Steel sheet and method for producing same

Abstract

To produce a steel sheet excellent in low temp. toughness and ammonia stress corrosion cracking resistance and to provide a method for producing it.SOLUTION: Provided are a steel sheet having a predetermined chemical composition, having a microstructure in which a hardness at a 1mm depth position is 210HV or less, a volume fraction of each of ferrite and bainite at a sheet thickness 1 / 4 depth position is 15% or more, and a total volume fraction of the ferrite and the bainite is 80% or more, and having a yield strength of 325MPa or more and 440MPa or less, a tensile strength of 440MPa or more, and a ductile-brittle transition temperature vTrs after imparting a prestrain of 5% of - 60 °C. or less, and a method for manufacturing the same.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel sheet 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.

[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 can be avoided.

[0005] The technologies for providing the low-temperature toughness and strength range required for liquefied gas storage tanks as described above are described in Patent Documents 1 and 2, which achieve high low-temperature toughness and predetermined strength properties. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 3802626 [Patent Document 2] Patent No. 3848415 Summary of the Invention [Problem to be solved by the invention]

[0007] When manufacturing a tank, the steel material is subjected to bending, but there is a concern that the toughness of the steel material deformed by the bending process may deteriorate, and it is considered necessary to suppress the deterioration of toughness from the viewpoint of safety. In this regard, the techniques described in Patent Documents 1 and 2 have the risk of insufficient toughness after bending.

[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, and a method for manufacturing the same, which is suitable for use in storage tanks used to store liquefied gas on energy transport ships, for example. [Means for solving the problem]

[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 steel plate. As a result, they have found that desired low-temperature toughness and strength properties can be exhibited by adding predetermined amounts of elements such as C, Mn, and Ti to a steel plate and controlling the microstructure so that the volume fractions of ferrite and bainite at a depth of 1 / 4 of the plate thickness of the steel plate are each 15% or more and the total volume fraction of ferrite and bainite is 80% or more.

[0010] Furthermore, the present inventors have found that by controlling the hardness of the steel plate at a depth of 1 mm to 210 HV 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. 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, 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 The composition of the alloy has 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: The hardness at a depth of 1 mm is 210 HV or less, At a depth position of 1 / 4 of the plate thickness, the volume fractions of ferrite and bainite are each 15% or more, and the total volume fraction of the ferrite and bainite is 80% or more, The yield strength is 325 MPa or more and 440 MPa or less, The tensile strength is 440 MPa or more, A steel plate having a ductile-brittle fracture transition temperature (vTrs) of -60°C or less after being pre-strained by 5%. 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 the element, and if the element is not contained, it is set to 0.

[0013] 2. The component composition further comprises, in mass%, Al: 0.060% or less, 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, Ca: 0.0050% or less, Mg: 0.0050% or less and REM: 0.0050% or less 2. The steel sheet according to 1 above, containing one or more selected from the following:

[0014] 3. 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, 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 a steel material having a chemical composition in which the carbon equivalent Ceq represented by the following formula (1) is 0.25 or more and 0.40 or less, and the balance is Fe and unavoidable impurities, Heat to a temperature of 950°C or higher and 1250°C or lower, Next, hot rolling is performed with a rolling start temperature of Ar3 point + 100°C or higher, a cumulative rolling reduction rate in the unrecrystallized region of 60% or higher, and a rolling end temperature of Ar3 point or higher. Then, start cooling from a temperature above the Ar3 point, and cool at an average cooling rate of 5°C / s or more until the temperature drops below the Ar3 point and reaches Ar3 point -90°C or more by 20°C or more. The cooling is stopped temporarily and the cooling is interrupted for 5 seconds or more and 600 seconds or less, Next, the steel plate is cooled at an average cooling rate of 5°C / s or more at a depth of 1 / 4 of the plate thickness, The cooling is completed when the temperature at a depth of 1 / 4 of the plate thickness of the steel plate is 580°C or less, Next, a heat treatment is performed at a temperature of 400°C or higher and 580°C or lower. 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 the element, and if the element is not contained, it is set to 0.

[0015] 4. The component composition further comprises, in mass%, Al: 0.060% or less, 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, Ca: 0.0050% or less, Mg: 0.0050% or less and REM: 0.0050% or less 4. The method for producing a steel sheet according to 3 above, wherein the steel sheet contains one or more selected from the following: [Effects of the Invention]

[0016] 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, the present invention has a significant industrial effect. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] [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 material. 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 material and the toughness of the joint HAZ decrease. Therefore, the C content is set to 0.120% or less. Furthermore, from the viewpoint of preventing a decrease in the toughness of the base material and weldability, the C content is preferably set to 0.115% or less, and more preferably set to 0.110% or less.

[0019] 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. Therefore, it is preferable to keep the Si content as low as possible, but 0.50% or less is acceptable. Since Al, Ti, and the like 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 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, more preferably 0.03% or more, and even more preferably 0.04% or more.

[0020] 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 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 furthermore, the alloy cost will become excessively high. Therefore, the Mn content is set to 2.00% or less. Furthermore, from the viewpoint of suppressing a decrease in the toughness and weldability of the base metal, the Mn content is preferably set to 1.80% or less, and more preferably 1.60% or less.

[0021] 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 preferable 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%. Since P is usually an element that is unavoidably contained in steel as an impurity, the P content may be greater than 0% industrially. Furthermore, because excessive reduction of P leads to increased refining costs, it is preferable to set the P content to 0.0005% or more.

[0022] 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 by acting as a fracture initiation point. Therefore, it is preferable to reduce the S content as much as possible. Furthermore, S 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 that is unavoidably contained in steel as an impurity, the S content may be more than 0% industrially. Furthermore, excessive reduction of S leads to an increase in refining costs, so from a cost perspective, it is preferable to set the S content to 0.0005% or more.

[0023] 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, adding 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% or less. Furthermore, the Ti content is preferably set to 0.040% or less, and more preferably 0.030% or less.

[0024] N: 0.0010% or more and 0.0100% or less N combines with Ti and precipitates as TiN, contributing to the refinement of the structure 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 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.

[0025] O: 0.0100% or less O is an element contained as an unavoidable impurity, and it adversely affects the toughness of the base material and the joint HAZ by forming oxides and becoming the initiation point of fracture. 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 unavoidably contained in steel as an impurity. Therefore, from an industrial standpoint, the O content may be greater than 0%. Because excessive reduction of O leads to an increase in refining costs, from a cost perspective, the O content is preferably 0.0020% or more.

[0026] Ceq: 0.25 or more and 0.40 or less In order to satisfy the specified strength and toughness of the base material in the steel plate, the following formula (1) Ceq=C+Mn / 6+(Cu+Ni) / 15+(V+Mo+Cr) / 5...(1) It is essential that the carbon equivalent (Ceq), defined as: (Ceq) be 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 a 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 ammonia-induced stress corrosion cracking cannot be satisfied, and the toughness of the base metal and the joint HAZ also deteriorates. To achieve a higher tensile strength (TS), Ceq is preferably 0.28 or more, and more preferably 0.31 or more. From the viewpoint of further improving the toughness of the joint HAZ, Ceq is preferably 0.37 or less, and more preferably 0.34 or less.

[0027] 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 or toughness, this basic composition may optionally further contain one or more selected from the group consisting of 0.060% or less Al, 1.00% or less Cu, 1.00% or less Ni, 1.00% or less Cr, 0.50% or less Mo, 0.50% or less V, 0.50% or less W, 0.50% or less Co, 0.0050% or less Ca, 0.0050% or less Mg, and 0.0050% or less REM.

[0028] 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, reducing the cleanliness, and therefore reducing the toughness of the base metal and the joint HAZ. Therefore, the Al content is preferably 0.060% or less. The Al content is more preferably 0.050% or less, and even more preferably 0.040% or less.

[0029] 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, more preferably 0.20% or more. On the other hand, if the Cu content exceeds 1.00%, it will result in a deterioration in toughness and an increase in alloy costs. Therefore, when Cu is added, the Cu content is preferably 1.00% or less, more preferably 0.50% or less.

[0030] Ni: 1.00% or less Ni, like Cu, is an element that improves the strength of steel sheets and can be added as desired. When Ni is added, the Ni content is preferably 0.01% or more, more preferably 0.20% or more, to achieve the above effect. On the other hand, if the Ni content exceeds 1.00%, it leads 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.

[0031] 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, and 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 preferably 1.00% or less.

[0032] 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 leads 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.

[0033] 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 will deteriorate the weldability and increase the alloy cost. Therefore, when V is added, the V content is preferably 0.50% or less, more preferably 0.25% or less.

[0034] 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 preferably 0.50% or less, more preferably 0.25% or less.

[0035] 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, if the Co content exceeds 0.50%, it will deteriorate the weldability and increase the alloy cost. Therefore, when Co is added, the Co content is preferably 0.50% or less, more preferably 0.25% or less.

[0036] 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, by including Ca, the morphology of sulfide-based inclusions can be controlled to be spherical, thereby improving the toughness of welds and other parts. When Ca is included, 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 Ca is included, the Ca content is preferably 0.0050% or less, more preferably 0.0040% or less, and even 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 welds and other parts. When Mg is included, in order 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 preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.

[0038] REM: 0.0050% or less Like Ca and Mg, REMs (rare earth metals) such as La and Ce are elements that bond with S and inhibit the formation of MnS and other compounds elongated in the rolling direction. Therefore, the inclusion of REMs controls the morphology of sulfide-based inclusions to be spherical, thereby improving the toughness of welds and other structures. 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 REMs are contained, the REM content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.

[0039] In addition to having the above-mentioned chemical composition, the steel sheet of the present invention has a microstructure in which the hardness at a 1 mm depth of the steel sheet is 210 HV or less, and the volume fractions of ferrite and bainite at a 1 / 4 depth of the steel sheet are each 15% or more, and the total volume fraction of ferrite and bainite is 80% or more at a 1 mm depth of the steel sheet. Here, "1 mm depth" means a position 1 mm deep from the surface of the steel sheet in the thickness direction. Also, "1 / 4 depth of the steel sheet" means a position 1 / 4 deep from the surface of the steel sheet in the thickness direction. The reasons for limiting the hardness and microstructure of the steel as described above will be explained below.

[0040] Hardness of steel plate at a depth of 1 mm is 210HV or less The hardness of the steel plate at a depth of 1 mm is set to 210 HV or less. If the hardness of the surface layer of the steel plate, specifically at a depth of 1 mm of the steel plate, is high, stress corrosion cracking in a liquid ammonia environment is promoted. In the present invention, by setting the hardness at a depth of 1 mm of the steel plate to 210 HV or less, excellent ammonia SCC (Stress Corrosion Cracking) resistance can be ensured. Furthermore, in order to improve ammonia SCC resistance, the hardness is preferably set to 208 HV or less, and more preferably 205 HV or less. The lower limit of the hardness at a depth of 1 mm in the steel plate is not particularly limited, but is preferably about 130 HV.

[0041] Generally, in a steel sheet that has been subsequently cooled after hot rolling, the surface, which cools faster than the interior, has a higher hardness. In the present invention, as described below in relation to the manufacturing conditions of the steel sheet, the cooling after hot rolling is temporarily stopped and the temperature at which the second cooling is stopped is set higher, thereby preventing excessive hardening of the steel sheet surface.

[0042] [Microstructure] The microstructure of the steel sheet of the present invention will be described.

[0043] [The volume fraction of ferrite and bainite at a depth of 1 / 4 of the plate thickness is 15% or more each, and the total volume fraction is 80% or more] In the structure at a depth of 1 / 4 of the plate thickness of the steel plate, the volume fractions of ferrite and bainite are each 15% or more, and the total volume fraction thereof is 80% or more. The microstructure at a depth of 1 / 4 of the plate thickness is specified because it has a strong effect on the strength characteristics of the entire steel plate.

[0044] If the volume fraction of ferrite is less than 15%, 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. From the viewpoint of increasing the toughness of the base material, the volume fraction of ferrite is preferably 20% or more, more preferably 25% or more. On the other hand, if the volume fraction of bainite is less than 15%, the yield strength (YS) and tensile strength (TS) of the base material decrease, making it impossible to achieve a yield strength (YS) of 325 MPa to 440 MPa and a tensile strength (TS) of 440 MPa or more. From the viewpoint of increasing the yield strength (YS) and tensile strength (TS) of the base material, the volume fraction of bainite is preferably 20% or more, more preferably 25% or more. On the other hand, if the volume fractions of ferrite and bainite are less than 80%, the toughness of the base material decreases, so the combined volume fraction of ferrite and bainite is set to 80% or more.

[0045] 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 be pearlite.

[0046] The volume fractions of various microstructures can be measured by the methods described in the examples below.

[0047] Next, a method for producing a steel sheet according to the present invention will be described. A steel material having the above-described chemical composition is heated and hot-rolled to obtain a hot-rolled steel sheet, which is then cooled to a cooling start temperature of Ar3 point or higher to obtain a steel sheet. Each manufacturing condition will be explained in detail below.

[0048] First, the conditions for producing the steel material do not need to be particularly limited, but it is preferable to melt molten steel having the above-mentioned composition using 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 using a known casting method such as a continuous casting method. Note that there is no problem with forming the slab of a predetermined size into a steel material using an ingot casting-blooming rolling method.

[0049] The resulting steel material is either hot-rolled directly after casting without cooling, or heated once before hot-rolling. Hot-rolling is performed at a rolling start temperature of (Ar3 point + 100°C) or higher and a rolling end temperature of Ar3 point or higher to produce a hot-rolled steel sheet. Next, cooling of the hot-rolled steel sheet is started when the temperature at a quarter-thickness depth of the steel sheet is equal to or higher than Ar3 point, and the steel sheet is cooled at an average cooling rate of 5°C / s or higher (first cooling) so that the temperature at the quarter-thickness depth of the steel sheet is equal to or lower than Ar3 point (Ar3 point - 90°C) or higher. The cooling is then stopped for 5 to 600 seconds, and the steel sheet is subsequently cooled at an average cooling rate of 5°C / s to 100°C / s (second cooling) at the quarter-thickness depth of the steel sheet, and the second cooling is completed when the temperature at the quarter-thickness depth of the steel sheet is equal to or higher than 400°C to 580°C. In the present invention, unless otherwise specified, the temperature of the steel plate is the temperature at a depth of 1 / 4 of the plate thickness of the steel plate.

[0050] (a) Heating temperature of steel material: 950℃ or higher and 1250℃ or lower If the heating temperature of the steel material is less than 950°C, the alloy will not dissolve sufficiently 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 set to 950°C or higher and 1250°C or lower. The heating temperature of the steel material is preferably 1000°C or higher, and more preferably 1150°C or lower.

[0051] (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 equal to or higher than the Ar3 point + 100°C. If the rolling start temperature is less than the Ar3 point + 100°C, the austenite grain size will become mixed, which will deteriorate the toughness of the base material and may make it impossible to achieve the subsequent hot rolling finish temperature and cooling start temperature. The upper limit of the rolling start temperature is determined by the heating temperature of the steel material, so it must be 1250°C or lower, and preferably 1150°C or lower. From the perspective of reducing deformation resistance and reducing the load on the hot rolling mill due to an increase in the rolling start temperature, the rolling start temperature is preferably 150°C or higher than the Ar3 point, and more preferably 200°C or higher than the Ar3 point.

[0052] Here, the Ar3 point can be determined, for example, by the following equation (2). Ar3(℃)=910-273×C-74×Mn-57×Ni-16×Cr-9×Mo-5×Cu...(2) However, the content (mass%) of each element is shown, and if the element is not contained, it is set to 0.

[0053] (c) Cumulative reduction in the unrecrystallized region: 60% or more If the cumulative reduction rate is less than 60% in the non-recrystallized region (which in the present invention means the region where the steel material is at a temperature less than the Ar3 point + 100°C), sufficient working effect on the austenite cannot be obtained, and the toughness of the base material in the product steel plate may decrease, as well as the toughness after imparting 5% pre-strain. Therefore, the cumulative reduction rate in the non-recrystallized region is specified to be 60% or more. From the viewpoint of further improving the base material toughness and the toughness after imparting 5% pre-strain, the cumulative reduction rate is preferably 65% ​​or more, and more preferably 70% or more. The upper limit of the cumulative reduction rate is not limited because it varies depending on the plate thickness of the steel plate, but is substantially 95% or less.

[0054] (d) Hot rolling finish 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. Furthermore, the load on the hot rolling mill increases. Therefore, the hot rolling completion temperature is set to be equal to or higher than the Ar3 point. Furthermore, the hot rolling completion temperature is preferably set to be equal to or higher than the Ar3 point + 20°C, and more preferably equal to or higher than the Ar3 point + 40°C. In order to satisfy the cumulative reduction rate in the non-recrystallized region, the upper limit of the hot rolling completion temperature is set to less than the Ar3 point + 100°C. In order to improve the base material toughness by reducing the non-recrystallized region, the hot rolling completion temperature is preferably set to be equal to or lower than the Ar3 point + 80°C.

[0055] (e) Cooling start temperature: Ar3 point or higher Next, the hot-rolled steel sheet is cooled from the Ar3 point or higher after hot rolling. If the cooling start temperature is below the Ar3 point, excessive ferrite is formed, resulting in a decrease in the yield strength (YS) and tensile strength (TS) of the base material. Therefore, the cooling start temperature is set to the Ar3 point or higher. The upper limit of the cooling start temperature is the hot rolling end temperature, which is set to less than the Ar3 point + 100°C. To improve the base material toughness by reducing the unrecrystallized region, the cooling start temperature is preferably set to the Ar3 point + 80°C or lower.

[0056] (f) Average cooling rate in the first cooling: The cooling rate at a depth of 1 / 4 of the steel plate thickness is 5°C / s or more. The average cooling rate in the first cooling step should be 5°C / s or more. If the average cooling rate is too low, excessive ferrite will be produced, resulting in a decrease in the yield strength (YS) and tensile strength (TS) of the base material. The average cooling rate is preferably 10°C / s or more. There is no particular upper limit to the average cooling rate, but excessive cooling increases cooling costs, so the average cooling rate is preferably 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).

[0057] (g) Cooling stop temperature in the first cooling: The temperature at the depth of 1 / 4 of the plate thickness is 20°C or more below Ar3 point (Ar3 point - 90°C). The stop temperature of the first cooling must be below the Ar3 point (Ar3 point - 90°C) to achieve a predetermined volume fraction of ferrite and bainite at a depth of 1 / 4 of the steel plate thickness. A temperature drop of at least 20°C from the start of cooling is required. If the cooling stop temperature exceeds the Ar3 point or does not involve a temperature drop of at least 20°C, ferrite will not be sufficiently formed, resulting in a decrease in toughness. Therefore, the cooling stop temperature must be below the Ar3 point. On the other hand, if the cooling stop temperature is below (Ar3 point - 90°C), excessive ferrite will be formed, resulting in a decrease in the yield strength (YS) and tensile strength (TS) of the base material. Furthermore, to reduce surface hardness and improve ammonia SCC resistance, the cooling stop temperature is preferably 700°C or higher, and more preferably 720°C or higher.

[0058] (h) Cooling interruption time: 5 seconds or more and 600 seconds or less After the first cooling cycle, the cooling is temporarily interrupted for 5 to 600 seconds. Interrupting the cooling allows ferrite to form. If the cooling interruption time is less than 5 seconds, ferrite does not form sufficiently, resulting in a decrease in toughness and an excessively high yield strength (YS) of the base material. On the other hand, if the cooling interruption time is more than 600 seconds, excessive ferrite forms, resulting in a decrease in the yield strength (YS) and tensile strength (TS) of the base material.

[0059] The hardness of the surface layer is reduced by setting the cooling stop temperature of the first cooling and the cooling stop temperature of the second cooling, which will be described later, higher.

[0060] (i) Second cooling rate: The average cooling rate at a depth of 1 / 4 of the steel plate thickness is 5°C / s or more. After the cooling is interrupted, cooling is resumed. The average cooling rate here is 5°C / s or more so that the microstructure has a predetermined volume fraction. 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 yield strength (YS) and tensile strength (TS) of the base material. From the perspective of increasing strength, the second cooling rate is preferably 10°C / s or more, and more preferably 20°C / s or more. On the other hand, although there is no particular upper limit to the average cooling rate, excessive cooling increases cooling costs, so the average cooling rate is preferably 100°C / s or less.

[0061] (j) Second cooling end temperature: The cooling end temperature at the depth of 1 / 4 of the steel plate thickness is 580°C or less The end temperature of the second cooling is set to 580°C or lower 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, although there is no particular lower limit for the cooling end temperature, excessive cooling increases the cooling cost, so it is preferable to set it to 200°C or higher.

[0062] (k) Heat treatment temperature: 400°C or higher and 580°C or lower The heat treatment temperature is set to 400°C or higher and 580°C or lower to adjust the hardness of the surface layer and the strength and toughness of the base material. If the heat treatment temperature exceeds 580°C, the yield strength (YS) and tensile strength (TS) of the base material decrease. On the other hand, if the heat treatment temperature is lower than 400°C, the toughness after applying a 5% prestrain does not meet the target characteristics. The heating rate and cooling rate are not particularly limited, but a rate exceeding 100°C / s increases the cost of heating or cooling, so a rate of 100°C / s or lower is preferable. The holding time at the heat treatment temperature is also not particularly limited, but from the perspective of improving toughness, a holding time of 300 seconds or higher is preferable, and a holding time of 600 seconds or higher is even more preferable. Holding at the heat treatment temperature for 86,400 seconds or longer increases the cost of heat treatment and reduces production efficiency, so a holding time at the heat treatment temperature of less than 86,400 seconds is preferable.

[0063] A steel plate having the above-described structure can be obtained by using a steel material having the above-described chemical composition and manufacturing conditions. The steel plate thus obtained has excellent strength properties and toughness. Here, "excellent strength properties" refers 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 325 MPa to 440 MPa and a tensile strength (TS) of 440 MPa or more, measured by a tensile test at room temperature. The yield strength (YS) is preferably 355 MPa or more. Of these, the 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 structural members of liquefied gas bulk carriers must be 440 MPa or less to minimize the risk of ammonia stress corrosion cracking.

[0064] Although the higher the tensile strength (TS) of steel sheets, the better, it is likely that a large amount of alloy will be added, which will increase costs. Furthermore, since 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, which is required to ensure ammonia stress corrosion cracking resistance, it is preferable that the tensile strength (TS) of steel sheets be 620 MPa or less. [Example]

[0065] Molten steel having the chemical composition shown in Table 1 was melted to prepare steel materials (slabs). These steel materials (slabs) were subjected to hot rolling and cooling under the conditions shown in Table 2.

[0066] The hardness of the obtained steel sheets was measured at a depth of 1 mm (surface layer), and the microstructure fraction was measured at a depth of 1 / 4 of the steel sheet thickness, and the tensile properties and toughness were evaluated.

[0067] [Hardness measurement at a depth of 1 mm] For a cross section perpendicular to the rolling direction, Vickers hardness (HV10) was measured at five points at a depth of 1 mm in accordance with JIS Z 2244, and the average value was calculated.

[0068] [Microstructure at 1 / 4 depth of plate thickness] 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 of the steel plate. The surface of the sample was mirror-polished and further subjected to nital etching, and then an area of ​​1000 μm × 1000 μm was photographed using a scanning electron microscope (SEM). The photographed image was analyzed using an image analyzer to determine the area fraction of the microstructure. When the anisotropy of the microstructure is small, the area fraction corresponds to the volume fraction, so in this specification, the area fraction is referred to as the volume fraction.

[0069] When calculating the area fraction of the microstructure, the distinction between each structure was made as follows. The steel material was mirror-polished, and then nital etched to reveal the structure, which was then observed under an SEM at 500 to 3000 times magnification. Ferrite was defined as an isotropically grown structure that does not contain carbides, with the grains appearing black. Pearlite was defined as a structure in which ferrite (black) and carbides (white) appear in a striped pattern. Bainite was defined as a structure with a long, thin, lath-shaped ferrite structure that contains carbides with a circle equivalent diameter of 0.05 μm or more, and this structure further contains carbides with a diameter of 1.0 × 10 4 pieces / mm 2When 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, there are multiple round carbides between the laths of bainite, so bainite and tempered bainite can be distinguished by looking at the carbides. Martensite, like bainite, has a lath-shaped ferrite structure that has grown long and thin, and is defined as a structure containing carbides of 0.05 μm or less in circle equivalent diameter, and this structure further contains carbides of 1.0 x 10 carbides or more. 4 pieces / mm 2 If the tempered martensite contains more than 0.50 μm of austenite, it is defined as tempered martensite. Note that carbides appear as white dots in the tempered martensite. Austenite is defined as a structure that exists between bainite and martensite, has a circle equivalent diameter of 0.50 μm or more, and is not a carbide structure.

[0070] [Strength characteristics] A JIS Z 2201 No. 1B test piece was taken from the full thickness of each steel plate so that the longitudinal direction of the test piece coincided with the plate width direction, and a tensile test was carried out in accordance with JIS Z 2241 to measure the yield strength YS (yield point YP if there was a yield point, or 0.2% proof stress σ0.2 if there was no yield point) and tensile strength (TS). Steel plates with a yield strength of 325 MPa or more and 440 MPa or less and a tensile strength of 440 MPa or more were evaluated as having excellent tensile strength.

[0071] [Toughness after 5% pre-strain] Tensile test specimens measuring 60 mm in width, 230 mm in length, and 500 mm in total length were taken from the entire thickness of each steel plate, with the longitudinal direction of the test specimen aligned with the rolling direction. They were then subjected to a 5% plastic strain using a tensile testing machine. Next, aging heat treatment was performed at 250°C for 1 h. Then, V-notch test specimens according to JIS Z 2242 were taken from a location 1 mm removed from the surface of each tensile test specimen, with the longitudinal direction of the test specimen aligned with the rolling direction. Charpy impact tests were performed according to 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 after pre-straining.

[0072] [Ammonia SCC resistance] Ammonia SCC resistance was evaluated by an accelerated test in which a four-point bending test was performed in the test solution and the test specimens were subjected to an anodic polarization to accelerate corrosion. Specifically, the ammonia SCC resistance was evaluated according to the following procedure. Test specimens measuring 5 mm thick x 15 mm x 115 mm were taken from the surface of the steel sheets, ultrasonically degreased in acetone for 5 minutes, and then subjected to four-point bending to apply a stress equal to the average of the yield strength and tensile strength of each steel sheet. The four-point bending test specimens were placed in a test cell, which was filled with a solution containing 12.5 g of ammonium carbamate and 1 L of liquid ammonia. The potentiostat was then used to apply a potential of +2.0 V vs. Pt to the test specimens, and the specimens were immersed in the solution at room temperature (25°C). If no cracks were observed in the test specimens after 168 hours of immersion, the ammonia SCC resistance was judged to be good; if cracks were observed, the specimens were judged to be poor. The evaluation results thus obtained are shown in Table 3.

[0073] [Joint HAZ toughness] Furthermore, V-groove processing was performed on test plates for joints taken from each steel plate, and CO2 welding was performed with a welding heat input of 15 kJ / cm using a commercially available welding wire for low-temperature steel to produce joints. The HAZ toughness of the obtained joints was then evaluated. The test method was as follows: NK U4 impact test specimens were taken from the surface of the joint with a depth of 1 mm as the test specimen surface layer, and a notch was made in the HAZ (fusion line). Charpy impact tests were performed on the taken test specimens at a test temperature of -60°C, and the average absorbed energy vE of three test specimens taken under the same conditions was calculated. -60℃ (unit: J) was taken as the toughness of the HAZ. The evaluation results thus obtained are shown in Table 3.

[0074] [Table 1] TIFF2026002201000002.tif255157

[0075] [Table 2] TIFF2026002201000004.tif255158TIFF2026002201000005.tif255163

[0076] [Table 3] TIFF2026002201000007.tif152170

[0077] As can be seen from Tables 1 and 2, all of the examples of the invention have a surface hardness of 210 HV10 or less, a yield strength (YS) of 325 MPa to 440 MPa, and a tensile strength (TS) of 440 MPa or more, and a ductile-brittle fracture transition temperature (vTrs) of -60°C or less after 5% pre-strain, resulting in steel plates with excellent toughness and ammonia stress corrosion cracking resistance at low temperatures.

[0078] On the other hand, steel plates Nos. 2 to 16, which correspond to comparative examples and whose manufacturing conditions are outside the scope of the present invention, are inferior to the invention examples in at least one of yield strength (YS), tensile strength (TS), low-temperature toughness, and ammonia SCC resistance. Steel plate No. 31, which corresponds to a comparative example, has a low C content, and its yield strength (YS) and tensile strength (TS) are inferior to the invention examples. Steel plate No. 32 has a high C content, and its low-temperature toughness is inferior to the invention examples. Steel plates Nos. 33, 35, 36, 37, 38, 40, 42, and 43 have higher contents of various elements than the invention examples, and their low-temperature toughness is inferior to the invention examples. Steel plate No. 34 has a low Mn content, and its tensile strength (TS) is inferior to the invention examples. Steel plates Nos. 39 and 41 have low amounts of nitrogen or titanium, and their low-temperature toughness is inferior to the invention examples. Steel plate No. 44 has a low Ceq and is inferior to the invention examples in yield strength YS and tensile strength TS. Steel plate No. 45 has a high Ceq and is inferior to the invention examples in surface hardness, yield strength YS, low-temperature toughness, and ammonia SCC resistance. [Industrial Applicability]

[0079] The steel plate of the present invention has excellent toughness and corrosion resistance, particularly excellent low-temperature toughness and resistance to ammonia stress corrosion cracking, and can be suitably used as a material for multipurpose tanks that carry a mixture of liquefied petroleum gas (hereinafter referred to as LPG) and liquid ammonia.

Claims

1. 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, Ti: 0.005% or more and 0.050% or less, N: 0.0010% or more and 0.0100% or less; O: 0.0100% or less The composition of the alloy has 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: The hardness at a depth of 1 mm is 210 HV or less, At a depth position of 1 / 4 of the plate thickness, the volume fractions of ferrite and bainite are each 15% or more, and the total volume fraction of the ferrite and the bainite is 80% or more, The yield strength is 325 MPa or more and 440 MPa or less, The tensile strength is 440 MPa or more, A steel plate having a ductile-brittle fracture transition temperature vTrs of -60°C or less after being pre-strained by 5%. 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 the element, and if the element is not contained, it is set to 0.

2. The component composition further comprises, in mass%, Al: 0.060% or less, 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, Ca: 0.0050% or less, Mg: 0.0050% or less and REM: 0.0050% or less The steel sheet according to claim 1, comprising one or more selected from the following:

3. 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, Ti: 0.005% or more and 0.050% or less, N: 0.0010% or more and 0.0100% or less; O: 0.0100% or less a steel material having a component composition in which a carbon equivalent Ceq represented by the following formula (1) is 0.25 or more and 0.40 or less, and the balance is Fe and unavoidable impurities, Heating to a temperature of 950°C or higher and 1250°C or lower, Next, the rolling start temperature was set to Ar ３ The rolling end temperature is set to 100°C or higher than the rolling point, the cumulative reduction rate in the unrecrystallized region is 60% or higher, and the rolling end temperature is Ar ３ Hot rolling is performed to a temperature of 1000°C or higher, Then, Ar ３ Cooling begins from a temperature above the Ar point. ３ Ar below point ３ Cool by 20°C or more at an average cooling rate of 5°C / s or more until the temperature reaches -90°C or higher, The cooling is stopped temporarily and the cooling is interrupted for 5 seconds or more and 600 seconds or less, Subsequently, cooling is performed at an average cooling rate of 5°C / s or more at a depth of 1 / 4 of the plate thickness of the steel plate, The cooling is completed when the temperature at a depth of 1 / 4 of the plate thickness of the steel plate is 580°C or less, Next, a heat treatment is performed at a temperature of 400°C or higher and 580°C or lower. 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 the element, and if the element is not contained, it is set to 0.

4. The component composition further comprises, in mass%, Al: 0.060% or less, 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, Ca: 0.0050% or less, Mg: 0.0050% or less and REM: 0.0050% or less The method for producing a steel sheet according to claim 3, wherein the steel sheet contains one or more selected from the following:

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