JPH11290905A - Manufacture of steel sheet free from scale defect - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of steel sheets on which scale defects are not generated in hot rolling. SOLUTION: In a hot finish rolling after descaling of steel having a composition consisting of, by weight, <=0.20% C, <=2.0% Si, 0.1-2.5% Mn, <=0.1% P, <=0.03% S, 0.01-0.1% sol. Al, <=0.01% N and the balance essentially Fe, the rolling is ended at a temp. of not lower than Ar3 without recuperating heat for >=5 sec between 920-970 deg.C of the surface temp. of a rough bar.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel sheet having no scale flaws in hot rolling of the steel sheet.

[0002]

2. Description of the Related Art If scales generated during the hot rolling process of a steel sheet are pushed during rolling, it causes surface flaws. Therefore, descaling has been conventionally performed immediately before each rolling mill. However, in a row of finish rolling mills in which a plurality of rolling mills are continuously arranged, the scale generated between stands is so thin that descaling is extremely difficult. Therefore, in the finish rolling of the steel sheet, it is inevitable that the steel sheet is rolled in a state where the scale is generated. Therefore, Japanese Patent Application Laid-Open
As disclosed in US Pat. No. 154301, there is a technique of performing water cooling on the finishing entry side as necessary and regulating the temperature of the finishing entry side to a predetermined temperature or less, thereby suppressing scale formation and preventing scale flaws. . Furthermore, CAMP
-As shown in ISIJ, 9 (1996), P972, it is possible to prevent the scale from being pushed into the ground steel by reducing the finishing inlet temperature to 1000 ° C and the finishing outlet temperature to 830 ° C. It has been reported. However, in the method of suppressing the occurrence of scale defects by extremely lowering the finishing entry side temperature and the finishing exit side temperature, it becomes difficult to produce a wide material, a thin material, a high tensile material, etc., particularly with an increase in the rolling load, and Not only the problem of sheet passing property occurs, but also in any steel sheet, it is necessary to secure the finishing outlet temperature of Ar3 transformation or higher from the viewpoint of the material properties, and there is a limit to measures that simply lower the rolling temperature. . In addition, it is estimated that the reason for this is that the occurrence of blisters, which is a phenomenon in which the scale rises from the ground iron between the finishing stands, can be suppressed. Iron and steel, 65 (1979), P599 show the relationship between the oxidation temperature and time on the generation of such a blister-like scale.
It has been reported that a blister-like scale is generated when the temperature is maintained at 50 to 1000 ° C. for about 25 seconds or more. Furthermore, it has been reported that in air cooling from a high temperature, a blister-like scale is generated by staying in a temperature range of 1000 to 1100 ° C. for about 10 seconds or more. As described above, although the oxidation temperature and time affecting the generation of blister-like scale have been investigated, in actual finish rolling, the residence time between the stands at 1000 ° C. or higher is almost as short as 10 seconds or less. No clear causal relationship with the flaw was found.

[0003]

For these reasons,
At present, no clue for effective scale flaw reduction has been found for any steel sheet including wide, thin and high-tensile materials.

[0004] The present invention has been made to solve the above problems, and an object of the present invention is to provide a method of manufacturing a steel sheet free of scale flaws in hot rolling.

[0005]

Means for Solving the Problems The present inventors have made intensive studies to overcome the above-mentioned problems. As a result of various studies on the formation conditions of blister scales for which a clear causal relationship has not been recognized at present, the generation of blister scales and the occurrence of indentation scale defects are considered by considering the relative flow velocity of the coarse bar and air in finish rolling. It was found that outbreaks could be clearly linked. The present invention has been made based on such findings, and the method for producing a steel sheet without scale flaws according to the present invention has a C: ≤ 0.20 wt.%
, Si: ≦ 2.0 wt.%, Mn: 0.1-2.5 wt.%
, P: ≦ 0.1 wt.%, S: ≦ 0.03 wt.%, So
l. Al: 0.01 to 0.1 wt.%, N: ≦ 0.01 w
% in the hot rolling of steel having a component composition substantially consisting of iron in the finish rolling step after descaling, the surface temperature of the rough bar is set to 920 to 970.
Rolling is completed at Ar3 or higher without reheating for 5 seconds or more between ° C.

Further, the method for producing a scale-free steel sheet according to the present invention comprises the steps of: C: ≤0.20 wt.%, Si: ≤2.0 w
t.%, Mn: 0.1 to 2.5 wt.%, P: ≦ 0.1 wt.
%, S: ≦ 0.03 wt.%, Sol. Al: 0.01 to
0.1 wt.%, N: ≦ 0.01 wt.%, And further, Ti: ≦ 0.20 wt.%, Nb: ≦ 0.10
wt.%, V: ≦ 0.10 wt.%, B: ≦ 0.005 wt.
%, Cu: ≦ 0.6 wt.%, Ni: ≦ 0.6 wt.%, C
r: ≦ 1.0 wt.%, Mo: ≦ 0.6 wt.%, containing one or more selected from the group consisting of: In hot rolling of steel, in the finish rolling process after descaling,
Rolling is completed at Ar3 or higher without reheating the surface temperature of the coarse bar to 920 to 970 ° C for 5 seconds or more.

Further, in the method for producing a steel sheet without scale flaws according to the present invention, in the above-mentioned method, in the finish rolling step after the descaling, between the descaling device and the first stand of the finish rolling, and in the finish rolling, Whether the surface temperature of the coarse bar is measured at all positions between the stands using a radiation thermometer, or whether one or more locations are measured using a radiation thermometer and the other positions are predicted by calculation Either, to grasp the surface temperature of the rough bar during finish rolling, based on the surface temperature of the rough bar thus grasped, between the descaling device and the first stand of finish rolling, and finish rolling The cooling water is supplied to the surface of the rough bar between the stands to adjust the surface temperature.

Further, in the method for producing a steel plate free of scale flaws according to the present invention, in the above method, in the finish rolling step after the descaling, between the descaling device and the first stand of the finish rolling, and in the finish rolling, 30 l / s.m ² (1 m ² on line)
(30 liters per second per second) or less is supplied to the surface of the rough bar, and only the surface of the rough bar is slowly cooled.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The steel sheet to which the present invention is applied includes, in addition to a soft steel sheet having a normal composition, a steel sheet provided with characteristics such as high strength and high corrosion resistance. Next, the reason for limiting the chemical composition of the steel slab used in the present invention will be described.

The main components C, Si, Mn, P, S,
sol. Al and N are as follows. C: C is an element effective for increasing the strength. Furthermore, Ti, N
When b and V are added, carbides are finely precipitated to suppress grain growth, thereby making the structure finer and strengthening the precipitation to contribute to an increase in strength. On the other hand, the addition of a large amount of C not only causes an increase in the amount of pearlite after winding, deteriorating workability, but also adversely affects weldability. Therefore, C
The upper limit of the content is defined as 0.20 wt.%.

Si: Si does not deteriorate workability,
It has the effect of solid solution strengthening ferrite and increasing the balance between strength and workability (enhancing strength and maintaining excellent workability). When used as a solid solution strengthening element, it should be added according to the required strength level. However, since a large amount of Si addition deteriorates toughness and weldability,
The upper limit of the Si content is defined as 2.0 wt.%. On the other hand, the lower limit is not particularly specified, but is 0.01
wt.% or more is desirable.

Mn: Mn is an element effective for increasing the strength. When used as a solid solution strengthening element, it should be added according to the required strength level. Furthermore, S in steel
Is fixed as MnS, it is possible to prevent slab cracking at the time of hot rolling caused by the grain boundary embrittlement of S. Therefore, the lower limit of the Mn content is 0.1%.
Defined in On the other hand, if the content exceeds 2.5 wt.%, The weldability deteriorates. Therefore, the upper limit of the Mn content is set to 2.5
Defined as wt.%.

P: P is an element effective for increasing strength and corrosion resistance. When used as a solid solution strengthening element, it should be added according to the required strength level. The lower limit is not specified, but 0.01 wt.%
The above is desirable. On the other hand, if P is added in excess of 0.1 wt.%, Segregation occurs at the grain boundaries and the secondary workability deteriorates. Therefore, P
The upper limit of the content is defined as 0.1 wt.%.

S: S may segregate at the grain boundaries during hot rolling, causing slab cracking and promoting the generation of surface flaws. Therefore, S is fixed as MnS by adding Mn. However, since excessive MnS becomes a starting point of voids during processing, ductility and stretch flangeability deteriorate. When Ti is added, Ti-based sulfide precipitates, but this precipitate is not only coarse and does not contribute to an increase in strength, but also serves as a starting point of a void at the time of processing, and the ductility and stretch flangeability decrease. . As described above, since S is an impurity element, it is desirable to reduce it as much as possible. Therefore,
The upper limit of the S content is specified as 0.03 wt.%.

Sol. Al: sol. Al acts as a deoxidizing element to reduce inclusions in the steel. so
l. If the Al content is less than 0.01 wt.%, This effect cannot be obtained. Therefore, sol. The lower limit of the Al content is defined as 0.01 wt.%. In addition, sol. If Al is added excessively in excess of 0.1 wt.%, Alumina-based inclusions increase and ductility decreases.
Defined as t.%.

N: When the N content exceeds 0.01 wt.%,
The upper limit of the N content is set to 0.01 wt.% Because there is a possibility that surface flaws may occur with slab cracks during hot rolling. The lower limit is not particularly specified, but is preferably 0.001 wt.% Or more from the viewpoint of manufacturing cost.

Next, if necessary, Ti, N
b, V, B, Cu, Ni, Cr and Mo are as follows. Ti: Ti forms fine Ti-based carbonitrides, refines the structure, and increases the strength by precipitation strengthening.
Therefore, it should be added according to the required strength level, but even if Ti is added in excess of 0.20 wt.%, The Ti-based carbonitride coarsens and does not contribute to the increase in strength. This leads to reduced ductility. Therefore, the upper limit of the amount of Ti added is set to 0.20 wt.%.

Nb: Nb is an element effective for refining the structure. In order to obtain high strength without impairing the workability, it is effective to make the structure finer. Further, the formation of Nb-based carbonitride contributes to an increase in strength. Therefore, Nb should be added according to the required strength level,
Even if it is added in a large amount exceeding t.%, not only the effect of refining the structure is saturated, but also the Nb-based carbonitride does not contribute to an increase in strength due to coarsening, and a decrease in ductility. Therefore, N
b The upper limit of the amount added is limited to 0.10 wt.%.

V: V forms fine V-based carbonitrides, refines the structure, and increases the strength by precipitation strengthening. Therefore, although it should be added according to the required strength level, even if V is added in excess of 0.10 wt.
The coarsening of the carbonitride not only does not contribute to an increase in strength but also lowers ductility. Therefore, the upper limit of the amount of V added is set to 0.10 wt.%.

B: Since B has an effect of suppressing the release of strain during hot working, it refines the structure and contributes to an increase in strength. Further, segregation at grain boundaries improves secondary workability. However, even if B is added in excess of 0.005 wt.%,
The grain refinement effect of B not only saturates, but also causes an increase in roll load due to accumulation of strain during hot rolling, making the rolling extremely difficult. Therefore, the upper limit of the amount of B added is 0.00
Specify 5 wt.%.

Cu: Cu is an element that contributes to an increase in strength by solid solution strengthening and is effective for corrosion resistance. However, the excess Cu is uneconomic because the strength increasing effect and the corrosion resistance effect are saturated, so the upper limit of the Cu addition amount is set to 0.6 watts.
Defined as t.%.

Ni: Ni contributes to an increase in strength by solid solution strengthening. Further, when Cu is added, slab cracks are caused during hot rolling, which causes surface flaws. On the other hand, by adding Ni in an amount equivalent to the atomic equivalent of Cu, it is possible to prevent the occurrence of surface defects. However, excessive Ni is uneconomic because the effect of increasing the strength and the effect of preventing surface flaws are saturated, so the upper limit of the amount of Ni added is set to 0.6 wt.%.

Cr: Cr contributes to an increase in strength by solid solution strengthening, and also refines carbides to improve workability. However, excessive Cr is uneconomical because the effect of increasing strength and the effect of improving workability are saturated, so the upper limit of the amount of added Cr is set to 1.0 wt.%.

Mo: Mo is an element that contributes to an increase in strength due to the formation of carbonitrides and is effective for corrosion resistance.
However, the excessive addition of Mo is uneconomical because the strength increasing effect and the corrosion resistance effect are saturated, so the upper limit of the amount of Mo added is set to 0.6 wt.%.

Next, the reason for limiting the surface temperature of the rough bar in the finish rolling process in the present invention will be described. Generally, when the scale grows, Fe becomes ions and diffuses and moves in the scale toward the outer surface, and combines with O ₂ in the atmosphere at the outermost layer of the scale. On the other hand, a part of O ₂ in the atmosphere diffuses at the scale grain boundaries, and Fe ions are combined with O ₂ at the scale grain boundaries, so that new scale is generated inside the scale. As described above, when a new scale is generated inside the scale, the scale tries to expand, but cannot expand because the scale is constrained by the bond with the ground iron. As a result, a compressive stress acts on the scale.

Furthermore, when O ₂ reaches the scale / iron interface through the scale grain boundary, C in steel having a lower oxide equilibrium dissociation pressure than Fe is preferentially linked to O _2, and the scale / iron CO gas will be generated at the interface. And compressive stress acting on the scale and C at the scale / iron interface
When the amount of generated O gas is large, a phenomenon occurs in which the scale peels off from the base iron and floats.

When such a blister-like scale is generated between stands in the finish rolling process, since the raised scale portion loses heat transfer from the ground iron, it is rapidly cooled by contact with the rolling roll. Will be. Then, since the cooled scale is very hard and brittle, cracks occur in the roll bite during rolling, and the scale is pushed into the base iron, and scale flaws are generated. Therefore, in order to prevent the occurrence of scale flaws, it is necessary to suppress the occurrence of blister-like scale between the finishing stands.

Here, in the normal finish rolling, the passing speed of the coarse bar after descaling is 0.3 m / s or more. However, since the Air flow rate greatly affects scale formation in the atmosphere, Air As a result of performing an oxidation test at a flow velocity of 0.3 m / s or more, the following findings were obtained.

As a test steel, a steel having a chemical composition shown in Table 1 was melted in a laboratory, and a thickness of 2.0 mm was used.
Was prepared.

[0030]

[Table 1]

The oxidation experiment was performed in a chamber equipped with a heating device. The sample was placed in an N ₂ atmosphere at 850-1050.
After heating to a temperature of ° C, the chamber is once evacuated and immediately
After ir substitution, while maintaining the heating temperature and isothermal temperature, 1
Air was introduced at a flow rate of m / s for 1 to 100 s to generate a scale. Thereafter, the pressure was again reduced, and the atmosphere was immediately replaced with N ₂ and then gradually cooled at 1 ° C./s. The reason for the slow cooling is to prevent the occurrence of blister-like scale due to the difference in thermal shrinkage between the scale and the ground iron during cooling. The sample after cooling was visually observed to determine whether or not blisters had occurred.

FIG. 1 shows the effect of oxidation time and oxidation temperature on the formation of blister scale. Here, in the figure, 条件 indicates a condition in which no blister was observed, and ×
Indicates the conditions under which blisters occurred. 5 seconds (s) at a temperature within the range of 920 to 970 ° C. which is outside the scope of the present invention.
When oxidized, blisters were observed.
On the other hand, when the oxidation temperature was lower than 920 ° C., when the oxidation temperature was higher than 970 ° C., or when the oxidation time was less than 5 seconds (s) even at a temperature in the range of 920 to 970 ° C., no blister was generated. .

When the oxidation temperature is lower than 920 ° C.,
_Since the diffusion coefficient of ₂ is small and the amount of O ₂ diffusing in the scale grain boundaries is small, the amount of new scale generation inside the scale and the generation of CO gas at the scale / iron interface are also reduced, and the generation of blister-like scale Is suppressed.

On the other hand, when the oxidation temperature exceeds 970 ° C., the formation of scale is greatly promoted, and most of O ₂ is consumed on the outermost layer side of the scale before diffusing in the scale grain boundaries. The amount of new scale generated inside the scale and the generation of CO gas at the scale / base iron interface are reduced, and the generation of blister scale is also suppressed in this case.

Further, when the oxidation temperature is in the range of 920 to 970 ° C., O ₂ is most likely to diffuse at the scale grain boundaries,
The generation of new scale inside the scale and the generation of CO gas at the scale / iron interface are promoted. Then, as the oxidation time increases, the amount of compressive stress or CO gas acting on the scale increases, and a blister-like scale is generated. Therefore, in the finish rolling process from the descaling to the end of the rolling, the surface temperature condition is 920 to 970 ° C.
Is not maintained for 5 seconds (s) or more at a temperature in the range of

In the finishing rolling process after the descaling, the surface temperature of the rough bar between the descaling apparatus and the first stand of the finish rolling, and between the stands of the finish rolling, is determined by measuring the surface temperature of the rough bar measured on the finishing side. The surface temperature may be determined by calculation in consideration of the roll peripheral speed and the gap of each finishing stand, or may be measured using a radiation thermometer.

Further, the method of controlling the temperature history of the surface of the rough bar in the finish rolling process is performed by a method of performing descaling immediately before finish rolling with high-pressure water, and the amount of water may be adjusted. The rolling distribution between the stands may be adjusted. Further, cooling water or the like may be applied between the descaling device and the first stand of the finish rolling and between the stands of the finish rolling to adjust. Further, when applying cooling water to the surface of the coarse bar, 30 l / s · m
² or less (30 liters per ^second per square meter per second) may be sprayed to slowly cool only the surface of the coarse bar to ensure the threading properties and the final temperature of the finish rolling.

If the finish temperature of the finish rolling is lower than Ar3, the surface layer of the steel sheet becomes coarse and the workability is significantly deteriorated. Therefore, the finish temperature of the finish rolling is specified to be Ar3 or more.

[0039]

Next, a method for manufacturing a steel plate free of scale flaws according to the present invention will be described with reference to examples. The present invention is not limited by these examples.

Table 2 shows the test pieces No. The chemical composition of 2 to 10 and Table 3 show the finish rolling conditions of the actual machine and the presence or absence of scale flaws.

[0041]

[Table 2]

[0042]

[Table 3]

The descaling on the finishing side is
Performed with a 150 kgf / cm ² hydraulic descaling device to completely remove surface scale before finish rolling. There are seven finishing mills, which are referred to as F1, F2,... F7 in order from the roll on the entry side. In Table 2, the time before F1 is the time required for the movement from the descaling machine to the F1 entry side. The temperature before F1 is a value obtained by actually measuring the surface of the rough bar using a radiation thermometer between the descaling device and F1. The temperatures between the stands F1 to F7 are values obtained by actually measuring the surface of the rough bar in the middle of each stand using a radiation thermometer. The cooling before F1 and between the stands was performed by applying 50 l / s · m ² (50 liters per ^second per m ² on the line per second) of cooling water to the surface of the coarse bar over a length of 0.1 m in the rolling direction on the line. Rapid cooling by spraying directly, or 5 l / s
The cooling was carried out by slow cooling by spraying m ² (5 liters per ^second per m ² on the line per second) in a rolling direction over the length of 2 m in the rolling direction on the line. The cooling before F1 was performed before the F1 roll, and the cooling between the stands was performed immediately after the F1 roll, for example, between F1-2.

The temperature of the rough bar before the finish rolling is reduced only by the surface due to the water at the time of descaling, and the heat transfer from the central portion of the sheet thickness in the subsequent rolling process and the heat generated during the rolling are factors that increase the surface temperature. On the other hand, cooling as time passes and the use of a cooling spray provided between stands acts as a factor for lowering the surface temperature.

Here, the specimen No. F in 2-10
The time and surface temperature before and after each stand were varied. Specimen No. No. 2 performed finish rolling without cooling between stands and reached the highest temperature between F2 and F3, but the temperature at that time was 913 ° C., which was within the temperature range of the present invention, and scale flaws were generated. There was no. Specimen No. No. 3 performs finish rolling without cooling between stands to obtain F1-2, F2-3, F3-4, and F4-.
Scale flaws occurred because the temperature between 5 was within the range of 923 to 958 ° C. and the time was out of the range of the temperature and time of the present invention, which was 6.0 seconds (s). Specimen No. No. 4 performed rapid cooling by performing rapid cooling between F1-2, and reached the maximum temperature between F2 and 3, but the temperature at that time was 923 ° C, and the time was 2.9 s, which was the time of the present invention. Within the range, no scale flaw was generated.
Specimen No. 5 performed finish rolling without cooling between stands, but the temperature in F1-2, F2-3, and F3-4 was in the range of 920 to 933 ° C., and the total time was 9.3 seconds (s). Scale flaws occurred because the temperature and time of the present invention were out of the range. Specimen No. No. 6 performs finish rolling without cooling between stands, and the temperature is in the range of 928 to 955 ° C between F2-3, F3-4, and F4-5, and the total time is 4.0.
Within the second (s) and the time range of the present invention, no scale flaw was generated. Specimen NO. 7 is F2-3, F3-4
Quenching, finish rolling, and F2-3
The maximum temperature was reached between
The temperature was 4 ° C., the time was 2.1 seconds (s), which was within the time range of the present invention, and no scale flaw was generated. Specimen No.
No. 8 performs rapid cooling in F2-3 and performs finish rolling, and the temperature between F2-3 and F3-4 is 923 to 93.
The temperature was within the range of 5 ° C., and the total time was 3.3 seconds (s), which was within the time range of the present invention, and no scale flaw was generated.
Specimen No. No. 9 performs finish rolling without cooling, and the temperature is 92 in F1-2, F4-5, and F5-6.
The temperature was in the range of 9 to 964 ° C, and the time was 4.4 s in total, which was within the time range of the present invention, and no scale flaw was generated. Specimen No. 10 is F1-2, F2-3, F3-
In step 4, the sample is slowly cooled and subjected to finish rolling.
The maximum temperature was reached during that time, but the temperature at that time was 91
The temperature was 2 ° C., which is within the temperature range of the present invention, and no scale flaw was generated.

[0046]

As described above, according to the method of the present invention,
In the finish rolling process after the descaling, the surface temperature is set to 920 to 970 ° C. for 5 s or more without reheating.
By terminating the rolling after 3 or more, a method for producing a steel sheet free of scale flaws is provided, and an industrially useful effect is brought about.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a view showing a state of generation of a blister scale when an oxidation time and an oxidation temperature in Air are variously changed.

Continuation of the front page (72) Inventor Noboru Shioya 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd.

Claims (4)

[Claims] 1. C: ≦ 0.20 wt.%, Si: ≦ 2.0 wt.%, Mn: 0.1-2.5 wt.%, P: ≦ 0.1 wt.%, S: ≦ 0.03 wt.%, Sol. Finish after descaling in hot rolling of steel containing Al: 0.01 to 0.1 wt.%, N: ≦ 0.01 wt.%, And the balance being substantially iron. A method for producing a steel plate having no scale flaws, wherein in a rolling step, rolling is completed at a temperature of Ar3 or more without reheating the surface temperature of the rough bar between 920 to 970 ° C for 5 seconds or more. 2. C: ≦ 0.20 wt.%, Si: ≦ 2.0 wt.%, Mn: 0.1-2.5 wt.%, P: ≦ 0.1 wt.%, S: ≦ 0.03 wt.%, Sol. Al: 0.01 to 0.1 wt.%, N: ≦ 0.01 wt.%, And further, Ti: ≦ 0.20 wt.%, Nb: ≦ 0.10 wt.% , V: ≦ 0.10 wt.%, B: ≦ 0.005 wt.%, Cu: ≦ 0.6 wt.%, Ni: ≦ 0.6 wt.%, Cr: ≦ 1.0 wt.% Mo: ≦ 0.6 wt.% Descaling in hot rolling of a steel containing one or more selected from the group consisting of: and a balance substantially consisting of iron. A method for producing a steel sheet without scale flaws, wherein in a subsequent finish rolling step, rolling is completed at Ar3 or more without reheating the surface temperature of the rough bar to 920 to 970 ° C for 5 seconds or more. 3. In the finish rolling step after descaling, the surface temperature of the rough bar is measured using a radiation thermometer between a descaling device and a first stand of finish rolling and between each stand of finish rolling. The surface temperature of the rough bar during finish rolling by either measuring at all positions or measuring at least one position using a radiation thermometer and predicting other positions by calculation. Based on the surface temperature of the rough bar grasped in this way, between the descaling device and the first stand of finish rolling, and between each stand of finish rolling, supply cooling water to the surface of the rough bar. The method for producing a steel plate free of scale flaws according to claim 1, wherein the surface temperature is adjusted by adjusting the surface temperature. 4. In the finish rolling process after descaling, 30 l / s · m ² (per 1 m ² on a line) between the descaling device and the first stand of finish rolling and between each stand of finish rolling. The cooling water of 30 liters or less per second) is supplied to the surface of the coarse bar to slowly cool only the surface of the coarse bar. A method for producing steel plates without scale flaws.