CN116171335A

CN116171335A - Steel plate

Info

Publication number: CN116171335A
Application number: CN202180062217.3A
Authority: CN
Inventors: 荒尾亮; 植田圭治; 友泽方成; 中岛孝一
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2020-09-30
Filing date: 2021-09-14
Publication date: 2023-05-26
Also published as: BR112023004211A2; TWI806170B; KR20230051276A; TW202217019A; WO2022070873A1; JP7272471B2; JPWO2022070873A1

Abstract

The purpose of the present invention is to provide a steel sheet having excellent toughness in a high heat input weld heat affected zone, wherein the weld line energy is 20.0kJ/mm or more. A steel sheet comprises the following components in mass percent: 0.030 to 0.120 percent of Si:0.01 to 0.15 percent of Mn: 0.80-2.00%, P: less than 0.020%, S:0.0005 to 0.0050 percent of Al: 0.005-0.100%, ti: 0.005-0.030%, N:0.0030 to 0.0080 percent, ca: 0.0005-0.0030%, O:0.0040% or less, the balance being Fe and unavoidable impurities, and S, ca, O being contained so as to satisfy the following formula (1), wherein the mass ratio of the precipitate exceeding 0.1 μm in terms of equivalent circle diameter among the TiN precipitates is 40% or more. 0 < (Ca- (0.18+130×Ca). Times.O)/1.25/S < 1 … (1) wherein each symbol of element represents the content (mass%) of each element.

Description

Steel plate

Technical Field

The present invention relates to steel materials used for various steel structures in the fields of ships, buildings, civil engineering and the like, and more particularly, to a steel sheet having excellent weld heat affected zone toughness even when large heat input welding with weld line energy exceeding 20.0kJ/mm is performed.

Background

With the increase in strength and thickness of steel materials, the application of high heat input welding, which is excellent in production efficiency such as submerged arc welding, gas welding and electroslag welding, is expected to increase in welding construction. Various steels for high heat input welding have been proposed because of a decrease in toughness of a weld heat affected zone (hereinafter, referred to as HAZ) in high heat input welding. For example, a technique of suppressing coarsening of austenite grains in a weld heat affected zone by finely dispersing TiN in steel and a technique of using TiN as ferrite transformation nuclei in the weld heat affected zone have been put into practical use.

It is economically useful to suppress coarsening of the structure by utilizing the precipitates of TiN, and widely used. On the other hand, in the weld heat affected zone, these effects are not obtained in a high temperature range such as TiN melting, and further, the molten TiN causes excessive amounts of solid solution Ti and solid solution N, which causes a problem that the base structure is embrittled and the toughness is significantly lowered.

Accordingly, patent document 1 proposes the following technique: tiOx (x: 0.65 to 1.3) having a grain size of 5 μm or less, which is a Ti oxide that is difficult to melt even in the high temperature range of the weld heat affected zone, is finely dispersed in steel to serve as nuclei for the formation of acicular ferrite in the weld heat affected zone, and the toughness of the weld heat affected zone is improved. Patent document 2 proposes the following technique: the composition of B, N and sol.Al content were adjusted to actively precipitate BN, which was refined in the weld heat affected zone, and to improve the toughness of the weld heat affected zone.

Patent document 3 proposes the following technique: the amount of Ti-B-N is adjusted in the composition so that the HAZ toughness is a high toughness region, and Ca or Ce is further added to impart a toughness improving effect by controlling the morphology of the inclusions. Patent document 4 also proposes the following technique: the toughness of the high heat input welded portion is improved by adding REM which forms stable sulfur oxide at the welded joint portion. Further, patent document 5 discloses the following technique: the Ca-based nonmetallic inclusion which becomes transformation nuclei and promotes ferrite transformation in the weld heat affected zone is finely dispersed in steel by appropriately controlling the contents of Ca, O and S, so that the toughness of the weld heat affected zone in high heat input welding exceeding 20.0kJ/mm is improved.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 57-51243

Patent document 2: japanese patent laid-open No. 62-170459

Patent document 3: japanese patent laid-open No. 60-204863

Patent document 4: japanese patent publication No. 4-14180

Patent document 5: japanese patent No. 3546308

Disclosure of Invention

Problems to be solved by the invention

However, the technique using Ti oxide described in patent document 1 has a problem that it is difficult to uniformly and finely disperse the oxide and the toughness of the weld heat affected zone cannot be stably ensured, particularly in mass production. In the technique described in patent document 2, cracks may be generated at the time of casting, with inclusions of the nitride main body as a starting point. In addition, in the techniques of patent documents 3 to 4, it is difficult to sufficiently suppress the grain growth of austenite in the weld heat affected zone in the high heat input welding exceeding 20.0kJ/mm, and therefore there is a problem that the toughness of the joint is unstable. In addition, the technique described in patent document 5 has a problem that it is difficult to secure stable toughness.

The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a steel sheet having excellent toughness in a high heat input weld heat affected zone having a weld line energy of 20.0kJ/mm or more.

Means for solving the problems

The present inventors have repeatedly conducted various studies to solve the above problems, and have found the following findings.

In order to suppress coarsening of the structure of the weld heat affected zone by utilizing TiN precipitates excellent in industrial productivity, it is important to appropriately control the size of TiN precipitates in the base steel sheet. That is, by ensuring that a certain amount or more of TiN precipitates having a residual size are melted even when exposed to a high temperature of a calculated thermodynamic melting temperature or higher, it is possible to obtain a steel sheet having excellent toughness in the weld heat affected zone while stably suppressing coarsening of the structure of the weld heat affected zone. In the present invention, by controlling the amounts of S, ca and O in addition to the TiN precipitates, it is possible to obtain a steel sheet having excellent toughness in the weld heat affected zone while suppressing coarsening of the structure of the weld heat affected zone.

The present invention has been further studied based on the findings obtained as described above, and the gist thereof is as follows.

[1] A steel sheet comprises the following components in mass percent: 0.030 to 0.120 percent of Si:0.01 to 0.15 percent of Mn: 0.80-2.00%, P: less than 0.020%, S:0.0005 to 0.0050 percent of Al: 0.005-0.100%, ti: 0.005-0.030%, N:0.0030 to 0.0080 percent, ca: 0.0005-0.0030%, O:0.0040% or less, the balance being Fe and unavoidable impurities, and S, ca, O being contained so as to satisfy the following formula (1), wherein the mass ratio of the precipitate exceeding 0.1 μm in terms of equivalent circle diameter among the TiN precipitates is 40% or more.

0＜(Ca-(0.18+130×Ca)×O)/1.25/S＜1…(1)

Wherein each element symbol represents the content (mass%) of each element.

[2] The steel sheet according to [1], wherein the composition further comprises, in mass%, a composition selected from the group consisting of Cu: less than 1.00%, ni: less than 1.50%, cr: less than 1.00%, mo: less than 0.50%, V:0.50% or less and Nb:0.05% or less.

[3] The steel sheet according to [1] or [2], wherein the composition further comprises, in mass%, a component selected from the group consisting of B: less than 0.0025%, mg: less than 0.0050%, zr: less than 0.0200%, REM:0.0200% or less.

Effects of the invention

According to the present invention, a steel sheet having excellent toughness in a heat affected zone of high heat input welding with a weld line energy of 20.0kJ/mm or more can be obtained, which is industrially extremely useful.

Detailed Description

Hereinafter, embodiments of the present invention will be described in detail. First, the composition of the steel sheet according to the present invention will be described. In the following description, the% expression of the chemical components refers to the mass% in total.

C：0.030～0.120％

C is an element for improving the strength of the steel material, and is required to be contained in an amount of 0.030% or more in order to secure the strength required as a structural steel. Therefore, the lower limit of the C content is set to 0.030%. The C content is preferably 0.040% or more, more preferably 0.050% or more, and still more preferably 0.060% or more. On the other hand, when C exceeds 0.120%, island martensite (hereinafter, also referred to as MA) is likely to be generated in the weld heat affected zone, and the toughness is lowered, so the upper limit is set to 0.120%. The C content is preferably 0.100% or less, more preferably 0.090% or less, and still more preferably 0.085% or less.

Si：0.01～0.15％

Si is an element added as a deoxidizer in melting steel, and is required to be contained in an amount of 0.01% or more. Therefore, the Si content is set to 0.01% or more. The Si content is preferably 0.02% or more, more preferably 0.03% or more, still more preferably 0.04% or more, and most preferably 0.06% or more. However, if the content exceeds 0.15%, in addition to the decrease in toughness of the base material, the heat affected zone may be welded with high heat input to increase the tendency of island-like martensite formation, resulting in a decrease in toughness. Therefore, the Si content is set to 0.15% or less. The Si content is preferably 0.13% or less, more preferably 0.10% or less, and still more preferably 0.09% or less.

Mn：0.80～2.00％

In order to secure strength of the base material, the Mn content is set to 0.80% or more. The Mn content is preferably 1.00% or more, more preferably 1.20% or more, further preferably 1.40% or more, and most preferably 1.50% or more. On the other hand, when the Mn content exceeds 2.00%, the toughness of the HAZ is significantly deteriorated, and thus is set to 2.00% or less. The Mn content is preferably 1.90% or less, more preferably 1.85% or less, further preferably 1.80% or less, and most preferably 1.70% or less.

P: less than 0.020%

P promotes MA generation in the HAZ near the joint, and the toughness is greatly reduced, so the P content is set to 0.020% or less. The P content is preferably 0.015% or less, more preferably 0.012% or less. The lower limit of the P content is not particularly limited. However, excessive P removal leads to an increase in cost, so that the P content is preferably 0.002% or more.

S：0.0005～0.0050％

S is an essential element for forming MnS or CaS functioning as nucleation sites for ferrite. Therefore, the S content is set to 0.0005% or more. The S content is preferably 0.0010% or more, more preferably 0.0015% or more. However, when S is excessively contained, the toughness of the base material is reduced, and thus the S content is set to 0.0050% or less. The S content is preferably 0.0040% or less, more preferably 0.0035% or less, and even more preferably 0.0030% or less.

Al：0.005～0.100％

Al is an element contained for deoxidizing the steel, and the Al content is set to 0.005% or more. The Al content is preferably 0.010% or more, more preferably 0.020% or more, and still more preferably 0.030% or more. However, when the content exceeds 0.100%, not only the toughness of the base material but also the toughness of the weld metal is lowered. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.085% or less, more preferably 0.070% or less, and still more preferably 0.065% or less.

Ti：0.005～0.030％

Ti becomes TiN and precipitates in the base material during solidification of the molten steel, and contributes to improvement of the toughness of the base material by suppressing coarsening of austenite grains. In addition, tiN suppresses coarse structure in the weld heat affected zone during welding, and becomes ferrite transformation nuclei, contributing to high toughness. In order to obtain this effect, it is necessary to contain 0.005% or more. Therefore, the Ti content is set to 0.005% or more. The Ti content is preferably 0.008% or more, more preferably 0.011% or more, and still more preferably 0.015% or more. On the other hand, when the Ti content exceeds 0.030%, the precipitated TiN is excessively coarsened, and the above-mentioned effects cannot be obtained. Therefore, the Ti content is set to 0.030% or less. The Ti content is preferably 0.027% or less, more preferably 0.024% or less, and still more preferably 0.020% or less.

N：0.0030～0.0080％

Since N forms TiN and contributes to improvement of toughness, the N content is set to 0.0030% or more. The N content is preferably 0.0035% or more, more preferably 0.0040% or more. On the other hand, if the content exceeds 0.0080%, the TiN melts while being kept at a high temperature by the welding heat cycle, and the solid solution N in the base structure may become excessive, thereby deteriorating the toughness. Thus, the N content is set to 0.0080% or less. The N content is preferably 0.0070% or less, more preferably 0.0065% or less, and still more preferably 0.0070% or less.

Ca：0.0005～0.0030％

Ca has the effect of fixing S and improving toughness. In order to obtain this effect, the Ca content is set to 0.0005% or more. The Ca content is preferably 0.0010% or more, more preferably 0.0015% or more. On the other hand, when the Ca content exceeds 0.0030%, the effect is saturated, and therefore the Ca content is set to 0.0030% or less. The Ca content is preferably 0.0025% or less, more preferably 0.0020% or less.

O:0.0040% or less

Since O indirectly affects the formation of a complex sulfide in which MnS is precipitated on CaS, the O content is set to 0.0040% or less. The O content is preferably 0.0030% or less, more preferably 0.0025% or less. The lower limit of the O content is not particularly limited. However, since excessive reduction in oxygen content leads to an increase in cost, the O content is preferably 0.0003% or more.

In addition, S, ca, O of the present invention are required to satisfy the following formula (1).

0＜(Ca-(0.18+130×Ca)×O)/1.25/S＜1…(1)

Wherein each element symbol represents the content (mass%) of each element.

(1) In the formula, "(Ca- (0.18+130×Ca). Times.O)/1.25/S" (hereinafter referred to as "A value") is 0 or less, caS is not crystallized, S is precipitated as a single component of MnS, and the toughness of the base material is lowered by elongation in the rolling direction at the time of producing a steel sheet. In addition, mnS melts in the weld heat affected zone, and thus excellent toughness is not obtained. Therefore, the a value is set to be greater than 0. The value a is preferably 0.1 or more, more preferably 0.2 or more, and still more preferably 0.3 or more. On the other hand, when the a value is 1 or more, most of S is fixed by Ca, mnS that becomes ferrite-generating nuclei does not precipitate on CaS, and therefore ferrite is not generated in the weld heat affected zone, and no toughness improvement effect is obtained. Therefore, the a value is set to be less than 1. The value of a is preferably 0.8 or less, more preferably 0.7 or less.

The above is the basic component composition of the present invention, and the balance is Fe and unavoidable impurities.

In the present invention, one or more selected from Cu, ni, cr, mo, V and Nb may be contained in the following ranges as optional elements for the purpose of improving the strength and the like in addition to the above-described components.

Cu: less than 1.00%

Cu is an element effective for increasing the strength of the steel sheet, and if it is excessively added, cracks of the cast steel block may be promoted, and the toughness of the steel sheet may be lowered. Therefore, in the case of containing Cu, the Cu content is set to 1.00% or less. The Cu content is preferably 0.50% or less, more preferably 0.30% or less. On the other hand, in order to obtain this effect, when Cu is contained, the Cu content is preferably set to 0.03% or more. The Cu content is more preferably set to 0.04% or more.

Ni: less than 1.50 percent

Ni increases toughness of the steel sheet and also increases strength, but excessive addition decreases toughness of the base material and HAZ and stresses manufacturing costs. Therefore, in the case of containing Ni, the Ni content is set to 1.50% or less. The Ni content is preferably 1.0% or less, more preferably 0.50% or less, and still more preferably 0.30% or less. On the other hand, in order to obtain this effect, when Ni is contained, the Ni content is preferably set to 0.03% or more. The Ni content is more preferably set to 0.04% or more.

Cr: less than 1.00%

Cr is an element that contributes to the enhancement of strength of the steel sheet, and excessive content reduces toughness of the base material and the HAZ. Therefore, when Cr is contained, the Cr content is set to 1.00% or less. The Cr content is preferably 0.80% or less, more preferably 0.50% or less, and still more preferably 0.30% or less. On the other hand, in order to obtain this effect, when Cr is contained, the Cr content is preferably set to 0.02% or more. The Cr content is more preferably set to 0.03% or more.

Mo: less than 0.50%

Mo is an element that contributes to the enhancement of strength of the steel sheet, but excessive content reduces toughness of the base material and HAZ. Therefore, when Mo is contained, the Mo content is set to 0.50% or less. The Mo content is preferably 0.40% or less, more preferably 0.30% or less, and still more preferably 0.20% or less. On the other hand, in order to obtain this effect, when Mo is contained, the Mo content is preferably set to 0.003% or more. The Mo content is more preferably set to 0.004% or more.

V: less than 0.50%

V is an element that contributes to the high strength of the steel sheet, but excessive content reduces the toughness of the base material and HAZ. Therefore, when V is contained, the V content is set to 0.50% or less. The V content is preferably 0.40% or less, more preferably 0.30% or less, and still more preferably 0.20% or less. On the other hand, in order to obtain this effect, when V is contained, the V content is preferably set to 0.003% or more. The V content is more preferably set to 0.004% or more.

Nb: less than 0.05%

Nb contributes greatly to the strength improvement of the steel sheet, but excessive inclusion sometimes causes an increase in upper bainite and island martensite in the weld heat affected zone structure, resulting in a decrease in toughness. Therefore, when Nb is contained, the Nb content is set to 0.05% or less. The Nb content is preferably 0.04% or less, more preferably 0.03% or less, and still more preferably 0.02% or less. On the other hand, in order to obtain this effect, when Nb is contained, the Nb content is preferably set to 0.002% or more. The Nb content is more preferably set to 0.003% or more.

In addition to the above components, the steel material of the present invention may contain one or more selected from the group consisting of B, mg, zr, and REM as optional elements in the following ranges.

B: less than 0.0025%

B generates BN in the weld heat affected zone, reduces solid solution N, and becomes ferrite transformation nuclei to generate ferrite, thereby improving toughness. In order to obtain this effect, when B is contained, the B content is set to 0.0003% or more. The content of B is preferably 0.0005% or more, more preferably 0.0008% or more. However, when B is contained in an amount exceeding 0.0025%, toughness of the base material and HAZ is lowered. Therefore, when B is contained, the B content is set to 0.0025% or less. The B content is preferably 0.0020% or less, more preferably 0.0018% or less.

Mg: less than 0.0050%, zr: less than 0.0200%, REM: less than 0.0200%

Mg, zr, and REM are elements having an effect of improving toughness by the dispersion of the oxide. In order to exhibit such effects, when Mg, zr, and REM are contained, the Mg content is preferably set to 0.0005% or more, and the Zr content and REM content are preferably set to 0.0010% or more, respectively. The Mg content is more preferably set to 0.0010% or more, and the Zr content and REM content are more preferably set to 0.0015% or more, respectively. On the other hand, even if Mg is contained in excess of 0.0050% and Zr and REM are contained in excess of 0.0200% respectively, only the effect thereof is saturated. Therefore, when these elements are contained, the Mg content is set to 0.0050% or less, and the Zr content and REM content are set to 0.0200% or less, respectively. Preferably, the Mg content is 0.0030% or less, and the Zr content and REM content are 0.01% or less, respectively.

Next, the structure of the steel sheet of the present invention will be described.

Of the TiN precipitates, those exceeding 0.1 μm in terms of equivalent circle diameter account for 40% or more in mass ratio

Regarding the precipitates of TiN in the steel sheet, by making the mass ratio (hereinafter also referred to as P value) of the precipitates exceeding 0.1 μm in terms of equivalent circle diameter in the total precipitation amount 40% or more, tiN is melted and remained even when large heat input welding exceeding 20.0kJ/mm is performed. As a result, the subsequent austenite grain growth is suppressed, contributing to the improvement of toughness of the heat affected zone and the steel sheet. Therefore, the mass ratio of the precipitate exceeding 0.1 μm in terms of the equivalent circle diameter to the total precipitation amount of the precipitate of TiN is set to 40% or more. The total deposition amount of the TiN precipitates is preferably 45% or more, more preferably 50% or more, of the total deposition amount of the precipitates exceeding 0.1 μm in terms of equivalent circle diameter. On the other hand, if the mass ratio of the precipitates having a large size (equivalent circle diameter) is excessively increased, the precipitates coarsen and may become the starting points of fracture, so that the mass ratio of the precipitates exceeding 0.1 μm in terms of equivalent circle diameter in the total precipitation amount of TiN is preferably set to 98% or less. The total precipitation amount of the precipitates in TiN exceeds 0.1 μm in terms of equivalent circle diameter, and is more preferably 98% or less, and still more preferably 95% or less in terms of mass ratio. In addition, the precipitates exceeding 2.0 μm in terms of equivalent circle diameter are likely to become the starting points of brittle fracture, and therefore are preferably reduced as much as possible.

In order to control the proportion of precipitates exceeding 0.1 μm in terms of equivalent circle diameter, for example, by adjusting the average cooling rate from 1450 ℃ to 1300 ℃ to 0.5 ℃/sec or less at the time of casting, the mass proportion of precipitates exceeding 0.1 μm in terms of equivalent circle diameter can be 40% or more due to ostwald ripening after precipitation. When the cooling rate is more than 0.5 ℃/sec, the proportion of precipitates having an equivalent circular diameter of 0.1 μm or less increases, and most of TiN melts during high heat input welding exceeding 20.0kJ/mm, and grain growth thereafter cannot be sufficiently suppressed.

Next, a method for manufacturing the steel sheet of the present invention will be described.

The steel sheet of the present invention can be produced by a conventionally known method for a production method other than the average cooling rate at the time of casting. For example, steel melted in a converter, an electric furnace, or the like is subjected to secondary refining such as RH degassing to adjust the steel composition to the above-described proper range, and then subjected to continuous casting or ingot-cogging to produce a steel material such as a billet. In the continuous casting or ingot casting, the average cooling rate may be controlled. Then, the steel sheet may be manufactured by reheating the steel material and hot-rolling the steel sheet to a desired size, and then cooling the steel sheet, or by subjecting the steel sheet to steps such as accelerated cooling, direct quenching-tempering, reheating normalizing-tempering, and the like after the hot rolling. The thickness of the sheet obtained by the present invention ranges from 9mm to 50mm.

The steel sheet of the present invention has excellent toughness in a large line energy affected zone where the weld line energy is 20.0kJ/mm or more. Specifically, in a large line energy influence region where the weld line energy is 20.0kJ/mm or more, when the Charpy impact test is performed at-40 ℃, an impact absorption value (vE) exceeding 100J can be obtained _-40℃ )。

Examples

The following describes embodiments of the present invention. The steel sheet and the method for producing the same according to the present invention are not limited to examples.

The steels No.1 to 18 having the composition shown in Table 1 were melted using a 150kg high-frequency melting furnace, cast at the average cooling rate shown in Table 2 to form steel blocks, and hot-rolled to form steel plates having a thickness of 50mm. The obtained steel sheet was subjected to quantification of TiN precipitates by the QUANPASS method (refer to Japanese patent application laid-open No. 2010-12778). Specifically, the following tests were performed: a10 mm square plate-shaped metal sample was cut from a 1/4 position of the plate thickness of the steel plate, the metal sample was electrolyzed in an electrolyte, precipitates and the like were extracted, filtered and quantitatively analyzed in different sizes, and the above-described operations were repeated. Based on the quantitative result, the mass ratio of TiN precipitates exceeding 0.1 μm in size in terms of equivalent circle diameter in the total TiN precipitates was set as the P value.

In addition, in order to evaluate toughness of the weld heat affected zone, a regenerative thermal cycle test simulating high heat input welding was performed. Cutting width 80m from 1/4 of plate thickness of steel plateTest pieces of m X length 80mm X thickness 15mm were subjected to a regenerative thermal cycle of heating to 1450℃and cooling at 800 to 500℃for 300 seconds, and then 2mmV notched Charpy test pieces were cut from these test pieces. The obtained Charpy test piece was subjected to a Charpy impact test at a test temperature of-40℃to evaluate toughness. Average impact absorption value (vE) of test results of three _-40℃ ) Samples exceeding 100J gave good results. The above-mentioned regenerative thermal cycle conditions were equivalent to thermal histories of the joint portion in the case of electro-gas welding in which the line energy of 1 pass welding at a plate thickness of 50mm was simulated to be 30.0 kJ/mm.

The average cooling rate, P value and toughness of the weld heat affected zone from 1450 ℃ to 1300 ℃ at the time of casting are also shown in table 2.

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TABLE 2

The underlined symbol of [ note 1] indicates that it is outside the scope of the present invention.

In the steel sheets nos. 1 to 10 and 18 as inventive examples, excellent toughness was exhibited in the heat affected zone of the high heat input welding. On the other hand, in steel sheets No.11 to 17 in which the composition of the steel or the P value is outside the range of the present invention, the toughness of the heat affected zone of the high heat input weld was lower than that of the invention example.

Claims

1.A steel sheet comprises the following components in mass percent: 0.030 to 0.120 percent of Si:0.01 to 0.15 percent of Mn: 0.80-2.00%, P: less than 0.020%, S:0.0005 to 0.0050 percent of Al: 0.005-0.100%, ti: 0.005-0.030%, N:0.0030 to 0.0080 percent, ca: 0.0005-0.0030%, O:0.0040% or less, the balance being Fe and unavoidable impurities, and S, ca, O being contained so as to satisfy the following formula (1), wherein the mass ratio of the precipitate exceeding 0.1 μm in terms of equivalent circle diameter among the TiN precipitates is 40% or more,

0＜(Ca-(0.18+130×Ca)×O)/1.25/S＜1…(1)

wherein each element symbol represents the content (mass%) of each element.

2. The steel sheet according to claim 1, wherein the composition further comprises, in mass%, a composition selected from the group consisting of Cu: less than 1.00%, ni: less than 1.50%, cr: less than 1.00%, mo: less than 0.50%, V:0.50% or less and Nb:0.05% or less.

3. The steel sheet according to claim 1 or 2, wherein the composition of the components further contains, in mass%, a composition selected from the group consisting of B: less than 0.0025%, mg: less than 0.0050%, zr: less than 0.0200%, REM:0.0200% or less.