Low-strength thick steel plate having excellent elongation and corrosion resistance

Abstract

A low-strength thick steel sheet having excellent elongation properties and corrosion resistance, wherein a prescribed composition group is provided, in the metal structure, 70% or more of ferrite is contained in terms of area ratio with respect to the entire metal structure, the balance contains one or more selected from the group consisting of pearlite, bainite, and martensite, the ferrite grain size is 3 μm or more and 40 μm or less, when a tensile test is performed using a test piece of NK-U1 No. of 200 mm gauge according to the Ship Classification Rule K of the Nippon Kaiji Kyokai (2019 edition), the tensile strength is 400 MPa or more and 520 MPa or less, the elongation at break is 16% or more, and the uniform elongation is 13.5% or more.

Description

TECHNICAL FIELD

[0001] The present invention relates to a low-strength thick steel sheet having excellent elongation properties and corrosion resistance. BACKGROUND

[0002] A ballast tank of a ship, because seawater is injected and discharged according to changes in the cargo state and the like, the steel material used is subjected to a very severe corrosion environment in which seawater immersion and exposure to a salt-containing humid atmosphere are repeated. When corrosion damage occurs in the ballast tank, sinking can occur due to perforation, and the like, and the loaded cargo such as crude oil or chemicals can flow out into the sea, and the like, so it is necessary to implement some kind of corrosion protection measure for the steel material, and as one of the measures, corrosion protection painting is performed. However, corrosion protection painting requires repainting due to deterioration of the painting, so maintenance costs are incurred, and environmental burdens are also increased. In recent years, in order to realize a sustainable society, reducing environmental burdens has become one of the important factors. Therefore, from the viewpoint of maintenance costs and reducing environmental burdens, it is also necessary to improve the corrosion resistance of the steel material. Steel materials for ships for the purpose of improving corrosion resistance are disclosed in, for example, Patent Literature 1 and Patent Literature 2.

[0003] PRIOR ART DOCUMENTS

[0004] PATENT LITERATURE

[0005] Patent Literature 1: Japanese Patent Application Publication No. 2015-151571

[0006] Patent Literature 2: Japanese Patent Application Publication No. 2010-126765 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] If it is an inland ship, depending on the hull structure, for example, in most cases, a low-strength material, so-called soft steel, having a tensile strength (TS) of 520 MPa or less is used. On the other hand, as a measure to improve corrosion resistance, an alloy element as a corrosion resistance improving element is generally added. However, if an alloy element is added, the strength generally increases, so it is difficult to balance corrosion resistance and low strength.

[0009] In addition, in recent years, as one of the means to realize a sustainable society, a low-carbon society is also being promoted. If the workability of a steel sheet can be improved, the operation time for forming parts and the like at the construction site is reduced, and ultimately, energy saving or reduction of CO2 emissions can be promoted. Therefore, it is also necessary to ensure the prescribed workability of the steel sheet (i.e., improvement of the elongation at break EL). Furthermore, when a ship or the like collides with an object, plastic deformation occurs in the member, and if a crack occurs, the maintenance burden increases, so it is not desirable from the viewpoint of realizing a low-carbon society. Therefore, from the viewpoint of uniform deformation when an external force is applied, it is also necessary to have the prescribed uniform elongation property (uEL).

[0010] The present application is made in view of such circumstances, and has an object to provide a thick steel sheet in which excellent corrosion resistance and low strength and high elongation characteristics coexist.

[0011] Means for solving the problem

[0012] Mode 1 of the present application is a low-strength thick steel sheet in which excellent elongation characteristics and corrosion resistance are obtained, wherein

[0013] C: 0.01 to 0.30 mass%,

[0014] Si: 0.01 to 2.0 mass%,

[0015] Mn: 0.85 to 2.00 mass%,

[0016] P: more than 0 to 0.015 mass%,

[0017] S: more than 0 to 0.005 mass%,

[0018] Al: 0.005 to 0.10 mass%,

[0019] Cr: 0.01 to 0.5 mass%,

[0020] Ti: 0.005 to 0.20 mass%,

[0021] Ca: 0.0001 to 0.005 mass%,

[0022] N: 0.0001 to 0.010 mass%, and

[0023] Cu + Ni: 0.50 to 0.85 mass%,

[0024] the balance including Fe and inevitable impurities,

[0025] in the metal structure, ferrite is contained at 70% or more in terms of area ratio with respect to the entire metal structure, and the balance includes one or more selected from the group consisting of pearlite, bainite, and martensite,

[0026] the ferrite grain size is 3 μm or more and 40 μm or less,

[0027] When a tensile test is performed using a test piece of NK-U1 No. with a gauge length of 200 mm according to the Ship Classification Rule K of the Nippon Kaiji Kyokai (2019 edition), the tensile strength is 400 MPa or more and 520 MPa or less, the elongation at break is 16% or more, and the uniform elongation is 13.5% or more.

[0028] Mode 2 of the present application, the thick steel sheet according to Mode 1, wherein the average corrosion depth of the painted scratch portion after the cyclic corrosion test for 168 days is 0.600 mm or less.

[0029] Mode 3 of the present application, the thick steel sheet according to Mode 1 or Mode 2, wherein it further contains one or more kinds selected from the group consisting of

[0030] B: 0.0001 mass% or more and 0.010 mass% or less,

[0031] V: 0.01 mass% or more and 0.50 mass% or less, and

[0032] Nb: 0.001 mass% or more and 0.50 mass% or less.

[0033] Mode 4 of the present application, the thick steel sheet according to any one of Modes 1 to 3, wherein it further contains one or more kinds selected from the group consisting of

[0034] Co: 0.005 mass% or more and 0.20 mass% or less,

[0035] REM: 0.005 mass% or more and 0.20 mass% or less,

[0036] Zr: 0.005 mass% or more and 0.20 mass% or less, and

[0037] Mg: 0.0001 mass% or more and 0.005 mass% or less.

[0038] Mode 5 of the present application is a manufacturing method of the thick steel sheet according to any one of Modes 1 to 4, wherein it includes the following steps:

[0039] a step of heating the steel satisfying the composition according to any one of Modes 1 to 4 to 1100°C or more;

[0040] a step of hot-rolling in a manner so that the rolling temperature is 1100°C or less and the recrystallization reduction is 80% or more, so as to satisfy the following formula (1);

[0041] a step of performing air cooling.

[0042] 0.08FRT + 0.1Tom - 0.5ε T +10 ≥ 69 … (1)

[0043] wherein,

[0044] FRT: finish rolling temperature (°C)

[0045] Tom: time (sec) between the final pass and the pass immediately before the final pass

[0046] ε T : total value of accumulated strain at each of the surface, t / 4 position, and t / 2 position (t: plate thickness) of the steel sheet, calculated by the following equation (2). Also, the accumulated strain at each position is calculated by the following equation (3). Further, the strain of each pass is calculated by the following equation (4).

[0047] ε T = ε yt (surface) + ε yt (t / 4) + ε yt (t / 2) … (2)

[0048] ε yt = ε y1 + ε y2 + ε y3 + … + ε yn … (3)

[0049] ε yi = C x (2y / h) 2 + 1.15 x ln(H / h) … (4)

[0050] In equations (2) to (4),

[0051] ε y : equivalent plastic strain at the y position in the plate thickness direction,

[0052] ε yi : equivalent plastic strain at the y position in the plate thickness direction of the i-th pass,

[0053] y: distance from the center of the plate thickness (mm), h: out-side plate thickness (mm), H: in-side plate thickness (mm),

[0054] C = B1 x (H / 2R) B2 x r e 2 … (5)

[0055] In equation (5),

[0056] B1 = 0.18381 + 0.34435 μ + 1.4086 x μ 2

[0057] B2 = 0.076669 - 2.0566 μ + 2.1128 x μ 2

[0058] wherein μ: coefficient of friction, R: roll radius (mm), r e : reduction

[0059] Inventive Effects

[0060] According to the embodiment of the present application, a thick steel plate having excellent corrosion resistance and low strength and high elongation properties can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0061] Figure 1 is a schematic view showing a corrosion test piece of the example.

[0062] Figure 2 is a view for explaining a corrosion test method of the example.

[0063] Figure 3 is a schematic view for explaining a measurement position of a corrosion depth of the example.

[0064] Figure 4 is a view showing a relationship between the left side of Equation (1) and the tensile strength.

[0065] Figure 5 is a view showing a relationship between the left side of Equation (1) and the uniform elongation. DETAILED DESCRIPTION

[0066] The present inventors, in order to ensure excellent corrosion resistance, have made intensive studies on the premise of containing a certain amount of Cu, Ni and Cr, in order to achieve low strength and excellent elongation properties. As a result, it has been found that a thick steel plate having excellent elongation properties and corrosion resistance and low strength can be obtained by manufacturing a steel having a prescribed chemical composition under recommended conditions, which has a prescribed chemical composition and contains 70% or more ferrite, the balance containing one or more selected from the group consisting of pearlite, bainite and martensite, in terms of area ratio of the entire metal structure, the ferrite grain size is 3 μm or more and 40 μm or less, the tensile strength is 400 MPa or more and 520 MPa or less, the elongation at break is 16% or more, and the uniform elongation is 13.5% or more.

[0067] 1. Chemical composition

[0068] The chemical composition of the thick steel plate of the embodiment of the present application will be described below. First, C, Si, Mn, P, S, Al, Cr, Ti, Ca, N, Cu and Ni as basic elements will be described, and then elements which can be selectively added will be described.

[0069] [C: 0.01 mass% or more and 0.30 mass% or less]

[0070] C is an element required to ensure the strength of the material. If C is contained in excess, the strength increases and low strength cannot be obtained. On the other hand, if the amount of C is excessively reduced, it is difficult to obtain the minimum strength, i.e., about 400 MPa or so, as a structural member. Therefore, the C content is 0.01 mass% or more and 0.30 mass% or less. Also, the above minimum strength varies depending on the wall thickness of the steel material used. The C content is preferably 0.05 mass% or more, and more preferably 0.10 mass% or more. In addition, the C content is preferably 0.20 mass% or less, and more preferably 0.15 mass% or less.

[0071] [Si: 0.01 mass% or more and 2.0 mass% or less]

[0072] Si is an element required to deoxidize and ensure the strength. Si is solid-solved in ferrite, thereby increasing the strength, but if the Si content is higher than 2.0 mass%, low strength cannot be obtained due to the increase in strength. On the other hand, if the Si content is lower than 0.01 mass%, the minimum strength as a structural member cannot be ensured. Therefore, the Si content is 0.01 mass% or more and 2.0 mass% or less. The Si content is preferably 1.00 mass% or less, more preferably 0.50 mass% or less, and further preferably 0.30 mass% or less. In addition, the Si content is preferably 0.10 mass% or more, and more preferably 0.15 mass% or more.

[0073] [Mn: 0.85 mass% or more and 2.00 mass% or less]

[0074] Mn, like Si, is an element required to deoxidize and ensure the strength, and if it is lower than 0.85 mass%, the minimum strength as a structural member cannot be ensured. However, if Mn is contained in excess, low strength cannot be obtained due to the increase in strength. Therefore, the Mn content is 0.85 mass% or more and 2.00 mass% or less. The Mn content is preferably 0.90 mass% or more, and more preferably 0.95 mass% or more. In addition, the Mn content is preferably 1.20 mass% or less, and more preferably 1.10 mass% or less.

[0075] [P: higher than 0 mass% and 0.015 mass% or less]

[0076] P, like Si, is an element that solid-solution strengthens a lot, and in the embodiment of the present application, which targets low strength, it is desirable to contain it as little as possible. In addition, because it also deteriorates toughness and weldability, the upper limit of the P content is set to 0.015 mass%.

[0077] [S: higher than 0 mass% and 0.005 mass% or less]

[0078] S is an element that reduces ductility if the content is increased, and thus, the content is preferably suppressed, with the upper limit of the allowable content of S being 0.005 mass%.

[0079] [Al: 0.005 mass% or more and 0.10 mass% or less]

[0080] Al is an element that forms a dense oxide film, thereby improving corrosion resistance. On the other hand, Al, although being smaller than the solid solution strengthening amount of Si and P, is also an element that contributes to an increase in strength. Therefore, the Al addition amount is 0.005 mass% or more and 0.10 mass% or less. The Al content is preferably 0.030 mass% or more, and more preferably 0.045 mass% or more. In addition, the Al content is preferably 0.090 mass% or less, and more preferably 0.080 mass% or less.

[0081] [Cu + Ni: 0.50 mass% or more and 0.85 mass% or less]

[0082] Cu and Ni are elements that are effective for improving corrosion resistance. Both Cu and Ni have an effect of suppressing a corrosion reaction that occurs under a corrosion-resistant coating film, and are elements that have an effect of suppressing bulging of a coating film due to corrosion under the coating film. In addition, Cu and Ni also have an effect of densifying rust when steel is corroded at a defect portion of a coating film, and are effective elements that exert an inhibitory effect on the progress of corrosion at a damaged portion of a coating film. In order to exert these effects, it is necessary to make the total content of Cu and Ni 0.50 mass% or more, but if the content is excessive, the strength grade of the soft steel is departed from, and thus, it is necessary to be 0.85 mass% or less. From the viewpoint of ensuring corrosion resistance, the total content of Cu and Ni is preferably 0.55 mass% or more, and more preferably 0.60 mass% or more. In addition, from the viewpoint of an increase in strength, the total content of Cu and Ni is preferably 0.80 mass% or less, and more preferably 0.75 mass% or less. In addition, the Cu content is preferably 0.20 mass% or more, and more preferably 0.25 mass% or more. In addition, the Cu content is preferably 0.35 mass% or less, and more preferably 0.33 mass% or less. The Ni content is preferably 0.30 mass% or more, and more preferably 0.32 mass% or more. In addition, the Ni content is preferably 0.40 mass% or less, and more preferably 0.37 mass% or less.

[0083] [Cr: 0.01 mass% or more and 0.5 mass% or less]

[0084] Cr is an element effective for improving corrosion resistance. Cr has an effect of adjusting the pH value under the coating film to an appropriate value, suppressing the consumption of the base coating, and in addition, is an effective element for suppressing the progress of corrosion at a damaged portion of the coating film. Further, an appropriate amount of Cr is also effective for improving toughness, and is an element required for obtaining the mechanical properties required for a raw material for a ballast tank. In order to exert these effects, it is necessary to contain Cr in an amount of 0.01 mass% or more. However, if Cr is contained in excess, the interface pH value is excessively lowered, the outflow of Fe is promoted, and this becomes a factor for deterioration of corrosion resistance. Therefore, the Cr content needs to be 0.5 mass% or less. The Cr content is preferably 0.3 mass% or less, and preferably 0.05 mass% or more.

[0085] [Ti: 0.005 mass% or more and 0.20 mass% or less]

[0086] Ti is an element effective for improving corrosion resistance. Ti has an effect of densifying rust generated in a chloride corrosion environment, and is an element for suppressing the progress of corrosion at a damaged portion of the coating film. In order to exert such an effect, Ti is contained in an amount of 0.005 mass% or more. However, if the Ti content is excessive, Ti carbonitride is excessively precipitated, the strength is increased, and thus the upper limit is 0.20 mass%. The Ti content is preferably 0.008 mass% or more, and more preferably 0.010 mass% or more. The Ti content is preferably 0.15 mass% or less, and more preferably 0.10 mass% or less.

[0087] [Ca: 0.0001 mass% or more and 0.005 mass% or less]

[0088] Ca is an element effective for improving corrosion resistance. Ca has an effect of moderating the decrease in pH value due to the hydrolysis of Fe and / or Cr by pH buffering, and effectively suppresses the promotion of corrosion due to a decrease in pH value. Such an effect is effectively exerted by containing Ca in an amount of 0.0001 mass% or more. However, if Ca is contained in excess of 0.005 mass%, the elongation properties are deteriorated. Therefore, the Ca content is 0.0001 mass% or more and 0.005 mass% or less. The Ca content is preferably 0.0005 mass% or more, and more preferably 0.0010 mass% or more. The Ca content is preferably 0.004 mass% or less, and more preferably 0.003 mass% or less.

[0089] [N: 0.0001 mass% or more and 0.010 mass% or less]

[0090] N is an element that increases the tensile strength when dissolved in Fe, and therefore the content is preferably suppressed, with the N content being 0.010 mass% or less, preferably 0.0075 mass% or less, and more preferably 0.0070 mass% or less. On the other hand, N is an element that contributes to an increase in HAZ toughness by forming TiN. From the viewpoint of exerting this effect, the N content is 0.0001 mass% or more, preferably 0.0010 mass% or more, and more preferably 0.0020 mass% or more.

[0091] [Balance]

[0092] The balance is Fe and unavoidable impurities. As the unavoidable impurities, the mixing of trace elements (e.g., As, Sb, Sn, etc.) that can be brought in due to the conditions of raw materials, materials, manufacturing equipment, etc. is permitted. Also, for example, like P and S, in general, the smaller the content, the more preferable, and therefore although they are unavoidable impurities, there are also elements for which the composition range is separately specified as described above. Therefore, in the present specification, the case where the "unavoidable impurities" that constitute the balance are such that the elements for which the composition range is separately specified are removed from the concept.

[0093] Any other element can be contained as long as the characteristics of the thick steel sheet of the embodiments of the present application can be maintained. Hereinafter, other elements that can be selectively contained like this are exemplified.

[0094] [one or more selected from the group consisting of B: 0.0001 mass% or more and 0.010 mass% or less, V: 0.01 mass% or more and 0.50 mass% or less, and Nb: 0.001 mass% or more and 0.50 mass% or less]

[0095] B, V, and Nb are all elements that are effective for improving the mechanical characteristics, but increase the strength with an increase in content, and therefore the content is preferably suppressed. Therefore, when B, V, and Nb are contained, the upper limit of B is preferably 0.010 mass%, the upper limit of V is 0.50 mass%, and the upper limit of Nb is 0.50 mass%. On the other hand, because a small amount of addition of B has the effect of improving the characteristics of the welded portion, it is preferable to contain B in an amount of 0.0001 mass% or more. Because the toughness is improved by a small amount of addition of V and Nb, it is preferable to contain V in an amount of 0.01 mass% or more and Nb in an amount of 0.001 mass% or more. The more preferable lower limits of these elements are 0.0003 mass% for B, 0.02 mass% for V, and 0.005 mass% for Nb. The more preferable upper limits are 0.0090 mass% for B, 0.45 mass% for V, and 0.45 mass% for Nb.

[0096] [one or more selected from the group consisting of Co: 0.005 to 0.20% by mass, REM: 0.005 to 0.20% by mass, Zr: 0.005 to 0.20% by mass, and Mg: 0.0001 to 0.005% by mass]

[0097] Co is an element effective for improving corrosion resistance. Co has the effect of densifying rust generated in a chloride corrosion environment, and is an element that suppresses the progress of corrosion at a coating film damage portion. In order to exert such an effect, it is preferable to contain Co in an amount of 0.005% by mass or more. However, if the Co content is excessive, the weldability and / or hot workability deteriorate, and thus it is preferable for the Co content to be 0.20% by mass or less. A more preferable lower limit when Co is contained is 0.02% by mass, and a more preferable upper limit is 0.15% by mass.

[0098] Zr is an element effective for improving corrosion resistance. Zr, like Ti, has the effect of densifying rust generated in a chloride corrosion environment, and is an element that suppresses the progress of corrosion at a coating film damage portion. In order to exert such an effect, it is preferable to contain Zr in an amount of 0.005% by mass or more. However, if the Zr content is excessive, the weldability and / or hot workability deteriorate, and thus it is preferable for the Zr content to be 0.20% by mass or less. A more preferable lower limit when Zr is contained is 0.008% by mass, and a more preferable upper limit is 0.15% by mass.

[0099] Mg is an element effective for improving corrosion resistance. Mg, like Ca, has the effect of mitigating a decrease in pH, exerts an effect of suppressing corrosion promotion due to a decrease in pH, and is effective in suppressing coating film bulging. Such an effect is effectively exerted by containing 0.0001% by mass or more, and is particularly effective when Co is co-present. However, if more than 0.005% by mass is contained, the workability and weldability deteriorate, and thus it is not preferable. A preferable lower limit when Mg is contained is 0.0005% by mass, and a more preferable upper limit is 0.004% by mass.

[0100] REM (Rare Earth Metal) has the effect of suppressing a decrease in pH in the vicinity of the surface of a steel material in a use environment, and is an element effective for further improving corrosion resistance. This effect is exerted by these elements reacting with hydrogen ions through corrosion dissolution. In order to effectively exert such an effect, it is preferable to contain REM in an amount of 0.005% by mass or more. However, if the REM content is excessive, the weldability and / or hot workability deteriorate, and thus it is preferable for the REM content to be 0.20% by mass or less when REM is contained. A more preferable lower limit when REM is contained is 0.008% by mass, and a more preferable upper limit is 0.15% by mass. As REM for use in the embodiments of the present application, Sc, Y, lanthanoid elements, and the like can be cited.

[0101] 2. Metal structure

[0102] Hereinafter, the metal structure of the thick steel sheet of the embodiment of the present application is described.

[0103] [Ferrite: 70 area% or more]

[0104] The thick steel sheet of the embodiment of the present application contains 70% or more of ferrite in terms of area ratio with respect to the entire metal structure, and the balance contains one or more selected from the group consisting of pearlite, bainite, and martensite. In the embodiment of the present application, as described later, by appropriately controlling the hot rolling conditions, the ferrite can be sufficiently ensured. By containing 70 area% or more of ferrite, the desired low strength and high elongation characteristics can be obtained. The area ratio of ferrite is preferably 75% or more. From the viewpoint of ensuring the minimum strength of the soft steel grade, the area ratio of ferrite is preferably 95% or less, more preferably 92% or less.

[0105] [Ferrite grain size: 3 μm or more and 40 μm or less]

[0106] The thick steel sheet of the embodiment of the present application has a ferrite grain size of 3 μm or more, preferably 5 μm or more, in order to suppress excessive strength increase. On the other hand, in order to ensure the minimum strength, the ferrite grain size is 40 μm or less, preferably 35 μm or less, more preferably 30 μm or less.

[0107] 3. Characteristics

[0108] The thick steel sheet of the embodiment of the present application is excellent in corrosion resistance, and has low strength and high elongation characteristics. Hereinafter, these characteristics of the thick steel sheet of the embodiment of the present application are described in detail.

[0109] [Tensile strength (TS): 400 MPa or more and 520 MPa or less]

[0110] For internal navigation vessels, depending on the hull structure, low strength material (so-called soft steel) is used in many cases. Therefore, when a tensile test is performed using a test piece of NK-U1 number with a gauge length of 200 mm according to the Ship Classification Rules K of the Nippon Kaiji Kyokai (2019 edition), the tensile strength is 400 MPa or more and 520 MPa or less. The tensile strength is preferably 410 MPa or more, more preferably 420 MPa or more, and is preferably 510 MPa or less, more preferably 500 MPa or less.

[0111] [Breaking elongation (EL): 16% or more]

[0112] The elongation at break (i.e., steel sheet workability) is 16% or more when a tensile test is performed using a test piece of NK-U1 No. 200 mm gauge in accordance with the Ship Classification Rule K Code Material (2019 edition) of the Nippon Kaiji Kyokai. Thus, the operation time for part forming and the like at a construction site can be reduced. The elongation at break is preferably 18% or more, more preferably 20% or more, and further preferably 22% or more. On the other hand, the upper limit of the elongation at break is not particularly limited, but is about 45% if the chemical composition described above and the manufacturing method described below are taken into consideration.

[0113] [Uniform elongation (uEL): 13.5% or more]

[0114] The uniform elongation (i.e., uniform deformation performance) is 13.5% or more when a tensile test is performed using a test piece of NK-U1 No. 200 mm gauge in accordance with the Ship Classification Rule K Code Material (2019 edition) of the Nippon Kaiji Kyokai. Thus, the maintenance burden at the time of collision of a ship or the like with an object can be reduced. The uniform elongation is preferably 14.0% or more, more preferably 14.5% or more, and further preferably 15.0% or more. On the other hand, the upper limit of the uniform elongation is not particularly limited, but is about 30% if the chemical composition described above and the manufacturing method described below are taken into consideration.

[0115] [Corrosion resistance: average corrosion depth of the painted scratch portion after 168 days of the combined cycle corrosion test (CCT) is 0.600 mm or less]

[0116] If a certain or more corrosion depth is formed, the pH value of the corrosion portion decreases, the corrosion depth progresses, and the maintenance burden at the time of collision of a ship or the like with an object increases, and thus it is preferable that the corrosion depth not progress higher than 0.600 mm. In the embodiment of the present application, the average corrosion depth of the painted scratch portion after 168 days of the combined cycle corrosion test (CCT) is preferably 0.600 mm or less, more preferably 0.550 mm or less, and further preferably 0.500 mm or less. Also, in the present embodiment, by satisfying the chemical composition described above, a thick steel sheet having an average corrosion depth of 0.600 mm or less, i.e., excellent corrosion resistance, can be obtained.

[0117] 4. Manufacturing method

[0118] Next, the manufacturing method of the thick steel sheet of the embodiment of the present application will be described.

[0119] The present inventors have found that, for a steel having the above chemical composition, when hot-rolling is performed under the prescribed conditions described later, and then air-cooling is performed, a thick steel sheet having the above desired metal structure, and as a result, the above intended characteristics is obtained. Details thereof are described below. Also, the embodiments of the present application relate to a thick steel sheet, and in this field, the so-called thick steel sheet generally refers to a steel sheet having a thickness of 3.0 mm or more. The thickness of the thick steel sheet targeted in the embodiments of the present application is preferably 6.0 mm or more.

[0120] (Steelmaking)

[0121] The steelmaking method is not particularly limited, and a general steelmaking method can be employed.

[0122] (Heating before hot-rolling)

[0123] The heating conditions before hot-rolling are not particularly limited, but it is preferable to be able to perform the hot-rolling described later within a prescribed temperature range, for example, heating to 1100°C or higher.

[0124] (Hot-rolling)

[0125] The hot-rolling is performed in a manner so as to satisfy that the rolling temperature is 1100°C or lower, and the recrystallization reduction is 80% or more, and satisfy the following equation (1). The recrystallization temperature region varies depending on the steel grade. In the case of the steel grade targeted in the present embodiment, the recrystallization temperature region is 850°C or higher. Therefore, the recrystallization reduction in the present embodiment means the cumulative reduction under rolling (including finish rolling) at 850°C or higher. Also, in the present embodiment, the above "rolling temperature" and the "finish rolling completion temperature" described later mean the surface temperature of the steel sheet. In addition, in the present specification, in the case of "rolling temperature", it means the rolling temperature in the entire rolling including the rough rolling and the finish rolling. In addition, in the case of "finish rolling completion temperature", it means the rolling temperature at the time when the final pass of the finish rolling is completed. In addition, in the following equation (2), "ε yt (surface)", means the cumulative strain on only one side of the two surfaces of the steel sheet. Also, in the following equation (2), "ε yt (t / 4)", means the cumulative strain at the position of t / 4 from the single side of the t / 4 positions from the front and back surfaces of the steel sheet. In addition, the roll radius R of the following equation (5) is the roll radius of the finish rolling mill, and if the finish rolling mill is a multi-type rolling mill, it means the radius of the roll (i.e., the work roll) that contacts the rolled material.

[0126] 0.08FRT + 0.1Tom - 0.5ε T + 10 ≥ 69... (1)

[0127] wherein,

[0128] FRT (Finish Rolling Temperature): Finish rolling temperature (°C)

[0129] Tom: Time (sec) between the final pass and the pass immediately before the final pass

[0130] ε T : Total value of accumulated strain at each position of the surface, t / 4 position and t / 2 position of the steel sheet (hereinafter, referred to as "total accumulated strain"), calculated by the following equation (2). Also, the accumulated strain at each position is calculated by the following equation (3). Further, the strain of each pass is calculated by the following equation (4). Also, the equation (4) is described in the paper of "Hiroshi Yoshida, Interaction Analysis of Temperature, Rolling Load, and Metallographic Properties of Hot Strip Mill", Journal of the JSTP, vol. 38, no. 437 (1997-6)".

[0131] ε T = ε yt (surface) + ε yt (t / 4) + ε yt (t / 2)... (2)

[0132] ε yt = ε y1 + ε y2 + ε y3 +... + ε yn ... (3)

[0133] ε yi = C x (2y / h) 2 + 1.15 x ln(H / h)... (4)

[0134] In the equations (2) to (4),

[0135] ε y : Equivalent plastic strain at the y position in the sheet thickness direction,

[0136] ε yi : Equivalent plastic strain at the y position in the sheet thickness direction of the i-th pass,

[0137] y: Distance from the center of the sheet thickness (mm), h: Exit side sheet thickness (mm), H: Entry side sheet thickness (mm),

[0138] C = B1 x (H / 2R) B2 x r e 2 ... (5)

[0139] In the equation (5),

[0140] B1 = 0.18381 + 0.34435μ + 1.4086 x μ 2

[0141] B2 = 0.076669 - 2.0566μ + 2.1128 x μ 2

[0142] wherein μ: coefficient of friction, R: roll radius (mm), r e : reduction

[0143] By making the rolling temperature be 1100°C or less, it is possible to suppress the strength increase due to the refinement of ferrite grain diameter. The rolling temperature is preferably 1050°C or less, and more preferably 1000°C or less. The lower limit of the rolling temperature is preferably 750°C or more, and more preferably 780°C or more, from the viewpoint of suppressing the strength increase. By making the recrystallization reduction be 80% or more, it is possible to suppress the excessive decrease in strength due to the coarsening of grain diameter. The recrystallization reduction is preferably 85% or more, and more preferably 90% or more. The recrystallization reduction is preferably 99% or less, from the viewpoint of suppressing the refinement of grain diameter.

[0144] Next, the above formula (1) is described. In order to ensure high uniform elongation characteristics, it is necessary to contain ferrite structures with little processing strain. In the hot rolling of steel, processing strain is introduced by rolling, and on the other hand, processing strain is reduced by recovery, recrystallization, and phase transformation. As a result of repeated research and investigation by the present inventors and others on various manufacturing conditions related to processing strain introduction and strain reduction, the present inventors and others have found that the finish rolling temperature (FRT), the time between the final rolling pass (i.e., the time between the final pass and the preceding rolling pass, Tom), and the total cumulative strain (ε T ) have a very large effect. Each parameter is described in detail below.

[0145] The finish rolling temperature (FRT) is a parameter related to the processing strain that is reduced by recovery and recrystallization. The higher the FRT, the greater the driving force for recovery and recrystallization, and the greater the reduction in processing strain. Therefore, from the viewpoint of obtaining ferrite structures with little processing strain, it is preferable for the FRT to be high. The finish rolling temperature can be, for example, in the range of 1100°C to 750°C.

[0146] Tom is the time from the end of the rolling of the one pass before the final pass to the start of the rolling of the final pass. Tom is a parameter related to the reduction of the working strain due to recovery and recrystallization and the introduction of working strain in the temperature range of ferrite generation. The greater Tom is, the longer the time for recovery and recrystallization, and the greater the reduction of the amount of working strain accumulated. Therefore, from the viewpoint of obtaining a ferrite structure with little working strain, it is preferable that Tom be large. Tom can be, for example, in the range of 5 seconds to 120 seconds.

[0147] The total cumulative strain (ε T ) is a parameter indicating the amount of working strain introduced by hot rolling. The working strain introduced in hot rolling is considered to remain in the structure after the phase transformation although it is alleviated by the phase transformation. In other words, the strain remaining in the ferrite after the phase transformation is affected by the cumulative strain introduced in hot rolling, and the greater the cumulative strain, the more the strain remains in the ferrite, which is considered to deteriorate the uniform elongation characteristics. Therefore, from the viewpoint of obtaining a ferrite structure with little working strain, it is preferable that the method be one in which the total cumulative strain is small. The total cumulative strain can be, for example, in the range of 1.0 to 30.

[0148] As explained above, the working strain remaining after hot rolling varies depending on the "total cumulative strain", "FRT", and "Tom" introduced in hot rolling. Therefore, the present inventors and others further conducted various studies and found the left side of Equation (1) using these as parameters. As a result, the present inventors and others found that if the left side of Equation (1) is less than 69, it is difficult to satisfy both low strength (i.e., 520 MPa or less) and high elongation characteristics (i.e., EL > 16% and uEL > 13.5%). Therefore, the value of the left side of Equation (1) is made 69 or more. The value of the left side of Equation (1) is preferably 75 or more, and more preferably 80 or more. From the viewpoint of ensuring the above characteristics, the upper limit of the value of the left side of Equation (1) is not particularly limited, but if the manufacturing conditions and the like of the embodiments of the present application are taken into consideration, the upper limit is approximately 100 or so.

[0149] (Cooling)

[0150] After the above hot rolling, no quenching such as water cooling is performed, and cooling to room temperature is performed by air cooling. This is because if water cooling or the like is performed, hard structures such as bainite, martensite, and island-shaped martensite (MA: Martensite-Austenite constituent) are generated, and the strength increases. In addition, if the hard structures such as MA are dispersed in the steel, the elongation characteristics deteriorate, so the phase transformation is performed by air cooling to ensure the desired ferrite fraction. Furthermore, if water cooling or the like with a large cooling rate is performed, the undercooling degree becomes large and the grains are refined. By performing air cooling, the refinement can be suppressed, and the increase in strength can be suppressed.

[0151] Example

[0152] (1) Preparation method of test material

[0153] A steel sheet having the chemical composition described in Table 1 was produced by converter melting. Thereafter, the steel sheet was heated to 1000°C or higher and 1230°C or lower before hot rolling, and hot rolling and cooling were performed under the conditions described in Table 2 to produce a steel sheet having the thickness described in Table 2. Also, the rolling temperature was 1100°C or lower. In addition, in the derivation of the formula (1) described in Table 2 in the present embodiment, the friction coefficient μ was "0.55". The reduction r e For the sake of simplifying the calculation, "0.3" as an average value can be used in the production of thick plate products. In addition, in Tables 1 and 2, and Tables 3 and 4 described later, the values underlined indicate a range departing from the embodiments of the present application. However, it should be noted that as for "-", even if it departs from the range of the embodiments of the present application, it is not underlined.

[0154] [Table 1]

[0155]

[0156] [Table 2]

[0157]

[0158] (2) Microstructure observation

[0159] For No. 1 and No. 6 to No. 8 of Tables 1 and 2, microstructure observation was performed in the following manner. A test piece was cut from the steel sheet such that the observation surface was parallel to the surface of the steel sheet and the observation position was at the t / 4 position (t: thickness of the steel sheet), and optical microscope observation was performed at 100x to 400x after nital etching. In each of 10 fields of view (field of view area: 150 μm x 200 μm) of the optical microscope, the area ratio of ferrite was analyzed using image analysis software (Image-J), and the average value of the area ratio of ferrite was calculated for each sample. In this way, the area ratio of ferrite was calculated for each sample.

[0160] The ferrite grain size was determined as follows. A grid at an interval of 25 μm was drawn in one field of view (field of view area: 150 μm x 200 μm) of the optical microscope image at 400x, the number of ferrite grains intersected by one straight line was counted, the length of the straight line in the field of view was divided by the number, and the grain size of each ferrite grain on the straight line was obtained. Then, the grain size of each ferrite grain was also obtained on other straight lines, and the average value thereof was calculated as the ferrite grain size.

[0161] The ferrite area ratios of No. 1, 6, 7, and 8 are 80%, 82%, 84%, and 83% respectively, meeting the requirements of the embodiments of the present invention. Additionally, the ferrite particle sizes of No. 1, 6, 7, and 8 are 7 μm, 22 μm, 16 μm, and 14 μm respectively, meeting the requirements of the embodiments of the present invention. Moreover, the remaining microstructure of No. 1, 6, 7, and 8 is all pearlite.

[0162] (3) Tensile test method

[0163] The test piece for the tensile test is extracted from the rolled material such that the longitudinal direction (tensile direction) is orthogonal to the rolling direction. The shape of the test piece is in accordance with NK-U1 of the ship classification rules K part of the Japan Maritime Association (2019 edition), with a gauge length (GL) of 200 mm and a total thickness shape, and thus is carried out with the plate thickness as described in Table 2. The yield point (YP: Yield Point) uses the upper yield point, but when the upper yield does not appear, the 0.2% yield strength is taken as the yield point. The tensile test conditions are carried out in accordance with the NK ship classification standard K part. Specifically, before reaching the yield point, the tensile speed is 20 N / mm 2 , and after reaching the yield point, the strain rate is 30 PL% per minute. Additionally, the uniform elongation rate is calculated as the elongation rate at the highest load in the stress-strain diagram. A specimen with a tensile strength (TS) of 400 MPa or more and 520 MPa or less, a fracture elongation rate (EL) of 16% or more, and a uniform elongation rate (uEL) of 13.5% or more is considered qualified. The test results are shown in Table 3. Moreover, the yield strength (YS) is also described in Table 3.

[0164] (4) Corrosion test method (CCT conditions)

[0165] The corrosion test piece, as [[ID=!5]] Figure 1 shown, a 15 μm Zn-rich coating is applied to the surface of a 5t×90W×120L (mm) steel, a 160 μm modified epoxy resin is applied and cured thereon, and then another 160 μm modified epoxy resin is applied and cured, resulting in a total coating thickness of 335 μm. In the central part of the coated test piece, a coating cut part (i.e., a coating scratch part) is artificially added using a plastic cutting machine, and masking tape is used on the outer periphery of the test piece to adjust the evaluation area to 80 cm 2 .

[0166] The corrosion test, as a laboratory evaluation test simulating the inside of a ballast tank, is carried out as follows. To simulate the environment on the back of the upper deck of a real ship's ballast tank, as Figure 2As shown, first, "spraying with artificial sea water at 35°C ± 1°C for 2 hours, drying at 60°C ± 1°C, 45% to 55% RH for 4 hours, and then maintaining at 50°C ± 1°C at a relative humidity higher than 95% RH for 2 hours" was performed for 7 days (hereinafter, this 7-day cycle will be referred to as "1st cycle"). Thereafter, "maintaining at 35°C ± 1°C at a relative humidity higher than 95% RH for 2 hours, drying at 60°C ± 1°C, 45% to 55% RH for 4 hours, and then maintaining at 50°C ± 1°C at a relative humidity higher than 95% RH for 2 hours" was performed for 7 days (hereinafter, this 7-day cycle will be referred to as "2nd cycle"). The 1st cycle and the 2nd cycle were alternately performed for 168 days. The test pieces were set to be inclined at 15° to 25° from the vertical direction. The test machine was adjusted so that the amount of artificial sea water sprayed was 1.5 ± 0.5 mL / 80 cm 2 / h.

[0167] Five pieces (total 10 pieces) of the above test pieces of the inventive steel (No. B of Table 1) and the conventional steel (No. A of Table 1) were prepared. Thereafter, the above corrosion test was performed, and the corrosion depth after the removal of the coating film was measured at 6 points at 10 mm intervals from the end of the scratch (refer to Figure 3 ). The average value of the 6 points was taken as the corrosion depth of each test piece, and the average value of 5 pieces was calculated to calculate the corrosion depth. The sample having a corrosion depth of 0.600 mm or less was qualified. The calculation results are shown in Table 4.

[0168] [Table 3]

[0169]

[0170] [Table 4]

[0171]

[0172] The above observation results, and the results of Table 3 and Table 4 were analyzed.

[0173] Example Nos. 1 and 6 to 8, because the requirements of the chemical composition and the manufacturing conditions of the embodiment of the present application are satisfied, the desired metal structure, the tensile strength (TS) and the elongation properties (EL, uEL) are good. In addition, Example Nos. 2 to 5, because the requirements of the chemical composition and the manufacturing conditions of the embodiment of the present application are satisfied, it can be considered that, as with Example Nos. 1 and 6 to 8, the desired metal structure can be obtained, as a result, the tensile strength (TS) and the elongation properties (EL, uEL) are good. In addition, Example Nos. 1 to 8, because Cu + Ni and Cr are contained in a prescribed amount, it is considered that the corrosion resistance is also excellent. In order to confirm this, as described above, corrosion tests using No. A and No. B were also performed. As a result, Example No. B, which satisfies the requirements of the chemical composition and the manufacturing conditions of the embodiment of the present application as with Example Nos. 1 to 8, it was confirmed that the corrosion resistance was good.

[0174] On the other hand, Comparative Example Nos. 9 to 11, because they do not satisfy formula (1), the tensile strength (TS) and the uniform elongation (uEL) are poor. In addition, although the elongation at break satisfies, if compared with the examples, it is a lower value. As a reason why formula (1) is not satisfied, it is considered that because FRT is low, Tom is low, and the cumulative strain εT is large in No. 9, and because FRT is low and the cumulative strain εT is large in No. 10, No. 11. Figure 4 is a graph showing the relationship of the left side of formula (1) with the tensile strength. Figure 5 is a graph showing the relationship of the left side of formula (1) with the uniform elongation. If referring to Figure 4 and Figure 5 , it is known that the left side of formula (1) is 69 or more, resulting in a decrease in the tensile strength and an increase in the uniform elongation. In addition, Comparative Example No. A, because it does not satisfy the requirements of the chemical composition and the manufacturing conditions of the embodiment of the present application, the corrosion depth exceeds 0.600 mm, and the corrosion resistance is poor.

[0175] Industrial applicability

[0176] The thick steel plate of the embodiment of the present application can be widely applied to the fields requiring improvement in the corrosion resistance and / or the field workability (that is, improvement in the elongation properties) of the coating, although not particularly limited, the thick steel plate for ships such as the upper deck of the ship's ballast tank can be cited as a specific application target.

[0177] This application claims priority based on Japanese Patent Application, No. 2020-168729, filed on October 5, 2020. Japanese Patent Application, No. 2020-168729 is incorporated by reference in the present specification.

Claims

1. A low-strength thick steel sheet excellent in elongation properties and corrosion resistance, comprising C: 0.01 mass% or more and 0.30 mass% or less, Si: 0.01 mass% or more and 2.0 mass% or less, Mn: 0.85 mass% or more and 2.00 mass% or less, P: more than 0 mass% and 0.015 mass% or less, S: more than 0 mass% and 0.005 mass% or less, Al: 0.005 mass% or more and 0.10 mass% or less, Cr: 0.01 mass% or more and 0.5 mass% or less, Ti: 0.005 mass% or more and 0.20 mass% or less, Ca: 0.0001 mass% or more and 0.005 mass% or less, 0.0001 mass% or more and 0.010 mass% or less, and Cu + Ni: 0.50 mass% or more and 0.85 mass% or less, with the balance consisting of Fe and unavoidable impurities, a metal structure comprising 70% or more of ferrite in terms of area ratio with respect to the entire metal structure, with the balance comprising one or more selected from the group consisting of pearlite, bainite, and martensite, a ferrite grain size of 3 μm or more and 40 μm or less, a tensile strength of 400 MPa or more and 520 MPa or less, an elongation at break of 16% or more, and a uniform elongation of 13.5% or more when a tensile test is performed using a test piece of NK-U1 No. of 200 mm in gauge length according to the Ship Classification Rules K of the Nippon Kaiji Kyokai, Inc. in 2019, and an average corrosion depth of 0.600 mm or less at a painted scratch portion after a cyclic corrosion test for 168 days. Further comprising at least one of (a) and (b) below: (a) one or more selected from the group consisting of B: 0.0001 mass% or more and 0.010 mass% or less, V: 0.01 mass% or more and 0.50 mass% or less, and Nb: 0.001 mass% or more and 0.50 mass% or less; and (b) one or more selected from the group consisting of REM: 0.005 mass% or more and 0.20 mass% or less, and Zr: 0.005 mass% or more and 0.20 mass% or less. including the following steps: a step of heating a steel satisfying the composition described in claim 1 or 2 to 1100°C or higher; a step of performing hot rolling so that the rolling temperature is 1100°C or lower and the recrystallization reduction is 80% or higher, in a manner satisfying the following formula (1): a step of performing air cooling, wherein FRT: finish rolling temperature, unit: °C, Tom: time between the final pass and the pass immediately before the final pass, unit: seconds, in formulas (2) to (4), y: distance from the center of the plate thickness, unit: mm, h: exit side plate thickness, unit: mm, H: entry side plate thickness, unit: mm, N： in formula (5), including the following steps: a step of heating a steel satisfying the composition described in claim 3 to 1100°C or higher; a step of performing hot rolling so that the rolling temperature is 1100°C or lower and the recrystallization reduction is 80% or higher, in a manner satisfying the following formula (1): a step of performing air cooling, wherein 2. The thick steel plate according to claim 1, wherein ​ 3. The thick steel plate according to claim 1 or 2, wherein ​ ​ ​ 4. A method of manufacturing a thick steel plate as claimed in claim 1 or 2, wherein, ​ ​ ​ ​ 0.08FRT + 0.1 Tom - 0.5 ε T + 10 ≥ 69... (1) ​ ​ ​ ε T : is a total value of accumulated strains at each position of the surface of the steel sheet, t / 4 position, and t / 2 position, calculated by the following equation (2), wherein t is the sheet thickness, further, the accumulated strain at each position is calculated by the following equation (3), and the strain of each pass is calculated by the following equation (4), ε T = ε yt (surface) + ε yt (t / 4) + ε yt (t / 2) …(2) ε yt = ε y1 + ε y2 + ε y3 +... + ε yn ... (3) ε yi = C x (2y / h) 2 + 1.15 x ln(H / h) … (4) ​ ε y : equivalent plastic strain in the plate thickness direction y position, ε yi : equivalent plastic strain at the y position in the sheet thickness direction of the i-pass, ​ C = B1 x (H / 2R) B2 x r e 2 … (5) ​ B1 = 0.18381 + 0.34435μ + 1.4086 x μ 2 B2 = 0.076669 - 2.0566μ + 2.1128μ2 2 wherein μ: friction coefficient, R: roll radius, unit: mm, r e : reduction.

5. A method of manufacturing a thick steel plate as set forth in claim 3, wherein ​ ​ ​ ​ 0.08FRT + 0.1 Tom - 0.5 ε T + 10 ≥ 69... (1) ​ FRT: finish rolling temperature, unit: °C Tom: time between the last pass and the pass before the last pass, unit: seconds ε T : is a total value of accumulated strains at each position of the surface of the steel sheet, t / 4 position, and t / 2 position, calculated by the following equation (2), wherein t is the sheet thickness, further, the accumulated strain at each position is calculated by the following equation (3), and the strain of each pass is calculated by the following equation (4), ε T = ε yt (surface) + ε yt (t / 4) + ε yt (t / 2) …(2) ε yt = ε y1 + ε y2 + ε y3 +... + ε yn ... (3) ε yi = C x (2y / h) 2 + 1.15 x ln(H / h) … (4) In formulas (2) to (4), ε y : equivalent plastic strain in the plate thickness direction y position, ε yi : equivalent plastic strain at the y position in the sheet thickness direction of the i-pass, y: distance from the center of the plate thickness, unit: mm, h: exit side plate thickness, unit: mm, H: entry side plate thickness, unit: mm C = B1 x (H / 2R) B2 x r e 2 … (5) In formula (5), B1 = 0.18381 + 0.34435μ + 1.4086 x μ 2 B2 = 0.076669 - 2.0566μ + 2.1128μ2 2 wherein μ: friction coefficient, R: roll radius, unit: mm, r e : reduction.

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