JPH05195144A - High tension steel sheet excellent in pitting corrosion resistance and pit expansion resistance - Google Patents

Abstract

PURPOSE:To obtain a high strength steel sheet excellent in pitting corrosion resistance and pit expansion resistance and capable of withstanding use as a chassis member for an automobile in the naked state. CONSTITUTION:This high tension steel sheet excellent in pitting corrosion resistance consists of, by weight, <=0.1% C, 0.10-2.0% Mn, 0.05-0.20% P, <=0.010% S, 0.05-1.0% Cu, 0.01-0.1% Al, <=0.008% N, Ti satisfying an inequality (48/32)XS<=Ti<=[(48/32)XS+(48/14)XN]X2 and the balance Fe with inevitable impurities or further contains one or more among 0.05-1.0% Si, 0.05-2.0% Ni, 0.05-5.0% Cr and 0.01-0.05% Nb.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet having excellent perforation corrosion resistance and hole expandability, and a method for producing the same, and more particularly, it is suitable for undercarriage members for automobiles and its reinforcing members. Hot rolled steel sheet.

[0002]

2. Description of the Related Art In recent automobiles, it is necessary to guarantee no perforation and corrosion for 10 years. For this reason,
The materials used are required to have improved corrosion resistance, but there is a strong demand for improved corrosion resistance, especially for underbody materials. For example, in cold regions such as North America and Canada, road surface is prevented from freezing and corrosion caused by salts sprayed for snow melting is promoted. Therefore, further improvement in corrosion resistance is strongly desired centering around suspension members.

Conventionally, as one of the methods for improving the corrosion resistance, it has been generally practiced to replace a portion where a bare steel plate is used with a plated steel plate mainly for hot dip galvanizing. Against this background, many plated steel sheets having excellent corrosion resistance have been developed. For example, JP-A-2-310354 describes a method for producing hot-dip galvanized steel sheets having excellent workability.

However, many of the underbody members and reinforcing members for automobiles are manufactured by arc-welding individual parts in many cases, and when a plated steel sheet is used as a material, defects such as blowholes occur during welding. However, there was the problem that a sound weld could not be obtained.

On the other hand, automobiles are required to have improved fuel efficiency in view of the preservation of the global environment, etc., and the steel materials used have been promoted to be lighter due to higher strength. The reduction in plate thickness due to this increase in strength is disadvantageous from the viewpoint of perforation corrosion resistance.

From this point of view, for example, Japanese Patent Laid-Open No. 2-2241
No. 6 discloses an alloyed galvanized steel sheet that improves the corrosion resistance of the plating base plate itself and exhibits excellent corrosion resistance even with a thin weight. However, even though it has a light weight, the presence of the plating layer inevitably causes defects during welding, and the workability such as hole expandability is not sufficient.

[0007]

SUMMARY OF THE INVENTION The present invention has been made for the purpose of solving the above problems, and can withstand bare use as a hot-rolled steel sheet for an automobile underbody member and a reinforcing member thereof. It is an object of the present invention to provide a high-strength steel sheet having excellent perforation corrosion resistance and workability, particularly excellent in hole expandability, and a method for producing the same.

[0008]

According to the present invention, the weight percent is
, C ≦ 0.1%, Mn: 0.10 to 2.0%, P: 0.05 to 0.20%,
S ≦ 0.010%, Cu: 0.05 to 1.0%, Al: 0.01 to 0.1%,
N ≦ 0.008%, and Ti in the range according to the following formula (1), and in some cases Si: 0.05 to 1.0%, N
i: 0.05 to 2.0%, Cr: 0.05 to 5.0%, Nb: 0.01 to 0.05
% High-strength steel sheet containing at least one of Fe and unavoidable impurities with the balance being Fe and unavoidable impurities and having excellent perforation corrosion resistance and hole expandability. (48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 (1).

Then, a steel billet having this chemical composition is
Since it is heated in the temperature range of 1100 to 1300 ℃ and hot-rolled, the hot-rolling is finished at the Ar ₃ transformation point or higher and in the temperature range of 800 to 950 ℃, and continuously wound at the temperature of 700 ℃ or lower. Provided is a method for producing a high-strength hot-rolled steel sheet having excellent puncture corrosion resistance and hole expandability.

[0010]

[Function] As a result of various test studies to solve the above-mentioned problems, the present inventors have found that among the basic components of the steel sheet,
By strictly adding C, Mn, P, Cu, and Ti in the above range, and by additionally adding at least one of Si, Ni, Cr, and Nb in the above range, high strength and excellent resistance can be obtained. It has been found that the addition of perforating corrosion properties and hole expanding properties is achieved.
From the viewpoint of manufacturing conditions, it was found that by appropriately controlling the conditions of hot rolling, it is possible to provide workability that sufficiently satisfies the properties required for automotive underbody parts.

The action of each component of the steel sheet according to the method of the present invention and the reason for limiting the content range thereof are as follows.

C is an element effective for increasing the strength. However, if the content exceeds 0.1%, the workability deteriorates, so the upper limit was made 0.1%.

Mn effectively acts to increase the strength, but 0.1
%, The effect is not observed, and if the content exceeds 2.0%, the workability deteriorates, so 0.10-2.0%
The range is.

P is an indispensable additional element in the steel of the present invention, which not only effectively acts to strengthen the steel, but also contains Cu in an amount of 0.05% or more in combination with Cu to provide a dense corrosion product with good adhesion. Which serves to improve the resistance to pitting corrosion. However, if P is contained in excess of 0.2%, the workability will deteriorate, so 0.05 to 0.
Add in the range of 2%.

S deteriorates the workability. Particularly, when MnS is combined with Mn to form MnS, the hole expandability is deteriorated. Therefore, it is preferable that S is as small as possible, but 0.010
% Is acceptable, so 0.010% or less.

Cu is an element that effectively acts to improve the resistance to pitting corrosion when added in combination with P as described above. If the addition amount is less than 0.05%, the effect is not observed, and if the addition amount exceeds 1.0%, not only the effect is saturated, but also the manufacturing cost increases, so the addition range is set to 0.05 to 1.0%.

N is an element which adversely affects the corrosion resistance and workability. Therefore, it is desirable that the amount is as small as possible, but 0.008% is acceptable in the present invention, so 0.008% is acceptable.
% Or less.

Al is added as a deoxidizer, but if it is less than 0.01%, the effect as a deoxidizer is not recognized, and if it is contained in excess of 0.1%, the effect is saturated, and Al is produced. Since the cost will be high, the upper limit was set to 0.1%.

Si is an element which is effective for increasing the strength and improving the corrosion resistance. If it is less than 0.05%, its effect is not observed, and if it exceeds 1.0%, the surface properties of the product are deteriorated and the workability is also deteriorated, so it is contained in the range of 0.05 to 1.0%.

Ni is an element that effectively acts to suppress the occurrence of hot cracking during hot rolling due to the addition of Cu, and is also effective in increasing the strength and improving the resistance to pitting corrosion. Addition amount is 0.05
If less than%, those effects are not recognized. Even if the content exceeds 2.0%, the effect is saturated and the cost is increased. For this reason, the Ni content range is 0.05 to 2.0%.

Cr is also an element effective for increasing the strength and improving the resistance to pitting corrosion. If less than 0.05%, the effect is not recognized, and if added over 5.0%, not only the effect is saturated, but also the manufacturing cost is significantly increased compared to the effect, so it is contained in the range of 0.05 to 5.0%. Let

Nb effectively contributes to high strength by refining the crystal grain size of the steel sheet. However, if the addition amount is less than 0.01%, the effect is not recognized, and if the addition amount exceeds 0.05%, precipitation of Nb carbonitrides increases and the strength increases, but the workability deteriorates significantly. It is contained in the range of%.

Ti is an element that fixes S and N and effectively acts to improve workability, particularly hole expandability. This effect is not observed when the Ti amount is less than (48/32) × S as shown in the equation (1). Also [(48/32) × S + (48/14) × N] × 2
Even if the content exceeds the above range, the effect is saturated and the cost increases, so it is necessary to contain within the range shown in the above formula (1).

By containing the above chemical components in the above range, it is possible to obtain a high-strength steel sheet having excellent perforation corrosion resistance and hole expandability. As a result of various examinations of the conditions, it was found that by appropriately controlling the extraction temperature, the hot rolling end temperature and the coiling temperature, excellent workability can be imparted to this steel sheet.

If the extraction temperature of the steel slab from the heating furnace during hot rolling (steel slab heating temperature) is less than 1100 ° C., the dissolution of carbonitride is insufficient and good workability cannot be obtained. Further, heating above 1300 ° C does not have any further effect on the dissolution of carbonitrides, rather leading to an increase in manufacturing costs.
Control to the range of.

Hot rolling is performed at an Ar ₃ transformation point or higher and 800 to 95.
Good workability can be obtained when finishing in the temperature range of 0 ° C.
If the hot rolling end temperature is less than the Ar ₃ transformation point or less than 800 ° C., equiaxed crystal grains cannot be obtained, and good workability cannot be obtained. Further, when the temperature exceeds 950 ° C, the crystal grain size becomes coarse and after the processing, rough skin called orange peel occurs. Also, when the winding temperature exceeds 700 ° C, the scale layer on the surface becomes thick and the pickling property deteriorates.
It is necessary to wind at a temperature below 700 ° C.

[0027]

【Example】

[Example 1] Steel having the chemical composition shown in Table 1 was melted to obtain a continuously cast slab. This slab was heated to 1230 ° C and hot-rolled at a hot rolling finish temperature of 900 ° C and a coiling temperature of 550 ° C to obtain a hot-rolled sheet with a thickness of 2.6 mm. The obtained hot-rolled sheet was pickled, and then test pieces were collected and subjected to a combined cycle corrosion test of salt spray, drying, and wet (3 cycles / day) under the conditions shown in FIG. Perform this corrosion test for 30 cycles, 60 cycles, 12 cycles
After 0 cycles and up to 240 cycles were performed, corrosion products were removed and the maximum erosion depth and corrosion weight loss were measured to evaluate the pitting corrosion resistance. The test results are shown in Table 2.

A tensile test and a hole expansion test were conducted on the hot rolled sheet. Tensile test was performed with JIS No. 5 test piece, and hole expandability test was punched out to a diameter of 10 mm with 20% clearance.
It was performed using a ball head punch having a diameter of 50 mm. The test results (λ%) are shown in Table 3.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

As is clear from Table 2, the steels of the present invention containing P and Cu in combination are compared with the steels of Comparative Examples containing no P and Cu or the steels of Nos. It can be seen that the corrosion loss is small, the maximum erosion depth is shallow, and the pitting corrosion resistance is excellent. Comparative Example No. 17 is a steel to which P and Cu are added but Ti is not added, and it can be seen that both the corrosion weight loss and the maximum erosion depth are improved by the combined addition of P and Cu. As can be seen, as compared with the steels of the present invention having the same strength level, the hole expandability is remarkably inferior.

[Example 2] Of the steels shown in Table 1, No. 3
As for the hot rolling conditions shown in Table 4,
The hot-rolled sheet was hot-rolled to 2.6 mm, and the obtained hot-rolled sheet was subjected to the same tensile test and hole expansion test as in Example 1, and the test results are shown in Table 5.
It was shown to.

[0034]

[Table 4]

[0035]

[Table 5]

As is apparent from Table 5, in the case of A, B and E which adopted the hot rolling conditions specified in the present invention, a hot rolled sheet showing good values in both tensile properties and hole expandability was obtained. ， No rough surface after processing. When the extraction temperature is lower than that specified in the present invention, and when the hot rolling end temperature is F, the rough surface after processing is not recognized, but the ductility and hole expandability are poor, and the underbody parts for automobiles It is not possible to satisfy the workability required as.
On the other hand, under the condition D in which the hot rolling finish temperature is high, the tensile properties and the hole expansibility are almost the same values as those of the examples of the present invention, but roughening of the surface occurs after processing.

[0037]

As described above, according to the present invention, a high-strength steel sheet excellent in perforation-corrosion resistance and hole-expansion property can be obtained. It also contributes to the weight reduction of automobiles due to higher strength.

[Brief description of drawings]

FIG. 1 is a test cycle diagram showing conditions of a corrosion test.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mikio Soge 11-11 Showa-cho, Kure City, Hiroshima Prefecture Nisshin Steel Co., Ltd.

Claims (3)

[Claims] 1. By weight%, C ≦ 0.1%, Mn: 0.10 to 2.0
%, P: 0.05 to 0.20%, S ≦ 0.010%, Cu: 0.05 to 1.0
%, Al: 0.01 to 0.1%, N ≦ 0.008%, and Ti in the range according to the following formula (1), with the balance being Fe and unavoidable impurities, which are excellent in perforation corrosion resistance and hole expansibility. Tension steel plate, (48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 ··· (1). 2. In% by weight, C ≦ 0.1%, Mn: 0.10 to 2.0
%, P: 0.05 to 0.20%, S ≦ 0.010%, Cu: 0.05 to 1.0
%, Al: 0.01 to 0.1%, N ≦ 0.008%, and Ti in the range according to the following formula (1), and Si: 0.05 to 1.0.
%, Ni: 0.05 to 2.0%, Cr: 0.05 to 5.0%, Nb: 0.01
To 0.05% of 1 or 2 or more, the balance being Fe and unavoidable impurities, and the high tensile strength steel sheet excellent in perforation corrosion resistance and hole expandability, (48/32) × S ≦ Ti ≦ { (48/32) × S + (48/14) × N} × 2 ··· (1). 3. By weight%, C ≦ 0.1%, Mn: 0.10 to 2.0
%, P: 0.05 to 0.20%, S ≦ 0.010%, Cu: 0.05 to 1.0
%, Al: 0.01 to 0.1%, N ≦ 0.008%, and Ti in the range according to the following formula (1), and in some cases, Si: 0.05 to 1.0%, Ni: 0.05 to 2.0%, Cr: 0.05 ~ 5.0
%, Nb: 0.01 to 0.05% of one or two or more, and the balance being steel and unavoidable impurities, a steel slab is heated in the temperature range of 1100 to 1300 ° C. and hot rolled. High-strength hot rolling with excellent perforation corrosion resistance and hole expandability, which consists of finishing hot rolling in the temperature range of 800 to 950 ° C above the Ar ₃ transformation point and then winding at a temperature below 700 ° C. Steel plate manufacturing method, (48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 ... (1).