JP7088235B2 - Wear-resistant steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a wear-resistant steel sheet having a plate thickness of less than 6.0 mm, which does not require heat treatment after rolling, is low-cost, and has excellent wear resistance, bendability, weldability, and toughness. Regarding the manufacturing method.

Parts of industrial machinery and transportation equipment used in construction, civil engineering, mining, etc., such as power shovels, bulldozers, hoppers, dump truck mounts, etc., wear over time due to earth and sand with a moth hardness of 100 or more during use. Therefore, wear-resistant steel plates are used.

It is known that the wear resistance of a steel sheet can be improved by increasing the hardness. Therefore, thick high-hardness steel sheets having improved quenching characteristics by adding a large amount of alloying elements such as Cr and Mo have been widely used as wear-resistant steel sheets.

For example, in Patent Document 1, a material containing 0.10 to 0.19% of C, further containing appropriate amounts of Si and Mn, and having an equivalent carbon content Ceq of 0.35 to 0.44 is hot. The surface hardness is 300 Hv (Vickers hardness) by rolling, directly from the hot rolling, or by cooling and reheating to 900 to 950 ° C, then quenching, and then tempering at 300 to 500 ° C. ) The above-mentioned method for manufacturing a wear-resistant steel sheet has been proposed.

Patent Document 2 contains 0.07 to 0.17% of C, further contains an appropriate amount of Si, Mn, V, B, Al, or is further one of Cu, Ni, Cr, and Mo. A method for producing a wear-resistant steel sheet having a surface hardness of 321 HB or more has been proposed by hot-rolling a material containing the above, directly from the hot-rolling, or by quenching after air-cooling and reheating. ing. The techniques disclosed in Patent Documents 1 to 3 improve wear resistance by adding a large amount of alloying elements, utilizing phenomena such as solid solution hardening, transformation hardening, and precipitation hardening to increase hardness. ..

On the other hand, Patent Document 3 contains 0.10 to 0.45% of C, 0.10 to 1.0% of Ti, and further contains appropriate amounts of Si, Mn, P, S, N, and Al. Or, by continuously casting molten steel containing one or more of Cu, Ni, Cr, Mo, and B, 400 deposits mainly composed of TiC having a size of 0.5 μm or more are produced per 1 mm ² . The wear-resistant steel deposited as described above has been proposed. The technique disclosed in Patent Document 3 produces coarse precipitates mainly composed of TiC having high hardness during solidification of continuous casting, and the precipitates improve wear resistance. There is no need to increase the hardness of the matrix. Therefore, it is possible to obtain wear-resistant steel having excellent workability and weldability.

Japanese Unexamined Patent Publication No. 62-142726 Japanese Unexamined Patent Publication No. 1-142023 Japanese Unexamined Patent Publication No. 6-256896

However, since Patent Documents 1 and 2 contain a large amount of alloying elements, the hardness of the steel material increases due to solid solution or precipitation of these elements or changes in transformation behavior, and the weldability and workability are remarkably improved. There is a problem of deterioration. Further, in Patent Document 3, since a large amount of Ti is contained, there is a problem that the toughness is deteriorated by the coarse precipitate.

In recent years, environmental regulations have become stricter, and there is a demand for lighter weight and longer life for dump trucks and the like, and demand for thin wear-resistant steel sheets with a thickness of less than 6 mm is increasing. Further, it is required to improve weldability such as relaxation of preheating when processing parts.

However, the wear-resistant steel sheets of Patent Documents 1 to 3 are all suitable manufacturing methods for steel sheets having a plate thickness of 6 mm or more, which are produced by thick plate rolling and quenching treatment. Therefore, when the product plate thickness is thinner than 6 mm, the temperature drops significantly during hot rolling, and there is a problem that a steel plate having a plate thickness of 4 mm or less cannot be manufactured. Another problem is that the quenching process causes cooling strain in the steel sheet, making it difficult to secure the shape (flatness).

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a wear-resistant steel sheet having a thickness of less than 6 mm and a method for manufacturing the same, which are excellent in wear resistance, bending workability and weldability. do.

The present inventors have made extensive studies on wear-resistant steel sheets having a thickness of less than 6 mm and methods for manufacturing the same. As a result, the following findings were obtained.
(1) Abrasion resistance is achieved by using martensite as the base phase of the base metal and setting the ratio (dZ / dL) of the rolling direction grain size dZ to the plate thickness direction grain size dL of the old austenite grains to 8.0 or more. Both bending workability can be achieved.
(2) In order to obtain the steel structure of (1), Nb is added to expand the austenite unrecrystallized temperature range, and in the hot-rolled steel strip process, finish rolling with a reduction ratio of 4.0 or more is performed in the unrecrystallized region. Then, it is accelerated and cooled using a runout cooling facility and wound on a coil. Next, by performing temper rolling of this coil by applying a high rolling load and forward-backward tension, a wear-resistant steel sheet having a plate thickness of less than 6 mm can be manufactured.

The present invention has been made based on such findings, and the gist thereof is as follows.
[1] In terms of mass%, C: 0.18 to 0.22%, Si: 0.01 to 0.45%, Mn: 0.30 to 2.00%, P: 0.020% or less, S: 0.008% or less, Al: 0.010 to 0.100%, Nb: 0.010 to 0.030%, Ti: 0.005 to 0.025%, B: 0.0004 to 0.0020% It contains, and the Ceq value represented by the formula (1) is 0.48% or less, and has a component composition consisting of the balance Fe and unavoidable impurities.
It has a structure in which the area ratio of martensite is 90% or more, the ratio (dZ / dL) of the grain size dZ in the rolling direction to the grain size dL in the plate thickness direction of the old austenite grain is 8.0 or more, and the Brinell hardness. A wear-resistant steel sheet having a hardness of 400 HBW5 / 750 or more.
Ceq = C + Mn / 6 + (Cu + Ni) / 15+ (Cr + Mo + V) / 5 ... (1)
In the above formula (1), the element symbol means the content (mass%) of each element. However, the element not contained is 0%.
[2] The composition of the components is Cu: 0.05 to 0.50%, Ni: 0.05 to 0.50%, Cr: 0.05 to 1.00%, Mo: 0.05 in mass%. The wear-resistant steel sheet according to [1], further containing one or more selected from ~ 0.50% and V: 0.005 to 0.080%.
[3] The component composition further contains one or two selected from Ca: 0.0005 to 0.0020% and REM: 0.0005 to 0.0020% in mass% [1]. Or the wear-resistant steel sheet according to [2].
[4] A steel slab having the composition according to any one of [1] to [3] is heated to 1180 to 1300 ° C., and the heated steel slab is subjected to a rolling ratio (reduction ratio) at a finishing rolling temperature of 980 to 850 ° C. Finish rolling with a finish rolling machine entry side thickness / finish rolling mill exit side thickness of 4.0 or more is performed, cooling is started from a temperature above the Ar ₃ transformation point, and the average cooling rate between 800 and 300 ° C. is 50. A method for manufacturing a wear-resistant steel sheet, which is accelerated and cooled under conditions of ° C./s or higher, wound up at a winding temperature of 250 ° C. or lower, and tempered and rolled.

According to the present invention, it is possible to manufacture a wear-resistant steel sheet having a plate thickness of less than 6.0 mm, which is excellent in wear resistance, bending workability, low temperature toughness and weldability.

The plate thickness is preferably 1.0 mm or more. Further, in the present invention, a steel sheet having a Brinell hardness of 400 HBW5 / 750 or more is considered to have excellent wear resistance.

Hereinafter, the present invention will be described in detail. The present invention is not limited to this embodiment.

[Ingredient composition]
First, the composition of the wear-resistant steel sheet of the present invention will be described. Unless otherwise specified, "%" representing the content of each component means "mass%".

C: 0.18 to 0.22%
C is an element useful for increasing the hardness of steel and ensuring wear resistance. In order to secure the desired surface hardness, 0.18% or more is required. On the other hand, excessive addition deteriorates weldability and low temperature toughness, so the content should be 0.22% or less. It is preferably 0.20% or less.

Si: 0.01-0.45%
Si acts as a deoxidizing agent and dissolves in steel to increase the strength of the base metal. To obtain the effect, 0.01% or more is required. It is preferably 0.10% or more. Further, if the content exceeds 0.45%, it causes scale defects and deteriorates the surface quality of the base material. Therefore, the Si content is set to 0.45% or less. It is preferably 0.40% or less.

Mn: 0.30 to 2.00%
Mn is an element that dissolves in solid solution and has the effect of increasing the hardness of the base metal. In addition, since it is inexpensive, it is also effective in minimizing the content of other expensive alloying elements. In the present invention, the content of 0.30% or more is required to secure the desired hardness. It is preferably 0.60% or more. On the other hand, if the content exceeds 2.00%, the toughness of the base metal and the delayed fracture characteristics are significantly reduced, so the content is set to 2.00% or less. It is preferably 1.60% or less.

P: 0.020% or less P is an impurity element and deteriorates the toughness of the base metal. If the content exceeds 0.020%, the above-mentioned adverse effects become remarkable, so the amount of P is set to 0.020% or less. It is preferably 0.015% or less. On the other hand, since it is desirable to make the P content as low as possible, the lower limit of the P content is not particularly limited, but since P is usually an element unavoidably contained in steel as an impurity, it is industrially industrialized. It may be more than 0%. Further, since excessive reduction causes an increase in refining cost, the P content is preferably 0.005% or more.

S: 0.008% or less S is an impurity element and exists as a sulfide-based inclusion such as MnS in steel, which deteriorates the toughness of the base metal and the welded portion and at the center of the plate thickness of the steel slab. It is unevenly distributed in large quantities and tends to cause defects. Such a tendency becomes remarkable when the content exceeds 0.008%. Therefore, the amount of S is set to 0.008% or less. It is preferably 0.005% or less. On the other hand, since it is desirable to make the S content as low as possible, the lower limit of the S content is not particularly limited, but since S is usually an element unavoidably contained in steel as an impurity, it is industrially industrialized. It may be more than 0%. Further, since excessive reduction causes an increase in refining cost, the S content is preferably 0.001% or more.

Al: 0.010 to 0.100%
Al is an element that acts as a deoxidizing agent, and is most commonly used as a deoxidizing agent in the molten steel deoxidizing process of steel. In order to obtain such an effect, 0.010% or more is required. On the other hand, if the content exceeds 0.100%, oxide-based inclusions increase and the toughness of the base metal decreases. It is preferably 0.060% or less.

Nb: 0.010 to 0.030%
Nb is an important element having the effect of expanding the unrecrystallized region, expanding the austenite grains in the hot rolling process, and improving the bending workability. In addition, Nb is finely deposited as an Nb-based carbide to make the base metal structure finer, so that it also has an effect of improving toughness. In order to obtain these effects, the content of 0.010% or more is required. On the other hand, if the content exceeds 0.030%, coarse Nb-based carbides increase and the toughness of the base metal deteriorates. Therefore, it should be 0.030% or less. It is preferably 0.025% or less.

Ti: 0.005 to 0.025%
Ti is an element that has a strong tendency to form a nitride and has an action of fixing N and reducing solid solution N. Therefore, the toughness of the base metal and the welded portion can be improved by containing Ti. Further, by containing both Ti and B, the precipitation of BN is suppressed by fixing N to Ti, and as a result, the hardenability improving effect of B is promoted. In order to obtain these effects, Ti is set to 0.005% or more. It is preferably 0.008% or more. On the other hand, when Ti exceeds 0.025%, a large amount of TiC is deposited and the toughness of the base metal is lowered. Therefore, Ti is set to 0.025% or less. It is preferably 0.015% or less.

B: 0.0004 to 0.0020%
B is an element having an action of remarkably improving hardenability even if it is contained in a small amount. Therefore, the inclusion of B promotes the formation of martensite, and the desired wear resistance can be obtained. In order to obtain such an effect, B is set to 0.0004% or more. On the other hand, if the content exceeds 0.0020%, the toughness of the base metal deteriorates, such as segregation at grain boundaries and the like, forming inclusions such as boride and serving as a starting point of fracture. Therefore, it is set to 0.0020% or less. It is preferably 0.0015% or less.

Further, the present invention needs to have a Ceq (%) value represented by the following equation (1) of 0.48% or less.
Ceq = C + Mn / 6 + (Cu + Ni) / 15+ (Cr + Mo + V) / 5 ... (1)
In the above formula (1), the element symbol means the content (mass%) of each element. However, the element not contained is 0%.

Ceq (carbon equivalent) is an index showing weldability. If it exceeds 0.48%, preheating is required before welding the gas metal arc with the solid wire, welding cracks occur, and the weldability is adversely affected. Therefore, the Ceq value is set to 0.48% or less. It is preferably 0.45% or less. There is no particular lower limit of the Ceq value, but if it is less than 0.40%, the desired hardness cannot be secured, so it is preferably 0.40% or more.

The above is the basic chemical composition of the present invention. In the present invention, one or more selected from Cu, Ni, Cr, Mo, and V may be further contained.

Cu: 0.05 to 0.50%
Cu is an element having an action of improving hardenability, and can be arbitrarily added in order to improve the hardness of the base metal. When Cu is added, the Cu content is set to 0.05% or more in order to obtain such an effect. On the other hand, if the Cu content exceeds 0.50%, the alloy cost will increase. Therefore, when Cu is added, the Cu content is set to 0.50% or less.

Ni: 0.05-0.50%
Like Cu, Ni is an element having an action of improving hardenability, and can be arbitrarily added in order to improve the hardness of the base metal. When Ni is added, the Ni content is set to 0.05% or more in order to obtain such an effect. On the other hand, if the Ni content exceeds 0.50%, the alloy cost will increase. Therefore, when Ni is added, the Ni content is set to 0.50% or less.

Cr: 0.05 to 1.00%
Cr is an element having an action of improving hardenability like Cu, and can be arbitrarily added in order to improve the hardness of the base metal. When Cr is added, the Cr content is set to 0.05% or more in order to obtain such an effect. On the other hand, if the Cr content exceeds 1.00%, the alloy cost will increase. Therefore, when Cr is added, the Cr content is set to 1.00% or less.

Mo: 0.05 to 0.50%
Mo is an element having an action of improving hardenability like Cu, and can be arbitrarily added in order to improve the hardness of the base metal. When Mo is added, the Mo content is set to 0.05% or more in order to obtain such an effect. On the other hand, if the Mo content exceeds 0.50%, the alloy cost will increase. Therefore, when Mo is added, the Mo content is set to 0.50% or less.

V: 0.005 to 0.080%
V is an element having an action of improving hardenability like Cu, and can be arbitrarily added in order to improve the hardness of the base metal. When V is added, the V content is set to 0.005% or more in order to obtain such an effect. On the other hand, if the V content exceeds 0.080%, the alloy cost will increase. Therefore, when V is added, the V content is set to 0.080% or less.

Further, in the present invention, one or two selected from Ca and REM may be contained.

Ca: 0.0005-0.0020%
Ca is an element that binds to S and has an effect of suppressing the formation of MnS or the like that extends long in the rolling direction. Therefore, by adding Ca, the morphology of the sulfide-based inclusions can be controlled so as to be spherical, and the toughness of the base metal can be improved. When Ca is added in order to obtain such an effect, the Ca content is set to 0.0005% or more. On the other hand, when the Ca content exceeds 0.0020%, the toughness of the base metal deteriorates due to the increase in inclusions. Therefore, when Ca is added, the Ca content is 0.0020% or less.

REM: 0.0005-0.0020%
Like Ca, REM (rare earth metal) is an element that binds to S and has an effect of suppressing the formation of MnS or the like that extends long in the rolling direction. Therefore, by adding Ca, the morphology of the sulfide-based inclusions can be controlled so as to be spherical, and the toughness of the base metal can be improved. When REM is added in order to obtain such an effect, the REM content is set to 0.0005% or more. On the other hand, when the REM content exceeds 0.0020%, the toughness of the base metal deteriorates due to the increase in inclusions. Therefore, when REM is added, the REM content is 0.0020% or less.

The rest other than the above consists of Fe and unavoidable impurities.

[Organization]
Next, the structure of the wear-resistant steel sheet of the present invention will be described. In the wear-resistant steel sheet of the present invention, the area ratio of martensite is 90% or more, and the ratio (dZ / dL) of the grain size dZ in the rolling direction to the grain size dL in the thickness direction of the old austenite grains is 8.0 or more. Has tissue. The surface integral of the structure in the present invention refers to the value at the 1/2 position (1 / 2t position) of the plate thickness t of the steel plate. The structure is measured by etching a cross section parallel to the rolling direction of the steel sheet and observing the 1 / 2t position. More specifically, the area ratio and dZ / dL can be obtained by the method described in the examples.

Area ratio of martensite: 90% or more In the present invention, the area ratio of martensite is 90% or more. When the area ratio of martensite is less than 90%, the hardness of the base structure of the steel sheet is lowered, so that the wear resistance is deteriorated. In addition, the low toughness of the upper bainite structure increases and the toughness deteriorates. Therefore, the area ratio of martensite is set to 90% or more.

The residual structure other than martensite is not particularly limited, but one or more other structures such as ferrite, pearlite, austenite, and bainite may be present. On the other hand, the higher the area ratio of martensite, the better, so the upper limit is not particularly limited and may be 100%.

Ratio of rolling direction grain size dZ to plate thickness direction grain size dL of old austenite grains (dZ / dL): 8.0 or more In order to improve bending workability, rolling of old austenite grains with respect to plate thickness direction grain size dL The ratio (dZ / dL) of the directional particle size dZ is 8.0 or more. By setting dZ / dL, the bending workability in the direction perpendicular to the rolling direction is particularly improved. The grain size in the plate thickness direction referred to here is the ferret diameter in the plate thickness direction, and the grain size in the rolling direction indicates the ferret diameter in the rolling direction. That is, the grain size in the rolling direction is 8.0 times or more the grain size in the plate thickness direction.

[surface hardness]
Brinell hardness: 400HBW5 / 750 or more The wear-resistant steel sheet of the present invention has a Brinell hardness of 400HBW5 / 750 or more. The wear resistance of the steel sheet can be improved by increasing the hardness of the surface layer portion of the steel sheet. In the present invention, if the hardness is Brinell hardness and less than 400 HBW, sufficient wear resistance cannot be obtained. Therefore, the hardness is Brinell hardness of 400 HBW or more. It is preferably 425 HBW or more. On the other hand, in consideration of bending workability, the upper limit is preferably 490 HBW or less. More preferably, it is 475 HBW or less. The hardness in the present invention refers to the Brinell hardness at a depth of 0.5 mm from the surface of the wear-resistant steel sheet. The Brinell hardness is a value (HBW5 / 750) measured with a load of 750 kgf using a tungsten hard ball having a diameter of 5.0 mm. More specifically, it can be measured by the method described in the examples.

[Production method]
Next, a method for manufacturing the wear-resistant steel sheet of the present invention will be described. The wear-resistant steel sheet of the present invention can be manufactured by heating a steel slab having the above-mentioned composition, hot rolling, accelerating cooling, winding on a coil, and tempering rolling. Unless otherwise specified, each temperature is the temperature at the center of the plate thickness.

Steel material (steel slab)
The method for producing the steel material is not particularly limited, and for example, molten steel having the above composition can be melted and cast by a conventional method. The melting can be carried out by any method such as a converter, an electric furnace, and an induction furnace. Further, the casting is preferably performed by the continuous casting method from the viewpoint of productivity, but it can also be performed by the ingot-decomposition rolling method. As the steel material, for example, a steel slab can be used.

Heating to 1180 to 1300 ° C. The obtained steel slab is heated to a heating temperature prior to hot rolling. The heating may be performed after the steel material obtained by a method such as casting is once cooled, or the obtained steel material may be directly subjected to heating without cooling. The heating temperature is 1180 to 1300 ° C. In order to obtain the effect of expanding the unrecrystallized region by Nb, it is necessary to dissolve Nb in solid solution before rolling. For that purpose, the heating temperature is set to 1180 ° C. or higher. When the heating temperature is less than 1180 ° C., not only the effect of expanding the unrecrystallized region by Nb cannot be obtained, but also the Nb-based precipitate becomes coarse and the toughness of the base metal deteriorates. On the other hand, when the heating temperature exceeds 1300 ° C., the oxidation of the steel slab becomes remarkable, and not only the loss due to the oxidation increases, but also surface defects due to the oxidation scale occur. Therefore, the temperature is set to the range of 1180 to 1300 ° C.

Hot Rolling Next, the heated steel slab is hot-rolled into a hot-rolled steel sheet.

Finish rolling temperature: 980-850 ° C
The finish rolling temperature is set to 980 ° C. or lower in order to make the ratio (dZ / dL) of the grain size in the rolling direction to the grain size in the plate thickness direction of the old austenite grains 8.0 or more. If the finish rolling temperature exceeds 980 ° C., dZ / dL cannot be 8.0 or more, and the desired bending workability cannot be obtained. Further, in order to start accelerated cooling from a temperature range above the Ar ₃ transformation point during cooling in the next step, the finish rolling temperature is set to 850 ° C. or higher.

Finish rolling reduction ratio (thickness on the entry side of the finish rolling mill / thickness on the exit side of the finish rolling mill): 4.0 or more In order to make dZ / dL 8.0 or more, the rolling reduction ratio in the finish rolling mill is 4.0 or more. Rolling is carried out. When the rolling reduction ratio of finish rolling is less than 4.0, dZ / dL cannot be 8.0 or more, and the desired bending workability cannot be obtained.

Accelerated cooling and winding Then, the obtained hot-rolled steel sheet is accelerated-cooled and wound around a coil. This accelerated cooling process is a runout cooling process in which the steel sheet is accelerated and cooled on a runout table from the end of rolling to winding in the hot-rolled steel strip manufacturing process in order to avoid shortening the manufacturing period and soaring manufacturing costs. It is preferable to do so.

Cooling start temperature: Ar ₃ transformation point or higher In order to start cooling before ferrite is formed, the cooling start temperature is set to Ar ₃ transformation point or higher. The Ar ₃ transformation point can be obtained, for example, by the following equation (2).
Ar ₃ (° C.) = 910-273 x C-74 x Mn-57 x Ni-16 x Cr-9 x Mo-5 x Cu ... (2)
However, each element symbol in the formula (2) is the content (mass%) of each element, and the element not contained is 0%.

If the cooling start temperature is less than the Ar ₃ transformation point, the steel sheet is not sufficiently hardened, so that the hardness of the steel sheet cannot be sufficiently improved, and as a result, the wear resistance of the finally obtained steel sheet is lowered. On the other hand, the upper limit of the cooling start temperature is not particularly limited, but is preferably 950 ° C. or lower.

Average cooling rate between 800 and 300 ° C: 50 ° C / s or more During accelerated cooling, the martensite area ratio after accelerated cooling should be 90% or more, and the desired base metal hardness and uniform steel plate shape should be obtained. The average cooling rate between 800 and 300 ° C. is 50 ° C./s or more. If the average cooling rate is less than 50 ° C./s, ferrite and bainite are formed and the desired base metal hardness cannot be obtained. In addition, cooling becomes non-uniform, and wavy distortion occurs in the steel sheet. When such distortion occurs, the shape cannot be corrected even by temper rolling by the skin pass in the next process. Therefore, in order to improve the hardness and shape, the average cooling rate is set to 50 ° C./s or more. On the other hand, the average cooling rate is preferably 95 ° C./s or less. When the average cooling rate is 95 ° C./s or less, a wear-resistant steel sheet having excellent flatness can be obtained without deteriorating the shape of the steel sheet.

Cooling stop (winding) temperature: 250 ° C or less The cooling stop temperature shall be 250 ° C or less, and the coil shall be wound at this temperature. If the cooling stop temperature (winding temperature) is higher than 250 ° C., the area ratio of martensite cannot be sufficiently increased, and the hardness of the steel sheet is lowered. The cooling shutdown temperature is preferably 200 ° C. or lower.

After winding, air cool to room temperature.

The steel sheet wound around the coil is temper-rolled for the purpose of correcting the steel sheet shape (flatness) defect due to cooling strain. It is preferable that the temper rolling is performed using a rolling mill and equipment equipped with a tension leveler with a rolling load of 700 tons or more, a forward-backward tension of 20 tons or more, and an elongation rate of 0.10% or more.

Hereinafter, examples of the present invention will be described.

Steels having the chemical components shown in Table 1 were melted to produce hot-rolled coils under the conditions shown in Table 2. Next, the hot-rolled coil was air-cooled, then inserted into the skin pass line, and tempered and rolled under the condition of an elongation rate of 0.10% or more. Next, the coil after temper rolling was rewound to prepare a sheet, and then a Brinell hardness test, a Charpy impact test, a weldability test, a microstructure observation, a bending test and a flatness were performed. Each test method is as follows.

<Brinell hardness test>
A test piece for measuring hardness was collected from each steel plate, and the Brinell hardness was measured in accordance with the provisions of JIS Z 2243 (1998). The measurement was carried out after grinding and removing the region from the surface of the steel sheet to a depth of 0.5 mm in order to remove the influence of the scale and the decarburized layer existing on the surface of the wear-resistant steel sheet. Therefore, the measured hardness is the surface hardness on the surface at a depth of 0.5 mm from the surface of the steel sheet. In the measurement, a tungsten hard ball having a diameter of 5.0 mm was used, and the load was 750 kgf. The measurement position was set at the center of the steel plate in the width direction, and the arithmetic average was obtained by measuring each of the five points.

<Charpy impact test>
A V-notch test piece was collected from the central portion of the width of each steel plate in accordance with JIS Z 2202. Using a V-notch test piece, a Charpy impact test was carried out in accordance with JIS Z 2242 to determine the Charpy absorption energy (vE-40 ° C) at _{−40 ° C.} and toughness was evaluated. For a steel plate having a plate thickness of 3.2 mm or more, a test piece having a test piece size of 2.5 mm thickness × 10 mm × 50 mm was used. For steel plates with a plate thickness of 2.5 mm or less, a test piece with a plate thickness of 10 mm x 55 mm (a test piece not specified by JIS, but a test piece according to JIS cannot be collected, so the cross section is set to a plate thickness of 10 mm. )It was used. The number of test pieces was three, and the arithmetic mean of the obtained absorbed energy values was calculated. For vE- _{40 ° C} , 10J or more was regarded as acceptable.

<Welding test>
A sample was taken from the center of each steel plate in the width direction. The presence or absence of cracks in the welded portion was investigated in accordance with JIS Z 3154 "Layered joint weld crack test method". The target material was used for the lower plate, and SS400 with a plate thickness of 19 mm was used for the upper plate. The welding method was gas shielded arc welding, and a test bead was placed at a welding heat input of 10 to 11 kJ / cm.

<Tissue observation>
Area ratio of martensite Samples were taken from the center of each steel sheet in the width direction. The sample is cut into 30 mm squares, embedded in resin so that the cross section parallel to the rolling direction becomes the observation surface, the surface is mirror-polished, and after Nital etching, the plate is used with a scanning electron microscope (SEM). The central part of the thickness was photographed at a magnification of 2000 times. The surface integral of martensite was obtained by analyzing the captured image using an image analysis device.

Ratio of rolling direction grain size dZ to plate thickness direction grain size dL of old austenite grains (dZ / dL)
A sample is taken from the center of each steel plate in the width direction, the sample is cut into 30 mm squares, embedded in resin so that the cross section parallel to the rolling direction becomes the observation surface, mirror-polished and etched, and then SEM is used. The central part of the plate thickness was photographed along the rolling direction. The captured image was analyzed using an image analysis device to obtain dZ / dL.

<Bending test>
Bending test pieces having a width of 40 mm and a length (rolling direction) of 200 mm are collected from the obtained steel sheet, and a 180 ° bending test is performed with a bending radius of 2.5 t (t is the plate thickness) in accordance with JIS Z 2248. Was performed, the presence or absence of cracks was confirmed, and the bendability was evaluated.

<Flatness>
After temper rolling, the sheet is made into a sheet with a steel plate width x rolling direction length of 6 meters using a recoil facility, and then the number of strains with a size of 3 mm or more is measured using a water thread by the method described in JIS G3193. did.

Table 2 shows the evaluation results obtained by the above method.

All of the steel sheets satisfying the conditions of the present invention had a Brinell hardness of 400HBW5 / 750 or more, vE _{−40 ° C} .: 10J or more, and had excellent weldability and bending workability. On the other hand, the steel sheet of the comparative example was inferior in at least one of the properties of hardness, toughness, weldability, and bending workability.

Claims (4)

By mass%, C: 0.18 to 0.22%, Si: 0.01 to 0.45%, Mn: 0.30 to 2.00%, P: 0.020% or less, S: 0.008 % Or less, Al: 0.010 to 0.100%, Nb: 0.010 to 0.030%, Ti: 0.005 to 0.025%, B: 0.0004 to 0.0020%, and The Ceq value represented by the formula (1) is 0.48% or less, and the component composition is composed of the balance Fe and unavoidable impurities.
It has a structure in which the area ratio of martensite is 90% or more, the ratio (dZ / dL) of the grain size dZ in the rolling direction to the grain size dL in the plate thickness direction of the old austenite grains is 8.0 or more, and the Brinell hardness. A wear-resistant steel sheet having a hardness of 400 HBW5 / 750 or more.
Ceq = C + Mn / 6 + (Cu + Ni) / 15+ (Cr + Mo + V) / 5 ... (1)
In the above formula (1), the element symbol means the content (mass%) of each element. However, the element not contained is 0%. The composition of the components is Cu: 0.05 to 0.50%, Ni: 0.05 to 0.50% , Mo: 0.05 to 0.50%, V: 0.005 to 0 in mass%. The wear-resistant steel sheet according to claim 1, further containing one or more selected from 080%. Claim 1 or 2 in which the component composition further contains one or two selected from Ca: 0.0005 to 0.0020% and REM: 0.0005 to 0.0020% in mass%. The wear resistant steel sheet described. The method for producing a wear-resistant steel sheet according to any one of claims 1 to 3, wherein the steel slab having the component composition is heated to 1180 to 1300 ° C., and the heated steel slab has a finish rolling temperature of 980. At ~ 850 ° C, finish rolling with a rolling reduction ratio (thickness on the entry side of the finish rolling mill / thickness on the exit side of the finish rolling machine) of 4.0 or more is performed, cooling is started from a temperature of Ar ₃ transformation point or higher, and 800 to 300 ° C. A method for manufacturing a wear-resistant steel sheet, in which accelerated cooling is performed under the condition that the average cooling rate is 50 ° C./s or more, winding is performed at a winding temperature of 250 ° C. or less, and temper rolling is performed.