JP7188180B2 - Corrosion-resistant and wear-resistant steel for holds of coal carriers and coal carriers - Google Patents

Description

TECHNICAL FIELD The present invention relates to a corrosion-resistant and wear-resistant steel for holds of a coal carrier or a combined coal carrier.

Cargo holds of coal carriers and coal carriers are wetted and corroded when they are loaded with coal in order to prevent spontaneous combustion. In addition, the hold is subject to wear from heavy machinery buckets and the like, particularly when coal is being unloaded. Corrosion-resistant and wear-resistant steel that achieves both corrosion resistance and wear resistance has therefore been proposed as a steel material for use in the holds of coal carriers and coal carriers (see, for example, Patent Documents 1 and 2).

JP 2010-100872 A JP 2017-128762 A

Conventionally, in the hold (cargo hold) of a coal carrier and a combined coal carrier, the sulfur contained in the coal dissolves into the condensation water formed on the side wall of the hold, and the rise in temperature causes the formation of sulfuric acid. It was thought that corrosion progresses due to the pH environment. However, as a result of analyzing the corroded steel material in the cargo hold by the present inventors, although chloride ions (Cl ⁻ ) were recognized from the base iron interface, sulfate ions (SO ₄ ²⁻ ) were not recognized. I didn't. Therefore, it is believed that the major corrosion factor in cargo holds is chloride ions (Cl ⁻ ) rather than sulfate ions (SO ₄ ²⁻ ).

As described above, conventionally, it was thought that corrosion of steel materials progressed by sulfuric acid in cargo holds, but it was found that chloride ions (Cl ⁻ ) are the main cause of corrosion. Therefore, a concept different from the conventional one is required for composition design of corrosion-resistant and wear-resistant steel for holds of coal carriers or coal carriers. Furthermore, the inside of the cargo hold, particularly the bottom plate, is subject to abrasion damage due to cargo such as coal and ore, and abrasion damage due to buckets of heavy machinery.

In view of such circumstances, the present invention provides a corrosion-resistant, wear-resistant cargo hold for a coal carrier or a combined coal carrier that is resistant to corrosion due to chloride ions, wear due to cargo such as coal and ore, buckets of heavy machinery, etc. The task is to provide steel.

The inventors of the present invention have found that the hold of a coal carrier or a combined coal carrier is required to have corrosion resistance against chloride ions rather than acid resistance, and that hardness is required to increase wear resistance. Therefore, attention was paid to Cr, which is an alloying element that increases the hardenability of steel and contributes to the improvement of hardness. In addition, in order to achieve both corrosion resistance and wear resistance of steel materials, the metallographic structure was also investigated. As a result, the Cr content was set to more than 0.05%, the cooling rate after hot rolling was set to more than 15 ° C./s, and the metal structure had a lath-like structure with an area ratio of 90% or more. It was found that both corrosion resistance and wear resistance can be achieved by setting the major axis / minor axis ratio of cementite present in the rice field. Here, "200HV5" is an indication based on JIS Z 2244:2009, and means that the Vickers hardness measured with a test force of 5 kgf (49.03 N) is 200 or more.

The present invention was made based on such findings, and the gist thereof is as follows.

[1] % by mass,
C: 0.01% or more and 0.15% or less,
Si: 0.05% or more and 1.00% or less,
Mn: 0.10% or more and 2.00% or less,
P: more than 0%, 0.020% or less,
S: more than 0%, 0.010% or less,
Cr: more than 0.05%, 3.00% or less,
Al: 0.01% or more and 0.10% or less,
B: 0.0003% or more and 0.0020% or less,
N: containing 0.0020% or more and 0.0100% or less, the balance being Fe and impurities,
The carbon equivalent Ceq (%) determined by the following formula (1) is 0.20% or more,
In a surface layer portion of 1 mm to 3 mm in the plate thickness direction from the surface, the area ratio of the lath-like structure is 90% or more, and the average value of the major axis/minor axis ratio of cementite present in the lath-like structure is 2.00 or more. having a metallographic structure of
Corrosion-resistant and wear-resistant steel for holds of coal carriers or coal carriers, wherein the surface layer has a Vickers hardness of 200HV5 or more.
Ceq(%)=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/4 (1)
Here, [X] is the content of the element X in mass %, and 0 is substituted for the element X that is not contained.

[2] Furthermore, in % by mass,
Mo: 1.00% or less,
W: 1.00% or less,
Ni: 1.00% or less,
Cu: 1.00% or less,
Sn: 0.20% or less,
Sb: Corrosion-resistant and wear-resistant steel for holds of a coal carrier or a coal carrier according to [1] above, containing one or more of 0.20% or less of Sb.

[3] Furthermore, in % by mass,
Ti: 0.025% or less,
V: 0.20% or less,
Nb: Corrosion-resistant and wear-resistant steel for holds of a coal carrier or a coal carrier according to [1] or [2] above, containing one or more of 0.05% or less of Nb.

[4] Furthermore, in % by mass,
Ca: 0.05% or less,
Mg: 0.05% or less,
REM: Corrosion-resistant and wear-resistant steel for holds of coal carriers or coal carriers according to any one of [1] to [3] above, containing one or more of 0.1% or less.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a corrosion-resistant and wear-resistant steel for holds of a coal carrier or a coal carrier, which is resistant to corrosion caused by chloride ions and damage caused by abrasion caused by cargo such as coal and ore, buckets of heavy machinery, and the like. It becomes possible, and the industrial effect is remarkable.

Hereinafter, the corrosion-resistant and wear-resistant steel for holds (hereinafter sometimes referred to as the corrosion-resistant and wear-resistant steel for holds) of a coal carrier or a combined coal carrier according to the present embodiment will be described in detail. First, the chemical composition of the corrosion-resistant and wear-resistant steel for cargo holds of this embodiment will be described. In addition, "%" which shows content of each element in a chemical composition means the mass %. In addition, in the numerical range in the chemical composition, the numerical range represented by using "~" means a range including the numerical values before and after "~" as lower and upper limits, unless otherwise specified. Therefore, for example, 0.01% to 0.15% means a range of 0.01% to 0.15%.

<C: 0.01% or more and 0.15% or less>
C is an effective element for ensuring hardness, and the C content is made 0.01% or more. The C content is preferably 0.02% or more, more preferably 0.03% or more. On the other hand, if the C content is excessive, the toughness is lowered, and depending on the manufacturing conditions, the cementite present in the lath-like structure grows, resulting in a major axis/minor axis ratio of less than 2.00 and a decrease in corrosion resistance. Therefore, the C content is made 0.15% or less. Preferably, the C content is 0.13% or less.

<Si: 0.05% or more and 1.00% or less>
Si is a deoxidizing agent and an element effective in improving hardness, and the Si content is made 0.05% or more. Preferably, the Si content is 0.10% or more. On the other hand, if the Si content exceeds 1.00%, the toughness decreases, so the Si content is made 1.00% or less. The Si content is preferably 0.80% or less, more preferably 0.50% or less.

<Mn: 0.10% to 2.00%>
Mn is an element that enhances hardenability and hardness, and should be contained in an amount of 0.10% or more. The Mn content is preferably 0.20% or more, more preferably 0.50% or more. On the other hand, excessive Mn content lowers the toughness and promotes the formation of cementite, which tends to result in lower corrosion resistance. Therefore, the Mn content is set to 2.00% or less. The Mn content is preferably 1.80% or less, more preferably 1.60% or less.

<P: more than 0%, 0.020% or less>
P is an impurity and lowers toughness and workability, so the P content is limited to 0.020% or less. The P content is preferably 0.015% or less, more preferably 0.010% or less. Although the lower limit of the P content is preferably 0%, it should be more than 0% from the viewpoint of manufacturing costs. The P content may be 0.0001% or more.

<S: more than 0%, 0.010% or less>
S is also an impurity and lowers toughness, so the S content is limited to 0.010% or less. The S content is preferably 0.007% or less, more preferably 0.005% or less, and still more preferably 0.003% or less. Although the lower limit of the S content is preferably 0%, it should be more than 0% from the viewpoint of manufacturing costs. The lower limit of the S content may be 0.0001%.

<Cr: more than 0.05%, 3.00% or less>
Cr is an important element that suppresses corrosion caused by chloride ions, enhances hardenability, and increases hardness. Chloride ions are the main cause of progress of corrosion of steel materials in cargo holds, and the Cr content is set to more than 0.05% in order to improve corrosion resistance. The Cr content is preferably 0.10% or more, and more preferably 0.20% or more in order to improve hardenability. On the other hand, if Cr is contained excessively, the toughness is lowered, so the Cr content is made 3.00% or less. The Cr content is preferably 2.00% or less, more preferably 1.70% or less, still more preferably 1.50% or less.

<Al: 0.01% or more and 0.10% or less>
Al is an element effective as a deoxidizing agent. In addition, Al forms AlN with N, refines crystal grains, and improves toughness, so the Al content is made 0.01% or more. The Al content is preferably 0.02% or more, more preferably 0.03% or more. On the other hand, if Al is contained excessively, the toughness is lowered, so the Al content is made 0.10% or less. The Al content is preferably 0.08% or less, more preferably 0.07% or less.

<B: 0.0003% or more and 0.0020% or less>
B is an element that significantly increases the hardenability of steel, and the B content is made 0.0003% or more to ensure hardness. The B content is preferably 0.0005% or more, more preferably 0.0007% or more. More preferably, it should be 0.0010% or more. On the other hand, if the B content is excessive, borides are formed, which lowers the hardenability and makes it impossible to secure the hardness. The B content is preferably 0.0018% or less, more preferably 0.0016% or less.

<N: 0.0020% or more, 0.0100% or less>
N is an element that forms nitrides with Al and Ti, refines crystal grains, and improves toughness, so the lower limit of the N content is made 0.0020%. The N content is preferably 0.0030% or more, more preferably 0.0040% or more. On the other hand, when N is contained excessively, coarse nitrides are formed and the toughness is lowered, so the N content is made 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0060% or less.

The rest of the chemical composition of the corrosion and wear resistant steel for holds according to the present embodiment is Fe and impurities. Here, the term "impurities" refers to components that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when steel is industrially manufactured. It means a permissible range that does not adversely affect the properties of the corrosion and wear resistant steel. However, in the corrosion-resistant and wear-resistant steel for ship holds according to the present embodiment, it is necessary to specify the upper limits of P and S among the impurities, as described above.

Furthermore, one or more of Mo, W, Ni, Cu, Sn and Sb can be contained in order to improve corrosion resistance.

<Mo: 1.00% or less>
Mo is an element that improves corrosion resistance and hardenability. The Mo content may be 0%, but is preferably 0.05% or more in order to promote formation of a lath-like structure and increase hardness. More preferably, the Mo content is 0.10% or more. On the other hand, if Mo is contained excessively, the hardness becomes too high and the toughness decreases, so the Mo content is made 1.00% or less. The Mo content is preferably 0.80% or less, more preferably 0.60% or less.

<W: 1.00% or less>
W, like Mo, is an element that improves corrosion resistance and hardenability. The W content may be 0%, but is preferably 0.05% or more in order to promote the formation of a lath-like structure and increase hardness. More preferably, the W content is 0.10% or more. On the other hand, if the W content is excessive, the hardness becomes too high and the toughness decreases, so the W content is made 1.00% or less. The W content is preferably 0.80% or less, more preferably 0.60% or less.

<Ni: 1.00% or less>
Ni is an element that enhances the hardenability of steel, contributes to the improvement of hardness, and improves corrosion resistance. The Ni content may be 0%, but is preferably 0.03% or more. More preferably, the Ni content is 0.10% or more, and still more preferably 0.20% or more. On the other hand, since Ni is an expensive alloying element, the Ni content is set to 1.00% or less from the viewpoint of cost. The Ni content is preferably 0.95% or less, more preferably 0.90% or less.

<Cu: 1.00% or less>
Cu is an element that enhances the hardenability of steel and improves the corrosion resistance. The Cu content may be 0%, but is preferably 0.03% or more. More preferably, the Cu content is 0.10% or more, and more preferably 0.20% or more. On the other hand, since Cu deteriorates hot workability and weldability, the Cu content is made 1.00% or less. The Cu content is preferably 0.95% or less, more preferably 0.90% or less.

<Sn: 0.20% or less>
Sn is an element that enhances corrosion resistance when the cargo hold becomes empty and chloride ions become concentrated and the pH drops. The Sn content may be 0%, but is preferably 0.01% or more. More preferably, the Sn content is 0.05% or more. On the other hand, if Sn is excessively contained, manufacturability and toughness are lowered, so the Sn content is made 0.20% or less. The Sn content is preferably 0.15% or less.

<Sb: 0.20% or less>
Sb, like Sn, is an element that enhances corrosion resistance when the cargo hold becomes empty and chloride ions become concentrated and the pH drops. The Sb content may be 0%, but is preferably 0.01% or more. More preferably, the Sb content is 0.05% or more. On the other hand, if the Sb content is excessive, the manufacturability and toughness are lowered, so the Sb content is made 0.20% or less. Preferably, the Sb content is 0.15% or less.

Furthermore, in order to improve mechanical properties such as toughness, one or more of Ti, V and Nb can be contained.

<Ti: 0.025% or less>
Ti is an element that forms TiN, refines crystal grains, and improves toughness. Although the content of Ti may be 0%, it is preferable to contain 0.005% or more. More preferably, the Ti content is 0.007% or more, and still more preferably 0.010% or more. On the other hand, excessive Ti content may reduce toughness, so the Ti content is made 0.025% or less. It is preferably 0.020% or less, more preferably 0.015% or less.

<V: 0.20% or less>
V is an element that increases hardenability, contributes to an improvement in hardness, and improves toughness. The V content may be 0%, but is preferably 0.01% or more. More preferably, the V content is 0.02% or more, and still more preferably 0.05% or more. On the other hand, if the V content is excessive, the toughness may be lowered, so the V content is made 0.20% or less. The V content is preferably 0.18% or less, more preferably 0.15% or less.

<Nb: 0.05% or less>
Nb is an element that contributes to refinement of crystal grains by suppressing formation of nitrides and recrystallization. The Nb content may be 0%, but is preferably 0.005% or more in order to improve toughness. More preferably, the Nb content is 0.007% or more, more preferably 0.01% or more. On the other hand, if the Nb content is excessive, the toughness may be lowered, so the Nb content is made 0.05% or less. The Nb content is preferably 0.03% or less, more preferably 0.02% or less.

Furthermore, one or more of Ca, Mg, and REM can be contained in order to control the morphology of inclusions.

<Ca: 0.05% or less>
Ca is an element that forms oxides and sulfides. Although the Ca content may be 0%, it is preferable to set the Ca content to 0.0005% or more in order to form inclusions that are difficult to stretch by hot rolling. More preferably, the Ca content is 0.0007% or more, more preferably 0.0010% or more. On the other hand, if Ca is contained excessively, coarse oxides may be formed and the toughness may be lowered, so the Ca content is made 0.05% or less. The Ca content is preferably 0.02% or less, more preferably 0.01% or less.

<Mg: 0.05% or less>
Mg is an element that forms oxides and sulfides. Although the Mg content may be 0%, the Mg content is preferably 0.0005% or more in order to form fine oxides and mainly improve toughness. More preferably, the Mg content is 0.0007% or more, more preferably 0.0010% or more. On the other hand, if Mg is contained excessively, coarse oxides may be formed and the toughness may be lowered, so the Mg content is made 0.05% or less. The Mg content is preferably 0.02% or less, more preferably 0.01% or less.

<REM: 0.1% or less>
REM is an element that forms oxides and sulfides. The REM content may be 0%, but the REM content is used to form inclusions that are difficult to stretch by hot rolling and to form fine oxides, mainly to improve toughness. is preferably 0.0005% or more. More preferably, the REM content is 0.001% or more, more preferably 0.002% or more. On the other hand, if the REM content is excessive, coarse oxides may be formed and the toughness may be lowered, so the REM content is made 0.1% or less. More preferably, the REM content is 0.02% or less, more preferably 0.01% or less. Here, REM refers to a total of 17 elements of Sc, Y and lanthanoids, and the REM content means the total content of these elements. REMs are added to molten steel using, for example, Fe--Si--REM alloys, which include, for example, Ce, La, Nd, Pr.

(Carbon equivalent Ceq: 0.20% or more)
The carbon equivalent Ceq is an index of hardenability that affects the hardness of steel, and is set to 0.20% or more to improve wear resistance. It is preferably 0.30% or more, more preferably 0.35% or more. The carbon equivalent Ceq is preferably 0.60% or less in order to ensure workability and toughness. More preferably 0.55% or less, more preferably 0.50% or less. The carbon equivalent Ceq is calculated by the following formula (1) depending on the contents of alloying elements.

Ceq(%)=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/4 (1)
Here, [X] is the content of the element X in mass %, and 0 is substituted for the element X that is not contained.

(metal structure)
Next, the metal structure of the corrosion-resistant and wear-resistant steel for holds according to this embodiment will be described.

In order to ensure the hardness of the corrosion-resistant and wear-resistant steel for cargo holds according to the present embodiment, the metal structure of the surface layer 1 mm to 3 mm from the surface in the plate thickness direction is important, and the area ratio of the lath-like structure is 90% or more. and The lath-like structure is a hard structure that is generated when the carbon equivalent Ceq is 0.20% or more and the cooling rate after hot rolling or the cooling rate in quenching treatment is set to more than 15°C/s. The lath-like structure is, for example, self-tempered martensite, lower bainite, or both, or tempered martensite. The remainder of the lath-like structure is generally one or more of polygonal ferrite, pearlite, and upper bainite. Self-tempered martensite is martensite in which cementite is formed during cooling after heating in a hot rolling or quenching process. In the metal structure other than the surface layer portion, the area ratio of the lath-like structure should be 50% or more, but it is preferably 90% or more as in the surface layer portion.

The area ratio of the lath-like structure is measured by mirror-polishing a sample whose observation surface is the plate thickness cross-section, etching with nital, observing the metal structure with an optical microscope at a magnification of 400, and using a photograph taken. do. It is recommended that the measurement of the area ratio of the lath-like structure of the surface layer be performed in the vicinity of the position 2 mm from the surface in the plate thickness direction. It is recommended that the measurement of the area ratio of the lath-like structure other than the surface layer be performed in the vicinity of the 1/2 position of the plate thickness in the plate thickness direction from the surface.

If the cooling rate is slow or if the tempering process is held at a high temperature for a long period of time, the cementite precipitated in the lath-like structure grows. At this time, if cementite grows excessively, the major axis/minor axis ratio decreases (approaches 1). When the major axis/minor axis ratio of cementite is less than 2.00, it functions as a cathodic site and accelerates melting of steel, resulting in deterioration of corrosion resistance. Therefore, in order to ensure corrosion resistance, the cementite present in the lath-like structure must have a major axis/minor axis ratio of 2.00 or more. That is, the major axis/minor axis ratio of cementite is an index indicating the degree of growth of cementite, and approaches 1 as cementite grows. .

The long axis/short axis ratio of cementite present in the lath-like structure can be measured using a scanning electron microscope (SEM). For example, when the corrosion-resistant and wear-resistant steel for cargo holds is a steel plate, the thickness cross-section of the steel plate is used as an observation surface, mirror polishing and electrolytic polishing are performed, and the range of 1 mm to 3 mm in the plate thickness direction from the surface is magnified 10000 times with an SEM. to shoot. The major axis/minor axis ratio of 100 cementites having a major axis of 30 to 300 nm is measured, and the arithmetic average is taken as the major axis/minor axis ratio of the cementite. An energy dispersive X-ray spectroscopy (EDS) attached to the SEM can be used to confirm whether or not the particles imaged by the SEM are cementite. It is recommended that the measurement of the major axis/minor axis ratio of cementite be performed in the vicinity of the position 2 mm from the surface in the plate thickness direction.

The Vickers hardness of the surface layer of the corrosion-resistant and wear-resistant steel for holds according to the present embodiment (hereinafter also referred to as "surface layer hardness") is set to 200HV5 or more in order to ensure wear resistance. 250HV5 or more is preferable. The hardness of the surface layer of the corrosion-resistant and wear-resistant steel for ship holds is preferably 450HV5 or less in consideration of workability. The surface layer hardness of the corrosion-resistant and wear-resistant steel for holds is measured in accordance with JIS Z 2244:2009 at a position 2 mm from the surface in the thickness direction using the thickness cross-section of the steel plate as a measurement surface. Specifically, at room temperature, with a test force of 49.03 N (5 kgf), at a position 2 mm from the surface in the thickness direction of the steel plate, 5 points every 1 mm in the direction perpendicular to the thickness direction, for a total of 25 points Vickers hardness is measured, and the average value (arithmetic mean) of these values is obtained.

In the present embodiment, the shape of the corrosion-resistant and wear-resistant steel for holds is not particularly limited, and may be a steel plate, a steel strip, a shaped steel, a steel pipe, a steel bar, a steel wire, or the like. Steel materials such as steel plates, steel strips, shaped steels, and steel pipes have a thickness of 7 to 50 mm. Considering the disappearance of the surface layer due to wear, it is preferably 10 mm or more. Although the upper limit is not particularly defined, it is preferably 40 mm or less, more preferably 30 mm or less.

(Production method)
Next, a preferred method for manufacturing the corrosion-resistant and wear-resistant steel for cargo holds according to this embodiment will be described. Corrosion- and wear-resistant steel for holds according to the present embodiment includes steel plates, shaped steels, steel pipes, and the like manufactured by hot rolling.

The corrosion-resistant and wear-resistant steel for ship holds according to the present embodiment is obtained by melting steel by a conventional method, adjusting the components, and then hot-rolling the obtained steel slab, followed by accelerated cooling as it is, or by air-cooling. After that, it is manufactured by reheating and quenching. Further, tempering treatment may be applied. Steel slabs can be manufactured by a known method such as continuous casting or ingot casting-blooming after being melted by a normal refining process such as a converter or an electric furnace, and there is no particular limitation. After hot rolling, it may be coiled.

In this embodiment, when manufacturing a steel pipe, a steel plate may be formed into a tubular shape and welded, and a UO steel pipe, an electric resistance welded steel pipe, a butt welded steel pipe, a spiral steel pipe, or the like may be formed. A seamless steel pipe manufactured by subjecting a steel billet to hot extrusion or piercing-rolling is also included in the corrosion-resistant and wear-resistant steel for ship holds of the present embodiment.

Water cooling after hot rolling and cooling for quenching treatment require a cooling rate of more than 15°C/s. This is because the metal structure of the corrosion-resistant and wear-resistant steel for holds according to the present embodiment is a lath-like structure with an area ratio of 90% or more. The faster the cooling rate, the better, but it is appropriately determined depending on the thickness of the plate and the capacity of the cooling equipment, and may be 50° C./s or less. The temperature at which accelerated cooling after hot rolling and cooling in quenching are stopped is preferably 400° C. or less in order to suppress precipitation of carbides.

Further, if workability and toughness are required for the corrosion-resistant and wear-resistant steel for cargo holds, a tempering treatment of heating to 400° C. or higher may be performed. The tempering temperature is preferably 600° C. or lower in order to suppress the growth of cementite. The holding time of the tempering treatment may be 10 minutes or more and 300 minutes or less.

Hereinafter, the present invention will be described in more detail with reference to examples of the corrosion-resistant and wear-resistant steel for cargo holds according to the present invention. The present invention is not limited to the following examples, and it is possible to make appropriate modifications within the scope of the present invention, and any of them are within the technical scope of the present invention. It is included.

Steel No. in Tables 1A and 1B. Steels having chemical compositions shown in A1 to A12 and B1 to B11 were melted. Here, Table 1B is a continuous table to the right of Table 1A, in which the chemical composition from C to N for each steel is listed in Table 1A, the chemical composition from Mo to REM for each steel, and the chemical composition from Mo to REM for each steel. are listed in Table 1B. The balance of the chemical composition of each steel shown in Tables 1A and 1B is Fe and impurities. After each steel was melted, the slab obtained by casting was heated, hot rolled, and cooled with water at a cooling rate of more than 15 ° C./s to produce a steel plate with a thickness of 10 mm (No in Table 2 .1 to No. 12, No. 101 to No. 111, No. 201). Here, as a comparison, No. 110 was allowed to cool after hot rolling. Furthermore, No. 12 and no. After cooling with water, No. 111 was tempered at 150° C. and 610° C. for 15 minutes.

The area ratio of the lath-like structure and the long axis/minor axis ratio of cementite present in the lath-like structure were measured using the plate thickness cross section as a test surface in the range of 1 mm to 3 mm from the surface in the plate thickness direction of the steel plate. The metal structure was observed by mirror-polishing the sample, etching it with nital, and magnifying it 400 times with an optical microscope. For samples with a lath-like structure area ratio of 90% or more in the surface layer portion, the metallographic structure at the central portion of the sheet thickness was also observed to measure the lath-like structure area ratio. The major axis/minor axis ratio of cementite present in the lath-like structure was measured by subjecting the sample to mirror polishing and electrolytic polishing, and imaging the sample at 10,000 times using SEM and EDS. The major axis/minor axis ratio of cementite having a major axis of 30 to 300 nm was measured, and the arithmetic mean of 100 measured values was obtained. When the area ratio of the lath-like structure was less than 90%, the cementite long axis/short axis ratio was not measured.

Further, a test piece was taken from the obtained steel sheet, and the Vickers hardness was measured at a position 2 mm in the thickness direction from the surface of the steel sheet using the thickness cross section as a test surface. Vickers hardness measurement is performed at room temperature with a test force of 49.03 N in accordance with JIS Z 2244: 2009, and the average value of 25 measurements in total, 5 points every 1 mm in the direction perpendicular to the thickness direction (arithmetic mean) was obtained. Furthermore, a full-size V-notch Charpy test piece was cut from the steel plate, and the Charpy absorbed energy (KV ₂ ) at −20° C. was measured according to JIS Z 2242:2018.

Furthermore, the surface of a test piece having a length of 100 mm, a width of 60 mm, and a thickness of 5 mm was subjected to blasting treatment so as to have a Sa of 2.5 (ISO 8501-1) or more, and was used for evaluation of corrosion resistance. Corrosion resistance is measured by placing coal powder containing artificial seawater on the test piece, holding it in a test chamber at a temperature of 40 ° C and a relative humidity of 98% for one week, removing it from the test chamber, and attaching it to the surface of the test piece The coal was lightly removed with a scraper, held again in the test tank for 1 week, and the corrosion test was carried out for 6 cycles, which was defined as 1 cycle, and evaluated. After 6 cycles of corrosion testing, the rust was removed with a scraper and derusted with ammonium citrate. After that, the change in weight before and after the test was defined as the amount of corrosion. Each corrosion amount is No. in Table 2. 201 ordinary steel was set to 100, and the relative amount was evaluated.

The judgment criteria for each evaluation item are as follows. The hardness of the surface layer was determined to be 200 HV5 or more from the viewpoint of wear resistance, and 450 HV5 or less from the viewpoint of cutting workability. A Charpy absorbed energy of 40 J or more and a corrosion resistance of 85 or less relative to the corrosion amount of ordinary steel (No. 201) being 100 were judged to be good.

Table 2 shows the evaluation results. No. 1 to 12 are within the scope of the present invention in terms of chemical composition, carbon equivalent Ceq, surface layer hardness, area ratio of lath-like structure, and cementite major axis/minor axis ratio. All of these steels are steel plates excellent in Charpy absorbed energy and corrosion resistance. On the other hand, No. Nos. 101 to 111 and 201 are comparative examples, and any one or more of chemical composition, surface layer hardness, Ceq, area ratio of lath-like structure, and cementite major axis/minor axis ratio is outside the scope of the present invention. Although not shown in Table 2, No. 1 to No. 12, No. 102-No. 106, No. It has been confirmed that 111 has an area ratio of 50% or more of the lath-like structure at the center of the plate thickness.

No. No. 101 is an example in which the C content was insufficient and the surface layer hardness and the area ratio of the lath-like structure were lowered. No. No. 105 is an example in which the Cr content was insufficient and the corrosion resistance was lowered. No. 102, No. 103, No. 104 and no. No. 106 is an example in which the C content, Si content, Mn content, and Cr content are excessive and the toughness is lowered. No. No. 107 lacks B content, and No. In No. 108, the B content was excessive and the hardenability was insufficient, so that the hardness of the surface layer, the area ratio of the lath-like structure, the toughness and the corrosion resistance were lowered.

No. No. 109 is an example in which the carbon equivalent Ceq is small, and the hardness of the surface layer, the area ratio of the lath-like structure, and the corrosion resistance are lowered. No. No. 110 is an example in which the cooling rate after hot rolling was slow, the area ratio of the lath-like structure decreased, and the corrosion resistance decreased. No. No. 111 is an example in which the temperature of the tempering treatment is high, the long axis/short axis ratio of cementite present in the lath-like structure is small, and the corrosion resistance is lowered. In addition, No. No. 201 is ordinary steel used as the evaluation standard for corrosion resistance as described above. No. 201 1 to No. It is clear that the corrosion resistance of No. 12 is remarkably excellent.

Thus, the comparative examples in which one or more of the chemical composition, surface layer hardness, Ceq, lath-like structure area ratio, and cementite major axis/minor axis ratio are outside the scope of the present invention are surface layer hardness and toughness. , At least one of the corrosion resistance did not reach the evaluation criteria judged to be good.

Preferred embodiments and experimental examples of the present invention have been described above, but these embodiments and experimental examples are merely examples within the gist of the present invention, and are within the scope of the gist of the present invention. Additions, omissions, substitutions, and other changes are possible. Accordingly, the present invention is not limited by the foregoing description, but only by the scope of the appended claims, and may be modified as appropriate within its scope.

Claims (4)

in % by mass,
C: 0.01% or more and 0.15% or less,
Si: 0.05% or more and 1.00% or less,
Mn: 0.10% or more and 2.00% or less,
P: more than 0%, 0.020% or less,
S: more than 0%, 0.010% or less,
Cr: more than 0.05%, 3.00% or less,
Al: 0.01% or more and 0.10% or less,
B: 0.0003% or more and 0.0020% or less,
N: containing 0.0020% or more and 0.0100% or less, the balance being Fe and impurities,
The carbon equivalent Ceq (%) determined by the following formula (1) is 0.20% or more,
In a surface layer portion of 1 mm to 3 mm in the plate thickness direction from the surface, the area ratio of the lath-like structure is 90% or more, and the average value of the major axis/minor axis ratio of cementite present in the lath-like structure is 2.00 or more. having a metallographic structure of
Corrosion-resistant and wear-resistant steel for holds of coal carriers or coal carriers, wherein the surface layer has a Vickers hardness of 200HV5 or more.
Ceq(%)=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/4 (1)
Here, [X] is the content of the element X in mass %, and 0 is substituted for the element X that is not contained. Furthermore, in mass %,
Mo: 1.00% or less,
W: 1.00% or less,
Ni: 1.00% or less,
Cu: 1.00% or less,
Sn: 0.20% or less,
2. The corrosion-resistant and wear-resistant steel for holds of coal carriers or coal carriers according to claim 1, containing one or more of Sb: 0.20% or less. Furthermore, in mass %,
Ti: 0.025% or less,
V: 0.20% or less,
3. The corrosion-resistant and wear-resistant steel for holds of coal carriers or coal carriers according to claim 1 or claim 2, containing one or more of Nb: 0.05% or less. Furthermore, in mass %,
Ca: 0.05% or less,
Mg: 0.05% or less,
REM: Corrosion-resistant and wear-resistant steel for holds of a coal carrier or a coal carrier according to any one of claims 1 to 3, containing one or more of 0.1% or less.