JP2008214681A - Galvannealed steel sheet superior in image clarity of coating and press formability, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a galvannealed steel sheet superior in the image clarity of a coating and press formability, and to manufacture the galvannealed steel sheet with an easy method without needing a roll having a special roughness and a control for the roll when temper-rolling a plated steel sheet. <P>SOLUTION: A composition of a plating bath includes, by mass%, 6 to 13% Fe, 0.25 to 1% Al, 0.25 to 1% Ni and the balance Zn with unavoidable impurities. The galvannealed steel sheet superior in the image clarity of the coating and the press formability has mean waviness (Wca) of 0.5 μm or less and mean roughness (Ra) of 0.4 μm or more. The galvannealed steel sheet preferably satisfies the relationship of Ra>2×Wca. The manufacturing method includes: preparing a cold-rolled steel sheet which has been annealed and temper-rolled; pre-plating the steel sheet with Ni; heating the steel sheet to 500°C or lower; galvanizing the steel sheet in a hot-dip galvanizing bath; wiping the plated film; heating the steel sheet to alloy the plated film; and temper-rolling the steel sheet at a rolling reduction of 0.1% or more. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an alloyed hot-dip galvanized steel sheet excellent in paint sharpness and press formability.

  Alloyed hot-dip galvanized steel sheet is known as a steel sheet for automobiles or buildings, and has excellent paint film adhesion and corrosion resistance after painting. The current situation is inferior in this respect, and this improvement is desired.

  In Patent Document 1, an alloyed hot-dip galvanized steel sheet is proposed in which the average waviness (Wca) is reduced and the average roughness (Ra) is increased in reverse to achieve both paint clarity and press formability. However, it is hard to say that the characteristics are sufficient. Also, since Wca and Ra are correlated, it is extremely difficult to make only one larger and the other smaller, and in order to realize this, plating is performed as disclosed in the example of Patent Document 1. Since it is necessary to combine surface polishing and temper rolling several times later, industrial practical application was difficult.

  Patent Documents 2 and 3 disclose a method of imparting roughness that achieves both paint clarity and press formability by subjecting an galvannealed steel sheet to temper rolling twice with a specific roll. However, this is also not a sufficient characteristic. Further, since the temper rolling is performed twice, the elongation of the steel sheet has to be high, and there is a problem that the material is greatly deteriorated accordingly.

  In Patent Document 4, the surface of a base steel plate having an average roughness (Ra) as small as 0.4 μm or less is alloyed and melted with Ra of 0.5 to 1.0 μm, Wca of 0.7 μm or less, and ppi of 150 or less. An alloyed hot-dip galvanized steel sheet characterized by forming a galvanized layer is disclosed, but because Wca is higher than Ra, it is possible to achieve both paint clarity and press formability. Not enough. Further, in order to obtain an appropriate post-plating roughness, it is necessary to strictly manage temper rolling conditions and rolls, and there is a problem in cost. Further, when the roughness of the base material is small, defects such as fine scratches in the plating process and the like are easily noticeable, and there is a problem that the yield is lowered.

  In addition, Patent Document 5 discloses a method in which the roughness of the surface-treated steel plate is 0.4 μm or less and ppi 100 or less in Ra, and temper rolling is performed at 0.5 to 1.8% in elongation after plating. It is disclosed. Patent Document 6 discloses a method in which a bright-finished steel sheet is subjected to alloying hot-dip galvanizing and subjected to temper rolling with a bright roll. Patent Document 7 discloses a method in which a cold rolled steel sheet rolled with a bright roll or a laser beam processing roll is used, and after the alloying hot dip galvanizing treatment, temper rolling is performed with the laser beam processing roll. All of the above examples have the same problems as in Patent Document 4.

JP-A-6-91303 Japanese Patent Laid-Open No. 5-15901 JP 7-124604 A JP 2004-156111 A JP-A-2-175007 JP-A-2-225562 JP-A-2-185959

  In view of the above, an object of the present invention is to obtain an alloyed hot-dip galvanized steel sheet excellent in paint sharpness and press formability. Another object of the present invention is to provide a simple manufacturing method that does not require special roll roughness or roll management in temper rolling after plating.

  All of the above-described conventional techniques are directed to a manufacturing method in which cold-rolled steel sheets are subjected to annealing, hot-dip galvanizing, alloying treatment, and temper rolling in a hot-dip galvanizing facility having a continuous annealing furnace. . To adjust the roughness after plating in this case, there is only a method that relies on adjusting the roughness of the final roll of temper rolling or cold rolling.

  However, in this method, no matter how the roughness is adjusted at the cold rolling stage, the alloying hot dip galvanizing process greatly changes the roughness (increases in Ra and Wca). Depending on the temper rolling, there was a limit to the properties that could be obtained.

  Therefore, when a method for suppressing the change in roughness (increase in Ra and Wca) after alloying hot dip galvanizing treatment and achieving both paint clarity and press formability was examined, an appropriate amount of Ni in the plating layer was investigated. In addition, it was found that if Ra and Wca are appropriately controlled, extremely good performance can be obtained. Further, when the cold-rolled steel sheet, which is a plating base material, is annealed, temper-rolled, Ni pre-plated and heated to 500 ° C. or lower, and then subjected to alloying hot-dip galvanizing treatment, the effect of Ni and tempering are performed. A uniform alloyed hot-dip galvanized layer is easily formed due to the effect of micro-strain on the surface layer applied by rolling, and the roughness change after plating is suppressed to a small level. The inventors have found that roughness can be easily obtained, and have reached the present invention.

  That is, the gist of the present invention is as follows.

  It is a plating composition consisting of Fe: 6-13%, Al: 0.25-1%, Ni: 0.25-1%, the balance Zn and inevitable impurities, and the average waviness (Wca) is 0. An alloyed hot-dip galvanized steel sheet excellent in paint vividness and press formability, characterized in that it is 5 μm or less and the average roughness (Ra) is 0.4 μm or more. The alloyed hot-dip galvanized steel sheet preferably satisfies the relationship Ra> 2 × Wca.

  In addition, the gist of the present invention is that Ni cold-rolled steel sheet that has been annealed and temper-rolled is subjected to Ni pre-plating, heated to a temperature of 500 ° C. or lower, then plated in a hot dip galvanizing bath, heated and alloyed after wiping. And a temper rolling of 0.1% or more, and a method for producing an alloyed hot-dip galvanized steel sheet excellent in paint sharpness and press formability. The average waviness (Wca) of the annealed and temper-rolled cold-rolled steel sheet is preferably 0.5 μm or less and the average roughness (Ra) is preferably 0.4 μm or more. The cold-rolled steel sheet that has been annealed and temper-rolled is desirably temper-rolled with an elongation of 0.2% or more.

  According to the present invention, an alloyed hot-dip galvanized steel sheet with excellent paint sharpness and press formability is obtained, and special roll roughness and roll management are required as in the past in temper rolling after plating. There is a remarkable effect that it can be manufactured by a simple method.

  The present invention is described in detail below.

  The alloyed hot-dip galvanized steel sheet of the present invention has a plating composition consisting of Fe: 6-13%, Al: 0.25-1%, Ni: 0.25-1%, the balance Zn and inevitable impurities in mass%. The average waviness (Wca) is 0.5 μm or less, and the average roughness (Ra) is 0.4 μm or more. First, the reason why the plating composition is limited in the present invention will be described.

  Fe: 6 to 13% is because the η and ζ phases are likely to remain in the surface layer below the lower limit of 6% and the slidability is inferior, and when the upper limit of 13% is exceeded, the piperability is inferior. It is. It is also a feature of the present invention that good slidability can be obtained even in a region of Fe 6 to 10%, which is generally considered to be poor in slidability. From the viewpoint of slidability and powdering property, it is more preferably 7 to 12%, still more preferably 8 to 11%.

  The reason why Al is set to 0.25 to 1% is that the powdering property is inferior when the content is less than the lower limit of 0.25%, and the paint vividness is inferior when the upper limit is 1%. Moreover, it is difficult to obtain an appropriate roughness in both cases of less than the lower limit and more than the upper limit.

  The reason why Ni is set to 0.25 to 1% is that paint sharpness is inferior when the content is less than the lower limit of 0.25%, and powdering properties are inferior when the upper limit is 1%. Moreover, it is difficult to obtain an appropriate roughness in both cases of less than the lower limit and more than the upper limit.

  Ni in the plating layer refines the plating crystal of the Zn-Fe alloy to make it microscopically smooth, and also has the effect of suppressing the concentration of Al oxide on the plating outermost layer and improving the chemical conversion processability. As a result of the combination of these effects, it is possible to obtain good paint vividness together with roughness described later.

  Further, by limiting Al and Ni in the plating layer as described above, good slidability can be obtained even in a low Fe% region of Fe6 to 10%, together with roughness described later, Good powdering properties can be obtained even in a high Fe% region of 13%, and good characteristics can be obtained in a wide Fe% range. In this respect, a more preferable Al concentration range is 0.3 to 0.6%, and a more preferable Ni concentration range is 0.3 to 0.8%.

  The reason why the average waviness (Wca) is 0.5 μm or less and the average roughness (Ra) is 0.4 μm or more is to achieve both paint clarity and press formability. The sharpness is inferior, and when Ra is less than 0.4 μm, the press formability is inferior. As for Ra, the upper limit is desirably set to 1.0 μm in order to achieve both better paint sharpness and press formability. The lower limit of Wca is not particularly limited.

  In order to achieve both better paint clarity and press formability, it is desirable to make Wca smaller and Ra larger, and in particular, Ra should be larger than twice Wca.

  Next, a method for manufacturing the above-described steel sheet will be described.

  The steel plate which is the plating original plate in the present invention is a product obtained by subjecting a cold-rolled and annealed steel plate to temper rolling. Although the steel plate component is not particularly limited, it is suitable for an ultra-low carbon steel plate that requires better paint clarity and press formability, and an ultra-low carbon steel plate to which Nb or Ti is added alone or in combination, or It can also be applied to an ultra-low carbon steel sheet or the like to which P or the like is further added to increase the strength.

  In the present invention, Ni pre-plating is applied to a cold-rolled steel sheet that has been annealed and temper-rolled, heated to a temperature of 500 ° C. or lower, then plated in a hot dip galvanizing bath, and heated and alloyed after wiping. By adopting such a method, it is possible to extremely reduce the increase in roughness during the galvannealing treatment. This point will be described in detail below with reference to the drawings.

The original plates a and b shown in FIG. 1 are 750 on a cold-rolled steel plate (Ti-added ultra-low carbon steel plate, plate thickness = 0.7 mm) adjusted to Ra = 0.30 μm and Wca = 0.25 μm by cold rolling. Recrystallization annealing at 0 ° C., temper rolling by shot dull roll, and adjustment of roughness (original plate a: Ra = 0.55 μm, Wca = 0.34 μm, original plate b: Ra = 1.12 μm, Wca = Adjusted to 0.48 μm), Ni pre-plating with an adhesion amount of 0.3 g / m ² , heated to 440 ° C. in a non-oxidizing atmosphere, immediately plated in a hot dip galvanizing bath containing 0.15% of Al, and after wiping again This shows the relationship between Ra and the elongation ratio of post-plating temper rolling when heating, alloying, and temper rolling with a shot dull roll once. Note that the original sheet c shown in FIG. 1 corresponds to a comparative prior art, and is a cold-rolled steel sheet (Ti-added ultra-low carbon steel sheet) adjusted to Ra = 0.30 μm and Wca = 0.25 μm by cold rolling. Plate thickness = 0.7 mm), recrystallization annealing at 750 ° C., cooling to 450 ° C., plating as it is with a hot dip galvanizing bath containing 0.11% Al, reheating after heating, alloying, The relationship between the elongation rate of Ra after-plating temper rolling at the time of temper rolling by a shot dull roll, and Ra is shown. In addition, the original plates a to c shown in FIG. 2 similarly show changes in Wca.

  As is apparent from FIGS. 1 and 2, compared to the prior art, the method of the present invention has a small increase in Ra and Wca after plating, and particularly a small increase in Wca. It can also be seen that Ra and Wca can be easily reduced by the method of the present invention even when the temper rolling ratio after plating is small. It is presumed that such an effect is obtained because a uniform strain is imparted to the surface layer by temper rolling before plating, and that the plated layer is easily made uniform by the effect of Ni.

  The temper rolling before plating in the present invention is desirably 0.2% or more from the viewpoint of imparting uniform strain to the surface layer. The upper limit is not particularly limited, but it is preferably as low as possible because there is a decrease in the steel sheet material, and it is usually preferable to set it to 1.0% or less. A roll for temper rolling is not particularly limited, and a roll for processing such as normal shot, laser, and electric discharge can be applied without limitation. Further, the roughness of the plated original sheet after the temper rolling is not particularly limited, but it is desirable that Wca is 0.5 μm or less and Ra is 0.4 μm or more. This is because if the Wca exceeds 0.5 μm, the Wca after plating tends to increase, and the paint sharpness tends to be poor. If the Ra is less than 0.4 μm, minute scratches are likely to be uneven plating, and the paint vividness tends to be poor.

In the present invention, the amount of Ni pre-plating is preferably 0.1 to 1 g / m ^{2 in} order to obtain more uniform plating. The Ni plating method is not particularly limited, and normal pretreatment such as degreasing and pickling may be performed as necessary.

  After Ni pre-plating, after heating to a temperature of 500 ° C. or lower, plating is performed in a hot dip galvanizing bath. At this time, if the heating exceeds 500 ° C., it becomes difficult to obtain a good surface state. This is presumably because the surface layer strain during temper rolling is released by heat. The lower limit of the heating temperature is preferably 400 ° C. or higher in consideration of plating appearance and wettability to a hot dip galvanizing bath. Since the heating atmosphere is poor in plating wettability in an oxidized state, a non-oxidizing or reducing atmosphere is desirable.

  The hot dip galvanizing bath uses a bath composed of Al 0.12 to 0.2%, unavoidable impurities and the balance Zn, but may further contain Pb, Sb, Sn, Mg, Ni or the like. If it is less than the lower limit of Al, powdering is poor, and if it exceeds the upper limit, the appearance tends to deteriorate. The bath temperature of hot dip galvanization is preferably about 430 to 450 ° C., but is not particularly limited thereto.

  After plating, the basis weight is adjusted by wiping, and then alloying is performed by reheating. The alloying temperature is preferably 500 to 600 ° C. from the viewpoint of slidability and powdering properties, but is not particularly limited thereto.

  The temper rolling after plating needs to be 0.1% or more. This is because it is difficult to obtain an appropriate roughness below this value. The upper limit is not particularly limited, but is preferably as low as possible because there is a decrease in the steel sheet material. For example, in the case of a material that does not have the possibility of yield point elongation, the upper limit is preferably about 0.1 to 0.5%. A material that may cause yield point elongation may be selected within a range of about 0.1 to 1.5% in accordance with the required material and roughness. A roll for temper rolling is not particularly limited, and a roll for processing such as normal shot, laser, and electric discharge can be applied without limitation.

  Hereinafter, the present invention will be described in detail by way of examples.

(Examples 1-11, Comparative Examples 1-5)
Using an Nb and Ti composite added ultra-low carbon steel sheet, after recrystallization annealing at 800 ° C., 1-pass temper rolling was performed with various rolls and elongation rates to adjust the raw sheet roughness. Next, after degreasing and pickling treatment, Ni pre-plating is performed by electroplating, heated to 440 ° C. in a non-oxidizing atmosphere, immediately immersed in a galvanizing bath with a bath temperature of 440 ° C., and after 3 seconds, the weight per unit area is 50 g / weight. It was adjusted to m ^2. Further, immediately above the wiping, it was heated to a predetermined temperature at a temperature rise of 30 ° C./sec, slowly cooled at 5 ° C./sec for 15 seconds, and then cooled with water. Furthermore, 1-pass temper rolling was performed.

(Examples 12 to 14)
The treatment was performed in the same manner as described above except that a 340 MPa class high-strength steel plate in which 0.03% of P was added to an Nb-added ultra-low carbon steel plate was used.

(Comparative Examples 6-8)
Using Nb and Ti composite added ultra-low carbon steel sheet, after recrystallization annealing at 800 ° C., it is cooled to 460 ° C., immersed in a galvanizing bath with a bath temperature of 455 ° C. as it is, and after 3 seconds, the weight per unit area is 50 g / m ^2. Adjusted. Further, immediately above the wiping, it was heated to a predetermined temperature at a temperature rise of 30 ° C./sec, slowly cooled at 5 ° C./sec for 15 seconds, and then cooled with water. Furthermore, temper rolling was performed twice so that specific roughness was obtained.

  Table 1 shows the manufacturing conditions of the samples of the examples and comparative examples.

  Table 2 shows the results of performance evaluation of each sample. The performance evaluation was performed as follows.

  Ra, Wca: measured with a roughness meter.

  Vividness: Unprocessed flat plate and after 5% tensile processing, automotive trication zinc phosphate treatment, cationic electrodeposition coating (10 μm), intermediate coating (30 μm), top coating (30 μm), and image clarity measuring instrument Measurements were made at More than 85 was evaluated as “◎”, 70 to 85 as “◯”, 60 to 70 as “Δ”, and less than 60 as “×”.

  Sliding property: U bending with W bead was performed with BHF1ton, and the limit molding height was measured. More than 60 mm was evaluated as “◎”, 55-60 as “◯”, 50-55 as “Δ”, and less than 50 as “x”.

  Powdering property: The side face at the above-described drawing height of 50 mm was peeled off with tape and evaluated by the degree of blackening. A degree of blackening of less than 10% was evaluated as “◎”, a value of less than 10-20% was evaluated as “◯”, a value of less than 20-30% was evaluated as “Δ”, and a value of 30% or more was evaluated as “x”.

  As described above, excellent characteristics were obtained in the scope of the present invention.

  According to the present invention, an alloyed hot-dip galvanized steel sheet excellent in paint sharpness and press formability and a method for producing the same can be obtained, which is extremely useful industrially.

It is a figure which shows the change of roughness Ra in this invention and a prior art. It is a figure which shows the change of roughness Wca in this invention and a prior art.

Claims (5)

It is a plating composition consisting of Fe: 6-13%, Al: 0.25-1%, Ni: 0.25-1%, the balance Zn and inevitable impurities, and the average waviness (Wca) is 0. An alloyed hot-dip galvanized steel sheet excellent in paint sharpness and press formability, characterized in that it is 5 μm or less and the average roughness (Ra) is 0.4 μm or more.   The alloyed hot-dip galvanized steel sheet according to claim 1, wherein Ra> 2 × Wca is satisfied.   Annealed and temper-rolled cold-rolled steel sheet is subjected to Ni pre-plating, heated to a temperature of 500 ° C. or lower, plated with a hot dip galvanizing bath containing 0.12 to 0.2% by mass of Al, and heated after wiping 3. The method for producing an alloyed hot-dip galvanized steel sheet having excellent paint clarity and press formability according to claim 1 or 2, wherein alloying is performed and temper rolling is performed at 0.1% or more.   4. The paint vividness according to claim 3, wherein the average waviness (Wca) of the annealed and temper-rolled cold-rolled steel sheet is 0.5 μm or less and the average roughness (Ra) is 0.4 μm or more. And manufacturing method of galvannealed steel sheet with excellent press formability.   The cold-rolled steel sheet annealed and temper-rolled is temper-rolled with an elongation of 0.2% or more. An excellent method for producing galvannealed steel sheets.

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