WO2022074933A1

WO2022074933A1 - Low-strength thick steel sheet having excellent elongation properties and corrosion resistance

Info

Publication number: WO2022074933A1
Application number: PCT/JP2021/029621
Authority: WO
Inventors: 亮太加藤; 雅人金子; 真司阪下
Original assignee: 株式会社神戸製鋼所
Priority date: 2020-10-05
Filing date: 2021-08-11
Publication date: 2022-04-14
Also published as: CN116057188A; JP2022060949A; KR20230051774A; JP7350705B2

Abstract

A low-strength thick steel sheet having excellent elongation properties and corrosion resistance, which has a specified compositional formula, in which a metal structure contains ferrite at an area ratio of 70% or more relative to the whole metal structure with a remainder comprising at least one component selected from the group consisting of perlite, bainite and martensite, the grain diameter of ferrite is 3 to 40 μm inclusive, and the tensile strength is 400 to 520 MPa inclusive, the elongation at break is 16% or more and the uniform elongation is 13.5% or more when a tensile test is performed using a NK-U1 test specimen having a distance between marking points of 200 mm in accordance with Rules and Regulations for the Classification of Ships, Part-K Materials (2019 ed.) published by Nippon Kaiji Kyokai.

Description

Low-strength thick steel sheet with excellent elongation characteristics and corrosion resistance

This disclosure relates to a low-strength thick steel sheet having excellent elongation characteristics and corrosion resistance.

Since the ballast tank of a ship injects and discharges seawater in response to changes in cargo conditions, etc., the steel material used is an extremely severe corrosive environment in which seawater is immersed and exposed to the moist atmosphere containing salt. Be exposed to. If the ballast tank is corroded and damaged, it may sink due to perforation or the cargo of crude oil or chemical substances may flow out into the ocean. There is anti-corrosion coating as. However, since the anticorrosion coating requires repainting due to deterioration of the coating, maintenance cost is high and the environmental load is also high. In recent years, reducing the environmental burden has become one of the important factors for the realization of a sustainable society. Therefore, it is necessary to improve the corrosion resistance of the steel material from the viewpoint of reducing the environmental load as well as the maintenance cost. Steel materials for ships for the purpose of improving corrosion resistance are disclosed in, for example, Patent Document 1 and Patent Document 2.

JP-A-2015-151571 Japanese Unexamined Patent Publication No. 2010-126765

In the case of coastal vessels, for example, low-strength materials with a tensile strength (TS) of 520 MPa or less, so-called mild steel, are often applied due to the hull structure. On the other hand, as a means for improving the corrosion resistance, it is usual to add an alloying element which is an element for improving the corrosion resistance. However, when an alloying element is added, the strength generally increases, so that it becomes difficult to achieve both corrosion resistance and low strength.

In recent years, the realization of a low-carbon society has also been emphasized as part of the realization of a sustainable society. If the workability of the steel sheet can be improved, it will lead to a reduction in the work time for forming parts at the site, and finally contribute to energy saving or reduction of CO ₂ emissions. Therefore, it is also necessary to secure a predetermined workability of the steel sheet (that is, improvement of the elongation at break EL). Further, when a ship or the like collides with an object, the member is plastically deformed, and when a crack occurs, the maintenance load increases, which 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, a predetermined uniform elongation characteristic (uEL) is also required.

The present invention has been made in view of such a situation, and an object thereof is to provide a thick steel sheet having 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% by mass, 0.015% by mass or less,
S: More than 0% by mass, 0.005% by mass or less,
Al: 0.005% by mass or more, 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, 0.20% by mass or less,
Ca: 0.0001% by mass or more, 0.005% by mass or less,
N: contains 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, and the balance consists of Fe and unavoidable impurities.
The metallographic structure contains ferrite with an area ratio of 70% or more with respect to the total metallic structure, and the balance consists of 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.
Tensile strength is 400 MPa or more and 520 MPa or less, fracture elongation when a tensile test is performed using a test piece of NK-U1 with a distance between gauge points of 200 mm in accordance with Nippon Kaiji Kyokai's ship class regulation K edition material (FY2019 version). Is a low-strength thick steel sheet having excellent elongation characteristics and corrosion resistance, having a uniform elongation of 13.5% or more and a uniform elongation of 16% or more.

Aspect 2 of the present invention is
The thick steel sheet according to aspect 1, wherein the average corrosion depth of the coated scratch portion after 168 days of the composite 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,
V: 0.01% by mass or more, 0.50% by mass or less, and Nb: 0.001% by mass or more, 0.50% by mass or less, further containing one or more selected from the group. The thick steel plate according to 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,
Aspects 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. It is a thick steel plate according to any one of.

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, and
A step of hot rolling so that the rolling temperature is 1100 ° C. or lower and the recrystallization reduction rate is 80% or more and the following formula (1) is satisfied.
The method for manufacturing a thick steel sheet according to any one of aspects 1 to 4, which comprises an air cooling step.
0.08FRT +0.1Tom-0.5ε _T +10 ≧ 69 ・・・ (1)
however,
FRT: Finish rolling completion temperature (° C)
Tom: Time between the last pass and the rolling pass one pass before the last pass (seconds)
ε _T : The total value of the cumulative strains at each position of the surface of the steel sheet, the t / 4 position and the t / 2 position (t: plate thickness), and is calculated by the following equation (2). The cumulative strain at each position is calculated by the following equation (3). Further, the distortion of each path 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 × (2y / h) ² + 1.15 × ln (H / h) ・・・ (4)
In equations (2) to (4)
ε _y : Equivalent plastic strain at y position in the plate thickness direction,
ε _yi : Equivalent plastic strain at the y position in the plate thickness direction of the i-pass,
y: Distance from the center of plate thickness (mm), h: Outer side plate thickness (mm), H: Enter side plate thickness (mm),
C = B ₁ × (H / 2R) ^B2 × _re ² ... (5)
In equation (5)
B ₁ = 0.18381 + 0.344435μ + 1.48066 × μ ²
B ₂ = 0.076669-2.0566μ + 2.1128 × μ ²
However, μ: friction coefficient, R: roll radius (mm), _re : reduction rate

According to the embodiment of the present invention, it is possible to provide a thick steel sheet having both excellent corrosion resistance, low strength and high elongation characteristics.

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

The present inventors have conducted diligent research to realize low strength and excellent elongation characteristics on the premise that a certain amount of Cu, Ni and Cr is contained in order to secure excellent corrosion resistance. As a result, by producing a steel having a predetermined chemical composition under the recommended conditions, the steel has a predetermined chemical composition and contains ferrite having an area ratio of 70% or more with respect to the total metal structure, and the balance is pearlite. It consists of one or more selected from the group consisting of bainite and martensite, has a ferrite particle size of 3 μm or more and 40 μm or less, a tensile strength of 400 MPa or more and 520 MPa or less, a breaking elongation of 16% or more, and a uniform elongation of 13.5. It has been found that a low-strength thick steel plate having excellent elongation characteristics and corrosion resistance, which is% or more, can be obtained.

1. 1. Chemical Composition The chemical composition of the thick steel sheet 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 described, and further, the elements that may be selectively added will be described.

[C: 0.01% by mass or more, 0.30% by mass or less]
C is an element necessary for ensuring the strength of the material. If C is excessively contained, 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, about 400 MPa. Therefore, the C content was set to 0.01% by mass or more and 0.30% by mass or less. The minimum strength varies depending on the wall thickness of the steel material used. The C content is preferably 0.05% by mass or more, and more preferably 0.10% by mass or more. The C content is preferably 0.20% by mass or less, and 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 its strength by being dissolved in ferrite as a solid solution, 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 secured. 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 further preferably 0.30% by mass or less. The Si content is preferably 0.10% by mass or more, and more preferably 0.15% by mass or more.

[Mn: 0.85% by mass or more, 2.00% by 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 secured. However, if Mn is excessively contained, low strength cannot be obtained due to an increase in 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, and more preferably 0.95% by mass or more. The Mn content is preferably 1.20% by mass or less, and more preferably 1.10% by mass or less.

[P: More than 0% by mass, 0.015% by mass or less]
Like Si, P is an element having a large amount of solid solution strengthening, and in the embodiment of the present invention aiming at low strength, the content should be small. Further, since the toughness and weldability are also deteriorated, the allowable upper limit of the P content is set to 0.015% by mass.

[S: More than 0% by mass, 0.005% by mass or less]
Since S is an element that reduces ductility as the content increases, it is preferable to suppress the content, and the allowable upper limit of the S content is 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, and more preferably 0.045% by mass or more. The Al content is preferably 0.090% by mass or less, and more preferably 0.080% by mass or less.

[Cu + Ni: 0.50% by mass or more, 0.85% by mass or less]
Cu and Ni are elements effective for improving corrosion resistance. Both Cu and Ni have the effect of suppressing the corrosion reaction that occurs under the anticorrosive coating film, and are elements that have the effect of suppressing the swelling of the coating film due to the corrosion under the coating film. In addition, Cu and Ni also have the effect of densifying the generated rust when the steel material is corroded in the coating film defect portion, and exert the effect of suppressing the corrosion progress of the coating film scratched portion. It is an effective element. In order to exert these effects, it is necessary to make the total content of Cu and Ni 0.50% by mass or more, but if it is excessively contained, it deviates from the strength class of mild steel. It should 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 the more preferable lower limit is 0.60% by mass or more. Further, from the viewpoint of increasing the strength, the total content of Cu and Ni is preferably 0.80% by mass or less, more preferably 0.75% by mass or less. The Cu content is preferably 0.20% by mass or more, more preferably 0.25% by mass or more. 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. 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 for improving corrosion resistance. Cr is an element that adjusts the pH under the coating film to a suitable value, has an effect of suppressing primer consumption, and further exerts an effect of suppressing the progress of corrosion of a scratched portion of the coating film. Further, an appropriate amount of Cr is effective for improving toughness, and is an element necessary for obtaining mechanical properties required as a ballast tank material. In order to exert these effects, it is necessary to contain Cr in an amount of 0.01% by mass or more. However, if Cr is excessively contained, the interfacial pH is lowered too much, the outflow of Fe is promoted, and it becomes a factor of 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 for improving corrosion resistance. Ti has an action of densifying the rust generated in a chloride-corroded environment, and is an element that suppresses the progress of corrosion in the scratched portion of the coating film. In order to exert such an effect, Ti is contained in an amount of 0.005% by mass or more. However, if the Ti content is excessive, Ti carbonitride is excessively precipitated and the strength is increased. Therefore, the upper limit is set to 0.20% by mass. The Ti content is preferably 0.008% by mass or more, and more preferably 0.010% by mass or more. The Ti content is preferably 0.15% by mass or less, and more preferably 0.10% by mass or less.

[Ca: 0.0001% by mass or more, 0.005% by mass or less]
Ca is an element effective for improving corrosion resistance. Ca has an action of alleviating a decrease in pH due to hydrolysis of Fe and / or Cr by a pH buffering action, and is effective in suppressing the promotion of corrosion due to a decrease in pH. Such an action is effectively exerted by containing 0.0001% by mass or more of Ca. However, if Ca is excessively contained in excess of 0.005% by mass, the elongation characteristics will be deteriorated. Therefore, the Ca content is 0.0001% by mass or more and 0.005% by mass or less. The Ca content is preferably 0.0005% by mass or more, and more preferably 0.0010% by mass or more. The Ca content is preferably 0.004% by mass or less, and 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 the 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 the improvement of HAZ toughness by forming TiN. From the viewpoint of exerting the 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.

[Remaining]
The balance is Fe and unavoidable impurities. As unavoidable impurities, it is permissible to mix trace elements (for example, As, Sb, Sn, etc.) brought in depending on the conditions of raw materials, materials, manufacturing equipment, and the like. In addition, for example, there are elements such as P and S, which are usually preferable as the content is smaller and are therefore unavoidable impurities, but the composition range thereof is separately defined as described above. Therefore, in the present specification, the term "unavoidable impurities" constituting the balance is a concept excluding the elements whose composition range is separately defined.

Any other element may be further contained as long as the characteristics of the thick steel sheet according to the embodiment of the present invention can be maintained. Other elements that can be selectively contained in this way are illustrated below.

[B: 0.0001% by mass or more, 0.010% by mass or less, V: 0.01% by mass or more, 0.50% by mass or less, and Nb: 0.001% by mass or more, 0.50% by mass or less. One or more selected from the group of
Although B, V and Nb are all effective elements for improving mechanical properties, it is preferable to suppress the content because the strength is improved by increasing the 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 characteristics of the welded portion, it is preferable to contain B in an amount of 0.0001% by mass or more. Since V and Nb are added in small amounts to improve toughness, it is preferable to contain V in an amount of 0.01% by mass or more and Nb in an amount of 0.001% by mass or more. The more preferable lower limit of these elements is 0.0003% by mass for B, 0.02% by mass for V, and 0.005% by mass for Nb. A more preferable upper limit is 0.0090% by mass for B, 0.45% by mass for V, and 0.45% by mass for Nb.

[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, Zr: 0.005% by mass or more, 0.20% by mass or less, and Mg: One or more selected from the group consisting of 0.0001% by mass or more and 0.005% by mass or less]
Co is an element effective for improving corrosion resistance. Co has an action of densifying the rust generated in a chloride corrosive environment, and is an element that suppresses the progress of corrosion in the scratched portion of the coating film. 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 becomes excessive, the weldability and / or the hot workability deteriorates, so the Co content is preferably 0.20% by mass or less. The more preferable lower limit when Co is contained is 0.02% by mass, and the more preferable upper limit is 0.15% by mass.

Zr is an element effective for improving corrosion resistance. Similar to Ti, Zr has an action of densifying rust generated in a chloride corrosive environment, and is an element that suppresses the progress of corrosion in a scratched portion of a coating film. 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 becomes excessive, the weldability and / or the hot workability deteriorates, so the Zr content is preferably 0.20% by mass or less. The more preferable lower limit when containing Zr is 0.008% by mass, and the more preferable upper limit is 0.15% by mass.

Mg is an element effective for improving corrosion resistance. Similar to Ca, Mg has an effect of alleviating a decrease in pH, exerts an effect of suppressing the promotion of corrosion due to a decrease in pH, and is effective in suppressing swelling of a coating film. Such an action is effectively exhibited by containing 0.0001% by mass or more, and is particularly effective when Co coexists. However, if it is contained in excess of 0.005% by mass, the workability and weldability will be deteriorated, which is not preferable. When Mg is contained, the preferable lower limit is 0.0005% by mass, and the more preferable upper limit is 0.004% by mass.

REM (Rare Earth Metal) has an effect of suppressing a decrease in pH near the surface of a steel material in a usage environment, and is an effective element for further improving corrosion resistance. This action is exhibited by the corrosion and dissolution of these elements and the reaction with hydrogen ions. In order to effectively exert such an action, 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 the hot workability is deteriorated. Therefore, when the REM content is contained, it is preferably 0.20% by mass or less. The more preferable lower limit when containing REM is 0.008% by mass, and the more preferable upper limit is 0.15% by mass. Examples of the REM used in the embodiment of the present invention include Sc, Y, and lanthanoids.

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

[Ferrite: 70 area% or more]
The thick steel sheet according to the embodiment of the present invention contains ferrite having an area ratio of 70% or more with respect to the total metal structure, and the balance is composed of one or more selected from the group consisting of pearlite, bainite and martensite. Further, in the embodiment of the present invention, as will be described later, the ferrite can be sufficiently secured by appropriately controlling the hot rolling conditions. By containing 70 area% or more of ferrite, 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 the minimum strength at the level of mild steel.

[Ferrite particle size: 3 μm or more and 40 μm or less]
The thick steel sheet 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 secure the minimum strength, the ferrite particle size is 40 μm or less, preferably 35 μm or less, and more preferably 30 μm or less.

3. 3. Characteristics The thick steel sheet 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 described in detail below.

[Tensile strength (TS): 400 MPa or more and 520 MPa or less]
When used for coastal ships, low-strength materials (so-called mild steel) are often applied due to the hull structure. Therefore, the tensile strength is 400 MPa or more and 520 MPa when a tensile test is performed using a test piece of NK-U1 with a distance between gauge points of 200 mm in accordance with the Nippon Kaiji Kyokai Ship Classification Rule K Knitting Material (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.

[Fracture elongation (EL): 16% or more]
Fracture elongation (that is, steel plate workability) is obtained when a tensile test is performed using a test piece of NK-U1 with a distance between gauge points of 200 mm in accordance with Nippon Kaiji Kyokai's ship class regulation K knitting material (2019 edition). , 16% or more. As a result, it is possible to reduce the work time for forming parts at the site. The elongation at break is preferably 18% or more, more preferably 20% or more, and 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-mentioned chemical composition and the production method described later.

[Uniform elongation (uEL): 13.5% or more]
Uniform elongation (that is, uniform deformation performance) is obtained when a tensile test is performed using a test piece of NK-U1 with a distance between gauge points of 200 mm in accordance with Nippon Kaiji Kyokai's ship class rule K knitting material (2019 edition). It is 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, and even more preferably 15.0% or more. On the other hand, the upper limit of the uniform elongation is not particularly limited, but is about 30% in view of the above-mentioned chemical composition and the production method described later.

[Corrosion resistance: The average corrosion depth of the painted scratch part after 168 days of the combined cycle test (CCT) is 0.600 mm or less]
When a certain corrosion depth is formed, the pH of the corroded part decreases, the corrosion depth increases, and the maintenance load when a ship or the like collides with an object increases. Therefore, the corrosion depth exceeds 0.600 mm. It is preferable not to let the corrosion progress. In the embodiment of the present invention, the average corrosion depth of the coated scratch portion after 168 days of the composite cycle test (CCT) is preferably 0.600 mm or less, more preferably 0.550 mm or less, still more preferably 0.500 mm or less. be. In this embodiment, by satisfying the above chemical composition, it is possible to obtain a thick steel sheet having an average corrosion depth of 0.600 mm or less, that is, an excellent corrosion resistance.

4. Manufacturing Method Next, a manufacturing method of a thick steel sheet according to an embodiment of the present invention will be described.
The present inventors have the above-mentioned desired metal structure by subjecting a steel having the above-mentioned chemical composition to hot rolling under predetermined conditions described later and then air-cooling, and as a result, the above-mentioned desired metal structure is obtained. It has been found that a thick steel sheet having the above-mentioned characteristics can be obtained. The details will be described below. It should be noted that the embodiment of the present invention relates to a thick steel sheet, and in this field, the thick steel sheet generally refers to a steel sheet having a plate thickness of 3.0 mm or more. The thickness of the thick steel plate targeted in the embodiment of the present invention is preferably 6.0 mm or more.

(Steelmaking)
The steelmaking method is not particularly limited, and a general steelmaking means may be adopted.

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

(Hot rolling)
In hot rolling, the rolling temperature is set to 1100 ° C. or lower and the recrystallization reduction rate is set to 80% or higher, and hot rolling is performed so as to satisfy the following formula (1). The recrystallization temperature region varies depending on the steel type. In the case of the steel grade targeted in this embodiment, the recrystallization temperature region is 850 ° C. or higher. Therefore, the recrystallization reduction rate according to the present embodiment means the cumulative reduction rate in rolling (including finish rolling) at 850 ° C. or higher. In the present embodiment, the above-mentioned "rolling temperature" and the "finish rolling completion temperature" described later mean the temperature on the surface of the steel sheet. Further, in the present specification, the term "rolling temperature" means the rolling temperature in all rolling including rough rolling and finish rolling. Further, the term "finish rolling completion temperature" means the rolling temperature at the time of completion of the final pass of finish rolling. Further, "ε _yt (surface)" in the following equation (2) means the cumulative strain on the surface of only one of both sides of the steel sheet. Similarly, "ε _yt (t / 4)" in the following equation (2) means the cumulative strain at only the t / 4 position on one side of the t / 4 positions from the front and back surfaces of the steel sheet. Further, the roll radius R in the following formula (5) is the roll radius of the finish rolling mill, and when the finish rolling mill is a multiple rolling mill, it means the radius of the roll (that is, the work roll) in contact with the rolled material. ..

0.08FRT +0.1Tom-0.5ε _T +10 ≧ 69 ・・・ (1)
however,
FRT (Finish Rolling Temperature): Finish rolling temperature (° C)
Tom: Time between the last pass and the rolling pass one pass before the last pass (seconds)
ε _T : The total value of cumulative strains at each position of the surface of the steel sheet, t / 4 position and t / 2 position (t: plate thickness) (hereinafter referred to as “total cumulative strain”), and is the following equation (2). It is calculated by. The cumulative strain at each position is calculated by the following equation (3). Further, the distortion of each path is calculated by the following equation (4). The following equation (4) is based on Hiroshi Yoshida, "Coupling analysis of temperature, rolling load, and metallurgical properties in a hot strip mill", Journal of the JSTP, vol.38, no.437 (1997-6). Refer to 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 equations (2) to (4)
ε _y : Equivalent plastic strain at y position in the plate thickness direction,
ε _yi : Equivalent plastic strain at the y position in the plate thickness direction of the i-pass,
y: Distance from the center of plate thickness (mm), h: Outer side plate thickness (mm), H: Enter side plate thickness (mm),
C = B ₁ × (H / 2R) ^B2 × _re ² ... (5)
In equation (5)
B ₁ = 0.18381 + 0.344435μ + 1.48066 × μ ²
B ₂ = 0.076669-2.0566μ + 2.1128 × μ ²
However, μ: friction coefficient, R: roll radius (mm), _re : reduction rate

By setting the rolling temperature to 1100 ° C or lower, it is possible to suppress an increase in strength due to the miniaturization of 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 the increase in strength. By setting the recrystallization reduction rate to 80% or more, it is possible to suppress an excessive decrease in strength due to coarsening of the particle size. The recrystallization reduction rate is preferably 85% or more, more preferably 90% or more. The recrystallization reduction rate is preferably 99% or less from the viewpoint of suppressing the miniaturization of the crystal grain size.

Next, the above equation (1) will be described. In order to secure high uniform elongation characteristics, it is necessary to contain a ferrite structure with less processing strain. During hot rolling of steel, machining strain is introduced by rolling, while machining strain is reduced by recovery, recrystallization and transformation. As a result of repeated research and investigation by the present inventors on various manufacturing conditions related to the introduction of machining strain and the reduction of strain, the present inventors have obtained the finish rolling completion temperature (FRT) and the time between final rolling passes (that is, the final pass). It was found that the influence of the time between rolling passes one pass before the final pass, Tom) and the total cumulative strain (ε _T ) was particularly large. Each parameter will be described in detail below.

Finish rolling completion temperature (FRT) is a parameter related to machining strain that decreases due to recovery and recrystallization. The higher the FRT, the larger the driving force for recovery and recrystallization, and the smaller the processing strain. Therefore, from the viewpoint of obtaining a ferrite structure with less processing strain, it is preferable that the FRT is high. The finish rolling completion temperature may be, for example, in the range of 1100 ° C to 750 ° C.

The time between final rolling passes (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 associated with recovery, reduction of machining strain due to recrystallization, and introduction of machining strain in the ferrite formation temperature range. The larger the Tom, the longer the time for recovery and recrystallization, and the smaller the amount of accumulated processing strain. Therefore, from the viewpoint of obtaining a ferrite structure with less processing strain, it is preferable that Tom is large. The time between final rolling passes may be, for example, in the range of 5 seconds to 120 seconds.

The total cumulative strain (ε _T ) is a parameter representing the amount of machining strain introduced by hot rolling. The machining strain introduced during hot rolling is considered to remain in the structure after transformation, although it is alleviated by transformation. That is, 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 strain is left in 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 cumulative strain is small. The total cumulative strain may be, for example, in the range of 1.0 to 30.

As explained above, the machining strain remaining after hot rolling changes depending on the "total cumulative strain", "FRT", and "Tom" introduced during hot rolling. Therefore, the present inventors have found the left side of the equation (1) using these as parameters after further studies. Then, the present inventors satisfy low strength (that is, 520 MPa or less) and high elongation characteristics (that is, EL ≧ 16%, uEL ≧ 13.5%) when the left side of the formula (1) is less than 69. Found to be difficult. Therefore, the lvalue in equation (1) is set to 69 or more. The lvalue in the 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 rvalue in the formula (1) is not particularly limited, but in view of the manufacturing conditions and the like according to the embodiment of the present invention, the upper limit is about 100.

(cooling)
After the hot rolling, so-called water cooling or the like is not performed for rapid cooling, but air cooling is performed to cool the product to room temperature. This is because when water cooling or the like is carried out, hard structures such as bainite, martensite, and island-like martensite (MA: Martensite-Austenite constituent) are generated, and the strength is increased. Further, when a hard structure such as MA is dispersed in steel, the elongation characteristics deteriorate, so that the steel is transformed by air cooling to secure a desired ferrite fraction. Further, if water cooling or the like having a high cooling rate is carried out, the degree of supercooling becomes large and the particles become finer. By air cooling, fine granulation can be suppressed and an increase in strength can be suppressed.

(1) Method for producing test material A steel piece having the chemical composition shown in Table 1 was produced by melting in a converter. Then, before hot rolling, it was heated to 1000 or more and 1230 ° C. or less, and hot rolling and cooling were carried out under the conditions shown in Table 2 to produce a steel sheet having the plate thickness shown in Table 2. The rolling temperature was 1100 ° C. or lower. Further, in the present embodiment, in the derivation of the formula (1) shown in Table 2, the friction coefficient μ is set to “0.55”. For the reduction rate 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 in Tables 3 and 4 described later, the underlined numerical values indicate that they are out of the scope of the embodiment of the present invention. However, it should be noted that "-" is not underlined even if it is out of the scope of the embodiment of the present invention.

(2) Tissue observation No. 1 and Table 2 No. 1 and No. 6-No. The tissue of No. 8 was observed as follows. A test piece is cut out from the steel plate so that the observation surface is parallel to the surface of the steel plate and the observation position is at the t / 4 position (t: plate thickness). Observed under a microscope. The area ratio of ferrite was analyzed with image analysis software (Image-J) for each of the 10 fields of view (field of view area: 150 μm × 200 μm) of the optical microscope, and the average value of the ferrite area ratio was calculated for each sample. In this way, the ferrite area ratio of each sample was calculated.
For the ferrite grain size, create a grid with 25 μm intervals in one field of view (field of view area: 150 μm × 200 μm) of a 400-fold optical microscope image, determine the number of ferrite grains crossed by one straight line, and determine the length of the straight line in the field of view. Was divided by the number to obtain the particle size per ferrite in the straight line. Then, in the other straight lines as well, the particle size per ferrite was obtained, the average value of these was calculated, and this average value was taken as the ferrite particle size.
No. The ferrite area ratios of 1, 6, 7 and 8 were 80%, 82%, 84% and 83%, respectively, which satisfied the requirements according to the embodiment of the present invention. In addition, No. The ferrite grain sizes of 1, 6, 7 and 8 were 7 μm, 22 μm, 16 μm and 14 μm, respectively, which satisfied the requirements according to the embodiment of the present invention. The remaining tissue is No. All of 1, 6, 7 and 8 were pearlite.

(3) Tensile test method The test piece for the tensile test was taken from the rolled material so that the longitudinal direction (tensile direction) was orthogonal to the rolling direction. The shape of the test piece is NK-U1 based on the Nippon Kaiji Kyokai's ship class regulation K edition material (2019 version), the distance between gauge points (GL) is 200 mm, and it is a total thickness shape, so it is shown in Table 2. It was carried out with the plate thickness of. As the yield point (YP: Yield Point), the upper yield point was adopted, but when the upper yield did not appear, the yield point was 0.2% proof stress. The tensile test conditions were carried out in accordance with the NK ship class standard K edition. Specifically, the tensile speed was set to 20 N / mm ² before reaching the yield point, and the strain rate was set to 30 PL% / min after reaching the yield point. The uniform elongation was calculated by the elongation up to the maximum load in the stress-strain diagram. Samples having a tensile strength (TS) of 400 MPa or more and 520 MPa or less, a breaking elongation (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. The yield strength (YS) is also shown in Table 3.

(4) Corrosion test method (CCT conditions)
As shown in FIG. 1, the corrosion test piece is prepared by applying 15 μm of Zn-rich paint on the surface of a 5t × 90W × 120L (mm) steel material, applying and curing 160 μm of a modified epoxy resin on the surface, and further 160 μm of the modified epoxy resin. Was applied and cured, and a coating having a total film thickness of 335 μm was applied. A coating film scratch portion (that is, a coating scratch portion) was artificially applied to the central portion of the painted test piece using a plastic cutter, and the outer periphery of the test piece was covered with masking tape, and the evaluation area was adjusted to 80 cm ² .

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

Five pieces (10 pieces in total) of each of the developed steel (No. B in Table 1) and the conventional steel (No. A in Table 1) were prepared. After that, the above corrosion test was carried out, and the corrosion depth after removing the coating film was measured at 6 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, the average value of 5 pieces was calculated, and the corrosion depth was calculated. Samples with a corrosion depth of 0.600 mm or less were accepted. The calculation results are shown in Table 4.

The results of the above tissue observation and the results of Tables 3 and 4 will be considered.
Example No. 1 and No. Since 6 to 8 satisfied the requirements for the chemical composition and the production conditions according to the embodiment of the present invention, a desired metal structure was obtained, and the tensile strength (TS) and elongation characteristics (EL, uEL) were good. there were. In addition, Example No. Since Nos. 2 to 5 satisfied the requirements for the chemical composition and the production conditions according to the embodiment of the present invention, Example No. 1 and No. It is considered that the desired metal structure was obtained as in 6 to 8, and as a result, the tensile strength (TS) and the elongation characteristics (EL, uEL) were good. In addition, Example No. 1 to No. Since No. 8 contains a predetermined amount of Cu + Ni and Cr, it is considered to be excellent in corrosion resistance. In order to demonstrate this, as described above, No. A and No. A corrosion test using B was also performed. As a result, Example No. 1 to No. Example No. 8 which satisfies the requirements for the chemical composition according to the embodiment of the present invention as in 8. B confirmed that the corrosion resistance was good.

On the other hand, Comparative Example No. 9-No. No. 11 was inferior in tensile strength (TS) and uniform elongation (uEL) because it did not satisfy the formula (1). In addition, although the elongation at break was satisfactory, the value was lower than that of the examples. The reason why the formula (1) was not satisfied was No. No. 9 had a relatively low FRT, a low Tom, and a large cumulative strain εT, and No. 10, No. It is considered that No. 11 was due to the relatively low FRT and the relatively large cumulative strain εT. FIG. 4 is a graph showing the relationship between the left side of equation (1) and the tensile strength. FIG. 5 is a graph showing the relationship between the left side of equation (1) and uniform elongation. With reference to FIGS. 4 and 5, it can be seen that when the left side of the equation (1) is 69 or more, the tensile strength is lowered and the uniform elongation is improved. In addition, Comparative Example No. Since A did not satisfy the requirements for the chemical composition according to the embodiment of the present invention, the corrosion depth exceeded 0.600 mm and the corrosion resistance was inferior.

The thick steel plate according to the embodiment of the present invention can be widely applied to fields such as shipbuilding, offshore structures, bridges, etc. where improvement of coating corrosion resistance and / or on-site workability (that is, improvement of elongation characteristics) is required, and is particularly limited. Although not to be applied, thick steel plates for ships such as the upper deck of a hull ballast tank can be mentioned as a specific application destination.

This application is accompanied by a priority claim based on Japanese Patent Application No. 2020-168729, the filing date of which is October 5, 2020. Japanese Patent Application No. 2020-168729 is incorporated herein by reference.

Claims

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% by mass, 0.015% by mass or less,
S: More than 0% by mass, 0.005% by mass or less,
Al: 0.005% by mass or more, 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, 0.20% by mass or less,
Ca: 0.0001% by mass or more, 0.005% by mass or less,
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, and the balance is composed of Fe and unavoidable impurities.
The metallographic structure contains ferrite with an area ratio of 70% or more with respect to the total metallic structure, and the balance consists of 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.
Tensile strength is 400 MPa or more and 520 MPa or less, fracture elongation when a tensile test is performed using a test piece of NK-U1 with a distance between gauge points of 200 mm in accordance with Nippon Kaiji Kyokai's ship class regulation K edition material (FY2019 version). Is 16% or more, and the uniform elongation is 13.5% or more, and is a low-strength thick steel sheet having excellent elongation characteristics and corrosion resistance.
The thick steel sheet according to claim 1, wherein the average corrosion depth of the painted scratch portion after 168 days of the composite cycle test is 0.600 mm or less.
The thick steel plate according to claim 1 or 2, further comprising at least one of the following (a) and (b).
(A) B: 0.0001% by mass or more, 0.010% by mass or less, V: 0.01% by mass or more, 0.50% by mass or less, and Nb: 0.001% by mass or more, 0.50% by mass. One or more selected from the group consisting of the following (b) 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, Zr: 0.005 One or more selected from the group consisting of mass% or more, 0.20 mass% or less, and Mg: 0.0001 mass% or more, 0.005 mass% or less.
A step of heating steel satisfying the component composition according to claim 1 or 2 to 1100 ° C. or higher, and
A step of hot rolling so that the rolling temperature is 1100 ° C. or lower and the recrystallization reduction rate is 80% or more and the following formula (1) is satisfied.
The method for manufacturing a thick steel sheet according to claim 1 or 2, which comprises an air cooling step.
0.08FRT +0.1Tom-0.5ε T +10 ≧ 69 ・・・ (1)
however,
FRT: Finish rolling completion temperature (° C)
Tom: Time between the last pass and the rolling pass one pass before the last pass (seconds)
ε T : The total value of the cumulative strains at each position of the surface of the steel sheet, the t / 4 position and the t / 2 position (t: plate thickness), and is calculated by the following equation (2). The cumulative strain at each position is calculated by the following equation (3). Further, the distortion of each path 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 × (2y / h) 2 + 1.15 × ln (H / h) ・・・ (4)
In equations (2) to (4)
ε y : Equivalent plastic strain at y position in the plate thickness direction,
ε yi : Equivalent plastic strain at the y position in the plate thickness direction of the i-pass,
y: Distance from the center of plate thickness (mm), h: Outer side plate thickness (mm), H: Enter side plate thickness (mm),
C = B 1 × (H / 2R) B2 × re 2 ... (5)
In equation (5)
B 1 = 0.18381 + 0.344435μ + 1.48066 × μ 2
B 2 = 0.076669-2.0566μ + 2.1128 × μ 2
However, μ: friction coefficient, R: roll radius (mm), re : reduction rate
A step of heating steel satisfying the component composition according to claim 3 to 1100 ° C. or higher,
A step of hot rolling so that the rolling temperature is 1100 ° C. or lower and the recrystallization reduction rate is 80% or more and the following formula (1) is satisfied.
The method for manufacturing a thick steel sheet according to claim 3, further comprising an air cooling step.
0.08FRT +0.1Tom-0.5ε T +10 ≧ 69 ・・・ (1)
however,
FRT: Finish rolling completion temperature (° C)
Tom: Time between the last pass and the rolling pass one pass before the last pass (seconds)
ε T : The total value of the cumulative strains at each position of the surface of the steel sheet, the t / 4 position and the t / 2 position (t: plate thickness), and is calculated by the following equation (2). The cumulative strain at each position is calculated by the following equation (3). Further, the distortion of each path 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 × (2y / h) 2 + 1.15 × ln (H / h) ・・・ (4)
In equations (2) to (4)
ε y : Equivalent plastic strain at y position in the plate thickness direction,
ε yi : Equivalent plastic strain at the y position in the plate thickness direction of the i-pass,
y: Distance from the center of plate thickness (mm), h: Outer side plate thickness (mm), H: Enter side plate thickness (mm),
C = B 1 × (H / 2R) B2 × re 2 ... (5)
In equation (5)
B 1 = 0.18381 + 0.344435μ + 1.48066 × μ 2
B 2 = 0.076669-2.0566μ + 2.1128 × μ 2
However, μ: friction coefficient, R: roll radius (mm), re : reduction rate