JP2800700B2 - Manufacturing method for galvannealed steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for alloying a hot-dip galvanized steel sheet using a P-added ultra-low carbon steel as a base metal, and particularly to a method in which a plating layer is less peeled (powdered) during press forming. The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet having good plating adhesion.

[0002]

2. Description of the Related Art A P-added ultra-low carbon steel sheet is a 40 kilogram steel sheet containing 0.1% or less of P in aluminum-killed steel utilizing the solid solution hardening action of P. This is a steel sheet having a high r value (plastic strain ratio) and suitable for drawing, and is frequently used as a high-tensile steel sheet for automobiles.

[0003] The galvannealed steel sheet has excellent corrosion resistance and weldability after painting, but has poor plating adhesion, and when used as an outer panel of an automobile body, peels off the plating by means of stone splashing and powders during press forming. Peeling of a plating layer called a ring easily occurs. Powdering properties tend to deteriorate as the Fe content in the plating film increases or as the alloying treatment temperature increases. Particularly, a P-added ultra-low carbon steel sheet is a material which is difficult to alloy, that is, a material having a slow alloying reaction, and therefore requires a higher alloying treatment temperature than usual due to the restriction of the heating zone length of the alloying treatment device. When the alloying treatment temperature is increased, there is a problem that the plating adhesion and the powdering resistance are deteriorated.

As a method for improving the powdering resistance,
Japanese Patent Publication No. 59-14541 discloses that the alloying treatment is performed by first heating and second heating.
There is disclosed a method in which primary heating is divided into primary heating and primary heating is rapidly performed to smooth the surface of the plating layer, and secondary heating is performed in a batch annealing furnace. However, in this method, although the effect of improving the powdering property is recognized, the improvement of the plating adhesion is not recognized, and the manufacturing process is complicated and the cost increases. Also, JP-A-2-106003, JP-A-4-341550
Each of the publications mentioned above, after plating in a zinc plating bath with a specific Al concentration, the first stage of the alloying treatment was brought to an unalloyed state by short-time heating at 500 to 600 ° C, and the 510 ° C
A method of completing alloying and rapidly cooling at a cooling rate of 30 ° C./s or more is disclosed. However, in this method, the first-stage heating is performed in a short time, that is, rapid heating is performed, so that no effect of improving the powdering property is recognized. Need to strengthen the seal.

On the other hand, in the Zn plating bath, growth of the Γ phase is suppressed,
That is, although Al is added for the purpose of improving powdering properties, when a difficult-to-alloy material is used as the base material, the alloying reaction rate is significantly reduced, and it is necessary to increase the alloying treatment temperature. However, it is difficult to produce only a single steel type in the actual processing line, and the P-containing steel type is also alloyed at the same temperature as other steel types. Therefore, Al addition to the Zn plating bath conversely reduces powdering resistance. It has caused deterioration.

[0006] Japanese Patent Application Laid-Open No. H5-247619 discloses that a temperature adjustment zone is provided between a heating zone for rapidly heating and a holding zone for generating an alloying reaction, and a rising gas flow passage extending from the temperature adjustment zone to the holding zone is provided. A vertical alloying furnace provided with a pressure loss resistor and a heat pattern of the alloying treatment are disclosed. The heat pattern was rapid heating to 470 ° C in the first heating zone and 504 in the second heating zone.
The temperature is reduced to ℃ and kept at that temperature. Therefore, even in this method, the effect of improving the powdering property is not recognized, and it is necessary to strengthen the seal with the second stage heating zone in order to increase the first stage heating to a high temperature.

Japanese Patent Application Laid-Open No. 61-207563 discloses that the heating zone of a plated steel sheet is divided into at least two or more zones, the heating in the last section on the exit side of the steel sheet is induction heating, and the input frequency to the induction heating coil is adjusted. Alloying treatment method of 10 kHz or more, if necessary, the first section is also induction heating, and the input frequency is 1 to
An alloying method at 10 kHz is disclosed. This method also rapidly heats the first compartment, and does not show any effect of improving the powdering resistance.

The heating patterns disclosed in these publications increase the heating rate in the initial stage, and do not achieve the desired effects of improving adhesion and powdering resistance.

[0009]

The above-mentioned various methods have been proposed to improve the alloying processability of difficult-to-alloy materials, but all of them are rapid heating systems, and the initial rapid heating is performed by a strong Fe-Al alloy. The subsequent alloying reaction becomes difficult to form a layer, and it is necessary to raise the alloying temperature of the holding zone or to use ultra-rapid heating (for example, 35 ° C / s or more) in the latter half of the heating furnace. In addition, significant equipment remodeling is required.

[0010] The present invention provides an alloying treatment which is excellent in adhesion and powdering resistance by specifying the heat pattern of the heating zone and the holding zone in the alloying treatment even for a very low carbon steel sheet containing P. An object of the present invention is to provide a method for manufacturing a hot-dip galvanized steel sheet.

[0011]

Means for Solving the Problems As a result of intensive studies on the alloying treatment temperature, the present inventors have conducted initial heating at a low speed, suppressed the formation of the Fe-Al layer, and performed rapid plating as the final heating. After securing the adhesion, it was found that alloying in a low-temperature holding band improves the adhesion and the powdering resistance, thus completing the present invention.

The gist of the present invention lies in a method for optimizing a heat pattern of a steel sheet in an alloying treatment heating furnace shown in FIG. 2 as shown in FIG.

This is a method for alloying a galvanized steel sheet (S) of ultra-low carbon steel containing 0.025% by weight or more of P, wherein the heating zone (1) of the alloying furnace (G) is divided into two sections. Divide as above, use induction heating method at least for the final heating zone (C), and set the steel sheet heating rate in the initial heating zone (A) to 5 to 10 ° C / s.
After heating to 460 to 470 ° C, heat the steel sheet in the final heating zone (C) to 490 to 510 ° C with the heating speed of the steel plate at 20 ° C / s or more, and set the temperature in the holding zone (2) to 470 to 510 ° C. Method.

[0014]

FIG. 1 is a view showing a heat pattern of the alloying treatment. The solid line shows the heat pattern of the present invention, and the chain line shows the conventional heat pattern. The alloying treatment method of the present invention improves the adhesion and the powdering resistance by performing initial heating at a low speed and final heating at a rapid rate, and performing alloying in a low-temperature holding zone.

In the following, with respect to the various conditions defined in the present invention, the effects and the reasons for limiting the conditions will be described.

I. About slow initial heating:
If the initial heating rate is higher than 10 ° C / s, a strong Fe-Al layer is formed, and the subsequent Fe-Zn alloying reaction becomes difficult. Therefore, raise the alloying treatment temperature or use ultra-rapid heating (35 ° C). / s
Above). As a result, the powdering property deteriorates or the equipment needs to be remodeled significantly. 5 ° C / s
If it is slower than the above, the 成長 phase grows and the slidability during press working deteriorates. Steel plate heating rate in the initial heating zone is 5-10 ° C / s
Within this range, there is no slidability deterioration particularly due to ζ phase growth, and no strong Fe—Al layer is formed.

The heating method is not particularly limited, but the induction heating method is preferable because the temperature can be easily controlled.

If the temperature reached in the initial heating zone is lower than 470 ° C., the ζ phase grows, and if it reaches 490 ° C. or higher, the Γ phase grows excessively, which is not preferable.

Here, the term "deterioration in slidability" refers to a phenomenon in which the presence of a soft に phase on the surface of a plating layer of a plated steel sheet increases the coefficient of friction during press forming, so that the plating layer peels off as if peeled off. Say. Further, "powderability" refers to a phenomenon in which a film (Fe-Zn alloy layer) peels off in a powder state when alloying of a plating layer proceeds excessively.

II. About rapid final heating:
Rapid heating is effective for completing alloying in a short time.
Rapid heating deteriorates slidability due to short low-temperature time.
This has the effect of suppressing the generation of a phase and the effect of activating the alloying reaction to impart irregularities in units of crystal grains to the interface of the plated steel sheet.

In particular, induction heating in the latter half of the heating is a particularly effective method because it is necessary to rapidly heat the latter half of the heating in order to form irregularities at the interface of the plated steel sheet. When the heating rate of the steel sheet in the final heating zone is 20 ° C./s or more, unevenness is easily provided at the interface of the coated steel sheet, and the plating adhesion by the anchor effect becomes good. When the heating rate is less than 20 ° C./s, unevenness is hardly provided, and improvement in the adhesion of plating cannot be expected.

On the other hand, if the temperature reached in the final heating zone is lower than 490 ° C., the alloying treatment becomes insufficient, and if the temperature reaches 510 ° C. or higher, the alloying treatment becomes excessive and powdering easily occurs, which is not preferable.

[0023]

FIG. 2 is a view showing an alloying furnace used in an embodiment of the present invention. 1 is a heating zone, 2 is a holding zone, 3 is a cooling zone, 4 is a partition plate, 5 is a sink roll, 6 is a top roll, 7 is a wiping nozzle, and S is a galvanized steel sheet. The heating zone 1 is divided into an initial heating zone A, an intermediate heating zone B, and a final heating zone C. This is a post induction heating type alloying furnace with a gas heating system in the initial heating zone and an induction heating system in the final heating zone.

A cold-rolled steel sheet having a thickness of 0.8 mm is passed through a continuous galvanizing hot-dip galvanizing line shown in FIG. 2, and the alloying processing hot-dip galvanized steel sheet is changed by changing the alloying processing conditions as shown in Table 1. Made. The obtained alloyed hot-dip galvanized steel sheet was evaluated for powdering resistance and plating adhesion. Further, as shown in Table 1, the chemical composition of the cold rolled steel sheet is P
Only the content was changed in four steps within the range of 0.015 to 0.055%, and the other components were C = 0.0017%, Si = 0.01%, and Mn = 0.
12%, S = 0.005%, balance Fe and unavoidable impurities. The material of P = 0.015% is a normal cold-rolled steel sheet and used as a reference example.

[0025]

[Table 1]

The plating conditions were such that the Al concentration in the bath was 0.11% by weight.
The hot-dip galvanizing bath was set to a bath temperature of 460 ° C and the basis weight was 45.
It was adjusted to ~65g / m ^2. In addition, the Fe concentration in the plating film after the alloying treatment was analyzed (the plating film was dissolved with hydrochloric acid containing an inhibitor, and the iron content in the solution was analyzed).
Those having an Fe concentration in the range of 8 to 12% were evaluated as having good alloying properties, and are indicated by a circle in Table 1.

(1) Powdering property A disc-shaped test piece having a diameter of 90 mm is cut out from a galvannealed steel sheet, and the disc-shaped test piece is subjected to cylindrical drawing. Adhesive tape is applied to the obtained cup side and then peeled off, the weight before and after peeling is measured, and the difference is taken as the amount of powdering
A powdering resistance of 10 mg / piece or less was determined to be good.

(2) Coating adhesion 100 mm long and 25 mm wide from galvanized steel sheet
Was cut out, and a 12.5 mm × 25 mm epoxy-based one-part adhesive was applied to the ends of the two test pieces as a glue allowance to produce a bonded sample. The thickness of the adhesive is 150 μm. The obtained sample was pulled in the shear direction, and the breaking strength was determined. Adhesion of 15 N / mm ² or more was regarded as good.

The results are shown in Table 1.

Nos. 1 to 5 of the present invention have a powdering amount of 9 mg /
The plating adhesion is 15.2 N / mm ² or more.

However, Comparative Examples Nos. 6 and 7 are poor in alloying processability because the heating rate in the initial heating zone is as low as 3 ° C./s and 4 ° C./s.

Nos. 8 and 9 have a heating rate of 15 ° C. in the initial heating zone.
As a result, the powdering amount is 18 mg / piece and 25 mg / piece, and the powdering resistance is poor.

No. 10 has a heating rate of 15 ° C./s in the final heating zone.
Therefore, the plating adhesion is 12.7 N / mm ² , which is inferior.

No. 11, the temperature reached in the initial heating zone was 458 ° C.
Since the heating rate of the final heating zone is as low as 15 ° C./s, the alloying processability is poor and the plating adhesion is 10.8 N / mm ² , which is poor.

In Nos. 12 and 13, the temperature reached in the final heating zone was 52.
Since the temperature is as high as 0 ° C., the powdering amount is 30 mg / piece and 31 mg / piece, and the powdering resistance is poor.

No. 14 shows that the temperature reached in the final heating zone is 470 ° C.
, The alloying processability is inferior.

No. 15 shows that the temperature reached in the final heating zone is 460 ° C.
And the temperature of the holding zone is as low as 460 ° C., so that the plating adhesion is inferior to 9.9 N / mm ² and the alloying property is inferior.

No. 16 is inferior in alloying property because the temperature of the holding zone is as low as 460 ° C.

In No. 17, since the temperature of the holding zone was as high as 520 ° C., the powdering amount was 31 mg / piece, and the powdering resistance was poor.

For No. 18, the temperature reached in the final heating zone was 520 ° C.
Since the temperature of the holding zone is as high as 520 ° C., the powdering amount is 40 mg / piece, and the powdering resistance is poor.

No. 19 has a low heating rate in the initial heating zone.
o.20 has a high heating rate in the initial heating zone, No.21 has a low reaching temperature in the final heating zone and a low holding zone temperature, and No.2
2 has a higher ultimate temperature in the final heating zone, but each has a P content of less than 0.025% by weight.
Good plating adhesion and alloying treatment.

[0042]

According to the alloying treatment method of the present invention, the heating zone is divided into a plurality of heating zones, the heating speed of the initial heating zone is made lower than the heating speed of the final heating zone, and the temperature of the holding zone is set on the exit side of the final heating zone. Since the temperature is lower than the temperature of the steel sheet, an alloyed hot-dip galvanized steel sheet excellent in adhesion and powdering resistance can be easily obtained even when a P-containing ultra-low carbon steel is used.

[Brief description of the drawings]

FIG. 1 is a view showing a heat pattern of an alloying treatment of the present invention.

FIG. 2 is a diagram showing an alloying treatment furnace used in an example of the present invention.

[Explanation of symbols]

1. Heating zone 2. Retention band 3.
Cooling zone 4. Partition plate 5. Sink roll 6.
Top roll 7. Wiping nozzle S. Hot-dip galvanized steel sheet G. Alloying furnace

Claims (1)

(57) [Claims] 1. A P content of not less than 0.025% by weight and not more than 0.1% by weight.
A method for alloying a galvanized steel sheet of ultra-low carbon steel containing (phosphorus), wherein the heating zone of the alloying furnace is divided into two or more sections, and at least a final heating zone uses an induction heating method. After heating the steel sheet in the initial heating zone to 460 to 470 ° C with a heating rate of 5 to 10 ° C / s, heating the steel sheet in the final heating zone to 20 ° C / s or more to 490 to 510 ° C, A method for producing a galvannealed steel sheet having excellent powdering resistance and plating adhesion, wherein the inside temperature is 470 to 510 ° C.