JP3263143B2 - Bake hardening type high strength alloyed hot-dip galvanized steel sheet excellent in workability and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention has a tensile strength of 390-69.
A high-strength galvannealed steel sheet having a low yield point, excellent elongation and stretch flangeability, baking hardening (BH property), and a cold rolled sheet galvannealed steel sheet of about 0 N / mm ² It relates to a manufacturing method.

[0002]

2. Description of the Related Art Conventionally, baking hardened steel sheets having a low yield point during press forming and exhibiting good workability as an outer panel for automobiles and exhibiting a higher yield point in a subsequent coating baking step to enhance dent resistance. Is used. Currently, 270
Steel sheets having a tensile strength of up to 370 N / mm ² are widely used as galvanized steel sheets from the viewpoint of improving the rust prevention ability of a vehicle body. Among them, alloyed hot-dip galvanized steel sheets have a high rust prevention ability. Because it can be thickened, it has been widely used.

[0003] Since the hot-dip galvanizing line is a continuous line equipped with a heating furnace capable of recrystallization annealing as a facility before the plating treatment, in the case of a cold-rolled original sheet, the steel sheet as cold-rolled is subjected to recrystallization annealing. Heating is also performed to perform plating.
In this case, in the low carbon steel, after the plating treatment or the alloying treatment, the cooling rate of the steel sheet is high, so that a large amount of solid solution C remains in the steel, thereby exhibiting a high BH property. There is a problem that it becomes significant.

On the other hand, an ultra-low carbon steel is used as an original sheet, and the aging at room temperature is performed by appropriately controlling the amount of solute C in the steel by adding a carbonitride forming element such as Ti, Nb, B, or Al. There has been proposed a method for ensuring the BH property without deterioration of the BH. For example, Japanese Patent Application Laid-Open No. 57-70258 discloses a method for producing a BH steel sheet to which Nb is added.
No. 3 discloses a method for producing a BH steel sheet by adding Ti, and JP-A-2-197549 discloses a method for producing a BH steel sheet to which Ti and Nb are added in combination.

[0005]

However, since these ultra-low carbon BH steel sheets are generally soft, they must rely on the addition of solid solution strengthening elements such as Si, P and Mn in order to increase the strength.

However, Si not only deteriorates the adhesion of the plating, but also tends to cause deterioration of the surface appearance due to scale, and P has a large strengthening ability but significantly delays alloying. Since the processing temperature must be increased or the sheet passing speed must be reduced, the addition of an excessive amount is limited.

On the other hand, Mn has few surface quality problems and production problems such as Si and P, but when used as a solid solution strengthening element, it causes finer crystal grains and raises the yield point. However, the advantage of the BH steel sheet that the yield point is low during press forming is lost.

As described above, in the case of an extremely low carbon BH steel sheet,
Even though a tensile strength of about 270 to 370 N / mm ² currently used is possible, it is very difficult to further increase the tensile strength. It is expected to be a big problem in the future.

In view of such circumstances, the present invention provides a bake hardenable alloyed hot-dip galvanized steel sheet having higher strength, a lower yield ratio, and excellent ductility than conventional ones, and a method of manufacturing the same. It is intended to do so.

[0010]

SUMMARY OF THE INVENTION In the manufacture of a bake hardening type high-strength galvannealed steel sheet having a tensile strength of about 390-690 N / mm ² , the present inventors have attempted to increase the strength of steel by solid solution strengthening. It has been found that workability can be increased to the same level as that of an ultra-low carbon BH steel sheet having a tensile strength of about 340 to 370 N / mm ² by optimizing the composition of the components and the state of the composite structure in order to strengthen the transformation structure. Thus, the present invention has been completed.

That is, the present invention relates to C: 0.04 to 0.1.
5%, Si: 0.20% or less, Mn: 1.0% to 2.5%, P:
0.050% or less, S: 0.020% or less, Al: 0.010
0.12%, Cr: 0.1-2.0%, with the balance being composed of iron and unavoidable impurities, 70%
The above ferrite phase and the balance at least 4% each
The gist of the present invention is a bake hardening type high strength alloyed hot-dip galvanized steel sheet having excellent workability, characterized by being composed of the above three-phase composite structure of bainite and martensite.

[0012] Further, the production method is as follows: steel having the above chemical composition is hot-rolled by a conventional method, then wound at a temperature of 700 ° C or less, pickled, and cold-rolled by 40% or more.
7 ° C / sec following soaking at 10 ° C or more for 10sec or more
After slowly cooling to a temperature range of 700 to 600 ° C. at the following cooling rate, cooling at 10 ° C./sec or more, 420 to 520 ° C.
Hot dip galvanizing, then alloying,
It is characterized by cooling to room temperature at a temperature of at least 0 ° C./sec and performing temper rolling of 0.3 to 0.8%.

[0013]

[Action]

Hereinafter, the present invention will be described more specifically. First, the reasons for limiting the chemical components in the present invention are as follows.

C: C has a great effect on the strength of steel, and affects elongation and stretch flangeability by changing the amount and form of low-temperature transformation products. However, if it is less than 0.04%, 390 N / m
Since it is difficult to obtain a high strength of m ² or more, this is the lower limit. On the other hand, if the content exceeds 0.15%, the weldability deteriorates, so this is made the upper limit.

Si: When Si is added in a large amount, the adhesion of the plating is remarkably deteriorated, so the upper limit is 0.20%. If it is 0.20% or less, the effect on the alloying behavior is very small and can be ignored.

Mn: Mn is an element that stabilizes austenite, changes the amount of solid solution C in austenite, and greatly affects the characteristics of a low-temperature transformation product generated in a cooling process. An appropriate amount of Mn is added for the formation of a transformation product. That is, in order to obtain the characteristics of a high-strength steel sheet having extremely excellent workability, it is necessary to add at least 1.0%. However, if the content exceeds 2.5%, not only becomes difficult to melt, but also F
e Since the concentration is excessively high and the rust preventive ability of the galvanization is deteriorated, the upper limit is set.

P: P is an element having a high solid solution strengthening ability, and can be added as an effective strengthening element when strengthening ferrite. However, if the addition exceeds 0.050%, the alloying treatment becomes difficult, and in some cases, plating unevenness tends to occur, so this is set as the upper limit.

S: S is fixed as a precipitate in steel, but if its amount increases, elongation and stretch flangeability deteriorate, so the upper limit is 0.020%.

Al: Al is at least 0.01 for deoxidation.
Add 0% or more. However, adding more than 0.120% not only increases the cost, but also deteriorates the surface properties. Therefore, the upper limit is set.

Cr: Cr enriches C in austenite, increases the stability and facilitates the formation of martensite. However, if the content is less than 0.1%, the effect is not obtained, and the low-temperature transformation product is only bainite, and it is not possible to achieve a low yield point and a high elongation. On the other hand, if it is added in excess of 2.0%, not only does the cost increase, but also martensite as a low-temperature transformation product, the amount of bainite decreases, the stretch flangeability deteriorates, and the zinc plating property deteriorates. Since this deteriorates, the upper limit is set.

It is desirable that inevitable impurities be as small as possible. For example, even if N (nitrogen) is present in the steel as solid solution N or as a precipitate in the form of AlN in the steel, the ductility is deteriorated. Therefore, N (nitrogen) is desirably 100 ppm or less.

The steel having the above-mentioned composition is manufactured as a galvannealed steel sheet in a hot-dip galvanizing line. It is important in the present invention that the composite has a three-phase composite structure of bainite and martensite, each of which has at least 4% or more of a balance.

By forming a composite structure of the ferrite phase and the hard phase, characteristics as a bake hardening type high-strength alloyed hot-dip galvanized steel sheet such as a low yield point, a high BH property and an ordinary temperature aging property can be obtained. Then, in order to improve elongation and stretch flangeability as workability, it is necessary to define the type and ratio of this hard low-temperature transformation product, and at least 4% or more of each of bainite and martensite.

That is, although only the hard martensite hard phase has excellent uniform elongation, the hardness difference between the matrix and the ferrite phase is large, so that when deformed, voids are easily generated at the interface, and the stretch flangeability is increased. Inferior in

On the other hand, bainite alone is excellent in stretch flangeability because it is softer than martensite, but has a small difference in hardness from the ferrite phase, and thus has insufficient characteristics as a composite structure. It is difficult to achieve improvement. Therefore, in order to make use of the features of both, it is necessary that each of them be present in 4% or more.

On the other hand, as an upper limit, the tensile strength is set to 390 to 390.
On the other hand, the upper limit of the hard phase is defined by setting the ratio of the ferrite phase to be equal to or more than a certain value so as to be about 690 N / mm ² .
That is, the ratio of the ferrite phase is set to 70% or more. In addition, the ratio referred to here is an area ratio of each tissue recognized at the time of tissue observation.

Next, as a manufacturing method for obtaining the above-mentioned structure,
The following method is recommended.

First, the hot rolling may be basically carried out according to a conventional method, with a heating temperature of 1000 to 1300 ° C., a finishing temperature of 800 to 950 ° C., and a cooling rate after the finish rolling of 30 to 12 ° C.
There is no problem if it is carried out under the ordinary condition range of 0 ° C./sec.

However, the winding temperature is specified to be 700 ° C. or lower. However, if the winding is performed at a temperature higher than this temperature, the scale of the surface becomes thick and the pickling property deteriorates. Although the lower limit is not particularly defined, if it is too low, the cold rolling property is reduced.
It is desirably at least 00 ° C.

After hot rolling, pickling and cold rolling are performed according to a conventional method, but cold rolling is preferably performed at 40% or more. If the cold rolling rate is lower than this, the hot rolled sheet for obtaining a desired product becomes thin and long, which undesirably affects productivity during pickling.

Next, recrystallization annealing is performed. This annealing is A
It may be heated by c ₁ transformation point or more, but it is preferable to heat above 780 ° C. is a component steel of the present invention range for stabilization of workability. There is no particular upper limit, but there is no problem if the temperature is 900 ° C. or lower. 10 seconds for soaking at high temperature
It is sufficient to hold the above, and a ferrite + austenite structure can be obtained.

After the soaking heat treatment, the temperature is cooled down to the plating bath temperature and plating treatment is performed.
At a cooling rate of 7 ° C / sec or less, 70% or more
Slowly cooled to a temperature range of 0 to 600 ° C, and thereafter 420 to 52
Cool at 10 ° C / sec or more to a plating bath of 0 ° C to obtain a bainite structure of 4% or more. Cooling rate after soaking is 7
When the temperature is higher than ℃ / sec, and the temperature exceeds the slow cooling end temperature of 700 to 600 ° C, bainite or pearlite is formed, and the ratio of the ferrite phase does not become 70% or more. On the other hand, if the cooling rate after the slow cooling is less than 10 ° C./sec, pearlite is formed and a desired structure cannot be obtained.

It should be noted that there is no problem in maintaining the temperature at a constant temperature until immersion in the plating bath. Alloying process is 500-7 as usual
It may be carried out at about 50 ° C., in the process from soaking to the end of alloying, a bainite structure of 4% or more is generated, C is concentrated in the remaining austenite, and a room temperature at a cooling rate of 10 ° C./sec or more. By cooling to above, a martensite structure of 4% or more is obtained. With the component composition and the production conditions falling within the range of the present invention, 4% or more of austenite remains at the end of the alloying treatment, so that it is transformed into martensite at a cooling rate of 10 ° C./sec or more as described above.

Next, temper rolling of 0.3 to 0.8% is performed. If the elongation is less than 0.3%, the roughness is coarse, and the sharpness after molding is impaired. If it exceeds 0.8%, the YP (yield point) rises remarkably, so this is set as the upper limit.

Next, examples of the present invention will be described.

[0037]

【Example】

Steels having the chemical components shown in Table 1 were melted to form slabs. The slab is heated to 1150 ° C.,
Hot rolled to 3.2mm thickness at 70-900 ° C, 50 ° C / s
Each was wound at 600 ° C. at an average cooling rate of ec. After pickling, cold-rolled to a thickness of 1.2 mm, annealed under the conditions shown in Table 2, and subsequently plated in a zinc bath at 440 ° C.
The alloying treatment was performed at a temperature of ℃. After the temper rolling at the elongation shown in Table 2, the mechanical properties were investigated. Table 3 shows the results.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

As is clear from Table 3, the examples of the present invention (No.
1 to No. 2, No. 5, and No. 7) show high strength and good strength-ductility balance, have a BH property of 30 N / mm ² or more, and have an extremely low yield ratio (YR). Is shown.

On the other hand, the comparative examples (No. 3 to No. 4, N
In No. 6, No. 8, No. 9 to No. 12), since the chemical composition or the production conditions are out of the range of the present invention, the structure does not become the optimal composite structure aimed at in the present invention, and the yield The ratio is 0.66
-0.80, which is higher than that of the examples of the present invention, and it is difficult to say that the strength-ductility balance is good. In addition, the comparative examples (No. 6, No. 1)
In Nos. 0 to 11), the BH property is also low and is lower than 30 N / mm ² .

[0044]

As described in detail above, according to the present invention,
A baking-hardened high-strength galvannealed steel sheet having a high tensile strength of about 390 to 690 N / mm ² , a low yield ratio and excellent ductility can be easily obtained.

────────────────────────────────────────────────── (5) Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI C23C2 / 28 C23C2 / 28 (56) References JP-A-58-39770 (JP, A) JP-A-4-128320 (JP) JP-A-4-128321 (JP, A) JP-A-56-69359 (JP, A) JP-A-57-123956 (JP, A) JP-A-55-122821 (JP, A) 6-65684 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C23C 2/00-2/40 C21D 9/46

Claims (2)

(57) [Claims] 1. C: 0.04% by weight (hereinafter the same)
0.15%, Si: 0.20% or less, Mn: 1.0% to 2.5
%, P: 0.050% or less, S: 0.020% or less, Al:
0.010 to 0.120%, Cr: 0.1 to 2.0%, the balance has a composition of iron and unavoidable impurities, 70% or more of a ferrite phase, and the balance is at least A bake hardening type high-strength galvannealed steel sheet having excellent workability, characterized by having a three-phase composite structure of bainite and martensite of 4% or more. 2. The steel having the chemical composition according to claim 1, which is hot-rolled by a conventional method, wound up at a temperature of 700 ° C. or less, pickled, and cold-rolled at 40% or more, and then 780 ° C. After the above soaking process for 10 seconds or more, and then gradually cooling to a temperature range of 700 to 600 ° C. at a cooling rate of 7 ° C./sec or less,
Cool at 10 ° C / sec or more, apply hot-dip galvanizing at 420 to 520 ° C, then perform alloying treatment,
A method for producing a bake hardenable high-strength galvannealed steel sheet having excellent workability, characterized in that the steel sheet is cooled to room temperature at a temperature equal to or higher than c and subjected to a temper rolling of 0.3 to 0.8%.