JP7127751B2 - Steel plate and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel sheet and a method for manufacturing the same.

Uses of steel plates include ships, high-rise buildings, other buildings, bridges, offshore structures, LNG storage tanks and other large tanks, and welded structures such as line pipes (see, for example, Patent Documents 1 to 5). . 2. Description of the Related Art In recent years, due to an increase in the loading weight of container ships, etc., the size of welded structures is being increased. Along with this, steel sheets are required to be thicker and stronger. In addition, from the viewpoint of further safety and reliability, further improvements in low-temperature toughness and fracture toughness have become issues for welded structures such as those described above.

Furthermore, welded structures are generally welded with a large heat input of, for example, 35 kJ/mm or more. In order to suppress brittle fracture, it is required to improve the toughness in the weld heat affected zone (HAZ: Heat Affected Zone) (hereinafter referred to as "HAZ toughness") generated when performing high heat input welding.

JP 2019-023322 A JP 2019-023323 A JP 2019-023324 A JP 2019-035107 A WO2019/069771

However, since there is generally a so-called trade-off relationship between strength, low-temperature toughness, and HAZ toughness, it has not been easy to achieve both. Furthermore, the current situation is that almost no investigation has been made so far regarding the improvement of fracture toughness.

An object of the present invention is to solve the above problems and to provide a steel sheet having high strength and excellent low temperature toughness, fracture toughness and HAZ toughness, and a method for producing the same.

The gist of the present invention is the following steel sheet and method for producing the same.

(1) The chemical composition of the steel sheet is % by mass,
C: 0.040 to 0.160%,
Si: 0.01 to 0.50%,
Mn: 0.70-2.50%,
P: 0.030% or less,
S: 0.020% or less,
Al: 0.010% or less,
N: 0.0010 to 0.0080%,
O: 0.0005 to 0.0040%,
Nb: 0.003 to 0.050%,
Ti: 0.003-0.024%,
Zr: 0.0007 to 0.0050%,
Insol. Zr: 0.0007 to 0.0040%,
Sol. Zr: 0.0010% or less,
B: 0.0003 to 0.0040%,
balance: Fe and impurities,
satisfying the following formula (I),
In a cross section perpendicular to the rolling direction of the steel plate, the metal structure at a position 1/4 t from the surface of the steel plate, where t is the thickness of the steel plate, is
80% or more of bainite is contained in terms of area%, and
The average length of the bainitic ferrite constituting the bainite in the major axis direction is 10 μm or less,
In a cross section parallel to the rolling direction and thickness direction of the steel sheet, the average length in the thickness direction of the prior austenite grains at a position 1/4 t from the surface of the steel sheet is 20 μm or less, and the average aspect ratio is 2. .5 or more,
Composite inclusions containing Zr and B, having an equivalent circle diameter of 0.5 μm or more, and having a ratio of Al ₂ O ₃ to the total of ZrO ₂ , Ti ₂ O ₃ and Al ₂ O ₃ of In mass%, the number density of the composite inclusions, which is 50% or less, is 5 to 300 pieces/mm ² ,
steel plate.
B _F ≤ 0.0030 (I)
however,
If B _F ′>B, then B _F =B
If 0<B _F '≦B then _BF = _BF '
If B _F ′≦0, then B _F =0
, and B _F ' is represented by the following formula (II).
_BF ′=B−(N−(Ti−(O—Insol.Zr×(32/91.224))×(95.73/48))×(14/47.867))×(10.811 / 14) ... (II)
The symbol of each element in the above formula represents the content (% by mass) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.

(2) the chemical composition, instead of part of the Fe, by mass%,
Cu: 1.50% or less,
Ni: 2.50% or less,
Cr: 1.00% or less,
Mo: 1.00% or less, and V: 0.150% or less,
It contains at least one or more selected from the group consisting of
The steel plate according to (1) above.

(3) the chemical composition, instead of part of the Fe, by mass%,
Te: 0.0100% or less,
which contains
The steel plate according to (1) or (2) above.

(4) the chemical composition, instead of part of the Fe, is mass %,
W: 1.00% or less, and Sn: 0.50% or less,
It contains at least one or more selected from the group consisting of
The steel sheet according to any one of (1) to (3) above.

(5) the chemical composition, instead of a part of the Fe, by mass%,
Total of Ca, Mg and REM: 0.0005% or less,
which contains
The steel sheet according to any one of (1) to (4) above.

(6) The method for manufacturing the steel sheet according to any one of (1) to (5) above,
A refining step of producing molten steel, and a continuous casting step of continuously casting the molten steel to produce a steel slab having the chemical composition according to any one of (1) to (5) above. In a method for manufacturing a steel sheet, in which a steel slab is subjected to a heating process, a hot rolling process, and an accelerated cooling process in this order,
In the refining step, the amount of deoxidized Al to be introduced is 0.2 to 1.3 kg per 1 t of the molten steel, Zr is added after the dissolved O concentration in the molten steel becomes 0.0050% by mass or less, and B is added after 1 minute or more from the addition of Zr,
In the continuous casting step, the average cooling rate is set to 0.5° C./second or less when the surface temperature of the steel slab is between 1200 and 900° C.,
In the heating step, the steel billet is heated to a heating temperature of 950 to 1080° C.,
The hot rolling process includes rough rolling and finish rolling,
The rough rolling is performed in a range where the surface temperature of the steel slab is equal to or higher than _Trex ,
The cumulative rolling reduction in the rough rolling is 10 to 75%,
The finish rolling is performed in a range where the surface temperature of the billet is Ar ₃ or more and less than _Trex ,
The cumulative rolling reduction in the finish rolling is 65 to 90%, and the time between passes is 15 seconds or less,
The time from the completion of the finish rolling to the start of cooling in the accelerated cooling step is set to 50 seconds or less,
In the accelerated cooling step, the cooling start temperature is T _rex -10 ° C. or less, and the average cooling rate from the start of cooling to the end of cooling is 5 to 50 ° C./sec, and the cooling stop temperature is 0 to 550 ° C. water-cooled to
A method of manufacturing a steel plate.
Here, Ar ₃ is obtained by the following formula (i), and _Trex is obtained by the following formula (ii). The symbol of each element in the following formula represents the content (% by mass) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.
Ar ₃ =910−310×C+65×Si−80×Mn−20×Cu−55×Ni−15×Cr−80×Mo (i)
T _rex =−91900 [Nb*] ² +9400 [Nb*]+770 (ii)
However, the solid-solution Nb amount (mass%) obtained by the following formula (iii) is calculated as sol. When Nb is
Nb≧sol. Nb, [Nb*]=sol. Nb
Nb<sol. For Nb, [Nb*]=Nb
and
sol. Nb=(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N) (iii)
Note that T in the above formula represents the heating temperature (° C.) of the steel slab in the heating process.

(7) After the accelerated cooling step, a tempering step of heating to a temperature range of 350 to 650 ° C. is further performed.
A method for producing a steel sheet according to (6) above.

According to the present invention, it is possible to obtain a steel sheet having high strength and excellent low temperature toughness, fracture toughness and HAZ toughness.

The present inventors have made detailed studies on the above problems, and as a result, have obtained the following findings.

As mentioned above, there is a so-called trade-off relationship between strength and low temperature toughness and HAZ toughness. In addition, as a result of studies by the present inventors, it has been found that it is not easy to achieve both strength and fracture toughness. Therefore, the present inventors first investigated a method for achieving both high strength and low temperature toughness and fracture toughness. As a result, by making the metal structure mainly bainite, the strength is increased, and in addition to refining and flattening the bainite structure, by refining the bainitic ferrite that constitutes the bainite, not only low-temperature toughness but also It was found that the decrease in fracture toughness can be suppressed.

In addition, by controlling the heating temperature before hot rolling to a low level and performing finish rolling at a high pressure reduction in the non-recrystallized region, the bainite structure is refined and flattened, and the bainitic ferrite is refined. I found what I can do.

Next, a method for improving HAZ toughness was investigated, and the following findings were obtained.

By containing Zr and B, B nitrides precipitate in the steel with the Zr-containing oxide as a nucleus. Such composite inclusions containing Zr and B (hereinafter also simply referred to as "composite inclusions") function effectively as intragranular ferrite formation sites and contribute to refinement of the HAZ structure.

On the other hand, HAZ toughness tends to deteriorate when Zr dissolved in steel increases. Therefore, the amount of Zr dissolved in steel, Sol. Zr is preferably 0.0010% by mass or less.

In order to obtain the effect of the composite inclusions described above, it is necessary to set the amount of B 2 _F , which is the amount of B dissolved in steel, to 0.0030% by mass or less.

Furthermore, when the ratio of Al ₂ O ₃ contained in the composite inclusions is low, the composite inclusions function more effectively as intragranular ferrite formation sites. Specifically, the equivalent circle diameter is 0.5 μm or more, and the ratio of Al ₂ O ₃ to the total of ZrO ₂ , Ti ₂ O ₃ and Al ₂ O ₃ is 50% or less in mass %. When the number density of the composite inclusions is 5 to 300 pieces/mm ² , a large amount of fine intragranular ferrite is generated in the HAZ, improving the HAZ toughness.

Excessive content of Al, which acts as a strong deoxidizing element, in steel inhibits the formation of composite inclusions. The Al content is preferably 0.010% by mass or less in order to secure the dissolved oxygen content in the molten steel and generate complex inclusions in the steel. Elements such as Ca, Mg, and REM, which have stronger deoxidizing power than Al, may be contained within a range of 0.0005% by mass or less in total.

The present invention has been made based on the above findings. Each requirement of the present invention will be described in detail below.

(A) Chemical composition The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %." In addition, in this specification, the term "to" indicating a numerical range is used to include the numerical values before and after it as a lower limit and an upper limit, unless otherwise specified.

C: 0.040-0.160%
0.040% or more of C is contained in order to ensure the strength of the steel sheet. On the other hand, if the C content exceeds 0.160%, it becomes difficult to ensure good low temperature toughness, fracture toughness and HAZ toughness, so the C content is made 0.160% or less. Therefore, the C content is 0.040% or more, preferably 0.050% or more or more than 0.050%, more preferably 0.060% or more or more than 0.075%. Also, the C content is 0.160% or less, preferably 0.140% or less, more preferably 0.120% or less.

Si: 0.01-0.50%
Since Si is effective as a deoxidizing element and a strengthening element, it is contained in an amount of 0.01% or more. On the other hand, if the Si content exceeds 0.50%, the low-temperature toughness, fracture toughness and HAZ toughness deteriorate significantly, so the Si content is made 0.50% or less. Therefore, the Si content is 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more. Also, the Si content is 0.50% or less, preferably 0.40% or less, more preferably 0.35% or less, still more preferably 0.30% or less.

Mn: 0.70-2.50%
Mn is contained in an amount of 0.70% or more in order to economically secure the strength of the steel sheet. On the other hand, if the Mn content exceeds 2.50%, the center segregation becomes remarkable, and the low temperature toughness, fracture toughness and HAZ toughness of the part where the center segregation occurs deteriorates, so the Mn content is 2.50%. Below. Therefore, the Mn content is 0.70% or more, preferably 0.90% or more, more preferably 1.20% or more. Also, the Mn content is 2.50% or less, preferably 2.00% or less, more preferably 1.80% or less, still more preferably 1.60% or less.

P: 0.030% or less P is an element present in steel as an impurity. In order to stably ensure low temperature toughness, fracture toughness and HAZ toughness, the P content is made 0.030% or less. It is preferably 0.020% or less, more preferably 0.015% or less. Although the lower limit is 0%, the P content may be 0.0001% or more in consideration of the cost for reducing the P content.

S: 0.020% or less S is an element present in steel as an impurity. If the S content exceeds 0.020%, a large amount of elongated MnS is generated in the center segregation portion, and the low temperature toughness, fracture toughness, HAZ toughness and ductility deteriorate. Therefore, the S content is set to 0.020% or less. Preferably, it is 0.010% or less. Since the lower the S content, the better, so the lower limit is not particularly defined, but from the viewpoint of production cost, the S content may be 0.0001% or more.

Al: 0.010% or less Al is generally an element positively contained as a deoxidizing element. However, when the Al content becomes excessive, the formation of desired composite inclusions becomes insufficient, and effective ferrite formation sites in the HAZ decrease. In addition, the formation of coarse cluster-like alumina (Al ₂ O ₃ )-based inclusions is promoted, which not only deteriorates the HAZ toughness, but also, in some cases, the low temperature toughness and fracture toughness. Therefore, the Al content is 0.010% or less, preferably 0.005% or less. Since the lower the Al content is, the better it is, the lower limit is not particularly defined, but from the viewpoint of production cost, the Al content may be 0.001% or more.

N: 0.0010 to 0.0080%
N forms Ti nitrides and has the effect of suppressing an increase in austenite grain size during heating of the billet, so 0.0010% or more is contained. However, if the N content exceeds 0.0080%, the steel sheet becomes embrittled, so the N content is made 0.0080% or less. Therefore, the N content is 0.0010% or more, preferably 0.0015% or more, more preferably 0.0020% or more. Also, the N content is 0.0080% or less, preferably 0.0065% or less, more preferably 0.0060% or less.

O: 0.0005 to 0.0040%
O is an element contained in steel and exists dissolved or as an oxide. Since it is difficult to clearly separate the two, the O content in the present invention is the total oxygen content (also referred to as T.O) that combines the two. If the O content is less than 0.0005%, the oxide dispersion number necessary for ensuring toughness cannot be obtained. On the other hand, when the O content exceeds 0.0040%, the cleanliness of the molten steel deteriorates, and nozzle clogging in the molten steel stage can be a factor in reducing productivity. In addition, since coarse oxides are formed and stress concentration occurs in the oxides, the low temperature toughness, fracture toughness and HAZ toughness are deteriorated. Therefore, the O content should be 0.0005 to 0.0040%.

Nb: 0.003-0.050%
Nb can improve the strength and toughness of the steel sheet. In addition, in order to obtain a predetermined microstructure, rolling in the non-recrystallized austenite region is required, and Nb is an element effective for expanding the non-recrystallized temperature region. It also contributes to productivity improvement. In order to obtain this effect, the content is 0.003% or more. However, if the Nb content exceeds 0.050%, the low temperature toughness, fracture toughness, HAZ toughness and weldability deteriorate, so the Nb content is made 0.050% or less. Therefore, the Nb content is 0.003% or more, preferably 0.005% or more, more preferably 0.008% or more. Also, the Nb content is 0.050% or less, preferably 0.025% or less, more preferably 0.018% or less.

Ti: 0.003-0.024%
Ti is an element that forms complex inclusions together with Zr. As described above, the composite inclusions function as intragranular ferrite formation sites in the HAZ and contribute to refinement of the HAZ structure. In order to obtain this effect, the content is 0.003% or more. However, when the Ti content exceeds 0.024%, a large amount of Ti nitrides are generated, the amount of B nitrides generated is suppressed, and the effect of improving HAZ toughness cannot be obtained. Also, excess Ti forms TiC, which degrades low temperature toughness, fracture toughness and HAZ toughness. Therefore, the Ti content is 0.003% or more, preferably 0.005% or more. Also, the Ti content is 0.024% or less, preferably 0.020% or less.

Zr: 0.0007-0.0050%
Zr is Sol. Zr and Insol. It is the sum with Zr. The Zr content is 0.0007% or more, preferably 0.0010% or more. Also, the Zr content is determined according to Insol. Zr upper limit and Sol. The sum with the upper limit of Zr, that is, 0.0050% or less, preferably 0.0040% or less.

Insol. Zr: 0.0007-0.0040%
Insol. Zr is acid-insoluble Zr and is Zr contained in inclusions such as composite inclusions. Zr is an important element that forms Zr-containing oxides that act as nuclei for intragranular transformation. However, Insol. If the Zr content is less than 0.0007%, the oxide composition required for securing toughness cannot be obtained. On the other hand, Insol. When the Zr content exceeds 0.0040%, most of it is ZrO ₂ produced during the molten steel stage, and the frequency of nozzle clogging increases. Also, Insol. When Zr is increased, excessive ZrO ₂ is produced and the stress is concentrated, which significantly deteriorates the HAZ toughness. For this reason, Insol. The Zr content should be 0.0007 to 0.0040%.

Sol. Zr: 0.0010% or less Sol. Zr represents acid-soluble Zr, that is, Zr dissolved in steel. Sol. When the Zr content increases, the HAZ toughness deteriorates significantly. Therefore, the content is made 0.0010% or less. Sol. Since Zr is preferably as small as possible, the lower limit is not particularly defined, and may be 0%.

Insol. Zr is measured by the following method. First, a test piece is cut out from a steel plate and electrolyzed with about 0.4 g of 10% acetylacetone-1% tetramethylammonium chloride/methanol at a current density of 20 mA/cm ² . The solution used for the electrolysis was filtered through a filter with a pore size of 0.2 μm, and the extraction residue collected on the filter was analyzed by a known chemical analysis method (e.g., ICP emission spectrometry). Zr content was measured and Insol. Let it be Zr. Insol. The Zr-subtracted value is the Sol. Let it be Zr.

B: 0.0003 to 0.0040%
B is an element that improves the hardenability of steel materials, precipitates as B nitrides around Zr-containing oxides to form complex inclusions, and improves intragranular transformation ability. In order to deposit B nitride around the Zr-containing oxide, the B content should be 0.0003% or more. However, even if the B content is excessive, the effect is saturated, so the B content is made 0.0040% or less. Therefore, the B content is 0.0003% or more, preferably 0.0005% or more, more preferably 0.0010% or more. Also, the B content is 0.0040% or less, preferably 0.0035% or less, more preferably 0.0030% or less.

In the chemical composition of the steel sheet of the present invention, in addition to the above elements, at least one selected from the group consisting of Cu, Ni, Cr, Mo and V is added in the following ranges for the purpose of improving strength. may be contained in The reason for limiting each element will be explained.

Cu: 1.50% or less Cu has the effect of improving the strength and toughness of the steel sheet, so it may be contained as necessary. However, if Cu is contained excessively, the improvement in performance corresponding to the increase in alloy cost cannot be seen, and rather it may cause surface cracks. Therefore, the Cu content is 1.50% or less, preferably 1.20% or less, more preferably 1.00% or less. In order to obtain the above effects more reliably, the Cu content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.050% or more.

Ni: 2.50% or less Ni is an element that has the effect of improving the strength of the steel sheet, so it may be contained as necessary. In addition, Ni is an element that has the effect of increasing the toughness of the steel matrix (material) in a solid solution state. However, excessive Ni content deteriorates low temperature toughness, fracture toughness, HAZ toughness and weldability. Therefore, the Ni content is 2.50% or less, preferably 1.00% or less, more preferably 0.50% or less, and even more preferably 0.30% or less. In order to obtain the above effects more reliably, the Ni content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.050% or more.

Cr: 1.00% or less Cr is an element that has the effect of improving the strength of the steel sheet, so it may be contained as necessary. However, excessive Cr content deteriorates low temperature toughness, fracture toughness, HAZ toughness and weldability. Therefore, the Cr content is 1.00% or less, preferably 0.80% or less, more preferably 0.50% or less, and even more preferably 0.30% or less. If the above effects are to be obtained more reliably, the Cr content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.050% or more.

Mo: 1.00% or less Mo is an element that has the effect of improving the strength of the steel sheet, so it may be contained as necessary. However, excessive Mo content deteriorates low temperature toughness, fracture toughness, HAZ toughness and weldability. Therefore, the Mo content is 1.00% or less, preferably 0.80% or less, more preferably 0.50% or less, and even more preferably 0.30% or less. In order to obtain the above effects more reliably, the Mo content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% or more.

V: 0.150% or less V is an element that has the effect of improving the strength of the steel sheet, so it may be contained as necessary. However, excessive V content deteriorates low temperature toughness, fracture toughness, HAZ toughness and weldability. Therefore, the V content is 0.150% or less, preferably 0.100% or less, more preferably 0.070% or less, and even more preferably 0.050% or less. In order to obtain the above effects more reliably, the V content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% or more.

In the chemical composition of the steel sheet of the present invention, in addition to the above elements, Te may be contained within the range shown below for the purpose of refining the metal structure. The reason for the limitation will be explained.

Te: 0.0100% or less Te is an element that contributes to the improvement of toughness by refining the structure of the steel sheet, so it may be contained as necessary. However, even if Te is contained excessively, the above effect is saturated. Therefore, the Te content is 0.0100% or less, preferably 0.0070% or less, more preferably 0.0050% or less. In order to obtain the above effects more reliably, the Te content is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more.

In the chemical composition of the steel sheet of the present invention, in addition to the above elements, for the purpose of improving corrosion resistance, at least one selected from the group consisting of W and Sn may be contained within the range shown below. . The reason for limiting each element will be explained.

W: 1.00% or less W is an element that dissolves and adsorbs to rust in the form of oxyacid ions _WO4- ^, suppresses permeation of chloride ions in the rust layer, and improves corrosion resistance. may be included depending on However, even if W is contained excessively, not only the above effect is saturated, but also the low temperature toughness, fracture toughness and HAZ toughness may decrease. Therefore, the W content is 1.00% or less, preferably 0.75% or less. In order to obtain the above effect more reliably, the W content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% or more.

Sn: 0.50% or less Sn is an element that dissolves as Sn ²⁺ and has the effect of inhibiting corrosion in an acidic chloride solution. In addition, Sn has the effect of suppressing the anodic dissolution reaction of steel and improving the corrosion resistance. Therefore, it may be contained as necessary. However, even if Sn is excessively contained, not only the above effect is saturated, but also the rolling cracks of the steel sheet tend to occur. Therefore, the Sn content is 0.50% or less, preferably 0.30% or less. If the above effects are to be obtained more reliably, the Sn content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

In the chemical composition of the steel sheet of the present invention, in addition to the above elements, Ca, Mg and REM may be contained within the following ranges for the purpose of reducing the concentration of dissolved oxygen in the molten steel. The reason for the limitation will be explained.

Total of Ca, Mg and REM: 0.0005% or less Ca, Mg and REM are elements that preferentially react with oxygen more than Al. Therefore, it may be contained as necessary. However, even if Ca, Mg and REM are contained excessively, the amount of dissolved oxygen in the molten steel cannot be ensured, and desired composite inclusions cannot be formed. In order to form desired composite inclusions, the total content of Ca, Mg and REM should be 0.0005% or less. More preferably, the Ca content is less than 0.0003%, the Mg content is less than 0.0003%, and the REM content is less than 0.0003%, and the total content is 0.0005% or less.

Here, in the present invention, REM refers to a total of 17 elements of Sc, Y and lanthanoids, and the REM content means the total content of these elements.

In addition, the chemical composition of the steel sheet according to the present invention satisfies the following formula (I).
B _F ≤ 0.0030 (I)
however,
If B _F ′>B then B _F =B
If 0<B _F '≦B then _BF = _BF '
If B _F ′≦0, then B _F =0
, and B _F ' is represented by the following formula (II).
_BF ′=B−(N−(Ti−(O—Insol.Zr×(32/91.224))×(95.73/48))×(14/47.867))×(10.811 / 14) ... (II)
The symbol of each element in the above formula represents the content (% by mass) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.

_BF is the B content present as solid solution B in the steel. Since it is difficult to directly measure the solid solution B amount, in the present invention, it is calculated by the above formula.

As described above, in the steel sheet according to the present invention, by precipitating B nitrides on the surface layer of the composite inclusions, it is possible to effectively promote the formation of intragranular ferrite during cooling after welding, thereby improving the microstructure. to improve HAZ toughness. In order to obtain this effect, _BF must be 0.0030% or less. On the other hand, when _BF exceeds 0.0030%, the hardenability of the steel becomes excessive, coarsening of bainite and excessive increase in hardness occur, thereby lowering the HAZ toughness. Therefore, _BF is 0.0030% or less, preferably 0.0020% or less, and more preferably 0.0010% or less.

In the chemical composition of the steel sheet of the present invention, the balance is Fe and impurities. Here, the term "impurities" refers to components mixed in by various factors in raw materials such as ores, scraps, etc., and in the manufacturing process when steel sheets are manufactured industrially. means something

(B) Metal structure of steel plate The metal structure of the steel plate of the present invention will be described. In addition, "%" means "area %" in the following description. Further, in the present invention, when the thickness of the steel plate is t, the position of 1/4t from the surface of the steel plate in the cross section perpendicular to the rolling direction of the steel plate is called "1/4t position in C section", The position of 1/4t from the surface of the steel plate in the cross section parallel to the rolling direction and thickness direction of the steel plate is called "1/4t position in the L cross section". Furthermore, the above "rolling direction" means the rolling direction in finish rolling.

Bainite: 80% or more In the present invention, the metal structure is mainly bainite. Specifically, the strength of the steel sheet can be ensured by setting the area ratio of bainite at the 1/4t position in the C section to 80% or more. The area ratio of bainite is preferably 90% or more. There is no need to set an upper limit on the area ratio of bainite, that is, a single bainite phase may be used.

Note that ferrite, pearlite, and martensite/austenite mixed phase (MA phase) may be mixed as a residual structure, but it is allowed if the total area ratio of these phases is 20% or less. The total area ratio is preferably 10% or less. The smaller the total area ratio of these, the better, and the lower limit is not particularly limited. For example, the total area ratio may be 0%. Moreover, it may be more than 0%, or it may be 1% or more.

As described above, in addition to using bainite as the main component, by making the bainite structure finer and flatter and further making the bainitic ferrite finer, it is possible to achieve both the strength, low temperature toughness, and fracture toughness of the steel sheet. can. Specifically, the bainite structure needs to satisfy the following regulations.

Average length of bainitic ferrite: 10 µm or less At the 1/4t position in the C cross section, the average length of the bainitic ferrite constituting the bainite in the longitudinal direction is set to 10 µm or less. Fracture toughness can be ensured by refining bainitic ferrite that constitutes bainite. The average length of bainitic ferrite is preferably 8 μm or less.

Average length in the thickness direction of prior austenite grains: 20 μm or less Average aspect ratio of prior austenite grains: 2.5 or more Refinement of the bainite structure is achieved by controlling the heating temperature before hot rolling to a low level and without recrystallization. It can be achieved by performing finish rolling at a high reduction rate in the region. That is, the prior austenite grains of bainite have a shape elongated in the rolling direction. Therefore, at the 1/4t position in the L cross section, the average length in the thickness direction of the prior austenite grains is set to 20 µm or less, and the average aspect ratio is set to 2.5 or more. The average length in the thickness direction of the prior austenite grains is preferably 15 μm or less. Also, the average aspect ratio of the prior austenite grains is preferably more than 2.5, more preferably 4.0 or more.

Here, in the present invention, the area ratio of the metal structure is obtained as follows. First, a sample is taken from the steel plate so that the 1/4t position in the C section becomes the observation surface. Then, the observation surface is etched with nital, and after the etching, 8 fields of view are photographed at 500 times using an optical microscope. Then, an image analysis is performed on the obtained structure photograph, and the area ratio of each of ferrite and black perlite is determined by taking white and pearlite.

Next, the nital-etched portion is subjected to repeller etching, and image analysis is performed on the gray-looking portion due to the nital-etching.

The average length of bainitic ferrite and the area ratio of bainite are calculated by KAM (Kernel Average Misorientation) analysis using EBSD (Electron Back Scatter Diffraction). In the KAM analysis, in the tissue determined to be ferrite, the region where the local misorientation exceeds 1.0° is bainitic ferrite. In addition, bainitic ferrite having a length of 1 μm or more in the major axis direction is used for the measurement. Also, the area ratio of bainite is the sum of the area ratios of bainitic ferrite.

The average length in the thickness direction and the average aspect ratio of the prior austenite grains are measured according to JIS G 0551:2013. First, a sample is taken from the steel plate so that the 1/4t position in the L cross section becomes the observation surface. Next, after the observation surface is mirror-polished, it is etched by the Bechet-Beaujard method using a picric acid saturated aqueous solution. Grains appearing black due to corrosion are referred to as prior austenite grains.

The observation surface on which prior austenite grains are exposed is observed with an optical microscope, and 8 or more fields of view with an area of 0.05 mm ² or more (a total of 0.40 mm ² or more) are photographed. Then, the thickness of the prior austenite grains is measured by a cutting method based on the structure photograph taken with an optical microscope, and the average value is taken as the average length in the thickness direction of the prior austenite grains. Prior austenite grains having a length of 1 μm or more in the thickness direction are used for the measurement.

Further, from the above structure photograph, for each prior austenite grain, the maximum length in the long axis direction and the maximum length in the short axis direction perpendicular to the long axis direction are measured, and the ratio (maximum long axis length / short maximum shaft length). And let the average value be an average of the aspect-ratio of a prior austenite grain. When finish rolling is performed at a high reduction rate in the non-recrystallized region, the prior austenite grains exhibit a shape elongated in the rolling direction. so-called ND direction).

If the prior austenite grains cannot be fully revealed by the above method, ``Study for higher accuracy reconstruction method of steel austenite structure'' (Kengo Hata, Masayuki Wakita, Tomoya Fujiwara, Kaori Kono, Nippon Steel & Sumikin Engineering Co., Ltd.) The prior austenite grains are identified by the reconstruction method described in Report No. 404 (2016), pp. 24-30), and the average length and aspect ratio in the thickness direction of the prior austenite grains are determined. .

Number density of composite inclusions: 5 to 300/mm ²
As described above, the steel sheet according to the present invention has composite inclusions containing Zr and B in the metallographic structure. This serves as intragranular ferrite generation sites during cooling after welding, improving HAZ toughness. At this time, too fine composite inclusions contribute little to the HAZ toughness. In addition, composite inclusions with a low Al ₂ O ₃ ratio function more effectively as intragranular ferrite formation sites.

That is, in the present invention, the equivalent circle diameter (diameter) is 0.5 μm or more, and the ratio of Al ₂ O ₃ to the total of ZrO ₂ , Ti ₂ O ₃ and Al ₂ O ₃ is 0.5% by mass. , focus on composite inclusions that are 50% or less, and control their number density. If the number density of the composite inclusions is less than 5/mm ² , the effect of improving HAZ toughness cannot be sufficiently obtained. The higher the number density of the composite inclusions, the ^more the number of intragranular ferrite formation sites increases. Therefore, the number density of the composite inclusions is set to 5 to 300/mm ² .

In the present invention, inclusions containing 5% by mass or more of Zr, 0.1% by mass or more of B, and 1% by mass or more of O are defined as complex inclusions containing Zr and B. Also, the equivalent circle diameter and number density of composite inclusions can be measured by observing a mirror-polished steel surface with an SEM.

Specifically, a range of 10 mm×10 mm (100 mm ² ) was observed by SEM, and by quantitative analysis using an energy dispersive X-ray spectrometer (EDX) attached to the SEM, Zr of 5% by mass or more and 0.5% by mass of Zr were found. Contains 1% by mass or more of B and 1% by mass or more of O, and the proportion of Al ₂ O ₃ in the total of ZrO ₂ , Ti ₂ O ₃ and Al ₂ O ₃ is 50% by mass or less Identify particles. Then, among the particles, the number of particles having an equivalent circle diameter of 0.5 μm or more is measured, and the number density is measured by dividing the number by the area of the observed field of view. A photograph taken by an SEM may be used for the measurement of the number density.

(C) Mechanical Properties of Steel Plate The mechanical properties of the steel plate according to the present invention are not particularly limited, but the steel plate according to the present invention has high strength and excellent low temperature toughness and HAZ toughness. Specifically, it is preferable that the yield stress (YS) is 460-860 MPa and the tensile strength (TS) is 570-980 MPa. Further, the fracture surface transition temperature (vTrs), which is an index of low temperature toughness, is preferably −60° C. or less. Furthermore, it is preferable that the Crack Tip Opening Displacement (CTOD) value at −10° C., which is an index of fracture toughness, is 0.50 mm or more.

The tensile strength (TS) and yield stress (YS) are measured using a No. 1B tensile test piece sampled in the direction perpendicular to the rolling direction from the center of the plate thickness based on JIS Z 2241:2011. Specifically, the yield stress (YS) is the yield strength of the elongation set method at 0.2% elongation set. The fracture surface transition temperature (vTrs) is evaluated according to JIS Z 2242:2005, the test piece is a V-notch test piece, and the sample is taken so as to include the 1/4t position of the steel plate. Furthermore, in accordance with ISO 15653:2018, a CTOD test piece is taken with the notch position of three-point bending for the total thickness of the base material in the plate thickness direction, and the CTOD value at -10°C is measured.

Furthermore, when welding is performed under the condition that the welding heat input is 35 kJ / mm, the Charpy absorbed energy of the HAZ at -20 ° C. is 100 J or more in the average value of three measurements, and 50 J or more in the minimum value. is preferred.

The Charpy absorption energy of HAZ is measured by the following method. First, a heat cycle that reproduces welding with a heat input of 35 kJ/mm (high heat input welding) is applied to the steel plate. As a specific heat cycle condition, after heating from room temperature to 1400° C., holding at 1400° C. for 5 seconds, after that, the temperature range from 800° C. to 500° C., which is the temperature range related to intragranular transformation, is set to 1.0. Cooling is controlled at a rate of °C/sec.

Three V-notch test pieces are taken from the steel plate after the heat cycle, and subjected to a Charpy impact test at -20°C to measure the absorbed energy (vE _-20 ). The V-notch test piece is prepared according to the V-notch test piece described in JIS Z 2242:2005, and the Charpy impact test is performed according to JIS Z 2242:2005. Then, the average and lowest values of vE _-20 of the three specimens are obtained.

(D) Thickness of steel plate The thickness of the steel plate according to the present invention is not particularly limited, but when used as a welded structure, the plate thickness is preferably 10 to 70 mm, more preferably 20 to 60 mm. is more preferred. Moreover, the effect of improving the low-temperature toughness and fracture toughness in the present invention is remarkably exhibited when the thickness is less than 50 mm.

(E) Steel sheet manufacturing method Although there is no particular limitation on the manufacturing conditions of the steel sheet according to the present invention, for example, a refining process, a continuous casting process, a heating process, a hot rolling process and an accelerated cooling process are performed in order under the conditions shown below. can be manufactured. Each step will be explained.

(a) Refining process A refining process is a process of manufacturing molten steel. In the refining process, the amount of deoxidized Al introduced is 0.2 to 1.3 kg per 1 ton of molten steel. By setting the deoxidizing Al amount to 0.2 kg/t or more, it becomes possible to reduce the dissolved O concentration and finely disperse the Zr-containing oxide. On the other hand, by setting the deoxidized Al amount to 1.3 kg/t or less, the ratio of Al ₂ O ₃ in the composite inclusions can be reduced. Deoxidizing Al can be charged using, for example, a converter.

Subsequently, vacuum degassing is performed, and Zr is added after the dissolved O concentration in the molten steel reaches 0.0050% by mass or less. If Zr is added in a state where the dissolved O concentration exceeds 0.0050% by mass, it becomes difficult not only to finely disperse the Zr-containing oxide, but also to reduce the proportion of Al ₂ O ₃ in the composite inclusions. may not be possible. In addition, the amount of ZrO ₂ produced increases, increasing the risk of blockage of the injection nozzle into the tundish during continuous casting of molten steel.

Next, B is added after one minute or more has passed since the addition of Zr. As a result, B segregates on the surface layer of the Zr-containing oxide, and B nitrides can be precipitated. There is no particular upper limit to the time from the addition of Zr to the addition of B, but it is preferably within 5 minutes. Addition of Zr and B can be performed, for example, in a reflux degasser.

(b) Continuous Casting Process The continuous casting process is a process of continuously casting molten steel to produce steel slabs having the chemical composition described above. In the continuous casting process, the average cooling rate is set to 0.5°C/second or less when the surface temperature of the steel slab is between 1200°C and 900°C. As a result, the separation of ZrO ₂ and Al ₂ O ₃ in the Zr-containing oxide proceeds, and the proportion of Al ₂ O ₃ in the composite inclusions can be reduced.

(c) Heating step The heating step is a step that contributes to the control of the structure of the austenitic phase by heating the steel slab. In the heating step, the steel slab is heated to a heating temperature of 950-1080°C. The heating step is preferably performed in a heating furnace. Note that heating the steel slab to 950 to 1080 ° C. means heating so that the average temperature of the entire thickness of the steel slab when extracted from the heating furnace is in the range of 950 to 1080 ° C. In this specification, , the total thickness average temperature of the steel billet is called the heating temperature of the steel billet. Further, the total thickness average temperature can be calculated from the temperature in the heating furnace, the heating time, and the surface temperature of the steel slab.

If the heating temperature is less than 950° C., austenitization becomes insufficient and the austenite grains become finer, resulting in lower hardenability. Furthermore, the refinement of the austenite grains promotes recrystallization during finish rolling, thereby lowering the aspect ratio of the prior austenite grains. On the other hand, if the heating temperature exceeds 1080° C., the austenite grains become coarse, making it difficult to refine the bainite structure in the final structure. A preferable heating temperature range is 1000 to 1050°C.

(d) Hot Rolling Process The hot rolling process includes rough rolling and finish rolling. Rough rolling is carried out in a range in which the surface temperature of the steel slab is equal to or higher than _Trex . That is, the rough rolling is started when the surface temperature of the billet is T _rex or more, and the rough rolling is finished when the surface temperature of the billet is T _rex or more. By performing rough rolling in the range of T _rex or higher, recrystallization of austenite grains enables refinement. The surface temperature at the end of rough rolling may be higher than the surface temperature at the start of rough rolling. This is thought to be due to the effect of heat generated during rough rolling and the effect of heat transfer in the thickness direction of the billet due to the fact that the internal temperature is higher than the surface temperature.

Also, the cumulative rolling reduction in rough rolling is in the range of 10 to 75%. The cumulative draft in rough rolling is a value obtained by subtracting the thickness after completion of rough rolling from the thickness at the start of rough rolling and dividing the thickness at the start of rough rolling by the thickness at the start of rough rolling. If the cumulative rolling reduction during rough rolling is less than 10%, refinement by recrystallization of austenite is difficult, and porosity may remain and internal cracks may occur, resulting in deterioration of ductility and toughness. In addition, when the cumulative rolling reduction exceeds 75%, the austenite grains are excessively refined, and recrystallization during finish rolling is promoted, thereby decreasing the aspect ratio of the prior austenite grains and increasing the number of passes. productivity. A preferred cumulative rolling reduction is 30 to 60%. In addition, in the following description, the steel slab after rough rolling is called a steel plate.

The subsequent finish rolling is carried out in a range where the surface temperature of the steel sheet is Ar ₃ or more and less than _Trex . That is, after the completion of rough rolling, the steel sheet is cooled, the finish rolling is started when the surface temperature of the steel sheet is Ar ₃ or more and less than _Trex , and the finish rolling is finished when the surface temperature of the steel sheet is Ar ₃ or more and less than _Trex . . By performing the finish rolling in the range of less than _Trex , it is possible to impart strain to the austenite grains without recrystallization. Thereby, the bainite in the final structure can be refined. If the finishing temperature is set within a range where the surface temperature is T _rex or higher, recrystallization will be promoted and the aspect ratio of the prior austenite grains will decrease. On the other hand, if the finish rolling is carried out at a surface temperature of less than ₃ Ar, deformed ferrite is generated, and the final structure may not be composed mainly of bainite.

Further, the cumulative rolling reduction in finish rolling is in the range of 65 to 90%. The cumulative rolling reduction in finish rolling is a value obtained by subtracting the thickness after finish rolling from the thickness at the start of finish rolling (after the end of rough rolling) and dividing it by the thickness at the start of finish rolling. By setting the cumulative rolling reduction in the finish rolling to 65% or more, it becomes possible to impart sufficient strain to the austenite grains. When the cumulative rolling reduction is less than 65%, the austenite grains are not sufficiently strained, and the flattening of the austenite grains is not promoted, resulting in a decrease in the aspect ratio. On the other hand, when the cumulative rolling reduction exceeds 90%, recrystallization is promoted, the aspect ratio of the prior austenite grains is lowered, and the number of passes is increased to lower the productivity. A preferred cumulative rolling reduction is 70 to 80%.

Furthermore, the time between passes in finish rolling shall be 15 seconds or less. If the time between passes exceeds 15 seconds, the strain imparted by working is recovered, and bainite in the final structure cannot be sufficiently refined, and recrystallization is promoted, and the aspect ratio of prior austenite grains is reduced. Since the shorter the time between passes, the better, so there is no need to set a lower limit, but from the viewpoint of operability, it is preferably 3 seconds or more. Finish rolling is generally performed by reverse rolling. The time between passes in finish rolling means that the steel plate is rolled by the rolling rolls while advancing forward, and after the trailing end of the steel plate leaves the rolling rolls, the direction of travel of the steel plate reverses backward, and the trailing end of the steel plate is reversed again. is the time until it is bitten into the rolling rolls.

The time from the completion of finish rolling to the start of cooling in the accelerated cooling step, which will be described later, is set to 50 seconds or less. If the time from the completion of finish rolling to the start of cooling exceeds 50 seconds, the strain imparted by working is recovered, and bainite in the final structure cannot be sufficiently refined, recrystallization is promoted, and prior austenite grains are reduced. aspect ratio is reduced. The shorter the time from the completion of finish rolling to the start of cooling, the better. Therefore, there is no need to set a lower limit, but from the viewpoint of workability, it is preferable to set the time to 5 seconds or more. The time from the completion of finish rolling to the start of cooling means the time from when the front end of the steel sheet traveling forward passes through the rolling rolls in the final pass until water cooling starts.

In the above description, Ar ₃ means the transformation start temperature at which the transformation from austenite grains to ferrite grains begins in the temperature-falling process, and is determined by the following formula (i). _Trex means a recrystallization temperature which is the lowest temperature at which equiaxed recrystallized grains can form and grow, and is determined by the following formula (ii). The symbol of each element in the following formula represents the content (% by mass) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.

Ar ₃ =910−310×C+65×Si−80×Mn−20×Cu−55×Ni−15×Cr−80×Mo (i)
T _rex =−91900 [Nb*] ² +9400 [Nb*]+770 (ii)
However, the solid-solution Nb amount (mass%) obtained by the following formula (iii) is calculated as sol. When Nb is
Nb≧sol. Nb, [Nb*]=sol. Nb
Nb<sol. For Nb, [Nb*]=Nb
and
sol. Nb=(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N) (iii)
Note that T in the above formula represents the heating temperature (° C.) of the steel slab in the heating process.

(e) Accelerated Cooling Step In the accelerated cooling step, the steel sheet after finish rolling is water-cooled. At this time, water cooling is performed to a cooling stop temperature of 0 to 550 ° C. under the conditions that the cooling start temperature is T _rex -10 ° C. or less and the average cooling rate from the start of cooling to the end of cooling is 5 to 50 ° C./sec. .

Even if finish rolling is performed in the range of Ar ₃ or more and less than T _rex , if the cooling start temperature exceeds T _rex -10 ° C. due to subsequent reheating, the recovery of the strain imparted by working is accelerated, and bainite in the final structure The bainitic ferrite that constitutes cannot be sufficiently refined.

In addition, by water cooling to a cooling stop temperature of 0 to 550° C. at an average cooling rate of 5 to 50° C./sec, the final structure can be made mainly of bainite. The average cooling rate and the cooling stop temperature are adjusted according to the value of Ceq in the chemical composition of the steel sheet, and the conditions are such that martensite transformation does not occur.

(f) Tempering Step A tempering step of heating to a temperature range of 350 to 650° C. may be further provided after the accelerated cooling step. By performing the tempering process, it is possible to reduce the dislocation density that has become excessively high due to cooling. When the cooling stop temperature in the accelerated cooling process is high, the self-tempering effect is obtained, so the tempering process does not have to be performed. On the other hand, in the accelerated cooling process, it is preferable to perform a tempering process when cooling to about room temperature, for example.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples.

Hot metal tapped from a blast furnace was desulfurized by hot metal pretreatment, deP and deC treated in a converter-type refining vessel, deoxidized Al was added, and received in a ladle. At the time of tapping, alloying elements were added, and cover slag for heat retention was added.

Subsequently, the molten steel in the ladle was decompressed by an RH vacuum degasser. During the melting process, molten steel samples were taken as needed and subjected to analysis to obtain molten steel components. The molten steel temperature changed from 1560°C to 1610°C. In the first half of the RH treatment, an alloy other than Zr and B was added to adjust the components, and vacuum degassing was performed to adjust the dissolved O concentration. Dissolved O concentration was measured using an oxygen concentration probe.

After that, Zr and B were added, and a reflux treatment was performed to uniformly mix them. Except for some examples, Zr was added, and B was added after a predetermined time had passed. In the example in which B was added before Zr was added, the time from the addition of Zr to the addition of B is indicated by minus.

Billets having the chemical compositions in Tables 1 and 2 were produced by a continuous casting method after being treated in an RH vacuum degasser. In continuous casting, the average cooling rate was appropriately adjusted when the surface temperature of the steel slab was between 1200 and 900°C. Tables 3 and 4 show the amount of deoxidized Al (kg / t) put into 1 ton of molten steel in the refining process, the dissolved O concentration (% by mass) in the molten steel when adding Zr, and after adding Zr The time (minutes) to B addition and the average cooling rate (° C./sec) between 1200 and 900° C. in the continuous casting process are shown. Furthermore, using the steel slabs described above, steel sheets with a thickness of 10 to 70 mm were experimentally manufactured under the manufacturing conditions shown in Tables 5 and 6.

The metal structure of the obtained steel plate was observed, and the area ratio of each structure was measured. Specifically, first, a sample was taken from the steel plate so that the 1/4t position in the C cross section was the observation surface. Then, the observation surface is etched with nital, and after etching, 8 fields of view are photographed at 500 times using an optical microscope, and image analysis is performed on the obtained structure photograph. Ferrite appears white, and black appears black. was defined as pearlite, and the area ratio of each was obtained.

Next, the nital-etched portion was subjected to repeller etching, and image analysis was performed on the portion that appeared gray due to the nital etching.

The average length of bainitic ferrite and the area ratio of bainite were calculated by KAM analysis using EBSD. In the KAM analysis, in the structure determined to be ferrite, the region where the local misorientation exceeded 1.0° was defined as bainitic ferrite. For the measurement, bainitic ferrite having a length of 1 μm or more in the major axis direction was used. Also, the area ratio of bainite was the sum of the area ratios of bainitic ferrite.

Furthermore, the average length in the thickness direction and the average aspect ratio of the prior austenite grains were measured according to JIS G 0551:2013. First, a sample was taken from the steel plate so that the 1/4t position in the L cross section would be the observation surface. Next, after the observation surface was mirror-polished, it was corroded by the Bechet-Beaujard method using a saturated aqueous solution of picric acid to reveal prior austenite grains.

The observation surface where the prior austenite grains were exposed was observed with an optical microscope, and 8 or more fields of view with an area of 0.05 mm ² or more (a total of 0.40 mm ² or more) were photographed. Then, the thickness of the prior austenite grains was measured by a cutting method based on the structure photograph taken with an optical microscope, and the average value was taken as the average length in the thickness direction of the prior austenite grains. Prior austenite grains having a length of 1 μm or more in the thickness direction were used for the measurement.

Further, from the above structure photograph, for each prior austenite grain, the maximum length in the long axis direction and the maximum length in the short axis direction perpendicular to the long axis direction are measured, and the ratio (maximum long axis length / short The maximum length of the axis) was obtained, and the average value was taken as the average aspect ratio of the prior austenite grains.

Subsequently, a test piece was cut out from the steel plate and electrolyzed with about 0.4 g of 10% acetylacetone-1% tetramethylammonium chloride/methanol at a current density of 20 mA/cm ² . The solution used for the electrolysis was filtered through a filter with a pore size of 0.2 μm, and the extraction residue collected on the filter was measured for the Zr content in the extraction residue by using ICP emission spectrometry. Zr. Insol. The Zr-subtracted value is the Sol. Zr.

In addition, the B content _BF present as solid solution B in the steel was calculated based on the following formula.
If B _F ′>B then B _F =B
If 0<B _F '≦B then _BF = _BF '
If B _F ′≦0, then B _F =0
, and B _F ' is represented by the following formula (II).
_BF ′=B−(N−(Ti−(O—Insol.Zr×(32/91.224))×(95.73/48))×(14/47.867))×(10.811 / 14) ... (II)
The symbol of each element in the above formula represents the content (% by mass) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.

Next, the surface of the steel sheet was mirror-polished, and the area of 10 mm × 10 mm (100 mm ² ) was observed by SEM. Particles containing B above and 1% by mass or more of O, and in which the proportion of Al ₂ O ₃ in the total of ZrO ₂ , Ti ₂ O ₃ and Al ₂ O ₃ is 50% by mass or less is specified. did. Then, among the particles, the number of particles having an equivalent circle diameter of 0.5 μm or more was measured, and the number density was measured by dividing the number by the area of the observed field of view.

These measurement results are shown in Tables 7 and 8. In the table, the area ratio of ferrite is “F fraction”, the area ratio of pearlite is “P fraction”, the area ratio of bainite is “B fraction”, and the area ratio of MA phase is “MA fraction”. ”, and the average length of the bainitic ferrite in the major axis direction is denoted as “BF length”.

Furthermore, tensile strength (TS) and yield stress (YS) were measured based on JIS Z 2241:2011. The test piece was measured using a No. 1B tensile test piece having a longitudinal direction (width direction) perpendicular to the rolling direction from the central portion of the plate thickness. Yield stress (YS) was the yield strength of the permanent elongation method when the elongation set was 0.2%. In this example, a steel having a YS of 460 MPa or more and a TS of 570 MPa or more was considered to have high strength.

Also, a V-notch test piece was taken so as to include the 1/4t position of the steel plate, and the fracture surface transition temperature (vTrs) was evaluated according to JIS Z 2242:2005. At this time, two V-notch test pieces were taken so that the longitudinal direction of each test piece coincided with the rolling direction and width direction of the steel plate. In this example, two specimens with vTrs of −60° C. or lower were regarded as having excellent low temperature toughness.

Then, in accordance with ISO 15653:2018, a CTOD test piece was taken with the total thickness of the base material in the plate thickness direction as the notch position of three-point bending, and the CTOD value at -10 ° C. was measured. The test was performed 3 times and the minimum values are listed in the table. In this example, a specimen having a minimum CTOD value of 0.50 mm or more at -10°C was considered to have excellent fracture toughness.

In addition, the Charpy absorbed energy of the HAZ at −20° C. when welding was performed under the condition of a welding heat input of 35 kJ/mm was measured by the following method. First, the steel plate was subjected to a heat cycle that reproduced welding with a heat input of 35 kJ/mm (high heat input welding). As a specific heat cycle condition, after heating from room temperature to 1400° C., holding at 1400° C. for 5 seconds, after that, the temperature range from 800° C. to 500° C., which is the temperature range related to intragranular transformation, is set to 1.0. Cooling was controlled at a rate of °C/sec.

Three V-notch test pieces were taken from each steel plate after the heat cycle was applied, and a Charpy impact test was performed at -20°C to measure the absorbed energy (vE _-20 ). The V-notch test piece was prepared according to the V-notch test piece described in JIS Z 2242:2005, and the Charpy impact test was performed according to JIS Z 2242:2005. Then, the average and minimum vE _-20 values of the three test pieces were determined. In this example, those having an average value of vE- ₂₀ of 100 J or more and a minimum value of 50 J or more were considered to have excellent HAZ toughness.

These measurement results are shown in Tables 9 and 10.

As can be seen from Tables 7 to 10, the examples of the present invention (test numbers 1 to 26) satisfying the provisions of the present invention had high strength and excellent low temperature toughness, fracture toughness and HAZ toughness. On the other hand, in the comparative examples (test numbers 27 to 67), at least one of strength, low temperature toughness, fracture toughness and HAZ toughness deteriorated.

Specifically, in Test No. 27, the C content was excessive, so the low temperature toughness, fracture toughness and HAZ toughness deteriorated. Test No. 28 had a low C content, resulting in insufficient strength and deterioration in low temperature toughness and fracture toughness. Since Test No. 29 had an excessive Si content, the low temperature toughness, fracture toughness and HAZ toughness were deteriorated. Since test number 30 had an excessive Mn content, the low temperature toughness, fracture toughness and HAZ toughness were deteriorated. Test No. 31 had a low Mn content and a low bainite area ratio, resulting in insufficient strength and deterioration in low temperature toughness and fracture toughness.

Test No. 32 had excessive P and S contents, so the low temperature toughness, fracture toughness and HAZ toughness were deteriorated. In Test No. 33, the Al content was excessive, and the formation of desired composite inclusions was insufficient, so the HAZ toughness deteriorated. Test No. 34 had an excessive N content and a low bainite area ratio, resulting in insufficient strength and deterioration in low temperature toughness and fracture toughness. Test No. 35 had a low N content and coarsened prior austenite grains, resulting in deterioration in low temperature toughness and fracture toughness.

Since test number 36 had an excessive O content, the low temperature toughness, fracture toughness and HAZ toughness were deteriorated. Test No. 37 had a low O content, resulting in insufficient formation of desired composite inclusions, resulting in deterioration of HAZ toughness. Since test number 38 had an excessive Nb content, the low temperature toughness, fracture toughness and HAZ toughness were deteriorated. In Test No. 39, the Nb content was low, the BF length was excessive, and the aspect ratio of the prior austenite grains was small, so the low temperature toughness and fracture toughness were deteriorated. Since test number 40 had an excessive Ti content, the low temperature toughness, fracture toughness and HAZ toughness deteriorated. In Test No. 41, the Ti content was low, and the BF length and prior austenite grains were coarsened, so the low temperature toughness and fracture toughness were deteriorated.

Test No. 42 had an excessive Ca content, and Test No. 43 had an excessive Mg and REM content, resulting in insufficient formation of desired composite inclusions, resulting in deterioration of HAZ toughness. Since test number 44 has an excessive Zr content, Insol. The Zr content became excessive and the HAZ toughness deteriorated. Test No. 45 has a low Zr content, and Insol. The HAZ toughness deteriorated due to the low Zr content and insufficient formation of the desired composite inclusions. Since Test No. 46 did not contain B, the desired composite inclusions were not formed at all and the HAZ toughness deteriorated. In addition, since the bainite area ratio was low, the strength was insufficient, and the low temperature toughness and fracture toughness were deteriorated.

Test No. 47 had an excessive deoxidized Al amount, while Test No. 48 had a low deoxidized Al amount. Test numbers 48 and 49 had an excessive dissolved O concentration when Zr was added. Also, in test number 50, the time from the addition of Zr to the addition of B was short, and in test number 51, B was added before the addition of Zr. Furthermore, Test No. 52 had a high average cooling rate in the continuous casting process. As a result, formation of the desired composite inclusions was insufficient in all examples, and the HAZ toughness deteriorated.

In Test No. 53, the heating temperature in the heating step was high, the BF length and prior austenite grains became coarse, and the low temperature toughness and fracture toughness deteriorated. In Test No. 54, the heating temperature was low and the bainite area ratio was low, so that the strength was insufficient and the low temperature toughness and fracture toughness were deteriorated. In Test No. 55, since the end temperature of rough rolling was lower than _Trex , the BF length and prior austenite grains became coarse, and the low temperature toughness and fracture toughness deteriorated.

Test No. 56 has a high cumulative rolling reduction in rough rolling, while Test No. 57 has a low cumulative rolling reduction. Toughness and fracture toughness deteriorated. In Test No. 58, the start temperature of the finish rolling was _Trex or higher, so the BF length and the prior austenite grains became coarse, the aspect ratio of the prior austenite grains decreased, and the low temperature toughness and fracture toughness deteriorated. In Test No. 59, since the final rolling temperature was less than Ar ₃ , deformed ferrite was excessively formed, resulting in insufficient strength. In addition, since the bainite area ratio was low, the strength was insufficient, and the low temperature toughness and fracture toughness were deteriorated.

Test No. 60 has a high cumulative rolling reduction in finish rolling, and the aspect ratio of the prior austenite grains is reduced. On the other hand, Test No. 61 has a low cumulative rolling reduction, coarsens the BF length and the prior austenite grains, and In both cases, the low temperature toughness and fracture toughness deteriorated because the aspect ratio of the steel decreased. Test No. 62 has a long interpass time, and Test No. 63 has a long time from the completion of finish rolling to the start of cooling, so the BF length is coarsened and the aspect ratio of the prior austenite grains is reduced, resulting in low temperature toughness and fracture toughness. has deteriorated.

In Test No. 64, the cooling rate in the accelerated cooling process was high, and the MA phase was excessively generated, resulting in deterioration of low temperature toughness and fracture toughness. Since the cooling rate was low in Test No. 65 and the cooling stop temperature was high in Test No. 66, none of them had a bainite-based structure, resulting in insufficient strength and deterioration in low-temperature toughness and fracture toughness. In Test No. 67, the cooling start temperature exceeded T _rex −10° C. and the BF length was coarsened, resulting in poor fracture toughness although good low temperature toughness.

According to the present invention, it is possible to obtain a steel sheet having high strength and excellent low temperature toughness, fracture toughness and HAZ toughness. Therefore, the steel plate according to the present invention can be suitably used as a material for welded structures such as ships, high-rise buildings, other buildings, bridges, offshore structures, LNG storage tanks and other large tanks, and line pipes. .

Claims (7)

The chemical composition of the steel sheet, in mass%,
C: 0.040 to 0.141 %,
Si: 0.01 to 0.50%,
Mn: 0.70-2.50%,
P: 0.030% or less,
S: 0.020% or less,
Al: 0.010% or less,
N: 0.0010 to 0.0080%,
O: 0.0005 to 0.0040%,
Nb: 0.007 to 0.050%,
Ti: 0.003-0.024%,
Zr: 0.0007 to 0.0050%,
Insol. Zr: 0.0007 to 0.0040%,
Sol. Zr: 0.0010% or less,
B: 0.0003 to 0.0040%,
balance: Fe and impurities,
satisfying the following formula (I),
In a cross section perpendicular to the rolling direction of the steel sheet, the metal structure at a position 1/4 t from the surface of the steel sheet, where t is the thickness of the steel sheet, is
80% or more of bainite is contained in terms of area%, and
The average length of the bainitic ferrite constituting the bainite in the major axis direction is 10 μm or less,
In a cross section parallel to the rolling direction and thickness direction of the steel sheet, the average length in the thickness direction of the prior austenite grains at a position 1/4 t from the surface of the steel sheet is 20 μm or less, and the average aspect ratio is 2. .5 or more,
Composite inclusions containing Zr and B, having an equivalent circle diameter of 0.5 μm or more, and having a ratio of Al ₂ O ₃ to the total of ZrO ₂ , Ti ₂ O ₃ and Al ₂ O ₃ of In mass%, the number density of the composite inclusions, which is 50% or less, is 16 to 300 pieces/mm ² ,
steel plate.
B _F ≤ 0.0030 (I)
however,
If B _F ′>B then B _F =B
If 0<B _F '≦B then _BF = _BF '
If B _F ′≦0, then B _F =0
, and B _F ' is represented by the following formula (II).
_BF ′=B−(N−(Ti−(O—Insol.Zr×(32/91.224))×(95.73/48))×(14/47.867))×(10.811 / 14) ... (II)
The symbol of each element in the above formula represents the content (% by mass) of each element contained in the steel sheet, and 0 is substituted when the element is not contained. wherein the chemical composition, instead of part of the Fe, is in % by mass,
Cu: 1.50% or less,
Ni: 2.50% or less,
Cr: 1.00% or less,
Mo: 1.00% or less, and V: 0.150% or less,
It contains at least one or more selected from the group consisting of
The steel plate according to claim 1. wherein the chemical composition, instead of part of the Fe, is in % by mass,
Te: 0.0100% or less,
which contains
The steel plate according to claim 1 or 2. wherein the chemical composition, instead of part of the Fe, is in % by mass,
W: 1.00% or less, and Sn: 0.50% or less,
It contains at least one or more selected from the group consisting of
The steel plate according to any one of claims 1 to 3. wherein the chemical composition, instead of part of the Fe, is in % by mass,
Total of Ca, Mg and REM: 0.0005% or less,
which contains
The steel plate according to any one of claims 1 to 4. A method for manufacturing a steel plate according to any one of claims 1 to 5,
The steel obtained by comprising a refining step of producing molten steel and a continuous casting step of continuously casting the molten steel to produce a steel slab having the chemical composition according to any one of claims 1 to 5. In a steel sheet manufacturing method in which a piece is sequentially subjected to a heating step, a hot rolling step and an accelerated cooling step,
In the refining step, the amount of deoxidized Al to be introduced is 0.2 to 1.3 kg per 1 t of the molten steel, Zr is added after the dissolved O concentration in the molten steel becomes 0.0050% by mass or less, and B is added after 1 minute or more from the addition of Zr,
In the continuous casting step, the average cooling rate is set to 0.5° C./second or less when the surface temperature of the steel slab is between 1200 and 900° C.,
In the heating step, the steel billet is heated to a heating temperature of 950 to 1080° C.,
The hot rolling process includes rough rolling and finish rolling,
The rough rolling starts when the surface temperature of the billet is equal to or higher than _Trex , and ends when the surface temperature of the billet is equal to or higher than _Trex ,
The cumulative rolling reduction in the rough rolling is 10 to 75%,
The finish rolling is started in a state where the surface temperature of the steel billet is Ar ₃ or more and less than _Trex , and finished in a state where the surface temperature of the steel billet is Ar ₃ or more and less than _Trex ,
The cumulative rolling reduction in the finish rolling is 65 to 90%, and the time between passes is 15 seconds or less,
The time from the completion of the finish rolling to the start of cooling in the accelerated cooling step is set to 50 seconds or less,
In the accelerated cooling step, the cooling start temperature is T _rex -10 ° C. or less, and the average cooling rate from the start of cooling to the end of cooling is 5 to 50 ° C./sec, and the cooling stop temperature is 0 to 550 ° C. water-cooled to
A method of manufacturing a steel plate.
Here, Ar ₃ is determined by the following formula (i), and _Trex is determined by the following formula (ii). The symbol of each element in the following formula represents the content (% by mass) of each element contained in the steel sheet, and 0 is substituted when the element is not contained.
Ar ₃ =910−310×C+65×Si−80×Mn−20×Cu−55×Ni−15×Cr−80×Mo (i)
T _rex =−91900 [Nb*] ² +9400 [Nb*]+770 (ii)
However, the solid-solution Nb amount (mass%) obtained by the following formula (iii) is calculated as sol. When Nb is
Nb≧sol. Nb, [Nb*]=sol. Nb
Nb<sol. For Nb, [Nb*]=Nb
and
sol. Nb = (10 ^{(-6770 / (T + 273) + 2.26)} ) / (C + 12 × N / 14 ) (iii)
Note that T in the above formula represents the heating temperature (° C.) of the steel slab in the heating process. After the accelerated cooling step, a tempering step of heating to a temperature range of 350 to 650 ° C. is further performed.
The method for manufacturing the steel sheet according to claim 6.