JP2000073122A - Production of hot dip galvanized steel sheet having high yield ratio - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a hot dip galvanized steel sheet having a high yield ratio required to use for building materials, structures, industrial equipment or the like and moreover, free from the generation of blackening in high temp.-high moisture environments even in the case that the surface of plating is applied to chromate treatment. SOLUTION: This plated steel sheet is produced by applying a hot rolled steel sheet of steel composed of 0.1 to 0.3% C, <=0.4% Si, 0.2 to 2.5% Mn, <=0.10% P, <=0.02% S, 0.005 to 0.10% sol.Al, <=0.01% N and the balance substantially consisting of Fe to skinpass rolling of 3.0-15.0% elongation percentage, thereafter, heating it at 500 to 800 deg.C in a continuous hot dip plating line, applying it to plating thereon and after the plating, applying it to skinpass rolling of <=2.0% elongation percentage. By executing the skinpass rolling of high elongation percentage before plating (the skinpass after plating is in low elongation percentage), high yield strength (>=0.7 yield ratio) and blackening resistance can simultaneously be secured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-dip galvanized steel sheet having a high yield ratio and useful for applications such as building materials, structures, and industrial machines.

[0002]

2. Description of the Related Art In terms of safety, buildings, structures, machinery and the like must be able to withstand external deformation stress caused by natural disasters such as earthquakes or accidents, and to stably maintain their original shapes (plastic deformation). Is unlikely to occur). For this reason, it is necessary that the steel sheets provided for these uses have a high yield strength. If the yield strength of the material is higher than the external force, plastic deformation does not occur and the original shape can be maintained. From these points, the yield ratio YR (= the ratio of yield strength / tensile strength) is generally used as a steel plate for buildings and structures.
Of 0.7 or more is used.

[0003] In order to guarantee the corrosion resistance required for the intended use, the steel sheet is coated with a hot-dip galvanized plating, and the plated surface is further subjected to a chromate treatment for the purpose of primary rust prevention. It is usually used. The general manufacturing process of the hot-dip galvanized steel sheet using a hot-rolled steel sheet as a base material is as follows. After being introduced, heated to a predetermined temperature, and passed through a hot-dip plating bath, a plating layer is formed. After plating, a skin pass (elongation rate of about 1%) is applied to eliminate the yield point elongation of the base steel sheet, Thereafter, a step of performing chromate treatment and coating film coating is performed.

[0004]

As means for making the hot-dip galvanized steel sheet have a high yield strength (high yield ratio), a steel sheet containing a carbide-forming element such as Nb, Ti, or W is used. Increase yield strength by precipitation strengthening action,
High elongation (approximately 3
% Or more) and increasing the yield strength by work hardening. However, a method of obtaining a high yield strength by utilizing the precipitation strengthening action of carbide is Nb, T
The use of expensive elements such as i and W greatly increases the production cost, and the precipitation of carbides proceeds in the hot rolling temperature range. There is a disadvantage that control is difficult.

On the other hand, hot-dip coated steel sheets have a high elongation (about 3%).
% Or more) is advantageous in terms of cost and operation because it does not have the above-mentioned disadvantages. However, a steel sheet obtained by subjecting the plated steel sheet to a chromate treatment has a problem that the appearance quality is easily deteriorated. That is, when a hot-dip galvanized steel sheet to which a skin pass with a high elongation rate is added is subjected to chromate treatment, the plating surface turns black in a short period of time in a high-temperature and high-humidity environment and loses commercial value. This phenomenon is attributable to the fact that the surface area of the skin pass roll (having a dull surface) and the contact area between the plating surface and the plating surface are increased and the plating surface is activated, as the skin pass elongation is increased. It is considered something.
An object of the present invention is to solve the above-mentioned problems in the production of a hot-dip galvanized steel sheet requiring a high yield ratio.

[0006]

The method for producing a hot-dip galvanized steel sheet according to the present invention is described below.
30%, Si: 0.40% or less, Mn: 0.20-2.
50%, P: 0.10% or less, S: 0.02% or less, so
l. A steel slab containing Al: 0.005 to 0.10% and N: 0.01% or less, with the balance substantially consisting of Fe, subjected to hot rolling, and a temperature of Ar ₃ or more. The hot rolling is finished at a temperature range of 750 ° C. or lower, and the hot rolled steel sheet is subjected to a skin pass with an elongation of 3.0 to 15.0%. And 50
It is heated to a temperature range of 0 to 800 ° C and plated with zinc.
After plating, a skin pass with an elongation of 2.0% or less is provided.

One of the most characteristic features of the present invention is that a skin pass having a high elongation of 3.0% or more is added to a base steel sheet at a stage before hot-dip plating. As the effect of the high elongation skin pass before plating, Nb, Ti, W
A high yield strength with a yield ratio of 0.7 or more can be ensured without using carbide precipitation strengthening elements such as. Moreover, the steel plate whose plating surface has been subjected to chromate treatment,
Unlike a steel sheet which is subjected to high elongation skin pass after plating and subjected to chromate treatment, even in a high temperature and high humidity environment, it has a good black discoloration resistance which does not easily cause black discoloration.
In addition, the plated steel sheet has good resistance to coarsening, and can stably maintain a homogeneous and fine crystal structure even under the influence of high-temperature heat.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reasons for limiting the components of the plated base steel sheet of the present invention will be described. C: 0.10 to 0.30% C is 0.10 to 0.30% in order to secure the mechanical properties of the steel sheet.
It is defined in the range of 0.30%. In the present invention in which a skin pass with a high elongation rate is performed on a base material steel sheet (hot-rolled steel sheet) before hot-dip plating, if the C content is less than 0.10%, a reheating process such as heat treatment and welding may be performed. In addition, the decrease in ductility, toughness, and strength of the steel sheet due to coarsening of the crystal grains increases. By setting the C content to 0.10% or more, the number and amount of carbides precipitated at the crystal grain boundaries increase, and as a pinning effect, the growth and coarsening of crystal grains are suppressed and prevented. The upper limit of C amount is 0.30
The reason for setting% is that if it exceeds this, the ductility of the steel sheet is greatly reduced due to excessive precipitation of carbides, and the weldability is also deteriorated.

Si: 0.40% or less Si is a deoxidizing element and has a solid solution strengthening effect on steel. The amount of Si for obtaining this effect is sufficient up to 0.40%. Addition of a large amount exceeding this causes a decrease in the wettability of the steel sheet surface against hot-dip galvanizing.

Mn: 0.20 to 2.50% Mn has the effect of solid-solution strengthening the steel and of fixing free S, which causes the embrittlement of the steel, as MnS to render it harmless. These effects can be obtained by containing 0.20% or more.
The reason for setting the upper limit to 2.5% is that if it exceeds this, the wettability of the steel sheet surface to the hot-dip coated metal decreases, the ductility of the steel sheet decreases, and the weldability decreases (hard embrittlement of the welded portion). is there.

P: 0.10% or less P has the effect of forming a solid solution in the ferrite phase of the steel structure to solid-solution strengthen the steel, but has an adverse effect on spot weldability. In addition, when producing an alloyed hot-dip galvanized steel sheet, the alloying reaction of the plating layer is hindered, and the operability of the plating line is deteriorated. To avoid these adverse effects, the content is limited to 0.10% or less. S: 0.02% or less S is a harmful impurity, and it is preferable to reduce as much as possible from the viewpoint of workability of the steel sheet. If it is 0.02% or less, there is no substantial adverse effect, so this is set as the upper limit.

Sol. Al: 0.005 to 0.10% Al requires 0.005% or more as a deoxidizing element. However, a large amount of addition lowers the workability of steel, so the upper limit is 0.10%. N: 0.01% or less N is an element effective for improving the yield strength of steel, but on the other hand, it embrittles the steel and promotes the generation of cracks in the slab in continuous casting. Therefore, in the present invention, 0.0
Limited to 1% or less.

Next, the manufacturing process of the present invention will be described in the order of steps.
I will tell. [Hot rolling] Steel having the above-mentioned composition is produced by continuous casting.
The slab is subjected to hot rolling. Hot rolling is the usual method
And Ar_ThreeTemperature range above the point
Finish rolling in the temperature range). Aus hot rolling
In the tenite temperature range, the uniformity of the material of the final product steel sheet
This is to ensure the nature. That is, the ferrite phase is mixed.
Hot rolling in ferrite + austenite two phase region
, A mixed grain structure occurs, and the material properties of the final product steel sheet,
This is particularly because moldability is generated. Ar of steel _ThreeThe point is
Depending on the chemical composition, the steel composition of the present invention
It is between 780 and 880 ° C, depending on the steel composition
It is set appropriately within the temperature range. Winding temperature of hot rolled steel sheet
Affects the amount of scale generated on the steel sheet surface. 750 ° C
If the hot rolled steel sheet is wound in a high temperature
The large amount of oil generated causes descaling in the subsequent pickling process.
Properties deteriorate. For this reason, the winding temperature is 750
It is good to be below ° C.

[Skin pass before hot-dip plating] One of the most important requirements of the present invention is that a skin pass with a high elongation is performed before a hot-rolled steel sheet is introduced into a hot-dip plating line. By setting the elongation percentage of the skin pass to 3% or more, it is possible to stably secure a high yield ratio (0.7 or more) of the product plated steel sheet. FIG. 1 shows the relationship between the elongation percentage of the skin pass before plating and the mechanical property values of the product plated steel sheet. The test steel sheets are as follows. Composition (wt%): C 0.172, Si 0.011, Mn 0.772, P 0.0192, S
0.008, sol.Al 0.0165, N 0.0017, Fe Bal Hot rolled sheet thickness: 5.0mm, Hot rolled winding temperature: 550 ° C,
Plating line heating temperature: 630 ° C, skin pass elongation after plating: 1.0%

As shown in FIG. 1, when a skin pass (normal skin pass) with an elongation of about 1% is performed, the yield strength (YP) and the yield ratio (YR) decrease, but 3%.
By applying the skin pass having the high elongation, the yield strength (YP) and the yield ratio (YR) increase, and the effect becomes more remarkable as the elongation increases. This is because the number of dislocations introduced into the steel increases as the skin pass elongation increases. This elongation is preferably set to a value of 5.0% or more.

As described above, the elongation percentage of the skin pass before plating needs to be 3% or more, but the value must not exceed 15%. This upper limit is related to the steel sheet heating temperature (500 to 800 ° C.) in the hot-dip plating line. This will be described with reference to FIG. FIG.
The relationship between the heating temperature of the hot-rolled steel sheet introduced into the hot-dip coating line after the skin pass before plating and the mechanical properties of the product-coated steel sheet in the same line is shown using the skin pass elongation rate before plating as a parameter. Chemical composition and hot rolling conditions of the test steel sheet (Hot rolled sheet winding temperature, thickness)
Is the same as that of the test steel plate of FIG.

As shown in FIG. 2, when a steel sheet having an excessively high skin pass elongation rate before plating is used, rapid softening of the steel occurs in line operation in which the heating temperature of the steel sheet is set to about 700 ° C. or more. In addition, high yield strength (high yield ratio) cannot be imparted to the product plated steel sheet. As described above, when the heating temperature of the steel sheet is about 650 ° C. or less, the skin pass elongation before plating can be increased and the yield strength (yield ratio) of the product plated steel sheet can be improved. When the temperature is about 700 ° C. or higher, the effect of increasing the yield strength is impaired. This is considered to be due to the following phenomenon.

It is considered that recrystallization of steel occurs when the total amount of heat energy by heating and strain energy by processing exceeds the energy required for recrystallization. If the elongation percentage of the skin pass before plating is relatively low at 15% or less (the workability of the steel is low), the amount of strain energy stored is insufficient for the energy required for recrystallization.
No recrystallization occurs and no rapid softening occurs. On the other hand, when the skin pass before plating has a high elongation of more than 15%, the degree of work received by the steel becomes sufficient, and as a result of the accumulation of the necessary amount of strain energy for recrystallization, recrystallization occurs, Steel softens rapidly. Also, the reason why the recrystallization temperature decreases as the elongation rate of the pre-plating skin path increases is that the amount of strain energy accumulated increases accordingly, so that the energy required for recrystallization can be reduced even with small heat energy (low heating temperature). This is because it satisfies the value.

As described above, if the elongation percentage of the skin pass performed before plating is too high, recrystallization and softening are caused by the thermal effect of heating the steel sheet in the hot-dip galvanizing line and by the thermal effect of welding during product processing. Is more likely to occur. For this reason, the present invention avoids the above-mentioned adverse effects due to the thermal influence by setting the elongation percentage of the skin pass before plating to 3% or more and setting the upper limit to 15%, thereby achieving high yield strength (high yield strength). Ratio). The pre-plating skin pass may be performed before or after the pickling treatment (surface scale removal treatment) of the hot-rolled steel sheet.

[Continuous hot-dip galvanizing] Hot-rolled steel sheet is subjected to high elongation skin pass and pickling treatment (irrespective of order).
After introducing this into a continuous hot-dip plating line,
The steel plate is heated to 500-800C. Heating temperature 500
The reason for setting the temperature to not less than ° C is as follows. When hot-dip galvanizing a steel sheet, if the temperature of the steel sheet is lower than the plating bath temperature, the plating wettability of the steel sheet surface becomes insufficient and non-plating defects occur. Since the temperature of the zinc-based plating bath is usually set to about 460 ° C., a steel sheet heating temperature of 500 ° C. or higher is required to avoid the above-mentioned problems.

On the other hand, the reason for setting the upper limit of the heating temperature to 800 ° C. is as follows. Since the hot-rolled steel sheet is not subjected to a large work hardening unlike the cold-rolled steel sheet, it is not necessary to perform annealing (recrystallization) unlike the case where the cold-rolled steel sheet is used for the base metal sheet. That is, it is only necessary to heat the steel sheet to a temperature (about 500 to 650 ° C.) necessary for ensuring the plating wettability of the surface of the steel sheet. Heating at a higher temperature than this is a waste of heat energy. Further, even if the elongation percentage of the skin pass before plating is specified in the range of 3 to 15% as described above,
When heating at a high temperature exceeding ℃, the yield strength (YP) of the steel sheet gradually decreases with the temperature rise, and it becomes difficult to secure a high yield ratio of 0.7 or more. Therefore, the upper limit is set to 800 ° C.

The steel sheet heated to a predetermined temperature is passed through a hot-dip plating bath to form a plating layer on the surface of the steel sheet. For the hot-dip plating bath, various zinc-based plating metal (alloy) compositions such as zinc, zinc-aluminum alloy, zinc-aluminum-magnesium alloy, and zinc-aluminum-silicon alloy are applied. The plating is performed according to a conventional method, and no special conditions or restrictions are added.

[Skin Pass after Plating] The skin pass after plating is to eliminate the yield elongation of the plated steel sheet, to improve the surface skin,
It is performed for the purpose of correcting the shape of the steel sheet. The elongation percentage of the skin pass for obtaining this effect is adjusted to 2.0% or less. When a skin pass with an elongation of more than 2% is added, deterioration of the surface quality of the product steel sheet obtained through the subsequent chromate treatment (primary rust prevention treatment), that is, the above-described blackening phenomenon in a high-temperature and high-humidity environment is avoided. You will not be able to do it. Therefore, the upper limit is 2.0%. The chromate treatment and the like on the hot-dip galvanized steel sheet thus obtained are appropriately performed under ordinary conditions.

[0024]

EXAMPLE A steel having a chemical composition (% by weight) shown in Table 1 was melted from a converter and continuously cast into a slab, which was hot-rolled (finishing temperature: 860-910 ° C.) to obtain a slab having a thickness of 5 mm. Obtain a hot rolled steel sheet. Next, a skin pass before plating, pickling treatment, heating and hot-dipping of a steel sheet in a plating line, and a skin pass after plating are sequentially performed. Further, a test steel sheet is obtained by performing a chromate treatment for primary rust prevention. For chromate treatment, total chromium amount: 10 g / L, Cr ³⁺ / (Cr
⁶⁺ + Cr ³⁺ ): Using a processing solution having a composition of 35 wt%,
The coating amount (Cr equivalent value) was adjusted to 15 mg / m ² . Table 1 shows the chemical composition (% by weight) of the hot-rolled steel sheet. In the table, steels A to C
Are steel materials satisfying the chemical composition of the present invention, steels D and E
Is a steel material (insufficient C content) that is out of the same range.

Tables 2 and 3 show the production conditions and various characteristics of the product plated steel sheet. In the table, the tensile properties (tensile strength
Yield point Yield ratio Elongation) was measured in accordance with JIS Z2241 using a JIS Z22015 No. 5 test piece sampled from the steel sheet in the L direction. Further, the evaluation of the coarse graining resistance and the black discoloration resistance was performed in the following manner.

[Evaluation of resistance to coarsening] Test pieces (40 × 40, m
m) was subjected to a heat treatment at 850 ° C. for 120 seconds, the metal structure was observed, and the ferrite grain size was measured based on JIS G0552. In the table, the symbols in the same column are as follows. …: Ferrite grain size number: N ≧ 8 Has a homogeneous fine grain structure (very good in structure). …: Ferrite crystal grain size number: N 6 to 7 It is a moderately fine grain and has a good structure that does not cause any practical problems. Δ: ferrite crystal grain size number: surface area N ≦ 5, inner area N ≧ 6 The toughness, ductility and strength of the surface area are low, and surface layer cracks easily occur during processing. ×: Ferrite grain size number: N ≦ 5 A coarse grain structure is exhibited over the entire region, and the toughness, ductility,
Low strength, not suitable for structural materials.

[Evaluation of resistance to black discoloration] A flat test piece (80 ×
80, mm), stack them, pack them in rust-proof paper, and then use a constant temperature and high humidity tester to measure the temperature at 60 ° C and the humidity at 90%.
And hold for 240 hours. After the test, the degree of black discoloration on the surface of the test piece in the center of the three-sheet stack is visually observed. In the table, the symbols in the same column are as follows. ○: No discoloration of black ×: Occurrence of black discoloration

As shown in Table 2, the invention examples have a high yield strength, a high yield ratio of 0.7 or more, and excellent black discoloration resistance, and can be easily used in a high-temperature and high-humidity environment. It does not discolor black and maintains sound surface quality. In addition, it has excellent resistance to coarsening, does not easily coarsen even under the influence of high-temperature heat, and maintains a good fine crystal structure. On the other hand, in Comparative Examples (Table 3), Nos. 51 to 54 are inferior in coarsening resistance due to an inappropriate composition of the base steel sheet (insufficient C content). No.56,
No. 57, No. 59 and No. 60 have good black discoloration resistance, but have low yield strength and yield ratio due to insufficient skin pass elongation before plating. Nos. 61 to 64 have good black discoloration resistance. However, since the skin pass elongation before plating is too high, the effect of improving yield strength and yield ratio is poor. No. 65 and No. 66 have good black discoloration resistance, but have low yield strength and yield ratio. The reason why the yield strength / yield ratio is at a low level although the elongation rate of the skin pass before plating is not excessive or insufficient is because the heating temperature of the steel sheet in the plating line is too high. Further, No. 55 and No. 58 have high yield strength (high yield ratio) as an effect of the skin pass before plating, but the black discoloration resistance is impaired because the skin pass elongation after plating is too high. Have been.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

The hot-dip galvanized steel sheet produced according to the present invention has a high yield strength of 0.7 or more in yield ratio, has excellent black discoloration resistance, and easily changes black color even in a high temperature and high humidity environment. Without this, good appearance quality can be maintained. According to the present invention, high yield strength can be obtained without using expensive carbide precipitation strengthening elements such as Nb, Ti, and W, and there is no operational difficulty when using these elements, and It is also advantageous in terms of cost. Further, the hot-dip galvanized steel sheet according to the present invention has excellent resistance to coarsening, maintains a uniform and fine crystal structure without being easily coarsened even under the influence of high-temperature heat, and is stable as a structural member. Excellent in nature.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between an elongation percentage of a skin pass before plating and a mechanical property value of a product plated steel sheet.

FIG. 2 is a graph showing a relationship between a skin pass elongation rate before plating, a heating temperature of a steel sheet in a plating line, and a mechanical property value of a product plated steel sheet.

Claims (1)

[Claims] 1. In% by weight, C: 0.10 to 0.30%, Si: 0.40% or less, Mn: 0.20 to 2.50%, P: 0.10% or less, S: 0.02% or less, sol. Al: 0.005 to 0.10%, N: 0.01% or less, the remainder being subjected to hot rolling a steel slab substantially made of Fe, At the end of hot rolling at a temperature of Ar ₃ or more,
After applying a skin pass having an elongation of 3.0 to 15.0% to a hot-rolled steel sheet obtained by winding at a temperature range of 750 ° C. or lower, the continuous hot-dip plating line performs a temperature range of 500 to 800 ° C. A hot-dip galvanized steel sheet having a high yield ratio, wherein the steel sheet is subjected to a zinc-based plating by heating, followed by a skin pass having an elongation of 2.0% or less.