JP3533960B2 - Galvanized steel sheet for automotive exterior materials and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zinc-plated steel sheet which is a base material for a surface-treated steel sheet used as an automobile exterior material and in which pinholes do not occur after painting, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, it has been known that grain boundary oxidation occurs in crystal grain boundaries on the surface of a steel sheet after hot rolling in the production of a cold rolled steel sheet having a high Si content.

The grain boundary oxidation is 50 to 50% from the surface of the steel sheet.
It occurs at a depth of about 100 μm and is difficult to remove by ordinary pickling. Therefore, if grain boundary oxidation occurs, the grain boundary oxidized portion is not removed by pickling, and the grain boundary oxidized portion is peeled off in cold rolling after pickling. Then, the peeled iron powder may cause uneven flaws on the surface of the steel sheet during cold rolling or continuous annealing. Further, there is a problem that workability is deteriorated in a high-strength steel sheet having high notch sensitivity due to microcracks in the grain boundary oxidized portion existing on the steel sheet surface.

In a steel sheet in which such grain boundary oxidation occurs,
Although measures such as slowing the pickling rate or using mechanical grinding and pickling together have been taken, there is a problem that the productivity is remarkably reduced.

In order to suppress such grain boundary oxidation, it is considered necessary to suppress the generation of scale in the hot rolled steel sheet as much as possible, and the atmosphere in the slab heating furnace is made non-oxidizing or the slab heating temperature is lowered. It is conceivable to suppress the oxidation of the rough rolled steel sheet as much as possible by such means and thin the grain boundary oxide layer to the extent that it can be removed by ordinary pickling.

However, such a method has problems that the cost of slab heating becomes high, the load in hot rolling becomes extremely high, and that grain boundary oxidation cannot be sufficiently prevented. There is also.

Therefore, even in the prior art, in hot rolling such a high silicon steel sheet, a method has been proposed for preventing the grain boundary oxidation by lowering the coiling temperature by quenching after the hot rolling. (Japanese Patent Laid-Open No. 2-50908).

[0008]

By the way, 0.5 wt% of Si
It is said that grain boundary oxidation does not occur in a steel sheet of less than%. However, in the ultra-low carbon steel sheet for deep drawing used for automobile steel sheet, as in the cold rolling steel sheet with high Si content described above, in order to secure its mechanical properties, the coiling temperature in hot rolling is generally used. Is set higher. Therefore, grain boundary oxidation is likely to occur in the extreme surface layer of the steel sheet base material during the scale formation process, grain boundary etching may be observed during pickling, and defects may occur in the steel sheet.

However, unlike the cold-rolled steel sheet having a high Si content as described above, in the case of ultra-low carbon steel, grain boundary oxidation occurs up to about 10 μm from the surface, that is, one grain of the surface layer. However, it does not occur in the lower layer.

Therefore, in the case of a steel sheet containing less than 0.5 wt% Si,
Grain boundary oxidation part is peeled off like the above-mentioned cold rolled steel sheet with high Si content, and the peeled iron powder causes uneven flaws on the steel sheet surface during cold rolling or continuous annealing, and further steel sheet There is no problem such as deterioration of workability in a high-strength steel sheet having high cutout sensitivity due to microcracks in the grain boundary oxidized portion existing on the surface.

However, particularly in the case of materials used for automobile exteriors, when such a material having a grain boundary oxidized surface layer is pickled, the grain boundary oxidized portion is selectively etched at the same time as the scale is removed, and the grain boundary oxidized portion is deeply etched. Grooves (depth about 10 μm) are formed. When this is cold-rolled, it becomes a flap (scab) -like defect, and when plating such as Zn is applied on this, this flap part becomes the starting point during press working by the user and cracks occur in the plating film. In addition, bubbles generated during electrodeposition coating by the user may be trapped in the cracks and not flow after baking, resulting in defects (pinholes).

Even if the pickling is strengthened, the grain boundary oxidized portion cannot be removed because the grain boundary oxidized portion is preferentially etched and remains as a deep groove after pickling, resulting in a pinhole after coating. Is inevitable.

The object of the present invention is to suppress grain boundary oxidation during hot rolling, to prevent flap formation during cold rolling, to prevent cracking in plating even after pressing, and to generate pinholes after coating. Not to provide a zinc-based plated steel sheet which is a base material for a surface-treated steel sheet used as an exterior material for automobiles, and a method for producing the same.

[0014]

SUMMARY OF THE INVENTION Although the present inventors have not so far been considered a problem, the present inventors have proposed a post-painting pin for a material having a very high surface quality requirement level such as an automobile exterior panel as described above. As a result of diligent research on the solution to the problem of hole formation, in order to suppress pickling etching, that is, grain boundary oxidation, which is the root cause of pinhole defects during coating, in the cooling process after winding during hot rolling, , Grain boundary P
It was found that cooling should be performed so that segregation does not occur.

The inventors of the present invention previously mentioned that Japanese Patent Application No. 10-52286.
In order to achieve the above-mentioned object, No. 6 proposes that the coiling temperature during hot rolling be 650 ℃, preferably 630 ℃ or less, but as a result of further diligent research, there is cooling after coiling. It was found that grain boundary P segregation can be suppressed by applying the conditions, and the present invention was completed.

The findings of the present invention are as follows. That is, in a surface-treated steel sheet used for automobile exteriors, a very high level of quality is required particularly for the appearance after painting. Generally, three coats are applied on the exterior of automobiles, namely, cationic electrodeposition, intermediate coating, and top coating. If defects such as pinholes occur in the first electrodeposition coating, the next intermediate coating is applied. At this time, the solvent in the intermediate coating partially dissolves the surface layer of the electrodeposition coating, and during baking, pinholes, that is, bubbles during the electrodeposition coating move to the intermediate coating, and pinholes also occur in the intermediate coating. Similarly, pinholes are also generated in the top coat, and eventually it becomes a defective material as an automobile exterior material. In order to solve this problem, it is necessary to eliminate the generation of pinholes during electrodeposition coating.

As a result of diligent research into the above problems, the present inventor has found that there is a plating crack at the bottom of the pinhole during this electrodeposition coating. That is, it is considered that the pinholes are the bubbles (H ₂ gas) generated during electrodeposition coating trapped in the cracks in the plating layer. Furthermore, under this crack in the plating layer, a flap is applied to the base metal.
There is (scab), and it is considered that cracks were generated in the plating layer during processing starting from this flap.

When the cause of the flap after the cold rolling was investigated in detail, it was found that the cause was the grain boundary etching in the pickling of the hot rolled material before the cold rolling. Grain boundary etching in this pickling has a groove shape from the surface layer to a depth of about 10 μm along the grain boundaries, and it is considered that when this portion collapses during cold rolling, a flap with a length of about 50 μm is formed.

As a result of further detailed investigation of the cause of this grain boundary etching, it was found that the grain boundary portion immediately below the black scale (scale) before pickling was oxidized. Once grain boundary oxidation occurs directly under the black skin, no matter how the pickling conditions are changed, the grain boundary oxidized part is selectively dissolved, so grain boundary etching occurs and it may lead to pinholes after painting. Deep groove.

Detailed examination of the grain boundary oxidized portion just below the black skin revealed that it was a P oxide. That is, it is considered that P was segregated at the grain boundaries in the cooling process after winding in the hot rolling.

As described above, when grain boundary oxidation occurs directly under the black skin of the hot rolled steel sheet, grain boundary etching occurs in the pickling step, deep grooves are formed, and a flap is formed on the cold rolled steel sheet. The surface-treated steel sheet on which a plating layer is provided has cracks in the plating layer starting from the flaps when pressed by an automobile manufacturer, and bubbles are trapped during electrodeposition coating, causing pinhole defects. Occurs.
Therefore, the present inventor thought that if this grain boundary oxidation could be prevented, a surface-treated steel sheet free from pinhole defects could be obtained, and as a result of further studies, the present invention was completed.

Therefore, the present invention is as follows. (1) C: 0.08 wt% or less, Si: 0.2 wt% or less, Mn: 0.1-
A cold-rolled steel sheet containing 1.5 wt% and P: 0.08 wt% or less is provided with zinc-based plating, and the number of flaps having a length of 10 μm or more is less than 10 in an arbitrary section of 30 mm in length of the cold-rolled steel sheet. A zinc-based plated steel sheet for automobile exterior materials that does not generate pinholes after painting.

(2) C: 0.08 wt% or less, Si: 0.2 wt% or less, Mn: 0.1 to 1.5 wt%, P: 0.08 wt% or less containing a low carbon or very low carbon Al killed steel slab hot After rolling and hot rolling, winding is performed under the cooling condition shown by the following formula (1), and the obtained hot rolled steel sheet is cold rolled and then annealed, and then zinc-based plating is performed. A method for producing a galvanized steel sheet for automobile exterior materials, which does not have pinholes after painting.

[0024]

[Equation 2]

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, according to the present invention, the grain boundary oxidation, the deep etching at the time of pickling, the flap after the cold rolling, the flap after the cold rolling, and the plating layer, which are found in the conventional Zn-plated steel sheet for automobile exteriors, are described. It prevents a series of defects such as cracks → pinholes after painting, and has an essential feature in preventing grain boundary oxidation of the hot-rolled steel sheet, which is the root cause of such defects. This point will be described in detail below. In this specification, "%" means "wt%" unless otherwise specified.
Represents

(Applicable steel type) C: 0.08 wt% or less,
Si: 0.2 wt% or less, Mn: 0.1 to 1.5 wt%, P: 0.08 wt%
The following low carbon or ultra low carbon Al killed steel plate,
A steel plate with good formability that is mainly used for automobile exteriors.

As described above, according to the present invention, a low carbon Al killed steel and an ultra low carbon Al killed steel are targeted because they are steel sheets having good formability. The reasons for limiting the steel composition in them are as follows. Is.

In the present invention, the content of C is limited to 0.08% or less in order to ensure moldability. It is preferably 0.05% or less. More preferably, it is 0.005% or less. Mn
Although 0.1 to 1.5% is compounded, if it is out of this range, it becomes difficult to secure the predetermined strength and toughness.

In the present invention, the steel composition may further contain S, Al, Ti, Nb, etc., which are contained in ordinary low carbon or ultra low carbon Al killed steel sheets. Is not particularly limited.

(Cooling after winding of hot rolling) The hot rolling in the present invention is not particularly limited, and for example, conventional hot rolling may be performed.

The grain boundary oxidation in the present invention is a grain boundary oxidation which occurs at about 10 μm from the surface layer, that is, at a portion corresponding to one grain boundary from the surface layer, which is caused by segregation of P to the grain boundary and its oxidation. it is conceivable that.

As a result of earnest studies on this phenomenon, the present inventors have found that it is possible to suppress the above two processes of segregation and oxidation as much as possible in the cooling process during hot rolling. Of deep etching → flaps after cold rolling → plating cracks → pinhole after painting It has been found that it is most effective not to occur.

That is, in the cooling process after winding at the time of hot rolling, cooling is performed under the conditions shown in the following formula (1), and P
It was found that the grain boundary oxidation can be suppressed by suppressing the segregation of the grain boundary to the grain boundary.

[TC-500 × (1 + 4 × P [wt%])] / CR <1000 / √P [wt%] + 5 × 10 ⁴ × C [wt%] ··· (1) TC: Winding Temperature (℃) CR: After winding, average cooling rate up to 500 × (1 + 4 × P [wt%]) ℃ (℃ / sec) Figure 1 shows P segregation at grain boundaries and after winding. It is a schematic diagram which showed the relationship of a cooling curve.

Regarding the P segregation to the grain boundary, it is estimated that the P segregation becomes the largest at a certain temperature as shown in FIG. This is because the driving force for P segregation increases as the temperature lowers, but conversely, as the temperature lowers, the diffusion rate becomes slower.
This is because the cooling process after winding in hot rolling is considered to have a segregation curve having a nose as shown in FIG.

This P segregation curve is affected by the P concentration and C concentration of the base metal, and when the P concentration is high, the segregation curve shifts to the left and the nose temperature tends to rise. Also,
If the C concentration is high, the segregation of P tends to be suppressed, so
The segregation curve of P shifts to the right. As a result of research conducted by the present inventors based on this idea, it was found that segregation of P does not occur if cooling is performed within the range shown in the equation (1).

That is, as shown in FIG. 1, when the coiling temperature is high and the cooling rate is slow as shown by the mark (○) in the figure, the segregation curve of P intersects, so that P segregates at the grain boundaries. Grain boundary oxidation occurs. On the other hand, when the winding temperature is low (△),
Even if the coiling temperature is high, if the subsequent cooling rate is fast, P
P segregation does not occur because it does not intersect with the segregation curve of. That is, the grain boundary oxidation is suppressed.

However, lowering the winding temperature also lowers the mechanical properties, so in order to secure the required mechanical properties,
It is necessary to set a lower limit and it is necessary to set it appropriately depending on the steel type.

(Pickling) In order to remove the black skin in the pickling step, it is necessary to remove only the black skin and not to etch the base material as much as possible. If so, that portion is selectively etched, so even if the pickling condition is mild, grain boundary etching is not hindered. However, it is preferable to make it as mild as possible, and therefore, in the present invention, it is preferable to treat with 3 to 15% HCl at 40 to 90 ° C. for 10 to 30 seconds.

(Depth of Pickling Grain Boundary Etching) In the preferred embodiment of the present invention, the grain boundary etching has a depth of 3 μm or less.
If the depth of grain boundary etching exceeds 3 μm, it may become a huge flap with a length of 20 μm or more depending on the cold rolling rate of cold rolling, which may result in pinholes after coating. More preferably, the depth of grain boundary etching is 2 μm or less.

(Cold Rolling) The cold rolling in the present invention may be carried out under the usual conditions so as to obtain desired strip thickness and characteristics, but it is desirable that the cold rolling reduction rate is 60% or more.

(Number of flaps) Here, with regard to the flaps after cold rolling in the present invention, the number of flaps having a length of 10 μm or more is less than 10 in an arbitrary cross section of 30 mm length. It is possible to obtain a surface-treated steel sheet in which cracks are suppressed and pinhole defects do not occur after coating. Preferably, there is no flap with a length of 10 μm or more.

Regarding (plating) plating, it means a rustproof steel plate for automobiles, and zinc-based plating, that is, electrogalvanizing (EG), Zn-Ni alloy electroplating (SZ), and alloyed hot dip galvanizing (GA) Etc.

[0044]

EXAMPLES Low carbon and ultra low carbon Al killed steel slabs having the steel compositions shown in Table 1 were hot rolled under conventional conditions and then Table 2
After winding at the winding temperature shown in, pickling (7% HCl x 80 sec,
0.3% inhibitor added), then cold rolling rate 85%
Cold-rolled steel sheet with a thickness of 0.8 mm was obtained by cold rolling in the above.

The cold-rolled steel sheet having a thickness of 0.8 mm was annealed and subjected to various platings to obtain a zinc-based plated steel sheet. The results of evaluating the obtained zinc-based plated steel sheet as follows are shown in Table 2.
Are also shown. The surface and cross section after pickling were observed by SEM (scanning electron microscope), and the grain boundary etching depth was measured.

Further, the number of flaps was counted and evaluated according to the following criteria by SEM observation of the cross section of the steel sheet after cold rolling. Ten regions with a length of 30 mm were arbitrarily selected, and the number of flaps with a length of 10 μm or more was counted in the cross section and averaged.

×: 50 or more flaps of 10 μm or more in 30 mm length △: 10 or more flaps of 10 μm or more in 30 mm length
Less than 50 ○: 1 to less than 10 flaps of 10 μm or more in 30 mm length ◎: No flap of 10 μm or more in 30 mm length.

Evaluation of pinhole after painting After applying various plating, in order to simulate the processing step in the automobile manufacturing process, a strain of 5% was applied in a tensile test to perform a chemical conversion treatment (chemical conversion treatment liquid: manufactured by Nippon Parkerizing Co., Ltd.). PB
-3080L standard conditions) was applied and electrodeposition coating was performed.

The electrodeposition coating conditions were the strict conditions shown below in order to reproduce pinholes. Electrodeposition paint: U-80 (manufactured by Nippon Paint Difference Co., Ltd.) Film thickness 25 μm Voltage 280V Docan energization (method of applying a specified voltage from the beginning of energization) Baking 250 ℃ × 20 seconds → 170 ℃ × 2 minutes initial rapid heating The number of pinholes was counted according to the following criteria by SEM observation of the cross section after coating baking, and the paintability was evaluated.

[0050] ×: 50 or more pinholes in 30mm length △: 10 or more to less than 50 pinholes in a 30 mm length ○: 1 or more and less than 10 pinholes in 30 mm length ◎: No pinholes in 30mm length.

[0051]

[Table 1]

[0052]

[Table 2]

FIG. 2 shows cases corresponding to Nos. 11 and 21 in Table 2, respectively. In steel type D in this example, the steel was wound at winding temperatures of 580 ° C. and 690 ° C., and after winding, 500 × (1 + 4 × P [wt
FIG. 3 is a schematic view showing SEM observation results of a steel sheet surface and a cross section after pickling a hot-rolled steel sheet with black skin, which was cooled at an average cooling rate of 0.01% / sec.

When the coiling temperature was 580 ° C. (No. 11), almost no grain boundary etching was observed and no flap was formed.
On the other hand, when the coiling temperature is 690 ° C (No. 21), grain boundary etching and flap formation are noticeable.

[0055]

According to the present invention, C: 0.08 wt% or less, S
i: 0.2 wt% or less, Mn: 0.1 to 1.5 wt%, P: 0.08 wt% or less of low carbon or ultra-low carbon Al killed steel slab is hot-rolled and cooled after winding as described in (1) above. By carrying out under the conditions shown by the formula, and then performing cold rolling, annealing, and zinc-based plating, a plated steel sheet that does not generate pinholes is manufactured in the coating process in the automobile manufacturing line.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the relationship between P segregation at grain boundaries and a cooling curve after winding.

FIG. 2 is a drawing of steel type D in the example with coiling temperatures of 580 ° C. and 690
After pickling the hot-rolled steel sheet with black skin, wound at ℃, after winding, cooled at an average cooling rate of 0.01 ℃ / sec up to 500 × (1 + 4 × P [wt%]) ℃ FIG. 3 is a schematic view showing SEM observation results of the steel plate surface and cross section of FIG.

Claims (2)

(57) [Claims] 1. C: 0.08 wt% or less, Si: 0.2 wt% or less,
A cold-rolled steel sheet containing Mn: 0.1 to 1.5 wt% and P: 0.08 wt% or less is provided with zinc-based plating, and 10 flaps having a length of 10 μm or more are provided in an arbitrary cross section of 30 mm in length of the cold-rolled steel sheet. A zinc-based plated steel sheet for automobile exterior materials, in which the number of pinholes is less than the number of coated pinholes after painting. 2. C: 0.08 wt% or less, Si: 0.2 wt% or less,
A low carbon or ultra low carbon Al killed steel slab containing Mn: 0.1 to 1.5 wt% and P: 0.08 wt% or less is hot-rolled, and after hot rolling, under the cooling condition shown by the following formula (1). Wind and pickle with 3-15% HCl at 40-90 ° C for 10-30 seconds,
A method for producing a zinc-based plated steel sheet for automobile exterior materials, which does not cause pinholes after painting, characterized in that the obtained hot-rolled steel sheet is cold-rolled, then annealed and then zinc-based plated. [Equation 1]