JP2546471B2 - Method for producing bake hardened high strength galvannealed steel sheet with excellent plating adhesion - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing a bake hardenable high strength galvannealed steel sheet which is excellent in workability and plating adhesion and does not cause powdering during press forming. It is about.

[0002]

2. Description of the Related Art As cold-rolled steel sheets for automobile outer panels, so-called bake hardening type cold-rolled steel sheets are used, which have a low yield point during press forming and have a higher yield point due to paint baking after forming, which improves dent resistance. It has been. In particular, with the aim of improving fuel efficiency and reducing exhaust gas volume caused by recent global environmental problems, bake-hardening high-strength cold that enables thinning in order to meet the demand for lighter vehicle bodies and the demand for higher production efficiency. Rolled steel sheets are widely used for door outers and the like.

The bake hardening type high strength cold rolled steel sheet as described above is
Conventionally, it was manufactured as a cold-rolled steel sheet product by box annealing using P-added low carbon Al killed steel as a material (for example, Japanese Patent Publication No. 58-303).
No. 74). However, in recent years, from the viewpoint of improving the anticorrosive ability of the vehicle body, there has been an increasing demand for alloying hot-dip galvanized steel sheets for bake hardening type high strength cold rolled steel sheets. In this case, the steel sheet is rapidly heated and then cooled in the hot dip galvanizing line, and it becomes difficult to obtain a high r value in the conventional P-added low carbon Al killed steel, and the solid solution C is remarkably large. Since it remains in the steel, the bake-hardening amount is high, but at the same time, the deterioration of the room temperature aging becomes very large, which may be a practical problem. Also, in order to obtain a high r value, a method of performing box annealing in advance to obtain a cold rolled steel sheet product and passing it through a hot dip galvanizing line to perform plating is conceivable, but it requires a long process, which is disadvantageous in terms of construction period and cost. Become.

On the other hand, a very small amount of Ti, Nb, B is added to ultra low carbon steel.
A method of adding a carbonitride forming element such as Mn and P and a strengthening element such as Mn and P to obtain a bake hardenable high-strength steel sheet excellent in workability even by annealing for a short time has been proposed (eg JP-A-61-276931 and JP-A-59-74232).

[0005]

However, when galvannealing is performed using a steel sheet containing Ti or P, these elements have a great influence on the alloying rate of Fe and Zn, and are not suitable. If you cannot obtain the proper alloying conditions,
Zn remains in an unalloyed state in the alloyed plating layer, or alloying is insufficient in the alloyed plated layer, or the alloyed plated layer is too fragile.
The phenomenon of excessive alloying in which the phase (Fe ₃ Zn ₁₀ ) grows thick becomes likely to appear, and the weldability, plating adhesion, or corrosion resistance after coating remarkably decreases. For example, it is said that Ti promotes the above alloying, and P delays alloying on the contrary,
When these elements are added in a complex manner, the actions of both elements are intricately entangled, so it becomes extremely difficult to quickly and accurately grasp the optimum operating conditions that do not cause insufficient alloying or excessive alloying, and due to alloying excess and deficiency A large reduction in yield is inevitable.

Japanese Patent Laid-Open No. 61-60860 discloses a plating bath temperature of 430 to 500 in order to obtain a plating layer having excellent quality characteristics.
℃, Al concentration in the bath is specified as 0.05% or more, but T
The same conditions are applied to all steels in the range of i: 0.03% or less and P: 0.02 to 0.1%, and it cannot be said that the optimum conditions are necessarily obtained according to the steel composition. More Ti and P
In the steel to which is added, since the influences of these elements in the opposite directions are complicatedly entangled as described above, it becomes more difficult to obtain a plating layer having a good degree of alloying.

On the other hand, Japanese Patent Publication No. 60-55589 discloses an alloy excellent in plating adhesion by adjusting the P content in steel and the effective Al concentration in the plating bath to values specified by a specific relational expression. A method of manufacturing a galvanized steel sheet is disclosed. However, the steel plate applied by this method has a C content of 0.02 to
It is a normal low carbon steel containing 0.12% and no Ti, not an ultra low carbon steel containing Ti. Therefore, no consideration is given to Ti, which has a great influence on the alloying behavior.

The present invention has been made by paying attention to the above circumstances, and an object thereof is to provide a bake hardenable high strength galvannealed steel sheet excellent in both workability and plating adhesion. A method of manufacturing is provided.

[0009]

The method for producing a high-strength galvannealed steel sheet according to the present invention, which has been capable of solving the above-mentioned problems, is C: 0.0005 to 0.004% by weight, Si: 0.2
Wt% or less, Mn: 1.5 wt% or less, P: 0.05 to 0.15 wt%, Al: 0.025 to 0.1 wt%, N: 0.005 wt% or less, S: 0.02 wt% or less, Ti: {0.5 × 3.43 [N] } ~
{3.43 [N] +1.5 [S] +4 ([C] -0.0005)} (However,
[C], [N], and [S] represent the weight% of C, N, and S, respectively.)
When the steels contained in each are hot-rolled, finish rolling is completed at a temperature of Ar ₃ transformation point to (Ar ₃ transformation point + 100 ° C), and after rolling at a temperature of 600 ° C or higher, pickling and cold rolling. After using the obtained steel sheet for plating and annealing it at a temperature equal to or higher than the recrystallization temperature, a hot dip galvanizing bath having an Al content adjusted so as to satisfy the requirements of the following formula [I] is used. It has a gist in that it is hot-dipped and then alloyed.

[0010]

(Equation 3)

Further, the upper limit of the Ti content is {0.5 × 3.43 [N]
}-{3.43 [N] +1.5 [S]} and using Nb added in the range of {0.3 × 7.75 [C]}-{7.75 ([C] -0.0005)}, The desired alloyed hot-dip galvanized steel sheet can be obtained by the method.

[0012]

The function 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: 0.0005 to 0.004 wt% C is the better for improving press workability, especially ductility. If the C content exceeds 0.004% by weight, sufficient ductility cannot be obtained. However, in order to impart bake hardenability, at least 0.0005% by weight of C must be present in the steel in a solid solution state.

Si: 0.2% by weight or less Si is an effective strengthening element, but if it is added in a large amount, the plating adhesion is significantly deteriorated, so 0.2% is made the upper limit. If it is 0.2% or less, the effect on the alloying behavior is very small and can be ignored.

Mn: 1.5 wt% 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. However, if the Mn content exceeds 1.5% by weight, the deep drawability is deteriorated and the workability is adversely affected. Therefore, the upper limit is 1.5% by weight.

P: 0.05 to 0.15% by weight P is less likely to deteriorate the deep drawability, while strengthening the steel. Therefore, in the present invention, the strength of the steel is increased mainly by adding P, and for that purpose, at least 0.
It is necessary to contain 05% by weight. However, if the content of P becomes excessive, the weldability of the steel sheet deteriorates, so it is necessary to make it 0.15 wt% or less.

Al: 0.025 to 0.1% by weight Al is added for deoxidation, but in order to sufficiently fix N as a precipitate in the hot rolling stage, at least 0.025%
It is necessary to add above. However, if more than 0.1% is added, not only the cost will increase, but also the surface properties will deteriorate, so this was made the upper limit.

N: 0.005% by weight or less If N is too much, Ti and A necessary for fixing and precipitating it
Since the amount of addition of 1 increases and not only the cost increases but also the workability deteriorates, the upper limit is 0.005% by weight.

S: 0.02 wt% or less S combines with Ti to form a sulfide, but if it is too much, it deteriorates the ductility of the steel sheet, so 0.02 wt% is the upper limit.

Ti: {0.5 × 3.43 [N]} to {3.43 [N] +1.
5 [S] +4 ([C] -0.0005)} (however, [C], [N], [S]
Indicates the weight% of C, N, and S, respectively.) Ti precipitates and fixes unnecessary C, N, S, etc. in the steel, so a sufficient amount of solid solution C is left to secure bake hardenability. Therefore, it is necessary to control the addition amount. The amount of Ti added is {0.5 × 3.43 [N]}
If it is less than the above, fine AlN is precipitated in the steel, causing deterioration in workability. On the other hand, the amount of Ti added is {3.43 [N] +1.5 [S] +4
If it exceeds ([C] −0.0005)}, the amount of solid solution C is too small to provide sufficient bake hardenability.

The steel sheet used in the present invention contains each of the above elements as a basic component, but it is possible to add a small amount of Nb. Nb remarkably raises the recrystallization temperature, so addition of a large amount should be avoided, but addition of a small amount effectively acts on the improvement of anisotropy. Therefore, instead of Ti, for the purpose of fixing C, {0.3 × 7.75 [ It is effective to add an amount corresponding to [C]}. When Nb is added, Ti 4 ([C] −0.00
05) = 48/12 ([C] −0.0005), 7.75
The upper limit is ([C] −0.0005) = 93/12 ([C] −0.0005). When Nb is added, the upper limit of Ti addition is 4
If it is not reduced by the amount corresponding to ([C] −0.005), sufficient bake hardenability cannot be imparted.

The reasons for defining 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 unfavorable for deep drawability after annealing develops, which is not preferable, and a hot rolled steel sheet with a random texture is obtained. Therefore, hot finishing may be performed at the Ar ₃ transformation point or higher. is necessary. 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 is set to about (Ar ₃ transformation point + 100 ° C). There is a need.

On the other hand, when the coiling temperature is less than 600 ° C., TiC
Since the precipitation of TiN and TiN is insufficient and solute C remains in the steel and adversely affects the deep drawability after annealing, the coiling temperature is 600
It must be above ℃. Although the upper limit of the winding temperature is not limited, it is preferably about 750 ° C. in consideration of maintenance and scale generation. Further, if the coiling temperature becomes too high, the crystal grains of the hot-rolled sheet may locally become granular, which may adversely affect the workability after cold-rolling annealing. From this viewpoint, the upper limit is preferably about 750 ° C.

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 desirable to carry out at 65-85%. From the viewpoint of workability, it is better to anneal at a temperature higher than the recrystallization temperature. However, if it is heated to a temperature above the 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 a 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, the value given by the above equation is less than 0.95 or
If it exceeds 1.05, corrosion resistance and weldability after coating due to insufficient alloying will deteriorate, or plating adhesion will deteriorate due to excessive alloying.

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.

[0029]

EXAMPLE Steels having the chemical compositions shown in Table 1 were melted and continuously cast into slabs. This slab was preheated at 1150 to 1250 ° C. for 3 hours, then hot-rolled to a thickness of 3.6 mm at a finishing temperature of 890 to 930 ° C. and wound at the winding temperatures shown in Table 2. After pickling, it was cold-rolled to a thickness of 0.8 mm, recrystallized and annealed under the soaking conditions shown in Table 2, followed by plating in a plating bath having an effective Al concentration in the bath shown in Table 2 to carry out alloying treatment.
After temper rolling with an elongation of 0.8%, the mechanical properties, plating adhesion, and corrosion resistance after painting were investigated. The results are also shown in Table 2.

The evaluation methods of plating adhesion and corrosion resistance after coating are as follows. <Plating Adhesion> Each of the test steel sheets that had been subjected to the alloying heat treatment was subjected to a V-shaped bend at an angle of 60 °, and the amount of plating peeling when the tape was peeled off on the compression side 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 + top coat paint 3
Layer coat and bake at 150 ° C for 20 minutes). Put a cross cut that reaches the surface of the plating layer on this coated steel sheet,
After conducting a salt spray test for 1000 hours, the degree of swelling of the coating film from the cross cut portion was evaluated in 3 levels. ○: excellent, △: good, × bad

[0032]

[Table 1]

[0033]

[Table 2]

As is apparent from Table 2, the invention examples (No.
1, 2, 5, 7) has a deep drawability with an r value of 1.7 or more and 30 N /
While having a BH amount (bake-hardening amount) of mm ² or more and exhibiting good characteristics of passing level in both corrosion resistance after coating and plating adhesion, in Comparative Examples (No. 3, 8-9) Since the hot rolling temperature or the chemical composition is different from the condition of the present invention, the r value is low, and the comparative examples (No. 10, 11) have T
Since the amounts of i and Nb are larger than those of the present invention, the r value is high, but bake hardenability is not obtained. On the other hand, a comparative example (No.
In Nos. 4, 6), since the effective Al concentration in the bath is not suitable for the component system, the corrosion resistance after coating and the plating adhesion are inferior to those of the examples of the present invention.

[0035]

As described above, according to the present invention, a bake hardenable high strength galvannealed steel sheet excellent in workability and plating adhesion can be realized.

Continuation of the front page (51) Int.Cl. ⁶ Identification code Reference number within the agency FI Technical indication location C22C 38/14 C22C 38/14 (56) References JP-A-2-185949 (JP, A) Japanese Patent Publication 3 -54186 (JP, B2)

Claims (2)

(57) [Claims] 1. C: 0.0005 to 0.004% by weight, Si: 0.2
% By weight, Mn: 1.5% by weight or less, P: 0.05 to 0.15% by weight, Al: 0.025 to 0.1% by weight, N: 0.005% by weight or less, S: 0.02% by weight or less, Ti: {0.5 × 3.43 [N] } ~ {3.
43 [N] +1.5 [S] +4 ([C] -0.0005)} (However, [C],
[N], [S], respectively C, N, the steel containing s husband shows the weight% of S), upon hot rolling, Ar ₃ transformation point -
Finishing rolling is completed at a temperature of (Ar ₃ transformation point + 100 ° C), and then rolled at a temperature of 600 ° C or higher, followed by pickling and cold rolling. After annealing at the above temperature, hot dip galvanizing is performed using a hot dip galvanizing bath whose Al concentration is adjusted so as to satisfy the requirements of the following formula [I], and then alloying treatment is performed. Of excellent bake hardening type high strength alloyed hot dip galvanized steel sheet of [Equation 1] 2. C: 0.0005 to 0.004% by weight, Si: 0.2
% By weight, Mn: 1.5% by weight or less, P: 0.05 to 0.15% by weight, Al: 0.025 to 0.1% by weight, N: 0.005% by weight or less, S: 0.02% by weight or less, Ti: {0.5 × 3.43 [N] } ~ {3.4
3 [N] +1.5 [S]}, Nb: {0.3 × 7.75 [C]} to {7.75 ([C] −0.
(Where [C], [N], and [S] represent the weight% of C, N, and S, respectively) during hot rolling of the steel containing Ar ₃ transformation point ~ ( Finishing rolling is completed at a temperature of (Ar ₃ transformation point + 100 ° C), and then rolled at a temperature of 600 ° C or higher, followed by pickling and cold rolling. After annealing at the temperature of
[I] Bake hardenable high strength alloy with excellent plating adhesion, characterized by hot-dip galvanizing using a hot dip galvanizing bath whose Al concentration is adjusted so as to satisfy the requirements of formula [I] Method for producing a hot-dip galvanized steel sheet. [Equation 2]