JPH0394018A - Production of high tensile hot dip galvanized steel sheet excellent in bendability - Google Patents

Abstract

PURPOSE:To produce a high tensile hot dip galvanized steel sheet having specific tensile strength and excellent in bendability by subjecting a steel having a specific composition consisting of C, Si, Mn, and iron to respectively specified hot rolling, cold rolling, and annealing and then applying hot dip galvanizing to the resulting steel sheet. CONSTITUTION:A steel which has a composition consisting of, by weight, 0.03-0.12% C, <=0.50% Si, 0.30-2.0% Mn, and the balance iron with inevitable impurities and further containing, if necessary, at least one kind among <=0.10% P, <=1.0% Cr, and <=0.5% Mo or at least one kind among 0.01-0.06% Ti, 0.01-0.06% Nb, and 0.01-0.06% V is hot-rolled. At this time, the resulting hot rolled coil is coiled while regulating finish rolling temp., average cooling rate to coiling, and coiling temp. to 800-1000 deg.C, 20-100 deg.C/sec, and 400-600 deg.C, respectively, by which a hot rolled steel plate having a dual-phase structure of ferrite and bainite is prepared. This hot rolled steel plate is pickled and cold-rolled at 30-60% draft. Then, the resulting cold rolled steel sheet is annealed at 400-680 deg.C for 2sec-10min and successively subjected to hot dip galvanizing. By this method, the high tensile hot dip galvanized steel sheet having >=60kgf/mm<2> tensile strength and excellent in bendability can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high-tensile galvanized plate, and particularly to a method for manufacturing a high-strength hot-dip galvanized plate, and in particular, for example, a method for producing a high-strength hot-dip galvanized plate, which has a high bendability suitable for use as a construction scaffolding plate. The present invention relates to a method for producing a tension hot-dip galvanized steel sheet.

．． Conventionally, the recovery annealing method, which is inexpensive to produce, has been widely used to produce high-strength hot-dip galvanized steel sheets for bending with a tensile strength of 60 kgf/mm2 or higher.

In general, the recovery annealing method is a method in which a thin steel plate is cold rolled to improve its strength, and then annealed to obtain bending workability without a significant decrease in strength.

However, when obtaining bending workability using this recovery annealing method,
If the recovery is insufficient, the steel sheet will lack bending workability, and if it is excessive, the steel sheet structure will recrystallize and become rapidly softened, resulting in insufficient strength. The tendency for such disadvantages in mechanical properties to occur is more significant as the cold rolling rate is higher. That is, the higher the cold rolling rate, the narrower the appropriate annealing temperature range for strength and bending workability. Therefore, in order to obtain a homogeneous cold-rolled steel sheet with excellent bending workability using the recovery annealing method, it is necessary to use equipment with small temperature changes depending on the position in the annealing furnace and excellent temperature control, or to add Nb or Zr. It is necessary to widen the appropriate annealing temperature range by sizing or the like.

For example, in Japanese Unexamined Patent Publication No. 51-110416, weight%
C content 0.02 to 0.10% and M n 0. L
~0. 0.02 to 0.02 Nb to algo-quilt steel containing 9%
Added in the range of 0.18%, the resulting steel becomes 40-70%
After cold rolling at a relatively low cold rolling rate, recovery annealing was performed for 7 minutes to 24 hours to achieve a yield strength of 63.
A method for obtaining a cold rolled steel sheet having a weight of 47 mm2 or more and a 2 inch elongation of 10% or more is described.

This method increases the recrystallization temperature, expands the recovery annealing temperature range, and obtains high yield strength due to the precipitates by precipitating the entire amount of Nb or Zr as carbides. However, regarding the improvement of bending workability,
There is no consideration given to old age.

The inventors of the present invention have undertaken extensive research to solve the problems described above in the production of conventional high-tensile galvanized steel sheets for moe processing. By appropriately regulating the chemistry, structure, cold rolling rate, and annealing conditions of the hot-dip galvanized steel sheet, it is possible to obtain a high-strength hot-dip galvanized steel sheet for bending that has sufficient strength and bending workability. This discovery led to the present invention.

That is, an object of the present invention is to provide a method for producing a high-tensile galvanized steel sheet, and in particular, an object of the present invention is to provide a method for producing a high-tensile alloyed galvanized steel sheet with excellent bending workability. .

One method for manufacturing a high-strength hot-dip galvanized steel sheet with excellent bending workability according to the present invention for reducing the amount of C 0.03 to 0 in weight percent
．． 12%, Si 0.50% or less, Mn 0.3.0 to 2.0%, and the balance iron and unavoidable impurities. The hot-rolled coil was wound up at an average cooling rate of 20 to 100°C/sec and a coiling temperature of 400 to 600'C to form a ferrite-bainite composite structure hot-rolled steel sheet, pickled, and rolled at a rolling reduction rate. By cold rolling at 30-60%, then annealing at a temperature in the range of 400-680°C for 2 seconds to 10 minutes, and then hot-dip galvanizing, it has a tensile strength of 60 kgf/mm2. The present invention is characterized by obtaining a high-strength hot-dip galvanized steel sheet having excellent bending workability as described above.

First, some chemistry of the steel used in the present invention will be explained.

C is an element necessary to ensure the strength of the steel plate, and when it is less than 0.03%, in the cold rolling described below, the rolling rate is 30 to 6'O%, and 60 kg
It is difficult to obtain a tensile strength of f/mm" or higher. However, when it exceeds 0.12%, even if high strength can be achieved, the bending workability deteriorates significantly. In the invention, the amount of C added is in the range of 0.03 to 0.12%.

Si is added to prevent deterioration of strength and bending workability, but when added in excess of 0.5%, it deteriorates the adhesion of hot-dip galvanizing, so the amount added is 0.5%.
% or less.

Mn is also an element necessary to ensure the strength of the steel plate. When the amount added is less than 0.03%, 60k
In order to obtain a tensile strength of more than 2.0%, it is not preferable to add other alloying elements, which increases manufacturing costs.However, when it exceeds 2.0%,
The band structure of steel becomes stronger and bending workability deteriorates.

Therefore, the amount of Mn added is in the range of 0.30 to 2.0%.

In the present invention, the steel used contains, in addition to the above-mentioned elements, at least one element selected from the group consisting of P 0.10% or less, Cr 1.0% or less, and Mo 0.5% or less. You can.

P is used to strengthen steel, but when added in excess of more than 0.1%, the steel becomes significantly brittle and the adhesion of zinc plating deteriorates.

Both Cr and Mo are added to improve the hardening performance of steel, but if too much is added, the above effect will be saturated and this is not preferable from an economic point of view.
r should be 1.0% or less, and MO should be 0.5
% or less.

Furthermore, in the present invention, the steel used has Ti0.01~
0.06%, Nb0.0.1-0.06%, and vo. It may contain at least one element selected from the group consisting of oi to 0.06%.

These elements have the effect of strengthening steel in small amounts, and also have the effect of expanding the temperature range of recovery annealing to the high temperature side. However, even if each element is added in an amount exceeding 0.6%, the above-mentioned effects will be saturated, which is not preferable from an economic point of view. Moreover, the amount of +Ji' products increases, deteriorating bending workability.

Next, manufacturing conditions in the method of the present invention will be explained.

According to the method of the present invention, a slab having the above-mentioned chemical properties is obtained by normal ingot forming or continuous casting, and then first, a ferrite-bainite composite structure hot-rolled steel sheet is obtained by hot rolling. In the case of a steel sheet with a ferrite/pearlite structure, since barlite has a very hard structure, cracks are likely to occur during bending at a smaller amount of strain than in a ferrite/bainite structure. That is, by forming the steel sheet with a ferrite-bainite structure, it is possible to increase local elongation and provide excellent bending and workability compared to a steel sheet with a ferrite-pearlite structure.

FIG. 1 shows the relationship between the tensile strength and hole expansion ratio (λ) of a ferrite-bainite steel plate and a ferrite-bainite steel plate having the same precepts (invention steel A in Table 1) and a ferrite-bainite steel plate.

In the method of the present invention, as mentioned above, in order to obtain a ferrite-bainite mum plate by hot rolling, the finishing temperature is set in the range of 800 to 1000°C, preferably in the range of 850 to 950°C. , the average cooling rate until winding is 20 to 100" C/sec, and the winding is 400 to 6" C/sec.
It is carried out in the range of 00°C.

When the finishing temperature is lower than 800°C, it is impossible to obtain the desired paynite structure, while when it exceeds 1000°C, the austenite grain size becomes large and the structure becomes coarse. Also, when the winding temperature is lower than 400°C,
Martensite grows and the deformation resistance during cold rolling becomes significantly high. When the temperature is higher than 600°C, a barite structure appears, which is not preferable.

Next, the thus obtained hot-rolled steel sheet having a ferrite 1 to payinite structure is pickled and cold-rolled at a rolling reduction of 30 to 60% to obtain a thin steel sheet.

As shown in Figures 2 and 3, in general, when hot-rolled steel sheets with the same composition are rolled at different cold rolling rates, the lower the cold rolling rate, the higher the recrystallization temperature and the wider the recovery annealing temperature range. do. In addition, the lower the cold rolling rate, the smaller the fluctuation in tensile strength with respect to the recovery annealing temperature, and the better the stress workability.

However, in order to obtain a tensile strength of 60 kgf/mm2 or more after recovery annealing, a cold rolling rate of 30% or more is required. On the other hand, when the cold rolling rate exceeds 60%, the recrystallization temperature decreases and recrystallization occurs rapidly, making it difficult to obtain a homogeneous material because the tensile strength is sensitive to the annealing temperature. Have difficulty. In the present invention, the cold rolling rate is particularly from 40 to 56
A range of % is preferred.

Thereafter, according to the present invention, recovery annealing is performed to obtain the desired high-tensile strength hot-dip galvanized steel sheet. Recovery annealing is 4
It is carried out by continuous annealing at a temperature in the range of 00 to 680 °C for 2 seconds 11 to 10 minutes, followed by
The steel strip is immersed in a hot-dip galvanizing bath at 400 to 500°C and galvanized to a tensile strength of 60 kgf/mm2.
It is possible to restore bending workability while maintaining the properties.

The high-strength hot-dip galvanized steel sheet according to the present invention thus obtained has a substantially unrecrystallized microstructure.

As mentioned above, according to the method of the present invention, the tensile strength is 60
It is possible to obtain a high-tensile galvanized steel sheet having a hole expansion rate (λ) of kgf/mm2 or more and having a high hole expansion rate (λ) that correlates with bending workability.

EXAMPLES The present invention will be described below with reference to Examples, but the present invention is not limited to these Examples in any way.

Using atmospheric melting steel having the chemical properties shown in Table 1, blooming, hot rolling and cold rolling are carried out according to conventional methods.
0. A steel plate with a thickness of 8 mm was manufactured. This steel plate was subjected to a heat treatment that resulted in hot-dip galvanizing.

In addition, for galvanizing, immerse it in a pot at 450°C for 5 seconds,
Processed. Thereafter, a JIS No. 5 tensile test piece and a hole expansion test piece were taken, and their mechanical properties were examined. The results are shown in Table 2.

Comparative steel A2 has a cold rolling rate of 80%, so it has high q-tensile strength, but since a large amount of processed structure remains, it is inferior in bending workability. Comparative steel A3 has a high annealing temperature of 7oo'c, so the black structure partially recrystallizes and softens.
Poor tensile strength. Comparative steel A4 has a coiling temperature of 700″C
Because of this, the microstructure is a ferrite/pearlite structure and has high tensile strength, but during bending, stress is concentrated at the ferrite/pearlite interface, resulting in poor bending workability.

Comparative steel B has an excessive amount of Mn, so it has high tensile strength but poor bending workability. Comparative steel C has an excessive amount of C, so it is inferior in bending workability. Comparative steel D has less Cil and too little Mnl, so it is inferior in tensile strength. Comparative steel E has tensile strength and bending workability, but the adhesion of the plating layer is poor because the amount of Si is excessive. Furthermore, comparative steel ◯ has sufficient strength and ductility, but because it contains too much Nb-t, it has a large amount of precipitates and is inferior in die workability.

Compared to these comparative steels, the hot-tempered bent lead-plated steel sheet produced by the method of the present invention has a tensile strength of at least 60 Jf/mm2, and a hole expansion ratio (λ) that correlates with the buffing processability. It is high and satisfies close bending after 45° bending with a punch (R = 0.5 mm).

FIG. 3 shows the relationship between the strength and hole expansion ratio (λ) of a steel plate produced by the conventional recovery annealing method and a steel plate produced by the present invention. According to the steel of the present invention, the hole expansion rate (λ) is improved compared to the conventional steel, and it is shown that the steel has excellent carving workability.

[Brief explanation of drawings]

Figure 1 shows Ferrite Barai 1, which has the same precepts.
A graph showing the relationship between tensile strength and hole expansion ratio (λ) for textured steel and ferrite Baini 1 textured steel. Figure 2 is a graph showing the relationship between annealing temperature and tensile strength. Figure 3 is a graph showing the relationship between annealing temperature and tensile strength. , tensile strength and hole expansion ratio (
λ). 17

Claims (3)

[Claims] (1) When hot rolling steel consisting of 0.03 to 0.12% C, 0.50% or less Si, 0.30 to 2.0% Mn, and the balance iron and unavoidable impurities in weight%, the finish rolling temperature Winding the hot rolled coil at a temperature of 800 to 1000 °C, an average cooling rate of 20 to 100 °C/sec, and a winding temperature of 400 to 600 °C,
A hot-rolled steel sheet with a ferrite-bainite composite structure was prepared, pickled, cold rolled at a rolling reduction of 30 to 60%, and then rolled at a rolling reduction of 400%.
Annealed at a temperature in the range of ~680°C for 2 seconds to 10 minutes,
A method for producing a high-tensile galvanized steel sheet having a tensile strength of 60 kgf/mm^2 or more and excellent bending workability, which comprises subsequently applying hot-dip galvanizing. (2) Contains (a) 0.03 to 0.12% of C, 0.50% or less of Si, and 0.30 to 2.0% of Mn in weight%, and (b) 0.10% or less of P, 1.0% of Cr. When hot rolling steel containing at least one element selected from the group consisting of 0.5% or less and Mo0.5% or less, with the balance consisting of iron and unavoidable impurities, the finish rolling temperature is set at 800 to 1000°C. Winding the hot-rolled coil at an average cooling rate of 20 to 100°C/sec and a winding temperature of 400 to 600°C,
A hot-rolled steel sheet with a ferrite-bainite composite structure was prepared, pickled, cold rolled at a rolling reduction of 30 to 60%, and then rolled at a rolling reduction of 400%.
Annealed at a temperature in the range of ~680°C for 2 seconds to 10 minutes,
A method for producing a high-tensile galvanized steel sheet having a tensile strength of 60 kgf/mm^2 or more and excellent bending workability, which comprises subsequently applying hot-dip galvanizing. (3) Contains (a) 0.03 to 0.12% of C, 0.50% or less of Si, and 0.30 to 2.0% of Mn in weight%, and (b) 0.01 to 0.06% of Ti. , Nb0.01-0.06%, and V0.01-0.06% When hot rolling steel containing at least one element selected from the group consisting of: Winding the hot-rolled coil at a finish rolling temperature of 800 to 1000°C, an average cooling rate of 20 to 100°C/sec, and a winding temperature of 400 to 600°C;
A hot-rolled steel sheet with a ferrite-bainite composite structure was prepared, pickled, cold rolled at a rolling reduction of 30 to 60%, and then rolled at a rolling reduction of 400%.
Annealed at a temperature in the range of ~680°C for 2 seconds to 10 minutes,
A method for producing a high-tensile galvanized steel sheet having a tensile strength of 60 kgf/mm^2 or more and excellent bending workability, which comprises subsequently applying hot-dip galvanizing.