CN114127322B

CN114127322B - Wear-resistant thin steel sheet and method for producing same

Info

Publication number: CN114127322B
Application number: CN202080052281.9A
Authority: CN
Inventors: 远藤贵之; 室田康宏
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2019-08-26
Filing date: 2020-06-09
Publication date: 2023-01-10
Anticipated expiration: 2040-06-09
Also published as: TW202108780A; TWI744952B; JP6874916B1; JPWO2021039021A1; WO2021039021A1; CN114127322A; KR20220032112A

Abstract

The present invention provides an advantageous method for producing a wear-resistant thin steel sheet having high flatness. A method for producing a wear-resistant thin steel sheet, comprising the steps of: a step of continuously casting molten steel having a composition containing predetermined amounts of C, si, mn, P, S, cr, al, ti, B and N, with the balance being Fe and unavoidable impurities, to obtain a slab; heating the plate blank to 1000-1300 ℃; thereafter subjecting the slab to hot rolling including finish rolling at a finish rolling temperature of 900 ℃ or higher to obtain a thin steel sheet; a step of cooling the steel sheet under the condition that the average cooling rate between 900 and 300 ℃ is more than 30 ℃/s; and thereafter winding the steel sheet at a winding temperature of 200 ℃ or lower.

Description

Wear-resistant thin steel sheet and method for producing same

Technical Field

The present invention relates to a wear-resistant steel sheet having high hardness, which is a thin material having a sheet thickness of less than 6.0mm, and a method for producing the same.

Background

Industrial machinery, parts, transportation equipment (e.g., an electric shovel, a bulldozer, a hopper, a bucket conveyor, a rock crushing apparatus), and the like used in the fields of construction, civil engineering, mining, and the like are exposed to abrasion such as abrasive abrasion, sliding abrasion, impact abrasion, and the like by rocks, sand, ore, and the like. Therefore, steels used for such industrial machines, parts, and transportation equipment are required to have excellent wear resistance to improve life.

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

For example, patent document 1 describes a method for producing a wear-resistant thick steel plate, the method including: a hot-rolled steel sheet is produced by hot-rolling a steel containing 0.10 to 0.19% of C and further containing an appropriate amount of Si, mn, and carbon, and having an equivalent Ceq of 0.35 to 0.44, and the hot-rolled steel sheet is quenched directly or after being reheated to 900 to 950 ℃, and is further tempered at 300 to 500 ℃, whereby the surface hardness is 300Hv (Vickers hardness) or more.

Patent document 2 describes a method for producing a wear-resistant thick steel plate, the method including: a hot-rolled steel sheet is produced by hot-rolling a steel billet containing 0.10 to 0.20% of C, an appropriate amount of Si, mn, P, S, N, al, O, and optionally one or more of Cu, ni, cr, mo, and B, and is quenched directly or after reheating to have a surface hardness of 340HB (Brinell hardness) or more.

Patent document 3 describes a method for producing a wear-resistant thick steel plate, the method including: a hot-rolled steel sheet is produced by hot-rolling a steel material containing 0.07 to 0.17% of C, an appropriate amount of Si, mn, V, B, al, and optionally one or more of Cu, ni, cr, and Mo, and is quenched directly or after temporary air-cooling reheating to have a surface hardness of 321HB or more.

In the techniques disclosed in patent documents 1 to 3, wear resistance is improved by adding a large amount of alloying elements to improve hardness by utilizing phenomena such as solid solution solidification, transformation solidification, and precipitation solidification.

Patent document 4 proposes a wear-resistant steel: continuously casting molten steel containing 0.10 to 0.45% of C, 0.10 to 1.0% of Ti, and further containing an appropriate amount of Si, mn, P, S, N, al, and further optionally containing at least one of Cu, ni, cr, mo, and B, at a rate of 1mm per molten steel ² 400 or more precipitates mainly comprising TiC having a size of 0.5 μm or more are precipitated. In the technique disclosed in patent document 4, coarse precipitates mainly composed of TiC having high hardness are generated during continuous casting solidification, and the precipitates improve the wear resistance.

Documents of the prior art

Patent literature

Patent document 1: japanese patent laid-open publication No. Sho 62-142726

Patent document 2: japanese patent laid-open publication No. Sho 63-169359

Patent document 3: japanese patent laid-open publication No. H1-142023

Patent document 4: japanese patent laid-open publication No. 6-256896.

Disclosure of Invention

In general, a slab is hot-rolled by a slab mill to produce a thick steel plate by a thick plate process, and the thick steel plate is quenched directly or after reheating and then optionally tempered. Patent documents 1 to 4 describe a method for producing a wear-resistant thick steel plate by a thick-plate process.

On the other hand, in recent years, demand for thin steel sheets has increased as wear-resistant steels. For example, from the viewpoint of environmental restrictions, the weight of the dump truck is required to be reduced. Therefore, thin steel sheets are desired as abrasion resistant steels used for the brackets of dump trucks that are loaded with high hardness materials such as sand and soil.

However, in the conventional thick plate process for producing wear-resistant steel, it is limited to industrially produce a thick steel plate having a thickness of about 6mm, and the thick plate process cannot be applied to the production of a thin steel plate having a thickness of less than 6.0 mm. That is, when a thin steel sheet having a thickness of less than 6.0mm is to be produced by a thick-plate process, there is a problem that the flatness specification cannot be satisfied due to cooling deformation in the characteristics of the thick-plate process.

In view of the above problems, an object of the present invention is to provide a wear-resistant thin steel sheet having high flatness and an advantageous method for producing the same.

In order to solve the above problems, the present inventors have conceived of manufacturing a wear-resistant thin steel sheet by a hot rolling process for manufacturing a general thin steel sheet. That is, a hot rolling mill including a roughing mill and a finishing mill used in a hot rolling process is used to hot-roll a slab to produce a steel sheet. Thereafter, the thin steel sheet is cooled at an average cooling rate of 30 ℃/s or more at 900 to 300 ℃, whereby a martensite-based structure can be obtained. Thereafter, the thin steel sheet is wound at a winding temperature of 200 ℃ or lower, whereby a wear-resistant thin steel sheet having a martensite-based structure and high hardness can be obtained. In addition, the wear-resistant thin steel sheet having high flatness can be manufactured by the hot rolling process.

The gist of the present invention completed based on the above-described situation is as follows.

(1) A wear resistant steel sheet having a composition of: contains, 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: 0.0001-0.0100% and N:0.01% or less, the remainder being Fe and unavoidable impurities, and having a structure in which the volume fraction of martensite is 90% or more throughout the thickness of the sheet, and the hardness at a depth of 0.5mm from the surface being 360 to 490HBW5/750 in terms of Brinell hardness.

(2) The wear resistant steel sheet as set forth in the above (1), wherein the above composition further contains, in mass%, a metal selected from the group consisting of 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% of the following, mg:0.0200% or less and REM:0.0500% or less.

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

(4) The wear-resistant thin steel sheet as claimed in any one of the above (1) to (3), wherein a maximum value of a gap between the surface of the steel sheet and the strip when a 2m strip is brought into contact with the surface of the steel sheet along the rolling direction is 10mm or less.

(5) A method for producing a wear-resistant thin steel sheet, comprising the steps of:

a step of continuously casting the molten steel having the composition of the above (1) or (2) to obtain a slab;

heating the plate blank to 1000-1300 ℃;

thereafter, subjecting the slab to hot rolling including finish rolling at a finish rolling temperature of 900 ℃ or higher to obtain a thin steel sheet;

a step of cooling the steel sheet under the condition that the average cooling rate between 900 and 300 ℃ is more than 30 ℃/s; and

and then, winding the steel sheet at a winding temperature of 200 ℃ or lower.

(6) The method for producing a wear-resistant thin steel sheet as set forth in the above (5), further comprising a step of subjecting the thin steel sheet obtained in the winding step to temper rolling.

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

Detailed Description

(abrasion-resistant thin steel sheet)

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

[ composition of ingredients ]

First, the composition of the wear-resistant steel sheet of the present invention and the reasons for the limitation thereof will be described. The unit of the content of each element in the component composition is "mass%", but hereinafter, unless otherwise specified, it is represented by "%".

C：0.10～0.30％

C is an element required to increase the hardness of the martensitic matrix. If the amount of C is too small, the amount of solid-solution C in the martensite phase becomes small, so that the hardness of the surface layer portion is reduced and the wear resistance is deteriorated. From this viewpoint, the C content is 0.10% or more, preferably 0.14% or more. On the other hand, if the amount of C is too large, weldability and toughness deteriorate remarkably. From this viewpoint, the C content is 0.30% or less, preferably 0.25% or less.

Si：0.01～1.0％

Si is an element effective for deoxidation, and contributes to increasing the hardness of steel by solid-solution strengthening. From the viewpoint of obtaining these effects, the Si content is 0.01% or more, preferably 0.10% or more. On the other hand, if the amount of Si is too large, the Si adheres to the surface of the steel sheet as scale, and deteriorates the surface roughness. From this viewpoint, the Si content is 1.0% or less, preferably 0.40% or less.

Mn：0.30～2.00％

Mn is an element effective for improving the hardenability of steel. By adding Mn, the hardness of the steel after quenching is improved, and as a result, the wear resistance is improved. From the viewpoint of obtaining this effect, the Mn content is 0.30% or more, preferably 0.50% or more, and more preferably 0.60% or more. On the other hand, if the Mn amount is too large, weldability and toughness deteriorate remarkably. From this viewpoint, the Mn content is 2.00% or less, preferably 1.50% or less.

P: less than 0.03%

P is an element that has an effect of improving the strength of steel, and is an element that reduces toughness, particularly toughness of a weld zone. Therefore, the P content is 0.03% or less, preferably 0.02% or less, and more preferably 0.01% or less. On the other hand, the lower limit is not particularly limited, and may be 0%, as the amount of P is preferably smaller. Among them, P is generally inevitably contained as an impurity in steel, and therefore the amount of P may exceed 0% industrially. From the viewpoint of steel production cost, the P content is preferably 0.001% or more.

S: less than 0.03%

S is present in steel as sulfide inclusions such as MnS, and deteriorates toughness. Therefore, the S content is 0.03% or less, preferably 0.02% or less, and more preferably 0.015% or less. On the other hand, the smaller the amount of S, the more preferable the lower limit is not particularly limited, and may be 0%. However, since S is usually unavoidable as an impurity in steel, the S content may be more than 0% in industry. From the viewpoint of steel production cost, the S content is preferably 0.0001% or more.

Cr：0.01～2.00％

Cr is an element effective for improving the hardenability of steel. By adding Cr, the hardness of the steel after quenching is increased, and as a result, the wear resistance is improved. From the viewpoint of obtaining this effect, the Cr content is 0.01% or more, 0.05% or more, and more preferably 0.10% or more. On the other hand, if the Cr amount is too large, weldability deteriorates. From this viewpoint, the Cr amount is 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 effective as a deoxidizer and has an effect of reducing austenite grain size by forming a nitride. From the viewpoint of obtaining this effect, the Al content is 0.001% or more, preferably 0.010% or more. On the other hand, if the amount of Al is too large, the toughness deteriorates. Therefore, the Al content is 0.100% or less, preferably 0.050% or less.

Ti：0.001～0.050％

Ti is an element having a strong affinity for N, and precipitates as TiN during solidification, thereby reducing dissolved N in the steel and reducing deterioration of toughness due to strain aging of N after cold working. In addition, ti also contributes to the improvement of toughness of the weld portion. From the viewpoint of obtaining these effects, the Ti content is 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 too large, the TiN particles become coarse, and the above-described effects cannot be sufficiently obtained. Therefore, from this viewpoint, the Ti content is 0.050% or less, and preferably 0.045% or less.

B：0.0001～0.0100％

B is an element having an effect of increasing the hardenability and thereby increasing the strength of the steel sheet by adding a very small amount of B. From the viewpoint of obtaining this effect, the amount of B is 0.0001% or more, preferably 0.0003% or more, and more preferably 0.0010% or more. On the other hand, if the amount of B is too large, the toughness, particularly the toughness of the weld portion, is reduced. Therefore, the amount of B is 0.0100% or less, preferably 0.0040% or less.

N: less than 0.01%

Since N is an element that reduces ductility and toughness, the amount of N is 0.01% or less. On the other hand, the smaller the amount of N, the more preferable, and therefore, the lower limit is not particularly limited and may be 0%. However, since N is inevitably contained as an impurity in steel, the amount of N may exceed 0% industrially. From the viewpoint of steel production cost, the N content is preferably 0.0005% or more.

In addition to the above-mentioned basic components, any of them may further contain, for the purpose of improving hardenability and weldability, a metal selected from the group consisting of 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.

Cu:2.00% or less

Cu is an element that improves hardenability without significantly reducing toughness. In order to obtain this effect, the amount of Cu is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if the amount of Cu is too large, cracking of the steel sheet due to a Cu-concentrated layer formed directly below the scale becomes a problem. Therefore, when Cu is added, the amount of Cu is 2.00% or less, preferably 1.50% or less.

Ni:5.00% or less

Ni is an element having an effect of improving hardenability and improving toughness. In order to obtain these effects, the amount of Ni is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if the amount of Ni is too large, the increase in production cost becomes a problem. Therefore, when Ni is added, the Ni content is 5.00% or less, 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, if the Mo amount is too large, weldability decreases. Therefore, when Mo is added, the Mo amount is 3.00% or less, preferably 2.00% or less.

V:1.000% or less

V is an element for improving the hardenability of steel. In order to obtain this effect, the amount of V is preferably 0.001% or more. On the other hand, if the V amount is too large, weldability decreases. Therefore, when V is added, the amount of V is 1.000% or less.

W:1.50% or less

W is an element for improving the hardenability of steel. In order to obtain this effect, the amount of W is preferably 0.01% or more. On the other hand, if the amount of W is too large, weldability decreases. Therefore, when W is added, the amount of W is 1.50% or less.

Ca:0.0200% or less

Ca is an element that improves weldability by forming oxysulfide having high stability at high temperatures. In order to obtain this effect, the amount of Ca is preferably 0.0001% or more. On the other hand, if the amount of Ca is too large, the cleanliness is reduced and the toughness of the steel is impaired. Therefore, when Ca is added, the amount of Ca is 0.0200% or less.

Mg:0.0200% or less

Mg is an element that improves weldability by forming oxysulfide having high stability at high temperature. In order to obtain this effect, the amount of Mg is preferably 0.0001% or more. On the other hand, when the amount of Mg is too large, the effect of Mg addition is saturated, and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when Mg is added, the Mg amount is 0.0200% or less.

REM: less than 0.0500%

REM (rare earth metal) is an element that improves weldability by forming oxysulfide having high stability at high temperature. In order to obtain this effect, the amount of REM is preferably 0.0005% or more. On the other hand, when the amount of REM is too large, the effect of REM addition is saturated, and an effect commensurate with the amount cannot be expected, which is economically disadvantageous. Therefore, when REM is added, the amount of REM is 0.0500% or less.

The remainder of the composition other than the above is composed of Fe and inevitable impurities. Sb, sn, co, as, pb, and Zn may be contained in an amount of 1.0% or less.

[ tissue ]

The wear-resistant thin steel sheet of the present invention has a structure in which the volume fraction of martensite throughout the sheet thickness from the front surface to the back surface is 90% or more.

Volume fraction of martensite: over 90 percent

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 fraction of martensite is 90% or more, preferably 95% or more. The structure of the remainder other than martensite is not particularly limited, and may be 1 or more selected from ferrite, pearlite, austenite, and bainite. On the other hand, the higher the volume fraction of martensite, the better, and therefore the upper limit of the volume fraction is not particularly limited, and may be 100%. The volume fraction of martensite is a value of the entire thickness of the wear-resistant thin steel sheet from the front surface to the back surface. The volume fraction of martensite can be measured by the method described in examples.

[ hardness ]

Brinell hardness: 360-490 HBW5/750

The wear resistance of the steel sheet can be improved by increasing the hardness of 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. When the brinell hardness of the surface layer portion of the steel sheet is less than 360HBW, sufficient abrasion resistance cannot be obtained. On the other hand, when the brinell hardness of the surface layer portion of the steel sheet exceeds 490HBW, the bending workability is deteriorated. Therefore, in the present invention, the hardness of the surface layer portion of the steel sheet is 360 to 490HBW in terms of Brinell hardness. Here, the "hardness of the surface portion" means the hardness of the wear-resistant thin steel sheet from the surface to a depth of 0.5 mm. This is to substantially remove the decarburized layer on the surface layer of the steel sheet and to reduce variations in measured values. In the present invention, the "Brinell hardness" is a value (unit: HBW 5/750) measured by a load of 750kgf using a tungsten steel ball having a diameter of 5 mm. The Brinell hardness can be measured by the method described in examples.

[ sheet thickness ]

The wear-resistant steel sheet of the present invention has a thickness of less than 6.0mm, preferably 4.5mm or less, and more preferably 4.0mm or less. The lower limit of the sheet thickness is not particularly limited, but is generally 2.0mm or more from the viewpoint of the limitation of the hot rolling process.

[ surface roughness ]

Surface roughness Ra: less than 40 μm

In the case of a wear-resistant thick steel sheet produced by a conventional thick-plate process, since the steel sheet is always in contact with the atmosphere during cooling (quenching) after hot rolling and is exposed to the atmosphere at a high temperature of 200 ℃ or higher for a period of time of approximately 20 hours, a large amount of scale is generated on the surface of the steel sheet, and the surface roughness Ra after cooling is approximately 50 to 150 μm. In contrast, since the wear-resistant steel sheet of the present invention is wound in a hot rolling process to form a hot rolled coil, and the surface of the steel sheet is exposed to the atmosphere in this state, the time of exposure to the atmosphere at a high temperature of 200 ℃ or higher is about 30 seconds from the finish rolling to the 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 applicable to the case where the wear-resistant thin steel sheet is coated and used. Further, by making the surface roughness small, the wear-resistant thin steel sheet of the present invention does not become a resistance to rotation when used mainly at a portion in contact with a rotating body, such as a cover at the shaft center for wind power generation. In the wear-resistant steel sheet of the present invention, the lower limit of the surface roughness Ra is not particularly limited, but is about 10 μm or more from the viewpoint of the limitation of the hot rolling process.

[ flatness ]

In the conventional thick plate process, the shape of the thick steel plate after quenching or after tempering is corrected by using a leveler. The shape correction by the leveler is based on the bauschinger effect, and basically, the shape correction is performed only by dispersing and homogenizing deformation, and the correctable area is narrow, so that the correction effect is limited. In the case of a thick steel plate, since the cooling deformation is small, high flatness can be obtained even if the shape is straightened by a leveler. However, in the case of a thin steel sheet which is greatly affected by cold deformation, high flatness cannot be obtained in shape correction by a leveler. That is, when a thin steel sheet having a thickness of less than 6.0mm is to be produced by a thick plate process, a thin steel sheet having a high flatness cannot be obtained. In contrast, the wear-resistant steel sheet of the present invention is manufactured by a hot rolling process. In the hot rolling process, a hot rolled coil is wound by a skin pass line, a front-rear tension is applied to the hot rolled coil to elongate the thin steel sheet, and a leveler is applied to the sheet, so that the leveling range is wide and the leveling effect is good. Therefore, the wear-resistant thin steel sheet of the present invention can obtain high flatness, and specifically, the maximum value of the gap between the surface of the steel sheet and the strip when a 2m strip is brought into contact with the surface of the steel sheet in the rolling direction can be 10mm or less, more preferably 5mm or less. The smaller the maximum value of the gap is, the more preferable it is, the 0mm or more is possible.

(method of manufacturing abrasion-resistant thin Steel sheet)

The method for producing a wear-resistant thin steel sheet of the present invention comprises the steps of: a step of continuously casting molten steel having the above-described composition to obtain a slab; heating the slab to a predetermined temperature; thereafter, hot rolling the slab under a predetermined condition to obtain a thin steel sheet; thereafter cooling the steel sheet under predetermined conditions; and then winding the steel sheet under a predetermined condition. The wear-resistant thin steel sheet of the present invention can be obtained by winding the hot rolled coil thus obtained and optionally temper rolling for the purpose of shape correction. Hereinafter, each step will be described in detail.

[ continuous casting ]

Steel having the above composition is melted by a conventional method using a melting facility such as a converter or an electric furnace, and continuously cast to obtain a slab. The conditions for continuous casting are not particularly limited, and the continuous casting may be carried out according to a usual method.

[ heating of the slab ]

Heating temperature: 1000-1300 deg.C

When the heating temperature is too low, the carbide is not completely melted and the solid solution C is insufficient, so that the strength is likely to be lowered. Further, hardenability is insufficient, and hardness of the surface layer portion of the thin steel sheet is lowered, and therefore, abrasion resistance is deteriorated. From this viewpoint, the heating temperature is 1000 ℃ or higher, preferably 1100 ℃ or higher, and more preferably 1200 ℃ or higher. On the other hand, if the heating temperature is too high, the structure becomes coarse and the toughness is lowered. Therefore, the heating temperature is 1300 ℃ or lower. The heating temperature of the slab is the temperature of the slab surface.

[ Hot Rolling ]

Thereafter, the slab is hot-rolled to obtain a steel sheet. This process is performed not by using a hot rolling mill (thick plate mill) used in the thick plate process but by using a hot rolling mill including a roughing mill and a finishing mill used in the hot rolling process for manufacturing a thin steel plate. The thickness of the thin steel sheet obtained in this step is as described above as the thickness of the wear-resistant thin steel sheet of the present invention.

The finishing temperature is as follows: above 900 deg.C

When the finish rolling temperature is too low, hardenability is insufficient, and hardness of the surface layer portion of the steel sheet is lowered, so that wear resistance is deteriorated. From this viewpoint, the finish rolling temperature is 900 ℃ or higher. The upper limit of the finish rolling temperature is not particularly limited, and when the finish rolling temperature is too high, the rolling efficiency deteriorates. From this viewpoint, the finish rolling temperature is preferably 1000 ℃ or lower. In the present invention, the "finish rolling temperature" is the temperature of the surface of the steel sheet, but in the case of the steel sheet, the temperature of the center portion of the sheet thickness is also substantially the same as the surface temperature.

[ Cooling ]

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

Subsequently, the thin steel sheet is cooled to obtain a structure mainly composed of martensite. At this time, the austenite grains in the finish rolling become martensite grains while maintaining their grain size by rapid cooling from the finish rolling temperature. Here, when the average cooling rate between 900 and 300 ℃ is less than 30 ℃/s, the volume fraction of martensite is less than 90%, and the hardness of the surface layer portion cannot be secured, and the wear resistance is deteriorated. Therefore, the average cooling rate between 900 and 300 ℃ is 30 ℃/s or more, preferably 50 ℃/s or more. On the other hand, the upper limit of the average cooling rate is not particularly limited, but from the viewpoint of restrictions on cooling equipment, the average cooling rate is generally 150 ℃/s or less. In the present invention, the "average cooling rate" is determined based on the decrease in the surface temperature of the steel sheet. The cooling means of the steel sheet is not particularly limited, but from the viewpoint of obtaining the above average cooling rate, water cooling is preferred.

[ winding ]

Winding temperature: below 200 deg.C

Next, the steel sheet is wound to obtain a hot-rolled coil. When the winding temperature exceeds 200 ℃, the volume fraction of martensite is less than 90%, and hardness of the surface layer portion cannot be secured, and abrasion resistance is deteriorated. Therefore, the winding temperature is 200 ℃ or less, preferably 150 ℃ or less. The lower limit of the winding temperature is not particularly limited, but the winding temperature is preferably 50 ℃ or higher for transporting the wound steel sheet. In the present invention, the "winding temperature" is the temperature of the surface of the steel sheet.

In the present invention, the steel sheet after finish rolling and after cooling can be directly wound without reheating (tempering). After the finish rolling, the time required for winding is preferably 30 to 90 seconds.

[ temper Rolling ]

It is preferable that the hot-rolled coil obtained in the winding step is wound, and the thin steel sheet is temper-rolled for the purpose of straightening the shape. Temper rolling is performed by elongating a steel sheet by about 0.1 to 1.0% to correct the shape. In the temper rolling, a tension leveler is preferably used.

Examples

Molten steel having a composition shown in table 1 was cast to obtain a slab. Steel sheets were produced by applying the "hot rolling process" or the "slab process" to each slab as shown in table 2. In this case, "slab heating temperature", "finish rolling temperature", and "average cooling rate" are shown in table 2 as parameters common to both steps. In addition, "winding temperature" is shown in table 2 as a parameter relating only to "hot rolling process". In either reference, reheating after cooling was not performed. The thicknesses of the sheets under the respective standards are also shown in table 2.

The "average cooling rate" as a reference in the hot rolling process means an average cooling rate between 900 and 300 ℃ on the basis of a finishing temperature of 900 ℃ or higher and a coiling temperature of 300 ℃ or lower, an average cooling rate between a finishing temperature and 300 ℃ on the basis of a finishing temperature of less than 900 ℃ and a coiling temperature of 300 ℃ or lower, and an average cooling rate between a finishing temperature and a coiling temperature on the basis of a finishing temperature of less than 900 ℃ and a coiling temperature of more than 300 ℃. The "average cooling rate" as a standard in the thick plate process means an average cooling rate between 900 and 300 ℃ on the basis of a finish rolling temperature of 900 ℃ or higher, and an average cooling rate between the finish rolling temperature and 300 ℃ on the basis of a finish rolling temperature of less than 900 ℃.

For the standard of the hot rolling process, temper rolling was performed. For the standard of the thick plate process, the shape of the cooled (quenched) thick steel plate is corrected by a leveler.

[ volume fraction of martensite ]

Samples were taken from the widthwise central portions of the steel sheets of each standard so as to expose a cross section in the thickness direction parallel to the rolling direction, and the cross section was mirror-polished and further subjected to nital etching. A total of 3 fields including the field of view of the steel plate surface (2 fields of view on one side and the other side) and the field of view of the plate thickness center in the cross section in the plate thickness direction were observed and photographed at a magnification of 400 times using a Scanning Electron Microscope (SEM). The obtained image was analyzed by an image analyzer to obtain the area fraction of martensite. In the present specification, when the area fraction of martensite is 90% or more in all 3 fields of view, the volume fraction of martensite in the total plate thickness is considered to be 90% or more. Therefore, the minimum value among the area fractions of martensite in 3 fields of view is shown in table 2 as "the volume fraction of martensite".

[ Brinell hardness ]

Samples were collected from each standard steel sheet or thick steel plate, the surface layer of each sample was polished to 0.5mm (thickness of 0.5mm from the surface), and after the surface was mirror-polished, the brinell hardness was measured at 5 points on the mirror-polished surface in accordance with JIS Z2243 (2008), and the average value of the 5 points is shown in the column "brinell hardness" in table 2. A tungsten steel ball having a diameter of 5mm was used for the measurement, and the load was 750kgf.

[ surface roughness ]

The arithmetic average height Ra specified in JIS B0601-2001 was determined for each standard steel sheet or thick steel sheet by a non-contact measurement method, and the results are shown in table 2.

[ flatness ]

The gap between the surface of the steel sheet and the strip was measured by a feeler gauge when the surface of each standard thin steel sheet or thick steel sheet was brought into contact with the strip of 2m in the rolling direction, and the maximum value was obtained. The measurement was performed at 3 total locations in the center and both ends of the steel sheet in the width direction, and the average of the 3 maximum values is shown in table 2.

Industrial applicability of the invention

Claims

1. A wear-resistant thin steel sheet having a sheet thickness of 4.5mm or less, characterized by having a composition of: contains, 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: 0.0001-0.0100% and N:0.01% or less, the remainder being Fe and inevitable impurities,

has a structure in which the volume fraction of martensite is 90% or more throughout the thickness of the sheet, and the hardness at a depth of 0.5mm from the surface is 360 to 490HBW5/750 in terms of Brinell hardness,

the surface roughness Ra is 40 μm or less.

2. The wear resistant steel sheet as set forth in claim 1, wherein the composition further contains, in mass%, a metal selected from the group consisting of 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% of the following, mg:0.0200% or less and REM:0.0500% or less.

3. The wear-resistant steel sheet as set forth in claim 1 or 2, wherein a maximum value of a gap between the surface of the steel sheet and the strip when a 2m strip is brought into contact with the surface of the steel sheet in the rolling direction is 10mm or less.

4. A method for producing a wear-resistant thin steel sheet having a thickness of 4.5mm or less, comprising the steps of:

a step of continuously casting molten steel having the composition according to claim 1 or 2 to obtain a slab;

heating the slab to 1000-1300 ℃;

thereafter, subjecting the slab to a hot rolling process including a finish rolling at a finish rolling temperature of 900 ℃ or higher using a hot rolling mill including a roughing mill and a finishing mill used in a hot rolling process for producing a steel sheet to obtain a steel sheet having a thickness of 4.5mm or less;

and a winding step of winding the steel sheet at a winding temperature of 200 ℃ or lower.

5. The method of producing a wear-resistant steel sheet as set forth in claim 4, further comprising the step of temper rolling the steel sheet obtained in the coiling step.