JPH04246128A - Production of cold rolled sheet of high tensile strength steel for deep drawing excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance, particularly pitting corrosion resistance, of a cold rolled steel sheet for deep drawing having >=35kgf/mm<2> tensile strength by controlling the components of a starting material and also the manufacturing conditions CONSTITUTION:The improvement of corrosion resistance can be attained by using, as a starting material, a slab of a steel which has a composition containing, by weight, <0.006% C, <=1.0% Si, <=1.5% Mn, 0.03-0.15% P, <=0.03% S, 0.01-0.10% Al, <=0.01% N, 0.18-1.5% Cu, and 0.001-0.2% Nb so that the relations in Nb/C>=3 and 0.01<=(Cu/64+P/31)X(Nb/C)<=0.10 are satisfied. Further, superior deep drawability can be attained by subjecting the above steel slab to hot rolling, to pickling and cold rolling, and then to recrystallization annealing in the temp. range between 700 deg.C and the Ac3 transformation point.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention has excellent deep drawability suitable for use in steel plates for automobiles, etc., and has a tensile strength of 3
The present invention relates to a method for producing a cold-rolled steel sheet having a high strength of 5 Kgf/mm2 or more and excellent corrosion resistance.

[0002] In recent years, with the aim of reducing the weight of automobile bodies and improving safety, tensile strength of 35 to 50 Kgf/mm2 has been developed.
Cold-rolled steel sheets that are large and have excellent deep drawability are now required. By adopting these high-strength cold-rolled steel sheets, it is possible to reduce the thickness of steel sheets for automobiles.
On the other hand, as the plate thickness decreases, corrosion resistance, especially pitting corrosion, becomes important. Therefore, there has been a demand for high-strength cold-rolled steel sheets that have excellent deep drawability and corrosion resistance.

0003

[Prior Art] Japanese Patent Publication No. 63-9579 describes a method for manufacturing high-strength cold-rolled steel sheets with excellent deep drawability.
A high-strength cold-rolled steel sheet has been disclosed in which the surface properties of the steel sheet are improved by the combined addition of Cu and Cu, but there is no suggestion regarding corrosion resistance.

0004

[Problems to be Solved by the Invention] Automotive steel plates are coated with a rust-preventing coating for the purpose of preventing corrosion, but the coating can be damaged by flying collisions with foreign objects that are rolled up while the car is running. The steel base is exposed and may be directly exposed to a corrosive environment due to salt damage. Therefore, it is important to impart excellent corrosion resistance to the steel sheet itself. Therefore, the purpose of this invention is to propose a method for producing cold-rolled steel sheets for deep drawing that has a tensile strength of 35 Kgf/mm2 or more and excellent corrosion resistance, especially pitting corrosion resistance, by regulating starting material components and production conditions. shall be.

[0005]

[Means for Solving the Problems] As a result of intensive research, the inventors have developed a high-tensile cooling material with extremely excellent corrosion resistance and deep drawability by limiting the starting material components and manufacturing conditions as follows. It was discovered that rolled steel plates can be manufactured. That is, in this invention, C: less than 0.006 wt%, S
i: 1.0wt% or less, Mn: 1.5wt% or less, P:
0.03-0.15wt% or less, S: 0.03wt%
Hereinafter, Al: 0.01 to 0.10 wt%, N: 0.
01 wt% or less, Cu: 0.18 to 1.5 wt% and Nb: 0.001 to 0.2 wt%, Nb/C≧3 and 0.01≦(Cu/64 + P/31)×(Nb/
C) A steel slab that satisfies the relationship of ≦0.10 is hot-rolled, then pickled and cold-rolled, followed by 7
This is a method for producing a high-strength cold-rolled steel sheet for deep drawing with excellent corrosion resistance, which is characterized by performing recrystallization annealing in a temperature range of 00° C. to Ac3 transformation point. The above steel slab further contains Ti: 0.01 to 0.2 wt% and B: 0.0001
~0.003wt%, Ni:0.01~1.5wt%,
Cr: 0.05-1.5 wt% and Mo: 0.00
Contains one or more types selected from 1 to 0.5 wt%, and the continuously cast slab is reheated or after continuous casting, immediately or heat-retaining treatment is performed without lowering the temperature below the Ar3 transformation point. are each advantageously suited for implementation.

[0006]

[Operation] First, the experimental results that formed the basis of this invention will be described below. That is, C: 0.001 to 0.006wt%
, Si: 0.02wt%, Mn: 0.1wt%, P:
0.01-0.15wt%, S: 0.010wt%,
Al: 0.05wt%, N: 0.002wt%, Nb
:0.001~0.2wt% and Cu:0.01~1
．． A steel sheet bar containing 5 wt% was heated to 1250 °C, soaked, hot rolled at a finishing temperature of 880 °C, followed by pickling, then cold rolled, and then heated to 850 °C.
Recrystallization annealing was performed at ℃ for 20 seconds. The results of investigating the corrosion resistance of the cold-rolled steel sheet thus obtained are shown in FIG. 1 in relation to the steel components. The evaluation of corrosion resistance is based on 0.5% N
aCl, 0.5% CaCl2 and 0.125% N
A corrosion cycle of immersion in a corrosive solution containing a2S2O5 for 8 hours and drying for 16 hours was repeated 30 times, and the maximum pitting depth after the corrosion test was measured. From the same figure,
Corrosion resistance strongly depends on the steel components, especially the amounts of C, Nb, P, and Cu; P≧0.03wt%, Cu≧0.18wt%
and 0.01≦(Cu/64+P/31) ×(N
It has been found that by satisfying the relationship b/C)≦0.10, the performance can be significantly improved.

Further, the inventors conducted various studies based on the above experimental results, and as a result, they discovered the following preferred ranges of the starting material composition and manufacturing conditions. (1) Steel composition (a) C: 0.006 wt% or less C is preferable because the lower its content, the better the deep drawability. However, if the content is 0.006 wt% or less, it has a very negative effect on the deep drawability. Since it is small, 0.006wt%
Limited to the following. (b) Si: 1.0wt% or less Si has the effect of strengthening steel, and is contained in the necessary amount depending on the desired strength, but if its content exceeds 1.0wt%, deep drawability and corrosion resistance deteriorate. Since it deteriorates, it is limited to 1.0 wt% or less. (c) Mn: 1.5 wt% or less Mn has the effect of strengthening steel and is included in the necessary amount depending on the desired strength, but if its content exceeds 1.5 wt%, deep drawability deteriorates. Therefore, the content was limited to 1.5 wt% or less. (d) P: 0.03 to 0.15 wt% P is an important element in this invention, and is contained in a necessary amount because it has the effect of strengthening steel and improving corrosion resistance. If its content is less than 0.03 wt%, it will have no effect on corrosion resistance, while if it exceeds 0.15 wt%, it will have an adverse effect on deep drawability, so it is limited to a range of 0.03 to 0.15 wt%. (e) S: 0.03 wt% or less S is preferable because the smaller the content, the better the deep drawability is. However, if the content is 0.03 wt% or less, there is very little negative effect on the deep drawability, so 0.03 wt% or less is preferable. 03wt%
Limited to the following. . (f) Al: 0.01 to 0.10wt% Al is contained as necessary to improve the yield of carbonitride forming elements by deoxidation, but if the content is 0.01w
If it is less than t%, there is no effect, while 0.10wt%
Since even more deoxidizing effect cannot be obtained even if the content exceeds 0.01 to 0.10 wt%. (g) N: 0.01wt% or less N is preferable because the smaller the content, the better the deep drawability is.However, if the content is 0.01wt% or less, there is very little negative effect on the deep drawability. 01wt%
Limited to the following. (h) Cu: 0.18 to 1.5 wt% Cu is an important element in this invention and is included to improve corrosion resistance. If the content is less than 0.18 wt%, it will have no effect on corrosion resistance, while if it exceeds 1.5 wt%, it will have a negative effect on deep drawability, so 0.18 to 1.5 wt%
limited to. (i) 0.01≦(Cu/64 + P/31)×(
Nb/C)≦0.10P and Cu are important elements in this invention and are contained to improve corrosion resistance. Its content depends on the content of C and Nb, (Cu/64 +P/31) × (Nb/C)<0.0
1 or (Cu/64 +P/31) × (Nb/C)>
At 0.10, corrosion resistance deteriorates, so 0.01≦(Cu/
64 +P/31) × (Nb/C)≦0.10. Although the details of the effects of C, Nb, P, and Cu are not clear, appropriate amounts of C, Nb, P,
It is thought that the inclusion of Cu changes the surface condition of the steel sheet, resulting in a change in surface quality that is advantageous for corrosion resistance, particularly pitting corrosion resistance. (j) Nb: 0.001 ~ 0.2wt%, Nb/
C≧3Nb is a carbide-forming element, and solid solution C in steel is replaced by Nb.
It is necessary to precipitate and fix as bC and preferentially form the {111} orientation, which is advantageous for deep drawability. If the content is less than 0.001 wt% or Nb/C<3, there is no effect, while if the content exceeds 0.2 wt%, the ductility deteriorates.
It was limited to 3. (k) Ti: 0.01-0.2wt%Ti is a carbonitride-forming element and converts solid solution (C,N) in steel into (TiC,T
{111} is precipitated and fixed as iN) and is advantageous for deep drawability.
Contained to form orientation. Its content is 0.0
If the content is less than 1 wt%, there will be no effect, while if the content exceeds 0.2 wt%, no effect will be obtained, and conversely it will lead to deterioration of the surface properties, so the content was limited to 0.01 to 0.2 wt%. (l) B: 0.0001 to 0.0030 wt% B is contained to improve the secondary processing brittleness of steel, but if the content is less than 0.0001 wt%, it has no effect; If the content exceeds 0.0001 to 0.0030wt%, it will have a negative effect on deep drawability.
limited to. (m) Ni: 0.01 to 1.5 wt% Ni is contained to improve the surface properties of the steel sheet when containing Cu, but if the content is less than 0.01 wt%, it has no effect; on the other hand, 1.
If the content exceeds 5 wt%, it will have an adverse effect on deep drawability, so it is limited to 0.01 to 1.5 wt%. (n) Cr: 0.05 to 1.5 wt% Cr is contained to improve corrosion resistance, but the content is 0.05 wt%.
If the content is less than 1.5 wt%, there is no effect, while if the content exceeds 1.5 wt%, it will have a negative effect on deep drawability, so 0.05 to 1.5
It was limited to wt%. (o) Mo: 0.001 to 0.5 wt% Mo is contained to improve corrosion resistance, but when the content is 0.001
If the content is less than 0.001 wt%, there is no effect, while if the content exceeds 0.5 wt%, it will have a negative effect on deep drawability.
It was limited to 0.5 wt%.

(2) Hot rolling conditions Hot rolling conditions are not particularly limited, but in the present invention, from the viewpoint of energy saving, the continuously cast slab is reheated or the temperature is lowered to below the Ar3 transformation point after continuous casting. It is preferable to subject the rolling stock to rough rolling immediately or after heat-insulating treatment. In addition, the hot rolling finishing temperature is Ar3
Although it is preferable for deep drawability to be above the transformation point, hot rolling at a low temperature below the Ar3 transformation point is also possible from the viewpoint of energy saving. Furthermore, the winding temperature is 300 to 80
A coiling temperature of 500°C or higher is suitable for promoting precipitation and improving deep drawability by coarsening.

(3) Cold rolling conditions Although the cold rolling conditions are not particularly limited, in order to obtain a high Lankford value (r value), it is necessary to ensure a cold rolling reduction of 60% or more. It is advantageous to desirably have a cold rolling reduction of 70% or more.

(4) Annealing conditions The annealing conditions are particularly important, and it is essential that the annealing be carried out in a temperature range of 700° C. or above and below the Ac3 transformation point. In other words, excellent deep drawability cannot be obtained at an annealing temperature of less than 700 °C, while when annealing is performed at a temperature exceeding the Ac3 transformation point, the texture becomes random due to α→γ transformation, resulting in poor deep drawability. is inferior. Therefore, the annealing temperature is set to 700
℃ or higher (more preferably 800℃ or higher) and lower than the Ac3 transformation point.

[0011] It is preferable to apply a continuous annealing line or a continuous hot-dip galvanizing line to the annealing step in the present invention. Further, as the continuous hot-dip plating method, non-alloyed hot-dip galvanizing and alloyed hot-dip galvanizing are suitable. Furthermore, after annealing or galvanizing, the steel sheet obtained according to the present invention can be subjected to special treatment to improve chemical conversion properties, weldability, press formability, corrosion resistance, etc.

0012

[Example] A steel slab having the composition shown in Table 1 was prepared using 1250
After heating and soaking at 0.degree. C. or without reheating after continuous casting, rough rolling was performed, followed by finish rolling. This hot-rolled sheet was subsequently pickled, cold-rolled at a rolling reduction of 75% to a thickness of 0.8 mm, and then recrystallized annealed at 850° C. for 20 seconds on a continuous annealing line. The tensile properties and corrosion resistance of the cold-rolled steel sheets thus obtained were investigated. Here, the tensile properties are measured using JIS No. 5 tensile test pieces, and the average Lankford value (hereinafter the average r
Values) are measured using a three-point method after applying 15% tensile prestrain, and are measured in the L direction (rolling direction), D direction (45° direction to the rolling direction), and C direction (direction at 45° to the rolling direction). Against 9
0° direction), the average r value = ( rL + 2
It was calculated as rD + r C )/4. Moreover, the corrosion resistance test method was evaluated using the same method as described above. As shown in Table 2, the evaluation results show that the steel plate obtained according to the present invention has a tensile strength of 35 kgf/mm2 or more and excellent corrosion resistance.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

[Effect of the invention] According to this invention, the tensile strength is 35K.
gf/mm2 or more, it is possible to provide a cold-rolled steel sheet having excellent corrosion resistance, particularly pitting corrosion resistance, and properties suitable for deep drawing, which greatly contributes to weight reduction of automobiles.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the corrosion resistance of a steel plate and the components of a starting material.

Claims (3)

[Claims] [Claim 1] C: less than 0.006 wt%, Si: 1
．． 0wt% or less, Mn: 1.5wt% or less, P: 0.0
3 to 0.15wt% or less, S: 0.03wt% or less, Al: 0.01 to 0.10wt%, N: 0.01w
t% below, Cu: 0.18-1.5wt% and Nb:
0.001 to 0.2 wt%, Nb/C≧3 and 0.001 to 0.2 wt%.
01≦(Cu/64 + P/31)×(Nb/C)≦
A steel slab that satisfies the relationship of 0.10 is hot-rolled, then pickled and cold-rolled, and then
A method for producing a high-strength cold-rolled steel sheet for deep drawing with excellent corrosion resistance, characterized by performing recrystallization annealing in a temperature range of ℃ to Ac3 transformation point. 2. In the method of claim 1, the steel slab contains Ti: 0.01-0.2 wt%, B: 0.0001-0.0001 wt%
0.003wt%, Ni: 0.01-1.5wt%, C
r: 0.05-1.5 wt% and Mo: 0.001
A method for producing a high-strength cold-rolled steel sheet for deep drawing with excellent corrosion resistance, further comprising one or more selected from 0.5 wt%. 3. In the method according to claim 1 or 2, the steel slab is heated by reheating the continuous casting slab or by applying Ar after continuous casting.
3. A method for producing a hot-rolled steel sheet with excellent deep drawability, characterized in that the sheet is immediately or heat-retained without being cooled below its transformation point.