JP2002275539A - Method for producing galvanized steel sheet having good appearance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a galvanized steel sheet which has good apperance, and in which the generation of stripy defects can advantageously be prevented. SOLUTION: A steel slab having a composition containing, by mass, <=0.0050% C, <=0.10% Si, <=0.30% Mn, 0.010 to 0.050% Ti, 0.001 to 0.030% Nb, 0.010 to 0.070% Al, <=0.0050% N, <=0.030% P and <=0.010% S, and the balance iron with inevitable impurities is hot-rolled. After that, strain is introduced into a surface layer region to a depth of 20 μm from the surface of the hot rolled sheet, and then, cold rolling is performed thereto. Subsequently, the steel sheet is heat-treated, and is next subjected to galvanizing treatment.

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, particularly a hot-dip galvanized steel sheet having a good appearance.

[0002]

2. Description of the Related Art A hot-dip galvanized steel sheet has a surface protected by a galvanized coating, and is excellent in a so-called sacrificial anticorrosion effect in which zinc, which is electrochemically lower than iron, is corroded and dissolved in preference to iron. It has excellent corrosion resistance. Therefore, hot-dip galvanized steel sheets are widely used, including materials for construction and automobiles where corrosion resistance is desired.

When such a hot-dip galvanized steel sheet is used for an outer panel of an automobile, not only excellent corrosion resistance but also beautiful surface of the steel sheet is strongly required. This is because the beauty of the steel plate surface is directly linked to the commercial value of the car. Furthermore, in recent years, the momentum for preserving the global environment has increased, and steel sheets for automobiles have been reduced in weight by increasing the tension. Therefore, there is a demand for a high-strength hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet having an excellent appearance for use as an automobile outer panel.

However, in the conventional hot-dip galvanized steel sheet, streaks extending in the rolling direction of the steel sheet occur on the surface on which the hot-dip galvanizing or alloyed hot-dip galvanizing is formed, and the appearance of the steel sheet is hindered. That was seen. With respect to such streak-like surface defects, it has been practiced in steel sheet consumers to make the streak-like defects inconspicuous by increasing the film thickness of a paint applied on the surface of the galvanized steel sheet. However, such a method is not an essential method for resolving streak defects. In recent years, the thickness of a coating film has been reduced with the aim of improving productivity and reducing costs. It has become necessary to eliminate shape defects themselves.

Japanese Patent Laid-Open Publication No. Hei 8-20852 discloses a method for preventing the occurrence of streak patterns on the surface of hot-dip galvanized steel by forming equiaxed crystal grains and elongated crystal grains on a surface layer ranging from the steel surface to 50 μm. An alloyed hot-dip galvanized steel sheet having an area ratio of 10% or less or 90% or more of mixed crystal grains has been proposed. However, the method of manufacturing an alloyed hot-dip galvanized steel sheet disclosed in Japanese Patent Application Laid-Open No.
Neither the component composition range of the material nor the manufacturing process is the same as a normal method of manufacturing a galvannealed steel sheet, and in such a normal method of manufacturing a galvannealed steel sheet, the steel sheet surface still remains. Streak-like defects sometimes occurred.

[0006]

SUMMARY OF THE INVENTION Accordingly, the present invention advantageously solves the above-mentioned problems, and provides an external appearance capable of advantageously preventing the occurrence of the above-mentioned streak defect by adding a device to the manufacturing process. It is an object of the present invention to propose a method for producing a hot-dip galvanized steel sheet having a good quality.

[0007]

Means for Solving the Problems The inventors of the present invention have investigated the structure of a steel sheet in order to solve the streak defect of a hot-dip galvanized steel sheet. It has been found that the alloying rate during the plating process is different from the steady portion because the unrecrystallized structure partially remains even after the heat treatment. Accordingly, the present inventors have made intensive studies to prevent the occurrence of an unrecrystallized structure in the surface layer of the steel sheet, and have completed the present invention.

That is, the gist of the present invention is as follows. (1) C: 0.0050 mass% or less, Si: 0.10 mass% or less, Mn:
0.30 mass% or less, Ti: 0.010 to 0.050 mass%, Nb: 0.00
1 to 0.030 mass%, Al: 0.010 to 0.070 mass%, N: 0.
0050 mass% or less, P: 0.030 mass% or less and S: 0.01
After hot rolling a steel slab consisting of iron and unavoidable impurities, the strain is introduced into the surface layer area up to a depth of 20 μm from the surface of the hot rolled sheet, A method for producing a hot-dip galvanized steel sheet having a good appearance, which comprises rolling, followed by heat treatment and then hot-dip galvanizing.

(2) C: 0.0050 mass% or less, Si: 0.10 mass
%, Mn: 0.30 mass% or less, Ti: 0.010 to 0.050 mass
%, Nb: 0.001 to 0.030 mass%, Al: 0.010 to 0.070 ma
ss%, N: 0.0050 mass% or less, P: 0.030 mass% or less, and S: 0.010 mass% or less, and B: 0.002 to 0.1%.
After hot rolling a steel slab containing iron and unavoidable impurities, the strain is introduced into the surface layer up to a depth of 20 μm from the surface of the hot rolled sheet, and then cold rolled. A hot-dip galvanized steel sheet having a good appearance, which is then subjected to a heat treatment and then to a hot-dip galvanizing treatment.

(3) The method for producing a hot-dip galvanized steel sheet having a good appearance according to the above (1) or (2), wherein shot blasting is performed to introduce distortion into the surface layer.

(4) The method for producing a hot-dip galvanized steel sheet having a good appearance according to any one of the above (1) to (3), wherein an alloying treatment is performed after the plating treatment.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have investigated the cause of the unrecrystallized structure of the surface layer of a steel sheet, which is the cause of streaky irregularities, and obtained the following inference. In the production process of a conventional hot-dip galvanized steel sheet, the finish rolling end temperature of hot rolling has been set to an Ar ₃ transformation point or higher. This is for the purpose of preventing the appearance of a transformed phase such as ferrite. Then, from the viewpoint of reducing the crystal grain size at the end of finish rolling and giving good workability to the final steel sheet, the finish rolling end temperature of hot rolling is usually set to just above the Ar ₃ transformation point. Was.

However, according to the findings of the inventors' research,
If the steel plate being rolled is allowed to cool to the atmosphere,
Due to heat transfer, contact with cooling water, etc.
The heat is removed preferentially over the central part in the
Finish rolling as in the production method of hot-dip galvanized steel sheet
Ar temperature_Three When placed just above the transformation point,
Ar at the end of finish rolling_Three The temperature is below the transformation point
There were cases. In other words, the conventional finish rolling end temperature is
The average temperature in the direction _Three Transformation point
Even if finish rolling is finished at the average temperature just above,
The temperature of the plate surface layer is partially Ar_Three May be below the transformation point
It was. Thus, the finish rolling end temperature is Ar_Three transformation
If it falls below the point, the steel sheet surface part will be
Orientation that is difficult to recrystallize by heat treatment (annealing)
Therefore, even if continuous annealing is performed, it remains unrecrystallized
I will. In addition, the finish rolling temperature is Ar_Three Transformation point
The lower part is the next cold area compared to other areas.
It is also a region where work hardening is easy during rolling. In this way,
Top rolling end temperature is Ar_Three The part below the transformation point is the other part
Since the crystal structure is different from the
The reaction rates in the processing and alloying process differ,
As a result, it was considered that the surface was uneven and became a streak defect.
available.

The reason why the unrecrystallized structure remains on the surface layer of the steel sheet is that the temperature of the surface layer of the steel sheet may partially fall below the Ar ₃ transformation point. It can be considered that if the heat treatment can be reduced, recrystallization in the surface layer of the steel sheet can be performed even by the conventional heat treatment, and the streak-like surface defects are eliminated. Therefore, when the inventors eagerly investigated how to lower the recrystallization temperature in the surface layer of the steel sheet, in addition to the strain caused by the cold rolling, it was found that introducing strain into the surface layer of the steel sheet after hot rolling was extremely effective. The present invention has been completed.

Hereinafter, the constituent features of the present invention will be described more specifically. First, the component composition range of the material is limited for the following reasons. C: 0.0050 mass% or less C is a component that deteriorates the workability, and among the properties required as a steel sheet for processing, the most important total elongation (El.),
In order to improve the Rankford value (r value), the smaller the C content, the better, and the content is set to 0.0050 mass% or less.

Si: 0.10 mass% or less When Si is increased, the wettability of the plating is reduced and the surface properties are deteriorated.

Mn: 0.30 mass% or less If Mn is contained in a large amount, the r value is reduced and the plating wettability is deteriorated. Therefore, the upper limit is set to 0.30 mass%.

Ti: 0.010 to 0.050 mass% Ti is a carbonitride forming component and fixes solid solution C and N in steel as precipitates, thereby advantageously preventing aging deterioration and stretcher strain. In order to effectively exert such an effect, Ti is contained in an amount of 0.010 mass% or more. However, if the addition is excessive, the improvement in quality is saturated and the cost increases, so the upper limit is set to 0.050 mass%.

Nb: 0.001 to 0.030 mass% Nb is a carbonitride forming component like Ti,
N is an effective component for fixing steel as a precipitate to clean the steel and improve the workability of the steel sheet. In order to sufficiently exhibit such an effect, the amount of Nb must be 0.001 mass% or more. However, if Nb is contained excessively, the recrystallization temperature of steel will increase, so the upper limit of the amount of Nb is 0.030 ma.
ss%.

Al: 0.010 to 0.070 mass% Al plays a role of a deoxidizing agent and is added to fix oxygen in steel. However, excessive addition not only saturates the effect but also increases the cost.
The range is 070 mass%.

N: 0.0050 mass% or less N is an impurity component, and is a component that deteriorates workability and causes aging deterioration and stretcher strain. Therefore, it is desirable to reduce N as much as possible. However, if the content is 0.0050 mass% or less, no excessive adverse effect is exerted. Therefore, up to 0.0050 mass% is allowed in consideration of N reduction cost.

P: 0.030 mass% or less Since P inhibits alloying, P is set to a range of 0.030 mass% or less.

S: 0.010 mass% or less S is preferable to be suppressed as much as possible because it deteriorates the ductility of steel. However, it is necessary to make S 0.010 mass% or less so as not to impair the effects of the present invention. .

Further, in addition to the above basic components, B can be added in the following range. B: 0.0002 to 0.0010 mass%, B is a component that increases the strength of the crystal grain boundary by concentrating at the crystal grain boundary and improves the brittleness. In particular, in the case of the ultra-low carbon P-added steel in the steel sheet of the present invention, P is concentrated at the grain boundaries, the grain boundary strength is reduced, and secondary work brittle cracking may occur. Therefore, in the case of such a component system, B
B is preferentially concentrated at the grain boundaries by adding
Secondary working brittleness can be effectively improved. In order to exert such an effect, B must be 0.0002 mass% or more. However, if added excessively, the recrystallization temperature of steel will be increased, so that the content of B is set in the range of 0.0002 to 0.0010 mass%.

The steel slab having the above composition is first subjected to hot rolling. The conditions for this hot rolling need not be particularly restricted, and may be in accordance with ordinary conditions. For example,
Heating furnace extraction temperature is 1000 ~ 1300 ℃, hot rolling end temperature is 8
It can be carried out at a temperature of 100 to 1000 ° C and a winding temperature of 500 to 800 ° C.

Next, after hot rolling, the surface of the hot rolled sheet is
It is important to reduce the recrystallization temperature in the surface layer of the steel sheet by introducing strain into the surface layer up to a depth of 20 μm, and to enable recrystallization in the surface layer of the steel sheet even by a conventional heat treatment. In other words, the streak defect has a problem that the crystal of the surface layer does not recrystallize even after annealing after cold rolling, and it is not necessary to apply extra strain to the inside of the steel sheet. By introducing strain into the surface layer area up to a depth of 20 μm from the surface of the hot rolled sheet, all are recrystallized by annealing after cold rolling.

As a method for introducing distortion, it is preferable to use a shot blast process. In this shot blasting treatment, steel particles having a diameter of 100 to 800 μm, more preferably 297 to 710 μm, are projected at an amount of 600 kg / m ² · min.
It is advantageous to carry out under the conditions described above and a projection time of 1 min or more.

After the introduction of the strain, cold rolling is performed. From the viewpoint of securing the deep drawability,
It is desirable to be 60% or more.

After the cold rolling, heat treatment for annealing is performed. This heat treatment is preferably performed in a continuous annealing line, and the lower limit of the annealing temperature is preferably about 800 ° C. as a temperature at which recrystallization is sufficiently completed. On the other hand, if the annealing temperature is too high, the grain growth will proceed too much and may soften more than necessary. Therefore, it is desirable to set the upper limit to about 900 ° C. Subsequent plating and alloying performed as necessary may be performed according to a conventional method.

[0030]

[Example] C: 0.0020 mass%, Si: 0.004 mass%, Mn:
0.12 mass%, P: 0.012 mass%, S: 0.005 mass%, A
l: 0.032 mass%, Ti: 0.029 mass%, Nb: 0.003 mass
% And N: 0.0020 mass%, the balance being iron and unavoidable impurities, steel slabs A and C: 0.0018 mass%,
Si: 0.002 mass%, Mn: 0.10 mass%, P: 0.010 mass
%, S: 0.006mass%, Al: 0.034mass%, Ti: 0.035m
ass%, Nb: 0.003 mass%, B: 0.0003 mass%, and N:
Steel slab B containing 0.0022 mass%, the balance being iron and unavoidable impurities, respectively, was extracted at a heating furnace extraction temperature of 1000 to 13
00 ° C, hot rolling end temperature 800-1000 ° C and winding temperature 5
After hot rolling at a temperature of 00 to 800 ° C., shot blasting is performed under the following conditions to introduce strain into the surface layer (a region from the surface to a depth of 20 μm) of the hot-rolled sheet, and then reduce the rolling rate to 80%. % Cold rolling, followed by annealing at 860 ° C. for 60 seconds in a continuous hot-dip galvanizing line, followed by a basis weight of 45 g / m ^2.
Was subjected to a hot-dip galvanizing treatment and subsequently an alloying treatment.
As a comparison, a hot-dip galvanized steel sheet was manufactured according to the same manufacturing process except that the shot blast treatment was not performed.

Note: Shot: steel ball (particle size: 297 to 710 mμm) Projection amount: 625 kg / m ² · min Projection time: 1 min

The surface of the alloyed hot-dip galvanized steel sheet thus obtained was visually observed for the presence or absence of streak defects. In any of the steel types, no streak defects were generated, but strain was introduced. Streak defects were observed on the surface of the steel sheet not subjected to the heat treatment.

[0033]

As described above, according to the present invention, a hot-dip galvanized steel sheet having no streak defect and having an excellent appearance can be stably provided with respect to a hot-dip galvanized steel sheet requiring a good appearance such as an automobile outer panel. Can be manufactured.

Continued on the front page F term (reference) 4K037 EA01 EA02 EA04 EA15 EA18 EA19 EA23 EA25 EA27 EA31 EB01 EB02 EB08 FA02 FA03 FC03 FC04 FE01 FE02 FE03 FG10 FJ06 GA05 HA05 JA06

Claims (4)

[Claims] C: 0.0050 mass% or less, Si: 0.10 mass% or less, Mn: 0.30 mass% or less, Ti: 0.010 to 0.050 mass%,
Nb: 0.001 to 0.030 mass%, Al: 0.010 to 0.070 mass
%, N: 0.0050 mass% or less, P: 0.030 mass% or less, and S: 0.010 mass% or less, and the rest is subjected to hot rolling on a steel slab composed of iron and unavoidable impurities, and then the surface of the hot-rolled sheet. A method for producing a hot-dip galvanized steel sheet having a good appearance, which comprises applying a strain to a surface layer from a depth of up to 20 μm, followed by cold rolling, heat treatment, and then hot-dip galvanizing. 2. C: 0.0050 mass% or less, Si: 0.10 mass% or less, Mn: 0.30 mass% or less, Ti: 0.010 to 0.050 mass%,
Nb: 0.001 to 0.030 mass%, Al: 0.010 to 0.070 mass
%, N: 0.0050 mass% or less, P: 0.030 mass% or less and S: 0.010 mass% or less, and B: 0.0002 to 0.00%
After hot rolling a steel slab consisting of iron and unavoidable impurities with the balance of 10 mass%, strain was introduced into the surface layer from the surface of the hot-rolled sheet to a depth of 20 μm, and then cold-rolled. A method for producing a hot-dip galvanized steel sheet having a good appearance, which comprises rolling, followed by heat treatment and then hot-dip galvanizing. 3. The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein a shot blasting treatment is performed to introduce distortion into a surface layer. 4. The method according to claim 1, wherein
A method for producing a hot-dip galvanized steel sheet having a good appearance, comprising performing an alloying treatment after the plating treatment.