JP2000034544A - Steel sheet excellent in pitting resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain excellent pitting resistance by preparing a steel sheet having a specified constitutional compsn. in which the contents of S, P and Cu are specified, and, moreover, the ratio of the Cu content to the S content is specified. SOLUTION: A steel sheet contg., by weight, 0.001 to 0.06% C, <=0.019% Si, <=0.3% Mn, <=0.05% Al, <=0.010%, preferably about 0.001 to 0.008% S, 0.02 to 0.05%, preferably about 0.02 to 0.04% P, 0.01 to 0.2%, preferably about 0.01 to 0.1% Cu and <=0.03% Cr, in which Cu (wt.%)/S (wt.%), i.e., the ratio of the Cu content to the S content is >=2.0, and the balance Fe with inevitable impurities prepd. It is possible that this steel sheet is furthermore subjected to various plantings as surface treatment, and the surface of this plating layer is subjected to chemical conversion treatment, coating treatment, or, organic film treatment or the like. In this way, the steel sheet excellent in press workability and having a long service life 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 steel sheet suitable for use in parts requiring corrosion resistance, such as outer and inner plates for automobiles, undercarriage members, and the like. The improvement of the perforation resistance at the steel sheet joining part.

[0002]

2. Description of the Related Art In regions where snow melting salt is sprayed, such as North America and Europe, where antifreezing agents such as NaCl and KCl are sprayed in winter,
^- since the state of the corrosive environment of wet and dry are repeated in the presence of ions, the automotive steel sheets are required to have excellent corrosion resistance. Normally, the body is coated with electrodeposition to extend the rust-prevention life of the body, but the electrodeposition coating does not sufficiently go around at the joints of steel plates such as welds and hems, and the exposed part of the base steel surface is easy. The problem is that the car body is corroded and the car body is perforated in a short time.

[0003] Conventionally, such uses are made of galvanized steel.
Various types of surface-treated steel sheets represented by
Known rust-preventive steel sheets added with Cu, Ni and P are used
ing. However, these steel sheets are subject to environmental
Is good, but Cl ^-Dry and wet in the presence of ions
In a repetitive corrosive environment, good results are not always
Could not be obtained. Steel plate matching for welded parts and hemmed parts
Even in galvanized steel sheets with excellent corrosion resistance,
^-In a corrosive environment where the wet and dry conditions are repeated in the presence of ions,
The sacrificial corrosion protection period due to zinc is also shorter, and the corrosion of the base steel plate
Proceed to.

[0004] As a steel sheet for improving corrosion resistance under such an environment, for example, Japanese Patent Application Laid-Open No. 6-306646 discloses P:
0.02 to 0.1 wt%, Cu: 0.2 to 0.5 wt%, Ni: 0.1 to 0.5
There has been proposed a highly corrosion-resistant steel material in which an ultra-thin galvanized film is adhered to the surface of a steel material to which wt% is added. In addition,
No. 2-156042 discloses that C: 0.1 wt% or less, S: 0.015 wt%
%, Cr: 0.04 to 0.1 wt%, Cu: 0.025 to 0.1 wt%, and a low-carbon cold-rolled steel sheet excellent in bare corrosion resistance and having an S / Cu of 0.5 or less has been proposed. Also, JP-A-9-6762
No. 6 discloses that C: 0.001 to 0.015 wt%, P: 0.03 wt%
Cu: 0.02 to 0.5 wt%, solid solution Ti: 0.02 to 0.3 wt%
%, Hot rolled, rolled at 600 ° C or more, pickled, cold rolled, annealed at a temperature of 750 ° C or more and _three points or less of Ac, and the average cooling rate up to 500 ° C after annealing is 20 ° C / A method for producing a cold-rolled steel sheet excellent in perforation corrosion resistance and deep drawability of at least s has been proposed.

[0005]

As in the case of weathering steel, by optimizing the composition of steel components, it has become possible to increase corrosion resistance to some extent compared to conventional steel sheets, but in all cases, a large amount of expensive alloy elements is used. , And the steel plate itself becomes expensive. However, in the automobile industry, reducing body manufacturing costs is an important issue, and low prices for steel materials are desired. Further, steel materials described in JP-A-6-306646 are Cu, Ni
Although excellent in corrosion resistance by addition, there is a problem in press workability when applied to, for example, automobile parts. In addition,
In the low-carbon cold-rolled steel sheet described in 2-156042, Cu,
Due to the addition of Cr, workability deteriorates, and there is a problem in application to automotive parts that require press workability. Further, in steel sheets manufactured by the technique described in Japanese Patent Application Laid-Open No. 9-67626, the amount of P added is reduced as much as possible, and Ti is added to improve the corrosion resistance and press workability of the steel sheet. With a C content in the low carbon region, the press workability is sufficiently excellent, but there is no applicability up to the low carbon region.

The present invention advantageously solves the problems of the prior art, and does not require the addition of a large amount of elements such as Cu and P, which improve corrosion resistance, and is inexpensive and has excellent press workability and excellent perforation resistance. The purpose is to provide.

[0007]

Means for Solving the Problems The present inventors have made intensive studies in order to achieve the above-mentioned object. First, the present inventors observed a rust layer during corrosion of Cu-added low carbon steel sheet in the presence of Cl ^- ions. As a result, it was found that Cu was concentrated in the rust portion of the steel sheet surface, and that the rust layer had a uniform thickness and a small number of cracks, and that Cu prevented perforated corrosion. Furthermore, it has been found that by adding Cu at least 2.0 times the S content, it is possible to prevent S from adversely affecting the corrosion resistance and preventing corrosion from becoming a starting point of corrosion. This is considered to result in the formation of a rust layer that is very dense and has strong adhesion to the steel sheet surface, and as a result, the rust layer becomes a protective film and prevents the penetration of corrosion promoting substances (water, oxygen, chlorine, etc.). .

The present inventors have further studied and found that
The low-carbon steel sheet contains a specific amount of P, further reduces the S content to a certain level or less, and significantly improves the perforation resistance by adding a Cu content more than twice the S content. It has been found that the amount of P added to improve the corrosion resistance can be reduced, and the decrease in press workability can be minimized. In addition, we found that by reducing the Si content to a certain level or less and adjusting the Cr content to an appropriate amount, press workability was further improved, and it became a steel sheet for automobiles that combines press workability and perforation resistance. did.

The present invention has been made based on the above findings. That is, the present invention provides a method for preparing C:
0.001 to 0.06%, Si: 0.019% or less, Mn: 0.3% or less,
Al: 0.05% or less, S: 0.010% or less, P: 0.02 to 0.05
%, Cu: 0.01-0.2%, Cr: 0.03% or less, and the ratio of Cu content to S content, Cu (wt%) / S (wt%)
Is 2.0 or more, and the balance is Fe and unavoidable impurities, the steel sheet having excellent puncture resistance, and in the present invention, the steel sheet preferably has a surface treatment layer on the surface.

[0010]

First, the reasons for limiting the composition of the steel sheet of the present invention will be described. C: 0.001 to 0.06% C is desirably as small as possible from the viewpoint of securing the workability of the steel sheet and improving the corrosion resistance. However, excessively low carbonization will increase costs, so the lower limit is 0.0
01%. On the other hand, if it exceeds 0.06%, the press workability deteriorates and, for example, it becomes impossible to withstand the processing of automobile parts, so the upper limit is made 0.06%.

Si: 0.019% or less Si acts as a deoxidizing agent and is an element that increases the strength of steel and is an effective element for increasing the strength, but deteriorates the press workability. In the present invention, Si is reduced to 0.019% or less in order to obtain excellent press workability. Mn: 0.3% or less Mn acts as a deoxidizing agent and is an element necessary for steelmaking, but when contained in a large amount, the workability is deteriorated. For this reason, in the present invention, Mn is limited to 0.3% or less. In addition, it is preferably 0.10 to 0.22%.

Al: 0.05% or less Al is an element that acts as a deoxidizing agent and improves the cleanliness of steel. However, if it exceeds 0.05%, the corrosion resistance deteriorates.
For this reason, Al was limited to 0.05% or less. S: 0.010% or less S is an important element in the present invention. S combines with metal elements and the like in steel to form sulfide-based inclusions. In addition, the sulfide becomes a minute anode portion and becomes a starting point of corrosion. S
If the content exceeds 0.010%, the corrosion resistance, particularly the puncture resistance, is remarkably deteriorated, and the amount of Cu required for improving the corrosion resistance is increased, thereby increasing the production cost. The S content is 0.01
If it exceeds 0%, problems such as segregation occur, and expensive Cr, Ni or the like must be added to improve the problem. For this reason, S is limited to 0.010% or less. S is
Although it is necessary to reduce the amount to as low as 0.010% or less, desulfurization technology is limited and desulfurization beyond necessity is costly and economically disadvantageous. Therefore, it is desirable to set the amount to 0.001% or more. In addition, S is preferably 0.001 to 0.008%.

P: 0.02 to 0.05% P is an important element for improving the corrosion resistance by adding it in combination with Cu. However, addition of a large amount of P is effective for improving strength and increasing strength, but deteriorates press workability. In the present invention, by adjusting the P content to 0.02 to 0.05%, deterioration of press workability is suppressed, and corrosion resistance is remarkably improved by addition of a composite with Cu. P content
If it is less than 0.02%, the effect of improving corrosion resistance cannot be sufficiently obtained. Further, if the annealing conditions fluctuate after the cold rolling, there is a possibility that P may be concentrated on the surface and the phosphate chemical conversion property may be deteriorated, so the P content is limited to 0.05% or less. In addition,
Preferably the P content is between 0.02 and 0.04%.

Cu: 0.01 to 0.2% Cu is an important element for improving the corrosion resistance together with P. However, a large amount of Cu may deteriorate the surface properties of the steel sheet. When the Cu content is less than 0.01%, the effect of improving corrosion resistance by adding composite with P is not seen, and no improvement in puncture resistance is observed. On the other hand, if it exceeds 0.2%, the press workability is deteriorated and the production cost is increased. In addition, preferably 0.01 to
0.1%.

Cr: 0.03% or less Cr is an element that suppresses the deterioration of the surface properties due to the addition of Cu, but deteriorates the press workability. Therefore, Cr is 0.03
% Or less. By suppressing Cr to 0.03% or less, it is possible to suppress the deterioration of press workability due to the addition of P,
The synergistic effect with the improvement of the press workability due to the reduced content is seen, and the press workability is improved. However, excessively low Cr content is accompanied by an increase in cost, so it is preferable to set the Cr content to 0.01% or more.

Cu (wt%) / S (wt%): 2.0 or more Cu: 0.01 to 0.2%, S: adjusted to 0.01% or less, and Cu
(Wt%) / S (wt%): By adjusting the ratio to 2.0 or more, the effect of improving corrosion resistance by adding composite with P is remarkably exhibited while suppressing the deterioration of press workability. Cu / S is 2.
If it is less than 0, the improvement of corrosion resistance is insufficient.

The balance of the steel sheet for automobiles of the present invention comprises Fe and inevitable impurities. As unavoidable impurities, N
i: 0.01 to 0.02% may be contained. Ni is an element that improves the corrosion resistance of the steel material, and may be contained at 0.01 to 0.02%. Ni: Content of 0.01 to 0.02% does not cause deterioration of formability of steel material. Next, a method for manufacturing a steel sheet according to the present invention will be described. The steel sheet of the present invention is a hot-rolled sheet or a cold-rolled sheet subjected to cold rolling and annealing after hot rolling.

The molten steel having the above-described composition is smelted by a commonly known smelting method such as a converter, an electric furnace, a vacuum melting furnace, and is continuously cast.
Alternatively, it is preferable to use a steel material (slab) by using an ingot making method. The steel material is heated or rolled by hot rolling without heating. The heating temperature of the hot rolling need not be particularly limited, but is preferably 1000 to 1300 ° C. In the hot rolling, the finish rolling temperature is preferably 800 ° C. or more, and the winding temperature is preferably 500 ° C. or more. If the finish rolling temperature is lower than 800 ° C., the deformation resistance increases, the load on the rolling mill increases, and the crystallinity becomes remarkably large, deteriorating the mechanical properties of the steel sheet. When the winding temperature of the hot rolled sheet is lower than 500 ° C., the shape and press workability of the steel sheet deteriorate.

Although the hot-rolled sheet may be used as it is, the hot-rolled sheet may be further subjected to cold rolling and annealing to use as a cold-rolled sheet. The hot rolled sheet is pickled, cold rolled, and then annealed. The rolling reduction of the cold rolling is desirably 50 to 95% from the viewpoint of obtaining a uniform cold worked structure. As the annealing, either continuous annealing or batch annealing is suitable, but continuous annealing is preferable from the viewpoint of productivity, control of a cooling rate, and the like. The annealing temperature is preferably set to 700 ° C. or higher from the viewpoint of improving the press workability.

The steel sheet (hot-rolled sheet,
Alternatively, the cold-rolled sheet) may be used as it is, or may be subjected to a surface treatment to form a surface-treated layer on the surface. Examples of the surface treatment include various types of plating, a chemical conversion treatment, a coating treatment, and an organic film treatment on the plating layer. Above all, zinc-based plating is preferable for improving corrosion resistance. When the hot-rolled sheet is further subjected to a surface treatment, it is preferable to remove the oxide layer such as black scale by pickling and then perform the surface treatment.

The zinc-based plating is preferably any of hot-dip plating, electroplating, electroplating and vacuum plating, and may be subjected to a heat treatment after plating. By applying the zinc-based plating, the corrosion resistance of the steel sheet is further improved due to the sacrificial corrosion prevention action of zinc against iron. Further, by applying zinc-based plating, the time until rust is generated is increased, the time until porosity is generated is increased, and the service life of the steel sheet can be increased. Further improvement in corrosion resistance is possible by increasing the amount of plating, but excessive deposition is economically disadvantageous. Therefore, it is desirable to adjust the amount of plating in accordance with the required corrosion resistance. Since the steel sheet of the present invention has good perforation resistance of the steel sheet itself, the coating weight can be reduced. In the present invention, the zinc-based coating weight is desirably 1 to 180 g / m ² per one side.

The surface treatment by hot-dip galvanizing may be carried out by an ordinary method. A steel sheet or a steel strip is immersed in a hot-dip zinc bath in which pure Zn or a metal to be eutectoid is melted together with Zn, and then adhesion control such as gas wiping is performed. The galvanized layer is formed on the surface of the steel sheet by controlling the coating weight by means and cooling. It should be noted that a Zn-Fe alloy plating layer can be formed by heating after applying pure zinc plating.

For the surface treatment by electrogalvanizing, any of a sulfuric acid bath, a chloride bath and a pyrophosphate bath can be suitably used. Form a layer. As the surface treatment by vacuum deposition zinc plating, any of chemical vapor deposition, physical vapor deposition, sputtering and the like can be suitably used, and a zinc plating layer is formed on the surface of the steel sheet using these.

[0024]

EXAMPLE Steel having the composition shown in Table 1 was melted in a vacuum melting furnace and slab was formed by continuous casting. These slabs
It was heated to 1250 ° C and hot rolled to a finish temperature of 910 ° C to obtain a hot-rolled 3.5 mm sheet. Next, these hot rolled sheets were
After removing the scale on the surface of the steel sheet by pickling in 10% hydrochloric acid at 10 ° C., the steel sheet was cold-rolled to a sheet thickness of 0.8 mm to obtain a cold-rolled sheet. Subsequently, the cold-rolled sheet was subjected to electrolytic degreasing treatment to remove the rolling oil adhering to the surface, and then heated to 800 ° C. in a 5 vol% H ₂ —N ₂ atmosphere (dew point −30 ° C.) for 1 hour. Then, annealing was performed at 30 ° C./sec in an N ₂ atmosphere. These cold-rolled sheets were subjected to hot-dip galvanizing. The amount of hot-dip galvanized coating was 45 g / m ^2, and for one part of the plate,
It was varied in the range of 1 ~180g / m ^2. Further, a plated steel sheet was produced by an electroplating method and a vacuum deposition zinc plating method. Further, for a part, the hot-rolled sheet was pickled as it was or the hot-rolled sheet was pickled and plated.

These cold rolled sheets, hot-dip galvanized sheets, electrogalvanized sheets, and vacuum-deposited plated sheets were subjected to the following puncture resistance test and press workability test to evaluate corrosion resistance and press workability. (A) Perforation resistance test After two steel sheets were stacked and welded together, phosphate treatment (Nippon Parker Co., Ltd., PB-L3020), electrodeposition coating (Nippon Paint Co., Ltd., U-600) ) To give a test body. After mounting the test piece under the car floor and conducting a 5-year actual running test in North America, the welded part of the test piece was dismantled, and the sheet thickness reduction value after removing the rust at the joint corrosion part (mm) Was measured to evaluate the puncture resistance. The evaluation is that the thickness reduction value is 0.5 mm
Less than ○ (good), 0.5mm or more × (poor)
And (B) Press workability Using an Erichsen deep drawing tester, punch diameter: 33 mmφ
The wrinkle holding force was set to 500 kg, and the test was performed in a state where a normal rust-preventive oil was applied. The limit draw ratio (LDR) of each plate was measured to evaluate press workability. In addition, LDR is defined as the maximum blank diameter / punch diameter that can be drawn out.
Evaluation of press formability is ○ (excellent), △ (normal), × (poor)
The LDR was evaluated as × when the LDR was less than 1.8, Δ when the LDR was less than 1.8 to 2.0, and ○ when the LDR was 2.0 or more.

Table 2 shows the test results.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

[Table 4]

As is evident from the results shown in Table 2, in the case of the present invention, both the puncture resistance and the press workability exhibited good values, whereas in the case of the comparative example, these characteristics were good. Was degraded.

[0032]

As described above, the cold-rolled steel sheet of the present invention is excellent in puncture resistance and has no problem in press workability. For example, the steel sheet of the present invention is required to have high corrosion resistance of a steel sheet for automobiles. When used in such a location, the service life of the vehicle is prolonged, which has a remarkable industrial effect. In addition, when the steel sheet of the present invention having been subjected to surface treatment such as galvanization is used, since the anticorrosion effect of the surface treatment layer such as galvanization is lost, the steel sheet itself exhibits excellent perforation resistance, and as a whole, There is also an effect that the life in a corrosive environment becomes extremely long.

 ──────────────────────────────────────────────────続 き Continued on front page (72) Inventor Kazuo Mochizuki 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Co., Ltd. (72) Shigeo Kurokawa 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki 4K024 AA05 AA19 AB01 BA03 BB02 BC01 CA16 DA03 DB04 GA04 GA08 4K027 AA02 AA05 AA23 AB02 AB05 AB42

Claims (2)

[Claims] 1. Weight%: C: 0.001 to 0.06%, Si: 0.019% or less, Mn: 0.3% or less, Al: 0.05% or less, S: 0.010% or less, P: 0.02 to 0.05%, Cu: 0.01 0.2%, Cr: 0.03% or less, and the ratio of Cu content to S content, Cu (wt%)
/ S (wt%) is 2.0 or more, and the balance is Fe and unavoidable impurities. 2. The steel sheet according to claim 1, wherein the steel sheet has a surface treatment layer on a surface.