JP3294322B2 - Method for producing hot-dip galvanized steel sheet for deep drawing with excellent paint bake hardenability and 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-dip galvanized steel sheet for deep drawing, which is mainly used for an automobile body and is excellent in paint bake hardening property and corrosion resistance against pitting.

[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 property that the yield stress of steel sheets increases when baking paint is used. 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 thereto, and high-strength steel with increased strength by adding Si, Mn, P, etc. There are many suggestions for.

For example, JP-A-57-98630, JP-A-58-107414 and JP-A-61-27692.
Japanese Patent Application Laid-Open No. 61-26757, Japanese Patent Application Laid-Open No. 63-276927, and Japanese Patent Application Laid-Open No. 2-118484 disclose a method of producing by continuous annealing using a carbon Al killed steel as a raw material.
No. 1 discloses a method of producing by ultra-low carbon Ti added steel 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, the baking-hardened cold-rolled steel sheet whose steel sheet is strengthened by paint baking after press forming It is a manufacturing method.

[0004] These are techniques for simultaneously imparting deep drawability and strength, and are intended to achieve weight reduction by reducing the thickness of a steel sheet for automobiles. However, from the viewpoint of strength, it is possible to reduce the thickness of the steel sheet. However, when the thickness of the steel sheet is reduced, there arises a problem that the corrosion-resistant life of a hole becomes short. For this reason, it is required that the steel sheet has good corrosion resistance.

In order to improve the corrosion resistance, it is effective to employ surface-treated steel sheets subjected to various surface treatments. At present, demand for various surface-treated steel sheets including alloyed hot-dip galvanized steel sheets is rapidly increasing. is there. For hot-dip galvanized steel sheets, low-carbon Al-killed steel and ultra-low-carbon steel Ti
Many proposals have been made for a hot-dip galvanized steel sheet based on an added steel and a high-tensile hot-dip galvanized steel sheet in which Si, Mn, P, and Cr are added thereto to increase the strength.

For example, Japanese Patent Publication No. 1-54413 discloses a hot-dip galvanized steel sheet obtained by adding P to a low-carbon Al-killed steel. A hot-dip galvanized steel sheet to which Mn is added is disclosed.

However, even with these plated steel sheets, corrosion resistance cannot be said to be sufficient, and depending on the environment in which they are used, perforated corrosion may occur. Therefore, the amount of plating adhesion is increased to further improve corrosion resistance. However, increasing the amount of plating causes a problem that the weldability is significantly deteriorated, so that it is practically difficult to increase the plating thickness more than ever.

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

However, the present invention mainly focuses on improving the baking hardenability of paint and the corrosion resistance, and has not been able to obtain sufficient properties for deep drawability. In addition, although it has excellent corrosion resistance as a cold-rolled steel sheet,
Corrosion resistance to the extent that perforated corrosion does not occur in severe corrosive environments where snow-melting salt is sprayed to prevent road freezing in winter has not been obtained.

[0010]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention provides a hot-dip galvanized steel sheet having excellent deep drawability and corrosion resistance based on a bake hardened cold-rolled steel sheet having improved corrosion resistance of the steel sheet itself. It is an object of the present invention to provide a method for producing the same.

[0011]

Means for Solving the Problems The present inventors relate to a method for producing a bake hardening type deep drawing hot-dip galvanized steel sheet with the addition of a very low carbon P and Cu composite in which the corrosion resistance of the steel sheet itself is improved.
As a result of a detailed study, it was found that excellent deep drawability can be obtained by adding a small amount of Nb and, if necessary, adding a small amount of Ti such that solid solution C cannot be fixed as TiC. Was.

In the present invention, C: 0.001-0.
008, Si: 1.5 or less, Mn: 0.05 to 1.8,
P: 0.03 to 0.20, S: 0.015 or less, Cu:
0.05-0.38 , Mo: 0.14-3.0, so
l. Al: 0.005 to 0.100, N: 0.005 or less, Nb: 0.005 to 0.03, 0.003 ≦ Ti ≦
(48/32) × S + (48/14) × N containing Ti in a range according to N, and if necessary, Ni 2 of 1.0 or less ;
7 . A slab of steel containing one or two types of Cr of 0 or less, the balance being Fe and unavoidable impurities: hot-rolled, pickled, cold-rolled, and then continuously molten zinc Perform 800-950 ° C. annealing on the plating line to galvanize, or use a continuous annealing line
Hot-dip galvanized zinc for deep drawing with excellent baking hardenability and corrosion resistance consisting of annealing at 0 to 950 ° C, applying Fe alloy plating in a continuous electroplating line, and then galvanizing in a continuous galvanizing line. Provided is a method for manufacturing a plated steel sheet.

The present invention also provides B: 0.0003 by weight%.
0.003 Income before Symbol of the method consists of containing,
Provided is a method for producing a hot-dip galvanized steel sheet for deep drawing having excellent paint bake hardenability and corrosion resistance.

First, the action of various components of the method for producing a steel sheet according to the present invention and the reason for limiting to the above range will be described. C is an essential element for obtaining paint bake hardenability, and is required to be 0.0 to obtain sufficient paint bake hardenability.
01% or more is required. On the other hand, when the content exceeds 0.008%, the amount of solid solution C increases and very high baking hardenability is obtained, but aging at room temperature occurs, leading to rapid deterioration of ductility. For this reason, C is limited to the range of 0.001 to 0.008%.

[0015] Si is an element that effectively acts to increase strength and improve perforation corrosion resistance. However, if it exceeds 1.5%, scale flaws are generated in the hot rolling step, and the surface properties of the product are reduced. The upper limit of the amount of addition was set to 1.5%, because it deteriorated the hardness and resulted in deterioration of ductility.

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

P is a characteristic element in the present invention, and significantly improves the pitting corrosion resistance when added in combination with Cu. An appropriate addition amount is required to be 0.03% or more from the viewpoint of improving corrosion resistance. However, when added in excess of 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 the deep drawability, and is preferably as small as possible. However, S is allowable up to 0.015%.

Cu is an element effective for improving corrosion resistance by being added in combination with P as described above.
If less than 0.05%, the effect is not recognized. Also,
If the addition exceeds 0.38%, the cost is increased. Therefore , the content is limited to the range of 0.05 to 0.38 %.

Al is added as a deoxidizing agent, and at least 0.005% is required to fulfill its role. However, if added in excess of 0.10%, Al ₂
Since inclusions such as O ₃ increase and degrade workability and surface quality, the range is limited to the range of 0.005 to 0.10%.

N is an element harmful to corrosion resistance and deep drawability, and is preferably as small as possible.

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

[0023] Ti is an element effective for securing the deep drawability, but its effect is not recognized if it is less than 0.003%. Also, Ti forms a compound with N, S, and C,
Precipitates as precipitates such as TiN, TiS, and TiC. If Ti is added in an amount exceeding that calculated as the total precipitation of TiN and TiS, sufficient paint 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, Ni of 1.0% or less, 3.0%
At least one of Mo and 7.0% or less of Cr
More than one species can be included.

Ni effectively acts to prevent hot brittleness due to Cu and to improve corrosive corrosion resistance. However, if the Ni content exceeds 1.0%, the effect is saturated and the production cost is increased. The upper limit of the amount added was 1.0%.

Mo effectively acts to increase the strength of the steel sheet and to improve the pitting corrosion resistance. However, if added in excess of 3.0%, the effect is saturated and becomes hard, deteriorates ductility, and further reduces cost. , The upper limit of the addition amount is set to 3.0%.

[0027] Cr is an element that effectively acts to increase the strength of the steel sheet and improve the corrosion resistance. However, if it exceeds 7%, the cost becomes very high. Therefore, the upper limit of the addition amount is set to 7.0%.

B is effective in improving the brittleness of secondary working,
For that purpose, it is necessary to add 0.0003% or more. However, the effect is saturated even if it is added in excess of 0.003%, so that the added amount is 0.0003 to 0.003%.
Limited to the range.

In the present invention, a steel sheet containing such a component is formed 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, the deep drawability deteriorates. Winding temperature is 5
If the temperature is lower than 00 ° C., the deep drawability deteriorates and the plate shape deteriorates. If it exceeds 750 ° C., the pickling properties will deteriorate and the coil will be deformed after winding. For this reason, the finishing temperature of hot rolling is higher than the Ar ₃ transformation point, and the winding temperature is 500 to
It is desirable that the temperature be 750 ° C.

In the cold rolling step, a cold-rolling ratio of 50 to 95% is required in order 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 on the cold rolling mill increases, and the productivity is inferior.

In the present invention, a method of passing a continuous hot-dip galvanizing line as it is after cold rolling is advantageous in terms of manufacturing cost. For example, in the case where Si is added in excess of 0.5%, In some cases, plating adhesion deteriorates.
Before passing through the hot-dip galvanizing line, annealing is performed in a continuous annealing line.
Alloy plating can also be applied.

In the present invention, Nb addition is essential,
Since the recrystallization temperature becomes high, it is necessary to perform annealing at a high temperature in order to improve workability at a temperature higher than the recrystallization temperature. Also, N
Since b is precipitated as NbC in the hot rolling step, it is necessary to dissolve the NbC precipitated by high-temperature annealing to form a solid solution in order to impart paint bake hardenability.
The above high temperature annealing is necessary. However, if annealing is performed at a temperature exceeding 950 ° C., surface flaws are likely to occur in a continuous galvanizing line or a continuous annealing line. Therefore, the annealing temperature in the continuous galvanizing line or the continuous annealing line was limited to the range of 800 to 950 ° C.

[0033]

DETAILED DISCLOSURE OF THE INVENTION

Example 1 A hot-rolled sheet having a thickness of 3.2 mm was formed by hot rolling using 10 steel grades having the compositions shown in Table 1 under the conditions shown in Table 2, and after pickling, cold rolling was performed. : 0.8 mm cold rolled steel sheet. After that, No. 11-14, 16-17
And 20 steel were subjected to annealing and hot dip galvanizing of 45 g / m ² per side in a continuous hot dip galvanizing line,
Alloying treatment of plating layer in-line, elongation:
A 0.8% skin pass rolling was performed. No. 18, 19
After annealing the cold-rolled steel sheet with a continuous annealing line, a continuous electroplating line is applied with ² g / m ² of Fe-B alloy plating on one side, and a continuous hot-dip galvanizing line is subjected to a heat reduction treatment at 700 ° C. And hot-dip galvanizing of 45 g / m ² per side, and furthermore, in-line plating layer alloying
And skin pass rolling at an elongation of 0.8%. The tensile properties, corrosion resistance, and secondary work brittleness resistance of the obtained hot-dip galvanized steel sheet were investigated, and the results are shown in Table 2.

A No. 5 test piece of JISZ2201 was used for examining the tensile properties. Corrosion resistance test is 70 × 150mm
Was cut out, the end face and the back face were sealed, and a composite corrosion test was performed to measure the maximum erosion depth. As shown in FIG. 1, the combined corrosion test was carried out in a salt spray test according to JISZ2371 for 2 hours, a drying test at 60 ° C. for 4 hours, and 50 ° C.
Humidity test of 95% or more for 2 hours, 8 hours in total
The test was performed under the condition of a cycle. In addition, the secondary work brittleness resistance, after blanking a steel plate, the cylindrical die drawing ratio is 2.0
Then, a 5 kg weight was dropped from a height of 1 m at various test temperatures to crush the test piece, and the critical temperature at which brittle cracking occurred was investigated. Bake hardenability (B
H): 2% tensile prestrain using JIS No. 5 tensile test piece
After artificial aging, apply artificial aging at 170 ° C for 20 minutes and pre-strain
BH is the difference between the stress at the time of application and the yield stress after artificial aging.
I asked.

[0035]

[Table 1]

[0036]

[Table 2]

As can be seen from the results in Table 2, C is high and P
No. and Cu are smaller than the amounts specified in the present invention, and the amount of Ti added is too high. Steel No. 11 has a low elongation (El) and does not provide sufficient bake hardenability (BH). Also, it has poor corrosion resistance. No. Cu is smaller than the amount specified in the present invention. Steel No. 12 exhibits good tensile properties equivalent to those of the steel of the present invention, but is inferior in corrosion resistance. On the other hand, No. 1 having a chemical composition within the range specified in the present invention. 18,1
Nine Steel has a high yield strength (YS) despite its high tensile strength (TS).
, The elongation (El) and the bake hardenability (BH) are good, and the corrosion resistance is excellent. In addition , N to which B was added
o. It can be seen that Steel No. 20 also has excellent secondary workability.

[Reference Experimental Example] Hot rolled steel sheet having a thickness of 3.2 mm by hot rolling under the conditions shown in Table 3 using 13 steel, cold-rolled by pickling and then cold-rolled steel sheet having a thickness of 0.8 mm, continuous Annealing at various temperatures in a hot dip galvanizing line, 4 per side
Hot-dip galvanizing of 5 g / m ² was performed, and alloying of the plating layer was performed in-line. After that, elongation: 0.8%
Was subjected to skin pass rolling. The tensile properties and corrosion resistance of the obtained steel sheet were investigated, and the results are shown in Table 3.

[0039]

[Table 3]

[0040] In the annealing temperature is 750 ° C., the elongation (El) and the average r value is somewhat better values are obtained, NbC
Is not sufficiently dissolved, and baking hardenability (BH) cannot be obtained. On the other hand, the annealing temperature becomes 800 ° C. or more .
And the elongation (El) and the average r value are good, and the coating bake hardenability (BH) is also excellent.

[0041]

As described above, the present invention clarifies a method for producing a hot-dip galvanized steel sheet having excellent baking 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 profit are extremely large.

[Brief description of the drawings]

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

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/00 301 C22C 38/00 301S 301T 38/48 38/48 C25D 3/20 C25D 3/20 5/26 5/26 M (72) Inventor Mikio Soshita 11-1, Showa-cho, Kure-shi, Hiroshima Prefecture Nisshin Steel Co., Ltd.Steel Research Laboratories Process and Steel Research Department (72) Inventor Toshiro Yamada 11-1 Showa-cho, Kure-shi, Hiroshima Prefecture No. Nisshin Steel Co., Ltd. Steel Research Laboratory Process and Steel Research Laboratory (56) References JP-A-2-173213 (JP, A) JP-A-3-281732 (JP, A) JP-A 5-655595 ( JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/48 C21D 8/04 C22C 38/00 C22C 38/48 C25D 3/20 C25D 5/26

Claims (2)

(57) [Claims] C: 0.001 to 0.008 by weight%;
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
-0.38, Mo: 0.14-3.0, sol. Al:
0.005 to 0.100, N: 0.005 or less, Nb:
0.005 to 0.03, 0.003 ≦ Ti ≦ (48/3
2) × S + (48/14) containing Ti in the range according to the × N, depending on further required than 1.0 Ni, 7. A slab of steel containing one or two types of Cr of 0 or less, the balance being Fe and unavoidable impurities: hot-rolled, pickled, cold-rolled, and then continuously molten zinc Galvanize by annealing at 800 to 950 ° C. in a plating line, or 800 to 95 in a continuous annealing line.
Hot-dip galvanized steel sheet for deep drawing with excellent baking hardenability and corrosion resistance consisting of annealing at 0 ° C, applying Fe alloy plating in a continuous electroplating line, and then galvanizing in a continuous galvanizing line Manufacturing method. 2. B: 0.0003 to 0.003 by weight%
The method for producing a hot-dip galvanized steel sheet for deep drawing according to claim 1, which is excellent in paint bake hardenability and corrosion resistance.