Steel sheet and method for manufacturing the same

Abstract

The present application provides a steel sheet and a method for manufacturing the same. The steel sheet of the present application 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, Ceq satisfies 0.300 to 0.550%, the remainder is Fe and inevitable impurities, the hardness of the center segregation portion of the steel sheet satisfies formula (3), the number density of segregation particles with an equivalent circle diameter of 100 μm or more is 2.0 pieces / mm 2 Hereinafter, the average effective crystal grain diameter of the center portion of the sheet thickness is 20 μm or less. Hv max / Hv ave ≤1.35+0.006 / [C]‑t / 500...(3)

Description

Technical Field

[0001] This invention relates to steel plates suitable for steel structures such as liquefied gas storage tanks, ships, and marine structures, and methods for manufacturing the same. Background Technology

[0002] Against the backdrop of accelerated efforts to achieve a decarbonized society, the requirements for steel used in liquefied gas (LNG) storage tanks are increasingly stringent. For example, CCS (Carbon Capture and Storage), the technology for recovering and storing emitted CO2, is indispensable for achieving carbon neutrality. To transport CO2 over long distances to storage sites, it needs to be liquefied and transported by ship. Liquefying CO2 requires pressurization at cryogenic temperatures; therefore, steel plates used for CO2 storage tanks, in addition to strength, must possess excellent toughness at low temperatures. Previously, 9% Ni steel, which exhibits good toughness even at low temperatures, was used to meet this requirement. However, the high cost of this alloy necessitates an industrial demand for steel plates that are inexpensive and possess excellent low-temperature toughness. Furthermore, during tank manufacturing, steel plates are bent to create the tanks. Typically, strain is introduced into the steel plates during processing, subsequently leading to a deterioration in toughness due to aging. Therefore, ensuring low-temperature toughness after strain aging is crucial from the perspective of ensuring the tank's safety in the event of a collapse.

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

[0004] This test involves introducing fatigue pre-cracks into the test piece in the toughness evaluation section and performing three-point bending at low temperature to measure the amount of crack opening (plastic deformation) before failure, thereby evaluating the resistance to brittle failure.

[0005] Because fatigue pre-cracks are introduced in the CTOD test, the extremely small area becomes the toughness evaluation area. If a local embrittlement zone exists, even if good toughness is obtained in the Charpy impact test, it may sometimes show lower toughness in the CTOD test.

[0006] As a means to improve the low-temperature toughness of steel, it is known to increase the Ni content. 9% Ni steel has been commercially applied in liquefied natural gas (LNG) storage tanks and other applications.

[0007] In addition, for steel used in ships and pipelines, a technique of improving low-temperature toughness by refining grains through the TMCP method was adopted.

[0008] For example, Patent Documents 1 and 2 propose techniques to improve low-temperature toughness by refining the crystal grain size through optimizing heating temperature and hot rolling conditions. Furthermore, Patent Document 3 proposes a technique to improve low-temperature toughness by adding more than 5.0% Ni.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent No. 5304924

[0012] Patent Document 2: Japanese Patent No. 5573265

[0013] Patent Document 3: Japanese Patent No. 7067628 Summary of the Invention

[0014] For liquefied CO2 storage tanks and other vessels used to transport CO2 over long distances from emission sites to storage sites, steel plates with a yield strength of 320 MPa or higher, and excellent CTOD characteristics and low-temperature toughness in the center of the plate thickness after strain aging are required.

[0015] Although patent documents 1 and 2 propose techniques for improving the low-temperature toughness of steel plates before processing, they do not study the CTOD characteristics and the toughness after strain aging at all. Therefore, it cannot be said that the CTOD characteristics and the toughness of the plate center after embrittlement due to strain aging are fully guaranteed.

[0016] Furthermore, Patent Document 3 proposes a technique to ensure low-temperature toughness by adding more than 5.0% Ni, but the increase in Ni content inevitably leads to a significant increase in alloy cost, making it difficult to apply to steel used in CO2 storage tanks that require low-cost transportation. Additionally, the CTOD characteristics and toughness after strain aging were not investigated.

[0017] As described above, according to the inventor's research, the prior art described in Patent Documents 1 to 3 did not study the CTOD characteristics and the low-temperature toughness of the plate thickness center after strain aging, and could not simultaneously suppress alloy costs while taking into account high strength, CTOD characteristics, and the toughness of the plate thickness center after strain aging.

[0018] The present invention was made in view of the above-mentioned problems existing in the prior art, and its purpose is to provide a steel plate with high strength, excellent CTOD characteristics, and excellent low-temperature toughness in the center of the plate after strain aging, and a method for manufacturing the same.

[0019] Here, "high strength" in this invention refers to a yield strength of 320 MPa or higher in a tensile test at the center of the plate thickness. "Excellent CTOD characteristics" refers to a crack tip displacement of 0.10 mm or higher in a CTOD test at a test temperature of -55°C. "Excellent low-temperature toughness" refers to the absorbed energy vE in a Charpy test at -60°C at the center of the plate thickness. -60 The average value is 50J or higher. It should be noted that in this invention, the center portion of the plate thickness refers to the area having a thickness of 10% of the plate thickness extending from the center of the plate towards both surfaces.

[0020] To address the aforementioned issues, the inventors conducted in-depth research on methods for improving CTOD characteristics and low-temperature toughness after strain aging while simultaneously increasing strength, resulting in the following insights.

[0021] (1) Since there is a strong correlation between strength and toughness and grain size, grain refinement is essential in order to balance high strength, CTOD characteristics, and low-temperature toughness in the center of the plate. Therefore, controlling the rolling conditions in the recrystallization temperature region and the non-recrystallization temperature region is effective. Specifically, rolling with a cumulative reduction of 40% or more in the recrystallization temperature region above 950°C with an average reduction / pass ratio of 3.5% or more promotes austenite recrystallization. Then, rolling with a cumulative reduction of 40% or more at Ar3 points and below 950°C with an average reduction / pass ratio of 3.5% or more introduces sufficient strain into the center of the plate. Thus, it was found that the average effective grain size in the center of the plate can be below 20 μm.

[0022] (2) In high-strength steel plates, the amount of alloying elements added to ensure necessary properties increases. Therefore, during continuous casting, element segregation regions (i.e., central segregation zones) are easily formed in the center of the slab thickness. In continuous casting, the solidification of molten steel proceeds from the surface to the interior. However, since the solid solubility of alloying elements such as C, P, S, and Mn in the liquid phase is greater than that in the solid phase, these alloying elements are enriched in the unsolidified liquid phase due to redistribution during solidification. Then, the molten steel with significantly enriched alloying elements solidifies at the center of the final solidification zone, thus forming a central segregation zone.

[0023] The hardness of the central segregated region is higher than that of the surrounding area, making it prone to becoming the starting point of failure and reducing CTOD properties and low-temperature toughness. Furthermore, low-temperature toughness decreases after strain aging, and the decrease increases with the increase of impurity elements such as phosphorus. Therefore, improving central segregation is crucial to ensuring low-temperature toughness after strain aging.

[0024] The inventors conducted further research and found that by performing two or more light reductions 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 suppressed, and the number of segregated particles is reduced. This resulted in an improvement in CTOD properties and low-temperature toughness after strain aging.

[0025] Furthermore, the inventors have discovered that by combining (1) and (2) above, not only is the strength and toughness of the base material excellent, but the toughness of the center of the plate after strain aging is also excellent. Specifically, it combines high strength, CTOD characteristics and low-temperature toughness after strain aging.

[0026] This invention was completed based on the above insights and through further research. Specifically, the main points of this invention are as follows.

[0027] [1] A steel plate, the composition of which, by mass%, contains:

[0028] C: 0.03~0.15%

[0029] Si: below 0.50%

[0030] Mn: 0.3–2.5%,

[0031] P: below 0.030%

[0032] S: Below 0.0050%

[0033] Ni: 0.01~5.00%,

[0034] Al: 0.005~0.100%

[0035] N: below 0.0100%, and

[0036] O: Below 0.0100%,

[0037] Furthermore, Ceq, as defined by equation (1), satisfies equation (2), with the remainder being Fe and unavoidable impurities.

[0038] The hardness of the central segregated portion of the steel plate satisfies equation (3).

[0039] The number density of segregating particles with an equivalent spherical diameter of 100 μm or more is 2.0 particles / mm. 2 the following,

[0040] The average effective crystal grain size in the center of the plate thickness is less than 20 μm.

[0041] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5...(1)

[0042] 0.300%≤Ceq≤0.550%...(2)

[0043] Hv max / Hv ave ≤1.35+0.006 / [C]-t / 500...(3)

[0044] In equations (1) and (3), [] represents the content (mass%) of the element within []. When the element is not present, the content is set to zero. Equation (3) shows Hv max Hv is the maximum Vickers hardness of the central segregated region. ave It is the average Vickers hardness at positions 1 / 8 to 3 / 8 and 5 / 8 to 7 / 8 of the plate thickness, where t is the plate thickness (mm).

[0045] [2] The steel plate according to [1], wherein the above-mentioned composition further contains, by mass %, a component selected from:

[0046] Cu: below 1.50%

[0047] Cr: less than 1.50%

[0048] Mo: 1.50% or less,

[0049] V: Below 0.20%

[0050] Ti: below 0.100%

[0051] Nb: below 0.100%

[0052] B: Below 0.0050%

[0053] Ca: below 0.0100%

[0054] W: Below 0.50%

[0055] REM: below 0.025%, and

[0056] Mg: below 0.0150%

[0057] One or more of them.

[0058] [3] A method for manufacturing a steel plate, which is the method for manufacturing a steel plate described in [1] or [2].

[0059] The steel sheet with the above-mentioned composition, manufactured by continuous casting, is heated to a heating temperature of 990°C to 1250°C. The continuous casting process involves two or more light presses on the upstream side of the final solidification position of the slab at a pressing speed of 0.3 mm / min to 2.5 mm / min.

[0060] Rolling is performed with an average reduction rate per pass of 3.5% or more and a cumulative reduction rate of 40% or more at a temperature of 950°C or higher in the center of the plate thickness. Then, rolling is performed with an average reduction rate per pass of 3.5% or more and a cumulative reduction rate of 40% or more at a temperature of Ar3 or higher but lower than 950°C in the center of the plate thickness.

[0061] The cooling is stopped when the average cooling rate at the center of the plate thickness is above 3.0℃ / s and the temperature at the center of the plate thickness is below 600℃.

[0062] [4] According to the steel plate manufacturing method described in [3], after cooling to the above-mentioned cooling stop temperature, tempering treatment is performed at a temperature of 700°C or below.

[0063] According to the present invention, a thick steel plate with a thickness of 8 to 70 mm, a high-strength steel plate with excellent CTOD characteristics and low-temperature toughness after strain aging, and a method for manufacturing the same are provided, which are extremely useful in industry. Detailed Implementation

[0064] The reasons for limiting the various constituent elements of the present invention will be explained below. It should be noted that the following description represents a preferred embodiment of the present invention, and the present invention is not limited to this embodiment.

[0065] [Steel plate]

[0066] [Ingredients]

[0067] First, the reasons for limiting the composition of the steel plates and sheets in this invention to the following numerical range will be explained. It should be noted that, unless otherwise specified, the "%" in the composition refers to "mass %".

[0068] C: 0.03~0.15%

[0069] Carbon (C) is an element that improves hardenability and strength of steel; to achieve these effects, it must contain at least 0.03%. However, if the C content exceeds 0.15%, the hardness of the C-enriched portion increases, the CTOD (Coefficient of Hardness) characteristics decrease, and the low-temperature toughness after strain aging decreases. Therefore, the C content is 0.03% or more, preferably 0.05% or more. Furthermore, the C content is 0.15% or less, preferably 0.12% or less.

[0070] Si: below 0.50%

[0071] Si is an element that can be used as a deoxidizer and is unavoidably present as an impurity. If the Si content exceeds 0.50%, it impairs the surface properties of the steel and reduces its CTOD characteristics and low-temperature toughness. Therefore, the upper limit of Si content is limited to 0.50%, preferably 0.40% or less. There is no particular limitation on the lower limit of Si content, but it is preferably 0.04% or more.

[0072] Mn: 0.3–2.5%

[0073] Mn is an element that improves the hardenability and strength of steel. However, if the Mn content exceeds 2.5%, the hardenability becomes excessive, reducing CTOD characteristics and low-temperature toughness. Therefore, the Mn content is 0.3% or more, preferably 0.8% or more. Furthermore, the Mn content is 2.5% or less, preferably 2.3% or less.

[0074] P: below 0.030%

[0075] Phosphorus (P) is an element that significantly affects grain boundary embrittlement. Excessive P content reduces CTOD properties and low-temperature toughness; therefore, the P content is limited to below 0.030%, preferably below 0.020%. While a lower limit for P content is not strictly defined, excessively reducing P content can lead to increased refining time and higher costs; therefore, a P content of 0.001% or higher is preferred.

[0076] S: Below 0.0050%

[0077] S is an element that reduces CTOD properties and low-temperature toughness; therefore, the upper limit of S content is limited to 0.0050%. The S content is preferably below 0.0030%. While it is desirable to reduce the S content as much as possible, there is no particular lower limit for S content. However, excessively reducing S will lead to increased refining time and higher costs; therefore, the S content is preferably above 0.0001%.

[0078] Ni: 0.01~5.00%

[0079] Ni is an effective element for improving the strength, CTOD properties, and low-temperature toughness of the base material, and must be present at a content of 0.01% or more. On the other hand, if the Ni content exceeds 5.00%, cost increases become a problem. Therefore, the Ni content is 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.

[0080] Al: 0.005~0.100%

[0081] Al is an element added to deoxidize molten steel, and therefore must contain at least 0.005% Al. However, if the content exceeds 0.100%, it reduces CTOD properties and low-temperature toughness. Therefore, the Al content is 0.100% or less, preferably 0.080% or less. The Al content is preferably 0.010% or more.

[0082] N: below 0.0100%

[0083] Nitrogen (N) is an element that reduces CTOD properties and low-temperature toughness; therefore, the upper limit of N content is limited to 0.0100%. The N content is preferably 0.0090% or less. While it is desirable to reduce the N content as much as possible, there is no particular lower limit for the N content. However, excessively reducing N will lead to increased refining time and higher costs; therefore, the N content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0084] O: Below 0.0100%

[0085] O is an element that reduces CTOD properties and low-temperature toughness, therefore the upper limit of O content is limited to 0.0100%. O content is preferably 0.0090% or less. It is desirable to reduce O content as much as possible. There is no particular limit to the lower limit of O content, but excessive reduction of O will lead to increased refining time and increased costs. Therefore, O content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0086] The basic composition of the steel plate of the present invention (hereinafter sometimes simply referred to as "thick steel plate") consists of the aforementioned elements, plus the remainder Fe and unavoidable impurities. With this basic composition, the steel plate of the present invention achieves the desired characteristics.

[0087] Furthermore, in this invention, to further improve strength, CTOD properties, low-temperature toughness, etc., in addition to the basic components described above, one or more of the following components selected from Cu, Cr, Mo, V, Ti, Nb, B, Ca, W, REM, and Mg may be included in any quantity as shown below. It should be noted that the components Cu, Cr, Mo, V, Ti, Nb, B, Ca, W, REM, and Mg shown below can be included as needed, and therefore their content can be 0%.

[0088] Cu: below 1.50%

[0089] Cu can be added as needed. Cu is an element that can increase the strength of thick steel plates without significantly deteriorating their low-temperature toughness; however, if the Cu content exceeds 1.50%, surface cracking caused by the Cu-rich layer formed immediately below the oxide scale becomes a problem. Therefore, when Cu is present, it is preferable to limit the Cu content to 1.50% or less. More preferably, the Cu content is 1.30% or less. When Cu is present, from the perspective of fully obtaining its effect, the Cu content is preferably 0.05% or more.

[0090] Cr: below 1.50%

[0091] Cr can 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 decrease. Therefore, when Cr is present, the Cr content is preferably 1.50% or less. More preferably, the Cr content is 1.30% or less. When Cr is present, from the perspective of fully obtaining its effect, the Cr content is preferably 0.01% or more, more preferably 0.10% or more.

[0092] Mo: 1.50% or less

[0093] Mo can 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 properties and low-temperature toughness decrease. Therefore, when Mo is present, the Mo content is preferably 1.50% or less. More preferably, the Mo content is 1.30% or less. When Mo is present, from the perspective of fully obtaining its effect, the Mo content is preferably 0.10% or more.

[0094] V: Below 0.20%

[0095] Vitamin V can be added as needed. Vitamin V is an element that improves the strength of the base material, but if the V content exceeds 0.20%, the CTOD properties and low-temperature toughness decrease. Therefore, when containing V, it is preferable to limit the V content to 0.20% or less. More preferably, the V content is 0.15% or less. When containing V, from the perspective of fully obtaining its effect, the V content is preferably 0.01% or more.

[0096] Ti: below 0.100%

[0097] Ti can be added as needed. Ti precipitates in the steel in the form of TiN. The precipitated TiN has the effect of suppressing the coarsening of austenite grains, improving CTOD properties and low-temperature toughness by refining the crystal grain size. On the other hand, if the Ti content exceeds 0.100%, the precipitation of dissolved Ti and coarse TiC will actually reduce CTOD properties and low-temperature toughness. Therefore, when Ti is present, 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 present, from the perspective of fully obtaining its effect, the Ti content is preferably 0.005% or more.

[0098] Nb: below 0.100%

[0099] Nb can be added as needed. Nb is an element that effectively improves the strength of the base material, but an Nb content exceeding 0.100% will reduce CTOD properties and low-temperature toughness. Therefore, when Nb is present, it is preferable to limit the Nb content to 0.100% or less. More preferably, the Nb content is 0.050% or less. When Nb is present, from the perspective of fully obtaining its effect, the Nb content is preferably 0.005% or more.

[0100] B: Below 0.0050%

[0101] Boolean (B) can be added as needed. Boolean is an element that can improve hardenability and thus increase the strength of steel plates even in trace amounts; however, if the B content exceeds 0.0050%, the CTOD properties and low-temperature toughness decrease. Therefore, when B is present, it is preferable to set the B content to 0.0050% or less, more preferably 0.0030% or less. When B is present, from the perspective of fully obtaining its effect, the B content is preferably 0.0005% or more.

[0102] Ca: below 0.0100%

[0103] Ca is an element that improves the toughness of the heat-affected zone of a weld by forming oxysulfides with high stability at high temperatures. The presence of Ca does not impair the effectiveness of this invention, and therefore it can be added as needed. However, if the Ca content exceeds 0.0100%, coarse inclusions are formed, deteriorating the CTOD characteristics and low-temperature toughness. Therefore, when Ca is present, the Ca content is preferably limited to 0.0100% or less. More preferably, the Ca content is 0.0080% or less. When Ca is present, from the perspective of fully obtaining its effect, the Ca content is preferably 0.0002% or more, more preferably 0.0015% or more.

[0104] W: below 0.50%

[0105] W can be added as needed. W is an element that improves the strength of the base material, but if the W content exceeds 0.50%, weldability decreases. Therefore, when W is present, it is preferable to limit the W content to 0.50% or less. More preferably, the W content is 0.40% or less. When W is present, from the perspective of fully obtaining its effect, the W content is preferably 0.02% or more, more preferably 0.10% or more.

[0106] REM: below 0.025%

[0107] REM can be added as needed. REM (rare earth metals) are elements that improve weldability by forming oxygen-sulfide inclusions with high stability at high temperatures. On the other hand, if the REM content exceeds 0.025%, the effect of adding REM becomes saturated, and the effect matching the content cannot be expected, which is economically disadvantageous. Therefore, when REM is present, it is preferable to limit the REM content to 0.025% or less. The REM content is more preferably 0.020% or less. When REM is present, from the perspective of fully obtaining its effect, the REM content is preferably 0.001% or more, and more preferably 0.010% or more.

[0108] Furthermore, REM is a collective term for 17 elements, including the 15 lanthanide elements plus Y and Sc. These elements can be present individually or in combinations. Therefore, the content of REM refers to the total content of these elements.

[0109] Mg: below 0.0150%

[0110] Mg can be added as needed. Mg is an element that improves weldability by forming oxygen-sulfide inclusions with high stability at high temperatures. On the other hand, if the Mg content exceeds 0.0150%, the effect of Mg addition becomes saturated, and the effect matching the content cannot be expected, which is economically disadvantageous. Therefore, when Mg is present, 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 present, from the perspective of fully obtaining its effect, the Mg content is preferably 0.0002% or more.

[0111] The composition of the steel plates and sheets described above must further meet the following conditions.

[0112] Ceq: 0.300%~0.550%

[0113] In this invention, to ensure high strength and good low-temperature toughness in the center of the plate thickness, it is necessary to contain appropriate components. Specifically, it is important that the components are contained in such a way that Ceq, as defined by equation (1), satisfies the relationship shown in equation (2), that is, 0.300% ≤ Ceq ≤ 0.550%. If Ceq is greater than 0.550%, the amount of poorly tough microstructures such as island martensite increases, and the CTOD characteristics and low-temperature toughness deteriorate. On the other hand, if Ceq is less than 0.300%, the target strength of this invention cannot be ensured. Therefore, the range of Ceq is set to 0.300% to 0.550%. In addition, Ceq is preferably 0.310% or more, preferably 0.530% or less. Ceq is more preferably 0.320% or more, more preferably 0.520% or less.

[0114] Ceq=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5...(1)

[0115] 0.300%≤Ceq≤0.550%...(2)

[0116] Here, in equation (1), [] represents the content (mass%) of the element shown in []. When the element is not present, the content is set to zero.

[0117] [Hardness of the central segregated portion]

[0118] In this invention, in order to ensure the CTOD characteristics and the low-temperature toughness of the central part of the plate after strain aging, it is important to stipulate that the hardness of the central segregated part of the steel plate satisfies Equation (3).

[0119] Hv max / Hv ave ≤1.35+0.006 / [C]-t / 500...(3)

[0120] Here, [C] in equation (3) is the content of [C] (mass%), Hv max Hv is the maximum Vickers hardness of the central segregated region. ave It is the average Vickers hardness at positions 1 / 8 to 3 / 8 and 5 / 8 to 7 / 8 of the plate thickness, where t is the plate thickness (mm).

[0121] That is, Hv max / Hv ave This is a dimensionless parameter representing the hardness of the central segregated region. If this value is higher than the value obtained from (1.35 + 0.006 / [C] - t / 500), the stress concentration in the hardened region, i.e., the central segregated region, increases, making it prone to failure. As a result, the CTOD value and low-temperature toughness decrease; therefore, it is set to be below (1.35 + 0.006 / [C] - t / 500). Hv max / Hvave Preferably, it is below (1.25+0.006 / [C]-t / 500).

[0122] Here, Hv max It is measured using a Vickers hardness tester within a region containing the central segregation zone, extending from the center of the plate towards both surfaces with a thickness equal to 10% of the plate thickness. The measurement is performed at 0.5 mm intervals along the thickness of the plate, and the maximum measured value is recorded. Hv ave It is measured using a Vickers hardness tester at 1mm intervals along the thickness of the steel plate in the regions of 1 / 8 to 3 / 8 and 5 / 8 to 7 / 8 of the plate thickness. The average value is recorded. It should be noted that the load of the Vickers hardness tester is 10 kgf.

[0123] [Number density of segregated particles]

[0124] Number density of segregating particles with an equivalent circular diameter of 100 μm or more: 2.0 particles / mm 2 the following

[0125] As mentioned above, the central segregation within the steel plate becomes the starting point of failure, thus deteriorating the CTOD properties and low-temperature toughness after strain aging. This is especially true for segregated particles with an equivalent circular diameter of 100 μm or more, per 1 mm. 2 The number of segregated particles (hereinafter referred to as "segregated particle number density") exceeds 2.0 particles / mm. 2 At that time, the possibility of the crack tip opening displacement (δ) in the CTOD test and the low-temperature toughness after strain aging becoming insufficient values ​​becomes very high, thus making the number density of segregated particles 2.0 particles / mm. 2 The following is crucial. The preferred number density of segregated particles is 1.8 particles / mm. 2 The number of segregated particles should be minimized as much as possible. While there is no specific lower limit for this number density, excessively reducing the number of segregated particles will increase the manufacturing load; therefore, 0.1 particles / mm is preferred. 2 above.

[0126] In this invention, segregated particles refer to [Mn]. EPMA For regions where [Mn] ≥ 1.33, [Mn] EPMA [Mn] is the Mn concentration (mass%) at the measurement location, while [Mn] is the Mn content (mass%) of the entire steel plate.

[0127] Furthermore, regarding the measurement frequency of this segregated particle number density, it is only necessary to measure one or two sections of any steel plate from which the steel sheets have the same melting and rolling conditions. As long as the melting method and rolling conditions of the steel sheets are not changed, the number density of segregated particles can be manufactured with good reproducibility. Therefore, the measurement results at the above measurement frequency can be considered representative of the whole.

[0128] Specifically, the "number density of segregated particles" can be measured using the method described in the examples. Segregated particles tend to be abundant in the central segregated portion, so if the number density of segregated particles is measured over the entire thickness, this value will be smaller. To meet the target characteristics, controlling the number density of segregated particles in the center of the plate thickness is crucial; therefore, the number density of segregated particles is evaluated within a 3mm thick region including the central segregated portion, extending 1.5mm from the center of the plate thickness towards both surfaces of the steel plate.

[0129] [Average effective crystal grain size]

[0130] Average effective crystal grain size at the center of the plate thickness: less than 20 μm

[0131] In this invention, the average effective grain size at the center of the plate thickness is set to 20 μm or less. By refining the grain size at the center of the plate thickness, the strength, CTOD characteristics, and low-temperature toughness after strain aging can be improved. On the other hand, if the average effective grain size is greater than 20 μm, the coarse grains become the starting point of failure, and therefore, even if center segregation is reduced, the target CTOD characteristics and low-temperature toughness after strain aging cannot be obtained. The average effective grain size is preferably 18 μm or less. The smaller the average effective grain size, the more advantageous it is, so there is no particular lower limit, but excessive grain refinement will lead to an increase in manufacturing load, so 1 μm or more is preferred.

[0132] In this invention, "effective crystal grain size" is defined as the equivalent circle diameter of a grain surrounded by a grain boundary with an orientation difference of 15° or more from an adjacent grain, i.e., a large-angle grain boundary. The average effective crystal grain size can be determined using the method described in the examples.

[0133] [Manufacturing Method]

[0134] Next, the method for manufacturing the steel plate of the present invention will be described. The thick steel plate of the present invention is preferably manufactured by the method described below. Unless otherwise specified, the temperatures described below refer to the center temperature of the plate thickness. It should be noted that the center temperature of the plate thickness can also be measured, but it can also be calculated by heat transfer calculations based on the surface temperature of the steel plate measured with a radiation thermometer in an actual production line or the like.

[0135] [Continuous casting]

[0136] Continuous casting conditions

[0137] In this invention, to reduce segregation in the center of the plate thickness, steel sheets (slabs) meeting the above-mentioned compositional composition are manufactured by continuous casting. This continuous casting involves at least two light reductions at a reduction rate of 0.3 mm / min to 2.5 mm / min upstream of the final solidification position. If the reduction rate is less than 0.3 mm / min, the reduction per unit time is insufficient, failing to suppress the flow of enriched molten steel and thus failing to alleviate center segregation. On the other hand, if the reduction rate exceeds 2.5 mm / min, the reduction per unit time is excessive, pushing the enriched molten steel in the center of the slab upstream in the casting direction, resulting in negative segregation with reduced solute elements in the center of the slab. The reduction rate is preferably 0.4 mm / min or higher, and preferably 2.3 mm / min or lower. Furthermore, if the number of light reductions at a reduction rate of 0.3 mm / min to 2.5 mm / min is less than once, the effect of pushing the unsolidified molten steel upstream is insufficient, and the segregation reduction effect brought about by the light reduction is insufficient. There is no particular limit to the number of light pressing cycles, but from a cost-effectiveness perspective, it is preferable to perform 30 cycles or less. More preferably, it is preferable to perform 10 cycles or less.

[0138] Heating conditions for steel sheets before rolling: 990℃~1250℃

[0139] In this invention, the steel sheet is heated to a temperature of 990°C to 1250°C. If the heating temperature is below 990°C, the deformation resistance during rolling increases, the load on the rolling mill increases, the number of passes increases, and thus the manufacturing efficiency decreases. The preferred heating temperature is above 1000°C.

[0140] On the other hand, if the heating temperature exceeds 1250°C, the austenite grains coarsen, making it impossible to obtain the desired fine-grained structure, and resulting in a decrease in strength, CTOD characteristics, and low-temperature toughness. Therefore, the heating temperature is set below 1250°C, preferably below 1230°C.

[0141] [Hot Rolled]

[0142] Hot rolling conditions

[0143] The purpose of hot rolling in this invention is to appropriately introduce strain into the center of the plate thickness to obtain the desired fine-grained structure. Therefore, controlling both the rolling temperature range above 950°C and the rolling temperature above the Ar3 point but below 950°C is crucial.

[0144] Hot rolling in the recrystallization temperature range

[0145] The heated steel sheet is hot-rolled at a temperature above 950℃. If the hot-rolling temperature is below 950℃, recrystallization is difficult to occur, and the austenite grain refinement is insufficient.

[0146] Furthermore, in rolling processes with an average reduction rate per pass of less than 3.5%, sufficient strain cannot be introduced into the center of the plate thickness. Even if the average reduction rate per pass is 3.5% or higher, if the cumulative reduction rate is less than 40%, recrystallization cannot proceed sufficiently, and austenite grain refinement is inadequate. Therefore, rolling is performed at 950°C or higher, with an average reduction rate per pass of 3.5% or higher and a cumulative reduction rate of 40% or higher. The higher the average reduction rate per pass and the higher the cumulative reduction rate, the easier it is to introduce strain into the center of the plate thickness; therefore, the average reduction rate per pass is preferably 4.0% or higher. Additionally, the cumulative reduction rate is preferably 45% or higher. However, excessively increasing the reduction rate per pass increases the load on the rolling mill; therefore, the average reduction rate per pass is preferably 12.0% or lower. Furthermore, if the cumulative reduction rate is excessively increased, the target plate thickness cannot be ensured; therefore, the cumulative reduction rate is preferably 95% or lower.

[0147] Hot rolling in the non-recrystallization temperature range

[0148] After hot rolling in the aforementioned recrystallization temperature range, rolling is performed at a temperature above Ar3 and below 950°C, with an average reduction rate per pass of 3.5% or more and a cumulative reduction rate of 40% or more. Rolling in the non-recrystallization temperature range makes recrystallization difficult, thus the strain introduced by rolling is not consumed by recrystallization but accumulates, acting as phase transformation nuclei during subsequent cooling. As a result, the microstructure of the final thick steel sheet can be refined. However, in rolling with an average reduction rate per pass of less than 3.5%, sufficient strain cannot be introduced to the center of the sheet thickness, and in rolling with a cumulative reduction rate of less than 40%, the grain refinement effect is insufficient. Therefore, in this invention, it is crucial to simultaneously satisfy an average reduction rate per pass of 3.5% or more and a cumulative reduction rate of 40% or more. The higher the average reduction rate per pass and the higher the cumulative reduction rate, the easier it is to introduce strain to the center of the sheet thickness; therefore, the average reduction rate per pass is preferably 4.0% or more. Furthermore, the cumulative reduction rate is preferably 45% or more.

[0149] Here, the Ar3 point mentioned above can be obtained through actual measurement, but it can also be calculated by equation (4).

[0150] Ar3 point (℃) = 910-273[C]-74[Mn]-5[Cu]-56[Ni]-16[Cr]-9[Mo]...(4)

[0151] In formula (4), [] represents the content (mass%) of the element shown in []. When the element is not present, the content is set to zero.

[0152] Rolling end temperature: Ar3 point or above (°C)

[0153] The heated steel sheet is hot-rolled at a temperature above Ar3. The rolling end temperature is above Ar3. If the rolling end temperature is below Ar3, the microstructure before cooling becomes a dual-phase microstructure of austenite and ferrite, resulting in poor uniformity of the steel microstructure and a significant decrease in manufacturing stability. Therefore, the rolling end temperature is set above Ar3, preferably above (Ar3 + 10°C).

[0154] Cooling after hot rolling

[0155] After the hot rolling process, the resulting hot-rolled steel sheet is cooled. This cooling can be performed by any method, such as water cooling, provided the following conditions are met.

[0156] Average cooling rate: 3.0℃ / s or higher

[0157] If the average cooling rate at the center of the plate thickness is less than 3.0℃ / s, coarse ferrite phases will appear in the microstructure of the thick steel plate, leading to deterioration in strength, CTOD characteristics, and low-temperature toughness. Therefore, the average cooling rate at the center of the plate thickness is set to 3.0℃ / s or higher. Preferably, the average cooling rate is 4.0℃ / s or higher, and more preferably 100℃ / s or lower.

[0158] It should be noted that the average cooling rate in this invention refers to the average cooling rate from 800°C to 500°C. However, when the cooling stop temperature is above 500°C (described later), the average cooling rate from 800°C to the cooling stop temperature is used as the average cooling rate. Furthermore, when the rolling end temperature is below 800°C, the average cooling rate from the rolling end temperature to 500°C is used as the average cooling rate. Additionally, when the cooling stop temperature is above 500°C and the rolling end temperature is below 800°C, the average cooling rate from the rolling end temperature to the cooling stop temperature is used as the average cooling rate.

[0159] Cooling stop temperature: below 600℃

[0160] In the above cooling process, the hot-rolled steel sheet is cooled to a cooling stop temperature below 600°C at the center of the sheet thickness. If the cooling stop temperature is higher than 600°C, the microstructure after phase transformation becomes coarser, the base material strength is insufficient, and the CTOD characteristics and low-temperature toughness deteriorate. Therefore, the cooling stop temperature is set below 600°C. Preferably, the cooling stop temperature is below 570°C.

[0161] [Tempering treatment]

[0162] Tempering temperature: below 700℃

[0163] After cooling stops, tempering can be performed at will. Tempering can improve the toughness of the base material. On the other hand, if the tempering temperature exceeds 700°C, various carbonitrides will precipitate in the steel, and the fine microstructure obtained from the phase transformation will disappear, resulting in a decrease in strength and toughness. Therefore, the tempering temperature is preferably below 700°C. More preferably, it is below 650°C. Furthermore, the tempering temperature is preferably above 300°C.

[0164] It should be noted that in the manufacturing method described in this invention, conventional methods can be used for items not described in this specification.

[0165] Example

[0166] The present invention will now be described in more detail based on embodiments. The following embodiments represent preferred examples of the invention, but the invention is not limited to these embodiments in any way.

[0167] Thick steel plates were manufactured using steel sheets with the compositions shown in Table 1, under the manufacturing conditions shown in Table 2. The continuous casting light reduction conditions during steel sheet manufacturing are also recorded in Table 2. It should be noted that during hot rolling, thermocouples were installed at the center positions along the length, width, and thickness directions of the rolled steel sheet, and the temperature at the center of the thickness of the sheet was measured. Simultaneously, the surface temperature of the steel sheet was measured using a radiation thermometer. The “-” in Table 1 indicates that the element was intentionally omitted, including both cases where the element was not present and cases where its presence was unavoidable.

[0168]

[0169]

[0170] For each of the obtained thick steel plates, the number density of segregated particles, Vickers hardness, average effective grain size, yield strength, tensile strength, low-temperature toughness after 5% strain aging, and CTOD value were determined using the following methods.

[0171] [Number density of segregated particles]

[0172] To prepare the test samples, a rectangular steel bar with a width of 500 mm and a thickness of 3 mm was cut from the central portion of the obtained thick steel plate in both the width and thickness directions. The cut steel bar was further divided into 20 equal parts along the width direction, resulting in 20 test samples with a width of 25 mm. The surface of the test sample perpendicular to the rolling direction (i.e., the surface with a width of 25 mm and a thickness of 3 mm) was mirror-polished, and immediately used as the test surface, quantitative analysis of Mn was performed using an electron probe microanalyzer (EPMA).

[0173] In this invention, the measured Mn concentration is defined as [Mn]. EPMA [Mn] EPMA Regions with a ratio of Mn ≥ 1.33 are defined as segregated particles. The number density of segregated particles is obtained by dividing the number of segregated particles with an equivalent circle diameter of 100 μm or more by the measured area.

[0174] [EPMA Measurement Conditions]

[0175] Accelerating voltage: 20kV

[0176] Irradiation current: 0.2μA

[0177] Integrating time: 0.05 seconds

[0178] Beam diameter: 10μm

[0179] Measurement range: 3mm high × 25mm wide × 20 samples

[0180] Vickers hardness

[0181] A section perpendicular to the width direction and parallel to the rolling direction of the steel plate was taken from the center of the width and mirror-polished. Then, in the prepared sample, a Vickers hardness tester was used to measure the hardness at 1mm intervals along the thickness direction of the steel plate, within a region containing the central segregation zone and extending from the center of the plate towards both surfaces, with a thickness of 10% of the plate thickness. The maximum value obtained was taken as Hv. max In addition, within the regions of 1 / 8 to 3 / 8 and 5 / 8 to 7 / 8 of the plate thickness, Vickers hardness tests were performed at 1 mm intervals along the thickness direction of the steel plate, and the average value of the obtained measurements was taken as Hv. ave The load on the Vickers hardness tester is 10 kgf.

[0182] [Average effective crystal grain size]

[0183] From the obtained thick steel plate, a sample is collected with the center of the plate in the length, width, and thickness directions as the measurement location, including the center of the plate thickness. The sample surface is then mirror-polished, followed by EBSD analysis under the following conditions. From the obtained crystal orientation diagram, the equivalent circle diameter of the microstructure surrounded by large-angle grain boundaries with an orientation difference of 15° or more from adjacent grains is determined, and the average value of the equivalent circle diameter in the analysis region is taken as the average effective grain size.

[0184] [EBSD analysis conditions]

[0185] Analysis area: 1mm x 1mm area at the center of the plate thickness

[0186] Step size: 0.4μm

[0187] [Tension Test]

[0188] JIS No. 4 test specimens were collected from half the thickness of the obtained thick steel plate, with the length direction of the specimen perpendicular to both the rolling direction and the thickness direction of the steel plate. Yield strength (YS) and tensile strength (TS) were determined according to JIS Z 2241. In the tensile test, if an upper yield point was observed, the upper yield stress was taken as the yield strength. Otherwise, if no upper yield point was observed, 0.2% of the yield strength was taken as the yield strength. A YS ≥ 320 MPa was considered high strength. Furthermore, a YS ≥ 320 MPa and TS ≥ 430 MPa were considered good tensile properties.

[0189] [Strain-Aging Charpy Impact Test]

[0190] Tensile test specimens (JIS 14B) were collected from the midpoint of the obtained thick steel plate, perpendicular to the rolling direction, and subjected to tensile testing. A 5% plastic strain was applied along the length of the specimen. After introducing the pre-strain, an aging treatment was performed at 250°C for 1 hour to provide a strain aging effect. Three JIS V-notch specimens were collected from the center of the aged tensile test plate, with the length of the specimen aligned with the longitudinal direction of the tensile test plate. The absorbed energy (vE) at -60°C was measured according to JIS Z 2242. -60 ), calculate its average value. Then, vE -60 An average value of 50J or higher indicates good Charpy impact characteristics.

[0191] [CTOD test]

[0192] A full-thickness CTOD test piece is collected with its length direction perpendicular to the rolling direction to measure the crack tip opening displacement. The test method is based on ISO 12135, evaluating the crack tip opening displacement [CTOD value (δ)] at a test temperature of -55°C. For each thick steel plate, three test pieces are used, and the minimum measured value is taken as δ. In this invention, a CTOD characteristic of δ ≥ 0.10 mm is considered good.

[0193] Table 3 also records the evaluation results. The inventive example meets the composition and manufacturing conditions of the present invention, and satisfies the conditions for hardness of the central segregated portion, number density of segregated particles, and average effective crystal grain size, exhibiting high strength and excellent CTOD characteristics and low-temperature toughness after strain aging. In contrast, the thick steel plate (comparative example) that does not meet the conditions of the present invention has poor performance in one or more of the following aspects: strength, CTOD characteristics, and low-temperature toughness after strain aging, and its properties are inferior to those of the inventive example.

[0194]

Claims

1. A steel plate, comprising, by mass percent: C：0.03～0.15%、 Si: below 0.50% Mn: 0.3–2.5%, P: below 0.030% S: Below 0.0050% Ni: 0.01~5.00% Al：0.005～0.100%、 N: below 0.0100%, and O: Below 0.0100%, Furthermore, Ceq, as defined by equation (1), satisfies equation (2), with the remainder being Fe and unavoidable impurities. Furthermore, the hardness of the central segregated portion of the steel plate satisfies equation (3). The number density of segregating particles with an equivalent spherical diameter of 100 μm or more is 2.0 particles / mm. 2 the following, The average effective crystal grain size at the center of the plate thickness is less than 20 μm. in, In equations (1) and (3), [] represents the content of the element within [] in terms of mass %; when the element is not present, the content is set to zero. Equation (3) shows Hv max Hv is the maximum Vickers hardness of the central segregated region. ave It is the average Vickers hardness at positions 1 / 8 to 3 / 8 and 5 / 8 to 7 / 8 of the plate thickness, where t is the plate thickness in mm.

2. The steel plate according to claim 1, wherein, The composition of the ingredients, expressed in % by mass, further contains ingredients selected from: Cu: below 1.50% Cr: less than 1.50% Mo: 1.50% or less, V: Below 0.20% Ti: below 0.100% Nb: below 0.100% B: Below 0.0050% Ca: below 0.0100% W: Below 0.50% REM: below 0.025%, and Mg: below 0.0150% One or more of them.

3. A method for manufacturing a steel plate, which is the method for manufacturing a steel plate according to claim 1 or 2. The steel sheet with the aforementioned composition, manufactured by continuous casting, is heated to a heating temperature of 990°C to 1250°C. The continuous casting involves two or more light presses at a pressing speed of 0.3 mm / min to 2.5 mm / min upstream of the final solidification position of the slab. Furthermore, rolling is performed where the average reduction rate per pass is 3.5% or more and the cumulative reduction rate is 40% or more when the temperature at the center of the plate thickness is 950°C or higher. Then, rolling is performed where the average reduction rate per pass is 3.5% or more and the cumulative reduction rate is 40% or more when the temperature at the center of the plate thickness is Ar3 or higher and lower than 950°C. Furthermore, the cooling stop temperature at the center of the plate thickness is set to below 600°C, with an average cooling rate of 3.0°C / s or higher.

4. The steel plate manufacturing method according to claim 3, wherein, After cooling to the cooling stop temperature, tempering is performed at a temperature below 700°C.

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