JP7315522B2 - Manufacturing method of hot-dip galvanized steel sheet with excellent material stability - Google Patents

Description

The present invention relates to a method for manufacturing hot-dip galvanized steel sheets (including so-called hot-dip Al—Zn alloy-coated steel sheets).

Painted steel sheets, which are made from unalloyed hot-dip galvanized steel sheets (including so-called hot-dip Al-Zn alloy-coated steel sheets), are mainly processed into roofs, walls, shutters, etc., and are widely used as thin building materials.
Typical plating components of this hot-dip galvanized steel sheet include hot-dip plating containing additive elements such as 0.1-11 mass% aluminum, 5-mass% or less magnesium, and less than 1-mass% Ni, with the balance being zinc and unavoidable impurities (hereinafter referred to as GI system); hereinafter referred to as the GL system) (for example, Patent Documents 1 and 2).

When a steel plate is press-formed, springback, which is elastic recovery deformation caused by uneven residual stress generated in the steel plate, becomes a problem. In particular, when the quality of each coated steel sheet before processing varies, there is a problem that the shape of the product after processing is not stable.

JP 2008-138285 A Japanese Patent Publication No. 46-7161

Here, a method for measuring the springback of a thin plate will be briefly described. For example, using a springback measuring device as shown in FIG. 1, a test piece with a width of 25 mm and a length of 150 to 200 mm (longitudinal direction of the steel plate) taken from a steel plate is inserted into the clamp portion, and the clamp screw and bending roller are set in the device by tightening. In this state, the roller handle rotates the test piece 180° along the mandrel. After holding for about 3 seconds, the roller handle is quickly returned to its original position, and the springback angle (return angle after bending 180°) of the test piece is read. Let this value be a springback amount (SBV).
Using this springback measuring device, the present inventors investigated the amount of springback of the hot-dip galvanized steel sheets (painted steel sheets) produced by the present inventors.

If the springback characteristics of the hot-dip galvanized steel sheet fluctuate greatly over time, there is a risk that the shape of the product after processing will not be stable, which is a problem in terms of quality.
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a manufacturing method capable of manufacturing a hot-dip galvanized steel sheet which is excellent in material stability and whose springback characteristics are suppressed from fluctuating over time.

As a result of repeated studies to solve the above problems, the present inventors have found that a steel sheet is immersed in a GI-based or GL-based hot-dip coating bath and then hot-dip plated, and then the plated steel sheet after the coating layer has solidified after coming out of the hot-dip coating bath is subjected to skin pass rolling and/or tension leveler treatment without reheating to 180 ° C. or higher to introduce dislocations into the steel plate. It was found that an excellent hot-dip galvanized steel sheet can be obtained.
The present invention was made based on such findings, and has the following gist.

[1] A steel sheet having a C content of 0.01 to 0.20 mass% is immersed in a hot-dip galvanizing bath containing 0.1 to 11 mass% Al and 0 to 5 mass% Mg to hot-dip plate, and the plated steel sheet after the coating layer has solidified after coming out of the hot-dip galvanizing bath is subjected to skin pass rolling and/or tension leveler treatment without reheating to 180 ° C. or higher to introduce dislocations into the steel plate. A method for producing a hot-dip galvanized steel sheet having excellent material stability, characterized by holding the steel sheet at 180 to 410° C. for 3 hours or longer and then cooling it to room temperature.
[2] A method for producing a hot-dip galvanized steel sheet excellent in material stability, wherein the hot-dip galvanizing bath contains Al: 0.1 to 11 mass%, Mg: 0 to 5 mass%, and the balance is zinc and unavoidable impurities in the manufacturing method of [1] above.
[3] A method for producing a hot-dip galvanized steel sheet excellent in material stability, wherein the hot-dip galvanizing bath further contains one or more selected from Ni: less than 1 mass% and misch metal containing Ce and/or La (total amount): less than 1 mass%.

[4] A method for producing a hot-dip galvanized steel sheet excellent in material stability, wherein the hot-dip galvanizing bath further contains at least one selected from Ni: 0.005 to 0.2 mass% and misch metal containing Ce and/or La (total amount): 0.005 to 0.05 mass%.
[5] A steel sheet having a C content of 0.01 to 0.20 mass% is immersed in a hot-dip galvanizing bath containing 40 to 70 mass% Al, 0.6 to 15 mass% Si, and 0 to 25 mass% Mg for hot-dip plating, and after the coating layer has solidified after coming out of the hot-dip galvanizing bath, the plated steel sheet is subjected to skin-pass rolling and/or tension leveler treatment without reheating to 180°C or higher to introduce dislocations into the steel sheet, and then, A method for producing a hot-dip galvanized steel sheet excellent in material stability, characterized by subjecting a steel sheet wound in a coil to a heat treatment in which the steel sheet is held at a maximum temperature of 180 to 370° C. for 3 hours or more and then cooled to room temperature.

[6] A method for producing a hot-dip galvanized steel sheet excellent in material stability, wherein the hot-dip galvanizing bath contains Al: 40-70 mass%, Si: 0.6-15 mass%, Mg: 0-25 mass%, and the balance is zinc and unavoidable impurities.
[7] A method for producing a hot-dip galvanized steel sheet excellent in material stability, wherein the hot-dip galvanizing bath further contains one or more selected from Cr, Ni, Co, Mn, Ca, V, Ti, B, Mo, Sn, Zr, Sr, Li, and Ag in less than 1 mass% of each element.
[8] A method for producing a hot-dip galvanized steel sheet with excellent material stability, wherein the average cooling rate between 350°C and 280°C is 70°C/second or less in the process of cooling the plated steel sheet after leaving the hot-dip galvanizing bath in any one of the above [1] to [7].

[9] A method for producing a hot-dip galvanized steel sheet with excellent material stability, wherein the average cooling rate between 350°C and 280°C is 50°C/sec or less in the process of cooling the plated steel sheet after leaving the hot-dip galvanizing bath in any one of the above [1] to [7].
[10] A method for producing a hot-dip galvanized steel sheet with excellent material stability, characterized in that in any one of the above [1] to [9], the total elongation of the galvanized steel sheet by skin-pass rolling and/or tension leveler treatment is 0.5% or more.
[11] In the manufacturing method of any one of the above [1] to [10], the steel sheet contains C: 0.01 to 0.20 mass%, Si: 0 to 0.04 mass%, Mn: 0 to 1.00 mass%, Al: 0 to 0.08 mass%, B: 0 to 0.002 mass%, P: 0.04 mass% or less, S: 0.30 mass% or less, N: 0.007 mass% or less. A method for producing a hot-dip galvanized steel sheet having excellent material stability, characterized by containing and the balance consisting of Fe and unavoidable impurities.

[12] A method for producing a chemical conversion treated steel sheet having excellent material stability, characterized by forming a chemical conversion coating on the surface of the hot-dip galvanized steel sheet obtained by the production method according to any one of the above [1] to [11].
[13] A method for producing a coated steel sheet with excellent material stability, characterized by forming a chemical conversion coating on the surface of the hot-dip galvanized steel sheet obtained by the manufacturing method of any one of the above [1] to [11], and then forming a single-layer or multiple-layer coating on the top layer.
[14] In the production method of [13] above, in the step of forming a coating film, after undercoating, baking treatment is performed at a maximum plate temperature of 150 to 270 ° C. for 10 to 60 seconds, and then topcoat is applied, followed by baking treatment at a maximum plate temperature of 150 to 280 ° C. for 15 to 90 seconds.

According to the present invention, it is possible to stably produce a hot-dip galvanized steel sheet that is excellent in material stability and that suppresses temporal fluctuations in springback characteristics.

Explanatory drawing showing the springback measuring device used in the present invention A graph showing the amount of springback (SBV) after each accelerated aging treatment and the amount of springback (SBV) before the accelerated aging treatment when 100° C. aging acceleration treatment is applied to a coated steel sheet based on a GI-based plated steel sheet manufactured in plating facility A and a coated steel sheet based on a GL-based coated steel sheet manufactured in plating facility B. Magnified microscope photograph of a cross-section in the plate thickness direction of the steel part of the coated steel plate based on the GI-based plated steel plate manufactured in the plating facility A (STEM bright field image, observation magnification 115000 times) Magnified microscope photograph of the cross section in the thickness direction of the steel portion of the painted steel sheet based on the GL-based plated steel sheet manufactured in plating facility B (STEM bright field image, observation magnification 115000 times) Graph showing changes in steel sheet temperature after coming out of the plating bath for the GI plated steel sheet hot-dip plated in the plating facility A and the GL plated steel sheet hot-dip plated in the plating facility B Graph showing the presence or absence of Fe ₃ C precipitation depending on the difference in cooling rate of the steel sheet leaving the plating bath Graph showing the relationship between the total elongation by skin-pass rolling and/or tension leveler treatment and the increase in springback amount (SBV) after 100°C aging acceleration treatment when the plated steel sheet is skin-pass rolled and then subjected to 100°C aging promotion treatment. Graph showing the relationship between the plate temperature holding time at 180°C or higher during heat treatment with a peak temperature of 200°C for the coated steel sheet produced in Example 1 and the amount of springback (SBV) after 100°C aging acceleration treatment. For the coated steel sheet produced in Example 1, a graph showing the relationship between the plate temperature holding time at 180 ° C. or higher during heat treatment with a peak temperature of 200 ° C. for the plated steel plate and the amount of increase in springback amount (SBV) after 100 ° C. aging acceleration treatment. For the coated steel sheet produced in Example 2, a graph showing the relationship between the plate temperature holding time at 180 ° C. or higher during heat treatment with a peak temperature of 200 ° C. and the springback amount (SBV) after 100 ° C. aging acceleration treatment. For the coated steel sheet produced in Example 2, a graph showing the relationship between the plate temperature retention time at 180 ° C. or higher during heat treatment with a peak temperature of 200 ° C. for the plated steel plate and the amount of increase in springback amount (SBV) after 100 ° C. aging acceleration treatment.

Using a cold-rolled steel sheet of low carbon steel as a base steel sheet, the following coated steel sheets were manufactured, and a springback reproduction experiment was performed on them.
(i) The GI-based (Al: 0.15 mass%, Mg: 0 mass%, the balance being zinc and unavoidable impurities) plated steel sheet obtained in plating facility A was coated in a coating facility to produce a coated steel sheet based on the GI-based hot dip plated steel sheet (hereinafter referred to as "GI-based coated steel sheet").
(ii) The GL-based (Al: 55 mass%, Si: 1.6 mass%, Mg: 0 mass%, balance being zinc and unavoidable impurities) plated steel sheet obtained in plating facility B was coated in a coating facility to produce a coated steel sheet based on the GL-based hot-dip coated steel sheet (hereinafter referred to as "GL-based base coated steel sheet").
The paint baking was carried out at a maximum plate temperature of 200°C and a baking time of 25 seconds for the undercoat, and at a maximum plate temperature of 230°C and a baking time of 35 seconds for the topcoat.

The coated steel sheets (i) and (ii) above were subjected to aging acceleration treatment (treatment to accelerate age hardening) under the conditions of 100°C x 1 hour and 100°C x 2 hours, respectively, to simulate the natural aging phenomenon. FIG. 2 shows the results, showing the amount of springback (SBV) after each aging acceleration treatment of 100° C.×1 hour and 100° C.×2 hours and the springback amount (SBV) before (initial) aging promotion treatment. Fig. 2(a) shows the results for the GI-based coated steel sheet, and Fig. 2(b) shows the results for the GL-based coated steel sheet.
As shown in FIG. 2, it was found that the springback amount of the GI base coated steel sheet based on the plated steel sheet manufactured in the plating facility A is stable before and after the aging acceleration treatment, whereas the springback amount of the GL based coated steel sheet based on the plated steel sheet manufactured in the plating facility B fluctuates greatly before and after the aging acceleration treatment. If the GL system can be manufactured in the plating facility A and the GI system can be manufactured in the plating facility B, it is easy to clarify whether the cause is due to the difference in the plating equipment or the difference in the plating type. Therefore, for each coated steel sheet, the interior of the steel was investigated using a scanning transmission electron microscope (STEM).

FIG. 3 is an enlarged microscope photograph (STEM bright field image, observation magnification of 115000 times) of a cross section in the thickness direction of a steel portion of a GI base coated steel sheet based on a plated steel sheet manufactured in plating facility A, and FIG. In both cases, those observed linearly are dislocations, and those observed granularly are carbides. In the GI base coated steel sheet based on the plated steel sheet manufactured in the plating facility A shown in FIG. 3, almost no carbides are present in the crystal grains (matrix). On the other hand, in the GL based coated steel sheet based on the plated steel sheet manufactured in the plating facility B shown in FIG.

In general, when solute carbon remains in the matrix, the solute carbon then migrates to stable precipitation sites such as dislocations and grain boundaries over time and hardens to pin the movement of dislocations, increasing springback. This is the age hardening phenomenon.
From the above, it was found that the reason why the GI base coated steel sheet based on the plated steel sheet manufactured in the plating facility A had a small age hardening and the GL based coated steel sheet based on the plated steel sheet manufactured in the plating facility B had a large age hardening is due to the difference in the final amount of dissolved carbon remaining due to the difference in the driving force for dissolved carbon. That is, since the GI base-coated steel sheet with a high supersaturated carbon concentration has a larger driving force for solute carbon than the GL base-coated steel sheet, it is thought that the movement of solute carbon to a stable precipitation site during the paint baking process is promoted.

Next, we investigated the cause of the difference in age hardening depending on the plating equipment. Presuming that the difference in cooling conditions after plating affects age hardening, the relationship between the time after the steel sheet leaves the plating bath and the steel sheet temperature was investigated. The results are shown in FIG. According to this, in the cooling process of 450° C. to 200° C., which is the carbide precipitation temperature, in the temperature range of 350° C. to 280° C., which has a particularly large effect, the GI system (plating facility A) cools rapidly at 70 to 80° C./sec.

Considering these facts, in the cooling process after the steel sheet leaves the plating bath, in the GI system (plating facility A), the amount of dissolved carbon increases after plating, but the energy for carbide precipitation is high. After dislocations were introduced by skin-pass rolling, the precipitation of carbides to the dislocations was promoted by the baking heat treatment in the coating facility, the amount of dissolved carbon in the matrix decreased, and age hardening decreased. On the other hand, in the GL system (plating facility B), precipitation of carbohydrates to dislocations is considered to be slow. A series of phenomena can be explained by considering that age hardening progressed by the paint baking process, which is the final process, and the subsequent room temperature aging.

Based on the above results, we considered remodeling the equipment to increase the cooling rate of the plating equipment B, but we had to give up because the cooling equipment is large and requires high equipment remodeling costs. Therefore, we investigated a method to stabilize springback without modifying the equipment. We focused on the fact that, regardless of whether the springback is large or small, as long as it is constant, it is possible to adjust the processing conditions to incorporate this and stabilize the shape of the product after processing. In other words, if the coated steel plate product is completely age-hardened before shipping, the springback value is stable, and by combining with the customer's adjustment of processing conditions, it is possible to manufacture stable thin-plate building material processed products.

As a result of conducting studies based on such an idea, the steel sheet is dipped in a GI-based or GL-based hot-dip coating bath and then hot-dip coated, and then the plated steel sheet after the coating layer has solidified after coming out of the hot-dip coating bath is skin-pass rolled or/and tension leveler treated without reheating to 180 ° C. or higher to introduce dislocations into the steel plate. It was found that hot-dip galvanized steel sheets with excellent material stability and excellent material stability in which variation in springback characteristics is suppressed over time can be stably produced by performing a heat treatment of holding at 0° C. for 3 hours or more and then cooling to room temperature, or in the case of the GL system, carrying out a heat treatment of holding at a maximum sheet temperature of 180° C. to 370° C. for 3 hours or more and then cooling to room temperature.

Details of the manufacturing method of the present invention will be described below.
The composition of the hot-dip zinc-based plating bath for producing a GI system contains Al: 0.1 to 11 mass%, Mg: 0 to 5 mass% (including the case of no addition), Ni: less than 1 mass%, misch metal containing Ce and / and La (total amount): less than 1 mass%, and the balance consists of zinc and unavoidable impurities.
If the Al content is less than 0.1 mass%, a thick Fe—Zn alloy layer is formed at the interface between the coating layer and the base steel sheet, which induces peeling of the coating layer during working, resulting in poor coating adhesion. On the other hand, if the Al content exceeds 11 mass %, the eutectic structure of Zn and Al cannot be obtained, and the Al-rich layer increases to reduce the sacrificial anti-corrosion action, resulting in poor corrosion resistance of the end faces.
Mg is added as necessary to improve corrosion resistance, but when Mg exceeds 5 mass%, magnesium oxide-based dross tends to occur in the plating bath. In order to obtain the effect of adding Mg as described above, the amount of addition is preferably 0.1 mass % or more.

When Ni is added in an appropriate amount in the range of less than 1 mass%, blackening resistance is improved, but when it is 1 mass% or more, Al--Mg dross containing Ni is produced, impairing the plating appearance. From this point of view, the preferable amount of Ni to be added is 0.005 to 0.2 mass%.
When the misch metal containing Ce and/or La is added in an appropriate amount in the range of less than 1 mass% (total amount), the fluidity of the plating bath is increased and the effect of smoothing the plating surface can be obtained. Also, from such a point of view, the preferable addition amount (total amount) of the misch metal containing Ce and/or La is 0.005 to 0.05 mass%.

On the other hand, the composition of the hot-dip zinc-based plating bath for producing a GL system contains Al: 40 to 70 mass%, Si: 0.6 to 15 mass%, and Mg: 0 to 25 mass% (including cases where no additives are added). and the balance consists of zinc and unavoidable impurities.
If the Al content is less than 40 mass%, the effect of improving the corrosion resistance due to Al cannot be sufficiently obtained. On the other hand, when Al exceeds 70 mass%, Zn becomes insufficient, so the sacrificial anti-corrosion function by Zn is lowered.
Si is added to suppress the formation of the interfacial alloy layer, but if the Si content is less than 0.6 mass%, the effect of suppressing the formation of the interfacial alloy layer due to the addition of Si cannot be sufficiently obtained, resulting in poor workability. On the other hand, when the Si content exceeds 15 mass%, the corrosion resistance also deteriorates. Furthermore, in order to suppress the growth of the interfacial alloy layer and improve the corrosion resistance at a higher level, the Si content is preferably 1.0 to 5 mass%.

Mg is added as necessary to improve corrosion resistance, but when Mg exceeds 25 mass%, the effect of improving corrosion resistance is saturated, and in addition, the production cost increases and the generation of magnesium oxide-based dross in the plating bath becomes significant. In order to obtain the effect of adding Mg as described above, the amount of addition is preferably 0.1 mass % or more. In order to improve corrosion resistance, reduce manufacturing costs, and suppress dross at a higher level, the Mg content is preferably 10 mass% or less, more preferably 5 mass% or less.
Cr, Ni, Co, Mn, Ca, V, Ti, B, Mo, Sn, Zr, Sr, Li, and Ag are known as stabilizing elements for corrosion products in hot-dip Al-Zn alloy plating, and if one or more of these elements are added in an appropriate amount in the range of less than 1 mass%, further improvement in corrosion resistance can be expected due to the effect of stabilizing corrosion products without impairing the effects of the present invention.

After the steel sheet is immersed in the hot-dip zinc-based plating bath described above to be hot-dip coated, the steel sheet is taken out of the plating bath and cooled to solidify the coating layer.
As shown in FIG. 6, when the cooling rate at 450° C. to 200° C., which is the carbide (Fe ₃ C) precipitation region, is fast, the carbide mainly composed of Fe ₃ C does not precipitate, and when it is slow, the carbide mainly composed of Fe ₃ C precipitates. As a result, when the cooling rate is high, the solute carbon in the crystal grains becomes supersaturated, and the drive energy that precipitates as carbides in the subsequent baking process of the coated steel sheet is retained, so the springback characteristic fluctuations are inherently small. Therefore, the present invention is likely to be effective when the cooling rate at 450° C. to 200° C. is slow, specifically when the average cooling rate is 70° C./sec or less (especially slow cooling of 50° C./sec or less). However, even with rapid cooling, the age hardening does not become zero, so the present invention is effective.
In addition, the temperature range of 350° C. to 280° C. is the region where the Fe ₃ C precipitation time is the shortest, so the present invention is particularly effective when the average cooling rate from 350° C. to 280° C. is 70° C./second or less, especially in the case of slow cooling of 50° C./second or less.

Next, the plated steel sheet (the plated steel sheet after the plating layer has solidified) is subjected to skin-pass rolling and/or tension leveler treatment without being reheated to 180° C. or higher to give strain to the steel sheet and introduce dislocations. If the steel sheet is strained while supersaturated with carbon and then age-hardened, carbides precipitate at dislocations, which are precipitation sites, which is advantageous for complete aging. Therefore, in order to sufficiently obtain the effects of the present invention, it is necessary not to heat the steel sheet to 180° C. (ageing start temperature) or higher before skin-pass rolling and/or tension leveler treatment of the plated steel sheet after the plating layer has solidified.

Skin-pass rolling and/or tension leveler treatment can be performed using equipment installed in a normal hot-dip plating line. In general, skin pass rolling is a low-elongation rolling performed to improve the surface properties and shape of steel sheets and to suppress stretcher strain during processing. After that, if necessary, a tension leveler is applied to improve the flatness of the steel sheets. The skin pass rolling and tension leveler treatment of the present invention can be performed online using such conventional equipment, but the same treatment may be performed offline.
In the present invention, any of skin pass rolling alone, tension leveler treatment alone, and skin pass rolling and tension leveler treatment may be selected as long as a predetermined elongation rate and strain can be introduced.

FIG. 7 shows how the springback amount (SBV) after aging acceleration treatment changes depending on the total elongation rate due to skin pass rolling and/or tension leveler treatment when aging acceleration treatment (100 ° C. x 1 hour, 100 ° C. x 2 hours) is performed after skin pass rolling and / and tension leveler treatment of the plated steel sheet. In this test, the steel composition (mass%) of the test material (steel plate) was 0.047% C-0.23% Mn-0.028% sol.Al-0.0033% N, and the steel plate was subjected to GL-based plating in plating equipment B. After that, skin pass rolling and / and tension leveler treatment were performed to make the total elongation rate 0.3% to 2.4%. Next, the undercoat was applied and baked at 200°C for 30 seconds, and then the topcoat was applied and baked at 230°C for 30 seconds. The coated steel sheet was subjected to aging acceleration treatment under the conditions of 100° C.×1 hour and 100° C.×2 hours. Using the springback test apparatus of FIG. 1, the springback amount of the steel sheet before and after the aging acceleration treatment was measured, and the increase in the springback amount (SBV) (= [springback amount after aging acceleration treatment] - [springback amount before aging acceleration treatment]) was investigated. The test under each condition was N number=2, and the amount of increase in the amount of springback in FIG. 7 is the average value of N number=2.

According to FIG. 7, dislocations can be introduced even when the total elongation by skin pass rolling and/or tension leveler treatment is relatively low, so the effect of the present invention can be obtained, but when the elongation is 1% or more, the effect becomes large, especially when the elongation is 1.5% or more. Therefore, the total elongation by skin pass rolling and/or tension leveler treatment is preferably 0.5% or more, more preferably 1.0% or more, and particularly preferably 1.5%.

Next, the plated steel sheet into which dislocations have been introduced by skin-pass rolling and/or tension leveler treatment as described above is heat-treated. Although the method of heat treatment is not particularly limited, for example, a method of winding the plated steel sheet into a coil and then performing heat treatment in an off-line batch-type heating furnace can be used. In this heat treatment, the diffusion rate of carbon must be sufficiently high, and therefore the heat treatment temperature must be 180° C. or higher as the highest sheet temperature. On the other hand, if the heat treatment temperature is too high, in addition to the decrease in the thermodynamic driving force due to the decrease in supersaturated solute carbon, interlayer adhesion of the plated steel sheet may occur. Therefore, the heat treatment temperature must be less than the complete solidification temperature of the plated metal. For this reason, the temperature is set to 410° C. or less (complete freezing point: 419° C.) for the GI system, and 370° C. or less (complete freezing point: 381° C.) for the GL system. In addition, the heat treatment time is set to 3 hours or more in order to ensure sufficient diffusion of carbon. Therefore, in the present invention, in the case of the GI system, the steel sheet wound into the coil is subjected to a heat treatment in which it is held at the maximum steel sheet temperature of 180 to 410° C. for 3 hours or more and then cooled to room temperature, and in the case of the GL system, it is subjected to heat treatment in which it is kept at the maximum steel sheet temperature of 180 to 370° C. (preferably 180 to 250° C.) for 3 hours or more and then cooled to room temperature. Here, the heat treatment time refers to the time during which the lowest temperature portion of the coil reaches 180° C. or higher.
A hot-dip galvanized steel sheet is obtained through the manufacturing process described above.

The hot-dip galvanized steel sheet produced by the present invention is usually used as a chemical conversion treated steel sheet with a chemical conversion film formed on its surface, or is used as a coated steel sheet with a chemical conversion treatment film formed on its surface and further coated with a single layer or multiple layers thereon.
The chemical conversion film on the chemically treated steel sheet can be formed, for example, by applying a chromate treatment liquid or a chromium-free chemical conversion treatment liquid to the plated steel sheet and performing drying treatment at 80 to 300 ° C. (steel plate temperature) without washing with water or by chromate-free chemical conversion treatment. The chemical conversion treatment film may be a single layer or multiple layers, and in the case of multiple layers, chemical conversion treatments may be performed sequentially.

When a coated steel sheet is manufactured, coating and baking treatment are performed on a chemically treated plated steel sheet in a coating line. The underlying chemical conversion coating is as described above. As the multi-layer coating film, for example, an undercoat film and a topcoat film having different base resin components are formed. Examples of methods for forming the coating film include roll coater coating, curtain flow coating, and spray coating. After applying the paint containing the organic resin, the paint film can be formed by heating and drying by means of hot air drying, infrared heating, induction heating, or the like. The baking conditions in this coating process are determined according to the type of paint, but are usually 10 to 60 seconds at a maximum plate temperature of 150 to 270° C. for undercoating, and 15 to 90 seconds at a maximum temperature of 150 to 280° C. for top coating.
The hot-dip galvanized steel sheet produced by the present invention has already been subjected to age hardening treatment by heat treatment, and since the above baking temperature is not in the temperature range where carbide redissolves, age hardening does not pose a problem. In addition, many painting lines have a leveler facility after the paint baking furnace, but the use of this leveler does not affect the amount of dissolved carbon, so the effect of the present invention is not reduced.

Next, the chemical composition of the steel sheet for plating used in the present invention will be described.
The present invention is intended to control the precipitation behavior of carbides, and since it does not exhibit the effect with ultra-low carbon steel (C content: less than 0.01 mass%), it is necessary to use a steel plate with a C content of 0.01 mass% or more as a steel plate for plating. In consideration of workability, the upper limit of the C content of the steel sheet is set to 0.20 mass%.
In addition, the hot-dip galvanized steel sheet produced in the present invention is particularly suitable as a base plated steel sheet for coated steel sheets for thin sheet building materials, and in this case, a base sheet (steel sheet) having a chemical composition generally applied to thin sheet building materials can be used. As a specific chemical composition of this steel sheet, in mass%, C: 0.01 to 0.20%, Si: 0 to 0.04%, Mn: 0 to 1.00%, Al: 0 to 0.08%, B: 0 to 0.002%, P: 0.04% or less, S: 0.30% or less, N: 0.007% or less. The balance is Fe and impurities.

C is preferably 0.01 to 0.20% for the reason described above.
The lower limit of Si and Mn is set to 0% because the addition amount should be small when high strength is not required. On the other hand, when high strength is required, it is preferable to add Mn as a solid-solution strengthening element, but when targeting thin plate building materials, it is preferable to use Mn, which is cheaper than Si. In addition, since Si deteriorates plating adhesion, it is preferable to suppress the amount of addition. These elements do not affect the precipitation of carbides, but from the viewpoint of economical industrial production, the upper limits of Si and Mn are preferably 0.04% and 1.00%, respectively.

Al and B are likely to combine with N to form AlN and BN, and when combined with N in this way, the age hardening change due to N can be suppressed. From the viewpoint of economical industrial production, it is preferable that Al is 0 to 0.08% and B is 0 to 0.002%.
Since P segregates at grain boundaries and causes embrittlement, it is preferably as small as possible, but the upper limit is preferably 0.04% from the viewpoint of economical industrial production.
S also causes embrittlement, so it is required to be as small as possible, but the upper limit is preferably 0.30% from the viewpoint of economical industrial production.
N, like C, is an interstitial solid solution and is more likely to cause age hardening changes over time than C, so it is preferably as small as possible, but the upper limit is 0.007% from the viewpoint of economical industrial production.

[Example 1]
The steel composition is C: 0.075 mass%, Si: 0.015 mass%, Mn: 0.5 mass%, Al: 0.025 mass%, B: 0.0001 mass%, P: 0.013 mass%, S: 0.015 mass%, N: 0.002 mass%, and the balance is Fe and inevitable impurities. , Si: 1.6 mass%, Mg: 0 mass%, GL-based plating is applied in a hot-dip zinc-based plating bath of 1.6 mass% Si and 0 mass% Mg, and the plated steel sheet after solidification of the coating layer is subjected to skin pass rolling with an elongation rate of 0.6% and tension leveler treatment with an elongation rate of 0.2% to introduce dislocations into the steel sheet. A hot-dip galvanized steel sheet was manufactured.

This hot-dip galvanized steel sheet was subjected to a chromate-based chemical conversion treatment, then subjected to an epoxy resin-based undercoat, baked at 200°C for 25 seconds, and further subjected to a melamine-cured polyester-based topcoat and baked at 230°C for 35 seconds to prepare a test material. This test material was subjected to aging acceleration treatment under the conditions of 100° C.×1 hour and 100° C.×2 hours. Using the springback test apparatus of FIG. 1, the springback amount (SBV) of the test material before and after the aging acceleration treatment was measured, and the amount of increase in the springback amount (SBV) was investigated. The results are shown in FIGS. 8 and 9. FIG. FIG. 8 shows the amount of springback (SBV) before (initial) aging-accelerating treatment and after aging-accelerating treatment under each condition of each test material, and FIG. The test under each condition was N number=2, and the amount of springback (SBV) and the amount of increase in springback amount (SBV) in FIGS. 8 and 9 are the average values of N number=2.
All of the above manufacturing examples satisfy the conditions of the present invention, and according to FIG. 8, the springback amount (SBV) is within the target value range of 34 to 40 degrees. Further, according to FIG. 9, the amount of increase in the amount of springback (SBV), which was about 5 degrees in the conventional example as shown in FIG. 2, is suppressed to 2 degrees or less.

[Example 2]
A steel sheet having the same chemical composition as in Example 1 was subjected to GL-based plating in a hot-dip zinc-based plating bath having a plating bath composition of Al: 55 mass%, Si: 1.6 mass%, and Mg: 0 mass% in plating equipment B. After the coating layer had solidified, the plated steel sheet was subjected to skin pass rolling with an elongation rate of 1.4% and tension leveler treatment with an elongation rate of 0.2% to introduce dislocations into the steel sheet. After holding at 180° C. or higher at 00° C. for 3 to 48 hours, a batch heat treatment was performed by cooling to room temperature to produce a hot-dip galvanized steel sheet.

This hot-dip galvanized steel sheet was subjected to a chromate-based chemical conversion treatment, then subjected to an epoxy resin-based undercoat, baked at 200°C for 25 seconds, and further subjected to a melamine-cured polyester-based topcoat and baked at 230°C for 35 seconds to prepare a test material. This test material was subjected to aging acceleration treatment under the conditions of 100° C.×1 hour and 100° C.×2 hours. Using the springback test apparatus of FIG. 1, the springback amount (SBV) of the test material before and after the aging acceleration treatment was measured, and the amount of increase in the springback amount (SBV) was investigated. The results are shown in FIGS. 10 and 11. FIG. FIG. 10 shows the amount of springback (SBV) before (initial) aging-accelerating treatment and after aging-accelerating treatment under each condition of each test material, and FIG. The test under each condition was N number=2, and the amount of springback (SBV) and the amount of increase in springback amount (SBV) in FIGS. 10 and 11 are the average values of N number=2.
All of the above manufacturing examples satisfy the conditions of the present invention, and according to FIG. 10, the springback amount (SBV) is within the target value range of 34 to 40 degrees. Further, according to FIG. 11, the amount of increase in the amount of springback (SBV), which was about 5 degrees in the conventional example as shown in FIG. 2, is suppressed to 2 degrees or less.

Claims (9)

A method for producing a hot-dip galvanized steel sheet, wherein the average cooling rate between 350° C. and 280° C. is 50° C./sec or less in the cooling process of the galvanized steel sheet leaving the hot-dip galvanizing bath,
A steel sheet having a C content of 0.01 to 0.20 mass% is immersed in a hot dip galvanizing bath containing 0.1 to 11 mass% Al and 0 to 5 mass% Mg to be hot-dip plated, and the plated steel sheet after the coating layer has solidified after coming out of the hot dip galvanizing bath is subjected to skin pass rolling and/or tension leveler treatment without reheating to 180 ° C. or higher to introduce dislocations into the steel plate. A method for producing a hot-dip galvanized steel sheet having excellent material stability, characterized by subjecting the steel sheet to heat treatment by holding the steel sheet at 0 to 410° C. for 3 hours or more and then cooling it to room temperature. 2. A method for producing a hot-dip galvanized steel sheet with excellent material stability according to claim 1, wherein the hot-dip galvanizing bath contains Al: 0.1-11 mass%, Mg: 0-5 mass%, and the balance is zinc and unavoidable impurities. 3. The method for producing a hot-dip galvanized steel sheet with excellent material stability according to claim 2, wherein the hot-dip zinc-based plating bath further contains one or more selected from less than 1 mass% of Ni and less than 1 mass% of misch metal containing Ce and/or La (total amount). 3. The method for producing a hot dip galvanized steel sheet with excellent material stability according to claim 2, wherein the hot dip galvanizing bath further contains one or more selected from Ni: 0.005 to 0.2 mass% and misch metal (total amount) containing Ce and/or La: 0.005 to 0.05 mass%. The method for producing a hot-dip galvanized steel sheet with excellent material stability according to any one of claims 1 to 4 , characterized in that the total elongation of the galvanized steel sheet by skin pass rolling and/or tension leveler treatment is 0.5% or more. The steel sheet contains C: 0.01 to 0.20 mass%, Si: 0 to 0.04 mass%, Mn: 0 to 1.00 mass%, Al: 0 to 0.08 mass%, B: 0 to 0.002 mass%, P: 0.04 mass% or less, S: 0.30 mass% or less, N: 0.007 mass% or less, and the balance consisting of Fe and unavoidable impurities. Item 6. A method for producing a hot-dip galvanized steel sheet having excellent material stability according to any one of Items 1 to 5 . A method for producing a chemically treated steel sheet having excellent material stability, comprising forming a chemically treated film on the surface of the hot-dip galvanized steel sheet obtained by the production method according to any one of claims 1 to 6 . A method for producing a coated steel sheet with excellent material stability, characterized by forming a chemical conversion coating on the surface of the hot-dip galvanized steel sheet obtained by the manufacturing method according to any one of claims 1 to 6 , and then forming a single-layer or multiple-layer coating on top of it. In the step of forming a coating film, after undercoating, baking treatment is performed at a maximum plate temperature of 150 to 270 ° C. for 10 to 60 seconds, and then topcoating is performed. A method for producing a coated steel sheet with excellent material stability according to claim 8 , wherein baking treatment is performed at a maximum plate temperature of 150 to 280 ° C. for 15 to 90 seconds.

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