Steel Plate and its Manufacturing Method

Abstract

This invention provides a high-strength steel plate with excellent low-temperature toughness and brittle crack initiation suppression performance, based on a simplified manufacturing process using direct quenching and tempering. The steel plate of this invention has a specified composition, with a retained austenite content of less than 1.7% by volume (1 / 4 t), and a Charpy absorption energy of over 200 J at -196°C when using a full-size test piece and over 100 J when using a half-size test piece.

Description

Technical Field

[0001] This invention relates to steel plates, and more particularly to high-strength thick steel plates suitable for applications at extremely low temperatures, capable of consistently ensuring excellent cryogenic toughness and brittle crack initiation inhibition across a wide range of plate thicknesses, as well as a method for manufacturing the same. The steel plates and manufacturing method of this invention can be applied, for example, to structures used in extremely low-temperature environments, such as liquefied gas storage tanks for marine and land-based applications. Background Technology

[0002] When hot-rolled steel plates are used in structures such as liquefied gas storage tanks, the extremely low temperatures necessitate not only high strength but also excellent low-temperature toughness (LTT). For example, when using hot-rolled steel plates in liquefied natural gas storage tanks, excellent toughness must be ensured at temperatures below the boiling point of liquefied natural gas, which is -164°C. Poor LTT of the steel may compromise the safety of the structure used for cryogenic storage, thus requiring high levels of LTT. For this purpose, 7% Ni or 9% Ni steel plates have traditionally been used.

[0003] For example, Patent Document 1 discloses a 7% Ni steel plate. Patent Document 1 also discloses a thick steel plate for extremely low temperatures containing Ni of more than 5.0% to less than 10.0% and specified amounts of C, Si, Mn, and Al. Furthermore, in the thick steel plate disclosed in Patent Document 1, within a thickness range of 6 to 50 mm, the average absorbed energy vE-196 per unit area is 1.25 J / mm². 2 above.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2011-219848 Summary of the Invention

[0007] The inventors conducted in-depth research on thick steel plates of so-called 7% Ni steel (hereinafter also referred to as 7% Ni steel plates) with Ni content of approximately 6.0 to 7.5%. The results showed that when the steel plates were manufactured by direct quenching and tempering after rolling to improve productivity, the risk of unstable failure (brittle fracture) increased due to reduced absorbed energy (toughness) and the generation of brittle cracks, as observed in Charpy tests. However, Patent Document 1 did not address these issues, particularly the generation of brittle fracture.

[0008] The present invention was made in view of the above circumstances, and its purpose is to provide a high-strength steel plate with excellent low-temperature toughness and brittle crack initiation inhibition performance, based on the premise of using a simplified process of direct quenching and tempering.

[0009] In order to solve the above problems, the inventors conducted in-depth research on the composition and microstructure of 7% Ni steel plates, and obtained the following insights:

[0010] (a) The reduction in the absorbed energy mentioned above is mainly due to the separation that occurs in the Mn-rich regions due to the formation of Mn-rich and depleted regions. Here, separation refers to the fracture that occurs as a result of brittle fracture, which is perpendicular to the cross-section and parallel to the rolling surface, as produced in toughness determination tests such as the Charpy impact test and the CTOD test.

[0011] (b) The aforementioned brittle cracks originate from Mn-rich regions, particularly from unstable austenite (hereinafter also referred to as γ) that is prone to form in these regions.

[0012] (c) In order to increase the absorption energy, Mn is limited to below 0.40%, reducing the Mn concentration in the Mn-rich region (Mn segregation band) formed by the band, and reducing the separation caused by the Mn segregation band.

[0013] (d) In order to reduce the formation of brittle cracks, Mn is limited to below 0.40%, reducing the Mn concentration in the banded Mn enrichment region, while suppressing the formation of unstable γ, which is the cause of brittle crack formation.

[0014] This invention is based on the above insights, and its main points are as follows.

[0015] [1] A steel plate having the following composition: containing, by mass %:

[0016] C: 0.01~0.15%

[0017] Si: 0.01~0.50%

[0018] Mn: 0.05~0.40%

[0019] Ni: 6.0% or more but less than 7.0%

[0020] Cr: 0.01~1.00%

[0021] Mo: 0.01–0.50%

[0022] P: below 0.030%

[0023] S: below 0.0050%

[0024] N: 0.0010~0.0080% and

[0025] Al: 0.008~0.100%

[0026] The remainder consists of Fe and unavoidable impurities;

[0027] The amount of retained austenite at a depth of 1 / 4 of the plate thickness along the thickness direction from the surface of the steel plate is less than 1.7% by volume.

[0028] Based on the Charpy impact test according to JIS Z 2242, the Charpy absorbed energy at -196°C is above 200J when using a full-size test piece and above 100J when using a half-size test piece.

[0029] [2] The steel plate according to [1] above, wherein the above composition further contains, by mass percent, a selection from:

[0030] Cu: less than 0.40%

[0031] Nb: below 0.05%

[0032] V: Below 0.05%

[0033] Ti: below 0.03% and

[0034] B: Below 0.0030%

[0035] One or more of them.

[0036] [3] The steel plate according to [1] or [2] above, wherein the above composition further contains, in mass %:

[0037] Ca: below 0.007%

[0038] REM: below 0.010% and

[0039] Mg: below 0.070%

[0040] One or more of them.

[0041] [4] A method for manufacturing a steel plate, wherein a steel billet having the composition described in any one of [1] to [3] above is hot-rolled to a cumulative reduction rate of 15 to 75% at 870°C or less and a final rolling end temperature of 830 to 700°C (based on the surface temperature of the steel plate) to produce a hot-rolled plate, the hot-rolled plate is then subjected to direct quenching at an average cooling rate of 3°C / s or more at a depth of 1 / 4 of the plate thickness along the thickness direction from the surface of the hot-rolled plate in a temperature range of 600°C to 300°C and a cooling stop temperature of 300°C or less, and then tempered at a temperature range of 550°C or more and less than the Ac1 phase transformation point, wherein the amount of retained austenite at a depth of 1 / 4 of the plate thickness along the thickness direction from the surface of the steel plate in the steel plate is less than 1.7% by volume, and, based on JIS Z The Charpy absorption energy of the Charpy impact test of 2242 at -196°C is above 200J when using a full-size test piece and above 100J when using a half-size test piece.

[0042] According to the present invention, high-strength steel plates with excellent low-temperature toughness and brittle crack initiation suppression properties can be provided with high productivity. By supplying the steel plates of the present invention and their manufacturing method to steel structures used in extremely low-temperature environments, such as liquefied gas storage tanks, the safety of these steel structures can be improved, resulting in significant industrial benefits. Detailed Implementation

[0043] The embodiments of the present invention will now be described in detail. It should be noted that the following description represents preferred embodiments of the present invention, and the present invention is not limited thereto.

[0044] [Ingredients]

[0045] The steel sheet of the present invention has a prescribed composition. Furthermore, the steel billet used in the manufacturing method of the present invention, suitable for manufacturing the steel sheet of the present invention, also has the above-described prescribed composition. Hereinafter, each element included in this composition will be described. It should be noted that, unless otherwise specified, in this specification, "%" as the unit for the content of each element means "mass %".

[0046] C: 0.01%~0.15%

[0047] Carbon (C) is an element that improves the strength of steel plates. To achieve this effect, the C content is 0.01% or more, preferably 0.03% or more. On the other hand, if the C content exceeds 0.15%, the extremely low temperature toughness of the steel plate decreases. Furthermore, the ability to suppress brittle crack formation also decreases. Therefore, the C content is 0.15% or less, preferably 0.12% or less.

[0048] Si: 0.01%~0.50%

[0049] Si is an element that helps improve the strength of steel sheets and also acts as a deoxidizer. To achieve these effects, the Si content is 0.01% or more, preferably 0.03% or more. On the other hand, if the Si content is too high, the toughness decreases. In addition, the ability to suppress brittle crack formation is also reduced. Therefore, the Si content is 0.50% or less, preferably 0.30% or less.

[0050] Mn: 0.05%~0.40%

[0051] Mn is an effective element for improving the hardenability of steel and increasing the strength of steel plates. To achieve this effect, Mn is added at 0.05% or more, preferably 0.10% or more. On the other hand, when the Mn content exceeds 0.40%, segregation is prone to occur, and toughness decreases; therefore, it is limited to 0.40% or less. Furthermore, when the Mn content exceeds 0.40%, the Mn concentration in the Mn segregation band tends to increase, and the ability to suppress brittle crack formation decreases; therefore, it is limited to 0.40% or less. The Mn content is preferably 0.35% or less, more preferably less than 0.35%, and even more preferably less than 0.20%.

[0052] Ni: 6.0% or more but less than 7.0%

[0053] Ni is an extremely effective element for improving the extremely low-temperature toughness of steel sheets and also helps to improve the inhibition of brittle crack formation. Therefore, the Ni content is 6.0% or more, preferably 6.5% or more. On the other hand, Ni is an expensive element, and the cost of steel sheets increases with its content. Therefore, in this invention, the Ni content is less than 7.0%.

[0054] Cr: 0.01%~1.00%

[0055] Cr is an element that can improve the strength of steel plates without severely compromising their low-temperature toughness. To achieve this effect, the Cr content is 0.01% or more, preferably 0.30% or more. However, if the Cr content exceeds 1.00%, the low-temperature toughness of the steel plate decreases. Furthermore, the ability to suppress brittle crack formation also decreases. Therefore, the Cr content is 1.00% or less, preferably 0.80% or less.

[0056] Mo: 0.01%–0.50%

[0057] Mo, like Cr, is an element that can improve the strength of steel plates without severely compromising their low-temperature toughness. Therefore, the Mo content is 0.01% or more, preferably more than 0.10%. On the other hand, if the Mo content exceeds 0.50%, the low-temperature toughness decreases. In addition, the ability to suppress brittle crack formation also decreases. Therefore, the Mo content is 0.50% or less, preferably 0.30% or less, and more preferably 0.25% or less.

[0058] P: below 0.030%

[0059] Phosphorus (P) is an unavoidable impurity and a harmful element that adversely affects the low-temperature toughness and brittle cracking inhibition performance of steel plates. For example, in order to obtain a sound base material and weld joint when steel plates are welded to form welded structures, it is preferable to minimize the P content as much as possible. Therefore, the P content is suppressed to 0.030% or less. Furthermore, from the viewpoint of low-temperature toughness, the lower the P content, the better; therefore, there is no particular limit to the lower limit, and it can be 0%, but in this case, its presence as an unavoidable impurity is also permissible. On the other hand, excessive reduction leads to increased costs; therefore, from a cost perspective, the lower limit of the P content is preferably 0.001%.

[0060] S: below 0.0050%

[0061] S forms MnS in steel, significantly deteriorating its low-temperature toughness and resistance to brittle cracking. Therefore, it is preferable to keep the S content as low as possible, with an upper limit of 0.0050%. The S content is preferably below 0.0020%. On the other hand, the lower the S content, the better, so there is no particular lower limit, and it can be 0%, but in this case, it is also permissible as an unavoidable impurity.

[0062] N: 0.0010%~0.0080%

[0063] Nitrogen (N) forms precipitates in steel, and if its content exceeds 0.0080%, it contributes to a decrease in the toughness of the base metal. Furthermore, its ability to inhibit brittle crack formation is also reduced. Nitrogen also contributes to the grain refinement of the base metal by forming AlN; this effect is achieved by keeping the N content at 0.0010% or higher. Therefore, the N content is 0.0010% to 0.0080%. The N content is preferably 0.0020% or higher, and more preferably 0.0060% or lower.

[0064] Al: 0.008%~0.100%

[0065] Al is an element included in deoxidizers. If the Al content is less than 0.008%, its effect as a deoxidizer is poor. Furthermore, Al also contributes to the grain refinement of the base material by forming AlN. Therefore, the Al content is 0.008% or more, preferably 0.020% or more. On the other hand, if the Al content exceeds 0.100%, the cleanliness of the steel is impaired, and its toughness, especially Charpy energy absorption under extremely low temperatures, decreases. Therefore, the Al content is 0.100% or less, preferably 0.050% or less.

[0066] In one embodiment of the present invention, the composition, in addition to the elements specified above, may consist of Fe and unavoidable impurities.

[0067] In other embodiments of the present invention, the above-described composition may preferably contain one or more of Cu, Nb, V, Ti and B in the amounts described below.

[0068] Cu: below 0.40%

[0069] Cu is an element that can improve the strength of steel plates by enhancing their hardenability. However, if the Cu content exceeds 0.40%, in addition to a decrease in the low-temperature toughness of the steel plate, the surface properties of the cast steel billet (billet) also deteriorate. Therefore, when adding Cu, it is preferable to keep the Cu content at 0.40% or less. More preferably, it is 0.30% or less. On the other hand, there is no particular limitation on the lower limit of the Cu content, but in order to obtain the above-mentioned effects, it is preferable to keep the Cu content at 0.10% or more.

[0070] Nb: below 0.05%

[0071] Nitrogen (Nb) is an effective element for improving the strength of steel plates through precipitation strengthening. However, if the Nb content is too high, the extremely low temperature toughness of the steel plate decreases. Therefore, when adding Nb, it is preferable to keep the Nb content at 0.05% or less, more preferably at 0.03% or less. On the other hand, there is no particular limitation on the lower limit of the Nb content, but in order to obtain the above-mentioned effect, it is preferable to keep the Nb content at 0.01% or more.

[0072] V: below 0.05%

[0073] V, like Nb, is an effective element for improving the strength of steel sheets through precipitation strengthening. However, if the V content is too high, the extremely low temperature toughness of the steel sheet decreases. Therefore, when adding V, it is preferable to keep the V content at 0.05% or less, more preferably at 0.04% or less. On the other hand, there is no particular limitation on the lower limit of the V content, but in order to obtain the above-mentioned effect, it is preferable to keep the V content at 0.01% or more.

[0074] Ti: below 0.03%

[0075] Ti is an element that improves the toughness of the weld without reducing the mechanical properties of the base material when steel plates are welded to form welded structures. Therefore, it is preferable to add 0.003% or more. On the other hand, if it exceeds 0.03%, it will reduce the toughness, so it is preferable that Ti is contained in the range of 0.03% or less.

[0076] B: Below 0.0030%

[0077] Bode (B) is an element that improves hardenability by being added in trace amounts. To effectively achieve this effect, it is preferable to contain 0.0003% or more of B. On the other hand, if the B content exceeds 0.0030%, the toughness deteriorates. Therefore, when B is present, it is preferable to keep its content below 0.0030%.

[0078] In other embodiments of the present invention, the above-described composition may preferably contain one or more of Ca, REM and Mg in the amounts described below.

[0079] Ca: below 0.007%

[0080] Ca is an element that improves the extremely low temperature toughness of steel plates by controlling the morphology of inclusions in the steel. However, if Ca is excessive, it impairs the cleanliness of the steel and reduces the Charpy absorption energy (hereinafter also referred to as Charpy toughness) under extremely low temperature conditions. Therefore, when adding Ca, it is preferable to keep the Ca content at 0.007% or less. More preferably, it is 0.004% or less. On the other hand, there is no particular limitation on the lower limit of the Ca content, but in order to obtain the above-mentioned effect, it is preferable to be 0.001% or more.

[0081] REM: below 0.010%

[0082] Rare earth metals (REMs) are elements similar to calcium (Ca) that improve the extremely low-temperature toughness of steel sheets by controlling the morphology of inclusions in the steel. However, if REM is added in excess, it impairs the cleanliness of the steel and reduces Charpy toughness. Therefore, when adding REM, it is preferable to keep the REM content at 0.010% or less, more preferably at 0.008% or less. On the other hand, there is no particular limitation on the lower limit of REM content, but in order to obtain the above-mentioned effect, it is preferable to keep the REM content at 0.001% or more.

[0083] Here, REM refers to the collective term for 17 elements obtained by combining Y and Sc with 15 lanthanide elements. These elements can be contained individually or in combination. It should be noted that the content of REM refers to the total content of these elements.

[0084] Mg: below 0.070%

[0085] Mg, like Ca and REM, is an element that improves the extremely low-temperature toughness of steel plates by controlling the morphology of inclusions in the steel. However, if Mg is excessive, it impairs the cleanliness of the steel and reduces Charpy toughness. Therefore, when adding Mg, it is preferable to keep the Mg content at 0.070% or less, more preferably at 0.004% or less. On the other hand, there is no particular limitation on the lower limit of Mg content, but in order to obtain the above-mentioned effect, it is preferable to keep the Mg content at 0.001% or more.

[0086] [Microstructure]

[0087] The steel plate of the present invention has a microstructure in which the amount of retained austenite (hereinafter also referred to as retained γ) at a depth of 1 / 4 of the plate thickness t (hereinafter also referred to as 1 / 4t) from the surface of the steel plate along the thickness direction is less than 1.7% by volume. As a representative location, if the amount of retained γ at said location is 1.7% or more, it can be seen that a greater amount of unstable γ is generated, particularly in the enriched region of the Mn segregation zone, which easily leads to brittle cracking. From the viewpoint of improving the performance of suppressing brittle crack formation, the amount of retained γ at said location is preferably as small as possible, preferably 1.5% or less, more preferably 1.0% or less, further preferably 0.5% or less, more preferably 0.1% or less, and even more preferably 0%.

[0088] It should be noted that the residual γ content can be determined by the method described in the examples below, and can be measured on either the surface or the back side of the steel plate.

[0089] Furthermore, the microstructure of the steel sheet is preferably composed mainly of martensite and bainite. Specifically, the total area fraction of martensite and bainite is 98.3% or more, more preferably 99.0% or more, and even more preferably 100%. As mentioned above, if the microstructure is mainly composed of martensite and / or bainite, sufficient strength can be easily obtained while ensuring excellent low-temperature toughness. It should be noted that the ratio of martensite to bainite can be arbitrary. In addition, there are no particular restrictions on the types of microstructure other than martensite and bainite.

[0090] There is no particular limitation on the thickness of the steel plate; it can be any thickness. For example, 6mm to 50mm is preferred.

[0091] [Mechanical Properties]

[0092] (Tensile strength)

[0093] The lower limit of the tensile strength of the steel plate does not need to be particularly limited, but it is preferable to have a lower limit of 690 MPa. More preferably, it is 720 MPa or higher. On the other hand, the upper limit of the tensile strength does not need to be particularly limited, but it is preferable to have an upper limit of 930 MPa. More preferably, it is 900 MPa or lower.

[0094] It should be noted that tensile strength can be determined by the method described in the examples described later.

[0095] (Extremely low temperature toughness)

[0096] For the toughness value of steel plate, the Charpy absorbed energy (vE) at -196℃ －196℃In Charpy impact tests using full-size test pieces, a J of 200 J or higher is required. On the other hand, 350 J or less is preferred, and 280 J or less is more preferable. Furthermore, in Charpy impact tests using half-size test pieces, vE... －196℃ The required J is 100J or higher. On the other hand, it is preferable to be less than 200J, and more preferably less than 150J.

[0097] It should be noted that the extremely low temperature toughness can be determined by the method described in the examples below.

[0098] (Suppression of brittle crack formation)

[0099] As a means of suppressing brittle cracking in steel plates, in the CTOD (Crack Tip Opening Displacement) test, it is preferable to reach the maximum load point without a sharp decrease in load, as described in the examples below.

[0100] [Manufacturing Method]

[0101] Next, the method for manufacturing the steel plate of the present invention will be described. The manufacturing method of the present invention uses a steel billet having the composition specified above, and controls the cumulative reduction rate and final rolling end temperature during hot rolling, the average cooling rate and cooling stop temperature during quenching, and the temperature range during tempering to specified conditions, thereby obtaining a steel plate that satisfies the specified residual γ content.

[0102] It should be noted that, unless otherwise specified, the temperature in the following description refers to the temperature at the center of the plate thickness. The temperature at the center of the plate thickness can be calculated, for example, from the surface temperature of the steel plate measured with a radiation thermometer through heat transfer calculations.

[0103] As an example, the steel plate of the present invention can be appropriately manufactured by performing the following steps (1) to (4) in sequence:

[0104] (1) Heating of steel billets;

[0105] (2) Hot rolling;

[0106] (3) Quenching (accelerated cooling);

[0107] (4) Tempering.

[0108] (1) Heating of steel billets

[0109] First, the steel billet needs to have the above-mentioned composition. The steel billet is preferably heated to a temperature of 900°C to 1200°C. The manufacturing method of the steel billet is not particularly limited; for example, it can be manufactured by smelting and casting molten steel with the above-mentioned composition using conventional methods. Smelting can be carried out by any method such as a converter, electric furnace, or induction furnace. Furthermore, from a productivity point of view, casting is preferably carried out by continuous casting, but it can also be carried out by agglomeration-disruption rolling. For example, a steel billet can be used as the steel billet material.

[0110] Here, the steel billet can be heated either by temporarily cooling the steel billet obtained by casting or other methods, or by directly heating the obtained steel billet without cooling it.

[0111] If the billet heating temperature is below 900°C, the billet exhibits high deformation resistance, thus increasing the load on the rolling mill during subsequent hot rolling and making hot rolling difficult. Therefore, the billet heating temperature is preferably above 900°C. On the other hand, if the billet heating temperature is above 1200°C, steel oxidation becomes significant, increasing losses due to the removal of the oxide film caused by oxidation, resulting in a lower yield. Therefore, the billet heating temperature is preferably below 1200°C.

[0112] (2) Hot rolling

[0113] [Reduction rate: The cumulative reduction rate below 870℃ is 15% to 75%]

[0114] In hot rolling, if the cumulative reduction rate within the austenite non-recrystallization temperature range below 870°C is less than 15%, sufficient microstructure refinement cannot occur, resulting in reduced toughness. Furthermore, strength and the ability to suppress brittle crack initiation 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 below 870°C is kept to be 15% to 75%. Preferably, the cumulative reduction rate is 30% or more, more preferably 70% or less, and even more preferably 30% to 70%.

[0115] [Final rolling temperature: 700℃~830℃ based on the surface temperature of the steel plate]

[0116] In hot rolling, if the final rolling temperature (finishing temperature) is less than 700°C (based on the surface temperature of the steel plate), separation due to aggregate structure easily occurs, resulting in reduced toughness. Furthermore, the ability to suppress brittle crack initiation is also reduced. On the other hand, if the final rolling temperature exceeds 830°C (based on the surface temperature of the steel plate), sufficient reduction in the non-recrystallized regions becomes difficult, preventing the formation of a fine microstructure and reducing toughness. Additionally, the ability to suppress brittle crack initiation is also reduced.

[0117] It should be noted that 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 preferably 6mm to 50mm.

[0118] (3) Quenching (accelerated cooling)

[0119] [Average cooling rate: over 3℃ / s in a 1 / 4t temperature range of 600~300℃]

[0120] The hot-rolled plate is directly quenched after hot rolling. The key to this direct quenching is that the average cooling rate at a position of 1 / 4 t of the steel plate, within a temperature range of 600℃ to 300℃, is above 3℃ / s.

[0121] That is, in direct quenching, if the average cooling rate is less than 3°C / s, it is difficult to obtain the desired phase transformation structure, and it is also difficult to obtain sufficient strength and toughness. Furthermore, it is difficult to obtain sufficient brittle crack suppression performance. The average cooling rate is preferably 5°C / s or more, more preferably 10°C / s or more. On the other hand, there is no particular upper limit to the average cooling rate, but if the average cooling rate exceeds 200°C / s, temperature control at various locations within the steel plate becomes difficult, and material deviations are prone to occur in the width direction and rolling direction. As a result, material properties such as tensile properties and toughness are prone to deviation. Therefore, the average cooling rate is preferably 200°C / s or less.

[0122] [Cooling stop temperature: below 300℃ at 1 / 4 t]

[0123] Furthermore, in direct quenching, if the cooling stop temperature (at 1 / 4 t) is higher than 300°C, unstable γ-rays are easily generated and remain, resulting in a failure to improve the suppression performance of brittle crack initiation. Additionally, toughness also decreases. Therefore, the cooling stop temperature (at 1 / 4 t) is below 300°C. Preferably, the cooling stop temperature is below 250°C, more preferably below 200°C. By directly and rapidly cooling the hot-rolled sheet under such conditions, the hot-rolled sheet is effectively quenched.

[0124] The cooling process during quenching can be carried out by any method without particular limitation, as long as the above conditions are met. For example, one or both of air cooling and water cooling can be used. For water cooling, any cooling method using water can be used (e.g., spray cooling, mist cooling, laminar flow cooling, etc.).

[0125] (4) Tempering

[0126] [Tempering temperature: Above 550℃ and below the Ac1 phase transition point]

[0127] Next, the quenched hot-rolled sheet is tempered. The tempering temperature is above 550°C but below the Ac1 phase transformation point. If the tempering temperature is below 550°C, the tempering is insufficient and Charpy toughness decreases. Furthermore, if the tempering temperature is above the Ac1 phase transformation point, the inhibition energy for brittle crack initiation decreases due to reduced strength and the formation of unstable γ phases.

[0128] It should be noted that the Ac1 phase transition point can be determined by the following equation (1).

[0129] A C1 Phase transition 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)

[0130] The element symbols in equation (1) above represent the content (mass%) of each element, which is 0 if the element is not present.

[0131] For heating in the tempering process, any heating method can be used as long as the heating temperature can be controlled as described above. Furnace heating is an example of such a method. There are no particular limitations on furnace heating; a general heat treatment furnace can be used.

[0132] It should be noted that after reaching the tempering temperature, cooling can begin at any time after maintaining the temperature at the tempering temperature. While there is no particular limitation on the holding time when maintaining the temperature at the tempering temperature, it is preferably 5 minutes or more.

[0133] Example

[0134] Manufacture steel plates according to the steps described below and evaluate their properties.

[0135] First, molten steel with the composition shown in Table 1 is smelted in a converter, and steel billets (thickness: 200 mm) are manufactured as billet materials by continuous casting. It should be noted that A, calculated using equation (1) above... C1 The phase transition point (°C) is marked in Table 1.

[0136]

[0137] Next, the obtained steel billets are heated and hot-rolled according to the conditions shown in Table 2 to produce hot-rolled plates with various thicknesses (final plate thicknesses). Then, the obtained hot-rolled plates are quenched and tempered according to the conditions shown in Table 2 to obtain steel plates.

[0138] Next, for each of the obtained steel plates, the microstructure (total area ratio of martensite + bainite), residual γ content, tensile strength (TS), and Charpy absorption energy (vE) at -196℃ were evaluated according to the following methods. －196℃ The evaluation results, including the brittle crack initiation suppression performance based on the CTOD test, are listed in Table 2.

[0139] [Microstructure]

[0140] Test pieces for microstructure observation were collected from each steel plate at a position of 1 / 4 t. The test piece was then embedded in resin with a cross-section perpendicular to the rolling direction as the observation surface and mirror-polished. Next, it was etched with nitric acid-ethanol solution and observed using a scanning electron microscope at 2000x and 10000x magnification, and images of the microstructure were captured. The obtained images were analyzed to identify the microstructure.

[0141] [Residual γ amount of 1 / 4t]

[0142] Five X-ray diffraction test pieces were collected from the 1 / 4t position of each steel plate, parallel to the plate surface. After immersion in liquid nitrogen at -196℃ for 30 minutes (deep cryogenic treatment), the test pieces were ground and chemically polished with the 1 / 4t position as the measurement surface, and then used for X-ray diffraction. The diffraction intensities of the (200) and (211) planes of α-Fe and the (200), (220), and (311) planes of γ-Fe appearing in the symmetrical reflection X-ray diffraction pattern were calculated. The volume fraction of γ-Fe was calculated, and the average value of the five test pieces was obtained as the residual γ amount (volume fraction).

[0143] It should be noted that the residual γ amount shown in Table 2 with a "-" indicates that the residual γ amount is 0% by volume.

[0144] (Tensile strength)

[0145] A JIS No. 4 tensile test specimen is collected from 1 / 4 t of the steel plate. Using this specimen, a tensile test is performed according to JIS Z2241 to evaluate the tensile strength (TS) of the steel plate. If the tensile strength is 690 MPa or higher, it is considered high strength and rated as acceptable.

[0146] (Extremely low temperature toughness)

[0147] According to JIS Z 2202, a V-notch test piece was collected from a position 1 / 4 t below the steel plate. Using this V-notch test piece, a Charpy impact test was conducted according to JIS Z 2242 to determine the Charpy absorbed energy (vE) at -196℃. －196℃The Charpy absorption energy can be considered as an indicator of the steel plate's low-temperature toughness. In the Charpy impact test, three test pieces were collected from each steel plate in the rolling direction for measurement. The average value of the measurement results is shown in Table 2. In this Charpy impact test, if the value is above 200 J when using a full-size test piece, and above 100 J when using a half-size test piece, the steel plate is evaluated as having excellent low-temperature toughness and is rated as qualified. It should be noted that in this embodiment, a half-size test piece is used only in case No. 41, and a full-size test piece is used in other cases.

[0148] (Suppression of brittle crack formation)

[0149] According to ISO 12135, when the plate thickness is greater than 11 mm but less than 20 mm, a 10×10×55 mm test piece is collected from the surface of the steel plate; when the plate thickness is greater than 20 mm, a 10×10×55 mm test piece is collected from the 1 / 4 t position; and when the plate thickness is less than 11 mm, a test piece of plate thickness × plate thickness × 55 mm is collected. The CTOD test is performed according to ISO 12135. The test temperature is -165℃. In the CTOD test, the test piece that reaches the maximum load point without visually confirming unstable failure (brittle failure) of the steel plate due to brittle cracking is considered acceptable. It should be noted that, for each test piece, whether the maximum load point was reached without confirming brittle failure can be clearly determined by referring to the above standard.

[0150] [Table 2]

[0151]

[0152] The underlined part indicates that it is outside the scope of this invention.

[0153] *Results using half-size test pieces.

Claims

1. A steel plate having the following composition: It contains, by mass%: C：0.01～0.15％、 Si: 0.01~0.50% Mn: 0.05~0.40% Ni: 6.0% or more but less than 7.0% Cr：0.01～1.00％、 Mo: 0.01–0.50% P: below 0.030% S: below 0.0050% N: 0.0010~0.0080% and Al：0.008～0.100％， The remainder consists of Fe and unavoidable impurities; Furthermore, the combined area ratio of martensite and bainite is 100.0%. The amount of retained austenite at a depth of 1 / 4 of the plate thickness along the thickness direction from the surface of the steel plate is less than 1.7% by volume. Based on the Charpy impact test according to JIS Z2242, the Charpy absorbed energy at -196℃ is above 200J when using a full-size test piece and above 100J when using a half-size test piece.

2. The steel plate according to claim 1, wherein, The composition further contains, by mass percent, a selection from: Cu: less than 0.40% Nb: below 0.05% V: Below 0.05% Ti: below 0.03% and B: Below 0.0030% One or more of them.

3. The steel plate according to claim 1 or 2, wherein, The composition further contains, by mass percent, a selection from: Ca: below 0.007% REM: below 0.010% and Mg: below 0.070% One or more of them.

4. A method for manufacturing a steel plate, comprising hot rolling a steel billet having the composition as described in any one of claims 1 to 3 at a cumulative reduction rate of 15% to 75% at a temperature below 870°C and a final rolling end temperature of 830% to 700°C (using a steel plate surface thermometer) to produce a hot-rolled plate, then subjecting the hot-rolled plate to direct quenching at an average cooling rate of 3°C / s or more at a depth of 1 / 4 of the plate thickness along the thickness direction from the surface of the hot-rolled plate and a cooling stop temperature of 300°C or less, and then tempering at a temperature above 550°C and below the Ac1 phase transformation point. The total area fraction of martensite and bainite in the steel plate is 100.0%, the amount of retained austenite at a depth of 1 / 4 of the plate thickness along the thickness direction from the surface of the steel plate is less than 1.7% by volume, and the Charpy absorption energy at -196°C based on the Charpy impact test according to JIS Z2242 is 200 J or more when using a full-size test piece and 100 J or more when using a half-size test piece.

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