JP2565054B2 - Method for producing galvannealed steel sheet with excellent deep drawability and plating adhesion - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention, workability particularly excellent in deep drawability, and a method for producing the alloy galvannealed steel sheet, such as not to cause excellent powdering during press molding in plating adhesion It is about.

[0002]

2. Description of the Related Art Recently, the amount of various surface-treated steel sheets used has rapidly increased as a measure for strengthening the rust prevention of various vehicles such as automobiles or outer panels for household electric appliances. In addition to its own high anti-corrosion ability, it is possible to make it thicker so that it has been put to practical use in large quantities.

However, as the demand for weight reduction of the vehicle body for the purpose of improving the fuel efficiency and reducing the amount of exhaust gas originated from the recent global environmental problems or the demand for the improvement of the production efficiency is gradually increasing, the strength and the formability are further improved. It is becoming necessary to improve the
Ultra low carbon steel plates (so-called IF steel plates) to which carbonitride forming elements such as Nb and Nb are added have come to be used as cold rolled base plates for galvannealed steel plates.

The ultra low carbon IF steel sheet is a machine after cold rolling annealing by fixing solid solution elements such as C and N in the steel as precipitates with strong carbonitride forming elements such as Ti and Nb. It is used in a large amount in cold-rolled steel sheets as ultra-deep steel sheets. In addition, from the viewpoint of improving strength, ultra-low carbon Ti-added steel and Ti-Nb-added steel have M
Steel sheets to which strengthening elements such as n and P are added are also known (for example, Japanese Patent Laid-Open No. 1-191748).

[0005]

However, Ti and P
When alloyed hot-dip galvanizing is performed using a steel sheet containing Al, these elements have a great influence on the alloying rate of Fe and Zn, and when appropriate alloying conditions cannot be obtained, alloying plating is performed. Insufficient alloying that Zn remains in the layer in an unalloyed state, or excessive alloying phenomenon in which the weak Γ phase (Fe ₃ Zn ₁₀ ) grows thick due to excessive alloying on the surface of the alloyed plating layer appears. In some cases, powdering or flaking in which the plating layer peels off during press molding may occur. In particular, the most commonly used Ti-added ultra-low carbon IF in which only Ti is added to fix C and N in steel
When steel is used, there is a strong tendency for excessive alloying to occur. On the other hand, such a drawback of the Ti-added ultra-low carbon IF steel is due to the fact that the steel contains a large amount of Ti that promotes alloying, and that Ti-containing Nb instead of Ti for forming carbides was used. A method for producing an alloyed hot-dip galvanized steel sheet which is excellent in powdering resistance with an Nb composite-added ultra-low carbon IF steel is proposed in, for example, Japanese Patent Publication No. Sho 61-32375. However, in this technique, C is precipitated as (Ti, Nb) C, and since fine NbC suppresses grain growth, a desired low yield strength (Y
P) and a high Rank Ford value (r value) are difficult to obtain.

By the way, as a method of obtaining a high-strength galvannealed steel sheet with good workability, S is added to the ultra low carbon IF steel.
A steel plate added with a strengthening element such as i, Mn, or P is used as a base plate,
A method of applying hot dip galvannealing to this is already known. For example, in Japanese Patent Laid-Open No. 61-60860, a P-added ultra-low carbon IF steel is used as a base plate, and hot-dip galvanizing is performed on the base plate by defining the plating bath temperature and the Al concentration in the bath. A method of manufacturing a galvanized steel sheet for deep drawing is disclosed. In this technology, in order to obtain a plating layer with excellent quality characteristics, the plating bath temperature is 430 to 500 ° C and the Al concentration in the bath is 0.05% or more.
Hereinafter, the same conditions are applied to all steels in the range of P: 0.02 to 0.1%, and it cannot be said that the optimum conditions are necessarily obtained according to the steel composition. On the other hand, in Japanese Patent Publication No. 60-55589, an alloyed galvanized steel sheet having excellent plating adhesion is manufactured by adjusting the P content in steel and the effective Al concentration in the plating bath to values defined by a relational expression. A method of doing so is disclosed. However, the steel plate applied by this method has a C content of
It is a normal low carbon steel containing 0.02 to 0.12% and containing no Ti, not an ultra low carbon steel containing Ti, and no consideration is given to Ti which has a great influence on the alloying behavior.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for producing an alloyed hot-dip galvanized steel sheet excellent in both deep drawability and plating adhesion. It is a thing.

[0008]

The method for producing an alloyed hot-dip galvanized steel sheet according to the present invention, which was able to solve the above problems, is C: 0.005% by weight or less, Si: 0.1% by weight or less, Mn: 1. % By weight, P: 0.03% by weight or less, S: 0.
02 wt% or less, Al: 0.01 to 0.1 wt%, Ti: {4 [C]
+3.43 [N] +1.5 [S]} to 0.1% by weight (however, [C], [N],
[S] represents the weight% of C, N, and S, respectively, and N: 0.005
During hot rolling of steel containing less than or equal to wt%, finish rolling is completed at a temperature of Ar ₃ transformation point to (Ar ₃ transformation point + 100 ° C), and after cooling at a cooling rate of 30 ° C / sec or more, 600 ° C. After pickling at the above temperature and cold rolling at a rolling rate of 65 to 90%, annealing was performed at a temperature of recrystallization temperature to Ac ₃ transformation point, and then Al was made to satisfy the requirements of the following formula [I]. It has a gist in that hot-dip galvanizing bath whose content is adjusted is used for hot-dip galvanizing and then alloying treatment is performed.

[0009]

[Equation 2]

[0010]

The present invention is constructed as described above, but in short,
By using a Ti-added ultra-low carbon steel sheet containing a large amount of Ti and strictly defining the optimum plating conditions that accurately correspond to the steel composition from the viewpoint of improving the quality of the plating layer, it is possible to achieve deep drawability and plating adhesion. It was also possible to realize excellent galvannealing. Each constituent element of the present invention will be described below. First, the reasons for limiting the chemical composition of the steel sheet used in the present invention are as follows.

C: 0.005 wt% or less C is preferably as small as possible for improving press workability, particularly ductility. If the C content exceeds 0.005% by weight, sufficient ductility cannot be obtained, and the amount of Ti added to precipitate and fix the solid solution C in steel increases, so the upper limit is 0.005% by weight. Si: 0.05 wt% or less Si is an effective strengthening element, but if it is added in a large amount, the plating adhesion is significantly deteriorated, so 0.05 wt% is set as the upper limit. If it is 0.05% by weight or less, the influence on the alloying behavior is very small and can be ignored.

Mn: 1% by weight or less Mn is a safe element in that it does not impair plating adhesion even if it is contained in a large amount, and although it cannot be expected so much, it is effective for the time being. It is positioned as a strengthening element. Therefore, Mn is also added to the steel of the present invention, but if the Mn content exceeds 1% by weight, the deep drawability is deteriorated and the workability is adversely affected, so the upper limit is 1% by weight. .

P: 0.03% by weight or less P has a high strengthening ability and can be added as an effective strengthening element when strengthening steel. However, if the P content becomes excessive, the deep drawability after annealing deteriorates due to the precipitation of a compound with Ti in the hot-rolled hot-rolled sheet, so the content must be 0.03% by weight or less.

S: 0.02 wt% or less S is fixed in steel as a precipitate mainly by Ti, but if it is too much, the workability of the steel sheet deteriorates, so 0.02 wt% is the upper limit. Al: 0.01 to 0.1% by weight At least 0.01% by weight or more of Al needs to be added for deoxidation. However, even if it is added in excess of 0.1% by weight, not only the cost will increase but also the surface properties will be deteriorated, so 0.1% by weight was made the upper limit.

Ti: {4 [C] +3.43 [N] +1.5 [S]} to 0.1
% By weight (however, [C], [N], and [S] represent the weight% of C, N, and S, respectively) Ti is used to precipitate and fix C, N, and S in steel as precipitates. , Add more than each equal amount. Therefore Ti is
Addition of 4 [C] +3.43 [N] +1.5 [S] or more is required, and if insufficient, it causes deterioration of deep drawability and aging. On the other hand, not only is the effect saturated when added in excess of 0.1% by weight,
This is the upper limit because it will increase the cost.

N: 0.005 wt% or less If N is too large, the amount of addition of Ti necessary for depositing and fixing the N increases, which not only increases cost but also deteriorates workability, so 0.005 wt% is the upper limit. . The steel sheet used in the present invention contains each of the above elements as a basic component, but it is possible to further add 0.001% by weight or more of Nb in order to improve anisotropy. However, Nb remarkably raises the recrystallization temperature. Therefore, when Nb is added, the addition amount should be 0.04
It is necessary to keep it below the weight percent. Further, 0.0001% by weight or more of B can be further added to improve the secondary workability. However, B deteriorates the deep drawability, so it is necessary to suppress it to 0.001% by weight or less.

Next, the reasons for limiting each manufacturing condition in the present invention are as follows. First, hot rolling may be basically performed according to a conventional method, but the hot rolling finish temperature is Ar ₃
It must be above the transformation point. Below the Ar ₃ transformation point, a texture, which is disadvantageous to deep drawability after annealing, develops, which is not preferable, and a hot-rolled steel sheet having a random texture is obtained. However, if it is finished at a too high temperature, the grain growth of austenite will be remarkable and the ferrite grain size after transformation will become coarse, which will have an unfavorable effect on the deep drawability after annealing, so it will be about (Ar ₃ transformation point + 100 ° C). There is a need to.

Immediately after finishing rolling, strong cooling is started at a cooling rate of 30 ° C./sec or more to suppress the growth of ferrite grains in favor of deep drawability after annealing, and to reduce the crystal grain size of the hot rolled steel sheet. It is preferable to measure At least 80 cooling
It may be performed at 0 ° C or lower. The upper limit of the cooling rate is not particularly limited, but it is 150 due to the facility restrictions.
℃ / sec or less.

On the other hand, it is preferable for the deep drawability after annealing to fix the unnecessary C and N in the steel as precipitates at the stage of hot rolling by setting the coiling temperature to 600 ° C. or higher. However, if the coiling temperature becomes too high, the ferrite grain size becomes coarse, which is not preferable for deep drawability after annealing.
It is recommended to wind at 750 ℃ or less.

After hot rolling, pickling, cold rolling and annealing are carried out in the usual manner. Cold rolling is a way to improve deep drawability.
It is good to carry out at 65-90%. From the viewpoint of workability, it is better to anneal at a temperature higher than the recrystallization temperature. But,
If it is heated to a temperature above the Ac ₃ transformation point, the workability deteriorates, which is not preferable.

After the recrystallization annealing, it is cooled, then hot dip galvanized and alloyed. At this time, the alloying of Fe-Zn is started by destroying the Fe-Al alloy layer generated by the reaction between Al in the plating bath and the base Fe by mutual diffusion of the base Fe and Zn. The properties of the Fe-Al alloy layer are strongly affected by Ti and P in steel and Al in the bath, Ti promotes alloying, and P and Al in the bath delay alloying. Here, Al in the bath means Al dissolved in the bath.
And Al that forms dross by combining with Fe in the bath at a weight ratio of 1: 1. However, the influence on the properties of the Fe-Al alloy layer is dissolution in the bath, which is said to be effective Al. Is Al.

In the present invention, this effective Al (A in the bath
(1 concentration-Fe concentration in the bath), the interaction between promotion of alloying by Ti and delay of alloying by P in the steel sheet is adjusted by the effective Al concentration to precisely control the alloying rate. . That is, the present inventors used various steel sheets having different Ti contents and P contents, performed a galvanizing treatment using hot dip galvanizing baths having different effective Al concentrations, and then immediately performed an alloying heat treatment. The conversion rate was investigated. As a result, it has been clarified that the Fe-Zn alloying rate becomes almost constant when the relationship of the following equation is established, and that an extremely accurate alloying operation with no excess or deficiency of alloying can be performed. Effective Al concentration in bath (%) = 0.100-1 / 12 x [P] +1/5 x [Ti] (where [P] and [Ti] are the amount of P and Ti in steel [wt%]
Represents)

That is, by adjusting the effective Al concentration in the plating bath to the value obtained from the term on the right side of the above formula according to the steel composition, the alloying treatment could be performed accurately and quickly. is there. Although it is optimal to operate under the conditions that satisfy the above formula, in practice, effective Al in the bath is used.
If the concentration is within the range of ± 5% of the optimum concentration, alloying will not be insufficient or over-alloying.
The relationship between the amount of P and the Al concentration in the bath was defined as in the above formula [I]. Therefore, if the above formula is less than 0.95 or more than 1.05, corrosion resistance and weldability after coating due to insufficient alloying or deterioration of plating adhesion due to excessive alloying will result.

As described above, according to the present invention, the effective Al concentration in the hot dip galvanizing bath is adjusted according to the Ti and P contents in the raw steel sheet to control the alloying rate by the mutual diffusion of the raw materials Fe and Zn. The hot-dip galvanizing bath at 430 to 500 ° C (this hot-dip galvanizing bath is not particularly limited, although the hot-dip galvanizing conditions and alloying heat treatment conditions are not particularly limited. Inside, a suitable amount of Al satisfying the above requirements is added) is a method of immersing and running the steel sheet to be treated, and the alloying heat treatment, the plated steel sheet 480 immediately after the plating treatment.
This is a method of heating at about 800 ° C for about 3 to 120 seconds. Needless to say, temper rolling may be performed after the galvannealing treatment.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and the type of steel sheet, plating pretreatment method, hot dip plating conditions, alloys, etc. It is within the technical scope of the present invention to carry out by appropriately changing the chemical heat treatment conditions and the like as necessary.

[0026]

EXAMPLE Steels having the chemical compositions shown in Table 1 were melted and continuously cast into slabs. This slab is heated to a slab temperature of 1100
It was hot-rolled at a temperature of ~ 1200 ° C and a finishing temperature of 890-930 ° C to a thickness of 3.6 mm, and cooled and wound under the conditions shown in Table 2. After pickling, cold rolling to 0.8mm thickness (cold rolling rate 78%)
Then, recrystallization annealing was performed under the soaking conditions shown in Table 2.
Plating is performed in a plating bath having an effective Al concentration in the bath shown in
Alloying treatment was performed. After temper rolling with 0.5% elongation,
The mechanical properties, plating adhesion, corrosion resistance after painting, etc. were investigated. The results are also shown in Table 2.

The evaluation methods of the plating adhesion, the corrosion resistance after coating, and the secondary work brittleness resistance are as follows. <Plating adhesion> For each test steel plate that has undergone alloying heat treatment,
V-shaped bending was performed at an angle of 60 °, and the amount of plating removal when the compression side was tape-peeled was evaluated in three levels. ○: excellent, △: good, ×: bad

<Corrosion resistance after coating> After each plated steel sheet is treated with phosphate, it is coated with 3 coats for automobiles (specifically, electrodeposition paint + intermediate coat paint + 3 coats of top coat, baking at 150 ° C for 20 minutes). ). A cross cut reaching the surface of the plating layer was put into this coated steel sheet, a salt water spray test was conducted for 1000 hours, and then a three-stage evaluation was made based on the degree of swelling of the coating film from the cross cut portion. ○: excellent, △: good, × bad

[0029]

[Table 1]

[0030]

[Table 2]

As is clear from Table 2, the invention examples (No.
2, 4, 6, 8) has a deep drawability with an r value of 2.0 or more,
Both the corrosion resistance after coating and the adhesion of plating show good characteristics of passing level, while Comparative Examples (No. 1, 3, 9-
In 12), since the hot rolling cooling condition, the coiling temperature or the chemical composition is different from the condition of the present invention, the r value is low,
Comparative examples (No. 5, 7) are effective A in the bath suitable for the component system.
Since the concentration is not l, the corrosion resistance after coating and the plating adhesion are inferior to those of the examples of the present invention.

[0032]

As described above, according to the present invention, a highly alloyed hot-dip galvanized steel sheet having good plating adhesion and extremely excellent press formability could be obtained.

Continuation of front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location C22C 38/06 C22C 38/06 (72) Inventor Masaaki Urai 1 Kanazawa-machi, Kakogawa-shi Kobe Steel Co., Ltd. Kakogawa (56) References JP 57-29555 (JP, A) JP 59-74231 (JP, A) JP 59-190332 (JP, A) JP 59-107029 (JP, A) ) JP-A-61-56245 (JP, A) JP-A-61-60860 (JP, A) JP-A-61-295324 (JP, A) JP-A-63-241122 (JP, A) JP-A-63- 290222 (JP, A) JP-A-1-191748 (JP, A) JP-A-3-150317 (JP, A) JP-A-3-232927 (JP, A) JP-A-5-59443 (JP, A) Japanese Patent Sho 61-32375 (JP, B2)

Claims (2)

(57) [Claims] 1. C: 0.005 wt% or less, Si: 0.1 wt% or less, Mn: 1 wt% or less, P: 0.03 wt% or less,
S: 0.02% by weight or less, Al: 0.01 to 0.1% by weight, Ti:
{4 [C] ＋3.43 [N] ＋1.5 [S]} to 0.1% by weight (however, [C]
, [N] and [S] represent the weight% of C, N and S, respectively,
N: When hot rolling steel containing 0.005 wt% or less,
Finishing rolling is completed at a temperature of Ar ₃ transformation point to (Ar ₃ transformation point + 100 ° C), and after cooling at a cooling rate of 30 ° C / sec or more, winding at 600 ° C or more, pickling and rolling rate 65 ~
After cold rolling at 90%, it was annealed at a recrystallization temperature to a temperature of Ac ₃ transformation point, and then A so that the requirements of the following formula [I] were satisfied.
1. A method for producing an alloyed hot-dip galvanized steel sheet having excellent deep drawability and plating adhesion, which comprises hot-dip galvanizing using a hot dip galvanizing bath having an adjusted 1 content, and then performing an alloying treatment. [Equation 1] 2. The production method according to claim 1, further comprising a steel containing Nb: 0.001 to 0.04% by weight and / or B: 0.0001 to 0.001% by weight.