JP3303929B2 - Manufacturing method of bake hardenable steel sheet for deep drawing with excellent late aging property - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a baking hardenable steel sheet for deep drawing, which reduces aging deterioration.

[0002]

2. Description of the Related Art Conventionally, with respect to a method of manufacturing a bake hardenable steel sheet for deep drawing, a required bake is controlled by controlling the value of Nb / C as disclosed in, for example, Japanese Patent Publication No. 61-11296. The curability is ensured.

[0003]

The effect of Nb / C on bake hardenability (BH property) in the conventional method as described above is as shown in FIG. 1, where Nb / C is within a predetermined range. It can be seen that stable high BH properties can be obtained. The BH amount in the figure is the rise amount (kg / mm ² ) of the yield point due to a heat treatment at 170 ° C. for 20 minutes after a 2% pre-strain is applied to the test piece.

On the other hand, when the effect of aging on the yield point elongation of a steel sheet manufactured by the conventional method is examined, Y
El (raw) ≦ 0.3%, but after accelerated aging treatment, YE
1 (aging) ≧ 0.3%, and deterioration due to aging occurs. Therefore, since the steel sheet obtained by the processor is a certain period (usually 6 months to 1 year) after the production, a stretcher strain may be generated at the time of press working. Further, there is a problem that the shape freezing property is likely to be deteriorated due to the springback accompanying the rise of the yield point.

The inventors of the present application have conducted experiments and studies in order to solve the above-mentioned problems, and found that by controlling the content ratio of Al and N to a predetermined range, printing for deep drawing was performed. It has been found that it is possible to reduce the aging deterioration of the hardenable steel sheet.

[0006]

SUMMARY OF THE INVENTION The present invention has been made based on the above findings , and has the following matters. 0.0010-0.005 C by weight%
0%, Al; 0.01 to 0.1%, N ≦ 0.0050
%, B; including 0.0002 to 0.0006 %, and N
b / C (weight% ratio) = 1.2 to 2.8, and A
1 / N (weight% ratio) = 20 to 45, the balance being Fe
After smelting a steel slab having a composition consisting of unavoidable impurities, the steel sheet was subjected to hot rolling at a finishing temperature of Ar ₃ transformation point or higher, wound around a coil at a temperature of 700 ° C. or lower, and then subjected to cold rolling. Thereafter, by performing continuous annealing at a temperature equal to or higher than the recrystallization temperature, a bake hardenable steel sheet for deep drawing having excellent delayed aging properties is manufactured.

First, the elemental composition of the steel slab used in the present invention and the reasons for limiting the components will be described. C: A minimum of 0.00100% is required to allow solid solution C to remain in the steel and have bake hardenability. On the other hand, in order to increase the r value (Rankford value), which is an index of deep drawability, it is advantageous to use a small amount of C, and the amount of Nb added for fixing as a carbide can be reduced. The upper limit was made 0.0050%.

In order to fix Al: N with Al, a lower limit of 0.01% is required.
It deteriorates ductility in the continuous annealing step. N: 0.00
When the content exceeds 50%, drawability and ductility are deteriorated in the continuous annealing step. B: r value can be improved by adding a very small amount of B in combination with Nb. On the other hand, excessive addition deteriorates ductility, so that 0.0002
To 0.0006 %.

Nb / C: 1.2 is required as the lower limit to ensure that YEl (raw) ≦ 0.3%. If it exceeds 2.8, bake hardenability will be lost. Al / N: A graph of FIG. 2 is obtained by plotting the results of an experiment performed by the present inventors on the effect of Al / N on YEl (aging)%. According to the results of this experiment,
When Al / N <20, a trace amount of N alone remains in the steel and N is fixed as NbN, so that Nb / C decreases and solute C increases, so that YEl (aging) occurs.
Exceeds 0.3%. When Al / N> 45,
Since N is fixed as AlN, YEl (aging) exceeds 0.3% due to solid solution B remaining in the steel.
The evaluation of YEl (aging) was determined by the yield point elongation after accelerated aging at 100 ° C for 30 minutes.

After smelting a steel slab having the above composition by a usual method, the steel slab is subjected to hot rolling at a finishing temperature not lower than the Ar ₃ transformation point, and is wound around a coil at a temperature of 700 ° C. or less.
If the winding temperature exceeds 700 ° C., the descaling property in the subsequent pickling step will deteriorate, and the surface properties of the steel sheet will deteriorate, which is not preferable. After pickling, cold rolling is performed, and a steel sheet having a predetermined thickness is continuously annealed at a recrystallization temperature or higher.

[0011]

EXAMPLE A steel having the composition shown in Table 1 was melted and formed into a slab by continuous casting, then hot-rolled at a finishing temperature not lower than the Ar ₃ transformation point, and wound up at 600 ° C. to form a hot-rolled coil. . After pickling the coil, the coil is cold-rolled (a reduction of 75%).
%) To obtain a cold-rolled steel sheet having a thickness of 0.8 mm. Subsequently, the steel sheet was subjected to continuous annealing (800 ° C.), cooled at 10 ° C./s or more, and then subjected to 0.9% skin pass rolling.

[0012] The mechanical properties of the thus prepared steel sheets were measured using a test piece in parallel to collect the rolling direction, the results shown in Table 2 were obtained.

[0013]

[Table 1]

[0014]

[Table 2]

As is evident from the results in Table 2, the ductility (total elongation) El and the bake hardenability BH of Examples 2 and 3 of the present invention are almost the same as Comparative Examples 1 and 4, but the yield point elongation is For YEl, see YEl of Examples 2 and 3.
Both (raw) and YEl (aged) are 0.3% or less, and aging deterioration is not recognized, whereas in Comparative Examples 1 and 4, YEl (raw) is 0.3% Below,
YEl (aging) shows a value far exceeding 0.3%, indicating that the effect of aging is large.

[0016]

As described above, according to the present invention, a steel slab in which the ratio of Al / N is controlled within a predetermined range is subjected to hot rolling and cold rolling, and then to continuous annealing, thereby deep drawing. It is possible to manufacture a steel sheet having little change over time and excellent in late aging property without impairing the properties and bake hardenability.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between Nb / C and changes with time in BH amount and YEl.

FIG. 2 is a graph showing the relationship between Al / N and YEl after aging treatment.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-93827 (JP, A) JP-A-62-278232 (JP, A) JP-B-61-11296 (JP, B1) (58) Field (Int.Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C22C 38/00-38/60

Claims (1)

(57) [Claims] C. 0.001 to 0.00% by weight in weight%
50%, Al; 0.01 to 0.1%, N ≦ 0.0050
%, B; including 0.0002 to 0.0006 %, and N
b / C (weight% ratio) = 1.2 to 2.8, and A
1 / N (weight% ratio) = 20 to 45, the balance being Fe
After smelting a steel slab having a composition consisting of unavoidable impurities, the steel was subjected to hot rolling at a finishing temperature of Ar ₃ transformation point or higher, wound around a coil at a temperature of 700 ° C. or lower, and then subjected to cold rolling. Thereafter, continuous annealing is carried out at a temperature equal to or higher than the recrystallization temperature.