JP3385089B2 - Method for producing high-strength hot-dip galvanized steel sheet with good plating appearance - Google Patents

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 high-strength galvanized steel sheet and a method for producing a high-strength galvannealed steel sheet.

[0002]

2. Description of the Related Art In recent years, steel sheets used as automobile body materials have excellent press workability, high strength, and excellent corrosion resistance from the viewpoints of weight reduction, safety improvement and corrosion resistance of the vehicle body. Is required. Therefore, there is a strong demand for surface treatment of high-strength steel sheets.

In general, high-strength steel sheets include Si, Mn,
Alloying elements such as P and Cr are added. When a steel sheet containing a large amount of these elements is annealed / plated in a hot dip galvanizing facility, these elements, particularly Si, are selectively oxidized by heating the surface of the steel sheet and diffused to the surface layer of the steel sheet. The oxide thickens and forms a film on the steel plate surface. These oxides are not reduced even by reduction annealing, significantly impair the wettability with molten zinc, and deteriorate the plating adhesion. As a result, so-called non-plating, in which molten zinc does not adhere to the steel sheet, often occurs, and a good plating appearance is not exhibited.

Further, in order to impart excellent press workability to a high-strength steel sheet to which alloy elements such as Si, Mn, P and Cr are added, it is necessary to anneal at a high temperature of 800 ° C. or higher. As the annealing temperature increases, these elements, especially S
The surface concentration of i becomes remarkable, the wettability of the plating is deteriorated, and significant non-plating occurs. For this reason, the hot-dip galvanized high-strength steel sheet cannot be used as a steel sheet for automobiles and the like to which press working and the like are applied.

As a conventional method of improving these, there is a method of suppressing the formation of an oxide film of an alloy element by oxidizing a steel sheet in advance and then reducing it, as in JP-A-55-122865. However, this method controls the air ratio of a NOF (non-oxidizing furnace), and the steel sheet surface is heated by the flame of a burner or exhaust gas, and is oxidized or reduced in the atmosphere.
Therefore, the amount of Fe oxide on the surface of the steel sheet changes due to a slight change in the air ratio, resulting in lack of stability.

[0006] As a countermeasure, Japanese Patent Laid-Open No. 4-202630
There is a method of oxidizing a steel sheet at a specific air ratio so that the oxide film growth rate is a constant value or more and a thickness of the oxide film within a certain range, and then performing reduction / plating, as in Japanese Patent Laid-Open Publication No. 2003-242242. However, even with this method, as long as the amount of Fe oxides on the surface of the steel sheet is controlled by the air ratio of NOF, it is inevitable that the amount of Fe oxides on the surface of the steel sheet will change due to a slight change in the air ratio, and thus lack of stability. Absent.

The conventional atmosphere is that O _{2 in} FIG. 1 is 0.2 vol.
%, The amount of Fe oxide in the steel sheet changes significantly if the amount of O ₂ changes slightly due to the change of the air ratio. In addition, these prior arts are
Although it has been described that the formation of the e oxide film suppresses the surface enrichment of the alloying elements in the subsequent reduction annealing, the Fe oxide film is peeled off by a roll or the like during passing, and It does not mention that the alloying elements are surface-enriched by reduction annealing. That is, when the air ratio is controlled by NOF, generally generated F
Although the e oxide film is easily peeled off, its adhesion is not mentioned.

[0008]

Problems to be Solved by the Invention Si, Mn,
In order to hot-dip galvanize a steel sheet containing a large amount of alloying elements such as P and Cr, it is necessary to suppress surface enrichment of these elements. As a method of suppressing this surface concentration, there is a method of suppressing the formation of an oxide film of an alloy element by previously oxidizing with NOF and then reducing it as described above. However, the stability of the reproducibility of the thickness of the oxide film formed by oxidation and the adhesiveness of the Fe oxide film are lacking.

The present invention relates to a high-strength steel sheet containing a large amount of alloying elements such as Si, Mn, P and Cr in the steel.
e A method for producing a high-strength hot-dip galvanized steel sheet and a high-strength hot-dip galvanized steel sheet with good plating appearance by improving the adhesion of the oxide film and suppressing the surface concentration of these alloy elements. It is intended to be provided.

[0010]

The present invention relates to Si, Mn,
A high-strength steel sheet containing 1.0 wt% or more of Cr in total,
O ₂ : 0.01 to 5 vol%, H ₂ O: ₂ vol% or more, and the balance is substantially N _{2 in} an oxidizing atmosphere, and the temperature rising rate is 15 ° C./s.
was heated to 400 to 800 ° C. at ec or after the oxidation treatment, H _2: 3 vol
% In a reducing atmosphere containing at least 800 to 880 ° C. in a reducing annealing, there is provided a process for the preparation of a good high-tensile galvanized steel sheet coating appearance, characterized in that the galvanizing, before
Serial and indirect heating the heating in the oxidizing atmosphere, it is desirable that during a previous SL oxidizing atmosphere does not contain CO and CO ₂ substantially.

Further, after hot-dip galvanizing by the above-mentioned method, a high-strength galvannealed steel sheet having a good plating appearance can be obtained even if heat alloying treatment is further applied.

[0012]

In the present invention, when annealing a steel sheet in a hot dip galvanizing facility, it is desirable to control the inside of the furnace to a non-NOF atmosphere containing substantially no CO or CO ₂ . This prevents selective oxidation of alloying elements and ensures good plating properties. That is, at the time of oxidation treatment, the burner is covered to prevent the steel sheet from being directly oxidized by the flame, and based on the measured value of the exhaust gas, the atmospheric gas is fed from a portion different from the combustion gas inlet. Then, it is desirable to indirectly control and directly control the atmosphere in the furnace to a very limited range that does not substantially contain CO or CO ₂ .

Further, the Fe oxide film formed by the oxidation treatment is peeled off by a roll or the like, and the oxidation treatment is carried out so that the selective oxidation of the additional element does not occur during the subsequent reduction annealing and the plating property is not hindered. This is a particularly important issue. In the present invention, a high-strength steel sheet containing alloy elements such as Si, Mn, P, and Cr in steel is provided with O ₂ : 0.01 to 5 vol.
%, H ₂ O: ₂ vol% or more, with the balance being substantially N _{2 in} an oxidizing atmosphere, at a temperature rising rate of 15 ° C./sec or more, 400 to 8
By heating to 00 ℃ and oxidizing,
It is set to 600 to 1800 mg / m ² .

O in the atmosphere during the oxidation treatment₂Content
Was limited to 0.01-5 vol% ₂Is less than 0.01 vol%
Since the amount of Fe oxide in the steel sheet is small, the surface of alloy elements
It cannot prevent thickening and deteriorates the plating property.
From, while O₂If the content exceeds 5vol%, steel plate
Fe oxide content in the
However, the unreduced material remains and deteriorates the plating property.
It

Further, in an atmosphere containing only O ₂ , it is not possible to obtain a sufficient amount of Fe oxide for suppressing the surface concentration of alloying elements, and it is necessary for H ₂ O to also be contained in a certain amount at the same time. Found out. FIG. 1 shows the relationship between the amount of oxygen and the amount of Fe oxide in the atmosphere. H ₂ O was hardly oxidized at 1 vol% and was significantly oxidized at 2 vol%. Conventionally, the amount of Fe oxide in the steel sheet is controlled by the air ratio, and the O ₂ concentration of the atmosphere is in the region of 0.2 vol% or less in FIG.
Even if the O ₂ amount slightly changed due to a subtle change in the air ratio, the Fe oxide amount in the steel sheet greatly changed, and the control lacked stability. On the other hand, in the present invention, since the atmospheric gas whose composition is adjusted in advance can be introduced, the amount of O ₂ can be kept stable.

In the present invention, ₂ % by volume of H ₂ O in an oxidizing atmosphere is used.
The reason for limiting the above is that the Fe oxide amount sufficient to suppress the surface concentration of the alloy element cannot be obtained unless the content is H ₂ O ₂ vol% or more from FIG. 1, and the plating property is deteriorated. Also,
Also in the present invention, as shown in FIG. 1, the preferable range of the O ₂ amount is such that when the range of H ₂ O in the oxidizing atmosphere is ₂ vol% or more, the Fe oxide amount of the steel sheet does not change even if the O ₂ amount slightly changes. It is a constant range, that is, a range of 0.3 to 2 vol%.

When the Fe oxide film formed in the oxidation treatment is peeled off, the alloy elements are concentrated on the surface during the subsequent reduction annealing and the plating property is deteriorated. Therefore, it is necessary to oxidize the Fe oxide film under the condition that the Fe oxide film is not peeled off. Under this condition, we found from Fig. 2 that the heating rate was 15 ° C / sec or more, and limited the heating rate to 15 ° C / sec or more. Temperature rising rate is 15 ℃ / sec
If it is less than, the oxidation rate locally progresses from the active point in the low temperature range with a slow heating rate, and the adhesion of the Fe oxide film becomes poor, so the Fe oxide film is easily peeled off by a roll or the like, This is because the alloy element surface-concentrates in the subsequent reduction annealing and deteriorates the plating property. On the other hand, the heating rate is 15
Above ℃ / sec, the rate of temperature rise is high and oxidation progresses from a large number of active points in the high temperature range, and the adhesion of the Fe oxide film is good, so the surface concentration of alloying elements is suppressed even in the subsequent reduction annealing. Because you can.

The desirable amount of Fe oxide is 600 to 1800.
It is in the range of mg / m ² . Fe oxide amount is 600 mg / m
^If it is less than ² , the amount of Fe oxide in the steel sheet is small and it is not possible to prevent the surface concentration of alloying elements, which deteriorates the plating property. If the amount of Fe oxide exceeds 1800 mg / m ² , Fe oxidation of the steel sheet will occur. This is because the amount of the substance is too large, and unreduced substances that have not been sufficiently reduced by the subsequent reduction annealing remain and deteriorate the plating property.

The reason why the oxidation temperature is limited to the range of 400 to 800 ° C. is that the amount of Fe oxide is 600 to 1800 mg / m ² when it is oxidized in the specified atmosphere of the present invention. In the present invention, CO or CO
It is desirable not to include ₂ substantially. Generally, when annealing with NOF, it is unavoidable that CO and CO ₂ are included, but as shown in FIG. 3, when the present invention does not substantially contain CO and CO ₂ , the present inventors do not include CO and CO ₂ . It has been found that the effect of suppressing the surface concentration of Mn and Si, which are alloying elements, is greater than that of the case where they are contained, and the plating property becomes better.

Therefore, in the present invention, N ₂ is the main component and O
Atmosphere gas substantially free of CO and CO ₂ adjusted with ₂ and H ₂ O needs to be prepared. After this oxidation treatment, reduction annealing is performed at 800 to 880 ° C. in an atmosphere of H ₂ 3 vol% or more, but if it is less than H ₂ 3 vol%, the amount of reduction is small and unreduced matter remains, resulting in non-plating. The annealing temperature is 800 ℃
The above is necessary for imparting pressability to the high-strength steel sheet and for sufficiently reducing Fe oxide. On the other hand, above 880 ° C, the Fe oxide is excessively reduced and Mn and Si are concentrated on the surface, so the upper limit was made 880 ° C.

Then, hot-dip galvanized steel sheet is manufactured by hot-dip galvanizing. Furthermore, in the present invention, the alloying treatment can be continued.

[0022]

[Examples] High-strength hot-dip galvanized steel sheets were manufactured by oxidizing, reducing and plating cold-rolled steel sheets having the compositions shown in Table 1 in a hot dip galvanizing facility. O ₂ : 0.001 to 8 vol%, H ₂ O: 1
~20vol%, in an atmosphere with the remainder substantially comprising N _2, heated to 400 to 800 ° C. at a heating rate 3 ° C. / sec or more after the oxidation treatment, H _2: Reduction in 2～20Vol% atmosphere annealing Hot-dip plated. For reduction annealing, Steel No. 1 is 830 ℃ and Steel No. 2 is 800.
℃, Steel No. 3 was performed at 860 ℃. The plating bath is Al
Was added at 0.14 wt% and the plating bath temperature was 480 ° C. The judgment of the appearance of the plating is shown in Table 2. When the oxidation treatment and the reduction annealing were carried out under the conditions of the present invention, dot-like non-plating occurred. However, when the oxidation treatment and the reduction annealing were carried out within the conditions of the present invention, the surface appearance was good and no plating occurred.

The high-strength hot-dip galvanized steel sheet produced in the same manner was heat-alloyed to produce a high-tensile hot-dip galvanized steel sheet. The determination of the appearance of the plating is shown in Table 3, and when oxidation treatment and reduction annealing were performed under the conditions of the present invention, dot-like non-plating occurred. However, the surface appearance was good when the oxidation treatment and reduction annealing were carried out within the conditions of the present invention.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

As described above, by performing the oxidation / reduction treatment under the temperature rising rate and atmospheric conditions according to the present invention,
Even with a high-strength steel sheet containing Si and Mn, a high-strength hot-dip galvanized steel sheet and a high-strength hot-dip galvannealed steel sheet having a good plating appearance can be produced as with ordinary steel.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the O ₂ content and the amount of Fe oxide with the H ₂ O content in the atmosphere as a parameter.

FIG. 2 is a graph showing the relationship between the temperature rising rate and the adhesion of an oxide film.

FIG. 3 is a graph showing the relationship between CO and CO ₂ in the atmosphere and the amount of surface concentration of Mn and Si.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI C22C 38/18 C22C 38/18 C23C 2/40 C23C 2/40 (56) Reference JP-A-6-41708 (JP, A) JP 5-51714 (JP, A) JP 4-48062 (JP, A) JP 4-202630 (JP, A) JP 55-122865 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (3)

(57) [Claims] 1. A Si, Mn, of high tensile strength steel containing a total of 1.0 wt% or more of _{Cr, O 2: 0.01~5vol%,} H 2 O:
In a reducing atmosphere containing H ₂ : 3vol% or more after heating at 400 to 800 ° C at a heating rate of 15 ° C / sec or more in an oxidizing atmosphere containing 2vol% or more and the balance substantially consisting of N ₂ and then H ₂ : 3vol% or more In 8
A method for producing a high-strength hot-dip galvanized steel sheet having a good plating appearance, which is characterized by performing reduction annealing at 00 to 880 ° C and hot-dip galvanizing. 2. A method heating in the oxidizing atmosphere, heated and indirect
The method for producing a high-strength hot-dip galvanized steel sheet having good plating appearance according to claim 1. 3. A method for producing a high-strength galvannealed steel sheet having a good plating appearance, which comprises hot-dip galvanizing by the method according to claim 1 or 2, and further heat-alloying treatment.