JPH07252590A - High tensile strength cold rolled steel plate for deep drawing excellent in balance of strength-ductility and its production - Google Patents

Abstract

PURPOSE:To produce a steel plate excellent in the balance of strength-ductility and workability by specifying its compsn. CONSTITUTION:Composition of this steel plate is constituted of, by weight, 0.002% C, 0.1 to 2.O% Si, 0.5 to 2.S% Mn, 0.03 to 0.20% P, <=0.003% S,% 0.1% Sol Al, 0.2% Ti, 0.001 to 0.1% Nb, 0.0002 to 0.005% B, <=0.002% N, and the balance Fe with inevitable impurities. If required, one or >=two kinds of metals selected from Ni, Cu, Mo and rare earths are suitably added thereto as well. At the time of producing the steel sheet, the steel slab having this compsn. is heated, its hot rolling is finished in the temp. range of the Ar3+ or -10 deg.C, and it is coiled at <=650 deg.C. This steel strip is subjected to cold rolling and is thereafter preferably annealed in the temp. range of 750 deg.C to (the Ac1+50 deg.C). Thus, the steel plate in which TS is regulated to 38 to 60kgf/mm<2>, the gamma value to -1.8 and TS X E1 to 1750(kgf/mm.%) can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength cold-rolled steel sheet mainly used for automobiles, particularly a deep-drawing high-tensile cold-rolled steel sheet for deep-drawing. The present invention relates to a steel plate and a manufacturing method thereof.

[0002]

2. Description of the Related Art Improving fuel efficiency (weight reduction) in recent automobiles
And improving safety is an important development theme.
Under such a background, as a cold-rolled steel sheet for deep drawing for automobiles, the tensile strength (TS) has a high tensile strength of 38 to 60 kgf / mm ² and a material having excellent press formability. Demand is growing.

As a promising technology for producing such high-strength cold-rolled steel sheets, conventionally, decarburization treatment has been sufficiently performed in the steelmaking process to obtain ultra-low carbon, and C, N in steel by adding Ti, Nb, etc. It has been proposed to use the solid solution strengthening action of Si, P, and Mn after fixing the above. For example, Japanese Examined Patent Publication (Kokoku) No. 63-190141 discloses a technique of adding a large amount of Mn, P to an extremely low carbon steel containing Ti.

[0004]

However, the characteristics of the ultra-low carbon cold-rolled steel sheet manufactured by the above-mentioned known method are such that although r value is about 1.8, the product of tensile strength and elongation is obtained. The strength-ductility balance represented by (TS x El) was 1750 (kgf / mm ² ·%) or less. This low strength-ductility balance means that higher tensile strength inevitably leads to lower ductility, and higher tensile strength of the cold-rolled steel sheet deteriorates press formability on the other hand. There was a problem of letting it. Further, in the ultra-low carbon cold-rolled steel sheet produced by the above-mentioned known technique, it is the fact that almost no strength increasing effect due to baking after press forming and coating, so-called bake hardenability, can be expected, and as a result, the above-mentioned final There is a problem that the product cannot be effectively and economically reduced in weight and safety.

Therefore, a main object of the present invention is to obtain a high-strength cold-rolled steel sheet for deep drawing which is effective for weight reduction and improvement of safety, and to establish an effective manufacturing technique thereof. And another one of the objects of the present invention is that TS is 38-60.
kgf / mm ² , r value is 1.8 or more, and TS × El is 1750 (kgf
/ mm ² ·%) strength having the above characteristics - ductility balance, i.e. to propose a deep drawing high-strength cold-rolled steel sheet for processing is excellent in press formability and manufacturing method thereof. Further, another object of the present invention is to provide a low-carbon steel having a bake hardenability, specifically, a deep bake hardenability having a bake hardenability of about 3.0 kgf / mm ² or more. It is to propose a high-strength cold-rolled steel sheet for drawing and a manufacturing method thereof.

[0006]

Under the recognition of such problems, the inventors have conducted diligent research to solve the problems, and as a result, have found that the range of the composition of the steel or the mutual relationship between the respective components, the hot work The inventors have found that the problems described above can be solved by controlling the rolling conditions and the annealing conditions for cold-rolled sheets, and have completed the present invention. That is, the gist of the present invention is as follows.

(1) C: 0.002 wt% or less, Si: 0.1-
2.0 wt%, Mn: 0.5-2.5 wt%, P: 0.03-0.20
wt%, S: 0.003 wt% or less, sol Al: 0.1 wt% or less, Ti: 0.2 wt% or less, Nb: 0.001 to 0.1 wt% B:
0.0002 to 0.005 wt%, N: 0.002 wt% or less,
The balance consists of Fe and unavoidable impurities. It is a high-strength cold-rolled steel sheet for deep drawing that has excellent strength-ductility balance and bake hardenability.

(2) C: 0.002 wt% or less, Si: 0.1-
2.0 wt%, Mn: 0.5-2.5 wt%, P: 0.03-0.20
wt%, S: 0.003 wt% or less, sol Al: 0.1 wt% or less, Ti: 0.2 wt% or less, Nb: 0.001 to 0.1 wt% B:
0.0002 to 0.005 wt%, N: 0.002 wt% or less, and Ni: 1.0 wt% or less, Cu: 1.0 wt% or less, and M
o: A high-strength cold-rolled steel sheet for deep drawing, which contains one or more selected from 0.5 wt% or less, and the balance is Fe and unavoidable impurities and is excellent in strength-ductility balance and bake hardenability.

(3) C: 0.002 wt% or less, Si: 0.1-
2.0 wt%, Mn: 0.5-2.5 wt%, P: 0.03-0.20
wt%, S: 0.003 wt% or less, sol Al: 0.1 wt% or less, Ti: 0.2 wt% or less, Nb: 0.001 to 0.1 wt% B:
0.2 wt% in total of 0.0002 to 0.005 wt%, N: 0.002 wt% or less, and any one or more elements selected from Y, La, Ce, Pr, Nd, Pm and Sm. A high-strength cold-rolled steel sheet for deep drawing, containing the following, the balance consisting of Fe and unavoidable impurities and having excellent strength-ductility balance and bake hardenability.

(4) C: 0.002 wt% or less, Si: 0.1-
2.0 wt%, Mn: 0.5-2.5 wt%, P: 0.03-0.20
wt%, S: 0.003 wt% or less, sol Al: 0.1 wt% or less, Ti: 0.2 wt% or less, Nb: 0.001 to 0.1 wt% B:
0.0002 to 0.005 wt%, N: 0.002 wt% or less, and Ni: 1.0 wt% or less, Cu: 1.0 wt% or less, and M
o: Contains one or more selected from 0.5 wt% or less, and further contains Y, La, Ce, Pr, Nd, Pm and Sm.
High tension cold drawing for deep drawing that contains 0.2 wt% or less in total of one or more elements selected from the following, with the balance consisting of Fe and unavoidable impurities and having excellent strength-ductility balance and bake hardenability. Rolled steel sheet.

(5) In producing the high-strength cold-rolled steel sheet according to any one of claims 1 to 4, a steel slab having the chemical composition described in each of the sections is heated to produce Ar _3-
Finish the hot rolling in the temperature range of Ar ₃ +100 ℃, wind it at 650 ℃ or less, and then remove the scale and perform the cold rolling, and then the temperature of 750 ℃ ~ (Ac ₁ +50 ℃) A method for producing a high-strength 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 among the respective components is preferably within the following range. Without adding Mo, Cu, Ni 0.2 ≤ (Si (wt%) + P (wt%)) / Mn (wt%) ≤ 1.0 When adding Mo, Cu, Ni 0.2 ≤ (Si (wt%) + P ( wt%) + Mo (wt%)) / (Mn (w
t%) + Ni (wt%) + Cu (wt%)) ≤1.0

[0013]

In the present invention, the reason why the composition and manufacturing conditions of steel are limited to the above constitution will be explained. C: 0.002 wt% or less; C deteriorates press workability, especially r value and elongation. If the content exceeds 0.002 wt%, the adverse effect thereof becomes large, so it is necessary to set it to 0.002 wt% or less, preferably 0.0012 wt% or less.

Si: 0.1 to 2.0 wt%; Si is a useful element for solid solution strengthening steel without significantly degrading the r value. If less than 0.1 wt%, the target strength cannot be obtained, while
If it exceeds 2.0 wt%, the plating property will deteriorate, so 0.1-2.
It is added in an amount of 0 wt%, preferably 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 grain refinement of the hot-rolled sheet due to the reduction of the transformation point as well as the improvement of the strength . If the added amount is less than 0.5 wt%, those effects cannot be obtained, while if it exceeds 2.5 wt%, a low temperature transformation phase appears and deteriorates the r value, so 0.5 to 2.5 wt%
%, Preferably 0.8 to 2.0 wt%.

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

S: 0.003 wt% or less; S forms MnS or the like to reduce the local ductility represented by stretch flangeability and also the total elongation, so the smaller the content, the better. 0.
When it exceeds 003 wt%, the effect becomes particularly remarkable, so 0.
It is 003 wt% or less, preferably 0.002 wt% or less. Figure 1
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%
Thickness of steel slabs with different chemical composition and different S content after hot rolling with hot rolling finish temperature of 900 ℃ and coiling temperature of 600 ℃, scale removal, cold rolling, and annealing at 860 ℃ 0.8
It shows the effect of S on the elongation (total elongation) of a steel sheet of mm. From this, the total elongation is 0.003 wt% S
It can be seen that the improvement is achieved when the percentage is below%.

Sol Al: 0.1 wt% or less; sol Al is an element necessary for deoxidation, but addition of more than 0.1 wt% not only saturates the effect, but also increases the formability by increasing inclusions. Therefore, its content must 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 function of fixing C, N and S to form a texture that is advantageous for deep drawability, but if it is added in excess of 0.2 wt%, it will be added to the steel.
It produces Ti phosphide, which causes elongation and deteriorates the r value. Therefore, the addition amount is 0.2 wt% or less, preferably 0.03 to 0.
It should be 12wt%.

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

B: 0.0002 to 0.005 wt%; B is added to improve the secondary work embrittlement resistance, but if it is less than 0.0002 wt%, its effect cannot be obtained, while if it is added in excess of 0.005 wt%. In addition to increasing the rolling load due to the delay of austenite recrystallization during hot rolling, it also deteriorates the elongation and r-value after cold rolling / annealing, so the addition amount is 0.0002 to 0.005 wt.
%, Preferably 0.0008 to 0.003 wt%.

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

Total amount of one or more elements selected from Y, La, Ce, Pr, Nd, Pm and Sm: 0.2
wt% or less; all of these elements have the effect of improving the surface treatment property, especially the hot dip galvanizing property, by concentrating on the surface of the steel sheet and suppressing the surface concentration of Si and Mn. However, if added excessively, the press workability deteriorates due to surface defects and inclusions, so the total amount of these elements added is 0.2 wt% or less, preferably 0.02 to 0.12 wt %.
Must be in the range.

Ni: 1.0 wt% or less, Cu: 1.0 wt% or less, M
o: 0.5 wt% or less; all of these elements are effective in strengthening the steel sheet, and the necessary amount is added depending on the strength.
If added in excess, the r value will decrease, so N
i: 1.0 wt% or less, Cu: 1.0 wt% or less, Mo: 0.5 wt% or less, preferably Ni: 0.1 to 0.6 wt%, Cu: 0.1 to 0.6 wt%
%, Mo: Add in the range of 0.05 to 0.3 wt%.

In the present invention, the mutual relationship among the respective components is preferably within 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
When 2) 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 are added.
Is a ferrite stabilizing element, and Mn, Ni, and Cu are austenite stabilizing elements. If the ratio of both elements is 0.2 or more, it is further improved. However, if this ratio exceeds 1.0, the transformation point of the steel becomes high, which is disadvantageous for the grain refinement of the hot-rolled sheet.

Next, the reasons for limiting the manufacturing method of the present invention, particularly the main requirements thereof, will be described. End temperature of hot rolling: Ar _{3 to} Ar ₃ + 100 ° C; When the end temperature of hot rolling is less than Ar ₃ , rolling occurs in the two-phase region, and a texture that is unfavorable for the r value after cold rolling and annealing is formed. Form. On the other hand, when the end temperature exceeds Ar ₃ + 100 ° C, the grain size of the hot-rolled sheet becomes coarse and the texture effective for improving the deep drawability does not develop. Therefore, the end temperature of hot rolling is Ar _{3 to} Ar ₃ +
It should be in the temperature range of 100 ° C.

Winding temperature: 650 ° C or lower; r value decreases when the winding temperature after hot rolling is 650 ° C or higher. r
The reason why the value decreases is that Ti and P form phosphide in this temperature range, and this phosphide deteriorates the r value. Therefore, the coiling temperature should be 650 ° C or lower, preferably 450 to 600 ° C.

Annealing temperature: 750 ° C. to (Ac ₁ + 50 ° C.); the cold-rolled sheet that has undergone the above steps is subjected to recrystallization annealing in the temperature range of 750 ° C. to (Ac ₁ + 50 ° C.) for {111} re-annealing. The crystal texture is highly developed, and a target high-strength cold-rolled steel sheet having a high r value can be obtained. That is, when the annealing temperature is less than 750 ° C, the recrystallization structure is undeveloped and the r value is low, while
If it exceeds (Ac ₁ + 50 ° C.), a transformation from austenite to ferrite occurs in the cooling process, so that the recrystallization texture is randomized and only a low r value can be obtained. In addition,
The equipment for carrying out the annealing of the present invention does not have to be specified in particular, but it is preferable to use the continuous annealing equipment from the viewpoint of productivity, cost and the like. Further, the annealing time is not particularly specified, but it is desirable to perform it in the range of 10 to 60 seconds.

As described above, in the method of the present invention, the slab having the component composition in the specific range is appropriately combined with the steps satisfying the above-mentioned respective conditions before the TS
Is 38 to 60 kgf / mm ² , r value is 1.8 or more, and TS × El is 17
High tensile cold-rolled steel sheet for deep drawing with excellent properties of strength-ductility balance and bake hardenability, with properties of 50 (kgf / mm ² ·%) or more and bake hardening amount of 3.0 kgf / mm ² or more. Can be manufactured. Further, even if the steel sheet obtained by the method of the present invention is subjected to surface treatment such as Zn-based electroplating or hot dip galvanizing, the effect is not impaired.

[0030]

EXAMPLES The composition of ingredients shown in Table 1, Table 2 and Table 3 was used.
Steel slab (values outside the scope of the invention are underlined
Is heated to 1250 ° C and the conditions shown in Table 4 and Table 5
Hot rolling according to the conditions, pickling, and then cold rolling with a rolling reduction of 80%
With a thickness of 0.8 mm, this cold-rolled sheet was
Annealing was performed for 40 seconds at the temperatures shown in Tables 4 and 5.
The tensile properties of the material properties of the cold-rolled steel sheet obtained in this way
Sought by. Here, the tensile properties are JIS No. 5 tensile test pieces.
Is measured using 3 points with r value of 15% tensile prestrain.
Method, L direction (rolling direction), D direction (rolling direction)
And r-value in C direction (perpendicular to rolling direction)
Then, the average r value is r value = (r_L+ 2r _D+ R_c) / 4
I asked from. Bake hardenability (BH) has a pre-strain of 2%
Depending on the amount of increase in deformation stress when processed at 170 ° C for 20 minutes
Furthermore, regarding the secondary processing brittleness resistance,
After ranking, the height of the cup squeezed with a 24.4 mmφ punch
After cutting it to 21 mm and keeping it at the specified temperature, weigh 5 kg-80 cm.
The temperature at which cracking occurred due to the weight drop was calculated as Tc.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Table 5]

The results of these evaluations are also 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 1750 (kgf / mm ² ·%) or more, and bake hardening amount of 3.0 kgf / mm ² or more. You can see that However, the comparative example is inferior in at least one of these properties.

[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 1750
It becomes possible to manufacture a high-strength cold-rolled steel sheet for deep drawing, which has an excellent strength-ductility balance and has a property of (kgf / mm ² ·%) or more. Further, according to the present invention, the bake hardening amount is 3.0 kgf / mm ²
It is possible to manufacture a high-strength cold-rolled steel sheet for deep drawing that has a bake hardenability of at least a certain level and is also excellent in 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 weight reduction and safety improvement of automobiles.

[Brief description of drawings]

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

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Sakata 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Technical Research Division, Kawasaki Steel Corporation (72) Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba (72) Inventor Eiko Yasuhara, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Iron & Steel Co., Ltd.

Claims (5)

[Claims] 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: 0.003 wt% or less, sol Al: 0.1 wt% or less , Ti: 0.2 wt% or less, Nb: 0.001 to 0.1 wt% B: 0.0002 to 0.005 wt%, N: 0.002 wt% or less, and the balance consisting of Fe and unavoidable impurities, strength-ductility balance and bake hardening. High-strength cold-rolled steel sheet with excellent properties for deep drawing. 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: 0.003 wt% or less, sol Al: 0.1 wt% or less , Ti: 0.2 wt% or less, Nb: 0.001 to 0.1 wt% B: 0.0002 to 0.005 wt%, N: 0.002 wt% or less, and Ni: 1.0 wt% or less, Cu: 1.0 wt% or less and Mo: A high-strength cold-rolled steel sheet for deep drawing, containing one or more selected from 0.5 wt% or less, and the balance consisting of Fe and unavoidable impurities and having excellent strength-ductility balance and bake hardenability. 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: 0.003 wt% or less, sol Al: 0.1 wt% or less , Ti: 0.2 wt% or less, Nb: 0.001 to 0.1 wt% B: 0.0002 to 0.005 wt%, N: 0.002 wt% or less, and selected from Y, La, Ce, Pr, Nd, Pm and Sm. 0.2 in total of one or more elements
A high-tensile cold-rolled steel sheet for deep drawing that contains less than wt% and the balance is Fe and unavoidable impurities and is excellent in strength-ductility balance and bake hardenability. 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: 0.003 wt% or less, sol Al: 0.1 wt% or less , Ti: 0.2 wt% or less, Nb: 0.001 to 0.1 wt% B: 0.0002 to 0.005 wt%, N: 0.002 wt% or less, and Ni: 1.0 wt% or less, Cu: 1.0 wt% or less and Mo: Contains one or more selected from 0.5 wt% or less, and further contains Y, La, Ce, Pr, Nd, Pm.
And Sm containing one or more elements selected from the total of 0.2 wt% or less, the balance being Fe and unavoidable impurities, which are excellent in strength-ductility balance and bake hardenability. Tension cold rolled steel sheet. 5. In producing the high-strength cold-rolled steel sheet according to any one of claims 1 to 4, a steel slab having the chemical composition described in each of the paragraphs is heated to produce Ar _{3 to} Ar _3. +10
Finish the hot rolling in the temperature range of 0 ℃, wind it at 650 ℃ or less, then remove the scale and perform the cold rolling, then anneal in the temperature range of 750 ℃ ~ (Ac ₁ +50 ℃) The method for producing a high-strength cold-rolled steel sheet for deep drawing which is excellent in strength-ductility balance and bake hardenability.