JPH1060589A - Galvanized steel sheet for working free from buckling wrinkly surface defect and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a galvanized thin steel sheet free from buckling wrinkly defects by subjecting an extra-low carbon steel contg. specified amounts of Ti, Nb, Zr, V or the like to hot rolling, coiling, cold rolling and annealing under specified temp. conditions and executing alloying treatment and skinpass rolling. SOLUTION: A slab having a compsn. contg., by weight, <0.0030% C, <0.05% Si, <0.50% Mn, <0.02% P, <0.020% S and <0.0030% N (including 0% in all cases), contg. 0.010 to 0.080% sol.Al and 0.0004 to 0.0015% B and furthermore contg. one or more kinds among Ti, Nb, Zr and V so as to satisfy the following inequality of Ti/48+Nb/96+Zr/91+V/51-N/14-S/32>4×C/12 is heated and is thereafter subjected to hot rolling to form into a steel sheet, which is furthermore coiled at >=550 deg.C, is cooled to an ordinary temp. so as to regulate the average cooling rate to 400 deg.c to 0.5 to 20 deg.C/min, is pickled and is thereafter subjected to cold rolling. Then, it is annealed at 830 deg.C in a continuous galvanizing line, is thereafter galvanized, is alloyed and is subsequently subjected to skinpass rolling to form into a thin sheet of <=1.2mm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inner and outer panel of an automobile,
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet having a favorable surface appearance suitable for electric equipment and building materials, and a method for producing the same.

[0002]

2. Description of the Related Art Since high workability is required for automobile inner and outer panels, a so-called IF steel in which Ti, Nb, etc. are added in an atomic ratio of not less than C and N to precipitate and fix these solute elements is used. Have been. This IF steel is suitable as a hot-dip galvanized steel sheet because a non-aged steel sheet can be produced without performing overaging treatment in continuous annealing, and many proposals have been made in connection with this. However, IF steel has many problems due to reduced impurity elements. For example, problems such as secondary working embrittlement due to weak grain boundaries are pointed out. Have been.

[0003]

In response to such a problem, in recent years, a B-added IF steel in which B which is easily segregated at grain boundaries is added to strengthen the grain boundaries has been used (for example, JP-A-59-140333). We consider this B
In the process of investigating and examining the properties of
The steel has a large yield point elongation at the unregulated stage compared with the B-free IF steel, and as a result, particularly in a thin material having a thickness of 1.2 mm or less, a wrinkle-like defect tends to occur in the manufacturing process. I found something. In recent years, demands from users for the steel sheet surface quality have become increasingly severe, and there has been a need to prevent or reduce the occurrence of such fine defects that were conventionally overlooked.
Therefore, an object of the present invention is to provide a hot-dip galvanized steel sheet for processing which does not have such a wrinkled surface defect and has excellent workability, and a method for producing the same.

[0004]

The constitution of the present invention for solving such a problem is as follows. (1) C: 0 to 0.0030 wt%, Si: 0 to 0.05
wt%, Mn: 0 to 0.50 wt%, P: 0 to 0.02
0 wt%, S: 0 to 0.020 wt%, sol. Al:
0.010-0.080 wt%, N: 0-0.0030
wt%, B: 0.0004 to 0.0015 wt%, and Ti, Zr, V, N in an amount satisfying the condition of the following formula (1).
1. A hot-dip galvanized steel sheet having a thickness of 1.2 mm or less and having no wrinkled wrinkle-like surface defects, wherein a steel sheet having a component composition containing one or more kinds of b is used as a base steel sheet. Ti / 48 + Nb / 96 + Zr / 91 + V / 51-N / 14-S / 32 ≧ 4 × (C / 12) (1) where Ti: Ti content (wt%), Zr: Zr content (wt%) , V: V content (wt%), Nb: Nb content (wt%), C: C content (wt%), N: N content (wt%), S: S content (wt%)

(2) C: 0 to 0.0030 wt%, Si:
0 to 0.05 wt%, Mn: 0 to 0.50 wt%, P:
0 to 0.020 wt%, S: 0 to 0.020 wt%, s
ol. Al: 0.010 to 0.080 wt%, N: 0 to 0
0.0030 wt%, B: 0.0004 to 0.0015
wt% and an amount of Ti, Zr,
A steel having a component composition containing one or more of V and Nb is hot-rolled to a winding temperature of 550 ° C. or higher, and an average cooling rate to at least 400 ° C. after winding is 0.5 to 20 ° C.
/ Min, followed by pickling and cold rolling, followed by annealing in a continuous galvanizing line at an annealing temperature of 830 ° C. or lower, followed by hot-dip galvanizing treatment. A method for producing a hot-dip galvanized steel sheet having a sheet thickness of 1.2 mm or less without surface defects. Ti / 48 + Nb / 96 + Zr / 91 + V / 51-N / 14-S / 32 ≧ 4 × (C / 12) (1) where Ti: Ti content (wt%), Zr: Zr content (wt%) , V: V content (wt%), Nb: Nb content (wt%), C: C content (wt%), N: N content (wt%), S: S content (wt%)

(3) In the manufacturing method of (2) above, after hot-dip galvanizing, alloying is performed, and then temper rolling is performed. A method for producing a hot-dip galvanized steel sheet for processing of 2 mm or less.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention and the reasons for limitation will be described below. The present inventors have investigated and investigated the cause of the occurrence of the above-described waistline wrinkle-like defect, and as a result, it was found that waistline wrinkles occurred near the top roll of the Sendzimir continuous galvanizing line, It originated from the iron-zinc alloy inevitably generated in the hot dipping line called our dross.
The added IF steel has a yield point elongation B at a temperature range of 300 ° C or lower.
The fact that it was larger than that of the unadded IF steel was ascertained.

Based on these facts, it is considered that the occurrence of the wrinkle-like defect is caused by the following mechanism. In other words, usually, in a continuous line, 1-5 kgf / mm ²
It is thought that dross adhering to the steel plate surface in the hot-dip galvanizing pot was pushed into the steel plate by the top roll due to the application of this tension, and this part would cause waist breakage. Can be And especially,
In the case of a thin steel plate with a thickness of 1.2 mm or less, in which the dross diameter is relatively large as compared with the steel plate thickness, it is considered that the steel plate receives more remarkable stress concentration, so that obvious wrinkle breakage occurs. Can be If the steel sheet yields continuously, even if it receives such stress concentration, it does not become a surface defect, but if the steel sheet yields discontinuously, it is considered that a wrinkle-like defect occurs.

A direct cause of the yield elongation of the B-added IF steel is that carbides such as TiC, ZrC, NbC, and VC re-dissolve in the annealing stage, but B segregates at grain boundaries.
Seems to be promoting this through some mechanism. Since the addition of B is indispensable for improving the performance of the product, a study was carried out to solve the above-mentioned problems on the premise of the addition of B. It was concluded that it was effective to add carbides to precipitate coarse carbides in the hot rolling step. further,
As a result of examining the preferable manufacturing method, it was concluded that it is efficient and effective to control the hot-rolling winding temperature and the cooling rate after winding, and to further suppress the annealing temperature.

The reasons for limiting the present invention will be described below. First, the reasons for limiting the component composition of the hot-dip galvanized base steel sheet are as follows. C: C is desirably reduced because it deteriorates the workability of the steel sheet, and is also an element that causes elongation at the yield point. Therefore, it is desirable to reduce C from the viewpoint of preventing the occurrence of creasing. Therefore, C is 0 to 0.0030 wt% as long as the effect of the present invention is not practically impaired.
(However, the case without addition is included). Si: Si not only deteriorates the workability of the steel sheet but also significantly deteriorates the adhesion between the plating film and the base steel sheet, so that it is desirable to reduce it. Therefore, as a range that does not impair the effects of the present invention, the content of Si is set to 0 to 0.05 wt% (including the case of no addition).

Mn: Mn is desirably reduced because Mn deteriorates the workability of the steel sheet.
t% (including the case of no addition). P: P is also desirably reduced because P deteriorates the workability of the steel sheet. Therefore, the content of P is set to 0 to 0.020 wt% (including the case of no addition). S: S is desirably reduced because S deteriorates the ductility of steel, and therefore, S is set to 0 to 0.020 wt% (including the case where no additive is added). sol. Al is added to steel as a deoxidizing agent. sol. If the content of Al is less than 0.010 wt%, the effect is not sufficient. On the other hand, if the content exceeds 0.080 wt%, the effect is saturated, and the adverse effect of inducing surface defects is caused. Therefore, sol. A
l is 0.010 to 0.080 wt%.

N: N is desirably reduced from the viewpoint of improving the workability of the steel sheet, and is set to 0 to 0.0030 wt% (including the case where it is not added) as a range that does not impair the effect of the present invention in practical use. B: B is an essential element for steel sheets used for working applications because B is an element effective for improving the brittleness resistance in secondary working and improving the r-value and effective for reducing the grain size of the hot-rolled sheet. However, if B is less than 0.0004 wt%, the effect cannot be sufficiently obtained. On the other hand, if B is added exceeding 0.0015 wt%, the effect is saturated and the workability is deteriorated. 0004 to 0.0015 wt%.

[0013] Ti, Zr, Nb, V: These elements precipitate and fix solid solution elements such as C and N and improve the workability of the steel sheet. In terms of stoichiometry, it is possible to precipitate all of these interstitial solid-solution elements by adding C, S, and N in an atomic equivalence ratio. ,
It is necessary to add these carbonitride forming elements in excess. Therefore, it is necessary to add one or more of these elements under the condition satisfying the following formula (1). If the addition amount of these elements falls below the lower limit defined by the lower limit expression (1), the above effects cannot be sufficiently obtained. Ti / 48 + Nb / 96 + Zr / 91 + V / 51-N / 14-S / 32 ≧ 4 × (C / 12) (1) where Ti: Ti content (wt%), Zr: Zr content (wt%) , V: V content (wt%), Nb: Nb content (wt%), C: C content (wt%), N: N content (wt%), S: S content (wt%)

Among these elements, Ti and Zr are coarser than carbides and are less likely to be re-dissolved in Ti ₄ C ₂ S ₂ , Zr ₄
Since a carbon sulfide such as C ₂ S ₂ is formed, there is an advantage that the precipitate can be easily coarsened in the hot rolling step.
Therefore, in order to obtain the effect of the present invention more reliably, T
It is desirable that i and / or Zr be added, and that the addition amount be N + S or more in terms of atomic weight ratio. The balance is substantially composed of Fe, but does not prevent the inclusion of other elements as long as the effects of the present invention are not impaired. The occurrence of the wrinkle-like defect is a problem peculiar to a plated steel sheet having a thickness of 1.2 mm or less. Therefore, the hot-dip galvanized steel sheet of the present invention is limited to a sheet thickness of 1.2 mm or less.

In the present invention, a hot-dip galvanized steel sheet free of creasing-like surface defects can be obtained irrespective of the production conditions, provided that the above-mentioned component conditions and the like are satisfied. It is on the street. Usually, the base steel sheet having the above component composition is manufactured through a series of steps of hot rolling, pickling, and cold rolling. In the manufacturing process, the winding temperature after hot rolling is 550 ° C. or higher. It is desirable that the average cooling rate to at least 400 ° C. after winding is 0.5 to 20 ° C./min. This is because the precipitate in the steel is coarsened by raising the winding temperature and gradually cooling after the winding to prevent the precipitate from re-dissolving during annealing. However, extreme slow cooling after winding causes enlargement of the scale and lowers the pickling efficiency. Therefore, the average cooling rate is desirably 0.5 to 20 ° C / min. The upper limit of the winding temperature is not particularly limited. However, if the temperature exceeds 750 ° C., the defect rate increases due to the enlargement of the surface oxide scale and the pickling efficiency decreases. Is preferred. Other manufacturing conditions are not particularly defined, but in order to obtain good workability, the finishing temperature is equal to or higher than the Ar ₃ transformation point, and the cooling pressure is 60 to 90%.
It is desirable to do.

The base steel sheet is annealed in a continuous hot-dip galvanizing line and subsequently hot-dip galvanized.
The annealing temperature is preferably 830 ° C. or less. This is because, when annealing is performed at a temperature exceeding 830 ° C., fine precipitates such as TiC precipitated in the temperature-raising stage re-dissolve in solution to increase the amount of solid solution C which causes a wrinkle-like defect. . Normally, a hot-dip galvanized steel sheet for processing is subjected to alloying treatment and temper rolling after plating to obtain a product. In the present invention, the iron making method (blast furnace method, electric furnace method, smelting reduction method, etc.), slab casting method (continuous casting method, slab casting method, thin slab casting method, etc.), and hot rolling method (direct rolling, slab casting) There is no special restriction on processes such as reheating rolling, and any of these processes does not affect the effects of the present invention.

[0017]

EXAMPLE A steel slab having the composition shown in Table 1 was prepared at 1200 ° C.
To finish hot rolling at 880 ° C., and then wound up at 650 ° C. After pickling, the hot-rolled steel sheet was cold-rolled at a rolling reduction of 80% to obtain a cold-rolled steel sheet. The cold-rolled steel sheet is annealed at 850 ° C. for 5 minutes in a continuous hot-dip galvanizing line of a Sendzimir type, and then hot-dip galvanizing, alloying and temper rolling are sequentially performed to form a 0.7 mm thick alloyed molten steel. Galvanized steel sheet was manufactured.
About the obtained alloyed hot-dip galvanized steel sheet, the presence or absence of a wrinkle-like wrinkle-like defect on the surface of the steel sheet, a ductility-brittle transition temperature as an index of secondary working brittleness resistance, total elongation, and a Rankford value were evaluated. Table 2 shows the results.

[0018] The evaluation of the buckling wrinkle-like defects, those that do not occur at all wrinkles "◎", 10cm ² per three or less of the things that minor wrinkle has occurred "○", 10cm ² per four or more teeth Those having wrinkles were evaluated as "x". The brittle transition temperature was defined as the lowest temperature at which brittle fracture did not occur when a steel sheet was punched into a blank of 105 mmφ, cup-formed at a drawing ratio of 2.1, and pressed in a refrigerant cooled to a predetermined temperature. According to Table 2, the hot-dip galvanized steel sheet of the present invention not only has excellent surface properties, but also has a brittle transition temperature of -90 ° C or less, a total elongation of 48% or more, and excellent properties such as a Rankford value of 2.0 or more. It shows that the steel sheet is suitable for press forming.

Example 2 Hot rolling was started by heating steel slabs of steel numbers 1-4 shown in Table 1 to 1200 ° C.
After finishing rolling at ℃, winding was carried out at the temperature shown in Table 3, and after winding, cooling was carried out up to 400 ° C at the average cooling rate shown in Table 3, followed by cooling to room temperature. After pickling, the hot-rolled steel sheet was cold-rolled at a rolling reduction of 80% to obtain a cold-rolled steel sheet. This cold rolled steel sheet was annealed in a continuous hot-dip galvanizing line of the Zenmigia type under the conditions shown in Table 3, and subsequently subjected to hot-dip galvanizing, alloying and temper rolling in order to form a 0.7 mm thick alloyed melt. Galvanized steel sheet was manufactured. About the obtained alloyed hot-dip galvanized steel sheet, the presence or absence of a crease-like wrinkle-like defect was determined by the same method and standard as in [Example 1].
The ductility-brittle transition temperature, total elongation and Rankford values were evaluated. Table 4 shows the results. Table 4 shows that the hot-dip galvanized steel sheet produced by the method of the present invention is a plated steel sheet having excellent surface properties and excellent workability and suitable for press forming.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

As described above, according to the present invention, it is possible to obtain a hot-dip galvanized steel sheet having no surface defects such as wrinkle-like surface defects and excellent workability.

Continuation of the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical indication C23C 2/28 C23C 2/28 (72) Inventor Shunsaku Node 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Sun (72) Inventor Seiji Nakamura 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan In-tube (72) Inventor Tomora Ohkita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Inside Steel Pipe Co., Ltd.

Claims (3)

[Claims] C: 0 to 0.0030 wt%, Si: 0
-0.05 wt%, Mn: 0-0.50 wt%, P: 0
-0.020 wt%, S: 0-0.020 wt%, so
l. Al: 0.010 to 0.080 wt%, N: 0 to 0
0.0030 wt%, B: 0.0004 to 0.0015
wt% and an amount of Ti,
A hot-dip galvanized steel sheet having a thickness of 1.2 mm or less and having no wrinkle-like surface defects, characterized in that a steel sheet having a component composition containing one or more of Zr, V, and Nb is used as a base steel sheet. . Ti / 48 + Nb / 96 + Zr / 91 + V / 51-N / 14-S / 32 ≧ 4 × (C / 12) (1) where Ti: Ti content (wt%), Zr: Zr content (wt%) , V: V content (wt%), Nb: Nb content (wt%), C: C content (wt%), N: N content (wt%), S: S content (wt%) 2. C: 0 to 0.0030 wt%, Si: 0
-0.05 wt%, Mn: 0-0.50 wt%, P: 0
-0.020 wt%, S: 0-0.020 wt%, so
l. Al: 0.010 to 0.080 wt%, N: 0 to 0
0.0030 wt%, B: 0.0004 to 0.0015
wt% and an amount of Ti, Zr,
A steel having a component composition containing one or more of V and Nb is hot-rolled to a winding temperature of 550 ° C. or higher, and an average cooling rate to at least 400 ° C. after winding is 0.5 to 20 ° C.
/ Min, followed by pickling and cold rolling, followed by annealing in a continuous galvanizing line at an annealing temperature of 830 ° C. or lower, followed by hot-dip galvanizing treatment. A method for producing a hot-dip galvanized steel sheet having a sheet thickness of 1.2 mm or less without surface defects. Ti / 48 + Nb / 96 + Zr / 91 + V / 51-N / 14-S / 32 ≧ 4 × (C / 12) (1) where Ti: Ti content (wt%), Zr: Zr content (wt%) , V: V content (wt%), Nb: Nb content (wt%), C: C content (wt%), N: N content (wt%), S: S content (wt%) 3. A hot-dip galvanizing treatment, an alloying treatment, and then a temper rolling.
2. A method for producing a hot-dip galvanized steel sheet having a thickness of 1.2 mm or less and having no wrinkle-like surface defects as described in 1).