JP2010077482A - Ultrathin cold-rolled steel sheet for building material, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrathin cold-rolled steel sheet which has strength and flatness required for building materials and is superior in its surface shape, particularly in its smoothness. <P>SOLUTION: The steel sheet has a component composition comprising, by mass%, 0.01-0.10% C, 0.03% or less Si, 0.005-0.5% Mn, 0.01-0.20% P, 0.03% or less S, 0.01-0.1% Al, 0.010% or less N and the balance Fe with unavoidable impurities; and also has a sheet thickness of 0.2 mm or less. Furthermore a surface shape of the steel sheet, which has been expressed by shape factors of A and B specified below, satisfies a relationship of an expression (1): B≤-0.08×A+1.7, wherein A represents the number of unevenness (pieces/1,500 mm) per unit length of a product; and B represents an average height (mm) of the unevenness. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an ultra-thin cold-rolled steel sheet for building materials and a method for producing the same, and particularly to an ultra-thin cold-rolled steel sheet having a thickness of 0.2 mm or less, in addition to the strength and flatness required for building materials, The surface smoothness is imparted.

In recent years, the demand for building materials for ultra-thin cold-rolled steel sheets has increased.
In other words, ultra-thin cold-rolled steel sheets are subjected to surface treatment such as hot dipping, electroplating, painting, etc., if necessary, and then bonded to, for example, wooden or resin boards, and the inner and outer walls of buildings, roofs, etc. It is used for
When bonding to the substrate, if the steel sheet is stretched in the ears or belly, gaps are likely to occur between the substrate and the steel sheet after bonding to the substrate, causing problems such as poor appearance and corrosion. It is desirable that the steel plate be flat without generating ear extension or belly extension as much as possible.

In the above applications, cold-rolled steel sheets are used with little processing.
In such applications, plate thickness accuracy, strength, and shape (flatness) are emphasized rather than formability such as ductility and r value. That is, it is required to be extremely thin, to have high strength, and to be excellent in flatness evaluated by ear elongation or belly elongation.
Furthermore, when used for such applications, the general-purpose material is required to be inexpensive.

Here, as a manufacturing method of the ultrathin cold-rolled steel sheet, for example, in Patent Document 1, C ≦ 0.010% and C + N ≦ 0.012%, Si ≦ 0.01%, Mn ≦ 0.15, P ≦ 0.02%, S ≦ 0.020%, It is disclosed that a coil after hot rolling of steel, the balance of which is Fe and inevitable impurities, is cold rolled to a thickness of 0.5 mm or less at a cold rolling rate of 80 to 99% without intermediate annealing. Yes. This technology reduces work hardening in cold rolling by reducing the C content and N content in steel, enables cold rolling at a high rolling rate, and tries to manufacture an ultrathin steel plate. Is.
However, in the application for building materials as described above, high strength, specifically, yield strength YS: 700 MPa or more is required. However, in the technique of Patent Document 1, high strength after cold rolling is required. It was difficult to achieve. Further, the above technique has a disadvantage that the manufacturing cost is high because both C and N need to be reduced.
JP-A-3-79726

In order to solve the above problems, the inventors previously described in Japanese Patent Application No. 2008-050920,
“In mass%, C: 0.01% to 0.10%, Si: 0.03% or less, Mn: 0.005% to 0.5%, P: 0.01% to 0.20%, S: 0.03% or less, Al: 0.01% or more 0.1 %, N: 0.010% or less, the balance is Fe and inevitable impurities, and the sheet thickness is a steel sheet cold-rolled to 0.2 mm or less, and the average hardness of the steel sheet after the cold rolling (HR30T) is 68 or more and 83 or less, and the ratio of the variation in hardness in the width direction within ± 2 of the average hardness is 90% or more of the whole steel sheet. steel sheet."
Proposed.
As a result, an ultra-thin cold-rolled steel sheet for building materials having a sheet thickness of 0.2 mm or less and excellent strength and flatness can be obtained at low cost.

  The present invention relates to the improvement of the technique disclosed in the above-mentioned Japanese Patent Application No. 2008-050920, and further improves the surface shape, particularly the smoothness, of an ultrathin cold-rolled steel sheet for building materials having a sheet thickness of 0.2 mm or less. The goal is to achieve.

Conventionally, as for the shape of steel plates for building materials, flatness such as ear stretch and belly stretch has been mainly used as an index. In such a case, there may be a problem in adhesion to the heat insulating material.
Therefore, as a result of various studies to solve this problem, the inventors have found that an extremely thin steel sheet having a thickness of 0.2 mm or less is generated on the entire surface of the steel sheet in order to further improve the adhesion to the heat insulating material. It was found that it is important to appropriately control and smooth the shape of the minute unevenness.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. In mass%, C: 0.01% to 0.10%, Si: 0.03% or less, Mn: 0.005% to 0.5%, P: 0.01% to 0.20%, S: 0.03% or less, Al: 0.01% to 0.1% And N: 0.010% or less, with the balance being the composition of Fe and inevitable impurities, plate thickness: 0.2 mm or less, and the surface shape of the steel sheet indicated by the shape indices A and B defined below is represented by the following formula ( An ultra-thin cold-rolled steel sheet for building materials characterized by satisfying the relationship 1).
Record
B ≦ −0.08 × A + 1.7 --- (1)
Where: A: Number of irregularities per product unit length (pieces / 1500mm)
B: Average height of irregularities (mm)

2. In mass%, C: 0.01% to 0.10%, Si: 0.03% or less, Mn: 0.005% to 0.5%, P: 0.01% to 0.20%, S: 0.03% or less, Al: 0.01% to 0.1% And N: 0.010% or less, with the balance being Fe and an inevitable impurity composition. After heating the steel material to a heating temperature of 1150 ° C or higher, the finish rolling temperature is 700 ° C or higher and the Ar ₃ transformation point temperature or lower. After performing hot rolling under the conditions, the coiling temperature is 550 ° C or higher and 700 ° C or lower, and the coil is wound on the coil. The average between the winding temperature and 400 ° C at the coil width direction end at the center of the winding thickness A hot rolled sheet with a cooling rate of 125 ° C./h or less, then pickled, and then cold rolled to a sheet thickness of 0.2 mm or less at a cold rolling reduction ratio of 85% to 95%, Elongation: A method for producing an ultra-thin cold-rolled steel sheet for building materials, which is subjected to a shape correction treatment of 0.15% or more.

  According to the present invention, it is needless to say that the strength and flatness required for building materials can be obtained, and an ultra-thin cold-rolled steel sheet for building materials having excellent surface shape, particularly smoothness can be obtained.

The present invention will be specifically described below.
First, the reason why the component composition of the steel sheet is limited to the above range in the present invention will be described. The unit of the content of each element is “% by mass”, but hereinafter, it is simply indicated by “%” unless otherwise specified.
C: 0.01% or more and 0.10% or less C has the effect of increasing the strength of the material by dissolving in steel, but when the content exceeds 0.10%, carbide is formed, and the load during cold rolling is extremely large. Therefore, it becomes difficult to obtain a cold-rolled steel sheet having a thickness of 0.2 mm or less, and the smoothness is also deteriorated. Therefore, in the present invention, the upper limit of the C amount is set to 0.10% from the viewpoint of cold rollability and smoothness. In addition, it is desirable to reduce the amount of C from the viewpoint of cold rollability, but a significant reduction leads to a decrease in the strength of the steel sheet, increases the cost for reducing C during steelmaking, and provides the material at low cost. It becomes difficult to do. Therefore, the lower limit of the C amount is set to 0.01% from the viewpoint of securing strength and cost. From the viewpoint of both cold rollability and cost, the preferred C content is 0.02% or more and 0.07% or less.
When the plate thickness is reduced to about 0.12 mm, the C content is preferably 0.045% or less from the viewpoint of preventing breakage during the leveler process.

Si: 0.03% or less
Si is effective as an element that increases the strength of steel. However, if a large amount is contained, not only cold rolling properties but also surface treatment properties, chemical conversion properties, and corrosion resistance will be reduced. Was limited to 0.03% or less.

Mn: 0.005% to 0.5%
Since Mn functions to suppress hot cracking due to S, 0.005% or more is contained in order to obtain this effect. More preferably, it is 0.01% or more, More preferably, it is 0.05% or more. However, addition of a large amount of Mn not only hardens the steel sheet material and decreases the cold rolling property, but also decreases the weldability and weld formability after welding, so the upper limit of Mn was set to 0.5%. In addition, when a better shape and corrosion resistance are required, the Mn content is desirably 0.30% or less.

P: 0.01% or more and 0.20% or less P has an effect of increasing the strength of the steel sheet material, so it is included in an amount of 0.01% or more. However, a large amount of addition decreases the cold rollability. Moreover, P has a strong tendency to segregate in steel and causes embrittlement of the weld. For this reason, in the present invention, P: 0.20% was made the upper limit. In addition, it is 0.10% or less more desirably.

S: 0.03% or less S is mainly present as an inclusion in the steel and lowers the corrosion resistance. Therefore, it is desirable to reduce it as much as possible, but up to 0.03% is acceptable. For this reason, in the present invention, the upper limit of the amount of S is set to 0.03%. The lower limit of the S amount is not particularly limited, and is preferably reduced as much as described above, but is preferably about 0.005% from the viewpoint of steelmaking capacity and cost.

Al: 0.01% or more and 0.1% or less
Al is added as a deoxidizer and is an element that improves the cleanliness of steel, so it is actively added. However, if the amount of Al is less than 0.01%, the effect of deoxidation is small, and inclusions remain and formability is reduced. However, if it exceeds 0.1%, the surface cleanliness of the steel sheet decreases, so in the present invention it is limited to 0.01% or more and 0.1% or less. From the viewpoint of material stability, it is desirable that Al: 0.02% or more and 0.080% or less.

N: 0.010% or less N is dissolved in the steel sheet, and when the content exceeds 0.010%, the steel sheet is remarkably hardened. The lower limit of the N amount is not particularly limited, but is preferably about 0.0010% in consideration of steelmaking ability and cost.

The balance consists of Fe and inevitable impurities.
Inevitable impurities include Cu, Ni, Cr, Mo, Nb, Ti, and B. Cu: 0.20% or less, Ni: 0.20% or less, Cr: 0.20% or less, Mo: 0.20 %, Nb: 0.02% or less, Ti: 0.02% or less, and B: 0.0010% or less.

By adjusting to the above-mentioned preferable component composition range, the average hardness of the steel sheet after cold rolling can be kept in the range of 68 to 83 in terms of Rockwell hardness (HR30T).
Further, after the leveler treatment, the flatness of the steel sheet can be 2 mm or less and the yield strength (YS) can be 700 MPa or more.

Since the hardness (hardness) of the steel sheet after cold rolling greatly affects the product shape, the hardness is important in the present invention. In the present invention, since the product plate thickness is as extremely thin as 0.2 mm or less, the hardness is obtained by measuring the plate surface and determining the plate surface hardness. The test method conforms to JIS Z 2245 “Rockwell hardness test method”.
If this average hardness (HR30T) is less than 68, it may be difficult to secure YS ≧ 700 MPa after leveler processing, and the product may be bent back. On the other hand, if it exceeds 83, shape correction by leveler processing becomes difficult Deterioration of product shape may be significant. More preferably, HR30T is 80 or less.

  Moreover, even if the average hardness of the steel sheet satisfies the range of 68 to 83 in HR30T, if the difference between the maximum hardness and the minimum hardness, that is, the hardness difference is too large, there is a concern that the shape will deteriorate after the leveler treatment. Therefore, it is desirable that the hardness difference is 6 or less, preferably 4 or less.

As described above, an ultrathin cold-rolled steel sheet having the strength and flatness required for building materials can be obtained.
However, as described above, when the strength and flatness only satisfy the above-described preferable ranges, good adhesiveness is not always obtained when the heat-resistant material is adhered.

Therefore, in the present invention, the shape indexes A and B defined below are used as the surface shape index of the steel sheet, and the relationship of the following formula (1) is satisfied for the surface shape of the steel sheet indicated by these A and B. It was.
Record
B ≦ −0.08 × A + 1.7 --- (1)
Where: A: Number of irregularities per product unit length (pieces / 1500mm)
B: Average height of irregularities (mm)

Conventionally, poor shape of cold-rolled steel sheet has been generally evaluated by poor flatness such as ear extension 2 and belly extension 3 generated in steel sheet 1 as shown in FIGS. 1 (a) and 1 (b). From the viewpoint of the smoothness of the steel sheet, the shape of the minute irregularities 4 generated on the entire surface of the steel sheet as shown in FIG. 2 is important.
Therefore, the inventors previously proposed in Japanese Patent Application No. 2008-074004,
In measuring the surface shape of a cold rolled steel sheet having a thickness of 0.4 mm or less, a distance meter was used to measure the distance from the surface of the cold rolled steel sheet over a certain length. A cold-rolled steel sheet characterized by extracting a component having a period of 1000 times to 2000 times the thickness of the steel sheet, and detecting minute irregularities on the surface of the steel sheet based on the number and height of the peaks in the extraction result The shape measurement method. "
Were used to study the shape of the fine irregularities required in the present invention, that is, the degree of smoothness.

  As a result, when the number of irregularities per unit length (number of peaks per unit length) is defined as A (pieces / 1500 mm), and the average height of irregularities (average peak height) is defined as B (mm) As for the shape indexes A and B, the desired smoothness can be obtained by satisfying the relationship of the above formula (1). As a result, extremely good adhesiveness can be obtained when an ultra-thin steel sheet is adhered to a heat insulating material. It has been determined that can be obtained.

Therefore, in the present invention, the relationship of the above formula (1) is satisfied with respect to the minute uneven shape on the steel sheet surface.
More preferably, it is in the range of the following formula (2).
B ≦ −0.08 × A + 1.2 --- (2)
Where: A: Number of irregularities per product unit length (pieces / 1500mm)
B: Average height of irregularities (mm)

Here, a method for measuring the shape of the cold-rolled steel sheet that detects the minute irregularities on the surface of the steel sheet will be described.
The occurrence of the above-described minute unevenness is considered to be caused by the stress acting on the steel sheet in the portion sandwiched between the upper and lower rolls during rolling, that is, in the roll bite. That is, the material compressed from above and below in the roll bite during rolling tends to spread in the width direction, and compressive stress is generated in the width direction. This compressive stress in the plate width direction is released on the roll bite exit side, and local width spread occurs, and in the case of a very thin steel sheet with a small thickness, this local width spread buckles and the entire surface of the steel plate It is considered that a minute uneven shape is formed.
In such a cold rolled steel sheet having a thickness of 0.4 mm or less, it has been impossible with a conventional shape measuring method or shape measuring apparatus to grasp a shape defect due to minute irregularities generated on the entire surface of the steel sheet.

Therefore, in a cold-rolled steel sheet with minute irregularities on the steel sheet surface, a non-contact distance meter is scanned in the longitudinal direction of the steel sheet to measure the displacement of the steel sheet surface, and the measurement data is subjected to spectrum analysis by Fourier transform. , Examined the period of small irregularities.
When investigating the period of minute irregularities for cold-rolled steel sheets of various thicknesses, there is a correlation between the thickness of the cold-rolled steel sheet and the period of minute irregularities, and the period of minute irregularities is not less than 1000 times and not more than 2000 times the thickness of the steel sheet. It was newly found out. It has also been found that the degree of shape defects due to minute irregularities increases as the peak height increases without changing the period of minute irregularities.
That is, in order to judge whether the surface shape of the cold-rolled steel sheet is good or not, a component corresponding to the period of minute irregularities is extracted from the displacement measurement data according to the thickness of the cold-rolled steel sheet, and a minute amount is extracted from the displacement data of the extracted component. What is necessary is just to judge the degree of unevenness. The degree of minute irregularities may be appropriately determined from the number of peaks per unit length in the minute irregularities and the average value of peak heights. As for the mountain height, by using the average value, it is possible to perform a stable evaluation regardless of the size of the measurement range.
Based on these examinations, in the present invention, as described above, the quality of the smoothness is judged based on the above formula (1) or the above formula (2).

Hereinafter, the measurement method will be described in more detail by taking a cold rolled steel sheet having a thickness of 0.15 mm and a width of 1000 mm as an example.
In the measurement method, first, the shape of the cold-rolled steel sheet is preferably measured using a non-contact distance meter. At this time, the measurement position in the width direction may be measured in detail, but since the target micro unevenness occurs on the entire surface of the steel plate, measurement over a certain length at an arbitrary position, for example, It is sufficient to measure the displacement over a length of 1500 mm at the center in the width direction of the steel sheet.
Next, from a measured value of a distance meter for a certain length in the longitudinal direction of the cold-rolled steel sheet, here, a length of 1500 mm, as a component corresponding to minute irregularities, 1000 times or more to the thickness of the cold-rolled steel sheet 2000 Extract a component with a period less than double. That is, since the thickness of the steel plate is 0.15 mm, the period of the small unevenness component is 150 mm or more and 300 mm or less. Note that a general signal processing method such as Fourier transform may be used to extract a component having a specific period from the measured value data.
With respect to the concavo-convex component having a cycle of 150 mm or more and 300 mm or less extracted as described above, the quality of the steel sheet is determined from the number of peaks and the height of the peaks.
The “constant length” for measuring the distance from the surface of the cold-rolled steel sheet needs to be at least long enough to measure a minute unevenness. For example, when the thickness of the steel sheet is 0.4 m or less, the minute unevenness component Since the upper limit is 2000 times the steel plate thickness, a length of 0.4 × 2000 = 800 (mm) or more is required. On the other hand, it is not necessary to set an upper limit, but about 2000 mm is sufficient.

Next, the manufacturing method of this invention is demonstrated.
The molten steel having the preferred component composition described above is melted using a known furnace such as a converter or an electric furnace, and then slabd by a known method such as a continuous casting method, an ingot-bundling method, or a thin slab casting method. And steel material. Among these known methods, the continuous casting method is more preferable for preventing macro segregation.

  Next, the steel material is heated and subjected to hot rolling. At this time, if the heating temperature of the raw material is less than 1150 ° C., the deformation resistance during hot rolling becomes high and the rolling load increases, making hot rolling difficult, so the heating temperature is set to 1150 ° C. or higher. In addition, 1150 ° C. or higher is preferable in order to make the material uniform. However, since heating becomes higher than 1300 ° C., crystal grains become coarse and ductility decreases, so the heating temperature is preferably 1300 ° C. or lower.

Subsequently, hot rolling is performed. In the present invention, the finishing temperature in this hot rolling is important.
That is, by setting the finish rolling temperature to 700 ° C. or more and the Ar ₃ transformation point temperature or less, a soft hot-rolled steel sheet can be obtained, and the load during cold rolling is reduced. The rolled material can be obtained with high plate thickness accuracy. In this respect, if the finishing temperature is less than 700 ° C., the hot-rolled sheet is too soft, so that the load during cold rolling is reduced, but the product is broken and the product shape is deteriorated. In addition, when the finishing temperature is lower than 700 ° C., the load during hot rolling increases. For this reason, the finishing temperature of the hot-rolled sheet is set to 700 ° C. or higher. More preferably, it is 750 ° C. or higher. On the other hand, if the finish rolling temperature is higher than the Ar ₃ transformation point temperature, the hot-rolled sheet becomes hard, the load in cold rolling increases, and the cold rolling property decreases. Therefore, the finish rolling temperature is less Ar ₃ transformation point temperature. Preferably it is 830 degrees C or less, More preferably, it is 800 degrees C or less, More preferably, it is less than 790 degreeC.
The Ar ₃ transformation point can be obtained by the following equation.
Ar ₃ transformation point = 901−325 [% C] −92 [% Mn] +33 [% Si] +287 [% P]
However, [] is the content of each element (mass%)

Winding temperature: 550 ° C. or higher and 700 ° C. or lower By setting the winding temperature to 550 ° C. or higher, the crystal grains after hot rolling are grown and coarsened, and the carbides are agglomerated and coarsened. Thereby, a soft hot-rolled sheet can be obtained, the load at the time of cold rolling can be reduced, the cold rolling property can be greatly improved, and the smoothness can be improved. However, if the coiling temperature is too high exceeding 700 ° C., surface scales are often generated, and the surface properties of the hot-rolled sheet and thus the surface properties after cold rolling may be deteriorated. Effect. For this reason, the coiling temperature should be between 550 ° C and 700 ° C. More preferably, it is 640 degreeC or more and 660 degreeC or less.

Average cooling rate between the coiling temperature end of the coil thickness direction at the coil width direction and 400 ° C: 125 ° C / h or less The average cooling rate in this temperature range is the minimum and maximum hardness of the steel sheet. It strongly affects the difference in hardness (hardness difference).
FIG. 3 shows a cold rolled steel sheet obtained by hot rolling, winding the coil at a coiling temperature of 660 ° C., cooling to 400 ° C. at various cooling rates (A), and then performing cold rolling. The results of examining the surface hardness variation in the plate width direction are shown using the cooling rate (A) as a parameter. Here, as shown in FIG. 4, the average cooling rate is obtained by measuring the coil width direction end portion of the winding thickness center portion, which is considered to be most suitable for measuring the average coil temperature, as a measurement point.
As shown in the figure, when the cooling rate (A) is 100 ° C / h, the variation in the surface hardness in the plate width direction is larger than when the cooling rate (A) is 200 ° C / h. In addition, when the cooling rate (A) is 80 ° C./h, the variation in surface hardness particularly in the vicinity of the edge portion is further improved.

As a result of repeated studies as described above, the inventors have found that when the average cooling rate exceeds 125 ° C./h, it is difficult to obtain satisfactory smoothness after the leveler treatment, and this average cooling rate is 125 ° C./h. It discovered that favorable smoothness could be ensured by setting it as follows.
Therefore, in the present invention, as shown in FIG. 4, the average cooling in the above temperature range with the coil width direction end portion of the central portion of the winding thickness considered to be most suitable for measuring the average temperature of the coil as a measurement point. The speed was limited to 125 ° C./h or less. Preferably it is 100 degrees C / h or less, More preferably, it is 80 degrees C / h or less.

  The lower limit of the cooling rate is not particularly limited, but is preferably about 10 ° C./h, for example, from the viewpoint of production efficiency. The reason why the lower limit of the temperature range to be subjected to such controlled cooling is 400 ° C. is that, in the temperature range below 400 ° C., the influence of the cooling rate on the hardness is small and the hardness does not change. .

Then, after pickling, it is cold-rolled by cold rolling.
The pickling conditions for the hot-rolled sheet need not be specified, and it is sufficient if the surface scale can be removed. For this purpose, the surface scale may be removed by a known method, for example, an acid such as hydrochloric acid or sulfuric acid.
Cold rolling is performed to a sheet thickness of 0.2 mm or less under a reduction ratio of 85% to 95%. Here, when the rolling reduction in cold rolling is less than 85%, it becomes necessary to make the thickness of the hot rolled sheet 1.3 mm or less, and it becomes difficult to secure a finishing temperature of a predetermined temperature or more. As the load increases and the temperature variation in the coil increases, the desired material cannot be obtained. On the other hand, the rolling exceeding 95% may deteriorate the shape, so the cold reduction ratio is 85 to 95%. It was limited to the range.

Next, the flatness and smoothness of the steel sheet are improved by applying a shape correction treatment with an elongation of 0.15% or more with a tension leveler or the like.
Here, if the elongation rate in the shape correction treatment is less than 0.15%, sufficient smoothness that satisfies the above-mentioned formula (1) cannot be obtained.
The upper limit of the elongation rate is not particularly limited, but if the elongation rate exceeds 0.5%, the steel sheet is liable to break during the leveler processing, so the elongation rate is preferably 0.5% or less. In order to reduce the probability of breakage, it is more preferably 0.3% or less.
In the above description, the leveler process is exemplified as the shape correction process. However, the present invention is not limited to this, and any shape correction process having the same effect as the leveler process is applicable. To do.

The steel sheet after the shape correction treatment may be subjected to a surface treatment as necessary.
Examples of the surface treatment to be applied include degreasing, drying, hot dip galvanizing, and subsequent chromate treatment, or degreasing, drying, electroplating, color coating, or drying and color coating. It is done. Furthermore, any surface treatment applied to ordinary cold-rolled steel sheets, such as plating such as tin plating and nickel plating, various alloy plating, and chemical conversion treatment, is suitable.

Example 1
Steels having the composition shown in Table 1 were melted in a converter, and a slab having a thickness of 260 mm was formed by a continuous casting method. Next, after heating these slabs to 1200 ° C, the hot rolling at a finish rolling temperature of 780 ° C, which is lower than the Ar ₃ transformation point temperature, was made into a 2.0 mm thick hot-rolled sheet, and then coiled at a temperature of 650 ° C. Next, after cooling at 400 ° C at a rate of 78 ° C / h with the end in the coil width direction at the center of the coil thickness as the measurement point, pickling and then cold rolling at a reduction rate of 94% The final sheet thickness was 0.12 mm cold rolled steel sheet. The plate width was 1000 mm. Furthermore, the obtained cold-rolled steel sheet was subjected to a leveler treatment at an elongation rate of 0.3%.

Table 1 shows the results of the investigation on the average surface hardness (HR30T) and hardness of the cold-rolled steel sheet after the cold rolling and before the leveler treatment.
Table 1 also shows the results of examining the surface shape of the steel plate after the leveler treatment by the above method using a non-contact laser distance meter in the longitudinal direction: 1500 mm.
In addition, about the surface shape (smoothness) of a steel plate, it judges whether the uneven | corrugated number A per product unit length and the average height B of an unevenness belong to which area | region of three areas shown in FIG. , ○, × evaluated.

  As is clear from the table, all the cold-rolled steel sheets produced according to the present invention had a good surface shape (smoothness).

Example 2
Steel containing C: 0.035%, Si: 0.010%, Mn: 0.14%, P: 0.011%, S: 0.006%, Al: 0.052% and N: 0.0018% with the balance being Fe and inevitable impurities The slab was made into a slab with a thickness of 260 mm by a continuous casting method. Next, after heating these slabs to 1200 ° C, the hot rolling at a finish rolling temperature of 780 ° C, which is lower than the Ar ₃ transformation point temperature, was made into a 2.0 mm thick hot-rolled sheet, and then coiled at a temperature of 650 ° C. Next, after cooling in various patterns up to 400 ° C using the coil width direction end of the coil thickness center as the measurement point, pickling, and then cold rolling at a reduction ratio of 94% to obtain the final sheet thickness : 0.12 mm cold-rolled steel sheet. The plate width was 1000 mm. Furthermore, the obtained cold-rolled steel sheet was subjected to a leveler treatment (TLV) at the elongation shown in Table 2.
In addition, the cooling pattern was made into the following five types as shown by ae in FIG.
a) Winding temperature: Cooling time from 650 ° C to 400 ° C: 1.3 hours → Average cooling rate: 192 ° C / h (conventional example)
b) Cooling time in the same temperature range: 2.0 hours → Average cooling rate: 125 ° C./h (Invention Example 1)
c) Cooling time in the same temperature range: 3.2 hours → Average cooling rate: 78 ° C./h (Invention Example 2)
d) Cooling time in the same temperature range: 4.6 hours → Average cooling rate: 54 ° C./h (Invention Example 3)
e) Cooling time in the same temperature range: 7.2 hours → Average cooling rate: 35 ° C./h (Invention Example 4)

Table 2 shows the results of the investigation on the average surface hardness (HR30T) and the hardness of the cold-rolled steel sheet after the cold rolling and before the leveler treatment.
Table 2 also shows the results of examining the surface shape and operational stability of the steel plate after the leveler treatment.
In addition, operation stability means the probability that a steel plate will fracture during leveler processing, and was evaluated below.
○: Probability of breaking 5% or less △: Probability of breaking exceeds 5%, 20% or less

As is clear from the table, all the cold-rolled steel sheets produced according to the present invention not only obtained a good surface shape (smoothness), but also had excellent operational stability.
Further, the sheet surface hardness after cold rolling satisfied an appropriate range.

It is a figure which shows the shape of the ear extension and belly extension which arose in the steel plate. It is a figure which shows the shape of the micro unevenness | corrugation peculiar to an ultra-thin steel plate. It is a figure which shows the variation in the surface hardness of the plate width direction of a cold-rolled steel plate, using a cooling rate (A) as a parameter. It is a figure which shows the measuring point of coil temperature. It is a figure which shows the evaluation content of the surface shape (smoothness) of the steel plate prescribed | regulated by the relationship between the uneven | corrugated number A and the average height B of an unevenness | corrugation. It is a figure which shows the cooling pattern after hot rolling.

Explanation of symbols

1 Steel plate 2 Ear extension 3 Abdominal stretch 4 Micro unevenness

Claims (2)

In mass%, C: 0.01% to 0.10%, Si: 0.03% or less, Mn: 0.005% to 0.5%, P: 0.01% to 0.20%, S: 0.03% or less, Al: 0.01% to 0.1% And N: 0.010% or less, with the balance being the composition of Fe and inevitable impurities, plate thickness: 0.2 mm or less, and the surface shape of the steel sheet indicated by the shape indices A and B defined below is represented by the following formula ( An ultra-thin cold-rolled steel sheet for building materials characterized by satisfying the relationship 1).
Record
B ≦ −0.08 × A + 1.7 --- (1)
Where: A: Number of irregularities per product unit length (pieces / 1500mm)
B: Average height of irregularities (mm) In mass%, C: 0.01% to 0.10%, Si: 0.03% or less, Mn: 0.005% to 0.5%, P: 0.01% to 0.20%, S: 0.03% or less, Al: 0.01% to 0.1% And N: 0.010% or less, with the balance being Fe and an inevitable impurity composition. After heating the steel material to a heating temperature of 1150 ° C or higher, the finish rolling temperature is 700 ° C or higher and the Ar ₃ transformation point temperature or lower. After performing hot rolling under the conditions, the coiling temperature is 550 ° C or higher and 700 ° C or lower, and the coil is wound on the coil. The average between the winding temperature and 400 ° C at the coil width direction end at the center of the winding thickness A hot rolled sheet with a cooling rate of 125 ° C./h or less, then pickled, and then cold rolled to a sheet thickness of 0.2 mm or less at a cold rolling reduction ratio of 85% to 95%, Elongation: A method for producing an ultra-thin cold-rolled steel sheet for building materials, which is subjected to a shape correction treatment of 0.15% or more.

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