JP3390084B2 - Bake hardenable thin steel sheet excellent in formability and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bake hardening type thin steel sheet suitable for use as a steel sheet for automobiles and the like, which is excellent in formability, particularly deep drawability, and a manufacturing method thereof.

[0002]

[Prior art]

Recently, environmental problems have been reexamined, and there has been an increasing demand for weight reduction of automobiles in order to improve fuel efficiency in accordance with automobile exhaust gas regulations. One of the means for reducing the weight of automobiles is to increase the tensile strength of thin steel sheets used for vehicle bodies. Further, as another means, it is possible to improve the formability of the thin steel sheet to change the structure formed by welding and assembling many parts to the structure formed by integral molding to achieve a reduction in the overlap margin for welding. .

By the way, in order to increase the tensile strength of a thin steel sheet, generally,
Since this involves the addition of a strengthening element, there is a problem that this strengthening element causes deterioration of press formability and deterioration of plating property. Therefore, there is a limit to increasing the tensile strength of the thin steel sheet by adding the strengthening element. On the other hand, there is a technique for achieving high tension by utilizing the effect of seizure. That is, this method is intended to increase the deformation strength of the final product and reduce the weight of the vehicle body by the so-called bake hardening action that utilizes the strain aging phenomenon of steel that occurs during paint baking after forming. According to this technique, although the amount of increase in strength is limited, the strength at the time of molding can be lowered, so that there is an advantage that it is possible to increase tensile strength while suppressing deterioration of moldability. Moreover, since the strengthening element as described above is not added, the plating property does not deteriorate.

Several proposals have been made so far for the manufacturing method of the bake hardening type thin steel sheet having such advantages. For example, the technique disclosed in JP-A-5-311246 is to obtain a steel sheet having bake hardenability and press formability by adding Zr. However, according to this known technique, as is clear from the examples, although the BH amount is about 30 MPa, the r value is only about 2.2 at most. It is difficult to apply a thin steel sheet having an r value of this degree to the above-mentioned integral molding of an automobile. On the other hand, regarding the method of improving the r value,
Several proposals have been made so far. For example, Japanese Unexamined Patent Publication (Kokai) No. 5-287448 discloses a technology for producing a cold-rolled steel sheet for deep drawing which has a high r value and is excellent in press formability. According to this, C and S are reduced as much as possible, and Al (wt%) / N
By setting (wt%) ≧ 20, it is possible to manufacture a cold-rolled steel sheet having a high r value and excellent strength-ductility balance. However, this method does not have bake hardenability, and the r value is, as is clear from the examples,
2.7 is the best. Therefore, even with this technique, integral molding is impossible. As described above, it is difficult to obtain a thin steel sheet that has bake hardenability and has a formability that can withstand processing by integral forming, specifically, an r value of about 2.2 or more. there were.

[0006]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above problems and provides a bake hardenable thin steel sheet having bake hardenability and having a high r value and excellent in formability. The purpose is to propose a manufacturing method thereof. Another object of the present invention is to propose a bake hardenable thin steel sheet excellent in formability having a r value of 2.7 or more and a bake hardening (BH) amount of 20 MPa or more, and a manufacturing method thereof. is there.

[0007]

Under the recognition of the above problems, the present inventors have studied in detail the composition of steel and the manufacturing conditions. As a result, in the bake hardening type thin steel sheet in which the Nb content and the C content were controlled, the addition of B was optimal for the precipitation fixation of N, and the precipitation of BN was achieved by optimizing the relationship between the B content and the N content. It was newly discovered that it is possible to form a recrystallized texture with which a high r value can be obtained by solid solution of appropriate amounts of B and N.

That is, the present invention achieves the above new findings by appropriately controlling the composition of steel and the production conditions, and its gist is as follows. (1) C: 0.0015 wt% or less, Si: 0.1 wt% or less, Mn:
0.5 wt% or less, Al: 0.5 wt% or less, P: 0.08 wt%
Below, S: 0.01 wt% or less N: 0.005 wt% or less, Nb: 0.03 wt% or less, B: 0.00
01-0.0050wt%, and Nb, C, B, N is 1 ≦ Nb
(wt%) / C (wt%) ≦ 30, and N (wt%) − (14 /
11) B (wt%) ≤ 0.001 (wt%) is contained,
The balance consists of Fe and inevitable impurities, and is a bake hardenable thin steel sheet with excellent formability.

(2) C: 0.0015 wt% or less, Si: 0.1 wt
% Or less, Mn: 0.5 wt% or less, Al: 0.5 wt% or less,
P: 0.08 wt% or less, S: 0.01 wt% or less N: 0.005 wt% or less, Nb: 0.03 wt% or less, B: 0.00
01-0.0050wt%, and Nb, C, B, N is 1 ≦ Nb
(wt%) / C (wt%) ≦ 30, and N (wt%) − (14 /
11) B (wt%) ≤ 0.001 (wt%) is contained,
The balance is 1050 steel slab consisting of Fe and unavoidable impurities.
After soaking and holding in the temperature range of 〜1300 ℃, finish the hot rolling at the Ar ₃ transformation point or higher, wind it at 800 ℃ or lower, and then pickle it, then cold reduction with the rolling reduction of 65 ~ 90%. Rolled, then
A method for producing a bake hardenable thin steel sheet excellent in formability, which comprises performing recrystallization annealing in a temperature range of 700 to 950 ° C.

After the recrystallization annealing of the above invention (2), hot dip galvanizing, one-layer or two-layer electroplating or the like can be performed. Further, in the above invention (2), it is also possible to perform recrystallization annealing in a temperature range of 700 to 950 ° C. in a continuous hot-dip galvanizing line, and subsequently apply alloying hot-dip galvanizing or non-alloy hot-dip galvanizing.

[0011]

First, the results of the research on which the present invention is based will be described. C: 0.0008 wt%, Si: 0.01 wt%, Mn: 0.1 wt%, A
l: 0.04 wt%, P: 0.01 wt%, S: 0.005 wt%, Nb: 0.0
1 wt%, N: 0.0009 to 0.004 wt%, B: 0 to 0.005 wt%
The slab consisting of the composition is heated to 1250 ° C and uniformly soaked, then 880 ° C
Then, the hot rolling was completed and the coil was wound at 600 ° C. Then
After pickling and cold rolling with a reduction rate of 85%, the temperature was 20 ° C at 860 ° C.
Continuous annealing with second soaking was performed to produce a specimen with a plate thickness of 0.7 mm. A test piece for r-value measurement was taken from this test material,
The relationship between the r value and N (wt%)-(14/11) B (wt%) was investigated. The result is shown in FIG. Here, the r value is a JIS No. 5 tensile test piece, pre-strained by 15%, and then measured by the three-point method.
The r values in the direction of 45 ° and the direction of 90 ° to the rolling direction are r _L , r _D and r _C respectively, and the average value thereof is calculated by (r _L + 2r _D + r _C ) / 4. It was From FIG. 1, it can be seen that the r value strongly depends on N (wt%)-(14/11) B (wt%), and this value is remarkably improved in the range of 0.001 wt% or less. The reason for this is not necessarily clear, but the precipitation and fixation of N by B and the decrease in the amount of solid solution N in the steel reduce the distribution of precipitates in the hot-rolled steel sheet with a decrease in the amounts of C and S. It is considered that this is because the recrystallization texture advantageous for the value has developed.

Next, in the present invention, the reason why the steel composition is limited to the above-mentioned constitution will be explained. C: 0.0015 wt% or less; C deteriorates the formability, and as the amount of C increases, the amount of Nb corresponding to this is required, and the recrystallization temperature rises. Therefore, it is preferable that the amount of C be as small as possible. If the amount exceeds 0.001 wt%, the adverse effect becomes remarkable, so 0.0015 wt% or less, preferably 0.0
01wt% or less.

Si: 0.1 wt% or less; Si is an element effective as a deoxidizing element in the steelmaking process and for strengthening the steel, and is added in a necessary amount according to the desired strength. However, if the addition amount exceeds 0.1 wt%, the workability deteriorates, so the content is made 0.1 wt% or less, preferably 0.05 wt% or less.

Mn: 0.5 wt% or less; Mn is necessary as a deoxidizing element in the steel making process, but if added in excess, it will embrittle the steel and increase the strength more than necessary. Therefore, the addition amount is 0.5 wt% or less, preferably 0.3 wt% or less.

Al: 0.5 wt% or less; Al is added as necessary for the deoxidizing action in the steel making process and for improving the yield of carbonitride forming elements.
Even if added over 5 wt%, the effect will be saturated, so 0.5
It should be less than wt%, preferably less than 0.1 wt%.

P: 0.08 wt% or less; P has an action of strengthening steel and is added in a required amount according to the desired strength, but if the added amount exceeds 0.08 wt%, workability deteriorates. ,
0.08 wt% or less, preferably 0.05 wt% or less.

S: 0.01 wt% or less; S deteriorates formability, so it is preferable to reduce S as much as possible with C.
If the S amount exceeds 0.01 wt%, the adverse effect thereof becomes particularly large, so it is necessary to set it to 0.01 wt% or less, preferably 0.005 wt% or less.

N: 0.005 wt% or less; N deteriorates formability, so it is desirable to reduce it as much as possible. If it exceeds 0.005 wt%, its adverse effects will be particularly large, so 0.005 wt%
% Or less, preferably 0.003 wt% or less.

Nb: 0.03 wt% or less; Nb is an element necessary for depositing and fixing C and improving moldability.
If it is added in excess of wt%, the formability is rather lowered. Therefore, the addition amount must be 0.03 wt% or less, preferably 0.02 wt% or less.

B: 0.0001 to 0.0050 wt%; B is an element effective for fixing N. Addition of less than 0.0001 wt% has no effect, while addition of more than 0.0050 wt% causes workability. Is deteriorated, so 0.0001 to 0.0050 wt%, preferably
It should be 0.0001 to 0.0030 wt%.

1≤Nb (wt%) / C (wt%) ≤30; In the present invention, Nb (wt%) / C (wt%) is important, and if the value is less than 1, the residual solid solution C remains. If the amount is too large, the formability deteriorates. However, when the value exceeds 30, all of C is precipitated and fixed by Nb, so bake hardenability (BH
Sex) is significantly reduced. Therefore, Nb (wt%) / C (w
The value of (t%) is 1 to 30, preferably 8 to 25.

N (wt%)-(14/11) B (wt%) ≤0.00
1 (wt%); In the present invention, it is important to precipitate and fix N with B to obtain a recrystallization texture effective for improving the r value. In order to obtain the effect, as shown in FIG. 1, the relation between N and B is N (wt%)-(14/11) B
The value of (wt%) must be 0.001 (wt%) or less.

Next, the reason for limiting the manufacturing conditions will be described. Soaking hold temperature: 1050-1300 ℃; If the soaking hold temperature of the slab is less than 1050 ℃, the FDT may be lower than the Ar ₃ transformation point, and if it exceeds 1300 ℃, the FDT becomes too high and the hot rolling Since the subsequent coarsening of crystal grains causes deterioration of the r value of the cold rolled annealed sheet, its temperature range is 1050 to 1300 ° C.
The temperature is preferably 1150 to 1250 ° C. From the viewpoint of energy saving, it is preferable to perform hot rolling immediately or after performing heat retention treatment without reheating the continuously cast slab or lowering the temperature below the Ar ₃ transformation point after continuous casting.

Hot rolling end temperature: Ar ₃ transformation point or higher; The hot rolling termination temperature needs to be Ar ₃ transformation point or higher from the viewpoint of formability. If the temperature is excessively high, the r value of the cold-rolled annealed sheet may be lowered due to the coarsening of crystal grains after hot rolling. Therefore, the Ar ₃ transformation point + 50 ° C. to Ar ₃ transformation point is preferable.

Winding temperature: 800 ° C. or less; If the winding temperature range after hot rolling exceeds 800 ° C., the material improving effect is saturated and the pickling property deteriorates, so the winding temperature is set to 800 ° C. or less. In order to promote the precipitation of AlN, MnS and NbC and to improve the formability due to the coarsening of the precipitate, the temperature range is preferably 500 to 700 ° C.

Cold rolling reduction: 65 to 90%; Cold rolling reduction requires at least 65% or more cold rolling to obtain a high r value. On the other hand, if the rolling reduction exceeds 90%, the moldability will rather decrease, so 65-
90%, preferably 70-85%.

Annealing temperature: 700 to 950 ° C .; if the annealing temperature for recrystallization after cold rolling is less than 700 ° C., an unrecrystallized structure remains, while if it exceeds 950 ° C., a transformed structure is formed. The temperature range is 700-950 ° C, preferably 800
It is recommended to set it to ~ 950 ° C. The annealing method may be either a continuous annealing method or a box annealing method. Further, the equipment for continuous annealing may be a normal continuous annealing line (CAL) or a continuous hot dip galvanizing line (CGL). As the hot dip galvanizing method after annealing by the CGL, any kind of galvanizing such as alloyed hot dip galvanization and non-alloyed hot dip galvanization may be used. When the above recrystallization annealing is performed by the box annealing method or CAL, hot dip galvanizing, one-layer or two-layer electroplating, or the like can be performed after the recrystallization annealing. Also, 700-950
When recrystallization annealing is performed in a CGL continuous hot-dip galvanizing line in the temperature range of ° C, alloying hot-dip galvanizing or non-alloy hot-dip galvanizing can be performed after the recrystallization annealing.

[0028]

EXAMPLE Steels having the compositional composition shown in Table 1 were smelted to form slabs, which were hot-rolled under the conditions shown in Table 2 by direct-feeding No. 1 and No. 2 and reheating the others, and then acid It was washed and cold-rolled at a reduction rate of 80% to obtain a 0.8 mm thick steel plate. Then, under the conditions shown in Table 2, annealing was performed on the CAL or CGL line, and further, some of them were plated to obtain test materials. The material properties of these test materials were examined, and the results are also shown in Table 2. here,
As described above, the r value was measured by using the JIS No. 5 tensile test piece, pre-straining 15%, and then measuring by the three-point method. The r-value was measured in the rolling direction, the direction of 45 ° with respect to the rolling direction, and the rolling direction. for
Let r values in the direction of 90 ° be r _L , r _D, and r _C , respectively, and calculate their average values from (r _L + 2r _D + r _C ) / 4.
Sought by. For the amount of BH, a JIS No. 5 tensile test piece was used, and after pre-straining 2%, heating at 170 ° C for 20 minutes,
It was re-tensioned and determined from the difference between the maximum load before aging and the load at the upper yield point after aging.

[0029]

[Table 1]

[0030]

[Table 2] From Table 2, all the thin steel sheets produced by the method of the present invention are
It has a bake hardenability and an excellent formability, and it can be seen that the bake hardenable (BH) amount is 20 MPa or more and the r value is 2.7 or more. On the other hand, the comparative example is inferior in at least one of moldability and bake hardenability.

[0031]

As described above, according to the manufacturing method of the present invention, it is possible to provide a bake hardenable thin steel sheet which is excellent in bake hardenability and has a high r value and excellent formability. Furthermore, the present invention has a bake hardening (BH) amount of 20 MPa or more and an r value of 2.7.
It is possible to provide a bake hardenable thin steel sheet having the above characteristics and excellent in formability.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between r value and N (wt%)-(14/11) B (wt%).

Continuation of the front page (56) Reference JP-A-7-62446 (JP, A) JP-A-5-287448 (JP, A) JP-A-5-263146 (JP, A) JP-A-5-112858 (JP , A) JP-A-3-211228 (JP, A) JP-A-61-281852 (JP, A) JP-A-61-26757 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB) Name) C22C 38/00 C21D 8/04 C21D 9/48

Claims (2)

(57) [Claims] 1. C: 0.0015 wt% or less, Si: 0.1 wt% or less, Mn: 0.5 wt% or less, Al: 0.5 wt% or less, P: 0.08 wt% or less, S: 0.01 wt% or less N: 0.005 wt % Or less, Nb: 0.03 wt% or less, B: 0.0001 to 0.0050 wt% included, and Nb, C, B, N
, 1 ≦ Nb (wt%) / C (wt%) ≦ 30, and N (wt
%)-(14/11) B (wt%) ≤ 0.001 (wt%), and the balance is Fe and inevitable impurities. Thin steel plate. 2. C: 0.0015 wt% or less, Si: 0.1 wt% or less, Mn: 0.5 wt% or less, Al: 0.5 wt% or less, P: 0.08 wt% or less, S: 0.01 wt% or less N: 0.005 wt % Or less, Nb: 0.03 wt% or less, B: 0.0001 to 0.0050 wt% included, and Nb, C, B, N
, 1 ≦ Nb (wt%) / C (wt%) ≦ 30, and N (wt
%)-(14/11) B (wt%) ≤ 0.001 (wt%), with the balance being Fe and unavoidable impurities, the steel slab is uniformly heated to a temperature range of 1050 to 1300 ℃. After holding, Ar
Hot rolling is completed at ₃ transformation points or higher, wound at 800 ° C or lower, followed by pickling, cold rolling with a reduction rate of 65 to 90%, and then re-rolling in the temperature range of 700 to 950 ° C. A method for manufacturing a bake hardenable thin steel sheet excellent in formability, characterized by performing crystal annealing.