KR20220146638A - Steel plate and its manufacturing method - Google Patents

Abstract

Chemical composition, in mass%, C: 0.040 to 0.160%, Si: 0.01 to 0.50%, Mn: 0.70 to 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 to 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, B _F ≤0.0030, the metal structure at the 1/4t position in the C section contains 80 area% or more of bainite, the long axis of bainitic ferrite constituting bainite The average length in the direction is 10 µm or less, the average length in the thickness direction of the old austenite grains at the 1/4t position in the L section is 20 µm or less, the average aspect ratio is 2.5 or more, and the predetermined diameter and component A steel sheet having a number density of 5 to 300 pieces/mm ² of composite inclusions with

Description

Steel plate and its manufacturing method

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

As a use of a steel plate, welded structures, such as a ship, a high-rise building, other buildings, a bridge, an offshore structure, LNG storage tanks, other large tanks, and a line pipe, are mentioned (for example, refer patent documents 1-5). In recent years, in order to increase the loading weight of a container ship, etc., the enlargement of a welded structure is progressing. In connection with this, thickness increase and high strength increase are requested|required of a steel plate. Furthermore, in the welded structure as described above, further improvement of low-temperature toughness and fracture toughness is a problem from the viewpoints of further safety and reliability.

In addition, it is common that welding of high heat input, such as 35 kJ/mm or more, is performed to a welded structure, for example. In order to suppress brittle fracture, improvement of the toughness (hereinafter referred to as "HAZ toughness") in the heat affected zone (HAZ: Heat Affected Zone) generated when high heat input welding is performed is required.

Japanese Patent Laid-Open No. 2019-023322 Japanese Patent Laid-Open No. 2019-023323 Japanese Patent Laid-Open No. 2019-023324 Japanese Patent Laid-Open No. 2019-035107 International Publication No. 2019/069771

However, in general, since there is a so-called trade-off relationship between strength and low-temperature toughness and HAZ toughness, it has not been easy to make them compatible. In addition, as for the improvement of fracture toughness, it is the present state that almost no examination was made so far.

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

This invention makes the following steel plate and its manufacturing method a summary.

(1) the chemical composition of the steel sheet, in mass%,

C: 0.040-0.160%,

Si: 0.01 to 0.50%,

Mn: 0.70 to 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,

It satisfies the following formula (I),

In a cross section perpendicular to the rolling direction of the steel sheet, when the thickness of the steel sheet is t, the metal structure at a position of 1/4t from the surface of the steel sheet is,

By area%, it contains 80% or more of bainite, and

The average length in the major axis direction of the bainitic ferrite constituting the bainite 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 old austenite grains at a position of 1/4t from the surface of the steel sheet is 20 µm or less, and the aspect the average of the rain is 2.5 or more,

As a composite inclusion containing Zr and B, the equivalent circle diameter is 0.5 μm or more, and the ratio of Al ₂ O ₃ in the total of ZrO ₂ , Ti ₂ O ₃ , and Al ₂ O ₃ is, in mass%, The number density of the composite inclusions of 50% or less is 5 to 300 pieces/mm ²

grater.

B _F ≤0.0030 … (I)

only,

If B _F '>B, then B _F =B

If 0<B _F '≤B, then B _F =B _F '

If B _F '≤0, then B _F =0

and B _F ' is represented by the following formula (II).

B _F '=B-(N-(Ti-(O-Insol.Zr×(32/91.224))×(95.73/48))×(14/47.867))×(10.811/14) … (II)

In addition, the element symbol in the said formula shows content (mass %) of each element contained in a steel plate, and shall substitute 0 when it does not contain.

(2) in (1) above,

The chemical composition replaces a part of the Fe, in 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

A steel sheet containing at least one selected from the group consisting of

(3) in (1) or (2) above,

The chemical composition replaces a part of the Fe, in mass%,

Te: 0.0100% or less

which contains, a steel plate.

(4) according to any one of (1) to (3),

The chemical composition replaces a part of the Fe, in mass%,

W: 1.00% or less, and

Sn: 0.50% or less

A steel sheet containing at least one selected from the group consisting of

(5) according to any one of (1) to (4),

The chemical composition replaces a part of the Fe, in mass%,

Total of Ca, Mg and REM: 0.0005% or less

which contains, a steel plate.

(6) The method for producing a steel sheet according to any one of (1) to (5),

A refining step of producing molten steel, and a continuous casting step of continuously casting the molten steel to produce a steel piece having the chemical composition according to any one of (1) to (5), and heating the obtained steel piece In the method for manufacturing a steel sheet, in which the hot rolling process and the accelerated cooling process are sequentially performed,

In the refining step, the amount of deoxidized Al to be charged is 0.2 to 1.3 kg per 1 t of the molten steel, and after the dissolved O concentration in the molten steel becomes 0.0050 mass% or less, Zr is added, and further 1 minute from the addition of Zr After the elapse of the above time, B is added,

In the continuous casting process, the average cooling rate in a surface temperature of the steel piece between 1200 and 900 °C is 0.5 °C/sec or less,

In the heating step, the steel piece is heated to a heating temperature of 950 ~ 1080 ℃,

The hot rolling process includes rough rolling and finish rolling,

The rough rolling is carried out in a range where the surface temperature of the steel piece is T _rex or more,

The cumulative reduction ratio in the rough rolling is 10 to 75%,

The finish rolling is carried out in a range where the surface temperature of the steel piece is Ar ₃ or more and less than _Trex ,

The cumulative reduction ratio in the finish rolling is set to 65 to 90%, and the time between passes is set to 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 cooling stop temperature of 0 to 550°C under the condition that the average cooling rate from the start of cooling to the end of cooling is 5 to 50°C/sec. water-cooled to

A method for manufacturing a steel plate.

Here, Ar ₃ is obtained by the following formula (i), and T _rex is obtained by the following formula (ii). In addition, the element symbol in the following formula represents content (mass %) of each element contained in a steel plate, and shall substitute 0 when it does not contain.

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, when the solid solution Nb amount (mass %) calculated|required by the following formula (iii) is made into sol.Nb,

When Nb≥sol.Nb, [Nb*]=sol.Nb

When Nb<sol.Nb, [Nb*]=Nb

do it with

sol.Nb=(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N) … (iii)

In addition, T in the said formula represents the heating temperature (degreeC) of the steel piece in a heating process.

(7) in (6) above,

After the accelerated cooling step, further performing a tempering step of heating to a temperature range of 350 to 650 ° C., the manufacturing method of a steel sheet.

ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to obtain the steel plate which has high intensity|strength and is excellent in low-temperature toughness, fracture toughness, and HAZ toughness.

MEANS TO SOLVE THE PROBLEM The present inventors came to acquire the following knowledge, as a result of conducting detailed examination about the said subject.

As described above, a so-called trade-off relationship exists between strength, low-temperature toughness, and HAZ toughness. Moreover, as a result of examination by the present inventors, it turned out that coexistence of strength and fracture toughness is also not easy. Then, first, the present inventors investigated the method of making high strength increase and the improvement of low-temperature toughness and fracture toughness compatible. As a result, not only low-temperature toughness but also decrease in fracture toughness can be suppressed by refining the bainitic ferrite constituting bainite in addition to refining and flattening the bainite structure and refining and flattening the bainite structure as well as increasing the strength by making the metal structure mainly of bainite. knew what could be

In addition, by controlling the heating temperature before hot rolling to be low and performing finish rolling at a high pressure reduction rate in the non-recrystallization region, refinement and flattening of the bainite structure and refinement of bainitic ferrite can be achieved. found

Next, the method of improving the HAZ toughness was studied, and the following findings were obtained.

By containing Zr and B, in steel, B nitride is precipitated with a Zr containing oxide as a nucleus. These composite inclusions containing Zr and B (hereinafter also simply referred to as "composite inclusions") function effectively as intragranular ferrite production sites and contribute to the miniaturization of the HAZ structure.

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

Moreover, in order to acquire the effect by said composite inclusion, it is necessary to make BF which is the amount of _B dissolved in steel into 0.0030 mass % or less.

In addition, when the ratio of Al ₂ O ₃ contained in the composite inclusion is low, the composite inclusion functions more effectively as an intragranular ferrite generating site. Specifically, the composite inclusions having an equivalent circle diameter of 0.5 μm or more, and wherein the ratio of Al ₂ O ₃ in the total of ZrO ₂ , Ti ₂ O ₃ , and Al ₂ O ₃ is 50% or less by mass% When the number density is 5 to 300 pieces/mm ² , fine and abundant intragranular ferrite is generated in the HAZ, and the HAZ toughness is improved.

When Al which acts as a strong deoxidation element is contained in steel excessively, the production|generation of a composite inclusion is inhibited. In order to ensure the amount of dissolved oxygen in molten steel and to generate|occur|produce a composite inclusion in steel, it is preferable that content of Al shall be 0.010 mass % or less. Further, elements having a stronger deoxidation power than Al, such as Ca, Mg, and REM, may be contained in a total of 0.0005 mass% or less.

The present invention has been made based on the above findings. Hereinafter, each requirement of this invention is demonstrated in detail.

(A) chemical composition

The reasons for limiting each element are as follows. In addition, in the following description, "%" with respect to content means "mass %". In addition, in this specification, "to" which shows a numerical range is used in the meaning which includes the numerical value described before and behind that as a lower limit and an upper limit, unless there is special mention.

C: 0.040-0.160%

C is contained in an amount of 0.040% or more in order to secure the strength of the steel sheet. On the other hand, when the C content exceeds 0.160%, it becomes difficult to ensure good low-temperature toughness, fracture toughness, and HAZ toughness. Therefore, the C content is made 0.160% or less. Accordingly, 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 0.075% or more. Moreover, C content is 0.160 % or less, Preferably it is 0.140 % or less, More preferably, it is 0.120 % or less.

Si: 0.01 to 0.50%

Since Si is effective as a deoxidation element and a strengthening element, it is made to contain 0.01% or more. On the other hand, when the Si content exceeds 0.50%, the low-temperature toughness, fracture toughness, and HAZ toughness greatly deteriorate, so the Si content is set to 0.50% or less. Therefore, the Si content is 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. Moreover, Si content is 0.50 % or less, Preferably it is 0.40 % or less, More preferably, it is 0.35 % or less, More preferably, it is 0.30 % or less.

Mn: 0.70 to 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, when the Mn content exceeds 2.50%, central segregation becomes significant, and the low-temperature toughness, fracture toughness, and HAZ toughness of the portion where central segregation occurs deteriorates. Therefore, the Mn content is set to 2.50% or less. Therefore, the Mn content is 0.70% or more, preferably 0.90% or more, and more preferably 1.20% or more. Moreover, Mn content is 2.50 % or less, Preferably it is 2.00 % or less, More preferably, it is 1.80 % or less, More preferably, it is 1.60 % or less.

P: 0.030% or less

P is an element which exists in steel as an impurity. In order to stably secure low-temperature toughness, fracture toughness and HAZ toughness, the content of P is set to 0.030% or less. Preferably, it is 0.020% or less, More preferably, it is 0.015% or less. Although the lower limit is 0%, in consideration of the cost for reducing the P content, the P content may be 0.0001% or more.

S: 0.020% or less

S is an element present in steel as an impurity. When the S content exceeds 0.020%, a large amount of elongated MnS is generated in the central segregation portion, and low-temperature toughness, fracture toughness, HAZ toughness, and ductility deteriorate. For this reason, the S content is made 0.020% or less. Preferably it is 0.010% or less. Although the lower limit is not particularly prescribed since it is so preferable that the S content is small, the S content may be 0.0001% or more from the viewpoint of manufacturing cost.

Al: 0.010% or less

Al is generally an element to be actively contained as a deoxidation element. However, when Al content becomes excessive, formation of a desired composite inclusion becomes inadequate, and the effective ferrite formation site in HAZ decreases. In addition, the formation of coarse cluster-shaped alumina (Al ₂ O ₃ )-based inclusions is promoted, and not only the HAZ toughness deteriorates, but in some cases, the low-temperature toughness and fracture toughness also deteriorate. Accordingly, the Al content is 0.010% or less, preferably 0.005% or less. Although the lower limit in particular is not prescribed|regulated since it is so preferable that the Al content is small, 0.001% or more of Al content may be sufficient from a viewpoint of manufacturing cost.

N: 0.0010 to 0.0080%

Since N forms Ti nitride and has the effect of suppressing the increase in the austenite grain size at the time of heating the steel piece, it is contained in an amount of 0.0010% or more. However, when the N content exceeds 0.0080%, the steel sheet becomes brittle, so the N content is made 0.0080% or less. Therefore, the N content is 0.0010% or more, preferably 0.0015% or more, and more preferably 0.0020% or more. Moreover, N content is 0.0080 % or less, Preferably it is 0.0065 % or less, More preferably, it is 0.0060 % or less.

O: 0.0005 to 0.0040%

O is an element contained in steel and is dissolved or exists as an oxide. Since it is difficult to separate the both clearly, the O content in the present invention is taken as the total oxygen content (also described as T.O.) in the present invention. When the O content is less than 0.0005%, the oxide dispersion water required for securing toughness cannot be obtained. On the other hand, when the O content exceeds 0.0040%, the cleanliness of the molten steel is deteriorated, and productivity such as nozzle clogging in the molten steel step may be reduced. Moreover, since a coarse oxide is produced|generated and a stress concentration generate|occur|produces in an oxide, low-temperature toughness, fracture toughness, and HAZ toughness deteriorate. For this reason, O content shall be 0.0005 to 0.0040%.

Nb: 0.003-0.050%

Nb can improve the strength and toughness of the steel sheet. Moreover, in order to obtain a predetermined microstructure, since rolling in a non-recrystallized austenite region is required, Nb is an effective element in order to expand a non-recrystallization temperature range, and it raises a rolling temperature and also contributes to productivity improvement. In order to acquire this effect, it is made to contain 0.003% or more. However, when the Nb content exceeds 0.050%, low-temperature toughness, fracture toughness, HAZ toughness, and weldability decrease. Therefore, the Nb content is set to 0.050% or less. Therefore, the Nb content is 0.003% or more, preferably 0.005% or more, and more preferably 0.008% or more. Moreover, Nb content is 0.050 % or less, Preferably it is 0.025 % or less, More preferably, it is 0.018 % or less.

Ti: 0.003-0.024%

Ti is an element that forms a composite inclusion together with Zr. As described above, the composite inclusion functions as an intragranular ferrite production site in the HAZ and contributes to the miniaturization of the HAZ structure. In order to acquire this effect, it is made to contain 0.003% or more. However, when the content of Ti exceeds 0.024%, a large amount of Ti nitride is produced, the amount of B nitride produced is suppressed, and the effect of improving the HAZ toughness cannot be obtained. In addition, excess Ti forms TiC and deteriorates low-temperature toughness, fracture toughness, and HAZ toughness. Therefore, the Ti content is 0.003% or more, preferably 0.005% or more. Moreover, Ti content is 0.024 % or less, Preferably it is 0.020 % or less.

Zr: 0.0007 to 0.0050%

Zr is the sum of Sol.Zr and Insol.Zr which will be described later. Zr content is 0.0007 % or more, Preferably it is 0.0010 % or more. Moreover, Zr content is the sum total of the upper limit of Insol.Zr and the upper limit of Sol.Zr, ie, 0.0050 % or less, Preferably it is 0.0040 % or less.

Insol.Zr: 0.0007 to 0.0040%

Insol.Zr is acid-insoluble Zr, and is Zr contained in inclusions, such as a composite inclusion. Zr is an important element for forming a Zr-containing oxide that becomes the nucleus of intragranular transformation. However, if the Insol.Zr content is less than 0.0007%, the oxide composition necessary for securing toughness is not obtained. On the other hand, when the Insol.Zr content exceeds 0.0040%, most of it is ZrO ₂ generated in the molten steel step, and the frequency of nozzle clogging increases. Moreover, when _Insol.Zr increases, since ZrO2 is produced|generated excessively and stress is concentrated, HAZ toughness deteriorates remarkably. For this reason, the Insol.Zr content is set to 0.0007 to 0.0040%.

Sol.Zr: 0.0010% or less

Sol.Zr represents acid-soluble Zr, that is, Zr dissolved in steel. As the content of Sol.Zr increases, the HAZ toughness deteriorates remarkably. Therefore, the content is made 0.0010% or less. Since it is so preferable that Sol.Zr is small, a lower limit is not specifically prescribed|regulated, and 0% may be sufficient.

In addition, Insol.Zr in the said Formula is measured by the following method. First, a test piece is cut out from a steel plate, and about 0.4 g of electrolysis is carried out in 10% acetylacetone-1% tetramethylammonium chloride/methanol at a current density of 20 mA/cm ² . The solution used for the electrolysis is filtered with a filter having a pore diameter of 0.2 µm, and the extraction residue collected on the filter is subjected to a known chemical analysis method (eg, ICP emission spectroscopy), whereby Zr content in the extraction residue is used. is measured, and let it be Insol.Zr. Moreover, let the value which subtracted Insol.Zr from the Zr content in steel (Total Zr) be Sol.Zr.

B: 0.0003 to 0.0040%

B is an element which improves the hardenability of a steel material, and precipitates as B nitride around a Zr containing oxide, forms a composite inclusion, and improves intragranular transformation ability. In order to precipitate as B nitride around the Zr-containing oxide, the B content is made 0.0003% or more. However, since the effect is saturated even if B is contained excessively, the B content is made 0.0040% or less. Accordingly, the B content is 0.0003% or more, preferably 0.0005% or more, and more preferably 0.0010% or more. Moreover, B content is 0.0040 % or less, Preferably it is 0.0035 % or less, More preferably, it is 0.0030 % or less.

In the chemical composition of the steel sheet of the present invention, in addition to the above elements, for the purpose of improving strength, at least one or more selected from the group consisting of Cu, Ni, Cr, Mo, and V are further selected from the following: You may make it contain in a range. The reason for limitation of each element is demonstrated.

Cu: 1.50% or less

Since Cu has the effect of improving the strength and toughness of a steel plate, you may contain it as needed. However, when Cu is contained excessively, the improvement of the performance suitable for an increase in alloy cost cannot be seen, but on the contrary, it may become a cause of surface cracking. Therefore, Cu content is 1.50 % or less, Preferably it is 1.20 % or less, More preferably, it is 1.00 % or less. When it is desired to obtain the above effect more reliably, Cu content becomes like this. Preferably it is 0.005 % or more, More preferably, it is 0.010 % or more, More preferably, it is 0.050 % or more.

Ni: 2.50% or less

Since Ni is an element having the effect of improving the strength of the steel sheet, it may be contained as needed. Moreover, Ni is an element which has the effect of raising the toughness of the matrix (dough) of steel in a solid solution state. However, when Ni is contained excessively, low-temperature toughness, fracture toughness, HAZ toughness and weldability deteriorate. Therefore, Ni content is 2.50 % or less, Preferably it is 1.00 % or less, More preferably, it is 0.50 % or less, More preferably, it is 0.30 % or less. When it is desired to obtain the above effect more reliably, the Ni content is preferably 0.005% or more, more preferably 0.010% or more, still more preferably 0.050% or more.

Cr: 1.00% or less

Since Cr is an element having the effect of improving the strength of the steel sheet, it may be contained as necessary. However, when Cr is contained excessively, low-temperature toughness, fracture toughness, HAZ toughness and weldability deteriorate. Therefore, Cr content is 1.00 % or less, Preferably it is 0.80 % or less, More preferably, it is 0.50 % or less, More preferably, it is 0.30 % or less. To obtain the above effect more reliably, the Cr content is preferably 0.005% or more, more preferably 0.010% or more, still more preferably 0.050% or more.

Mo: 1.00% or less

Since Mo is an element having an effect of improving the strength of the steel sheet, it may be contained as necessary. However, when Mo is contained excessively, low-temperature toughness, fracture toughness, HAZ toughness and weldability deteriorate. Therefore, Mo content is 1.00 % or less, Preferably it is 0.80 % or less, More preferably, it is 0.50 % or less, More preferably, it is 0.30 % or less. When it is desired to acquire the said effect more reliably, Mo content becomes like this. Preferably it is 0.001 % or more, More preferably, it is 0.005 % or more, More preferably, it is 0.010 % or more.

V: 0.150% or less

Since V is an element having an effect of improving the strength of the steel sheet, it may be contained as necessary. However, when V is contained excessively, low-temperature toughness, fracture toughness, HAZ toughness and weldability deteriorate. Therefore, V content is 0.150 % or less, Preferably it is 0.100 % or less, More preferably, it is 0.070 % or less, More preferably, it is 0.050 % or less. When it is desired to obtain the above-mentioned effect more reliably, V content becomes like this. Preferably it is 0.001 % or more, More preferably, it is 0.005 % or more, More preferably, it is 0.010 % or more.

The chemical composition of the steel sheet of this invention WHEREIN: In addition to said element, you may contain Te in the range shown below for the purpose of refinement|miniaturization of a metal structure. The reason for limitation is explained.

Te: 0.0100% or less

Since Te is an element that contributes to the improvement of toughness by refining the structure of the steel sheet, 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, and more preferably 0.0050% or less. When it is desired to obtain the above effect more reliably, the Te content is preferably 0.0001% or more, more preferably 0.0005% or more, 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 or more selected from the group consisting of W and Sn may be contained within the range shown below. . The reason for limitation of each element is demonstrated.

W: 1.00% or less

Since W is an element which melt|dissolves and adsorb|sucks to rust in the form of oxygen acid ion WO ₄ ^- , suppresses permeation|transmission of the chloride ion in a rust layer, and improves corrosion resistance, you may contain it as needed. However, even when W is contained excessively, not only the above effects are saturated, but also low-temperature toughness, fracture toughness, and HAZ toughness may decrease. Therefore, the W content is 1.00% or less, preferably 0.75% or less. To obtain the above effect more reliably, the W content is preferably 0.001% or more, more preferably 0.005% or more, still more preferably 0.010% or more.

Sn: 0.50% or less

Sn is an element which melt|dissolves as Sn ^<2+ >, and has the effect|action which suppresses corrosion by the inhibitor action in an acidic chloride solution. Moreover, Sn has an effect|action which suppresses the anode dissolution reaction of steel and improves corrosion resistance. Therefore, you may make it contain as needed. However, even if Sn is contained excessively, not only the said effect is saturated, but it becomes easy to generate|occur|produce the rolling crack of a steel plate. Therefore, Sn content is 0.50 % or less, Preferably it is 0.30 % or less. When it is desired to obtain the above-mentioned effect more reliably, Sn content becomes like this. Preferably it is 0.001 % or more, More preferably, it is 0.005 % or more, More preferably, it is 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 further contained within the ranges shown below for the purpose of reducing the dissolved oxygen concentration in the molten steel. The reason for limitation is explained.

Total of Ca, Mg and REM: 0.0005% or less

Ca, Mg, and REM are elements more likely to react with oxygen more preferentially than Al. Therefore, you may make it contain as needed. However, even if Ca, Mg and REM are contained excessively, the amount of dissolved oxygen in the molten steel cannot be ensured, and a desired composite inclusion cannot be formed. In order to form a desired composite inclusion, the sum of the contents of Ca, Mg and REM is made 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 thereof 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 content of REM means the total content of these elements.

Further, in the chemical composition of the steel sheet according to the present invention, the following formula (I) is satisfied.

B _F ≤0.0030 … (I)

only,

If B _F '>B, then B _F =B

If 0<B _F '≤B, then B _F =B _F '

If B _F '≤0, then B _F =0

and B _F ' is represented by the following formula (II).

B _F '=B-(N-(Ti-(O-Insol.Zr×(32/91.224))×(95.73/48))×(14/47.867))×(10.811/14) … (II)

In addition, the element symbol in the said formula shows content (mass %) of each element contained in a steel plate, and shall substitute 0 when it does not contain.

B _F is the B content present as a solid solution B in steel. Since it is difficult to directly measure the amount of solid solution B, 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 nitride on the surface layer of the composite inclusion, the generation of intragranular ferrite during cooling after welding can be effectively promoted, and the HAZ toughness can be improved by refining the structure. In order to acquire this effect, it is necessary to make B _F into 0.0030% or less. Moreover, when BF exceeds _0.0030 %, the hardenability of steel materials becomes excessive, and HAZ toughness falls by coarsening of bainite and excessive hardness increase generate|occur|produce. Accordingly, _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, "impurity" is a component that is mixed by various factors of raw materials such as ore and scrap, and various factors in the manufacturing process when a steel sheet is industrially manufactured, and is allowed in a range that does not adversely affect the present invention.

(B) the metal structure of the steel plate

The metal structure of the steel plate of this invention is demonstrated. In addition, in the following description, "%" means "area %". Further, in the present invention, when the thickness of the steel sheet is t, the position of 1/4 t from the surface of the steel sheet in the cross section perpendicular to the rolling direction of the steel sheet is called "the position of 1/4 t in the C section", , a position of 1/4t from the surface of the steel sheet in a cross section parallel to the rolling direction and thickness direction of the steel sheet is referred to as a “1/4t position in the L section”. In addition, said "rolling direction" shall mean the rolling direction in finish rolling.

Bainite: 80% or more

In the present invention, the metal structure is mainly bainite. Specifically, by setting the area ratio of bainite at the 1/4t position in the C section to 80% or more, it becomes possible to ensure the strength of the steel sheet. It is preferable that the area ratio of bainite is 90% or more. In addition, it is not necessary to provide an upper limit to the area ratio of bainite, that is, a bainite single phase may be sufficient.

In addition, although ferrite, pearlite, and a martensite-austenite mixed phase (MA phase) may be mixed as a remainder structure, if these total area ratios are 20 % or less, it is permissible. It is preferable that the said total area ratio is 10 % or less. It is preferable that there are few these total area ratios, and a lower limit is not specifically limited. For example, the total area ratio may be 0%. Moreover, more than 0 % may be sufficient and 1 % or more may be sufficient as it.

As described above, in addition to having bainite as the main component, fine and flattening the bainite structure, and further refining bainitic ferrite, it is possible to achieve both strength and low-temperature toughness and fracture toughness of the steel sheet. Specifically, the bainite structure needs to satisfy the following requirements.

Average length of bainitic ferrite: 10 μm or less

The average length in the major axis direction of the bainitic ferrite constituting bainite at the 1/4t position in the C section is set to 10 µm or less. By refining the bainitic ferrite constituting bainite, it becomes possible to secure fracture toughness. 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 old austenite particles: 2.5 or more

The refinement|miniaturization of a bainite structure can be achieved by controlling the heating temperature before hot rolling to be low, and also performing finish rolling at a high pressure reduction rate in a non-recrystallization 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 section, the average length in the thickness direction of the prior austenite particles is set to 20 µm or less, and the average of the aspect ratio is set to 2.5 or more. It is preferable that the average length in the thickness direction of a prior austenite particle is 15 micrometers or less. Moreover, it is preferable that it is more than 2.5, and, as for the average of the aspect-ratio of prior austenite particle|grains, it is more preferable that it is 4.0 or more.

Here, in this invention, the area ratio of a metal structure is calculated|required 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 nital-etched, and after etching, 8 fields of view are photographed at 500 magnification using an optical microscope. And image analysis is performed about the obtained structure|tissue photograph, and what appears white is ferrite, and what appears black is pearlite, and each area ratio is calculated|required.

Next, the nital-etched part is repera-etched, image analysis is performed with respect to the part shown gray by the nital etching, and the area ratio is calculated|required as the MA phase which looks white.

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 structure judged to be ferrite, the region where the local orientation difference exceeds 1.0° is bainitic ferrite. In the measurement, bainitic ferrite having a length in the major axis direction of 1 µm or more is targeted. In addition, the area ratio of bainite is the sum of the area ratios of bainitic ferrite.

The measurement of the average length and the average of the aspect-ratio in the thickness direction of prior austenite particle|grains is performed according to JISG0551:2013. First, a sample is taken from the steel plate so that the 1/4t position in the L section becomes the observation surface. Next, after mirror-polishing the observation surface, it is corroded by the Bechet-Beaujard method using a saturated aqueous solution of picric acid. Let the particle|grains which appeared in black by corrosion be old austenite particle|grains.

The observation surface in which the old austenite particle|grains were made to appear is observed with the optical microscope, and 8 fields or more (total of 0.40 mm< ² > or more) are image|photographed for the field of area 0.05 mm< ² > or more. And based on the micrograph of the structure image|photographed with the optical microscope, the thickness of the prior austenite particle is measured by the cutting method, and let the average value be the average length in the thickness direction of the prior austenite particle. In addition, in a measurement, the length of thickness direction makes object 1 micrometer or more old austenite particle|grains.

In addition, from the above structure photograph, for each old austenite particle, the maximum length in the major axis direction and the maximum length in the minor axis direction orthogonal to the major axis direction are measured, respectively, and the ratio (maximum major length / maximum minor axis length) save And let the average value be the average of the aspect-ratio of prior austenite particle|grains. In addition, when finish rolling is performed at a high pressure reduction in the non-recrystallization region, the old austenite grains exhibit a shape elongated in the rolling direction, so the major axis direction is the rolling direction, and the minor axis direction is the plate thickness direction (so-called ND direction).

When the old austenite grains cannot be sufficiently expressed by the above method, "Examination of high-precision methods for reconstructing steel austenite structure" (Kengo Hata, Masayuki Wakita, Kazuki Fujiwara, Kaori Kawano, and Nittetsu Sumikin) The prior austenite grains are specified according to the reconstruction method described in Publication No. 404 (2016), p.24-30), and the average length and aspect ratio of the prior austenite grains in the thickness direction are obtained. .

Number density of composite inclusions: 5-300 pieces/mm ²

As described above, the steel sheet according to the present invention has a composite inclusion containing Zr and B in the metal structure. Thereby, it becomes an intragranular ferrite formation site at the time of cooling after welding, and HAZ toughness is improved. At this time, too fine composite inclusions have little contribution to the HAZ toughness. Moreover, _the composite inclusion with a low ratio _of Al2O3 functions still more effectively as an intragranular ferrite formation site.

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 in the total of ZrO ₂ , Ti ₂ O ₃ , and Al ₂ O ₃ is in mass%, Pay attention to the composite inclusions of 50% or less, and control the number density. When the number density of the said composite inclusion is less than 5 pieces/mm< ² >, the improvement effect of HAZ toughness cannot fully be acquired. Although it is preferable because the number density of the said composite inclusion is large, since the intragranular ferrite formation site increases, even if the number density exceeds 300 pieces/mm< ² >, the effect is saturated. Accordingly, the number density of the composite inclusions is 5 to 300 pieces/mm ² .

In addition, in this invention, the inclusion containing 5 mass % or more of Zr, 0.1 mass % or more, B, and 1 mass % or more of O is defined as a composite inclusion containing Zr and B. In addition, the equivalent circle diameter and number density of a composite inclusion can be measured by observing the mirror-polished steel surface with SEM.

Specifically, in the range of 10 mm × 10 mm (100 mm ² ), observed by SEM, and by quantitative analysis by an energy dispersive X-ray analyzer (EDX) attached to the SEM, Zr of 5 mass % or more, Particles containing 0.1 mass % or more of B and 1 mass % or more of O, and wherein the ratio of Al ₂ O ₃ in the total of ZrO ₂ , Ti ₂ O ₃ , and Al ₂ O ₃ is 50 mass % or less, is specified. . Then, among the particles, the number of particles having an equivalent circle diameter of 0.5 μm or more is measured and divided by the area of the observed field to measure the number density. In the measurement of a number density, you may use the photograph image|photographed by SEM.

(C) Mechanical properties of the steel plate

The mechanical properties of the steel sheet according to the present invention are not particularly limited, but the steel sheet according to the present invention has high strength and is excellent in low-temperature toughness and HAZ toughness. Specifically, it is preferable that the yield stress (YS) is 460 to 860 MPa and the tensile strength (TS) is 570 to 980 MPa. Moreover, it is preferable that the fracture|rupture transition temperature (vTrs) used as an index|index of low-temperature toughness is -60 degreeC or less. Moreover, it is preferable that the Crack Tip Opening Displacement (CTOD) value in -10 degreeC which becomes an index|index of fracture toughness is 0.50 mm or more.

In addition, tensile strength (TS) and yield stress (YS) are measured using the No. 1B tensile test piece extract|collected in the direction orthogonal to the rolling direction from the plate thickness center based on JIS Z 2241:2011. Specifically, the yield stress (YS) is the proof stress of the permanent elongation method at a permanent elongation of 0.2%. In addition, evaluation of fracture|rupture transition temperature (vTrs) is based on JIS Z 2242:2005, let the test piece be a V-notch test piece, and collect|collect so that the 1/4t position of a steel plate may be included. Moreover, according to ISO 15653:2018, the CTOD test piece which makes the full thickness of the plate|board thickness direction of a base material the notch position of 3-point bending is extract|collected, and the CTOD value in -10 degreeC is measured.

Moreover, it is preferable that the Charpy absorbed energy at -20 degreeC of HAZ when welding heat input is welded under the conditions of 35 kJ/mm is 100 J or more as an average value of three measurements, and 50 J or more as a minimum value.

The Charpy absorbed energy of HAZ is measured by the following method. First, a heat cycle in which a heat input of 35 kJ/mm welding (large heat input welding) is reproduced is applied to a steel sheet. As specific thermal cycle conditions, after heating from room temperature to 1400°C, the temperature is maintained at 1400°C for 5 seconds, and thereafter, the temperature range from 800°C to 500°C, which is a temperature range related to intragranular transformation, is performed at a rate of 1.0°C/sec. Controlled cooling.

Three V-notch test pieces are taken each from the steel plate after heat cycle, and a Charpy impact test is performed at _{-20 degreeC, and absorbed energy (vE-20} ) is measured. In addition, a V-notch test piece is produced according to the V-notch test piece of JIS Z 2242:2005, and a Charpy impact test is performed based on JIS Z 2242:2005. And the average value and the minimum value of vE- ₂₀ of three test pieces are calculated|required.

(D) the thickness of the steel plate

Although there is no restriction|limiting in particular about the thickness of the steel plate which concerns on this invention, When using as a welded structure, it is preferable that it is 10-70 mm, and, as for plate thickness, it is more preferable that it is 20-60 mm. Further, 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) Manufacturing method of steel plate

Although there is no particular limitation on the manufacturing conditions of the steel sheet according to the present invention, for example, it can be manufactured by sequentially performing the refining process, continuous casting process, heating process, hot rolling process, and accelerated cooling process under the conditions shown below. have. Each process is demonstrated.

(a) refining process

A refining process is a process of manufacturing molten steel. In the refining step, the amount of deoxidized Al to be charged is 0.2 to 1.3 kg per 1 t of molten steel. When the amount of deoxidized Al is 0.2 kg/t or more, the dissolved O concentration is reduced and it becomes possible to finely disperse the Zr-containing oxide. On the other hand, when the amount of deoxidized Al is 1.3 kg/t or less, the ratio of Al ₂ O ₃ in the composite inclusion can be reduced. The introduction of deoxidized Al can be performed using, for example, a converter.

Then, vacuum degassing is performed, and after the dissolved O concentration in molten steel becomes 0.0050 mass % or less, Zr is added. When Zr is added in a state where the dissolved O concentration exceeds 0.0050 mass %, not only it becomes difficult to finely disperse the Zr-containing oxide, but there is a fear that the ratio of Al ₂ O ₃ in the composite inclusions cannot be reduced. Moreover, _the amount of ZrO2 produced|generated increases and the risk of the blockage of the injection|pouring nozzle to the tundish at the time of continuously casting molten steel increases.

Next, B is added after 1 minute or more has elapsed from the addition of Zr. Thereby, B segregates in the surface layer of a Zr containing oxide, and it becomes possible to precipitate B nitride. Regarding the time from addition of Zr to addition of B, although it is not necessary to provide an upper limit in particular, it is preferable to set it as 5 minutes or less. Zr and B can be added, for example, in a reflux degassing device.

(b) continuous casting process

A continuous casting process is a process of continuously casting molten steel and manufacturing the steel piece which has the above-mentioned chemical composition. In a continuous casting process, the average cooling rate in the surface temperature of a steel piece between 1200-900 degreeC shall be 0.5 degreeC/sec or less. Thereby, in _a Zr containing oxide, separation _of ZrO2 and Al2O3 advances, _{and it} becomes possible to reduce the ratio _of Al2O3 in a composite inclusion.

(c) heating process

The heating step is a step that contributes to the control of the structure of the austenite phase by heating the steel piece. In a heating process, the said steel piece is heated to the heating temperature of 950-1080 degreeC. What is necessary is just to perform a heating process with a heating furnace. In addition, heating a steel piece to 950-1080 degreeC means heating so that the total thickness average temperature of the steel piece at the time of extraction from a heating furnace may become the range of 950-1080 degreeC, and, in this specification, the total thickness average of this steel piece The temperature is called the heating temperature of the steel piece. Moreover, the total thickness average temperature can be calculated|required by calculation from the temperature in a heating furnace, a heating time, and the surface temperature of a steel piece.

If the heating temperature is less than 950°C, austenitization becomes insufficient and hardenability decreases due to refining of the austenite grains, so it is difficult to obtain a steel sheet having a thick sheet thickness and high strength. In addition, since recrystallization at the time of finish rolling is accelerated|stimulated by refinement|miniaturization of austenite particle|grains, the aspect-ratio of prior austenite particle|grains falls. Moreover, when a heating temperature exceeds 1080 degreeC, austenite grains will coarsen and it will become difficult to refine|miniaturize a bainite structure in a final structure. The range of preferable heating temperature is 1000-1050 degreeC.

(d) hot rolling process

The hot rolling process includes rough rolling and finish rolling. Rough rolling is performed in the range where the surface temperature of a steel piece is T _rex or more. That is, rough rolling is started in a state where the surface temperature of the steel piece is T _rex or higher, and rough rolling is finished in a state where the surface temperature of the steel piece is T _rex or higher. By performing rough rolling in the range of T _rex or more, refinement|miniaturization becomes possible by recrystallization of austenite grains. Moreover, the surface temperature at the time of completion|finish of rough rolling may be higher than the surface temperature at the time of the start of rough rolling. This is considered to be an effect of generating heat from processing due to rough rolling, and an effect of heat transfer in the sheet thickness direction of the steel piece due to the fact that the internal temperature is higher than the surface temperature.

Moreover, the cumulative reduction ratio in rough rolling is made into the range of 10-75 %. The cumulative reduction ratio in rough rolling is a value obtained by subtracting the plate thickness after the end of rough rolling from the plate thickness at the start of rough rolling by the plate thickness at the start of rough rolling. If the cumulative reduction ratio at the time of rough rolling is less than 10%, it is difficult to refine the austenite by recrystallization, and the pores remain and internal cracks may occur, possibly resulting in deterioration of ductility and toughness. In addition, when the cumulative reduction ratio exceeds 75%, since the austenite grains are excessively refined, recrystallization at the time of finish rolling is promoted, thereby reducing the aspect ratio of the old austenite grains and increasing the number of passes, resulting in lower productivity do. A preferable cumulative reduction ratio is 30 to 60%. In addition, in the following description, the steel piece after rough rolling is called a steel plate.

The continuous finish rolling is performed in a range where the surface temperature of the steel sheet is Ar ₃ or more and less than _Trex . That is, after completion of rough rolling, cooling is performed to start finish rolling in a state where the surface temperature of the steel sheet is Ar ₃ or more and less than T _rex , and finish rolling is finished in a state in which the surface temperature of the steel sheet is Ar ₃ or more and less than T _rex . By performing finish rolling in a range less than T _rex , it becomes possible to impart strain to the austenite grains without recrystallization. Thereby, bainite in the final structure can be refined. When the finishing temperature is performed in a range where the surface temperature is T _rex or more, recrystallization is promoted and the aspect ratio of the prior austenite grains is lowered. On the other hand, when the finish rolling is performed in a range where the surface temperature is less than Ar ₃ , deformed ferrite is generated, and there is a possibility that the final structure cannot be made into a structure mainly composed of bainite.

Moreover, the cumulative reduction ratio in finish rolling is made into the range of 65 to 90%. The cumulative rolling reduction in finish rolling is a value obtained by subtracting the plate thickness after finish rolling from the plate thickness at the start of finish rolling (after rough rolling), divided by the plate thickness at the start of finish rolling. By setting the cumulative reduction ratio in the finish rolling to 65% or more, it becomes possible to provide sufficient strain to the austenite grains. When the cumulative reduction ratio is less than 65%, the provision of strain to the austenite grains becomes insufficient, the flattening of the austenite grains is not promoted, and the aspect ratio decreases. Moreover, when the cumulative reduction ratio exceeds 90 %, recrystallization is accelerated|stimulated, while the aspect-ratio of an old austenite particle|grains falls, the number of passes increases, and productivity falls. A preferable cumulative reduction ratio is 70 to 80%.

In addition, the time between passes in finish rolling shall be 15 second or less. When the time between passes exceeds 15 seconds, the strain imparted by processing is restored, and while it becomes impossible to sufficiently refine the bainite in the final structure, recrystallization is promoted, and the aspect ratio of the former austenite grains decreases. Since it is so preferable that the time between passes is short, it is not necessary to form a lower limit, but it is preferable to set it as 3 second or more from a viewpoint of operability. In addition, in general, finish rolling is performed by reverse rolling. The time between passes in finish rolling means that the steel sheet is rolled by a rolling roll while advancing forward, and after the rear end of the steel sheet comes out of the rolling roll, the advancing direction of the steel sheet is reversed backward, and the rear end of the steel sheet is rolled again. It means the time until it is rolled up into a roll.

In addition, the time from the completion of finish rolling to the start of cooling in the accelerated cooling process mentioned later shall be 50 second or less. When the time from the completion of the finish rolling to the start of cooling exceeds 50 seconds, the strain imparted by the processing is recovered, and while it becomes impossible to sufficiently refine the bainite in the final structure, recrystallization is promoted, and the former austenite grains the aspect ratio of Since the shorter the time from the completion of finish rolling to the start of cooling, the shorter the better. Therefore, it is not necessary to provide a lower limit, but it is preferably set to 5 seconds or longer from the viewpoint of operability. In addition, the time from the completion of finish rolling to the start of cooling means the time from when the front-end|tip of the steel plate advancing forward exits the rolling roll in the last pass until water cooling is started.

In the above description, Ar ₃ means a transformation initiation temperature at which transformation from austenite particles to ferrite particles starts during the temperature decrease process, and is obtained by the following formula (i). In addition, T _rex means the recrystallization temperature, which is the lowest temperature at which equiaxed recrystallized grains can be generated and grown, and is obtained by the following formula (ii). In addition, the element symbol in the following formula represents content (mass %) of each element contained in a steel plate, and shall substitute 0 when it does not contain.

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, when the solid solution Nb amount (mass %) calculated|required by the following formula (iii) is made into sol.Nb,

When Nb≥sol.Nb, [Nb*]=sol.Nb

When Nb<sol.Nb, [Nb*]=Nb

do it with

sol.Nb=(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N) … (iii)

In addition, T in the said formula represents the heating temperature (degreeC) of the steel piece in a heating process.

(e) accelerated cooling process

In the accelerated cooling step, the steel sheet after finishing rolling is cooled with water. At this time, water cooling is carried out to a cooling stop temperature of 0 to 550 °C under the condition 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 the 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 recuperation, recovery of the strain imparted by processing is accelerated, and the final structure It becomes impossible to sufficiently refine the bainitic ferrite constituting bainite.

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 a bainite-based structure. In addition, an average cooling rate and cooling stop temperature are adjusted according to the value of Ceq in the chemical composition of a steel plate, and let it be the conditions which do not undergo martensitic transformation.

(f) tempering process

You may further provide the tempering process of heating to the temperature range of 350-650 degreeC after an accelerated cooling process. By performing a tempering process, the dislocation density which became excessively high by cooling can be reduced. Moreover, since a self-tempering effect is acquired when the cooling stop temperature in an accelerated cooling process is high, it is not necessary to perform a tempering process. On the other hand, in an accelerated cooling process, when cooling to about room temperature, for example, it is preferable to perform a tempering process.

Hereinafter, the present invention will be described in more detail by way of Examples, but the present invention is not limited to these Examples.

Example

Molten iron tapped from the blast furnace was desulfurized by molten iron preliminary treatment, deP and deC were treated in a converter-type refining vessel, and deoxidized Al was further added, and then taken to a ladle. At the time of steel tapping, an alloying element was added, and cover slag for heat preservation was added.

Then, the molten steel in a ladle was pressure-reduced by the RH vacuum degassing apparatus. In the solvent, a molten steel sample was appropriately collected and used for analysis to obtain a molten steel component. The molten steel temperature changed from 1560 degreeC to 1610 degreeC. While performing component adjustment by adding alloys except Zr and B throughout the RH treatment, vacuum degassing was performed to adjust the dissolved O concentration. The dissolved O concentration was measured using an oxygen concentration probe.

Thereafter, Zr and B were added and a reflux treatment was performed to uniformly mix. With a few exceptions, Zr was added, and after a certain time had elapsed, B was added. In addition, in the example in which B was added before Zr was added, the time from adding Zr to adding B is expressed as a minus sign.

After processing in the RH vacuum degassing apparatus, steel pieces having the chemical compositions shown in Tables 1 and 2 were produced by continuous casting. In continuous casting, the surface temperature of a steel piece adjusted the average cooling rate in 1200-900 degreeC suitably. In Tables 3 and 4, the amount of deoxidized Al added per 1 t of molten steel (kg/t) in the refining process, the dissolved O concentration in the molten steel when Zr is added (mass %), and B after adding Zr The time (minutes) until addition of , and the average cooling rate (°C/sec) between 1200 and 900°C in the continuous casting process are shown. In addition, using the above steel pieces, steel sheets having a sheet thickness of 10 to 70 mm were prepared according to the manufacturing conditions in Tables 5 and 6.

The metal structure of the obtained steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, a sample was taken from the steel sheet so that the 1/4t position in the C section was the observation surface. Then, the observation surface is nital-etched, and after etching, 8 fields of view are photographed at 500 magnification using an optical microscope, and image analysis is performed on the obtained tissue photograph. Each area ratio was calculated.

Next, the nital-etched part was subjected to repera etching, and image analysis was performed on the part shown gray by nital etching, and the area ratio of what appeared white was calculated|required as MA phase.

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 judged to be ferrite, the region in which the local orientation difference exceeds 1.0° was defined as bainitic ferrite. In the measurement, bainitic ferrite having a length in the major axis direction of 1 µm or more was targeted. In addition, the area ratio of bainite was made into the sum total of the area ratios of bainitic ferrite.

In addition, the measurement of the average length and the average of the aspect-ratio in the thickness direction of prior austenite particle|grains was performed according to JISG0551:2013. First, samples were taken from the steel sheet so that the 1/4t position in the L section was the observation surface. Next, after the observation surface was mirror-polished, it was corroded by the Bechet-Beaujard method using saturated picric acid aqueous solution, and the old austenite particle|grains were made to surface.

The observation surface in which the old austenite particle|grains were made to appear was observed with the optical microscope, and the visual field of 0.05 mm< ² > or more was image|photographed 8 fields or more (total 0.40 mm< ² > or more). And the thickness of the prior austenite grains was measured by the cutting method based on the structure photograph image|photographed with the optical microscope, and the average value was made into the average length in the thickness direction of the prior austenite grains. In the measurement, the prior austenite particles having a length in the thickness direction of 1 µm or more were targeted.

In addition, from the above structure photograph, for each old austenite particle, the maximum length in the major axis direction and the maximum length in the minor axis direction orthogonal to the major axis direction are measured, respectively, and the ratio (maximum major length / maximum minor axis length) It calculated|required and the average value was made into the average of the aspect-ratios of prior austenite particle|grains.

Then, the test piece was cut out from the steel plate, and about 0.4g electrolysis was carried out in 10% acetylacetone-1% tetramethylammonium chloride/methanol at a current density of 20 mA/cm< ² >. The solution used for the electrolysis was filtered with a filter with a pore diameter of 0.2 µm, and the Zr content in the extraction residue was measured by using ICP emission spectroscopy for the extraction residue collected on the filter, and it was set as Insol.Zr. Moreover, the value which subtracted Insol.Zr from Zr content contained in steel was made into Sol.Zr.

Moreover, B content B _F which exists as solid solution B in steel was computed based on the following formula.

If B _F '>B, then B _F =B

If 0<B _F '≤B, then B _F =B _F '

If B _F '≤0, then B _F =0

and B _F ' is represented by the following formula (II).

B _F '=B-(N-(Ti-(O-Insol.Zr×(32/91.224))×(95.73/48))×(14/47.867))×(10.811/14) … (II)

In addition, the element symbol in the said formula shows content (mass %) of each element contained in a steel plate, and shall substitute 0 when it does not contain.

Next, the surface of the steel sheet is mirror polished, and the range of 10 mm × 10 mm (100 mm ² ) is observed by SEM, and by quantitative analysis by EDX attached to SEM, Zr of 5 mass % or more, and 0.1 mass % or more of B and ₁ mass% or more _of O, _{and the} ratio _of Al2O3 to the _{total of} ZrO2, Ti2O3, and Al2O3 is 50 mass % or less was identified. Then, among the particles, the number of particles having an equivalent circle diameter of 0.5 µm or more was measured, divided by the area of the observed field of view, and the number density was measured.

These measurement results are shown in Tables 7 and 8. In addition, 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", the area ratio of MA phase is "MA fraction", and bainitic ferrite is The average length in the major axis direction is expressed as "BF length".

In addition, tensile strength (TS) and yield stress (YS) were measured based on JISZ2241:2011. The test piece was measured using the No. 1B tensile test piece which extract|collected as a longitudinal direction the direction (width direction) which goes straight to a rolling direction from a plate|board thickness center part. The yield stress (YS) was taken as the proof strength of the permanent elongation method at the time of permanent elongation of 0.2%. In the present Example, YS of 460 MPa or more and TS of 570 MPa or more were considered to have high strength.

Moreover, the V-notch test piece was extract|collected so that the 1/4t position of the steel plate might be included, and based on JISZ 2242:2005, fracture|fracture transition temperature (vTrs) was evaluated. At this time, two V-notch test pieces were each sampled so that the longitudinal direction of a test piece might correspond to the rolling direction and the width direction of a steel plate, respectively. In this example, in both of the two test pieces, those having vTrs of -60°C or lower were said to be excellent in low-temperature toughness.

And according to ISO 15653:2018, the CTOD test piece which makes the full thickness of the plate|board thickness direction of a base material the notch position of three-point bending was extract|collected, and the CTOD value in -10 degreeC was measured. The test was performed 3 times, and the minimum value was described in the table|surface. In the present Example, the thing with a minimum CTOD value of 0.50 mm or more at -10 degreeC was said to be excellent in fracture toughness.

Moreover, the Charpy absorbed energy in -20 degreeC of HAZ at the time of welding under the conditions of 35 kJ/mm of welding heat input was measured with the following method. First, a heat cycle in which a heat input of 35 kJ/mm welding (large heat input welding) was reproduced was applied to the steel sheet. As specific thermal cycle conditions, after heating from room temperature to 1400°C, the temperature is maintained at 1400°C for 5 seconds, and then, the temperature range from 800°C to 500°C, which is a temperature range related to intragranular transformation, is performed at a rate of 1.0°C/sec. controlled and cooled.

Three V-notch test pieces were taken each from the steel plate after giving a thermal cycle, the Charpy impact test was done at _{-20 degreeC, and absorbed energy (vE-20} ) was measured. In addition, the V-notch test piece was produced according to the V-notch test piece described in JIS Z 2242:2005, and the Charpy impact test was performed based on JIS Z 2242:2005. And the average value and the minimum value of vE- ₂₀ of three test pieces were calculated|required. In the present Example, the average value of vE- ₂₀ was 100 J or more, and the thing with a minimum value of 50 J or more was said to be excellent in HAZ toughness.

These measurement results are shown in Tables 9 and 10.

As can be seen from Tables 7 to 10, in the examples of the present invention (Test Nos. 1-26) satisfying the regulations of the present invention, high strength was obtained, and the result was excellent in low-temperature toughness, fracture toughness, and HAZ toughness. In contrast, in the comparative examples (Test Nos. 27 to 67), at least any one of strength, low-temperature toughness, fracture toughness, and HAZ toughness deteriorated.

Specifically, in Test No. 27, since the C content was excessive, the low-temperature toughness, fracture toughness, and HAZ toughness deteriorated. In Test No. 28, the C content was low and the strength was insufficient, and the low-temperature toughness and fracture toughness deteriorated. In Test No. 29, since the Si content was excessive, the low-temperature toughness, fracture toughness, and HAZ toughness deteriorated. In Test No. 30, since the Mn content was excessive, the low-temperature toughness, fracture toughness, and HAZ toughness deteriorated. In Test No. 31, since the Mn content was low and the bainite area ratio was low, the strength was insufficient and the low-temperature toughness and fracture toughness deteriorated.

In Test No. 32, since the contents of P and S were excessive, low-temperature toughness, fracture toughness, and HAZ toughness deteriorated. In Test No. 33, since the Al content was excessive and formation of a desired composite inclusion became insufficient, the HAZ toughness deteriorated. In Test No. 34, since the N content was excessive and the bainite area ratio was low, the strength was insufficient, and the low-temperature toughness and fracture toughness deteriorated. In Test No. 35, the low-temperature toughness and fracture toughness deteriorated because the N content was low and the prior austenite particles became coarse.

In Test No. 36, since the O content was excessive, the low-temperature toughness, fracture toughness, and HAZ toughness deteriorated. In Test No. 37, the HAZ toughness deteriorated because the O content was low and formation of a desired composite inclusion became insufficient. In Test No. 38, since the Nb content was excessive, the low-temperature toughness, fracture toughness, and HAZ toughness deteriorated. In Test No. 39, the low-temperature toughness and fracture toughness deteriorated because the Nb content was low, the BF length was excessive, and the aspect ratio of the prior austenite particles became small. In Test No. 40, since the Ti content was excessive, the low-temperature toughness, fracture toughness, and HAZ toughness deteriorated. In Test No. 41, the Ti content was low and the BF length and the prior austenite grains were coarsened, so that the low-temperature toughness and fracture toughness deteriorated.

In Test No. 42, the Ca content was excessive, and in Test No. 43, the Mg and REM contents were excessive, and formation of the desired composite inclusions became insufficient, so that the HAZ toughness deteriorated. In Test No. 44, since the Zr content was excessive, the Insol.Zr content became excessive, and the HAZ toughness deteriorated. In Test No. 45, since the Zr content was low, the Insol.Zr content was low, and formation of a desired composite inclusion became insufficient, the HAZ toughness deteriorated. Since Test No. 46 did not contain B, the desired composite inclusion was not formed at all, and the HAZ toughness deteriorated. Moreover, since the bainite area ratio became low, while it became insufficient strength, low-temperature toughness and fracture toughness deteriorated.

In Test No. 47, the amount of deoxidized Al was excessive, while in Test No. 48, the amount of deoxidized Al was low. In Test Nos. 48 and 49, the dissolved O concentration at the time of addition of Zr was excessive. In addition, in Test No. 50, the time from addition of Zr to addition of B was short, and in Test No. 51, B was added before addition of Zr. In addition, Test No. 52 had a high average cooling rate in the continuous casting process. Therefore, in any example, formation of a desired composite inclusion became inadequate, and HAZ toughness deteriorated.

In Test No. 53, the heating temperature in the heating step was high, the BF length and the prior austenite particles were coarsened, and the low-temperature toughness and fracture toughness deteriorated. In Test No. 54, since the heating temperature was low and the bainite area ratio was low, the strength was insufficient, and the low-temperature toughness and fracture toughness deteriorated. In Test No. 55, since the end temperature of rough rolling was less than T _rex , BF length and prior austenite grains coarsened, and low-temperature toughness and fracture toughness deteriorated.

Test No. 56 had a high cumulative rolling reduction in rough rolling, while Test No. 57 had a low cumulative rolling reduction, and both BF length and prior austenite grains were coarsened, and the aspect ratio of prior austenite grains was lowered, Low-temperature toughness and fracture toughness deteriorated. In Test No. 58, since the starting temperature of finish rolling was T _rex or higher, the BF length and the prior austenite grains were coarsened, and the aspect ratio of the prior austenite grains decreased, and the low-temperature toughness and fracture toughness deteriorated. In Test No. 59, since the finishing temperature of finish rolling was less than Ar ₃ , excessively deformed ferrite was generated, resulting in insufficient strength. Moreover, since the bainite area ratio became low, while it became insufficient strength, low-temperature toughness and fracture toughness deteriorated.

In Test No. 60, the cumulative reduction ratio of the finish rolling is high, and the aspect ratio of the prior austenite grains is lowered, while in Test No. 61, the cumulative reduction rate is low, the BF length and the prior austenite grains are coarsened, and the prior austenite grains are coarsened. Since the aspect-ratio of the knight particle|grains fell, low-temperature toughness and fracture toughness all deteriorated. In Test No. 62, the time between passes is long, and in Test No. 63, the time from completion of finish rolling to the start of cooling is long, so the BF length is coarse, and the aspect ratio of the prior austenite grains is lowered, so that low-temperature toughness and fracture toughness This has deteriorated.

In Test No. 64, the low-temperature toughness and fracture toughness deteriorated because the MA phase was excessively generated due to the high cooling rate in the accelerated cooling process. Since Test No. 65 had a low cooling rate and Test No. 66 had a high cooling stop temperature, neither did a bainite-based structure, but lacked strength, and the low-temperature toughness and fracture toughness deteriorated. In Test No. 67, the cooling initiation temperature exceeded _Trex -10°C and the BF length was coarse, so the low-temperature toughness was good, but the fracture toughness deteriorated.

Industrial Applicability

ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to obtain the steel plate which has high intensity|strength and is excellent in low-temperature toughness, fracture toughness, and HAZ toughness. Accordingly, the steel sheet 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, other large tanks, and line pipes.

Claims (7)

The chemical composition of the steel sheet, in mass%,
C: 0.040-0.160%,
Si: 0.01 to 0.50%,
Mn: 0.70 to 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,
It satisfies the following formula (I),
In a cross section perpendicular to the rolling direction of the steel sheet, when the thickness of the steel sheet is t, the metal structure at a position of 1/4t from the surface of the steel sheet is,
By area%, it contains 80% or more of bainite, and
The average length in the major axis direction of the bainitic ferrite constituting the bainite 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 old austenite grains at a position of 1/4t from the surface of the steel sheet is 20 µm or less, and the aspect the average of the rain is 2.5 or more,
As a composite inclusion containing Zr and B, the equivalent circle diameter is 0.5 μm or more, and the ratio of Al ₂ O ₃ in the total of ZrO ₂ , Ti ₂ O ₃ , and Al ₂ O ₃ is, in mass%, The number density of the composite inclusions of 50% or less is 5 to 300 pieces/mm ²
grater.
B _F ≤0.0030 … (I)
only,
If B _F '>B, then B _F =B
If 0<B _F '≤B, then B _F =B _F '
If B _F '≤0, then B _F =0
and B _F ' is represented by the following formula (II).
B _F '=B-(N-(Ti-(O-Insol.Zr×(32/91.224))×(95.73/48))×(14/47.867))×(10.811/14) … (II)
In addition, the element symbol in the said formula shows content (mass %) of each element contained in a steel plate, and shall substitute 0 when it does not contain. The method according to claim 1,
The chemical composition replaces a part of the Fe, in 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
A steel sheet containing at least one selected from the group consisting of The method according to claim 1 or 2,
The chemical composition replaces a part of the Fe, in mass%,
Te: 0.0100% or less
which contains, a steel plate. 4. The method according to any one of claims 1 to 3,
The chemical composition replaces a part of the Fe, in mass%,
W: 1.00% or less, and
Sn: 0.50% or less
A steel sheet containing at least one selected from the group consisting of 5. The method according to any one of claims 1 to 4,
The chemical composition replaces a part of the Fe, in mass%,
Total of Ca, Mg and REM: 0.0005% or less
which contains, a steel plate. A method for manufacturing a steel sheet according to any one of claims 1 to 5, comprising:
A refining step of manufacturing molten steel, and a continuous casting step of continuously casting the molten steel to produce a steel piece having the chemical composition according to any one of claims 1 to 5; In the method for manufacturing a steel sheet, in which the step and the accelerated cooling step are sequentially performed,
In the refining step, the amount of deoxidized Al to be charged is 0.2 to 1.3 kg per 1 t of the molten steel, and after the dissolved O concentration in the molten steel becomes 0.0050 mass% or less, Zr is added, and further 1 minute from the addition of Zr After the elapse of the above time, B is added,
In the continuous casting process, the average cooling rate in a surface temperature of the steel piece between 1200 and 900 °C is 0.5 °C/sec or less,
In the heating step, the steel piece is heated to a heating temperature of 950 ~ 1080 ℃,
The hot rolling process includes rough rolling and finish rolling,
The rough rolling is carried out in a range where the surface temperature of the steel piece is T _rex or more,
The cumulative reduction ratio in the rough rolling is 10 to 75%,
The finish rolling is carried out in a range where the surface temperature of the steel piece is Ar ₃ or more and less than _Trex ,
The cumulative reduction ratio in the finish rolling is set to 65 to 90%, and the time between passes is set to 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 cooling stop temperature of 0 to 550°C under the condition that the average cooling rate from the start of cooling to the end of cooling is 5 to 50°C/sec. water-cooled to
A method for manufacturing a steel plate.
Here, Ar ₃ is obtained by the following formula (i), and T _rex is obtained by the following formula (ii). In addition, the element symbol in the following formula represents content (mass %) of each element contained in a steel plate, and shall substitute 0 when it does not contain.
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, when the solid solution Nb amount (mass %) calculated|required by the following formula (iii) is made into sol.Nb,
When Nb≥sol.Nb, [Nb*]=sol.Nb
When Nb<sol.Nb, [Nb*]=Nb
do it with
sol.Nb=(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N) … (iii)
In addition, T in the said formula represents the heating temperature (degreeC) of the steel piece in a heating process. 7. The method of claim 6,
After the accelerated cooling step, further performing a tempering step of heating to a temperature range of 350 to 650 ° C., the manufacturing method of a steel sheet.