CN115398019B

CN115398019B - Steel sheet and method for producing same

Info

Publication number: CN115398019B
Application number: CN202180026274.6A
Authority: CN
Inventors: 菅泽早帆; 中岛清孝; 新宅祥晃; 宫崎洪太郎; 今城大贵; 木下昌彦
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2020-08-31
Filing date: 2021-08-31
Publication date: 2023-07-07
Anticipated expiration: 2041-08-31
Also published as: CN115398019A; JPWO2022045353A1; JP7099654B1; WO2022045353A1; KR20220145393A

Abstract

A steel sheet, wherein the chemical composition is C: 0.040-0.160%, si:0.01 to 0.50 percent of Mn: 0.70-2.50%, P: less than 0.030%, S: less than 0.020%, al:0.001 to 0.100 percent, N:0.0010 to 0.0080 percent, nb:0.003 to 0.050 percent, B:0.0001 to 0.0050 percent, the balance: fe and impurities, ceq is 0.25 to 0.60%, a metallographic structure at a position of 1/4t in a C section contains 80% or more of bainite in terms of area%, an average length in a major axis direction of bainitic ferrite constituting the bainite is 10 μm or less, an average length in a thickness direction of prior austenite grains at a position of 1/4t in an L section is 20 μm or less, and an aspect ratio is 2.5 or more on average.

Description

Steel sheet and method for producing same

Technical Field

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

Background

Examples of the application of the steel sheet include welded structures such as ships, high-rise buildings, other buildings, bridges, marine structures, other large tanks of LNG storage tanks, and line pipes (see patent documents 1 to 5, for example). In recent years, in order to increase the loading weight of container ships, the size of welded structures has been increased. Accordingly, the thickness of the steel sheet is required to be thick and high in strength. Further, in the welded structure as described above, further improvement in low-temperature toughness and fracture toughness is a problem from the viewpoint of further safety and reliability.

Prior art literature

Patent literature

Patent document 1: japanese patent application laid-open No. 2019-0232322

Patent document 2: japanese patent laid-open publication No. 2019-023223

Patent document 3: japanese patent laid-open publication No. 2019-0232324

Patent document 4: japanese patent application laid-open No. 2019-035107

Patent document 5: international publication No. 2019/069771

Disclosure of Invention

Problems to be solved by the invention

However, since there is a so-called trade-off relationship between strength and low-temperature toughness, it is not easy to achieve both high strength and low-temperature toughness improvement. Further, the improvement of fracture toughness has been hardly studied so far.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a steel sheet having high strength and excellent low-temperature toughness and fracture toughness, and a method for producing the same.

Solution for solving the problem

The present invention has been made based on the above findings, and aims to provide a steel sheet and a method for producing the same.

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

C：0.040～0.160％、

Si：0.01～0.50％、

Mn：0.70～2.50％、

P: less than 0.030 percent,

S: less than 0.020%,

Al：0.001～0.100％、

N：0.0010～0.0080％、

Nb：0.003～0.050％、

B：0.0001～0.0050％、

The balance: fe and impurities are mixed in the alloy,

ceq defined by the following formula (i) is 0.25 to 0.60%,

when t is the thickness of the steel sheet in a section perpendicular to the rolling direction of the steel sheet, 80% or more of bainite is contained in area% in a metallographic structure at a position 1/4t away from the surface of the steel sheet, and the average length in the long axis direction of bainitic ferrite constituting the bainite is 10 μm or less,

In a cross section of the steel sheet parallel to the rolling direction and the thickness direction, the average length in the thickness direction of the prior austenite grains at a distance of 1/4t from the surface of the steel sheet is 20 μm or less, the aspect ratio is 2.5 or more on average,

Ceq＝C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 (i)

the symbol of the element in the above formula represents the content (mass%) of each element contained in the steel sheet, and if not, 0 is substituted.

(2) The steel sheet according to the above (1), wherein the chemical composition is contained in a mass% in place of a part of the Fe selected from the group consisting of

Ti:0.050% or less,

Cu: less than 1.50 percent,

Ni: less than 2.50 percent,

Cr: less than 1.00%,

Mo:1.00% or less, and

v:0.150% or less.

(3) The steel sheet according to the above (1) or (2), wherein the chemical composition contains, in mass%, a component selected from the group consisting of

Mg:0.0100% or less,

Ca:0.0100% or less, and

REM:0.0100% or less.

(4) The steel sheet according to any one of the above (1) to (3), wherein the chemical composition contains, in mass%, a component selected from the group consisting of

Zr:0.0100% or less, and

Te:0.0100% or less.

(5) The steel sheet according to any one of the above (1) to (4), wherein the chemical composition contains, in mass%, a component selected from the group consisting of

W:1.00% or less, and

sn:0.50% or less of at least one kind selected from the group consisting of.

(6) The steel sheet according to any one of the above (1) to (5), wherein B is calculated by the following formula (I) _F ' more than 0%,

B _F ’＝B-(N-Ti×(14/47.867))×(10.811/14) (I)

(7) A method for producing a steel sheet according to any one of the above (1) to (6),

the method for manufacturing a steel billet having a chemical composition according to any one of (1) to (6) above, wherein the steel billet is subjected to a heating step, a hot rolling step and an accelerated cooling step in this order,

in the heating step, the billet is heated to a heating temperature of 950 to 1080 ℃,

the hot rolling process includes rough rolling and finish rolling,

the rough rolling is performed on the billet with the surface temperature T _rex The above-mentioned range is implemented,

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

The finish rolling is performed at a surface temperature Ar of the billet ₃ Above and below T _rex Is carried out in a range of (a) to (b),

the cumulative rolling reduction in the finish rolling is set to 65 to 90%, and the inter-pass time is set to 15 seconds or less,

the time from the completion of finish rolling to the start of cooling in the accelerated cooling step is 50 seconds or less,

in the above-described accelerated cooling step, the cooling start temperature is set to T _rex Water-cooling to a cooling stop temperature of 0 to 550 ℃ under the conditions that the temperature is below 10 ℃ and the average cooling speed from the start of cooling to the end of cooling is 5 to 50 ℃/s,

wherein Ar is ₃ T is obtained by the following formula (ii) _rex The content (mass%) of each element contained in the steel sheet is determined by the following formula (iii), and if not, 0 is substituted,

Ar ₃ ＝910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo (ii)

T _rex ＝-91900[Nb*] ² +9400[Nb*]+770 (iii)

wherein, when the solid-solution Nb amount (% by mass) obtained by the following expression (iv) is set as sol.Nb,

in the case where Nb is equal to or greater than sol.nb, [ Nb x ] = sol.nb,

in the case of Nb < sol.nb, [ Nb x ] =nb,

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

in the above formula, T represents a heating temperature (c) of the billet in the heating step.

(8) The method for producing a steel sheet according to (7) above, wherein a tempering step of heating to a temperature in the range of 350 to 650 ℃ is further performed after the accelerated cooling step.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a steel sheet having high strength and excellent low-temperature toughness and fracture toughness can be obtained.

Detailed Description

The present inventors have studied the above problems in detail and as a result, have found the following.

As described above, there is a so-called trade-off relationship between strength and low-temperature toughness. Moreover, as a result of the study by the present inventors, it was found that it was not easy to achieve both strength and fracture toughness. Accordingly, the present inventors have studied a method for improving both high strength and low temperature toughness and fracture toughness. As a result, it was found that by making the metallographic structure a main body of bainite, and by making bainitic ferrite constituting bainite finer in addition to the fine and flattened bainitic structure, not only the low-temperature toughness but also the fracture toughness can be suppressed from being lowered.

Further, by controlling the heating temperature before hot rolling to be low and finish rolling the non-recrystallized region at a high reduction rate, it is possible to achieve fine and flattened bainitic microstructure and fine bainitic ferrite.

The present invention has been made based on the above findings. The following describes each essential condition of the present invention in detail.

(A) Chemical composition

The reasons for limiting the elements are as follows. In the following description, "%" for the content refers to "% by mass". In the present specification, "to" indicating a numerical range "is used in the sense that the numerical values described before and after the numerical range are included as a lower limit value and an upper limit value unless otherwise specified.

C：0.040～0.160％

C is contained in an amount of 0.040% or more in order to secure strength of the steel sheet. On the other hand, if the C content exceeds 0.160%, it is difficult to ensure good low-temperature toughness and fracture toughness, and therefore the C content is set to 0.160% or less. Accordingly, the C content is 0.040% or more, preferably 0.050% or more than 0.050%, more preferably 0.060% or more than 0.075%. The C content is 0.160% or less, preferably 0.140% or less, and more preferably 0.120% or less.

Si：0.01～0.50％

Si is effective as a deoxidizing element and a strengthening element, and therefore 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 and fracture toughness are greatly deteriorated, 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, more preferably 0.05% or more. The Si content is 0.50% or less, preferably 0.40% or less, more preferably 0.35% or less, and 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 and fracture toughness of the portion where the center segregation occurs deteriorate, so the Mn content is set to 2.50% or less. Therefore, the Mn content is 0.70% or more, preferably 0.90% or more, more preferably 1.20% or more. The Mn content is 2.50% or less, preferably 2.00% or less, more preferably 1.80% or less, and still more preferably 1.60% or less.

P: less than 0.030 percent

P is an element present in the steel as an impurity. In order to stably secure low-temperature toughness and fracture toughness, the content of P is set to 0.030% or less. Preferably 0.020% or less, and more preferably 0.015% or less. The lower limit is 0%, but the P content may be set to 0.0001% or more in view of the cost for reducing the P content.

S: less than 0.020%

S is an element present in the steel as an impurity. If the S content exceeds 0.020%, mnS extending in the center segregation portion is largely produced, and low-temperature toughness, fracture toughness and ductility deteriorate. Therefore, the S content is set to 0.020% or less. Preferably 0.010% or less. The lower limit is not particularly limited as the S content is smaller, but the S content may be set to 0.0001% or more from the viewpoint of manufacturing cost.

Al：0.001～0.100％

Al is an element that is normally positively contained as a deoxidizing element, and the Al content is set to 0.001% or more.However, when the Al content is excessive, coarse cluster alumina (Al ₂ O ₃ ) The formation of the inclusion is promoted, and the low-temperature toughness and fracture toughness are deteriorated. The Al content is thus 0.100% or less, preferably 0.050% or less.

N：0.0010～0.0080％

N has an effect of forming Ti nitride and suppressing increase in austenite grain diameter when the billet is heated, and therefore is required to be contained at 0.0010% or more. However, if the N content exceeds 0.0080%, the steel sheet becomes brittle, and the N content is set to 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. The N content is 0.0080% or less, preferably 0.0065% or less, and more preferably 0.0060% or less.

Nb：0.003～0.050％

Nb can improve strength and toughness of the steel sheet. In addition, in order to obtain a predetermined microstructure, rolling of an unrecrystallized austenite region is required, while Nb is an element effective for expanding the unrecrystallized temperature range, and increasing the rolling temperature contributes to improvement of productivity. In order to obtain such an effect, it is necessary to contain 0.003% or more. However, if the Nb content exceeds 0.050%, the low-temperature toughness, fracture toughness, and weldability are reduced, and thus 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. The Nb content is 0.050% or less, preferably 0.025% or less, and more preferably 0.018% or less.

B：0.0001～0.0050％

B is an element that improves hardenability and contributes to improvement in strength of the steel sheet. In order to obtain such an effect, it is necessary to contain 0.0001% or more. However, if B is contained in excess, the low-temperature toughness and fracture toughness are reduced, and therefore the B content is set to 0.0050% or less. Therefore, the B content is 0.0001% or more, preferably 0.0005% or more, more preferably 0.0010% or more. The B content is 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

In addition to the above elements, the steel sheet of the present invention may contain at least one or more selected from the group consisting of Ti, cu, ni, cr, mo and V in the following ranges for improving strength. The reason for limiting each element will be described.

Ti: less than 0.050%

Ti has an effect of improving strength and toughness of the steel sheet, and thus may be contained as needed. However, if the Ti content is excessive, the weld portion is hardened, and the toughness is significantly deteriorated. Therefore, the Ti content is 0.050% or less, preferably 0.035% or less, and more preferably 0.020% or less. In order to more reliably obtain the above-described effects, the Ti content is preferably 0.003% or more, more preferably 0.006% or more, and still more preferably 0.010% or more

Cu: less than 1.50 percent

Cu has an effect of improving strength and toughness of the steel sheet, and thus may be contained as needed. However, if Cu is contained in an excessive amount, improvement in performance corresponding to an increase in alloy cost is not found, and instead, the surface cracking may be caused. Therefore, the Cu content is 1.50% or less, preferably 1.20% or less, and more preferably 1.00% or less. In order to obtain the above effect more reliably, the Cu content is preferably 0.10% or more, more preferably 0.20% or more.

Ni:2.50% or less

Ni is an element having an effect of improving the strength of the steel sheet, and thus may be contained as needed. Ni is an element that has an effect of improving toughness of a steel matrix (billet) in a solid solution state. However, if Ni is contained excessively, low-temperature toughness, fracture toughness and weldability are deteriorated. Therefore, the Ni content is 2.50% or less, preferably 1.00% or less, more preferably 0.50% or less, and still more preferably 0.30% or less. In order to obtain the above-described effect more reliably, the Ni content is preferably 0.10% or more, more preferably 0.20% or more.

Cr: less than 1.00%

Cr is an element having an effect of improving the strength of the steel sheet, and thus may be contained as needed. However, when Cr is contained in an excessive amount, low-temperature toughness, fracture toughness and weldability are deteriorated. Therefore, the Cr content is 1.00% or less, preferably 0.80% or less, more preferably 0.50% or less, and still more preferably 0.30% or less. In order to obtain the above-described effect more reliably, the Cr content is preferably 0.10% or more, more preferably 0.20% or more.

Mo: less than 1.00%

Mo is an element having an effect of improving the strength of the steel sheet, and thus may be contained as needed. However, if Mo is contained in excess, low-temperature toughness, fracture toughness and weldability are deteriorated. Therefore, the Mo content is 1.00% or less, preferably 0.80% or less, more preferably 0.50% or less, and still more preferably 0.30% or less. In order to obtain the above-described effect more reliably, the Mo content is preferably 0.01% or more, more preferably 0.02% or more.

V: less than 0.150%

V is an element having an effect of improving the strength of the steel sheet, and thus may be contained as needed. However, if V is contained in excess, low-temperature toughness, fracture toughness and weldability are deteriorated. Therefore, the V content is 0.150% or less, preferably 0.100% or less, more preferably 0.070% or less, and still more preferably 0.050% or less. In order to obtain the above-described effect more reliably, the V content is preferably 0.010% or more, more preferably 0.020% or more.

In addition to the above elements, the steel sheet of the present invention may contain at least one or more selected from the group consisting of Mg, ca and REM in the ranges shown below in order to control inclusions. The reason for limiting each element will be described.

Mg:0.0100% or less

Mg is a deoxidizing element, and is an element that suppresses the formation of coarse inclusions and fine oxides by forming sulfides, and suppresses the formation of harmful inclusions. Therefore, it may be contained as needed. However, if Mg is contained excessively, coarse oxides, sulfides and oxysulfides are easily formed, and low-temperature toughness and fracture toughness are lowered. Therefore, the Mg content is 0.0100% or less, preferably 0.0070% or less, more preferably 0.0050% or less. In order to obtain the above effect more reliably, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more.

Ca:0.0100% or less

Ca is a deoxidizing element, and is an element that suppresses the formation of coarse inclusions and fine oxides by forming sulfides, and suppresses the formation of harmful inclusions. Therefore, it may be contained as needed. However, when Ca is excessively contained, coarse oxides, sulfides and oxysulfides are easily formed, and low-temperature toughness and fracture toughness are lowered. Therefore, the Ca content is 0.0100% or less, preferably 0.0070% or less, more preferably 0.0050% or less. In order to obtain the above-described effect more reliably, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more.

REM:0.0100% or less

REM is a deoxidizing element, and is an element that suppresses the formation of coarse inclusions by forming sulfides, and suppresses the formation of harmful inclusions by forming fine oxides. Therefore, it may be contained as needed. However, when REM is contained in excess, coarse oxides, sulfides and oxysulfides are easily formed, and low-temperature toughness and fracture toughness are lowered. Therefore, the REM content is 0.0100% or less, preferably 0.0070% or less, more preferably 0.0050% or less. In order to obtain the above-described effect more reliably, the REM content is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more.

In the present invention, REM means 17 elements in total of Sc, Y and lanthanoid, and the content of REM means the total content of these elements. The lanthanoid element is industrially added as a misch metal alloy.

In addition to the above elements, the steel sheet of the present invention may contain at least one or more elements selected from the group consisting of Zr and Te in the following ranges in order to achieve miniaturization of the metallographic structure. The reason for limiting each element will be described.

Zr:0.0100% or less

Zr is an element contributing to improvement of toughness by refining the structure of the steel sheet. Zr also functions as a deoxidizing element. Therefore, it may be contained as needed. However, if Zr is contained in excess, low-temperature toughness and fracture toughness are reduced. Accordingly, the Zr content is 0.0100% or less, preferably 0.0070% or less, more preferably 0.0050% or less. In order to obtain the above-mentioned effect more reliably, the Zr content is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more.

Te:0.0100% or less

Te is an element contributing to improvement of toughness by refinement of the structure of the steel sheet, and thus may be contained as needed. However, even if Te is contained in excess, 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. In order to obtain the above effect 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 addition to the above elements, the chemical composition of the steel sheet of the present invention may contain at least one or more selected from the group consisting of W and Sn in the ranges shown below in order to improve corrosion resistance. The reason for limiting each element will be described.

W: less than 1.00%

W is dissolved by oxo acid ion WO ₄ ^- The form (c) of (c) is an element that adsorbs to rust, inhibits permeation of chloride ions in the rust layer, and improves corrosion resistance, and thus may be contained as needed. However, even if W is contained in an excessive amount, the above effects are saturated, and there is a possibility that the low-temperature toughness and fracture toughness are lowered. Therefore, the W content is 1.00% or less, preferably 0.75% or less. In order to obtain the above-described effect more reliably, the W content is preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.05% or more.

Sn: less than 0.50%

Sn is Sn having a form of ²⁺ And an element which dissolves and inhibits corrosion by the inhibitor action in the acid chloride solution. In addition, sn has an effect of suppressing anodic dissolution reaction of steel and improving corrosion resistance. Therefore, it may be contained as needed. However, even if Sn is contained in an excessive amount, the above effect is saturated, and rolling cracks of the steel sheet are easily generated. Thus, the first and second substrates are bonded together,the Sn content is 0.50% or less, preferably 0.30% or less. In order to obtain the above effect more reliably, the Sn content is preferably 0.03% or more, more preferably 0.05% or more.

In the chemical composition of the steel sheet of the present invention, ceq defined by the following formula (i) is required to be 0.25 to 0.60%.

Ceq＝C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 (i)

By setting the Ceq value to 0.25% or more, the strength required for the steel sheet can be ensured. In addition, when Ceq is 0.60% or less, excellent low-temperature toughness and fracture toughness can be ensured. Ceq is 0.25% or more, preferably 0.28% or more, more preferably 0.31% or more, and still more preferably 0.34% or more. The Ceq is 0.60% or less, preferably 0.57% or less, more preferably 0.54% or less, and still more preferably 0.51% or less.

Further, in the chemical composition of the steel sheet of the present invention, B is calculated by the following formula (I) _F ' preferably exceeding 0%.

B _F ’＝B-(N-Ti×(14/47.867))×(10.811/14) (I)

B _F ' is a value that becomes an index of the amount of solid solution B. Since it is difficult to measure the amount of solid solution B, in the present invention, it is calculated by the above formula. The presence of solid solution B in the steel contributes to improvement of strength by segregation of solid solution B in the original γ grain boundary and suppression of ferrite formation.

In the chemical composition of the steel sheet of the present invention, the balance is Fe and impurities. The term "impurities" as used herein refers to components which are mixed in by various factors such as raw materials including ores and scraps and production steps in the industrial production of steel sheets, and which are allowed within a range not adversely affecting the present invention. In the steel sheet, O may be mixed as an impurity, but if the O content is 0.0040% or less, O is allowed.

(B) Metallographic structure of steel plate

The metallurgical structure of the steel sheet of the present invention will be described. In the following description, "%" means "% by area". In the present invention, when the thickness of the steel sheet is t, the position of the steel sheet in the cross section perpendicular to the rolling direction, which is 1/4t from the surface of the steel sheet, is referred to as "1/4 t position in the C cross section", and the position of the steel sheet in the cross section parallel to the rolling direction and the thickness direction, which is 1/4t from the surface of the steel sheet, is referred to as "1/4 t position in the L cross section". The term "rolling direction" as used herein refers to the rolling direction in finish rolling.

Bainite: 80% or more

In the invention, the main body of the metallographic structure is bainite. Specifically, the area ratio of bainite at the 1/4t position in the C section is 80% or more, whereby the strength of the steel sheet can be ensured. The area ratio of bainite is preferably 90% or more. The area ratio of bainite may be a single phase of bainite without setting an upper limit.

It is noted that ferrite, pearlite, and a mixed martensite/austenite phase (MA phase) may be mixed as the balance of the structure, but if the total area ratio is 20% or less, it is allowable. The total area ratio is preferably 10% or less. The total area ratio is preferably small, and the lower limit value is not particularly limited. For example, the total area ratio may be 0%. Further, the content may be more than 0% or 1% or more.

As described above, by making the bainitic structure fine and flattened, and further making the bainitic ferrite fine in addition to the bainitic structure as a main body, the strength of the steel sheet and the low-temperature toughness and fracture toughness can be simultaneously achieved. Specifically, the bainitic structure needs to satisfy the following specifications.

Average length of bainitic ferrite in the long axis direction: less than 10 mu m

The average length of bainitic ferrite constituting bainite in the long axis direction at the 1/4t position in the C section is set to 10 μm or less. By miniaturizing bainitic ferrite constituting bainite, fracture toughness can be ensured. The average length of bainitic ferrite is preferably 8 μm or less.

Average length of prior austenite grains in thickness direction: 20 μm or less

Aspect ratio average of prior austenite grains: 2.5 or more

The refinement of the bainitic structure can be achieved by controlling the heating temperature before hot rolling to be low and finish rolling the non-recrystallized region at a high reduction rate. That is, the prior austenite grains of bainite are formed in a shape elongated in the rolling direction. Therefore, the average length in the thickness direction of the prior austenite grains at the 1/4t position in the L section is 20 μm or less, and the aspect ratio is 2.5 or more on average. The average length of the prior austenite grains in the thickness direction is preferably 15 μm or less. The average aspect ratio of the prior austenite grains is preferably more than 2.5, more preferably 4.0 or more.

In the present invention, the area ratio of the metallographic structure was determined as follows. First, a sample was collected from a steel plate so that the 1/4t position in the C section became the observation plane. The observation surface was then etched with an aqueous nitric acid-ethanol etching solution, and after etching, 8 fields of view were photographed at 500 x using an optical microscope. Then, the obtained tissue photograph was subjected to image analysis, and the area ratios were obtained as ferrite when white was seen and as pearlite when black was seen.

Next, the portion etched with the nitrate ethanol etching solution was subjected to the lepra etching, and the portion in gray was subjected to image analysis, and the portion in white was found to be the MA phase, to determine the area ratio.

The average length of bainitic ferrite and the area ratio of bainite were calculated by KAM (Kernel Average Misorientation) analysis using electron back scattering diffraction (EBSD, electron Back Scatter Diffraction). In KAM analysis, the bainitic ferrite is a region having a difference in local orientation exceeding 1.0 ° in the structure determined to be ferrite. In the measurement, bainitic ferrite having a length of 1 μm or more in the long axis direction was used as an object. The area ratio of bainite is obtained by summing up the area ratios of bainitic ferrite.

The average length and aspect ratio in the thickness direction of the prior austenite grains were measured according to JIS G0551: 2013. First, a sample was collected from a steel plate so that the 1/4t position in the L section became the observation plane. The observation surface was mirror polished, and then etched by the Bechet-Beaujard method using a saturated aqueous solution of picric acid. Black grains appear as prior austenite grains by corrosion.

The observation surface on which the prior austenite grains appeared was observed by an optical microscope, and 8 or more fields of view (total 0.40mm were observed ² Above) 0.05mm ² The above field of view. Then, based on the tissue photograph taken by an optical microscope, the thickness of the prior austenite grains was measured by an intercept method, and the average value thereof was taken as the average length in the thickness direction of the prior austenite grains. In the measurement, the prior austenite grains having a length of 1 μm or more in the thickness direction were targeted.

The ratio (major axis maximum length/minor axis maximum length) of the maximum length in the major axis direction and the maximum length in the minor axis direction perpendicular to the major axis direction was determined from the above-mentioned structure photograph for each prior austenite grain. The average value is then used as the aspect ratio average of the prior austenite grains. In the case where 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, and therefore the long axis direction is the rolling direction and the short axis direction is the plate thickness direction (so-called ND direction).

In the case where the prior austenite grains cannot sufficiently appear by the above method, by the research of "high accuracy of a reconstruction method for austenitic structure of steel (melting back, coating of the coating of the coating hypochondriac Tian Changxing, known rattan, hejia, new japanese iron and gold technical report No. 404 (2016), p.24-30), the prior austenite grains were specified, and the average length and aspect ratio in the thickness direction of the prior austenite grains were determined.

(C) Mechanical properties of steel sheet

The mechanical properties of the steel sheet of the present invention are not particularly limited, and the steel sheet of the present invention has high strength and excellent low-temperature toughness and fracture toughness. Specifically, it is preferable that the Yield Stress (YS) is 460 to 860MPa and the Tensile Strength (TS) is 570 to 980MPa. Further, it is preferable that the fracture transition critical temperature (vTrs) which is an index of low temperature toughness is-60 ℃ or lower. Further, it is preferable that the crack tip opening displacement (Crack Tip Opening Displacement: CTOD) value at-10℃which is an index of fracture toughness is 0.50mm or more.

The Tensile Strength (TS) and the Yield Stress (YS) were set according to JIS Z2241: 2011, a tensile test piece No. 1B collected from the center portion of the sheet thickness in a direction perpendicular to the rolling direction. Specifically, the Yield Stress (YS) is the endurance of the permanent elongation method at 0.2% permanent elongation. Further, the fracture transition critical temperature (vTrs) was evaluated in accordance with JIS Z2242: 2005, the test piece was a V-notch test piece and was collected so as to include the 1/4t position of the steel plate. And further according to ISO 15653:2018, collecting CTOD test pieces with the total thickness of the base material in the plate thickness direction set as the notch position of 3-point bending, and measuring CTOD values at-10 ℃.

(D) Thickness of steel sheet

The thickness of the steel sheet of the present invention is not particularly limited, but when used as a welded structure, the thickness is preferably 10 to 70mm, more preferably 20 to 60mm. In addition, the effect of improving low-temperature toughness and fracture toughness in the present invention is remarkably exhibited when the thickness is less than 50 mm.

(E) Method for manufacturing steel sheet

The conditions for producing the steel sheet of the present invention are not particularly limited, and the steel sheet can be produced, for example, by sequentially subjecting a steel slab having the above chemical composition to a heating step, a hot rolling step, and an accelerated cooling step under the conditions shown below. Each step will be described.

(a) Heating process

The heating step is a step of heating the billet to contribute to control of the structure of the austenite phase. In the heating step, the billet is heated to a heating temperature of 950 to 1080 ℃. The heating step may be performed by a heating furnace. The heating of the billet to 950 to 1080 ℃ means heating such that the average temperature of the total thickness of the billet when the billet is drawn out from the heating furnace is in the range of 950 to 1080 ℃, and in this specification, the average temperature of the total thickness of the billet is referred to as the heating temperature of the billet. The total thickness average temperature can be calculated from the temperature in the heating furnace, the heating time, and the surface temperature of the billet.

When the heating temperature is lower than 950 ℃, the austenitization is insufficient and austenite grains are refined, whereby the hardenability is lowered, and it is difficult to form a steel sheet having a high thickness and strength. Further, recrystallization at the time of finish rolling is promoted by refinement of austenite grains, whereby the aspect ratio of the prior austenite grains is reduced. In addition, if the heating temperature exceeds 1080 ℃, austenite grains coarsen, and it is difficult to refine the bainite structure in the final structure. The preferred heating temperature range is 1000 to 1050 ℃.

(b) Hot rolling process

The hot rolling process includes rough rolling and finish rolling. Rough rolling to make the surface temperature of billet be T _rex The above range is implemented. Namely, the surface temperature of the billet is T _rex The rough rolling is started under the above state, and the surface temperature of the billet is T _rex The rough rolling is ended in the above state. By at T _rex Rough rolling in the above range can achieve refinement by recrystallization of austenite grains. The surface temperature at the end of rough rolling may be higher than the surface temperature at the beginning of rough rolling. This is considered to be an influence of heat generation in the working due to rough rolling and an influence of heat transfer in the thickness direction of the billet due to the internal temperature being higher than the surface temperature.

Further, the cumulative rolling reduction in rough rolling is in the range of 10 to 75%. The cumulative rolling reduction in rough rolling is a value obtained by dividing a value obtained by subtracting the plate thickness after the completion of rough rolling from the plate thickness at the start of rough rolling by the plate thickness at the start of rough rolling. When the cumulative rolling reduction at the time of rough rolling is less than 10%, it is difficult to make the austenite finer by recrystallization, and internal cracks may be generated due to residual pores, which may deteriorate ductility and toughness. In addition, if the cumulative reduction exceeds 75%, austenite grains are excessively miniaturized, and therefore recrystallization at the time of finish rolling is promoted, whereby the aspect ratio of the prior austenite grains is reduced, and the number of passes increases, and productivity is reduced. The cumulative reduction is preferably 30 to 60%. In the following description, a steel slab after rough rolling is referred to as a steel sheet.

The surface temperature of the steel sheet after the subsequent finish rolling is Ar ₃ Above and below T _rex Is implemented within a range of (2). That is, after finishing the rough rolling, the steel sheet is cooled to a surface temperature of Ar ₃ Above and below T _rex Finish rolling is started in a state of (2) at a surface temperature of Ar ₃ Above and below T _rex Finish rolling is finished in the state of (2). By being below T _rex The range of (2) is subjected to finish rolling, and strain can be imparted to austenite grains without recrystallization. This can refine bainite in the final structure. If the final processing temperature is T at the surface temperature _rex When the above range is used, recrystallization is promoted, and the aspect ratio of the prior austenite grains is reduced. On the other hand, if the surface temperature is lower than Ar ₃ If finish rolling is performed in the range of (2), processed ferrite is generated, and a bainitic structure may not be formed in the final structure.

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 dividing a value obtained by subtracting the plate thickness after finish rolling from the plate thickness at the start of finish rolling (after finish rolling) by the plate thickness at the start of finish rolling. By setting the cumulative rolling reduction in finish rolling to 65% or more, a sufficient strain can be imparted to austenite grains. If the cumulative reduction is less than 65%, the strain imparted to the austenite grains is insufficient, flattening of the austenite grains is not promoted, and the aspect ratio is reduced. If the cumulative reduction exceeds 90%, recrystallization is promoted, the aspect ratio of the prior austenite grains decreases, the number of passes increases, and the productivity decreases. The preferred cumulative reduction is 70 to 80%.

Further, the inter-pass time in finish rolling is 15 seconds or less. If the inter-pass time exceeds 15 seconds, the strain imparted by the working is recovered, so that bainite in the final structure cannot be sufficiently refined, recrystallization is promoted, and the aspect ratio of the prior austenite grains is reduced. The shorter the inter-pass time, the more preferable, and therefore, the lower limit is not required, but from the viewpoint of operability, it is preferably 3 seconds or more. In general, finish rolling is performed by reversing rolling. The inter-pass time in finish rolling means the time from when the steel sheet is rolled by the rolling roller while traveling forward, when the rear end of the steel sheet passes through the rolling roller, until the traveling direction of the steel sheet is reversed to the rear, and when the rear end of the steel sheet is again caught by the rolling roller.

Then, the time from completion of finish rolling to the start of cooling in an accelerated cooling step described later is set to 50 seconds or less. If the time from completion of finish rolling to start of cooling exceeds 50 seconds, the strain imparted by working is recovered, bainite in the final structure cannot be sufficiently refined, recrystallization is promoted, and the aspect ratio of the prior austenite grains is reduced. The shorter the time from completion of finish rolling to start of cooling is, the more preferable, and therefore, the lower limit is not required, but from the viewpoint of operability, it is preferably 5 seconds or more. The time from finishing rolling to the start of cooling refers to the time from the leading end of the steel sheet traveling forward through the rolling roll in the final pass to the start of water cooling.

In the above description, ar ₃ The transformation start temperature is obtained by the following formula (ii) from the transformation start temperature of austenite grains to ferrite grains during the cooling process. In addition, T _rex The minimum temperature at which equiaxed recrystallized grains can be formed and grown, that is, the recrystallization temperature, is determined by the following formula (iii). The symbol of the element in the following formula represents the content (mass%) of each element contained in the steel sheet, and if not, 0 is substituted.

Ar ₃ ＝910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo (ii)

T _rex ＝-91900[Nb*] ² +9400[Nb*]+770 (iii)

in the case where Nb is equal to or greater than sol.nb, [ Nb x ] = sol.nb,

in the case of Nb < sol.nb, [ Nb x ] =nb.

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

(c) Accelerated cooling process

In the accelerated cooling step, the finish-rolled steel sheet is water-cooled. At this time, the cooling start temperature is set to T _rex And cooling the mixture to a cooling stop temperature of 0 to 550 ℃ at a temperature of 10 ℃ or lower and an average cooling rate of 5 to 50 ℃/sec from the start of cooling to the end of cooling.

Even in Ar ₃ Above and below T _rex If the cooling start temperature exceeds T due to the subsequent reheating _rex At-10 ℃, recovery of strain imparted by working is promoted, and bainitic ferrite constituting bainite in the final structure cannot be sufficiently refined.

The final structure can be formed into a bainitic structure by cooling the solution to a cooling stop temperature of 0 to 550 ℃ at an average cooling rate of 5 to 50 ℃/sec. The average cooling rate and the cooling stop temperature were adjusted according to the value of Ceq in the chemical composition of the steel sheet, and the conditions were set so as not to undergo martensitic transformation.

(d) Tempering process

After the accelerated cooling step, a tempering step of heating to a temperature range of 350 to 650 ℃ may be further provided. By performing the tempering process, the dislocation density excessively increased due to cooling can be reduced. In the case where the cooling stop temperature in the accelerated cooling step is high, the self-tempering effect can be obtained, and thus the tempering step may not be performed. On the other hand, in the accelerated cooling step, for example, when cooling to about room temperature, the tempering step is preferably performed.

The present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

Examples

Steel sheets having a thickness of 10 to 70mm were produced by trial using billets having the chemical compositions shown in Table 1 under the production conditions shown in Table 2.

TABLE 1

TABLE 2

The metallographic structure of the obtained steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, a sample is collected from a steel plate so that the 1/4t position in the C section becomes the observation plane. Then, the observation surface was etched with an aqueous nitric acid-ethanol etchant, and after the etching, 8 visual fields were taken at 500 x using an optical microscope, and the obtained tissue photograph was subjected to image analysis, and the area ratios were determined as ferrite when white was seen and as pearlite when black was seen.

The average length of bainitic ferrite and the area ratio of bainite were calculated by KAM analysis using EBSD. In KAM analysis, a region having a difference in local orientation exceeding 1.0 ° in the structure determined to be ferrite is regarded as bainitic ferrite. In the measurement, bainitic ferrite having a length of 1 μm or more in the long axis direction was used as the target. The area ratio of bainite is obtained by summing up the area ratios of bainitic ferrite.

Further, the average length in the thickness direction and the average aspect ratio of the prior austenite grains were measured in accordance with JIS G0551: 2013. First, a sample was collected from a steel plate so that the 1/4t position in the L section became the observation plane. The observation surface was mirror polished, and then etched by the Bechet-Beaujard method using a saturated aqueous solution of picric acid. The prior austenite grains appear.

The maximum length in the major axis direction and the maximum length in the minor axis direction perpendicular to the major axis direction were measured for each prior austenite grain from the above-mentioned structure photograph, and the ratio (major axis maximum length/minor axis maximum length) was obtained, and the average value was used as the aspect ratio average of the prior austenite grains.

Further, the Tensile Strength (TS) and the Yield Stress (YS) are based on JIS Z2241: 2011. The test piece was measured using a tensile test piece No. 1B collected from the center portion of the sheet thickness in a direction (width direction) perpendicular to the rolling direction as a longitudinal direction. The Yield Stress (YS) is the endurance of the permanent elongation method at 0.2% permanent elongation. In this example, the case where YS is 460MPa or more and TS is 570MPa or more is regarded as having high strength.

Further, V notch test pieces were collected so as to include 1/4t position of the steel sheet, and according to JIS Z2242: 2005, fracture transition critical temperature (vTrs) was evaluated. At this time, 2V-notch test pieces were collected so that the longitudinal direction of the test piece matches the rolling direction and the width direction of the steel sheet. In this example, when the vTrs was-60℃or lower for all of the 2 test pieces, the low-temperature toughness was excellent.

Next, according to ISO 15653:2018, collecting CTOD test pieces with the total thickness of the base material in the plate thickness direction set as the notch position of 3-point bending, and measuring CTOD values at-10 ℃. The test was performed 3 times and their minimum values are shown in the table. In this example, the case where the minimum CTOD value at-10℃is 0.50mm or more is regarded as excellent fracture toughness.

The measurement results are shown in Table 3. In the table, the area ratio of ferrite is described as "F fraction", the area ratio of pearlite is described as "P fraction", the area ratio of bainite is described as "B fraction", the area ratio of MA phase is described as "MA fraction", and the average length in the long axis direction of bainitic ferrite is described as "BF length".

TABLE 3

TABLE 3 Table 3

B _F '＝B-(N-Ti×(14/47.867))×(10.811/14)

As is clear from table 3, the present invention examples (test numbers 1 to 25) satisfying the regulations of the present invention gave results that have high strength and excellent low-temperature toughness and fracture toughness. In contrast, in the comparative examples (test nos. 26 to 55), at least one of the strength and the low-temperature toughness was deteriorated.

Specifically, for test No. 26, the C content was excessive, and thus the low temperature toughness and fracture toughness were deteriorated. For test No. 27, the C content was low and the strength was insufficient. For test No. 28, si content was excessive, and thus low temperature toughness and fracture toughness were deteriorated. For test No. 29, the Mn content was excessive, and thus the low temperature toughness and fracture toughness were deteriorated. For test number 30, the Mn content was low and the strength was insufficient.

The contents of P and S were excessive for test number 31, the Al content was excessive for test number 32, the N content was excessive for test number 33, the bainitic area ratio was lowered, and thus strength was insufficient, and the low temperature toughness and fracture toughness were deteriorated. In test No. 34, the prior austenite grains became coarse with a low N content, and therefore the low-temperature toughness and fracture toughness deteriorated. For test No. 35, nb content was excessive, and thus low temperature toughness and fracture toughness were deteriorated. For test No. 36, nb content was low, BF length was excessive, and the aspect ratio of prior austenite grains was reduced, so that low-temperature toughness and fracture toughness were deteriorated.

For test No. 37, the B content was excessive, and thus the low temperature toughness and fracture toughness were deteriorated. For test number 38, B was not contained, and for test number 39, ceq was reduced. As a result, the bainite area ratio was reduced for these examples, so that the strength was insufficient, and the low-temperature toughness and fracture toughness were deteriorated. For test number 40, ceq was excessive in value, and therefore low temperature toughness and fracture toughness deteriorated.

In test No. 41, the heating temperature in the heating step was high, BF length and prior austenite grains were coarsened, while in test No. 42, the heating temperature was low, the aspect ratio of the prior austenite grains was lowered, and both low-temperature toughness and fracture toughness were deteriorated. For test number 43, the end temperature of the roughing was below T _rex Therefore, BF length and prior austenite grain coarsening, low temperature toughness and fracture toughness deteriorate.

In test No. 44, the cumulative reduction was low, BF length and prior austenite grains coarsened, and further the aspect ratio of the prior austenite grains was lowered, so that the low-temperature toughness and fracture toughness were deteriorated. On the other hand, in test No. 45, the cumulative rolling reduction in rough rolling was high, and therefore the aspect ratio of the prior austenite grains was lowered, and the low-temperature toughness was deteriorated. For test number 46, the finish rolling temperature was lower than Ar ₃ Thus, excessive processed ferrite is generated, strength is insufficient, and low-temperature toughness and fracture toughness are deteriorated. For test No. 47, the finish rolling starting temperature was T _rex As described above, BF length and prior austenite grains coarsen, and the aspect ratio of the prior austenite grains decreases, and low-temperature toughness and fracture toughness deteriorate.

In test No. 48, the cumulative reduction of finish rolling was high, the bainitic area ratio was low, and the aspect ratio of prior austenite grains was low, so that the strength was insufficient, and the low-temperature toughness and fracture toughness were deteriorated. For test No. 49, the cumulative reduction was low, and therefore BF length and prior austenite grain coarsening, low temperature toughness, and fracture toughness were deteriorated. In test No. 50, the inter-pass time was long, and in test No. 51, the time from completion of finish rolling to start of cooling was long, so BF length coarsened, and the aspect ratio of the prior austenite grains was lowered, and low-temperature toughness and fracture toughness were deteriorated. In addition, for test number 51, the bainite area ratio was reduced, so that the strength was insufficient, and the low temperature toughness and fracture toughness were deteriorated.

In test No. 52, the cooling rate in the accelerated cooling step was high, and therefore MA phase was excessively generated, and low-temperature toughness and fracture toughness were deteriorated. For test No. 53, the cooling rate was low, the structure in which the bainite main body was not formed, the strength was insufficient, and the low-temperature toughness was deteriorated. In test number 54, the cooling stop temperature was high, and therefore, the structure of the bainite main body was not formed, and the low-temperature toughness and fracture toughness were deteriorated. For test number 55, the cooling start temperature exceeded T _rex As a result, the fracture toughness was deteriorated although the low-temperature toughness was good, since the BF length was roughened at-10 ℃.

Industrial applicability

According to the present invention, a steel sheet having high strength and excellent low-temperature toughness and fracture toughness can be obtained. Therefore, the steel sheet of the present invention can be suitably used as a material for welded structures such as ships, high-rise buildings, other buildings, bridges, marine structures, LNG storage tanks, other large tanks, and piping.

Claims

1. A steel sheet, wherein the chemical composition of the steel sheet is C:0.040 to 0.160 percent,

Si：0.01～0.50％、

Mn：0.70～2.50％、

P: less than 0.030 percent,

S: less than 0.020%,

Al：0.001～0.100％、

N：0.0010～0.0080％、

Nb：0.003～0.050％、

B：0.0001～0.0050％、

Ti:0.050% or less,

Cu: less than 1.50 percent,

Ni: less than 2.50 percent,

Cr: less than 1.00%,

Mo: less than 1.00%,

V: less than 0.150 percent,

Mg:0.0100% or less,

Ca:0.0100% or less,

REM:0.0100% or less,

Zr:0.0100% or less,

Te:0.0100% or less,

W: less than 1.00%,

Sn: less than 0.50 percent,

The balance: fe and impurities are mixed in the alloy,

ceq defined by the following formula (i) is 0.25 to 0.60%,

when the thickness of the steel sheet is t in a cross section perpendicular to the rolling direction of the steel sheet, a metallographic structure at a position 1/4t away from the surface of the steel sheet contains 80% or more of bainite in terms of area%, and the average length in the long axis direction of bainitic ferrite constituting the bainite is 10 μm or less,

In a cross section of the steel sheet parallel to the rolling direction and the thickness direction, the average length in the thickness direction of the prior austenite grains at a position 1/4t away from the surface of the steel sheet is 20 μm or less, the aspect ratio is 2.5 or more on average,

Ceq＝C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 (i)

the symbol of the element in the above formula represents the mass% content of each element contained in the steel sheet, and if not, 0 is substituted.

2. The steel sheet according to claim 1, wherein B is calculated by the following formula (I) _F ' more than 0%, B _F ’＝B-(N-Ti×(14/47.867))×(10.811/14)(I)

3. A method for producing a steel sheet according to claim 1 or claim 2,

the manufacturing method comprises sequentially performing a heating step, a hot rolling step, and an accelerated cooling step on a steel billet having the chemical composition according to claim 1 or claim 2,

the hot rolling process includes rough rolling and finish rolling,

the surface temperature of the rough rolling on the steel billet is T _rex The above-mentioned range is implemented,

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

The surface temperature of the finish rolling on the billet is Ar ₃ Above and below T _rex Is carried out in a range of (a) to (b),

the cumulative rolling reduction in the finish rolling is set to 65-90%, and the inter-pass time 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 set to T _rex Water-cooling to a cooling stop temperature of 0 to 550 ℃ under the conditions that the temperature is below 10 ℃ and the average cooling speed from the start of cooling to the end of cooling is 5 to 50 ℃/s,

wherein Ar is ₃ T is obtained by the following formula (ii) _rex The content of each element contained in the steel sheet is represented by the following formula (iii), and if not, 0 is substituted,

Ar ₃ ＝910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo (ii)

T _rex ＝-91900[Nb*] ² +9400[Nb*]+770 (iii)

wherein, when the solid-solution Nb amount in mass% obtained by the following formula (iv) is taken as sol.Nb,

in the case where Nb is equal to or greater than sol.nb, [ Nb x ] = sol.nb,

in the case of Nb < sol.nb, [ Nb x ] =nb,

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

in the above formula, T represents the heating temperature of the billet in the heating step, and the unit of the heating temperature is ℃.

4. The method for producing a steel sheet according to claim 3, wherein a tempering step of heating to a temperature in the range of 350 to 650 ℃ is further performed after the accelerated cooling step.