KR20020042154A - method of manufacturing melting galvanized steel iron with good workability and chipping resistance - Google Patents

Abstract

PURPOSE: A method of manufacturing a galvannealed steel sheet with good workability and chipping resistance is provided. CONSTITUTION: In a method of manufacturing a galvannealed steel sheet, the method is characterized in that before coagulation of zinc, a galvanized steel sheet drawn up out of zinc bath containing Al 0.13-0.14 wt.% is rapidly heated by an induction heat furnace to inhibit the crystallization of zeta and gamma phase wherein electricity power ratio of lower zone to upper zone of the induction heat furnace is regulated to 2:1-4:1. After heat treatment in the induction heat furnace, galvannealed steel sheet is rapidly cooled to 30-50°C/sec to inhibit the growth of gamma phase.

Description

Method of manufacturing melting galvanized steel iron with good workability and chipping resistance

The present invention is a rapid heating by adjusting the output ratio of the induction heating furnace 2: 1 to 4: 1 during the alloying hot dip galvanizing heat treatment by the high frequency induction heating furnace, and by rapid cooling to 30 ~ 50 ℃ / sec after the alloying heat treatment A method for producing an alloyed hot-dip galvanized steel sheet having excellent chipping properties. More specifically, a lower zone and an upper zone of a high frequency induction furnace before solidifying a galvanized steel plate lifted in a hot dip galvanizing bath having an Al concentration of 0.13 to 0.14 wt%. The ratio of the output power is 2: 1 to 4: 1 for rapid heating to suppress the growth of the zeta phase and the gamma phase and the rapid cooling to 30 to 50 ° C / sec using the air mist to suppress the growth of the weak gamma phase. It is a manufacturing method of alloyed hot-dip galvanized steel sheet with improved chipping property. Since alloyed hot-dip galvanized steel sheet has excellent corrosion resistance, paintability and weldability, and is low in price, Has been expanded using a car shell uses a shim. 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 galvanized 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. In addition, chip chipping, which evaluates the degree of plating layer dropping by colliding gravel at high speed after electrodeposition coating, is improved by minimizing gamma and zeta phase. However, these alloy phases are produced by thermal diffusion of zinc in the plating layer with the iron component of the base material. The distribution of the alloy phases according to the steel composition, alloying temperature, alloying time, and components in the hot dip galvanizing bath is performed. Will be different.

Alloying 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 or gamma phase. Therefore, recently, as a result of adopting the alloying heat treatment method by the high frequency or low frequency induction heating method, it is reported that the temperature distribution is uniform and the alloy phase control is easy. However, when the output ratio of the induction furnace and the alloying conditions are inadequate, the problem of deterioration of workability and chipping resistance has existed. Therefore, in the present invention, an alloyed hot-dip galvanized steel sheet excellent in workability and chipping resistance was prepared by controlling the output ratio of the induction furnace and the cooling rate after the alloying treatment.

As a method for improving the processability of conventional alloyed hot-dip galvanized steel sheet (NSC) has produced a flash alloyed hot-dip galvanized steel sheet with a thin electroplated iron on the alloyed hot-dip galvanized layer. In addition, Kawasaki, Japan (KSC) has developed an alloyed hot-dip galvanized steel sheet that is flash-plated with iron-phosphorus alloy plating. 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 coated with 0.5 ~ 1g / m2 of phosphate film (Zn3-xMx (PO4) 2 · 4H2O) on the alloyed hot-dip steel sheet, which improves workability by reducing the molding load during press work and extending the life of the press die and reducing the friction coefficient. There is a problem in that the powdering property 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 100 ~ 200mg / m2 nickel coated lubricating coating on alloyed hot-dip galvanized steel sheet, which reduces the coefficient of friction and improves press formability. 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. In addition, although a small amount of Si was added to the steel component in Corrus, it was found that the chipping resistance was improved by increasing the bonding force between the plated layer and the base iron, but the experimental results showed that the alloying was greatly suppressed when Si was added.

The present invention has been made to solve the above problems, alloying hot-dip galvanized steel sheet excellent in workability and chipping resistance without the after-treatment and steel composition of the separate flash plating, ginseng treatment, lubricating coating, etc. described above An attempt was made to develop a method for preparing the same. In particular, the inventors found that the alloy phase distribution in the alloyed hot-dip galvanized layer is greatly changed according to the heat treatment conditions and the cooling rate after the initial alloying process, and thus the workability and chipping resistance of the plated layer are greatly affected. The evaluation revealed it.

The present invention is rapid heating by adjusting the output ratio of the induction heating furnace 2: 1 to 4: 1 during the alloying hot dip galvanizing heat treatment by the high frequency induction heating furnace, by rapid cooling to 30 ~ 50 ℃ / sec after alloy heat treatment It is a method for producing an alloyed hot dip galvanized steel sheet having excellent chipping properties. More specifically, before the solidified zinc plated steel sheet is solidified in a hot dip galvanizing bath having an Al concentration of 0.13 to 0.14 wt%, the output ratio of the lower zone and the upper zone of the high frequency induction furnace is rapidly heated to 2: 1 to 4: 1. A method for producing an alloyed hot-dip galvanized steel sheet which suppresses the growth of the zeta phase and the gamma phase and suppresses the growth of the vulnerable gamma phase by rapid cooling at 30 to 50 ° C./sec using an air mist. It aims to provide.

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

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 inhibition 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, if the initial alloying heating rate is high, the diffusion suppression layer is quickly extinguished, and the growth of the zeta phase is suppressed. On the other hand, if the heating rate is low, the initial alloying reaction occurs unevenly in the form of outburst. In addition, growth of the zeta phase that is stable at low temperatures is promoted, and flaking properties are lowered. In addition, if the cooling rate after the alloying heat treatment is slow to 10 ~ 25 ℃ / second, the growth of the vulnerable gamma phase is promoted, the powdering resistance and chipping resistance was greatly deteriorated.

The reason for limiting the output ratio of the lower zone and the upper zone of the high frequency induction furnace in the present invention is 2: 1 to 4: 1.

Conventional alloying heat treatment furnaces consist of heating, soaking and cooling. The heating table by the induction heating method is composed of several induction heating furnaces, but a conventional heating furnace is composed of two induction heating furnaces of a lower zone and an upper zone. The alloying temperature of the alloyed hot-dip galvanized steel sheet is usually controlled by the output of the induction furnace because the emissivity of the galvanized steel sheet is so severe that accurate temperature measurement is difficult.

Therefore, in the present invention, it was intended to suppress the growth of the zeta phase and the gamma phase by differentiating the initial heating rate and the late heating rate in the induction heating furnace before entering the crack zone. If the induction heating output of the lower zone before entering the cracking zone is less than 2: 1 or less than the upper zone, the initial diffusion inhibiting layer is unevenly destroyed, the outburst structure is developed, and the zeta phase grows thick, resulting in the final alloy. The layer has a zeta phase as a main component, and the friction coefficient is increased during press working, and the plating layer is dropped in the form of flaking at the interface between the base iron and the plating layer. On the other hand, if the induction heating output of the lower zone exceeds 4: 1, the gamma image grows thick due to excessive alloying, and the powdering resistance and the chipping resistance are degraded, and the shutdown of the induction furnace due to the electromagnetic collision is caused. Appeared a lot. However, when the output ratio of the lower zone and the upper zone of the present invention is adjusted to 2: 1 to 4: 1, the diffusion suppression layer is quickly extinguished, the growth of the zeta phase and the gamma phase is suppressed, and the workability and chipping resistance of the plating layer are greatly improved. It became.

Next, the reason why the cooling rate after the alloying treatment is limited to 30 to 50 ° C / sec will be explained. If the cooling rate after the alloying treatment is less than 30 ℃ / seconds, the alloying proceeds during the cooling process, the weak gamma phase grows thick, greatly deteriorated powdering and chipping resistance. On the other hand, it was difficult to increase the cooling rate above 50 ° C / sec by the air mist method, and there was no effect of increasing the growth inhibition of the weak gamma phase.

In addition, the aluminum concentration in the plating bath needs to be limited to 0.13 to 0.14%. When the Al concentration in the plating bath was less than 0.13%, 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, thereby forming an unalloyed zinc plating layer composed of zeta phase, thereby deteriorating the flaking resistance.

Powder resistance and chipping resistance are closely related to the thickness of the gamma phase in the alloy phase. In particular, it is necessary to manage the gamma phase thickness to be less than 0.5 μm in terms of powder resistance and chipping resistance, and as a result of investigating the correlation between the gamma phase thickness and the alloying degree in the alloy phase, the alloying treatment is performed at a low temperature of 480 to 500 ° C. for a long time. 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 0.6 µm can be obtained. That is, when the alloying degree is less than 9wt%, flaking occurs to the alloy phase mainly composed of zeta phase, and when it exceeds 11wt%, the gamma phase thickness exceeds 0.6 µm, so that the powdering resistance and the chipping resistance decrease.

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

<Example 1>

Table 1 shows an embodiment according to the present invention disclosed in the evaluation of the flaking amount, powdering amount, chipping resistance according to the output ratio of the induction furnace furnace 60g based on the plated amount by using the ultra-low carbon cold-rolled steel sheet having a thickness of 0.8mm It was / m2. After heating with different output ratios of the lower zone and the upper zone of the high-frequency induction furnace, the substrate is maintained for 15 seconds at 500 ° C and the cooling rate is 40 ° C / sec using an air mist. The amount of king, amount of powdering and chipping resistance were evaluated and shown. 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. The chipping resistance was measured by impacting 50 g of gravel at a pressure of 3 kgf / cm 2 at -20 ° C after electrodeposition coating to measure the area where the plating layer was peeled off.

Lower / Top Output Ratio Flaking amount (mg) Powder amount (mg) Chipping resistance (mm2 / cm2) Remarks 0.5: 1 35 2 0.16 Comparative Example 1 1: 1 20 4 0.27 Comparative Example 2 2: 1 5 5 0.35 Inventive Example 1 4: 1 3 8 0.41 Inventive Example 2 5: 1 One 25 1.35 Comparative Example 3

As shown in Table 1, Inventive Example (1-2) that satisfies the conditions of the present invention, the growth of the zeta phase and the gamma phase is suppressed, so that the amount of flaking of the plating layer (pass <10 mg), the amount of powdering (pass <10 mg) and chipping resistance This (pass <0.5 mm <2> / cm <2>) was excellent. However, when the output ratio of the lower / upper zone of the induction furnace is less than 2: 1 (Comparative Example 1, Comparative Example 2), the amount of zeta phase in the alloy phase is high, so that the amount of powdering and chipping is reduced, but the amount of flaking due to the increase of the friction coefficient. Increasingly, press formability continuous workability was greatly reduced. In addition, when the output ratio of the lower / upper zone exceeded 4: 1 (Comparative Example 3), the thickness of the vulnerable gamma phase was increased to increase the amount of powdering and greatly deteriorate chipping resistance.

<Example 2>

Table 2 shows the embodiment according to the present invention disclosed the flaking amount, powdering amount and chipping resistance according to the cooling rate after the alloying treatment, using the ultra-low carbon cold rolled steel plate having a thickness of 0.8mm 60g / m2 was set. The output ratio of the lower zone and the upper zone of the high frequency induction furnace is 2: 1, and the amount of flaking is applied to the alloyed hot-dip galvanized steel sheet manufactured by maintaining the cooling rate at 500 ° C. for 15 seconds and varying the cooling rate using an air mist. Powdering amount and chipping resistance were evaluated and shown.

Cooling rate (℃ / sec) Flaking amount (mg) Powder amount (mg) Chipping resistance (mm2 / cm2) Remarks 10 One 45 2.15 Comparative Example 1 20 3 25 1.27 Comparative Example 2 30 5 7 0.35 Inventive Example 1 40 8 5 0.32 Inventive Example 2 60 10 4 0.31 Comparative Example 3

As can be seen in Table 2, Inventive Example 1 and Inventive Example 2, in which the alloying cooling rate was adjusted to satisfy the conditions of the present invention, an appropriate degree of alloying and a gamma phase thickness of 0.5 μm or less, flaking resistance, powdering resistance, and The chipping resistance was very good. However, if the cooling rate is less than 30 ° C / sec (Comparative Example 1, Comparative Example 2), because the alloying proceeds further during the cooling process to increase the gamma phase, the amount of powdering and chipping area increased significantly. On the other hand, even though the cooling rate exceeded 50 ° C / sec (Comparative Example 3), the amount of powdering and chipping area was insignificant, which is sufficient to prevent further alloying at 30 ° C to 50 / sec cooling rate of the present invention. It is assumed that cooling is possible. In addition, in order to secure a cooling rate of more than 50 / sec, the amount of cooling water must be further increased, or the cooling medium must be changed from the existing nitrogen at room temperature to liquid nitrogen.

As described above, the present invention can greatly improve the workability and chipping resistance of the alloyed hot-dip galvanized steel sheet when the output ratio of the induction furnace is controlled and the cooling rate after the alloying treatment is adjusted to 30 ° C. to 50 / sec. The industrial use effect is very big.

Claims (3)

In the method for producing an alloyed hot-dip galvanized steel sheet, rapid heating by adjusting the output ratio of the lower zone and the upper zone of the high frequency induction furnace to 2: 1 to 4: 1 before solidifying the galvanized steel sheet lifted in the hot dip galvanizing bath. To suppress the growth of the zeta phase and the gamma phase to produce an alloyed hot-dip galvanized steel sheet excellent in workability and chipping resistance. The method for producing an alloyed hot-dip galvanized steel sheet according to claim 1, wherein after the alloying heat treatment, an air mist is used for rapid cooling at 30 to 50 ° C./sec to suppress the growth of vulnerable gamma phases. The method of claim 1, wherein the Al concentration of the hot dip galvanizing bath is adjusted to 0.13-0.14% by weight.