JP3180569B2 - Bake hardening type cold rolled steel sheet with excellent surface properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-low carbon bake hardening type cold rolled steel sheet having excellent surface properties and used for automobile parts and electric equipment parts.

[0002]

2. Description of the Related Art With the progress of steelmaking degassing technology today, ultra-low carbon steels in which the amount of carbon in steel has been reduced to 30 ppm or less have become relatively inexpensive and mass-produced. Widely used for parts and other applications. In addition, carbon-nitride forming elements such as Nb and Ti are added to ultra-low carbon steel, and they are soft and have excellent formability during press forming, and the so-called bake hardening type, which increases strength during subsequent baking of paint. Ultra-low carbon cold rolled steel sheets have also been widely used in order to meet needs such as weight reduction of automobile bodies.

[0003] For example, Japanese Patent Publication No. Sho 60-17004 (Prior Art 1) and the like relate to a bake hardening type cold rolled steel sheet obtained by adding Nb to an ultra-low carbon steel, and focus on improvement of formability and bake hardenability. It is the one that applied.

[0004] Japanese Patent Application Laid-Open No. Hei 2-232342 (hereinafter referred to as Prior Art 2) and Japanese Patent Application Laid-Open No. 60-92453 (hereinafter referred to as Prior Art 3) disclose ultra-low carbon materials to which Ni or Cr is individually added. This is related to steel sheets, and Ni or Cr is added to improve brazing weldability.

Further, Japanese Patent Application Laid-Open Nos. Hei 6-25753 (hereinafter referred to as Prior Art 4) and Japanese Patent Laid-Open Publication No. Hei 6-25754 (hereinafter referred to as Prior Art 5) disclose, although not essential components, Ni and Cr.
A technology relating to ultra-low carbon cold-rolled steel sheets to which a composite addition is added. In the example, after adding Nb-Ti-V in a composite manner, Ni and Cr were added in 0.5% and 0.2%, respectively. The addition of Ni and Cr aims at increasing corrosion resistance and strength.

[0006]

A steel sheet used for automobile parts, electric equipment parts and the like often requires good surface properties. However, ultra-low carbon steel sheets tend to have poor surface properties due to the high purity of the steel.

[0007] One of the surface property defects that occur in extremely low carbon cold rolled steel sheets is surface unevenness. The unevenness remains as unevenness even after plating or chemical conversion treatment, and the commercial value is reduced.
In particular, baking-hardened cold-rolled steel sheets are often used as automobile outer panels, so if there are surface defects, even if they have excellent internal properties such as baking hardenability, their commercial value will be significantly reduced. There is a problem. This surface unevenness has not always been clarified.

[0008] In view of such circumstances, the present invention aims to reduce surface unevenness which often occurs in ultra-low carbon cold-rolled steel sheets, and has an ultra-low carbon bake hardening type cold-rolling having a preferable component composition for improving surface properties. Provide steel sheet.

[0009]

Means for Solving the Problems The present inventors have studied the surface unevenness which occurs in an ultra-low carbon bake hardening type cold rolled steel sheet, and have found the following.

[0010] The cold-rolled steel sheet is subjected to alkali cleaning for the purpose of removing rolling oil after cold rolling and before annealing. However, since the surface of the steel sheet is extremely activated by the alkali cleaning, local cold-rolled steel sheets are locally localized. Rust is generated on the steel sheet surface by the oxidation reaction. In particular, extremely low-carbon cold-rolled steel sheets are frequently generated because the steel is highly purified. The starting point of this rust is
It is an oxide present on the grain boundaries that have been cleaned or on the steel sheet surface. This rust does not disappear even after annealing in a reducing atmosphere, and remains after plating or chemical conversion treatment, resulting in uneven surface of the steel sheet.

Therefore, in order to improve the surface unevenness,
It is important to suppress the non-uniformity of rust generation after alkali washing. Specifically, while suppressing the purification of grain boundaries that are the starting point of rust, reducing oxides and further suppressing rust generation Therefore, it is effective to add Ni and Cr in combination according to the amount of O 2.

Incidentally, when comparing the components of the present invention with the above-mentioned prior art, the amount of Ni overlaps with that of Prior Art 2 and the amount of Cr overlaps with that of Prior Art 3. However, these prior arts all contain Ni or Cr. The present invention is different from the single addition in that it is a composite addition of Ni and Cr. Prior arts 4 and 5 are Ni
Although the disclosure of composite addition of Cr is disclosed, the example is Ti-Nb-V
And the basic component system is different from the components of the present invention, and the amounts of Ni and Cr are both out of the range of the present invention. Further, all of these prior arts have different purposes from the improvement of surface cleaning aimed at by the present invention.

The present invention has been made on the basis of such knowledge, and the characteristic configuration thereof is as follows.

(1) By weight%, C: 0.001 to 0.003%, Mn: 0.
05-2.2%, Si: 0.8% or less, P: 0.1% or less, S: 0.015% or less, s
ol.Al: 0.01-0.1%, N: 0.003% or less, Nb: 0.002-0.01%, N
i: 0.01 to 0.15%, Cr: 0.01 to 0.2%, O: 0.0002% or more, and said Ni, Cr, O is [% O] ≦ 0.002 × [% Ni +% C
r] +0.0018, baking hardened cold rolled steel sheet with excellent surface properties consisting of iron and unavoidable impurities.

(2) C: 0.001 to 0.003%, Mn: 0.
05-2.2%, Si: 0.8% or less, P: 0.1% or less, S: 0.015% or less, s
ol.Al: 0.01-0.1%, N: 0.003% or less, Nb: 0.002-0.01%, N
i: 0.01 to 0.15%, Cr: 0.01 to 0.2%, O: 0.0002% or more, and said Ni, Cr, O is [% O] ≦ 0.002 × [% Ni +% C
r] +0.0018 is satisfied, and Ti: 0.002% to (4
8/14) × [% N] + (48/32) × [% S], baking hardened cold rolled steel sheet with excellent surface properties consisting of balance iron and unavoidable impurities.

[0016]

The present invention will be described below in detail. O exists as an oxide (mainly alumina) in steel, but when the oxide is present on the steel sheet surface, it becomes a starting point of rust generation after alkali washing. If O 2 is high, the amount of oxides present on the surface of the steel sheet increases, and this leads to a large amount of rust. Conversely, if O 2 is too low, the number of rusts generated decreases, but the size is large and conspicuous. Therefore, in order to exert the effects of the present invention, O 2 is required to be 0.0002% or more. Also, the upper limit of O is Cr and Ni shown below.
It is necessary to set the amount according to the amount.

The combined addition of Ni and Cr has the effect of suppressing the formation of rust after alkali washing. this is
Since Ni and Cr densify the oxide film after alkali cleaning,
It is presumed that this is because the rust does not grow to such an extent that it can be visually confirmed. In addition, the effect is not recognized by single addition. To obtain the desired effects, 0.01% or more is required for each. Also, if the addition amount is too large, the reason is not always clear, but since rust is locally concentrated on the end of the steel sheet and the like, the upper limit is 0.15 each.
% And 0.2%.

When the amount of O 2 is large, the amount of oxides present on the surface of the steel sheet increases. In order to suppress the generation of rust originating from this oxide, the total amount of Ni and Cr needs to be adjusted according to the amount of O 2. FIG. 1 shows the relationship between the amount of Ni + Cr and the amount of O and the number of occurrences of surface unevenness. As shown in the figure, the amount of Ni + Cr is [% O] ≦ 0.002 × [% N
i +% Cr] + 0.0018. Incidentally, preferably, Ni + Cr: 0.14% or less and O:
By setting the content to 0.0015% or less, the number of occurrences of surface unevenness can be further reduced.

The lower the content of C, the more advantageous the moldability. However, 0.001% or more is required to secure bake hardenability. If the amount exceeds 0.003%, the desired formability cannot be achieved. Therefore, C is 0.001
Limited to ~ 0.003%.

Mn is effective in suppressing surface cracks during hot rolling that promotes rust formation. If Mn is less than 0.05%, the effect is not sufficient. Further, an excessive amount of Mn exceeding 2.2% deteriorates the material. Therefore, Mn is limited to 0.05 to 2.2%.

The lower the material of Si is, the better.
0.8%. P is preferably lower in terms of material, and the upper limit is set to 0.1%.

If the content of S is too large, the generation of rust is promoted. Therefore, the content of S is preferably small. The upper limit is set to 0.015%.

Al must be added for deoxidizing molten steel. If the amount is less than 0.01% as sol.Al in steel, the object cannot be sufficiently achieved. If the amount exceeds 0.1%, the amount of alumina as a starting point of rust increases, and the surface unevenness increases. Therefore, sol.Al is limited to 0.01 to 0.1%.

N is desirably low in terms of moldability, and its upper limit is made 0.003%. Nb needs to be added because it has a function of improving formability on a very low carbon base. However, if Nb is less than 0.002%, the object cannot be sufficiently achieved. On the other hand, if it is added in excess of 0.01%, baking hardenability cannot be obtained. Therefore, Nb is 0.002
Limited to ~ 0.01%.

Although the above is the basic component system of the present invention, an appropriate amount of Ti may be further added to suppress the aging at room temperature.

Ti combines with N and has the effect of suppressing the room-temperature aging due to N. However, if it is added in excess, it bonds with S 2 and then with C 2. When combined with C, bake hardenability decreases, so to obtain the desired bake hardenability, Ti should be (48 /
14) It is necessary not to exceed × [% N] + (48/32) × [% S]. Therefore, Ti: 0.002% to (48/14) × [% N] + (4
8/32) x Limited to [% S].

The cold-rolled steel sheet to which the present invention is applied is not limited to a normal cold-rolled steel sheet, but may be subjected to a surface treatment such as a hot-dip coated steel sheet, an electroplated steel sheet, or the above-mentioned steel sheet with an organic composite film or a chemical conversion treatment. It broadly includes surface-treated steel sheets and the like applied alone or in combination.

Further, the effect of the present invention can be obtained in any of the above steel sheets. The steel sheet having the above-mentioned chemical components is usually melted and cast in a converter or an electric furnace according to a conventional method, passed through hot rolling, pickled, and then cold-rolled into a steel sheet having a desired thickness. Although it is not particularly limited, the heating temperature is 1150 ° C. or higher, the finishing temperature is Ar ₃ points or higher, and the hot rolling is performed, and the cold rolling that secures a reduction of 50% or more is performed. Is maximized. Note that the effects of the present invention are not impaired even in the case of direct rolling, in which reheating is not performed during hot rolling.

The annealing after the cold rolling may be any of box annealing, a continuous annealing line or a continuous hot-dip galvanizing line. In the latter case, the presence or absence of alloying treatment does not matter.
In addition, surface treatment such as electroplating, an organic composite film, or a chemical conversion treatment can be performed alone or in combination after temper rolling after annealing. Even when these processes are performed, the effects of the present invention are not impaired.

[0030]

EXAMPLES Table 1 shows the compositions of the steels of the present invention and the comparative steels and the number of surface irregularities caused by rust at the final product stage.

[0031]

[Table 1]

The basic production conditions were as follows: a hot-rolled steel sheet having a thickness of 2.8 to 3.6 mm was obtained by subjecting a continuously cast slab to hot rolling, cooling on a run-out table, and winding. The average finishing temperature is 900 ℃ and the winding temperature is 660 ℃
It is. Furthermore, cold rolling is performed after pickling (sheet thickness 0.6 to 1.6 m
m), 800-870 ° C continuous annealing-temper rolling of 1.4% (steel 1-6, 13-20, 25-28). Note that the galvanized steel sheet, continuously at 820 ° C. after cold rolling annealing, plating the molten zinc per side 55 g / m ² at the stage of cooling to 460 ° C., subsequently subjected to alloying treatment at 500 ° C., Temper rolling of 1.0% was performed (Steels 7 to 9, 21). Some parts were further subjected to upper layer electrogalvanizing of 80% Fe-Zn alloy of 3 g / m ² per one side to obtain hot-dip + electroplated double-layer plated steel sheets (steel 10, 22). The electroplated steel sheet, after temper rolling, subjected to 88% Zn-Ni alloy electroplating in per side 30 g / m ² (steel 11, 23), for the organic coated steel sheet, further reckoned as metal chromium on the electroplating Was applied to form a 50 mg / m ² chromate coating and a 1 μm resin layer composite coating (Steel 12, 24).

The degree of rust generation depends on the final product stage.
The evaluation was made based on the number of uneven surfaces having a diameter of 2 mm or more distributed per 1 m ² . The bake hardenability was evaluated by performing a low-temperature baking treatment at 145 ° C. for 20 minutes after applying 2% tensile strain, and evaluating the increase in yield strength (BH amount) after baking before applying strain.

Steels 17 to 17 in which the amounts of Ni and Cr added to the comparative steels are not appropriate
21: 4.3-6.7 / m ^Two, Ni and Cr are composite additives
Non-added steels 25 and 26 are 6.2-7.3 pieces / m^Two, O amount is not appropriate
2.5 to 16.3 pieces / m for steel 16, 22 to 24^TwoIn contrast, the present invention
0.8 for steel 1 ~ 15^TwoIt is as follows. In particular, 0: 0.00
15% or less and Cr + Ni: 0.02 to 0.14% 0.6 pieces / m^TwoAnd
It is lower. Also, Nb and Ti addition amounts are not appropriate
Steels 27 and 28 have a BH content of 1 to 3 MPa, compared to the steel of the present invention of 32 to 54 MPa.
It is lower than that.

[0035]

As described above, according to the present invention, surface unevenness due to rust, which is a problem in extremely low carbon cold rolled steel sheets, can be reduced without installing new equipment or changing the manufacturing process. It is possible to obtain a bake hardening type cold rolled steel sheet having excellent heat resistance.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amount of O, the amount of Ni + Cr, and the number of uneven surfaces.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Seiji Nakamura 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-5-188851 (JP, A) (58) Survey Field (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] [Claim 1] By weight%, C: 0.001-0.003%, Mn: 0.05-
2.2%, Si: 0.8% or less, P: 0.1% or less, S: 0.015% or less, sol.A
l: 0.01-0.1%, N: 0.003% or less, Nb: 0.002-0.01%, Ni: 0.
01-0.15%, Cr: 0.01-0.2%, O: 0.0002% or more, and the above-mentioned Ni, Cr, O 2 is [% O] ≦ 0.002 × [% Ni +% Cr] +
A bake hardenable cold rolled steel sheet that satisfies the relationship of 0.0018 and has excellent surface properties consisting of the balance of iron and unavoidable impurities. 2. In% by weight, C: 0.001 to 0.003%, Mn: 0.05 to
2.2%, Si: 0.8% or less, P: 0.1% or less, S: 0.015% or less, sol.A
l: 0.01-0.1%, N: 0.003% or less, Nb: 0.002-0.01%, Ni: 0.
01-0.15%, Cr: 0.01-0.2%, O: 0.0002% or more, and the above-mentioned Ni, Cr, O 2 is [% O] ≦ 0.002 × [% Ni +% Cr] +
Satisfies the relationship of 0.0018, and further, Ti: 0.002% ~ (48/14)
A bake-hardened cold-rolled steel sheet containing × [% N] + (48/32) × [% S] and excellent in surface properties consisting of the balance of iron and unavoidable impurities.