JP2587725B2 - Method for producing P-containing high tensile alloyed hot-dip galvanized steel sheet - Google Patents

Description

The present invention relates to a method for producing a P-containing high-tensile alloyed hot-dip galvanized steel sheet. More specifically, the present invention relates to a method for producing a P-containing high-strength galvannealed steel sheet.
Pre-annealing by rapid oxidation to ensure the properties of galvannealed steel sheet for P-containing steel sheet of 04% or more, then hot-dip coating after annealing in hydrogen-containing atmosphere.
The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet.

(Prior Art) In recent years, cold-rolled steel sheets used as automotive steel sheets include:
From the standpoint of weight reduction and safety improvement and corrosion resistance of the car body, in addition to being excellent in press workability, it has high strength,
In addition, corrosion resistance is required, and surface treatment of the supplied steel material is strongly required. Among these, particularly recently, the demand for surface treatment of high-tensile steel sheets is increasing. As a surface treatment method mainly aimed at improving the rust-preventive force of the high-tensile steel sheet, there is a sendzimer-type hot-dip zinc plating method capable of easily performing thick plating from the viewpoint of productivity. In the case of this Zenzimer hot-dip galvanizing method, after the iron oxide film generated in the non-oxidizing furnace is reduced in the subsequent reduction zone, S, which is an easily oxidizable element in steel,
i, Mn, P, and Al are selectively oxidized on the surface of the steel sheet, thereby causing a concentration gradient. Of these elements, P forms a PO _x film on the steel sheet surface,
The alloying characteristics are significantly impaired (uneven alloying occurs locally). As a conventional method of improving this, prior to annealing, a method of pre-plating Fe, Zn or Ni and removing the rolling oil on the steel sheet surface in an oxidation furnace containing oxygen, and after forming an appropriate oxide film, After reduction annealing in an atmosphere containing hydrogen,
A method of adjusting the plate temperature in a furnace and performing plating has already been disclosed in JP-A-55-1.
It is known from 22865. In other words, in the non-oxidizing furnace system that does not contain oxygen, oil on the steel surface can be removed, but since the oxidizing atmosphere is weak, Si, Mn, and Al that are easily oxidized are diffused and oxidized on the surface. The object forms the steel surface. These oxides are not reduced in the reduction furnace and cause poor plating wetting and poor plating adhesion. For this purpose, the surface of the steel sheet is oxidized so as to have an oxide film thickness of 400 to 10,000 °, then annealed in an atmosphere containing hydrogen, and hot-dip plated.

(Problems to be Solved by the Invention) However, in the prior art method of pre-plating Fe, Zn or Ni before annealing, the pre-plating film is unstable, the atmosphere in the furnace is contaminated, and plating is performed depending on the type of steel. There are difficulties such as having to use different liquids,
Further, as for the latter JP-A-55-122865, it is said that, as described above, heating is performed by increasing the air ratio of the non-oxidizing furnace, forming an Fe oxide film, and then reducing and heating, thereby obtaining good plating properties. Only knowledge, no disclosure of the relationship between P in steel and the galvannealed layer.
Furthermore, there is a method of alloying the entire steel sheet by lowering the line speed in the actual operation line and lengthening the furnace time in the alloying furnace, but this method is used.
The line speed must be reduced by 20 to 30%, and there is a problem that productivity is remarkably reduced.

Therefore, the present invention provides a method for obtaining an alloyed hot-dip galvanized steel sheet having a stable quality and an excellent uniform appearance without any alloy unevenness by a method different from the conventional method in a line of high productivity. It is in.

(Means for Solving the Problems) In order to solve the above problems and achieve the object, the gist of the present invention is to provide a high-tensile galvanized steel sheet containing P having a P concentration of 0.04% or more in steel. In the production method of the above, after rapid oxidation to an oxidation zone combustion air ratio of 0.95 to 1.10 and an average oxidation rate of 30 酸化 / sec or more in the oxidation zone excluding the pre-tropical zone, annealing in an atmosphere having a reduction zone hydrogen concentration of 10% or less is performed. And hot-dip plating. Hereinafter, the present invention will be described in detail.

In the present invention, the reason why the P concentration in the steel is 0.04% or more is a desirable element because it has the effect of solid solution strengthening as a high-tensile steel, the action of refining the crystal grains of a hot-rolled sheet, and the cost is low. In particular, it is an important element because it has a large effect of reducing ductile deterioration due to skin pass. To achieve this effect, 0.04% or more is required. On the other hand, if it exceeds 0.15%, excessive P becomes brittle, The optimum value is desirably 0.04 to 0.15 in order to cause deterioration in weldability. On the other hand, in the case of a P-containing steel having a P concentration of 0.04% or more in the steel, it is generally called a non-alloyable material, and Si, Mn, Al, P, etc. in the steel are heated by heating the steel sheet surface.
Since these oxides are diffused into the surface layer of the steel sheet, these oxides are concentrated and form a steel surface. These oxides are not reduced even in a reduction furnace, impair alloying properties, and deteriorate plating adhesion. Accordingly, it is an object of the present invention to provide hot-dip galvanizing for steel materials intended for these difficult-to-alloy materials in a high-productivity line without alloy unevenness and excellent uniform appearance. First, the oxidation air combustion ratio was 0.95 to 1.10. Second, the average oxidation rate in the oxidation zone, which is the most important feature of the present invention, is set to 30 ° / sec or more. Third, annealing and hot-dip plating are performed in an atmosphere in which the hydrogen concentration in the reduced zone is 10% or less. That is, the first
The condition that the oxidation air combustion air ratio was 0.95 to 1.10,
If the air ratio is less than 0.95, the oxidation behavior changes greatly due to a slight change in the gas composition, and it is not possible to adjust the optimal target oxide film of 200 to 1000 mm. When the air ratio exceeds 1.10, an oxide film is rapidly generated, easily exceeds the target oxide film, and it is difficult to adjust this too. Therefore, the air ratio should be within the range where the production of the oxide film as the optimum target can be easily controlled, and then the second condition, the average oxidation rate in the oxidation zone excluding the pre-tropical zone, which is the largest feature, should be 30Å. / sec or more. The reason for this is as shown in the conventional method A as shown in FIG.
For example, in the case of 20 mm / sec or less, since the formation rate of the oxide film is slow, Si, Mn, Al, and P, which are easily oxidized, are diffused and oxidized on the surface before forming an appropriate oxide film on the steel sheet surface. These oxides form on the steel surface. Therefore, they have found that rapid oxidation by heating for a short time without generating these oxides is necessary. In addition, considering the necessary oxide film, it is necessary to have a straight line B of 30 ° / sec or more shown in FIG. Above this level, Si, Mn,
Al, P, etc. Especially on iron surface before P is not diffused and oxidized on the surface
A Fe oxide film is formed, and this Fe oxide film is
Prevents oxidation of n, Al, P, etc. Thereafter, the oxide film is annealed in an atmosphere having a reduced hydrogen concentration of 10% or less, reduced, passed through a molten zinc bath, and adjusted in plating amount by air wiping. In order to achieve rapid oxidation at an average oxidation rate of 30Å / sec or more, a so-called direct-fired heating burner, which is a burner that injects a flame perpendicular to the steel sheet, controls the more optimal average oxidation rate. be able to.

(Function) In the present invention, the Fe oxide, which is an easily oxidizable element due to rapid oxidation, in particular, an oxide of P is not concentrated on the surface layer of the steel sheet, and only Fe oxide is generated. Thus, the Fe oxide film which is reduced by the reduction zone and hinders the alloying characteristics is eliminated, and the uniformity of the appearance after the galvannealing can be achieved.

(Example) Example 1 High tension 60 of high tension 60 kg / cm ² class (C 0.10%, Si 0.
6%, Mn 1.10%, P 0.04%, Al 0.03%) is heated and oxidized to 500 ° C at an oxidizing zone combustion air ratio of 1.05 and an average oxidizing rate of 400Å / 6sec, then reduced at a reducing zone hydrogen concentration of 10%, and then melted After passing through a zinc bath (0.15% Al, 450 ° C.), the amount of plating was adjusted to 180 g / m ² by air wiping, and then an alloying treatment was performed. The alloying properties of the galvannealed steel sheet were examined. The alloying property was evaluated by a cross-sectional photograph of the plated portion. As a result, as shown in FIG. 2, when compared with the conventional case where the average oxidation rate was 20 ° C./sec under the same conditions, according to the evaluation shown in Table 1, some alloy unevenness or alloy unevenness was found. Was seen a lot. On the other hand, according to the present invention, uniform appearance and alloy unevenness were not observed at all.

Example 2 High tension 80 kg / cm ² Hyten 80 (C 0.15%, Si 1.0%, Mn
1.5%, P 0.10%, Al 0.02%)
After oxidizing by heating to 500 ° C at an average oxidation rate of 500Å / 12sec, it is reduced at a reducing zone hydrogen concentration of 5%, and then molten zinc bath (0.10Al
460 ° C) and air wiping to reduce the plating amount
After adjusting to 180 g / cm ² , alloying treatment was performed. The alloying properties of the galvannealed steel sheet were examined. As a result, in the alloying of the plating, there was no alloy unevenness at all from the cross-sectional photograph, and four points were obtained as a result.

(Effect of the Invention) As described above, by performing rapid heating and oxidation at the average oxidation rate according to the present invention, even in the case of a P-containing high-strength steel sheet, the alloying hot-dip galvanizing conditions are not changed unnecessarily. Another advantage is that the alloying properties similar to those of ordinary steel are obtained, and the performance to be provided as an alloyed hot-dip galvanized steel sheet having a stable quality and a uniform appearance can be obtained sufficiently efficiently without uneven alloy plating.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the formed oxide film thickness and time according to the present invention and the conventional method, and FIG. 2 is a diagram showing the relationship between the average oxidation rate and the alloying property evaluation in the oxidation zone excluding the pre-tropical zone. It is.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical indication location C23C 8/14 C23C 8/14 (72) Inventor Katsunori Kawaguchi 1 Kimitsu, Kimitsu-shi, Chiba Prefecture Made in New Japan Inside the Kimitsu Works of Steel Corporation (72) Inventor Masaji Aiba 1 Kimitsu Works, Kimitsu City, Chiba Prefecture Inside the Kimitsu Works of Nippon Steel Corporation (56) References JP-A-55-131167 (JP, A) Tokiko Sho 53-44141 (JP, B2)

Claims (2)

(57) [Claims] 1. A method for producing a high-tensile alloyed hot-dip galvanized steel sheet having a P concentration of 0.04% or more in steel, comprising:
After rapid oxidation at 0.95 to 1.10 and an average oxidation rate of 30 酸化 / sec or more in the oxidized zone excluding the pre-tropical zone, the reduced zone hydrogen concentration was 10
% Of P-containing high-strength alloyed hot-dip galvanized steel sheet, which is annealed in an atmosphere of not more than 10% and hot-dip coated. 2. The heating of the oxidation zone is rapidly oxidized using a burner that injects a flame perpendicularly to the steel sheet, and then annealed in an atmosphere having a hydrogen concentration in the reduction zone of 10% or less and hot-dip plating. 2. The method for producing a P-containing high tensile alloyed hot-dip galvanized steel sheet according to claim 1.