KR102674055B1 - Wear-resistant steel sheet and manufacturing method thereof - Google Patents

Abstract

An advantageous method for manufacturing wear-resistant steel sheets with high flatness is provided. A process of obtaining a slab by continuously casting molten steel having a composition containing a predetermined amount of C, Si, Mn, P, S, Cr, Al, Ti, B, and N, with the remainder being Fe and inevitable impurities; A process of heating a slab to 1000 to 1300°C, and then subjecting the slab to hot rolling including finish rolling performed under conditions where the finish rolling temperature is 900°C or higher, to obtain a steel sheet, and the steel sheet A process of cooling under conditions where the average cooling rate between 900 and 300°C is 30°C/s or more, and thereafter, a process of winding the steel sheet under conditions where the coiling temperature is 200°C or lower. Method for manufacturing wear-resistant sheet steel.

Description

Wear-resistant steel sheet and manufacturing method thereof

The present invention relates to thin wear-resistant steel with a plate thickness of less than 6.0 mm, that is, a high-hardness wear-resistant steel sheet and a method for manufacturing the same.

Industrial machinery, parts, and transportation equipment (e.g., power excavators, bulldozers, hoppers, bucket conveyors, rock crushing devices) used in fields such as construction, civil engineering, and mining are subject to abrasive damage by rocks, sand, ore, etc. Exposure to wear such as abrasion, sliding wear, and impact wear. Therefore, steels used in such industrial machines, parts, and transportation equipment are required to have excellent wear resistance in order to improve their lifespan.

It is known that the wear resistance of steel can be improved by increasing hardness. Therefore, high-hardness steel obtained by subjecting alloy steel to which a large amount of alloying elements such as Cr and Mo are added and subjected to heat treatment such as quenching has been widely used as wear-resistant steel.

For example, in Patent Document 1, steel containing 0.10 to 0.19% C, additionally containing an appropriate amount of Si and Mn, and having a carbon equivalent Ceq of 0.35 to 0.44 is hot rolled to obtain a hot rolled steel sheet, This hot-rolled steel sheet is quenched directly or after reheating to 900 to 950°C, and then tempered at 300 to 500°C to produce a wear-resistant thick steel plate whose surface hardness is 300Hv (Vickers hardness) or more. It is done.

Patent Document 2 contains 0.10 to 0.20% of C, and further contains an appropriate amount of Si, Mn, P, S, N, Al, and O, and optionally one of Cu, Ni, Cr, Mo, and B. A method for manufacturing a wear-resistant thick steel sheet is described by hot-rolling a steel material containing the above to obtain a hot-rolled steel sheet, reheating the hot-rolled steel sheet directly or by allowing it to cool, and then quenching the surface hardness to 340 HB (Brinnell hardness) or more. It is done.

Patent Document 3 discloses a steel material containing 0.07 to 0.17% C, further containing an appropriate amount of Si, Mn, V, B, and Al, and optionally containing one or more of Cu, Ni, Cr, and Mo. A method for manufacturing a wear-resistant thick steel sheet is described by hot rolling to obtain a hot-rolled steel sheet, and quenching the hot-rolled steel sheet directly or once by cooling it in the air and then reheating it, so that the surface hardness is 321HB or higher.

In the technology disclosed in Patent Documents 1 to 3, wear resistance is improved by adding a large amount of alloy elements and increasing hardness by utilizing phenomena such as solid solution hardening, transformation hardening, and precipitation hardening.

Patent Document 4 contains 0.10 to 0.45% C and 0.10 to 1.0% Ti, and further contains appropriate amounts of Si, Mn, P, S, N, and Al, and optionally Cu, Ni, Cr, and Mo. , A wear-resistant steel has been proposed in which molten steel containing at least one type of B is continuously cast, and more than 400 precipitates mainly composed of TiC with a size of 0.5 ㎛ or more are deposited per 1 mm2. In the technology disclosed in Patent Document 4, during solidification of continuous casting, coarse precipitates mainly composed of TiC with high hardness are generated, and wear resistance is improved by the precipitates.

Japanese Patent Publication No. 62-142726 Japanese Patent Publication No. 63-169359 Japanese Patent Publication No. 1-142023 Japanese Patent Publication No. 6-256896

Generally, in the production of wear-resistant steel, a thick plate process is adopted in which a slab is hot-rolled using a thick plate mill to form a thick steel plate, the thick steel plate is quenched directly or after reheating, and then optionally tempered. Patent Documents 1 to 4 also describe manufacturing a wear-resistant thick steel plate using a method for manufacturing a wear-resistant steel plate using a thick plate process.

On the other hand, recently, demand for thin steel sheets as wear-resistant steel has been increasing. For example, from the viewpoint of environmental regulations, the weight of dumpers is required to be lighter. Therefore, there is a demand to use thin steel plates as wear-resistant steel used in the trestles of dumps that load high hardness materials such as earth and sand.

However, the thick plate process that has been used to manufacture conventional wear-resistant steel has an industrial limit to producing thick steel plates with a plate thickness of about 6 mm, and the thick plate process is applied to the production of steel plates with a plate thickness of less than 6.0 mm. I couldn't do it. That is, when attempting to manufacture a thin steel plate with a plate thickness of less than 6.0 mm using a thick plate process, there was a problem that the flatness specifications could not be met due to cooling deformation due to the characteristics of the thick plate process.

Therefore, in consideration of the above problems, the purpose of the present invention is to provide a wear-resistant steel sheet with high flatness and an advantageous manufacturing method thereof.

In order to solve the above problems, the present inventors got the idea of producing a wear-resistant steel sheet by a hot rolling process for manufacturing general steel sheets. That is, using a hot rolling mill including a rough rolling mill and a finishing mill used in the hot rolling process, the slab was hot rolled to form a steel sheet. After that, a martensite-based structure was obtained by cooling the steel sheet under conditions where the average cooling rate was 30°C/s or more between 900 and 300°C. Thereafter, by winding the steel sheet under the condition that the coiling temperature was 200°C or lower, a wear-resistant steel sheet with high hardness due to the martensite-based structure was obtained. And, a wear-resistant steel sheet with high flatness could be manufactured through the hot rolling process.

The main structure of the present invention completed based on the above recognition is as follows.

(1) In mass%,

C: 0.10 to 0.30%,

Si: 0.01 to 1.0%,

Mn: 0.30 to 2.00%,

P: 0.03% or less,

S: 0.03% or less,

Cr: 0.01 to 2.00%,

Al: 0.001 to 0.100%,

Ti: 0.001 to 0.050%,

B: 0.0001 to 0.0100%, and

N: 0.01% or less

It contains and has a component composition where the balance consists of Fe and inevitable impurities,

It has a structure with a volume ratio of martensite of 90% or more in the overall plate thickness,

A wear-resistant thin steel sheet characterized in that the hardness at a depth of 0.5 mm from the surface is 360 to 490 HBW5/750 in Brinell hardness.

(2) The above component composition is expressed in mass%,

Cu: 2.00% or less,

Ni: 5.00% or less,

Mo: 3.00% or less,

V: 1.000% or less,

W: 1.50% or less,

Ca: 0.0200% or less,

Mg: 0.0200% or less, and

REM: 0.0500% or less

The wear-resistant steel sheet according to (1) above, further comprising at least one member selected from the group consisting of.

(3) The wear-resistant steel sheet according to (1) or (2) above, wherein the surface roughness Ra is 40 μm or less.

(4) Any one of (1) to (3) above, wherein when a 2 m long object is placed on the surface of the steel sheet along the rolling direction, the maximum gap between the surface of the steel sheet and the long object is 10 mm or less. Wear-resistant steel sheet described in clause.

(5) a process of obtaining a slab by continuously casting molten steel having the composition described in (1) or (2) above,

A process of heating the slab to 1000-1300°C,

Thereafter, a process of subjecting the slab to hot rolling including finish rolling performed under conditions where the finish rolling temperature is 900°C or higher to obtain a steel sheet;

A step of cooling the steel sheet under conditions where the average cooling rate is 30°C/s or more between 900 and 300°C,

Thereafter, a process of winding the steel sheet under conditions where the coiling temperature is 200° C. or lower.

A method of manufacturing a wear-resistant steel sheet, characterized in that it has a.

(6) The method for producing a wear-resistant steel sheet according to (5) above, further comprising a step of performing temper rolling on the steel sheet obtained through the coiling process.

According to the present invention, a wear-resistant steel sheet with high flatness and an advantageous manufacturing method thereof can be provided.

(Form for carrying out the invention)

(Wear-resistant sheet steel)

Hereinafter, the wear-resistant thin steel sheet (hot rolled steel sheet) of the present invention will be described.

[Ingredients Composition]

First, the component composition of the wear-resistant steel sheet of the present invention and the reason for its limitation will be explained. In addition, the unit of content of each element in the component composition is all "% by mass", but hereinafter, unless otherwise specified, it is simply expressed as "%".

C: 0.10 to 0.30%

C is an essential element to increase the hardness of the martensite matrix. When the amount of C is too small, the amount of dissolved C in the martensite phase decreases, so the hardness of the surface layer decreases and the wear resistance deteriorates. From this point of view, the amount of C is set to 0.10% or more, and preferably 0.14% or more. On the other hand, when the amount of C is excessive, weldability and toughness are significantly deteriorated. From this point of view, the amount of C is set to 0.30% or less, and preferably 0.25% or less.

Si: 0.01 to 1.0%

Si is an element effective in deoxidation, and is also an element that contributes to increasing the hardness of steel through solid solution strengthening. From the viewpoint of obtaining these effects, the amount of Si is set to 0.01% or more, and preferably 0.10% or more. On the other hand, when the amount of Si is excessive, it adheres as scale to the surface of the thin steel plate, deteriorating the surface roughness. From this viewpoint, the amount of Si is set to 1.0% or less, and preferably 0.40% or less.

Mn: 0.30 to 2.00%

Mn is an effective element for improving the hardenability of steel. By adding Mn, the hardness of the steel after quenching increases, and as a result, the wear resistance improves. From the viewpoint of obtaining this effect, the Mn amount is set to 0.30% or more, preferably 0.50% or more, and more preferably 0.60% or more. On the other hand, when the amount of Mn is excessive, weldability and toughness are significantly deteriorated. From this viewpoint, the amount of Mn is set to 2.00% or less, and preferably 1.50% or less.

P: 0.03% or less

P is an element that has the effect of increasing the strength of steel, but is an element that reduces the toughness, especially the toughness of the weld zone. Therefore, the amount of P is set to 0.03% or less, preferably 0.02% or less, and more preferably 0.01% or less. On the other hand, since the smaller the amount of P, the more desirable it is, the lower limit is not particularly limited and may be 0%. However, since P is usually unavoidably contained in steel as an impurity, the amount of P should be more than 0% for industrial purposes. Additionally, from the viewpoint of steelmaking costs, it is preferable that the P amount is 0.001% or more.

S: 0.03% or less

S exists as sulfide-based inclusions such as MnS in steel and deteriorates toughness. Therefore, the amount of S is set to 0.03% or less, preferably 0.02% or less, and more preferably 0.015% or less. On the other hand, since the smaller the amount of S, the more desirable it is, the lower limit is not particularly limited and may be 0%. However, since S is usually unavoidably contained in steel as an impurity, the amount of S may be more than 0% for industrial purposes. Additionally, from the viewpoint of steelmaking costs, it is preferable that the amount of S is 0.0001% or more.

Cr: 0.01 to 2.00%

Cr is an effective element for improving the hardenability of steel. By adding Cr, the hardness of the steel after quenching increases, and as a result, the wear resistance improves. From the viewpoint of obtaining this effect, the Cr amount is set to 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, when the amount of Cr is excessive, weldability deteriorates. From this viewpoint, the Cr amount is set to 2.00% or less, preferably 1.80% or less, and more preferably 1.00% or less.

Al: 0.001∼0.100%

Al is an element that is effective as a deoxidizer and has the effect of forming nitrides and reducing the austenite grain size. From the viewpoint of obtaining this effect, the Al amount is set to 0.001% or more, and preferably 0.010% or more. On the other hand, when the amount of Al is excessive, toughness deteriorates. Therefore, the Al amount is set to 0.100% or less, and preferably 0.050% or less.

Ti: 0.001 to 0.050%

Ti is an element with a strong affinity for N, which precipitates as TiN during solidification, reduces dissolved N in the steel, and has the effect of reducing toughness deterioration due to strain aging of N after cold working. Additionally, Ti also contributes to improving the toughness of the weld zone. From the viewpoint of obtaining these effects, the amount of Ti is set to 0.001% or more, preferably 0.005% or more, and more preferably 0.007% or more. On the other hand, if the amount of Ti is excessive, the TiN particles become coarse and the above effects cannot be sufficiently obtained. Therefore, from this viewpoint, the amount of Ti is set to 0.050% or less, and preferably 0.045% or less.

B: 0.0001 to 0.0100%

B is an element that has the effect of improving quenchability by adding a very small amount and thereby improving the strength of the steel sheet. From the viewpoint of obtaining this effect, the amount of B is set to 0.0001% or more, preferably 0.0003% or more, and more preferably 0.0010% or more. On the other hand, when the amount of B is excessive, the toughness, especially the toughness of the weld zone, decreases. Therefore, the amount of B is set to 0.0100% or less, and preferably 0.0040% or less.

N: 0.01% or less

Since N is an element that reduces ductility and toughness, the amount of N is set to 0.01% or less. On the other hand, since the smaller the amount of N, the more preferable it is, the lower limit is not particularly limited and may be 0%. However, since N is usually unavoidably contained in steel as an impurity, the amount of N may be more than 0% for industrial purposes. Additionally, from the viewpoint of steelmaking costs, it is preferable that the N amount is 0.0005% or more.

In addition to the basic components described above, as optional components, for the purpose of improving quenchability and weldability, Cu: 2.00% or less, Ni: 5.00% or less, Mo: 3.00% or less, V: 1.000% or less, W: 1.50% Hereinafter, it may further include at least one selected from the group consisting of Ca: 0.0200% or less, Mg: 0.0200% or less, and REM: 0.0500% or less.

Cu: 2.00% or less

Cu is an element that can improve quenching properties without significantly deteriorating toughness. In order to obtain this effect, the Cu amount is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, when the amount of Cu is excessive, cracking of the steel sheet caused by the Cu-enriched layer formed immediately below the scale becomes a problem. Therefore, when adding Cu, the amount of Cu is set to 2.00% or less, and preferably 1.50% or less.

Ni: 5.00% or less

Ni is an element that has the effect of increasing hardenability and improving toughness. In order to obtain these effects, the Ni amount is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, when the amount of Ni is excessive, an increase in manufacturing cost becomes a problem. Therefore, when adding Ni, the amount of Ni is set to 5.00% or less, and preferably 4.50% or less.

Mo: 3.00% or less

Mo is an element that improves the hardenability of steel. In order to obtain this effect, the Mo amount is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, when the amount of Mo is excessive, weldability decreases. Therefore, when adding Mo, the amount of Mo is set to 3.00% or less, and preferably 2.00% or less.

V: 1.000% or less

V is an element that improves the hardenability of steel. In order to obtain this effect, the amount of V is preferably set to 0.001% or more. On the other hand, when the amount of V is excessive, weldability decreases. Therefore, when adding V, the amount of V is set to 1.000% or less.

W: 1.50% or less

W is an element that improves the hardenability of steel. In order to obtain this effect, the amount of W is preferably set to 0.01% or more. On the other hand, when the amount of W is excessive, weldability decreases. Therefore, when adding W, the amount of W is set to 1.50% or less.

Ca: 0.0200% or less

Ca is an element that improves weldability by forming an acid sulfide with high stability at high temperatures. In order to obtain this effect, it is preferable that the Ca amount is 0.0001% or more. On the other hand, when the amount of Ca is excessive, the cleanliness decreases and the toughness of the steel is impaired. Therefore, when adding Ca, the amount of Ca is set to 0.0200% or less.

Mg: 0.0200% or less

Mg is an element that improves weldability by forming an acid sulfide with high stability at high temperatures. In order to obtain this effect, the amount of Mg is preferably set to 0.0001% or more. On the other hand, if the amount of Mg is excessive, the effect of adding Mg is saturated and an effect corresponding to the content cannot be expected, which becomes economically disadvantageous. Therefore, when adding Mg, the amount of Mg is set to 0.0200% or less.

REM: 0.0500% or less

REM (rare earth metal) is an element that improves weldability by forming oxysulfides with high stability at high temperatures. In order to obtain this effect, it is preferable that the REM amount is 0.0005% or more. On the other hand, if the amount of REM is excessive, the effect of adding REM is saturated and an effect corresponding to the content cannot be expected, which becomes economically disadvantageous. Therefore, when adding REM, the amount of REM is set to 0.0500% or less.

The remainder of the component composition other than the above consists of Fe and inevitable impurities. Additionally, Sb, Sn, Co, As, Pb, and Zn may each be contained in amounts of 1.0% or less.

[group]

The wear-resistant steel sheet of the present invention has a structure in which the volume ratio of martensite is 90% or more over the entire sheet thickness including from the surface to the back surface.

Volume ratio of martensite: 90% or more

If the volume fraction of martensite is less than 90%, the hardness of the matrix structure of the steel sheet decreases, and therefore the wear resistance decreases. Therefore, the volume ratio of martensite is set to 90% or more, and preferably 95% or more. The remaining structure other than martensite is not particularly limited, but may be one or more selected from the group consisting of ferrite, pearlite, austenite, and bainite. On the other hand, since the higher the volume ratio of martensite, the better, the upper limit of the volume ratio is not particularly limited, and 100% is sufficient. In addition, the volume ratio of martensite is taken as a value for the entire sheet thickness including from the surface to the back surface of the wear-resistant steel sheet. The volume fraction of martensite can be measured by the method described in the Examples.

[Hardness]

Brinell hardness: 360∼490HBW5/750

The wear resistance of a steel sheet can be improved by increasing the hardness in the surface layer portion of the steel sheet. Here, in the present invention, Brinell hardness is used as an index for evaluating wear resistance characteristics. If the Brinell hardness of the surface layer of the steel sheet is less than 360 HBW, sufficient wear resistance cannot be obtained. On the other hand, when the Brinell hardness of the surface layer of the steel sheet exceeds 490 HBW, bending workability deteriorates. Therefore, in the present invention, the hardness of the surface layer portion of the steel sheet is set to 360 to 490 HBW in terms of Brinell hardness. In addition, here, “hardness in the surface layer portion” refers to the hardness at a depth of 0.5 mm from the surface of the wear-resistant steel sheet. This is to substantially remove the decarburization layer on the surface of the steel sheet and reduce the deviation of the measured values. In addition, in the present invention, “Brinnell hardness” is a value measured under a load of 750 kgf (unit: HBW5/750) using a tungsten steel ball with a diameter of 5 mm. This Brinell hardness can be measured by the method described in the Examples.

[Plate thickness]

The plate thickness of the wear-resistant steel sheet of the present invention is less than 6.0 mm, preferably 4.5 mm or less, and more preferably 4.0 mm or less. In addition, the lower limit of the sheet thickness is not particularly limited, but is approximately 2.0 mm or more due to limitations in the hot rolling process.

[Surface roughness]

Surface roughness Ra: 40㎛ or less

In the case of wear-resistant thick steel plates manufactured by the conventional thick plate process, they are always in contact with the atmosphere during the cooling (quenching) process after hot rolling, and the exposure time to the atmosphere at high temperatures above 200°C is approximately 20 hours, so the steel plates A lot of scale grew on the surface, and the surface roughness Ra immediately after cooling was about 50 to 150 μm. In contrast, the wear-resistant steel sheet of the present invention is wound in a hot rolling process to become a hot-rolled coil, and in this state, the surface of the steel sheet is not exposed to the atmosphere, so the time exposed to the atmosphere at a high temperature of 200 ° C. or higher is, It takes approximately 30 seconds from finish rolling until winding, and the amount of scale on the surface of the steel sheet is small. As a result, the wear-resistant steel sheet of the present invention can have a surface roughness Ra of 40 μm or less. The lower the surface roughness, the more beautiful the surface of the steel sheet and the better the paintability. Therefore, the present invention is also suitable when using a coated wear-resistant steel sheet. In addition, because the surface roughness is small, when the wear-resistant steel sheet of the present invention is used in areas that contact a rotating body, such as the cover of the shaft for wind power generation, there is no resistance to rotation. In addition, in the wear-resistant steel sheet of the present invention, the lower limit of the surface roughness Ra is not particularly limited, but is approximately 10 μm or more due to limitations in the hot rolling process.

[flatness]

In the conventional thick plate process, shape correction of the thick steel plate after quenching or subsequent tempering is performed using a leveler. Shape correction using a leveler is based on the Bauschinger effect. In principle, it only disperses and equalizes the deformation, and the area that can be corrected is narrow, so the effect of correction is limited. In the case of thick steel plates, cooling deformation is small, so high flatness can be obtained even with shape correction using a leveler. However, in the case of steel sheets that are greatly affected by cooling deformation, high flatness cannot be obtained through shape correction using a leveler. In other words, when attempting to manufacture a steel plate with a plate thickness of less than 6.0 mm using a thick plate process, it is not possible to obtain a steel plate with high flatness. In contrast, the wear-resistant steel sheet of the present invention is manufactured by a hot rolling process. In the hot rolling process, the hot rolled coil is rewound to the skin pass line, front-to-back tension is applied to stretch the steel sheet, and then a leveler is applied, so the range that can be corrected is wide, and the correction effect is high. Therefore, the wear-resistant steel sheet of the present invention can obtain high flatness, and specifically, when a 2 m long object is placed on the surface of the steel sheet along the rolling direction, the maximum value of the gap between the surface of the steel sheet and the long object is 10. It can be set to 5 mm or less, and more preferably 5 mm or less. The smaller the maximum value of the gap, the more desirable it is, and can be 0 mm or more.

(Method of manufacturing wear-resistant thin steel sheet)

The method for manufacturing a wear-resistant steel sheet of the present invention includes the steps of obtaining a slab by continuously casting molten steel having the above-described chemical composition, the step of heating the slab to a predetermined temperature, and thereafter, hot-heating the slab under predetermined conditions. It includes a step of performing rolling to obtain a steel sheet, a step of cooling the steel sheet under predetermined conditions, and a step of winding the steel sheet under predetermined conditions. The wear-resistant steel sheet of the present invention can be obtained by rewinding the hot-rolled coil obtained in this way and performing optional temper rolling for the purpose of shape correction. Hereinafter, each process will be described in detail.

[Continuous casting]

Steel having the above-described composition is melted by a conventional method in a melting facility such as a converter or electric furnace, and then continuously casted to obtain a slab. The conditions for continuous casting are not particularly limited, and it may be performed by a conventional method.

[Slab heating]

Heating temperature: 1000∼1300℃

If the heating temperature is too low, the carbide does not completely dissolve and the solid solution C is insufficient, so the strength is likely to decrease. In addition, quenching properties become insufficient and the hardness of the surface layer of the steel sheet decreases, so wear resistance deteriorates. From this viewpoint, the heating temperature is set to 1000°C or higher, preferably 1100°C or higher, and more preferably 1200°C or higher. On the other hand, if the heating temperature is too high, the structure becomes coarse and the toughness decreases. For this reason, the heating temperature is set to 1300°C or lower. In addition, the heating temperature of the slab is set to the temperature of the slab surface.

[Hot rolling]

Afterwards, hot rolling is performed on the slab to obtain a thin steel plate. This process is performed using a hot rolling mill including a rough rolling mill and a finish rolling mill used in the hot rolling process for manufacturing thin steel plates, rather than the hot rolling mill (thick plate mill) used in the thick plate process. And, the sheet thickness of the steel sheet obtained through this process is as already described as the sheet thickness of the wear-resistant steel sheet of the present invention.

Finish rolling temperature: 900℃ or higher

When the finish rolling temperature is too low, quenching properties become insufficient and the hardness of the surface layer portion of the steel sheet decreases, thereby deteriorating wear resistance. From this point of view, the finish rolling temperature is set to 900°C or higher. The upper limit of the finish rolling temperature is not particularly limited, but if the finish rolling temperature is too high, rolling efficiency deteriorates. From this viewpoint, it is preferable that the finish rolling temperature is 1000°C or lower. In addition, in the present invention, the “finish rolling temperature” is the temperature of the surface of the steel sheet. However, in the case of the steel sheet, the temperature at the center of the sheet thickness is also almost equal to the surface temperature.

[Cooling]

Average cooling rate between 900 and 300℃: 30℃/s or more

Subsequently, by cooling the steel sheet, a martensite-based structure is obtained. At this time, by rapid cooling from the finish rolling temperature, the austenite grains during finish rolling become martensite grains while maintaining their grain size. Here, when the average cooling rate between 900 and 300°C is less than 30°C/s, the volume ratio of martensite becomes less than 90%, the hardness of the surface layer cannot be secured, and wear resistance deteriorates. Therefore, the average cooling rate between 900 and 300°C is 30°C/s or more, and is preferably 50°C/s or more. On the other hand, the upper limit of the average cooling rate is not particularly limited, but due to restrictions on cooling equipment, the average cooling rate is approximately 150°C/s or less. In addition, in the present invention, the “average cooling rate” is determined based on the decrease in the surface temperature of the steel sheet. The means for cooling the steel sheet is not particularly limited, but water cooling is preferable from the viewpoint of obtaining the above-mentioned average cooling rate.

[Winding]

Winding temperature: 200℃ or less

Subsequently, the steel sheet is wound to obtain a hot-rolled coil. When the coiling temperature exceeds 200°C, the volume ratio of martensite becomes less than 90%, the hardness of the surface layer cannot be secured, and wear resistance deteriorates. Therefore, the coiling temperature is set to 200°C or lower, and preferably 150°C or lower. The lower limit of the coiling temperature is not particularly limited, but in order to coil and transport the steel sheet, the coiling temperature is preferably 50°C or higher. In addition, in the present invention, the “coiling temperature” is taken as the temperature of the surface of the steel sheet.

Additionally, in the present invention, the steel sheet may be cooled after finish rolling and then rolled as is, and reheating (tempering) is unnecessary. The time from finish rolling to winding is preferably 30 to 90 seconds.

[Temp rolling]

It is preferable to rewind the hot-rolled coil obtained through the winding process and perform temper rolling for the purpose of shape correction on the steel sheet. Temper rolling performs shape correction by stretching the steel sheet by about 0.1 to 1.0%. Additionally, in temper rolling, it is preferable to use a tension leveler.

Example

Molten steel having the component composition shown in Table 1 was cast to obtain a slab. For each slab, a “hot rolling process” or a “thick plate process” was applied as shown in Table 2 to manufacture a steel plate. At that time, “slab heating temperature”, “finish rolling temperature”, and “average cooling rate” as parameters common to both processes are shown in Table 2. In addition, as a parameter related only to the “hot rolling process”, “coiling temperature” is shown in Table 2. At no level is reheating after cooling performed. The plate thickness at each level is also shown in Table 2.

In addition, regarding the “average cooling rate” at the level of the hot rolling process, at a level where the finish rolling temperature is 900°C or higher and the coiling temperature is 300°C or lower, the average cooling rate is indicated between 900 and 300°C, and the finish rolling temperature is 900°C or higher. At a level where the rolling temperature is less than 900°C and the coiling temperature is 300°C or lower, the average cooling rate is expressed between the finish rolling temperature and 300°C, and when the finish rolling temperature is less than 900°C and the coiling temperature is more than 300°C. At the phosphorus level, the average cooling rate between the finish rolling temperature and the coiling temperature was shown. In addition, regarding the “average cooling rate” at the level of the thick plate process, at a level where the finish rolling temperature is 900°C or higher, the average cooling rate is between 900 and 300°C, and at a level where the finish rolling temperature is below 900°C, the average cooling rate is indicated. The average cooling rate from the rolling temperature to 300°C was shown.

Regarding the level of the hot rolling process, temper rolling was performed. Regarding the level of the thick plate process, the shape of the thick steel plate after cooling (quenching) was corrected using a leveler.

[Volume ratio of martensite]

A sample exposing a cross section in the sheet thickness direction parallel to the rolling direction was taken from the central portion in the width direction of the steel plate at each level, and the cross section was mirror polished and further subjected to nital corrosion. Using a scanning electron microscope (SEM), a total of 3 views including the steel sheet surface (2 views on one side and the other side) and the center of the plate thickness were observed at a magnification of 400x among the cross sections in the sheet thickness direction. , filmed. The area fraction of martensite was determined by analyzing the obtained image using an image analysis device. In this specification, when the area fraction of martensite is 90% or more in all three views, the volume fraction of martensite is considered to be 90% or more in the entire plate thickness. Therefore, the minimum value among the area fractions of martensite in the three fields of view was set as the “volume fraction of martensite” and is listed in Table 2.

[Brinnell hardness]

Samples are taken from each level of thin steel plate or thick steel plate, 0.5 mm of the surface layer (thickness of 0.5 mm from the surface) of each sample is ground, and the surface is then mirror polished, in accordance with JIS Z2243 (2008). On the surface after mirror polishing, the Brinell hardness was measured at 5 points, and the average of the 5 points is shown in the “Brinnell hardness” column in Table 2. A tungsten steel ball with a diameter of 5 mm was used for the measurement, and the load was 750 kgf.

[Surface roughness]

For each level of thin steel plate or thick steel plate, the arithmetic mean height Ra specified in JIS B 0601-2001 was obtained by non-contact measurement method, and the results are shown in Table 2.

[flatness]

When a 2 m long piece was placed on the surface of each level of thin steel plate or thick steel plate along the rolling direction, the gap between the steel plate surface and the long piece was measured with a skimmy gauge, and the maximum value was obtained. The measurement was performed at a total of three locations in the center and both ends of the steel plate in the width direction, and the average of the three maximum values is shown in Table 2.

(Industrial applicability)

According to the present invention, a wear-resistant steel sheet with high flatness and an advantageous manufacturing method thereof can be provided.

Claims (6)

In mass%,
C: 0.10 to 0.30%,
Si: 0.01 to 1.0%,
Mn: 0.30 to 2.00%,
P: 0.03% or less,
S: 0.03% or less,
Cr: 0.01 to 2.00%,
Al: 0.001 to 0.100%,
Ti: 0.001 to 0.050%,
B: 0.0001 to 0.0100%, and
N: 0.01% or less
It contains and has a component composition where the balance consists of Fe and inevitable impurities,
It has a structure with a volume ratio of martensite of 90% or more in the overall plate thickness,
The hardness at a depth of 0.5 mm from the surface is 360 to 490 HBW5/750 in Brinell hardness,
A wear-resistant steel sheet, characterized in that when a 2 m long piece is placed on the surface of the steel sheet along the rolling direction, the maximum gap between the surface of the steel sheet and the long piece is 10 mm or less. According to paragraph 1,
The component composition is expressed in mass%,
Cu: 2.00% or less,
Ni: 5.00% or less,
Mo: 3.00% or less,
V: 1.000% or less,
W: 1.50% or less,
Ca: 0.0200% or less,
Mg: 0.0200% or less, and
REM: 0.0500% or less
A wear-resistant steel sheet, further comprising at least one selected from the group consisting of. According to claim 1 or 2,
Wear-resistant steel sheet with a surface roughness Ra of 40㎛ or less. delete A process of obtaining a slab by continuously casting molten steel having the composition described in claim 1 or 2;
A process of heating the slab to 1000-1300°C,
Thereafter, a process of subjecting the slab to hot rolling including finish rolling performed under conditions where the finish rolling temperature is 900°C or higher to obtain a steel sheet;
A process of cooling the steel sheet under conditions where the average cooling rate is 30°C/s or more between 900 and 300°C;
Thereafter, there is a process of winding the steel sheet under conditions where the coiling temperature is 200° C. or lower,
It has a structure in which the volume ratio of martensite is 90% or more in the overall plate thickness, the hardness at a depth of 0.5 mm from the surface is 360 to 490 HBW5/750 in Brinell hardness, and a 2 m long strip along the rolling direction is formed on the surface of the steel plate. A method for producing a wear-resistant steel sheet, characterized in that the maximum value of the gap between the surface of the steel sheet and the elongated piece when touched is 10 mm or less. According to clause 5,
A method for producing a wear-resistant steel sheet, further comprising the step of performing temper rolling on the steel sheet obtained by the coiling process.