclusions. Further, out of the 30 oxides analyzed, the ratio of the length of contact between the oxides constituting the region A and the oxides constituting the region B with respect to the outer periphery of the oxides constituting the region A is 30% to 100%. The number of oxides that satisfy the range of% and the ratio of the length of contact between the oxides constituting the region B and the base iron to the outer periphery of the oxides constituting the region B is in the range of 30% to 100%. Was measured, and the number ratio was determined.

The toughness of the base metal was in accordance with JIS Z 2242 (2005), and 2 mm V notch Charpy impact test pieces were taken from the direction parallel to the rolling direction at 5 mm and 1/4 positions below the surface in the plate 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). Those having vTrs of -40 ° C or less were considered to have excellent base material toughness.
The strength of the base metal was in accordance with JIS Z 2241 (2011), and tensile test pieces were collected from the direction perpendicular to the rolling direction at a position of 1/4 in the plate thickness direction. Each two tensile test pieces were tested and measured, and the average value was calculated. The tensile test piece was JIS Z 2241 (2011) No. 4 test piece.
The NRL drop test was performed in accordance with ASTM E208. The test piece was collected from the direction parallel to the rolling direction so that the embrittlement bead could be applied to the 1/2 position (t / 2) in the plate thickness direction, and the Nil-Ductility Transition temperature (hereinafter referred to as “non-ductility transition)” (Sometimes referred to as "NDTT").
The brittle ductile transition temperature (vTrs) at a position 5 mm below the table, the NDTT at a position 1/2 in the plate thickness direction, and the plate thickness obtained by the above test are substituted into the above equation (4) to arrest. The index was calculated.

HAZ toughness conforms to NK Ship Class Steel Ship Regulation M Welding (2015), so that the welding direction is parallel to the width direction (to be perpendicular to the rolling direction), 2-electrode simple electro Gas arc welding was performed. Welding is performed using SB-60VT (manufactured by Nippon Steel & Sumikin Welding & Co., Ltd.) as a backing material under the conditions that the groove angle of the groove shape is 20 ° and the distance between the tips of the groove shape is 8 mm. As a 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.
Then, in accordance with the NK Ship Class Steel Ship Regulation K Knitting Material (2015), the U4 test piece is placed at a position 6 mm from the front side of the plate thickness to the center direction of the plate thickness (bottom of the table) from the direction perpendicular to the welding line direction. Three pieces were collected from the center of the plate thickness (t / 2) and the position of 6 mm from the back side of the plate thickness in the center direction of the plate thickness (lower back), and a 2 mm V notch was machined on the fusion line (boundary). Created. The test was carried out three times under the condition of a test temperature of −20 ° C., and the absorbed energy (vE-20) of HAZ was determined from this average value at each position of the lower front, t / 2, and lower back. Those having HAZ absorption energy (vE-20) of 100 J or more at each of the lower front, t / 2, and lower back positions were evaluated as having excellent HAZ toughness.

For the evaluation of 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) "Brittality crack arrest toughness test method". The temperature gradient type ESSO test was performed. The temperature at which the arrest toughness value Kca became 6000 N / mm ^1.5 , that is, T _{Kca 6000} was determined. Then, it was evaluated that the _TKca6000 having a temperature of −10 ° C. or lower was excellent in _{arrestability} .

As is clear from Tables 1 to 6, all of the steels 1 to 27 had excellent HAZ toughness when subjected to high heat input welding. In addition, the base metal, which is a portion other than the HAZ and the weld metal portion, had excellent mechanical properties.
On the other hand, since the steels 28 to 56 are outside the range specified by the steel sheet according to the present embodiment, the excellent HAZ toughness was inferior when the large heat input welding was performed. Further, even if the HAZ toughness was excellent, the mechanical properties of the base material, which is a portion other than the HAZ and the weld metal portion, were inferior.

The steel sheet according to the present embodiment is a steel sheet having excellent toughness in the heat-affected zone of welding when large heat input welding is performed and also having excellent mechanical properties in the base metal. Therefore, according to the steel plate according to the present embodiment, safety is improved, highly efficient welding is possible, and the construction cost of the welded structure can be dramatically reduced.

11 Ground iron, 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: having a chemical composition of Fe and impurities.
_BF represented by the following formula (1) is 0.0005% to 0.0030%.
Carbon equivalent Ceq. Represented by the following formula (3). However, it is 0.35% to 0.50%,
In the crystal orientation analysis using the electron backscatter diffraction method (EBSD) at the 1/4 position in the plate 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 expressed by the following formula (4) is -10 or less,
As an average composition, the Ti, the Zr, and the said Ti, the said Zr, and the said Ti, the said Zr, and the said | said Ti, the said Zr, and the said Ti The content ratio of the mass conversion value of Al oxide is 5% to 70% or less, and the content ratio of the mass conversion value of Zr oxide is 5% to 70% with respect to the total mass conversion value of the oxides of each element of Al. Oxide in which the total content of Ti oxides in terms of mass is 5% to 70% or less, the equivalent circle diameter is 0.5 μm to 10 μm, and the number density is 10 pieces / mm ² or more. Contains things,
Of the oxides, 10% or more of the oxides as a whole has an oxide A having the following composition A and an oxide B having the following composition B, and the oxide A and the oxidation It is in contact with object B and
Composition A: The content ratio of the mass-converted value of Al oxide to the total mass-converted value of the oxide of each element is 0% to 60% or less, and the content ratio of the mass-converted value of Zr oxide is 15% to 95. % Or less, and the content ratio of the mass conversion value of Ti oxide is 15% to 95% or less Composition B: The content ratio of the mass conversion value of Al oxide to the total mass conversion value of the oxides of the above elements is 15. % To 95% or less, the content ratio of the mass-converted value of Zr oxide is 0% to 60% or less, and the content ratio of the mass-converted value of Ti oxide is 15% to 95% or less. Zr content of the oxide A. Is Zr (A), the mass ratio when the Zr content of the oxide B is Zr (B) is Zr (A) / Zr (B)> 1, and the Al content of the oxide A is Al (A). ), The mass ratio when the Al content of the oxide B is Al (B) satisfies Al (A) / Al (B) <1.
The ratio of the length of contact between the oxide A and the oxide B (the length of contact between the oxide A and the oxide B / the outer circumference of the oxide A) with respect to the outer circumference of the oxide A is 30%. ~ 100%,
The ratio of the length of contact between the oxide B and the base iron (the length of contact between the oxide B and the base iron / the outer circumference of the oxide B) with respect to the outer circumference of the oxide B is 30% to 100%. A steel plate.

(However, in the formula (1), _BasBN is represented by the formula (2). B is the content (mass%) of the element B contained in the steel sheet, and has a relationship of 0 ≦ _BF ≦ 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 (however, Insol.Zr represents the content of acid-insoluble Zr (% by mass).)
Ceq. = C + Mn / 6 + (Cu + Ni) / 15+ (Cr + Mo + V) / 5 ... (3)
(However, C, Mn, Cu, Ni, Cr, Mo and V in the formula (3) represent the content (mass%) of each element contained in the steel sheet.)
Arrest index = 0.34 x t + 0.40 x vTrs (table) + 0.12 x NDTT (t / 2) ... (4)
(However, in equation (4), t is the plate thickness [mm], vTrs (table) is the brittle ductility transition temperature [° C] in the Charpy impact test at the position 5 mm below the table and in the direction parallel to the rolling direction, and is NDTT. (T / 2) indicates the Nil-Ductility Transition (non-ductility transition) temperature in the Naval Research Laboratory drop test at a position 1/2 of the steel plate surface in the plate thickness direction.) The temperature at which the plate thickness is 55 mm or more, the yield stress of the base metal, which is the part other than the weld heat affected zone and the weld metal part, 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, wherein the temperature is 10 ° C. or lower. When the plate thickness is 55 mm to 80 mm, the absorbed energy of the Charpy impact test is performed at the test temperature of -40 ° C for 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. However, in the plate thickness direction, it is 100 J or more at all points on the front side of the plate thickness, the position of the center of the plate thickness (t / 2), and the back side of the plate thickness, 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 ductivity transition temperature of the base metal, which is a portion, is −40 ° C. or lower. The method for manufacturing a steel sheet according to any one of claims 1 to 3.
In the secondary refining in a reduced pressure atmosphere, at least one of Al and C is added to the molten steel, and the amount of dissolved oxygen in the molten steel is adjusted by mass% to 0.0005% to 0.0100% to adjust the amount of dissolved oxygen. After adding Ti, Al, and Zr to the molten steel in the order of Ti, Al, and Zr, the molten steel after adding Ti, Al, and Zr is cast to obtain a slab.
A heating step of heating the steel pieces after the casting step in a temperature range of 1000 ° C. to 1150 ° C.
Rolling of the steel pieces after the heating step is started in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction ratio is 50% or more, and the temperature 1 sec after the completion of finish rolling is the rolling start temperature -80 ° C to the rolling start temperature. A rolling process that carries out rolling at + 80 ° C and
Water cooling is started when the steel sheet after the rolling step is in the temperature range of 650 ° C. to 850 ° C., and the cooling rate at the position 5 mm from the surface of the steel sheet and the 1/4 position in the plate thickness direction is 3 ° C./sec to 30 ° C. A cooling step that stops water cooling in a temperature range of / sec and a surface temperature of 500 ° C or less,
A method for manufacturing a steel sheet having. The method for manufacturing a steel sheet according to any one of claims 1 to 3.
In the secondary refining in a reduced pressure atmosphere, the amount of dissolved oxygen in the molten steel is adjusted to 0.0005% to 0.0100% by mass% without adding Al and C to the molten steel, and then the amount of dissolved oxygen is adjusted. Ti, Al, and Zr are added to the molten steel 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 pieces after the casting step in a temperature range of 1000 ° C. to 1150 ° C.
Rolling of the steel pieces after the heating step is started in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction ratio is 50% or more, and the temperature 1 sec after the completion of finish rolling is the rolling start temperature -80 ° C to the rolling start temperature. A rolling process that carries out rolling at + 80 ° C and
Water cooling is started when the steel sheet after the rolling step is in the temperature range of 650 ° C. to 850 ° C., and the cooling rate at the position 5 mm from the surface of the steel sheet and the 1/4 position in the plate thickness direction is 3 ° C./sec to 30 ° C. A cooling step that stops water cooling in a temperature range of / sec and a surface temperature of 500 ° C or less,
A method for manufacturing a steel sheet having. The method for producing a steel sheet according to claim 4 or 5, further comprising a heat treatment step of reheating the steel sheet after the cooling step to a temperature of 300 ° C. to 600 ° C.