KR20030049335A - Manufacturing method of hot-dip galvannealed steel sheets with good stone chipping resistance - Google Patents

Abstract

PURPOSE: A method of manufacturing hot-dip galvannealed steel sheets with good stone chipping resistance is provided, which is characterized in that crater formed inside galvannealed layer plays roles both as crack arrest and as locking that improves binding force between galvannealed layer and matrix. CONSTITUTION: The method is characterized in that the temperature of steel sheet to be fed into galvanizing bath with aluminum concentration of 0.13 to 0.14 wt.% is kept between 460 and 480 deg.C; alloying degree of galvanized layer is controlled within 10.5 to 12.5 wt%Fe; crater portion in galvannealed layer is regulated in the range of 11 to 20 %.

Description

Manufacturing method of hot-dip galvannealed steel sheets with good stone chipping resistance

The present invention relates to a method for manufacturing an alloyed hot-dip galvanized steel sheet, and more particularly, to a method for manufacturing an alloyed hot-dip galvanized steel sheet having improved chip chipping resistance by adjusting the crater fraction in the plating layer.

Recently, galvanized galvanized steel (GA) has excellent corrosion resistance, paintability, and weldability, and is economical, and is recently being used for automotive exteriors mainly in Japan and the United States. However, in order to use the GA steel sheet as an automobile shell, the chipping resistance or the plating layer must be very excellent so that the coating film or the plating layer does not fall off due to the flying stones or gravel. This property is exhibited by an alloyed plating layer of zinc and iron in 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 directly above the zinc plated steel sheet before the surface of the zinc plated layer is completely hardened. Done. In the alloying heat treatment reaction, the zinc in the molten state and the iron component of the base heat-diffuse reaction to form an alloy layer and the reaction is stopped as it is cooled to room temperature.

Next, each phase present in the alloyed hot-dip galvanized layer and its characteristics are as follows. First, the keptal gamma (Γ) phase and the capital gamma source (Γ1) phase present at the interface with the base iron are 24 to 24, respectively. 31 wt% and 18.5-23.5 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 iron content of 6.7 ~ 7.2wt% and the lattice structure is monoclinic system, which has the best plating adhesion among 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. These alloy phases are produced by thermal diffusion of zinc in the plating layer and an iron component of the base material. The distribution of the alloy phases varies depending on the alloying temperature, the alloying time, and the components in the hot dip galvanizing bath. .

Here, looking at the causes of chipping resistance degradation of the GA material coated with a medium-definition coating can be largely divided into material factors and coating factors. Material factors include alloying degree, coating weight, alloy phase structure, adhesion between base iron / plating layer interface, crater in plating layer, thickness of gamma phase and steel components, etc. Coating factors include antirust oil degreasing and paint type. And thickness. However, in the present invention, the coating factors are fixed and the plating layer factors are investigated.

One of the important properties observed in alloyed hot-dip galvanized steel sheet is the crater formation of the plated layer. Craters are produced by a violent zinc-iron outburst reaction starting along the grain boundaries of ferrous iron. These heterogeneous alloy phases grow faster by inhaling liquid zinc in adjacent regions by capillarity and surface tension effects, and form craters in the form of craters where zinc is depleted. The craters exhibit very large differences in crater fractions due to the inherent properties observed in most alloyed hot-dip galvanized steel sheets, especially in continuous hot-dip galvanized steel sheets with high aluminum content in the plating bath.

However, the study of the effect of craters on the chipping resistance in the plating layer is very weak, and the researchers have published conflicting results.

In order to improve the chipping resistance of conventional alloyed hot-dip galvanized steel sheet, Nippon Nippon Steel (NSC) has produced flash-alloyed hot-dip galvanized steel sheet which is thinly electroplated with iron on the alloyed hot-dip galvanized layer. . In addition, Kawasaki Steel (KSC) has developed an alloyed hot dip galvanized steel sheet that is flash-plated with iron-phosphorus alloy plating (GALVATECH '95). However, in response to demand for cost reduction by automobile companies, recently developed phosphate-alloyed hot-dip galvanized steel sheets produced by LTV in the United States and nickel-based inorganic lubricated coated steel sheets of NKK (Japan KAL) were developed (GALVATECH '98).

The former is a phosphate coating _{(Zn 3_x M x (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 on alloyed hot-dip coated steel strip Although chipping resistance is improved, there is a problem in that powdering resistance is deteriorated and 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 100 ~ 200mg / m ² nickel coated lubricating coating on alloyed hot-dip galvanized steel sheet, which reduces the coefficient of friction and improves workability and chipping resistance. Phosphate treatment, paintability and corrosion resistance It is reported to have equivalent performance. However, it is rarely adopted by automobile companies due to the necessity of new facility to apply a separate lubricating film and an increase in manufacturing cost. In addition, Hoogensens, the Netherlands, reported that the addition of 0.05% Si to the steel component increased the shear strength of the plated layer and the ferrous iron, thereby improving the chipping resistance (GALVATECH '98). There is a problem that is greatly suppressed.

Accordingly, the present inventors have repeatedly conducted several experiments to develop a method for producing an alloyed hot-dip galvanized steel sheet having excellent chipping resistance without post-treatment such as the above-described separate flash plating, phosphate treatment, lubricating coating, and change of steel components. And various quality evaluations revealed that the chipping resistance is greatly changed according to the crater fraction and the degree of alloying present in the alloyed hot dip galvanized layer. That is, the present invention is to provide a method for manufacturing an alloyed hot-dip galvanized steel sheet with improved chipping resistance by appropriately adjusting the crater fraction in the alloyed hot-dip galvanized layer.

In the present invention for achieving the above object in the method of manufacturing an alloyed hot-dip galvanized steel sheet excellent in chipping resistance by adjusting the crater fraction in the alloyed hot-dip galvanizing layer,

The chipping resistance characterized by maintaining the temperature of the steel sheet introduced into the hot dip galvanizing bath having an Al concentration of 0.13 to 0.14 wt% at the plating bath and adjusting the alloying degree of the plating layer to 10.5 to 12.5 wt% Fe. It provides a method for producing alloyed hot-dip galvanized steel sheet with improved properties.

In addition, the alloying treatment is characterized in that the crater fraction in the alloyed hot-dip galvanizing layer is adjusted to 11 to 20%.

Hereinafter, the present invention will be described in detail.

First, the present inventors have observed the damage of the doped coating film several times during the chipping resistance test at low temperature (-20 ° C) of the alloyed hot-dip galvanized steel sheet subjected to medium-phase treatment to identify the cause of chipping resistance degradation of the GA steel sheet It was found that the plating layer was dropped to about 1 mm at the plating layer / ferrous iron interface. Therefore, in order to improve the chipping resistance, it was found that the adhesion between the plating layer and the base steel is very important. In addition, according to the results of observation of the surface of the GA plating layer having poor chipping resistance with a scanning electron microscope (SEM), some zeta phase remained on the surface of the plating layer, the iron content in the plating layer was less than 10.5wt%, and the crater fraction was less than 10%. Appeared as low. Therefore, the chipping resistance of the GA plating layer was found to be closely correlated with the alloying degree and the crater fraction of the GA plating layer.

As a result of investigating the cause of crater formation in the plating layer, the Fe ₂ Al ₅ alloy layer is formed before the steel sheet enters the alloying furnace in the plating bath. These initial alloy layers are called diffusion suppression layers because they delay the iron-zinc transformation to form a complete alloy layer. These diffusion inhibitory layers become unstable during the alloying process to form a complete iron-zinc alloy layer. At this time, since the alloying reaction proceeds somewhat uniformly and rapidly on the highly reactive surface, an alloy layer having almost no craters is formed.

On the other hand, if the surface reactivity is moderate or nonuniform, these alloying reactions occur unevenly in the form of outburst. Therefore, the adjacent residual zinc is sucked up by capillary action or surface tension effect and grows faster. At this time, the zinc is depleted to form craters.

Obviously, the outburst grows while consuming the iron at the bottom, and the result of observing the base iron cross-sectional structure shows that the interface has decayed at least 15% of the average plating layer. Such a device was more clearly seen when the surface of the alloy layer was removed by chemical method. The depressed part observed under the scanning electron microscope after the chemical removal clearly corresponds to the initial outburst, it can be seen that located in the crater generating part.

In addition, the results of the high magnification show that the ferrite grain boundary is always observed in the lower portion, and it is found that the outburst coincides well with the theory that the outburst follows the ferrite grain boundary of the ultra low carbon steel.

In the present invention, the reason for limiting the crater fraction in the plating layer to 11 to 20% will be described. The crater fraction in the plated layer was observed by the optical microscope of the plated layer structure subjected to the electrodeposition coating, measured the length of the portion of the plated layer to the base iron interface, divided by 1cm, the length of the entire observation portion was measured as a percentage. This method takes a lot of time, but it was used as a criterion because it can accurately determine the presence or absence of craters and has excellent reproducibility.

A crater fraction of less than 11% is mainly an unalloyed state that is not sufficiently alloyed to the surface of the plating layer.The zeta phase is the main component, the friction coefficient increases during press work, and the plating layer falls off in the form of flaking at the interface between the base iron and the plating layer. It was. According to the experimental results of the present inventors, the chipping resistance has a low correlation with the powdering, which is the plating layer dropping phenomenon due to the compressive stress, but it has a very close correlation with the flaking dropped over the thickness of the plating layer by the shear stress.

When the crater fraction exceeded 20%, the outburst reaction was excessively progressed, so that the vulnerable gamma image developed thickly, powdering occurred severely, and the corrosion resistance after coating was also reduced.

However, when the crater fraction of the present invention is adjusted to 11-20%, it acts as a crack arrest for chipping resistance evaluation and increases the bonding force with the coating layer due to surface irregularities (locking effect). It was observed that the chipping resistance was improved.

The following explains the reason for limiting the operation factors related to crater formation.

First, the steel sheet temperature introduced into the plating bath is limited to 460 to 480 ° C because the outburst reaction between iron and zinc increases as the steel sheet temperature increases. In other words, when the steel plate temperature exceeds 480 ℃, the alloy phase initially formed during the deposition in the plating bath is mostly outburst structure appeared to exceed the crater generation rate 20%. In addition, the problem of deterioration of the powdering property by promoting the formation of a weak gamma phase. On the other hand, below 460 ° C, all of the initial alloy phases were formed as zeta phases, which are columnar crystals, and the outburst reaction was greatly suppressed.

The reason for limiting the aluminum concentration in the plating bath to 0.13 to 0.140% is that when the Al concentration in the plating bath is less than 0.13%, the initial diffusion suppression layer of iron-aluminum does not cover the surface of the base iron, so that the diffusion suppression layer does not exist or is thin. The alloy layer grows preferentially at the site, indicating that the amount of crater generation is high. In addition, the amount of powdering was greatly increased by promoting formation of a weak gamma phase.

On the other hand, when the aluminum concentration exceeds 0.14%, the initial diffusion suppression layer is formed thick, and the alloying reaction between iron and zinc is very suppressed, thus forming an alloy layer made of zeta phase and the crater formation is suppressed to less than 10%.

The reason for limiting the alloying degree of the plating layer to 10.5 to 12.5wt% Fe is that when the alloying degree is less than 10.5wt% Fe, the zeta phase of the columnar phase remains on the surface of the plating layer, so that the friction coefficient is increased and the flaking property is not only deteriorated but also the chipping resistance is remarkably increased. It was shown to degrade. On the other hand, when the alloying degree exceeds 12.5wt% Fe, the zeta phase disappears, but the thickness of the vulnerable gamma phase increases, resulting in an increase in powdering amount.

The amount of powdering is closely related to the thickness of the gamma phase in the alloy phase. In particular, it is necessary to manage the thickness of the gamma phase to be 0.6 µm or less in terms of powder resistance, and when the alloy phase is less than 10.5 wt%, flaking occurs to an alloy phase mainly composed of zeta phase. The amount of powdering increases because the thickness exceeds 0.6 µm.

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

<Example 1>

Table 1 shows the results of chipping resistance, flaking resistance and powdering evaluation according to the crater fraction according to the present invention. The coating deposition amount was 50 g / m ² based on the cross section using a cold rolled steel sheet having a thickness of 0.8 mm. The alloying degree was different from about 11wt%, but powdering, chipping resistance, and flaking resistance were evaluated for steel sheets with varying crater fractions in the plating layer. At this time, powdering property was quantitatively evaluated by measuring the amount of dropping of the plating layer using a cup forming tester.

In order to evaluate the chipping resistance of the steel sheet, electrodeposition coating was first performed on a GA steel sheet in an automobile coating line with a thickness of about 10 μm. After the electrodeposition coating, the specimens were subjected to intermediate and top coat thicknesses of 30 µm, respectively. The chipping resistance test method used a SUGA chipping tester, and was held at a low temperature of -20 ° C for at least 3 hours, and then hit the specimen at an angle of 90 ° at an air pressure of 2 Kg / cm ² . . The pebbles used at this time were about 5 mm in diameter prescribed in JIS A5001 according to Japanese automobile test regulations and crashed 50 ± 1 g onto the specimen.

After the chipping test, the specimens were quantitatively evaluated by peeling using an adhesive cellophane tape until the swollen coating film completely dropped. At this time, the area that is hard to fall off with the tape was completely removed using a sharp knife. In the quantitative evaluation method, the peeling area (mm ² / cm ² ) per evaluation area was calculated by measuring the area of the peeling part of 0.5mm or more in the 50mmX50mm part in the center of the specimen by an optical microscope.

0.45 mm²/ cm²Less than The chipping resistance was shown to be excellent.

The flaking property used a self-made U-channel flaking tester. Test specimens (50x240mm) are drawn through beads at a speed of 80mm / sec to make a 66mm high channel. In order to give workability according to the thickness of the specimen without breaking the material, the curvature of the bead was fixed at 0.5 mm and the load at 0.5 ton. Test specimens were graded from the 1st grade (very good) to the 5th grade (very poor) by determining the internal test standard through visual inspection. It was judged that the plating layer of grades 1-3 was passed.

Crater fraction (%) Chipping resistance peeling area (mm ² / cm ² ) Flicking (Grade) Powdering amount (mg) Remarks 3 1.35 5 15 Comparative Example 1 6 0.85 4 10 Comparative Example 2 12 0.35 2 8 Inventive Example 1 18 0.25 One 5 Inventive Example 2 25 0.18 One 35 Comparative Example 3

As shown in Table 1, the invention examples (1, 2) in which the crater fraction satisfies 11 to 20% of the conditions of the present invention act as a crack arrest against chipping crack propagation, Peeling area is increased by 0.25 ~ 0.35mm²/ cm²Low standard value 0.45 mm²/ cm²Under Excellent chipping resistance was observed.

However, the amount of powdering was good in Comparative Examples (1, 2) having a crater fraction of less than 10%, but chipping resistance and flaking resistance were greatly deteriorated. On the other hand, in the comparative example (3) having a crater fraction of more than 20%, the chipping resistance and flaking resistance were good, but the thickness of the vulnerable gamma phase was increased to 1 µm or more, thereby greatly increasing the amount of powdering.

<Example 2>

Thickness of 0.8mm in the coating weight by using a cold-rolled steel sheet with a 50g / m ² based on the cross-section, the coated steel strip is immersed in a bath temperature bath, and the plating yoknae aluminum concentration, otherwise the alloy plating layer be within a fraction of craters and chipping resistance And the results of measuring the amount of powdering are shown in Table 2.

Alloying parameters Crater fraction (%) Chipping Resistance (mm ² / cm ² ) Powdering amount (mg) Remarks Steel plate bathing temperature (℃) Al concentration (wt%) Alloying degree (wt% Fe) 470 0.135 11.5 16 0.25 6 Inventive Example 1 440 0.135 11.3 5 1.35 15 Comparative Example 1 490 0.135 11.8 30 0.18 45 Comparative Example 2 470 0.121 11.8 25 0.20 37 Comparative Example 3 470 0.150 11.4 2 0.91 11 Comparative Example 4 470 0.135 9.5 0 1.47 3 Comparative Example 5 470 0.135 13.5 35 1.08 75 Comparative Example 6

As can be seen in Table 2, Inventive Example (1), in which the alloying parameters were adjusted to satisfy the conditions of the present invention, satisfies the appropriate crater fraction and was very excellent in chipping resistance and powdering resistance.

However, in Comparative Example (1) having a steel plate bathing temperature (steel plate temperature introduced into the plating bath) of less than 460 ° C, the crater fraction was decreased and chipping resistance was deteriorated, and the steel sheet bathing temperature of Comparative Example (2) exceeded 480 ° C. In this case, the chipping resistance was good, but the powdering property was deteriorated due to the formation of a weak gamma phase.

As in Comparative Example (3), when the aluminum concentration in the plating bath was less than 0.13 wt%, the heterogeneous alloying reaction was promoted and the chipping resistance was good due to the increase of the crater fraction, but the gamma phase thickness was greatly increased and the powdering resistance was lowered.

In addition, when the aluminum concentration in the plating bath exceeds 0.14 wt% as in Comparative Example (4), the generation rate of craters is very small and chipping resistance is degraded.

In addition, in Comparative Example (5) having an alloying degree of less than 9.5 wt% Fe with different alloying times, zeta phase was present on the surface, and the crater fraction was zero, and chipping resistance was greatly reduced.

In addition, when the alloying degree exceeded 12.5wt% Fe as in Comparative Example (6), the gamma phase thickness was increased due to excessive alloying, so that the amount of powdering was increased, and the plating layer was dropped at the interface between the plating layer and the base iron, thereby deteriorating the chipping resistance. .

As described above, the present invention can greatly improve the chipping resistance of the alloyed hot-dip galvanized steel sheet when the crater fraction in the plated layer is properly adjusted without any post-treatment or change of steel components, so that the present invention can be used for automobile exterior plates. very big.

Claims (2)

In the manufacturing method of alloyed hot-dip galvanized steel sheet excellent in chipping resistance by adjusting the crater fraction in the alloyed hot-dip galvanized layer, The chipping resistance characterized by maintaining the temperature of the steel sheet introduced into the molten zinc plating bath in which the Al concentration of the plating bath is 0.13 to 0.14 wt% and adjusting the alloying degree of the plating layer to 10.5 to 12.5 wt% Fe. Method for producing alloyed hot-dip galvanized steel sheet with improved properties. The method of claim 1, A method of producing an alloyed hot-dip galvanized steel sheet with improved chipping resistance, characterized in that the alloying treatment so that the crater fraction in the plating layer is adjusted to 11 to 20%.