JP3241075B2 - High strength plated steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength plated steel sheet.

[0002]

2. Description of the Related Art Conventionally, hot-rolled steel sheets and cold-rolled steel sheets have been used as steel plates for suspension parts such as frames and suspensions of automobiles or as inner plates for reinforcement. Even if an attempt is made to reduce the thickness of these parts to reduce the weight, it has been difficult to reduce the thickness because the fatigue strength of the welded portion is low or the corrosion resistance is insufficient. In order to ensure corrosion resistance, for example, JP-A-62-243738,
JP-A-63-255341 and the like.

[0003] These reduce the content of C, Cu and P
Is characterized by the addition of To ensure corrosion resistance,
Although these countermeasures are effective, all of these steel materials have a disadvantage that the strength of the weld heat affected zone is reduced and the fatigue strength of this portion is lower than that of the base metal. From this point,
Even if an attempt was made to reduce the sheet thickness by improving the corrosion resistance, there was naturally a limit. Also, the corrosion resistance was not always sufficient.

[0004]

An object of the present invention is to provide a high-strength plated steel sheet which solves the above-mentioned drawbacks, suppresses a decrease in the strength of the heat affected zone, and has sufficient corrosion resistance.

[0005]

The features of the present invention are as follows: C: 0.005 to 0.20%, Si: 0.005%
To 1.0%, Mn: 0.1 to 2.5%, P: 0.001
0.10%, S: 0.001 to 0.010%, Al:
0.005 to 0.1%, N: 0.0005 to 0.010
0%, Nb: 0.01 to 0.05%, Mo: 0.1 to
After plating with a sacrificial anticorrosion effect on the surface of a steel sheet containing 0.50% and the balance of Fe and unavoidable impurities , a strain of 1.0% or more and less than 10.0% in elongation is reduced.
In the added high-strength plated steel sheet.

The corrosive environment of automobiles is most severe in North America and Europe. In this area, road deicing agents (sodium chloride, calcium chloride, etc.) are used in winter, so that wet and dry are repeated in the presence of chloride ions. It is. In such an environment, addition of Cu and P is effective as disclosed in JP-A-62-243738.

[0007] However, the corrosion resistance of this steel is insufficient, and higher corrosion protection performance has been required. Galvanization is effective against this corrosion. In other words, various types of arc welding are performed around the automobile's feet, and even if the corrosion resistance of the steel plate is sufficient, the corrosion resistance of the molten metal part deteriorates depending on the material of the welding rod. Is valid.

Next, as a result of studying a method for preventing softening of the weld heat affected zone, it was found that a composite addition of Nb and Mo was essential. Although the reason is not clear, even if the temperature is increased by welding by adding Nb and Mo in combination,
It is considered that Mo suppresses the disappearance of dislocations introduced by molding or the like, and the dislocations serve as precipitation nuclei, whereby (Nb, Mo) C precipitates in a short time, thereby suppressing the softening of the weld heat affected zone. In order to more effectively exert this effect, it has been found that it is preferable to apply a plastic strain of 1.0% or more in terms of elongation.

The reasons for limiting each component and the amount of distortion will be described below.
When C becomes a large precipitate like cementite, it becomes a cathode portion during corrosion, and a couple current increases, thereby deteriorating the corrosion resistance. Therefore, it is desirable that C is low, but when Mo is added as in the present invention, May be high, and can be contained up to 0.20%. If it exceeds 0.20%, even if Mo is added, the corrosion resistance deteriorates.
0% is the upper limit. If less than 0.005%, (N
b, Mo) Since C does not precipitate and the softening of the heat affected zone cannot be prevented, the lower limit is 0.005%.

[0010] Si is used as an auxiliary element for obtaining strength. If the content is less than 0.005%, the production cost is high and it is economically disadvantageous. Therefore, the lower limit is 0.005%, and if it exceeds 1.0%, the removal of scale at the hot rolling stage is costly and economical. Therefore, the upper limit is 1.0%.

Mn is an element mainly for obtaining strength. If it is less than 0.1%, it is costly to melt and economically disadvantageous.
%, The workability is degraded and it becomes impossible to withstand the processing. Therefore, the upper limit is set to 2.5%.

If P is less than 0.001%, the cost is industrially high. Therefore, the lower limit of P is 0.001%, and 0.10%
If it exceeds 300, the secondary workability is deteriorated and cracks occur during press molding, or cracks occur with a slight force after press molding.
The upper limit is 10%.

Since S is a starting point of rust, it is better to be small, and it is necessary to be 0.01% or less. For this reason, the upper limit is set to 0.01%, and if it is less than 0.001%, cost is required. Therefore, the lower limit is set to 0.001%.

Al is an element necessary for the deoxidation of steel.
If it is less than 05%, deoxidation will be insufficient and defects such as pinholes will occur. Therefore, the lower limit is 0.005%, and if it exceeds 0.1%, inclusions such as alumina increase and the ductility of steel is impaired. The upper limit is 1%.

N is involved in the precipitation of (Nb, Mo) C,
It is slightly contained in these precipitates and is necessary for securing strength.
0005% or more. If the content exceeds 0.0100%, NbN precipitates in the hot rolling stage, and the amount of Nb effective for preventing the softening of the weld heat affected zone during welding is reduced, so the upper limit is 0.0100%. And

Nb, together with Mo, has the effect of preventing the heat affected zone from softening, and is an essential element in the present invention.
If it is less than 0.01%, the effect of preventing softening of the weld heat affected zone is lost, so the lower limit is 0.01%. If it exceeds 0.05%, the workability deteriorates, so the upper limit is 0.05%.

Mo not only improves the corrosion resistance but also improves the N
It is an element effective for preventing the softening of the heat affected zone by addition in combination with b, and is an essential element in the present invention. M
If o is less than 0.1%, both corrosion resistance and the effect of preventing softening of the weld heat affected zone are lost, so 0.1% is the lower limit,
If it exceeds 0.5%, the effect is saturated and the number of inclusions causing flaws increases, so the upper limit is 0.5%.

The steel adjusted to the above components is formed into a steel sheet according to the following method. First, steel is melted in a converter and formed into a slab by a continuous casting method. This slab is kept in a high temperature state, or cooled to room temperature, and then charged into a heating furnace, and the
で 1250 ° C., then 800-95
Finish rolling is performed in a temperature range of 0 ° C., and then rolled at a temperature of 700 ° C. or less to form a hot-rolled steel sheet.

After pickling, plating is performed directly, or plating is performed after cold rolling and annealing. The type of plating is zinc plating,
Aluminum plating, zinc-aluminum alloy plating,
Plating with a sacrificial anticorrosive action, such as iron-zinc alloy plating or a combination thereof. The plating having a sacrificial anticorrosion action refers to a plating that is electrochemically lower than the potential of the steel sheet.

Thus, the corrosion resistance of the weld can be improved. This is because if the plating has a sacrificial anticorrosion effect, even if a part of the filler metal and the base metal is exposed in the arc welded portion, the corrosion resistance of the part can be sufficiently covered. These platings are applied to the steel sheet surface at a basis weight of ³ to 200 g / m ² . If it is 3 g / m ² or less, any of the platings loses the sacrificial corrosion protection effect, and if it exceeds 200 g / m ² , defects such as blowholes occur during arc welding.

A plastic strain having an elongation of 1.0% or more and less than 10.0% is applied to the steel sheet thus manufactured. As a method of applying strain, a method such as skin pass rolling, or cutting into a steel sheet and then applying tensile strain is used. The amount of distortion is
If it is less than 1.0%, the amount of dislocations introduced is small and there is no effect of suppressing the softening of the weld heat affected zone. Therefore, the lower limit is 1.0%, and if it exceeds 10.0%, the ductility of the steel sheet is reduced and forming is difficult. Therefore, the upper limit is 10.0%. 1.0% elongation
~ 10.0% means that the score marked on the steel sheet surface is 1.0%
% To 10.0%. Thus, a high-strength plated steel sheet excellent in the fatigue strength and corrosion resistance of the weld is obtained.

[0022]

EXAMPLE Steels having the chemical components A to O shown in Table 1 were melted in a converter and formed into a slab by continuous casting, and then hot-rolled under the conditions shown in Table 1, and a hot-rolled steel sheet (sheet thickness 2) was obtained. 0.3 mm), then cold-rolled (sheet thickness: 1.2 mm), annealed at a recrystallization temperature or higher, and electrogalvanized (20 g / m ² , A to D, J to L); Plating (45 g / m ² , E ~ I, M ~
O) was produced. Plastic strain was applied by skin pass rolling.

These steel plates were subjected to arc welding, and the hardness of the base metal and the re-softened portion of the heat affected zone was measured. Arc welding conditions are: current; 250 A, voltage; 25 V, speed;
00 mm / min, atmosphere; mag gas. The softening property of the heat affected zone was evaluated based on the difference between the hardness of the softest part and the hardness of the base metal.

The corrosion resistance is determined by subjecting a steel sheet to a phosphate treatment (Nippon Parker BTL 3080), cationic electrodeposition coating (Nippon Paint Powertop D-30, coated with 20 μm), applying a flaw reaching the substrate, and spraying salt water. 35 ° C / 6 hours-dry 70 ° C / RH60% / 4 hours-wet 49 ° C / R
Evaluation was made based on the erosion depth of the flaw when 80 cycles of an acceleration test were performed in which one cycle of H95% / 4 hours-cooling 20 ° C./4 hours.

The results are shown in Table 2. It can be seen that the steel of the present invention is superior to the comparative steel in both the softening characteristics and the corrosion resistance of the weld heat affected zone.

[0026]

[Table 1]

[Table 2]

[0027]

As described above, it is possible to provide a high-strength galvanized steel sheet having excellent corrosion resistance and less softening of the weld heat affected zone, that is, excellent fatigue characteristics of the weld heat affected zone. it can.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 9/46 C23C 2/26 C25D 3/22

Claims (1)

(57) [Claims] C. 0.005 to 0.20%, Si: 0.005 to 1.0%, Mn: 0.1 to 2.5%, P: 0.001 to 0. 10%, S: 0.001 to 0.010%, Al: 0.005 to 0.1%, N: 0.0005 to 0.0100%, Nb: 0.01 to 0.05%, Mo: 0 .1 to 0.50%, on the surface of the steel sheet consisting of the balance Fe and inevitable impurities,
After plating with sacrificial corrosion protection ,
It is noted that a strain of 1.0% or more and less than 10.0% was applied.
High-strength plated steel sheet and butterflies.