JP2011144427A - Cold rolled steel sheet excellent in formability and shape-fixability after aging, and producing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet excellent in a formability and a shape-fixability after aging, and a producing method therefor. <P>SOLUTION: The cold-rolled steel sheet has a composition containing 0.01-0.05% C, &le;0.05% Si, 0.1-0.5% Mn, &le;0.05% P, &le;0.02% S, 0.02-0.10% Al, &le;0.005% N and the balance Fe with inevitable impurities, and has a structure consisting mainly of a ferrite-phase, wherein the average grain diameter of the ferrite-phase is 10-20 &mu;m and the standard deviation &sigma;<SB>A</SB>of natural logarithm to the value of the individual ferrite grain diameter divided with average value is &ge;0.30. In order to obtain the steel sheet, when an annealing is applied after cold-rolling, the steel sheet is heated with 1-30&deg;C/s the average heating speed in the temperature range from 600&deg;C to the soaking temperature, and is subjected to soaking treatment at the soaking temperature of 800-900&deg;C, and for the soaking time of 30-200s, and is cooled at the average cooling speed of 3-30&deg;C/s in the temperature region from the soaking temperature to 550&deg;C, and held at 500-300&deg;C for &ge;30s, and the strain of elongation of 0.5-2.0% is applied at the room temperature. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a cold-rolled steel sheet excellent in formability and shape freezing property after aging, 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 manufacturing method thereof.

  Thin flat panel TVs and office automation equipment use a large number of flat-plate parts formed by processing mainly bending and stretching. And in manufacturing the member (thin steel plate) used for these parts, in order to correct the shape of the plate or to eliminate the yield point elongation, a light reduction of about several percent elongation is performed in the temper rolling. . However, when time elapses after temper rolling, a characteristic called so-called strain aging is deteriorated, such as recovery of yield point elongation and reduction of ductility. In particular, in recent years, in order to reduce costs, the number of cases in which coils are exported and pressed overseas has become extremely large, and since it takes time from coil manufacturing to pressing, strain aging has become unavoidable. Yes. Further, even in Japan, steel sheets are strain-aged when it takes time in the coil distribution process or when the coils are stocked. Thus, when strain aging occurs and the characteristics of the steel sheet deteriorate, it is necessary to readjust the pressing conditions and the mold, which is one of the causes of cost increase.

  Furthermore, recently, there is a great demand for reducing the amount of steel sheets used by reducing the thickness of the members in order to reduce costs. When the plate thickness is reduced, problems such as deterioration of shape freezing property during processing and occurrence of cracks during processing are likely to occur. Furthermore, in order to compensate for the decrease in part rigidity due to thinning, the shape of the part may be changed, such as adding a bead or being close to the closed cross-sectional structure by bending, etc., and the processing conditions become increasingly severe. As a result, cracking and shape defects during pressing are promoted. In particular, in the case of bending, a shape defect called a ridge line warp occurs, causing problems such as part warping. Further, in the case of overhang processing, problems such as cracks occur when the overhang height is large, and wrinkles occur when wrinkle suppression is weak.

  It is advantageous to lower the r value for such ridge warpage. However, lowering the r value causes a decrease in elongation, which is disadvantageous for overhanging. Furthermore, when the yield point elongation is revived or the ductility is lowered due to strain aging, it becomes impossible to cope with cracks, wrinkles, etc. by changing the press conditions.

Strain aging is known to be caused by C and N dissolved in the steel sheet, and IF steel with C and N added as a precipitate by adding carbonitride-generating elements such as Ti and Nb Known as a steel plate that is less susceptible to strain aging. However, the conventional IF steel has a high manufacturing cost and a high r value, which is disadvantageous when it includes bending.
From the above, there is a great demand for an inexpensive member (thin steel plate) having a low r value, low yield point elongation and high elongation even after aging.

As a steel sheet having a low r value and excellent shape freezing property, for example, Patent Document 1 discloses that in finish rolling in hot rolling, the reduction ratio of Ar3 to (Ar3 + 100) is 25% or more and the friction coefficient during rolling is as follows. Finish rolling with Ar3 or less at 0.2 or less, or finish rolling with a rolling reduction of Ar3 or less at 25% or more and a friction coefficient during rolling of 0.2 or less, thereby controlling the texture and rolling in the rolling direction. A steel sheet is disclosed in which at least one of r values in the perpendicular direction is 0.7 or less.
Patent Document 2 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 3, 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 JP 2008-255491 A JP 2003-55739 A

  However, the steel sheets described in Patent Documents 1 to 3 have poor workability after aging, and problems such as press cracking occur.

  In view of such circumstances, an object of the present invention is to provide a cold-rolled steel sheet excellent in formability and shape freezing property after aging, and a method for producing the same.

Inventors repeated earnest research investigation in order to solve the said subject.
As a result, the average r value in the rolling direction, the 45 ° direction of rolling, and the direction perpendicular to the rolling direction is 1.2 or less, the elongation after aging is 40% or more, and the yield point elongation after aging is 1.0% or less. It has been found that a cold-rolled steel sheet having excellent formability and shape freezing property can be obtained. Here, the average r value _{(r m)} is the rolling direction, the rolling direction of 45 °, respectively r values in the rolling direction perpendicular when the _{r L, r D, r C} , r m = (r _L + 2r _D + r _C ) / 4.
Moreover, the mechanism which can ensure the moldability and shape freezing property after aging by this invention is considered as follows. In general, in order to eliminate the yield point elongation, a method of introducing strain at a room temperature and applying strain is employed. However, when the amount of strain is small, the movable dislocation is fixed by C and N due to aging, and the yield point elongation is restored. On the other hand, when the amount of strain at room temperature is increased, the yield point is increased and the elongation is reduced, so that the formability is lowered. Therefore, in the present invention, attention is paid to the distribution of the ferrite particle diameter. By increasing the distribution of the ferrite grain size, even with a small amount of strain, the strain can be concentrated by making the strain introduction position non-uniform. As a result, yield point elongation can be suppressed even after aging. In addition, since the grains with little strain introduction have little hardening due to aging, a decrease in elongation can be suppressed. Such non-uniform introduction of strain can be achieved by increasing the standard deviation of the ferrite grain size distribution.
In addition, the cold rolled steel sheet having an r value of 1.2 or less, an elongation after aging of 40% or more, and a yield point elongation after aging of 1.0% or less has finished finish rolling in the ferrite region in hot rolling. It is obtained by unwinding at a low temperature so that it is not recrystallized in the hot rolling stage, and controlling the thermal history during annealing to control the ferrite grain size and grain size distribution and the strain after cooling.
This invention is made | formed based on the above knowledge, The summary is as follows.
[1] By mass%, C: 0.01 to 0.05%, Si: 0.05% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.02 to 0.10%, N: 0.005% In the following, the balance is iron and inevitable impurities and a structure mainly composed of ferrite phase, and the average particle diameter of the ferrite phase is 10 to 20 μm, and the individual ferrite particle diameters are divided by the average value. _A cold-rolled steel sheet having excellent formability and shape freezing property after aging, wherein σ _A ≧ 0.30, where σ _A is the standard deviation of natural logarithm.
[2] Formability and shape freezing after aging characterized in that, in [1], further containing at least one of Ti: 0.005 to 0.02% and B: 0.0003 to 0.0030% by mass% Cold-rolled steel sheet with excellent properties.
[3] A cold-rolled steel sheet excellent in formability and shape freezing property after aging, characterized in that in [1] or [2] above, a zinc-based plating layer is provided on the steel sheet surface.
[4] A steel slab having the composition described in [1] or [2] above is hot when the final delivery temperature of finish rolling is (Ar3-100 ° C) to Ar3 ° C, and the coiling temperature is less than 550 ° C. After rolling, pickling, cold rolling at a rolling reduction of 40-80%, and annealing, the temperature range from 600 ° C to the soaking temperature is 1-30 ° C / s average heating Heating at a rate, soaking temperature is 800-900 ° C, soaking time is 30-200s, temperature range from the soaking temperature to 550 ° C at an average cooling rate of 3-30 ° C / s A method for producing a cold-rolled steel sheet having excellent formability and shape freezing property after aging, characterized by cooling, holding at 500 to 300 ° C. for 30 seconds or more, and applying a strain of 0.5 to 2.0% elongation at room temperature.
[5] In the above [4], after the soaking, the temperature range from the soaking temperature to 550 ° C. is cooled at an average cooling rate of 3 to 30 ° C./s, and subsequently cooled to a temperature range of 500 ° C. or less. Then, it is reheated to a temperature range of 500 to 550 ° C., then held at 500 to 300 ° C. for 30 s or more, and an elongation rate of 0.5 to 2.0% is applied at room temperature. A method for producing a cold-rolled steel sheet having excellent freezing properties.
In addition, in this specification,% which shows the component of steel is mass% altogether. Further, the cold-rolled steel sheet targeted by the present invention includes a steel sheet for subjecting the cold-rolled steel sheet to a zinc-based plating treatment (for example, electrogalvanizing treatment, hot-dip galvanizing treatment, alloying hot-dip galvanizing treatment). . 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 as a general member for home appliances that bends, projects, and slightly draws flat surfaces such as a large-screen TV backlight chassis, refrigerator panel, and air conditioner outdoor unit. . Furthermore, if this invention is used, the backlight chassis about 650x500mm (32V type) or more can be manufactured, for example with the steel plate of plate thickness 0.8mm.

  According to the present invention, a cold-rolled steel sheet excellent in formability after aging and shape freezing property can be obtained. Thereby, a flat plate shape required for a large component can be secured, and a member such as a backlight chassis of a large liquid crystal television can be manufactured.

It is a figure which shows the influence of (sigma) _{A on} the yield elongation (YP-El) and elongation (El) after aging. It shows the effect of σ on _A (the finish rolling final delivery temperature (FT) -Ar3). It is a figure which shows the influence of coiling temperature (CT) which has on (sigma) _A.

  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: 0.01-0.05%
C forms cementite, can reduce solute C, and can reduce yield strength. When there is little C, the formation of cementite is suppressed, and solid solution C increases, making it easier to age harden, and in hot rolling, when transforming from austenite to ferrite in the finishing stand, the two-phase region is small In addition, the deformation resistance rapidly decreases, and the rolling becomes unstable. Therefore, C needs to be 0.01% or more. On the other hand, when C is increased, the grain growth is suppressed and the grain size is reduced, so that the steel sheet is hardened and the elongation is lowered. Therefore, C needs to be 0.05% or less.

Si: 0.05% or less
When Si is added in a large amount, the formability deteriorates due to hardening, and the plating property is hindered due to the formation of Si oxide during annealing. Therefore, Si needs to be 0.05% or less.

Mn: 0.1-0.5%
Mn needs to be 0.1% or more in order to detoxify harmful steel S as MnS. On the other hand, a large amount of Mn deteriorates formability due to hardening by solid solution strengthening or generation of a low temperature transformation phase. In addition, Mn lowers the transformation point and makes rolling in the ferrite region in hot rolling difficult. In addition, the structure becomes finer by suppressing the recrystallization of ferrite during annealing. Therefore, Mn needs to be 0.5% or less, preferably 0.3% or less.

P: 0.05% or less
P segregates at the grain boundary and deteriorates ductility and toughness, so it is necessary to make it 0.05% or less. Preferably it is 0.03% or less.

S: 0.02% or less
S significantly decreases the hot ductility, thereby inducing hot cracking and remarkably deteriorating the surface properties. Further, S hardly contributes to the strength, but also reduces the ductility by forming coarse MnS as an impurity element. These problems become significant when the S content exceeds 0.02%, and it is desirable to reduce them as much as possible. Therefore, the amount of S needs to be 0.02% or less.

Al: 0.02-0.10%
Al can suppress age hardening due to solute N by fixing N as a nitride. In order to obtain such an effect, Al needs to be 0.02% or more. On the other hand, the addition of a large amount of Al not only increases strength and decreases formability, but also increases costs. Therefore, Al needs to be 0.10% or less.

N: 0.005% or less
When N is contained in a large amount, there is a risk that surface flaws occur due to slab cracking during hot rolling. Moreover, when it exists as solid solution N after cold rolling and annealing, age hardening will be caused. Therefore, N must be 0.005% or less.

  In addition to the above elements, in the present invention, for the purpose of improving aging and shape freezing property, at least one of Ti and B is within the range of Ti: 0.005 to 0.02% and B: 0.0003 to 0.0030%. Can be contained.

Ti: 0.005-0.02%
Ti combines with N at high temperatures to form nitrides, and aging can be improved by reducing the solid solution N. In order to obtain such an effect, Ti needs to be 0.005% or more. On the other hand, if the content of Ti is large, it further combines with C to produce carbides and carbonitrides, so the strength increases and the formability decreases. Therefore, when it contains Ti, it is 0.005% or more and 0.02% or less.

B: 0.0003 to 0.0030%
B combines with N at a high temperature to form a nitride, and the aging can be improved by reducing the solid solution N. Further, B can improve the shape freezing property by suppressing the grain growth of ferrite in the annealing process after cold rolling and controlling the r value. In order to obtain such an effect, B needs to be 0.0003% or more. On the other hand, when B is present in a large amount, the recrystallization of ferrite during annealing is suppressed, and the structure becomes finer. Therefore, when it contains B, it is 0.0003% or more and 0.0030% or less.

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

The structure of the steel sheet of the present invention is mainly composed of a ferrite phase. The average particle size of the ferrite phase is 10 to 20 μm. Furthermore, σ _A ≧ 0.30, where σ _A is the standard deviation of the natural logarithm of the value obtained by dividing the individual ferrite grain size by the average value.
In order to ensure moldability, a soft ferrite phase is mainly used. Here, “mainly composed of a ferrite phase” refers to a case where the ratio of the ferrite phase to the entire structure is 95% or more in terms of area ratio. By mainly using a ferrite structure, it is possible to achieve an elongation after aging of 40% or more. It is preferable that the ferrite structure is 100% because elongation is improved. The second phase other than the main phase is a cementite phase or a pearlite phase, and can be contained in an area ratio of 5% or less. When the content exceeds 5%, the ductility is significantly reduced. The area ratio of the ferrite phase can be obtained by image processing after identifying the ferrite phase and the other phases by structure observation.
The average particle diameter is 10 μm or more in order to ensure moldability. On the other hand, when the particle size is increased, appearance defects such as orange peel are generated during molding and the particle size distribution is reduced, so the upper limit of the average particle size is 20 μm. The average particle diameter is measured by a cutting method, and is calculated by 2 / [(1 / Ll) + (1 / Lc)] from the average section lengths Ll and Lc in the rolling direction and the plate thickness direction.
In the present invention, by increasing the distribution of the ferrite grain size, even with a small amount of strain, the strain introduction position is made non-uniform so that the strain is concentrated, and the occurrence of yield point elongation is suppressed even after aging. In addition, since the grains with less strain introduction are less hardened by aging, they suppress the decrease in elongation. For that purpose, when the standard deviation of the natural logarithm of the value obtained by dividing the individual ferrite grain size by the average value is σ _A , it is necessary to satisfy σ _A ≧ 0.30. This will be described below.
Considering the actual use of steel sheets, it is sufficient to consider 6 months as the aging period at room temperature (20 ° C). FIG. 1 shows the effect of σ _{A on} yield elongation (YP-El) and elongation (El) when aged at 20 ° C. for 6 months. FIG. 1 shows C: 0.01 to 0.05%, Si: 0.05% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.02 to 0.10%, N: 0.005% or less. And the balance is iron and inevitable impurities, the ratio of the ferrite phase is 95% or more in area ratio, and the average diameter of the ferrite phase is 10 to 20 μm. JIS No. 5 tensile test piece was measured. Here, by setting the yield elongation after aging to 1.0% or less, wrinkles after molding can be eliminated or suppressed to a level that is hardly discernable visually. Further, by setting the elongation after aging to 40% or more, the wall angle at the time of overhanging molding can be molded without cracking to about 45 °, and it can cope with most press molding. As shown in FIG. 1, by setting σ _A to 0.30 or more, the yield elongation can be reduced to 1.0% or less and the elongation can be increased to 40% or more. Therefore, σ _A is set to 0.30 or more.

  Next, manufacturing conditions for the steel sheet of the present invention will be described. In the present invention, the steel slab having the above composition is hot-rolled at a final delivery temperature of finish rolling (Ar3-100 ° C) to Ar3 ° C, a winding temperature of less than 550 ° C, and then pickled. After performing cold rolling at a rolling reduction of 40 to 80%, and annealing, the temperature range from 600 ° C to the soaking temperature is heated at an average heating rate of 1 to 30 ° C / s, and the soaking temperature. Is soaked at 800-900 ° C, soaking time is 30-200s, and the temperature range from the soaking temperature to 550 ° C is cooled at an average cooling rate of 3-30 ° C / s, and at 500-300 ° C for 30s. By maintaining the above and applying a strain of 0.5 to 2.0% at room temperature, the ferrite particle size distribution can be increased, and a low yield point strength after aging, a low r value and excellent elongation can be obtained.

Finishing finish temperature: (Ar3-100 ℃) ~ Ar3
By finishing the finish rolling in the hot rolling in the ferrite region, strain accumulates in the ferrite structure and the recovery proceeds non-uniformly depending on the crystal orientation. As a result, the accumulation of strain becomes non-uniform, and the ferrite grain size distribution after annealing can be increased. Further, nonuniformity due to crystal orientation can randomize the development of texture, lower the r value, and improve the shape freezing property. Therefore, the final temperature of finish rolling needs to be Ar3 or less. More preferably, the temperature is lower than Ar3. The amount of reduction at Ar 3 or lower is not particularly limited, but is preferably 10% or more, more preferably 20% or more. On the other hand, when the finish rolling finish temperature is lowered, the recovery of crystals introduced with strain does not progress and the accumulation of strain does not become uneven. Furthermore, the rolling load is increased, which causes operational difficulties. Therefore, the finish rolling finish temperature needs to be (Ar3-100 ° C.) or higher.
Ar3 can be obtained by the following equation.
When Mn content <0.4%: Ar3 = 880-1000 x C content (%)
When Mn content ≧ 0.4%: Ar3 = 870−1000 × C content (%)
Winding temperature: less than 550 ° C If the winding temperature after finish rolling is high, ferrite will recrystallize and it will not be possible to introduce non-uniform strain, so the winding temperature should be less than 550 ° C. There is. The lower limit of the coiling temperature is not particularly specified, but if the temperature is too low, the coil winding shape deteriorates, so 300 ° C. or higher is preferable. The cooling rate from the finish rolling to the winding is not particularly specified, but is preferably 10 ° C./S or more, more preferably 30 ° C./s or more, and further preferably 100 ° C./s or more.

Rolling ratio during cold rolling: 40-80%
When the rolling reduction in cold rolling after pickling hot-rolled sheet is large, the introduction of strain is made uniform, the ferrite grain size distribution after annealing is reduced, and the strength is increased by refinement by increasing the amount of strain. And formability is reduced. In addition, as the texture develops, the r value increases and the shape freezing property decreases. From the above, the rolling reduction needs to be 80% or less. On the other hand, when the rolling reduction is small, the amount of strain introduced is small, so that recrystallization at the time of annealing is suppressed and a recovery structure is formed, so that formability is lowered. Therefore, the rolling reduction needs to be 40% or more.

Average heating rate from 600 ° C to soaking temperature: 1-30 ° C / s
After cold rolling, annealing is performed. In the present invention, the ferrite grain size and grain size distribution are controlled by controlling the thermal history during annealing, and the strain after cooling is controlled. Therefore, the manufacturing conditions for annealing are an important requirement.
When the average heating rate from 600 ° C. to the soaking temperature is small, the recovery proceeds and recrystallization is suppressed. Therefore, the average heating rate needs to be 1 ° C./s or more. On the other hand, if the average heating rate is high, recrystallization nucleation during heating is suppressed, and nucleation occurs simultaneously during soaking, resulting in finer grains. Therefore, the average heating rate needs to be 30 ° C./s or less.

Soaking temperature: 800-900 ° C, Soaking time: 30-200s
In the soaking process after heating, it is necessary to complete recrystallization and increase the grain size to improve the formability. Therefore, the soaking temperature needs to be 800 ° C. or higher. On the other hand, if the soaking temperature is too high, the transformation from ferrite to austenite proceeds, and the particle size becomes small due to reverse transformation after cooling. Therefore, the soaking temperature needs to be 900 ° C. or less.
Further, if the soaking time is short, recrystallization will not be completed, or even if it is completed, the time for grain growth is short, so that the particles are finely granulated and the moldability is lowered. Therefore, the soaking time during heating needs to be 30 seconds or longer. On the other hand, when the soaking time is lengthened, large grains grow while eroding the small grains and become larger, so the ferrite grain size distribution becomes smaller and the grain size becomes larger, so that the appearance such as orange peel at the time of press molding Bring about defects. Therefore, the soaking time needs to be 200 s or less.

Average cooling rate in the temperature range from soaking temperature to 550 ° C: 3-30 ° C / s
When the cooling rate after soaking is small, the growth of ferrite grains is promoted, and large grains grow and grow while eroding the small grains, so the ferrite grain size distribution becomes smaller and the grain size becomes larger. In press molding, it causes poor appearance such as orange peel. Therefore, the average cooling rate in the temperature range from the soaking temperature to 550 ° C. needs to be 3 ° C./s or more. On the other hand, if the cooling rate is too large, the strength increases and the moldability decreases, so the average cooling rate needs to be 30 ° C./s or less.
In addition, what is necessary is just to cool suitably according to a manufacturing facility during the holding | maintenance to 500-300 degreeC after cooling from above-mentioned soaking to 550 degreeC. Preferably, after cooling from the soaking temperature to 550 ° C., cooling is continued in the same cooling rate range, that is, 3 to 30 ° C./s.

Holding time at 500 to 300 ° C .: 30 s or longer Solid solution C is precipitated as cementite, thereby improving aging properties. Therefore, it is necessary to hold for 30 seconds or more in a temperature range of 300 to 500 ° C. at which cementite is likely to precipitate. Although the upper limit of the time is not particularly defined, it is preferable that the upper limit of the holding time is about 300 s because holding for a long time decreases the production efficiency.
In addition, although it cools to room temperature after holding | maintenance, the said cooling conditions do not need to prescribe | regulate in particular and should just carry out suitably according to manufacturing equipment.

Strain application at room temperature, elongation: 0.5-2.0%
After annealing, the yield point can be eliminated by applying strain at room temperature. Therefore, the strain applied at room temperature must be 0.5% or more in terms of elongation. On the other hand, when the elongation increases, the yield point rises and the formability decreases, so it is necessary to make it 2.0% or less. Preferably it is 1.5% or less. The application of strain at room temperature may be rolling with a roll or tension, or a combination of roll and tension. In rolling, it may or may not be lubricated.

In carrying out the present invention, a melting method can be appropriately applied, such as a normal converter method and 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, after finishing under the above-mentioned finishing conditions, winding is performed at the above-described winding temperature. Then, after the usual pickling, the above-mentioned cold rolling is performed. About the annealing process after cold rolling, heating, holding | maintenance, and cooling are performed on the above-mentioned conditions. If necessary, plating with molten zinc may be performed at around 480 ° C. Further, after plating, the plating may be alloyed by reheating to 500 ° C. or higher. When reheating is performed, the temperature should be 550 ° C. or lower so that ferrite does not grow. In addition, regarding the above-described holding at 500 to 300 ° C., cementite may be dissolved by reheating to 500 ° C. or higher, so when reheating to 500 ° C. or higher and 550 ° C. or lower, The holding time at 300 ° C. is preferably 30 seconds or longer. The lower limit of the plating bath temperature is about 460 ° C. That is, when hot dip galvanization is performed after the soaking and further alloying is performed, the heat history may be as follows. After the soaking, the temperature range from the soaking temperature to 550 ° C. is cooled at an average cooling rate of 3 to 30 ° C./s, subsequently cooled to a temperature range of 500 ° C. or less, and hot dip galvanized, and then 500 It is reheated to a temperature range of ˜550 ° C. and subjected to alloying treatment, and then held at 500 to 300 ° C. for 30 s or more. The holding time is preferably about 300 s for the same reason as described above. In addition, what is necessary is just to cool suitably to room temperature after holding | maintenance. Furthermore, temper rolling is performed at an elongation of about 0.5 to 2.0%. Preferably it is 0.5 to 1.5%. 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.
From the above, a cold-rolled steel sheet having excellent formability and shape freezing property after aging can be obtained. The cold-rolled steel sheet obtained as described above has an average r value of 1.2 or less in the rolling direction, the 45 ° direction of rolling, and the direction perpendicular to the rolling, an elongation after aging of 40% or more, and an elongation at yield point after aging of 1.0%. It is as follows. These characteristics are the average r-value, elongation, and yield point elongation after aging treatment at 20 ° C. for 6 months.
The r value correlates with the warp that occurs after bending. In bending, the r-value in the bending direction increases, so that saddle-shaped warpage occurs remarkably along the bending line. Therefore, for the purpose of improving the shape freezing property after press forming by lowering the r value, the average r value is set to 1.2 or less in the present invention.
Elongation has a good correlation with moldability, and the larger the elongation, for example, the higher the stretch molding. Therefore, the larger the required elongation, the better, and by setting the elongation after aging to 40% or more, drawing or overhanging can be performed, and the shape required for the part can be ensured.

  In addition to the above, the steel sheet of the present invention has a yield point elongation after aging of 1.0% or less. By reducing the yield point elongation after aging as well as immediately after the production of the steel sheet, it is possible to suppress the stretcher strain after molding and to produce a molded product having an excellent surface appearance.

After melting the slab having the chemical composition shown in Table 1, reheat and hot-roll at the final delivery temperature (FT) shown in Table 1, and after cooling at an average cooling rate of 10 ° C / s, The winding process was performed at the winding temperature (CT) shown in Table 1. Next, pickling was performed, and cold rolling was performed at the rolling reduction shown in Table 1, and annealing was performed under the conditions shown in Table 1. Next, reduction was performed at room temperature at the elongation shown in Table 1 to produce a test material.
In Table 1, the average heating rate from 600 ° C to the soaking temperature is HR, the soaking temperature is AT, the soaking time is Ht1, the average cooling rate from the soaking temperature to 550 ° C is CR, 500 ° C to 300 ° C. The residence time at ° C. was Ht2. Sample No. 4 was plated with hot galvanized at 480 ° C. midway, and the surface was hot dip galvanized (GI). Specimen No. 3 was plated with hot dip zinc at 480 ° C., then reheated to 540 ° C., and the surface was alloyed hot dip galvanized (GA). Sample No. 2 was electroplated and the surface was electroplated (EG). The samples other than sample No. 4 were continuously cooled from 550 ° C. to 500 ° C. at the same cooling rate as CR shown in Table 1.
The structure and mechanical properties of the specimens obtained as described above were investigated. The structure was obtained by observing the thickness cross section in the rolling direction with an optical microscope, and obtaining the average particle size and particle size distribution of the structure by a cutting method. As a result, in this example, the structure of all the test materials had a ferrite phase of 99% or more. In addition, a JIS No. 5 tensile test piece with the rolling direction as the tensile direction was cut out from the specimen, and after aging treatment at 20 ° C for 6 months, a tensile test was conducted at a tensile rate of 10 mm / min, yield point elongation (YP -El) and total elongation (El) were measured. Further, r value, the rolling direction of the test piece, the rolling direction of 45 °, cut a JIS5 No. tensile specimens from each direction perpendicular to the rolling direction, it was measured with prestrain 15%, the rolling direction of the r value (r _L) , the rolling direction of 45 ° of the r value (r _d), from the direction perpendicular to the rolling direction C in r value (r _C), average of r values _{(r m) r m = (} r L + 2r D + r C) / Obtained in 4.
The obtained results are shown in Table 1 together with the component composition and production conditions. Further, in FIG. 2, the test pieces Nos. 1-8, on sigma _A the effect of (FT-Ar3), in FIG. 3, the specimen Nanba1～4,9, of CT on sigma _A Each impact is shown.

According to Table 1, the steel sheet (invented steel) having the composition of the present invention and manufactured by the manufacturing method of the present invention has a ferrite average particle diameter in the range of 10 to 20 μm and a standard deviation (σ _A ). Is 0.30 or more. As a result, the average r value in the rolling direction, the 45 ° direction, and the direction perpendicular to the rolling direction is 1.2 or less, the yield point elongation after aging is 1.0% or less, and the elongation (Elm) after aging is 40% or more. A cold-rolled steel sheet excellent in later formability and shape freezing property was obtained.
On the other hand, the steel sheet (comparative steel) whose manufacturing method is outside the scope of the present invention has an average ferrite value or standard deviation (σ _A ) outside the range, and the average r value, yield point after aging. Either elongation or elongation after aging (El) was inferior.
Further, FIG. 2 shows that the standard deviation (σ _A ) can be set to 0.30 or more by setting the final delivery temperature (FT) to (Ar3-100 ° C.) to Ar3.
FIG. 3 shows that the standard deviation (σ _A ) can be set to 0.30 or more by setting the coiling temperature (CT) to less than 550 ° C.

Claims (5)

In mass%, C: 0.01 to 0.05%, Si: 0.05% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.02 to 0.10%, N: 0.005% or less, The balance is iron and inevitable impurities and a structure mainly composed of a ferrite phase, and the average particle diameter of the ferrite phase is 10 to 20 μm, and the natural logarithm of the value obtained by dividing the individual ferrite particle diameter by the average value. _A cold-rolled steel sheet having excellent formability and shape freezing property after aging, wherein σ _A ≧ 0.30 when the standard deviation is σ _A.   Furthermore, it contains at least one of Ti: 0.005-0.02% and B: 0.0003-0.0030% by mass%, and is excellent in formability and shape freezing property after aging according to claim 1 Cold rolled steel sheet.   The cold-rolled steel sheet excellent in formability and shape freezing property after aging according to claim 1 or 2, wherein the steel sheet surface has a zinc-based plating layer.   A steel slab having the composition according to claim 1 or 2 is hot-rolled at a final delivery temperature of finish rolling of (Ar3-100 ° C) to Ar3 ° C and a winding temperature of less than 550 ° C, After pickling and cold rolling at a rolling reduction of 40 to 80%, when annealing, the temperature range from 600 ° C to the soaking temperature is heated at an average heating rate of 1 to 30 ° C / s, The soaking temperature is 800 to 900 ° C., the soaking time is 30 to 200 s, and the temperature range from the soaking temperature to 550 ° C. is cooled at an average cooling rate of 3 to 30 ° C./s. A method for producing a cold-rolled steel sheet having excellent formability and shape freezing property after aging, characterized by holding at 300 ° C. for 30 s or more and applying a strain of 0.5 to 2.0% elongation at room temperature.   After the soaking, the temperature range from the soaking temperature to 550 ° C. is cooled at an average cooling rate of 3 to 30 ° C./s, subsequently cooled to a temperature range of 500 ° C. or lower, and then the temperature range of 500 to 550 ° C. 5) After that, it is held at 500 to 300 ° C. for 30 seconds or more, and strain at 0.5% to 2.0% elongation is applied at room temperature. A method for producing an excellent cold-rolled steel sheet.

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