JP2001303179A - Hot dip metal coated steel sheet excellent in roll- formability and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot dip metal coated steel sheet excellent in roll- formability hardly developing both pocket waves and periodical wavy defects and fluting and to provide a producing method by which this steel can be produced at a low cost. SOLUTION: This hot dip metal coated steel sheet is composed of a steel sheet having the composition consisting of <=0.12% C, <=0.10% Si, 0.10-2.00% Mn, <=0.10% P, 0.0050-0.0200% N, <=0.100% Al and the balance Fe with inevitable impurities, and the structure of >=10.0 μm average crystal grain diameter, and a plating layer coating this steel sheet, and has <=6.0% yield elongation. The hot dip coated steel sheet is produced by successive processes of hot-rolling, cold-rolling, annealing, plating and leveller working, in this order. In these processes, it is important to regulate the FDT in the hot-rolling to lower than the Ar1 transformation point and the elongation ratio in the leveller working after plating to >0.3% to 2.0%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet having excellent roll formability and a method for producing the same, and more particularly to a method for removing shape defects such as pocket waves (oil cans) and fluting (bending) during roll forming. The present invention relates to a hot-dip galvanized steel sheet and a method of manufacturing the same.

[0002] In the present invention, the percentage relative to the chemical component content
Is% by mass. The coil means a steel sheet that has undergone a hot-rolling winding process.

[0003]

2. Description of the Related Art Zinc-based hot-dip galvanized steel sheets have excellent corrosion resistance and paintability and are inexpensive compared to stainless steel sheets. . Most of the plated steel sheets are color-painted, and some are shipped unpainted, making use of the spangle pattern, which is the metal structure of the plating layer.
The product is formed by roll forming such as round waves.

In such roll forming, when a spandrel and square wave forming are performed, a periodic wave-like defect called a pocket wave or an oil can occurs in a flat portion called a web, while a round wave forming is performed. However, there is a problem in that a bending defect extending in the rolling direction called fluting or waist bending occurs, which significantly reduces the commercial value.

[0005] The pocket wave is considered to be a phenomenon in which, when a bent portion is pulled in the coil width direction at the time of molding, the longitudinal direction slightly shrinks and deforms, and the residual compressive stress generated as a result buckles the web portion. It is generally known that the higher the yield stress and yield elongation of a steel sheet, and the thinner the web width and the smaller the web width, the less likely it is to occur. Furthermore, Hira et al. (Plasticity and processing, 1997, Volume 20,
933), it was clarified that the larger the proportional limit (elastic limit stress) measured from the stress-strain curve of the tensile test of the steel sheet, the more difficult it is for a pocket wave to occur.

On the other hand, fluting is based on the fact that the Luders band is observed to be concentrated along the bend,
It is considered to be non-uniform deformation called stretcher strain, and it is generally known that the smaller the yield elongation is, the less likely it is to occur, contrary to the pocket wave. Pocket wave prevention measures that have been proposed so far
After flattening by leveler processing, a method of raising the yield limit and the proportionality limit by low-temperature annealing for the purpose of age hardening (Japanese Patent Publication No. 62-2035, JP-A-1-319635), the elongation of leveler processing by 0.3 % Or less to suppress the reduction in yield stress (Japanese Patent Application Laid-Open No. 54-78353,
No. 18, JP-A-9-206831), and a method of suppressing the shrinkage deformation by giving a larger amount of shrinkage in the rolling direction by rolling during roll forming (JP-A-57-52129). You.

However, according to the method, in the case of the coating specification, the existing baking treatment equipment can be utilized for the aging treatment, which is advantageous. However, in the case of the non-painting specification, a heating equipment for aging is separately required, which increases the cost. There's a problem. In the method, there is a problem that the yield elongation does not sufficiently decrease and fluting cannot be prevented. In the method, there is a problem that versatility is poor due to restrictions on roll forming equipment.

As a method for preventing both the pocket wave and the fluting, a method of increasing the N content of the material to 50 ppm or more and performing cold working of 0.1 to 10% after plating (Japanese Patent Laid-Open No. 3-281729). Has been proposed. This is to increase the amount of solid solution N to facilitate aging at room temperature and increase the proportional limit / yield stress to prevent pocket waves, while reducing the yield stress / yield elongation by cold working after plating to prevent fluting. However, in order to achieve a sufficient anti-fluting effect, a deformation of as much as 2 to 10% is required, and since a normally used tension leveler has insufficient equipment capacity, it is necessary to perform skin pass rolling. However, skin pass rolling has a problem in that the spangle pattern on the plating surface is impaired and the commercial value in non-painting applications is reduced.

[0009]

As described above, as described above,
In order to prevent both pocket wave and fluting, it is desired to take measures to improve the material properties without deteriorating versatility and inexpensiveness. There are no effective measures for both.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned conventional problems, and to provide a hot-dip galvanized steel sheet which is excellent in roll formability in which both pocket wave and fluting hardly occur, and a method of manufacturing the same at a low cost. Is to provide.

[0011]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and have found that the average grain size of the steel structure (recrystallized grain size) of a hot-dip galvanized steel sheet having a specific steel composition is determined. If the average grain size is above a certain critical value and the yield elongation of the plated steel sheet is below a certain critical value, both fluting and pocket wave can be sufficiently suppressed. In addition, such performance is finished at less than the Ar ₁ transformation point. The present inventors have found that the present invention can be achieved by hot rolling, cold rolling, recrystallization annealing, and leveling with a relatively small elongation after plating, and have made the present invention as described below.

(1) C: 0.12% or less, Si: 0.10% or less,
Mn: 0.10 to 2.00%, P: 0.10% or less, N: 0.0050 to 0.02
A steel sheet containing 00%, Al: 0.100% or less, having a composition comprising the balance of Fe and unavoidable impurities, a structure having an average crystal grain size of 10.0 μm or more, and a plating layer covering the steel sheet. A hot-dip galvanized steel sheet having excellent roll formability, characterized in that the content is 6.0% or less.

(2) The galvanized steel sheet according to (1), wherein the plating layer is a zinc plating layer or an Al-Zn alloy plating layer. (3) C: 0.12% or less, Si: 0.10% or less, Mn: 0.10-2.
00%, P: 0.10% or less, N: 0.0050 to 0.0200%, Al: 0.
Hot rolling, cold rolling, annealing at recrystallization temperature or higher, hot-dip metal plating, steel material containing 100% or less, with the balance of Fe and unavoidable impurities, where the rolling end temperature is less than the Ar ₁ transformation point. A method for producing a hot-dip galvanized steel sheet excellent in roll formability, wherein leveling is performed in this order so that the total elongation is more than 0.3% to 2.0% or less.

(4) The method according to (3), wherein the hot-dip metal plating is hot-dip galvanizing or hot-dip Al-Zn alloy.

[0015]

First, the reasons for limiting the steel composition will be described. C is an element originally effective in increasing the yield stress of the steel sheet and preventing pocket waves, but if it exceeds 0.12%, the crystal grains become fine and carbides are precipitated at high density. Since the yield point is greatly reduced and a pocket wave is generated, the content is set to 0.12% or less. In addition, it is preferably 0.02 to 0.08%.

[0016] Si is an element that can increase the yield stress without forming precipitates,
If it exceeds 0.10%, the plating property is deteriorated and non-plating is caused. Incidentally, preferably 0.070
% Or less. Mn is an element effective in preventing hot cracking by S and strengthening steel. To obtain this effect, 0.10%
The above is required, but the addition exceeding 2.00% is excessive with respect to the required strength level and only increases the cost.

P is an element effective for increasing the strength of steel by solid solution strengthening. However, if it exceeds 0.10%, it causes embrittlement. Therefore, P is set to 0.10% or less for practical use. Incidentally, the content is preferably 0.060% or less. N is an essential element for promoting room temperature strain aging in the present invention. N is 0.0050
%, The proportional limit due to strain aging and the rise in yield stress do not occur in a short time at room temperature, and pocket waves cannot be prevented. However, if N exceeds 0.0200%, the yield stress becomes too high, and it becomes difficult to suppress fluting by leveler processing. Further, excessive N addition promotes excessive precipitation of AlN in the hot rolling step (including the winding step), so that the growth of recrystallized grains during annealing is suppressed, and the predetermined average crystal grain size is reduced. Also has the disadvantage of making it difficult to achieve Therefore, N
Was set to 0.0050 to 0.0200%. In addition, preferably 0.0050 ~
It is 0.0150%.

Al is an element used as a deoxidizing agent in the steelmaking stage. In the present invention, if Al exceeds 0.100%, AlN excessively precipitates in the hot rolling step, and the recrystallized grains in the annealing are formed. Growth is suppressed, and a predetermined average crystal grain size cannot be obtained. Therefore, the content of Al is set to 0.100% or less. Incidentally, the content is preferably 0.01 to 0.050%. The steel composition of the present invention contains O, S, and the like as unavoidable impurities in addition to the above-mentioned component elements and Fe. O is 0.050% or less, and S is 0.050% or less in consideration of the allowable level of cleanliness and deoxidation cost. It is preferable to limit the content to 0.05% or less in consideration of the allowable level of high-temperature brittleness such as hot cracking and desulfurization cost.

Next, the reasons for limiting the average grain size of the steel sheet and the yield elongation of the plated steel sheet will be described. In the present invention, in addition to the steel composition being within the above-mentioned limited range, the steel sheet has an average crystal grain size of 10.0 μm or more and a yield elongation of 6.0% or less.
This is an essential requirement that can prevent both fluting and pocket waves. When the average crystal grain size is less than 10.0 μm, one or both of fluting and pocket waves tend to occur.

The reason for this is not necessarily clear, but the inventors think as follows. It is presumed that a large amount of dislocations are generated inside the steel sheet due to tension in the coil width direction applied to the steel sheet during roll forming, and these dislocations are deposited on carbides and grain boundaries to generate internal stress. The finer the crystal grain, the higher the grain boundary density, the greater the number of dislocations to be deposited, and the greater the internal stress. Therefore, the yield elongation, which is considered to be the cause of fluting, increases. This internal stress is considered to be the cause of the so-called Bauschinger effect that lowers the yield limit and the proportional limit in repeated deformation of elongation / shrinkage generated by leveler processing. Therefore, as the crystal grains become finer, the reduction of the yield stress and the proportional limit by the leveler processing become larger, so that pocket waves are more likely to occur.

On the other hand, even if the average crystal grain size is 10.0 μm or more, if the yield elongation after leveler processing exceeds 6.0%, the internal stress increases during roll forming and fluting is likely to occur. . Since the present invention has a large effect when applied to a hot-dip galvanized steel sheet and an Al-Zn-based alloy hot-dip steel sheet, the plating layer on the steel sheet surface may be a galvanized layer or an Al-Zn-based alloy plated layer. preferable.

Next, the reasons for limiting the manufacturing method will be described. The method of smelting and solidifying the steel material, the method of cold rolling after hot rolling, and the method of hot-dip plating are not particularly limited, and any conventional method may be used. Note that, if necessary, pickling may be performed before cold rolling or before plating. In hot rolling, the finishing temperature (rolling end temperature;
FDT) is less than the Ar ₁ transformation point. This is because at least the final stage of the finish rolling is performed in the α range so that the crystal grain size of the hot-rolled sheet is coarsened, and in annealing after cold rolling (recrystallization annealing), the annealing temperature is so high. This is because a recrystallized structure having an average particle size of 10.0 μm or more can be obtained without performing. If the FDT is equal to or higher than the Ar ₁ transformation point, the hot-rolled sheet has a fine grain structure, and in order to make the average grain size of the recrystallized structure 10.0 μm or more, recrystallization annealing must be performed at a high temperature of 850 ° C. or more. Will not get. In this temperature range, a large amount of solute N precipitates as AlN, and it becomes impossible to cause room temperature strain aging. Therefore, FDT was set to be lower than the Ar ₁ transformation point. The Ar ₁ transformation point is determined from the steel composition and rolling conditions.

Annealing after cold rolling is performed at a recrystallization temperature or higher in order to remove a rolling strain in cold rolling and obtain a recrystallized structure.
The recrystallization temperature is determined from the steel composition. Note that annealing at 850 ° C. or higher makes it impossible to cause room-temperature strain aging as described above. Therefore, it is desirable to set the annealing temperature to less than 850 ° C. The hot-dip galvanized after annealing is preferably hot-dip galvanizing or hot-dip Al-Zn alloy alloy from the viewpoint of the manifestation of the effect of the present invention.

The leveling process after hot-dip plating must be performed so that the total elongation (hereinafter simply referred to as elongation) is more than 0.3% to 2.0% or less. When the elongation is less than 0.3%, the yield elongation exceeds 6.0%, so that fluting cannot be prevented. On the other hand, if the elongation is more than 2.0%, the effect of preventing fluting is saturated, and the risk of coil breakage is significantly increased in a thin steel plate. Therefore, the growth rate is 0.3%
Ultra-2.0% or less. Since the leveler processing can be performed with a normal tension leveler, there is no possibility that the spangle pattern on the plating surface is spoiled.

In the present invention, after the leveler processing, it is preferable to perform a resin chromate treatment for the purpose of lubricity during roll forming and primary rust prevention of plating. Also, the present invention
Although it is intended for zinc-based hot-dip coated steel sheets for non-painting applications, it is also possible to ship them as color steel sheets after paint baking after leveler processing.

[0026]

[Example 1] Table cast by continuous casting method
250 mm thick steel slabs A to E having the composition shown in FIG.
After heating to 1100 ° C in a soaking furnace, the FDT was measured according to the values and specifications shown in Table 2.
Hot-rolled to a thickness of 2.3mm and rolled at 550 ° C
It was taken as a coil. After pickling the coil, cold rolling
A 0.4mm thick cold rolled sheet is used, and this is used for in-line annealing.
The base plate was annealed at the annealing temperature shown in Table 2 and
150g / m ^Two Hot-dip 55% Al-Zn alloy
Then, adjust the tension so that the elongation becomes the value shown in Table 2.
Perform shape correction with a tension leveler while setting
Further, a resin chromate treatment was performed. The resulting plated steel sheet
Was left at 25 ° C. for 14 days, and then subjected to the following investigation.

Occurrence of pocket wave: A steel sheet was roll-formed and the center of the web was scanned along the longitudinal direction with an eddy current displacement meter to measure the profile of the pocket wave in the longitudinal direction. As a pocket wave weight, calculated sum of the wave height per unit length (h _W = Σh _i). In addition,
If the amount of pocket waves is 1.5 mm / m or less, it can be judged that it is hardly visually recognized.

Occurrence of fluting: One groove is cut perpendicularly to the bottom surface on the side surface of a round bar steel having a diameter of 50 mm, and the groove is cut into the longitudinal direction (L direction) of 100 mm and the width direction (C direction).
The short side of the steel sheet cut to 250 mm was inserted and wound along the side face, and the generated fluting defects were evaluated by visual judgment. In addition, the fluting occurrence status is indicated by ◎ when no visual observation is observed, ○ when almost non-visible.
The case where it was slightly recognized was marked with Δ, the case where it was clearly recognized was marked with X, and ○ and ◎ were regarded as acceptable levels of fluting resistance.

Average grain size (d _GS ): 500 in the cross section in the L direction
An optical microscope photograph of × magnification was taken, the total number N of crystal grains in the visual field of the total area S was measured, and it was obtained by the formula: d _GS = √ (S / (N · π)) (π: pi) . Proportional limit: Determined by attaching a strain gauge having a gauge length of 2 mm to the center of a JIS No. 5 tensile test piece and measuring the stress-strain curve in the L direction while pulling in the C direction at a strain rate of 0.0017 mm / s.

Yield elongation: JI in C direction at a strain rate of 0.1667 mm / s
S No. 5 tensile test was performed and read from the stress-strain curve.
Table 2 shows the results of the above investigation.

[0031]

[Table 1]

[0032]

[Table 2]

As shown in Table 2, the invention examples were manufactured by a manufacturing method satisfying the requirements of the present invention, and all three items of steel composition, d _GS and yield elongation were within the scope of the present invention. Both properties and fluting resistance are at acceptable levels. On the other hand, in the comparative example, one or more of the three items deviated from the scope of the present invention, and either or both of the pocket wave resistance and the fluting resistance were rejected.

Example 2 Steel slabs F to Q having a composition shown in Table 1 and having a thickness of 250 mm, which were cast by a continuous casting method.
After heating to 1100 ° C in a soaking furnace, hot rolling was performed so that the FDT had the value shown in Table 3 and the finished plate thickness was 2.3 mm.
And wound up as a coil. After pickling, the coil was cold-rolled to form a 0.35 mm-thick cold-rolled sheet, which was annealed at an annealing temperature shown in Table 3 by an in-line annealing method to obtain a base sheet, and a basis weight for the base sheet. performed galvanized 130 mg / m ^2,
Next, the shape was corrected by a tension leveler while adjusting the tension so that the elongation ratio became a value shown in Table 3, and a resin chromate treatment was further performed. 25 plated steel sheets obtained
After leaving at 14 ° C. for 14 days, the same inspection as in Example 1 was performed.

Table 3 shows the results of the above investigation.

[0036]

[Table 3]

As shown in Table 3, the invention examples were manufactured by a manufacturing method satisfying the requirements of the present invention, and all three items of steel composition, d _GS and yield elongation were within the scope of the present invention. Both properties and fluting resistance are at acceptable levels. On the other hand, in the comparative example, one or more of the three items deviated from the scope of the present invention, and either or both of the pocket wave resistance and the fluting resistance were rejected.

[0038]

According to the present invention, the FD is increased by increasing the N content.
A low-carbon steel sheet having a mean grain size after recrystallization annealing of 10.0 μm or more after hot rolling with T controlled to less than the Ar ₁ transformation point is used as a base plate, and an elongation of more than 0.3% to 2.0% or less after plating. Since the yield elongation is limited to 6.0% or less by leveler processing, hot-dip galvanized steel sheet for building materials with excellent roll formability (pocket wave resistance and fluting resistance) can be manufactured at low cost. It has a remarkable effect.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C23C 2/12 C23C 2/12 2/40 2/40 (72) Inventor Keiichi Yoshioka Chuo-ku, Chiba-shi, Chiba No. 1, Kawasaki-cho, Kawasaki Steel Corp. (72) Inventor Hiroshi Hashimoto 8252-11, Oshima, Tamashima, Kurashiki City, Okayama Prefecture Kawashima Steel Plate Co., Ltd. F-term (reference) 4K027 AA02 AA05 AA23 AB02 AB28 AB42 AB48 AC12 AC62 AC82 AC86 4K037 EA01 EA04 EA05 EA06 EA15 EA18 EA27 EB02 EB06 EB08 FA02 FC02 FC03 FE02 FH01 FJ05 FM01 GA05

Claims (4)

[Claims] 1. C: 0.12% or less, Si: 0.10% or less, Mn:
0.10-2.00%, P: 0.10% or less, N: 0.0050-0.0200
%, Al: 0.100% or less, comprising a steel sheet having a composition comprising the balance of Fe and unavoidable impurities, a structure having an average crystal grain size of 10.0 μm or more, and a plating layer covering the steel sheet, and having a yield elongation of A hot-dip galvanized steel sheet excellent in roll formability, characterized in that the content is 6.0% or less. 2. The method according to claim 2, wherein the plating layer is a zinc plating layer or A
The hot-dip galvanized steel sheet according to claim 1, which is an l-Zn-based alloy plating layer. 3. C: 0.12% or less, Si: 0.10% or less, Mn:
0.10-2.00%, P: 0.10% or less, N: 0.0050-0.0200
%, Al: 0.100% or less, hot rolling, cold rolling, annealing at a recrystallization temperature or higher where the rolling end temperature is lower than the Ar ₁ transformation point, with the balance being Fe and unavoidable impurities. Hot-dip metal plating, total elongation is more than 0.3% to 2.0
%. A method for producing a hot-dip galvanized steel sheet having excellent roll formability, wherein leveler processing is performed in this order. 4. The method according to claim 3, wherein the hot-dip metal plating is hot-dip galvanizing or hot-dip Al-Zn alloy.