JP3318385B2 - Alloyed hot-dip galvanized steel sheet with excellent press workability and plating resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanized steel sheet which is excellent in press workability and peeling resistance and is used for an outer panel of an automobile body.

[0002]

2. Description of the Related Art In recent years, rust prevention of automobile bodies has been required to be enhanced, and zinc-based surface-treated steel sheets have been used as an improvement measure. Among them, the hot-dip galvanized steel sheet is particularly economically excellent, and further, the coating layer is made of Fe-Z
By using an n-alloy (alloyed hot-dip galvanized steel sheet), weldability and corrosion resistance after painting can be improved.

An alloyed hot-dip galvanized (hereinafter referred to as GA) steel sheet used for an outer panel of an automobile body is subjected to a high degree of press working for the purpose of enhancing design properties. ) And drawability (Rankford value, r value) are higher. As a solution to these problems, C, N, P, S
Numerous reductions have been developed along with optimal hot rolling and cold rolling processes.

On the other hand, the characteristics required for the plating layer of the GA steel sheet include powdering, which cannot follow the deformation of the steel sheet at the time of processing, powders and peels, and flaking, which is squeezed into a press die and peels in a scale-like manner. It is required that it does not easily occur. When these occur, they accumulate in the press die and generate press flaws. Alternatively, there is a problem in that the original anticorrosion property of the plating is lost. It is said that the plating layer of GA steel sheet generally consists of three Zn-Fe alloy phases of ζ, δ ₁ and Г in order from the low Fe content, and it is said that powdering is caused by the Г phase and flaking is caused by the ζ phase. ing.

By the way, when a GA steel sheet is manufactured by subjecting a C, N, P, S reduction material to hot dip galvanizing, the material of ductility and r value is satisfied, but alloying at the steel sheet grain boundary is excessively accelerated. It became clear that the amount of phase formation increased and the powdering resistance decreased. Then, in order to ensure powdering resistance, it is necessary to suppress the alloying degree (Fe concentration) that does not substantially generate a Г phase. However, in this case, for example, as disclosed in Japanese Patent Application Laid-Open No. Hei 2-11745, if only the Fe content is specified so that the Г phase is hardly generated, the ζ phase exists thickly on the plating surface layer depending on the manufacturing conditions, When ironing at the time of processing becomes strong, a new problem that flaking property easily occurs is caused.

[0006]

SUMMARY OF THE INVENTION In the present invention, a steel sheet obtained by reducing C, N, P, and S in steel to obtain a steel material is used as an alloy having both powdering resistance and flaking resistance. The purpose is to obtain a hot-dip galvanized steel sheet.

[0007]

According to the present invention, C: 0.00
15% by weight or less, Si: 0.1% by weight or less, Mn: 0.1% by weight or less.
03 wt% to 0.3 wt%, Al: 0.01 wt% to 0.1 wt%, P: 0.01 wt% or less,
S: 0.005% by weight or less; O: 0.005% by weight or less; N: 0.005% by weight or less;
3% by weight or less of Ti or 0.03% by weight or less of Nb
Fe: 9 wt% or more and 12 wt% or less, Al: 0.3 wt% or more, and 1 wt% or more of Fe: 9 wt% or more and 12 wt% or less on the steel sheet surface containing C / 12 ≦ Ti * / 48 + Nb / 93 ≦ C / 2. Excellent in press workability and plating peeling resistance, characterized in that an alloyed hot-dip galvanized layer containing 0.5% by weight or less and Pb: 0.1% by weight or less is formed in a range of 25 g / m ² or more and 70 g / m ² or less. Alloyed hot-dip galvanized steel sheet.

However, Ti * is Ti- (48N / 14 +
When 48S / 32) ≧ 0, Ti * = Ti− (48N /
14 + 48S / 32), Ti- (48N / 14 + 48S)
/ 32) <0, Ti * = 0. Further, the steel sheet composition may further contain B in an amount of 0.001% by weight or less.

[0009]

The following is a description of an alloyed hot-dip galvanized steel sheet which is an object of the present invention and is excellent in workability and plating peeling resistance. First, the components in the steel of the steel plate as the plating material are determined as follows in order to satisfy the required material and to produce economically.

C: C is an element which directly determines the strength of steel, and has extremely high workability (high El, r) which is the object of the present invention.
In order to obtain (Value), the smaller the content, the better, and it is necessary that the content is 0.0015% by weight or less. N, P, S: N, P, S are all dissolved in steel
Decrease l and r values. Therefore, as with C, the smaller the content, the better, respectively, 0.005% by weight, 0.01% by weight,
It is considered that 0.005% by weight or less is necessary.

O: If O is excessively present in steel, it precipitates as an oxide and lowers the El and r values. Therefore, 0.0
The upper limit is 05%. Mn: Mn is effective for compounding with S which forms a solid solution in steel, precipitating and rendering it harmless, and the effect of the material is small when a small amount of solid solution forms. However, when the content exceeds 0.3% by weight, the values of El and r gradually decrease. Therefore, it is set to 0.03% by weight or more and 0.3% by weight or less.

Si: Si, when present in a large amount in steel, like Mn, lowers the El and r values and inhibits plating wettability. Therefore, the upper limit is set to 0.1% by weight. Ti, Nb: Ti and Nb combine with C to form TiC, NbC
And improve workability. Therefore, the atomic ratio to C is required to be 1 or more. However, an excessive addition impairs economic efficiency, so the upper limit is set at 6 in atomic ratio. Also, the upper limit of each addition amount is preferably set to 0.03% by weight. However, since Ti is more easily combined with N and S than C, it is necessary to use a Ti amount obtained by subtracting N and S equivalents.

That is, the amounts of Ti and Nb satisfy the following equation. C / 12 ≦ Ti * / 48 + Nb / 93 ≦ C / 2 (where Ti * is Ti− (48N / 14 + 48S /
32) ≧ 0, Ti * = Ti− (48N / 14 + 4
8S / 32), (Ti-48N / 14 + 48S / 32)
When <0, Ti * = 0. ) Al: Al is required to be 0.01% by weight or more in order to prevent oxidation and loss of Ti and Nb when adding Ti and Nb, and to be harmless by combining with N and S in steel. effective. However, even if added in excess of 0.1% by weight, the effect is saturated and not economical.

Further, the steel sheet of the present invention more preferably further contains B in an amount of 0.001% by weight or less in addition to the above basic composition. B is effective for strengthening grain boundaries and improving spot weldability and secondary work brittleness. However, if added in excess of 0.001% by weight, the drawability is impaired, so the upper limit is made 0.001% by weight. next,
The reasons for limiting the components of the plating layer will be described.

An alloyed hot-dip galvanized steel sheet is prepared by immersing the steel sheet in a hot-dip galvanizing bath and then heating the steel sheet to diffuse Fe in the base steel sheet into the coating layer to form a Zn-Fe alloy layer. Thus, the corrosion resistance, the chemical conversion property, and the spot weldability are remarkably improved, and such a function is suitably achieved by setting the Fe content in the plating layer to 9% by weight or more. In addition, 9% by weight or more is necessary in order not to develop a ζ layer which causes flaking. On the other hand, if the Fe content exceeds 12% by weight, a hard and brittle Г layer develops even if the Al content in the plating layer is controlled in the range described below, and the press formability is impaired. Therefore, the Fe content in the plating layer is 9 to 12% by weight.
Must be:

The Al content in the plating layer affects the phase structure of the Zn—Fe alloy formed during the alloying treatment. If the amount is less than 0.3% by weight, the Г layer develops and powdering occurs. And more than 1.5% by weight makes it difficult to obtain a sufficient alloying. Therefore, the Al content in the plating layer is set to 0.3 to 1.5% by weight. Pb in the plating layer has an adverse effect on the corrosion resistance, and is therefore limited to 0.02% by weight or less.

The basis weight of the plating layer is 25 g from the viewpoint of corrosion resistance.
/ M ² or more, but if it is too thick, it will not be able to follow the deformation of the steel sheet during press forming and powdering will occur, so the upper limit is 70 g / cm ² . Although the method for producing the steel sheet of the present invention is not particularly limited, a preferred production example will be described below.

The molten steel adjusted to the above composition is formed into a slab by a continuous casting method, and is subjected to hot rolling and cold rolling to form a cold steel sheet. In hot rolling, the finishing temperature is Ar to obtain high workability.
_The temperature is preferably 850 to 920 ° C before and after the _three transformation points, and the winding temperature is preferably 600 ° C or higher. Further, it is preferable that the rolling reduction is 50% or more even in cold rolling. In hot-dip galvanizing, the surface of a steel sheet is first cleaned prior to annealing reduction, and this method may be degreasing, pickling, or the like, or may be combustion. The steel sheet is then annealed and reduced, but it is appropriate to use H ₂ containing several to several tens of percent of N ₂ as the atmosphere, and the dew point is preferably 0 ° C. or less. The annealing reduction temperature must be equal to or higher than the recrystallization temperature to secure the material, but is preferably 780 ° C. or higher in consideration of productivity.

After the steel sheet after the annealing reduction is cooled in a reducing gas, the steel sheet is directly introduced into a hot-dip galvanizing bath. The bath components and the temperature are determined as follows. Al concentration in bath: The present invention relates to Al-F formed in a plating bath.
The amount of the e-alloy layer is controlled to form an alloy mainly composed of the δ ₁ phase, and the powdering resistance and the anti-flaking property are ensured. Fifteen
g / m ² or more, and the amount of Al in the bath is 0.13.
% Or more is required, and 0.145% by weight or more is preferable in order to efficiently form an Al—Fe alloy layer. on the other hand,
If the amount of the Al—Fe layer exceeds 0.5 g / m ² in the amount of Al, the suppression of alloying becomes excessive and the productivity may be impaired. That is, in the plating after alloying, it is preferable to set the upper limit to 1.5% by weight of Al including Al in the plating other than the Al-Fe layer. Therefore, the upper limit of the Al concentration in the bath is 0.2% by weight.
Becomes

Pb concentration in bath: Unlike Al, Pb in the bath does not concentrate during plating during hot-dip plating. But,
If the concentration in the plating layer exceeds 0.1% by weight, corrosion resistance is concerned, and the upper limit of the Pb concentration in the bath is set to 0.1% by weight.
Further, the steel sheet of the present invention can be used as it is, in various forms such as automobiles, home appliances, building materials, etc., in a form subjected to various chemical conversion treatments such as pre-coating, post-coating, lamination, or chromate treatment, phosphate treatment, It is preferable to further apply a plating containing at least one of Fe, Zn, and Ni to the upper layer of the alloyed hot-dip galvanized layer because the corrosion resistance and the paintability are further improved.

The steel sheet after immersion in the plating bath is subjected to alloying treatment to obtain a GA steel sheet having an alloying degree (Fe) of 9 to 12%. As described above, an alloyed hot-dip galvanized steel sheet having excellent press workability and plating peeling resistance can be manufactured.

[0022]

EXAMPLES The effects of the present invention will be described below with reference to examples. A vertical hot-dip galvanizing test apparatus was used for plating, and a steel sheet of 70 mm × 200 mm was plated using 5% hydrogen-containing nitrogen as an annealing reducing gas. For the alloying treatment of the plating, a heating furnace of a type in which electric current was directly applied to the plated steel sheet to control the heating value of the resistance was used. The test steel sheet was melted in a vacuum melting furnace, hot rolled and cold rolled in advance to a sheet thickness of 0.7 mm, and subjected to electrolytic degreasing and hydrochloric acid pickling before being inserted into the plating apparatus. The finishing temperature change of the hot rolling was 900 ° C., and after the coil was once cooled, the heat history after the coil was wound was soaked at 700 ° C. for 1 hour, and the cold rolling was performed at a rolling reduction of 75% after the cooling and pickling.
Tables 1 and 2 show the components of the test steel sheet, and Table 3 shows the plating conditions, the composition of the plating layer before the alloying treatment, and the properties of the plated steel sheet after the alloying. In addition, the steel sheet material is CGL, and the steel sheet after cold rolling is annealed at 850 ° C. for 20 seconds and cooled during cooling for 5 seconds.
It was determined by heat treatment in a galvannealing cycle maintained at 00 ° C. for 30 seconds and is shown in Table 2 together with the components in the steel.

In the measurement of the amount of Al—Fe in Table 3, the plated steel sheet before the alloying treatment was immersed in fuming nitric acid to remove the zinc (η) phase, and the passivated Al—Fe alloy layer remaining undissolved was removed. After dissolving in hydrochloric acid, the amount of Al was measured by an atomic absorption method. In the evaluation of the plated steel sheet, elongation (El) and r value were determined from a tensile test as a steel sheet material, and powdering resistance and flaking resistance were determined as characteristics of the plated layer. The evaluation of the powdering property was performed by bending the alloyed plated steel sheet by 90 degrees, bending it back, collecting the peeled plating with a cellophane tape attached beforehand, and evaluating the amount in 1 to 5 stages based on the amount. 1 is good, 5
Is evil. Flaking resistance is 10% for steel sheets after alloying.
It was cut to a width of mm and tested by a bead type pull-out tester shown in FIG. The bead-type pull-out tester has a concave member 1 and a convex member 3
The test piece 2 is pulled out through a curved path between the test piece 2 and the test piece 2. The test piece was subjected to a pull-out test with no oil coating at a holding load of 100 kgf and a pull-out speed of 500 mm / min. The peeled plating was collected with a cellophane tape, and the presence or absence of flaking was visually evaluated in two stages of Δ ×.

As is evident from Table 3, according to the present invention, an alloyed hot-dip galvanized steel sheet having high workability and excellent plating peeling resistance can be manufactured.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[0029]

[0030]

According to the present invention, an alloyed hot-dip galvanized steel sheet having high press workability and excellent plating peeling resistance can be manufactured.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a bead type pull-out test.

[Explanation of symbols]

 Reference Signs List 1 concave material 2 test piece 3 convex member

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-9694 (JP, A) JP-A-4-116124 (JP, A) JP-A-4-276027 (JP, A) JP-A-3-3 281732 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.0015% by weight or less, Si: 0.1% by weight or less, Mn: 0.03% by weight or more and 0.3% by weight or less, Al: 0.01% by weight or more and 0.1% by weight %: P: 0.01% by weight or less, S: 0.005% by weight or less, O: 0.005% by weight or less, N: 0.005% by weight or less, and 0.03% by weight or less Ti or 0.
Fe: 9 wt% or more and 12 wt% or less, Al: 0 .1 wt% or less on the steel sheet surface containing at least one kind of Nb of not more than 03 wt% in a range satisfying C / 12 ≦ Ti * / 48 + Nb / 93 ≦ C / 2. Press workability, characterized in that an alloyed hot-dip galvanized layer containing 3% by weight or more and 1.5% by weight or less and Pb: 0.1% by weight or less is formed in an amount of 25 g / m ² or more and 70 g / m ² or less. Alloyed hot-dip galvanized steel sheet with excellent plating resistance. However, Ti * is Ti- (48N / 14 + 48S /
32) ≧ 0, Ti * = Ti− (48N / 14 + 4
8S / 32), Ti- (48N / 14 + 48S / 32)
When <0, Ti * = 0. 2. An alloyed hot-dip galvanized steel sheet having excellent press workability and plating releasability, further comprising 0.001% by weight or less of B in the steel sheet composition according to claim 1.