JPH05195142A - High-strength hot-rolled steel sheet excellent in corrosion resistance and fatigue characteristic at weld zone - Google Patents

Abstract

PURPOSE:To produce a high-strength hot-rolled steel sheet excellent in corrosion resistance and fatigue characteristic at the weld zone by applying a strain of specified elongation percentage to a steel sheet having specified contents of Cu, P, Nb and Mo. CONSTITUTION:A plastic strain of 1.0 to 10.0% of the elongation percentage is applied by skin-pass rolling, etc., to a hot-rolled steel sheet contg., by weight, 0.005-0.20% C, 0.005-1.0% Si, 0.1-2.5% Mn, 0.050-0.10% P, 0.001-0.010% S, 0.005-0.1% Al, 0.0005-0.0100% N, 0.10-0.50% Cu, 0.01-0.005% Nb, 0.1-0.50% Mo, 0.05-0.50% Ni and the balance Fe with inevitable impurities. The objective high-strength hot-rolled steel sheet excellent in corrosion resistance and with the softening of the weld heat-affected zone reduced is obtained in this way.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled steel sheet which is excellent in corrosion resistance and weld fatigue characteristics.

[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 it is attempted to reduce the plate thickness of these parts in order to reduce the weight, it is difficult to reduce the plate 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-62-2
No. 43738 and JP-A-63-255341.

[0003] These reduce the content of C, Cu and P
It is characterized by the addition of. To ensure corrosion resistance,
Although these measures are effective, all of these steel materials have the drawback that the strength of the weld heat affected zone is reduced and the fatigue strength of this area is inferior to that of the base metal. From this point,
Even when trying to improve the corrosion resistance and reduce the plate thickness, the limit naturally occurred.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned drawbacks, suppresses the decrease in strength of the heat-affected zone of welding, and has a sufficient corrosion resistance, which is a high-strength hot-rolled sheet for automobile suspensions and the like. It provides a steel plate.

[0005]

The features of the present invention are that C: 0.005 to 0.20% by weight and Si:
0.005-1.0%, Mn: 0.1-2.5%, P:
0.050 to 0.10%, S: 0.001 to 0.010
%, Al: 0.005-0.1%, N: 0.0005-
0.0100%, Cu: 0.10 to 0.50%, Nb:
0.01 to 0.05%, Mo: 0.1 to 0.50%, N
i: 0.05-0.50%, balance Fe and unavoidable impurities, and elongation 1.0% or more 1
It is a high-strength hot-rolled steel sheet having a strain of less than 0.0% and excellent in corrosion resistance and weld fatigue characteristics.

[0006] The corrosive environment around automobiles is the most severe in North America and Europe. In this region, road deicing agents (sodium chloride, calcium chloride, etc.) are used. The drying is repeated. Under such an environment, JP-A-62-243738
Addition of Cu and P is effective, as disclosed in Japanese Patent Publication No. However, this steel needs to have a low carbon content, which is disadvantageous for obtaining strength.

In order to obtain a high-strength steel sheet, it is preferable that the C content is high, and it has been a problem to secure the corrosion resistance in the high C material. Therefore, as a result of further detailed study by the present inventors, it was found that the inclusion of Mo is indispensable for improving the corrosion resistance in a high C material. Although the reason for this is not clear, it is considered that Mo acid is likely to be generated around the carbides, thereby preventing the corrosion resistance from being deteriorated even in the presence of the carbides.

Further, as a result of studying a method of preventing softening of the heat-affected zone of welding, it was found that the combined addition of Nb and Mo is essential. The reason for this is not clear, but by adding Nb and Mo in combination, even if the temperature rises due to welding,
It is considered that Mo suppresses the disappearance of dislocations introduced by molding, etc., and these dislocations serve as precipitation nuclei to precipitate (Nb, Mo) C in a short time, thereby suppressing softening of the weld heat affected zone. As described above, in the present invention, Cu, P, Nb, Mo
Is an essential element.

The reasons for limiting each component will be described below. C is
When it becomes a large precipitate like cementite, it becomes a cathode part during corrosion and couple current increases and deteriorates the corrosion resistance. Therefore, it is preferable that it is low, but it is high when Mo is added as in the present invention. It doesn't matter,
It can contain up to 0.20%. If it exceeds 0.20%, the corrosion resistance deteriorates even if Mo is added, so 0.20% is made the upper limit. If less than 0.005%, (Nb, M
o) Since C does not precipitate and the softening of the weld heat affected zone cannot be prevented, the lower limit is 0.005%.

Si is used as an auxiliary element for obtaining strength. If it is less than 0.005%, it is economically disadvantageous because of the manufacturing cost, so 0.005% is the lower limit, and if it exceeds 1.0%, the scale removal in 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 will be costly and economically disadvantageous to produce it.
%, The workability deteriorates and the workability cannot be endured, so 2.5% is made the upper limit.

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

Since S is a starting point of rust, it is preferable that S is small, and it is necessary to set S to 0.01% or less. For this reason, the upper limit is set to 0.01%, and it is costly to make it less than 0.001%, so 0.001% is made the lower limit.

Al is an element necessary for deoxidizing steel, and is 0.0
If it is less than 05%, deoxidation becomes insufficient and defects such as pinholes occur, so 0.005% is the lower limit, and if it exceeds 0.1%, inclusions such as alumina increase and the ductility of the steel is impaired. The upper limit is 1%.

N is involved in the precipitation of (Nb, Mo) C,
It is slightly contained in this precipitate and is necessary for securing strength.
Include at least 0005%. Further, if it is contained in excess of 0.0100%, NbN will be precipitated in the hot rolling stage, and the amount of Nb effective in preventing softening of the heat affected zone during welding will be reduced, so 0.0100% is the upper limit. And

Cu is an essential element for the present invention. 0.
If it is less than 10%, the effect of improving the corrosion resistance is lost, so 0.10% is the lower limit, and if it exceeds 0.5%, the effect of addition is saturated and defects such as whiskers occur even when Ni or the like is added. Since it is easy to do so, the upper limit is 0.05%.

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

Mo improves corrosion resistance and, at the same time, N
It is an element effective in preventing softening of the heat-affected zone of welding in a complex addition with b and is an essential element in the present invention. M
If o is less than 0.1%, both the corrosion resistance and the effect of preventing softening of the weld heat affected zone are lost, so 0.1% is made the lower limit,
If it exceeds 0.5%, the effect is saturated and the number of inclusions that cause defects increases, so 0.5% is made the upper limit.

Ni is necessary to prevent the baldness at the time of hot rolling due to the addition of Cu. If it is less than 0.05%, this effect is lost.
If it exceeds, this effect is saturated, so 0.5% is made the upper limit.

The steel adjusted to the above composition is made into a steel sheet according to the following method. First, steel is melted in a converter and made into a slab by the continuous casting method. This slab is kept in a high temperature state or cooled to room temperature and then charged into a heating furnace to
Heating in the temperature range of ˜1250 ° C., then 800-95
Finish rolling is performed in a temperature range of 0 ° C., and then wound at a temperature of 600 ° C. or lower to obtain a hot rolled steel sheet. Pickling is carried out if necessary depending on the usage conditions.

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 for applying strain, there is a method such as skin pass rolling or a method of applying tensile strain after cutting into a steel plate. 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, so 1.0% is the lower limit, and if it exceeds 10.0%, the ductility of the steel sheet decreases and forming is difficult. Therefore, the upper limit is 10.0%. Elongation rate 1.0%
~ 10.0% means that the marked points on the steel plate surface are 1.0
% To 10.0% elongation amount.

[0022]

EXAMPLE Steels having chemical compositions A to M shown in Table 1 were melted in a converter and made into a slab by continuous casting, followed by hot rolling under the conditions shown in Table 2 and skin pass rolling for applying strain. Done,
The hot rolled steel sheet was 2.3 mm.

Arc welding was applied to these steel sheets, and the hardness of the base material and the softening points of the heat affected zone were measured. Arc welding conditions are: current; 250A, voltage; 25V, speed; 10
00 mm / min, atmosphere; mag gas. The softness of the welding heat affected zone was evaluated by the difference between the hardness of the softest zone and the hardness of the base material.

Corrosion resistance is determined by subjecting a steel sheet to a phosphate treatment (BTL3080 manufactured by Nippon Parker), followed by cation electrodeposition coating (Nippon Paint power top D-30, 20 μm applied), and then cross cutting to reach the base material, and then salt water. Spray 3
5 ° C / 6 hours-drying 70 ° C / RH 60% / 4 hours-wet 49 ° C / RH 95% / 4 hours-cooling 20 ° C / 4 hours 1
Evaluation was made by the erosion depth of the cross-cut portion when 80 cycles of the accelerated test as a cycle were performed. 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 of the heat-affected zone of welding and the corrosion resistance.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet which is excellent in corrosion resistance and softening of the welding heat-affected zone, that is, excellent in fatigue characteristics of the welding heat-affected zone, and has a great industrial effect. it can.

Claims (1)

[Claims] 1. By weight%, 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: 0.001-0.010%, Al: 0.005-0.1%, N: 0.0005-0.0100%, Cu: 0.10-0.50%, Nb: 0.01 to 0.05%, Mo: 0.1 to 0.50%, Ni: 0.05 to 0.50% The balance consists of Fe and unavoidable impurities, and the elongation is 1.0% or more. A high-strength hot-rolled steel sheet having excellent corrosion resistance and weld fatigue characteristics to which a strain of less than 0% is added.