JPH0756069B2 - Method for manufacturing hot-dip galvanized hot rolled steel sheet - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a hot-dip galvanized steel sheet of descaled hot-rolled steel sheet and a method for producing the hot-rolled steel sheet by low-temperature heating, and in particular, a difficult-to-plate steel sheet typified by high Si-added steel. In addition, depending on the component system, for high-strength hot-rolled steel sheets that inevitably have different surface cleanliness, without changing the hot-dip galvanizing conditions unnecessarily, the same galvanizing appearance, adhesion, etc. as ordinary steel,
It is intended to efficiently obtain the performance to be provided as a hot-dip galvanized steel sheet.

(Prior Art) Conventionally, in order to extend the life or improve the design of bare steel, which is often used as a structural member in building materials, etc., after a certain forming process, it is necessary to apply plating or painting or some other post-treatment. However, there is a strong demand for surface treatment of the supplied steel material in order to reduce the cost of the steel material used due to labor saving in the process.

Among these, recently, the demand for surface treatment of thick high-tensile hot-rolled steel sheets with a plate thickness of 2 to 6 mm is increasing. As a surface treatment method whose main purpose is to improve the rust-preventive property of this high-strength hot-rolled steel sheet, there is a Zenzimer hot-dip galvanizing method capable of easily thickening from the viewpoint of productivity.

However, in this case, from the viewpoint of material strength of the steel sheet, the maximum heating plate temperature before hot dip galvanizing is premised on the heating of a preheating image slightly higher than the bath temperature of hot dip galvanizing. 600 which is one of the features of
The cleaning effect (sponge effect) on the steel surface by non-oxidative heating at a high temperature of ℃ or more cannot be expected so much, and conversely, there is a drawback that non-plating is likely to occur due to insufficient reduction in the low temperature heating reduction step.

As a conventional technique to improve this, for high-strength steel sheets by cold rolling, under the conventional heating method, when finishing the hot-dip galvanizing of uniform appearance, the surface of the substrate is pre-plated with Fe. Japanese Patent Publication No. 60-56418, which features the same, and Japanese Patent Publication No. 62-56949, which also features Zn pre-plating.
No. etc. have been proposed.

However, in the former, stable Fe plating is difficult because the Fe pre-plating technology itself produces Fe ³⁺ by electrolytic oxidation, and in the latter, in the heating after pre-plating, Zn is selected from the pre-plating layer. Due to vaporization, severe pollution of the reducing gas and its atmosphere in the furnace, and growth of the Fe-Zn binary alloy layer that is fragile in processing, there is a problem in lack of operational stability under a high productivity line. Further problems remain in securing the appearance of hot-dip galvanizing or adhesion at an industrial level.

(Problems to be Solved by the Invention) The present invention provides a highly productive hot dip galvanizing line,
By only slightly grinding the surface and surface layer of the hot-rolled steel sheet that has been descaled, under low temperature heating within the range that does not impair the strength of the steel sheet, stable appearance and uniform appearance without the occurrence of non-plating To obtain the hot-dip galvanized steel sheet.

(Means for Solving the Problems) The present invention is to perform uniform hot dip galvanizing on the surface of a high-strength hot-rolled steel sheet without appearance of non-plating. The present invention is characterized in that a specific amount of surface grinding is performed and a low temperature heating condition thereafter is specified. The configuration is as follows. That is, the surface of a hot rolled steel sheet pickled and descaled by pickling and descaling in a Zenzimer type continuous hot dip galvanizing process was ground to 0.1 to 5 μm and dried, and then the maximum reached plate temperature was 450 in an N ₂ gas atmosphere having a dew point of −5 ° C. or lower. ~ 600 ℃ is a manufacturing method of hot-dip galvanized hot rolled steel sheet, characterized by heating in the range, after hot dip galvanizing, the plate temperature 450 ~ 600 in the atmosphere while the galvanized layer is in a molten state ℃, heat retention time 10
Including the heating for less than a second, Si content in the steel sheet to be plated
It includes using 0.5 to 1.0 wt% hot rolled steel sheet, and includes using high-strength hot rolled steel sheet as the steel sheet to be plated.

Further, if necessary, the hot dip galvanizing bath has an Al content of 0.2-10 wt.
%, And 0.05 to 1.0 wt% of at least one of Mg, Si and Sb as the third alloying element, and the total amount of Pb, Cd, Sn, etc. mixed as unavoidable impurities is less than 0.02 wt% and the balance is Zn.
Including that.

(Function) The technical purpose of the surface grinding in the present invention is to enhance the surface strength of steel and other additive elements in the steel in order to strengthen the material strength of the steel sheet, and to remove the residue (pickling mud) during descaling and pickling. ) At the same time to increase the wettability of the surface of the steel sheet to the hot dip galvanizing bath and the tensile tension, and improve the wettability by improving the uniformity of the appearance of hot dip galvanizing and the coating adhesion. It is a thing.

Also, regarding this surface grinding method, either mechanical or chemical method may be used originally, but from the viewpoint of productivity,
A mechanical method such as a brush roll containing discharge particles is preferable.

The surface grinding effect is proportional to the grinding amount, but 0.1
If it is less than μm, the above-mentioned grinding effect is small, and if it exceeds 5 μm, the effect is saturated and it is not economical. Preferably 0.3 to 3
μm is good.

The heating of the steel sheet referred to in the present invention refers to heating conditions that do not reduce the material strength of the steel sheet, suppress surface oxidation, and do not hinder the interfacial reaction with the hot dip galvanizing bath.

Therefore, in the present invention, it is necessary to control the plate temperature and the heating atmosphere. If the plate temperature is less than 450 ° C, it is difficult to maintain and manage the hot dip galvanizing bath temperature by lowering the plate temperature, high cost boiling, and difficult bath temperature control during line acceleration / deceleration.
Mainly lacks operational stability of the line and lacks practicality.

On the other hand, above 600 ° C, Fe diffusion from the steel sheet proceeds excessively,
Especially in the pinhole part of the pre-plated layer and its periphery,
This is not preferable because uneven formation of a superalloy layer that is brittle during processing causes poor adhesion of the hot-dip coating layer and also causes a decrease in initial material strength of the steel sheet. Therefore, the preferable low temperature heating plate temperature in the present invention is preferably 450 to 550 ° C.

Further, as the heating atmosphere, it is necessary to control the dew point in an N ₂ gas atmosphere.

As the dew point in the present invention, it is necessary to remove oxygen and moisture adsorbed on the surface of the steel sheet and prevent re-adsorption in the furnace, and N ₂ gas having a lower dew point is required to be −5 ° C. or lower,
It is preferably -10 ° C or lower.

The hot-dip galvanizing bath in the present invention, Zn-based or Zn-Al
System alloy plating can be applied, and it is necessary to control bath components for the purpose of improving corrosion resistance as a galvanized steel sheet.

Al suppresses abnormal growth of the Fe-Zn binary alloy layer, which is fragile during processing, by forming a barrier of the Fe-Al-Zn ternary alloy layer generated at the interface of the galvanized layer, and at the same time improves the adhesion of the plated layer. , Zn-Al and eutectic alloy crystals electrochemically induce Z
It is used for the purpose of appropriately suppressing the excessive anode reaction (dissolution reaction) of n, and thereby improving the corrosion resistance of the galvanized steel sheet.

When Al is less than 0.2 wt%, the formation of the Fe-Al-Zn ternary alloy layer is not sufficient, which causes abnormal growth of the Fe-Z binary alloy layer and lowers the plating adhesion. On the other hand, A10wt
If the content exceeds%, there is abnormal elution due to Al in the pre-plated layer described above, and the eluted pre-plating components react with Al to disperse or float as dross (intermetallic compound) in the hot dip galvanizing bath. It is not preferable because it redeposits in the plating layer or on the surface thereof, and the commercial value of the hot-dip galvanized steel sheet is greatly impaired, such as the uniformity of the plating appearance and the frequent occurrence of dents and scratches due to rolls.

A preferable Al concentration is 0.3 to 7 wt%.

The unavoidable impurities referred to in the present invention refer to Pb, Cd, Sn, etc., and are localized in the hot-dip galvanized layer to segregate in the grain boundaries and the like, and to prevent delamination of the plated layer from local corrosion with Zn. It must be excluded from the plating bath as much as possible.

The total amount of inevitable impurities such as Pb is 0.
When it exceeds 02 wt%, it is not preferable because it promotes the intergranular corrosion as described above and greatly impairs the commercial value. The preferable unavoidable impurities are 0.01 wt% or less.

In the hot-dip galvanizing bath applied to the present invention, any one of Mg, Si and Sb is added as the third alloying element.
More than one seed is used. The purpose of applying this third element is to improve the corrosion resistance and improve the uniformity of the appearance of the galvanized steel sheet.

If one or more of the above-mentioned third elements is less than 0.05 wt%, high corrosion resistance cannot be expected. On the other hand, if it exceeds 1.0 wt%, the excessive dispersion in the hot dip galvanizing bath reacts with Al in the bath and floats as dross on the bath surface, which reattaches to the plating layer and has a uniform appearance. Since the galvanized layer is not easily damaged or brittle fractures easily, it is difficult to obtain sufficient workability, and the commercial value is greatly impaired, which is not preferable. Therefore, the preferable addition amount of the third element is 0.1 to 0.5.
wt% is good.

Post-heating treatment applied to the present invention, by diffusion of Fe from the steel sheet base material caused by heating, react with each alloy component in the hot-dip galvanized layer, the entire hot-dip galvanized layer iron-based composite dispersion-type plating layer The purpose of this is to improve the surface treatment property of the coating, the adhesion of the top coating and the corrosion resistance.

The heating atmosphere is in the air, and if the maximum attainable plate temperature is less than 450 ° C as the heating condition, the above-mentioned effect of modifying the hot-dip galvanized layer due to the diffusion of iron ions can hardly be expected, and exceeds 600 ° C. However, due to excessive diffusion of iron ions, it is difficult to dramatically improve the paintability of the top coat and the corrosion resistance after painting. Therefore, the preferable maximum plate temperature is preferably 500 ° C to 550 ° C.

Further, if the holding time at the maximum reached plate temperature is less than 2 seconds, a uniform diffusion layer cannot be formed over the entire hot-dip plated layer due to insufficient diffusion of Fe, which is outside the scope of the present invention.
On the other hand, if it exceeds 10 seconds, excessive diffusion of Fe causes abnormal development of a hard iron-based alloy layer that is brittle during processing, and the plating adhesion tends to decrease, which is not preferable. More preferable retention time is 3 to 7 seconds.

(Example) A high-strength hot-rolled steel sheet composed of the specific steel components shown in Table 2 which was descaled by hydrochloric acid pickling was first surface-ground by a predetermined amount of Scotch-Brite roll specified in Table 1 in a Sendzimer type continuous hot dip galvanizing line. After being washed with water, it is once drained and dried.

Immediately thereafter, heat treatment is performed under the heating atmosphere and heat cycle defined in Table 1, and hot dip galvanization is performed as it is without exposing to the atmosphere. Next, the steel sheet hot-dipped in the plating bath of the component system specified in Table 1 was gas-wiped in the atmosphere, and the amount of zinc deposited was 100 to 120 g / m ^{2 on} one side.
Controlled by.

After that, the manufacturing process is divided into two systems in terms of the required degree of plating surface function, and if it is required for a normal hot-dip galvanized steel sheet, it is water-cooled and dried as it is to obtain a product.

In addition, in the case of a request for further improving the paintability and the corrosion resistance after coating, the above-mentioned hot-dip coating layer with controlled deposition amount is aimed at the molten or semi-molten state, and again in the atmospheric heating furnace, After being heated under the specific heating conditions shown in Table 1, it is dried with water to obtain a product.

The plating performance of the examples according to the present invention will be summarized in Table 1 below based on comparative examples.

(1) From Table 1, regarding the surface grinding effect on the raw steel plate type, Examples according to the present invention are shown in No. 1 to No. 21, and comparative examples thereof are shown in No. 22 to No. 30.

As a result, the surface grinding effect for improving the uniformity of the hot-dip galvanized appearance is clear even if the base steel type changes, and the grinding amount is 0.
It turns out that 1 μm or more is enough.

(2) Next, similarly from Table 1, Examples under No. 39 to No. 41 according to the present invention are shown with respect to an appropriate dew point in a nitrogen gas atmosphere under appropriate heating conditions after surface grinding of an original plate, and comparative examples thereof. Shown in No.42 to No.43.

Further, regarding the proper heating plate temperature, examples of the present invention are shown in No.44 to No.47, and comparative examples thereof are shown in No.48 to No.50.

Accordingly, if the amount of surface grinding is within the proper range, the dew point in the nitrogen gas atmosphere should be controlled within the proper dew point range of the present invention because it is necessary to prevent the surface oxidation of the original plate during heating. I understand.

Furthermore, with regard to the maximum attainable plate temperature, in the present invention intended to maintain the galvanizability in a range that does not reduce the strength of the original plate material, if the example is deviated, the plating adhesion is mainly hindered. I understand.

(3) With respect to the hot-rolled steel sheet in which the pretreatment is contained in the proper range in this way, the examples of the proper Al concentration range are No. 50 to No. 50 for the hot dip galvanizing bath composition of the present invention.
55, and comparative examples thereof are shown in No. 56 to No. 57.

Further, regarding the proper addition amount range of the third alloying element, Examples of the present invention are shown in No. 58 to No. 64, and comparative examples thereof are No. 66 to No.
Shown in 67.

Furthermore, regarding the allowable range for inevitable impurities, Examples of the present invention are shown in No. 33 and No. 65, and the comparative example is N.
Shown in o.68 to No.69.

As is clear from this, Al in the zinc plating bath,
It can be seen that, by keeping the alloying elements and the unavoidable impurities within the proper range referred to in the present invention, it is possible to further improve the corrosion resistance mainly.

(4) In order to further improve the coating adhesion to the hot-dip galvanized hot-rolled steel sheet of the present invention thus formed, the coating layer of the hot-dip galvanized steel sheet is aimed at a molten or semi-molten state, and The zinc plating layer is heated to allow proper diffusion of Fe from the steel plate base,
-It is necessary to exert the surface anchoring effect by modifying the Zn diffusion plating layer.

As an example of the present invention in this regard, the appropriate range of the heating plate temperature after reaching the maximum is No. 70 to No. 73, the comparative example No. 74.
~ No.75 shows. In addition, the applicability of the present invention to raw steel plates is shown in Nos. 76 to 79 of the examples. Furthermore, regarding the plate temperature holding time at the time of post-heating, Examples of the present invention No. 80 to No.
83, and the comparative example is shown in No. 84.

As is clear from these results, when the required heat energy for post-heating becomes excessive or insufficient, the Fe-Zn alloying reaction of the zinc plating layer becomes insufficient or excessive, and the target paint anchoring effect cannot be expected so much. It is clear that it is necessary to maintain the proper post-heating conditions referred to in the present invention.

Note that the notes * 1 to * 9 shown in Table 1 are described below.

* 2 Surface grinding roll Scotch bright roll (fine type manufactured by Sumitomo 3M) is used.

* 3 Pre-plating alloy element eutectoid rate and plating adhesion amount measurement, after dissolution and peeling with aqua regia, IC of the peeling solution alloy element
The alloy eutectoid ratio is obtained by dividing the measured P value into the numerator and dividing the coating weight obtained by the gravimetric method into the denominator.

* 4 Uniformity of hot-dip galvanized appearance (visual judgment) ◎ Smooth and rich in gloss, ○ Matte, △ Pore-like non-plating occurred,
× Partial non-plating occurred * 5 Plating adhesion 180 degree adhesion After bending, it was evaluated by peeling the tape tape.

◎ No peeling at all, ○ Very slight spot peeling, △ Spot peeling, ×
Delamination * 6 Phosphate treatment PB37SS treatment (manufactured by Nippon Parker Rising), 65 ℃ × 10se
cEvaluated from the state of deposition of the phosphate-forming film after spray treatment.

◎ Uniform fine crystals, ○ Uniform and slightly coarse-grained crystals mixed, △ Fine-grained, coarse-grained mixed crystals partially precipitated, × coarse grains and outstanding.

* 7 Paint adhesion, PB37SS treatment + Pn62 treatment (Nippon Parker Rising)
After that, melamine alkyd resin paint (made by Kansai Paint)
20 μm, and bake at 125 ° C for 20 minutes. Then, after soaking in pure water boiling water for 30 minutes, 24 hours later
× 100 □) Evaluated by the secondary adhesion after peeling the cellophane tape. The evaluation shows the number of remaining coating film squares with respect to 100 squares as a ratio.

◎ 100/100, ○ 90/100, △ 70/100, × 50 or less / 100 * 8 Unpainted corrosion resistance Cr ⁶⁺ -Cr ³⁺ system coating type chromate treatment (T.Cr 30mg / m ² ) and then outdoor Evaluated by the ratio of rusted area one year after the test.

◎ White rust ≤ 5%, ○ White rust ≤ 10%, △ White rust ≥ 50%, some red rust, × Red rust ≥ 5% * 9 Corrosion resistance after painting * 7 Chemical conversion treatment-Paint was treated under the same conditions After that, a cross cut is carved on the painted surface, and salt spray test (JIS Z-237
1) Evaluate the film blistering width from the cross cut after 7 days.

◎ No blistering, ○ Blistering width ≤ 3mm, △ Blistering width ≥ 5m
m, × blistering width ≧ 10 mm (effect of the invention) As described above in detail with reference to Examples and Comparative Examples,
The method of the present invention is an epoch-making method that can easily apply hot-dip galvanizing to hot-rolled steel sheets including high Si-containing steel that is difficult to coat by low-temperature heating without deteriorating the original plate material. It is a method for producing hot-dip galvanized hot rolled steel sheet.

 ─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-56-47554 (JP, A) JP-A-49-134531 (JP, A) JP-A-53-128539 (JP, A)

Claims (5)

[Claims] 1. A method of manufacturing a hot-dip galvanized hot-rolled steel sheet, which comprises hot-rolling a hot-rolled steel sheet using a Zenzimer type continuous hot-dip galvanizing process, wherein the surface of the hot-rolled steel sheet pickled and descaled is ground by 0.1 to 5 μm, and Dew point -5 after drying
A method for producing a hot-dip galvanized hot-rolled steel sheet, which comprises heating in a N ₂ gas atmosphere at a temperature equal to or lower than 0 ° C to achieve a maximum attainable plate temperature in the range of 450 to 600 ° C and hot-dip galvanizing. 2. A method for producing a hot-dip galvanized hot-rolled steel sheet, which comprises hot-rolling a hot-rolled steel sheet using a Zenzimer type continuous hot-dip galvanizing process, wherein the surface of the pickled and descaled hot-rolled steel sheet is ground by 0.1 to 5 μm, Dew point -5 after drying
In the N ₂ gas atmosphere below ℃, the maximum plate temperature is heated in the range of 450 to 600 ℃, hot dip galvanizing is performed, and the plate temperature is 450 to 600 ℃ in the air while the galvanized layer is in a molten state. A method for producing a hot-dip galvanized hot-rolled steel sheet, which comprises heating at a heat retention time of 10 seconds or more. 3. The method for producing a hot-dip galvanized hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet having a Si content of 0.5 to 1.0 wt% is used as the sheet-plated steel sheet. 4. A method for producing a hot-dip galvanized hot-rolled steel sheet according to claim 1 or 2, wherein a high-strength hot-rolled steel sheet is used as the steel sheet to be plated. 5. The hot dip galvanizing bath has an Al content of 0.2 to 10 wt%.
And 0.05 to 1.0 wt% of at least one of Mg, Si and Sb as the third alloy element, the total amount of Pb, Cd, Sn, etc. mixed as unavoidable impurities is less than 0.02 wt% and the balance is Zn. The method for producing a hot-dip galvanized hot-rolled steel sheet according to claim 1 or 2, characterized in that.