Thick steel sheet and manufacturing method therefor

Abstract

Provided is a thick steel sheet that has a thickness exceeding 100 mm, and that has high strength, as well as excellent low temperature toughness, excellent deformation performance in the sheet thickness center section, and excellent crack tip opening displacement (CTOD) property of a multi-layer weld joint. The thick steel sheet comprises, in mass %, C: 0.03-0.20%, Si: 0.50% or less, Mn: 0.3-3.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005-0.100%, B: 0.0001-0.0050%, N: 0.0100% or less, and O: 0.0100% or less, and at least one selected from the group consisting of Cr: 0.01-2.00%, Mo: 0.01-2.00% and Ni: 0.01-5.00%, with the remainder being Fe and inevitable impurities. The steel sheet has a component composition satisfying formula (1). The maximum prior austenite grain size of the sheet in the section at the center of the sheet thickness is 150 μm or less. The number density of porosities having an equivalent circle diameter of 100 μm or more is 0.10 pcs / mm2 or less. The number density of MnS having a thickness of 30 μm or more in the plate thickness direction is 10 pcs / mm2 or less. The sheet thickness exceeds 100 mm.

Description

Heavy steel plate and its manufacturing method

[0001] The present invention relates to a steel plate and a method for manufacturing the same.

[0002] As steel structures such as ships, marine structures, pressure vessels, penstocks, and offshore wind turbine installation vessels become larger, there is an increasing demand for higher strength and thicker steel materials. Furthermore, since the steel materials used in these steel structures are welded together to form structures of desired shapes, from the perspective of structural safety, the base material must not only have high strength and excellent toughness, but also be excellent in deformability at the center of the plate thickness and in toughness at welded joints.

[0003] Conventionally, the Charpy test has been mainly used to evaluate the toughness of steel, but in recent years, as a method for evaluating fracture resistance with higher accuracy, the Crack Tip Opening Displacement Test (hereinafter referred to as the "CTOD test") has been increasingly applied to thick steel plates used in steel structures. In this test, a test specimen with a fatigue pre-crack introduced in the toughness evaluation section is subjected to three-point bending at low temperature, and the amount of crack opening (amount of plastic deformation) just before fracture is measured to evaluate the resistance to brittle fracture.

[0004] 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.

[0005] When thick steel plates are applied to steel structures such as ships, marine structures, pressure vessels, penstocks, and ships for installing offshore wind turbines, multi-layer welding is used. In the heat affected zone (Heat Affected Zone: hereinafter also referred to as "Multi-layer Weld HAZ") of this multi-layer welding, a region near the weld line that has become a coarse structure due to a preceding welding pass (Coarse Grain Heat Affected Zone: hereinafter also referred to as "CGHAZ") is reheated to a two-phase region of ferrite + austenite by a subsequent welding pass, and an island martensite (Martensite-Austenite Constituent: hereinafter also referred to as "MA") structure is mixed in the coarse base structure. It is known that this region (Inter-Critically Reheated Coarse Grain Heat Affected Zone: hereinafter also referred to as "ICCGHAZ") becomes a locally embrittled region.

[0006] Conventionally, techniques for improving the toughness of a weld heat affected zone (HAZ) include suppressing the coarsening of austenite grains in the CGHAZ by finely dispersing TiN, and utilizing TiN as ferrite transformation nuclei.

[0007] In addition, techniques have been used to inhibit the growth of austenite grains by dispersing REM-based oxysulfides formed by adding REM (rare earth metals), to inhibit the growth of austenite grains by dispersing Ca-based oxysulfides formed by adding Ca, and to combine the ferrite nucleation ability of BN with oxide dispersion.

[0008] For example, Patent Documents 1 and 2 disclose techniques for improving the toughness of welds by adding REM together with Ti to disperse fine particles in steel, thereby suppressing the grain growth of austenite. Patent Document 3 also proposes a technique for improving HAZ toughness by using CaS and a technique for improving base metal toughness by hot rolling.

[0009] In recent years, steel structures such as ships, marine structures, pressure vessels, penstocks, and ships for installing offshore wind turbines have tended to become larger, and as a result, the steel plates used in these structures have become thicker and stronger. To achieve both thicker and stronger steel plates, the amount of alloying elements added must be increased. However, adding large amounts of alloying elements makes it more difficult to ensure the toughness of the multi-pass weld HAZ. To address this issue, Patent Document 4 discloses a technology for improving low-temperature toughness by controlling the hardness of the central segregation region.

[0010] JP 60-152626 A JP 60-184663 A JP 2012-184500 A JP 2013-91845 A

[0011] Recent offshore wind turbine installation ships and the legs of jack-up rigs for marine structures use thick steel plates with a yield strength of 650 MPa or more and a thickness of over 100 mm, and these thick steel plates are required to have excellent CTOD properties in the weld heat-affected zone.

[0012] However, the techniques for improving the CTOD characteristics of the weld heat affected zone described in Patent Documents 1 to 3 are intended for steel materials with relatively low strength or steel materials with a small content of alloy elements due to a thin plate thickness, and cannot be applied to steel materials with higher strength or steel materials with a thick plate thickness and a high content of alloy elements because the HAZ structure does not contain ferrite.

[0013] Furthermore, Patent Document 4 proposes a technology for satisfying the joint CTOD characteristics in a low temperature range for thick steel plates having a plate thickness of 150 mm or less, but does not pay attention to the deformability at the center of the plate thickness, and it cannot be said that the deformability at the center of the plate thickness is sufficiently ensured. Furthermore, no consideration has been given to thick steel plates having a plate thickness of more than 150 mm, for which it is more difficult to ensure material properties.

[0014] As described above, according to the investigations of the present inventors, the conventional techniques described in Patent Documents 1 to 4 have been unable to simultaneously and sufficiently improve the low-temperature toughness, deformation performance in the plate thickness direction, and CTOD characteristics of joints subjected to multi-pass welding (hereinafter also referred to as "multi-pass welded joint CTOD characteristics") in high-strength, thick steel plates having a plate thickness of more than 100 mm, which are in demand in recent years.

[0015] The present invention has been made in consideration of the above-mentioned problems of the prior art, and aims to provide a thick steel plate having a thickness of more than 100 mm, which has high strength, excellent low-temperature toughness, excellent deformability in the thickness center, and excellent CTOD characteristics of multi-pass welded joints, together with a manufacturing method thereof. Here, in the present invention, "high strength" refers to a yield strength of 650 MPa or more in a tensile test at the thickness center, "excellent low-temperature toughness" refers to an absorbed energy of 100 J or more in a Charpy test at -40°C at the thickness center, "excellent deformability in the thickness center" refers to a reduction of area of ​​25% or more in a tensile test through the thickness, and "excellent CTOD characteristics of multi-pass welded joints" refers to a crack tip opening displacement of 0.10 mm or more in a CTOD test at a test temperature of -10°C with the weld bond as the notch position. In the present invention, the "center 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 the deformation performance at the center of the plate thickness and the CTOD characteristics of multi-pass welded joints while achieving both high strength and excellent low-temperature toughness in thick steel plates with a plate thickness of more than 100 mm. As a result, they have obtained the following findings.

[0017] (A) When attempting to increase the strength and thickness of steel plates, the amount of alloying elements added to ensure the necessary properties increases, which makes it more likely that porosity will increase and become larger during slab production. If porosity generated during slab production remains unbonded during rolling, it can become the origin of fracture within the steel plate. In order to bond porosity in the center of the plate thickness, it is necessary to appropriately introduce strain into the center of the plate thickness during rolling. However, this is difficult for thick steel plates with a plate thickness of over 100 mm, which makes the problem of unbonded remaining porosity more likely to become apparent. However, as a result of the inventors' studies, it was found that sufficient strain can be introduced into the center of the plate thickness and porosity can be sufficiently bonded by controlling the temperature of the center and surface of the steel plate in each rolling pass to set the average X to 0.70 or less, and further by rolling with an average reduction rate / pass of 3.5% or more so that the cumulative reduction rate is 35% or more.

[0018] (B) When manufacturing high-strength steel plates with a thickness of over 100 mm, the addition of large amounts of alloying elements is necessary, which tends to result in element segregation regions in the center of the slab. The concentration of alloying elements in these regions leads to the formation of coarse inclusions such as MnS. These coarse inclusions act as fracture initiation sites, reducing the through-thickness tensile properties (reduction of area) and CTOD properties. However, it has been discovered that by controlling the temperatures at the center and surface of the steel plate in each rolling pass to set the average X to 0.70 or less, the deformation resistance at the center and surface of the steel plate can be controlled, and by rolling with an average reduction rate / pass of 3.5% or more so that the cumulative reduction rate is 35% or more, the strain applied to the center of the plate thickness can be increased, and the coarse inclusions can be made sufficiently thin, thereby reducing the adverse effects on the material properties.

[0019] (C) In thick steel plates with a thickness of more than 100 mm, the cooling rate at the center of the plate thickness is small, which causes the problem of coarsening of crystal grains in such locations. However, it has been found that by controlling the temperatures at the center and surface of the steel plate in each rolling pass to set the average of X to 0.70 or less, the deformation resistance at the center and surface of the steel plate can be controlled, and further, by performing rolling with an average reduction rate / pass of 3.5% or more so that the cumulative reduction rate is 35% or more, it is possible to introduce sufficient strain into the center of the plate thickness, promote austenite recrystallization, enable austenite refinement and grain regulation, and make it possible to set the maximum prior austenite grain size to 150 μm or less.

[0020] In addition, the inventors have found that the combination of (A) and (B) above contributes greatly to improving the CTOD characteristics of multi-layer welded joints, and the combination of (A) and (C) above contributes greatly to improving the reduction of area, and that these together make it possible to solve the problem.

[0021] The present invention has been completed based on the above findings and further investigations. That is, the gist of the present invention is as follows.

[0022] 1. A steel sheet comprising, by mass%, one or more elements selected from the group consisting of C: 0.03 to 0.20%, Si: 0.50% or less, Mn: 0.3 to 3.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, and Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, and Ni: 0.01 to 5.00%, with the balance consisting of Fe and unavoidable impurities, and having a chemical composition that satisfies the following formula (1), wherein the maximum prior austenite grain size in the center of the sheet thickness is 150 μm or less, The number density of porosity having a circle equivalent diameter of 100 μm or more is 0.10 pieces / mm ２ The number density of MnS particles having a thickness of 30 μm or more in the plate thickness direction is 10 particles / mm ２A thick steel plate having a composition ratio of 0.1 to 1.0, and a thickness of more than 100 mm, where Ceq (= [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5) ≥ 0.580% (1) In the formula, [ ] represents the content (mass%) of the element in [ ], and is set to zero if the element is not contained.

[0023] 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: 2.00% or less, V: 0.20% or less, Ti: 0.10% or less, Nb: 0.10% or less, Ca: 0.010% or less, W: 0.50% or less, REM: 0.025% or less, and Mg: 0.0150% or less.

[0024] 3. A method for producing a steel plate according to the above 1 or 2, comprising the steps of: ３ After heating to 1250°C, the average of X calculated by the following formula (2) is 0.70 or less, and the average reduction rate / pass is 3.5% or more. ３ The rolling is performed so that the cumulative rolling reduction is 35% or more at a temperature above the rolling point, and then the rolled steel sheet is subjected to the following steps: (i) subjecting the rolled steel sheet to Ar ３ (ii) a step of quenching the steel plate by rapidly cooling it from a temperature above the quenching point to 300°C or less; ３ The steel plate is quenched by rapidly cooling from a temperature above the temperature point to 300°C or below, and then ３ (iii) a process of reheating the steel sheet to a temperature of 1000 to 1050°C and then rapidly cooling it to 300°C or less once or twice or more times, and then quenching it again; or ３ and (c) subjecting the steel plate to a tempering treatment at a temperature of 450 to 700°C. In the formula, [C] is the C content (mass%) of the steel slab, and T ｋ(板厚中心) is the absolute temperature (K) at the center of the plate thickness just before the start of rolling, and T ｋ(鋼板表面)is the absolute temperature (K) of the steel plate surface immediately before the start of rolling. 4. The method for producing a steel plate according to 3 above, wherein the tempering treatment is carried out under conditions in which the steel plate is held at a temperature of 450 to 750°C for less than 2.3t minutes, where t is the thickness of the steel plate (mm).

[0025] According to the present invention, it is possible to provide a thick steel plate having a thickness of more than 100 mm, which has high strength, low-temperature toughness, excellent deformation performance in the center of the plate thickness, and excellent CTOD characteristics of multi-pass welded joints, together with a manufacturing method thereof, and this is extremely useful in industry.

[0026] The reasons for limiting each of the constituent elements of the present invention will be explained below.

[0027] [Composition] First, the reasons for limiting the composition of the steel plate and steel billet to the above ranges in the present invention will be explained. Note that "%" regarding the composition means "mass %" unless otherwise specified.

[0028] C: 0.03 to 0.20% C is an element that improves hardenability and improves the strength of steel, and a content of 0.03% or more is required. However, if the C content exceeds 0.20%, the hardness of the C-enriched portion increases, and the joint CTOD characteristics deteriorate. Therefore, the C content is set to 0.03% or more, preferably 0.05% or more, and 0.20% or less, preferably 0.15% or less.

[0029] 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%, the joint CTOD characteristics will deteriorate. 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.

[0030] Mn: 0.3 to 3.0% Mn is an element that has the effect of improving the strength of the base metal and welds by improving the hardenability of the steel. However, if the Mn content exceeds 3.0%, not only does the weldability decrease, but the hardenability also becomes excessive, reducing the toughness of the base metal and welds and deteriorating the joint CTOD characteristics. Therefore, the Mn content is set to 0.3% or more, preferably 0.8% or more, and 3.0% or less, preferably 2.8% or less.

[0031] 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 HAZ toughness and joint CTOD characteristics, so the P content is limited to 0.030% or less, 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 higher costs, so the P content is preferably 0.001% or more.

[0032] S: 0.0050% or less Since S is an element that deteriorates the CTOD characteristics of a joint, the S content is limited to 0.0050% or less, and 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, but excessively reducing the S content increases the refining time and costs, so the S content is preferably 0.0001% or more.

[0033] Al: 0.005 to 0.100% Al is an element added to deoxidize molten steel. When it is contained at 0.005% or more, it forms nitrides in the steel, reducing the amount of dissolved nitrogen and suppressing the precipitation of BN, thereby ensuring the B necessary for quenching and improving hardenability. On the other hand, if the Al content exceeds 0.100%, the base material toughness and joint CTOD characteristics decrease, so 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.

[0034] B: 0.0001 to 0.0050% B is an element that can improve hardenability even with a very small amount, thereby improving the strength of the steel sheet, and this effect can be obtained with a content of 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, the HAZ toughness decreases and the joint CTOD characteristics deteriorate, so the B content is set to 0.0050% or less, and preferably 0.0030% or less. The B content is preferably 0.0005% or more.

[0035] N: 0.0100% or less N is an element that reduces HAZ toughness and deteriorates joint CTOD characteristics, so the upper limit of the N content is limited to 0.0100%, 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, but excessively low N content leads to increased refining time and higher costs, so the N content is preferably 0.0005% or more.

[0036] O: 0.0100% or less Since O is an element that reduces HAZ toughness and deteriorates joint CTOD characteristics, the upper limit of the O content is limited to 0.0100%, preferably 0.0090% or less. It is desirable to reduce the O content as much as possible, and there is no restriction on the lower limit of 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.

[0037] One or more elements selected from the group consisting of Cr: 0.01-2.00%, Mo: 0.01-2.00%, and Ni: 0.01-5.00%. The component composition includes one or more elements selected from the group consisting of Cr: 0.01-2.00%, Mo: 0.01-2.00%, and Ni: 0.01-5.00%. Cr and Mo are elements that improve the hardenability and strength of steel, and Ni is an element that is effective in improving the strength of the base metal and the CTOD characteristics of the joint. The Cr content is preferably 0.10% or more, the Mo content is preferably 0.10% or more, and the Ni content is preferably 0.10% or more. The Cr content is 2.00% or less, preferably 1.80% or less, the Mo content is 2.00% or less, preferably 1.80% or less, and the Ni content is 5.00% or less, preferably 4.80% or less. When the Cr content and the Mo content are within the above ranges, deterioration of the joint CTOD characteristics can be easily avoided. Furthermore, when the Ni content is within the above range, an increase in cost can also be suppressed.

[0038] The basic chemical composition of the steel plate and steel slab of the present invention consists of the above elements with the balance being Fe and inevitable impurities. In addition to the above chemical composition, one or more elements selected from the group consisting of Cu, V, Ti, Nb, Ca, W, REM, and Mg may be optionally further contained in the amounts shown below, for the purpose of further improving strength, base metal toughness, joint toughness, etc.

[0039] Cu: 2.00% or less Cu is an element that can increase the strength of steel plates without significantly deteriorating the base material toughness and joint toughness, but if the Cu content exceeds 2.00%, surface cracking due to the Cu-enriched layer that forms directly below the scale becomes a problem. Therefore, when Cu is contained, the Cu content is limited to 2.00% or less, preferably 1.50% or less. When Cu is contained, in order to fully obtain its effects, the Cu content is preferably 0.05% or more.

[0040] V: 0.20% or less V is an element that improves the strength of the base metal, but if the V content exceeds 0.20%, the HAZ toughness decreases and the joint CTOD characteristics deteriorate. Therefore, when V is contained, the V content is limited to 0.20% or less, and preferably 0.15% or less. When V is contained, in order to fully obtain its effects, the V content is preferably 0.01% or more.

[0041] Ti: 0.10% or less Ti precipitates in steel as TiN. The precipitated TiN has the effect of suppressing the coarsening of austenite grains in the base material and HAZ, thereby refining the HAZ structure and improving joint CTOD characteristics. On the other hand, if the Ti content exceeds 0.10%, the precipitation of solute Ti and coarse TiC actually reduces HAZ toughness and deteriorates joint CTOD characteristics. Therefore, when Ti is contained, the Ti content is limited to 0.10% or less, preferably 0.05% or less, and more preferably 0.04% or less. When Ti is contained, the Ti content is preferably 0.005% or more in order to fully obtain its effects.

[0042] Nb: 0.10% or less Nb is an element that is effective in improving the strength of the base metal, but a content exceeding 0.10% reduces the joint CTOD characteristics. Therefore, when Nb is contained, the Nb content is limited to 0.10% or less, and preferably 0.05% or less. When Nb is contained, in order to fully obtain its effects, the Nb content is preferably 0.005% or more.

[0043] Ca: 0.010% or less Ca is an element that improves the toughness of multi-pass welding HAZ by forming oxysulfides that are highly stable at high temperatures, but a content of more than 0.010% actually reduces the joint CTOD characteristics. Therefore, when Ca is contained, the Ca content is limited to 0.010% or less, and preferably 0.008% or less. When Ca is contained, in order to fully obtain its effects, the Ca content is preferably 0.0002% or more.

[0044] W: 0.50% or less W is an element that improves the strength of the base metal, but if the W content exceeds 0.50%, the HAZ toughness decreases and the joint CTOD characteristics deteriorate. Therefore, when W is contained, the W content is limited to 0.50% or less, and preferably 0.40% or less. When W is contained, in order to fully obtain its effects, the W content is preferably 0.05% or more.

[0045] REM: 0.025% or less REM (rare earth metals) form oxysulfide inclusions, thereby suppressing austenite grain growth in the HAZ and improving HAZ toughness, but if the REM content exceeds 0.025%, the base material toughness and HAZ toughness actually decrease, and the joint CTOD characteristics deteriorate. Therefore, when REM is contained, the REM content is limited to 0.025% or less, and preferably 0.020% or less. When REM is contained, in order to fully obtain its effects, the REM content is preferably 0.001% or more.

[0046] Mg: 0.0150% or less Mg is an element that forms oxide-based inclusions to suppress the growth of austenite grains in the weld heat-affected zone and improve HAZ toughness. However, if the Mg content exceeds 0.0150%, the effect of adding it becomes saturated, and no effect commensurate with the content can be expected, which is economically disadvantageous. Therefore, if Mg is contained, the Mg content is limited to 0.0150% or less, and preferably 0.0100% or less. If Mg is contained, the Mg content is preferably 0.0002% or more in order to fully obtain its effect.

[0047] The chemical compositions of the steel plates and billets must also satisfy the following conditions.

[0048] Ceq: 0.580% or more In the present invention, appropriate elements must be contained to ensure high strength and good toughness at the center of the plate thickness. It is important that the elements are contained so as to satisfy the relationship Ceq ≥ 0.580% defined by formula (1). Preferably, Ceq ≥ 0.600%. Ceq (= [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5) ≥ 0.580% (1) In the formula, [ ] represents the content (mass%) of the element shown in [ ], and is set to zero if the element is not contained.

[0049] [Maximum prior austenite grain size] Maximum prior austenite grain size at the center of plate thickness: 150 μm or less In the present invention, in a thick steel plate having a thickness of more than 100 mm, the maximum prior austenite grain size at the center of plate thickness is set to 150 μm or less. By refining and regulating the prior austenite grains at the center of plate thickness, where segregation is likely to exist, as described above, it is possible to improve the toughness of the base material and also improve drawing characteristics. The maximum prior austenite grain size is preferably 130 μm or less. In the present invention, the "prior austenite grain size" refers to the circle-equivalent diameter of a prior austenite grain, and the "maximum prior austenite grain size" refers to the maximum value of the measured "prior austenite grain size." The "maximum prior austenite grain size" can be measured by the method described in the Examples.

[0050] [Number density of porosity] Number density of porosity having a circle equivalent diameter of 100 μm or more: 0.10 pieces / mm ２ As mentioned above, the porosity remaining in the steel sheet becomes the starting point of fracture, and therefore deteriorates the CTOD characteristics and reduction of area. ２ The number of porosity particles per square meter (hereinafter also referred to as "porosity density") is set to 0.10 particles / mm ２ By setting the number density of porosity to 0.10 pieces / mm or less, the target crack tip opening displacement and reduction characteristics can be obtained. ２ It is important that the porosity density is preferably 0.09 pieces / mm ２The lower limit of the porosity number density is not particularly limited. In the present invention, the porosity number density refers to the average number density across the entire thickness x width in a thickness direction cross section (cross section perpendicular to the rolling direction) parallel to the width direction of the thick steel plate. Furthermore, the measurement frequency of the porosity number density can be measured by measuring one or two cross sections of any one steel plate among steel plates that have the same slab melting conditions and rolling conditions. Since the porosity number density can be produced with good reproducibility unless the slab melting method or rolling conditions are changed, it can be said that the measurement results at the above measurement frequency represent the whole. Specifically, the "porosity number density" can be measured by the method described in the Examples.

[0051] [Number density of MnS] Number density of MnS having a thickness of 30 μm or more in the plate thickness direction: 10 pieces / mm ２ As mentioned above, coarse MnS remaining in the steel sheet becomes the starting point of fracture, and reduces the drawing characteristics and CTOD characteristics. ２ The number of particles per square millimeter (hereinafter also referred to as "MnS particle density") is set to 10 particles / mm ２ By setting the number density of MnS to 10 particles / mm or less, the target crack tip opening displacement and reduction characteristics can be obtained. ２ It is important that the number density of MnS is preferably 9 particles / mm ２ The number density of MnS in the present invention refers to the average number density across the entire thickness x width in a thickness direction cross section parallel to the width direction of the steel plate (cross section perpendicular to the rolling direction). Furthermore, the frequency of measurement of the number density of MnS can be determined by measuring one or two cross sections of any one steel plate among steel plates that have the same slab melting conditions and rolling conditions. Since the number density of MnS can be produced with good reproducibility unless the slab melting method or rolling conditions are changed, it can be said that the measurement results at the above measurement frequency represent the whole. Specifically, the "number density of MnS" can be measured by the method described in the Examples.

[0052] The steel plate of the present invention has a thickness of more than 100 mm. From the viewpoint of application to large steel structures, the thickness is preferably 150 mm or more. The thickness is preferably 300 mm or less, since it can be easily avoided that the thickness becomes too thick and it becomes difficult to obtain the desired reduction during rolling.

[0053] [Manufacturing method] The steel plate of the present invention can be 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 production line, it may also be determined by heat transfer calculation from the steel plate surface temperature measured with a radiation thermometer.

[0054] The slabs used in the manufacturing method of the present invention are not particularly limited as long as they satisfy the above-mentioned conditions of chemical composition. The method for producing the slabs is not particularly limited, and any known method such as a converter, an electric furnace, or a vacuum melting furnace is suitable. The slabs are produced, for example, by a continuous casting method. Furthermore, the molten steel produced from such slabs may be further subjected to secondary refining such as ladle refining.

[0055] Heating conditions for steel billets: Ac ３ In the present invention, the steel billet is ３ Heat to 1250°C. ３ If the heating temperature is lower than this point, the austenite single phase region is not achieved, resulting in poor uniformity of the steel structure and a significant decrease in manufacturing stability. In addition, the deformation resistance during rolling increases, which increases the load on the rolling mill and increases the number of passes, resulting in a decrease in manufacturing efficiency. Furthermore, it becomes difficult to compress porosity and reduce coarse MnS. Therefore, the heating temperature is set to Ac ３ point or more, preferably (Ac ３ 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 toughness and drawing characteristics of the base material deteriorate. Therefore, the heating temperature is set to 1250°C or lower, and preferably 1200°C or lower. ３ The points can be obtained by actual measurement, but may also be calculated from the following formula (3): ３Point (°C) = 937.2 - 436.5 [C] + 56 [Si] - 19.7 [Mn] - 16.3 [Cu] - 26.6 [Ni] - 4.9 [Cr] + 38.1 [Mo] + 124.8 [V] + 136.3 [Ti] - 19.1 [Nb] + 198.4 [Al] + 3315 [B] ... (3) In the formula, [ ] represents the content (mass%) of the element shown in [ ], and is set to zero if the element is not contained.

[0056] Hot rolling conditions: The purpose of hot rolling in this invention is to introduce appropriate strain into the center of the sheet thickness, refine the structure by recrystallization, regulate the grain size, thin out coarse MnS, and compress the porosity. Therefore, it is important to perform rolling with an average reduction / pass of 3.5% or more so that the cumulative reduction is 35% or more under the condition that the average value of X calculated by equation (2) is 0.70 or less.

[0057] 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 The temperature is equal to or higher than the temperature point +10°C. ３ The points can be obtained by actual measurement, but may also be calculated from the following formula (4): Ar ３ Point (°C) = 910 - 273 [C] - 74 [Mn] - 5 [Cu] - 56 [Ni] - 16 [Cr] - 9 [Mo] ... (4) In the formula, [ ] represents the content (mass%) of the element shown in [ ], and is set to zero if the element is not contained.

[0058] Average value of X obtained by formula (2) in each rolling pass: 0.70 or less In the present invention, it is important to obtain X for each rolling pass by the following formula (2), and to control the temperatures of the center part of the thickness and the surface of the steel plate and to control the hot rolling so that the average value of X (average value for all rolling passes) is 0.70 or less.

[0059] Specifically, by performing rolling in each rolling pass at a timing when the temperature difference between the center of plate thickness and the surface of the steel plate becomes an appropriate value according to the C content of the steel slab, the average value of X in each rolling pass can be controlled to be 0.70 or less.

[0060] In the formula, [C] is the C content (mass%) of the steel slab, and T ｋ(板厚中心) is the absolute temperature (K) at the center of the plate thickness just before the start of rolling, and T ｋ(鋼板表面) is the absolute temperature (K) of the steel sheet surface immediately before the start of rolling.

[0061] Here, T ｋ（板厚中心） is the absolute temperature (K) at the center of the steel plate thickness just before the start of rolling, and T ｋ（鋼板表面） is the absolute temperature (K) of the steel sheet surface immediately before the start of rolling. The temperature of the steel sheet surface can be measured by a radiation thermometer, and the temperature at the center of the sheet thickness can also be measured actually, but in an actual production line, the temperature may also be determined by heat transfer calculation from the steel sheet surface temperature measured by the radiation thermometer.

[0062] Under the condition that the average value of X in each rolling pass exceeds 0.70, it is not possible to introduce sufficient strain into the center of the plate thickness for a thick steel plate having a plate thickness of more than 100 mm, and porosity remains, resulting in a porosity density of 0.10 pieces / mm ２ It is not possible to make the average value of X smaller than 1.0 μm. Furthermore, since the MnS cannot be thinned and recrystallization is insufficient, the maximum prior austenite grain size cannot be made 150 μm or smaller. The average value of X is preferably 0.68 or less. Furthermore, the smaller the average value of X, the easier it is to introduce strain into the center of the sheet thickness, and therefore the smaller the value, the more preferable it is. However, since an excessively small value of X imposes a large burden on manufacturing, it is preferable to make it 0.20 or more.

[0063] Average reduction rate per pass is 3.5% or more, cumulative reduction rate is 35% or more In the present invention, the average reduction rate per pass is 3.5% or more. The reduction rate per pass is the reduction rate per pass, and corresponds to the reduction rate of each rolling pass. The steel plate thickness (mm) at the rolling entry side is expressed as h ０ , the steel plate thickness (mm) at the rolling exit side is h １ As such, (h０ -h １ ) / h ０ × 100. In rolling where the average reduction / pass (average of all rolling passes) is less than 3.5%, sufficient strain cannot be introduced into the center of the plate thickness. In addition, in rolling where the cumulative reduction is less than 35%, porosity cannot be sufficiently compressed, and coarse MnS cannot be sufficiently thinned by rolling. Therefore, in the present invention, it is important to simultaneously satisfy the average reduction / pass of 3.5% or more and the cumulative reduction of 35% or more.

[0064] The larger the average reduction rate / pass and the cumulative reduction rate, the easier it is to introduce strain into the center of the plate thickness, so the average reduction rate / pass is preferably 4.0% or more, and the cumulative reduction rate is preferably 40% or more. However, since an excessive increase in the reduction rate / pass increases the load on the rolling mill, the average reduction rate / pass is set to 12.0% or less. Furthermore, if the cumulative reduction rate is increased too much, the desired plate thickness cannot be secured, so the cumulative reduction rate is set to 90% or less. The number of rolling passes in hot rolling can be set as appropriate, for example, to two or more.

[0065] The final thickness of the steel plate after hot rolling is more than 100 mm, and from the viewpoint of application to large steel structures, it is preferably 150 mm or more. In order to avoid the plate thickness becoming too thick and making it difficult to obtain the desired reduction during rolling, the plate thickness is preferably 300 mm or less.

[0066] [Quenching process] In order to obtain excellent strength and toughness at the center of the plate thickness, the hot-rolled steel plate is subjected to the following steps: (i) quenching with Ar ３ (ii) a step of quenching the steel plate by rapidly cooling it from a temperature above the quenching point to 300°C or less; ３ The steel plate is quenched by rapidly cooling from a temperature above the temperature point to 300°C or below, and then ３ (iii) a process of reheating the steel sheet to a temperature of 1000 to 1050°C and then rapidly cooling it to 300°C or less once or twice or more times, and then quenching it again; or ３ The steel is then subjected to one of the following steps: a) reheating the steel to a temperature of 1000 to 1050°C, and then rapidly cooling the steel to 300°C or less, once or twice or more times, thereby quenching the steel.

[0067] In order to improve the strength and toughness of the base material, it is preferable to perform quenching again. Specifically, in the above step (ii) or in the above step (iii), ３ It is preferable to carry out the process of reheating to a temperature of 1000 to 1050°C and then quenching to 300°C or less two or more times.

[0068] The rapid cooling can be carried out by, for example, water cooling, but it is preferable to cool the material to a temperature of 300° C. or less by natural cooling.

[0069] In the above steps (i) and (ii), when quenching after hot rolling, the quenching start temperature is set to Ar ３ The reason for setting the temperature at or above this point is that by making the structure before the start of rapid cooling an austenite single-phase structure, the steel sheet structure and material properties after quenching and tempering are homogenized, and desired strength and toughness are obtained.

[0070] In the above steps (ii) and (iii), the reason why the reheating temperature is set to 1050°C or less when rapidly cooling after reheating is that austenite grains become coarse at temperatures exceeding this range, and the toughness of the base material decreases. The reheating temperature is preferably 1000°C or less. ３ The reason for setting the temperature at or above this point is to make the steel sheet before quenching into an austenite single phase structure, thereby homogenizing the structure and material of the steel sheet after quenching and tempering, and obtaining the desired strength and toughness. ３ It is preferably +10°C or higher.

[0071] The reason for rapidly cooling to 300°C or less during quenching (including re-quenching) is to sufficiently quench the entire steel plate, cause bainite or martensitic transformation throughout the entire plate thickness, and obtain the desired strength and toughness.

[0072] [Tempering Treatment] Tempering temperature: 450 to 700°C. To achieve both the desired base material strength and toughness, the hot-rolled steel sheet, which has been subjected to one of the above steps (i) to (iii) and quenched, must be reheated and tempered at a temperature of 450 to 700°C. A tempering temperature below 450°C does not provide sufficient tempering effects, resulting in reduced base material toughness and reduced drawing characteristics in through-thickness tensile tests. The tempering temperature is 450°C or higher, preferably 500°C or higher. On the other hand, a tempering temperature above 700°C precipitates various carbonitrides in the steel, and the microstructure obtained by transformation disappears, significantly reducing strength and toughness. The tempering temperature is 700°C or lower. Tempering Holding Time: To achieve both the desired strength and toughness, the holding time at the tempering temperature is preferably less than 2.3t (where t is the thickness of the steel sheet (mm)) minutes. If the holding time at the tempering temperature is 2.3 t minutes or more, coarse carbonitrides may precipitate in the steel, resulting in a significant decrease in strength and toughness. Therefore, the holding time at the tempering temperature is preferably less than 2.3 t minutes. The lower limit of the tempering holding time is not particularly limited, but from the viewpoint of obtaining a sufficient tempering effect, 10 minutes or more is preferable. The tempering treatment results in a steel sheet structure consisting of a tempered bainite structure and / or a tempered martensite structure. Ferrite, pearlite, retained austenite, etc. may also be present as the remaining structure. As described above, it is important to subject the steel sheet of the present invention to a tempering treatment after quenching. By performing these heat treatments, a steel sheet with high strength and excellent low-temperature toughness can be produced.

[0073] In the production method according to the present invention, any item not described in this specification can be performed in a conventional manner.

[0074] 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.

[0075] Steel billets having the chemical compositions shown in Table 1 were used to produce steel plates under the production conditions shown in Table 2. During hot rolling, thermocouples were attached to the center positions of the steel material being rolled in the longitudinal, width and thickness directions, and the temperature at the center of the thickness direction was measured. In addition, the surface temperature of the steel material was measured with a radiation thermometer.

[0076] For each of the obtained steel plates, the maximum prior austenite grain size, the number density of porosity, the number density of MnS, the yield strength, the tensile strength, the reduction in area in the thickness direction tensile test, the base material toughness, and the CTOD value were measured by the following methods.

[0077] [Maximum Prior Austenite Grain Size] A sample was taken from an as-quenched steel sheet at a thickness cross section parallel to the rolling direction (a cross section perpendicular to the sheet width direction) so that the measurement positions were the centers of the steel sheet in the longitudinal direction, width direction, and thickness direction. The surface of the sample was then mirror-polished, and the prior austenite grain boundaries were revealed with picric acid. A 900 μm × 1500 μm region was observed with an optical microscope, and the circle-equivalent diameters of the prior austenite grains were evaluated by image analysis, and their maximum value was calculated.

[0078] [Number Density of Porosity and MnS] Ultrasonic testing is often used to detect defects inside steel sheets because it allows non-destructive testing. However, direct observation was performed to accurately confirm the size and number of porosities, and the number density of porosity was measured. In addition, the number density of MnS was also measured. First, one or two observation samples were taken from the thickness direction cross section (cross section perpendicular to the rolling direction) parallel to the sheet width direction of the tempered rolled material, and mirror polished. Next, the prepared samples were observed with an optical microscope and photographed. The obtained photographs were subjected to image analysis to determine the circle-equivalent diameter of each porosity present and the thickness of MnS in the sheet thickness direction. The number of porosity with a circle-equivalent diameter of 100 μm or more and MnS with a thickness in the sheet thickness direction of 30 μm or more were divided by the measurement area (total sheet thickness × total sheet width) to determine the average number density of porosity with a circle-equivalent diameter of 100 μm or more and MnS with a thickness in the sheet thickness direction of 30 μm or more in the total thickness × total width.

[0079] [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. A tensile test was conducted in accordance with JIS Z 2241 to measure the yield strength (YS) and tensile strength (TS). If an upper yield point appeared in the tensile test, the upper yield stress was taken as the yield strength. If no upper yield point appeared, the 0.2% proof stress was taken as the yield strength. If YS ≥ 650 MPa, the specimen was evaluated as having high strength. If YS ≥ 650 MPa and TS ≥ 720 MPa, the tensile properties were evaluated as being good.

[0080] [Charpy Impact Test] Three JIS V-notch 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 absorbed energy at −40° C. (vE-40° C.) was measured in accordance with JIS Z 2242, and the average value was calculated. Then, when vE-40° C. ≧ 100 J, the low temperature toughness was evaluated as good.

[0081] [Tensile test in the thickness direction] Three round bar tensile test specimens (φ10 mm) were taken from each of the obtained thick steel plates in the thickness direction, and the reduction of area (RA) after fracture was measured and the average value was calculated in accordance with JIS G 3199. Then, when RA ≥ 25%, the tensile properties in the thickness direction were evaluated as good.

[0082] Multi-layer welded joints were fabricated using each of the obtained thick steel plates. For each of the obtained multi-layer welded joints, a joint CTOD test was performed with the weld bond on the straight side of the K groove as the notch position, and the crack tip opening displacement was measured. The fabrication conditions of the multi-layer welded joints and the joint CTOD test conditions are described below.

[0083] [Joint CTOD Test] The welded joints used in the joint CTOD test were prepared by submerged arc welding (multi-pass welding) with a K groove shape and a heat input of 5.0 kJ / mm. The test method conformed to EN ISO 15653, and the crack tip opening displacement [CTOD value (δ)] at a test temperature of -10°C was evaluated. For each thick steel plate, the test was conducted using three test pieces for each notch position, and the minimum measured value was taken as δ. In the present invention, multi-pass welded joints that satisfied δ≧0.10 mm were evaluated as having good CTOD characteristics.

[0084] The evaluation results are also shown in Table 3. The invention examples satisfied the chemical composition and manufacturing conditions of the present invention, and all of the conditions for maximum prior austenite grain size, porosity, and MnS number density were satisfied, and they had high strength and excellent low-temperature toughness, and had a reduction of area of ​​25% or more and a CTOD value of 0.10 mm or more at -10°C, providing excellent CTOD properties. In contrast, the steel plates (comparative examples) that did not satisfy the conditions of the present invention had CTOD values ​​of less than 0.10 mm and / or poor base material properties, and were therefore inferior in properties to the invention examples.

[0085]

[0086]

[0087]

[0088] According to the present invention, there is provided a thick steel plate having a thickness of more than 100 mm, which has high strength, low-temperature toughness, excellent deformation performance at the center of the plate thickness, and excellent CTOD characteristics of multi-pass welded joints, together with a manufacturing method thereof. The thick steel plate of the present invention can be suitably used for steel structures such as ships, marine structures, pressure vessels, penstocks, and ships for installing offshore wind turbines, and is therefore highly useful in industry.

Claims

1. A steel sheet containing, by mass%, C: 0.03 to 0.20%, Si: 0.50% or less, Mn: 0.3 to 3.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, and one or more elements selected from the group consisting of Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, and Ni: 0.01 to 5.00%, with the balance consisting of Fe and unavoidable impurities, and having a chemical composition that satisfies the following formula (1), wherein the maximum prior austenite grain size in the center of the sheet thickness is 150 μm or less, The number density of porosity having a circle equivalent diameter of 100 μm or more is 0.10 pieces / mm ２ The number density of MnS particles having a thickness of 30 μm or more in the plate thickness direction is 10 particles / mm ２ A thick steel plate having a composition ratio of 0.1 to 1.0, and a thickness of more than 100 mm, where Ceq (= [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5) ≥ 0.580% (1) In the formula, [ ] represents the content (mass%) of the element in [ ], and is set to zero if the element is not contained.

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: 2.00% or less, V: 0.20% or less, Ti: 0.10% or less, Nb: 0.10% or less, Ca: 0.010% 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 claim 1 or 2, comprising the steps of: ３ After heating to 1250°C, the average of X calculated by the following formula (2) is 0.70 or less, and the average reduction rate / pass is 3.5% or more. ３ The rolling is performed so that the cumulative rolling reduction is 35% or more at a temperature above the rolling point, and then the rolled steel sheet is subjected to the following steps: (i) subjecting the rolled steel sheet to Ar ３ (ii) a step of quenching the steel plate by rapidly cooling it from a temperature above the quenching point to 300°C or less; ３ The steel plate is quenched by rapidly cooling from a temperature above the temperature point to 300°C or below, and then ３ (iii) a process of reheating the steel sheet to a temperature of 1000 to 1050°C and then rapidly cooling it to 300°C or less once or twice or more times, and then quenching it again; or ３ and (c) subjecting the steel plate to a tempering treatment at a temperature of 450 to 700°C. In the formula, [C] is the C content (mass%) of the steel slab, and T ｋ(板厚中心) is the absolute temperature (K) at the center of the plate thickness just before the start of rolling, and T ｋ(鋼板表面) is the absolute temperature (K) of the steel sheet surface immediately before the start of rolling.

4. A method for manufacturing thick steel plate according to claim 3, wherein the tempering treatment is carried out under conditions in which the steel plate is held at a temperature of 450 to 750°C for a time of less than 2.3t minutes, where t is the thickness of the steel plate (mm).

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