JPH04297551A - Automobile thin steel sheet excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain an automobile high tensile strength steel sheet excellent in corrosion resistance by forming a clad steel sheet of low carbon steel having specified chemical compsns. on the surface layer part and the inside. CONSTITUTION:A clad steel sheet in which the surface layer part contains, by weight, <=0.1% C, <=0.1% Si, 0.1 to 1.5% Mn, <=0.1% P, 0.01 to 0.07% Al and the balance Fe with inevitable impurities, and the inside contains <=0.1% C, <=1.0% Si, 0.1 to 1.5% Mn, <=0.1% P, 0.01 to 0. 07% Al and one or two kinds of 0.5 to 5.0% Cr and 0.1 to 1.0% Cu and the balance Fe with inevitable impurities is formed. In this way, the automobile steel sheet excellent in corrosion resistance from a soft steel sheet to a 60 kilo class high tensile strength steel sheet can be obtd.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin steel sheet for automobiles which has excellent corrosion resistance and is suitable for automobile panels and the like.

0002

BACKGROUND OF THE INVENTION The number of years of use of steel sheets for automobiles and the like is increasing, and customers' demands for longer lifespans are also becoming stronger. On the other hand, the corrosive environment is becoming more severe due to the use of magnesium chloride for snow melting in winter. Usually a sufficient coating is applied, but this coating breaks down due to the crushing action of these salts, and the corrosion then progresses by attacking the steel itself. Therefore, the corrosion resistance of the base material is important for the total corrosion life of the steel plate until a hole is formed.

There is a prior art technique for increasing the corrosion resistance of the base material (Japanese Patent Application Laid-Open No. 186850/1983). This prior art has high P
- It is intended to provide corrosion resistance with high S sulfide and high P. Due to high P, it becomes hard and machinability cannot be ensured. Furthermore, high S results in an increase in inclusions, which induces hot embrittlement of the steel and further deteriorates the workability of the product. From this point of view, it is natural that it is not suitable for advanced processing applications such as automobiles.

Conventionally, methods for manufacturing composite metal materials by continuous casting have been known, for example, as disclosed in Japanese Patent Application Laid-open No. 10894-1983.
It is disclosed in Publication No. 7. However, this method is related to a continuous casting method for composite metal materials, and is not related to a thin steel sheet for automobiles having excellent corrosion resistance as the present invention.

[0005]

[Problems to be Solved by the Invention] The present inventors have conducted extensive research on automotive thin steel sheets with excellent corrosion resistance, and have found that composite steel sheets made of multiple layers of steel sheets have good corrosion resistance.

[0006]

[Means for Solving the Problems] The gist of the present invention is as follows. (1) Surface layer part: C: 0.1% or less, Si: 0.1% or less, Mn: 0.1 in weight%
~1.5%, P: 0.1% or less, Al: 0.01~0.
07%, the balance consisting of Fe and unavoidable impurities,
Inside is C: 0.1% or less, Si: 1.0% or less, Mn: 0.1 to 1.5%, P: 0.1% or less, Al:
0.01-0.07%, Cr: 0.5-5.0%, C
u: Contains any one or two of 0.1 to 1.0%,
A thin steel sheet for automobiles with excellent corrosion resistance, characterized in that the remainder is Fe and unavoidable impurities.

(2) Surface layer has C: 0.1% by weight
Below, Si: 0.1% or less, Mn: 0.1 to 1.5%,
Contains P: 0.1% or less, Al: 0.01 to 0.07%, the balance consists of Fe and inevitable impurities, the inside is % by weight
C: 0.1% or less, Si: 1.0% or less, Mn: 0.
1-1.5%, P: 0.1% or less, Al: 0.01-0
．． 07%, Cr: 0.5-5.0%, Ti: 0.01
~0.1%, Nb:0.01~0.1%, V:0.01
A thin steel sheet for automobiles having excellent corrosion resistance, characterized in that it contains up to 0.2% of one or more kinds, and the balance consists of Fe and unavoidable impurities.

(3) Surface layer has C: 0.1% by weight
Below, Si: 0.1% or less, Mn: 0.1 to 1.5%,
Contains P: 0.1% or less, Al: 0.01 to 0.07%, the balance consists of Fe and inevitable impurities, the inside is % by weight
C: 0.1% or less, Si: 1.0% or less, Mn: 0.
1-1.5%, P: 0.1% or less, Al: 0.01-0
．． 07%, Cu: 0.1-1.0%, Ti: 0.01
~0.1%, Nb:0.01~0.1%, V:0.01
A thin steel sheet for automobiles having excellent corrosion resistance, characterized in that it contains up to 0.2% of one or more kinds, and the balance consists of Fe and unavoidable impurities.

(4) Surface layer has C: 0.1% by weight
Below, Si: 0.1% or less, Mn: 0.1 to 1.5%,
Contains P: 0.1% or less, Al: 0.01 to 0.07%, the balance consists of Fe and inevitable impurities, the inside is % by weight
C: 0.1% or less, Si: 1.0% or less, Mn: 0.
1-1.5%, P: 0.1% or less, Al: 0.01-0
．． 07%, Cr: 0.5-5.0%, Cu: 0.1-1
．． 0%, Ti: 0.01-0.1%, Nb: 0.01
-0.1%, V: 0.01-0.2%, and the remainder consists of Fe and unavoidable impurities.

0010

[Operation] First, the reason for numerically limiting the main constituent elements of the present invention will be described. Describe the constituent elements of the surface layer. C is limited in terms of processability, and if it exceeds 0.1%, the desired formability cannot be obtained. Since Si is an element that inhibits chemical conversion treatment properties if added too much, it needs to be kept at 0.1% or less.

[0011] Since Mn deteriorates spot weldability, the upper limit needs to be 1.5%. If it is less than 0.1%, the effect cannot be expected. The preferred range is 0.1-1.4%
shall be. Since P deteriorates secondary workability, the upper limit is 0.1.
It needs to be in %. The preferred range is 0.005-0.
08%.

Al is an element necessary for deoxidation, but 0.0
If it is less than 1%, the effect cannot be expected. However, if it is included in a large amount, it may cause the formation of inclusions, so it should be kept at 0.07% or less. The surface layer can be made of ultra-low carbon steel and further have good workability by adding carbide-forming elements.

Ti and Nb are important elements for forming carbides and improving workability. However, adding a large amount increases the cost, so the upper limit is set to 0.1%. Furthermore, the lower limit is set to 0.01% since the effect becomes smaller as the content decreases. V is also an important element for forming carbides and improving workability. However, if added in large quantities, the cost will be high, so the upper limit is 0.2%.
And so. The lower limit is 0.
01%.

[0014] When Ti, Nb, and V are added, it is desirable to keep C at 0.01% or less in order to obtain the desired effect. The interior is made of Al-killed steel and its constituent elements are described below. C is limited in terms of processability, and if it exceeds 0.1%, the desired formability cannot be obtained. If Si increases, it hardens and deteriorates workability, so it should be kept at 1.0% or less.

1. Mn deteriorates spot weldability.
Keep it below 5%. If it is less than 0.1%, the effect cannot be expected. The preferred range is 0.1 to 1.4%. P
Since it deteriorates secondary workability, it should be kept at 0.1% or less. The preferred range is 0.005 to 0.08%. Al is an element necessary for deoxidation, but if it is less than 0.01%, the effect cannot be expected. However, if it is contained in a large amount, it may cause the formation of inclusions, so it should be kept at 0.07% or less.

[0016] Cr is an element necessary for workability, but 0.
If it is less than 5%, there is no effect of addition, and if it exceeds 5.0%, it becomes hard and press formability is impaired. Furthermore, Cu can be actively used to improve corrosion resistance. Cu is an element that increases corrosion resistance, and if it is less than 0.1%, it has no effect, and if it is more than 1.0%, it will strengthen the solid solution of Cu or ε
- Precipitation of Cu hardens the steel and reduces workability.

[0017] Steel having the above-mentioned composition is usually produced by a continuous casting method and sent to a hot rolling process, but in the present invention, the finishing temperature of hot rolling is 800°C or higher and the winding temperature is 750°C. The following is desirable. After hot rolling, the pickled steel sheet is subjected to temper rolling of about 0.3 to 2%, if necessary, and then used as a product.

[0018] Further, cold rolling is performed after hot rolling and pickling, and the rolling reduction ratio is preferably 75% or more for improving deep drawability. Next, regarding the annealing conditions, continuous annealing is more preferable. The annealing temperature should be higher than the recrystallization temperature. In the case of continuous annealing, cooling after annealing may be performed by any method (gas jet method, air/water method, roll cooling method, water quenching method, etc.) without any particular problem. In the case of continuous annealing, it is preferable to perform overaging treatment at 200 to 500°C. Annealed steel plate: 0.3-2% if necessary
It is subjected to a certain degree of temper rolling and then offered as a product.

As described above, the object of the present invention is a hot-rolled steel plate and a cold-rolled steel plate, which are composite steel plates made into multiple layers by a continuous casting method. The thickness of the surface layer of the multilayer steel plate in the present invention is 10 μm
20% or less of the total plate thickness is preferable.

[0020]

[Example] Steel having the composition shown in Tables 1 and 2 (continued from Table 1) was produced by continuous casting. Of these, steel code 1, 2, 3, 4
, 5, 6, 7, and 8 are steels according to the present invention (steel codes 1 to 4 correspond to claim 1, steel code 5 corresponds to claim 2, steel code 6 corresponds to claim 3, and steel code 6 corresponds to claim 3). Items corresponding to item 4 are steel codes 7 and 8). Steel code 9 is a uniform steel plate without the addition of Cu or Cr. Steel code 10 is a uniform steel plate with too much Cr added and
There is no addition of u. Steel code 11 is a uniform steel plate with insufficient Cr addition and no Cu addition.

[0021] The hot-rolled steel sheet and cold-rolled steel sheet of the steel of the present invention will be described. Heating temperature 1150-1180℃, finishing temperature 900℃
~920°C, hot rolling at a winding temperature of 550~700°C,
The pickled steel sheet is subjected to temper rolling of about 0.3 to 2%, if necessary, to obtain a hot rolled steel sheet. Further, the steel plate that is cold rolled after hot rolling and pickling, and subjected to continuous annealing and box annealing is subjected to temper rolling of about 0.3 to 2.0%, if necessary, to obtain a cold rolled steel plate.

The obtained steel plates were sampled and examined for mechanical test values and corrosion resistance. Mechanical test values were conducted using JIS No. 5 test pieces. The results are shown in Table 3. Next, for the corrosion resistance test, in order to simulate an actual car, we performed bonding treatment with zinc phosphate, then applied electrodeposition coating, and then
I applied an intermediate coat and a top coat. A cross cut was then made and a corrosion test was conducted. Corrosion test is CCT (cycliccor)
rosion test). This consists of the following cycle (SST: Salt Spray Test).

[0023] In this test, the condition after 200 hours was investigated. As an index, after CCT, the rust layer was removed and the plate thickness was measured. Ten points were taken from the one with the largest decrease in plate thickness, and the average plate thickness was taken as the remaining plate thickness. The test was conducted with 3 repetitions.

The corrosion resistance of the products of the present invention (test steel Nos. 1 to 8) is extremely good, and the reduction in plate thickness is generally within 20%. Test steel No. 9, No. No. 10 is a cold-rolled steel sheet of a comparative example, and has poor corrosion resistance. Test steel No. No. 11 is a hot-rolled steel sheet of a comparative example, and has poor corrosion resistance.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[Effects of the Invention] By forming a steel plate into multiple layers according to the present invention, a thin steel plate for automobiles having extremely excellent corrosion resistance can be provided. According to the present invention, the corrosion resistance of the inner and outer panels of an automobile can be improved, and the life of the automobile can be significantly extended.

Claims (4)

[Claims] Claim 1: The surface layer has C: 0.1% or less by weight;
Si: 0.1% or less, Mn: 0.1 to 1.5%, P: 0
．． 1% or less, Al: 0.01 to 0.07%, the balance consists of Fe and inevitable impurities, the interior is C:
0.1% or less, Si: 1.0% or less, Mn: 0.1-1
．． 5%, P: 0.1% or less, Al: 0.01-0.07
%, Cr: 0.5-5.0%, Cu: 0.1-1.0
A thin steel sheet for automobiles having excellent corrosion resistance, characterized in that it contains one or both of the following: % and the remainder is Fe and unavoidable impurities. Claim 2: The surface layer part has C: 0.1% or less by weight;
Si: 0.1% or less, Mn: 0.1 to 1.5%, P: 0
．． 1% or less, Al: 0.01 to 0.07%, the balance consists of Fe and inevitable impurities, the interior is C:
0.1% or less, Si: 1.0% or less, Mn: 0.1-1
．． 5%, P: 0.1% or less, Al: 0.01-0.07
%, Cr: 0.5-5.0%, Ti: 0.01-0.
1%, Nb: 0.01-0.1%, V: 0.01-0.
A thin steel sheet for automobiles having excellent corrosion resistance, characterized in that it contains 2% of one or more kinds, and the balance consists of Fe and unavoidable impurities. Claim 3: The surface layer has C: 0.1% or less by weight;
Si: 0.1% or less, Mn: 0.1 to 1.5%, P: 0
．． 1% or less, Al: 0.01 to 0.07%, the balance consists of Fe and inevitable impurities, the interior is C:
0.1% or less, Si: 1.0% or less, Mn: 0.1-1
．． 5%, P: 0.1% or less, Al: 0.01-0.07
%, Cu: 0.1-1.0%, Ti: 0.01-0.
1%, Nb: 0.01-0.1%, V: 0.01-0.
A thin steel sheet for automobiles having excellent corrosion resistance, characterized in that it contains 2% of one or more kinds, and the balance consists of Fe and unavoidable impurities. 4. The surface layer part has C: 0.1% or less by weight,
Si: 0.1% or less, Mn: 0.1 to 1.5%, P: 0
．． 1% or less, Al: 0.01 to 0.07%, the balance consists of Fe and inevitable impurities, the interior is C:
0.1% or less, Si: 1.0% or less, Mn: 0.1-1
．． 5%, P: 0.1% or less, Al: 0.01-0.07
%, Cr: 0.5-5.0%, Cu: 0.1-1.0%
, Ti: 0.01-0.1%, Nb: 0.01-0.
1%, V: 0.01 to 0.2%, and the remainder is Fe and unavoidable impurities.