JPH0841587A - Baking hardening type steel plate excellent in surface characteritic - Google Patents

Abstract

PURPOSE:To reduce scale defects by incorporating specific amounts of C, Mn, P, S, Al, N, Nb, Si, O, and Fe. CONSTITUTION:A composition, containing, by weight, 0.001-0.003% C, 0.05-2.2% Mn, <=0.1% P, <=0.015% S, 0.01-0.08% sol. Al, <=0.003% N, and 0.002-0.01% Nb, also containing 20-80ppm Si and 5-20ppm O in the range satisfying O<=-0.1XSi+25, and having the balance essentially Fe, is provided. Scale defects are caused by fayalite (SiO2-FeO) formed into wedge shape by the preferential oxidation of grain boundaries in the slab heating stage at the time of hot rolling. The amounts of Si and O in the steel is reduced, and the grain boundary oxidation inhibiting scale peeling property is prevented. By this method, the steel plate excellent in surface characteristic can be obtained while obviating the necessity of setting up new equipment or changing the manufacturing process.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a steel sheet having excellent bake hardenability (including steel strips, original sheets for surface-treated steel sheets, surface-treated steel sheets and steel strips), for example, excellent dent resistance for automobile outer panels. TECHNICAL FIELD The present invention relates to a steel sheet suitable for use in a thin steel sheet that requires high properties.

[0002]

2. Description of the Related Art Due to the progress of steelmaking degassing technology today, ultra-low carbon steel with a carbon content in the steel reduced to 30 ppm or less has become relatively inexpensive and mass-produced. Widely used in.

In recent years, attention has been paid to global environmental problems, and reducing exhaust gas from automobiles has become an important issue. As one of the measures for reducing exhaust gas, the strength of automobile outer panel and the like is increased, and the fuel consumption is improved by reducing the weight of the vehicle body by thinning the members.
Since thinning of steel sheets makes it easier to dent products, it is necessary to raise the yield point and secure dent resistance (hardness of dents in steel sheets) in parts such as outer panel panels where appearance is important. However, considering press forming, it is necessary to lower the yield point in order to suppress the surface strain.

As a steel sheet that can meet this need, by adding carbonitride forming elements such as Nb and Ti to ultra-low carbon steel, the yield point is low at the time of forming, but the yield point is high at the time of baking after painting. The so-called bake hardening type ultra low carbon cold rolled steel sheet has come to be widely used for outer panel of automobiles.

For example, Japanese Examined Patent Publication No. 17004/1985 relates to a bake hardenable cold rolled steel sheet in which Nb is added to an ultra-low carbon steel, which focuses on improving formability and bake hardenability. is there.

Japanese Unexamined Patent Publication (Kokai) No. 4-323345, Japanese Patent Publication No. 61-45689, and Japanese Patent Publication No. 3-72134 relate to Nb-based or Nb-Ti ultra-low carbon bake hardening type steel sheets, and are similarly formed. The purpose is to improve the heat resistance and bake hardenability.
Although these technologies disclose data on low-Si materials of 0.010% or less, none disclose the importance of reducing both Si and O elements. Further, each of them has a different technical content from the present invention for improving the surface property as described later.

[0007]

By the way, ultra low carbon steel sheets such as ultra low carbon cold rolled steel sheets have a problem that surface defects are likely to occur due to high purity of the steel. One of the surface defects that is likely to occur in this thin steel sheet is a scale defect.

This scale defect remains not only on the hot rolled steel sheet but also on the surface of the steel sheet after pickling, cold rolling, or after further surface treatment such as plating or chemical conversion treatment, resulting in poor surface appearance. .

In particular, since most of the bake hardening type steel sheets are used as outer plates of automobiles, if they have surface defects, even if they have excellent internal properties such as bake hardenability,
The product value is significantly reduced.

In view of such circumstances, the present invention aims to reduce the scale defects that often occur in the ultra low carbon bake hardening type steel sheet, and has an extremely low carbon bake hardening type having a preferable component composition for improving the surface texture. It provides a steel plate.

[0011]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies as to the scale peelability of an ultra-low carbon bake hardening type steel sheet, and as a result, the cause of this scale defect is slab heating during hot rolling. The wedge-shaped firelite (SiO ₂ -FeO) that inhibits the scale peeling by the preferential oxidation of the grain boundaries at the stage inhibits the scale peeling by further reducing Si and O in the steel. It was found that grain boundary oxidation was suppressed.

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

(1) C: 0.001 to 0.003% by weight, Mn: 0.
05-2.2%, P: 0.1% or less, S: 0.015% or less, sol.Al: 0.01-
0.08%, N: 0.003% or less, Nb: 0.002-0.01%, Si: 20-80pp
m, O: contains 5 to 20 ppm, and the above Si and O are [ppmO] ≦
A bake hardenable steel plate that satisfies the relationship of -0.1 x [ppmSi] +25, and has the balance of iron and unavoidable impurities with excellent surface properties.

(2) C: 0.001 to 0.003% by weight, Mn: 0.
05-2.2%, P: 0.1% or less, S: 0.015% or less, sol.Al: 0.01-
0.08%, N: 0.003% or less, Nb: 0.002-0.01%, Si: 20-80pp
m, O: 5 to 20 ppm is included, and the above Si and O are [ppmO] ≦ −
Satisfies the relationship of 0.1 × [ppmSi] + 25, and Ti: 0.002
%-(48/14) x [% N] + (48/32) x [% S], with the balance being iron and unavoidable impurities, and a bake hardening type steel sheet with excellent surface properties.

[0015]

The details of the present invention will be explained below together with the reasons for limitation.

First, grain boundary oxidation will be described. Figure 2
Is a photograph showing the state of grain boundary oxidation of the base metal found in the ultra-low carbon steel after heating the slab.The oxidation structure of the metal of the slab cross section after heating the ultra-low carbon steel slab with Si of 0.02% in the heating furnace. FIG. The scale bites like a wedge along the grain boundary of the base metal. This intergranular oxidation is due to the formation of firelite (SiO ₂ -FeO), and the Si content is 0.01 to 0.04%.
Even with a relatively low content, it is observed in a wide heating temperature range of 1100-1300 ° C. For ultra low carbon steel, Si is 0.01
Even with a relatively low content of ~ 0.04%, the concentration of Si in the grain boundaries occurs and sharp and deep grain boundary oxidation is generated, so the anchor effect hinders scale exfoliation during descaling. , The scale defect occurrence rate is higher than that of low carbon steel.

The present invention improves the scale releasability and reduces the scale defects of the steel sheet by reducing Si and O 2 in the steel sheet. This point will be described with reference to FIG. FIG. 1 is a diagram showing the relationship between the amount of Si and the amount of O 2 in steel and the scale defect occurrence rate.

As shown in FIG. 1, when the Si content is 80 ppm or less, the defect occurrence rate is drastically reduced. When Si is 80 ppm or less, the number of grain boundary oxidations is the same as when the number exceeds, but the depth of grain boundary oxidation is significantly reduced. Therefore, the scale releasability at the time of descaling is improved, and the incidence of scale defects is drastically reduced. However, if the Si content is less than 20 ppm, the scale becomes thin and dense, and conversely, the scale releasability decreases and the scale defect rate increases. Therefore, Si is 20
It is necessary to make it above the ppm and below 80 ppm. In addition, the defect generation rate is per defect of 15 cm or more in the final process.
It is set to 1.5 m and is a value obtained by dividing the total defect length by the coil length.

Further, as shown in FIG. 1, O 2 needs to be 5 ppm or more and 20 ppm or less. O exists as an oxide in the steel, but when it is distributed in the surface layer, it becomes the nucleus of internal oxidation in the slab heating stage, and the product of internal oxidation extends to the surface layer and is linked to the oxide of the surface layer, forming grains. It takes on a form similar to that of field oxidation. When O 2 exceeds 20 ppm, the distribution of oxides in the surface layer increases, and a large number of products of internal oxidation extend to the surface layer and connect with the oxides in the surface layer, resulting in a morphology similar to grain boundary oxidation and descaling. In this case, the scale releasability at that time decreases, and the scale defect occurrence rate increases. 20 pp O
By reducing it to m or less, it is possible to suppress the occurrence rate of this scale defect.

If the austenite grains are large, scale defects due to intergranular cracks tend to occur during hot rolling.
O suppresses grain growth of austenite and suppresses generation of scale defects due to intergranular cracks during hot rolling. O is 5p
If it is less than pm, the effect of suppressing the grain growth of austenite is reduced, and the occurrence of scale defects due to grain boundary cracking during hot rolling increases. Therefore, O must be 5 ppm or more.

Even if Si and O 2 are in the above ranges, if both are near the upper limit, the defect occurrence rate becomes high.
In order to reduce the defect occurrence rate, [ppm
It is necessary to further satisfy the relation of [O] ≦ −0.1 × [ppmSi] +25.

The range of limitation of other elements is as follows. The lower the content of C, the more advantageous the moldability. However, 0.001% or more is necessary to secure the bake hardenability. Further, if the amount exceeds 0.003%, the moldability decreases. Therefore, the range is limited to 0.001 to 0.003%.

Mn suppresses cracking during hot rolling and contributes to the improvement of surface properties, so Mn must be added. The amount is 0.05%
If it is less than the above range, the above effects cannot be sufficiently exhibited. Also,
Excess Mn decreases the formability, so the amount must be 2.2% or less. Therefore, Mn is limited to 0.05 to 2.2%.

P is an intergranular embrittlement element, and is preferably low in terms of suppressing surface cracking during hot rolling, and its upper limit is set to 0.
1%

If the content of S is too large, the grain boundaries become brittle and cause cracking during hot rolling. Therefore, the smaller the content of S, the more desirable and the upper limit is 0.015%.

Al must be added for deoxidizing molten steel. If the amount is less than 0.01% as sol.Al in steel, the purpose cannot be sufficiently achieved. In addition, the amount is 0.08%
If it exceeds, the amount of Al ₂ O ₃ increases, which hinders the scale releasability. Therefore, 0.01 ~ 0.08% as sol.Al
Limited to

N is preferably low in terms of aging at room temperature and moldability, and its upper limit is 0.003%.

Nb is an extremely low carbon base and has the function of improving the formability, so it must be added. If Nb is less than 0.002%, the purpose cannot be sufficiently achieved. In addition, 0.01%
If it is added in excess, the bake hardenability cannot be obtained. Therefore, Nb is set to 0.002 to 0.01%.

Although the above is the basic component range of the present invention, an appropriate amount of Ti may be added in order to suppress the room temperature aging. Ti is
It has the effect of binding to N and suppressing the room temperature aging due to N. It is necessary to set it to 0.002% or more in order to achieve the desired effect. However, when added in excess, S, then C
Combine with. When combined with C, the bake hardenability decreases, so to obtain the desired bakeability, Ti should be (48/14)
It is necessary not to exceed × [% N] + (48/32) × [% S]. Therefore, Ti: 0.002% ~ (48/14) x [% N] + (48/3
2) Limited to × [% S].

The steel sheet of the present invention is a hot-rolled steel sheet, a heat-treated steel sheet thereof, a steel sheet descaled by pickling, a steel sheet subjected to heat treatment of a descaled steel sheet, a steel sheet subjected to descaling such as pickling after heat treatment, or Includes steel plates obtained by subjecting these steel plates to surface treatment such as plating.

Further, the steel sheet of the present invention includes a cold-rolled steel sheet obtained by hot rolling, pickling, cold rolling, and then recrystallization annealing.

Further, the steel sheet of the present invention includes a surface-treated steel sheet and may be a hot-rolled steel sheet or a cold-rolled steel sheet which has been treated.

The steel sheet of the present invention can be manufactured by a conventional method. That is, steel can be melted, cast, and hot rolled to produce a hot rolled steel sheet. The steel may be melted in either a converter or an electric furnace. In addition, outside-furnace refining can be applied if necessary. Casting may be either ordinary ingot or continuous casting. The hot rolling may be performed after reheating the cold slab or after lightly reheating (including heat retention and retention) the hot slab after casting. Moreover, the case where hot rough rolling is not performed may be performed.

If necessary, the steel sheet after hot rolling can be subjected to heat treatment, descaling such as pickling, heat treatment of the descaled steel sheet, and after heat treatment, such as pickling such as descaling. Alternatively, these steel plates can be further subjected to surface treatment such as plating.

After hot rolling, pickling, cold rolling and recrystallization annealing can be carried out to produce a cold rolled steel sheet. In this case, although not particularly limited, by performing hot rolling with a heating temperature of 1150 ° C. or more and a finishing temperature of Ar ₃ points or more, and further performing cold rolling with a rolling reduction of 50% or more, The effect can be maximized. Recrystallization annealing may be performed in any of a box-type annealing furnace, a continuous annealing furnace, and a hot-dip galvanizing line having a continuous annealing furnace.

The surface-treated steel sheet of the present invention may be a hot-rolled steel sheet or a cold-rolled steel sheet that has been treated. As the surface treatment, hot metal plating (zinc, aluminum, zinc-aluminum alloy), hot-dip galvanizing alloy, electrogalvanizing, electrozinc alloy plating, chemical conversion treatment, organic composite plating, painting, tin plating, etc. are used. The treatments can be applied alone or in combination as appropriate. Even if these treatments are performed, the effect of the present invention is not impaired.

[0037]

EXAMPLES Table 1 shows the compositions of the present invention steel and comparative steel and the occurrence rate of scale defects. In addition, in the table, the indication of the component is% by weight, provided that C, N, Si, 0, and 25-0.1Si are
It is in ppm display. Further, the display of the defect occurrence rate is%.
Further, the thick line frame in the comparative steel indicates that it is outside the scope of the present invention.

[0038]

【table 1 】

The basic manufacturing conditions are as follows:
A slab cooled from 600 ° C. to room temperature was reheated to 1150 to 1280 ° C. and hot rolled, cooled on a runout table, and wound up to obtain a hot rolled steel sheet having a thickness of 2.8 to 3.6 mm. Descaling was performed twice before rough rolling and finish rolling by injecting high-pressure water with a collision pressure of 3 kgf / cm ² , and the average finishing temperature was 900 ° C and the winding temperature was 660 ° C. And Furthermore, after pickling, cold rolling is performed.
(Sheet thickness 0.6 to 1.6 mm), continuous annealing at 800 to 870 ° C and temper rolling of 1.4% (steel 1 to 5, 8 to 10, 14 to 19, 23 to 26).
For hot-dip galvanized steel sheets, after cold rolling in the same manner as above, annealed at 850 ° C in a continuous hot-dip galvanizing line, and immediately after cooling to 460 ° C, 55 g per side
/ m ² of hot dip galvanized, followed by alloying treatment at 500 ° C and temper rolling of 1.4% (steel 6). A part of the above was further electroplated with 3 g / m ² of 80% Fe-Zn alloy on one side to obtain a double-layer galvanized steel sheet of hot dipping and electroplating (steels 7, 11, 20, 21). The electroplated steel sheet, cold rolling - annealing - after temper rolling, per side 20 g / m ²
88% Zn-Ni alloy electroplating (Steels 12 and 22) .For organic coated steel sheets, a chromate coating of 50 mg / m ^{2 in} terms of metallic chromium and a composite coating of 1 μm resin layer are applied on the electroplated steel sheet. Went (steel 13).

As a parameter of the scale defect generation, one defect of 15 cm or more at the final stage is set to 1.5 m, and a value obtained by dividing the total by the cold rolling coil length is defined as the scale defect generation rate. The surface properties were evaluated. The bake hardenability was evaluated by increasing the yield strength (BH amount) after baking after applying a 2% tensile strain and then performing a low temperature baking treatment at 145 ° C. for 20 minutes.

Steels 17, 23, 24, in which the Si content of the comparative steels is not appropriate,
Steels 18 to 20 and 22 with improper O content and steels 20 and 21 with improper Si-O balance have a defective rate due to scale defects.
In contrast to 0.61% or more, the invention steels 1 to 16 have an incidence of 0.
It is less than 39%. In addition, steels 25 and 26 in which the Nb or Ti addition amount is not proper have BH amounts of 13 to 16 MPa and 35 to 53 M of the present invention steel.
It is lower than Pa.

[0042]

As described above, according to the present invention, scale defects, which are problems in bake hardening type ultra-low carbon steel sheet, can be reduced without installing new equipment or changing the manufacturing process, and have excellent surface properties. It is possible to obtain a steel plate.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a Si content and an O 2 content and a scale defect defect rate.

FIG. 2 is a drawing-substituting photograph of an oxidized structure of a metal of a slab cross section showing a state of grain boundary oxidation after heating of an ultra-low carbon steel slab containing 0.02% Si.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Issei Okimoto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Toshiaki Matsuura 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd. (72) Inventor Jiro Tanabe 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. C: 0.001 to 0.003% by weight, Mn: 0.05 to
2.2%, P: 0.1% or less, S: 0.015% or less, sol.Al: 0.01 to 0.08
%, N: 0.003% or less, Nb: 0.002 to 0.01%, Si: 20 to 80 ppm, O:
5 to 20ppm, and the above Si, O is [ppmO] ≦ −0.1
X [ppmSi] +25 The bake hardenable steel sheet that satisfies the relationship of [25] and the balance is iron and unavoidable impurities and has excellent surface properties. 2. C: 0.001 to 0.003% by weight, Mn: 0.05 to
2.2%, P: 0.1% or less, S: 0.015% or less, sol.Al: 0.01 to 0.08
%, N: 0.003% or less, Nb: 0.002 to 0.01%, Si: 20 to 80 ppm, O:
5 to 20ppm, and the above Si and O are [ppmO] ≦ −0.1
X [ppmSi] +25 is satisfied, and Ti: 0.002% ~
A bake hardenable steel sheet containing (48/14) x [% N] + (48/32) x [% S], with the balance being iron and unavoidable impurities, with excellent surface properties.