JP6926774B2 - 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.

Applications of steel sheets include, for example, 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 crystal 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 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 4). 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 dispersing an oxide containing Ti as a main component and a composite precipitate of TiN, MnS and BN in a steel sheet containing substantially no Al. In this method, in addition to the intragranular transformation due to Ti oxide, B suppresses the formation of ferrite from the grain boundaries and improves HAZ toughness.

The technique disclosed in Patent Document 4 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. 03-162522 Japanese Unexamined Patent Publication No. 2007-177327

When the present inventors examined the techniques disclosed in the above-mentioned Patent Documents 1 to 4, 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 between the amount of Ti, the amount of Zr and the amount of O 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 Al and 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 it is difficult to secure the number of Ti oxides in the steel sheet only by adding Ti to the molten steel containing no Al.

The techniques disclosed in Patent Documents 1 to 3 are substantially Al-free. In order to realize this, it is necessary to suppress the mixing of Al in any of the steps in the manufacturing process. Further, if the deoxidizing treatment is performed without using Al in order to suppress the mixing of Al, it is necessary to secure a long reflux time and the production time becomes long, so that the production efficiency decreases.

Further, as a result of examining the technique disclosed in Patent Document 4, 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 as a solid solution. , It was found that the deterioration of HAZ 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 steel sheets have been used to improve HAZ toughness. However, in order to improve the efficiency of welding, it has been difficult to stably improve the HAZ toughness of the steel sheet when a 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 a 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 an Al-containing steel sheet. , 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: 0.0005% 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: 0.0001% to 0.0050%,
Insol. Al: 0.0001% to 0.0030% ,
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%,
Mg: 0% to 0.0005%,
Ca + REM: 0% to 0.0005%, and
Remaining: Has a chemical composition consisting of Fe and impurities,
_BF represented by the following formula (1) is 0.0003% to 0.0030%.
_{The BasBN} represented by the following formula (2) is 0.0010% or less, and the carbon equivalent Ceq. Represented by the following formula (3). However, it is 0.35% to 0.50%.
The arrestability index AI represented by the following formula (4) is 65 or less.
In the crystal orientation analysis using electron backscatter diffraction (EBSD) at the 1/4 position in the plate thickness direction of the cross section perpendicular to the rolling direction and the position 5 mm in the plate thickness direction from the steel plate surface, both regions were found. 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% , the pearlite fraction is 0% to 5% , and the MA fraction. The rate is 0% to 1% , and the sum of the ferrite fraction, the bainite fraction, the pearlite fraction, and the MA fraction is 100% .
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 Ti The content ratio of the mass-converted value of Al oxide to the total mass-converted value of the oxide of each element of Al is 5% to 50%, and the Zr oxidation with respect to the total of the mass-converted value of the oxide of each element of Al. The first oxide in which the total content ratio of the substance and the Ti oxide in terms of mass satisfies 50% to 95%, the equivalent circle diameter is 0.5 μm to 10 μm, and the number density is 10 pieces / mm ² or more. Contains,
The first oxide satisfies the content ratio of the mass conversion value of the Al oxide is 20% or less, and the total content ratio of the mass conversion value of the Zr oxide and the Ti oxide is 80% or more. A steel plate having a second oxide having a length in contact with iron of 5% or more, and having a number ratio of the second oxide to the first oxide of 30% or more.

(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, and O are the contents (mass%) of each element of N, Ti, and O contained in the steel plate. Insol.Zr represents the content of acid-insoluble Zr (% by mass), and Insol.Al represents the content of acid-insoluble Al (% 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.)

AI = 0.69 x Deff (table) -5.15 x fα (table) / 100
+4.55 x Deff (t / 4) -82.1 x fα (t / 4) / 100
−0.92 × ΔHv ・ ・ ・ (4)
(However, in the formula (4), Deff (table) is the effective crystal grain size [μm] at a position 5 mm in the plate thickness direction from the surface of the steel plate having a cross section perpendicular to the rolling direction, and fα (table) is the surface of the steel plate. The ferrite fraction [%] at a position of 5 mm in the plate thickness direction, and Def (t / 4) is the effective crystal grain size [%] at a position 1/4 of the steel plate surface in the cross section perpendicular to the rolling direction. μm], fα (t / 4) is the ferrite fraction [%] in the region at a position 1/4 in the plate thickness direction from the surface of the steel sheet, and ΔHv is represented by the formula (5).)
ΔHv = a × t / 2 + b ... (5)
(However, in the formula (5), a and b are coefficients a and b when the hardness distribution of the steel plate in the plate thickness direction is approximated by the following formula (6), and t is the plate thickness [mm]. .)
y = a × x ² + b × x + c ... (6)
(However, in equation (6), y is the Vickers hardness of the cross section perpendicular to the rolling direction, and x is a predetermined position [mm] in the plate thickness direction. The minimum value of x is the plate thickness direction from the steel plate surface. The maximum value of x is the position of 5 mm from the position of 1/2 in the plate thickness direction toward the surface of the steel sheet, and c is a coefficient.)

(2)
The plate thickness is 55 mm or more, the yield stress of the base metal, which is a part other than the weld heat affected zone and the weld metal part, is 460 MPa or more, the brittle ductility transition temperature is -40 ° C or less, and the brittle ductility transition temperature is -10 ° C. The steel sheet according to (1), wherein the arrest toughness value Kca is 6000 N / mm ^{1.5 or more.}

(3)
Thickness is 55Mm～80mm, carried out at a test temperature -10 ° C. in the heat affected zone generated when performing the high-heat-input welding heat input 40kJ / mm~60kJ / mm, at the crack opening displacement test, opening The steel sheet according to (1) or (2), wherein the minimum displacement value is 0.15 mm or more.

(4)
The method for manufacturing a steel sheet according to any one of (1) to (3).
Pre-deoxidation by adding Al is performed before the secondary refining, and then in the secondary refining in a reduced pressure atmosphere, after adding Ti to the molten steel in which the amount of dissolved oxygen is adjusted to 0.0005% to 0.0050% by mass%. Add Ti and Zr in the order of Zr, add Ti in the order of Zr, or add Ti and Zr at the same time. After adding Ti and Zr, cast molten steel after adding Ti and Zr. And the casting process to obtain the 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 of the steel sheet after the rolling step was started when the temperature range was 650 ° C. to 850 ° C., and the cooling rates at 5 mm and 1/4 positions from the surface of the steel sheet were 3 ° C./sec to 30 ° C./sec. A cooling process that stops water cooling in a temperature range where the surface temperature is 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).
Pre-deoxidation by adding Al is performed before the secondary refining, and then Ti is added to the molten steel whose dissolved oxygen content is adjusted to 0.0005% to 0.0050% by mass% in the secondary refining in a reduced pressure atmosphere. Then, after adjusting the amount of dissolved oxygen in the molten steel after the addition of Ti to 0.0005% to 0.0050% by mass%, Zr is added, and the molten steel after the addition of Ti and Zr is cast and slabs are formed. And the casting process to get
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 of the steel sheet after the rolling step was started when the temperature range was 650 ° C. to 850 ° C., and the cooling rates at 5 mm and 1/4 positions from the surface of the steel sheet were 3 ° C./sec to 30 ° C./sec. A cooling process that stops water cooling in a temperature range where the surface temperature is 500 ° C or less,
A method for manufacturing a steel sheet having.

(6)
The method for manufacturing a steel sheet according to any one of (1) to (3).
Preliminary deoxidation by adding Al is performed before the secondary refining, and then Zr is added to the molten steel whose dissolved oxygen content is adjusted to 0.0005% to 0.0050% by mass% in the secondary refining in a reduced pressure atmosphere. Then, after adjusting the amount of dissolved oxygen in the molten steel after adding Zr to 0.0005% to 0.0050% by mass%, Ti is added, and the molten steel after adding Ti and Zr is cast into slabs. And the casting process to get
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 of the steel sheet after the rolling step was started when the temperature range was 650 ° C. to 850 ° C., and the cooling rates at 5 mm and 1/4 positions from the surface of the steel sheet were 3 ° C./sec to 30 ° C./sec. A cooling process that stops water cooling in a temperature range where the surface temperature is 500 ° C or less,
A method for manufacturing a steel sheet having.

(7)
The method for producing a steel sheet according to any one of (4) to (6), 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. can.

It is a photograph which shows an example which image | photographed the steel plate (as mirror polished) of this embodiment by a scanning electron microscope. It is a photograph which shows an example which took the steel plate (etching process) of this embodiment by a scanning electron microscope. It is a graph which shows the relationship between the arrest property index and the arrest toughness value Kca 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 at the time of 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 an oxide containing Zr (particularly an oxide containing Zr and Ti) is dispersed in a steel sheet has not been investigated.

In the steel sheet to which Al has been added, the present inventors have actually produced a steel sheet to which Zr has been added, and oxides, solid solution B, and B which are nuclei for forming intragranular ferrite capable of refining the structure of HAZ. We focused on nitrides and conducted a diligent study. Specifically, pre-deoxidation by adding Al is performed before the secondary refining, molten steel to which Zr and Ti are added is cast by the secondary refining in a reduced pressure atmosphere, and the obtained steel pieces are rolled to obtain the above-mentioned steel pieces. We examined the points. As a result, mainly about the following composition and number density of (A) oxide, (B) solid solution Zr, (C) solid solution B, (D) deoxidation method, (E) Al, and (F) microstructure. , New findings were obtained.
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, the steel plate has a specific range with respect to the total mass conversion value of Ti oxide, Zr oxide, and Al oxide, and corresponds to a specific circle-equivalent diameter (corresponding to the diameter of a circle assuming a circle). HAZ toughness is achieved by having a first oxide having a specific composition, and having a second oxide having a specific composition in which the length in contact with the ground iron is at least a specific ratio among the first oxides. It became clear that it would improve.

Specifically, the steel plate has an average composition of 5% to 50% of Al oxide and Ti oxidation with respect to the total mass conversion value of Ti oxide, Zr oxide, and Al oxide. The total content ratio of the substance and the Zr oxide is an oxide satisfying the range of 50% to 95%, and the number density of oxides having a circle equivalent diameter of 0.5 μm to 10 μm is 10 pieces / mm ² or more. Contains the first oxide that is.
Further, the first oxide satisfies the content ratio of the mass conversion value of the Al oxide is 20% or less, and the total content ratio of the mass conversion value of the Zr oxide and the Ti oxide is 80% or more. It has a second oxide having a length of 5% or more in contact with the base metal. The number ratio of the second oxide to the first oxide is 30% or more.
It has been found that when the oxide contained in the steel sheet satisfies this condition, the HAZ toughness is improved through the miniaturization of the structure.
In the present specification, the length of contact between the second oxide and the base iron is such that the second oxide and the base iron are in contact with each other with respect to the peripheral length (peripheral length) of the second oxide. It is expressed as a percentage of the length [(the length of contact between the second oxide and the ground iron / the circumference of the second oxide (peripheral length))].

The average composition of the first oxide has a content ratio of Al oxide of 5% with respect to the total mass conversion value of Ti oxide, Zr oxide, and Al oxide in terms of further improvement of HAZ toughness. It is preferably ~ 40%, preferably 5% to 30%, and even more preferably 5% to 20%. Further, the total content ratio of the Zr oxide and the Ti oxide in terms of mass is preferably 60% to 95%, preferably 70% to 95%, and preferably 80% to 95%. More preferred.

Further, when the equivalent circle diameter of the first oxide is smaller than 0.5 μm, the function of the intragranular ferrite as a nucleation nucleation (IGF nuclei) deteriorates, and when it is larger than 10 μm, the coarse oxide itself is the starting point of fracture. Is more likely to act as. 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). , More preferably 50 pieces / mm ² or more, most 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.

The second oxide has a composition of 30% or more, which satisfies the above composition and has a length ratio of 5% or more in contact with the base iron, in terms of further improving the HAZ toughness. It is preferably 50% or more, and more preferably 50% or more.
The number ratio is the number of the second oxides to the number of the first oxides to be analyzed among the first oxides [(the number of the second oxides / the first oxide to be analyzed). The number of oxides)] is expressed as a percentage.

Here, the oxide in which the content ratio of the mass-converted value of Al oxide exceeds 20% and the total content ratio of the mass-converted value of Zr oxide and Ti oxide is less than 80% comes into contact with the base iron. When the length ratio was 5% or more, it did not become a formation nucleus of intragranular ferrite. The total content ratio of the Zr oxide and the Ti oxide in terms of mass includes the composite oxide of Ti and Zr.

An oxide containing any one of Al, Ti, and Zr (including the case where both Ti and Zr are contained) contained in the steel plate according to the present embodiment (the first oxide and the first oxide are The equivalent circle diameter, number density, and composition of the second oxide), and the ratio of the length of the second oxide in contact with the base iron, and the number ratio are analyzed using a scanning electron microscope (SEM). Determined by.

Specifically, first, a thermodynamic 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 the test piece was heated and held 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 was obtained by SEM / EDX (scanning electron microscope / energy dispersive type). Observe by X-ray spectroscopy) 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. 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. Of 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 sum 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 mass conversion values of Ti 2} O ₃ , ZrO ₂ , and Al ₂ O ₃ obtained from the formulas (5) to (7) are obtained, and the ratio of the oxide of each element to the total is 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, the number density was measured, and the content ratio of the mass-converted value of Al oxide was 5% to 50%, and the total content ratio of the mass-converted value of Zr oxide and Ti oxide was 50% to 95%. Oxide in which the content ratio of the mass-converted value of Al oxide is 20% or less and the total content ratio of the mass-converted value of Zr oxide and Ti oxide is 80% or more among the satisfied oxides. Observe the object and measure the ratio of the length in contact with the ground iron and the number ratio to the peripheral length of this oxide. The number of this oxide analyzed is at least 20 of the oxides whose number density has been measured. Then, the sum of the O content, the Ti content, the Zr content, and the mass% of 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. Obtain the amount of Ti oxide. If it is difficult to observe the surface of inclusions with mirror polishing, sample adjustment such as electrolytic polishing or electrolytic etching such as selective constant potential may be performed.

(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 total amount of the Zr oxide and the Ti oxide is contained in the oxide in a certain amount or more. On the other hand, Zr that does not form an oxide and remains on the steel sheet (solid solution Zr (solid solution Zr is referred to as “Sol.Zr”) significantly deteriorates the toughness of not only HAZ but also the steel sheet itself, and therefore in the steel sheet. 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 ferrite during welding and improves the HAZ toughness. In a steel sheet containing an oxide in which the total amount of Zr oxide and Ti oxide is a certain amount or more, the amount of solid solution B increases as compared with a steel sheet in which an oxide containing Ti but not containing Zr is dispersed. I found. _{The mass% (BF} ) of the solid-dissolved B in the steel sheet containing an oxide in which the total amount of the Zr oxide and the Ti oxide is equal to or more than a certain amount becomes B nitride from the content of B contained in the steel sheet. It is obtained by subtracting the mass% of B. That is, _BF is represented by the following equation (1). When this value is 0.0003% or more (preferably 0.0005% or more, 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 B _asBN 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 according to this embodiment, _BasBN is 0.0010% or less.

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.

The steel sheet according to this embodiment contains Al. However, since Al acts as a strong deoxidizing element in the steel sheet, if it is contained in the steel sheet in a large amount, it tends to inhibit the oxide formation of Zr and Ti. Therefore, the Al content is preferably within a predetermined range.

Considering that Al is contained in the steel sheet, it is considered that the steps for producing oxides and nitrides are as follows. 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 forms 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, and O in the formula (2) are the contents (mass%) of each element of N, Ti, and O contained in the steel sheet. Insol. Zr is the content (% by mass) of acid-insoluble Zr. Insol. Al is the content (% by mass) of acid-insoluble Al. 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 the Zr content (% by mass) dissolved in the 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.

(D): Deoxidizing method Oxide particles are generated when deoxidizing molten steel. This is called a primary oxide. Further, during casting and solidification, as the temperature of the molten steel decreases, Al oxide, Ti oxide, Zr oxide, and an oxide containing Ti and Zr 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 → pan → secondary refining → continuous casting, oxide-based inclusions remaining in the slab are pre-deoxidized by adding Al, especially before secondary refining. Then, the amount of dissolved oxygen in the molten steel before the start of deoxidation in the secondary refining is 0.0005% to 0.0050% in mass% (preferable upper limit is 0.0040% or less, more preferably 0.0030% or less). ), And by adding the deoxidizing elements Ti and Zr, it was found that the average particle size of the oxides became remarkably finer and the number of oxides increased.

The order of addition of the deoxidizing elements Ti and Zr may be any of the order of Ti and Zr, the order of Zr and Ti, or the simultaneous addition of Ti and Zr. When Ti and Zr are added separately in the order of Ti and Zr, the amount of dissolved oxygen in the molten steel is 0.0005% to 0.0050% in mass% (preferably the upper limit is 0.0040% or less, more). After controlling to 0.0030% or less, Ti is added, and the amount of dissolved oxygen in the molten steel is 0.0005% to 0.0050% in mass% (preferably the upper limit is 0.0040% or less, more). It is preferable to add Zr after making the content preferably 0.0030% or less, more preferably 0.0020% or less). When Zr and Ti are added in this order, it is preferable to control the amount of dissolved oxygen in the same manner as when Ti and Zr are added separately.

By this step, the oxide particles finally remaining in the steel sheet have an average composition of 5% to 50% of the mass conversion value of Al oxide, and the mass conversion of Zr oxide and Ti oxide. It has a first oxide having a total value content of 50% to 95%, a circle equivalent diameter (diameter) of 0.5 μm to 10 μm, and a number density of 10 particles / mm ^{2 or more.} Further, the first oxide has a mass-converted content of Al oxide of 20% or less, and a total of 80% or more of Zr oxide and Ti oxide in terms of mass, and is a base iron. It was found that it had a second oxide having a length of contact with 5% or more.
Here, the secondary refining shows a step performed by a vacuum refining device or a refining device in an inert gas after converter refining. Zr and Ti may be added in either the form of a single metal or an alloy.

(E): Al
Since Al acts as a strong deoxidizing element in a steel sheet, when it is contained in a large amount in the steel sheet, it inhibits the formation of oxides of Zr and Ti. After preliminary deoxidation by adding Al, the upper limit of Al content is limited to 0.0050% by mass or less in order to secure the dissolved oxygen content in the molten steel and generate a composite oxide containing Zr and Ti in the steel sheet. It is important to. Further, it is important to limit the upper limit of the content as acid-insoluble Al (Insol.Al) to 0.0030% by mass or less.

(F): Microstructure This embodiment targets a steel sheet having excellent HAZ toughness, base metal toughness, base material strength, and arrest property.
Here, 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.
The base metal structure is a mixed structure of ferrite, bainite, pearlite, and MA (a mixture of martensite and austenite). Further, the base metal structure may be a mixed structure of ferrite and bainite that does not contain pearlite and MA. 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, in addition to the optical observation, the present inventors performed crystal orientation analysis using an electron backscatter diffraction method (EBSD: Electron Backscatter Diffraction pattern) to analyze the microstructure.

More specifically, a 5 mm position in the plate thickness direction from the steel plate surface (hereinafter, may be referred to as "steel plate surface 5 mm portion") and a plate thickness 1/4 position in the plate thickness direction from the steel plate surface (hereinafter, "t"). A sample for microstructure observation is taken from "/ 4 parts"), and the cross section in the direction perpendicular to the main rolling direction (width direction) is mirror-polished. Then, nital corrosion was carried out 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 taken as the pearlite component of t / 4 parts. It was a rate. In addition, repeller corrosion was carried out, and four fields of view were photographed at a magnification of 500 using an optical microscope, the MA fraction of each field of view was measured, and the average value was taken as the MA fraction of t / 4 parts.
Further, for each portion of the steel sheet surface 5 mm portion and t / 4 portion, a region of 500 μm × 500 μm is formed 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. It was measured. A boundary with a crystal orientation difference of 15 ° or more from adjacent grains is defined as a 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. bottom.
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 of the 5 mm portion and the t / 4 portion of the steel sheet surface.
The bainite fraction of the t / 4 part was the balance other than the pearlite fraction, the MA fraction and the ferrite fraction of the t / 4 part. That is, the total of the bainite fraction, the pearlite fraction, the MA fraction, and the ferrite fraction of t / 4 parts is 100% in terms of 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 (13).

As a result of investigating the relationship between the toughness of the base material of 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%. Further, when the MA fraction exceeded 1.0%, 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 MA fraction is low, and the MA 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. ..

Next, FIG. 3 shows the results of investigating the relationship between the arrest property, the microstructure, and the arrest property index calculated from the hardness distribution of the steel sheet. In FIG. 3, the horizontal axis is the arrest toughness index AI calculated by the microstructure and the plate thickness by the following formula (4), and the vertical axis is the arrest toughness value Kca (K _{ca-10) at −10 ° C.} ). In FIG. 3, the result of approximating the arrest property index AI and K _ca-10 with a linear function is shown by a dotted line. As shown in FIG. 3, it was found that there is a strong correlation between _{the arrest property index AI and K ca-10.} From this first-order approximation curve, it was found that the AI must be 65 or less in order for _{K ca-10} to be 6000 N / mm ^{1.5 or more.} Considering the variation in measurement, the arrest property index AI is preferably 60 or less, and more preferably 55 or less. The lower limit of the arrest property index AI is not particularly limited, but in consideration of the increase in rolling load during rolling to improve the arrest property, the production load such as the decrease in productivity, etc., for example, -10 or more (preferably-). 8 or more, more preferably -1 or more).
AI = 0.69 x Deff (table) -5.15 x fα (table) / 100
+4.55 x Deff (t / 4) -82.1 x fα (t / 4) / 100
−0.92 × ΔHv ・ ・ ・ (4)

Here, in the formula (4), the expressions of Deff (table), fα (table) / 100, 4.55 × Deff (t / 4), and fα (t / 4) / 100 are as follows. be.
Deff (table): Effective crystal grain size [μm] at a position 5 mm in the plate thickness direction from the surface of the steel plate with a cross section perpendicular to the rolling direction.
fα (table): Ferrite fraction [%] at a position 5 mm in the plate thickness direction from the surface of the steel plate.
Deff (t / 4): Effective crystal grain size [μm] at a position 1/4 of the sheet thickness direction from the surface of the steel sheet with a cross section perpendicular to the rolling direction.
fα (t / 4): Ferrite fraction [%] in the region 1/4 of the steel sheet surface in the plate thickness direction.

Further, in the formula (4), ΔHv is represented by the following formula (5).
ΔHv = a × t / 2 + b ... (5)
However, in the formula (5), the ones represented by a and b are as follows.
a, b: Coefficients a and b when the hardness distribution of the steel sheet in the plate thickness direction is approximated by the following equation (6).
t: Plate thickness [mm]

y = a × x ² + b × x + c ... (6)
However, in the formula (6), the ones represented by y and x are as follows.
y: Vickers hardness of the cross section perpendicular to the rolling direction x: A predetermined position [mm] in the plate thickness direction. The minimum value of x is a position of 1 mm in the plate thickness direction from the surface of the steel plate, and the maximum value of x is a position of 5 mm from a position of 1/2 in the plate thickness direction toward the surface of the steel plate.
c: Coefficient

Vickers hardness is measured according to JIS Z 2244 (2009). The Vickers hardness is measured every 1 mm in the plate thickness direction in the region between the position of 1 mm in the plate thickness direction from the front side of the steel plate and the position of 5 mm from the position of 1/2 in the plate thickness direction toward the surface of the steel plate. The load is 9.8 N.
Then, based on the measurement position in the plate thickness direction and the result of the measured value of Vickers hardness, the hardness distribution is approximated by the above equation (6), and the arrestability index is obtained by the equations (4) and (5). Ask for.

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 material in a large heat-affected zone welded joint. Specifically, the yield stress of the base material is 460 MPa or more (for example, 460 MPa to 600 MPa), the brittle ductility transition temperature (vTrs) of the base material is -40 ° C or less, and the arrest toughness value K _{ca- at -10 ° C.} A steel plate having a value of ₁₀ of 6000 N / mm and ^{1.5 or more can be obtained.} Further, when the plate thickness is 55 mm to 80 mm, the opening of the crack opening displacement test at −10 ° 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. It was found that the minimum displacement value was 0.15 mm or more.

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 of the steel sheet (base material) 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, which 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 is changed, the HAZ structure is not miniaturized, and the HAZ toughness is lowered. Therefore, the upper limit of the amount of Si is set to 0.50% or less. 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 metal and HAZ, resulting in toughness. Deteriorates. 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 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 lower limit of the amount of N is preferably 0.0020% or more, more preferably 0.0025% or more. The upper limit of the amount of N is preferably 0.0070% or less, more preferably 0.0060% or less.

(O: 0.0005% 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 0.0005% 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 the ductility of stretch drawing and the like. Therefore, the upper limit of the amount of O is set to 0.0050% or less. The preferable upper limit of the amount of O is 0.0040% or less, and the more preferable upper limit is 0.0030% or less. The preferable lower limit of the amount of O is 0.0010% or more, and the more preferable lower limit is 0.0015% or more.

(Zr: 0.0005% to 0.0100%)
Zr is an element indispensable for fine dispersion of oxides and increase of solid solution B, and must be contained in an amount of 0.0005% or more. The Zr oxide and the composite oxide of Zr and Ti function as intragranular ferrite nucleation nuclei 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 that is solid-solved 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 dissolved by electrolysis in a non-aqueous solvent, and the residue (precipitate and inclusions) is extracted with a filter having a pore size of 0.1 μm to 0.2 μm and separated. Is. After separation, the amount of Zr contained in the solution is Sol. It is the amount of Zr. Insol. Zr is an acid-insoluble Zr, and Insol. Zr amount and Sol. The sum of the Zr amount is the Zr amount.

(Al: 0.0001% to 0.0050%)
Al is an element that is positively added as a deoxidizing element in general. However, since Al tends to react with oxygen preferentially, if the content is excessive, the formation of a desired composite oxide containing Zr and Ti becomes insufficient, and an effective ferrite nucleation in HAZ is produced. Decrease. Further, excessive addition of Al _{promotes the formation of coarse clustered alumina (Al 2} O ₃ ) -based inclusions, which deteriorates the toughness of the base metal and HAZ. Therefore, it is preferable to reduce the Al content as much as possible. The upper limit of the allowable amount of Al is 0.0050%. It is preferably 0.0040% or less, more preferably 0.0030% or less. The smaller the amount of Al, the more preferable. However, in the present embodiment, since Al is used for preliminary deoxidation, the lower limit of the amount of Al is 0.0001% or more. When the Al amount exceeds 0.0001%, the lower limit of the Al amount may be 0.0003% or more, 0.0005% or more, or 0.0008% or more.

(Insol.Al: 0.0001% to 0.0030%)
Insol. Al represents acid-insoluble Al. Insol. When the Al content increases, the amount of Al oxide in the oxide increases, and the HAZ toughness is significantly deteriorated. Therefore, it is necessary to limit the upper limit to 0.0030% or less. Insol. The preferable upper limit of Al is 0.0025% by mass or less, more preferably 0.0020% or less. Insol. The smaller the amount of Al, the more preferable, but in the present embodiment, the lower limit of the amount of Al is 0.0001% because it is used for preliminary deoxidation. When the Al amount exceeds 0.0001%, the lower limit of the Al amount may be 0.0003% or more, 0.0005% or more, or 0.0008% or more.
Insol. Al can be measured by the electrolytic extraction residue analysis method. The electrolytic extraction residue analysis method is a method in which a steel plate is dissolved by electrolysis in a non-aqueous solvent, and the residue (precipitate and inclusions) is extracted with a filter having a pore size of 0.1 μm to 0.2 μm and separated. Is. After separation, the amount of Al contained in the solution is Sol. The amount of Al. Insol. Al is an acid-insoluble Al, and Insol. Al amount and Sol. The sum of the Al amounts is the Al amount.

(Cu: 0.1% to 1.5%)
Cu is an element having an effect of improving strength and corrosion resistance. In order to obtain the effect of containing Cu, 0.1% or more of Cu is contained. Preferably, the lower limit of 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, more preferably 0.5% or less.

(Ni: 0.1% to 3.0%)
Ni is an element that 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 1.5% 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 of the present embodiment may contain one or more of the following elements instead of a part of Fe.

(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, more preferably 0.015% or less.

(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%)
Mo has the effect of improving the strength and toughness of the base material, and may be contained in the steel sheet as needed. 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.

(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 to reduce them as much as possible without intentionally containing them.
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.

(Mg: 0% to 0.0005%)
Since Mg tends to react with oxygen preferentially, if the content thereof is excessive, the formation of a desired composite oxide containing Zr and Ti becomes insufficient. Then, the number of effective ferrite nuclei in HAZ is reduced, and the toughness of HAZ is deteriorated. Therefore, the Mg content is limited to 0.0005% or less. Since the smaller the amount of Mg, the more preferable it is, the lower limit value is not particularly specified and may be 0%.

(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 plate cannot be satisfied. On the other hand, 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% or more, more preferably 0.39% or more. The upper limit of carbon equivalent is preferably 0.48% or less, more preferably 0.46% or less, still more preferably 0.44% or less.

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 ores and scraps 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, and it may be 55 mm to 80 mm.

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
Pre-deoxidation by adding Al is performed before the secondary refining, and then in the secondary refining in a reduced pressure atmosphere, after adding Ti to the molten steel in which the amount of dissolved oxygen is adjusted to 0.0005% to 0.0050% by mass%. Add Ti and Zr in the order of Zr, add Ti in the order of Zr, or add Ti and Zr at the same time. After adding Ti and Zr, cast molten steel after adding Ti and Zr. And the casting process to obtain the 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 of the steel sheet after the rolling step was started when the temperature range was 650 ° C. to 850 ° C., and the cooling rates at 5 mm and 1/4 positions from the surface of the steel sheet were 3 ° C./sec to 30 ° C./sec. A cooling process that stops water cooling in a temperature range where the surface temperature is 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, as described above, preliminary deoxidation by adding Al is performed, and the amount of dissolved oxygen before the start of deoxidation thereafter is controlled.
Specifically, Ti and Zr are added to molten steel in which the amount of dissolved oxygen is adjusted to 0.0005% to 0.0050% by mass in the secondary refining in a reduced pressure atmosphere. The order in which Ti and Zr are added is not particularly limited.

For example, in the order of adding Ti and Zr, in the case of adding Zr after adding Ti, Ti is added to the molten steel in which the amount of dissolved oxygen is adjusted to 0.0005% to 0.0050% in mass%. After adjusting the amount of dissolved oxygen in the molten steel after the addition of Ti to 0.0005% to 0.0050% by mass, Zr is added.
When the order of adding Ti and Zr is the order of adding Ti after adding Zr, Zr is added to the molten steel in which the amount of dissolved oxygen is adjusted to 0.0005% to 0.0050% in mass%. After adjusting the amount of dissolved oxygen in the molten steel after adding Zr to 0.0005% to 0.0050% or less in mass%, Ti is added.
Further, when the order of adding Ti and Zr is the order of adding Ti and Zr at the same time, Ti and Zr are added to the molten steel in which the amount of dissolved oxygen is adjusted by mass% by 0.0005% to 0.0050%. Add at the same time.
The method for performing the secondary refining is not particularly limited, and examples thereof include a method using RH (Rhesus factor-Heraeus).

Examples of the method for obtaining the slab (steel piece) include the method for obtaining the slab (steel piece) as follows.
For example, after refining in a converter, preliminary deoxidation by adding Al is performed. Then, in the secondary refining under a reduced pressure atmosphere performed by a vacuum refining device or a refining device in an inert gas, the dissolved oxygen content of the molten steel is adjusted in the range of 0.0005% to 0.0050% in mass%. .. After that, Ti and Zr are added in a predetermined 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.
The method for 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 refine 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, finish rolling is continuously performed on the steel sheet. This step 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 plate surface temperature 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 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 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. NS.

(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 rate at the position 5 mm and the position 1/4 from the surface of the steel sheet is 3 ° C./sec or more, it becomes easy to secure the strength of the base metal. 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 cooling rate can be calculated by calculation from the amount of cooling water, the plate passing speed, the thermal conductivity of the steel plate, and the like.

Further, when the cooling shutdown temperature is 500 ° C. or lower, the strength is easily secured and the effective crystal particle 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 by way of examples, but the following examples are not of a nature limiting the present invention, and may be changed to an 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 present invention.

Tables 1 and 2 show the chemical composition of the steel sheet. Here, Sol. Zr represents an acid-soluble Zr. 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. Insol. Al indicates that it is acid-insoluble Al. Insol. Al is Insol. It was measured by the same method as Zr. That is, Sol. Al and the amount of Al were measured, and Sol. It was obtained by subtracting the amount of Al. B _asBN is obtained by the above-mentioned equation (2), _BF is obtained by the above-mentioned equation (1), and Ceq. Was obtained by the above-mentioned equation (3).

Tables 3 and 4 show the amount of dissolved oxygen 1 minute before the addition of Ti in the RH vacuum refining facility, the amount of dissolved oxygen 1 minute before the addition of Zr, the amount of dissolved oxygen 1 minute before the simultaneous addition of Ti and Zr, and Ti. The order of addition of Zr is shown. In the order of addition of Ti and Zr, when Zr was added after Ti for Ti and Zr, when Ti was added after Zr for Zr and Ti, Zr and Ti were added at the same time for simultaneous addition. Shows the case. In Tables 3 and 4, the steel 55 was not pre-deoxidized with Al. That is, all the steels other than the steel 55 were pre-deoxidized with Al.
Further, heating conditions, rolling conditions, cooling conditions, and heat treatment conditions (temper temperature) are shown.

Tables 5 and 6 show the plate thickness, effective grain size, ferrite fraction, bainite fraction, pearlite fraction, MA fraction, arrest property index AI, number density of the first oxide, and second oxidation. Shows the number ratio of objects. It also shows the strength of the base metal, the toughness of the base metal, the welding conditions (heat input), and the HAZ toughness.
In Tables 5 and 6, "α fraction" is the area fraction of ferrite, "B fraction" is the area fraction of bainite, "P fraction" is the area fraction of pearlite, and "MA fraction". "Ratio" represents the fraction of the martensite-austenite mixture, respectively.
Further, "0.5 μm to 10 μm oxide of 5% ≤ Al ₂ O ₃ ≤ 50% and 50% ≤ Ti ₂ O ₃ + ZrO ₂ ≤ 95 %" represents the first oxide, and "number density" is Represents the number density of the first oxide.
The "specific oxide" represents the second oxide of the first oxide, and the "number ratio of the specific oxide" represents the number ratio of the second oxide to the first oxide.
“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 are subjected to the conditions (heating, rolling, cooling, heat treatment) shown in Tables 3 and 4 so that the chemical composition of the steel sheet has the values shown in Tables 1 and 2. Each steel plate of 55 mm to 80 mm was manufactured.

Steels 1 to 27 are examples, and steels 28 to 55 are comparative examples.
Steel is melted in a 400-ton converter, pre-deoxidized by adding Al, and then vacuum degassed by secondary refining by RH (Rhesstahl-Heraeus) so as to have the values shown in Tables 3 and 4. Occasionally deoxidized. Then, the dissolved oxygen was adjusted before the addition of Ti and Zr, and then Ti and Zr were added to perform deoxidation, and the slab was cast into a slab having a thickness of 280 mm to 360 mm by continuous casting. Then, under the conditions shown in Tables 3 and 4, the steel sheet was manufactured as a steel sheet having a thickness of 55 mm to 80 mm through each step of heating, rolling, and cooling. Then, heat treatment was carried out as necessary to adjust the material. The temper temperature during the heat treatment was in the range of 300 ° C. to 600 ° C.
The obtained steel sheet was welded and subjected to each test. The heat input under the welding conditions was 40 kJ / mm to 60 kJ / mm.

The effective grain size, pearlite fraction, ferrite fraction, bainite fraction, and MA fraction were measured by the following procedure.
First, a method for measuring the effective crystal grain size will be described. Specimens were collected 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 was 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 (13).

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.
For the MA fraction, the test piece is taken from the center of the width of the steel sheet and 1/4 of the thickness direction, the surface perpendicular to the rolling direction is mirror-polished, the repeller is corroded, and the magnification is 500 times using an optical microscope. 4 fields of view were photographed with, the MA fraction of each field of view was obtained, and the average value was taken as the MA fraction. The size of one field of view is 200 μm × 200 μm. Further, MA was determined by performing image analysis on the assumption that it looks like a lumpy white color when the repeller is corroded.
Ferrite is a portion where the KAM (Kernel Average Measurement) value is 1 ° or less when the measurement points measured by the above EBSD method are first close to each other, and the surface integral of this ferrite is defined as 5 mm portion of the steel plate surface. It was calculated for each part of t / 4 part.
The bainite fraction was the balance of the pearlite fraction and the ferrite fraction.

The inclusion survey was measured by the following procedure. First, a thermodynamic cycle test piece having a thickness direction of 12 mm, a plate width direction of 12 mm, and a rolling direction of 70 mm was collected from the center of the width of the steel plate and a 1/4 position in the plate thickness direction. Then, after heating and holding at 1400 ° C. for 23 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 number density of inclusions was measured by SEM / EDX (scanning electron microscope / energy dispersive X-ray spectroscopy) analysis using "JXA-8530F" manufactured by JEOL on the surface of the thermodynamic cycle test piece as it was mirror-polished. .. 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, Ti, Zr, and Al.
FIG. 1 shows an example of the observation result. In FIG. 1, 12 is the observed inclusions. Table 7 shows the mass% of each target element when the inclusions shown in FIG. 1 were analyzed. 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 (7) to (9). Calculated.
Ti ₂ O ₃ = Ti × 3.003 ... (7)
ZrO ₂ = Zr × 1.351 ... (8)
Al ₂ O ₃ = Al × 3.779 ... (9)
Table 8 shows the mass conversion values of the oxides of each element.

As an average composition with respect to these totals, _{the content ratio (%) of Al 2} O ₃ (Al oxide) is 5% to 50%, and ZrO ₂ (Zr oxide) and Ti ₂ O ₃ (Ti oxide). The number density of oxides in which the total content ratio (%) of the oxides is 50% to 95% and the equivalent circle diameter of the oxides is 0.5 μm or more and 10 μm or less was determined.
Ti ₂ O ₃ content ratio (%) = Ti ₂ O ₃ / (Ti ₂ O ₃ + ZrO ₂ + Al ₂ O ₃ ) ... (10)
ZrO ₂ content ratio (%) = ZrO ₂ / (Ti ₂ O ₃ + ZrO ₂ + Al ₂ O ₃ ) ... (11)
Al ₂ O ₃ content ratio (%) = Al ₂ O ₃ / (Ti ₂ O ₃ + ZrO ₂ + Al ₂ O ₃ ) ... (12)
The calculation results are shown in Table 9.

Next, as an average composition, _{the content ratio (%) of Al 2} O ₃ (Al oxide) is 5% to 50%, and the content ratio of ZrO ₂ (Zr oxide) and Ti ₂ O ₃ (Ti oxide) ( %) The composition of the surface of the oxide satisfying 50% to 95% in total was analyzed.

The thermal cycle test piece was subjected to a selective constant potential electrolytic etching method, and 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 30. The elements to be analyzed were O, Ti, Zr, and Al.

FIG. 2 shows an example of the observation result. In FIG. 2, 11 represents a base iron, 12 represents an inclusion, A represents a region A in the inclusion 12, B represents a region B in the inclusion 12, and C represents a region C in the inclusion 12 (hereinafter, reference numerals are omitted). do.). Various contrasts are also observed in inclusions that appear to be granular due to the difference in color tone (contrast) with respect to the ground iron (background). This means that the composition of inclusions varies from place to place. Therefore, the composition of each contrast region was analyzed.

Table 10 shows the mass% of each target element when the inclusions shown in FIG. 2 were analyzed. Table 11 shows the results obtained by using equations (7) to (9) for the mass conversion values of the oxides of each element assuming _{Ti 2} O ₃ , ZrO ₂ , and Al ₂ O _3. Is shown. Table 12 shows the results obtained by calculating the mass conversion value of the oxide of each element for _{the content ratio (%) of Al 2} O ₃ , ZrO ₂ , and Ti ₂ O ₃ using the formulas (10) to (12). Is shown. In the oxides shown in FIG. 2, the oxides constituting the region A and the region C are 20% or less as the content ratio of the mass conversion value of Al oxide, and the content ratio of the mass conversion value of Zr oxide and Ti oxide. The total was 80% or more, and the ratio of the length in contact with the base iron was 5% or more.

A similar analysis was performed on the remaining 29 inclusions, and the mass-converted content of Al oxide was 20% or less, and the total mass-converted content of Zr oxide and Ti oxide was 80. The ratio of the length of the oxide in contact with the base iron of% or more was calculated. Further, among the 30 oxides analyzed, the content ratio of the mass-converted value of Al oxide is 20% or less, and the total content ratio of the mass-converted value of Zr oxide and Ti oxide is 80% or more. , The number of oxides in which the ratio of the length in contact with the base iron satisfies 5% or more was measured, and the number ratio was determined.

The toughness of the base metal was in accordance with JIS Z 2242 (2005), and a 2 mm V notch Charpy test piece was collected from a direction parallel to the rolling direction at a 1/4 position 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 of the two tensile test pieces was tested and measured, and the average value was calculated. The tensile test piece was JIS Z 2241 (2011) No. 4 test piece.

HAZ toughness was evaluated according to ISO 15653 (2010). First, groove processing was performed so that the welding direction was parallel to the width direction (so that the direction was perpendicular to the rolling direction), and two-electrode simple electrogas 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 10 mm. As a wire, EG-47T (manufactured by Nippon Steel & Sumikin Welding Co., Ltd.) was used. The amount of heat input during welding was 40 kJ / mm to 60 kJ / mm.
Then, according to ISO 15653 (2010), the weld line is bent at three points, and a CTOD test piece as the notch position of the crack opening displacement test (CTOD; Crack Tip Opening Displacement test) piece is collected, and the opening displacement at −10 ° C. The value (CTOD value δ _-10 ) was measured. The test was carried out on three test pieces, and _{those having a minimum value of δ-10} of 0.15 mm or more were evaluated as having excellent HAZ toughness of the welded joint. In the table, the minimum value of _{δ-10 is shown.}

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) "Brittle crack arrest toughness test method". _{Then, the arrest toughness value K ca-10} at a test temperature of -10 ° C. in the temperature gradient type ESSO test was measured.

As is clear from Tables 1 to 6, Steels 1 to 27 have excellent HAZ toughness. Further, it has excellent mechanical properties in the base material which is a portion other than the HAZ and the weld metal portion.
On the other hand, the steels 28 to 55 were inferior in HAZ toughness because they were out of the range specified by the steel sheet according to the present embodiment. Further, even though it had excellent HAZ toughness, 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 (7)

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: 0.0005% 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: 0.0001% to 0.0050%,
Insol. Al: 0.0001% to 0.0030% ,
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%,
Mg: 0% to 0.0005%,
Ca + REM: 0% to 0.0005%, and
Remaining: Has a chemical composition consisting of Fe and impurities,
_BF represented by the following formula (1) is 0.0003% to 0.0030%.
_{The BasBN} represented by the following formula (2) is 0.0010% or less, and the carbon equivalent Ceq. Represented by the following formula (3). However, it is 0.35% to 0.50%.
The arrestability index AI represented by the following formula (4) is 65 or less.
In the crystal orientation analysis using electron backscatter diffraction (EBSD) at the 1/4 position in the plate thickness direction of the cross section perpendicular to the rolling direction and the position 5 mm in the plate thickness direction from the steel plate surface, both regions were found. 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% , the pearlite fraction is 0% to 5% , and the MA fraction. The rate is 0% to 1% , and the sum of the ferrite fraction, the bainite fraction, the pearlite fraction, and the MA fraction is 100% .
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 Ti The content ratio of the mass-converted value of Al oxide to the total mass-converted value of the oxide of each element of Al is 5% to 50%, and the Zr oxidation with respect to the total of the mass-converted value of the oxide of each element of Al. The first oxide in which the total content ratio of the substance and the Ti oxide in terms of mass satisfies 50% to 95%, the equivalent circle diameter is 0.5 μm to 10 μm, and the number density is 10 pieces / mm ² or more. Contains,
The first oxide satisfies the content ratio of the mass conversion value of the Al oxide is 20% or less, and the total content ratio of the mass conversion value of the Zr oxide and the Ti oxide is 80% or more. A steel plate having a second oxide having a length in contact with iron of 5% or more, and having a number ratio of the second oxide to the first oxide of 30% or more.

(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, and O are the contents (mass%) of each element of N, Ti, and O contained in the steel plate. Insol.Zr represents the content of acid-insoluble Zr (% by mass), and Insol.Al represents the content of acid-insoluble Al (% 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.)
AI = 0.69 x Deff (table) -5.15 x fα (table) / 100
+4.55 x Deff (t / 4) -82.1 x fα (t / 4) / 100
−0.92 × ΔHv ・ ・ ・ (4)
(However, in the formula (4), Deff (table) is the effective crystal grain size [μm] at a position 5 mm in the plate thickness direction from the surface of the steel plate having a cross section perpendicular to the rolling direction, and fα (table) is the surface of the steel plate. The ferrite fraction [%] at a position of 5 mm in the plate thickness direction, and Def (t / 4) is the effective crystal grain size [%] at a position 1/4 of the steel plate surface in the cross section perpendicular to the rolling direction. μm], fα (t / 4) is the ferrite fraction [%] in the region at a position 1/4 in the plate thickness direction from the surface of the steel sheet, and ΔHv is represented by the formula (5).)
ΔHv = a × t / 2 + b ... (5)
(However, in the formula (5), a and b are coefficients a and b when the hardness distribution of the steel plate in the plate thickness direction is approximated by the following formula (6), and t is the plate thickness [mm]. )
y = a × x ² + b × x + c ... (6)
(However, in equation (6), y is the Vickers hardness of the cross section perpendicular to the rolling direction, and x is a predetermined position [mm] in the plate thickness direction. The minimum value of x is the plate thickness direction from the steel plate surface. The maximum value of x is the position of 5 mm from the position of 1/2 in the plate thickness direction toward the surface of the steel sheet, and c is a coefficient.) The plate thickness is 55 mm or more, the yield stress of the base metal, which is a part other than the weld heat affected zone and the weld metal part, is 460 MPa or more, the brittle ductility transition temperature is -40 ° C or less, and the brittle ductility transition temperature is -10 ° C. The steel sheet according to claim 1, wherein the arrest toughness value Kca is 6000 N / mm ^{1.5 or more.} Thickness is 55Mm～80mm, carried out at a test temperature -10 ° C. in the heat affected zone generated when performing the high-heat-input welding heat input 40kJ / mm~60kJ / mm, at the crack opening displacement test, opening The steel sheet according to claim 1 or 2, wherein the minimum displacement value is 0.15 mm or more. The method for manufacturing a steel sheet according to any one of claims 1 to 3.
Pre-deoxidation by adding Al is performed before the secondary refining, and then in the secondary refining in a reduced pressure atmosphere, after adding Ti to the molten steel in which the amount of dissolved oxygen is adjusted to 0.0005% to 0.0050% by mass%. Add Ti and Zr in the order of Zr, add Ti in the order of Zr, or add Ti and Zr at the same time. After adding Ti and Zr, cast molten steel after adding Ti and Zr. And the casting process to obtain the 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 of the steel sheet after the rolling step was started when the temperature range was 650 ° C. to 850 ° C., and the cooling rates at 5 mm and 1/4 positions from the surface of the steel sheet were 3 ° C./sec to 30 ° C./sec. A cooling process that stops water cooling in a temperature range where the surface temperature is 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.
Pre-deoxidation by adding Al is performed before the secondary refining, and then Ti is added to the molten steel whose dissolved oxygen content is adjusted to 0.0005% to 0.0050% by mass% in the secondary refining in a reduced pressure atmosphere. Then, after adjusting the amount of dissolved oxygen in the molten steel after the addition of Ti to 0.0005% to 0.0050% by mass%, Zr is added, and the molten steel after the addition of Ti and Zr is cast and slabs are formed. And the casting process to get
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 of the steel sheet after the rolling step was started when the temperature range was 650 ° C. to 850 ° C., and the cooling rates at 5 mm and 1/4 positions from the surface of the steel sheet were 3 ° C./sec to 30 ° C./sec. A cooling process that stops water cooling in a temperature range where the surface temperature is 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.
Preliminary deoxidation by adding Al is performed before the secondary refining, and then Zr is added to the molten steel whose dissolved oxygen content is adjusted to 0.0005% to 0.0050% by mass% in the secondary refining in a reduced pressure atmosphere. Then, after adjusting the amount of dissolved oxygen in the molten steel after adding Zr to 0.0005% to 0.0050% by mass%, Ti is added, and the molten steel after adding Ti and Zr is cast into slabs. And the casting process to get
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 of the steel sheet after the rolling step was started when the temperature range was 650 ° C. to 850 ° C., and the cooling rates at 5 mm and 1/4 positions from the surface of the steel sheet were 3 ° C./sec to 30 ° C./sec. A cooling process that stops water cooling in a temperature range where the surface temperature is 500 ° C or less,
A method for manufacturing a steel sheet having. The method for producing a steel sheet according to any one of claims 4 to 6, further comprising a heat treatment step of reheating the steel sheet after the cooling step to a temperature of 300 ° C. to 600 ° C.

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