JPH07197189A - Steel sheet having excellent pitting corrosion resistance - Google Patents

Abstract

PURPOSE:To produce a steel sheet having excellent corrosion resistance (pitting corrosion resistance) and also enabling to simply use as scrap. CONSTITUTION:This steel sheet is the one consisting of <=0.08% C, <=2.0% Si, 0.02%<Mn<=2.50%, <0.03% P, <=0.01% S, 0.003-0.05% Al, 0.02-0.25% solid solution Ti, <=0.006% N, and as necessary, further containing one or more kinds among <=0.50% Cu, <=0.50% Ni, <=0.20% Cr, <=0.10% Nb, <=0.10 V, <=0.05% Zr, <=0.25% Mo, <=0.0060% B, <=0.0100% Ca and <=0.010% REM, and the balance iron with inevitable impurities and having excellent pitting corrosion resistance. The steel sheet can be used as hot-rolled steel sheet, cold-rolled steel sheet or the steel sheet capable of executing various kinds of platings, coatings and coating substrate treatment, organic film treatment, etc., thereto after executing the hot-rolling or the cold-rolling followed by annealing thereto.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet having excellent corrosion resistance and can be widely used in industrial fields such as automobiles, construction and shipbuilding where corrosion is a problem using steel sheets.

[0002]

2. Description of the Related Art Iron corrodes even in the atmosphere, and when steel sheets are used industrially,
At present, a large amount of cost is consumed in order to prevent corrosion and to secure sufficient characteristics even if corrosion occurs.

Among them, automobiles are used in extremely severe corrosive environments such as large temperature changes, stones flying at high speed, snow melting agents in cold regions, and the like. In addition, from the viewpoint of protecting the global environment in recent years, improving fuel efficiency of cars, and improving riding comfort,
Steel sheets used for automobiles have a strong tendency to have higher strength and thinner thickness.

Particularly, in important safety parts such as suspensions of automobiles, it is necessary that the steel sheet has no holes due to corrosion, or that the plate thickness necessary for securing the strength in design remains even before the holes are formed. Is. For this reason, it is necessary to improve the rust preventive ability when thinning parts.

Further, from the viewpoint of high quality and high texture, it is required to use a steel sheet which is less likely to cause rust and has excellent corrosion resistance. Especially in North America, Northern Europe, etc.
(Cl, KCl, MgCl, etc.), and in areas where gravel is sprayed on roads to prevent slipping, it will repeatedly dry and wet in the presence of gravel that destroys the coating film and Cl ^- ions that promote corrosion of the steel sheet. , Especially good anti-corrosion ability is needed.

On the other hand, in steel sheets for automobiles, scrap produced when punched by a press is melted by an automobile manufacturer and recycled as a raw material for castings for engines and the like, which is an element that deteriorates the characteristics of the castings, particularly toughness. When a large amount of is contained in the steel sheet, there is a problem that recycling is limited.

Conventionally, it has been known that the addition of P, Cu or the like alone or in combination is effective for improving the corrosion resistance of a steel sheet (JP-A-2-22416). This technique shows that the formation of a dense rust layer of P and Cu improves the corrosion resistance. However, in a scrap produced from such a steel sheet, when the P content and the Cu content in the casting are large, the toughness of the casting is deteriorated, and thus it is necessary to remove these when melting the scrap. However, Cu cannot be removed by the current refining technology, and in order to remove P, expensive melting equipment such as a cupola furnace is required.

Under these circumstances, the present invention has an object of providing a steel sheet which has excellent corrosion resistance and can be easily used as scrap.

[0009]

[Means for Solving the Problems] In order to solve the above problems, as a result of diligent investigations and studies by the present inventors, as a result, it was excellent in a corrosive environment in which dry and wet conditions are repeated in the presence of Cl ⁻ ions that promote corrosion of iron. We have succeeded in developing a steel plate that has corrosion resistance and high strength, promotes weight reduction of automobile suspension parts in particular, and can be easily used for scrap.

That is, according to the present invention, C ≦ 0.08%, Si
≦ 2.0%, 0.02% <Mn ≦ 2.50%, P <0.03
%, S ≦ 0.01%, 0.003% ≦ Al ≦ 0.05%,
0.02% ≤ solid solution Ti ≤ 0.25%, N ≤ 0.006%, and if necessary, Cu ≤ 0.50%, Ni ≤ 0.5%.
50%, Cr≤0.20%, Nb≤0.10%, V≤0.1
0%, Zr ≦ 0.05%, Mo ≦ 0.25%, B ≦ 0.00
60%, Ca ≦ 0.0100%, rare earth (REM) ≦ 0.0
The gist is a steel sheet containing 100% of one or two kinds and the balance of iron and unavoidable impurities and having excellent perforation corrosion resistance.

[0011]

The present invention will be described in more detail below.

[0012] In addition to strength, the steel sheet is required to have excellent perforation and corrosion resistance as properties required for the field to which the steel sheet is applied, and it is required that it can be easily used as scrap.
Satisfaction with (scrap reusability) also means
A very difficult material design will be required.

With respect to this point, the inventors of the present invention have conducted intensive studies, and as a result, have obtained an original knowledge that has not been known so far. That is, the perforation corrosion resistance of the steel sheet has a close relationship with the amount of solid solution Ti dissolved in the steel sheet, and it is possible to remarkably improve the perforation corrosion resistance by controlling the amount of solid solution Ti. There was found.

Regarding solid solution Ti, I having an extremely low C than before is used.
In F steel, Ti is added to fix C and N as TiC or TiN precipitates. The main purpose of this addition of Ti is to improve press formability, and since it is not preferable to add more Ti than the amount required for precipitation and fixation of C and N, Ti is hardly solid-dissolved. However, even if a small amount of Ti (solid solution) is solid-dissolved depending on the manufacturing conditions, it is actually at most about 0.01%.

As described above, conventionally, Ti is added to a specific steel type, but even if Ti is added, it has not been intentionally made to cause a relatively large amount of solid solution Ti to exist in the steel sheet. It was

However, according to the research conducted by the present inventors, as shown in FIG. 1, which shows the relationship between the solid solution Ti content and the corrosion resistance (that is, the maximum perforation depth), the solid solution Ti content is 0.02% or more. It was found that by controlling the perforation depth began to decrease significantly. This tendency is that the amount of solid solution Ti is 0.04 to 0.0.
If it is 05% or more, and further 0.07 to 0.08% or more, the perforation depth can be reduced to about half that of conventional steel.

The present invention has been completed on the basis of such novel and original knowledge. The reasons for limiting the chemical components in the present invention will be described below.

C: C is an element that strengthens steel. However, when the C content exceeds 0.08%, a large amount of carbide such as cementite which becomes a cathode at the time of corrosion in a normal manufacturing process is generated, and the carbide and the carbide are formed. Corrosion is promoted by the potential difference between the base metals, and corrosion resistance is reduced, which is not preferable. Therefore, the C content is 0.08% or less. Although there is a balance between strength and improvement of corrosion resistance against pitting, it is preferably 0.05% or less, and more preferably 0.005% or less.

Si: Si is an effective element for adjusting strength while ensuring deoxidation and press workability, but 2.0
%, The content of Si is 2.0% or less, because it is concentrated on the surface of the steel sheet during hot rolling and the pickling property of the steel sheet is deteriorated.

Mn: Mn is an effective element for preventing hot cracking due to S and adjusting strength, but 0.02%
Below, the effect of preventing hot cracking of S is low, and the effect of strengthening is small. Further, if added over 2.5%, the total elongation is remarkably reduced, which is not preferable from the viewpoint of processing. Therefore, the amount of Mn is made 0.02% <Mn ≦ 2.5%.

P: P is an effective element for adjusting the strength while ensuring press workability, but when it is contained in an amount of 0.03% or more, it causes embrittlement after working and scrap casting. In order to deteriorate the toughness, P <0.03%.

S: S is combined with metallic elements in steel,
It exists as a sulfide inclusion. This sulfide-based inclusion causes a potential difference between the sulfide-based inclusion and the metal, and becomes a starting point of corrosion. Therefore, the lower the S concentration, the better. Particularly when the S concentration exceeds 0.01%, the corrosion resistance is extremely deteriorated due to an increase in the amount of sulfide-based inclusions, so the S content is set to 0.01% or less.

Al: Al is added for the purpose of deoxidation, but
If it is less than 003%, deoxidation is not sufficiently carried out, and the O content in the steel cannot be reduced. Also, if it is added in excess of 0.05%, the effect is saturated, so the range of the addition amount is 0. 0
It is set to 03 to 0.05%.

Ti: Iron rust is that Fe is first converted into Fe ² + (Fe ³ +) ions and eluted, and then converted into iron hydroxide or oxide. When Fe becomes Fe ² + (Fe ³ +) ions and is eluted, solid solution elements are eluted at the same time as iron. As shown in FIG. 1, the presence of solid solution Ti improves the corrosion resistance (perforation corrosion resistance), so Ti is added. The mechanism is not clear, but the presence of solid solution Ti by the addition of Ti significantly enhances the passivation ability and improves the structure and morphology of iron oxyhydroxide by Ti ions.
Specifically, stabilization of dense α-FeOOH, formation of a dense rust layer of TiO ₂ and the like can be considered. The effect of improving the corrosion resistance of solid solution Ti is that the effect does not appear unless the amount of solid solution Ti is 0.02% or more, and the amount of Ti added is increased when the amount of solid solution Ti exceeds 0.25%. As a result, the size of Ti-based inclusions increases and the workability deteriorates.
The lower limit of the amount of i is 0.02% and the upper limit is 0.25%. It is more preferably at least 0.04%, and even more preferably at least 0.07%.

N: When the amount of N is large, aging occurs, and a part of N is combined with Ti to form TiN, the amount of solid solution Ti is reduced, and the corrosion resistance is deteriorated, so the N content is 0. .00
6% or less. It is preferably 0.0035% or less.

Although the above-mentioned components are indispensable, one or more of the following components may be contained in appropriate amounts, if necessary.

Cu: Cu is an element that densifies the generated rust and improves the corrosion resistance. However, even if added in an amount exceeding 0.50%, the effect of the corrosion resistance is saturated and the workability is deteriorated.
The addition range is 0.50% or less.

Ni: a steel with a high Cu content is likely to have a bald defect on the surface, but when Ni is added to a steel with a high Cu content, this defect can be prevented, so that the surface of the product can be prevented. It is added to improve the properties. Ni is a component that contributes to the improvement of corrosion resistance. However, even if added in excess of 0.50%, the effect of improving the surface properties and corrosion resistance saturates, and Ni is expensive, so its content should be 0.50% or less. From the viewpoint of surface properties, when the Cu addition amount exceeds 0.20%, it is desirable to add Ni in the same amount as half the Cu content.

Cr: Cr is an element effective for strengthening steel, but when added in an amount of more than 0.20%, the strengthening effect saturates and the corrosion resistance deteriorates by promoting uneven corrosion. Therefore, the addition amount is 0.20% or less.

Nb: Nb is added to strengthen and improve the workability of the steel and to increase the amount of solid solution Ti by adding Nb, that is, to improve the corrosion resistance, but when it exceeds 0.10%, it is added to the steel. Is brittle and expensive, so the addition amount is set to 0.10% or less.

V: V is added to strengthen the steel and improve workability, but if it is added in an amount of more than 0.10%, the effect is saturated, the steel becomes brittle, and it becomes expensive. The amount should be 0.10% or less.

Zr: Zr is added to strengthen the steel and improve workability, but if added in excess of 0.05%, the steel becomes brittle and expensive, so the addition amount is 0.05. % Or less.

B: B is added in order to improve the embrittlement of the steel after working, but if it is added in excess of 0.0060%, the steel becomes rather brittle, so the addition amount is made 0.0060% or less. .

Mo: Mo is an extremely effective element for strengthening the steel and improving the workability, but even if it is added in an amount of more than 0.25%, the effect is saturated and the cost becomes high. The amount should be 0.25% or less.

Ca: At the stage where the corrosion of iron is progressing,
The following reactions occur inside the pitting corrosion, the inside of the pitting corrosion is acidified, and the corrosion of iron is further promoted. However, when Ca is present, Ca is dissolved at the same time as iron, and since Ca is an alkali metal, It is added because it makes the inside of pitting corrosion basic and reduces the progress of pitting corrosion. However, if added in excess of 0.0100%, not only the effect is saturated, but also embrittlement of the steel occurs, so the Ca addition amount is made 0.0100% or less.

[0036]

[Chemical 1]

[0037]

[Chemical 2]

[0038] Rare earth element: A rare earth element (REM) is added as well as Ca because it basifies the inside of the pit and reduces the progress of the pit, but if it is added in excess of 0.0100%. , The effect is not only saturated, but also causes embrittlement of the steel, so the amount of rare earth elements added is 0.0100%.
Below.

By adjusting the chemical composition as described above, a steel sheet having excellent corrosion resistance can be obtained. The production method thereof is the same as in the case of ordinary steel sheets. A hot rolled steel sheet can be manufactured by performing hot rolling after the ingot casting. that time,
It is also possible to perform degassing treatment after melting, and decomposition rolling after casting. Furthermore, after hot rolling, cold rolling and annealing are performed,
It is also possible to manufacture cold rolled steel sheets. In addition, hot-rolled steel sheet, cold-rolled steel sheet, or hot-rolled or cold-rolled and then annealed steel sheet, various plating such as hot dipping, electroplating, vapor deposition plating, various coatings, coating It is also possible to perform a base treatment, an organic film treatment, and the like.

Next, examples of the present invention will be shown.

[0041]

【Example】

For steels having the chemical composition shown in Table 1,
Both the steel sheet of the present invention and the conventional steel sheet are subjected to actual level melting, and the hot rolled steel sheet has a finishing temperature of 900 ° C and a winding temperature of 500 ° C
Of the actual machine hot rolling, after pickling, used for evaluation of corrosion resistance,
For cold-rolled steel sheets, the finishing temperature is 900 ° C and the coiling temperature is 6
After hot rolling at an actual temperature of 00 ° C., pickling, cold rolling at a cold rolling rate of 70%, continuous annealing at 800 ° C. for 1 minute, and evaluation of corrosion resistance were performed. The results are shown in Table 1.

The corrosion resistance was determined by subjecting the steel sheet to a phosphate treatment (SD5000 made by Nippon Paint) and then by cationic electrodeposition coating (PT-U-80 made by Nippon Paint, 15 μm coating).
Cross-cut to reach the base and spray with salt water 50 ℃ 16
Erosion depth (maximum perforation depth) of the cross cut part when 100 cycles of corrosion acceleration test with time → dry 70 ° C 4 hours → wet 50 ° C humidity 85% 4 hours as 1 cycle
Expressed in mm.

As can be seen from Table 1, in all the examples of the present invention, the perforation depth was remarkably reduced as compared with the comparative examples, and the excellent corrosion resistance was obtained.
(Perforated corrosion resistance) is shown. Figure 1 shows the relationship between the amount of solid solution Ti and the maximum perforation depth.

[0045]

[Table 1]

[0046]

As described in detail above, according to the present invention,
A steel sheet with excellent perforation and corrosion resistance can be obtained, and it is most suitable not only for automobiles but also for industrial fields such as construction and shipbuilding where corrosion of steel is a problem. This steel sheet exhibits excellent performance when used bare or coated, but exhibits further excellent effect when combined with an appropriate surface treatment such as plating or coating an organic film. Moreover, the scraps can be recycled and used as raw materials for casting without deteriorating the characteristics of the casting.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the amount of solid solution Ti and corrosion resistance (maximum perforation depth).

Front page continued (72) Inventor Shunjiro Takeuchi 1 Kanazawa-machi, Kakogawa-shi, Hyogo Kobe Steel Works Kakogawa Steel Works (72) Inventor Takenori Nakayama 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo Kobe Steel Works, Kobe Research Institute

Claims (3)

[Claims] 1. In% by weight (the same applies hereinafter), C ≦ 0.08%, Si ≦ 2.0%, 0.02% <Mn ≦ 2.50%, P <0.03%, S ≦ 0. 01%, 0.003% ≤ Al ≤ 0.05%, 0.02% ≤ solid solution Ti ≤ 0.25%, N ≤ 0.006%, with the balance being iron and inevitable impurities. Steel plate with excellent perforation and corrosion resistance. 2. Further, Cu ≦ 0.50% and Ni ≦ 0.50.
%, Cr ≦ 0.20%, Nb ≦ 0.10%, V ≦ 0.10
%, Zr ≦ 0.05%, Mo ≦ 0.25%, B ≦ 0.006
0%, Ca ≦ 0.0100%, rare earth (REM) ≦ 0.01
The steel plate according to claim 1, which contains one or two of 00%. 3. The steel sheet according to claim 1, which is excellent in scrap reusability as a casting raw material.