JP3347152B2 - Method for producing cold-rolled high-strength hot-dip galvanized steel sheet with excellent resistance to pitting corrosion - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled high-strength hot-dip galvanized steel sheet having a low yield ratio, a high ductility, and a low yield ratio, which is excellent in pitting corrosion resistance.

[0002]

2. Description of the Related Art In the automotive industry, high-strength steel sheets having excellent corrosion resistance and workability have been demanded for the purpose of preventing rust and reducing the weight of a vehicle body. Generally, ductility such as elongation and bending decreases with increasing strength such as yield point and tensile strength. Therefore, a high-strength steel sheet whose strength is enhanced by utilizing solid solution strengthening, precipitation strengthening, or the like becomes insufficient for processing applications.

[0003] As a high-strength steel sheet developed from such a background, there is a cold rolled high-strength hot-dip galvanized steel sheet with a composite structure.
For example, JP-B-63-3930 and JP-A-56-47555 disclose that the strength and elongation are both improved by using a C-high Mn steel and forming the steel structure into a composite structure of ferrite and martensite. Base material steel plates have been proposed.

[0004]

The manufacturing technology of the composite structure steel sheet has been developed to simultaneously impart the strength and elongation to the high strength steel sheet for automobiles, and the thinning, that is, the weight reduction of the steel sheet for automobiles will be achieved. It is assumed that. Although the use of these steel sheets makes it possible to reduce the thickness of the steel sheet from the viewpoint of strength, if the thickness is reduced, corrosion due to perforation of the steel sheet becomes a problem due to corrosion. This problem also becomes apparent with hot-dip galvanized steel sheets.

That is, in the hot-dip galvanized steel sheet, the alloy layer formed by the reaction of the hot-dip zinc and the ground iron bonds the zinc layer and the ground iron. An alloy layer having improved adhesion is often used, but this alloy layer is hard and brittle. For this reason, when the plating layer becomes thicker, the workability of the hot-dip galvanized steel sheet is determined not by the workability of the ground iron but by the workability of the plating layer. Therefore, even if the workability of the base metal is improved, high workability cannot be obtained, and peeling and damage of the plating layer become noticeable during processing, exposing the base iron to rust and easily corroding corrosion. The problem that it arises becomes apparent.

Accordingly, the present invention improves the corrosion resistance and workability of the composite structure steel sheet itself, and obtains a low yield ratio cold-rolled high-tensile hot-dip galvanized steel sheet having excellent workability with excellent pitting corrosion resistance. The purpose is to:

[0007]

According to the present invention, the weight%
C: 0.02 to 0.25%, Si: 2.0% or less, Mn: 1.6 to 3.
5%, P: 0.03-0.20%, S: 0.02% or less, Cu: 0.05-
2.0%, sol. Al: 0.005 to 0.100%, N: 0.008% or less,
In some cases, Ti: 0.005-0.06% or N
b: contains at least one or more of 0.005 to 0.06% and / or at least one or more of Ni: 2.0% or less, Mo: 3.0% or less, and Cr: 3.0% or less, and in some cases, further contains B: 0.0003 to 0.005%. A steel slab containing iron and unavoidable impurities is hot-rolled, pickled, cold-rolled to the target thickness, and then passed through an in-line annealing type continuous hot-dip galvanizing line. In the line
Provided is a method for producing a hot-dip galvanized steel sheet having a low yield ratio and a low yield ratio, which is excellent in pitting corrosion resistance, which is performed by continuous annealing at a temperature of 720 to 950 ° C and hot-dip galvanizing.

[0008]

[Action] Mn, Ni, Cr which improves the hardenability of low carbon steel
The addition of alloying elements such as these can provide a composite structure steel of ferrite and martensite through continuous heat treatment in a continuous hot-dip galvanizing line of the in-line annealing type, and the inclusion of appropriate amounts of Cu and P reduces the corrosion resistance of this steel. Improve. Furthermore, the inclusion of trace amounts of Ti and Nb can improve ductility,
To increase the strength and improve the corrosion resistance, it is effective to add Si, Mn and Ni, Mo, Cr. By these combined actions, it is possible to obtain a cold-rolled high-tensile galvanized steel sheet having a low yield ratio and excellent workability and corrosion resistance.

Further, if the cold-rolled coil before passing through the continuous hot-dip galvanizing line is subjected to iron plating in advance by continuous electroplating, non-plating seen in the case of adding Si can be further avoided. A hot-dip galvanized steel sheet having good adhesion can be obtained.

After hot-dip galvanizing, the plating layer is
When the alloy layer is formed between the base steel sheet and the temperature range of 0 to 650 ° C., the coating film adhesion and the lap resistance weldability can be improved.

The actions of the various components of the hot-rolled base steel sheet according to the method of the present invention and the reasons for limiting the content range thereof are as follows.

C is an element effective for obtaining a composite structure composed of ferrite and martensite and improving the strength. In order to obtain the target composite structure, C should be set at 0.
More than 02% is required, but if it exceeds 0.25%, ductility and weldability deteriorate. Therefore, the lower limit is 0.02% and the upper limit is 0.25%
And

Si is a preferable element for improving the strength of the steel without impairing the workability, and has an effect that a composite structure can be obtained by promoting the formation of ferrite in a cooling process from high-temperature austenite during annealing. However, when Si exceeds about 2.0%, this effect saturates and becomes hard and ductility deteriorates. Therefore, the upper limit is set to 2.0%. On the other hand, according to the study of the present inventors, in the in-line annealing type continuous galvanizing line, when Si in the steel is set to about 0.1 or more, non-plating is easily caused. Therefore, Si is 0.1%
It is desirable in this sense to make it less than this. However, this problem can be completely eliminated if iron plating with an adhesion amount of about 2 g / m ² is applied by electroplating prior to the continuous galvanizing line passing.

Mn is an element effectively acting to improve the hardenability of steel and obtain a composite structure. 1.6% of Mn content
If it is less than 3.5%, a composite structure having low yield ratio characteristics cannot be obtained, while if it exceeds 3.5%, workability and weldability are reduced.
Therefore, the lower limit is set to 1.6% and the upper limit is set to 3.5%.

P and Cu are characteristic elements in the present invention, and the corrosion resistance is significantly improved by the combined addition of these elements. In order to improve the corrosion resistance, P must be at least 0.03% and Cu must be at least 0.05%. On the other hand, P is 0.20%, C
Even if u exceeds 2.0%, the effect of improving corrosion resistance is saturated,
Ductility deteriorates. Therefore, P is 0.03-0.2%, Cu is 0.05
To 2.0%.

S is an element which is essentially harmful to the steel of the present invention, and is desirably as small as possible. However, up to 0.02% can be tolerated.

Al must be present in an amount of 0.005% or more to play a role as a deoxidizing agent, but if it exceeds 0.10%, Al ₂ O ₃
The lower limit was set to 0.005%, and the upper limit was set to 0.10% because inclusions such as these increased and deteriorated workability and surface quality.

N is an essentially harmful element for the steel of the present invention, and the smaller the amount, the better. However, N is allowable up to 0.008%, so N is set to 0.008% or less.

In the present invention, the N content of up to 2.0%
The strength and corrosion resistance of the steel sheet can be improved by adding one or more of i, Mo up to 3.0%, and Cr up to 3.0%.

Ni effectively acts to prevent hot brittleness and improve corrosion resistance due to Cu, but if it exceeds 2.0%, the effect is saturated and the manufacturing cost becomes high. Therefore, the upper limit is set to 2.0%.

Mo effectively acts to increase the strength of the steel sheet and to improve the corrosion resistance. However, if the Mo content exceeds 3.0%, the effect is saturated and the production cost becomes expensive. Therefore, the upper limit is made 3.0%.

[0022] Cr effectively acts to improve the perforation corrosion resistance, but if it exceeds 3.0%, the production cost increases, so the upper limit is made 3.0%.

B is an element that improves hardenability and strengthens grain boundaries. 0.00 to achieve this effect
Addition of more than 03% is necessary, but the effect saturates even if it exceeds 0.005%. Therefore, the lower limit is 0.0003% and the upper limit is 0.005%.

Ti and Nb refine ferrite crystal grains,
It is an element that improves ductility. To obtain such an effect, 0.005% or more must be added. However, if it exceeds 0.06%, the amount of fine TiC and NbC deposited increases and ductility is deteriorated. Therefore, the lower limit of each element is 0.005
%, The upper limit is 0.06%.

In the present invention, a steel slab containing such components is hot-rolled, pickled, then cold-rolled to a target sheet thickness, and the cold-rolled coil is transferred to an in-line annealing type continuous hot-dip galvanizing line. The sheet is passed to obtain a hot-dip galvanized steel sheet.
At that time, the finishing temperature in hot rolling is preferably set to the Ar ₃ transformation point or higher from the viewpoint of improving workability.
It may be in the range of 500-750 ° C. Further, the cold rolling rate in the cold rolling step is preferably 50 to 95%.

In the continuous galvanizing line, in-line annealing is performed in a temperature range of 720 to 950 ° C. The lower limit of the annealing temperature is set to 720 ° C. or higher because 720 ° C. is required as a minimum temperature required to obtain a composite structure. Limit
The reason why the temperature is set to 950 ° C. or lower is that even if the temperature exceeds this temperature, the effect of improving workability is saturated and surface flaws are liable to occur in the continuous galvanizing line.

If the cooling rate is low in the cooling process after annealing in the continuous galvanizing line, it may be difficult to obtain a composite structure. This can be avoided by the addition of the above-described hardenability improving element, but it is desirable that the cooling rate be high. There is no problem if the average cooling rate from the in-line annealing temperature to the hot-dip galvanizing bath (about 460 ° C) is 3 ° C / sec or more. The obtained plated steel sheet is subjected to a slight skin pass as necessary to produce a product steel sheet.

In the hot-dip galvanizing treatment, it is also desirable to perform an alloying treatment on the plating layer. This alloying treatment does not impair the material of the material of the present invention, and improves the coating adhesion and lap resistance weldability of the galvanized steel sheet. It can be said that. Usually, the alloying treatment is performed after the plating bath in the continuous hot-dip galvanizing line.
If the temperature is within the above range, the achievement of alloying becomes necessary and sufficient. If the temperature is lower than this temperature range, alloying is insufficient, and if the temperature is higher, alloying becomes excessive, and conversely, the adhesion of the plating layer may be impaired.

[0029]

Example 1 A steel sheet having the chemical composition shown in Table 1 was hot-rolled into a hot-rolled steel sheet having a thickness of 3.0 mm by hot rolling under the conditions shown in Table 2. After pickling, a cold-rolled steel sheet having a thickness of 0.8 mm was obtained. A rolled sheet was obtained. Subsequently, in-line annealing at the annealing temperatures shown in Table 2 was followed by hot-dip galvanizing with a coating weight of 30 g / m ² , followed by elongation.
A 0.3% skin pass rolling was performed. The properties of the obtained plated steel sheet were examined, and the results are shown in Table 2.

For tensile properties, a JIS Z 2201 No. 5 test piece was used. In the corrosion resistance test, 70 × 150 mm test pieces were cut out and subjected to a composite corrosion test. The combined corrosion test is based on the salt spray test of JIS Z 2371, and the salt spray test with a salt water concentration of 5% is performed for 2 hours →
4 hours of hot air drying at 60 ° C → 2 hours of wet test, for a total of 8
Evaluation was made by measuring the maximum erosion depth due to corrosion after 240 cycles, with the time treatment as one cycle.

[0031]

[Table 1]

[0032]

[Table 2]

As can be seen from the results in Table 2, Comparative Example No. 1 in which Mn, P was lower than the amount specified in the present invention and Cu was not added.
The hot dip galvanized steel sheet manufactured using
R) The composite structure steel sheet having a high yield and a low yield ratio has not been obtained. It has low elongation and poor corrosion resistance. Similarly, in the comparative example of No. 2 in which P is low and Cu is not added (Mn is increased from No. 1), the yield ratio (YR) is low and the elongation (El) is low.
Is also a good composite structure steel, but the corrosion resistance is poor.

On the other hand, the hot-dip galvanized steel sheets of Nos. 3 to 11 according to the method of the present invention all have a composite structure steel having a low yield ratio (YR) and a good elongation (El).
Has good strength-elongation balance (TS × El). And the corrosion resistance is much better than that of the comparative example.

[0035]

Example 2 A steel sheet having the chemical composition shown in Table 3 was hot-rolled to a thickness of 3.0 mm by hot rolling under the conditions shown in Table 4, and after pickling, cold-rolled to a thickness of 0.8 mm by cold rolling. A rolled sheet was obtained. This cold rolled sheet is Fe-0.05% with a coating weight of 2 g / m ² by continuous electroplating.
B was subjected to iron plating, then continuously annealed at the annealing temperatures shown in Table 4, and subsequently subjected to hot-dip galvanizing with an adhesion amount of 30 g / m ² . Thereafter, skin pass rolling was performed at an elongation of 0.3%. The properties of the obtained steel sheet were examined in the same manner as in Example 1, and the results are shown in Table 4.

[0036]

[Table 3]

[0037]

[Table 4]

As can be seen from the results in Table 4, the hot dip galvanized steel sheet manufactured using the comparative steel No. 12 having a lower P than specified in the present invention and containing no Cu has a yield ratio (YR) of Although the elongation (El) is low and good, the corrosion resistance is poor.

On the other hand, the galvanized steel sheets of Nos. 13 to 15 according to the present invention have a low yield ratio (YR) and a low elongation (E
l) is also good and corrosion resistance is excellent.

[0040]

As described above, according to the present invention, a high tensile galvanized steel sheet which maintains a low yield ratio and high ductility and has excellent corrosion resistance can be manufactured. This steel sheet can greatly contribute to weight reduction and corrosion prevention of a vehicle body, particularly a reinforcing member.

Continuation of the front page (56) References JP-A-55-104429 (JP, A) JP-A-61-157625 (JP, A) JP-A-55-122821 (JP, A) JP-A-2-101117 (JP) , A) (58) Surveyed fields (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C22C 38/00-38/60

Claims (6)

(57) [Claims] C .: 0.02 to 0.25% by weight, Si: 2.0% by weight.
% Or less, Mn: 1.6 to 3.5%, P: 0.03 to 0.20%, S: 0.0
2% or less, Cu: 0.05 to 2.0%, sol. Al: 0.005 to 0.100
%, N: 0.008% or less, with the balance being iron and inevitable impurities, hot rolled steel, pickled, cold rolled to the target thickness, and then in-line annealed continuous molten zinc Passing through the plating line, within the line 720-950
A method for producing a hot-dip galvanized steel sheet with a low yield ratio and excellent cold-rolled corrosion resistance, comprising continuous annealing at a temperature of ° C and hot-dip galvanizing. 2. In% by weight, C: 0.02 to 0.25%, Si: 2.0
% Or less, Mn: 1.6 to 3.5%, P: 0.03 to 0.20%, S: 0.0
2% or less, Cu: 0.05 to 2.0%, sol. Al: 0.005 to 0.100
%, N: 0.008% or less, and Ti: 0.
005 to 0.06% or Nb: 0.005 to 0.06% or more, and / or Ni: 2.0% or less, Mo: 3.0% or less or Cr: 3.0% or less, the balance being iron and inevitable A steel slab consisting of impurities is hot-rolled, pickled, cold-rolled to the target thickness, and then passed through a continuous hot-dip galvanizing line of in-line annealing type. A method for producing a cold-rolled high-tensile hot-dip galvanized steel sheet with excellent low-yield-ratio, excellent in pitting corrosion resistance, comprising performing continuous annealing in hot dip galvanizing. 3. In% by weight, C: 0.02 to 0.25%, Si: 2.0
% Or less, Mn: 1.6 to 3.5%, P: 0.03 to 0.20%, S: 0.0
2% or less, Cu: 0.05 to 2.0%, sol. Al: 0.005 to 0.100
%, N: 0.008% or less, B: 0.0003 to 0.005%, and Ti: 0.005 to 0.06% or Nb: 0.005 to 0.005%
At least one of 0.06% and / or Ni: 2.0
%, Mo: 3.0% or less: Cr: 3.0% or less, with the balance being steel and slabs consisting of iron and unavoidable impurities, hot-rolled, pickled and then cooled to the target thickness Rolling, then passing through a continuous hot-dip galvanizing line of the in-line annealing type, and performing continuous hot-dip galvanizing at a temperature of 720 to 950 ° C in the line.
A method for producing cold-rolled high-strength hot-dip galvanized steel sheet with excellent yield resistance and low yield ratio. 4. The method according to claim 1, wherein the Si content is less than 0.1%.
4. The method for producing a cold-rolled high-strength hot-dip galvanized steel sheet according to 2 or 3, which is excellent in perforation corrosion resistance. 5. The Si content is 0.1 to 2.0%.
4. The steel sheet according to claim 1, wherein the cold-rolled sheet having the content is subjected to iron plating in a continuous electroplating line before being passed through a continuous hot-dip galvanizing line. A method for producing a cold rolled high tensile galvanized steel sheet with a low yield ratio. 6. The hot-dip galvanizing method according to claim 1, 2, 3, 4, or 5, wherein the hot-dip galvanizing includes a process of alloying the adhered galvanized layer with the base steel sheet in a temperature range of 400 to 650 ° C. A method for producing cold-rolled high-strength hot-dip galvanized steel sheets with excellent corrosion resistance.