JPH05140653A - Manufacture of low yield ratio cold rolled high tensile strength galvanized steel sheet excellent in pitting corrosion resistance - Google Patents

Abstract

PURPOSE:To manufacture a low yield ratio cold rolled high tensile strength galvanized steel sheet excellent in pitting corrosion resistance by subjecting the cold rolled sheet of steel having a specified compsn., in which the contents of P and Cu are specified to continuous annealing at a specified temp. in a continuous galvanizing line, and executing plating. CONSTITUTION:A steel slab contg., by weight, 0.02 to 0.25% C, <=2.0% Si, 1.6 to 3.5% Mn, 0.03 to 0.20% P, <=0.02% S, 0.05 to 0.20% Cu, 0.005 to 0.100% solAl and <=0.008% N and the balance iron with inevitable impurities is subjected to hot rolling in such a manner that its finishing temp. is regulated to the Ar3 transformation point or above, and pickling is executed. Next, it is subjected to cold rolling into the objective sheet thickness, is passed through a continuous galvanizing line of an inline annealing type, is subjected to continuous annealing in the temp. range of 720 to 950 deg.C in the line and is galvanized. In this way, the plated steel sheet having a low yield and high ductility and excellent in pitting corrosion resistance can be obtd.

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 low yield ratio cold-rolled high-strength hot-dip galvanized steel sheet having a low yield ratio, high ductility, and excellent perforation corrosion resistance.

[0002]

2. Description of the Related Art In the automobile industry, high-strength steel sheets having excellent corrosion resistance and workability are required for the purpose of preventing rust and weight of the vehicle body. Generally, steel has lower ductility such as elongation and bending as the strength such as yield point and tensile strength increases. Therefore, high-strength steel sheets, which have been strengthened by utilizing solid solution strengthening, precipitation strengthening, etc., are insufficient for processing applications.

As a high-strength steel sheet developed from such a background, there is a composite structure cold-rolled high-strength hot-dip galvanized steel sheet.
For example, in Japanese Patent Publication No. 63-3930 and Japanese Patent Publication No. 56-47555, both strength and elongation are improved by using C-high Mn steel and making the steel structure a composite structure of ferrite and martensite. Base material steel sheets have been proposed.

[0004]

[Problems to be Solved by the Invention] The manufacturing technique of a composite microstructure steel sheet has been developed to impart strength and elongation at the same time especially for high-strength steel sheets for automobiles, and to achieve thinning, that is, weight reduction of automobile steel sheets. It is what Although it is possible to reduce the thickness of the steel sheet from the viewpoint of strength by adopting these steel sheets, if the thickness is reduced, corrosion due to perforation of the steel sheet becomes a problem due to corrosion. This problem also becomes apparent in hot-dip galvanized steel sheets.

That is, in the hot-dip galvanized steel sheet, the alloy layer formed by the reaction between hot-dip zinc and base iron binds the zinc layer and base iron, and the alloy layer is positively alloyed to form the entire plated layer. Although an alloy layer with improved adhesion is often used, this alloy layer is hard and brittle. For this reason, when the coating layer becomes thicker, the workability of the hot-dip galvanized steel sheet is defined not by the workability of the base steel but by the workability of the coating layer. Therefore, even if the machinability of the base metal is improved, high machinability cannot be obtained, and the peeling and damage of the plating layer become prominent during machining, the base metal is exposed and the part rusts, and pitting corrosion easily occurs. Problems such as emergence become apparent.

Therefore, the present invention improves the corrosion resistance and workability of the composite structure steel sheet at the same time, and obtains a low yield ratio cold rolled high-strength hot-dip galvanized steel sheet having excellent perforation corrosion resistance and excellent workability. The purpose is to

[0007]

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

[0008]

[Operation] Mn, Ni, Cr for improving hardenability in low carbon steel
When alloy elements such as the above are added, it is possible to form a ferrite + martensite composite structure steel by continuous heat treatment in an in-line annealing type hot-dip galvanizing line, and the inclusion of appropriate amounts of Cu and P can improve the corrosion resistance of this steel. Improve. Furthermore, ductility can be improved by adding a small amount of Ti and Nb.
Addition of Si and Mn and additions of Ni, Mo and Cr are effective for increasing strength and improving corrosion resistance. Due to these combined actions, it is possible to obtain a cold-rolled high-strength hot-dip galvanized steel sheet with 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 by a continuous electroplating process, it is possible to avoid the non-plating which may occur when Si is added. A hot-dip galvanized steel sheet with good adhesion can be obtained.

After the hot dip galvanizing, the plating layer is 40
Holding the temperature range of 0 to 650 ° C. to form an alloy layer with the base steel sheet can improve coating film adhesion and lap resistance weldability.

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

C is an element effective in obtaining a composite structure composed of ferrite and martensite and improving the strength. In order to obtain the target composite structure, C is 0.
02% or more is necessary, 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 steel without impairing workability, and has an effect that a composite structure can be obtained by promoting the formation of ferrite in the cooling process from austenite at high temperature during annealing. However, when Si exceeds about 2.0%, this effect saturates and becomes hard and the ductility deteriorates. Therefore, the upper limit was set to 2.0%. On the other hand, according to the study by the present inventors, in the in-line annealing continuous galvanizing line, when the Si in the steel is about 0.1 or more, non-plating is likely to occur. Therefore, Si is 0.1%
From this point of view, it is preferable that the number is less than 1. However, this problem can be completely solved by applying iron plating with an adhesion amount of about 2 g / m ^{2 by} electroplating prior to passing the continuous galvanizing line.

Mn is an element that acts effectively to improve the hardenability of steel and obtain a composite structure. Mn amount is 1.6%
If it is less than 3.5%, a composite structure having a low yield ratio characteristic cannot be obtained, while if it exceeds 3.5%, the workability and weldability deteriorate.
Therefore, the lower limit is 1.6% and the upper limit is 3.5%.

P and Cu are characteristic elements in the present invention, and the corrosion resistance is remarkably improved by the combined addition of these elements. To improve the corrosion resistance, P must be 0.03% or more and Cu must be 0.05% or more. On the other hand, P is 0.20%, C
Even if u exceeds 2.0%, the effect of improving corrosion resistance saturates,
Ductility deteriorates. Therefore, P is 0.03 to 0.2% and Cu is 0.05.
~ 2.0%

S is an element that is essentially harmful to the steel of the present invention, and the smaller the amount, the more desirable, but 0.02% is acceptable, so S was made 0.02% or less.

Al needs to be 0.005% or more in order to function as a deoxidizer, but if it exceeds 0.10%, Al ₂ O ₃
Increasing inclusions such as these deteriorate workability and surface quality, so the lower limit was made 0.005% and the upper limit was made 0.10%.

N is an element which is essentially harmful to the steel of the present invention, and the smaller the amount, the more desirable, but up to 0.008% is allowable, so N was made 0.008% or less.

Further, in the present invention, N up to 2.0%
If one or more of i, Mo up to 3.0% and Cr up to 3.0% are contained, the strength and corrosion resistance of the steel sheet can be improved.

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 expensive. Therefore, the upper limit is 2.0%.

Mo effectively acts to increase the strength and improve the corrosion resistance of the steel sheet, but if it exceeds 3.0%, the effect is saturated and the manufacturing cost becomes high, so the upper limit is made 3.0%.

Cr is effective in improving the resistance to pitting corrosion, but if it exceeds 3.0%, the manufacturing cost becomes high, so the upper limit is made 3.0%.

B is an element that improves hardenability and strengthens grain boundaries. 0.00 for this effect
It is necessary to add more than 03%, but even if added over 0.005%, the effect is saturated. Therefore, the lower limit is 0.0003% and the upper limit is 0.005%.

Ti and Nb refine the ferrite grains,
It is an element that improves ductility. To obtain such an effect, 0.005% or more must be added, but if added over 0.06%, the fine TiC and NbC precipitation amounts increase and ductility deteriorates. Therefore, the lower limit of 0.005 is
%, The upper limit is 0.06%.

In the present invention, a steel slab containing such components is hot-rolled, pickled, and then cold-rolled to a target plate thickness, and this cold-rolled coil is subjected to an in-line annealing continuous hot dip galvanizing line. It is passed through a plate to obtain a hot-dip galvanized steel plate.
At that time, it is preferable that the finishing temperature in hot rolling is not less than the Ar ₃ transformation point in order to improve workability, and the winding temperature is
It may be in the range of 500 to 750 ° C. The cold rolling rate in the cold rolling step is preferably 50 to 95%.

In the continuous hot-dip galvanizing line, in-line annealing is performed in the 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 the minimum temperature required to obtain the composite structure. The upper limit
The reason why the temperature is 950 ° C. or less is that the effect of improving the workability is saturated and the surface flaw is likely to occur in the continuous hot-dip galvanizing line even if the temperature is exceeded.

When the cooling rate is slow in the cooling process after annealing in the continuous hot-dip galvanizing line, it may be difficult to obtain a composite structure. This can be avoided by adding the above-mentioned 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 lightly skin-passed as necessary to obtain a product steel sheet.

In the hot dip galvanizing process, it is also desirable to alloy the plated layer. This alloying treatment does not impair the material of the present invention material and improves the coating adhesion and lap resistance weldability of the galvanized steel sheet. Can be said. Usually, alloying treatment is performed after the plating bath in the continuous hot-dip galvanizing line, but 400-650 ℃
It is necessary and sufficient to achieve alloying in the temperature range of. If the temperature is lower than this temperature range, the alloying is insufficient, and if the temperature is higher than the above range, the alloying is excessive and, on the contrary, the adhesion of the plating layer may be impaired.

[0029]

Example 1 Steels having the chemical composition values shown in Table 1 were hot-rolled under the conditions shown in Table 2 into hot-rolled steel sheets having a thickness of 3.0 mm. After pickling, cold rolling was performed to obtain cold-rolled steel sheets having a thickness of 0.8 mm. A rolled plate was obtained. Then, in-line annealing at the annealing temperature shown in Table 2 was followed by hot dip galvanizing with an adhesion amount of 30 g / m ² , and then the elongation percentage.
A 0.3% skin pass rolling was performed. The properties of the obtained plated steel sheet were investigated, and the results are shown in Table 2.

For the tensile properties, JIS Z 2201 No. 5 test piece was used. For the corrosion resistance test, a 70 × 150 mm test piece was cut out and a complex corrosion test was performed. The complex corrosion test conforms to the JIS Z 2371 salt spray test, and a salt spray test with a salt water concentration of 5% is performed for 2 hours.
Hot air drying at 60 ℃ for 4 hours → Wet test for 2 hours, total 8
It was evaluated by measuring the maximum erosion depth due to corrosion after 240 cycles with one cycle of time treatment.

[0031]

[Table 1]

[0032]

[Table 2]

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

On the other hand, in the hot-dip galvanized steel sheets of Nos. 3 to 11 according to the method of the present invention, composite structure steels having a low yield ratio (YR) and a good elongation (El) were obtained,
It has a good strength / elongation balance (TS x El). The corrosion resistance is far superior to that of the comparative example.

[0035]

Example 2 Steel having the chemical composition values shown in Table 3 was hot-rolled under the conditions shown in Table 4 into a hot-rolled sheet having a thickness of 3.0 mm. After pickling, cold rolling was performed to obtain a cold-rolled sheet having a thickness of 0.8 mm. A rolled plate was obtained. Fe-0.05% with an adhesion amount of 2 g / m ^{2 on} this cold-rolled sheet by continuous electroplating
After the iron plating of B was applied, it was continuously annealed at the annealing temperature shown in Table 4, followed by hot dip galvanizing with an adhesion amount of 30 g / m ² . After that, skin pass rolling with an elongation of 0.3% was performed. The properties of the obtained steel sheet were examined by the same method 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 prepared by using the comparative steel No. 12 in which P is lower than that specified in the present invention and which does not contain Cu has a yield ratio (YR) of It has a low elongation (El) and good corrosion resistance.

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) and an elongation (E
l) is also good and has excellent corrosion resistance.

[0040]

As described above, according to the present invention, it is possible to manufacture a high-strength hot-dip galvanized steel sheet which maintains a low yield ratio and high ductility and is excellent in corrosion resistance. This steel sheet can greatly contribute to the weight reduction and corrosion prevention of the automobile body, especially the reinforcing member.

Claims (6)

[Claims] 1. By weight%, C: 0.02-0.25%, 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, the balance being steel and the balance being iron and unavoidable impurities, hot-rolled steel slab, pickled, cold-rolled to the target thickness, and then in-line annealed continuous molten zinc Pass through the plating line and within the line 720-950
A method for producing a low-yield ratio cold-rolled high-strength hot-dip galvanized steel sheet having excellent pitting corrosion resistance, which comprises performing continuous galvanizing at a temperature of ℃. 2. 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% at least one or more, and / or Ni: 2.0% or less, Mo: 3.0% or less or Cr: 3.0% or less at least one or more, and the balance is iron and unavoidable A steel slab of impurities is hot-rolled, pickled, cold-rolled to a target thickness, and then passed through an in-line annealed continuous hot-dip galvanizing line at a temperature of 720-950 ℃. A method for producing a low-yield ratio cold-rolled high-strength hot-dip galvanized steel sheet having excellent puncture corrosion resistance, which comprises continuously annealing and hot-dip galvanizing. 3. By weight%, C: 0.02-0.25%, 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%, Ti: 0.005 to 0.06% or Nb: 0.005 to
0.06% of at least one and / or Ni: 2.0
%, Mo: 3.0% or less, or Cr: 3.0% or less, at least one of which is steel and the balance is iron and unavoidable impurities, hot-rolled, pickled, and then cold-rolled to the target thickness. Rolling, then passing through an in-line 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.
A method for producing a low-yield ratio cold-rolled high-strength hot-dip galvanized steel sheet having excellent perforation corrosion resistance. 4. The Si content is less than 0.1%.
The method for producing a low-yield ratio cold-rolled high-strength hot-dip galvanized steel sheet having excellent perforation corrosion resistance according to 2 or 3. 5. The Si content is 0.1 to 2.0%.
The cold-rolled sheet having the content is excellent in perforation corrosion resistance according to claim 1, 2 or 3, wherein iron plating is applied in a continuous electroplating line before being passed through a continuous hot-dip galvanizing line. Manufacturing method of cold-rolled high-strength hot-dip galvanized steel sheet with low yield ratio. 6. The hot-dip galvanizing method according to claim 1, 2, 3, 4 or 5, which comprises a treatment of alloying the attached galvanized layer with a base steel sheet in a temperature range of 400 to 650 ° C. A method for producing a cold-rolled high-strength hot-dip galvanized steel sheet having a low yield ratio and excellent corrosion resistance.