WO2014091724A1

WO2014091724A1 - Hot-dip-galvanized steel sheet

Info

Publication number: WO2014091724A1
Application number: PCT/JP2013/007149
Authority: WO
Inventors: 善継鈴木; 麻衣宮田; 長滝　康伸
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2012-12-14
Filing date: 2013-12-05
Publication date: 2014-06-19
Also published as: KR20150093227A; CN104838035B; JP5907055B2; JP2014118584A; CN104838035A; TWI502082B; TW201430144A; KR101721483B1; MX2015007449A

Abstract

Provided is a hot-dip-galvanized steel sheet which exhibits excellent appearance, post-pressing plating adhesion, spot-welding properties, and post-pressing/post-coating corrosion resistance. This hot-dip-galvanized steel sheet has a yield stress (YS) of 260-350MPa, inclusive, and has: a steel sheet comprising a composition containing, in mass%, C in the amount of 0.05-0.1%, inclusive, Si in the amount of 0.10% or less, Mn in the amount of 0.30-0.70%, inclusive, P in the amount of 0.040% or less, S in the amount of 0.010% or less, N in the amount of 0.005% or less, and Al in the amount of 0.10% or less, with Fe and inevitable impurities constituting the remainder; a hot-dip galvanized layer containing Al in the amount of 0.3-0.6 mass%, inclusive, and formed on at least part of the surface of the steel sheet; and an intermetallic compound present between the steel sheet and the hot-dip galvanized layer, and containing Al in the amount of 0.12gm^-2 to 0.22gm^-2, inclusive, and Fe₂Al₅ having an average particle diameter of 1μm or less.

Description

Hot-dip galvanized steel sheet

The present invention relates to a hot dip galvanized steel sheet that can be suitably used for an outer plate and an inner plate of an automobile.

In recent years, surface-treated steel sheets in which rust resistance is imparted to material steel sheets, particularly hot-dip galvanized steel sheets that are excellent in rust resistance have been used in fields such as automobiles, home appliances, and building materials. In particular, European and American automakers are considering improving rust prevention performance by applying hot-dip galvanized steel sheets that can easily increase the plating thickness. In addition, there is a great demand for automotive steel sheets in the East Asian region where economic growth is remarkable.

In addition, in the case of automotive steel sheets that are required to have good workability, the durability of the product cannot be maintained unless the plating adhesion after press working and the corrosion resistance after painting after press working are good.

In particular, so-called high-strength steel sheets used as strength members are also required to have severe press workability and rust prevention of the processed parts after press processing, and thus the plating adhesion of the processed parts is extremely important.

Patent Document 1 discloses a method for producing a hot-dip galvanized steel sheet having excellent slidability at the time of press work, which regulates the amount of Al in the plating layer and the amount of Al at the plating / steel sheet interface. However, Patent Document 1 does not sufficiently consider the durability of the product such as plating adhesion of a processed portion after press working and corrosion resistance after press working. Therefore, it cannot be said that these problems are completely absent.

As described above, conventionally, there is no durable steel sheet with good plating adhesion after press working and post-paint corrosion resistance after press working.

Moreover, since the hot dip galvanized steel sheet is used in the fields of automobiles, home appliances, building materials, etc., it is also required to have an excellent post-painting appearance and spot weldability.

JP 2004-315965 A

The present invention has been made in view of such circumstances, and has excellent plating adhesion after press working, excellent spot weldability, excellent post-paint corrosion resistance after press working, and excellent post-paint appearance. An object is to provide a galvanized steel sheet.

The present inventors have intensively studied to solve the above problems. Instead of simply performing the hot dip galvanizing treatment as in the prior art, the hot dip galvanizing treatment is performed in which the structure of the hot dip galvanized layer is controlled and an intermetallic compound is formed with a predetermined property between the steel sheet and the hot dip galvanized layer. Preferably, the hot dip galvanizing treatment is performed to control the solidification structure and surface texture of the hot dip galvanized layer and to control the state of internal oxidation in the surface iron surface layer of the steel sheet surface. By such hot dip galvanizing treatment, hot dip galvanized steel sheet having excellent plating adhesion after press working, excellent spot weldability, excellent post-coating corrosion resistance of processed parts after press working, and excellent post-painting appearance As a result, the present invention has been completed. More specifically, the present invention provides the following.

The hot-dip galvanized steel sheet of the present invention is, in mass%, C: 0.05% or more and 0.10% or less, Si: 0.10% or less, Mn: 0.30% or more and 0.70% or less, P: 0 0.040% or less, S: 0.010% or less, N: 0.005% or less, Al: 0.10% or less, the balance being the composition of Fe and inevitable impurities, and the surface of the steel plate Formed in at least a part of the galvanized layer containing Al in an amount of 0.3% or more and 0.6% or less, and 0.12 g / m ² or more present between the steel sheet and the galvanized layer. An intermetallic compound containing 0.22 g / m ² or less of Al and Fe ₂ Al ₅ having an average particle size of 1 μm or less, and a yield stress (YS) of 260 MPa or more and 350 MPa or less.

In the hot dip galvanized steel sheet of the present invention, the surface roughness Ra of the surface of the hot dip galvanized layer is 0.8 μm or more and 1.6 μm or less, and the glossiness (G value) of the surface of the hot dip galvanized layer is 550. Zinc basal plane orientation ratio (Zn), which is the ratio of the crystal orientation of the (002) plane of Zn crystal to the crystal orientation of the (004) plane of Zn crystal on the surface of the hot dip galvanized layer. (002) / (004)) is 0.60 or more and 0.90 or less, and the amount of internal oxidation of the surface layer portion in contact with the hot-dip galvanized layer on the surface of the steel sheet is 0.05 g / m ² or less. Are preferred.

The hot-dip galvanized steel sheet of the present invention has excellent plating adhesion after press working, excellent spot weldability, excellent post-paint corrosion resistance after press working, and excellent post-paint appearance.

Hereinafter, the present invention will be specifically described. In addition, this invention is not limited to the following embodiment.

The hot dip galvanized steel sheet of the present invention has a steel sheet, a hot dip galvanized layer formed on at least a part of the surface of the steel sheet, and a metal compound existing between the steel sheet and the hot dip galvanized layer.

<Steel plate>
The steel sheet used in the present invention is in mass%, C: 0.05% or more and 0.10% or less, Si: 0.10% or less, Mn: 0.30% or more and 0.70% or less, P: 0.040. %: S: 0.010% or less, N: 0.005% or less, Al: 0.10% or less, with the balance being composed of Fe and inevitable impurities. Hereinafter, the component composition will be described. In the present specification, “%” in the component composition means “mass%” unless otherwise specified.

C: 0.05% or more and 0.10% or less Increasing the C content can contribute to increasing the strength of the steel sheet. In order to increase the strength, the C content needs to be 0.05% or more. On the other hand, a large amount of C causes an increase in solute C, greatly increasing yield strength and elongation, and also greatly reducing weldability. Therefore, the C content needs to be 0.10% or less.

Si: 0.10% or less When a large amount of Si is added, the plating adhesion of the steel sheet after press working is inhibited due to the formation of Si oxide during annealing. Therefore, the Si content needs to be 0.10% or less. A preferable Si content is 0.03% or less.

Mn: 0.30 to 0.70%
Mn contributes to high strength by solid solution strengthening. Moreover, Mn suppresses the diffusion of C and refines cementite to reduce the solid solution C, thereby reducing the yield strength and the elongation. Furthermore, Mn also has an effect of detoxifying S in harmful steel as MnS. In order to obtain such an effect, the Mn content needs to be 0.30% or more. On the other hand, the inclusion of a large amount of Mn not only causes a decrease in ductility due to hardening, but also causes the formation of Mn oxide during annealing, thereby inhibiting the plating adhesion of the steel sheet after press working. Therefore, the Mn content needs to be 0.70% or less.

P: 0.040% or less P contributes to high strength as a solid solution strengthening element. However, P deteriorates ductility and toughness. For this reason, the content of P needs to be 0.040% or less. A preferable P content is 0.015% or less.

S: 0.010% or less When there is much content of S, the toughness of a welded part will deteriorate. For this reason, the upper limit of the S content is 0.010%. A preferable S content is 0.007% or less.

N: 0.005% or less, Al: 0.10% or less Al (sol. Al) and N do not impair the effects of the present invention as long as they are contained in a normal steel sheet. N combines with Ti to form TiN, or N combines with Al to form AlN. Therefore, the Al content is specified to be 0.10% or less, and the N content is specified to be 0.005% or less. In addition, when content of Al exceeds 0.10%, formation of the intermetallic compound mentioned later will be inhibited. On the other hand, when the Al content exceeds 0.10%, nucleation is suppressed and each crystal in the steel sheet structure is coarsened, resulting in deterioration of plating adhesion after press working. Further, the N content exceeding 0.005% is dispersed in the ferrite grains and the work hardening rate is lowered. The preferable Al content is 0.04% or less, and the preferable N content is 0.002% or less.

<Hot galvanized layer>
The hot dip galvanized layer is a hot dip galvanized layer formed by a normal hot dip galvanizing process. The hot-dip galvanized layer contains Al in an amount of 0.3% to 0.6% by mass. In the present invention, the hot dip galvanized layer may contain components other than Zn and Al as long as the effects of the present invention are not impaired. Examples of components other than Al include Fe, Mg, and Cr.

When the Al content is less than 0.3%, it is necessary to reduce the Al concentration in the plating bath. When the Al concentration is lowered, Fe is eluted, so that dross is deposited and appearance is deteriorated, or hard dross is dispersed in the hot dip galvanized layer. When the dross is dispersed in the hot dip galvanized layer, the press workability of the hot dip galvanized steel sheet deteriorates due to contact with the mold during press working. If the Al content exceeds 0.6%, a large amount of an oxide film of Al is formed on the surface of the hot dip galvanized layer, and the spot weldability of the hot dip galvanized steel sheet deteriorates.

The hot-dip galvanized layer preferably has a surface roughness Ra of 0.8 to 1.6 μm. If the surface roughness Ra is less than 0.8, the oil may not be retained on the surface of the hot dip galvanized layer during pressing of the hot dip galvanized steel sheet, and press workability may be poor. If the surface roughness Ra exceeds 1.6 μm, the sharpness after coating and the plating adhesion after press working are inferior, and an excellent appearance may not be imparted to the hot-dip galvanized steel sheet after coating. In addition, the said surface roughness Ra means surface roughness Ra measured by the method as described in an Example.

The glossiness (G value) of the surface of the hot dip galvanized layer is preferably 550 or more and 750 or less. If the glossiness (G value) is less than 550, the sharpness after coating may be inferior, and an excellent appearance may not be imparted to the hot-dip galvanized steel sheet after coating. When the glossiness (G value) exceeds 750, it may be too smooth, and the oil may not be retained on the surface of the hot dip galvanized layer during pressing of the hot dip galvanized steel sheet, resulting in poor formability. In addition, the said glossiness (G value) means the glossiness (G value) measured using the gloss meter as described in an Example.

Zinc basal plane orientation ratio (Zn (002) / (004)), which is the ratio of the crystal orientation of the (002) plane of Zn crystal to the crystal orientation of the (004) plane of Zn crystal on the surface of the hot dip galvanized layer. ) Is preferably 0.60 or more and 0.90 or less. If the orientation ratio of the bottom surface of the zinc base is less than 0.60, the orientation of the zinc crystals is relatively random, and the crystal size when the zinc solidifies immediately after plating becomes fine, so the surface of the hot dip galvanized layer is too smooth and pressed. Sometimes the oil is not retained on the surface and press workability is poor. If the orientation ratio of the basal plane of the zinc base exceeds 0.90, the orientation of the basal plane of the Zn crystal is too high and the crystal grains are likely to grow. As a result, a dendritic arm develops, resulting in poor sharpness after coating and melting after coating. The appearance of the galvanized steel sheet may be deteriorated. Moreover, if the zinc base bottom surface orientation ratio exceeds 0.90, the corrosion resistance may be deteriorated. Here, the zinc base bottom surface orientation ratio can be defined by the following formula.

The zinc base bottom orientation ratio (Zn (002) / (004)) represents {(002) plane Zn crystal orientation} / {(004) plane Zn crystal orientation}. I _(xyz) is Zn intensity measured by X-ray on the (xyz) plane of the sample, I _{std (xyz)} is Zn intensity measured by X-ray on the (xyz) plane of the standard sample (pure Zn powder), Σ Means the sum of Zn intensities in all directions.

If the degree of orientation of the basal plane of the zinc base is defined as described above, Zn has an hcp structure and is usually easily oriented on the basal plane, and it can be seen how much the crystals are oriented randomly. The degree of orientation of the solidified structure affects the gloss, crystal size, and surface roughness (surface roughness). For this reason, accurately controlling the zinc base orientation rate is important in controlling not only the surface properties of the hot-dip galvanized steel sheet but also the press workability.

Further, the hot dip galvanized layer may be formed on at least a part of the steel plate surface. Since the hot dip galvanized layer is formed on the surface of the steel plate by a method of immersing the steel plate in a plating bath, the hot dip galvanized layer is usually formed on the entire surface of the steel plate.

Further, the thickness of the hot dip galvanized layer is not particularly limited, but the thickness of the hot dip galvanized layer can be adjusted by controlling the plating adhesion amount during the hot dip galvanizing treatment.

<Intermetallic compound>
Intermetallic compound is composed of an intermetallic compound containing Fe ₂ Al ₅ of average particle size below 1 [mu] m, it is present in the steel sheet and hot-dip galvanizing layer. The intermetallic compound contains 0.12 g / m ² or more and 0.22 g / m ² or less of Al. The presence of an intermetallic compound containing Fe ₂ Al ₅ suppresses the formation of the FeZn alloy phase and ensures good plating adhesion. This effect cannot be obtained in the case of an intermetallic compound other than the intermetallic compound containing Fe ₂ Al ₅ because it is often hard and brittle. In addition, in an intermetallic compound other than an intermetallic compound containing Fe ₂ Al ₅ , a hard and brittle FeZn intermetallic compound may be generated, and when this is generated, plating adhesion deteriorates. The content of Fe ₂ Al ₅ may be appropriately adjusted so that the effect of the present invention is obtained. Note that the presence of the intermetallic compound can be confirmed by a method in which the vicinity of the interface with the steel plate in the cross section of the hot dip galvanized layer is analyzed and detected by electron beam diffraction in a transmission electron microscope.

If the average particle size of Fe ₂ Al ₅ exceeds 1 μm, the hard intermetallic compound is excessively grown and the impact resistance characteristics of the hot dip galvanized steel sheet deteriorate. For this reason, the upper limit of the average particle diameter is 1 μm.

If the Al content in the intermetallic compound is less than 0.12 g / m ^2, it is necessary to set the Al concentration in the molten zinc bath of the plating low. If the Al concentration is set low, dross precipitates and the molten zinc The appearance and press workability of the plated steel sheet, the plating adhesion after the press work, and the corrosion resistance of the hot dip galvanized steel sheet are deteriorated. If the Al content in the intermetallic compound exceeds 0.22 g / m ^2, it is necessary to set the Al concentration in the plating bath high. If the Al concentration is set high, an oxide film of Al is formed on the surface of the galvanized layer. As a result, spot weldability deteriorates.

<Physical properties of hot-dip galvanized steel sheet>
The hot dip galvanized steel sheet of the present invention is excellent in plating adhesion after press working and excellent in corrosion resistance after painting of a processed part after press working. And the hot-dip galvanized steel sheet of the present invention has an excellent appearance after coating. For this reason, the hot dip galvanized steel sheet of the present invention can be applied to products having very severe processing parts such as a back door and a hood.

Moreover, the hot dip galvanized steel sheet of the present invention has a yield stress (YS) of 260 MPa or more and 350 MPa or less. If the yield stress is in the above range, the hot dip galvanized steel sheet can be preferably applied mainly to applications in which severe processing of the inner plate or the like and shape freezing property must be ensured. A preferable yield stress is 270 MPa or more and 310 MPa or less.

Moreover, in order to ensure the further favorable plating adhesiveness, it is preferable that the amount of internal oxidation in the surface layer portion after removing the plating layer is 0.05 g / m ² or less per side. Internal oxidation occurs when oxidizable elements such as Si, Mn, Al, and P added to steel are oxidized in a hot rolling process, an annealing process in CGL, and the like. In order to suppress the amount of internal oxidation, it is necessary not to increase the coiling temperature during hot rolling excessively or to increase the dew point in the annealing atmosphere in CGL excessively. If the amount of internal oxidation is large, the grain boundary becomes brittle in the processed part after press working, the plating adhesion after press working may deteriorate, and the spot weldability may also deteriorate. In addition, a surface iron surface layer part points out the range to 50 micrometers in the thickness direction of a steel plate from the interface of a hot-dip galvanized layer and a steel plate.

The method for removing the plating layer for measuring the amount of internal oxidation is not particularly limited, and removal by acid or alkali is possible. However, care should be taken not to remove the ground iron and to oxidize the surface after removal due to the combined use of an inhibitor (ground iron dissolution inhibitor). As an example, the present invention can be carried out with 20% by weight NaOH-10% by weight triethanolamine aqueous solution 195 cc + 35% by weight H ₂ O ₂ aqueous solution 7 cc. In addition, a dilute HCl solution containing an inhibitor is also possible.

The amount of internal oxidation can be obtained by measuring the amount of oxygen in the surface layer of the steel after the plating layer is removed. The amount of internal oxide in the surface layer portion of the steel is measured by, for example, “impulse furnace melting-infrared grade method”. However, in order to accurately estimate the amount of internal oxidation immediately below the plating layer, it is necessary to subtract the amount of oxygen contained in the base material itself. For this reason, the oxygen content in the steel was separately measured for a sample obtained by mechanically polishing 100 μm or more of the front and back surface portions of the sample from which the plating layer had been removed, and the above sample was obtained from the amount of oxide in the surface layer portion after removing the plating layer. By subtracting the amount of oxygen, the amount of increase in oxidation only in the surface layer portion is calculated, and converted into the amount per unit area to obtain a value.

<Method for producing hot-dip galvanized steel sheet>
Then, the manufacturing method of a hot dip galvanized steel plate is demonstrated. For example, a hot-dip galvanized steel sheet can be manufactured by the following method. First, steel having the above component composition is made into a slab by continuous casting, the slab is heated, and scale removal and rough rolling are performed. Next, after cooling, finish rolling, cooling, winding, pickling, and cold rolling are performed. Next, the steel sheet is annealed and hot-dip galvanized in a continuous hot-dip galvanizing facility.

The heating time, heating temperature, rough rolling conditions, cooling conditions, finish rolling conditions, winding conditions, etc. when heating the slab can be set as appropriate. However, in the present invention, in order to adjust the internal oxidation amount in the surface layer portion of the ground iron to the above range, it is preferable to adjust the conditions of finish rolling (hot rolling) and the winding speed.

Also, the annealing conditions of the steel sheet affect the yield stress of the hot dip galvanized steel sheet. In the present invention, in order to set the yield stress in the above range, it is preferable to set the heating temperature during annealing (the annealing temperature, which means the maximum steel sheet temperature) to 760 ° C. or higher and 840 ° C. or lower.

Moreover, the annealing atmosphere may be adjusted as appropriate. In the present invention, it is preferable to adjust the dew point to −55 ° C. or more and 0 ° C. or less. If the dew point exceeds 0 ° C., it is not preferable because the surface of the furnace body tends to become brittle, and if the dew point is less than −55 ° C., it is not preferable because it is technically difficult to ensure airtightness.

Further, the hydrogen concentration in the annealing atmosphere is preferably 1 vol% or more and 50 vol% or less. If the hydrogen concentration is 1 vol% or more, it is preferable because the surface of the steel sheet is activated, and if the hydrogen concentration is 50 vol% or more, it is not preferable because it is economically disadvantageous.

In the present invention, it is necessary to adjust the conditions of the hot dip galvanizing treatment in order to control the Al content of the hot dip galvanized layer and allow an intermetallic compound to exist between the steel plate and the hot dip galvanized layer. Moreover, in order to make the surface state (surface roughness Ra, glossiness (G value), zinc base bottom orientation ratio) of a hot dip galvanized layer into a desired state, it is necessary to adjust the conditions of the hot dip galvanizing treatment. . Hereinafter, conditions for the hot dip galvanizing process will be described.

The infiltration plate temperature, which is the temperature of the steel plate when the annealed steel plate enters the plating bath, is not particularly limited. In the present invention, the immersion plate temperature is preferably a temperature of the plating bath (bath temperature) of −20 ° C. or higher and a bath temperature of + 20 ° C. or lower. If the infiltration plate temperature is in the above range, the change in bath temperature is small, and it is easy to perform desired hot dip galvanization continuously. The Al content in the hot-dip galvanized layer and the Al content in the intermetallic compound tend to decrease by increasing the bath temperature. Moreover, the glossiness of the surface of the hot dip galvanized layer tends to increase as the bath temperature is increased.

The composition of the plating bath into which the annealed steel sheet enters is not limited as long as it contains Al in addition to Zn, and may contain other components as necessary. The concentration of Al in the plating bath is not particularly limited. The concentration of Al in the plating bath is preferably 0.16% by mass or more and 0.25% by mass or less. An Al concentration within the above range is preferable because an FeAl alloy phase is formed and formation of an FeZn alloy phase is suppressed. The glossiness can be adjusted by the Al concentration in the plating bath. When the Al concentration in the plating bath decreases, FeZn crystals, not FeAl, are formed slightly at the interface, which forms Zn solidification nucleation sites, thereby generating a large number of zinc crystals and randomizing the zinc crystal orientation. The orientation rate tends to decrease. As a result, as the Al concentration is lower, dendritic Zn crystal growth is suppressed, and surface unevenness is reduced and smoothed, so that the glossiness increases. A more preferable Al concentration is 0.19% by mass or more and 0.22% by mass or less. Since the Al concentration also affects the Al content in the hot-dip galvanized layer and the Al content in the intermetallic compound, it is preferable to determine the Al concentration in consideration of these contents.

Further, the temperature of the plating bath (bath temperature) is not particularly limited. In the present invention, the bath temperature is preferably 430 ° C. or higher and 470 ° C. or lower. A bath temperature of 430 ° C. or higher is preferred for the reason that the zinc bath dissolves stably without solidification, and a bath temperature of 470 ° C. or lower is preferred for the reason that Fe elution is small and dross defects are reduced. A more preferable range of the bath temperature is 450 ° C. or higher and 465 ° C. or lower.

The immersion time when the steel sheet is immersed in the plating bath is not particularly limited. In the present invention, the immersion time is preferably 0.1 seconds or more and 5 seconds or less. When the immersion time is in the above range, a desired hot dip galvanized layer is easily formed on the surface of the steel plate.

Immediately after lifting the steel plate from the plating bath, the amount of plating adhesion is adjusted by gas jet wiping or the like. In the present invention, the plating adhesion amount is not particularly limited. In the present invention, the plating adhesion amount is preferably in the range of 20 g / m ² or more and 120 g / m ² or less. If the plating adhesion amount is less than 20 g / m ² , it may be difficult to ensure corrosion resistance. On the other hand, when the plating adhesion amount exceeds 120 g / m ² , the plating peel resistance may deteriorate.

After adjusting the plating adhesion amount as described above, temper rolling (SK treatment) is performed. The type of roll used for the SK treatment is not particularly limited, and an Electro-Discharge Texture roll (EDT roll), an Electron Beam Texture roll (EBT roll), a shotdal roll, a topochrome roll, or the like can be used.

The rolling reduction rate during SK treatment (SK rolling reduction rate (%)) is not particularly limited. In the present invention, the SK rolling reduction is preferably 0.7 to 0.9%. If the SK rolling reduction is in the above range, the surface roughness can be easily adjusted to the above preferable range. Further, if the SK rolling reduction is outside the above range, the dullness that holds the rolling oil may not be obtained, and the formability may be reduced, and the yield strength may be reduced.

The cooling rate after the steel sheet is lifted from the plating bath is preferably −5 ° C./second or more and −30 ° C./second or less.

As described above, the hot dip galvanized steel sheet of the present invention has been described. Hereinafter, the use of the hot dip galvanized steel sheet of the present invention will be described.

The hot dip galvanized steel sheet of the present invention is preferably used for applications in which a coating film is formed on the surface of the hot dip galvanized layer because it has excellent post-coating corrosion resistance after press working. Moreover, the hot-dip galvanized steel sheet of the present invention is excellent in plating adhesion even when applied to applications requiring strict workability, and does not significantly reduce corrosion resistance and mechanical properties. Examples of applications in which strict processability is required and a coating film is formed include automotive steel plates such as automobile outer plates and inner plates. The method for forming the coating film is not particularly limited. In the present invention, it is preferable that a chemical conversion treatment is performed on the surface of the hot dip galvanized layer to form a chemical conversion film, and then a coating film is formed on the chemical conversion film.

Either a coating type or a reaction type can be used as the chemical conversion treatment liquid. Moreover, the component contained in a chemical conversion liquid is not specifically limited, either a chromate processing liquid may be used and a chromium free chemical conversion liquid may be used. Moreover, the chemical conversion film may be a single layer or a multilayer.

The coating method for forming the coating film is not particularly limited. Examples of the coating method include electrodeposition coating, roll coater coating, curtain flow coating, and spray coating. Moreover, in order to dry a coating material, means, such as hot air drying, infrared heating, induction superheating, can be used.

Hereinafter, the present invention will be specifically described based on examples. The present invention is not limited to the following examples.

It has a steel composition as shown in Table 1 and is wound at a finish rolling temperature shown in Table 2 (Table 2 and Table 2-2 are taken as Table 2) at a winding temperature of 650 ° C. or lower. After removing the black scale of the manufactured hot-rolled steel sheet by pickling, it was cold-rolled at a rolling reduction of 65%, and the plate thickness was set to 0.8 mm and 1.8 mm. Thereafter, the surface was degreased on the entry side of a CGL (continuous hot dip galvanizing line) and annealed and hot dip galvanized under the conditions shown in Table 2 to produce a hot dip galvanized steel sheet. The bath temperature and the Al concentration in the bath were appropriately changed. As the roll, an EDT processing roll was used and the rolling reduction ratio was appropriately changed. The amount of adhesion was 55 g / m ² per side. In addition, after pulling up the steel plate from the plating bath and adjusting the plating adhesion amount by gas jet wiping, SK treatment was performed under the conditions shown in Table 2 before cooling.

The following measurements were performed on the hot-dip galvanized steel sheet obtained as described above.

The intermetallic compound composition was identified by the X-ray diffraction method on the surface of the galvanized layer removed with fuming nitric acid. Regarding the amount, the surface of the intermetallic compound on the sample surface prepared in the same manner was dissolved in dilute hydrochloric acid and quantified by ICP. Similarly, the amount of Al in the plating layer was dissolved in dilute hydrochloric acid and determined by ICP.

The particle size of the intermetallic compound was measured by the following method. A test piece was collected from the steel plate, and the metal structure of a cross section parallel to the rolling direction was observed with a scanning electron microscope (SEM) at a magnification of 5000 to measure the average particle size of the intermetallic compound. The results are shown in Table 2.

The amount of internal oxidation was measured by first removing the hot dip galvanized layer with 195 cc of a 20% by weight NaOH-10% by weight triethanolamine aqueous solution and 7 cc of a 35% by weight H ₂ O ₂ aqueous solution. The amount was measured by impulse furnace melting-infrared grade technique. However, in order to accurately estimate the amount of internal oxidation immediately below the plating layer, it is necessary to subtract the amount of oxygen contained in the base material itself. For this reason, the oxygen content in the steel is separately measured for a sample obtained by mechanically polishing 100 μm or more of the surface iron surface layer on the front and back of the sample from which the plating layer has been removed, and the amount of oxide in the surface iron surface layer after removing the plating layer is measured. By subtracting the amount of oxygen of the sample, the amount of increase in oxidation only at the surface layer portion of the railway was calculated and converted to the amount per unit area. The results are shown in Table 2.

The surface roughness Ra of the hot dip galvanized layer was measured by the following method. In accordance with the provisions of JIS B 0601, the arithmetic average roughness Ra was measured using a stylus type surface roughness meter. The measurement results are shown in Table 2.

Glossiness (G value) was measured using a gloss meter. The measurement results are shown in Table 2.

Using an X-ray diffractometer, the crystal orientation of the (002) plane of the Zn crystal on the surface of the hot dip galvanized layer and the crystal orientation of the (004) plane of the Zn crystal are measured, and the zinc base bottom orientation ratio (Zn (002 ) / (004)). The zinc base bottom surface orientation ratio is shown in Table 2.

As for the plating adhesion of the processed part after press working, the punch with a frustum diameter of 5/8 inch is heightened at 1843 g for the part subjected to frustoconical overmolding (molding equivalent to press molding) under the condition of a plate thickness reduction rate of 10%. An impact resistance test of dropping from 1 m was performed, and evaluation was performed by a method of peeling cellophane tape. Those with peeling were considered poor adhesion (x), and those without peeling were good adhesion (◯). The evaluation results are shown in Table 2.

A JIS No. 5 tensile test piece is taken from the hot dip galvanized steel sheet in the direction of 90 ° with respect to the rolling direction, and a tensile test is performed under the condition that the crosshead speed is 10 mm / min (constant) in accordance with the provisions of JIS Z 2241. It was. Yield stress (YS (MPa) was measured, and those with YS of 260 to 350 MPa were considered good.

スポット Spot weldability was evaluated by spot welding continuous spotting. Specifically, after degreasing the steel plate, using a DR6 electrode with a tip diameter of 6 mm, a pressing force of 250 kgf, an initial pressurization time of 35 cy / 60 Hz, an energization time of 18 cy / 60 Hz, a holding time of 1 cy / 60 Hz, a rest time of 16 cy / 60 Hz, welding Under the welding conditions of a current of 10 kA and a nugget diameter of ≧ 4√t, the number of continuous dots at the time of spot welding was investigated for a 0.8 mm material. The number of continuous hit points ≧ 2000 points was good, and less than 2000 was bad. The results are shown in Table 2.

Next, the hot-dip galvanized steel sheet obtained by the above method was subjected to chemical conversion treatment, electrodeposition coating, intermediate coating, and overall coating, and the appearance after coating was visually evaluated. When there was no appearance defect such as uneven plating, it was evaluated as good, and when it was present, it was evaluated as defective. The evaluation results are shown in Table 2.

¡Cone overhang molding part was subjected to chemical treatment, electrodeposition coating, intermediate coating, and top coating, and the corrosion resistance after coating was evaluated by the following method. A salt spray test based on JIS Z 2371 (2000) was conducted for 10 days, and the presence or absence of swelling in the processed part after press working was evaluated. Those with blisters were judged as bad (x), and those without blisters were judged as good (◯). The evaluation results are shown in Table 2.

As apparent from Table 2, the hot-dip galvanized steel sheet of the present invention has good appearance, spot weldability, and yield stress. Moreover, although the hot dip galvanized steel sheet of the present invention is pressed, it has excellent plating adhesion and good corrosion resistance after coating.

Claims

In mass%, C: 0.05% or more and 0.10% or less, Si: 0.10% or less, Mn: 0.30% or more and 0.70% or less, P: 0.040% or less, S: 0.0. Steel plate containing 010% or less, N: 0.005% or less, Al: 0.10% or less, and the balance being composed of Fe and inevitable impurities;
Formed on at least a part of the surface of the steel sheet, a hot-dip galvanized layer containing Al by 0.3% to 0.6% by mass,
An intermetallic compound containing 0.12 g / m 2 or more and 0.22 g / m 2 or less of Al and Fe 2 Al 5 having an average particle size of 1 μm or less, existing between the steel sheet and the hot dip galvanized layer. Have
A hot-dip galvanized steel sheet having a yield stress (YS) of 260 MPa to 350 MPa.
The surface roughness Ra of the surface of the hot dip galvanized layer is 0.8 μm or more and 1.6 μm or less,
The glossiness (G value) of the surface of the hot dip galvanized layer is 550 or more and 750 or less,
Zinc basal plane orientation ratio (Zn (002) / (004), which is the ratio of the crystal orientation of the (002) plane of Zn crystal to the crystal orientation of the (004) plane of Zn crystal on the surface of the hot dip galvanized layer. )) Is 0.60 or more and 0.90 or less,
2. The hot-dip galvanized steel sheet according to claim 1, wherein an internal oxidation amount of a surface iron surface layer portion in contact with the hot-dip galvanized layer on the surface of the steel sheet is 0.05 g / m 2 or less.