JPH10152756A - Plasted steel sheet small in secular deterioration in impact resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a plated steel sheet in which secular deterioration in impact resistance is effectively suppressed and, by being utilized as an automotive steel sheet, capable of suitable securing collision stability even after the passage of a long period from the production the car body. SOLUTION: The cleanliness (dt) at least in the surface layer of this steel sheet is regulated to <=0.05%, and the skewness (Rsk) which is the index of deviation in the height direction of the roughness curve in the plated surface is regulated to -1.5 to +1.0. The plating in the steel sheet is composed of galvanizing, galvannealing or electrogalvanizing. By regulating the cleanliness in the surface layer of the steel sheet, the starting points of rust generation are extremely reduced, and by regulating the surface roughness (Rsk) in the plated surface to the prescribed range, its lubricity at the time of press working is improved to suppress the peeling of the plating, by which secular deterioration in its impact resistance can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plated steel sheet having excellent impact resistance, and more particularly to a plated steel sheet having a small deterioration with time in impact resistance after pressing.

[0002]

2. Description of the Related Art In recent years, demands for collision safety of automobiles have been increasing due to the tightening of laws and regulations on automobile safety, and thin steel sheets for automobiles having excellent impact resistance have been actively developed. . As one of such technologies,
JP-A-8-3678 discloses that under a specific component composition,
2. Description of the Related Art A steel sheet for automobiles has been proposed in which the volume ratio of each metal phase in the structure composed of a ferrite phase and a martensite phase and the dislocation density in the ferrite phase are specified.

[0003]

This technique is intended to improve the impact resistance of a steel sheet by controlling the composition and structure of the steel sheet. The impact resistance of the material pressed after forming the steel sheet into a surface-treated steel sheet deteriorates over time, and even if the composition and structure are controlled as in the above-mentioned conventional technology, substantial impact resistance that contributes to the collision safety of automobiles It was found that the characteristics (that is, the impact resistance after a long period of time from the manufacture of the vehicle body) were not sufficiently obtained. That is, the present inventors conducted a corrosion acceleration test on a sample obtained by press-molding a surface-treated steel sheet for an automobile as a model, and investigated a high-speed deformation characteristic (average collapse load when crushed) described later for this sample. result,
It was found that the average decay load of the sample that rusted in the corrosion promotion test was significantly inferior to that of the sample before the corrosion promotion test, that is, the impact resistance deteriorated with time.

Conventionally, there is no known technique for improving the impact resistance of a surface-treated steel sheet from the viewpoint of suppressing such deterioration of the impact resistance over time. Therefore, an object of the present invention is to effectively suppress the deterioration of the impact resistance over time, and to use the steel sheet as a steel sheet for automobiles to appropriately secure the collision stability even after a long period of time from the manufacture of the vehicle body. It is to provide a plated steel sheet that can be formed.

[0005]

Means for Solving the Problems In view of the above-mentioned problems, the present inventors have conducted repeated studies to obtain a surface-treated steel sheet in which the deterioration of impact resistance over time is suppressed. As a method for evaluating the presence or absence of deterioration over time, it has been found that a method of measuring an average collapse load when a molded member is crushed in high-speed deformation is most suitable. For this reason,
Especially for galvanized steel sheets with good sacrificial corrosion protection,
By variously changing the inner quality, surface texture and plating conditions of steel sheets having various metal structures (metal phases), performing a corrosion promotion test on each plated steel sheet after press working, and measuring the above average collapse load The presence or absence of deterioration with time of the impact resistance was examined. As a result, the time-dependent deterioration of the impact resistance is caused by the occurrence of rust on the surface of the steel sheet, and in order to suppress the time-dependent deterioration of the impact resistance due to such rust, the following (1) It has been found that the conditions of ~ (3) need to be satisfied.

(1) It is necessary to minimize the starting point of rusting by minimizing the cleanliness (dt) of at least the surface layer of the steel sheet to a predetermined level or less. (2) It is necessary to control the surface roughness (Rsk) of the plating surface within a predetermined range to improve lubricity at the time of press working and to suppress plating peeling. (3) In order to properly obtain the function and effect of the regulation of the cleanliness (dt) of the steel sheet surface layer, the metal phase of the steel sheet is mainly composed of a ferrite phase, a ferrite phase + a pearlite phase, and a
It is preferable to use any of bainite phase, ferrite phase + martensite phase, bainite phase, ferrite phase + austenite phase + bainite phase, ferrite phase + austenite phase + bainite phase + martensite phase. (4) As the galvanized steel sheet, a zinc-based galvanized steel sheet having particularly good sacrificial corrosion prevention action is preferable.

The present invention has been made on the basis of such findings, and the structure is such that at least the cleanliness (dt) in the surface layer of the steel sheet is 0.05% or less and the roughness of the plating surface in the height direction of the roughness curve. A plated steel sheet having a small deterioration over time in impact resistance, characterized in that a skewness (Rsk) as a deviation index is -1.5 to +1.0.
Here, the skewness (Rsk) indicates the symmetry of the roughness curve with respect to the center line of the amplitude distribution curve distribution, and is given by the following equation.

(Equation 1)

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention and the reasons for limitation will be described below. The following test was conducted to examine the relationship between the cleanliness (dt) of the steel sheet surface layer and the deterioration over time of the impact resistance. As test steel sheets, thin steel sheets (sheet thickness: 1.6 mm) having the same chemical composition, metal phase and strength and different surface cleanliness (dt) were prepared, and these were subjected to hot-dip galvanizing (coating weight: 60 g / weight). m ² / 60g / m ^2). after that,
An alloying treatment was performed, and then temper rolling was performed to adjust the skewness (Rsk) of the plating surface to be -1.5 to +1.0. Cleanliness (dt) of the surface layer of these steel sheets
Was adjusted by controlling the refining and casting conditions of the steel, and by making the manufacturing conditions after rolling the same, the metal phase and the strength were made substantially the same.

After pressing these coated steel sheets,
Complex cycle corrosion test (“0.5% NaCl aqueous solution (3
5 ° C) SST test for 3 hours → 65 ° C, humidity 10
Drying in a 15% atmosphere for 6 hours → 3 hours in an atmosphere of 55 ° C. and a humidity of 90% or more is performed as one cycle, and a total of 200 cycles are implemented). : 11 m / sec), and the average collapse load at the time of crushing was measured. The average collapse load was also measured for the as-pressed plated steel sheet under the same conditions. Based on the measured values of the average collapse load, ΔFave, which is an index of the degree of deterioration of the impact resistance over time, is used.
(= [Average collapse load of plated steel sheet after combined cycle corrosion test] − [Average collapse load of plated steel sheet as pressed]), and this ΔFave (the larger the negative value of this ΔFave value, the greater the impact resistance property). (Deterioration over time is large) and the cleanliness (dt) of the steel sheet surface layer were examined. The result is shown in FIG. The cleanliness (dt) of the steel sheet surface layer
Is a measured value of a portion from the surface of the steel sheet to a depth of t / 8 (where t: sheet thickness) in the thickness direction, measured by the method of JIS G 0555.

According to FIG. 1, the cleanliness (dt) of the surface layer of the steel sheet is shown.
Is less than 0.05%, the negative value of ΔFave indicates a low level, and almost no deterioration of the impact resistance is observed. However, when the cleanness (dt) exceeds 0.05%, ΔFave exceeds 0.05%.
Negative value sharply increases, and the impact resistance is deteriorated. The cleanliness (dt) of such a surface layer is 0.05%
As a result of observing the surface of the coated steel sheet after the combined cycle corrosion test, a large number of spot rusts were observed, and cracks were also observed from the rusted portions on the cross-sectional microscope. From the above results, in the present invention, the cleanliness (dt) of the surface layer of the steel sheet is set to 0.05% or less as a condition that does not cause the deterioration of the impact resistance over time. The “surface layer of the steel sheet” that regulates the cleanliness in this way is at most t / 8 in the thickness direction from the steel sheet surface.
(However, it is preferable to target a steel sheet surface layer within a range not exceeding t: steel sheet thickness).

The cleanliness (dt) of the surface layer of such a steel sheet is as follows:
0.05% or less can be achieved by appropriately performing secondary refining after smelting of steel and measures against inclusion of inclusions during casting. In the present invention, the cleanliness (dt) of the central part of the steel sheet in the thickness direction (the part other than the “surface layer of the steel sheet” in the thickness direction).
Although not particularly specified, for a steel sheet for use in which particularly severe press forming is performed, the cleanliness (dt) at the center of the sheet thickness is set to 0.05% or less from the viewpoint of suppressing the above-described generation of rust. Is preferred.

Next, from the viewpoint of suppressing the generation of rust through the peeling of the plated steel sheet after the press working of the plated steel sheet and the accompanying deterioration of the impact resistance property over time, the roughness pattern of the surface of the plated steel sheet and the aging of the impact resistance property are considered. The following test was performed to examine the relationship with the thermal degradation. As test steel sheets, galvanized steel sheets (sheet thickness 1.6 mm) having the same chemical composition, metal phase, and strength and having various surface roughness patterns were produced. These plated steel sheets, galvanized thin steel sheet (plating unit ^{weight: 60g / m 2 / 60g /} m 2) and then, subjected to alloying treatment, was produced by rolling subsequently temper. The roughness pattern of the surface of the plated steel sheet was adjusted by the plating conditions and the temper rolling at the final stage, and the other production conditions after the rolling were made the same so that the metal phase and the strength were almost the same. Further, the cleanliness (dt) of each steel sheet surface layer was adjusted to 0.05% or less by controlling the refining and casting conditions of the steel.

After pressing these coated steel sheets,
High-speed deformation (deformation speed: 11) was performed on the plated steel sheet subjected to the combined cycle corrosion test (the test conditions were also the same as the test of FIG. 1) and the as-pressed plated steel sheet as in the test of FIG.
m / sec), the average collapse load at the time of crushing was measured. Based on the measured values of these average collapse loads, ΔFave (= [average collapse load of plated steel sheet after combined cycle corrosion test], which is an index of the degree of deterioration of impact resistance over time, is −
[Average collapse load of as-pressed plated steel sheet]) was determined, and the relationship between ΔFave and skewness (Rsk) of the surface of the plated steel sheet was examined. The result is shown in FIG.

According to FIG. 2, when the skewness (Rsk) of the plating surface is less than -1.5 and more than +1.0, Δ
Fave increases sharply, and the impact resistance is significantly deteriorated with time. In particular, the skewness (R
When sk) is less than -1.5, the surface of the plating is peeled off by press working, and a lot of rust is observed at the site. Therefore, the impact resistance is deteriorated at Rsk <-1.5. It is considered that the reason for this is that the lubricity at the time of press working deteriorated due to an excessive decrease in skewness (Rsk), and plating peeling occurred. Also, the cause of such plating peeling is that if the skewness (Rsk) is too low, the oil pool of the lubricating oil during press working is reduced and the sliding resistance is increased. As a result, the surface pressure during press working is increased. It is considered that plating peeling occurs.
On the other hand, when the skewness (Rsk) is more than +1.0, spot-like peeling of the plating occurs, and rusting is observed at the site. Is considered to be due to such local plating exfoliation. The cause of such local plating exfoliation is that if the skewness (Rsk) is too high, the pattern of the roughness curve becomes convex and locally sharpened. This is considered to be because the surface pressure with the mold in contact with the portion increases, and the plating is easily peeled off at that portion.

From the above results, in the present invention, the skewness (Rsk), which is an index of deviation in the height direction of the roughness curve of the plating surface, is set to -1.5 to less than 1.5 as a condition for preventing the deterioration of impact resistance over time. +1.0. The surface roughness specified by the skewness (Rsk) is determined by adjusting the plating conditions and rolling the steel sheet using a dulled roll before or after plating. Can be obtained by
Further, in order to properly obtain the above-mentioned effect by regulating the cleanliness (dt) of the surface layer of the steel sheet, the metal structure of the steel sheet mainly includes a ferrite phase, a ferrite phase + a pearlite phase,
It is preferably any of ferrite phase + bainite phase, ferrite phase + martensite phase, bainite phase, ferrite phase + austenite phase + bainite phase, and ferrite phase + austenite phase + bainite phase + martensite phase. In addition, these metal phases do not prevent the inclusion of phases such as intermetallic compounds and nonmetallic inclusions generally inevitably present in steel.

The present invention can be applied to plated steel sheets having various strengths. That is, plated steel sheets having the above-described various metal structures are used as automotive steel sheets. For example, the following uses are made according to the desired strength. Ferrite phase → 270-500N / mm ² Ferrite phase + pearlite phase → 400-800N / mm
² Ferrite phase + bainite phase → 450 to 750 N / mm
² Ferrite phase + martensite phase → 500 to 900 N /
mm ² bainite phase → 500~900N / mm ² ferrite phase + austenite phase + bainite phase → 70
0 to 900 N / mm ² ferrite phase + austenite phase + bainite phase + martensite phase → 700 to 900 N / mm ²

It is preferable that the metallographic structure of the plated steel sheet of the present invention is appropriately selected according to the type of workability required even at the same strength level. That is, when a steel sheet is required to have high stretch flangeability (burring property), a ferrite phase + martensite phase, a ferrite phase + austenite phase + bainite phase or a ferrite phase + austenite phase + bainite phase + maltenite rather than a ferrite phase. The site phase is more preferable, and the ferrite phase + bainite phase is more preferable than the ferrite phase + martensite phase.
The bainite phase is more preferable than the bainite phase.
Further, when high ductility (total elongation) is required for the steel sheet, the ferrite phase is more preferable than the bainite phase,
A ferrite phase + martensite phase or a ferrite phase + bainite phase is more preferable than the ferrite phase, and a ferrite phase + austenite phase + bainite phase or a ferrite phase + austenite phase + bainite phase is more preferable than the ferrite phase + bainite phase. + The martensite phase is preferred. The effects of the present invention can be obtained particularly effectively in a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, or an electro-galvanized steel sheet.
Ni plating, Al plating), Zn-Ni alloy plating, Z
It can also be obtained in various plated steel sheets such as n-Mn alloy plating and Zn-Al alloy plating.

[0018]

【Example】

[Example 1] The chemical components are 0.09% C-0.01% Si
Hot-dip galvanized steel sheet (sheet thickness: 1.6 mm, made of 440 MPa class ferrite + pearlite steel of −0.50% Mn−0.100% P and having a surface cleanliness (dt) as shown in Table 1. plating basis weight: 60g / m ² / 60g
/ M ² ) was subjected to alloying treatment, followed by temper rolling to adjust the surface roughness (Rsk) shown in the table. After press-working these plated steel sheets, a combined cycle corrosion test (test conditions were the same as the test in FIG. 1) was performed, and the occurrence of rust was observed. Also, as in the test of FIG. 1, the above-described plated steel sheet subjected to the combined cycle corrosion test and the plated steel sheet as pressed are subjected to high-speed deformation (deformation speed: 11 m / sec).
The average collapse load at the time of crushing was measured. Based on the measured values of these average collapse loads, ΔFave (= [average collapse load of plated steel sheet after combined cycle corrosion test]-[average collapse load of plated steel sheet as pressed] indicating the degree of deterioration of impact resistance over time ]). Table 1 shows the results.

According to Table 1, No. 1 of the present invention was used. 1 to
No. In No. 3, no rust was observed even after the combined cycle corrosion test, and thus ΔFave was a small value, indicating that deterioration with time of the impact resistance was effectively suppressed. On the other hand, in Comparative Example No. 4-No. 6
Does not satisfy any of the cleanliness (dt) of the surface layer of the steel sheet and the skewness (Rsk) of the plating surface specified by the present invention, and therefore rusts after the combined cycle corrosion test, and thus ΔFave is also large. It can be seen that the impact resistance has deteriorated with time.

[0020]

[Table 1]

Example 2 0.003% C-
0.25% Si-2.00% Mn-0.075% P-
Hot-dip galvanized steel sheet made of 0.07% Ti 440 MPa class ferritic steel and having a surface cleanliness (dt) as shown in Table 2 (sheet thickness: 1.6 mm, plating weight: 6)
^{0g / m 2 / 60g / m} 2) rolled to temper, and adjusted to a surface roughness (Rsk) shown in the same table. After press-working these plated steel sheets, a combined cycle corrosion test (test conditions were the same as the test in FIG. 1) was performed, and the occurrence of rust was observed. Also, as in the test of FIG. 1, the average collapse load when the above-mentioned plated steel sheet subjected to the combined cycle corrosion test and the plated steel sheet as pressed was crushed by high-speed deformation (deformation speed: 11 m / sec). . Based on the measured values of these average collapse loads, ΔFave (= [average collapse load of plated steel sheet after combined cycle corrosion test]-[average collapse load of plated steel sheet as pressed] indicating the degree of deterioration of impact resistance over time ]). Table 2 shows the results.
Shown in

According to Table 2, No. 1 which is an example of the present invention. In No. 7, no rust was observed even after the combined cycle corrosion test, and thus ΔFave was also a small value, indicating that deterioration with time of the impact resistance was effectively suppressed. On the other hand, in Comparative Example No. No. 8 rusted after the combined cycle corrosion test because the skewness (Rsk) of the plating surface exceeded the upper limit specified in the present invention, and thus ΔF
The ave is also large, which indicates that the impact resistance has deteriorated with time.

[0023]

[Table 2]

Example 3 Chemical component is 0.10% C-
0.50% Si-1.40% Mn-0.015% P5
Electrogalvanized steel sheet (sheet thickness: 1.6 mm, basis weight: 30) which is made of 90 MPa class ferrite + pearlite steel and has a surface cleanliness (dt) as shown in Table 3.
^{g / m 2 / 30g / m} 2) by temper rolling was adjusted to a surface roughness (Rsk) shown in the same table. After press-working these plated steel sheets, a combined cycle corrosion test (test conditions were the same as the test in FIG. 1) was performed, and the occurrence of rust was observed.
Also, as in the test of FIG. 1, the average collapse load when the above-mentioned plated steel sheet subjected to the combined cycle corrosion test and the plated steel sheet as pressed was crushed by high-speed deformation (deformation speed: 11 m / sec). . Based on the measured values of these average collapse loads, ΔFave (= [average collapse load of plated steel sheet after combined cycle corrosion test]-[average collapse load of plated steel sheet as pressed] indicating the degree of deterioration of impact resistance over time ]). Table 3 shows the results.

According to Table 3, No. 1 of the present invention was used. No. 9 showed no rust even after the combined cycle corrosion test, and thus ΔFave was a small value, indicating that the deterioration with time of the impact resistance was effectively suppressed. On the other hand, in Comparative Example No. In No. 10, since the cleanliness (dt) of the steel sheet surface layer exceeded the upper limit specified by the present invention, rust occurred after the combined cycle corrosion test, and thus ΔFave was also large. You can see that it is occurring.

[0026]

[Table 3]

Example 4 The chemical composition was 0.09% C-
0.08% Si-1.55% Mn-0.010% P5
Electrogalvanized steel sheet made of 90 MPa class ferrite + bainite steel and having a surface cleanliness (dt) as shown in Table 4 (sheet thickness: 3.0 mm, plating weight: 30 g)
^{/ M 2 / 30g / m 2} ) by temper rolling was adjusted to a surface roughness (Rsk) shown in the same table. After press working these plated steel sheets, a combined cycle corrosion test (test conditions are shown in FIG.
(Similar to the above test), and the occurrence of rust was observed. Also, as in the test of FIG. 1, the average collapse load when the above-mentioned plated steel sheet subjected to the combined cycle corrosion test and the plated steel sheet as pressed was crushed by high-speed deformation (deformation speed: 11 m / sec). . Based on the measured values of these average collapse loads, ΔFave (= [average collapse load of plated steel sheet after combined cycle corrosion test]-[average collapse load of plated steel sheet as pressed] indicating the degree of deterioration of impact resistance over time ]). Table 4 shows the results.

According to Table 4, No. 1 of the present invention was used. 11
No rusting was observed even after the combined cycle corrosion test, and thus ΔFave was a small value, indicating that deterioration with time of the impact resistance was effectively suppressed. On the other hand, in Comparative Example No. Sample No. 12 rusted after the combined cycle corrosion test because the skewness (Rsk) of the plating surface exceeded the upper limit specified in the present invention, and therefore Δ
Fave is also large, and it can be seen that the impact resistance has deteriorated with time.

[0029]

[Table 4]

Example 5 The chemical composition is 0.09% C-
Electrogalvanized steel sheet made of 590 MPa class ferrite and martensite steel composed of 0.20% Si-0.75% Mn-0.070% P and having a surface cleanliness (dt) as shown in Table 5 ( thickness: 1.6 mm, plating weight per unit area: after ^{30g / m 2 / 30g / m} 2) was treated alloyed,
Subsequently, temper rolling was performed to adjust the surface roughness (Rsk) shown in the table. After press-working these plated steel sheets, a combined cycle corrosion test (test conditions were the same as the test in FIG. 1) was performed, and the occurrence of rust was observed. Also, as in the test of FIG. 1, the average collapse load when the above-mentioned plated steel sheet subjected to the combined cycle corrosion test and the plated steel sheet as pressed was crushed by high-speed deformation (deformation speed: 11 m / sec). . Based on the measured values of these average collapse loads, ΔFav indicating the degree of deterioration of the impact resistance over time is shown.
e (= [average collapse load of plated steel sheet after combined cycle corrosion test] − [average collapse load of plated steel sheet as pressed]) was determined. Table 5 shows the results.

According to Table 5, No. 1 of the present invention was used. 13
No rusting was observed even after the combined cycle corrosion test, and thus ΔFave was a small value, indicating that deterioration with time of the impact resistance was effectively suppressed. On the other hand, in Comparative Example No. In No. 14, since the cleanliness (dt) of the steel sheet surface layer exceeded the upper limit specified in the present invention, rust occurred after the combined cycle corrosion test, and thus ΔFave was also large. You can see that it is occurring.

[0032]

[Table 5]

Example 6 The chemical composition is 0.10% C-
1.08% Si-1.64% Mn-0.010% P5
Cleanliness (dt) of the surface layer as shown in Table 6 consisting of 90 MPa class ferrite + austenite + bainite steel
Electro-galvanized steel sheet having a (plate thickness: 1.6 mm, plating basis ^{weight: 30g / m 2 / 30g /} m 2) by temper rolling was adjusted to a surface roughness (Rsk) shown in the same table. After press-working these plated steel sheets, a combined cycle corrosion test (test conditions were the same as the test in FIG. 1) was performed, and the occurrence of rust was observed. Also, as in the test of FIG. 1, the above-described plated steel sheet subjected to the combined cycle corrosion test and the plated steel sheet as pressed were subjected to high-speed deformation (deformation speed: 11 m).
/ Sec), the average collapse load at the time of crushing was measured. Based on the measured values of these average collapse loads, ΔFave (= [average collapse load of plated steel sheet after combined cycle corrosion test]-[average collapse load of plated steel sheet as pressed] indicating the degree of deterioration of impact resistance over time ]).
Table 6 shows the results.

According to Table 6, No. 1 of the present invention example. Fifteen
No rusting was observed even after the combined cycle corrosion test, and thus ΔFave was a small value, indicating that deterioration with time of the impact resistance was effectively suppressed. On the other hand, in Comparative Example No. No. 16 rusted after the combined cycle corrosion test because the skewness (Rsk) of the plating surface exceeded the upper limit specified in the present invention, and therefore Δ
Fave is also large, and it can be seen that the impact resistance has deteriorated with time.

[0035]

[Table 6]

Example 7 The chemical component is 0.15% C-
Electrogalvanized steel sheet (sheet) made of 780 MPa class ferrite + pearlite steel consisting of 0.15% Si-2.30% Mn-0.100% P and having a surface cleanliness (dt) as shown in Table 7 Thickness: 3.0mm, plating weight:
^{30g / m 2 / 30g / m} 2) by temper rolling was adjusted to a surface roughness (Rsk) shown in the same table. After pressing these steel plates, a combined cycle corrosion test (test conditions are shown in FIG.
(Similar to the above test), and the occurrence of rust was observed. Also, as in the test of FIG. 1, the average collapse load when the above-mentioned plated steel sheet subjected to the combined cycle corrosion test and the plated steel sheet as pressed was crushed by high-speed deformation (deformation speed: 11 m / sec). . Based on the measured values of these average collapse loads, ΔFave (= [average collapse load of plated steel sheet after combined cycle corrosion test]-[average collapse load of plated steel sheet as pressed] indicating the degree of deterioration of impact resistance over time ]). Table 7 shows the results.

According to Table 7, No. 1 of the present invention was used. 17
No rusting was observed even after the combined cycle corrosion test, and thus ΔFave was a small value, indicating that deterioration with time of the impact resistance was effectively suppressed. On the other hand, in Comparative Example No. In No. 18, since the cleanliness (dt) of the surface layer of the steel sheet exceeded the upper limit specified in the present invention, rust occurred after the combined cycle corrosion test, and thus ΔFave also increased. You can see that it is occurring.

[0038]

[Table 7]

[0039]

As described above, according to the present invention, the cleanliness (dt) of the surface layer of the steel sheet and the surface roughness (Rsk) of the plated surface are obtained.
By optimizing the impact resistance, it is possible to provide a surface-treated steel sheet with little deterioration in impact resistance over time. Improvement can be achieved.

[Brief description of the drawings]

FIG. 1 shows ΔFave (=) which is an index of deterioration with time of the cleanliness (dt) of the surface layer of a steel sheet and the impact resistance of a galvanized steel sheet
Graph showing the relationship between [average collapse load of plated steel sheet after combined cycle corrosion test]-[average collapse load of plated steel sheet as pressed])

FIG. 2 is a graph showing a skewness (Rsk) of a plating surface and ΔF which is an index of a deterioration with time of an impact resistance property of a galvanized steel sheet.
ave (= [Average collapse load of plated steel sheet after combined cycle corrosion test]-[Average collapse load of plated steel sheet as pressed])

 ────────────────────────────────────────────────── ─── Continued from the front page (72) Inventor Takeshi Fujita 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd.

Claims (2)

[Claims] 1. The cleanness (d) of at least the surface layer of a steel sheet
t) is 0.05% or less, and a skewness (Rs) which is a deviation index in the height direction of the roughness curve of the plating surface.
k) is -1.5 to +1.0, a plated steel sheet having a small deterioration over time in impact resistance. 2. The coated steel sheet according to claim 1, wherein the plating is hot-dip galvanized, alloyed hot-dip galvanized, or electrogalvanized.