WO2022025135A1

WO2022025135A1 - Wear-resistant steel

Info

Publication number: WO2022025135A1
Application number: PCT/JP2021/027960
Authority: WO
Inventors: 雄三川本; 仁秀吉村; 学星野
Original assignee: 日本製鉄株式会社
Priority date: 2020-07-28
Filing date: 2021-07-28
Publication date: 2022-02-03
Also published as: CN115989326A; KR20230024381A; JP2024015532A; JPWO2022025135A1

Abstract

Wear-resistant steel which contains, in mass%, from 0.08% to 0.20% of C, from 0.01% to 0.50% of Si, from 0.10% to 2.00% of Mn and from 2.10% to 8.00% of Cr, while containing 0.5% by area or less of carbides in the metal structure. With respect to this wear-resistant steel, the average circle-equivalent diameter of the carbides is 500 nm or less; the average grain size of ten crystal grains in descending order of grain size among the crystal grains surrounded by large angle grain boundaries of 15° or more is 40 μm or less; the Charpy absorbed energy at -40°C is 27 J or more; and the surface hardness is 360 Hv or more and 634 × (C)1/2 + 140 HV10 or more.

Description

Wear resistant steel

This disclosure relates to wear-resistant steel. It was

Since wear-resistant steel is required to have hardness near the surface layer, it contains an element that enhances hardenability. Cr, Ni, and Mo are elements that enhance hardenability. Conventionally, wear-resistant steels containing Cr, Ni, Mo and the like have been proposed (see, for example, Patent Documents 1 to 4).
Further, a steel sheet having high strength and improved toughness has been proposed (see, for example, Patent Document 5).

Patent Document 1: Japanese Patent Application Laid-Open No. 10-204575 Patent Document 2: Japanese Patent Application Laid-Open No. 10-102185 Patent Document 3: Japanese Patent Application Laid-Open No. 59-129724 Patent Document 4: Japanese Patent Application Laid-Open No. 59-707421 Patent Document 5: International Publication No. 2019/050010

The wear-resistant steels of Patent Document 1 and Patent Document 2 contain 1.0% by mass or more of Mo. The wear-resistant steels of Patent Documents 3 and 4 contain 2.0% by mass or more of Ni. Ni and Mo are expensive elements, and reduction is desired from the viewpoint of alloy cost.
Further, the high-strength steel plate of Patent Document 5 is intended to improve toughness, but wear resistance is not taken into consideration.
In view of such circumstances, an object of the present disclosure is to provide a wear-resistant steel having excellent wear resistance and low-temperature toughness while suppressing the use of expensive elements such as Ni and Mo.

The gist of this disclosure is as follows.

(1) The chemical composition is mass%.
C: 0.08% or more, 0.20% or less,
Si: 0.01% or more, 0.50% or less,
Mn: 0.10% or more, 2.00% or less,
P: 0.015% or less,
S: 0.0300% or less,
Cr: 2.10% or more, 8.00% or less,
N: 0.008% or less,
Cu: 0% or more, 0.50% or less,
Ni: 0% or more, 0.50% or less,
Mo: 0% or more, 0.50% or less,
V: 0% or more, 0.500% or less,
W: 0% or more, 0.50% or less,
B: 0% or more, 0.0050% or less,
Al: 0% or more, 0.300% or less,
Ti: 0% or more, 0.100% or less,
Nb: 0% or more, 0.100% or less,
Ca: 0% or more, 0.0100% or less,
Mg: 0% or more, 0.0100% or less,
REM: 0% or more, 0.0100% or less, and the balance: Fe and impurities.
In the cross section in the thickness direction, when the thickness is t, when t is less than 16 mm, it is 1 / 2t from the surface, and when t is 16 mm or more, it is 1 / 4t from the surface.
The carbide contained in the metal structure is 0% or more and 0.5% or less in area%, and the average circle equivalent diameter of the carbide is 500 nm or less.
In the region of 400 μm × 400 μm, among the crystal grains surrounded by large tilt angle grain boundaries of 15 ° or more, the average grain size of 10 crystal grains is 40 μm or less in descending order of grain size, and at -40 ° C. Crystall absorption energy is 27J or more,
When the C content in% by mass is [C], the surface hardness at a position 0.7 mm in the thickness direction from the surface is 360 HV10 or more and 634 × [C] ^1/2 + 140 HV10 or more. Worn steel.
(2) The wear-resistant steel according to claim 1, wherein the hardness of the central portion in the central portion in the thickness direction is 360 HV10 or more and 634 × [C] ^1/2 + 140 HV10 or more.
(3) By mass%,
Cu: 0.01% or more, 0.50% or less,
Ni: 0.01% or more, 0.50% or less,
Mo: 0.01% or more, 0.50% or less,
V: 0.003% or more, 0.500% or less,
W: 0.01% or more, 0.50% or less,
B: 0.0003% or more, 0.0050% or less,
Al: 0.005% or more, 0.300% or less,
Ti: 0.003% or more, 0.100% or less,
Nb: 0.003% or more, 0.100% or less,
Ca: 0.0003% or more, 0.0100% or less,
Mg: 0% or more, 0.0100% or less, and REM: 0.0003% or more, 0.0100% or less,
The wear-resistant steel according to (1) or (2), which comprises at least one selected from the group consisting of.
(4) The wear-resistant steel according to any one of (1) to (3), which is a steel plate.
(5) The wear-resistant steel according to (4), wherein the plate thickness is 8 mm or more and 50 mm or less.

According to the present disclosure, wear-resistant steel having excellent wear resistance and low-temperature toughness is provided while suppressing the use of expensive elements such as Ni and Mo. By reducing the alloy cost of wear-resistant steel, it becomes possible to provide inexpensive wear-resistant steel. Therefore, this disclosure is extremely significant in terms of industrial contribution.

Hereinafter, as the wear-resistant steel according to one embodiment of the present disclosure (hereinafter, referred to as “the present embodiment”), a steel sheet will be mainly described.
In the numerical range described stepwise in the present specification, the upper limit of the numerical range described in one step may be replaced with the upper limit value of the numerical range described in another step. The lower limit of the range may be replaced with the lower limit of the numerical range described in other steps. The upper limit value or the lower limit value may be replaced with the value shown in the embodiment.
The term "process" is included in this term not only in an independent process but also in the case where the intended purpose of the process is achieved even if it cannot be clearly distinguished from other processes.
First, the new findings obtained as a result of the studies by the present inventors who have completed the present disclosure will be described in detail.

Cr is an element that is cheaper than Ni and Mo. By suppressing the Ni and Mo contents and increasing the Cr content, it is possible to reduce the cost of wear-resistant steel. As a result of the studies by the present inventors, Cr required for alloy-saving wear-resistant steel when Ni and Mo are not intentionally contained or the Ni and Mo contents are suppressed to 0.50% or less, respectively. It was found that the content was 2.10% or more. Further, it was found that by increasing the Cr content in this way, the corrosion resistance is also improved, which contributes to the extension of the life of the wear-resistant steel used in the wet environment.
On the other hand, in order to secure the surface hardness of the wear-resistant steel, it is preferable to increase the C content, but there is a problem that the toughness is lowered.

The present inventors have succeeded in achieving both surface hardness and toughness of wear-resistant steel by suppressing precipitation and coarsening of carbides. As a result of the studies by the present inventors, it was found that the precipitation and coarsening of carbides are suppressed and the toughness is ensured by limiting the C content and the quenching stop temperature. Specifically, it was found that the precipitation and coarsening of carbides can be effectively suppressed by setting the C content to 0.20% or less and the quenching stop temperature to 200 ° C. or less. When the area ratio of carbide is 0.5% or less, the average circle equivalent diameter is 500 nm or less, and the C content is [C], the surface hardness is 360 HV10 or more and 634 × [C] in Vickers hardness. When it was ^1/2 + 140 HV10 or more, the total area ratio of one or both of martensite and lower bainite was found to be 95% or more.

<Chemical composition>
Next, the alloying elements constituting the chemical composition of the steel sheet according to the present embodiment will be described. In the following description of alloying elements, "%" of content means "mass%".

[C: 0.08% or more, 0.20% or less]
C (carbon) is an element that enhances the hardenability of steel and increases its hardness. The C content is 0.08% or more from the viewpoint of ensuring hardness. The C content is preferably 0.10% or more. On the other hand, C is an element that produces carbides. The C content is 0.20% or less from the viewpoint of ensuring toughness. The C content is preferably 0.18% or less, more preferably 0.15% or less.

[Si: 0.01% or more, 0.50% or less]
Si (silicon) is a deoxidizing element. The Si content is 0.01% or more from the viewpoint of obtaining the effect of deoxidation. The Si content is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, the Si content is 0.50% or less from the viewpoint of ensuring toughness. The Si content is preferably 0.40% or less, more preferably 0.30% or less.

[Mn: 0.10% or more, 2.00% or less]
Mn (manganese) is an element that enhances the hardenability of steel. The Mn content is 0.10% or more from the viewpoint of ensuring hardness. The Mn content is preferably 0.50% or more, more preferably 0.80% or more, still more preferably 1.00% or more. On the other hand, Mn is an element that embrittles the grain boundaries of steel. The Mn content is 2.00% or less from the viewpoint of ensuring toughness. The Mn content is preferably 1.80% or less, more preferably 1.60% or less.

[P: 0.015% or less]
P (phosphorus) is an element mixed with steel in raw materials and manufacturing processes. The P content is 0.015% or less from the viewpoint of ensuring toughness. The P content is preferably 0.012% or less, more preferably 0.010% or less. The P content is preferably reduced, and the lower limit is not limited, but it may be more than 0% from the viewpoint of manufacturing cost. The P content may be 0.001% or more.

[S: 0.0300% or less]
S (sulfur) is an element mixed with steel in raw materials and manufacturing processes. The S content is 0.0300% or less from the viewpoint of ensuring toughness. The S content is preferably 0.0050% or less, more preferably 0.0030% or less. The S content is preferably reduced, and the lower limit is not limited, but it may be more than 0% from the viewpoint of manufacturing cost. The S content may be 0.0001% or more.

[Cr: 2.10% or more, 8.00% or less]
Cr (chromium) is an element that enhances the hardenability of steel. Cr is an important element from the viewpoint of suppressing the content of expensive elements such as Ni and Mo and ensuring the hardness and toughness of wear-resistant steel. The Cr content is 2.10% or more from the viewpoint of ensuring hardenability. The Cr content is preferably 2.20% or more, more preferably 2.40% or more. On the other hand, the Cr content is 8.00% or less and may be 7.50% or less from the viewpoint of cost. The Cr content is preferably 5.00% or less, more preferably 3.00% or less.

[N: 0.0080% or less]
N (nitrogen) is an element mixed with steel in raw materials and manufacturing processes. The N content is 0.0080% or less from the viewpoint of ensuring toughness. The N content is preferably 0.0070% or less, more preferably 0.0060% or less. The N content is preferably reduced, and the lower limit is not limited, but it may be more than 0% from the viewpoint of manufacturing cost. When Al or Ti is contained, it is combined with N to form fine nitrides such as AlN and TiN. The N content is preferably 0.0010% or more, more preferably 0.0020% or more, from the viewpoint of miniaturization of the metal structure by the nitride.

In the wear-resistant steel according to the present embodiment, in order to improve hardenability without excessively increasing the manufacturing cost, one or more of Cu, Ni, Mo, V, W and B are described below as necessary. It may be contained within the range of.

[Cu: 0% or more, 0.50% or less]
Cu (copper) is an element that enhances the hardenability of steel. The lower limit of the Cu content is not limited and may be 0%. In order to surely obtain the effect, the Cu content is preferably 0.01% or more, more preferably 0.03% or more. On the other hand, the Cu content is 0.50% or less from the viewpoint of cost. The Cu content is preferably 0.30% or less, 0.10% or less, and may be 0.05% or less.

[Ni: 0% or more, 0.50% or less]
Ni (nickel) is an element that enhances the hardenability of steel. The lower limit of the Ni content is not limited and may be 0%. In order to obtain the effect, the Ni content is preferably 0.01% or more, more preferably 0.03% or more. On the other hand, the Ni content is 0.50% or less from the viewpoint of cost. The Ni content is preferably 0.30% or less, more preferably 0.10% or less, still more preferably 0.05% or less.

[Mo: 0% or more, 0.50% or less]
Mo (molybdenum) is an element that enhances the hardenability of steel. The lower limit of the Mo content is not limited and may be 0%. In order to obtain the effect, the Mo content is preferably 0.01% or more, more preferably 0.03% or more. On the other hand, the Mo content is 0.50% or less from the viewpoint of cost. It is preferably 0.30% or less, more preferably 0.10% or less, still more preferably 0.05% or less.

[V: 0% or more, 0.500% or less]
V (vanadium) is an element that forms a precipitate such as a carbide or a nitride, and has an effect of enhancing hardenability. The lower limit of the V content is not limited and may be 0%. In order to obtain the effect, the V content is preferably 0.003% or more, more preferably 0.005% or more. On the other hand, the V content is 0.500% or less from the viewpoint of cost. It is preferably 0.300% or less, more preferably 0.100% or less, and further preferably 0.050% or less.

[W: 0% or more, 0.50% or less]
W (tungsten) is an element that enhances the hardenability of steel. The lower limit of the W content is not limited and may be 0%. In order to surely obtain the effect, the W content is preferably 0.01% or more, more preferably 0.03% or more. On the other hand, the W content is 0.50% or less from the viewpoint of cost. The W content is preferably 0.30% or less, more preferably 0.10% or less, still more preferably 0.05% or less.

[B: 0% or more, 0.0050% or less]
B (boron) is an element that significantly enhances the hardenability of steel even in a trace amount. The lower limit of the B content is not limited and may be 0%. In order to obtain the effect, the B content is preferably 0.0003% or more, more preferably 0.0005% or more, still more preferably 0.0008% or more. On the other hand, even if B is excessively contained, the effect is saturated, so that the B content is 0.0050% or less. The B content is preferably 0.0045% or less, more preferably 0.0040% or less.

The wear-resistant steel according to the present embodiment contains one or more of Al, Ti, Nb, Ca, Mg and REM that form compounds such as carbides, nitrides, oxides and sulfides, if necessary. It may be contained.

[Al: 0% or more, 0.300% or less]
Al (aluminum) is a deoxidizing element and is also an element forming a nitride. The lower limit of the Al content is not limited and may be 0%. In order to obtain the effect of deoxidation, the Al content is preferably 0.005% or more. From the viewpoint of miniaturization of the metal structure by AlN, the Al content is more preferably 0.010% or more. On the other hand, from the viewpoint of suppressing the formation of coarse inclusions, the Al content is 0.300% or less. The Al content is preferably 0.100% or less, more preferably 0.070% or less.

[Ti: 0% or more, 0.100% or less]
Ti (titanium) is a deoxidizing element and is also an element forming a nitride. Further, when Ti is contained in steel, Ti is used to suppress the formation of BN and improve hardenability. The lower limit of the Ti content is not limited and may be 0%. In order to obtain the deoxidizing effect, the Ti content is preferably 0.003% or more. From the viewpoint of miniaturization of the metal structure by TiN, the Ti content is more preferably 0.005% or more, still more preferably 0.010% or more. On the other hand, the Ti content is 0.100% or less from the viewpoint of suppressing the formation of coarse inclusions. The Ti content is more preferably 0.050% or less, still more preferably 0.030% or less.

[Nb: 0% or more, 0.100% or less]
Nb (niobium) is an element that forms a precipitate such as a carbide or a nitride. The lower limit of the Nb content is not limited and may be 0%. From the viewpoint of miniaturization of the metal structure due to the precipitate, the Nb content is preferably 0.003% or more, more preferably 0.005% or more, still more preferably 0.010% or more. On the other hand, even if Nb is excessively contained, the effect is saturated, so that the Nb content is 0.100% or less. The Nb content is preferably 0.050% or less, more preferably 0.030% or less.

[Ca: 0% or more, 0.0100% or less]
Ca (calcium) is an element that forms oxides or sulfides and controls the morphology of inclusions. The lower limit of the Ca content is not limited and may be 0%. In order to obtain the effect, the Ca content is preferably 0.0003% or more. The Ca content is more preferably 0.0005% or more, still more preferably 0.0010% or more. On the other hand, from the viewpoint of suppressing the formation of coarse inclusions, the Ca content is 0.0100% or less. The Ca content is preferably 0.0080% or less, more preferably 0.0060% or less.

[Mg: 0% or more, 0.0100% or less]
Mg (magnesium) is an element that forms oxides or sulfides and controls the morphology of inclusions. The lower limit of the Mg content is not limited and may be 0%. In order to obtain the effect, the Mg content is preferably 0.0003% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more. On the other hand, from the viewpoint of cost, the Mg content is 0.0100% or less. The Mg content is preferably 0.0080% or less, more preferably 0.0060% or less.

[REM: 0% or more, 0.0100% or less]
REM (rare earth element) is a general term for a total of 17 elements including two elements Sc and Y and 15 lanthanoid elements such as La, Ce and Nd. The REM content means the total content of the 17 elements.
REM is an element that forms oxides or sulfides and controls the morphology of inclusions. The lower limit of the REM content is not limited and may be 0%. In order to obtain the effect, the REM content is preferably 0.0003% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more. On the other hand, from the viewpoint of cost, the REM content is 0.0100% or less. The REM content is preferably 0.0080% or less, more preferably 0.0060% or less.

The rest of the chemical composition of the steel according to this embodiment is Fe and impurities. Here, the impurities are components that are not intentionally contained when the wear-resistant steel according to the present embodiment is industrially manufactured, but are mixed by raw materials such as ore and scrap or various factors in the manufacturing process. .. The inclusion of impurities is permitted within a range that does not adversely affect the characteristics of the wear-resistant steel according to the present embodiment.

As an impurity, for example, O (oxygen) is an element mixed in steel in raw materials and manufacturing processes. Since O is an impurity, the O content is preferably 0.0060% or less. The O content is more preferably 0.0050% or less, still more preferably 0.0040% or less. The O content is preferably reduced, and the lower limit is not limited, but it may be more than 0% from the viewpoint of manufacturing cost. The O content may be 0.0020% or more.

<Metal structure>
Next, the metallographic structure of the wear-resistant steel according to the present embodiment will be described. In the following, "%" of the metallographic structure is "area%".
The wear-resistant steel according to the present embodiment contains 0% or more and 0.5% or less of carbides in the metal structure in terms of area%. The average circle-equivalent diameter of the carbide is 500 nm or less. Of the regions (considered as crystal grains) surrounded by large-angle grain boundaries (sometimes called "high-angle grain boundaries") of 15 ° or more in the region of 400 μm × 400 μm. The average grain size of the 10 crystal grains is 40 μm or less in descending order of grain size. Examples of the carbide include cementite (Fe ₃ C), Cr carbide, and Fe ₂ C, but the carbide in the present disclosure is mainly cementite.

The metallographic structure of the wear-resistant steel according to the present embodiment may be such that the area ratio of carbides, the average diameter equivalent to a circle, and the average particle size of crystals are within the above-mentioned ranges. However, when upper bainite or pearlite is formed, the area ratio of carbides increases and it tends to be coarsened. When ferrite is formed, the hardness tends to be insufficient, which may reduce toughness. Retained austenite may become hard martensite (work-induced martensite) due to deformation and reduce toughness. Therefore, it is preferable that the metallographic structure of the wear-resistant steel according to the present embodiment is composed of one or both of martensite and lower bainite. The total area ratio of martensite and lower bainite is preferably 95% or more. The rest, excluding martensite and lower bainite, consists of one or more of ferrite, pearlite, retained austenite and upper bainite.

The upper bainite is a structure having a particle size of 40 μm or more in which cementite or retained austenite is precipitated at the grain boundaries without containing carbides in the grains. Martensite and lower bainite are lath-like hard phases and are generally discriminated by the presence or absence of carbides. When the metal structure of the wear-resistant steel according to the present embodiment consists of martensite and lower bainite, the metal structure containing carbides is regarded as lower bainite. In this embodiment, it is not necessary to distinguish between martensite and lower bainite. If ferrite, pearlite, retained austenite and upper bainite are not observed by light microscopy, it is determined that the metallographic structure consists of one or both of martensite and lower bainite.
The wear-resistant steel according to the present embodiment may contain a metal structure other than martensite and lower bainite as long as the area ratio of carbides, the diameter equivalent to an average circle, and the average particle size of crystals are within the above-mentioned ranges.

When the thickness of the wear-resistant steel is less than 16 mm, the metallographic structure is observed at a portion halved in the thickness direction from the surface (hereinafter, also referred to as "1 / 2t portion"). "1 / 2t" is synonymous with "t / 2"). When the thickness of the wear-resistant steel is 16 mm or more, a portion of 1/4 of the thickness (hereinafter, also referred to as "1 / 4t portion") in the thickness direction from the surface is referred to as "1 / 4t". Is synonymous with "t / 4"), where the metallographic structure is observed. The observation surface of the sample used for observing the metallographic structure is a cross section cut in the thickness direction along the L (longitudinal) direction (rolling direction) of the sample (hereinafter referred to as "L (longitudinal) cross section of the sample"). It may be referred to as), and is subjected to wet polishing and etching with Nital. The metallographic structure is observed at a magnification of 400 times, and the presence or absence of ferrite, pearlite, retained austenite and upper bainite is determined by observing 5 fields of view. Retained austenite is observed by repeller corrosion performed in a 1% to 10% sodium metabisulfite solution.

[Area ratio of carbides: 0% or more, 0.5% or less]
When the metal structure of the wear-resistant steel according to the present embodiment is made of martensite, the area ratio of carbides is 0%. When the metal structure of the wear-resistant steel according to the present embodiment is composed of martensite and lower bainite, the metal structure containing carbides is regarded as lower bainite. The area ratio of carbides is 0.5% or less from the viewpoint of ensuring toughness. The area ratio of the carbide is preferably 0.4% or less, more preferably 0.3% or less. When the quenching stop temperature is lowered, the formation of upper bainite and pearlite is suppressed, and the area ratio of carbides is reduced.

The area ratio of charcoal is measured by a scanning electron microscope (hereinafter referred to as "SEM"). The area ratio of carbides is measured in 1 / 2t portions when the thickness of the wear-resistant steel is less than 16 mm. When the thickness of the wear-resistant steel is 16 mm or more, the area ratio of carbides is measured at 1 / 4t portion. The observation surface of the sample used to measure the area ratio of carbides is the L (longitudinal) cross section of the sample and is subjected to electrolytic etching. The area ratio of carbides is measured by image analysis (binarization by image processing) of photographs taken at 30,000 times. Here, electrolytic etching is carried out at 20 to 40 V for 10 to 30 seconds in a solution in which 62 cc of perchloric acid, 700 cc of ethanol, 137 cc of distilled water and 100 cc of Brucelsolve are mixed. For binarization by image processing, a threshold is set so that carbides deposited in the martensite block can be visually separated.

[Average circle-equivalent diameter of carbide: 500 nm or less]
The average circle-equivalent diameter of the carbide is 500 nm or less from the viewpoint of ensuring toughness. The average circle-equivalent diameter of the carbide is preferably 300 nm or less. The average circle-equivalent diameter of the carbide may be 50 nm or more, or 100 nm or more. The average circle-equivalent diameter of the carbide is calculated from the area (total) of the carbide obtained by image analysis of the SEM photograph used for measuring the area ratio, and the number of carbides. That is, the average circle-equivalent diameter is calculated from the average area of carbides obtained by dividing the total area of carbides by the number of carbides. When the area ratio of the carbide is 0%, the diameter corresponding to the average circle is 0 nm. When the quenching stop temperature is lowered, the formation of upper bainite and pearlite is suppressed, and the diameter corresponding to the average circle of carbide becomes smaller. The carbides contained in the upper bainite and pearlite have a large diameter and area ratio equivalent to an average circle. If the area ratio of the carbide is 0.5% or less and the diameter corresponding to the average circle is 500 nm or less, it may be determined that the total area ratio of the upper bainite and pearlite is less than 5%.

[Average of the top 10 grain sizes surrounded by large tilt angle grain boundaries of 15 ° or more: 40 μm or less]
The average of the top 10 grain sizes (10 crystal grain sizes in descending order of grain size) surrounded by large tilt angle grain boundaries of 15 ° or more in the region of 400 μm × 400 μm is 40 μm or less. Hereinafter, the grain size surrounded by the large tilt angle grain boundaries having a crystal orientation difference of 15 ° or more is referred to as a large tilt angle grain size. When the large tilt grain size is large, fracture is likely to occur, and when the large tilt grain size is small, the toughness is improved. The large tilt angle grain size is evaluated by the average value of the top 10 crystals (10 from the largest) among the crystal grains in the region of 400 μm × 400 μm. The average of the top 10 particles having a large tilt angle particle size is more preferably 30 μm or less.

In this embodiment, the large tilt angle particle size is measured by an electron backscattered diffraction pattern (hereinafter, also referred to as “EBSD”). Measurements by EBSD are performed with a field of view of 400 μm × 400 μm and a pitch of 0.4 μm. The particle size distribution is displayed by commercially available analysis software (OIM-Anysis manufactured by TSL), and the average of the top 10 particle sizes of the crystal grains surrounded by the large tilt angle grain boundaries of 15 ° or more is calculated. The measurement by EBSD is performed in the 1 / 2t portion when the thickness of the wear-resistant steel is less than 16 mm. When the thickness is 16 mm or more, the measurement by EBSD is performed in the 1 / 4t portion. The sample used for the measurement by EBSD has an observation surface of 10 mm square, and is cut out from a position (1/4 width) of the plate width from the end portion in the width direction of the steel plate. The observation surface is a cross section in the L (longitudinal) direction of the sample, and is subjected to electrolytic polishing.

Next, the surface hardness of the wear-resistant steel, the hardness at the 1 / 2t portion, and the low-temperature toughness of the wear-resistant steel according to the present embodiment will be described.

[Surface hardness: 360 HV10 or more and 634 x [C] ^1/2 + 140 HV10 or more]
The surface hardness of the wear-resistant steel according to the present embodiment is 360 HV10 or more and 634 × [C] ^1/2 + 140 HV10 or more in Vickers hardness in order to secure wear resistance. [C] is the C content in the wear-resistant steel. The surface hardness of the wear-resistant steel is preferably 360 HV10 or more and 634 × [C] ^1/2 + 160 HV10 or more, and more preferably 360 HV10 or more and 634 × [C] 1/2 + 180 HV10 or more. The higher the surface hardness, the more preferable, and the upper limit is not limited. The surface hardness of the wear-resistant steel may be 634 × [C] ^1/2 + 230HV10 or less from the viewpoint of ensuring toughness.
The surface hardness of the wear-resistant steel is measured at a position of 0.7 mm in the thickness direction from the surface of the wear-resistant steel in consideration of the influence of decarburization. The Vickers hardness test is performed according to JIS Z 2244: 2009, and the load is 10 kgf. Vickers hardness is the average value of three points measured in the L (longitudinal) cross section of the sample. When the surface hardness is 360 HV10 or more and 634 × [C] ^1/2 + 140 HV10 or more in Vickers hardness, it may be determined that the total area ratio of ferrite and retained austenite is less than 5%.
The surface hardness of the wear-resistant steel according to the present embodiment is determined from the viewpoint of ensuring absolute wear resistance regardless of the C content, rather than the relative wear resistance in consideration of the C content. It may be 380 HV10 or more, 400 HV10 or more, or 450 HV10 or more.

[Center hardness in the central part in the thickness direction: 360 HV10 or more and 634 × [C] ^1/2 + 140 HV10 or more]
The central portion in the thickness direction of the wear-resistant steel is synonymous with the 1 / 2t portion, and hereinafter, the hardness of the central portion in the central portion in the thickness direction of the wear-resistant steel is also referred to as the 1 / 2t portion hardness. From the viewpoint of preventing deterioration of wear resistance due to use, the 1 / 2t portion hardness is preferably 360HV10 or more and 634 × [C] ^1/2 + 140HV10 or more, more preferably 360HV10 or more and 634 × [Vickers hardness. C] ^1/2 +160HV10 or more. The 1 / 2t portion hardness is more preferably 360 HV10 or more and 634 × [C] ^1/2 + 180 HV10 or more, and the higher the hardness, the more preferable, and the upper limit is not limited. The hardness of the 1 / 2t portion may be 634 × [C] ^1/2 + 230HV10 or less from the viewpoint of ensuring toughness. The Vickers hardness test is performed according to JIS Z 2244: 2009, and the load is 10 kgf. Vickers hardness is the average value of three points measured in the L (longitudinal) cross section of the sample. The hardness of 1 / 2t portion is ensured by increasing the content of the alloy that enhances hardenability and increasing the cooling rate of quenching.
From the viewpoint of ensuring absolute wear resistance regardless of the C content, rather than the relative wear resistance in consideration of the C content, the central portion of the wear resistant steel according to the present embodiment. The hardness may be 380 HV10 or more, 400 HV10 or more, or 450 HV10 or more.

[Average value of Charpy absorption energy at -40 ° C: 27J or more]
Abrasion resistant steel may be used in cold or highlands. Wear-resistant steel is required to have toughness at -40 ° C in order to suppress fracture due to impact during processing and use. From such a viewpoint, the wear-resistant steel according to the present embodiment has an average value of Charpy absorption energy at −40 ° C. of 27 J or more. The average value of Charpy absorption energy at −40 ° C. is preferably 50 J or more. The higher the average value of the Charpy absorption energy at −40 ° C. is, the more preferable it is, and the upper limit is not limited, but it may be 100 J or less.

The Charpy test is carried out in accordance with JIS Z 2242: 2018 using a full-size test piece provided with a V notch. When the thickness of the wear-resistant steel is 12 mm or less, a test piece having a subsize of 5 mm is used. The collection position of the Charpy test piece is 1 / 4t part for wear-resistant steel having a plate thickness of 16 mm or more, and 1 / 2t part for wear-resistant steel having a plate thickness of less than 16 mm. The longitudinal direction of the Charpy test piece is the rolling direction. Considering the variation of the measured values, the average value of the Charpy absorbed energy is the arithmetic mean of the measured values of the three test pieces.

The shape of the wear-resistant steel according to the present embodiment is not particularly limited, and is a steel plate, a steel strip, a shaped steel, a steel pipe, a steel bar, a steel wire, or the like. The thickness of steel materials such as steel plates, strips, shaped steels, and steel pipes is not particularly limited, and is, for example, 8 mm or more and 50 mm or less. The thickness of the steel material may be 10 mm or more, and may be 16 mm or more. The thickness of the steel material may be 45 mm or less, and may be 40 mm or less.
When the wear-resistant steel according to the present embodiment is a steel bar, the Vickers hardness measurement and the Charpy test may be performed by regarding the diameter as the thickness.

The wear-resistant steel according to this embodiment is also excellent in corrosion resistance. When evaluating the corrosion resistance of the wear-resistant steel according to the present embodiment, the solution is carried out with a 1/100 diluted artificial sea aqueous solution based on the test methods of JASO M609 and JASO M610. Two hours of salt spraying, four hours of drying, and two hours of wetting are defined as one cycle. After corrosion treatment by 504 cycles, rust is removed by pickling treatment, and the corrosion rate is compared from the weight change before and after corrosion. In such a corrosion resistance test, the wear-resistant steel according to the present embodiment can be suppressed to a corrosion rate of less than 80% as compared with the SM490 of ordinary steel.

<Manufacturing method>
Next, a method for manufacturing wear-resistant steel according to this embodiment will be described.

The wear-resistant steel according to the present embodiment is manufactured using steel pieces obtained by melting steel, adjusting the components, and then casting as a steel material. The steel material is hot rolled and either quenched or air cooled as is. After air cooling, it is reheated and quenched. After hot rolling and quenching, reheating and quenching may be performed.

The method for producing the steel material used for producing the wear-resistant steel according to the present embodiment is not limited, and the steel material is produced by a known method.
For example, steel pieces are melted by a normal refining process such as a converter or an electric furnace, and then manufactured by a known method such as a continuous casting method or an ingot-breaking method.
The steel pieces are preferably cooled after casting, reheated to a temperature equal to or higher than the _Ac3 transformation point, and hot-rolled. When the steel pieces after continuous casting are charged into a heating furnace by hot charging without being cooled to 400 ° C. or lower, coarse austenite generated during casting may remain in the steel pieces after heating. In order to promote the miniaturization of the structure of the wear-resistant steel, it is preferable that the steel pieces after continuous casting are once cooled to 400 ° C. or lower.

Hot rolling is performed in a temperature range in which the temperature of the surface of the material to be rolled is equal to or higher than the _Ar3 transformation point. The _Ar3 transformation point is the temperature at which the transformation from austenite to ferrite begins during cooling. The heating temperature of the steel material before hot rolling is preferably 900 ° C. or higher, more preferably 1000 ° C. or higher, and further preferably 1100 ° C. or higher. The heating temperature is preferably 1330 ° C. or lower in order to suppress the coarsening of crystal grains. The heating temperature is more preferably 1200 ° C. or lower, still more preferably 1150 ° C. or lower.

In hot rolling of wear-resistant steel, it is preferable to promote recrystallization and reduce the grain size of austenite from the viewpoint of ensuring low temperature toughness. In order to promote recrystallization, the reduction rate in the temperature range of 1000 ° C. or higher is preferably 50% or higher. The reduction rate in the temperature range of 1000 ° C. or higher (simply referred to as reduction rate) is obtained by the following formula from the thickness of the steel material before hot rolling and the thickness of the material to be rolled at 1000 ° C.
Rolling ratio (%) = 100 × {(thickness of steel material)-(thickness of material to be rolled at 1000 ° C)} / thickness of steel material

The end temperature of hot rolling is equal to or higher than the _Ar3 transformation point from the viewpoint of preventing the formation of ferrite. When the material to be rolled is quenched as it is after the completion of hot rolling, the end temperature of hot rolling is 770 ° C. or higher. On the other hand, the end temperature of hot rolling is preferably 900 ° C. or lower, more preferably 850 ° C. or lower, from the viewpoint of miniaturization of the metal structure.
The thickness of the material to be rolled after the completion of hot rolling is the thickness of the wear-resistant steel, for example, 8 mm or more and 50 mm or less. If the rolling reduction in hot rolling is small and the thickness is too large, the γ grain size becomes coarse and the toughness is insufficient, and there are areas where the hardenability is insufficient and upper bainite is generated and the toughness is insufficient. there is a possibility. When the thickness is 50 mm or less, insufficient toughness is suppressed. On the other hand, if the thickness is 8 mm or more, sufficient strength can be ensured as wear-resistant steel.

The material to be rolled after hot rolling is either quenched or air-cooled as it is. The air-cooled material to be rolled is reheated and quenched. The process of quenching the material to be rolled after hot rolling as it is is called direct quenching. The process in which the material to be rolled after hot rolling is air-cooled, reheated, and quenched is called reheating quenching. The material to be rolled, which has been hardened after hot rolling, may be reheated and hardened. The reheating temperature may be a temperature equal to or higher than the _Ac3 transformation point from the viewpoint of preventing the formation of ferrite. The reheating temperature is preferably 850 ° C. or higher, more preferably 900 ° C. or higher. From the viewpoint of miniaturization of the metal structure, the reheating temperature is preferably 1150 ° C. or lower. The reheating temperature is more preferably 1050 ° C. or lower, still more preferably 1000 ° C. or lower.

The following description of the quenching start temperature, cooling rate, and stopping temperature does not differ depending on the direct quenching and reheating quenching steps. The quenching start temperature is 770 ° C. or higher from the viewpoint of preventing the formation of ferrite. The cooling rate of quenching is 3 ° C./sec or more at the surface temperature of the material to be rolled from the viewpoint of preventing the formation of ferrite and suppressing the precipitation and growth of carbides. The faster the cooling rate is, the more preferable it is, but there is a limit depending on the capacity of the cooling equipment, the thickness of the material to be rolled, and the like, and the cooling rate may be 50 ° C./sec or less. The cooling rate is a value calculated by the following formula.
Cooling rate = (Surface temperature at the start of cooling-Surface temperature at the stop of cooling) / Cooling time Here, the "surface temperature at the stop of cooling" is the surface temperature of the material to be rolled at the time of the stop of water cooling (at the reheat temperature). No).

The quenching stop temperature is 200 ° C. or lower at the surface temperature of the material to be rolled from the viewpoint of preventing the formation of ferrite and suppressing the precipitation and growth of carbides. The quenching stop temperature is preferably 100 ° C. or lower at the surface temperature of the material to be rolled, particularly from the viewpoint of suppressing the growth of carbides.

Here, _Ac3 is the transformation start temperature during heating calculated below, and _Ar3 is the transformation start temperature during cooling calculated below, which is calculated using the chemical composition of steel.

A _c3 (° C.) = 902-255 x C + 19 x Si-11 x Mn-5 x Cr + 13 x Mo-20 x Ni + 55 x V
Ar3 (° C.) = _868-396 × C + 24.6 × Si-68.1 × Mn-24.8 × Cr
C, Si, Mn, Cr, Mo, Ni, and V in the above formula mean the content expressed in% by mass.

Examples of the present disclosure are shown below. However, the examples shown below are examples of the present disclosure, and the present disclosure is not limited to the examples described below. Example 1 is an example of direct quenching, and Example 2 is an example of reheating quenching.

<Example 1>
The thickness of the steel pieces produced by melting and continuous casting of steel by a converter is 245 mm. A sample is taken from a piece of steel and chemically composed using fluorescent X-ray analysis, combustion-infrared absorption, inert gas melting, inductively coupled plasma mass spectrometry (ICP mass spectrometry), etc. Was analyzed. The results are shown in Table 1. In Table 1, the blank means that it was less than the lower limit of analysis. Although not shown in Table 1, the content of O, which is an impurity, was 20 ppm to 60 ppm. In Tables 1 to 3, underline means values or conditions outside the scope of the present disclosure.

The steel pieces were hot-rolled, and the material to be rolled was hardened as they were to produce steel sheets (wear-resistant steel). The heating temperature of hot rolling is above the _Ac3 transformation point, and the end temperature is above the _Ar3 transformation point. In the hot rolling step, the material to be rolled was rolled so that the rolling reduction ratio was 50% or more in a temperature range of 1000 ° C. or higher.
Steel No. of steel pieces used for hot rolling. The quenching start temperature, cooling rate, stop temperature and sheet steel thickness are shown in Table 2.

(Observation of metal structure)
Observation of the metallographic structure of each of the obtained thick steel sheets was carried out in the 1 / 2t portion when the thickness of the wear-resistant steel was less than 16 mm, and in the 1 / 4t portion when the thickness of the wear-resistant steel was 16 mm or more. It was conducted. The observation surface is a cross section in the L (longitudinal) direction of the sample, and was subjected to wet polishing and etching with nital. The metallographic structure was observed at a magnification of 400 times, and the presence or absence of ferrite, pearlite, upper bainite, and retained austenite was determined by observing 5 fields of view. When tissues other than lower bainite and martensite were observed, they are shown in Table 2 by the following symbols, and when the following tissues were not observed, "-" was marked.
α: Ferrite P: Pearlite uB: Upper bainite γ: Retained austenite

(Measurement of area ratio of carbides and diameter equivalent to average circle)
The sample used for observing the metallographic structure was electropolished, and the area ratio of carbides and the equivalent diameter of the circle were measured by SEM. The area of carbides and the number of carbides were measured by image analysis of photographs taken at 30,000 times, and the area ratio of carbides and the diameter corresponding to the average circle were calculated. The area ratio of carbides and the diameter equivalent to the average circle are shown in Table 2.
In addition to cementite (Fe ₃ C), Cr carbide and Fe ₂ C can be considered as carbides. The Cr carbide is a carbide that is deposited when it is held at a high temperature for a long time, but in the production process in the present disclosure, it is cooled before the precipitation of the Cr carbide. When Cr carbide is deposited, the solid solution Cr decreases, the hardenability decreases, and the amount of the solid solution C decreases, so that it is considered that the hardness as a wear-resistant steel does not appear.
Fe ₂ C is considered to be precipitated by low temperature tempering (100 to 200 ° C.), but it is a minute size observed by TEM (transmission electron microscope) and is not observed by SEM.
From these findings, the carbide observed in the examples is cementite.

(Measurement of effective crystal grain size)
In addition, EBSD measured the average of the top 10 grain sizes surrounded by large grain boundaries of 15 ° or higher. Measurements by EBSD were performed with a field of view of 400 μm × 400 μm and a pitch of 0.4 μm. The average of the top 10 grain sizes surrounded by large tilt angle grain boundaries of 15 ° or more was determined by OIM-Analysis manufactured by TSL. The average of the top 10 grain sizes surrounded by the large grain boundaries of 15 ° or more is shown in Table 2 as the effective grain size.

(Vickers hardness test)
In the L (longitudinal) cross section of the sample, the Vickers hardness of the wear-resistant steel was measured at a position 0.7 mm in the thickness direction from the surface of the wear-resistant steel and at 1 / 2t portion. The Vickers hardness test was performed in accordance with JIS Z 2244: 2009 with a load of 10 kgf. Vickers hardness is the average value of any three measured points. If the surface hardness is 360 HV10 or more and 634 × [C] ^1/2 + 140 HV10 or more, the wear resistance is good, and if the hardness of the 1 / 2t portion is 360 HV10 or more and 634 × [C] ^1/2 + 140 HV10 or more. If so, the deterioration of wear resistance is also suppressed well.

(Charpy test)
The Charpy test was carried out in accordance with JIS Z 2242: 2018 using a full size test piece provided with a V notch. The average value of Charpy absorbed energy is the arithmetic mean of the measured values of the three test pieces. The longitudinal direction of the Charpy test piece is the rolling direction of the steel sheet.
When the thickness of the wear-resistant steel was 12 mm or less, a test piece having a subsize of 5 mm was used. The collection position of the Charpy test piece is 1 / 4t part for wear-resistant steel having a plate thickness of 16 mm or more, and 1 / 2t part for wear-resistant steel having a plate thickness of less than 16 mm. Vickers hardness (surface hardness and 1 / 2t hardness) and Charpy absorption energy (KV ₂ ) are shown in Table 2. If the Charpy absorption energy is 27 J or more, the low temperature toughness is good.

(Corrosion resistance test)
The corrosion resistance test was carried out by diluting the solution with a 1/100 diluted artificial sea aqueous solution based on the test methods of JASO M609 and JASO M610. Two hours of salt spraying, four hours of drying, and two hours of wetting were set as one cycle, and after corrosion treatment by 504 cycles, rust was removed by pickling treatment, and the corrosion rate was compared from the weight change before and after corrosion. Compared with SM490 of ordinary steel, the steel material having a corrosion rate of less than 80% was evaluated by A, and the steel material having a corrosion rate of 80% or more was evaluated by B.

<Example 2>
The steel pieces whose components are shown in Table 1 were hot-rolled, air-cooled, and then reheat-quenched to the material to be rolled to produce a steel sheet. The heating temperature of hot rolling is above the _Ac3 transformation point, and the end temperature is above the _Ar3 transformation point. In the hot rolling step, the material to be rolled was rolled so that the rolling reduction ratio was 50% or more in a temperature range of 1000 ° C. or higher. The heating temperature for reheating and quenching is equal to or higher than the _Ac3 transformation point.
Steel No. of steel pieces used for hot rolling. The quenching start temperature, cooling rate, stop temperature and sheet steel thickness are shown in Table 3.

Similar to Example 1, observation of the metal structure of each obtained thick steel plate, measurement of carbide area ratio and average circle equivalent diameter, measurement of effective crystal grain size, measurement of Vickers hardness and Charpy absorption energy, and corrosion resistance. The test was done. The results are shown in Table 3.

As shown in Tables 2 and 3, the steel sheets satisfying the requirements of the present disclosure do not contain Ni and Mo or have a content of 0.50% or less, respectively, and have wear resistance, corrosion resistance and low temperature. Has excellent toughness.
Manufacturing No. In 101, since the C content is high, the carbides are coarsened and the target low temperature toughness is not obtained.
Manufacturing No. In 102, since the Cr content is low, the carbides are coarsened and the target low temperature toughness is not obtained.
Manufacturing No. In 103, since the C content is low, the surface hardness is insufficient, and the target wear resistance is not obtained.
Manufacturing No. In 104, since the Mn content is high, a large amount of MnS, which is the starting point of brittle fracture, is deposited, and the target low temperature toughness is not obtained.
Manufacturing No. In 105, since the quenching start temperature is low, ferrite is generated, and the target wear resistance and low temperature toughness are not obtained.
Manufacturing No. In 106, since the cooling rate is slow, ferrite is generated, carbides are coarsened, and the target wear resistance and low temperature toughness are not obtained.
Manufacturing No. In 107, since the quenching stop temperature is high, the carbides are coarsened and the target low temperature toughness is not obtained.
Manufacturing No. In No. 108, since the C content is high, the carbides are coarsened and the target low temperature toughness is not obtained.
Manufacturing No. In 109, since the Cr content is low, the carbides are coarsened and the target low temperature toughness is not obtained.
Manufacturing No. In 110, since the C content is low, the surface hardness is insufficient to obtain the target wear resistance, and the low temperature toughness is also insufficient.
Manufacturing No. In 111, the target low temperature toughness is not obtained because the Mn content is high.
Manufacturing No. In 112, the target low temperature toughness is not obtained because the quenching stop temperature is high.
Manufacturing No. In 113, since the quenching stop temperature is low, ferrite is generated and the surface hardness is insufficient, the target wear resistance cannot be obtained, and the low temperature toughness is also insufficient.
Manufacturing No. At 114, since the cooling rate is slow, ferrite is generated, carbides are coarsened, and the target wear resistance and low temperature toughness are not obtained.
Manufacturing No. In 115, since the quenching stop temperature is high, the carbides are coarsened and the target low temperature toughness is not obtained.

The entire disclosure of Japanese patent application 2020-127724, filed July 28, 2020, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are referenced herein to the same extent as if individual documents, patent applications, and technical standards were specifically and individually described. Is taken in by.

Claims

The chemical composition is by mass%,
C: 0.08% or more, 0.20% or less,
Si: 0.01% or more, 0.50% or less,
Mn: 0.10% or more, 2.00% or less,
P: 0.015% or less,
S: 0.0300% or less,
Cr: 2.10% or more, 8.00% or less,
N: 0.0080% or less,
Cu: 0% or more, 0.50% or less,
Ni: 0% or more, 0.50% or less,
Mo: 0% or more, 0.50% or less,
V: 0% or more, 0.500% or less,
W: 0% or more, 0.50% or less,
B: 0% or more, 0.0050% or less,
Al: 0% or more, 0.300% or less,
Ti: 0% or more, 0.100% or less,
Nb: 0% or more, 0.100% or less,
Ca: 0% or more, 0.0100% or less,
Mg: 0% or more, 0.0100% or less,
REM: 0% or more, 0.0100% or less, and the balance: Fe and impurities.
In the cross section in the thickness direction, when the thickness is t, when t is less than 16 mm, it is 1 / 2t from the surface, and when t is 16 mm or more, it is 1 / 4t from the surface.
The carbide contained in the metal structure is 0% or more and 0.5% or less in area%, and the average circle equivalent diameter of the carbide is 500 nm or less.
In the region of 400 μm × 400 μm, among the crystal grains surrounded by large tilt angle grain boundaries of 15 ° or more, the average grain size of 10 crystal grains is 40 μm or less in descending order of grain size, and at -40 ° C. Crystall absorption energy is 27J or more,
When the C content in% by mass is [C], the surface hardness at a position 0.7 mm in the thickness direction from the surface is 360 HV10 or more and 634 × [C] 1/2 + 140 HV10 or more. Worn steel.
The wear-resistant steel according to claim 1, wherein the hardness of the central portion in the central portion in the thickness direction is 360 HV10 or more and 634 × [C] 1/2 + 140 HV10 or more.
By mass%,
Cu: 0.01% or more, 0.50% or less,
Ni: 0.01% or more, 0.50% or less,
Mo: 0.01% or more, 0.50% or less,
V: 0.003% or more, 0.500% or less,
W: 0.01% or more, 0.50% or less,
B: 0.0003% or more, 0.0050% or less,
Al: 0.005% or more, 0.300% or less,
Ti: 0.003% or more, 0.100% or less,
Nb: 0.003% or more, 0.100% or less,
Ca: 0.0003% or more, 0.0100% or less,
Mg: 0% or more, 0.0100% or less, and REM: 0.0003% or more, 0.0100% or less,
The wear-resistant steel according to claim 1 or 2, which comprises at least one selected from the group consisting of.
The wear-resistant steel according to any one of claims 1 to 3, which is a steel plate.
The wear-resistant steel according to claim 4, wherein the plate thickness is 8 mm or more and 50 mm or less.