JP7350705B2 - Low-strength thick steel plate with excellent elongation properties and corrosion resistance - Google Patents

Description

The present invention relates to a low-strength thick steel plate with excellent elongation properties and corrosion resistance.

Ships' ballast tanks inject and discharge seawater in response to changes in loading conditions, so the steel used is exposed to an extremely harsh corrosive environment that involves repeated immersion in seawater and exposure to humid air containing salt. exposed to If corrosion damage occurs in a ballast tank, there is a risk of sinking due to holes or the spillage of cargo such as crude oil or chemicals into the ocean, so it is necessary to apply some kind of anti-corrosion measure to the steel material, and one of the methods is There is also anti-corrosion coating. However, anticorrosive coating requires repainting as the coating deteriorates, resulting in high maintenance costs and environmental burden. In recent years, reducing environmental impact has become one of the important factors for realizing a sustainable society. Therefore, it is necessary to improve the corrosion resistance of steel materials from the viewpoint of reducing not only maintenance costs but also environmental loads. Steel materials for ships aimed at improving corrosion resistance are disclosed in, for example, Patent Document 1 and Patent Document 2.

Japanese Patent Application Publication No. 2015-151571 Japanese Patent Application Publication No. 2010-126765

In the case of coastal ships, low-strength materials with a tensile strength (TS) of 520 MPa or less, for example, so-called mild steel, are often used due to their hull structure. On the other hand, as a means to improve corrosion resistance, it is common to add alloying elements that serve as corrosion resistance improving elements. However, adding alloying elements generally increases strength, making it difficult to achieve both corrosion resistance and low strength.

Furthermore, in recent years, the realization of a low-carbon society has become important as part of the realization of a sustainable society. If steel plate workability can be improved, it will lead to a reduction in work time for parts forming on site, and ultimately contribute to energy savings and CO ₂ emissions reduction. Therefore, it is also necessary to ensure a predetermined steel sheet workability (that is, improvement in elongation at break EL). Furthermore, when a ship or the like collides with an object, the parts undergo plastic deformation and cracks occur, which increases the maintenance load, which is undesirable from the perspective of realizing a low-carbon society. Therefore, from the viewpoint of uniform deformation when external force is applied, a predetermined uniform elongation characteristic (uEL) is also required.

The present invention has been made in view of these circumstances, and its purpose is to provide a thick steel plate that has both excellent corrosion resistance, low strength, and high elongation characteristics.

Aspect 1 of the present invention is
C: 0.01% by mass or more, 0.30% by mass or less,
Si: 0.01% by mass or more, 2.0% by mass or less,
Mn: 0.85% by mass or more, 2.00% by mass or less,
P: more than 0 mass%, 0.015 mass% or less,
S: more than 0% by mass, 0.005% by mass or less,
Al: 0.005% by mass or more and 0.10% by mass or less,
Cr: 0.01% by mass or more, 0.5% by mass or less,
Ti: 0.005% by mass or more and 0.20% by mass or less,
Ca: 0.0001% by mass or more, 0.005% by mass or less,
Contains N: 0.0001% by mass or more and 0.010% by mass or less, and Cu + Ni: 0.50% by mass or more and 0.85% by mass or less, the remainder consisting of Fe and inevitable impurities,
The metal structure contains ferrite in an area ratio of 70% or more with respect to the total metal structure, and the remainder consists of one or more types 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 was performed using a test piece of NK-U1 with a gauge length of 200 mm in accordance with Nippon Kaiji Kyokai's Classification Rules, Part K Materials (2019 edition), the tensile strength was 400 MPa or more and 520 MPa or less, and the elongation at break was It is a low-strength thick steel plate with excellent elongation characteristics and corrosion resistance, with a uniform elongation of 16% or more and a uniform elongation of 13.5% or more.

Aspect 2 of the present invention is
The thick steel plate according to Aspect 1, wherein the average corrosion depth of the painted scratch portion after 168 days of the combined cycle test is 0.600 mm or less.

Aspect 3 of the present invention is
B: 0.0001% by mass or more, 0.010% by mass or less,
Embodiment 1 or embodiment further containing one or more selected from the group consisting of V: 0.01% by mass or more and 0.50% by mass or less, and Nb: 0.001% by mass or more and 0.50% by mass or less It is the thick steel plate according to No. 2.

Aspect 4 of the present invention is
Co: 0.005% by mass or more, 0.20% by mass or less,
REM: 0.005% by mass or more, 0.20% by mass or less,
Embodiments 1 to 3 further containing one or more selected from the group consisting of Zr: 0.005% by mass or more and 0.20% by mass or less, and Mg: 0.0001% by mass or more and 0.005% by mass or less. The thick steel plate according to any one of the above.

Aspect 5 of the present invention is
A step of heating steel satisfying the composition according to any one of aspects 1 to 4 to 1100 ° C. or higher,
A step of hot rolling at a rolling temperature of 1100° C. or lower and a recrystallization reduction rate of 80% or higher to satisfy the following formula (1);
The method for manufacturing a thick steel plate according to any one of aspects 1 to 4, including the step of air cooling.
0.08FRT+0.1Tom-0.5ε _T +10≧69...(1)
however,
FRT: Finish rolling completion temperature (℃)
Tom: Time between rolling passes from the last pass to the last pass (seconds)
ε _T : Total value of cumulative strain at each position on the surface of the steel plate, t/4 position and t/2 position (t: plate thickness), and is calculated by the following formula (2). Note that the cumulative distortion at each position is calculated using the following equation (3). Further, the distortion of each path is calculated using 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×(2y/h) ² +1.15×ln(H/h)...(4)
In formulas (2) to (4),
ε _y : Equivalent plastic strain at the y position in the plate thickness direction,
ε _yi : Equivalent plastic strain at the y position in the thickness direction of the i-th pass,
y: Distance from the center of plate thickness (mm), h: Output side plate thickness (mm), H: Inlet side plate thickness (mm),
C=B ₁ × (H/2R) ^B2 × r _e ² ... (5)
In formula (5),
B ₁ =0.18381+0.34435μ+1.4086× ^μ2
B ₂ =0.076669-2.0566μ+2.1128×μ ²
However, μ: friction coefficient, R: roll radius (mm), r _e : rolling reduction rate

According to the present invention, it is possible to provide a thick steel plate that has both excellent corrosion resistance, low strength, and high elongation characteristics.

FIG. 1 is a schematic diagram showing a corrosion test piece according to an example. FIG. 2 is a diagram for explaining the corrosion test method according to the example. FIG. 3 is a schematic diagram for explaining the corrosion depth measurement position according to the example. FIG. 4 is a graph showing the relationship between the left side of equation (1) and tensile strength. FIG. 5 is a graph showing the relationship between the left side of equation (1) and uniform elongation.

In order to ensure excellent corrosion resistance, the present inventors conducted extensive research in order to realize low strength and excellent elongation properties on the premise that certain amounts of Cu, Ni, and Cr are contained. As a result, by manufacturing steel with a predetermined chemical composition under the recommended conditions, it is possible to produce steel with a predetermined chemical composition and contain ferrite with an area ratio of 70% or more based on the total metal structure, with the remainder being pearlite. , bainite, and martensite, 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. It has been found that a low-strength thick steel plate with excellent elongation properties and corrosion resistance can be obtained.

1. Chemical Composition The chemical composition of the thick steel plate according to the embodiment of the present invention will be described below. First, the basic elements C, Si, Mn, P, S, Al, Cr, Ti, Ca, N, Cu, and Ni will be explained, and further elements that may be added selectively will be explained.

[C: 0.01% by mass or more, 0.30% by mass or less]
C is an element necessary to ensure the strength of the material. If C is contained excessively, low strength cannot be obtained due to an increase in strength. On the other hand, if the amount of C is reduced too much, it becomes difficult to obtain the minimum strength as a structural member, that is, approximately 400 MPa. Therefore, the C content was set to 0.01% by mass or more and 0.30% by mass or less. Note that the above minimum strength varies depending on the thickness of the steel material used. The C content is preferably 0.05% by mass or more, more preferably 0.10% by mass or more. Further, the C content is preferably 0.20% by mass or less, more preferably 0.15% by mass or less.

[Si: 0.01% by mass or more, 2.0% by mass or less]
Si is an element necessary for deoxidation and ensuring strength. Si increases the strength by being dissolved in ferrite, but if the Si content exceeds 2.0% by mass, the strength increases and low strength cannot be obtained. On the other hand, if the Si content is less than 0.01% by mass, the minimum strength as a structural member cannot be ensured. Therefore, the Si content was set to 0.01% by mass or more and 2.0% by mass or less. The Si content is preferably 1.00% by mass or less, more preferably 0.50% by mass or less, and even more preferably 0.30% by mass or less. Further, the Si content is preferably 0.10% by mass or more, more preferably 0.15% by mass or more.

[Mn: 0.85 mass% or more, 2.00 mass% or less]
Like Si, Mn is also necessary for deoxidation and ensuring strength, and if it is less than 0.85% by mass, the minimum strength as a structural member cannot be ensured. However, if Mn is contained excessively, the strength will increase, making it impossible to obtain low strength. Therefore, the Mn content was set to 0.85% by mass or more and 2.00% by mass or less. The Mn content is preferably 0.90% by mass or more, more preferably 0.95% by mass or more. Further, the Mn content is preferably 1.20% by mass or less, more preferably 1.10% by mass or less.

[P: more than 0 mass%, 0.015 mass% or less]
Like Si, P is an element that exhibits a large amount of solid solution strengthening, and in the present invention, which aims at low strength, the smaller the content, the better. Furthermore, since toughness and weldability are also deteriorated, the upper limit of the allowable P content is set to 0.015% by mass.

[S: more than 0 mass%, 0.005 mass% or less]
Since S is an element that decreases ductility when the content increases, it is preferable to suppress the content, and the allowable upper limit of the S content is up to 0.005% by mass.

[Al: 0.005% by mass or more, 0.10% by mass or less]
Al is an element that forms a dense oxide film and improves corrosion resistance. On the other hand, Al is an element that contributes to an increase in strength, although the amount of solid solution strengthening is smaller than that of Si and P. Therefore, the amount of Al added was set to 0.005% by mass or more and 0.10% by mass or less. The Al content is preferably 0.030% by mass or more, more preferably 0.045% by mass or more. Further, the Al content is preferably 0.090% by mass or less, more preferably 0.080% by mass or less.

[Cu+Ni: 0.50 mass% or more, 0.85 mass% or less]
Cu and Ni are elements effective in improving corrosion resistance. Both Cu and Ni have the effect of suppressing the corrosion reaction that occurs under the anticorrosive coating, and are elements that have the effect of suppressing coating blistering due to corrosion under the coating. In addition, Cu and Ni also have the effect of densifying the rust formed when the steel material is corroded in the defective parts of the paint film, and they have the effect of suppressing the progress of corrosion in the scratched parts of the paint film. It is a valid element. In order to exhibit these effects, it is necessary that the total content of Cu and Ni be 0.50% by mass or more; however, if excessively contained, the strength class of mild steel will be exceeded. It needs to be 85% by mass or less. From the viewpoint of ensuring corrosion resistance, the total content of Cu and Ni is preferably 0.55% by mass or more, and a more preferable lower limit is 0.60% by mass or more. Further, from the viewpoint of increasing strength, the total content of Cu and Ni is preferably 0.80% by mass or less, more preferably 0.75% by mass or less. Further, the Cu content is preferably 0.20% by mass or more, more preferably 0.25% by mass or more. Further, the Cu content is preferably 0.35% by mass or less, more preferably 0.33% by mass or less. The Ni content is preferably 0.30% by mass or more, more preferably 0.32% by mass or more. Further, the Ni content is preferably 0.40% by mass or less, more preferably 0.37% by mass or less.

[Cr: 0.01% by mass or more, 0.5% by mass or less]
Cr is an element effective in improving corrosion resistance. Cr has the effect of adjusting the pH under the paint film to a suitable value and suppressing primer consumption, and is an effective element for suppressing the progress of corrosion in scratched parts of the paint film. In addition, an appropriate amount of Cr is effective in improving toughness, and is also a necessary element in order to obtain the necessary mechanical properties as a ballast tank material. In order to exhibit these effects, it is necessary to contain Cr in an amount of 0.01% by mass or more. However, when Cr is contained excessively, the interfacial pH is lowered too much, which promotes the outflow of Fe and causes deterioration of corrosion resistance. Therefore, the Cr content needs to be 0.5% by mass or less. The Cr content is preferably 0.3% by mass or less, and preferably 0.05% by mass or more.

[Ti: 0.005% by mass or more, 0.20% by mass or less]
Ti is an element effective in improving corrosion resistance. Ti has the effect of densifying rust generated in a chloride corrosion environment, and is an element that suppresses the progress of corrosion in scratched parts of the paint film. In order to exhibit such effects, Ti is contained in an amount of 0.005% by mass or more. However, if the Ti content becomes excessive, Ti carbonitrides will precipitate excessively and increase the strength, so the upper limit is set to 0.20% by mass. The Ti content is preferably 0.008% by mass or more, more preferably 0.010% by mass or more. The Ti content is preferably 0.15% by mass or less, more preferably 0.10% by mass or less.

[Ca: 0.0001 mass% or more, 0.005 mass% or less]
Ca is an element effective in improving corrosion resistance. Due to its pH buffering effect, Ca has the effect of alleviating the pH drop caused by the hydrolysis of Fe and Cr, and is effective in suppressing corrosion acceleration due to the pH drop. These effects are effectively exhibited by containing 0.0001% by mass or more of Ca. However, if Ca is contained excessively in excess of 0.005% by mass, the elongation properties will deteriorate. Therefore, the Ca content is set to 0.0001% by mass or more and 0.005% by mass or less. The Ca content is preferably 0.0005% by mass or more, more preferably 0.0010% by mass or more. The Ca content is preferably 0.004% by mass or less, more preferably 0.003% by mass or less.

[N: 0.0001% by mass or more, 0.010% by mass or less]
Since N is an element that increases tensile strength when dissolved in Fe, it is preferable to suppress the content, and the N content is 0.010% by mass or less, preferably 0.0075% by mass or less, more preferably It is 0.0070% by mass or less. On the other hand, N is an element that contributes to improving HAZ toughness by forming TiN. From the viewpoint of exhibiting this effect, the N content is 0.0001% by mass or more, preferably 0.0010% by mass or more, and more preferably 0.0020% by mass or more.

[Remainder]
The remainder is Fe and inevitable impurities. As unavoidable impurities, trace elements (for example, As, Sb, Sn, etc.) brought in depending on the conditions of raw materials, materials, manufacturing equipment, etc. are allowed. Note that, for example, there are elements such as P and S, which are preferably contained in smaller amounts and are therefore unavoidable impurities, but whose composition ranges are separately specified as described above. Therefore, in this specification, the term "inevitable impurities" constituting the remainder is a concept excluding elements whose composition range is separately defined.

Any other element may be further included as long as the characteristics of the thick steel plate according to the embodiment of the present invention can be maintained. Examples of other elements that can be selectively included are shown below.

[B: 0.0001 mass% or more, 0.010 mass% or less, V: 0.01 mass% or more, 0.50 mass% or less, and Nb: 0.001 mass% or more, 0.50 mass% or less One or more types selected from the group consisting of]
B, V, and Nb are all elements that are effective in improving mechanical properties, but an increase in their content leads to an improvement in strength, so it is preferable to suppress their content. Therefore, when B, V and Nb are contained, it is preferable that the upper limit of B is 0.010% by mass, the upper limit of V is 0.50% by mass, and the upper limit of Nb is 0.50% by mass. On the other hand, since the addition of a small amount of B has the effect of improving the properties of the weld zone, it is preferable that B is contained in an amount of 0.0001% by mass or more. Since V and Nb improve toughness when added in small amounts, it is preferable to contain V at 0.01% by mass or more and Nb at 0.001% by mass or more. More preferable lower limits for these elements are 0.0003% by mass for B, 0.02% by mass for V, and 0.005% by mass for Nb. More preferable upper limits are 0.0090% by mass for B, 0.45% by mass for V, and 0.45% by mass for Nb.

[Co: 0.005 mass% or more, 0.20 mass% or less, REM: 0.005 mass% or more, 0.20 mass% or less, Zr: 0.005 mass% or more, 0.20 mass% or less, and Mg: one or more types selected from the group consisting of 0.0001% by mass or more and 0.005% by mass or less]
Co is an element effective in 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 in scratched parts of the paint film. In order to exhibit such effects, it is preferable that Co be contained in an amount of 0.005% by mass or more. However, if the Co content becomes excessive, weldability and hot workability deteriorate, so the Co content is preferably 0.20% by mass or less. When Co is contained, a more preferable lower limit is 0.02% by mass, and a more preferable upper limit is 0.15% by mass.

Zr is an element effective in 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 in scratched parts of the paint film. In order to exhibit these effects, Zr is preferably contained in an amount of 0.005% by mass or more. However, if the Zr content becomes excessive, weldability and hot workability deteriorate, so the Zr content is preferably 0.20% by mass or less. When containing Zr, a more preferable lower limit is 0.008% by mass, and a more preferable upper limit is 0.15% by mass.

Mg is an element effective in improving corrosion resistance. Mg, like Ca, has the effect of alleviating a pH drop, exhibits the effect of suppressing corrosion acceleration due to a pH drop, and is effective in suppressing paint film blistering. These effects are effectively exhibited by containing 0.0001% by mass or more, and are particularly effective when Co coexists. However, if the content exceeds 0.005% by mass, it is not preferable because it deteriorates workability and weldability. When Mg is contained, a preferable lower limit is 0.0005% by mass, and a more preferable upper limit is 0.004% by mass.

REM (Rare Earth Metal) has the effect of suppressing a decrease in pH near the surface of steel materials in the usage environment, and is an effective element for further improving corrosion resistance. This effect is achieved when these elements corrode and dissolve and react with hydrogen ions. In order to effectively exhibit these effects, it is preferable to contain REM in an amount of 0.005% by mass or more. However, an excessive REM content deteriorates weldability and hot workability, so when REM is included, it is preferably 0.20% by mass or less. A more preferable lower limit for containing REM is 0.008% by mass, and a more preferable upper limit is 0.15% by mass. Examples of REM used in embodiments of the present invention include Sc, Y, lanthanoids, and the like.

2. Metal structure The metal structure of the thick steel plate according to the embodiment of the present invention will be explained below.

[Ferrite: 70 area% or more]
The thick steel plate according to the embodiment of the present invention contains ferrite in an area ratio of 70% or more with respect to the total metal structure, and the remainder consists of one or more types selected from the group consisting of pearlite, bainite, and martensite. Furthermore, in the present invention, as will be described later, by appropriately controlling the hot rolling conditions, a sufficient amount of ferrite can be ensured. By containing ferrite in an area of 70% or more, desired low strength and high elongation characteristics can be obtained. The area ratio of ferrite is preferably 75% or more. The area ratio of ferrite is preferably 95% or less, more preferably 92% or less, from the viewpoint of ensuring a minimum strength comparable to mild steel.

[Ferrite grain size: 3 μm or more and 40 μm or less]
The thick steel plate according to the embodiment of the present invention has a ferrite grain size of 3 μm or more, preferably 5 μm or more, in order to suppress an excessive increase in strength. 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, and more preferably 30 μm or less.

3. Properties The thick steel plate according to the embodiment of the present invention has excellent corrosion resistance, low strength, and high elongation characteristics. These characteristics of the thick steel plate according to the embodiment of the present invention will be explained in detail below.

[Tensile strength (TS): 400 MPa or more, 520 MPa or less]
When used in domestic vessels, low-strength materials (so-called mild steel) are often used due to the hull structure. Therefore, the tensile strength is 400 MPa or more and 520 MPa or more when a tensile test is performed using a test piece of NK-U1 with a gauge distance of 200 mm in accordance with Nippon Kaiji Kyokai's Classification Rules K Edition Materials (2019 edition). It is as follows. The tensile strength is preferably 410 MPa or more, more preferably 420 MPa or more, preferably 510 MPa or less, and more preferably 500 MPa or less.

[Elongation at break (EL): 16% or more]
The elongation at break (i.e., steel plate workability) was determined when a tensile test was performed using a test piece of NK-U1 with a gauge length of 200 mm in accordance with Nippon Kaiji Kyokai's Classification Rules, Part K Materials (2019 edition). , 16% or more. Thereby, it is possible to reduce the working time for parts molding and the like on site. The elongation at break is preferably 18% or more, more preferably 20% or more, even more preferably 22% or more. On the other hand, the upper limit of the elongation at break is not particularly limited, but is about 45% in view of the above chemical composition and the manufacturing method described below.

[Uniform elongation (uEL): 13.5% or more]
Uniform elongation (that is, uniform deformation performance) was determined when a tensile test was performed using a test piece of NK-U1 with a gauge distance of 200 mm in accordance with Nippon Kaiji Kyokai's Classification Rules, Part K Materials (2019 edition). , 13.5% or more. This makes it possible to reduce the maintenance load when a ship or the like collides with an object. The uniform elongation is preferably 14.0% or more, more preferably 14.5% or more, even more preferably 15.0% or more. On the other hand, the upper limit of uniform elongation is not particularly limited, but is about 30% in view of the above chemical composition and the manufacturing method described below.

[Corrosion resistance: Average corrosion depth of paint scratch area after 168 days of combined cycle test (CCT) is 0.600 mm or less]
When a corrosion depth exceeds a certain level, the pH of the corroded area decreases, the corrosion depth progresses, and the maintenance load increases when a ship or the like collides with an object. It is preferable not to let the disease progress. In an embodiment of the present invention, the average corrosion depth of the painted scratch area after 168 days of combined cycle testing (CCT) is preferably 0.600 mm or less, more preferably 0.550 mm or less, even more preferably 0.500 mm or less. be. In addition, in this embodiment, by satisfying the above chemical composition, it is possible to obtain a thick steel plate with an average corrosion depth of 0.600 mm or less, that is, excellent corrosion resistance.

4. Manufacturing method Next, a method for manufacturing a thick steel plate according to an embodiment of the present invention will be described.
The present inventors hot-rolled steel having the above chemical composition under predetermined conditions described below, and then air-cooled the steel to obtain the desired metal structure described above. It was found that a thick steel plate having the following characteristics could be obtained. The details will be explained below. The present invention relates to a thick steel plate, and in this field, a thick steel plate generally refers to a plate having a thickness of 3.0 mm or more. The thickness of the thick steel plate targeted by the present invention is preferably 6.0 mm or more.

(steel manufacturing)
The steel manufacturing method is not particularly limited, and any general steel manufacturing method may be used.

(Heating before hot rolling)
The heating conditions before hot rolling are not particularly limited, but it is preferable to heat to 1100° C. or higher, for example, so that the hot rolling described below can be carried out in a predetermined temperature range.

(hot rolling)
Hot rolling is performed at a rolling temperature of 1100° C. or less and a recrystallization reduction rate of 80% or more so as to satisfy the following formula (1). The recrystallization temperature range varies depending on the steel type. In the case of the steel type targeted in this embodiment, the recrystallization temperature range is 850° C. or higher. Therefore, the recrystallization reduction rate according to the present embodiment means the cumulative reduction rate in rolling at 850° C. or higher (including finish rolling). In addition, in this embodiment, the said "rolling temperature" and the below-mentioned "finish rolling completion temperature" mean the temperature on the surface of a steel plate. Furthermore, in this specification, the term "rolling temperature" refers to the rolling temperature in the total rolling including rough rolling and finish rolling. Further, the term "finish rolling completion temperature" means the rolling temperature at the completion of the final pass of finish rolling. Moreover, "ε _yt (surface)" in the following formula (2) means the cumulative strain on the surface of only one of both surfaces of the steel plate. Similarly, "ε _yt (t/4)" in the following formula (2) means the cumulative strain at only the t/4 position on one side among the t/4 positions from the front and back surfaces of the steel plate. In addition, the roll radius R in the following formula (5) is the roll radius of the finishing rolling mill, and when the finishing rolling mill is a multiple rolling mill, it means the radius of the roll (i.e., work roll) that comes into contact with the rolled material. .

0.08FRT+0.1Tom-0.5ε _T +10≧69...(1)
however,
FRT (Finish Rolling Temperature): Finish rolling completion temperature (℃)
Tom: Time between rolling passes from the last pass to the last pass (seconds)
ε _T : Total value of cumulative strain at each position on the surface of the steel plate, t/4 position and t/2 position (t: plate thickness) (hereinafter referred to as "total cumulative strain"), expressed by the following formula (2) It is calculated by Note that the cumulative distortion at each position is calculated using the following equation (3). Further, the distortion of each path is calculated using the following equation (4). The following equation (4) is based on Hiroshi Yoshida, "Coupled analysis of temperature, rolling load, and metallurgical properties in a hot strip mill," Journal of the JSTP, vol. 38, no. 437 (1997-6). The calculation formula described in .
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×(2y/h) ² +1.15×ln(H/h)...(4)
In formulas (2) to (4),
ε _y : Equivalent plastic strain at the y position in the plate thickness direction,
ε _yi : Equivalent plastic strain at the y position in the thickness direction of the i-th pass,
y: Distance from the center of plate thickness (mm), h: Output side plate thickness (mm), H: Inlet side plate thickness (mm),
C=B ₁ × (H/2R) ^B2 × r _e ² ... (5)
In formula (5),
B ₁ =0.18381+0.34435μ+1.4086× ^μ2
B ₂ =0.076669-2.0566μ+2.1128×μ ²
However, μ: friction coefficient, R: roll radius (mm), r _e : rolling reduction rate

By setting the rolling temperature to 1100° C. or lower, it is possible to suppress an increase in strength due to refinement of the ferrite grain size. The rolling temperature is preferably 1050°C or lower, more preferably 1000°C or lower. The lower limit of the rolling temperature is preferably 750°C or higher, more preferably 780°C or higher, from the viewpoint of suppressing an increase in strength. By setting the recrystallization reduction ratio to 80% or more, excessive decrease in strength due to coarsening of grain size can be suppressed. The recrystallization reduction ratio is preferably 85% or more, more preferably 90% or more. The recrystallization reduction rate is preferably 99% or less from the viewpoint of suppressing grain size refinement.

Next, the above formula (1) will be explained. In order to ensure high uniform elongation characteristics, it is necessary to contain a ferrite structure with little processing strain. During hot rolling of steel, working strain is introduced by rolling, while working strain is reduced by recovery, recrystallization, and transformation. As a result of repeated research and investigation by the present inventors regarding various manufacturing conditions related to the introduction of processing strain and strain reduction, the present inventors have determined that the finishing rolling completion temperature (FRT) and the time between final rolling passes (i.e., the final rolling pass time) It has been found that the influence of the rolling pass time before the final pass (Tom) and the total cumulative strain (ε _T ) is particularly large. Each parameter will be explained in detail below.

Finish rolling completion temperature (FRT) is a parameter related to processing strain that decreases due to recovery and recrystallization. The higher the FRT, the greater the driving force for recovery and recrystallization, which reduces processing strain. Therefore, from the viewpoint of obtaining a ferrite structure with less processing strain, a higher FRT is preferable. The finish rolling completion temperature is, for example, within the range of 1100°C to 750°C.

The final rolling pass time (Tom) is the time from the end of rolling one pass before the final pass to the start of rolling of the final pass. The time between final rolling passes is a parameter related to recovery, reduction of working strain due to recrystallization, and introduction of working strain in the ferrite formation temperature range. The larger Tom is, the longer the time for recovery and recrystallization becomes, and the amount of accumulated processing strain is reduced. Therefore, from the viewpoint of obtaining a ferrite structure with less processing strain, it is preferable that Tom be larger. The time between final rolling passes is, for example, within a range of 5 seconds to 120 seconds.

The total cumulative strain (ε _T ) is a parameter representing the amount of processing strain introduced by hot rolling. Although the processing strain introduced during hot rolling is alleviated by transformation, it is thought that it remains in the structure after transformation. In other words, the strain remaining in the ferrite after transformation is affected by the cumulative strain introduced during hot rolling, and the larger the cumulative strain, the more residual strain introduced into the ferrite, which deteriorates the uniform elongation characteristics. Conceivable. Therefore, from the viewpoint of obtaining a ferrite structure with less processing strain, it is preferable that the total accumulated strain is small. The total cumulative strain may be within a range of 1.0 to 30, for example.

As explained above, the processing strain remaining after hot rolling changes depending on the "total cumulative strain", "FRT", and "Tom" introduced during hot rolling. Therefore, after further various studies, the present inventors found the left side of equation (1) using these as parameters. The present inventors also found that when the left side of formula (1) is less than 69, low strength (i.e., 520 MPa or less) and high elongation properties (i.e., EL≧16%, uEL≧13.5%) are satisfied. I found that it becomes difficult. Therefore, the left-hand side value of equation (1) is set to 69 or more. The left-hand side value of formula (1) is preferably 75 or more, more preferably 80 or more. From the viewpoint of ensuring the above characteristics, the upper limit of the left-hand side value of formula (1) is not particularly limited, but the upper limit is approximately 100 in view of the manufacturing conditions according to the embodiment of the present invention.

(cooling)
After the above-mentioned hot rolling, rapid cooling such as so-called water cooling is not performed, but cooling is performed to room temperature by air cooling. This is because when 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 is increased. Furthermore, if hard structures such as MA are dispersed in steel, the elongation properties will deteriorate, so transformation is carried out by air cooling to ensure a desired ferrite fraction. Furthermore, if water cooling or the like with a high cooling rate is performed, the degree of supercooling will increase and the particles will become fine. By air cooling, grain refinement can be suppressed and strength increase can be suppressed.

(1) Method for producing test materials Steel slabs having the chemical composition shown in Table 1 were produced by melting in a converter furnace. Thereafter, the steel sheets were heated to 1000 to 1230° C. before hot rolling, and hot rolling and cooling were performed under the conditions listed in Table 2 to produce steel plates having the thicknesses listed in Table 2. Note that the rolling temperature was 1100°C or lower. Further, in the present embodiment, in deriving equation (1) shown in Table 2, the friction coefficient μ was set to “0.55”. For the rolling reduction ratio r _e , in order to simplify the calculation, "0.3", which is an average value in the manufacture of thick plate products, may be used. Further, in Tables 1 and 2, and Tables 3 and 4 described later, underlined numerical values indicate that they are outside the scope of the embodiments of the present invention. However, it should be noted that "-" is not underlined even if it is outside the scope of the embodiment of the present invention.

(2) Structure observation No. in Tables 1 and 2. 1 and no. 6~No. Regarding No. 8, the structure was observed as follows. A test piece is cut from the steel plate so that the observation surface is parallel to the steel plate surface and the observation position is at the t/4 position (t: plate thickness), and after being subjected to nital corrosion, it is optically inspected at 100x to 400x. Observed under a microscope. The area ratio of ferrite was analyzed using image analysis software (Image-J) for each of the 10 fields of view (field area: 150 μm x 200 μm) of the optical microscope, and the average value of the area ratio of ferrite was calculated for each sample. In this way, the ferrite area ratio of each sample was calculated.
The ferrite grain size is determined by creating a grid with 25 μm intervals in one field of view of an optical microscope image magnified 400 times (viewing area: 150 μm x 200 μm), calculating the number of ferrite grains that one straight line crosses, and calculating the length of the straight line within the field of view. was divided by the number of particles to determine the grain size per ferrite on the straight line. Then, the grain size per ferrite was similarly determined for the other straight lines, and the average value of these was calculated, and this average value was taken as the ferrite grain size.
No. The ferrite area ratios of Samples Nos. 1, 6, 7, and 8 were 80%, 82%, 84%, and 83%, respectively, and satisfied the requirements according to the embodiment of the present invention. Also, No. The ferrite grain sizes of Samples Nos. 1, 6, 7, and 8 were 7 μm, 22 μm, 16 μm, and 14 μm, respectively, and satisfied the requirements according to the embodiment of the present invention. Note that the remaining tissue is No. 1, 6, 7 and 8 were all pearlite.

(3) Tensile test method A test piece for the tensile test was taken from a rolled material so that the longitudinal direction (tensile direction) was perpendicular to the rolling direction. The test piece shape is No. NK-U1, which complies with Nippon Kaiji Kyokai's Classification Rules, Part K Materials (2019 edition), the gauge distance (GL) is 200 mm, and it is a full thickness shape, so it is listed in Table 2. The test was carried out with a plate thickness of . As the yield point (YP), the upper yield point was adopted, but if the upper yield did not appear, 0.2% proof stress was used as the yield point. The tensile test conditions were conducted in accordance with Part K of the NK Marine Classification Standard. Specifically, before the yield point was reached, the tensile rate was 20 N/mm ² , and after the yield point was reached, the strain rate was 30 PL%/min. Further, uniform elongation was calculated by elongation up to the maximum load in the stress-strain diagram. Samples with a tensile strength (TS) of 400 MPa or more and 520 MPa or less, an elongation at break (EL) of 16% or more, and a uniform elongation (uEL) of 13.5% or more were accepted. The test results are shown in Table 3. Note that yield strength (YS) is also listed in Table 3.

(4) Corrosion test method (CCT conditions)
As shown in Fig. 1, the corrosion test piece was prepared by applying 15 μm of Zn-rich paint to the surface of a 5t x 90W x 120L (mm) steel material, applying and curing 160 μm of modified epoxy resin on top of it, and then applying 160 μm of modified epoxy resin. was applied and cured to give a total film thickness of 335 μm. A paint film flaw (ie, paint scratch) was artificially created in the center of the painted test piece using a plastic cutter, and the outer periphery of the test piece was adjusted to an evaluation area of 80 cm ² with masking tape.

The corrosion test was conducted as a laboratory evaluation test simulating the inside of a ballast tank as follows. In order to simulate the environment behind the upper deck of an actual ship's ballast tank, as shown in Figure 2, we first sprayed artificial seawater at 35°C ± 1°C for 2 hours, then sprayed it at 60°C ± 1°C, 45% to 55%. A cycle of drying at RH for 4 hours and then holding at 50°C ± 1°C for 2 hours at a relative humidity higher than 95% RH was performed for 7 days (hereinafter, this 7-day cycle is referred to as the ``first cycle''). . Then, it was kept at 35°C ± 1°C for 2 hours at a relative humidity higher than 95% RH, dried at 60°C ± 1°C, 45% to 55% RH for 4 hours, and then at 50°C ± 1°C for 2 hours. , a cycle of holding at a relative humidity higher than 95% RH for 7 days (hereinafter, this 7-day cycle will be referred to as the "second cycle"). The first cycle and the second cycle were conducted alternately for 168 days. The test piece was installed at an angle of 15° to 25° from the vertical direction. The test machine was adjusted so that the amount of artificial seawater sprayed was 1.5±0.5 mL/80 cm ² /h.

Five test pieces (total 10 pieces) each of the developed steel (No. B in Table 1) and the conventional steel (No. A in Table 1) were prepared. Thereafter, the above corrosion test was carried out, and the corrosion depth after removal of the coating film was measured at six points at 10 mm intervals from the scratch end (see FIG. 3). The average value of 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. Samples with a corrosion depth of 0.600 mm or less were accepted. The calculation results are shown in Table 4.

The above tissue observation results and the results in Tables 3 and 4 will be considered.
Example No. 1 and no. Samples Nos. 6 to 8 satisfied the chemical component composition requirements and manufacturing conditions according to the embodiment of the present invention, so the desired metal structure was obtained and the tensile strength (TS) and elongation properties (EL, uEL) were good. there were. Moreover, Example No. Examples Nos. 2 to 5 satisfied the chemical component composition requirements and manufacturing conditions according to the embodiment of the present invention, so they were designated as Example Nos. 1 and no. It is thought that the desired metal structure was obtained similarly to Examples 6 to 8, and as a result, the tensile strength (TS) and elongation properties (EL, uEL) were good. Moreover, Example No. 1~No. No. 8 is considered to have excellent corrosion resistance because it contains a predetermined amount of Cu+Ni and Cr. To demonstrate this, as mentioned above, No. A and No. A corrosion test using B was also conducted. As a result, Example No. 1~No. Example No. 8, which satisfies the chemical component composition requirements according to the embodiment of the present invention, as well as Example No. 8. It was confirmed that B had good corrosion resistance.

On the other hand, comparative example No. 9~No. Sample No. 11 did not satisfy the formula (1) and was therefore inferior in tensile strength (TS) and uniform elongation (uEL). In addition, although the elongation at break was satisfactory, the value was lower than that in Examples. The reason why formula (1) was not satisfied is that No. No. 9 had a relatively low FRT, a low Tom, and a large cumulative strain εT; 10, No. No. 11 is considered to be because the FRT was relatively low and the cumulative strain εT was relatively large. FIG. 4 is a graph showing the relationship between the left side of equation (1) and tensile strength. FIG. 5 is a graph showing the relationship between the left side of equation (1) and uniform elongation. Referring to FIGS. 4 and 5, it can be seen that when the left side of equation (1) is 69 or more, the tensile strength decreases and uniform elongation improves. Moreover, comparative example No. Sample A did not satisfy the chemical component composition requirements according to the embodiment of the present invention, so the corrosion depth exceeded 0.600 mm and the corrosion resistance was poor.

The thick steel plate of the present invention can be widely applied to fields such as shipbuilding, offshore structures, bridges, etc., where improvements in coating corrosion resistance and field workability (i.e., improvements in elongation properties) are required, and are not particularly limited. Specific applications include thick steel plates for ships such as the upper deck of ship ballast tanks.

Claims (5)

C: 0.01% by mass or more, 0.30% by mass or less,
Si: 0.01% by mass or more, 2.0% by mass or less,
Mn: 0.85% by mass or more, 2.00% by mass or less,
P: more than 0 mass%, 0.015 mass% or less,
S: more than 0% by mass, 0.005% by mass or less,
Al: 0.005% by mass or more and 0.10% by mass or less,
Cr: 0.01% by mass or more, 0.5% by mass or less,
Ti: 0.005% by mass or more and 0.20% by mass or less,
Ca: 0.0001% by mass or more, 0.005% by mass or less,
Contains N: 0.0001% by mass or more and 0.010% by mass or less, and Cu + Ni: 0.50% by mass or more and 0.85% by mass or less, the remainder consisting of Fe and inevitable impurities,
The metal structure contains ferrite in an area ratio of 70% or more with respect to the total metal structure, and the remainder consists of one or more types 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 was performed using a test piece of NK-U1 with a gauge length of 200 mm in accordance with Nippon Kaiji Kyokai's Classification Rules, Part K Materials (2019 edition), the tensile strength was 400 MPa or more and 520 MPa or less, and the elongation at break was A low-strength thick steel plate with excellent elongation characteristics and corrosion resistance, with a uniform elongation of 16% or more and a uniform elongation of 13.5% or more. The thick steel plate according to claim 1, wherein the average corrosion depth of the painted scratch portion after 168 days of the combined cycle test is 0.600 mm or less. B: 0.0001% by mass or more, 0.010% by mass or less,
V: 0.01% by mass or more and 0.50% by mass or less; and Nb: 0.001% by mass or more and 0.50% by mass or less. The thick steel plate according to claim 2. Co: 0.005% by mass or more, 0.20% by mass or less,
REM: 0.005% by mass or more, 0.20% by mass or less,
Claims 1 to 3 further contain one or more selected from the group consisting of Zr: 0.005% by mass or more and 0.20% by mass or less, and Mg: 0.0001% by mass or more and 0.005% by mass or less. 3. The thick steel plate according to any one of 3. A step of heating steel satisfying the composition according to any one of claims 1 to 4 to 1100 ° C. or higher,
A step of hot rolling at a rolling temperature of 1100° C. or lower and a recrystallization reduction rate of 80% or higher to satisfy the following formula (1);
The method for producing a thick steel plate according to any one of claims 1 to 4, comprising the step of air cooling.
0.08FRT+0.1Tom-0.5ε _T +10≧69...(1)
however,
FRT: Finish rolling completion temperature (℃)
Tom: Time between rolling passes from the last pass to the last pass (seconds)
ε _T : Total value of cumulative strain at each position on the surface of the steel plate, t/4 position and t/2 position (t: plate thickness), and is calculated by the following formula (2). Note that the cumulative distortion at each position is calculated using the following equation (3). Further, the distortion of each path is calculated using 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×(2y/h) ² +1.15×ln(H/h)...(4)
In formulas (2) to (4),
ε _y : Equivalent plastic strain at the y position in the plate thickness direction,
ε _yi : Equivalent plastic strain at the y position in the thickness direction of the i-th pass,
y: Distance from the center of plate thickness (mm), h: Output side plate thickness (mm), H: Inlet side plate thickness (mm),
C=B ₁ × (H/2R) ^B2 × r _e ² ... (5)
In formula (5),
B ₁ =0.18381+0.34435μ+1.4086× ^μ2
B ₂ =0.076669-2.0566μ+2.1128×μ ²
However, μ: friction coefficient, R: roll radius (mm), r _e : rolling reduction rate