JP3911075B2 - Manufacturing method of steel sheet for ultra deep drawing with excellent bake hardenability - Google Patents

Description

【００２９】
【表２】

表２より、発明鋼（Ａ〜Ｆ）では強度ＴＳが３００〜４５０ＭＰａの範囲において形状凍結性に優れるＹＳ≦２５０ＭＰａを満足し、かつｒ値＞２．２の深絞り性を得ているにも関わらず、３０〜５０ＭＰａのＢＨ性を熱延および焼鈍条件に関わらず安定して得られていることがわかる。なお、鋼Ｄおよび鋼Ｆについては、箱焼鈍形式で焼鈍した参考例である。しかし、Ｃ量が適合範囲外の比較鋼Ｇ、Ｈ、及びＴｉ量が適合範囲外の比較鋼Ｉ、及びＣ、Ｎ量と炭化物形成元素の原子モル比が適合範囲外の比較鋼Ｊでは、適正範囲内のＢＨ性が得られていない。また、製造条件の影響を見ると１次冷延率の低い比較鋼Ｋ、及び２次冷延率の低い比較鋼Ｌにおいては、適度なＢＨ性は得られているものの、深絞り性に劣る。
【００３０】
【発明の効果】
この発明によれば、製鋼、熱延、焼鈍の制約条件の影響を受けにくく、安定した焼付硬化性を有し、かつ一体成形化を可能とする超深絞り性を有する自動車の外板パネル用鋼板が製造できる。
【図面の簡単な説明】
【図１】ｒ値およびＢＨ性に及ぼす１次冷延率の影響を示す図である。
【図２】ｒ値およびＢＨ性に及ぼす２次冷延率の影響を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a steel sheet for ultra deep drawing, which is mainly used for an outer panel of an automobile and has excellent bake hardenability.
[0002]
[Prior art]
Outer panels used for automobile roofs, door outers, etc. have low yield strength with excellent shape when molded, and high yield with excellent dent resistance (characteristic that dents cannot be easily depressed after molding). The contradictory characteristics which become a point are required. Furthermore, recently, from the viewpoint of high productivity, there has been a demand for excellent deep drawability that enables the outer panel to be integrally formed. In response to these demands, Japanese Patent Application Laid-Open No. 53-114717 discloses a bake-hardening type steel sheet having increased strength after working by age hardening using a paint baking process for automobiles after pressing. This bake hardenability (hereinafter referred to as BH property) is an effective characteristic that can achieve both a low yield point during forming and a high yield point after forming by solid solution C or solid solution N in steel. Furthermore, in Japanese Patent Application Laid-Open Nos. 57-70258 and 59-31827, Nb-added steel or Ti / Nb composite-added steel is manufactured by continuous annealing, assuming that BH properties and deep drawability are imparted. A method is disclosed. This is to fix C and N in the steel, which inhibits the formation of a texture with good deep drawability before continuous annealing, and to decompose carbonitride after continuous annealing, leaving solute C and N. This is a method of imparting BH properties.
[0003]
[Problems to be solved by the invention]
As described above, in the Nb addition or Ti / Nb composite addition technology that satisfies the required characteristics of the outer panel by simultaneously imparting BH property and deep drawability, the solid solution generated by decomposition of carbonitride during annealing In order to impart BH properties by leaving C to room temperature, a high annealing temperature of 850 ° C. or higher is required for decomposition of carbonitride, and rapid cooling is required to prevent reprecipitation during cooling to room temperature. For this reason, in principle, it is difficult to obtain a stable BH property that is largely dependent on the annealing temperature and the cooling rate, and that a high annealing temperature is required. There are problems such as deterioration in surface quality and surface quality due to wrinkles caused by rolls. Further, the BH property needs to be controlled in a very small amount of ppm level with respect to the solute C and N amounts, but is easily influenced by variations in the C and N amounts in the steel making process, and this influence is also large in the above method. On the other hand, the method of leaving the solid solution C before the continuous annealing does not require high-temperature annealing because the BH property can be imparted without using the dissolution of carbonitride, and also due to variations in the amount of C and N in the steelmaking stage. Although the influence of variation in BH property is small, stable production is possible, but this method has a problem that good deep drawability cannot be obtained. Accordingly, an object of the present invention is to manufacture a steel sheet for an outer panel of an automobile having stable BH properties and ultra-deep drawing properties that enable integral molding in order to solve the above problems.
[0004]
[Means for Solving the Problems]
The gist of the present invention is as follows.
(1) C: 0.001 to 0.01% by weight%,
Al: 0.01 to 0.1%,
N: 0.005% or less,
Mn: 0.05 to 2.0%,
Si: ≦ 1.5%
P: ≦ 0.15%,
S: ≦ 0.015%,
Further, at least one of Ti, Nb, and V is 0.05% or less and (Ti / 48 + Nb / 93 + V / 51) / (C / 12 + N / 14) <1
After the normal hot rolling, the steel pieces that are inevitable impurities are pickled as a hot-rolled steel strip, and then cold-rolled at a reduction rate of 50% or more, and above the recrystallization temperature. After continuous annealing and cold rolling again at a reduction rate of 50% or more, continuous annealing is performed at a recrystallization temperature of 840 ° C. or less (excluding a temperature range of 50 ° C. or more than intermediate annealing) , and pressure regulation Bake hardenability having a bake hardenability of 30 to 50 MPa and deep drawability with an r value of 2.2 or more, characterized by performing temper rolling at a rate of 0.5 to 3.0% A method for producing a steel sheet for an outer panel of an automobile having excellent deep drawability.
(2) Further, the steel slab is weight% and B: ≦ 0.005%
The bake hardenability of 30-50 MPa as described in the above (1), and a deep drawability with an r value of 2.2 or more is excellent in bake hardenability and deep drawability. Manufacturing method of steel plate for outer panel.
[0005]
DETAILED DESCRIPTION OF THE INVENTION
The inventors added carbonitride-forming elements such as Ti, Nb and V to ultra-low carbon steel in order to develop a steel sheet that satisfies the properties excellent in deep drawability and bake hardenability, The manufacturing method was examined in detail, including high-tensile steel sheets with P and Mn added as strengthening elements. As a result, a steel containing C in an atomic molar amount in excess of the carbon nitride forming element required amount is made into a hot-rolled steel strip by ordinary hot rolling, cold-rolled at a reduction rate of 50% or more after pickling, After annealing at the crystal temperature, cold-roll again at a reduction rate of 50% or more, and recrystallize at a recrystallization temperature of 840 ° C or less (excluding a temperature range of 50 ° C or more than the intermediate annealing). Thus, it was found that a steel sheet having a BH property of 30 MPa or more and a Rankford value of 2.2 or more can be obtained.
[0006]
The experiment which obtained this knowledge is described below.
[0007]
C; 0.0040 wt% (hereinafter simply referred to as%), Nb; 0.025%, Mn; 0.5%, Si; 0.02%, P; 0.06%, S; 0.006%, A steel containing Al; 0.03%, N; 0.0018% is hot-rolled to a thickness of 6.0 mm and then rolled at various cold rolling rates, and this is continuously annealed at 775 ° C. or 680 ° C. After performing box annealing for 12 hours to produce uniform recrystallized grains, cold rolling was performed again. This was annealed in a continuous annealing cycle soaking at 800 ° C. for 1 minute, then subjected to temper rolling with a reduction rate of 0.8%, and then a tensile test was conducted as a JIS No. 5 test piece, and the material was investigated. The BH property was evaluated by measuring the amount of increase in yield point by aging treatment corresponding to a baking process at 170 ° C. for 20 minutes after giving 2% pre-strain.
[0008]
FIG. 1 shows the first cold rolling rate (hereinafter referred to as the primary cold rolling rate), r value and BH property at 70% of the re-cold rolling rate after cold rolling annealing (hereinafter referred to as the secondary cold rolling rate). The relationship is shown. Compared with the conventional method in which the primary cold rolling rate is 0%, in this method, the r value is remarkably improved in the range of the primary cold rolling rate of 50% or more. At this time, the BH property is stable regardless of the cold rolling rate. FIG. 2 shows the relationship between the secondary cold rolling rate at the primary cold rolling rate of 70%, the r value, and the BH property. The secondary cold rolling rate is 0%, that is, compared with the conventional method, the r value is remarkably improved in this method in the range of the cold rolling rate of 50% or more. The reason why the r value is improved is not necessarily clear, but is estimated as follows. The primary cold rolling annealing causes a fine and uniform recrystallized structure to be produced on the steel sheet, and further grows and disappears fine carbonitrides that inhibit formation of a texture effective for deep drawability. Therefore, it is considered that the density for developing a texture effective for deep drawability during secondary cold rolling annealing has increased. On the other hand, when the primary cold rolling rate is 40% or less, the r value does not improve because when the structure after the first annealing is observed, a uniform recrystallized structure is not formed and the secondary cold rolling annealing occurs. This is probably because a recrystallization texture advantageous to deep drawability could not be obtained. The same applies to the case where the secondary cold rolling rate is 40% or less. Further, it can be seen that the BH property is stably obtained without depending on the primary and secondary cold rolling rates. About this cause, since the coarse precipitate produced | generated by the primary cold rolling annealing, it is thought that the reprecipitation site density at the time of cooling was low, and the stable solid solution C remained easily after annealing.
[0009]
The reason for limiting the component composition range in the present invention will be described below.
[0010]
C: 0.001 to 0.01%
The lower the C, the better the ductility and deep drawability, but if it is too low, the BH property cannot be obtained, so the lower limit is 0.0010%. The presence of a large amount of solute C causes damage to the appearance such as the occurrence of stretcher strain at the time of pressing. Therefore, a carbonitride-forming element as described later is added to optimize the amount of solute C. The larger the amount, the more precipitates, and the presence of a large amount of precipitates significantly impairs ductility. Therefore, the upper limit of the C amount is 0.01%.
[0011]
Al: 0.01 to 0.1%
Al is used as a deoxidizing material, and 0.01% or more is necessary for sufficient deoxidation. Further, the dissolved N is made harmless as AlN. However, the addition of a large amount causes inclusions in the steel to increase, leading to a decrease in productivity in continuous casting, and causes problems such as reducing the cleanliness of the steel and degrading the ductility. Therefore, the amount of Al in steel is 0.1% or less.
[0012]
N: 0.005% or less N, which is an interstitial element in steel, expresses BH properties like C. However, the aging characteristics at room temperature are inferior to C. Therefore, in the present invention, nitride forming elements such as Al and Ti are added to render N harmless. In addition, when N is contained excessively, the amount of alloy addition for fixing is large, which is disadvantageous in terms of cost, and the presence of a large amount of precipitates deteriorates ductility. Therefore, the upper limit of the N amount is set to 0.005%.
[0013]
Ti, Nb, V
Ti, Nb, and V are added to fix C and N in the steel by forming carbonitride. In the present invention, since the main BH property is expressed by leaving an appropriate amount of C in the steel, addition of (C + N) or more in atomic molar amount is avoided. That is, at least one of Ti, Nb, and V is 0.05% or less, and the range of (Ti / 48 + Nb / 93 + V / 51) / (C / 12 + N / 14) <1 is the content.
[0014]
By satisfying the above component ranges, excellent deep drawability and bake hardenability can be ensured by this production method, but even if the following elements are contained, excellent characteristics can be obtained. They are described below.
[0015]
Mn: 0.05 to 2.0%
Mn is an effective element for increasing the strength of a steel sheet without impairing formability. However, if 2.0% or more is added, the ductility is extremely lowered, so that sufficient formability cannot be obtained. If 0.05% or more is not included, the effect of suppressing hot cracking due to S is suppressed. Fades. Therefore, 0.05 to 2.0% is appropriate for the content range.
[0016]
Si: ≦ 1.5%
Si, like Mn, is an effective element that increases the strength of the steel sheet without impairing ductility. However, if 1.5% or more is added, the ductility is drastically lowered, so that sufficient moldability cannot be ensured. Therefore, the upper limit is 1.5%.
[0017]
P ≦ 0.15%
P, like Si and Mn, is an element effective for increasing the strength of the steel sheet, and is an element that can increase the strength at a low cost because the amount of increase in strength per added amount is large. However, addition exceeding 0.15% deteriorates press formability, and also deteriorates secondary workability due to grain boundary segregation. Therefore, the upper limit is made 0.15%.
[0018]
S: ≦ 0.015%
Since S is easy to generate cracks in the hot process, the smaller the addition amount, the better. However, the reduction of the S content has problems such as increasing the refining cost and inhibiting the productivity. However, in this method, S is detoxified because a sulfide-forming element such as Mn or Ti is added. Therefore, the amount of S is allowed up to 0.015%.
[0019]
B: ≦ 0.005%
Since B in steel has a strong affinity with N, it has an effect of fixing solute N in steel. Further, B segregates at the grain boundaries to increase the grain boundary strength, and has the effect of improving the secondary workability of a steel sheet or the like containing a large amount of P. However, the effect on secondary workability is saturated at about 15 ppm, and beyond this, the moldability is adversely affected. Therefore, considering the B content for fixing N, the upper limit is made 0.005%.
[0020]
Next, production conditions for the steel of the present invention will be described.
[0021]
The hot-rolling conditions may be normal hot-rolling and may be reduced to a predetermined thickness determined from conditions for performing cold-rolling annealing described later twice and a final product thickness.
[0022]
If the primary cold rolling rate after pickling is 50% or less, a fine and uniform recrystallized structure cannot be obtained after annealing, and good deep drawability cannot be expected at the time of cold rolling annealing twice. Therefore, the lower limit is made 50%.
[0023]
The first annealing temperature (hereinafter referred to as the intermediate annealing temperature) may be equal to or higher than the recrystallization temperature because uniform recrystallized grains may be generated .
[0024]
As the secondary cold rolling rate is higher, the deep drawability is improved, but at a low pressure reduction rate, a uniform recrystallized structure is not obtained as in the first cold rolling, and the deep drawability is not improved. Therefore, the secondary cold rolling rate is set to 50% or more.
[0025]
In the present invention, the second annealing temperature (hereinafter referred to as final annealing temperature) is obtained by using excessive solid solution C to obtain BH properties, so it is not necessary to dissolve carbide by high-temperature annealing, and the recrystallization temperature is 840 or more. It is sufficient that the temperature is not higher than ° C (however, excluding the temperature range of 50 ° C or higher than the intermediate annealing) .
[0026]
The pressure regulation rate may be in a normal range, and may be 0.5 to 3.0%.
[0027]
【Example】
Steels having the composition shown in Table 1 are melted by a converter-degassing method, made into a slab by a continuous casting method, then subjected to normal hot rolling, pickled as a 6.0 mm plate, and then shown in Table 2. Under the conditions, cold rolling annealing was performed twice, and temper rolling was performed on a steel sheet having a sheet thickness of 0.7 mm at a rolling reduction of 0.7% to 1.5%. The obtained steel sheet was processed into a JIS No. 5 test piece, and the results of examining the mechanical properties of the test piece are shown in Table 2. The BH property was evaluated by the amount of increase in the yield point after 2% pre-strain as described above and after baking for 20 minutes at 170 ° C.
[0028]
[Table 1]

[0029]
[Table 2]

From Table 2, the invention steels (A to F) satisfy YS ≦ 250 MPa, which is excellent in shape freezing property in the range of strength TS of 300 to 450 MPa, and have deep drawability of r value> 2.2. Regardless, it can be seen that a BH property of 30 to 50 MPa is stably obtained regardless of hot rolling and annealing conditions. In addition, about the steel D and the steel F, it is the reference example which annealed with the box annealing type. However, in comparative steels G, H, and Ti in which the C amount is outside the conforming range, and in comparative steel J in which the atomic molar ratio of the C, N amount and the carbide forming element is outside the conforming range, BH property within the proper range is not obtained. Moreover, when the influence of manufacturing conditions is seen, in comparative steel K with a low primary cold rolling rate and comparative steel L with a low secondary cold rolling rate, although moderate BH property is obtained, it is inferior to deep drawability. .
[0030]
【The invention's effect】
According to the present invention, it is difficult to be affected by the constraints of steelmaking, hot rolling, and annealing, has stable bake hardenability, and has an ultra-deep drawability that enables integral molding. Steel sheets can be manufactured.
[Brief description of the drawings]
FIG. 1 is a diagram showing the influence of a primary cold rolling rate on r value and BH property.
FIG. 2 is a diagram showing the influence of secondary cold rolling rate on r value and BH property.

Claims (2)

C: 0.001 to 0.01% by weight%,
Al: 0.01 to 0.1%,
N: 0.005% or less,
Mn: 0.05 to 2.0%,
Si: ≦ 1.5%
P: ≦ 0.15%,
S: ≦ 0.015%,
Further, at least one of Ti, Nb, and V is 0.05% or less and (Ti / 48 + Nb / 93 + V / 51) / (C / 12 + N / 14) <1
After the normal hot rolling, the steel pieces that are inevitable impurities are pickled as a hot-rolled steel strip, and then cold-rolled at a reduction rate of 50% or more, and above the recrystallization temperature. After continuous annealing and cold rolling again at a reduction rate of 50% or more, continuous annealing is performed at a recrystallization temperature of 840 ° C. or less (excluding a temperature range of 50 ° C. or more than intermediate annealing) , and pressure regulation Bake hardenability having a bake hardenability of 30 to 50 MPa and deep drawability with an r value of 2.2 or more, characterized by performing temper rolling at a rate of 0.5 to 3.0% A method for producing a steel sheet for an outer panel of an automobile having excellent deep drawability. In addition, the steel slab is in weight% B: ≦ 0.005%
The bake hardenability of 30 to 50 MPa according to claim 1 and a deep drawability having an r value of 2.2 or more. A method for producing a steel plate for a panel.

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