JP2946135B2 - High-strength steel plate for car suspension - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate for automobile underbody parts.

[0002]

2. Description of the Related Art Conventionally, hot-rolled steel sheets have been used as steel sheets for suspension parts such as automobile frames and suspensions. 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.

[0003]

In order to ensure corrosion resistance, there are, for example, those described in JP-A-62-243738 and JP-A-63-255341. These are characterized in that the content of C is reduced and Cu and P are added. These measures are effective to ensure corrosion resistance, but these steels all have the disadvantage that the strength of the weld heat affected zone is reduced and the fatigue strength of this part is inferior to that of the base metal. . From this point, even if an attempt is made to reduce the sheet thickness by improving the corrosion resistance, there is naturally a limit.

An object of the present invention is to provide a steel plate for a vehicle undercarriage which solves the above-mentioned drawbacks, suppresses a decrease in the strength of the weld heat affected zone, and has sufficient corrosion resistance.

[0005]

Means for Solving the Problems The gist of the present invention is that the weight%
, C: 0.022 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
0.05%, Mo: 0.1 to 0.50%, Ni: 0.0
This is a high-strength steel sheet for automobile undercarriage containing 5 to 0.50%, the balance being Fe and unavoidable impurities.

[0006]

[Effect] The corrosive environment around the car's undercarriage is the most severe in North America and Europe. In this region, road deicing agents (sodium chloride, calcium chloride, etc.) are used in winter, so it can become wet in the presence of chlorine ions. Drying is repeated.
In such an environment, addition of Cu and P is effective as disclosed in JP-A-62-243738. However, this steel requires low C, 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. Then, as a result of further detailed examination by the present inventors, it was found that the inclusion of Mo is indispensable in order to increase the corrosion resistance with a high C material. The reason for this is not clear,
It is considered that Mo acid is easily generated around carbides, thereby preventing corrosion resistance from being deteriorated even if carbides are present. Furthermore, as a result of examining a method for preventing softening of the weld heat affected zone, N
It has been found that composite addition of b and Mo is essential. The reason for this is not clear, but by adding Nb and Mo in combination, even if the temperature rises by welding, Mo suppresses the disappearance of dislocations introduced by molding or the like, and the dislocations become precipitation nuclei. It is considered that (Nb, Mo) C precipitates in a short time and suppresses the softening of the heat affected zone.

As described above, according to the present invention, Cu, P,
Nb and Mo are essential elements.

The reasons for limiting each component are described below.

When C becomes a large precipitate like cementite, it becomes a cathode portion during corrosion, and the couple current increases and the corrosion resistance is deteriorated. Therefore, it is desirable that C is low. However, when C is added as in the present invention, If it is present, it may be high, and can be contained up to 0.20%. 0.20%
If Mo exceeds Mo corrosion resistance deteriorates even if Mo is added, so the upper limit is 0.20%. If the content is less than 0.022 %, (Nb, Mo) C will not precipitate and the softening of the heat affected zone cannot be prevented, so the lower limit is made 0.022 %.

[0010] Si is used as an auxiliary element for obtaining strength. Since the production cost and the economic disadvantage are caused to be less than 0.005%, the lower limit is 0.005%,
If it exceeds 1.0%, removing the scale at the hot rolling stage is costly and disadvantageous economically. 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 it is economically disadvantageous. Therefore, the lower limit is 0.1%, and if it exceeds 2.5%, the workability deteriorates and it becomes impossible to withstand the processing. The upper limit is 2.5%.

P is an essential element, and is contained at 0.05% or more. If it is less than this, the effect of improving the corrosion resistance is lost, so that the lower limit is set. If it exceeds 0.1%, the secondary workability is degraded, cracking during press molding or cracking with a slight force after press molding. .10% as the upper limit.

Since S is a starting point of rust, it is better to reduce it.
It must be 0.01% or less. Therefore, the upper limit is set to 0.
Since it is costly to make it 01% and less than 0.001%, the lower limit is made 0.001%.

Al is an element necessary for deoxidation of steel,
If it is less than 5%, 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, and is slightly contained in the precipitate and is necessary for securing the strength.
005% or more. If the content exceeds 0.0100%, NbN precipitates in the hot rolling stage, and the amount of Nb effective for preventing the welding heat-affected zone softening at the time of welding is reduced, so the upper limit is 0.0100%. .

Cu is an essential element. If it is less than 0.05%, the effect of improving the corrosion resistance is lost, so that 0.05%
% Is set as the lower limit, and when it exceeds 0.5%, the effect of addition is saturated, and even when Ni or the like is added, defects such as barbs tend to occur. 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. If it is less than 0.01%, the effect of preventing softening of the weld heat affected zone is lost.
The lower limit is set to 0.01%, and if it exceeds 0.05%, the workability deteriorates. Therefore, the upper limit is set to 0.05%.

Mo improves the corrosion resistance and improves the Nb
It is an element effective for preventing the softening of the weld heat affected zone in the addition of a composite with, and is an essential element. If it is less than 0.1%, both the corrosion resistance and the effect of preventing softening of the weld heat affected zone will be lost. Therefore, the upper limit is 0.5%.

Ni is necessary in order to prevent scorching during hot rolling by adding Cu. If the content is less than 0.05%, this effect is lost, so the lower limit is 0.05% and the content exceeds 0.5%. Since this effect is saturated, the upper limit is 0.5%.

The steel adjusted to the above components is made into a steel sheet according to the following method. First, steel is melted in a converter and made into a slab by a continuous casting method. The slab is kept in a high temperature state or cooled to room temperature, and then charged into a heating furnace,
Heating in the temperature range of 1250 ° C., then 800-950
Finish rolling is performed in a temperature range of ℃, and then wound at a temperature of 600 ℃ or less to obtain a hot-rolled steel sheet. The pickling is performed if necessary according to the use situation.

[0021]

Embodiments of the present invention will be described below.

Steels having the chemical compositions of C to M shown in Table 1 were melted in a converter and made into slabs by continuous casting, and then hot-rolled under the conditions shown in Table 2, and hot-rolled to a sheet thickness of 2.6 mm. A steel plate was used.

[0023]

[Table 1]

[0024]

[Table 2]

These steel sheets were subjected to arc welding, and the hardness of the softened portions of the base metal and the heat affected zone was measured. The arc welding conditions were as follows: current: 250 A, voltage: 25 V, speed: 10
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 of the steel sheet was determined by subjecting the steel sheet to a phosphate treatment (Nippon Parker BTL3080), applying a cationic electrodeposition coating (Nippon Paint Power Top D-30, coated with 20 μm), and then performing a cross-cut to reach the base, Spray 3
5 ° C./6 hours—dry 70 ° C./RH 60% / 4 hours—wet 49 ° C./RH 95% / 4 hours—cooling 20 ° C./4 hours
Evaluation was made based on the erosion depth of the cross-cut portion when the acceleration test was performed for 80 cycles.

The results are shown in Table 3. 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.

[0028]

[Table 3]

[0029]

According to the present invention, it is possible to provide a steel sheet for automobile undercarriage excellent in corrosion resistance and fatigue characteristics of the heat affected zone by welding, and has a great industrial effect.

Claims (1)

(57) [Claims] 1. C: 0.022 to 0.20% by weight
%, Si: 0.005 to 1.0%, Mn: 0.1 to 2.%.
5%, P: 0.050 to 0.10%, S: 0.001
0.010%, Al: 0.005 to 0.1%, N: 0.
0005-0.0100%, Cu: 0.10-0.50
%, Nb: 0.01-0.05%, Mo: 0.1-0.
50%, Ni: 0.05 to 0.50%, the balance F
High-strength steel sheet for automobile undercarriage composed of e and unavoidable impurities.