JP2001295017A - High strength hot-dip zincing steel sheet having good corrosion resistance and press-workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet plated with hot-dip zincing having metallurgical structure where martensite and retained austenite coexist in ferrite and good press-workability with 490-880 MPa tensile strength TS by the strengthening of the composite structure. SOLUTION: The high strength hot-dip zincing steel sheet having good corrosion resistance and press-workability is characterized in that on the high strength steel sheet containing by mass % 0.05-0.15% C, 0.3-2.00% Si, 1.5-2.8% Mn, <=0.03% P, <=0.02% S, 0.05-0.5% Al, <=0.006% N and if necessary, containing 0.0002-0.002% B and the balance Fe with inevitable impurities and further, when %C, %Si and %Mn show C, Si, and Mn contents, respectively, satisfying (%Mn)/(%C)>=15 and (%Si)/(%C)>=4, the hot-dip zincing layer containing 0.05-0.5 mass % Al, 5-15 mass % Fe and further, one or more kinds among 0.1-10 mass % Ni, 0.05-3 mass % Cu and 0.05-1 mass % Mg and the balance Zn with inevitable impurities, is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength galvannealed steel sheet and a method for producing the same, and more particularly, has excellent workability, and is applicable to various uses, for example, steel sheets for building materials and automobiles. It relates to a plated steel sheet that can be formed.

[0002]

2. Description of the Related Art A galvannealed steel sheet is known as a coated steel sheet having good corrosion resistance. This alloyed hot-dip galvanized steel sheet is usually degreased, then preheated in a non-oxidizing furnace, subjected to reduction annealing in a reducing furnace to clean the surface and secure the material, and immersed in a hot-dip zinc bath, It is manufactured by alloying after controlling the amount of adhesion. It is widely used mainly in automobiles, building materials and the like because of its excellent corrosion resistance and plating adhesion.

In recent years, particularly in the field of automobiles, it has become necessary to increase the strength of plated steel sheets in order to ensure a function of protecting an occupant in the event of a collision and to achieve a reduction in weight for the purpose of improving fuel efficiency. In order to increase the strength of the steel sheet without deteriorating the workability, it is effective to add elements such as Si, Mn, and P. However, the addition of these elements generally deteriorates the wettability of the steel sheet surface, It becomes difficult to apply hot-dip galvanizing. Therefore, in order to perform hot-dip galvanizing on a steel sheet to which elements such as Si, Mn, and P are added,
For example, as disclosed in Japanese Patent Application Laid-Open No. 55-122865, Si or M
A method of suppressing generation of an oxide layer of elements such as n and P and improving plating adhesion has been used. Further, as a high-strength galvanized steel sheet manufactured by using such a plating method, JP-A-6-192807 and JP-A-6-192807 are known.
192854, etc. are listed.

[0004] Further, since galvannealed steel sheets are generally used after being coated from the viewpoint of design and corrosion resistance, corrosion resistance after coating is important.

[0005]

However, in the above-mentioned and other high-strength galvanized steel sheets disclosed above, neither the strength nor the corrosion resistance of the steel sheet is sufficiently ensured.

Accordingly, the present invention solves the above-mentioned problems and has a metal structure in which martensite and retained austenite are mixed in ferrite, and a tensile strength TS of 490 to 880 MPa by strengthening the composite structure. An object of the present invention is to provide a steel sheet subjected to alloyed hot-dip galvanizing with good workability.

[0007]

Means for Solving the Problems The present inventors disclosed in Japanese Patent Laid-Open No.
Japanese Patent Application Publication No. 1-279691 proposes a high-strength galvannealed steel sheet having good workability and a method for producing the same. Further, the present inventors have conducted intensive studies on the plating treatment of a high-strength steel sheet, and as a result, as a plating layer, Al: 0.
0.5 to 0.5% by mass, Fe: 5 to 15% by mass, and N
i: 0.1 to 10% by mass, Cu: 0.05 to 3% by mass,
Mg: one or two or more of 0.05 to 1 mass%, and the balance has an alloyed hot-dip galvanized layer composed of Zn and unavoidable impurities. The inventor of the present invention has found that a plated steel sheet can be obtained.

That is, the gist of the present invention is as follows.
It is as shown below.

(1) In mass%, C: 0.05 to 0.1
5%, Si: 0.3 to 2.0%, Mn: 1.5 to 2.8
%, P: 0.03% or less, S: 0.02% or less, Al:
005-0.5%, N: 0.006% or less, the balance consisting of Fe and unavoidable impurities,
When Si and% Mn are C, Si and Mn contents, respectively, (% Mn) / (% C) ≧ 15 and (% Si) / (%
C) On a high-strength steel sheet satisfying ≧ 4, Al: 0.0
5 to 0.5% by mass, Fe: 5 to 15% by mass, Ni: 0.1 to 10% by mass, Cu: 0.05 to 3%
Good corrosion resistance and press working characterized by having an alloyed hot-dip galvanized layer containing one or more of 0.05 to 1% by mass of Mg and 0.05 to 1% by mass, with the balance being Zn and unavoidable impurities. High strength alloyed hot-dip galvanized steel sheet with good properties.

(2) B: 0.0002% by mass
The high-strength galvannealed steel sheet according to the above (1), wherein 0.002% is contained in the steel sheet with good corrosion resistance and good press workability.

(3) The above-mentioned (1) or (2), wherein the metal structure of the steel sheet contains martensite and retained austenite in a volume ratio of 3% or more and 20% or less.
2. A high-strength galvannealed steel sheet having good corrosion resistance and good press workability as described in 1).

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.

First, C, Si, Mn, P, S, Al,
The reasons for limiting the numerical values of N and B will be described. C is an essential element when the steel sheet is to be strengthened by strengthening the structure by martensite or retained austenite. The reason why the content of C is set to 0.05% or more is that if C is less than 0.05%, cementite or pearlite is difficult to be rapidly cooled from the annealing temperature by using mist or jet water as a cooling medium in a galvanizing line. This is because it is easy to produce and it is difficult to secure the required tensile strength. On the other hand, the reason why the content of C is 0.15% or less is that if C exceeds 0.15%, it becomes difficult to form a sound welded portion by spot welding, and at the same time, segregation of C becomes remarkable and workability is increased. Is deteriorated.

Si is added in an amount of 3 to 2.0% as an element for increasing the strength without significantly impairing the workability of the steel sheet, particularly the elongation, and is set to be at least 4% by mass of the C content. The reason why the content of Si is set to 0.3% or more is that if the content of Si is less than 0.3%, it is difficult to secure the required tensile strength.
The reason for setting the content of i to 2.0% or less is that the content of Si is 2.0% or less.
%, The effect of increasing the strength is saturated and the ductility is reduced. Further, by setting the content by weight of 4 times or more of the C content, the progress of pearlite and bainite transformation is remarkably slowed by reheating for the alloying treatment performed immediately after plating, and the volume ratio is 3 to 3 even after cooling to room temperature. A metal structure in which 20% of martensite and retained austenite are mixed in ferrite can be obtained.

Mn and C are added in an amount of 1.5% or more for the purpose of stabilizing austenite before immersing the steel strip in the plating bath in order to lower the free energy of austenite. Further, by adding 15% by weight or more of the C content, the progress of pearlite and bainite transformation is remarkably slowed by reheating for the alloying treatment performed immediately after plating, and 3% by volume even after cooling to room temperature. A metal structure in which about 20% of martensite and retained austenite are mixed in ferrite can be obtained. However, if the amount is too large, the slab is liable to crack and the spot weldability deteriorates, so the upper limit is 2.8%.

P is generally contained in steel as an unavoidable impurity, but if its amount exceeds 0.03%, the spot weldability deteriorates remarkably and the tensile strength as in the present invention exceeds 490 MPa. In a high-strength steel sheet, the cold rolling property is significantly deteriorated together with the toughness.
% Or less. S is generally contained in steel as an unavoidable impurity. However, if the amount exceeds 0.02%, the presence of MnS elongated in the rolling direction becomes remarkable, which adversely affects the bendability of the steel sheet. 0.02% or less.

Al is used as a deoxidizing element of steel, and is used in order to improve the material quality by suppressing grain refinement of a hot-rolled material by AlN and suppressing coarsening of crystal grains in a series of heat treatment steps.
It is necessary to add at least 05%. However, if it exceeds 0.5%, not only the cost is increased, but also the surface properties are deteriorated, so the content is made 0.5% or less.

N is also generally contained as an unavoidable impurity in steel, but if its amount exceeds 0.006%, the brittleness is deteriorated as well as the elongation. Therefore, its content is made 0.006% or less.

B is generally known as an element that increases hardenability, and delays the transformation of pearlite and bainite during reheating for alloying treatment, so that after cooling to room temperature, 3 to 20% by volume of 0.0002% or more is added for the purpose of facilitating the formation of a metal structure in which sites are mixed in ferrite. However, if the addition amount exceeds 0.002%, the ferrite does not grow to a sufficient volume ratio even when the temperature is slowly cooled from the temperature range of two phases of ferrite and austenite to 650 ° C, and the ferrite and austenite do not grow from 650 ° C to the plating bath. Austenite is transformed into martensite during the cooling process, and then martensite is tempered by reheating for alloying treatment to precipitate cementite. Therefore, it is difficult to achieve both high strength and good press workability.

In addition, Nb, T
Even if i, Mo, Cu, Sn, Zn, Zr, W, Cr, and Ni are contained in a total amount of 1% or less, the effect of the present invention is not impaired, and depending on the amount, the corrosion resistance may be improved.

Next, the alloyed hot-dip galvanized layer will be described.

In the present invention, the reason why the Al composition of the galvannealed layer is limited to 0.05 to 0.5% by mass is as follows.
If the content is less than 0.05% by weight, Zn-F
e alloying proceeds too much and a brittle alloy layer develops too much at the interface of the base iron, and the plating adhesion deteriorates.
This is because, if it exceeds 300, the Fe—Al—Zn-based barrier layer is formed too thick and alloying does not proceed during the alloying treatment, so that plating with a desired iron content cannot be obtained.

The reason for limiting the Fe composition to 5 to 15% by mass is that if the Fe content is less than 5% by mass, soft Zn
This is because a -Fe alloy is formed to deteriorate press formability, and if it exceeds 15% by mass, a brittle alloy layer develops too much at the interface of the base iron and the plating adhesion deteriorates.

Further, for the purpose of improving corrosion resistance after painting, N
i: 0.1 to 10% by mass, Cu: 0.05 to 3% by mass,
Mg: One or two or more of 0.05 to 1% by mass are added. The reason for the addition of these elements to improve the corrosion resistance after coating is that the thin film formed on the surface of the plating layer further exhibits a passivation tendency and slows the corrosion of the plating layer under the coating film. The passivation tendency described above suppresses the reaction at the interface between the plating layer and the coating film, and contributes to the stabilization of the coating film. It is considered that the plating layer surface has an anchoring effect on the coating film due to the presence of fine irregularities.

The effect of improving corrosion resistance after painting is as follows: Ni,
For Cu and Mg, the effect starts to be remarkable at 0.1, 0.05, and 0.05% by mass or more, respectively. However, when the amount of addition increases, the appearance after plating becomes coarse and poor appearance occurs due to, for example, dross or oxide adhesion. Therefore, the upper limit of each element is 10, 3, or 1% by mass.

The method of adding Ni, Cu, and Mg to the plating is not particularly limited. A method of adding Ni, Cu, or Mg to a plating bath, or a method of adding these metals or Zn containing these metals in advance to the surface. A method in which a steel sheet is immersed in a hot-dip Zn plating bath and mixed in an alloying process can be used. In addition, the plating layer may contain Sb, Pb, or the like, alone or in combination, in an amount of 0.5% by mass or less. There is no particular limitation on the amount of the galvannealed galvanized coating, but it is preferably 20 g / m ² or more from the viewpoint of corrosion resistance and 150 g / m ² or less from the viewpoint of economy.

In the present invention, the method for producing a plated steel sheet is not particularly limited, and a usual hot-dip plating method using a non-oxidizing furnace can be applied. However, when annealing in a continuous hot-dip galvanizing equipment of an in-line annealing method, the annealing temperature is in a region where two phases of ferrite and austenite coexist.
The conditions for the alloying treatment are not particularly defined.
A range of 60 to 550 ° C and a processing time of 10 to 40 seconds is appropriate for practical operation.

[0028]

The present invention will be described below in detail with reference to examples.

(Example 1) First, test materials shown in Table 1 were prepared, and an alloyed hot-dip galvanized steel sheet having a plating composition as shown in Table 2 was prepared using an in-line annealing continuous hot-dip galvanizing equipment. Manufactured. Plating is the amount of Mg in the bath at 460 ° C,
Al content, for 3 seconds dip plating with Zn-Mg-Al plating bath was varied and was adjusted coating weight on one side 40 g / m ² by N ₂ wiping. Thereafter, an alloying treatment was performed in an alloying furnace for 20 seconds. The addition of Cu and Ni during plating was carried out by using a steel sheet plated with these metals in advance and diffusing it with Fe in the alloying process.

The tensile strength and elongation were determined by cutting a JIS No. 5 test piece from the plated steel strip produced in this manner and performing a tensile test at room temperature. 490 tensile strength
MP and above were accepted, and elongation was 25% or more.

The test piece for evaluating corrosion resistance is 150 × 70
mm, and was first subjected to a zinc phosphate-based dip-type chemical conversion treatment. Subsequently, a cationic electrodeposition coating of 20 μm is applied.
Further, after a middle coat 35 μm and a top coat 35 μm were applied, the back surface and the end were sealed with an insulating tape. For corrosion resistance test, SS
CCT using one cycle of T6 hr → dry 4 hr → wet 4 hr → frozen 4 hr was used. The post-painting corrosion resistance was evaluated by applying a crosscut to the painted surface with a cutter to reach the base steel, and measuring the blister width after 60 cycles of CCT. The rating was "good" or more. (Blistering width) ： １: less than 1 mm ： １: 1 mm or more and less than 3 mm Δ: 3 mm or more and less than 5 mm ×: 5 mm or more

The evaluation results are as shown in Table 2, and the materials of the present invention were all high-strength galvannealed steel sheets having good corrosion resistance and good press workability.

[0033]

[Table 1]

[0034]

[Table 2]

(Example 2) First, a test material shown in H of Table 1 was prepared, and alloyed hot-dip galvanizing having a plating composition as shown in Table 3 was performed using an in-line annealing continuous hot-dip galvanizing equipment. A steel plate was manufactured. Plating is performed using Mg in a bath at 460 ° C.
Hot-dip plating was performed for 3 seconds in a Zn—Mg—Al plating bath in which the amount and the amount of Al were changed, and the amount of plating was adjusted to 40 g / m ² on one side by N ₂ wiping. Then, 2 in the alloying furnace
An alloying treatment was performed for 0 seconds. Cu, Ni during plating
Was added by using a steel sheet plated with these metals with a different amount of adhesion in advance and by diffusing it with Fe in the alloying process.

The test piece for evaluating the corrosion resistance is 150 × 70
mm, and was first subjected to a zinc phosphate-based dip-type chemical conversion treatment. Subsequently, a cationic electrodeposition coating of 20 μm is applied.
Further, after a middle coat 35 μm and a top coat 35 μm were applied, the back surface and the end were sealed with an insulating tape. For corrosion resistance test, SS
CCT using one cycle of T6 hr → dry 4 hr → wet 4 hr → frozen 4 hr was used. The post-painting corrosion resistance was evaluated by applying a crosscut to the painted surface with a cutter to reach the base steel, and measuring the blister width after 60 cycles of CCT. The rating was "good" or more. (Blistering width) ： １: less than 1 mm ： １: 1 mm or more and less than 3 mm Δ: 3 mm or more and less than 5 mm ×: 5 mm or more

As to the plating adhesion, the powdering property was examined, and the case where the peel width exceeded 3 mm was rejected.

The press workability was evaluated by pulling out a steel plate at a constant speed while applying a load by sandwiching the steel plate with a mold imitating a bead formed by press forming. In the evaluation, the pressing load was set to 600 kg, and the one that pulled out was accepted, and the one that generated galling and the plate was broken was rejected. The evaluation results are as shown in Table 3. The materials of the present invention were all high-strength galvannealed steel sheets having good corrosion resistance and good press workability.

[0039]

[Table 3]

[0040]

As described above, according to the method of the present invention, it is possible to produce a high-strength galvannealed steel sheet having good corrosion resistance and good press workability.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Koji Sakuma 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu Works F-term (reference) 4K027 AA05 AB02 AB43 AC52 AC73

Claims (3)

[Claims] 1. A mass% of C: 0.05 to 0.15%,
Si: 0.3 to 2.0%, Mn: 1.5 to 2.8%,
P: 0.03% or less, S: 0.02% or less, Al: 00
5 to 0.5%, N: 0.006% or less, the balance F
e and unavoidable impurities, further containing% C,% S
When i and% Mn are respectively C, Si and Mn contents, (% Mn) / (% C) ≧ 15 and (% Si) / (% C)
On a high-strength steel sheet satisfying ≧ 4, Al: 0.05 to
0.5% by mass, Fe: 5 to 15% by mass, Ni: 0.1 to 10% by mass, Cu: 0.05 to 3% by mass, Mg: 0.05 to 1% by mass Alternatively, a high-strength galvannealed steel sheet having good corrosion resistance and good press workability, characterized by having an alloyed hot-dip galvanized layer containing two or more kinds and the balance consisting of Zn and unavoidable impurities. 2. B: 0.0002 to 0.00% by mass.
The high-strength galvannealed steel sheet according to claim 1, wherein the steel sheet contains 2% of good corrosion resistance and good press workability. 3. The metal structure of the steel sheet has a volume ratio of 3% or more to 20% or more.
The high-strength galvannealed steel sheet having good corrosion resistance and good press workability according to claim 1 or 2, wherein the steel sheet contains martensite and retained austenite of not more than 3%.