JP3304747B2 - Cold rolled steel sheet excellent in balance between bake hardenability, ductility and normal temperature aging, and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet which is suitable for use as an outer panel for automobiles and has an excellent balance between bake hardenability, ductility and aging at ordinary temperature, and a method for producing the same.

[0002]

2. Description of the Related Art A steel sheet used for an outer panel of an automobile is required to have good formability at the time of press working and to have a predetermined strength at the time of a final product. However, ductility generally decreases when the steel sheet itself is strengthened. As a steel sheet having the conflicting required performance, a paint bake hardenable steel sheet (BH steel sheet) based on an ultra-low carbon system has been conventionally known. Since this steel sheet is a very low carbon steel, it is soft at the time of forming. Further, since solid solution C remains, it has a property of hardening after molding and baking, that is, baking hardenability (bake hardness: BH property). Since the BH property of the steel sheet is caused by the strain aging phenomenon of C and N dissolved in the steel, it is necessary to increase the solute C and the solute N in the steel in order to improve the BH property.
On the other hand, when the amount of solute C and the amount of solute N in steel are increased, the BH property is improved, but the ductility and the aging resistance at room temperature are deteriorated. Therefore, a steel sheet having an excellent balance between BH property, ductility, and aging resistance at room temperature is desired.

In response to such a request, for example,
Japanese Patent Application Laid-Open No. -10855 and Japanese Patent Application Laid-Open No. 57-89437 propose a technique for producing a cold-rolled steel sheet having excellent deep drawability by using ultra-low carbon steel to which Nb or Ti is added.

However, this technique requires high-temperature annealing at 850 ° C. or higher, which is disadvantageous not only in cost, but also in deterioration of the surface properties and shape of the plate, and roughening due to coarsening of crystal grains. Problem.

[0005] Japanese Patent Publication No. 61-45689 discloses a technique for producing a cold-rolled steel sheet for deep drawing excellent in BH property from an ultra-low carbon steel to which Nb and Ti are added in combination. In this technique, Ti is added to fix N as TiN to avoid deterioration at room temperature and to prevent deterioration of the material. Nb partially fixes C as NbC and forms a solid solution that contributes to BH properties and room temperature aging. It is added to control C.

However, even in this technique, BH
It is hard to say that the balance between ductility, ductility and room temperature aging is still sufficient.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and does not cause problems such as deterioration of surface properties and shape and rough skin, and has bake hardenability, ductility, and room temperature aging. It is an object of the present invention to provide a cold-rolled steel sheet excellent in balance with the above, and a method for producing the same.

[0008]

In order to control the BH property, that is, to control the amount of solid solution C, carbon nitrides such as Nb and Ti are used in the above-mentioned BH steel sheet based on an ultra-low carbon system. Forming elements are added. Therefore, in such a BH steel sheet, Nb-based and Ti-based precipitates and solid solution C coexist. Such precipitates are repeatedly melted and precipitated in a series of processes such as cast solidification, hot rolling, cold rolling, and annealing, and thus are likely to be unevenly distributed in the thickness direction and the width direction of the steel sheet. In general, it is known that such Nb and Ti-based precipitates affect recrystallization and grain growth of ferrite similarly to AlN. For this reason, if these precipitates are unevenly distributed in the steel before the ferrite is recrystallized, the ferrite grains after the recrystallization become uneven. The ultra-low carbon steel has a clean grain boundary, so that the grain boundary is weak, so that stress is likely to concentrate near the grain boundary during press forming, and local cracks are likely to occur. In this case, if the ferrite grains are non-uniform as described above, local cracks are further promoted and ductility is reduced.

Further, when an element having a relatively high affinity for C, such as a carbonitride forming element, exists in a solid solution state in steel, it is considered that the following effects are exerted on the strain aging of C. . That is, solid solution C is densely distributed around elements that could not be carbonitride due to their affinity. Then, in an atmosphere in which the diffusion of C is not active, such as at room temperature aging, solid solution C is trapped due to its affinity. For this reason, C cannot fix dislocations. However, in an atmosphere in which C diffusion is extremely active, as in the case of paint baking, solid solution C can move without being affected by the affinity of the mononitride forming element, thereby fixing dislocations.

[0010] The present inventors have studied the effect of such a carbonitride on the ferrite structure and the fact that the carbonitride forming element is C
As a result of diligent studies in consideration of the effect of steel on strain aging, based on Nb, V composite added ultra-low carbon steel, seizure that could not be obtained with the ultra-low carbon steel manufactured by the conventional technology was obtained. The inventors have found that a cold-rolled steel sheet excellent in balance between curability, ductility, and normal temperature aging can be stably manufactured, and have completed the present invention.

The present invention has been completed based on such findings, and C: 0.001 to 0.008 wt.
%, Si: 0.3 wt% or less, Mn: 1.5 wt%
Hereinafter, P: 0.1 wt% or less, S: 0.015 wt% or less, Al: 0.1 wt% or less, N: 0.005 wt% or less, Nb: 0.003 to 0.06 wt%, V: 0.03
5 to 0.14 wt% , the balance being substantially composed of Fe, and further satisfying the formulas (1) and (2) shown below. It is intended to provide a cold-rolled steel sheet having an excellent balance. 0.0002 ≦ C− (12/93) Nb ≦ 0.002 (wt%) (1) V ≧ 0.55Nb + 0.001 (wt%) (2)

C: 0.001 to 0.008 wt%
Less than, Si: 0.3 wt% or less, Mn: 1.5 wt% or less, P: 0.1 wt% or less, S: 0.015 wt% or less, Al: 0.1 wt% or less, N: 0.005 wt% or less, Nb: 0.003 to 0.06 wt%, V: 0.03
A steel containing 5 to 0.14 wt% , the balance being substantially Fe, and further satisfying the formulas (1) and (2) shown below is melted, then hot-rolled, and then exceeds 600 ° C. 7
An object of the present invention is to provide a method for producing a cold-rolled steel sheet excellent in the balance between bake hardenability, ductility and room-temperature aging, characterized by winding at a temperature of 50 ° C. or lower, followed by cold rolling and continuous annealing.

0.0002 ≦ C− (12/93) Nb ≦ 0.002 (wt%) (1) V ≧ 0.55Nb + 0.001 (2)

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described specifically.

The steel sheet of the present invention has a C content of 0.001 to 0.00.
Less than 8 wt%, Si: 0.3 wt% or less, Mn: 1.5
wt% or less, P: 0.1 wt% or less, S: 0.015 w
t% or less, Al: 0.1 wt% or less, N: 0.005 w
t% or less, Nb: 0.003 to 0.06 wt%, V:
0.035-0.14 wt% is contained.

The reasons for defining each component in this manner are as follows.

C: 0.001 to less than 0.008 wt% C is an effective element for securing bake hardenability.
However, if it is less than 0.001% by weight, the desired bake hardenability cannot be obtained. If the content is 0.008 wt% or more, the baking hardenability-ductility and the normal temperature aging balance are not improved by the addition of Nb and V as described later. Therefore, C
The content is in the range of 0.001 to less than 0.008 wt%.

Si: 0.3 wt% or less Si is a strengthening element for steel, and is added to secure strength. However, when the content exceeds 0.3 wt%, remarkable deterioration of ductility and deterioration of surface properties are caused. Therefore,
The Si content is set to 0.3 wt% or less.

Mn: 1.5 wt% or less Mn is added to secure the strength of steel and to avoid hot brittleness due to S. However, when the content exceeds 1.5 wt%, remarkable deterioration of ductility is caused. Therefore, the Mn content is set to 1.5 wt% or less.

P: 0.1 wt% or less P is an element effective for strengthening steel. However, when the content exceeds 0.1 wt%, ductility is significantly deteriorated. Therefore, the P content is set to 0.1 wt% or less.

S: 0.015 wt% or less If the content of S exceeds 0.015 wt%, remarkable red hot embrittlement is caused, so S is set to 0.015 wt% or less.

Al: 0.1 wt% or less Al is added for deoxidizing steel. However, 0.1w
If it exceeds t%, the ductility and the surface properties deteriorate. Therefore, the Al content is set to 0.1 wt% or less. In the present invention, since N is fixed as Nb, V-based carbonitride, AlN is not substantially formed.

N: 0.005 wt% or less N is fixed as Nb, V-based carbonitride. However, if the amount of N exceeds 0.005 wt%, there is a concern that the room-temperature aging property is deteriorated.

Nb: 0.003 to 0.06 wt% Nb is an element having a strong bonding force with C, and is added in order to partially fix C and secure baking hardenability. In this case, by coexisting with V, Nb, V-based precipitates are uniformly formed at the stage of winding treatment and annealing heating after hot rolling. This precipitate affects the recrystallization and grain growth of the ferrite grains during the annealing step, and makes the ferrite grains after recrystallization uniform. As a result, local stress concentration on the ferrite grain boundary caused when the ferrite grains are non-uniform is alleviated, and ductility is improved. However, if the Nb content is less than 0.003 wt%, such an effect cannot be obtained,
On the other hand, if the content exceeds 0.06%, the precipitates become excessive, so that the ductility deteriorates. Therefore, in order to stably obtain such an effect, the content of Nb is set to 0.003 to 0.06.
wt% range.

V: 0.035 to 0.14 wt% V coexists with Nb to significantly improve the balance between bake hardenability and ductility as described above. Further, V has a weaker bonding force with C than Nb.
It is not preferable to secure the baking hardenability by partially fixing C only by itself. Therefore, in order to stably secure bake hardenability, it is necessary to add Nb which is an element having a stronger bonding force with C than V. By the way, since V has a weak bonding force with C, most of them exist in a solid solution state. However, since V has a bonding force with C, V cannot form a carbide around the solid solution V. Are distributed. This solid solution C
In an atmosphere in which the diffusion of C is active, such as in the case of baking paint, the C moves away from the bonding force with V to fix dislocations. Therefore, baking hardenability is obtained. On the other hand, in an atmosphere in which C diffusion is relatively inactive, such as normal temperature aging, C is trapped in V. By adding V in this manner, the effect that the aging resistance at room temperature is not deteriorated and the bake hardenability is obtained can be obtained. However, such an effect is obtained when V is less than 0.035 wt%, and when V exceeds 0.14 wt%, not only such an effect is not obtained, but also the ductility is not improved due to an excessive amount of precipitates. Therefore, the content of V is set to 0.035 to
The range is 0.14 wt% .

Others are substantially Fe, but unavoidable impurities and other minor additives are acceptable.

In the present invention, it is also required that the following formulas (1) and (2) be satisfied in addition to the above individual component ranges.

0.0002 ≦ C− (12/93) Nb ≦ 0.002 (wt%) (1) V ≧ 0.55Nb + 0.001 (wt%) (2) FIG. It is a figure which shows C content and Nb content on a vertical axis | shaft, and shows the relationship between these content and the balance of baking hardenability, ductility, and normal temperature aging. This figure shows that V: in the range of 0.01 to 0.1 wt% and V:
tr. This shows the results of determining the balance BH + El between bake hardenability and ductility, and the balance BH-AI between bake hardenability and room temperature aging, as described below.
Other components were within the scope of the present invention. In the figure, ○ indicates B: BH + E in the range of 0.01 to 0.1 wt%.
l was 90 to 120, BH-AI was 30 to 50, and both balances were excellent, and Δ was V: 0.01 to 0.1 wt%.
Where BH + El is 60 to 100 and BH-AI is 1
0 to 20: poor balance between bake hardenability, ductility, and aging at normal temperature, x: V: tr. belongs to. From this figure, when V is in the range of 0.01 to 0.1 wt% of the present invention, the value of C- (12/93) Nb is 0.0002.
It is confirmed that the content of baking hardenability, ductility and aging at room temperature is excellent when the content is 0.002 wt%.

That is, when the value of C- (12/93) Nb is less than 0.0002 wt%, the solid solution C is too small, so that not only the bake hardenability is not obtained but also the Nb as described above. And V cannot be obtained. On the other hand, if this value exceeds 0.002 wt%, the amount of solid solution C becomes excessive, so that the effects of Nb and V as described above cannot be obtained. Therefore, in the present invention, it is required that the above expression (1) is satisfied.

As shown in FIG. 1, V: tr. , The value of BH-AI is 1 even if equation (1) is satisfied.
It is as low as 0 to 20, and good balance between bake curability and aging at room temperature cannot be obtained.

FIG. 2 shows the relationship between the Nb content on the horizontal axis and the V content on the vertical axis, and the balance between these contents and bake hardenability, ductility and room temperature aging. And the balance between bake hardenability and ductility described below BH + E
l and the balance between bake hardenability and room temperature aging BH-
It shows AI. Other components were within the scope of the present invention. In the figure, ○ indicates that BH + El is 90 to 1
20, BH-AI having a good balance between 30 and 50, and B having a BH + El of 60-100 and a BH-AI of 10-20, the balance between bake hardenability, ductility and room temperature aging was poor. Things. From this figure, V is w
It is confirmed that the balance between baking hardenability, ductility, and aging at room temperature is excellent at 0.55 Nb + 0.001 or more at t%.

That is, as described above, when V coexists with Nb, a good balance between bake hardenability, ductility and normal temperature aging can be obtained. 0.55Nb + 0.001 in wt%
It cannot be obtained with less than. Therefore, it is required that the above expression (2) be satisfied.

Next, a method for manufacturing a cold-rolled steel sheet according to the present invention will be described.

In the present invention, a steel having the composition described above is melted, hot-rolled, then wound at a temperature of more than 600 ° C. and 750 ° C., and subsequently subjected to cold rolling and continuous annealing. Manufacture of cold rolled steel sheets with excellent balance between bake hardenability, ductility and normal temperature aging.

In this case, the method of melting steel may be any of the converter method and the electric furnace method which are usually used, and the casting of steel may be any of the ingot casting method and the continuous casting method.

Subsequently, the cast slab is hot-rolled, and the hot rolling at this time may be either direct rolling or rolling after reheating. The finishing temperature is desirably at least _three points of Ar from the viewpoint of making the structure of the hot-rolled sheet uniform.

The subsequent winding is preferably carried out at a temperature higher than 600 ° C. and not higher than 750 ° C., in order to uniformly form Nb, V-based precipitates. This is because if the temperature is lower than 600 ° C., it is difficult to deposit.

The hot-rolled sheet is subjected to cold rolling after pickling, and the rolling reduction at this time is desirably 60 to 90%. By performing cold rolling in this range, Nb, V-based precipitates can be uniformly strain-induced precipitated in the subsequent heating step of annealing.

The soaking temperature during continuous annealing is desirably not less than the recrystallization temperature and not more than 900 ° C. This temperature is 90
If the temperature exceeds 0 ° C., the ferrite grains become not only non-uniform, but also coarse, and there is a fear of rough skin.

In the present invention, it is not hindered that the steel sheet produced in this way is further subjected to a surface treatment such as electroplating and chemical conversion treatment. The effect is not impaired.

The effects of the addition of Nb and V on the bake hardenability, ductility and room temperature aging as described above have been found for the first time by the present inventors, and Ti, Zr, M
It cannot be obtained in a system to which o, W, etc. are added.

Further, according to the present invention, a cold-rolled steel sheet having an excellent balance of baking hardenability, ductility and aging at ordinary temperature can be obtained without problems such as deterioration of surface properties and shape, and surface roughness.

[0043]

EXAMPLES (Example 1) Steels having the compositions shown in Tables 1 and 2 were melted and slabs were produced by continuous casting. After heating these slabs to 1250 ° C., hot rolling was performed at a finishing temperature of 890 ° C. to obtain a sheet thickness of 2.8 m.
m was prepared. Subsequently, it is wound at 650 ° C.
After pickling, cold rolling was performed to a sheet thickness of 0.7 mm. The soaking temperature is 830 ° C and the soaking time is 60 sec
And then temper rolling of 1.0% was performed. Note that, in Table 1, No. 1 to 30 (however, steel No. 2,
Nos. 3, 6, 9, 13, and 16) are examples of the present invention. 31 to 58 are comparative examples.

[0044]

[Table 1]

[0045]

[Table 2]

The steel sheet thus obtained was subjected to a tensile test. The tensile test was performed using an JIS No. 5 test piece specified in JIS Z2201 using an Instron type testing machine.

The bake hardenability (BH property) and the aging property at room temperature (aging index AI) were also evaluated. For the former,
After applying 2% prestrain, heat treatment is performed at 170 ° C for 20 minutes, and the amount of increase in yield strength after heat treatment is evaluated. For the latter, after 8% prestrain is applied, heat treatment is performed at 100 ° C for 60 minutes. Evaluation was made based on the amount of increase in yield strength after heat treatment.

Further, the uniformity of the ferrite grains was evaluated as follows. First, samples were randomly sampled from 25 locations in each of the width direction of the annealed plate and the longitudinal direction of the plate, a total of 50 places, and the structure of the longitudinal cross section of the plate and the cross section in a direction perpendicular thereto were observed. The ferrite grain size was measured. The particle size is measured by a cutting method specified in JIS G 0552. The evaluation was made by the equivalent circle diameter. The frequency distribution of 100 data obtained in this way is obtained,
The deviation from the average particle size is 2 μm out of 100 data.
Within 90 or more, ◎, 80 to 89, ○, 70 to 79, △, 69 or less, ×, ○ and ○, ferrite grains In the case of Δ and ×, it was determined that the ferrite grains were non-uniform.

Tables 3 and 4 show the evaluation results.
In the table, YP represents the yield point, TS represents the tensile strength, El represents the elongation, and BH and AI are the bake hardenability and the room temperature aging index determined as described above, respectively. The values of BH + El and BH-AI were also determined as values for evaluating the balance between bake hardenability and ductility and between bake hardenability and room temperature aging. The meaning of these values is as follows. That is, the higher the BH and the larger the El, the better the balance between the bake hardenability and the ductility. Therefore, the larger the BH + El, the better the balance. In addition, the higher the BH and the lower the AI, the better the balance between bake hardenability and normal temperature aging.
The larger the value of is, the better these balances are.

[0050]

[Table 3]

[0051]

[Table 4]

As shown in Table 3, No. 1 of the present invention was used .
1 to 30 (However, Nos. 2, 3, 6, 9, 13, and 16
Steel plate of the missing number) is, BH + El is 90~120, BH-A
1 was 30 to 50, and it was confirmed that the balance between bake hardenability and ductility and the balance between bake hardenability and room temperature aging were both good. The evaluation of the uniformity of the ferrite grains was ○ or ◎, and it was confirmed that the ferrite grains were uniform.

On the other hand, as shown in Table 4, the composition of Comparative Example having a composition out of the range of the present invention. 31 to 58 had poor balance of these. That is, No. 31-
34, 36, 38, 43, 46 and 52 have BH + El of 9
The value of BH-Al was less than 30, and the balance between bake hardenability and ductility, and the balance between bake hardenability and room temperature aging were both poor. In addition, No. 35, 37, 3
9 to 42, 44, 45, 47 to 51, 53 to 58 are B
The H-AI was less than 30, and the balance between bake hardenability and room temperature aging was poor. In addition, the evaluation results of the uniformity of these ferrite grains were all × or ×, indicating that the ferrite grains were non-uniform.

Example 2 In Tables 1 and 2 above, N
o. After heating a steel slab having a composition of 4,44,52,54,58, the sheet thickness was 2.8 m under the hot rolling conditions shown in Tables 5 to 7.
m was prepared. Subsequently, cold rolling was performed to a thickness of 0.7 mm, continuous annealing was performed under the conditions shown in Table 5, and then temper rolling was performed at an elongation of 1.0%. The mechanical properties of the obtained steel sheet are also shown in these tables.

[0055]

[Table 5]

[0056]

[Table 6]

[0057]

[Table 7]

No. 1 having a composition within the scope of the present invention. The steel sheet No. 4 has a BH + El of 90 to 1 regardless of the manufacturing conditions.
20, BH-AI was 30 to 50, and it was confirmed that the balance between bake hardenability and ductility and the balance between bake hardenability and room temperature aging were both good. Among them, No. 4a to j, the winding temperature is over 600 ° C. to 750
Because of the temperature range, both balances were particularly excellent. In each case, the evaluation of the uniformity of the ferrite grains was evaluated as ○ or ◎.
And it was confirmed that the ferrite grains were uniform.

On the other hand, No. 1 having a composition outside the scope of the present invention. Regarding the steel sheets of Nos. 44, 52, 54, and 58, the BH-AI value was less than 30 irrespective of the production conditions, and the balance between bake hardenability and normal temperature aging was poor. In addition, the evaluation results of the uniformity of these ferrite grains were all × or ×, indicating that the ferrite grains were non-uniform.

The results of Example 1 and Example 2 are shown in FIGS. 3 and 4. That is, FIG. 3 is a diagram showing the relationship between BH and El of each sample, and FIG.
FIG. From these figures, in the case of the present invention, the value of BH + El is excellent at 90 to 120, and the value of BH-AI is excellent at 30 to 50. Particularly, when the winding temperature exceeds 600 ° C to 720 ° C or less, these values are high. It is clear that these values, especially BH-AI, are low in the case of the comparative example.

[0061]

As described above, according to the present invention,
A cold-rolled steel sheet which does not suffer from problems such as deterioration of surface properties and shape and surface roughness, and has an excellent balance between bake hardenability, ductility and room temperature aging, and stable production of such a cold-rolled steel sheet. Is provided. For this reason, the cold rolled steel sheet of the present invention is suitable for applications such as automobile outer panels.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the contents of C and Nb and the balance between bake hardenability, ductility, and aging at room temperature.

FIG. 2 is a graph showing the relationship between Nb and V contents and the balance between bake hardenability, ductility and room temperature aging.

FIG. 3 is a diagram showing the balance between bake hardenability and ductility in Examples 1 and 2.

FIG. 4 is a diagram showing the balance between bake hardenability and normal temperature aging in Examples 1 and 2.

Continued on the front page (72) Inventor Yutaka Horiuchi 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Hiroshi Baba 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-5-171351 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: less than 0.001 to 0.008 wt%, Si: 0.3 wt% or less, Mn: 1.5 wt% or less, P: 0.1 wt% or less, S: 0.015 wt% or less, Al : 0.1 wt% or less, N: 0.005 wt% or less, Nb: 0.003 to 0.06 wt%, V: 0.035 to 0.14 wt%, the balance being substantially composed of Fe. A cold-rolled steel sheet excellent in balance between bake hardenability, ductility, and aging at room temperature, characterized by satisfying the formulas (1) and (2) shown in the following. 0.0002 ≦ C− (12/93) Nb ≦ 0.002 (wt%) (1) V ≧ 0.55Nb + 0.001 (wt%) (2) 2. C: 0.001 to less than 0.008 wt%, Si: 0.3 wt% or less, Mn: 1.5 wt% or less, P: 0.1 wt% or less, S: 0.015 wt% or less, Al : 0.1 wt% or less, N: 0.005 wt% or less, Nb: 0.003 to 0.06 wt%, V: 0.035 to 0.14 wt%, the balance being substantially composed of Fe. After smelting steel satisfying the formulas (1) and (2) shown in the following, hot rolling, winding at a temperature of 600 ° C. or more and 750 ° C. or less, and subsequent cold rolling and continuous annealing are performed. A method for producing a cold-rolled steel sheet having an excellent balance between bake hardenability, ductility, and normal temperature aging. 0.0002 ≦ C− (12/93) Nb ≦ 0.002 (wt%) (1) V ≧ 0.55Nb + 0.001 (wt%) (2)