Steel sheet and method for manufacturing same

Abstract

Provided are a steel sheet and a method for manufacturing the same. A steel sheet according to the present invention has a component composition, in mass%, that includes 0.03-0.15% of C, 0.50% or less of Si, 0.3-2.5% of Mn, 0.030% or less of P, 0.0050% or less of S, 0.01-5.00% of Ni, 0.005-0.100% of Al, 0.0100% or less of N, and 0.0100% or less of O, where the Ceq satisfies 0.300-0.550% and the remainder is Fe and inevitable impurities, wherein: the hardness of a center segregation part of the steel sheet satisfies formula (3); the number density of segregated particles having an equivalent circle diameter of 100 μm or more is 2.0 particles / mm2 or less; and the average effective crystal grain diameter of a plate thickness center part is 20 μm or less. Formula (3): Hvmax / Hvave ≤ 1.35 + 0.006 / [C] - t / 500

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a steel plate suitable for use in steel structures such as liquefied gas storage tanks, ships, and marine structures, and a method for manufacturing the same.

[0002] As efforts to realize a decarbonized society accelerate, there is a growing demand for steel materials for liquefied gas storage tanks. ２ Carbon Capture and Storage (CCS), which captures and stores CO2, is an essential technology for achieving carbon neutrality. ２ To transport CO over long distances to a storage site, ２ It is necessary to liquefy it and transport it by ship. ２ When liquefying CO, it is necessary to pressurize it at a low temperature. ２ Steel plates used in storage tanks require not only strength but also excellent toughness at low temperatures. To meet this requirement, 9% Ni steel, which has good toughness even at low temperatures, has traditionally been used. However, due to the high alloying costs, there is an industrial demand for inexpensive steel plates with excellent low-temperature toughness. Furthermore, when manufacturing storage tanks, steel plates are bent to produce the tanks. Generally, strain is introduced into steel plates during processing, and the toughness deteriorates as a result of subsequent aging. Therefore, ensuring low-temperature toughness after strain aging is important from the perspective of ensuring the tank's fracture safety.

[0003] Conventionally, the Charpy test has been mainly used to evaluate the toughness of steel. However, in recent years, as a method for evaluating fracture resistance with higher accuracy, the Crack Tip Opening Displacement Test (hereinafter referred to as "CTOD test") has been increasingly applied to thick steel plates used in steel structures.

[0004] In this test, a test piece with a fatigue pre-crack introduced in the toughness evaluation section is subjected to three-point bending at low temperature, and the opening of the crack (amount of plastic deformation) just before fracture is measured to evaluate the resistance to brittle fracture.

[0005] Since the CTOD test introduces a fatigue pre-crack, an extremely small area is used as the toughness evaluation area, and if a localized embrittlement area is present, the CTOD test may show low toughness even if good toughness is obtained in the Charpy impact test.

[0006] A known method for improving the low-temperature toughness of steel is to increase the Ni content, and 9% Ni steel is used on a commercial scale for storage tanks for liquefied natural gas (LNG) and the like.

[0007] Furthermore, for steel materials used in ships and line pipes, a technology is used to improve low-temperature toughness by refining the crystal grains using the TMCP method.

[0008] For example, Patent Documents 1 and 2 propose techniques for improving low-temperature toughness by optimizing the heating temperature and hot rolling conditions to refine the crystal grain size, while Patent Document 3 proposes a technique for improving low-temperature toughness by adding 5.0% or more of Ni.

[0009] Patent No. 5304924 Patent No. 5573265 Patent No. 7067628

[0010] CO ２ Liquefied CO loaded onto a ship for long-distance transport from the emission site to the storage site. ２ Storage tanks and the like require steel plates with a yield strength of 320 MPa or more, excellent CTOD characteristics, and low-temperature toughness at the center of the plate thickness after strain aging.

[0011] Patent Documents 1 and 2 propose techniques for improving the low-temperature toughness of steel sheets before processing, but do not consider at all the CTOD characteristics and the toughness after strain aging, and it cannot be said that the toughness of the center of the sheet thickness after embrittlement due to the CTOD characteristics and strain aging is sufficiently ensured.

[0012] In addition, Patent Document 3 proposes a technology for ensuring low-temperature toughness by adding 5.0% or more Ni, but an increase in the Ni content necessitates a significant increase in alloy costs, and therefore, CO ２It is difficult to apply this method to steel materials for storage tanks. In addition, the CTOD characteristics and toughness after strain aging have not been investigated.

[0013] As described above, according to the inventors' investigations, the conventional technologies described in Patent Documents 1 to 3 did not consider the CTOD characteristics and the low-temperature toughness in the center of the plate thickness after strain aging, and it was not possible to achieve both high strength, CTOD characteristics, and toughness in the center of the plate thickness after strain aging while suppressing alloy costs.

[0014] The present invention has been made in consideration of the above-mentioned problems of the conventional technology, and has an object to provide a steel plate having high strength, excellent CTOD characteristics, and excellent low-temperature toughness in the center part of the plate thickness after strain aging, together with a manufacturing method thereof.

[0015] Here, "high strength" in the present invention means that the yield strength in a tensile test at the center of the plate thickness is 320 MPa or more. "Excellent CTOD characteristics" means that the crack tip opening displacement is 0.10 mm or more in a CTOD test at a test temperature of -55°C. "Excellent low-temperature toughness" means that the absorbed energy vE －６０ In the present invention, the central part of the plate thickness refers to a region having a thickness of 10% of the plate thickness from the center of the plate thickness toward both surfaces of the steel plate.

[0016] In order to solve the above problems, the present inventors have conducted extensive research into methods for improving both the CTOD characteristics and the low-temperature toughness after strain aging while increasing the strength, and as a result have obtained the following findings.

[0017] (1) Since strength and toughness are strongly correlated with grain size, in order to achieve high strength at the center of the plate thickness, CTOD characteristics, and low-temperature toughness at the same time, it is essential to refine the grains at this location. To achieve this, it is effective to control the rolling conditions in the recrystallization temperature range and the non-recrystallization temperature range. Specifically, austenite recrystallization can be promoted by rolling in the recrystallization temperature range of 950°C or higher with an average reduction rate / pass of 3.5% or more so that the cumulative reduction rate is 40% or more. ３By rolling at a temperature above 950°C and below with an average reduction / pass value of 3.5% or more so that the cumulative reduction is 40% or more, sufficient strain can be introduced into the center of the sheet thickness. It has been found that this makes it possible to make the average effective grain size in the center of the sheet thickness 20 μm or less.

[0018] (2) In high-strength steel plates, the amount of alloying elements added increases to ensure the required properties, which tends to result in element segregation regions (i.e., center segregation regions) occurring in the center of the slab's thickness during continuous casting. In continuous casting, the solidification of molten steel progresses from the surface to the interior. However, since the solid solubility limits of alloying elements such as C, P, S, and Mn are greater in the liquid phase than in the solid phase, these alloying elements are redistributed during solidification and concentrated in the unsolidified liquid phase. Furthermore, at the center of the plate thickness, which is the final solidification point, the molten steel solidifies with significant enrichment of alloying elements, forming center segregation regions.

[0019] Since the center segregation area is harder than the surrounding area, it becomes the starting point of fracture, reducing the CTOD characteristics and low-temperature toughness. Furthermore, low-temperature toughness decreases after strain aging, and the amount of decrease increases with the content of impurities such as P. Therefore, improving the center segregation is important to ensure low-temperature toughness after strain aging.

[0020] As a result of further investigation, the inventors have found that by performing soft reduction two or more times at a reduction rate of 0.3 mm / min to 2.5 mm / min upstream of the final solidification position of the slab during continuous casting, center segregation is reduced, hardening of the center segregated portion is inhibited, and the number of segregated grains is reduced, thereby improving the CTOD characteristics and low-temperature toughness after strain aging.

[0021] In addition, the inventors have discovered that the combination of (1) and (2) above not only provides excellent strength and toughness to the base material, but also provides excellent toughness in the center of the plate thickness after strain aging; specifically, it combines high strength, CTOD characteristics, and low-temperature toughness after strain aging.

[0022] The present invention was completed based on the above findings and further investigations. That is, the gist of the present invention is as follows: [1] The component composition contains, in mass %, C: 0.03 to 0.15%, Si: 0.50% or less, Mn: 0.3 to 2.5%, P: 0.030% or less, S: 0.0050% or less, Ni: 0.01 to 5.00%, Al: 0.005 to 0.100%, N: 0.0100% or less, and O: 0.0100% or less, and Ceq defined by formula (1) satisfies formula (2), with the balance being Fe and inevitable impurities, the hardness of the center segregation portion of the steel sheet satisfies formula (3), and the number density of segregated grains having an equivalent circle diameter of 100 μm or more is 2.0 grains / mm ２ and the average effective grain size at the center of the sheet thickness is 20 μm or less. Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 (1) 0.300%≦Ceq≦0.550% (2) Hv ｍａｘ / Hv ａｖｅ ≦1.35+0.006 / [C]−t / 500 (3) Here, the brackets [ ] in formulas (1) and (3) represent the content (mass%) of the element in the brackets, and if the element is not contained, the content is set to zero. ｍａｘ is the maximum value of Vickers hardness at the center segregation, and Hv ａｖｅis the average value of Vickers hardness at 1 / 8 to 3 / 8 positions and 5 / 8 to 7 / 8 positions of the sheet thickness, and t is the sheet thickness (mm) of the steel sheet. [2] The steel sheet according to [1], wherein the chemical composition further includes, in mass%, one or more selected from the group consisting of Cu: 1.50% or less, Cr: 1.50% or less, Mo: 1.50% or less, V: 0.20% or less, Ti: 0.100% or less, Nb: 0.100% or less, B: 0.0050% or less, Ca: 0.0100% or less, W: 0.50% or less, REM: 0.025% or less, and Mg: 0.0150% or less. [3] A method for producing a steel plate according to [1] or [2], wherein a slab having the above-mentioned composition is produced by continuous casting in which soft reduction is performed twice or more at a reduction rate of 0.3 mm / min to 2.5 mm / min upstream of the final solidification position of the slab, and the slab is heated to a heating temperature of 990°C or higher and 1250°C or lower, and the slab is rolled so that the average value of the reduction rate / pass at the temperature of the center of the plate thickness of 950°C or higher is 3.5% or higher and the cumulative reduction rate is 40% or higher, and then the temperature of the center of the plate thickness is reduced by Ar. ３ [4] A method for manufacturing a steel sheet according to [3], comprising rolling the steel sheet at a temperature between 100°C and 950°C so that the average value of the reduction / pass is 3.5% or more and the cumulative reduction is 40% or more, and cooling the steel sheet at an average cooling rate of 3.0°C / s or more at the center of the thickness to a cooling stop temperature at which the temperature at the center of the thickness is 600°C or less.

[0023] According to the present invention, it is possible to provide a thick steel plate having a thickness of 8 to 70 mm, which has high strength and excellent CTOD characteristics and low-temperature toughness after strain aging, together with a manufacturing method thereof, and which is extremely useful industrially.

[0024] The reasons for limiting each of the constituent elements of the present invention will be explained below. Note that the following explanation shows a preferred embodiment of the present invention, but the present invention is not limited to this embodiment.

[0025] [Steel Sheet] [Composition] First, the reasons for limiting the composition of the steel sheet and steel billet in the present invention to the following numerical ranges will be explained. Note that "%" regarding the composition means "mass %" unless otherwise specified.

[0026] C: 0.03 to 0.15% C is an element that improves hardenability and improves the strength of steel, and a content of 0.03% or more is required to achieve these effects. However, if the C content exceeds 0.15%, the hardness of the C-enriched portions increases, the CTOD characteristics deteriorate, and the low-temperature toughness after strain aging deteriorates. Therefore, the C content is set to 0.03% or more, preferably 0.05% or more. Furthermore, the C content is set to 0.15% or less, preferably 0.12% or less.

[0027] Si: 0.50% or less Si is also used as a deoxidizer and is an element that is inevitably contained as an impurity, but if the Si content exceeds 0.50%, not only will the surface properties of the steel be impaired, but the CTOD properties and low-temperature toughness will also decrease. Therefore, the upper limit of the Si content is limited to 0.50%, and preferably 0.40% or less. The lower limit of the Si content is not particularly limited, but is preferably 0.04% or more.

[0028] Mn: 0.3 to 2.5% Mn is an element that has the effect of improving the hardenability and strength of steel. However, if the Mn content exceeds 2.5%, the hardenability becomes excessive, deteriorating the CTOD characteristics and low-temperature toughness. Therefore, the Mn content is set to 0.3% or more, preferably 0.8% or more. Furthermore, the Mn content is set to 2.5% or less, preferably 2.3% or less.

[0029] P: 0.030% or less P is an element that has a large effect of embrittling grain boundaries, and if contained in large amounts, it reduces CTOD properties and low-temperature toughness, so the P content is limited to 0.030% or less. The P content is preferably 0.020% or less. It is desirable to reduce the P content as much as possible, and there is no particular lower limit for the P content, but excessively low P content leads to increased refining time and costs, so the P content is preferably 0.001% or more.

[0030] S: 0.0050% or less S is an element that reduces CTOD characteristics and low-temperature toughness, so the upper limit of the S content is limited to 0.0050%. The S content is preferably 0.0030% or less. It is desirable to reduce the S content as much as possible, and there is no lower limit for the S content. However, excessively low S content increases refining time and costs, so the S content is preferably 0.0001% or more.

[0031] Ni: 0.01 to 5.00% Ni is an element effective in improving the strength, CTOD characteristics, and low-temperature toughness of the base metal, and a content of 0.01% or more is required. On the other hand, if the Ni content exceeds 5.00%, an increase in cost becomes a problem, so the Ni content is set to 5.00% or less, preferably 4.80% or less, and more preferably 3.50% or less. The Ni content is preferably 0.10% or more.

[0032] Al: 0.005 to 0.100% Al is an element added to deoxidize molten steel, and for this purpose, an Al content of 0.005% or more is required. However, if the Al content exceeds 0.100%, the CTOD properties and low-temperature toughness will deteriorate. Therefore, the Al content is set to 0.100% or less, and preferably 0.080% or less. The Al content is preferably 0.010% or more.

[0033] N: 0.0100% or less N is an element that reduces CTOD characteristics and low-temperature toughness, so the upper limit of the N content is limited to 0.0100%. The N content is preferably 0.0090% or less. It is desirable to reduce the N content as much as possible, and there is no lower limit for the N content. However, excessive reduction in N content increases refining time and costs, so the N content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0034] O: 0.0100% or less Since O is an element that reduces CTOD characteristics and low-temperature toughness, the upper limit of the O content is limited to 0.0100%. The O content is preferably 0.0090% or less. It is desirable to reduce the O content as much as possible, and there is no lower limit for the O content, but excessive reduction in O content leads to an increase in refining time and an increase in costs, so the O content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0035] The basic composition of the steel plate of the present invention (hereinafter sometimes simply referred to as "thick steel plate") consists of the above elements with the balance being Fe and unavoidable impurities. With this basic composition, the steel plate of the present invention can achieve the desired properties.

[0036] Furthermore, in the present invention, for the purpose of further improving strength, CTOD characteristics, low-temperature toughness, etc., in addition to the above-mentioned basic component composition, one or more elements selected from the group consisting of Cu, Cr, Mo, V, Ti, Nb, B, Ca, W, REM, and Mg can be optionally contained in the amounts shown below. Note that, since each of the components Cu, Cr, Mo, V, Ti, Nb, B, Ca, W, REM, and Mg shown below can be contained as needed, the content of these elements may be 0%.

[0037] Cu: 1.50% or less Cu may be added as needed. Cu is an element that can increase the strength of steel plates without significantly deteriorating low-temperature toughness, but if the Cu content exceeds 1.50%, surface cracking due to a Cu-enriched layer formed directly below the scale becomes a problem. Therefore, when Cu is contained, it is preferable to limit the Cu content to 1.50% or less. The Cu content is more preferably 1.30% or less. When Cu is contained, the Cu content is preferably 0.05% or more in order to fully obtain its effects.

[0038] Cr: 1.50% or less Cr may be added as needed. Cr is an element that improves the hardenability and strength of steel, but if the Cr content exceeds 1.50%, the CTOD characteristics and low-temperature toughness will deteriorate. Therefore, when Cr is contained, the Cr content is preferably 1.50% or less. The Cr content is more preferably 1.30% or less. When Cr is contained, in order to fully obtain its effects, the Cr content is preferably 0.01% or more, more preferably 0.10% or more.

[0039] Mo: 1.50% or less Mo may be added as needed. Mo is an element that improves the hardenability and strength of steel, but if the Mo content exceeds 1.50%, the CTOD characteristics and low-temperature toughness decrease. Therefore, when Mo is contained, the Mo content is preferably 1.50% or less. The Mo content is more preferably 1.30% or less. When Mo is contained, the Mo content is preferably 0.10% or more in order to fully obtain its effects.

[0040] V: 0.20% or less V may be added as needed. V is an element that improves the strength of the base metal, but if the V content exceeds 0.20%, the CTOD characteristics and low-temperature toughness decrease. Therefore, when V is contained, the V content is preferably limited to 0.20% or less. The V content is more preferably 0.15% or less. When V is contained, the V content is preferably 0.01% or more in order to fully obtain its effects.

[0041] Ti: 0.100% or less Ti may be added as needed. Ti precipitates in steel as TiN. The precipitated TiN has the effect of suppressing coarsening of austenite grains, and by refining the crystal grain size, improves the CTOD properties and low-temperature toughness. On the other hand, if the Ti content exceeds 0.100%, the precipitation of solute Ti and coarse TiC will actually deteriorate the CTOD properties and low-temperature toughness. Therefore, when Ti is contained, it is preferable to limit the Ti content to 0.100% or less. The Ti content is more preferably 0.080% or less. When Ti is contained, the Ti content is preferably 0.005% or more in order to fully obtain its effects.

[0042] Nb: 0.100% or less Nb may be added as needed. Nb is an element that is effective in improving the strength of the base material, but a Nb content exceeding 0.100% reduces the CTOD characteristics and low-temperature toughness. Therefore, when Nb is contained, the Nb content is preferably limited to 0.100% or less. The Nb content is more preferably 0.050% or less. When Nb is contained, the Nb content is preferably 0.005% or more in order to fully obtain its effects.

[0043] B: 0.0050% or less B may be added as needed. B is an element that can improve hardenability even with a very small amount of B, thereby improving the strength of the steel sheet. However, if the B content exceeds 0.0050%, the CTOD characteristics and low-temperature toughness decrease. Therefore, when B is contained, the B content is preferably 0.0050% or less, and more preferably 0.0030% or less. When B is contained, the B content is preferably 0.0005% or more in order to fully obtain its effects.

[0044] Ca: 0.0100% or less Ca is an element that improves the toughness of the weld heat-affected zone by forming oxysulfides that are highly stable at high temperatures. Since the effects of the present invention are not impaired even when Ca is contained, it may be added as needed. However, if Ca is contained in an amount exceeding 0.0100%, coarse inclusions are formed, deteriorating the CTOD characteristics and low-temperature toughness. Therefore, when Ca is contained, the Ca content is preferably limited to 0.0100% or less. The Ca content is more preferably 0.0080% or less. When Ca is contained, in order to fully obtain its effects, the Ca content is preferably 0.0002% or more, more preferably 0.0015% or more.

[0045] W: 0.50% or less W may be added as needed. W is an element that improves the strength of the base metal, but if the W content exceeds 0.50%, weldability decreases. Therefore, when W is contained, it is preferable to limit the W content to 0.50% or less. The W content is more preferably 0.40% or less. When W is contained, in order to fully obtain its effects, the W content is preferably 0.02% or more, more preferably 0.10% or more.

[0046] REM: 0.025% or less. REM may be added as needed. REM (rare earth metal) is an element that improves weldability by forming oxysulfide inclusions that are highly stable at high temperatures. On the other hand, if the REM content exceeds 0.025%, the effect of adding REM saturates, and no effect commensurate with the content can be expected, resulting in economic disadvantage. Therefore, when REM is contained, the REM content is preferably limited to 0.025% or less. The REM content is more preferably 0.020% or less. When REM is contained, in order to fully obtain its effect, the REM content is preferably 0.001% or more, more preferably 0.010% or more. Here, REM is a collective term for 17 elements, including 15 lanthanoid elements plus Y and Sc. These elements can be contained alone or in combination. Therefore, the REM content refers to the total content of these elements.

[0047] Mg: 0.0150% or less Mg may be added as needed. Mg is an element that improves weldability by forming oxysulfide inclusions that are highly stable at high temperatures. On the other hand, if the Mg content exceeds 0.0150%, the effect of adding Mg becomes saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when Mg is contained, it is preferable to limit the Mg content to 0.0150% or less. The Mg content is more preferably 0.0100% or less. When Mg is contained, the Mg content is preferably 0.0002% or more in order to fully obtain its effect.

[0048] The chemical compositions of the above steel plates and steel billets must also satisfy the following conditions.

[0049] Ceq: 0.300% or more and 0.550% or less In the present invention, appropriate elements are required to ensure high strength and good low-temperature toughness in the center of the plate thickness. Specifically, it is important to contain elements so that Ceq, as defined by formula (1), satisfies the relationship shown in formula (2), i.e., 0.300%≦Ceq≦0.550%. If Ceq is greater than 0.550%, the amount of structures with poor toughness, such as island martensite, increases, deteriorating the CTOD properties and low-temperature toughness. On the other hand, if Ceq is less than 0.300%, the strength targeted in the present invention cannot be ensured. Therefore, the range of Ceq is set to 0.300% or more and 0.550% or less. Note that Ceq is preferably 0.310% or more and preferably 0.530% or less. Ceq is more preferably 0.320% or more and more preferably 0.520% or less. Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 (1) 0.300%≦Ceq≦0.550% (2) Here, [ ] in formula (1) represents the content (mass%) of the element shown in [ ], and if the element is not contained, the content is set to zero.

[0050] [Hardness of the central segregation portion] In the present invention, in order to ensure the CTOD characteristics and the low-temperature toughness after strain aging at the center of the plate thickness, it is important to specify the hardness of the central segregation portion of the steel plate so as to satisfy the formula (3). ｍａｘ / Hv ａｖｅ ≦1.35+0.006 / [C]−t / 500 (3) Here, [C] in formula (3) is the content (mass%) of [C], and Hv ｍａｘ is the maximum value of Vickers hardness at the center segregation, and Hv ａｖｅ is the average value of the Vickers hardness at the 1 / 8 to 3 / 8 position and the 5 / 8 to 7 / 8 position of the plate thickness, and t is the plate thickness (mm) of the steel plate.

[0051] That is, Hv ｍａｘ / Hv ａｖｅis a dimensionless parameter that represents the hardness of the center segregation. If this value is higher than the value calculated by (1.35 + 0.006 / [C] - t / 500), stress concentration in the center segregation, which is the hardened part, increases, making fracture more likely. As a result, the CTOD value and low-temperature toughness decrease, so it is set to (1.35 + 0.006 / [C] - t / 500) or less. Hv ｍａｘ / Hv ａｖｅ is preferably set to be equal to or less than (1.25+0.006 / [C]-t / 500).

[0052] Here, Hv ｍａｘ The hardness is measured in an area having a thickness of 10% of the plate thickness from the center of the plate thickness in both surface directions of the steel plate, including the central segregation portion, at intervals of 0.5 mm in the plate thickness direction of the steel plate using a Vickers hardness tester, and is the maximum value of the measured values ​​obtained. ａｖｅ is the average value of values ​​measured at 1 mm intervals in the thickness direction of the steel plate in the regions of 1 / 8 to 3 / 8 and 5 / 8 to 7 / 8 of the plate thickness using a Vickers hardness tester. Note that the load of the Vickers hardness tester is 10 kgf in both cases.

[0053] [Number density of segregated grains] Number density of segregated grains having a circle equivalent diameter of 100 μm or more: 2.0 grains / mm ２ As mentioned above, the center segregation in the steel sheet becomes the fracture origin, and deteriorates the CTOD characteristics and the low-temperature toughness after strain aging. In particular, the center segregation grains with a circle equivalent diameter of 100 μm or more within 1 mm ２ The number of particles per unit area (hereinafter simply referred to as "segregation particle number density") is 2.0 particles / mm ２ If the number density of segregated grains exceeds 2.0 grains / mm, the crack tip opening displacement (δ) in the CTOD test and the low temperature toughness after strain aging are likely to be insufficient. ２ It is important to make the number density of segregated grains below 1.8 grains / mm. ２ It is desirable to reduce the number of segregated grains as much as possible, and there is no particular lower limit for the number density, but an excessive reduction in the number of segregated grains leads to an increase in the manufacturing load, so the lower limit is set to 0.1 grains / mm ２ It is preferable that the above is set.

[0054] The segregated grains in the present invention are [Mn] ＥＰＭＡ / [Mn] ≧ 1.33, and [Mn] ＥＰＭＡ is the Mn concentration (mass%) at the measurement position, and [Mn] is the Mn content (mass%) of the entire steel sheet.

[0055] Furthermore, the frequency of measurement of the number density of such segregation grains may be such that one or two cross sections of any one steel plate are measured among steel plates that have been produced under the same slab melting conditions and the same rolling conditions. As long as the slab melting method and rolling conditions are not changed, the number density of segregation grains can be produced with good reproducibility, and therefore the measurement results at the above measurement frequency can be said to represent the whole.

[0056] Specifically, the "number density of segregated grains" can be measured by the method described in the Examples. Since segregated grains tend to be present in large numbers in the central segregation region, when the number density of segregated grains is measured over the entire thickness, the number density of segregated grains is small. Since it is important to control the number density of segregated grains in the center of the sheet thickness in order to achieve desired properties, the number density of segregated grains is evaluated in a region having a total thickness of 3 mm, including the central segregation region, which is 1.5 mm from the center of the sheet thickness in both surface directions of the steel sheet.

[0057] [Average Effective Grain Size] Average effective grain size at the center of the sheet thickness: 20 μm or less In the present invention, the average effective grain size at the center of the sheet thickness is set to 20 μm or less. Refining the grain size at the center of the sheet thickness can improve strength, CTOD characteristics, and low-temperature toughness after strain aging. On the other hand, if the average effective grain size is larger than 20 μm, coarse grains become fracture origins, so even if center segregation is reduced, the desired CTOD characteristics and low-temperature toughness after strain aging cannot be obtained. The average effective grain size is preferably 18 μm or less. Since the smaller the average effective grain size, the more advantageous it is, and no lower limit is particularly specified. However, excessive grain refinement increases the manufacturing load, so it is preferable to set it to 1 μm or more.

[0058] In the present invention, the "effective grain size" is defined as the circle-equivalent diameter of a grain surrounded by a grain boundary having an orientation difference of 15° or more with adjacent grains, i.e., a high-angle grain boundary. The average effective grain size can be measured by the method described in the Examples.

[0059] [Manufacturing Method] Next, a manufacturing method for the steel plate of the present invention will be described. The steel plate of the present invention is preferably manufactured by the method described below. In the following description, temperatures refer to temperatures at the center of the plate thickness unless otherwise specified. The temperature at the center of the plate thickness can be measured, but in an actual manufacturing line, it may also be determined by heat transfer calculation from the steel plate surface temperature measured with a radiation thermometer.

[0060] [Continuous Casting] Continuous Casting Conditions In the present invention, in order to reduce segregation in the center of the plate thickness, a steel slab satisfying the above-mentioned composition is produced by continuous casting, in which soft reduction is performed two or more times upstream of the final solidification position at a reduction rate of 0.3 mm / min to 2.5 mm / min. If the reduction rate is less than 0.3 mm / min, the reduction amount per unit time is insufficient, the flow of concentrated molten steel cannot be suppressed, and center segregation cannot be reduced. On the other hand, if the reduction rate exceeds 2.5 mm / min, the reduction amount per unit time becomes too large, pushing the concentrated molten steel in the center of the slab upstream in the casting direction, resulting in negative segregation in the center of the slab due to reduced solute elements. The reduction rate is preferably 0.4 mm / min or more and preferably 2.3 mm / min or less. Furthermore, if the number of times soft reduction is performed at a reduction rate of 0.3 mm / min to 2.5 mm / min is one or less, the effect of sweeping the molten steel in the unsolidified portion upstream becomes insufficient, and the effect of reducing segregation by soft reduction becomes insufficient. There is no particular upper limit on the number of times soft reduction is performed, but from the viewpoint of cost-effectiveness of installing soft reduction rolls, it is preferably 30 times or less, and more preferably 10 times or less.

[0061] Heating conditions for steel slabs before rolling: 990°C or higher and 1250°C or lower In the present invention, steel slabs are heated to a heating temperature of 990°C or higher and 1250°C or lower. If the heating temperature is lower than 990°C, deformation resistance during rolling increases, which increases the load on the rolling mill and increases the number of passes, resulting in a decrease in production efficiency. The heating temperature is preferably 1000°C or higher.

[0062] On the other hand, if the heating temperature is higher than 1250°C, the austenite grains become coarse, the desired fine grain structure cannot be obtained, and the strength, CTOD characteristics, and low-temperature toughness decrease. Therefore, the heating temperature is set to 1250°C or lower, and preferably 1230°C or lower.

[0063] [Hot rolling] Hot rolling conditions The purpose of hot rolling in the present invention is to properly introduce strain into the center of the plate thickness to obtain a desired fine grain structure. To achieve this, rolling in the recrystallization temperature range of 950°C or higher and rolling in Ar ３ It is important to control both the rolling temperature above 950°C and below 950°C.

[0064] Hot rolling in the recrystallization temperature range The heated steel slab is hot rolled at a temperature of 950° C. or higher. If the hot rolling temperature is less than 950° C., recrystallization does not occur easily, and the austenite grains are not sufficiently refined.

[0065] Furthermore, rolling with an average reduction rate per pass of less than 3.5% fails to introduce sufficient strain into the center of the sheet thickness. Even if the average reduction rate per pass is 3.5% or more, if the cumulative reduction rate is less than 40%, recrystallization does not proceed sufficiently, resulting in insufficient refinement of austenite grains. Therefore, rolling is performed at 950°C or higher with an average reduction rate per pass of 3.5% or more and a cumulative reduction rate of 40% or more. Since the larger the average reduction rate per pass and the cumulative reduction rate, the easier it is to introduce strain into the center of the sheet thickness, the average reduction rate per pass is preferably 4.0% or more. Furthermore, the cumulative reduction rate is preferably 45% or more. However, since an excessive increase in the reduction rate per pass increases the load on the rolling mill, the average reduction rate per pass is preferably 12.0% or less. Furthermore, since an excessive increase in the cumulative reduction rate makes it impossible to ensure the desired sheet thickness, the cumulative reduction rate is preferably 95% or less.

[0066] Hot rolling in the non-recrystallization temperature range After hot rolling in the recrystallization temperature range, Ar ３Rolling is performed at a temperature between 1000°C and 950°C, with an average reduction / pass value of 3.5% or more and a cumulative reduction of 40% or more. Because recrystallization is difficult to occur during rolling in the non-recrystallization temperature range, strain introduced by rolling accumulates without being consumed by recrystallization and functions as a transformation nucleus in the subsequent cooling process. As a result, the structure of the final steel plate can be refined. However, rolling with an average reduction / pass value of less than 3.5% fails to introduce sufficient strain into the center of the plate thickness, and rolling with a cumulative reduction rate of less than 40% results in insufficient grain refinement. Therefore, in the present invention, it is important to simultaneously satisfy the average reduction / pass value of 3.5% or more and the cumulative reduction rate of 40% or more. Since the larger the average reduction / pass value and the cumulative reduction rate, the easier it is to introduce strain into the center of the plate thickness, the average reduction / pass value is preferably 4.0% or more. Furthermore, the cumulative reduction rate is preferably 45% or more.

[0067] Here, the above Ar ３ The points can be obtained by actual measurement, but they may also be calculated from equation (4). ３ Point (°C) = 910 - 273 [C] - 74 [Mn] - 5 [Cu] - 56 [Ni] - 16 [Cr] - 9 [Mo] (4) In formula (4), [ ] represents the content (mass%) of the element shown in [ ], and if the element is not contained, the content is set to zero.

[0068] Rolling end temperature: Ar ３ The steel billet heated to above the temperature (℃) is ３ The hot rolling is performed at a temperature equal to or higher than the rolling temperature. ３ The rolling end temperature is Ar ３ If the rolling temperature is lower than this point, the structure before the start of cooling will be a two-phase structure of austenite and ferrite, the uniformity of the steel structure will be poor, and manufacturing stability will be significantly reduced. ３ point or more, preferably (Ar ３ point +10℃ or higher.

[0069] [Cooling after Hot Rolling] After the hot rolling is completed, the obtained hot-rolled steel sheet is cooled. This cooling can be performed by any method, for example, water cooling, as long as the following conditions are met.

[0070] Average cooling rate: 3.0°C / s or more If the average cooling rate at the center of the plate thickness is less than 3.0°C / s, a coarse ferrite phase will form in the structure of the steel plate, deteriorating the strength, CTOD characteristics, and low-temperature toughness. Therefore, the average cooling rate at the center of the plate thickness is set to 3.0°C / s or more. The average cooling rate is preferably 4.0°C / s or more and preferably 100°C / s or less.

[0071] The average cooling rate in the present invention refers to the average value of the cooling rate from 800°C to 500°C. However, when the cooling stop temperature described below is 500°C or higher, the average cooling rate refers to the average value of the cooling rate from 800°C to the cooling stop temperature. Furthermore, when the rolling end temperature is 800°C or lower, the average cooling rate refers to the average value of the cooling rate from the rolling end temperature to 500°C. Furthermore, when the cooling stop temperature is 500°C or higher and the rolling end temperature is 800°C or lower, the average cooling rate refers to the average value of the cooling rate from the rolling end temperature to the cooling stop temperature.

[0072] Cooling stop temperature: 600°C or less In the cooling, the hot-rolled steel sheet is cooled to a cooling stop temperature of 600°C or less at the center of the sheet thickness. If the cooling stop temperature is higher than 600°C, the structure after transformation becomes coarse, the base material strength becomes insufficient, and the CTOD characteristics and low-temperature toughness deteriorate. For this reason, the cooling stop temperature is set to 600°C or less. The cooling stop temperature is preferably 570°C or less.

[0073] [Tempering Treatment] Tempering temperature: 700°C or less After cooling is stopped, further tempering can be performed as desired. Tempering can improve the toughness of the base material. On the other hand, if the tempering temperature exceeds 700°C, various carbonitrides precipitate in the steel, and the microstructure obtained by transformation disappears, resulting in a decrease in strength and toughness. Therefore, the tempering temperature is preferably 700°C or less. The tempering temperature is more preferably 650°C or less. The tempering temperature is preferably 300°C or more.

[0074] In the production method according to the present invention, any item not described in this specification can be performed in a conventional manner.

[0075] Next, the present invention will be described in more detail based on examples. The following examples are intended to illustrate preferred examples of the present invention, and the present invention is not limited to these examples in any way.

[0076] Steel billets having the chemical compositions shown in Table 1 were used to produce steel plates under the production conditions shown in Table 2. Table 2 also shows the soft reduction conditions during continuous casting when producing the billets. During hot rolling, thermocouples were attached to the center positions in the longitudinal, width, and thickness directions of the steel plate being rolled, and the temperature at the center of the plate thickness was measured. In addition, the surface temperature of the steel plate was measured with a radiation thermometer. In Table 1, "-" indicates that no element was intentionally added, and includes not only cases where no element is contained, but also cases where an element is unavoidably contained.

[0077]

[0078]

[0079] For each of the obtained steel plates, the number density of segregated grains, Vickers hardness, average effective grain size, yield strength, tensile strength, low temperature toughness after 5% strain aging, and CTOD value were measured by the following methods.

[0080] [Number Density of Segregated Grains] To prepare a measurement sample, a rectangular parallelepiped steel piece with a width of 500 mm in the width direction and a thickness of 3 mm was cut out from the center of the obtained thick steel plate in both the width and thickness directions. The cut-out steel was further cut into 20 equal parts in the width direction to obtain 20 measurement samples with a width of 25 mm in the width direction. A surface of each measurement sample perpendicular to the rolling direction (i.e., a surface with a width of 25 mm in the width direction and a thickness of 3 mm in the thickness direction) was mirror-polished, and then, the mirror-polished surface was used as the measurement surface and quantitatively analyzed for Mn using an electron probe microanalyzer (EPMA).

[0081] In the present invention, the measured Mn concentration is expressed as [Mn] ＥＰＭＡand [Mn] ＥＰＭＡ The area where / [Mn]≧1.33 was defined as a segregated grain. The number density of segregated grains was calculated by dividing the number of segregated grains with a circle equivalent diameter of 100 μm or more by the measurement area. [EPMA measurement conditions] Acceleration voltage: 20 kV Probe current: 0.2 μA Integration time: 0.05 seconds Beam diameter: 10 μm Measurement range: Height 3 mm × Width 25 mm × 20 samples

[0082] [Vickers hardness] A cross section in the thickness direction parallel to the rolling direction of the steel sheet (i.e., a cross section perpendicular to the width direction) was taken from the center of the width of the steel sheet and mirror-polished. Next, in the prepared sample, regions having a thickness of 10% of the sheet thickness from the center of the sheet thickness to both surface directions of the steel sheet, including the central segregation portion, were measured in the thickness direction of the steel sheet with a Vickers hardness tester at intervals of 1 mm in the thickness direction, and the maximum value of the obtained measurements was designated as Hv ｍａｘ The average value of the values ​​measured at 1 mm intervals in the thickness direction of the steel plate in the regions of 1 / 8 to 3 / 8 positions and 5 / 8 to 7 / 8 positions in the thickness direction with a Vickers hardness tester was taken as Hv ａｖｅ The load of the Vickers hardness tester was set to 10 kgf in all cases.

[0083] [Average effective grain size] A sample was taken from the obtained thick steel plate so that the measurement positions were the center of the steel plate in the longitudinal direction, width direction, and plate thickness direction, and the plate thickness center was included. Next, the surface of the sample was mirror-polished, and then EBSP analysis was performed under the following conditions. From the obtained crystal orientation map, the circle equivalent diameter of the structure surrounded by high-angle grain boundaries with an orientation difference of 15° or more from adjacent crystal grains was determined, and the average value of the circle equivalent diameter in the following analysis region was taken as the average effective grain size. [EBSP analysis conditions] Analysis region: 1 mm x 1 mm region at the plate thickness center Step size: 0.4 μm

[0084] [Tensile Test] JIS No. 4 test pieces were taken from the obtained thick steel plate at a position halfway through the plate thickness so that the longitudinal direction of the test piece was perpendicular to the rolling direction and plate thickness direction of the steel plate, and the yield strength (YS) and tensile strength (TS) were measured according to JIS Z 2241. If an upper yield point appeared in the tensile test, the upper yield stress was taken as the yield strength. If an upper yield point did not appear, the 0.2% proof stress was taken as the yield strength. If YS ≥ 320 MPa, the specimen was evaluated as having high strength. If YS ≥ 320 MPa and TS ≥ 430 MPa, the tensile properties were evaluated as being good.

[0085] [Strain-Aging Charpy Impact Test] JIS No. 14B flat tensile test specimens were taken from the obtained thick steel plate at a position halfway through the plate thickness perpendicular to the rolling direction, and a tensile test was performed to apply a 5% plastic strain in the longitudinal direction of the tensile test specimen. After the pre-strain was introduced, an aging treatment was performed by holding the specimen at 250°C for 1 hour to impart the effect of strain aging. Three JIS V-notch test specimens were taken from the center of the plate thickness of the tensile test specimen after the aging treatment so that the longitudinal direction of the test specimen coincided with the longitudinal direction of the tensile test specimen, and the absorbed energy (vE) at -60°C was measured according to JIS Z 2242. －６０ ) was measured and the average value was calculated. －６０ When the average value of 50 J or more was evaluated as good, the Charpy impact properties were evaluated.

[0086] [CTOD Test] Full-thickness CTOD test specimens were taken so that the longitudinal direction of the test specimen was perpendicular to the rolling direction, and the crack tip opening displacement was measured. The test method conformed to ISO 12135, and the crack tip opening displacement [CTOD value (δ)] was evaluated at a test temperature of -55°C. For each thick steel plate, the test was performed using three test specimens, and the minimum measured value was taken as δ. In the present invention, specimens satisfying δ≧0.10 mm were evaluated as having good CTOD properties.

[0087] The evaluation results are also shown in Table 3. The inventive examples satisfied the component composition and manufacturing conditions of the present invention, and also satisfied the conditions for the hardness of the center segregation region, the number density of segregated grains, and the average effective grain size, and were high in strength and excellent in CTOD properties and low-temperature toughness after strain aging. In contrast, the steel plates (comparative examples) that did not satisfy the conditions of the present invention were poor in one or more of the strength, CTOD properties, and low-temperature toughness after strain aging, and were inferior in properties to the inventive examples.

[0088]

Claims

1. The chemical composition contains, in mass%, C: 0.03 to 0.15%, Si: 0.50% or less, Mn: 0.3 to 2.5%, P: 0.030% or less, S: 0.0050% or less, Ni: 0.01 to 5.00%, Al: 0.005 to 0.100%, N: 0.0100% or less, and O: 0.0100% or less, and Ceq defined by formula (1) satisfies formula (2), with the balance being Fe and unavoidable impurities, the hardness of the central segregation portion of the steel plate satisfies formula (3), and the number density of segregated grains with an equivalent circle diameter of 100 μm or more is 2.0 grains / mm ２ and the average effective grain size at the center of the sheet thickness is 20 μm or less. Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5 (1) 0.300%≦Ceq≦0.550% (2) Hv ｍａｘ / Hv ａｖｅ ≦1.35+0.006 / [C]−t / 500 (3) Here, the brackets [ ] in formulas (1) and (3) represent the content (mass%) of the element in the brackets, and if the element is not contained, the content is set to zero. ｍａｘ is the maximum value of Vickers hardness at the center segregation, and Hv ａｖｅ is the average value of the Vickers hardness at the 1 / 8 to 3 / 8 position and the 5 / 8 to 7 / 8 position of the plate thickness, and t is the plate thickness (mm) of the steel plate.

2. The steel plate according to claim 1, wherein the chemical composition further includes, in mass%, one or more elements selected from the group consisting of Cu: 1.50% or less, Cr: 1.50% or less, Mo: 1.50% or less, V: 0.20% or less, Ti: 0.100% or less, Nb: 0.100% or less, B: 0.0050% or less, Ca: 0.0100% or less, W: 0.50% or less, REM: 0.025% or less, and Mg: 0.0150% or less.

3. A method for manufacturing a steel plate according to claim 1 or 2, comprising heating a slab having the above-mentioned chemical composition, produced by continuous casting in which soft reduction is performed twice or more at a reduction rate of 0.3 mm / min to 2.5 mm / min upstream of the final solidification position of the slab, to a heating temperature of 990°C or higher and 1250°C or lower, rolling is performed so that the average value of the reduction rate per pass at the temperature of the center of the plate thickness is 3.5% or higher and the cumulative reduction rate is 40% or higher, and then, when the temperature of the center of the plate thickness is 950°C or higher, the temperature is reduced by Ar. ３ a rolling step in which the rolling reduction per pass is 3.5% or more and the cumulative rolling reduction is 40% or more at a temperature between 950°C and 950°C, and the rolling step is performed at an average cooling rate of 3.0°C / s or more at the center of the plate thickness to a cooling stop temperature at which the temperature at the center of the plate thickness is 600°C or less.

4. The method for producing a steel sheet according to claim 3, wherein after cooling to the cooling stop temperature, tempering is carried out at a temperature of 700°C or less.

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