JP2910497B2 - Cold rolled steel sheet and surface treated steel sheet with excellent bake hardenability - 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 for processing and a surface-treated steel sheet having excellent bake hardenability suitable for use as, for example, an outer panel of an automobile.

[0002]

2. Description of the Related Art In recent years, problems related to the environment of the earth have been attracting public attention, and global warming due to an increase in the concentration of CO _{2 in} the atmosphere has become a particular problem. Therefore, in order to reduce the exhaust gas of automobiles, which causes an increase in CO ₂ , fuel efficiency is being improved by reducing the weight of the vehicle body. As one of the countermeasures, the outer panel of an automobile is made thinner and lighter. A particularly required property of the outer panel is dent resistance (the dent resistance of the steel sheet).

The dent resistance decreases as the thickness of the steel sheet decreases, and improves as the yield point of the steel sheet increases. Therefore,
In order to reduce the thickness of the steel sheet, it is necessary to increase the yield point. Therefore, in press forming, it is soft and easy to form, and the yield point rises due to strain aging due to solid solution C in steel in the coating baking process after pressing, that is, a bake hardening type steel plate having bake hardening property is used for the outer panel. In this case, it is possible to reduce the thickness of the panel while maintaining the same dent resistance even when the thickness is reduced.

Japanese Patent Application Laid-Open No. 63-171832 discloses a technique of adding Sn to a low-carbon aluminum killed steel as one method of improving the bake hardenability of a steel sheet. As low as less than 1.7, overaging is indispensable. Japanese Patent Application Laid-Open Nos. 4-41658 and 4-131357 disclose techniques for adding Sn.
Does not mention improving the bake hardenability. In JP-A-4-41658, embrittlement of the steel sheet becomes a problem because the amount of Sn added is increased. In addition, JP-A-4-131357
In the publication, it is indispensable to add Mo and B in combination, and the r value is low.

[0005]

The reuse of scrap as a raw material for steelmaking is expected to increase in the future, and it is conceivable that Sn inevitably mixes with steel. Therefore, Sn
There is a demand for a technology for positively adding and using phenol. However, conventional Sn not only reduces the elongation and r-value of steel sheets,
The problem of embrittlement of steel sheets is known. But,
The present inventors have found a cold-rolled steel sheet having good workability and high bake hardenability by adding Sn, and a surface-treated steel sheet obtained by performing a surface treatment on the cold-rolled steel sheet, and have reached the present invention.

[0006]

The gist of the present invention is a trace amount of Nb.
A very small amount of Sn is added to an ultra-low carbon steel containing Ti and Ti to obtain excellent bake hardenability, and specifically has the following configuration. (1) The invention of claim 1 has the following component composition (component composition is wt.
%, Which is excellent in bake hardenability. (A) C: 0.001 to 0.006%, Si: 1% or less, M
n: 2% or less, P: 0.1% or less, Sol.Al:
0.01-0.08%, S: 0.01% or less, N: 0.
004% or less, Nb: 0.005% or more and Ti: 0.005
% Or more, and (b) C * represented by the following formula (1) is 0 to 0.0015%, and C * = C− (12/93) Nb− (12/48) ) Ti * (1) where Ti * = Ti- (48/14) N- (48/32)
S However, when Ti * ≦ 0, Ti * = 0, and (c) Sn is set to 2.
7C * + 0.001 ≦ Sn ≦ 0.02%
(D) The balance consists of Fe and inevitable impurities.

(2) The invention of claim 2 is a cold-rolled steel sheet for processing excellent in bake hardenability, which further contains B in an amount of 0.0002 to 0.002% in addition to the component composition described in claim 1. (3) A third aspect of the invention is a cold-rolled steel sheet according to the first or second aspect, which is a surface-treated steel sheet for processing excellent in bake hardenability.

[0008]

Next, the reasons for limiting the additional elements in the present invention will be described. C: 0.001 to 0.006%. C is an element necessary for obtaining bake hardenability, but if the amount is less than 0.001%, bake hardenability cannot be expected, so the lower limit is made 0.001%. If the content exceeds 0.006%, the r value and ductility decrease, so the upper limit is made 0.006%.

Si: 1% or less. Si is effective for improving the strength of the steel sheet by solid solution strengthening, but when added in a large amount, it causes a scale defect on the surface of the steel sheet.

Mn: 2% or less. Mn is added because it avoids hot brittleness and is effective for increasing the strength of the steel sheet. However, if it exceeds 2%, the formability and surface properties of the steel sheet deteriorate, so the upper limit is made 2%.

P: 0.1% or less. P is added because it is the most effective element for increasing the strength of the steel sheet.
If added in a large amount, the ductility of the steel sheet deteriorates, which causes cracking during press forming, so the upper limit is made 0.1%.

S: 0.01% or less. S forms sulfides in the steel and deteriorates the ductility of the steel sheet.
% Or less.

Sol. Al: 0.01 to 0.08%. Al
Is added to deoxidize molten steel. N is added to fix N as AlN. However, if Al is added excessively, inclusions in the steel increase, which causes the press formability to deteriorate. If the amount of Al is too small, N cannot be fixed sufficiently. Therefore, Sol.Al is 0.01-0.08%
Limited to.

N: Not more than 0.004%. N is effective in obtaining bake hardenability like C, but at room temperature, the diffusion rate in steel is faster than that of C, and the deterioration of the room temperature aging of the material is remarkable. Therefore, the above range is set.

Sn: 2.7C * + 0.001 ≦ Sn ≦ 0.0
It is contained in the range of 2%. Here, C * = C− (12/93) Nb− (12/48) Ti * (1) Ti * = Ti− (48/14) N− (48/32) S However, when Ti * ≦ 0; Ti * = 0. Sn is added to improve bake hardenability. In order to enhance bake hardenability, it is necessary to increase the amount of solid solution C in steel.However, it is difficult to achieve both bake hardenability and a high r value because increase of solid solution C significantly lowers the r value. .

However, if Sn is added to steel having an appropriate component, bake hardenability can be improved without impairing workability. The reason will be described below. C: 0.0022
%, Si: 0.07%, Mn: 0.65%, Sol. Al: 0.0
43%, P: 0.034%, S: 0.006%,.
Steel containing N: 0.0018%, Nb: 0.008%, and Sn in the range of 0 to 0.056% is continuously cast, and the slab is heated to 1200 ° C .; 6 mm, hot-rolled at a finishing temperature of 880 ° C., wound up at 680 ° C., pickled the hot-rolled steel sheet, cold-rolled to 0.7 mm, heated to 830 ° C., annealed, and stretched 1.2%. % Temper rolling was performed. Fig. 1 shows the BH (bake hardenability) and rm (average Rankford) values of these cold rolled steel sheets. BH was measured using a JIS No. 5 test piece in accordance with JIS G 3135 attached documents.

The rm value is 0 °, 45 °, 9 ° with respect to the rolling direction.
The r value at 0 ° (referred to as r ₀ , r ₄₅ , and r ₉₀ ) was measured, and was calculated as rm value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4. BH
Rises sharply by adding a trace amount of Sn,
Up to 02%, BH increases with an increase in Sn content, but 0.02%.
If it exceeds%, the effect is saturated. The reason for the increase in BH is that the grain boundary of the ultra-low carbon steel is extremely clean and is likely to be a precipitation site of C. However, when Sn is added, Sn segregates at the grain boundary, and C
Is presumed to be caused by increase of solid solution C due to difficulty in precipitation. The rm value hardly deteriorates even when Sn is added up to 0.02%, but drops sharply when it exceeds 0.02%.

Therefore, the upper limit of Sn is set to 0.02% so that the r value does not decrease. The effect of Sn increasing bake hardenability depends on the amount of solid solution C in steel. Figure 2 shows C * expressed by the following formula (1) as an indicator of the amount of solid solution C, and plots ΔBH when a steel sheet having various C * s is annealed at 830 ° C on a matrix of C * and Sn amounts. This is the result. ΔBH is the BH amount increased by adding Sn (ΔBH = BH (Sn added) −BH (Sn not added)).

C * = C− (12/93) Nb− (12/48) Ti * (1) where Ti * = Ti− (48/14) N− (48/32)
S, where Ti * ≤0; Ti * = 0. From FIG. 2, it is necessary to add 2.7 C * + 0.001% or more of Sn in order to obtain the bake hardenability of 10 N / mm ² or more. The reason why the amount of Sn required to improve bake hardenability as C * increases is that the larger the value of C *, the higher the driving force for C to precipitate at the grain boundary, and the more It is estimated that Sn is required. Therefore, from the viewpoint of bake hardenability, the lower limit of Sn is set to 2.7C * +
0.001%. The effect of Sn is C * from 0 to 0.001
Only observed in the range of 5%.

If C * exceeds 0.0015%, even if Sn is added, B precipitates at the grain boundaries, so that BH does not increase. Also C
If * is less than 0%, C bonds with Nb and Ti to form a carbide, and solid solution C disappears. Therefore, even if Sn is added, BH does not increase.
Therefore, in order to improve the bake hardenability by Sn,
It is necessary to limit the addition amounts of Nb and Ti, and to set C * within the above range.

Nb and Ti: Nb: 0.005% or more and Ti:
Contains 0.005% or more of one or two kinds, and
0 ≦ C- (12/93) Nb- (12/48) Ti * ≦ 0.
Limited to a range that satisfies 0015%.

Nb and Ti not only optimize the amount of C * but also
This is added to refine the structure of the hot-rolled sheet to increase the r-value. However, if the content is less than 0.005%, the effect of reducing the grain size cannot be expected. The upper limits of these elements are limited by the above formula.

B: 0.0002 to 0.002% B is added as necessary in order to prevent the embrittlement of secondary working which becomes apparent with high-strength IF steel. However, if the amount is too large, the r value and ductility decrease, so the upper limit is set to 0.00.
2%. Further, since the effect cannot be expected if it is less than 0.0002%, the lower limit is made 0.0002%. In addition, even if B is added to a steel plate having a relatively low strength, there is no obstacle on the material.

If the steel in the range of the present invention is made into a cold-rolled steel sheet under ordinary conditions, a cold-rolled steel sheet having excellent bake hardenability can be obtained. It is desirable to manufacture a steel sheet under the conditions described in (1).
Regardless of whether the steel is melted in a converter or an electric furnace, the performance of the steel sheet is not affected at all. The casting method is continuous casting method,
Either the ingot making method may be used, and the hot rolling may be direct rolling or thin rolling of a thin slab in addition to the usual method.

The finishing temperature of hot rolling is coarsened particle size near the surface of the hot rolled sheet below the Ar ₃ point, since r value which is an index of press formability is degraded, or the Ar ₃ point is desirable .
If the winding temperature of hot rolling is too low, solid solution N
And solute C remain, and the precipitate does not become coarse.
Since a favorable texture is not formed during recrystallization annealing after cold rolling, the temperature is desirably 560 ° C or more, preferably 630 ° C or more. In order to develop a texture after annealing, the cold rolling is preferably performed at a cold rolling rate of 70% or more.

The recrystallization annealing may be performed in any of a box annealing furnace, a continuous annealing line, and a continuous hot-dip galvanizing line. In the case of box annealing, continuous annealing is performed at 600 ° C. or more (including a continuous hot-dip galvanizing line). In this case, annealing at 750 ° C. or more is desirable. Temper rolling is preferably performed at an elongation of 0.8% or more to prevent the occurrence of stretch strain during press forming and to suppress the deterioration of the material at room temperature, but if the elongation exceeds 3%. Material deterioration such as an increase in yield strength and a decrease in ductility is remarkable.

The cold-rolled steel sheet of the present invention is also suitable for use after being subjected to various surface treatments. Surface treatment results in a surface-treated steel sheet having excellent bake hardenability.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The operation and effect of the present invention will be described below based on embodiments. Example 1 Steel having the composition shown in Table 1 was continuously cast, and the slab was 1200
C. and hot-rolled to a finish thickness of 3.2 mm at a finish temperature of 890.degree. C. and wound up at 660.degree. After pickling, the hot-rolled steel sheet was cold-rolled to 0.7 mm, continuously annealed, and temper-rolled at an elongation of 1.3%. Table 2 shows annealing temperatures and materials. ΔBH in the table was increased by adding Sn
(ΔBH = BH (with Sn added) −BH (without Sn added)).

[0029]

[Table 1]

The steels 1,4,10,13,16,22
For, in addition to continuous annealing, annealing and alloyed hot-dip galvanizing were performed in a continuous hot-dip galvanizing line. The plating conditions were as follows: a plate temperature of 485 ° C. and a bath temperature of 470 ° C. were immersed in a hot-dip galvanizing bath, and an alloying treatment was performed at 480 ° C. Temper rolling was performed at an elongation of 1.3%.

The lower part of Table 2 shows the annealing temperature, material, and plating adhesion. The plating adhesion was evaluated by measuring the plating releasability by a draw bead test, and was evaluated on a scale of 1 to 5 (1: extremely good to 5: poor). As is clear from the above table, the cold rolled steel sheet of the present invention has a BH of 11 N / mm ² or more increased by adding Sn, and a BH of 42 N / mm ^2.
It has an rm value of at least ² and an rm value of at least 1.8, has excellent bake hardenability and excellent workability, and has good adhesion of galvannealed alloyed zinc.

[0032]

[Table 2]

[0033]

As described above, according to the present invention, a cold rolled steel sheet for processing and a surface-treated steel sheet having excellent bake hardenability and excellent workability can be manufactured, and the outer plate of an automobile and the like can be manufactured. It is intended to provide a steel sheet suitable for use in a steel sheet.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the Sn content and bake hardenability (BH) and the relationship between moldability (rm value).

FIG. 2 is a diagram showing the relationship between ΔBH and the relationship between C * and Sn content.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Tomoyoshi Ohkita 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-4-131357 (JP, A) JP-A Sho 63-171832 (JP, A) JP-A-4-41658 (JP, A) JP-A-3-285022 (JP, A) JP-A-62-112731 (JP, A) JP-A-3-253543 (JP, A A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C22C 38/00 301 C22C 38/14

Claims (3)

(57) [Claims] 1. A cold-rolled steel sheet for processing excellent in bake hardenability having the following component composition (the component composition is wt%). (A) C: 0.001 to 0.006%, Si: 1% or less, Mn: 2% or less, P: 0.1% or less, Sol.Al: 0.01 to 0.08%, S: 0 0.01% or less, N: 0.004% or less, Nb: 0.005% or more and Ti: 0.005% or more, containing one or two kinds, and (b) C * represented by the following formula (1) Is 0 to 0.0015%, and C * = C− (12/93) Nb− (12/48) Ti * (1) where Ti * = Ti− (48/14) N− ( 48/32)
S However, when Ti * ≦ 0, Ti * = 0, and (c) Sn is contained in the range of 2.7 C * + 0.001 ≦ Sn ≦ 0.02%. (D) The balance is Fe and inevitable impurities. Consists of 2. A cold-rolled steel sheet for processing having excellent bake hardenability having the following component composition (the component composition is wt%). (A) C: 0.001 to 0.006%, Si: 1% or less, Mn: 2% or less, P: 0.1% or less, Sol.Al: 0.01 to 0.08%, S: 0 0.01% or less, N: 0.004% or less, B: 0.0002 to 0.002
%, Nb: 0.005% or more and Ti: 0.005% or more. (B) C * represented by the following formula (1) is 0 to 0.0015%; * = C- (12/93) Nb- (12/48) Ti * ... (1) where Ti * = Ti- (48/14) N- (48/32)
S However, when Ti * ≦ 0, Ti * = 0, and (c) Sn is contained in the range of 2.7 C * + 0.001 ≦ Sn ≦ 0.02%. (D) The balance is Fe and inevitable impurities. Consists of 3. A surface treated steel sheet having excellent bake hardenability, wherein the cold rolled steel sheet according to claim 1 or 2 is subjected to a surface treatment.