KR101528014B1 - Cold-rolled steel plate and method for producing same - Google Patents

Abstract

A cold-rolled steel sheet excellent in moldability and shape-mobility and a method for producing the same. The steel sheet according to any one of claims 1 to 3, wherein the steel has a composition of C: not less than 0.010%, not more than 0.030%, Si: not more than 0.05%, Mn: not more than 0.3%, P: not more than 0.05%, S: not more than 0.02% And the remainder is iron and inevitable impurities, and the average r value is 1.2 or less and the average total elongation is 41% or more. (C amount + N amount x 12/14)} - 1850] C - > [0156] The hot rolled steel sheet was then subjected to hot rolling at a cooling rate of 20 DEG C / s or lower, (C content and N content in the steel are in the range of C content (ppm) and N content (ppm) in the steel) (0.5 x (C amount + N amount x 12/14) , Cold rolling at a reduction ratio of 55% or more, and then annealing at an annealing temperature of 650 ° C to 800 ° C.

Description

Technical Field [0001] The present invention relates to a cold rolled steel sheet,

The present invention relates to a cold-rolled steel sheet which is most suitable as a member of a large flat plate-like component such as a backlight chassis of a large-size liquid crystal television, and is excellent in moldability and shape-mobility, and a manufacturing method thereof.

BACKGROUND ART [0002] Many thin flat-panel liquid crystal TVs, OA appliances, and the like use flat plate components formed by bending and elongating molding. A member (material) to be used for these parts is often required to have a flatness in addition to the degree of processing in the form of a part from the viewpoints of the design and thinness of the product. However, when bending and extending molding are performed on the flat surface of the member (material), the flatness tends to deteriorate. Such deterioration of the flatness is a phenomenon that occurs because the shape of the member (material) when the member (press) is formed by molding is poor, so that the steel sheet as the member (material) is required to have shape- do.

As a factor for deteriorating the flatness, ridge warping occurring at the time of bending is well known. One of them, bending occurring at the bent end, is said to be reduced by lowering the r value of the material, and a technique has been established to give a low r value and a low yield strength to the material.

For example, Patent Document 1 has established a technique for developing a cold-rolled steel sheet having a yield strength of 150 MPa and an r-value of 0.67 (1.45 in a direction perpendicular to the rolling direction) in the rolling direction.

Patent Document 2 discloses a technique for developing a steel sheet having excellent shape-formability by adjusting the coiling temperature.

Patent Document 3 discloses an automotive ferritic thin steel sheet excellent in shape-formability having a ratio of a {100} plane and a {111} plane parallel to a plate surface of 1.0 or more.

In Patent Document 4, the strength of {100} <011> to {223} <110> bearing groups and the strengths of {112} <110>, {554} 111} &lt; 112 &gt;, and {111} &lt; 110 &gt;, at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is set to 0.7 or less.

Japanese Patent Publication No. 3532138 Japanese Patent Publication No. 4126007 Japanese Patent Application Laid-Open No. 2008-255491 Japanese Patent Application Laid-Open No. 2003-55739

In recent years, however, processing of steel sheets has been complicated with the increase in size and function of thin-type liquid crystal TVs and OA devices. In order to cope with such a situation, it is necessary to improve the shape fixability of the member (material) and it is necessary to improve the shape fixability of the member (material) and to change the r value in the entire rolling direction (L direction) . For example, the r values in the rolling direction and the direction perpendicular to the rolling direction are reduced by the crystal grains having orientations of {100} <011> to {223} <110> specified in Patent Documents 1 and 4, When the r value in the direction is increased, the shape mismatch can not be improved after straight extensional forming with the rolling direction at 45 deg.

In Patent Document 2, although the r value of the steel sheet is reduced by lowering the coiling temperature, excessive reduction of the coiling temperature lowers the entire degree of drawing, and therefore is not preferable from the viewpoint of material cracking and breaking prevention at the time of molding.

In Patent Document 3, it is necessary to strictly control the reduction amount and the friction coefficient at a predetermined hot rolling temperature range, and it is necessary to control the coiling temperature specified by the composition of the components such as Mn, Si, P, and Al It is difficult to stably produce the film. As for the cold-rolled steel sheet, a case where the cold-rolled steel sheet is manufactured with a cold-rolling rate of about 53% is disclosed. However, at such a low cold rolling rate, it is difficult to manufacture a thin steel sheet of about 1.0 mm or less, .

SUMMARY OF THE INVENTION An object of the present invention is to provide a cold-rolled steel sheet excellent in moldability and shape-formability, and a method of manufacturing the same, in view of such circumstances.

SUMMARY OF THE INVENTION The inventors of the present invention have conducted extensive research in order to solve the above problems.

As a result, the following points were found. It is important to lower the r value while maintaining high ductility in the annealed plate after cold rolling in order to achieve both workability without occurrence of warpage and complicated shape machining. That is, by increasing the average degree of drawing, it is possible to secure the shape required for the parts after securing the workability when drawing or extending processing. Further, by reducing the r value, occurrence of springback or warping after machining can be suppressed, and shape fixability can be ensured.

The high ductility and low r value are realized by controlling the coiling temperature for the purpose of optimizing the amount of solid solution C and the solid solution N contained in the steel after hot rolling and the hot rolled steel sheet during hot rolling.

The present invention has been made based on the above findings, and its gist of the invention is as follows.

The steel according to any one of claims 1 to 3, which is characterized by comprising: C: not less than 0.010%, not more than 0.030%, Si: not more than 0.05%, Mn: not more than 0.3%, P: not more than 0.05%, S: not more than 0.02% : 0.005% or less, and the balance being iron and unavoidable impurities, and has an average r value of 1.2 or less and an average total elongation of 41% or more.

[2] The cold-rolled steel sheet according to the above item [1], further comprising at least one of Ti and B in an amount of 0.02% or less of Ti and 0.005% or less of B in mass%.

[3] The steel sheet according to any one of [1] to [2] above, which is subjected to hot rolling and then cooled to an average cooling rate of 20 ° C / s or less, (C amount + N amount 占 12/14)} - 1850] 占 폚 - {{0.5 占 (C amount + N amount 占 12/14)} + 520 占 폚 (Ppm) and N content (ppm)), cold rolling at a reduction ratio of 55% or higher, and then annealing at an annealing temperature of 650 ° C to 800 ° C. &Lt; / RTI &gt;

[4] The method for producing a cold-rolled steel sheet according to [3] above, wherein after the annealing, an overexposure treatment is performed at a temperature of 300 ° C to 400 ° C for 60 s to 300 s.

In the present specification,% and ppm indicating the steel components are all% by mass and mass ppm, respectively. The cold-rolled steel sheet to which the present invention is applied includes a steel sheet obtained by subjecting a cold-rolled steel sheet to surface treatment such as electro-galvanizing or hot-dip galvanizing. It also includes a steel sheet on which a film is adhered by chemical conversion treatment or the like.

Further, the steel sheet of the present invention can be widely used for members of general household appliances that perform flat surfaces, bending, elongation, hardness drawing, and the like, such as a backlight chassis of a large TV, a panel of a refrigerator, and an air conditioner outdoor unit. Further, by using the present invention, it is possible to manufacture a backlight chassis of about 650 x 500 mm (32 V type) or more with a steel sheet having a thickness of 0.8 mm, for example.

According to the present invention, it is possible to obtain a cold-rolled steel sheet which is capable of obtaining a high degree of elongation, a low r-value, and is capable of performing drawing, bending, As a result, it is possible to secure a flat plate shape required for a large-sized component, and for example, a member such as a backlight chassis of a large liquid crystal television can be manufactured.

Fig. 1 is a graph showing the relationship between the addition C and N amount, the optimum winding temperature (CT), the average total elongation (El) of the steel sheet, and the average r value.

Hereinafter, the present invention will be described in detail. First, the chemical composition of the steel sheet of the present invention will be described. In the following description, the content% of the element elements means the entire mass%.

C: 0.010% or more and less than 0.030%

When C is 0.030% or more, the deposition amount of the carbide is increased and the ductility is lowered. On the other hand, if it is less than 0.010%, the manufacturing cost is increased. Therefore, C is made 0.010% or more and less than 0.030%.

Si: not more than 0.05%

Si improves yield strength and deteriorates ductility with solid solution strengthening elements. Therefore, it should be 0.05% or less.

Mn: not more than 0.3%

Mn is an element which forms a sulfide and improves hot brittleness, but it is also a solid solution strengthening element, thereby increasing yield strength and deteriorating ductility. Therefore, Mn should be 0.3% or less.

P: not more than 0.05%

P improves the yield strength and deteriorates ductility with solid solution strengthening elements. Therefore, it should be 0.05% or less.

S: not more than 0.02%

S forms a sulfide at the stage of the hot-rolled sheet and causes anisotropy of the crystal structure after cold annealing to increase. Therefore, it is set to 0.02% or less.

Al: 0.02% or more and 0.10% or less

Al is a deoxidizing element and requires an addition of 0.02% or more. On the other hand, when it is excessively added, it causes fine precipitation at the time of annealing and increases the anisotropy of the crystal structure. Therefore, the upper limit is set to 0.10%.

N: 0.005% or less

When N is dissolved in steel, it causes stressor strain. Further, in the case of fine precipitation, the anisotropy of the crystal structure is increased. In view of the above, N is preferably as small as 0.005% or less.

In addition to the above elements, in the present invention, at least one of Ti and B may be contained in an amount of not more than 0.02% of Ti and not more than 0.005% of B, for the following purpose.

Ti: not more than 0.02%

Ti has a strong affinity with N and has the effect of forming precipitates at high temperatures and alleviating the adverse effects of N. In order to obtain such an effect, it is preferable that the content is 0.005% or more. On the other hand, excessive addition causes an increase in manufacturing cost. From the above point of view, if it is contained, it should be 0.02% or less.

B: not more than 0.005%

B is an invasive employment element that functions to lower the r value when employed in a steel. Further, there is an effect that the solid solution N is fixed as a precipitate and the strain strain is suppressed. In order to obtain such an effect, it is preferable to contain 0.0002% or more. On the other hand, an excessive addition causes a high yield strength and low ductility, so that the content is 0.005% or less.

The other components are composed of iron and inevitable impurities. As the inevitable impurities, for example, Cu, Cr of not more than 0.05% and Sn of less than 0.01%, Mo, W, V, Ni and the like which are likely to be mixed from scrap can be mentioned.

In the cold-rolled steel sheet of the present invention, the average r value in the rolling direction, the rolling direction of 45 ° and the direction perpendicular to the rolling direction is 1.2 or less.

The higher the r value is, the larger the warping that occurs in the bent portion of the bending forming member becomes. By setting the average r value to 1.2 or less, the shape fixability after molding can be sufficiently improved. In addition, the r value of the average can be obtained by measuring by the following method. JIS No. 5 tensile test specimens are cut from the rolling direction, the rolling direction at 45 ° and the direction perpendicular to the rolling direction, and the plastic deformation ratio test according to JIS Z 2254 is conducted at a preliminary strain of 15%. Then, it is obtained by the following formula (a).

The average r value of _{r m = (r L + 2r} D + r C) / 4 ... (a)

Where r _L is the r value in the rolling direction, r _D is the r value in the direction of the rolling 45 °, r _C is the r value in the direction perpendicular to the rolling direction

The steel sheet of the present invention has an average total elongation of at least 41%. In addition to the above characteristics, by drawing the average total drawing degree to 41% or more, it is possible to perform drawing or elongation processing, thereby securing the shape required for the parts. In the extensional forming, an excessive increase in the extension height causes cracking of the material. In order to prevent such a material from cracking and to increase the forming height of the elongation as much as possible, it is effective to increase the ductility of the material. When the average total degree of drawing is 41% or more as a steel sheet for home appliances and building materials, the shape required for parts can be secured sufficiently when the work is performed. Therefore, in the present invention, the average degree of drawing is set to 41% or more. And preferably at least 44%.

The total elongation of the average can be determined by the following method. JIS No. 5 tensile test specimens are cut out from the rolling direction, the rolling direction at 45 ° and the direction perpendicular to the rolling direction, and a tensile test according to JIS Z 2241 is carried out. Then, it is obtained by the following formula (b).

The total elongation of the average El _m = (El _L + 2 El _D + El _C ) / 4 ... (b)

Here, El _L: elongation in the rolling direction also, El _D: elongation in the rolling direction is also 45 °, El _C: elongation in the rolling direction at right angles is also

Next, the method of manufacturing the steel sheet of the present invention will be described. In the present invention, the steel slab having the above composition is hot-rolled, then cooled to an average cooling rate of 20 DEG C / s or lower, (C), (C), (C), (N), and (C) )) ° C, and then subjected to cold rolling at a reduction ratio of 55% or more and then annealing at an annealing temperature of 650 ° C to 800 ° C to obtain a high total elongation and a low r value have.

Heating temperature: 1200 占 폚 or higher (preferable condition)

Since the precipitate such as AlN should be once solidified during the hot rolling and finely precipitated after winding, it is preferable that the heating temperature of hot rolling is 1200 占 폚 or higher.

Finish rolling finish temperature: 880 DEG C or higher (preferable condition)

If a phase potential is generated in the ferrite phase during finish rolling, the structure of the hot rolled steel sheet may become uneven and the material may become unstable. Therefore, the finish rolling is preferably carried out in a single austenite phase, and the finishing rolling finishing temperature is preferably 880 DEG C or more.

Average cooling rate: 20 ° C / s or less

After finishing rolling, cooling is carried out at an average cooling rate of 20 DEG C / s or lower. Further, in the case of air cooling (cold cooling), since the cooling rate is about 10 ° C / s or less, the average cooling rate is 20 ° C / s or less. Although water cooling can be carried out, excessive cooling causes the crystal grains of the hot-rolled sheet to become finer, and accompanying this, the crystal grains after annealing become finer, the material strength increases and the ductility deteriorates. Therefore, the average cooling rate is set to 20 ° C / sec or less.

(C) + (N) x 12/14) - 1850] C to {{0.5 x (C amount + N amount x 12 / 14)} + 520] C

(Where C and N are the C content (ppm) and N content (ppm) in the steel)

Then, the hot rolled sheet is wound in a coil shape. In this case, the coiling temperature is not lower than 0 占 폚 and not higher than [{8 占 (C amount + N amount 占 12/14)} - 1850] 占 폚 - {{0.5 占 (C amount + N amount 占 12/14)} + Deg.] C. The coiling temperature range is obtained by analyzing the relationship between the average r value, the total elongation of the average, the content of C and N, and the CT based on the results obtained by the inventors. The reason why the r value of the average, the total elongation of the average, the content of C and N, and the relationship of CT were analyzed as follows. That is, when the coiling temperature is too high, cementite or AlN precipitates coarsely, and the growth of recrystallized grains, which is unfavorable to reduce the r value at the time of annealing, is promoted. In addition to this, the amount of solid solution C and solid solution N in the hot-rolled steel sheet, which generates a recrystallization direction favorable to reduction of the r value at the time of annealing, decreases. On the other hand, when the coiling temperature is too low, the amount of solid solution C and N is excessively increased, so that a large deformation band is formed at the time of cold rolling, and the recrystallized particles generated therefrom at the time of annealing are increased to miniaturize the crystal grain size of the annealing plate, do. Further, as the crystal grain size of the hot-rolled sheet becomes smaller, the grain size of the annealing sheet becomes finer and the ductility deteriorates. In order to prevent the cooling water from solidifying and damaging the surface of the hot-rolled plate, the temperature is set to 0 캜 or higher. When the average r value is 1.2 or less and the total elongation degree of the average is 44%, the coiling temperature is preferably not lower than 500 占 폚 and not higher than [{8 占 (C amount + N amount 占 12/14)} - 1850] {0.5 占 (C amount + N amount 占 12/14)} + 520 占 폚.

As described above, the coiling temperature is important because it coarsens the crystal grain size at the time of winding the coil and coagulates the carbonitride to make the solid C and N contents of the hot-rolled sheet appropriate, To achieve this, controlling the coiling temperature is an important requirement in the present invention.

Reduction rate (cold pressing rate) during cold rolling: 55% or more

After winding, pickling is carried out by a conventional method, and cold rolling is carried out at a reduction ratio of 55% or more to form a desired sheet thickness. However, when the rolling reduction is too low, a driving force sufficient for recrystallization can not be obtained, and the deformation remains after the annealing, so that the structure becomes high strength and low ductility. Further, It is necessary to excessively thin the hot-rolled sheet, resulting in an increase in the hot-rolling cost. From the above point of view, the reduction rate is 55% or more. In addition, if the reduction rate is excessively high, the rolling load becomes excessively large, which makes it difficult to roll and the production efficiency may be lowered. Therefore, the reduction rate is preferably about 85% or less.

Annealing temperature: 650 to 800 ° C

And annealing is performed at an annealing temperature of 650 ° C to 800 ° C to promote grain growth. If the annealing temperature is excessively high, AlN dissolves, and solid N appears in the steel, causing stress strain. On the other hand, if it is excessively low, deformation introduced at the time of cold rolling will not be released, resulting in high YP and low ductility. Therefore, the annealing temperature is set to be 650 ° C or higher and 800 ° C or lower.

Holding time: 30 to 200 s (desirable condition)

If the holding time at the annealing temperature is short, the recrystallization is not completed or the grain growth is suppressed even if the completion is completed, so that the degree of drawing is lowered. Further, the diffusion of solid solution N becomes insufficient, solid solution N remains in the ferrite particles, the yield strength becomes large, and the degree of drawing is lowered. Therefore, the holding time at annealing is preferably 30 s or more. On the other hand, if the soaking time is prolonged, the particles excessively grow and become large, causing problems of surface roughness during processing, and the surface properties are deteriorated. Therefore, the cracking time at the time of heating is preferably 200 s or less.

Further, in order to precipitate the solid solution C to improve the total degree of stretching, it is preferable to conduct the over-treatment treatment at a temperature of 300 ° C to 400 ° C for 60 to 300 s in the cooling process after the annealing.

Setting the overheating treatment temperature to 400 캜 or less is to increase the driving force of cementite precipitation to promote precipitation. Setting the temperature at 300 캜 or higher is intended to promote diffusion of Fe and C and precipitation of cementite.

If the overaging effect treatment time is less than 60 s, the effect is small. If the overshoot effect time exceeds 300 s, the effect is saturated and the production efficiency is lowered.

After annealing, temper rolling and leveling may be performed for the purpose of calibrating the plate shape or the like. Further, the surface of the steel sheet may be plated with an element for improving corrosion resistance such as zinc, chromium, and nickel, or may be subjected to a chemical treatment for improving corrosion resistance, sliding properties, and the like.

In the practice of the present invention, the solvent method can be suitably applied to a conventional converter method, a converter furnace method, or the like. The molten steel, after casting into a slab, is left as it is, or cooled, heated and hot-rolled. In the hot rolling, the steel sheet is cooled at the above-mentioned cooling rate and wound at the above-mentioned winding temperature. Then, after the usual pickling, cold rolling and annealing described above are carried out. If necessary, hot-dip galvanizing may be performed in the vicinity of 480 ° C. After the plating, the substrate may be reheated to a temperature of 500 ° C or higher to alloy the plating. Alternatively, a thermal history to be held during cooling may be taken. If necessary, temper rolling may be performed at an elongation of about 0.5 to 2%. In the case where plating is not carried out during annealing, electro-galvanizing or the like may be performed to improve corrosion resistance. Further, a coating film may be adhered to the cold-rolled steel sheet or the coated steel sheet by chemical conversion treatment or the like. As described above, a cold-rolled steel sheet excellent in moldability and shape-formability can be obtained.

[Example 1]

Rolled at the finishing temperature (FT) shown in Table 2, cooled under the conditions shown in Table 2, and wound up at the winding temperature (CT) shown in Table 2 Respectively. Next, the obtained hot rolled sheet was pickled, cold-rolled at a reduction ratio of 70%, annealed at an annealing temperature (AT) shown in Table 2 in the range of 730 DEG C to 800 DEG C, tempered at an elongation of 1% Followed by rolling to produce a sealant. The plate thickness after cold rolling was 0.6 to 0.8 mm and the holding time at annealing was 50 to 200 s. In addition, some of the steel sheets were annealed and subjected to overexposure treatment under the conditions shown in Table 2.

The average r value and the total elongation (El) of the average were measured for the specimens obtained as described above. The tensile test specimens of JIS No. 5 were cut out from the rolling direction (L direction), the rolling direction (D direction) and the rolling direction (C direction) of the specimens respectively and were subjected to a tensile test according to JIS Z 2241 and a JIS The plastic deformation ratio test according to Z 2254 was carried out at 15% preliminary strain. Then, the r value of the average and the total elongation (El) of the average were determined by the above-mentioned expressions (a) and (b).

The obtained results are shown in Table 2 together with the conditions. Fig. 1 shows the relationship between the amount of C and N, the optimum coiling temperature (CT), the average total elongation (El) of the steel sheet, and the average r value.

The results are shown in Table 2. In Table 2, as the winding conditions, the coiling temperature is 0 DEG C or more and [{8 x (C amount + N amount x 12/14 (ppm)占 12/14 (ppm))} + 520 占 폚 was evaluated as?, And when it satisfied 500 占 폚 or higher was evaluated as?, And when it was not satisfied, marked as 占.

According to Table 2, the steel sheet having the composition of the present invention and produced by the production method of the present invention had an average total elongation (El) of 41% or more and an average r value of 1.2 or less. On the other hand, in the case of the steel sheet whose composition is out of the range of the present invention or whose composition is within the range of the present invention, the production method is out of the range of the present invention, El or the average r value is decreased.

In Fig. 1, the optimum winding temperature range described in the present invention is surrounded by a dotted line in the figure. A steel sheet having an average total elongation (El) of 41% or more and an average r value of 1.2 or less is obtained by production within this range. When the coiling temperature is 500 ° C or more and the amount of C + N amount x 12/14 is 300 ppm or less, a steel sheet having an average total elongation (El) of 44% or more can be obtained.

As a result, in the cold-rolled steel sheet of the present invention, the average overall degree of drawing and the r value of the average are controlled, and the formability and shape-crystallinity are excellent.

Claims (4)

C: not less than 0.010%, not more than 0.030%, Si: not more than 0.05%, Mn: not more than 0%, not more than 0.3%, P: not less than 0% : Not less than 0.02% and not more than 0.10%, N: not less than 0% and not more than 0.005%, and the balance being iron and inevitable impurities, and has an average r value of 1.2 or less, defined by the following formula (a) Is not less than 41%. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt;
The average r value of _{r m = (r L + 2r} D + r C) / 4 ... (a)
The total elongation of the average El _m = (El _L + 2 El _D + El _C ) / 4 ... (b)
Where r _L is the r value in the rolling direction, r _D is the r value in the rolling direction of 45 °, r _C is the r value in the direction perpendicular to the rolling direction, El _L is the elongation in the rolling direction, El _D is the elongation in the rolling direction El _C : Drawing degree in the direction perpendicular to the rolling direction The method according to claim 1,
Further comprising, in mass%, at least one of Ti and B in an amount of 0.02% or less of Ti and 0.005% or less of B, respectively. A steel slab having a composition according to claim 1 or 2 is used for hot rolling, followed by cooling to an average cooling rate of 20 DEG C / s or lower, coiling temperature: 0 DEG C or higher, and [{8 x (C content + N content 占 12/14)} - 1850 占 폚 - [{0.5 占 (C content + N content 占 12/14)} + 520 占 폚 (ppm) and N content (ppm)), cold rolling at a reduction ratio of 55% or more, and annealing at an annealing temperature of 650 ° C to 800 ° C. Gt; The method of claim 3,
Wherein the annealing is followed by an overexposure treatment at a temperature of 300 ° C to 400 ° C for 60s to 300s.

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