JP3142922B2 - Manufacturing method of hot-rolled high-strength hot-dip galvanized steel sheet with low yield ratio and excellent pitting corrosion resistance - 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 hot-rolled high-strength hot-dip galvanized steel sheet having a low yield ratio and a high ductility and excellent perforation 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 composite-structure high-strength steel sheet, and a hot-dip galvanized composite-structure hot-rolled high-strength galvanized steel sheet is known. For example, JP-A-58-58264 and JP-A-56-1
The 08830 discloses a C-Si- high Mn steel A ₁ transformation point or more A ₃
A method of producing a hot-rolled high-tensile hot-dip galvanized steel sheet with improved strength and elongation by applying box annealing at a temperature below the transformation point to obtain a composite structure steel of ferrite and martensite and subjecting it to hot-dip galvanizing. Is described.

[0004]

The manufacturing technology of the composite structure steel sheet has been developed to simultaneously impart the strength and the elongation particularly 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. However, in the production method described in the above publication, box annealing is required to obtain a composite structure, and the former cannot provide sufficient elongation.

Although the use of these steel sheets makes it possible to reduce the thickness of the steel sheet in terms of strength, the problem of perforated corrosion of the steel sheet becomes apparent when the thickness is reduced.

Accordingly, an object of the present invention is to provide a hot-rolled, high-tensile hot-dip galvanized steel sheet having a low yield ratio, excellent workability and excellent corrosion resistance and workability, simultaneously improving the corrosion resistance and workability of the composite structure steel sheet itself. With the goal.

[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, and in some cases, further, at least one or more of Ti: 0.005 to 0.06% or Nb: 0.005 to 0.06%, and / or Ni: 2.0 % Or less, Mo: 3.0% or less or Cr:
Hot-rolling a steel slab containing at least one or more of 3.0% or less, and in some cases further containing B: 0.0003 to 0.005%, with the balance being iron and unavoidable impurities,
After pickling, the hot-rolled coil is passed through a continuous hot-dip galvanizing line of in-line annealing type, and hot-dip galvanizing is performed in the line by continuous annealing at a temperature of 720 to 950 ° C. Provided is a method for producing a hot-rolled high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and a low yield ratio.

[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 addition of trace amounts of Ti and Nb can improve the hole-expanding property, which is a disadvantage of the composite structure steel, and the addition of Si, Mn and the addition of Ni, Mo, and Cr are effective in increasing the strength and corrosion resistance. It is. By these combined actions, a low-yield-ratio hot-rolled high-tensile hot-dip galvanized steel sheet having excellent workability and corrosion resistance can be obtained.

Further, if the hot-rolled coil before being passed 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.

Hereinafter, the contents of the present invention will be described in detail. The effects of the various components of the base material hot-rolled steel sheet according to the present invention and the reasons for limiting the content range 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 which effectively acts to improve the hardenability of steel and obtain a composite structure. Mn amount is 1.6
%, A composite structure having low yield ratio characteristics cannot be obtained.
On the other hand, if it exceeds 3.5%, the 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 perforated corrosion resistance, but if it exceeds 3%, 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 are effective in refining the metal structure, improving the ductility, and improving the hole expandability which is a disadvantage of the composite structure steel. To obtain such an effect, it is necessary to add 0.005% or more, but if it exceeds 0.06%, fine T
The precipitation amount of iC and NbC increases, and the ductility deteriorates. Therefore, the lower limit of each element is set to 0.005% and the upper limit is set to 0.06%.

In the present invention, a steel slab containing such components is hot-rolled, pickled, and then the hot-rolled coil is passed through a continuous hot-dip galvanizing line of an in-line annealing type.
Galvanizing is performed by continuous annealing at a temperature of 720 to 950 ° C in this line.
It is preferable to set the temperature to r ₃ transformation point or more from the viewpoint of improving workability, and the winding temperature may be set within a range of 500 to 750 ° C.

The lower limit of the annealing temperature in the continuous hot-dip galvanizing line is set to 720 ° C. because 720 ° C. is required as the minimum temperature required to obtain a composite structure. Up to 9
The reason why the temperature is set to 50 ° C. is that even if the temperature is exceeded, the effect of improving the workability is saturated, and the surface flaw is easily generated in the continuous hot-dip 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.

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 a chemical composition shown in Table 1 was hot-rolled to a thickness of 2.5 mm by hot rolling under the conditions shown in Table 2, and after pickling, continuously subjected to an annealing temperature shown in Table 2. Annealing, adhesion amount
Subjecting the galvanized 30 g / m ^2, then, elongation of 0.3%
Was subjected to skin pass rolling. The properties of the obtained hot-rolled annealed galvanized steel sheet were examined, and the results are shown in Table 2.

For the tensile properties, a JIS Z 2201 No. 5 test piece was used. In the hole expansion test, a punched hole with a diameter of 10 mm was made in the center of a 150 × 150 mm test piece, and then a hole expansion test was performed with a 50 mm diameter ball-head punch. 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 water concentration is 5%.
2 hours of salt spray test → 4 hours of hot-air drying at 60 ° C → 2 hours of wet test, with a total of 8 hours of treatment as one cycle, evaluated by measuring the maximum erosion depth due to corrosion after 240 cycles did.

[0031]

[Table 1]

[0032]

[Table 2]

As can be seen from the results in Table 2, the hot-dip galvanized steel sheet of Comparative Example No. 1 manufactured using a steel having Mn, P lower than the amount specified in the present invention and containing no Cu was yielded. ratio
A composite structure steel sheet having a high (YR) and a low yield ratio has not been obtained. Therefore, the strength-elongation balance (TS × El), and the strength-hole expansion balance (TS × λ) are inferior, and the corrosion resistance is inferior. Similarly, the comparative example of No. 2 in which P is low and Cu is not added (Mn is increased from No. 1) is a composite structure steel having a low yield ratio (YR) and a good elongation (El). However, the corrosion resistance is poor. In addition, strength-elongation balance (TS × E
l) and strength-hole expansion balance (TS × λ) are not enough.

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).
High strength and high hole expansion ratio (λ)
Elongation balance (TS × El) and strength / hole expansion balance (TS ×
λ), 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 2.5 mm by hot rolling under the conditions shown in Table 4, and after pickling, a coating weight of 2 g / kg was obtained by electroplating. of m ² Fe-0.05%
B, and continuously annealed at the annealing temperatures shown in Table 4.
Hot-dip galvanizing with a coating weight of 30 g / m ² , then elongation
A 0.3% skin pass rolling was performed. The properties of the obtained hot-rolled annealed galvanized 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 of Comparative Example No. 12 manufactured using a steel in which P is lower than the amount specified in the present invention and no Cu is added has a yield ratio (Y
R) is low and elongation (El) is good, but corrosion resistance is poor. On the other hand, the hot-dip galvanized steel sheets of Nos. 13 to 15 according to the present invention have a low yield ratio (YR), a good elongation (El), and a high hole expansion ratio (λ) despite high strength. And its corrosion resistance is much better than that of No. 12.

[0039]

As described above, according to the present invention, it is possible to produce a high-strength hot-dip galvanized steel sheet which maintains a low yield ratio, high ductility and good hole expandability and is excellent in corrosion resistance. This steel sheet can greatly contribute to the weight reduction and corrosion prevention of the vehicle body, especially the underbody parts.

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-5-112832 (JP, A) JP-A-5-140652 (JP, A) JP-A-5-140653 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/46, 8/02 C22C 38/00-38/60 C23C 2 / 28,2 / 40

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 consisting of iron and unavoidable impurities, hot-rolled, pickled, and then passed the hot-rolled coil through a continuous hot-dip galvanizing line of in-line annealing type And a method for producing a hot-dip galvanized steel sheet having a low yield ratio and excellent hot-rolled corrosion resistance, comprising continuously annealing at a temperature of 720 to 950 ° C. in the line to perform 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 After hot rolling a steel slab consisting of impurities and pickling, the hot-rolled coil is passed through a continuous hot-dip galvanizing line of in-line annealing type, and continuously annealed at a temperature of 720 to 950 ° C in the line. A method for producing a hot-rolled, high-strength hot-dip galvanized steel sheet with excellent yield resistance and low yield ratio, comprising 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
% Or less, Mo: 3.0% or less: Cr: 3.0% or less, with the balance being iron and unavoidable impurities, the steel slab is hot-rolled, and after pickling, the hot-rolled coil is inlined. Low-yield specific hot rolling with excellent perforation corrosion resistance consisting of passing through an annealing-type continuous hot-dip galvanizing line, and performing continuous annealing at a temperature of 720 to 950 ° C in the line to perform hot-dip galvanizing. Manufacturing method of high tension hot-dip galvanized steel sheet. 4. The method according to claim 1, wherein the Si content is less than 0.1%.
4. The method for producing a hot-rolled high-strength hot-dip galvanized steel sheet having a low yield ratio and excellent in pitting corrosion resistance according to 2 or 3. 5. The Si content is 0.1 to 2.0%.
The hot-rolled coil having a high 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 high-strength hot-dip galvanized steel sheets 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 hot-rolled, high-strength hot-dip galvanized steel sheets with excellent yield corrosion resistance.