JP3347166B2 - Manufacturing method of paint bake hardening type hot-dip galvanized steel sheet excellent in deep drawability 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 galvanized steel plate which is mainly used for an automobile body and which is excellent in deep drawability 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 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 a low-carbon Al-killed steel by continuous annealing.
No. 1 discloses a method of producing by ultra-low carbon Ti added steel by continuous annealing. These are paint bake-hardened cold-rolled steel sheets that not only increase the strength of the steel sheet, but also have excellent formability, and are soft at the time of forming, but are strengthened by paint baking after press forming. It is a manufacturing method of.

[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 corrosion-resistant hole opening life is shortened. 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 on hot-dip galvanized steel sheets based on added steel and on high-strength hot-dip galvanized steel sheets having increased strength by adding Si, Mn, P, and Cr thereto.

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-mentioned problems, the present inventors have made various studies on a method for producing a hot-dip galvanized steel sheet which has improved corrosion resistance of the steel sheet itself and has excellent paint bake hardenability and deep drawability. Based on ultra-low carbon steel or ultra-low carbon + trace amount of Ti-added steel, composite addition of P and Cu, and further addition of Si, Mn and Ni, Mo, Cr, etc., make the paint bake hardenability and corrosion resistance Of a method for producing a hot-dip galvanized steel sheet having excellent processing properties.
173924 and JP-A-4-173925.

However, the present invention mainly focuses on the improvement of bake hardenability and corrosion resistance, and has not been able to obtain sufficient deep drawability.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides a method for producing a very excellent deep drawability in a paint-bake hardened galvanized steel sheet having improved corrosion resistance of the steel sheet itself. The purpose is to:

[0011]

Means for Solving the Problems The present inventors have conducted a detailed study on a method for producing a paint-baking-hardened hot-dip galvanized steel sheet with the addition of ultra-low carbon P and Cu composites having improved corrosion resistance of the steel sheet itself. As a result, of the added elements, the amount of P is appropriately regulated, and a small amount of Ti that cannot fix solid solution C as TiC.
It was found that excellent deep drawability can be obtained by adding a small amount of Nb as needed and controlling the hot rolling conditions.

That is, in the present invention, C: 0.00% by weight.
1 to 0.008, Si: 0.02 to 1.5, Mn: 0.05
~ 1, P: 0.05-0.18, S: 0.015 or less, C
u: 0.05 to 1, sol. Al: 0.005 to 0.1,
N: 0.005 or less, 0.003 ≦ Ti ≦ (48/14) ×
After casting a steel slab containing Ti in the range according to N + (48/32) × S and the balance being Fe and unavoidable impurities, the steel was once cooled to 700 ° C. or lower and reheated to a temperature lower than 1200 ° C. Hot rolling, pickling, cold rolling, and then 700 to 9 in a continuous galvanizing line
Zinc plating by annealing at 00 ° C, or annealing at 700 to 900 ° C in a continuous annealing line, and applying Fe alloy plating in a continuous electroplating line, followed by galvanizing in a continuous galvanizing line The present invention provides a method for producing a paint-bake-hardened hot-dip galvanized steel sheet having excellent deep drawability and corrosion resistance.

[0013] The present invention also provides C: 0.001 to
0.008, Si: 0.02 to 1.5, Mn: 0.05 to 5
1, P: 0.05 to 0.18, S: 0.015 or less, Cu:
0.05 to 1.0, sol. Al: 0.005 to 0.1,
N: 0.005 or less, 0.003 ≦ Ti ≦ (48/14) ×
Ti in the range according to N + (48/32) × S, and further N
b: containing 0.005 to 0.03, with the balance being 70% after casting a steel slab consisting of Fe and unavoidable impurities.
Cooled to 0 ° C or lower, reheated to a temperature of less than 1200 ° C, hot-rolled, pickled, cold-rolled,
After performing 800 to 920 ° C annealing in the continuous galvanizing line and performing galvanizing, or performing 800 to 920 ° C annealing in the continuous annealing line and performing Fe alloy plating in the continuous electroplating line, Provided is a method for producing a paint-baking-hardened hot-dip galvanized steel sheet which is excellent in deep drawability and corrosion resistance by galvanizing with a continuous hot-dip galvanizing line.

The present invention also provides C: 0.001 to 0.1% by weight.
0.008, Si: 0.02 to 1.5, Mn: 0.05 to 5
1, P: 0.05 to 0.18, S: 0.015 or less, Cu:
0.05 to 1, sol. Al: 0.005 to 0.1, N: 0.1.
005 or less, 0.003 ≦ Ti ≦ (48/14) × N + (4
8/32) × S, and 0.02 to 1.
After casting a steel slab containing one or two of Ni of 0 and Mo of 0.02 to 2.0, and the balance being Fe and unavoidable impurities, the steel was once cooled to 700 ° C.
Hot-rolled by reheating to a temperature below 0 ° C., after pickling,
Perform cold rolling, and then perform galvanizing by annealing at 700 to 900 ° C in a continuous hot-dip galvanizing line,
Alternatively, the steel sheet is annealed at a temperature of 700 to 900 ° C. in a continuous annealing line, is subjected to Fe alloy plating in a continuous electroplating line, and is excellent in deep drawability and corrosion resistance, which is galvanized in a continuous galvanizing line. Provided is a method for producing a paint bake hardening type galvanized steel sheet.

The present invention also provides C: 0.001% by weight.
0.008, Si: 0.02 to 1.5, Mn: 0.05 to 5
1, P: 0.05 to 0.18, S: 0.015 or less, Cu:
0.05 to 1, sol. Al: 0.005 to 0.1, N: 0.1.
005 or less, 0.003 ≦ Ti ≦ (48/14) × N + (4
8/32) × S, and 0.02 to 1.
0 or Ni and one or two kinds of Mo of 0.02 to 2 and further Nb: 0.005 to 0.03, with the balance being Fe
After casting a steel slab consisting of unavoidable impurities, the steel is once cooled to 700 ° C. or lower, reheated to a temperature lower than 1200 ° C., hot-rolled, pickled, cold-rolled, and then continuously. 800-920 ° C in the hot-dip galvanizing line
To perform galvanizing or annealing at 800 to 920 ° C. in a continuous annealing line, and applying an Fe alloy plating in a continuous electroplating line, and then galvanizing in a continuous galvanizing line. Provided is a method for producing a paint-bake-hardened hot-dip galvanized steel sheet having excellent drawability and corrosion resistance.

According to the present invention, the steel has excellent deep drawability and corrosion resistance according to any one of the preceding claims, wherein the steel contains B: 0.0003 to 0.003% by weight in addition to the above-mentioned respective compositions. Provided is a method for producing a paint-bake-hardened hot-dip galvanized steel sheet.

First, the operation of various components of the steel sheet used in the manufacturing method according to the present invention and the reason for limiting the above range will be described. C is an element indispensable for obtaining baking hardenability of the coating, and 0.001% or more is necessary to obtain sufficient bake hardenability. On the other hand, when the content exceeds 0.008%, the amount of solid solution C increases and very high baking hardenability can be obtained, but aging at room temperature occurs, causing a rapid deterioration of ductility.
For this reason, C was limited to the range of 0.001 to 0.008%.

Si is an element that effectively acts to increase strength and improve pitting corrosion resistance. However, if less than 0.02%, the effect is not recognized.
Scale flaws are generated in the hot rolling step to deteriorate the surface properties of the product, and the product becomes hard and deteriorates in ductility. Therefore, the range of the addition amount is set to 0.02 to 1.5%.

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 1.0%, the deep drawability deteriorates.
It was limited to the range of 0.05 to 1%.

P is a characteristic element in the present invention, and significantly improves the corrosion resistance to pitting by adding it with Cu, and adding Fe and Fe by adding it with Ti.
Precipitates compounds of Ti and P to improve deep drawability. From the viewpoint of improving corrosion resistance, the appropriate amount of addition is 0.03.
~ 0.20%, but less than 0.05% or 0.2%.
If the addition exceeds 18%, the deep drawability deteriorates. Therefore, the addition amount was limited to 0.05 to 0.18%.

S is an element harmful to the deep drawability, and the smaller the amount, the better. However, since it is acceptable up to 0.015%,
0.015% or less.

Cu is an element effective for improving corrosion resistance by being added in combination with P as described above.
If it is less than 0.05%, the effect is not recognized. 1%
Is added, the effect is saturated and the cost is increased. Therefore, the content is limited to the range of 0.05 to 1%.

Al is added as a deoxidizing agent, and 0.005% or more is required to fulfill its role. However, if added in excess of 0.10%, Al ₂ O
_Since inclusions such as ₃ increase and degrade workability and surface quality, the range is limited to 0.005 to 0.1%.

N is an element which is harmful to corrosion resistance and deep drawability, and is preferably as small as possible. However, up to 0.005% is acceptable, so it is set to 0.005% or less.

[0025] Ti is an element effective for securing the deep drawability, but its effect is not recognized if it is less than 0.003%. Further, Ti forms a compound with N, S, C or P and precipitates as a precipitate such as TiN, TiS, TiC, FeTiP.
If it is added in an amount exceeding that calculated as total precipitation, sufficient paint bake hardenability cannot be obtained. Therefore, the lower limit of the addition amount is 0.003%, and the upper limit is (48 /
14) × N + (48/32) × S%.

Nb is an element effective for improving the deep drawability, but its effect is not recognized at less than 0.005%. Also, if the content exceeds 0.03%, not only the effect is saturated, but also the recrystallization temperature is unnecessarily increased, and the deep drawability and the ductility are rather deteriorated.
It was limited to the range of 5 to 0.03%.

The steel of the present invention has a Ni content of 0.02 to 1% and a 0.02 to 1.0% content in order to increase the strength of the steel sheet and to improve the corrosion resistance.
It may contain one or two of 2% Mo.

Ni effectively prevents hot brittleness by Cu and improves pitting corrosion resistance. However, if less than 0.02%, the effect is not recognized. If more than 1%, the effect is not observed. Since the effect is saturated and the production cost is increased, the range of the addition amount is set to 0.02 to 1%.

Mo effectively acts to increase the strength of the steel sheet and to improve the corrosion resistance to perforation. However, if less than 0.02%, the effect is not recognized, and if more than 2%, the effect is reduced. In addition to being saturated, it becomes hard and deteriorates ductility, further increasing the cost.
2%.

Further, in the present invention, in order to improve the resistance to secondary work cracking, 0.0003 to 0.003 is added to steel.
B can be added.

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 slab heating conditions in the hot rolling step are controlled. And very good deep drawability can be obtained. That is, after the slab is cast, it is once cooled to a temperature of 700 ° C. or less, and then reheated to a temperature of less than 1200 ° C. to perform hot rolling. It is presumed that this affects the precipitates and precipitates during the subsequent cold rolling annealing, and forms a texture that is advantageous for deep drawability.

The finishing temperature in the hot rolling 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 to be 750 ° C.

In the cold rolling step, a cold-rolling rate 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 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 directly 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.

The annealing temperature in the continuous galvanizing line or the continuous annealing line is as follows when Nb is not added.
In order to impart workability above the recrystallization temperature, 70
It must be at least 0 ° C. However, when the temperature exceeds 900 ° C., the crystal grain size becomes coarse, and after the product is processed, a surface defect called orange peel is generated.
The range was limited to 900 ° C.

On the other hand, since the recrystallization temperature rises when Nb is added, it is necessary to perform annealing at a higher temperature to improve the workability at a temperature higher than the recrystallization temperature. Is deposited as NbC in order to impart the seizure effect.
Since C must be dissolved, high-temperature annealing at 800 ° C. or higher is required. However, even if annealing is performed at a temperature exceeding 920 ° C., the effect is not only saturated, but also a surface flaw is easily generated in a continuous galvanizing line or a continuous annealing line.

[0037]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be illustrated below by way of examples. Example 1 A slab of 17 steels having the composition shown in Table 1 was cast, then cooled to 700 ° C. or lower, hot-rolled under the conditions shown in Table 2 into a hot-rolled sheet having a thickness of 3.2 mm, and pickled. Thereafter, a cold-rolled steel sheet having a thickness of 0.8 mm is applied by applying a cold pressure, and is annealed in a continuous galvanizing line except for No. 13 to obtain a 45 mm per side.
g / m ² of hot-dip galvanized. Then, No.1 ~
Sample No. 10 was subjected to in-line alloying treatment of the plating layer, and skin pass rolling was performed at an elongation of 0.8%. No. 13 is
After the cold-rolled steel sheet is annealed in a continuous annealing line, a continuous electroplating line is coated with 2 g / m ² of Fe-B alloy per side, and a continuous hot-dip galvanizing line is used.
g / m ² hot-dip galvanizing, elongation: 0.8%
Was subjected to skin pass rolling. The tensile properties and corrosion resistance of the obtained hot-dip galvanized steel sheet were investigated, and the results are shown in Table 2.

For the investigation of the tensile properties, a No. 5 test piece of JISZ2201 was used. 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. Composite corrosion test, salt spray test according to J ISZ2371 (5% N
aCl, 35 ° C. ± 1 ° C., humidity 98% or more) for 2 hours, 6
A dry test at 0 ° C. was performed for 4 hours, and a wet test at 50 ° C. and a humidity of 95% or more was performed under a condition that one cycle was a total of 8 hours of 2 hours.

No. 1 to No. 4 are comparative steels in which the amount of P was changed without adding Ti. These indicate that the corrosion resistance becomes better with an increase in the P content, but the average r value, which is an index of deep drawability, decreases with an increase in the P content. In addition, each shows a value lower than the level of the steel of the present invention. No. 6-N
No. 9 is a steel in which the amount of P was changed by adding Ti in the range specified in the present invention, and the average r-value showed a peak at P: 0.08%, which was within the range of the present invention. 0.05 to 0.18%
It can be seen that a high average r value can be obtained. Corrosion resistance,
It can be seen that the steel in the range of the present invention exhibits excellent corrosion resistance as compared with the comparative steels No. 1 and No. 5 in which P is less than the range of the present invention.

In No. 11, Ti was added beyond the range specified in the present invention, but the average r-value showed a good value, but no bake hardenability (BH) was obtained. No.12
Although Nb is added, a high average r value is obtained without performing baking hardenability (BH) by annealing at a high temperature. No. 13 to No. 17 are Si,
Although Ni and Mo were added in the range specified in the present invention, both of them effectively act to increase the strength and improve the corrosion resistance.

Example 2 A slab having the composition shown in No. 7 and No. 12 in Table 1 was cast, cooled once to 700 ° C. or less, and hot-rolled under the conditions shown in Table 3 to obtain a sheet having a thickness of 3.2 mm. Hot-rolled sheet, pickled and then cold-pressed to form a cold-rolled steel sheet with a thickness of 0.8 mm, annealed at various temperatures in a continuous hot-dip galvanizing line, and melted at 45 g / m ² per side. Galvanized. afterwards,
Elongation: skin pass rolling at 0.8% was performed. The tensile properties and corrosion resistance of the obtained steel sheet were investigated, and the results are shown in Table 3.

No. 7 shows that when the slab heating temperature is as high as 1250 ° C., the average r-value is inferior to that of the example of the present application where other manufacturing conditions are equal. Also, if the annealing temperature is 7
When the temperature is lower than 00 ° C., recrystallization and grain growth are not sufficient, the yield point (YS), the strength (TS) are high, and the elongation (El) is high.
And the average r value is inferior. On the other hand, when the temperature exceeds 900 ° C., the crystal grain size becomes coarse, so that orange peel was generated on the surface of the sample after the tensile test. In addition, in No. 12, a good value of the average r value was obtained by annealing at 750 ° C., but the solid solution of NbC was not sufficient, and the bake hardenability (BH) was not obtained.

[0043]

As described above, the present invention has clarified a method for producing a steel sheet which imparts excellent deep drawability to a paint-baked hardened galvanized steel sheet having excellent corrosion resistance.
INDUSTRIAL APPLICABILITY The present invention greatly contributes to weight reduction and long life of automobiles, and its industrial significance and profit are extremely large.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

Continued on the front page (72) Inventor Seiichi Hamanaka 11-1 Showa-cho, Kure-shi, Hiroshima Prefecture Nisshin Steel Manufacturing Co., Ltd. Steel Research Laboratory Process and Steel Research Laboratory (72) Inventor Mikio Soshita Kure-shi, Hiroshima 11-1 Showa-cho Nisshin Steel Co., Ltd. Steel Research Laboratory, Process and Steel Research Laboratory (56) References JP-A-4-80323 (JP, A) JP-A-4-173925 (JP, A) JP-A-2-173213 (JP, A) JP-A-4-168246 (JP, A) JP-A-3-107426 (JP, A) JP-A-4-246128 (JP, A) (58) Fields investigated (Int) .Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C22C 38/00-38/60

Claims (5)

(57) [Claims] C: 0.001 to 0.008 by weight%, S
i: 0.02 to 1.5, Mn: 0.05 to 1, P: 0.05
~ 0.18, S: 0.015 or less, Cu: 0.05-1, s
ol. Al: 0.005 to 0.1; N: 0.005 or less;
003 ≦ Ti ≦ (48/14) × N + (48/32) × S containing Ti in a range according to S, the remainder being cast after casting a steel slab consisting of Fe and unavoidable impurities, and then temporarily cooled to 700 ° C. or less, Hot-rolled by reheating to a temperature of less than 1200 ° C, cold-rolled after pickling, and then galvanized by annealing at 700 to 900 ° C in a continuous hot-dip galvanizing line, or 700 ~ in continuous annealing line
After annealing at 900 ° C, the continuous electroplating line
A method for producing a paint-bake hardened hot-dip galvanized steel sheet having excellent deep drawability and corrosion resistance comprising galvanizing a continuous hot-dip galvanizing line after applying an alloy plating. 2. C: 0.001 to 0.008 by weight%, S
i: 0.02 to 1.5, Mn: 0.05 to 1, P: 0.05
~ 0.18, S: 0.015 or less, Cu: 0.05-1, s
ol. Al: 0.005 to 0.1; N: 0.005 or less;
003 ≦ Ti ≦ (48/14) × N + (48/32) × S. A steel slab containing Ti in the range according to S and Nb: 0.005 to 0.03, with the balance being Fe and unavoidable impurities. After casting, once cooled to 700 ° C or less, 12
Hot-rolled by reheating to a temperature of less than 00 ° C, hot-rolled, pickled, cold-rolled, and then galvanized by annealing at 800 to 920 ° C in a continuous galvanizing line, or A coating excellent in deep drawability and corrosion resistance consisting of performing annealing at 800 to 920 ° C. in a continuous annealing line, applying Fe alloy plating in a continuous electroplating line, and then galvanizing in a continuous galvanizing line. Manufacturing method of bake hardening type hot-dip galvanized steel sheet. 3. C: 0.001 to 0.008 by weight%, S
i: 0.02 to 1.5, Mn: 0.05 to 1, P: 0.05
~ 0.18, S: 0.015 or less, Cu: 0.05-1, s
ol. Al: 0.005 to 0.1; N: 0.005 or less;
003 ≦ Ti ≦ (48/14) × N + (48/32) × range according to S Ti, and 0.02 to 1 of Ni, 0.02 to
2 contains one or more Mo, and the remainder is Fe
After casting a steel slab consisting of unavoidable impurities, the steel is once cooled to 700 ° C. or lower, reheated to a temperature lower than 1200 ° C., hot-rolled, pickled, cold-rolled, and then continuously. 700-900 ° C in hot-dip galvanizing line
To perform galvanizing, or to anneal at 700 to 900 ° C in a continuous annealing line, apply Fe alloy plating in a continuous electroplating line, and then galvanize in a continuous galvanizing line. A method for producing a paint-bake-hardened hot-dip galvanized steel sheet having excellent deep drawability and corrosion resistance. 4. C: 0.001 to 0.008 by weight%, S
i: 0.02 to 1.5, Mn: 0.05 to 1, P: 0.05
~ 0.18, S: 0.015 or less, Cu: 0.05-1, s
ol. Al: 0.005~0.1, N: 0.005 or less, 0.
003 ≦ Ti ≦ (48/14) × N + (48/32) × S
Range of Ti according to, and from 0.02 to 1 of Ni, 0.02
1 to 2 or more Mo, and Nb: 0.
After casting a steel slab containing 005 to 0.03, the balance being Fe and unavoidable impurities, the steel was once cooled to 700 ° C or lower, reheated to a temperature lower than 1200 ° C, hot-rolled, After washing, cold rolling is performed, and thereafter, galvanizing is performed by annealing at 800 to 920 ° C. in a continuous galvanizing line, or 800 in a continuous annealing line.
~ 920 ° C annealing, and F in continuous electroplating line
A method for producing a paint-bake hardened hot-dip galvanized steel sheet having excellent deep drawability and corrosion resistance, which is performed by galvanizing a continuous hot-dip galvanizing line after e-alloy plating. 5. The steel has a weight percentage of B: 0.0003 to 0.0.
The method for producing a paint-baking-hardened hot-dip galvanized steel sheet having excellent deep drawability and corrosion resistance according to any one of the preceding claims, which contains 0.3%.