JPH11246960A - Hot dip galvanized steel tube excellent in high strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the strength of a steel tube by applying hot dip galvanizing on the surface of a steel tube produced by subjecting a steel sheet contg. a specified comps. of C, Si, Mn and N to skinpass rolling at a specified draft. SOLUTION: The compsn. of a steel is composed of 0.01 to 0.25% C, <=0.5% Si, 0.10 to 1.50% Mn, 0.005 to 0.02% N, and the balance Fe with inevitable impurities. The steel sheet having this compsn. is subjected to skinpass rolling at a draft of 0.5 to 3%, and strains are applied to the steel sheet. In this way, dislocations to form a precipitated region of C and N of solid solution in a supersaturated state con be increased, and the age hardening therein is increased, whereby its strength can be higher. This steel sheet is made into a steel tube by electric resistance welded tube equipment or the like and is next treated with hot dip galvanizing. The plating is executed in such a manner that the steel tube is preheated to 320 to 600 deg.C and is thereafter dipped to pass in a plating bath heated at 430 to 480 deg.C for 0.5 to 8 sec.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel pipe having excellent high strength.

[0002]

2. Description of the Related Art As a hot-dip galvanized steel pipe, it is known that a steel pipe is immersed in a hot-dip galvanizing bath to be galvanized, then cooled and cut to a predetermined length.
No. 607.

[0003]

The hot dip galvanized steel pipe manufactured by the above method has been mainly applied to electric conduits. When applying such a hot-dip galvanized steel pipe to other uses, there are difficulties in tensile strength and the like,
There are problems such as difficulty in application to a wide range.
The present invention has been made to advantageously solve such a problem, and has as its object to provide a hot-dip galvanized steel pipe excellent in high strength.

[0004]

The features of the present invention are as follows: C: 0.01 to 0.25%, Si: 0.5% or less, Mn: 0.10 to 1.50%, N: 0.005-
High strength characterized in that the surface of a steel pipe produced by subjecting a steel sheet comprising 0.02%, the remaining Fe and inevitable impurities to temper rolling at a rolling reduction of 0.5 to 3%, and then subjecting the surface to galvanizing. Excellent galvanized steel pipe.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the composition of a steel pipe as described above will be described. If C is less than 0.01%, the amount of C dissolved in the ferrite phase in a supersaturated state decreases, and the intended increase in strength due to age hardening cannot be obtained. Also,
If it exceeds 0.25%, the amount of C dissolved in supersaturation hardly changes, and the increase in strength due to age hardening does not change. Therefore, the content is set in the range of 0.01 to 0.25%. If Si exceeds 0.5%, oxides having a high melting point are likely to be generated during electric resistance welding at the time of pipe making, and remain in the electric resistance welded portion to cause welding defects, which is not preferable. Therefore, the lower limit is preferably 0.5% or less, and the lower limit is preferably as small as possible. Mn is
If it is less than 0.10%, it becomes difficult to secure the material strength. On the other hand, when the content exceeds 1.5%, oxides having a high melting point are liable to be generated during electric resistance welding at the time of pipe making and remain in the electric resistance welded portion to cause welding defects, which is not preferable. For these reasons, the range is 0.10 to 1.5%. N
If the content is less than 0.005%, the amount of N dissolved in a supersaturated state in the ferrite phase will be small, and the desired increase in strength due to age hardening cannot be obtained. Further, even if N is added in excess of 0.02%, the amount of C dissolved in supersaturation hardly changes, and the increase in strength due to age hardening does not change. For these reasons, the content is set in the range of 0.005 to 0.02%.

[0006] As described above, by subjecting a steel sheet having a composition to temper rolling and imparting a strain to the steel sheet, dislocations serving as precipitation regions of supersaturated solid solution of C and N can be increased, and the aging effect can be obtained. To increase the strength. When the rolling reduction is less than 0.5%, dislocations which become precipitation regions of supersaturated solid solution C and the like are reduced, and it is difficult to increase strength by age hardening. The effect of aging hardly changes even if a reduction of more than 3% is applied. Therefore, the rolling reduction of the temper rolling is set to 0.5 to 3%. A steel sheet (belt) having such a composition is processed into a steel pipe by, for example, an electric resistance welded steel pipe facility, and then subjected to hot-dip galvanizing as described below to improve corrosion resistance and the like, and to reduce the hot-dip galvanized coating amount. It is intended to accurately control the thickness from the thickness to the thickness according to the application.

That is, as a method of applying hot-dip galvanizing, for example, the above-mentioned steel pipe conveyed from a continuous steel pipe manufacturing line is continuously immersed and passed into a hot-dip galvanizing bath without cutting, so that hot-dip galvanizing is applied to the outer surface of the steel pipe. Apply plating. As described above, the steel pipe is preheated to 320 to 600 ° C. (electric induction heating or the like) prior to hot-dip galvanizing. However, if the preheating temperature is lower than 320 ° C., the surface of the steel pipe in the galvanizing bath is not heated. The temperature does not reach the temperature required for forming the galvanized layer, which is not preferable. On the other hand, when the temperature exceeds 600 ° C., the limit as the facility capacity is exceeded, which is disadvantageous.

In this way, the steel pipe is preheated and immersed and passed through a hot-dip galvanizing bath. The hot-dip galvanizing bath temperature is considered in consideration of fluidity and the like since the melting point of zinc is about 420 ° C. A lower limit of 430 ° C. is sufficient. On the other hand, if the upper limit exceeds 480 ° C., the fluidity of zinc increases, making it difficult to stably control the thickness to be thick, and the range of application of the hot-dip galvanized steel pipe as a product is undesirably narrow.

If the immersion passage time of the steel pipe in the hot dip galvanizing bath is less than 0.5 seconds, the diffusion of zinc into the steel is not sufficient, so that it is difficult to form a plated layer. In addition, a dipping passage time of up to 8 seconds is sufficient to obtain a plating adhesion amount of up to 400 g / m ² . Therefore, as the immersion passage time of the steel pipe in the hot dip galvanizing bath,
0.5 to 8.0 seconds. Such adjustment of the immersion passage time of the steel pipe can be adjusted by the length of the hot dip galvanizing bath and the pipe passing speed.

After the hot-dip galvanizing as described above,
Gas is injected from the gas injection nozzle on the outlet side of the hot-dip galvanizing bath to control the amount of zinc deposition to a predetermined amount.The gas injection nozzle is a ring-shaped nozzle because it injects gas all around the steel pipe. When the gap between the nozzle and the steel pipe is less than 3.0 mm, the steel pipe may come into contact with the nozzle due to vibration. Further, if the gap exceeds 16 mm, zinc wrinkles are generated on the surface of the steel pipe, and the appearance is unfavorably deteriorated. Further, by changing the gap between the upper side and the lower side of the steel pipe, it is also possible to uniformly control the amount of plating applied over the entire surface of the steel pipe. Next, when the gas injection pressure from the gas injection nozzle is less than 0.3 kg / cm ² , zinc wrinkles are generated on the surface of the steel pipe, and the appearance is undesirably deteriorated. Also, 5.0
Even if it exceeds Kg / cm ² , the effect of removing zinc hardly changes, and therefore the amount of adhesion hardly changes, and the gas injection pressure is set to 0.3 to 5.0 Kg / cm ² . The slit width of the gas injection nozzle is 0.2 to 1.0 m.
m is sufficient. Thus, according to the preheating temperature of the steel pipe, the temperature of the hot-dip galvanizing bath, the immersion time of the steel pipe in the bath, the gap between the gas injection nozzle and the steel pipe, and the thin adhesion of the galvanized coating of 20 to 400 g / m ² depending on the gas injection pressure. From the amount to the thick attachment amount, and the pipe speed is 20 to 120 m /
It is possible to perform plating at a high speed while maintaining high productivity. After the hot-dip galvanizing is performed in this manner, the product is cooled by immersion in, for example, cooling water, and then cut into a predetermined length to obtain a product.

Further, if necessary, the steel pipe which has been hot-dip galvanized as described above is continuously heated and diffused into the hot-dip galvanized layer by thermal diffusion of the steel pipe (iron) to perform an alloying treatment. It can be a plating layer. Thus, the amount of iron in the alloy plating layer is preferably a zinc-iron alloy plating layer of 7 to 13%. That is, if the iron content is less than 7%, the plating layer is easily peeled, and the coating adhesion is poor, which is not preferable. On the other hand, if it exceeds 13%, the plating layer (alloy layer) becomes brittle, and the workability may be reduced, which is not preferable.

As the heating temperature for treating such an alloy plating layer, iron can be thermally diffused at 450 to 500 ° C., whereby the alloy plating layer comprising the δ ₁ phase can be treated. -Cracking and peeling of the iron alloy plating layer can be prevented. If the heating temperature is less than 450 ° C., it takes a long time to alloy the iron content in the alloy plating layer to 7% or more, and the pipe passing speed is reduced to sacrifice productivity, or However, the length of the heating device is undesirably increased. On the other hand, when the temperature exceeds 500 ° C., a ζ phase is generated in the alloy plating layer, and the workability is deteriorated, which is not preferable. The heating time is 450 to
The heating temperature is 500 ° C., and the alloying process can be accurately performed to reduce the iron content in the plating layer to 7 to 13% in 5 to 50 seconds. After processing to zinc-iron alloy plating in this way,
For example, the product is cooled by immersion in, for example, cooling water, and then cut into a predetermined length to obtain a product.

As described above, after cooling such as galvanizing or galvanizing-iron alloy plating, the interstitial elements such as C and N in the steel pipe (in the steel) in the steel are cooled during cooling to solidify the plating layer. It can form a supersaturated solid solution in the ferrite phase, and then age hardens over time to increase the strength of the galvanized steel pipe. That is, as described above, since strain is generated at the time of forming a steel pipe into a steel pipe, the interstitial elements such as C and N can be easily dissolved in the ferrite phase in the steel in a supersaturated manner. Thus, for example, prior to forming a steel pipe, the steel sheet is reduced at a reduction rate of 0.5 to
By subjecting the steel sheet to 3.0% temper rolling to generate strain in the steel sheet, it is possible to increase the number of dislocations in the precipitation region of supersaturated solid solution of C and N, thereby reducing the effect of aging. Can be raised to higher strength. In addition, the age hardening can be performed by aging the steel after cooling, if necessary, by heating the hot-dip galvanized steel pipe to 100 to 300 ° C. in addition to aging.

Such hot-dip galvanizing or hot-dip zinc-iron alloy plating can be effectively used for undercarriage parts for automobiles as well as electric conduits, and is extremely significant.

Next, an example of the method of the present invention will be described with reference to the drawings. Example 1 In FIG. 1, a steel pipe 1 conveyed from a continuous steel pipe production line (not shown) is preheated by a preheating device 2 and then continuously immersed and passed through a hot-dip galvanizing bath 4 of a hot-dip galvanizing bath 3. Hot-dip galvanized on the outer surface of. A hot-dip galvanizing bath generator 5 having a hot-dip galvanizing bath corresponding to the consumption of the hot-dip galvanizing bath 4 in the hot-dip galvanizing bath 3 is heated and melted by a hot-dip galvanizing bath generator 5, and supplied to a supply pipe 7 via a pump 6.
To the hot-dip galvanizing bath 3. In the figure, reference numeral 8 denotes a sealing material.

Thus, the plated steel pipe 1a is discharged from the hot-dip galvanizing bath 3 by the gas injection nozzle 9 at the exit side.
A gas is sprayed onto the outer surface of a to control the amount to be adhered, and then cooled by passing through cooling water 11 in a water tank 10 and then cut to a predetermined length by a cutting machine 12 to form a hot-dip galvanized steel pipe. Products (hot-dip galvanized steel pipe products).

When manufacturing a steel pipe not subjected to hot-dip galvanizing, a valve 13 at the bottom of the hot-dip galvanizing bath 3 is used.
Is opened and the hot-dip galvanizing bath 4 is extracted into the hot-dip galvanizing bath generator 5, and then the discharged steel pipe 1 from the continuous steel pipe production line is passed through the preheating device 2 without preheating, and then the hot-dip galvanizing bath 3 and the gas The cutting machine 1 passes through the gas injection nozzle 9 whose injection has been stopped and the water tank 10 from which the cooling water 11 has been extracted.
It is cut into a predetermined length in 2 to obtain a steel pipe product. By doing so, the hot dip galvanized steel pipe and the steel pipe can be separately manufactured, and the operation rate of the continuous steel pipe production line can be improved.

Example 2 In FIG. 2, after a steel pipe 1 conveyed from a continuous steel pipe production line (not shown) is preheated by a preheating device 2, it is continuously immersed and passed through a hot dip galvanizing bath 4 of a hot dip galvanizing bath 3. Then, hot dip galvanizing is applied to the outer surface of the steel pipe 1. A hot-dip galvanizing bath generator 5 having a hot-dip galvanizing bath corresponding to the consumption of the hot-dip galvanizing bath 4 in the hot-dip galvanizing bath 3 is heated and melted by a hot-dip galvanizing bath generator 5, and supplied to a supply pipe 7 via a pump 6.
To the hot-dip galvanizing bath 3. In the figure, reference numeral 8 denotes a sealing material.

Thus, the galvanized steel pipe 1a is separated from the hot-dip galvanizing bath 3 by the gas injection nozzle 9 at the outlet side.
A gas is sprayed onto the outer surface of a to control the amount to be adhered, and then heated by a heating device 10 (high-frequency heating or the like) to alloy the hot-dip galvanized layer. After passing and cooling, it is cut to a predetermined length by the cutting machine 13 to obtain a hot-dip galvanized steel pipe product (hot-dip galvanized steel pipe product).

When producing a steel pipe not subjected to hot-dip galvanizing, the valve 14 at the bottom of the hot-dip galvanizing bath 3 is opened and the hot-dip galvanizing bath 4 is extracted into the hot-dip galvanizing bath generator 5. Then, the dispensed steel pipe 1 from the continuous steel pipe production line is passed through the preheating device 2 without preheating, then the hot-dip galvanizing bath 3, the gas injection nozzle 9 for which gas injection was stopped, the heating device 10 for which heating was stopped, and the cooling water 12. Is cut into a predetermined length by a cutting machine 13 through a water tank 11 from which the steel pipe is extracted to obtain a steel pipe product. By hiding,
Hot-dip galvanized steel pipes and steel pipes can be separately manufactured, and the operation rate of a continuous steel pipe production line can be improved.

[0021]

Next, examples of the present invention will be described together with comparative examples.

[Table 1]

[0022]

[Table 2] (continuation of Table 1)

[0023]

[Table 3] (Continued from Table 2)

Note 1: The test was carried out in an in-line plating facility in which a plating apparatus was connected to the production line of the ERW steel pipe. Note 2: For the steel pipe, a hot rolled steel sheet was made as described above. Note 3: Temper rolling of steel sheet is the temper reduction rate for imparting distortion. Note 4: For hot-dip galvanization, zinc, a commonly used steel tube plating component, was used. Note 5: Attachment amount control was performed by injecting room-temperature air from the gas injection nozzle having a slit formed all around the inside of the ring-shaped nozzle all around the outer surface of the steel pipe after galvanizing. Note 6: The gap is the gap between the steel pipe and the gas injection nozzle. Note 7: The cooling temperature of the coated steel pipe is the temperature at which the coated steel pipe after the coating weight control or the heat-alloying treatment was immersed and passed through cooling water at room temperature. Note 8: Age hardening: Heating was performed by heat treatment immediately after the production of hot-dip galvanized steel pipe and hardened. The aging was hardened by the passage of the aging days after the production of the hot-dip galvanized steel pipe. Note 9: The strength is indicated by a tensile load at the time when the steel pipe after plating cut into a length of 300 mm was tensilely broken by a tensile tester.

[0025]

Industrial Applicability According to the galvanized steel pipe of the present invention, it is excellent in high strength and has a great industrial effect, such as being applicable to various uses.

[Brief description of the drawings]

FIG. 1 is a side flow diagram showing an example of the method of the present invention.

FIG. 2 is a side flow diagram showing an example of the method of the present invention.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Takehiko Sakagami 5-3 Tokai-cho, Tokai-shi, Aichi Pref.

Claims (1)

[Claims] 1. C: 0.01 to 0.25%, Si: 0.1%
5% or less, Mn: 0.10 to 1.50%, N: 0.00
A steel pipe made by subjecting a steel sheet composed of 5 to 0.02% and the balance Fe and inevitable impurities to temper rolling at a rolling reduction of 0.5 to 3% is subjected to hot-dip galvanizing. Hot-dip galvanized steel pipe with excellent strength.