Steel plate with excellent resistance to ammonia stress corrosion cracking and method for manufacturing the same

Abstract

To provide a cost-effective high-strength steel sheet that exhibits excellent low-temperature toughness and excellent resistance to ammonia stress corrosion cracking, and a method for producing the same.SOLUTION: The present invention provides a steel sheet and a method for producing the same, wherein the steel sheet has a predetermined composition with a carbon equivalent Ceq value of 0.30 or more and 0.40 or less; the steel sheet includes a metallographic structure in which the volume fraction of tempered bainite at a position 0.5 mm from the surface in the thickness direction is 90% or more, the volume fraction of tempered bainite at the 1 / 2 thickness position is 20% or more and the total volume fraction of ferrite and the tempered bainite is 60% or more; the average Vickers hardness at a position 0.5 mm from the surface in the thickness direction is 230 HV1 or less, and the variation in Vickers hardness is 50 HV1 or less; and the Vickers hardness in the weld heat-affected zone is 230 HV1 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel sheet for low-temperature use excellent in low-temperature toughness and ammonia stress corrosion cracking resistance, which is suitable for structural members such as tanks used in low-temperature liquid ammonia environments, and a method for producing such a steel sheet. [Background Art]

[0002] In liquid ammonia environments, carbon steel is concerned about the occurrence of stress corrosion cracking caused by liquid ammonia. Therefore, for carbon steel structures such as pipes, storage tanks, tank trucks and line pipes that handle liquid ammonia, measures have been taken including the application of steel materials excellent in ammonia stress corrosion cracking resistance and operational measures to suppress ammonia stress corrosion cracking.

[0003] It is known that ammonia stress corrosion cracking correlates with the strength and hardness of a material. In particular, it is known that ammonia stress corrosion cracking frequently occurs in heat-affected zones of welding. Therefore, when high-strength steel is used for structures used in liquid ammonia environments, measures to prevent ammonia stress corrosion cracking, such as performing post-weld heat treatment through full annealing to adjust the hardness of the welded part, are required.

[0004] On the other hand, in recent years, liquid ammonia has attracted attention as a clean energy because it does not generate CO2 when combusted, and large-scale demand is expected for it. Along with this, there is a demand for larger-scale equipment for transporting and storing liquid ammonia. As the equipment becomes larger, it becomes extremely difficult to perform post-weld heat treatment through full annealing.

[0005] Generally, when increasing the size of a tank, from the viewpoint of weight reduction and reduction in construction cost, thinning of the steel material to be used is pursued, so the use of high-strength steel is desired.

[0006] Furthermore, since liquid ammonia is transported and stored at low temperatures, steel sheets used for such applications are required to have excellent low-temperature toughness.

[0007] As a method for manufacturing steel that achieves both high strength and low-temperature toughness as described above, the technologies described in Patent Documents 1 and 2 are disclosed. These technologies achieve high low-temperature toughness and high strength by heat-treating a thick steel plate that has been hot-rolled and cooled several times, or by heat-treating a thick steel plate that has been hot-rolled, water-cooled, and then heat-treating it several times. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 10-140235 [Patent Document 2] Japanese Patent Application Publication No. 10-168516 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, the manufacturing methods described in the above-mentioned Patent Documents 1 and 2 require multiple heat treatments, which presents economic problems due to the high costs associated with the equipment and energy required for these treatments.

[0010] The present invention aims to solve the above problems and provide an inexpensive, high-strength steel sheet with excellent low-temperature toughness and resistance to ammonia stress corrosion cracking, suitable for use in tanks for transporting or storing liquid ammonia, and a method for manufacturing the same. [Means for solving the problem]

[0011] To achieve the above objective, the inventors diligently investigated the influence of various factors on the low-temperature toughness and strength characteristics of steel sheets using the TMCP process and an online induction heating device.

[0012] As a result, we found that by appropriately controlling the composition of the steel sheet, as well as the metallographic structure and hardness near the surface and in the center of the sheet thickness, stress corrosion cracking resistance in a liquid ammonia environment can be effectively obtained, eliminating the need for costly multiple heat treatments.

[0013] This invention is based on the above findings, and the gist of this invention is as follows: [1] Contains, by mass%, C: 0.010~0.200%, Si: 0.01~1.00%, Mn: 0.20~2.30%, P: 0.030% or less, S: 0.0100% or less, Al: 0.010~0.100%, N: 0.0010~0.0100%, and O: 0.0100% or less, and the Ceq value shown in the following formula (1) is 0.30 or more and 0.40 or less. Ceq=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 ···(1) (In the formula, [X] represents the mass %) content of element X in the steel, and is 0 if element X is not present.) The composition consists of Fe and unavoidable impurities. The volume fraction of tempered bainite at a position 0.5 mm from the surface in the thickness direction is 90% or more. The metal structure has a volume fraction of tempered bainite of 20% or more at the 1 / 2 thickness position, and a total volume fraction of ferrite and said tempered bainite of 60% or more. The average Vickers hardness at a position 0.5 mm from the surface in the thickness direction is 230 HV1 or less, and the variation in said Vickers hardness is 50 HV1 or less. A steel plate characterized by having a Vickers hardness of 230 HV1 or less in the heat-affected zone of the weld.

[0014] [2] The steel sheet according to [1], wherein the component composition further contains, in mass%, one or more elements selected from Group A) Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 1.80% or less, Sn: 0.50% or less, Sb: 0.50% or less and W: 1.00% or less, Group B) Mo: 0.50% or less, V: 0.200% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.100% or less and B: 0.0100% or less, Group C) Ca: 0.0200% or less, Mg: 0.0200% or less and REM: 0.0200% or less.

[0015] [3] A method for manufacturing a steel sheet, comprising: hot rolling a steel material having the component composition of [1] or [2] above to a rolling end temperature of Ar3 transformation point or higher to form a steel sheet; cooling from a temperature above the Ar3 transformation point at an average cooling rate of 10 to 120 °C / s to a cooling stop temperature in the range of 50 to 600 °C; and reheating until the temperature at a position 0.5 mm from the surface of the steel sheet in the thickness direction is in the range of 550 to 750 °C, and the temperature at a position half the thickness is in the range of more than 500 °C and 700 °C or less. [Effects of the Invention]

[0016] According to the present invention, steel plates with excellent low-temperature toughness and resistance to ammonia stress corrosion cracking, and high strength suitable for structural members such as tanks used in low-temperature and liquid ammonia environments, can be provided through an inexpensive process. [Modes for carrying out the invention]

[0017] Embodiments of the present invention are described below. The present invention is not limited to the embodiments described below, and can be implemented in various ways within the scope of its gist. In addition, the "%" used to represent the content of the constituent elements below means "mass%" unless otherwise specified. Also, A (numerical value) to B (numerical value) means A or greater and B or less.

[0018] (1) Ingredient composition C: 0.010~0.200% Carbon (C) is the most effective element for increasing the strength of steel plates. To achieve this effect, the C content is specified to be 0.010% or higher. Furthermore, from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost, it is preferable that the C content be 0.020% or higher. On the other hand, if the C content exceeds 0.200%, it leads to a deterioration of the toughness and weldability of the steel plate. Therefore, the C content is specified to be 0.200% or lower. Furthermore, from the viewpoint of toughness and weldability, it is preferable that the C content be 0.170% or lower.

[0019] Si: 0.01~1.00% Si is added for deoxidation. To obtain such an effect, the Si content is specified to be 0.01% or more. Further, the Si content is preferably 0.03% or more. On the other hand, when the Si content exceeds 1.00%, it causes deterioration of the toughness and weldability of the steel sheet. Therefore, the Si content is specified to be 1.00% or less. Further, from the viewpoints of toughness and weldability, the Si content is preferably 0.60% or less.

[0020] Mn: 0.20 to 2.30% Mn is an element that has the effect of increasing the hardenability of steel, and is one of the important elements that require addition to satisfy high strength as in the present invention. To obtain such an effect, the Mn content is specified to be 0.20% or more. Further, from the viewpoint of reducing the content of other alloying elements and achieving production at lower cost, the Mn content is preferably 0.50% or more. On the other hand, when the Mn content exceeds 2.30%, in addition to reducing toughness and weldability, the alloy cost becomes excessively high. Therefore, the Mn content is specified to be 2.30% or less. Further, from the viewpoint of suppressing the reduction in toughness and weldability, the Mn content is preferably 1.80% or less.

[0021] Al: 0.010 to 0.100% Al acts as a deoxidizer. To obtain such an effect, the Al content is specified to be 0.010% or more. On the other hand, when the Al content exceeds 0.100%, oxide inclusions increase, which reduces cleanliness and lowers toughness. Therefore, the Al content is specified to be 0.100% or less. Further, from the viewpoint of preventing toughness deterioration, the Al content is preferably 0.050% or less.

[0022] N: 0.0010 to 0.0100% N combines with Ti to precipitate as TiN, contributing to microstructure refinement and improving the toughness of the steel sheet. To achieve this effect, the N content is specified to be 0.0010% or more. Preferably, it is 0.0020% or more. On the other hand, if the N content exceeds 0.0100%, it can actually lead to a decrease in toughness. Therefore, the N content is specified to be 0.0100% or less. Furthermore, from the viewpoint of suppressing a decrease in toughness and weldability, it is preferable to set the N content to 0.0080% or less.

[0023] P:0.030% or less P is an element that is inevitably present as an impurity, and it has adverse effects such as reducing toughness and weldability by segregating at grain boundaries. Therefore, it is desirable to keep the P content as low as possible, but it is acceptable if it is 0.030% or less. There is no particular lower limit to the P content, and it may be 0%, but since P is usually an element that is inevitably present in steel as an impurity, it may be greater than 0% industrially. Furthermore, since reducing excess P leads to a rise in refining costs, it is preferable to keep the P content at 0.001% or more.

[0024] S: 0.0100% or less S is an element that is inevitably present as an impurity, existing in steel as sulfide inclusions such as MnS, and is an element that has adverse effects, such as reducing the toughness of steel sheets by becoming the starting point for fracture. For this reason, it is desirable to keep the S content as low as possible, but it is acceptable if it is 0.0100% or less. There is no particular lower limit to the S content, and it may be 0%. Normally, since S is an element that is inevitably present in steel as an impurity, it may be greater than 0% industrially. Furthermore, since reducing excess S leads to a rise in refining costs, from a cost perspective, it is preferable to have an S content of 0.0001% or more.

[0025] O: 0.0100% or less O is an element that is inevitably present as an impurity, and it has adverse effects such as forming oxides, becoming the starting point for fracture, and reducing the toughness of steel sheets. Therefore, its content should be limited to 0.0100% or less. Preferably, the O content should be 0.0050% or less, and more preferably 0.0030% or less. On the other hand, there is no particular lower limit to the O content, and it may be 0%, but since O is usually an element that is inevitably present in steel as an impurity, it may be greater than 0% industrially. Furthermore, since reducing excess O leads to a rise in refining costs, from a cost perspective, it is preferable to have an O content of 0.0005% or more.

[0026] The remainder of the steel material of the present invention, other than the above-mentioned components, consists of Fe and unavoidable impurities. However, if necessary, one or more elements selected from those listed below may be further included.

[0027] Cu: 1.00% or less Cu forms a protective film with appropriate properties, improving resistance to ammonia stress corrosion cracking. However, if the Cu content exceeds 1.00%, this effect saturates, and the alloy cost increases. Therefore, when Cu is included, the Cu content should be 1.00% or less. Furthermore, from the viewpoint of suppressing an excessive increase in alloy cost, it is preferable to keep the Cu content at 0.50% or less.

[0028] Ni: 2.00% or less Ni is an element that improves the strength and toughness of steel plates and can be included as desired. However, if the Ni content exceeds 2.00%, the resistance to ammonia stress corrosion cracking deteriorates. Therefore, when Ni is included, the Ni content should be 2.00% or less. Furthermore, from the viewpoint of suppressing an excessive increase in alloy costs, it is preferable to keep the Ni content at 1.00% or less.

[0029] Co:2.00% or less Co is an element that improves the strength and toughness of steel sheets and can be included as desired. However, if the Co content exceeds 2.00%, its effect saturates, and the alloy cost increases. Therefore, when Co is included, the Co content should be 2.00% or less. Furthermore, from the viewpoint of suppressing an excessive increase in alloy costs, it is preferable to keep the Co content at 1.00% or less.

[0030] Cr:1.80% or less Cr forms a protective film with appropriate properties, improving resistance to ammonia stress corrosion cracking. However, when the Cr content exceeds 1.80%, this effect saturates, and the alloy cost increases. Therefore, when Cr is included, the Cr content should be 1.80% or less. Furthermore, from the viewpoint of suppressing an excessive increase in alloy cost, it is preferable to keep the Cr content at 1.00% or less.

[0031] Sn: 0.50% or less Sn forms a protective film with appropriate properties, improving resistance to ammonia stress corrosion cracking. However, if the Sn content exceeds 0.50%, the effect saturates, and the alloy cost increases. Therefore, when Sn is included, the Sn content should be 0.50% or less. Furthermore, from the viewpoint of suppressing an excessive increase in alloy cost, it is preferable to keep the Sn content at 0.20% or less.

[0032] Sb: 0.50% or less Sb forms a protective film with appropriate properties, improving resistance to ammonia stress corrosion cracking. However, if the Sb content exceeds 0.50%, the effect saturates, and the alloy cost increases. Therefore, when Sb is included, the Sb content should be 0.50% or less. Furthermore, from the viewpoint of suppressing an excessive increase in alloy cost, it is preferable to keep the Sb content at 0.20% or less.

[0033] W: 1.00% or less W forms a protective film with appropriate properties, improving resistance to ammonia stress corrosion cracking. However, if the W content exceeds 1.00%, the effect saturates, and the alloy cost increases. Therefore, when W is included, the W content should be 1.00% or less. Furthermore, from the viewpoint of suppressing an excessive increase in alloy cost, it is preferable to have a W content of 0.50% or less.

[0034] Mo: 0.50% or less Mo is an element that improves the strength of steel plates and can be included as desired. However, if the Mo content exceeds 0.50%, it degrades the resistance to ammonia stress corrosion cracking. Therefore, when Mo is included, the Mo content should be 0.50% or less. Furthermore, from the viewpoint of suppressing an excessive increase in alloy costs, it is preferable to keep the Mo content at 0.25% or less.

[0035] V:0.200% or less V is an element that improves the strength of steel plates and can be included as desired. However, if the V content exceeds 0.200%, the hardness increases excessively, leading to a deterioration in resistance to ammonia stress corrosion cracking. Therefore, when V is included, the V content should be 0.200% or less. Furthermore, from the viewpoint of suppressing an excessive increase in alloy costs, it is preferable to keep the V content at 0.100% or less.

[0036] Ti:0.100% or less Ti is an element that has a strong tendency to form nitrides, forming TiN and improving toughness, and can be included as desired. However, if the Ti content exceeds 0.100%, toughness actually decreases. Therefore, when Ti is included, the Ti content should be 0.100% or less. Furthermore, from the viewpoint of suppressing an excessive increase in alloy costs, it is preferable to keep the Ti content at 0.050% or less.

[0037] Nb: 0.100% or less Nb is an element that improves toughness by reducing the size of the prior austenite grain when precipitated as carbonitrides. However, if the Nb content exceeds 0.100%, a large amount of NbC precipitates, reducing toughness. Therefore, when Nb is included, the Nb content should be 0.100% or less. Furthermore, from the viewpoint of suppressing an excessive increase in alloy costs, it is preferable to keep the Nb content at 0.050% or less.

[0038] Zr: 0.100% or less Zr is an element that improves the strength of steel plates and can be included as desired. However, if the Zr content exceeds 0.100%, the hardness increases excessively, leading to a deterioration in resistance to ammonia stress corrosion cracking. Therefore, when Zr is included, the Zr content should be 0.100% or less. Furthermore, from the viewpoint of suppressing an excessive increase in alloy costs, it is preferable to keep the Zr content at 0.050% or less.

[0039] B: 0.0100% or less B is an element that significantly improves hardenability even in trace amounts. In other words, it can improve the strength of steel plates. However, if the B content exceeds 0.0100%, the weldability decreases. Therefore, when B is included, the B content should be 0.0100% or less. Furthermore, from the viewpoint of suppressing the decrease in weldability, it is preferable to have a B content of 0.0050% or less.

[0040] Ca:0.0200% or less Ca is an element that combines with S and suppresses the formation of MnS and other minerals that elongate in the rolling direction. In other words, by including Ca, the morphology of sulfide inclusions can be controlled so that they take on a spherical shape, and stress concentration on the inclusions can be suppressed, thereby improving the toughness of the heat-affected zone of the weld. On the other hand, if the Ca content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in the toughness of the base material. Therefore, when Ca is included, the Ca content should be 0.0200% or less. Furthermore, from the viewpoint of suppressing a decrease in the cleanliness of the steel, it is preferable to have a Ca content of 0.0100% or less.

[0041] Mg: 0.0200% or less Like Ca, Mg is an element that combines with S and suppresses the formation of MnS and other elements that elongate in the rolling direction. In other words, by including Mg, the morphology of sulfide inclusions can be controlled so that they take on a spherical shape, and stress concentration on the inclusions can be suppressed, thereby improving the toughness of the heat-affected zone of the weld. On the other hand, if the Mg content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in the toughness of the base material. Therefore, when Mg is included, the Mg content should be 0.0200% or less. Furthermore, from the viewpoint of suppressing a decrease in the cleanliness of the steel, it is preferable to have a Mg content of 0.0100% or less.

[0042] REM: 0.0200% or less Rare earth metals (REMs), like Ca and Mg, are elements that bond with sulfur and suppress the formation of MnS and other elements that elongate in the rolling direction. In other words, by including REMs, the morphology of sulfide inclusions can be controlled to become spherical, and stress concentration in the inclusions can be suppressed, thereby improving the toughness of the heat-affected zone of the weld. On the other hand, if the REM content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to a decrease in the toughness of the base material. Therefore, when REMs are included, the REM content should be 0.0200% or less. Furthermore, from the viewpoint of suppressing a decrease in the cleanliness of the steel, it is preferable to have a REM content of 0.0100% or less.

[0043] Ceq: 0.30% to 0.40% If the carbon equivalent (Ceq) of the steel sheet is less than 0.30%, the desired strength cannot be obtained. On the other hand, if the Ceq of the steel sheet exceeds 0.40%, the hardness of the heat-affected zone during welding increases excessively, leading to a deterioration in resistance to ammonia stress corrosion cracking. Therefore, the Ceq of the steel sheet should be between 0.30% and 0.40%.

[0044] Furthermore, Ceq is calculated according to the following formula (1), where [X] represents the mass %) of element X in the steel, and 0 is used if element X is not present.

[0045] Ceq(%)=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (%)...(1)

[0046] (2) Metal structure In addition to having the above-mentioned component composition, the steel sheet of the present invention has a metal structure in which the average hardness at a depth of 0.5 mm from the surface of the steel sheet in the thickness direction (also referred to as the 0.5 mm position in the present invention) is 230 HV1 or less and the variation in such hardness is 50 HV1 or less, and furthermore, the volume fraction of tempered bainite at such 0.5 mm position is 90% or more. The steel sheet of the present invention also has a metal structure in which the volume fraction of tempered bainite at the 1 / 2 thickness position is 20% or more and the total volume fraction of ferrite and said tempered bainite is 60% or more, and the hardness of the weld heat-affected zone obtained by welding is 230 HV1 or less.

[0047] The reason for limiting the metal structure of the steel plate as described above is explained below.

[0048] [The volume fraction of tempered bainite at the 0.5mm position is 90% or more] To satisfy the strength characteristics and ammonia stress corrosion cracking resistance of steel sheets, the volume fraction of tempered bainite in the metal structure at the 0.5 mm position must be 90% or more. When hard phases such as martensitic structures and island martensite (MA) structures are formed in the surface layer of the steel sheet, the surface hardness increases, and the variation in hardness within the steel sheet increases, hindering material uniformity. If the total volume fraction of tempered bainite at the 0.5 mm position is less than 90%, the volume fraction of other structures, namely ferrite, island martensite, martensite, pearlite, and austenitic structures, increases, and sufficient strength and / or ammonia stress corrosion cracking resistance cannot be obtained.

[0049] The remaining tissue, which accounts for 10% or less by volume, may include ferrite, pearlite, and austenite, as well as martensite. The proportion of each tissue in the remaining tissue does not need to be particularly limited, but it is preferable that the remaining tissue is pearlite. The volume fractions of various microstructures can be measured by the methods described in the examples below.

[0050] [At the 1 / 2 thickness position, the volume fraction of tempered bainite is 20% or more, and the total volume fraction of ferrite and tempered bainite is 60% or more.] The microstructure at the half-thickness position must have a volume fraction of tempered bainite of 20% or more, and a total volume fraction of ferrite and tempered bainite of 60% or more. This is because excessive ferrite formation leads to a decrease in strength, and if the total volume fraction of ferrite and tempered bainite is less than 60%, the volume fraction of other microstructures, namely island martensite, martensite, pearlite, and austenite, will increase, resulting in insufficient strength or toughness and failure to satisfy the mechanical properties. The total volume fraction of ferrite and tempered bainite at the half-thickness position may be 100%.

[0051] [Average hardness at 0.5mm position is 230HV1 or less, and variation is 50HV1 or less] The average hardness at the 0.5 mm position is 230 HV1 or less, and its variation is 50 HV1 or less. If a high-hardness region exists in the outermost layer of the steel plate, specifically at the 0.5 mm position, stress corrosion cracking in a liquid ammonia environment is promoted. Furthermore, if a localized high-hardness region exists, stress concentration occurs when stress is applied to the steel plate, promoting stress corrosion cracking. In this invention, by setting the hardness at the 0.5 mm position to 230 HV1 or less and adjusting its variation to 50 HV1 or less, excellent resistance to ammonia stress corrosion cracking can be ensured. The lower limit of the average hardness at the 0.5 mm position is not particularly limited, but around 130 HV1 is preferred. The lower limit of the variation may be 0 HV1, but industrially it is around 10 HV1.

[0052] Here, the above-mentioned variation refers to the variation in Vickers hardness at the 0.5 mm position, and means the standard deviation of the Vickers hardness measured to determine the average hardness. These can be measured by the method described in the examples below.

[0053] [Vickers hardness of the heat-affected zone of the welded steel plate is 230HV1 or less] The Vickers hardness of the heat-affected zone of the welded steel plate should be 230 HV1 or less. The presence of a high-hardness region in the surface layer of the steel plate promotes ammonia stress corrosion cracking. In particular, ammonia stress corrosion cracking occurs in the heat-affected zone. That is, if the hardness of the heat-affected zone of the welded steel plate exceeds 230 HV1, the desired resistance to ammonia stress corrosion cracking cannot be obtained. The Vickers hardness of the heat-affected zone can be measured by the method described in the examples below.

[0054] The welding method for obtaining the heat-affected zone having the Vickers hardness is preferably one of shielded metal arc welding, gas metal arc welding, or submerged arc welding, with a welding heat input of 10 kJ / cm or more and 100 kJ / cm or less. Furthermore, the thickness of the steel plate to be welded is preferably 10 mm or more and 40 mm or less.

[0055] (3) Manufacturing conditions The manufacturing method in this invention involves heating and hot-rolling a steel slab with the aforementioned component composition, followed by accelerated cooling and immediate heat treatment. The reasons for limiting the manufacturing conditions of the steel sheet are explained below.

[0056] First, there is no particular need to limit the manufacturing conditions of the steel material, but it is preferable to, for example, melt molten steel having the aforementioned component composition using a known melting method such as a converter, and then use a known casting method such as continuous casting to produce steel material such as slabs of a predetermined size. There is no problem in producing steel material such as slabs of a predetermined size by the ingot-part rolling method.

[0057] The resulting steel material is either hot-rolled directly without cooling, or hot-rolled after reheating. Hot rolling is performed above the Ar3 transformation point. From a temperature above the Ar3 transformation point, accelerated cooling is performed to a cooling stop temperature in the range of 50 to 600°C, with an average cooling rate of 10 to 120°C / s at the 1 / 2 position of the steel plate thickness. Furthermore, reheating is performed until the temperature at a position 0.5 mm from the surface of the steel plate is in the range of over 550°C and under 700°C, and the temperature at the 1 / 2 position of the plate thickness is over 500°C and under 700°C.

[0058] Steel material heating temperature: 950~1300℃ While there are no particular limitations on the heating temperature of the steel material, if the heating temperature is too low, the deformation resistance will increase, increasing the load on the hot rolling mill and potentially making hot rolling difficult. On the other hand, if the temperature exceeds 1300°C, oxidation will be significant, increasing oxidation loss and potentially reducing yield. For these reasons, it is preferable to heat the material to a temperature between 950°C and 1300°C. The heating temperature of the steel material should be the temperature of the core of the steel material.

[0059] Rolling completion temperature: Above the Ar3 transformation point In this invention, after heating the steel material to the above temperature, hot rolling is started and terminated above the Ar3 transformation point. If the rolling termination temperature falls below the Ar3 transformation point, ferrite is formed, hindering the uniformity of the material in the surface layer of the steel sheet and increasing the variation in hardness, thus degrading the resistance to ammonia stress corrosion cracking. Furthermore, the formed ferrite is affected by the processing, resulting in a deterioration of toughness. In addition, the load on the hot rolling mill increases.

[0060] Therefore, the hot rolling completion temperature in the present invention is set to be above the Ar3 transformation point. More preferably, the rolling completion temperature is above the Ar3 transformation point + 10°C. On the other hand, if the rolling completion temperature exceeds 950°C, the microstructure may coarseen and the toughness may deteriorate, so it is preferable to set it to 950°C or lower.

[0061] Here, the Ar3 transformation point can be determined, for example, by the following equation. Ar3 transformation point (°C) = 910 - 273 × C - 74 × Mn - 57 × Ni - 16 × Cr - 9 × Mo - 5 × Cu

[0062] In the above formula, each element represents the mass %) content of that element in the steel, and elements that are not present are represented as 0. The rolling completion temperature is the temperature at the point where the steel plate is half its thickness.

[0063] Cooling start temperature: Above the Ar3 transformation point Next, the steel sheet obtained by hot rolling is cooled from a temperature above the Ar3 transformation point. If the cooling start temperature is below the Ar3 transformation point, excessive ferrite will be formed, resulting in insufficient strength, deterioration of toughness, and further deterioration of resistance to ammonia stress corrosion cracking. Therefore, the cooling start temperature should be above the Ar3 transformation point. The cooling start temperature should be the temperature at the point where the steel sheet is half its thickness.

[0064] Average cooling rate for accelerated cooling: 10-120°C / s Cooling at an average cooling rate of 10°C / s or higher at the half-thickness position is an essential process for obtaining high-strength and high-toughness steel sheets, as high cooling rates provide a strength-enhancing effect through transformation strengthening. Therefore, to obtain this effect, the average cooling rate at the half-thickness position during cooling according to the present invention is specified to be 10°C / s or higher. From the viewpoint of obtaining high-strength and high-toughness steel sheets with low Ceq, it is preferable to set the average cooling rate to over 25°C / s. On the other hand, if the average cooling rate at the half-thickness position exceeds 120°C / s, the volume fraction of martensite becomes too large, reducing toughness. Therefore, the average cooling rate is specified to be 120°C / s or lower. Furthermore, when the average cooling rate is high, the difference in cooling rates in the thickness direction becomes large, making the steel sheet more prone to warping. From the viewpoint of preventing this, it is preferable to set the average cooling rate to 80°C / s or lower.

[0065] The above cooling rate is controlled by controlled cooling through intermittent cooling, including cooling stop periods. Furthermore, although it is difficult to directly measure the temperature at the 1 / 2 thickness position, the temperature distribution within the cross-section of the plate can be determined in real time by performing a difference calculation using, for example, a process computer, based on the steel plate surface temperature at the start of cooling and the target steel plate surface temperature at the end of cooling, as measured by a radiation thermometer.

[0066] Cooling stop temperature: 50~600℃ The present invention allows ferrite and tempered bainite to be distributed in a predetermined volume ratio at the center of the plate thickness by cooling according to the present invention to a cooling stop temperature arbitrarily set in the range of 50 to 600°C after the completion of rolling.

[0067] If the cooling stop temperature is set below 50°C, cooling will take a long time, raising concerns about reduced manufacturing efficiency. On the other hand, if the cooling stop temperature exceeds 600°C, excessive ferrite and pearlite structures will form, leading to insufficient strength and deterioration of toughness. Therefore, the cooling stop temperature is specified to be in the range of 50 to 600°C. Furthermore, from the viewpoint of preventing deterioration of toughness due to an excessively high volume fraction of martensite, it is preferable to set the cooling stop temperature to 200°C or higher. Also, from the viewpoint of achieving both high strength and high toughness, it is preferable to set the cooling stop temperature to below 450°C. Note that the cooling stop temperature is the temperature at the 1 / 2 thickness position of the plate.

[0068] Reheating temperature: The temperature at a point 0.5 mm from the surface of the steel plate should be between 550°C and 750°C, and the temperature at half the plate thickness should be between 500°C and 700°C. In this invention, it is necessary to reheat (temper) the steel plate after accelerated cooling. When a thick steel plate is accelerated, the cooling rate of the surface layer of the steel plate increases, and the surface layer cools to a lower temperature than the interior of the steel plate. As a result, hard structures such as martensite are easily formed in the surface layer of the steel plate, which may degrade its resistance to ammonia stress corrosion cracking. Therefore, in this invention, the surface layer of the steel plate is reheated after accelerated cooling. This is because it is possible to reduce the hardness of the surface layer of the steel plate. The time from the cessation of cooling to reheating should be between 5 seconds and 300 seconds.

[0069] Here, if the reheating temperature at a position 0.5 mm from the surface of the steel plate is less than 550°C, the reduction in hardness is insufficient. On the other hand, if the reheating temperature at a position 0.5 mm from the surface of the steel plate exceeds 750°C, the overall strength of the steel plate decreases, making it difficult to obtain the required strength. Therefore, the reheating temperature at a position 0.5 mm from the surface of the steel plate during reheating after accelerated cooling is specified to be in the range of 550 to 750°C. The average heating rate of reheating can be 5°C / s or more and 100°C / s or less at a position 0.5 mm from the surface of the steel plate.

[0070] Furthermore, if the temperature at the half-thickness point during reheating is below 500°C, the reheating temperature of the surface layer will also be low, resulting in insufficient reduction in the hardness of the surface layer. On the other hand, if the temperature exceeds 700°C, a decrease in the overall strength and deterioration of toughness will occur in the steel plate. Therefore, the temperature at the half-thickness point during reheating after accelerated cooling is specified to be between 500°C and 700°C. The average heating rate at the half-thickness point during reheating can be between 5°C / s and 100°C / s.

[0071] As a means of reheating after accelerated cooling, induction heating is preferably used. In particular, high-frequency induction heating is preferred so that the heating is concentrated on the surface layer of the steel plate, including the 0.5 mm position. Once the temperature at the 0.5 mm position and the temperature at the 1 / 2 thickness position of the steel plate reach the above reheating temperature range, it is not necessary to maintain the temperature within that range, and cooling may be started immediately.

[0072] Furthermore, while there are no particular limitations on the cooling method after reheating, in the case of thick steel plates exceeding approximately 40 mm in thickness, the cooling rate may be slow, raising concerns about deterioration of toughness due to the coagulation and coarsening of carbides. In such cases, water cooling or mist cooling may be performed after the reheating treatment. The average cooling rate at the 0.5 mm position is preferably 0.1°C / s or more and 20°C / s or less. The average cooling rate at the 1 / 2 thickness position is preferably 0.05°C / s or more and 10°C / s or less.

[0073] By manufacturing a steel material having the above-described component composition according to the above-described manufacturing conditions, a steel sheet having the component composition and structure according to the present invention can be obtained. The steel sheet thus obtained according to the present invention possesses excellent strength properties and toughness, and excellent resistance to liquid ammonia stress corrosion cracking. Here, excellent strength properties mean a yield strength YS (yield point YP if there is a yield point, or 0.2% proof stress σ0.2 if there is no yield point) of 450 MPa or more and a tensile strength (TS) of 570 MPa or more. Excellent toughness means that the transition temperature according to JIS Z 2242 is -30°C or lower.

[0074] Furthermore, in the manufacturing method according to the present invention, conventional methods can be used for items not described herein. [Examples]

[0075] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to the following examples.

[0076] Steel with the component composition shown in Table 1 (steel Nos. 1-39, the remainder being Fe and unavoidable impurities) was cast into slabs using a continuous casting method, and thick steel plates (steel Nos. 1-39) with a thickness of 30 mm were manufactured using these slabs under the manufacturing conditions shown in Table 2. Reheating was performed by induction heating. For the obtained steel plates, the microstructure fraction of the metall structure at a position 0.5 mm from the surface of the steel plate was measured, the Vickers hardness at a position 0.5 mm from the surface of the steel plate and in the heat-affected zone of the weld was measured, the strength properties and toughness were evaluated, and the resistance to ammonia stress corrosion cracking in a liquid ammonia environment was evaluated. The test methods for each test are as follows.

[0077] [Measurement of hardness in the heat-affected zone of a weld] Test specimens were taken from steel plates and bead-on welding was performed. The size of the welded test specimens was 20mmt × 75mmw × 200mmL. The welding method was shielded metal arc welding, and the welding material was TB-43 manufactured by Kobe Steel, Ltd., with a wire diameter of 4.0mm. There was no preheating, the current was 170A, the voltage was 24V, and the welding speed was 150mm / min. The weld length was 125mm. Five test pieces for cross-sectional macro observation were taken from the welded test specimens. After mirror polishing the collected cross-sectional macro observation test pieces, they were etched with a 3% Nital solution so that the boundary between the weld metal and the laminate could be observed. Subsequently, the Vickers hardness (HV1) of the laminate was measured. The measurement points were 1mm from the surface of the laminate, at 0.5mm intervals from the boundary between the weld metal and the laminate towards the laminate, with 20 points on each side of the weld metal, for a total of 40 points. The hardest value among the measurement results was defined as the hardness of the heat-affected zone during welding.

[0078] [Measurement of structural fraction at a distance of 0.5 mm from the surface of the steel plate] The microstructure of this embodiment was investigated by taking samples from each obtained steel plate so that the observation surface was at a position of 0.5 mm. Specifically, these samples were mirror-polished and then etched with nital to reveal the microstructure, and then a scanning electron microscope (SEM) was used to image a 10 mm × 10 mm area at a magnification of 500 to 3000 times. In the captured SEM images, polygonal ferrite was identified as ferrite, and structures with elongated, lath-like ferrite and carbides with an equivalent circle diameter of 0.05 μm or more were identified as tempered bainite. The area ratio of the microstructure was determined by analyzing the images using an image analysis device. The microstructure of the present invention has low anisotropy, and when the anisotropy of the microstructure is low, the area ratio corresponds to the volume ratio; therefore, in this invention, the surface ratio was used as the volume ratio.

[0079] [Measurement of Vickers hardness at a distance of 0.5 mm from the surface of the steel plate] For a cross-section perpendicular to the rolling direction, the Vickers hardness (HV1) was measured at 100 points at a 0.5 mm position in accordance with JIS Z 2244, and the average value was calculated. The standard deviation of these 100 Vickers hardness values ​​was also calculated to represent the hardness variability at the 0.5 mm position.

[0080] [Strength characteristics] From the total thickness of each steel plate, a 1B test specimen according to JIS Z 2241:2022 was taken so that the direction perpendicular to the rolling direction and the thickness direction of the steel plate coincided with the longitudinal direction of the test specimen. Tensile tests were performed according to the procedure described in JIS Z 2241:2022, and the yield strength (YS) (yield point YP if there is a yield point, or 0.2% proof stress σ0.2 if there is no yield point) and tensile strength (TS) were measured. In this example, steel plates with a yield strength of 450 MPa or higher and a tensile strength of 570 MPa or higher were evaluated as having excellent strength characteristics.

[0081] [Toughness] Furthermore, V-notch test specimens were taken from a 1 mm thick section of each steel sheet, with the rolling direction of the steel sheet aligned with the longitudinal direction of the test specimen, according to JIS Z 2242:2023. A Charpy impact test was then performed according to JIS Z 2242:2023, and the transition temperature was evaluated. In this example, steel sheets with a transition temperature of -30°C or lower were evaluated as having excellent toughness.

[0082] [Ammonia stress corrosion cracking resistance] Ammonia stress corrosion cracking resistance was evaluated by performing a four-point bending test in a test solution and then conducting an accelerated test using constant potential anodic electrolysis to promote corrosion.

[0083] Specifically, the following steps were taken.

[0084] Test specimens measuring 5 mm thick × 15 mm × 115 mm were taken from the surface of a steel plate, ultrasonically degreased in acetone for 5 minutes, and subjected to a stress (100% YS) equal to the yield strength of each steel plate by four-point bending. A test cell containing these four-point bent test specimens was filled with a solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia, and then immersed at room temperature (25°C) while a potentiostat controlled the potential to be applied to the test specimens using the three-electrode method so that a potential of +2.0 V vs Pt was applied. In this example, if no cracks were observed in the test specimens after 168 hours of immersion, the ammonia stress corrosion cracking resistance was judged to be good and evaluated as "○", and if cracks occurred, it was judged to be poor and evaluated as "×".

[0085] The evaluation results obtained are listed in Table 2.

[0086] [Table 1] TIFF0007913571000002.tif245167

[0087] [Table 2] TIFF0007913571000004.tif245116

[0088] All of the steel sheets produced according to the manufacturing method of the present invention exhibited excellent strength properties, toughness, and resistance to ammonia stress corrosion cracking. On the other hand, the comparative steel sheets that did not meet the component composition or manufacturing conditions showed that one or more of the strength properties, toughness, and resistance to ammonia stress corrosion cracking were impaired.

Claims

1. In mass percent, C: 0.010-0.200%, Si: 0.01-1.00%, Mn: 0.20-2.30%, P: 0.030% or less, S: 0.0100% or less, Al: 0.010-0.100%, N: 0.0010 to 0.0100% and O: 0.0100% or less It contains, The value of Ceq shown in the following formula (1) is 0.30 or more and 0.40 or less, Ceq=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14...(1) (In the formula, [X] represents the mass %) content of element X in the steel, and is 0 if element X is not present.) The composition consists of Fe and unavoidable impurities. The volume fraction of tempered bainite at a position 0.5 mm from the surface in the thickness direction is 90% or more. The metal structure has a volume fraction of tempered bainite of 20% or more at the 1 / 2 position of the plate thickness, and a total volume fraction of ferrite and said tempered bainite of 60% or more. The average Vickers hardness at a position 0.5 mm from the surface in the thickness direction is 230 HV1 or less, and the variation in Vickers hardness is 50 HV1 or less. The Vickers hardness of the heat-affected area during welding is 230 HV1 or less. A steel plate characterized by having a yield strength of 450 MPa or more and a tensile strength of 570 MPa or more.

2. The aforementioned component composition is further expressed in mass%, Group A: Cu: 1.00% or less, Ni: 2.00% or less, Co: 2.00% or less, Cr: 1.80% or less, Sn: 0.50% or less, Sb: 0.50% or less and W: 1.00% or less, Group B: Mo: 0.50% or less V: 0.200% or less, Ti: 0.100% or less, Nb: 0.100% or less, Zr: 0.100% or less and B: 0.0100% or less, Group C: Ca: 0.0200% or less, Mg: 0.0200% or less and REM: 0.0200% or less The steel sheet according to claim 1, comprising one or more elements selected from the following.

3. A steel material having the above-mentioned component composition, wherein the rolling end temperature is Ar ３ After being hot-rolled above the transformation point to form a steel sheet, Ar ３ A method for manufacturing a steel sheet according to claim 1 or 2, comprising cooling from a temperature above the transformation point at an average cooling rate of 10 to 120 °C / s to a cooling stop temperature in the range of 50 to 600 °C, and then reheating until the temperature at a position 0.5 mm from the surface of the steel sheet in the thickness direction is in the range of 550 to 750 °C, and the temperature at a position half the thickness is in the range of more than 500 °C and less than or equal to 700 °C.

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