JP2002212698A - Galvannealed steel sheet having excellent powdering resistance and low temperature chipping resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a galvannealed steel sheet which exhibits excellent powdering resistance when it is subjected to press processing and has excellent low temperature chipping resistance after being coated. SOLUTION: Iron content in the plated film is <=13 mass %, and when an average value of Vickers hardnesses at ten positions of the surface of the steel sheet after removing plating is defined as Hav., the maximum value of the Vickers hardnesses is defined as Hmax., and the minimum value of the Vickers hardnesses is defined as Hmin., the value of (Hmax.-Hmin.)/Hav. is adjusted to be <0.3. Further, the ratio of the particle diameter in the rolling direction to the particle diameter in the direction perpendicular to the rolling direction of each crystalline particle on the uppermost surface of the steel sheet is controlled to be within a prescribed range, and the iron content and/or the aluminum content in the plated film are also each controlled to be within a prescribed range.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet used as a rust-proof surface-treated steel sheet for automobiles, and more particularly to a galvannealed steel sheet having excellent powdering resistance and low-temperature chipping resistance.

[0002]

2. Description of the Related Art Galvannealed steel sheets (hereinafter GA)
Steel plates are abundant and widely used as automotive steel plates because of their low cost and excellent corrosion protection. However, GA steel sheet is
Since a Zn-Fe alloy layer is formed by heat alloying (Fe) and Zn, a hard and brittle layer is generated at the interface between the steel sheet and the plating layer. Therefore, the deformation of the steel sheet due to the press working or the contact with the metal mold causes the plating layer to become a powder of plating of about 100 μm or less, which is called powdering, and a defect of peeling off easily occurs. It is known that this powdering is likely to occur when the plating layer of the GA steel sheet is subjected to compressive stress (see “Press Forming Difficulty Handbook”, 2nd edition, Chapter 5). Further, as an evaluation method of the powdering resistance, it is studied to perform a draw bead test, a bending back test, a cup drawing test, and the like.

By the way, several methods for improving powdering resistance have been proposed. For example, a method of defining the amount of Si and the amount of P in steel (JP-A-6-41707)
Japanese Patent Application Laid-Open No. 9-291349), and a method of defining a heating rate and a cooling rate during alloying (Japanese Patent Application Laid-Open No. 2-170959). However, Si and P used for high strength
Restricting the amount of addition of the steel is the strength and elongation of the GA steel sheet, r
Therefore, it is difficult to obtain a steel sheet that satisfies the target material and has good powdering resistance. Further, in order to regulate the temperature rise and the cooling rate during alloying, it is necessary to enhance the equipment and the like, which leads to an increase in cost.

[0004] On the other hand, chipping refers to a phenomenon in which a painted GA steel sheet used for an automobile outer panel or the like peels off from an interface of a plating layer due to collision of scattering of pebbles or the like. Is referred to as low temperature chipping.
It is said that low-temperature chipping also involves interface adhesion, similar to powdering, and that the formation of a hard and brittle Γ phase deteriorates low-temperature chipping resistance. As a method for improving the low-temperature chipping resistance of GA steel sheets which has been proposed so far, a method of defining the amounts of Si and P in steel (Japanese Patent Laid-Open No.
No. 49), a method of having a Fe alloy plating film as a second layer on the GA steel sheet (Japanese Patent Application Laid-Open No. 5-339751), grinding the base plate surface before plating, and further defining the rate of temperature rise during alloying (JP-A-6-57391).

However, restricting the amount of Si or P used for increasing the strength imposes restrictions on the strength, elongation, and r-value of the steel sheet. It is difficult to get good ones. In addition, a method of regulating the rate of temperature rise and cooling during alloying,
The method of plating the upper layer of A and the method of grinding the mother plate
This requires an increase in equipment and the like, and also complicates the process, resulting in an increase in cost.

[0006]

SUMMARY OF THE INVENTION The present invention causes the above problems of the prior art, in particular, the restriction of the material due to the regulation of the amount of reinforcing element added to the steel, and the increase in the V cost due to complicated processes. It is an object of the present invention to improve the powdering resistance and the low-temperature chipping resistance after coating of an alloyed hot-dip galvanized steel sheet without pressing.

[0007]

The gist of the present invention is as follows. That is, (1) The iron content in the plating is 13% by mass or less, and the 10-point average of the Vickers hardness of the steel sheet surface after the plating is removed is Ha.
v. When the maximum value is Hmax. and the minimum value is Hmin. (Hma
x.-Hmin.) / Hav. <0.3, an alloyed hot-dip galvanized steel sheet having excellent powdering resistance and low-temperature chipping resistance. (2) The galvannealed steel sheet according to the above (1), wherein the ratio of the grain diameter in the direction perpendicular to the rolling direction of the outermost crystal grains of the steel sheet after plating is 3.0 or less. (3) The iron content in the plating is 8 to 13% by mass, and the Al content in the plating is the following formula: Al content in the plating (g / m ² )-0.0012 × Zn content in the plating (g /
m ² ) The galvannealed steel sheet according to the above (1) or (2), which satisfies ≧ 0.06.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION As a simulation of press working, the inventors investigated powdering resistance by a cup drawing test which combines bending return, contact and sliding with a mold, and shrinkage and elongation deformation of a steel plate. did. The low-temperature chipping resistance was investigated by scattering pebbles at -20 ° C after coating. By this method, the GA steel sheet prepared under various conditions and the GA steel sheet after painting were investigated in detail. In addition to the plating properties, the surface properties of the steel sheet after plating removal had a great effect on powdering resistance and low-temperature chipping resistance. It was found that it was exerting. That is, the peeling of the plating (powdering) accompanied by the shrinkage deformation of the steel sheet and the peeling of the plating (chipping) due to the impact on the steel sheet are caused by the iron content of the plating layer and the
It was found that among the phase thickness and the properties of the steel sheet, the uniformity of the hardness was particularly affected on the outermost surface of the steel sheet immediately below the plating layer. Hereinafter, this will be described in detail.

[0009] When the GA steel sheet is subjected to compression or tensile deformation by press forming or the like, the plating layer can hardly be deformed. Therefore, a crack (crack) is formed from the surface of the plating layer to the steel sheet interface. From the interface of
At this time, since the steel sheet under the plating layer has ductility, the steel sheet follows the deformation to some extent, but if the deformation is large, cracks also occur at the grain boundaries of the steel sheet surface layer. The inventors have investigated this phenomenon in detail, and as a result, the size of the crack by the steel plate,
It was found that the difference in depth and density and the separation of the plating layer involved grain boundary cracks in the surface layer of the steel sheet. When a small but large crack is generated on the surface of the steel sheet, the large crack is a starting point, and the plating layer is peeled off. It was also found that it was difficult to occur.

Next, the influence of the size, depth, and density of grain boundary cracks on the surface layer of the steel sheet was examined using steel sheets having various characteristics.
As a result, it was found that the variation in the hardness of the surface layer of the steel sheet after the removal of the plating contributed to the above characteristics. Hereinafter, the variation in hardness is represented by the difference between the maximum value and the minimum value of Vickers hardness. When the hardness difference between the surface layers of the steel sheet is large, the number of cracks on the surface of the steel sheet is small but large and deep cracks are generated. On the other hand, when the hardness difference is relatively small, the number is large but small and shallow cracks are generated. It is considered that the reason why such a phenomenon appears is that when the surface layer portion of the steel sheet is deformed, if the hardness difference of the surface is large, local uneven deformation is likely to occur. In other words, it is considered that a large deep crack is generated at a grain boundary near a boundary between the soft part and the hard part because the soft part is easily deformed and the hard part is hardly deformed. When the variation in hardness is small, cracks are small and shallow because every crystal grain undergoes the same deformation.

According to the results of the investigations by the inventors, in the GA steel sheet having a large surface hardness difference, cracks are 100 to 30%.
In the GA steel sheet having a length of 0 μm and a depth of about 10 μm, but having a small difference in hardness, cracks are several tens μm in length and 2 in depth.
It stops at about μm. Therefore, as a result of investigating various GA steel sheets in more detail, assuming that the average of 10 points of the Vickers hardness of the steel sheet surface after plating removal is Hav., The maximum value is Hmax., And the minimum value is Hmin. Hmin.) / Ha
v. It was found that cracks in the surface layer of the GA steel sheet were small and the powdering resistance was good when the control was performed within the range of <0.3.

Further, it has been found that the steel sheet having the reduced hardness variation has good low-temperature chipping resistance after coating. The mechanism is considered as follows. Low-temperature chipping occurs when the impact of pebbles or the like colliding with the painted surface reaches the interface between the plating and the steel sheet, and the plating peels off from the interface. When the difference in hardness of the steel sheet surface layer is large, the hard part and the soft part exist locally, so the impact applied from the outside is easy to be absorbed in the soft part, but it is easy to propagate along the hard part. It is considered that the plating is easily peeled off.

The powdering resistance and low-temperature chipping resistance are also affected by the shape of the crystal grains on the steel sheet surface, and the grain diameter in the direction perpendicular to the rolling direction of the outermost surface crystal grains after the plating is removed. It is desirable that the ratio be 3.0 or less. It is more preferably 2.0 or less. In the recrystallization annealing after cold rolling, if the crystal grains in the outermost layer recrystallize and grow uniformly regardless of the direction, the crystal grains are relatively circular, and the grain ratio in the rolling direction and the direction perpendicular to the rolling direction. Becomes closer to 1. On the other hand, when recrystallization and grain growth become non-uniform due to factors such as a large amount of precipitates on the surface layer of the steel sheet, the structure of the structure extending in the rolling direction in the cold-rolled state remains and the grain size ratio increases. When the grain size ratio of the crystal grains in the surface layer of the steel sheet thus formed is small and recrystallized uniformly, the powdering resistance and the low-temperature chipping resistance are further improved. On the other hand, when the grain size ratio of the crystal grains becomes non-uniform, such as 3.0 or more, when deformation or impact is applied to the steel sheet, a part with a large amount of deformation or a part where the impact is likely to propagate Will exist,
Plating becomes easy to peel off.

[0014] The iron content in the plating layer is preferably 13% by mass or less. If the iron content exceeds 13% by mass, the hard layer tends to increase in the plating layer, the plating itself is easily broken, and the powdering resistance and the low-temperature chipping resistance are inferior. Further, the iron content in the plating layer is 8 to 13% by mass, and the amount of Al in the plating is as follows: Al amount in plating (g / m ² ) −0.0012 × Zn amount in plating (g
/ M ² ) ≧ 0.06, both the powdering resistance and the low-temperature chipping resistance are further improved. If the iron content in the plating is less than 8% by mass, a soft η phase remains,
Powdering resistance and low temperature chipping resistance do not matter. Therefore, when the iron content in the plating is in the range of 8 to 13% by mass, the improvement of the powdering resistance and the low-temperature chipping resistance by the above measures is remarkable.

The amount of Al during plating affects the uniformity of alloying and the amount of Γ phase generated. When the amount of Al is small, alloying occurs uniformly, so that the interface between the plating and the steel sheet becomes smooth, and the adhesion between the plating and the steel sheet is weakened. Conversely, if the amount of Al is large, non-uniform alloying occurs such that alloying at the grain boundaries is suppressed, and the interface between the plating and the steel sheet becomes uneven. Strength and adhesion are improved. Further, if Al is present in the plating, the stable region of the Γ phase changes thermodynamically, so that a hard and brittle Γ phase is hardly generated. The effect of Al as described above is based on the relationship between the amount of Al in the plating layer and the amount of Zn in the plating. The amount of Al in the plating (g / m ² ) −0.0012 × the amount of Zn in the plating (g / m ² ) ≧
It is exhibited by controlling to the range of 0.06.

Hereinafter, a method for measuring the hardness and the crystal grain size of the steel sheet surface described above will be described. As the plating removal method for measuring the hardness, an Sb-added hydrochloric acid aqueous solution, an inhibitor-added hydrochloric acid aqueous solution, or an alkali solution specified in JIS H 0401 can be used. In such plating removal with an acid or alkali, the steel sheet may be taken out at the stage when the dissolution reaction is completed. In the plating removal method by electrolysis, it is only necessary to dissolve until the iron potential is reached using an electrolytic solution. The specific method of mixing the liquid is described below. SbCl ₃ 32 g, hydrochloric acid (density 1.18 g / cm ³ or more) 1 L stock solution 5 ml hydrochloric acid (density 1.18 g / cm ³ or more) 100 ml bit 710 N (Asahi Chemical Industry Co., Ltd.) 5 ml 10% hydrochloric acid aqueous solution 1 L 20 % NaOH aqueous solution 80 ml 10% aqueous triethanolamine _{40 ml 35% H 2 O 2} 7 ml H 2 O 75 ml NaCl 200 g ZnSO 4 100 g H 2 O 1 L

The Vickers hardness of the surface of the steel sheet from which the plating has been dissolved and removed is preferably measured with a load of about 0.049 N to 0.98 N in order to reduce the depth of the indentation to 1/5 or less of the crystal grain size. The number of Vickers hardness measurement points may be ten. Thus, by setting the load smaller than the hardness measurement of a normal steel plate, it becomes possible to measure the hardness of the crystal grains only on the steel plate surface. The measurement interval of Vickers hardness is set to 5 times or more and 10 times or less of the size (diagonal length) of the indentation so that the subsequent indentation is not affected by the change in hardness around the indentation. That's why. FIG. 1 schematically shows a method for measuring Vickers hardness employed in the present invention. In addition, the method of observing the crystal grains on the surface of the steel sheet after plating is removed is as follows: the surface of the steel sheet from which plating has been removed is polished to a thickness of about 1 μm with a buff coated with a diamond paste, and etched with nital. Observe with an optical microscope. The grain size ratio between the rolling direction and the direction perpendicular to the rolling direction is determined by counting the number of crystal grains existing at a length of 250 μm in the rolling direction and the direction perpendicular to the rolling direction, and defining the respective grain sizes as the respective grain sizes. . Normally, it is preferable to average the particle size ratios determined in three visual fields.

For example, the temperature and time of slab reheating are related to the surface hardness of the steel sheet. When the temperature is high and heating is performed for a long time, the surface layer of the slab is locally nitrided or oxidized, and a large amount of precipitates and oxides are present in the surface layer. In a portion where many of these exist, dissipation (strain) disappears slowly during recrystallization annealing after hot rolling and cold rolling, resulting in a hard portion. On the other hand, a portion where the amount of precipitates and oxides is small is soft, so that a difference in hardness occurs. At this time, if the recrystallization does not proceed sufficiently and the form of the cold-rolled structure remains, the particle size ratio increases. As a method for controlling these, Sb may be added to steel. The amount of Al in the plating layer is greatly affected by the Al concentration in the plating bath and the plating time. Therefore, during plating
In order to satisfy Al−0.0012 × Zn ≧ 0.06, the Al concentration in the bath should be 0.132% by mass or more, and the plating time should be 0.5 seconds or more.

[0019]

EXAMPLE A test steel having the components shown in Table 1 was melted in a converter,
The slab was 250 mm thick by continuous casting. The slab reheating temperature was changed as shown in Table 2 and the finishing temperature (FDT) was changed.
Hot rolling to 900mm at 4mm, winding temperature (CT)
It was wound up at 500 ° C. on a hot-rolled coil. Next, the iron oxide film was mainly dissolved and removed in the pickling line, and cold rolling was performed to reduce the thickness to 0.7 mm. This was subjected to a continuous hot-dip galvanizing line (CGL) at a dew point of -30 ° C and an annealing temperature of 800 ° C.
And then hot-dip galvanized, and a heat-alloying treatment to form a Zn-Fe alloy layer.

[0020]

[Table 1]

With respect to the obtained GA steel sheet, Vickers hardness and crystal grain size were measured, and powdering resistance and low-temperature chipping resistance were evaluated. Each measurement,
The evaluation method is as follows. -Vickers hardness of the steel sheet surface: After dissolving and removing the plating layer with an aqueous solution of Sb-added hydrochloric acid, the hardness of the steel sheet surface was measured. As shown in FIG. 1, at a position having a pitch of 20 μm in the rolling direction and a pitch of 20 μm in a direction perpendicular to the rolling direction, a load of 0.147 N
(15 gf).・ Grain size ratio of the steel sheet surface: The steel sheet from which the plating layer has been dissolved and removed with an aqueous solution of Sb-added hydrochloric acid is polished to a thickness of 1 μm with a buff coated with diamond paste, and then nital (nitric acid-nitric acid).
(Ethanol solution), and the diameter was measured in each direction by optical microscope observation to determine the ratio. -Powdering resistance: 70mmφ blank is 33mm
Using a φ punch, a cylinder was drawn with a wrinkle pressing pressure of 4900 N (500 kgf) to obtain a cup having a height of 25 mm. As a peeling amount for one round of the outside of the cup, a tape of 20 mm width was attached and peeled from the shoulder part in contact with the mold to the opening part, and the Zn count number was measured by X-ray fluorescence with respect to the adhered part. evaluated. -Low temperature chipping resistance: After applying a chemical treatment of ² g / m2 to GA steel sheet, cationic electrodeposition coating for automobiles (20 µm)
, Middle coat (35μm), top coat (35μm)
A baking treatment was performed at 170 ° C. for 20 minutes. These chemicals were all manufactured by Nippon Paint Co., Ltd. After keeping this painted GA steel plate at -25 ° C or lower for 30 minutes or more,
The temperature was set to −20 ° C. with a gravelo tester, 15 granites of 0.3 to 0.5 g / piece, and air pressure of 196 kPa (2 kg / cm
^{In 2} ), they were scattered one by one and collided with the steel plate. After the temperature was returned to room temperature after the test, a gum tape was stuck and peeled off, and image processing was performed on the adhered portion to determine a peeling area ratio.

[0022]

[Table 2]

Table 2 shows that all of the examples of the present invention have excellent powdering resistance and low-temperature chipping resistance. Moreover, in the present invention example, Si: 0.5% by mass, Mn: 0.
Even in a steel composition of 5 mass%, powdering resistance and low-temperature chipping resistance can be achieved by appropriately selecting the slab heating temperature and the like. An increase in cost due to material restrictions, complicated processes, and the like can be eliminated.

[0024]

As described above, according to the present invention,
To provide an alloyed hot-dip galvanized steel sheet having excellent powdering resistance and low-temperature chipping resistance without increasing the cost due to restrictions on the material due to the regulation of the amount of the reinforcing element added to the steel and complicated processes. Becomes possible. Therefore, the present invention is expected to contribute to the improvement of the yield in the press forming process and the improvement of the durability of the automobile in cold regions.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a method for measuring Vickers hardness of a steel sheet surface.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Chiaki Kato 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. AE27

Claims (3)

[Claims] When the iron content in plating is 13% by mass or less, the average of 10 points of the Vickers hardness of the steel sheet surface after removing the plating is Hav., The maximum value is Hmax., And the minimum value is Hmin.
(Hmax.-Hmin.) / Hav. An alloyed hot-dip galvanized steel sheet having excellent powdering resistance and low-temperature chipping resistance, characterized by having a value of less than 0.30. 2. The galvannealed steel sheet according to claim 1, wherein the ratio of the grain diameter in the direction perpendicular to the rolling direction to the rolling direction of the outermost crystal grains of the steel sheet after plating is 3.0 or less. . 3. The iron content in the plating is 8 to 13 mass%, and the amount of Al in the plating is as follows: Al amount in the plating (g / m ² ) −0.0012 × Zn amount in the plating (g / m ² )
^{3. The} galvannealed steel sheet according to claim 1 or 2, which satisfies m ² ) ≧ 0.06.