JP5533730B2 - Method for producing galvannealed steel sheet - Google Patents

Description

  The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet, and more particularly, to a method for producing an alloyed hot-dip galvanized steel sheet in which pre-plating is performed.

  Alloyed hot-dip galvanized steel sheets are extremely used in automobiles, home appliances, building materials and the like because they are excellent in coating adhesion, coating corrosion resistance, weldability, and the like. An alloyed hot-dip galvanized steel sheet forms a Zn-Fe alloy by coating hot-dip zinc on the surface of the steel sheet and immediately holding it at a temperature equal to or higher than the melting point of zinc and diffusing Fe from the steel sheet into the zinc. However, since the alloying speed varies greatly depending on the composition and structure of the steel sheet, the control thereof requires a considerably advanced technique.

  Automotive steel sheets pressed into a complicated shape are required to have very high formability, and the degree of alloying of the alloyed hot-dip galvanized layer greatly affects the formability. Further, when applied to an automobile body, the plating appearance unevenness caused by the unevenness of the alloying degree also affects the appearance of the automobile, and therefore, the demand for the plating appearance is becoming stricter.

  In response to these requirements, equipment measures in the production line for alloyed hot dip galvanizing, and improvements and developments regarding the components and structure of steel materials and plating layers are being promoted. There are various types of production lines that are characteristic in terms of cleaning and annealing of steel sheets, but generally the following two types. These are the conventional non-oxidation furnace-reduction furnace system and the total reduction furnace system. The characteristic of the non-oxidation furnace-reduction furnace system is that the steel sheet with the rolling oil adhered is burned and removed in the non-oxidation furnace without having a degreasing section on the entry side before annealing. Then, the rolling oil is incinerated, and then annealed in a reduction furnace, so that continuous hot dip plating is possible. Although it is reasonable as a method of hot dip plating, it is not suitable for applications requiring high appearance quality, such as automobile bodies, because plating appearance stains due to carbon on the steel sheet remaining after annealing occur. For this reason, in recent years, instead of a non-oxidizing furnace, there is a degreasing section on the entry side, so that the all-reduction furnace method that cleans the oil adhering to the steel plate and prevents it from being brought into the furnace has become the mainstream. Currently.

  On the other hand, a manufacturing method completely different from the above process has been proposed. For example, in Patent Document 1, a Ni pre-plated layer is plated on the surface of a steel sheet by 0.2 to 2 g / m2 and then rapidly heated to a plate temperature of 430 to 500 ° C in a non-oxidizing or reducing atmosphere at a heating rate of 30 ° C / sec or more. Alloyed hot-dip Zn-plated steel sheet, which is hot-dip plated in a Zn-plating bath containing 0.05-0.25% Al and then alloyed at 470-550 ° C for 10-40 seconds immediately after wiping The manufacturing method is shown. In this process, steel plate annealing and surface activation to ensure plating wettability are performed separately, and pre-plating is necessary to activate the surface, although there are restrictions, corrosion resistance and workability, It is possible to obtain good characteristics that cannot be obtained by general methods in various respects such as paintability.

  However, the technique disclosed in Patent Document 1 alone cannot be said to be sufficient for the recent stricter requirements for workability and plating appearance. In particular, ultra-low carbon steel sheets with Ti added can be widely used as cold steel sheets for automobiles or electrogalvanized steel sheets for automobiles because excellent deep drawability and ductility can be obtained stably over a wide range of components. It has been. However, because the grain boundaries are cleaned by the influence of Ti in the steel, the alloying reaction is promoted at the grain boundaries, and as a result, the outburst reaction is likely to occur, the overalloy is likely to proceed, and the powdering property is improved. Has the problem of getting worse.

  With respect to this problem, in Patent Documents 2, 3, and 4, using an original plate including a Ti-containing ultra-low carbon steel plate, optimizing the plating conditions, and setting the appropriate range, such as appearance and workability The characteristics are improved. However, it has not yet reached a level where there are no problems in terms of appearance and powdering. Moreover, in patent document 3, it is necessary to set the range of bath temperature narrowly, and there exists a problem on operativity. In Patent Document 4, it is necessary to limit Al and Ni to a narrow range, and there is a problem in operability.

Patent No. 2784352 JP 2007-84913 A JP 2008-195987 A JP 2009-280859 A

  In view of the above, the present invention is a manufacturing method in which pre-plating is performed on an annealed steel sheet and then rapidly heated to perform hot dip galvanizing and alloying. Another object of the present invention is to provide a method for producing a galvannealed steel sheet. The present invention also provides an alloyed hot-dip galvanized steel sheet that provides excellent powdering and plating appearance even when a Ti-added ultra-low carbon steel sheet, which is prone to problems in terms of uneven plating and powdering properties, is used as a base plate. It aims to provide a method.

The gist of the present invention is that
(1) The first pre-plating including one or more of Ni, Co, and Fe is performed on the surface of the steel plate, and after annealing the steel plate, one or more of Ni, Co, Cu, and Sn are further formed on the surface of the steel plate. After performing the second pre-plating including, and then performing rapid heating at a temperature rising rate of 30 ° C./sec or higher up to a plate temperature of [plating bath temperature −20] ° C. or higher and 500 ° C. or lower in a non-oxidizing or reducing atmosphere, A method for producing an alloyed hot-dip galvanized steel sheet, which comprises dipping in a Zn plating bath containing 0.05 to 0.25% by mass of Al to perform hot-dip plating, followed by heat alloying treatment.
(2) Said 1st pre-plating or said 2nd pre-plating contains P further, The manufacturing method of the galvannealed steel plate as described in (1) characterized by the above-mentioned.
(3) The galvannealed steel sheet according to (1) or (2), wherein the first pre-plating performed before the annealing is Co plating of 0.005 to 1 g / m ^2. Manufacturing method.
(4) The alloying according to any one of (1) to (3), wherein the second pre-plating applied to the annealed steel plate is 0.01 to 1 g / m ² Ni plating. Manufacturing method of hot dip galvanized steel sheet.
(5) The alloyed hot-dip galvanized steel sheet according to any one of (1) to (4), wherein the steel sheet is an ultra-low carbon steel sheet having a C content of 0.004% by mass or less. Production method.
(6) The method for producing an galvannealed steel sheet according to (5), wherein the ultra-low carbon steel sheet contains 0.01 to 0.1% by mass of Ti.

  By this invention, the manufacturing method of the galvannealed steel plate which was excellent in workability, was excellent in the appearance without streaky plating unevenness, and obtained.

  Hereinafter, the present invention will be described in detail. In the present invention, a first pre-plating (hereinafter referred to as “pre-plating before annealing” or “pre-plating before annealing”) is applied to a steel plate before annealing, and after the steel plate is annealed, a second pre-plating ( (Hereinafter referred to as “pre-plating before annealing” or “pre-plating before annealing”), and in a non-oxidizing or reducing atmosphere, the plating bath temperature is -20 ° C. or higher and a plate temperature of 500 ° C. or lower is 30 ° C./sec or higher. An alloyed hot-dip galvanized steel sheet characterized by performing rapid heating at a rate of temperature rise, followed by hot-dip plating in a Zn plating bath containing 0.05 to 0.25% Al, and performing heat alloying treatment It is a manufacturing method.

  The greatest feature of the present invention is that pre-plating is performed before annealing. As the type of pre-plating before annealing, Ni, Co, Fe, or alloy plating thereof is used. The effects of pre-annealing pre-plating performed before annealing are the following four points.

  First, pre-plating before annealing suppresses unevenness of alloyed hot dip galvanizing and contributes to the improvement of appearance. Many plating irregularities are caused by non-uniformity of the surface crystal grains of the steel sheet during the annealing process due to some unevenness such as dirt, wrinkles, segregation, etc. before the annealing process on the surface of the steel sheet used as the plating plate. doing. Therefore, by pre-plating Ni, Co, Fe or these alloys on the steel sheet surface and annealing, these diffuse into the steel sheet and uniformize the surface structure, so that the plating appearance can be improved. Presumed.

The second is to greatly expand the kind of pre-plating and the amount of adhesion after annealing described later. In order to obtain an extremely good appearance and powdering property using an ultra-low carbon steel plate without pre-annealing pre-plating, the pre-plating type after annealing is preferably limited to Ni, and the amount of adhesion is preferably small. Without such a limitation, the form of the initial alloy layer formed in the plating bath changes, and in some cases, Zn-Fe alloying progresses unevenly in the bath, and the appearance and powdering properties are impaired. is there. However, if the adhesion amount of Ni pre-plating is too small, the plating wettability of hot dip zinc decreases, so the adhesion amount range is limited to a narrow range of about 0.05 to 0.25 g / m ² . On the other hand, if it is a steel plate that has been subjected to pre-plating before annealing and annealed to make the surface layer structure uniform, it is uniform regardless of the pre-plating conditions after annealing, and the Zn-Fe alloy in the bath The formation of an initial alloy layer that suppresses crystallization can be expected. From this point of view, Ni or Co is desirable as pre-plating before annealing.

  Thirdly, the appropriate range of Al concentration and bath temperature in the hot dip galvanizing bath is greatly expanded. When trying to obtain an extremely good appearance and powdering property using an ultra-low carbon steel plate without pre-annealing before annealing, the Al concentration and bath temperature in the galvanizing bath are limited to a narrow range. If not limited in this way, the form of the initial alloy layer formed in the plating bath changes, and in some cases, Zn-Fe alloying progresses unevenly in the bath, and the appearance and powdering properties are impaired. is there. If the steel sheet is annealed after pre-plating before annealing and the surface layer is made uniform, formation of an initial alloy layer that is uniform and suppresses Zn—Fe alloying in the bath can be expected regardless of the conditions of the molten zinc bath. From such a viewpoint, Ni or Co is desirable as the pre-plating before annealing.

  Fourth, the effect of improving the powdering properties by suppressing local and rapid alloying reaction called so-called outburst, which is likely to occur in steel plates with extremely clean grain boundaries, especially Ti-containing ultra-low carbon steel plates. It is. This effect is particularly remarkable when Co is used for pre-plating before annealing. In order to make the surface layer structure uniform as described above, uniform reaction is likely to occur, and in addition to the action of Co that is selectively diffusing to the grain boundaries in the steel, abrupt in the grain boundary portion is caused. It is estimated that the Zn—Fe alloying is effectively suppressed.

The pre-annealing pre-plating method of the present invention is not limited at all, and known methods such as electroplating, displacement plating, electroless plating, and vapor phase plating are used. The appropriate adhesion amount of pre-plating before annealing is about 0.005 to 5 g / m ² . The type of element effects no effect of appearance improvements in different but less than 0.005 g / m ² may be any element, tends to unevenness exceeds 5 g / m ² tends to occur. Ni, Co whereas the effect even at a coverage more small Do have compared to Fe, so prone to unevenness when the adhesion amount is large, it is desirable that the upper limit is made 1 g / m ^2, 0.5 g / M ² is more desirable.

  The annealing method and conditions of the present invention are not limited at all, and can be carried out without limitation using ordinary equipment and methods. After annealing, treatments performed after normal annealing such as pickling and temper rolling can be performed without any limitation.

  Next, pre-plating after annealing will be described.

The primary purpose of pre-plating after annealing is to ensure the wettability of molten zinc, and Ni, Co, Cu, Sn, or alloy plating thereof is used as the type. Depending on the effect of pre-plating before annealing, the plating wettability can be ensured without any problem regardless of the type of pre-plating after annealing, but Ni is desirable particularly when workability and corrosion resistance are taken into consideration. A desirable adhesion amount of pre-plating after annealing is 0.01 to 1 g / m ² . If it is less than 0.01 g / m ² , non-plating tends to occur, and if it exceeds 1 g / m ² , the appearance and powdering properties tend to deteriorate. In the present invention, since pre-plating before annealing is performed, even if the pre-plating amount after annealing is as small as 0.01 g / m ² , an excellent non-plating suppressing effect can be obtained.

  The first pre-plating before annealing and the second pre-plating after annealing of the present invention can contain P. By adding P, the effect of further uniforming the progress of alloying and improving the appearance can be expected. This effect can be enjoyed particularly when a Ti-containing ultra-low carbon steel plate is used as the original plate.

  As described above, after pre-plating before annealing, annealing, and pre-plating after annealing, in a non-oxidizing or reducing atmosphere, a plating bath temperature of −20 ° C. or higher and a plate temperature of 500 ° C. or lower is 30 ° C./sec or higher. After rapid heating at a rate of temperature increase, hot dip galvanization is performed and a heat alloying treatment is performed.

  The reason why the heating temperature is set to [Plating bath temperature −20] ° C. or higher and 500 ° C. or lower is that the appearance tends to deteriorate when the temperature is less than [Plating bath temperature −20] ° C., and if it exceeds 500 ° C., diffusion of pre-plating after annealing occurs. This is because the Γ phase formed at the iron-iron interface at the time of the alloying process easily develops due to the influence of alloy formation with the pre-plating element before annealing diffused on the steel sheet surface layer. Also, the rate of temperature increase was set to 30 ° C./sec or more because, if it is less than 30 ° C., the diffusion of pre-plating after annealing becomes significant, and due to the influence of alloy formation with the pre-plating element before annealing diffused in the steel sheet surface layer, This is because the Γ phase formed at the iron-iron interface at the time of alloying treatment tends to develop. Although there is no upper limit for the rate of temperature rise to exert the effect of the present invention, it is difficult to raise the temperature at about 100 ° C./sec or more industrially and industrially, and this is a practical upper limit value.

  Thereafter, hot dip plating is performed in a Zn plating bath containing 0.05 to 0.25% Al, but the plating bath may contain Fe or steel such as Ni resulting from pre-plating. good. When the Al content in the hot dip galvanizing bath is less than 0.05% by mass, the appearance and powdering properties are poor, and when it exceeds 0.25% by mass, the degree of alloying tends to be insufficient, and the appearance, slidability, Corrosion resistance is reduced. The bath temperature can be used without any particular limitation as long as it is in a normal range from the melting point of the plating composition alloy to about 500 ° C. or less.

  Although normal conditions can be used without limitation for the alloying treatment after plating, if one example is shown, it is heated to a plate temperature of 470 to 600 ° C. and does not take a soaking time or has a soaking time of 40 seconds or less. Cool later.

  After the heat alloying treatment, a commonly used treatment such as temper rolling can be used without limitation.

  The components of the steel sheet used in the present invention are not particularly limited, but considering that the present invention is characterized by workability and plating appearance, the C content used for automobile outer plates and the like is 0.004% by mass or less. It is most effective to apply to the ultra-low carbon steel sheet. Furthermore, among ultra-low carbon steel plates, Ti-added ultra-low carbon steel plates containing 0.01 to 0.1% by mass of Ti are difficult to obtain good powdering properties and plated appearance by other methods. Most appropriate to apply.

  Cold-rolled steel sheets having various components shown in Table 1 were used as original sheets. These are materials that have been found to be susceptible to uneven alloying due to surface non-uniformity after casting in the conventional method of producing a galvannealed steel sheet. A to F are Ti-added ultra-low carbon steel sheets, G is an ultra-low carbon steel sheet, and H is a general steel.

Pre-plating treatment was performed under various conditions shown in Table 2, and then annealed. The annealing conditions were as follows: 5% hydrogen (residual nitrogen) atmosphere at 10 ° C./sec to 750 ° C. (steel type No. A, B, C) or 800 ° C. (other than the above steel type No.) Until cooled. Thereafter, pre-plating treatment after annealing is performed under various conditions shown in Table 2, and then heated to various plate temperatures at various heating rates in an atmosphere of 5% hydrogen (residual nitrogen). After immersing and plating in an Al-containing zinc bath and adjusting the plating adhesion amount to 50 g / m ² by air wiping, the alloying treatment was performed by further raising the temperature to a predetermined temperature and holding it for 10 seconds and then cooling.

  Tables 3 and 4 show the types of steel used, the type and amount of pre-plating before and after annealing, molten zinc, the heating rate, the heating plate temperature, the plating bath temperature, the Al concentration of the plating bath, and the alloying temperature. The conditions of are shown.

Each sample was evaluated by the following method. The results are shown in Tables 5 and 6.
Alloying degree
The composition of the plating layer was subjected to chemical analysis (ICP analysis after dissolving hydrochloric acid) to determine the degree of alloying (plating layer Fe%) and evaluated with the following indices.
A: 9.5% ≦ Fe% ≦ 11%
○: 9% ≦ Fe% <9.5% or 11% <Fe% ≦ 11.5%
Δ: 8% ≦ Fe% <9% or 11.5% <Fe% ≦ 12.5%
X: Fe% <8% or 12.5% <Fe%
appearance
The appearance was confirmed visually, and the uniformity of the alloying reaction was evaluated based on whether or not there was a streak pattern. The scores are as follows.
◎: No streak pattern and uniform appearance ○: No streak pattern but slight unevenness △: Partial streak pattern generated ×: Streak pattern on the entire surface
The powdering test material was cut out to 50 mm × 200 mm, coated with press oil, and then subjected to a draw bead test at a load of 4.9 kN. The bead passing part was peeled off with tape, and the degree of blackening of the tape (measured by a color difference meter to measure the decrease in L value) was measured. The larger the degree of blackening, the more intense the powdering and the poorer the plating adhesion. Evaluation was made according to the following criteria.
◎: Blackening degree <2 ○: Blackening degree 2 or more, less than 4 △: Blackening degree 4 or more, less than 6 ×: Blackening degree 6 or more
A slidability 30 mm width sample was used to conduct a flat plate continuous sliding test (40 mm length, crimped from both sides with a flat plate mold having a shoulder r2 mm). The continuous sliding was performed 5 times at a pressure bonding load of 4.9 kN, and the evaluation was made by the fifth friction coefficient.
A: Friction coefficient <0.13
○: 0.13 ≦ friction coefficient <0.15
Δ: 0.15 ≦ friction coefficient <0.2
×: 0.2 ≦ coefficient of friction
Cut out the corrosion-resistant test material to 70 mm x 150 mm, alkaline degreasing (Nihon Parkerizing, FC-E2001, 40 ° C, 3 minutes immersion), chemical conversion (Nihon Parkerizing, PB-SX35, 40 ° C, 3 minutes immersion) After performing cathodic electrodeposition coating (Nippon Paint, PN120M, film thickness 20 μm, baking at 160 ° C. for 20 minutes), a cut wound reaching the base iron was made in the center. This was subjected to a corrosion acceleration test for 4 weeks by the JASO M609-91 method, the film swelling width from the cut flaw was measured, and the maximum swelling width was evaluated according to the following criteria.
A: Coating film swelling width less than 1 mm B: Coating film swelling width of 1 mm or more and less than 2 mm Δ: Coating film swelling width of 2 mm or more and less than 4 mm x: Coating film swelling width of 4 mm or more

As described above, the inventive examples showed good characteristics. In particular, the one using Co for pre-annealing pre-annealing was good in various properties such as powdering property in all steel types including Ti-added ultra-low carbon steel plate. In Comparative Example 1, there was no pre-plating before annealing, and in Comparative Example 2, since there was no pre-plating after annealing, the appearance was streaked and the powdering property was not good. In Comparative Example 3, since the Al content in the hot dip galvanizing bath was low, the appearance showed a streak pattern and the powdering characteristics were poor. In Comparative Examples 4 and 5, since the Al content in the molten zinc bath was too high, Zn-Fe alloying became non-uniform, the slidability was poor, and the appearance was partially unsatisfactory. In Comparative Example 6, the powdering property was not good because the heating rate of rapid heating was slow. In Comparative Example 7, since the plate temperature after rapid heating was 420 ° C. of less than [plating temperature -20] = 43 0 ℃, appearance was not good exhibit some streaky pattern.

  By this invention, the manufacturing method of the galvannealed steel plate which is remarkably excellent in workability, has no plating unevenness, and was excellent in the external appearance is obtained. According to the present invention, even when a Ti-added ultra-low carbon steel sheet that has been not widely developed in terms of plating characteristics while being excellent in terms of material properties can be obtained as an original sheet, extremely good characteristics can be obtained. It is a thing.

Claims (6)

  A first pre-plating containing one or more of Ni, Co, and Fe is applied to the surface of the steel plate, and after annealing the steel plate, a second containing one or more of Ni, Co, Cu, and Sn on the surface of the steel plate. Next, rapid heating was performed at a rate of temperature increase of 30 ° C./sec or higher up to a plate temperature of not lower than [plating bath temperature −20] ° C. and not higher than 500 ° C. in a non-oxidizing or reducing atmosphere. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises dipping in a Zn plating bath containing 0.05 to 0.25% by mass to perform hot-dip plating, followed by heat alloying treatment.   The method for producing an galvannealed steel sheet according to claim 1, wherein the first pre-plating or the second pre-plating further contains P. The method for producing an galvannealed steel sheet according to claim 1 or 2, wherein the first pre-plating performed before the annealing is 0.005 to 1 g / m ² of Co plating. The second pre-plating to be applied to the steel plate after annealing is 0.01 to 1 g / m ² of Ni plating. A method of manufacturing a steel sheet.   The said steel plate is an ultra-low carbon steel plate whose C content is 0.004 mass% or less, The manufacturing method of the galvannealed steel plate of any one of Claims 1-4 characterized by the above-mentioned.   The said ultra-low carbon steel plate contains 0.01-0.1 mass% of Ti, The manufacturing method of the galvannealed steel plate of Claim 5 characterized by the above-mentioned.