JP2020132994A - Corrosion resistant and abrasion resistant steel for coal exclusive ship or ore and coal combined ship - Google Patents

Abstract

To provide a steel sheet excellent in corrosion resistance and abrasion resistance, suitable for a hold of a coal-exclusive ship and an ore-coal combined ship.SOLUTION: Corrosion resistant and abrasion resistant steel for a hold of a coal-exclusive ship or an ore and coal combined ship contains, by mass%, C:0.01 to 0.15%, Si:0.05 to 1.00%, Mn:0.10 to 2.00%, Cr: exceeding 0.05% and 3.00% or smaller, Al:0.01 to 0.10%, B:0.0003 to 0.0020%, has a metallographic texture having a carbon equivalent Ceq (%) obtained by the following formula (1) of 0.20% or larger, an area rate of a lath texture of a superficial part of 90% or larger, a ratio of major axis/minor axis of cementite present in the lath texture of 2.00 or larger, and hardness of the superficial part is 200 HV or larger. Ceq(%)=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/4 (1). Here, [X] is a content by mass% of an element X, and in a term of an element [X] that is not contained, 0 is substituted.SELECTED DRAWING: None

Description

The present invention relates to corrosion-resistant and wear-resistant steel for holds of coal-only ships or mining and coal-combined ships.

When coal is loaded in the hold of a coal-only vessel and a coal-combined vessel, water is sprinkled to prevent spontaneous combustion, and the vessel becomes moist and corrodes. In addition, the hold is worn by heavy machinery buckets and the like, especially when coal is carried out. Therefore, as a steel material used in the hold of a coal-only ship and a ship that also serves as mining and coal, corrosion-resistant and wear-resistant steel that has both corrosion resistance and wear resistance has been proposed (see, for example, Patent Documents 1 and 2).

Japanese Unexamined Patent Publication No. 2010-100872 Japanese Unexamined Patent Publication No. 2017-128762

Conventionally, in the hold (inside the cargo hold) of a coal-only ship and a ship that also serves as mining coal, sulfur contained in coal dissolves in the condensed water generated on the side wall of the hold, and sulfuric acid is generated as the temperature rises. It was thought that corrosion would proceed due to the pH environment. However, the present inventors have, as a result of the analysis of corroded steel in the cargo hold, from base steel interface chloride ion (Cl ^-) but was noted, sulfate ion (SO 4 ^_2-) was observed There wasn't. Therefore, the main corrosion factors in the cargo hold rather than sulfate ion _(SO ^{4 2-),} chloride ions ^- believed to be (Cl).

As described above, it has been conventionally considered that the corrosion of steel materials is promoted by sulfuric acid in the cargo hold, but it has been found that chloride ion (Cl ⁻ ) is the main cause of the corrosion. Therefore, a concept different from the conventional one is required for the component design of the corrosion-resistant and wear-resistant steel for the hold of a coal-only ship or a ship that also serves as mining coal. Further, the inside of the cargo hold, particularly the bottom plate, suffers wear damage due to coal and ore as cargo, and wear damage due to a bucket of heavy machinery.

In view of these circumstances, the present invention is resistant to corrosion due to chloride ions and damage due to wear caused by cargo such as coal and ore and buckets of heavy machinery. Corrosion resistance and wear resistance for the yard of a coal-only ship or a coal-combined ship. The issue is the provision of steel.

The present inventors are required that the hold of a coal-only ship or a coal-combined ship is not acid-resistant but corrosion-resistant to chloride ions, and that hardness is required to enhance wear resistance. Therefore, we focused on Cr, which is an alloying element that enhances the hardenability of steel and contributes to the improvement of hardness. Then, in order to achieve both corrosion resistance and abrasion resistance of the steel material, the metal structure was also studied repeatedly. As a result, the Cr content is more than 0.05%, the cooling rate after hot rolling is more than 15 ° C./s, the metal structure has a lath-like structure with an area ratio of 90% or more, and the lath-like structure. It was found that it is possible to achieve both corrosion resistance and abrasion resistance by assuming that the major axis / minor axis ratio of cementite present in is 2.00 or more and the Vickers hardness of the surface layer is 200 HV5 or more. It was. Here, "200HV5" is a display based on JIS Z 2244: 2009, and means that the Vickers hardness measured with a test force of 5 kgf (49.03N) is 200 or more.

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

[1] By mass%
C: 0.01% or more, 0.15% or less,
Si: 0.05% or more, 1.00% or less,
Mn: 0.10% or more, 2.00% or less,
P: Over 0%, 0.020% or less,
S: Over 0%, 0.010% or less,
Cr: Over 0.05%, 3.00% or less,
Al: 0.01% or more, 0.10% or less,
B: 0.0003% or more, 0.0020% or less,
N: Contains 0.0020% or more and 0.0100% or less, and the balance consists of Fe and impurities.
The carbon equivalent Ceq (%) obtained by the following formula (1) is 0.20% or more.
The area ratio of the lath-like structure is 90% or more in the surface layer portion of 1 mm to 3 mm in the plate thickness direction from the surface, and the average value of the major axis / minor axis ratio of cementite existing in the lath-like structure is 2.00 or more. Has a metallographic structure that is
A corrosion-resistant and wear-resistant steel for the hold of a coal-only ship or a coal-combined ship having a Vickers hardness of 200 HV5 or more on the surface layer.
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 term of the element X that is not contained.

[2] Furthermore, 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-only ship or a coal-combined ship according to the above [1], which contains one or more of 0.20% or less.

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

[4] Furthermore, by mass%,
Ca: 0.05% or less,
Mg: 0.05% or less,
REM: Corrosion-resistant and wear-resistant steel for the hold of a coal-only ship or a coal-combined ship according to any one of the above [1] to [3], which contains one or more of 0.1% or less.

According to the present invention, it is possible to provide corrosion-resistant and wear-resistant steel for the yard of a coal-only ship or a coal-combined ship, which is resistant to corrosion due to chloride ions and damage due to wear caused by cargo such as coal and ore and buckets of heavy machinery. It is possible and the industrial effect is remarkable.

Hereinafter, the corrosion-resistant and wear-resistant steel for the hold of the coal-only ship or the ship that also serves as mining coal (hereinafter, may be referred to as the corrosion-resistant and wear-resistant steel for the hold) according to the present embodiment will be described in detail. First, the chemical composition of the corrosion-resistant and wear-resistant steel for holds of the present embodiment will be described. In addition, "%" indicating the content of each element in a chemical composition means mass%. Further, 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 the lower limit value and the upper limit value unless otherwise specified. Therefore, for example, 0.01 to 0.15% means a range of 0.01% or more and 0.15% or less.

<C: 0.01% or more, 0.15% or less>
C is an element effective for ensuring hardness, and the C content is 0.01% or more. The C content is preferably 0.02% or more, more preferably 0.03% or more. On the other hand, when the C content becomes excessive, the toughness decreases, and depending on the production conditions, cementite existing in the lath-like structure grows, the semimajor axis / minor axis ratio becomes less than 2.00, and the corrosion resistance decreases. Therefore, the content of C is set to 0.15% or less. Preferably, the C content is 0.13% or less.

<Si: 0.05% or more, 1.00% or less>
Si is an antacid and an element effective for improving hardness, and has a Si content of 0.05% or more. The Si content is preferably 0.10% or more. On the other hand, if the Si content exceeds 1.00%, the toughness decreases, so the content is set to 1.00% or less. The Si content is preferably 0.80% or less, more preferably 0.50% or less.

<Mn: 0.10% or more, 2.00% or less>
Mn is an element that enhances hardenability and hardness, and it is necessary to contain 0.10% or more. The Mn content is preferably 0.20% or more, more preferably 0.50% or more. On the other hand, if Mn is excessively contained, the toughness is lowered, the formation of cementite is promoted, and as a result, the corrosion resistance is likely to be lowered. 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: Over 0%, 0.020% or less>
Since P is an impurity and reduces toughness and workability, 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. The lower limit of the P content is preferably 0%, but it is more than 0% from the viewpoint of manufacturing cost. The P content may be 0.0001% or more.

<S: Over 0%, 0.010% or less>
Since S is also an impurity and reduces toughness, 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 even more preferably 0.003% or less. The lower limit of the S content is preferably 0%, but it is more than 0% from the viewpoint of manufacturing cost. The lower limit of the S content may be 0.0001%.

<Cr: Over 0.05%, 3.00% or less>
Cr is an important element that suppresses corrosion due to chloride ions, enhances hardenability, and increases hardness. Chloride ions are the main cause of the progress of corrosion of steel materials in the cargo hold, 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 excessively contained, the toughness is lowered, so the Cr content is set to 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, 0.10% or less>
Al is an effective element as an antacid. Further, Al has an Al content of 0.01% or more in order to form N and AlN, refine the crystal grains, and improve the toughness. The Al content is preferably 0.02% or more, more preferably 0.03% or more. On the other hand, if Al is excessively contained, the toughness is lowered, so the Al content is set to 0.10% or less. The Al content is preferably 0.08% or less, more preferably 0.07% or less.

<B: 0.0003% or more, 0.0020% or less>
B is an element that remarkably enhances the hardenability of steel, and the B content is set to 0.0003% or more in order to secure the hardness. The B content is preferably 0.0005% or more, more preferably 0.0007% or more. Even more preferably, it is 0.0010% or more. On the other hand, if B is excessively contained, boride is formed and the hardenability is lowered, and the hardness cannot be secured. Therefore, the B content is set to 0.0020% or less. The B content is preferably 0.0018% or less, more preferably 0.0016% or less.

<N: 0.0020% or more, 0.0100% or less>
Since N is an element that forms a nitride with Al and Ti to refine crystal grains and improve toughness, the lower limit of the N content is set to 0.0020%. The N content is preferably 0.0030% or more, more preferably 0.0040% or more. On the other hand, when N is excessively contained, coarse nitrides are formed and the toughness is lowered. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0060% or less.

The balance of the chemical composition of the corrosion-resistant and wear-resistant steel for holds according to the present embodiment is Fe and impurities. Here, the impurity is a component mixed by various factors in the manufacturing process, including raw materials such as ore and scrap, when steel is industrially manufactured, and is used for the shipyard according to the present embodiment. It means a material that is acceptable as long as it does not adversely affect the characteristics of corrosion-resistant and wear-resistant steel. However, in the corrosion-resistant and wear-resistant steel for holds according to the present embodiment, it is necessary to specify the upper limit for P and S among the impurities as described above.

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

<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 the formation of a lath-like structure and increase the hardness. More preferably, the Mo content is 0.10% or more. On the other hand, if Mo is excessively contained, the hardness becomes too high and the toughness decreases, so the Mo content is set to 1.00% or less. The Mo content is preferably 0.80% or less, more preferably 0.60% or less.

<W: 1.00% or less>
Like Mo, W is an element that improves corrosion resistance and hardenability. The content of W may be 0%, but is preferably 0.05% or more in order to promote the formation of lath-like structure and increase the hardness. More preferably, the W content is 0.10% or more. On the other hand, if W is excessively contained, the hardness becomes too high and the toughness decreases, so the W content is set to 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 the 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, still more preferably 0.20% or more. On the other hand, since Ni is an expensive alloy 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 hardenability of steel and improves 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, still more preferably 0.20% or more. On the other hand, Cu has a Cu content of 1.00% or less in order to reduce hot workability and weldability. 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, chloride ions are concentrated, and the pH is lowered. 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, the manufacturability and toughness are lowered, so the Sn content is set to 0.20% or less. The Sn content is preferably 0.15% or less.

<Sb: 0.20% or less>
Similar to Sn, Sb is an element that enhances corrosion resistance when the cargo hold is emptied, chloride ions are concentrated, and the pH is lowered. The content of Sb 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 Sb is excessively contained, the manufacturability and toughness are lowered, so the Sb content is set to 0.20% or less. Preferably, the Sb content is 0.15% or less.

Further, 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. The Ti content may be 0%, but is preferably 0.005% or more. More preferably, the Ti content is 0.007% or more, still more preferably 0.010% or more. On the other hand, if Ti is excessively contained, the toughness may be lowered, so the Ti content is set to 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 enhances hardenability, contributes to the improvement of 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, still more preferably 0.05% or more. On the other hand, if V is excessively contained, the toughness may be lowered, so the V content is set to 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 the refinement of crystal grains by suppressing the 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, still more preferably 0.01% or more. On the other hand, if Nb is excessively contained, the toughness may be lowered, so the Nb content is set to 0.05% or less. The Nb content is preferably 0.03% or less, more preferably 0.02% or less.

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

<Ca: 0.05% or less>
Ca is an element that forms oxides and sulfides. The Ca content may be 0%, but the Ca content is preferably 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, still more preferably 0.0010% or more. On the other hand, if Ca is excessively contained, coarse oxides may be formed to reduce toughness. Therefore, the Ca content is set to 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. The Mg content may be 0%, but 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, and even more preferably 0.0010% or more. On the other hand, if Mg is excessively contained, coarse oxides may be formed to reduce toughness. Therefore, the Mg content is set to 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 mainly for improving toughness by forming inclusions that are difficult to stretch by hot rolling and forming fine oxides. Is preferably 0.0005% or more. The REM content is more preferably 0.001% or more, still more preferably 0.002% or more. On the other hand, if the REM content is excessively contained, coarse oxides may be formed to reduce the toughness. Therefore, the REM content is set to 0.1% or less. More preferably, the REM content is 0.02% or less, still more preferably 0.01% or less. Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of the REM means the total content of these elements. REM is added to molten steel using, for example, a Fe-Si-REM alloy, which alloys 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 in order 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. It is more preferably 0.55% or less, and more preferably 0.50% or less. The carbon equivalent Ceq is calculated by the following equation (1) according to the content of the alloying element.

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 term of 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 the present embodiment will be described.

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

The area ratio of the lath-shaped structure is measured by mirror-polishing a sample whose observation surface is a thick cross section, etching with nital, magnifying it 400 times with an optical microscope, observing the metal structure, and using photographs taken. To do. It is recommended that the area ratio of the lath-shaped structure of the surface layer be measured in the vicinity of a position 2 mm in the plate thickness direction from the surface. It is recommended to measure the area ratio of the lath-like structure other than the surface layer portion in the vicinity of the position of 1/2 of the plate thickness in the plate thickness direction from the surface.

When the cooling rate is slow, or when the tempering process is carried out at a high temperature for a long time, cementite precipitated in the lath-like structure grows. At this time, if the growth of cementite progresses excessively, the semimajor axis / minor axis ratio becomes smaller (approaches 1). When the major axis / minor axis ratio of cementite is less than 2.00, it functions as a cathode site, accelerates the melting of steel, and lowers the corrosion resistance. Therefore, in order to ensure corrosion resistance, the major axis / minor axis ratio of cementite present in the lath-like structure needs to be 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 the growth of cementite progresses. Therefore, a larger ratio is preferable, but it may be 5.00 or less. ..

The major / minor 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 a hold is a steel plate, the thickness cross section of the steel plate is used as an observation surface, and mirror polishing and electrolytic polishing are performed to expand the range of 1 mm to 3 mm from the surface in the plate thickness direction by SEM 10,000 times. And take an image. The major axis / minor axis ratio of 100 cementites having a major axis of 30 to 300 nm is measured, and the arithmetic mean is taken as the major axis / minor axis ratio of cementite. Whether or not the particles imaged by the SEM are cementite can be confirmed by an energy dispersive X-ray Spectroscopy (EDS) attached to the SEM. It is recommended that the major axis / minor axis ratio of cementite be measured in the vicinity of a position 2 mm in the plate thickness direction from the surface.

The Vickers hardness of the surface layer portion 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 200 HV5 or more in order to ensure wear resistance. It is preferably 250 HV5 or more. The surface hardness of the corrosion-resistant and wear-resistant steel for holds is preferably 450 HV5 or less in consideration of workability. The surface 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 plate thickness direction with the plate thickness cross section of the steel plate as the measurement surface. Specifically, at room temperature, with a test force of 49.03N (5 kgf), at a position 2 mm from the surface in the plate thickness direction of the steel sheet, 5 points every 1 mm in the direction perpendicular to the plate thickness direction, for a total of 25 points. Vickers hardness is measured and calculated as the average value (arithmetic mean) of these.

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

(Production method)
Next, a preferable manufacturing method of the corrosion-resistant and wear-resistant steel for holds according to the present embodiment will be described. The corrosion-resistant 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.

For the corrosion-resistant and abrasion-resistant steel for shipyards according to the present embodiment, the steel is melted by a conventional method, the components are adjusted, the steel pieces obtained by casting are hot-rolled, and accelerated cooling is performed as it is, or air cooling is performed. After that, it is manufactured by reheating and quenching. Further, a tempering treatment may be performed. The steel pieces can be produced by a known method such as a continuous casting method or a lump-incubation method 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 wound into a coil.

In the present embodiment, when manufacturing a steel pipe, the steel plate may be formed into a tubular shape and welded, and may be a UO steel pipe, an electrosewn steel pipe, a forge welded steel pipe, a spiral steel pipe, or the like. A seamless steel pipe produced by hot extrusion or perforation rolling of a steel piece is also included in the corrosion-resistant and wear-resistant steel for a hold of the present embodiment.

For water cooling after hot rolling and cooling of quenching treatment, it is necessary to make the cooling rate 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 has a lath-like structure having an area ratio of 90% or more. The faster the cooling rate is, the more preferable it is, but it is appropriately determined depending on the plate thickness and the capacity of the cooling equipment, and may be 50 ° C./s or less. The stop temperature of accelerated cooling after hot rolling and cooling of quenching treatment is preferably 400 ° C. or lower in order to suppress precipitation of carbides.

Further, when the corrosion-resistant and wear-resistant steel for holds is required to have workability and toughness, a tempering treatment for heating to 400 ° C. or higher may be performed. The temperature of the tempering treatment 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 corrosion-resistant and wear-resistant steel for holds according to the present invention. The present invention is not limited to the following examples, and can be carried out with appropriate modifications within a range that can be adapted to the gist of the present invention, all of which fall within the technical scope of the present invention. It is included.

Steel Nos. Of Table 1A and Table 1B. Steels having the chemical compositions shown in A1 to A12 and B1 to B11 were melted. Here, Table 1B is a continuous table on the right side of Table 1A, and the chemical components C to N for each steel are listed in Table 1A, and the chemical components Mo to REM for each steel and each steel. The carbon equivalent chems of are listed in Table 1B. The rest of the chemical composition of each steel shown in Tables 1A and 1B is Fe and impurities. After melting each steel, the steel pieces obtained by casting were heated and hot-rolled, and water-cooled at a cooling rate of more than 15 ° C./s to produce a steel sheet with a thickness of 10 mm (No. in Table 2). .1-No12, No.101-No111, No.201). Here, as a comparison, No. 110 was hot rolled and then allowed to cool. Furthermore, No. 12 and No. After cooling with water, 111 was tempered by holding it at 150 ° C. and 610 ° C. for 15 minutes, respectively.

The area ratio of the lath-shaped structure and the long-axis / short-axis ratio of cementite present in the lath-shaped structure were measured in the range of 1 mm to 3 mm from the surface in the plate thickness direction of the steel sheet with the plate thickness cross section as the test surface. The metallographic structure was observed by mirror-polishing the sample, etching it with nital, magnifying it 400 times with an optical microscope, and measuring the area ratio of the lath-shaped structure using the photograph taken. For the sample in which the area ratio of the lath-like structure in the surface layer portion was 90% or more, the metal structure in the central portion of the plate thickness was also observed, and the area ratio of the lath-like structure was measured. 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 at 10000 times using SEM and EDS. The semimajor / 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. If the area ratio of the lath-like structure is less than 90%, the major axis / minor axis ratio of cementite is not measured.

Further, a test piece was collected from the obtained steel sheet, and the Vickers hardness was measured at a position 2 mm in the plate thickness direction from the surface of the steel plate with the plate thickness cross section as the test surface. The Vickers hardness is measured in accordance with JIS Z 2244: 2009, and the test force is 49.03N at room temperature. (Arithmetic mean) was calculated. Further, a full-size V-notch Charpy test piece was cut out from the steel plate, and the Charpy absorption energy (KV ₂ ) at −20 ° C. was measured according to JIS Z 2242: 2018.

Further, the surface of the test piece having a length of 100 mm, a width of 60 mm and a thickness of 5 mm was blasted so as to have Sa2.5 (ISO 8501-1) or more, and used for evaluation of corrosion resistance. Corrosion resistance is obtained by holding coal powder containing artificial seawater on a test piece in a test tank having a temperature of 40 ° C. and a relative humidity of 98% for one week, removing the coal powder from the test tank, and adhering to the surface of the test piece. The coal was lightly removed with a scraper, held again in a test tank for 1 week, and a corrosion test with this as 1 cycle was carried out for 6 cycles for evaluation. After 6 cycles of corrosion testing, rust was removed with a scraper and rusted with ammonium citrate. After that, the weight change before and after the test was defined as the amount of corrosion. The amount of each corrosion is No. 2 in Table 2. It was evaluated by a relative quantity with 201 ordinary steel as 100.

The judgment criteria for each evaluation item are as follows. The surface layer hardness was judged to be good at 200 HV5 or more from the viewpoint of wear resistance and 450 HV5 or less from the viewpoint of cutting workability. Further, it was judged that the Charpy absorption energy was 40 J or more, and the corrosion resistance was 85 or less in terms of the relative amount of corrosion with ordinary steel (No. 201) as 100.

The evaluation results are shown in Table 2. No. In Nos. 1 to 12, the chemical composition, carbon equivalent Ceq, surface hardness, lath-like structure area ratio, and cementite major / minor axis ratio are within the scope of the present invention. All of these steels are steel sheets having excellent Charpy absorption energy and corrosion resistance. On the other hand, No. 101 to 111 and 201 are comparative examples, and any one or more of the chemical composition, surface hardness, Ceq, area ratio of lath-like structure, and cementite major / minor axis ratio is outside the scope of the present invention. Although not shown in Table 2, No. 1-No12, No. 102 to No. 106, No. It has been confirmed that the area ratio of the lath-shaped structure at the center of the plate thickness of 111 is 50% or more.

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

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

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

Although the preferred embodiments and experimental examples of the present invention have been described above, these embodiments and experimental examples are merely examples within the scope of the gist of the present invention and do not deviate from the gist of the present invention. Allows you to add, omit, replace, and make other changes to the configuration. That is, the present invention is not limited by the above description, but is limited only by the appended claims, and can be appropriately modified within the scope.

Claims (4)

By mass%
C: 0.01% or more, 0.15% or less,
Si: 0.05% or more, 1.00% or less,
Mn: 0.10% or more, 2.00% or less,
P: Over 0%, 0.020% or less,
S: Over 0%, 0.010% or less,
Cr: Over 0.05%, 3.00% or less,
Al: 0.01% or more, 0.10% or less,
B: 0.0003% or more, 0.0020% or less,
N: Contains 0.0020% or more and 0.0100% or less, and the balance consists of Fe and impurities.
The carbon equivalent Ceq (%) obtained by the following formula (1) is 0.20% or more.
The area ratio of the lath-like structure is 90% or more in the surface layer portion of 1 mm to 3 mm in the plate thickness direction from the surface, and the average value of the major axis / minor axis ratio of cementite existing in the lath-like structure is 2.00 or more. Has a metallographic structure that is
A corrosion-resistant and wear-resistant steel for the hold of a coal-only ship or a coal-combined ship having a Vickers hardness of 200 HV5 or more on the surface layer.
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 term of the element X that is not contained. In addition, 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-only ship or a coal-combined ship according to claim 1, which contains one or more of 0.20% or less. In addition, 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-only ship or a coal-combined ship according to claim 1 or 2, which contains one or more of 0.05% or less. In addition, in% by mass,
Ca: 0.05% or less,
Mg: 0.05% or less,
REM: Corrosion-resistant and wear-resistant steel for the hold of a coal-only ship or a coal-combined ship according to any one of claims 1 to 3, which contains one or more of 0.1% or less.