0001]The present invention relates to a wear resistance excellent steel sheet (abrasion steel).
　This application is June 21, 2017, claiming priority on Japanese Patent Application No. 2017-121641 filed in Japan, the contents of which are incorporated herein.
BACKGROUND
[0002]Construction machinery, in applications such as industrial machines, even under severe wear environments, are demanded abrasion steel sheet can be used over a long period of time, from the viewpoint of the wear allowance secured by the plate thickness increases, the abrasion resistance improvement of is required. In general, in order to improve the wear resistance of the steel sheet, it is necessary to increase the hardness of the steel sheet. Especially in heavy wear steel or plate thickness is 40 mm, the hardness near the surface of the steel sheet (hereinafter sometimes referred to as "surface layer portion hardness". The surface layer portion, from the surface of the steel sheet in the thickness direction of 1 mm ~ 5 mm an area.) as well, the hardness (hereinafter in the central portion of the hardness is difficult to obtain the thickness direction may be referred to as "plate thickness center portion hardness". the central part, the plate thickness from the surface of the steel sheet in the thickness direction T 1/2 (i.e., T / 2) away (i.e., the thickness of the center) is a region from ± 5 mm (total 10mm thickness).) is an issue secure.
[0003]
　Abrasion steel is exposed to local temperature above room temperature, since the case is used in a severe environment, a small decrease in hardness even higher temperature range than room temperature (e.g. a temperature range of about 0.99 ~ 300 ° C.) (excellent hot hardness) that in some cases is required. Hardness at higher temperature range than room temperature (hereinafter, sometimes referred to as "hot hardness".) To ensure a steel sheet obtained by increasing the content of Si has been proposed (e.g., Patent Documents 1 to 3, reference ).
CITATION
Patent Document
[0004]
Patent Document 1: Japanese Patent Laid-Open 8-41535 discloses
Patent Document 2: Japanese Patent 2001-49387 JP
Patent Document 3: Japanese Patent 2002-235144 JP
Summary of the Invention
Problems that the Invention is to Solve
[0005]
　For example, Patent Document 1, the content of Si 0.40 ~ 1.50% by mass (hereinafter, "mass%" is simply referred to as "%".), Is a steel sheet containing Nb have been proposed. However, in Patent Document 1, the plate thickness of the steel sheet is not more 40mm or less, no description about the mid-thickness portion hardness, it has not been studied in terms of securing the wear allowance by thickening of the steel sheet.
[0006]
　In Patent Document 2, assuming a locally severe wear environments exposed to a temperature higher than room temperature, in order to ensure the hot hardness of the steel, and containing 0.5% ultra to 1.2% Si, steel using precipitation strengthening due to V carbide has been proposed. However, the steel tends to occur a cast halves containing a large amount and V, lowering of productivity is concerned.
[0007]
　In Patent Document 3, in order to ensure the hot hardness of the steel sheet, the steel sheet containing 1.00 to 1.50% of Si has been proposed. In Patent Document 3, it is also ensured with a thickness of the central portion hardness of the steel sheet is considered, the difference between the surface layer portion hardness and mid-thickness portion hardness (hereinafter, "difference in hardness between the surface part and the mid-thickness portion", or simply referred to as "hardness difference". not described for), not been studied in terms of securing the wear allowance by thickening of the steel sheet.
[0008]
　In view of the use environment or use form abrasion steel, even under a high temperature environment of about not without 0.99 ~ 300 ° C. room temperature only, maintenance and high hardness, sufficient in the central portion in the thickness direction (plate thickness central portion) there is a case in which the hardness is required. The increase in the content of alloy components, although the hardness of the plate thickness central portion can be easily ensured, since the weldability decreases, it is necessary to provide an upper limit of carbon equivalent. In order to ensure the hardness of the steel sheet in a high temperature environment, Si added 1.00 percent is effective. However, the present inventors have found that in steel sheet containing 1.00% of Si, that the difference between the surface layer portion hardness and mid-thickness portion hardness is remarkably large, undesirably tend to wear resistance of the steel sheet It was found that there.
[0009]
　Previously, no report on the relationship between the steel sheet and the difference in hardness containing 1.00% of Si, consideration for reducing the difference in hardness at room temperature has not been sufficiently performed. In view of such circumstances, not at room temperature only, it is possible to maintain high hardness under a high temperature environment, especially the thickness is in the above steel sheet 40 mm, the carbon equivalent is less than 0.800% , the difference between the surface layer portion hardness and mid-thickness portion hardness is equal to or less than 15.0% of the surface layer portion hardness at room temperature, and to provide a steel sheet excellent in wear resistance.
Means for Solving the Problems
[0010]
　Steel containing 1.00% super to 2.00% Si is in that it can secure the hardness at room temperature and at elevated temperatures, which is advantageous in the wear resistance. On the other hand, the studies by the present inventors, containing 1.00% of Si, the steel plate thickness of over 40 mm, at room temperature, is the difference between the surface layer portion hardness and mid-thickness portion hardness is likely to occur all right. This is because the central portion of the thickness direction of the steel sheet, the surface and the cooling rate is lowered in comparison with the surface layer portion, but the formation of martensitic structure is caused to become insufficient, the increase in the content of Si impact is not always clear.
[0011]
　The present inventors have further result of extensive investigations, a plate thickness of 40mm or more, the steel sheet containing 1.00% of Si, reduce the difference between the surface layer portion hardness and mid-thickness portion hardness at room temperature the index Q for derived. Index Q is calculated by the following equation considering the hardenability of the alloy elements, and a plate thickness (1). However, the following equation (1), is required to reduce the difference between the surface layer portion hardness and mid-thickness portion hardness of the steel sheet containing 1.00% of Si, other than Si alloying elements (C , Mn, Ni, Cr, since attention is paid to Mo), does not consider the amount of Si. In the following, sometimes referred to as "room temperature hardness" the hardness at room temperature. In the following, simply referred to as "hardness" indicates the hardness at room temperature, it shows a 22 ± 5 ℃ (17 ~ 27 ℃) Room temperature.
[0012]
　Steel sheet according to the present invention is the plate thickness of 40mm or more, under the influence of such residual stress due to welding because the delayed crack is concerned with hydrogen, carbon equivalent is determined by the following formula (2) Ceq (%) a is less than 0.800%. The index Q obtained by the following formula (1) by 0.00 or more, the hardness difference between the surface portion and the plate thickness center in the room temperature, becomes less 15.0% of the surface layer portion hardness, the hardness difference is less and carbon equivalent low, then the thickness is 40mm or more, it is possible and obtain a steel sheet excellent in wear resistance. Incidentally, by substituting the thickness T, the content of [X] of each element X in the following equation (1) as a number of dimensionless units of indicators determined Q is dimensionless. The unit of the carbon equivalent Ceq obtained by the following equation (2) is "%".
[0013]
　Q = 0.18-1.3 (logT) +0.75 ( 2.7 × [C] + [Mn] + 0.45 × [Ni] + 0.8 × [Cr] + 2 × [Mo]) ··· (1)
　Ceq (%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 4 · · · (2)
　where , index Q in the formula (1) is calculated by substituting the numerical values and the content by mass percent of the element X thickness T (mm) [X], if not containing the element X 0 substituting. Carbon equivalent Ceq of the formula (2) (%) is calculated by substituting the numerical value of the content by mass percent of the element X [X], the case of not containing the element X 0 is substituted.
[0014]
　The present invention has been made based on such knowledge, its gist the following.
[0015]
[1] steel sheet according to one embodiment of the present invention, in
mass%, C: 0.20
~ 0.35%, Si: 1.00% Super
2.00% ~, Mn: 0.60 ~ 2.00
%,
Cr:
0.10 ~ 2.00%, Mo: 0.05 ~
1.00%, Al: 0.010 ~ 0.100%, N: 0.0020 ~
0.0100%, B: 0.
~ 0.0020% 0003, P: 0.0200% or less,
S: less than%
0.0100,
Cu: 0 ~ 0.500%,
Ni: 0 ~ 1.00%, Nb: 0 ~ 0.050%, V:
0 ~ 0.120%, Ti: 0 ~ 0.025%, Ca: 0 ~ 0.050%, Mg: 0 ~ 0.050%, REM: 0 ~ 0.100%, and balance: Fe and an impurity, 　index Q obtained by the following formula (1) is not less 0.00 or more, 　the carbon equivalent Ceq (%) obtained by the following formula (2) It has the chemical composition is less than 0.800%,

　Is 400 or more in Vickers hardness surface layer portion hardness at room temperature with the ratio of the difference between the surface layer portion hardness and mid-thickness portion hardness of 15.0% of the surface layer portion hardness at room temperature,
　the plate thickness T is more than 40mm it is.
　Q = 0.18-1.3 (logT) +0.75 ( 2.7 × [C] + [Mn] + 0.45 × [Ni] + 0.8 × [Cr] + 2 × [Mo]) ··· (1)
　Ceq (%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 4 · · · (2)
　the formula index Q of (1) is calculated by substituting the numerical values and the content by mass percent of the element X thickness T (mm) [X], the case of not containing the element X 0 is substituted for . Carbon equivalent Ceq (%) of the formula (2) is calculated by substituting the numerical value of the content by mass percent of the element X [X], the case of not containing the element X 0 is substituted.
[2] The steel sheet according to [1], the index Q is 0.04 or more,
　the ratio may be less 13.0%.
[3] The steel sheet according to [1] or [2], in
mass%, Ni: may have a chemical composition which is 0.05 to 1.00%.
[4] The steel sheet according to any one aspect of [1] to [3], in mass%,
Mn: may have a chemical composition which is 0.63 to 2.00%.
Effect of the invention
[0016]
　According to the above aspect of the invention, not at room temperature only, it is possible to maintain high hardness under a high temperature environment, especially the thickness is in the above steel sheet 40 mm, the carbon equivalent Ceq (%) is 0.800% less than, a difference between the surface layer portion hardness and mid-thickness portion hardness at room temperature is equal to or less than 15.0% of the surface layer portion hardness, provides a steel sheet excellent in wear resistance. Steel sheet according to the present invention, even under severe environments where the temperature is about 0.99 ~ 300 ° C., such as may be used for a long period of time, the contribution of the industry is extremely remarkable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017]
It is a diagram illustrating a temperature change in the difference between [1] the surface hardness and the reference hardness of the steel sheet.
Is a diagram illustrating a FIG. 2 the thickness direction of the hardness distribution of the steel plate.
3 is a diagram illustrating the relationship between the hardness difference ratio Delta] HV / Hvs and index Q of the steel sheet.
DESCRIPTION OF THE INVENTION
[0018]
　The relationship between the temperature change of the Si content and hardness of the steel sheet will be described with reference to FIG. Figure 1 is a diagram illustrating a temperature change in the difference between the surface hardness and the reference hardness of the steel sheet. The C content is constant, the quenching treatment applied to steel sheet having a thickness of 40mm with varying Si contents, FIG results of measuring up to 400 ° C. from room temperature, the Vickers hardness at the surface of the steel sheet (surface hardness) HV5 It is shown in 1. The vertical axis of FIG. 1, the difference between the Vickers hardness (surface hardness) HV5 at each temperature for each steel, the Vickers hardness at room temperature of the steel sheet Si content of 0.25% (reference hardness) HV5 is there. Note that Vickers hardness HV5 is cut out a sample from a position of a depth of 5mm from the surface of the steel sheet conforms to JIS Z 2252-1991, the test force and: 49.03 N (5 kgf), was measured by a high temperature Vickers hardness test . Measurement of the reference hardness, the conditions other than the control of the temperature was carried out in the same high temperature Vickers hardness test described above.
[0019]
　From Figure 1, an increase in room temperature hardness and high-temperature hardness by increasing the Si content, and (the difference between the surface hardness and the reference hardness) Hardness decrease in a high temperature environment is also can be seen small. Thus, the steel sheet containing 1.00% super to 2.00% Si is in that it can secure the hardness at room temperature and at elevated temperatures, are excellent in wear resistance.
[0020]
　Next, the hardness distribution in the thickness direction after quenching the steel plate (thickness 40 mm) containing 1.00% of Si a (Vickers hardness) in FIG. Vickers hardness HV5 the JIS Z 2244: conforms to 2009, measured at room temperature the test force as: 49.03 N (5 kgf). As shown in FIG. 2, the plate thickness central portion hardness is lower than the surface layer portion hardness. Furthermore, from the results of Vickers hardness test, (the average value of Vickers hardness was measured in the range of 1 mm ~ 5 mm from the surface of the steel sheet in the thickness direction) surface layer portion hardness Hvs and at mid-thickness portion hardness HVC (thickness direction seeking a central portion from ± 5 mm of the steel sheet average value of Vickers hardness was measured in the range of (total 10mm thickness)) was calculated difference (hardness difference) Delta] HV of plate thickness central portion hardness and the surface layer portion hardness at room temperature. That, Delta] HV is represented by the following formula (a).
[0021]
　ΔＨｖ＝Ｈｖｓ－Ｈｖｃ　・・・　（ａ）
[0022]
　Table 1 shows the results of the Vickers hardness test. Table 1, it can be seen that ΔHv increases with increasing Si content. Thus, the present inventors found that a thick steel plate Si content is large to obtain a knowledge that tends to occur a difference between the surface layer portion hardness and mid-thickness portion hardness at room temperature.
[0023]
[Table 1]

[0024]
　Accordingly, the present inventors, contains 1.00% of Si, the thickness is more than the steel plate 40 mm, it was investigated how to reduce the hardness difference between the surface portion and the plate thickness central portion at room temperature . The present inventors, in consideration of the hardenability and the plate thickness of the alloying elements, extensive investigations in order to reduce the difference in hardness of the steel sheet.
[0025]
　In order to ensure the hardness of the steel sheet, the hot rolling, the transformation to austenite at Atsushi Nobori Ac ending 3 after reheating the steel sheet to a temperature above points, performs like water cooling (quenching) is normally line are we. At this time, the surface layer portion of the steel plate is fast cooling rate, sufficient hardness can be secured. On the other hand, in the mid-thickness portion of the steel sheet, cooling rate to lower than the surface portion, the generation of martensite is insufficient, the hardness decreases.
[0026]
　The mid-thickness portion of the steel sheet as described above, the cooling rate is lowered. Therefore, in order to ensure a sufficient hardness in the sheet thickness center part of the steel sheet, it increased amounts of alloy elements, it is necessary to increase the hardenability. However, when a certain amount of content of alloying elements, or insufficient hardenability by thickness, costs or increased by the inclusion of unnecessary amounts of alloying elements, also including impairs weldability It occurs problems. Therefore, in order to control the appropriate range and the content of alloying elements, the cooling rate of the sheet thickness central part it is necessary to consider that the influence of thickness.
[0027]
　The present inventors, contains 1.00% of Si, on the hardness difference ratio Delta] HV / Hvs of various steel materials described above thickness 40 mm, the relationship between the content and the thickness of the alloying elements having a hardenability to organize, to derive the index Q represented by the following formula (1). Here, the hardness difference ratio ΔHv / Hvs (%), which represents the percentage obtained by dividing the difference between the surface layer portion hardness and mid-thickness portion hardness at room temperature at the surface portion hardness in percentage. Incidentally, the hardness difference ratio ΔHv / Hvs (%) is represented by the following formula (b). In Formula (b), Hvs is surface portion hardness (average value of Vickers hardness was measured in a range from the surface of the steel sheet in the thickness direction of 1mm ~ 5mm), Hvc the plate of the plate thickness central portion hardness (steel plate it is the thickness direction of the central portion from ± 5 mm average value of Vickers hardness was measured in the range of (total 10mm thickness)).
[0028]
　ΔＨｖ／Ｈｖｓ（％）＝１００×（Ｈｖｓ－Ｈｖｃ）／Ｈｖｓ　・・・　（ｂ）
[0029]
　Conventionally, in a steel containing 1.00% of Si, hardenability when the cooling rate is slow it has been considered to decrease. However, the present inventors have found that steel containing 1.00% of Si, an alloy element other than Si (C, Mn, Ni, Cr, Mo) when securing the hardenability contain a cooling rate There was found that Si be reduced to contribute to improvement in hardenability. Formula (1), in order to increase the plate thickness central portion hardness, alloy elements other than Si (C, Mn, Ni, Cr, Mo) present inventors that contain a need to ensure the hardenability is based on al findings, the index Q does not contain the term of the Si content.
[0030]
　Q = 0.18-1.3 (logT) +0.75 ( 2.7 × [C] + [Mn] + 0.45 × [Ni] + 0.8 × [Cr] + 2 × [Mo]) ··· (1)
　here, the index Q of the above formula (1) is calculated by substituting the numerical values and the content by mass percent of the element X thickness T (mm) [X], the element X If you do not want to contain substitutes 0. That is, in the above formula (1), the plate thickness T, as a dimensionless numerical content [X] of each element, to calculate the index Q. Incidentally, log of the above formula (1), the bottom is logarithmic, i.e. common logarithm is 10.
[0031]
　3, the hardness difference ratio ΔHv / Hvs (%) and shows the relationship between the index Q. From Figure 3, as a reference which can be life of a thick steel plate, the hardness difference ratio ΔHv / Hvs (%), when setting below 15.0% of the surface layer portion hardness Hvs, it needs to be Q ≧ 0.00 it was found that there is. Also, when setting the hardness difference ratio Delta] HV / Hvs (percent) below 13.0% of the surface layer portion hardness Hvs, it was found that it is necessary to make the Q ≧ 0.04.
[0032]
　Further, the steel sheet according to the present embodiment, since the plate thickness because it is 40mm or more, hydrogen embrittlement cracks under residual stress effects due to welding are concerned, the carbon equivalent is represented by the following formula (2) Ceq (percent) is less than 0.800%. Incidentally, the following formula (2), it is necessary to consider weldability of the steel sheet, which includes a term of the Si content.
[0033]
　Ceq (%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 4 ··· (2)
　the formula (2) the carbon equivalent Ceq (%) is calculated by substituting the numerical value of the content by mass percent of the element X [X], the case of not containing the element X 0 is substituted. Units of the carbon equivalent Ceq obtained by the equation (2) is "%".
[0034]
　The indicator Q of the above formula (1) by 0.00 or more, and 15.0% of the hardness difference ΔHv the surface layer portion hardness Hvs between the surface layer portion and the plate thickness center part of the steel sheet at room temperature, the hardness difference is small, carbon is equivalent is less than 0.800%, it is possible to plate thickness is at 40mm or more, and obtain a steel sheet excellent in wear resistance.
[0035]
　It will now be described in detail the steel sheet according to the present embodiment. First described the chemical composition of the steel sheet according to the present embodiment. Unless otherwise specified,% on chemical composition means mass%.
[0036]
　
　C is an element effective in improving the hardness, the C content in order to ensure the hardness of the steel sheet to 0.20% or more. Preferably 0.22% or more C content, more preferably 0.24% or more. On the other hand, when the C content exceeds 0.35%, increased resistance to hydrogen embrittlement susceptibility by increasing the hardness, since the occurrence of cracks is concerned by hydrogen embrittlement, the C content to 0.35% or less . Preferably less 0.32% of C content, and more preferably 0.30% or less.
[0037]
　 Si is a deoxidizer, and is an effective element in improving the hardness of the steel sheet. In the present embodiment, Si is a very important element for maintaining the hardness of the steel sheet in a high temperature environment. In order to obtain the effect of Si containing, the Si content is 1.00 percent. Preferably the Si content 1.10% or more, more preferably 1.20% or more, or 1.30% or more. On the other hand, when the Si content exceeds 2.00%, since there may inhibit the toughness of the steel sheet, the Si content is set to 2.00% or less. Preferably less 1.90% of Si content, more preferably less 1.80%.
[0038]
　 Mn enhances the hardenability, is an element for improving the hardness, in order to ensure the hardness of the steel sheet, it is necessary to contain more than 0.60% . Preferably the Mn content 0.70% or more, more preferably 0.80% or more. On the other hand, if excessively contained the Mn, toughness is lowered and also promote the formation of cementite, resulting in sometimes results in a decrease in hot hardness of the steel sheet. Therefore, the Mn content is set to 2.00% or less. Preferably the Mn content 1.50% or less or 1.35% or less, more preferably 1.20% or less or 1.00% or less.
[0039]
　 Cr increases the hardenability, is an element for improving the toughness and hardness of the steel sheet. To ensure the toughness and hardness of the steel sheet, the Cr content is set to 0.10% or more. Preferably the Cr content of 0.50% or more, more preferably 0.80% or more. Meanwhile, since the toughness of the steel sheet when the Cr content exceeds 2.00% decreases, the Cr content is set to 2.00% or less. Preferably less 1.70% of Cr content, more preferably less 1.50%.
[0040]
　 Mo also enhances hardenability, is an element for improving the hardness of the steel sheet. Further, Mo is an element effective to also maintain the hardness of the steel sheet in a high temperature environment. Therefore, the Mo content is 0.05% or more. Preferably the Mo content of 0.10% or more, more preferably 0.20% or more. Meanwhile, since the Mo content is reduced toughness of the steel sheet exceeds 1.00%, a Mo content to 1.00% or less. Preferably 0.60 percent Mo content, more preferably 0.40% or less.
[0041]
　 Al is an element effective as a deoxidizer. Further, Al forms a N and AlN, the crystal grains by fine, improve the toughness of the steel sheet. Therefore, the Al content is 0.010% or more. Preferably the Al content 0.020% or more, more preferably 0.030% or more. On the other hand, if excessively contained the Al, to produce a decrease in the toughness of the steel sheet, the Al content is 0.100% or less. Preferably less 0.080% of Al content, more preferably less 0.070%.
[0042]
　
　N forms Al or Ti and a nitride, a crystal grain by fine, is an element for improving the toughness of the steel sheet. Therefore, the N content is 0.0020% or more. Preferably the N content of 0.0030% or more, more preferably 0.0040% or more. On the other hand, if excessively containing N, the coarse nitrides are produced, to reduce the toughness of the steel sheet, the N content to 0.0100% or less. Preferably less 0.0080% of N content, more preferably 0.0060% or less.
[0043]
　
　B is significantly enhanced hardenability of steel, is an element effective to especially improve the hardness of the plate thickness center part of the steel sheet. Therefore, B content of 0.0003% or more. Preferably the B content of 0.0005% or more, more preferably 0.0007% or more, even more preferably 0.0010% or more. On the other hand, if it contains B in excess forms a boride, it reduces the hardenability, it becomes impossible to secure the hardness of the steel sheet, B content and to 0.0020%. Preferably less 0.0018% of B content, more preferably less 0.0016%.
[0044]
　
　P is an impurity, to reduce the toughness and workability of the steel sheet, to limit the P content below 0.0200%. Preferably less 0.0150% of P content, more preferably 0.0100% or less. The lower limit of the P content is preferably 0%, from the viewpoint of production cost, the P content may be 0.0001% or more.
[0045]
　:
　As with S also P, it is an impurity, because of lowering the toughness of the steel sheet, to limit the S content to less than 0.0100%. Preferably less 0.0070% of S content, more preferably 0.0050% or less, even more preferably to 0.0030% or less. The lower limit of the S content is preferably 0%, from the viewpoint of production cost, the S content may be 0.0001% or more.
[0046]
　In the steel sheet according to the present embodiment can be for the purpose of improving the mechanical properties such as hardness and toughness of the steel sheet, Cu, Ni, Nb, selectively containing V, and Ti with one or more . The lower limit of the content of these components is 0%.
[0047]
　 Cu forms fine precipitates is an element contributing to the improvement of the strength of the steel sheet may contain more than 0.001%. More preferably the Cu content 0.050% or more, even more preferably 0.100% or more. On the other hand, if excessively contained a Cu, in order to degrade the wear resistance of the steel sheet, the upper limit of the Cu content is at most 0.500%. More preferably less 0.450% of Cu content, and even more preferably at most 0.400%.
[0048]
　 Ni has increases the hardenability of steel, is an element contributing to the improvement of hardness of the steel sheet may contain more than 0.05%. More preferably the Ni content of 0.10% or more, even more preferably 0.20% or more. On the other hand, since Ni is an expensive alloying element, from the viewpoint of cost, Ni content is at most 1.00%. More preferably 0.70 percent Ni content, and even more preferably 0.50% or less.
[0049]
　 Nb is by inhibition of the formation and recrystallization of a nitride, an element contributing to grain refinement of the crystal grains, in order to improve the toughness of the steel, 0.005% it may contain more. More preferably the Nb content of 0.007% or more, even more preferably 0.010% or more. On the other hand, if excessively contained the Nb, because it can reduce the toughness of the steel sheet, Nb content is 0.050% or less. More preferably 0.030% or less of Nb content, and even more preferably to 0.020% or less.
[0050]
　
　V is an element contributing to the improvement of hardness of the steel sheet may contain more than 0.010%. More preferably V content of 0.020% or more, even more preferably 0.040% or more. On the other hand, if excessively contained the V, because they may impair the manufacturability and cracked of the slab, V content is at most 0.120%. More preferably 0.100% or less of V content, and even more preferably at most 0.070%.
[0051]
　 Ti has, TiN is formed and the crystal grains by fine, an element improving the toughness of the steel sheet may contain more than 0.005%. More preferably the Ti content of 0.007% or more, even more preferably 0.010% or more. On the other hand, if excessively contained a Ti, because it can reduce the toughness of the steel sheet, Ti content is at most 0.025%. More preferably 0.020% or less of Ti content, and even more preferably 0.015% or less.
[0052]
　To control the morphology of inclusions in the steel or the like, it is possible to Ca, Mg, selectively contain one or more of REM. The lower limit of the content of these components is 0%.
[0053]
　 Ca, Mg, REM are all combined with S to form sulfides, an element which forms inclusions hardly stretched by hot rolling, which contributes mainly to the improvement of the toughness of the steel sheet. On the other hand, Ca, Mg, when the excessively contained REM, there is a case where these elements together to form a coarse oxide O, toughness of the steel sheet is lowered. Therefore, Ca content, respectively Mg content 0.050% or less, REM content is 0.100% or less. More preferably Ca content, Mg content, respectively REM content 0.020% or less, even more preferably 0.010% or less, or 0.005% or less. Meanwhile, in order to obtain the toughness improving effect of the steel sheet, Ca content, respectively Mg content 0.0005% or more, REM content is preferably 0.001% or more. More preferably the Ca content, respectively Mg content 0.0007% or more, the REM content is 0.002% or more.
　Incidentally, REM (rare earth metal element) is, Sc, means a total of 17 elements consisting of Y and lanthanoid. The content of REM, means the total content of these 17 elements.
[0054]
　The remainder of the chemical composition of the steel sheet according to the present embodiment is Fe and impurities. Here, the impurities, in producing the steel sheet industrially, starting raw materials such as ores and scrap, a component mixed by various factors of the manufacturing process, the steel sheet according to the embodiment It means what is allowed in a range that does not adversely affect the properties. However, in the steel sheet according to the present embodiment, among the impurities, for P and S, as described above, it is necessary to define the upper limit.
[0055]
　Further, as an impurity in the steel, there is a case O, Sb, Sn, and the As are more than one or two or mixed. Even if these impurities are mixed, if the normal contamination levels of abrasion steel (range of the content), there is no particular problem. Therefore, the normal contamination levels of wear steel below, to limit the content thereof. The lower limit of the content of these impurities is 0%.
[0056]

　O is sometimes mixed as impurities in the steel, because it is an element to form a coarse oxide, O content is preferably small. In particular, when the O content exceeds 0.006% forms coarse oxides in the steel, because the deterioration of the wear resistance of the steel sheet, the O content is set to 0.006% or less. Preferably the O content 0.005% or less, even more preferably to 0.004% or less.
[0057]
　 Sb is an element which is mixed from scrap as a steel material. In particular, the excessive content of Sb, since the wear resistance of the steel sheet deteriorates, the Sb content is 0.01% or less. Preferably 0.007% or less of Sb content, 0.005% or less.
[0058]
　 Sn, like Sb, is an element which is mixed from scrap as a steel material. In particular, the excessive content of Sn, since the wear resistance of the steel sheet deteriorates, the Sn content is 0.01% or less. Preferably 0.007% or less of Sn content, 0.005% or less.
[0059]
　 As is, Sb, similarly to Sn, is an element which is mixed from scrap as a steel material. In particular, the excessive content of As, since wear resistance of the steel sheet deteriorates, the As content is 0.01% or less. Preferably 0.007% or less of As content, 0.005% or less.
[0060]
　Steel sheet according to the present embodiment has a small hardness difference between the surface portion and the plate thickness center part of the steel sheet at room temperature, as the percentage difference in hardness relative to the surface layer portion hardness becomes 15.0% or less, the following equation (1) the index Q sought to 0.00 or more. Indicators Q is numerically the thickness T (mm), was calculated by substituting the numerical value of the content by mass percent of the respective elements X [X] as a number of dimensionless, if not containing the element X, [X] and 0. In order to reduce the hardness difference between the surface portion and the plate thickness center part of the steel sheet, the index Q is preferably 0.01 or more, more preferably 0.04 or more, more preferably 0.05 or more, even more preferably and 0.10 or more. While index upper limit of Q is not particularly defined, increasing the index Q, the carbon equivalent Ceq (%) also becomes large, it is naturally limited. To ensure weldability and carbon equivalent Ceq (%) of less than 0.800%, index Q is preferably 1.10 or less. More preferably the index Q 0.80 or less or 0.50 or less, even more preferably 0.30 or less or 0.20 or less.
[0061]
　Q = 0.18 to 1.3 (logT) +0.75 (2.7 × [C] + [Mn] + 0.45 × [Ni] + 0.8 × [Cr] + 2 × [Mo]) · (1)
[0062]
　Steel sheet according to the present embodiment can suppress the weld cracking, in order to ensure the weldability of the steel sheet, the carbon equivalent Ceq (%) of the less than 0.800%. Carbon equivalent Ceq (%) is also calculated by substituting the numerical value of the content by mass percent of the respective elements [X], if not containing the element X, [X] is set to 0. The lower limit of carbon equivalent Ceq (%) is not specified in particular, since the index Q becomes smaller as decreasing the carbon equivalent Ceq (%), it is naturally limited. In order to reduce the hardness difference by the index Q to 0.00 or more, the carbon equivalent Ceq (%) is preferably at least 0.507%. To increase the abrasion resistance of the steel sheet, more preferably carbon equivalent Ceq (%) of 0.600% or more, even more preferably at 0.650% or more. Even more more preferably at least 0.700% carbon equivalent Ceq (%). Order to improve the weldability of the steel sheet, the following 0.785% carbon equivalent Ceq (%), may be less or 0.760% or less 0.770%.
[0063]
　Ceq (%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 4 · · · (2)
[0064]
　Steel sheet according to the present embodiment, the ratio of the difference between the difference (difference in hardness) is small, the surface layer portion hardness and the plate thickness central portion hardness for the surface layer portion hardness of the surface layer portion hardness and mid-thickness portion hardness at room temperature 15. it is 0% or less, it is possible to exhibit excellent wear resistance for a long period of time. More preferable hardness difference ratio ΔHv / Hvs (%) is small, it is difficult to below 0% or less than 1.0%. Therefore, it may be the lower limit 0% or 1.0%. Considering the increase in cost due to increase in the content of alloying elements, the hardness difference ratio ΔHv / Hvs (%) may be 3.0% or more. Surface layer portion hardness and mid-thickness portion hardness is a Vickers hardness HV5 at room temperature, JIS Z 2244: measured according to the 2009. Surface portion hardness, the cross section parallel to the rolling direction and the thickness direction of the steel sheet and the measurement surface, an average value of Vickers hardness HV5 measured in a range from the surface in the thickness direction of the steel sheet of 1 mm ~ 5 mm. In the measurement of the surface layer portion hardness of the steel sheet, in the range of 5 to at least 1mm per measures the Vickers hardness at a total of 25 points. Mid-thickness portion hardness in the measuring surface, the average value of Vickers hardness HV5 measured in a range of ± 5 mm from the center of the plate thickness direction of the steel sheet (total 10mm thickness). In the measurement of the central portion hardness of the steel sheet in the range of at least 1mm every 5 points, measuring the Vickers hardness at a total 55 points.
[0065]
　Steel sheet according to the present embodiment is that the surface layer portion hardness Hvs at room temperature Vickers hardness (HV5) at 400 or more. When the surface layer portion hardness Hvs is less than 400 in Vickers hardness (HV5), the strength of the surface layer portion of the steel sheet is insufficient, it can not be used for construction machines, applications such as industrial machinery. For improving wear resistance of Vickers the surface layer portion hardness Hvs at room temperature (HV5), 440 or more, 460 or more, may be 480 or more or 500 or more.
　Incidentally, the steel sheet according to the present embodiment, the surface portion has a very high hardness to a thickness of the central portion, a very high tensile strength. If necessary, the tensile strength at room temperature (TS) of 1000MPa or more, or 1200 MPa, may be more than or 1500 MPa 1350 MPa. It is not particularly necessary to define the upper limit of the tensile strength may be below 2300 MPa. The tensile strength, total thickness specimens (i.e., plate-shaped test piece) or a round bar specimens taken from 1/4 away the thickness T (T / 4) from the surface of the steel sheet, JIS Z 2241 : it is measured in accordance with 2011.
[0066]
　Steel sheet according to the present embodiment is a steel plate produced by hot rolling, the thickness is 40mm or more, preferably 42mm or more or at least 50mm, more preferably 60mm or more or 80mm more steel. The thickness of the upper limit is not particularly defined, it may be a 150mm depending on the application. Taking into account the homogenization of the thickness direction of the characteristics of the steel sheet may be a sheet thickness as less than 100mm.
[0067]
　A method for manufacturing a steel sheet according to the present embodiment. In the present embodiment, the steel strip having the above chemical composition, after melting in a conventional refining process, such as the converter and electric furnace, a continuous casting method or an ingot-making - produced by a known method such as blooming method it can be, not particularly limited.
[0068]
　In the present embodiment, a steel slab obtained by casting hot rolling, it is either cooled or after cooling, and quenching and re-heating, to produce a steel sheet. However, steel sheet and as quenched, and shall not subjected to heat treatment such as tempering.
[0069]
　After smelted steel was cast, may be performed as it is hot rolled, a steel strip, once cooled to room temperature, Ac 3 and then re-heated above the point of temperature, performing hot rolling it may be. Ac 3 points, the structure of the steel becomes an austenite by heating (austenite transformation is completed) is temperature. The heating temperature of hot rolling, to reduce the deformation resistance, preferably 900 ° C. or higher, more preferably 1000 ° C. or higher. On the other hand, if the heating temperature of hot rolling is too high, the tissue becomes coarse, for low temperature toughness of the steel sheet may be deteriorated, preferably 1250 ° C. or less. More preferably 1200 ° C. The heating temperature or less, even more preferably to 1150 ° C. or less.
[0070]
　Hot rolling, Ar ferrite transformation is a temperature for starting the cooling 3 is preferably terminate at points or more. Ac 3 point and Ar 3 points, a test specimen was sampled from the steel strip, it can be obtained from the thermal expansion behavior during heating and during cooling. Or Ru quenching immediately to 250 ° C. below the temperature after hot rolling, or hot rolling after cooling steel sheet Ac 3 were reheated to a temperature above points, Yakiireru to a temperature of 250 ° C. or less.
Example
[0071]
　Hereinafter, examples of the steel sheet according to the present invention, illustrating the present invention more specifically. However, the invention is not intended to be well limited to the following examples, it is also possible to practice after appropriate modifications or variations within the scope adaptable to the gist of the present invention, the technical of both their invention it is intended to be included within the scope.
[0072]
　Steels having the chemical compositions shown in Table 2 were melted, the steel plate thickness shown in Table 3 by performing the hot rolling after casting were air-cooled to room temperature. Then, after heated to the quenching temperature shown in Table 3, by performing quenching, plate thickness to produce a more steel 40 mm. The obtained test pieces from the steel plate was taken as a test plane section parallel to the rolling direction and the thickness direction of the steel sheet, the Vickers hardness of the surface layer portion and the mid-thickness portion JIS Z 2244: conforms to 2009, room temperature in, was measured test force as: 49.03 N (5 kgf). Surface portion of the Vickers hardness (surface layer portion hardness) Hvs is in the range of 1 mm ~ 5 mm from the surface in the thickness direction of the steel sheet (surface layer portion), 5 points per 1 mm, and measuring the Vickers hardness at a total of 25 points, from these the mean value (arithmetic mean). Vickers hardness (mid-thickness portion hardness) HVC of mid-thickness portion is in the range of steel sheet thickness direction of the central portion from ± 5 mm (total 10mm thickness), 5 points per 1 mm, the Vickers hardness in the total of 55 points of it was measured and from these the mean value (arithmetic mean). By using the thus obtained value of the surface layer portion hardness Hvs and mid-thickness portion hardness Hvc to give a hardness difference ratio shows the difference in hardness between the surface layer portion and the central portion of the steel plate △ Hv / Hvs (%) at room temperature It was. Incidentally, the hardness difference ratio ΔHv / Hvs (%) is represented by the following formula (b).
[0073]
　△Ｈｖ／Ｈｖｓ（％）＝１００×（Ｈｖｓ－Ｈｖｃ）／Ｈｖｓ　・・・　（ｂ）
[0074]
　Also, cut out a sample from the steel plate, conforming to JIS Z 2252-1991, at 400 ° C., was subjected to high temperature Vickers hardness test the test force as 9.807N (1kgf). This gave the hot hardness of the surface layer portion of the steel plate (HV1). The measurement of the high temperature hardness of the surface portion, the conditions other than the control and the test force temperature was performed in the same as the surface layer portion Vickers hardness test of the (room temperature). Furthermore, 1/4 away the thickness T from the surface of the steel sheet (T / 4), cut out V-notch Charpy test piece full size in a direction parallel to the rolling direction, JIS Z 2242: conforms to 2005 , 0 ℃ of Charpy absorbed energy (VE 0 was measured).
[0075]
　Criteria of each evaluation items are as follows. The surface layer portion hardness Hvs (HV5), both mid-thickness portion hardness HVC (HV5) is wear-resistant from the viewpoint of more than 400 were judged to be good 600 or less from the viewpoint of cutting processability. The surface portion of the hot hardness (HV5) was determined to be good over 300 in terms of the wear resistance at high temperatures. 0 Charpy absorbed energy of ℃ was judged to be good for more than 15J.
[0076]
　The results are shown in Table 3. No. 1-18, the chemical composition comprising an indicator Q and the carbon equivalent Ceq (%), the parameters of the thickness T is in the range of the present invention, the hardness difference ratio of the surface layer portion and the central portion △ Hv / Hvs also 15. is less than or equal to 0%. These steels are all, the surface layer portion hardness Hvs, sheet thickness central hardness HVC, hot hardness of the surface layer portion, and a steel sheet excellent in Charpy absorbed energy of 0 ° C..
[0077]
[Table 2]

[0078]
[table 3]

[0079]
　On the other hand, No. of Table 3 101-115 are comparative examples, the chemical composition comprising a Q value is outside the range of the present invention. No. 101-103, Q value is lowered in relation to the plate thickness, the hardness difference ratio △ Hv / Hvs (%) is an example in excess of 15.0%.
　No. 106 insufficient Si content, an example of reduced high-temperature hardness of the surface layer portion. On the other hand, No. 107 is often Si content, an example in which toughness is lowered.
[0080]
　No. 104,108 and 114, is, C content, respectively, insufficient Mn content and B content, the surface layer portion hardness Hvs, sheet thickness central hardness HVC, and high-temperature hardness of the surface layer portion is an example of reduction.
　No. of Cr content is insufficient 110, the surface layer portion hardness Hvs, sheet thickness central hardness HVC, and is an example of drops toughness in addition to high-temperature hardness of the surface layer portion.
　No. of Mo content is insufficient 112 is an example in which the plate thickness central portion hardness HVC, hot hardness of the surface layer portion, and the toughness is lowered.
[0081]
　No. 105 is often C content, an example in which the surface layer portion hardness Hvs becomes excessively high.
　Mn content is more No. 109, Cr content is more No. 111, Mo content is large No. 113 is an example in which toughness is lowered.
　B content is excessive No. 115 is an example of the surface layer portion hardness Hvs, sheet thickness central hardness HVC, and high-temperature hardness of the surface layer portion was reduced.
　In all embodiments, O content is 0.006% or less, and, Sb content, Sn content, and As content was in all 0.01% or less.
[0082]
　Thus, Comparative Example any one or more is out of the scope of the present invention the chemical composition and the Q value No. 101-115, the hardness difference ratio △ Hv / Hvs, the surface layer portion hardness Hvs, sheet thickness central hardness HVC, hot hardness of the surface layer portion, at least one of toughness, did not reach the criteria which are determined to be good.

claims

[Requested item 1]
　By
mass%, C: 0.20
~ 0.35%, Si: 1.00% Super
2.00% ~,
Mn: 0.60 ~ 2.00%, Cr: 0.10 ~ 2.00%, Mo:
0.05 ~ 1.00%, Al: 0.010 ~ 0.100%, N: 0.0020 ~ 0.0100%, B: 0.0003 ~ 0.0020%, P: 0.0200% hereinafter, S: less than% 0.0100, Cu: 0 ~ 0.500%, Ni: 0 ~ 1.00%, Nb: 0 ~ 0.050%, V: 0 ~ 0.120%, Ti: 0 ~ % 0.025, Ca: 0 ~ 0.050%, Mg: 0 ~ 0.050%, REM: 0 ~ 0.100%, and balance: a Fe and impurities, 　indication determined by the following formula (1) Q is 0.00 or more, 　have a chemical composition carbon equivalent Ceq obtained by the following formula (2) (%) is less than 0.800% ,

　Is 400 or more in Vickers hardness surface layer portion hardness at room temperature with the ratio of the difference between the surface layer portion hardness and mid-thickness portion hardness of 15.0% of the surface layer portion hardness at room temperature,
　the plate thickness T is more than 40mm steel plate is.
　Q = 0.18-1.3 (logT) +0.75 ( 2.7 × [C] + [Mn] + 0.45 × [Ni] + 0.8 × [Cr] + 2 × [Mo]) ··· (1)
　Ceq (%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 4 · · · (2)
　the formula index Q of (1) is calculated by substituting the numerical values and the content by mass percent of the element X thickness T (mm) [X], the case of not containing the element X 0 is substituted for . Carbon equivalent Ceq (%) of the formula (2) is calculated by substituting the numerical value of the content by mass percent of the element X [X], the case of not containing the element X 0 is substituted.
[Requested item 2]
　The index Q is 0.04 or more,
　the steel sheet according to claim 1 wherein the ratio is less than 13.0%.
[Requested item 3]
　By
mass%, Ni: 0.05 ~ 1.00%
steel sheet according to claim 1 or claim 2 having a chemical composition is.
[Requested item 4]
　By
mass%, Mn: 0.63 ~ 2.00%
steel sheet according to any one of claims 1 to 3 having a chemical composition is.