JP5481920B2 - Cold-rolled steel sheet excellent in formability and shape freezing property, and manufacturing method thereof - Google Patents

Description

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

  In recent years, along with the increase in size of flat-screen TVs, the backlight chassis of LCD TVs has also increased in size. In addition, there are great demands for lighter TVs and lower material costs, and the backlight chassis is also becoming thinner. However, on the other hand, the backlight chassis has the rigidity to support the light, the flatness to prevent the light from hitting or breaking the liquid crystal part, and the so-called “sticky” feeling that does not easily cause bending. Therefore, as the backlight chassis becomes larger and thinner, problems such as rigidity and flatness are becoming obvious.

  In order to ensure rigidity, it is effective to form a bead by overmolding on the flat surface of the backlight chassis. However, when the flat surface is processed, the flatness is inferior or the feeling of stickiness increases. Problems such as these also arise. Furthermore, the same problem arises when the rigidity is secured by bending the end portion. Such deterioration of flatness is a phenomenon that occurs due to poor shape freezing properties during press molding, so steel plates as members are increasingly required to have shape freezing properties as well as workability. ing.

  Conventionally, as a steel sheet excellent in shape freezing property, for example, as disclosed in Patent Document 1, the texture is controlled, and at least one of the r values in the rolling direction or the direction perpendicular to the rolling is 0.7 or less. Thus, there is a steel plate manufactured by a method of reducing the amount of spring back during bending. Patent Document 2 discloses a method for suppressing springback and wall warping during bending by controlling the anisotropy of local elongation and uniform elongation. Furthermore, Patent Document 3 discloses a method of suppressing spring back during bending by setting the ratio of the {100} plane to the {111} plane to 1.0 or more.

  In addition, there is a problem of stickiness after molding in the backlight molding of a large-screen TV backlight chassis. This is due to the uneven flow of the plate in the drawing, and the uneven thickness of the molded part. , Which is what happens.

Japanese Patent No. 3532138 JP 2004-183057 A International Publication No. 00/066791 Pamphlet

  However, the techniques described in Patent Documents 1 to 3 have a certain effect at the time of bending, but there is a problem that the shape freezing property is not always maintained in the case of, for example, overhanging. Furthermore, in recent years, the backlight chassis of large-sized liquid crystal televisions having a size of 32 inches or more, for which the market is extremely expanding, has a problem that it is difficult to obtain the required workability to ensure the required component shape and rigidity. It is known that the stickiness seen as a phenomenon of shape freezing occurs due to ridge warping caused by bending or overhanging. This can be suppressed by lowering the r value, but on the other hand, drawing at a low r value becomes difficult.

  It is known that yield strength affects the suppression of springback. In general, low carbon steel has a high yield strength and is not sufficiently stretched. Therefore, an extremely low carbon steel is often used in a region where processing is severe. On the other hand, in order to lower the yield strength, it is effective to soften the steel, and as its method, increasing the annealing temperature and increasing the cold rolling reduction ratio are effective. However, due to the softening of steel, a (111) -oriented texture develops and becomes a high r value. Thus, by solving all of the softening of steel for suppressing springback and the reduction of r value for suppressing distortion caused by bending, it becomes the first time to become a component such as a backlight chassis of a large TV. It is thought that the application of low carbon steel is possible. However, in conventional low carbon steels, measures have been taken to aim for softening of the steel and a high r value.

  Therefore, the object of the present invention is to solve such problems of the prior art, and in low carbon steel, it is possible to carry out drawing processing, bending processing, and overhanging processing while achieving both workability and shape freezing, and large parts. An object of the present invention is to provide a cold-rolled steel sheet excellent in formability and shape freezing property, and a method for producing the same.

The features of the present invention for solving such problems 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 %, And the balance is iron and inevitable impurities, the area ratio of the ferrite structure is 80% or more, and the average yield strength (YS _m ) represented by the following formula (a) is 230 MPa or less. and elongation of the average represented by the following equation (b) (El _m) is not less than 40%, cold-rolled steel sheet, wherein the r value in the rolling direction and the direction perpendicular to the rolling direction is 0.7 to 1.4.
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)
Where YS _L : Yield strength in rolling direction
YS _D : Yield strength in 45 ° direction of rolling
YS _C : Yield strength in the direction perpendicular to rolling
El _L : Elongation in rolling direction
El _D : Rolling 45 ° direction elongation
El _C : Elongation in the direction perpendicular to rolling (2), hot slab having a composition described in (1) is heated at a heating temperature of 1200 ° C. or higher, and then hot rolling is performed to finish finish rolling at 850 to 950 ° C. After winding at 700 to 800 ° C., pickling, and after performing cold rolling at a reduction rate of 50 to 90%, annealing is performed from 600 ° C. (Ac1 transformation point + 100 ° C. ) The temperature range up to the above soaking temperature is heated at an average heating rate of 1 to 30 ° C / s, soaking is performed at the soaking temperature for 30 to 200s, and then the average cooling rate to 600 ° C is set to 3 A method for producing a cold-rolled steel sheet, characterized by cooling at -30 ° C / s.

  The present invention has been made as a result of intensive studies to solve the above-described problems. Here, after taking a large rectangular flat plate from a steel plate, when processing into the required parts, the long side of the rectangle is taken in parallel with the rolling direction of the steel plate or the direction perpendicular to the rolling direction. This is advantageous in terms of yield and operation. The present invention can achieve both workability and surface properties even in a large part when taking the material in such a manner. That is, by increasing the average elongation, drawing and overhanging can be performed, and the shape required for the part can be ensured. Furthermore, by reducing the yield strength, it is possible to suppress the occurrence of springback after processing and to secure the shape freezing property. Moreover, shape freezing property is securable by making r value of a rolling direction and a rolling orthogonal direction into 0.7-1.4.

  In the present invention, the mechanism for improving elongation and reducing yield strength is considered as follows. That is, during hot rolling, by controlling the coiling temperature, carbide is generated and the crystal grain size of the ferrite structure is coarsened. Furthermore, by setting the annealing temperature to a high temperature (Ac1 transformation point + 100 ° C.) or higher, the remaining solid solution C is precipitated as carbides in the ferrite grains and at the grain boundaries, and the solid solution C is reduced. By optimizing growth, high elongation and low yield strength of 230 MPa or less can be achieved. The lower the yield strength, the better. In the present invention, the yield strength is more preferably 200 MPa or less. However, since the r value is improved by crystal grain growth in the high temperature annealing in the α region, the r value is increased by annealing in the γ region above (Ac1 transformation point + 100 ° C.) in order to maintain a low r value. It is necessary to reduce the (111) orientation and develop the (110) and (100) orientations. Thereby, it came to complete this invention by ferrite grain coarsening and carbide | carbonized_material formation with low r value.

  In addition, the cold-rolled steel plate which this invention makes object also includes the steel plate which gave surface treatments, such as electrogalvanization and hot-dip galvanization, to the cold-rolled steel plate. Further, it includes a steel plate having a film formed thereon by chemical conversion treatment or the like.

  Moreover, the steel plate of the present invention is widely used not only for the backlight chassis of large TVs, but also for general members for home appliances such as refrigerator panels, air conditioner outdoor units, etc. that bend, project, lightly draw, etc. with flat portions. be able to. If this invention is used, the backlight chassis about 650x500mm (32V type) or more can be manufactured, for example with the steel plate of plate thickness 0.8mm.

  According to the present invention, excellent elongation and low yield strength can be obtained, both workability and shape freezing properties can be achieved, and drawability, bending processing, and overhanging processing can be performed, and formability and shape freezing properties are excellent. Cold rolled steel sheet is obtained. Thereby, a flat plate shape required for a large component can be secured, and a member such as a backlight chassis of a large liquid crystal television can be manufactured.

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

C: 0.030 to 0.060%
In order to reduce the yield strength to 230 MPa or less, it is necessary to precipitate cementite in the grains and at the grain boundaries during annealing to reduce the solid solution C as much as possible. For this purpose, it is necessary to leave an appropriate amount of C in the cold-rolled sheet before annealing. If the amount of C is less than 0.030%, the supersaturation degree for carbide precipitation is small and the carbide precipitation is not sufficient, so the yield strength cannot be made 230 MPa or less. Therefore, the lower limit was made 0.030%. On the other hand, when it exceeds 0.060%, workability is remarkably deteriorated. Therefore, the upper limit was made 0.060%.

Si: 0.05% or less
If Si is contained in a large amount, the workability deteriorates due to hardening, and the plating property is hindered due to the generation of Si oxide during annealing. Also, during hot rolling, the temperature at which the steel transforms from austenite to ferrite rises, making it difficult to finish rolling in the austenite region. Therefore, Si needs to be 0.05% or less.

Mn: 0.1-0.3%
Mn needs to be 0.1% or more in order to detoxify harmful steel S as MnS. On the other hand, a large amount of Mn needs to be 0.3% or less because deterioration of workability due to hardening and recrystallization of ferrite during annealing are suppressed.

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

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

Al: 0.02-0.10%
Al can suppress age hardening due to solute N by fixing N as a nitride. In order to obtain such an effect, the Al amount needs to be 0.02% or more. On the other hand, a large amount of Al increases the temperature at which the steel transforms from austenite to ferrite during hot rolling, making it difficult to finish rolling in the austenite region. Therefore, the 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 risk that surface flaws occur due to slab cracking during hot rolling. Moreover, when it exists as solid solution N after cold rolling and annealing, age hardening will be caused. Therefore, the N amount needs to be 0.005% or less.

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

  The metal structure of the steel sheet of the present invention is ferrite with an area ratio of 80% or more. While the area ratio of the ferrite structure is 80% or more, hard structures other than the ferrite structure such as cementite, pearlite, martensite, and bainite are contained in an area ratio of 20% or less. Yield strength of 230 MPa or less can be achieved by mainly using a ferrite structure. On the other hand, if the ferrite structure is 100%, the r value may be too high, which is not preferable.

  The steel sheet of the present invention has an average yield strength obtained by the above formula (a) of 230 MPa or less. If the average yield strength exceeds 230 MPa, shape defects such as springback may occur. For this reason, the average yield strength is 230 MPa or less.

  The steel sheet of the present invention has an r value of 0.7 to 1.4 in the rolling direction and the direction perpendicular to the rolling direction. As described above, it is known that the unevenness seen as a phenomenon of shape freezing is caused by ridge line warping by bending and overhanging, but this can be suppressed by lowering the r value. On the other hand, when the r value is low, drawing is difficult. The present inventors have found that 0.7 to 1.4 is required as an index of r value that suppresses ridge warping and enables drawing.

  As described above, when processing a large rectangular flat plate into a required part, the long side of the rectangle is taken in parallel with the rolling direction of the coil, or the direction perpendicular to the rolling direction, and the yield of the material, It is advantageous from the point of operation. Even in the steel plate of the present invention, the material is taken as such to make a part. However, in order to achieve both workability and flatness even in a large part, the rolling direction and the right angle of rolling are used. By restricting the upper limit of the r value of the direction to 1.4, in particular, when bending the long side and short side ends of a rectangular flat plate, the inflow of material at the corner can be suppressed, The flatness can be maintained. Further, by restricting the lower limit of the r value to 0.7, it is possible to suppress a decrease in the rigidity of the component due to a decrease in the thickness of the corner portion. More preferably, the lower limit of the r value is more than 0.7 and is preferably 0.75 or more.

  The steel sheet of the present invention has an average elongation of 40% or more determined by the equation (b). In addition to the above characteristics, when the average elongation is increased to 40% or more, drawing and overhanging can be performed, and the shape required for the parts can be ensured.

  Next, manufacturing conditions for the steel sheet of the present invention will be described. In the present invention, the low carbon steel slab having the above composition is subjected to a soaking process by controlling the coiling temperature by controlling the coiling temperature and heating at a soaking temperature of 900 ° C. or higher. Thus, it is possible to obtain excellent elongation and low yield strength by agglomerating carbides to reduce the amount of solid solution C in the ferrite and promoting the grain growth of ferrite.

Heating temperature: 1200 ° C or higher When hot-rolling, precipitates such as AlN must be dissolved once during heating, and fine precipitation is required after winding, so the heating temperature for hot rolling should be 1200 ° C or higher There is a need.

Finishing rolling finish temperature: 850-950 ° C
If the austenite region is changed to the ferrite region in the course of finish rolling, the rolling load is drastically decreased, and the load control of the rolling mill becomes difficult, so that there is a risk that breakage or the like occurs in the plate. Although this risk can be avoided if the sheet is passed through the ferrite region from the entry side of finish rolling, the rolling temperature decreases, the structure of the hot rolled sheet becomes non-recrystallized ferrite, and the rolling load during cold rolling. There is also a problem that increases. Therefore, finish rolling needs to be finished in the austenite region, and is finished at 850 ° C. or higher. On the other hand, if the finishing temperature of finish rolling becomes too high, the crystal grains become coarse, the r value of the cold-rolled annealed plate is lowered, and processing becomes impossible. Therefore, finish rolling must be completed at 950 ° C. or lower.

Winding temperature: 700 ℃ ~ 800 ℃
The coiling temperature is important in the present invention in order to coarsen the crystal grain size and coagulate the carbides and reduce the solid solution C during coil winding.

  When the coiling temperature after finish rolling is low, the steel plate becomes hard due to the generation of acicular ferrite, and the rolling load at the time of subsequent cold rolling becomes high, resulting in operational difficulties. Further, the agglomeration of carbides is insufficient and a large amount of solute C remains, and the yield strength cannot be reduced. Therefore, the winding temperature needs to be 700 ° C. or higher. On the other hand, if the coiling temperature rises above 800 ° C, seizure occurs between the steel sheets wound in a coil shape, or a large amount of scale is generated, resulting in insufficient scale peeling during pickling before cold rolling. And may become defective during cold rolling. Therefore, the winding temperature needs to be 800 ° C. or less.

Reduction ratio during cold rolling (cold reduction ratio): 50% to 90%
When the rolling reduction in the cold rolling after pickling the hot-rolled sheet is small, the crystal rotation due to rolling becomes insufficient, so that the texture cannot be developed, the r value becomes low, and deep drawing Can not be. In addition, the higher the cold pressure ratio, the more recrystallized at a lower temperature, and the crystal grain size is likely to become coarser. Therefore, the cold pressure ratio needs to exceed 50%. On the other hand, in order to perform cold rolling at a reduction ratio exceeding 90%, for example, to obtain a cold-rolled annealed sheet having a thickness of 0.6 mm, the thickness of the hot-rolled sheet is set to 6 mm (cold pressure ratio 90%). Therefore, the load on the cold rolling mill is large and the operation is difficult.

Average heating rate from 600 ℃ to soaking temperature: 1-30 ℃ / s
In the annealing of cold-rolled sheets, if the heating rate from 600 ° C to the soaking temperature is small, the carbide generated in the hot-rolled plate is dissolved and the solid solution C increases, so the average heating rate is 1 ° C / s or more. There is a need to. On the other hand, if the heating rate is large, a large amount of ferrite remains until the soaking temperature, the concentration of C in the precipitated carbide becomes insufficient, a large amount of solid solution C remains, the yield strength increases, and the elongation increases. Also decreases. For this reason, an average heating rate shall be 30 degrees C / s or less.

Soaking temperature (annealing temperature): (Ac1 transformation point + 100 ° C.) or more In order to reduce the yield strength, it is important to coarsen the crystal grains and to coarsen the carbides. For this reason, the soaking temperature at the time of soaking needs to be (Ac1 transformation point + 100 ° C.) or more. This is because by setting the Ac1 transformation point or higher, the solid solution C diffuses quickly and precipitates as carbides at the grain boundaries. For solubilization C diffusion, a soaking temperature of Ac1 transformation point + 100 ° C (about 820 ° C) or higher is required. On the other hand, the upper limit of the soaking temperature is 1000 ° C. If it exceeds 1000 ° C, the crystal grain size becomes coarse, and it tends to cause defects such as rough skin during processing.

Soaking time: 30 ~ 200s
If the soaking time (soaking time) for maintaining the soaking temperature is short, recrystallization will not be completed, or even if it is completed, grain growth will be suppressed and elongation will be reduced. Further, the diffusion of the solid solution C from the ferrite grains to the austenite grains becomes insufficient, so that the solid solution C remains in the ferrite grains, the yield strength increases, and the elongation decreases. Therefore, the soaking time at the time of soaking needs to be 30 s or more. On the other hand, when the soaking time is long, the grains grow and become large, so that the problem of rough skin occurs during processing, and the surface properties deteriorate. Therefore, the soaking time during heating needs to be 200 s or less.

Cooling rate from soaking temperature to 600 ° C: 3-30 ° C / s
After maintaining the soaking temperature, the steel sheet is cooled, but when the cooling rate is slower than 3 ° C./s, C precipitated as carbide re-dissolves and raises the yield strength. For this reason, the average cooling rate from the soaking temperature to 600 ° C. needs to be 3 ° C./s or more. On the other hand, when the cooling rate exceeds 30 ° C./s or more, transformation from austenite grains to ferrite grains becomes insufficient, r value and elongation decrease, and ferrite grain growth is insufficient, yield strength. Is high and hard. For this reason, a cooling rate shall be 30 degrees C / s or less.

  In carrying out the present invention, a melting method can be appropriately applied, such as a normal converter method and an electric furnace method. The molten steel is cast into a slab and then heated as it is or after cooling and hot rolling. In hot rolling, after finishing under the above-mentioned finishing conditions, winding is performed at the above-described winding temperature. The cooling rate from finish rolling to winding is not particularly limited, but a cooling rate higher than air cooling is sufficient. Moreover, you may perform rapid cooling of 100 degree-C / s or more as needed. Then, after the usual pickling, the above-mentioned cold rolling is performed. About annealing (heating-uniform heat treatment-cooling), heating to cooling is performed under the above-described conditions. The cooling rate in the region lower than 600 ° C. is arbitrary, and if necessary, plating with hot dip zinc may be performed at around 480 ° C. Further, after plating, the plating may be alloyed by reheating to 500 ° C. or higher. Alternatively, a heat history such as holding during cooling may be taken. Furthermore, if necessary, temper rolling may be performed at an elongation of about 0.5 to 2%. In addition, when plating is not performed during annealing, electrogalvanization or the like may be performed in order to improve corrosion resistance. Further, a film may be formed on the cold-rolled steel plate or the plated steel plate by chemical conversion treatment or the like.

  Examples of the present invention will be described.

Table 1 shows the chemical composition, production conditions, and characteristic values of the specimen. The Ac1 transformation point was determined from the components of each steel according to the following formula.
Ac1 transformation point = 723 + 29.1 × [Si%]-10.7 × [Mn%] + 16.9 × [Cr%]

After melting the slab having the chemical composition shown in Table 1, the slab is heated for 1 hour at the heating temperature (RT) in the table, and after rough rolling, the finishing temperature (FT) and winding temperature (CT) shown in the table are shown. ). The thickness of the hot rolled sheet was 2.0 to 3.5 mm. After pickling, cold rolling, heating, soaking, and cooling were performed under the conditions shown in Table 1. The plate thickness after cold rolling was 0.6 to 1.0 mm. Here, the heating rate is an average heating rate from 600 ° C. to the soaking temperature, and the cooling rate is an average cooling rate from the soaking temperature to 600 ° C. In addition, after 600 degreeC, it cooled to room temperature with the same cooling rate. After annealing, temper rolling with a rolling reduction of 1.0% was performed, and the area ratio and mechanical properties of the ferrite structure were investigated. Here, as for the tensile properties, a JIS No. 5 tensile test piece was cut out from the rolling direction (L direction), the 45 ° direction (D direction), and the direction perpendicular to the rolling direction (C direction), and a tensile test was performed at a tensile speed of 10 mm / min. . The r value was measured at a pre-strain of 15% by cutting a JIS No. 5 tensile test piece from each of the L, C, and D directions. Then, 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 obtained.

  The measurement results are also shown in Table 1.

According to Table 1, the steel sheet having the composition of the present invention and manufactured by the manufacturing method of the present invention has an area ratio of ferrite structure of 80% or more, and the average of rolling direction, rolling 45 ° direction, and rolling perpendicular direction. Yield strength (YSm) was 230 MPa or less, average elongation (Elm) was 40% or more, and r values (r _L , r _C ) in the rolling direction and the direction perpendicular to the rolling were 0.7 to 1.4. On the other hand, a steel sheet whose composition is outside the scope of the present invention or whose manufacturing method is outside the scope of the present invention even if the composition is within the scope of the present invention is any of YSm, Elm, r _L , and r _C. It was inferior.

  In the steel sheet of the present invention, hard structures such as cementite, pearlite, and bainite were observed in addition to the ferrite structure.

Claims (2)

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, The balance is iron and inevitable impurities, the area ratio of the ferrite structure is 80% or more, the average yield strength (YS _m ) represented by the following formula (a) is 230 MPa or less, and the following ( b) A cold-rolled steel sheet having an average elongation (El _m ) represented by the formula of 40% or more and an r value in a rolling direction and a direction perpendicular to the rolling of 0.7 to 1.4.
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)
Where YS _L : Yield strength in rolling direction
YS _D : Yield strength in 45 ° direction of rolling
YS _C : Yield strength in the direction perpendicular to rolling
El _L : Elongation in rolling direction
El _D : Rolling 45 ° direction elongation
El _C : Elongation in the direction perpendicular to rolling After the steel slab having the composition according to claim 1 is heated at a heating temperature of 1200 ° C. or higher, hot rolling is performed to finish finish rolling at 850 to 950 ° C., and the steel slab is wound at 700 to 800 ° C. After pickling, cold rolling at a reduction rate of 50 to 90%, and annealing, the temperature range from 600 ° C to the soaking temperature of (Ac1 transformation point + 100 ° C) or higher is 1 Heating at an average heating rate of ˜30 ° C./s, performing a soaking treatment at the soaking temperature for 30 to 200 s, and then cooling at an average cooling rate up to 600 ° C. at 3 to 30 ° C./s. The manufacturing method of the cold-rolled steel plate of Claim 1 .