JPH11106883A - Hot dip galvanized steel sheet for punching - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip galvanized steel sheet for punching having formability free from problems in practical use and capable of suppressing the generation of burrs as much as possible without hardening the steel sheet and its surface. SOLUTION: On a soft steel sheet 1 having <=390 N/mm<2> tensile strength, an oxide scale layer 2 is formed, on the oxidzed scale layer 2, plating alloying layer 4 is formed via a reduced iron layer 3 or directly without interposing the reduced iron layer 3, and on the plating alloying layer 4, a hot-dip galvanizing layer 5 is formed. The thickness of the oxidized scale layer 2 is regulated to 1 to 10 μm, and the volume ratio of wustite in the oxidized scale is regulated to >=70%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a tensile strength of 390 N
The present invention relates to a hot-dip galvanized steel sheet having a thickness of / mm ² or less.

[0002]

^2. Description of the Related Art A mild steel sheet having a tensile strength of 390 N / mm ² or less, which is widely used in building materials, automobiles, home appliances, etc.
In order to meet the demand for longer life of products, hot dip galvanizing is applied to improve rust prevention ability. Since this hot-dip galvanized steel sheet is soft, it is customary to perform various forming and punching processes.
In the punching process, if the steel sheet is soft, a large amount of burr occurs on the end face after the punching, which causes a problem in the work process.

[0003] As means for improving the punching workability, optimization of the clearance and improvement of the shape of the punch and the die have been carried out in the processing technology, and the steel plate to be processed is, for example, a steel plate itself. And a method of hardening the surface of a steel sheet have been proposed.

[0004] As a method of hardening the steel sheet itself,
A method of adding a solid solution strengthening element such as P or Si has been proposed. As a method of hardening the surface of a steel sheet, a method of introducing plastic strain into a surface layer portion of an annealed steel sheet by rolling or the like (Japanese Unexamined Patent Application Publication No. Japanese Patent Application Laid-Open No. 1-255626), or a method using a nitriding treatment or a carburizing treatment (JP-A-1-255626)
JP, JP-A-2-133561, JP-A-3-1
No. 99343, JP-A-3-202442) are known.

[0005]

However, in the case of a hot-dip galvanized steel sheet to be subjected to punching, the method of hardening the steel sheet itself can improve the punching workability to some extent, but with it, the inherent required characteristics of the steel sheet However, there is a problem that mechanical properties such as ductility and toughness are remarkably deteriorated, and moldability is deteriorated. In addition, the method of hardening the steel sheet surface requires special equipment and steps for nitriding treatment and carburizing treatment, and is difficult in terms of equipment and productivity.

The present invention has been made in view of such a problem, and has a practically problem-free formability and suppresses the generation of burrs as much as possible without hardening the steel sheet and its surface. An object of the present invention is to provide a hot-dip galvanized steel sheet for punching.

[0007]

The hot-dip galvanized steel sheet for stamping according to the present invention has an oxide scale layer formed on a mild steel sheet of 390 N / mm ² or less, and a reduced iron layer formed on the oxide scale layer. A galvannealed layer is formed on the plated alloyed layer, or the galvanized layer is formed on the plated alloyed layer, and the oxide scale layer has a thickness of 1 to 10 μm, and wustite in the oxide scale has a volume ratio of 70%. % Or more. In the present invention, “thickness” means an average thickness.

FIG. 1 is a schematic sectional view showing the structure of a hot-dip galvanized steel sheet according to the present invention, in which an oxide scale layer 2 is formed on a mild steel sheet 1 having a tensile strength of 390 N / mm ² or less.
Is formed on the oxide scale layer 2 via a reduced iron layer 3 to form a plating alloyed layer 4 formed by a reaction between reduced iron and a plating metal, and a hot-dip galvanized layer 5 is further formed thereon. Are laminated.

The mild steel sheet 1 is made of ordinary low carbon or ultra low carbon Al-killed steel, or ultra low carbon Al to which carbonitride forming elements such as Ti, Nb and B are added individually or in combination.
A mild steel plate having a tensile strength of 390 N / mm ² or less, such as killed steel, is used. In order to ensure good workability, it is preferable that each of P and Si, which are solid solution strengthening elements, be less than 0.04%.

When a hot rolled steel sheet is used as the mild steel sheet 1, the oxide scale layer 2 may be an oxide scale layer formed on the steel sheet surface in the hot rolling step. Of course, mild steel plate 1
A cold-rolled steel sheet may be used as the material, and in this case, heating may be performed in an appropriate oxidizing atmosphere before plating to form an oxide scale layer. It is advantageous to use a hot-rolled steel sheet in terms of productivity and economy.

The reduced iron layer 3 is formed by reducing an oxide scale layer formed on a mild steel sheet 1. By plating, the reduced iron and the plating metal react with each other, and the plating alloying layer 4 is formed on the surface of the reduced iron layer 3.
Is formed. When the reduced iron layer before plating is very thin or when the reaction between the reduced iron and the plating metal is accelerated by the hot-dip plating conditions, the entire reduced iron layer may become a plating alloy layer. In this case, the oxide scale layer 2
Plating alloyed layer 4 without a reduced iron layer 3
Is directly formed, and such a structure is also included in the present invention.

According to the hot-dip galvanized steel sheet for stamping of the present invention, the stamping workability is improved for the following reasons. Microscopic observation of the punched part of the steel sheet,
After the contact between the punch and the steel sheet, a shear fracture surface is first formed across the front and back surfaces of the steel sheet with the lowering of the punch, and the rest is locally subjected to strong shear deformation, resulting in voids Cracks are formed, and the cracks unite from the front and back surfaces to complete the processing. Therefore, in order to reduce burrs after punching, it is effective to promote the formation of voids and the progress of cracks. In the hot-dip galvanized steel sheet of the present invention, since the hard oxide scale layer 2 is formed on the mild steel sheet 1, voids are easily formed, and cracks develop quickly. It is considered that almost no occurrence occurs.

When a conventional hot-dip galvanized steel sheet is manufactured by immersing a base plate in a hot-dip galvanizing bath,
Since the oxide scale layer present on the surface of the steel sheet hinders the adhesion of the plating, it has been removed by pickling or the like before the plating treatment. For this reason, in the conventional hot-dip galvanized steel sheet, the effect of promoting the formation of voids by the oxide scale layer cannot be expected, the voids in the punched portion are small, and the progress of cracks is slow, so the punched portion is pulled in at the end of the processing. The burr was getting bigger.

In the present invention, since the oxide scale layer 2 is present, if processing such as bending is performed, peeling or cracking may occur in the hot-dip galvanized layer 5 depending on the thickness and properties of the oxide scale layer 2. Then, plating adhesion becomes a problem.

First, the relationship between the thickness of the oxide scale layer 2 and the plating adhesion will be described. The hardness of the oxide scale is about 3 to 5 times higher than that of the mild steel sheet 1 and its thickness is 1
If the thickness exceeds 0 μm, cracks easily occur in the oxide scale layer itself, and the adhesion of plating deteriorates. However, if the thickness of the oxide scale layer 2 is less than 1 μm, the formation of voids and the development of cracks due to the oxide scale layer cannot be expected, and there is no burr suppression effect. Therefore, in the present invention, the thickness of the oxide scale layer 2 is set to 1 to 10 μm.

Next, the relationship between the properties of the oxide scale layer and the plating adhesion will be described. Generally, the oxide scale layer on the surface of the hot-rolled steel sheet, wustite (FeO), magnetite (Fe ₃ O _4), it is known that hematite (Fe ₂ O ₃₎ is present. It is said that magnetite is very dense and has higher breaking strength than wustite and hematite. Therefore, it is generally considered that the higher the volume ratio (constituent ratio) of magnetite, the smaller the cracks in the oxide scale itself, and the better the plating adhesion.

However, as a result of intensive studies by the present inventors, in order to obtain good plating adhesion, it is preferable to increase the volume ratio of wustite in the oxide scale as much as possible. While maintaining good properties,
In order to ensure a certain degree of workability, the volume ratio (composition ratio) of wustite in the oxide scale adhered to the hot-dip coated steel sheet should be 70% or more, preferably 80% or more, more preferably 90% or more. It has been found that it is more desirable that the oxide scale is composed of wustite alone.

The reason for this is not clear at present, but is considered as follows. That is, when the volume ratio of magnetite in the oxide scale is high, the cracks in the oxide scale itself are small, but the plating layer is peeled off from the interface between the oxide scale and the ground iron (steel body). On the other hand, as the volume ratio of wustite increases, the consistency of the crystal structure at the atomic level with ferrous iron becomes better than that of magnetite or hematite. Peeling of the plating layer is reduced, and the plating adhesion is improved.

Wustite (FeO) in the above oxide scale
Is formed as follows. Fe generated in the hot rolling process
O is gradually cooled by cooling after winding around 400 ° C. where the transformation rate of 4FeO → Fe + Fe ₃ O ₄ is maximum, so that all of the FeO is transformed and no FeO exists in the hot-rolled steel sheet at room temperature. On the other hand, when the hot-rolled steel sheet is passed through a hot-dip galvanizing line and heated in a reduction treatment step before plating, an eutectoid reaction of Fe + Fe ₃ O ₄ → 4FeO proceeds at a temperature of 560 ° C. or more and FeO
Is generated. After reduction, the steel sheet is cooled to a plating temperature of about 460 ° C and plated, and then cooled to room temperature. However, since the cooling rate at this time is high, most of the generated FeO exists at room temperature. Become. Regarding Fe ₂ O ₃ in the oxide scale generated in the hot rolling process, it is reduced to FeO during the reduction treatment in the hot-dip plating line, and then the cooling rate is high.

FIG. 3 is a graph showing the relationship between the volume ratio of wustite in the oxide scale layer and the plating adhesion in the hot-dip galvanized steel sheet, wherein the volume ratio of wustite is 70%.
From the above, it can be seen that good plating adhesion is obtained at preferably 80% or more, more preferably 90% or more.

The hot-dip galvanized steel sheet used in the investigation of FIG. 3 is a hot-rolled low-carbon steel (C: 0.05 wt%), and a hot-rolled steel sheet having a 5 to 8 μm-thick oxide scale adhered thereto is reduced. After heating to 500 to 800 ° C. in a gas atmosphere to form a reduced iron layer of about 0.1 to 1 μm on the surface of the oxide scale layer, an average cooling rate of 3 ° C./sec to plating temperature of 460 ° C.
The above was cooled and subjected to hot-dip galvanizing. At this time, the thickness of the oxide scale layer was adjusted by adjusting the hot rolling end temperature, the cooling rate of the steel sheet from the end of hot rolling to winding, and the winding temperature. Further, the volume ratio of FeO 2 in the oxide scale was adjusted by adjusting the reduction temperature and the reduction time during the reduction in the hot-dip plating line.

The evaluation of the plating adhesion in FIG.
Mechanical properties of SG-3302 hot-dip galvanized steel sheet and steel strip: Performed based on bendability, ◎: excellent (no plating peeling), ○: good (slight peeling), ×: poor (large peeling) ). The volume fraction of FeO in the oxide scale was calculated by the following equation based on the diffraction intensity of each iron oxide obtained by X-ray diffraction.

[0023]

[Equation 1] Volume% of FeO = I (FeO) × 100 / ｛I (FeO) + I
(Fe ₃ O ₄ ) + I (Fe ₂ O ₃ )｝ where, I (FeO): Diffraction intensity of (200) plane I (Fe ₃ O ₄ ): Diffraction intensity of (104) plane I (Fe ₂ O ₃₎ ): Diffraction intensity of (220) plane

Regarding the adhesion of the hot-dip galvanized layer 5 to the oxide scale layer 2, the hot-dip galvanized layer 5 is placed on the oxide scale layer 2 via the reduced iron layer 3 or directly without the reduced iron layer 3. Since it is bonded to the plating alloying layer 4, the plating adhesion is good, and the uniformity of the coating amount is also improved. The reduced iron layer 3 is formed by heating a steel sheet having an oxide scale layer in a reducing gas atmosphere so that the surface portion of the oxide scale layer is reduced. Is formed by the reaction between the reduced iron having good wettability and the plating metal.

The reduced iron layer 3 and the plated alloy layer 4 after plating preferably have a total thickness (approximately equivalent to the thickness of the reduced iron layer before plating) of 0.1 to 5 μm.
If the thickness is less than 0.1 μm, the thickness of the reduced iron layer and thus the thickness of the plated alloy layer is likely to be non-uniform, and the plating adhesion is reduced.
On the other hand, it is not practical to form a reduced iron layer having a thickness of more than 5 μm because the reduction treatment requires time and cost and the effect of improving plating adhesion is saturated. Preferably 0.2
.About.5 .mu.m, more preferably 0.2-2 .mu.m.

FIG. 4 shows that the thickness of the oxide scale layer is 5 μm,
FIG. ₄ is a graph showing the relationship between the total thickness of the reduced iron layer and the plating alloyed layer and the plating adhesion in a hot-dip galvanized steel sheet having a volume fraction of FeO in the oxide scale of 90% and a balance of Fe ₃ O _4. When the thickness is 0.1 μm or more, good adhesion is obtained, and when it is 0.2 μm or more, excellent adhesion is stably obtained. The plating alloy layer was extremely thin, and its thickness was negligible. The plating adhesion was evaluated in the same manner as described above.

To further improve the plating adhesion, as shown in FIG. 2, countless cracks 6 penetrating through the oxide scale layer 2, the reduced iron layer 3, and the plating alloy layer 4 and reaching the mild steel sheet 1 are formed. It is preferable to form and fill the cracks 6 with a plating metal during hot-dip plating. According to this configuration, the adhesion of the hot-dip galvanized layer 5 to the mild steel sheet 1 is further improved by the anchor effect of the plated metal penetrating and filling the crack 6.

In order to form the cracks 6, the hot-rolled steel sheet on which the oxide scale layer has been formed may be passed through a skin pass rolling mill or a leveler before plating. The size and amount of cracks can be adjusted by adjusting the outer shape of.

In the industrial production method of the above-mentioned hot-dip galvanized steel sheet, when a hot-rolled steel sheet having an oxide scale layer (tensile strength of 390 N / mm ² or less) is used as a base sheet for plating, the heating temperature of the slab is usually increased. Is set to about 1250 to 1050 ° C., and finish rolling is completed at a temperature equal to or higher than the Ar ₃ transformation point. The thickness of the oxide scale layer is such that the hot-rolling finishing temperature is low, the cooling rate after finishing rolling is high, for example, 50 ° C./sec or more, and the winding temperature is 700 ° C. or less, preferably 500 ° C. or less. Can be made thinner.

The heating and reduction of the oxide scale layer of the hot-rolled steel sheet is performed by continuously passing the plating base sheet under a reducing gas atmosphere in the continuous hot-dip galvanizing line without passing the plating base sheet through the pickling step. be able to. By increasing the reduction temperature, extending the reduction time or increasing the hydrogen concentration in the reducing gas atmosphere, it is possible to increase the volume fraction of FeO and promote the reduction of the oxide scale surface.

The reduction temperature is 560-900 ° C., preferably 650-800 ° C. Fe below 560 ° C
The volume ratio of O 2 does not increase, and a sufficient reduced iron layer cannot be formed on the surface of the oxide scale layer. Although the reduction time is not particularly limited, the time that can be realized in a normal continuous hot-dip galvanizing line is about 10 to 80 seconds. Thereby, 560-900 ° C, preferably 650
In the reduction temperature range of up to 800 ° C., a reduced iron layer having a volume ratio of FeO of 70% or more and a surface area of the oxide scale layer of 0.1 μm or more can be formed. As described above, the total thickness of the reduced iron layer after plating and the plating alloy layer is substantially equal to the thickness of the reduced iron layer before plating.

After the reduction, hot-dip galvanizing is performed. If the coating is kept at a low temperature (400 to 560 ° C.) for a long time before the plating, a part of FeO generated during the reduction is transformed into Fe ₃ O _4. The average cooling rate is preferably 2 ° C./sec or more, and more preferably 5 ° C./sec or more, for facilitating the cooling.

As described above, as a plating base plate,
Using a hot-rolled steel sheet from which the oxide scale layer has been removed by pickling or a cold-rolled steel sheet subjected to cold rolling, the steel sheet surface is oxidized by heating in air or an oxidizing atmosphere (including heating during annealing). One having a scale layer formed thereon can also be used.

As the hot-dip galvanizing, the plating bath composition is pure Zn, or a very small amount of Al (0.08 to 0.3
0 wt%), more Al
(E.g., Zn-5% Al, Z
n-5% Al-0.1% Mg, Zn-55% Al-1.
6% Si, etc.), and contains one or more auxiliary components such as Si, Pb, Fe, Ti, Cr, Ni, and rare earth elements. You may. The plating adhesion amount is not particularly limited as long as it is within the adhesion amount range obtained under normal operating conditions.

[0035]

EXAMPLE After mild steel having the components shown in Table 1 was hot-rolled and wound up, the hot-rolled coil was passed through a continuous hot-dip galvanizing line without passing through a pickling line, and was placed in a reducing gas atmosphere. After reducing the surface of the oxide scale layer by heating in the above, it was immersed in a plating bath to produce a hot-dip galvanized steel sheet. Further, as a conventional example, the hot-rolled coil is pickled,
A hot-dip galvanized steel sheet was manufactured under the same conditions as in the original sheet, from which the oxide scale layer had been removed, under the same conditions.

The thickness and composition of the oxidized sour layer were adjusted by controlling the finishing rolling temperature and the winding temperature in the hot rolling step. Further, the composition of the oxide scale layer was readjusted by controlling the reheating time in the hot-dip galvanizing step as needed. Further, the thickness of the reduced iron layer was adjusted by adjusting the hydrogen concentration in the reducing gas atmosphere and the reduction time.

For the obtained hot-dip galvanized steel sheet, the total thickness of the oxide scale layer, the reduced iron layer, and the plated alloy layer was determined by observing the cross section, and the composition of iron oxide in the oxide scale (by volume%) was determined by X-ray diffraction. ). Note that the plating alloyed layer was extremely thin, and its thickness was negligible.

Further, punching workability, non-plating property, and plating adhesion were examined. The punching workability was evaluated by measuring the burr height at the edge of the hole punched into 10 mmφ. The non-plating property was visually determined based on the presence or absence of a non-plated portion. The plating adhesion was evaluated in the same manner as described above. Table 2 shows the results of these investigations.

[0039]

[Table 1]

[0040]

[Table 2]

As is clear from Table 2, in the present invention,
The burr height at the time of punching is 16 μm at the maximum, there is no non-plating, and the plating adhesion is good.
On the other hand, in the comparative example (sample No. 4) in which the thickness of the oxide scale layer is out of the range of the present invention and the wustite is less than 70%, the punching property is relatively good, but the plating adhesion is poor. In addition, a conventional example having no oxide scale layer (sample N
o. 12 to 15), in the comparative example (sample No. 16) in which the thickness of the oxide scale layer was less than the range of the present invention, although the plating adhesion was good, the burr height exceeded 25 μm, It turns out that it is inferior in sex.

[0042]

As described above, the hot-dip galvanized steel sheet for punching according to the present invention has a predetermined thickness on a mild steel sheet.
Since the oxide scale layer of the composition is formed, not only the formability but also the punching workability and the plating adhesion are excellent despite being a mild steel plate. Further, the total thickness of the reduced iron layer and the plating alloy layer is 0.1 to 5 μm.
By this, the adhesion of the hot-dip galvanized layer can be stabilized and further improved. Also, it can be manufactured by continuous galvanizing line,
Excellent productivity.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing the structure of a hot-dip galvanized steel sheet for punching according to the present invention.

FIG. 2 is a schematic cross-sectional view showing a structure of a hot-dip galvanized steel sheet for punching according to the present invention in which a crack formed in an oxide scale layer is infiltrated and filled with a plating metal.

FIG. 3 is a graph showing the relationship between the volume fraction of FeO in an oxide scale and plating adhesion.

FIG. 4 is a graph showing the relationship between the total thickness of a reduced iron layer and a plating alloying layer and plating adhesion.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mild steel plate 2 Oxide scale layer 3 Reduced iron layer 4 Plating alloy layer 5 Hot-dip galvanized layer 6 Crack

Continued on the front page (72) Inventor Masatoshi Iwai 1 Kanazawa-cho, Kakogawa City, Hyogo Prefecture Inside Kobe Steel Works Kakogawa Works (72) Inventor Tetsuo Toyoda 1 Kanazawacho, Kakogawa City, Hyogo Prefecture Kobe Steel Ltd Inside Kakogawa Works (72) Inventor Tatsuya Asai 1 Kanazawacho, Kakogawa City, Hyogo Prefecture Inside Kobe Steel Works Kakogawa Works (72) Inventor Takafumi Iwai 1 Kanazawacho, Kakogawa City, Hyogo Prefecture Kakogawa Works Inside the steelworks

Claims (3)

[Claims] An oxide scale layer is formed on a mild steel plate having a tensile strength of 390 N / mm ² or less, and a plated alloyed layer is formed on the oxide scale layer via a reduced iron layer or directly. A hot-dip galvanized steel sheet for punching, wherein a hot-dip galvanized layer is formed on the plated alloyed layer, and the oxide scale layer has a thickness of 1 to 10 μm and a whistite in the oxide scale has a volume ratio of 70% or more. 2. The hot-dip galvanized steel sheet for punching according to claim 1, wherein the total thickness of the reduced iron layer and the plated alloy layer is 0.1 to 5 μm. 3. The hot-dip galvanized steel sheet for punching according to claim 1, wherein a crack formed so as to penetrate the oxide scale layer, the reduced iron layer, and the plating alloy layer is filled with a plating metal. .