MX2011010580A - Cold-rolled steel sheet with excellent formability, shape retentivity, and surface appearance and process for producing same. - Google Patents

Abstract

A cold-rolled low-carbon steel sheet having excellent formability, shape retentivity, and surface appearance, the steel sheet combining processability with shape retentivity, being able to be processed by drawing, bending, or bulging and to ensure a shape required of a large part, and having such a high degree of flatness that no appearance defects generate. Provided is a process for producing the cold-rolled steel sheet. The cold-rolled steel sheet is characterized by having a composition which comprises, in terms of mass%, 0.030-0.060% C, up to 0.05% Si, 0.1-0.3% Mn, up to 0.05% P, up to 0.02% S, 0.02-0.10% Al, up to 0.005% N, and iron and incidental impurities as the remainder, having an r value of 0.7-1.4 when examined in the direction of rolling and in the direction perpendicular to the rolling direction, having an in-plane r-value anisotropy (∆r) of -0.2≤∆r≤0.2, having an average yield strength and an average elongation in three directions, i.e., the rolling direction, a direction forming an angle of 45º with the rolling direction, and the direction perpendicular to the rolling direction, of 230 MPa or lower and 40% or longer, respectively, and having a yield elongation after 60-minute holding at 170ºC of 2% or shorter in each of the three directions.

Description

COLD LAMINATED STEEL SHEET WITH TRAINING CAPACITY, SHAPE RETENTIVITY AND APPEARANCE OF EXCELLENT SURFACES AND PROCESS FOR ITS PRODUCTION TECHNICAL FIELD The present invention relates to a sheet of cold-rolled steel which is more suitable for members of large tubular parts such as rear lighting chassis for televisions with large liquid crystal display and which has excellent formability, retentivity of form, and superficial appearance and also refers to a method for its manufacture.

In recent years, the increase in the size of thin-screen televisions has led to the increase in the size of the backlight chassis for televisions with liquid crystal display. In addition, there are needs for lighter televisions and reduced material costs and that they use thinner backlight chassis. However, the rear lighting chassis need to have rigidity to support the lights, they need to have good flatness, and they need to be strongly deformed so that the lights do not hit the sections of the liquid crystal or they crack, that is, the rear lighting chassis need to be free of the so-called "torsion". The requirements for stiffness and flatness have become more severe because of the larger size and thinning of the backlight chassis.

In order to ensure rigidity, it is effective to form a sphere on a flat surface of a back lighting chassis by stretching. However, the machining of the flat surface causes new problems such as the deterioration of flatness and the increase of "torsion". In the case of securing the stiffness by bending an extreme portion, similar problems occur. Since the deterioration of flatness is a phenomenon caused by the deficient retentivity of form, it is increasingly required that the steel sheets used as members have malleability and shape retentivity.

An example of a conventional steel sheet with excellent shape retentivity is a steel sheet in which the texture is controlled and in which at least one of the Lankford value in the rolling direction and the Lankford value in a perpendicular direction to the rolling direction is 0.7 or less as described in Patent Literature 1. It is disclosed that the elastic return of this steel sheet is small during bending. Patent Literature 2 describes a method for suppress the elastic return or warping during bending by controlling the anisotropy of local elongation or uniform elongation. In addition, Patent Literature 3 discloses a method for suppressing the elastic return during bending by adjusting the plane ratio. { 100.}. to the plane. { 111.}. to 1.0 or more.

The backlight chassis, formed by stretching, for large size televisions have the problem of "twisting". This is because the feeding of a steel sheet is uneven during stretching and therefore the thickness of a formed portion is uneven.

In addition, marks called formations of creep figures are caused, which lead to the problem of poor flatness and poor appearance of these backlight chassis. Patent Literature 4 discloses a method for reducing the yield strength of low carbon steel sheet which is responsible for the formations of creep figures. In this method, an appropriate amount of B is added and the depth of the valley of the central line (Rv) and the average of the central line (Ra), which are parameters of the surface roughness, are adjusted to 0.5 to 10 m and 0.5 μp? or more, respectively, during cold rolling.

Appointment list Patent Literature PTL 1: Japanese Patent No. 3532138 PTL2: Publication of Patent Application Japanese Unexamined No. 2004-183057 PTL3: International Publication No. WO 00/06791 PTL: Publication of Patent Application Japanese Unexamined No. 4-276023 BRIEF DESCRIPTION OF THE INVENTION Technical problem In the rear lighting chassis for televisions with liquid screen of 81.28 cm (32 inches) and larger, for which the market has expanded markedly in recent years, usually increases the height of the dome or the number of portions flared in order to re rigidity despite the reduction in thickness; therefore, greater elongation is needed to machine these portions. However, there is a problem that the techniques described in Patent Literatures 1 to 3 are incapable of achieving malleability to re the required part geometry and stiffness.

It is known that a reduction in the elongation resistance is effective to suppress the elastic return.

In general, steels with low carbon content have great resistance to elongation and insufficient elongation. Therefore, ultra-low carbon steels are used for portions that are difficult to machine. The softening of the steel is effective to reduce the resistance to elongation and an increase in the annealing temperature and an increase in the reduction of the cold rolling are effective as techniques for this. However, the softening of the steel develops an oriented texture (111) to increase the Lankford value. Probably, low carbon steel can be applied to parts such as backlight chassis for large size TVs only by achieving both the softening of the steel in order to suppress the elastic return and the Lankford value reduction. in order to suppress the ton caused by the bend. However, in steels with low carbon content, steel softening and high Lankford values have been mainly required.

For the problem of geometric flatness and poor appearance, it is important that the strength of a sheet of steel be small not only right after the steel sheet is made but also until the steel sheet is formed, ie , your Properties after aging are important. However, the method described in Patent Literature 4 needs to fulfill all the requirements of the surface roughness of a cold-rolled sheet, the requirement of its cooling rate during annealing with recrystallization and the requirements of over-aging conditions and by therefore it has the problem that the control of manufacturing conditions is complicated.

DETAILED DESCRIPTION OF THE INVENTION An object of the present invention is to solve the problems associated with these conventional techniques. That is, in steel with low carbon content, an object of the present invention is to provide a cold rolled steel sheet and a method for its manufacture. Cold rolled steel sheet satisfies both malleability and shape retentivity; it can be subjected to traction, bending and stretching; can assure required shapes for large parts; has high flatness; does not show appearance defects; and it is excellent in its malleability, shape retentivity and appearance of the surfaces.

Solution to the problem The characteristics of the present invention that are designed to solve these problems are those described below: (1) A sheet of cold rolled steel having a composition of 0.030% at 0.060% C, 0.05% or less Si, from 0.1% to 0.3% Mn, 0.05% or less of P, 0.02% or less of S, from 0.02% to 0.10% of Al, and 0.005% or less of N based on the mass, the rest is constituted by iron and unavoidable impurities, where its resistance to medium elongation (YSm) is 230 MPa or less as determined by the following equation (a); its average elongation (Elm) is 40% or more as determined by the following equation (b); its Lankford value is from 0.7 to 1.4 in a rolling direction and a direction perpendicular to the rolling direction; the anisotropy in the plane (Ar) of its Lankford value satisfies the inequality -0.2 < R = 0.2 as determined by the following equation (c); and the elastic limit of the cold-rolled steel sheet maintained at 170 ° C for 60 minutes is 2% or less in the rolling direction, a 45 degree direction to the rolling direction, and the direction perpendicular to the direction of laminate: YSm = (YSL + 2YSD + YSC) / 4 (a) Elm = (E1L + 2E1D + Elc) / 4 (b) Ar = (rL - 2rD + rc) (c) where YSm is the resistance to elongation mean, Elm is the average elongation, Ar is the anisotropy in the Lankford value plane, YSL is the elongation resistance in the rolling direction, YSD is the elongation resistance in the 45 degree direction to the rolling direction, YSC is the elongation resistance in the direction perpendicular to the rolling direction, E1L is the elongation - in the rolling direction, E1D is the elongation in the direction at 45 degrees to the rolling direction, Elc is the elongation in the direction perpendicular to the rolling direction, rL is the Lankford value in the rolling direction, rD is the Lankford value in the 45 degree direction to the rolling direction, and rc is the Lankford value in the direction perpendicular to the direction laminate direction. (2) A method for manufacturing a cold rolled steel sheet includes heating a piece of steel having the composition specified in Clause (1) to a heating temperature of 1200 ° C or higher, performing the hot rolling so that the final laminate is finished at a temperature of (transformation temperature of Al - 50 ° C) at (Al transformation temperature + 100 ° C), perform cooling at 550 ° C up to 680 ° C, perform pickling, laminate cold at a rolling reduction of 50% to 85%, perform heating at an annealing temperature of 700 ° C or higher at a speed of average heating from 1 to 30 ° C / s, and then cooling to 600 ° C at an average cooling speed of 3 ° C / s or more.

The present invention is the result of intensive investigations carried out to solve the problems mentioned above. In the case where a large rectangular flat plate of a steel sheet is taken and then machine processed in a required part, in view of the material produced and the operation, it is advantageous that the plate is taken from it in such a way that the long sides of the rectangular plate are parallel to the rolling direction of the steel sheet or to a direction perpendicular to the rolling direction thereof. In the case of taking materials in such a manner, the present invention allows even large-sized parts to satisfy the malleability and the quality of the surfaces. That is to say, the traction and the stretch can be realized increasing the average lengthening, with what the required forms for the parts can be assured. The appearance of the elastic return can be suppressed after machining by reducing the elongation resistance, thereby ensuring shape retentivity. The Lankford value is adjusted to 0.7 to 1.4 in the rolling direction and the direction perpendicular to the rolling direction and the inequality -0.2 < ? G < 0.2; therefore, retention of form can be ensured. In addition, it is most important that the adjustment of the elongation resistance with aging at 2% or less allows the formation of creep figures to be suppressed during machining, the appearance of the surfaces becomes excellent, the appearance of return Elastic is removed after machining, and shape retentivity is ensured.

In the present invention, it is believed that the mechanism of an increase in elongation and a reduction in elongation resistance are as described below. That is, the hot rolled, the finished temperature is adjusted to (Al-50 ° C transformation temperature) up to (Al + 100 ° C transformation temperature) and the laminate is finished so that no austenite is formed but ferrite, so the grain size of a ferrite microstructure becomes thick. This allows the size of the grains to be roughened after cold rolling or annealing with recrystallization and allows softening.

Meanwhile, the hot rolling is finished in a range from (Al-50 ° C transformation temperature) to (Al + 100 ° C transformation temperature), thereby forming the orientation (110) in a surface layer of a hot-rolled steel sheet. The hot rolled steel sheet is cold rolled and then annealed with recrystallization, whereby the Lankford value is kept low because of the orientation development (110). This allows the softening due to the thickening of the ferrite grains with the Lankford value kept low. In addition, the elastic limit is completely eliminated and the resistance to elongation with aging is reduced; therefore, a steel sheet can be obtained in which the formation of creep shapes after forming and having excellent appearance of surfaces is suppressed. In the present invention, the details of the reason for the elimination of elongation resistance are not clarified, but it is believed that its mechanism is as described below. That is, it is known that the orientation (110) is an orientation in which tension is likely to accumulate and the development of this orientation in the surface layer allows tension due to cold rolling or the laminate is easily introduced with annealing. This probably causes the so-called dislocation; therefore, it is assumed that the formation of creep figures is difficult.

Even if the cold rolled steel sheet according to the present invention becomes a sheet Made of cold-rolled steel with a thickness of 1.0 mm to 0.5 mm, this steel sheet has no creep pattern formation and has excellent surface appearance. The cold-rolled steel sheets contemplated by the present invention include steel sheets manufactured by subjecting cold-rolled steel sheets to treatment of surfaces such as electrogalvanized or galvanized and steel sheets fabricated by the provision of coatings on those steel sheets. .

The steel sheet according to the present invention can be widely used not only for backlight chassis for large size TVs but also as members, such as refrigerator panels, outdoor air conditioning units, for use in household appliances, Common limbs have flat portions and are subjected to bending, stretching, or light traction. The present invention can be used to manufacture a backlight chassis with a size of approximately 850 mm x 650 mm (type 42V) from a steel sheet with a thickness, for example of 0.8 mm.

Advantageous effects of the invention According to the present invention, the following sheet can be obtained: a sheet of cold-rolled steel that is capable of achieving low elastic limit, excellent elongation, low resistance to elongation, and low elastic limit with aging; which satisfies both the malleability and the retentivity of form; that can be subjected to traction, bending and stretching; and that it is excellent in terms of training capacity, retentivity of shape and appearance of the surfaces. This allows tabular shapes required for large parts to be secured; therefore, large-screen liquid crystal displays can be manufactured.

Description of Modalities The chemical components of a steel sheet according to the present invention will now be described. In the following description, the% content of each component element is expressed as mass percent.

C: 0.030 to 0.060% During annealing with recrystallization, the solute C is reduced by the formation of cementite. In this operation, when the content of C is less than 0.030%, the resistance to elongation can not be adjusted at 230 MPa or less because the degree of supersaturation is too small to precipitate carbides and therefore, the precipitation of the carbides is insufficient. Thus, the lower limit is 0.030%. When the content of the same is greater than 0.060%, the malleability seriously deteriorates. Therefore, the upper limit is 0.060%. Yes: 0.05% or less When a large amount of Si is contained, the workability deteriorates due to hardening or the ability to form plaques is affected by the formation of silicon oxide during annealing. In addition, tartar forms on the surface due to Si concentration and therefore the appearance of the surfaces can probably deteriorate. Thus, the upper limit is 0.05%.

Mn: from 0.1 to 0.3% The Mn converts the harmful S into steel in nS, which is harmless, and therefore its content needs to be 0.1% or more. However, a large amount of Mn causes the deterioration of the malleability due to hardening or suppresses the recrystallization of the ferrite during annealing; therefore, its content needs to be 0.3% or less.

P: 0.05% or less Since the P is separated in the grain boundaries to deteriorate the ductility and hardness, its content needs to be 0.05% or less and preferably 0.03% or less.

S: 0.02% or less S significantly reduces hot ductility and therefore causes hot cracking, to seriously deteriorate the quality of the surfaces. In addition, S contributes significantly to strength and reduces ductility because the S serves as an impurity element to form a coarse nS. These problems are serious when the content of S is greater than 0.02%; therefore, it is preferable that its content be reduced as much as possible. Therefore, the content of S needs to be 0.02% or less.

Al: from 0.02 to 0.10% By fixing the nitrogen in the form of a nitride, so that the hardening with aging can be suppressed because of the N solute. To obtain such an effect, the content of Al needs to be 0.02% or higher. However, a large amount of Al causes deterioration of malleability. In this way, the content of Al needs to be 0.10% or less.

N: 0.005% or less When a large amount of N is contained, Surface imperfections can possibly occur because cracking occurs during hot rolling. The presence of N solute causes hardening with aging after cold rolling or annealing. Therefore, the content of N must be 0.005% or less.

The components different from the previous ones are the iron and the inevitable impurities. Examples of unavoidable impurities include 0.05% or less of Cu and Cr, which are probably contained in scrap, and 0.01% or less of other elements such as Sn, Mo, W, V, Ti, Nb, Ni and B.

The metallographic structure of the steel sheet according to the present invention is constituted substantially of ferrite and cementite. The average grain size of the ferrite of a ferrite microstructure is 7 μm or greater. Larger ferrite grains are achieved in a hot rolling step, as described below.

The steel sheet according to the present invention has an average elongation resistance of 230 MPa or less as determined by the above Equation (a). When the resistance to the average elongation of the same exceeds 230 MPa, the shape fails so that elastic returns are caused in some cases. Thus, the resistance to the average elongation of the same is 230 MPa or less.

The steel sheet according to the present invention has a Lankford value of 0.7 to 1.4 in the rolling direction thereof and a direction perpendicular to the rolling direction. The phenomenon of "torsion" that is due to retentivity in the manner described above, is known to have been caused by buckling of the edge during bending or stretching and can be suppressed by reducing the Lankford value. However, when the value of Lankford is small, traction is difficult. The inventors have found that the Lankford value needs to be from 0.7 to 1.4 as an indicator showing that buckling of the edge is suppressed and traction is possible.

In the case of machining large rectangular flat plates in required parts, in view of the material produced and the operation, it is advantageous that the plates are taken from steel sheets in such a way that the long sides of the rectangular plates are parallel to the rolling direction of the steel sheets or a direction perpendicular to the rolling direction thereof. In the steel sheet according to the present invention, the materials are taken in such a way that they are machined into parts. In order to balance the malleability and flatness of large-sized parts, the upper limit of Lankford's value is limited to 1.4 in the rolling direction and a direction perpendicular to the rolling direction, so in the case of bending an extreme portion of a long side and an end portion of a short side of a rectangular flat plate, it can be prevented that a material is fed to a corner portion thereof and the flatness of the parts can be maintained. In addition, the lower limit of the Lankford value is limited to 0.7, whereby the rigidity of the parts can be prevented from being reduced because of the reduction in the thickness of the corner portion. The lower limit of the Lankford value is preferably greater than 0.7 and more preferably 0.75 or higher.

The steel sheet according to the present invention has an average elongation of 40% or more as determined by Equation (b) above. In addition to the above properties, the average elongation of the same increases to 40% or more, whereby the steel sheet can be pulled and stretched and the necessary shapes for the parts can be secured.

In the steel sheet according to the present invention, the anisotropy in the plane (Ar) of the Lankford value satisfies the inequality -0.2 = Ar = 0.2 as determined by Equation (c) above. In the case of forming backlight chassis for large or similar TVs by traction, "twisting" occurs after stretching in some cases. This occurs because the feeding of a plate is uneven during traction and therefore the thickness of a stretched portion is uneven. Accordingly, the anisotropy in the (Ar) plane of the Lankford value is preferably almost "0" and the feeding of a plate is preferably even; therefore, Ar is limited to the range of -0.2 to 0.2.

In addition to the above, the steel sheet according to the present invention has an elastic limit of 2% or less in each of the lamination direction thereof, an address at 45 ° to the lamination direction thereof, and a direction perpendicular to the rolling direction thereof after the steel sheet is subjected to aging at 170 ° C for 60 minutes. The elastic limit is reduced not only after the manufacture of the steel sheet but also of its aging, so that the formations of creep shapes are suppressed after forming and the steel sheet can be manufactured so that it has excellent superficial appearance.

The conditions for manufacturing the steel sheet according to the present invention are described below. In the present invention, a piece having the The above composition is hot rolled such that its finished rolling temperature is adjusted to (Al-50 ° C transformation temperature) up to (Al + 100 ° C transformation temperature), whereby a sheet of hot rolled steel so that the grain size of the ferrite is increased and the orientation (110) develops in a surface layer of the hot rolled steel sheet during hot rolling. The hot-rolled steel sheet is cold-rolled and then annealed with recrystallization, whereby coarse ferrite grains are formed. This allows low resistance to elongation and excellent elongation to be achieved and the elastic limit to be completely eliminated, whereby an appropriate Lankford value can be obtained.

Heating temperature: 1200 ° C or higher Since it is necessary that carbides such as A1N are converted once into solid solutions during heating, before hot rolling and then finely precipitated after rolling, the heating temperature during hot rolling needs to be 1200 ° C or higher.

Final finished rolling temperature: (Al-50 ° C transformation temperature) a (Al + 100 ° C transformation temperature) The hot lamination needs to be carried out at a termination temperature of (Al-50 ° C transformation temperature) at (Al + 100 ° C transformation temperature), which is a key point of the present invention. This terminates the lamination so that the microstructure of the steel sheet is not austenite but ferrite. The lamination is terminated with a ferrite microstructure, which completes the transformation of austenite to ferrite and stress is caused by rolling at approximately 700 ° C to 800 ° C; therefore, the ferrite grains become thick. This allows the hot-rolled steel sheet to have a coarse grain size. At present, the transformation temperature of Al is about 720 ° C.

Winding temperature: 550 ° C to 680 ° C.

During coiling, the grain size is increased, the carbides are added, and the amount of C solute is reduced.

When the winding temperature after finishing winding is low, the formation of acicular ferrite hardens the steel sheet to cause an increase in the winding force during the subsequent cold rolling and therefore difficulty of the operation is caused. In addition, the aggregation of carbides is insufficient and therefore a large amount of C solute; therefore, the resistance to elongation can not be reduced. Thus, the winding temperature needs to be 550 ° C or higher and preferably is 600 ° C or higher. However, when the winding temperature is higher than 680 ° C, the temperature of an edge portion of a roll of steel sheet (the steel sheet wound into the shape of a roll) relatively decreases, temperature control in the roll it is difficult, and the elongation is reduced. Then, the roll of steel sheet is gripped, a large amount of scale is caused, it is insufficient to remove the scale by pickling before cold rolling, and defects are caused during cold rolling. In this way, the winding temperature needs to be 680 ° C or lower.

Reduction of the lamination (reduction of cold rolling) during the cold rolling: from 50% to 85% The reduction of the cold rolling can be within a common interval. When the reduction of cold rolling is low, the thickness of a hot-rolled steel sheet to obtain a steel sheet with a desired thickness is extremely small and the load during hot rolling is large. Therefore, the lower limit of the reduction of cold rolling It is 50%. The upper limit of it can be about 85%, which is common for cold-rolled smelters.

Average heating speed at 600 ° C or more: from 1 to 30 ° C / s In the annealing of the cold rolled steel sheet, when the heating rate from 600 ° C to the annealing temperature is small, the carbides produced in the hot rolled steel sheet dissolve and the C solute increases. In this way, the average heating speed from 600 ° C up to the annealing temperature needs to be 1 ° C / s or higher. However, when the heating rate is high, the concentration of C in the precipitated carbides is insufficient; therefore, a large amount of solute C remains and the elastic limit can not be reduced. In this way, the average heating speed is 30 ° C / s or less.

Annealing temperature: 700 ° C or higher The annealing temperature can be a suitable temperature for recrystallization. For steels with low carbon content, recrystallization occurs at 700 ° C or more and therefore the annealing temperature is 700 ° C or more. Since the steel sheet is hard when the annealing temperature exceeds the temperature of transformation of AC3, the annealing temperature is preferably not lower than the transformation temperature of Ac3 and more preferably not lower than 800 ° C.

When the time (soaking time) to maintain the annealing temperature (also known as the soaking temperature) is short, the growth of the grains is suppressed whether the recrystallization is complete or not; therefore, in some cases sufficient elongation can not be ensured. In this way, the soaking time is preferably 30 seconds or more. However, when the soaking time is too long, the grains become larger; therefore, the problem of surface roughness occurs during machine work and therefore the quality of the surface is likely to deteriorate. In this way, the soaking time is preferably 200 seconds or less.

Average cooling speed to lower up to 600 ° C: 3 ° C / s or more After heating to the annealing temperature, the steel sheet is cooled. The average cooling speed from the annealing temperature to 600 ° C is less than 3 ° C / s, the carbon precipitated in the form of carbides forms solid solutions again to increase the resistance to elongation. In this way, The average cooling speed from the annealing temperature to 600 ° C needs to be 3 ° C / s or more. However, when the cooling rate exceeds 30 ° C / s, the growth of the ferrite grains will probably be insufficient; therefore, the resistance to elongation is likely to be high and the steel sheet is likely to be hard. In this way, the average cooling speed is preferably 30 ° C / s or less.

A production process such as a common converter process or electric oven process can be used to carry out the present invention. The produced steel is melted into a piece, which is directly hot rolled or cooled, heated and then hot rolled. The hot-rolled steel sheet is terminated under the aforementioned finishing conditions and then rolled up to the aforesaid winding temperature. The cooling speed of the finished roll for rolling is not particularly limited and may be a speed equal to or greater than that obtained by air cooling. Shutdown can be performed at 100 ° C / s or more, as required. After the pickling is carried out, the cold rolling is carried out as described above. The annealing is carried out after the cold rolling in such a way that the heating and then the cooling is carried out under the conditions mentioned above. The cooling rate at temperatures below 600 ° C is arbitrary. The galvanization can be carried out at approximately 480 ° C as required. After the galvanization is performed, a coating can be alloyed by reheating the coating to 500 ° C or more. Alternatively, the thermal history can be preserved by maintenance or the like in the course of cooling. In addition, temper rolling can be performed at an elongation of about 0.5% to 2% as required. If the veneering is not carried out during the annealing, electrogalvanization or similar can be carried out in order to improve the corrosion resistance. In addition, a coating may be provided on a cold rolled steel sheet or a sheet of plated steel by chemical conversion or the like.

EXAMPLE 1 Now examples of the present invention will be described.

Table 1 shows the chemical composition, manufacturing conditions and properties of each specimen.

OR TABLE 1 The underlines show the values that are outside the scope of the present invention.

After the pieces of the chemical compositions shown in Table 1 were produced, each piece was heated to a heating temperature (RT) for one hour, rolled crude and then treated at a termination temperature (FT) and winding temperature (CT). The steels according to the present invention had an Al transformation temperature of about 720 ° C. The hot rolled sheets had a thickness of 2.0 mm to 3.5 mm. After the hot rolled sheets were pickled, the hot rolled sheets were cold rolled and then annealed under the conditions shown in Table 1. The cold rolled sheets had a thickness of 0.6 mm to 1.0 mm. At present, the heating rate is the average heating rate from 600 ° C to the soaking temperature and the cooling rate is the average cooling speed from the soaking temperature to 600 ° C. Cooling was performed from 600 ° C to room temperature at a similar cooling rate. After the annealing was carried out, the lamination was carried out with tempering at a lamination reduction of 1.0%. The ferrite grain size and the mechanical properties of ferrite microstructures were investigated. For tensile strength properties, specimens were cut from Traction according to Japanese Industrial Standard (JIS) No. 5 in a rolling direction (direction L), a rolling direction (direction D) at 45 degrees to the rolling direction, and one direction (direction C) perpendicular to the rolling direction and then subjected to a tensile test at a cross-bar speed of 10 mm / min. The JIS No. 5 traction specimens were cut in the L direction, the D direction and the C direction and then measured to obtain the Lankford value with a pre-tension of 15%. The Lankford value (rL) in the L direction, the Lankford value (rc) in the C direction, the Lankford value (rD) in the D direction and the medium elongation resistance (YSm) were determined. In addition, the elongation resistance in each of the rolling direction (direction L), the direction (direction D) at 45 degrees to the rolling direction and the direction was measured. (direction C) perpendicular to the rolling direction and the aging resistance was measured after keeping at 170 ° C for 60 minutes. In the present, the following equation sustains: Elm = (E1L + 2E1D + Elc) / where each of the subscripts L, D and C indicates El in a corresponding address of the previous directions.

The average ferrite grain size was determined according to JIS G 0551 (2005).

The results of the measurement are summarized in Table 1. The resistance to elongation is the maximum value of the measurements in each direction.

According to Table 1, the steel sheets that were manufactured by a method according to the present invention and having compositions according to the present invention have an average ferrite grain size of 7 μm or more; a resistance to average elongation (YSm) of 230 MPa or less in each of the rolling direction, the direction at 45 degrees to the rolling direction, and the direction perpendicular to the rolling direction; an average elongation (Elm) of 40% or more; a Lankford value (rL, rc) of 0.7 to 1.4 in each of the rolling direction and the direction perpendicular to the rolling direction; and a resistance to aged elongation of 0%. The anisotropy in the plane (Ar) of the Lankford value of the steel sheets satisfies the inequality -0.2 = Ar = 0.2. In contrast, steel sheets having compositions outside the scope of the present invention or having compositions within the scope of the present invention and manufactured by methods outside the scope of the present invention are inferior in any of the YSm, Elm values. , rL, rc, Ar and elastic limit.

Claims (2)

1. Sheet of cold rolled steel that has a composition of 0.030% to 0.060% C, 0.05% or less of Si, 0.1 to 0.3% Mn, 0.05% or less of P, 0.02% or less of S, of 0.02 % to 0.10% of Al, and 0.005% or less of N based on the mass, the rest is constituted by iron and unavoidable impurities, where the resistance to the average elongation (YSm) of the same is 230 MPa or less according to what is determined by the following equation (a); its average elongation (Elm) is 40% or more as determined by the following equation (b); its Lankford value is from 0.7 to 1.4 in a rolling direction and a direction perpendicular to the rolling direction; the anisotropy in the plane (Ar) of its Lankford value satisfies the inequality -0.2 < Ar < 0.2 as determined by the following equation (c); and the yield strength of the cold-rolled steel sheet maintained at 170 ° C for 60 minutes is 2% or less in the rolling direction, a 45 degree direction to the rolling direction, and the direction perpendicular to the direction Rolling: YSm = (YSL + 2YSD + YSC) / 4 (a) Elm = (E1L + 2E1D + Elc) / 4 (b) Ar = (rL - 2rD + rc) / 2 (c) where YSm is the resistance to elongation mean, Elm is the average elongation, ñr is the anisotropy in the Lankford value plane, YSL is the elongation resistance in the rolling direction, YSD is the elongation resistance in the 45 degree direction to the rolling direction, YSC is the resistance to elongation in the direction perpendicular to the direction of rolling, E1L is the elongation in the direction of rolling, E1D is the elongation in the direction at 45 degrees to the direction of rolling, Elc is the elongation in the perpendicular direction to the rolling direction, rL is the Lankford value in the rolling direction, rD is the Lankford value in the 45 degree direction to the rolling direction, and rc is the Lankford value in the direction perpendicular to the direction of lamination.

2. Method for manufacturing a cold rolled steel sheet comprising heating a piece of steel having the composition specified in claim 1 to a heating temperature of 1200 ° C or more, performing the hot rolling so that the final rolling is finish at a temperature of (transformation temperature from Al - 50 ° C) to (transformation temperature of Al + 100 ° C), perform cooling at 550 ° C up to 680 ° C, perform pickling, perform cold rolling at a reduction of lamination from 50% to 85%, perform heating to a annealing temperature of 700 ° C or higher from 600 ° C or more at an average heating rate of 1 to 30 ° C / s, and then cooling to 600 ° C at an average cooling rate of 3 ° C / sec plus. ARY A sheet of cold rolled steel with low carbon content that has excellent formability, shape retentivity and appearance of the surfaces, the steel sheet that combines processing capacity with shape retentivity, is capable of being processed by Stretch, fold, or bulge and ensure a required shape of a large part, and has a high degree of flatness that does not generate appearance flaws. A process for producing the cold rolled steel sheet is provided. The cold rolled steel sheet is characterized by having a composition that comprises, in terms of% by mass, from 0.030 to 0.060% C, up to 0.05% Si, from 0.1 to 0.3% Mn, up to 0.05% of P, up to 0.02% of S, from 0.02 to 0.10% of Al, up to 0.005% of N, and the rest consisting of iron and unavoidable impurities, has an r value of 0.7 to 1.4 when examined in the direction of rolling and in the direction perpendicular to the direction of rolling, has an anisotropy of value r in the plane (Ar) of -0.2 = Ar = 0.2, has a resistance to average elongation and an average elongation in three directions, ie the direction of rolling , an address forming an angle of 45 ° with the direction of rolling, and the direction perpendicular to the direction of rolling, of 230 MPa or less and 40% or more, respectively, and has an elastic limit after 60 minutes keeping at 170 ° C of 2% or less in each of the three directions.