JP3255765B2 - Method for producing high-strength hot-dip or alloyed hot-dip galvanized steel sheet - 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 hot-dip galvanized high-strength steel sheet and a galvannealed high-strength steel sheet for automotive materials.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets are widely used as automotive materials such as inner and outer plates of automobile bodies and structural parts because of their excellent corrosion resistance after painting.
There are various methods for producing galvannealed alloys. In order to reduce cost and production efficiency, an in-line annealing method (annealing to obtain the material and annealing to reduce the Fe oxide on the surface, followed by hot dip galvanizing) Plating and alloying by heating) are the mainstream (Steel Handbook, 3rd edition, Vol. 6, 421p)
(1982) Maruzen). Among the in-line annealing methods, in the early Sendzimir method, oil and the like were cleaned by oxidizing the flame of the burner by direct flame, and then reduction annealing was performed (a so-called oxidation furnace). There was a problem that the heating speed was slow and the line length was long. For this reason, the Sendzimir method has been abolished at present, and the non-oxidizing furnace method and the electrolytic degreasing method have become mainstream. In other words, the surface is cleaned to remove oil and the like, and the Fe on the steel sheet surface is not oxidized.
With these methods, a normal steel sheet is hot-dip galvanized or alloyed hot-dip galvanized without any problem.

[0003] On the other hand, in order to reduce the weight of an automobile body, it is required to increase the tensile strength of a steel material. In particular, CAFE has demanded an improvement in fuel efficiency of automobiles, and there is a strong need for that. Therefore, an alloyed hot-dip galvanized high-strength steel sheet was required. To increase the tension, there are methods such as solid solution strengthening and structure strengthening, but in any case, additional elements other than Fe are required.

[0004]

The reinforcing elements include:
There are C, Si, Mn, P, Cr, Ni, Mo, Ti, Nb, etc. Among them, it has been found that Si, Mn, Cr significantly impairs the plating property. That is, Si, M
n or Cr is added at least 1.0% in total, and
When an alloyed hot-dip galvanized steel sheet of a high-strength steel sheet of kg / m ² or more was to be manufactured, the hot-dip galvanizing did not cover the entire surface, and non-plating occurred.

According to the present invention, the above-mentioned non-plating is suppressed, in particular, S
One or more additional elements such as i, Mn, and Cr
An object of the present invention is to provide a method for producing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized high-strength steel sheet of 45 kg / m ² or more containing not less than wt%.

[0006]

As a prior art for solving the above-mentioned problem, Japanese Patent Application Laid-Open No. 55-122865 (4) discloses a method of depositing an oxide film on the surface of a steel sheet in an oxidationless furnace system (NOF).
00-10000 °). However, this method has a fatal defect that the effect is not stable because NOF is used as an apparent method. Japanese Patent Laid-Open No. 4-202 is a further development of this technology.
No. 630, No. 202632, No. 202633, No. 2
There are a number of improved technologies such as No. 02631, No. 254531, and No. 254532. These techniques are disclosed in
Since the effect cannot be obtained stably in 122865, for example, the oxidation rate and the reduction amount are specified, the oxide film thickness in the oxidation zone is actually measured, and the oxidation condition and the reduction condition are controlled from here on to stabilize the effect. It was intended to be. However, the results of our studies showed that the effects were still unstable with these technologies. These prior arts are all based on a NOF type heating system.

[0007] As a heating method, a CGL having an all-radiant furnace instead of a NOF has recently been developed. In this case, a different oxidation method is required instead of the NOF.
Then, the present inventors pursued the cause of why the effect is not stable in the oxidation method using NOF, and as a result of examining an oxidation method capable of obtaining a stable effect other than NOF,
This has led to the present invention.

That is, the present invention relates to a method for producing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet by using a continuous hot-dip galvanizing equipment having an annealing furnace . Its atmosphere
Contains O ₂ : 0.1% or more, and H ₂ O: 1%
And / or O containing 0.1% or more of CO ₂ :
High tension melting or alloying characterized by performing a normal reduction annealing, hot-dip Zn plating, and alloying treatment after oxidizing Fe by heating to a plate temperature of 400 to 650 ° C. using a _2- N ₂ -based gas . It is intended to provide a method for producing a hot-dip galvanized steel sheet.

[0009] Here, the heating method for oxidation are all radiant, inductive heating, gas jet, energization heating, or electric furnace type is preferred.

[0010]

The present invention will be described below in more detail. NOF
The reason why the improvement effect of the plating property is unstable is that the oxidation amount fluctuates. For this reason, the amount of oxidation is insufficient or excessive, and F
Non-plating occurs due to non-plating due to the e-oxide or surface concentration of additional elements such as Si and Mn due to excessive reduction.

Therefore, the effect of the composition in the atmosphere on the amount of oxidation was investigated. The result is shown in FIG. As is clear from FIG. 1, when the O ₂ concentration is 0.1% or less, the fluctuation of the oxidation amount is remarkably large depending on the O ₂ concentration. The air ratio (the theoretical ratio of air / fuel), which is the operating condition of NOF, is 1.
If it is less than 0, the O ₂ concentration is theoretically 0%, but actually, an O ₂ concentration of about 0.01 to 0.08% is measured. At an air ratio of 1.0 to 1.1, an O ₂ concentration of about 0.01 to 0.5% is measured. That is, in the oxidation by NOF, the operation is usually performed at an air ratio of 0.90 to 0.99, and the oxidation amount is liable to fluctuate essentially in the range of the above-mentioned conventional technology of 0.9 to 1.1.

The NOF is heated by the burner flame or its exhaust gas, and is not atmospherically uniform in the width direction and the longitudinal direction of the surface of the steel sheet. This also increases the fluctuation of the amount of oxidation and destabilizes the effect. Cause. Furthermore, the O ₂ concentration in the supplied fuel gas can sufficiently fluctuate by about 0.1%, which also causes a fluctuation in the oxidation amount. In the first place, it is not easy to accurately measure an O ₂ amount of 0.1% or less.

On the other hand, if the O ₂ concentration is 0.1% or more,
_{(2) The} amount of oxidation hardly changes with the concentration. The reason is that the O ₂ supply is rate-determined when the O ₂ concentration is 0.1% or less,
If it is 0.1% or more, O ₂ is sufficiently supplied.
This is because the O ₂ concentration does not affect the oxidation rate.
Therefore, the O ₂ concentration of 0.1 to 100% is the range of the atmosphere of the present invention.

When the oxygen concentration is low, depending on the type of the steel sheet to be annealed, the supply of O ₂ may be limited by the influence of water or an oxide film originally present on the surface or by fluctuations in the line speed. Therefore, the O ₂ concentration is preferably 0.5% or more. Furthermore, preferably 1.0% to 2%
1% is good. Free air can be used below 21%, but cheaper but relatively expensive O ₂ above 21%
Must be supplied.

Of course, the use of NOF is unsuitable as described above, and in the present invention, oxidation is performed by controlling the atmosphere in a preheating section (pre-tropical zone) of an all-radiant tube type annealing furnace. The temperature is 400
650 ° C. is appropriate. If the temperature exceeds 650 ° C., FeO is generated, and the adhesion of the oxide film is significantly deteriorated. If the temperature is lower than 400 ° C., a sufficient amount of the oxide film cannot be obtained by the continuous furnace method.
Preferably, 500 to 600 ° C is good.

[0016] subsequent reduction annealing galvanizing, alloying may under normal conditions, the reduction annealing such as H ₂ concentration 2-5
0%, dew point -10 to -50 ° C, annealing temperature 700 to 950
° C. The hot-dip galvanizing conditions are, for example, Al concentration 0.1
2-0.2%, bath temperature 450-500 ° C, penetration plate temperature 400
６００600 ° C. Alloying conditions are, for example, a temperature of 400 ° C.
600 ° C.

When performing the oxidation treatment in the presence of O _{2 in the} above-mentioned preheat section, it is conceivable to use a burner or use a combustion waste gas. However, in that case, instability similar to NOF occurs. This is O ₂
NOF even if O ₂ concentration is increased to 0.1% or more
However, there are cases in which the effect is caused by impurities (N and S) in the fuel and reducing gases such as CO and H ₂ . Reducing gas such as CO or H ₂ by reacting with O _2, it is believed to be due to imbalance of the reaction between O ₂ and Fe.

FIG. 2 shows the influence of dew point and CO in the presence of 1% of O ₂ . When the dew point is low, the amount of oxidation is significantly reduced due to the influence of CO. CO when dew point is high
Not affected by When there is no CO, it is not affected by the dew point. C in a conventional oxidation furnace or NOF atmosphere
Since it contains O, H ₂ , dew point (H ₂ O), CO ₂ , and O ₂ , not only O ₂ concentration but also CO, H ₂ , H ₂ O,
It can be said that the amount of oxidation is extremely difficult to control because it is affected by CO ₂ .

Therefore, in the present invention, it is preferable that the burner, NOF and its exhaust gas are not used, and the atmosphere is free from CO and H ₂ O. Further, nitrogen compounds and sulfur compounds as impurities contained in the fuel gas have a decomposing action and a poisoning action, and have an action of inhibiting or remarkably accelerating the oxidation of Fe, so that control becomes impossible. It is inappropriate.

In the Sendzimir-type oxidizing furnace, oil is burned and removed by a burner frame.
The amount of oxidation cannot be controlled due to the influence of H ₂ or N compounds and S compounds. In fact, it is a method that has been pursued on the issue of oxidation. In fact, when a burner was used, the reproducibility of the oxidation amount was extremely poor.

Therefore, it is preferable that the atmosphere be a gas of only O ₂ -N ₂ system. However, Ar, which is an inert gas, and a small amount of CO ₂ or H ₂ O contained in the atmosphere are irrelevant and may be contained.

As described above, unlike the conventional oxidation, the oxidation treatment of the present invention does not use a burner, a flame, and waste gas, and is not affected by CO, H ₂ , N compounds, and S compounds. Is obtained.

On the other hand, since a burner or a flame is not used, volatile components evaporate and are thermally decomposed to some extent, but there is no effect of burning and removing oil which is a technical idea of an oxidation furnace. Therefore, degreasing such as electrolytic degreasing is required as pretreatment, and activation by pickling is also preferable for stabilizing oxidation.

Since the atmosphere substantially requires an O ₂ —N ₂ system, it is better not to use a conventional burner, a frame, or waste gas. Therefore, any of an all-radiant heating method, an electric heating method, an induction heating method, and an electric furnace method is appropriate. In particular, the induction heating method is the best because the heating rate is fast, and the all-radiant type is excellent from the viewpoint of cost.

Further, when the O ₂ -N ₂ system by adding a small amount of H ₂ O and CO ₂ in the atmosphere, but the oxidation amount hardly changes, since there is a tendency to improve the adhesion of the oxide film, the present invention
_In, O 2 -N ₂ system gas H ₂ O 1% or more, good
Preferably 2% or more, more preferably 5% or more and / or
Other CO ₂ 0.1% or more, preferably to contain more than 2%
You .

By the oxidation according to the present method, there is usually no problem in the adhesion of the oxide film. However, in the case of a high-lane material containing a large amount of Si and Mn, the ability to prevent the surface concentration from being reduced due to the deterioration of the adhesion of the oxide film. Occurrence of defects in the line due to oxide peeling may be a problem. In this case, reduction of the oxidation temperature and addition of H ₂ O and CO ₂ are effective.

The oxidation in the present method may be carried out simply by maintaining the sheet temperature in an atmosphere, and a suitable holding time is about instantaneously to about 5 seconds. In addition, the oxidation amount is almost determined by the temperature,
The influence of time is an error range in the above range. This is because diffusion is controlled by an oxide film.

The amount of oxidation is affected by the type of steel.
200 to 700 mg / m as almost O at 0 to 650 ° C
² When the addition amount of Si and Mn is large, it is necessary to increase the oxidation temperature and increase the oxidation amount to enhance the ability to prevent surface concentration, and may be set according to the steel component. Si, Mn
Since there is a tendency that the more the alloying element is, the more difficult it is to be oxidized and reduced, the reduction conditions hardly need to be changed, and the subsequent reduction annealing may be performed in a normal N ₂ -H ₂ atmosphere.

As described above, the present invention can be applied to almost all types of steels only by changing the oxidation temperature to change the oxidation amount, and a stable effect can be obtained by controlling the oxidation temperature. The total amount of Si and Mn added is 2.
When the content is 7% or more and Si is 0.7% or more, although a large surface concentration suppressing effect is obtained, the surface concentration cannot be completely suppressed. In this case, it is effective to suppress the non-plating when the zinc bath temperature is increased, the Al concentration is decreased, and the penetration plate temperature is increased.

In the present invention, the amount of oxidation (mg /
m ² ) is a value obtained by quantifying O by fluorescent X-ray,
The amount of oxide film increases about 3.6 times.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. (Example) A high-strength steel sheet having the components shown in Table 1
Then, reduction annealing, hot-dip galvanizing, and alloying treatment were performed. The results were evaluated in terms of surface concentration and plating properties, and are shown in Table 1. (1) Surface Concentration The surface concentration of Si and Mn, which are particularly harmful to plating properties, was evaluated by GDS as follows. ＝= No surface thickening-significantly suppressed △ = Surface thickening inhibitory effect × = No surface thickening inhibitory effect (2) Plating properties Plating properties were visually evaluated as follows. ○ = No plating △ = Slightly non-plating (can be improved by penetration plate temperature, bath temperature, etc.) × = Unplated (cannot be improved under plating conditions)

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

According to the conventional NOF or oxidation furnace, it is difficult to control the amount of oxidation. By specifying the O ₂ concentration, which is the cause, and eliminating reductive gases such as CO and H ₂ and the sulfur compounds and nitrogen compounds in the fuel gas, which are the causes of the remarkable fluctuation of the oxidation amount, a stable oxidation amount can be obtained. can get. As a result, the surface concentration of Si and Mn during the subsequent reduction annealing can be suppressed,
Stable plating properties can be secured.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of oxygen concentration in the atmosphere on the amount of oxidation.

FIG. 2 is a diagram showing the influence of an atmospheric dew point and a CO concentration on an oxidation amount.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-51714 (JP, A) JP-A-4-2022631 (JP, A) JP-A-3-134147 (JP, A) (58) Field (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] When producing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet using a continuous hot-dip galvanizing equipment having an annealing furnace , the atmosphere in a pre-tropical zone (preheat section) of the annealing furnace is O ₂ : 0.1% or more
And H ₂ O: 1% or more and / or CO
₂ : O ₂ -N ₂ -based gas containing 0.1% or more , heated to a plate temperature of 400 to 650 ° C. to oxidize Fe, followed by ordinary reduction annealing, hot-dip galvanizing, and alloying if necessary A method for producing a high tension hot-dip or alloyed hot-dip galvanized steel sheet, comprising performing a treatment. 2. The high-tension molten or alloyed molten zinc according to claim 1, wherein the heating system for oxidation is any of an all-radiant system, an induction heating system, a gas jet system, an electric heating system, and an electric furnace system. Manufacturing method of plated steel sheet.