JPH0987799A - Thick steel plate excellent in scale peeling resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thick steel plate, excellent in scale peeling resistance, and to provide a method for producing it. SOLUTION: This thick steel plate has a composition containing 0.03-0.25% C, 0.05-<0.50% Si, 0.05-1.60% Mn, and 0.005-0.10% Al, and moreover, the color tone of scale at the surface of the steel plate is <=0.5 by a*-value. Further, at least one element, selected from a second element group consisting of Nb, V, Ti, Cu, Ni, Cr, Mo, and B and a third element group consisting of Ca and REM, can be incorporated into the above steel. This steel plate can be produced by subjecting a steel, where the above components are added, to heating at 1050-1300 deg.C and then to rolling under specific conditions where temp. and draft are respectively controlled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thick steel plate having excellent scale peeling resistance and a method for manufacturing the same.

[0002]

2. Description of the Related Art In the fields of construction machinery, concrete formwork, shipbuilding, and bridges, major issues for the industry as a whole are reduction of manufacturing costs (automation of manufacturing lines, thorough efficiency of processing work), improvement of product appearance, workplace There are three measures to improve the sanitary condition of the country (especially measures against this due to the seriousness of the young manufacturing industry leaving the manufacturing industry). A large amount of red fine scale called red scale adheres to the surface of thick steel plates (mainly plate thickness 6 to 25 mm) used in large quantities in these fields. This red scale easily peels off from the steel sheet during the processing of the steel sheet and rises up as fine powder, which reduces the work efficiency due to the disappearance of the scoring lines drawn on the steel sheet surface, hinders the automation of the production line, and impairs occupational health. This is a big problem because it causes deterioration of the appearance of the product when it is used as a countermeasure for concrete formwork. Therefore, in the above-mentioned field, there is a strong demand for a thick steel sheet which has almost no red scale that easily peels off on the surface of the steel sheet and has excellent scale peeling resistance.

Further, laser cutting is rapidly prevailing in cutting thick steel plates in the above-mentioned fields centering on construction machinery, but red scale thick steel plates cannot be stably laser cut. This is because the red scale thick steel plate has a large number of parts where the scale has peeled off when viewed microscopically,
This is because self-burning occurs during laser cutting,
A stable laser cutting process can be expected if it is a thick steel plate having a uniform scale even in a microscopic scale and excellent in scale peeling resistance.

However, in the manufacturing process of the thick steel plate, the residence time at high temperature during rolling is long, and the scale and base iron are easily oxidized during cooling because they are cooled without rolling after rolling. Characteristically, the scale on the steel surface is very likely to become red scale, and a method capable of inexpensively and stably producing a thick steel sheet excellent in scale peeling resistance has not been established.

As a conventional main measure against the red scale, improvement of the capacity of the descaling device, such as increasing the pressure of the descaling water during rolling, is being studied. As a conventional technique other than improving the capacity of a descaling device, Japanese Patent Laid-Open No. 5-3
Japanese Patent No. 9523 discloses a technique for suppressing the generation of red scale by mainly controlling the amount of Si added and the rolling temperature, but in addition, it is considered that a thick steel sheet excellent in scale peeling resistance can be manufactured conventionally. No technology is found.

A large amount of capital investment is required to improve and newly install the descaling device, resulting in an increase in manufacturing cost. As a measure other than improving the capacity of the descaling device,
The technique disclosed in Japanese Patent Laid-Open No. 5-39523 is 0.
Since it is necessary to add Si in an amount of 5% or more, it is not possible to meet the demand for an inexpensive thick steel sheet having excellent scale peeling resistance.

[0007]

The problem to be solved by the present invention is that it can greatly contribute to the reduction of manufacturing cost, which is a major problem in the fields of construction machinery, concrete formwork, shipbuilding, and bridges. Steel plate with excellent properties,
Another object of the present invention is to provide a method for inexpensively and stably manufacturing the steel sheet.

[0008]

The present invention has been made to solve the above problems, and its gist is as follows. (1) C: 0.03 to 0.25%, S in weight%
i: 0.05 to less than 0.50%, Mn: 0.05 to 1.
60%, Al: 0.005 to 0.10%, the balance consisting of Fe and inevitable impurities, and the color tone of the scale on the surface of the steel sheet is 0.5 or less in a ^* value, which is excellent in scale peeling resistance. Thick steel plate.

(2) In weight%, (a) C: 0.03
~ 0.25%, Si: 0.05-less than 0.50%, M
n: 0.05-1.60%, Al: 0.005-0.1
0%, (b) Nb: 0.001 to 0.20%, V: 0.
001 to 0.30%, Ti: 0.001 to 0.20%,
Cu: 0.05 to 1.50%, Ni: 0.05 to 1.5
0%, Cr: 0.05 to 1.00%, Mo: 0.05 to
1.00%, B: 0.0003 to 0.003%, containing at least one element selected from the group consisting of, the balance consisting of Fe and inevitable impurities, and the scale of the steel sheet surface having a color tone of a. ^{* A} thick steel plate with a value of 0.5 or less and excellent scale peeling resistance.

(3) In% by weight, (a) C: 0.03
~ 0.25%, Si: 0.05-less than 0.50%, M
n: 0.05-1.60%, Al: 0.005-0.1
0%, (b) Nb: 0.001 to 0.20%, V: 0.
001 to 0.30%, Ti: 0.001 to 0.20%,
Cu: 0.05 to 1.50%, Ni: 0.05 to 1.5
0%, Cr: 0.05 to 1.00%, Mo: 0.05 to
At least one element selected from the group consisting of 1.00%, B: 0.0003 to 0.003%, and (c) C.
a: 0.0003 to 0.010%, REM: 0.001
~ 0.030%, at least 1 selected from the group consisting of
A thick steel sheet which contains various elements, the balance consisting of Fe and unavoidable impurities, and has a scale color tone on the steel sheet surface of 0.5 or less in terms of a ^* value, which is excellent in scale peeling resistance.

(4) C: 0.03 to 0.
25%, Si: 0.05 to less than 0.50%, Mn: 0.
05 to 1.60%, Al: 0.005 to 0.10%, the balance consisting of Fe and unavoidable impurities, and a heating temperature of 10
When the rolling biting temperature of the final pass of rolling is _TF (° C.) and the average rolling reduction of the final three passes of finishing rolling is R _F (%), 880 ≦ T _F ≦ 980 ( ℃) 21 ≤ R _F ≤ 40 (%) R _F ≥ 155-3T _F / 20 (%) Rolling under the conditions that satisfy the three formulas at the same time, and air-cooling after rolling to obtain a scale with a thickness of 20 to 60 µm. A method for producing a thick steel sheet having excellent scale peeling resistance, comprising:

(5) In% by weight, (a) C: 0.03
~ 0.25%, Si: 0.05-less than 0.50%, M
n: 0.05-1.60%, Al: 0.005-0.1
0%, and (b) Nb: 0.001 to 0.
20%, V: 0.001 to 0.30%, Ti: 0.00
1 to 0.20%, Cu: 0.05 to 1.50%, Ni:
0.05 to 1.50%, Cr: 0.05 to 1.00%,
Mo: 0.05-1.00%, B: 0.0003-0.
003%, containing at least one element selected from the group consisting of, the balance consisting of Fe and unavoidable impurities,
When the heating temperature is 1050 to 1300 ° C., the roll biting temperature in the final pass of rolling is T _F (° C.), and the average rolling reduction of the final three passes in finish rolling is R _F (%), 880 ≦ T _F ≦ 980 (° C.) 21 ≦ R _F ≦ 40 (%) R _F ≧ 155-3T _F / 20 (%) Rolling is carried out under the condition of simultaneously satisfying the three formulas, and after air-cooling after rolling, 20-60 μm A method for producing a thick steel sheet having excellent scale peeling resistance, characterized by having a thickness scale.

(6) In weight%, (a) C: 0.03
~ 0.25%, Si: 0.05-less than 0.50%, M
n: 0.05-1.60%, Al: 0.005-0.1
0%, and (b) Nb: 0.001 to 0.
20%, V: 0.001 to 0.30%, Ti: 0.00
1 to 0.20%, Cu: 0.05 to 1.50%, Ni:
0.05 to 1.50%, Cr: 0.05 to 1.00%,
Mo: 0.05-1.00%, B: 0.0003-0.
003%, at least one element selected from the group consisting of (c) Ca: 0.0003 to 0.010%, RE
M: 0.001 to 0.030%, containing at least one element selected from the group consisting of, the balance consisting of Fe and unavoidable impurities, and heating temperature of 1050 to 1300
℃ and the roll entrapment temperature of the final pass of rolling is T
_F (° C), the average rolling reduction of the final 3 passes in finish rolling is R _F
(%), 880 ≤ T _F ≤ 980 (° C) 21 ≤ R _F ≤ 40 (%) R _F ≥ 155-3T _F / 20 (%) rolling under the conditions that simultaneously satisfy the three formulas, A method for producing a thick steel sheet having excellent scale peeling resistance, which comprises a scale having a thickness of 20 to 60 μm by rolling and then air cooling.

(7) C: 0.03 to 0.
25%, Si: 0.05 to less than 0.50%, Mn: 0.
05 to 1.60%, Al: 0.005 to 0.10%, the balance consisting of Fe and unavoidable impurities, and a heating temperature of 10
When the rolling biting temperature of the final pass of rolling is _TF (° C.) and the average rolling reduction of the final three passes of finish rolling is R _F (%), 880 ≦ T _F ≦ 1020 ( C.) 21 ≦ R _F ≦ 40 (%) R _F ≧ 155-3T _F / 20 (%) 3 conditions are simultaneously rolled, and after rolling, 3
By starting accelerated cooling at a cooling rate of 5 ° C / sec or more within 0 seconds, stopping accelerated cooling in a temperature range of the steel sheet surface temperature of 500 ° C or more and 750 ° C or less, and then performing air cooling, 3 to A method for producing a thick steel sheet having excellent scale peeling resistance, which has a scale having a thickness of 20 μm.

(8) In% by weight, (a) C: 0.03
~ 0.25%, Si: 0.05-less than 0.50%, M
n: 0.05-1.60%, Al: 0.005-0.1
0%, and (b) Nb: 0.001 to 0.
20%, V: 0.001 to 0.30%, Ti: 0.00
1 to 0.20%, Cu: 0.05 to 1.50%, Ni:
0.05 to 1.50%, Cr: 0.05 to 1.00%,
Mo: 0.05-1.00%, B: 0.0003-0.
003%, containing at least one element selected from the group consisting of, the balance consisting of Fe and unavoidable impurities,
When the heating temperature is 1050 to 1300 ° C., the roll biting temperature in the final pass of rolling is T _F (° C.), and the average rolling reduction of the final three passes in finish rolling is R _F (%), 880 ≦ T _F ≦ 1020 (° C.) 21 ≦ R _F ≦ 40 (%) R _F ≧ 155-3T _F / 20 (%) Rolling is performed under the condition that the three formulas are simultaneously satisfied, and after rolling, 3
By starting accelerated cooling at a cooling rate of 5 ° C / sec or more within 0 seconds, stopping accelerated cooling in a temperature range of the steel sheet surface temperature of 500 ° C or more and 750 ° C or less, and then performing air cooling, 3 to A method for producing a thick steel sheet having excellent scale peeling resistance, which has a scale having a thickness of 20 μm.

(9) In weight%, (a) C: 0.03
~ 0.25%, Si: 0.05-less than 0.50%, M
n: 0.05-1.60%, Al: 0.005-0.1
0%, and (b) Nb: 0.001 to 0.
20%, V: 0.001 to 0.30%, Ti: 0.00
1 to 0.20%, Cu: 0.05 to 1.50%, Ni:
0.05 to 1.50%, Cr: 0.05 to 1.00%,
Mo: 0.05-1.00%, B: 0.0003-0.
003%, at least one element selected from the group consisting of (c) Ca: 0.0003 to 0.010%, RE
M: 0.001 to 0.030%, containing at least one element selected from the group consisting of, the balance consisting of Fe and unavoidable impurities, and heating temperature of 1050 to 1300
℃ and the roll entrapment temperature of the final pass of rolling is T
_F (° C), the average rolling reduction of the final 3 passes in finish rolling is R _F
(%), 880 ≤ T _F ≤ 1020 (° C) 21 ≤ R _F ≤ 40 (%) R _F ≥ 155-3T _F / 20 (%) Rolling under the conditions that simultaneously satisfy the three formulas, After rolling 3
By starting accelerated cooling at a cooling rate of 5 ° C / sec or more within 0 seconds, stopping accelerated cooling in a temperature range of the steel sheet surface temperature of 500 ° C or more and 750 ° C or less, and then performing air cooling, 3 to A method for producing a thick steel sheet having excellent scale peeling resistance, which has a scale having a thickness of 20 μm.

The inventors of the present invention have conducted a number of experiments to solve the above problems. As a result, there is a strong correlation between the scale peeling resistance and the color tone of the scale on the surface of the steel sheet, and the scale peeling resistance of the thick steel sheet is confirmed. It was newly found that it is effective to make the color tone of the scale of the steel sheet surface to be 0.5 or less in terms of the color difference value (a ^* value) by a color difference meter in order to improve the property.

Further, as a method for producing such a thick steel sheet having excellent resistance to scale delamination, it is effective to increase the temperature during rolling and increase the rolling reduction.
According to this manufacturing method, it is possible to manufacture a thick steel plate that is stable, excellent in scale peeling resistance at low cost, without requiring a special device that causes an increase in manufacturing cost, and addition of a special alloy element. The present invention was made by newly discovering the above.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the components in the present invention will be described. C needs to be added in an amount of 0.03% or more to secure the strength, and a large amount of addition deteriorates the base metal toughness and hardens the weld heat affected zone to deteriorate the weldability, so the upper limit is set to 0.25%. To do. Since Si needs to be added in an amount of 0.05% or more for deoxidizing the steel, a large amount of addition deteriorates the descaling property during rolling, scale indentation flaws are likely to occur, and cost rises. The upper limit is less than 0.50%.

Mn needs to be added in an amount of 0.05% or more in order to fix S contained in steel and improve toughness, and addition of a large amount causes an increase in manufacturing cost and also deteriorates toughness. The upper limit is 1.60%. Al is required to be added in an amount of 0.005% or more for deoxidizing steel, and addition of a large amount significantly deteriorates toughness, so the upper limit is set to 0.10.
%.

In the present invention, in addition to the above-mentioned elements, the steel used is Nb:
0.001 to 0.20%, V: 0.001 to 0.30
%, Ti: 0.001 to 0.20%, Cu: 0.05 to
1.50%, Ni: 0.05 to 1.50%, Cr: 0.
05-1.00%, Mo: 0.05-1.00%, B:
At least one element selected from the group consisting of 0.0003 to 0.003% can be contained.

Nb, V, and Ti have the effect of precipitating in the steel as carbonitrides and increasing the strength, and by making the microstructure of the steel finer, the strength, the base metal toughness, and the toughness of the heat-affected zone of welding are both Has the effect of improving. For each element, 0.001% or more must be added to obtain these effects, and addition of a large amount significantly deteriorates the toughness of the weld heat affected zone.
The upper limits of Nb are 0.20%, V is 0.30%, and Ti is 0.20%.

Cu has the effect of improving strength and corrosion resistance. In order to obtain these effects, addition of 0.05% or more is necessary, and addition of a large amount causes the occurrence of hot cracking of the slab, so the upper limit is made 1.50%. Ni has the effect of improving both strength and toughness. To obtain this effect, it is necessary to add 0.05% or more, and addition of a large amount significantly impairs economic efficiency, so the upper limit is made 1.50%.

Cr has the effect of enhancing the hardenability, the effect of enhancing the resistance to temper softening, and the effect of improving the corrosion resistance. To obtain these effects, addition of 0.05% or more is necessary, and addition of a large amount deteriorates the toughness of the weld heat affected zone, so the upper limit is made 1.00%.

Mo has the effect of increasing the hardenability and the effect of increasing the temper softening resistance. In order to obtain these effects, addition of 0.05% or more is necessary, and addition of a large amount deteriorates the toughness of the weld heat affected zone, so the upper limit is made 1.00%.

B has the effect of significantly improving the hardenability and increasing the strength. 0.000 to obtain this effect
It is necessary to add 3% or more. If a large amount is added, the weld heat-affected zone is hardened and the weldability is significantly deteriorated, so 0.003% is made the upper limit.

Further, in the present invention, in the steel, in addition to the above elements or separately from the above elements, mainly for improving the toughness, Ca: 0.0003 to 0.010%, and R
EM: At least one element selected from the group consisting of 0.001 to 0.030% can be contained.

Ca and REM have the effects of improving the anisotropy of the mechanical properties of steel and improving the anti-lamella tear property. In order to obtain these effects, it is necessary to add Ca in an amount of 0.0003% or more and REM in an amount of 0.001% or more. Since the addition of a large amount of both elements deteriorates the toughness, the upper limit of Ca is 0. 010% and REM 0.030%.

FIG. 1 shows the influence of the color tone of the scale on the surface of the steel sheet on the scale peeling resistance of the thick steel sheet. The color tone of the scale on the surface of the steel sheet was evaluated using an average value of a ^* values (color display method conforms to JIS Z8729) at any 5 to 10 points on the surface of the steel sheet measured by a color difference meter according to JIS Z8722. did.

The scale peeling resistance is 5.
Affix the transparent adhesive tape to the rectangular area of 00.0x10.0cm and 50.0cm2, and apply 0.1kg from the top of the tape.
A pressure of f / cm2 was applied to bring the adhesive surface of the tape into good contact with the scale, and then the tape was peeled off. The area ratio of the peeled scale adhering to the tape was measured, and this area ratio was evaluated as the scale peeling rate. From FIG. 1, it can be seen that the scale peeling resistance is remarkably improved by setting the color tone of the scale on the surface of the steel sheet to 0.5 or less in terms of a ^* value.

Below, the color tone of the scale on the surface of the steel sheet is a
^A thick steel plate with an ^* value of 0.5 or less and excellent scale peeling resistance is expressed as "blue scale thick steel plate" based on its external appearance, and the color tone of the scale is 0.5 in terms of a ^* value. The following is referred to as "blue scale conversion".

Next, heating conditions and rolling conditions will be described. The most important point of the manufacturing method in the present invention is the control of rolling conditions, and heating at a temperature of 1050 ° C. or higher is necessary to satisfy this. The upper limit of the heating temperature is set to 1300 ° C. in order to suppress the generation of scale indentation. Regarding the slab in-furnace time at the time of heating, it takes about 120 minutes to uniformly bring the slab temperature to 1050 ° C. or higher, and the heating for a long time causes an increase in manufacturing cost, so it is set to about 300 minutes. Is desirable.

The rolling conditions in the case of manufacturing by air cooling after rolling will be described with reference to FIG. FIG. 2 is a diagram showing the influence of the rolling temperature and the rolling reduction on the scale state of the thick steel plate in the case of manufacturing after cooling by air cooling. As the rolling temperature, the roll biting temperature of the final pass of finish rolling (hereinafter, referred to as rolling finish temperature T _F ) is the most important factor, and as the rolling reduction, the average rolling reduction of the final three passes of finishing rolling is used. (Hereinafter, referred to as finish rolling reduction R _F ) is the most important factor.

For example, as finish rolling, 7-pass rolling is 20%, 25%, 30%, 30%, 30%,
When the rolling reductions of 35% and 35% are performed, the finish rolling reduction R _F is (30% + 35% + 35%) / 3 =
It becomes 33.3%. The four types of marks (○, △,
X, □) mean the scale states shown in Table 1, respectively,
The range surrounded by a straight line is the rolling condition range of the present invention.

[0035]

[Table 1]

The effect of the rolling finish temperature T _F on the scale state is large, and blue scale can be obtained by increasing the temperature. Conventionally, rolling is performed at a finish reduction R _F of about 20% or less, but in this case, the rolling finish temperature T _F is 930.
If it is not higher than ℃, blue scale thick steel plate cannot be obtained. If the rolling finish temperature T _F is higher than 980 ° C., minute blisters are generated on the scale during air cooling after rolling, and the surface appearance of the steel sheet deteriorates. Therefore, the upper limit of the rolling finish temperature T _F is 980 ° C.
And

The effect of the finish reduction rate R _F on the scale color tone is even greater, and by increasing the finish reduction rate R _F , a remarkable blue scale can be obtained. In order to produce a blue scale thick steel plate with a finish reduction R _F of about 20% or less, a rolling finish temperature T _F of 930 ° C. or higher is required, but among the thick steel plates, the plate thickness is about 6 to 25 mm. Since a steel plate having a relatively small plate thickness rapidly lowers the temperature of the steel plate surface during rolling, it was very difficult to stably set the rolling finishing temperature T _F to 930 ° C. or higher. However, by increasing the finishing reduction ratio R _F to about 30%, the rolling finish temperature range in which the blue scale thick steel sheet can be obtained can be significantly expanded to 880 ° C. or higher, and thus the stable production of the blue scale thick steel sheet can be achieved. It has become possible.

Furthermore, since the rolling finishing temperature T _F can be made lower, a desired thin scale thickness can be obtained,
The scale thickness can be set to 20 μm even in the case of manufacturing by air cooling after rolling. From the rolling ability,
The upper limit of the finish rolling reduction R _F is set to 40%.

Regarding the scale thickness of the thick steel plate in the case of manufacturing by air cooling after rolling, the lower limit is 20 μm, and the upper limit is 60 μm, which is the upper limit scale thickness without minute blistering. In addition, as shown by the circle mark in FIG.
880 ≦ T _F ≦ 980 (° C.), 21 ≦ R _F ≦ 40
(%) And R _F ≧ 160 −3 T _F / 20 (%) are the rolling condition ranges that satisfy the three conditions at the same time, which are more preferable condition ranges for stable production of blue scale thick steel sheet.

Next, within 30 seconds after completion of rolling, 5 ° C./sec
About the rolling condition and the accelerated cooling condition when starting the accelerated cooling at the above cooling rate, stopping the accelerated cooling in the temperature range of the steel sheet surface temperature of 500 ° C. or more and 750 ° C. or less, and manufacturing in the air cooling step Describe. When forced cooling is performed immediately after rolling, no blistering occurs on the scale even when the rolling finishing temperature is 1020 ° C. Since it is difficult to stably raise the rolling finish temperature to higher than 1020 ° C due to the temperature decrease of the steel sheet during rolling, the upper limit of the rolling finish temperature is set to 1020 ° C.

This accelerated cooling suppresses scale growth,
The purpose is to obtain a blue scale thick steel plate with a thin scale thickness. At the end of rolling, the temperature is 880 ° C or higher, and since the scale growth rate is high, accelerated cooling is started within 30 seconds after the end of rolling, and the steel plate surface temperature is rapidly reduced at a cooling rate of 5 ° C / sec or higher. Scale growth rate is slow enough 7
The accelerated cooling is stopped at a temperature of 50 ° C or lower.

If the cooling rate is less than 5 ° C./sec, the effect of suppressing scale growth is insufficient, so the lower limit of the cooling rate is set to 5 ° C./sec. If the stop temperature of the accelerated cooling is lower than 500 ° C, a good steel plate shape cannot be obtained, so the lower limit is set to 500 ° C. Specific methods of this accelerated cooling include water cooling and forced air cooling with a fan.

Next, the reason why it is important to control the rolling temperature and the rolling reduction in order to stably produce a steel material provided with a blue scale, and a specific method for providing scale peeling resistance will be described. . The reason why the control of the rolling temperature and the rolling reduction is important for stable production of blue scale steel will be described below. As a result of extensive studies on the cause of the generation of red scale, the red scale is fine powder hematite (Fe ₂ O ₃ ), and this fine powder hematite has a fineness of about 0.1 to 10 μm during rolling. It is clear that wustite (FeO) and magnetite (Fe ₃ O ₄ ) crushed to various sizes were oxidized into fine powder of hematite during cooling after rolling, and hematite crushed during rolling. Became.

Then, as a result of extensive research on the method of suppressing the crushing of the scale during the rolling, the present invention has been achieved, and the gist thereof is the following 6 points. In order to prevent the scale from being crushed during rolling, it is effective to set the rolling finishing temperature T _F to a high temperature. The reason for this is that if the temperature during rolling is sufficiently high,
This is because the scale has the same ductility as that of base steel, so that it can be plastically deformed before crushing.

However, in the conventional rolling with a rolling reduction of about 20% or less, the rolling finishing temperature T _F is 93
If it is not higher than 0 ° C, blue scale thick steel plate cannot be obtained.
The relatively thickness of thin steel plate of about a thickness 6～25Mm, because of making the stable 930 ° C. or higher finishing rolling temperature is very difficult, than the finish rolling temperature T _F only the hot blue It is difficult to manufacture scale thick steel plate stably.

Increasing the rolling reduction at the time of rolling is very effective for forming a blue scale. The mechanism is to increase the heat reduction of the steel during rolling by increasing the rolling reduction, suppress the temperature decrease of the scale on the surface of the steel sheet and maintain the ductility of the scale high, and make the scale thickness thinner. However, it is difficult to crush during rolling.

By increasing the finishing reduction ratio R _F as compared with the conventional one, the finishing temperature T _F required for making the blue scale is 88.
It is possible to lower the temperature to 0 ° C., which makes it possible to stably manufacture a blue scale thick steel plate having a plate thickness of 6 to 25 mm. Further, the blue scale thick steel plate produced by the above mechanism, the scale is uniformly stretched in the same manner as the base metal during rolling, and then grows uniformly during cooling after rolling, so that the scale thickness is uniform, Also, its surface roughness is small.

[0048]

[Examples] The influence of the steel plate surface scale color tone on the scale peeling resistance is shown in Table 3 using thick steel plates having a chemical composition of steel Nos. A to F shown in Table 2 and having a thickness of 6 to 25 mm. The scale peeling resistance and the scale color tone evaluation method on the surface of the steel sheet are the same as those described above with reference to FIG.

[0049]

[Table 2]

[0050]

[Table 3]

Steels Nos. 1 to 11 of the present invention are the steels of the present invention, and the scale of the scale on the surface of the steel sheet is 0.5 or less in a ^* value, so the scale peeling rate is less than 5%, which is good. Exhibits excellent scale peeling resistance. It is more preferable that the color tone of the scale on the surface of the steel sheet is 0 or less in terms of a ^* value, because the peeling rate of the scale can be less than 1%. On the other hand, sample steel No. 12
Up to 22 are comparative steels, and the scale of the steel plate surface has a color tone of more than 0.5, so the scale peeling rate is 5%.
As described above, the scale peeling resistance is remarkably deteriorated.

Next, using the test steels having the chemical compositions of steel Nos. A to F shown in Table 2, heating under the conditions shown in Table 4, air-cooling after rolling, the plate thickness of 6 to 25 mm was produced. Table 4 shows the scale properties of the steel sheet. Scale thickness is any 10 on the steel plate measured from the optical micrograph of the scale cross section.
The average value of the scale thickness at the location was used. As an index of the scale thickness uniformity, the minimum and maximum values of the measured scale thickness at 10 locations are also shown. The surface roughness is an average value of Ra values measured using a surface roughness meter according to JIS B 0601 over a length of 25 mm for the roughness of the scale surface at arbitrary 3 to 5 places.

[0053]

[Table 4]

[0054]

[Table 5]

Specimen Nos. 1 to 19 are steels manufactured according to the present invention, and the color difference value is 0.5 or less in a ^* value, and the scale thickness is 20 to 60 μm. Further, the scale thickness is almost uniform, and the surface roughness is 0.5 at Ra value.
It is a blue scale thick steel plate having a uniform thickness and a small surface roughness of about 1.5 μm. On the other hand, sample steel No. 20-30
Is, the rolling conditions deviate from the condition range of the present invention,
It is a red scale thick steel plate with a value of 0.5 <a ^* . Further, in the case of the test steel Nos. 31 to 32, the rolling conditions deviate from the condition range of the present invention, and the rolling finishing temperature is 980 <T _F. Therefore, minute blisters occur on the scale and the steel plate surface appearance deteriorates. It was done.

The effects of rolling conditions and accelerated cooling after rolling on the scale properties are shown in Table 5 by using thick steel plates having the chemical compositions of steel Nos. A to F shown in Table 2 and having a thickness of 6 to 25 mm. A thick steel plate having a plate thickness of 6 to 25 mm was manufactured by heating and rolling under the conditions shown in Table 5 and then water cooling at a cooling rate of 5 to 30 ° C./sec. Table 5 shows the scale properties of the thick steel plate. The water cooling start time is the time from immediately after the end of rolling until the water cooling is started, and the water cooling stop temperature is the steel plate surface temperature when the water cooling is stopped and the air cooling is started. The scale color tone, scale thickness, and surface roughness were evaluated by the methods described above in Table 4.

[0057]

[Table 6]

[0058]

[Table 7]

Specimen Nos. 1 to 20 are thick steel plates of the present invention, the color difference value is 0.5 or less in a ^* value, and the scale thickness is 3 to 20 μm. Further, the scale thickness is substantially uniform, and the surface roughness is a blue scale thick steel plate with a small surface roughness having a Ra value of 0.5 to 1.5 μm and a uniform thickness. Regarding the test steel Nos. 21 to 31, the rolling conditions deviate from the condition range of the present invention, and the red scale thick steel plate with 0.5 <a ^* value is obtained. Regarding the accelerated cooling conditions, the cooling rate can be increased to about 20 to 30 ° C./sec, the cooling start time can be set to 5 seconds or less, and the cooling stop temperature can be set to 700 ° C. or lower to reduce the scale thickness. It is possible.

[0060]

According to the present invention, it is possible to greatly contribute to reduction of manufacturing cost, improvement of product appearance, and improvement of hygiene at work, which are major problems in the fields of construction machinery, concrete formwork, shipbuilding, and bridges. The thick steel plate having excellent scale peeling resistance can be stably supplied at low cost, which is extremely useful in industry.

[Brief description of drawings]

FIG. 1 is a chart showing the influence of the color tone of the scale on the surface of a steel sheet on the scale peeling resistance of a thick steel sheet.

FIG. 2 is a chart showing the influence of rolling temperature and reduction rate on the scale state of thick steel plates in the case of manufacturing after cooling by air cooling.

Claims (9)

[Claims] 1. By weight%, C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10. % A thick steel plate having a balance of Fe and inevitable impurities, and having a scale color tone on the surface of the steel plate of 0.5 or less in a ^* value, which is excellent in scale peeling resistance. 2. In weight%, (a) C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10% (b) Nb: 0.001 to 0.20% V: 0.001 to 0.30% Ti: 0.001 to 0.20% Cu: 0.05 to 1.50% Ni: 0 0.05 to 1.50% Cr: 0.05 to 1.00% Mo: 0.05 to 1.00% B: 0.0003 to 0.003% at least one element selected from the group consisting of However, the balance consists of Fe and unavoidable impurities, and the color tone of the scale on the steel plate surface is 0.5 or less in a ^* value,
Thick steel plate with excellent scale peeling resistance. 3. In weight%, (a) C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10% (b) Nb: 0.001 to 0.20% V: 0.001 to 0.30% Ti: 0.001 to 0.20% Cu: 0.05 to 1.50% Ni: 0 0.05 to 1.50% Cr: 0.05 to 1.00% Mo: 0.05 to 1.00% B: 0.0003 to 0.003% and at least one element selected from the group consisting of: (C) Ca: 0.0003 to 0.010% REM: 0.001 to 0.030% and at least one element selected from the group consisting of, and the balance consisting of Fe and inevitable impurities, and The color tone of the scale on the surface of the steel sheet is 0.5 or less in a ^* value,
Thick steel plate with excellent scale peeling resistance. 4. In% by weight, C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10. % The balance consists of Fe and inevitable impurities, and the heating temperature is 1
When the rolling biting temperature of the final pass of rolling is _TF (° C.) and the average rolling reduction of the final three passes in finishing rolling is R _F (%), 880 ≦ T _F ≦ 980 ( ℃) 21 ≤ R _F ≤ 40 (%) R _F ≥ 155-3T _F / 20 (%) Rolling under the conditions that satisfy the three formulas at the same time, and air-cooling after rolling to obtain a scale with a thickness of 20 to 60 µm. A method for producing a thick steel sheet having excellent scale peeling resistance, comprising: 5. In weight%, (a) C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10%, and (b) Nb: 0.001 to 0.20% V: 0.001 to 0.30% Ti: 0.001 to 0.20% Cu: 0.05 to 1 .50% Ni: 0.05 to 1.50% Cr: 0.05 to 1.00% Mo: 0.05 to 1.00% B: 0.0003 to 0.003% at least selected from the group consisting of Containing one element, the balance consisting of Fe and unavoidable impurities, heating temperature of 1050 to 1300 ° C, roll biting temperature of the final pass of rolling T _F (° C), final 3 passes of finish rolling the average reduction rate of when the _{R F (%), 880 ≦} T F ≦ 980 (℃) 21 ≦ R F ≦ 4 (%) Was rolled at _{R F ≧ 155-3T F / 20 (} %) of three equations simultaneously satisfy conditions, by air cooling after rolling, characterized by having a scale thickness of 20 to 60 [mu] m, A method for manufacturing a thick steel sheet having excellent scale peeling resistance. 6. In weight%, (a) C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10%, and (b) Nb: 0.001 to 0.20% V: 0.001 to 0.30% Ti: 0.001 to 0.20% Cu: 0.05 to 1 .50% Ni: 0.05 to 1.50% Cr: 0.05 to 1.00% Mo: 0.05 to 1.00% B: 0.0003 to 0.003% at least selected from the group consisting of One element and (c) Ca: 0.0003 to 0.010% and at least one element selected from the group consisting of REM: 0.001 to 0.030%, the balance being Fe and unavoidable. Made of mechanical impurities, the heating temperature is set to 1050 to 1300 ℃, and the roll is caught in the final pass of rolling. Assuming that the temperature is T _F (° C.) and the average rolling reduction of the final three passes in finish rolling is R _F (%), 880 ≦ T _F ≦ 980 (° C.) 21 ≦ R _F ≦ 40 (%) R _F ≧ 155-3T _F / 20 (%), which is characterized by having a scale of 20 to 60 μm by rolling under the conditions that simultaneously satisfy the three formulas, and then air-cooling after rolling. An excellent method for manufacturing thick steel plates. 7. In wt%, C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10. % The balance consists of Fe and inevitable impurities, and the heating temperature is 1
When the rolling biting temperature of the final pass of rolling is _TF (° C.) and the average rolling reduction of the final three passes in finish rolling is R _F (%), 880 ≦ T _F ≦ 1020 ( C.) 21 ≦ R _F ≦ 40 (%) R _F ≧ 155-3T _F / 20 (%) 3 conditions are simultaneously rolled, and after rolling, 3
By starting accelerated cooling at a cooling rate of 5 ° C / sec or more within 0 seconds, stopping accelerated cooling in a temperature range of the steel sheet surface temperature of 500 ° C or more and 750 ° C or less, and then performing air cooling, 3 to A method for producing a thick steel sheet having excellent scale peeling resistance, which has a scale having a thickness of 20 μm. 8. In weight%, (a) C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10%, and (b) Nb: 0.001 to 0.20% V: 0.001 to 0.30% Ti: 0.001 to 0.20% Cu: 0.05 to 1 .50% Ni: 0.05 to 1.50% Cr: 0.05 to 1.00% Mo: 0.05 to 1.00% B: 0.0003 to 0.003% at least selected from the group consisting of Containing one element, the balance consisting of Fe and unavoidable impurities, heating temperature of 1050 to 1300 ° C, roll biting temperature of the final pass of rolling T _F (° C), final 3 passes of finish rolling the average reduction rate of when the _{R F (%), 880 ≦} T F ≦ 1020 (℃) 21 ≦ R F ≦ _{0 (%) R F ≧ 155-3T} F / 20 was rolled at the same time satisfying the condition 3 Expressions (%), the finish rolling after 3
By starting accelerated cooling at a cooling rate of 5 ° C / sec or more within 0 seconds, stopping accelerated cooling in a temperature range of the steel sheet surface temperature of 500 ° C or more and 750 ° C or less, and then performing air cooling, 3 to A method for producing a thick steel sheet having excellent scale peeling resistance, which has a scale having a thickness of 20 μm. 9. In weight%, (a) C: 0.03 to 0.25% Si: 0.05 to less than 0.50% Mn: 0.05 to 1.60% Al: 0.005 to 0.10%, and (b) Nb: 0.001 to 0.20% V: 0.001 to 0.30% Ti: 0.001 to 0.20% Cu: 0.05 to 1 .50% Ni: 0.05 to 1.50% Cr: 0.05 to 1.00% Mo: 0.05 to 1.00% B: 0.0003 to 0.003% at least selected from the group consisting of One element and (c) Ca: 0.0003 to 0.010% and at least one element selected from the group consisting of REM: 0.001 to 0.030%, the balance being Fe and unavoidable. Made of mechanical impurities, the heating temperature is set to 1050 to 1300 ℃, and the roll is caught in the final pass of rolling. Assuming that the temperature is T _F (° C.) and the average rolling reduction of the final three passes in finish rolling is R _F (%), 880 ≦ T _F ≦ 1020 (° C.) 21 ≦ R _F ≦ 40 (%) R _F ≧ 155-3T _F / 20 (%) 3 formulas are simultaneously rolled under the conditions that are satisfied, and after the completion of rolling, 3
By starting accelerated cooling at a cooling rate of 5 ° C / sec or more within 0 seconds, stopping accelerated cooling in a temperature range of the steel sheet surface temperature of 500 ° C or more and 750 ° C or less, and then performing air cooling, 3 to A method for producing a thick steel sheet having excellent scale peeling resistance, which has a scale having a thickness of 20 μm.