JP3403869B2 - Method for producing hot-rolled hot-dip galvanized steel sheet without blister - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing a blister-free hot-dip galvanized steel sheet by continuous hot dipping.

[0002]

2. Description of the Related Art Hot-dip Zn-plated hot-rolled steel sheet is pickled and descaled from a hot-rolled steel sheet as a plating base plate, then reduced and heated in a hydrogen-containing atmosphere according to the Cirrus method, Zenzimer method, etc., and immersed in a molten Zn bath. Is manufactured by However, since the hot-rolled steel sheet is used as the plating base sheet, blisters called blisters are generated in the plating layer when several minutes to several days have passed after hot dipping. The blister is a phenomenon in which hydrogen dissolved in steel at the time of reduction heating diffuses at the interface between the hot-dip coating layer and the steel substrate after plating, and becomes hydrogen molecules at the interface to push up the plating layer by gas pressure. The blister not only impairs the surface appearance of the plated steel sheet, but also deteriorates the corrosion resistance and damages the coating film when it is formed into a hot-dip Zn-coated steel sheet, resulting in a marked deterioration in quality. In this respect, unlike hot-dip galvanizing of cold-rolled steel sheets in which blisters do not occur, a process peculiar to hot-rolled steel sheets used as a plating base sheet is required.

Various methods have heretofore been proposed to suppress blisters. For example, JP-A-54-13
In the 0443 publication, after the steel strip is reduced and heated, 8-20
Steel strip temperature 45 in a hydrogen-nitrogen atmosphere containing volume% hydrogen
It is maintained at 0 to 550 ° C. In this method, hydrogen stored in the steel strip diffuses into the atmosphere to the extent that it does not adversely affect plating. Also, JP-A-56-16325
In Japanese Patent No. 0, the occurrence of blisters is suppressed by the combined addition of N and B. The mechanism by which the combined addition of B and N acts to suppress blister is considered to be due to the fact that the BN generated in the steel by the combined addition traps hydrogen and transforms the hydrogen into a state in which it is difficult to diffuse at room temperature.

[0004]

However, in the method of Japanese Patent Laid-Open No. 54-130443, the steel strip is exposed to an atmosphere containing 8 to 20% by volume of hydrogen until immediately before being immersed in the plating bath. Therefore, it is inevitable that a hydrogen-containing atmosphere of at least 8 to 20 volume% and a concentration of hydrogen that equilibrates at that temperature remain in the base steel. As a result, this method can reduce blistering, but cannot completely prevent blistering. On the other hand, in the method of Japanese Patent Application Laid-Open No. 56-163250, which suppresses blisters by the combined addition of B and N, hydrogen trapped in BN is present in the steel in a thermally unstable state. Therefore, when the plated steel sheet is heated to a temperature higher than room temperature, hydrogen is released from BN, and blisters are likely to occur. In addition, the steel containing B and N tends to have flaws on the surface of the steel sheet during hot rolling, which requires maintenance such as flaw removal, resulting in high cost. The present invention has been devised to solve such a problem, and is heated by providing an inert gas zone whose atmospheric conditions are adjusted in a part between the reduction zone and the hot dip bath. The object of the present invention is to stably produce hot-dip Zn-plated hot-rolled steel sheet without blisters.

[0005]

The method for producing hot-dip galvanized hot-rolled steel sheet according to the present invention, in order to achieve the object, is a hydrogen-containing
When introducing a hot-rolled steel strip into the hot dip coating bath from the reduction zone of the nitrogen atmosphere through the cooling zone, some or all of the cooling zone is an inert gas zone, and the hydrogen concentration in the reduction zone is H (volume%), reduction When the steel strip temperature of the strip is T (° C.) and the stay time of the steel strip in the inert gas strip is t (seconds), the reduction zone and the inert gas are satisfied under the conditions that satisfy the formulas (1) to (3). Characterized by maintaining the obi. H ² ≦ (400−T / 3) × t (1) 450 ≦ T ≦ 700 (2) 5 ≦ H ≦ 100 (3) Width in the inert gas zone It is preferable to cool the surface of the steel strip by spraying the inert gas with a nozzle capable of spraying the inert gas with a uniform flow rate distribution in the direction of travel and the direction of travel. Examples of the inert gas include nitrogen, helium, argon and the like. Considering the cost, it is preferable to use nitrogen as an inert gas for steel types that do not have a harmful effect due to nitrogen occlusion.

[0006]

In the hot dip coating line in which the steel strip activated by the reduction heating is introduced into the hot dip coating bath, the reduction heating furnace is arranged upstream of the hot dip coating bath. The steel strip is heated in a reduction heating furnace whose inside is maintained in a hydrogen-nitrogen atmosphere, whereby the oxide film on the surface of the steel strip is decomposed. In this hot dip coating line, the inventors of the present invention, when the steel strip is reduced and heated in a hydrogen-nitrogen atmosphere, held and cooled in an inert gas atmosphere, and then introduced into the hot dip bath as it is, the occurrence of blisters occurs. It was found to be suppressed. The suppression of the blister is due to the phenomenon that hydrogen that has penetrated into the steel during reduction heating is released outside the steel during cooling using an inert gas. As a result of various investigations and studies conducted by the present inventors, the intrusion of hydrogen into the steel and the release of hydrogen to the outside of the steel are caused by the hydrogen concentration during reduction, the steel strip temperature during reduction, and the inert gas atmosphere. It was found that the cooling time (in other words, the staying time of the steel material in the inert gas zone) was greatly influenced.

Based on these findings, the present invention provides a high-quality hot-dip Zn-plated hot-rolled steel sheet with suppressed blisters, in order to obtain hydrogen concentration in the reduction zone, steel strip temperature and steel strip in an inert gas zone. The length of stay is optimized. The inert gas zone in the present invention is provided in a part or all of the cooling zone between the outlet side of the reduction heating furnace and the inlet side of the hot dip coating bath. This inert gas zone may exhibit a weak reducing property due to hydrogen mixed from the reduction zone,
When the hydrogen concentration is regulated to 5% by volume or less, generation of blisters due to hydrogen is suppressed. The present inventors
As a result of various experiments, appropriate conditions of parameters such as hydrogen concentration H (volume%) in the reduction zone, steel strip temperature T (° C.), and steel strip residence time t (seconds) in an inert gas zone that can prevent the occurrence of blisters. It was clarified that satisfying the conditions of formulas (1) to (3) is effective in preventing the occurrence of blisters.

Equation (1) is an approximate mathematical expression of the boundary condition for the presence / absence of blister occurrence, obtained from experimental results. Formula (2) shows that the steel strip temperature T (° C.) of the reduction zone is 450 to
It is a condition specified at 700 ° C. Steel strip temperature T is 450 ° C
If it is less than the above range, the wettability between the plating bath and the steel strip is poor and non-plating is likely to occur. On the contrary, reduction heating exceeding 700 ° C
It is unnecessary for hot-rolled steel sheet and consumes heat energy unnecessarily. Formula (3) is a condition for defining the hydrogen concentration H in the reduction zone to 5% by volume or more in order to obtain a sufficient reduction reaction. 5
When the hydrogen concentration H is less than volume%, the reducing ability of the atmosphere is insufficient, and non-plating due to poor reduction easily occurs.

Residence time t of the steel strip in the inert gas zone
Is determined by the length of the cooling zone and the strip running speed. In any of the hot-dip galvanizing lines such as Cirrus type, Sendzima type, NOF type, and US steel type, it is necessary to slow the strip running speed in order to obtain the residence time t of 60 seconds or more in the cooling zone. And is economically disadvantageous. When the reducing zone and the inert gas zone are operated under the conditions satisfying the above equations (1) to (3), the hot-dip galvanized steel sheet without blisters is stably produced. FIG. 1 shows an appropriate range of the hydrogen concentration H in the reduction zone and the residence time t in the inert gas zone when the steel strip temperature T (° C.) in the reduction zone is 600 ° C., for example. A region S surrounded by the curve a defined by the equation (1) and the straight lines b and c defined by the equation (3) is an appropriate condition in this case.

Further, in order to eliminate the influence of stored hydrogen or adsorbed hydrogen which causes blisters, blowing an inert gas onto the surface of the steel sheet is also an effective means. For spraying the inert gas, a nozzle capable of spraying the inert gas at a uniform flow rate in the width direction and the traveling direction of the steel strip is used. As such a nozzle, specifically, a nozzle assembly in which a plurality of circular nozzles are arranged in the plate width and the advancing direction of the plate, or a set of slit nozzles in which a plurality of slit nozzles spread in the plate width direction are arranged in the advancing direction The body etc. are used.
The inert gas blown directly onto the surface of the steel sheet reduces the adsorbed hydrogen on the surface of the steel sheet and promotes the release of hydrogen in the steel to the outside of the steel. Further, cooling the steel strip by spraying the cooling gas also promotes the release of hydrogen. Thus, according to the present invention, the hot-dip Zn-plated hot-rolled steel sheet without blisters is continuously and stably manufactured. Also, since hydrogen is actively removed from the steel,
Unlike the method disclosed in JP-A-56-163250, there is no risk of blister formation even when the hot-dip Zn-plated hot-rolled steel sheet is used in a temperature atmosphere of room temperature or higher. Specifically, about 2
Even when the coating film is baked and heated at 00 ° C, a hot-dip Zn-plated hot-rolled steel sheet with no blister is obtained.

[0011]

EXAMPLES Example 1: Hot-rolled steel sheets of steel types A and B having the compositions shown in Table 1 were hot-dipped by a Cirrus type hot-dip Zn plating line.
The steel strip temperature in the reduction zone was set to a constant value of 540 ° C. for steel type A having a plate thickness of 3.0 mm and 690 ° C. for steel type B having a plate thickness of 1.5 mm. Nitrogen gas at a temperature of 50 ° C. was allowed to flow into the surface of the steel strip at a flow rate of 2 Nm ³ / min. The steel strip, which had been cooled to 480 ° C by the flow of an inert gas, was introduced into the molten Zn plating bath maintained at a temperature of 460 ° C. The plate passing speed was set to 0.6 m / sec. After hot-dip galvanizing, a part of the galvanized steel strip is sampled and placed in air at 250
After heating for 3 hours at 0 ° C and further for 4 weeks at room temperature, the presence or absence of blisters was visually observed. FIG. 2 shows the determination result of steel type A, and FIG. 3 shows the determination result of steel type B. In FIGS. 2 and 3, samples in which blisters were not observed are indicated by ◯, samples in which blisters occurred were indicated by x, and samples in which non-plating occurred regardless of the presence or absence of blisters were indicated by Δ. As is clear from FIGS. 2 and 3, when the hydrogen concentration H in the reduction zone and the residence time t in the inert gas zone are in the proper region S ₂ or S ₃ in both the steel types A and B, blister is generated at all. It was confirmed that a good hot-dip Zn-plated hot-rolled steel sheet without any plating and without unplating was obtained.

Example 2: Hot-rolled steel sheets of steel types A and C having the compositions shown in Table 1 were hot-dipped with a hot-dip galvanizing line of NOF type. The steel strip temperature in the reduction zone was set to a constant value of 600 ° C. for steel type A having a plate thickness of 3.0 mm and 480 ° C. for steel type C having a plate thickness of 6.0 mm. In the inert gas zone, nitrogen gas at a temperature of 70 ° C. was blown onto the surface of the steel strip at a flow rate of 5 Nm ³ / min. A steel strip cooled to 450 ° C by spraying an inert gas was melted Zn
It was introduced into the plating bath. The plate passing speed was set to 0.4 m / sec.

[0013]

[Table 1]

A part of the hot-dip steel strip is sampled,
The presence or absence of blisters was determined under the same conditions as in Example 1. The determination result of the steel type A is shown in FIG. 4, and the determination result of the steel type C is shown in FIG. As is clear from FIGS. 4 and 5,
For both steel types A and C, when the hydrogen concentration H in the reduction zone and the residence time t in the inert gas zone are in the proper region S ₄ or S ₅ , no blister occurs and good melting without unplating occurs. It was confirmed that a Zn-plated hot rolled steel sheet was obtained.

[0015]

As described above, in the present invention, by controlling the hydrogen concentration in the reduction zone, the temperature of the steel strip in the reduction zone and the residence time of the steel strip in the inert gas zone,
Hydrogen in steel is diffused into the atmosphere and a steel strip without hydrogen adsorption is sent to the hot dip bath. In hot-dip galvanized steel strips, hydrogen does not condense and gasify at the interface between the plating layer and the base steel, so even if it is heated to a high temperature in the painting process etc. Occurrence is suppressed. Thus, according to the present invention, a good hot-dip galvanized steel sheet without blisters is produced. Particularly in the case of the steel sheet used for coating, blisters are suppressed during baking and drying of the coating film, so that a coated steel sheet having excellent coating film adhesion and corrosion resistance under the coating film and having a good appearance can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing the effects of hydrogen concentration in the reduction zone and steel material residence time in the inert gas zone on the occurrence of blisters when the temperature of the reduction zone is 600 ° C.

FIG. 2 is a graph showing effects of hydrogen concentration in the reduction zone and steel material residence time in the inert gas zone on the occurrence of blisters when the temperature of the reduction zone is 540 ° C.

FIG. 3 is a graph showing the effects of hydrogen concentration in the reduction zone and steel material residence time in the inert gas zone on the occurrence of blisters when the temperature of the reduction zone is 690 ° C.

FIG. 4 is a graph showing the effects of hydrogen concentration in the reduction zone and steel material residence time in the inert gas zone on the occurrence of blisters when the temperature of the reduction zone is 600 ° C.

FIG. 5 is a graph showing the effects of hydrogen concentration in the reduction zone and the steel material residence time in the inert gas zone on the occurrence of blisters when the temperature of the reduction zone is 480 ° C.

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-6-81097 (JP, A) JP-A-4-276051 (JP, A) JP-A-62-54069 (JP, A) JP-A-58- 64367 (JP, A) JP-A-54-130443 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] 1. When introducing a hot-rolled steel strip into a hot-dip galvanizing bath from a hydrogen-nitrogen atmosphere reduction zone through a cooling zone, a part or all of the cooling zone is made an inert gas zone, and the hydrogen concentration in the reduction zone is adjusted. H
(Volume%), the temperature of the steel strip in the reduction zone is T (° C.), and the residence time of the steel strip in the inert gas zone is t (seconds), under the conditions that satisfy equations (1) to (3). A method for producing a blister-free hot-dip galvanized steel sheet, characterized by maintaining a reduction zone and an inert gas zone. H ² ≦ (400−T / 3) × t (1) 450 ≦ T ≦ 700 (2) 5 ≦ H ≦ 100 (3) 2. The inert gas zone according to claim 1, wherein
A method for producing a hot-dip Zn-plated hot-rolled steel sheet, which comprises spraying an inert gas onto the surface of a steel strip at a uniform flow rate distribution in a width direction and a traveling direction through a nozzle.