Thick steel plate and method for producing the same

Abstract

To provide a thick steel plate exceeding 100 mm in thickness, excellent in high strength, low-temperature toughness, deformability in the central thickness region of the plate, and CTOD characteristics at a multi-layer welded joint.SOLUTION: A thick steel plate having a thickness exceeding 100 mm, in which the maximum prior austenite grain size in the central thickness region of the plate is 150 μm or less, the number density of porosities with an equivalent circle diameter of 50 μm or more is 0.08 pieces / mm2 or less, and the number density of MnS with a thickness in a plate-thickness direction of 20 μm or more is 8 pieces / mm2 or less, comprises by 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, O: 0.0100% or less, and one or more 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 comprising Fe, etc.SELECTED DRAWING: None

Description

[Technical Field]

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

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

[0004] Since the CTOD test introduces a fatigue pre-crack, the toughness evaluation area is an extremely small 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] Multi-pass welding is used when steel plates are applied to steel structures such as ships, marine structures, pressure vessels, penstocks, and offshore wind turbine installation vessels. The heat-affected zone (HAZ) of multi-pass welding is a locally embrittled region in the area near the weld line where a coarse grain structure was formed by a previous welding pass (Coarse Grain Heat Affected Zone: CGHAZ). Subsequent welding passes reheat the CGHAZ to the ferrite-austenite two-phase region, resulting in the coarse grain structure containing island martensite (MA). This significantly reduced toughness results in the inter-critically reheated HAZ (ICCGHAZ).

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

[0007] In addition, technologies 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 suppressing austenite grain growth and improving the toughness of welds by adding REM together with Ti to disperse fine particles in steel. Furthermore, Patent Document 3 proposes a technique for improving HAZ toughness by using CaS and a technique for improving base material toughness by hot rolling.

[0009] Recently, 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 addition of more alloying elements is necessary. However, adding large amounts of alloying elements makes it more difficult to ensure the toughness of multi-pass weld HAZs. To address this issue, Patent Document 4 discloses a technology for improving low-temperature toughness by controlling the hardness of the center segregation region. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 152626 / 1983 [Patent Document 2] Japanese Patent Application Publication No. 184663 / 1983 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-184500 [Patent Document 4] Japanese Patent Application Laid-Open No. 2013-91845 Summary of the Invention [Problem to be solved by the invention]

[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 weld heat-affected zones described in Patent Documents 1 to 3 are intended for steel materials with relatively low strength or steel materials with a thin plate thickness and therefore a low content of alloying elements, and cannot be applied to steel materials with higher strength or steel materials with a thick plate thickness and a high content of alloying elements, because the HAZ structure does not contain ferrite.

[0013] Furthermore, Patent Document 4 proposes a technology for satisfying the joint CTOD characteristics at low temperatures for thick steel plates of 150 mm or less, but does not focus on 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 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 multi-pass welded joints (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 conventional technology, and has an object 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. Here, in the present invention, "high strength" refers to a yield strength of 650 MPa or more in a tensile test at the center of the plate thickness, "excellent low-temperature toughness" refers to an absorbed energy of 100 J or more in a Charpy test at -60°C at the center of the plate thickness, "excellent deformation performance at the center of the plate thickness" refers to a reduction in area of ​​35% or more in a tensile test through the plate thickness, and "excellent CTOD characteristics of multi-pass welded joints" refers to a crack tip opening displacement of 0.20 mm or more in a CTOD test at a test temperature of -10°C with the weld bond at 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. [Means for solving the problem]

[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 over 100 mm, and have obtained the following findings.

[0017] (A) When steel plates are made stronger and thicker, the amount of alloying elements added to ensure the necessary properties increases, which makes it more likely that the porosity generated during slab production will increase and become larger. If the porosity generated during slab production is not pressed into place during rolling and remains, it can become the starting point for fracture within the steel plate. In order to press the porosity in the center of the plate thickness, it is necessary to introduce appropriate strain into the center of the plate thickness during rolling, but this is difficult with thick steel plates over 100 mm thick, so the problem of unpressed remaining porosity is easily apparent. However, as a result of studies by the present inventors, it was found that sufficient strain can be introduced into the thickness center and that porosity can be fully compressed by controlling the temperatures of the steel plate at the thickness center and surface in each rolling pass to set the average X at 0.70 or less, thereby controlling the deformation resistance of the steel plate at the thickness center and surface, and further performing rolling with an average reduction / pass of 3.5% or more so that the cumulative reduction is 35% or more. In addition, it was found that the porosity compression ability during rolling can be further improved by reducing the width direction of the steel slab by Y / 20 mm or more before rolling, and then performing hot forging at 950°C or higher with a strain rate of 3 / s or less and a cumulative reduction of 10% or more.

[0018] (B) When manufacturing high-strength steel plates with thicknesses exceeding 100 mm, the addition of large amounts of alloying elements is necessary. This poses a problem: element segregation regions tend to form 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, we found that 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 controls the deformation resistance of the center and surface of the steel plate. Furthermore, by rolling with an average reduction / pass of 3.5% or more and a cumulative reduction of 35% or more, we were able to increase the strain applied to the center of the plate, thereby sufficiently thinning the coarse inclusions and reducing their adverse effects on the material properties. In addition, it was discovered that before rolling, the width of the steel billet is set to Y mm, and the width direction of the steel billet is reduced by Y / 20 mm or more, and then hot forging is performed at 950°C or higher with a strain rate of 3 / s or less, resulting in a cumulative reduction of 10% or more. This further increases the strain applied to the center of the plate thickness, and makes the coarse inclusions sufficiently thin, making them harmless.

[0019] (C) In thick steel plates over 100 mm thick, the cooling rate at the center of the plate thickness is slow, which causes the problem of coarsening of the crystal grains in those areas. However, it was discovered that by controlling the temperature at the center and surface of the plate in each rolling pass to set the average X at 0.70 or less, it is possible to control the deformation resistance at the center and surface of the plate, and by 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 at the center of the plate thickness, which promotes austenite recrystallization and enables austenite refinement and grain regulation, resulting in a maximum prior austenite grain size of 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-pass 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. In 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 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% and the balance being Fe and unavoidable impurities, and having a component composition that satisfies the following formula (1): The maximum prior austenite grain size at the center of the plate thickness is 150 μm or less, The number density of porosity with a circular equivalent diameter of 50 μm or more is 0.08 pieces / mm 2 is as follows: The number density of MnS particles with a thickness of 20 μm or more in the plate thickness direction is 8 particles / mm 2 is less than or equal to: Thick steel plate with a thickness of over 100 mm. Note Ceq(=[C]+[Mn] / 6+([Cu]+[Ni]) / 15+([Cr]+[Mo]+[V]) / 5)≧0.580% …(1) In the formula, [ ] indicates the content (mass%) of the element in [ ], and is set to zero if the element is not contained.

[0023] 2. The component composition further comprises, in mass%, 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 2. The steel plate according to claim 1, further comprising one or more elements selected from the group consisting of Mg: 0.0150% or less.

[0024] 3. A method for producing a steel plate according to 1 or 2, A billet having the composition 1 or 2 is heated to 1200 to 1350°C, and the width of the billet is set to Y mm, and the billet is reduced in the width direction by Y / 20 mm or more. Hot forging is performed at 950°C or higher with a strain rate of 3 / s or less so that the cumulative reduction is 10% or more, and then the steel is allowed to cool. After heating to Ac3 point to 1250°C, rolling is performed at Ar3 point or higher so that the average X calculated by the following formula (2) is 0.70 or less and the average reduction rate / pass is 3.5% or more so that the cumulative reduction rate is 35% or more, and a rolled steel sheet is obtained. The rolled steel plate, (i) a process of quenching by rapidly cooling from a temperature above the Ar3 point to 300°C or below; (ii) A process of quenching the steel sheet by rapidly cooling it from a temperature above the Ar3 point to 300°C or less, and then reheating it to between the Ac3 point and 1050°C and rapidly cooling it to 300°C or less once or twice or more times, and then quenching it again; or (iii) After cooling, the steel is reheated to Ac3 to 1050°C and then rapidly cooled to 300°C or less, and this process is repeated once or twice or more to harden the steel. and then subjecting the resulting mixture to one of the steps A method for manufacturing thick steel plates, including tempering at a temperature of 450 to 700°C. Note

number

[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 at 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. DETAILED DESCRIPTION OF THE INVENTION

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

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

[0028] C: 0.03 to 0.20% C is an element that improves hardenability and increases the strength of steel, and a C 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%, 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 hardenability of steel and thereby improving the strength of the base metal and weld. 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 weld 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 increases refining time and 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 joints, the S content is limited to 0.0050% or less, 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 low S content increases 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, and a content of 0.005% or more forms nitrides in the steel, reducing the amount of dissolved nitrogen and suppressing the precipitation of BN, thereby improving hardenability by ensuring the B necessary for hardening. On the other hand, an Al content exceeding 0.100% reduces the base material toughness and joint CTOD characteristics, so the Al content is set to 0.100% or less, 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, preferably 0.0030% or less. The B content is preferably 0.0005% or more.

[0035] N: 0.0100% or less Since N is an element that reduces HAZ toughness and deteriorates joint CTOD characteristics, 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 increases refining time and 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 lower limit for the O content, but excessively reducing the O content increases refining time and costs, so the O content is preferably 0.0005% or more.

[0037] One or more 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% The chemical composition includes one or more elements selected from the group consisting of 0.01-2.00% Cr, 0.01-2.00% Mo, and 0.01-5.00% Ni. 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 and joint CTOD characteristics of the base metal. 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 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, increases in costs can 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 unavoidable impurities. Furthermore, for the purpose of further improving strength, base metal toughness, joint toughness, etc., 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.

[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 a Cu-enriched layer formed 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 metal and HAZ, thereby refining the HAZ structure and improving the joint CTOD characteristics. On the other hand, if the Ti content exceeds 0.10%, the precipitation of solute Ti and coarse TiC will actually reduce the HAZ toughness and deteriorate the 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 weld 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, preferably 0.008% or less. When Ca is contained, the Ca content is preferably 0.0002% or more in order to fully obtain its effects.

[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, 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 to inhibit austenite grain growth in the HAZ and improve HAZ toughness. However, if the REM content exceeds 0.025%, the base material toughness and HAZ toughness decrease, and the joint CTOD characteristics deteriorate. Therefore, if REM is contained, the REM content is limited to 0.025% or less, preferably 0.020% or less. If REM is contained, the REM content is preferably 0.001% or more to fully obtain its effects.

[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 additive effect saturates, and the effect commensurate with the content cannot 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, it is necessary to contain appropriate elements to ensure high strength and good toughness at the center of the plate thickness, and it is important to contain elements 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 when the element is not contained.

[0049] [Maximum prior austenite grain size] Maximum prior austenite grain size at the center of the 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 the plate thickness is set to 150 μm or less. By refining and regulating the prior austenite grains at the center of the plate thickness, where segregation is likely to occur, as described above, it is possible to improve the toughness of the base material and also the 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] [Porosity density] Number density of porosity with a circle equivalent diameter of 50 μm or more: 0.08 pieces / mm 2 below As mentioned above, 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 particles per square meter (hereinafter referred to as "porosity density") is 0.08 particles / mm 2 The target crack tip opening displacement and reduction characteristics can be obtained by keeping the porosity density at 0.08 pieces / mm 2 It is important that the porosity density is preferably 0.07 pieces / mm 2 The lower limit of the number density of porosity is not particularly limited. The number density of porosity in the present invention refers to the average number density across the total thickness x total width in a thickness direction cross section parallel to the plate width direction of the thick steel plate (cross section perpendicular to the rolling direction). Furthermore, the frequency of measuring the porosity number density is such that one or two cross sections of any one steel plate are measured among steel plates that have been produced under the same slab melting conditions and the same rolling conditions. As long as the slab melting method and rolling conditions are not changed, the porosity number density can be produced with good reproducibility, and therefore the measurement results at the above measurement frequency can be said to represent the whole. Specifically, the "number density of porosity" can be measured by the method described in the Examples.

[0051] [MnS number density] Number density of MnS particles with a thickness of 20 μm or more in the plate thickness direction: 8 particles / mm 2 below As mentioned above, coarse MnS particles remaining in the steel sheet act as fracture initiation points, reducing the drawing capacity and CTOD characteristics. 2 The number of particles per unit area (hereinafter referred to as "MnS particle density") is 8 particles / mm 2 By setting the number density of MnS to 8 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 7 particles / mm 2 The following is the result. In the present invention, the number density of MnS refers to the average number density across the total thickness and total width in a thickness direction cross section parallel to the plate width direction of the thick steel plate (cross section perpendicular to the rolling direction). Furthermore, the measurement frequency for the number density of MnS can be as low as measuring one or two cross sections of any one steel plate among steel plates that have been produced under the same slab melting conditions and rolling conditions. As long as the slab melting method and rolling conditions are not changed, the number density of MnS can be produced with good reproducibility, and therefore the measurement results at the above measurement frequency can be said to 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 easily avoid the situation where 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 produced 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 calculations 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 billets before hot forging: 1200~1350℃ In the present invention, the steel billet is heated to 1200 to 1350°C before hot forging. If the heating temperature is lower than 1200°C, the desired cumulative reduction amount of hot forging cannot be ensured. On the other hand, if the heating temperature is higher than 1350°C, surface defects are likely to occur due to scale formed during heating, increasing the maintenance load after hot forging, so the upper limit is set to 1350°C. Preferably, the temperature is 1220°C or higher and 1300°C or lower.

[0056] Width reduction before hot forging: Y / 20mm or more By reducing the width of the billet before hot forging and increasing the billet thickness, the amount of reduction in hot forging can be secured, and the effects of compressing porosity and thinning MnS can be increased. Here, the width of the billet refers to the direction perpendicular to the plate thickness direction and the rolling direction. If the width of the slab is Y mm, and the widthwise reduction is less than Y / 20 mm, the thickness of the slab will not increase sufficiently, and the effect of increasing the reduction in hot forging will not be obtained. Therefore, the widthwise reduction before hot forging is set to Y / 20 mm or more. To further increase the thickness of the slab, the widthwise reduction is preferably set to Y / 15 mm or more. Although the thickness of the slab can be further increased by increasing the widthwise reduction, taking into account the production load, the widthwise reduction is preferably 1000 mm or less. The width of the slab, Y, can be set to 1000 mm or more and 4000 mm or less.

[0057] Hot forging temperature: 950℃ or higher If the hot forging temperature is less than 950°C, the deformation resistance during hot forging increases, placing a heavy load on the forging machine and preventing sufficient porosity compression and thinning of the MnS. Therefore, the hot forging temperature is set to 950°C or higher. The hot forging temperature is preferably 1000°C or higher. The hot forging temperature can be set to a temperature equal to or lower than the heating temperature of the steel slab before hot forging, and a higher temperature within that range is preferred in terms of reducing the load on the forging machine. During hot forging, the steel slab may be reheated to adjust the hot forging temperature to 950°C or higher, but it is preferable to continue hot forging under width direction compression without reheating so that the temperature at the end of hot forging is 950°C.

[0058] Cumulative reduction in hot forging: 10% or more If the cumulative reduction amount of hot forging is less than 10%, it will not be able to sufficiently contribute to porosity compaction in the slab or thinning the MnS, so the cumulative reduction amount of hot forging should be 10% or more. The cumulative reduction amount of hot forging is the cumulative reduction amount from the slab thickness (thickness) increased by hot working the slab in the width direction.

[0059] Hot forging strain rate: 3 / s or less If the strain rate of hot forging exceeds 3 / s, the deformation resistance during hot forging increases, the load on the hot working machine increases, and the hot forging cannot sufficiently contribute to the compaction of porosity and the thinning of MnS, so the strain rate of hot forging is set to 3 / s or less. Furthermore, in order to easily avoid a decrease in productivity due to a longer hot forging time, the strain rate of hot forging is preferably set to 0.01 / s or more. More preferably, it is in the range of 0.01 / s to 1 / s.

[0060] Heating conditions for steel billets before rolling: Ac3 point to 1250℃ In the present invention, the steel slab is heated to a temperature between the Ac3 point and 1250°C. If the heating temperature is lower than the Ac3 point, the austenite single phase region is not reached, resulting in poor uniformity of the steel structure and significantly reduced 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 reduced manufacturing efficiency. Furthermore, it becomes difficult to compress porosity and reduce coarse MnS. Therefore, the heating temperature is set to the Ac3 point or higher, preferably (Ac3 point + 100°C) or higher. 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 base material toughness and drawing characteristics deteriorate. Therefore, the heating temperature is set to 1250°C or lower, and preferably 1200°C or lower. Here, the Ac3 point can be determined by actual measurement, but it may also be calculated from the following formula (3). Ac3 point (℃)=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 when the element is not contained.

[0061] 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 and regulate the grain size through recrystallization, thin out the 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.

[0062] Rolling end temperature: Ar3 point (℃) or higher The heated steel slab is hot rolled at a temperature equal to or higher than the Ar3 point. The rolling end temperature for hot rolling is equal to or higher than the Ar3 point. If the rolling end temperature is lower than the Ar3 point, the structure before the start of cooling will be a two-phase structure of austenite and ferrite, which will result in poor uniformity in the steel structure and significantly reduced manufacturing stability. Therefore, the rolling end temperature is set to equal to or higher than the Ar3 point, and preferably equal to or higher than (Ar3 point + 10°C). Here, the Ar3 point can be determined by actual measurement, but it may also be calculated from the following formula (4). Ar3 point (℃)=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 when the element is not contained.

[0063] Average value of X calculated by formula (2) for each rolling pass: 0.70 or less In the present invention, it is essential to calculate 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 so that the average value of X (average value for all rolling passes) is 0.70 or less, and to control the hot rolling.

[0064] Specifically, by performing rolling in each rolling pass at a timing that allows the temperature difference between the center of thickness and the surface of the steel sheet to reach an appropriate value depending on 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.

[0065]

number

[0066] where 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 plate surface just before the start of rolling. The temperature of the steel plate surface can be measured with a radiation thermometer, and the temperature at the center of the plate thickness can also be measured, but in an actual production line, it can also be determined by heat transfer calculation from the steel plate surface temperature measured with a radiation thermometer.

[0067] When the average value of X in each rolling pass exceeds 0.70, sufficient strain cannot be introduced into the center of the plate thickness for thick steel plates over 100 mm, and porosity remains, resulting in a porosity density of 0.08 pieces / mm 2 It is not possible to make the average value of X smaller than 1.0 μm. Furthermore, the MnS cannot be thinned and recrystallization is insufficient, so 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, so the smaller the value is, but making it too small imposes a heavy burden on manufacturing, so it is preferably 0.20 or more.

[0068] Average reduction / pass is 3.5% or more, cumulative reduction is 35% or more In the present invention, the average reduction rate per pass is set to 3.5% or more. The reduction rate per pass is the reduction rate per pass, corresponds to the reduction rate of each rolling pass, and is expressed as (h0-h1) / h0×100, where h0 is the steel sheet thickness (mm) at the rolling entry side and h1 is the steel sheet thickness (mm) at the rolling exit side. Rolling with an average reduction rate per pass (average of all rolling passes) of less than 3.5% fails to introduce sufficient strain into the center of the sheet thickness. Furthermore, in rolling with a cumulative reduction rate of less than 35%, the porosity cannot be sufficiently compressed, and the coarse MnS cannot be sufficiently thinned by rolling. Therefore, in the present invention, it is important to simultaneously satisfy an average rolling reduction / pass value of 3.5% or more and a cumulative rolling reduction of 35% or more.

[0069] 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 the hot rolling can be set appropriately to, for example, two or more.

[0070] The final thickness of the steel plate after hot rolling is more than 100 mm, and is preferably 150 mm or more from the viewpoint of application to large steel structures, and is preferably 300 mm or less from the viewpoint of avoiding the difficulty of obtaining the desired reduction during rolling due to the plate thickness becoming too thick.

[0071] [Quenching process] In order to obtain excellent strength and toughness at the center of the plate thickness, the hot-rolled steel plate is (i) a process of quenching the steel sheet by rapidly cooling it from a temperature of Ar3 or higher to 300°C or lower; (ii) A process of quenching the steel sheet by rapidly cooling it from a temperature above the Ar3 point to 300°C or less, and then reheating it to between the Ac3 point and 1050°C and rapidly cooling it to 300°C or less once or twice or more times, and then quenching it again; or (iii) After cooling, the steel is reheated to Ac3 point to 1050°C and then rapidly cooled to 300°C or less, and this process is repeated once or twice or more to harden the steel. The mixture is subjected to one of the following steps.

[0072] To improve the strength and toughness of the base material, it is preferable to quench the material again. Specifically, it is preferable to carry out the process of reheating the material to between the Ac3 point and 1050°C in step (ii) above, or reheating the material to 300°C or less, and then quenching the material again, two or more times in step (iii) above.

[0073] 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.

[0074] In the above steps (i) and (ii), the reason why the quenching start temperature is set to Ar3 point or higher when quenching after hot rolling is performed is to make the structure before quenching a single-phase austenite structure, thereby homogenizing the structure and material properties of the steel sheet after quenching and tempering, and thereby obtaining the desired strength and toughness.

[0075] In the above steps (ii) and (iii), the reheating temperature when rapidly cooling after reheating is set to 1050°C or less because austenite grains become coarse at temperatures higher than this, reducing the toughness of the base material. The reheating temperature is preferably 1000°C or less. Furthermore, the reheating temperature is set to a temperature of the Ac3 point or higher because turning the steel sheet into an austenite single-phase structure before quenching homogenizes the steel sheet structure and material properties after quenching and tempering, thereby achieving the desired strength and toughness. The reheating temperature is preferably Ac3 + 10°C or higher.

[0076] The reason for rapidly cooling to 300°C or below during quenching (including re-quenching) is to thoroughly quench the entire steel plate, causing bainite or martensitic transformation throughout the entire thickness of the plate, thereby obtaining the desired strength and toughness.

[0077] [Tempering treatment] Tempering temperature: 450~700℃ In order to achieve both the desired base material strength and toughness, after hot rolling, the steel sheet that has been subjected to any of the above steps (i) to (iii) and quenched must be reheated and tempered at a temperature of 450 to 700°C. If the tempering temperature is less than 450°C, a sufficient tempering effect cannot be obtained, and the base material toughness and the drawing characteristics in a through-thickness tensile test will decrease. The tempering temperature is 450°C or higher, and preferably 500°C or higher. On the other hand, if the tempering temperature exceeds 700°C, various carbonitrides will precipitate in the steel, and the fine structure obtained by transformation will disappear, resulting in a significant decrease in strength and toughness.Tempering temperatures should be kept below 700°C. Tempering holding time The tempering holding time can be set as appropriate, but in order to achieve both the desired strength and toughness, it is preferable that the holding time at the tempering temperature be less than 2.3t minutes, where t is the thickness of the steel plate (mm). If the holding time is less than 2.3t minutes, the precipitation of coarse carbonitrides in the steel is suppressed, and good strength and toughness can be easily ensured. There is no particular lower limit to the tempering holding time, but from the viewpoint of obtaining a sufficient tempering effect, it is preferably 10 minutes or more. The tempering treatment results in a steel sheet structure consisting of a tempered bainite structure and / or a tempered martensite structure. The remaining structure may include a ferrite structure, a pearlite structure, a retained austenite structure, etc. As described above, it is important that the steel sheet of the present invention is tempered after quenching, and by performing these heat treatments, a steel sheet with high strength and excellent low-temperature toughness can be produced.

[0078] In the production method according to the present invention, any item not described in this specification can be performed in a conventional manner. [Example]

[0079] 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.

[0080] Steel billets having the chemical compositions shown in Table 1 were used to produce steel plates under the manufacturing 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 to measure the temperature at the center of the thickness direction. In addition, the surface temperature of the steel material was measured with a radiation thermometer.

[0081] 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.

[0082] [Maximum prior austenite grain size] A sample was taken from the 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. Next, the surface of the sample was 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 values ​​were calculated.

[0083] [Porosity and MnS number density] Ultrasonic testing is often used to detect internal defects in steel plates because it allows for nondestructive testing. However, to accurately confirm the size and number of porosities, direct observation was performed to measure the porosity number density. In addition, the number density of MnS was also measured. First, one or two observation samples were taken from the thickness cross section (cross section perpendicular to the rolling direction) of the tempered rolled material parallel to the plate width direction and mirror-polished. The prepared samples were then observed with an optical microscope and photographed. The resulting photographs were analyzed to determine the circle-equivalent diameter of each porosity and the thickness of MnS in the plate thickness direction. The number of porosity with a circle-equivalent diameter of 50 μm or more and MnS with a thickness in the plate thickness direction of 20 μm or more were divided by the measured area (total plate thickness × total plate width) to determine the average number density of porosity with a circle-equivalent diameter of 50 μm or more and MnS with a thickness in the plate thickness direction of 20 μm or more across the total thickness × total width.

[0084] [Tensile test] JIS No. 4 test pieces were taken from the obtained 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. Tensile tests were conducted in accordance with JIS Z 2241 to measure yield strength (YS) and tensile strength (TS). If an upper yield point was observed in the tensile test, the upper yield stress was taken as the yield strength. If no upper yield point was observed, 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 good.

[0085] [Charpy impact test] Three JIS V-notch test pieces were taken from the obtained 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 -60°C (vE-60°C) was measured in accordance with JIS Z 2242 and the average value was calculated. Then, when vE-60°C ≥ 100J, the low-temperature toughness was evaluated as good.

[0086] [Tensile test through the plate thickness] Three round bar tensile test pieces (φ10 mm) were taken from each of the obtained thick steel plates in the plate thickness direction, and the reduction of area (RA) after fracture was measured and averaged in accordance with JIS G 3199. RA ≥ 35% was evaluated as having good tensile properties in the plate thickness direction.

[0087] Multi-pass welded joints were fabricated using each of the obtained thick steel plates. For each of the obtained multi-pass 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 for the multi-pass welded joints and the joint CTOD test conditions are described below.

[0088] [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)] was evaluated at a test temperature of -10°C. 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 ≥ 0.20 mm were evaluated as having good CTOD properties.

[0089] 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. They also had high strength and excellent low-temperature toughness, and had excellent CTOD properties, with reduction of area of ​​35% or more and CTOD values ​​of 0.20 mm or more at -10°C. In contrast, the steel plates (comparative examples) that did not satisfy the conditions of the present invention had CTOD values ​​of less than 0.20 mm and / or poor base material properties, and were therefore inferior in properties to the invention examples.

[0090] [Table 1]

[0091] [Table 2]

[0092] [Table 3] [Industrial Applicability]

[0093] 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 method for manufacturing the same. 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 industrially.

Claims

1. In mass%, C: 0.03-0.20%, Si: 0.50% or less, Mn: 0.3 to 3.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005-0.100%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, and One or more 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% and the balance being Fe and unavoidable impurities, and having a component composition that satisfies the following formula (1): The maximum prior austenite grain size at the center of the plate thickness is 150 μm or less, The number density of porosity having a circle equivalent diameter of 50 μm or more is 0.08 pieces / mm ２ is as follows: The number density of MnS particles with a thickness of 20 μm or more in the plate thickness direction is 8 particles / mm ２ is less than or equal to, and A thick steel plate having a thickness of more than 100 mm. Note 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 component composition further comprises, in mass%, 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 The steel plate according to claim 1, further comprising one or more selected from the group consisting of Mg: 0.0150% or less.

3. A method for producing a steel plate according to claim 1 or 2, A steel slab having the chemical composition according to claim 1 or 2 is heated to 1200 to 1350°C, and the width of the steel slab is set to Y mm, and the steel slab is reduced in the width direction by Y / 20 mm or more. Hot forging is performed at 950 ° C. or higher with a strain rate of 3 / s or less and a cumulative reduction of 10% or more, followed by cooling. A.C. 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 the rolling point or higher, and after obtaining a rolled steel sheet, The rolled steel plate, (i) Ar ３ a process of rapidly cooling from a temperature above the temperature point to 300°C or less and quenching; (ii) Ar ３ The steel plate is quenched by rapidly cooling from a temperature above the temperature point to 300°C or below, and then ３ a process of reheating the steel to 1050°C or higher and then rapidly cooling it to 300°C or lower once or twice or more times, and then quenching the steel again; or (iii) After cooling, ３ This is a process in which the material is reheated to between 1000°C and 1050°C and then rapidly cooled to 300°C or below once or twice or more times to harden the material. and then subjecting the resulting mixture to one of the steps A method for producing a steel plate, comprising tempering the steel plate at a temperature of 450 to 700°C. Note [Equation 1] During the ceremony, [C] is the C content (mass%) of the steel slab, T ｋ(板厚中心) is the absolute temperature (K) at the center of the plate thickness just before the start of rolling, T ｋ(鋼板表面) is the absolute temperature (K) of the steel sheet surface immediately before the start of rolling.

4. The method for producing a thick steel plate according to claim 3, wherein the tempering treatment is carried out under conditions of holding the steel plate 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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