JP2003089842A - High tensile strength steel sheet with excellent workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength steel sheet combining excellent elongation with excellent stretch-flange formability both as indices to workability. SOLUTION: The steel sheet has a structure which is composed essentially of ferrite single-phase structure and in which precipitates containing Nb and Mo in the amounts within the range satisfying Mo/(Nb+Mo)>=0.25 by atomic % are dispersedly precipitated. Further, this steel sheet has >=590 MPa tensile strength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet which is suitable as a material for automobile members and has excellent workability.

[0002]

2. Description of the Related Art From the viewpoint of improving fuel efficiency leading to environmental protection,
Although there is a strong demand for high-strength, thin steel sheets for automobiles,
Many automobile members have a complicated shape obtained by press molding, and a material having both excellent stretchability and stretch flangeability, which is an index of workability, is required.

Conventionally, various kinds of steel sheets of this kind have been proposed. For example, Japanese Patent Application Laid-Open No. 6-172924 proposes a steel sheet having a stretch flangeability which is formed by a bainitic ferrite structure having a high dislocation density. ing. However, this steel sheet has a drawback that it has poor elongation because it contains a bainitic ferrite structure with a high dislocation density.

Japanese Unexamined Patent Publication No. 6-200351 proposes a steel sheet having a structure in which the majority of the structure is polygonal ferrite, which is mainly TiC and is precipitation strengthened and solid solution strengthened and has excellent stretch flangeability. However, generally well-known precipitates used for this steel sheet have a drawback that precipitates with large dimensions are easily generated and elongation and stretch-flangeability are low when compared with the same strength.

Japanese Unexamined Patent Publication (Kokai) No. 7-11382 discloses an acicular material in which fine TiC and / or NbC are deposited.
A steel sheet having a ferrite structure and excellent in stretch flangeability has been proposed. However, this steel sheet, like the above-mentioned steel sheet proposed in Japanese Patent Laid-Open No. 6-172924, has a structure with a high dislocation density called acicular ferrite, and therefore sufficient elongation cannot be obtained.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and is an index of workability suitable for applications such as automobile parts having a complicated sectional shape at the time of pressing. It is an object of the present invention to provide a high-strength steel sheet having excellent stretch flangeability and stretch flangeability.

[0007]

Means for Solving the Problems The inventors of the present invention have earnestly studied in order to achieve the above object, and have obtained the following findings. (1) When the structure has a low dislocation density and is strengthened by fine precipitates, the strength-elongation balance is improved. (2) When the structure is made substantially single-phase and strengthened with fine precipitates, the strength-stretch flangeability balance is improved. (3) When a composite precipitate containing Mo is used, the precipitate is finely precipitated. (4) When the proportion of Mo in the composite precipitate becomes low, the precipitate becomes coarse, so that both elongation and stretch-flange formability decrease.

The present invention has been completed based on these findings and provides the following (1) to (5).

(1) A ferrite single-phase structure is substantially formed, and precipitates containing Nb and Mo are dispersed and deposited in a range satisfying Mo / (Nb + Mo) ≧ 0.25 in atomic%. , A high-strength steel sheet with a tensile strength of 590 MPa or more and excellent workability.

(2) In the above (1), the precipitate further contains V, and Mo / (Nb + V + M in atomic%).
o) A high-tensile steel plate excellent in workability, characterized by satisfying ≧ 0.25.

(3) In the above item (1), in weight%,
C: 0.02-0.06%, Si: 0.3% or less, M
n: 0.5 to 2.0%, P: 0.06% or less, S: 0.
005% or less, Al: 0.06% or less, N: 0.006
% Or less, Mo: 0.1 to 0.5%, Nb: more than 0.06% and 0.35% or less, and the balance is substantially Fe, which is a high-tensile steel sheet excellent in workability. .

(4) In the above (1) or (2), C: 0.02 to 0.06%, Si: 0.
3% or less, Mn: 0.5 to 2.0%, P: 0.06% or less, S: 0.005% or less, Al: 0.06% or less,
N: 0.006% or less, Mo: 0.1 to 0.5%, N
b: A high-tensile steel plate excellent in workability, characterized by containing more than 0.06% and 0.35% or less, V: 0.15% or less, and the balance being substantially Fe.

(5) In the above (1) or (2), C: 0.02 to 0.06%, Si: 0.
3% or less, Mn: 0.5 to 2.0%, P: 0.06% or less, S: 0.005% or less, Al: 0.06% or less,
N: 0.006% or less, Mo: 0.1 to 0.5%, N
b: more than 0.06% and 0.35% or less, V: 0.15% or less, Ti: 0.005% or more and less than 0.03%, and the balance is substantially Fe High strength steel plate with excellent properties.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. The high-strength steel sheet according to the present invention has a substantially ferrite single-phase structure, and in atomic%, Mo / (Nb + Mo) ≧
Precipitates containing Nb and Mo are dispersed and deposited in a range satisfying 0.25. This precipitate contains V in addition to Nb and Mo.
May be included, in which case, in atomic%, Mo /
(Nb + V + Mo) ≧ 0.25 is preferable. The following is a description of each item.

Substantially ferrite single-phase structure: The matrix is made to have a substantially ferrite single-phase structure because ferrite having a low dislocation density is effective for improving elongation, and for improving stretch flangeability. This is because it is effective to have a single-phase structure, and the effect is particularly remarkable in a ferrite single-phase structure that is rich in ductility. However, the matrix does not necessarily have to have a completely ferrite single-phase structure, and may have a substantially ferrite single-phase structure. Here, the term "substantially ferrite single-phase structure" means that a small amount of another phase or precipitate other than the precipitate of the present invention is allowed, and the area ratio of ferrite is preferably 95% or more.

Mo / (Nb + Mo) ≧ 0.
Precipitate containing Nb and Mo in the range of 25: Nb and Mo
Precipitates containing and become fine, which is effective for strengthening steel. Conventionally, it has been the mainstream to use NbC as a precipitate, but Nb has a strong tendency to form precipitates, so Mo
If it does not contain, it tends to coarsen and the effect on strengthening becomes low. Therefore, in order to obtain the required strengthening amount, a precipitate is required until the workability is deteriorated. On the other hand, N
The composite precipitate containing b and Mo finely precipitates and can strengthen the steel without deteriorating the workability. This is Mo
Since the tendency of the precipitate to form is weaker than that of Nb, the precipitate can exist stably and finely, so that the effect of strengthening is high, and the necessary amount of strengthening can be obtained with the amount of the precipitate that can maintain good workability. It is thought to be because. In particular, by setting the average particle size of the composite precipitate to be less than 10 nm, the strain around the precipitate becomes more effective for the resistance to the movement of dislocations, and good steel strengthening can be obtained. The particle size is preferably less than 10 nm. More preferably, the average particle size is 5 nm or less. In order for the precipitates to exist stably and finely, the composition of the precipitates influences, and the composition of the precipitates has an atomic ratio of Mo / (Nb + Mo) ≧
When it is 0.25, the effect of suppressing coarsening of precipitates becomes high, and desired fine precipitates can be obtained.

.V in addition to Nb and Mo, atomic%
And precipitates in which Mo / (Nb + V + Mo) ≧ 0.25: The precipitates may be those in which V is compounded and precipitated in addition to Nb and Mo. Since the tendency of Mo to form precipitates is weaker than that of V, the composite precipitates can exist stably and finely, like the composite precipitates of Nb and Mo. In this case, the composition is Mo / (Nb + V + M) in atomic ratio.
It is preferable that o) ≧ 0.25. Within this range, the effect of suppressing the coarsening of the composite precipitate is high, and the required amount of strengthening can be obtained with the amount of the precipitate capable of maintaining good workability.

In the present invention, if the high-strength steel filled with the above-mentioned metal structure is obtained, desired elongation and stretch-flangeability and a strength of 590 MPa or more can be obtained, and the chemical composition is not particularly limited, but the weight percentage is C: 0. 0.02-0.06%, S
i: 0.3% or less, Mn: 0.5 to 2.0%, P: 0.
06% or less, S: 0.005% or less, Al: 0.06%
Below, N: 0.006% or less, Mo: 0.1 to 0.5
%, Nb: more than 0.06% and 0.35% or less, and the balance is preferably Fe. Further, as described above, when V is contained in the composite precipitate to further refine the structure, in addition to the above components, V: 0.15% or less is further contained, and the balance is substantially Fe. Is more preferable. When V is contained, Ti:
It is more preferable to contain 0.005% or more and less than 0.03%. Hereinafter, each of these components will be described.

C: 0.02-0.06% C forms carbides and is effective in strengthening steel. However, if it is less than 0.02%, the strengthening of the steel is insufficient,
If added in excess of 0.06%, pearlite is formed and precipitates are coarsened, which may impair the elongation and stretch flangeability. Therefore, the C content is preferably 0.02 to 0.06%.

Si: 0.3% or less Si is an effective element for solid solution strengthening, but if added in excess of 0.3%, precipitation of C from ferrite is promoted and coarse iron carbide is present at grain boundaries. Tend to precipitate, the stretch-flange formability tends to deteriorate, and the steel sheet surface quality deteriorates. Therefore, the Si content is preferably 0.3% or less.

Mn: 0.5 to 2.0% Mn is 0.5% from the viewpoint of strengthening steel by solid solution strengthening.
The above is preferable, but if added in excess of 2.0%, segregation occurs, a hard phase is formed, and stretch flangeability deteriorates. Therefore, the Mn content is preferably 0.5 to 2.0%.

P: 0.06% or less P is effective for solid solution strengthening, but if added in excess of 0.06%, segregation may occur and stretch flangeability may be deteriorated. preferable.

S: 0.005% or less S is preferably as small as possible. If it exceeds 0.005%, stretch flangeability may be deteriorated.
% Or less is preferable.

Al: 0.06% or less Al is added as a deoxidizing agent. However, if it exceeds 0.06%, both the elongation and the stretch flangeability tend to decrease, so 0.06% or less is preferable.

N: 0.006% or less N is preferably as small as possible, and if it exceeds 0.006%, coarse nitrides increase and stretch flangeability deteriorates.
06% or less is preferable.

Mo: 0.1-0.5% Mo is an important element in the present invention, and by containing 0.1% or more, pearlite transformation is suppressed and Nb is reduced.
It is possible to strengthen the steel while forming excellent composite elongation precipitates and stretch-flangeability by forming fine composite precipitates with and or fine composite precipitates containing V in addition to Nb. But,
If added in excess of 0.5%, a hard phase is formed and stretch flangeability is deteriorated. Therefore, the Mo content is 0.1 to
0.5% is preferable.

Nb: more than 0.06% and not more than 0.35% Nb is an important element in the present invention, and by forming a complex precipitate with Mo, while ensuring excellent elongation and stretch flangeability, steel can be obtained. Can be strengthened. But,
If it is less than 0.06%, the effect of strengthening steel is insufficient,
If it exceeds 0.35%, the elongation deteriorates. Therefore, Nb
The content is preferably more than 0.06% and 0.35% or less.

V: 0.15% or less V is effective for the fine graining of the structure, and Nb and M
Since it forms a composite precipitate with O and contributes to obtaining excellent elongation and stretch flangeability, it is added as necessary. However, if it exceeds 0.15%, the elongation deteriorates. Therefore, when V is contained, its content is preferably 0.15% or less.

Ti: 0.005% or more and less than 0.03% Ti contributes to obtaining excellent elongation and stretch flangeability by fixing N when V is added, and is therefore added as necessary. However, if it is less than 0.005%, the effect of fixing N is insufficient, and if it is 0.03% or more, the effect is saturated. Therefore, when Ti is contained, its content is preferably 0.005% or more and less than 0.03%.

In addition to the above components, Cr: 0.15% or less, Cu: 0.15% or less, Ni: 0.15% or less 1
There is no problem in characteristics even if more than one type is included.

The steel sheet of the present invention can be manufactured under the general manufacturing conditions for hot-rolled steel sheet.

[0032]

EXAMPLES Steel pieces having the chemical composition shown in Table 1 were
Heat to 0 ° C and finish at a finishing temperature of 8 by the usual hot rolling process.
A plate thickness of 3.2 mm was finished at 80 to 930 ° C. After that, the cooling rate and the winding temperature were changed at a winding temperature of over 600 ° C. to manufacture steel sheets having various structures.

After pickling the obtained steel sheets, the steel sheets were subjected to J
IS5 tensile test pieces and hole expansion test pieces were sampled and subjected to a tensile test and a hole expansion test. Tensile test pieces were taken from the vertical direction of rolling, and in the hole expanding test, a clearance of 12 mm was provided in the center of a 130 mm square steel plate by a 10 mmφ punch.
A test piece having a hole punched with 5% was prepared and pushed up from the direction opposite to the burr side of the punched hole by a 60 ° conical punch, and the hole diameter d at the time when the crack penetrated the steel plate was measured, and the hole expansion ratio λ Was calculated from the following formula. λ (%) = [(d-10) / 10] × 100

Further, the thin film produced from the steel sheet was observed by a transmission electron microscope (TEM) to measure the size of the precipitate. The composition of Nb, V, and Mo in the precipitate was determined by analysis by EDX equipped with TEM.

Table 2 shows tensile strength (TS) and elongation (E
1), hole expansion ratio (λ), structure, average particle size of precipitates, and composition of precipitates.

As shown in Table 2, Nos. 1-9
Both have a ferrite structure, the average grain size of the precipitates is less than 10 nm, the atomic percentage is Mo, and the Mo ratio is 0.25 or more. Therefore, the tensile strength (TS) is 590 MPa or more, which is excellent in elongation and elongation. Has flangeability.

On the other hand, the comparative steel No. In No. 10, since Mo is not added, precipitates are coarsened, and both elongation and stretch flangeability are low, and particularly stretch flangeability is low. In addition, No. In No. 11, since the amount of C was too large, pearlite was generated and the precipitate was coarsened, and both elongation and stretch flangeability were low, and particularly stretch flangeability was low. N
o. In No. 12, since the Mn content is too large, segregation is remarkable, and martensite is generated in the structure and the dislocation density is high, so that both elongation and stretch flangeability are low. No. 1
Since No. 3 has a small amount of Nb, the precipitates necessary for strengthening the steel are insufficient and the tensile strength (TS) is less than 590 MPa. No. Since No. 14 has an excessively large amount of Nb, both elongation and stretch flangeability are low, and particularly elongation is low. No. 1
In the case of No. 5, the amount of V is too large, so that the elongation is particularly low. No. 16
Since the amount of Si is too large, the precipitate tends to coarsen, and both elongation and stretch flangeability are low.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

As described above, according to the present invention,
It is possible to provide a high-strength steel sheet having excellent elongation and stretch flangeability, which are indicators of workability, and the effect of contributing to weight reduction of automobile members is remarkable.

Continued front page    (72) Inventor Kunikazu Tomita             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Tetsuo Yamamoto             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Eiji Maeda             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd.

Claims (5)

[Claims] 1. A ferrite single-phase microstructure, which is characterized in that precipitates containing Nb and Mo are dispersed and deposited in a range satisfying Mo / (Nb + Mo) ≧ 0.25 in atomic%. A high-strength steel sheet with excellent workability with a tensile strength of 590 MPa or more. 2. The precipitate further contains V and is in atomic%.
Satisfying Mo / (Nb + V + Mo) ≧ 0.25, the high-strength steel sheet with excellent workability according to claim 1. 3. By weight%, C: 0.02-0.06%, Si: 0.3% or less, Mn: 0.5-2.0%, P: 0.06% or less, S: 0. 0.005% or less, Al: 0.06% or less, N: 0.006% or less, Mo: 0.1 to 0.5%, Nb: more than 0.06% and 0.35% or less, and the balance is substantially The high-strength steel sheet with excellent workability according to claim 1, wherein the high-strength steel sheet is Fe. 4. By weight%, C: 0.02 to 0.06%, Si: 0.3% or less, Mn: 0.5 to 2.0%, P: 0.06% or less, S: 0. 0.005% or less, Al: 0.06% or less, N: 0.006% or less, Mo: 0.1 to 0.5%, Nb: more than 0.06% and 0.35% or less, V: 0.15 % Or less, and the balance is substantially Fe. The high-tensile steel sheet excellent in workability according to claim 1 or 2, wherein 5. By weight%, C: 0.02 to 0.06%, Si: 0.3% or less, Mn: 0.5 to 2.0%, P: 0.06% or less, S: 0. 0.005% or less, Al: 0.06% or less, N: 0.006% or less, Mo: 0.1 to 0.5%, Nb: more than 0.06% and 0.35% or less, V: 0.15 %, Ti: 0.005% or more and less than 0.03%, the balance being substantially Fe. The high-tensile steel sheet having excellent workability according to claim 1 or 2.