TWI429758B - Cold rolled steel sheet having excellent formability, shape fixability after aging and method for manufacturing the same - Google Patents

Description

Cold rolled steel sheet excellent in formability and shape freezeability after aging and method for producing the same

The present invention relates to a cold-rolled steel sheet which is excellent in moldability and shape freezeability after aging of a component member having a large flat plate shape, such as a backlight casing of a large liquid crystal television, and a method for producing the same.

In a thin liquid crystal TV, an OA machine, or the like, a plurality of flat-shaped members formed by processing by bending and forming a main body are used. Therefore, in the manufacture of the members (thin steel sheets) for the parts, in order to correct the shape of the sheets and eliminate the yield point elongation, light rolling shrinkage of about several percent elongation is performed in the temper rolling. However, the lapse of the yield point elongation and the decrease in ductility after the lapse of time after temper rolling are called deterioration of the characteristics of the so-called strain aging. In particular, in recent years, in order to reduce the cost, a large number of cases have been used to output coils and press processing overseas, and it takes time to manufacture coils to press processing, and it is impossible to avoid deformation aging. Moreover, even in China, it takes time to circulate the coil material, or the steel sheet is strain-aged when the coil material is stored in an inventory type. As described above, when the strain aging deteriorates the characteristics of the steel sheet, it is necessary to perform the pressurization conditions and the re-adjustment of the metal mold or the like, which is one of the factors for increasing the cost.

Furthermore, recently, in order to reduce costs, it is highly desirable to reduce the thickness of the member and to reduce the amount of steel used. When the thickness is reduced, problems such as deterioration of shape freezeability during processing and cracking during processing tend to occur. In addition, in order to compensate for the decrease in the rigidity of the member due to the thinning, the bead is added, the shape of the part is changed to the closed cross-sectional structure by bending or the like, and the processing conditions become more severe. As a result, the processing is further enhanced. Cracking and poor shape during pressing. In particular, in the case of bending processing, a shape defect called ridge bending occurs, and problems such as bending of a part occur. Further, in the case of the protruding processing, cracking occurs when the protruding height is large, and wrinkles are generated when the wrinkles are suppressed from being weak.

For such ridge bending, it is advantageous to reduce the r value. However, a decrease in the r value leads to a decrease in elongation, which has an adverse effect on the protrusion processing. In addition, when the resurrection and the ductility of the yield point elongation are lowered by strain aging, it is impossible to cope with cracking, wrinkles, and the like by changing the pressurization conditions and the like.

Strain aging is known to be caused by C and N which are solid-melted in a steel sheet, and an IF steel in which a nitride-forming element such as Ti or Nb is added and the C and N are fixed as precipitates is a steel sheet which is hard to cause strain aging. However, the previous IF steel is expensive to manufacture and has a high r value, which is disadvantageous in the case of including bending.

From the above, there is a great expectation of an inexpensive member (thin steel sheet) which is low in r value, small in yield point elongation, and high in elongation after aging.

As a steel sheet having a low r value and excellent shape freezeability, for example, Patent Document 1 discloses that in the finish rolling of hot rolling, the rolling reduction ratio of Ar3 to (Ar3+100) is 25% or more, and at the time of rolling. The friction coefficient is 0.2 or less, and finish rolling is performed at Ar3 or more, or the rolling reduction ratio of Ar3 or less is 25% or more, and the friction coefficient at the time of rolling is 0.2 or less, and finish rolling is performed, and the assembly is controlled while rolling is performed. At least one of the direction or the r value of the rolling vertical direction is set to a steel plate of 0.7 or less.

Patent Document 2 discloses that an automobile having an excellent shape freezeability in which the ratio of the parallel {100} plane to the {111} plane of the plate surface is 1.0 or more is made of a ferrite-based iron-based steel sheet.

Patent Document 3 discloses that in order to obtain a ferrite-based iron-based steel sheet excellent in shape freezeability, the intensity of the {100}<011>~{223}<110> orientation group is controlled and {112}<110>, {554} <225>, {111}<112>, {111}<110> the intensity of each of the positions; and at least one of the r value in the rolling direction and the r value perpendicular to the rolling direction is set to 0.7 or less.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent No. 3532138

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2008-255491

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2003-55739

However, in the steel sheets described in Patent Documents 1 to 3, workability after aging is lowered, and problems such as pressure cracking occur.

In view of such circumstances, the present invention has an object of providing a cold-rolled steel sheet excellent in moldability and shape freezeability after aging and a method for producing the same.

Inventors and others have repeatedly worked hard to investigate and solve the above problems. As a result, it was found that the average r value in the rolling direction and the rolling direction in the rolling direction was set to 1.2 or less, the elongation after aging was set to 40% or more, and the yield point elongation after aging was set to 1.0%. In the following, a cold-rolled steel sheet excellent in moldability and shape freezeability even after aging can be obtained. Here, the average r value (r _m ) means that the r values of the rolling direction, the rolling 45° direction, and the rolling vertical direction are set to r _L , r _D , and r _C , respectively, and r _m = ( r _L +2r _D +r _C )/4.

Further, the mechanism for ensuring moldability and shape freezeability after aging according to the present invention is considered as follows. In general, in order to eliminate the yield point elongation, a method of adding strain at room temperature and introducing a movable index is employed. However, when the strain is small, the movable index is fixed by C and N through aging, and the yield point elongation is revived. On the other hand, when the amount of strain at room temperature is increased, the elongation is lowered and the formability is lowered because the yield point is increased. Thus, in the present invention, attention is paid to the distribution of the particle size of the ferrite. When the distribution of the particle size of the ferrite iron is increased, even if the amount of strain is small, the introduction position of the strain can be made uneven and the strain can be concentrated. As a result, the occurrence of the yield point elongation can be suppressed even after aging. Further, since a small amount of strain introduced into the strain is less hardened by aging, the elongation can be suppressed from being lowered, and the hardening by aging is also small, so that the elongation can be suppressed. Therefore, the introduction of such uneven strain can achieve a standard deviation of increasing the particle size distribution of the ferrite.

Further, as described above, 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 is subjected to finish rolling in the ferrite-iron region during hot rolling. The coil is taken at a low temperature, and is not recrystallized in the hot rolling stage. When the heat history is controlled during the annealing, the strain after cooling can be controlled while controlling the particle size and particle size distribution of the ferrite.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

[1] A cold-rolled steel sheet having excellent formability and shape-freezing property after aging, which is contained in terms of % by mass, C: 0.01 to 0.05%, Si: 0.05% or less, and 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 the composition of iron and unavoidable impurities and the structure of the ferrite phase iron phase main body, and the ferrite iron The average particle diameter of the phase is 10 to 20 μm, and the natural logarithmic standard deviation of the particle diameters of the respective ferrite grains divided by the average value is set to σ _A , and σ _A ≧ 0.30.

[2] The cold-rolled steel sheet having excellent properties after aging and the shape-freezing property of the above-mentioned [1], further containing, by mass%, Ti: 0.005 to 0.02%, and B: 0.0003 to 0.0030%. More than one species.

[3] The cold-rolled steel sheet having excellent formability and shape-freezing property after aging according to the above [1] or [2], wherein the steel sheet has a galvanized layer on the surface thereof.

[4] A method for producing a cold-rolled steel sheet having excellent formability and shape-freezing property after aging, characterized in that the steel preform formed by the composition described in the above [1] or [2] is finally finished. The exit side temperature is (Ar3-100 ° C) to Ar 3 ° C, the coiling temperature is hot rolled at less than 550 ° C, followed by pickling, and after cold rolling at a rolling reduction of 40 to 80%, annealing is performed. The temperature range from 600 ° C to soaking temperature is heated at an average heating rate of 1 to 30 ° C / s, the soaking temperature is 800 to 900 ° C, and the soaking time is 30 to 200 seconds for soaking, and the above The temperature range from the thermal temperature to 550 ° C is cooled at an average cooling rate of 3 to 30 ° C / s, maintained at 500 to 300 ° C for more than 30 seconds, and the elongation at room temperature is 0.5 to 2.0% strain.

[5] The method for producing a cold-rolled steel sheet having excellent formability and shape-freezing property after aging according to the above [4], wherein after the soaking treatment, the temperature range from the soaking temperature to 550 ° C is 3 to 30 Cool at an average cooling rate of °C/s, then cool to a temperature range below 500 °C, then reheat to a temperature range of 500-550 °C, then hold at 500-300 °C for more than 30 seconds, and extend at room : 0.5~2.0% strain.

In addition, in this specification, all the % of steel components are shown by mass %. Further, the cold-rolled steel sheet to which the present invention is applied also includes a steel sheet which is subjected to a galvanizing treatment (for example, an electrogalvanizing treatment, a hot-dip galvanizing treatment, or an alloying hot-dip galvanizing treatment) on the cold-rolled steel sheet. Further, a steel sheet on which a film is added via a chemical conversion treatment or the like is also included.

Moreover, the steel plate of the present invention can be widely used as a flat member such as a large-sized TV backlight case, a refrigerator panel, a cold air outdoor unit, and the like, and a general member for performing home appliances such as bending, protrusion, and mild deep drawing. Further, according to the present invention, for example, a steel plate having a thickness of 0.8 mm can be used to manufacture a backlight casing of about 650 × 500 mm (32 V type) or more.

According to the present invention, a cold-rolled steel sheet excellent in moldability and shape freezeability after aging can be obtained. This ensures the shape of the flat plate required for large parts and can be used to manufacture components such as the backlight casing of large LCD TVs.

The chemical composition of the steel sheet according to the present invention will be described. In addition, in the following description, the content % of a component element all means mass %.

C: 0.01~0.05%

C can form cementite, reduce solid solution C, and lower the yield strength. If C is small, the formation of iron carbide is suppressed, and the solid solution C is increased, and it becomes easy to age harden. In the hot rolling, in the completion stage, the Tiansi iron is transformed into the ferrite iron, and the two-phase region is small, so The strain resistance drops sharply and the rolling becomes unstable. Therefore, C must be set to 0.01% or more. On the other hand, when C is increased, grain growth is suppressed and finely granulated, so that the steel plate is hardened and the ductility is lowered. Therefore, C must be set to 0.05% or less.

Si: 0.05% or less

When Si is added in a large amount, the moldability is deteriorated by hardening, and the plating property is inhibited by the Si oxide formed during annealing. Therefore, Si must be set to 0.05% or less.

Mn: 0.1~0.5%

Since Mn is harmless in the MnS type in the harmful steel, it must be set to 0.1% or more. On the other hand, a large amount of Mn is hardened by strengthening solid solution and generating a low-temperature metamorphic phase, and the formability is deteriorated. Further, Mn lowers the deformation point and is difficult to roll in the ferrite iron region during hot rolling. Further, at the time of annealing, recrystallization of the ferrite iron is inhibited to coarsen the structure. Therefore, Mn must be set to 0.5% or less, preferably 0.3% or less.

P: 0.05% or less

P segregates at the grain boundary to deteriorate ductility and toughness, so it must be set to 0.05% or less. It is preferably 0.03% or less.

S: 0.02% or less

S significantly reduces ductility during heating, induces thermal cracking, and significantly deteriorates surface properties. Further, S not only contributes little to strength, but also reduces ductility by forming coarse MnS as an impurity element. These problems are remarkable because the amount of S exceeds 0.02%, and it is expected to be reduced as much as possible. Therefore, the amount of S must be set to 0.02% or less.

Al: 0.02~0.10%

In the Al system, N is fixed as a nitride, and aging hardening can be suppressed by the solid solution N. In order to obtain such an effect, it is necessary to set Al to 0.02% or more. On the other hand, the addition of a large amount of Al not only increases the strength but also lowers the formability, and increases the cost. Therefore, Al must be set to 0.10% or less.

N: 0.005% or less

If a large amount of N is contained, the steel sheet is cracked in the hot rolling, and surface flaws occur. Further, in the case of being solid-melted in the N-type after cold rolling and annealing, aging hardening is caused. Therefore, N must be set to 0.005% or less.

In the present invention, in the present invention, one or more of Ti and B may be contained in the range of Ti: 0.005 to 0.02% and B: 0.0003 to 0.0030% for the purpose of improving aging property and shape freezing property.

Ti: 0.005~0.02%

Ti combines with N to form a nitride at a high temperature, and the reduction of solid solution N improves aging. In order to obtain such an effect, Ti must be set to 0.005% or more. On the other hand, when the content of Ti is large, carbides and carbonitrides are further formed in combination with C, so that the strength is increased and the formability is lowered. Therefore, the case where Ti is contained is set to 0.005% or more and 0.02% or less.

B: 0.0003~0.0030%

B combines with N to form a nitride at a high temperature, and the reduction of solid solution N improves aging. Furthermore, B can suppress the grain growth of the ferrite iron during the annealing process after cold rolling, and can suppress the r value to improve the shape freezing property. In order to obtain such an effect, B must be set to 0.0003% or more. On the other hand, in the case where B is present in a large amount, since the recrystallization of the ferrite iron during annealing is suppressed, the structure is finely granulated. Therefore, when B is contained, it is set to 0.0003% or more and 0.0030% or less.

The components other than the above are composed of iron and unavoidable impurities. Examples of the unavoidable impurities include Cu and Cr which are easily mixed with 0.05% or less of scrap iron, and other 0.01% or less of Sn, Mo, W, V, Nb, Ni, and the like.

The steel sheet structure of the present invention is formed into a ferrite-rich iron phase body. Further, the average particle diameter of the ferrite iron phase is 10 to 20 μm. Further, when the natural logarithmic standard deviation of the particle diameters of the respective ferrite grains divided by the average value is set to σ _A , σ _A ≧ 0.30.

In order to ensure formability, a soft ferrite iron phase is mainly used. Here, the term "fertilizer iron phase as the main component" refers to a case where the ratio of the ferrite-particle iron phase to the entire structure is 95% or more. With the ferrite iron structure as the main body, the elongation after aging can reach 40% or more. If the ferrite iron structure is 100%, the elongation is improved. The second phase other than the main phase is equal to the iron carbide phase and the ferrite, and the area ratio may be in the range of 5% or less. If it is more than 5%, the decrease in ductility is remarkable. In addition, the area ratio of the ferrite grain iron phase can be determined by observing the structure, distinguishing the ferrite grain iron phase from the other phases, and obtaining it by image processing.

The average particle diameter is set to 10 μm or more in order to secure moldability. On the other hand, when the particle size is increased, the upper limit of the average particle diameter is set to 20 μm, in addition to the appearance defect such as orange peel during molding. Further, the average particle diameter can be calculated from the cutting method and the average slice lengths L1 and Lc in the rolling direction and the thickness direction by 2/[(1/L1)+(1/Lc)].

In the invention, the distribution of the particle size of the ferrite iron is increased, and even if the strain is small, the strain introduction position is uneven and the strain is concentrated, and the elongation of the yield point can be suppressed after the aging. Further, since the particles having less strain are introduced, since the hardening due to aging is small, the decrease in the elongation can be suppressed. Therefore, when the natural logarithmic standard deviation of the particle diameter of each fertilized iron divided by the average value is set to σ _A , it must be σ _A ≧ 0.30. Hereinafter, it will be explained.

Considering the actual use of the steel sheet, it is considered that the aging period at room temperature (20 ° C) is 6 months. Figure 1 is a graph showing the effect of σ _A on yield elongation (YP-E1) and elongation (E1) at 6 months of aging at 20 °C. Further, in Fig. 1, 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%, and N: 0.005% are used. Hereinafter, the remaining part is a composition of iron and unavoidable impurities, and the proportion of the ferrite grain iron phase is 95% or more, and the average grain size of the ferrite grain iron phase is 10-20 μm, and the The steel sheet was processed into a JIS No. 5 tensile test piece and measured. Here, by setting the yield elongation after aging to 1.0% or less, the wrinkles after molding can be suppressed to the extent that they are not or almost impossible to visually judge. Further, when the elongation after aging is set to 40% or more, the corner angle at the time of the protrusion molding can be molded so as not to be cracked by about 45°, and almost all of the press molding can be performed. As shown in Fig. 1, when σ _{A is} set to 0.30 or more, the yield elongation can be as small as 1.0% or less, and the elongation can be as large as 40% or more. Therefore, σ _{A is} set to 0.30 or more.

Next, the manufacturing conditions of the steel sheet according to the present invention will be described. In the present invention, the steel preform having the above composition is hot-rolled at a final exit temperature of (Ar3-100 ° C) to Ar 3 ° C, and at a coiling temperature of less than 550 ° C, followed by pickling, and 40~80% of the rolling reduction after cold rolling, 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, and the soaking temperature is 800 ~ 900 ° C The soaking time is 30 to 200 seconds for soaking, and the temperature range from the soaking temperature to 550 ° C is cooled at an average cooling rate of 3 to 30 ° C / s, and maintained at 500 to 300 ° C for more than 30 seconds. And adding 0.5~2.0% strain at room temperature can increase the distribution of the ferrite iron particle size, and obtain low yield point strength and low r value after aging and excellent elongation.

Finishing finish temperature: (Ar3-100 °C) ~ Ar3

When the finish rolling in the hot rolling is completed in the ferrite iron region, the strain can be accumulated in the ferrite iron structure, and the recovery can be performed according to the uneven orientation of the crystal. As a result, the accumulation of strain becomes uneven, and the particle size distribution of the ferrite iron after annealing can be increased. Further, the development of the aggregate structure is irregularized according to the unevenness of the crystal orientation, and the r value can be reduced and the shape freezeability can be improved. Therefore, the final temperature of finish rolling must be set to be less than Ar3. More preferably, the temperature is less than Ar3. The amount of rolling reduction below Ar3 is not particularly limited, but is preferably 10% or more, and more preferably 20% or more. On the other hand, if the finish rolling completion temperature is lowered, the strain-introduced crystals are not recovered, and the strain accumulation is unlikely to be uneven. Further, since the rolling load is increased and work is difficult, the finish rolling completion temperature must be set to (Ar3 - 100 ° C) or more.

Further, Ar3 can be obtained by the following formula.

When the Mn content is <0.4%: Ar3 = 880-1000 × C content (%)

When the Mn content is %0.4%: Ar3=870-1000×C content (%)

Coiling temperature: less than 550 ° C

If the coiling temperature after finish rolling is high, the ferrite iron is recrystallized, and uneven strain cannot be introduced, so the coiling temperature must be set to less than 550 °C. The lower limit of the coiling temperature is not particularly limited. However, if the temperature is too low, the coiling shape of the coil is deteriorated, so that it is preferably 300 ° C or higher. The cooling rate until completion of the finish rolling is not particularly limited, but is preferably 10 ° C / s or more, more preferably 30 ° C / s or more, and still more preferably 100 ° C / s or more.

Rolling reduction rate during cold rolling: 40~80%

When the rolling reduction ratio in the cold rolling after pickling the hot-rolled sheet is large, the strain introduction is made uniform, and the particle size distribution of the ferrite-grained iron after annealing is reduced, and the amount of the strain is increased and finely granulated and high. The strength is increased and the formability is lowered. Further, the collection organization is also developed, and the r value becomes high and the shape freezeability is lowered. From the above, the rolling reduction ratio must be set to 80% or less. On the other hand, when the rolling reduction ratio is small, the amount of strain introduced is small, the recrystallization at the time of annealing is suppressed, and the structure is restored, and the formability is lowered. Therefore, the rolling reduction rate must be set to 40% or more.

Average heating rate from 600 ° C to soak temperature range: 1~30 ° C / s

After cold rolling, annealing is performed. In the present invention, under the control of the heat history during annealing, the amount of strain after cooling is controlled while controlling the particle size and particle size distribution of the ferrite. Therefore, the manufacturing conditions at the time of annealing are important requirements.

When the average heating rate from 600 ° C to the soaking temperature is small, recovery is performed to suppress recrystallization. Therefore, the average heating rate must be set to 1 ° C / s or more. On the other hand, when the average heating rate is large, the occurrence of nucleation of recrystallization is suppressed during heating, and the core is generated at the time of soaking, so that the particles are finely granulated. Therefore, the average heating rate must be set to 30 ° C / s below.

Soaking temperature: 800~900°C, soaking time: 30~200 seconds

After the soaking treatment after heating, it is necessary to increase the particle size and improve the formability while completing the recrystallization. Therefore, the soaking temperature must be set to 800 ° C or higher. On the other hand, if the soaking temperature is too high, the ferrite iron is subjected to the metamorphosis of the Worthite iron, and the particle size is reduced in the inverted state after cooling. Therefore, the soaking temperature must be set to 900 ° C or higher.

Moreover, if the soaking time is short, recrystallization is not completed, or even if it is completed, since the time of grain growth is short, it is fine-grained, and moldability falls. Therefore, the soaking time at the time of heating must be set to 30 seconds or more. On the other hand, when the soaking time is long, the large particles erode the small particles and grow larger. Therefore, the distribution of the particle size of the ferrite grains becomes smaller, and the particle size becomes larger, which causes the formation during press forming. The appearance of orange peel is poor. Therefore, the soaking time must be set to 200 seconds or less.

Average cooling rate in the temperature range from soaking temperature to 550 °C: 3~30 °C/s

When the cooling rate after the soaking treatment is small, the growth of the ferrite particles is promoted, and the large particles erode the small particles and grow larger. Therefore, the distribution of the particle size of the ferrite grains becomes smaller, and the particle size becomes larger. At the time of press molding, the appearance of orange peel or the like is poor. Therefore, the average cooling rate in the temperature range from the soaking temperature to 550 ° C must be set to 3 ° C / s or more. On the other hand, when the cooling rate is too large, the strength is increased and the moldability is lowered. Therefore, the average cooling rate must be set to 30 ° C / s or less.

In addition, after cooling by the soaking to 550 ° C, the holding between 500 and 300 ° C may be appropriately cooled in accordance with the production equipment. Preferably, after cooling from a soaking temperature to 550 ° C, it is then cooled in the same cooling rate range, that is, at 3 to 30 ° C / s.

Hold time of 500~300 °C: 30 seconds or more

The solid solution C is precipitated in the form of iron carbide to improve aging. Therefore, it must be maintained in the temperature range of 300 to 500 ° C where iron carbide is easily precipitated for more than 30 seconds. The upper limit of the time is not particularly limited, but since the production efficiency is lowered for a long period of time, the upper limit of the holding time is preferably set to about 300 seconds.

Further, the cooling condition is not particularly limited as long as it is cooled to room temperature after holding, and may be appropriately carried out in accordance with the production equipment.

Strain at room temperature, elongation: 0.5~2.0%

After annealing, additional strain is applied at room temperature to eliminate the yield point. Therefore, the strain to be added at room temperature must be set to 0.5% or more. On the other hand, when the elongation is increased, the yield point is increased and the formability is lowered. Therefore, it is necessary to set it to 2.0% or less. It is preferably 1.5% or less. Further, the strain is applied at room temperature, and it may be rolled by a roll and may be stretched, or may be combined with a roll and stretch. Moreover, even if it is not lubricated during rolling.

In the practice of the present invention, a usual converter method, an electric furnace method, or the like can be suitably applied to the melting method. The molten steel is cast into a steel preform and directly or cooled and heated to perform hot rolling. After hot rolling is processed under the above-described processing conditions, it is taken up at the above-mentioned coiling temperature. Next, after the general pickling, the above-described cold rolling is performed. The annealing treatment after cold rolling is performed by heating, holding, and cooling under the above conditions. If necessary, it is plated with molten zinc at around 480 °C. Further, after plating, the plating may be alloyed by reheating at 500 ° C or higher. In addition, when reheating is performed, it is necessary to be 550 ° C or less in which the ferrite iron does not grow. In addition, in the above-mentioned holding of 500 to 300 ° C, the iron carbide may be melted by reheating at 500 ° C or higher. Therefore, after heating at 500 ° C or higher and 550 ° C or lower, 500 to 300 after reheating. It is preferable to set the holding time of °C to 30 seconds or more. Further, the lower limit of the plating bath temperature is about 460 °C. In other words, after the above-described soaking treatment, the hot-dip galvanization is performed, and the alloying treatment is further performed, and the heat history may be treated as follows. After the soaking treatment, the temperature range from the soaking temperature to 550 ° C is cooled at an average cooling rate of 3 to 30 ° C / s, followed by cooling in a temperature range of 500 ° C or lower to carry out hot-dip galvanizing, followed by 500 to 550. The alloying treatment is carried out by reheating in a temperature range of °C, and thereafter maintained at 500 to 300 ° C for 30 seconds or more. The holding time is preferably set to about 300 seconds for the same reason as described above. Further, it may be cooled to room temperature after being held, and further subjected to temper rolling at an elongation of about 0.5 to 2.0%. It is preferably 0.5 to 1.5%. Further, in the case where plating is not performed during the annealing, plating or galvanizing may be performed to improve corrosion resistance. Further, on the cold-rolled steel sheet and the plated steel sheet, the additional film may be treated by chemical conversion.

From the above, a cold-rolled steel sheet excellent in moldability and shape freezeability after aging is obtained. According to the cold-rolled steel sheet obtained as described above, the average r value in the rolling direction, the rolling 45° direction, and the rolling vertical direction is 12.1 or less, the elongation after aging is 40% or more, and the yield point elongation after aging is 1.0% or less. In addition, these characteristics are the average r value, elongation, and yield point elongation after aging treatment at 20 ° C for 6 months.

The value of r is related to the bending generated after bending. In the bending forming, the r value in the bending direction becomes high, and the saddle bending along the bending line remarkably occurs. Therefore, for the purpose of improving the shape freezeability after press molding by low r value, in the present invention, the average r value is set to 1.2 or less.

The degree of elongation is highly dependent on the formability, and the greater the degree of elongation, for example, the protrusion can be formed up to the height. Therefore, the greater the elongation as needed, the better the elongation after aging is set to 40% or more, and the deep drawing processing and the protruding processing are performed to ensure the desired shape of the part.

In the above-described steel sheet according to the present invention, the yield point elongation after aging is set to 1.0% or less, and the tensile strain after molding is suppressed not only after the steel sheet is produced, but also at the yield point elongation after aging is reduced, and the surface can be produced. A molded article with excellent appearance.

[Example 1]

The steel slab having the chemical composition shown in Table 1 was melted, heated, and hot rolled at the final exit temperature (FT) shown in Table 1, and cooled at an average cooling rate of 10 ° C / s. The coiling temperature (CT) shown in Fig. 1 is taken up. Subsequently, the mixture was pickled, cold rolled at the rolling reduction rates shown in Table 1, and annealed under the conditions shown in Table 1. Subsequently, the test piece was produced by rolling at room temperature at an elongation shown in Table 1.

In addition, 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, and the average cooling rate from the soaking temperature to 550 ° C is CR, 500. The stop time from °C to 300 °C is Ht2. Further, the test material No. 4 was subjected to a plating treatment with molten zinc at 480 ° C in the middle, and the surface was made into hot-dip galvanizing (GI). The test material No. 3 was plated with molten zinc at 480 ° C in the middle, and then heated at 540 ° C to form a surface of alloyed hot-dip galvanizing (GA). The test material No. 2 was not subjected to electroplating treatment, and the surface was plated (EG). Further, the test material No. 4 was cooled from 550 ° C to 500 ° C and then cooled at the same cooling rate as CR shown in Table 1.

For the test materials obtained according to the above, the tissue and mechanical properties were investigated. The microstructure was observed by an optical microscope in the thickness profile of the rolling direction, and the average particle diameter and particle size distribution of the structure were determined according to the cutting method. As a result, in the present embodiment, the microstructure of all the test materials was 99% or more of the ferrite iron phase. Moreover, the JIS No. 5 tensile test piece was cut out from the test material in the rolling direction, and after aging treatment at 20 ° C for 6 months, the tensile test was performed at a tensile speed of 10 mm/min. Yield point elongation (YP-E1) and full elongation (E1) were measured. Further, the r value was measured by cutting the JIS No. 5 tensile test piece from the rolling direction of the test material, the rolling direction of 45°, and the rolling vertical direction, and measured by a pre-strain of 15%, and was measured by the rolling direction. r value (r _L ), r value (r _d ) in the 45° direction of rolling, r value (r _C ) in the direction of C in the vertical direction of the rolling, and the average r value (r _m ) as r _m =(r _L + 2r _D +r _C )/4. The results obtained are shown in Table 1 in combination with the composition and manufacturing conditions.

According to Table 1, according to the steel sheet (invention steel) having the composition of the present invention and produced by the production method of the present invention, the average particle diameter of the ferrite iron is in the range of 10 to 20 μm, and the standard deviation (σ _A ) is 0.30 or more. As a result, the average r value of the rolling direction, the rolling 45° direction, and the rolling vertical direction was 1.2 or less, the yield point elongation after aging was 1.0% or less, and the elongation (E1m) after aging was 40. A cold-rolled steel sheet excellent in moldability and shape freezeability after aging of % or more.

In contrast, the manufacturing method is a steel sheet (comparative steel) outside the scope of the invention, and the average grain size or standard deviation (σ _A ) of the ferrite grain is outside the range, the average r value, the yield point elongation after aging, and the extension after aging. Any degree (E1) is poor.

Fig. 2 shows the influence of the test materials No. 1 to 8, (FT-Ar3) on σ _A , and Fig. 3 shows the influence of CT on the σ _A in the test materials No. 1 to 4.9.

As can be seen from Fig. 2, the final exit temperature (FT) is (Ar3-100 °C) to Ar3, and the standard deviation (σ _A ) can be set to 0.30 or more.

As can be seen from Fig. 3, the coiling temperature (CT) can be set to less than 550 ° C, and the standard deviation (σ _A ) can be set to 0.30 or more.

Figure 1 shows the effect of σ _A on the yield elongation (YP-E1) and elongation (E1) after aging.

Figure 2 shows the effect of the final exit side temperature (FT)-Ar3 of the finish rolling on σ _A .

Figure 3 shows the effect of the coiling temperature (CT) on σ _A .

Claims (6)

A cold-rolled steel sheet excellent in formability and shape-freezing property after aging, characterized by being contained by mass%, C: 0.01 to 0.05%, Si: 0.05% or less, Mn: 0.1 to 0.5%, P: 0.05% Hereinafter, S: 0.02% or less, Al: 0.02 to 0.10%, N: 0.005% or less, and the balance is the composition of iron and unavoidable impurities and the structure of the ferrite phase iron phase main body, and the average of the ferrite grain iron phase When the particle diameter is 10 to 20 μm, and the natural logarithmic standard deviation of the particle diameter of each fertilized iron divided by the average value is set to σ _A , σ _A ≧ 0.30. The cold-rolled steel sheet having excellent formability and shape-freezing property after aging according to the first aspect of the patent application is further contained in the amount of Ti: 0.005 to 0.02%, and B: 0.0003 to 0.0030%. . The cold-rolled steel sheet having excellent formability and shape-freezing property after aging in the first or second aspect of the patent application has an average r value of 1.2 or less in the rolling direction, the rolling 45° direction, and the rolling vertical direction. A cold-rolled steel sheet having excellent formability and shape-freezing property after aging in the first or second aspect of the patent application, wherein the steel sheet has a galvanized layer on the surface thereof. A method for producing a cold-rolled steel sheet having excellent formability and shape-freezing property after aging, characterized in that the steel preform formed by the composition described in the first or second aspect of the patent application has a final exit temperature of the finish rolling It is (Ar3-100 ° C) to Ar 3 ° C, the coiling temperature is less than 550 ° C, hot rolling, followed by pickling, and cold rolling at a shrinkage of 40 to 80%, and then annealing, 600 The average temperature range from °C to soaking temperature is 1~30°C/s Speed heating, the soaking temperature is 800-900 ° C, the soaking time is 30-200 seconds to be soaked, and the average cooling rate of 3~30 ° C / s in the temperature range from the soaking temperature to 550 ° C Cool, hold at 500~300 °C for more than 30 seconds, and apply elongation at room temperature: 0.5~2.0% strain. A method for producing a cold-rolled steel sheet having excellent formability and shape-freezing property after aging according to the fifth aspect of the patent application, wherein after the soaking treatment, the temperature range from the soaking temperature to 550 ° C is 3 to 30 ° C The average cooling rate of /s is cooled, then cooled to a temperature range of 500 ° C or less, and then heated to a temperature range of 500 to 550 ° C, followed by 500 to 300 ° C for more than 30 seconds, and an elongation is applied at room temperature. : 0.5~2.0% strain.