KR20020046709A - method of manufacturing hot-dip galvannealed steels with good anti-flaking properties - Google Patents

Abstract

PURPOSE: A method of manufacturing hot-dip galvannealed steels with excellent anti-flaking and anti-powdering properties is provided. CONSTITUTION: The method of manufacturing hot-dip galvannealed steels with excellent anti-flaking properties is characterized in that by adding 0.05-0.1 wt.% of nickel in a conventional plating bath for hot-dip galvannealing that contains 0.13 to 0.14 wt.% of Al and controlling the heating rate of hot-dip galvannealing at 30 to 50°C/sec, the growth of zeta-phase and gamma-phase in steel can be restricted.

Description

Method of manufacturing hot-dip galvannealed steels with good anti-flaking properties

The present invention uses a plating bath in which nickel is added to 0.05 to 0.1 wt% in a molten zinc plating bath having an aluminum content of 0.13 to 0.14 wt%, and rapidly heated to 30 to 50 ° C./sec during alloying heat treatment to prevent flaking resistance (Anti- The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet having excellent flaking, and more particularly, to a conventional alloyed hot-dip galvanized bath having an Al concentration of 0.13 to 0.14 wt%. Method for manufacturing alloyed hot-dip galvanized steel sheet which improves flaking resistance and powdering resistance of plating layer by suppressing growth of zeta phase and gamma phase by rapid heating at 30 ~ 50 ℃ / second in heating zone of alloying furnace before the galvanized steel sheet is solidified to be.

The alloyed hot-dip galvanized steel sheet has excellent corrosion resistance, paintability and weldability, and is inexpensive, and has been recently used in automobile and exterior plates mainly in Japan and the United States. This property is exhibited by an alloy plating layer of zinc and iron of an alloyed hot dip galvanized steel sheet. The alloyed plating layer is manufactured by passing the hot dip galvanizing bath in the continuous hot dip plating process and heating the plated layer in an alloy heat treatment furnace installed in the upper part before the zinc plated layer of the surface layer is completely hardened, followed by rapid cooling in an air cooling zone. do. During such an alloying heat treatment, the zinc in the molten state and the base iron are thermally diffused to form an alloy layer, and the reaction is stopped as it is cooled to room temperature. The phases present in the alloyed hot-dip galvanized layer and the characteristics thereof will be described. First, the kepital gamma (Γ) phase and the kepital gamma source (Γ1) phase present at the interface with the base iron have a content of 24 to 31 wt% of iron in the alloy layer, respectively. And 18.5 to 23.5 wt%, and the metallic lattice structure is a body centered cubic system and a face centered cubic system. Among them, the capital Gamma phase is the weakest phase and is the main cause of powdering of the alloy layer during processing. Next, the delta phase (δ) present in the upper layer has an iron component of 8.5 to 13 wt% and has a hexagonal lattice structure, which is superior in workability and low coefficient of friction as compared to the capacitive gamma layer. The zeta (ζ) phase present in the uppermost layer has the iron component of 6.7-7.2wt% and the lattice structure consists of monoclinic system, which has the best workability among the alloy phases, but has a high coefficient of friction. Will cause. Therefore, in view of workability, it is advantageous to have an alloy phase in which the gamma phase and the zeta phase are very thin and formed only in the delta phase. However, these alloy phases are produced by the thermal diffusion of zinc in the plating layer and the iron component of the base alloy. The distribution of the alloy phases according to the steel composition, alloying temperature, alloying time and components in the hot dip galvanizing bath Will be different. In addition, since the flaking resistance and the powder resistance have properties that are incompatible with the Fe content in the plating layer, it is very difficult to satisfy the operating conditions satisfying the two characteristics.

On the other hand, the alloy heat treatment by the direct heating method using a conventional burner has a lot of problems in that the temperature distribution of the plated steel sheet is uneven and rapid heating is difficult to form a thick zeta phase or gamma phase. Therefore, as a result of adopting an alloying heat treatment method using a high frequency or low frequency induction heating method, it has been reported that the rapid heating is possible and the temperature distribution is uniform so that the alloy phase control is easy. However, if the heating speed of the induction furnace is inadequate, the flaking resistance and the powder resistance deteriorate. Therefore, in the present invention, an alloyed hot-dip galvanized steel sheet excellent in flaking resistance and powdering resistance was controlled by controlling the heating rate of the induction furnace.

The flaking of the alloyed hot dip galvanized layer is known to occur mainly when the iron content in the plated layer is lower than 9%. Therefore, in order to improve the flaking resistance of conventional alloyed hot-dip galvanized steel sheet, the iron content in the plating layer should be increased to 10 to 12 wt%, but there is a problem in that the powder resistance is deteriorated relatively. As a method for improving the powder resistance of alloyed hot-dip galvanized steel sheet, Japan Shin-Il Iron Co., Ltd. (NSC) has produced flash alloyed hot-dip galvanized steel sheet in which iron is electroplated thinly on the alloyed hot-dip galvanized layer. However, in response to the cost reduction demands of automobile companies, phosphate-treated alloyed hot-dip galvanized steel sheets produced by US LTV and nickel-based inorganic lubricated coated steel sheets of NKK were developed. The former is alloyed hot-dip coated steel strip over the phosphate coating _{(Zn 3-x Mx (PO} 4) 2 · 4H 2 O) a 0.5~1g / m ² is applied to extended life and reduced coefficient of friction of the press die to reduce the molding force during press-forming Furnace resistance is improved, but there is a problem that the powdering resistance is deteriorated and the coating adhesion is deteriorated because the phosphate coating is not degreased well in the pretreatment process during the electrodeposition coating of automobiles. The latter is a 100-200mg / m ² nickel-coated lubricating film coated on an alloyed hot-dip galvanized steel sheet, which reduces frictional coefficients and improves flaking resistance. It is reported to have. However, it is rarely adopted by automobile companies due to the necessity of new facility to apply a separate lubricating film and the increase of manufacturing cost.

Therefore, the present invention has been made in accordance with the above requirements, and the alloying hot-dip galvanized steel sheet excellent in the flaking resistance and powdering resistance without the post-treatment, such as the separate flash plating, ginseng salt treatment, lubrication coating and changing the steel composition described above The purpose is to provide a method which can be manufactured.

Hereinafter, the present invention will be described in more detail.

The inventors have found that the alloy phase distribution in the alloyed hot dip galvanized layer is greatly changed according to the plating bath component and the alloying heating rate, and thus, through numerous repeated experiments and various quality evaluations, the flaking resistance and the powdering resistance of the plating layer are greatly affected. Revealed. The present invention uses a plating bath in which nickel is added to 0.05 to 0.1 wt% in a molten zinc plating bath having an aluminum content of 0.13 to 0.14 wt%, and rapidly heated to 30 to 50 ° C./sec during alloying heat treatment to prevent flaking resistance (Anti- It is a manufacturing method of alloying hot-dip galvanized steel sheet excellent in flaking). More specifically, 30 to 50 ° C. in an alloying induction furnace before the galvanized steel sheet solidified in a plating bath containing 0.05 to 0.1 wt% of nickel is added to a conventional alloyed hot dip galvanizing bath having an Al concentration of 0.13 to 0.14 wt%. It is a manufacturing method of alloyed hot-dip galvanized steel sheet which improves the flaking resistance and the powdering resistance of a plating layer by suppressing the growth of a zeta phase and a gamma phase by rapid heating at / second.

The flaking of the alloyed hot dip galvanized layer refers to a phenomenon in which the plated layer is dropped into particles having a size of 50 to 200 mm at the base iron / plated layer interface due to shear stress during press working. Powdering, on the other hand, refers to a phenomenon in which a plating layer is dropped into small particles of 10 mm or less at a plating layer or a plating layer / ferrous iron interface due to compressive stress during press working. Therefore, the flaking and powdering have a common point in that the working stress and the dropping particle size are different in the press working, but are greatly affected by the adhesive force on the crab surface between the plating layer and the base iron. Therefore, the present inventors have increased the bonding force between the plating layer and the base iron, and derived a method capable of producing an alloy phase consisting only of the delta phase.

The Fe ₂ Al ₅ alloy layer is formed when the steel plate is lifted in a conventional hot dip galvanizing bath containing 0.13 to 0.14 wt% of aluminum. These initial alloy layers are called diffusion inhibition layers because they retard iron-zinc transformation to form a complete alloy layer. If the diffusion suppression layer is too thick, the alloying reaction is suppressed to form an alloy layer mainly composed of a zeta phase, thereby lowering the flaking resistance. In order to solve this problem, in the present invention, by adding 0.05 to 0.1wt of nickel, a ternary system of iron-zinc-aluminum-nickel, which does not interfere with the alloying reaction instead of the ternary alloy layer of iron-zinc-aluminum, which hinders the alloying reaction It forms an alloy layer. The formation of the quaternary alloy layer was analyzed by using a glow discharge spectrometer (GDS) in the depth direction of the plating layer obtained during hot dip plating. As a result, it was found that nickel was concentrated at the same time as the aluminum concentration decreased at the plating layer / ferrous iron interface. Able to know. In addition, in order to determine the nickel distribution and bonding state of the four-membered alloy layer, a replica sample was prepared from the plated layer section and observed with a transmission electron microscope (TEM). The tissue could be observed. At this time, according to the result of EDAX analysis of components, Al, Ni, Fe, and Zn are agglomerated and mainly composed of a combination of aluminum and nickel. From the results of diffraction patterns measured by binary alloy state diagram of aluminum and nickel and transmission electron microscope, precipitates of AlNi intermediate phase appeared. Therefore, the alloying speed is increased by forming a quaternary alloy layer of porous iron-zinc-aluminum nickel that does not inhibit the alloying reaction at the plating layer / ferrous iron interface due to the addition of nickel in the plating bath. In addition, the intermediate phase compound of aluminum-nickel, which is stable on the plating layer / ferrous iron interface, inhibits the growth of vulnerable gamma phase and acts as a barrier that prevents crack formation and propagation of the plating layer during press working, thereby preventing flaking resistance and resistance. It is estimated to improve powdering property.

In addition, if the heating rate is fast to reach the crack during alloying heat treatment, the diffusion inhibiting layer is quickly extinguished, and the growth of the zeta phase is suppressed, whereas if the heating rate is low, the initial alloying reaction occurs unevenly in the form of outburst. That is, the growth of the stable zeta phase at low temperature is promoted, and flaking resistance is lowered.

The reason why the Ni content in the plating bath is limited to 0.05 to 0.1 wt in the present invention will be described. When the Ni content in the plating bath is less than 0.05wt%, it is insufficient to form the iron-zinc-nickel-aluminum quaternary alloy phase at the plating layer / ferrous iron interface, and thus partially form the iron-zinc-aluminum tertiary alloy layer. The alloy phase of zeta phase is developed, and when it exceeds 0.1wt%, the alloying reaction is promoted, but the dross generation amount due to nickel is increased on the plating bath, so that the surface quality of the product is deteriorated and it is not advantageous in terms of cost.

Next, the reason why the heating rate was limited to 30 to 50 ° C / sec during the alloying treatment will be explained. If the alloying heating rate is less than 30 ° C./sec, the initial alloying reaction occurs unevenly in the form of outburst. That is, the growth of the zeta phase which is stable at low temperature is promoted, and the flaking resistance is lowered. On the other hand, if the heating rate exceeds 50 ° C / sec, the initial diffusion inhibiting layer disappears rapidly and the growth of the zeta phase is suppressed, while the gamma phase grows rapidly, deteriorating the powdering resistance, and the heating rate is increased by 50 induction heating. It was difficult in terms of equipment to be higher than or equal to ° C / sec.

Flaking resistance and powder resistance are closely related to iron concentration and gamma phase thickness in the plating layer. When the alloying degree (iron content in the plating layer) is narrowly controlled to 9 to 11 wt%, a gamma-phase thickness of less than 0.5 μm can be obtained. That is, when the alloying degree is less than 9wt%, the amount of flaking is increased due to the alloy phase mainly composed of zeta phase. When the alloying degree is more than 11wt%, the gamma phase thickness exceeds 0.6 µm and the powdering resistance is lowered.

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

(Example)

Table 1 shows an embodiment according to the present invention using a very low carbon cold rolled steel sheet having a thickness of 0.8mm to set the coating deposition amount of 60g / m ² based on the cross-sectional basis. The alloyed hot-dip galvanized steel sheet prepared by coating the 460 ° C plating bath containing 0.135wtAl and 0.02% Fe with different nickel content, heating at 40 ° C / sec and maintaining at 480 ° C to produce 9wt% Fe in the plating layer. The amount of flaking, the amount of powdering and the surface appearance were evaluated by the subjects, and are shown in Table 1 below. The amount of flaking was measured using the U-bead tester to measure the amount of dropping of the plated layer by the shear stress, and the amount of powdering was measured by the cup forming tester to determine the amount of falling of the plated layer by the compressive stress. Measured and evaluated.

As shown in Table 1, Inventive Examples (1 to 2) satisfying the conditions of the present invention suppress the growth of the zeta phase and the gamma phase so that the amount of flaking (passing <10 mg), powdering amount (passing <10 mg), and surface appearance of the plating layer is reduced. Excellent. However, when the Ni content in the plating bath is less than 0.05 wt% (Comparative Examples 1 and 2), the zeta phase fraction in the alloy phase is high, so that the amount of powdering decreases but the amount of flaking increases due to the increase of the friction coefficient. It was greatly degraded. In addition, when the Ni addition amount exceeded 0.10 wt% (Comparative Example 3, Comparative Example 4), the flaking amount was good, but the thickness of the vulnerable gamma phase increased, resulting in an increase in powdering amount and adhesion of zinc-nickel dross to the steel sheet. The surface appearance was also greatly degraded. On the other hand, when the Al content in the plating bath exceeds 0.14 wt% (Comparative Example 5), even if Ni is added within the range of the present invention, an alloy layer made of a zeta phase is formed due to the thick academic acid suppression layer to significantly deteriorate the flaking resistance. It became.

division Plating bath composition (wt%) Flaking amount (mg) Powdering amount (mg) Surface appearance Al Ni Inventive Example 1 0.13 0.06 3 8 Good Inventive Example 2 0.14 0.09 5 4 Good Comparative Example 1 0.13 0 24 5 Good Comparative Example 2 0.14 0.04 27 2 Good Comparative Example 3 0.13 0.15 One 56 Bad Comparative Example 4 0.14 0.20 2 43 Bad Comparative Example 5 0.15 0.07 38 3 Bad

(Example 2)

Table 2 shows an embodiment according to the present invention using a very low carbon cold rolled steel sheet having a thickness of 0.8mm to set the coating deposition amount of 60g / m ² on a cross-sectional basis. After plating in a 460 ° C plating bath containing 0.135wtAl, 0.02% Fe, 0.06% Ni, alloying produced 10wt% of Fe in the plating layer by maintaining the heating rate of the alloying induction furnace at 480 ° C for 15 seconds. The amount of flaking and powdering of the hot-dip galvanized steel sheet was evaluated and shown in Table 2 below.

As can be seen in Table 2, Inventive Example 1 and Inventive Example 2, in which the alloying heating rate was adjusted to satisfy the conditions of the present invention, were very excellent in flaking resistance and powdering resistance. However, when the heating rate was less than 30 ° C./sec (Comparative Example 1), growth of the zeta phase stable at low temperature was promoted and the flaking resistance greatly decreased. On the other hand, if the heating rate exceeds 50 ° C / sec (Comparative Example 2), the initial diffusion inhibitor layer disappears rapidly to suppress the growth of the zeta phase, whereas the gamma phase grows rapidly, increasing the amount of powdering and frequent electrical shorts. In terms of equipment, there were many problems.

division Heating rate (℃ / sec) Flaking amount (mg) Powdering amount (mg) Inventive Example 1 35 3 7 Inventive Example 2 45 2 5 Comparative Example 1 15 25 3 Comparative Example 2 55 One 25

As described above, the present invention can greatly improve the flaking resistance and the powdering resistance of the alloyed hot-dip galvanized steel sheet when the plating bath component control and alloying heating rate is adjusted to 30 ° C. to 50 / sec. Use effect is very big.

Claims (2)

Alloyed hot-dip galvanized steel sheet having excellent flaking resistance and powdering resistance by suppressing growth of zeta phase and gamma phase by adding nickel to 0.05 to 0.1 wt% in a conventional alloyed hot dip galvanizing bath having an Al concentration of 0.13 to 0.14 wt%. Manufacturing method. A method for producing an alloyed hot-dip galvanized steel sheet which improves the flaking resistance and the powdering resistance of the plating layer by rapidly heating the alloying heating rate at 30 to 50 ° C./sec to suppress the growth of the zeta phase and the gamma phase.