JPH07268537A - Steel plate for structure - Google Patents

Abstract

PURPOSE:To produce a steel plate for structures small in red scales and excellent in scale adhesion, in a steel contg. specified ratios of C, Si, Mn, Al and N, by prescribing the average grain size and the area ratio of Fe2O3 on the surface of the steel plate and the thickness of scales. CONSTITUTION:A steel plate contg., by weight, 0.04 to 0.20% C, 0.03 to 1.00% Si, 0.30 to 2.00% Mn, 0.005 to 0.10% Al and 0.001 to 0.01% N, and the balance Fe with inevitable impurities is prepd. This steel plate of a high temp. is subjected to reciprocating rolling by a rolling mill and is subjected to rolling in such a manner that descaling and cooling are suitably combined to regulate the ratio of the amt. of Fe2O3 having <=2mum average grain size on the surface of the steel plate to <20% by area and the thickness of scales to <=5.0mum. Thus, the steel plate for structures small in red scales and excellent in scale adhesion can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structural steel plate which can be manufactured economically and with high productivity.

[0002]

2. Description of the Related Art In recent years, the demand for thick steel plates in the environment of use has increased, but in the field of industrial machines where there are many paintability and aesthetics of steel plates, especially in the case of many forming operations, it is easy to peel off from the viewpoint of work environment and easily become powder. The demand for steel sheets with a small amount of red scale is increasing, and steel materials having good scale properties are desired. Further, as a characteristic of the scale of the steel sheet from the viewpoint of the use performance of the steel material, there is an increasing demand for prevention of coating unevenness and adhesion of the scale when the coating is applied as it is on a bent member.

Regarding the mechanism of discoloration of the oxide scale on the surface, materials and processes CAMP-ISIJ VOL. 6 (199
3) -357. That is, it has been clarified that the steel sheet looks reddish brown due to the presence of fine hematite (Fe ₂ O ₃ ) powder on the surface of the steel sheet, and the color is governed by the amount of hematite powder of 2 μm or less. However, although the size of the Fe ₂ O ₃ powder for appearing red is being clarified in principle, the relationship between the amount of hematite adhering to the surface of the actual steel sheet and the color is still unclear.

Further, as a method for improving the scale adhesion, in the field of hot-rolled wire rods, there has been proposed a method of reducing the scale thickness as described in, for example, "Iron and Steel" 65 (1979), S390. ing. In the field of hot-rolled steel strips, as an example of reducing the scale thickness, for example, as described in JP-A-58-157517, a method of covering a finishing rolling mill and a water-cooling device with a laminar water-cooling to shut off from the atmosphere, As described in JP-A-60-24320 and JP-A-60-77922, a method of cooling a low carbon aluminum killed steel after completion of rolling to a low temperature in a non-acid atmosphere, and JP-A-61-123403. As described above, a method of cooling to a low temperature in an inert gas or reducing gas atmosphere immediately after finish rolling, or JP-A-61-1957.
As described in Japanese Unexamined Patent Publication No. 02-202, a method of cooling a low carbon aluminum killed steel containing Cr immediately after rolling is proposed.

However, this method requires equipment for shutting off steel strips or wires that pass at high speed from the atmosphere, or equipment for rapidly cooling these steel strips to a low temperature immediately after rolling. Therefore, there is a drawback that a great equipment cost is brought about. The hot-rolled steel strip manufactured by the above continuous rolling process has a significantly shorter high-temperature residence time than a thick steel sheet, and is therefore extremely advantageous in thinning the scale.

However, in the production of thick steel plates, a slab is usually charged into a heating furnace, heated in a temperature range of 1200 to 1250 ° C., and then sent to a descaling device to remove scales generated during heating, and then Width-rolling rolling or finish rolling for rolling to a predetermined product thickness is performed by a base or two reversible rolling mills. Therefore, the thick steel plate after finish rolling is sent to a hot leveler, and after hot straightening, air-cooled or controlled-cooled so that the surface of the steel plate becomes a predetermined temperature or lower.

In such a conventional method, the slab rolled by the reversible rolling mill is at a high temperature and the rolling time is relatively long, so that a thick scale is unevenly generated at the end of finish rolling. When the next hot leveler passes through the plate or becomes a product, the scale peels off during press forming, resulting in uneven coating, and the thick steel plate is formed into a bent member and painted as it is. In this case, the characteristics of the scale on the surface of the thick steel plate cause many problems in various aspects, such as uneven coating caused by unevenness of the scale. As described above, in the case of a thick steel plate, since the content heat capacity is large and the rolling time in reverse rolling is relatively long, little useful method is disclosed regarding the method of controlling the color and thickness of the scale of the thick steel plate. .

[0008]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned drawbacks of the conventional manufacturing method and is adaptable to various applications, has excellent surface properties, is excellent in productivity, and can be manufactured economically. It is an object to provide a thick steel plate.

[0009]

The gist of the present invention is as follows. (1) C: 0.04 to 0.20% by weight, Si:
0.03 to 1.00%, Mn: 0.30 to 2.00%,
Al: 0.005 to 0.10%, N: 0.001 to 0.
Steel containing 0.1% and the balance Fe and unavoidable components, and the amount of Fe ₂ O ₃ having an average grain size of 2 μm or less on the surface of the steel sheet is less than 20% in area ratio.

(2) C: 0.04 to 0.20 by weight%
%, Si: 0.03 to 1.00%, Mn: 0.30
2.00%, Al: 0.005 to 0.10%, N: 0.
001-0.01%, and Ti: 0.003-
0.10%, Cr: 0.01 to 0.50%, Ni: 0.0
1 to 3.00%, Mo: 0.01 to 0.50%, Cu:
0.01 to 1.50%, V: 0.005 to 0.20%,
Nb: 0.003 to 0.05%, B: 0.0003 to
A steel containing 0.0020% of one kind or two or more kinds and the balance being Fe and inevitable components, and the amount of Fe ₂ O ₃ having an average particle diameter of 2 μm or less on the surface of a steel sheet is less than 20% in terms of area ratio.

(3) C: 0.04 to 0.20 by weight%
%, Si: 0.03 to 1.00%, Mn: 0.30
2.00%, Al: 0.005 to 0.10%, N: 0.
Steel containing 001 to 0.01% and the balance being Fe and unavoidable components, and having an average grain size of 2 μm or less on the surface of the steel sheet.
The size of the colony containing e ₂ O ₃ is 500 circle equivalent diameter.
The area ratio of the colonies is less than 50% and less than 50%.

(4) C: 0.04 to 0.20 by weight%
%, Si: 0.03 to 1.00%, Mn: 0.30
2.00%, Al: 0.005 to 0.10%, N: 0.
001-0.01%, and Ti: 0.003-
0.10%, Cr: 0.01 to 0.50%, Ni: 0.0
1 to 3.00%, Mo: 0.01 to 0.50%, Cu:
0.01 to 1.50%, V: 0.005 to 0.20%,
Nb: 0.003 to 0.05%, B: 0.0003 to
It contains one or more 0.0020%, the dimension equivalent circle diameter of colonies balance exists an average particle size of 2μm or less of the amount of Fe ₂ O ₃ in the steel sheet surface with a steel consisting of Fe and unavoidable component And the area ratio of the colony is less than 50%.

(5) C: 0.04 to 0.20 by weight%
%, Si: 0.03 to 1.00%, Mn: 0.30
2.00%, Al: 0.005 to 0.10%, N: 0.
Steel containing 001 to 0.01% and the balance being Fe and unavoidable components, and having an average grain size of 2 μm or less on the surface of the steel sheet.
The amount of e ₂ O ₃ is less than 20% in area ratio, and the thickness of the scale is 5.0 μm or less.

(6) C: 0.04 to 0.20 by weight%
%, Si: 0.03 to 1.00%, Mn: 0.30
2.00%, Al: 0.005 to 0.10%, N: 0.
001-0.01%, and Ti: 0.003-
0.10%, Cr: 0.01 to 0.50%, Ni: 0.0
1 to 3.00%, Mo: 0.01 to 0.50%, Cu:
0.01 to 1.50%, V: 0.005 to 0.20%,
Nb: 0.003 to 0.05%, B: 0.0003 to
Steel containing 0.0020% of one kind or two kinds or more, the balance being Fe and unavoidable components, and having an area ratio of less than 20% of Fe ₂ O ₃ having an average particle diameter of 2 μm or less on the surface of the steel sheet, And the thickness of the scale is 5.0 μm or less.

(7) C: 0.04 to 0.20 by weight%
%, Si: 0.03 to 1.00%, Mn: 0.30
2.00%, Al: 0.005 to 0.10%, N: 0.
Steel containing 001 to 0.01% and the balance being Fe and unavoidable components, and having an average grain size of 2 μm or less on the surface of the steel sheet.
The size of the colony containing e ₂ O ₃ is 500 circle equivalent diameter.
The area ratio of the colonies is less than 50%, and the thickness of the scale is less than 5.0 μm.

(8) C: 0.04 to 0.20 by weight%
%, Si: 0.03 to 1.00%, Mn: 0.30
2.00%, Al: 0.005 to 0.10%, N: 0.
001-0.01%, and Ti: 0.003-
0.10%, Cr: 0.01 to 0.50%, Ni: 0.0
1 to 3.00%, Mo: 0.01 to 0.50%, Cu:
0.01 to 1.50%, V: 0.005 to 0.20%,
Nb: 0.003 to 0.05%, B: 0.0003 to
A steel containing 0.0020% of one or more kinds and the balance being Fe and unavoidable components and having Fe ₂ O ₃ having an average particle diameter of 2 μm or less on the surface of the steel sheet, the colony size is equivalent to a circle. The area ratio of the colonies is 500 μm or less, the area ratio of the colonies is less than 50%, and the thickness of the scale is 5.0 μm.
m or less.

Further, the rolled steel for structural use which is the subject of the present invention has been conventionally confirmed in the field of application in order to obtain the required material for ordinary welded structural steel, as described below. The same action and effect can be obtained by similarly using the types and amounts of the additional elements defined based on the action-effect relationship. Therefore, the steel containing these is the subject steel of the present invention. The reason and amount of addition of each of these component elements are shown below.

[0018] C is the 0.04% as an effective ingredient for improving the strength of the steel is intended to be added, the excess amount of more than 0.20%, HAZ (H eat A ffected Z on
Island-like martensite precipitates in e) and significantly deteriorates HAZ toughness, so the content is controlled to 0.20% or less. Si is
It is necessary as a deoxidizing element for molten steel and is added as a strength increasing element, but if it is less than 0.03%, the deoxidizing effect is insufficient, and if it exceeds 1.00%, the workability of the steel deteriorates. However, since the HAZ toughness decreases, the addition amount is 0.0
Restrict to 3 to 1.0%.

Mn is also necessary as a deoxidizing component element,
If it is less than 0.30%, the cleanliness of the steel is lowered and the workability is impaired. Further, it is necessary to add 0.30% or more as a component for improving the strength of the steel material. However, Mn significantly deteriorates the weldability due to excessive addition, so 2.00% is made the upper limit. Al is necessary because the grain size of steel made of Al nitride can be refined. However, when the addition amount is small, the effect is not exerted, and when it is excessive, the toughness of the steel is deteriorated, so the addition amount is restricted to 0.005 to 0.10%.
N combines with Al and Ti to effectively work for the refinement of austenite grains, but in order to clarify the effect, 0.0
It is necessary to contain O.sub.1% or more, but if it is added excessively in excess of 0.01%, solute N will increase and adversely affect toughness, so 0.01% is made the upper limit.

The basic components of the structural steel targeted by the present invention are as described above. Based on this, according to the properties required for the purpose of increasing the base metal strength or joint toughness, Ti,
One or two of Cr, Ni, Mo, Cu, V, Nb, B
More than one species can be included. First, Ti contributes to the improvement of the base metal strength by precipitation strengthening, and it is an extremely effective element for the joint toughness of the weld zone by refining the austenite grains due to the formation of TiN. % Or more must be added. On the other hand, 0.1
The upper limit is set to 0.10% in order to form Ti carbide exceeding 0% to deteriorate toughness and ductility. Cr and Mo are both effective elements for increasing the strength of the base material, but 0.01% or more is necessary for producing a clear effect, while if added over 0.50%, the toughness deteriorates. Therefore, the range is 0.01 to 0.50%.

Further, Ni is a very effective element because it can improve the strength and toughness of the base material at the same time, but it is necessary to contain Ni in an amount of 0.01% or more in order to exert the effect. Strength and toughness improve as the content increases, but 3.
If added in excess of 00%, the transformation behavior will change and the appropriate manufacturing conditions will change, so the upper limit is 3.00% in the range of the present invention. Next, Cu has almost the same effect as Ni, but if added in excess of 1.50%, a problem of precipitation hardening occurs, so the range is limited to 0.01 to 1.50%.

Both V and Nb mainly contribute to improving the strength of the base material by precipitation strengthening, but if added excessively, HAZ is added.
The toughness deteriorates. Therefore, V is 0.005 to 0.20 as a range in which the effect can be exhibited without deteriorating the toughness.
% And Nb are 0.003 to 0.05%. B is 0.0
Addition of an extremely small amount of 003% or more is very effective for increasing the hardenability of steel and increasing the strength, but if added in excess, the toughness is greatly deteriorated, so the upper limit is made 0.0020%.
In the present invention, the heating temperature of the slab has an upper limit of 1200 ° C. to prevent coarsening of austenite, and a lower limit temperature is preferably 900 ° C. or higher in consideration of rolling work. Also, Nb
The steel material containing the element is 110 for completely dissolving Nb.
Heating above 0 ° C is required.

[0023]

In order to solve the problems of the above-mentioned prior art and to achieve the object of the present invention, the present inventors have set C: 0.
05-0.15%, Si: 0.15-0.25%, M
n: 0.8 to 2.6%, Al: 0.01 to 0.05%,
N: Various experimental studies were repeated using a general structural steel having a chemical composition of 0.0020 to 0.0050%.

First, the color of the scale and the state of the surface were examined. The surfaces of the steel plates in which the color of the scale appeared red and the steel plate in which the color of the scale appeared black were observed with a scanning electron microscope. On the surface of the steel plate that looks red, there is a level difference that looks like a scale is sculpted, and powdered hematite (Fe ₂ O ₃ ) is present in the sculpted part. The surface is smooth. It was also found that the larger the area of the scale that was scooped and broken, the more the amount of powdered hematite increased, and as a result, the steel sheet appeared red.

It has been found that it is important to associate the results of quantitatively measuring and evaluating the size and area ratio of the powdery hematite with the color of the steel sheet.

Therefore, materials and processes CAMP-IS
IJ VOL. 6 (1993) -357, the results of evaluating the area ratio of the powdered hematite (Fe ₂ O ₃ ) of ₂ μm or less in the steel sheet and the color of the steel sheet are shown in FIG. 1. The area ratio shown in FIG. 1 was measured from a scanning electron microscope photograph, and the color of the steel sheet was visually judged. Figure 1
From the results, it was found that when the area ratio of the region where the powdery hematite of 2 μm or less remains is less than 20%, the color of the steel sheet surface cannot be identified as red.

Further, the relationship between the size and area ratio of the colonies in which the powdery hematite of 2 μm or less remains and the color of the steel sheet by visual judgment was investigated. As a result, it was found that the permissible range of the area ratio could be increased to 50% when the size of the colony having powdery hematite of 2 μm or less remaining was 500 μm or less in equivalent circle diameter. Therefore,
In order to prevent the red color from appearing, the size of the colonies in which the powdered hematite of 2 μm or less remains is 500 μm or less in equivalent circle diameter, and the area ratio is controlled to 50% or less.

As a condition for adjusting the amount of hematite on the scale surface as described above, the reason why the rolling pass condition of 950 ° C. or lower is defined is that the peeling effect due to descaling is remarkably lowered at 950 ° C. or lower, and the scale is rolled. When it is destroyed and carried out for 10 passes or more, the amount of hematite of 2 μm or less exceeds 20%, or the colonies with a circle equivalent diameter of 500 μm or less in which powdery hematite of 2 μm or less remains exceeds the area ratio of 50%. I understood it.

The sample for measuring the scale condition on the surface of the steel sheet was sampled from each position divided into five parts in the width direction at a pitch of 1000 mm in the longitudinal direction of the product steel sheet, and the average value of the measured values was obtained and evaluated. There is. Further, when the steel plate scale is peeled off by working, it may remain as powdery hematite having a size of 2 μm or less, and the part may become red. Therefore, when peeling is expected due to bending or the like, it is necessary to have scale adhesion at the same time.

Therefore, the limit scale thickness, which greatly affects the scale adhesion, was examined. In the experiment, various steel plates with different scale thicknesses were manufactured and evaluated by the method described below. That is, the scale adhesion is often lacking in quantitativeness because it is often subjected to a sensitivity test such as visual judgment. So this time,
After a 90-degree bending test with a bending radius r = 1.5 × t (t: plate thickness), 10 samples were taken from the surface and the scale was peeled off with a scanning electron microscope to show the area ratio (10 The average value of the individual pieces) was measured and evaluated by the exposed area ratio of the base iron shown in Table 1. It should be noted that the rating ranks 1 and 2 were passed, and the adhesiveness was good.

[0031]

[Table 1]

FIG. 2 shows the relationship between the scale thickness and the rating rank. From FIG. 2, it was found that the adhesiveness was good when the scale thickness was 5.0 μm or less. Next, rolling was carried out to make the scale thickness 5.0 μm or less, cooling conditions and cooling conditions after rolling were variously changed, and an examination was conducted.
The examination result is shown in FIG. In addition, the scale thickness is 5 mm in the width direction at a pitch of 1000 mm in the longitudinal direction of the product steel sheet.
It is the average scale thickness obtained by averaging the values measured from optical micrographs by sampling from each divided position.

From FIG. 3, a descaling collision pressure of 1.2 kg / cm ² or more is required to reduce the scale thickness to 5.0 μm or less, and the high temperature residence time up to 650 ° C. on the surface of the steel sheet after rolling is 30 seconds. It is necessary to be within the range, and it was found that cooling after the completion of rolling is effective in suppressing the growth of scale during high temperature retention. It is known from the results of experiments conducted separately that the scale hardly grows at 650 ° C or lower. Regarding the conditions during rolling in FIG. 3, Table 2 shows the cooling conditions for each case.

[0034]

[Table 2]

Cases 1 to 4 are examples of the present invention, and cases 5 and 6 are comparative examples. Case 5 is pass no. Only the descaling in 2 and 3 did not inject the cooling water in all the remaining passes, and the case 6 is an example in which the collision pressure of the hydraulic descaling header was set to less than 1.2 kg / cm ² . Further, the formulas and in Table 2 are values obtained by the following. Formula Collision pressure p = 68 × (P ^0.5 × Q (90−θ2) /
(H ² × θ1) P: Injection pressure kg / cm ² Q: Injection water amount L / min h: Nozzle height mm θ1: Nozzle injection angle θ2: Nozzle attack angle type Cooling capacity Log = 2.358 +0.663 Log W-0 .
00147 × T _s W: Water density L / m ² · min T _s : Steel plate surface temperature ℃ (using 800 ℃) Source: Special Report No. issued by the Iron and Steel Institute of Japan. 29 According to "Forced cooling of steel", page 16.

A rolling method for reducing the thickness of the scale before cooling will be described. First, an example of equipment used for the present invention will be described with reference to FIG. In a housing 8 of the reversible hot rolling mill 1, an upper work roll 2, a vertical rolling reinforcing roll 4 in contact with the roll, a lower work roll 3 and a lower rolling reinforcing roll 5 in contact with the roll are arranged. Top guides 9 and 9-1 are provided on the biting side (or biting side) and the biting side (or biting side) of the rolling mill, and the top guides 9 and 9-1 are looked into the hole portions of the top guide, and are placed on the steel plate surface. Descaling nozzles 14 and 14-1 for jetting water and cooling jet nozzle 1
2, 12-1 are provided.

The descaling nozzles 14 and 14-1 are provided closest to the upper work roll 2 and are connected to the hydraulic descaling headers 13 and 13-1 so as to have an attack angle to the moving steel plate surface. There is.

The cooling water injection nozzles 12 and 12-1 are installed adjacent to the descaling nozzles 14 and 14-1,
And the cooling header 1 so that the steel sheet surface is jetted almost at right angles.
1, 11-1. Nozzle 12, 12-1
The injection direction may be directed to the rolling direction to improve drainage. Further, the roller tables 10 and 10-1 having the same feed surface as the feed surface of the lower work roll 3 are the bite side (or bite side) and the bite side (or bite side) of the rolling mill.
And the descaling nozzle 2 between the lower work roll 3 and the roller of the roller table.
Nos. 0 and 20-1 are provided on the descaling headers 18 and 18-1 through nozzle holders 19 and 19-1 so as to have an attack angle with respect to the steel plate surface, and cooling water injection nozzles 17 are provided between the rollers of the roller table. 17-1
Nozzle holder 1 so that is sprayed almost at right angles to the steel plate surface
It is provided in the cooling header 15, 15-1 via 6, 16-1. In the figure, 6-1, 6-1 is a guide on the rolling mill, 7,
Reference numeral 7-1 is a rolling mill lower guide.

In the above apparatus, when the high-temperature thick steel plate S is rolled, the descaling nozzle 14, immediately before the steel plate to be rolled on the front surface A of the rolling mill 1 is caught in the work rolls 2, 3 as the first pass. The upper and lower surface oxides are removed by the high-pressure jet cooling water from 20 and the upper and lower surfaces of the rolled steel sheet rolled and bitten by the rear surface b are cooled by the water jet nozzle 12-.
Cool from 1, 17-1 with cooling water. Next, as a second pass, immediately before the rolled steel plate on the rear surface B of the rolling mill 1 is caught in the work rolls 2 and 3, the descaling nozzle 14
-1, 20-1 removes the scale generated on the upper and lower surfaces of the steel sheet with high-pressure jet water, and the upper and lower surfaces of the steel sheet to be rolled, which have been rolled and bitten by the front surface A of the rolling mill, are cooled by water jet nozzles 12, 17. Cool with cooling water from. Then, the reciprocal rolling is repeated a plurality of times until the desired plate thickness is obtained.

As a method of thinning the scale, descaling before hot leveling may be used together after rolling under the above-mentioned rolling conditions. In order to improve the strength of the steel sheet obtained as described above, the effect of the present invention will not be impaired by using any coolant such as water, steam, or a water-vapor mixture after rolling.

[0041]

EXAMPLES Examples of the present invention are shown below together with comparative examples. The composition of the test steel of the present invention may be any combination as long as it is the above-mentioned elements and addition amounts, and the chemical composition of the steel used in this example as a typical structural steel having different strength levels is shown in Table a-. Shown in 1.

As the test steels shown in Table a-1, seven kinds of steels having different strength levels are selected, and V, Nb and N are selected as required.
Alloying elements such as i, Ti, Cu, Ni, Cr and Mo are added.

Table a-2 shows the production conditions, the obtained materials, the area ratio of Fe ₂ O ₃ , the thickness of the scale, the color and the adhesion. No. in Table a-2. The invention examples A1 to A14 were all black steel plates, and structural steel plates with excellent scale adhesion were obtained depending on the conditions.

On the other hand, the number of passes where the number of passes at 950 ° C. or lower during rolling was 10 or more was determined. The steel plates of B1 to B7 were red in color.

Further, in Comparative Examples in which the cooling conditions during rolling were not satisfied, No. 1 using steel types 1, 2, 3, 4, and 7 were used. B1
In B4 and B7, the scale thickness was large and the adhesion was poor. Further, although the cooling conditions during rolling were satisfied, the No. 3 test piece which had a high temperature residence time from the end of rolling to 650 ° C. for 30 seconds or more was used. In B5, the scale thickness did not satisfy the predetermined value. In addition, the application case K of the cooling conditions in Table a-2 shows Table a-
Shown in 3.

[0046]

[Table 3]

[0047]

[Table 4]

[0048]

[Table 5]

In Table a-2, <K>: The application cases of the cooling conditions are in accordance with Table a-3. <L>: The average roughness of the surface of the steel sheet is evaluated by sampling from each position divided into 5 in the width direction at a pitch of 1000 mm in the longitudinal direction of the product steel sheet and obtaining the average value of the measured values of the average roughness. . The area ratio of Fe ₂ O _{3 is} the area ratio of the powder of 2 μm or less on the surface of the steel sheet. <M>: The scale thickness was sampled from each position of the product steel sheet divided into 5 parts in the width direction at a pitch of 1000 mm in the longitudinal direction, obtained from an optical micrograph, and the average scale thickness was obtained from the average value. <N>: Scale adhesion is evaluated as r = 1.5 × t
(T: plate thickness) Ten samples were taken from a test piece bent 90 degrees, the sample was observed with a scanning electron microscope to obtain the exposure rate of the base metal, and the value was evaluated according to the evaluation criteria shown in Table 1. Is shown. <Q>: The color of the steel sheet was visually determined by an appearance inspection. Note 1) to 4) indicate the area ratio of Fe ₂ O ₃ colonies of 500 μm or less of ₂ μm or less.

[0050]

[Table 6]

In Table a-3, cases 1 to 4 are examples of the present invention, and cases 5 and 6 are comparative examples. Case 5 is pass no. Only the descaling in 2 and 3 did not inject the cooling water in all the remaining passes, and the case 6 is an example in which the collision pressure of the hydraulic descaling header was set to less than 1.2 kg / cm ² . Further, the expressions in Table a-3 and are the values obtained by the following. Formula Collision pressure p = 68 × (P ^0.5 × Q (90−θ2) /
(H ² × θ1) P: Injection pressure kg / cm ² Q: Injection water amount L / min h: Nozzle height mm θ1: Nozzle injection angle θ2: Nozzle attack angle type Cooling capacity Log = 2.358 +0.663 Log W-0 .
00147 × T _s W: Water quantity density L / m ² · min T _s : Steel plate surface temperature ℃ (using 800 ° C.) T _s : Steel plate surface temperature ℃ (using 800 ° C.) Source: Special Report issued by the Iron and Steel Institute of Japan No. 29 According to "Forced cooling of steel", page 16.

[0052]

INDUSTRIAL APPLICABILITY According to the present invention, by controlling the cooling conditions during rolling and the cooling conditions after rolling, it is possible to produce structural steel sheets that are smoothly and stably adapted to various uses under high productivity. It is possible to do so, and the effect brought to the industrial world is extremely large, especially in this kind of field.

[Brief description of drawings]

FIG. 1 shows the relationship between the area of hematite of 2 μm or less on the surface of a steel sheet and the color of the steel sheet.

FIG. 2 is a diagram showing the relationship between scale thickness and scale adhesion (rating rank).

FIG. 3 is a chart showing the relationship between rolling-cooling conditions for each cooling case, high temperature residence time after rolling and scale thickness.

FIG. 4 is a schematic front view showing a rolling method of the present invention.

Claims (8)

[Claims] 1. By weight%, C: 0.04 to 0.20%, Si: 0.03 to 1.00%, Mn: 0.30 to 2.00%, Al: 0.005 to 0. 10%, N: 0.001 to 0.01%, a balance of Fe and unavoidable components in the balance, and the amount of Fe ₂ O ₃ having an average particle size of 2 μm or less on the surface of the steel sheet is less than 20% in area ratio. Structural steel plate with less scale. 2. By weight%, C: 0.04 to 0.20%, Si: 0.03 to 1.00%, Mn: 0.30 to 2.00%, Al: 0.005 to 0. 10%, N: 0.001 to 0.01%, Ti: 0.003 to 0.10%, Cr: 0.01 to 0.50%, Ni: 0.01 to 3.00%, Mo: 0.01 to 0.50%, Cu: 0.01 to 1.50%, V: 0.005 to 0.20%, Nb: 0.003 to 0.05%, B: 0.0003 to 0. Steel containing 0020% of one or more kinds and the balance of Fe and unavoidable components, and having a mean particle size of 2 μm or less on the surface of the steel plate of Fe ₂ O ₃ of less than 20% in area ratio Structural steel plate with small scale. 3. By weight%, C: 0.04 to 0.20%, Si: 0.03 to 1.00%, Mn: 0.30 to 2.00%, Al: 0.005 to 0. 10%, N: 0.001 to 0.01%, the balance is Fe and inevitable components, and the size of the colony in which Fe ₂ O ₃ having an average grain size of 2 μm or less on the surface of the steel sheet is present is a circle equivalent diameter. A structural thick steel plate having a red area of less than 500 μm and having a colony area ratio of less than 50%. 4. In% by weight, C: 0.04 to 0.20%, Si: 0.03 to 1.00%, Mn: 0.30 to 2.00%, Al: 0.005 to 0. 10%, N: 0.001 to 0.01%, Ti: 0.003 to 0.10%, Cr: 0.01 to 0.50%, Ni: 0.01 to 3.00%, Mo: 0.01 to 0.50%, Cu: 0.01 to 1.50%, V: 0.005 to 0.20%, Nb: 0.003 to 0.05%, B: 0.0003 to 0. A steel containing 0020% of one or more kinds and the balance of Fe and unavoidable components, and the size of a colony containing Fe ₂ O ₃ having an average particle size of 2 μm or less on the surface of the steel plate is 50 in terms of circle equivalent diameter.
A structural thick steel plate having a red scale of less than 0 μm and having an area ratio of colonies of less than 50%. 5. By weight%, C: 0.04 to 0.20%, Si: 0.03 to 1.00%, Mn: 0.30 to 2.00%, Al: 0.005 to 0. 10%, N: 0.001 to 0.01%, the balance being steel consisting of Fe and inevitable components, the amount of Fe ₂ O ₃ having an average particle size of 2 μm or less on the surface of the steel sheet is less than 20% in area ratio, In addition, a structural thick steel plate having a small scale thickness of 5.0 μm or less and excellent red scale scale adhesion. 6. C: 0.04 to 0.20% by weight%, Si: 0.03 to 1.00%, Mn: 0.30 to 2.00%, Al: 0.005 to 0.10. %, N: 0.001 to 0.01%, Ti: 0.003 to 0.10%, Cr: 0.01 to 0.50%, Ni: 0.01 to 3.00%, Mo: 0 0.01 to 0.50%, Cu: 0.01 to 1.50%, V: 0.005 to 0.20%, Nb: 0.003 to 0.05%, B: 0.0003 to 0.0020 %, One or two or more, and the balance being Fe and unavoidable components, the amount of Fe ₂ O ₃ having an average particle size of 2 μm or less on the surface of the steel sheet is less than 20% in area ratio, and scale There are few red scales whose thickness is less than 5.0 μm,
Structural steel plate with excellent scale adhesion. 7. In% by weight, C: 0.04 to 0.20%, Si: 0.03 to 1.00%, Mn: 0.30 to 2.00%, Al: 0.005 to 0. 10%, N: 0.001 to 0.01%, the balance is Fe and inevitable components, and the size of the colony in which Fe ₂ O ₃ having an average grain size of 2 μm or less on the surface of the steel sheet is present is a circle equivalent diameter. A structural thick steel plate having an area ratio of colonies of less than 500 μm and less than 50%, a red scale of 5.0 μm or less and less red scale, and excellent scale adhesion. 8. By weight%, C: 0.04 to 0.20%, Si: 0.03 to 1.00%, Mn: 0.30 to 2.00%, Al: 0.005 to 0. 10%, N: 0.001 to 0.01%, Ti: 0.003 to 0.10%, Cr: 0.01 to 0.50%, Ni: 0.01 to 3.00%, Mo: 0.01 to 0.50%, Cu: 0.01 to 1.50%, V: 0.005 to 0.20%, Nb: 0.003 to 0.05%, B: 0.0003 to 0. A steel containing 0020% of one or more kinds and the balance of Fe and unavoidable components, and the size of a colony containing Fe ₂ O ₃ having an average particle size of 2 μm or less on the surface of the steel plate is 50 in terms of circle equivalent diameter.
A structural thick steel plate having an area ratio of the colonies of 0 μm or less, less than 50%, a scale thickness of 5.0 μm or less, a small red scale, and excellent scale adhesion.