JP2017008344A - Wear-resistant steel plate and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wear-resistant steel plate combining excellent wear resistance with excellent workability.SOLUTION: There is provided the wear-resistant steel plate which contains, by mass%, C:0.12 to 0.30%, Si:0.01 to 1.0%, Mn:0.5 to 2.0%, P:0.015% or less, S:0.005% or less, Cr:0.3 to 1.5%, Al:0.03 to 0.08%, Ti:0.005 to 0.02%, B:0.0005 to 0.003%, N:0.005% or less, O:0.003% or less, Mo:0 to 1.5%, Ni:0 to 1.5%, Cu:0 to 1.0%, Nb:0 to 0.1%, V:0 to 0.1%, Ca:0 to 0.01%, Mg:0 to 0.01% and the balance Fe with impurities, and in which difference (H2-H1) between Vickers hardness H1 at a position 0.5 mm away from at least one surface and Vickers hardness H2 at a position 1.5 mm away from the one surface is 30 or more, Brinell hardness at a position 0.7 mm away from each surface is 350 or more, 90% or more of a metallographic structure in a region 0.5 mm to 0.25t (t is plate thickness (mm)) away from each surface is martensite structure.SELECTED DRAWING: None

Description

  The present invention relates to a wear-resistant steel plate and a method for producing the same.

  For example, wear resistance is required for members used in civil engineering, mining construction machines, large industrial machines, and the like. As it is known that the wear resistance is strongly controlled by the surface hardness of the member, high-hardness steel is applied to the structural member of the machine that requires the wear resistance.

  International Publication No. 2011/066182 (Patent Document 1) has a predetermined chemical composition, and the ratio (DI / t) between the hardenability index DI and the steel sheet thickness t (mm) is 0.5 to 15.0, martensite transformation start temperature Ms is 430 or less, martensite ratio M in the microstructure is 70% or more, M × C is 23 or less, surface hardness is Brinell hardness, HBW 400-500 An invention relating to wear steel is disclosed.

  In JP-A-10-8186 (Patent Document 2), the surface layer portion of the steel sheet is composed of a martensite structure or a mixed structure of martensite and bainite, and the Vickers hardness of the surface layer portion of the steel sheet is in the range of 300 to 550. And the relationship between the Vickers hardness HVs at the boundary between the surface layer portion and the inside of the steel sheet, the Vickers hardness HVc at the center position inside the steel sheet, and the sheet thickness t (mm) of the steel sheet, Is less than 30 mm, HVs−HVc ≧ 0.4 × t, and when the plate thickness t of the steel sheet is 30 mm or more, an invention relating to a wear-resistant steel sheet having HVs−HVc ≧ 0.6 × t-6 is disclosed. Yes.

International Publication No. 2011/0661812 Japanese Patent Laid-Open No. 10-8186

  In the invention described in Patent Document 1, the C content is controlled in order to control the surface hardness within a range in which wear resistance, toughness, and workability can be aligned. However, attention is focused only on the surface hardness, and it may be difficult to achieve both wear resistance and workability.

  Although the invention described in Patent Document 2 focuses on the hardness of the surface layer position and the center position, the hardness at the center position is lower than the surface layer position, so it is difficult to maintain excellent wear resistance. is there.

  As described above, when the surface hardness of the steel sheet is increased, the wear resistance is increased while the workability is deteriorated. When the surface hardness is decreased, the workability is increased while the wear resistance is deteriorated. For this reason, in the prior art, steel materials having excellent wear resistance and workability cannot be obtained.

  The present invention has been made to solve the problems of the prior art, and an object thereof is to provide a wear-resistant steel plate having excellent wear resistance and workability.

  In order to solve this problem, the present inventors have studied in detail a mechanism for improving workability, and obtained the following knowledge.

  (A) When the steel plate is processed, the load is most applied to the steel plate and the processing tool at the initial stage of processing, that is, when the surface of the steel plate is processed. And once a process advances, a process can be easily advanced compared with the initial stage of a process. Therefore, in order to ensure excellent wear resistance, the hardness inside the steel plate is maintained, while a soft layer with high workability may be formed only on a part of the front and back surfaces of the steel plate.

  (B) In order to form such a soft phase on the front and back surfaces of the steel sheet, it is only necessary to locally temper only a part of the front and back surfaces of the steel sheet. For example, a short-time tempering process is conceivable. . However, simply shortening the tempering time makes it difficult to form a soft layer having a sufficient thickness to improve workability and to secure the hardness inside the steel sheet.

  Therefore, as a result of further studies on a method of providing a soft layer with high workability only on a very small part of the front and back surfaces of the steel sheet without reducing the hardness inside the steel sheet, the inventors have found the following knowledge Got.

  (C) In the direct quenching process after hot rolling, once cooling is interrupted, recuperation occurs due to the latent heat inside the steel sheet. By using this double heat, it is possible to temper locally on only a part of the front and back surfaces of the steel sheet, and as a result, a soft layer with high workability is formed on only a part of the front and back surfaces of the steel sheet. can do.

  (D) On the other hand, if the cooling interruption time is too long, quenching becomes insufficient, and the hardness inside the steel sheet decreases. For this reason, it is important to maintain a minimum average cooling rate as a quenching treatment.

  The present invention has been made on the basis of the above findings, and the gist of the present invention is the following wear-resistant steel plate and a method for producing the same.

(A) The chemical composition is mass%,
C: 0.12 to 0.30%,
Si: 0.01 to 1.0%,
Mn: 0.5 to 2.0%
P: 0.015% or less,
S: 0.005% or less,
Cr: 0.3 to 1.5%,
Al: 0.03-0.08%,
Ti: 0.005 to 0.02%,
B: 0.0005 to 0.003%,
N: 0.005% or less,
O: 0.003% or less,
Mo: 0 to 1.5%,
Ni: 0 to 1.5%,
Cu: 0 to 1.0%
Nb: 0 to 0.1%,
V: 0 to 0.1%
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
The balance: a wear-resistant steel plate that is Fe and impurities,
In the thickness direction of the steel sheet, the difference (H2−H1) between the Vickers hardness H1 at a position of 0.5 mm from at least one surface of the steel sheet and the Vickers hardness H2 at a position of 1.5 mm from the surface is 30 That's it,
Brinell hardness at 0.7 mm position from each surface of the steel sheet is 350 or more,
The metal structure in the region of 0.5 mm or more and 0.25 t (t: plate thickness (mm)) or less from each surface of the steel sheet is a martensite structure of 90% or more.
Wear-resistant steel plate.

(B) In mass%,
Mo: 1.5% or less, Ni: 1.5% or less, Cu: 1.0% or less, Nb: 0.1% or less and V: containing at least one selected from 0.1% or less,
The wear-resistant steel sheet of (A) above.

(C)% by mass,
Containing one or more selected from Ca: 0.01% or less and Mg: 0.01% or less,
The wear-resistant steel sheet according to (A) or (B) above.

ただし、上記式中の各元素記号は、それぞれの元素の含有量（質量％）を意味する。 (D) A method for producing a wear-resistant steel sheet, comprising the following steps (1) to (3).
(1) A heating step of heating a slab having the chemical composition of any one of (A) to (C) to 1000 to 1200 ° C.
(2) When the steel sheet is obtained by hot rolling the slab, the rolling reduction is completed at a temperature range of 750 ° C. or higher with a cumulative reduction ratio of 50% or more with respect to the slab thickness in a temperature range of 850 ° C. or higher. Hot rolling process, and
(3) When cooling the steel sheet from a temperature range of 680 ° C. or more to a temperature range of 500 ° C. or less, the average cooling rate at the position of 0.25 t (t: plate thickness (mm)) from each surface of the steel plate is as follows: A quenching step that is equal to or higher than Vc (° C./s) obtained from the equation and that undergoes a thermal history in which at least one surface temperature of the steel sheet is 300 ° C. or lower during cooling, and then double-heats to 400 ° C. or higher. .

However, each element symbol in the above formula means the content (% by mass) of each element.

ADVANTAGE OF THE INVENTION According to this invention, the abrasion-resistant steel plate which has the outstanding abrasion resistance and workability can be provided. This wear-resistant steel plate is suitable for use as a machine component that requires wear resistance, such as civil engineering, mining construction machines, and large industrial machines.

  Hereinafter, the constituent requirements of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

(A) Chemical composition of steel sheet C: 0.12 to 0.30%
C is the most effective and inexpensive element for improving the surface hardness. If the C content is less than 0.12%, it is necessary to contain other alloy elements to compensate for the decrease in hardness, resulting in an increase in cost. On the other hand, when the content exceeds 0.30%, workability is remarkably deteriorated and delayed fracture resistance is remarkably inhibited. For this reason, C content shall be 0.12-0.30%. A preferred lower limit is 0.15% and a preferred upper limit is 0.28%.

Si: 0.01 to 1.0%
Si contributes to improvement in surface hardness and delayed fracture resistance. If the Si content is less than 0.01%, the above effect is insufficient. On the other hand, if the content exceeds 1.0%, the workability is remarkably deteriorated and the toughness that affects the heat cracking resistance is deteriorated. For this reason, Si content is made into 0.01 to 1.0%. A preferred lower limit is 0.05% and a preferred upper limit is 0.8%.

Mn: 0.5 to 2.0%
Mn improves surface hardness through improved hardenability. If the Mn content is less than 0.5%, it is necessary to supplement the hardness by containing other alloy elements, resulting in an increase in cost. On the other hand, when the content exceeds 2.0%, workability is remarkably deteriorated and delayed fracture resistance is remarkably impaired. For this reason, Mn content shall be 0.5 to 2.0%. A preferred lower limit is 0.6% and a preferred upper limit is 1.6%.

P: 0.015% or less P is present as an inevitable impurity in the steel and segregates at the grain boundaries to degrade the delayed fracture resistance and toughness of the steel. Therefore, its content is desirably as low as possible. In particular, since the deterioration is significant when the P content exceeds 0.015%, the P content is limited to 0.015% or less. Preferably it is 0.012% or less.

S: 0.005% or less S is an inevitable impurity element that deteriorates the ductility and toughness of steel. If the content exceeds 0.005%, such adverse effects become obvious, so the S content is limited to 0.005% or less. Preferably it is 0.003% or less.

Cr: 0.3 to 1.5%
Cr is effective in improving both hardness and toughness through the work of improving hardenability. When the Cr content is less than 0.3%, the above effect is not sufficient. On the other hand, when the content exceeds 1.5%, workability is remarkably deteriorated and toughness is remarkably deteriorated. For this reason, Cr content shall be 0.3-1.5%. A preferred lower limit is 0.5% and a preferred upper limit is 1.2%.

Al: 0.03-0.08%
Al effectively suppresses overgrowth of initial austenite grains by generating AlN during slab heating. If the Al content is less than 0.03%, the effect is small. On the other hand, when the content exceeds 0.08%, workability is remarkably deteriorated and toughness is remarkably deteriorated. For this reason, Al content is made into 0.03 to 0.08%. A preferred lower limit is 0.04% and a preferred upper limit is 0.07%. The Al content of the present invention refers to acid-soluble Al (so-called “sol.Al”).

Ti: 0.005-0.02%
Ti becomes fine TiN and fixes N, exhibits a pinning effect at the time of heating, not only suppresses the growth of austenite grains, but also helps segregation of effective B to austenite grain boundaries when B is added, Combined with the effect of improving hardenability. These effects cannot be obtained when the content is less than 0.005%. On the other hand, when the content exceeds 0.02%, the workability is remarkably deteriorated and the coarsening of TiN becomes remarkable and the toughness is lowered. For this reason, Ti content shall be 0.005-0.02%. A preferred lower limit is 0.008% and a preferred upper limit is 0.016%.

B: 0.0005 to 0.003%
B is an extremely important element that remarkably improves hardenability. If the content is less than 0.0005%, the effect is small. When the content exceeds 0.003%, workability is remarkably deteriorated and toughness is remarkably deteriorated. For this reason, B content shall be 0.0005 to 0.003%. A preferred lower limit is 0.001% and a preferred upper limit is 0.002%.

N: 0.005% or less N is an impurity inevitably contained in steel. When present in a large amount, the HAZ toughness is deteriorated. On the other hand, if the N content exceeds 0.005%, workability is remarkably deteriorated and it is inevitable that the toughness is deteriorated in both the base material and the HAZ. Therefore, the N content is 0.005% or less. Preferably it is 0.004% or less.

O: 0.003% or less O is also an impurity inevitably contained in the steel. When the O content increases, nonmetallic inclusions in the steel increase, which significantly deteriorates workability and impairs low-temperature toughness. In order to avoid this, the O content is set to 0.003% or less.

Mo: 0 to 1.5%,
Since Mo has an effect of improving the strength and toughness of the base material, it may be contained as necessary. When the content is excessive, the hardness of the HAZ is increased, and the toughness and SSC resistance are impaired. Therefore, when it contains Mo, the content shall be 1.5% or less. In order to effectively improve the strength and toughness of the base material, the content is preferably 0.05% or more.

Ni: 0 to 1.5%,
Ni has the effect of increasing the toughness of the steel matrix (dough) in the solid solution state, and may be contained as necessary. When the content is excessive, the improvement of the characteristics commensurate with the increase in the alloy cost cannot be obtained. Therefore, when it contains Ni, the content shall be 1.5% or less. In order to effectively improve the toughness of the base material, the content is preferably 0.05% or more.

Cu: 0 to 1.0%
Since Cu has an effect of further improving the strength, it may be contained as necessary. When the content is excessive, improvement in characteristics commensurate with an increase in alloy cost cannot be obtained. Therefore, when it contains Cu, the content shall be 1.0% or less. In order to effectively improve the strength of the base material, the content is preferably 0.05% or more.

Nb: 0 to 0.1%,
Nb has the effect of suppressing the coarsening of crystal grains during slab heating, and also exhibits the same effect during quenching and is effective in refining the structure. Furthermore, it precipitates as Nb (C, N) in the grains during tempering and has a function of contributing to improvement in yield strength. For this reason, you may make it contain as needed. When the content is excessive, the coarsening of the precipitate becomes remarkable and the toughness is lowered. Therefore, when Nb is contained, the content is made 0.1% or less. In order to obtain these effects effectively, the content is preferably 0.01% or more.

V: 0 to 0.1%
V has an effect of improving the hardenability of the steel, and can further increase the strength of the steel sheet due to the precipitation effect during the tempering treatment. Therefore, V may be contained as necessary. When the content is excessive, the coarsening of the precipitate becomes remarkable and the toughness is lowered. Therefore, when V is contained, the content is made 0.1% or less. In order to obtain these effects effectively, the content is preferably 0.01% or more.

Ca: 0 to 0.01%,
When Ca is contained, non-metallic inclusions are spheroidized and low temperature toughness can be improved, so Ca may be contained as necessary. When the content is excessive, inclusions such as CaO and CaS are generated in a large amount to impair the toughness of the steel, and the workability of the steel is remarkably deteriorated. Therefore, when Ca is contained, its content is set to 0.01% or less. In order to effectively obtain this effect, the Ca content is preferably 0.001% or more.

Mg: 0 to 0.01%,
When Mg is also contained, nonmetallic inclusions can be spheroidized and low temperature toughness can be improved, so it may be contained as required. When the content is excessive, inclusions such as MgO and MgS are produced in a large amount to impair the toughness of the steel, and the workability of the steel is remarkably deteriorated. For this reason, when it contains Mg, the content shall be 0.01% or less. In order to effectively obtain this effect, the Mg content is preferably 0.001% or more.

  The wear-resistant steel sheet according to the present invention contains the elements listed above in appropriate ranges, and the balance has a chemical composition of Fe and impurities. Here, an impurity means the component mixed by raw materials and other factors, such as an ore and a scrap, when manufacturing steel materials industrially.

(B) Hardness and structure of steel plate (B-1) Softening layer In order to obtain a steel plate having both excellent wear resistance and workability, the front and back surfaces of the steel plate are not reduced without reducing the hardness inside the steel plate. It is effective to provide a soft layer with high workability only in a very small part of the film. Here, in the case of a steel sheet obtained by adopting a normal quenching process, the region from the surface to about 0.2 mm tends to soften by decarburization, but in a deeper region, a hard structure that has been quenched. Is formed. And with such a thin softened layer, the workability cannot be improved.

  As a result of examining the thickness of the softening layer necessary for improving the bending workability in detail, the present inventors have confirmed that the area of 1.0 mm or more from each surface of the steel sheet is required to ensure wear resistance. While it is necessary to ensure hardness, it has been found that it is important to provide a softened layer with a sufficient thickness in the region on the surface side of the surface. The inventors of the present invention have made extensive studies, and in the thickness direction of the steel sheet, the difference between the Vickers hardness H1 at a position of 0.5 mm from the surface of the steel sheet and the Vickers hardness H2 at a position of 1.5 mm from the surface ( H2-H1) has a correlation with bending workability, and in particular, it has been found that by making this difference (H2-H1) 30 or more, workability can be improved without reducing wear resistance. The difference (H2−H1) is preferably 35 or more, and more preferably 40 or more.

  As for the above-mentioned hardness, if at least one surface of the steel sheet satisfies this condition, a steel sheet having excellent workability can be obtained. However, both surfaces of the steel sheet satisfy this condition. May be.

  As described above, the wear-resistant steel plate of the present invention forms a soft layer only on a very small part of at least one surface of the steel plate, and thus exhibits excellent workability when processing various structural members. When steel plates are used as various structural members, the softened layer disappears in the initial stage, and thereafter, good wear resistance can be exhibited.

(B-2) Hardness inside the steel sheet As described above, the present invention improves the workability by providing a soft layer only on a very small part of at least one surface of the steel sheet. In order to ensure wear resistance, it is important that the inside of the steel plate is sufficiently hard. In particular, if the hardness at a position 0.7 mm from each surface of the steel sheet is too low, it is difficult to ensure wear resistance. Therefore, the Brinell hardness at 0.7 mm from each surface of the steel plate needs to be 350 or more. The Brinell hardness at this position is preferably 380 or more, and more preferably 400 or more.

(B-3) Metal structure In order to ensure the wear resistance of the steel sheet, it is necessary to make the inside of the steel sheet a martensite-based metal structure. Specifically, the metal structure in the region of 0.5 mm or more and 0.25 t (t: plate thickness (mm)) or less from each surface of the steel sheet needs to be a martensite structure of 90% or more. Here, the martensite includes tempered martensite. This is because when the structure other than martensite (bainite and ferrite structure) exceeds 10%, it becomes difficult to control the wear resistance. The metal structure in this region is preferably 93% or more of martensite structure, and more preferably 95% or more of martensite structure.

(C) Steel plate manufacturing method (C-1) Slab heating First, a slab having the chemical composition described in (A) above is prepared. A normal method may be adopted as a method for manufacturing the slab. For example, an ingot method may be employed, but it is preferable to employ a continuous casting method from the viewpoint of cost reduction.

  The heating temperature of the slab needs to be 1000 ° C. or higher in order to dissolve coarse inclusions such as Ti and B precipitated during casting to expand the fine crystal temperature range. On the other hand, when the heating temperature is too high, coarsening of the prior austenite is caused and the toughness is lowered. Therefore, the heating temperature of a slab shall be 1000-1200 degreeC. A preferred lower limit is 1050 ° C. and a preferred upper limit is 1160 ° C.

  Here, “slab” is a general term for steel ingots, blooms, billets and the like. The thickness of the slab is not limited, but when the steel sheet is manufactured, the reduction is performed while controlling the temperature. Therefore, the thickness is preferably at least three times the finished thickness. More preferably, the thickness is at least 5 times the finished thickness. Further, from the viewpoint of steel plate production efficiency, the thickness of the slab is preferably 10 times or less of the finished thickness.

(C-2) Hot rolling In order to ensure the hardness inside the steel sheet, it is effective to refine the steel sheet structure during rolling. For this reason, it is better to carry out controlled rolling in the recrystallization region. Note that the refinement of the steel sheet structure is also effective in improving toughness.

  In the method for producing a wear-resistant steel sheet according to the present invention, rolling is performed in a temperature range of 850 ° C. or higher which is a recrystallization region. Rolling in the recrystallization region needs to recrystallize the steel sheet structure and reduce the austenite grain size. In order to sufficiently reduce the austenite grain size, it is necessary to set the cumulative reduction ratio with respect to the slab thickness to 50% or more. The upper limit of the cumulative rolling reduction is not particularly defined, but the upper limit of the cumulative rolling reduction in the recrystallization region is 75% considering the thickness of the finally obtained thick steel plate.

  In addition, when the temperature of hot rolling is too low, the rolling load is increased and quenching that is the next step cannot be performed by direct quenching. Therefore, the completion temperature of hot rolling needs to be 750 ° C. or higher. The completion temperature of hot rolling is preferably 800 ° C. or higher.

(C-3) About quenching In quenching, so-called direct quenching is performed and the steel sheet after hot rolling is quenched. That is, the pre-rolling process is completed at 750 ° C. or higher, and cooling is performed as it is from a temperature range of 680 ° C. or higher to a temperature range of 500 ° C. or lower. In order to ensure hardenability, it is preferable to perform quenching from a higher temperature range, and quenching is preferably performed from a temperature range of 750 ° C. or higher.

ただし、上記式中の各元素記号は、それぞれの元素の含有量（質量％）を意味する。 At this time, the average cooling rate at the position of 0.25 t (t: plate thickness (mm)) from each surface of the steel plate needs to be not less than Vc (° C./s) obtained from the following formula. By cooling under such conditions, the metal structure in the region of 0.5 mm or more and 0.25 t (t: plate thickness (mm)) or less from each surface of the steel sheet is made a martensite structure of 90% or more. Is possible.

However, each element symbol in the above formula means the content (% by mass) of each element.

  Here, after quenching, it is possible to form a softened layer on the surface layer by a normal heat treatment that performs tempering, but softening occurs to the inside of the steel plate, hardness is reduced, and it is difficult to ensure wear resistance. It is expected to be. Therefore, in the direct quenching process after hot rolling, it is effective to perform partial tempering using recuperation generated when cooling is temporarily interrupted.

  That is, during the cooling, it is important that at least once, the surface temperature of at least one of the steel sheets becomes 300 ° C. or lower, and then undergoes a heat history of double heating to 400 ° C. or higher. The steel sheet surface that has undergone such a thermal history has a soft tempered martensite structure on the surface and a martensite structure on the inside. As a result, it is possible to obtain a steel sheet having the constituent elements described in “(B) Hardness and structure of steel sheet”. The above double heat may be performed a plurality of times.

  As a method of realizing the heat history of the recuperation, cooling control during direct water cooling is effective. That is, the hot-rolled steel sheet is passed through a water-cooling apparatus installed at intervals and cooled. At this time, in the cooling process, even if the surface temperature of the steel sheet once becomes 300 ° C. or less, the inside of the steel sheet remains at a high temperature close to the rolling temperature. For this reason, it becomes possible to reheat to 400 degreeC or more of the surface of a steel plate by once stopping cooling between the cooling devices after the surface temperature of a steel plate once became 300 degrees C or less. As a result, the steel sheet surface on which the above double heat is generated has a tempered martensite structure on the surface layer, and a softened layer is formed. The double heat temperature is preferably limited to 460 ° C. or lower in order to maintain the hardness inside the steel plate.

  After the above-mentioned double heat, cooling by the cooling device is performed again. At this time, by cooling so that the average cooling rate at the position of 0.25 t (t: plate thickness (mm)) from each surface of the steel plate becomes Vc (° C./s) or more, the inside of the steel plate is affected by the latent heat. It is not tempered and becomes a martensite structure. As a result, it is possible to form a softening layer only on the extreme surface layer of the steel plate and to secure the hardness inside the steel plate.

  If the above conditions are satisfied, the surface of the steel plate on which double heat is generated becomes a tempered martensite structure, and the ratio of the martensite structure increases toward the inside. However, depending on the quenching conditions, the metal structure inside the steel sheet may become tempered martensite, but the hardness conditions listed in the above (B-1) and (B-2) and (B-3) are listed. As long as the conditions of the metal structure are satisfied, there is no problem.

  A slab having the chemical composition shown in Table 1 was obtained. The obtained slab was subjected to heating, hot rolling and direct quenching under the conditions shown in Table 2 to obtain a steel plate. The performance of the obtained steel sheet was evaluated by the following method. The results are shown in Table 3.

<Vickers hardness test>
According to JIS Z 2244, Vickers hardness HV (1 kg) was measured for the cross section in the C direction of the steel sheet. Specifically, a Vickers hardness H1 at a position of 0.5 mm from one surface and a Vickers hardness H2 at a position of 1.5 mm from the surface were measured.

<Brinell hardness test>
According to JIS Z 2243, the Brinell hardness HBW (10/3000) at a position of 0.7 mm from the steel sheet surface was measured for the Z-direction cross section of the steel sheet.

<Bending test (workability evaluation)>
Since there is almost no difference in characteristics between the front and back surfaces of the steel plate, a JIS No. 1 test piece was sampled in parallel with the rolling direction only from one side of the steel plate and subjected to a bending test. It was judged that a crack was not generated at a bending radius of 2.5 t (t is the plate thickness) (O), and a crack was determined as a failure (X).

<Martensite fraction>
For the L direction cross section of the steel sheet, the microstructure is observed at 500 times after etching with Nital, and the martensite content in the region of 0.5 mm or more and 0.25 t (t: plate thickness (mm)) or less from the steel sheet surface. The rate was measured.

  As shown in Table 3, test no. 26 to 32 are examples in which the production conditions deviate from the range defined by the present invention, although having the chemical composition within the range defined by the present invention. In these examples, the difference in martensite fraction and / or Vickers hardness (H2−H1) is outside the range defined in the present invention. As a result, no. In Nos. 27, 28 and 32, workability was deteriorated. No. In Nos. 26 and 29 to 32, the Brinell hardness was out of the range defined in the present invention, and in particular, the wear resistance was deteriorated. In addition, Test No. 33 to 43 are examples in which the production conditions are within the range defined by the present invention but do not have the chemical composition defined by the present invention. In these examples, the workability was deteriorated in all cases.

  On the other hand, test No. 1 satisfying all the conditions defined in the present invention. 1 to 25 had excellent wear resistance and workability.

ADVANTAGE OF THE INVENTION According to this invention, the abrasion-resistant steel plate which has the outstanding abrasion resistance and workability can be provided. This wear-resistant steel plate is suitable for use as a machine component that requires wear resistance, such as civil engineering, mining construction machines, and large industrial machines.

Claims (4)

Chemical composition is mass%,
C: 0.12 to 0.30%,
Si: 0.01 to 1.0%,
Mn: 0.5 to 2.0%
P: 0.015% or less,
S: 0.005% or less,
Cr: 0.3 to 1.5%,
Al: 0.03-0.08%,
Ti: 0.005 to 0.02%,
B: 0.0005 to 0.003%,
N: 0.005% or less,
O: 0.003% or less,
Mo: 0 to 1.5%,
Ni: 0 to 1.5%,
Cu: 0 to 1.0%
Nb: 0 to 0.1%,
V: 0 to 0.1%
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
The balance: a wear-resistant steel plate that is Fe and impurities,
In the thickness direction of the steel sheet, the difference (H2−H1) between the Vickers hardness H1 at a position of 0.5 mm from at least one surface of the steel sheet and the Vickers hardness H2 at a position of 1.5 mm from the surface is 30 That's it,
Brinell hardness at 0.7 mm position from each surface of the steel sheet is 350 or more,
The metal structure in the region of 0.5 mm or more and 0.25 t (t: plate thickness (mm)) or less from each surface of the steel sheet is a martensite structure of 90% or more.
Wear-resistant steel plate. % By mass
Mo: 1.5% or less, Ni: 1.5% or less, Cu: 1.0% or less, Nb: 0.1% or less and V: containing at least one selected from 0.1% or less,
The wear-resistant steel sheet according to claim 1. % By mass
Containing one or more selected from Ca: 0.01% or less and Mg: 0.01% or less,
The wear-resistant steel sheet according to claim 1 or 2. The manufacturing method of an abrasion-resistant steel plate provided with the process of following (1)-(3).
(1) A heating step of heating a slab having the chemical composition according to any one of claims 1 to 3 to 1000 to 1200 ° C,
(2) When the steel sheet is obtained by hot rolling the slab, the rolling reduction is completed at a temperature range of 750 ° C. or higher with a cumulative reduction ratio of 50% or more with respect to the slab thickness in a temperature range of 850 ° C. or higher. Hot rolling process, and
(3) When cooling the steel sheet from a temperature range of 680 ° C. or more to a temperature range of 500 ° C. or less, the average cooling rate at the position of 0.25 t (t: plate thickness (mm)) from each surface of the steel plate is as follows: A quenching step that is equal to or higher than Vc (° C./s) obtained from the equation and that undergoes a thermal history in which at least one surface temperature of the steel sheet is 300 ° C. or lower during cooling, and then double-heats to 400 ° C. or higher. .

However, each element symbol in the above formula means the content (% by mass) of each element.