KR940000873B1 - Method for making a hot-dipped zinc coating steel sheet of high strength steel - Google Patents

Abstract

The method is characterised by comprises cold rolling the hot rolled steel sheet in the cold reduction rate of above 75 %, recovery annealing the cold steel sheet at 580-620 deg.C for 30-60 sec., and conventionally gavalnizing the annealed steel sheet. The steel sheet comprises (in wt.%) up to 0.01 % C, 0.2-0.5 % Mn, 0.02-0.08 % P, up to 0.006 % N, up to 0.03 % Si, up to 0.02 % S, 0.02-0.06 % Ti and balance Fe with inevitable impurities. The galvanized steel sheet has excellent workability, stable adhesion property of the galvanized layer, and tensile strength of above 70 kg/mm2.

Description

Manufacturing method of high tensile hot dip galvanized steel sheet

1 and 2 are graphs showing the change in hardness according to the annealing temperature.

3 is a graph showing the change in tensile strength according to the cold rolling rate.

The present invention relates to a method for producing a high tensile hot dip galvanized steel sheet used for building exterior materials, and more particularly, to a method for manufacturing a high tensile hot dip galvanized steel sheet having a tensile strength of 70 kg / mm 2 or more having excellent processability and stable plating adhesion. It is about.

High-strength materials of hot-dip galvanized steel sheet are mainly used for building exterior materials and in recent years, the trend of favoring thin-walled products with beautiful surface and strict shape quality is due to the high quality of life. In addition to securing the processability is difficult.

In general, the method of producing high-strength material is a method of producing martensite and ferrite by annealing the steel sheet in an ideal region (θ + γ) and then quenching it to give high strength and ductility to the steel sheet. It is difficult to secure stable plating adhesion during hot dip galvanizing by low temperature treatment.

Secondly, there is a partial recrystallization annealing manufacturing method that secures the necessary material by completing annealing and plating after re-crystallization below the recrystallization completion temperature after cold rolling. Management is required, and material deviations occur largely even at the slight annealing temperature, which is undesirable in many manufacturing mills, which are accompanied by productivity and workability load during production or operation.

Thirdly, there is a method of adding a large amount of strength increasing elements (C, Mn, P, Nb, Ti, V) to obtain desired strength after complete recrystallization annealing. However, there is a problem in that the application of the mass production process, which emphasizes commerciality, such as workability and productivity decrease in the preceding process, has limitations.

Fourth, it is also possible to prevent the decrease in strength by performing recovery annealing (550-580 ° C) on steel sheets that have been worked at very low cold pressure (15-25%) in a range where the size of hot-rolled ferrite grains does not change in general low carbon steel However, this method has a problem that it is difficult to apply in the actual production process due to the limitation of the thickness of the material used to apply the cold rolled thickness with a cold reduction rate of 15-25%.

Thus, the present inventors conducted research and experiments to improve the problem, and based on the results, the present invention is proposed, the present invention is a high strength hot-dip galvanized steel sheet with a tensile strength of 70kg / ㎜ or more excellent in workability and plating adhesion To provide a method for producing, the purpose is.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is a method for producing a high-strength hot-dip galvanized steel sheet, by weight% C: 0.01% or less, Mn: 0.2-0.5%, P: 0.020-0.080%, N: 0.006% or less, Si: 0.03% or less, S: 0.02% or less Ti: 0.02-0.06%, remainder Fe and other unavoidably contained steels are hot rolled by a conventional method, cold rolled at a cold reduction rate of 75% or more, and then at a temperature of 580-620 ° C. The present invention relates to a method for producing high tensile hot dip galvanized steel sheet by recovering annealing heat treatment for 30-60 seconds and hot-dip galvanizing in a conventional manner.

The reason for the numerical limitation will be described below.

When the carbon (C) is added to 0.01% or less, the strength can be sufficiently secured, but since the ductility is poor workability worsens, the content is preferably limited to 0.01% or less.

Since the sulfur (S) is a component that causes redness brittleness, it is preferable to contain as little as possible, but even if contained up to about 0.02%, the effect of the present invention is shown, the content is preferably limited to 0.02% or less.

When the manganese (Mn) is 0.2% or less, it is not enough to remove S in the form of sulfide, so 0.2% or more should be added, and the upper limit thereof is limited to 0.5% considering that the maximum content of S is 0.02%. Even if S can be sufficiently removed, the Mn content is preferably limited to 0.2-0.5%. The silicon (Si) is preferably limited to 0.03% or less because it lowers the ductility and plating adhesion.

Since phosphorus (P) is first deposited at the surface grain boundary during annealing, it is a component that prevents deterioration of plating adhesion due to excessive alloy layer growth due to Ti segregation of the surface grain boundary, and should be contained at least 0.020% to obtain such an effect. In the case of more than 0.080%, the ductility is poor and the workability deteriorates. Therefore, the phosphorus content is preferably limited to 0.020-0.080%.

The nitrogen (N) is a component that reduces the ductility, the content is preferably limited to 0.0060%.

The titanium (Ti) is a component added to obtain a recrystallization temperature increase effect, the addition amount is less than 0.02% or less, and when it is more than 0.06%, the recrystallization temperature increase effect width is slowed and uneconomical, the addition amount is It is desirable to limit it to 0.02-0.06%.

If the cold reduction rate is less than 75%, since the amount of deformation is too small to secure sufficient accumulated energy, it is difficult to obtain a tensile strength of 70kg / mm2 or more after recovery annealing, so the cold reduction rate during cold rolling is limited to 75% or more. It is preferable.

When the annealing temperature is 580 ° C or lower, considering the optimum temperature of the direct heating cleaning is about 600-700 ° C, it is difficult to secure adhesion, and when it is 620 ° C or higher, the recrystallization temperature is higher. Therefore, the annealing temperature is preferably limited to 580-620 ℃. The annealing time is preferably maintained at about 30-60 seconds at a temperature of 580-620 ℃ in consideration of plating adhesion and mechanical properties.

On the other hand, the steel having a composition range of the present invention described above is hot rolled by a conventional method, cold-rolled at a cold reduction rate of 50-70%, and then subjected to recovery annealing heat treatment for 30-60 seconds at 580-620 ° C. By hot-dip galvanizing by the method of the above, it is also possible to obtain a hot-dip galvanized steel sheet of 50-60kg / mm2 tensile strength.

Hereinafter, the present invention will be described in detail through examples.

Example 1

The steel composition as shown in Table 1 is hot-rolled and cold-rolled under the conditions of hot rolling, cold rolling and annealing of Table 1, recovery and annealing are performed, and mechanical properties and plating deposition amount are measured. 2 is shown.

TABLE 1

TABLE 2

As shown in Table 2, the present invention (1-5) is cold-rolled steel having a composition range in accordance with the present invention at a cold reduction rate of 79-83% included in the cold reduction rate range of the present invention The mechanical properties of yield strength 62-70kg / mm2, tensile strength 70-74kg / mm2, and total elongation 8-9% are shown, and also it can be seen that it is good also in plating adhesion.

On the other hand, the steel outside the composition range of the present invention is cold rolled at a reduction ratio of 80% [Comparative Material (2-3)], or the steel having a composition range in accordance with the present invention is outside the cold reduction rate range of the present invention. In case of cold rolling at 60% rolling reduction [Comparative Material (1)], it shows mechanical properties of yield strength 40-50kg / mm2, tensile strength 46-60kg / mm2, and total elongation 7-12%. In the case of (2), it can be seen that cracks are generated in the microplating layer.

Example 2

Steel having a chemical composition as shown in Table 3 below was hot rolled at a hot finish rolling temperature of 910 ° C. and a coiling temperature condition of 680 ° C., and cold rolled at a cold rolling reduction ratio shown in Table 3, followed by FIGS. 1 and 2. The annealing heat treatment was carried out while varying the annealing temperature as shown in the drawing. In order to observe the recrystallization temperature change, the change in hardness value according to the annealing temperature was measured and the results are shown in FIGS. 1 and 2.

TABLE 3

As shown in FIG. 1 and FIG. 2, unlike the present invention material (6) and the comparative material (4-7) to which Ti was added, it can be seen that the recrystallization temperature was increased to 580-650 ° C by the addition of Ti. This is because recrystallization is suppressed by precipitation of denitride at grain boundaries during annealing.

Example 3

C: 0.004%, Si: 0.02%, Mn: 0.26%, P: 0.07%, S: 0.01%, Ti: 0.05%, remainder Fe, and other inevitable hot-rolled steel at 910 ° C Hot-rolled at the temperature and winding temperature of 680 ℃ and cold-rolled with varying cold reduction rate as shown in FIG. 3, then annealing heat treatment at pre-heating temperature of 570 ℃ and heating temperature of 590 ℃ and tensile strength The measurement result is shown in FIG.

As shown in FIG. 3, it can be seen that the tensile strength of 70 kg / mm 2 or more is obtained only when the cold reduction rate is 75% or more.

As described above, the present invention provides a yield strength of 60-70kg / mm 2, a tensile strength of 70-80kg / mm 2; And it is effective to provide a high-strength hot-dip galvanized steel sheet excellent in workability and plating adhesion of 7% or more of the total elongation.

Claims (1)

In the method of manufacturing high tensile hot-dip galvanized steel sheet, C: 0.01% or less, Mn: 0.2-0.5%, P: 0.02-0.080%, N: 0.006% or less, Si: 0.03% or less, S: 0.02 by weight % Or less, Ti: 0.02-0.06%, remainder Fe and other unavoidably contained steels are hot rolled by a conventional method, cold rolled to a cold reduction rate of 75% or more, and then 30- at a temperature of 580-620 ° C. A method for producing a high tensile strength hot dip galvanized steel sheet, characterized in that the recovery annealing heat treatment for 60 seconds and hot-dip galvanizing in a conventional manner.