KR101263612B1

KR101263612B1 - Cold-rolled steel sheet with excellent formability, shape retentivity, and surface appearance and process for producing same

Info

Publication number: KR101263612B1
Application number: KR1020117025095A
Authority: KR
Inventors: 에이코 야스하라; 고이치로 후지타; 다로 기즈; 가즈히로 하나자와; 시게히로 다카조
Original assignee: 제이에프이 스틸 가부시키가이샤
Priority date: 2009-04-28
Filing date: 2010-02-19
Publication date: 2013-05-10
Also published as: EP2431490A1; KR20110137816A; EP2431490B1; JP4962527B2; EP2431490A4; WO2010125848A1; PL2431490T3; MY150597A; CN102414334A; CN102414334B; JP2010255069A; MX2011010580A

Abstract

In low carbon steel, both workability and shape freezing can be achieved, drawing processing, bending processing, and elongation processing can be performed, the shape required for large parts can be ensured, flatness is high, and appearance defects are generated. A cold rolled steel sheet excellent in moldability, shape freezing property and surface appearance, and a manufacturing method thereof. The composition is% by mass, C: 0.030 to 0.060%, Si: 0.05% or less, Mn: 0.1 to 0.3%, P: 0.05% or less, S: 0.02% or less, Al: 0.02 to 0.10%, N: 0.005% or less And remainder are iron and unavoidable impurities, r values in the rolling direction and the rolling right angle direction are 0.7 to 1.4, in-plane anisotropy (Δr) of the r value is -0.2 ≦ Δr ≦ 0.2, rolling direction, rolling 45 ° direction, rolling The average yield strength in the three directions in the right angle direction is 230 MPa or less, the average elongation is 40% or more, and the yield elongation after holding 170 ° C. for 60 minutes in all three directions is 2% or less.

Description

Cold rolled steel sheet excellent in formability, shape freezing, surface appearance, and manufacturing method thereof {COLD-ROLLED STEEL SHEET WITH EXCELLENT FORMABILITY, SHAPE RETENTIVITY, AND SURFACE APPEARANCE AND PROCESS FOR PRODUCING SAME}

The present invention relates to a cold rolled steel sheet having excellent moldability, shape freezing property and surface properties as a member of a part having a large flat plate shape, such as a backlight chassis of a large liquid crystal television, and a manufacturing method thereof.

In recent years, with the increase of the size of a thin television, the backlight chassis of a liquid crystal television is also enlarged. In addition, the demand for weight reduction and material cost reduction of TVs is large, and the backlight chassis is also thinned. On the other hand, in the backlight chassis, since the rigidity for supporting the light and the light does not collide with or crack in the liquid crystal part, the flatness is good and the deformation is hard to occur, and there is no so-called "twist feeling". Required. As the size and thickness of the backlight chassis increase, the demand for rigidity and flatness is becoming more stringent.

In order to secure rigidity, it is effective to form a bead by elongate molding on the flat surface of a backlight chassis. However, processing of the flat surface causes new problems such as deterioration of flatness and increase in "twist feeling". Moreover, the same problem arises also when securing rigidity by bending process of the edge part. Such deterioration of flatness is a phenomenon which occurs because of poor shape freezing property at the time of press molding, and therefore, the shape freezing property is further required for the steel sheet as a member together with workability.

Conventionally, as a steel plate excellent in shape freezing property, as disclosed in Patent Literature 1, at least one of the r values in the rolling direction and the rolling right angle direction is at least 0.7, while controlling the texture of the assembly. There is a steel sheet to be used. It is disclosed that the springback amount at the time of bending of this steel plate is small. In addition, Patent Document 2 discloses a method of suppressing spring back and wall warping at the time of bending by controlling the anisotropy of local stretching and uniform stretching. In addition, Patent Document 3 discloses a method of suppressing spring back during bending by setting the ratio of the {100} plane and the {111} plane to 1.0 or more.

Moreover, also in the drawing shaping | molding of the backlight chassis of a large sized TV, there exists a problem of "twisting" after shaping | molding, and this arises because the plate | board thickness of a shaping | molding part becomes nonuniform because the inflow of the plate in drawing shaping is nonuniform.

In addition, at the time of these moldings, wrinkles called stretcher strains occur, causing problems such as poor flatness of the backlight chassis and poor appearance. Patent Document 4 adds an appropriate amount of B as a method of reducing yield elongation of low carbon steel, which is the cause of the stretcher strain, and in the cold rolling, the centerline valley depth Rv, which is an index of surface roughness, is 0.5 to 10 µm. The method of making centerline average roughness Ra into 0.5 micrometer or more is disclosed.

Japanese Patent No. 3532138 Japanese Laid-Open Patent Publication 2004-183057 International Publication No. 00/06791 Pamphlet Japanese Patent Application Laid-Open No. 04-276023

In recent years, in the backlight chassis of a large-size liquid crystal television of 32 inches or more, the market is expanding very much, in order to secure rigidity even if it is thinned, in many cases, it responds to the increase in the elongation height and the increase in the elongation site. High stretching is necessary. However, in the technique described in patent documents 1-3, there exists a problem that workability for ensuring the required part shape and rigidity is not acquired.

It is known that lowering the yield strength is effective for suppressing springback. In general, low-carbon steels have a high yield strength and insufficient stretching, and thus extremely low carbon steels are used in difficult-to-work parts. In order to lower the yield strength, softening the steel is effective. As a technique, an increase in annealing temperature and an increase in cold rolling reduction rate are effective. However, due to softening of the steel, the aggregate structure of the (111) orientation develops and the r value increases. It is thought that low carbon steel can be applied to components such as backlight chassis of large TVs by achieving both softening of the steel for springback suppression and reduction of r value for suppression of distortion caused by bending. . However, in the low carbon steel up to now, only soft nitriding of steel and high r value have been aimed at.

In addition, in the problem of shape flatness and poor appearance, the yield stretching is small not only immediately after steel sheet production but until the steel sheet is molded, that is, the characteristic after aging is important. However, in the technique described in Patent Document 4, it is necessary to satisfy both the cooling rate and re-aging conditions during recrystallization annealing together with the definition of the surface roughness of the cold rolled material, which leads to a problem that the control of manufacturing conditions is complicated. have.

An object of the present invention is to solve such a problem of the prior art. That is, in low carbon steel, both workability and shape freezing can be achieved, drawing processing, bending processing, and elongation processing can be performed, the shape required for large parts can be ensured, and the flatness is high. It is to provide a cold rolled steel sheet excellent in moldability, shape freezing property, and surface appearance which does not occur, and a production method thereof.

The characteristics of the present invention for solving such a problem are as follows.

(1) In mass%, C: 0.030 to 0.060%, Si: 0.05% or less, Mn: 0.1 to 0.3%, P: 0.05% or less, S: 0.02% or less, Al: 0.02 to 0.10%, N: 0.005% or less, and the balance of to iron and inevitable with impurities of a composition having, (a) expression as the average yield strength (YS _m) the average elongation (El _m) is not more than 230 ㎫, also represented by the following (b) expression representing the It is 40% or more, the r value of a rolling direction and a rolling right angle direction is 0.7-1.4, the in-plane anisotropy ((D) r) of r value represented by following (c) formula is -0.2 <(DELTA) r <= 0.2, The rolling direction, rolling 45 degrees Direction and the rolling elongation after hold | maintaining for 60 minutes at 170 degreeC in all the directions of a right angle direction of a rolling is 2% or less, The cold rolled sheet steel characterized by the above-mentioned.

Average yield strength YS _m = (YS _L + 2YS _D + YS _C ) / 4. (a)

Average elongation El _m = (El _L + 2El _D + El _C ) / 4... (b)

in-plane anisotropy Δr = (r _L -2r _D + r _C ) / 2. (c)

Here, YS _L : yield strength in the rolling direction

YS _D : Yield strength in rolling 45 ° direction

YS _C : Yield strength in rolling right direction

El _L : Stretching in the rolling direction

El _D : stretching in the rolling 45 ° direction

El _C : Stretch in the rolling right direction

r _L : r value in the rolling direction

r _D : r value in the rolling 45 ° direction

r _C : r value in the right angle direction of rolling

After heating the slab of the steel which consists of (2) and the composition of (1) at the heating temperature of 1200 degreeC or more, hot rolling which complete | finishes finish rolling at (Al transformation point -50 degreeC)-(Al transformation point +100 degreeC) is performed. After winding up at 550 to 680 ° C, pickling was performed, and after cold rolling at a reduction ratio of 50 to 85%, the average heating rate at 600 ° C or higher was 1 to 30 ° C / s. Heating at an annealing temperature of 700 ° C. or higher, and then cooling the average cooling rate up to 600 ° C. to 3 ° C./s or more.

The present invention has been made as a result of intensive studies to solve the above problems. Here, in taking out a large rectangular flat plate from a steel plate, and processing into the required part, the long side of the rectangle is collected in parallel with the rolling direction or the rolling right angle direction of the steel sheet, and the working surface. Is advantageous in In the case of taking such a material, the present invention can achieve both workability and surface properties even in a large part. In other words, by increasing the average stretching, the drawing processing and the elongation processing can be performed, and the shape required for the component can be secured. In addition, by reducing the yield strength, it is possible to suppress the occurrence of springback after processing and ensure shape freezing property. Moreover, shape freezing property can be ensured by making r value of a rolling direction and a rolling right angle direction into 0.7-1.4, and setting it as -0.2 <= r <= 0.2.

In addition, the most important point is that the yield elongation after aging is 2% or less, thereby suppressing the generation of the strainer strain during processing and excellent surface appearance, and suppressing the occurrence of springback after processing to ensure the shape freezing property. will be.

In the present invention, the mechanism of improving the stretching and reducing the yield strength is considered as follows. That is, at the time of hot rolling, finish temperature is set to (Al transformation point -50 degreeC)-(Al transformation point +100 degreeC), and the grain size of a ferrite structure is coarsened by finishing rolling by ferrite instead of austenite. Thereby, the crystal grain size after cold rolling and recrystallization annealing can be coarsened and softened.

On the other hand, by finishing finishing rolling in the temperature range of (Al transformation point -50 degreeC)-(Al transformation point +100 degreeC) in hot rolling, (110) orientation is formed in a hot-rolled sheet surface layer, and this is cold-rolled and recrystallized annealing. As a result, the (110) orientation develops, thereby maintaining a low r value. This makes it possible to soften by coarsening of the ferrite particles while keeping a low r value. In addition, since the yield stretching completely disappears and the yield stretching after aging also becomes small, generation | occurrence | production of the stretcher strain after shaping | molding can be suppressed, and the steel plate excellent in surface appearance can be obtained. Although the details of the reason why the yield elongation disappears in the present invention are not clear, the mechanism is considered as follows. That is, the (110) orientation is known as an orientation in which deformation easily accumulates. As this orientation develops in the surface layer, deformation due to cold rolling or temper rolling can be easily introduced. Since so-called dislocation is easy to generate | occur | produce by this, it is estimated that a stretcher strain hardly arises.

In addition, the cold rolled steel sheet of the present invention is a steel sheet excellent in surface appearance that does not generate strain strain when a cold rolled steel sheet having a sheet thickness of 1.0 to 0.5 mm is used. It also includes a steel plate subjected to surface treatment such as plating or hot dip galvanizing. Moreover, the steel plate in which the film was formed by chemical conversion treatment etc. is also included on it.

In addition, the steel sheet of the present invention has not only a backlight chassis of a large TV but also a flat panel such as a panel of a refrigerator, an air conditioner outdoor unit, and the like, and a member of a general household appliance use that performs bending, elongation, hardness drawing, or the like. Can be widely used in By using the present invention, a backlight chassis of about 850 mm × 650 mm (42 V type) can be manufactured, for example, from a steel plate having a plate thickness of 0.8 mm.

According to the present invention, it is possible to obtain low yield stretching and excellent stretching, low yield strength, and yield stretching after low aging, and both formability and shape freezing can be achieved to achieve drawing processing, bending processing, and elongation processing. A cold rolled steel sheet having excellent shape freezing property and excellent surface appearance is obtained. Thereby, the flat plate shape required for a large component can be ensured, and members, such as a backlight chassis of a large liquid crystal television, can be manufactured.

The chemical component of the steel plate of this invention is demonstrated. In addition, in the following description, all content% of a component element means the mass%.

C: 0.030% to 0.060%

Cementite is formed during recrystallization annealing to reduce the solid solution C. However, if the amount of C is less than 0.030%, the supersaturation degree for carbide precipitation is small and the precipitation strength of carbide is not sufficient, so the yield strength is set to 230 MPa or less. Can't. Therefore, the lower limit was made into 0.030%. On the other hand, when it adds exceeding 0.060%, workability becomes remarkably bad. Therefore, the upper limit was made into 0.060%.

Si: not more than 0.05%

When a large amount of Si is contained, workability deteriorates due to hardening or plating property is inhibited by generation of Si oxide during annealing. Moreover, at the time of hot rolling, the scale in which Si was concentrated is formed in the surface, and there exists a possibility of inhibiting surface appearance. Therefore, the upper limit was made into 0.05%.

Mn: 0.1% to 0.3%

Since Mn is made harmless by making S into MnS in harmful steel, it is necessary to set it as 0.1% or more. On the other hand, since a large amount of Mn suppresses workability deterioration by hardening and recrystallization of ferrite at the time of annealing, it is necessary to make it 0.3% or less.

P: not more than 0.05%

P segregates at grain boundaries and deteriorates ductility and toughness, so it is required to be 0.05% or less. Preferably it is 0.03% or less.

S: not more than 0.02%

S significantly lowers the ductility in hot, causing hot cracking and significantly deteriorating the surface properties. In addition, S hardly contributes to the strength and reduces ductility by forming coarse MnS as an impurity element. These problems become remarkable when the amount of S exceeds 0.02%, and it is preferable to reduce as much as possible. Therefore, the amount of S needs to be 0.02% or less.

Al: 0.02% to 0.10%

Al can suppress aging hardening by solid solution N by fixing N as nitride. In order to acquire such an effect, Al amount needs to be 0.02% or more. On the other hand, a large amount of Al deteriorates workability. Therefore, Al amount needs to be 0.10% or less.

N: 0.005% or less

When N is contained in a large amount, there is a fear that scratches may occur on the surface along with slab cracking during hot rolling. Moreover, when it exists as solid solution N after cold rolling and annealing, an age hardening is produced. Therefore, N amount needs to be 0.005% or less.

Components other than the above consist of iron and unavoidable impurities. As unavoidable impurities, for example, 0.05% or less of Cu, Cr, which are easily mixed from scraps, and 0.01% or less of Sn, Mo, W, V, Ti, Nb, Ni, and B may be mentioned.

The metal structure of the steel plate of this invention consists of a ferrite and cementite generally. Moreover, the average ferrite particle diameter of a ferrite structure is 7 micrometers or more. Coarse ferrite particles are realized by a hot rolling process as shown below.

The steel plate of this invention makes the average yield strength calculated | required by the said (a) formula into 230 Mpa or less. When the average yield strength exceeds 230 MPa, shape defects, such as a springback, may arise. For this reason, average yield strength shall be 230 Mpa or less.

The steel plate of this invention makes r value of 0.7-1.4 in a rolling direction and a rolling right angle direction. It is known that "twist" which appears as a phenomenon resulting from shape freezing as mentioned above arises by ridge bending at the time of bending process and elongation process, but this can be suppressed by making r value low. On the other hand, at low r values, drawing molding becomes difficult. The present inventors have found that 0.7 or more and 1.4 or less are required as an index of the r value capable of suppressing the ridge bending state and allowing drawing processing.

As described above, in processing a large rectangular flat plate into the required parts, the long side of the rectangle is advantageously obtained in parallel with the rolling direction or the rolling right angle direction of the steel sheet in terms of the yield of the material and the operation aspect. In the steel sheet of the present invention, the material is taken as such, and in order to achieve good workability and flatness even in large parts, the upper limit of the r value in the rolling direction and the rolling right angle direction is regulated to 1.4, in particular, the long side and In the case of bending the end portion on the short side, the inflow of the material at the corner portion can be suppressed, and the flatness of the part can be maintained. In addition, by regulating the lower limit of the r value to 0.7, it is possible to suppress the decrease in the rigidity of the component accompanying the decrease in the thickness of the corner portion. More preferably, the minimum of r value is more than 0.7, and it is preferable to set it as 0.75 or more.

The steel plate of this invention makes 40% or more of average stretch calculated | required by the said (b) formula. In addition to the above characteristics, by increasing the average stretching to 40% or more, drawing processing and elongation processing can also be performed, and the shape required for the component can be ensured.

The steel plate of this invention makes in-plane anisotropy ((D) r) of r value calculated | required by said (c) formula to be -0.2 <= (D) r <= 0.2. When drawing-molding the backlight chassis of a large-sized TV, etc., a "twist" may generate | occur | produce after shaping | molding. This is caused by the nonuniformity of the plate thickness in the forming part, which is caused by the nonuniformity of the plate thickness in the forming part. For this purpose, the anisotropy (Δr) of the r value is around "0" and the inflow of the plate from each direction This uniform thing is preferable and (DELTA) r is prescribed | regulated to -0.2 or more and 0.2 or less range.

In addition to the above, the steel sheet of this invention makes yield stretching after aging after hold | maintaining for 60 minutes at 170 degreeC in all the directions of a rolling direction, the rolling 45 degree direction, and a rolling right angle direction to 2% or less. By reducing yield stretching not only immediately after steel sheet production but also after aging, the strainer strain after shaping | molding can be suppressed and the steel plate excellent in surface appearance can be manufactured.

Next, the manufacturing conditions of the steel plate of this invention are demonstrated. In the present invention, the low-carbon steel slab having the above-mentioned composition is subjected to a finish rolling temperature of (Al transformation point -50 ° C) to (Al transformation point + 100 ° C) in hot rolling to coarsen the ferrite grain size during hot rolling, In addition, by developing a (110) orientation in the surface layer of the hot rolled sheet to produce a hot rolled steel sheet, and cold rolling and recrystallization annealing this to form coarse ferrite particles, low yield strength and excellent elongation can be obtained. Yield elongation can be completely extinguished to obtain an appropriate value of r.

Heating temperature: above 1200 ℃

When hot rolling, it is necessary to solidify carbides, such as AlN, once during heating, and to make it precipitate fine after winding, and the heating temperature of hot rolling needs to be 1200 degreeC or more.

Finish rolling finish temperature: (Al transformation point -50 ° C)-(Al transformation point + 100 ° C)

It is an important point of this invention, and the finishing temperature at the time of hot rolling needs to be performed at (Al transformation point -50 degreeC)-(Al transformation point +100 degreeC). This completes the rolling of the steel structure with ferrite instead of austenite. By finishing rolling with a ferrite structure, the transformation from austenite to ferrite is completed by finishing rolling, and the deformation | transformation by rolling is provided at about 700 to 800 degreeC, and a ferrite particle coarsens. As a result, the grain size of the hot rolled sheet is coarsened. Al transformation point here is about 720 degreeC.

Winding temperature: 550 ℃ to 680 ℃

At the time of coil winding, a grain size is coarsened, carbides are aggregated, and solid solution C is reduced.

When the coiling temperature after finishing rolling is low, steel sheet becomes hard by generation | occurrence | production of acyclic ferrite, and the rolling load at the time of subsequent cold rolling will become high, and it is accompanied by operational difficulty. In addition, the aggregation of carbides is insufficient, so that a large amount of solid solution C remains, so that the yield strength cannot be reduced. Therefore, it is necessary to make winding temperature 550 degreeC or more, Preferably it is 600 degreeC or more. On the other hand, when the coiling temperature becomes higher than 680 ° C, the temperature of the edge portion of the steel sheet coil (steel sheet wound in coil shape) is relatively lowered, making it difficult to control the temperature in the coil and the yield is lowered. In addition, aging of the steel sheet coil may occur, or a large amount of scale may be generated, resulting in insufficient scale peeling due to pickling performed before cold rolling, resulting in defects during cold rolling. Therefore, the coiling temperature needs to be 680 degrees C or less.

Rolling reduction rate (cold rolling ratio) at cold rolling: 50% to 85%

Although the cold-rolling rate should just be a range currently performed, when the cold-rolling rate is low, since the thickness of the hot rolled sheet for obtaining the steel plate of desired thickness becomes too thin, and the load at the time of hot rolling becomes large, the minimum of cold-rolling rate shall be 50%. In addition, an upper limit may be 85% of the grade performed with a normal cold rolling mill.

Average heating rate above 600 ℃: 1 ~ 30 ℃ / s

In the annealing of the cold rolled sheet, if the heating rate from 600 ° C. to the annealing temperature is small, the average heating rate from 600 ° C. to the annealing temperature is 1 ° C./s because the carbide produced in the hot rolled sheet dissolves and the solid solution C increases. It is necessary to do the above. On the other hand, when the heating rate is large, the concentration of C to the precipitated carbide is insufficient, so that a large amount of solid solution C remains, and yield elongation cannot be reduced. For this reason, an average heating rate shall be 30 degrees C / s or less.

Annealing Temperature: Above 700 ℃

The annealing temperature may be a temperature to be recrystallized. In the case of low carbon steel, the annealing temperature is set to 700 ° C or higher because the recrystallization is usually performed at 700 ° C or higher. In addition, when annealing temperature exceeds Ac3 transformation point temperature, since a steel plate becomes hard, it is preferable to set it as Ac3 transformation point temperature or less, and it is more preferable to set it as 800 degrees C or less.

If the time (cracking time) to be maintained at the annealing temperature (also referred to as the cracking temperature) is short, re-crystallization may not be completed or grain growth may be suppressed even when it is completed, and thus sufficient stretching may not be ensured. For this reason, it is preferable to make crack time into 30 s or more. On the other hand, when the crack time is too long, the particles grow and become large, which causes a problem that the surface becomes rough during processing, which tends to deteriorate the surface properties. For this reason, it is preferable to make a crack time into 200 s or less.

Average cooling rate up to 600 ℃: 3 ℃ / s or more

After heating to the annealing temperature, the steel sheet is cooled. When the average cooling rate from the annealing temperature to 600 ° C. is slower than 3 ° C./s, C precipitated as carbide is reclaimed to increase the yield strength. For this reason, the average cooling rate from annealing temperature to 600 degreeC needs to be 3 degreeC / s or more. On the other hand, if the cooling rate is more than 30 ° C / s or more, the growth of the ferrite particles tends to be insufficient, and the yield strength is high, making it easy to become hard. For this reason, it is preferable to make an average cooling rate into 30 degrees C / s or less.

In the practice of the present invention, the solvent method can be suitably applied, for example, a conventional converter method, a converter method, or the like. The molten steel is hot-rolled after casting into a slab, or as it is cooled and heated. In hot rolling, it winds up at the winding temperature mentioned above after finishing on the finishing conditions mentioned above. Although the cooling rate until winding after finishing rolling is not specifically prescribed | regulated, it is sufficient if there exists a cooling rate more than air cooling. Moreover, you may perform rapid cooling of 100 degreeC / s or more as needed. Thereafter, the above-mentioned cold rolling is performed after normal pickling. About annealing after cold rolling, heating and cooling are performed on the conditions mentioned above. The cooling rate in the area | region below 600 degreeC is arbitrary, and you may plating with molten zinc in the vicinity of 480 degreeC as needed. Moreover, after plating, you may reheat at 500 degreeC or more and alloy plating. Or you may implement thermal history, such as holding in the middle of cooling. Moreover, you may perform temper rolling at the elongation of about 0.5 to 2% as needed. Moreover, when plating is not performed in the middle of annealing, in order to improve corrosion resistance, you may give electro zinc plating etc. Moreover, you may form the film by a chemical conversion process etc. on a cold rolled sheet steel or a plated steel sheet.

Example 1

An embodiment of the present invention will be described.

Table 1 shows the chemical composition, production conditions, and characteristic values of the specimens.

After the slab having the chemical composition shown in Table 1 was dissolved, the slab was heated for 1 hour at the heating temperature (RT) in the table, and after rough rolling, the finishing temperature (FT) and the winding temperature (CT) shown in the table were used. In addition, Al transformation point of the steel of this invention was about 720 degreeC. The thickness of the hot rolled sheet was 2.0 to 3.5 mm. After pickling a hot rolled sheet, it cold-rolled on the conditions shown in Table 1, and then performed the annealing process. In addition, the plate | board thickness after cold rolling was 0.6-1.0 mm. Here, a heating rate is an average heating rate from 600 degreeC to a crack temperature, and a cooling rate is an average cooling rate from a crack temperature to 600 degreeC. Furthermore, after 600 degreeC, it cooled to room temperature at the same cooling rate. After annealing, temper rolling of 1.0% of the reduction ratio was performed, and the ferrite grain size and the mechanical properties of the ferrite structure were examined. Here, the tensile characteristics cut out the JIS No. 5 tensile test piece from the rolling direction (L direction), rolling 45 degrees (D direction), and the rolling right angle direction (C direction), and performed the tensile test at the tensile speed of 10 mm / min. . The r value cut out the JIS No. 5 tensile test piece from each direction of L, C, and D, and measured it by 15% of deformation. The r value (r _L ) in the _L direction, the r value (r _C ) in the _C direction, the r value (r _d ) in the D direction, the average yield strength (YSm), and the average elongation (Elm) were determined. In addition, yield stretching was measured in all directions of a rolling direction (L direction), rolling 45 degree (D direction), and a rolling right angle direction (C direction), and also the yield extension after aging after hold | maintaining at 170 degreeC for 60 minutes was also measured.

From here,

El _m = (El _L + 2El _D + El _C ) / 4

And the subscripts of L, D and C represent El in each direction.

In addition, the ferrite average particle diameter was calculated | required based on JISG0555 (2005).

The measurement results are shown in Table 1 together. Yield elongation showed the maximum value of the measurement result in each direction.

According to Table 1, the steel plate which has the composition of this invention and manufactured by the manufacturing method of this invention has a ferrite average particle diameter of 7 micrometers or more, and the average yield strength (YSm) of a rolling direction, the rolling 45 degree direction, and a rolling right angle direction is is not more than 230 ㎫, an average elongation (Elm) is not less than 40%, the r value in the rolling direction and at right angles the rolling direction _(L r, _C r) is 0.7 ~ 1.4. the planar anisotropy (Δr) of the r value is -0.2 ≤ Δr ≤ 0.2 and the yield stretching after aging was 0%. The composition or out of the scope of the present invention for a composition of the range is outside the steel sheet of the present invention in even the manufacturing method scope of the present invention, YSm, Elm, r _L, r _C, Δr, fell to yield any of the stretching.

Claims

In mass%, C: 0.030 to 0.060%, Si: 0.05% or less (but not including 0%), Mn: 0.1 to 0.3%, P: 0.05% or less (but not including 0%), S: 0.02% or less (but not including 0%), Al: 0.02 to 0.10%, N: 0.005% or less (but not including 0%), and the balance has iron and inevitable impurities. Together, the average yield strength (YS _m ) represented by the following (a) formula is 230 MPa or less, and the average elongation (El _m ) represented by the following (b) formula is 40% or more, r in the rolling direction and the rolling right angle direction The value is 0.7-1.4, the in-plane anisotropy (Δr) of the r value represented by the following formula (c) is -0.2 ≦ Δr ≦ 0.2, and is 170 ° C. in all directions in the rolling direction, the rolling 45 ° direction, and the rolling right angle direction. The yield stretch after 60-minute hold | maintain is 2% or less, The cold rolled steel plate characterized by the above-mentioned.
Average yield strength YS _m = (YS _L + 2YS _D + YS _C ) / 4. (a)
Average elongation El _m = (El _L + 2El _D + El _C ) / 4... (b)
in-plane anisotropy Δr = (r _L -2r _D + r _C ) / 2. (c)
Here, YS _L : yield strength in the rolling direction
YS _D : Yield strength in rolling 45 ° direction
YS _C : Yield strength in rolling right direction
El _L : Stretching in the rolling direction
El _D : stretching in the rolling 45 ° direction
El _C : Stretch in the rolling right direction
r _L : r value in the rolling direction
r _D : r value in the rolling 45 ° direction
r _C : r value in the right angle direction of rolling

After heating the slab of the steel which consists of a composition of Claim 1 at the heating temperature of 1200 degreeC or more, hot rolling which complete | finishes finish rolling at (Al transformation point -50 degreeC)-(Al transformation point +100 degreeC) is performed, and 550- After winding at 680 ° C, pickling was performed, and after cold rolling at a reduction ratio of 50 to 85%, an annealing at 700 ° C or higher was performed at an average heating rate of 600 ° C or higher as 1 to 30 ° C / s. It heats to temperature, and it cools by making the average cooling rate up to 600 degreeC into 3 degreeC / s or more after that, The manufacturing method of the cold rolled sheet steel characterized by the above-mentioned.