[Document Name] Description [Title of Invention] HIGH-TOUGHNESS WEAR RESISTANT STEEL AND METHOD FOR PRODUCING THE SAME [Technical Field] [0001] The present invention relates to a high-toughness wear resistant steel suitable for use as constituent members, required to have wear resistance, of machines such as construction machines for civil engineering or mines, or large-scale industrial machines, and relates to a method for producing the high-toughness wear resistant steel. [Background Art] [0002] The wear resistance of a constituent member of a machine is strongly controlled by the surface hardness of the constituent member, and hence high hardness steels are applied to the constituent members, required to have wear resistance, of machines such as construction machines for civil engineering or mines or large-scale industrial machines. Recently, mine development in cold regions has been actively promoted, and accordingly, demand for construction machines used in cold regions has been increasing. In consideration of such use in cold regions, wear resistant steel is also required to have low temperature toughness. Additionally, need for such wear resistant steels provided with excellent workability has also been being enhanced. [0003] For the purpose of solving the problem of achieving high toughness, for example, Patent Document I proposes a method for ensuring the compatibility between high hardness and high toughness by optimizing the component system, heat rolling and heat treatment. - 1- [0004] Patent Document 2 proposes an achievement of high toughness based on the use of the morphological control of the austenite particle through the rolling reduction of the unrecrystallized region and the use of direct quenching. [Citation List] [Patent Document] [0005] [Patent Document 1] JP09-118950A [Patent Document 2] JP2002-80930A [Summary of Invention] [Technical Problem] [0006] However, the method proposed in Patent Document I does not include in the scope thereof the use in cold regions; no sufficient toughness is ensured when use in cold regions is assumed. [0007] The method proposed in Patent Document 2 is required to increase the rolling reduction in the unrecrystallized region and thus imposes a significant constriction on the production conditions. This method is also unsuitable for the production of thick-wall products hardly susceptible to penetration of the rolling reduction. [0008] Moreover, in neither of these methods, the improvement of the workability of wear resistant steels is considered. [0009] In view of such circumstances, an object of the present invention is to provide a high-toughness wear resistant steel which has a toughness allowing the aforementioned -2steel to be usable even in cold regions and is satisfactory in workability, and is hardly dependent on production conditions with respect to the properties thereof, and to provide a method for producing the high-toughness wear resistant steel. [Solution to Problem] [0010] The present inventors performed a series of diligent studies for the purpose of solving the aforementioned problems, and obtained the following findings (a) to (h). [0011] (a) In general, with the increase of the hardness, the toughness tends to decrease; however, wear resistant steel is required to have certain hardness for the purpose of ensuring wear resistance. Accordingly, the present inventors have investigated wear resistance, toughness and workability, in various ways, and consequently have found that there is a hardness range in which wear resistance, toughness and workability can be compatible with each other. [0012] (b) The control of the hardness only requires the control of the content of C. However, for the purpose of obtaining a more stable toughness, it is not sufficient to control only the hardness, and it is also necessary to control the hardenability. Specifically, when a wear resistant steel is intended to be produced inexpensively, generally the martensitic microstructure is utilized; however, in the case where the hardenability is insufficient and the upper bainitic microstructure is produced, the toughness is significantly degraded, and hence the steel is required to have a certain or higher level of hardenability. In this connection, when the plate thickness is increased, it is difficult to achieve quenching, and hence the hardenability is not only required to be increased to a certain level, but also hardenability appropriate to the plate thickness is required. [0013] -3 - (c) It has been found that, as described above, for the purpose of obtaining a hardness and an intended microstructure, by imparting a hardenability appropriate to the plate thickness to the steel material, the wear resistance, low-temperature toughness and workability can be made to be compatible with each other. [0014] Specifically, the steel composition including the content of C is regulated, the surface hardness is regulated to fall within a predetermined range, the ratio of hardenability to plate thickness is regulated and the martensitic transformation initiation temperature is regulated. [0015] The ratio between the hardenability and the plate thickness is regulated to fall within a required range for the purpose of ensuring an adequate hardenability as wear resistant steel according to the plate thickness. This is because although with the increase of the plate thickness t, the hardenability of the central portion in the plate thickness direction is decreased, the hardenability can be maintained by increasing the content of the alloy component in the steel; however, there is an adverse possibility that the weldability and the workability are impaired. [0016] The reason for regulating the martensitic transformation initiation temperature is that with the decrease of the martensitic transformation initiation temperature, the temperature at which martensite is produced can be decreased, and when the bainitic microstructure as a microstructure other than martensite is produced, the lower bainitic microstructure tends to be produced, and hence it is facilitated to obtain a high toughness. [0017] (d) Specifically, the steel composition consists, by mass%, of C: 0.15 to 0.25%, Si: 0.1 to 1.0%, Mn: 0.4 to 1.3%, P: 0.015% or less, S: 0.005% or less, Cr: 0.2 to 0.9%, Nb: 0.005 to 0.03%, Ti: 0.005 to 0.03%, B: 0.0003 to 0.004%, Al: 0.005 to 0.08% and N: 0.005% or less, the balance being Fe and inevitable impurities. The steel composition -4may optionally contain, by mass%, one or more of the elements among Cu: 0.5% or less, Ni: 0.5% or less, Mo: 0.5% or less and V: 0.08% or less. [0018] (e) The surface hardness of steel is specifically required to be HBW 400 to 500 in terms of the Brinell hardness, as the hardness facilitating the mechanical working and allowing the steel to be used as wear resistant steel. [0019] (f) With respect to the ratio of hardenability to plate thickness and the martensitic transformation initiation temperature, the ratio DI/t of hardenability index DI to plate thickness t (mm) is required to satisfy the following formula (1), and at the same time, the martensitic transformation initiation temperature Ms (*C) is required to satisfy the following formula (2). [0020] DI/t = 0.5 to 15.0 ... (1) Ms s 430 ... (2) wherein t represents the plate thickness (mm) of the steel, DI represents the hardenability index, and Ms represents the martensitic transformation initiation temperature ( 0 C). [0021] The hardenability index DI depends on the chemical composition of the steel, and can be calculated on the basis of the following formula (3). Originally, DI means the ideal critical diameter; DI is a diameter within which 50% of the central portion of the round rod becomes the martensitic microstructure, when the round rod is quenched by an ideal cooling. Accordingly, DI can be diverted as the hardenability index. [0022] DI=9.238IC(1+0.64Si)(1+4.1Mn)(1+0.27Cu)(1+0.5Ni)(1+2.33Cr)(1+3.14Mo)... (3) wherein the symbols of the elements in the formula respectively represent the contents (mass%) of the elements. -5- [0023] The martensitic transformation initiation temperature Ms is the martensitic transformation initiation temperature (*C) during quenching and cooling, is also dependent on the chemical composition of the steel, and can be calculated on the basis of the following formula (4): [0024] Ms = 521 -353xC - 22xSi -24xMn -27xNi -18xCr -8xCu -16xMo ... (4) wherein the symbols of the elements in the formula respectively represent the contents (mass%) of the elements. [0025] (g) Next, for the purpose of obtaining an excellent toughness, it is preferable to produce a microstructure mainly comprising martensite, in particular, a microstructure in which the martensite ratio is 70% or more. [0026] However, the martensitic microstructure offers a cause for degrading the workability. The content of carbon in the steel also offers a cause for degrading the workability. Accordingly, for the purpose of producing a high-toughness wear resistant steel having an excellent workability, it is preferable to set the product between the martensite ratio M and the content of carbon to be 23 or less. [0027] (h) A steel having such hardenability according to hardness, a microstructure and a plate thickness can be produced from a slab having the aforementioned steel composition by either of the following methods (i) and (ii). [0028] (i) A method based on "reheating quenching" in which a slab is heated to a temperature of 900 to 1200*C, successively hot rolled, and rolled at a temperature of I 000*C or lower, and the rolling of the slab is completed at a temperature between the Ar 3 point -100*C or higher and the Ar 3 point + 150*C or lower and then the slab is cooled, -6then the rolled slab is again heated to a temperature between the Ac 3 point or higher and 950*C or lower, and then the rolled slab is water cooled. [0029] (ii) A method based on "direct quenching" in which a slab is heated to a temperature of 900 to 1200'C, successively hot rolled, and rolled at a temperature of 1000*C or lower, and the rolling is completed at a temperature between the Ar 3 point or higher and the Ar 3 point + 150*C or lower, and then the rolled slab is cooled from a temperature of the Ar 3 point or higher, at a cooling rate of 3.0*C/sec or more, to 200*C or lower in terms of the surface temperature of the resulting steel plate. [0030] The present invention has been achieved on the basis of the aforementioned findings, and the gist of the present invention is as shown in the following items (1) to (5). [0031] (1) A high-toughness wear resistant steel consisting, by mass%, of C: 0.15 to 0.25%, Si: 0.1 to 1.0%, Mn: 0.4 to 1.3%, P: 0.015% or less, S: 0.005% or less, Cr: 0.2 to 0.9%, Nb: 0.005 to 0.03%, Ti: 0.005 to 0.03%, B: 0.0003 to 0.004%, Al: 0.005 to 0.08% and N: 0.005% or less, the balance being Fe and inevitable impurities, wherein the high toughness wear resistant steel satisfies the following formulas (1) and (2), and the surface hardness is HBW 400 to 500 in terms of Brinell hardness: [0032] DI/t = 0.5 to 15.0... (1) Ms 5 430 ... (2) wherein t represents the plate thickness (mm), DI represents the hardenability index, Ms represents the martensitic transformation initiation temperature (*C), DI and Ms are calculated on the basis of the following formulas (3) and (4), respectively, and the symbols of the elements in these formulas respectively mean the contents (mass%) of the elements in the steel: [0033] -7- DI = 9.238C(1+0.64Si)(1+4.1Mn)(1+0.27Cu)(1+0.5Ni)(1+2.33Cr)(1+3.14Mo)... (3) Ms = 521 -353xC - 22xSi -24xMn -27xNi -18xCr -8xCu -16xMo ... (4) [0034] (2) The high-toughness wear resistant steel according to the item (1) above, wherein the martensite ratio M in the microstructure is 70% or more and satisfies the following formula (5): MxC 5 23 ... (5) wherein M represents the martensite ratio (%) and C represents the content (mass%) of carbon in the steel. [0035] (3) The high-toughness wear resistant steel according to the item (1) or (2) above, further containing, by mass%, one or more of the elements among Cu: 0.5% or less, Ni: 0.5% or less, Mo: 0.5% or less and V: 0.08% or less. [0036] (4) A method for producing a high-toughness wear resistant steel, wherein a slab having the chemical composition according to any one of the items (1) to (3) above is heated to a temperature of 900 to 1200*C, the slab is rolled at a temperature of 1 000*C or lower, the rolling of the slab is completed at a temperature between the Ar 3 point - I 00*C or higher and the Ar 3 point + 150*C or lower and then the slab is cooled, then the rolled slab is again heated to a temperature between the Ac 3 point or higher and 950*C or lower, and then the rolled slab is water cooled. [0037] (5) A method for producing a high-toughness wear resistant steel, wherein a slab having the chemical composition according to any one of the items (1) to (3) above is heated to a temperature of 900 to 1200 0 C, the slab is rolled at a temperature of 1000 0 C or lower, the rolling is completed at a temperature between the Ar 3 point or higher and the Ar 3 point + 150*C or lower, and then the rolled slab is cooled from a temperature of the -8- Ar 3 point or higher, at a cooling rate of 3.0*C/sec or more, to 200*C or lower in terms of the surface temperature of the resulting steel plate. [Advantageous Effect of Invention] [0038] According to the present invention, there is obtained a high-toughness wear resistant steel which has a toughness allowing the aforementioned steel to be usable even in cold regions and is satisfactory in workability, and the properties of which are hardly dependent on production conditions. [Description of Embodiments] [0039] Hereinafter, the present invention is described in detail. [0040] 1. On the Chemical Composition of the High-Toughness Wear resistant Steel According to the Present Invention First, detailed description is made on the reasons for regulating as described above the chemical composition of the high-toughness wear resistant steel according to the present invention. Here, it is noted that "%" in each of the contents of the individual elements represents "mass%." [0041] C: 0.15 to 0.25% C is the most effective element for improving the surface hardness and is inexpensive. However, when the content of C is less than 0.15%, the hardness is required to be compensated by increasing the contents of other alloying elements to result in cost increase. On the other hand, the content of C exceeds 0.25%, the hardness comes to be too high, and hence the toughness is degraded. Consequently, the content of C is set at -9- 0.15 to 0.25%. The lower limit of the content of C is preferably 0.17%. The upper limit of the content of C is preferably 0.22%. [0042] Si: 0.1 to 1.0% Si is an element contributing to the improvement of the surface hardness. However, when the content of Si is 0.1% or less, the improvement effect of the surface hardness is insufficient. On the other hand, when the content of Si exceeds 1.0%, the toughness is degraded. Consequently, the content of Si is set at 0.1 to 1.0%. The lower limit of the content of Si is preferably 0.2%. The upper limit of the content of Si is preferably 0.8%. [0043] Mn: 0.4 to 1.3% Mn is an element improving the surface hardness through the improvement of the hardenability. However, when the content of Mn is less than 0.4%, the hardness is required to be compensated by increasing the contents of other alloying elements to result in cost increase. On the other hand, when the content of Mn exceeds 1.3%, the toughness is remarkably impaired. Consequently, the content of Mn is set at 0.4 to 1.3%. The lower upper limit of the content of Mn is preferably 0.6%. The upper limit of the content of Mn is preferably 1.2%. [0044] P: 0.015% or less P is an element present as an impurity in the steel. P is segregated in grain boundary to degrade the delayed fracture resistance and the toughness of the steel, and hence the content of P is preferably as small as possible. In particular, when the content of P exceeds 0.015%, such adverse effects as aforementioned is remarkable, and consequently, the content of P is limited to 0.015% or less. [0045] S: 0.005% or less - 10 - S is an element present as an impurity in the steel. S degrades the ductility and the toughness of the steel, and hence the content of S is preferably as small as possible. In particular, when the content of S exceeds 0.005%, such adverse effects as aforementioned is remarkable, and consequently, the content of S is limited to 0.005% or less. [0046] Cr: 0.2 to 0.9% Cr is an element effective in improving both of the hardness and the toughness, through the function to enhance the hardenability. However, when the content of Cr is less than 0.2%, such effects are not sufficient. On the other hand, when the content of Cr exceeds 0.9%, the toughness is remarkably degraded. Consequently, the content of Cr is set at 0.2 to 0.9%. The lower limit of the content of Cr is preferably 0.3% . The upper limit of the content of Cr is preferably 0.8%. [0047] Nb: 0.005 to 0.03% Nb is an element to suppress the coarsening of the grains not only during heating a slab but also during quenching the slab, and hence is an element effective in producing steel having a fine unit of fracture surface. However, when the content of Nb is less than 0.005%, such effects are not sufficient. On the other hand, when the content of Nb exceeds 0.03%, such effects are saturated and the weldability is remarkably disturbed. Consequently, the content of Nb is set at 0.005 to 0.03%. The lower limit of the content of Nb is preferably 0.010%. The upper limit of the content of Nb is preferably 0.025%. [0048] Ti: 0.005 to 0.03% Ti is effective as a deoxidizing element, and an element effective in grain refinement during heating through the production of a nitride. For the purpose of obtaining this effect, the total content of Ti in the steel is required to be 0.005% or more. However, when Ti is contained in a content exceeding 0.03%, the toughness degradation due to the carbide formed by Ti comes to be remarkable. Consequently, the content of Ti - 11 is set at 0.005 to 0.03%. The lower limit of the content of Ti is preferably 0.008%. The upper limit of the content of Ti is preferably 0.025%. [0049] B: 0.0003 to 0.004% B is an extremely important element that remarkably improves the hardenability. However, when the content of B is less than 0.0003%, the improvement effect of the hardenability is not sufficient. On the other hand, when the content of B exceeds 0.004%, and the toughness is remarkably degraded. Consequently, the content of B is set at 0.0003 to 0.004%. The lower limit of the content of B is preferably 0.0005%. The upper limit of the content of B is preferably and 0.003%. [0050] Al: 0.005 to 0.08% Al is an element capable of effectively suppressing the overgrowth of initial austenite grains through the production of AlN during heating of a slab. However, when the content of Al is less than 0.005%, this effect is not sufficient. On the other hand, the content of Al exceeds 0.08%, the toughness is remarkably degraded. Consequently, the content of Al is set at 0.005 to 0.08%. The lower limit of the content of Al is preferably 0.010%. The upper limit of the content of Al is preferably 0.07%. [0051] N: 0.005% or less N is an element present as an impurity in the steel. N offers a cause for degrading the toughness, and hence the content of N is preferably as small as possible. In particular, when the content of N exceeds 0.005%, the adverse effect on the toughness comes to be remarkable. Consequently, the content of N is limited to 0.005% or less. [0052] The high-toughness wear resistant steel according to the present invention contains Fe and impurities in addition to the aforementioned components. The impurities as referred to herein mean the components that contaminate the steel when the steel is - 12 industrially produced, due to the raw materials such as ores and scraps, and due to various other factors in the production process, and are allowed to contaminate within the ranges not adversely affecting the present invention. [0053] The high-toughness wear resistant steel according to the present invention may contain, as elements to be optionally added, one or more of the elements listed below. [0054] Cu: 0.5% or less Cu is an element to be optionally added, and can be contained if necessary. When Cu is contained, the contained Cu has an effect to more improve the strength and corrosion resistance. However, when Cu is contained in a content exceeding 0.5%, the performance improvement corresponding to the cost increase is not found. Consequently, when Cu is contained, the upper limit of the content of Cu is set at 0.5%. When the effect due to Cu of improving the strength and the corrosion resistance is intended to be obtained, it is preferable to contain Cu in a content of 0.2% or more. [0055] Ni: 0.5% or less Ni is an element to be optionally added, and can be contained if necessary. When Ni is contained, the contained Ni has an effect to increase the toughness of the matrix (base metal) of the steel in a solid solution condition. However, when Ni is contained in a content exceeding 0.5%, the performance improvement corresponding to the cost increase is not found. Consequently, when Ni is contained, the upper limit of the content of Ni is set at 0.5%. When the effect due to Ni of improving the toughness is intended to be obtained, it is preferable to contain Ni in a content of 0.2% or more. [0056] Mo: 0.5% or less Mo is an element to be optionally added, and can be contained if necessary. When Mo is contained, the contained Mo has an effect to improve the strength and the toughness - 13 of the base metal. However, when Mo is contained in a content exceeding 0.5%, in particular the hardness of the heat affected zone (HAZ) is increased, and the toughness and the weldability are impaired. Consequently, when Mo is contained, the upper limit of the content of Mo is set at 0.5%. When the effect due to Mo of improving the strength and the toughness of the base metal is intended to be obtained, it is preferable to contain Mo in a content of 0.1% or more. [0057] V: 0.08% or less V is an element to be optionally added, and can be contained if necessary. When V is contained, the contained V has an effect to improve the strength of the base metal mainly through the precipitation of carbon-nitride during tempering. However, when V is contained in a content exceeding 0.08%, the effect of improving the performance of the base metal is saturated, and the toughness degradation is caused. Consequently, when V is contained, the upper limit of the content of V is set at 0.08%. When the effect due to V of improving the strength of the base metal is intended to be obtained, it is preferable to contain V in a content of 0.01% or more. [0058] 2. On the Microstructure of the High-Toughness Wear resistant Steel According to the Present Invention For the purpose of allowing the high-toughness wear resistant steel according to the present invention to exhibit an excellent high toughness, it is necessary to extend a microstructure mainly comprising martensite to the central portion in the plate thickness direction of the steel material. [0059] First, for the purpose of allowing the microstructure mainly comprising martensite to be extended to the central portion in the plate thickness direction of the steel material, it is necessary to control the ratio DI/t of hardenability index DI to plate thickness (mm) of the steel to be 0.5 to 15.0. This is because when the ratio DI/t is less than 0.5, no - 14 sufficient martensite ratio is obtained and the toughness is degraded, and on the other hand, when the ratio DI/t exceeds 15.0, the addition of a large amount of alloying elements is required, the cost for the alloy is increased and the toughness is also significantly degraded. [0060] Next, for the purpose of obtaining an excellent toughness as quenched, it is necessary to suppress as much as possible the by-production of the upper bainite poor in toughness that is a by-produced microstructure in addition to mainly produced martensite. For that purpose, the martensitic transformation initiation temperature Ms (*C) is controlled to be 430*C or lower, and thus the by-production of the upper bainitic microstructure poor in toughness is suppressed, wherein a lower bainitic structure excellent in toughness tends to be produced in addition to mainly produced martensite. Consequently, by controlling the martensitic transformation initiation temperature Ms (*C) to be 430 or lower, it is possible to obtain an excellent toughness as quenched. [0061] The high-toughness wear resistant steel according to the present invention is required to be a microstructure mainly comprising martensite, but may also contain other microstructures such as retained austenite and the aforementioned lower bainitic microstructure. However, the retained austenite offers a cause for degradation of the toughness of the base metal, and hence the content of the retained austenite is preferably set at less than 5%. [0062] 3. On the Workability of the High-Toughness Wear resistant Steel According to the Present Invention When the high-toughness wear resistant steel according to the present invention is used, for example, for the shovel of a loading shovel, the steel itself is required to be - 15 formed into a shovel shape. For the purpose of being excellent in mechanical workability involving lathe turning, piercing and others, the hardness of the surface is important. [0063] Accordingly, the surface hardness of the steel is required to be HBW 400 to 500 in terms of Brinell hardness. This is because when the hardness is less than HBW 400, the steel is soft and it is difficult to use the steel as wear resistant steel, and on the other hand, when the hardness exceeds HBW 500, the steel is too hard and the mechanical working of the steel is difficult. A preferable range of the hardness is from HBW 410 to 470. [0064] Next, for the purpose of obtaining an excellent toughness, it is preferable to form a microstructure mainly comprising martensite, in particular, martensite ratio of 70% or more. [0065] However, the martensitic microstructure offers a cause for degrading the workability. The carbon content in the steel also offers a cause for degrading the workability. Accordingly, when the martensite ratio M and the carbon content are both too large, and the product between the martensite ratio M and the carbon content exceeds 23, the workability is remarkably degraded. [0066] Accordingly, for the purpose of allowing the high-toughness wear resistant steel of the present invention to be a high-toughness wear resistant steel having excellent workability, it is preferable that the following formula (5) is satisfied: [0067] M x C:5 23 ... (5) wherein M represents the martensite ratio (%) and C represents the carbon content (mass%) in the steel. - 16 - [0068] 4. On the Method for Producing the High-Toughness Wear resistant Steel According to the Present Invention The invention steel can be produced from a slab having the aforementioned steel composition according to either of the following methods (i) and (ii). [0069] (i) A method based on "reheating quenching" in which a slab is heated to a temperature of 900 to 1200*C, and rolled at a temperature of 1000*C or lower; the rolling of the slab is completed at a temperature between the Ar 3 point - 1 00*C or higher and the Ar 3 point + 150*C or lower; and then the rolled slab is cooled, then again heated to a temperature between the Ac 3 point or higher and 950*C or lower, and then water cooled. [0070] (ii) A method based on "direct quenching" in which a slab is heated to a temperature of 900 to 1200*C, and rolled at a temperature of 1000*C or lower; the rolling is completed at a temperature between the Ar 3 point or higher and the Ar 3 point + 150*C or lower; and then the rolled slab is cooled from a temperature of the Ar 3 point or higher, at a cooling rate of 3.0*C/sec or more, to 200*C or lower in terms of the surface temperature of the resulting steel plate. [0071] Hereinafter, description is made on the individual steps of the method for producing a high-toughness wear resistant steel. The steps common to the methods (i) and (ii) are described collectively. [0072] (1) On the Heating Step In either of the aforementioned (i) reheating quenching method (RD) or the aforementioned (ii) direct quenching method (DQ), the slab having the aforementioned composition is heated at a temperature of 900 to 1200*C. The production method of the - 17 slab itself does not matter particularly. The slab can be produced by a usually used production method such as a continuous casting method. [0073] The heating of the slab at 900*C or higher is for the purpose of causing the austenitic transformation to obtain a uniform microstructure. With the increase of the slab heating temperature, the slab is softened and the deformation resistance of the slab is decreased to facilitate the rolling in the rolling step as the subsequent step. However, the high heating temperature increases the energy consumption in the heating furnace to be unpreferable for the production cost and the natural environment; thus, the upper limit of the heating temperature is set at 1200*C. The preferable upper limit of the slab heating temperature is 1150*C. The preferable lower limit of the slab heating temperature is 1 000*C, respectively. [0074] The heating time in the aforementioned temperature range is preferably set at 2 hours or more for the purpose of isothermalizing the slab inclusive of the central portion thereof. [0075] (2) On the Hot Rolling Step The slab heated under the aforementioned conditions is subjected to a hot working to be finished into an intended shape; in this hot working, rolling is performed at a temperature of 1000*C or lower. The rolling performed at 1000*C or lower is for the purpose of promoting the grain refinement by recrystallization. When the heating temperature of the slab is higher, after the slab temperature is decreased to 1000*C or lower, the rolling is started. [0076] In the case where the reheating quenching of (i) is performed, the rolling is completed at a temperature between the Ar 3 point - 100*C or higher and the Ar 3 point + 150*C or lower. In the case where the rolling completion temperature is lower, that is, - 1, 8 the rolling completion temperature is lower than the Ar 3 point, even when water cooling is performed successively, quenching is not achieved, and no sufficient martensitic microstructure can be obtained. In this case, by performing quenching through reheating after the work piece has been once cooled, the martensitic microstructure can be obtained. By processing in such a way, even when the rolling completion temperature is lower than the Ar 3 point, the martensitic microstructure can be obtained with the proviso that quenching through reheating is performed after the work piece has been once cooled. However, when the rolling completion temperature is too low, the deformation resistance of the slab is too large and the rolling is made difficult; accordingly, the lower limit of the rolling completion temperature is set at the Ar 3 point - 100*C. The preferable lower limit of the rolling completion temperature is the Ar 3 point. [0077] On the other hand, when the rolling completion temperature is the Ar 3 point or higher, the direct quenching of (ii) can be performed, and hence it is not necessary to reheat after cooling is performed purposely. However, reheating makes the quenching more easily achieved, and accordingly, makes the martensitic microstructure be more easily obtained. Accordingly, when the reheating quenching is performed, the upper limit of the rolling completion temperature is set at the Ar 3 point + 150*C. When the rolling completion temperature is the Ar 3 point or higher, the direct quenching of (ii) may be performed; thus, the preferable upper limit of the rolling completion temperature is the Ar 3 point from the viewpoint of being capable of omitting the reheating. When the direct quenching of (ii) is performed, the rolling is completed at a temperature between the Ar 3 point or higher and the Ar 3 point + 150*C or lower. In the below-described water cooling step, the water cooling start temperature is set at the Ar 3 point or higher, and hence the lower limit of the rolling completion temperature is set at the Ar 3 point. There is some lag time between the rolling completion time and the water cooling, and the temperature of the steel is possibly decreased during the lag time. Accordingly, the preferable lower limit of the rolling completion temperature is set at the - 19 - Ar 3 point + 50*C. On the other hand, for the purpose of achieving the toughness improvement through the grain refinement, the upper limit of the rolling completion temperature is set at the Ar 3 point + 150'C. [0078] (3) On the Cooling Step In the case where the reheating quenching of (i) is performed, the completion of the rolling is followed by cooling, then the reheating at a temperature between the Ac 3 point or higher and 950*C or lower is followed by water cooling. The cooling after the completion of the rolling does not particularly matter with respect to the involved cooling method, and it suffices to allow the work piece to stand to cool in the air. The cooling following the rolling renders unnecessary the cooling of the work piece to room temperature, and it suffices to cool the work piece to approximately 400*C. After the cooling, the work piece is reheated to a temperature between the Ac 3 point or higher and 950*C or lower, and then water cooled. The reheating temperature is set at the Ac 3 point or higher because the water cooling start temperature is intended to be set at the Ac 3 point or higher, and because unless the cooling is started from the austenite single phase range, no sufficient martensitic microstructure fraction is obtained, and the hardness and the toughness are both decreased. From the viewpoint of the lag time from the reheating to the water cooling, the lower limit of the reheating temperature is preferably set at the Ac 3 point + 50*C. On the other hand, from the viewpoint of reducing the cost for the energy consumed for heating and the time consumed for heating, the upper limit of the reheating temperature is set at 950*C. In the water cooling, the work piece need not be cooled to room temperature, but it suffices to cool the work piece to approximately 200*C. [0079] When the direct quenching of (ii) is performed, water cooling is performed from a temperature of the Ar 3 point or higher at a cooling rate of 3.0*C/sec or more to 200*C or lower in terms of the surface temperature of the steel plate. In this case, the cooling is also performed from the temperature of the Ar 3 point or higher because, similarly to the - 20 case where the reheating quenching of (i) is performed, the work piece is cooled from the austenite single phase range and thus a sufficient martensitic microstructure is intended to be ensured. From the viewpoint of achieving quenching, the cooling rate is preferably as fast as possible; the cooling is preferably performed at a rate of 5.
0 *C/sec or more. The upper limit of the cooling rate is not particularly regulated; however, in view of the maximum cooling rate of currently available cooling apparatuses, the cooling rate is at most approximately 60*C/sec. The cooling method is not particularly limited; examples of the cooling method include water cooling and mist cooling. The cooling is regulated to be performed to 200*C or lower in terms of the surface temperature of the steel plate; this is for the purpose of obtaining a sufficient quenched microstructure. [0080] Hereinbefore, the method for producing the invention steel has been described; between the respective steps or during the respective steps, the treatments such as descaling, strain straightening, and heating for isothermalizing may also be performed. After the production of the invention steel by the production method as described above, the invention steel can be used as wear resistant steel without being subjected to tempering. [0081] The wearing-resistant steel excellent in workability and low-temperature toughness, according to the present invention, and the method for producing the aforementioned wearing-resistant steel are more specifically described with reference to Examples. However, these Examples are construed not to limit the present invention. [Examples] [0082] To the slabs having the chemical compositions and properties shown in Table 1 were subjected to heating, soaking, hot rolling, cooling to room temperature, reheating and - 21 quenching to yield samples (Sample Nos. I to 32) of 12 to 50 mm in plate thickness. None of the samples were subjected to tempering. [0083] [Table 1] - 22 e, 0' 0o w0 w0 w0 N ~ toDN.4 N 0D V) Go 00 o 00 . No N" N4 f N- 0 N *G co No 0 Go-C N,. CR NW00f D1 D1 t CR R CIN! 0. Nd Ni a; Go0 t r .- N- 0i N,% 0s (6 00, i D CD6 00) No 4D 4 r N CD -o( r -- M MCDDD C M4Q Q MQ T-N m m Q Q V W4N! o W C CDIo0WCDCDOO 4 M N r 4C hCD 44 0CNW1CMNrCDO3rW-M o o 0(GJ oc o 0 2 00 0 0 0 0 00 00 00 cc 0 00 0 0 o 0 00 0 00 0m 0 0 0 G0 0 0 Co 101100 N4 1 CDNNO.40lCDN CDn4IOI m r00P0CDq4 1D 000 UD Wi W3 Nc N NNNN N No M W N N N- No W N cN N N N N N N Co W))W N 0 N > N D 00 fi 0> .- ' NZ 0N1 0N N N -- N00N1 0 1 0 1 0D 0 0 -0 0 0D 0 0 -0 0 0 0. 0D 0 0D 0 0 0 0 0 0 co 0 0D 0 0 0 0 0D 0D 0D 0D C CD 00 0000000 0 0D 0D 0D 0D 0D 0 0 0 0 0 0 00 N5 1001 CC00Di( 0 C) 0 N= 10 CD N 0NDt CD) CD N 0 C0D N C Co D CD 0 D r D N r D r D N D r D 0 D N D r . N D 0 r r D N D r D N D r D N > r D N D r D r D r D N > coo Nr D CD C q R RCo q RoRo!ooo RR ooqoo Ro RC!oRoooRooooRoC Z ; C D C D C D C D C D C ) C 0 D C D C D C D C D C D C D 0---- -- , (I m m v ), m v- " -4 w " -q ) r- m r o " m co, m D c cm - N T..W) .. ) CDC NC 4 C Y -* I . ~ D . DC . N C1C 0> 0 DC DC CDU DC D4 DC DC DC DC DC DC DC DC DC o DC DC 2.------------------ r VP o ) C W N 01 0 Em C0 CC 00 - ~ ~ ~ ~ ~ ~~C rCDN1 r 0 0 D r 4N ~ - 4 D 0 rr N~ E m r 0 r r 0 r r3 0o 0 D r N r- r 0m r0 0 .0 r r 0 0D ro 0D0D0D'o00omCD6 6 ' 6 C.6 Co M. -D6 666 66CD.0C6 ,ss00 c r N N *0 00 0 N Nf 00 N. 0 N N 000ND I-~ 004 [Tale 2] E ~ ~~~~~~~~~~ 23 M- *q ~ NM %I MML % % - --- Ni -S-c-w-:-w - to to--- r- W F oP- -o -D N. - - -, ad 0 te *E 00OCOOOOOOOO0oooooooocoo 1!OOOOO00 =Z ~ I aNN IcoW)N nflc -Ca M f~ tor aotmL c 0 1:Na N0NCDNmraao In03 o c m o N co . N r. 0 0~ o 0b0 o U 0n 0t ot U0 0U 0.3 - 0 0 0 0 0 00 0 00 0 00 0 00 00 0 0 0 0 o~ u ui ui liU u u rn oiu.uucicw 4)u 01 R~E 00 0 0 0 0 0 00 0 0 0 0 o Oo 4 o 'N 4 'ON ' oO 'COO 'W0No - ' 'O cu CIIa a5 ama ama ama am am ma ND8 l1 l c E aE cy A C 0 m cU 00 -~~~~ ~~~~ coa am m m m a m a a m m a a a m m :11 c o cD c c C, 2 C. o c g w "I " " e oi mw mCD cDm c f Go o r w a c cDF, n ' m ) 0 w ww wm ww 27 cDa V) a. ---- -- 2. 4D o- - - - c5- -~ ------ 5- a 0 E* - -- - - -- -- c5 C3c 5nc 5c Dc c Dc D=c Dc mc 3 Lo c cl cl- - w ' cDo m co, g 0 N ID a) 'o o ama C 30C YC YC a' WION (awt NIlN . N 3000 01~N N a* ) .0 ~ ~ ~ ~ ~ ~ ~ 1 w N0fN:C~~ 00l nn a n Nl m ri-E 0 C'C I oN.4= LD N LoC N0oOoO0I T3 E i-N N. Rq~ rflfllN (D .- flN t- f l * z~ ~ 24 M- [0085] These samples were subjected to the Brinell surface hardness test, and also to the Charpy impact test at the portions inwardly away by 1/4 the plate thickness from the surface of the steel plates, namely, the locations of (1/4)t the plate thickness at -40*C. In the Charpy impact test, the case where the absorbed energy of yE4o of 27 J or more was exhibited was determined to be satisfactory in low-temperature toughness. Additionally, the bending test was performed to evaluate the workability. In the bending test, JIS No. 1 specimens were sampled in the direction parallel to the rolling direction, and the case where no crack occurred for the bend radius of 3t (t: plate thickness) was determined to be acceptable (marked with 0). The microstructure of each of the specimens was observed at a magnification of 500 after etching with nital, and the martensite ratio was measured. The test results are collectively shown in Table 2. [0086] Consequently, it is shown that Sample Nos. 1 to 24 all fall within the scope of the present invention, and are all excellent in hardness, toughness and workability. [0087] On the contrary, it is shown that Sample No. 25, a Comparative Example, has a content of C exceeding the scope of the present invention, and hence is too high in hardness to be degraded in workability and toughness. [0088] It is shown that Sample Nos. 26 and 27, Comparative Examples, have a content of Si falling out of the scope of the present invention and a content of Mn falling out of the scope of the present invention, respectively, and are degraded in toughness. [0089] It is shown that Sample No. 28, a Comparative Example, has a content of Cr falling out of the scope of the present invention and has a direct quenching (DQ) start temperature lower than the Ar 3 point, and hence is degraded in toughness. [0090] - 25 - It is shown that Sample No. 29, a Comparative Example, has a high Ms value and a low DI/t value, hence has a low martensite ratio, and is consequently degraded in toughness. [0091] It is shown that Sample No. 30, a Comparative Example, has a content of Ti falling out of the scope of the present invention, and is degraded in toughness. [0092] It is shown that Sample No. 31, a Comparative Example, has a direct quenching (DQ) start temperature lower than the Ar 3 point, hence is incapable of obtaining a sufficient martensite ratio, and is degraded in hardness and toughness. [0093] It is shown that Sample No. 32, a Comparative Example, has a low reheating temperature during the reheating quenching, hence is incapable of obtaining a sufficient martensite ratio, and is degraded in hardness and toughness. [Industrial Applicability] [0094] According to the present invention, there is obtained a high-toughness wear resistant steel which has a toughness allowing the aforementioned steel to be usable even in cold regions and is satisfactory in workability, and the properties of which are hardly dependent on production conditions. The invention steel can be used as the constituent members, required to have wear resistance, of machines such as construction machines for civil engineering or mines, or large-scale industrial machines. - 26 -