Wear resistant steel plate and manufacturing process therefor

Abstract

A wear resistant steel plate that exhibits excellent impact wear resistant properties and that is suitable for use in construction machinery, shipbuilding, steel pipes or tubes, civil engineering, construction and so on, and a method for manufacturing the same. The wear resistant steel plate includes a specific steel composition, where DI* defined by Formula 1 is 100-250, and has a surface layer part containing 90% or more in area ratio of martensite, a Brinell hardness of 450 HBW 10/3000 or more, and a central part in thickness direction of the steel plate containing 70% or more in area ratio of lower bainite, the central part representing a zone extending from a ½ position of the steel plate thickness to distances of 0.5 mm toward both surfaces of the steel plate.
DI*=33.85×(0.1×C).sup.0.5×(0.7×Si+1)×(3.33×Mn+1)×(0.35×Cu+1)×(0.36×Ni+1)×(2.16×Cr+1)×(3×Mo+1)×(1.75×V+1)×(1.5×W+1)  Formula 1
where the symbols of elements represent the contents by mass % of the elements, respectively.

Claims

1. A wear resistant steel plate having a chemical composition comprising, by mass %: C: 0.25% to 0.33%; Si: 0.1% to 1.0%; Mn: 0.40% to 1.3%; P: 0.010% or less; S: 0.004% or less; Al: 0.06% or less; N: 0.007% or less; at least one selected from the group consisting of Cu: 1.5% or less, Ni: 2.0% or less, Cr: 3.0% or less, Mo: 1.5% or less, W: 1.5% or less, and B: 0.0030% or less; and the balance including Fe and incidental impurities, wherein DI* defined by the following Formula 1 is in a range of 100 to 250,
DI*=33.85×(0.1×C).sup.0.5×(0.7×Si+1)×(3.33×Mn+1)×(0.35×Cu+1)×(0.36×Ni+1)×(2.16×Cr+1)×(3×Mo+1)×(1.75×V+1)×(1.5×W+1)  Formula 1, where the symbols of elements represent the respective contents by mass % of the elements, a surface layer part of the steel plate includes 90% or more in area ratio of martensite, the surface layer part representing a zone extending up to a depth of 1 mm from a surface of the steel plate, the surface of the steel plate having a Brinell hardness of 450 HBW 10/3000 or more, and a central part of the steel plate in a thickness direction includes 70% or more in area ratio of lower bainite having an average grain size of 25 μm or less, the central part representing a zone extending from a ½ position of the steel plate in the thickness direction up to 0.5 mm toward both surfaces of the steel plate.

2. The wear resistant steel plate according to claim 1, wherein the chemical composition further comprises, by mass %, at least one selected from the group consisting of Nb: 0.005% to 0.025%, V: 0.01% to 0.1%, and Ti: 0.005% to 0.03%.

3. The wear resistant steel plate according to claim 1, wherein the chemical composition further comprises, by mass %, at least one selected from the group consisting of REM: 0.02% or less, Ca: 0.005% or less, and Mg: 0.005% or less.

4. A method for manufacturing a wear resistant steel plate, the method comprising: heating a slab having the chemical composition according to claim 1 to 1000° C. to 1200° C.; subjecting the slab to hot rolling to obtain a hot-rolled steel plate; air cooling the steel plate to room temperature; reheating the steel plate to a temperature in a range of Ac.sub.3 point to 950° C.; and then quenching the plate to obtain a central part of the steel plate in a thickness direction includes 70% or more in area ratio of lower bainite having an average grain size of 25 μm or less, the central part representing a zone extending from a ½ position of the steel plate in the thickness direction up to 0.5 mm toward both surfaces of the steel plate.

5. A method for manufacturing a wear resistant steel plate, the method comprising: heating a slab having the chemical composition according to claim 1 to 1000° C. to 1200° C.; subjecting the slab to hot rolling in a temperature range of Ar.sub.3 point or higher to obtain a hot-rolled steel plate; and then quenching the steel plate from a temperature in a range of Ar.sub.3 point to 950° C. to obtain a central part of the steel plate in a thickness direction includes 70% or more in area ratio of lower bainite having an average grain size of 25 μm or less, the central part representing a zone extending from a ½ position of the steel plate in the thickness direction up to 0.5 mm toward both surfaces of the steel plate.

6. The method for manufacturing a wear resistant steel plate according to claim 5, further comprising, after the quenching, reheating the steel plate to a temperature in a range of Ac.sub.3 point to 950° C. and subsequently quenching the steel plate.

7. The wear resistant steel plate according to claim 2 wherein the chemical composition further comprises, by mass %, at least one selected from the group consisting of REM: 0.02% or less, Ca: 0.005% or less, and Mg: 0.005% or less.

8. A method for manufacturing a wear resistant steel plate, the method comprising: heating a slab having the chemical composition according to claim 2 to 1000° C. to 1200° C.; subjecting the slab to hot rolling to obtain a hot-rolled steel plate; air cooling the steel plate to room temperature; reheating the steel plate to a temperature in a range of Ac.sub.3 point to 950° C.; and then quenching the steel plate to obtain a central part of the steel plate in a thickness direction includes 70% or more in area ratio of lower bainite having an average grain size of 25 μm or less, the central part representing a zone extending from a ½ position of the steel plate in the thickness direction up to 0.5 mm toward both surfaces of the steel plate.

9. A method for manufacturing a wear resistant steel plate, the method comprising: heating a slab having the chemical composition according to claim 3 to 1000° C. to 1200° C.; subjecting the slab to hot rolling to obtain a hot-rolled steel plate; air cooling the steel plate to room temperature; reheating the steel plate to a temperature in a range of Ac.sub.3 point to 950° C.; and then quenching the steel plate to obtain a central part of the steel plate in a thickness direction includes 70% or more in area ratio of lower bainite having an average grain size of 25 μm or less, the central part representing a zone extending from a ½ position of the steel plate in the thickness direction up to 0.5 mm toward both surfaces of the steel plate.

10. A method for manufacturing a wear resistant steel plate, the method comprising: heating a slab having the chemical composition according to claim 7 to 1000° C. to 1200° C.; subjecting the slab to hot rolling to obtain a hot-rolled steel plate; air cooling the steel plate to room temperature; reheating the steel plate to a temperature in a range of Ac.sub.3 point to 950° C.; and then quenching the steel plate to obtain a central part of the steel plate in a thickness direction includes 70% or more in area ratio of lower bainite having an average grain size of 25 μm or less, the central part representing a zone extending from a ½ position of the steel plate in the thickness direction up to 0.5 mm toward both surfaces of the steel plate.

11. A method for manufacturing a wear resistant steel plate, the method comprising: heating a slab having the chemical composition according to claim 2 to 1000° C. to 1200° C.; subjecting the slab to hot rolling in a temperature range of Ar.sub.3 point or higher to obtain a hot-rolled steel plate; and then quenching the steel plate from a temperature in a range of Ar.sub.3 point to 950° C. to obtain a central part of the steel plate in a thickness direction includes 70% or more in area ratio of lower bainite having an average grain size of 25 μm or less, the central part representing a zone extending from a ½ position of the steel plate in the thickness direction up to 0.5 mm toward both surfaces of the steel plate.

12. A method for manufacturing a wear resistant steel plate, the method comprising: heating a slab having the chemical composition according to claim 3 to 1000° C. to 1200° C.; subjecting the slab to hot rolling in a temperature range of Ar.sub.3 point or higher to obtain a hot-rolled steel plate; and then quenching the steel plate from a temperature in a range of Ar.sub.3 point to 950° C. to obtain a central part of the steel plate in a thickness direction includes 70% or more in area ratio of lower bainite having an average grain size of 25 μm or less, the central part representing a zone extending from a ½ position of the steel plate in the thickness direction up to 0.5 mm toward both surfaces of the steel plate.

13. A method for manufacturing a wear resistant steel plate, the method comprising: heating a slab having the chemical composition according to claim 7 to 1000° C. to 1200° C.; subjecting the slab to hot rolling in a temperature range of Ar.sub.3 point or higher to obtain a hot-rolled steel plate; and then quenching the steel plate from a temperature in a range of Ar.sub.3 point to 950° C. to obtain a central part of the steel plate in a thickness direction includes 70% or more in area ratio of lower bainite having an average grain size of 25 μm or less, the central part representing a zone extending from a ½ position of the steel plate in the thickness direction up to 0.5 mm toward both surfaces of the steel plate.

14. The method for manufacturing a wear resistant steel plate according to claim 11, further comprising, after the quenching, reheating the steel plate to a temperature in a range of Ac.sub.3 point to 950° C. and subsequently quenching the steel plate.

15. The method for manufacturing a wear resistant steel plate according to claim 12, further comprising, after the quenching, reheating the steel plate to a temperature in a range of Ac.sub.3 point to 950° C. and subsequently quenching the steel plate.

16. The method for manufacturing a wear resistant steel plate according to claim 13, further comprising, after the quenching, reheating the steel plate to a temperature in a range of Ac.sub.3 point to 950° C. and subsequently quenching the steel plate.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The present invention will be further described below with reference to the accompanying drawings, wherein:

(2) FIGS. 1A and 1B illustrate the positions from which impact wear test pieces are collected; and

(3) FIG. 2 illustrates an impact wear tester.

DESCRIPTION OF EMBODIMENTS

(4) The chemical composition and microstructure according to the present invention are defined below.

(5) [Chemical Composition]

(6) Hereinbelow, all contents are provided in mass % of the composition.

(7) C: 0.25% to 0.33%

(8) Carbon (C) is an element that is important for increasing hardness of martensite and increasing quench hardenability, so as to provide a predetermined microstructure in a central part in thickness direction of a steel plate, and to thereby guarantee excellent wear resistance. To obtain this effect, 0.25% or more of C needs to be contained in steel. On the other hand, if the content of C exceeds 0.33%, weldability worsens and, when exposed to repetitive load caused by impact, a white layer tends to form easily in a steel plate, which promotes wear due to exfoliation and/or cracking resulting in a deterioration in impact wear resistant properties. Therefore, the content of C is limited to 0.25% to 0.33%, and preferably 0.26% to 0.31%.

(9) Si: 0.1% to 1.0%

(10) Silicon (Si) is an element that acts as a deoxidizer, is necessary for steelmaking, and is effective for increasing hardness of a steel plate by solid solution strengthening when dissolved in steel. To obtain this effect, 0.1% or more of Si needs to be contained in steel. On the other hand, if the content of Si exceeds 1.0%, weldability and toughness significantly worsen. Therefore, the content of Si is limited to 0.1% to 1.0%, and preferably 0.2% to 0.8%.

(11) Mn: 0.40% to 1.3%

(12) Manganese (Mn) is an element that is effective for increasing quench hardenability of steel. To guarantee sufficient hardness of base steel, 0.40% or more of Mn needs to be contained in steel. On the other hand, if the content of Mn exceeds 1.3%, the toughness, ductility, and weldability of base steel worsen and any central segregation part becomes susceptible to grain boundary segregation of phosphorus, promoting the occurrence of a delayed fracture. Further, the amount and size of MnS which forms in a central part in thickness direction of a steel plate increase, so that stress concentrates near the MnS regions and a white layer forms more easily when a cross-sectional part of the steel plate is exposed to an impact wear environment, causing the impact wear properties to deteriorate. Therefore, the content of Mn is limited to 0.40% to 1.3%, and preferably 0.50% to 1.2%.

(13) P: 0.010% or Less

(14) Phosphorus (P) segregates at grain boundaries, serves as an origin from which a delayed fracture occurs, and lowers toughness when contained in steel in an amount of more than 0.010%. Therefore, the upper limit of P content is set to be 0.010%, and desirably, the P content is kept as small as possible. Note that the content of P is desirably set to 0.002% or more, since excessive reduction thereof can increase refining cost and be economically disadvantageous.

(15) S: 0.004% or Less

(16) Sulfur (S) is an element that deteriorates the low temperature toughness and ductility of base steel. Further, the amount and size of MnS which forms in a central part in thickness direction of a steel plate increase, so that stress concentrates near the MnS regions and a white layer forms more easily when a cross-sectional part of the steel plate is exposed to an impact wear environment, causing the impact wear properties to deteriorate. Therefore, the upper limit of S content is set to be 0.004%, and desirably, the S content is kept as small as possible.

(17) Al: 0.06% or Less

(18) Aluminum (Al) is an element that acts as a deoxidizer and is used most commonly in molten steel deoxidizing processes to obtain a steel plate.

(19) Al is also effective for suppressing coarsening of crystal grains by fixing solute N in steel in the form of AlN, and for mitigating deterioration of toughness and occurrence of a delayed fracture by virtue of reduced solute N. On the other hand, if the amount of Al exceeds 0.06%, the amount and size of AlN and Al.sub.2O.sub.3 which form in a central portion in thickness direction of a steel plate, so that stress concentrates near the AlN and Al.sub.2O.sub.3 regions and a white layer forms more easily when a cross-sectional part of the steel plate is exposed to an impact wear environment, causing the impact wear properties to deteriorate. Therefore, the content of Al is limited to 0.06% or less.

(20) N: 0.007% or Less

(21) Nitrogen (N) is an element that is contained in steel as an incidental impurity. If the content of N exceeds 0.007%, the amount and size of AlN which forms in a central part in thickness direction of a steel plate increase, so that stress concentrates near the AlN regions and a white layer forms more easily when a cross-sectional part of the steel plate is exposed to an impact wear environment, causing the impact wear properties to deteriorate. Therefore, the content of N is limited to 0.007% or less.

(22) At Least One of Cu, Ni, Cr, Mo, W, and B

(23) Copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), tungsten (W), and boron (B) are elements that all contribute to increased quench hardenability and increased hardness of steel, and may be contained in steel as appropriate for desired strength.

(24) When Cu is added to steel, the content of Cu is preferably 0.05% or more, but 1.5% or less because containing over 1.5% of Cu causes hot shortness in the steel plate, deteriorating the surface texture.

(25) When Ni is added to steel, the content of Ni is preferably 0.05% or more, but 2.0% or less because containing over 2.0% of Ni does not increase the effect, rather becomes economically disadvantageous.

(26) When Cr is added to steel, the content of Cr is preferably 0.05% or more, but 3.0% or less because containing over 3.0% of Cr deteriorates toughness and weldability.

(27) Mo is an element that significantly increases quench hardenability and is useful for increasing the hardness of base steel. To obtain this effect, the content of Mo is preferably 0.05% or more, but 1.5% or less because containing over 1.5% of Mo adversely affects the toughness, ductility, and weld cracking resistance of the base steel.

(28) W is an element that significantly increases quench hardenability and is useful for increasing the hardness of base material. To obtain this effect, the content of W is preferably 0.05% or more, but 1.5% or less because containing over 1.5% of W adversely affects the toughness, ductility, and weld cracking resistance of the base steel.

(29) B is an element that significantly increases quench hardenability with a very small amount of addition and is useful for increasing the hardness of base steel. To obtain this effect, the content of B is preferably 0.0003% or more, but 0.0030% or less because containing over 0.0030% of B adversely affects the toughness, ductility, and weld cracking resistance of the base steel.
DI*=33.85×(0.1×C).sup.0.5×(0.7×Si+1)×(3.33×Mn+1)×(0.35×Cu+1)×(0.36×Ni+1)×(2.16×Cr+1)×(3×Mo+1)×(1.75×V+1)×(1.5×W+1)=100 to 250
DI* is defined for the purpose of achieving excellent wear resistance by providing a microstructure such that a surface layer part of base steel contains 90% or more in area ratio of martensite and a central part in thickness direction contains 70% or more in area ratio of lower bainite. DI* is set to be 100 to 250. If DI* is less than 100, the quenching depth from a surface layer in thickness direction of a steel plate is reduced and a central part in thickness direction of the steel plate cannot have a desired microstructure, which results in a shorter lifetime of the wear resistant steel. On the other hand, if DI* exceeds 250, toughness and delayed fracture properties significantly worsen. Therefore, DI* is set in the range of 100 to 250, and preferably in the range of 120 to 230.

(30) The basic chemical composition of the present invention has been described, where the balance includes Fe and incidental impurities.

(31) In addition to the aforementioned basic chemical system, the present invention may contain at least one of Nb, V, Ti, REM, Ca, and Mg, in order to have even better properties.

(32) Nb: 0.005% to 0.025%

(33) Niobium (Nb) is an element that precipitates as a carbonitride, refines a microstructure, and fixes solute N, and that has the effect of improving toughness and the effect of suppressing delayed fracture. To obtain such effects, 0.005% or more of Nb needs to be contained in steel. On the other hand, if the content of Nb exceeds 0.025%, a coarse carbonitride precipitates and a white layer forms more easily, causing the impact wear resistant properties to deteriorate. Therefore, the content of Nb is limited to 0.005% to 0.025%.

(34) V: 0.01% to 0.1%

(35) V (vanadium) is an element that precipitates as a carbonitride, refines a microstructure, and fixes solute N, and that has the effect of improving toughness and the effect of suppressing delayed fracture. To obtain such effects, 0.01% or more of V needs to be contained in steel. On the other hand, if the content of V exceeds 0.1%, a coarse carbonitride precipitates and a white layer forms more easily, causing the impact wear resistant properties to deteriorate. Therefore, the content of V is limited to 0.01% to 0.1%.

(36) Ti: 0.005% to 0.03%

(37) Ti (titanium) is an element that is effective for suppressing coarsening of crystal grains by fixing solute N in the form of TiN, and for mitigating deterioration of toughness and occurrence of a delayed fracture by virtue of reduced solute N. To obtain such effects, 0.005% or more of Ti needs to be contained in steel. On the other hand, if the content of Ti exceeds 0.03%, a coarse carbonitride precipitates and a white layer forms more easily, causing the impact wear resistant properties to deteriorate. Therefore, the content of Ti is limited to 0.005% to 0.03%.

(38) REM (rare earth metal), calcium (Ca), and magnesium (Mg) are elements that all contribute to improving toughness and are selectively added to steel depending on desired properties.

(39) When REM is added, the content of REM is preferably 0.002% or more, yet the upper limit is set to be 0.02% since containing over 0.02% of REM does not increase the effect.

(40) When Ca is added, the content of Ca is preferably 0.0005% or more, yet the upper limit is set to be 0.005% since containing over 0.005% of REM does not increase the effect.

(41) When Mg is added, the content of Mg is preferably 0.001% or more, yet the upper limit is set to be 0.005% since containing over 0.005% of REM does not increase the effect.

(42) [Microstructure]

(43) To improve the impact wear resistant properties in a cross-sectional part, a steel plate according to the present invention has a microstructure in a central part in thickness direction thereof contains 70% or more in area ratio of lower bainite having an average grain size of 25 μm or less in equivalent circular diameter. The central part represents a zone extending from a ½ position of the steel plate thickness up to 0.5 mm toward both surfaces of the steel plate. In this case, an average grain size exceeding 25 μm in equivalent circular diameter deteriorates toughness and causes a delayed fracture. In addition, when martensite is formed in steel as a phase other than lower bainite, a white layer forms more easily and cracking happens via a non-metal inclusion and the like, causing the impact wear resistant properties to deteriorate. The effect is negligible, however, if the content of martensite is 10% or less. Moreover, in the presence of lower bainite, ferrite, pearlite or the like, hardness is reduced and impact wear resistant properties deteriorate. The effect is also negligible, however, if the content thereof is 20% or less.

(44) In addition, a surface layer part of the steel material contains 90% or more in area ratio of martensite phase, in terms of impact wear resistant properties. The surface layer part represents a zone extending up to a depth of 1 mm from a surface of the steel material. Excellent impact wear resistant properties may be obtained by guaranteeing the surface layer part containing 90% or more of martensite phase and the surface of the steel plate having a Brinell hardness of 450 HBW 10/3000 or more. Note that microstructure observation will be described later with reference to examples of the present invention.

(45) [Hardness of Surface Layer Part of Steel Plate]

(46) If a surface of a steel plate has a Brinell hardness of less than 450 HBW 10/3000, sufficient impact wear resistant properties cannot be obtained, which results in a shorter lifetime of the wear resistant steel. Therefore, the surface hardness is set to be 450 HBW 10/3000 or more in Brinell hardness.

(47) [Method for Manufacturing Wear Resistant Steel Plates]

(48) The wear resistant steel according to the present invention may be manufactured under the following conditions.

(49) As used herein, the temperatures presented below in “° C.” represent temperatures at the ½ position of the steel plate thickness.

(50) Firstly, a molten steel having the aforementioned composition is prepared by a well-known steelmaking process and subjected to, for example, continuous casting or ingot casting and blooming to obtain a semi-finished casting product such as a slab of a predetermined dimension.

(51) The resulting semi-finished casting product is reheated to 1000° C. to 1200° C. immediately after being casted without being cooled, or alternatively after being cooled, and then subjected to hot rolling to obtain a steel plate having a desired thickness. At a reheating temperature lower than 1000° C., deformation resistance becomes so high during hot rolling that a high rolling reduction ratio per pass cannot be achieved. This may result in an increased number of rolling passes and lower rolling efficiency, making it impossible to remove casting defects from a semi-finished casting product (slab) by pressure bonding. On the other hand, at a reheating temperature higher than 1200° C., scales form during heating and tend to cause surface defects, increasing work to remove surface defects after rolling. Therefore, the reheating temperature for the semi-finished casting product is set in the range of 1000° C. to 1200° C.

(52) The reheated semi-finished casting product is subjected to hot rolling until it reaches a desired thickness. Limitations are not particularly placed on the hot rolling conditions, as long as the desired thickness and shape are obtained. For ultra-thick steel plates having a thickness greater than 70 mm, however, it is desirable to carry out at least one rolling pass at a rolling reduction ratio of 15% or more per pass for removing porous shrinkage cavities by pressure bonding. The finisher delivery temperature is preferably equal to or higher than Ar.sub.3 point.

(53) When the finisher delivery temperature is lower than Ar.sub.3 point, deformation resistance and rolling load increase, thus, an increased burden is placed on the rolling mill, and a thick steel plate should be held on standby in the course of rolling before it can be cooled to a rolling temperature equal to or lower than Ar.sub.3 point. This significantly impairs productivity.

(54) The steel plate is air-cooled, reheated, and quenched after completion of hot rolling, or is alternatively subjected to direct quenching immediately after completion of hot rolling.

(55) When the steel plate is subjected to reheating and quenching after completion of rolling, it is reheated to and held for a certain period of time at a temperature from Ac.sub.3 point to 950° C. before quenching. If the heating temperature exceeds 950° C., the surface texture of the steel plate degrades and the crystal grains coarsen, causing the toughness and delayed fracture properties to deteriorate.

(56) Limitations are not particularly placed on the holding time, yet if it exceeds one hour, austenite grains coarsen and the toughness of base steel decreases, and therefore the holding time is desirably within one hour. A short holding time may suffice, given a good uniformity of temperature in a heat treatment furnace. For example, Ac.sub.3 point (° C.) can be derived by substituting the contents of the components of the steel material into the relation defined by:
Ac.sub.3=854−180C+44Si−14Mn−17.8Ni−1.7Cr
, where the symbols of elements represent the contents by mass % of the elements in the steel material, respectively.

(57) When the steel plate is subjected to direct quenching after completion of rolling, the semi-finished casting product is subjected to hot rolling at a temperature range of Ar.sub.3 point or higher, and after completion of the rolling, the steel plate is quenched from a temperature in the range of Ar.sub.3 point to 950° C.

(58) For example, Ar.sub.3 point (° C.) can be derived by substituting the contents of the components of the steel material into the relation defined by:
Ar.sub.3=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo
, where the symbols of elements represent the contents by mass % of the elements in the steel material, respectively.

(59) Quenching may be performed by injecting a high-pressure, high-speed water stream onto the surface of the steel plate, or by immersing the steel plate in water. In this case, the cooling rate at a ½ position of the steel plate thickness is set to be approximately 20° C./s for a steel plate thickness of 35 mm, approximately 10° C. is for a steel plate thickness of 50 mm, and approximately 3° C./s for a steel plate thickness of 70 mm. With these cooling rates, the central part in thickness direction of the steel plate may have a microstructure containing 70% or more in area ratio of lower bainite. Note that if quenching is conducted by water cooling for a steel plate thickness of 30 mm or less, the cooling rate becomes too high to provide the central part in thickness direction of the steel plate with a microstructure containing 70% or more in area ratio of lower bainite,

(60) After being subjected to direct quenching after hot rolling, the steel plate may further be subjected to a reheating and quenching process, by which it is reheated to a temperature from Ac.sub.3 point to 950° C. As a result, the microstructure of the steel plate is further homogenized and refined and the strength and toughness of base steel are improved.

EXAMPLES

(61) Examples of the present invention will be described below.

(62) Steel slabs were prepared by a process for refining with converter and ladle and continuous casting. The chemical compositions thereof are shown in Table 1. The steel slabs were heated to temperatures from 1000° C. to 1200° C. under the conditions shown in Table 2, and then subjected to hot rolling. Some of the steel plates were subjected to direct quenching (DQ) immediately after the rolling. Some of the steel plates subjected to direct quenching (DQ) were reheated to 900° C. and then subjected to quenching (RQ). Some of the steel plates that were subjected to hot rolling and cooling were reheated to 900° C. and then subjected to quenching (RQ).

(63) The steel plates thus obtained were subjected to microstructure observation, surface hardness measurement, base steel toughness measurement, and impact wear test as stated below.

(64) Test pieces were collected from the respective steel plates. Each test piece was subjected to microstructure observation under an optical microscope and a transmission electron microscope (TEM), at a ½ position of the steel plate thickness in thickness direction of the steel plate (t) in a cross section in the direction parallel to the rolling direction, to determine the microstructure proportion (proportion of lower bainite) and the average grain size of prior austenite grains (prior γ grains). Lower bainite transforms from austenite without long range diffusion and thus has the same grain size as prior austenite. In addition, lower bainite and martensite can be distinguished generally by using an optical microscope and precisely by using a transmission electron microscope (TEM) to determine the difference in the form of precipitation of cementite.

(65) Surface hardness measurement was made in accordance with JIS Z2243 (1998) to measure the surface hardness blow the surface layer. The measurement was performed under a load of 3000 kgf using tungsten hard balls with a diameter of 10 mm.

(66) V-notch test pieces were collected from steel plates at ¼ positions of the thickness of the steel plates in a direction orthogonal to the rolling direction, in accordance with JIS Z 2202 (1998). Then, the test pieces of the steel plates were subjected to Charpy impact test in accordance with JIS Z 2242 (1998), where three test pieces were used for each temperature, to determine absorption energy at 0° C. and evaluate the toughness of base steel. Those steel plates were determined to have good toughness of base steel if three test pieces thereof showed an average absorption energy (vE.sub.0) of 30 J or more.

(67) For impact wear test, test pieces of 10 mm×25 mm×75 mm were collected from steel plates, as shown in FIG. 1, from a surface layer part of each steel plate and from a ½ position of the steel plate thickness (t) in a cross section of the steel plate. A target steel and a SS400 steel test piece were fixed to the rotor of the impact wear tester shown in FIG. 2, 1500 cm.sup.3 of silica stones of 100% SiO.sub.2 (average grain size: 30 mm) were placed and sealed in the drum, and the drum was rotated under the conditions of rotor rotational speed of 600 rpm, drum rotational speed of 45 rpm, and total number of rotor rotations of 10000.

(68) The surface of each test piece after completion of the test was observed using a projector, and those steel plates without cracks of 3 mm long or more were determined to have good cracking resistance. In addition to this, measurement was also made to determine the changes in weight of each test piece before and after the test. The wear resistance ratio was determined by (weight reduction of SS400 test piece)/(weight reduction of target test piece). Those steel plates were determined to have good impact wear resistant properties if the wear resistance ratio of the surface layer part of the steel plate was 3.0 or more and the wear resistance ratio of a cross-sectional part of the steel plate at the ½ position of the steel plate thickness (t) was 2.5 or more.

(69) Table 3 shows the test results.

(70) It can be seen from Table 3 that in the examples of the present invention, the surface hardness is 450 HBW 10/3000 or more, the toughness of base steel at 0° C. is 30 J or more, no cracks formed during the impact wear test, and the wear resistant ratio with respect to the SS400 test piece is 3.0 or more in the surface layer part and 2.5 or more in the ½ t cross-sectional part thereof. In contrast, it was found that none of the comparative examples out of the scope of the present invention satisfy the desired performance, in terms of any one or more of surface hardness, toughness of base steel, and impact wear test results.

(71) TABLE-US-00001 TABLE 1 Slab Chemical Composition (mass %) No. C Si Mn P S Al Nb V Ti Cu Ni Cr Mo W 1 0.293 0.36 1.15 0.007 0.0008 0.048 1.37 2 0.276 0.22 0.63 0.009 0.0021 0.039 0.57 0.62 1.09 3 0.308 0.51 0.97 0.006 0.0012 0.055 1.47 0.36 4 0.258 0.40 1.06 0.005 0.0014 0.031 0.012 0.65 0.37 5 0.312 0.76 0.50 0.006 0.0029 0.022 0.022 1.52 0.21 0.64 6 0.288 0.36 0.88 0.003 0.0007 0.042 0.04 0.67 1.02 0.51 7 0.289 0.37 0.76 0.005 0.0010 0.031 0.021 0.04 0.014 1.12 0.13 8 0.267 0.54 0.90 0.008 0.0034 0.031 0.014 0.019 1.05 0.55 0.64 9 0.303 0.29 0.65 0.004 0.0021 0.034 0.07 0.012 0.84 0.59 10 0.238 0.36 0.72 0.007 0.0025 0.036 0.55 0.96 0.59 0.18 11 0.344 0.26 0.61 0.007 0.0019 0.021 0.06 0.011 0.73 0.51 12 0.286 0.38 1.56 0.009 0.0012 0.047 0.014 0.69 0.24 13 0.301 0.44 0.94 0.015 0.0030 0.024 0.017 0.013 0.71 0.48 14 0.294 0.22 0.89 0.008 0.0047 0.026 0.05 0.008 0.47 1.05 0.47 0.11 15 0.274 0.31 1.03 0.006 0.0017 0.068 0.018 0.51 0.49 16 0.284 0.25 0.62 0.009 0.0018 0.032 0.012 0.36 0.55 0.57 0.12 17 0.295 0.38 0.99 0.004 0.0025 0.033 0.021 0.05 1.07 0.46 Chemical Slab Composition (ppm) Ar.sub.3 Ac.sub.3 No. N B REM Ca Mg DI* (° C.) (° C.) Remarks 1 15 138.7 707 799 Inventive Example 2 20 12 126.0 641 794 Inventive Example 3 29 219.3 715 805 Inventive Example 4 27 22 159.9 706 809 Inventive Example 5 27 8 120.7 673 797 Inventive Example 6 20 14 129.2 640 788 Inventive Example 7 44 12 19 130.1 732 806 Inventive Example 8 38 83 180.0 689 797 Inventive Example 9 35 22 196.3 704 802 Inventive Example 10  26 10 124.8 692 799 Comparative Example 11  30 20 162.0 703 794 Comparative Example 12  38 23 50 152.1 686 796 Comparative Example 13  30 11 14 196.1 692 805 Comparative Example 14  18 10 20 130.1 643 780 Comparative Example 15  20 10 132.6 702 804 Comparative Example 16  27 70  84.1 717 794 Comparative Example 17  53 271.0 687 802 Comparative Example Note 1: Values underlined if outside the range of the present invention. Note 2: For chemical composition, contents of N, B, REM, Ca, and Mg are provided in ppm. Note 3: DI* = 33.85 × (0.1 × C).sup.0.5 × (0.7 × Si + 1) × (3.33 × Mn + 1) × (0.35 × Cu + 1) × (0.36 × Ni + 1) × (2.16 × Cr + 1) × (3 × Mn + 1) × (1.75 × V + 1) × (1.5 × W + 1)

(72) TABLE-US-00002 TABLE 2 Semi- finished Hot Rolling Casting Finisher Heat Treatment Steel Product Steel Plate Heating Delivery Heating Plate Slab Thickness Thickness Temp. Temp. Temp. Cooling ID No. (mm) (mm) (° C.) (° C.) Cooling Method (° C.) Method Remarks A 1 250 50 1150 850 air-cooling 900 water-cooling Inventive Example B 1 250 50 1150 850 water-cooling from 800° C. n/a — Inventive Example C 1 250 50 1150 850 water-cooling from 800° C. 900 water-cooling Inventive Example D 1 250 50 1150 850 air-cooling n/a — Comparative Example E 1 250 50 1150 850 water-cooling from 700° C. n/a — Comparative Example F 1 250 50 1150 850 air-cooling 770 water-cooling Comparative Example G 1 250 50 1150 850 air-cooling 1000  water-cooling Comparative Example H 2 200 35 1050 800 water-cooling from 750° C. n/a — Inventive Example I 3 300 120 1150 900 water-cooling from 870° C. 930 water-cooling Inventive Example J 4 250 75 1100 870 air-cooling 850 water-cooling Inventive Example K 5 200 40 1030 740 air-cooling 900 water-cooling Inventive Example L 6 250 50 1110 870 water-cooling from 850° C. n/a — Inventive Example M 7 300 60 1180 950 water-cooling from 900° C. 850 water-cooling Inventive Example N 8 300 80 1150 870 air-cooling 870 water-cooling Inventive Example O 9 300 100 1120 790 air-cooling 930 water-cooling Inventive Example P 10  250 50 1150 850 air-cooling 900 water-cooling Comparative Example Q 11  250 75 1100 870 air-cooling 850 water-cooling Comparative Example R 12  250 50 1110 870 water-cooling from 850° C. 850 water-cooling Comparative Example S 13  300 80 1150 870 air-cooling 870 water-cooling Comparative Example T 14  250 50 1150 850 air-cooling 900 water-cooling Comparative Example U 15  250 60 1150 850 air-cooling 930 water-cooling Comparative Example V 16  250 50 1180 900 water-cooling from 850° C. n/a — Comparative Example W 17  250 75 1100 870 air-cooling 900 water-cooling Comparative Example Note: Text and values underlined if outside the range of the present invention.

(73) TABLE-US-00003 TABLE 3 Microstructure in Central Part in Thickness Direction Microstructure in Area Ratio Surface Layer Part of Area Ratio Surface Toughness of Steel Average Lower of Hardness Base Steel Plate Slab Grain Size Bainite Martensite HBW vE.sub.0 ID No. Microstructure (μm) (%) Microstructure (%) 10/3000 (J) A 1 LB 12 100  M 100 531 39 B 1 LB 23 100  M 100 521 33 C 1 LB 10 100  M 100 530 44 D 1 F + P + UB 31  0 UB + F + P  0 324 11 E 1 F + LB + M 20 23 LB + F + P  85 415 8 F 1 F + LB + M  9 45 M + F  80 440 19 G 1 LB + M 34 84 M 100 535 10 H 2 LB + M 22 90 M 100 513 47 I 3 LB + UB 18 77 M 100 542 35 J 4 LB  8 100  M + LB  92 460 53 K 5 LB 11 100  M 100 556 34 L 6 LB + UB 23 92 M 100 504 37 M 7 LB  8 100  M 100 526 42 N 8 LB + UB 12 86 M + LB  95 481 67 O 9 LB + UB 16 79 M + LB  97 492 35 P 10  LB 13 100  M 100 439 70 Q 11  LB 11 100  M 100 612 20 R 12  LB + UB + M 13 66 M 100 527 33 S 13  LB + UB + M 15 64 M 100 524 10 T 14  LB + UB 11 88 M 100 530 9 U 15  LB + UB 14 93 M + LB  96 492 23 V 16  F + P + UB 27  0 UB + LB  0 426 8 W 17  M + LB 19 23 M 100 535 7 Impact Wear Test Wear Wear Resistance Resistance Steel Ratio of Ratio of Plate Crack in Surface Surface Layer Crack in Cross- Cross- ID Layer Part Part sectional Part sectional Part Remarks A no crack 3.8 no crack 3.4 Inventive Example B no crack 3.5 no crack 3.3 Inventive Example C no crack 3.9 no crack 3.5 Inventive Example D no crack 1.9 no crack 1.6 Comparative Example E no crack 2.0 no crack 1.7 Comparative Example F no crack 2.2 no crack 1.9 Comparative Example G crack observed 2.6 crack observed 1.9 Comparative Example H no crack 3.5 no crack 3.2 Inventive Example I no crack 3.8 no crack 3.4 Inventive Example J no crack 3.3 no crack 2.9 Inventive Example K no crack 4.1 no crack 3.6 Inventive Example L no crack 3.7 no crack 3.3 Inventive Example M no crack 3.5 no crack 3.1 Inventive Example N no crack 3.4 no crack 3.0 Inventive Example O no crack 3.3 no crack 2.8 Inventive Example P no crack 2.6 no crack 2.1 Comparative Example Q crack observed 2.7 no crack 2.2 Comparative Example R no crack 2.3 crack observed 1.4 Comparative Example S no crack 2.6 crack observed 1.5 Comparative Example T crack observed 2.5 crack observed 1.4 Comparative Example U no crack 2.4 no crack 1.5 Comparative Example V no crack 2.0 no crack 1.4 Comparative Example W crack observed 2.8 crack observed 1.6 Comparative Example Note 1: Text or values underlined if out of the scope of the present invention. Note 2: Abbreviations for microstructure phases: ferrite—F, pearlite—P, upper bainite—UB, lower bainite—LB, martensite—M.