Steel sheet and producing method thereof

Abstract

To provide a steel sheet that is provided for a storage tank used for accommodating liquefied gas in, for example, an energy transportation ship, is excellent in ammonia stress corrosion cracking resistance and in mother material toughness and HAZ toughness, and has excellent strength property, and has excellent flatness, and a producing method thereof.SOLUTION: A steel sheet has a specific chemical composition, and a microstructure in which the volume fraction of ferrite at a depth position of one-quarter of the plate thickness from the surface is 20% or more, the total volume fraction of ferrite and bainite is 80% or more, and island-like martensite is 5% or less by volume fraction. A yield strength is 235 MPa or more and 440 MPa or less, a tensile strength is 400 MPa or more, a base metal toughness vE-55 is 100 J or more, a maximum gap is 14 mm or less between the steel surface and a 2-meter long straightedge placed along the rolling direction on the steel surface, and a plate thickness is less than 13 mm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel plate having excellent toughness (base material toughness, HAZ toughness) and corrosion resistance, in particular a steel plate having excellent low-temperature toughness (base material toughness, HAZ toughness) and ammonia stress corrosion cracking resistance, which is used for multipurpose tanks that carry a mixture of liquefied petroleum gas (hereinafter referred to as LPG) and liquid ammonia, and a method for manufacturing the same. [Background technology]

[0002] With the recent increase in energy demand, the transportation of liquefied gas by energy carriers has become popular. For efficient operation of energy carriers, tanks may carry not only LPG but also liquid ammonia. Since these liquefied gases are transported at low temperatures, the steel plates used in the storage tanks for these liquefied gases are required to have excellent low-temperature toughness (base material toughness).

[0003] The steel plates used in these tanks are required to have a yield strength (YS) of 235 MPa or more. Furthermore, liquefied ammonia is known to cause stress corrosion cracking, so by controlling the yield strength (YS) to 440 MPa or less, stress corrosion cracking caused by ammonia can be avoided.

[0004] Patent Documents 1 and 2 describe techniques for providing the low-temperature toughness (base material toughness) required for liquefied gas storage tanks and for satisfying a specified strength range, and by heat treating a thick steel plate that has been cooled after hot rolling, or by heat treating a thick steel plate that has been water-cooled after hot rolling, high low-temperature toughness (base material toughness) and specified strength characteristics are achieved. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 3802626 [Patent Document 2] Patent No. 3848415 Summary of the Invention [Problem to be solved by the invention]

[0006] The methods described in Patent Documents 1 and 2 above have had the problem that steel sheets with a thickness of less than 13 mm in particular are distorted due to cooling, resulting in a decrease in flatness of the steel sheets.

[0007] The present invention aims to solve the above problems and to provide a steel plate having excellent ammonia stress corrosion cracking resistance, base material toughness and HAZ toughness, as well as excellent strength properties and excellent flatness, and a manufacturing method thereof, which is suitable for use in storage tanks used to store liquefied gas in, for example, energy transport ships.

[0008] "Excellent strength characteristics" refers to the yield strength (YS) of the steel plate (yield point (YP) when there is a yield point, or 0.2% proof stress σ when there is no yield point). 0.2 ) is 235 MPa or more and 440 MPa or less, and the tensile strength is 400 MPa or more.

[0009] Furthermore, "excellent resistance to ammonia stress corrosion cracking" means that the yield strength (YS) of the steel plate is 440 MPa or less. [Means for solving the problem]

[0010] In order to achieve the above object, the present inventors have conducted extensive research into various factors affecting the low-temperature toughness (base material toughness, HAZ toughness) and strength properties of steel plates. As a result, they have found that, by adding elements such as C, Si, Mn, etc. in predetermined amounts or more, rolling and cooling using a predetermined hot rolling method, and then controlling the microstructure so that the volume fraction of ferrite at a depth of 1 / 4 of the plate thickness is 20% or more, the total volume fraction of ferrite and bainite is 80% or more, and the volume fraction of island martensite is 5% or less, the desired strength properties and low-temperature toughness (base material toughness, HAZ toughness) can be exhibited, and further, deterioration in the flatness of steel plates can be suppressed even in steel plates with a thickness of less than 13 mm.

[0011] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows. [1] In mass %, C: 0.045% or more, 0.100% or less, Si: 0.50% or less, Mn: 0.50% or more, 1.80% or less, P: 0.010% or less, S: 0.0100% or less, Al: 0.060% or less, Ti: 0.005% or more, 0.050% or less, N: 0.0100% or less and O: 0.0100% or less, and has a component composition in which the carbon equivalent Ceq represented by the following formula (1) is 0.250 or more and 0.380 or less, and the balance is Fe and unavoidable impurities, The microstructure has a volume fraction of ferrite of 20% or more at a depth of 1 / 4 of the plate thickness from the surface, a total volume fraction of ferrite and bainite of 80% or more, and a volume fraction of island martensite of 5% or less, The yield strength is 235 MPa or more and 440 MPa or less and the tensile strength is 400 MPa or more, Base material toughness vE -55 is 100J or more, When a 2 m long piece is applied to the steel plate surface along the rolling direction, the maximum value of the gap between the steel plate surface and the long piece is 14 mm or less, Steel plate with a thickness of less than 13 mm. Ceq=C+Mn / 6+(Cu+Ni) / 15+(V+Mo+Cr) / 5...(1) In the above formula (1), each element symbol represents the content (mass%) of each component, and if no component is contained, it is set to 0. [2] The component composition further includes, in mass%, Cu: 1.00% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 0.50% or less V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.050% or less, B: 0.0050% or less, Ca: 0.0050% or less, Mg: 0.0050% or less and The steel sheet according to [1], containing one or more selected from REM: 0.0050% or less. [3] The steel sheet according to [1] or [2], wherein, in the chemical composition, Ti / N is, in mass%, 2.00 or more and 4.00 or less, and the following formula (2) is satisfied: 169≦5158×Ti+25563×N≦360 (2) In the above formula (2), each element symbol represents the content (mass %) of each component. [4] A steel material having a chemical composition according to any one of [1] to [3], Heat to a temperature above Ac3 and below 1250°C, Next, hot rolling is performed with a rolling start temperature of Ar3 point + 100°C or higher and a rolling end temperature of Ar3 point or higher. After that, the first cooling is performed under the condition that the cooling start temperature exceeds Ar3 point and the cooling stop temperature is between Ar3 point - 200°C and Ar3 point. The cooling is stopped for 5 seconds or more, Next, a second cooling step is performed in which the average cooling rate from the surface of the steel plate to a depth of 1 / 4 of the plate thickness is 100°C / s or less, and cooling is stopped at a temperature of 400°C or higher. A series of cooling steps, including the first cooling step and the second cooling step, is carried out at a sheet threading speed of 45 mpm or more using online cooling equipment. Steel plate manufacturing method. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a steel plate with excellent strength, excellent base material toughness and HAZ toughness at low temperatures, and excellent resistance to ammonia stress corrosion cracking, and with high flatness suitable for tanks used in low-temperature, corrosive environments, and therefore, it is extremely effective in industry. [Brief explanation of the drawings]

[0013] [Figure 1] 1 shows a schematic diagram (cross-sectional view) of measuring the maximum value of the gap between a long piece and the steel sheet surface by applying the long piece to the surface of the steel sheet along the rolling direction. DETAILED DESCRIPTION OF THE INVENTION

[0014] In the present invention, it is important that the steel sheet and the steel material used for its production have the above-mentioned chemical composition. Therefore, the reason for limiting the chemical composition of the steel in the present invention to the above-mentioned will first be explained. Note that "%" regarding the chemical composition means "mass %" unless otherwise specified.

[0015] [Component composition] C: 0.045% or more, 0.100% or less C is an element that improves the strength of steel and is one of the important elements that must be added to achieve high strength. To achieve this effect, the C content is set to 0.045% or more. From the viewpoint of increasing the strength of the base metal (yield strength (YS) and tensile strength (TS)), the C content is preferably set to 0.050% or more, and more preferably set to 0.055% or more. On the other hand, C is an element that increases the hardenability of steel. If the C content exceeds 0.100%, the strength becomes excessive, reducing the toughness of the base metal and the toughness of the joint HAZ (heat affected zone) during welding. Therefore, the C content is set to 0.100% or less. Furthermore, from the viewpoint of suppressing a decrease in the toughness of the base metal and the joint HAZ, the C content is preferably set to 0.095% or less, and more preferably set to 0.090% or less.

[0016] Si:0.50% or less Although Si acts as a deoxidizer, it also reduces the toughness of the base material and the HAZ of the joint during welding. Therefore, it is preferable to keep the Si content as low as possible, but a content of 0.50% or less is acceptable. Since Al, Ti, and other elements can sufficiently deoxidize steel, the lower limit of the Si content is not particularly limited and may be 0%. From the viewpoint of the toughness of the base material and the HAZ of the joint, the Si content is preferably 0.40% or less, and more preferably 0.30% or less. On the other hand, Si is a component that can suppress the formation of coarse carbides and increase the strength of the base material. From the viewpoint of reducing the content of other alloying elements and achieving lower manufacturing costs, the Si content is preferably 0.10% or more. Furthermore, from the viewpoint of lower manufacturing costs, the Si content is more preferably 0.15% or more, and even more preferably 0.20% or more.

[0017] Mn: 0.50% or more, 1.80% or less Mn is an element that increases the strength of steel and is one of the important elements that must be added to achieve high strength. To achieve this effect, the Mn content is set to 0.50% or more. From the viewpoint of reducing the content of other alloying elements that increase the strength (YS, TS) of the base metal and manufacturing at lower cost, the Mn content is preferably set to 0.70% or more, and more preferably 0.90% or more. On the other hand, if the Mn content exceeds 1.80%, the toughness of the base metal and the toughness of the joint HAZ during welding will decrease, and further, the alloy cost will become excessively high. Therefore, the Mn content is set to 1.80% or less. Furthermore, from the viewpoint of suppressing deterioration of weldability, the Mn content is preferably set to 1.70% or less, and more preferably 1.60% or less.

[0018] P:0.010% or less P has adverse effects, such as reducing the toughness of the base material and the toughness of the joint HAZ during welding by segregating at grain boundaries. Therefore, it is desirable to keep the P content as low as possible, but a P content of 0.010% or less is acceptable. The lower limit of the P content is not particularly limited and may be 0%. However, since P is an element that is usually inevitably contained in steel as an impurity, industrially, the P content may be greater than 0%. Furthermore, since excessive reduction of P leads to increased refining costs, the P content is preferably 0.0005% or more, and more preferably 0.001% or more.

[0019] S: 0.0100% or less S is present in steel as sulfide-based inclusions such as MnS, and is an element that adversely affects the toughness of the base material, for example by acting as a fracture initiation site. Therefore, it is desirable to keep the S content as low as possible. S also has an adverse effect, such as reducing the toughness of the joint HAZ during welding. Therefore, the S content is set to 0.0100% or less. The S content is preferably set to 0.0080% or less, and more preferably set to 0.0060% or less. The lower limit of the S content is not particularly limited and may be 0%. Since S is usually an element that is inevitably contained in steel as an impurity, the S content may exceed 0% industrially. Excessive reduction of S leads to increased refining costs, so from a cost perspective, the S content is preferably set to 0.0005% or more, and more preferably set to 0.0010% or more.

[0020] Al: 0.060% or less Al acts as a deoxidizer, reducing oxide-based inclusions and improving the toughness of the joint HAZ, while also refining the grain size. To achieve these effects, the Al content is preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, if the Al content exceeds 0.060%, the oxide-based inclusions increase and the cleanliness decreases, resulting in a decrease in the toughness of the base metal and the toughness of the joint HAZ in high-heat-input welding. Therefore, the Al content is set to 0.060% or less. The Al content is preferably 0.050% or less, and more preferably 0.040% or less.

[0021] Ti: 0.005% or more and 0.050% or less Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing the amount of solute N. Therefore, the inclusion of Ti can improve the toughness of the base material and weld. To achieve this effect, the Ti content is set to 0.005% or more. The Ti content is preferably set to 0.012% or more. On the other hand, if the Ti content exceeds 0.050%, the toughness (base material toughness, HAZ toughness) decreases. Therefore, the Ti content is set to 0.050% or less. The Ti content is preferably set to 0.040% or less, and more preferably 0.030% or less.

[0022] N: 0.0100% or less N is an element that should be particularly reduced, and therefore its content is specified. N has adverse effects, such as forming nitrides and becoming the starting point for brittle fracture. Therefore, the N content is limited to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0060% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0%. Since N is usually an element that is inevitably contained in steel as an impurity, industrially, it may be greater than 0%. Furthermore, excessive reduction of N leads to an increase in refining costs, and therefore, from a cost perspective, the N content is preferably 0.0020% or more.

[0023] Ti and N precipitate as TiN during the solidification of steel, and can improve the toughness of the HAZ by suppressing the coarsening of austenite grains in the HAZ or by acting as nuclei for ferrite transformation.

[0024] O: 0.0100% or less O is an element that forms oxides and acts as a fracture initiation point, adversely affecting the toughness of the base material and the joint HAZ. Therefore, the O content is limited to 0.0100% or less. Furthermore, the O content is preferably 0.0050% or less, and more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0%, but O is usually an element that is inevitably contained in steel as an impurity. Therefore, from an industrial standpoint, the O content may be greater than 0%. Excessive reduction of O leads to increased refining costs, so from a cost perspective, the O content is preferably 0.0020% or more.

[0025] Carbon equivalent Ceq shown by the following carbon equivalent (1) formula: 0.250 or more, 0.380 or less Ceq=C+Mn / 6+(Cu+Ni) / 15+(V+Mo+Cr) / 5 ···(1) In the above formula (1), each element symbol represents the content (mass%) of each component, and if no component is contained, it is set to 0. To achieve a yield strength (YS) of 235 MPa or more and a tensile strength (TS) of 400 MPa or more, which are suitable for tanks, Ceq must be 0.250 or more. To achieve a higher tensile strength (TS), Ceq is preferably 0.270 or more, and more preferably 0.290 or more. On the other hand, if Ceq exceeds 0.380, the yield strength (YS) of 440 MPa or less, which is necessary to avoid stress corrosion cracking due to ammonia, cannot be satisfied, and the toughness of the base metal and the joint HAZ also decreases. To improve the toughness of the base metal and the joint HAZ, Ceq is preferably 0.370 or less, and more preferably 0.360 or less.

[0026] The basic composition of the present invention contains the above components, with the balance being Fe and unavoidable impurities. For the purpose of improving strength characteristics or toughness (base metal toughness, HAZ toughness), this basic composition may optionally further contain one or more selected from Cu: 1.00% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.050% or less, B: 0.0050% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less.

[0027] Cu:1.00% or less Cu is an element that has the effect of increasing the hardenability of steel and improving the strength of the steel sheet, and can be added as desired. When Cu is added, in order to obtain the above effect, the Cu content is preferably 0.01% or more, and more preferably 0.20% or more. On the other hand, if the Cu content exceeds 1.00%, it will deteriorate the toughness (base material toughness, HAZ toughness) and increase the alloy cost. Therefore, when Cu is added, the Cu content is set to 1.00% or less, preferably 0.50% or less, and more preferably 0.30% or less.

[0028] Ni: 1.00% or less Ni, like Cu, is an element that improves the strength of steel sheet and can be added as desired. When Ni is added, in order to obtain the above effect, the Ni content is preferably 0.01% or more, and more preferably 0.20% or more. On the other hand, if the Ni content exceeds 1.00%, it leads to deterioration of weldability and an increase in alloy cost. Therefore, when Ni is added, the Ni content is set to 1.00% or less, preferably 0.50% or less, and more preferably 0.30% or less.

[0029] Cr:1.00% or less Cr, like Cu, is an element that improves the strength of steel sheets and can be added as desired. When Cr is added, in order to obtain the above effect, the Cr content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, a Cr content exceeding 1.00% leads to deterioration of weldability and an increase in alloy costs. Therefore, when Cr is added, the Cr content is set to 1.00% or less, preferably 0.50% or less, and more preferably 0.30% or less.

[0030] Mo: 0.50% or less Mo, like Cu, is an element that improves the strength of steel sheet and can be added as desired. When Mo is added, in order to obtain the above effect, the Mo content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, if the Mo content exceeds 0.50%, it leads to deterioration of weldability and an increase in alloy cost. Therefore, when Mo is added, the Mo content is set to 0.50% or less, preferably 0.25% or less, and more preferably 0.10% or less.

[0031] V: 0.50% or less V, like Cu, is an element that improves the strength of the steel sheet and can be added as desired. When V is added, in order to obtain the above effect, the V content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, if the V content exceeds 0.50%, it leads to deterioration of weldability and an increase in alloy cost. Therefore, when V is added, the V content is set to 0.50% or less, preferably 0.25% or less, and more preferably 0.10% or less.

[0032] W: 0.50% or less W, like Cu, is an element that improves the strength of the steel sheet and can be added as desired. When W is added, in order to obtain the above effect, the W content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, if the W content exceeds 0.50%, it will deteriorate the weldability and increase the alloy cost. Therefore, when W is added, the W content is set to 0.50% or less, preferably 0.25% or less, and more preferably 0.10% or less.

[0033] Co:0.50% or less Co, like Cu, is an element that improves the strength of the steel sheet and can be added as desired. When Co is added, in order to obtain the above effect, the Co content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, a Co content exceeding 0.50% leads to deterioration of weldability and an increase in alloy costs. Therefore, when Co is added, the Co content is set to 0.50% or less, preferably 0.25% or less, and more preferably 0.10% or less.

[0034] Nb: 0.050% or less Nb is an element that reduces the prior austenite grain size by precipitating as carbonitrides and has the effect of improving toughness (base material toughness, HAZ toughness). When Nb is contained, in order to obtain the above effect, the Nb content is preferably 0.005% or more, more preferably 0.007% or more. On the other hand, when the Nb content exceeds 0.050%, a large amount of NbC precipitates, and the toughness (base material toughness, HAZ toughness) decreases. Therefore, when Nb is contained, the Nb content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less.

[0035] B: 0.0050% or less B is an element that has the effect of significantly improving hardenability even in small amounts. Therefore, by including B, the strength of the steel sheet can be improved. When B is included, in order to obtain the above effect, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more. On the other hand, if the B content exceeds 0.0050%, the weldability decreases. Therefore, when B is included, the B content is 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0036] Ca: 0.0050% or less Ca is an element that bonds with S and inhibits the formation of MnS and other compounds that elongate in the rolling direction. Therefore, the inclusion of Ca can control the morphology of sulfide-based inclusions to be spherical, thereby improving the toughness of the HAZ. When Ca is added, the Ca content is preferably 0.0005% or more, more preferably 0.0020% or more, to achieve the above effect. On the other hand, if the Ca content exceeds 0.0050%, the cleanliness of the steel decreases. A decrease in cleanliness leads to deterioration of surface properties due to an increase in surface defects and a decrease in bending workability. Therefore, when Ca is added, the Ca content is set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0037] Mg: 0.0050% or less Like Ca, Mg is an element that bonds with S and inhibits the formation of MnS and other compounds that elongate in the rolling direction. Therefore, by including Mg, the morphology of sulfide-based inclusions can be controlled to be spherical, thereby improving the toughness of the HAZ. When Mg is included, to achieve this effect, the Mg content is preferably 0.0005% or more, more preferably 0.0020% or more. On the other hand, if the Mg content exceeds 0.0050%, the cleanliness of the steel decreases. A decrease in cleanliness leads to deterioration of surface properties due to an increase in surface defects and a decrease in bending workability. Therefore, when Mg is included, the Mg content is set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0038] REM: 0.0050% or less Like Ca and Mg, REM (rare earth metals) are elements that bond with S and inhibit the formation of MnS and other compounds that elongate in the rolling direction. Therefore, adding REM controls the morphology of sulfide-based inclusions so that they assume a spherical shape, thereby improving HAZ toughness. When REM is added, to achieve this effect, the REM content is preferably 0.0005% or more, more preferably 0.0020% or more. On the other hand, if the REM content exceeds 0.0050%, the cleanliness of the steel decreases. A decrease in cleanliness leads to deterioration of surface properties due to an increase in surface defects and a decrease in bending workability. Therefore, when REM is added, the REM content is limited to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0039] Ti / N: 2.00 or more and 4.00 or less If the Ti / N ratio is less than 2.00, the amount of TiN produced decreases, and the solute N that does not become TiN reduces HAZ toughness. Therefore, Ti / N is set to 2.00 or more. Note that Ti / N is preferably set to 2.10 or more, and more preferably set to 2.20 or more. Furthermore, if Ti / N exceeds 4.00, TiN coarsens, reducing HAZ toughness. Therefore, Ti / N is set to 4.00 or less. Furthermore, from the viewpoint of improving HAZ toughness, Ti / N is preferably set to 3.90 or less, and more preferably set to 3.80 or less. Note that Ti / N refers to the content (mass%) of each element in the steel, and Ti / N refers to the ratio of the Ti content (mass%) to the N content (mass%).

[0040] 169≦5158×Ti+25563×N≦360 (2) In the above formula (2), each element symbol represents the content (mass %) of each component. Conventional TiN-based HAZ toughness improvement techniques for high-heat-input welding have had problems with insufficient HAZ toughness improvement, such as the decomposition of TiN in the HAZ formed during high-heat-input welding, which results in a loss of grain refinement, and the formation of solute Ti and N by TiN decomposition that embrittle the steel matrix. Therefore, to suppress TiN decomposition, it is essential to set the value of 5158 × Ti + 25563 × N to 169 or greater. From the perspective of further improving HAZ toughness, the value of 5158 × Ti + 25563 × N is preferably 175 or greater, more preferably 180 or greater, and even more preferably 185 or greater. On the other hand, if the value of 5158 × Ti + 25563 × N exceeds 360, a large amount of TiN is formed, which actually reduces HAZ toughness. Therefore, the value of 5158 × Ti + 25563 × N is set to 360 or less. From the viewpoint of further improving toughness (HAZ toughness), 5158×Ti+25563×N is preferably set to 330 or less, more preferably 300 or less, and even more preferably 270 or less.

[0041] The balance consists of Fe and unavoidable impurities. These impurities are unavoidably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are permitted to be present to the extent that they do not impair the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap.

[0042] In addition to having the above-mentioned chemical composition, the steel sheet of the present invention has a microstructure in which the volume fraction of ferrite at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet is 20% or more, the total volume fraction of ferrite and bainite is 80% or more, and the volume fraction of island martensite is 5% or less.

[0043] The reasons for limiting the microstructure of the steel as described above will be explained below.

[0044] [Microstructure] The microstructure of the steel sheet of the present invention will be described.

[0045] [The volume fraction of ferrite at a depth of 1 / 4 of the plate thickness from the surface of the steel plate is 20% or more, the total volume fraction of ferrite and bainite is 80% or more, and the volume fraction of island martensite is 5% or less] The structure at a depth of 1 / 4 of the plate thickness from the surface of the steel plate has a ferrite volume fraction of 20% or more, a total of ferrite and bainite volume fraction of 80% or more, and island martensite contained in the remainder volume fraction of 5% or less. The microstructure at a depth of 1 / 4 of the plate thickness from the surface of the steel plate is specified because it has a strong influence on the strength characteristics of the entire steel plate.

[0046] If the volume fraction of ferrite is less than 20%, the yield strength (YS) of the base material increases, making it impossible to achieve a yield strength (YS) of 440 MPa or less. Furthermore, the toughness of the base material decreases. Therefore, the volume fraction of ferrite is set to 20% or more. The volume fraction of ferrite is preferably 30% or more, and more preferably 40% or more. While there is no particular upper limit, it is preferable that the volume fraction of ferrite be 60% or less, since a decrease in the volume fraction of bainite would result in a decrease in strength.

[0047] On the other hand, if the total volume fraction of ferrite and bainite is less than 80%, the toughness of the base material decreases, so the total volume fraction of ferrite and bainite is set to 80% or more. The total volume fraction of ferrite and bainite is preferably 85% or more, and more preferably 90% or more. There is no particular upper limit, but the upper limit of the total volume fraction of ferrite and bainite may be 100% or less.

[0048] In order to achieve a tensile strength (TS) of 400 MPa or more, the volume fraction of bainite is preferably 20% or more.

[0049] The remaining structure other than the ferrite and bainite accounts for 20% or less in volume fraction. The remaining structure may contain martensite or island martensite in addition to pearlite and austenite. The island martensite in the remaining structure has higher strength but lower toughness than normal martensite, and can become the starting point of fracture. If the volume fraction of island martensite exceeds 5%, the toughness of the base material decreases significantly, so the volume fraction of island martensite is set to 5% or less, preferably 3% or less, and can of course be 0%. The volume fraction of each structure in the remaining structure other than island martensite does not need to be particularly limited, but pearlite is preferred. The volume fractions of various microstructures can be measured by the methods described in the Examples below.

[0050] Next, a method for producing a steel sheet according to the present invention will be described.

[0051] A steel material having the above-described chemical composition is heated and hot-rolled to obtain a hot-rolled steel sheet, and then cooled to a starting temperature equal to or higher than the Ar3 transformation point to obtain a steel sheet. Each manufacturing condition will be explained in detail below. In the present invention, the temperatures of the steel material and the steel sheet are those at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet, unless otherwise specified.

[0052] First, the conditions for producing the steel material do not need to be particularly limited, but it is preferable to produce molten steel having the above-mentioned composition by a known melting method such as a converter, and then form the molten steel into a steel material such as a slab of a predetermined size by a known casting method such as a continuous casting method. Note that there is no problem with producing a steel material such as a slab of a predetermined size by an ingot casting-decomposition rolling method.

[0053] The resulting steel material is either hot-rolled directly without cooling, or is reheated after cooling and then hot-rolled. Hot rolling is performed at a starting temperature of at least Ar3 transformation point (hereinafter also referred to as "Ar3 point") + 100°C and an ending temperature of at least Ar3 point to produce a hot-rolled steel sheet. Next, cooling of the hot-rolled steel sheet is started at a temperature above the Ar3 point, and the steel sheet is cooled until the temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel sheet is at least Ar3 point - 200°C and at most Ar3 point. The cooling is then stopped for at least 5 seconds, and the steel sheet is subsequently cooled (second cooling) at an average cooling rate of 100°C / s or less to the temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel sheet. The cooling (second cooling) is stopped when the temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel sheet is at least 400°C.

[0054] (a) Heating temperature of steel material: Ac3 point or higher, 1250°C or lower If the heating temperature of the steel material exceeds 1250°C, austenite will coarsen, the toughness of the base material will decrease, and oxidation will become severe, which may increase oxidation loss and reduce yield. For these reasons, the heating temperature is set to 1250°C or lower. The heating temperature of the steel material is preferably set to 1150°C or lower, and more preferably to 1100°C or lower. On the other hand, if the heating temperature of the steel material is lower than the Ac3 point, reverse transformation to austenite will not occur, and deformation resistance will increase, which may increase the load on the hot rolling mill and make hot rolling difficult. For these reasons, the heating temperature of the steel material is set to 950°C or higher, and more preferably to 1000°C or higher, from the viewpoint of improving the yield strength (YS) and tensile strength (TS) of the base material.

[0055] Here, the Ac3 point can be determined, for example, by the following equation (3). Ac3(℃)=937-476.5×C+56×Si-19.7×Mn-16.3×Cu-26.6×Ni-4.9×Cr+38.1×Mo+124.8×V-136.3×Nb-19×Ti+3315×B...(3) However, the content (mass%) of each element is shown, and if the element is not contained, it is set to 0.

[0056] (b) Hot rolling start temperature: Ar3 point + 100°C or higher After the steel material is heated to the above temperature, hot rolling is initiated at a temperature of Ar3 point + 100°C or higher. In other words, if the rolling start temperature is less than Ar3 point + 100°C, the austenite grain size will become mixed grains, which will deteriorate the toughness of the base material and may make it impossible to satisfy the subsequent rolling end temperature and cooling start temperature. The rolling start temperature is preferably Ar3 point + 150°C or higher, and more preferably Ar3 point + 200°C or higher. There is no particular upper limit, but to avoid high heating temperatures, it is preferably 1100°C or lower, and more preferably 1050°C or lower.

[0057] Here, the Ar3 point can be determined, for example, by the following equation (4). Ar3(℃)=910-273×C-74×Mn-57×Ni-16×Cr-9×Mo-5×Cu...(4)

[0058] (c) Rolling end temperature: Ar3 point or higher Rolling is completed at a temperature equal to or higher than the Ar3 point. That is, if the rolling completion temperature is lower than the Ar3 point, ferrite is generated, and the generated ferrite is affected by processing, resulting in a deterioration in toughness (base material toughness) and an increased load on the hot rolling mill. Therefore, the rolling completion temperature is set to the Ar3 point or higher. Furthermore, the rolling completion temperature is preferably set to the Ar3 point + 20°C or higher. It is more preferable that the rolling completion temperature is set to the Ar3 point + 40°C or higher. There is no particular upper limit, but it is preferably set to 1000°C or lower to avoid an increase in the heating temperature.

[0059] (d) Cooling start temperature in the first cooling step: above Ar3 point Next, the hot-rolled steel sheet is cooled from a temperature above the Ar3 point after hot rolling. From the viewpoint of improving the yield strength (YS) and tensile strength (TS) of the base material, the cooling start temperature in the first cooling step is set to be above the Ar3 point. The cooling start temperature is preferably set to the Ar3 point + 20°C or more, and more preferably to the Ar3 point + 40°C or more. There is no particular upper limit, but to avoid a high heating temperature, it is preferably set to the Ar3 point + 100°C or less.

[0060] (e) Average cooling rate in the first cooling pass: The average cooling rate in the first cooling is not particularly limited. From the viewpoint of improving the yield strength (YS) and tensile strength (TS) of the base material, the average cooling rate is preferably 5°C / s or more, and more preferably 10°C / s or more. There is no particular upper limit to the cooling rate, but since excessive cooling increases cooling costs, it is preferable to set the average cooling rate to 200°C / s or less. The average cooling rate is calculated by dividing the difference between the temperature (°C) at the start of cooling and the temperature (°C) at the end of cooling by the cooling time (s).

[0061] (f) Cooling stop temperature in the first cooling: Ar3 point - 200°C or higher and Ar3 point or lower The stop temperature of the first cooling pass is set to the Ar3 point or below so that the volume fraction of ferrite at a depth of 1 / 4 of the plate thickness from the surface of the steel plate and the combined volume fraction of ferrite and bainite are predetermined. If the cooling stop temperature exceeds the Ar3 transformation point, ferrite will not be sufficiently formed, resulting in an excessively large YS. Therefore, the cooling stop temperature is set to the Ar3 point (Ar3 transformation point) or below. The cooling stop temperature is preferably set to the Ar3 point - 50°C or below, and more preferably to the Ar3 point - 100°C or below. Furthermore, to increase the yield strength (YS) and tensile strength (TS) of the base metal and achieve a bainite volume fraction of 20% or more, the cooling stop temperature is set to the Ar3 point - 200°C or above. The cooling stop temperature is preferably set to the Ar3 point - 180°C or above, and more preferably to the Ar3 point - 160°C or above.

[0062] (g) Cooling stop time: 5 seconds or more After the first cooling, the cooling is temporarily stopped for 5 seconds (s) or more. By stopping the cooling, ferrite can be generated. If the cooling stop time is less than 5 seconds, ferrite will not be generated sufficiently, and the yield strength (YS) of the base material will become excessively high. Therefore, the cooling stop time is set to 5 seconds or more. The cooling stop time is preferably 10 seconds or more, more preferably 20 seconds or more, and even more preferably 30 seconds or more. On the other hand, although there is no particular upper limit for the cooling stop time, in order to increase the volume fraction of bainite to 20% or more in order to increase the yield strength (YS) and tensile strength (TS) of the base material, the cooling stop time is preferably 600 seconds or less. It is more preferably 300 seconds or less, and even more preferably 100 seconds or less.

[0063] (h) Second cooling rate: The average cooling rate at a depth of 1 / 4 of the plate thickness from the surface of the steel plate is 100°C / s or less. After the cooling is stopped, cooling is resumed. The average cooling rate is set to 100°C / s or less so that the microstructure reaches a predetermined volume fraction. If the average cooling rate exceeds 100°C / s, the volume fraction of island martensite becomes too high, resulting in a decrease in toughness (base material toughness, HAZ toughness). Therefore, the average cooling rate is set to 100°C / s or less. The average cooling rate is preferably set to 80°C / s or less, and more preferably to 60°C / s or less. On the other hand, to increase the yield strength (YS) and tensile strength (TS) of the base material and achieve a bainite volume fraction of 20% or more, the average cooling rate is preferably set to 5°C / s or more. As mentioned above, the average cooling rate is calculated by dividing the difference between the temperature (°C) at the start of cooling and the temperature (°C) at the end of cooling by the cooling time (s).

[0064] The second cooling start temperature is not particularly limited, but is preferably equal to or lower than the first cooling end temperature and equal to or higher than (first cooling end temperature - 200°C).

[0065] (i) Second cooling stop temperature: Cooling stop temperature is 400°C or higher The end temperature of the second cooling is set to 400°C or higher to adjust the hardness of the surface layer and the strength and toughness of the base material. If the cooling stop temperature is less than 400°C, the yield strength (YS) of the base material will be excessively high, the toughness (base material toughness) will decrease, and the hardness of the surface layer will also increase. Therefore, the cooling stop temperature is set to 400°C or higher. The cooling stop temperature is preferably 450°C or higher, and more preferably 500°C or higher. In order to increase the tensile strength (TS) of the base material, the cooling stop temperature is preferably set to 580°C or lower.

[0066] (j) Threading speed: 45mpm or more The series of cooling steps (d) to (i) above (including the first and second cooling steps) is performed in an online cooling facility at a sheet threading speed of 45 mpm (m / min) or more. If cooling is performed at a sheet threading speed of less than 45 mpm, the cooling of the steel sheet becomes uneven, resulting in a decrease in the longitudinal flatness of the sheet. Therefore, the sheet threading speed is set to 45 mpm or more. To improve flatness, the sheet threading speed is preferably set to 50 mpm or more, and more preferably 60 mpm or more. There is no upper limit to the sheet threading speed, but a speed exceeding 200 mpm increases the length of the cooling equipment, increasing the cost of the equipment, so it is preferably set to 200 mpm or less.

[0067] A steel plate having the above-described structure can be obtained by manufacturing a steel material having the above-described chemical composition according to the above-described manufacturing conditions. The steel plate thus obtained has strength characteristics and toughness (base material toughness and HAZ toughness) suitable for a cryogenic tank storing ammonia. Yield strength (YS) is closely related to ammonia stress corrosion cracking resistance, and the IMO Gas Code and classification rules of the International Maritime Organization (IMO) stipulate that the yield point of a structural member of a liquefied gas bulk carrier must be 440 MPa or less to minimize the risk of ammonia stress corrosion cracking. Therefore, the yield strength in the present invention is set to 440 MPa or less. Preferably, the yield strength is 430 MPa or less, and more preferably 420 MPa or less. Furthermore, the yield strength is set to 235 MPa or more.

[0068] Basically, the higher the tensile strength (TS) of a steel plate, the better, and steel plates of 400 MPa or more are usually used. Therefore, the tensile strength is set to 400 MPa or more. The tensile strength (TS) of a steel plate is preferably 440 MPa or more, and more preferably 490 MPa or more. However, if the tensile strength (TS) of a steel plate is too high, it will not be possible to satisfy the yield strength (YS) (yield point (YP) when there is a yield point, or 0.2% proof stress σ0.2 when there is no yield point) of 440 MPa or less to ensure ammonia stress corrosion cracking resistance, so the tensile strength (TS) of a steel plate is preferably 620 MPa or less.

[0069] Furthermore, it is preferable that the yield ratio, which is the yield strength (YS) divided by the tensile strength (TS), is 85% or less. Furthermore, by setting the yield ratio to 80% or less, it is possible to omit post-weld heat treatment (PWHT) that is performed to remove residual stress after welding, so it is more preferable that the yield ratio is 80% or less.

[0070] In addition, the toughness of the base material of the steel plate vE -55 is over 100J.

[0071] When a 2m long piece is placed on the surface of a steel plate along the rolling direction, the maximum gap between the surface of the steel plate and the long piece is 14mm or less Figure 1 shows a schematic diagram (cross-sectional view) of measuring the maximum gap between a 2 m long piece and the steel sheet surface in the rolling direction by placing the piece against the surface of the steel sheet. Reference numeral 1 denotes the steel sheet, reference numeral 2 denotes the long piece, reference numeral 3 denotes the maximum gap between the steel sheet surface and the long piece, and reference numeral 4 denotes the rolling direction. In order to reduce the flatness of the steel sheet, when a 2 m long piece is placed against the surface of the steel sheet in the rolling direction, the maximum value of the gap between the steel sheet surface and the long piece needs to be 14 mm or less. The maximum value is preferably 12 mm or less, and more preferably 10 mm or less. The lower limit is not particularly limited, and may be 0 mm. The maximum value of the gap between the steel sheet surface and the long piece refers to the value at which the difference in height of the steel sheet surface in the thickness direction is maximum, as shown in Figure 1. When the steel plate is a steel strip, the measurement position may be any of the leading end, center, or tail end of the steel strip, but it is preferable to measure at 1 / 4 width of the leading end. Furthermore, the length may be longer than that of a steel plate, and the above measurement can also be applied to a cut plate.

[0072] The thickness of the steel sheet of the present invention is less than 13 mm. Although there is no particular lower limit, it is preferable to target a steel sheet having a thickness of 8 mm or more. [Example]

[0073] Molten steel having the chemical composition shown in Table 1 was melted to prepare steel materials (slabs). These steel materials (slabs) were heated, hot rolled and cooled under the conditions shown in Table 2 to obtain steel sheets.

[0074] [Microstructure at a depth of 1 / 4 of the plate thickness from the surface] A sample was taken from each of the obtained steel plates so that the observation surface was at a depth of 1 / 4 of the plate thickness from the surface of the steel plate. The surface of the sample was mirror-polished and further subjected to nital etching, and then an image of an area of ​​1000 μm × 1000 μm was taken using a scanning electron microscope (SEM). Note that the SEM observation was performed at an acceleration voltage of 20 kV and a magnification of 1000 times. The area fraction of the microstructure was determined by analyzing the photographed image using an image analyzer. When the anisotropy of the microstructure is small, the area fraction corresponds to the volume fraction, and therefore, in this specification, the area fraction is referred to as the volume fraction.

[0075] In either case, when determining the area fraction of the microstructure, the distinction between each structure was made as follows. The steel material was mirror-polished and etched with nital to reveal the structure, which was then observed under an SEM at 500 to 3000 times magnification. Ferrite was defined as a structure that did not contain isotropically grown carbides and in which the grains appeared black, while pearlite was defined as a structure in which ferrite (black) and carbides (white) appeared in a striped pattern. Bainite was defined as a structure that had a long, thin, lath-shaped ferrite structure and contained carbides with a circle equivalent diameter of 0.05 μm or more, and in which carbides were further defined as 1.0 × 10 4 pieces / mm 2 When it contains more than 1.0 x 10 carbides, it is defined as tempered bainite. In a tempered structure, the carbides are divided, and for example, the long and thin carbides that appear between the laths of bainite have become multiple round carbides, so it is easy to distinguish by looking at the carbides. Martensite has a lath-shaped ferrite structure that has grown long and thin like bainite, and is a structure that contains carbides of less than 0.05 μm in equivalent circle diameter, and further, carbides of 1.0 x 10 carbides are defined as 1.0 x 10 carbides. 4 pieces / mm 2If the structure contains more than 0.50 μm in diameter, it is defined as tempered martensite. Note that the carbides appear as white dots. Austenite is defined as a structure that does not contain carbides and has a circle equivalent diameter of 0.50 μm or more, and exists between bainite and martensite structures.

[0076] The total of ferrite and bainite is the combined volume fraction of ferrite, which is a structure that does not contain isotropically grown carbides and in which the grains appear black, the bainite, and tempered bainite.

[0077] Furthermore, a structure that does not contain carbides and appears white was defined as island martensite.

[0078] [Tensile properties of base material] From the entire thickness of each steel plate, JIS Z 2241 (2022) No. 1B test pieces are taken in the direction perpendicular to the rolling direction, and a tensile test is carried out in accordance with JIS Z 2241 (2022). The yield strength (YS) (if there is a yield point, it is the yield point YP, and if there is no yield point, it is the 0.2% proof stress σ) is measured. 0.2 The steel sheets were evaluated as having excellent tensile strength properties if they had a tensile strength of 400 MPa or more and a yield strength of 235 MPa or more, and as having excellent ammonia stress corrosion cracking resistance if they had a yield strength of 440 MPa or less.

[0079] [Base material toughness] In addition, V-notch test pieces according to JIS Z 2242 (2023) were taken from the area shaved 1 mm from the surface of each steel plate in the rolling direction, and a Charpy impact test was performed according to the method of JIS Z 2242 (2023). The test pieces were then subjected to a Charpy impact test at a test temperature of -55°C, and the average value of the absorbed energy of three test pieces performed under the same conditions was calculated as vE -55℃ (Unit: J) was taken as the toughness of the base material. And, regarding the toughness of the base material, vE -55℃ When the strength was 100 J or more, it was evaluated as having excellent toughness (base material toughness).

[0080] [Joint HAZ toughness] Furthermore, test plates for joints taken from each of the steel plates were subjected to Y-groove processing, and submerged arc welding was performed with a welding heat input of 60 kJ / cm using commercially available welding wire for low-temperature steel to produce joints using high heat input welding. The toughness (HAZ toughness) of the obtained joints was then evaluated. The test methods are as follows: NK U4 impact test specimens were taken so that the surface layer of the test specimen was from the surface of the joint to a depth of 1 mm, and the HAZ was the notch position. Charpy impact tests were performed on the taken test specimens at a test temperature of -55°C, and the average absorbed energy vE of three test specimens taken under the same conditions was calculated. -55℃ (Unit: J) was taken as the toughness of the HAZ. -55℃ When the HAZ toughness was 100 J or more, it was evaluated as excellent HAZ toughness.

[0081] [Flatness] A 2m long piece was placed against the surface of each steel plate in the rolling direction, and the gap between the steel plate surface and the long piece was measured with a feeler gauge to determine the maximum value. The measurement was performed at three locations, the center and both ends in the width direction of the steel plate, and the average of the three maximum values ​​was evaluated.

[0082] The evaluation results thus obtained are shown in Table 2.

[0083] [Table 1]

[0084] [Table 2]

[0085] As can be seen from Tables 1 and 2, all of the inventive examples have a yield strength (YS) of 235 MPa or more and 440 MPa or less, a high tensile strength (TS) of 400 MPa or more, a base metal toughness of 100 J or more, a HAZ toughness of 100 J or more, and a flatness of 14 mm or less.

[0086] Steel plates Nos. 15 and 18, in which Ti and N were controlled within the preferred ranges, had HAZ toughness that was even better than that of the other invention examples.

[0087] On the other hand, steel plates Nos. 2 to 11, which correspond to comparative examples and whose manufacturing conditions deviate from those of the present invention, are inferior to the invention examples in any one of yield strength (YS), tensile strength (TS), base metal toughness, HAZ toughness, and flatness. Steel plate No. 26, which corresponds to a comparative example, has a low carbon content and its tensile strength (TS) is inferior to the invention examples. Steel plates Nos. 27 to 37 contain various elements in amounts lower or higher than the invention examples, and at least one of base metal tensile strength (TS), base metal toughness, and HAZ toughness is inferior to the invention examples. Steel plate No. 38 has a low Ceq and its tensile strength (TS) is inferior to the invention examples. Steel plate No. 39 has a high Ceq, resulting in excessively high yield strength (YS), and its base metal toughness and HAZ toughness are inferior to the invention examples. Steel plate No. 40 has inferior base metal toughness to the invention examples. [Explanation of symbols]

[0088] 1 steel plate 2 Long length 3 Maximum gap between steel plate surface and length 4. Rolling direction

Claims

1. In mass%, C: 0.045% or more, 0.100% or less, Si: 0.50% or less, Mn: 0.50% or more, 1.80% or less, P: 0.010% or less, S: 0.0100% or less, Al: 0.060% or less, Ti: 0.005% or more, 0.050% or less, N: 0.0100% or less; and O: 0.0100% or less, a carbon equivalent Ceq represented by the following formula (1) is 0.250 or more and 0.380 or less, and the balance is Fe and unavoidable impurities, The microstructure has a volume fraction of ferrite of 20% or more at a depth of 1 / 4 of the plate thickness from the surface, a total volume fraction of ferrite and bainite of 80% or more, and a volume fraction of island martensite of 5% or less, The yield strength is 235 MPa or more and 440 MPa or less and the tensile strength is 400 MPa or more, Base material toughness vE －５５ is 100 J or more, When a 2 m long piece is applied to the surface of the steel sheet along the rolling direction, the maximum value of the gap between the steel sheet surface and the long piece is 14 mm or less, A steel plate having a thickness of less than 13 mm. Ceq=C+Mn / 6+(Cu+Ni) / 15+(V+Mo+Cr) / 5...(1) In the formula (1), each element symbol represents the content (mass %) of each component, and is set to 0 when the component is not contained.

2. The component composition further includes, in mass %, Cu: 1.00% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.050% or less, B: 0.0050% or less, Ca: 0.0050% or less, Mg: 0.0050% or less and The steel sheet according to claim 1, further comprising at least one selected from the group consisting of REM: 0.0050% or less.

3. The steel sheet according to claim 1, wherein the component composition further has a Ti / N ratio in mass% of 2.00 or more and 4.00 or less, and satisfies the following formula (2): 169≦5158×Ti+25563×N≦360 (2) In the above formula (2), each element symbol represents the content (mass %) of each component.

4. The steel sheet according to claim 2, wherein the component composition further has a Ti / N ratio in mass% of 2.00 or more and 4.00 or less, and satisfies the following formula (2): 169≦5158×Ti+25563×N≦360 (2) In the above formula (2), each element symbol represents the content (mass %) of each component.

5. A steel material having the component composition according to any one of claims 1 to 4, A.C. ３ Heating to a temperature of 1250°C or higher, Next, the rolling start temperature was set to Ar ３ point + 100 ° C or more, the rolling end temperature is Ar ３ Hot rolling is performed to a temperature of 1000°C or higher, After that, the cooling start temperature is set to Ar ３ point, the cooling stop temperature is Ar ３ Point -200℃ or higher Ar ３ The first cooling is performed under conditions below the The cooling is stopped for 5 seconds or more, Subsequently, a second cooling step is performed in which the average cooling rate at a depth of ¼ of the plate thickness from the surface of the steel plate is set to 100°C / s or less, and cooling is stopped at a temperature of 400°C or higher; A series of cooling steps including the first cooling step and the second cooling step is carried out at a sheet threading speed of 45 mpm or more using online cooling equipment. Steel plate manufacturing method.

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