JP3319332B2 - Method of manufacturing hot-dip galvanized steel sheet free from wrinkle-like surface defects - Google Patents

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,
Melting for processing with good surface appearance suitable for electrical equipment, building materials, etc.
The present invention relates to a method for manufacturing a galvanized steel sheet .

[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 IF-added steel,
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.
Accordingly, an object of the present invention is to provide a molten zinc for processing which has no such wrinkle-like surface defects and has excellent workability.
An object of the present invention is to provide a method for manufacturing a plated steel sheet .

[0004]

The constitution of the present invention for solving such a problem is as follows .

[1] In a continuous hot-dip galvanizing line,
C: 0.0030 wt% or less , Si: 0.05 wt% or less
Bottom , Mn: 0.5 wt% or less , P: 0.02 wt% or less
Bottom , S: 0.020 wt% or less , sol. Al: 0.0
10 to 0.080 wt %, N: 0.0030 wt% or less
Below , B: 0.0004-0.0015 wt%, Cu:
A steel sheet having a component composition of 0.01 to 0.05 wt% and containing one or more of Ti, Zr, V, and Nb in an amount satisfying the following equation (1) satisfies the following equation (2). A method for producing a hot-dip galvanized steel sheet having a thickness of 1.2 mm or less without wrinkle-like surface defects, comprising annealing under conditions and subsequently performing hot-dip galvanizing treatment. C / 12 ≦ Ti / 48 + Nb / 96 + Zr / 91 + V /
51-N / 14-S / 32 ≦ 4 × (C / 12)
... (1) ts ≦ exp {(890-Ta) / 100} × (Ti /
48 + Nb / 96 + Zr / 91 + V / 51-N / 14-
S / 32) × (12 / C) (2) where Ti: Ti content (wt%), Zr: Zr content (wt%), V: V content (wt%), Nb: Nb content Amount (wt%), C: C content (wt%), N: N content (wt%), S: S content (wt%), Ta: annealing temperature (° C), ts: soaking time ( min)

[2] In the manufacturing method of the above [1] , after hot-dip galvanizing treatment, alloying treatment 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 has been found that waistline wrinkles are occurring 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 to give rise to waist breakage from this part due to this tension application. 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.

Although the reason why the B-added IF steel exhibits a large yield point elongation is not always clear, it is considered that B segregated at the grain boundaries has some adverse effect. Since the addition of B is indispensable for improving the product performance, the study was carried out to solve the above problems on the premise of the addition of B. As a result, it was found that the reduction of the solute C in the production stage was to prevent the wrinkle-like defect. Was found to be effective. As a result of diligent study of this measure, it was concluded that if Cu is effectively used, solid solution C can be effectively reduced, and good surface properties free from buckling can be obtained. This is because the sulfide is precipitated by the addition of Cu and the sulfide effectively functions as a precipitation nucleus of carbide, so that the solid solution C in the steel sheet is reduced.
It is considered that the waistline wrinkle-like defect is improved. Further, regarding the production conditions of the plated steel sheet, it was found that lowering the annealing temperature as much as possible to prevent the solid solution of the carbide again was more effective in preventing wrinkle-like wrinkle-like defects.

The reasons for limiting the component conditions and production conditions of the present invention will be described below. First, the reasons for limiting the composition of the base steel sheet of the hot-dip galvanized 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. For this reason, C is 0.0030 wt% or less within a range that does not impair the effects of the present invention practically.
And 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, the content of Si is set to 0.05 wt% or less as long as the effect of the present invention is not impaired.

Mn: Mn is desirably reduced because it hardens the steel sheet and deteriorates workability.
0.5 wt% or less . P: P is also desirably reduced because it hardens the steel sheet and deteriorates the workability.
And S: S is desirably reduced because S deteriorates the ductility of steel, and therefore is set to 0.020 wt% or less . sol. Al is added to steel as a deoxidizing agent. so
l. 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. Al is 0.010-0.
080 wt%.

N: N is desirably reduced from the viewpoint of improving the workability of the steel sheet, and is set to 0.0030 wt% or less as a range that does not impair the effect of the present invention practically. 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%. Cu: Cu is effective not only for generating CuS and detoxifying S but also for contributing to the improvement of buckling wrinkles because the generated CuS functions effectively as a precipitation nucleus of carbide. If Cu is less than 0.01 wt%, the effect of the addition cannot be sufficiently obtained, while if it exceeds 0.05 wt%, surface defects called Cu flaws are easily induced. Therefore, Cu is set to 0.01 to 0.05 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. However, excessive addition not only increases the cost, but also degrades the surface quality of the steel sheet. Therefore, it is necessary to add one or more of these elements under the condition satisfying the following formula (1). If the amount of these elements falls below the lower limit defined by the following formula (1), the above effects cannot be sufficiently obtained. C / 12 ≦ Ti / 48 + Nb / 96 + Zr / 91 + V / 51−N / 14−S / 3 2 ≦ 4 × (C / 12) (1) where Ti: Ti content (wt%), Zr: Zr content Amount (wt%), V: V content (wt%), Nb: Nb content (wt%), C: C content (wt%), N: N content (wt%), S: S content Amount (wt%)

Among these elements, Ti and Zr form carbosulfides which are coarser than carbides and are less likely to form a solid solution again, and thus have the advantage that the precipitates can be easily coarsened in the hot rolling stage. . Therefore, in order to more reliably obtain the effects of the present invention, it is desirable to add Ti and / or Zr and to make the addition amount at least the N + S amount in atomic ratio. The balance is substantially composed of Fe, and does not prevent the inclusion of other elements as long as the effects of the present invention are not impaired. For example, Cr up to 0.1 wt% as an impurity element, 0.1 wt%
Up to 0.01 wt%, Sn and the like. The occurrence of a wrinkled surface defect is a problem specific to a plated steel sheet having a thickness of 1.2 m or less. Therefore, the hot-dip galvanized steel sheet of the present invention is limited to a plated steel sheet having a thickness of 1.2 mm or less.

According to the present invention, a hot-dip galvanized steel sheet having no wrinkle-like surface defects can be obtained irrespective of the production conditions provided that the above-mentioned component conditions are satisfied. is there. The base steel sheet having the above component composition is manufactured through a series of steps of hot rolling, pickling, and cold rolling, and then subjected to annealing and hot dip galvanizing in a continuous hot dip galvanizing line. If the annealing is performed under the following conditions, better surface properties can be obtained. ts ≦ exp {(890-Ta) / 100} × (Ti / 48 + Nb / 96 + Zr / 91 + V / 51-N / 14-S / 32) × (12 / C) (2) 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%), Ta: annealing temperature (° C), ts: soaking time (min)

The ease of re-dissolution of carbide during annealing is governed by the annealing temperature, soaking time, the amount of carbonitride forming element added, and the amount of carbon. In other words, when the annealing temperature is high, carbide re-dissolves even in a short annealing time (soaking time), but when the annealing temperature is low or when the amount of carbonitride forming elements is large, the carbide re-dissolves. It is difficult. On the other hand, in order to enhance the workability of the steel sheet, annealing at a high temperature for a long time is desirable. The present inventors have studied the appropriate annealing conditions for obtaining good materials and surface properties, and as a result, if the conditions given by the above equation (2) are satisfied,
It was found that a steel sheet excellent in workability and free of wrinkle-like defects was obtained. Long-time annealing exceeding the condition of the above-mentioned formula (2) promotes re-dissolution of carbides to increase the amount of solute C, thereby causing a wrinkle-like wrinkle-like defect. FIG. 1 shows the ranges of the annealing temperature and the soaking time specified by the above equation (2) for steel No. 1 in Table 1.

The hot rolling heating temperature, finishing temperature, winding temperature, and cold pressure ratio are not particularly limited, but in order to obtain good workability and surface quality, the heating temperature is 1200 ° C. or less and the finishing temperature is Ar ₃ transformation. It is preferable that the winding temperature is set to 550 to 750 ° C. and the cooling rate is set to 60 to 90%. 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, bulk casting method, thin slab casting method, etc.), hot rolling method (direct feed 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.

[0018]

【Example】

[Example 1] A steel slab having the composition shown in Table 1 was used in 1200
After heating to ℃ to start hot rolling, finishing rolling at 890 ° C, winding was performed at 650 ° C. Further, the hot-rolled steel sheet was pickled and then cold-rolled at a reduction of 75% to obtain a cold-rolled steel sheet. This cold-rolled steel sheet is annealed at 850 ° C. in a Sendzimir-type continuous hot-dip galvanizing line, and subsequently subjected to hot-dip galvanizing treatment, alloying treatment and temper rolling in order to obtain a 0.7 mm thick alloyed hot-dip galvanized steel plate. Was manufactured.

The obtained alloyed hot-dip galvanized steel sheet was evaluated for the presence or absence of wrinkle-like wrinkle-like defects on the steel sheet surface, the presence or absence of plating unevenness, the ductility-brittle transition temperature, which is an index of secondary work brittleness resistance, and the total elongation. . Table 2 shows the results. The evaluation of buckling wrinkles defects, those which do not occur at all wrinkles "◎", those wrinkles is less than ² per one 10 cm "○", also more than a ² per two 10 cm "× Was evaluated. The brittle transition temperature is 105 mm
After punching into a blank of φ, a cup was formed at a drawing ratio of 2.1, and this was specified at the lowest temperature at which brittle fracture did not occur when pressed in a refrigerant cooled to a predetermined temperature. According to Table 2, the plated steel sheet manufactured by the method of the present invention not only has excellent surface properties, but also has a sufficiently low brittle transition temperature of −100 ° C. or less, and an excellent total elongation of 45% or more. It turns out that it is a plated steel sheet suitable for press forming.

Example 2 The cold-rolled steel sheets Nos. 2 and 5 obtained in the above-mentioned [Example 1] were annealed in a Sendzimir-type continuous hot-dip galvanizing line under the conditions shown in Table 3, and subsequently hot-dip zinc. Plating process, alloying process and temper rolling are sequentially performed,
An alloyed hot-dip galvanized steel sheet having a thickness of 0.7 mm was manufactured. The obtained alloyed hot-dip galvanized steel sheet was evaluated for the presence or absence of wrinkle-like defects, the presence or absence of plating unevenness, the ductility-brittle transition temperature, and the total elongation by the same method and criteria as in [Example 1]. Table 4 shows the results. According to Table 4,
It can be seen that satisfying the production conditions of the present invention makes it possible to produce a hot-dip galvanized steel sheet that has no wrinkle-like defects and has extremely excellent surface properties.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

[Table 4]

[0025]

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.

[Brief description of the drawings]

FIG. 1 is a graph showing ranges of an annealing temperature and a soaking time that satisfy a formula (2) defined by the present invention for steel sheet No. 1 shown in Table 1.

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Seiji Nakamura 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Shunsaku 1-1-1-2 Marunouchi, Chiyoda-ku, Tokyo Japan JP Steel Co., Ltd. (56) References JP-A-8-199300 (JP, A) JP-A-2-267231 (JP, A) JP-A-6-25754 (JP, A) JP-A-8-241303 (JP, A) JP-A-10-60541 (JP, A) JP-A-9-227951 (JP, A) JP-A-10-265902 (JP, A) (58) Fields studied (Int. Cl. ⁷ , DB Name) C22C 38/00-38/60

Claims (2)

(57) [Claims] In a continuous galvanizing line,
C: 0.0030 wt% or less , Si: 0.05 wt% or less
Bottom , Mn: 0.5 wt% or less , P: 0.02 wt% or less
Bottom , S: 0.020 wt% or less , sol. Al: 0.0
10 to 0.080 wt %, N: 0.0030 wt% or less
Below , B: 0.0004-0.0015 wt%, Cu:
A steel sheet having a component composition of 0.01 to 0.05 wt% and containing one or more of Ti, Zr, V, and Nb in an amount satisfying the following equation (1) satisfies the following equation (2). A method for producing a hot-dip galvanized steel sheet having a thickness of 1.2 mm or less without wrinkle-like surface defects, comprising annealing under conditions and subsequently performing hot-dip galvanizing treatment. C / 12 ≦ Ti / 48 + Nb / 96 + Zr / 91 + V /
51-N / 14-S / 32 ≦ 4 × (C / 12)
... (1) ts ≦ exp {(890-Ta) / 100} × (Ti /
48 + Nb / 96 + Zr / 91 + V / 51-N / 14-
S / 32) × (12 / C) (2) where Ti: Ti content (wt%), Zr: Zr content (wt%), V: V content (wt%), Nb: Nb content Amount (wt%), C: C content (wt%), N: N content (wt%), S: S content (wt%), Ta: annealing temperature (° C), ts: soaking time ( min) After wherein galvanizing treatment is performed alloyed, then and performing temper rolling, claim 1
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).