JP3157888B2 - 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 due to the low fatigue strength of the welded portion or insufficient corrosion resistance. In order to ensure corrosion resistance, there are, for example, JP-A-62-243738 and JP-A-63-255341.

[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%
1.0%, Mn: 0.1-2.5%, P: 0.050
0.10%, S: 0.001 to 0.010%, Al:
0.005 to 0.1%, N: 0.0005 to 0.010
0%, Cu: 0.10 to 0.50%, Nb: 0.01 to
A high-strength plated steel sheet containing 0.05%, Mo: 0.1 to 0.50%, and having a surface having a sacrificial anticorrosion effect applied to the surface of the steel sheet composed of the balance of Fe and inevitable impurities;
And a high-strength plated steel sheet additionally containing Ni: 0.05 to 0.50%, and a high-strength plated steel sheet in which the plating is zinc.

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.

However, in this steel, it is necessary to lower the carbon content, which is disadvantageous for obtaining strength. In order to obtain a high-strength steel sheet, it is preferable that C is high, and it has been a problem to secure corrosion resistance with a high C material.

Accordingly, the present inventors have conducted further detailed studies and found that Mo is essential to increase the corrosion resistance of a high C material. Although the reason is not clear, it is considered that Mo acid is easily generated around the carbide, thereby preventing the corrosion resistance from being deteriorated even if the carbide is present.

Further, it has been found that zinc plating is effective for the corrosion resistance of the arc welded portion. That is, for arc welding, various welding rods are used,
Even if the corrosion resistance of the steel sheet is sufficient, the corrosion resistance of the molten metal part is deteriorated depending on the material of the welding rod, so that the sacrificial corrosion prevention effect of galvanization is effective as a method to compensate for this.

Next, as a result of examining a method for preventing softening of the heat affected zone by welding, it was found that the combined addition of Nb and Mo is 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.

As described above, in the present invention, Cu,
P, Nb, and Mo are essential elements, and plastic strain is also essential. Hereinafter, reasons for limiting each component and the amount of distortion will be described.

[0012] When C becomes a large precipitate like cementite, it becomes a cathode portion during corrosion and the couple current increases, deteriorating the corrosion resistance. Therefore, it is desirable that C is low. However, as in the present invention, Mo is added. , The content may be as high as 0.20%. 0.20
%, The corrosion resistance deteriorates even if Mo is added, so the upper limit is 0.20%. If it is less than 0.005%, (Nb, Mo) C will not be precipitated, and it will not be possible to prevent softening of the heat affected zone by welding, so the lower limit is made 0.005%.

[0013] Si is used as an auxiliary element for obtaining strength. If it is less than 0.005%, the production cost is high and it is economically disadvantageous. Therefore, the lower limit is 0.005%. 1.0% is set as the upper limit because it is disadvantageous.

Mn is an element for mainly 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%.

P is an essential element of the present invention, and 0.0
More than 5% is contained. If it is less than this, the effect of improving the corrosion resistance is lost, so this is set as the lower limit. If it exceeds 0.1%, the secondary workability is deteriorated and cracks occur during press molding,
0.10% because it breaks with a slight force after press molding
Is the upper limit.

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 deoxidizing 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

Cu is an essential element in the present invention. 0.1
If it is less than 0%, the effect of improving corrosion resistance is lost, so the lower limit is 0.10%. If it exceeds 0.5%, the effect of addition is saturated, and defects such as scabs occur even when Ni or the like is added. Therefore, the upper limit is set to 0.5%.

Nb has an effect of preventing softening of the heat affected zone together with Mo, 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
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%.

Ni is necessary to prevent scabbing during hot rolling by adding Cu when the heating temperature of hot rolling is as high as 1200 ° C. or higher. If less than 0.05%, this effect is lost. , 0.05% is the lower limit, and if it exceeds 0.5%, this effect is saturated, so the upper limit is 0.5%. At the time of manufacturing, if the heating temperature of the hot rolling can be made lower than 1200 ° C., Ni need not be particularly contained.

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. Thereby, the corrosion resistance of the welded portion can be improved.

[0025] If the plating has a sacrificial anticorrosion action, even if a part of the filler metal and the base metal is exposed in the arc welded portion, the corrosion resistance of the portion 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 the amount is less than 3 g / m ² , any of the platings loses the sacrificial anticorrosion effect. If the amount 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. Examples of the method of applying strain include skin pass rolling, or a method of cutting out a steel sheet and then applying a tensile strain. If the amount of strain 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, so the lower limit is 1.0%. If it exceeds 10.0%, the ductility of the steel sheet is reduced. Therefore, the upper limit is set to 10.0% because molding becomes difficult. Elongation 1.0
% To 10.0% means that the score marked on the steel plate surface is 1.%.
It means the amount of strain extending from 0% to 10.0%. Thus, a high-strength plated steel sheet excellent in the fatigue strength and corrosion resistance of the weld is obtained.

[0027]

EXAMPLE Steels having the chemical components A to O shown in Table 1 were melted in a converter and converted into slabs by continuous casting, and then hot-rolled under the conditions shown in Table 1 to obtain a hot-rolled steel sheet (sheet thickness 2). 0.3 mm), then cold-rolled (sheet thickness: 1.2 mm), annealed above the normal recrystallization temperature, and electrogalvanized (20 g / m ² , A to D, J to L)
And hot-dip galvanized (45 g / m ² , E ~
I, MO) were produced. Plastic strain was applied by skin pass rolling.

These steel sheets 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 (BTL3080 manufactured by Nippon Parker), applying a cationic electrodeposition coating (Powertop D-30 manufactured by Nippon Paint, applying 20 μm), applying a flaw reaching the substrate, and spraying with 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 flaws when 80 cycles of the accelerated test in which H 95% / 4 hours-cooling 20 ° C./4 hours were performed as one cycle were performed. Table 2 shows the results. 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.

[0030]

[Table 1]

[Table 2]

[0031]

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.

Claims (3)

(57) [Claims] 1. A weight ratio of C: 0.005 to 0.20%, Si: 0.005 to 1.0%, Mn: 0.1 to 2.5%, P: 0.050 to 0. 10%, S: 0.001 to 0.010%, Al: 0.005 to 0.1%, N: 0.0005 to 0.0100%, Cu: 0.10 to 0.50%, Nb: 0 0.1 to 0.05%, Mo: 0.1 to 0.50%, The surface of a steel sheet comprising the balance of Fe and unavoidable impurities is plated with sacrificial corrosion protection, and has an elongation of 1.0% or more and 10% or more. High-strength plated steel sheet to which less than 0.0% strain is applied. 2. The high-strength plated steel sheet according to claim 1, which contains 0.05 to 0.50% of Ni. 3. The high-strength plated steel sheet according to claim 1, wherein the plating is zinc.