JP2018123415A - Nickel-containing steel material for low temperatures and tank for low temperatures therewith - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nickel-containing steel material for low temperature having excellent stress corrosion crack resistance without damaging the strength or base matrix toughness and a tank for low temperatures therewith.SOLUTION: A nickel-containing steel material for low temperature characterized in that predetermined components are contained, a ratio of surface layer hardness and t/4 hardness is set to 1.10 or lower, retained austenite at 1.5 mm position from a surface layer is 3.0 vol.% or larger and 20.0 vol.% or smaller, the yield strength is 590 MPa or larger and 800 MPa or smaller, the tensile strength is 690 MPa or larger and 830 MPa or smaller, and an average value of JIS4 Charpy impact absorptive energy (vE-196) at -196°C is 150 J or larger, and a tank for low temperatures therewith.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a nickel-containing steel material for low temperature having a Ni content of more than 8.00 to 9.50% by mass, suitable for a material for storing a liquefied gas, and a low-temperature tank using the same.

  The main purpose of the present invention is a storage tank for storing liquefied natural gas (boiling point: −164 ° C., hereinafter referred to as LNG). Excellent low temperature toughness is required for low temperature nickel-containing steel used for storage tanks. As such a steel material, for example, there is steel containing Ni in the above range (hereinafter referred to as 9% Ni steel).

  As conventional technologies for low-temperature nickel-containing steel materials used in storage tanks, Patent Documents 1 and 2 disclose steels with a thickness of 40 mm or more and containing 9% by mass of Ni. In Patent Document 1, HAZ characteristics are improved by adding an appropriate amount of Mo simultaneously with the reduction of Si. In Patent Document 2, stable precipitation of austenite is obtained by reducing the Si content and appropriately controlling the cumulative rolling reduction. In order to improve low temperature toughness.

  Patent Document 3 proposes a steel material containing more than 11.0 to 13.0% Ni for a steel material that contains a large amount of Ni and requires high strength and toughness, and resistance to stress corrosion cracking against seawater. Has been.

  To date, 9% Ni steel has been widely used for onshore LNG tank applications, but there is almost no track record for marine use.

Japanese Patent Laid-Open No. 04-371520 Japanese Patent Laid-Open No. 06-184630 JP 09-137253 A

One of the causes of almost no use of 9% Ni steel for marine use is stress corrosion cracking in a chloride environment. In the case of marine LNG tanks, there have been cases in which cracks have occurred in 9% Ni steel tanks in ships that have been in service for about 25 years, and aluminum alloys and stainless steel are mainly used at present. In the future, countermeasures against stress corrosion cracking have become an important issue in order to use Ni steel for cryogenic use for ships. In the past, investigation reports have already been published on cases where stress corrosion cracking has occurred in 9% Ni steel tanks. The cause of stress corrosion cracking in the tank is as follows. However, there is a description that (2) the weld heat-affected zone (HAZ) where cracking occurred had a hardness as high as about 420 Hv, and the opinion that it was cracked by hydrogen was stated. However, since no trace of S (sulfur) content is found in the corrosion product, there is a description that there is no basis for the influence of hydrogen sulfide. Thus, there are many unclear points about the cause of stress corrosion cracking that actually occurred.
The present invention provides a low-temperature nickel-containing steel material excellent in stress corrosion cracking resistance and a low-temperature tank using the same without impairing strength and base metal toughness.

  The present inventors examined a nickel-containing steel material for low temperature that can be used for a marine LNG tank or the like. Since the steel is manufactured within the range of the specified values of yield strength and tensile strength due to the design standard specification, a steel material having a yield strength at normal temperature of 590 MPa to 800 MPa and a tensile strength of 690 MPa or more was assumed.

Considering the process from construction to operation of the ship LNG tank, the stress acting on the corrosive environment was organized and the cause of stress corrosion cracking was examined. As a result, cases where stress corrosion cracking actually occurred occurred after a long period of about 25 years after construction, and in ship LNG tanks regularly (about once every 5 years) On the other hand, since there is no such problem of stress corrosion cracking in LNG tanks for land use without open inspection, there is no adhesion of salt and dew condensation in the tank during open inspection. It can be considered the cause. Therefore, a test method capable of reproducing chloride stress corrosion cracking was established by simulating the residual stress of the weld and applying stress, and measures for the material were examined. As a result, the following findings (a) to (b) were found.
(a) When the ratio between the surface layer hardness and the t / 4 hardness is 1.10 or less, the occurrence of chloride stress corrosion cracking is remarkably suppressed.
(b) When the retained austenite at a position of 1.5 mm from the surface is 3.0% by volume or more, the progress of chloride stress corrosion cracking is remarkably suppressed.

  The present invention has been completed based on such findings. The gist of the present invention is as follows.

(1) By mass%, C: 0.010 to 0.150%, Si: 0.010 to 0.600%, Mn: 0.20 to 2.00%, P: 0.0100% or less, S: 0.0100% or less, Ni: more than 8.00% and 9.50% or less, Al: 0.005 to 0.100%, N: 0.0010 to 0.0100%, the balance from Fe and impurities The ratio of surface hardness to t / 4 hardness is 1.10 or less, the retained austenite at a position of 1.5 mm from the surface layer is 3.0 volume% or more and 20.0 volume% or less, and the yield strength is 590 MPa or more. A nickel-containing steel material for low temperature, characterized in that the average value of JIS No. 4 Charpy impact absorption energy (vE-196) at -196 ° C is 150 J or more, 800 MPa or less, tensile strength of 690 MPa or more and 830 MPa or less.

(2) Further, by mass, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, W: 0.01 to 1.00%, V: 0.01 to 1.00 %, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.100%, one or two of them are contained, The nickel-containing steel material for low temperature according to claim 1 .

(3) A low-temperature tank manufactured using the low-temperature nickel-containing steel material according to (1) or (2).
Here, the steel material may be a steel pipe, a steel tube, a bar wire (bar steel, wire rod) in addition to a steel plate, and its manufacturing method is not particularly limited, and for example, rolling, forging, rolling, etc. Examples include manufacturing, extrusion, drawing, and pressing. In the case of a steel plate, it may be a thick steel plate, a hot rolled steel plate (hot coil), or a cold rolled steel plate. The plate thickness of the steel plate may be more than 50 mm (for example, 80 mm). In the case of a steel pipe, it may be a seamless steel pipe or a welded steel pipe (such as an electric resistance steel pipe or UO steel pipe). The section steel may be H-section steel, I-section steel, T-section steel, angle steel, channel steel, steel sheet pile, and the like. Further, t is the thickness of the steel material, which is a plate thickness in the case of a steel plate, a wall thickness in the case of a steel pipe, and a cross-sectional diameter in the case of a bar wire.

  According to the present invention, even if the incoming chloride cannot be managed during open inspection of the low temperature tank, or even if there is a deficiency in the humidity management in the tank and the inside of the tank is condensed, stress corrosion cracking due to chloride will occur. The present invention makes a significant contribution to the industry.

It is a graph which shows the influence of the hardness ratio which gives to stress corrosion cracking sensitivity. It is a graph which shows the influence of a retained austenite on stress corrosion cracking sensitivity. It is a figure explaining a chloride stress corrosion cracking test method. It is a figure explaining the influence of the tempering temperature rising rate ratio lower limit which has on stress corrosion cracking sensitivity. It is a figure explaining the influence of the tempering temperature rising rate ratio upper limit which acts on yield stress.

  The present inventors have studied to ensure the resistance to chloride stress corrosion cracking while securing the base material strength toughness of the nickel-containing steel material for low temperature, and obtained the following knowledge (A) to (B). .

  The steel material for low temperature according to the present invention will be described below. Hereinafter, “%” display of the content of each chemical component means “mass%”.

(A) Chemical composition C: 0.010 to 0.150%
C is an element necessary for securing strength and is an element that stabilizes retained austenite. In the present invention, the C content is 0.010% or more. On the other hand, if the C content is less than 0.010%, the strength may decrease, the amount of retained austenite may decrease, and the stress corrosion cracking resistance may decrease. Preferably, the C content is 0.030% or more. On the other hand, if the amount of C exceeds 0.150%, the tensile strength becomes excessive and the base material toughness is significantly lowered. Further, the surface layer hardness is likely to increase, and the stress corrosion cracking resistance is lowered. Therefore, the upper limit of the C amount is 0.150% or less. The upper limit with preferable C amount is 0.120% or less.

Si: 0.010 to 0.600%
Si is an element for deoxidizing and ensuring strength, and in order to obtain an effect, the amount of Si is set to 0.010% or more. Si is an element that suppresses the decomposition and precipitation reaction from cemented martensite into cementite in the tempering step. By suppressing the cementite, the carbon concentration in the retained austenite increases and the retained austenite is stabilized. As a result, the stress corrosion cracking resistance is improved by increasing the amount of retained austenite. Preferably, the Si content is 0.020% or more, more preferably 0.030% or more. On the other hand, if the Si content exceeds 0.600%, the tensile strength becomes excessive and the base material toughness decreases, so the upper limit is made 0.600% or less. Preferably, the upper limit of the Si amount is 0.500% or less.

Mn: 0.20 to 2.00%
Mn is a deoxidizer and is an element necessary for improving hardenability and ensuring strength. In the present invention, the Mn content is 0.20% or more in order to ensure the yield and tensile strength of the base material. Preferably the amount of Mn is 0.30% or more, more preferably 0.50% or more. On the other hand, if the amount of Mn exceeds 2.00%, the base material characteristics in the thickness direction become non-uniform due to center segregation, and the base material toughness is lowered, and in addition, the origin of corrosion in steel. MnS is formed, the corrosion resistance is lowered, and the stress corrosion cracking resistance is lowered. For this reason, the upper limit of the amount of Mn is made 2.00% or less. Preferably, the amount of Mn is 1.50% or less.

P: 0.0100% or less P is an impurity, and segregates at the grain boundary to lower the base material toughness. Therefore, the P content is limited to 0.0100% or less. Preferably, the P content is 0.0080% or less. The lower the amount of P, the better. Therefore, the lower limit is not particularly specified, but 0.0010% or more may be contained from the viewpoint of manufacturing cost.

S: 0.0100% or less S is an impurity, and forms MnS as a starting point of corrosion in steel, thereby reducing corrosion resistance and stress corrosion cracking resistance. In addition, the amount of S is limited to 0.0100% or less because it may promote center segregation or cause stretched MnS to be the starting point of brittle fracture and cause the base metal toughness to decrease. Preferably, the S amount is 0.0050% or less. Since the lower the amount of S, the better. Therefore, the lower limit is not particularly specified, but 0.00010% or more may be contained from the viewpoint of manufacturing cost.

Ni: more than 8.00% and not more than 9.50% Ni is the most basic element necessary for securing the toughness of the base metal as a low-temperature steel. In the present invention, the Ni content is more than 8.00%. Preferably, the Ni content is more than 8.20%, and a more preferable range is more than 8.40%. The higher the amount of Ni, the higher the low-temperature toughness is obtained, but not only the cost is increased, but the corrosion resistance in a chloride environment is remarkably increased. However, due to the high corrosion resistance, local corrosion marks (local pits) are easily formed. Further, chloride stress corrosion cracking is likely to occur due to stress concentration at the local pit portion. Therefore, the upper limit of the Ni amount is set to 9.50% or less.

Al: 0.005 to 0.100%
Al is a deoxidizer, and the amount of Al is set to 0.005% or more to prevent an increase in inclusions such as alumina and a decrease in base material toughness due to insufficient deoxidation. Al is also an element that suppresses the formation of cementite. By suppressing the cementite, the carbon concentration in the retained austenite increases and the retained austenite is stabilized. As a result, the stress corrosion cracking resistance is improved by increasing the amount of retained austenite. Preferably, the Al content is 0.010% or more. On the other hand, if the Al content exceeds 0.100%, the base material toughness is reduced due to inclusions, so the upper limit is made 0.100% or less. Preferably, the Al content is 0.070% or less.

N: 0.0010 to 0.0100% or less N is bonded to Al, and has the effect of refining crystal grains and improving base material toughness by forming AlN. This effect is obtained by adding 0.001% or more. However, if the content exceeds 0.010%, the toughness of the base material is lowered, so the content is limited to 0.010% or less. A preferable upper limit is 0.008%.

  The nickel-containing steel material for low temperature of the present invention is composed of Fe and impurities in the balance in addition to the above components. Here, impurities are components that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing steel for low temperature, and have an adverse effect on the present invention. It means what is allowed in the range not given.

  Furthermore, you may contain 1 type, or 2 or more types of Cr, Mo, W, V, Nb, and Ti as needed.

Cr: 0.01-2.00%
Cr has the effect of increasing the strength, and also has the effect of reducing the corrosion resistance of the steel material in a thin-film water environment where chloride is present, thereby suppressing the formation of local pits and suppressing the occurrence of chloride stress corrosion cracking. An effect is acquired by containing 0.01% or more of Cr. More preferably, the Cr amount is 0.40% or more. When the content exceeds 2.00%, not only the effect is saturated but also the toughness is lowered. The upper limit of the Cr amount is preferably 2.00% or less. More preferably, the Cr content is 1.20% or less.

Mo: 0.01 to 1.00%
Mo has an effect of increasing strength, and Mo eluted in a corrosive environment forms molybdate ions. Chloride stress corrosion cracking of Ni steel for low temperature progresses due to the dissolution of steel at the crack tip, but the presence of molybdate ions suppresses dissolution at the crack tip due to its inhibitor action, greatly increasing crack resistance. To be high. The effect can be obtained by containing 0.01% or more of Mo. More preferably, the Mo amount is 0.20% or more. If the Mo content exceeds 1.00%, not only the dissolution suppressing effect is saturated, but also the toughness is remarkably reduced. Therefore, the Mo content is preferably 1.00% or less. More preferably, the Mo amount is 0.50% or less.

W: 0.01-1.00%
W also has the same effect as Mo. In the corrosive environment, W eluted in the corrosive environment forms tungstate ions, thereby suppressing dissolution at the crack tip and improving chloride stress corrosion cracking resistance. An effect is acquired by containing 0.01% or more of W. More preferably, the W amount is 0.20% or more. When the content exceeds 1.00%, not only the effect is saturated but also the toughness is lowered. Therefore, the upper limit of the W amount is 1.00% or less, and more preferably 0.50% or less.

V: 0.01-1.00%
V also has the same action as Mo. In the corrosive environment, V eluted in the corrosive environment forms vanadate ions, thereby suppressing dissolution at the crack tip and improving chloride stress corrosion cracking resistance. An effect is acquired by making V contain 0.01% or more. More preferably, the V amount is 0.20% or more. When the content exceeds 1.00%, not only the effect is saturated but also the toughness is lowered. Therefore, the upper limit of the V amount is 1.00%. Preferably it is 0.50% or less.

Nb: 0.001 to 0.100%
Nb is an element having the effect of suppressing the occurrence of stress corrosion cracking by strengthening the oxide film formed in the atmosphere in addition to improving the strength and base material toughness by refining the structure, You may contain 0.001% or more. More preferably, the Nb amount is 0.030% or more. On the other hand, when Nb is added excessively, coarse carbides and nitrides are formed, and the toughness of the base metal may be lowered. Therefore, the Nb content is preferably 0.100% or less. More preferably, the Nb amount is 0.080% or less.

Ti: 0.001 to 0.100%
When Ti is used for deoxidation, it forms an oxide phase composed of Al, Ti, and Mn, refines the structure and improves strength and base material toughness, and combines with S in steel to form sulfide. Is an element that has the effect of significantly reducing MnS as a starting point of corrosion and suppressing the occurrence of stress corrosion cracking, and may contain 0.001% or more of Ti. More preferably, the Ti amount is 0.010% or more.
On the other hand, if the Ti content exceeds 0.100%, Ti oxide or Ti-Al oxide may be formed and the base metal toughness may be lowered. Therefore, the Ti content is preferably 0.100% or less. More preferably, the Ti amount is 0.080% or less.

(B) Metallographic structure B-1. The ratio of the hardness of the surface layer to the hardness at the t / 4 portion is 1.10 or less. The hardness is closely related to the occurrence of chloride stress corrosion cracking, and the higher the hardness, the easier the cracking occurs. Further, stress corrosion cracking is a phenomenon on the steel material surface, and the hardness of the surface layer is more important than the hardness at t / 4 which usually evaluates the strength. If the surface hardness is high, the dislocation density is high and stress corrosion cracking is easy to occur. Specifically, when the ratio between the surface layer hardness and the t / 4 hardness is 1.10 or less, the occurrence of chloride stress corrosion cracking is suppressed while securing the steel material strength. FIG. 1 shows the influence of the hardness ratio on the occurrence of stress corrosion cracking in steel materials containing 3.0% or more of retained austenite. Even if 3.0% of retained austenite is contained, if the hardness ratio exceeds 1.10, the surface layer hardness increases and stress corrosion cracking occurs. Surface hardness is measured at 1.0 mm from the surface in the plate thickness direction.

B-2. Residual austenite at a position of 1.5 mm from the surface in the thickness direction is 3.0% by volume or more. Residual austenite in the steel suppresses the progress of cracking and significantly improves the resistance to chloride stress corrosion cracking. Since retained austenite contains a lot of Ni, dissolution in a chloride thin film water environment is greatly suppressed. Since stress corrosion cracking is a phenomenon that occurs on the steel surface, the amount of retained austenite on the steel surface layer is important. FIG. 2 shows the effect of retained austenite on the stress corrosion cracking susceptibility of steel materials satisfying the hardness ratio of the surface layer and t / 4 of 1.10 or less. Even if the hardness ratio between the surface layer and t / 4 is 1.10 or less, if the retained austenite is less than 3.0% by volume, stress corrosion cracking easily occurs. As the amount of retained austenite increases, the resistance to chloride stress corrosion cracking is improved. However, if the amount is too large, the strength decreases, and the required strength cannot be ensured. Therefore, the amount of retained austenite is preferably 20.0% by volume or less, and more preferably 15% by volume or less. The amount of retained austenite is a test piece having an observation surface at a position of 1.5 mm in the plate thickness direction from the surface of the steel (plate thickness direction 1.5 mm × width direction 25 mm × longitudinal rolling direction 25 mm, and the observation surface is a 25 mm square surface. And measured by an integral intensity method from X-ray diffraction measurement.

  The above strength (yield strength is 590 MPa to 800 MPa, tensile strength is 690 MPa to 830 MPa) and toughness (JIS No. 4 Charpy impact absorption at -196 ° C) because the LNG tank has sufficient fracture resistance against large earthquakes The average value of energy (vE-196) is 150 J or more). The low-temperature thick steel material of the present invention having the above component composition and metal structure is excellent in toughness in a low temperature region of −60 ° C. or lower, particularly in a low temperature environment of −165 ° C. or lower, and further resistant to chloride stress cracking. It has excellent properties and is suitable for applications in which liquefied gases such as LPG and LNG are stored at low temperatures.

  As an example of the manufacturing method of the nickel-containing steel material for low temperature of this invention, the case where steel materials are steel plates is demonstrated below. The steel sheet is subjected to homogenization heat treatment after casting. Thereafter, the steel slab is reheated and hot-rolled, and then heat-treated at a predetermined temperature for manufacturing (Steps 1 to 5). Details will be described below. In addition, about the steel piece which uses for hot rolling, if it is the component range of this invention, it does not prescribe | regulate the casting condition exceptionally, you may use an ingot-breaking slab as a steel ingot, or continuous. A cast slab may be used. From the viewpoint of production efficiency, yield, and energy saving, it is preferable to use a continuously cast slab.

Homogenization heat treatment process (process 1)
The steel slab is heated for homogenization before rolling. It is preferable to heat at 1200 to 1350 ° C. for 10 hours or more. If there are few impurity elements in the steel slab and the base material toughness can be sufficiently secured, it may be omitted.

Heating process (process 2)
The steel slab is heated to 1000-1250 ° C. Thereby, the rolling roll load can be reduced while suppressing the coarsening of the structure.

Rolling process (process 3)
It is preferable to hot-roll at a total rolling reduction of 50% or more and finish the hot rolling at a finishing temperature of 600 to 850 ° C. As a result, while suppressing deformation resistance, the deformation band can be positively introduced into the tissue and the tissue can be refined.

Quenching process (process 4)
After finish rolling, it is cooled and quenched. Preferably, the step of cooling to 200 ° C. or lower at a cooling rate of 3 ° C./s or higher after hot rolling, or once cooling to 150 ° C. or lower after hot rolling and reheating to Ac 3 points or higher, Cool to 200 ° C. or less at a cooling rate of sec or more Thereby, while obtaining a hardened structure, the formation of coarse carbides is suppressed and the base material toughness is improved. The cooling rate is preferably 5 ° C./sec or more, and it is preferable to cool the steel plate by injecting water onto the front and back surfaces.

Tempering process (process 5)
Temper after quenching. Preferably, the steel sheet is heated to Ac 1 + 80 ° C. or lower and then cooled to 200 ° C. or lower at a cooling rate of 1 ° C./sec or higher. This improves toughness.
The difference in hardness between the surface layer and t / 4 occurs because the cooling rate differs in the plate thickness direction during the quenching heat treatment. This is because the surface layer has a high cooling rate and a hard structure is formed. Therefore, in the tempering heat treatment step after the quenching heat treatment, the ratio of the temperature rise rate between the surface layer and t / 4 ((temperature rise rate of the surface layer) / (temperature rise rate of t / 4)) is 1.3 times or more and 4.0. By setting it to be twice or less, the surface layer portion can reach the target temperature earlier than t / 4, and the holding time at the target tempering temperature can be made longer than t / 4. FIG. 4 shows the result. The surface layer is tempered more than t / 4, the hardness ratio between the surface layer hardness and t / 4 is 1.10 or less, and the retained austenite of the surface layer is 3.0 volume% or more and 20.0 volume% or less. Can do. When the heating rate ratio of the surface layer and t / 4 is less than 1.3 times, the tempering of the surface layer does not proceed sufficiently and the stress corrosion cracking resistance is lowered. When the heating rate ratio of the surface layer and t / 4 exceeds 4.0 times, retained austenite increases, and the required lower limit of tensile strength of 690 MPa cannot be secured. FIG. 5 shows the result.
Thus, in order to increase the temperature rising rate of the surface layer in comparison with t / 4 in the tempering heat treatment process, for example, heat treatment with precisely controlled temperature of the heat treatment furnace or heat treatment using an induction heating device should be employed. Can do. Since this process changes only the texture characteristics of the surface layer, the characteristics at the 1/4 thickness position, which is the strength evaluation site, will not deteriorate. Thus, by controlling the structure of the surface layer portion, the chloride stress corrosion cracking resistance can be remarkably improved while ensuring the necessary strength.

  In addition, between the above-mentioned process 4 and process 5, you may heat to Ac1-Ac3 point, and may cool to 200 degrees C or less with the cooling rate of 3 degrees C / sec or more. This improves toughness. However, if sufficient tempering can be performed in step 5, it may be omitted because it is softened and sufficient base material toughness is ensured.

  Hereinafter, the present invention will be described in more detail by way of examples.

  42 types of steel materials having chemical compositions shown in Table 1 were dissolved, and rolling and heat treatment were performed by the manufacturing methods shown in Table 2 to produce steel plates having a thickness of 5 to 80 mm. The mechanical properties are shown in Table 3. The evaluation was made by measuring three cases of JIS No. 4 Charpy impact absorption energy (vE-196) at −196 ° C. when the yield strength was less than 590 MPa or more than 800 MPa, and the tensile strength was less than 690 MPa or more than 830 MPa. The case of less than 150 J was rejected. From the outermost surface of the obtained steel plate, a stress corrosion cracking test piece having a width of 10 mm, a length of 75 mm, and a thickness of 1.5 mm was collected. The test piece was polished up to polishing paper No. 600, set in a four-point bending test jig as shown in FIG. 3, and a stress of 590 MPa was applied. The test surface is a surface on the surface side of the steel plate. Next, a sodium chloride aqueous solution was applied to the test surface so that the amount of adhered salt per unit area was 5 g / m 2 and was corroded in an environment of a temperature of 60 ° C. and a relative humidity of 80% RH. The test period is 1000 hours. This method is a chloride stress corrosion cracking test that simulates an environment in which salt adheres to the tank and thin film water is formed on the steel surface. An aqueous solution was applied to the surface of the test piece and held in a high-temperature and high-humidity furnace for the test period. The corrosion product was removed from the test piece after the test by a physical method and a chemical method, and the presence or absence of a crack was evaluated by observing the cross section of the corrosion part under a microscope. Nittal-etched 500 times optical microscope photograph (270 μm × 350 μm) was observed in 20 fields of view, taking into account irregularities due to corrosion, and those that progressed in the depth direction by 50 μm or more from the surface were rejected as cracked “Yes” did.

  In Tables 1 to 3, it can be seen that the low-temperature nickel-containing steel sheets according to the examples of the present invention are excellent in base material strength, toughness, and stress corrosion cracking resistance, and are excellent as low-temperature materials.

  On the other hand, in the comparative example that does not satisfy the conditions specified in the present invention, it can be seen that the desired properties cannot be obtained in the base material strength, toughness, and stress corrosion cracking resistance.

Claims (3)

  In mass%, C: 0.010 to 0.150%, Si: 0.010 to 0.600%, Mn: 0.20 to 2.00%, P: 0.0100% or less, S: 0.0100 %: Ni: more than 8.00% and not more than 9.50%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0100%, the balance being Fe and impurities, The ratio of hardness to t / 4 hardness is 1.10 or less, the retained austenite at a position of 1.5 mm from the surface is 3.0 vol% or more and 20.0 vol% or less, and the yield strength is 590 MPa or more and 800 MPa or less, A nickel-containing steel material for low temperature, characterized in that an average value of JIS No. 4 Charpy impact absorption energy (vE-196) at a tensile strength of 690 MPa to 830 MPa and -196 ° C is 150 J or more.   Further, by mass, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, W: 0.01 to 1.00%, V: 0.01 to 1.00%, Nb The low-temperature nickel-containing steel material according to claim 1, wherein one or two of Ti: 0.001 to 0.100% and Ti: 0.001 to 0.100% are contained.   A low-temperature tank manufactured using the low-temperature nickel-containing steel material according to claim 1.