Steel plate and production method therefor

Abstract

A high-strength steel sheet with excellent cryogenic toughness and performance in suppressing brittle cracking is provided, based on the premise of manufacturing with process savings using direct quenching tempering. The steel sheet of the present invention has a predetermined composition, the amount of retained austenite in 1 / 4t is less than 1.7% by volume, and the Charpy absorption energy at -196°C is 200J or more when using a full-size test specimen and 100J or more when using a half-size test specimen.

Description

Technology Field

[0001] The present invention relates to steel plates, and in particular, to a high-strength thick steel plate suitable for cryogenic applications and a method for manufacturing the same, which can reliably secure excellent cryogenic toughness and performance in suppressing brittle cracking over a wide range of plate thicknesses. The steel plate and manufacturing method of the present invention can be suitably utilized in structures used in cryogenic environments, such as, for example, liquefied gas storage tanks for ships and land. Background Technology

[0002] When hot-rolled steel sheets are used for structures such as liquefied gas storage tanks, excellent toughness under cryogenic conditions (cryogenic toughness) is required in addition to the strength of the steel sheets, as the operating environment becomes cryogenic. For example, when hot-rolled steel sheets are used for liquefied natural gas storage tanks, it is necessary to ensure excellent toughness under cryogenic conditions of -164°C or lower, which is the boiling point of liquefied natural gas. Since there is a concern that safety as a cryogenic storage structure cannot be maintained if the cryogenic toughness of the steel is poor, there is a high demand for improved cryogenic toughness of the steel sheets used. In response to this demand, conventional steel sheets containing 7% Ni or 9% Ni have been used.

[0003] A 7% Ni steel sheet is proposed, for example, in Patent Document 1. Patent Document 1 discloses a thick steel sheet for cryogenic applications containing Ni: greater than 5.0% to less than 10.0% and a predetermined amount of C, Si, Mn, and Al. In the thick steel sheet disclosed in Patent Document 1, the average value of the absorbed energy vE-196 per unit area is 1.25 J / mm² or higher over a thickness of 6 to 50 mm. Prior art literature

[0004] Japanese Patent Publication No. 2011-219848 The problem to be solved

[0005] As a result of the inventors' careful investigation into so-called 7% Ni steel sheets (hereinafter also referred to as 7% Ni steel sheets) with Ni: 6.0 to 7.5%, it was found that when manufactured by performing quenching and tempering directly after rolling for the purpose of improving productivity, the risk of unstable fracture of the steel sheet (brittle fracture) caused by a decrease in absorbed energy (toughness) in the Charpy test, etc. and the occurrence of brittle cracks increases. However, Patent Document 1 does not examine these problems, particularly the occurrence of brittle fracture.

[0006] The present invention was made in consideration of the above circumstances and aims to provide a high-strength steel plate with excellent cryogenic toughness and performance in suppressing brittle cracking, based on the premise of manufacturing with process savings using direct quenching tempering. means of solving the problem

[0007] In order to solve the above problem, the inventors conducted a thorough study on the composition and microstructure of 7% Ni steel sheets and obtained the following findings. That is,

[0008] (a) The above-mentioned reduction in absorbed energy is attributed to separation occurring mainly in the Mn enrichment region due to the formation of Mn enrichment and leanness regions. Here, separation refers to fracture resulting from brittle fracture that occurs perpendicular to the fracture surface and parallel to the rolling plane during toughness measurement tests, such as the Charpy impact test and CTOD test.

[0009] (b) The above-mentioned brittle cracking is caused by Mn enrichment zones, particularly unstable austenite (hereinafter also referred to as γ) that is prone to forming therein.

[0010] (c) To increase absorption energy, limit Mn to 0.40% or less and reduce the Mn concentration in the band-shaped Mn enrichment region (Mn segregation band) to reduce the occurrence of separation caused by the Mn segregation band.

[0011] (d) To reduce the occurrence of brittle cracks, limit Mn to 0.40% or less, and reduce the Mn concentration in the band-shaped Mn enrichment zone, while suppressing the formation of unstable γ that causes brittle cracks.

[0012] The present invention is completed based on the above recognition, and its gist is as follows.

[0013] [1] Mass %,

[0014] C: 0.01∼0.15%,

[0015] Si: 0.01∼0.50%,

[0016] Mn: 0.05∼0.40%,

[0017] Ni: 6.0% or more and less than 7.0%,

[0018] Cr: 0.01∼1.00%,

[0019] Mo: 0.01∼0.50%,

[0020] P: 0.030% or less,

[0021] S: 0.0050% or less,

[0022] N: 0.0010∼0.0080% and

[0023] Al: 0.008∼0.100%

[0024] It has a composition containing, and the remainder consisting of Fe and unavoidable impurities, and

[0025] The amount of retained austenite at a depth of 1 / 4 of the plate thickness in the plate thickness direction from the surface of the steel plate is less than 1.7% in volume percentage, and

[0026] A steel plate having a Charpy impact energy at -196°C in accordance with JIS Z 2242, which is 200 J or more when using a full-size test specimen and 100 J or more when using a half-size test specimen.

[0027] [2] The above composition of components is additionally, in mass%,

[0028] Cu: 0.40% or less,

[0029] Nb: 0.05% or less,

[0030] V: 0.05% or less,

[0031] Ti: 0.03% or less and

[0032] B: 0.0030% or less

[0033] A steel plate described in [1] containing 1 or 2 or more selected from.

[0034] [3] The above composition of components is additionally, in mass%,

[0035] Ca: 0.007% or less,

[0036] REM: 0.010% or less and

[0037] Mg: 0.070% or less

[0038] A steel plate described in [1] or [2] containing 1 or 2 or more selected from.

[0039] [4] A steel material having the compositional composition described in any of [1] to [3] above is hot-rolled to produce a hot-rolled plate with a cumulative reduction rate of 15 to 75% at 870°C or less and a final rolling end temperature of 830 to 700°C as the steel plate surface temperature, and then direct quenching is performed on the hot-rolled plate in which the average cooling rate in the temperature range of 300°C or less and the cooling stop temperature is 3°C or more and the temperature at a depth of 1 / 4 of the plate thickness from the surface of the hot-rolled plate in the plate thickness direction is 600°C or less and the cooling stop temperature is 300°C or less, and then tempering is performed in the temperature range of 550°C or more and below the Ac1 transformation point, such that the amount of residual austenite at a depth of 1 / 4 of the plate thickness from the surface of the steel plate in the plate thickness direction is less than 1.7% by volume, and at -196°C according to the Charpy impact test in accordance with JIS Z 2242 A method for manufacturing a steel plate in which the Charpy absorption energy is 200 J or more when using a full-size test specimen and 100 J or more when using a half-size test specimen. Effects of the invention

[0040] According to the present invention, a high-strength steel plate with excellent cryogenic toughness and performance in suppressing brittle cracking can be provided under high productivity. By providing the steel plate and the method of manufacturing the same according to the present invention to steel structures used in cryogenic environments, such as liquefied gas storage tanks, the safety of said steel structures can be improved, thereby bringing about significant industrial effects. Specific details for implementing the invention

[0041] (Form for carrying out the invention)

[0042] Embodiments of the present invention will be described in detail below. Furthermore, the following description represents suitable embodiments of the present invention and is not limited thereto.

[0043] [Ingredient Composition]

[0044] The steel sheet of the present invention has a predetermined composition. In addition, the steel material used in the manufacturing method of the present invention, which is suitable for manufacturing the steel sheet of the present invention, also has the above-mentioned predetermined composition. Hereinafter, each element included in this composition will be described. Furthermore, unless specifically stated otherwise, in this specification, "%" as a unit of the content of each element means "mass%".

[0045] C: 0.01% or more and 0.15% or less

[0046] C is an element that has the effect of improving the strength of steel sheets. To obtain this effect, the C content is set to 0.01% or more. Preferably, it is 0.03% or more. On the other hand, if the C content exceeds 0.15%, the cryogenic toughness of the steel sheet decreases. In addition, the performance of suppressing the occurrence of brittle cracks also decreases. Therefore, the C content is set to 0.15% or less. Preferably, it is 0.12% or less.

[0047] Si: 0.01% or more and 0.50% or less

[0048] Si is an element that contributes to improving the strength of steel sheets and also acts as a deoxidizer. To exhibit these effects, the Si content is set to 0.01% or more. Preferably, it is 0.03% or more. On the other hand, if the Si content becomes excessively high, toughness decreases. In addition, the performance in suppressing the occurrence of brittle cracks also decreases. Therefore, the Si content is set to 0.50% or less. Preferably, it is 0.30% or less.

[0049] Mn: 0.05% or more, 0.40% or less

[0050] Mn is an element effective for increasing the hardenability of steel and thereby increasing the strength of steel sheets. To obtain this effect, Mn is added in an amount of 0.05% or more. Preferably, it is 0.10% or more. On the other hand, if Mn is contained in an amount exceeding 0.40%, separation is prone to occur and toughness decreases, so it is limited to 0.40% or less. Furthermore, if Mn is contained in an amount exceeding 0.40%, the Mn concentration in the Mn segregation band is prone to increase, and the performance in suppressing brittle cracking also decreases, so it is limited to 0.40% or less. The amount of Mn is preferably 0.35% or less, more preferably less than 0.35%, and even more preferably less than 0.20%.

[0051] Ni: 6.0% or more and less than 7.0%

[0052] Ni is an element that is highly effective for improving the cryogenic toughness of steel sheets and also contributes to the improvement of performance in suppressing brittle cracking. To this end, the Ni content is set to 6.0% or more. Preferably, it is 6.5% or more. On the other hand, since Ni is an expensive element, the cost of the steel sheet increases as its content increases. Therefore, in the present invention, the Ni content is set to less than 7.0%.

[0053] Cr: 0.01% or more and 1.00% or less

[0054] Cr is an element that can improve the strength of steel sheets without significantly impairing cryogenic toughness. To obtain the above effect, the Cr content is set to 0.01% or more. Preferably, it is 0.30% or more. However, if the Cr content exceeds 1.00%, the cryogenic toughness of the steel sheet decreases. In addition, the performance in suppressing the occurrence of brittle cracks also decreases. Therefore, the Cr content is set to 1.00% or less. Preferably, it is 0.80% or less.

[0055] Mo: 0.01% or more and 0.50% or less

[0056] Mo, like Cr, is an element that can improve the strength of steel sheets without significantly impairing cryogenic toughness. To achieve this, the Mo content is set to 0.01% or more. Preferably, it is greater than 0.10%. On the other hand, if the Mo content exceeds 0.50%, cryogenic toughness actually decreases. In addition, the performance in suppressing the occurrence of brittle cracks also decreases. Therefore, the Mo content is set to 0.50% or less. Preferably, it is 0.30% or less. More preferably, it is 0.25% or less.

[0057] P: 0.030% or less

[0058] Phosphorus (P) is an unavoidable impurity and a harmful element that adversely affects the cryogenic toughness and the performance of suppressing brittle cracking in steel plates. For example, to obtain sound base material and welded joints when steel plates are welded into a welded structure, it is desirable to reduce the P content as much as possible. Therefore, the P content is kept below 0.030%. Furthermore, from the perspective of cryogenic toughness, the lower the P content, the better; thus, the lower limit is not specifically restricted and even 0% is acceptable, although its inclusion as an unavoidable impurity is permitted even in that case. On the other hand, since excessive reduction leads to increased costs, from a cost perspective, it is desirable to set the lower limit of the P content to 0.001%.

[0059] S: 0.0050% or less

[0060] Since S forms MnS in steel, it significantly deteriorates cryogenic toughness and the ability to suppress brittle cracking, so it is desirable to limit the upper limit to 0.0050% and reduce it as much as possible. The S content is preferably 0.0020% or less. Meanwhile, since the lower the S content, the better, the lower limit is not particularly limited and 0% is acceptable, but even in that case, it is permitted to contain it as an unavoidable impurity.

[0061] N: 0.0010% or more, 0.0080% or less

[0062] N forms precipitates in steel, and if its content exceeds 0.0080%, it causes a decrease in the toughness of the base material. In addition, the performance in suppressing brittle crack formation also decreases. However, N is also an element that contributes to the fineness of the base material by forming AlN, and this effect is obtained by making the N content 0.0010% or more. Therefore, the N content is set to 0.0010% or more and 0.0080% or less. The N content is preferably 0.0020% or more. Preferably, it is 0.0060% or less.

[0063] Al: 0.008% or more, 0.100% or less

[0064] Al is an element included in deoxidizers. If the Al content is less than 0.008%, the effect as a deoxidizer is insufficient. In addition, Al is an element that contributes to the fineness of the base material by forming AlN. Therefore, the Al content is set to 0.008% or more. Preferably, it is set to 0.020% or more. On the other hand, if the Al content exceeds 0.100%, the cleanliness of the steel is impaired, and toughness, particularly the Charpy absorption energy at cryogenic temperatures, decreases. Therefore, the Al content is set to 0.100% or less. Preferably, it is set to 0.050% or less.

[0065] In one embodiment of the present invention, the composition of the components may consist of the remainder being Fe and unavoidable impurities in addition to the predetermined amount of the element described above.

[0066] In addition, in another embodiment of the present invention, the component composition may optionally additionally contain one or more selected from Cu, Nb, V, Ti and B, preferably in an amount as described below.

[0067] Cu: 0.40% or less

[0068] Cu is an element that has the effect of increasing the strength of steel sheets by improving quenchability. However, if the Cu content exceeds 0.40%, in addition to a decrease in the cryogenic toughness of the steel sheet, the surface properties of the steel material (slab) after casting deteriorate. Therefore, when adding Cu, it is desirable to keep the Cu content 0.40% or less. More preferably, it should be 0.30% or less. Meanwhile, although the lower limit of the Cu content is not particularly limited, it is desirable to keep the Cu content 0.10% or more to obtain the above effect.

[0069] Nb: 0.05% or less

[0070] Nb is an effective element for increasing the strength of steel sheets through precipitation strengthening. However, if the Nb content becomes excessively high, the cryogenic toughness of the steel sheet decreases. Therefore, when adding Nb, it is desirable to keep the Nb content 0.05% or less. More preferably, it should be 0.03% or less. Meanwhile, although the lower limit of the Nb content is not particularly limited, it is desirable to keep the Nb content 0.01% or more to obtain the above effect.

[0071] V: 0.05% or less

[0072] V, like Nb, is an effective element for increasing the strength of steel sheets through precipitation strengthening. However, if the V content becomes excessively high, the cryogenic toughness of the steel sheet decreases. Therefore, when adding V, it is desirable to keep the V content 0.05% or less. More preferably, it should be 0.04% or less. Meanwhile, although the lower limit of the V content is not particularly limited, it is desirable to keep the V content 0.01% or more to obtain the above effect.

[0073] Ti: 0.03% or less

[0074] Ti is an element that has the effect of increasing the toughness of the weldment without degrading the mechanical properties of the base material when steel plates are welded to form a welded structure. To achieve this, it is desirable to add 0.003% or more. On the other hand, since exceeding 0.03% actually lowers toughness, it is desirable to include Ti within a range of 0.03% or less.

[0075] B: 0.0030% or less

[0076] B is an element that increases quenchability when added in small amounts. To effectively exert this effect, it is desirable to contain 0.0003% or more of B. On the other hand, if the content of B exceeds 0.0030%, toughness deteriorates. Therefore, when containing B, it is desirable to keep its content at 0.0030% or less.

[0077] In addition, in another embodiment of the present invention, the above component composition may optionally additionally contain one or more selected from Ca, REM, and Mg, preferably in the amounts described below.

[0078] Ca: 0.007% or less

[0079] Ca is an element that has the effect of improving the cryogenic toughness of steel sheets by controlling the form of inclusions in the steel. However, if Ca is in excess, it impairs the cleanliness of the steel and lowers the Charpy absorption energy at cryogenic temperatures (hereinafter also referred to as Charpy toughness). Therefore, when adding Ca, it is desirable to keep the Ca content 0.007% or less. More preferably, it should be 0.004% or less. Meanwhile, although the lower limit of the Ca content is not particularly limited, it is desirable to keep it 0.001% or more to obtain the above effect.

[0080] REM: 0.010% or less

[0081] REM (rare earth metals), like Ca, is an element that has the effect of improving the cryogenic toughness of steel sheets by controlling the form of inclusions in the steel. However, if REM is in excess, it impairs the cleanliness of the steel and reduces Charpy toughness. Therefore, when adding REM, it is desirable to keep the REM content 0.010% or less. More preferably, it should be 0.008% or less. Meanwhile, although the lower limit of the REM content is not particularly limited, it is desirable to keep the REM content 0.001% or more to obtain the above effect.

[0082] Here, REM is a collective term for 17 elements, comprising the 15 elements of lanthanoids plus Y and Sc, which may be contained individually or in combination. Additionally, the content of REM refers to the total content of these elements.

[0083] Mg: 0.070% or less

[0084] Mg, like Ca or REM, is an element that improves the cryogenic toughness of steel sheets by controlling the form of inclusions in the steel. However, if Mg is in excess, it impairs the cleanliness of the steel and reduces Charpy toughness. Therefore, when adding Mg, it is desirable to keep the Mg content 0.070% or less. More preferably, it should be 0.004% or less. Meanwhile, although the lower limit of the Mg content is not particularly limited, it is desirable to keep the Mg content 0.001% or more to obtain the above effect.

[0085] [Microtissue]

[0086] The steel sheet of the present invention has a microstructure in which the amount of residual austenite (hereinafter also referred to as the amount of residual γ) is less than 1.7% by volume at a depth position of 1 / 4 of the thickness t (hereinafter also referred to as 1 / 4t) in the thickness direction from the surface of the steel sheet. As a representative position, if the amount of residual γ at the above position is 1.7% or more, it is indicated that a relatively large amount of unstable γ was generated, particularly in the enrichment region of the Mn segregation band, and consequently, brittle cracks are prone to occur. From the perspective of improving the performance of suppressing brittle cracks, it is preferable that the amount of residual γ at the above position be as small as possible, preferably 1.5% or less, more preferably 1.0% or less, even more preferably 0.5% or less, even more preferably 0.1% or less, and even more preferably 0%.

[0087] In addition, the residual γ amount can be measured by the method described in the examples to be described later, and it is preferable to measure it on either the front or back surface of the steel plate.

[0088] In addition, the microstructure of the steel sheet is preferably a microstructure mainly composed of martensite and bainite, specifically, the total area percentage of martensite and bainite is preferably 98.3% or more, more preferably 99.0% or more, and even more preferably 100%. This is because, as described above, a microstructure mainly composed of martensite and / or bainite makes it easy to obtain sufficient strength while securing excellent cryogenic toughness. Furthermore, the ratio of martensite to bainite is not a problem and can be arbitrary. In addition, the type of microstructure other than martensite and bainite is not particularly limited.

[0089] The thickness of the steel plate is not particularly limited and can be of any thickness. For example, it is preferable to make it 6 mm or more and 50 mm or less.

[0090] [Mechanical properties]

[0091] (tensile strength)

[0092] Although there is no need to specifically limit the lower limit of the tensile strength of the steel plate, it is preferable to set the lower limit to 690 MPa. More preferably, it should be 720 MPa or higher. Meanwhile, although there is no need to specifically limit the upper limit of the tensile strength, it is preferable to set the upper limit to 930 MPa. More preferably, it should be 900 MPa or lower.

[0093] In addition, tensile strength can be measured by the method described in the examples below.

[0094] (Cryogenic Toughness)

[0095] The toughness value of the steel sheet is the Charpy absorption energy (vE) at -196℃ －196℃ ) needs to be 200J or more in a Charpy impact test using a full-size specimen. Meanwhile, it is preferable that it be 350J or less, and more preferably 280J or less. In addition, in a Charpy impact test using a half-size specimen, vE －196℃ It needs to be 100J or more. Meanwhile, it is preferable that it be less than 200J, and more preferably 150J or less.

[0096] In addition, cryogenic toughness can be measured by the method described in the examples below.

[0097] (Performance to suppress brittle crack formation)

[0098] Regarding the performance of suppressing brittle cracking in steel plates, it is desirable to reach the maximum load point without a sudden drop in load during the CTOD (Crack Tip Opening Displacement) test, as described in the method of the examples described below.

[0099] [Manufacturing Method]

[0100] Next, a method for manufacturing a steel sheet according to the present invention will be described. The manufacturing method of the present invention is a method for obtaining a steel sheet that satisfies the aforementioned predetermined residual γ amount by using a steel material having the aforementioned predetermined compositional composition; controlling the cumulative reduction rate and the final rolling end temperature during hot rolling; the average cooling rate and the cooling stop temperature during quenching; and the temperature range during tempering under predetermined conditions.

[0101] In addition, in the following description, unless otherwise specified, temperature refers to the temperature at the center of the plate thickness. The temperature at the center of the plate thickness can be obtained, for example, by heat transfer calculations from the surface temperature of the steel plate measured with a radiation thermometer.

[0102] As an example, the steel plate of the present invention can be suitably manufactured by sequentially performing the following processes (1) to (4).

[0103] (1) Heating of steel material

[0104] (2) Hot rolling

[0105] (3) Quenching (accelerated cooling)

[0106] (4) Tempering

[0107] (1) Heating of steel material

[0108] First, the steel material needs to have the aforementioned composition. It is preferable to heat the steel material to a temperature of 900°C or higher and 1200°C or lower. The method of manufacturing the steel material is not particularly limited, but, for example, it can be manufactured by melting molten steel having the aforementioned composition by a conventional method and casting it. The melting can be carried out by any method, such as a converter, an electric furnace, or an induction furnace. Furthermore, while it is preferable to perform casting by a continuous casting method from the perspective of productivity, it may also be carried out by an ingot-blowing rolling method. As the steel material, for example, a steel slab can be used.

[0109] Here, the heating of the steel material may be performed after cooling the steel material obtained by a method such as casting, or the obtained steel material may be provided to heating directly without cooling.

[0110] If the heating temperature of the steel material is below 900°C, the deformation resistance of the steel material is high, which increases the load on the rolling mill during subsequent hot rolling, making it difficult to perform hot rolling. Therefore, it is desirable to heat the steel material to 900°C or higher. On the other hand, if the heating temperature of the steel material is higher than 1200°C, oxidation of the steel becomes significant, and as a result of increased loss due to the removal of the oxide film caused by oxidation, the yield decreases. Therefore, it is desirable to heat the steel material to 1200°C or lower.

[0111] (2) Hot rolling

[0112] [Reduction rate: Cumulative reduction rate at 870℃ or lower is 15% or more and 75% or less]

[0113] In hot rolling, if the cumulative reduction rate in the austenite unrecursed temperature range of 870°C or lower is less than 15%, the refinement of the microstructure does not occur sufficiently, and toughness decreases. In addition, strength and the performance of suppressing brittle crack formation also decrease. On the other hand, if the cumulative reduction rate exceeds 75%, rolling at the final rolling end temperature described later becomes difficult. Therefore, the cumulative reduction rate of 870°C or lower is set to 15 to 75%. The above cumulative reduction rate is preferably 30% or more, preferably 70% or less, and more preferably 30 to 70%.

[0114] [Final rolling end temperature: Steel plate surface temperature of 700℃ or higher and 830℃ or lower]

[0115] In hot rolling, if the final rolling end temperature (finishing temperature) is less than 700°C in terms of the steel sheet surface temperature, separation caused by the texture is prone to occur, and toughness decreases. In addition, the performance of suppressing brittle cracking also decreases. On the other hand, if the final rolling end temperature exceeds 830°C in terms of the steel sheet surface temperature, it becomes difficult to achieve sufficient reduction in the unrecrystallized region, and a fine structure is not obtained, resulting in a decrease in toughness. In addition, the performance of suppressing brittle cracking also decreases.

[0116] In addition, the thickness of the hot-rolled plate obtained after hot rolling (equivalent to the final thickness of the steel plate) is not particularly limited, but as mentioned above, it is preferable to make it 6 mm or more and 50 mm or less.

[0117] (3) Quenching (accelerated cooling)

[0118] [Average cooling rate: 1 / 4t, 3℃ / s or more in the temperature range of 600–300℃]

[0119] Quenching is performed directly on the hot-rolled plate after the above hot rolling. For this direct quenching, it is important that the average cooling rate in the temperature range of 300°C or lower and 600°C or higher at the 1 / 4th position of the steel plate is 3°C / s or higher.

[0120] In other words, in direct quenching, if the above average cooling rate is less than 3℃ / s, it becomes difficult to obtain the desired transformation structure, making it difficult to obtain sufficient strength and toughness. Furthermore, it becomes difficult to obtain sufficient performance to suppress the occurrence of brittle cracks. The above average cooling rate is preferably 5℃ / s or higher, and more preferably 10℃ / s or higher. On the other hand, although there is no specific upper limit for the average cooling rate, if the average cooling rate is higher than 200℃ / s, temperature control at each location within the steel sheet becomes difficult, and material deviations are prone to occur in the sheet width direction and the rolling direction. As a result, deviations in material properties such as tensile strength and toughness are prone to occur. Therefore, it is desirable to keep the average cooling rate at 200℃ / s or lower.

[0121] [Cooling stop temperature: 300℃ or lower at 1 / 4t]

[0122] In addition, in direct quenching, if the cooling stop temperature is higher than 300°C at 1 / 4t, unstable γ is prone to being generated and retained, and consequently, the performance of suppressing brittle crack formation cannot be improved. Furthermore, toughness is also reduced. Therefore, the cooling stop temperature is set to 300°C or lower at 1 / 4t. The above cooling stop temperature is preferably 250°C or lower, and more preferably 200°C or lower. Under these conditions, by directly accelerating cooling the hot-rolled plate after hot rolling, the hot-rolled plate is quenched well.

[0123] The cooling treatment in quenching can be performed in any way without being particularly limited, as long as the aforementioned conditions are satisfied. For example, either or both of air cooling and water cooling may be used. As for water cooling, any cooling method using water (e.g., spray cooling, mist cooling, lamination cooling, etc.) may be used.

[0124] (4) Tempering

[0125] [Tempering temperature: 550℃ or higher, below Ac1 transformation point]

[0126] Next, tempering is performed on the hot-rolled plate after quenching. The tempering temperature is set to 550°C or higher and below the Ac1 transformation point. If the tempering temperature is below 550°C, the tempering is insufficient and Charpy toughness decreases. In addition, if the tempering temperature exceeds the Ac1 transformation point, the ability to suppress brittle cracking decreases due to a decrease in strength and the formation of unstable γ.

[0127] In addition, the Ac1 transformation point can be calculated by the following equation (1).

[0128] Ac1 transformation point (°C) = 750.8 - 26.6 × C + 17.6 × Si - 11.6 × Mn - 22.9 × Cu - 23 × Ni + 24.1 × Cr + 22.5 × Mo - 39.7 × V - 5.7 × Ti + 232.4 × Nb - 169.4 × Al … (1)

[0129] However, the element symbols in the above (1) represent the content (mass%) of each element, and if the element is not contained, they are set to 0.

[0130] For heating in the tempering process, any heating method can be used as long as the heating temperature can be controlled as described above. One example of a heating method is furnace heating. For the furnace heating, a general heat treatment furnace can be used without any particular limitations.

[0131] In addition, after reaching the tempering temperature, cooling may be initiated after maintaining the tempering temperature for any amount of time. When maintaining the tempering temperature, the holding time is not particularly limited, but it is preferable to make it at least 5 minutes.

[0132] (Example)

[0133] Steel plates were manufactured following the procedure described below, and their characteristics were evaluated.

[0134] First, molten steel having the composition shown in Table 1 was melted in a converter, and a steel slab (thickness: 200 mm) was produced as a steel material by a continuous casting method. In addition, the Ac1 transformation point (°C) obtained by the aforementioned equation (1) is listed in Table 1.

[0135]

[0136] Next, the obtained steel slab was heated and hot-rolled according to the conditions shown in Table 2 to produce a hot-rolled plate having each plate thickness (final plate thickness). In addition, the obtained hot-rolled plate was subjected to quenching and tempering according to the conditions shown in Table 2 to obtain a steel plate.

[0137] Next, for each of the obtained steel sheets, the microstructure (total area ratio of martensite + bainite), residual γ amount, tensile strength (TS), and Charpy absorption energy (vE) at -196°C －196℃ ) and, the performance of suppressing brittle crack formation by CTOD testing was evaluated according to the following method. The evaluation results are shown in Table 2.

[0138] [Microtissue]

[0139] From each steel plate, a specimen for microstructure observation was taken at a 1 / 4t position. This specimen was embedded in resin with a cross-section perpendicular to the rolling direction as the observation surface and mirror-polished. Subsequently, after Nital etching, images of the microstructure were captured by observing with a scanning electron microscope at magnifications of 2,000 and 10,000. The obtained images were analyzed to identify the microstructure.

[0140] [Residual γ amount at 1 / 4t]

[0141] Five X-ray diffraction test specimens were taken parallel to the plate surface from a 1 / 4t position of each steel plate, and after immersion in liquid nitrogen at -196°C for 30 minutes (deep cryogenic treatment), the specimens were ground and chemically polished so that the 1 / 4t position became the measurement surface, and then provided for X-ray diffraction. The diffraction intensities of the (200), (211) planes of α-Fe and the (200), (220), and (311) planes of γ-Fe appearing in the symmetric reflection X-ray diffraction pattern were determined, the volume fraction of γ-Fe was calculated, and the average value of the five test specimens was determined and used as the residual γ amount (volume fraction).

[0142] In addition, the residual γ amount “-” shown in Table 2 means that the residual γ amount is 0 volume%.

[0143] (tensile strength)

[0144] A JIS No. 4 tensile test specimen was taken from a 1 / 4 ton position of the steel plate. Using this tensile test specimen, a tensile test was performed in accordance with the specifications of JIS Z 2241 to evaluate the tensile strength (TS) of the steel plate. If the tensile strength is 690 MPa or higher, it was deemed to have passed as high strength.

[0145] (Cryogenic Toughness)

[0146] A V-notch test specimen was taken from the 1 / 4t position of the steel plate in accordance with the specifications of JIS Z 2202. Using this V-notch test specimen, a Charpy impact test was performed in accordance with the specifications of JIS Z 2242, and the Charpy absorbed energy (vE) at -196℃ －196℃ ) was calculated. This Charpy impact energy can be considered an indicator of the cryogenic toughness of the steel sheet. In the Charpy impact test, three specimens were taken in the rolling direction for each steel sheet, and measurements were taken. The average value of the measurement results is shown in Table 2. In this Charpy impact test, if a full-size specimen was used, the value was 200J or higher; and if a half-size specimen was used, the value was 100J or higher. In this example, the cryogenic toughness was evaluated as excellent and the test was deemed acceptable. Furthermore, in this embodiment, a half-size specimen was used only for No. 41, while a full-size specimen was used for all other cases.

[0147] (Performance to suppress brittle crack formation)

[0148] In accordance with ISO 12135, a test specimen of 10×10×55 mm was taken from the surface of the steel plate when the plate thickness was greater than 11 mm and less than 20 mm, and from a 1 / 4 t position when the plate thickness was 20 mm or more, and a test specimen of plate thickness × plate thickness × 55 mm was taken when the plate thickness was 11 mm or less, and a CTOD test was performed in accordance with ISO 12135. The test temperature was -165°C. In the CTOD test, it was deemed acceptable if the maximum load point was reached without any visual confirmation of unstable fracture (brittle fracture) of the steel plate caused by the occurrence of brittle cracks. Furthermore, regarding whether each test specimen reached the maximum load point without confirmation of brittle fracture, it is possible to determine this unilaterally by referring to the above standard.

[0149]

Claims

Claim 1 It has a composition in mass% containing C: 0.01–0.15%, Si: 0.01–0.50%, Mn: 0.05–0.40%, Ni: 6.0% or more and less than 7.0%, Cr: 0.01–1.00%, Mo: 0.01–0.50%, P: 0.030% or less, S: 0.0050% or less, N: 0.0010–0.0080%, and Al: 0.008–0.100%, with the remainder being Fe and unavoidable impurities; the total of martensite and bainite is 100.0% in area percentage; the amount of retained austenite at a depth of 1 / 4 of the plate thickness in the plate thickness direction from the surface of the steel sheet is 0% in volume percentage; and according to the Charpy impact test in accordance with JIS Z 2242 A steel plate having a Charpy absorbed energy of 200 J or more when using a full-size specimen and 100 J or more when using a half-size specimen, and reaching the maximum load point without unstable failure of the steel plate due to brittle cracking being visually confirmed in a CTOD test with a test temperature of -165°C in accordance with ISO 12135. Claim 2 A steel plate according to claim 1, wherein the composition of the above components further comprises, in mass%, one or more selected from Cu: 0.40% or less, Nb: 0.05% or less, V: 0.05% or less, Ti: 0.03% or less, B: 0.0030% or less, Ca: 0.007% or less, REM: 0.010% or less, and Mg: 0.070% or less. Claim 3 A steel material having the compositional composition described in claim 1 or 2 is subjected to hot rolling at a cumulative reduction rate of 15–75% at 870°C or lower and a final rolling end temperature of 830–700°C as the steel sheet surface temperature to produce a hot-rolled sheet; then, direct quenching is performed on the said hot-rolled sheet, wherein the average cooling rate in a temperature range of 300°C or lower and a cooling stop temperature of 300°C or higher is 600°C or lower at a depth of 1 / 4 of the plate thickness from the surface of the said hot-rolled sheet in the plate thickness direction is 3°C / s or higher and the cooling stop temperature is 300°C or lower; and subsequently, by direct quenching tempering, tempering is performed on the said hot-rolled sheet after direct quenching in a temperature range of 550°C or higher and below the Ac1 transformation point, such that the total of martensite and bainite is 100.0% in area ratio and the amount of residual austenite at a depth of 1 / 4 of the plate thickness from the surface of the steel sheet in the plate thickness direction is A method for manufacturing a steel plate having a volume fraction of 0%, a Charpy impact energy at -196°C in accordance with JIS Z 2242 that is 200J or more when using a full-size test specimen and 100J or more when using a half-size test specimen, and a CTOD test in accordance with ISO 12135 at a test temperature of -165°C in which unstable fracture of the steel plate due to brittle cracking is not visually confirmed, and reaching the maximum load point. Claim 4 delete

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