Steel sheet and method for manufacturing same

Abstract

Provided are: a steel sheet that has exceptional ammonia stress corrosion cracking resistance and low-temperature toughness, and is used for storage tanks used to store liquefied gas in energy transport vessels, for example; and a method for manufacturing the steel sheet. A steel sheet according to the present invention has a prescribed component composition, has a hardness at a depth position of 1 mm from the surface of 210 HV or less, and has a microstructure in which the volume ratio of each of ferrite and bainite is 20% or more at a depth position of 1 / 4 of the sheet thickness from the surface, the volume ratio of the total of ferrite and bainite is 80% or more, and the volume ratio of martensite islands is 5% or less. The steel sheet has a yield strength of 355-440 MPa, a tensile strength of 440 MPa or more, and a ductility-brittleness fracture surface transition temperature vTrs of -60°C or less.

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a steel plate having excellent toughness and corrosion resistance, in particular to a steel plate having excellent low-temperature toughness and ammonia stress corrosion cracking resistance for use in multipurpose tanks for carrying a mixture of liquefied petroleum gas (hereinafter referred to as LPG) and liquid ammonia, and a method for manufacturing the same.

[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, not only LPG but also liquid ammonia may be transported in the tanks.

[0003] 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.

[0004] The steel plates used in these tanks are required to have a tensile strength (TS) of 440 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 is avoided.

[0005] The techniques for providing the low-temperature toughness and satisfying the strength range required for liquefied gas storage tanks as described above are described in Patent Documents 1 and 2, and high low-temperature toughness and predetermined strength characteristics are achieved by a method in which a thick steel plate that has been cooled after hot rolling is heat-treated several times, or a thick steel plate that has been water-cooled after hot rolling is heat-treated several times.

[0006] Patent No. 3802626 Patent No. 3848415

[0007] The methods described in Patent Documents 1 and 2 above require multiple heat treatments, which poses an economic problem due to the high costs of equipment and energy required for the treatment. Furthermore, when the first quenching temperature is high, the improvement in toughness at a depth of ¼ of the plate thickness from the surface of the steel plate is small, which may result in unstable toughness in the surface layer of the steel plate exposed to high temperatures for a long time.

[0008] An object of the present invention is to solve the above problems and to provide a steel plate having excellent ammonia stress corrosion cracking resistance and low-temperature toughness, which can be used for storage tanks used to store liquefied gas on energy transport ships, for example, and a method for manufacturing the same.

[0009] In order to achieve the above object, the present inventors have conducted extensive research using an online cooling device to examine various factors affecting the low-temperature toughness and strength properties of a steel plate. As a result, they have found that desired low-temperature toughness and strength properties can be exhibited by adding elements such as C, Si, Mn, Ti, etc. to a steel plate in predetermined amounts or more and controlling the microstructure so that the volume fractions of ferrite and bainite at a depth of ¼ of the plate thickness from the surface of the steel plate are 20% to 80%, respectively, and the volume fraction of island martensite is 5% or less.

[0010] Furthermore, the present inventors have found that by controlling the hardness at a depth of 1 mm from the surface of the steel plate to Hv 210 or less, stress corrosion cracking resistance in a liquid ammonia environment is improved.

[0011] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows.

[0012] 1. A steel sheet having a component composition containing, in mass%, C: 0.045% or more and 0.120% or less, Si: 0.50% or less, Mn: 0.50% or more and 2.00% or less, P: 0.020% or less, S: 0.0100% or less, Al: 0.060% or less, Ti: 0.005% or more and 0.050% or less, N: 0.0010% or more and 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities, wherein the carbon equivalent Ceq represented by the following formula (1) is 0.25 or more and 0.40 or less, and the hardness at a depth of 1 mm from the surface is HV210 or less, A steel plate having a microstructure at a depth of 1 / 4 of the plate thickness from the surface, in which the volume fractions of ferrite and bainite are each 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 steel plate having a yield strength of 355 MPa or more and 440 MPa or less, a tensile strength of 440 MPa or more, and a ductile-brittle fracture transition temperature vTrs of -60°C or less: Ceq=C+Mn / 6+(Cu+Ni) / 15+(V+Mo+Cr) / 5 (1) where, in the above formula (1), each element symbol represents the content (mass%) of each element, and if the element is not contained, it is set to 0.

[0013] 2. The steel sheet according to 1 above, wherein the chemical composition further contains, in mass%, one or more elements selected from Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.10% or less, Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less.

[0014] 3. The steel plate according to 1 or 2 above, wherein the hardness at a depth of 1 mm from the surface is HV208 or less.

[0015] 4. A steel material containing, by mass%, C: 0.045% or more and 0.120% or less, Si: 0.50% or less, Mn: 0.50% or more and 2.00% or less, P: 0.020% or less, S: 0.0100% or less, Al: 0.060% or less, Ti: 0.005% or more and 0.050% or less, N: 0.0010% or more and 0.0100% or less, and O: 0.0100% or less, and having a carbon equivalent Ceq represented by the following formula (1) of 0.25 or more and 0.40 or less, with the balance being Fe and unavoidable impurities, is heated to a temperature of 950°C or more and 1250°C or less, and then, the rolling start temperature is set to Ar ３ The rolling end temperature is set to Ar ３ After that, hot rolling is performed to a temperature of 1000°C or more. ３ Cooling begins from a temperature above the Ar point. ３ Ar below point ３ A method for manufacturing a steel plate, comprising: cooling the steel plate at an average cooling rate of 5°C / s or more to a temperature of -90°C or higher; stopping the cooling once and suspending the cooling for 5 to 600 seconds; subsequently, cooling the steel plate at an average cooling rate of 5°C / s or more and 100°C / s or less at a depth of 1 / 4 of the plate thickness from the surface; and completing the cooling when the temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel plate is 400°C or more and 580°C or less. Ceq=C+Mn / 6+(Cu+Ni) / 15+(V+Mo+Cr) / 5 (1) where, in the above formula (1), each element symbol represents the content (mass%) of each element, and is set to 0 if the element is not contained.

[0016] 5. The method for producing a steel sheet according to 4 above, wherein the chemical composition further contains, in mass%, one or more selected from Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.10% or less, Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less.

[0017] 6. The Ar ３ 6. The method for producing a steel sheet according to 4 or 5 above, wherein the cooling, which starts from a temperature equal to or higher than the above temperature, is stopped at a temperature of 700°C or higher.

[0018] According to the present invention, it is possible to provide, at low cost, a steel plate which has excellent low-temperature impact resistance (Charpy toughness value) and resistance to ammonia stress corrosion cracking, and which is suitable for tanks to be used in low-temperature, corrosive environments, and therefore, it is extremely effective in industry.

[0019] First, the steel sheet of the present invention will be specifically described. In the present invention, it is important that the steel sheet and the steel material used for producing it have the above-mentioned composition. Therefore, first, the reason for limiting the composition of the steel to the above-mentioned composition in the present invention will be explained. Note that "%" regarding the composition means "mass %" unless otherwise specified.

[0020] [Component Composition] C: 0.045% or More and 0.120% or Less C is an element that increases the hardenability of steel and is one of the important elements that must be added to achieve the desired strength (YS, TS) of the base metal. To achieve this effect, the C content is set to 0.045% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and achieving lower manufacturing costs, the C content is preferably set to 0.050% or more, and more preferably set to 0.055% or more. On the other hand, if the C content exceeds 0.120%, the toughness of the base metal and the toughness of the joint HAZ decrease. Therefore, the C content is set to 0.120% or less. Furthermore, from the viewpoint of suppressing a decrease in the toughness and weldability of the base metal, the C content is preferably set to 0.115% or less, and more preferably set to 0.110% or less.

[0021] Si: 0.50% or less Si acts as a deoxidizer, but it also reduces the toughness of the base material and the joint HAZ. Therefore, it is preferable to keep the Si content as low as possible, but a content of 0.50% or less is acceptable. Since steel can be sufficiently deoxidized with Al, Ti, or the like, the lower limit of the Si content is not particularly limited and may be 0%. From the viewpoint of the toughness and weldability of the base material, 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 0.02% or more. Furthermore, from the viewpoint of lower manufacturing costs, the Si content is more preferably 0.03% or more, and even more preferably 0.04% or more.

[0022] Mn: 0.50% or more and 2.00% or less Mn is an element that increases the hardenability of steel and is one of the important elements that must be added to satisfy the tensile strength (TS) of the base metal. To achieve this effect, the Mn content is set to 0.50% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and achieving lower manufacturing costs, the Mn content is preferably set to 0.70% or more, and more preferably set to 0.90% or more. On the other hand, if the Mn content exceeds 2.00%, the toughness of the base metal and the toughness of the joint HAZ will decrease, and further, the alloy cost will become excessively high. Therefore, the Mn content is set to 2.00% or less. Furthermore, from the viewpoint of suppressing deterioration in the toughness and weldability of the base metal, the Mn content is preferably set to 1.80% or less, and more preferably set to 1.60% or less.

[0023] P: 0.020% or less P is an element contained as an unavoidable impurity, and by segregating at grain boundaries, it has adverse effects such as reducing the toughness of the base material and the toughness of the joint HAZ. Therefore, it is desirable to keep the P content as low as possible, but a content of 0.020% or less is acceptable. The lower limit of the P content is not particularly limited and may be 0%, but since P is usually an element that is inevitably contained in steel as an impurity, the P content may be more than 0% industrially. Moreover, since excessive reduction leads to an increase in refining costs, it is preferable to set the P content to 0.0005% or more.

[0024] S: 0.0100% or less S is an element contained as an unavoidable impurity. It exists in steel as sulfide-based inclusions such as MnS, and has an adverse effect on the toughness of the base material, such as becoming the origin of fracture. Therefore, it is desirable to reduce the S content as much as possible. S also has an adverse effect, such as reducing the toughness of the joint HAZ. Therefore, the S content is set to 0.0100% or less. The lower limit of the S content is not particularly limited and may be 0%. Since S is usually an element inevitably contained in steel as an impurity, the S content may exceed 0% industrially. In other words, excessive reduction of S leads to an increase in refining costs, so from a cost perspective, it is more preferable to set the S content to 0.0005% or more.

[0025] Al: 0.060% or less Al acts as a deoxidizer, reducing oxide-based inclusions, improving the toughness of the joint HAZ, and refining the grains. 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, thereby reducing the toughness of the base material and the toughness of the joint HAZ. 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.

[0026] 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 solute N. Therefore, the addition of Ti can improve the toughness of the base metal and the joint HAZ. To achieve this effect, the Ti content is set to 0.005% or more. Furthermore, the Ti content is preferably set to 0.012% or more. On the other hand, if the Ti content exceeds 0.050%, the toughness of the base metal and the joint HAZ will actually decrease. Therefore, the Ti content is set to 0.050%. Furthermore, the Ti content is preferably set to 0.040% or less, and more preferably 0.030% or less.

[0027] N: 0.0010% or more and 0.0100% or less N combines with Ti to precipitate as TiN, contributing to microstructural refinement and improving the toughness of the base metal and joint HAZ. To achieve this effect, the N content is set to 0.0010% or more. Furthermore, the N content is preferably set to 0.0020% or more. On the other hand, an N content exceeding 0.0100% actually leads to a decrease in the toughness of the base metal and joint HAZ. Therefore, from the viewpoint of suppressing a decrease in the toughness of the base metal and the joint HAZ, the N content is set to 0.0100% or less. Furthermore, the N content is preferably set to 0.0080% or less, and more preferably to 0.0060% or less.

[0028] O: 0.0100% or less O is an element contained as an unavoidable impurity. It forms oxides and becomes the origin of fracture, adversely affecting the toughness of the base material and the joint HAZ. Therefore, the O content is limited to 0.0100% or less. 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 unavoidably contained in steel as an impurity. Therefore, industrially, the O content may be more than 0%. That is, excessive reduction of O leads to an increase in refining costs, so from a cost perspective, the O content is preferably 0.0020% or more.

[0029] Ceq: 0.25 or more and 0.40 or less In order to satisfy the specified strength and toughness of the base metal in the steel plate, it is essential to set the carbon equivalent, defined by the following formula (1): Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (V + Mo + Cr) / 5 (1), to 0.25 or more and 0.40 or less. If Ceq is less than 0.25, the yield strength (YS) of 355 MPa or more and the tensile strength (TS) of 440 MPa or more required for the tank cannot be achieved. On the other hand, if Ceq exceeds 0.40, the yield strength (YS) of 440 MPa or less required to avoid stress corrosion cracking due to ammonia cannot be satisfied, and the toughness of the base metal and the toughness of the joint HAZ also decrease. In order to achieve a higher tensile strength (TS), Ceq is preferably 0.30 or more, and more preferably 0.35 or more.

[0030] The basic composition of the present invention contains the above components, with the balance being Fe and unavoidable impurities. This basic composition may optionally further contain one or more elements selected from Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.10% or less, Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less, for the purpose of improving strength characteristics or toughness.

[0031] 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, a Cu content exceeding 1.00% leads to a deterioration in toughness and an increase in alloy costs. Therefore, when Cu is added, the Cu content is preferably 1.00% or less, and more preferably 0.50% or less.

[0032] Ni: 1.00% or less Ni is an element that, like Cu, has the effect of improving the strength of the 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, more preferably 0.20% or more. On the other hand, if the Ni content exceeds 1.00%, it will lead to deterioration of weldability and an increase in alloy costs. Therefore, when Ni is added, the Ni content is preferably 1.00% or less, more preferably 0.50% or less.

[0033] Cr: 0.10% or less Cr is an element that, like Cu, has the effect of improving the strength of the steel sheet 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, and more preferably 0.05% or more. On the other hand, if the Cr content exceeds 0.10%, it will lead to deterioration of weldability and an increase in alloy costs. Therefore, when Cr is added, the Cr content is preferably 0.10% or less.

[0034] Mo: 0.50% or less Mo, like Cu, is an element that improves the strength of steel sheets 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 will lead to deterioration of weldability and an increase in alloy costs. Therefore, when Mo is added, the Mo content is preferably 0.50% or less, more preferably 0.25% or less.

[0035] V: 0.50% or less V, like Cu, is an element that improves the strength of steel sheets 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 will lead to deterioration of weldability and an increase in alloy costs. Therefore, when V is added, the V content is preferably 0.50% or less, more preferably 0.25% or less.

[0036] W: 0.50% or less W, like Cu, is an element that improves the strength of steel sheets 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, a W content exceeding 0.50% leads to deterioration of weldability and an increase in alloy costs. Therefore, when W is added, the W content is preferably 0.50% or less, more preferably 0.25% or less.

[0037] 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 cost. Therefore, when Co is added, the Co content is preferably 0.50% or less, more preferably 0.25% or less.

[0038] Ca: 0.0050% or less Ca is an element that bonds with S and inhibits the formation of MnS and other elements that elongate in the rolling direction. Therefore, adding Ca can control the shape of sulfide-based inclusions so that they assume a spherical shape, thereby improving the toughness of welds and other structures. When adding Ca, in order to achieve this effect, the Ca content is preferably 0.0005% or more, and more preferably 0.0020% or more. 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 adding Ca, the Ca content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.

[0039] Mg: 0.0050% or less Like Ca, Mg is an element that bonds with S and suppresses the formation of MnS and other compounds that elongate in the rolling direction. Therefore, adding Mg can control the shape of sulfide-based inclusions so that they are spherical, thereby improving the toughness of welds and other structures. When adding Mg, to achieve this effect, the Mg content is preferably 0.0005% or more, and 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 adding Mg, the Mg content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.

[0040] REM: 0.0050% or less. Rare earth metals (REMs) such as La and Ce, like Ca and Mg, bond with S and inhibit the formation of MnS and other compounds elongated in the rolling direction. Therefore, adding REMs can control the shape of sulfide-based inclusions to be spherical, thereby improving the toughness of welds and other structures. When adding REMs, 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 adding REMs, the REM content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.

[0041] In addition to having the above-mentioned chemical composition, the steel sheet of the present invention has a hardness of Hv 210 or less at a depth of 1 mm from the surface of the steel sheet, and a microstructure in which, at a depth of ¼ of the sheet thickness from the surface of the steel sheet, the volume fractions of ferrite and bainite are each 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 reasons for limiting the hardness and microstructure of the steel as described above will be explained below.

[0042] Hardness at a depth of 1 mm from the surface of the steel plate is Hv 210 or less The hardness at a depth of 1 mm from the surface of the steel plate is Hv 210 or less. If the hardness at the surface layer of the steel plate, specifically at a depth of 1 mm from the surface of the steel plate, is high, stress corrosion cracking in a liquid ammonia environment is promoted. Therefore, in the present invention, by setting the hardness at a depth of 1 mm from the surface of the steel plate to Hv 210 or less, excellent ammonia SCC resistance can be ensured. Furthermore, in order to improve ammonia SCC resistance, the hardness is preferably Hv 208 or less, and more preferably Hv 205 or less. The lower limit of the hardness at a depth of 1 mm from the surface of the steel plate is not particularly limited, but is preferably about Hv 130.

[0043] Generally, in a steel sheet that has been continuously cooled after hot rolling, the surface has a higher hardness because the cooling rate is faster than that of the interior. In the present invention, as described below in relation to the manufacturing conditions of the steel sheet, excessive hardening of the steel sheet surface is prevented by temporarily halting the cooling after hot rolling and setting a higher temperature at which the second cooling is stopped.

[0044] [Microstructure] The microstructure of the steel plate of the present invention will be described. [Volume fraction of ferrite and bainite at a depth of ¼ of the plate thickness from the surface of the steel plate is 20% or more, their total volume fraction is 80% or more, and the volume fraction of island martensite is 5% or less] The structure at a depth of ¼ of the plate thickness from the surface of the steel plate has a volume fraction of ferrite and bainite of 20% or more, their total volume fraction is 80% or more, and the volume fraction of island martensite contained in the remainder is 5% or less. The microstructure at a depth of ¼ 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. 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. In addition, the toughness of the base material decreases. On the other hand, if the volume fraction of bainite is less than 20%, the yield strength (YS) and tensile strength (TS) of the base material decrease, and it becomes impossible to achieve a yield strength (YS) of 355 MPa or more and 440 MPa or less, and a tensile strength (TS) of 440 MPa or more.

[0045] The remainder other than the ferrite and bainite may contain microstructures such as pearlite and austenite, but the island martensite in the remainder has higher strength but lower toughness than normal martensite, and can serve as the initiation point of fracture. If the island martensite is contained in an amount of more than 5 volume %, the toughness of the base material will be significantly reduced, so the volume fraction of the island martensite is set to 5% or less, preferably 3% or less, and can of course be set to 0%. In other words, the island martensite in the remainder has higher strength but lower toughness than normal martensite, and can serve as the initiation point of fracture, so the volume fraction of the structure is specified.

[0046] On the other hand, the remaining structure, which accounts for 20% or less in volume fraction, may contain martensite in addition to pearlite and austenite. The volume fraction of each structure in the remaining structure does not need to be particularly limited, but it is preferable that the remaining structure is pearlite. The volume fractions of various microstructures can be measured by the method described in the Examples below.

[0047] Next, a method for manufacturing a steel sheet according to the present invention will be described. A steel material having the above-described composition is heated and hot-rolled to form a hot-rolled steel sheet. ３ The steel sheet is then cooled to a temperature equal to or higher than the transformation point. Each manufacturing condition will be described in detail below.

[0048] 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.

[0049] The obtained steel material is either directly hot rolled without cooling, or is once heated and then hot rolled. ３ Transformation point (hereinafter abbreviated as "Ar ３ The starting temperature is above 100°C (also called "temperature point"), ３ The temperature at a depth of 1 / 4 from the surface of the steel sheet is then adjusted to Ar. ３ Cooling of the hot-rolled steel plate is started from a temperature above the temperature point, and the temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel plate reaches Ar ３ Point or below (Ar ３ The steel plate is cooled at an average cooling rate of 5°C / s or more to a temperature (temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel plate) or higher (point −90°C) (first cooling), the cooling is stopped once and interrupted for a period of 5 seconds to 600 seconds, and then cooling is performed at an average cooling rate of 5°C / s to 100°C / s at a depth of 1 / 4 of the plate thickness from the surface of the steel plate (second cooling), and the cooling is completed when the temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel plate is 400°C to 580°C. In the present invention, unless otherwise specified, the temperature of the steel plate refers to the temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel plate.

[0050] (a) Heating temperature of steel material: 950°C or higher and 1250°C or lower. The heating temperature of the steel material is not particularly limited. However, if the heating temperature of the steel material is lower than 950°C, the alloy will not be sufficiently dissolved in the austenite, resulting in a decrease in the yield strength (YS) and tensile strength (TS) of the base material. Furthermore, if the heating temperature is too low, the deformation resistance will increase, which may increase the load on the hot rolling mill and make hot rolling difficult. On the other hand, if the heating temperature of the steel material is higher than 1250°C, the 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 of the steel material is preferably set to 950°C or higher and 1250°C or lower. Furthermore, the heating temperature of the steel material is more preferably set to 1000°C or higher and 1150°C or lower.

[0051] (b) Hot rolling start temperature: Ar ３ Point +100°C or higher After heating to the above temperature, Ar ３ Hot rolling is started at a temperature of Ar + 100 ° C. or higher. ３ If the temperature is less than the hot rolling point +100°C, the austenite grain size will become mixed grains, which may deteriorate the toughness of the base material and make it impossible to achieve the subsequent hot rolling finish temperature and cooling start temperature.

[0052] Here, Ar ３ The point can be obtained, for example, by the following equation (2): Ar ３ (°C) = 910 - 273 x C - 74 x Mn - 57 x Ni - 16 x Cr - 9 x Mo - 5 x Cu (2) where each element indicates the content (mass%) of that element.

[0053] (c) Hot rolling finish temperature: Ar ３ Above transformation point Ar ３ The rolling is terminated at a temperature equal to or higher than the Ar temperature. ３ If the temperature is lower than this point, ferrite will be generated and the generated ferrite will be affected by the processing, resulting in a deterioration in toughness. Furthermore, the load on the hot rolling mill will increase. Therefore, the hot rolling finishing temperature is set to Ar ３ The hot finish temperature is set to Ar ３ It is preferable to set the temperature at the point +20°C or higher.

[0054] (d) Cooling start temperature: Ar ３ Next, the hot-rolled steel sheet after hot rolling is ３ Cooling is started from the Ar point. ３ Below this point, excessive ferrite is generated, which reduces the yield strength (YS) and tensile strength (TS) of the base material. ３ Points or more.

[0055] (e) Average cooling rate in the first cooling: cooling rate at a ¼ depth from the surface of the steel plate is 5°C / s or more The average cooling rate in the first cooling is 5°C / s or more. If the average cooling rate is low, excessive ferrite is formed, resulting in a decrease in the yield strength (YS) and tensile strength (TS) of the base material. The average cooling rate is 10°C / s or more. There is no particular upper limit to the average cooling rate, but excessive cooling increases cooling costs, so 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).

[0056] (f) First cooling stop temperature: The temperature at a depth of 1 / 4 from the surface of the steel plate is Ar ３ Point or below (Ar ３ The stop temperature of the first cooling is set to Ar so that the ferrite or bainite at a depth of 1 / 4 from the surface of the steel plate becomes a predetermined volume fraction. ３ Point or below (Ar ３ The cooling stop temperature is Ar ３ If the cooling temperature exceeds this point, ferrite does not form and the toughness decreases. ３ On the other hand, the cooling stop temperature is set to (Ar ３ If the cooling stop temperature is lower than the cooling stop temperature (point −90°C), excessive ferrite is produced, which reduces the yield strength (YS) and tensile strength (TS) of the base material. Furthermore, in order to reduce the surface hardness and increase the ammonia SCC resistance, it is preferable to set the cooling stop temperature to 700°C or higher.

[0057] (g) Cooling stop time: 5 seconds or more and 600 seconds or less After the first cooling, cooling is temporarily stopped for 5 seconds or more and 600 seconds or less. By stopping cooling, ferrite can be generated. If the cooling time is less than 5 seconds, ferrite is not generated, resulting in a decrease in toughness and an excessively high yield strength (YS) of the base material. On the other hand, if the cooling stop time exceeds 600 seconds, excessive ferrite is generated, resulting in a decrease in the yield strength (YS) and tensile strength (TS) of the base material. In addition, by setting the stop temperature of this cooling stop and the second cooling described below higher, the hardness of the surface layer is reduced.

[0058] (h) Second cooling rate: An average cooling rate of 5°C / s or more and 100°C / s or less at a depth of 1 / 4 from the surface of the steel plate. After the cooling is stopped, cooling is resumed. The average cooling rate here is set to 5°C / s or more and 100°C / s or less so that the microstructure has a predetermined volume fraction. That is, if the average cooling rate is less than 5°C / s, the volume fraction of the bainite structure becomes too small, resulting in a decrease in the tensile strength (TS) of the base material. On the other hand, if the average cooling rate exceeds 100°C / s, the volume fraction of island martensite becomes too large, resulting in a decrease in toughness.

[0059] (i) Second cooling end temperature: The cooling end temperature at a ¼ depth position from the surface of the steel plate is 400°C or higher and 580°C or lower. The second cooling end temperature is 400°C or higher and 580°C or lower in order to adjust the hardness of the surface layer and the strength and toughness of the base material. If the cooling end temperature exceeds 580°C, the yield strength (YS) and tensile strength (TS) of the base material will decrease. On the other hand, if the cooling end temperature is less than 400°C, the yield strength (YS) of the base material will increase, the toughness will decrease, and the hardness of the surface layer will also increase.

[0060] By manufacturing a steel material having the above-described chemical composition according to the above-described manufacturing conditions, a steel plate having the above-described structure can be obtained. The steel plate thus obtained has excellent strength properties and toughness. Here, "excellent strength properties" refer to a yield strength (YS) (yield point (YP) when a yield point exists, or 0.2% proof stress σ0.2 when no yield point exists) of 355 MPa to 440 MPa and a tensile strength (TS) of 440 MPa or more. Of these, the yield strength (YS) is closely related to ammonia stress corrosion cracking susceptibility, and the IMO Gas Code and classification rules of the International Maritime Organization (IMO) specify that the yield point must be 440 MPa or less for structural members of liquefied gas bulk carriers in order to minimize the risk of ammonia stress corrosion cracking.

[0061] Although the higher the tensile strength (TS) of a steel plate, the better, it is highly likely that a large amount of alloy will be added, which will increase the cost. Furthermore, since it will be impossible to simultaneously achieve a yield strength (YS) of 440 MPa or less (yield point (YP) when there is a yield point, or 0.2% proof stress σ0.2 when there is no yield point) to ensure ammonia stress corrosion cracking resistance, it is desirable that the tensile strength (TS) of the steel plate be 620 MPa or less.

[0062] Molten steel having the chemical composition shown in Table 1 was produced as steel materials (slabs). These steel materials (slabs) were subjected to hot rolling and cooling under the conditions shown in Table 2.

[0063] The hardness of the obtained steel sheets was measured at a depth of 1 mm from the surface of the steel sheets (surface layer), and the structure fraction in the microstructure at a depth of ¼ of the sheet thickness from the surface of the steel sheets was measured, and the tensile properties and toughness were evaluated.

[0064] [Hardness Measurement at a Depth of 1 mm from the Surface] Vickers hardness (HV10) was measured at five points at a depth of 1 mm from the surface in accordance with JIS Z 2244 on a cross section perpendicular to the rolling direction, and the average value was calculated.

[0065] [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 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). The photographed image was analyzed using an image analyzer to determine the area fraction of the microstructure, and this value was taken as the volume fraction. When the anisotropy of the microstructure is small, the area fraction corresponds to the volume fraction, so in this specification, the area fraction was taken as the volume fraction.

[0066] 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 then subjected to nital etching 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 the carbides are contained in the bainite laths, the carbides are divided, and for example, the elongated carbides appearing between the laths of the bainite are transformed into multiple round carbides, making it easy to distinguish between the two. Martensite has a lath-shaped ferrite structure that has grown elongated like bainite, and is defined as a structure containing carbides with a circle equivalent diameter of 0.05 μm or less, and further, carbides with a circle equivalent diameter of 1.0 × 10 ４ pieces / mm ２ When the carbides contained more than 0.50 μm in equivalent circle diameter, the carbides were defined as tempered martensite. The carbides appeared as white dots. Austenite was defined as a structure that was present between bainite and martensite structures and did not contain carbides with a circle equivalent diameter of 0.50 μm or more.

[0067] [Strength properties] A JIS Z 2201 No. 1B test piece was taken from the entire thickness of each steel plate in the direction perpendicular to the rolling direction, and a tensile test was carried out in accordance with JIS Z 2241 to measure 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) and tensile strength (TS). Steel plates with a yield strength of 355 MPa or more and 440 MPa or less and a tensile strength of 440 MPa or more were evaluated as having excellent tensile strength.

[0068] [Toughness] Furthermore, a V-notch test piece according to JIS Z 2202 was taken from a site 1 mm removed from the surface of each steel plate in the rolling direction, and a Charpy impact test was carried out in accordance with JIS Z 2242 to measure the ductile-brittle fracture transition temperature vTrs. Steel plates with a vTrs of −60° C. or less were evaluated as having excellent toughness.

[0069] [Ammonia SCC Resistance] Resistance to stress corrosion cracking caused by ammonia (also referred to as "ammonia SCC resistance") was evaluated by an accelerated test in which a four-point bending test was performed in a test solution and constant-potential anodic electrolysis was performed to accelerate corrosion. Specifically, the test was performed as follows. Test specimens measuring 5 mm thick x 15 mm x 115 mm were taken from the surface of the steel sheet and ultrasonically degreased in acetone for 5 minutes. Then, an average stress of the yield strength and tensile strength of each steel sheet was applied by four-point bending. A test cell containing the four-point bending test specimens was filled with a solution containing 12.5 g of ammonium carbamate and 1 L of liquid ammonia. The test specimens were then immersed at room temperature (25°C) while controlling the flow of +2.0 V vs. Pt using a potentiostat. If no cracks were observed after 168 hours of immersion, the ammonia SCC resistance was judged to be good and marked with ○, and if cracks were observed, the ammonia SCC resistance was judged to be poor and marked with ×. The evaluation results thus obtained are also shown in Table 2.

[0070] [Joint HAZ Toughness] Furthermore, a V-groove was formed on a joint test plate taken from each of the steel plates, and a CO ２Welding was performed to produce joints by small heat input welding. Then, toughness was evaluated using the obtained joints. The test methods are as follows. NK U4 impact test pieces were taken so that the surface layer of the test piece 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 pieces at a test temperature of -60°C, and the average value of absorbed energy vE of three test pieces run under the same conditions was calculated. －６０℃ The evaluation results thus obtained are shown in Table 2.

[0071]

[0072]

[0073] As can be seen from Tables 1 and 2, all of the invention examples have a surface hardness of 210 HV or less, a yield strength (YS) of 355 MPa or more and 440 MPa or less, a tensile strength (TS) of 440 MPa or more, and a ductile-brittle fracture transition temperature vTrs of -60°C or less, and thus provide steel plates excellent in toughness and ammonia stress corrosion cracking resistance at low temperatures.

[0074] On the other hand, steel plates Nos. 2 to 15, which correspond to comparative examples and were manufactured under different conditions, are inferior to the invention examples in terms of surface hardness, yield strength (YS), tensile strength (TS), low-temperature toughness, and ammonia SCC resistance. Steel plate No. 28, which corresponds to a comparative example, has a low carbon content, and its yield strength (YS) and tensile strength (TS) are inferior to the invention examples. Steel plate No. 29 has a high carbon content, and its low-temperature toughness is also inferior to the invention examples. Steel plates Nos. 30, 32, 33, 34, 35, 37, 39, and 40 contain more elements than the invention examples, and their low-temperature toughness is inferior to the invention examples. Steel plate No. 31 has a low manganese content, and its tensile strength (TS) is inferior to the invention examples. Steel plate No. 40 has a high manganese content, and its low-temperature toughness is inferior to the invention examples. Steel Plate No. 36 and 38 have low amounts of nitrogen or titanium and are inferior in low-temperature toughness compared to the invention examples. Steel Plate No. 41 has a low Ceq and is inferior in yield strength YS and tensile strength TS compared to the invention examples. Steel Plate No. 42 has a high Ceq and is inferior in surface hardness, yield strength YS, low-temperature toughness, and ammonia SCC resistance compared to the invention examples.

Claims

1. A composition containing, by mass, C: 0.045% or more and 0.120% or less, Si: 0.50% or less, Mn: 0.50% or more and 2.00% or less, P: 0.020% or less, S: 0.0100% or less, Al: 0.060% or less, Ti: 0.005% or more and 0.050% or less, N: 0.0010% or more and 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities, a carbon equivalent Ceq represented by the following formula (1) is 0.25 or more and 0.40 or less, and a hardness at a depth of 1 mm from the surface is HV210 or less, A steel plate having a microstructure at a depth position of 1 / 4 of the plate thickness from the surface, in which the volume fractions of ferrite and bainite are each 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, a yield strength of 355 MPa or more and 440 MPa or less, a tensile strength of 440 MPa or more, and a ductile-brittle fracture transition temperature vTrs of -60°C or less. Ceq=C+Mn / 6+(Cu+Ni) / 15+(V+Mo+Cr) / 5... (1) where, in the above formula (1), each element symbol represents the content (mass%) of each element, and 0 is used when the element is not contained.

2. The steel plate according to claim 1, wherein the chemical composition further contains, in mass%, one or more selected from the following: Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.10% or less, Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Ca: 0.0050% or less, Mg: 0.0050% or less and REM: 0.0050% or less.

3. The steel plate according to claim 1 or 2, having a hardness of HV208 or less at a depth of 1 mm from the surface.

4. A steel material containing, by mass%, C: 0.045% or more and 0.120% or less, Si: 0.50% or less, Mn: 0.50% or more and 2.00% or less, P: 0.020% or less, S: 0.0100% or less, Al: 0.060% or less, Ti: 0.005% or more and 0.050% or less, N: 0.0010% or more and 0.0100% or less, and O: 0.0100% or less, and having a carbon equivalent Ceq represented by the following formula (1) of 0.25 or more and 0.40 or less, with the balance being Fe and unavoidable impurities, is heated to a temperature of 950°C or more and 1250°C or less, and then rolled at a rolling start temperature of Ar. ３ The rolling end temperature is set to Ar + 100 ° C or higher. ３ Then, the steel is subjected to hot rolling to a temperature of 1000° C. or higher. ３ Cooling is started from a temperature above the Ar point. ３ Ar below the point ３ A method for manufacturing a steel plate, comprising the steps of: cooling at an average cooling rate of 5°C / s or more until the temperature reaches a temperature point -90°C or higher; once stopping the cooling for a period of 5 to 600 seconds; subsequently cooling at an average cooling rate of 5°C / s or more and 100°C / s or less at a depth position of 1 / 4 of the plate thickness from the surface of the steel plate; and ending the cooling when the temperature at a depth position of 1 / 4 of the plate thickness from the surface of the steel plate is 400°C or more and 580°C or less. Ceq=C+Mn / 6+(Cu+Ni) / 15+(V+Mo+Cr) / 5 (1) where, in the above formula (1), each element symbol represents the content (mass%) of each element, and is set to 0 if not contained.

5. A method for manufacturing steel plate as described in claim 4, wherein the chemical composition further contains, by mass%, one or more selected from the following: Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.10% or less, Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Ca: 0.0050% or less, Mg: 0.0050% or less and REM: 0.0050% or less.

6. The Ar ３ The method for producing a steel sheet according to claim 4 or 5, wherein the cooling, which starts from a temperature equal to or higher than the above temperature point, is stopped at a temperature of 700°C or higher.

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