JPH0631467B2 - Metallic steel plate for automobile undercarriage - Google Patents

Description

TECHNICAL FIELD The present invention relates to a plated steel sheet which is excellent in weldability, corrosion resistance and chemical conversion treatment characteristics and is suitable for an automobile underbody member.

<Background Art> In order to meet the demand for improving the rust preventive performance of automobiles, which has become stronger in recent years, the introduction of plated steel sheets into body panel materials was first studied, and as a result, the durability thereof has been significantly improved. For this reason, as the next subject, the improvement of the corrosion resistance of the chassis of the automobile and the arms around the wheels, that is, the "underbody member" has been attracting much attention.

Since the undercarriage portion of this automobile is directly related to the safety of the vehicle body, measures are currently taken to increase the thickness of the underbody member when the corrosive environment is severe. However, on the other hand, there is also a strong demand for the thickness of the underbody member to be as thin as possible from the viewpoint of weight reduction, which leads to a reduction in fuel consumption.

Therefore, in order to satisfy both of them, measures were taken to improve the corrosion resistance by applying sufficient coating so as to enhance the durability without increasing the weight. However, automobile undercarriage members generally have a "closed structure" manufactured by forming and processing steel plates, and it is extremely difficult to apply sufficient coating to the inner surface, and the method called "painting" provides satisfactory corrosion resistance. It became clear that the improvement effect could not be obtained.

For this reason, it was considered to apply Zn-based plating to the underbody members as well as the body panel materials.However, since many underbody members are usually assembled by arc welding after forming, the evaporation of Zn during the welding. Therefore, it was found that the inconvenience that welding defects are likely to occur cannot be avoided.

<Means for Solving Problems> The inventors of the present invention have the above-mentioned problems, and have no fear of occurrence of welding defects (blow holes) due to evaporation of plating metal during welding. Moreover, we focused on “A-based plating,” which is said to be relatively good in terms of corrosion resistance, and examined the possibility of applying it to improve the corrosion resistance of automobile underbody parts. From the viewpoint of "the A-type plated alloy, which has been known in the past, is one step farther from the viewpoint that it has sufficient workability to withstand the corrosive environment that the car body receives, and has workability that is satisfactory as an automotive underbody material." Although it was dissatisfied and hesitated for automobile underbody parts, a specific amount of Mn was added to A or a specific amount of Fe, Co, Ni or Cr was added. The composition of the plating alloy If the slag is hardened, a plated alloy material capable of producing an A alloy-plated steel sheet having more excellent corrosion resistance and workability with good workability can be obtained. (B) However, the A alloy with the specific composition as described above is obtained. Almost no phosphating film is formed on the surface of the gold plating layer, and therefore the corrosion resistance after coating is not sufficient. (C) However, an extremely thin coating base plating on the surface of the A alloy plating layer of the above specific composition (Zinc plating, zinc-based alloy plating such as Zn-Ni alloy and Zn-Fe alloy, Fe-P alloy, Fe-
When plated with Ni-based alloys or iron-based alloys such as Fe-Mn alloys), the phosphate treatment is improved, the blister resistance after coating is improved, and the corrosion resistance is further improved. Almost no defects occur,
The knowledge was obtained.

Moreover, in this case, if the component composition of the base steel sheet is adjusted to "a material containing a small amount of Cr and containing Cu or Cu and Ni and further containing a trace amount of Ti or Nb", the two-layer plated steel sheet It was also confirmed that the corrosion resistance is further improved.

The present invention has been made based on the above findings, in which a steel plate for an automobile underbody member has Mn: 1 to 30% (hereinafter,% representing a component ratio is% by weight) on the surface of a base steel plate, Fe. , One or more of Co, Ni and Cr: 0.5 to 20% in total, A and unavoidable impurities: having a two-layer plating coating consisting of an alloy plating lower layer consisting of the rest and a coating base plating upper layer of 1 to 10 g / m ² . It has excellent corrosion resistance due to the structure, improves the durability of the automobile underbody without increasing the weight, and completely eliminates the concern about the occurrence of welding defects during welding assembly. Is to have.

The above-mentioned "coating undercoat" is generally known zinc plating, zinc-based alloy plating such as Zn-Ni alloy or Zn-Fe alloy, Fe-P alloy, Fe-Ni alloy or Fe-Mn alloy. Needless to say, it refers to a plating with good chemical conversion treatment, which is carried out for improving the adhesion of the coating film and suppressing the corrosion under the coating film, such as the iron-based alloy plating.

Next, in the steel sheet of the present invention, the reason why the component composition of the plating underlayer and the basis weight of the coating base plating layer are numerically limited as described above will be described.

A) Component composition of lower layer plating The component composition of the lower layer plating is adjusted so as not to cause welding defects at the time of welding and yet to provide sufficient corrosion resistance to the steel sheet, and the content ratio of each constituent component is as follows. It was limited for the reason. The thickness of the lower layer plating is not particularly limited, but if the basis weight is less than 5 g / m ² , the desired corrosion resistance may not be secured,
On the other hand, when the weight per unit area exceeds 40 g / m ² , welding defects are likely to occur, so that the preferable weight per unit area is 5 to 40 g.
/ M ² can be said.

(a) Mn The Mn component is added to improve the properties of the A-based plating layer, but if the content ratio is less than 1%, the current density of 20 A / dm ² or more is generally required today. In the high-efficiency plating, it is not possible to secure satisfactory plating properties, while if the content exceeds 30%, the workability becomes poor. Therefore, the Mn content ratio is set to 1 to 30%.

(b) Fe, Co, Ni and Cr These components have an equal effect to improve the corrosion resistance of the plating material, so they are added alone or in combination of two or more, but their total content is If less than 0.5%, the desired effect is not obtained on the other hand, while if it exceeds 20%, workability is deteriorated, so Fe, Co, Ni
And the content ratio of one or more of the Cr components is 0.5 to 20% in total.
I decided.

B) Coating undercoat plating layer The coating undercoat plating layer provided on the lower layer of the A alloy plating is a coating undercoating treatment (link improvement) to "improve adhesion of coating film" and "suppress corrosion under coating film" (link). Chemical conversion treatment such as acid salt treatment)
Zinc plating, which has excellent chemical conversion treatability and is sufficient for coating surface treatment, zinc-based alloy plating such as Fe-Zn alloy and Ni-Zn alloy, Fe-based alloy such as Fe-P alloy and Fe-Ni alloy Gold plating is preferred.

However, if the basis weight is less than 1 g / m ² , the plating layer becomes non-uniform, and the plating layer may disappear during the chemical conversion treatment, so that a sound chemical conversion coating may not be formed.
On the other hand, when the basis weight exceeds 10 g / m ² , defects such as blowholes are likely to occur during welding. Therefore, the basis weight of the coating base plating is limited to 1 to 10 g / m ² .

The steel sheet having the plating layer having the above-mentioned structure has sufficient performance for automobile underbody, but the base steel sheet is C: 0.03% or less, Cr: 2.0 to 7.0%, Cu: 0.05 to 1.0%. In addition to containing 1 type or 2 types of Ti: 0.05 to 1.0% and Nb: 0.05 to 1.0%, or additionally containing Ni: 0.05 to 1.0%, the balance is substantially Fe
When the composition of the component is adjusted, the corrosion resistance is further improved, and it is a very preferable material. In this case, the reason for adjusting the components of the base steel sheet as described above is as follows.

(A) C When the C content in the steel sheet is large, not only chromium carbide is formed and the mechanical properties and corrosion resistance are deteriorated, but also the corrosion resistance of the heat-affected zone formed during welding is adversely affected. However, by limiting the content to 0.03% or less, the above-mentioned adverse effects can be suppressed to a practically acceptable level. Therefore C
The content is preferably 0.03% or less, more preferably 0.01% or less.

(B) Cr Cr component has the effect of improving the corrosion resistance in the complex corrosive environment that the car body receives, especially when added together with Cu or Cu-Ni, the corrosion resistance improving effect becomes more remarkable, and the corrosion resistance Cr The content range is expanded, but C of less than 2.0%
With r content, the effect of improving corrosion resistance is insufficient, while 7.0%
If the content is more than%, the material after coating will show pitting corrosion, which is not preferable as a structural member.

(C) Cu Cu component has the effect of significantly improving the corrosion resistance of the steel sheet by adding it in the presence of Cr, but 0.05
When the content is less than%, a sufficient effect cannot be obtained for the above action,
On the other hand, if the content of Cu exceeds 1.0%, the effect of improving corrosion resistance is saturated and the cost is increased.
It is better to adjust it to 05-1.0%.

(D) Ti and Nb Since these components have the effect of improving the corrosion resistance of the steel sheet, it is preferable to contain 0.05 to 1.0% of each, either alone or in combination. In particular, Ti has a remarkable effect of stabilizing C in the steel and improving the corrosion resistance of the welded portion, and Nb has a function of stabilizing C in the steel and preventing the deterioration of the corrosion resistance due to the precipitation of chromium carbide. However, it also has the effect of significantly suppressing the progress of corrosion by precipitating as a corrosion product on the corroded surface, but in each case less than 0.05%, a sufficient addition effect cannot be obtained, while in all cases, it exceeds 1.0%. Even if it is contained, the effect is saturated and the cost is increased.

(E) Ni Ni component has the effect of further significantly improving the corrosion resistance of the steel sheet by adding it in combination with Cu in the presence of Cr, so it is recommended to add it when particularly high corrosion resistance is required. Preferable, however, if the content is less than 0.05%, sufficient effects cannot be obtained for the above-mentioned action, while if the content exceeds 1.0%, the corrosion resistance improving effect is saturated and the cost is increased. The amount should be adjusted to 0.05-1.0%.

Next, the present invention will be described by way of Examples in comparison with Comparative Examples.

<Example> Example 1 First, an ordinary hot-rolled steel sheet (Fe-0.01% C- with a thickness of 2.3 mm) was prepared.
0.05% Si-0.15% Mn-0.010% P-0.010% S) under the following molten salt bath electroplating conditions: A-Mn-X (X is Fe, Co,
One or more of Ni and Cr) alloy was plated at a basis weight of 10 g / m ² .

Bath composition: AC ₃ -NaC-KC, bath temperature: 210 ° C., additive material in bath: Mn (MnC _{2 with an} ionic concentration of 5 in the bath)
0 to 6000 ppm), X (as XC _n , 50 to 6000 pp in ion concentration in the bath)
m), current density: 30 A / dm ² .

Subsequently, various kinds of upper layer plating (coating base plating) was performed on the plated layer using the following acidic aqueous solution electroplating bath.

Zinc plating: Zinc sulfate-sodium sulfate based bath, Zn-Ni alloy plating ... Zinc sulfate-nickel sulfate-sodium sulfate based bath, Fe-Zn alloy plating ... Zinc sulfate-iron sulfate-sodium sulfate based bath, Fe-P alloy plating … Iron sulfate-sodium hypophosphite-based bath, Fe-Ni alloy plating ... Iron sulfate-nickel sulfate-sodium acetate-based bath.

The "bare corrosion resistance" and "corrosion resistance after cationic electrodeposition coating" of the two-layer plated steel sheet thus obtained were examined, and also the "blown hole property of the welded portion" was examined, and the results were analyzed. It is shown in Table 1 together with the composition.

The cationic electrodeposition coating was carried out after the usual zinc phosphate treatment, and the epoxy resin was coated to a coating film thickness of 20 μm.

Corrosion resistance test was carried out under the assumption of corrosive environment of automobile body, salt spray (35% 5% NaC, 4 hours) → dry (50
We performed a cycle test (CCT) consisting of (° C, 2 hours) → wetting (relative humidity: 95% or more, 50 ° C, 18 hours). The bare corrosion resistance was the corrosion depth after 40 days, and the corrosion resistance after painting was 8
The evaluation was made by "the blister width of coating film (blister width) at the coating cut flaw portion" after the lapse of 0 days.

In addition, the blowhole property of the welded portion was obtained by stacking plated steel sheets as shown in FIG. 1 and welding current: 130A, After performing butt welding with carbon dioxide gas arc or MIG under the conditions of welding voltage: 22 V, welding speed: 60 cm / min, the presence or absence of blowholes was evaluated by microscopic observation of the cross section of weld 1 and evaluated. … No blowhole, ×… confirmed blowhole, and displayed. The reference numeral 2 in FIG. 1 is a plated steel plate.

As is clear from the results shown in Table 1, the steel sheet of the present invention has excellent corrosion resistance in a corrosive environment to which an automobile body is subjected, and does not cause a defect in a welded portion, which is extremely excellent for an automobile underbody member. It can be seen that it has performance.

Example 2 A hot-rolled steel sheet was produced from a steel having a composition as shown in Table 2 by a conventional method, and then a coating of A-20% Mn-5% Fe alloy was applied on the surface as shown in Table 3. :
Apply at 10 g / m ² and further Zn-80% Fe alloy plating is applied to the surface at a weight of 5 g / m ² , the surface is not plated, and A-20% Mn-5% Fe alloy is applied to the surface.
It was applied at 10 g / m ² and further Zn-80% Fe alloy plating was applied thereon at a basis weight of 15 g / m ² , under the condition that the corrosion resistance and the blowhole property of the welded portion were investigated in the same manner as in Example 1. .

The results are also shown in Table 3.

From the results shown in Table 3, it is clear that, in the plated steel sheet according to the present invention, if the components of the base steel sheet are adjusted, even more excellent characteristics are exhibited for automobile underbody.

<Summary of Effects> As described above, according to the present invention, a steel sheet that exhibits excellent corrosion resistance in the environment in which an automobile is used and that also sufficiently satisfies other properties required for the automobile underbody member. It can be provided, and can contribute to the further improvement of the durability of the automobile, resulting in industrially useful effects.

[Brief description of drawings]

FIG. 1 is a schematic life diagram of welding means carried out for investigating the blowhole property of a welded portion. In the drawing, 1 ... welded portion, 2 ... plated steel sheet.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical display location C25D 5/26 E (56) References JP-A-58-11771 (JP, A) JP-A-63 -65061 (JP, A) JP-A-54-142133 (JP, A)

Claims (3)

[Claims] 1. An alloy plating lower layer consisting of Mn: 1 to 30% by weight, one or more of Fe, Co, Ni and Cr: 0.5 to 20% by weight in total, A and inevitable impurities: the rest on the surface of the base steel sheet. And a plated steel sheet for an automobile undercarriage, which has a two-layer plating coating of 1 to 10 g / m ² of a coating base plating upper layer. 2. The base steel sheet contains C: 0.03% or less, Cr: 2.0 to 7.0%, Cu: 0.05 to 1.0% by weight, Ti: 0.05 to 1.0%, Nb: 0.05 to 1.0. % Of one or two, and the balance of the composition is substantially Fe. The plated steel sheet for automobile underbody according to claim 1, wherein 3. The base material steel sheet contains C: 0.03% or less, Cr: 2.0 to 7.0%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0% by weight, and Ti: 0.05 to 1.0. %, Nb: 0.05 to 1.0% of 1 type or 2 types, and the balance is composed of a component composition consisting essentially of Fe. The plated steel sheet for an automobile underbody according to claim 1.