JPH06306531A - Cold rolled steel sheet for machining excellent in baking hardenability and surface treated steel sheet - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet excellent in baking hardenability by preparing a cold rolled steel sheet for machining having a specified componental compsn. in which the content of C, P and Sn is prescribed. CONSTITUTION:A cold rolled steel sheet having a compsn. contg., by weight, 0.001 to 0.006% C, <=1% Si, <=2% Mn, <=0.1% P, 0.01 to 0.08% SolAl, <=0.01% S, <=0.004% N and one or two kinds of >=0.005% Nb and >=0.005% Ti, in which C* shown by the formula C*=C-(12/93)Nb-(12/48)Ti* (wherein, Ti*=Ti-(48/14) N-(48/32)S, but if Ti*<=0, then Ti*=0) is regulated to 0 to 0.0015% and contg. Sn by 2.7C*+0.001<=Sn<=0.02%, and the balance Fe with inevitable impurities is prepd. In this way, the cold rolled steel sheet for machining having excellent baking hardenability and workability can be obtd.

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 which have excellent bake hardenability and are suitable for use in, for example, outer panels of automobiles.

[0002]

2. Description of the Related Art In recent years, the problems relating to the environment of the earth have attracted public attention, and the global warming due to the increase of CO ₂ concentration 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 , the weight of the vehicle body has been reduced to improve fuel efficiency. As one of the countermeasures, the outer panel of an automobile is made thinner and lighter. The property that is particularly required for the outer panel is dent resistance (durability of the steel sheet).

The dent resistance decreases as the thickness of the steel sheet decreases, and increases as the yield point of the steel sheet increases. Therefore,
It is necessary to raise the yield point in order to reduce the thickness of steel sheets. Therefore, a bake hardening type steel sheet having a property of being soft and easy to form at the time of press forming and having a yield point increased due to strain aging due to solid solution C in the steel in a paint baking process after pressing, that is, a bake hardening type steel plate having bake hardenability If it is used for, it is possible to reduce the thickness of the panel while maintaining the same dent resistance even when the thickness is reduced.

As one of the methods for improving the bake hardenability of a steel sheet, Japanese Patent Laid-Open No. 63-171832 discloses a technique of adding Sn to a low carbon aluminum killed steel, but the r value (rank field value) is It is as low as less than 1.7, and overaging treatment is essential. Further, Japanese Patent Application Laid-Open No. 4-41658 and Japanese Patent Application Laid-Open No. 4-131357 disclose a technique of adding Sn.
Has not been mentioned to improve bake hardenability. In JP-A-4-41658, since the amount of Sn added is increased, embrittlement of the steel sheet becomes a problem. In addition, JP-A-4-131357
In the publication, the combined addition of Mo and B is indispensable, and the r value is low.

[0005]

[Problems to be Solved by the Invention] It is expected that the reuse of scrap as a steelmaking raw material will increase in the future, and it is conceivable that Sn will inevitably be mixed into steel. Therefore, Sn
There is a demand for a technique for actively adding and utilizing. However, conventional Sn not only lowers the elongation and r value of the steel sheet,
The problem of embrittlement of steel sheets is known. But,
The present inventors have found a cold-rolled steel sheet having good bakeability and high bake hardenability by addition of Sn, and a surface-treated steel sheet obtained by subjecting this cold-rolled steel sheet to a surface treatment, and arrived at the present invention.

[0006]

Means for Solving the Problems The skeleton 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, and excellent bake hardenability is obtained. Specifically, it has the following constitution. (1) The invention of claim 1 has the following component composition (component composition is wt.
%), Which is a cold-rolled steel sheet for working having excellent 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 to 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)
However, when Ti * ≦ 0, Ti * = 0 and (c) Sn is 2.
7C * + 0.001 ≦ Sn ≦ 0.02% contained,
(D) The balance consists of Fe and inevitable impurities.

(2) The invention of claim 2 is a cold-rolled steel sheet for working, which has B of 0.0002 to 0.002% in addition to the component composition described in claim 1 and which is excellent in bake hardenability. (3) The invention of claim 3 is a surface-treated steel sheet for working, which is obtained by subjecting the cold-rolled steel sheet according to claim 1 or 2 to a surface treatment and which has excellent bake hardenability.

[0008]

Next, the reasons for limiting the additive 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 it is less than 0.001%, bake hardenability cannot be expected, so the lower limit is made 0.001%. Further, if it exceeds 0.006%, the r value and the ductility decrease, so the upper limit is made 0.006%.

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

Mn: 2% or less Mn is added to avoid hot brittleness and to increase the strength of the steel sheet, but is added, but 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 and causes cracking during press forming, so the upper limit is made 0.1%.

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

Sol.Al: 0.01 to 0.08%. Al
Is added to deoxidize molten steel. In addition, N is added to fix it as AlN. However, if Al is added excessively, inclusions in the steel will increase, which will be a cause of impairing press formability. Further, if Al is too small, N cannot be sufficiently fixed. Therefore, Sol.Al is 0.01-0.08%
Limited to

N: 0.004% or less. N is effective for obtaining bake hardenability like C, but at room temperature, the diffusion rate in steel is faster than C, and the room temperature aging of the material is significantly deteriorated. Therefore, the above range is set.

Sn: 2.7 C * + 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, if Ti * ≦ 0; Ti * = 0. Sn is added to improve the bake hardenability. In order to improve the bake hardenability, it is necessary to increase the solid solution C in the steel, but when the solid solution C increases, the r value decreases significantly, so it was difficult to achieve both bake hardenability and high r value. .

However, if Sn is added to steel having an appropriate composition, the bake hardenability can be improved without impairing the 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, the slab is heated to 1200 ° C., and finished thickness: 3. 6 mm, finishing temperature: hot rolled at 880 ° C., wound at 680 ° C., the hot rolled steel sheet is pickled, cold rolled to 0.7 mm, annealed by heating to 830 ° C., elongation rate 1.2. The temper rolling was performed in%. The BH (bake hardenability) and rm (average Rankford) values of these cold rolled steel sheets are shown in FIG. A JIS No. 5 test piece was used for the measurement of BH, and the measurement was performed according to the JIS G 3135 Annex.

The rm values are 0 °, 45 °, 9 with respect to the rolling direction.
The r value at 0 ° (represented as r ₀ , r ₄₅ , and r ₉₀ , respectively) was measured and calculated by rm value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4. BH
Is rapidly increased by adding a small amount of Sn, and
BH increases up to 02% with increasing Sn content, but 0.02
If it exceeds%, the effect will be saturated. The reason why BH increases is that the grain boundaries of ultra-low carbon steel are extremely clean and tend to become precipitation sites for C. However, when Sn is added, Sn segregates at the grain boundaries and C
It is presumed that this is due to the fact that it becomes difficult to precipitate and the solid solution C increases. The rm value hardly deteriorates even when Sn is added up to 0.02%, but sharply decreases when it exceeds 0.02%.

Therefore, the upper limit of Sn is set to 0.02% so that the r value is not lowered. The effect of Sn for improving the bake hardenability depends on the amount of solid solution C in the steel. In Fig. 2, C * expressed by the following equation (1) is used as an index of the amount of solid solution C, and ΔBH when steel sheets with various C * are annealed at 830 ° C are plotted on a matrix of C * and Sn content. This is the result. ΔBH is the amount of BH 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, However, if Ti * ≦ 0; Ti * = 0. From FIG. 2, it is necessary to add 2.7 C * + 0.001% or more Sn in order to obtain an improvement in bake hardenability of 10 N / mm ² or more. The reason why the amount of Sn required to improve the bake hardenability increases with the increase in C * is that the larger the value of C *, the higher the driving force for C to precipitate at the grain boundaries, It is estimated that Sn is required. Therefore, from the viewpoint of bake hardenability, the lower limit of Sn is 2.7C * +
It is set to 0.001%. The effect of Sn is that C * is 0 to 0.001.
Only allowed in the 5% range.

When C * exceeds 0.0015%, BH does not rise even if Sn is added because C precipitates at grain boundaries. Also C
When the content of * is less than 0%, C bonds with Nb and Ti to form a carbide, and solid solution C disappears, so even if Sn is added, BH does not rise.
Therefore, in order to improve the bake hardenability by Sn,
It is necessary to limit the amount of Nb and Ti added and 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, and
0 ≦ C− (12/93) Nb− (12/48) Ti * ≦ 0.
It is limited to the range that satisfies 0015%.

In addition to optimizing the amount of C * for Nb and Ti,
This is because the structure of the hot-rolled sheet is finely grained and added to increase the r value, but if both are less than 0.005%, the grain-refining effect cannot be expected. Further, 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 secondary work embrittlement that is manifest in high-strength IF steel. However, if the amount is too large, the r value and the ductility decrease, so the upper limit is set to 0.00
2% Further, if less than 0.0002%, the effect cannot be expected, so the lower limit is made 0.0002%. In addition, even if B is added to a steel plate having a relatively low strength of steel, there is no obstacle in terms of material.

A cold-rolled steel sheet having excellent bake hardenability can be obtained by making the steel within the scope of the present invention into a cold-rolled steel sheet under normal conditions. However, if more severe workability is required, It is desirable to manufacture the steel sheet under the conditions described in.
Whether the steel is melted in a converter or an electric furnace, there is no problem with the performance of the steel sheet. The casting method is continuous casting,
Either of the ingot making methods may be used, and the hot rolling may be a direct feed rolling or a thin cast piece may be directly rolled 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 coiling temperature of hot rolling is too low, solid solution N in the hot rolled sheet
And solid solution C remain, and the precipitate does not become coarse.
A desirable texture is not formed during recrystallization annealing after cold rolling, so 560 ° C. or higher, preferably 630 ° C. or higher is desirable. Since cold rolling develops a texture after annealing, it is desirable that the cold rolling rate be 70% or more.

The recrystallization annealing may be performed in either a box annealing furnace, a continuous annealing line, or a continuous hot dip galvanizing line. In the box annealing, continuous annealing is performed at 600 ° C. or higher (including the continuous hot dip galvanizing line). Then, it is desirable to anneal at 750 ° C. or higher. Temper rolling is preferably performed at an elongation rate of 0.8% or more in order to prevent the occurrence of stretcher strain during press forming and to suppress the deterioration of the material at room temperature aging, but when the elongation rate exceeds 3%. 3% or less is desirable because the material deterioration such as increase in yield strength and 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, such as hot-dip galvanizing, hot-dip galvanizing, electrogalvanizing, electrogalvanizing alloy plating, and organic composite plating. When surface-treated, it becomes a surface-treated steel sheet having excellent bake hardenability.

[0028]

EXAMPLES The operational effects of the present invention will be described below based on examples. Example 1 A steel having the composition shown in Table 1 was continuously cast into a slab of 1200
It was heated to ℃ and hot-rolled to a finish temperature of 890 ℃ to a finish thickness of 3.2 mm and wound at 660 ℃. The hot-rolled steel sheet was pickled, cold-rolled to 0.7 mm, continuously annealed, and temper-rolled at an elongation of 1.3%. Table 2 shows the annealing temperature and materials. ΔBH in the table is BH increased by adding Sn
The amount (ΔBH = BH (Sn added) −BH (Sn not added)).

[0029]

[Table 1]

Steels 1, 4, 10, 13, 16, 22
In addition to continuous annealing, annealing and alloying hot dip galvanizing were performed in a continuous hot dip galvanizing line. The plating conditions were such that the plate temperature was 485 ° C., the bath temperature was 470 ° C., and the alloy was immersed in a hot dip galvanizing bath at 480 ° C. for alloying treatment. The temper rolling was performed at an elongation rate 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 evaluated on a scale of 1 to 5 (1: extremely good to 5: poor). As is clear from the above table, in the cold rolled steel sheet of the present invention, BH is increased by 11 N / mm ² or more by adding Sn, and BH is 42 N / mm ^2.
It has an rm value of ² or more and an rm value of 1.8 or more, has excellent bake hardenability and excellent workability, and has good adhesion for galvannealing.

[0032]

[Table 2]

[0033]

As described above, according to the present invention, it is possible to produce cold-rolled steel sheets for processing and surface-treated steel sheets which have excellent bake hardenability and excellent workability, and are used for automobile outer panels and the like. The present invention provides a steel plate suitable for use in.

[Brief description of drawings]

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

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

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tomoyoshi Okita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (3)

[Claims] 1. A cold-rolled steel sheet for processing, which has the following component composition (the component composition is wt%) and is excellent in bake hardenability. (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, and contains one or two kinds. (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)
However, if Ti * ≦ 0, Ti * = 0, (c) Sn is contained in the range of 2.7C * + 0.001 ≦ Sn ≦ 0.02%, (d) The balance is Fe and unavoidable impurities. Consists of. 2. A cold-rolled steel sheet for working, which has the following component composition (the component composition is wt%) and is excellent in bake hardenability. (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 of 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)
However, if Ti * ≦ 0, Ti * = 0, (c) Sn is contained in the range of 2.7C * + 0.001 ≦ Sn ≦ 0.02%, (d) The balance is Fe and unavoidable impurities. Consists of. 3. A surface-treated steel sheet for working, which is obtained by subjecting the cold-rolled steel sheet according to claim 1 or 2 to a surface treatment and which has excellent bake hardenability.