KR102109272B1 - A hot rolled steel sheet having excellent blanking properties and uniforminty, and a manufacturing method thereof - Google Patents

Abstract

The present invention is by weight, C: 0.10 to 0.30%, Si: 0.001 to 1.0%, Mn: 0.5 to 2.5%, Cr: 0.001 to 1.5%, Mo: 0.001 to 0.5%, Al: 0.001 to 0.5%, P : 0.001 ~ 0.01%, S: 0.001 ~ 0.01%, N: 0.001 ~ 0.01%, B: 0.0001 ~ 0.004%, Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1% and the balance contains iron and unavoidable impurities , Satisfies the following relation (1),
The microstructure, the main phase is composed of a martensite phase and a bainite phase, the fraction of the martensite phase is 50% or more and less than 90%, the fraction of the bainite phase is 5% or more and 50% or less, and the martensite phase The sum of the fractions of the bainite phase is 90% or more, and the remainder provides a high-strength hot-rolled steel sheet made of a ferrite phase.
[Relational formula (1)] CL <1
CL = -0.692 - 0.158 × [Mn ] + 0.121 × [Mn] 2 + 0.061 × [Cr] 2 - 0.319 × [Mo] + 0.035 × [Hardness_HRC]
(Wherein, CL is an effective crack generation index, [Mn], [Cr], [Mo] is the weight percent of the alloy element, and [Hardness_HRC] is the Rockwell hardness (HRC).)

Description

High strength hot rolled steel sheet with excellent punchability and material uniformity and its manufacturing method {A HOT ROLLED STEEL SHEET HAVING EXCELLENT BLANKING PROPERTIES AND UNIFORMINTY, AND A MANUFACTURING METHOD THEREOF}

The present invention relates to a high-strength hot-rolled steel sheet having a high tensile strength of 1100 MPa or more, a surface hardness of 35 HRC or more, and excellent material uniformity, and a method of manufacturing the same.

Conventional chain and mechanical parts are manufactured by spheroidizing heat treatment and QT (Quenching and Tempering) heat treatment using high carbon steel and high carbon alloy steel. However, this repetitive heat treatment process causes carbon dioxide emission and pollution, and increases the manufacturing cost of chains and mechanical parts. Therefore, to improve this, a technique capable of securing the target strength and hardness without additional heat treatment has been proposed by using low-carbon steel and manufacturing low-temperature transformed structure steel using bainite and martensite as a base structure.

Patent Document 1 proposes a technique to ensure the target strength and hardness by manufacturing bainite and martensite according to specific cooling conditions immediately after hot rolling the steel. In addition, Patent Document 2 proposed a method for securing the surface hardness based on the C-Si-Mn-Ni-B component system.

However, high-strength steels as described above have a problem in that cracks are generated in the rolled sheet after punching when punching in the process of manufacturing chains and machine parts. In particular, alloying components such as Si, Mn, Mo, Cr, V, Cu, and Ni, which are mainly used to secure high strength and hardness, are locally segregated or result in non-uniformity of the microstructure, resulting in inferior punching characteristics, When used, fatigue breakdown easily occurs in areas where component segregation and microstructure are non-uniform. In addition, the high hardenability steel is sensitive to changes in microstructure when cooled, so the low temperature transformation phase is formed non-uniformly, further reducing the punching characteristics. In order to improve this, it is possible to consider introducing an additional heat treatment process, but the introduction of such an additional heat treatment process is economically disadvantageous, and it is difficult to apply it since there is no discrimination from existing high carbon steel and high carbon alloy steel processes. .

European Patent Publication No. 1375694 Japanese Patent Publication No. 1999-302781

In the present invention, a high-strength hot-rolled steel sheet is obtained by optimizing alloy composition, rolling temperature, and cooling speed to obtain a high-strength microstructure having excellent punchability uniformly over the entire length and width, so that the punchability and material uniformity are excellent. It is intended to provide a high-strength hot-rolled steel sheet characterized in that and a manufacturing method thereof.

In addition, the subject of this invention is not limited to the above-mentioned content. The subject matter of the present invention will be understood from the entire contents of the present specification, and those skilled in the art to which the present invention pertains will have no difficulty in understanding the additional subject matter of the present invention.

One aspect of the present invention in weight percent, C: 0.10 to 0.30%, Si: 0.001 to 1.0%, Mn: 0.5 to 2.5%, Cr: 0.001 to 1.5%, Mo: 0.001 to 0.5%, Al: 0.001 to 0.5 %, P: 0.001 ~ 0.01%, S: 0.001 ~ 0.01%, N: 0.001 ~ 0.01%, B: 0.0001 ~ 0.004%, Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1% and the balance iron and unavoidable impurities And satisfies the following relational expression (1),

The microstructure, the main phase is composed of a martensite phase and a bainite phase, the fraction of the martensite phase is 50% or more and less than 90%, the fraction of the bainite phase is 5% or more and 50% or less, and the martensite phase The sum of the fractions of the bainite phase is 90% or more, and the remainder provides a high-strength hot-rolled steel sheet made of a ferrite phase.

[Relational formula (1)] CL <1

CL = -0.692 - 0.158 × [Mn ] + 0.121 × [Mn] 2 + 0.061 × [Cr] 2 - 0.319 × [Mo] + 0.035 × [Hardness_HRC]

(Wherein, CL is an effective crack generation index, [Mn], [Cr], [Mo] is the weight percent of the alloy element, and [Hardness_HRC] is the Rockwell hardness (HRC).)

In the high strength hot rolled steel sheet, the average packet size on the martensite is 1 to 7 µm in diameter per circle, and the aspect ratio of the packet structure on the martensite is in the thickness direction center (t / 4 to t / 2) may be 1 to 5, the thickness direction of the surface layer portion (surface layer ~ t / 8) may be 1.1 to 6, the aspect ratio of the surface layer divided by the aspect ratio of the center may be 0.9 to 2.

The high-strength hot-rolled steel sheet has a tensile strength of 1100 MPa or more, and a surface hardness of 35 HRC or more.

When the tensile strength and the surface hardness of the coiled coiled hot-rolled steel sheet are measured at 9 parts in full width and 3 parts in full length, the difference between the maximum and minimum values of each measurement result may be within 140 MPa of tensile strength and 4 HRC of surface hardness. .

Another aspect of the present invention in weight percent, C: 0.10 to 0.30%, Si: 0.001 to 1.0%, Mn: 0.5 to 2.5%, Cr: 0.001 to 1.5%, Mo: 0.001 to 0.5%, Al: 0.001 to 0.5%, P: 0.001 ~ 0.01%, S: 0.001 ~ 0.01%, N: 0.001 ~ 0.01%, B: 0.0001 ~ 0.004%, Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1% and the balance iron and inevitable Reheating the steel slab containing impurities and satisfying the above relation (1) to 1180 ~ 1350 ℃; Hot rolling the reheated steel slab to satisfy the following relational expression (2); Cooling the hot rolled steel sheet to a temperature in the range of 0 to 400 ° C. to satisfy the following relationship (3); And winding the cooled steel sheet at a temperature in the range of 0 to 400 ° C. It provides a method of manufacturing a high-strength hot-rolled steel sheet comprising a.

[Relational Formula (2)] Tn-70 ≤ FDT ≤ Tn

Tn = 967-280 × [C] + 35.7 × [Si]-28.1 × [Mn]-11.4 × [Cr] + 11.4 × [Mo]-62 × [Ti] + 46.2 × [Nb]

(Wherein, Tn is the critical rolling temperature (℃), FDT is the rolling finish temperature (℃), [C], [Si], [Mn], [Cr], [Mo], [B], [Nb], [Ti] is the weight percent of the alloy element.)

[Relational Formula (3)] LCR ≤ CR ≤ HCR

LCR = 2000 / (-1076 + 2751 × [C] + 17 × [Si] + 301 × [Mn] + 330 × [Cr] + 355 × [Mo] + 42939 × [B])

HCR = 2500 / (-70.3 + 198 × [C] + 32.0 × [Si] + 16.7 × [Mn] + 18.4 × [Cr] + 42.1 × [Mo] + 5918 × [B])

(Where CR is the cooling rate in the cooling zone (℃ / s), LCR is the minimum critical cooling rate (℃ / s), the minimum value is 5, the maximum value is 45, and the HCR is the maximum critical cooling rate (℃ / s) and its minimum value is 50 and its maximum value is 200. [C], [Si], [Mn], [Cr], [Mo], [B] are weight percentages of the alloy element.)

After the winding step, the high-strength hot-rolled steel sheet may be oiled after pickling.

According to the present invention, by optimizing the alloy composition, rolling temperature and cooling rate, microstructures having excellent punchability compared to high strength are obtained uniformly over the entire length and width, and high strength hot-rolled steel sheet excellent in punchability and material uniformity, and It can provide a manufacturing method.

1 is an EBSD photograph showing the microstructure of the surface layer portion and the central portion of Invention Steel 3.

High strength hot rolled steel sheet

Hereinafter, a high-strength hot-rolled steel sheet according to an aspect of the present invention will be described in detail.

High-strength hot-rolled steel sheet according to an aspect of the present invention, by weight, C: 0.10 to 0.30%, Si: 0.001 to 1.0%, Mn: 0.5 to 2.5%, Cr: 0.001 to 1.5%, Mo: 0.001 to 0.5% , Al: 0.001 ~ 0.5%, P: 0.001 ~ 0.01%, S: 0.001 ~ 0.01%, N: 0.001 ~ 0.01%, B: 0.0001 ~ 0.004%, Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1% and The balance contains iron and unavoidable impurities, satisfies the following relational expression (1), the microstructure, the main phase is composed of a martensite phase and a bainite phase, the fraction of the martensite phase is 50% or more and less than 90%, The fraction of the bainite phase is 5% or more and 50% or less, the sum of the fractions of the martensite phase and the bainite phase is 90% or more, and the balance may be made of a ferrite phase.

[Relational formula (1)] CL <1

CL = -0.692 - 0.158 × [Mn ] + 0.121 × [Mn] 2 + 0.061 × [Cr] 2 - 0.319 × [Mo] + 0.035 × [Hardness_HRC]

(Wherein, CL is an effective crack generation index, [Mn], [Cr], [Mo] is the weight percent of the alloy element, and [Hardness_HRC] is the Rockwell hardness (HRC).)

First, the alloy composition of the high-strength hot-rolled steel sheet according to an aspect of the present invention will be described in detail. Hereinafter, the unit of each alloy element is% by weight.

C: 0.10 to 0.30%

C is the most economical and effective element for reinforcing steel, and as the amount added increases, the fraction of ferrite phase decreases and the bainite and martensite phases with high hardness can be obtained due to the solid solution strengthening effect. However, if the content is less than 0.10%, it is difficult to obtain a sufficient strengthening effect, and if it exceeds 0.30%, there is a problem in that the martensitic phase is too hard and has low brittleness and deteriorates the punchability. Therefore, the content of C is preferably included in 0.10 ~ 0.30%.

Si: 0.001 ~ 1.0%

Si deoxidizes the molten steel and has a solid solution strengthening effect, and is advantageous in improving the punchability by delaying the formation of coarse carbides. However, if the content is less than 0.001%, it is difficult to obtain the above effect, and when it exceeds 1.0%, a red scale by Si is formed on the surface of the steel sheet during hot rolling, resulting in very poor quality of the steel sheet surface and deteriorating surface hardness. Therefore, it is preferable to limit the content to 1.0% or less. Therefore, the content of Si is preferably included in 0.001 ~ 1.0%.

Mn: 0.5-2.5%

Mn is an effective element for solid solution strengthening of steel and increases the hardenability of steel to suppress the formation of ferrite upon cooling to increase the strength and hardness of steel. However, if the content is less than 0.5%, the above effects due to addition cannot be obtained, and if it exceeds 2.5%, the segregation part is greatly developed at the thickness center during slab casting in the playing process. By forming the tissue non-uniformly, the punching characteristics are inferior. Therefore, the content of Mn is preferably limited to 0.5 to 2.5%.

Cr: 0.001 ~ 1.5%

Cr hardens the steel and increases the hardenability of the steel to suppress ferrite formation, thereby increasing the strength and hardness of the steel. However, if the Cr content is less than 0.001%, the above effect due to addition cannot be obtained, and if it exceeds 1.5%, the segregation part in the center of the thickness is greatly developed, and the microstructure in the thickness direction is non-uniform to deteriorate the punching property. Therefore, the content of Cr is preferably limited to 0.001 to 1.5%.

Mo: 0.001 ~ 0.5%

Mo strengthens the grain boundary to improve the punchability, and improves the hardenability of the steel to increase the strength. However, if the content is less than 0.001%, the effect is incomplete, and when it exceeds 0.5%, the effect is saturated and the production cost of steel is greatly increased. Therefore, it is preferable to limit the content of Mo to 0.001 to 0.5%. Do.

Al: 0.001 to 0.5%

Al is a component added for deoxidation, and in the dissolved state, if its content is less than 0.001%, the deoxidation effect is insufficient, and if it exceeds 0.5%, defects due to inclusion formation are likely to occur and there is a problem that nozzle clogging occurs when playing. . Therefore, it is desirable to limit the content to 0.001 to 0.5%.

P: 0.001 to 0.01%

P is an imperatively contained in the steel, and it is advantageous to control its content as low as possible. However, in order to reduce the P content to less than 0.001%, manufacturing costs are high, which is economically disadvantageous, and when the content exceeds 0.01%, brittleness by grain boundary segregation occurs, thereby reducing the punchability of steel. Therefore, it is preferable to limit the P to 0.001 to 0.01%.

S: 0.001 ~ 0.01%

S is an impurity present in the steel, and when its content exceeds 0.01%, it is easy to form a non-metallic inclusion by combining with Mn, etc., which causes deterioration of the steel's punchability. In addition, in order to manufacture less than 0.001%, time and cost are excessively consumed during the steelmaking operation, resulting in poor productivity. Therefore, it is preferable to limit the content to 0.001 to 0.01%.

N: 0.001 to 0.01%

N is a solid solution strengthening element. In order to manufacture this to less than 0.001%, the steelmaking operation takes a lot of time and cost, which decreases productivity, and if it exceeds 0.01%, a large amount of inclusions that adversely affect the punchability during production is generated. Therefore, in the present invention, it is preferable to limit the content of N to 0.001 to 0.01%.

B: 0.0001 ~ 0.004%

B is an element that increases the hardenability of steel to facilitate the securing of martensite and bainite phases, and its effect is known to be superior to that of other elements. However, if the content is less than 0.0001%, it is difficult to obtain a sufficient curing ability synergistic effect, and if it exceeds 0.004%, the curing ability synergistic effect is saturated and it is difficult to expect an increase in curing ability by additional addition. Therefore, the content of B is preferably 0.0001 to 0.004%.

Ti: 0.001 ~ 0.1%

Ti has a precipitation strengthening effect through the production of TiC, and has a strong affinity with N to form coarse TiN among steels, and suppresses the formation of BN to improve the hardenability of steel. However, if the content of Ti is less than 0.001%, the above effect cannot be sufficiently obtained, and if the content of Ti exceeds 0.1%, there is a problem in that the punching characteristics are deteriorated when coarsening the precipitate. Therefore, in the present invention, it is preferable to limit the Ti content to 0.001 to 0.1%.

Nb: 0.001 to 0.1%

Nb is a representative precipitation-enhancing element and contributes to the improvement of strength, hardness, and punchability of the steel through precipitation during hot rolling and the effect of grain refinement due to recrystallization delay. At this time, when the content of Nb is less than 0.001%, the above effect cannot be sufficiently obtained, and when the content of Nb exceeds 0.1%, the punchability is reduced due to the formation of coarse composite precipitates. Therefore, in the present invention, it is preferable to limit the Nb content to 0.001 to 0.1%.

The high-strength hot-rolled steel sheet of the present invention is an iron (Fe) component other than the above-mentioned alloying elements. However, in the normal manufacturing process, unintended impurities may be inevitably mixed from the raw material or the environment of the environment, and thus cannot be excluded. Since these impurities are known to anyone skilled in the art, they are not described in detail.

In addition, the high-strength hot-rolled steel sheet according to an aspect of the present invention not only satisfies the above-described alloy composition, but also satisfies the following relationship (1) in order to secure the punchability.

[Relational formula (1)] CL <1

CL = -0.692 - 0.158 × [Mn ] + 0.121 × [Mn] 2 + 0.061 × [Cr] 2 - 0.319 × [Mo] + 0.035 × [Hardness_HRC]

(Wherein, CL is an effective crack generation index, [Mn], [Cr], [Mo] is the weight percent of the alloy element, and [Hardness_HRC] is the Rockwell hardness (HRC).)

In the relational expression (1), the effective crack initiation index (CL) is an index indicating the punching characteristics of steel, and when this value is 1 or more, when the steel is punched, cracks of an effective size leading to fatal defects occur on the punching surface of the steel. You can judge that. The punching properties of steel materials are affected by segregation depending on the alloying element content, and are mainly included in a large amount in the steel materials, and the content of Mn and Cr, which is known to cause segregation in the continuous casting process, is a major index related to this. As the content of Mn and Cr increases, the linearity tends to exceed the linear tendency, resulting in a decrease in punchability due to segregation, so CL increases in proportion to the squared value of Mn and Cr, thereby controlling segregation by controlling the content of the two components. It should be avoided. In addition, as the hardness of the steel material increases, the toughness decreases, and as a result, the punching property tends to deteriorate, so that an optimum component system that does not deteriorate the punching properties of the steel material is produced while making a high-strength hot-rolled product at the target level. It is necessary, and it is expressed by reflecting it in relation (1). Particularly, when Mo was added, it was confirmed that the hardenability of the steel material was greatly increased, and the uniformity in the steel material was increased, so that it was possible to secure a higher punchability even at the same hardness, and this was added to the relational expression (1).

On the other hand, the microstructure of the high-strength hot-rolled steel sheet according to an aspect of the present invention is composed of a martensite phase and a bainite phase, the fraction of the martensite phase is 50% or more and less than 90%, and the fraction of the bainite phase is 5 % Or more and 50% or less, the sum of the fractions of the martensite phase and the bainite phase is 90% or more, and the balance may be made of a ferrite phase. In addition, the average packet size on the martensite is 1 to 7 μm in diameter per circle, and the aspect ratio of the packet structure on the martensite is in the center of the thickness direction (t / 4 to t / It may be 1 to 5 in 2), and may be 1.1 to 6 in a thickness direction surface layer (surface ~ t / 8), and a value obtained by dividing the aspect ratio of the surface layer by the aspect ratio of the center may be 0.9 to 2.

First, in the microstructure of the high-strength hot-rolled steel sheet of the present invention, the main phase is composed of a martensite phase and a bainite phase, and the fraction of the martensite phase may be 50% or more and less than 90%. When the fraction of the martensite phase is less than 50%, the fraction of the ferrite / bainite phase having a relatively low hardness is increased, so that the target hardness cannot be secured. On the other hand, if the fraction of the martensite phase is 90% or more, the toughness of the steel material is excessively insufficient, and it is difficult to secure a target punching property. Therefore, it is preferable to limit the fraction of martensite phase to 50% or more and less than 90%.

Meanwhile, the fraction of bainite phase may be 5% or more and 50% or less. The bainite phase has a slightly lower hardness than the martensite phase, but has a similar level, and the degree of contributing to the punchability during formation is excellent compared to the martensitic phase, so at least 5% should be included to maintain the balance between hardness and punchability. have. However, if the fraction exceeds 50%, it is difficult to satisfy the target hardness, so the maximum value is limited to 50% or less. Therefore, it is preferable to limit the fraction of bainite phase to 5% or more and 50% or less.

In addition, the sum of the fractions of the martensite phase and the bainite phase is 90% or more, and the balance may be made of a ferrite phase. When the fraction of the ferrite phase, which is the balance excluding the martensite phase and the bainite phase, is 10% or more, the punchability decreases due to the difference in hardness between the phases at the interface of the ferrite-martensite, so the fraction of the ferrite phase is 10% It is preferred to limit to less than.

On the other hand, it is more preferable that the martensite phase is the main phase among the martensite phase and the bainite phase, and the fraction is 75% or more. In addition, the microstructure of the hot-rolled steel sheet of the present invention may be made of only a martensite phase and a bainite phase without a ferrite phase.

The average packet size on martensite among the microstructures of the present invention may be 1 to 7 μm in diameter per circle. Here, the packet on the martensite refers to the tissues adjacent to each other having the same azimuth-assembled tissue within the martensite, and the average size is obtained by obtaining the diameter per circle of the tissues in the same direction through SEM measurement to obtain the average value. Or, it can be defined by specifying the size of organizations having the same defense relationship through EBSD measurement. The average packet size is preferably measured at the center of the steel sheet. It can also be measured through other well-known methods known in the art. By controlling the average packet size on the martensite among the microstructures of the prepared steel to be 1 to 7 µm in diameter per circle, it is possible to increase the punchability of the steel through grain refinement. When the average packet size is less than 1 μm, an excessive rolling load occurs in the hot rolling process for grain refinement, whereas when it exceeds 7 μm, it is difficult to expect an effect of increasing hardness through grain refinement. Therefore, the average packet size on the martensite is preferably 1 to 7 μm in diameter per circle.

In addition, the aspect ratio of the packet structure on the martensite among the microstructures of the present invention (aspect ratio) is 1 to 5 in the thickness direction central portion (t / 4 to t / 2), the thickness direction surface layer portion (surface layer-t / 8 ) Is 1.1 to 6, and the value of the aspect ratio of the surface layer divided by the aspect ratio of the center may be 0.9 to 2. Here, the aspect ratio of the packet structure on the martensite may be defined as a value obtained by dividing a long axis of a short axis by simplifying adjacent tissues having an aggregate structure having the same azimuth relationship within the martensite in the form of an ellipse.

If the aspect ratio is less than 1 in the central portion in the thickness direction (t / 4 to t / 2), the grain refining effect due to recrystallization delay is insufficient to increase the hardness, whereas if it exceeds 5, partial recrystallization occurs to the center of the steel material. The punching characteristics are deteriorated by the material deviation in the thickness direction of the steel material.

On the other hand, if the aspect ratio in the thickness direction surface layer portion (surface ~ t / 8) is less than 1.1, the surface layer hardening effect to achieve the target hardness is insufficient because the retardation of recrystallization due to rolling hardly occurs even in the surface layer. If the value exceeds 6, excessive partial recrystallization occurs in the surface layer, which may cause deterioration of punching characteristics due to material deviation in the thickness direction.

In addition, if the value obtained by dividing the aspect ratio of the surface layer portion by the aspect ratio of the center portion is less than 0.9, the surface hardening effect due to recrystallization delay becomes insufficient, and when the value exceeds 2, the punching characteristics are deteriorated due to material deviation in the thickness direction.

Therefore, the aspect ratio of the packet structure on the martensite is 1 to 5 at the center portion in the thickness direction (t / 4 to t / 2), and 1.1 to 6 at the thickness surface portion (surface to t / 8), and the aspect ratio of the surface portion is the center aspect ratio. It is preferable that the value divided by is 0.9-2.

Meanwhile, the high-strength hot-rolled steel sheet according to an aspect of the present invention has a tensile strength of 1100 MPa or more and a surface hardness of 35 HRC or more. In particular, when the tensile strength and the surface hardness of the coiled coiled hot-rolled steel sheet were measured at 9 parts of the entire width and 3 parts of the total length, the difference between the maximum value and the minimum value of each measurement result was within 140 MPa of tensile strength and within 4 HRC of surface hardness. desirable. Here, the 9 parts of the full width means to select 9 parts in the width direction of the coiled hot-rolled steel sheet, and the 3 parts of the total length means to select 3 parts in the longitudinal direction of the coiled hot-rolled steel sheet.

Manufacturing method of high strength hot rolled steel sheet

Hereinafter, a method for manufacturing a high-strength hot-rolled steel sheet according to another aspect of the present invention will be described in detail.

Method of manufacturing a high-strength hot-rolled steel sheet according to another aspect of the present invention in weight%, C: 0.10 to 0.30%, Si: 0.001 to 1.0%, Mn: 0.5 to 2.5%, Cr: 0.001 to 1.5%, Mo: 0.001 ~ 0.5%, Al: 0.001 ~ 0.5%, P: 0.001 ~ 0.01%, S: 0.001 ~ 0.01%, N: 0.001 ~ 0.01%, B: 0.0001 ~ 0.004%, Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ Reheating the steel slab with 0.1% and the balance containing iron and unavoidable impurities, and satisfying the following relation (1) to 1180 to 1350 ° C; Hot rolling the reheated steel slab to satisfy the following relation (2); Cooling the hot rolled steel sheet to a temperature in the range of 0 to 400 ° C. to satisfy the following relationship (3); And winding the cooled steel sheet at a temperature in the range of 0 to 400 ° C. It includes.

[Relational formula (1)] CL <1

CL = -0.692 - 0.158 × [Mn ] + 0.121 × [Mn] 2 + 0.061 × [Cr] 2 - 0.319 × [Mo] + 0.035 × [Hardness_HRC]

(Wherein, CL is an effective crack generation index, [Mn], [Cr], [Mo] is the weight percent of the alloy element, and [Hardness_HRC] is the Rockwell hardness (HRC).)

[Relational Formula (2)] Tn-70 ≤ FDT ≤ Tn

Tn = 967-280 × [C] + 35.7 × [Si]-28.1 × [Mn]-11.4 × [Cr] + 11.4 × [Mo]-62 × [Ti] + 46.2 × [Nb]

(Wherein, Tn is the critical rolling temperature (℃), FDT is the rolling finish temperature (℃), [C], [Si], [Mn], [Cr], [Mo], [B], [Nb], [Ti] is the weight percent of the alloy element.)

[Relational Formula (3)] LCR ≤ CR ≤ HCR

LCR = 2000 / (-1076 + 2751 × [C] + 17 × [Si] + 301 × [Mn] + 330 × [Cr] + 355 × [Mo] + 42939 × [B])

HCR = 2500 / (-70.3 + 198 × [C] + 32.0 × [Si] + 16.7 × [Mn] + 18.4 × [Cr] + 42.1 × [Mo] + 5918 × [B])

(Where CR is the cooling rate in the cooling zone (℃ / s), LCR is the minimum critical cooling rate (℃ / s), the minimum value is 5, the maximum value is 45, and the HCR is the maximum critical cooling rate (℃ / s) and its minimum value is 50 and its maximum value is 200. [C], [Si], [Mn], [Cr], [Mo], [B] are weight percentages of the alloy element.)

Steps to reheat the slab

First, a steel slab having the above-described alloy composition and satisfying the relation (1) is reheated at a temperature of 1180 to 1350 ° C. At this time, if the reheating temperature is less than 1180 ° C, the precipitates are not sufficiently re-used, so that the formation of precipitates is reduced in the process after hot rolling, coarse TiN remains, and it is difficult to resolve the segregation generated during performance by diffusion. In addition, when the temperature exceeds 1350 ° C, strength decreases and texture unevenness occur due to abnormal grain growth of austenite grains. Therefore, it is preferable to limit the reheating temperature to 1180 to 1350 ° C.

Step of hot rolling

The reheated slab is hot rolled at a temperature in the range of 750 to 1000 ° C. When hot rolling is started at a temperature higher than 1000 ° C, the temperature of the hot-rolled steel sheet becomes high, the grain size becomes coarse and the descaling is insufficient, and thus the surface quality of the hot-rolled steel sheet is inferior. In addition, when rolling is finished at a temperature of less than 750 ° C, the recrystallization behavior of the steel is different for each location, so that the material is not uniform and the punching characteristics are deteriorated.

In addition, in the hot rolling step, the rolling finish temperature (FDT) is hot rolled to satisfy the following relational expression (2).

[Relational Formula (2)] Tn-70 ≤ FDT ≤ Tn

Tn = 967-280 × [C] + 35.7 × [Si]-28.1 × [Mn]-11.4 × [Cr] + 11.4 × [Mo]-62 × [Ti] + 46.2 × [Nb]

 (Wherein, Tn is the critical rolling temperature (℃), FDT is the rolling finish temperature (℃), [C], [Si], [Mn], [Cr], [Mo], [B], [Nb], [Ti] is the weight percent of the alloy element.)

The relational expression (2) is a formula showing the relationship between the rolling finish temperature and the component of the steel material. In general, when the temperature of the steel material is lowered below a certain critical temperature during hot rolling, a recrystallization delay phenomenon occurs in the steel material, thereby improving the punching characteristics of the steel material through a grain refining effect. Therefore, if the rolling finish temperature (FDT) of the steel is controlled to be below the critical rolling temperature (Tn), the average packet size on the martensite among the microstructures of the manufactured steel becomes 1 to 7 µm in diameter per circle, resulting in grain refinement. Through it can increase the punchability of the steel.

However, if the rolling finish temperature (FDT) is excessively lowered, a problem arises in the plateability in the rolling process, and excessive partial recrystallization occurs only in the surface layer portion, which causes the punching property to be deteriorated due to the difference in physical properties in the thickness direction of the steel. Therefore, by adjusting the rolling finish temperature (FDT) of the steel to Tn-70 or higher, the aspect ratio of the packet structure on the martensite is 1 to 5 at the center of the thickness direction (t / 4 to t / 2). It is 1.1 to 6 in the thickness direction surface layer part (surface layer ~ t / 8), the aspect ratio of the surface layer divided by the aspect ratio of the center is adjusted to be 0.9 to 2, it is possible to improve the punchability and material uniformity of the steel.

Cooling and winding steps

The rolled steel sheet is cooled to an average cooling rate of 5 to 200 ° C / sec to a temperature in the range of 0 to 400 ° C, wound at a temperature in the range of 0 to 400 ° C, and the cooling rate of the steel sheet at this time depends on the components of the steel grade. Accordingly, the following relational expression (3) is set.

[Relational Formula (3)] LCR ≤ CR ≤ HCR

LCR = 2000 / (-1076 + 2751 × [C] + 17 × [Si] + 301 × [Mn] + 330 × [Cr] + 355 × [Mo] + 42939 × [B])

HCR = 2500 / (-70.3 + 198 × [C] + 32.0 × [Si] + 16.7 × [Mn] + 18.4 × [Cr] + 42.1 × [Mo] + 5918 × [B])

(Where CR is the cooling rate in the cooling zone (℃ / s), LCR is the minimum critical cooling rate (℃ / s), the minimum value is 5, the maximum value is 45, and the HCR is the maximum critical cooling rate (℃ / s) and its minimum value is 50 and its maximum value is 200, and [C], [Si], [Mn], [Cr], [Mo], [B] are weight percentages of the alloy element.)

The relational expression (3) is an expression for the cooling condition of the steel material. Cooling conditions in the cooling zone determine the microstructure of the steel and have a dominant effect on strength and hardness. In addition, at this time, the cooling condition of the steel material should consider a change in hardenability according to the amount of alloying elements added. Therefore, it is essential to cool by applying the optimum cooling rate according to the alloying elements included in the steel.

To this end, in the present invention, the maximum critical cooling rate (HCR) and the minimum critical cooling rate (LCR) according to the amount of alloying elements are respectively obtained, and the cooling rate (CR) in the cooling zone is the maximum critical cooling rate (HCR) and minimum critical cooling. It was made to satisfy between the speeds (LCR). When the steel is cooled at a rate faster than the maximum critical cooling rate (HCR), a hard but brittle martensite structure is formed, resulting in deterioration in punchability, deterioration of the shape of the steel, and due to excessive rapid cooling in the cooling zone. Material uniformity decreases because the water intake is not the same across all regions. Conversely, when the cooling rate of the steel material is slower than the minimum critical cooling rate (LCR), a ferrite phase with a relatively low hardness is generated by more than 10% to degrade the hardness of the steel material, and the amount of ferrite reacts too sensitively to the change in the cooling rate. Material uniformity is deteriorated. Therefore, it is preferable to set the cooling rate CR in the cooling zone to a value between the maximum critical cooling rate HCR and the minimum critical cooling rate LCR.

(Example)

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by matters described in the claims and reasonably inferred therefrom.

First, a steel slab satisfying the component system shown in Table 1 was heated to 1200 ° C., and high-strength hot-rolled steel sheets were prepared under the hot-rolling conditions shown in Table 2. The high-strength hot-rolled steel sheet thus manufactured was tested to measure microstructure, strength, hardness and punchability, and summarized in Tables 2 and 4 below.

The fraction of each component element in Table 1 below is% by weight, and the meanings of FDT, Tn, CR, LCR, and HCR in Table 2 are as follows. In addition, in the fraction of the microstructure, Fer means ferrite, Bai means bainite, Mar means martensite, and if the fraction of each microstructure satisfies the target level, an 'O' is displayed on satisfaction, or ' X '.

-FDT: Rolling finishing temperature (℃)

-Tn: Critical rolling temperature (℃)

-CR: Cooling speed in cooling zone (℃ / s)

-LCR: Minimum critical cooling rate (℃ / s)

-HCR: Maximum critical cooling rate (℃ / s)

In addition, for the inventive steel and the comparative steel, the packet structure on the martensite at the central portion in the thickness direction and the surface portion in the thickness direction was observed to simplify each packet in the shape of an ellipse, and among them, the length of the long axis was shortened. The divided aspect ratio is measured and shown in Table 3 below, and when the packet size and aspect ratio on the martensite satisfies the target level, 'O' is indicated as satisfied or not, and 'X' is indicated as otherwise. When the manufacturing conditions shown in Table 2 did not satisfy the target relational expression, the defects in shape resulted in the martensitic structure becoming too fine / coarse or the thickness direction deviating.

Tensile strength of Table 4 is the total average of the values of tensile strength or Rockwell hardness measured at uniform intervals in the entire width of the coiled hot-rolled steel sheet at 9 parts and 3 parts in total length.The tensile strength is once for each position, Hardness was measured 10 times for each location. The variation in tensile strength represents the difference between the maximum and minimum values of the measured values.

CL represents the effective crack initiation index, and when cracks of an effective size occurred when steel was punched, it was denoted as 'O' for satisfactory punchability, and 'X' otherwise.

Psalter C Si Mn Cr Mo Al P S N B Ti Nb Comparative Steel 1 0.080 0.100 1.400 0.400 0.002 0.002 0.003 0.003 0.002 0.002 0.015 0.001 Comparative Steel 2 0.295 0.050 1.200 0.300 0.100 0.002 0.003 0.003 0.003 0.002 0.015 0.001 Comparative Steel 3 0.160 0.040 1.800 0.200 0.200 0.200 0.002 0.002 0.002 0.001 0.002 0.001 Comparative steel 4 0.180 0.500 1.350 0.060 0.200 0.010 0.002 0.004 0.003 0.001 0.010 0.050 Comparative Steel 5 0.195 0.150 1.500 0.100 0.100 0.010 0.003 0.003 0.003 0.001 0.020 0.010 Comparative steel 6 0.170 0.300 2.200 0.100 0.050 0.002 0.003 0.002 0.004 0.001 0.015 0.015 Comparative Steel 7 0.190 0.300 1.500 1.480 0.010 0.002 0.003 0.002 0.002 0.001 0.015 0.015 Comparative Steel 8 0.270 0.100 1.600 0.700 0.010 0.002 0.003 0.002 0.002 0.002 0.015 0.020 Invention Steel 1 0.210 0.002 1.400 0.002 0.200 0.002 0.003 0.002 0.002 0.0015 0.025 0.002 Invention Steel 2 0.210 0.002 1.800 0.002 0.002 0.003 0.002 0.003 0.003 0.0015 0.025 0.002 Invention Steel 3 0.195 0.100 1.250 0.600 0.200 0.002 0.003 0.003 0.002 0.0015 0.015 0.020 Invention Steel 4 0.195 0.100 1.100 0.800 0.200 0.003 0.003 0.004 0.002 0.0015 0.015 0.020 Invention Steel 5 0.210 0.003 1.250 0.800 0.200 0.003 0.004 0.002 0.003 0.0015 0.015 0.020 Invention Steel 6 0.210 0.002 1.400 0.400 0.200 0.004 0.002 0.002 0.003 0.0015 0.015 0.020 Invention Steel 7 0.210 0.003 1.400 0.800 0.200 0.002 0.002 0.001 0.002 0.0015 0.015 0.002 Invention Steel 8 0.230 0.100 1.400 0.800 0.200 0.002 0.001 0.003 0.002 0.0015 0.015 0.020

Psalter Rolling condition (relationship 2) Cooling condition (relationship 3) Microstructure fraction Tn-70 FDT Tn LCR CR HCR Fer Bai Mar Satisfaction
Whether Comparative Steel 1 833 880 903 5 45 50 0.05 0.08 0.87 O Comparative Steel 2 779 860 849 5 65 80 0.08 0.12 0.80 O Comparative Steel 3 803 790 873 23 145 200 0.02 0.10 0.88 O Comparative steel 4 830 850 900 5 140 129 0.00 0.02 0.98 X Comparative Steel 5 805 840 875 45 40 200 0.15 0.25 0.60 X Comparative steel 6 797 830 867 13 100 128 0.00 0.11 0.89 O Comparative Steel 7 795 840 865 5 65 70 0.01 0.14 0.85 O Comparative Steel 8 772 830 842 5 60 65 0.01 0.09 0.89 O Invention Steel 1 800 850 870 34 80 200 0.01 0.10 0.89 O Invention Steel 2 786 850 856 18 120 200 0.02 0.15 0.83 O Invention Steel 3 806 870 876 12 110 121 0.01 0.11 0.88 O Invention Steel 4 808 870 878 10 80 114 0.01 0.20 0.79 O Invention Steel 5 796 800 866 7 95 103 0.00 0.12 0.88 O Invention Steel 6 797 800 867 10 100 129 0.00 0.13 0.87 O Invention Steel 7 791 830 861 6 80 93 0.01 0.11 0.89 O Invention Steel 8 790 820 860 5 70 74 0.01 0.16 0.84 O

Psalter Martensite tops
Average packet size (㎛) Aspect ratio of packet structure on martensite Satisfaction
Whether Thickness direction center
(t / 4 ~ t / 2) Thickness direction surface layer part
(Surface ~ t / 8) Aspect ratio of the surface layer / aspect ratio of the center Comparative Steel 1 3.14 3.71 4.00 1.07 O Comparative Steel 2 7.08 2.89 3.21 1.10 X Comparative Steel 3 2.37 3.14 8.44 2.68 X Comparative steel 4 4.47 3.88 4.28 1.10 O Comparative Steel 5 3.74 4.54 4.81 1.06 O Comparative steel 6 4.88 4.98 5.14 1.0 O Comparative Steel 7 6.14 3.04 5.12 1.68 O Comparative Steel 8 5.77 4.87 5.87 1.20 O Invention Steel 1 4.15 3.81 4.11 1.08 O Invention Steel 2 5.12 4.11 4.51 1.09 O Invention Steel 3 4.36 4.12 4.71 1.14 O Invention Steel 4 4.87 3.71 4.72 1.27 O Invention Steel 5 3.54 4.12 5.11 1.24 O Invention Steel 6 3.81 4.47 5.64 1.26 O Invention Steel 7 4.12 3.81 5.07 1.33 O Invention Steel 8 3.94 4.24 4.41 1.04 O

Psalter The tensile strength
(MPa) Tensile strength deviation
(ΔMPa) Surface hardness
(HRC) Hardness deviation
(ΔHRC) CL
(Relational formula 1) Repellency
Satisfaction Comparative Steel 1 984 51 35.1 1.8 0.56 O Comparative Steel 2 1901 121 52.9 5.1 1.12 X Comparative Steel 3 1336 131 42.0 7.2 0.82 O Comparative steel 4 1345 98 42.1 5.2 0.73 O Comparative Steel 5 1085 54 37.1 2.1 0.81 O Comparative steel 6 1436 66 43.9 2.3 1.07 X Comparative Steel 7 1476 72 44.7 2.2 1.04 X Comparative Steel 8 1776 124 50.5 6.7 1.16 X Invention Steel 1 1443 62 44.0 1.8 0.80 O Invention Steel 2 1459 55 44.3 1.9 0.97 O Invention Steel 3 1406 68 43.3 2.2 0.77 O Invention Steel 4 1389 41 43.0 1.4 0.76 O Invention Steel 5 1488 66 44.9 2.4 0.85 O Invention Steel 6 1485 31 44.8 1.5 0.84 O Invention Steel 7 1527 52 45.7 1.9 0.90 O Invention Steel 8 1631 67 47.7 2.1 0.97 O

As can be seen in Tables 1 to 4, the invention steels 1 to 8 satisfy the alloy composition suggested in the present invention, and it can be seen that all have tensile strength of 1100 MPa or more and surface hardness of 35 HRC or more.

However, since the comparative steel 1 has a carbon concentration of 0.08%, which is less than the component range, the solid solution strengthening effect by C is insufficient, and thus, the hardness and strength compared to the target are insufficient.

On the other hand, as a result of analyzing the comparative steel and the invention steel using the relationship (2), all the invention steels satisfied the relationship (2), and accordingly, the average packet size on martensite is 1 ~ as the diameter per circle. 7㎛, the aspect ratio of the packet structure on the martensite is 1 to 5 in the thickness direction central portion (t / 4 to t / 2), 1.1 to 6 in the thickness direction surface layer portion (surface layer ~ t / 8), the aspect ratio of the surface layer portion The value divided by the aspect ratio of the center satisfied 0.9-2. This was also confirmed through observation of actual microstructure, and representatively, the results of EBSD analysis on the microstructure of the surface layer and the center of Invention Steel 3 are attached to FIG. 1.

However, the component range of each alloying component of Comparative Steel 2 satisfies the conditions of the present invention, but the Tn value is lower than that of the normal, and thus the FDT is higher than Tn, which does not satisfy the relation (2). Due to this high rolling finish temperature, the martensitic structure of the surface layer and the deep layer was coarse, resulting in a decrease in punchability. In addition, in the case of Comparative Steel 3, the rolling was finished at an excessively low temperature, and thus the FDT temperature was lower than Tn-70, so that the relationship (2) was not satisfied. As a result, an excessively deformed microstructure was formed in the surface layer, resulting in reduced punchability due to variations in microstructure between the surface layer portion and the center portion, and reduced material uniformity.

As a result of analyzing the comparative steel and the invention steel using the relational formula (3), it was confirmed that all invention steels satisfy the relational expression (3) and are summarized in Table 2. Therefore, all of the inventive steels did not generate a hard but highly brittle martensite phase without generating more than 10% of the ferrite phase, which lowers the strength and hardness, and the phenomenon of deterioration in punchability did not occur.

However, in the case of Comparative Steel 4, since the cooling rate was faster than the HCR value, the amount of ferrite phase or bainite phase was insufficient and only the martensite phase with low brittleness was produced in large quantity. Accordingly, the punchability is reduced, and it is difficult to uniformly control the cooling speed in the width direction in the cooling zone due to the excessively fast cooling speed, thereby reducing the uniformity in the width direction material. In addition, in the case of Comparative Steel 5, since the cooling rate was slower than the LCR value, relational expression (2) was not satisfied. Therefore, the cooling rate compared to the curing ability was too slow, and a large amount of ferrite phase was contained, so that the strength and hardness were less than the target.

On the other hand, as a result of analyzing the comparative steel and the inventive steel using the relational expression (1), it was confirmed that all inventive steels satisfy the relational expression (1) and are summarized in Table 4. Therefore, it was confirmed that all invention steels secured the target level of punchability, and that an effective level of cracking, which had a fatal effect on product quality, did not occur during punching for production of real parts.

However, in the case of comparative steel 6, the content of Mn was too high, resulting in intensification of Mn segregation, thereby deteriorating punching characteristics. As a result, since the relational expression (1) is not satisfied, it can be confirmed that the punchability is poor. In the case of Comparative Steel 7, the Cr content was too high to satisfy the relational expression (1), and as a result, Cr segregation intensified and the punching characteristics were deteriorated.

On the other hand, in the case of the comparative steel 8, a large amount of component systems such as C for hardening the steel material is included, and thus the component system having a very high hardness value. As a result, due to the excessive increase in hardness, relation (1) was not satisfied, and a number of effective cracks having a fatal effect on product quality occurred during punching.

Claims (6)

In weight percent, C: 0.10 to 0.30%, Si: 0.001 to 1.0%, Mn: 0.5 to 2.5%, Cr: 0.001 to 1.5%, Mo: 0.001 to 0.5%, Al: 0.001 to 0.5%, P: 0.001 to 0.01%, S: 0.001 ~ 0.01%, N: 0.001 ~ 0.01%, B: 0.0001 ~ 0.004%, Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1% and the balance contains iron and unavoidable impurities, and the following relationship (1) is satisfied,
The microstructure, the main phase is composed of a martensite phase and a bainite phase, the fraction of the martensite phase is 50% or more and less than 90%, the fraction of the bainite phase is 5% or more and 50% or less, and the martensite phase A high-strength hot-rolled steel sheet in which the sum of the fractions of the bainite phase is 90% or more, and the balance is made of a ferrite phase.
[Relational formula (1)] CL <1
CL = -0.692 - 0.158 × [Mn ] + 0.121 × [Mn] 2 + 0.061 × [Cr] 2 - 0.319 × [Mo] + 0.035 × [Hardness_HRC]
(Wherein, CL is an effective crack generation index, [Mn], [Cr], [Mo] is the weight percent of the alloy element, and [Hardness_HRC] is the Rockwell hardness (HRC).) According to claim 1,
The average packet size on the martensite is 1 to 7 µm in diameter per circle, and the aspect ratio of the packet structure on the martensite is in the center of the thickness direction (t / 4 to t / 2). ) 1 to 5, the thickness direction of the surface layer portion (surface ~ t / 8) is 1.1 to 6, the thickness ratio of the aspect ratio of the surface layer portion divided by the aspect ratio of the central portion in the thickness direction is high strength hot-rolled steel sheet. According to claim 1,
The high-strength hot-rolled steel sheet is a high-strength hot-rolled steel sheet, characterized in that the tensile strength is 1100MPa or more, and the surface hardness is 35HRC or more. The method according to any one of claims 1 to 3,
When the tensile strength and the surface hardness of the coiled coiled hot-rolled steel sheet are measured at 9 parts in total width and 3 parts in total length, the difference between the maximum and minimum values of each measurement result is within 140 MPa of tensile strength and 4 HRC of surface hardness. High-strength hot-rolled steel sheet. In weight percent, C: 0.10 to 0.30%, Si: 0.001 to 1.0%, Mn: 0.5 to 2.5%, Cr: 0.001 to 1.5%, Mo: 0.001 to 0.5%, Al: 0.001 to 0.5%, P: 0.001 to 0.01%, S: 0.001 ~ 0.01%, N: 0.001 ~ 0.01%, B: 0.0001 ~ 0.004%, Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1% and the balance contains iron and unavoidable impurities, and the following relationship (1) re-heating the steel slab satisfying 1180 ~ 1350 ℃;
Hot rolling the reheated steel slab to satisfy the following relational expression (2);
Cooling the hot rolled steel sheet to a temperature in the range of 0 to 400 ° C. to satisfy the following relationship (3); And
Winding the cooled steel sheet at a temperature in the range of 0 to 400 ° C;
Method of manufacturing a high-strength hot-rolled steel sheet comprising a.
[Relational formula (1)] CL <1
CL = -0.692 - 0.158 × [Mn ] + 0.121 × [Mn] 2 + 0.061 × [Cr] 2 - 0.319 × [Mo] + 0.035 × [Hardness_HRC]
(Wherein, CL is an effective crack generation index, [Mn], [Cr], [Mo] is the weight percent of the alloy element, and [Hardness_HRC] is the Rockwell hardness (HRC).)
[Relational Formula (2)] Tn-70 ≤ FDT ≤ Tn
Tn = 967-280 × [C] + 35.7 × [Si]-28.1 × [Mn]-11.4 × [Cr] + 11.4 × [Mo]-62 × [Ti] + 46.2 × [Nb]
(Where, Tn is the critical rolling temperature (℃), FDT is the rolling finish temperature (℃), [C], [Si], [Mn], [Cr], [Mo], [B], [Nb], [Ti] is the weight percent of the alloy element.)
[Relational Formula (3)] LCR ≤ CR ≤ HCR
LCR = 2000 / (-1076 + 2751 × [C] + 17 × [Si] + 301 × [Mn] + 330 × [Cr] + 355 × [Mo] + 42939 × [B])
HCR = 2500 / (-70.3 + 198 × [C] + 32.0 × [Si] + 16.7 × [Mn] + 18.4 × [Cr] + 42.1 × [Mo] + 5918 × [B])
(Where CR is the cooling rate in the cooling zone (℃ / s), LCR is the minimum critical cooling rate (℃ / s), the minimum value is 5, the maximum value is 45, and the HCR is the maximum critical cooling rate (℃ / s) and its minimum value is 50 and its maximum value is 200, and [C], [Si], [Mn], [Cr], [Mo], [B] are weight percentages of the alloy element.) The method of claim 5,
After the winding step, the high-strength hot-rolled steel sheet is a method of manufacturing a high-strength hot-rolled steel sheet characterized in that it is oiled after pickling treatment.

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