JP3132377B2 - Galvannealed steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an alloyed hot-dip galvanized steel sheet exhibiting excellent formability by exhibiting excellent sliding characteristics between a mold and a plating layer during press forming, and such an alloy. The present invention relates to an alloyed hot-dip galvanized steel sheet from which a galvannealed steel sheet can be obtained.

[0002]

2. Description of the Related Art In recent years, the use of surface-treated steel sheets in which the surface of a steel sheet has been plated has been increasing. Among these, alloyed hot-dip galvanized steel sheets have been used for their weldability, paintability, corrosion resistance after painting, and the like. Because of their superiority, they are frequently used in automobiles and home appliances. These products are manufactured through steps such as press forming, assembling, and painting. However, during press forming, cracks may occur in the alloyed hot-dip galvanized steel sheet, which is a material, and the product may not be a product.

[0003] These problems are caused not only by the insufficient mechanical properties of the material, but also by the poor sliding condition between the plating layer surface and the press die, and the material not flowing smoothly into the die. No. In order to cope with such a situation, the surface of a mold is conventionally subjected to chrome plating or the like. On the other hand, on the material side, improvements such as improvement of lubricating oil and uniform application have been made, and the phase structure of the plating layer has been improved.

The alloyed hot-dip galvanized layer has a Γ phase (Fe
₃ Zn ₁₀ ) or Γ ₁ phase (hereinafter collectively referred to as “Γ phase”), δ ₁ phase (FeZn ₇ ), and ζ phase (FeZn ₁₃ ). Of these alloy phases, the Γ phase has the highest iron content and is hard,
Then a [delta] _1-phase, zeta-phase is the smallest and most soft iron content. These alloy phases basically have a three-phase structure of Γ phase, δ ₁ phase and ζ phase from
When the degree of alloying is further advanced, the ζ phase disappears and the Γ phase and δ ₁
The phase changes to a two-phase structure.

If the softest ζ phase is formed on the surface of the plating layer, adhesion occurs due to sliding with the mold, frictional resistance increases, and material inflow into the mold is hindered. It is said that the material cracks easily. For these reasons, the purpose of changing the ζ phase to [delta] ₁ phase, as further advance the degree of alloying, and increasing the set temperature of the alloying furnace, the or to heat treatment conditions reduce the line speed Control has been implemented.

[0006]

However, in an alloyed hot-dip galvanized steel sheet manufactured by the above-mentioned method so as to increase the degree of alloying, it is the hardest in the coating layer.
There is a danger that the thickness of the phase is increased and the so-called powdering in which the plating layer peels off in a powdery state during press molding is caused. For these reasons, instead of excessively increasing the degree of alloying, galvannealed steel sheets that are appropriately controlled to suppress the formation of Γ phase and that also suppress the formation of ζ phase that causes material cracking The reality is that the realization of is strongly desired.

The present invention has been made in view of the above circumstances, and an object of the present invention is to form a good plating process by forming a good plating layer with stable sliding characteristics with a mold during press forming. An object of the present invention is to provide a galvannealed steel sheet exhibiting heat resistance.

[0008]

The alloyed hot-dip galvanized steel sheet of the present invention, which can solve the above-mentioned problems, comprises:
0.01% or less, Si: 0.5% or less, Mn: 2.0%
Below, S: 0.02% or less, sol. Al: 0.08% or less, each containing 0.005 to 0.1% of Ti and / or 0.005 to 0.1% of Nb, and P satisfying the following formula (1) So that the balance is Fe
And an alloyed hot-dip galvanized layer is formed on the surface of a steel sheet composed of unavoidable impurities, and the plated layer is
The gist is that the number of concave portions having a plating layer thickness of 2 μm or less is 70 or less per 3 mm in the horizontal length of the plating layer surface. [P] ≧ ([Ti] + [Nb]) × １ ／ (1) where [P], [Ti] and [Nb] are P and T, respectively.
The content (% by mass) of i and N is shown.

[0009] The alloyed hot-dip galvanized steel sheet having the above-described plating layer exhibits excellent sliding properties between the mold and the plating layer during press forming, and exhibits good formability. . In the alloyed hot-dip galvanized steel sheet of the present invention, B: 0.001 to 0.
003% may be contained, and the plating layer preferably has an Al content of 0.15 to 0.7%.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The body of an automobile, in particular, is press-formed through a complicated deformation mode, so that the alloyed hot-dip galvanized steel sheet used for it is required to have excellent formability. Therefore, carbon or nitride (meaning carbide, nitride and carbonitride) forming element such as Ti or Nb is compounded or added alone to ultra-low carbon steel, and C and N are fixed to form a solid solution state. A so-called IF steel sheet in which C and N are reduced as much as possible to improve the formability is often used as a base steel sheet of an alloyed hot-dip galvanized steel sheet. In addition, depending on the part, high strength is required in addition to excellent formability, but in such a case, an IF steel sheet to which Si, Mn, or the like is positively added is used as the base steel sheet.

Therefore, the present inventors first made the IF steel sheet a base steel sheet, and applied various alloyed hot-dip galvanizing to the base steel sheet.
The material cracking of the obtained galvannealed steel sheet was investigated. At this time, the mechanical properties of the base steel sheet and the degree of alloying of the plating layer were adjusted to be substantially the same. First, the cross-sectional structure of the plating layer that governs the sliding characteristics was examined in detail.

As a result, the uniformity of the thickness of the plating layer differs for each plated steel sheet. For example, as shown in FIG. Steel plate [FIG. 1 (a)] having fine irregularities peculiar to the layer [FIG. 1 (a)] and plating in which the thickness of the plating layer is extremely thin locally and the plating thickness is not uniform A plated steel sheet having a layer [FIG. 1 (b)] was observed, and some of the materials had a concave portion at an intermediate level between them.

Next, the present inventors have examined the relationship between the concave portion and the material crack. At this time, in order to quantify the generation amount of the concave portion, the number of the concave portion where the plating film thickness was 2 μm or less was observed per 3 mm in the horizontal direction length of the plating layer surface, and the relationship with the material crack was investigated. The reason why the portion having a plating film thickness of 2 μm or less is defined as a concave portion is that the portion having a plating film thickness of 2 μm or less can be clearly distinguished as an extremely thin portion with respect to a film thickness of about 4 to 8 μm of a normal galvannealed steel sheet. That's why.

FIG. 2 shows the result. As is clear from FIG. 2, as the number of recesses increases, material cracking increases,
When this number is 70 or less, cracking of the material is not recognized,
If it exceeds this, it will be understood that material cracking starts to occur.
Therefore, it is understood that the number of the concave portions needs to be 70 or less in order to effectively prevent the material crack.

The inventors of the present invention have conducted intensive studies on the mechanism of generation of the concave portions and specific means for preventing the generation of the concave portions, and have found the following. First, in the alloyed hot-dip galvanized steel sheet with a large number of recesses, a detailed observation of the cross-section of the coating layer from the plating stage to the alloying process showed that the ground iron crystal grain boundary was in the initial stage of plating or alloying. (A Zn-Fe alloy layer grown by an outburst reaction (a Zn-Fe alloying reaction that causes local abnormal growth of the Zn-Fe alloy layer) occurring in the (grain boundaries of the base steel sheet)) has a slow alloying reaction rate. It has been found that the above-mentioned concave portions are formed because zinc existing in the base iron crystal grains is diffused and moved by the action of alloying heating.
That is, the plating layer has a convex shape on the grain boundary of the base iron having a high alloying reaction rate, and has a concave portion within the crystal grain of the base iron having a low alloying reaction rate.

As described above, since the surrounding zinc diffuses and moves to the convex plating layer, the amount of zinc in this portion increases, and a large amount of ζ phase having a low iron concentration is generated on the plating layer surface in this portion. However, it is considered that the plating layer surface is in a state in which adhesion is likely to occur due to sliding with the mold, frictional resistance is increased, material inflow into the mold is inhibited, and material cracking is likely to occur. In addition, the reason why the alloying plating rate on the grain boundary of the base iron becomes large is that T
By the addition of carbon and nitride forming elements such as i and Nb, there is no solid solution C or N at the crystal grain boundaries, so that impurities are reduced and the state becomes extremely clean, and the reactivity with molten zinc is reduced. This is considered to be because the Zn—Fe alloy layer is generated and grows early when it becomes high. Therefore, in order to suppress the formation of the concave portions, it is considered necessary to reduce the difference in the alloying reaction rate between the base iron crystal grain boundary and the base iron crystal grain.

The present inventors have studied specific means based on such findings. First, in order to reduce the purity of the base iron grain boundaries in order to reduce the alloying rate at the base iron grain boundaries, attention was paid to P, which is one of the elements that are easily segregated at the grain boundaries, and various angles were set. Considered from. That is,
A steel sheet obtained by adding various amounts of P to various IF steels having different contents of Ti and Nb is used as a base steel sheet, and an alloyed hot-dip galvanized steel sheet is produced by forming an alloyed hot-dip galvanized coating on the surface of this steel sheet. A press forming test was performed on the plated steel sheet. The result is shown in FIG.

As is clear from FIG. 3, Ti in the base steel sheet
And the content of Nb (either Ti or Nb alone, or the total content of Ti and Nb) and the P content, a clear relationship is recognized.
As the content of P increases, the P content also needs to be increased. It can be seen that press cracking can be prevented if the P content is set so as to satisfy the above equation (1). As described above, it is necessary to set the P content in an appropriate range according to the content of Ti and Nb, because the cleanliness of the grain boundaries of the ground iron becomes higher and higher as the concentration of Ti and Nb increases. Therefore, it can be considered that the content of P needs to be increased in accordance with the content of Ti or Nb in order to reduce the purity.

The present invention is most characterized in that the P content in the steel sheet for galvannealed steel is appropriately adjusted in accordance with the content of Ti or Nb. To improve the sliding characteristics during press molding and prevent the occurrence of material cracks.
b, C, Si, Mn, S and so
l. The content of elements such as Al also needs to be appropriately adjusted. That is, as described above, the alloyed hot-dip galvanized steel sheet according to the present invention needs to have excellent press formability that can withstand severe press forming such as that performed in automobiles. However, some parts need to have high strength together with good press formability. The reasons for limiting the chemical components such as C, Si, Mn, S and sol.Al in the present invention are as follows.

C: 0.01% or less C is an element occupying a very important position in achieving excellent press formability. For this purpose, it is better that the content is as small as possible. If the amount exceeds the above range, the moldability is greatly reduced. Therefore, the upper limit of the content of C is set to 0.01%.

Si: 0.5% or less, Mn: 2.0% or less Si and Mn are elements necessary for increasing the strength of the IF steel sheet, but when these elements are excessive, the formability is reduced. In particular, Si forms an oxide on the surface of the steel sheet, prevents a plating reaction during hot-dip plating, and easily generates a so-called non-plated portion where a plated layer is not formed. From these viewpoints, Si is 0.5
% And Mn must be 2.0% or less.

S: not more than 0.02% If the content of S is excessive, the formability is reduced. Therefore, the content of S must be not more than 0.02%. sol. Al: 0.08% or less sol. Al acts as a deoxidizing agent, but if this amount is excessive, press formability is reduced.

Ti: 0.005 to 0.1% and / or Nb: 0.005 to 0.1% Ti and Nb are both extremely important elements for obtaining excellent press formability. If it is less than 0.05%, sufficient press formability cannot be obtained, and if it exceeds 0.1%, the recrystallization temperature increases, the crystal grains become finer, and the formability decreases. All amounts are 0.00
It was specified as 5 to 0.1%. These may be contained alone or in combination.

The chemical composition of the steel sheet for galvannealed steel according to the present invention is as described above, and the balance is composed of Fe and unavoidable impurities. It is also effective to contain B. The reasons for limiting the component range when B is contained are as follows.

B: 0.001% to 0.003% B is an element effective for preventing secondary working embrittlement caused by clean grain boundaries of the base iron of the IF steel sheet, and such properties are required. It is included when used for parts that use
In order to exert such an effect, the content needs to be 0.001% or more. However, if it is excessive, the press formability is reduced. Therefore, the content needs to be 0.003% or less.

By the way, the inventors of the present invention have proposed that
It is clear that the Al content in the plating layer is also related. FIG. 4 is a graph showing the relationship between the Al content in the plating layer and the number of recesses formed. As is clear from this figure, the number of concave portions greatly increases as the Al content increases, and when the Al content exceeds 0.7%, the number of concave portions decreases to 70, which causes material cracking during pressing. However, when the content is 0.7% or less, the number of concave portions generated is 70 or less, and it can be seen that the number of concave portions generated decreases as the Al content decreases. Therefore, in order to achieve good moldability by setting the number of concave portions formed on the surface of the plating layer to 70 or less, it is preferable that the Al content of the plating layer be 0.7% or less. However, when the Al content in the plating layer is small and becomes less than 0.15%, the reaction speed of alloying becomes extremely fast, and adversely affects plating resistance.
Phase growth cannot be suppressed. From such a viewpoint, the Al content in the plating layer is preferably 0.15 to 0.7%, and more preferably about 0.2 to 0.6%.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following Examples do not limit the present invention.
Modifications and alterations that do not depart from the spirit described below are all included in the technical scope of the present invention.

[0028]

EXAMPLES C: 0.0022%, Si: 0.01%, M
n: 0.15%, S: 0.006% and sol. Al:
As shown in Table 1 below, a steel sheet containing 0.025% as a basic component and having varied contents of Ti, Nb, and P was used as a steel sheet for galvannealing. Hot dip galvanizing treatment and alloying treatment were performed in a plating line to produce an alloyed hot dip galvanized steel sheet. About the obtained alloyed hot-dip galvanized steel sheet,
The number of recesses formed on the surface of the plating layer (the number per 3 mm length in the horizontal direction of the surface of the plating layer) was measured. The results are also shown in Table 1, and all the examples satisfying all the requirements specified in the present invention show excellent press formability and that the occurrence of material cracks is effectively prevented. I understand.

[0029]

[Table 1]

[0030]

According to the present invention, there is provided an alloy capable of improving the sliding characteristics of a plating layer with a mold to improve press formability and preventing the occurrence of material cracks during press forming. A hot-dip galvanized steel sheet can be realized, and this steel sheet is particularly useful for automobiles.

[Brief description of the drawings]

FIG. 1 is a view schematically showing the surface properties of an alloyed hot-dip galvanized steel sheet.

FIG. 2 is a graph showing the relationship between the number of concave portions present in a plating layer and material cracking.

FIG. 3 is a graph showing the effect of Ti, Nb and P concentrations in an original plate on material cracking.

FIG. 4 is a graph showing the relationship between the Al content in a plating layer and the number of recesses formed.

Continuation of front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 2/00-2/40 C22C 38/00 301 C22C 38/14

Claims (3)

(57) [Claims] 1. C: 0.01% or less (meaning by mass, the same applies hereinafter), Si: 0.5% or less, Mn: 2.0% or less,
S: 0.02% or less, sol. Al: 0.08% or less, and Ti: 0.005 to 0.1% and / or
Or Nb: 0.005 to 0.1%, P is contained so as to satisfy the following formula (1), and the balance is formed on the surface of a steel sheet composed of Fe and unavoidable impurities by alloying molten zinc.
The plating layer is formed by plating
The concave part whose layer thickness is 2μm or less is in the horizontal direction of the plating layer surface.
It is characterized by being 70 or less per 3 mm length
Alloyed hot-dip galvanized steel sheet. [P] ≧ ([Ti] + [Nb]) × １ ／ (1) where [P], [Ti] and [Nb] are P and T, respectively.
The content (% by mass) of i and N is shown. 2. The steel sheet further comprises: B: 0.001 to 0.001.
2. The composition according to claim 1, which contains 0.003%.
Metallized hot-dip galvanized steel sheet. 3. The plating layer has an Al content of 0.15.
The alloyed hot-dip galvanized steel sheet according to claim 1 or 2, wherein the content is 0.7% to 0.7%.