JP2017031452A - Alloy galvanized steel sheet for outer sheet panel of automobile and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an alloy galvanized steel sheet for outer sheet panel having excellent surface quality, powdering resistance and shape freezing property and 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%, Al:0.001 to 0.050%, N≤0.0035%, Ti:0.001 to 0.015%, Nb:0.001 to 0.015%, B:0.0007 to 0.0018% and the balance Fe with impurities, a metallic structure mainly containing ferrite having crystal grain size defined in JIS G 0551 of 9.5 or less and mechanical properties such as tensile strength of 340 MPa or more, yield strength of 200 to 250 MPa and coating/baking cured amount defined in JIS G 3135 of 30 to 80 MPa.SELECTED DRAWING: None

Description

  The present invention relates to an alloyed hot-dip galvanized steel sheet for an outer panel of an automobile and a method for producing the same. Specifically, the present invention relates to an alloyed hot-dip galvanized sheet for an outer panel of an automobile having an excellent shape freezing property and a tensile strength of 340 MPa or more. It is related with a steel plate and its manufacturing 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.

  The high-tensile alloyed hot-dip galvanized steel sheet used for the outer panel is also required to have a property (dent resistance) that does not cause permanent deformation when pressed with a finger. 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 can maintain the dimensions and shape as 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. The shortage of 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% or less, 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 (0.035% or less) and the low Mn content that the surface quality and powdering resistance of the galvannealed steel sheet are not problematic In an amount (0.74% or less), 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, in the outer panel such as the door outer panel and the hood outer panel, surface distortion occurs near a minute uneven portion (for example, a recessed portion for mounting the door handle in the case of the door outer panel). Since it may occur, it is required to ensure the shape freezing property by the low yield strength. 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 The initial yield strength is controlled to be lower (initial yield strength: 200 to 250 MPa).

  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 for an outer panel having a tensile strength of 340 MPa or more and having excellent surface quality, powdering resistance and shape freezing property.

  The present inventors have intensively studied to use an alloyed hot-dip galvanized steel sheet having a tensile strength of 340 MPa class for an outer panel of an automobile, and obtained the knowledge (i) to (iii) listed below. Completed the invention.

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

  (Ii) The shape freezing property can be improved by controlling the yield strength to be low, specifically by controlling the yield strength to 250 MPa or less. The yield strength can also be controlled by adding B. B precipitates as BN during hot rolling to reduce solid solution N in the steel, so that precipitation of fine nitride during subsequent cold rolling and recrystallization annealing can be suppressed, and moderate grain growth occurs. The yield strength can be suppressed to 250 MPa or less.

  (Iii) Thus, in order to ensure all the surface quality, powdering resistance and shape freezing property in the alloyed hot-dip galvanized steel sheet having a tensile strength of 340 MPa, Mn content, P content and B content Although it is important to optimize the balance of the amount, it is not possible to ensure the surface quality and the powdering resistance only by optimizing the balance. Furthermore, by adding Ti and Nb in combination, surface quality and 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 ferrite having a chemical composition consisting of 0007 to 0.0018%, the balance Fe and impurities, and having a crystal grain size of 9.5 or less in the grain size number defined in JIS G 0551 (2013). It has a mechanical property that the tensile strength is 340 MPa or more, the yield strength is 200 to 250 MPa, and the paint bake hardening amount specified in JIS G 3135 (2013) is 30 to 80 MPa. Car skin Alloyed hot-dip galvanized steel sheet for panels.

  (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 having a chemical composition consisting of 0007 to 0.0018%, the balance Fe and impurities is subjected to hot finish rolling at 880 to 930 ° C., wound at 450 to 800 ° C., and then subjected to a rolling reduction of 60 to 95%. A method for producing an alloyed hot-dip galvanized steel sheet for an outer panel of an automobile, characterized by performing cold rolling 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, excellent surface quality and powdering resistance with a low P content (0.035% or less) and a low Mn content (0.74% or less) so that surface quality and powdering resistance do not become a problem. It is possible to provide an alloyed hot-dip galvanized steel sheet for an outer panel having a tensile strength of 340 MPa or more, which has both properties and shape freezing property.

  Therefore, according to the present invention, it is possible to reduce the thickness (weight reduction) of, for example, a door outer panel that particularly requires excellent shape freezing properties.

  The reasons for limiting the chemical composition, metal structure, mechanical properties, application, and production conditions in the present invention will be described.

1. Chemical composition (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 0.0010% 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.

(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.

(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.

(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.

(5) S: 0.015% or less S is contained as an impurity, and has the effect 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.

(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 terms of cost. Therefore, the Al content is 0.050% or less. The Al content is preferably 0.040% or less.

(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.

(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.

(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.

(10) B: 0.0007 to 0.0018%
B reacts with N contained as an impurity during hot rolling, precipitates as BN, and reduces solid solution N. When the B content is less than 0.0007%, BN precipitation during hot rolling is insufficient, and fine nitride is precipitated due to residual solid solution N during annealing. For this reason, the metal structure becomes too fine, the yield strength increases, and the desired shape freezing property cannot be ensured. For this reason, B content shall be 0.0007% or more. However, when the B content exceeds 0.0018%, the deep drawability deteriorates remarkably due to an increase in the recrystallization temperature. For this reason, B content shall be 0.0018% or less.

(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 The metal structure of the galvannealed steel sheet according to the present invention 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 200 MPa or more and 250 MPa or less, and a paint bake hardening amount defined in JIS G 3135 (2013) is 30 MPa. It is 80 MPa or less. If the yield strength exceeds 250 MPa, the desired shape freezing property 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 galvanization, 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 200 MPa or more and 250 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.

  Sample No. On the surface appearances of 11 and 22, extremely slight plating unevenness was observed, but there was no practical problem.

  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% which exceeds the upper limit of the range of the present invention, x2 has poor powdering resistance.

  Sample No. Since x3 has an Mn content of 0.89%, which exceeds the upper limit of the range of the present invention, powdering resistance was unsatisfactory.

  Sample No. x4 has an Nb content of 0.017%, which exceeds the upper limit of the range of the present invention. Therefore, grain growth is delayed during annealing, and the grain size exceeds the upper limit of the range of the present invention in terms of grain size number, yield strength. Became unpleasant.

  Sample No. Since x5 exceeded the upper limit of the range of the present invention with Ti content of 0.018%, powdering resistance was unsatisfactory. Moreover, the surface appearance also deteriorated.

  Sample No. For x6, the B content was 0.0022%, which exceeded the upper limit of the range of the present invention, and the grain size exceeded the upper limit of the range of the present invention in terms of grain size number, so the yield strength was excessively high at 285 MPa.

  Sample No. Since x7 exceeds 0.05% and the range of the present invention, x7 has poor powdering resistance. Moreover, the surface appearance also deteriorated.

  Furthermore, sample no. x8 has an N content of 0.0040%, which exceeds the upper limit of the range of the present invention, and the crystal grain size exceeds the upper limit of the range of the present invention in terms of the grain size number. In addition, the amount of paint baking and curing became 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 A metal structure mainly composed of ferrite having a chemical composition composed of .0007 to 0.0018%, the balance Fe and impurities, and having a grain size number of 9.5 or less as defined in JIS G 0551 (2013) The tensile strength is 340 MPa or more, the yield strength is 200 to 250 MPa, and the paint bake hardening amount specified in JIS G 3135 (2013) is 30 to 80 MPa. To auto Galvannealed steel sheet for outer panels of.   The alloyed hot-dip galvanized steel sheet for an outer panel of an automobile according to claim 1, wherein the chemical composition has Mo: 0.080% 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 having a chemical composition consisting of .0007 to 0.0018%, the balance Fe and impurities is subjected to hot finish rolling at 880 to 930 ° C., wound at 450 to 800 ° C., and then reduced to 60 to 95%. A method for producing an alloyed hot-dip galvanized steel sheet for an outer panel of an automobile, characterized in that the steel sheet is cold-rolled and annealed 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.