CN115398018B

CN115398018B - Steel sheet and method for producing same

Info

Publication number: CN115398018B
Application number: CN202180026262.3A
Authority: CN
Inventors: 中井启介; 今城大贵; 中村真吾; 新宅祥晃; 中岛清孝
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: 2023-07-21
Anticipated expiration: 2041-08-31
Also published as: JP7127753B2; JPWO2022045352A1; WO2022045352A1; CN115398018A; KR20220147126A

Abstract

A steel sheet has a chemical composition of 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 of Ti:0.003 to 0.050 percent, the balance: fe and impurities, wherein the metallographic structure at 1/4t of the C section contains 80 area% or more of bainite, the average length in the long axis direction of bainitic ferrite constituting the bainite is 10 μm or less, the average length in the thickness direction of prior austenite grains at 1/4t of the L section is 20 μm or less, the aspect ratio is 2.5 or more on average, and the grain boundary density in the C section is 500 to 1100mm/mm at 1/10t ² 400-1000 mm/mm at 1/4t position ² 300-900 mm/mm at 1/2t position ² 。

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.

Further, even when a brittle crack is generated at a welded joint portion, a brittle crack propagation stopping characteristic (hereinafter referred to as "crack stopping property") is required for stopping the brittle crack at the base material.

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 combine them. In addition, improvement of the crack resistance is not easy and becomes an important problem. 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, fracture toughness and crack resistance, and a method for producing the same.

Solution for solving the problem

The gist of the present invention is the following 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％、

Ti：0.003～0.050％、

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

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,

in the section of the steel sheet perpendicular to the rolling direction,

the grain boundary density at a position 1/10t away from the surface of the steel sheet is 500-1100 mm/mm ² 、

A distance of 1 +.Grain boundary density at the position of 4t is 400-1000 mm/mm ² 、

The grain boundary density at a position 1/2t away from the surface of the steel sheet is 300-900 mm/mm ² 。

(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

Cu: less than 1.50 percent,

Ni: less than 2.50 percent,

Cr: less than 1.00%,

Mo: less than 1.00%,

V:0.150% or less, and

b:0.0050% or less of at least one kind selected from the group consisting of.

(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 the chemical composition satisfies the following formula (i),

1.7≤Ti/N≤3.4(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.

(7) The steel sheet according to any one of the above (1) to (6), wherein the chemical composition satisfies the following formula (ii),

in a cross section of the steel sheet perpendicular to a rolling direction, tiN particles having an average equivalent circle diameter of 60nm or less at a position at a distance of 1/10t from a surface of the steel sheet, and an area ratio of the TiN particles being 0.0001% or more,

Ti×N≥3.0×10 ^-5 (ii)

(8) 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 1050 ℃,

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 Average cooling at-10deg.C or lower from start of cooling to end of coolingWater-cooling to a cooling stop temperature of 0-550 ℃ under the condition of a speed of 5-50 ℃/s,

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

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

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

wherein, when the solid solution Nb amount (% by mass) obtained by the following expression (v) 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)(v)

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

(9) A method for producing a steel sheet according to (7) above,

the manufacturing method comprises the following steps: a refining step of producing molten steel; and a continuous casting step of continuously casting the molten steel to produce a billet having the chemical composition according to any one of (1) to (6), and sequentially performing a heating step, a hot rolling step and an accelerated cooling step on the obtained billet,

In the refining step, ti is added after the dissolved O concentration in the molten steel is 0.0050% or less,

in the continuous casting step, the average cooling rate during the period when the surface temperature of the billet is 1200 to 900 ℃ is set to 0.1 to 0.5 ℃/sec,

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 steel billetIs of surface temperature T _rex The above-mentioned range is implemented,

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

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,

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

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

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

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)(v)

(10) The method for producing a steel sheet according to (8) or (9), 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, fracture toughness and crack resistance 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. Therefore, 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.

Next, as a result of examining a method for improving crack resistance, it was found that crack resistance in a direction parallel to the surface of a steel sheet, for example, in a direction perpendicular or parallel to the rolling direction, can be improved by controlling the grain boundary density in the plate thickness direction of the steel sheet.

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 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 this effect, the content is 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.

Ti：0.003～0.050％

Ti can improve strength and toughness of the steel sheet. Further, by containing Ti, tiN is formed, and an increase in austenite grain diameter is suppressed when the billet is heated. When the austenite grain diameter is increased, the grain diameter of the transformation structure is also increased, and therefore, it is difficult to obtain a predetermined grain boundary density, and toughness and crack resistance are reduced. In order to obtain the effect achieved by TiN, 0.003% or more of Ti is contained.

However, if the Ti content exceeds 0.050%, tiC is formed and HAZ toughness is reduced, so the Ti content is set to 0.050% or less. Therefore, the Ti content is 0.003% or more, preferably 0.006% or more, more preferably 0.008% or more. The Ti content is 0.050% or less, preferably 0.020% or less, and more preferably 0.015% or less.

In addition, the Ti content preferably satisfies the following formula (i) in relation to the N content. By setting the Ti/N value to 1.7 or more, the crack resistance can be improved by fixing the solid solution N. When the solid solution N is excessive, it is considered that cracking resistance is lowered due to phenomena such as promotion of fracture sensitivity by the solid solution N, promotion of grain boundary embrittlement by the solid solution N, formation of MA by the solid solution N, embrittlement by Fe nitride to which dislocation is fixed, and the like.

On the other hand, by setting the Ti/N value to 3.4 or less, formation of coarse TiN, tiC, or the like is suppressed, and the crack arrest property can be improved. The value of Ti/N is preferably 2.0 to 3.0, more preferably 2.3 to 2.7.

1.7≤Ti/N≤3.4(i)

Further, in the relation between the Ti content and the N content, the following formula (ii) is preferably satisfied. By setting the value of Ti×N to 3.0X10 ^-5 As described later, tiN particles having an average equivalent circle diameter of 60nm or less and an area ratio of 0.0001% or more are obtained at a position at a distance of 1/10t from the surface of the steel sheet, contributing to improvement of crack resistance. The value of Ti×N is preferably 4.0X10 ^-5 ～10.0×10 ^-5 More preferably 5.0X10 ^-5 ～8.0×10 ^-5 。

Ti×N≥3.0×10 ^-5 (ii)

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 Cu, ni, cr, mo, V and B in the following ranges for improving strength. The reason for limiting each element will be described.

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 0.005% or more, more preferably 0.010% or more, and still more preferably 0.050% 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 0.005% or more, more preferably 0.010% or more, and still more preferably 0.050% 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 0.005% or more, more preferably 0.010% or more, and still more preferably 0.050% 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 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% 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 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% or more.

B: less than 0.0050%

B is an element that improves hardenability and contributes to improvement of strength of the steel sheet, and thus may be contained as needed. However, if B is contained in excess, the low-temperature toughness and fracture toughness are lowered. Accordingly, the B content is 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less. In order to obtain the above-described effect more reliably, the B 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 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 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% 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. Therefore, the Sn content is 0.50% or less, preferably 0.30% or less. In order to obtain the above-described effect more reliably, the Sn content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.010% or more.

In the chemical composition of the steel sheet of the present invention, 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: 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.

Grain boundary density at 1/10t position in C section: 500-1100 mm/mm ²

Grain boundary density at 1/4t position in C section: 400-1000 mm/mm ²

Grain boundary density at 1/2t position in C section: 300-900 mm/mm ²

As a dominant factor in improvement of crack resistance, the contribution of grain boundaries, which are an obstacle to brittle crack propagation, is large. In the grain boundaries, the crystal orientation is different between adjacent crystal grains, and thus the crack propagation direction is changed in this portion. Accordingly, an unbroken region is generated, and stress is dispersed in the unbroken region, thereby forming a crack closure stress. Therefore, the driving force for crack propagation is reduced, and crack resistance is improved. In addition, the unbroken areas eventually ductile fail, thus absorbing the energy required for brittle fracture. Thus, the crack resistance is improved.

It has hitherto been thought that in order to increase the grain boundaries, it is necessary to thin the grain diameters. As described above, the ferrite-based structure is indispensable for the use of bainite in steel having a thick plate and high strength. The bainite is different from ferrite, and the shape of the lower structure is complex, so that the definition of crystal grains is extremely difficult. Therefore, even if the relationship between the grain diameter and the crack resistance is calculated as equivalent circle diameter, the deviation is large, and it is difficult to determine the grain diameter required for improvement of the crack resistance.

Therefore, it is understood that the correlation is most favorable when the total length of grain boundaries per unit area (hereinafter referred to as "grain boundary density") is defined by the basic principle that returning to the grain boundaries becomes an obstacle to crack propagation, and the correlation is most favorable by using the relationship between the grain boundaries and crack stopping properties. Here, "grain boundary density" refers to "total length per unit area of grain boundaries having a difference in crystal orientation of 15 ° or more". The reason why the difference in crystal orientation is 15 ° or more is that when the difference is smaller than 15 °, the grain boundary is less likely to be an obstacle to the propagation of brittle cracks, and the crack resistance improvement effect is reduced.

By setting the grain boundary density in the C section at 1/10t position to 500mm/mm ² The above was set at 400mm/mm at 1/4t position ² The above was set at 300mm/mm at 1/2t position ² As described above, excellent crack resistance can be obtained. Furthermore, in order to stably improve crack resistance, the grain boundary density in the C section is preferably 600mm/mm at the 1/10t position ² Above, preferably 500mm/mm at 1/4t position ² The above, preferably 400mm/mm at 1/2t position ² The above.

The crack resistance improves as the grain boundary density increases, but an excessive increase leads to an increase in rolling load, and further, productivity decreases. Therefore, the grain boundary density in the C section was set to 1100mm/mm at the 1/10t position ² The following is set to 1000mm/mm at 1/4t position ² The following is set to 900mm/mm at 1/2t position ² The following is given. The grain boundary density in the C section is preferably 1000mm/mm at the 1/10t position ² Below, preferably 900mm/mm at the 1/4t position ² Below, preferably 800mm/mm at 1/2t position ² The following is given.

In order to improve the crack resistance of an extremely thick material, it is necessary to increase the grain boundary density of the entire plate thickness. In the production method described later, the grain boundary density at the 1/2t position is mainly controlled. In addition to this, the plate thickness position is inevitably low in temperature and increases in cooling rate, so that the grain boundary density tends to increase. Therefore, it is often sufficient to define only the grain boundary density at the 1/2t position. However, according to the heating method, a large temperature gradient is generated in the plate thickness direction, for example, at the 1/4t position and the 1/2t position, and the grain boundary density may be reversed. Therefore, in the present invention, as representative values of the grain boundary densities of the plate thickness averages, the grain boundary densities at the 1/10t position, the 1/4t position, and the 1/2t position are specified.

In the present invention, the grain boundary density is measured by an electron beam back scattering diffraction (EBSD) method. Specifically, by measuring a region of 500 μm×500 μm at 1/10t position, 1/4t position, and 1/2t position at 1 μm pitch by the EBSD method, a boundary having a crystal orientation difference of 15 ° or more from adjacent crystal grains is defined as a grain boundary, and the total length of the grain boundary at this time is divided by the measurement area, whereby the grain boundary density can be obtained.

TiN particles at 1/10t position

Average equivalent circle diameter: 60nm or less

Area ratio: 0.0001% or more

At 1/10t, if TiN particles are finely dispersed, the pinning effect achieved by the TiN particles is effectively exhibited, and coarsening of prior austenite is suppressed. As a result, the grain boundary density at the 1/10t position increases and the crack resistance of the steel sheet further improves. Therefore, the average equivalent circle diameter of TiN particles present at the 1/10t position is preferably 60nm or less and the area ratio is preferably 0.0001% or more.

The average equivalent circle diameter of the TiN particles is more preferably 50nm or less, still more preferably 40nm or less. The lower limit of the average equivalent circle diameter of the TiN particles is not particularly limited, and may be, for example, 10nm or more. The area ratio of TiN particles is more preferably 0.0002% or more, and still more preferably 0.0003% or more. The upper limit of the area ratio of TiN particles is not particularly limited, and may be, for example, 0.0020% or less.

The average equivalent circle diameter and area ratio of TiN particles were measured by the following methods. First, an extraction replica was prepared from a 1/10t position of a steel plate, and the observation area of 1 field of view was set to 15 μm by a TEM with an energy dispersive X-ray analysis device (EDX) with a magnification of 3 ten thousand times or more ² As described above, particles having a size of 15 to 200nm were observed. All particles observed were analyzed using EDX and will contain 1 massParticles of Ti in an amount of not less than 1% by mass of O (oxygen), and N in an amount of not less than 1% by mass are distinguished as TiN particles.

The electron beam diameter of the TEM used for quantitative analysis of the particles was 1 to 20nm, and the observation magnification was 5 to 100 tens of thousands of times, so that quantitative analysis was performed at any position in the particles. The average equivalent circle diameter of TiN particles is obtained by arithmetic averaging of equivalent circle diameters (diameters) that form the same area as that of each TiN particle identified above. The area ratio of TiN particles is a value obtained by dividing the sum of the areas of the TiN particles identified above by the area of the field of view observed.

Here, the observation area of 1 field of view was set to 15 μm by using a magnification of 3 ten thousand times or more ² In the above, when the number of TiN particles to be detected is less than 100 by observing particles having a size of 15 to 200nm, other visual fields are confirmed, and the observation is continued until the total number of TiN particles is 100 or more. At this time, the average equivalent circle diameter of the TiN particles is an arithmetic average of the equivalent circle diameters (diameters) of the respective discriminated TiN particles as described above. The area ratio of TiN particles is a value obtained by dividing the sum of areas of TiN particles observed continuously until 100 or more by the total area of the field of view observed so far. Further, the number of the continuous visual field was 50, and the cumulative viewing area was 750. Mu.m ² In the case where the sum of the number of TiN particles identified is less than 100 at the above time, it is considered that no TiN particles exist, and the TiN particles are out of the scope of the present application.

(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 is excellent in low-temperature toughness, fracture toughness and crack resistance. 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 ℃.

Further, the brittle crack growth stopping toughness value Kca at a test temperature of-10℃in the temperature gradient ESSO test (hereinafter referred to as "crack stopping toughness value Kca) _-10℃ ") is preferably 6000N/mm ^1.5 Above, more preferably 8000N/mm ^1.5 The above. By satisfying this characteristic, the steel sheet has excellent crack resistance.

Crack arrest toughness value Kca _-10℃ The test results were measured according to the "test results of temperature gradient ESSO test and temperature gradient double tensile test (temperature hook ESSO test and temperature hook double primer by NK class association steel ship rule test key K code K3.12.2-1 (2016)", ken by , of ken by means of the "test results of temperature hook ESSO test and temperature hook double primer by NK class association steel ship rule test key K.

The non-ductile transition temperature (hereinafter referred to as "NDT temperature") in the NRL drop test is preferably-100 ℃ or lower, more preferably-110 ℃ or lower. By satisfying this characteristic, the steel sheet has excellent crack resistance.

NDT temperature was determined by testing according to the NRL drop hammer test method specified in ASTM E208-06. The NRL drop hammer test method will be described in detail. First, the outermost surface of the steel sheet was included to collect a type P3 test piece specified in ASTM E208. The type P3 test piece refers to a test piece 130mm in length, 50mm in width and 16mm in thickness. At this time, the test piece was collected so that the thickness direction of the test piece was aligned with the thickness direction of the steel sheet and the longitudinal direction of the test piece was aligned with the rolling direction of the steel sheet.

Then, NRL drop test was performed according to ASTM E208-06 using the test pieces described above. Specifically, first, a weld extending in a direction parallel to the longitudinal direction of the test piece is formed on the outermost surface of the steel sheet perpendicular to the thickness direction of the test piece. In this case, a welding material having low toughness as specified in ASTM E208 is used as the welding material. The length of the weld is adjusted to be in the range of 60-70 mm and the width is adjusted to be in the range of 12-16 mm. Then, a slit parallel to the width direction of the test piece was formed in the weld. At this time, the width of the notch was 1.5mm or less, and the distance between the bottom of the notch and the test piece was adjusted so as to be within the range of 1.8 to 2.0 mm.

Then, the surface of the test piece on which the weld was formed was directed downward, and after supporting both ends in the longitudinal direction, an impact bending load by a drop weight was applied to the surface on the opposite side to the side on which the weld was formed. Then, the state of brittle cracks generated by the cuts in the test piece was examined, and thus Break (with crack growth) or No Break (without crack growth) was determined. When the brittle crack generated by the notch spreads on the surface of the test piece in the width direction of the test piece and progresses to the end portion thereof, the test result is determined as Break (crack spread). When the crack did not reach the end in the width direction, the test result was determined as No Break (No crack propagation).

For the drop weight test, for example, starting from a condition of-100℃each 2 test pieces were used, the test temperature was changed at 5℃intervals (5℃lower in the case of No Break and 5℃higher in the case of Break), and the temperature at which the lowest test temperature of No Break was obtained from each 2 test pieces was set as the non-ductile transition temperature.

(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 performing a refining step, a continuous casting step, a heating step, a hot rolling step, and an accelerated cooling step under the conditions shown below. Each step will be described.

(a) Refining process

The refining step is a step of producing molten steel. The conditions of the refining step are not particularly limited, and a conventional method may be used. However, it is intended to suppress Ti ₂ O ₃ When the average equivalent circle diameter of TiN particles at the 1/10t position is 60nm or less and the area ratio is 0.0001% or more, it is preferable to perform vacuum degassing so that the dissolved O concentration in molten steel becomes 0.0050 mass% or less, and then to add Ti. If Ti is added in a state where the dissolved O concentration exceeds 0.0050 mass%, it is difficult to suppress Ti ₂ O ₃ Is generated. The addition of Ti can be performed in a reflux type deaerator, for example.

(b) Continuous casting process

The continuous casting step is a step of continuously casting molten steel to produce a billet having the chemical composition described above. The conditions of the continuous casting step are not particularly limited, and a conventional method may be used. However, when the average equivalent circle diameter of the TiN particles at the 1/10t position is set to 60nm or less and the area ratio is set to 0.0001% or more, the average cooling rate during the period when the surface temperature of the billet is 1200 to 900 ℃ is preferably set to 0.1 to 0.5 ℃/sec. When the average cooling rate is less than 0.1 ℃/sec, the TiN particles may coarsen, and when it exceeds 0.5 ℃/sec, the area ratio of TiN may be reduced.

(c) 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 ℃.

As described above, by properly controlling the timing of Ti addition in the refining step and properly controlling the average cooling rate during the period of 1200 to 900 ℃ in the continuous casting step, tiN can be finely dispersed, and thus the grain boundary density can be controlled within the above-described range. In this case, the heating temperature of the billet may be 1080 ℃ or lower.

On the other hand, even in the case where TiN is not actively and effectively used, the grain boundary density can be controlled within the above range by controlling the heating temperature of the billet in the heating step to be low and suppressing coarsening of austenite. At this time, the heating temperature of the billet is 1050 ℃ or lower.

(d) 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 expression (iii) 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 (iv). The symbol of the element in the following formula represents each element contained in the steel sheetContent (mass%) is substituted into 0 when not contained.

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

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

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)(v)

(e) 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.

(f) 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

The molten iron discharged from the blast furnace is subjected to desulfurization treatment by pretreatment of the molten iron, and is subjected to P-removing and C-removing treatment by a converter type refining vessel, and then is received in a ladle. During tapping, alloy elements are added, and surface slag for heat preservation is added.

Then, the molten steel in the ladle was subjected to a pressure reduction treatment by an RH vacuum degassing device. And properly collecting a molten steel sample in smelting, and analyzing to obtain molten steel components. The temperature of molten steel is shifted from 1560 ℃ to 1610 ℃. Vacuum degassing was performed in the first half of RH treatment to adjust the dissolved O concentration. The dissolved O concentration was measured using an oxygen concentration probe. Then, ti was added and a reflow treatment was performed for uniform mixing.

After treatment with an RH vacuum degasser, billets having the chemical compositions shown in tables 1 and 2 were produced by continuous casting. In continuous casting, the surface temperature of the billet is suitably adjusted to an average cooling rate during 1200 to 900 ℃. Tables 3 and 4 show the concentration (mass%) of dissolved O in molten steel when Ti is added, and the average cooling rate (. Degree.C/sec) during 1200 to 900℃in continuous casting. Further, steel sheets having a thickness of 10 to 70mm were produced by trial using the above billets under the production conditions shown in tables 5 and 6.

TABLE 1

TABLE 2

TABLE 3

TABLE 3 Table 3

TABLE 4

TABLE 4 Table 4

TABLE 5

TABLE 6

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 average equivalent circle diameter and area ratio of TiN particles were measured using TEM with EDX. First, an extraction replica was prepared from a 1/10t position of a steel plate, and the observation area of 1 field of view was set to 15 μm by TEM at a magnification of 3 ten thousand times or more ² As described above, particles having a size of 15 to 200nm were observed. All the particles observed were analyzed by EDX, and particles containing 1 mass% or more of Ti, less than 1 mass% of O (oxygen), and 1 mass% or more of N were identified as TiN particles.

The electron beam diameter of the TEM was 1 to 20nm, and the observation magnification was 5 to 100 tens of thousands of times, so that quantitative analysis was performed at any position in the particles. The average equivalent circle diameter of TiN particles is obtained by arithmetic averaging of equivalent circle diameters (diameters) that form the same area as that of each TiN particle identified above. The area ratio of TiN particles is a value obtained by dividing the sum of the areas of the TiN particles identified above by the area of the field of view observed.

Subsequently, the grain boundary density was measured by the EBSD method. Specifically, by measuring a region of 500 μm×500 μm at 1/10t position, 1/4t position, and 1/2t position at 1 μm pitch by the EBSD method, a boundary having a crystal orientation difference of 15 ° or more from adjacent crystal grains is defined as a grain boundary, and the total length of the grain boundary at this time is divided by the measurement area, whereby the grain boundary density can be obtained.

The measurement results are shown in tables 7 and 8. 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 7

TABLE 8

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.

In addition, the check requirement (temperature-hooking type ESSO test and temperature-hooking type double-lead by the same, by the NK boat level society steel boat rule check requirement K code appendix K3.12.2-1 (2016) ", for the temperature gradient type ESSO test and the temperature gradient type double-tensile test, by the same applicant as the current support of , was measured to determine the crack-stopping toughness value Kca _-10℃ . Next, the NDT temperature was determined by performing a test according to the NRL drop hammer test method specified in ASTM E208-06. In this example, crack arrest toughness value Kca _-10℃ 6000N/mm ^1.5 The above-mentioned NDT is excellent in crack-stopping property when the NDT temperature is-100 ℃ or lower.

The measurement results are shown in tables 9 and 10.

TABLE 9

TABLE 10

Table 10

As is clear from tables 7 to 10, the inventive examples (test nos. 1 to 29) satisfying the regulations of the present invention gave results that have high strength and are excellent in low-temperature toughness, fracture toughness and crack resistance. In contrast, in the comparative examples (test nos. 30 to 61), at least one of the strength, the low-temperature toughness and the crack resistance was deteriorated.

Specifically, in test No. 30, the dissolved O concentration at the time of Ti addition in the refining step was high, and the heating temperature in the heating step was high, and in test No. 31, the average cooling rate in the continuous casting step was high, no TiN particles were precipitated, and the grain boundary density could not be optimized, so that the crack resistance was deteriorated. In test No. 32, since the average cooling rate in the continuous casting step was low, coarse TiN particles were precipitated and the grain boundary density could not be optimized, so that the crack resistance was deteriorated.

For test number 33, the C content was excessive, and thus the low temperature toughness and fracture toughness were deteriorated. For test number 34, the C content was low, the structure where the bainite main body was not formed, the strength was insufficient, and the low temperature toughness and fracture toughness were deteriorated. For test No. 35, si content was excessive, and thus low temperature toughness and fracture toughness were deteriorated. For test No. 36, the Mn content was excessive, and thus the low temperature toughness and fracture toughness were deteriorated. For test number 37, the Mn content was low and the strength was insufficient.

The contents of P and S were excessive for test number 38, the Al content was excessive for test number 39, and the N content was excessive for test number 40, so that the low temperature toughness and fracture toughness were deteriorated. In test No. 41, the low-temperature toughness, fracture toughness and crack resistance were deteriorated because the N content was low, BF length and prior austenite grains became coarse, and the grain boundary density could not be optimized.

For test number 42, the Nb content was excessive, and thus the low temperature toughness and fracture toughness were deteriorated. In test No. 43, nb content was low, BF length and prior austenite grains were coarsened, and the aspect ratio of prior austenite grains was reduced, and further, the grain boundary density could not be appropriately adjusted, so that low temperature toughness, fracture toughness and crack resistance were deteriorated. For test number 44, the Ti content was excessive, and thus the low temperature toughness and fracture toughness were deteriorated. Further, since TiN particles coarsen and the heating temperature in the heating step is also high, the grain boundary density cannot be properly adjusted and the crack resistance is also deteriorated. In test No. 45, the low-temperature toughness, fracture toughness and crack resistance were deteriorated because the BF length and prior austenite grains were coarsened and the grain boundary density could not be optimized.

In test numbers 46 and 47, the heating temperature in the heating step was high, and the BF length and prior austenite grains were coarsened, and further, the grain boundary density could not be appropriately adjusted, so that the low-temperature toughness, fracture toughness and crack-stopping property were deteriorated. For test number 48, the heating temperature was low, the bainite area ratio was reduced, the strength was insufficient, and the low temperature toughness and fracture toughness were deteriorated. For test number 49, the end temperature of the roughing was below T _rex BF length and coarsening of prior austenite grains, and further, the grain boundary density cannot be optimized, and therefore low-temperature toughness, fracture toughness and crack resistance deteriorate.

In test No. 50, the cumulative rolling reduction of rough rolling was high, BF length and prior austenite grains were coarsened, the aspect ratio of prior austenite grains was reduced, and the grain boundary density could not be properly adjusted, so that low temperature toughness, fracture toughness and crack resistance were deteriorated. On the other hand, in test No. 51, the cumulative reduction was low, BF length and prior austenite grains were coarsened, and the grain boundary density could not be optimized, so that the low-temperature toughness, fracture toughness and crack resistance were deteriorated.

For test number 52, 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 further, the grain boundary density cannot be appropriately adjusted, so that low-temperature toughness, fracture toughness, and crack resistance deteriorate. For test No. 53, the finish temperature of the finish rolling was lower than Ar ₃ Therefore, processed ferrite is excessively generated, strength is insufficient, and low-temperature toughness and fracture toughness are deteriorated.

In test No. 54, the cumulative reduction of the finish rolling was high, and in test No. 55, the cumulative reduction was low, BF length and prior austenite grains were coarsened, and the aspect ratio of the prior austenite grains was reduced, and further, the grain boundary density could not be optimized, so that the low-temperature toughness, fracture toughness and crack arrest property were deteriorated. In test No. 56, the inter-pass time was long, and in test No. 57, the time from completion of finish rolling to start of cooling was long, and BF length and prior austenite grains were coarsened, and the aspect ratio of the prior austenite grains was reduced, and grain boundary density could not be optimized, so that low-temperature toughness, fracture toughness, and crack resistance were deteriorated.

For test No. 58, the cooling rate in the accelerated cooling step was high, so that MA phase was excessively generated, and low-temperature toughness and fracture toughness were deteriorated. For test number 59, 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 and fracture toughness were deteriorated. For test number 60, the cooling stop temperature was high, so that the structure of the bainite main body was not formed, the strength was insufficient, and the low temperature toughness, fracture toughness and crack resistance were deteriorated. For test number 61, 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, fracture toughness and crack resistance 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％、

Ti：0.003～0.050％、

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,

B: less than 0.0050%,

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,

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,

in a section of the steel sheet perpendicular to the rolling direction,

the grain boundary density at a position at a distance of 1/10t from the surface of the steel sheet is 500 to 1100mm/mm ² 、

The grain boundary density at a position at a distance of 1/4t from the surface of the steel sheet is 400 to 1000mm/mm ² 、

The grain boundary density at a position at a distance of 1/2t from the surface of the steel sheet is 300 to 900mm/mm ² 。

2. The steel sheet according to claim 1, wherein the chemical composition satisfies the following formula (i),

1.7≤Ti/N≤3.4 (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.

3. The steel sheet according to claim 1 or claim 2, wherein the chemical composition satisfies the following formula (ii),

in a cross section of the steel sheet perpendicular to a rolling direction, an average equivalent circle diameter of TiN particles at a position at a distance of 1/10t from a surface of the steel sheet is 60nm or less, and an area ratio of the TiN particles is 0.0001% or more,

Ti×N≥3.0×10 ^-5 (ii)

4. 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 (iii) _rex The content of each element contained in the steel sheet is represented by the following formula (iv), and if not, 0 is substituted,

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

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

wherein, when the solid-solution Nb amount in mass% obtained by the following expression (v) 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) (v)

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

5. A method for producing a steel sheet according to claim 3,

the manufacturing method comprises the following steps: a refining step of producing molten steel; and a continuous casting step of continuously casting the molten steel to produce a steel slab having the chemical composition according to claim 1 or claim 2, and sequentially performing a heating step, a hot rolling step and an accelerated cooling step on the obtained steel slab,

the hot rolling process includes rough rolling and finish rolling,

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

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

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

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

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) (v)

6. The method for producing a steel sheet according to claim 4 or claim 5, wherein a tempering step of heating to a temperature range of 350 to 650 ℃ is further performed after the accelerated cooling step.