JP5549232B2 - Cold rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a cold-rolled steel sheet excellent in formability and shape freezing property, which is optimal as a member of a large flat plate-like component such as a backlight chassis of a large-sized liquid crystal television, and a method for manufacturing the same.

Thin flat panel TVs and office automation equipment use a large number of flat-plate parts formed by processing mainly bending and stretching. The members (materials) used for these parts often require flatness in addition to the degree of processing into the part shape from the viewpoints of product design and thickness reduction. However, flatness tends to deteriorate when bending / extrusion molding is performed on a flat plate surface of a member (material). Such deterioration of flatness is a phenomenon that occurs due to poor shape freezing property of the member (material) when the member (material) is press-molded. Shape freezing property is required.
Further, as a factor that deteriorates the flatness, ridge warpage that occurs during bending is well known. It is said that the warp that occurs at the bending end, which is one of them, is reduced by lowering the r value of the material. Conventionally, there is a technique for imparting a low r value and low yield strength to the material. Has been established.
For example, Patent Literature 1 establishes a technique for developing a cold-rolled steel sheet having a yield strength of 150 MPa and an r value of 0.67 in the rolling direction (a direction perpendicular to the rolling direction of 1.45).
Patent Document 2 discloses a technique for developing a steel sheet having excellent shape freezing property by adjusting a winding temperature.
Patent Document 3 discloses a ferritic steel sheet for automobiles excellent in shape freezing property, in which the ratio of {100} plane and {111} plane parallel to the plate surface is 1.0 or more.
In Patent Document 4, in order to obtain a ferritic steel sheet having excellent shape freezing property, the strength of {100} <011> to {223} <110> orientation group and {112} <110>, {554} <225>, {111} <112>, {111} <110>, controlling the strength of each direction, at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is 0.7 or less Is disclosed.

Japanese Patent No. 3532138 Japanese Patent No. 4126007 JP 2008-255491 A JP 2003-55739 A

However, in recent years, the processing of steel sheets has become more complicated with the increase in size and functionality of thin LCD TVs and office automation equipment. In order to cope with such a situation, it is essential to improve the shape freezing property of the member (material), and all of the rolling direction (L direction), the rolling 45 ° direction (D direction), and the direction perpendicular to the rolling direction (C direction). It is necessary to reduce the r value in the direction. For example, although the r value in the rolling direction and the direction perpendicular to the rolling direction is reduced by the crystal grains having the {100} <011> to {223} <110> orientations defined in Patent Documents 1 and 4, r in the rolling 45 ° direction is reduced. When the value is increased, it is not possible to improve the shape freezing property after linear overhang forming in which the rolling 45 ° direction is the longitudinal direction.
In Patent Document 2, the r value of the steel sheet is reduced by lowering the coiling temperature. However, excessive reduction of the coiling temperature reduces the total elongation, which is preferable from the viewpoint of preventing material cracking and fracture during molding. Absent.
In Patent Document 3, it is necessary to strictly control the amount of reduction and the friction coefficient in a predetermined hot rolling temperature range, and further, the coiling is defined by the composition of components in steel such as Mn, Si, P, and Al. Since it is necessary to control to temperature, it is difficult to manufacture stably. In addition, for cold-rolled steel sheet, a case where the cold rolling rate is substantially manufactured at about 53% is disclosed, but such a low cold rolling rate improves shape freezing property, but about 1.0 mm or less. It becomes difficult to manufacture the thin steel plate.

  In view of such circumstances, it is an object of the present invention to provide a cold-rolled steel sheet excellent in formability and shape freezing property and a manufacturing method thereof.

Inventors repeated earnest research investigation in order to solve the said subject.
As a result, the following points were found. In order to achieve both workability that does not cause warpage even when complicated processing is performed and high shape freezeability, it is important to lower the r value while maintaining high ductility in the annealed sheet after cold rolling. is there. That is, by increasing the average elongation, it is possible to ensure the shape required for the part while ensuring the workability when drawing or overhanging. Furthermore, by reducing the r value, it is possible to suppress the occurrence of springback and warping after processing and to secure the shape freezing property.
The above high ductility and low r value is a coiling temperature for the purpose of optimizing the amount of solid solution C and the amount of solid solution N contained in the hot rolled sheet structure and the steel after hot rolling in hot rolling. This is realized by controlling
This invention is made | formed based on the above knowledge, The summary is as follows.
[1] By mass%, C: 0.010% or more and less than 0.030%, Si: 0.05% or less, Mn: 0.3% or less, P: 0.05% or less, S: 0.02% or less, Al: 0.02% or more, 0.10% or less, N A cold-rolled steel sheet having a composition of 0.005% or less, the balance being iron and inevitable impurities, 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 [1], further comprising, in mass%, at least one of Ti and B within a range of Ti: 0.02% or less and B: 0.005% or less. .
[3] Hot rolling is performed using the steel described in [1] or [2], then cooled at an average cooling rate of 20 ° C./s or less, a winding temperature of 0 ° C. or more and [{ 8 x (C amount + N amount x 12/14)}-1850] ° C to [{0.5 x (C amount + N amount x 12/14)} + 520] (where C amount and N amount are C in steel) After rolling in the range of content (ppm), N content (ppm) ° C, cold rolling at a reduction rate of 55% or more, then annealing at 650 ° C to 800 ° C A method for producing a cold-rolled steel sheet.
[4] The method for producing a cold-rolled steel sheet according to [3], wherein after the annealing, an overaging 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 components of steel are mass% and mass ppm, respectively. Moreover, the cold-rolled steel sheet targeted by the present invention includes a steel sheet obtained by subjecting the cold-rolled steel sheet to surface treatment such as electrogalvanizing or hot dip galvanizing. Further, it includes a steel plate having a film formed thereon by chemical conversion treatment or the like.

  Further, the steel sheet of the present invention can be widely used for general household appliances such as backlight chassis of large TVs, refrigerator panels, and air conditioner outdoor units, which are bent, extended, and slightly drawn, etc. . Furthermore, if this invention is used, the backlight chassis about 650x500mm (32V type) or more can be manufactured, for example with a steel plate with a plate thickness of 0.8 mm.

  According to the present invention, it is possible to obtain a cold-rolled steel sheet that can obtain high elongation and low r value, and can be drawn, bent, and stretched, and has excellent formability and shape freezing property. Thereby, a flat plate shape required for a large component can be secured, and for example, a member such as a backlight chassis of a large liquid crystal television can be manufactured.

It is a figure which shows the relationship between addition C and N amount, appropriate coiling temperature (CT), the average total elongation (El), and average r value of a steel plate.

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

C: 0.010% or more and less than 0.030%
When C is 0.030% or more, the amount of precipitated carbide increases and ductility decreases. On the other hand, if it is less than 0.010%, the production cost is increased. Therefore, C is set to 0.010% or more and less than 0.030%.

Si: 0.05% or less
Si is a solid solution strengthening element that increases yield strength and degrades ductility. Therefore, it is 0.05% or less.

Mn: 0.3% or less
Mn is an element that forms sulfides and improves hot brittleness, but is also a solid solution strengthening element and increases yield strength and degrades ductility. Therefore, Mn is set to 0.3% or less.

P: 0.05% or less
P is a solid solution strengthening element that increases yield strength and degrades ductility. Therefore, it is 0.05% or less.

S: 0.02% or less
S forms a sulfide at the stage of hot-rolled sheet, and causes an increase in the anisotropy of the crystal structure after cold rolling annealing. Therefore, 0.02% or less.

Al: 0.02% to 0.10%
Al is a deoxidizing element and requires addition of 0.02% or more. On the other hand, if added excessively, it is finely precipitated during annealing and causes an increase in the anisotropy of the crystal structure, so the upper limit is made 0.10%.

N: 0.005% or less
N, when dissolved in steel, causes stretcher strain. Moreover, when it precipitates finely, it becomes a cause which the anisotropy of a crystal structure increases. From the above, N is preferably as small as possible, and is 0.005% or less.

  In addition to the above elements, the present invention can contain one or more of Ti and B within the ranges of Ti: 0.02% or less and B: 0.005% or less for the following purposes.

Ti: 0.02% or less
Ti has a strong affinity for N, and has the effect of reducing the adverse effects of N by forming precipitates at high temperatures. In order to obtain such an effect, the content is preferably 0.005% or more. On the other hand, excessive addition causes an increase in manufacturing cost. Based on the above, the content is 0.02% or less.

B: 0.005% or less
B is an interstitial solid solution element, and acts to lower the r value when dissolved in steel. Moreover, there exists an effect which fixes solid solution N as a precipitate and suppresses a stretcher strain. In order to acquire such an effect, it is preferable to contain 0.0002% or more. On the other hand, excessive addition leads to high yield strength and low ductility, so when it is contained, the content is made 0.005% or less.

  Components other than the above consist of iron and inevitable impurities. Examples of the inevitable impurities include 0.05% or less of Cu and Cr that are easily mixed from scrap, and 0.01% or less of Sn, Nb, Mo, W, V, Ni, and the like.

The cold rolled steel sheet of the present invention has an average r value of 1.2 or less in the rolling direction, the 45 ° direction, and the direction perpendicular to the rolling direction.
The warp generated in the bent part of the bending material increases as the r value increases. By setting the average r value to 1.2 or less, the shape freezing property after molding can be sufficiently improved.
The average r value can be measured and determined by the following method. JIS No. 5 tensile specimens are cut out from the rolling direction, the 45 ° direction and the direction perpendicular to the rolling direction, respectively, and a plastic strain ratio test based on JIS Z 2254 is performed at a pre-strain of 15%. And it calculates | requires by the following (a) formula.
Average r value r _m = (r _L + 2r _D + r _C ) / 4 (a)
Here, r _L : r value in rolling direction, r _D : r value in rolling 45 ° direction, r _C : r value in perpendicular direction of rolling The steel sheet of the present invention has an average total elongation of 41% or more.
In addition to the above characteristics, when the average total elongation is increased to 41% or more, drawing and overhanging can be performed, and the shape required for the part can be ensured. In the overhang forming, if the overhang height is excessively increased, the material is cracked. Increasing the ductility of the material is effective in preventing such cracking of the material and increasing the overhang forming height as much as possible. If the average total elongation is 41% or more as a steel sheet for home appliances and building materials, the shape required for the parts can be sufficiently ensured when processing is performed. Therefore, in the present invention, the average elongation is set to 41% or more. Preferably it is 44% or more.
The average total elongation can be measured and determined by the following method. A JIS No. 5 tensile test piece is cut out from the rolling direction, the 45 ° direction, and the direction perpendicular to the rolling direction, and a tensile test based on JIS Z 2241 is performed. And it calculates | requires by the following (b) formula.
Average total elongation El _m = (El _L + 2El _D + El _C ) / 4 (b)
Here, El _L : Elongation in the rolling direction, El _D : Elongation in the 45 ° direction of rolling, El _C : Elongation in the direction perpendicular to the rolling Next, the method for producing the steel sheet of the present invention will be described. In the present invention, a steel slab having the above composition is hot-rolled and then cooled at an average cooling rate of 20 ° C./s or less, and a winding temperature of 0 ° C. or more and [{8 × (C amount + N Amount × 12/14)} − 1850] ° C. to [{0.5 × (C amount + N amount × 12/14)} + 520] (where C amount and N amount are the C content (ppm) in steel, After rolling in the range of N content (ppm) ° C., cold rolling at a reduction ratio of 55% or more, and then annealing at an annealing temperature of 650 ° C. to 800 ° C., high total elongation, A low r value can be obtained.

Heating temperature: 1200 ° C or higher (preferred conditions)
In hot rolling, since it is necessary to once dissolve precipitates such as AlN during heating and finely precipitate after winding, the heating temperature for hot rolling is preferably 1200 ° C. or higher.

Finishing rolling finish temperature: 880 ° C or higher (preferred conditions)
If phase transition occurs in the ferrite region during finish rolling, the structure of the hot-rolled sheet becomes non-uniform and the material may become unstable. Therefore, the finish rolling is preferably performed in an austenite single phase, and the finish rolling finish temperature is preferably 880 ° C. or higher.

Average cooling rate: 20 ° C./s or less After finish rolling, cooling is performed at an average cooling rate: 20 ° C./s or less. In the case of air cooling (cooling), the cooling rate is about 10 ° C./s or less, so the average cooling rate is 20 ° C./s or less. Although water cooling can be performed, excessive cooling refines the crystal grains of the hot-rolled sheet, and accordingly, crystal grains after annealing become finer, the material strength increases, and ductility deteriorates. Therefore, an average cooling rate shall be 20 degrees C / sec or less.

Winding temperature (hereinafter sometimes referred to as CT): 0 ° C. or higher and [{8 × (C amount + N amount × 12/14)} − 1850] ° C. to [{0.5 × (C amount + N amount × 12 / 14)} + 520] (where C content and N content are C content (ppm) and N content (ppm) in steel) ° C
Next, the hot rolled plate is wound into a coil shape. The winding temperature at this time is 0 ° C or higher and [{8 × (C amount + N amount × 12/14)}-1850] ° C to [{0.5 × (C amount + N amount × 12/14)} + 520] ° C. To do. This coiling temperature range was obtained by analyzing the relationship between the average r value, average total elongation, C and N content, and CT based on the results obtained by experiments by the inventors. The reason for analyzing the relationship between the average r value, the average total elongation, the C and N contents, and CT is because of the following consideration. That is, when the coiling temperature is too high, cementite and AlN are coarsely precipitated, and the growth of recrystallized grains, which is disadvantageous for reducing the r value, is promoted during annealing. In addition, the amount of solute C and solute N in the hot-rolled steel sheet, which generates a recrystallization orientation that is advantageous for reducing the r value during annealing, is reduced. On the other hand, if the coiling temperature is too low, the amount of dissolved C and N will increase excessively, creating many deformation bands during cold rolling, and increasing the number of recrystallized grains that are generated from annealing, thereby annealing annealing. The crystal grain size becomes finer and ductility decreases. Furthermore, when the crystal grain size of the hot-rolled sheet is reduced, the crystal grain size of the annealed sheet is refined and the ductility is lowered. Further, in order to prevent the cooling water from solidifying and damaging the surface of the hot rolled plate, the temperature is set to 0 ° C. or higher. In order to obtain an average r value of 1.2 or less and an average total elongation of 44%, the coiling temperature is 500 ° C. or higher and [{8 × (C amount + N amount × 12/14)} − 1850] ° C. It is preferable to set to [{0.5 × (C amount + N amount × 12/14)} + 520] ° C.
As described above, the coiling temperature is important for coarsening the crystal grain size during coil winding and at the same time agglomerating carbonitride and optimizing the amount of solute C and N in the hot rolled sheet. It is an important requirement in the present invention to control the coiling temperature in order to obtain a cold-rolled steel sheet having excellent properties and shape freezing properties.

Rolling ratio during cold rolling (cold rolling ratio): After winding 55% or more, pickling is performed by a normal method, cold rolling is performed at a rolling reduction of 55% or more, and a desired sheet thickness is formed. A lower rolling reduction is preferable for lowering the r value, but if it is excessively low, sufficient driving force for recrystallization cannot be obtained, resulting in a structure in which strain remains even after annealing, resulting in higher strength and lower ductility. In addition, when trying to finish the plate to a thickness of about 1.0 mm or less, the hot-rolled plate must be made excessively thin, resulting in an increase in hot-rolling cost. From the above, the rolling reduction is 55% or more. If the rolling reduction is too high, the rolling load becomes too large and rolling may be difficult and the production efficiency may be reduced. Therefore, the rolling reduction is preferably about 85% or less.

Annealing temperature: 650-800 ° C
Grain growth is promoted by annealing at an annealing temperature of 650 ° C or higher and 800 ° C or lower. If the annealing temperature is excessively high, AlN dissolves and solid solution N appears in the steel, causing stretcher strain. On the other hand, if it is too low, the strain introduced during cold rolling is not released, leading to high YP and low ductility. Therefore, the annealing temperature is set to 650 ° C. or higher and 800 ° C. or lower.

Holding time: 30-200s (preferred conditions)
If the time for maintaining the annealing temperature is short, recrystallization will not be completed, or even if it is completed, grain growth will be suppressed and elongation will be reduced. Further, the diffusion of the solid solution N becomes insufficient, the solid solution N remains in the ferrite grains, the yield strength increases, and the elongation decreases. Therefore, the holding time during annealing is preferably 30 s or longer. On the other hand, if the soaking time is long, the grains grow excessively and become large, which causes a problem of rough skin at the time of processing, resulting in poor surface properties. Therefore, the soaking time during heating is preferably set to 200 s or less.

In order to precipitate solid solution C and improve the total elongation, it is preferable to perform an overaging treatment at 300 ° C. to 400 ° C. for about 60 to 300 seconds in the cooling process after annealing.
The over-aging temperature is set to 400 ° C or lower to increase the driving force of cementite precipitation and promote precipitation, and the temperature to 300 ° C or higher is to promote diffusion of Fe and C and precipitation of cementite. .
Further, the overaging treatment time is less than 60 seconds, the effect is small, and even if it exceeds 300 seconds, the effect is saturated and the production efficiency is lowered.
After annealing, temper rolling and leveling may be performed for the purpose of correcting the plate shape. Further, the surface of the steel plate may be plated with an element that improves corrosion resistance such as zinc, chromium, or nickel, or chemical conversion treatment may be performed to improve the corrosion resistance or slidability.

In carrying out the present invention, a melting method can be appropriately applied, such as a normal converter method or an electric furnace method. The molten steel is cast into a slab and then heated as it is or after cooling and hot rolling. In hot rolling, the steel sheet is cooled at the above cooling rate and wound at the above winding temperature. Then, after the usual pickling, the above-mentioned cold rolling and annealing are performed. If necessary, plating with hot dip zinc may be performed at around 480 ° C. Further, after plating, the plating may be alloyed by reheating to 500 ° C. or higher. Alternatively, a heat history such as holding during cooling may be taken. Furthermore, if necessary, temper rolling may be performed at an elongation rate of about 0.5 to 2%. In addition, when plating is not performed during annealing, electrogalvanization or the like may be performed in order to improve corrosion resistance. Further, a film may be formed on the cold-rolled steel plate or the plated steel plate by chemical conversion treatment or the like.
As described above, a cold-rolled steel sheet having excellent formability and shape freezing property is obtained.

After the slab having the chemical composition shown in Table 1 is melted, it is hot-rolled at the finishing temperature (FT) shown in Table 2, cooled under the conditions shown in Table 2, and the coiling temperature (CT) shown in Table 2 The winding process was performed. Next, after pickling the obtained hot-rolled sheet, it was cold-rolled at a reduction rate of 70%, and annealed at the annealing temperature (AT) shown in Table 2 in the range of 730 ° C to 800 ° C. Samples were prepared by temper rolling at an elongation rate of%. The plate thickness after cold rolling was 0.6 to 0.8 mm, and the holding time during annealing was 50 to 200 s. In addition, some steel plates were over-aged under the conditions shown in Table 2 after annealing.
The average r value and average total elongation (El) were measured for the specimens obtained as described above. The measuring method is to cut out JIS No. 5 tensile specimens from the rolling direction (L direction), rolling 45 ° direction (D direction), and perpendicular direction (C direction) of the specimen, A plastic strain ratio test based on JIS Z 2254 was performed at a pre-strain of 15%. And average r value and average total elongation (El) were calculated | required by the above-mentioned (a) Formula and (b) Formula.
The results obtained are shown in Table 2 together with the conditions. Fig. 1 shows the relationship between the amount of C and N, the appropriate coiling temperature (CT), the average total elongation (El) of the steel sheet, and the average r value.
In Table 2, as the winding conditions, the winding temperature is 0 ° C. or higher and [{8 × (C amount + N amount × 12/14 (ppm))} − 1850] ° C. to [{0.5 × (C amount + N (Quantity × 12/14 (ppm))} + 520] ° C. is indicated by Δ, the case where 500 ° C. or more is satisfied is indicated by ○, and the case where these are not satisfied is indicated by ×.

  According to Table 2, the steel sheet having the composition of the present invention and manufactured by the manufacturing 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, a steel sheet whose composition is outside the scope of the present invention or whose manufacturing method is outside the scope of the present invention even if the composition is within the scope of the present invention is inferior in either El or average r value. It was.

  Further, in FIG. 1, an appropriate winding temperature range described in the present invention is surrounded by a dotted line in the figure. By manufacturing within this range, a steel sheet having an average total elongation (El) of 41% or more and an average r value of 1.2 or less can be obtained. Furthermore, by setting the coiling temperature to 500 ° C. or more and the C content + N content × 12/14 to 300 ppm or less, a steel sheet having an average total elongation (El) of 44% or more can be obtained.

  As a result of the above, the cold rolled steel sheet of the present invention is controlled in average total elongation and average r value, and is excellent in formability and shape freezeability.

Claims (4)

By mass%, C: 0.010% or more and less than 0.030%, Si: 0.05% or less, Mn: 0.3% or less, P: 0.05% or less, S: 0.02% or less, Al: 0.02% or more, 0.10% or less, N: 0.005% In the following, the balance has a composition of iron and inevitable impurities,
The average r value (r _m ) represented by the following formula (a) is 1.2 or less,
Cold-rolled steel sheet, wherein a total elongation of average (El _m) is 41% or more and represented by the following formula (b).
r _m = (r _L + 2r _D + r _C ) / 4 (a)
Here, r _L : r value in rolling direction, r _D : r value in rolling 45 ° direction, r _C : r value in rolling perpendicular direction.
El _m = (El _L + 2El _D + El _C ) / 4 (b)
Here, El _L : elongation in the rolling direction, El _D : elongation in the 45 ° direction of rolling , El _C : elongation in the direction perpendicular to the rolling.   The cold-rolled steel sheet according to claim 1, further comprising, in mass%, at least one of Ti and B within a range of Ti: 0.02% or less and B: 0.005% or less. Using the steel slab having the composition according to claim 1 or 2, hot rolling is performed, and then cooling is performed at an average cooling rate of 20 ° C./s or less, a winding temperature of 0 ° C. or more and [{ 8 x (C amount + N amount x 12/14)}-1850] ° C to [{0.5 x (C amount + N amount x 12/14)} + 520] (where C amount and N amount are C in steel) After rolling in the range of content (ppm), N content (ppm) ° C, cold rolling at a reduction rate of 55% or more, then annealing at 650 ° C to 800 ° C A method for producing a cold-rolled steel sheet.   The method for producing a cold-rolled steel sheet according to claim 3, wherein after the annealing, an overaging treatment is performed at a temperature of 300 ° C to 400 ° C for 60 s to 300 s.

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