KR20120099144A - Cold-rolled steel plate and method for producing same - Google Patents

Abstract

Provided are a cold rolled steel sheet excellent in formability and shape freezing property and excellent in practical production stability, and a method for producing the same. The composition is 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: It is 0.010 to 0.030%, B: 0.0010 <= B-11 / 14 * N <= 0.0050%, and remainder is iron and an unavoidable impurity. And average elongation (El _m ) is 42% or more, and average r value (r _m ) is 1.2-1.6. Furthermore, according to the reduction ratio CR (%), the amount of Nb (mass ppm), and the amount of B (mass ppm), the annealing after cold rolling is carried out at a crack temperature of (820 + Nb / 15 + B-CR)-860 ° C at 30? By keeping it for 200 sec, it can be manufactured stably with a real machine without controlling the crack temperature with a pin point.

Description

Cold rolled steel sheet and its manufacturing method {COLD-ROLLED STEEL PLATE AND METHOD FOR PRODUCING SAME}

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

BACKGROUND ART A thin plate-shaped part molded by a process mainly for bending and ejecting molding is used in thin liquid crystal TVs and OA devices. The member (material) used for these components often requires flatness in addition to the degree of work for the shape of the component from the viewpoint of designability, thickness reduction, and the like of the product. However, when bending and elongation molding are performed on the flat surface of the member (material), flatness tends to be deteriorated. Such deterioration of flatness is a phenomenon which occurs because the shape freezing property of the member (material) at the time of press forming the member (material) is bad. Required.

Moreover, as a factor which worsens flatness, the ridgeline bending which arises at the time of bending process is well known. It is known that the curvature which generate | occur | produces in one of the bending edge | tips which is one of them is reduced by making r value of a material low, and the technique which gives a low r value and a resistive strength to a material is established conventionally.

For example, Patent Literature 1 discloses a technique for developing a cold rolled steel sheet having a yield strength of 150 MPa and a r value of 0.67 (rolling right angle direction 1.45) in the rolling direction.

Patent Literature 2 discloses an automotive ferritic steel sheet excellent in shape freezing property in which the ratio of the # 100 'surface to the # 111' surface parallel to the plate surface is 1.0 or more.

In Patent Document 3, in order to obtain a ferritic thin steel sheet excellent in shape freezing property, the strength of the # 100 ｛<011>? 223223 <110> bearing group and the 112 ｛<110>, ｛554｝ <225>, ｛ It is disclosed to control at least one of the directions of 111 <112> and # 111 ｛<110> to at least one of r value in the rolling direction and r value in the direction perpendicular to the rolling direction.

Japanese Patent No. 3532138 Japanese Laid-Open Patent Publication 2008-255491 Japanese Laid-Open Patent Publication 2003-55739

However, in the technique described in Patent Documents 1 and 3, the r values in the rolling direction and the rolling right angle direction are reduced by the crystal grains having the orientations of # 100｝ <011>? ｛223｝ <110>, but in the rolling 45 ° direction. In order to make r value high, flatness may be impaired after the press molding of the steel which has many # 100 << 011>? # 223 << 110> azimuth grains. For example, when the elongate bead-shaped elongation molding in which the longitudinal direction becomes the rolling 45 degree direction is performed to the backlight chassis used for a thin liquid crystal television, the ridgeline bending which arises in the bead length direction, and near the elongate part There is a problem that warpage of the backlight chassis occurs due to the inflow of material from the flange portion.

In addition, Patent Literatures 1 to 3 disclose a case where the cold rolling rate is lowered, specifically 40%, and the r value is lowered for the cold rolled steel sheet. When rolling at an extremely low cold rolling rate of about 40%, There is a problem that the shape worsens and the flatness of the final product is degraded. Moreover, such a low cold rolling rate improves shape freezing, but makes it difficult to manufacture a thin steel sheet of about 1.0 mm or less.

Therefore, the present inventors have focused on ultra-low carbon steel, which is easy to obtain high ductility and hardly produces a strainer strain which causes molding breakage and wrinkles during press molding. In general, ultra-low carbon steel sheets such as Ti-based IF steels require low r-value in the annealing process because recrystallized grains that are advantageous for high r-value are easy to grain grow. For low r value, it is necessary to anneal at a low temperature where recrystallization is difficult but grains are difficult to grow, and in order to do so, it is necessary to control the annealing conditions under conditions in which the cracking temperature only shows fluctuations of several degrees Celsius. However, in actual practice, since materials having different set cracking temperatures during annealing sequentially pass through the continuous annealing facility, there are cases where the cracking temperature fluctuates by several tens of degrees Celsius, and the cracking temperature is controlled within a range of several degrees Celsius. It is difficult. Accordingly, there has been a problem that it is annealed at a temperature higher than the desired temperature, so that low r value cannot be achieved. The same problem exists in patent documents 1-3, and although the conditions of annealing temperature are described, the difficulty of controlling annealing temperature by a pin point is not considered.

In view of such circumstances, the present invention solves the problems of the prior art, and can perform drawing processing, bending processing, and elongation processing, and is capable of securing the shape required for large parts, and the formability and the shape freezing property. An object of the present invention is to provide a cold rolled steel sheet excellent in practical production stability and a method for producing the same.

The inventors made intensive research and investigation in order to solve the said subject. As a result, the following points were found. Firstly, it is important to control the r value in the annealing plate after cold rolling in a state of maintaining high ductility, in order to achieve both workability that does not cause warping even when complicated processing is performed, and high- That is, by increasing the average elongation, the shape required for the component can be secured after securing workability at the time of drawing processing or elongation processing. Further, by reducing the r-values in the three directions of the rolling direction, the rolling right angle direction, and the rolling 45 ° direction, and setting the average r value within the range of defining the average r value, the elongated bead shape in which the longitudinal direction becomes the rolling 45 ° direction is obtained. Even when elongate molding is performed, warpage is prevented, the occurrence of spring back and warpage can be suppressed after processing, and shape freezing property can be ensured.

By forming the crack temperature range in an annealing process according to a reduction ratio, Nb amount, and B amount, it becomes possible to manufacture stably, without controlling crack temperature by a pin point.

This invention is made | formed based on the above knowledge, The summary is as follows.

(1) 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% Hereinafter, Nb: 0.010-0.030%, B: 0.0010 <= B-11 / 14 * N <= 0.0050%, and remainder has the composition which is iron and an unavoidable impurity, and the average elongation (El _m ) represented by following formula (a) is It is 42% or more, and the average r value (r _m ) represented by following (b) formula is 1.2-1.6, The cold rolled steel plate characterized by the above-mentioned.

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 rolling right angle 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

(2) In the above (1), further, in terms of mass%, Ti: 0.005% to 0.020%, and B: 0.0015 <B-11 / 14 x N≤0.0050% instead of B: 0.0015 < B-11x (| N / 14-Ti / 48 | + (N / 14-Ti / 48)) / 2≤0.0050%, The cold rolled steel plate characterized by the above-mentioned.

(3) After heating the steel slab which consists of a composition as described in said (1) or said (2) at the heating temperature of 1150 degreeC or more, hot rolling which completes finish rolling at the finishing temperature of 880 degreeC or more is performed, and 700 degreeC After winding up and pickling, cold rolling at a reduction ratio of 55 to 80%, and depending on the reduction ratio CR (%), Nb amount (mass ppm) and B amount (mass ppm), (820 + Nb) / 15 + B-CR) annealing held at a cracking temperature of 860 ° C. for 30 to 200 seconds, followed by cooling, to produce a cold rolled steel sheet.

In addition, in this specification, all% and ppm which show the component of steel are mass% and mass ppm. Moreover, the cold rolled steel sheet which this invention makes object includes the steel plate which surface-treated, such as electrogalvanization and hot dip galvanization, to the cold rolled steel sheet. The steel sheet further includes a steel sheet having a film deposited thereon by chemical conversion treatment or the like.

Moreover, the steel plate of this invention can be used widely as a general member for home appliances which performs a flat part, bending, extension, drawing of hardness, etc., such as a backlight chassis of a large TV, a panel of a refrigerator, and an air conditioner outdoor unit. Moreover, using this invention, the backlight chassis can be manufactured more than about 650x500 mm (32V type) with the steel plate of plate thickness 0.8mm, for example.

ADVANTAGE OF THE INVENTION According to this invention, the cold rolled sheet steel which is excellent in the moldability and shape freezing property which can perform a drawing process, a bending process, and an elongation process is obtained. And the cold rolled sheet steel excellent in moldability and shape freezing property can be manufactured stably with a real machine, without controlling the crack temperature at the time of annealing by a pin point. 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.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the relationship between cold rolling rate (rolling reduction rate) CR, a crack temperature, and a performance.
2 is a diagram showing a relationship between Nb [mass ppm] / 15 + B [mass ppm], crack temperature, and performance.
3 is a diagram showing the effect of the crack temperature at the time of annealing on the average r value.
4 is a diagram showing a relationship between AT-ATO and an average r value, which is an indicator of whether a crack temperature at annealing is appropriate.
FIG. 5 is a diagram showing the relationship between AT-ATO and average elongation, which are indicators of whether a crack temperature at annealing is appropriate.

Hereinafter, the present invention will be described in detail. First, the chemical component of the steel plate of this invention is demonstrated. In the following description, the content% and ppm of the constituent elements mean% by mass and ppm by mass, respectively.

C ： 0.0010 ~ 0.0030%

Since C has the effect of reducing r value, it is preferable to add a lot from a viewpoint of low r-valued, and it may be 0.0010% or more. However, excessive addition results in a stretcher strain which causes wrinkles at the time of press molding, and the steel sheet is strengthened by dispersion strengthening by solid solution strengthening and carbide formation, and the elongation is lowered. Therefore, C is made into 0.0010% or more and 0.0030% or less.

Si: 0.05% or less

Si is an element with high solid solution strengthening ability, increases yield strength and greatly reduces elongation. Therefore, you may be 0.05% or less.

Mn ： 0.1% to 0.3%

Mn is an element which forms sulfides and improves hot brittleness. In order to acquire this effect, 0.1% or more of addition is required. On the other hand, in order to improve the yield strength and deteriorate the ductility, it is an element having high solid solution strengthening ability, and an upper limit is made into 0.3%.

P: 0.05% or less

P is a solid solution strengthening element, increases the yield strength and deteriorates the ductility. Therefore, you may be 0.05% or less.

S ： 0.02% or less

S forms a sulfide at the stage of a hot rolled sheet, and causes deterioration of ductility. Therefore, you may be 0.02% or less.

Al: 0.02% to 0.10%

Al has affinity with N, has the effect of reducing the amount of solid solution N in the cold rolling process, and suppressing aging hardening. In addition, the precipitated AlN has a high tendency to precipitate finely and suppresses grain growth in the annealing step. In order to acquire these effects, addition of 0.02% or more is required. However, excessive addition causes an increase in the manufacturing cost and increases the temperature of transformation from austenite to ferrite during hot rolling, making it difficult to finish rolling in the austenite region. Therefore, Al needs to be 0.10% or less.

N ： 0.005% or less

When N is contained in a large amount, it will solidify in steel and will cause a stretcher strain. Therefore, you may be 0.005% or less.

Nb ： 0.010 ~ 0.030%

Nb fixes solid solution C as a precipitate, and suppresses a strainer strain. Moreover, NbC which is a precipitate tends to be finely precipitated and suppresses grain growth at the time of annealing. In order to acquire these effects, it is necessary to add 0.010% or more. However, when excessively added, the recrystallization temperature is excessively raised. It also causes an increase in cost. Therefore, an upper limit is made into 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 becomes an important requirement in the present invention, and by being present as solid solution B in steel, it is possible to suppress grain growth of ferrite in the annealing process after cold rolling and to control the r value even at high cracking temperature. In order to acquire such an effect, after BN precipitates at high temperature at the time of hot rolling, solid solution B needs to exist 0.0010% or more. However, excessive addition forms precipitate with C and reduces elongation. It is set as 0.0010 <= B-11 / 14 * N <= 0.0050% for the above reason. Preferably 0.0015 &lt; B-11/14 x N? 0.0050%.

Furthermore, when Ti is added, coarse TiC precipitates compared with NbC, and since grain growth at the time of annealing is improved, the amount of solid solution B having a grain growth inhibiting effect needs to be more than 0.0015%. However, excessive addition forms precipitate with C and reduces elongation. For these reasons, instead of the relationship of 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, it is 0.0020% <= B-11x (| N / 14-Ti / 48 | + (N / 14-Ti / 48)) / 2 <0.0050%.

In addition to said element, in this invention, it can contain in the range of Ti: 0.005%-0.020% for the following purpose.

Ti has strong affinity with N and forms precipitates at a high temperature, and has an effect of suppressing strengthening of solid solution of solid solution N and enhancement of dispersion due to micro-precipitation of AlN. Moreover, when it wants to improve elongation especially, it can also add. In order to exhibit these effects, it is preferable to add at least 0.005%. However, excessive addition accelerates the precipitation of TiC, not only reduces the effect of inhibiting grain growth during annealing with NbC, but also increases the manufacturing cost, so the upper limit when adding is set to 0.020%.

Components other than the above consist of iron and unavoidable impurities. As an unavoidable impurity, 0.05% or less of Cu, Cr which is easy to mix from scrap, and 0.01% or less of Sn, Mo, W, V, Ni, etc. are mentioned, for example.

The steel plate of this invention makes the average elongation calculated | required by the following (a) formula into 42% or more. Elongation has a good correlation with moldability, and it can elongate shape | molding to high as elongation rate is large, for example. Therefore, the larger the elongation required, the better. By increasing the average elongation to 42% or more, drawing processing and elongation processing can also be performed, and the shape required for the component can be ensured.

In addition, an average total elongation can be measured and calculated | required by the following method. The JIS No. 5 tensile test piece is cut out from the rolling direction, the rolling 45 ° direction, and the rolling right angle direction, respectively, and a tensile test in accordance with JIS Z 2241 is performed. And it calculates by the following (a) formula.

Average Elongation El _m = (El _L + 2El _D + El _C ) / 4 ??? (a)

El _L : Elongation in the rolling direction

El _D : Elongation in rolling 45 ° direction

El _C ： Elongation in the rolling perpendicular direction

In the steel sheet of the present invention, the average r value determined by the following (b) formula is set to 1.2 to 1.6. r value is correlated with the curvature which arises after bending shaping | molding and elongation shaping | molding. In bending shaping | molding, when the r value of a bending direction becomes high, a curvature of a saddle type arises remarkably along a bending line. Moreover, in elongate molding, the use of a high r value material makes material inflow from the flange part around the elongate part remarkable, and residual stress and deformation generate | occur | produce in a flange part. Therefore, in order to improve the shape freezing property after press molding by low r value, in this invention, an average r value is made 1.6 or less. On the other hand, excessive low r value extremely reduces the elongation, so the lower limit of the average r value is set to 1.2.

In addition, an average r value can be measured and calculated | required by the following method. JIS No. 5 tensile test pieces are cut out from the rolling direction, the rolling 45 ° direction, and the rolling right angle direction, respectively, and a plastic strain ratio test in accordance with JIS Z 2254 is performed at 15% of pre-strain. And it calculates by the following (b) formula.

Average r value r _m = (r _L + 2r _D + r _C ) / 4 ??? (b)

Where r _L : r value in the rolling direction

r _D : Rolling r value in 45 ° direction

r _C : r value at right angle to rolling direction

For example, when the elongate bead-shaped elongation molding in which the longitudinal direction becomes the rolling 45 degree direction is performed to the backlight chassis used for a thin liquid crystal television, the ridgeline bending which arises in the bead longitudinal direction, and the elongate part The curvature of the backlight chassis which arises by the inflow of the material of the flange part of the vicinity becomes large when r value of rolling 45 degree direction is high. Therefore, reduction of r values in three directions of the rolling direction, the rolling right angle direction, and the rolling 45 ° direction is important, and in addition to the average r value of 1.2 to 1.6, preferably, the rolling direction, the rolling right angle direction, and the rolling 45 It is preferable that the maximum r value is less than 2.0 among three directions of a (degree) direction.

Next, the manufacturing method of the cold rolled sheet steel of this invention is demonstrated.

In the present invention, the steel slab having the composition described above is heated at a heating temperature of 1150 ° C or higher, and then hot rolled to finish finish rolling at a finishing temperature of 880 ° C or higher, wound at 700 ° C or lower, and pickled. Then, after cold rolling at a reduction ratio of 55 to 80%, depending on the reduction ratio CR (%), Nb amount (mass ppm) and B amount (mass ppm), (820 + Nb / 15 + B-CR) High elongation and low r value can be obtained by performing annealing maintained at a crack temperature of? 860 ° C. for 30 to 200 sec, and then cooling.

Heating temperature: more than 1150 ℃

The steel material which adjusted the component is heated to 1150 degreeC or more, and the precipitate is dissolved. In heating below 1150 degreeC, N and C remain unemployed, carbide and nitride do not precipitate fine at the time of a winding process or annealing, and the particle growth suppression effect at the time of annealing is not fully exhibited. Therefore, heating temperature is so good that it is good, and it is 1150 degreeC or more, Preferably it is 1200 degreeC or more. However, when excessively heated, a thick oxidation scale is formed on the steel surface and the cost of pickling treatment is increased, so 1300 ° C or less is preferable.

Finishing temperature: More than 880 degrees Celsius

Then, hot rolling is performed. In the final rolling stand at the time of finish rolling, it is necessary to roll in the austenite region single phase. In the case of rolling in the two-phase region of austenite and ferrite, the rolling load is greatly changed along with the transformation, which makes it difficult to control the load on the rolling stand and cause breakage of the plate. Moreover, rolling to a ferrite reverse single phase may increase the manufacturing cost by promoting the remaining of unrecrystallized crystals and excessively increasing the rolling load in subsequent cold rolling. As mentioned above, let finishing temperature be 880 degreeC or more, and finish-rolling shall be performed to an austenite area | region single phase. In addition, when the finishing temperature is too high, the oxidation scale becomes thick and the cost of the pickling treatment increases, so the temperature is preferably 950 ° C or lower.

It is preferable to perform water cooling after finish rolling. In order to suppress the precipitation of C and N, which is known to reduce the r value by solidifying C and N before cold rolling, and to refine the precipitate carbide and nitride and to suppress grain growth during annealing, the above cooling is performed. It is preferable to carry out.

Winding temperature: 700 degrees Celsius or less

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

Rolling rate (CR) during cold rolling: 55 to 80%

The hot rolled sheet is pickled in a conventional manner. Next, cold rolling of 55% or more and 80% or less of rolling reduction CR is performed, and it shape | molds to desired plate | board thickness. If the reduction ratio CR is less than 55%, the structures become mixed and the ductility is extremely reduced. On the other hand, when the reduction ratio (CR) is higher than 80%, an aggregate structure which is disadvantageous for reducing the r value after annealing is likely to be formed.

Crack temperature: (820 + Nb / 15 + B-CR)? 860 degrees Celsius, holding time: 30-200 sec

Next, as an annealing process, a crack temperature is 30 degreeC in the range of (820 + Nb / 15 + B-CR)-860 degreeC according to reduction ratio CR (%), Nb amount (mass ppm), and B amount (mass ppm). Hold for 200 s. Thereafter, cooling is performed.

In the annealing step, the strain introduced by cold rolling is lost by recrystallization to soften the steel sheet. The temperature at which recrystallization is completed is lower as the reduction ratio (CR) is higher and the amount of additional elements, in particular, Nb and B is smaller. Therefore, since the fall of elongation by the remainder of a non-recrystallized structure is prevented, it is necessary to make a crack temperature more than (820 + Nb / 15 + B-CR). The reason for limitation of this cracking temperature is mentioned later. On the other hand, if the recrystallization temperature is excessively high, in addition to the increase in manufacturing cost, the ferrite is transformed into austenite, and the transformation from austenite to ferrite during subsequent cooling results in an excessively fine structure and high strength. Accompanyingly, the elongation is lowered. Therefore, the crack temperature needs to be 860 degrees C or less.

If the crack holding time is too short, unrecrystallized structure remains, and if too long, grain growth proceeds excessively and the r value increases. Therefore, the holding time needs to be 30 sec or more and 200 sec or less. Moreover, since cooling after a crack prevents r value becoming high by excessive grain growth, it is preferable to cool at the speed of 3 degree-C / sec or more.

Here, the reason which made the crack temperature at the time of annealing more than (820 + Nb / 15 + B-CR) is demonstrated concretely. In general, when the reduction ratio is increased, the driving force of the recrystallization is increased, and the temperature at which recrystallization is completed at the time of annealing (hereinafter referred to as recrystallization temperature) moves to the low temperature side. On the other hand, when the amount of added Nb and B is increased, recrystallization is remarkably suppressed and the recrystallization temperature moves to the high temperature side. According to the experiments of the present inventors, the recrystallization temperature has a correlation with the reduction ratio (CR), the amount of Nb added, and the amount of B. 1 and 2 show the relationship between the reduction ratio (CR), the crack temperature and the performance, respectively, and the relationship between the Nb [mass ppm] / 15 + B [mass ppm], the crack temperature and the performance. Here, in FIG. 1, the chemical composition of the test material is Nb: 150 ppm and B: 30 ppm, and the reduction ratio CR is 70% in FIG. Except for the crack temperature at the time of annealing, all are characteristic values of the annealing plate created within the scope of the present invention. Moreover, in each drawing, when an average elongation was 42% or more and an average r value became 1.2-1.6, (circle) was made into x, and when it did not, it was made into x. The straight line in each figure is a straight line which becomes a crack temperature = (820 + Nb / 15 + B-CR) [degreeC], and it turns out that it is a good boundary line which divides (circle) and x. 1 and 2, when the annealing crack temperature is higher than (820 + Nb / 15 + B-CR) [° C.], the average elongation is 42% or more and the average r value is 1.2 to 1.6, which is excellent in formability and shape freezing property. Cold rolled steel sheet is manufactured. As mentioned above, the crack temperature at the time of annealing is made into (820 + Nb / 15 + B-CR) or more.

Moreover, at the time of implementing this invention, a solvent method can be suitably applied, such as a conventional converter method and a converter method. The molten steel is heated to the above-mentioned heating temperature as it is or after being cast to the slab, or cooled to perform hot rolling. In hot rolling, after finishing at the finishing temperature mentioned above, it winds up at the above-mentioned winding temperature. Although the cooling rate after finishing rolling to winding up is not specifically defined, the cooling rate more than air cooling is sufficient. Moreover, you may perform quenching more than 100 degree-C / s as needed. Thereafter, the above-mentioned cold rolling is performed after normal pickling. About annealing (heat-cracks-cooling), heating and cooling are performed on the conditions mentioned above. As needed, you may perform plating by molten zinc in the vicinity of 480 degreeC for the purpose of improving corrosion resistance. Moreover, after plating, you may reheat at 500 degreeC or more and alloy plating. Or you may take the heat history which hold | maintains in the middle of cooling. Furthermore, you may perform temper rolling and leveling at the elongation to 2% as needed. Moreover, when plating is not performed in the middle of annealing, you may perform electro zinc plating etc. Moreover, you may make a film adhere on a cold rolled sheet steel or a plated steel sheet by chemical conversion treatment. As mentioned above, the cold rolled sheet steel of this invention is manufactured.

Example 1

An embodiment of the present invention will be described. The steel which has a chemical composition shown in Table 1 was melted, and the test material was manufactured on the manufacturing conditions shown in Tables 2 and 3. In Table 1, solid solution B points out the value of B-11 / 14 * N. However, when Ti is added, the value of B-11x (| N / 14-Ti / 48 | + (N / 14-Ti / 48)) / 2 is pointed out. Moreover, when the calculated value is 0 or less, it shows as 0. The detail of manufacturing conditions is as follows. First, in the hot rolling step, the molten steel was heated at 1250 ° C. for 1 hr, rough rolling and finish rolling were performed. FT and CT shown in Tables 2 and 3 are finish temperature and winding temperature, respectively. The thickness of the hot rolled sheet was 2.0 to 3.5 mm. Next, it pickled before the cold rolling process, and rolled at room temperature until plate | board thickness became 0.6-1.0 mm. Subsequently, as an annealing process, it heated to each crack temperature at the heating rate of 20 degree-C / sec, hold | maintained at the crack temperature for 30-200 sec, and then cooled to room temperature at the cooling rate of 20 degree-C / sec. CR, AT, and ATO shown in Tables 2 and 3 each show a reduction ratio of cold rolling, a crack temperature, and (820 + Nb / 15 + B-CR). After annealing, temper rolling of 1.0% of elongation was performed and the test material was obtained. About the obtained test material, average elongation (El _m ) and average r value (r _m ) were investigated. In addition, average elongation (El _m ) and average r value (r _m ) are JIS 5 tensile test pieces from the rolling direction (L direction), the rolling 45 degree direction (D direction), and the rolling right angle direction (C direction) of a test material. Each was cut out and the elongation was measured by a tensile test based on JIS Z 2241, and the r value was measured at 15% of pre-deformation based on JIS Z 2254 as described above. And the average elongation (El _m ) and the average r value (r _m ) were calculated | required by the above-mentioned (a) formula and (b) formula. The obtained result is shown with a manufacturing condition to FIG. 3, FIG. 4, FIG. 5 and Table 2, Table 3. FIG.

In the example of this invention, an average elongation is 42% or more, the average r value is 1.2-1.6, and it is excellent in moldability and shape freezing property. In addition, as can be seen from the manufacturing conditions and mechanical properties of the symbols 32 to 35 in Table 2 and 41 to 44 in Table 3, in the examples of the present invention, the r value is 1.2 to 1.6 and the elongation is 42% or more. Although the crack temperature range was 800 ° C to 850 ° C, in the comparative example, in the component system having a small amount of solid solution B, a crack temperature showing a value of r value of 1.2 to 1.6 and elongation of 42% or more was not confirmed. From this, in the example of this invention, low r value and high elongation were obtained also in high temperature annealing, and it was confirmed that it has the outstanding actual machine | machine manufacturing stability.

3 is a diagram showing the effect of the crack temperature at the time of annealing on the average r value. In this invention example with a large amount of solid solution B, the average r value is 1.2-1.6 also in the area | region where the crack temperature at the time of annealing is high.

4 is a diagram showing a relationship between AT-ATO and an average r value, which is an indicator of whether a crack temperature during annealing is appropriate. When AT-ATO is negative, the crack temperature is not appropriate, and as a result of insufficient recrystallization, the average r value is less than 1.2.

Fig. 5 is a diagram showing the relationship between AT-ATO and average elongation, which are indices of whether the cracking temperature during annealing is appropriate. When AT-ATO is negative, the crack temperature is not appropriate and the recrystallization is not sufficiently performed. As a result, the average elongation is 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 having iron and an unavoidable impurity, and the average elongation (El _m ) represented by the following formula (a) is 42% or more And the average r value (r _m ) represented by the following (b) formula is 1.2-1.6.
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)
El _L : Elongation in the rolling direction
El _D : Elongation in the rolling 45 ° direction
El _C : Elongation 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 The method of claim 1,
Furthermore, in mass%, Ti: 0.005%-0.020% is contained, and instead of said B: 0.0010 <= B-11 / 14 * N <0.0050%, B: 0.0015 <B-11x (| N / 14- Ti / 48 | + (N / 14-Ti / 48)) / 2≤0.0050%. A cold rolled steel sheet characterized by the above-mentioned. After heating the steel slab which consists of a composition of Claim 1 or 2 at the heating temperature of 1150 degreeC or more, hot rolling which complete | finishes finish rolling at the finishing temperature of 880 degreeC or more, it winds up at 700 degrees C or less, After pickling and cold rolling at a reduction ratio of 55 to 80%, depending on the reduction ratio CR (%), Nb amount (mass ppm) and B amount (mass ppm), (820 + Nb / 15 + B-CR) Annealing is maintained at a cracking temperature of? 860 ° C. for 30 to 200 seconds, and then cooled to produce a cold rolled steel sheet.

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