JPH05195077A - Production of high tensile strength plated steel sheet excellent in pit corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain a high strength plated steel sheet suitable for automobile under carriage member, capable of withstanding severe corrosive environment, and excellent in workability. CONSTITUTION:A slab of a steel which has a composition consisting of, by weight, <=0.1% C, 0.10-2.0% Mn, 0.05-0.20 P, <=0.010% S, 0.05-1.0% Cu, 0.05-2.0 Mo, 0.01-0.1 Al, <=0.008% N, and the balance Fe with inevitable impurities and further containing, if necessary, one or >=2 kinds among 0.05-1.0% Si, 0.05-2.0% Ni, 0.05-5.0% Cr, and 0.01-0.05% Nb and/or Ti by the amount in the range satisfying an inequality (48/32)XS<=Ti<=[(48/32))<S+(48/140)XN]-m2 is heated at 1100-1250 deg.C and hot rolling is started and is then finished at a temp. not lower than the Ar transformation point and between 800 and 950 deg.C. Successively, the steel is coiled at <=700 deg.C, pickled, and subjected to plating treatment. By this method, the high tensile strength plated steel sheet excellent in pit corrosion resistance can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength plated steel sheet having excellent corrosion resistance against perforation, which is particularly suitable for underbody members for automobiles and its reinforcing member applications.

[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.

On the other hand, automobiles are required to have improved fuel efficiency in order to preserve the global environment, 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
Japanese Unexamined Patent Publication No. 6 discloses an alloyed galvanized steel sheet which improves the corrosion resistance of the plating base plate itself and exhibits excellent corrosion resistance even in a thin weight. However, even this cannot be said to be able to sufficiently withstand today's severe corrosive environment conditions, and further improvement in pitting corrosion resistance is required.

[0006]

Therefore, the present invention is
It is an object of the present invention to provide a high-strength plated steel sheet that can sufficiently withstand today's severe corrosive environmental conditions, particularly for automobile underbody members and their reinforcing members.

[0007]

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%, Mo: 0.05 to 2.0%,
Al: 0.01-0.1%, N: 0.008% or less, and in some cases Si: 0.05-1.0%, Ni: 0.05-2.0
%, Cr: 0.05 to 5.0%, Nb: 0.01 to 0.05%, and / or two or more kinds and / or Ti in the range according to the following formula (1), with the balance being Fe and unavoidable impurities. 1
The hot rolling is started by heating in the temperature range of 100 to 1250 ° C., the hot rolling is finished at the temperature of Ar ₃ transformation point or higher and in the temperature range of 800 to 950 ° C., and continuously wound at a temperature of 700 ° C. or lower, Provided is a method for producing a high-strength plated steel sheet excellent in perforation corrosion resistance, which comprises performing a plating treatment after pickling. (48/32) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 (1).

[0008]

The present inventors have conducted various test studies in order to achieve the above-mentioned object, and as a result, among the basic components of the steel plate of the plating base material, C, Mn, P, Cu, Mo are within the above range. Strictly, and by adding at least one of Si, Ni, Cr, and Nb in the above range, and by adding a trace amount of Ti satisfying the formula (1), high strength and excellent resistance are obtained. Perforation and corrosiveness are achieved, and from the viewpoint of manufacturing conditions, by appropriately controlling the hot rolling conditions, it is possible to provide workability that satisfies the characteristics required for automobile underbody parts. all right.

By performing zinc plating, aluminum plating or alloy plating of these steel plates as a base material, the resistance to perforation corrosion is further improved.

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

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

[0012] Mn also acts effectively to increase the strength of steel, but if it is less than 0.1%, its effect is not recognized, and if it exceeds 2.0%, the workability deteriorates, so 0.10-2.
The range is 0%.

P is an indispensable additional element in the base steel sheet of the present invention, which not only effectively acts to increase the strength, but also contains Cu in an amount of 0.05% or more in combination with Cu, thereby providing a dense and good adhesion. It produces corrosion products, which serve to contribute to the improvement of pitting corrosion resistance. But P is 0.2
%, The workability will be deteriorated, so 0.05 to 0.2% is added.

S deteriorates 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 perforation corrosion resistance of the steel sheet by adding it 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%.

Mo is also an essential element in the steel of the present invention,
It is effective in increasing the strength and improving the resistance to pitting corrosion. 0.05
If less than%, those effects are not recognized. If it is added in excess of 2.0%, the effect of improving the pitting corrosion resistance is saturated and the workability deteriorates, which is also a cost disadvantage. For this reason, the addition range of Mo is set to 0.05 to 2.0%.

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).

It has been found that when a steel slab containing the above chemical components in the above range is hot-rolled under appropriate hot rolling conditions, excellent workability can be imparted to this steel. That is, this steel slab is heated in a temperature range of 1100 to 1250 ° C. to start hot rolling, and the hot rolling is performed at an Ar ₃ transformation point or higher and 800
High-strength hot-rolled steel sheets with excellent perforation corrosion resistance and workability can be obtained by finishing in the temperature range of up to 950 ° C and then winding at a temperature of 700 ° C or lower.

If the extraction temperature of the billet from the heating furnace (the billet heating temperature) for starting hot rolling is less than 1100 ° C., the carbonitride is not sufficiently melted and good workability cannot be obtained. The upper limit of the heating temperature for melting this carbonitride is 1250 ° C, and even if it exceeds this temperature, the manufacturing cost rises.
It is better to control in the range of 1250 ℃.

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.

The hot-rolled steel sheet thus obtained is pickled and then plated to obtain a high-strength plated steel sheet excellent in perforation corrosion resistance according to the present invention. The plating treatment may be zinc plating, aluminum plating, or alloy plating of these, depending on the application. The plating method is not particularly limited, and any method such as hot dipping, electroplating and vapor deposition plating may be used.

[0028]

【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 steel sheet with a thickness of 2.6 mm. After pickling the obtained hot-rolled steel sheet, using a Zenzimer type hot dip Zn plating equipment,
Hot dip galvanizing was applied with a coating weight of 45 g / m ² .

Test pieces were taken from the obtained galvanized steel sheet and subjected to a combined cycle corrosion test of salt spray, dryness, and wetness (3 cycles / day) under the conditions shown in FIG. This corrosion test was carried out for 30, 60, 120, and up to 240 cycles, the corrosion products were removed, and the maximum erosion depth was measured to evaluate the resistance to pitting corrosion of the plated steel sheet. The test results are shown in Table 2.

[0030]

[Table 1]

[0031]

[Table 2]

From the results of Table 2, the following is clear.
At the initial stage of the corrosion test up to 30 cycles, the steel of the present invention having P, Cu and Mo added as a base material and the steel of the comparative example (steel lacking any or all of P, Cu and Mo) were used. There is no significant difference in the maximum erosion depth between the material with). It can be considered that this is because the plating layer was not attacked at the beginning of the test.

However, when the surface plating layer is eroded over 30 cycles, the hot-rolled steel sheet lacking any or all of P, Cu and Mo, which are the main additive elements of the present invention, is used as the base material. It can be seen that the galvanized steel sheets of Comparative Examples Nos. 17 to 21 described above have a large maximum erosion depth, whereas the galvanized steel sheets according to the present invention have a very slow progress of corrosion and are excellent in perforation corrosion resistance.

In Comparative Example No. 21, steel containing P and Cu added but no Mo added was used as a base material, and the combined addition of P and Cu provided better resistance than other Comparative Examples. Although it shows pitting corrosion, it cannot be said to be a sufficient value as compared with those of the examples of the present invention.

Example 2 Of the steels shown in Table 1, No.
2, 8 and 15 were hot-rolled under the hot-rolling conditions shown in Table 3 up to a plate thickness of 2.6 mm, the obtained hot-rolled steel sheet was pickled, and then electroplated with Zn. ² electric Zn plated steel sheet was obtained. Specimens were taken from this electric Zn plated steel sheet and subjected to a tensile test and a hole expansion test. Tensile test is performed with JIS No. 5 test piece, and hole expandability test is clearance
It was punched to a diameter of 10 mm at 20%, and a ball head punch with a diameter of 50 mm was used. The results of these tests are shown in Table 4.

[0036]

[Table 3]

[0037]

[Table 4]

As is clear from Table 4, in the cases of A, B and E which adopted the hot rolling conditions specified in the present invention, the tensile properties and the hole expandability both showed good values and the surface roughness after processing was rough. Is not admitted. In the case where the steel billet heating temperature is lower than that specified in the present invention, and the hot rolling end temperature is F, roughening after processing is not observed, but ductility and hole expandability are poor, and the foot for automobiles is inferior. It is not possible to satisfy the workability required for the surrounding member. On the other hand, under the condition D in which the hot rolling finish temperature is high, the tensile properties and the hole expandability are almost the same values as those of the examples of the present invention, but surface roughening occurs after processing.

[0039]

As described above, according to the present invention, a high-strength plated steel sheet having excellent corrosion resistance against pitting can be obtained, and this steel sheet exhibits an effect which has not been obtained by conventional materials as a suspension member or a reinforcing member for automobiles. . That is, not only is it effective in reducing the weight of automobiles due to its high strength, but it is also excellent in workability and perforation corrosion resistance, so it is possible to provide a material suitable for automobile underbody members that can withstand today's severe corrosive environments. .. Further, the plated steel sheet of the present invention exerts an effect which is not obtained by conventional materials even when applied to outdoor building materials and the like due to its excellent characteristics.

[Brief description of drawings]

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

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location // C22C 38/00 301 A 7217-4K F 7217-4K 38/16 (72) Inventor Muneshita Mikio 11-1 Showa-cho, Kure-shi, Hiroshima Nisshin Steel Co., Ltd. Steel Research Laboratory

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
%, Mo: 0.05 to 2.0%, Al: 0.01 to 0.1%, N: 0.008%
Hereinafter, a steel slab having a balance of Fe and inevitable impurities is heated in a temperature range of 1100 to 1250 ° C to start hot rolling, and the hot rolling is performed at an Ar ₃ transformation point or higher and at 800 to 950 ° C. A method for producing a high-strength plated steel sheet excellent in perforation corrosion resistance, which comprises finishing in a temperature range, continuously winding at a temperature of 700 ° C or lower, pickling, and then plating treatment. 2. 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
%, Mo: 0.05 to 2.0%, Al: 0.01 to 0.1%, N: 0.008
% Or less, 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
%, One or more, and / or Ti in the range according to the following formula (1), the balance consisting of Fe and inevitable impurities is heated in a temperature range of 1100 to 1250 ° C and hot rolled. And the hot rolling is performed at an Ar ₃ transformation point or higher and at 800 to 950
A method for producing high-strength plated steel sheet with excellent perforation corrosion resistance, which consists of finishing in the temperature range of ℃, rolling up at a temperature of 700 ℃ or lower, pickling, and then plating treatment, (48/32 ) × S ≦ Ti ≦ {(48/32) × S + (48/14) × N} × 2 ... (1). 3. The method for producing a high-strength plated steel sheet according to claim 1, wherein the plating treatment is hot dip galvanizing treatment.