JP2003129168A - High bh steel sheet and method for using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet which can impart itself a BH (bake hardening) quantity of 50 MPa or more, without causing a stretcher strain after press- formed, and a using method thereof. SOLUTION: The high BH steel sheet is characterized by having a bake hardening (BH) quantity of 50 MPa or more, and yield point elongation (YP-El) of 0.4% or more, after artificial aging at 100 deg.C for 1 hr. The method for using the high BH steel sheet is characterized by rewinding the steel sheet, skin-pass- rolling it at a reduction of 4.0% or less, cutting it to the final shape, and press- forming it.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet which is excellent in bake hardenability and non-aging after press forming and a method of using the same, and is particularly suitable for use in press forming for automobiles.
The steel sheet according to the present invention is used by press forming automobiles, household electric appliances, buildings and the like. Further, it includes hot-rolled steel sheets, cold-rolled steel sheets in a narrow sense not subjected to surface treatment, and cold-rolled steel sheets subjected to surface treatment such as Zn plating or alloyed Zn plating for rust prevention.

[0002]

2. Description of the Related Art When a thin steel plate is pressed, surface defects called local stretchers or dendritic non-uniform patterns called stretcher strains may occur in a flat portion where the workability is not so high. is there. It is considered that the cause of the stretcher strain is that solid solution C / N existing in the steel sheet fixes dislocations. The larger the yield point elongation (yield elongation), the larger the stretcher strain. Since most thin steel sheets have a yield point elongation in the as-annealed state, a steel material manufacturer performs temper rolling to perform yield point elongation. The current situation is to turn it off and ship it to the automobile manufacturer. However, even if the yield point elongation disappears temporarily by temper rolling, in the steel sheet in which the solid solution C / N remains, the solid solution C / N fixes dislocations again with the passage of time, and the yield point elongation increases. Since it recovers (strain aging), most automobile manufacturers usually press work within 3 months after the arrival of steel sheets.

Further, the BH amount of a paint bake hardenable (BH) steel sheet using solid solutions C and N is usually set to about 60 MPa as an upper limit in order to prevent the above-mentioned stretcher strain from occurring. The present inventors are concerned about the occurrence of stretcher strain, because if the automobile manufacturer performs temper rolling before press working, the yield point elongation will disappear due to temper rolling even if yield elongation occurs. It became possible to easily manufacture a steel plate that can obtain a BH amount of 50 MPa or more without any problem, and it was thought that automakers could carry out press working without worrying about the period from the delivery of steel materials to the press working. There is no such prior art.

A part of the inventors of the present invention is disclosed in JP-A-6-11665.
No. 0, the low carbon steel is annealed in the γ region, and the annealing temperature is (Ar ₁ -50 ° C) to (Ar ₁ + 50 ° C) 30 ° C.
Disclosed is a method for producing a cold-rolled steel sheet having a low-temperature transformation product single-phase structure capable of imparting a BH amount of up to about 100 MPa by cooling at a cooling rate of 1 / s or more. A low-temperature transformation product has a single-phase structure to reduce the elongation to 0.2% or less, which is different from the present invention in which the yield point elongation after artificial aging is intentionally generated.

[0005]

SUMMARY OF THE INVENTION According to the present invention, stretcher strain does not occur after press molding, and
An object of the present invention is to provide a steel sheet capable of imparting an H amount of 50 MPa or more and a method of using the steel sheet.

[0006]

In order to solve the above problems, the gist of the present invention is (1) 100 ° C. × 1
Yield point elongation (YP-El) after artificial aging treatment of hr is 0.3% or more, and bake hardening (BH) amount is 50 MPa or more, high BH steel plate, (2)% by mass,
C: 0.0001% or more and 0.25% or less, Si: 0.
001% or more, 2.5% or less, Mn: 0.01% or more,
2.5% or less, P: 0.15% or less, S: 0.03% or less, Al: 1.5% or less, N: 0.0005 to 0.01
5% or less, O: 0.015% or less, and the high BH steel sheet according to (1) above, which comprises the balance iron and unavoidable impurities, (3) Further, Ti: 0.20% or less, Nb: 0. Two
0% or less, V: 0.20% or less, Zr ≦ 0.2%, B:
The high BH steel sheet according to (2) above, which contains one or more of 0.007% or less, (4) further Mo ≦ 1%, C
The high BH steel sheet according to (2) or (3), containing one or more of u ≦ 2% and Ni ≦ 1%, (5) further Sn ≦ 0.2%, Cr ≦ 2.0. %, W ≦ 1.5%,
The high BH steel sheet according to any one of (2) to (4), which contains one or more of Mg ≦ 0.1% and Ca ≦ 0.1%, (6) above (1) ~ Use of a high BH steel sheet, characterized in that after rewinding the steel sheet according to any one of (5), skin pass rolling of 4.0% or less, cutting into a final shape, and press forming are performed. Method, (7) above (1)
Use of a high BH steel sheet, characterized in that the steel sheet according to any one of (5) is unwound, cut into a final shape, and then skin pass rolling of 4.0% or less and press forming are performed. Method, (8) Method of using the high BH steel sheet according to (6) or (7), characterized in that skin pass rolling and press forming are performed in the same line, (9) Leveler rolling, shot instead of skin pass rolling Distortion is introduced by ball projection or punch and die (6)
~ Usage method of the high BH steel sheet according to any one of (8),
It is in.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. In the present invention according to (1) above, YP-El after artificial aging at 100 ° C. × 1 hr is 0.3% or more. Conventionally, when skin pass rolling is not performed before press forming,
To ensure non-aging at room temperature, YP-
Although it was necessary to reduce El to 0.2% or less, in the present invention, since the skin pass rolling is performed before the press forming, even if the strainer strain occurs, the stretcher strain should be eliminated by the skin pass rolling. Therefore, it is not necessary to keep YP-El after artificial aging low.
On the contrary, in order to secure a high BH amount of 50 MPa or more, it is preferable to leave a large amount of solid solution C.N. Therefore, YP-El after artificial aging is set to 0.3% or more. Y after artificial aging
The effect of the present invention can be obtained without particularly defining the upper limit of P-El, but if it is too high, it is necessary to give a large amount of strain in order to prevent stretcher strain, resulting in deterioration of ductility and equipment. Therefore, it is preferable to set it to 5.0% or less.

The amount of BH is set to 50 MPa or more. B
The effect of the present invention can be obtained without particularly defining the upper limit of the amount of H, but if it is too high, it is necessary to give a lot of strain in order to prevent stretcher strain from occurring.
As a result, ductility deteriorates and an excessive load is applied to the equipment, so the pressure is preferably 200 MPa or less. The effect of the present invention is most effective when the BH is in the range of 60 to 150 MPa. In addition, BH is subjected to a heat treatment for 20 minutes at 170 ° C. after 2% tensile deformation evaluated as described below, and then subjected to a tensile test again. The value obtained by subtracting the flow stress during 2% tensile deformation from the upper yield point during re-tension is BH. In addition, since the heating temperature of the BH treatment is decreasing in recent years, 170 ° C., 20
Instead of minutes, 160 ° C., 10 minutes or 150 ° C., 10 minutes may be adopted. In this case, the lower limit of BH is 4
It becomes 5 MPa and 40 MPa.

Next, the reasons for limiting the components of the inventions (2) to (5) will be described. The lower limit of C is set to 0.0001% because the lower limit obtained in practical steel is used. If the upper limit exceeds 0.25%, workability and weldability deteriorate, so this value is set. Si and Mn are deoxidizing elements, and are also effective for improving the workability such as ductility by increasing the strength of steel and obtaining a composite structure of ferrite and retained austenite, martensite, bainite and the like. The respective contents are 0.001% and 0.01% or more, but the upper limits are 2.5% or less, respectively, because workability and weldability are deteriorated when the upper limits are exceeded. P and S are set to 0.15% or less and 0.03% or less, respectively, in order to prevent deterioration of workability and hot brittleness.

In addition to being effective as a deoxidizing element, Al is also useful for obtaining retained austenite.
% Or more, but if too much, workability and weldability deteriorate, so the upper limit is made 1.5%. N is B
0.0005 because it is an effective element for imparting H property
% Or more. On the other hand, if the amount is too large, it becomes difficult to completely eliminate the stretcher strain or the weldability deteriorates, so 0.015% is made the upper limit. C + N is B
In order to secure the H property, it is preferable to add 0.002% or more in total.

O is an impurity and is set to 0.015% or less so as not to deteriorate workability. Ti, Nb, V, Z
Since r and B improve the material through mechanisms such as carbon and nitrogen fixing, precipitation strengthening, and fine grain strengthening, 0.0001 of each is used.
% Or more is desirable, and excessive addition deteriorates workability, so an upper limit was set for each. Mo, C
u and Ni are 0.001 for securing strength.
%, 0.001%, 0.001% or more is desirable, and excessive addition deteriorates workability, so the upper limits are made 1.0%, 2.0%, and 1.0%, respectively.

[0012] Sn, Cr, W in order to ensure strength,
0.001% or more may be added, Mg is a deoxidizing element, and Ca is useful for controlling the morphology of sulfides. Therefore, 0.0001% or more may be added. Therefore, the upper limits are set to 0.2%, 2.0%, 1.5%, 0.1% and 0.1%, respectively.

Next, a method of using the steel sheet of the present invention will be described. In the invention according to (6), the above (1) to
3. After rewinding the steel sheet according to any one of (5),
Skin pass rolling of 0% or less, cut into the final shape,
Press molding will be performed. The skin pass rolling is performed in order to prevent dislocations fixed and solidified by solid solution C / N to be movable dislocations, and to prevent stretcher strains that may occur after forming. In order to exert this effect, it is preferable to perform skin pass rolling with a rolling reduction of 0.2% or more. On the other hand, if the rolling reduction exceeds 4.0%, ductility deteriorates due to work hardening.
Is the upper limit. After skin pass rolling, it is cut into a final shape called blanking, and then press-molded. Skin pass rolling and press forming may be performed on separate lines,
You may implement on the same line at a time. In any case, it is preferable to perform press molding as soon as possible after performing skin pass rolling. It is preferable to press-mold within 1 week, and at most within 1 month.

By carrying out the skin pass rolling before the press forming as described above, the solid solution C is temporarily assumed before the skin pass rolling.
・ Even if N has fixed dislocations, stretcher strain can be avoided by skin pass rolling, and a high BH of 50 MPa or more can be achieved by the subsequent press forming and subsequent bake hardening without generating stretcher strain.
Amount can be given. In the invention (7), the steel sheet is rewound, cut into a final shape, and then skin pass rolling of 4.0% or less is performed, and then press forming is performed. In the present invention, the final shape is cut before the skin pass rolling. The reason why the skin pass rolling is performed before the press forming is the same as in the invention of (6) above, and the reason for limiting the rolling reduction of the skin pass rolling is the same as in the invention of (6) above.

In the above invention (8), the skin pass rolling and the press forming are performed on the same line. As a result, press molding can be performed efficiently in a short time without generating stretcher strain. In the invention of (9) above, strain is introduced by leveler rolling, shot ball projection, or punch and die instead of skin pass rolling. A known method can be used for the leveler rolling and the like. The purpose of leveler rolling or the like is to make dislocations fixed and immobilized by solid solution C · N to be mobile dislocations as in the case of skin pass rolling. Therefore, the equivalent strain is 0.2%
It is preferable to introduce a strain of 0.5% or more. When introducing distortion with a punch and a die,
It is preferable that a small amount of embossing or protrusion is provided on the punch or die, or both, and the strain is introduced by transferring it to the steel plate. At this time, it is preferable to introduce strain so that the steel plate macroscopically maintains a two-dimensional shape. This is because stretcher strains occur in deep drawing and overhang forming. However, when a method other than skin pass rolling is adopted, it is not easy to estimate the strain amount, so a tensile test is performed after the strain is introduced, and it is possible to introduce a strain sufficient for the yield point elongation to disappear. preferable.

The steel sheet of the present invention is hot-rolled or hot-rolled,
Cold rolling, annealing process (may be plated)
Can be manufactured by. Skin pass may be applied immediately after annealing. This skin pass is from (6) above.
Unlike the skin pass performed immediately before the press molding described in (8), it is a skin pass that has been conventionally performed. The manufacturing method is not particularly limited,
It is necessary to select a condition for imparting a predetermined BH of 50 MPa or more, in other words, a condition that can secure 0.002% or more of solid solution C + solid solution N in total. The structure of the hot-rolled steel sheet or the cold-rolled steel sheet obtained by the present invention has ferrite or bainite as the main phase, but both phases may be mixed, and martensite, austenite, carbide, and nitride are present in them. You can do it. That is, it suffices to create different organizations according to the required characteristics.

[0017]

EXAMPLE Steels having the compositions shown in Table 1 were melted and hot-rolled at a slab heating temperature of 1250 ° C., a finishing temperature of 900 ° C. and a winding temperature of 650 ° C. to obtain a steel strip having a thickness of 3.5 mm. After pickling some of the steel sheets, they are cold-rolled at a reduction rate of 80% to make a 0.7 mm-thick cold-rolled sheet, and then a heating rate of 10 ° C / s,
Soaking 780-820 ° C x 50s, from annealing temperature to 650 ° C
Continuous anneal was performed at 4 ° C./s for the above, an average cooling rate of 60 ° C./s from 650 ° C. to the over-aging temperature shown in Table 1, and then for 180 s at the same aging temperature. Then, the obtained cold-rolled steel sheet was subjected to artificial aging treatment at 100 ° C. for 1 hr, and then (a)
Final cutting-skin pass rolling (1.2% reduction),
(B) Skin pass rolling (1.2% rolling reduction) -final cutting was performed, and JIS No. 5 tensile test pieces were sampled and subjected to a tensile test. Before performing the above skin pass rolling,
That is, the yield point elongation of the steel sheet after artificial aging was 0.4% or more in all cases. For some steel sheets, leveler rolling, shot ball projection, or punch and dies were used to introduce an average strain of 1.4% instead of skin pass rolling.
As a comparative example, one in which skin pass rolling was omitted was carried out. The results of the tensile test are summarized in Table 2.

Here, the amount of BH is the stress when a tensile test is conducted again after a heat treatment equivalent to coating baking of 170 ° C. × 20 minutes is applied to a 2% prestrained material to which a tensile strain of 2% is added in the rolling direction. (The value obtained by subtracting 2% deformation stress from the upper yield stress during the re-tension test). In all of the examples of the present invention, stretcher strain did not occur, and BH of 50 PMa or more was used.
The amount could be given. On the other hand, in the comparative example, the strainer strain was generated and the surface quality was deteriorated.

[0019]

EFFECTS OF THE INVENTION According to the present invention, a BH amount of 50 MP can be obtained without causing stretcher strain after press molding.
It is possible to obtain a steel plate capable of imparting a or more.

[0020]

[Table 1]

[0021]

[Table 2]

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Natsuko Sugiura             20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel shares             Company Technology Development Division (72) Inventor Akihiro Miyasaka             20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel shares             Company Technology Development Division

Claims (9)

[Claims] 1. A high BH steel sheet having a yield point elongation (YP-El) of 0.3% or more and a bake hardening (BH) amount of 50 MPa or more after artificial aging treatment at 100 ° C. × 1 hr. . 2. In mass%, C: 0.0001% or more,
0.25% or less, Si: 0.001% or more, 2.5% or less, Mn: 0.01% or more, 2.5% or less, P: 0.15% or less, S: 0.03% or less, The high BH steel sheet according to claim 1, which contains Al: 1.5% or less, N: 0.0005 to 0.015% or less, O: 0.015% or less, and the balance iron and unavoidable impurities. 3. One kind of Ti: 0.20% or less, Nb: 0.20% or less, V: 0.20% or less, Zr: 0.20% or less, B: 0.007% or less, or The high BH according to claim 2, which contains two or more kinds.
steel sheet. 4. The high BH steel sheet according to claim 2, further comprising one or more of Mo ≦ 1%, Cu ≦ 2% and Ni ≦ 1%. 5. Further, one or more of Sn ≦ 0.2%, Cr ≦ 2.0%, W ≦ 1.5%, Mg ≦ 0.1%, and Ca ≦ 0.1% are contained. The high BH steel plate according to any one of claims 2 to 4. 6. The steel sheet according to claim 1 is rewound and then skin pass rolled at 4.0% or less, cut into a final shape, and press-formed. How to use high BH steel sheet. 7. The steel sheet according to any one of claims 1 to 5 is rewound, cut into a final shape, and then skin pass rolling of 4.0% or less is performed, followed by press forming. How to use high BH steel sheet. 8. The high B according to claim 6 or 7, wherein the skin pass rolling and the press forming are performed in the same line.
How to use H steel plate. 9. A method of using the high BH steel sheet according to claim 6, wherein the strain is introduced by leveler rolling, shot ball projection, or punch and die instead of skin pass rolling. .