JP2017031453A - Alloy galvanized steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an alloy galvanized steel sheet for outer sheet panel of an automobile having low P content (0.035% or less) and low Mn content (0.74% or less) that surface quality and powdering resistance do not cause a problem and excellent in dent resistance and having tensile strength of 340 MPa or more.SOLUTION: There is provided an alloy galvanized steel sheet for outer sheet panel of an automobile having a chemical composition consisting of C:0.0010 to 0.0030%, Si:0.005 to 0.10%, Mn:0.30 to 0.74%, P:0.010 to 0.035%, S:0.015% or less, Al:0.001 to 0.050%, N:0.0035% or less, Ti:0.001 to 0.015%, Nb:0.001 to 0.015%, B:0.0001 to less than 0.0007% and the balance Fe with impurities, a metallic structure mainly containing ferrite having crystal grain size defined in JIS G 0551 (2013) of over 9.5 and mechanical properties such as tensile strength of 340 MPa or more, yield strength of 230 to 280 MPa and coating/baking cured amount defined in JIS G 3135 (2013) of 30 to 80 MPa.SELECTED DRAWING: None

Description

  TECHNICAL FIELD The present invention relates to an alloyed hot-dip galvanized steel sheet and a method for producing the same. Specifically, the present invention relates to an alloyed hot-dip galvanized steel sheet having an excellent dent resistance and a tensile strength of 340 MPa or more for automobile outer panel. Regarding the method.

  Automotive steel plates used in press forming have high tensile strength of, for example, 340 MPa or higher and high rust prevention properties in order to improve fuel efficiency by reducing the weight of the vehicle body and improve safety in the event of a collision. Is required. For this reason, a high tensile alloyed hot dip galvanized steel sheet having a tensile strength of 340 MPa is used for an outer panel such as a door outer panel or a hood outer panel of an automobile.

  Outer panels such as automobile doors and hoods are parts that are the appearance of the final product and require aesthetics. For this reason, the high-tensile alloyed hot-dip galvanized steel sheet used for the outer panel must have a property (dent resistance) that is unlikely to cause dents that are permanently deformed by pressing with a finger. Is also required. The dent resistance is improved as the yield strength after the coating baking process is performed after press molding is higher, and as the plate thickness of the steel sheet is thicker. For this reason, it is possible to reduce the thickness by using a steel plate having a high yield strength after being subjected to paint baking after press forming.

  In addition, when forming into an outer panel by performing press processing by hot forming on a high-tensile alloyed hot-dip galvanized steel sheet, it has excellent powdering resistance (resistance to plating peeling during processing) and high shape freezing property. (Properly adaptable to the press die, and less likely to generate spring back when the molded product is removed from the press die, and has the property of ensuring the dimensions and shape as in the die) is also required. For this reason, the steel plate used as the raw material of the outer panel is also required to have a low yield strength before press forming.

  Therefore, a steel plate having a low yield strength before press forming and having a high yield strength after coating and baking after press forming is suitable as a high-tensile alloyed hot-dip galvanized steel plate for an outer panel.

  Furthermore, the steel plate used for the outer panel is required to have a very high level of appearance after painting. Insufficient mechanical properties such as press formability can be compensated to some extent by adjusting the manufacturing conditions such as molds, but there is no way to compensate for surface defects present in the steel sheet. In other words, it leads directly to a decrease in yield. For this reason, the surface property of the steel plate itself to be used is strictly controlled, and a steel plate without surface defects is required. However, many of the various alloy elements added to ensure the desired high strength in the high-tensile steel sheet are generally easily oxidizable elements. For this reason, high-strength steel sheets tend to easily generate scale defects (surface defects), and it is not easy to stably produce high-tensile galvannealed steel sheets having high-quality surface properties.

  Conventionally, a bake hardenable steel plate (BH steel plate) is known as a steel plate having dent resistance. A BH steel sheet is a steel sheet that utilizes a so-called strain age hardening phenomenon in which solute C and N atoms segregate on dislocations to fix the dislocations and increase yield strength. In the process of press forming and baking coating using BH steel plate, dislocations introduced during press forming are fixed and fixed by solid solution C and N during paint baking, so baking coating compared to press forming Later yield strength increases. Many proposals have been made regarding BH steel sheets.

  In Patent Document 1, C: 0.0010 to 0.0020% (in the present specification, “%” for chemical composition means “mass%” unless otherwise specified), Si: 0.03% or less , Mn: 0.1 to 1.0%, P: 0.005 to 0.06%, S: 0.003 to 0.02%, sol. Al: 0.03 to 0.2%, N: 0.0020% or less, Nb: 0.005 to 0.010%, high temperature strength at a temperature T (° C.) of 350 to 450 ° C. is high temperature strength (MPa) ≦ 120 or 285-0.3T ≦ High temperature strength (MPa), and the amount of paint bake hardening measured by the falling yield stress is 30 to 45 MPa, so that bake hardening with excellent surface quality after press working is achieved. A high strength cold-rolled steel sheet is disclosed.

  In Patent Document 2, C: 0.0005% or more and less than 0.030%, Si: 0.1% or less, Mn: 0.05 to 2.0%, P: 0.005 to 0.06%, S : 0.020% or less, sol. Al: 0.0005 to 0.08% and N: 0.005% or less, ferrite area ratio of 90% or more, particle size ratio of 0.20 μm or less, MnS number ratio of 10% or less, cleanliness d0.05% Below, and having a mechanical property of a yield ratio of 75% or less, the yield strength is low before press molding and the press moldability is excellent, and after press molding, the yield strength is increased by heat treatment and good dent resistance. A cold-rolled steel sheet exhibiting properties is disclosed.

JP 2005-187835 A JP 2011-246767 A

  As in the inventions disclosed in Patent Documents 1 and 2, by containing a large amount of P and Mn, a high strength of a tensile strength of 340 MPa or more can be easily obtained, but in the case of an galvannealed steel sheet, Surface quality and powdering resistance deteriorate. For this reason, in the inventions disclosed in Patent Documents 1 and 2, the low P content (P ≦ 0.035%) and the low Mn so that the surface quality and powdering resistance of the galvannealed steel sheet are not problematic. With the content (Mn ≦ 0.74%), an alloyed hot-dip galvanized steel sheet for an outer panel that has a tensile strength of 340 MPa or more and a yield strength of 250 MPa or less cannot be obtained.

  In addition, an outer panel such as a door outer panel or a hood outer panel is required to have excellent dent resistance, but depending on the outer panel panel, there is an outer panel having a shape with a shallow drawing depth. In such an outer panel, since there is little work hardening by press molding, the dent resistance may be insufficient, and the yield strength before being subjected to press molding is required to be high. However, in the inventions disclosed in Patent Documents 1 and 2, the Japan Iron and Steel Federation Standard JFS A 3011: 2014 340BH standard (JAC340H, YS = 195 MPa or more, 295 MPa or less, however, plate thickness 0.4 mm or more, 0.8 mm In the case of less than 2), the yield strength before being subjected to press molding is increased (230 to 270 MPa) and the desired dent resistance cannot be obtained.

  The present invention has been made in view of the problems of the prior art, and has a low P content (0.035% or less) and a low Mn content (0 It is an object of the present invention to provide an alloyed hot-dip galvanized steel sheet having an excellent dent resistance and a tensile strength of 340 MPa or more, and a method for producing the same, for automotive outer panel.

  The inventors of the present invention have intensively studied to use a TS340 MPa class hot dip galvanized steel sheet for an automobile outer panel, and obtained the knowledge (i) to (iii) listed below to complete the present invention.

  (I) A hot-dip galvanized steel sheet used for an outer panel of an automobile is required to have surface quality and powdering resistance. In order to improve the surface quality and the powdering resistance, the Mn content, the P content and the Si content may be decreased. However, a decrease in the Mn content and the P content leads to a decrease in tensile strength. In order to obtain the target tensile strength of 340 MPa or more, B may be added to refine the steel structure. By adding B and lowering the finish rolling temperature in finish rolling to perform rolling, recrystallized austenite grains are refined, and grain growth can be suppressed by cooling after finish rolling.

  (Ii) The dent resistance can be improved by controlling the yield strength to be higher, specifically by controlling the yield strength to 230 MPa or more. The yield strength can be controlled by the amount of B added. That is, by precipitating B as BN during hot rolling to reduce solid solution N in the steel, precipitation of fine nitride during subsequent cold rolling and recrystallization annealing is suppressed, grain growth occurs, The yield strength is low. For this reason, the yield strength can be increased to 230 MPa or more by suppressing the addition of B and leaving the solid solution N to remain. However, as described above, B also has an effect of increasing the tensile strength, and the B content is controlled to an appropriate amount, specifically, the B content is controlled to less than 7 ppm while balancing the tensile strength and the yield strength. .

  (Iii) Thus, in order to ensure the desired surface quality, powdering resistance and dent resistance (yield strength of 230 MPa or more) in the galvannealed steel sheet having a tensile strength of 340 MPa, the Mn content It is important to properly balance the P content and the B content. However, just by optimizing the balance of the Mn content, the P content and the B content, the surface quality and the powdering resistance can be ensured. It cannot be secured. Further, by adding Ti and Nb in combination, the surface quality and the powdering resistance can be reliably ensured. Patent Documents 1 and 2 described above do not describe examples in which Ti, Nb and B are added in combination.

The present invention is listed below.
(1) C: 0.0010 to 0.0030%, Si: 0.005 to 0.10%, Mn: 0.30 to 0.74%, P: 0.010 to 0.035%, S: 0 0.15% or less, Al: 0.001 to 0.050%, N: 0.0035% or less, Ti: 0.001 to 0.015%, Nb: 0.001 to 0.015%, B: 0.0. A metal structure mainly composed of a ferrite structure having a chemical composition composed of 0001 to less than 0.0007%, the balance Fe and impurities, and having a grain size number of more than 9.5 as defined in JIS G 0551 (2013) It has mechanical properties such that the tensile strength is 340 MPa or more, the yield strength is 230 to 280 MPa, and the paint bake hardening amount specified in JIS G 3135 (2013) is 30 to 80 MPa. Characterize Alloyed hot-dip galvanized steel sheet for automotive outer panel.

  (2) The alloyed hot-dip galvanized steel sheet for an outer panel of an automobile described in item 1, wherein the chemical composition has Mo: 0.050% or less.

  (3) C: 0.0010 to 0.0030%, Si: 0.005 to 0.10%, Mn: 0.30 to 0.74%, P: 0.010 to 0.035%, S: 0 0.15% or less, Al: 0.001 to 0.050%, N: 0.0035% or less, Ti: 0.001 to 0.015%, Nb: 0.001 to 0.015%, B: 0.0. A slab containing 0001 to less than 0.0007% and having a chemical composition consisting of the balance Fe and inevitable impurities is hot-finished at 880 to 930 ° C. and wound at 450 to 800 ° C., and then 60 to A method for producing an alloyed hot-dip galvanized steel sheet for an outer panel of an automobile, characterized by performing cold rolling at a reduction rate of 95% and annealing at 720 to 860 ° C in a continuous hot-dip galvanizing line.

  (4) The said chemical composition is a manufacturing method of the galvannealed steel plate for the outer panel of the motor vehicle described in 3 which has Mo: 0.050% or less.

  According to the present invention, a low P content (0.035% or less) and a low Mn content (0.74% or less) that the surface quality and powdering resistance are not problematic, An alloyed hot-dip galvanized steel sheet having an excellent dent resistance and a tensile strength of 340 MPa or more can be provided. For this reason, according to the present invention, it is possible to secure a tensile strength of 340 MPa or more while suppressing the alloy cost, and contribute to the reduction of the plate thickness (weight reduction) by using it for a flat door or the like particularly requiring dent resistance. Can do.

  The reasons for limiting the chemical composition, metallographic structure, mechanical properties, application, and manufacturing method of the alloyed hot-dip galvanized steel sheet for an automotive outer panel according to the present invention will be described.

1. Chemical composition (1-1) C: 0.0010 to 0.0030%
C has an effect of exerting bake hardening ability by being present in the steel in a solid solution state. When the C content is less than 0.0010%, it is difficult to obtain the effect by the above action. Therefore, the C content is 0.010% or more. On the other hand, if the C content exceeds 0.0030%, the moldability may be significantly reduced. Therefore, the C content is 0.0030% or less. Preferably it is 0.0028% or less.

(1-2) Si: 0.005 to 0.10%
Si is an element effective for increasing the strength while suppressing a decrease in ductility, and has an effect of increasing the plating adhesion. Therefore, the Si content is 0.005% or more. Preferably it is 0.010% or more, More preferably, it is 0.020% or more. However, when the Si content exceeds 0.10%, the surface quality and powdering resistance of the galvannealed steel sheet are deteriorated. Therefore, the Si content is 0.10% or less. Preferably it is 0.08% or less, More preferably, it is 0.06% or less.

(1-3) Mn: 0.30 to 0.74%
Mn is an element effective for easily increasing the strength of steel. In the galvannealed steel sheet according to the present invention, when the Mn content is less than 0.30% in relation to other elements (P, B) for increasing the strength, the tensile strength of the steel is set to 340 MPa or more. It cannot be increased. Therefore, the Mn content is 0.30% or more. On the other hand, if the Mn content exceeds 0.74%, desired surface quality and powdering resistance cannot be ensured. Therefore, the Mn content is 0.74% or less.

(1-4) P: 0.010 to 0.035%
P is also an element effective for easily increasing the strength of steel. If the P content is less than 0.010% in relation to other elements (Mn, B) for increasing the strength of the steel, the tensile strength of the steel cannot be increased to 340 MPa or more. Therefore, the P content is 0.010% or more. On the other hand, if the P content exceeds 0.035%, the surface quality and the powdering resistance cannot be ensured. Therefore, the P content is 0.035% or less. The P content is preferably 0.030% or less.

(1-5) S: 0.015% or less S is contained as an impurity and has an action of segregating at grain boundaries and embrittlement of steel. In addition, even if Mn is contained and fixed as MnS in order to suppress embrittlement, if the absolute amount of MnS is excessive, MnS starts as a starting point and induces cracking during press molding. Therefore, the S content is 0.015% or less. The S content is preferably 0.012% or less. The smaller the S content, the better. Therefore, it is not necessary to limit the lower limit, but it is preferably 0.002% or more from the viewpoint of melting cost.

(1-6) Al: 0.001 to 0.050%
Al has the effect | action which makes steel healthy by deoxidation. Moreover, it has the effect | action which suppresses normal temperature aging by solid solution N by fixing N in steel as AlN. If the Al content is less than 0.001%, it is difficult to obtain the effect by the above action. Therefore, the Al content is 0.001% or more. Preferably it is 0.005% or more. On the other hand, even if the Al content exceeds 0.050%, the effect of the above action is saturated, which is disadvantageous in cost. Therefore, the Al content is 0.050% or less. The Al content is preferably 0.040% or less.

(1-7) N: 0.0035% or less N is an element inevitably contained in steel, and deteriorates ductility, deep drawability, and normal temperature aging resistance. For this reason, N content shall be 0.0035% or less. Preferably it is 0.0030% or less. The smaller the N content, the better. Therefore, it is not necessary to define the lower limit of the N content. However, excessively low nitrogen is accompanied by a significant increase in steelmaking costs. Therefore, the N content is preferably 0.001% or more.

(1-8) Ti: 0.001 to 0.015%
Ti has an effect of improving surface quality and powdering resistance. In order to appropriately obtain this effect, the Ti content is set to 0.001% or more. The Ti content is preferably 0.02% or more. However, when the Ti content increases, a so-called outburst reaction is likely to occur, in which abnormally fast iron diffusion occurs from the grain boundary in the steel near the interface with zinc during the alloying heat treatment. As a result, surface irregularities are formed on the surface of the hot dip galvanized steel sheet due to abnormal growth of the alloy phase crystal partially when the alloy phase crystal is formed during the alloying process, and the appearance of the plating is improved. Unevenness and surface roughness increase. Therefore, the Ti content is 0.015% or less. The Ti content is preferably 0.010% or less.

(1-9) Nb: 0.001 to 0.015%
Nb also has the effect of improving surface quality and powdering resistance in the same manner as Ti. It also has an effect of suppressing the outburst reaction due to Ti. In order to appropriately obtain this effect, the Nb content is set to 0.001% or more. The Nb content is preferably 0.002% or more. However, when the Nb content increases, not only the effect is saturated, but also the grain growth is inhibited during annealing, and the crystal grains are refined, so that the formability may be deteriorated. Therefore, the Nb content is 0.015% or less. The Nb content is preferably 0.010% or less.

(1-10) B: 0.0001 to less than 0.0007% B reacts with N contained as an impurity during hot rolling, precipitates as BN, and reduces solid solution N. If the B content is less than 0.0001%, crystal grains easily grow during annealing, and the yield strength decreases to less than 230 MPa, making it impossible to obtain desired dent resistance. Further, if the B content is less than 0.0001% in relation to other elements (Mn, P) for increasing the strength of the steel, the tensile strength of the steel cannot be increased to 340 MPa or more. For this reason, B content shall be 0.0001% or more. The B content is desirably 0.0003% or more. However, if the B content is 0.0007% or more, the BN produced by reaction with nitrogen becomes coarse, the fine nitride is reduced, and crystal grains grow. In this case as well, the yield strength is lowered and the desired dent resistance cannot be obtained. For this reason, the B content is less than 0.0007%.

(1-11) Mo: 0.080% or less Mo is an optional element contained as necessary, and has an effect of improving aging resistance by interaction with C at room temperature. Therefore, you may contain Mo. However, even if the Mo content exceeds 0.080%, the effect of the above action is saturated and disadvantageous in cost. Therefore, the Mo content is 0.080% or less. The Mo content is preferably 0.050% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable to contain Mo content 0.005% or more.

  The balance other than the above is Fe and impurities.

2. Metal structure In the galvannealed steel sheet according to the present invention, the metal structure is mainly composed of ferrite. If the metal structure is ferrite, the formability (ductility) and weldability necessary for an alloyed hot-dip galvanized steel sheet for an outer panel of an automobile can be ensured.

  Although the alloyed hot-dip galvanized steel sheet according to the present invention is basically a structure made of ferrite, inevitable bainite and pearlite may be included in addition to ferrite. However, when the structure other than ferrite exceeds 5.0 area%, the strength to be obtained by the present invention is not satisfied. Therefore, the structure other than ferrite inevitably contained is 5 area% or less.

  The crystal grain size of the ferrite structure in the present invention is 9.5 or less in the grain size number defined in JIS G 0551 (2013). When the crystal grain size exceeds 9.5 in grain size number, the yield strength increases and the desired shape freezing property cannot be obtained.

3. Mechanical Properties The alloyed hot-dip galvanized steel sheet according to the present invention has a tensile strength of 340 MPa or more, a yield strength of 230 MPa or more and 280 MPa or less (preferably 230 to 270 MPa, more preferably 235 to 270 MPa), and JIS G 3135. The coating bake hardening amount specified in (2013) is 30 MPa or more and 80 MPa or less. If the yield strength is below 230 MPa, the desired dent resistance cannot be ensured.

4). Application The alloyed hot-dip galvanized steel sheet for outer panel according to the present invention is used for outer panels such as automobile outer door panels, outer hood panels, trunk lid outer panels, and front fender panels.

5). Manufacturing method The alloyed hot-dip galvanized steel sheet for outer panel according to the present invention is not manufactured by a specific method, and can be manufactured by any method as long as it can have the above chemical composition, metal structure and mechanical properties. An example of a suitable manufacturing method that takes into account mass productivity and the like will be described below.

  The alloyed hot-dip galvanized steel sheet for outer panel according to the present invention is applied to a steel ingot or steel slab having the above chemical composition, for example, a slab heating process, a hot rolling process, a pickling process, a cold rolling process, and a continuous annealing. It is manufactured through a process, a hot dip galvanizing process and an alloying process.

  In the slab heating step, it is exemplified that the slab having the above-described chemical composition is charged into a heating furnace so that the surface temperature is, for example, 1150 ° C. or higher and 1350 ° C. or lower. The slab can be manufactured by continuously casting a molten steel produced by a conventional method, or by subjecting the molten steel to a steel ingot and then subjecting it to a piece rolling. The slab charged into the heating furnace may be cooled to room temperature, or may be in a high temperature state after continuous casting or ingot rolling. In order to further improve the surface properties of the final product, it is preferable to cool or warm the slab before charging into the heating furnace.

  In the hot rolling step, the slab extracted from the heating furnace is hot-rolled at a finishing temperature of 880 ° C. or higher and 930 ° C. or lower to obtain a hot rolled steel sheet at a winding temperature of 450 to 800 ° C.

  In order to obtain the metal structure after cold rolling, it is preferable to make the crystal grain size of the hot-rolled steel sheet fine. Therefore, it is preferable that the finishing temperature in the hot rolling is a low temperature range of the austenite range. If the finishing temperature is higher than 930 ° C or less than 880 ° C, the hot-rolled steel sheet becomes coarse. Therefore, the finishing temperature of hot rolling is set to 880 ° C. or more and 930 ° C. or less.

  Also, if the coiling temperature is less than 450 ° C, a phenomenon that water partially gets on the steel sheet during cooling tends to occur, resulting in poor flatness, while if the coiling temperature exceeds 800 ° C, the scale becomes thick. Descaling during pickling becomes difficult. For this reason, the coiling temperature after hot rolling shall be 450 degreeC or more and 800 degrees C or less.

  In the pickling process, the hot-rolled steel sheet obtained by the hot rolling process is pickled to obtain a pickled steel sheet. Pickling may be performed by a conventional method. In order to further reduce scale wrinkles, surface grinding may be performed before or after pickling.

  In the cold rolling process, the pickled steel sheet obtained by the pickling process is subjected to cold rolling with a rolling reduction of 60 to 95% to obtain a cold rolled steel sheet.

  In order to obtain the above-described metal structure, it is preferable that the rolling reduction in cold rolling is 60% or more. On the other hand, when the rolling reduction in cold rolling exceeds 95%, the rolling load increases, and the load on the rolling mill may be excessive. Therefore, the rolling reduction in cold rolling is preferably 95% or less. The cold-rolled steel sheet is subjected to a treatment such as degreasing according to a known method as necessary. In order to further reduce scale wrinkles, surface grinding may be performed after cold rolling and before annealing.

  Further, the cold-rolled steel sheet thus obtained is subjected to annealing, hot dip galvanizing and alloying treatment in a continuous hot dip galvanizing line.

  Annealing is performed at 720 ° C. or higher and 860 ° C. or lower. If the annealing temperature is less than 720 ° C or more than 860 ° C, the metal structure cannot be obtained.

  The cold-rolled steel sheet thus obtained is subjected to hot dip galvanization and alloying treatment according to a conventional method. The plating method and the chemical composition of the plating film are not limited.

  The alloyed hot-dip galvanized steel sheet for an outer panel according to the present invention thus obtained may be subjected to temper rolling according to a conventional method. However, the yield ratio increases and the elongation decreases by temper rolling. In order to suppress this, it is preferable that the elongation of temper rolling is 1.8% or less. More preferably, it is 1.6% or less.

  Steel having the chemical composition shown in Table 1 was melted in a test converter, and a 250 mm thick slab was manufactured using a continuous casting tester.

  The obtained slab was heated and hot rolled to a thickness of 4.0 mm using a hot rolling tester. Table 1 shows finish rolling temperature (hot rolling completion temperature) and coiling temperature as hot rolling conditions.

  After removing the scale of the obtained hot-rolled steel sheet by hydrochloric acid pickling, it was cold-rolled to a thickness of 0.65 mm at a rolling reduction shown in Table 1.

  The obtained cold-rolled steel sheet was annealed at the annealing temperature shown in Table 1. About hot dip galvanization, it immersed in the hot dip galvanization bath of 460 degreeC in the middle of cooling after annealing, performed hot dip galvanization, and it heated after alloying and performed the alloying process. After hot dip plating, temper rolling with an elongation of 1.4% was performed.

  Sample No. obtained in this way. About 1-25 (invention example) and sample Nos. X1-X8 (comparative example), yield strength (YS) and Tensile strength (TS) was determined.

  Further, a JIS No. 5 tensile test piece was sampled from the 90 ° direction of rolling, and a paint bake hardening amount (BH amount) was determined by a paint bake hardening amount test method defined in JIS G 3135 (2013).

  Furthermore, while visually observing the surface of the obtained steel plate, the surface quality was evaluated and the powdering resistance was evaluated as follows. That is, after performing deep drawing with (cylindrical drawing condition blank diameter: 90 mm, drawing height: 30 mm, lubricating oil: general rust preventive oil (Nox-Rust550HN; Parker Kosan)), affixing tape to the side wall portion Peeling resistance was evaluated according to the criteria shown in Table 2 from the difference in weight before and after peeling.

  The crystal grain size of the ferrite structure was determined by image processing based on JIS G 0551 (2013) by taking a structure photograph in the vicinity of the center of the plate thickness with an optical microscope.

The results are summarized in Table 1.
Sample No. in Table 1 Nos. 1 to 25 are examples of the present invention that satisfy all the conditions defined by the present invention. X1 to X8 are comparative examples that do not satisfy this condition.

Sample No. 1-25 all have a tensile strength of 340 MPa or more, a yield strength of 230 MPa or more and 280 MPa or less, and a coating bake hardening amount of 30 MPa or more and 80 MPa or less, and a plating adhesion amount of 45 to 52 g / m ² per side. In addition, the powdering resistance and surface appearance were good. For this reason, sample no. Nos. 1 to 25 are suitable for use in, for example, door outer panels and the like that particularly require excellent shape freezing properties and dent resistance.

  Sample No. On the surface appearances of 18 and 25, extremely slight plating unevenness was observed, but it was not problematic in practical use.

  In contrast, sample no. x1 had a C content of 0.0035%, exceeding the upper limit of the range of the present invention, and thus the coating bake hardening amount was excessively 84 MPa, and the moldability was poor.

  Sample No. Since x2 is 0.105% and exceeds the upper limit of the range of the present invention, x2 has poor powdering resistance and appearance of plating.

  Sample No. Since x3 has a Mn content of 0.89%, which exceeds the upper limit of the range of the present invention, the powdering resistance and the appearance of plating were unsatisfactory.

  Sample No. Since x4 has an Nb content of 0.017%, which exceeds the upper limit of the range of the present invention, grain growth progressed during annealing, and the amount of paint bake hardened became unsatisfactory.

  Sample No. Since x5 has a Ti content of 0.017%, which exceeds the upper limit of the range of the present invention, the powdering resistance and the appearance of plating were unsatisfactory.

  Sample No. For x6, the B content was 0.0008%, which exceeded the upper limit of the range of the present invention, and the crystal grain size was smaller than the lower limit of the range of the present invention in terms of grain size number, so the yield strength was too low at 225 MPa.

  Sample No. In x7, the P content was 0.045%, which exceeded the range of the present invention, so that the surface quality and the powdering resistance were lowered.

  Furthermore, sample no. In x8, the N content was 0.0040%, exceeding the upper limit of the range of the present invention, so that the normal temperature aging resistance was deteriorated and the coating bake hardening amount was unsatisfactory.

Claims (4)

In mass%, C: 0.0010 to 0.0030%, Si: 0.005 to 0.10%, Mn: 0.30 to 0.74%, P: 0.010 to 0.035%, S: 0.015% or less, Al: 0.001 to 0.050%, N: 0.0035% or less, Ti: 0.001 to 0.015%, Nb: 0.001 to 0.015%, B: 0 0.0001 to less than 0.0007%, having a chemical composition consisting of the balance Fe and impurities,
The crystal grain size has a metal structure mainly composed of a ferrite structure with a grain size number specified in JIS G 0551 (2013) and exceeding 9.5,
An automobile characterized by a tensile strength of 340 MPa or more, a yield strength of 230 to 280 MPa, and mechanical properties such that the amount of paint bake hardening specified in JIS G 3135 (2013) is 30 to 80 MPa. Alloyed hot-dip galvanized steel sheet for outer panel.   2. The alloyed hot-dip galvanized steel sheet for an outer panel of an automobile according to claim 1, wherein the chemical composition has Mo: 0.050% or less.   In mass%, C: 0.0010 to 0.0030%, Si: 0.005 to 0.10%, Mn: 0.30 to 0.74%, P: 0.010 to 0.035%, S: 0.015% or less, Al: 0.001 to 0.050%, N: 0.0035% or less, Ti: 0.001 to 0.015%, Nb: 0.001 to 0.015%, B: 0 A slab containing 0.0001 to less than 0.0007% and having a chemical composition consisting of the balance Fe and inevitable impurities is hot-finished at 880 to 930 ° C. and wound at 450 to 800 ° C. A method for producing an alloyed hot-dip galvanized steel sheet for an outer panel of an automobile, characterized by performing cold rolling at a reduction rate of ~ 95% and annealing at 720 to 860 ° C in a continuous hot-dip galvanizing line.   The said chemical composition is Mo: 0.080% or less The manufacturing method of the galvannealed steel plate for the outer panel of a motor vehicle described in Claim 3.