JP2514298B2 - Method for producing galvannealed steel sheet with excellent press formability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is excellent in ductility and deep drawability and has small anisotropy of material, and is used for molding difficult-to-mold parts such as oil pans of automobiles and for integrally molding large parts such as side panels. The present invention relates to a method for producing a suitable galvannealed steel sheet having excellent press formability.

[0002]

2. Description of the Related Art In recent years, automobile parts have been conventionally formed by pressing a plurality of parts as an integrated product from the molding stage in order to reduce the number of molds and the number of assembly and welding steps. It's coming. In addition, the shape of the parts themselves is more complicated than in the past due to the design of the vehicle body.

In order to enable the formation of such difficult-to-form parts, carbonitride forming elements such as Ti and Nb are added to ultra-low carbon steel to fix the solid solution C and N in the steel as precipitates. IF steel having excellent deep drawability was developed and is now widely used. In addition, recently, due to the demand for improvement of rust preventive ability for automobiles, the use of cold-rolled steel sheets for automobiles has been promoted to surface-treated steel sheets. Therefore, although it has been widely used, the ultra-low carbon IF steel that can withstand the above-mentioned strong working is no exception and has come to be widely used as a cold-rolled base plate for galvannealed steel sheets.

Further, in recent years, high-purity steel has been developed aiming at further improvement of deep drawability and ductility, and for example, Japanese Patent Laid-Open No. 62-54058 has been proposed.

[0005]

However, when the steel is highly purified, the crystal grain growth is improved, so that the press formability is improved, but on the other hand, the crystal grains become too large, and the surface called rough during forming is called. In many cases, defects occur and the appearance quality is significantly impaired.

As a method for solving such a problem, there is a method of refining the crystal grain of the hot rolled sheet to reduce the grain size of the annealed sheet without deteriorating the formability after annealing, but high purity In order to refine the crystal grains of the hot rolled steel sheet,
It is necessary to achieve a remarkably high cooling rate, or a cooling facility capable of rapidly cooling immediately after finish rolling and a facility such as a hot rolling mill capable of obtaining a high pressure reduction rate are required,
There is a problem that equipment costs increase.

For example, the method proposed in JP-A-1-177322 is a method for obtaining fine equiaxed grains by hot cooling after hot rolling to increase the r-value after annealing. To achieve this, a very high cooling rate of 110 to 400 ° C./sec is required.

Also, a very low carbon IF containing a large amount of Ti
When an alloyed hot-dip galvanized steel sheet is manufactured using steel as an original plate, alloying may proceed excessively, and powdering or flaking may occur in which the plating layer peels off during press forming.

The present invention solves the above-mentioned problems of the prior art and is capable of reducing the grain size of the hot-rolled sheet even if the existing hot-rolling cooling equipment is used. An object of the present invention is to provide a method capable of producing a galvannealed steel sheet having press formability and plating adhesion.

[0010]

As a result of earnest studies to solve the above problems, the present inventor utilized conventional hot rolling and hot rolling cooling equipment by utilizing the Nb grain refining effect. Even if it is used, the grain size of the hot-rolled sheet is refined to obtain a fine-grained steel that does not cause surface roughening during forming even after cold-rolling annealing, a large amount of Nb is added and a compound of Ti is added, and high-purity steel is used as a raw material. It has excellent deep drawability and ductility with small anisotropy, and by controlling the content of P that suppresses alloying to the contrary to Ti that promotes alloying within a certain range, plating adhesion It was found that an alloyed hot-dip galvanized steel sheet excellent in both formability and press formability can be obtained, and the present invention has been completed here.

That is, according to the present invention, C ≦ 0.0044.
%, Si ≦ 0.2%, Mn ≦ 1.5%, P ≦ 0.070%,
S ≦ 0.015%, 0.020% ≦ Al ≦ 0.120%, N
≤ 0.007%, further 3.43N ≤ Ti ≤ 0.06
0% and P / Ti in the range of 0.2 to 2.0, 7.75
× C ≦ Nb and 0.025% ≦ Nb ≦ 0.060% Ti and Nb are added together, and the balance is steel and iron is inevitable impurities. Steel is heated at 1100-1250 ° C in a heating furnace. Then, during hot rolling, finish rolling is completed at the Ar ₃ transformation point or higher, and immediately cooled at 30 ° C./sec or higher to 650 ° C.
After winding at the above temperature, then pickling, cold rolling at 60% or more, continuous annealing at 800 ° C or more and Ac ₃ point or less for 5 minutes or less, cooling, and 420 to 530 ° C The gist is a method for producing an alloyed hot-dip galvanized steel sheet having excellent press formability, which is characterized by dipping in a hot-dip galvanized bath, then reheating, and alloying at 550 to 750 ° C.

[0012]

[Action]

The present invention will be described in more detail below. First, the reasons for limiting the chemical components of steel in the present invention will be described.

C: C is preferably as small as possible in order to improve press formability, particularly ductility, and is made 0.0044% or less.

Si: If a large amount of Si is added, the plating adhesion is significantly deteriorated, so the content is set to 0.20% or less. If it is 0.20% or less, the effect on the alloying behavior is very small and can be ignored.

Mn: Mn does not impair plating adhesion even if added in a large amount, and although it has a slightly low strengthening ability, it can be added as an effective strengthening element. However, if the content exceeds 1.5%, the deep drawability will start to deteriorate and the workability will be significantly adversely affected, so the content is made 1.5% or less.

P: P has little deterioration in deep drawability and can strengthen steel, so in the present invention, it can be added up to 0.070%. Addition of more than 0.070% deteriorates the vertical cracking resistance of the steel sheet, so the upper limit is defined as this.

S: S is fixed as a precipitate by Mn and Ti, but is set to 0.015% or less in order to prevent an increase in the amount of Mn and Ti added and an increase in cost.

Al: Al is added in an amount of 0.020% or more for deoxidation. However, the addition of more than 0.120% not only increases the cost but also deteriorates the surface properties, so the upper limit is set.

N: If the amount of N is too large, the amount of Ti required for precipitation and fixation increases, which not only increases the cost but also deteriorates the workability, so the content is made 0.007% or less.

Ti: Ti precipitates and fixes unnecessary solid solution elements C, N and S in the steel as precipitates, but in order to improve the deep drawability, at least an equivalent amount of N or more is added. Need to be kept. Further, if it is less than 3.43N, fine AlN is precipitated in the steel, causing deterioration in workability. Therefore, the Ti amount is 3.43N≤Ti. In the usual process, since the compound of Ti starts to precipitate nitrides and sulfides before hot rolling, in order to stably precipitate nitrogen as TiN, the amount of Ti should be more than the equivalent amount of N. ,
For example, it is desirable to add at least (equivalent amount of N) + (equivalent amount of S × 0.3) or more. On the other hand, Ti is 0.
Even if added in excess of 060%, the effect will be saturated and the cost will increase, so this is the upper limit.

Further, in order to adjust the alloying degree, T is used in the present invention.
The content of i and P is defined as a range of 0.2 to 2.0 in terms of P / Ti ratio. Ti greatly influences the alloying rate of Fe and Zn, promotes alloying, and is brittle in the steel containing a large amount of Γ layer.
A phenomenon of excessive alloying in which (Fe ₃ Zn ₁₀ ) grows thick is likely to occur, and a plating layer peeling phenomenon called powdering is likely to occur during press molding. On the other hand, P suppresses alloying, and in a steel containing a large amount, alloying may not be completed unless kept at high temperature. That is, Ti and P have mutually opposite effects on alloying, and by appropriately controlling the contents of both, stable and excellent galvannealed galvannealed steel sheet. Is obtained. P / T
If i is less than 0.2, alloying tends to be excessive, and if it exceeds 2.0, alloying is insufficient, and sufficient corrosion resistance cannot be obtained.

Nb: Nb, together with Ti added in combination,
In order to completely precipitate and fix the solid solution C in the steel as a carbide, to prevent the deep drawability from deteriorating, to refine the crystal grains, and to improve the anisotropy of the material, at least 7.75C or more,
And 0.025% or more is added to make the effect effective. However, addition of 0.060% or more not only saturates the effect but also leads to cost increase. Furthermore, it leads to a remarkable increase in the recrystallization temperature.

Next, the reasons for limiting the manufacturing conditions in the present invention will be described.

After the steel slab having the above-mentioned composition is cast, it may be heated in a heating furnace as it is, or may be once cooled to room temperature and then heated in a heating furnace at a temperature of 1100.
~ 1250 ° C. If the temperature is lower than 1100 ° C, the carbides become coarse and the grain growth is remarkable, and it is difficult to obtain fine grains. On the other hand, 1
At 250 ° C. or higher, not only the heat energy cost increases, but also good characteristics in terms of material cannot be obtained.

The hot rolling finishing temperature is specified to be the Ar ₃ transformation point or higher. Below the Ar ₃ transformation point, an unfavorable texture develops for deep drawability after annealing, which is not preferable. In order to obtain a hot rolled sheet having a random texture, it is necessary to finish at the Ar ₃ transformation point or higher. In addition, when finishing at a too high temperature, the grain growth of austenite is remarkable and the ferrite grain size after transformation is coarsened, which is not preferable for deep drawability after annealing.

After finishing rolling, cooling is started immediately to suppress the grain growth of ferrite. For that purpose, a cooling rate of at least 30 ° C./sec or more is required. The upper limit is not particularly specified, but in the present invention, since it is a component system that can obtain fine particles without special rapid cooling, about 30 to 80 ° C./sec which can be achieved by ordinary equipment is sufficient.

It is preferable for the deep drawability after annealing to set the coiling temperature at 650 ° C. or higher and fix unnecessary C and N in the steel as precipitates as much as possible at the stage of hot rolling. It should be noted that winding at an excessively high temperature causes deterioration in pickling property, and therefore, winding at 750 ° C. or lower is desirable.

After hot rolling, pickling and cold rolling are performed according to a conventional method. Cold rolling is preferably carried out at 60% or more in order to improve the deep drawability. Note that it is desirable to set the upper limit to about 90% due to equipment restrictions.

From the viewpoint of workability, it is better to perform continuous annealing at a temperature higher than the recrystallization temperature, and the lower limit of the annealing temperature is defined as 800 ° C. in the present invention. However, heating to a temperature higher than the Ac ₃ transformation point deteriorates the workability, which is not preferable. In addition, from the viewpoint of productivity, the time required for annealing is preferably within 5 minutes at the longest.

After the recrystallization annealing, it is cooled and then 420
Immerse in a hot dip zinc bath at ~ 530 ° C, apply plating, and
Reheat to 50 to 750 ° C. for alloying treatment. If the alloying temperature is less than 550 ° C, no matter how much Ti or P is adjusted, alloying does not proceed sufficiently in the short time possible in a continuous line, while if it exceeds 750 ° C, alloying proceeds in a short time. I will pass. When the temperature of the molten zinc bath is lower than 420 ° C, the molten zinc solidifies, while when it exceeds 530 ° C, the plating property is deteriorated.

There is no problem in the present invention even if the cooling rate after soaking or alloying treatment and the constant temperature holding time before immersion in the plating bath are not particularly specified in the present invention. After annealing and plating, temper rolling may be performed, but from the viewpoint of workability, it is desirable to control it to about 1.5% or less.

Next, examples of the present invention will be shown.

[0034]

【Example】

Steel having the chemical composition shown in Table 1 was melted and continuously cast into a slab. This slab was hot-rolled under the conditions shown in Table 2 to a thickness of 3.6 mm and wound at the winding temperature shown in Table 2. After pickling, cold rolling to 0.8 mm thickness, recrystallization annealing plating under the conditions shown in Table 2, alloying treatment, temper rolling with 0.5% elongation, mechanical properties investigated. Table 3 shows the results.

As is clear from Table 3, the invention examples (No.
2, 3, 6, 7, 10) shows an excellent deep drawability with an r value of 2.0 or more and a good strength ductility balance, and Δr is ±
It is in the range of 0.2, indicating that the anisotropy is small. Also, the powdering property is good.

On the other hand, in Comparative Examples No. 1 and 5, the slab heating temperature in hot rolling or the cooling conditions are different from the conditions of the present invention, so that the crystal grain size (FGc) of the annealed sheet is large (FGc).
c is 7.0 or less), which is at a level where there is concern about rough skin during press molding. In addition, Comparative Examples No. 4, 8, 9, 12 to 15
Because the chemical composition or the manufacturing conditions are different from the conditions of the present invention, the r value and the strength ductility balance are inferior to those of the present invention examples.
Alternatively, the anisotropy of Δr is large. In addition, Comparative Example No. 11
Has a higher alloying treatment temperature than the conditions of the present invention, resulting in excessive alloying and poor powdering properties.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

As described above in detail, according to the present invention,
To obtain an alloyed hot-dip galvanized steel sheet with excellent ductility, deep drawability, small material anisotropy, excellent press formability suitable for forming difficult-to-form parts and integral molding of large parts, and good plating adhesion. You can

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Claims (1)

(57) [Claims] 1. In weight% (hereinafter, the same), C ≦ 0.004
4%, Si ≦ 0.2%, Mn ≦ 1.5%, P ≦ 0.070
%, S ≦ 0.015%, 0.020% ≦ Al ≦ 0.120
%, N ≦ 0.007%, and further 3.43 N ≦ Ti ≦
0.060% and P / Ti in the range of 0.2 to 2.0,
7.75 × C ≦ Nb, and 0.025% ≦ Nb ≦ 0.060
In the range of 100%, Ti and Nb are added together, and the balance is steel consisting of iron and inevitable impurities.
When hot-rolled by heating at ℃, finish rolling is completed at the Ar ₃ transformation point or higher, and immediately cooled at 30 ℃ / sec or higher,
Winding at a temperature of 650 ° C or higher, then pickling, cold rolling of 60% or higher, continuous annealing at a temperature of Ac ₃ points or lower at 800 ° C or higher for 5 minutes or less, and then cooling, 420 to 530 ℃
Immersed in a molten zinc bath at, then reheated to 550-7
A method for producing an alloyed hot-dip galvanized steel sheet having excellent press formability, which comprises performing an alloying treatment at 50 ° C.