KR102246956B1 - Blackened hot rolled steel sheet and its manufacturing method - Google Patents

Abstract

A black-colored hot-rolled steel sheet having sufficient blackness even when viewed from any direction and excellent in adhesion to scales is provided. A base material hot-rolled steel _{sheet, comprising a scale on the surface of the base material hot-rolled steel sheet, consisting of Fe 3} O ₄ and Fe, and having a thickness of 3.0 to 20 μm, wherein the average particle diameter in the surface layer of the scale is 3.0 Μm or less, and the Fe area ratio in the cross section of the scale is less than 1.0% in an area of 0 to 1.0 µm in the thickness direction of the scale from the outermost layer of the scale, and between the scale and the base material hot rolled steel sheet A scaled hot-rolled steel sheet having 1.0% or more in an area of 0 to 1.0 µm in the thickness direction of the scale from the interface therebetween.

Description

Blackened hot rolled steel sheet and its manufacturing method

The present invention relates to a scaled hot-rolled steel sheet, and more particularly, to a scaled hot-rolled steel sheet, which is optimal for structural members used without removing scale, such as building materials, pipes, agricultural equipment, and the like. In addition, the present invention relates to a method of manufacturing the above-mentioned blackened hot-rolled steel sheet.

In order to remove scale, that is, oxide scale from a non-pickled hot-rolled steel sheet, it is necessary to perform hydrochloric acid pickling or shot blasting.裸材) is very expensive. Therefore, even now, for cost reduction, there has been a great demand for a blackened hot-rolled steel sheet having good scale adhesion at low cost. In addition, in recent years, a new demand for a black-colored hot-rolled steel sheet is also increasing.

As a blackened hot-rolled steel sheet excellent in scale adhesion, the steel sheets described in Patent Documents 1 and 2 are exemplified.

In Patent Document 1, C: 0.001 to 0.20%, Si: 0.01 to 0.50%, Mn: 0.05 to 2.0%, P: 0.05% or less, S: 0.05% or less, sol.Al: 0.01 to 0.10%, N: 0.020 % Or less, has a scale of 4 µm or less in thickness on the surface, and a hot-rolled steel sheet having a surface roughness of 0.8 µm or less has been proposed.

In Patent Document 2, C: 0.01 to 0.4%, Si: 0.001 to 2.0%, Mn: 0.01 to 3.0%, P: 0.05% or less, S: 0.05% or less, Al: 0.3% or less, N: 0.01% or less. A hot-rolled steel sheet containing and having a scale of 20 µm or less in thickness has been proposed. In the hot-rolled steel sheet, the ratio of the contact length of the base iron and the magnetite to the contact length of the base iron and the scale in the rolling direction of the steel sheet is 80% or more, and the average particle diameter of the magnetite is 3 μm or less.

On the other hand, as a scaled hot-rolled steel sheet excellent in adhesion and blackness, the hot-rolled steel sheets described in Patent Documents 3 to 5 are exemplified.

In Patent Document 3, a hot-rolled steel sheet has been proposed which has a scale having a thickness exceeding 4 µm, and the scale contains 50% or more of Fe ₃ O _{4 by volume.} The scale of the hot-rolled steel sheet does not contain precipitated Fe in a region from the surface to a depth of at least 2 µm in the thickness direction.

In Patent Document 4, a method for producing a hot-rolled steel sheet is proposed in which rolling is performed immediately after each reduction or immediately after each reduction with water with high pressure water, and the rolling end temperature is 850°C or less.

In Patent Document 5, a method for producing a hot-rolled steel sheet is proposed in which rolling is performed immediately after each reduction with water cooling with high pressure water, and water-cooled to a temperature of 800°C or less and 700°C or more immediately after the rolling is completed.

Japanese Published Patent Publication No. Hei 09-067649 Japanese Published Patent Publication No. 2014-031537 Japanese Published Patent Publication No. 2004-043888 Japanese Published Patent Publication No. Hei 07-048622 Japanese Published Patent Publication No. Hei 07-048623

However, in the techniques described in Patent Documents 1 and 2, there was a problem that sufficient blackness was not obtained. In addition, in the technique described in Patent Document 3, there is a problem that the blackness greatly changes in the direction of light, so that it looks black only in a specific direction, and does not look black in other directions. Further, in the techniques described in Patent Documents 4 and 5, the scale obtained was bluish-black, and was insufficient in terms of blackness.

The present invention has been made in view of the above-described reality, and an object of the present invention is to provide a black coated hot-rolled steel sheet having sufficient blackness when viewed from any direction and excellent in adhesion to scales, and a method for producing the same.

The summary structure of the present invention is as follows.

1. With the base material hot rolled steel plate,

As a blackened hot-rolled steel sheet comprising a scale on the surface of the base material hot-rolled steel sheet, consisting of _{Fe 3} O _{4 and Fe, and having a thickness of 3.0 to 20 μm,}

The average particle diameter in the surface layer of the scale is 3.0 μm or less,

Fe area ratio in the cross section of the scale,

It is less than 1.0% in a region of 0 to 1.0 µm in the thickness direction of the scale from the outermost layer of the scale,

1.0% or more in a region of 0 to 1.0 µm in the thickness direction of the scale from the interface between the scale and the base material hot rolled steel sheet,

Black skin hot rolled steel sheet.

2. The base material hot-rolled steel sheet, in mass%,

C: 0.20% or less,

Si: 1.5% or less,

Mn: 0.1 to 3.0%,

P: 0.10% or less,

S: 0.10% or less,

Al: 1.0% or less and,

N: contains 0.020% or less,

The blackened hot-rolled steel sheet according to 1 above, wherein the balance has a chemical composition consisting of Fe and inevitable impurities.

3. With respect to the steel slab having the chemical composition described in 2 above, hot rolling including rough rolling and finish rolling is performed to obtain a hot rolled steel sheet,

Cooling the hot-rolled steel sheet at an average cooling rate of 10°C/s or more,

As a method for producing a scaled hot-rolled steel sheet, in which the cooled hot-rolled steel sheet is wound at a winding temperature of 400°C or more and less than 600°C,

After the rough rolling is finished, descaling is performed using high-pressure water having an impact pressure of 1.0 MPa or higher at a temperature of 1100°C or less,

After the descaling is finished, the finish rolling is started within 10 s,

The end temperature of the finish rolling is 800 to 950°C,

Initiating the cooling within 2.0 s from the end of the finish rolling,

Manufacturing method of black-skinned hot-rolled steel sheet.

Advantageous Effects of Invention According to the present invention, it is possible to obtain a blackened hot-rolled steel sheet that has sufficient blackness even when viewed from any direction, and has excellent scale adhesion.

1 is a diagram showing a relationship between an impact pressure in descaling and an average particle diameter of a surface layer of a scale.
2 is a diagram showing the relationship between the average particle diameter of the surface layer of the scale and L*.

(Form for carrying out the invention)

Next, a method of carrying out the present invention will be described in detail. In addition, the following description shows a suitable embodiment of this invention, and this invention is not limited at all by the following description.

The blackened hot-rolled steel sheet of the present invention includes a hot-rolled steel sheet as a base material, and a scale (black skin) formed on the surface of the hot-rolled steel sheet. In addition, the scale has the above-described thickness, composition, and Fe area ratio. Hereinafter, the reason for the limitation will be described.

Thickness: 3.0∼20㎛

If the thickness of the scale is less than 3.0 µm _{, a so-called magnetite seam in which FeO is transformed into Fe 3} O ₄ at the scale/base material interface is not formed, and as a result, the adhesion of the scale is deteriorated. Therefore, the scale thickness is set to 3.0 µm or more. On the other hand, if the scale is too thick, the scale is liable to be destroyed inside, so the adhesion is deteriorated. Therefore, the scale thickness is set to 20 µm or less. The scale thickness is preferably 15 µm or less, and more preferably 10 µm or less. The scale thickness can be measured by the method described in Examples.

Composition: Fe ₃ O ₄ and Fe

Fe ₃ O ₄ (magnetite) is the most excellent in blackness among iron oxides that can be a component of oxidized scale. In addition, Fe is a component generated with a reaction in which FeO (ustite) undergoes eutectoid transformation and _{becomes Fe 3} O _4. Therefore, the scale in the present invention consists only of _{Fe 3} O _{4 and Fe.} The composition of the scale can be evaluated by the method described in Examples.

Surface average particle diameter: 3.0㎛ or less

By reducing the particle size of the scale in the surface layer, the degree of blackness when viewed from all directions can be increased. Therefore, the average particle diameter in the scale surface layer is set to 3.0 µm or less. It is preferable that the said average particle diameter is 2.0 micrometers or less. On the other hand, the lower limit of the average particle diameter is not particularly limited, and may be an arbitrary value. However, when the average particle diameter is 0.5 µm or more, destruction of the scale can be further prevented. Therefore, it is preferable that the average particle diameter is 0.5 µm or more. In addition, the "surface layer" here refers to a region from the outermost layer of the scale to 0.5 µm in the thickness direction of the scale. In addition, the average particle diameter can be measured by the method described in Examples.

Surface Fe area ratio: less than 1.0%

If there is a large amount of precipitation of Fe in the surface layer of the scale, the blackness decreases. Therefore, the Fe area ratio (hereinafter referred to as "surface Fe area ratio") of the scale cross section in a region of 0 to 1.0 µm in the thickness direction of the scale from the outermost layer of the scale is set to less than 1.0%. The lower limit of the surface layer Fe area ratio is not particularly limited, but may be 0%. The surface layer Fe area ratio is preferably less than 0.5%, and more preferably 0%. The surface layer Fe area ratio can be measured by the method described in Examples.

Interface Fe area ratio: 1.0% or more

At the interface between the scale and the base hot-rolled steel sheet, the adhesion of the scale can be improved by depositing Fe along with the formation of the magnetite core. Therefore, from the interface between the scale and the base hot-rolled steel sheet, the Fe area ratio (hereinafter referred to as "interface Fe area ratio") of the scale cross section in a region of 0 to 1.0 µm in the thickness direction of the scale is 1.0%. Do it as above. It is preferable to set it as 2.0% or more, and, as for the interface Fe area ratio, it is more preferable to set it as 3.0% or more. On the other hand, the upper limit of the interfacial Fe area ratio is not particularly limited, but when the interfacial Fe area ratio is 10% or less, destruction of the scale can be further prevented. Therefore, it is preferable that the interface Fe area ratio is 10% or less.

It is preferable that the said base material hot-rolled steel sheet has a chemical composition described below. In addition, "%" in the following description shall mean "mass%" unless otherwise noted.

C: 0.20% or less

C combines with O at the interface between the scale and the base material hot-rolled steel sheet to generate CO gas, thereby reducing the adhesion of the scale. Therefore, it is preferable that the amount of C is made 0.20% or less. On the other hand, the lower limit of the amount of C is not particularly limited, and may be 0%, but also in that case, it is allowed to contain it as an unavoidable impurity. However, since excessive reduction causes an increase in cost, the amount of C is preferably 0.0010% or more.

Si: 1.5% or less

While Si greatly suppresses the formation of scale, it segregates at the interface between the scale and the base material, thereby deteriorating the adhesion of the scale. In addition, Si is a ferrite-generating element, and by increasing the grain size of the base material, the grain size of the scale is also increased. In addition, Si is also a cause of red scale, and when red scale is generated, blackness and design properties are greatly reduced. Therefore, it is preferable that the amount of Si is 1.5% or less. The amount of Si is more preferably 1.0% or less, and still more preferably 0.5% or less. On the other hand, the lower limit of the amount of Si is not particularly limited, and may be 0%, but also in that case, it is allowed to contain it as an inevitable impurity. However, since excessive reduction causes an increase in cost, the amount of Si is preferably 0.005% or more.

Mn: 0.1 to 3.0%

Mn contributes to fine-grained scale by reducing the particle diameter of the base material. In order to obtain the above effect, the amount of Mn is preferably 0.1% or more. On the other hand, since Mn is an austenite-generating element, when Mn is contained in a large amount, the transformation temperature of the base material decreases in cooling after hot rolling. When the transformation temperature decreases, transformation distortion occurs at a low temperature, and therefore, cracks occur in the scale and the adhesion of the scale may decrease. Therefore, it is preferable that the amount of Mn is 3.0% or less. It is more preferable that the amount of Mn is 2.0% or less.

P: 0.10% or less

P segregates at the interface between the scale and the base material and reduces the adhesion of the scale. Therefore, it is preferable that the amount of P is 0.10% or less. The amount of P is more preferably 0.03% or less, and even more preferably 0.01% or less. On the other hand, the lower limit of the amount of P is not particularly limited, and may be 0%, but also in that case, it is allowed to contain it as an inevitable impurity. However, since excessive reduction causes an increase in cost, the amount of P is preferably set to 0.005% or more.

S: 0.10% or less

S remarkably lowers the ductility of the steel during hot, thereby causing hot cracking, and remarkably deteriorating the surface properties of the steel sheet. Therefore, it is preferable that the S amount is 0.10% or less. It is more preferable that S amount is made 0.03% or less. On the other hand, the lower limit of the amount of S is not particularly limited, and may be 0%, but also in that case, it is allowed to contain it as an inevitable impurity. However, since excessive reduction causes an increase in cost, the S amount is preferably 0.0001% or more.

Al: 1.0% or less

Al is a ferrite-generating element, and by increasing the grain size of the base material, the grain size of the scale is also increased. Therefore, it is preferable that the amount of Al is 1.0% or less. The amount of Al is more preferably 0.5% or less. On the other hand, the lower limit of the amount of Al is not particularly limited and may be 0%, but also in that case, it is allowed to contain it as an unavoidable impurity. However, since excessive reduction causes an increase in cost, the amount of Al is preferably 0.01% or more.

N: 0.020% or less

N remarkably lowers the ductility of the steel during hot, thereby causing hot cracking, and remarkably deteriorating the surface properties of the steel sheet. Therefore, it is preferable that the amount of N be 0.020% or less. The amount of N is more preferably 0.010% or less. On the other hand, the lower limit of the amount of N is not particularly limited and may be 0%, but also in that case, it is allowed to contain it as an unavoidable impurity. However, since excessive reduction causes an increase in cost, the amount of N is preferably 0.001% or more.

The chemical composition of the base hot-rolled steel sheet in one embodiment of the present invention can be made of the above elements and the remainder of Fe and unavoidable impurities. In addition, "the balance consists of Fe and unavoidable impurities" here means that the case containing other trace elements is included in the scope of the present invention, as long as the function and effect of the present invention are not impaired. For example, in one embodiment of the present invention, the chemical composition is Ti, Nb, V, Mo, Ta, W, Cr, Ni, Cu, Ca, REM (rare earth metal), Sb, B, Sn, One or two or more selected from the group consisting of Mg, Co, As, Pb, Zn, and O may be contained in a total of 1.0% or less, further optionally.

Next, a method of manufacturing a scaled hot-rolled steel sheet in an embodiment of the present invention will be described.

The blackened hot-rolled steel sheet of the present invention can be manufactured by sequentially performing the following treatments on a steel slab having the above chemical composition.

(1) Hot rolling including rough rolling and finish rolling

(2) cooling

(3) winding

Then, descaling using high-pressure water is performed between the rough rolling and finish rolling.

Hereinafter, the reasons for limiting the conditions in each step will be described.

Descaling temperature: 1100℃ or less

When the descaling temperature is high, the grain size of the base material increases, and the grain size of the scale newly generated after descaling increases. Therefore, the descaling temperature needs to be 1100°C or less. The descaling temperature is preferably 1050°C or less. On the other hand, the lower limit of the descaling temperature is not particularly limited, but from the viewpoint of securing the steel sheet temperature in finish rolling, it is preferably set to 950°C or higher. In addition, "descaling temperature" here refers to the surface temperature of the steel sheet at the time of descaling start.

Collision pressure: 1.0 MPa or more

By increasing the collision pressure of the high-pressure water used for descaling, the surface layer of the base material is cooled, and the particle diameter at the time of generation of the scale after descaling can be reduced. Therefore, the collision pressure needs to be 1.0 MPa or more. The collision pressure is preferably 2.0 MPa or more, and more preferably 3.0 MPa or more. On the other hand, the upper limit of the collision pressure is not particularly limited, but from the viewpoint of securing the steel sheet temperature in finish rolling, it is preferably set to 10 MPa or less.

Time from end of descaling to start of finish rolling: within 10s

After descaling is finished, finish rolling is performed. At that time, if the time from the end of descaling to the start of finish rolling is long, the scale thickness increases and the scale particle diameter also increases. Therefore, it is necessary to start finish rolling within 10 s after completion of descaling, and it is preferable to start finish rolling within 5 s after completion of descaling.

Finish rolling end temperature: 800 to 950°C

When the finishing temperature of the finish rolling is high, the thickness of the scale and the size of the scale will increase. Therefore, the finish rolling end temperature needs to be 950°C or less, and is preferably 900°C or less. On the other hand, if the end temperature of the finish rolling is too low, the scale breaks during rolling and the scale becomes too thin. Therefore, the finish rolling temperature needs to be 800°C or higher, and is preferably 830°C or higher.

Time from finish rolling to start of cooling: 2.0s or less

If the time from the end of finish rolling to the start of cooling is long, the scale thickness and the scale particle diameter increase. Therefore, the time from the end of the finish rolling to the start of cooling needs to be 2.0 s or less, and is preferably 1.0 s or less. On the other hand, the lower limit of the time from the end of the finish rolling to the start of cooling is not particularly limited, and may be 0 s, but is preferably 0.1 s or more.

Average cooling rate: 10℃/s or more

After finishing the finish rolling, the obtained hot-rolled steel sheet is cooled. In that case, if the cooling rate is small, the scale thickness and the scale particle diameter will increase. Therefore, it is necessary to set the average cooling rate from the start of cooling to the start of the next winding up to 10°C/s or more. It is preferable that the average cooling rate is 20°C/s or more. The upper limit of the average cooling rate is not particularly limited. However, if the average cooling rate is excessively large, the scale is liable to break. Therefore, the average cooling rate is preferably set to 100° C./s or less.

In addition, the method of cooling is not particularly limited, and any method can be used as long as it satisfies the above conditions. Suitably, the cooling can be performed by water cooling.

Winding temperature: 400℃ or more and less than 600℃

When the coiling temperature is high, Fe ₃ O ₄ in the scale surface layer is transformed into FeO _{and then retransformed into Fe 3} O ₄ and Fe at the time of cooling, whereby Fe precipitates on the surface layer, resulting in a decrease in blackness. Therefore, the coiling temperature needs to be less than 600°C. It is preferable that the coiling temperature is less than 580°C. On the other hand, when the coiling temperature is excessively low, precipitation of Fe at the interface between the base hot-rolled steel sheet and the scale is suppressed, the adhesion of the scale decreases, and FeO remains in the scale, resulting in a decrease in blackness. Therefore, the coiling temperature needs to be 400°C or higher, and preferably 450°C or higher.

Example

Next, examples of the present invention will be described.

Rough rolling, descaling, finish rolling, cooling, and winding were sequentially performed on the steel slab having the chemical composition shown in Table 1 to produce a black-skinned hot-rolled steel sheet having a thickness of 3 mm. The cooling was performed by water cooling, and the manufacturing conditions in each step were as shown in Table 2.

Subsequently, about each of the obtained blackened hot-rolled steel sheets, the thickness of the scale, the composition, the average surface layer particle diameter, the surface layer Fe area ratio, the interface Fe area ratio, the blackness, and the adhesion were evaluated by the following methods. Table 3 shows the evaluation results.

(Scale thickness)

The scale thickness was measured in the following procedure. The cross section in the rolling direction-plate thickness direction of the black-skin hot-rolled steel sheet was embedded-polished, and a reflection electron image of the cross-section after polishing was photographed using a scanning electron microscope (SEM). In the above photographing, three photographs of a region having a width of 100 μm in the rolling direction were photographed at a magnification of 1000 times so that the entire thickness of the scale fits within the field of view. Subsequently, the obtained reflected electron image was subjected to image processing to determine the area of the scale portion, and divided by 300 µm, which is the total width of the observation area, to obtain an average scale thickness.

(Furtherance)

The component (phase) constituting the scale was identified using an X-ray diffraction apparatus using a Co tube. In addition, since the Fe peak of the hot-rolled steel sheet of the base material is also detected by X-rays, the determination of Fe was performed based on the presence or absence of a light gray portion similar to that of the base material in the reflected electron image of the SEM.

(Average surface particle diameter)

The average particle diameter of the scale in the surface layer was evaluated by backscattering electron diffraction (Electron BackScatter Diffraction: EBSD) using the same sample as the scale thickness measurement. The EBSD measurement was carried out in a measurement step of 0.1 µm in three areas with a width of 100 µm in the rolling direction so that the scale surface layer enters. With an orientation difference of 15° or more as the grain boundary, the grain coefficient in the entire width direction of the measurement area at a position of 0.5 μm in the thickness direction of the scale from the outermost layer of the scale to the hot-rolled steel sheet of the base material is counted, and the observation area is 300 μm. It was obtained by dividing by the coefficient of grain.

(Fe area ratio)

The surface layer Fe area ratio and the interface Fe area ratio were determined by image-processing the same SEM photograph as in the above scale thickness measurement. In addition, even when there are irregularities at the interface between the scale and the base material hot-rolled steel sheet, a range of 0 to 1.0 µm was measured based on the interface.

(Blackness)

For quantification of blackness, L* specified in JIS Z8729 (CIE1976) was used. In addition, the smaller L* is, the closer it is to black. The measurement of L* was performed under the conditions of geometric condition c (de: 8°) specified in JIS Z8722 in order to reflect the degree of blackness in all directions, and D65 was used as a light source.

(Adhesion)

The adhesiveness of the scale was evaluated by performing a tape peeling test after applying bending to the blackened hot-rolled steel sheet. The provision of the bending was performed by bending at 170° of 2t (t: plate thickness) of the inner punch in accordance with the pressing bend method of JIS Z 2248. The tape peeling test was performed at the outer periphery of the bending test piece, and the peeling state was evaluated visually. The evaluation criteria were as follows.

○: Almost no peeling was observed.

△: Some peeling was confirmed.

X: All surface peeling is confirmed.

As can be seen from the results shown in Tables 1 to 3, the blackened hot-rolled steel sheet satisfying the conditions of the present invention had both sufficient blackness and excellent scale adhesion even when viewed from any direction. Further, the obtained results are summarized in Figs. 1 and 2.

1 is a plot showing a relationship between an impact pressure in descaling and an average particle diameter of a surface layer of a scale. In addition, in FIG. 1, only the result when manufacturing conditions other than the impact pressure satisfy|fill the conditions of this invention were plotted. From the results shown in Fig. 1, it can be seen that by making the descaling impact pressure within the range of the present invention, the average particle diameter of the surface layer of the scale can be within the range of the present invention.

2 is a plot showing the relationship between the average particle diameter of the surface layer of the scale and L*. In addition, in FIG. 2, only the result when the parameter other than the surface layer average particle diameter of a scale satisfies the conditions of this invention was plotted. From the results shown in Fig. 2, it can be seen that L* can be made 40 or less by making the average particle diameter of the scale surface layer within the range of the present invention.

Claims (3)

A base material hot rolled steel sheet,
As a _{blackened hot-rolled steel sheet consisting of Fe 3} O ₄ and Fe and comprising a scale on the surface of the base material hot-rolled steel sheet having a thickness of 3.0 to 20 μm,
The average particle diameter in the surface layer of the scale is 0.5 µm or more and 3.0 µm or less,
Fe area ratio in the cross section of the scale,
It is less than 1.0% in a region of 0 to 1.0 µm in the thickness direction of the scale from the outermost layer of the scale,
1.0% or more in a region of 0 to 1.0 µm in the thickness direction of the scale from the interface between the scale and the base hot rolled steel sheet. The method of claim 1,
The base material hot-rolled steel sheet, in mass%,
C: 0.20% or less,
Si: 1.5% or less,
Mn: 0.1 to 3.0%,
P: 0.10% or less,
S: 0.10% or less,
Al: 1.0% or less and,
N: contains 0.020% or less,
Black-skinned hot-rolled steel sheet having a chemical composition in which the balance consists of Fe and inevitable impurities. The steel slab is subjected to hot rolling including rough rolling and finish rolling to obtain a hot rolled steel sheet,
Cooling the hot-rolled steel sheet at an average cooling rate of 10°C/s or more,
As a method for producing a scaled hot-rolled steel sheet, in which the cooled hot-rolled steel sheet is wound at a winding temperature of 400°C or more and less than 600°C,
The steel slab, in mass%,
C: 0.20% or less,
Si: 1.5% or less,
Mn: 0.1 to 3.0%,
P: 0.10% or less,
S: 0.10% or less,
Al: 1.0% or less and,
N: contains 0.020% or less,
The balance has a chemical composition consisting of Fe and unavoidable impurities,
After the rough rolling is finished, descaling is performed using high-pressure water having an impact pressure of 1.0 MPa or higher at a temperature of 1100°C or less,
After the descaling is finished, the finish rolling is started within 10 s,
The end temperature of the finish rolling is 800 to 950°C,
Initiating the cooling within 2.0 s from the end of the finish rolling,
Manufacturing method of black-skinned hot-rolled steel sheet.

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