JP6926773B2 - 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, even if a brittle crack is generated at a welded joint, the ability to stop the brittle crack with a base material (hereinafter, may be referred to as "arrest property") is required. As the plate thickness increases, the arrest property tends to be inferior, but on the other hand, the required arrest property increases.

For example, in Non-Patent Document 1, the minimum required arrest toughness value (Kca value) at -10 ° C for an extra-thick arrest steel sheet having a thickness of more than 80 mm and 100 mm or less is 8000 N / mm ^{1.5 under the strictest conditions.} It is stated that.

In order to ensure the arrest property, it is effective to increase the strength of the steel sheet and reduce the thickness of the steel sheet. However, in general, when the strength of the steel sheet is increased, the hardness of the weld heat-affected zone is increased, and the toughness of the weld heat-affected zone is deteriorated. In order to further ensure the safety and reliability of the welded structure, the toughness of the heat-affected zone of welding (hereinafter, may be referred to as "HAZ") (hereinafter, "welding") while increasing the strength of the steel sheet. It is important to ensure the "heat-affected zone toughness" (sometimes referred to as "HAZ toughness").

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 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.

July 27, 2016 Nippon Kaiji Kyokai (Class NK) Press Release "World's First Knowledge on Minimum Arest Toughness Required for Extra-Thick Arest Steel Sheets with a Thickness of 100 mm Used in Ultra-Large Container Ships"

Japanese Unexamined Patent Publication No. 1-159356 Japanese Unexamined Patent Publication No. 2008-291347 Japanese Unexamined Patent Publication No. 3-162522 Japanese Unexamined Patent Publication No. 2007-177327

When the present inventors examined the steel sheet described in Non-Patent Document 1 above, the following findings were obtained.

In order to stably secure the arrest toughness value Kca of a steel sheet having a thickness of more than 80 mm described in Non-Patent Document 1 of 8000 N / mm ^1.5 or more, for example, an increase in Ni content and an effective crystal grain size are required. There is a method such as application of a process having a high manufacturing load in order to miniaturize. However, these are costly.

Further, 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.

As a result of examining the technique disclosed in Patent Document 4, since the portion adjacent to the weld metal is exposed to a high temperature, TiN utilizing the pinning effect disappears by solid solution, and the deterioration of HAZ toughness cannot be suppressed. Do you get it.

By the way, the welding construction cost accounts for a large amount of the total construction cost of the welded structure. In order to reduce this cost, it is effective to reduce the number of welding passes by increasing the strength and thinning the steel sheet. However, simply increasing the strength of the steel sheet will inevitably cure the HAZ and deteriorate its toughness.

Conventionally, dispersed particles such as inclusions in steel sheets have been used to improve HAZ toughness. However, it has been difficult to stably improve the HAZ toughness by increasing the strength of the steel sheet. It is considered that the cause of this is, for example, that inclusions such as oxides tend to aggregate in the molten steel and are difficult to uniformly disperse in the steel sheet, so that it is difficult to secure a sufficient number of nuclei for intragranular transformation.

As described above, the actual situation is that the technique for improving the HAZ toughness of the high-strength steel sheet has not been established.
The present invention has been made in view of such circumstances, and has excellent toughness in HAZ when welding is performed, and is an excellent machine 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 a toughness.

The present inventor has solved the above-mentioned problems as 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 the HAZ portion. We found that we could do it, and completed the present invention.

The gist of the present invention is as follows.

(1)
By mass%
C: 0.01% to 0.20%,
Si: 0.02% to 0.50%,
Mn: 0.30% to 2.50%,
Ti: 0.003% to 0.024%,
B: 0.0005% to 0.0030%,
N: 0.0010% to 0.0090%,
O: 0.0010% to 0.0050%,
Zr: 0.0005% to 0.0100%,
Sol. Zr: 0% to 0.0020%,
Cu: 0.1% to 1.5%,
Ni: 0.1% to 3.0%,
Al: 0% to 0.0050%,
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 0.0005% to 0.0030%.
_BasBN represented by the following formula (2) is 0% or less.
Carbon equivalent Ceq. Represented by the following formula (3). Is 0.45% to 0.55%,
In crystal orientation analysis using electron backscatter diffraction (EBSD) with a cross section perpendicular to the rolling direction, 5 mm from the front side of the steel sheet in the plate thickness direction and 1/2 position in the plate thickness direction toward the surface of the steel sheet 5 mm. The average value of the effective crystal grain size in the region between the position and the position is 30 μm or less.
The average value of the effective crystal grain size at each measurement position in the plate thickness direction in the region is from the average value of the effective crystal particle size of the entire region of -15 μm to the average value of the effective crystal particle size of the entire region + 15 μm. Satisfy the range,
The microstructure in the entire region has a bainite fraction of 80.0% to 100.0%, a ferrite fraction of 0% to 20.0%, and a pearlite fraction of 0% to the average value of the area ratio. 1.0%, the MA fraction is 0% to 1.0% , and the total of the bainite fraction, the ferrite fraction, the pearlite fraction, and the MA fraction is 100% .
The bainite fraction at each measurement position in the plate thickness direction in the region satisfies the range of the average value of the bainite fraction in the entire region of -15% to the average value of the bainite fraction in the entire region + 15%. ,
The oxide analyzed at the 1/4 position in the plate thickness direction is a single oxidation by the elements of Ti, Zr, and Al, which is obtained from the measured values of the amount of O, the amount of Ti, the amount of Zr, and the amount of Al in the oxide. The content ratio of the mass-converted value of Al oxide to the total mass-converted value of the oxide of each element of Ti, the Zr, and Al when it is assumed to be a thing is 20% or less, and the mass-converted Zr oxide. An oxide having a value of 5% or more and a total mass conversion value of Zr oxide and Ti oxide of 80% or more, a circle equivalent diameter of 0.5 μm to 10 μm, and a number density of 10 pieces / mm ² or more. Steel plate that is.

However, in the formula (1), _BasBN is represented by the following formula (2). Further, B is the content (mass%) of the element B contained in the steel sheet and satisfies the relationship of _{0 ≦ BF ≦ B.}

However, in the formula (2), 0 ≦ B _asBN ≦ B (when B _asBN <0, B _asBN = 0), 0 ≦ Insol. Satisfying the relationship of Zr, N, Ti, O, and Al are the contents (mass%) of each element contained in the steel sheet, and Insol. Zr indicates the content (% by mass) of acid-insoluble Zr.

Ceq. = C + Mn / 6 + (Cu + Ni) / 15+ (Cr + Mo + V) / 5 ... (3)
However, C, Mn, Cu, Ni, Cr, Mo and V in the formula are the contents (mass%) of each element contained in the steel sheet.

(2)
The temperature at which the plate thickness is 50 mm or more, the yield stress of the base metal, which is the part other than the weld heat affected zone and the weld metal part, is 550 MPa or more, and the arrest toughness value Kca is 6000 N / mm ^{1.5 is-.} A crack opening displacement test performed at a test temperature of -10 ° C on a weld heat-affected zone that occurs when welding is performed at a heat input of 4.5 kJ / mm to 6.0 kJ / mm at 10 ° C or lower. The steel plate according to (1), which has an opening amount of 0.15 mm or more.

(3)
The method for manufacturing the steel sheet according to (1) or (2).
In the secondary refining in a reduced pressure atmosphere, to the molten steel in which the amount of dissolved oxygen is adjusted to 0.0005% to 0.0050% in mass%, it is added in the order of Zr after adding Ti, in the order of Ti after adding Zr, or Ti. And Zr are added at the same time.
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
A cooling step of starting the water cooling of the steel sheet after the rolling step when the temperature range is 650 ° C. to 850 ° C. and stopping the water cooling in the temperature range of the surface temperature of 500 ° C. or lower.
A method for manufacturing a steel sheet having.

(4)
The method for manufacturing the steel sheet according to (1) or (2).
In the secondary refining in a reduced pressure atmosphere, Ti was added to the molten steel in which the amount of dissolved oxygen was adjusted to 0.0005% to 0.0050% by mass%, and the amount of dissolved oxygen in the molten steel after the addition of Ti was adjusted by mass%. After adjusting to 0.0005% to 0.0050%, Zr is added, and molten steel after addition of Ti and Zr is cast to obtain a slab.
A heating step of heating the steel pieces after the casting step in a temperature range of 1000 ° C. to 1150 ° C.
Rolling of the steel pieces after the heating step is started in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction ratio is 50% or more, and the temperature 1 sec after the completion of finish rolling is the rolling start temperature -80 ° C to the rolling start temperature. A rolling process that carries out rolling at + 80 ° C and
A cooling step of starting the water cooling of the steel sheet after the rolling step when the temperature range is 650 ° C. to 850 ° C. and stopping the water cooling in the temperature range of the surface temperature of 500 ° C. or lower.
A method for manufacturing a steel sheet having.

(5)
The method for manufacturing the steel sheet according to (1) or (2).
In the secondary refining in a reduced pressure atmosphere, Zr was added to the molten steel in which the amount of dissolved oxygen was adjusted to 0.0005% to 0.0050% by mass%, and the amount of dissolved oxygen in the molten steel after the addition of Zr was adjusted by mass%. After adjusting to 0.0005% to 0.0050%, Ti is added, and the molten steel after addition of Ti and Zr is cast to obtain a slab.
A heating step of heating the steel pieces after the casting step in a temperature range of 1000 ° C. to 1150 ° C.
Rolling of the steel pieces after the heating step is started in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction ratio is 50% or more, and the temperature 1 sec after the completion of finish rolling is the rolling start temperature -80 ° C to the rolling start temperature. A rolling process that carries out rolling at + 80 ° C and
A cooling step of starting the water cooling of the steel sheet after the rolling step when the temperature range is 650 ° C. to 850 ° C. and stopping the water cooling in the temperature range of the surface temperature of 500 ° C. or lower.
A method for manufacturing a steel sheet having.

(6)
The method for producing a steel sheet according to any one of (3) to (5), further comprising a heat treatment step of reheating the steel sheet after the cooling step to a temperature of 400 ° C. to 600 ° C.

According to this embodiment, it is possible to provide a steel sheet having excellent toughness in HAZ when welded and having excellent mechanical properties in a base material which is a portion other than HAZ and the weld metal part.

It is a photograph which shows an example which image | photographed the steel plate of this embodiment by a scanning electron microscope.

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 grain boundary ferrite and improves the strength of the steel sheet.
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.

The present inventors have focused on solid solution B, which is an oxide that becomes an intragranular ferrite nucleation nucleus (intragrain) capable of miniaturizing the structure of the HAZ portion, and conducted a diligent study. As a result, mainly the following composition and number density of (A) oxide, (B) solid solution Zr, (C) solid solution B, (D) deoxidation method, (E) Al, (F) microstructure, and (G) The following new findings were obtained regarding the method for manufacturing steel sheets.
Hereinafter, these new findings will be described.

(A): Oxide composition and number density The present inventors actually produced a steel sheet to which Zr was added, investigated each oxide in detail for the oxide that is the core of the intragranular ferrite, and HAZ. We investigated the effect on improving toughness.
As a result, the content ratio of the mass-converted value of Al oxide is 20% or less and the mass-converted value of Zr oxide is 5% or more with respect to the total mass-converted value of Ti oxide, Zr oxide, and Al oxide. , And the total content ratio of Zr oxide and Ti oxide in terms of mass is 80% or more, and the equivalent circle diameter of the oxide containing (corresponding to the diameter of the circle assuming a circular shape) is 0. It was clarified that the HAZ toughness was improved through the micronization of the structure when the oxide having a thickness of .5 μm to 10 μm was ^{contained at a number density of 10 pieces / mm 2 or more.}

If the content ratio of the mass conversion value of Al oxide exceeds 20%, or if the total content ratio of the mass conversion value of Zr oxide and Ti oxide is less than 80%, it will be a formation nucleus of intragranular ferrite. There wasn't. If the content ratio of the mass-converted value of Al oxide is within the range of 20% or less, the oxide that is the core of the formation of intragranular ferrite is the content ratio of the mass-converted value of Zr oxide and Ti oxide. The total includes a single Zr oxide and a composite oxide of Ti and Zr.
The reason for limiting the mass conversion value of Zr oxide to 5% or more will be described in "(C): Solid solution B ( _BF )" described later.

The content ratio of the Al oxide in terms of mass is preferably 15% or less, more preferably 10% or less, still more preferably 5% or less. The content ratio of the mass-converted value of Al oxide may be 0%.
The total content ratio of the Zr oxide and the Ti oxide in terms of mass is preferably 85% or more, more preferably 90% or more, still more preferably 95% or more. The total content ratio of the Zr oxide and the Ti oxide in terms of mass may be 100%. Although the mass-converted value of Zr oxide is 5% or more, the upper limit of the content ratio of the mass-converted value of Zr oxide is not particularly limited, but may be, for example, 98% or less.

In addition, if the equivalent circle diameter is smaller than 0.5 μm, the function as an IGF nuclei that is the nucleation of intragranular ferrite deteriorates, and if it is larger than 10.0 μm, the coarse oxide itself may act as a starting point of fracture. Increase. When 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, the HAZ structure is finer than that of steel containing no Zr. It was clarified that the HAZ toughness was improved by the conversion.
The number density of oxides having the above composition having a circle equivalent diameter of 0.5 μm to 10 μm is preferably 20 pieces / mm ² or more, more preferably 30 pieces / mm ² or more, still more preferably 50 pieces / mm ^2. As mentioned above, most preferably 60 pieces / mm ² or more. The upper limit of the number density of oxides is not particularly limited, and examples thereof include 200 oxides / mm ² or less.

Here, a method for observing oxides will be described.
The equivalent circle diameter, number density, and composition of the oxide containing any of Al, Ti, and Zr (including the case where both Ti and Zr are contained) contained in the steel sheet according to the present embodiment are scanned. Determined by analysis using a scanning electron microscope (SEM). Specifically, 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 1/4 in the plate thickness direction. This test piece was heated and held at 1400 ° C. for 23 seconds, cooled at a cooling speed of 1 ° C./sec, cut in a direction perpendicular to the rolling direction, and the obtained cross section was obtained by SEM / EDX (scanning electron microscope). / Energy dispersive X-ray spectroscopy), and quantitative analysis and measurement of 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 (because the morphology of the oxide is granular) due to the difference in color tone (contrast) with respect to the base iron 11 (background), each of these inclusions Quantitative analysis of the overall composition of inclusions.
Here, the heat treatment for heating the test piece to 1400 ° C. does not affect the size, number, and composition of oxides regardless of whether or not the heat treatment is performed. When heated to 1400 ° C., precipitates other than oxides such as TiN are solid-solved, so that the oxide to be observed becomes easy to see and the time required for analysis can be reduced.

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. I made it a thing. Then, for this oxide, the mass conversion value of the oxide of each element when a single oxide of these elements is assumed is calculated from the mass% of each element using the following formulas (5) to (7). calculate.
Ti ₂ O ₃ = Ti × 3.003 ... (5)
ZrO ₂ = Zr × 1.351 ... (6)
Al ₂ O ₃ = Al × 3.779 ... (7)

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 value of Ti 2} O ₃ , ZrO ₂ , and Al ₂ O ₃ obtained from the formulas (5) to (7) is obtained, and the ratio of the mass conversion value of the oxide of each element to the total is calculated. The content ratio (%) of the oxide of each element contained in the oxide was used.

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)

(B): Solid solution Zr (Sol.Zr)
Zr (solid solution Zr (Sol. Zr)) that does not form an oxide and remains on the steel sheet significantly deteriorates the toughness of not only HAZ but also the steel sheet itself. It is necessary to reduce Zr. Sol. The toughness tends to improve as the amount of Zr decreases, and in order to obtain a steel sheet having excellent HAZ toughness, Sol. It is important to limit Zr to 0.0020% by weight or less.
In order to further improve the HAZ toughness, it is preferable to limit the HAZ toughness to 0.0010% by mass or less (more preferably 0.0005% by mass or less). Here, Sol. Zr is acid-soluble Zr and 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 described in Insol. It is Zr (Insol.Zr in the formula (2)), and the sum of the acid-soluble Zr and the acid-insoluble Zr is the amount of Zr in the steel sheet.

(C): Solid solution B ( _BF )
The solid solution B of the steel sheet segregates at the old austenite grain boundaries to suppress the formation of grain boundary ferrite and improve the strength of the base metal. It has been found that the solid solution B increases in the steel sheet containing Zr oxide and the total of Zr oxide and Ti oxide in a certain amount or more. It was found that the mass conversion value of Zr oxide is required to be 5% or more in order to obtain this effect. The mass% ( _BF ) of the solid solution B is obtained by subtracting the mass% of B, which is a B nitride, from the content of B contained in the steel sheet. That is, _BF is represented by the following equation (1). When this value is 0.0005% or more (preferably 0.0010% or more), the effect of improving the strength of the base material due to the solid solution B can be obtained. If _BF becomes excessive, there is a concern that the base metal toughness and HAZ toughness deteriorate. Therefore, _{the upper limit of BF} is set to 0.0030% or less. It is more preferably 0.0025% or less, still more preferably 0.0020% or less.

However, in the formula (1), _BasBN is represented by the following formula (2). Further, B is the content (mass%) of the element B contained in the steel sheet and satisfies the relationship of _{0 ≦ BF ≦ B.}
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.

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.

It is preferable that the steel sheet according to this embodiment does not contain Al. This is because Al acts as a strong deoxidizing element in the steel sheet, and when it is contained in the steel sheet in a large amount, it inhibits the formation of oxides of Zr and Ti. However, in actual production, when Al is mixed as an impurity, or when the molten steel temperature becomes too low, Al has to be heated, and the steel sheet may contain Al.

Considering that Al is contained, it is considered that the steps for producing oxides and nitrides are as follows. Since an oxide is formed from an element having a strong deoxidizing power, first, in molten steel, Zr, which has a stronger deoxidizing power than Al, becomes an oxide preferentially and a Zr oxide is formed. Then, 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. Next, 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. In order to utilize the added B as a solid solution B in its entirety, this value is set to 0%. Further, since BN becomes a nucleation of ferrite, it is preferable that _{BasBN is 0% in order to stably secure the strength of the base metal.}

However, N, Ti, O, and Al in the formula are the contents (mass%) of each element contained in the steel sheet, and Insol. Zr is the content (% by mass) of acid-insoluble Zr. 0 ≦ B _asBN ≦ B, and when B _asBN <0, B _asBN = 0.
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, as the molten steel temperature decreases during casting and solidification, 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 → ladle → secondary refining → continuous casting, the oxide-based inclusions remaining in the slab are particularly the dissolved oxygen in the molten steel before the start of deoxidation in the secondary refining. The amount is controlled to 0.0005% to 0.0050% (preferable upper limit is 0.0040% or less, more preferable upper limit is 0.0030% or less) in mass%, and the deoxidizing elements Ti and Zr are added. It was found that the average particle size of the oxide was significantly reduced and the number of oxides was increased.
The order of addition of the deoxidizing elements Ti and Zr may be the order of Ti, Zr, the order of Zr, Ti, or the simultaneous addition of Ti, 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% by mass (preferably the upper limit is 0.0040% or less, more). After controlling to a preferable upper limit of 0.0030% or less), Ti is added. Then, the amount of dissolved oxygen in the molten steel was set to 0.0005% to 0.0050% (a preferable upper limit is 0.0040% or less, a more preferable upper limit is 0.0030% or less, and a further preferable upper limit is 0.0020% or less). Later, Zr may be added.
When Zr and Ti are added separately in this order, Zr is added after controlling to 0.0005% to 0.0050% (a preferable upper limit is 0.0040% or less, and a more preferable upper limit is 0.0030% or less). Then, the amount of dissolved oxygen in the molten steel is set to 0.0005% to 0.0050% (a preferable upper limit is 0.0040% or less, a more preferable upper limit is 0.0030% or less, and a further preferable upper limit is 0.0020% or less). After that, Ti may be added.
When Ti and Zr are added at the same time, the amount of dissolved oxygen in the molten steel is controlled to 0.0005% to 0.0050% (the preferable upper limit is 0.0040% or less, and the more preferable upper limit is 0.0030% or less), and then Ti. And Zr may be added at the same time.

By this step, the oxide particles finally remaining in the steel plate have a mass-equivalent ratio of Al oxide of 20% or less, a mass-equivalent value of Zr oxide of 5% or more, and Zr oxide and Ti oxidation. It was found that the total mass-equivalent ratio of the objects is 80% or more, and the number density of the oxide particles having a circle-equivalent diameter (diameter) of 0.5 μm to 10 μm is 10 particles / mm ² or more. ..
Here, the secondary refining shows a process performed by a vacuum refining device, a refining device in an inert gas, or the like 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. 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 limit the Al content to 0.0050% by mass or less.

(F): Microstructure The steel sheet according to the present embodiment is intended for a steel sheet having excellent HAZ toughness, base material 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 mainly bainite, and is a mixed structure of bainite, ferrite, pearlite, and MA. However, in a structure in which bainite and ferrite are mixed, the basic structure unit is objectively defined and its size is measured only by observing the structure with a normal optical microscope (hereinafter, may be referred to as "light microscopic observation"). It's very difficult to do. Therefore, the present inventors performed crystal orientation analysis using EBSD (Electron Backscatter Diffraction pattern) in addition to optical observation, and analyzed the microstructure.

More specifically, a position 5 mm in the plate thickness direction from the front side of the steel plate (hereinafter, may be referred to as "5 mm portion below the table") and a position 5 mm from the 1/2 position in the plate thickness direction toward the steel plate surface (hereinafter, may be referred to as "5 mm portion below the table"). Hereinafter, a sample for tissue observation for observing the tissue is collected in the region between the area and the “t / 2-5 mm portion”). The structure observation sample is collected so that the structure can be observed every 5 mm in the plate thickness direction, a cross section perpendicular to the rolling direction is cut, and mirror polishing is performed. Then, nital corrosion was carried out, and the region between the 5 mm portion and the t / 2-5 mm portion below the table was photographed every 5 mm in the plate thickness direction at 500 times for 4 fields using an optical microscope, and the pearlite component of each field was taken. The rate is measured, and the average value is taken as the pearlite fraction at each measurement position in the plate thickness direction (position measured every 5 mm in the plate thickness direction).
Furthermore, repeller corrosion was carried out, and the region between the 5 mm portion and the t / 2-5 mm portion below the table was photographed every 5 mm in the plate thickness direction at 500 times for 4 fields using an optical microscope, and the MA portion of each field was taken. The rate is measured, and the average value is taken as the MA fraction at each measurement position in the plate thickness direction.
Ferrite was defined as a portion having a KAM (Kernel Average Measurement) value of 1 ° or less when the measurement points measured by the above EBSD method were first close to each other. The surface integral of this ferrite was determined at each measurement position in the plate thickness direction. The bainite fraction was the balance of the pearlite fraction, the ferrite fraction, and the MA fraction. That is, the total of the bainite fraction, the pearlite fraction, the ferrite fraction, and the MA fraction is 100% in terms of area fraction.
If the measurement position every 5 mm in the plate thickness direction does not include the 1/4 position in the plate thickness direction from the front side of the steel plate (hereinafter, may be referred to as "t / 4 part"), t / 4 part. Is also measured separately in the same manner as above. That is, it is shown that the region between the 5 mm portion and the t / 2-5 mm portion below the table is the region between the 5 mm portion and the t / 2-5 mm portion below the table, including the t / 4 portion.

Next, the effective crystal grain size will be described.
For the mirror-polished surface, the area between the 5 mm portion and the t / 2-5 mm portion below the surface is formed every 5 mm in the plate thickness direction, and the region of 500 μm × 500 μm is formed by the EBSD method with respect to the cross section in the direction perpendicular to the rolling direction. Measure at a pitch of 1 μm. If t / 4 parts are not included, the t / 4 parts are measured separately in the same manner.
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 5 mm below the table including the t / 4 part. The effective crystal grain size of the entire region between the portion and the t / 2-5 mm portion was taken. 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 grain is A ₁ , A ₂ , A ₃ , ... A _i , ... _AN , and the equivalent circle diameter (diameter) of each grain. but _{D 1, D 2, D 3} , ··· D i, and a · · · _{D N.} In that case, the effective crystal grain size (Deff) is calculated by the following formula (11).

_{The average value (Def t} ) of the effective crystal grain size in the entire region between 5 mm and t / 2-5 mm below the table is the effective crystal grain size at each measurement position in the plate thickness direction including t / 4 part. Is Def _j , and the measured number of visual fields is M, which is expressed by the following equation (12). Def _j is a value obtained by the equation (11).

As a result of investigating the relationship between the base metal toughness and the microstructure in the region between 5 mm and t / 2-5 mm below the table (including t / 4), the average value of the effective crystal grain size in this region as a whole was found. As the miniaturization progressed, the brittle ductile transition temperature (hereinafter, may be referred to as “vTrs”) became lower. Specifically, it has been clarified that vTrs is -60 ° C. or less when the effective crystal grain size is 30 μm or less.
The preferable upper limit of the effective crystal grain size (average value) in the entire region between the 5 mm portion and the t / 2-5 mm portion below the table is 25 μm or less, more preferably 20 μm or less, and further preferably 15 μm or less. The average value of the effective crystal grain size in the entire region between the 5 mm portion and the t / 2-5 mm portion below the table should be as small as possible, and the lower limit value is not particularly limited, but may be, for example, 5 μm or more. Further, 1 μm or more can be mentioned.

As a result of investigating the relationship between the strength of the base metal and the microstructure, the bainite fraction increased as the average value of the entire region (including t / 4 part) between the 5 mm part and the t / 2-5 mm part below the table. As the ferrite fraction decreased, the strength of the base metal at t / 4 parts improved. In this respect, the ferrite fraction is 20% or less. The ferrite fraction is preferably 15.0% or less, preferably 10.0% or less, and more preferably 5.0% or less. The ferrite fraction may be 0%.
On the other hand, when the bainite fraction was less than 80.0%, the strength of the base metal in this region decreased. The bainite fraction is preferably 85.0% or more, more preferably 90.0% or more, and even more preferably 95.0% or more. The bainite fraction may be 100%. As the bainite fraction increases, the strength increases and the toughness deteriorates, but if the average value of the effective crystal grain size in the entire region is 30 μm or less, the toughness of the base metal can be ensured.

Here, even if the effective crystal grain size is 30 μm or less, if at least one of the pearlite fraction and the MA fraction in the above region exceeds 1.0%, vTrs exceeds −60 ° C. and the mother. It was found that it becomes difficult to secure the toughness of the material. In order to ensure the toughness of the base metal, it is preferable that the pearlite fraction and the MA fraction are low, and these fractions may be 0%.

As a result of further investigation of the relationship between arrest property and microstructure, the average value of the effective crystal grain size in the entire region (including t / 4 part) between the 5 mm part and the t / 2-5 mm part below the table was 30 μm or less. Even if the bainite fraction is 80.0% or more, the _{temperature of TKca6000} may be high.
Therefore, we investigated the variation in microstructure.
As a result, in the region between the 5 mm portion and the t / 2-5 mm portion (including the t / 4 portion) below the table, the effective crystal grain size and the bainite fraction were found at least at least a part of each measurement position in the plate thickness direction. It was found that if the variation is large, the _{temperature of TKca6000 may become high.}

In _{order to keep TKca6000} stable at -10 ° C or lower, the effective crystal grain size (average value) at each measurement position in the plate thickness direction is between 5 mm and t / 2-5 mm below the table. It was found that the average value of the effective crystal grain size in the entire region (including t / 4 part) should be in the range of -15 μm to the average value of the effective crystal particle size in the entire region + 15 μm. The average value of the effective crystal particle size in the entire region is preferably in the range of −10 μm to the average value of the effective crystal particle size + 10 μm, and more preferably, the average value of the effective crystal particle size in the entire region is −5 μm to. It is in the range of the average value of the effective crystal grain size + 5 μm.
Further, the bainite fraction (average value) at each measurement position in the plate thickness direction is the bainite fraction in the entire region (including t / 4 part) between the 5 mm portion and the t / 2-5 mm portion below the table. It was found that the average value should be in the range of -15% to the average value of the bainite fraction + 15%. The average value of the bainite fraction in the entire region is preferably in the range of -10% to the average value of the bainite fraction + 10%, and more preferably the average value of the bainite fraction in the entire region is -5% to the bainite fraction. It is in the range of the average value of the rate + 5%. This is because if there is a region (particularly a region) in which the effective crystal grain size and the bainite fraction vary widely locally, that region becomes the starting point of fracture.

(G): Steel sheet manufacturing conditions In order to form the above microstructure, a process for manufacturing a steel sheet was examined.

As a result of investigating the relationship between the rolling conditions and the microstructure, it was found that it is effective to manufacture by the following steps in order to form the above microstructure.
The heating temperature of the slab is 1000 ° C to 1150 ° C. In finish rolling, rolling is started in a temperature range where the surface temperature of the steel sheet 1 sec before finish rolling (hereinafter, may be referred to as “rolling start temperature”) is 650 ° C to 850 ° C, and the cumulative rolling reduction rate is 50% or more. implement. Then, the temperature 1 sec after the completion of rolling (hereinafter, may be referred to as “finishing temperature”) is in the range of rolling start temperature −80 ° C. to rolling start temperature + 80 ° C. Then, the rolled steel sheet is started to be water-cooled from a temperature of 650 ° C. to 850 ° C., and water cooling is stopped when the surface temperature is 500 ° C. or lower. By these steps, the above-mentioned microstructure was effectively obtained.
Further, the steel sheet after the cooling step may be further reheated to a temperature of 400 ° C. to 600 ° C., if necessary. It is preferable to carry out the step of reheating to a temperature of 400 ° C. to 600 ° C. because the toughness may be improved.
In order to stably secure the characteristics of the steel plate (HAZ toughness, base metal toughness, base material strength, and arrest property), the rolling start temperature is 700 ° C to 800 ° C, and the finishing temperature is the rolling start temperature -50 ° C to 50 ° C. The rolling start temperature is preferably in the range of + 50 ° C., and the cooling start temperature is preferably 700 ° C. to 800 ° C.
When the carbon equivalent Ceq is low (for example, 0.45% to 0.50%), the heating temperature of the slab may be suppressed so as not to be too high, and the rolling start temperature may be set to a higher temperature. When heat treatment (temper) is performed, it is preferable to keep the heat treatment temperature (temper temperature) low or not to perform heat treatment (temperless). For example, specifically, the heating temperature of the slab is 1000 ° C. to 1100 ° C., the rolling start temperature is 700 ° C. to 850 ° C., and the temper temperature is 560 ° C. or lower when heat treatment is performed.

When the heating temperature of the slab was less than 1000 ° C. and the rolling start temperature was less than 650 ° C., the bainite fraction was less than 80.0%, and the strength of the base metal was insufficient.
On the other hand, when the heating temperature exceeds 1150 ° C. and the rolling start temperature exceeds 850 ° C., the effective crystal grain size at each measurement position in the plate thickness direction (position measured every 5 mm in the plate thickness direction) becomes more than 30 μm, and the base metal. Lack of toughness and arrestability.
Further, if the finishing temperature does not satisfy the range of the rolling start temperature of −80 ° C. to the rolling start temperature of + 80 ° C., the variation in the plate of the microstructure becomes large, and the arrest property cannot be stably ensured.
When reheating is performed after the cooling step, if the reheating temperature is less than 400 ° C, the toughness of the base metal is not improved, and if it exceeds 600 ° C, the strength of the base metal decreases, and it is difficult to secure the strength of the base metal. became.

It has been clarified that the steel sheet according to the present embodiment satisfying these conditions improves the HAZ toughness through the miniaturization of the HAZ structure and is excellent in the mechanical properties of the base material.

Further, the reason for limiting the chemical composition of the steel sheet according to the present embodiment will be described. In the following description, "%" in the description of each element means "mass%".

(C: 0.01% to 0.20%)
C is an element necessary for ensuring strength. If the amount of C is less than 0.01%, the required strength cannot be secured. However, if the amount of C exceeds 0.20%, it becomes difficult to secure toughness for both the base material and HAZ. The preferable lower limit of the amount of C is 0.03% or more, more preferably 0.05% or more. The preferred upper limit 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. 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 increased toughness. to degrade. The preferable lower limit of the amount of Mn is 0.60% or more, and the preferable upper limit is 2.00% or more.

(Ti: 0.003% to 0.024%)
Ti forms a composite oxide together with Zr, and this composite oxide functions as an intragranular ferrite nucleation nucleus 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 formation of B nitrides is suppressed, and the desired effect cannot be obtained with the steel sheet according to 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 0.0030%)
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 grain boundary ferrites, which contributes to the improvement of the base metal strength. In order to secure the strength of the base material, 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 0.0030% or less. The preferable upper limit of the amount of B is 0.0025% or less, more preferably 0.0020% or less.

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

(O: 0.0010% to 0.0050%)
O (oxygen) is an element indispensable for the formation of a composite oxide containing Zr and Ti, and it is necessary to contain 0.0010% or more of O. However, when the amount of O is excessive, oxides are excessively generated, which deteriorates the cleanliness of the steel sheet and adversely affects the toughness of the base material and 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 lower limit of the amount of O is 0.0015% or more, and the preferable upper limit is 0.0040% or less.

(Zr: 0.0005% to 0.0100%)
Zr is an element indispensable for fine dispersion of oxides and increase of solid solution B, and 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 0.0030% 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. Zr. Insol. Zr is an acid-insoluble Zr, and Insol. Zr and Sol. Zr is the sum of Zr.

(Cu: 0.1% to 1.5%)
Cu is an element effective for ensuring the strength of steel. 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 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 effective for increasing the toughness of a steel matrix (fabric) in a 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 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, but if the content 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.

(Al: 0% 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. Since the smaller the amount of Al, the more preferable it is, the lower limit is not particularly specified and may be 0%.

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. The amount of Nb is more preferably 0.025% or less, still 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. The amount of Cr is preferably 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 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. The amount of V is preferably 0.05% or less.

(Ca + REM [total of Ca and REM]: 0% to 0.0005% or less)
Ca and REM are elements that are more likely to react with oxygen more preferentially than Al. 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. 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 0% may be used.
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% or less)
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 is not particularly specified, and 0% may be used.

(Carbon equivalent Ceq .: 0.45% to 0.55%)
The steel sheet according to this embodiment has a carbon equivalent Ceq. Obtained by the following formula (3). Is 0.45% to 0.55%.
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. The additive-free element (element having a content of 0% by mass) is calculated by substituting zero (0% by mass) as the content of the corresponding element in the formula (3).

If the carbon equivalent is less than 0.45%, the strength required for the high-strength steel plate cannot be satisfied. If the carbon equivalent exceeds 0.55%, the hardenability becomes excessive and the joint toughness cannot be satisfied. The preferable lower limit of carbon equivalent is 0.46% or more, and the more preferable lower limit is 0.47% or more. The preferable upper limit of carbon equivalent is 0.53% or less, and the more preferable upper limit is 0.50% or less.

The steel sheet according to this 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, so it is necessary to add an extra element in consideration of the yield. Moreover, the addition method is not specified in particular. Any method may be used as long as it 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 50 mm or more, and it may be 50 mm to 80 mm.

The steel sheet according to the present embodiment has the following physical properties, for example, when the plate thickness is 50 mm or more (for example, 50 mm to 80 mm).
The yield stress is 550 MPa or more (for example, 550 MPa to 750 MPa).
In a welded joint with a heat input of 4.5 kJ / mm to 6.0 kJ / mm (for example, a multi-layer welded joint), a notch is formed in the fusion wire (hereinafter, may be referred to as "FL") on the straight side of the displacement groove. In the crack opening displacement test (hereinafter, sometimes referred to as “CTOD test”) conducted at a test temperature of −10 ° C., the amount of crack opening immediately before fracture (hereinafter, “CTOD”) was introduced. (Sometimes referred to as "value") is 0.15 mm or more.
The brittle ductile transition temperature (vTrs) of the base metal is −60 ° C. or lower.
The temperature at which the arrest toughness value Kca reaches 6000 N / mm ^1.5 (hereinafter, may be referred to as “TKca 6000”) is −10 ° C. or lower.

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

A preferred method for producing a steel sheet according to this embodiment is
In the secondary refining in a reduced pressure atmosphere, to the molten steel in which the amount of dissolved oxygen is adjusted to 0.0005% to 0.0050% in mass%, it is added in the order of Zr after adding Ti, in the order of Ti after adding Zr, or Ti. And Zr are added at the same time.
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
A cooling step of starting the water cooling of the steel sheet after the rolling step when the temperature range is 650 ° C. to 850 ° C. and stopping the water cooling in the temperature range of the surface temperature of 500 ° C. or lower.
Have. The slab (steel piece) has the above-mentioned chemical composition.
Hereinafter, each step will be described.

(Casting process)
In order to obtain the steel sheet according to the present embodiment, the amount of dissolved oxygen before the start of deoxidation is controlled as described above.
Specifically, 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, in secondary refining under a reduced pressure atmosphere performed by a vacuum refining device or a refining device in an inert gas after converter refining, the amount of dissolved oxygen in the molten steel is 0.0005% to 0.0050% by mass. Adjust to the range of. 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 uniform solution of the trace alloy element (for example, Nb when Nb is contained), 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.
When the carbon equivalent Ceq is low in the above range (for example, 0.45% to 0.50%), the slab heating temperature is set to, for example, 1000 ° C. to 1100 ° C. to stabilize the arrest property. Can be secured.

(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 important step for determining the effective crystal grain size and bainite fraction.

In the finish rolling, rolling is started in a temperature range in which the surface temperature of the steel sheet (rolling start temperature) 1 sec before the finish rolling is 650 ° C to 850 ° C. Then, in this temperature range, finish rolling is performed so that the rolling reduction rate is 50% or more and the temperature (finishing temperature) 1 sec after the completion of rolling is from the rolling start temperature of −80 ° C. to the rolling start temperature of + 80 ° C.

The lower limit of the rolling start temperature is preferably 680 ° C. or higher, more preferably 700 ° C. or higher. The upper limit of the rolling start temperature is preferably 830 ° C. or lower, more preferably 800 ° C. or lower. 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.
When the carbon equivalent Ceq is low within the above range (for example, 0.45% to 0.50%), selecting a higher range (preferably 700 ° C. to 850 ° C.) for the rolling start temperature is selected. The steel plate according to this embodiment can be easily obtained.

The preferred range of the finishing temperature is the range of rolling temperature −50 ° C. to rolling temperature + 50 ° C., and the more preferable range is the range of rolling temperature −40 ° C. to rolling 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 water cooling is stopped when the surface temperature is 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.

When the cooling shutdown temperature is 500 ° C. or lower, the strength is easily secured and the effective crystal grain size is easily refined. Alternatively, by generating pearlite in the range of 1.0% 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 400 ° 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 400 ° 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. When the carbon equivalent Ceq is low in the above range (for example, 0.45% to 0.50%), when heat treatment is performed, the heat treatment temperature (temper temperature) should be set in a lower range (for example, 560 ° C. or lower). When selected, the steel sheet according to the present embodiment can be easily obtained. Further, in this case, the steel sheet according to the present embodiment can be easily obtained without heat treatment.

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 is acid-soluble Zr, and the matrix is dissolved by electrolysis of steel in a non-aqueous solvent by electrolytic extraction residue analysis method, and the residue (precipitate and inclusions) is extracted with a filter having a pore size of 0.1 μm. It was separated and the amount of Zr contained in the separated solution was measured. Sol. In Table 1. Where Zr is "-", Sol. Indicates that Zr was not measured. And Insol. Zr is acid-insoluble Zr, and Zr to Sol. It can be obtained by subtracting Zr. B _AsBN is determined by equation (2), _{B F} is calculated by the equation (1), Ceq. Was calculated by equation (3).

Tables 3 and 4 show the amount of dissolved oxygen 1 minute before the addition of Ti in the RH vacuum refining facility, the amount of dissolved oxygen 3 minutes before the addition of Zr, the amount of dissolved oxygen 3 minutes before the simultaneous addition of Ti and Zr, Ti, Zr. The order of addition, heating conditions, rolling conditions, cooling conditions, and heat treatment conditions (temper temperature) are shown.
In Tables 3 and 4, in the "Ti, Zr addition order column", Ti and Zr are the same when Zr is added next to Ti, and Zr and Ti are the same when Ti is added next to Zr. The addition shows the case where Zr and Ti are added at the same time.

Tables 5 and 6 show the plate thickness, effective grain size, bainite fraction, ferrite fraction, pearlite fraction, and MA fraction. The effective crystal grain size and bainite fraction are the average values of the entire region between 5 mm and t / 2-5 mm below the table, and the average values at each measurement position in the plate thickness direction in the region. Indicates the maximum and minimum values of. Further, the maximum value-average value and the average value-minimum value calculated from these average value, maximum value and minimum value are shown.
Further, the ratio of the mass conversion value of Al oxide is 20% or less, the ratio of the mass conversion value of Zr oxide is 5% or more, and the total of the mass conversion value of Zr oxide and Ti oxide is 80% or more. Indicates the number density of oxides having an equivalent circle diameter of 0.5 μm to 10 μm, which satisfies the above.
Then, the toughness of the base metal, the strength of the base metal, the welding conditions (heat input), and the HAZ toughness are shown.
In Tables 5 and 6, the "region between the 5 mm portion and the t / 2-5 mm portion below the table" is from the front side of the steel plate to the surface of the steel plate from the position of 5 mm in the plate thickness direction and the position of 1/2 in the plate thickness direction. The area between the position and the position of 5 mm is shown.

Steels 1 to 20 are examples, and steels 21 to 48 are comparative examples.

The steel was melted in a 400-ton converter and deoxidized during the vacuum degassing treatment of the secondary refining by RH (Rhesstahl-Heraeus). Dissolved oxygen was adjusted before adding Ti and Zr so that the values shown in Tables 3 and 4 were obtained, and then Ti and Zr were added to deoxidize and cast into 280 mm to 360 mm thick slabs 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 50 mm to 80 mm through each step of heating, rolling, and cooling. Then, heat treatment was performed as necessary to adjust the material. The temper temperature during the heat treatment was between 400 ° C. and 600 ° C. The heat input under the welding conditions is 5.0 kJ / mm.

The effective grain size, bainite fraction, ferrite fraction, pearlite fraction and MA fraction were measured by the following procedure.

First, a method for measuring the effective crystal grain size will be described. A sample is taken so that the structure of the center of the width of the steel sheet, between 5 mm and t / 2-5 mm below the surface, can be observed every 5 mm in the thickness direction, and the surface perpendicular to the rolling direction is mirror-polished. .. If 1/4 part of the steel plate thickness (hereinafter, may be referred to as "t / 4 part") is not included in the above-mentioned sampling procedure, t / 4 part may be separately used for microstructure observation. A sample is taken and the surface perpendicular to the rolling direction is mirror-polished. The mirror-polished surface has a region between 5 mm and t / 2-5 mm below the surface that is perpendicular to the rolling direction every 5 mm in the plate thickness direction, and a region of 500 μm × 500 μm is pitched by 1 μm by the EBSD method. Measured in. 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. The weighted average was obtained by the above-mentioned equation (11).

The pearlite fraction is perpendicular to the rolling direction of the center of the width of the steel sheet, in the area between 5 mm below the surface and t / 2-5 mm, so that the structure can be observed every 5 mm in the plate thickness direction. The surface was mirror-polished. If 1/4 part of the steel plate thickness (hereinafter, may be referred to as "t / 4 part") is not included in the above-mentioned sampling procedure, t / 4 part may be separately used for microstructure observation. A sample was taken and the surface perpendicular to the rolling direction was mirror-polished. The mirror-polished surface was corroded with nital, and four fields of view were photographed at a magnification of 500 times using an optical microscope to obtain the pearlite fraction of each field of view, and the average value was taken as the pearlite fraction. The size of one field of view was 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.

As for the MA fraction, the mirror-polished surface was corroded by a repeller, and four fields of view were photographed at a magnification of 500 times using an optical microscope to obtain the pearlite fraction of each field, and the average value was taken as the pearlite fraction. The size of one field of view was 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 was defined as a portion having a KAM (Kernel Average Measurement) value of 1 ° or less when the measurement points measured by the above EBSD method were first close to each other. The surface integral of this ferrite was determined for each 5 mm in the plate thickness direction in the region between the 5 mm portion and the t / 2-5 mm portion below the table. When t / 4 parts were not included, the sample of t / 4 parts was separately prepared, and the ferrite fraction was determined by the above method.

The bainite fraction was the balance of the pearlite fraction, the MA fraction and the ferrite fraction.

The inclusion survey was measured by the following procedure. First, a thermodynamic cycle test piece of 12 mm in the thickness direction × 12 mm in the plate width direction × 70 mm in the rolling direction was collected from the t / 4 position in the plate thickness direction at the center of the width of the steel plate. Next, 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 surface of the thermodynamic cycle test piece as it was mirror-polished was measured by SEM / EDX (scanning electron microscope / energy dispersive X-ray spectroscopy) using "JXA-8530F" manufactured by JEOL. The observation conditions were an acceleration voltage of 15 kV, a current of 89 μA to 91 μA, an observation field area of 90 mm ^{2 to} 100 ^{mm 2} , and an number of analyzes of 500 or more. The elements to be analyzed were O, 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 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 value of the oxide of each element assuming the single oxides of these elements, Ti ₂ O ₃ , ZrO ₂ , and Al ₂ O ₃ , is calculated from the following formulas (5) to (7). calculate.
Ti ₂ O ₃ = Ti × 3.003 ... (5)
ZrO ₂ = Zr × 1.351 ... (6)
Al ₂ O ₃ = Al × 3.779 ... (7)

_{The content ratio (%) of Al 2} O ₃ (Al oxide) is 20% or less with respect to the total of these, that is, _{the content ratio of ZrO 2} (Zr oxide) and Ti ₂ O ₃ (Ti oxide) ( The number density of oxides in which the total of%) satisfies 80% or more and the equivalent circle diameter of this oxide 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 ) ... (8)
ZrO ₂ content ratio (%) = ZrO ₂ / (Ti ₂ O ₃ + ZrO ₂ + Al ₂ O ₃ ) ... (9)
Al ₂ O ₃ content ratio (%) = Al ₂ O ₃ / (Ti ₂ O ₃ + ZrO ₂ + Al ₂ O ₃ ) ... (10)
The calculation results are shown in Table 8.

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 t / 4 position in the plate thickness direction. The test piece was subjected to the test three times in the range of 0 ° C. to −140 ° C. to determine the brittle ductile transition temperature (vTrs). Those having vTrs of -60 ° C or lower 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 taken from the direction perpendicular to the rolling direction at the t / 4 position 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, a test piece is sampled so that the welding direction is parallel to the width direction (so that it is perpendicular to the rolling direction), a submerged groove with a groove angle of 30 ° is provided, and a gap of 10 mm is provided. Assembled with, welding wire: Y-DM3 (manufactured by Nippon Steel & Sumikin Welding & Co., Ltd.), welding flux: NB-60L, welded joint by performing multi-layer welding by submerged arc welding with welding heat input of 5.0 kJ / mm. Was produced. Then, according to ISO 15653 (2010), a CTOD test piece in which the melting line on the straight side of the re-shaped groove is bent at three points and used as the notch position of the CTOD test piece is collected, and the CTOD value (opening displacement δ) at −10 ° C. _-10 ) was measured. Three of the above tests were carried out, _{and those having a minimum value of δ-10} of 0.15 mm or more were considered to have 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". The temperature gradient type ESSO test was performed. The temperature at which the arrest toughness value Kca became 6000 N / mm ^1.5 _{, that is, TKca 6000} was determined. Then, _{it was evaluated that the TKca6000} having a temperature of −10 ° C. or lower was excellent in arrestability.

As is clear from Tables 1 to 6, Steels 1 to 20 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 21 to 48 of the comparative example were inferior in HAZ toughness because they were outside the range specified by the steel sheet according to the present embodiment. Further, even those having excellent HAZ toughness had inferior mechanical properties in the base metal, which is a portion other than the HAZ and the weld metal part.

Since the steel sheet according to the present embodiment has high strength and is excellent in base material toughness, arrest property, and weld heat-affected zone toughness, safety is improved and the steel sheet can be thinned. , It is possible to dramatically reduce the construction cost of welded structures.

Claims (6)

By mass%
C: 0.01% to 0.20%,
Si: 0.02% to 0.50%,
Mn: 0.30% to 2.50%,
Ti: 0.003% to 0.024%,
B: 0.0005% to 0.0030%,
N: 0.0010% to 0.0090%,
O: 0.0010% to 0.0050%,
Zr: 0.0005% to 0.0100%,
Sol. Zr: 0% to 0.0020%,
Cu: 0.1% to 1.5%,
Ni: 0.1% to 3.0%,
Al: 0% to 0.0050%,
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 0.0005% to 0.0030%.
_BasBN represented by the following formula (2) is 0% or less.
Formula (3) represented Ru coal Mototo amount Ceq. Is 0.45% to 0.55%,
In crystal orientation analysis using electron backscatter diffraction (EBSD) with a cross section perpendicular to the rolling direction, 5 mm from the front side of the steel sheet in the plate thickness direction and 1/2 position in the plate thickness direction toward the surface of the steel sheet 5 mm. The average value of the effective crystal grain size in the entire region between the position and the position is 30 μm or less.
The average value of the effective crystal grain size at each measurement position in the plate thickness direction in the region is from the average value of the effective crystal particle size of the entire region of -15 μm to the average value of the effective crystal particle size of the entire region + 15 μm. Satisfy the range,
The microstructure in the entire region has a bainite fraction of 80.0% to 100.0%, a ferrite fraction of 0% to 20.0%, and a pearlite fraction of 0% to the average value of the area ratio. The total of the bainite fraction, the ferrite fraction, the pearlite fraction, and the MA fraction is 100%, with 1.0% and the MA fraction being 0% to 1.0%.
The bainite fraction at each measurement position in the plate thickness direction in the region satisfies the range of the average value of the bainite fraction in the entire region of -15% to the average value of the bainite fraction in the entire region + 15%. ,
The oxide analyzed at the 1/4 position in the plate thickness direction is a single oxidation by the elements of Ti, Zr, and Al, which is obtained from the measured values of the amount of O, the amount of Ti, the amount of Zr, and the amount of Al in the oxide. The content ratio of the mass-converted value of Al oxide to the total mass-converted value of the oxide of each element of Ti, the Zr, and Al when it is assumed to be a thing is 20% or less, and the mass-converted Zr oxide. An oxide having a value of 5% or more and a total mass conversion value of Zr oxide and Ti oxide of 80% or more, a circle equivalent diameter of 0.5 μm to 10 μm, and a number density of 10 pieces / mm ² or more. Steel plate that is.

(However, in the formula (1), _BasBN is represented by the following formula (2). B is the content (mass%) of the element B contained in the steel sheet, and the relationship of _{0 ≦ BF ≦ B.} Satisfy.)

(However, in the formula (2), 0 ≦ B _asBN ≦ B (when B _asBN <0, B _asBN = 0), 0 ≦ Insol.Zr is satisfied, and N, Ti, O, and Al are , The content (mass%) of each element contained in the steel plate, and Insol.Zr indicates the content (mass%) of acid-insoluble Zr.)
Ceq. = C + Mn / 6 + (Cu + Ni) / 15+ (Cr + Mo + V) / 5 ... (3)
However, C, Mn, Cu, Ni, Cr, Mo and V in the formula are the contents (mass%) of each element contained in the steel sheet. The temperature at which the plate thickness is 50 mm or more, the yield stress of the base metal, which is the part other than the weld heat affected zone and the weld metal part, is 550 MPa or more, and the arrest toughness value Kca is 6000 N / mm ^{1.5 is-.} A crack opening displacement test performed at a test temperature of -10 ° C on a weld heat-affected zone that occurs when welding is performed at a heat input of 4.5 kJ / mm to 6.0 kJ / mm at 10 ° C or lower. The steel plate according to claim 1, wherein the opening amount is 0.15 mm or more. The method for manufacturing a steel sheet according to claim 1 or 2.
In the secondary refining in a reduced pressure atmosphere, to the molten steel in which the amount of dissolved oxygen is adjusted to 0.0005% to 0.0050% in mass%, it is added in the order of Zr after adding Ti, in the order of Ti after adding Zr, or Ti. And Zr are added at the same time.
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
A cooling step of starting the water cooling of the steel sheet after the rolling step when the temperature range is 650 ° C. to 850 ° C. and stopping the water cooling in the temperature range of the surface temperature of 500 ° C. or lower.
A method for manufacturing a steel sheet having. The method for manufacturing a steel sheet according to claim 1 or 2.
In the secondary refining in a reduced pressure atmosphere, Ti was added to the molten steel in which the amount of dissolved oxygen was adjusted to 0.0005% to 0.0050% by mass%, and the amount of dissolved oxygen in the molten steel after the addition of Ti was adjusted by mass%. After adjusting to 0.0005% to 0.0050%, Zr is added, and molten steel after addition of Ti and Zr is cast to obtain a slab.
A heating step of heating the steel pieces after the casting step in a temperature range of 1000 ° C. to 1150 ° C.
Rolling of the steel pieces after the heating step is started in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction ratio is 50% or more, and the temperature 1 sec after the completion of finish rolling is the rolling start temperature -80 ° C to the rolling start temperature. A rolling process that carries out rolling at + 80 ° C and
A cooling step of starting the water cooling of the steel sheet after the rolling step when the temperature range is 650 ° C. to 850 ° C. and stopping the water cooling in the temperature range of the surface temperature of 500 ° C. or lower.
A method for manufacturing a steel sheet having. The method for manufacturing a steel sheet according to claim 1 or 2.
In the secondary refining in a reduced pressure atmosphere, Zr was added to the molten steel in which the amount of dissolved oxygen was adjusted to 0.0005% to 0.0050% by mass%, and the amount of dissolved oxygen in the molten steel after the addition of Zr was adjusted by mass%. After adjusting to 0.0005% to 0.0050%, Ti is added, and the molten steel after addition of Ti and Zr is cast to obtain a slab.
A heating step of heating the steel pieces after the casting step in a temperature range of 1000 ° C. to 1150 ° C.
Rolling of the steel pieces after the heating step is started in a temperature range of 650 ° C. to 850 ° C., the cumulative rolling reduction ratio is 50% or more, and the temperature 1 sec after the completion of finish rolling is the rolling start temperature -80 ° C to the rolling start temperature. A rolling process that carries out rolling at + 80 ° C and
A cooling step of starting the water cooling of the steel sheet after the rolling step when the temperature range is 650 ° C. to 850 ° C. and stopping the water cooling in the temperature range of the surface temperature of 500 ° C. or lower.
A method for manufacturing a steel sheet having. The method for producing a steel sheet according to any one of claims 3 to 5, further comprising a heat treatment step of reheating the steel sheet after the cooling step to a temperature of 400 ° C. to 600 ° C.

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