JP2011189394A - Method for manufacturing hot rolled steel sheet having excellent surface property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a hot rolled steel sheet having excellent surface property, which prevents the generation of red scale and flaws due to scale biting so as to minimize surface defects in manufacturing the hot rolled steel sheet. <P>SOLUTION: In hot-rolling after continuously casting a steel containing 0.001-0.30% C, 0.10% or less Si, 1.0% or less Mn, 0.04% or less P, 0.02% or less S, 0.005-0.10% acid soluble Al and the balance being Fe and unavoidable impurities, the temperature of the steel sheet is made T1(°C) or higher denoted by formula (1) below and the impingement pressure of the high-pressure water is made 15.7 MPa or less, when the surface of the steel sheet is descaled with high-pressure water at the entry side of a finish rolling mill and the surface temperature of the steel sheet between the second-row and third-row stands of the finish rolling mill is made T2(°C) or lower denoted by formula (2) below. Formula (1) is expressed by T1≥998×Si(%)-1,283×P(%)+1,010 and formula (2) is expressed by T2≤599×Si(%)-770×P(%)+976. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for producing a hot-rolled steel sheet (hereinafter referred to as a hot-rolled steel sheet) having a good surface property, which is used for automobile parts, building members, general processing, or various plated steel sheets. .

A hot-rolled steel sheet is generally manufactured with equipment having a configuration as shown in FIG. In FIG. 1, the conveying direction of the hot-rolled steel sheet is from the right to the left in the figure.
A slab having a thickness of about 200 mm, which is a material, is heated to, for example, 1200 to 1300 ° C. in a heating furnace (not shown). Thereafter, the scale generated on the surface is descaled by a high-pressure water descaling device (not shown), and then thinned by the rough rolling mill 1 to a thickness of about 25 to 45 mm. Subsequently, after finishing rolling with a finish rolling mill 2 to a thickness of about 1.2 to 6.0 mm, the steel sheet is cooled on a hot run table 6 and wound by a coiler 7 to obtain a hot-rolled steel sheet. ing.

The scale generated on the surface of the steel strip in the hot rolling process includes a primary scale generated by oxidizing the surface of the slab in the heating furnace, a secondary scale generated during rough rolling, and further during finish rolling and There is a cubic scale to be generated later.
The primary scale is removed by a high-pressure water descaling device (not shown) installed on the entry side of the roughing mill 1. The secondary scale generated thereafter is removed in most cases by the high-pressure water descaling device 3 installed on the entry side of the finish rolling mill 2, and a uniform tertiary scale is generated on the surface of the rolled material.

However, depending on the raw material components and the temperature of the rolled material, the secondary scale may not be completely removed by the high-pressure water descaling device before finish rolling, and may remain in that case. It may be reduced by the roll of the machine and a red scale may occur.
The red scale is a secondary scale that has not been removed by high-pressure water descaling and remains on the surface of the steel strip in the form of strips or stripes and is observed as a pattern.
On the other hand, the tertiary scale generated during finish rolling is rolled together with the steel sheet, but when the steel sheet surface temperature is high, the tertiary scale peels off during finish rolling, and the second row stand roll of the finish rolling mill. In some cases, the steel sheet is wound around the surface of the steel sheet, resulting in scale wrinkles on the surface of the steel sheet and rough surface of the steel sheet.

Thus, it is well known that the red scale or the peeled tertiary scale adversely affects the surface properties of the hot-rolled steel sheet. Various measures have been studied and proposed.
For example, in Patent Document 1, C: 0.02 to 0.60%, Si: 0.02 to 2.0%, Mn: 0.2 to 3.0%, P: 0.05 to 0.11% Containing and satisfying 0.04 ≦ P / Si ≦ 0.55, rough rolling according to the soaking time of the heating furnace, the collision pressure of high pressure water descaling before the start of rough rolling and the value of P / Si By controlling the end temperature, a method for producing a hot-rolled steel sheet excellent in surface properties without a red scale has been proposed.

  Further, in Patent Document 2, as a method for producing a thin hot-rolled steel sheet that prevents scale wrinkles and has excellent surface properties, C: 0.03 to 0.20%, Si: less than 0.05%, Mn: 0.10 to 2.0%, Al: 0.08% or less, P / Si ≦ 2.0, steel sheet surface temperature on the entry side of the second row and third row stands of the finish rolling mill is 830 ° C. The manufacturing method by carrying out finish rolling below the temperature defined by the surface skin of the work rolls of the second row and third row stands of the finish rolling mill and the P / Si ratio of the steel sheet is disclosed above.

  In Patent Document 3, C: 0.01 to 0.2%, Si: 0.05 to 0.7%, Mn: 0.01 to 2.0%, P: 0.001 to 0.1%, S : 0.001 to 0.1%, Cu: 0.01 to 0.15%, Al: 0.001 to 0.1%, N: 0.0005 to 0.01%, Ni: 0.005 to 0 By producing a steel containing 0.03% and having an atomic ratio Mn / S of Mn / S of 5 or more under predetermined conditions, a method for producing a hot-rolled steel sheet having a small red scale and excellent surface properties is provided. Proposed.

In Patent Document 4, C: 0.01 to 0.2%, Si: 0.05% or less, Mn: 0.1 to 1.0%, P: 0.02% or less, S: 0.02% or less , Al: 0.08% or less, Ni: 0.05-0.2%, Cu is a hot rolled steel sheet containing 1/3 × Ni <Cu <2 / 3Ni with respect to Ni, and the scale thickness of the surface Has proposed a method for producing a hot-rolled steel sheet excellent in surface properties and scale adhesion, characterized in that the ratio of magnetite (Fe ₃ O ₄ ) in the scale is 80% or more.
Furthermore, Patent Documents 5 to 7 propose a method of manufacturing a steel sheet having excellent surface properties in which generation of red scale in Si-added steel is suppressed.

JP-A-8-206723 Japanese Patent Laid-Open No. 11-77108 JP 11-302783 A JP 2000-87185 A JP 2000-319730 A JP 2007-39774 A Japanese Patent Laid-Open No. 5-279734

In each of the above-mentioned patent documents, various measures are individually examined for a means for suppressing red scale due to insufficient descaling before finish rolling and a means for preventing scale wrinkles due to the peeled tertiary scale. For this reason, even though the red scale can be suppressed, the bite scale wrinkle cannot be prevented. Moreover, even if the biting scale wrinkles can be prevented, the red scale cannot be suppressed. Therefore, a method for producing a hot-rolled steel sheet having neither a red scale nor a bite scale is desired.
The present invention has been devised in order to solve such problems. When manufacturing a hot-rolled steel sheet, the generation of red scale is suppressed, and the scale flaw is prevented and surface flaws are prevented. It aims at providing the manufacturing method which can obtain the hot-rolled steel plate excellent in surface properties with few.

In order to achieve the object, the method for producing a hot-rolled steel sheet having excellent surface properties according to the present invention is, in mass%, C: 0.001 to 0.30%, Si: 0.10% or less, Mn: 1. Continuous casting of steel containing 0% or less, P: 0.04% or less, S: 0.02% or less, acid-soluble Al: 0.005-0.10%, and the balance of Fe and inevitable impurities After the hot rolling, the steel plate temperature when performing high pressure water descaling on the steel plate surface on the hot rolling finish rolling mill entry side is equal to or higher than T1 (° C) indicated by the following formula (1), and the high pressure water collision pressure is The steel sheet surface temperature between the second row and third row stands of the hot finish rolling mill is T2 (° C.) or less represented by the following equation (2).
T1 ≧ 998 × Si (%) − 1283 × P (%) + 1010 (1)
T2 ≦ 599 × Si (%) − 770 × P (%) + 976 (2)
However,
Si (%): Si content (mass%) contained in hot-rolled steel sheet
P (%): P content (mass%) contained in hot-rolled steel sheet
T1: Descaling temperature (° C) on the finishing mill entry side
T2: Steel plate surface temperature (° C) between the second row and third row stands of the finish rolling mill

When performing high-pressure water descaling on the steel sheet surface on the hot rolling finishing rolling mill entrance side, the steel sheet temperature may be equal to or higher than T1 ′ (° C.) expressed by the following equation (3), and the high-pressure water collision pressure may be equal to or higher than 24.5 MPa.
T1 '≧ 776 × Si (%) − 998 × P (%) + 980 ・ ・ ・ ・ （3）
However,
T1 ′: Descaling temperature (° C.) on the finishing mill entry side

Moreover, it is good also as below T2 '(degreeC) which shows the steel plate surface temperature between the 2nd row | line | column and 3rd row | line | column stand of a hot finish rolling mill by following (4) Formula.
T2 ′ ≦ 599 × Si (%) − 770 × P (%) + 915 (4)
However,
T2 ′: Steel plate surface temperature (° C.) between the second row and third row stands of the finish rolling mill

  In addition, as steel composition, Cu: 0.01-0.30%, Ni: 0.01-0.20%, Ti: 0.01-0.20%, Nb: 0.01-0.20 %, B: 0.0005 to 0.005%, one or two or more of them are preferable.

In the method for producing a hot-rolled steel sheet according to the present invention, the steel temperature and the hot finish when performing high-pressure water descaling on the steel sheet surface on the hot-rolling finish rolling mill entrance side are in steels with specified contents of Si and P. By regulating the steel plate surface temperature between the second row and third row stands of the rolling mill and the impact pressure of high-pressure water, the primary scale and the secondary scale are improved in peelability, and the generation of the red scale and the tertiary Peeling during finish rolling of the scale can be suppressed.
Therefore, according to the present invention, it is possible to obtain a hot-rolled steel sheet having excellent surface properties with less surface wrinkles by preventing generation of red scale and scale biting during finish rolling.

Schematic of a hot rolling apparatus to which the hot rolling method of the present invention is applied The figure which shows the relationship between the temperature and the f value which influence the scale peelability at the time of high temperature bending test Figure showing the relationship between the descaling temperature on the finishing entry side and the f value for the occurrence of red scale The figure which shows the relationship between the steel plate surface temperature between the 2nd row | line | column and 3rd row | line stand of a finish rolling mill, and f value with respect to generation | occurrence | production of a scale flaw. The figure which shows the relationship between the descaling temperature and the f value on the finishing side when the descaling impact pressure on the finishing side of the finishing mill is set to a high level against the occurrence of red scale When the steel plate surface temperature between the second row and the third row stand of the finish rolling mill is set to a low level against the occurrence of scale wrinkles, the steel plate surface temperature between the second row and the third row stand and f Diagram showing value relationship

1: Rough rolling mill 2: Finish rolling mill 3: High pressure water descaling device 4: Finish rolling roll F2
5: Finishing roll F3 6: Hot run table 7: Coiler
8: Measurement position of descaling temperature (measures the front side of the steel plate)
9: Measurement position of temperature between F2 and F3 (measures the front side of the steel plate)
10: Measurement position of finished delivery side steel plate temperature (measures the front side of the steel plate)

In general, in Si-added steel, a Si-based oxide layer is formed at the interface between the base iron and the scale, and Si is considered to exist in the form of stable firelite (Fe ₂ SiO ₄ ).
If the scale including this firelight is not removed by descaling on the finishing mill entry side, it is pushed into the steel sheet surface by the finishing mill, resulting in unevenness on the steel sheet surface, which is considered to be the cause of red scale. .
In particular, Si and P are concentrated at the interface between the scale and the base iron in a temperature range of 900 ° C. or higher, and form a Fe—Si—P-based oxide, thereby affecting the peelability of the scale layer.

Therefore, the inventors investigated the peelability of the scale in the high temperature range in relation to the Si content and the P content.
First, five types of test steels having alloy components shown in Table 1 were prepared, and a high temperature bending test was performed in the atmosphere at a temperature of 920 to 1100 ° C. to determine the scale peeling rate of the test steel. The test piece for the high temperature bending test shall be 1.5 thickness x 12 width x 50 length, JIS
This was performed in accordance with Z2248.
The results are shown in Table 2.

The relationship between the temperature of the high-temperature bending test and the scale peel rate is determined by multiple regression analysis based on the relationship between the Si content and the P content among the obtained scale peel rate and alloy components. F value = −665 × Si (%) + 855 We found that xP (%) can be used as an index.
And in the manufacturing conditions of the hot-rolled steel sheet, the steel sheet surface temperature at the position of the high-pressure water descaling device on the finishing rolling mill entrance side and the presence or absence of red scale generation, and between the second row and third row stands of the finishing rolling mill It was found that the surface temperature of the steel sheet and the occurrence of scale wrinkles can be clearly arranged by using this f value.

As a result, as shown in FIG. 2, it was found that the scale peelability deteriorates with an increase in the amount of Si. In order to remove the scale of the hot-rolled steel sheet in the descaling on the entry side of the finish rolling mill, it has been found that it is effective to increase the descaling temperature on the finish entry side as the Si amount increases.
Moreover, about P amount, it turned out that scale peelability improves with the increase in P amount. Therefore, even when the amount of P is small, it is possible to suppress biting flaws or rough surface skin (hereinafter abbreviated as scale flaws) by increasing the descaling temperature on the finish entry side because scale peelability deteriorates. I found out that

In finish rolling, when the tertiary scale generated after descaling peels between rolls of the finish rolling mill, it adheres to the roll surface and causes scale wrinkles.
Accordingly, it has been found that the third scale is more effective for suppressing the generation of scale wrinkles when it is more difficult to peel off, and for that purpose, it is effective to lower the finish rolling temperature.
However, since the descaling temperature on the finishing entry side is set by the Si amount and P amount, the actual finishing rolling temperature is determined depending on the cooling capacity and the thickness of the rolled material, and should be set to an arbitrary temperature. I can't.

Therefore, the present invention sets the appropriate descaling temperature on the finishing rolling entry side and the steel sheet surface temperature between the second row and third row stands of the finish rolling mill in accordance with the Si amount and the P amount, thereby hot rolling. It has been estimated that a hot-rolled steel sheet having excellent surface properties can be produced by suppressing red scale and scale wrinkles of the steel sheet, and the method of the present invention has been achieved.
The details will be described below.
In addition, this invention relates to the surface property of the cold rolled steel plate and plated steel plate which used the hot rolled steel plate and this raw material as base steel, and does not specify the kind of the annealing method or plating method of a cold rolled steel plate.

The effects of the chemical components of the base steel in the present invention, the reason for limiting the content, and the hot rolling conditions are individually described as follows.
C: 0.30% or less C is an element effective for securing strength. As the amount added increases, the workability decreases and the scale peeling becomes non-uniform and wrinkles are likely to occur, so the upper limit was set to 0.30%.

Si: 0.10%
Si is generally added for deoxidation and strengthening of the steel. As the amount of Si increases, the concentration of Fe-Si oxides generated at the interface between the scale layer and the base iron advances, and the eutectic temperature rises. Scaling makes it difficult to remove the scale. However, although it works effectively on the scale peeling between the second row stand (hereinafter referred to as F2) and the third row stand (hereinafter referred to as F3) of the finish rolling mill, the surface texture is caused by the occurrence of red scale. Since the deterioration becomes significant, the upper limit is set to 0.10% or less.

Mn: 1.0% or less Mn is an element added to improve the strength. The strength improvement effect by Mn increases as the Mn content increases. However, even if the Mn content exceeds 1.0%, the workability deteriorates although the strength increases with the addition amount. Therefore, 1.0% was made the upper limit.

P: 0.04% or less P is an alloy element effective for increasing the strength. As P increases, Fe-Si-P-based oxides are generated and concentrated at the interface between the scale layer and the ground iron. Since the P-based oxide lowers the eutectic temperature and increases the liquid phase, the scale peelability is good. However, even if the content exceeds 0.04%, the effect on the peelability of the scale is saturated, and grain boundary embrittlement easily occurs and the workability deteriorates. Therefore, 0.04% was made the upper limit.

S: 0.02% or less Since a large amount of S impairs cold or hot workability, it is preferably as small as possible. There is no problem above.
Al: 0.005-0.10%
Al is added as a deoxidizer, but in order to obtain a sufficient deoxidation effect, 0.005% or more of acid-soluble Al needs to be added. The effect of Al deoxidation is saturated at 0.10%, and even if it is added more than that, the cost of the steel is increased.

Cu: 0.01 to 0.30%
Since Cu has the property of diffusing from the ground iron into the scale interface and in the scale layer, it is effective in dispersing the firelite (Fe ₂ SiO ₄ ) concentrated layer and has a low melting point. Can be improved. For this reason, it adds as needed. If the added amount is less than 0.01%, the concentrated layer will not be concentrated sufficiently on the side of the ground iron interface, so an addition of 0.01% or more is necessary. However, if it exceeds 0.30%, hot brittleness may deteriorate.

Ni: 0.01-0.20%
Ni is concentrated at the interface between the iron base and the scale, and has the effect of preventing embrittlement of the slab due to Cu melting during slab heating, so it is added as necessary. If the Ni content is less than 0.01%, the concentrated layer does not sufficiently concentrate on the side of the ground iron interface, so addition of 0.01% or more is necessary. However, if it exceeds 0.20%, hot brittleness deteriorates and surface properties deteriorate.

Ti: 0.01-0.20%
Ti combines with C, S and N to form precipitates, and is an element effective for increasing the strength of steel strip by precipitation strengthening. Further, these precipitates suppress the recovery of processing strain in the weld heat affected zone and can prevent softening of the heat affected zone due to solid solution and reprecipitation during welding heating. If the addition amount is less than 0.01%, a remarkable effect is not exhibited. However, even if added over 0.20%, the effect is saturated and the manufacturing cost is increased.

Nb: 0.01-0.20%
Nb combines with C in the same way as Ti to form precipitates, and is an element effective for increasing the strength of the steel strip by precipitation strengthening, and refines the metal structure of the steel sheet to improve the strength. Further, in the welded portion, similarly to the effect of Ti, the recovery of the processing strain of the weld heat affected zone can be suppressed by the precipitate, and softening of the heat affected zone can be prevented by solid solution and reprecipitation. If the addition amount is less than 0.01%, a remarkable effect is not exhibited. However, even if added over 0.20%, the effect is saturated and the manufacturing cost is increased.

B: 0.0005 to 0.005%
B is added to improve aging and hardenability as required. If the addition amount of B is less than 0.0005%, there is no effect of addition, and if it exceeds 0.005%, the effect is saturated and economically disadvantageous.

Finishing mill entry-side descaling temperature: T1 (° C.) satisfying the formula (1) or more When the concentration of Fe-Si-based or Fe-Si-P-based oxides generated at the interface between the scale layer and the ground iron proceeds, The eutectic temperature changes and affects the peelability of the scale.
If the descaling temperature on the finish rolling mill entry side is lower than T1 (° C.) in the formula (1), the secondary scale peelability will deteriorate and the scale will remain on the surface of the rolled material, causing the occurrence of red scale. T1 (° C.) or higher. The equation (1) is obtained from FIG.

The removal of the secondary scale is performed by causing high-pressure water to collide with the steel plate surface on the entrance side of the finish rolling mill. For this reason, the descaling impact pressure is preferably as low as possible in the range where the secondary scale can be removed from the viewpoint of manufacturing cost, but it is usually 15.7 MPa or less.
T1 ≧ 998 × Si (%) − 1283 × P (%) + 1010 (1)
T2 ≦ 599 × Si (%) − 770 × P (%) + 976 (2)
However,
Si (%): Si content (mass%) contained in hot-rolled steel sheet
P (%): P content (mass%) contained in hot-rolled steel sheet

The secondary scale is removed by increasing the high-pressure water descaling collision pressure from 15.7 MPa or less to 24.5 MPa or more, so that the descaling temperature on the finish rolling mill entry side is up to T1 ′ in equation (3). Decreasing the scale completely removes the scale and suppresses the generation of red scale.
T1 '≧ 776 × Si (%) − 998 × P (%) + 980 ・ ・ ・ ・ （3）

Steel plate surface temperature between the second row and third row stands of the finish rolling mill: T2 (° C) or less satisfying the formula (2) If the tertiary scale generated after descaling does not peel during finish rolling, Occurrence can be suppressed. And in order to prevent the peeling of the scale during the finish rolling, the steel sheet surface temperature between the second row and the third row stand of the finish rolling mill should be lower than T2 (° C.) in the formula (2). It is effective for improvement.

Here, the reason for regulating the steel sheet surface temperature between the second row and third row stands of the finish rolling mill is that the rolling speed is high after the fourth stand, the amount of scale generated between the rolling mill stands is reduced, and the scale thickness This is because the scale becomes thin, so even if the scale is pushed into the railway, it does not become a scale ridge.
T2 ≦ 599 × Si (%) − 770 × P (%) + 976 (2)

Furthermore, by reducing the surface temperature of the steel plate between the second row and third row stands at a finish rolling temperature T2 ′ (° C.) or less that satisfies the expression (4), scale wrinkles can be reduced.
T2 ′ ≦ 599 × Si (%) − 770 × P (%) + 915 (4)
In addition, (2) Formula, (3) Formula, and (4) Formula are calculated | required by the below-mentioned Example similarly to (1) Formula.

Steel slabs having the components shown in Table 3 were prepared and hot-rolled under the conditions shown in Tables 4 to 6, respectively, to evaluate the presence or absence of red scale and the occurrence of scale wrinkles.
Details are as follows.

Each slab was heated in a heating furnace and unloaded at the extraction temperature shown in Table 4.
The primary scale is removed with a high-pressure water descaling device before the roughing mill, and after the rough rolling is finished at about 1120 ° C., the secondary scale is removed with a high-pressure water descaling device before the finishing mill, and then finish rolling. Rolling was performed with a machine.
At that time, the descaling temperature on the entry side of the finishing mill is 940 ° C. to 1105 ° C., the descaling collision pressure is 9.8 to 15.7 MPa, and the surface temperature of the steel plate between the second and third row stands of the finishing mill is 920 C. to 1045.degree. C., and the exiting temperature of the finishing mill was 850.degree. C. to 970.degree.
Then, it water-cooled on the hot run table, and manufactured the hot-rolled coil with a plate | board thickness of 2.0 mmt at the coiling temperature of 500 to 650 degreeC.

No. Each specific condition of 1-27 is as having described in Table 4.
The collision pressure of the high pressure water descaling device is shown in No. 4 of Table 4. Nos. 1, 4, 7, and 8 were 9.8 MPa, No. 2,3,5,6,9 was 15.7 MPa. Also, the descaling temperature on the finish rolling entry side, the steel plate temperature between F2 and F3, and the finish delivery side temperature are the positions of 9, 9, 10 in FIG. Measured using a meter.

After pickling the obtained hot-rolled coil, the presence or absence of red scale on the coil surface and biting flaws or rough surface (hereinafter abbreviated as scale wrinkles) was visually determined.
The pickling conditions are a liquid temperature of 80 ° C., a hydrochloric acid concentration of 6.4%, and an immersion time of 20 seconds.
In Table 4, No. About each of 1-27, a visual determination result is shown about the hot rolling conditions, the presence or absence of generation | occurrence | production of a red scale, and the presence or absence of generation | occurrence | production of a scale flaw.
No. 1, 4, 7, 10, 13, 16, 19, 22, and 25, the finishing entry side descaling temperature is higher than T1 (° C.) determined by the equation (1) corresponding to each test steel. Scaling easily removes the scale.
In finish rolling, the surface temperature of the steel plate between the second row and third row stands of the finish rolling mill is lower than T2 (° C.) determined by equation (2), and the scale is difficult to peel off. Does not occur.

On the other hand, no. 3, 6, 9, 12, 15, 18, 21, 24, and 27 have a finishing entry-side descaling temperature lower than T1 (° C.), and the scale is not removed by descaling on the finish rolling entry side.
In this case, where the scale could not be removed by descaling, the remaining scale was pushed into the steel sheet surface by finish rolling, and a red scale was generated (by finish rolling).
No. 2, 5, 8, 11, 14, 17, 20, 23, and 26, the finish entry side descaling temperature is higher than T1 (° C.) determined by the equation (1) corresponding to each test steel. The secondary scale was removed by high pressure water descaling and removed. However, the surface temperature of the steel plate between the second row and third row stands of the finish rolling mill is higher than the formula (2), and the tertiary scale is easily peeled off.

Table 5 shows the visual determination result of red scale wrinkles when the high pressure water descaling pressure on the finishing rolling mill entry side is set to 15.7 MPa and 24.5 MPa.
Here, no. In 28 and 29, the extraction temperature was 1230 ° C., the finish outlet temperature was 885 ° C., and the winding temperature was 550 ° C.
No. 30 and 31, respectively, 1230 ° C., 880 ° C., 550 ° C., No. For 32 and 33, they were 1240 ° C, 880 ° C and 600 ° C.

No. Nos. 28, 30, and 32 have a high pressure water descaling collision pressure of 15.7 MPa, but the final entry side descaling temperature is lower than T1 (° C.) determined by the equation (1), so that the secondary scale remains and red. Scale generation was observed.
On the other hand, no. Nos. 29, 31, and 33 have a finishing entry side descaling temperature lower than the temperature T1 determined by the equation (1), but higher than T1 ′ determined by the equation (3), and the high pressure water descaling impact pressure is 24. Since it is 5 MPa, the scale is completely removed and no red scale wrinkles occur.

Table 6 shows the visual judgment result of scale wrinkles when the steel plate temperature between the second row and third row stands of the finish rolling mill is set according to the equation (4).
Here, the test steel is No. In the case of No. 6 (No. 34 and No. 35), the extraction temperature is 1230 ° C. 2 (No. 36 to 38), the extraction temperature is 1250 ° C. In the case of No. 5 (No. 39 to 41), No. In the case of No. 4 (No. 42 and 43), No. 1240 ° C. In the case of No. 8 (No. 44 and 45), it was set to 1220 ° C.
Moreover, the collision pressure of the high pressure water descaling on the finishing rolling mill entrance side is 15.7 MPa in all cases.

  No. In 34, 36, 39, 42, and 44, the surface temperature of the steel plate between the second row and third row stands is higher than the formula (2), and therefore, the tertiary scale is easily peeled off and scale wrinkles are likely to occur. By setting the steel sheet surface temperature between the second row and third row stands based on the equation (4) instead of the equation (2), No. As in 35, 37, 38, 40, 41, 43, and 45, the generation of scale wrinkles is further reduced.

Claims (4)

In mass%, C: 0.001 to 0.30%, Si: 0.10% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.02% or less, acid-soluble Al : When the steel containing 0.005 to 0.10% and having the composition of the balance Fe and unavoidable impurities is continuously cast and then hot-rolled, high-pressure water The steel plate temperature at the time of scaling is set to T1 (° C.) or higher expressed by the following formula (1), the collision pressure of high pressure water is set to 15.7 MPa or lower, and between the second row and third row stands of the hot finish rolling mill. A method for producing a hot-rolled steel sheet having excellent surface properties, wherein the steel sheet surface temperature is set to T2 (° C) or less represented by the following formula (2).
T1 ≧ 998 × Si (%) − 1283 × P (%) + 1010 (1)
T2 ≦ 599 × Si (%) − 770 × P (%) + 976 (2)
However,
Si (%): Si content (mass%) contained in hot-rolled steel sheet
P (%): P content (mass%) contained in hot-rolled steel sheet
T1: Descaling temperature (° C) on the finishing mill entry side
T2: Steel plate surface temperature (° C) between the second row and third row stands of the finish rolling mill In mass%, C: 0.001 to 0.30%, Si: 0.10% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.02% or less, acid-soluble Al : When the steel containing 0.005 to 0.10% and having the composition of the balance Fe and unavoidable impurities is continuously cast and then hot-rolled, high-pressure water The steel plate temperature at the time of scaling is T1 ′ (° C.) or higher expressed by the following equation (3), the collision pressure of high pressure water is 24.5 MPaMPa or less, and between the second row and third row stands of the hot finish rolling mill. The manufacturing method of the hot-rolled steel plate excellent in the surface property characterized by making the steel plate surface temperature into T2 (degreeC) or less shown by following (2) Formula.
T1 '≧ 776 × Si (%) − 998 × P (%) + 980 ・ ・ ・ ・ （3）
T2 ≦ 599 × Si (%) − 770 × P (%) + 976 (2)
However,
Si (%): Si content (mass%) contained in hot-rolled steel sheet
P (%): P content (mass%) contained in hot-rolled steel sheet
T1 ′: Descaling temperature (° C.) on the finishing mill entry side
T2: Steel plate surface temperature (° C) between the second row and third row stands of the finish rolling mill The hot rolling excellent in surface properties according to claim 1 or 2, wherein the surface temperature of the steel plate between the second row and third row stands of the hot finish rolling mill is T2 '(° C) or less represented by the following formula (4). A method of manufacturing a steel sheet.
T2 ′ ≦ 599 × Si (%) − 770 × P (%) + 915 (4)
However,
T2 ′: Steel plate surface temperature (° C.) between the second row and third row stands of the finish rolling mill   Steel composition is further Cu: 0.01 to 0.30%, Ni: 0.01 to 0.20%, Ti: 0.01 to 0.20%, Nb: 0.01 to 0.20%, B The method for producing a hot-rolled steel sheet having excellent surface properties according to any one of claims 1 to 3, wherein the method comprises one or more of 0.0005 to 0.005%.

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