KR20050069307A - Heat treatment method in the alloying heat treatment furnace - Google Patents

Abstract

       BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method in an alloying heat treatment furnace during a manufacturing process of an alloyed hot dip plated steel sheet in a method of manufacturing an alloyed hot dip plated steel sheet. It is to provide a heat treatment method for preventing the occurrence of the unalloy layer on the surface of the steel sheet.

According to the present invention, in the heat treatment method of the alloying heat treatment furnace in the manufacturing process of the alloyed hot-dip steel sheet, the galvanized steel sheet passed through the hot-dip galvanizing bath for 2 to 4 seconds at a temperature of 485 ± 25 ℃ In the alloying heat treatment furnace consisting of a heating step, the step of maintaining the end temperature of the crack stage to 440 ± 20 ℃ and the step of holding the heated steel sheet for 20 to 25 seconds, and the step of cooling the heated steel sheet in the cooling zone A heat treatment method is provided.

According to the present invention, it is possible to reduce the amount of powdering, which is one of the most important matters in the GA steel sheet for automobiles, which has recently increased rapidly, and enables stable operation without unalloying, and alloying by rapid heating. It is possible to reduce the occurrence of overalloy due to difficulty in temperature control.

Description

Heat treatment method in the alloying heat treatment furnace}

 The present invention relates to a method for producing an alloyed hot-dip steel sheet, and more particularly, to a heat treatment method in an alloy heat treatment furnace during the manufacturing process of an alloyed hot-dip steel sheet.

An alloyed hot-dip steel sheet is a steel sheet manufactured by hot-dip galvanizing and then heating the hot-dip galvanized layer to diffuse the iron component of the steel sheet into the plating layer, and is widely used as an automotive and household rustproof steel sheet because of its excellent weldability and paintability. When the steel sheet is used as an automobile exterior material, there is a case that the plating layer is broken when it is pressed and the powder is peeled off, which is called powdering. When powdering occurs, the powder falls on the press die and wears the press die, Therefore, in order to maintain the corrosion resistance of the steel sheet and reduce the wear loss of the press die, the powder resistance is important in the press working of the alloyed hot-dipped steel sheet.

In general, alloyed hot-dip steel sheet is mainly degreased Ti or Ti-Nb IF (interstitial free) steel, and after annealing at 750 ~ 900 ° C, the steel sheet temperature is cooled to 450 ~ 500 ° C, aluminum content 0.125 ~ 0.165wt It is obtained by heating a galvanized steel sheet with a plating amount of 30 to 80 g / m 2 through a molten zinc plating bath of% at a temperature range of 450 to 620 ° C. in an alloying heat treatment furnace.

According to Japanese Patent Publication No. 62196364, the alloying treatment is performed at an alloying temperature of 550 ± 5 ° C for 10 to 15 seconds, or at 690 to 820 ° C for 2 to 3 seconds, and according to Korean Laid-Open Publication No. 2000-045508, 490 to 520. The alloy is heated by heating for 6-8 seconds in the temperature range of ℃. In general, heating in the alloy heat treatment furnace is usually heated for 8 to 15 seconds in the temperature range of 450 ~ 620 ℃.

The alloy heat treatment furnace is conventionally used a gas heating furnace, but recently, an induction heating furnace capable of uniform heating is used. The heating pattern is heated in a heating zone, maintained for a certain time in a cracking zone, and then cooled in a cooling zone. . At this time, the known methods have completed the alloying heat treatment process in a short time by taking a heating pattern that rapidly heats up to the target temperature in the heating zone as much as possible and in fact, rapid cooling starts already in the cracking zone.

According to well-known heating patterns in an alloying heat treatment furnace, when the alloying temperature is relatively high, the tendency of powdering to occur during pressing (powdering amount) increases, whereas when the alloying temperature is low, unalloyed parts tend to occur. There is a growing problem. Low alloying temperature results in a fatal adverse effect on the corrosion resistance of the steel sheet when unalloyed.

 SUMMARY OF THE INVENTION An object of the present invention is to develop a heat treatment method for preventing the formation of an unalloy layer on the surface of a steel sheet while reducing the amount of powdering of the steel sheet finally obtained after the alloying heat treatment in the alloying heat treatment furnace during the manufacturing process of the alloyed hot-dip steel sheet. will be.

In order to achieve the above object, the present invention provides a heat treatment method in an alloying heat treatment furnace during a manufacturing process of an alloyed hot-dip steel sheet, wherein the steel plate galvanized through a hot dip galvanizing bath is heated at a temperature of 485 ± 25 ° C. on a heating table. An alloying heat treatment consisting of a heating step of heating for a second time, maintaining a crack stage end temperature at 440 ± 20 ° C. and holding the heated steel sheet for 20 to 25 seconds, and cooling the heated steel sheet in a cooling zone. Provided is a method for heat treatment in a furnace.

Hereinafter, the present invention will be described in detail.

In general, the plating layer in an alloyed hot-dip steel sheet is composed of Γ (bcc), Γ ₁ (fcc), δ ₁ (hexagonal), ξ (monoclinic) phase of the iron-zinc intermetallic compound, depending on their composition ratio It is known to change. In terms of press formability, a plating layer composed of δ ₁ (hexagonal) and ξ (monoclinic) phase is advantageous while suppressing the formation of brittle Γ (bcc) and Γ ₁ (fcc) phases. If it remains, there is a problem that the weldability worsens because the alloying reaction speed with the welding rod in the assembly line is high.

In order to investigate the relationship between the plating layer microstructure and powdering, the present inventors add a small amount of carbide generating elements Ti and Nb to improve the processability, that is, C and N in steel. Using ultra-low carbon steel precipitated in the form of TiC, TiN, NbC, etc., and setting the operating conditions in the Al concentration of 0.125 ~ 0.165 wt% in the zinc bath and the alloying temperature of 450 ~ 620 ℃, and producing more than 100 products. Test samples with thicknesses of 0.6 to 0.8 mm were taken. Al concentrations in zinc baths were quantified by inductively coupled plasma (ICP) method (effective number of decimal places up to 3 digits) The cold-mounted alloyed hot dip galvanized steel sheet (hereinafter referred to as GA steel sheet) was mirror polished and etched with 1% hydrochloric acid solution for 40 seconds. During press processing, the plating layer microstructure and the coating defoliation relationship were plated. Was determined by the release evaluation amount (deep drawing test), ringryang powder was characterized in terms of the experiment before and after the GA steel sheet according to the weight difference between the amount of plating layer peeling using a measuring device within the powdering in mg / m ² unit.

Table 1 shows the amount of powder obtained by plating layer microstructure, Γ phase thickness, and deep drawing test according to the alloying temperature. If the alloying temperature is below 525 ° C, as the alloying temperature increases, the thickness of the Γ phase increases and the amount of powdering increases, but above 540 ℃, no correlation is found between them. Although the thickness of the Γ phase was thin, the amount of powdering was the highest. From this fact, the powdering amount, as is generally known, was influenced by other factors in addition to the thickness of the Γ phase, and was highly dependent on the surface structure of the plating layer constituting the plating layer. It became.

Table 1

Figure 1 is a change in the surface structure of the coating layer with an increase in the alloying heating temperature. When the alloying temperature is 540 ℃ or more as shown in Fig. ₁ , in addition to the block δ ₁ phase plate-shaped alloy phase of the size of 10 ~ 15 mm appears, these phase analysis results as described later It was found to be a brittle δ ₁ phase containing more iron than the bulk δ ₁ phase. These plate-shaped δ ₁ phase textures were found to be higher with increasing alloying temperature. That is, when the alloying temperature is 600 ° C or more (Fig. 1 (c)), most of the surface of the plated layer is composed of a plate-shaped δ ₁ phase structure, where the bulk structure is partially scattered.

FIG. 2 shows the XRD results on the surface of each sample observed in FIG. 1. In the surface structure of FIG. 1, although the proportion of the plate-shaped δ ₁ phase increases as the alloying temperature increases, X-ray observation showed no difference as shown in FIG. 2. SEM / EDS analysis blocky-type organization 90.3 at% plate-shaped tissue, but that these organizations are the X-ray observation no difference to show the average zinc content of 87.1 at% of iron - they each hexagonal from zinc phase diagram δ _1p phase (zinc Side) and superlattice hexagonal crystals were found to belong to the δ _1k phase (iron side). Therefore it is seen that the plate-shaped convex-side _1k δ differs significantly brittle compared to the bulk type δ _1p side of zinc, and thus more brittle plate-shaped δ _1k differs increase could be seen, increasing the amount of plating layer peeling during press working.

Or more results produced from the press formability is excellent GA steel sheet conditions, and found that the coated tissue is to inhibit the formation of the plate-shaped δ ₁ to form a composite structure on a bulk-type δ ₁ phase, alone or blocky-type δ ₁ + columnar ξ.

In order to achieve this, the inventors adjusted the heating pattern of the galvannealing furnace heating part and the cracking part. That is, the galvanized steel sheet passed through the hot dip galvanizing bath was heated to a temperature of 485 ± 25 ° C. in the heating table. Heating step for heating for 2 to 4 seconds, maintaining the crack stage end temperature at 440 ± 20 ℃ and the heat holding step for holding the heated steel plate for 20 to 25 seconds, and the step of cooling the heated steel plate in the cooling zone The pattern was constructed.

Figure 3 shows the change in the heating pattern of the alloy plated steel sheet used in the present invention for the deep steel sheet. In the conventional GA alloying heat treatment, the heating temperature was performed at about 550 ~ 600 ° C, but in the present invention 485 ± 25 At this time, the heating time should be 2 ~ 4 seconds. In the conventional case, after the rapid heating, the cracking zone was relatively rapidly cooled (cracking end temperature is 380 to 400 ° C.), but in the present invention, the cooling was performed slowly and the cracking end temperature was controlled at 440 ± 20 ° C. At this time, the holding time is 20 to 25 seconds. After that, the alloying process is terminated by cooling the steel sheet in the cooling zone.

Figure 4 shows the SEM surface structure of the plated steel sheet prepared at the alloy heating pattern temperature is carried out by lowering the temperature of the heating table according to the present invention and the temperature of the end of the cracking zone observed in 1500 times to determine the composition ratio of the alloy phase to be. At the alloying temperature of 510 ° C. (FIG. 4 (a)), the surface of the plating layer is mostly composed of the bulky δ ₁ phase alone, but in some cases, the ξ phase is also observed. On the other hand, in the case where the alloying temperature is lowered to 460 ° C., as shown in FIG. 4 (c), a large amount of ξ phase of the columnar shape is observed along with the δ ₁ phase of the bulk shape on the surface of the plating layer. This fact crack versus temperature due to the rise gayeoldae the temperature can be down-I plating surface differs from the conventional plate-shaped δ ₁ phase + blocky-type δ bulk type in the mixed structure on the ₁ δ ₁ dandoksang or blocky-type δ ₁ + main phase in GA alloying induction Type ξ converted to mixed tissue. From the series of tissue photographs of FIGS. 1 and 4, it can be seen that the upper limit temperature of the heating stand should be limited to 485 + 25 ° C or 510 ° C. The lower limit temperature of the heating stand is limited to 485-25 ° C or 460 ° C. This is because unalloyment will occur below.

FIG. 5 shows the XRD results on the surface of each sample observed in FIG. 4. The X-ray results for FIG. 5 (a) show that the peaks 221, 401, 132, and 321 of the ξ phase are indicated by arrows. In other words, it can be seen that the flexible columnar ξ phase can be stably obtained by lowering the alloying temperature in the alloying furnace.

Hereinafter, embodiments of the present invention will be described.

In the embodiment, in order to improve the workability, a small amount of carbide, which is added to Ti and Nb, is added to precipitate C and N in the form of TiC, TiN, NbC, etc. A galvanized steel sheet was used as a basic material through a hot dip galvanizing bath of 0.125 to 0.165 wt%. The heating pattern is maintained at 420 ~ 460 ℃ temperature for 21.8 seconds at 225m length and 29.1m length in the cracking zone of the heating table, and the heating temperature is controlled from 460 ℃ to 530 while cooling for 8 seconds at 8m long cooling table. The alloying was carried out by changing to ℃. Specimens obtained after alloying were evaluated for alloying quality by measuring the amount of powdering.

6 shows the amount of powdering and the amount of Fe in the alloy layer according to the alloying temperature change.

6 (a) shows the powdering amount (plating layer peeling amount) obtained as a result of the deep drawing test of the GA steel sheet obtained by varying the alloying temperature with respect to the alloying temperature. Referring to the plating layer powdering amount according to the alloying degree, It can be seen that the sharp decrease. Considering that the amount of powdering required by the automobile company is 20% or less, it can be seen that the powdering amount of the GA steel sheet produced in the alloying heating pattern according to the present invention managing the alloying temperature at 510 ° C or less shows a target value or less.

6 (b) shows the change in alloying degree of the plating layer according to the change in alloying temperature. The alloying degree is 11% above the alloying temperature of 540 ℃, but when the alloying temperature is less than 510 ℃ it can be seen that the decrease to less than 10%. In other words, it can be considered that the alloying reaction was reduced as a result of suppressing the iron-zinc diffusion reaction with the temperature drop. In addition, the thickness of the Γ phase formed at the plated layer / ferrous iron interface also decreases as the alloying temperature decreases and decreases from 750 nm to 550 nm.

It can be seen from the above results that the degree of alloying degree of the coating layer and the thickness of the brittle Γ phase decreases as the alloying temperature decreases, the generation of the plate-shaped δ ₁ phase is suppressed, and as a result, the amount of powdering reacting sensitively to this decreases.

According to the present invention, it is possible to reduce the amount of powdering, which is one of the most important matters in the GA steel sheet for automobiles, which has recently increased rapidly, and enables stable operation without unalloying, and alloying by rapid heating. It is possible to reduce the occurrence of overalloy due to difficulty in temperature control.

1 is a surface texture photograph of the plating layer when the alloying temperature is high,

2 is an X-ray diagram of the samples of FIG. 1;

3 is a heat treatment pattern diagram in an alloying furnace;

4 is a texture picture of the plated layer surface when the alloying temperature is low,

5 is an X-ray diagram of the samples of FIG. 4;

Figure 6 is a change in the amount of powder and the amount of Fe in the alloying layer according to the alloying temperature changes.

Claims (1)

In the heat treatment method in the alloying heat treatment furnace of the alloying hot-dip steel sheet manufacturing process, A heating step of heating the galvanized steel sheet through a hot dip galvanizing bath at a temperature of 485 ± 25 ° C. for 2 to 4 seconds in a heating table; A thermal insulation step of maintaining the crack zone end temperature at 440 ± 20 ° C. for 20-25 seconds; And Cooling the heated steel sheet in a cooling table; Heat treatment method in the alloy heat treatment furnace, characterized in that consisting of.