KR101898567B1 - Thick steel sheet and method for manufacturing the same - Google Patents

Abstract

Provided are a steel sheet suitable for members requiring abrasion resistance against rocks, sand, ores, slurry materials, etc. of industrial machines, transportation and transportation equipment, and a manufacturing method thereof. Ni, Cr, and Cr in an amount of 0.2 to 0.350% by mass, 0.05 to 0.45% by mass, 0.05 to 0.45% by mass, 0.50 to 2.00% by mass, 0.0020 to 0.10% A composition comprising one or more of Mo, V, Nb, Ti, B, REM, Ca and Mg, a CI of 40 or more as defined by a specific formula, the balance Fe and inevitable impurities, 60% or more, an area fraction of island-shaped martensite of 5% or more and less than 20%, and the remainder being a ferrite phase, a pearlite or a martensite phase. The steel of the above composition is hot-rolled and then accelerated and cooled to 400 to 650 占 폚.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a steel sheet,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet suitable for members requiring abrasion resistance against rocks, sand, ores, slurry materials, etc. of industrial machines, transportation and transportation equipment, and a manufacturing method thereof.

Industrial machines such as power shovels, bulldozers, hoppers, buckets, and dump trucks used in the fields of construction, civil engineering and mining, and steel pipes for transporting slurry materials The absence of transporting and transporting equipment wears off by gravel or the like during use.

Conventionally, it has been known that the abrasion resistance is improved by increasing the hardness of a steel material, and steel materials having increased hardness by adding a large amount of alloying elements as a member for which some abrasion resistance is required have heretofore been used.

However, it is known that when the hardness of the steel material is increased to improve the abrasion resistance, the workability is largely lowered, and there is a problem that it is difficult to apply a high hardness material to a member application requiring machining.

Therefore, there is a demand for a steel material which is excellent in abrasion resistance and excellent in workability. For example, Patent Document 1 discloses a method for producing a steel sheet which comprises 0.13 to 0.18% of C, and contains a proper amount of Si, Mn, P, S, Al, B and N as Cr, 0.5 to 2.0% Characterized in that it contains 0.03 to 0.3% of Mo and 0.03 to 0.1% of Nb and has a composition of HI of 0.7 or more and Ceq of more than 0.50 and HB of 360 or more and 440 or less at 25 캜 Is proposed. Here, HI = [C] + 0.59 [Si] -0.58 [Mn] + 0.29 [Cr] +0.39 [Mo] +2.11 ([Nb] -0.02) -0.72 [Ti] +0.56 (% By mass), Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + to be.

Patent Document 1 discloses that according to the above technique, it is possible to improve abrasion resistance at a high temperature by forming a martensite structure of HB400 class by quenching treatment and further increasing the amount of solid solution Nb.

Patent Document 2 discloses a titanium alloy containing 0.10% to 0.45% of C, a proper amount of Si, Mn, P, S and N, 0.10% to 1.0% of Ti, Of TiC precipitates or a complex precipitate of TiC, TiN and TiS in an amount of not less than 400 per 1 mm 2 and Ti * which can be expressed in a specific formula is less than 0.05% to 0.4%.

Patent Document 3 discloses that DI * which contains 0.05 to 0.35% of C and contains a proper amount of Si, Mn and Al, further contains 0.1% to 1.2% of Ti, Is less than 60, and the ferrite phase-bainite phase is in a gaseous phase, and a hard phase is dispersed in the matrix, and a wear-resistant steel sheet excellent in workability has been proposed.

In Patent Documents 2 and 3, it has been described that the above-mentioned technique can improve the abrasion resistance at low cost by producing a precipitate mainly composed of TiC as a large grain diameter at the time of solidification.

Japanese Patent Publication No. 4590012 Japanese Patent Publication No. 3089882 Japanese Laid-Open Patent Publication No. 2010-222682

However, the technique described in Patent Document 1 is difficult to say that the hardness is not less than HB360 and the hardness is high and the workability is good, because it is subjected to a quenching process and made into a martensite structure. Further, since the technology described in Patent Document 1 adds a large amount of alloying elements, the cost of the alloy increases.

Since the techniques described in Patent Documents 2 and 3 form a coarse TiC at the time of solidification, it is necessary to perform the surface treatment of the slab before rolling, thereby increasing the manufacturing cost. Further, the high temperature abrasion resistance of the techniques described in Patent Documents 2 and 3 is unknown.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a post-warp sheet which is inexpensive, has excellent processability and is excellent in wear resistance, and a method for producing the same.

In order to achieve the above object, the inventors of the present invention have studied intensively on the influence of various factors on wear resistance. As a result, the composition of the steel material was optimized, and a value defined by the sum of the content of a plurality of alloying elements in the composition was set to a constant value, and the area fraction of the bainite phase was 60% It is possible to obtain a steel structure having an area fraction of not less than 5% and less than 20% and a remainder of ferrite phase, pearlite, martensite phase or more, Wear resistance can be provided.

The present invention has been completed based on the above-described recognition. That is, the gist of the present invention is as follows.

[1]

C: 0.200 to 0.350%

Si: 0.05 to 0.45%

Mn: 0.50 to 2.00%

P: 0.020% or less,

S: 0.005% or less,

Al: 0.005 to 0.100%

And a CI defined by the following expression (1) satisfies 40 or more,

The balance Fe, and inevitable impurities,

Wherein the area fraction of the bainite phase is 60% or more, the island-shaped martensite in the bainite phase is 5% or more and less than 20%

And the remainder has a steel structure composed of at least one of ferrite phase, pearlite and martensite phase.

CI = 60C + 8Si + 22Mn + 10 (Cu + Ni) + 14Cr + 21Mo + 15V (One)

In the formula, each alloy element is defined as a content (mass%). However, the content of the element which does not contain is 0.

[2]

Further, in terms of% by mass,

Cu: 0.03 to 1.00%

Ni: 0.03 to 2.00%

Cr: 0.05 to 2.00%

Mo: 0.05 to 1.00%

V: 0.005 to 0.100%,

Nb: 0.005 to 0.100%,

Ti: 0.005 to 0.100%

B: 0.0003 to 0.0030%,

By weight based on the total weight of the steel sheet.

[3]

Further, in terms of% by mass,

REM: 0.0005 to 0.0080%,

Ca: 0.0005 to 0.0050%,

Mg: 0.0005 to 0.0050%

, Wherein the steel sheet contains at least one member selected from the group consisting of iron oxide and iron oxide.

[4]

A cast steel or a steel piece having a steel composition according to any one of [1] to [3] is hot rolled at a temperature of 950 to 1250 캜 and ending at a temperature of Ar ₃ or higher. Immediately after hot rolling, Wherein the steel sheet is subjected to accelerated cooling at a cooling rate of 400 占 폚 to 650 占 폚.

[5]

A cast or slab having a steel composition according to any one of [1] to [3] is heated to 950 to 1250 캜, hot rolled, air cooled to less than 400 캜, reheated to a temperature of Ac _{3 to} 950 캜 And immediately after reheating, cooling is carried out at a cooling rate of 5 ° C / sec or more to 400 ° C to 650 ° C.

According to the present invention, it is possible to easily and stably produce a wear-resistant steel sheet having excellent workability and stably exhibiting excellent abrasion resistance, and has remarkable effects in industry.

1 is a view for explaining an abrasion tester.

(Mode for carrying out the invention)

The present invention defines the composition of the components and the steel structure.

[Composition of ingredients]

In the description,% is mass%.

C: 0.200 to 0.350%

C is an element contributing to island-shaped martensite generation and is an important element for obtaining excellent abrasion resistance. When the C content is less than 0.200%, the above-mentioned effect can not be sufficiently obtained. On the other hand, if the C content exceeds 0.350%, the weldability and workability deteriorate. For this reason, the C content was limited to the range of 0.200 to 0.350%. Further, it is preferably 0.210 to 0.300%.

Si: 0.05 to 0.45%

Si is an effective element that acts as a deoxidizing agent of molten steel and is an effective element that improves quenching and has an action contributing to the formation of island-shaped martensite. In order to secure such effect, the Si content is set to 0.05% or more. On the other hand, when the Si content exceeds 0.45%, the weldability decreases. Therefore, the Si content is limited to the range of 0.05 to 0.45%. Further, it is preferably 0.15 to 0.40%.

Mn: 0.50 to 2.00%

Mn is an effective element that improves quenching and has an action contributing to the formation of island-shaped martensite. In order to secure such effect, it is necessary to set the Mn content to 0.50% or more. On the other hand, when the Mn content exceeds 2.00%, the weldability is lowered, and a large amount of MnS, which is a starting point of fracture at the time of processing such as bending, is produced. For this reason, the Mn content is limited to the range of 0.50 to 2.00%. Further, it is preferably 0.60 to 1.70%.

P: not more than 0.020%

P, when contained in a large amount in the steel, causes a decrease in toughness. Therefore, it is preferable that the P content is reduced as much as possible. In the present invention, the P content can be allowed up to 0.020%. Therefore, the P content is limited to 0.020% or less. In addition, excessively reducing the P content leads to an increase in the refining cost, so that the P content is preferably 0.005% or more.

S: not more than 0.005%

When S is contained in a large amount in the steel, Mn precipitates as MnS, resulting in deterioration of toughness and a starting point of fracture at the time of processing. Therefore, it is preferable to reduce the S content as much as possible. In the present invention, it is acceptable if the S content is up to 0.005%. Therefore, the S content is limited to 0.005% or less. In addition, excessively reducing the S content leads to an increase in refining cost, and therefore, it is preferable that the S content is 0.0005% or more.

Al: 0.005 to 0.100%

Al is an effective element serving as a deoxidizing agent for molten steel. In order to obtain such an effect, a content of not less than 0.005% is required. When the Al content is less than 0.005%, these effects are not sufficiently obtained. On the other hand, if the Al content exceeds 0.100%, the weldability and toughness decrease. Therefore, the Al content is limited to the range of 0.005 to 0.100%. Further, it is preferably 0.015 to 0.040%.

CI = 60C + 8Si + 22Mn + 10 (Cu + Ni) + 14Cr + 21Mo + 15V? 40

In the formula, each alloy element represents the content (% by mass), and the element that does not contain is calculated as 0.

When the CI is less than 40, there is insufficient quenching, and the above-mentioned steel structure can not be obtained, and good wear resistance can not be obtained. Therefore, the CI was limited to 40 or more. Further, it is preferably 44 or more. If the CI is excessively large, the quenching becomes too high and the amount of martensite produced increases, so that the steel structure may not be formed. Therefore, the CI is preferably 80 or less, and more preferably 75 or less.

The components are the basic component composition and the balance Fe and inevitable impurities. In the present invention, one or more of Cu, Ni, Cr, Mo, V, Nb, Ti, B, REM, Ca and Mg may be selectively contained as the selective element to improve the characteristics.

Cu: 0.03 to 1.00%

Cu is an element having an effect of improving quenching and contributing to the formation of island-shaped martensite. In order to obtain such an effect, it is necessary to contain 0.03% or more. On the other hand, if the Cu content exceeds 1.00%, the hot workability decreases and the manufacturing cost increases. Therefore, when Cu is contained, it is preferable to limit the Cu content to a range of 0.03 to 1.00%. From the viewpoint of suppressing lowering of the hot workability and reducing the cost, it is more preferable to be limited to the range of 0.03 to 0.50%.

Ni: 0.03 to 2.00%

Ni is an element which contributes to improvement in low temperature toughness as well as improvement in quenching. In order to obtain such an effect, a content of 0.03% or more is required. On the other hand, if the Ni content exceeds 2.00%, the manufacturing cost increases. Therefore, when Ni is contained, it is preferable to limit the Ni content to a range of 0.03 to 2.00%. Further, from the viewpoint of cost reduction, it is more preferable to be limited to a range of 0.03 to 0.50%.

Cr: 0.05 to 2.00%

Cr is an element having an effect of improving quenching and contributing to the formation of island-shaped martensite. In order to obtain such an effect, a content of not less than 0.05% is required. On the other hand, when the Cr content exceeds 2.00%, the weldability is lowered and the manufacturing cost is increased. Therefore, when Cr is contained, the Cr content is limited to a range of 0.05 to 2.00%. Further, it is preferably in the range of 0.07 to 1.50%, more preferably 0.20 to 1.00%.

Mo: 0.05 to 1.00%

Mo is an element having an effect of improving quenching and contributing to the formation of island-shaped martensite. In order to obtain such an effect, a content of 0.05% or more is required. On the other hand, if the Mo content exceeds 1.00%, the weldability decreases and the manufacturing cost increases. Therefore, when Mo is contained, the Mo content is limited to the range of 0.05 to 1.00%. Further, it is preferably 0.10 to 0.80%, more preferably 0.20 to 0.50%.

V: 0.005 to 0.100%,

V is an element contributing to improvement in toughness through an effect of improving the atomicity and precipitating as carbonitride and making the structure finer. In order to obtain such an effect, it is necessary to contain 0.005% or more. On the other hand, if the V content exceeds 0.100%, the weldability decreases. Therefore, when V is contained, the V content is limited within the range of 0.005 to 0.100%.

Nb: 0.005 to 0.100%,

Nb is an element that precipitates as carbonitride and contributes effectively to improvement in toughness through microstructure. In order to obtain such an effect, a content of not less than 0.005% is required. On the other hand, if the Nb content exceeds 0.100%, the weldability decreases. Therefore, when Nb is contained, the content of Nb is limited to a range of 0.005 to 0.100%. Further, from the viewpoint of microstructure, it is preferable to set the range of 0.010 to 0.030%.

Ti: 0.005 to 0.100%

Ti precipitates as TiN and is an element contributing to improvement in toughness through fixing of solid solution N. In order to obtain such an effect, it is necessary to contain 0.005% or more. On the other hand, when the Ti content exceeds 0.100%, coarse carbonitrides precipitate and the toughness decreases. Therefore, when Ti is contained, the Ti content is limited to a range of 0.005 to 0.100%. Further, from the viewpoint of cost reduction, it is preferable to be limited to a range of 0.005 to 0.030%.

B: 0.0003 to 0.0030%,

B is an element contributing to improvement of traceability by trace amount. In order to obtain such an effect, it is necessary to contain 0.0003% or more. On the other hand, if the B content exceeds 0.0030%, the toughness decreases. Therefore, when B is contained, the B content is limited to a range of 0.0003 to 0.0030%.

REM: 0.0005 to 0.0080%,

In the REM, S is fixed to inhibit toughness and generation of MnS which is a cause of fracture at the time of processing. In order to obtain such an effect, it is necessary to contain 0.0005% or more. On the other hand, when the REM content exceeds 0.0080%, the amount of intermetallic compounds increases, resulting in a decrease in toughness. Therefore, when REM is contained, the REM content is limited to a range of 0.0005 to 0.0080%. Further, it is preferably 0.0005 to 0.0020%.

Ca: 0.0005 to 0.0050%,

Ca fixes S to inhibit toughness and MnS formation which is a cause of fracture at the time of processing. In order to obtain such an effect, it is necessary to contain 0.0005% or more. On the other hand, when the Ca content exceeds 0.0050%, the amount of intercalated material in the steel increases, resulting in a decrease in toughness. Therefore, when Ca is contained, the Ca content is limited to a range of 0.0005 to 0.0050%. Further, it is preferably 0.0005 to 0.0030%.

Mg: 0.0005 to 0.0050%,

Mg fixes S to inhibit the toughness and the generation of MnS which is a cause of fracture at the time of processing. In order to obtain such an effect, it is necessary to contain 0.0005% or more. On the other hand, if the Mg content exceeds 0.0050%, the amount of intermetallics increases, resulting in a decrease in toughness. Therefore, when Mg is contained, it is preferable to limit the Mg content to a range of 0.0005 to 0.0050%. Further, it is preferably 0.0005 to 0.0040%.

[Steel organization]

The bainite phase is contained in an area fraction (sometimes referred to as an area ratio) of 60% or more, and the island martensite in the bainite phase is contained in an area fraction of 5% or more and less than 20% An additional ferrite phase, a pearlite and a martensite phase. By using such a fraction, the plastic deformation capability of the steel sheet is improved, and good workability is obtained. In addition, excellent abrasion resistance can be obtained even if the steel sheet is not excessively hardened.

Bainite phase: 60% or more in area fraction

When the bainite phase fraction is less than 60% by area, desired abrasion resistance and processability can not be ensured. Therefore, the content of the bainite phase is set to 60% or more in terms of the area fraction. Preferably 80% or more.

Island-shaped martensite: 5% to less than 20% in area fraction

Since the island-shaped martensite is finely dispersed in the bainite phase and has a high hardness, it contributes to improvement of wear resistance. If the fraction of the island-shaped martensite is less than 5% in terms of the area fraction with respect to the whole structure, the desired abrasion resistance can not be secured. On the other hand, when the above-mentioned area fraction is 20% or more, the effect of improving the wear resistance is saturated and the hardness of the steel sheet is excessively increased, and the workability and toughness are deteriorated. Therefore, the above-mentioned area fraction should be 5% or more and less than 20%. Since the island-shaped martensite is generated between the laths on the bainite or the grain boundaries on the bainite and is minute, it is difficult to separate the bainite phase and the island-shaped martensite with the optical microscope Do. For this reason, the island-shaped martensite is regarded as a part of the bainite phase. That is, in the calculation of the area fraction of the bainite phase, the area of the bainite phase includes the area of the island-shaped martensite. However, the area fraction of island-shaped martensite is calculated for the entire structure.

The balance other than the bainite phase of the steel structure is one or more of ferrite phase, pearlite and martensite phase.

Next, a method of manufacturing a steel sheet according to the present invention will be described.

When the steel material having the above-mentioned composition is kept at a predetermined temperature after casting, the steel material is cooled, heated, and hot-rolled without cooling to obtain a desired steel sheet. The production method of the steel material is not particularly limited, but it is preferable to melt the molten steel by a known solvent method such as a converter to obtain a slab of a predetermined size by a known casting method such as a continuous casting method desirable. The slab may be formed by ingot casting-slabbing method.

The slab heating temperature is limited to a range of 950 to 1250 ° C. Below 950 占 폚, the deformation resistance is high and the rolling load becomes excessive, which hinders the rolling efficiency. In order to stably obtain the abrasion resistance, it is necessary to uniformly produce island-shaped martensite over the entire steel sheet. Below 950 占 폚, the diffusion of segregating elements such as C and Mn present in the micro segregation portion in the steel material is insufficient, and island-shaped martensite is preferentially formed in the segregation portion, causing a deviation in the distribution. On the other hand, at a high temperature exceeding 1250 占 폚, the yield is reduced due to excessive scale generation and the energy consumption is increased. Therefore, the heating temperature is limited to 950 to 1250 占 폚. The slab heating temperature is an average temperature in the thickness direction of the slab calculated by heat transfer-heat conduction calculation. The average temperature in the thickness direction of the slab is almost equal to the temperature at the plate thickness 1/4 position.

For hot rolling, the rolling finishing temperature should be at least Ar ₃ . When the rolling finishing temperature is lower than Ar ₃ , ferrite is produced and a sufficient amount of bainite is not produced. Therefore, the rolling finishing temperature should be at least Ar ₃ . If the rolling finishing temperature is too high, the austenite grain size grows and the austenite grain size becomes large. As a result, the quenching becomes too high and the amount of martensite produced becomes excessive, making it difficult to obtain a desired structure. Therefore, the upper limit of the rolling finishing temperature is preferably 930 캜 or lower. The Ar ₃ transformation point can be measured from the thermal expansion curve when cooling from the austenite. The rolling finishing temperature is the surface temperature of the steel sheet.

Immediately after completion of the hot rolling, accelerated cooling is started. &Quot; Immediate " is within 30 seconds after completion of hot rolling. The cooling rate is 5 ° C / sec or more, and the cooling stop temperature is 400 ° C to 650 ° C. When the cooling rate is less than 5 占 폚 / sec, ferrite is generated and a sufficient amount of bainite is not generated, so that the temperature is set to 5 占 폚 / sec or more. The upper limit of the cooling rate is not particularly limited, but the upper limit of the cooling rate in the accelerated cooling is determined by the heat transfer on the surface of the steel sheet, so that the cooling rate is practically 80 DEG C / sec or less. The cooling rate means an average cooling rate from the start of the acceleration cooling to the end of the acceleration cooling at the plate thickness 1/4 position. In the present invention, the cooling start temperature, the cooling rate, and the cooling stop temperature are specified by the temperature at the plate thickness 1/4 position, but the temperature at the 1/4 plate thickness is the temperature at the surface of the steel plate, This is because it can be considered that the average temperature of the entire steel sheet is represented by the temperature in the middle of the temperature.

When the cooling stop temperature is lower than 400 캜, bainite transformation is completed, so that a sufficient amount of island-shaped martensite is not produced. On the other hand, when the cooling stop temperature exceeds 650 ° C, C is consumed in the pearlite formed at the subsequent air cooling, and a sufficient amount of island-shaped martensite is not produced. Therefore, the cooling stop temperature is 400 to 650 占 폚. The cooling stop temperature is the temperature at the end of accelerated cooling at the plate thickness 1/4 position.

After the completion of hot rolling, in place of a step of performing accelerated cooling, after the end of hot rolling, after the ferrite transformation or the bainite temperature of the sheet thickness 1/4 position transformation is complete, cooling to less than 400 ℃, at least Ac ₃ 950 Deg.] C or lower, and then accelerated cooling may be performed. The initiation of accelerated cooling must be performed before the temperature of the steel sheet is lowered to start the ferrite transformation. Therefore, it is preferable that the steel sheet is taken out from the reheating furnace within 30 seconds.

When the reheating temperature is less than Ac ₃ , reverse transformation from ferrite to austenite does not occur sufficiently. In reheating, since it is necessary to transform the entire steel sheet into austenite, it is heated from the 1 / 2t position of the steel sheet to Ac ₃ or higher. If the reheating temperature exceeds 950 占 폚, the austenite grain size coarsens and adversely affects toughness, resulting in an increase in energy consumption. Therefore, the reheating temperature should be not less than Ac _{3 and not} more than 950 ° C. The reheating temperature is the temperature at the 1 / 2t position of the steel plate and is obtained by heat transfer-heat conduction calculation. Further, the Ac ₃ transformation temperature can be measured from the thermal expansion curve when heating from ferrite to austenite.

Example

Molten steel having the composition shown in Table 1 was dissolved in a vacuum melting furnace and cast into a casting mold to obtain a 150 kg steel ingot (slab). The obtained slab was heated, hot-rolled, and then subjected to accelerated cooling. Further, in some of the steel sheets, after the end of the hot rolling, the steel sheets were air-cooled, further reheated, and then accelerated and cooled.

From the steel sheet thus obtained, a test piece was taken and subjected to tissue observation and abrasion test. The test method was as follows.

(1) Tissue observation

A test piece for tissue observation was taken from the 1/4 position of the plate thickness of the obtained steel sheet so that the observation plane was in parallel with the rolling direction, and then the specimen was polished to the mirror surface and the structure was represented by nital etching. Thereafter, the three fields of view were observed and photographed at a magnification of 400 times using an optical microscope, and the bainite image was visually identified to calculate the area ratio (bainite fraction). In addition, the same test piece for microstructure observation was mirror-polished again, and island-shaped martensite was represented by a two-step etching method. Thereafter, 10 fields of view were observed and photographed from a portion having a bainite structure at a magnification of 2000 times using a scanning electron microscope, and the area ratio (island-shaped martensite fraction) of the island-shaped martensite was measured by an image analysis software . The area ratio of the bainite phase and the island-shaped martensite is an area ratio with respect to the whole structure.

(2) Abrasion test

A wear test piece (size: 10 mm thickness x 25 mm width x 75 mm length) was taken from the obtained steel sheet so that the position of 0.5 mm from the steel sheet surface was the test surface (abrasive surface) And a wear test was conducted.

The abrasion test piece was attached so that a surface of 25 mm x 75 mm perpendicular to the rotation axis of the rotor of the tester was in the circumferential tangential direction of the rotation circle, and then the abrasive was introduced therein. The abrasive used was a silica having an average particle diameter of 30 mm.

The test conditions were as follows: rotor: 600 revolutions per minute, drum: 45 revolutions per minute. The number of revolutions of the rotor was rotated until the total number of times reached 10,000, and then the test was terminated. After completion of the test, the weight of each test piece was measured. The abrasion resistance ratio (= (reference value) / (weight reduction amount of the test piece)) was calculated by using the weight reduction amount of SS400 (JIS G3101 general structure rolled steel) as a reference value, Respectively. When the abrasion resistance ratio was 1.5 or more, it was evaluated as "excellent abrasion resistance".

(3) Bending workability

(T = sheet thickness) under the conditions of a bending radius of 2.0t (t = sheet thickness) using a steel sample (width 100 mm 占 300 mm length x original thickness of steel sheet; tmm) based on JIS Z2248 a 180-degree bend test was conducted by the bend method. When the sample after the bending test had no torn scratches or other defects due to visual observation, it was said that the bending workability was good.

Table 2 shows the results of the test items in accordance with the manufacturing conditions. In the examples of Nos. 1 to 15, 17, 18 and 20, the abrasion resistance ratio was 1.5 or more, and excellent abrasion resistance was confirmed. On the other hand, in No. 16 of the comparative example, the bainite fraction of the steel structure and the island-shaped martensite fraction do not satisfy the requirements of the present invention and the bending workability is poor. In No. 19 of the comparative example, the bainite fraction of the steel structure and the island-shaped martensite fraction do not satisfy the requirements of the present invention, and the abrasion resistance is poor. In Nos. 21 to 23, the island-shaped martensite fractions in the steel structure did not satisfy the requirements of the present invention, and the wear resistance was poor.

Claims (7)

In terms of% by mass,
C: 0.200 to 0.350%
Si: 0.05 to 0.45%
Mn: 0.50 to 2.00%
P: 0.020% or less,
S: 0.005% or less,
Al: 0.005 to 0.100%
(CI) defined by the following formula (1) satisfies 40 or more,
The balance Fe, and inevitable impurities,
Wherein the area fraction of the bainite phase is 60% or more and the island-shaped martensite in the bainite phase is 5% or more and less than 20%
And the remainder has a steel structure composed of at least one of ferrite phase, pearlite and martensite phase.
CI = 60C + 8Si + 22Mn + 10 (Cu + Ni) + 14Cr + 21Mo + 15V (One)
In the formula, each alloy element is defined as a content (mass%). However, the content of the element which does not contain is 0. The method according to claim 1,
Further, in terms of% by mass,
Cu: 0.03 to 1.00%
Ni: 0.03 to 2.00%
Cr: 0.05 to 2.00%
Mo: 0.05 to 1.00%
V: 0.005 to 0.100%,
Nb: 0.005 to 0.100%,
Ti: 0.005% or more and less than 0.100%
B: 0.0003 to 0.0030%,
By weight of the steel sheet. 3. The method according to claim 1 or 2,
Further, in terms of% by mass,
REM: 0.0005 to 0.0080%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005 to 0.0050%
By weight of the steel sheet. A method for producing a steel sheet according to any one of claims 1 to 3, characterized in that a steel sheet or a steel sheet having a steel composition according to any one of claims 1 to ₃ is heated to a temperature of 950 to 1250 캜, And after the hot rolling, accelerated cooling is carried out at a temperature of 1/4 plate thickness at a cooling rate of 5 DEG C / sec or more to 400 DEG C to 650 DEG C . A method for producing a steel sheet according to any one of claims 1 to 3, wherein the steel sheet or the steel sheet having the steel composition according to claim 1 or 2 is heated to 950 to 1250 캜, hot rolled, / 4, and then reheated to a temperature of Ac _{3 to} 950 캜 at a plate thickness of ½, and immediately after reheating, a temperature of 5 And cooling the steel sheet at a cooling rate of 400 캜 to 650 캜 at a cooling rate of at most 0 캜 / sec. 20. A manufacturing method of a steel sheet after described in 3, wherein, after heating the slab or slabs made of a steel composition according to 3, wherein a 950~1250 ℃, subjected to hot rolling ending temperature to the Ar ₃ or more in the steel sheet surface temperature, Immediately after hot rolling, accelerated cooling is carried out at a temperature of 1/4 plate thickness at a cooling rate of 5 占 폚 / sec or higher to 400 占 폚 to 650 占 폚. A method for manufacturing a steel sheet according to claim 3, wherein the steel sheet or the steel sheet having the steel composition according to claim 3 is heated to 950 to 1250 캜, hot-rolled, and heated to a temperature of 400 Deg.] C and then reheated to Ac _{3 to} 950 [deg.] C at a plate thickness of 1/2 position, immediately after reheating, at a cooling rate of 5 [deg.] C / And cooling the steel sheet to a temperature ranging from 0 to 650 캜.

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