KR20190132494A - Black skin hot rolled steel sheet and manufacturing method thereof - Google Patents

Abstract

The black skin hot rolled sheet steel which has sufficient blackness and is excellent in the adhesiveness of a scale also seen from any direction is provided. A black skin hot rolled steel sheet comprising a base material hot rolled steel sheet, a Fe ₃ O ₄ and Fe, and a scale on the surface of the base material hot rolled steel sheet having a thickness of 3.0 to 20 µm, wherein the average particle diameter in the surface layer of the scale is 3.0. The area ratio of Fe in the cross section of the said scale is less than 1.0% in the area | region of 0-1.0 micrometer in the thickness direction of the said scale from the outermost layer of the said scale, and of the said scale and the said base material hot rolled sheet steel The black skin hot rolled steel sheet which is 1.0% or more in the area | region of 0-1.0 micrometer in the thickness direction of the said scale from the interface between them.

Description

Black skin hot rolled steel sheet and manufacturing method thereof

TECHNICAL FIELD The present invention relates to a black skin hot rolled steel sheet, and more particularly, to a black skin hot rolled steel sheet that is optimal for structural members used without leaving scales, such as building materials, piping, and agricultural balls. Moreover, this invention relates to the manufacturing method of the said black skin hot rolled sheet steel.

In order to remove the scale, that is, the oxide scale from the non-pickled hot-rolled steel sheet, it is necessary to perform hydrochloric acid cleaning, shot blasting, etc. I) is very expensive. Therefore, until now, for the cost reduction, the demand for the black-skinned hot rolled sheet steel with good scale adhesiveness inexpensively was large. Also, in recent years, new demands for obtaining black design for black skin hot rolled steel sheets have also increased.

As a black hot rolled sheet steel excellent in scale adhesiveness, the steel plates of patent document 1 and 2 are mentioned, for example.

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

In patent document 2, C: 0.01-0.4%, Si: 0.001-2.0%, Mn: 0.01-3.0%, P: 0.05% or less, S: 0.05% or less, Al: 0.3% or less, N: 0.01% or less Hot rolled steel sheet which contains and has a scale of 20 micrometers or less in thickness is proposed. In the said hot-rolled steel sheet, the ratio of the contact length of the said iron-iron and magnetite with respect to the contact length in the rolling direction of the steel plate and scale of a steel plate is 80% or more, and the average particle diameter of magnetite is 3 micrometers or less.

On the other hand, as a hot-rolled steel sheet excellent in the adhesiveness and blackness of a scale, the hot rolled sheet steel of patent documents 3-5 is mentioned, for example.

In Patent Document 3, has a scale thickness exceeding 4㎛, the art has the scale hot rolled steel sheet containing at least 50% Fe ₃ O ₄ by volume have been proposed. The scale of the hot rolled steel sheet does not contain precipitated Fe in a region from the surface to the depth of at least 2 μm.

In patent document 4, the manufacturing method of the hot rolled sheet steel which rolls, water-cooling with high pressure water immediately after each press reduction or just after each 2 press reduction, and makes rolling end temperature 850 degrees C or less is proposed.

In patent document 5, the manufacturing method of the hot rolled sheet steel which rolls, water-cooling with high pressure water immediately after each pressure reduction, and water-cools to the temperature of 800 degrees C or less and 700 degrees C or more immediately after completion | finish of rolling is proposed.

Japanese Patent Application Laid-Open No. 09-067649 Japanese Laid-Open Patent Publication No. 2014-031537 Japanese Laid-open Patent Publication 2004-043888 Japanese Patent Application Laid-Open No. 07-048622 Japanese Patent Application Laid-Open No. 07-048623

However, in the technique of patent documents 1 and 2, there existed a problem that sufficient blackness was not obtained. Moreover, in the technique of patent document 3, since blackness changes largely in the direction of light, there existed a problem that it looked black only in a specific direction, and not black in other directions. Furthermore, in the technique of patent documents 4 and 5, the scale obtained was blue black and was insufficient at the point of blackness.

This invention is made | formed in view of the said actual state, and an object of this invention is to provide the black skin hot rolled sheet steel which has sufficient blackness, and is excellent in the adhesiveness of a scale from any direction, and its manufacturing method.

The summary structure of this invention is as follows.

1. Base material hot rolled steel plate,

A black skin hot rolled steel sheet comprising a scale on the surface of the base material hot rolled steel sheet, which is composed of Fe ₃ O ₄ and Fe and has a thickness of 3.0 to 20 μm,

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

The Fe area ratio in the cross section of the scale,

From the outermost layer of the said scale, it is less than 1.0% in the area | region of 0-1.0 micrometer in the thickness direction of the said scale,

It is 1.0% or more in the area | region of 0-1.0 micrometer in the thickness direction of the said scale from the interface between the said scale and the said base material hot rolled sheet steel,

Black skin hot rolled steel plate.

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

C: 0.20% or less,

Si: 1.5% or less,

Mn: 0.1-3.0%,

P: 0.10% or less,

S: 0.10% or less,

Al: 1.0% or less and,

N: contains 0.020% or less,

The black skin hot rolled steel sheet as described in said 1 whose remainder has a chemical composition which consists of Fe and an unavoidable impurity.

3. The steel slab having the chemical composition according to the above 2 is subjected to hot rolling including rough rolling and finish rolling to obtain a hot rolled steel sheet,

The hot rolled steel sheet is cooled at an average cooling rate of 10 ° C./s or more,

As a manufacturing method of a black skin hot rolled sheet steel which winds the cooled said hot rolled sheet steel at the winding temperature of 400 degreeC or more and less than 600 degreeC,

After completion of the rough rolling, descaling is performed using high pressure water of 1.0 MPa or more at a temperature of 1100 ° C. or less,

After finishing the descaling, the finish rolling is started within 10 seconds,

End temperature of the said finish rolling shall be 800-950 degreeC,

Starting the cooling within 2.0 s from the end of the finish rolling;

Method for producing black skin hot rolled steel sheet.

According to the present invention, it is possible to obtain a black skin hot rolled steel sheet having sufficient blackness even in any direction and excellent in adhesion of scales.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the relationship between the collision pressure in descaling, and the surface-average particle diameter of a scale.
2 is a diagram showing the relationship between the surface layer average particle diameter of the scale and L *.

(Form to carry out invention)

Next, the method of implementing this invention is demonstrated concretely. In addition, the following description shows suitable embodiment of this invention, and this invention is not limited at all by the following description.

The black skin hot rolled steel sheet of this invention is equipped with the hot rolled steel sheet as a base material, and the scale (black skin) formed in the surface of the said hot rolled steel sheet. And the said scale has the thickness, composition, and Fe area ratio which were mentioned above. Hereinafter, the reason for limitation is demonstrated.

Thickness: 3.0-20㎛

When the thickness of the scale is less than 3.0 µm, a so-called magnetite seam in which FeO transforms into Fe ₃ O ₄ at the scale / base metal interface is not formed, and as a result, the adhesion of the scale is reduced. Therefore, the scale thickness is made 3.0 micrometers or more. On the other hand, when the scale is too thick, the scale is easily broken inside, and thus the adhesiveness is lowered. Therefore, the scale thickness is made 20 micrometers or less. It is preferable to set it as 15 micrometers or less, and it is more preferable to set it as 10 micrometers 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 most excellent in blackness among iron oxides that can be components of an oxidizing scale. In addition, Fe is a vacancy by transformation (eutectoid transformation) FeO (Ust-tight) is a component that is generated accompanying the reaction of the Fe ₃ O _4. Therefore, the scale of the present invention comprises only Fe ₃ O ₄ and Fe. The composition of the scale can be evaluated by the method described in Examples.

Surface layer average particle size: 3.0 µm or less

By reducing the particle size of the scale in the surface layer, the blackness when viewed from all directions can be increased. Therefore, the average particle diameter in a scale surface layer is made into 3.0 micrometers or less. It is preferable that the said average particle diameter shall be 2.0 micrometers or less. In addition, the minimum of the said average particle diameter is not specifically limited, It can be set as arbitrary values. However, when the average particle diameter is 0.5 µm or more, breakage of the scale can be further prevented. Therefore, it is preferable that the said average particle diameter shall be 0.5 micrometer or more. In addition, a "surface layer" refers here to the area | region from the outermost layer of a scale to 0.5 micrometer in the thickness direction of the said scale. In addition, the said average particle diameter can be measured by the method of describing in an Example.

Surface Fe Area Ratio: Less than 1.0%

If there is much precipitation of Fe in a scale surface layer part, blackness will fall. Therefore, from the outermost layer of the scale, the Fe area ratio (hereinafter referred to as "surface layer Fe area ratio") of the scale cross section in an area of 0 to 1.0 µm in the thickness direction of the scale is made less than 1.0%. The lower limit of the surface layer Fe area ratio is not particularly limited, but may be 0%. The surface Fe area ratio is preferably less than 0.5%, more preferably 0%. The said surface layer Fe area ratio can be measured by the method as described in an Example.

Interface Fe area ratio: 1.0% or more

At the interface between the scale and the base material hot rolled steel sheet, adhesion of the scale can be improved by depositing Fe with formation of the magnetite core. Therefore, from the interface of the scale and the base material hot-rolled steel sheet, the Fe area ratio (hereinafter referred to as "interface Fe area ratio") of the scale cross section in the region of 0 to 1.0 µm in the thickness direction of the scale is 1.0%. Do as above. It is preferable to set it as 2.0% or more, and, as for 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, breakage of the scale can be further prevented. Therefore, it is preferable to make the said interface Fe area ratio into 10% or less.

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

C: 0.20% or less

C couple | bonds with O at the interface of a scale and a base material hot rolled sheet steel, and produces | generates CO gas, and the adhesiveness of a scale falls. Therefore, it is preferable to make C amount into 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 in that case, it is acceptable to contain it as an unavoidable impurity. However, since excessive reduction causes a cost increase, it is preferable to make C amount into 0.0010% or more.

Si: 1.5% or less

Si greatly suppresses the generation of the scale, segregates at the interface between the scale and the base metal, and degrades the adhesion of the scale. In addition, Si is a ferrite generating element, and the particle size of a scale also becomes large by increasing the particle diameter of a base material. In addition, Si also causes a red scale, and when red scale is produced, blackness and designability are greatly reduced. Therefore, it is preferable to make Si amount 1.5% or less. The amount of Si is more preferably 1.0% or less, and even 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 in that case, it is acceptable to contain it as an unavoidable impurity. However, since excessive reduction causes a cost increase, it is preferable to make Si amount into 0.005% or more.

Mn: 0.1-3.0%

Mn contributes to the refinement of the scale by reducing the particle size of the base material. In order to acquire the said effect | action, it is preferable to make Mn amount into 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 a base material falls in cooling after hot rolling. When transformation temperature falls, transformation distortion will generate | occur | produce at low temperature, a crack may arise in a scale and adhesiveness of a scale may fall. Therefore, it is preferable to make Mn amount into 3.0% or less. As for Mn amount, it is more preferable to set it as 2.0% or less.

P: 0.10% or less

P segregates at the interface between the scale and the base material, thereby decreasing the adhesiveness of the scale. Therefore, P amount is preferably 0.10% or less. As for P amount, it is more preferable to set it as 0.03% or less, and it is more preferable to set it as 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 in that case, it is acceptable to contain it as an unavoidable impurity. However, since excessive reduction causes a cost increase, it is preferable to make P amount into 0.005% or more.

S: 0.10% or less

S remarkably lowers the ductility of the steel in hot, causing hot cracking and significantly deteriorating the surface properties of the steel sheet. Therefore, it is preferable to make S amount into 0.10% or less. As for S amount, it is more preferable to set it as 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 in that case, it is acceptable to contain it as an unavoidable impurity. However, since excessive reduction causes a cost increase, it is preferable to make S amount into 0.0001% or more.

Al: 1.0% or less

Al is a ferrite generating element, and the particle size of the scale is also increased by increasing the particle size of the base material. Therefore, it is preferable to make Al amount 1.0% or less. As for Al amount, it is more preferable to set it as 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 in this case, it is acceptable to contain it as an unavoidable impurity. However, since excessive reduction causes a cost increase, it is preferable to make Al amount 0.01% or more.

N: 0.020% or less

N significantly lowers the ductility of the steel in hot, thereby causing hot cracking and significantly deteriorating the surface properties of the steel sheet. Therefore, it is preferable to make N amount into 0.020% or less. As for N amount, it is more preferable to set it as 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 in that case, it is acceptable to contain it as an unavoidable impurity. However, since excessive reduction causes a cost increase, it is preferable to make N amount into 0.001% or more.

The chemical composition of the base material hot-rolled steel sheet in one embodiment of the present invention can be such that the above elements and the balance are made of Fe and unavoidable impurities. In addition, the "residue consists of Fe and an unavoidable impurity" here means that the case containing another trace element is included in the scope of the present invention, unless the action and effect of this invention are 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, 1 or 2 or more selected from the group consisting of Mg, Co, As, Pb, Zn, and O may be optionally 1.0% or less in total and further optionally.

Next, the manufacturing method of a black skin hot rolled sheet steel in one Embodiment of this invention is demonstrated.

The black skin hot rolled sheet steel of this invention can be manufactured by performing the following processes sequentially about the steel slab which has the said chemical composition.

(1) Hot rolling, including rough rolling and finish rolling

(2) cooling

(3) winding

And descaling using high pressure water is performed between the said rough rolling and finish rolling.

Hereinafter, the reason for limitation of the conditions in each said process is demonstrated.

Descaling Temperature: Below 1100 ℃

If the descaling temperature is high, the particle size of the base material becomes large, and the particle diameter of the scale newly generated after descaling becomes large. Therefore, the descaling temperature needs to be 1100 degrees C or less. It is preferable to make descaling temperature 1050 degrees C or less. In addition, although the minimum of descaling temperature is not specifically limited, From a viewpoint of ensuring the steel plate temperature in finish rolling, it is preferable to set it as 950 degreeC or more. In addition, "descaling temperature" shall refer to the surface temperature of the steel plate at the time of descaling start here.

Collision pressure: 1.0 MPa or more

By raising the collision pressure of the high pressure water used for descaling, the base material surface layer can be cooled and the particle size at the time of scale generation after descaling can be made small. 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. In addition, although the upper limit of the said collision pressure is not specifically limited, It is preferable to set it as 10 Mpa or less from a viewpoint of ensuring the steel plate temperature in finish rolling.

Time from descaling to finish rolling: within 10 s

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

Finish rolling finish temperature: 800-950 degreeC

When the finishing temperature of finish rolling is high, the thickness of the scale and the scale particle size become large. Therefore, the finish rolling finish temperature needs to be 950 degrees C or less, and it is preferable to set it as 900 degrees C or less. On the other hand, when the finishing temperature of finish rolling is too low, the scale will be broken at the time of rolling, and the scale will become too thin. Therefore, the finishing temperature of finish rolling needs to be 800 degreeC or more, and it is preferable to set it as 830 degreeC or more.

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

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

Average cooling rate: 10 ℃ / s or more

After finishing rolling, the obtained hot rolled steel sheet is cooled. At that time, if the cooling rate is small, the scale thickness and the scale particle size become large. Therefore, it is necessary to make the average cooling rate from the cooling start until the next winding start into 10 degreeC / s or more. It is preferable that the said average cooling rate shall be 20 degreeC / s or more. Although the upper limit of the said average cooling rate is not specifically limited, Since the scale becomes fragile easily when the said average cooling rate is excessively large, it is preferable to make the said average cooling rate into 100 degrees C / s or less.

In addition, the method of cooling is not specifically limited, As long as the said conditions are satisfied, it can be performed by arbitrary methods. Suitably, the cooling can be performed by water cooling.

Winding temperature: 400 ° C or more and less than 600 ° C

If the coiling temperature is high, after the Fe ₃ O ₄ of the scale surface layer is transformed into FeO, the Fe is precipitated on the surface layer by re-transforming into Fe ₃ O ₄ and Fe at the time of cooling, and blackness falls. Therefore, it is necessary to make winding temperature less than 600 degreeC. It is preferable to make winding temperature less than 580 degreeC. On the other hand, when the coiling temperature is excessively low, precipitation of Fe at the interface between the base material hot-rolled steel sheet and the scale is suppressed, the adhesiveness of the scale is lowered, and FeO remains in the scale and the blackness is reduced. Therefore, it is necessary to make winding temperature into 400 degreeC or more, and it is preferable to set it as 450 degreeC or more.

Example

Next, the Example of this invention is described.

Rough rolling, descaling, finish rolling, cooling, and winding were sequentially performed on the steel slab having the chemical composition shown in Table 1, thereby producing a black skin hot rolled steel sheet having a plate thickness of 3 mm. The said cooling was performed by water cooling, and the manufacturing conditions in each process were as showing in Table 2.

Next, the thickness, composition, surface layer average particle diameter, surface layer Fe area ratio, interfacial Fe area ratio, blackness, and adhesiveness of each of the obtained black skin hot rolled steel sheets were evaluated by the following methods. Table 3 shows the results of the evaluation.

(Scale thickness)

Scale thickness was measured in the following procedure. The roll direction-plate thickness direction cross section of the black-skinned hot rolled sheet steel was embedded-polished, and the reflection electron image of the said cross section after grinding | polishing was image | photographed using the scanning electron microscope (SEM). In the said imaging | photography, three photos of the area | region of the width | variety 100 micrometers in a rolling direction were image | photographed so that the whole thickness of a scale might fit in a visual field at 1000 times magnification. Next, the area of a scale part was calculated | required by image-processing the obtained reflected electron image, and it divided | segmented into 300 micrometers which is the total width of an observation area | region, and calculated the average scale thickness.

(Furtherance)

The component (phase) which comprises a scale was identified using the X-ray-diffraction apparatus which used Co tube. In addition, since the Fe peak of a base material hot-rolled steel sheet is also detected in X-ray, Fe discrimination was performed based on the presence or absence of the light gray part similar to a base material in the reflecting electron image of SEM.

(Surface average particle diameter)

The average particle diameter of the scale in surface layer was evaluated by the backscatter electron diffraction (EBSD) using the same sample as the said scale thickness measurement. EBSD measurement was performed in three areas of width 100 micrometers of a rolling direction so that a scale surface layer might enter in measurement step 0.1 micrometer. The grain boundary in the whole width direction of the measurement area | count is counted in the position of 0.5 micrometer base material hot-rolled steel plate side from the scale outermost layer in the thickness direction of a scale from a scale outermost layer as a grain boundary, and 300 micrometers which is an observation area | region Obtained by dividing by.

(Fe area ratio)

The surface Fe area ratio and the interface Fe area ratio were calculated | required by image-processing the SEM photograph similar to the said scale thickness measurement. Moreover, even when there was an unevenness | corrugation in the interface of a scale and a base material hot rolled sheet steel, 0-1.0 micrometer area | region was measured on an interface reference | standard.

(Black degree)

L * prescribed | regulated to JISZ8729 (CIE1976) was used for quantification of blackness. Also, the smaller L * means the closer to black color. The measurement of L * was performed on the conditions of geometric condition c (de: 8 degrees) prescribed | regulated to JIS Z8722 in order to reflect blackness in all directions, and D65 was used as a light source.

(Adhesiveness)

The adhesiveness of the scale evaluated by performing a tape peeling test, after providing a bending to a black skin hot rolled sheet steel. The bending was performed by a 170 ° bending of the inner punch 2t (t: sheet thickness) according to the pressing bend method of JIS Z 2248. The tape peeling test was performed in the outer peripheral part of the bending test piece, and the peeling state was evaluated visually. Evaluation criteria were as follows.

(Circle): Peeling is hardly confirmed.

(Triangle | delta): Some peeling is confirmed.

X: The whole surface peeling is confirmed.

As can be seen from the results shown in Tables 1 to 3, the black skin hot rolled steel sheet that satisfies the conditions of the present invention had sufficient blackness and excellent scale adhesion even when viewed from any direction. In addition, the results obtained are summarized in FIGS. 1 and 2.

1 is a plot showing the relationship between the collision pressure in descaling and the surface-average particle size of the scale. 1, only the result when manufacturing conditions other than an impact pressure satisfy | fills the conditions of this invention was plotted. The results shown in FIG. 1 show that the surface-average particle size of the scale can be in the range of the present invention by bringing the descaling collision pressure into the range of the present invention.

2 is a plot showing the relationship between the surface layer average particle diameter of the scale and L *. In addition, only the result in the case where parameters other than the surface layer average particle diameter of a scale satisfy | fills the conditions of this invention was plotted in FIG. From the result shown in FIG. 2, it turns out that L * can be 40 or less by making the scale surface layer average particle diameter into the range of this invention.

Claims (3)

Base material hot rolled steel plate,
Made of a Fe ₃ O ₄ and Fe, as mill scale hot rolled steel sheet including a scale on the surface of the base material hot-rolled steel sheet having a thickness of 3.0~20㎛,
The average particle diameter in the surface layer of the said scale is 3.0 micrometers or less,
The Fe area ratio in the cross section of the scale,
From the outermost layer of the said scale, it is less than 1.0% in the area | region of 0-1.0 micrometer in the thickness direction of the said scale,
The black skin hot rolled sheet steel which is 1.0% or more in the area | region of 0-1.0 micrometer in the thickness direction of the scale from the interface between the said scale and the said base material hot rolled sheet steel. The method of claim 1,
The base material hot rolled steel sheet is in mass%,
C: 0.20% or less,
Si: 1.5% or less,
Mn: 0.1-3.0%,
P: 0.10% or less,
S: 0.10% or less,
Al: 1.0% or less and,
N: contains 0.020% or less,
The black skin hot-rolled steel sheet whose chemical balance consists of Fe and an unavoidable impurity. The steel slab having the chemical composition according to claim 2 is subjected to hot rolling including rough rolling and finish rolling to obtain a hot rolled steel sheet.
The hot rolled steel sheet is cooled at an average cooling rate of 10 ° C./s or more,
As a manufacturing method of a black skin hot rolled sheet steel which winds the said cooled hot rolled sheet steel at the winding temperature of 400 degreeC or more and less than 600 degreeC,
After completion of the rough rolling, descaling is performed using high pressure water of 1.0 MPa or more at a temperature of 1100 ° C. or less,
After finishing the descaling, the finish rolling is started within 10 seconds,
End temperature of the said finish rolling shall be 800-950 degreeC,
Starting the cooling within 2.0 s from the end of the finish rolling;
Method for producing black skin hot rolled steel sheet.

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