WO2022224458A1

WO2022224458A1 - Wear-resistant steel sheet

Info

Publication number: WO2022224458A1
Application number: PCT/JP2021/016546
Authority: WO
Inventors: 仁秀吉村; 仁志古谷; 拓海三宅; 大司中田; 康哲 ▲高▼橋
Original assignee: 日本製鉄株式会社
Priority date: 2021-04-23
Filing date: 2021-04-23
Publication date: 2022-10-27
Also published as: JPWO2022224458A1; JP7239056B1

Abstract

Provided is a wear-resistant steel sheet having a predetermined chemical composition, wherein: the Vickers hardness HV10 (Hv) thereof at a position 1 mm deep from the surface satisfies expression 1: HV10 ≥ 634 x √[C] + 140; crystal grains surrounded by boundaries having a misorientation of 15˚ or greater at the 1 mm deep position from the surface have an average circle equivalent diameter of 25 μm or less; and carbides at the 1 mm deep position from the surface after a heat-treatment test at 300˚C for 10 minutes have a major axis average value of 0.25 to 5.00 μm.

Description

Wear-resistant steel plate

The present invention relates to a wear-resistant steel plate, and more particularly to a wear-resistant steel plate that is useful for use in members of machinery that requires wear resistance, such as industrial machinery and transportation machinery.

Industrial machinery and transportation machinery (e.g., dump trucks, bulldozers, power shovels) used in mining excavation and civil engineering construction work use steel materials with excellent wear resistance in order to extend their service life. ing. In order to improve the wear resistance of steel, it is necessary to harden the structure by quenching the steel. However, when the steel material becomes hard, it becomes difficult to cut, for example, and there is a problem that the productivity is lowered and the processing cost is increased.

In relation to improving the machinability of steel, for example, in Patent Document 1, in mass%, C: 0.3 to 0.6%, Si: 1.0% or less, Mn: 1.0% or less, P : 0.04% or less, S: 0.005-0.2%, Cr: 4.0-11.0%, Al: 0.001-0.1%, Ca: 0.0005-0.02% , O: 3.3 mm ² of a sulfide having an equivalent circle diameter of 5 μm or more containing 0.0005 to 0.01% O, the balance being substantially Fe, and containing 0.1 to 10% Ca A martensitic stainless steel part is described which contains 5 or more per part and is characterized by induction hardening on the surface to be used. In addition, in Patent Document 1, S improves the machinability of steel, and a film that protects the tool during cutting is formed by setting the Ca content in the sulfide to the range of 0.1 to 10%. It is taught that

In Patent Document 2, in terms of % by mass, C: 0.005-0.2%, Mn: 0.3-2.0%, P: 0.005-0.2%, S: 0.01-0. 7%, Pb: 0.03 to 0.5%, N: 0.004 to 0.02%, and O: 0.003 to 0.03%, the balance consists of Fe and impurities, in the steel of the MnS inclusions, Pb inclusions, and Pb—MnS inclusions containing Pb and MnS, the MnS inclusions, the Pb inclusions, and the Pb—MnS inclusions having an equivalent circle diameter of 5 μm or more The ratio of the number of the Pb--MnS inclusions having an equivalent circle diameter of 5 μm or more to the total number is 5% or more, and the lengths of the MnS inclusions, the Pb inclusions and the Pb--MnS inclusions are 200 μm or less. The average length of the MnS inclusions, the Pb inclusions and the Pb--MnS inclusions having an equivalent circle diameter of 5 μm or more is 50 μm or less, and the MnS inclusions, the Pb inclusions and the Pb- A lead free-cutting steel is described in which 500 inclusions/mm ² or more of MnS inclusions have an equivalent circle diameter of 2 μm or more and an aspect ratio of 10 or less. In addition, in Patent Document 2, machinability is affected by the built-up edge that adheres to the cutting edge of the cutting tool during cutting, and free-cutting inclusions in steel (MnS inclusions, Pb inclusions, Pb-Mn inclusions ) is 200 μm or less, and the average length of the free-cutting inclusions having an equivalent circle diameter of 5 μm or more is 50 μm or less, the built-up edge becomes fine, and 10 or less of the free-cutting inclusions It is taught that when the number of elements having an aspect ratio is 500/mm ² or more, a large number of fine built-up edges are uniformly formed, and the machinability is enhanced.

JP-A-2000-282185 WO2014/125779

Wear-resistant steel plates used in parts of industrial machinery and transportation machinery often require low-temperature toughness in consideration of harsh usage environments. However, neither of Patent Literatures 1 and 2 has necessarily made sufficient studies on achieving both machinability and low-temperature toughness in addition to wear resistance.

The present invention has been made in view of such circumstances, and its object is to have improved machinability while maintaining good wear resistance and low temperature toughness by a novel structure. An object of the present invention is to provide a wear-resistant steel plate.

In order to achieve the above objectives, the inventors investigated the chemical composition and metallographic structure of the steel sheet. As a result, the present inventors improved both wear resistance and low-temperature toughness by adjusting the Vickers hardness at a depth of 1 mm from the surface of the steel sheet while keeping the chemical composition of the steel sheet within a predetermined range. Also, by suppressing the Si content of the steel sheet to 0.09% or less and making the metal structure at a depth of 1 mm from the steel plate surface fine, the metal structure at a relatively high temperature during cutting. By promoting the precipitation of carbides from the steel and keeping the size of the carbides precipitated at such high temperatures within a predetermined range, the deformation resistance during cutting is alleviated and the machinability is improved. and completed the present invention.

The present invention, which has achieved the above objects, is as follows.
(1) in mass %,
C: 0.140 to 0.250%,
Si: 0.09% or less,
Mn: 1.20-2.00%,
P: 0.0200% or less,
S: 0.0050% or less,
Cr: 0.10 to 1.00%,
Mo: 0.05-0.29%,
Ti: 0.005 to 0.030%,
B: 0.0003 to 0.0050%,
Al: 0.0030 to 0.1000%,
N: 0.0010 to 0.0080%,
O: 0.0050% or less,
Ni: 0 to 0.50%,
Nb: 0 to 0.050%,
Cu: 0-0.50%,
V: 0 to 0.050,
W: 0 to 0.50%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
REM: 0 to 0.0050%,
Balance: having a chemical composition of Fe and impurities,
The Vickers hardness HV ₁₀ (Hv) at a depth of 1 mm from the surface satisfies the following formula 1,
The average circle equivalent diameter of crystal grains surrounded by boundaries with a misorientation of 15° or more at a depth of 1 mm from the surface is 25 μm or less,
The average value of the long axis of the carbide at a depth of 1 mm from the surface after a heat treatment test at 300 ° C. for 10 minutes. A wear-resistant steel plate in which the average value of the long diameters of 100 carbides selected from the largest ones is 0.25 to 5.00 μm.
HV ₁₀ ≧634×√[C]+140 Formula 1
Here, [C] is the C content (% by mass) of the wear-resistant steel plate.
(2) the chemical composition, in mass %,
Ni: 0.01 to 0.50%,
Nb: 0.001 to 0.050%,
Cu: 0.01-0.50%,
V: 0.001 to 0.050,
W: 0.01 to 0.50%,
Ca: 0.0001 to 0.0050%,
Mg: 0.0001-0.0050%, and REM: 0.0001-0.0050%
The wear-resistant steel plate according to (1) above, comprising one or more selected from the group consisting of:
(3) The wear-resistant steel plate according to (1) or (2) above, which has a plate thickness of 6 to 150 mm.

According to the present invention, it is possible to provide a wear-resistant steel plate having improved machinability while maintaining good wear resistance and low-temperature toughness. Therefore, by using the wear-resistant steel plate according to the present invention, it is possible to improve the productivity of cutting and significantly reduce the processing cost.

<Abrasion-resistant steel plate>
The wear-resistant steel plate according to the embodiment of the present invention is mass%,
C: 0.140 to 0.250%,
Si: 0.09% or less,
Mn: 1.20-2.00%,
P: 0.0200% or less,
S: 0.0050% or less,
Cr: 0.10 to 1.00%,
Mo: 0.05-0.29%,
Ti: 0.005 to 0.030%,
B: 0.0003 to 0.0050%,
Al: 0.0030 to 0.1000%,
N: 0.0010 to 0.0080%,
O: 0.0050% or less,
Ni: 0 to 0.50%,
Nb: 0 to 0.050%,
Cu: 0-0.50%,
V: 0 to 0.050,
W: 0 to 0.50%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
REM: 0 to 0.0050%,
Balance: having a chemical composition of Fe and impurities,
The Vickers hardness HV ₁₀ (Hv) at a depth of 1 mm from the surface satisfies the following formula 1,
The average circle equivalent diameter of crystal grains surrounded by boundaries with a misorientation of 15° or more at a depth of 1 mm from the surface is 25 μm or less,
The average value of the long axis of the carbide at a depth of 1 mm from the surface after a heat treatment test at 300 ° C. for 10 minutes. It is characterized in that the average value of the major diameters of a total of 100 carbides selected from the largest 10 is 0.25 to 5.00 μm.
HV ₁₀ ≧634×√[C]+140 Formula 1
Here, [C] is the C content (% by mass) of the wear-resistant steel plate.

As mentioned above, when the steel material becomes hard, it becomes difficult to cut it. Therefore, it is difficult to obtain a wear-resistant steel plate that maintains wear resistance, improves machinability, and further improves low-temperature toughness. Generally difficult. Therefore, the present inventors first set the chemical composition of the steel sheet within a predetermined range, and set the Vickers hardness at a depth of 1 mm from the steel sheet surface to an appropriate hardness particularly according to the C content of the steel sheet. More specifically, the Vickers hardness HV ₁₀ (Hv) at a depth of 1 mm from the steel plate surface is expressed by formula 1: HV ₁₀ ≥ 634 × √ [C] + 140 (where [C] is the C content of the wear-resistant steel plate (% by mass)), it is possible to improve the low-temperature toughness while maintaining the hardness of the surface layer portion of the steel sheet to ensure wear resistance. In order to improve the low-temperature toughness, it is effective to eliminate coarse grains by making the metallographic structure uniform. Although it is not intended to be bound by any particular theory, the metal structure of the steel sheet is achieved by realizing a hardness that satisfies the above formula 1 while keeping the chemical composition of the steel sheet within the range described above. In particular, the metal structure of the surface layer of the steel sheet can be made, for example, a uniform structure that is mainly composed of martensite and is closer to a single phase, so both wear resistance and low temperature toughness can be improved. It is considered to be.

On the other hand, when cutting a steel plate, heat is generated due to friction between the cutting tool and the steel plate, and the cut portion of the steel plate reaches a high temperature of, for example, about 300°C. Therefore, in relation to the improvement of the machinability of steel sheets, the present inventors paid attention to the metal structure under high temperature conditions during cutting, and studied the effect of the metal structure on the machinability of steel sheets. As a result, the present inventors have found that the solute carbon (C) contained in the metal structure of the steel sheet precipitates as carbides at such high temperatures, and the precipitation of the carbides relaxes the deformation resistance during cutting. I found out. As a result of further studies based on such knowledge, the present inventors suppressed the Si content of the steel sheet to 0.09% or less and made the metal structure at a depth of 1 mm from the steel sheet surface finer. More specifically, by setting the average circle equivalent diameter of the crystal grains surrounded by the boundaries with a misorientation of 15° or more to 25 μm or less, the precipitation of carbides from the metal structure at a relatively high temperature during cutting is promoted. Furthermore, the size of the carbides precipitated at such high temperatures is within a predetermined range, more specifically, the average value of the major axis of the carbides is 0.25 to 5.00 μm. It has been found that by setting the content within the range, the deformation resistance during cutting can be reliably alleviated, and as a result, the machinability is remarkably improved.

Although it is not intended to be bound by any particular theory, by setting the average circle equivalent diameter of the crystal grains to 25 μm or less, the metal structure becomes finer, which is advantageous for the diffusion of solute carbon in the steel sheet. Since the number of fine grain boundaries can be increased, it is thought that the combination of such a fine structure and a low Si content can further promote the precipitation of carbides. The carbides precipitated at high temperatures in this way reduce the hardness of the structure of the cut portion by precipitating within a specific size range, that is, in the direction of alleviating the deformation resistance during cutting. considered to work. Therefore, it has a predetermined chemical composition with a low Si content, has appropriate hardness and a fine metal structure at a depth of 1 mm from the steel plate surface corresponding to the cut part, and has an appropriate size at high temperatures. According to the wear-resistant steel sheet according to the present invention in which is precipitated, it is considered possible to significantly improve machinability while maintaining both wear resistance and low-temperature toughness.

The wear-resistant steel plates according to the embodiments of the present invention will be described in more detail below. In the following description, the unit of content of each element, "%", means "% by mass" unless otherwise specified. In addition, in this specification, the term "to" indicating a numerical range is used to include the numerical values before and after it as a lower limit and an upper limit, unless otherwise specified. Also, in this specification, "√" applies only to the letter or number immediately following it.

[C: 0.140 to 0.250%]
Carbon (C) is an element necessary to increase hardness and improve wear resistance. In order to sufficiently obtain such effects, the C content is made 0.140% or more. The C content may be 0.150% or more, 0.160% or more, or 0.170% or more. On the other hand, when C is contained excessively, the deterioration of toughness may become remarkable, and the strength tends to be excessive. Therefore, the C content should be 0.250% or less. The C content may be 0.240% or less, 0.230% or less, or 0.220% or less.

[Si: 0.09% or less]
Silicon (Si) is an element whose content in the steel sheet is suppressed in order to promote the precipitation of carbides during cutting and alleviate the deformation resistance during cutting. In order to sufficiently obtain the effect of accelerating the precipitation of carbides, it is extremely important to reduce the Si content in the steel sheet. If the Si content is high, the carbides are not sufficiently precipitated at high temperatures during cutting, so the average major axis of the carbides becomes small, and as a result, the deformation resistance during cutting cannot be sufficiently alleviated. From this point of view, the Si content is set to 0.09% or less. The Si content may be 0.08% or less, 0.07% or less, 0.06% or less, 0.05% or less, or 0.04% or less. For example, the Si content may be 0% because the lower the Si content, the higher the effect of accelerating the precipitation of carbides. However, from the viewpoint of manufacturing cost, the Si content may be 0.005% or more or 0.01% or more.

[Mn: 1.20 to 2.00%]
Mn (manganese) is an element that deoxidizes molten steel, and is also an element that improves hardenability and increases hardness. In order to sufficiently obtain these effects, the Mn content should be 1.20% or more. The Mn content may be 1.25% or more, 1.30% or more, or 1.40% or more. On the other hand, when Mn is contained excessively, segregation increases and hardenability becomes excessive, so strength may excessively increase and toughness may decrease. Therefore, the Mn content should be 2.00% or less. The Mn content may be 1.90% or less, 1.80% or less, or 1.70% or less.

[P: 0.0200% or less]
Phosphorus (P) is an impurity, and if it is contained excessively, it may segregate at grain boundaries and reduce toughness. Therefore, the P content should be 0.0200% or less. The P content is preferably 0.0180% or less, more preferably 0.0150% or less, most preferably 0.0100% or less or 0.0080% or less. The P content is preferably as small as possible, and therefore may be 0%. However, from the viewpoint of manufacturing cost, the P content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[S: 0.0050% or less]
Sulfur (S) is an impurity, and when contained excessively, it promotes center segregation and may cause the formation of elongated MnS, which is the starting point of brittle fracture. Therefore, the S content should be 0.0050% or less. The S content is preferably 0.0040% or less, more preferably 0.0030% or less, and most preferably 0.0020% or less. The lower the S content, the better, so it may be 0%. However, from the viewpoint of manufacturing cost, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[Cr: 0.10 to 1.00%]
Chromium (Cr) is an element that improves hardenability and increases hardness. In order to sufficiently obtain such effects, the Cr content is set to 0.10% or more. The Cr content may be 0.20% or more, 0.30% or more, or 0.40% or more. On the other hand, if Cr is contained excessively, the hardenability becomes excessive, and the toughness may decrease. Therefore, the Cr content should be 1.00% or less. The Cr content may be 0.90% or less, 0.80% or less, or 0.70% or less.

[Mo: 0.05-0.29%]
Molybdenum (Mo) is an element that improves hardenability and increases hardness. In order to sufficiently obtain such effects, the Mo content is set to 0.05% or more. Mo content may be 0.10% or more, 0.12% or more, or 0.15% or more. On the other hand, if Mo is contained excessively, the hardenability becomes excessive, and toughness may decrease. Therefore, Mo content shall be 0.29% or less. The Mo content may be 0.28% or less, 0.27% or less, or 0.25% or less.

[Ti: 0.005 to 0.030%]
Titanium (Ti) is an element that forms titanium nitride (TiN) as pinning particles, refines crystal grains, and improves toughness. Moreover, in order for B to function effectively in improving hardenability, it is necessary to prevent B from precipitating as boron nitride (BN), that is, to fix N. In addition to contributing to refinement of crystal grains as pinning particles, Ti consumes solute nitrogen in steel by forming TiN, so B combines with the solute nitrogen to form BN. It also has the function of inhibiting In order to sufficiently obtain these effects, the Ti content should be 0.005% or more. The Ti content may be 0.007% or more, 0.010% or more, or 0.015% or more. On the other hand, when Ti is contained excessively, TiC may precipitate and the toughness may decrease. Therefore, the Ti content should be 0.030% or less. The Ti content may be 0.027% or less, 0.025% or less, or 0.022% or less.

[B: 0.0003 to 0.0050%]
Boron (B) is an element that segregates at grain boundaries in a very small amount to improve hardenability and increase hardness. In order to sufficiently obtain such effects, the B content should be 0.0003% or more. The B content may be 0.0005% or more, 0.0007% or more, 0.0010% or more, or 0.0015% or more. On the other hand, when B is contained excessively, BN may precipitate and the toughness may be lowered. Therefore, the B content should be 0.0050% or less. The B content may be 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[Al: 0.0030 to 0.1000%]
Aluminum (Al) is an element necessary for deoxidizing molten steel. In order to sufficiently obtain such effects, the Al content is made 0.0030% or more. The Al content may be 0.0050% or more, 0.0100% or more, 0.0150% or more, or 0.0300% or more. On the other hand, if Al is contained excessively, the cleanliness of the steel may be lowered and the toughness may be lowered. Therefore, the Al content should be 0.1000% or less. The Al content may be 0.0900% or less, 0.0800% or less, 0.0700% or less, or 0.0500% or less.

[N: 0.0010 to 0.0080%]
Nitrogen (N) is an element necessary for refining crystal grains and improving toughness by precipitating as TiN. In order to sufficiently obtain such effects, the N content is made 0.0010% or more. The N content may be 0.0015% or more, 0.0020% or more, or 0.0030% or more. On the other hand, when N is contained excessively, TiN is excessively precipitated, which may act as starting points of fracture and lower the toughness. Therefore, the N content should be 0.0080% or less. The N content may be 0.0070% or less, 0.0060% or less, or 0.0050% or less.

[O: 0.0050% or less]
Oxygen (O) is an impurity, so its content is made 0.0050% or less. The O content is preferably 0.0045% or less, more preferably 0.0040% or less, most preferably 0.0035% or less. It is preferable to reduce the O content as much as possible, so it may be 0%. However, from the viewpoint of deoxidation cost, the O content may be 0.0001% or more, 0.0002% or more, or 0.0003% or more.

The basic chemical composition of the wear-resistant steel sheet according to the embodiment of the present invention is as described above. Furthermore, the wear-resistant steel plate may contain one or more selected from the group consisting of the following optional elements, if necessary. For example, the wear-resistant steel plate may contain one or two selected from the group consisting of Ni: 0-0.50% and Nb: 0-0.050%. In addition, the wear-resistant steel plate contains one or more selected from the group consisting of Cu: 0 to 0.50%, V: 0 to 0.050%, and W: 0 to 0.50%. You can stay. In addition, the wear-resistant steel plate contains one or more selected from the group consisting of Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, and REM: 0 to 0.0050%. You can stay. These optional elements are described in detail below.

[Ni: 0 to 0.50%]
Nickel (Ni) is an element that contributes to improving toughness. Although the Ni content may be 0%, the Ni content is preferably 0.01% or more in order to obtain such effects. The Ni content may be 0.03% or more, 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, even if Ni is contained excessively, the effect is saturated. Therefore, containing more Ni than necessary in the steel sheet may simply increase the alloy cost. Therefore, the Ni content should be 0.50% or less. The Ni content may be 0.45% or less, 0.40% or less, or 0.35% or less.

[Nb: 0 to 0.050%]
Niobium (Nb) is an element that contributes to the improvement of toughness by refining crystal grains by precipitating NbCN as pinning particles. Although the Nb content may be 0%, the Nb content is preferably 0.001% or more in order to obtain such effects. The Nb content may be 0.005% or more, 0.008% or more, 0.010% or more, or 0.015% or more. On the other hand, even if Nb is contained excessively, the grain refining effect may be saturated, and/or coarse NbCN may be precipitated to act as a starting point of fracture and reduce toughness. Therefore, the Nb content should be 0.050% or less. The Nb content may be 0.045% or less, 0.040% or less, or 0.035% or less.

[Cu: 0 to 0.50%]
Copper (Cu) is an element that improves hardenability and increases hardness. The Cu content may be 0%, but in order to obtain these effects, the Cu content is preferably 0.01% or more. The Cu content may be 0.03% or more, 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, when Cu is contained excessively, the toughness may decrease. Therefore, the Cu content is set to 0.50% or less. The Cu content may be 0.45% or less, 0.40% or less, or 0.35% or less.

[V: 0 to 0.050%]
Vanadium (V) is an element that improves hardenability and increases hardness. Although the V content may be 0%, the V content is preferably 0.001% or more in order to obtain these effects. The V content may be 0.005% or more, 0.008% or more, 0.010% or more, or 0.015% or more. On the other hand, if V is contained excessively, carbonitrides may be formed and the toughness may be lowered. Therefore, the V content should be 0.050% or less. The V content may be 0.045% or less, 0.040% or less, or 0.035% or less.

[W: 0 to 0.50%]
Tungsten (W) is an element that improves hardenability and increases hardness. Although the W content may be 0%, the W content is preferably 0.01% or more in order to obtain these effects. The W content may be 0.03% or more, 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, an excessive W content may reduce the toughness. Therefore, the W content should be 0.50% or less. The W content may be 0.45% or less, 0.40% or less, or 0.35% or less.

[Ca: 0 to 0.0050%]
Calcium (Ca) is an element that controls the forms of oxides and sulfides. Although the Ca content may be 0%, the Ca content is preferably 0.0001% or more in order to obtain such effects. The Ca content may be 0.0005% or more, 0.0008% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if Ca is contained excessively, the effect is saturated, and the formation of inclusions may impair the toughness. Therefore, the Ca content should be 0.0050% or less. The Ca content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.

[Mg: 0 to 0.0050%]
Magnesium (Mg) is an element that controls the forms of oxides and sulfides. Although the Mg content may be 0%, the Mg content is preferably 0.0001% or more in order to obtain such effects. The Mg content may be 0.0005% or more, 0.0008% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if Mg is contained excessively, the effect is saturated, and the formation of inclusions may impair the toughness. Therefore, the Mg content should be 0.0050% or less. The Mg content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.

[REM: 0 to 0.0050%]
Rare earth metals (REMs) are elements that control the morphology of oxides and sulfides. Although the REM content may be 0%, the REM content is preferably 0.0001% or more in order to obtain such effects. The REM content may be 0.0005% or greater, 0.0008% or greater, 0.0010% or greater, or 0.0015% or greater. On the other hand, even if the REM content is excessive, the effect is saturated, and the formation of inclusions may impair the toughness. Therefore, the REM content should be 0.0050% or less. The REM content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less. REM in this specification refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum with atomic number 57 (La) to lutetium with atomic number 71 (Lu ), and the REM content is the total content of these elements.

In the wear-resistant steel sheet according to the embodiment of the present invention, the balance other than the above elements is Fe and impurities. Impurities are components and the like that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the wear-resistant steel sheet is industrially manufactured.

[Carbon equivalent (Ceq): 0.86 or less]
Carbon equivalent (Ceq) is an index of hardenability. In general, the higher the Ceq, the higher the hardness of the wear-resistant steel plate. In the wear-resistant steel plate according to the embodiment of the present invention, Ceq is calculated by Equation 2 below.
Ceq=[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Cu]+[Ni])/15 Equation 2
Here, [C], [Mn], [Cr], [Mo], [V], [Cu] and [Ni] are the content (% by mass) of each element, and 0 when no element is contained. is. In the wear-resistant steel plate according to the embodiment of the present invention, the appropriate hardness of the wear-resistant steel plate (for example, Brinell hardness 360-490 HB) can be achieved. Therefore, the Ceq of the wear-resistant steel plate is not particularly limited, but from the viewpoint of obtaining more preferable hardness, the Ceq is preferably 0.86 or less, for example. Ceq may be 0.82 or less, 0.80 or less, or 0.75 or less. Although the lower limit of Ceq is not particularly limited, Ceq may be 0.37 or more, 0.40 or more, 0.45 or more, or 0.55 or more, for example.

[Vc90: 2.0 to 30.0]
Vc90 is a value known as the critical cooling rate (° C./sec) at which a 90% martensite structure is obtained, and is an index of hardenability as in the case of Ceq. Therefore, in general, the smaller the value of Vc90, the higher the hardenability of the steel, which tends to increase the hardness of the wear-resistant steel plate. Vc90 is calculated according to the following formula 3 or 4 according to the B content of the steel plate, and in the wear resistant steel plate according to the embodiment of the present invention, the B content is 0.0003 to 0.0050%. It is calculated by the following formula 3.
(B content ≥ 0.0003%)
logVc90=2.94−0.75×(2.7[C]+0.4[Si]+[Mn]+0.45[Ni]+0.45[Cu]+0.8[Cr]+2[Mo])・・・Formula 3
(B content <0.0003%)
logVc90=3.69−0.75×(2.7[C]+0.4[Si]+[Mn]+0.45[Ni]+0.45[Cu]+0.8[Cr]+[Mo])・・・Equation 4
In formulas 3 and 4, [C], [Si], [Mn], [Ni], [Cu], [Cr] and [Mo] are the contents (mass%) of each element, and contain the elements 0 if not. In the wear-resistant steel plate according to the embodiment of the present invention, the appropriate hardness of the wear-resistant steel plate (for example, Brinell hardness 360-490 HB) can be achieved. Therefore, Vc90 of the wear-resistant steel plate is not particularly limited, but from the viewpoint of obtaining more preferable hardness, Vc is preferably 2.0 to 30.0, for example. Vc90 may be greater than or equal to 2.2, greater than or equal to 2.4, or greater than or equal to 2.6, and/or less than or equal to 28.0, less than or equal to 25.0, less than or equal to 20.0, or less than or equal to 15.0.

[Vickers hardness HV ₁₀ at a depth of 1 mm from the surface]
In the wear-resistant steel plate according to the embodiment of the present invention, the Vickers hardness HV ₁₀ (Hv) at a depth of 1 mm from the surface of the wear-resistant steel plate satisfies Formula 1 below.
HV ₁₀ ≧634×√[C]+140 Formula 1
Here, [C] is the C content (% by mass) of the wear-resistant steel plate. Sufficient wear resistance can be maintained by setting the hardness of the surface layer portion of the steel sheet so as to satisfy Expression 1 above. For example, simply increasing the hardness of the steel sheet may lead to a decrease in low-temperature toughness, although the wear resistance can be improved. On the other hand, in the embodiment of the present invention, while the chemical composition of the steel sheet is within the range described above, the Vickers hardness at a position 1 mm deep from the steel sheet surface is increased according to the C content that particularly contributes to the hardness of the steel sheet. By setting the hardness to an appropriate level, the metal structure of the steel plate, particularly the metal structure of the surface layer of the steel plate, can be made, for example, a uniform structure that is closer to a single phase than is mainly composed of martensite, and is therefore wear resistant. It is thought that it becomes possible to improve both toughness and low temperature toughness. When the right side of the above formula 1 is represented as f(C), that is, when f(C)=634×√[C]+140, the Vickers hardness HV ₁₀ is, for example, HV ₁₀ ≧1.01×f(C ), HV ₁₀ ≧1.02×f(C), or HV ₁₀ ≧1.05×f(C).

In the present invention, the Vickers hardness HV ₁₀ (Hv) at a depth of 1 mm from the steel plate surface is determined by first mechanically polishing the L cross section of the wear resistant steel plate (the cross section parallel to the rolling direction and thickness direction of the steel plate), and then Vickers hardness test conforming to JIS Z2244: 2009 is performed at a position 1 mm deep from the surface in the plate thickness direction, Vickers hardness (measurement load 10 kgf = 98.07 N) is measured at three points, and the values are averaged. It is determined.

[Metal structure at a depth of 1 mm from the surface]
As described above, in the wear-resistant steel plate according to the embodiment of the present invention, while the chemical composition of the steel plate is within the range described above, the Vickers hardness HV at a position ₁ mm deep from the steel plate surface is such that it satisfies the above formula 1. Both wear resistance and low temperature toughness can be improved by Therefore, the metal structure at a depth of 1 mm from the steel plate surface is not particularly limited. On the other hand, in order to reliably obtain a wear-resistant steel sheet that satisfies the above formula 1 within the chemical composition range described above, the metallographic structure is preferably hard and uniform. From this point of view, it is preferable that the metal structure at a depth of 1 mm from the steel sheet surface is mainly composed of martensite. More specifically, the metal structure preferably contains 80% or more martensite in area ratio, more preferably 85% or 90% or more martensite, and 92% or more or 95% or more It is most preferable to contain martensite, particularly preferably 80% or more in terms of area ratio of martensite in observation with an optical microscope, more preferably 85% or 90% or more martensite, 92% or more or Most preferably it contains 95% or more martensite. Although the upper limit of the martensite area ratio is not particularly limited, it may be 100%, for example. In particular, the metal structure at a depth of 1 mm from the steel plate surface is composed of 90% or more martensite, so that the Vickers hardness HV ₁₀ (Hv) at the depth position satisfies the above formula 1 more reliably. be able to.

The area ratio of martensite is determined as follows. First, the L cross section (cross section parallel to the rolling direction and thickness direction of the wear resistant steel plate) at a depth of 1 mm from the surface of the wear resistant steel plate is mirror-polished, then corroded with nital, and an optical microscope is used at 500 times. It is determined by observing, randomly selecting three observation fields of 200 μm×500 μm, measuring the area ratio of martensite in each observation field, and averaging them. In observation with an optical microscope, for example, it is sometimes difficult to clearly distinguish between so-called autotempered martensite in which cementite is precipitated by autotempering and such cementite-free martensite. Thus, the term martensite as used herein includes autotempered martensite.

[Average circle equivalent diameter of crystal grains at a depth of 1 mm from the surface: 25 μm or less]
In the wear-resistant steel plate according to the embodiment of the present invention, the average equivalent circle diameter of grains surrounded by boundaries with a misorientation of 15° or more at a depth of 1 mm from the surface of the wear-resistant steel plate is 25 μm or less. By setting the average equivalent circle diameter of the crystal grains to 25 μm or less, the metal structure becomes finer, and the number of grain boundaries that are advantageous for the diffusion of solute carbon in the steel sheet can be increased. Therefore, by combining such a fine structure with a low Si content, it is possible to further promote the precipitation of carbides from the metal structure at high temperatures during cutting, thereby reliably alleviating the deformation resistance during cutting. becomes. The average equivalent circle diameter of the crystal grains is preferably 24 μm or less, more preferably 22 μm or less, and most preferably 20 μm or less. Since the smaller the average equivalent circle diameter of the crystal grains, the better, the lower limit is not particularly defined. However, the average equivalent circle diameter of the grains may be, for example, 1 μm or more, or 3 μm or more. Since the wear-resistant steel sheet according to the present invention is basically quenched, the surface layer, which is sufficiently quenched, is the hardest and most difficult part to cut compared to the inside of the steel sheet. . Therefore, when examining the machinability of a steel sheet, it is important to improve the machinability at a position 1 mm deep from the surface of the steel sheet, which corresponds to such a site that is most difficult to cut. This is because the hardness becomes softer than that of the surface layer portion toward the inner side of the steel sheet, and the machinability is improved. The same can be said for the "average value of the major axis of carbides" which will be described later.

In the present invention, the average equivalent circle diameter of crystal grains is determined as follows. First, the L cross section (cross section parallel to the rolling direction and thickness direction of the wear resistant steel plate) at a depth of 1 mm from the surface of the wear resistant steel plate is mirror-polished, and then any rolling is performed by electron beam backscatter diffraction (EBSD). The crystal orientation of a region of 1.0 mm in the direction × 0.4 mm in the depth direction is measured at one point, and the region where the orientation difference between adjacent grains is 15° or more is defined as one crystal grain, and the grain of each crystal grain. Calculate the diameter as a circle equivalent diameter. The area average calculated based on all the calculated crystal grains is determined as the "average circle-equivalent diameter of the crystal grains." More specifically, the area average (d) is calculated from the area occupied by each crystal grain (a _i ) and the circle-equivalent diameter (d _i ) of each crystal grain by Equation 5 below.
d=Σ(a _i ×d _i )/Σa _i Expression 5

[Aspect ratio of prior austenite grains at a depth of 1 mm from the surface: 1.0 to 1.8]
In the wear-resistant steel plate according to the embodiment of the present invention, the average circle equivalent diameter of the crystal grains surrounded by the boundaries with a misorientation of 15 ° or more at a depth of 1 mm from the surface may be specified. Other grain morphologies such as the aspect ratio are not particularly limited. However, for example, the aspect ratio of the prior austenite grains at a depth of 1 mm from the surface may be 1.0 to 1.8. The aspect ratio of the prior austenite grains is determined as follows. First, the L cross section (cross section parallel to the rolling direction and thickness direction of the wear resistant steel plate) at a depth of 1 mm from the surface of the wear resistant steel plate is mirror-polished, and then any rolling is performed by electron beam backscatter diffraction (EBSD). One crystal orientation in a region of 1.0 mm in the direction × 0.4 mm in the depth direction is measured, and the region where the orientation difference between adjacent grains is 20 ° or more and 45 ° or less is defined as one prior austenite grain, and each The rolling direction length and plate thickness direction length of the prior austenite grains are measured, and the aspect ratio of each prior austenite grain is calculated. The arithmetic mean of the aspect ratios of all the calculated prior austenite grains is determined as the “aspect ratio of prior austenite grains”.

[Average major diameter of carbide at 1 mm depth from surface after heat treatment test at 300° C. for 10 minutes: 0.25 to 5.00 μm]
In the wear-resistant steel plate according to the embodiment of the present invention, after the heat treatment test at 300 ° C. for 10 minutes, the average value of the major axis of the carbide at a depth of 1 mm from the surface of the wear-resistant steel plate is 0.25 to 5.00 μm. . When cutting a steel plate, heat is generated during processing due to friction between the cutting tool and the steel plate, and the cut portion of the steel plate reaches a high temperature of, for example, about 300°C. In the wear-resistant steel plate according to the embodiment of the present invention, after a heat treatment test at 300 ° C. for 10 minutes simulating cutting, the long diameter of carbide precipitated from the metal structure, for example, as-quenched martensite under such a high temperature is within the range of 0.25 to 5.00 μm, the deformation resistance during cutting can be alleviated by the precipitation of carbides of appropriate size. As a result, machinability can be significantly improved. For example, the average major axis of the carbide may be 0.27 μm or more, 0.28 μm or more, or 0.30 μm or more. On the other hand, if the average value of the major axis of the carbide is too large, the hardness of the steel sheet may be excessively decreased, resulting in decreased wear resistance. Therefore, the average major axis of the carbide is 5.00 μm, and may be, for example, 4.00 μm or less, 3.00 μm or less, 2.00 μm or less, or 1.00 μm or less.

In the present invention, after a heat treatment test at 300°C for 10 minutes, the average value of the major axis of carbide at a depth of 1 mm from the surface is determined as follows. First, the L cross section (cross section parallel to the rolling direction and thickness direction of the wear resistant steel plate) at a depth of 1 mm from the surface of the wear resistant steel plate after the heat treatment test is mirror-polished, then corroded with nital, and scanned electron Observe at 10,000 times using a microscope (SEM), and randomly select 10 observation fields of 10 μm×6 μm. Next, select 10 carbides with the largest major axis (maximum line segment length across the carbide) in each observation field, and average the major axis of the carbides by averaging the major axis of a total of 100 carbides in the 10 fields of view. value is determined.

[Thickness: 6 to 150mm]
The wear-resistant steel plate according to the embodiment of the present invention is not particularly limited, but may have a plate thickness of 6 to 150 mm, for example. By setting the chemical composition and structure of the wear-resistant steel plate within the ranges described above, it is possible to achieve excellent machinability while maintaining wear resistance and low-temperature toughness even with such a thick steel plate. becomes. The plate thickness of the wear-resistant steel plate may be, for example, 8 mm or more, 10 mm or more, 15 mm or more, 20 mm or more, 25 mm or more, or 30 mm or more. Similarly, the thickness of the wear-resistant steel plate may be, for example, 120 mm or less, 100 mm or less, 90 mm or less, or 80 mm or less.

[Mechanical properties]
The wear-resistant steel plate according to the embodiment of the present invention can achieve excellent mechanical properties, for example, a Brinell hardness of 360 to 490 HB at a depth of 1 mm from the surface. The Brinell hardness is preferably 380 HB or higher, more preferably 400 HB or higher. Similarly, the Brinell hardness may be 480 HB or less, or 460 HB or less. In addition, according to the wear-resistant steel plate according to the embodiment of the present invention, excellent low-temperature toughness can be achieved, and more specifically, the Charpy impact absorption energy (vE _{- 20} ) is 27 J or more, preferably 30 J or more, more preferably 40 J or more, and most preferably 45 J or more. The upper limit of the average value of the Charpy impact absorption energy (vE ₋₂₀ ) is not particularly limited, but may be 100J, for example. Furthermore, according to the wear-resistant steel plate according to the embodiment of the present invention, excellent machinability can be achieved. Cutting speed is 200m/min, depth of cut is 1mm, tool feed is 0.1mm/rev, and dry (no cutting oil). Excellent machinability of 200m or more, preferably 220m or more, more preferably 250m or more, most preferably 280m or more can be achieved when measured. Although the upper limit of the cutting distance is not particularly limited, it may be 500 m, for example. With conventional wear-resistant steel, the cutting distance to tool failure under the same conditions is less than 200 m. It can be seen that it has machinability. All of the above Brinell hardness, Charpy impact absorption energy, and cutting distance to tool failure are measured on wear-resistant steel plates that have not been subjected to a heat treatment test at 300° C. for 10 minutes.

The Brinell hardness is measured under a load of 3000 kgf using a hard tungsten ball with a diameter of 10 mm on the surface of the wear-resistant steel plate with 1 mm removed by grinding. In addition, the average value of the Charpy impact absorption energy (vE _-20 ) is based on the JIS No. 4 2 mm V-notch test piece taken from the 1/4 position of the plate thickness in the L direction of the wear-resistant steel plate, based on the provisions of JIS Z2242: 2005. Then, using an impact blade with a radius of 2 mm, three Charpy impact absorption energies are measured at -20°C, and calculated by averaging them.

As described above, the wear-resistant steel plate according to the embodiment of the present invention exhibits excellent wear resistance, low-temperature toughness and machinability. It is particularly suitable for use in beveling and drilling with a milling tip.

[Method for producing wear-resistant steel plate]
Next, a preferred method for manufacturing a wear-resistant steel plate according to an embodiment of the present invention will be described. The following description is intended to exemplify the characteristic method for manufacturing the wear-resistant steel plate according to the embodiment of the present invention, and the wear-resistant steel plate is manufactured by the manufacturing method described below. It is not intended to be limited to

The manufacturing method of wear-resistant steel plates includes a hot rolling process and a quenching process. Each step will be described in more detail below. The steel slab to be subjected to this manufacturing method is not particularly limited as long as it has the chemical composition described above, and a steel slab manufactured under any appropriate casting conditions known to those skilled in the art can be used. can be done. For example, the billet may be an ingot-blooming slab or a continuously cast slab. From the viewpoint of production efficiency, yield and energy saving, it is preferable to use a continuously cast slab as the billet.

[Hot rolling process]
First, a cast steel slab is reheated in a hot rolling process, and then hot rolled at a rolling reduction of 50% or more, for example. The reheating temperature is preferably 1000° C. or higher from the viewpoint of reducing the load on the rolling rolls, and is preferably 1250° C. or lower from the viewpoint of suppressing coarsening of the metal structure, particularly the martensitic structure. The rolling end temperature is preferably 1000° C. or higher from the viewpoint of productivity.

[Quenching process]
After the hot rolling process, the steel sheet is once cooled to 200 ° C. or less, then reheated to a reheating temperature T ° C. that satisfies the following formulas 6 and 7, and finally an average cooling rate of 5.0 ° C./sec or more. is water-cooled to 200°C or less.
0.00<R<1.00 Formula 6
R=(T-AC)/130 Expression 7
Here, AC is a coefficient determined from the alloying elements in the steel sheet, and is obtained by Equation 8 below.
AC=937-476[C]+56[Si]-20[Mn]-16[Cu]-27[Ni]-5[Cr]+38[Mo]+125[V]+136[Ti]-19[Nb]+3315 [B] Formula 8
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Ti], [Nb] and [B] are It is the content (% by mass), and is 0 when no element is contained.

In this quenching treatment step, by reheating to a reheating temperature T ° C. where the parameter R determined by the alloying element and the reheating temperature is in the range of more than 0.00 and less than 1.00, the metal structure is improved. It is possible to suppress coarsening, that is, it is possible to reliably make the average equivalent circle diameter of grains surrounded by boundaries with an orientation difference of 15° or more at a depth of 1 mm from the steel plate surface to 25 μm or less. If the parameter R is out of the above range, the grains become coarse, precipitation of carbides during cutting is not promoted, and sufficient machinability cannot be achieved. In particular, when the parameter R is 0.00 or less, austenitization during reheating is insufficient, so that the hardness of the steel sheet is lowered and the wear resistance is also lowered.

In addition, in the quenching treatment step in the present manufacturing method, as described above, quenching (RQ) is performed after reheating, so the old steel is elongated compared to the case of direct quenching (DQ) by water cooling immediately after hot rolling. There are no or almost no austenite grains, and therefore the prior austenite grains at a depth of 1 mm from the steel plate surface are relatively spherical. Therefore, the aspect ratio of the prior austenite grains is, for example, in the range of 1.0 to 1.8. On the other hand, in the case of direct quenching, since flat prior austenite grains are formed, the aspect ratio of the prior austenite grains at a depth of 1 mm from the steel plate surface is relatively large, exceeding 2.0.

After reheating to the reheating temperature T (° C.), water cooling to 200° C. or less at an average cooling rate of 5.0° C./sec or more as described above, thereby appropriately transforming the metal structure. It becomes possible to achieve sufficient hardness in the steel plate. If the average cooling rate is less than 5.0°C/sec or if the cooling stop temperature is higher than 200°C, the transformation of the metallographic structure is insufficient, resulting in a decrease in the hardness of the steel sheet and a decrease in wear resistance. Sometimes. The measurement position of the average cooling rate is the plate thickness 1/2 position, and it can be calculated by a calculation simulation that considers the heat generated during processing from the line speed, the reduction rate, and the like.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

In the following examples, wear-resistant steel plates according to embodiments of the present invention were produced under various conditions, and the mechanical properties of the obtained wear-resistant steel plates were investigated.

[Manufacture of wear-resistant steel plate]
First, slabs having chemical compositions shown in Table 1 were cast by a continuous casting method. Next, after these slabs were hot rolled under the conditions shown in Table 2, the steel sheets were once cooled to 200° C. or less, and then quenched under the conditions shown in Table 2 to achieve a thickness of 10 to 10. A 100 mm wear-resistant steel plate was produced. Table 3 shows the results. In addition, the Vickers hardness HV ₁₀ (Hv) at a depth of 1 mm of the manufactured wear-resistant steel plate, the average circle equivalent diameter of the grains, the average value of the long diameter of the carbide after the heat treatment test at 300 ° C. for 10 minutes, and the mechanical Properties were determined by the following methods.

[Vickers hardness HV ₁₀ (Hv) at a depth of 1 mm from the surface]
The Vickers hardness HV ₁₀ (Hv) is determined by mechanically polishing the L cross section of the wear-resistant steel plate (the cross section parallel to the rolling direction and thickness direction of the steel plate), and then at a depth of 1 mm from the surface in the thickness direction. A Vickers hardness test based on JIS Z2244:2009 was performed to measure Vickers hardness (measurement load 10 kgf=98.07 N) at three points, and the values were averaged. Table 3 also shows the value of f(C)=634×√[C]+140 in relation to Equation 1 below.
HV ₁₀ ≧634×√[C]+140 Formula 1

[Average circle equivalent diameter of crystal grains at a depth of 1 mm from the surface]
The average circle-equivalent diameter of crystal grains was determined as follows. First, the L cross section (cross section parallel to the rolling direction and thickness direction of the wear resistant steel plate) at a depth of 1 mm from the surface of the wear resistant steel plate is mirror-polished, and then any rolling is performed by electron beam backscatter diffraction (EBSD). The crystal orientation of a region of 1.0 mm in the direction × 0.4 mm in the depth direction is measured at one point, and the region where the orientation difference between adjacent grains is 15° or more is defined as one crystal grain, and the grain of each crystal grain. The diameter was calculated as a circle equivalent diameter. The area average calculated based on all the calculated crystal grains is determined as the “average circle equivalent diameter of the crystal grains”, and more specifically, the area average (d) is the area occupied by each crystal grain (a _i ) and the circle-equivalent diameter (d _i ) of each crystal grain were calculated by the following formula 5.
d=Σ(a _i ×d _i )/Σa _i Expression 5

[Average major diameter of carbide at a depth of 1 mm from the surface after heat treatment test]
After a heat treatment test at 300° C. for 10 minutes, the average major axis of carbide at a depth of 1 mm from the surface was determined as follows. First, the L cross section (cross section parallel to the rolling direction and thickness direction of the wear resistant steel plate) at a depth of 1 mm from the surface of the wear resistant steel plate is mirror-polished, then corroded with nital, and scanned with a scanning electron microscope (SEM). Observation was performed at a magnification of 10,000 using the microscope, and 10 fields of observation of 10 μm×6 μm were randomly selected. Next, select 10 carbides with the largest major axis (maximum line segment length across the carbide) in each observation field, and average the major axis of the carbides by averaging the major axis of a total of 100 carbides in the 10 fields of view. determined the value.

[Mechanical properties]
The Brinell hardness was measured under a load of 3000 kgf using a hard tungsten ball with a diameter of 10 mm on the surface of the wear-resistant steel plate with 1 mm removed by grinding. The average value of Charpy impact absorption energy (vE _-20 ) is based on a JIS No. 4 2 mm V-notch test piece taken from the 1/4 position of the plate thickness in the L direction of the wear-resistant steel plate, in accordance with the provisions of JIS Z2242: 2005. , Charpy impact absorption energy at −20° C. was measured three times using an impact blade with a radius of 2 mm, and calculated by averaging them. The cutting distance to tool failure was measured using a cemented carbide tool P class (JIS B4053: 2013) chip at a cutting speed of 200 m / min, a depth of cut of 1 mm, a tool feed of 0.1 mm / rev, and a dry process (no cutting oil). It was determined by measuring the cutting distance from the surface of the wear-resistant steel plate to chipping by milling under the conditions of .

Good wear resistance and low-temperature toughness when the Brinell hardness is 360 to 490 HB, the average value of Charpy impact absorption energy (vE _-20 ) is 27 J or more, and the cutting distance to tool failure is 200 m or more. was evaluated as a wear-resistant steel sheet having improved machinability while maintaining

Referring to Tables 1 to 3, in Comparative Examples 23 to 28, the C, Mn, Cr, Mo, Ti or B content was low, so sufficient Brinell hardness was not obtained and the wear resistance decreased. Especially in Comparative Example 27, since the Ti content was low, the solute nitrogen in the steel could not be sufficiently fixed, and BN precipitated, and it is considered that the effect of improving the hardenability of B was not sufficiently obtained. In Comparative Example 29, since the N content was low, precipitation of TiN as pinning particles was insufficient, and a sufficient grain refining effect was not obtained, resulting in a decrease in low-temperature toughness.

In Comparative Examples 30, 32, 35 and 36, the low temperature toughness decreased due to the high C, Mn, Cr or Mo content. In Comparative Example 31, since the Si content was high, precipitation of carbides during cutting was insufficient, and the machinability deteriorated. In Comparative Examples 33, 34 and 37-40, the low temperature toughness decreased due to the high P, S, Ti, B, Al or N content. In Comparative Examples 41 and 42, since the reheating temperature T in the quenching treatment step was not appropriate, the value of the parameter R determined by the alloy elements and the reheating temperature deviated from the desired range. became coarse and the machinability decreased. In particular, in Comparative Example 41, the value of the parameter R was small, and thus the austenitization during reheating was considered to be insufficient. In Comparative Example 43, since the average cooling rate after the reheating temperature in the quenching treatment step was slow, the transformation of the metal structure did not proceed sufficiently, resulting in a decrease in hardness and wear resistance of the steel sheet.

In contrast, in all examples, the chemical composition of the wear-resistant steel plate, the Vickers hardness HV ₁₀ at a depth of 1 mm from the surface and the average equivalent circle diameter of the grains, and the heat treatment test at a depth of 1 mm from the surface A wear-resistant steel sheet having improved machinability while maintaining good wear resistance and low-temperature toughness was obtained by optimizing the average value of the major axis of the carbides at the slant position. Although not shown in Table 3, the aspect ratio of the prior austenite grains at a depth of 1 mm from the surface was within the range of 1.0 to 1.8 in all the examples in Table 3. Furthermore, from the results of structural analysis with an optical microscope, the metal structure at a depth of 1 mm from the steel plate surface is mainly composed of martensite in all the wear-resistant steel plates according to the examples, and more specifically, the area ratio contained more than 90% martensite.

Claims

in % by mass,
C: 0.140 to 0.250%,
Si: 0.09% or less,
Mn: 1.20-2.00%,
P: 0.0200% or less,
S: 0.0050% or less,
Cr: 0.10 to 1.00%,
Mo: 0.05-0.29%,
Ti: 0.005 to 0.030%,
B: 0.0003 to 0.0050%,
Al: 0.0030 to 0.1000%,
N: 0.0010 to 0.0080%,
O: 0.0050% or less,
Ni: 0 to 0.50%,
Nb: 0 to 0.050%,
Cu: 0-0.50%,
V: 0 to 0.050,
W: 0 to 0.50%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
REM: 0 to 0.0050%,
Balance: having a chemical composition of Fe and impurities,
The Vickers hardness HV 10 (Hv) at a depth of 1 mm from the surface satisfies the following formula 1,
The average circle equivalent diameter of crystal grains surrounded by boundaries with a misorientation of 15° or more at a depth of 1 mm from the surface is 25 μm or less,
The average value of the long axis of the carbide at a depth of 1 mm from the surface after a heat treatment test at 300 ° C. for 10 minutes. A wear-resistant steel plate in which the average value of the long diameters of 100 carbides selected from the largest ones is 0.25 to 5.00 μm.
HV 10 ≧634×√[C]+140 Formula 1
Here, [C] is the C content (% by mass) of the wear-resistant steel plate.
The chemical composition, in mass %,
Ni: 0.01 to 0.50%,
Nb: 0.001 to 0.050%,
Cu: 0.01-0.50%,
V: 0.001 to 0.050,
W: 0.01 to 0.50%,
Ca: 0.0001 to 0.0050%,
Mg: 0.0001-0.0050%, and REM: 0.0001-0.0050%
The wear-resistant steel plate according to claim 1, comprising one or more selected from the group consisting of
The wear-resistant steel plate according to claim 1 or 2, having a plate thickness of 6 to 150 mm.