JP2011144428A - Cold rolled steel sheet and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet which has excellent moldability and shape fixability, and has excellent machine production stability, and a method for producing the same. <P>SOLUTION: The cold rolled steel sheet has a composition comprising 0.0010 to 0.0030% C, ≤0.05% Si, 0.1 to 0.3% Mn, ≤0.05% P, ≤0.02% S, 0.02 to 0.10% Al, ≤0.005% N, 0.010 to 0.030% Nb and B: 0.0010≤B-11/14×N≤0.0050%, and the balance iron with inevitable impurities. Then, the mean elongation (El<SB>m</SB>) is ≥42%, and the mean r value (r<SB>m</SB>) is 1.2 to 1.6. Further, in annealing after cold rolling, the steel sheet is held at a soaking temperature of (820+Nb/15+B-CR) to 860°C for 30 to 200 sec in accordance with a draft CR(%), the Nb content (mass ppm) and the B content (mass ppm), thus the steel sheet can be stably produced by an actual machine without controlling the soaking temperature by pinpoints. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

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

Thin flat panel TVs and office automation equipment use a large number of flat-plate parts formed by processing mainly bending and stretching. The members (materials) used for these parts often require flatness in addition to the degree of processing into the part shape from the viewpoints of product design and thickness reduction. However, flatness tends to deteriorate when bending / extrusion molding is performed on a flat plate surface of a member (material). Such deterioration of flatness is a phenomenon that occurs due to poor shape freezing property of the member (material) when the member (material) is press-molded. Shape freezing property is required.
Further, as a factor that deteriorates the flatness, ridge warpage that occurs during bending is well known. It is said that the warp that occurs at the bending end, which is one of them, is reduced by lowering the r value of the material. Conventionally, there is a technique for imparting a low r value and low yield strength to the material. Has been established.
For example, Patent Document 1 discloses a technology for developing a cold-rolled steel sheet having a yield strength of 150 MPa and an r value of 0.67 in the rolling direction (a direction perpendicular to the rolling direction of 1.45).
Patent Document 2 discloses a ferritic steel sheet for automobiles excellent in shape freezing property, in which the ratio of {100} plane and {111} plane parallel to the plate surface is 1.0 or more.
In Patent Document 3, the strength of {100} <011> to {223} <110> orientation group and {112} <110>, {554} <225>, {111} <112>, {111} <110>, controlling the strength of each direction, at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is 0.7 or less Is disclosed.

Japanese Patent No. 3532138 JP 2008-255491 A JP 2003-55739 A

However, in the techniques described in Patent Documents 1 and 3, although the r value in the rolling direction and the direction perpendicular to the rolling direction is reduced by the crystal grains having the {100} <011> to {223} <110> orientation, the rolling 45 ° In order to increase the r value in the direction, the steel having many {100} <011> to {223} <110> oriented grains may lose flatness after press forming. For example, when a long bead-shaped overhang molding is applied to a backlight chassis used in a thin-screen liquid crystal television so that the longitudinal direction is a 45 ° rolling direction, ridge line warpage that occurs in the bead longitudinal direction and the vicinity of the overhanging portion There is a problem that the backlight chassis is warped due to the material inflow of the flange portion.
Patent Documents 1 to 3 disclose a case where the cold rolling rate of the cold-rolled steel sheet is low, specifically, the case where the r value is low as 40%. When rolling at a rate, there is a problem that the plate shape deteriorates and the flatness of the final product deteriorates.
In addition, such a low cold rolling rate improves the shape freezing property, but makes it difficult to produce a thin steel sheet of about 1.0 mm or less.

  Therefore, the present inventors have paid attention to ultra-low carbon steel, which is easy to obtain high ductility and hardly causes stretcher strain that causes molding breakage and wrinkling during press molding. In general, extremely low carbon steel sheets such as Ti-based IF steels need to have a low r value because recrystallized grains that are advantageous for increasing the r value easily grow in the annealing process. In order to lower the r value, annealing is performed at a low temperature at which recrystallization is difficult but grain growth is difficult, and in order to achieve this, it is necessary to control the annealing conditions under conditions where the soaking temperature exhibits a variation of only a few degrees C. . However, in actual machines, materials with different soaking temperatures set during annealing pass through the continuous annealing equipment in sequence, so the soaking temperature may vary by several tens of degrees Celsius, and the soaking temperature may be within several degrees Celsius. It is difficult to control with a range. Therefore, there is a problem that annealing is performed at a temperature higher than the target temperature, and a low r value cannot be achieved. In Patent Documents 1 to 3, there is a similar problem, and although there is a description regarding the condition of the annealing temperature, the difficulty of controlling the annealing temperature to a pinpoint is not considered.

  In view of such circumstances, the present invention solves such problems of the prior art, can perform drawing, bending, and overhanging, and can secure the shape required for large parts, An object of the present invention is to provide a cold-rolled steel sheet having excellent shape freezing property and excellent actual machine manufacturing stability, and a method for manufacturing the same.

Inventors repeated earnest research investigation in order to solve the said subject.
As a result, the following points were found. First, in order to achieve both workability in which warpage does not occur even when complicated processing is performed and high shape freezing property, the r value is controlled while maintaining high ductility in the annealed sheet after cold rolling. is important. That is, by increasing the average elongation, it is possible to ensure the shape required for the part while ensuring the workability during drawing and overhanging. Furthermore, by reducing the r value in the three directions of the rolling direction, the direction perpendicular to the rolling direction, and the 45 ° direction of rolling, and within the range that defines the average r value, the longitudinal direction becomes the 45 ° direction of rolling. In addition, even when elongated bead-shaped overhang molding is performed, warpage is prevented, and the occurrence of springback and warpage after processing is suppressed, and the shape freezing property can be secured.
By providing the soaking temperature range in the annealing process in accordance with the rolling reduction, the Nb amount, and the B amount, it is possible to stably manufacture the soaking temperature without pinpointing the soaking temperature.
This invention is made | formed based on the above knowledge, The summary is as follows.
(1) By mass%, C: 0.0010 to 0.0030%, 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% In the following, Nb: 0.010 to 0.030%, B: 0.0010 ≦ B-11 / 14 × N ≦ 0.0050%, the balance is iron and inevitable impurities, and the average elongation represented by the following formula (a) (El _m) is not less 42% or more, cold-rolled steel sheet, wherein the r value of the average represented by the following equation (b) (r _m) is 1.2 to 1.6.
Average elongation El _m = (El _L + 2El _D + El _C ) / 4 (a)
Average r value r _m = (r _L + 2r _D + r _C ) / 4 (b)
Here, El _L : Elongation in the rolling direction, El _D : Elongation in the rolling 45 ° direction, El _C : Elongation in the direction perpendicular to the rolling, r _L : R value in the rolling direction, r _D : R value in the rolling 45 ° direction, r _C : r value in the direction perpendicular to rolling (2) In the above (1), Ti: 0.005% to 0.020% is further contained in mass%, and B: 0.0010 ≦ B−11 / 14 × N ≦ 0.0050% Instead of B: 0.0015 <B-11 × (| N / 14−Ti / 48 | + (N / 14−Ti / 48)) / 2 ≦ 0.0050% cold-rolled steel sheet,
(3) After heating the steel slab having the composition described in (1) or (2) above at a heating temperature of 1150 ° C or higher, hot rolling is performed to finish finishing rolling at a finishing temperature of 880 ° C or higher. After rolling at 700 ° C or lower, pickling, and cold rolling at a rolling reduction of 55-80%, the rolling reduction CR (%), Nb amount (mass ppm) and B amount (mass ppm) A method for producing a cold-rolled steel sheet, characterized in that annealing is carried out for 30 to 200 seconds at a soaking temperature of (820 + Nb / 15 + B-CR) to 860 ° C., followed by cooling.

  In the present specification, “%” and “ppm” indicating the components of steel are mass% and mass ppm, respectively. Moreover, the cold-rolled steel sheet targeted by the present invention includes a steel sheet obtained by subjecting the cold-rolled steel sheet to surface treatment such as electrogalvanizing or hot dip galvanizing. Further, it includes a steel plate having a film formed thereon by chemical conversion treatment or the like.

  Further, the steel sheet of the present invention can be widely used as a general member for home appliances that bends, projects, and slightly draws flat surfaces such as a large-screen TV backlight chassis, refrigerator panel, and air conditioner outdoor unit. . Furthermore, if this invention is used, the backlight chassis about 650x500mm (32V type | mold) or more can be manufactured, for example with the steel plate of plate | board thickness 0.8mm.

  ADVANTAGE OF THE INVENTION According to this invention, the cold rolled steel plate excellent in the moldability and shape freezing property which can perform a drawing process, a bending process, and an overhanging process is obtained. And the cold-rolled steel plate excellent in formability and shape freezing property can be stably manufactured with an actual machine, without controlling the soaking temperature at the time of annealing pinpoint. Thereby, a flat plate shape required for a large component can be secured, and a member such as a backlight chassis of a large liquid crystal television can be manufactured.

It is a figure which shows the relationship between cold rolling rate (rolling rate) CR, soaking temperature, and performance. It is a figure which shows the relationship between Nb [mass ppm] / 15 + B [mass ppm], soaking temperature, and performance. It is a figure which shows the influence which the soaking temperature at the time of annealing has on the average r value. It is a figure which shows the relationship between AT-AT0 which is a parameter | index of whether the soaking | uniform-heating temperature at the time of annealing is appropriate, and the average r value. It is a figure which shows the relationship between AT-AT0 which is a parameter | index of whether the soaking | uniform-heating temperature at the time of annealing is appropriate, and average elongation.

  Hereinafter, the present invention will be described in detail. First, the chemical components of the steel sheet of the present invention will be described. In the following description, the content% and ppm of the component elements mean mass% and mass ppm, respectively.

C: 0.0010 to 0.0030%
Since C has the effect of reducing the r value, it is desirable to add a large amount from the viewpoint of lowering the r value, and it is 0.0010% or more. However, excessive addition leads to stretcher strain that causes wrinkles during press molding, and at the same time, the steel sheet becomes stronger due to solid solution strengthening and dispersion strengthening due to the formation of carbides, and the elongation decreases. Therefore, C is 0.0010% or more and 0.0030% or less.

Si: 0.05% or less
Si is an element having a high solid solution strengthening ability, increases the yield strength, and greatly reduces the elongation. Therefore, it is 0.05% or less.

Mn: 0.1-0.3%
Mn is an element that forms sulfides and improves hot brittleness. In order to obtain this effect, addition of 0.1% or more is necessary. On the other hand, the upper limit is made 0.3% in order to increase yield strength and degrade ductility because it is an element with high solid solution strengthening ability.

P: 0.05% or less
P is a solid solution strengthening element that increases yield strength and degrades ductility. Therefore, it is 0.05% or less.

S: 0.02% or less
S forms sulfides at the stage of hot rolling and causes ductility to deteriorate. Therefore, 0.02% or less.

Al: 0.02-0.10%
Al has a strong affinity with N, and has the effect of reducing the amount of solute N during the cold rolling process and suppressing age hardening. Moreover, the precipitated AlN has a high tendency to precipitate finely and suppresses the grain growth in the annealing process. In order to obtain these effects, addition of 0.02% or more is necessary. However, excessive addition leads to an increase in production cost, and during hot rolling, the temperature at which austenite is transformed into ferrite is raised, so that it is difficult to finish rolling in the austenite region. Therefore, Al needs to be 0.10% or less.

N: 0.005% or less
If N is contained in a large amount, it dissolves in the steel and causes stretcher strain. Therefore, it is 0.005% or less.

Nb: 0.010 to 0.030%
Nb fixes solute C as precipitates and suppresses stretcher strain. Furthermore, NbC, which is a precipitate, tends to precipitate finely and suppress grain growth during annealing. In order to obtain these effects, it is necessary to add 0.010% or more. However, when excessively added, the recrystallization temperature is excessively increased. In addition, the cost increases. Therefore, the upper limit is made 0.030%.

B: 0.0010 ≦ B-11 / 14 × N ≦ 0.0050% (in the case of Ti addition, 0.0015 <B−11 × (| N / 14−Ti / 48 | + (N / 14−Ti / 48)) / 2 ≦ 0.0050%)
B is an element which is an important requirement in the present invention. By making it exist as a solid solution B in steel, B suppresses the grain growth of ferrite in the annealing process after cold rolling, and controls the r value even at a high soaking temperature. It is possible to do. In order to obtain such an effect, it is necessary that solute B is present in an amount of 0.0010% or more after BN is precipitated at a high temperature during hot rolling. However, excessive addition forms precipitates with C and reduces elongation. For the above reasons, 0.0010 ≦ B−11 / 14 × N ≦ 0.0050%. Preferably 0.0015 <B-11 / 14 × N ≦ 0.0050%.
Further, when Ti is added, coarse TiC is precipitated as compared with NbC, and the grain growth property at the time of annealing is improved. Therefore, the amount of solute B having an effect of suppressing grain growth needs to exceed 0.0015%. However, excessive addition forms precipitates with C and reduces elongation. For these reasons, instead of the relational expression B: 0.0010 ≦ B−11 / 14 × N ≦ 0.0050%, 0.0015% <B−11 × (| N / 14−Ti / 48 | + (N / 14− Ti / 48)) / 2 ≦ 0.0050%. Preferably, 0.0020% ≦ B−11 × (| N / 14−Ti / 48 | + (N / 14−Ti / 48)) / 2 ≦ 0.0050%.

In addition to the above elements, in the present invention, Ti can be contained in the range of 0.005% to 0.020% for the following purpose.
Ti has a strong affinity for N and forms precipitates at high temperatures, and has the effect of suppressing solid solution strengthening of solid solution N and dispersion strengthening due to fine precipitation of AlN. Moreover, it can also be added when it is desired to particularly improve the elongation. In order to exert these effects, it is desirable to add 0.005% or more. However, excessive addition promotes the precipitation of TiC and not only reduces the effect of suppressing grain growth during annealing with NbC, but also increases the manufacturing cost, so the upper limit when adding is 0.020% .

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

The steel sheet of the present invention has an average elongation calculated by the following formula (a) of 42% or more. Elongation has a good correlation with moldability, and the larger the elongation, for example, the higher the stretch molding. Therefore, the larger the required elongation, the better, and by increasing the average elongation to 42% or more, drawing processing and overhanging processing can be performed, and the shape required for the parts can be ensured.
The average total elongation can be measured and determined by the following method. A JIS No. 5 tensile test piece is cut out from the rolling direction, the 45 ° direction, and the direction perpendicular to the rolling direction, and a tensile test based on JIS Z 2241 is performed. And it calculates | requires by the following (a) formula.
Average elongation El _m = (El _L + 2El _D + El _C ) / 4 (a)
Where El _L : Elongation in rolling direction
El _D : Rolling 45 ° direction elongation
El _C : Elongation in the direction perpendicular to rolling The steel sheet of the present invention has an average r value of 1.2 to 1.6 determined by the following formula (b). The r value correlates with the warpage that occurs after bending or stretch forming. In bending, the r-value in the bending direction increases, so that saddle-shaped warpage occurs remarkably along the bending line. In the stretch forming, the use of a high r-value material makes the material inflow from the flange portion around the stretch portion remarkable, and residual stress and distortion occur in the flange portion. Therefore, for the purpose of improving the shape freezing property after press molding by lowering the r value, the average r value is set to 1.6 or less in the present invention. On the other hand, excessively lowering the r value extremely reduces the elongation, so the lower limit of the average r value is 1.2.

The average r value can be measured and determined by the following method. JIS No. 5 tensile specimens are cut out from the rolling direction, the 45 ° direction and the direction perpendicular to the rolling direction, respectively, and a plastic strain ratio test based on JIS Z 2254 is performed at a pre-strain of 15%. And it calculates | requires by the following (b) formula.
Average r value r _m = (r _L + 2r _D + r _C ) / 4 (b)
Here, r _L : r value in the rolling direction r _D : r value in the 45 ° direction of rolling r _C : r value in the direction perpendicular to the rolling direction In addition, for example, in a backlight chassis used in a thin LCD TV, the longitudinal direction is 45 When an elongated bead-shaped bulge forming is performed, the ridge line warpage that occurs in the longitudinal direction of the bead and the warpage of the backlight chassis caused by the material inflow of the flange portion in the vicinity of the bulging portion are 45 ° in the rolling direction. The higher the r value, the larger. Therefore, it is important to reduce the r value in the three directions of the rolling direction, the perpendicular direction of rolling, and the 45 ° direction of rolling, and the average r value is preferably 1.2 to 1.6, preferably the rolling direction, the perpendicular direction of rolling, and the rolling. The maximum r value in the three directions of 45 ° is preferably less than 2.0.

Next, the manufacturing method of the cold rolled steel sheet of this invention is demonstrated.
In the present invention, the steel slab having the above composition is heated at a heating temperature of 1150 ° C. or higher, then hot rolled to finish the finish rolling at a finishing temperature of 880 ° C. or higher, and wound at 700 ° C. or lower, After pickling, and then cold rolling at a reduction rate of 55 to 80%, according to the reduction rate CR (%), Nb amount (mass ppm) and B amount (mass ppm), (820 High elongation and a low r value can be obtained by annealing for 30 to 200 seconds at a soaking temperature of + Nb / 15 + B-CR) to 860 ° C., followed by cooling.

Heating temperature: 1150 ° C or higher steel material with adjusted components is heated to 1150 ° C or higher to precipitate precipitates. With heating below 1150 ° C, N and C remain undissolved, and during graining or annealing, carbide and nitride do not precipitate finely and the grain growth suppression effect during annealing is sufficient. Is not demonstrated. Accordingly, the higher the heating temperature is, the better, 1150 ° C. or higher, desirably 1200 ° C. or higher. However, if heating is performed excessively, a thick oxide scale is formed on the steel surface and the cost of pickling treatment is increased.

Finishing temperature: 880 ° C or higher followed by hot rolling. In the final rolling stand at the time of finish rolling, it is necessary to perform rolling in an austenite region single phase. When rolling in the two-phase region of austenite and ferrite, the rolling load changes greatly with the transformation, so that it becomes difficult to control the load at the rolling stand, and the plate may be broken. In addition, rolling in a ferrite region single phase promotes the remaining of non-recrystallization and may increase the manufacturing cost by excessively increasing the rolling load in the subsequent cold rolling. From the above, the finish temperature is set to 880 ° C. or higher, and finish rolling is performed in an austenite single phase. If the finishing temperature is too high, the oxide scale becomes thick and the cost of pickling treatment increases, so it is preferable to set the finishing temperature to 950 ° C. or lower.
It is preferable to perform water cooling after finish rolling. It is known that when C and N are dissolved before cold rolling, the r value is reduced. In order to suppress the precipitation of C and N, the precipitated carbides and nitrides are refined, and the grains during annealing are reduced. In order to suppress growth, the cooling is preferably performed.

Winding temperature: 700 ° C. or lower As the final step of hot rolling, the hot rolled coil is wound at 700 ° C. or lower. When the coiling temperature exceeds 700 ° C., Al, Nb, and Ti form carbides and nitrides at a high temperature, which is not preferable from the viewpoint of securing solid solution C and solid solution N before cold rolling, which affects the r value reduction. In addition, since the winding shape of a coil will worsen when coiling temperature becomes low too much, it is preferable to set it as 400 degreeC or more.

Rolling ratio during cold rolling (CR): 55-80%
The hot-rolled sheet is pickled by a normal method. Next, cold rolling with a rolling reduction (CR) of 55% or more and 80% or less is performed to form a desired plate thickness. When the rolling reduction (CR) is less than 55%, the structure becomes mixed and the ductility is extremely reduced. On the other hand, when the rolling reduction (CR) is higher than 80%, it is easy to form a texture that is disadvantageous for reducing the r value after annealing.

Soaking temperature: (820 + Nb / 15 + B-CR) ~ 860 ℃, Retention time: 30 ~ 200sec
Next, as the annealing process, the soaking temperature is (820 + Nb / 15 + B-CR) to 860 ° C depending on the rolling reduction CR (%), Nb amount (mass ppm), B amount (mass ppm) In the range of 30-200s. Thereafter, cooling is performed.

In the annealing step, the strain introduced by cold rolling is eliminated by recrystallization, and the steel sheet is softened. The temperature at which recrystallization is completed becomes lower as the rolling reduction (CR) is higher and the amount of additive elements, particularly Nb and B, is smaller. Therefore, the soaking temperature needs to be (820 + Nb / 15 + B-CR) or higher in order to prevent a decrease in elongation due to remaining unrecrystallized structure. The reason for limiting the soaking temperature will be described later. On the other hand, when the recrystallization temperature is excessively increased, the manufacturing cost is increased, and ferrite is transformed into austenite, and the structure is excessively refined and increased in strength by transformation from austenite to ferrite during subsequent cooling. Along with this, the elongation decreases. Therefore, the soaking temperature needs to be 860 ° C. or lower.
If the soaking time is too short, an unrecrystallized structure remains, and if it is too long, grain growth proceeds excessively and the r value increases. Therefore, the holding time needs to be 30 seconds or more and 200 seconds or less. In addition, the cooling after soaking is preferably performed at a rate of 3 ° C./sec or more in order to prevent the r value from increasing due to excessive grain growth.
Here, the reason why the soaking temperature during annealing is set to (820 + Nb / 15 + B-CR) or higher will be specifically described. Generally, when the rolling reduction is increased, the driving force for recrystallization increases, and the temperature at which recrystallization is completed during annealing (hereinafter referred to as the recrystallization temperature) shifts to the low temperature side. On the other hand, when the amounts of added Nb and B are increased, recrystallization is remarkably suppressed, and the recrystallization temperature is shifted to a higher temperature side. According to the experiments by the present inventors, the recrystallization temperature is correlated with the rolling reduction (CR), the added Nb amount, and the B amount. Figures 1 and 2 show the relationship between reduction ratio (CR) and soaking temperature and performance, and the relationship between Nb [mass ppm] / 15 + B [mass ppm] and soaking temperature and performance, respectively. is there. Here, in FIG. 1, the chemical composition of the test material is Nb: 150 ppm and B: 30 ppm, and in FIG. 2, the rolling reduction (CR) is 70%. Except for the soaking temperature at the time of annealing, all are the characteristic values of the annealed plate prepared within the scope of the present invention. Moreover, in each figure, when the average elongation was 42% or more and the average r value was 1.2 to 1.6, ○ was indicated, and otherwise X was indicated. The straight line in each figure is a straight line where the soaking temperature = (820 + Nb / 15 + B-CR) [° C.], and it can be seen that this is a good boundary line for dividing ◯ and X. 1 and 2, when the soaking temperature during annealing is (820 + Nb / 15 + B-CR) [° C] or higher, the average elongation is 42% or more and the average r value is 1.2 to 1.6. A cold-rolled steel sheet having excellent formability and shape freezing property has been manufactured. From the above, the soaking temperature during annealing is set to (820 + Nb / 15 + B-CR) or higher.

In carrying out the present invention, a melting method can be appropriately applied, such as a normal converter method or an electric furnace method. The melted steel is cast into a slab and then directly or cooled and heated at the above-mentioned heating temperature and subjected to hot rolling. In hot rolling, after finishing at the above-described finishing temperature, 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. If necessary, for the purpose of improving the corrosion resistance, 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 and leveling may be performed at an elongation of up to 2%. Further, when plating is not performed during annealing, electrogalvanization or the like may be performed. Further, a film may be formed on the cold-rolled steel plate or the plated steel plate by chemical conversion treatment or the like.
As described above, the cold-rolled steel sheet of the present invention is manufactured.

Examples of the present invention will be described.
Steel having the chemical composition shown in Table 1 was melted, and specimens were produced under the manufacturing conditions shown in Tables 2 and 3. In Table 1, solid solution B indicates a value of B-11 / 14 × N. However, when Ti is added, it indicates a value of B-11 × (| N / 14-Ti / 48 | + (N / 14-Ti / 48)) / 2. If the calculated value is 0 or less, it is represented as 0. Details of the manufacturing conditions are as follows. First, in the hot rolling process, the molten steel was heated at 1250 ° C. for 1 hour to perform rough rolling and finish rolling. FT and CT shown in Tables 2 and 3 are finishing temperature and winding temperature, respectively. The thickness of the hot rolled sheet was 2.0 to 3.5 mm. Subsequently, it pickled before the cold rolling process and rolled at room temperature until the plate thickness became 0.6 to 1.0 mm. Then, as an annealing process, it heated to each soaking temperature at a heating rate of 20 ° C./sec, held at the soaking temperature for 30 to 200 sec, and then cooled to room temperature at a cooling rate of 20 ° C./sec. CR, AT, and AT0 shown in Tables 2 and 3 represent the cold rolling reduction ratio, the soaking temperature, and (820 + Nb / 15 + B-CR), respectively. After annealing, temper rolling with an elongation rate of 1.0% was performed to obtain a test material.
The average elongation (El _m ) and average r value (r _m ) of the obtained specimens were investigated. The average elongation (El _m ) and the average r value (r _m ) are JIS5 from the rolling direction (L direction), 45 ° direction (D direction), and right angle direction (C direction) of the specimen. Each tensile test piece was cut out, the elongation was measured by a tensile test in accordance with JIS Z 2241, and the r value was measured at a pre-strain of 15% in accordance with JIS Z 2254 as described above. Then, to determine the elongation of the average by the above formula (a) and (b) formula (El _m) and the average of the r value _{(r m).} The obtained results are shown in FIGS. 3 to 5 and Tables 2 and 3 together with the production conditions.

In the examples of the present invention, the average elongation is 42% or more, the average r value is 1.2 to 1.6, and the moldability and shape freezing property are excellent. Further, as can be seen from the manufacturing conditions and mechanical characteristics of symbols 32 to 35 in Table 2 and 41 to 44 in Table 3, in the present invention example, the r value is 1.2 to 1.6 and the elongation is a value showing a value of 42% or more. While the thermal temperature range is 800 ° C to 850 ° C, the comparative example shows a soaking temperature that shows a value of 1.2 to 1.6 and elongation of 42% or more in a component system with a small amount of dissolved B. There wasn't. From this, in the example of this invention, it was confirmed that the low r value and high elongation were obtained even at high temperature annealing, and it had excellent actual machine manufacturing stability.
FIG. 3 is a diagram showing the influence of the soaking temperature during annealing on the average r value. In the present invention example having a large amount of solute B, the average r value is 1.2 to 1.6 even in a region where the soaking temperature during annealing is high.
FIG. 4 is a diagram showing the relationship between AT-AT0, which is an indicator of whether the soaking temperature at the time of annealing is appropriate, and the average r value. When AT-AT0 is negative, the soaking temperature is not appropriate and recrystallization is not performed sufficiently. As a result, the average r value is less than 1.2.
FIG. 5 is a diagram showing the relationship between AT-AT0, which is an indicator of whether the soaking temperature during annealing is appropriate, and average elongation. When AT-AT0 is negative, the soaking temperature is not appropriate and recrystallization is not sufficient, resulting in an average elongation of less than 42%.

Claims (3)

In mass%, C: 0.0010 to 0.0030%, 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, Nb : 0.010 to 0.030%, B: 0.0010 ≦ B-11 / 14 × N ≦ 0.0050%, the balance is iron and inevitable impurities, and the average elongation (El _m ) shown by the following formula (a) is A cold-rolled steel sheet which is 42% or more and has an average r value (r _m ) represented by the following formula (b) of 1.2 to 1.6.
Average elongation El _m = (El _L + 2El _D + El _C ) / 4 (a)
Average r value r _m = (r _L + 2r _D + r _C ) / 4 (b)
Where El _L : Elongation in rolling direction
El _D : Rolling 45 ° direction elongation
El _C : Elongation in the direction perpendicular to the rolling r _L : r value in the rolling direction r _D : r value in the 45 ° direction of rolling r _C : r value in the direction perpendicular to the rolling   Further, Ti: 0.005% to 0.020% by mass%, and B: 0.0015 <B-11 × (| N / 14− in place of B: 0.0010 ≦ B−11 / 14 × N ≦ 0.0050% The cold-rolled steel sheet according to claim 1, wherein Ti / 48 | + (N / 14−Ti / 48)) / 2 ≦ 0.0050%.   The steel slab having the composition according to claim 1 or 2 is heated at a heating temperature of 1150 ° C or higher, and then hot-rolled to finish the finish rolling at a finishing temperature of 880 ° C or higher. Winding, pickling, and after cold rolling at a reduction rate of 55-80%, according to the reduction rate CR (%), Nb amount (mass ppm) and B amount (mass ppm) ( 820 + Nb / 15 + B-CR) A method for producing a cold-rolled steel sheet, comprising annealing at a soaking temperature of 860 ° C. for 30 to 200 seconds and then cooling.

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