JPH06240367A - Production of hot dip galvanized steel sheet for deep drawing excellent in coating/baking hardenability and corrosion resistance - Google Patents

Abstract

PURPOSE:To produce a hot dip galvanized steel sheet for deep drawing excellent in coating/baking hardenability and pitting corrosion resistance by using steel obtd. by adding specified trace amounts of Nb and Ti to dead soft P-Cu compositely added steel as stock. CONSTITUTION:The slab of steel having a compsn. contg., by weight, 0.001 to 0.008% C, <1.5% Si, 0.05 to 1.8% Mn, 0.03 to 0.20% P, <0.015% S, 0.05 to 1.5% Cu, 0.005 to 0.10% sol.Al, <0.005% N and 0.005 to 0.03% Nb or contg. Ti by the amt. satisfying 0.003%<=Ti<=(48/32)XS+(48/14)XN% and 0.0003 to 0.003% Ti independently or compositely is subjected to hot rolling, oxidized scales on the surface are pickled away, and after that, cold rolling is executed. This cold rolled steel sheet is annealed at 800 to 950 deg.C in a continuous hot dip galvanizing line and is galvanized, or it is subjected to continuous annealing at 800 to 950 deg.C, is applied with Fe-B allay plating and is thereafter galvanized.

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-dip galvanized steel sheet for deep drawing, which is mainly used for automobile bodies and which is excellent in paint bake hardenability and perforation corrosion resistance.

[0002]

2. Description of the Related Art In the field of cold-rolled steel sheets used for automobiles, in addition to being excellent in deep drawability, in order to improve dent resistance, the yield stress of the steel sheet is increased during baking. That is, paint bake hardenability is often required. Conventionally, cold-rolled steel sheets of this type include low-carbon Al-killed steel, ultra-low-carbon steel with Ti added, and high-strength steel sheets with added Si, Mn, P, etc. to increase the strength. There are many suggestions for.

For example, JP-A-57-98630, JP-A-58-107414 and JP-A-61-27692.
No. 7, a method of continuously annealing using extremely carbon Al killed steel as a raw material is also disclosed in JP-A-61-26757, JP-A-63-276927 and JP-A-2-11184.
No. 1 discloses a method of producing ultra-low carbon Ti-added steel as a raw material by continuous annealing. These not only increase the strength of the steel sheet, but also have excellent formability, while being soft at the time of forming, it is a bake hardening type cold rolled steel sheet that has high strength due to coating baking after press forming. It is a manufacturing method.

By the way, these are techniques for imparting deep drawability and strength at the same time, and are intended to achieve weight reduction by making automobile steel sheets thinner. However, from the viewpoint of strength, it is possible to reduce the thickness of the steel sheet, but if the thickness of the steel sheet is reduced, there arises a problem that the perforation resistance life due to corrosion is shortened. Therefore, it is required that the steel sheet has good corrosion resistance.

In order to improve the corrosion resistance, it is effective to adopt surface-treated steel sheets having various surface treatments, and at present, demand for various surface-treated steel sheets including galvannealed steel sheets is rapidly increasing. is there. Regarding hot-dip galvanized steel sheets, low carbon Al killed steel and ultra low carbon steel Ti
Many proposals have been made on hot-dip galvanized steel sheets based on additive steel and high-strength hot-dip galvanized steel sheets obtained by adding Si, Mn, P, and Cr to these to increase strength.

For example, in Japanese Examined Patent Publication No. 1-54413, a hot dip galvanized steel sheet in which P is added to a low carbon Al killed steel, and Japanese Unexamined Patent Publication No. 57-43974 discloses ultra-low carbon Ti-added steel in a large amount of P. The hot dip galvanized steel sheet to which Mn of is added is disclosed.

However, even these plated steel sheets do not have sufficient corrosion resistance, and pitting corrosion may occur depending on the environment in which they are used. Therefore, the amount of plating adhered is increased to further improve the corrosion resistance. However, increasing the coating weight causes a problem that the weldability is significantly deteriorated, so it is practically difficult to further increase the plating thickness.

In order to solve the above problems, the present inventors have made various studies on a method for producing a cold-rolled steel sheet which has improved corrosion resistance of the steel sheet itself and is excellent in paint bake hardenability and deep drawability. , Ultra low carbon steel, or ultra low carbon + trace Ti
Composite addition of P and Cu based on additive steel, and further Si,
By adding Mn and Ni, Mo, Cr, etc.,
A method for obtaining a cold-rolled steel sheet for deep drawing which is excellent in paint bake hardenability and corrosion resistance was found out and disclosed in Japanese Patent Application No. 3-212713.

However, the present invention mainly focuses on the improvement of paint bake hardenability and corrosion resistance, and sufficient deep drawability has not been obtained. In addition, although it has excellent corrosion resistance as a cold rolled steel sheet,
Corrosion resistance has not been obtained to the extent that perforation corrosion does not occur in a severe corrosive environment where snow melting salts are sprayed to prevent road freezing in winter.

[0010]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention is a hot-dip galvanized steel sheet having excellent deep drawability and corrosion resistance based on a bake hardening type cold rolled steel sheet having improved corrosion resistance of the steel sheet itself. It aims at providing the manufacturing method of.

[0011]

[Means for Solving the Problems] The inventors of the present invention relate to a method for producing a bake-hardening hot-dip galvanized steel sheet for baking, which is a combination of ultra-low carbon P and Cu with improved corrosion resistance of the steel sheet itself.
As a result of a detailed study, it was found that excellent deep drawability can be obtained by adding a trace amount of Nb and, if necessary, by adding a trace amount of Ti to the extent that solid solution C cannot be fixed as TiC. It was

That is, in the present invention, C: 0.00% by weight.
1 to 0.008, Si: 1.5 or less, Mn: 0.05 to
1.8, P: 0.03 to 0.20, S: 0.015 or less, Cu: 0.05 to 1.5, sol. Al: 0.00
5 to 0.100, N: 0.005 or less, Nb: 0.00
5 to 0.03, and further contains one or two or more of Ni of 1.0 or less, Mo of 3.0 or less, and Cr of 7.0 or less, and the balance of Fe and unavoidable as necessary. Steel slab consisting of static impurities: hot-rolled, pickled,
Cold rolling is performed, and then 800 to 950 ° C. annealing is performed in a continuous hot dip galvanizing line for galvanizing,
Alternatively, it is annealed at 800 to 950 ° C. in a continuous annealing line, is subjected to Fe alloy plating in a continuous electroplating line, and is then galvanized in a continuous hot dip galvanizing line, which is excellent in coating bake hardenability and corrosion resistance. Provided is a method for manufacturing a hot-dip galvanized steel sheet for deep drawing.

The present invention also provides C: 0.001-wt%.
0.008, Si: 1.5 or less, Mn: 0.05 to 1.
8, P: 0.03 to 0.20, S: 0.015 or less, C
u: 0.05 to 1.5, sol. Al: 0.005-
0.100, N: 0.005 or less, Nb: 0.005
0.03, 0.003 ≦ Ti ≦ (48/32) × S +
It contains Ti in the range according to (48/14) × N, and further contains Ni of 1.0 or less, Mo of 3.0 or less, if necessary.
A steel slab containing one or more Cr of 7.0 or less and the balance Fe and unavoidable impurities:
Hot rolling is performed, and after pickling, cold rolling is performed, and then 800 to 950 ° C. is annealed in a continuous hot dip galvanizing line for galvanizing, or 800 to 950 ° C. in a continuous annealing line. A method for producing a hot-dip galvanized steel sheet for deep drawing, which is excellent in paint bake hardenability and corrosion resistance, which comprises performing annealing, performing Fe alloy plating on a continuous electroplating line, and then galvanizing it on a continuous galvanizing line. I will provide a.

The present invention also provides B: 0.0003 in weight percent.
Provided is a method for producing a hot-dip galvanized steel sheet for deep drawing, which is excellent in paint bake hardenability and corrosion resistance by any one of the above-mentioned methods comprising the addition of 0.003 to 0.003.

First, the action of various components of the method for producing a steel sheet according to the present invention and the reason why the above range is limited will be described. C is an essential element for obtaining paint bake hardenability, and is 0.0 for obtaining sufficient paint bake hardenability.
01% or more is required. On the other hand, if it exceeds 0.008%, the amount of solid solution C increases and a very high paint bake hardenability is obtained, but aging at room temperature occurs and ductility rapidly deteriorates. Therefore, C is limited to the range of 0.001 to 0.008%.

Si is an element that effectively acts to increase the strength and improve the resistance to pitting corrosion, but if it exceeds 1.5%, scale flaws occur during the hot rolling process, and the surface properties of the product are increased. Therefore, the upper limit of the amount added is set to 1.5%.

Mn is an element effective for improving the strength, and for this purpose, at least 0.05% or more is necessary.
On the other hand, if it exceeds 1.8%, the ductility and deep drawability deteriorate, so the range was limited to 0.05 to 1.8%.

P is a characteristic element in the present invention, and when it is added together with Cu, P resistance is significantly improved. An appropriate amount of addition is required to be 0.03% or more from the viewpoint of improving corrosion resistance. However, if added over 0.20%, ductility and deep drawability deteriorate. Therefore, the addition amount is limited to 0.03 to 0.20%.

S is an element harmful to deep drawability, and the smaller the amount, the more desirable, but 0.015% is acceptable, so S was made 0.015% or less.

Cu is an element effective in improving the corrosion resistance when added in combination with P as described above.
If it is less than 0.05%, the effect is not recognized. Also,
Even if added over 1.5%, the effect is saturated and the cost is increased, so the range is limited to 0.05 to 1.5%.

Al is added as a deoxidizing agent, and 0.005% or more is necessary to fulfill its role. However, if added over 0.10%, Al ₂
Inclusions such as O ₃ increase and deteriorate the workability and surface quality, so the range is limited to 0.005 to 0.10%.

N is an element detrimental to corrosion resistance and deep drawability, and the smaller the amount, the more preferable. However, N is 0.005% or less because it is allowable up to 0.005%.

Nb is an element effective in improving the deep drawability, but if it is less than 0.005%, its effect is not recognized.
Moreover, even if the content exceeds 0.03%, not only the effect is saturated, but also the recrystallization temperature is unnecessarily increased,
On the contrary, since the deep drawability and the ductility are deteriorated, 0.
It was limited to the range of 005 to 0.03%.

Ti is an effective element for ensuring deep drawability, but if it is less than 0.003%, its effect is not recognized. Also, Ti forms a compound with N, S, and C,
Precipitates as TiN, TiS, TiC, etc., but if Ti is added in excess of the amount calculated as TiN, TiS, the coating bake hardenability cannot be obtained. Therefore, the lower limit of the addition amount is 0.003% and the upper limit is (48/32) × S + (4
8/14) × N%.

Further, in the present invention, in order to increase the strength of the steel sheet and improve the corrosion resistance, Ni of 1.0% or less and 3.0% or less are used.
One or two of Mo below and Cr below 7.0%
It may contain more than one species.

Ni effectively prevents hot embrittlement due to Cu and improves pitting corrosion resistance, but if it exceeds 1.0%, the effect is saturated and the manufacturing cost is increased. The upper limit of the amount added was 1.0%.

Mo acts effectively to increase the strength of the steel sheet and to improve the resistance to pitting corrosion, but if it is added in excess of 3.0%, the effect saturates and becomes hard, and the ductility deteriorates. Therefore, the upper limit of the addition amount is set to 3.0%.

[0028] Cr is an element that effectively acts to increase the strength of the steel sheet and improve the corrosion resistance, but if it exceeds 7%, the cost becomes very high, so the upper limit of the addition amount was made 7.0%.

B is effective in improving the secondary processing brittleness,
For that purpose, 0.0003% or more must be added. However, even if added over 0.003%, the effect is saturated, so the addition amount is 0.0003-0.003%.
Limited to the range.

In the present invention, the steel containing such components is made into a thin steel sheet through a hot rolling step and a cold rolling step. In this case, the finishing temperature in the hot rolling step is A
Below the r ₃ transformation point, deep drawability deteriorates. Winding temperature is 5
If it is less than 00 ° C, the deep drawability is deteriorated and the plate shape is deteriorated. If it exceeds 750 ° C, the pickling property is deteriorated and the coil is deformed after winding. Therefore, the finish temperature of hot rolling is at or above the Ar ₃ transformation point, but the winding temperature is 500 to
It is desirable that the temperature be 750 ° C.

In the cold rolling process, a cold rolling ratio of 50 to 95% is required to secure deep drawability. If the cold rolling ratio is less than 50%, the deep drawability is inferior, and if it exceeds 95%, the load of the cold rolling mill is large and the productivity is inferior.

In the present invention, a method of passing the continuous hot dip galvanizing line as it is after cold rolling is advantageous in terms of manufacturing cost. For example, when Si is added in an amount of more than 0.5%. Has poor plating adhesion,
Before passing through the hot-dip galvanizing line, it is annealed in a continuous annealing line, and further in a continuous electroplating line, Fe
Alloy plating can also be applied.

In the present invention, the addition of Nb is essential,
Since the recrystallization temperature becomes high, it is necessary to anneal at a high temperature in order to improve the workability above the recrystallization temperature. Also, N
Since b is precipitated as NbC in the hot rolling process, it is necessary to form a solid solution with NbC precipitated by high temperature annealing in order to impart coating bake hardenability.
The above high temperature annealing is required. However, when annealing is performed at a temperature higher than 950 ° C., surface defects are likely to occur in the continuous hot-dip galvanizing line or the continuous annealing line. Therefore, the annealing temperature in the continuous hot-dip galvanizing line or the continuous annealing line is limited to the range of 800 to 950 ° C.

[0034]

[Detailed Disclosure of the Invention]

[Example 1] A hot rolled sheet having a thickness of 3.2 mm was formed by hot rolling under the conditions shown in Table 2 using 10 steel types having the compositions shown in Table 1, and after pickling, cold rolling was performed to obtain the sheet thickness. : 0.8 mm cold rolled steel sheet. After that, No. Steels 1 to 5 and 10 are continuous hot dip galvanizing line, and annealed and 4 per side
5g / m ² hot-dip galvanized, elongation: 0.8%
Skin pass rolling was performed. No. For 6 to 9 steel, after cold-rolled steel sheet is annealed in a continuous annealing line, Fe-B alloy plating of ² g / m ² per side is applied in a continuous electroplating line,
After heat reduction treatment at 700 ° C. on a continuous hot dip galvanizing line, hot dip galvanizing was performed at 45 g / m ^{2 on each} side, and skin pass rolling was performed at an elongation rate of 0.8%. The tensile properties, corrosion resistance, and secondary work embrittlement resistance of the obtained hot-dip galvanized steel sheet were investigated, and the results are also shown in Table 2.

[0035]

[Table 1]

[0036]

[Table 2]

A JISZ2201 No. 5 test piece was used to investigate the tensile properties. Corrosion resistance test is 70 x 150 mm
The test piece was cut out, the end surface and the back surface were sealed, and a composite corrosion test was performed to measure the maximum erosion depth. As shown in FIG. 1, the complex corrosion test is a salt spray test according to JIS Z2371 for 2 hours, a drying test at 60 ° C. for 4 hours, and a temperature of 50 ° C.
Wet test with humidity of 95% or more for 2 hours for a total of 8 hours
It was carried out under the condition of a cycle. Further, the secondary processing brittleness resistance is that the cylindrical die drawing ratio is 2.0 after blanking the steel sheet.
Then, a 5 kg weight was dropped from a height of 1 m at various test temperatures, the test piece was crushed, and the critical temperature at which brittle cracking occurred was investigated.

As can be seen from the results in Table 2, C is high and P is high.
And Cu whose amount is less than the amount specified in the present invention. Comparative steel No. 1 has a low elongation (El) and a too high paint bake hardenability (BH), and therefore is aged at room temperature. Also, it has poor corrosion resistance. No. 3 whose Cu is less than the amount specified in the present invention. The No. 2 steel exhibits good tensile strength values comparable to those of the steel of the present invention, but is inferior in corrosion resistance. On the other hand, No. 3 having a chemical composition within the range specified in the present invention. Despite having a high tensile strength (TS), the 3 to 10 steels have a low yield point (YS), good elongation (El) and paint bake hardenability (BH), and excellent corrosion resistance. In addition, No. It can be seen that the No. 10 steel is also excellent in secondary work embrittlement resistance.

[0039]

Example 2 Ten steel grades having the composition shown in Table 3 were hot-rolled under the conditions shown in Table 4 to obtain a hot-rolled sheet having a thickness of 3.2 mm, which was pickled and then cold-rolled. : 0.8 mm cold rolled steel sheet. After that, No. Steel Nos. 11 to 17 and 20 were subjected to annealing and hot dip galvanizing at 45 g / m ² per side in a continuous hot dip galvanizing line, and in-line alloying treatment was performed to obtain an elongation of 0.8%. Skin pass rolling was performed. For No. 18 and 19 steels, after cold-rolled steel sheets were annealed in a continuous annealing line, Fe-B alloy plating of ² g / m ² per side was applied in a continuous electroplating line and heated at 700 ° C in a continuous hot dip galvanizing line. After the reduction treatment, hot dip galvanizing 45 g / m ² per side, and in-line alloying treatment of the plated layer,
Elongation: 0.8% skin pass rolling was performed. The tensile properties, corrosion resistance and secondary work embrittlement resistance of the obtained hot dip galvanized steel sheet were investigated, and the results are also shown in Table 4.

[0040]

[Table 3]

[0041]

[Table 4]

As can be seen from the results in Table 4, C is high and P is high.
And Cu are less than the amounts specified in the present invention, and the addition amount of Ti is too high. No. 11 steel has a low elongation (El) and cannot obtain sufficient paint bake hardenability (BH). Also, it has poor corrosion resistance. No. 3 whose Cu is less than the amount specified in the present invention. The No. 12 steel has good tensile properties equivalent to those of the steel of the present invention, but is inferior in corrosion resistance. On the other hand, No. 3 having a chemical composition within the range specified in the present invention. 13-2
No. 0 steel has a high tensile strength (TS) but a yield point (YS).
Is low, the elongation (El) and the paint bake hardenability (BH) are good, and the corrosion resistance is excellent. Also, N added with B
o. It can be seen that the No. 20 steel is also excellent in secondary workability.

[0043]

[Embodiment 3] Table 3 No. 13 steel was hot-rolled under the conditions shown in Table 5 into a hot-rolled sheet with a thickness of 3.2 mm, and after pickling, cold-rolled to obtain a cold-rolled steel sheet with a thickness of 0.8 mm,
Annealed at various temperatures in a continuous hot-dip galvanizing line to apply hot-dip galvanizing of 45 g / m ² per side,
The plating layer was alloyed in-line. afterwards,
Elongation: 0.8% skin pass rolling was performed. The tensile properties and corrosion resistance of the obtained steel sheet were investigated, and the results are also shown in Table 5.

The comparative example having an annealing temperature of 750 ° C. has an elongation (E
l) and the average r value have been obtained to some good values,
The solid solution of NbC is not sufficient and coating bake hardenability (BH) cannot be obtained. On the other hand, the invention examples having an annealing temperature of 800 ° C. or higher have good elongation (El) and average r value, and are also excellent in coating bake hardenability (BH).

[0045]

As described above, the present invention clarifies a method for producing a hot-dip galvanized steel sheet having excellent paint bake hardenability and corrosion resistance while maintaining workability such as deep drawability and ductility. INDUSTRIAL APPLICABILITY The present invention greatly contributes to weight reduction and long life of automobiles, and its industrial significance and benefits are extremely large.

[Brief description of drawings]

FIG. 1 is a test cycle diagram showing conditions of a corrosion test.

[Table 5]

[Procedure amendment]

[Submission date] January 5, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Delete

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Internal reference number FI Technical indication C22C 38/00 FS 38/48 C25D 3/20 5/26 M (72) Inventor Toshiro Yamada Hiroshima 11-1 Showa-cho, Kure, Niishin Steel Co., Ltd.

Claims (3)

[Claims] 1. C: 0.001-0.008 by weight%,
Si: 1.5 or less, Mn: 0.05 to 1.8, P: 0.
03-0.20, S: 0.015 or less, Cu: 0.05
~ 1.5, sol. Al: 0.005 to 0.100,
N: 0.005 or less, Nb: 0.005 to 0.03, and if necessary, 1.0 or less Ni, 3.0
A steel slab containing one or more of the following Mo and Cr of 7.0 or less, and the balance of Fe and inevitable impurities: hot-rolled, pickled, and cold-rolled. , Then 800-9 in continuous hot dip galvanizing line
It is annealed at 50 ° C for galvanizing, or it is annealed at 800-950 ° C in a continuous annealing line, and Fe alloy plating is applied in a continuous electroplating line, followed by galvanizing in a continuous hot dip galvanizing line. A method for producing a hot-dip galvanized steel sheet for deep drawing which is excellent in paint bake hardenability and corrosion resistance. 2. C: 0.001-0.008 by weight%,
Si: 1.5 or less, Mn: 0.05 to 1.8, P: 0.
03-0.20, S: 0.015 or less, Cu: 0.05
~ 1.5, sol. Al: 0.005 to 0.100,
N: 0.005 or less, Nb: 0.005 to 0.03,
0.003 ≦ Ti ≦ (48/32) × S + (48/1
4) contains Ti in the range according to N, and further contains one or two or more of Ni of 1.0 or less, Mo of 3.0 or less, and Cr of 7.0 or less, if necessary, and the balance is A steel slab consisting of Fe and unavoidable impurities: hot-rolled, pickled, cold-rolled, and then annealed at 800 to 950 ° C. in a continuous hot dip galvanizing line for galvanizing. , Or 800-95 in continuous annealing line
Hot-dip galvanized steel sheet for deep drawing, which is excellent in paint bake hardenability and corrosion resistance, which consists of annealing at 0 ° C, performing Fe alloy plating on a continuous electroplating line, and then galvanizing it on a continuous galvanizing line. Manufacturing method. 3. B: 0.0003 to 0.003 by weight%.
A method for producing a hot-dip galvanized steel sheet for deep drawing which is excellent in coating bake hardenability and corrosion resistance according to claim 1 or 2.