JP3451703B2 - High tensile cold rolled steel sheet for deep drawing with excellent strength-ductility balance and bake hardenability, 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 high-tensile cold-rolled steel sheet mainly used for automobiles, and in particular, a high-strength cold-rolled steel sheet for deep drawing subjected to deep drawing and provided for an automobile body. The present invention relates to a steel plate and a method for manufacturing the same.

[0002]

2. Description of the Related Art Recent automobiles have improved fuel efficiency (weight reduction).
Improvement of safety is an important development theme.
Under these circumstances, as the deep drawing for cold rolled steel sheets for automobiles, with tensile strength (TS) has a high tension of about 38~60kgf / mm ^2, the material having excellent press formability Demands are growing.

[0003] As such a promising production technique of a high-tensile cold-rolled steel sheet, conventionally, decarbonization has been sufficiently performed in the steel making process to obtain extremely low carbon, and C, N in the steel has been added by adding Ti, Nb and the like. In addition, there has been proposed a method in which the solid solution strengthening effect of Si, P, and Mn is utilized after the solid solution is fixed. For example, Japanese Patent Publication No. 63-190141 discloses a technique for adding a large amount of Mn and P to ultra-low carbon steel to which Ti is added.

[0004]

However, the characteristics of the ultra-low carbon cold rolled steel sheet manufactured by the method of the above-mentioned known technique have an r value of about 1.8, but the product of the tensile strength and the elongation. The strength-ductility balance represented by (TS × El) was only 1750 (kgf / mm ² ·%) or less. A low strength-ductility balance means that a higher tensile strength inevitably leads to a decrease in ductility, and a higher tensile strength of a cold-rolled steel sheet, on the other hand, degrades press formability. There was a problem of letting it. In addition, in the ultra-low carbon cold rolled steel sheet manufactured by the above-mentioned known technology, it is a fact that almost no so-called bake hardenability can be expected due to the effect of increasing the strength due to baking after press forming and painting, and as a result, There was a problem that the product could not be effectively and economically reduced in weight and improved in safety.

Accordingly, a main object of the present invention is to obtain a high-strength cold-rolled steel sheet for deep drawing that is effective for reducing weight and improving safety, and to establish an effective manufacturing technique therefor. One of the other objects of the present invention is that the TS is 38 to 60.
kgf / mm ² , r value is 1.8 or more and TS × El is 1792 (kg
f / mm ²・%)
That is, it is to propose a high-tensile cold-rolled steel sheet for deep drawing excellent in press formability and a method for producing the same. Further, another object of the present invention is to provide a baking hardenable material having a bake hardenability of about 3.0 kgf / mm ² or more even if it is a very low carbon steel. It is to propose a high-tensile cold-rolled steel sheet for drawing and a method for producing the same.

[0006]

Under the recognition of such a problem, the present inventors have conducted intensive studies to solve the problem, and as a result, have found that the range of the composition of the steel or the mutual relationship between the components, the hot work, etc. It has been found that the above problems can be solved by controlling the rolling conditions and the annealing conditions of the cold rolled sheet, and the present invention has been completed. That is, the gist configuration of the present invention is as follows.

(1) C: 0.002 wt% or less, Si: 0.1 to 2.0 wt%
%, Mn: 0.5 to 2.5 wt%, P: 0.03 to 0.20 wt%, S: 0.00
3 wt% or less, sol Al: 0.1 wt% or less, Ti: 0.2 wt% or less, N
b: 0.001 to 0.1 wt% , B: 0.0002 to 0.005 wt%, N: 0.002
wt% or less , and the above Si, P and Mn are represented by the following formula (1)
The meet, with the remainder Ri Do from Fe and unavoidable impurities, T
S × El is 1792kgf / mm ² ·% or more der Ru strength - ductility balance and deep drawn high-strength cold-rolled steel sheet for processing excellent bake hardenability. Serial 0.2 ≦ (Si (wt%) + P (wt%)) / Mn (wt%) ≦ 1.0 ...... (1)

(2) C: 0.002 wt% or less, Si: 0.1 to 2.0 wt%
%, Mn: 0.5 to 2.5 wt%, P: 0.03 to 0.20 wt%, S: 0.00
3 wt% or less, sol Al: 0.1 wt% or less, Ti: 0.2 wt% or less, N
b: 0.001 to 0.1 wt% , B: 0.0002 to 0.005 wt%, N: 0.002
wt% or less, and Ni: 0.1 to 1.0 wt%, Cu: 0.1 to
1.0 wt% and Mo: one or more selected from 0.05 to 0.5 wt%, and the Si, P, M
n, Mo, Ni, Cu satisfies the following equation (2), the balance Ri Do of Fe and unavoidable impurities, TS × El is 1792kgf / mm ² ·% or more
On Der Ru strength - ductility balance and deep drawn high-strength cold-rolled steel sheet for processing excellent bake hardenability. Serial 0.2 ≦ (Si (wt%) + P (wt%) + Mo (wt%)) / (Mn (wt%) + Ni
(wt%) + Cu (wt%)) ≦ 1.0 …… (2)

(3) C: 0.002 wt% or less, Si: 0.1 to 2.0 wt%
%, Mn: 0.5 to 2.5 wt%, P: 0.03 to 0.20 wt%, S: 0.00
3 wt% or less, sol Al: 0.1 wt% or less, Ti: 0.2 wt% or less, N
b: 0.001 to 0.1 wt% , B: 0.0002 to 0.005 wt%, N: 0.002
wt% or less, and Y, La, Ce, Pr, Nd, Pm and
One or more elements selected from Sm are contained in a total amount of 0.2 wt% or less , and the above Si, P, and Mn are
Serial satisfying the expression (1), the remainder Ri Na <br/> of Fe and unavoidable impurities, TS × El is 1792kgf / mm ² ·% or more der Ru intensity - deep excellent ductility balance and bake hardenability machining High-strength cold-rolled steel sheet. Serial 0.2 ≦ (Si (wt%) + P (wt%)) / Mn (wt%) ≦ 1.0 ...... (1)

(4) C: 0.002 wt% or less, Si: 0.1 to 2.0 wt%
%, Mn: 0.5 to 2.5 wt%, P: 0.03 to 0.20 wt%, S: 0.00
3 wt% or less, sol Al: 0.1 wt% or less, Ti: 0.2 wt% or less, N
b: 0.001 to 0.1 wt% , B: 0.0002 to 0.005 wt%, N: 0.002
wt% or less, and Ni: 0.1 to 1.0 wt%, Cu: 0.1 to
1.0 wt% and Mo: one or more selected from 0.05 to 0.5 wt% , further containing Y, La, Ce, Pr,
One or more elements selected from Nd, Pm and Sm are contained in a total amount of 0.2 wt% or less , and
i, P, Mn, Mo, Ni and Cu satisfy the following equation (2) , and the balance is Fe
And Ri Do unavoidable impurities, the TS × El 1792kgf /
mm ² ·% or more der Ru intensity - deep drawing high-strength cold-rolled steel sheet for processing excellent ductility balance and bake hardenability. Serial 0.2 ≦ (Si (wt%) + P (wt%) + Mo (wt%)) / (Mn (wt%) + Ni
(wt%) + Cu (wt%)) ≦ 1.0 …… (2)

(5) In producing the high-tensile cold-rolled steel sheet according to any one of claims 1 to 4, a steel slab having the chemical composition described in each section is heated, and Ar _{3 to} A
r ₃ Finish hot rolling in the temperature range of + 100 ° C, take up at 600 ° C or less, then remove the scale, perform cold rolling, and then run at a temperature of 750 ° C to (Ac ₁ + 50 ° C). A method for producing a high-tensile cold-rolled steel sheet for deep drawing, which is excellent in strength-ductility balance and bake hardenability, characterized by performing annealing in a range.

In each of the above inventions, the mutual relationship between the components is preferably within the following range. 0.2 ≦ (Si (wt%) + P (wt%)) / Mn (wt%) ≦ 1.0 when Mo, Cu, Ni is not added 0.2 ≦ (Si (wt%) + P ( wt%) + Mo (wt%)) / (Mn (w
t%) + Ni (wt%) + Cu (wt%)) ≦ 1.0

[0013]

Next, the reason why the composition of steel and the production conditions in the present invention are limited as described in the above summary will be described. C: 0.002 wt% or less; C deteriorates press workability, particularly r value and elongation. If the content exceeds 0.002% by weight, the adverse effect increases, so the content needs to be 0.002% by weight or less, preferably 0.0012% by weight or less.

Si: 0.1 to 2.0 wt%; Si is an element useful for solid solution strengthening of steel without significantly deteriorating the r value. If it is less than 0.1 wt%, the desired strength cannot be obtained.
If it exceeds 2.0 wt%, the plating properties will deteriorate, so 0.1 to 2.
0 wt%, preferably in the range of 0.2 to 1.2 wt%.

Mn: 0.5 to 2.5 wt%; Mn is an element that contributes to the improvement of the r value through the refinement of the hot-rolled sheet due to the reduction of the transformation point as well as the improvement of the strength . If the addition amount is less than 0.5 wt%, these effects cannot be obtained. On the other hand, if it exceeds 2.5 wt%, a low-temperature transformation phase appears and deteriorates the r value.
%, Preferably in the range of 0.8 to 2.0% by weight.

P: 0.03 to 0.20 wt%; P has the effect of increasing the strength by solid solution strengthening and improving the r-value. The effect appears when the addition is 0.03 wt% or more. However, when it exceeds 0.20 wt%, it segregates at the grain boundary to cause grain boundary embrittlement and worsens center segregation at the time of solidification. Therefore, the addition amount is 0.03 to 0.20%. wt%, preferably 0.05-0.12wt
% Range.

S: 0.003 wt% or less; S forms MnS or the like, reduces local ductility represented by stretch flangeability, and also reduces total elongation. 0.
If the content exceeds 003 wt%, the effect becomes particularly noticeable.
003 wt% or less, preferably 0.002 wt% or less. FIG.
Is: C: 0.0006 wt%, Si: 0.51 wt%, Mn: 1.02 wt%,
P: 0.080 wt%, solAl: 0.026 wt%, Ti: 0.005 wt
%, Nb: 0.020 wt%, B: 0.0020 wt%, N: 0.0018 wt%
After hot rolling at a finish temperature of 900 ° C and a winding temperature of 600 ° C from a steel slab with a different chemical composition of S, the scale was removed, after cold rolling, and annealed at 860 ° C 0.8
FIG. 3 shows the effect of S on elongation (total elongation) for a steel sheet of mm. From this, the total elongation is 0.003 wt%
%, It can be seen that it is improved.

Sol Al: 0.1 wt% or less; sol Al is an element necessary for deoxidation, but if added in excess of 0.1 wt%, not only the effect is saturated, but also the formability due to the increase in inclusions Therefore, its content needs to be 0.1 wt% or less, preferably 0.02 to 0.06 wt%.

Ti: 0.2 wt% or less; Ti is a solid solution in steel
It has the effect of fixing C, N, and S to form a texture that is advantageous for deep drawing, but if added in excess of 0.2 wt%,
Produces Ti phosphide, deteriorating elongation and r-value. Therefore, the amount of addition is 0.2 wt% or less, preferably 0.03 to 0.
Need to be 12wt%.

Nb: 0.001 to 0.1 wt%; Nb converts C in steel to Nb
By fixing as C and making the structure before finish rolling finer, there is an effect of forming a texture advantageous for deep drawability. Further, in the ultra-low carbon steel of the present invention, it is an extremely effective element for exhibiting bake hardenability. Nb
If the content of is less than 0.001wt%, there is no effect of its addition,
On the other hand, if it exceeds 0.1 wt%, hot rolling becomes rolling in an austenite non-recrystallized state and adversely affects the formability after cold rolling and annealing, so the addition amount is 0.001 to 0.1 wt%,
Preferably, it must be 0.003 to 0.05 wt%.

B: 0.0002 to 0.005 wt%; B is added to improve the resistance to secondary working embrittlement. However, if B is less than 0.0002 wt%, the effect cannot be obtained. In addition to increasing the rolling load caused by the delay of austenite recrystallization during hot rolling, the elongation after cold rolling / annealing and the r value are deteriorated.
%, Preferably in the range of 0.0008 to 0.003 wt%.

N: 0.002 wt% or less; N has a bad effect on both the r value and elongation characteristics, so the smaller the content, the better. However, if it exceeds 0.002 wt%, the effect becomes remarkable, so 0.002 wt%. Or less, preferably 0.0015 wt% or less.

The total amount of any one or more of the elements selected from Y, La, Ce, Pr, Nd, Pm and Sm: 0.2
wt% or less; All of these elements have an effect of improving the surface treatment properties, particularly the hot-dip galvanizing property, by concentrating on the steel sheet surface and suppressing the surface concentration of Si and Mn. However, excessive addition causes a reduction in press workability due to surface defects and inclusions. Therefore, the total amount of these elements is 0.2 wt% or less, preferably 0.02 to 0.12 wt %.
Must be in the range.

Ni: 0.1 to 1.0 wt%, Cu: 0.1 to 1.0 wt%, M
o: 0.05-0.5wt% ; All of these elements are effective for strengthening steel sheets.
Add the required amount according to the strength, but if you add in excess,
r : Ni content: 0.1 to 1.0 wt%, C
u: 0.1 to 1.0 wt%, Mo: 0.05 to 0.5 wt% , preferably Ni:
0.1-0.6wt%, Cu: 0.1-0.6wt%, Mo: 0.05-0.3wt%
Add within the range.

In the present invention, the mutual relationship between the components is preferably set in the following range from the viewpoint of improving the r value. 1) When Mo, Cu, and Ni are not added, 0.2 ≦ (Si (wt%) + P (wt%)) / Mn (wt%) ≦ 1.0
2) When Mo, Cu and Ni are added, 0.2 ≦ (Si (wt%) + P (wt%) + Mo (wt%)) / (Mn (w
t%) + Ni (wt%) + Cu (wt%)) ≦ 1.0 That is, in the above formulas 1) and 2), Si, P and Mo
Is a ferrite stabilizing element, and Mn, Ni and Cu are austenite stabilizing elements, and can be further improved by setting the ratio of both elements to 0.2 or more. However, if the ratio exceeds 1.0, the transformation point of the steel becomes high, which is disadvantageous for making the hot-rolled sheet finer.

Next, the reasons for limiting the manufacturing method of the present invention, particularly the main requirements, will be described. Hot rolling end temperature: Ar _{3 to} Ar ₃ + 100 ° C .; If the hot rolling end temperature is lower than Ar ₃ , rolling will be in the two-phase region, and a texture disadvantageous to the r value after cold rolling and annealing will be obtained. Form. On the other hand, if the end temperature exceeds Ar ₃ + 100 ° C., the grain size of the hot-rolled sheet becomes coarse, and a texture effective for improving the deep drawability cannot be developed. Accordingly, the finishing temperature of hot rolling is Ar ₃ to Ar ₃ +
It must be in the 100 ° C temperature range.

Winding temperature: 600 ° C. or less; When the winding temperature after hot rolling is 600 ° C. or more, the r value decreases. The reason why the r value decreases is considered that Ti and P form phosphide in this temperature range, and the phosphide deteriorates the r value. Therefore, the winding temperature is 60
The temperature must be below 0 ° C, preferably between 450 and 600 ° C.

Annealing temperature: 750 ° C. to (Ac ₁ + 50 ° C.); {111} re-annealing is performed on the cold-rolled sheet having undergone the above-mentioned steps in a temperature range of 750 ° C. to (Ac ₁ + 50 ° C.). The crystal texture is highly developed, and a high-strength cold-rolled steel sheet having a high target r value can be obtained. That is, when the annealing temperature is lower than 750 ° C, the recrystallized structure is not developed and the r-value is low.
If the temperature exceeds (Ac ₁ + 50 ° C.), transformation from austenite to ferrite occurs in the cooling process, so that the recrystallization texture is randomized and only a low r value is obtained. In addition,
The equipment for performing the annealing of the present invention does not need to be particularly specified, but is preferably a continuous annealing equipment from the viewpoint of productivity, cost, and the like. Although the annealing time is not particularly specified, it is preferable to perform the annealing in the range of 10 to 60 sec.

As described above, in the method of the present invention, the slab having a specific range of the component composition can be properly combined with the steps satisfying the above-mentioned conditions to obtain the TS.
Is 38-60 kgf / mm ² , r value is 1.8 or more, and TS × El is
^{1792 (kgf / mm 2 ·%} ) have the above properties, further strength bake hardening amount has a 3.0 kgf / mm ² or more characteristics - high-strength cold-rolled steel sheet for deep drawing excellent in ductility balance and bake hardenability Can be manufactured. Further, even if the steel sheet obtained by the method of the present invention is subjected to a surface treatment such as Zn-based electroplating or hot-dip galvanizing, its effect is not impaired.

[0030]

EXAMPLES The compositions shown in Tables 1, 2 and 3 were used.
Steel slab (Values outside the range of the present invention are underlined.
Was heated to 1250 ° C. and the conditions shown in Tables 4 and 5 were obtained.
Hot rolling, pickling, and cold rolling with a rolling reduction of 80%
The thickness of the cold rolled sheet is 0.8 mm.
Annealing was performed at the temperatures shown in Tables 4 and 5 for 40 seconds.
The material properties of the cold-rolled steel sheet obtained in this way were subjected to a tensile test.
Determined by Here, the tensile properties are JIS No. 5 tensile test pieces.
The r value is measured at 3 points with a tensile prestrain of 15%.
L direction (rolling direction), D direction (rolling direction)
And 45 degrees) and r value in C direction (perpendicular to rolling direction)
Then, the average r value is calculated as r value = (r_L+ 2r _D+ R_c) / 4
Asked from. The bake hardenability (BH) is 2% pre-strain
After 170 ° C for 20 minutes
And the secondary processing embrittlement resistance
After the ranking, height of the cup squeezed with 24.4mmφ punch
Cut to 21mm, keep at the specified temperature, and weigh 5kg-80cm
The crack generation temperature due to the falling of the weight was determined as Tc.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Table 5]

The evaluation results are shown in Tables 4 and 5. As is clear from Tables 4 and 5, the steel sheet obtained by the method of the present invention has a TS of 38 to 60 kgf / mm ² ,
Excellent deep drawing and tensile properties with r value of 1.8 or more, TS × El of 1792 (kgf / mm ² ·%) or more, and baking hardening amount of about 3.0 kgf / mm ² or more. You can see that it is doing. However, the comparative example is inferior in at least one of these characteristics.

[0037]

As described above, according to the present invention,
TS is 38-60 kgf / mm ² , r value is 1.8 or more, TS × El is 1792
(Kgf / mm ² ·%) High strength cold-rolled steel sheets for deep drawing with excellent strength-ductility balance can be manufactured. Further, according to the present invention, it is possible to produce a high-tensile cold-rolled steel sheet for deep drawing, which has a bake hardening amount of about 3.0 kgf / mm ² or more and has excellent aging resistance. Therefore, according to the present invention, it is possible to provide a cold-rolled steel sheet having both high strength and high workability, which greatly contributes to reducing the weight and improving the safety of an automobile.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between elongation and S content.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Engineering Co., Ltd. (72) Inventor Eiko Yasuhara 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki (56) References JP-A-62-109927 (JP, A) JP-A-5-263146 (JP, A) JP-A-6-25755 (JP, A) 9698 (JP, A) JP-A-5-247540 (JP, A) JP-A-4-247827 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38 / 60 C21D 9/46-9/48

Claims (5)

(57) [Claims] C: 0.002 wt% or less, Si: 0.1 to 2.0 wt%, M
n: 0.5 to 2.5 wt%, P: 0.03 to 0.20 wt%, S: 0.003 wt%
Below, sol Al: 0.1 wt% or less, Ti: 0.2 wt% or less, Nb: 0.0
01-0.1wt% , B: 0.0002-0.005wt%, N: 0.002wt%
In addition to the following, Si, P and Mn satisfy the following formula (1).
Tashi, the remainder Ri Do from Fe and unavoidable impurities, TS ×
El is 1792kgf / mm ² ·% or more der Ru strength - ductility balance and deep drawn high-strength cold-rolled steel sheet for processing excellent bake hardenability. Serial 0.2 ≦ (Si (wt%) + P (wt%)) / Mn (wt%) ≦ 1.0 ...... (1) 2. C: 0.002% by weight or less, Si: 0.1 to 2.0% by weight, M
n: 0.5 to 2.5 wt%, P: 0.03 to 0.20 wt%, S: 0.003 wt%
Below, sol Al: 0.1 wt% or less, Ti: 0.2 wt% or less, Nb: 0.0
01-0.1wt% , B: 0.0002-0.005wt%, N: 0.002wt%
The following is included, and Ni: 0.1 to 1.0 wt%, Cu: 0.1 to 1.0 wt%
% And Mo: one or more selected from 0.05 to 0.5 wt%, and the above Si, P, Mn, M
o, Ni, Cu satisfies the following equation (2), the balance Ri Do of Fe and unavoidable impurities, in TS × El is 1792kgf / mm ² ·% or more
Oh Ru intensity - deep drawing high-strength cold-rolled steel sheet for processing excellent ductility balance and bake hardenability. Serial 0.2 ≦ (Si (wt%) + P (wt%) + Mo (wt%)) / (Mn (wt%) + Ni
(wt%) + Cu (wt%)) ≦ 1.0 …… (2) 3. C: 0.002% by weight or less, Si: 0.1 to 2.0% by weight, M
n: 0.5 to 2.5 wt%, P: 0.03 to 0.20 wt%, S: 0.003 wt%
Below, sol Al: 0.1 wt% or less, Ti: 0.2 wt% or less, Nb: 0.0
01-0.1wt% , B: 0.0002-0.005wt%, N: 0.002wt%
Hereinafter, include, and Y, La, Ce, Pr, Nd, together containing less 0.2 wt% in a total amount of any one or two or more elements selected from the Pm and Sm, the Si, P, Mn Is below
Equation (1) is satisfied , and the remainder is made up of Fe and inevitable impurities.
Ri, TS × El is 1792kgf / mm ² ·% or more der Ru strength - ductility balance and deep drawn high-strength cold-rolled steel sheet for processing excellent bake hardenability. Serial 0.2 ≦ (Si (wt%) + P (wt%)) / Mn (wt%) ≦ 1.0 ...... (1) 4. C: 0.002 wt% or less, Si: 0.1 to 2.0 wt%, M
n: 0.5 to 2.5 wt%, P: 0.03 to 0.20 wt%, S: 0.003 wt%
Below, sol Al: 0.1 wt% or less, Ti: 0.2 wt% or less, Nb: 0.0
01-0.1wt% , B: 0.0002-0.005wt%, N: 0.002wt%
The following is included, and Ni: 0.1 to 1.0 wt%, Cu: 0.1 to 1.0 wt%
% And Mo: one or more selected from 0.05 to 0.5 wt%, and further containing Y, La, Ce, Pr, Nd,
One or more elements selected from Pm and Sm are contained in a total amount of 0.2 wt% or less , and the Si,
P, Mn, Mo, Ni, Cu satisfies the following equation (2), the balance Ri Do of Fe and unavoidable impurities, TS × El is 1792kgf / mm
² ·% or more der Ru strength - ductility balance and deep drawn high-strength cold-rolled steel sheet for processing excellent bake hardenability. Serial 0.2 ≦ (Si (wt%) + P (wt%) + Mo (wt%)) / (Mn (wt%) + Ni
(wt%) + Cu (wt%)) ≦ 1.0 …… (2) Upon 5. producing high strength cold-rolled steel sheet according to any one of claims 1 to 4, heating the steel slab having a chemical composition described in each claim, Ar ₃ to Ar ₃ +1
Finish hot rolling in a temperature range of 00 ° C., take up at 600 ° C. or less, then perform cold rolling after removing scale, and then anneal in a temperature range of 750 ° C. to (Ac ₁ + 50 ° C.) A method for producing a high-tensile cold-rolled steel sheet for deep drawing, which is excellent in strength-ductility balance and bake hardenability.