AU2013319621A1 - Wear-resistant steel plate having excellent low-temperature toughness and corrosion wear resistance - Google Patents

Abstract

Provided is a wear-resistant steel plate having excellent wear resistance, low-temperature toughness, and corrosion wear-resistance. The wear-resistant steel plate is characterized in: containing, in terms of mass percent, 0.23-0.35% C, 0.05-1.00% Si, 0.1-2.0% Mn, no more than 0.020% P, no more than 0.005% S, 0.005-0.100% Al, 0.03-2.0% Cr, and 0.03-1.0% Mo, so as to satisfy the condition that DI* defined in formula (1) below is at least 45; having a component structure comprising a remainder of Fe and unavoidable impurities; using an as-quenched martensite phase as a main phase; having a structure in which the prior austenite grain size is no larger than 30 µm; and having the surface hardness be at least 450 in terms of a Brinell hardness of HBW 10/3000.

Description

DESCRIPTION Title of the Invention: ABRASION RESISTANT STEEL PLATE HAVING EXCELLENT LOW-TEMPERATURE TOUGHNESS AND EXCELLENT CORROSIVE WEAR RESISTANCE Technical Field [0001] The present invention relates to an abrasion resistant steel plate suitably used for parts of industrial machines, transporting machines and the like. The abrasion resistant steel plate according to the present invention has excellent low temperature toughness and can be suitably used as parts which are used in places where wear or abrasion generated due to a contact of the abrasion resistant steel plate with earth and sand containing water must be particularly taken into consideration. Background Art [0002] Conventionally, with respect to parts for industrial machines, transporting machines and the like such as, for example, a power shovel, a bulldozer, a hopper, a bucket or a dump truck used in a construction site, a civil engineering site, a mine or the like, abrasion is generated due to a contact of the part with earth, sand or the like. Accordingly, in manufacturing the above-mentioned parts, a steel material having excellent abrasion resistance is used for extending lifetime of the parts. Inan actual in-use environment, various states such as a dry state or a wet state are considered as a state of earth, sand or the like. Particularly, there may be a case where earth, sand or the like in a wet state contain a corrosive material. In this case, the wear due to earth, sand or the like in a wet state becomes wear in an environment which contains the corrosive material, that is, so-called corrosive wear. This corrosive wear has been known as an extremely severe wear environment. In view of the above, there has been a demand for an abrasion resistant steel material having excellent corrosive wear resistance. [0003] The use of these industrial machines, transporting machines and the like in a low-temperature zone of 0 0 C or below is also considered. Accordingly, a steel material which is used for parts of these industrial machines, transporting machines and the like is requested to possess the excellent low-temperature toughness in addition to the abrasion resistance and corrosive wear resistance. [0004] To satisfy such a request, for example, patent literature 1 proposes a method of manufacturing a high-hardness abrasion resistant steel having excellent low-temperature toughness, 2 wherein hot rolling is applied to a steel slab having the composition containing by mass%: 0.30% to 0.50% C, proper amounts of Si, Mn, Al, N, Ti, Nb and B respectively, and 0.10% to 0.50% Cr and 0.05% to 1.00% Mo, thereafter, quenching treatment is applied to the hot rolled plate from a temperature of Ar 3 transformation point or above and, subsequently, the quenched plate is tempered thus obtaining high-strength abrasion resistant steel. According to the description of the technique described in patent literature 1, the improvement of hardenability of the steel and the improvement of low-temperature toughness through strengthening of grain boundaries are achieved by allowing the steel to contain a large amount of Cr and a large amount of Mo. Further, according to the description of the technique described in patent literature 1, the further enhancement of low-temperature toughness is achieved by applying tempering treatment to the steel. [0005] Patent literature 2 proposes a high toughness abrasion resistant steel plate which has the composition containing by mass%: 0.18% to 0.25% C, 0.10% to 0.30% Si, 0.03% to 0.10% Mn, proper amounts of Nb, Al, N and B respectively, 1.00% to 2.00% Cr, and Mo more than 0.50% to 0.80%, and exhibits excellent toughness and excellent delayed fracture resistance after water quenching and tempering. According to the description of a technique described in patent literature 2, by suppressing the 3 content of Mn to a low level, and by allowing the steel plate to contain a large amount of Cr and a large amount of Mo, hardenability of the steel plate can be enhanced so that predetermined hardness can be ensured and, at the same time, toughness and delayed fracture resistance can be enhanced. Further, according to the description of the technique described in patent literature 2 further improves low-temperature toughness by applying tempering. [0006] Patent literature 3 proposes a high toughness and abrasion resistant steel which has the composition containing by mass%: 0.30% to 0.45% C, 0.10% to 0.50% Si, 0.30% to 1.20% Mn, 0.50% to 1.40% Cr, 0.15% to 0.55% Mo, 0.0005% to 0.0050% B, 0.015% to 0.060% sol. Al, and proper amounts of Nb and/or Ti. According to the description of the technique described in patent literature 3, the steel contains a large amount of Cr and a large amount of Mo and hence, hardenability of the steel is enhanced and, at the same time, grain boundaries are strengthened thus enhancing low-temperature toughness. [0007] Patent literature 4 proposes a method of manufacturing an abrasion resistant steel, wherein hot-rolling is applied to steel having the composition containing bymass%: 0.05% to 0.40% C, 0.1% to 2.0% Cr, further, proper amounts of Si, Mn, Ti, B, AlandNrespectivelyand, further, Cu, Ni, Mo, and Vas arbitrary 4 components at a cumulative reduction ratio of 50% or more in an austenitic non-recrystallized temperature range at a temperature of 900'C or below, thereafter, quenching is applied to a hot-rolled plate from a temperature of Ar 3 transformation point or above and, subsequently, the quenched plate is tempered, thus abrasion resistant steel being obtained. According to the description of this technique, directly quenching and tempering elongated austenite grains result the tempered martensitic structure where prior austenite grains are elongated. The tempered martensitic structure of the elongated grains remarkably enhances low-temperature toughness. [0008] Further, patent literature 5 proposes an abrasion resistant steel plate having excellent low-temperature toughness and having the composition containing by mass%: 0. 10% to 0.30% C, 0.05% to 1.0% Si, 0.1% to 2.0% Mn, 0.10% to 1.40% W, 0.0003% to 0.0020% B, 0.005% to 0.10% Ti and/or 0.035% to 0.1% Al. In the description of the technique described in patent literature 5, the abrasion resistant steel plate may further contain one or more kinds of elements selected from a group consisting of Cu, Ni, Cr and V. Due to such composition, it is considered that the abrasion resistant steel plate has high surface hardness and exhibits excellent abrasion resistance and excellent low-temperature toughness. [0009] 5 Further, in patent literature 6, an abrasion resistant steel plate having excellent bending property is described. The technique described in patent literature 6 is related to an abrasion resistant steel plate having the composition containing by mass%: 0.05% to 0.30% C, 0.1% to 1.2% Ti, and not more than 0.03% solute C, and having the structure wherein a matrix is formed of a ferrite phase and a hard phase is dispersed in the matrix. The abrasion resistant steel plate described in patent literature 6 may further contain one or two kinds of components selected from a group consisting of Nb and V, one or two kinds of components selected from a group consisting of Mo and W, one or two kinds of components selected from a group consisting of Si, Mn and Cu, one or two kinds of components selected from a group consisting of Ni and B, and Cr. Due to such composition, regarding the abrasion resistant steel plate described in patent literature 6, it is considered that both abrasion resistance against abrasion caused by earth and sand and bending property can be enhanced without inducing remarkable increase of hardness. Citation List Patent Literature [0010] PTL 1: JP-A-H08-41535 PTL 2: JP-A-H02-179842 PTL 3: JP-A-S61-166954 6 PTL 4: JP-A-2002-20837 PTL 5: JP-A-2007-92155 PTL 6: JP-A-2007-197813 Summary of Invention Technical Problem [0011] The respective techniques described in patent literatures 1 to 5 aim at the acquisition of the steel plates having low-temperature toughness and abrasion resistance. Further, the technique described in patent literature 6 aims at the acquisition of the steel plate having both bending property and abrasion resistance. However, in none of these patent literatures, the wear in an environment which contains a corrosive material such as earth and sand in a wet state has been studied and hence, there exists a drawback that consideration has not been made sufficiently with respect to corrosive wear resistance. [0012] Further, in the respective techniques described in patent literatures 1 to 4, tempering is a requisite and hence, there exists a drawback that a manufacturing cost is increased. In the technique described in patent literature 5, the steel plate contains W as an indispensable component and hence, there exists a drawback that a manufacturing cost is increased. In the technique described in patent literature 6, the main phase is 7 formed of ferrite and hence, there is a problem that surface hardness is low whereby the steel plate cannot acquire sufficient abrasion resistance. [0013] The present invention has been made to overcome the above-mentioned drawbacks of the related art, and it is an object of the present invention to provide an abrasion resistant steel plate which can be manufactured at a low cost, possesses excellent abrasion resistance, and has both of excellent low-temperature toughness and excellent corrosive wear resistance. Solution to Problem [0014] To achieve the above-mentioned object, inventors of the present invention have made extensive studies on the influence of various factors exerted on abrasion resistance, low-temperature toughness and corrosive wear resistance of the steel plate. As a result of the studies, the inventors have found that the corrosive wear resistance of a steel plate can be remarkably enhanced by making the steel plate have the composition containing proper amounts of Cr and Mo as indispensable components. It is supposed that by allowing the steel plate to contain Cr and Mo, even when the steel plate is exposed to earth and sand in a wet state having pH in a various range, Cr and Mo exist as an oxyacid and hence, corrosive wear 8 is suppressed. [0015] The inventors also have found that abrasion resistance and corrosive wear resistance against abrasion caused by earth and sand can be remarkably enhanced by maintaining surface hardness of the steel plate at a high level provided that the steel plate has the above-mentioned composition. [0016] The inventors also have found that the excellent low-temperature toughness of the steel plate can be surely acquired while the excellent abrasion resistance being assured by allowing the steel plate to contain proper amounts of Cr and Mo as indispensable components and to contain proper amounts of at least C, Si, Mn, P, S, Al, Cr, Mo in a state where DI* defined by the following formula (1) is satisfied 45 or more to enhance hardenability of the steel plate, then by making the structure where an as-quenched martensitic phase forms a main phase with ensuring surface hardness of 450 or more at Brinel hardness HBW 10/3000 and further by making the as-quenched martensitic phase finer so that a grain size of prior austenite (y) grains is 30 pm or less. DI*=33.85x(0.lxC) 0 5 x(0 .7xSi+l)x(3.33xMn+l)x(0.35xCu+1)x(o.3 6xNi+l) x (2. 16xCr+1) x (3xMo+l)x (1. 75xV+l) ..... (1) (where, C, Si, Mn, Cu, Ni, Cr, Mo and V denote the contents (mass%) of respective elements) 9 The present invention has been made based on the above-mentioned findings and has been completed after further study of the findings. That is, the gist of the invention is as follows. [0017] (1) An abrasion resistant steel plate having excellent low temperature toughness and excellent corrosive wear resistance, the steel plate having the composition containing by mass%: 0.23% to 0.35% C, 0.05% to 1.00% Si, 0.1% to 2. 0% Mn, 0.020% or less P, 0.005% or less S, 0.005% to 0.100% Al, 0.03% to 2. 0% Cr, and 0, 03% to 1.0% Mo in a state where DI* defined by the following formula (1) is satisfied 45 ormore, and further containing remaining Fe and unavoidable impurities as a balance, the steel plate having a structure where an as-quenched martensitic phase forms a main phase and a grain size of prior austenite grains is 30 gm or less, and surface hardness of the steel plate being 450 or more at Brinel hardness HBW10/3000. (Formula) DI*=33.85x (0.1xC)0 5 x (O.7xSi+1) x (3.33xMn+l)x (0.35xCu+l)x (0.3 6xNi+l) x(2 .16xCr+l)x (3xMo+1) x (l.75xV+1) ...... (1) (where, C, Si, Mn, Cu, Ni, Cr, Mo and V in the formula (1) refer to the contents (mass%) of respective elements.) (2) In the abrasion resistant steel plate described in (1), the steel composition further contains by mass% one or two or more kinds of components selected from a group consisting 10 of 0.005% to 0.1% Nb, 0.005% to 0.1% Ti, and 0.005% to 0.1% V. [0018] (3) In the abrasion resistant steel plate described in (1) or (2), the steel composition further contains by mass% one or two kinds of components selected from a group consisting of 0.005% to 0.2% Sn and 0.005% to 0.2% Sb. {0019] (4) In the abrasion resistant steel plate described in any of (1) to (3), the steel composition further contains by mass% one or two or more kinds of components selected from a group consisting of 0.03% to 1.0% Cu, 0.03% to 2.0% Ni, and 0.0003% to 0.0030% B. [0020] (5) In the abrasion resistant steel plate described in any of (1) to (4), the steel composition further contains by mass% one or two or more kinds of components selected from a group consisting of 0.0005% to 0.008% REM, 0.0005% to 0.005% Ca, and 0.0005% to 0.005% Mg. [0021] (6) In the abrasion resistant steel plate described in any of (1) to (5), wherein the content of the as-quenched martensitic phase is 98% or more in terms of volume fraction. Advantageous Effects of Invention [0022] According to the present invention, it is possible to 11 manufacture, easily and in a stable manner, an abrasion resistant steel plate having especially excellent corrosive wear resistance in an earth-and-sand abrasion environment in a wet state, having excellent low temperature toughness, and excellent abrasion resistance in a stable manner without lowering surface hardness. Description of Embodiments [0023] Firstly, the reasons for limiting the composition of the abrasion resistance steel plate of the present invention, which is also called "the steel plate" in this specification, are explained. In the explanation made hereinafter, mass% is simply expressed by % unless otherwise specified. [0024] C: 0.23% to 0.35% C is an element for increasing hardness of the steel plate and for enhancing abrasive resistance. When the content of C is less than 0.23%, the steel plate cannot acquire sufficient hardness. On the other hand, when the content of C exceeds 0.35%, weldability, low-temperature toughness and workability of the steel plate are lowered. Accordingly, the content of C is limited to a value which falls within a range from 0.23% to 0.35%. The content of C is preferably limited to a value which falls within a range from 0.25% to 0.30%. [0025] 12 Si: 0.05% to 1.00% Si is an effective element acting as a deoxidizing agent for molten steel. Si is also an element which contributes to the enhancement of strength of the steel plate by increasing solid solution strengthening. The content of Si is set to 0.05% or more to ensure such effects. When the content of Si is less than 0.05%, a deoxidizing effect cannot be sufficiently acquired. On the other hand, when the content of Si exceeds 1.00%, ductility and toughness of the steel plate are lowered, and the content of inclusions in the steel plate is increased. Accordingly, the content of Si is limited to a value which falls within a range from 0.05% to 1.00%. The content of Si is preferably limited to a value which falls within a range from 0.15% to 0.45%. [0026] Mn: 0.1% to 2. 0% Mn is an element having an action of enhancing hardenability. To ensure such an effect, the content of Mn is set to 0.1% or more. On the other hand, when the content of Mn exceeds 2.0%, temper embrittlement is occurred and weld heat-affected zone become hardened, weldability being lowered. Accordingly, the content of Mn is limited to a value which falls within a range from 0.1% to 2.0%. The content of Mn is preferably limited to a value which falls within a range from 0.4% to 1.7%. It is more preferable that the content of Mn is 13 limited to a value which falls within a range from 0. 5% to 1. 0%. [0027] P: 0.020% or less When the content of P in steel is large, lowering of low-temperature toughness of the steel plate is induced and hence, it is desirable that the content of P be as small as possible. In the present invention, the permissible content of P is 0.020%. The excessive reduction of the content of P induces the sharp rise in a refining cost. Accordingly, it is desirable to set the content of P to 0.005% or more. [0028] S: 0.005% or less When the content of S in steel is large, S is precipitated as MnS. In high strength steel, MnS becomes an initiation point of the occurrence of fracture and induces deterioration of toughness of the steel plate and hence, it is desirable that the content of S be as small as possible. In the present invention, the permissible content of S is 0. 005%. Accordingly, the content of S is limited to 0.005% or less. The excessive reduction of the content of S induces the sharp rise of a refining cost. Accordingly, it is desirable to set the content of S to 0.0005% or more. [0029] Al: 0.005% to 0.100% Al is an element acting as a deoxidizing agent for molten 14 steel. Further, Al contributes for the enhancement of low-temperature toughness due to refining of crystal grains. To acquire such an effect, the content of Al is set to 0.005% or more. When the content of Al is less than 0.005%, such an effect cannot be sufficiently acquired. On the other hand, when the content of Al exceeds 0.100%, weldability of the steel plate is lowered. Accordingly, the content of Al is limited to a value which falls within a range from 0.005% to 0.100%. The content of Al is preferably limited to a value which falls within a range from 0.015% to 0.050%. [0030] Cr: 0.03% to 2.0% Cr has an effect of increasing hardenability. Cr has also an effect of enhancing low-temperature toughness due to refining of a martensitic phase. Accordingly, in the present invention, Cr is an important element. Further, in a corrosive wear environment where a contact between a steel plate and earth and sand or the like in a wet state becomes a problem, Cr is dissolved as chromate ion due to an anodic reaction, and suppresses corrosion due to an inhibitor effect thus giving rise to an effect of enhancing corrosive wear resistance of the steel plate. To acquire such an effect, the content of Cr is set to 0.03% or more. When the content of Cr is less than 0.03%, the steel plate cannot exhibit such an effect sufficiently. On the other hand, when the content of Cr exceeds 2.0%, weldability 15 is lowered and a manufacturing cost is sharply increased. Accordingly, the content of Cr is limited to a value which falls within a range from 0.03% to 2.0%. The content of Cr is preferably limited to a value which falls within a range from 0.07% to 1.0%. It is more preferable that the content of Cr is limited to a value which falls within a range from 0.2% to 0.9%. [0031] Mo: 0.03% to 1.0% Mo has an effect of increasing hardenability. Mo has also an effect of enhancing low-temperature toughness due to refining of a martensitic phase. Accordingly, in the present invention, Mo is an important element. Further, in a corrosive wear environment where a contact between a steel plate and earth and sand or the like in a wet state becomes a problem, Mo is dissolved as molybdate ion due to an anodic reaction, and suppresses corrosion by an inhibitor effect thus giving rise to an effect of enhancing corrosive wear resistance. To acquire such an effect, the content of Mo is set to 0.03% or more. When the content of Mo is less than 0.03%, the steel plate cannot exhibit such an effect sufficiently. On the other hand, when the content of Mo exceeds 1.0%, weldability of the steel plate is lowered and a manufacturing cost is sharply increased. Accordingly, the content of Mo is limited to a value which falls within a range from 0.03% to 1.0%. The content of Mo is 16 preferably limited to a value which falls within a range from 0.10% to 0.50%. It is more preferable that the content of Mo is limited to a value which falls within a range from 0.20% to 0.40%. [0032] By containing Cr and Mo in a combined manner in the steel plate, it is expected that corrosive wear resistance can be enhanced remarkably. It is based on the estimation that Cr and Mo have different pH regions where Cr or Mo can exist as an oxygen acid and hence, corrosive wear caused by earth and sand or the like in a wet state having pH in a wide range can be suppressed. [0033] The above-mentioned components are the basic components of the steel. The abrasion resistant steel plate according to the present invention further may optionally contain, in addition to the above-mentioned basic components, as an optional element or optional elements, one or two or more kinds of components selected from a group consisting of 0. 005% to 0.1% Nb, 0.005% to 0.1% Ti, and 0.005% to 0.1% V, and/or one or two kinds of components selected from a group consisting of 0.005% to 0.2% Sn and 0.005% to 0.2% Sb, and/or one or two or more kinds of components selected from a group consisting of 0.03% to 1.0% Cu, 0.03% to 2.0% Ni, and 0.0003% to 0.0030% B, and/or one or two or more kinds of components selected from a group consisting of 0.0005% to 0.008% REM, 0.0005% to 0.005% Ca, and 0.0005% to 17 0.005% Mg. [0034] One or two or more kinds of components selected from a group consisting of 0.005% to 0.1% Nb, 0.005% to 0.1% Ti, and 0.005% to 0.1% V All of Nb, Ti and V are elements which precipitate as precipitates, and enhance toughness of steel through refining of the structure. The abrasion resistant steel plate according to the present invention, when necessary, contains one or two or more kinds of components selected from a group consisting of Nb, Ti and V. [0035] Nb is an element which precipitates as carbonitride and contributes to the enhancement of toughness through refining of the structure. The content of Nb may be set to 0.005% or more for obtaining such an effect. On the other hand, when the content of Nb exceeds 0.1%, weldability may be lowered. When the steel contains Nb, the content of Nb is preferably limited to a value which falls within a range from 0. 005% to 0.1%. The content of Nb is more preferably set to a value which falls within a range from 0.012% to 0.03% from a view point of refining of the structure. [0036] Ti is an element which precipitates as TiN and contributes to the enhancement of toughness through fixing solid solute N. 18 The content of Ti is set to 0.005% or more for acquiring such an effect. On the other hand, when the content of Ti exceeds 0.1%, coarse carbonitride precipitates so that toughness is lowered in some cases. When the steel contains Ti, the content of Ti is preferably limited to a value which falls within a range from 0.005% to 0.1%. The content of Ti is preferably limited to a value which falls within a range from 0.005% to 0.03% from a view point of the reduction of a manufacturing cost. {0037] V is an element which precipitates as carbonitride and contributes to the enhancement of toughness through an effect of refining the structure. The content of V is set to 0.005% or more for acquiring such an effect. On the other hand, when the content of V exceeds 0.1%, weldability is lowered in some cases. Accordingly, when the steel contains V, the content of V is preferably limited to a value which falls within a range from 0.005% to 0.1%. [0038] One or two kinds of components selected from a group consisting of 0.005% to 0.2% Sn and 0.005% to 0.2% Sb Both Sn and Sb are elements which enhance corrosive wear resistance. The abrasion resistant steel plate according to the present invention, when necessary, contains one or two kinds of elements selected from a group consisting of Sn and Sb. [0039] 19 Sn is dissolved as Sn ion due to an anodic reaction, and suppresses corrosion by an inhibiter effect thus enhancing corrosive wear resistance of a steel plate. Further, Sn forms an oxide film containing Sn on a surface of the steel plate and hence, an anodic reaction and a cathode reaction of the steel plate are suppressed whereby corrosive wear resistance of the steel plate is enhanced. The content of Sn is set to 0.005% or more for acquiring such an effect. On the other hand, when the content of Sn exceeds 0.2%, the deterioration of ductility and toughness of the steel plate may be induced. Accordingly, when the steel contains Sn, the content of Sn is preferably limited to a value which falls within a range from 0.005% to 0.2%. The content of Sn is more preferably set to a value which falls within a range from 0.005% to 0.1% from a view point of reducing tramp elements. [0040] Sb suppresses corrosion of a steel plate by suppressing an anodic reaction of the steel plate and also by suppressing a hydrogen generation reaction which is a cathode reaction thus enhancing corrosive wear resistance of the steel plate. The content of Sb is set to 0.005% or more for sufficiently acquiring such an effect. On the other hand, when the content of Sb exceeds 0.2%, the deterioration of toughness of the steel plate may be induced. Accordingly, when the steel contains Sb, the content of Sb is preferably set to a value which falls within 20 a range from 0.005% to 0.2%. It is more preferable that the content of Sb is set to a value which falls within a range from 0.005% to 0.1%. [0041] One or two or more kinds of components selected from a group consisting of 0.03% to 1.0% Cu, 0.03% to 2.0% Ni, and 0.0003% to 0.0030% B All of Cu, Ni and B are elements which enhance hardenability. The abrasion resistant steel plate according to the present invention, when necessary, may contain one or two or more kinds of elements selected from a group consisting of Cu, Ni and B. [0042] Cu is an element which contributes to the enhancement of hardenability. The content of Cu may be 0.03% or more for acquiring such an effect. On the other hand, when the content of Cu exceeds 1.0%, hot workability is lowered, and a manufacturing cost also sharply rises. Accordingly, when the steel contains Cu, the content of Cu is preferably limited to a value which falls within a range from 0.03% to 1.0%. The content of Cu is more preferably limited to a value which falls within a range from 0.03% to 0.5% from a view point of further reduction of a manufacturing cost. [0043] Ni is an element which contributes also to the enhancement 21 of hardenability and the enhancement of low-temperature toughness of the steel plate. The content of Ni may be 0.03% or more for acquiring such an effect. On the other hand, when the content of Ni exceeds 2.0%, a manufacturing cost may rise. When the steel contains Ni, the content of Ni is preferably limited to a value which falls within a range from 0.03% to 2. 0%. The content of Ni is more preferably limited to a value which falls within a range from 0.03% to 0.5% from a viewpoint of further reduction of a manufacturing cost. [0044] B is an element which contributes to the enhancement of hardenability with a small amount in steel. The content of B may be 0.0003% or more for acquiring such an effect. On the other hand, when the content of B exceeds 0.0030%, toughness of the steel plate may be lowered. Accordingly, when the steel contains B, the content of B is preferably limited to a value which falls within a range from 0.0003% to 0.0030%. The content of B more preferably falls within a range from 0.0003% to 0.0015% from a viewpoint of suppressing cold cracking at a welded part formed by low-heat input welding such as CO 2 welding or the like used in general in welding of an abrasion resistant steel plate. [0045] One or two or more kinds of components selected from a group consisting of 0.0005% to 0.008% REM, 0.0005% to 0.005% Ca, and 0.0005% to 0.005% Mg 22 All of REM, Ca and Mg are elements which form sulfide inclusions by combining with S and hence, these elements are elements which suppress the formation of MnS. The abrasion resistant steel plate according to the present invention, when necessary, contains one or two or more kinds of components selected from a group consisting of REM, Ca and Mg. [0046] REM fixes S thus suppressing the formation of MnS which causes lowering of toughness of the steel plate. The content of REM may be 0.0005% or more for acquiring such an effect. On the other hand, when the content of REM exceeds 0.008%, the contents of inclusions in the steel plate are increased so that toughness is lowered in some cases. When the steel contains REM, the content of REM is preferably limited to a value which falls within a range from 0.0005% to 0.008%. The content of REM is more preferably set to a value which falls within a range from 0.0005% to 0.0020%. [0047] Ca fixes S thus suppressing the formation of MnS which causes lowering of toughness. The content of Ca may be 0.0005% or more for acquiring such an effect. On the other hand, when the content of Ca exceeds 0.005%, the content of inclusions in the steel is increased and toughness may be lowered to the contrary. When the steel contains Ca, the content of Ca is preferably limited to a value which falls within a range from 23 0.0005% to 0.005%. The content of Ca is more preferably set to a value which falls within a range from 0.0005% to 0.0030%. [0048] Mg fixes S thus suppressing the formation of MnS which causes lowering of toughness of the steel plate. The content of Mg may preferably be 0.0005% or more for acquiring such an effect. On the other hand, when the content of Mg exceeds 0.005%, the content of inclusions in the steel plate is increased and toughness may be lowered to the contrary. When the steel contains Mg, the content of Mg is preferably limited to a value which falls within a range from 0.0005% to 0.005%. It is more preferable that the content of Mg is set to a value which falls within a range from 0.0005% to 0.0040%. [0049] The abrasion resistant steel plate according to the present invention has the above-mentioned components within the above-mentioned rages and in a state where DI* is satisfied 45 or more. DT* is defined by the following formula (1) . In the calculation for DI*, regarding the elements described in the formula (1), elements not contained in the steel are calculated as Zero. DI*=33.85x(0.lxC)O x(0.7xSi+l)x(3.33xMn+l)x(0.35xCu+l)x(o.3 6xNi+l)x(2.16xCr+1)x(3xMo+l)x(l.75xV+l)...... (1) (where, C, Si, Mn, Cu, Ni, Cr, Mo and V are the contents (mass%) of respective elements.) 24 When DI* is set to less than 45, a quenching depth from a surface of the steel plate becomes less than 10 mm and hence, a lifetime of the steel plate as the abrasion resistant steel plate is shortened. Accordingly, DI* is limited 45 or more. The range of DI* is preferably set to 75 or more. [0050] Remaining other than the above-mentioned compositions are Fe and unavoidable impurities as a balance. [0051] Next, the structure and the property of the abrasion resistant steel plate of the present invention are explained. [0052] The abrasion resistant steel plate according to the present invention has the above-mentioned composition and the structure wherein an as-quenched martensitic phase forms a main phase and a grain size of prior austenite (7) grains is 30 pm or less. Further, the abrasion resistant steel plate according to the present invention has surface hardness of 450 or more at Brinel hardness HBW 10/3000. Here, a phase which occupies 90% or more in an area ratio is defined as "main phase". [0053] As-quenched martensitic phase: 90% or more in area ratio When the phase fraction of the as-quenched martensitic phase is less than 90% in an area ratio, the steel plate cannot ensure desired hardness. Accordingly, when the area ratio is 25 less than 90%, wear resistance of the steel plate is lowered so that desired wear resistance cannot be ensured. Further, the steel plate cannot ensure the sufficient low-temperature toughness. Further, in tempered martensite phase, Cr and Mo form carbide together with Fe when cementite is formed in tempering. Due to the formation of carbide, solute Cr and solute Mo, which are effective to ensure corrosion resistance, are decreased. Accordingly, the martensitic phase is held in the as-quenched martensitic phase where the martensitic phase is not tempered. A phase fraction of the as-quenched martensitic phase is preferably set to 95% or more in area ratio, and it is more preferable that the phase fraction of the as-quenched martensitic phase is set to 98% or more in area ratio. [0054] Grain size of prior austenite (7) grains: 30 ptm or less Even when the phase fraction of the as-quenched martensitic phase can ensure the area ratio of 90% or more, when a grain size of prior austenite (y) grains becomes coarse exceeding 30 m, the low-temperature toughness of the steel plate is lowered. As the grain size of prior austenite (y) grains, values which are obtained in accordance with JIS G 0551 after microscopically observing the structure etched by a picric acid using an optical microscope (magnification: 400 times)are used. 26 [0055) The abrasion resistant steel plate according to the present invention having the above-mentioned composition and structure has surface hardness of 450 or more at Brinel hardness HBW 10/3000. [0056] Surface hardness: 450 or more at Brinel hardness HBW 10/3000 When the surface hardness of steel is less than 450 at Brinel hardness HBW 10/3000, the lifetime of the abrasion resistant steel plate becomes short. Accordingly, the surface hardness is set to 450 or more at Brinel hardness HBW 10/3000. Brinel hardness is measured in accordance with the stipulation described in JIS Z 2243. [0057] Next, the preferred method of manufacturing the abrasion resistant steel plate of the present invention is explained. [0058] The steel material having the above-mentioned composition is produced by casting and then subjected to hot rolling without cooling when the steel material holds a predetermined temperature or subjected to hot rolling after cooling and reheating, thus manufacturing a steel plate having a desired size and a desired shape. [0059] The method of manufacturing the steel material is not 27 particularly limited. It is desirable that molten steel having the above-mentioned composition is produced using a known refining method such as using a converter, and a steel material such as a slab having a predetermined size is manufactured by a known casting method such as a continuous casting method. It goes without saying that a steel material can be manufactured by an ingot casting-blooming method. [0060] Reheating temperature: 950 to 12504C When the reheating temperature is below 950 0 C, the deformation resistance becomes excessively high so that a rolling load becomes excessively large whereby hot rolling may not be performed. On the other hand, when the reheating temperature becomes high exceeding 1250*C, the crystal grains become excessively coarse so that steel may not ensure desired high toughness. Accordingly, the reheating temperature is preferably limited to a value which falls within a range from 950 to 1250*C. [0061] The reheated steel material or the steel material which holds a predetermined temperature without being reheated is, then, subjected to hot rolling so that a steel plate having a desired size and a desired shape is manufactured. The hot rolling condition is not particularly limited. After the hot rolling is finished, it is preferable that direct quenching 28 treatment where the steel plate is immediately quenched is applied to the steel plate. It is preferable that a quenching start temperature is set to a temperature not below an Ar3 transformation point. To set the quenching start temperature to the Ar3 transformation point or higher, it is preferable that the hot rolling finish temperature is set to 800'C or more not below the Ar3 transformation point. When the hot rolling finish temperature is excessively high, there may be a case where crystal grains become coarse. Accordingly, it is preferable that the hot rolling finish temperature is set to 950 0 C or below. A quenching cooling rate is not particularly limited provided that the quenching cooling rate is equal to or higher than a cooling rate at which a martensitic phase is formed. It is desirable that the quenching cooling rate is as high as possible to prevent a martensitic phase from being self-tempered. The solute Cr and the solute Mo, which are effective for corrosion resistance, form carbide along with Fe when cementite is formed in the self-tempering, so that the amount of solute Cr and solute Mo is reduced. The self-tempering also reduces a volume fraction of martensite. It is desirable that the quenching cooling rate is set to 65 to 75 0 C/s when a plate thickness is 5 to 15 mm, the quenching cooling rate is set to 40 to 55*C/s when the plate thickness is 16 to 22 mm, the quenching cooling rate is set to 30 to 40*C/s when the plate thickness is 22 to 28 mm, and the quenching cooling rate is set to 20 to 30 0 C/s when the plate 29 thickness is 29 to 35 mm. Further, it is preferable that the cooling stop temperature is set to 300 0 C or below. It is more preferable that the cooling stop temperature is 200'C or below. In this specification, "cooling rate" is a cooling rate obtained by calculating a temperature of a center portion of a steel plate by heat transfer-heat conduction calculation. [0062] After hot rolling is finished, in place of the direct quenching treatment where a steel plate is immediately quenched, treatment may be performed where the steel plate is gradually cooled by air after the hot rolling is finished (air cooling) and, thereafter, the steel plate is reheated to a predetermined heating temperature and, thereafter, the steel plate is quenched. It is desirable that the reheating temperature is set to a value which falls within a range from 850 to 950*C. A quenching cooling rate after reheating is not particularly limited provided that the quenching cooling rate after reheating is equal to or higher than a cooling rate at which a martensitic phase is formed. It is desirable that the quenching cooling rate is as high as possible to prevent a martensitic phase from being self-tempered. The solute Cr and the solute Mo, which are effective for corrosion resistance, form carbide along with Fe when cementite is formed in the self-tempering, so that the amount of solute Cr and solute Mo is reduced. The self-tempering also reduces a volume fraction 30 of martensite. It is desirable that the quenching cooling rate is set to 65 to 75"C/s when a plate thickness is 5 to 15 mm, the quenching cooling rate is set to 40 to 55 0 C/s when the plate thickness is 16 to 22 mm, the quenching cooling rate is set to 30 to 40 0 C/s when the plate thickness is 22 to 28 mm, and the quenching cooling rate is set to 20 to 30*C/s when the plate thickness is 29 to 35 mm. Further, to prevent a martensitic phase from being self-tempered, it is preferable that the cooling stop temperature is set to 300*C or below. It is more preferable that the cooling stop temperature is set to 200 0 C or below. [0063] To acquire the as-quenched martensite structure, tempering treatment is not performed after performing the above-mentioned treatment. [0064] Hereinafter, the present invention is further explained based on examples. Example [0065] Molten steel having the composition described in Table 1 was produced by a vacuum melting furnace, and was cast into a mold so that ingots (steel material) having a weight of 150 kgf respectively were manufactured. These steel materials were reheated at heating temperatures described in Tables 2 31 (Table 2-1, Table 2-2, and Table 2-3) and, thereafter, the steel materials were subjected to hot rolling under conditions described in Table 2. Then, with respect to some steel plates, direct quenching treatment (DQ) where quenching (direct quenching) is immediately performed after hot rolling is finished was performed under conditions described in Tables 2. With respect to other steel plates, reheating quenching treatment (RQ) where a steel plate is cooled by air after hot rolling is finished on the respective conditions described in Table 2 and the steel plate is reheated at a temperature described in Tables 2 and, thereafter, is quenched was performed. In the examples described in Table 2-3, cooling rates from 800*C to 500*C at DQ or RQ were also indicated. In general, with respect to an ordinary C-Mn steel, the transformation during cooling is started at a temperature of approximately 800*C and is completed at a temperature around 500*C. Therefore, a cooling rate from 800*C to 500*C largely influences the transformation behavior of steel. Accordingly, the cooling rate from 800 0 C to 500'C has been generally used as a representative cooling rate for estimating the transformation behavior of steel. [0066] Specimens were sampled from the manufactured steel plates, and the specimens were subject to an observation of the structure, a surface hardness test, a Charpy impact test, and 32 a corrosive wear resistance test. The following test methods were adopted. The results of the observation of the structure, the surface hardness test, the Charpy impact test, and the corrosive wear resistance test are shown in Table 3 (Table 3-1, Table 3-2, and Table 3-3). (1) Structure observation Specimens for structure observation were sampled from manufactured steel plates at a position of 1/2 plate thickness of the steel plate such that an observation surface becomes a cross section parallel to the rolling direction. The observation surface of the specimens for structure observation was polished and was etched by a picric acid thus exposing prior y grains. Thereafter, the observation surfaces were observed by an optical microscope (magnification: 400 times). Equivalent circle diameters of respective 100 views of prior y grains were measured, an arithmetic mean was calculated based on obtained equivalent circle diameters, and the arithmetic mean was set as the prior y grain size of the steel plate. [0067] Thin film specimens (specimens for observation of structure by transmission electron microscope) were sampled from the manufactured steel plates at a position of 1/4 plate thickness of the steel plate in the same way. Next, the thin film specimen was grinded and polished (mechanical polishing, electrolytic polishing) thus forming a thin film. Next, each 33 20 fields of vision of the thin film were observed by a transmission electron microscope (magnification: 20000 times), a region where cementite does not precipitate was recognized as a martensitic phase region, and the area of the region was measured. The area of the martensitic phase region was indicated by a ratio (%) with respect to the whole structure, and this ratio was set as a martensitic fraction (area ratio). Also, a kind of a phase where cementite precipitates was determined. [0068] (2) Surface hardness test Specimens for surface hardness measurement were sampled from the manufactured steel plates, and surface hardness HBW 10/3000 was measured in accordance with JIS Z 2243 (1998) . In the hardness measurement, a tungsten hard ball having a diameter of 10 mm was used, and a weight was set to 3000 kgf. [0069] (3) Charpy impact test V-notched specimens were sampled from manufactured steel plates at a position of 1/4 plate thickness of the steel plate, in the direction (C direction) perpendicular to the rolling direction, and a Charpy impact test was performed in accordance with the stipulation of JIS Z 2242(1998) . Absorbed energy vE- 40 (J) was obtained under the condition of a test temperature at -40'C. The number of specimens was three for each of the steel 34 plates, and an arithmetic mean of the obtained vales of three specimens is respectively set as the absorbed energy vE- 40 of the steel plate. The steel plate having the absorbed energy vE- 40 of 30 J or more was evaluated as the steel plate having excellent toughness. [0070] (4) corrosive wear resistance test Wear specimens (size: thickness of 10 mm, width of 25 mm and length of 75 mm) were sampled from manufactured steel plates at a position 1 mm away from a surface of the manufactured steel plate. These wear specimens were mounted on a wear tester, and a wear test was carried out. [0071] The wear specimen was mounted on the wear tester such that the wear specimen was perpendicular to an axis of rotation of a rotor of the tester and a surface of 25 mm x 75 mm was parallel to the circumferential tangential direction of a rotating circle, the specimen and the rotor were covered with an outer vessel, and a wear material was introduced into the inside of the outer vessel. As the wear material, a mixture is used where silica sand having an average grain size of 0. 65 mm and an NaCl aqueous solution which was prepared such that the concentration becomes 15000 mass ppm were mixed together such that a weight ratio between silica sand and the NaCl aqueous solution becomes 3:2. 35 [0072] Test conditions were set such that the rotor was rotated at 600 rpm and the outer vessel was rotated at 45 rpm. The test was finished at the revolutions of the rotor became 10800 times in total. After the test was finished, weights of the respective specimens were measured. The difference between the weight after test and the initial weight (=an amount of reduction of weight) was calculated, and a wear resistance ratio (= (reference value)/(amount of reduction of weight of specimen)) was calculated using an amount of reduction of weight of the steel plate SS400 stipulated in Rolled steels for general structure, Tensile strength 400 MPa class (JIS G3101) (conventional example) as a reference value. When the wear resistance ratio was 1.5 or more, the steel plate was evaluated as the steel plate "having excellent corrosive wear resistance". 36 0 0 0 0 0 0 0 0 0 0 0 C) C C C C C C C C C C) -4-15 -4- - - - - 4 eN C) C 3 C) w) to uj c C) C) C) W e e e e e e e e e e e e E C ) C ) ca . C ) C L C ) C) C2) C) C ) C ) E 0-0- 0- 0- 0 0 0- 0- 0 - 0- 0-0 6t 15 15 '6 'd 5 O 5 5 tt C C) C C) C C C) C) C C C 0-0- 0- - n. c- 0- 0 0- C cL 008 0 0 0 0 0 0 0 00 U 0 0 0 0 80 0 0 en Ce On e n en e en W en en Sd ~ a ca a a a a a a L Lm 0D C Lo U-) o U.) 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Eo to M to M LO c O P W - c 0 u)a:= a z a C _____ *** a z d E E 2 2 E E E E E E E E E Sm m o ca co w cu m M m fu w m m Xxxx x x x x x x x x x x CC C C C C C C C C C C C C C C C E.C )C C< C C )C )C )< < C )< C < >C )< C 0)CD a) 0(D0 W) Q)00 C) W0 0))) V a a = a = a = =c a a = a ac 05 0 0 0 00 0 0 0 0 0 000a00a coo =3o o o o to oto o o o o o a) ) Q) 0 00) 0 a)C ) 0 ) C C 0 )0 (D 0 4D W ( a o o o a a a a a a a ccw a 2 2 2 'D 2 2 2 2 2 2 2 2 '- 2 2 2 a. a. a. a a. a. a. a. a. a. a. a. a. a. a. a. C) C ) 0 ) C ) C)o C) C) C CL c) M C) 0 C C) CL U))Q 0) U ) U ) U) U ) U ) LO Ly L' LO 0. LO I- t l L m5 C14 T - 0 0 > - CO CO -r 00 osq 0 1 00)l 0)2 - r0 S. ou Ro o o o o o 0) 0 i2 o 0 0 8 o: Co-: g 0 caL O1 o O o o c EI ow o5 o o a) ooo ooooeoo oZ 0 0 o o o o 00) 0o 0 0 0 ow co co oo co o coo 0i o oEa CD 0 a a 0 0 a a a a o com o of co1 to I ontto 00).2 rr w o. o. o o r0. cL e 00co o L o 0)0)0) Co E o C\1 CCI CC ~C CoC <0) = CD (a Tl co mt)o M to (0 Cfl00 -t - O O O O O c OD 0 oo 0o 02 0)r0 L CD0 (0 o- oM N c CO ) 0) o0 to N o IC 0 0)e E6oz. 0 ) 0 0( co co to ( o co c0 to 0) co co ( z C [0077) [Table 3-1] Structure Surface Low-temperature Corrosive Wear Hardness Toughness Resistance Steel Wear Plate Steel Grain Size of Mart Resistance Remarks Number Prior renstic HBW vE4o Ratio Austenite Fract"/n 10/3000 ("C) (Reference: 1.0 Grains (pm) (area%) (conventional example)) 1 A 25 93 486 33 1.5 present invention example 2 A 27 92 491 32 1.5 present invention example 3 A 27 91 493 31 1.5 present invention example 4 A 28 85 432 30 1.3 comparison example 5 A 32'._ 83 430 17 1.2 comparison example 6 B 22 96 469 38 1.9 present invention example 7 B 25 93 468 34 1.9 present invention example 8 B 26 92 459 33 1.9 present invention example 9 B 36 92 466 17 1.9 comparison example 10 B 35 94 471 14 2.0 comparison example 11 C 16 97 465 39 1.9 present invention example 12 C 18 95 469 36 2.0 present invention example 13 C 19 93 472 34 2.1 present invention example 14 D 15 95 455 45 2.0 present invention example 15 D 12 96 460 46 2.1 present invention example 16 D 10 94 465 50 2.3 present invention example 17 E 15 95 470 45 2.0 present invention example 18 E 14 96 475 46 2.1 present invention example 19 F 12 94 490 52 2.4 present invention example 20 F 16 95 470 42 2.0 present invention example 21 F 13 95 489 46 2.1 present invention example 22 G 12 94 498 47 2.0 present invention example 23 G 18 94 470 46 2.0 present invention example 24 G 17 93 478 45 2.1 present invention example 25 G 15 95 498 48 2.1 present invention example 26 H 25 95 515 35 1.5 present invention example 27 H 27 93 519 33 1.5 present invention example 28 H 28 91 521 32 1.5 present invention example 29 I 22 96 493 33 1.6 present invention example 30 1 24 94 503 36 1.6 present invention example 31 1 25 92 505 32 1.6 present invention example 32. J 21 97 521 38 2.0 present invention example 33 J 17 95 534 40 2.1 present invention example 34 J 16 93 539 42 2.0 present invention example 35 K 23 96 465 36 2.1 present invention example 36 K 20 93 470 37 2.1 present invention example 37 K 24 92 481 34 2.1 present invention example 44 [00781 [Table 3-2] Structure Surface Low-temperature Corrosive Wear Hardness Toughness Resistance Steel Wear N ber NSteel Grain Size of Martensitic Resistance Remarks Plate Number Prior Frac HBW VEa4 Ratio Austenite Fraction 10/3000 (*C) (Reference: 1.0 Grains (Pm) (area%) (conventional example)) 38 L 12 97 557 49 2.4 present invention example 39 L 13 95 545 57 2.4 present invention example 40 L 13 93 550 52 24 present invention example 41 M 11 93 508 45 1.6 present invention example 42 M 12 94 512 42 1.6 present invention example 43 M 10 92 505 45 1.5 present invention example 44 N 13 99 490 73 2.5 present invention example 45 N 10 98 493 62 2.5 present invention example 46 N 8 97 488 66 2.5 present invention example 47 0 32 92 482 27 0.8 comparison example 48 0 34 91 491 25 0.8 comparison example 49 0 31 93 493 24 0.8 comparison example 50 P 38 95 531 17 0.9 comparison example 51 P 36 92 524 22 0.9 comparison example 52 P 32 93 519 24 0.9 comparison example 53 Q 33 94 521 28 1.2 comparison example 54 a 32 92 532 25 1.2 comparison example 55 Q 34 92 530 27 1.2 comparison example 56 R 15 96 413 51 1.4 comparison example 57 R 16 93 410 48 1.4 comparison example 58 R 16 91 409 44 1.4 comparison example 59 S 22 52 420 15 1.2 comparison example 60 S 21 55 425 20 1.2 comparison example 61 S 25 47 413 12 1.2 comparison example 62 T 27 94 507 34 1.6 present invention example 63 T 26 94 509 36 1.6 present invention example 64 T 25 93 506 37 1.6 present invention example 65 U 23 96 511 37 2.1 present invention example 66 U 26 95 510 35 2.1 present invention example 67 U 22 96 507 40 2.1 present invention example 68 V 20 97 520 40 2.4 present invention example 69 V 19 96 523 43 2.4 present invention example 70 V 21 97 519 38 2.5 present invention example 71 W 21 97 528 45 2.4 present invention example 72 W 17 97 531 48 2.4 present invention example 73 W 15 96 521 51 2.4 present invention example 4 5 [0079] [Table 3-3] Structure Surface Low-temperature Corrosive Wear Hardness Toughness Resistance Steel Wear Plate Steel Grain Size of Martensitic Resistance Remarks Number Number Prior Fraction W vE.4 Ratio Austenite (area 10/3000 (*C) (Reference: 1.0 Grains (gm) (conventional example)) 74 A 25 93 486 33 1.5 present invention example 75 A 27 94 493 34 1.5 present invention example 76 A 26 97 500 32 1.6 present invention example 77 A 25 98 501 31 1.7 present invention example 78 A 26 99 504 32 1.8 present invention example 79 A 27 92 491 32 1.5 present invention example 80 A 27 92 492 33 1.5 present invention example 81 A 26 96 498 34 1.6 present invention example 82 A 27 91 493 31 1.5 present invention example 83 A 26 95 496 30 1.7 present invention example 84 B 22 96 469 38 1.9 present invention example 85 B 21 98 473 39 2.0 present invention example 86 B 20 99 477 38 2.1 present invention example 87 B 25 93 468 34 1.9 present invention example 88 B 25 96 472 36 2.0 present invention example 89 B 26 92 459 33 1.9 present invention example 46 [00801] All of the present invention examples exhibit high surface 'hardness of 450 or more in HBW 10/3000, excellent low-temperature toughness of vE.

40 of 30 J or more, and excellent corrosive wear resistance of the wear resistance ratio of 1.5 or more. Moreover, the steel plate cooled with higher cooling rate has a higher martensitic fraction. Particularly, the steel plate having martensitic fraction of 98% or more exhibits excellent corrosive wear resistance in particular, as compared with the steel plate having martensitic fraction of less than 98% and having same composition. On the other hand, the comparative examples which fall outside the scope of the present invention exhibit lowering of surface hardness, lowering of low-temperature toughness, lowering of corrosive wear resistance or lowering of two or more of these properties. 47