JP2904809B2 - Method for producing hot-dip galvanized steel sheet - Google Patents

Description

The present invention relates to a hot-dip galvanized steel sheet of a descaled hot-rolled steel sheet and a method for producing the hot-rolled steel sheet by low-temperature heating, and particularly to a hard-to-plate steel sheet represented by a high Si-added steel sheet. Including high-strength hot-rolled steel sheets with different surface cleanliness inevitably depending on the component system,
The present invention can efficiently obtain the same galvanized appearance and adhesion as ordinary steel and the performance to be provided as a hot-dip galvanized steel sheet without changing the hot-dip galvanizing conditions unnecessarily.

(Prior art) Conventionally, in order to extend the service life or improve the design of bare steel materials that are frequently used as structural members in building materials and the like, after certain forming processes, some post-processing such as plating and painting is performed by consumers. However, there has been a strong demand for surface treatment of the supplied steel material in order to reduce the cost of the steel material used by labor saving of the process. Among these, recently, the demand for surface treatment of a thick high-tensile hot-rolled steel sheet having a thickness of 2 to 6 mm is increasing.

As a surface treatment method mainly aimed at improving the rust resistance of the high-strength hot-rolled steel sheet, there is a sendzimer-type hot-dip galvanizing method capable of easily forming a thick plating from the viewpoint of productivity.

(Problems to be solved by the invention) However, in this case, from the viewpoint of the material strength of the steel sheet, the maximum heating sheet temperature before hot-dip galvanizing is premised on heating a preheating image slightly higher than the hot-dip galvanizing bath temperature. Therefore, one of the features of the Sendzimer type line is usually 600
The cleaning effect (sponge effect) of the steel surface due to the non-oxidative heating at a high temperature of not less than ℃ cannot be expected much, and conversely, there is a disadvantage that non-plating is apt to occur due to insufficient reduction in the low-temperature heating reduction step.

As an improved prior art, a high-strength steel sheet formed by cold rolling is subjected to hot-dip galvanizing with a uniform appearance under a conventional heating method.
Japanese Patent Publication No. Sho 56-56418, which is characterized by applying Fe plating, and Japanese Patent Publication No. Sho 62-5569, which also features pre-plating of Zn.
No. 9 publication has been proposed.

However, in the former, it is difficult for the Fe pre-plating technology itself to produce stable Fe plating due to the formation of Fe ³⁺ by electrolytic oxidation.In the latter, selective vaporization of Zn from the pre-plated layer during heating after pre-plating In addition, there is a disadvantage that the reducing gas and its atmosphere in the furnace are severely contaminated, and the stability of the operation is lacking under a high productivity line due to the growth of a Fe-Zn binary alloy layer that is brittle for processing. There is still a problem in securing the hot-dip galvanized appearance or adhesion at the level.

Therefore, the present invention is directed to a high-productivity line, in which a low-temperature heating within a range that does not impair the steel sheet strength is achieved by applying a surface grinding and a pre-plating system different from the conventional pre-plating of Fe or Zn. The purpose of the present invention is to obtain a hot-dip galvanized steel sheet having a stable quality and a uniform appearance without occurrence of non-plating.

(Means for Solving the Problems) The method for producing a hot-dip galvanized steel sheet according to the present invention is to grind a hot-rolled steel sheet or a high-tensile hot-rolled steel sheet having an Si content of 0.5 to 1.0 wt% pickled and descaled by 0.3 to 5 μm. The surface of the steel sheet is coated with Ni plating or Ni-based alloy plating containing 0.1 to 20 wt% of at least one of Fe, Cr, Co, Mo, Ti, Zr, V, W, B, and P in a range of 0.1 to 10%.
After forming g / m ² , H ₂ gas concentration is 15% or less, dew point is -5 ° C
Heating is performed in the following N ₂ gas atmosphere with the maximum attained sheet temperature in the range of 450 to 600 ° C, and hot-dip galvanizing is performed.

In addition, the method of the present invention has an Si content of 0.5
~ 1.0wt% hot rolled steel sheet or high tensile hot rolled steel sheet 0.3 ~ 5μ
m, Ni plating or Fe, Cr, Co, Mo, Ti, Z
r, V, W, B, after the Ni-based alloy plated with 0.1 to 10 g / m ² formed which contains 0.1-20 weight% of at least one or more of P, H ₂ gas concentration than 15%, dew point -5 The maximum temperature of the plate is heated in the range of 450 to 600 ° C in an N ₂ gas atmosphere at a temperature of 400 ° C or lower, and hot-dip galvanizing is applied.The plate temperature is 400 to 600 ° C in the air while the galvanized layer is in a molten state. Heat for 10 seconds or less.

(Function) The outline of the structure of the present invention is as follows. In performing a uniform hot-dip galvanizing process on the surface of a high-strength hot-rolled steel sheet without appearance of non-plating, as a specific pretreatment, after descaling pickling, Alternatively, a specific amount of surface grinding is performed thereafter, a specific pre-plating is performed, and then low-temperature heating conditions are specified, and the configuration is as follows.

In the configuration of the present invention described above, the suitable range and the effect of the constituent factors will be described below.

Regarding the surface grinding amount of the steel sheet before the pre-plating, the technical purpose of the surface grinding in the present invention is performed as a pre-treatment of the pre-plating, and in order to strengthen the material strength of the steel sheet, the steel including Si Simultaneously removes the surface layer concentration of the added elements and the residue (pickling mud) during descaling pickling, increases the wet tensile strength of the steel sheet surface to the hot dip galvanizing bath, and increases the wettability. The purpose is to improve the uniformity of plating appearance and the adhesion of plating adhesion.

In addition, this surface grinding method may be any of elemental, mechanical, and chemical methods, but from the viewpoint of productivity, a mechanical method such as a spouted brush roll is preferred.

Incidentally, the surface grinding effect is proportional to the grinding amount, but 0.
If it is less than 3 μm, the above-mentioned grinding effect is small, and
Above, the effect saturates and is not economic. Therefore, the thickness is preferably 0.5 to 3 μm.

Regarding the pre-plating system used in the present invention, the pre-plating system is particularly concentrated on the surface of a high-tensile hot-rolled steel sheet with an increased Si or Mn level, and segregation of the elements in the steel by heating and formation of its oxide by a trace amount of oxygen in the heating atmosphere By this, the surface tension of the steel sheet increases, preventing a significant decrease in wettability to molten zinc,
The main purpose is to improve the uniform finish of the hot-dip appearance such as preventing the occurrence of non-plating, and in addition, it is based on the knowledge that a combined effect such as improvement in corrosion resistance or coating property of the hot-dip coated steel sheet by heat diffusion can be expected, In applying it, it is based on the following reasons.

When applying Ni plating or Ni-based alloy plating containing at least one kind of alloy elements of Fe, Co, Cr, Ti, Zr, V, Mo, W, P, and B to the steel sheet, the content is 0 wt%, By pre-plating only Ni only, the wettability of the steel sheet with the hot-dip galvanizing bath by Ni plating is greatly improved, and it is possible to obtain a uniform hot-dip galvanized appearance. Since it is difficult to improve the corrosion resistance, it is preferable that the Ni plating layer be plated with a Ni-based alloy.

The higher the eutectoid rate of the alloy element in the Ni plating layer, the higher the corrosion resistance. However, if the eutectoid rate exceeds 20 wt%, the Ni-based alloy plating layer itself becomes hardened, hair cracks due to internal stress, etc. In order to cause the non-uniform generation of the Fe-Zn binary alloy layer due to the partial diffusion of iron from the steel sheet base that occurs through the crack at the interface with the hot-dip galvanized layer, the hot-dip galvanized appearance gloss is non-uniform or It is not preferable because the pre-plated layer is caused to undergo cohesive failure in severe press working or the like. For the reasons described above, the content of the alloy element in the preferred Ni-based pre-plated layer is 0.5 to 10 w
t% is good.

Further, the coating weight is less than 0.1 g / m ² of Ni-based alloy pre-coating layer is an alloy ratio as described above, skin covering rate decreases for steel sheet surface during heating in a pinhole portion and the pre-plating unit In the subsequent hot-dip galvanizing process, non-plating may occur or Fe-Zn2
It is not preferable because the uniformity of the plating appearance is hindered due to the non-uniform formation of the original alloy layer and the like, which departs from the gist of the present invention.

On the other hand, the amount of deposition of the pre-plating layer exceed 10 g / m ^2,
The degree of improvement in the plating quality of the hot-dip coated steel sheet is not very economical because of the saturation and the soaring pre-plating cost including equipment. As described above, the amount of the Ni-based alloy pre-plated layer is preferably 0.5 to 5 g / m ² .

The method for obtaining the Ni-based alloy pre-plated layer of the present invention may be any of known techniques such as electroplating, chemical plating, vapor deposition plating, vapor phase plating, and molten salt electrolysis. Is preferably electroplating or vapor deposition plating.

The heating condition of the steel sheet before the hot-dip galvanizing referred to in the present invention means that the heating of the steel sheet does not impair the interfacial reaction with the hot-dip galvanizing bath without reducing the material strength of the steel sheet and suppressing the surface oxidation. The heating conditions.

Therefore, in the present invention, it is necessary to control the sheet temperature and the heating atmosphere. If the plate temperature is lower than 450 ° C, the maintenance cost of the hot-dip galvanizing bath temperature rises due to the low plate temperature, and it is difficult to control the bath temperature during line acceleration / deceleration. Lacking in reality. On the other hand, if the temperature exceeds 600 ° C, the diffusion of Fe from the steel sheet excessively progresses, and in particular, non-uniform formation of an overalloy layer which is brittle in processing in and around the pinhole portion of the pre-plated layer causes poor adhesion of the hot-dip plated layer. This is not preferable because it causes the reduction of the initial material strength of the steel sheet. Therefore,
The preferred low-temperature heating plate temperature in the present invention is 450 to 550
° C is good. As the heating atmosphere, it is necessary to control the H ₂ gas concentration and the dew point in an N ₂ gas atmosphere.

Role of H ₂ gas in the present invention, the adsorbed oxygen to be introduced from the inevitably steel sheet heating furnace under high productivity line H ₂
To prevent excessive oxidative contamination of the steel sheet surface, so that dew point control is necessary but excess H ₂
It is not advisable to maintain the gas concentration.

In this sense, the concentration of H ₂ gas in the N ₂ gas in the low-temperature heating atmosphere of the present invention is sufficient to be 15% or less. In particular, by performing the pre-plating, the critical activity at the interface of the steel sheet is expanded toward the low H ₂ gas concentration side, and depending on the dew point, the hot-dip galvanizing is sufficient even with 0% H ₂ gas.

On the other hand, as described above, the adsorbed oxygen generates moisture by a combustion reaction with H ₂ , causing an increase in the dew point in the furnace. Required. In this sense, the dew point in the present invention is preferably −5 ° C. or less, more preferably −10 ° C.

For the hot-dip galvanizing bath composition in the present invention, the hot-dip galvanizing bath is a Zn-based Zn-Al-based alloy plating,
It is necessary to control bath components for the purpose of improving corrosion resistance as a hot-dip galvanized steel sheet.

Al suppresses abnormal growth of the Fe-Zn ternary alloy layer which is brittle in processing by forming a barrier of the Fe-Al-Zn ternary alloy layer generated at the interface of the hot-dip galvanized layer, while improving the adhesion of the plated layer, and -The formation of a eutectic alloy with Al reduces electrochemically the excessive anode reaction (elution reaction) of Zn,
This is used for the purpose of achieving high corrosion resistance as a galvanized steel sheet.

If the Al content is less than 0.2 wt%, the formation of the Fe-Al-Zn ternary alloy layer is not sufficient, so that abnormal growth of the Fe-Zn binary alloy layer is caused and the plating adhesion is reduced. On the other hand, Al10wt
%, There is abnormal elution of Al in the pre-plated layer described above, and the eluted pre-plated component reacts with Al, disperses or floats as dross (intermetallic compound) in the hot-dip galvanizing bath, and this is dispersed in the hot-dip layer. Or, it re-adheres to the surface, and the commercial value as a hot-dip galvanized steel sheet is greatly impaired, such as uniformity of plating appearance, occurrence of dents and scratches due to rolls, which is not preferable. The preferred Al concentration is 0.
3-7 wt% is good.

The unavoidable impurities referred to in the present invention refer to Pb, Cd, Sn, etc., which are segregated at the crystal grain boundaries and the like in the hot-dip galvanized layer and minimize layer peeling of the plated layer from local corrosion with Zn. Must be excluded outside the plating bath system.

The total amount of inevitable impurities such as Pb
If it exceeds 0.02 wt%, it promotes intergranular corrosion as described above,
This is not very desirable because it greatly impairs the commercial value. Preferred unavoidable impurities are preferably 0.01% by weight or less.

In the hot dip galvanizing bath applied to the present invention, any one of Mg, Si and Sb is used as the third alloy element to be added.
More than one species is used. The purpose of application of the third element is to increase the corrosion resistance of the hot-dip galvanized steel sheet and to improve the uniformity of the plating appearance.

If at least one of the third elements is less than 0.05 wt%, high corrosion resistance cannot be expected. On the other hand, if the content exceeds 1.0 wt%, the material excessively dispersed in the hot-dip galvanizing bath reacts with Al in the bath and floats on the bath surface as dross. However, it is difficult to obtain a sufficient workability because it becomes a galvanized layer that easily damages or breaks brittlely, and both are not preferable because they greatly impair the commercial value. Therefore, the preferable addition amount of the third element is 0.1 to
0.5wt% is good.

The post-heating treatment for the heating conditions of the steel sheet after hot-dip galvanizing applied to the present invention is performed by diffusing Fe ions from the steel sheet base caused by heating, the pre-plated layer or each alloy component in the hot-dip galvanized layer. By reacting and modifying the entire hot-dip galvanized layer into an iron-based composite dispersion-type plated layer, the aim is to improve the coating base treatment property, or the adhesion and corrosion resistance of the topcoat paint. The heating atmosphere is the atmosphere, and the maximum sheet temperature is 450
If the temperature is less than ℃, the effect of modifying the hot-dip galvanized layer by the diffusion of thermal ions as described above cannot be expected very much, and if it exceeds 600 ℃, the overdiffusion of iron ions due to the excessive diffusion of iron ions and the corrosion resistance after coating. Dramatic improvement is difficult.

Therefore, the preferred maximum plate temperature is preferably 500 ° C to 550 ° C. If the holding time at the highest temperature is less than 2 seconds, the diffusion of iron ions is insufficient, so that a uniform diffusion layer cannot be formed over the entire hot-dip coating layer, which is outside the scope of the present invention. On the other hand, over 10 seconds, due to the excessive diffusion of iron ions, the abnormal development of a hard iron-based alloy layer
This is not preferable because plating adhesion tends to decrease. From the above, a preferable retention time is 3 to 7 seconds.

The effects of the present invention configured as described above will be described in more detail based on the embodiments described below.

(Example) A high-strength hot-rolled steel sheet composed of the specific steel components shown in Table 2 and descaled by hydrochloric acid pickling is first surface-ground by a predetermined amount of scotch bright roll specified in Table 1 in a Sendzimer-type continuous galvanizing line. After rinsing, pre-plating specified in Table 1 is performed by an electroplating method in a sulfate-based bath (however, Ti-based is a vapor-deposited plating method), washed with water, and once drained and dried. Then, it is immediately heat-treated under the heating atmosphere and heat cycle specified in Table 1 and hot-dip galvanized without being exposed to the air.

Next, the steel sheet hot-dip coated with the component-based plating bath specified in Table 1 is subjected to gas wiping in the atmosphere, and the zinc deposition amount is controlled to 100 to 120 g / m ² on one side. Thereafter, the production process is divided into two systems in terms of the required degree of plating surface function, and if a normal hot-dip galvanized steel sheet is required, the product is directly cooled with water and dried to obtain a product. In addition, in the case of a request for further improving the coating property or the corrosion resistance after coating, the above-mentioned hot-dip coating layer in which the coating amount is controlled is aimed at a molten or semi-molten state, and again in an air heating furnace. After being heated under the specific heating conditions in Table 1, it is dried with water and cooled to obtain a product. The plating performance of the examples according to the present invention thus configured is summarized in Table 1 based on Comparative Examples.

[1] Surface Grinding Effect The present invention is shown in Nos. 1 to 7 with respect to hot-dip galvanizing properties when a fixed amount of Ni pre-plating is performed while changing the grinding amount. As is clear from this, the surface grinding effect before the pre-plating is improved in accordance with the grinding amount in the high Si-added steel, which is said to be the most difficult to hot-dip, and is particularly effective in improving the appearance of hot-dip galvanized. It can be seen that the grinding amount of 0.3 μm or more is sufficient.

[2] Pre-plating effect of Ni-based alloy plating The combined effect of surface grinding and pre-plating and the effect of alloy element system of Ni-based alloy plating are shown in Examples Nos. 6 to 26 of the present invention.

In any case, within the proper range indicated by the present invention, even if the steel type of the original sheet is different, there is no difference in the degree of the effect, and the effect is effectively exhibited.

[3] as a proper condition prior to treatment in the heating atmosphere for the previous hot dipping low-temperature heating conditions after relates proper hydrogen gas concentration in the N ₂ gas atmosphere, the present invention No2 and No27～29,
Regarding the appropriate dew point, comparison with No27 and No30-No32
This is shown in comparison with No33 to No34. Further, with respect to the appropriate range of the highest attained sheet temperature upon heating, the present invention examples are shown in No. 35 to No. 38 and the comparative examples are shown in No. 39 to No. 40.

As is clear from this, the heating atmosphere for maintaining the active state of the steel sheet surface is at least non-oxidizing to reducing, and an oxidizing atmosphere is not preferable. Further, under this atmosphere, low-temperature heating that does not hinder the hot-dip galvanizing property is possible as long as the material strength is not reduced.

[4] Hot-dip galvanizing bath composition The hot-dip galvanizing is applied to the surface of the steel sheet which has been subjected to the pretreatment of the present invention described above. The effects of Al are No. 41 to No. 46 of the present invention, and Comparative Examples No. 47 to No. 48.
Shown together. In addition, regarding the effect of addition of Si, Mg and Sb, the inventive examples were No. 49 to No. 56 and No. 59 to No.
Shown with No58.

As is clear from the above, it is found that the alloy element in the hot-dip galvanizing bath according to the present invention has sufficiently achieved the high corrosion resistance of the steel sheet subjected to the specific pretreatment as a hot-dip galvanized steel sheet.

[5] About unavoidable impurities in hot-dip galvanizing bath The unavoidable impurities in the bath in the present invention are those that form local Zn and bulk batteries in the hot-dip galvanized layer, and suppress intergranular corrosion and corrosion resistance life. It is an essential component, and Pd, Cd, Sn and the like correspond to this. The present invention is shown in No. 27 and the comparative examples are shown in No. 63 to No. 64 in this proper range.

As is evident from the above, it is understood that the above-mentioned adverse effects can be eliminated if the present invention is within the scope of the present invention.

[6] About heating conditions after hot-dip galvanizing Post-heating in the present invention is based on the diffusion of Fe from the steel sheet interface.
In order to improve the plating layer, particularly as a hot-dip coated steel sheet, to improve the phosphatability, paintability, etc., the results are as follows. No. to No. 68 and No. 71 to No. 74, and the comparative examples are No.
69 to No70. Regarding the proper holding time, the examples of the present invention are shown in No. 66 and No. 75 to No.
Shown in As is clear from this, it is understood that the above-mentioned excellent coating base function is improved within the proper range according to the present invention.

The annotations * 1 to * 9 shown in Table 1 are described below.

* 1 Steel plate component system of applicable base plate (Table 2) * 2 Surface grinding roll Scotch Bright Roll (Fine type made by Sumitomo 3M) * 3 Measurement of eutectoid rate of pre-plated alloy elements and plating adhesion amount Alloy elements in stripping liquid after dissolution and separation in aqua regia The ic
The value obtained by P measurement is used as a numerator, and the value obtained by dividing the plating adhesion amount obtained by the gravimetric method as a denominator is defined as the alloy eutectoid rate.

* 4 Uniformity of hot-dip galvanized appearance (visual judgment) ◎ Smooth and uniform glossy, ○ Glossy, △ Polishing non-plating, × Partial non-plating * 5 Plating adhesion Cello after 180 degree close contact bending Evaluation by taping peeling ◎ No peeling at all, ○ Very slight point peeling, △ Point peeling,
× Laminate peeling, * 6 Phosphate treatment PB37SS treatment (manufactured by Nippon Parkerizing), 65 ° C × 10se
c Evaluated from the state of deposition of the phosphate-forming film after spraying ◎ Uniform fine crystals, ○ Uniform and slightly coarse-crystal mixed, △ Fine and coarse-particle mixed crystals precipitated, but some invisibility occurred, ×
Coarse-grained crystals are noticeable.

* 7 Paint adhesion PB37SS treatment + Pn62 treatment (manufactured by Nippon Parkerizing) followed by melamine alkyd resin paint (manufactured by Kansai Paint)
Paint 20μm and bake at 125 ℃ for 20 minutes. Then, after immersing in pure water boiled water for 30 minutes, after 24 hours, a bang (1 mm □ × 100
□) Evaluated by secondary adhesion after cellophane tape peeling. Evaluation is square
It is indicated by the ratio of the number of squares of the remaining coating film to 100 □.

◎ 100/100, ○ 90/100, △ 70/100, × 50 or less / 100 * 8 Unpainted corrosion resistance Cr ⁶⁺ -Cr ³⁺ coating type chromate treatment (T, Cr, 30mg / m ² )
After that, it was evaluated by the rust area ratio one year after the outdoor backpack test. ◎ White rust ≦ 5%, ○ White rust ≦ 10%, △ White rust ≧ 50%, partially red rust, × red rust ≧ 5% * 9 Paint Post-corrosion resistance * Chemical treatment with * 7-After treating the coating under the same conditions,
Cut a cross cut on the painted surface and spray with salt water (JIS Z-237
1) Evaluate the coating blister width from the cross cut after 7 days ◎ No blister at all, ○ Blur width ≦ 3mm, △ Blur width ≦ 5m
m, × blister width ≧ 10 mm (Effect of the Invention) As described in the above embodiments, according to the method of the present invention,
As a pre-treatment of the hot-rolled steel sheet after the pickling descaling treatment, by performing surface grinding and / or pre-plating of a Ni-based alloy, it has been conventionally referred to as difficult plating. Even with high Si-added steel, uniform plating appearance and plating adhesion can be simultaneously improved by low-temperature heat treatment under a specific atmosphere that does not affect the material strength, and low-temperature that was not obtained with conventional technology This is an epoch-making product that can provide a high-strength hot-dip galvanized steel sheet on the industrial level on the premise of heating.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-61-44168 (JP, A) JP-A-57-76176 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] 1. An Si content of 0.5 to 1.0 wt% after descaling by pickling.
Hot-rolled steel sheet or high-strength hot-rolled steel sheet is ground by 0.3 to 5 μm, and the surface of the steel sheet is Ni-plated or Fe, Cr, Co, Mo, Ti, Zr, V, W,
After forming a Ni-based alloy plating containing 0.1 to 20 wt% of at least one of B and P at 0.1 to 10 g / m ² , the H ₂ gas concentration becomes 15%.
%. A hot-dip galvanized steel sheet is produced by heating in a N ₂ gas atmosphere having a dew point of −5 ° C. or less and a maximum attained sheet temperature in a range of 450 to 600 ° C. to perform hot-dip galvanizing. 2. A pickled and descaled Si content of 0.5 to 1.0 wt%.
Hot-rolled steel sheet or high-strength hot-rolled steel sheet is ground by 0.3 to 5 μm, and the surface of the steel sheet is Ni-plated or Fe, Cr, Co, Mo, Ti, Zr, V, W,
After forming a Ni-based alloy plating containing 0.1 to 20 wt% of at least one of B and P at 0.1 to 10 g / m ² , the H ₂ gas concentration becomes 15%.
% And a maximum temperature of 450-600 ° C in a N ₂ gas atmosphere with a dew point of -5 ° C or less, hot-dip galvanizing, and while the galvanized layer is in a molten state, A hot-dip galvanized steel sheet characterized by heating at a sheet temperature of 400 to 600 ° C. and a heat retention time of 10 seconds or less.