JPH05140654A - Manufacture of cold rolled steel sheet for deep drawing excellent in baking hardenability and corrosion resistance - Google Patents

Abstract

PURPOSE:To provide the method for manufacturing a cold rolled steel sheet for deep drawing improved in baking hardenability and corrosion resistance. CONSTITUTION:The slab of steel contg., by weight, 0.001 to 0.008% C, <=1.5% Si, 0.05 to 1.8% Mn, 0.03 to 0.20% P, <=0.015% S, 0.05 to 1.5% Cu, 0.005 to 0.100% solAl and <=0.005% N as well as 0.003 to 0.05% Ti, satisfying the relationship in which [Ti] conforming to [Ti]=Ti-48/14.N-48/32.S is regulated to <= zero, furthermore constituted of one or >= two kinds among <=1% Ni, <=3% Mo and <=7% Cr and the balance iron with inevitable impurities is subjected to hot rolling, is pickled, is thereafter subjected to cold rolling and is subsequently annealed at 700 to 950 deg.C in a continuous annealing line.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a cold-rolled steel sheet for deep drawing which has bake hardenability and is excellent in corrosion resistance.

[0002]

2. Description of the Related Art In the field of cold-rolled steel sheets used as steel sheets for automobiles, in addition to being excellent in deep drawability, the yield stress of the steel sheet increases during baking to improve dent resistance. In many cases, the characteristic, that is, the bake hardenability is required. Conventionally, cold-rolled steel sheets of this type are low-carbon aluminum killed steel sheets, ultra-low carbon steel with Ti added, and high-strength steel sheets with Si, Mn, P, and Cr added to increase strength. There are many suggestions.

For example, JP-A-57-98630, JP-A-58-107414 and JP-A-61-27692.
No. 7, a method of continuously annealing using ultra-low carbon Al killed steel as a raw material is also disclosed in JP-A 61-26757, JP-A 63-276927 and JP-A 2-111841.
Discloses a method of producing ultra-low carbon Ti-added steel as a raw material by continuous annealing. These are the production of bake hardened cold rolled steel sheets that not only increase the strength of the steel sheet but also have excellent formability while being soft at the time of forming, but strengthening the steel sheet by paint baking after press forming. On the way.

By the way, these are techniques for imparting deep drawability and strength at the same time, and are intended to achieve thinning, that is, weight reduction of a steel sheet for automobiles. However, although it is possible to reduce the thickness of the steel sheet from the viewpoint of strength, if the thickness of the steel sheet is reduced, pitting corrosion due to corrosion becomes a problem. Therefore, it is required that the steel sheet has good corrosion resistance.

[0005]

SUMMARY OF THE INVENTION In view of the above-mentioned circumstances, the present invention has an object of providing a method for producing a cold-rolled steel sheet which improves the corrosion resistance of the base material and is satisfactory in bake hardenability and deep drawability. And

[0006]

[Means for Solving the Problems] The inventors of the present invention have conducted various studies on a method for producing a cold-rolled steel sheet excellent in bake hardenability, deep drawability and corrosion resistance. Bake hardenability can be imparted by properly controlling and adding C and Ti contents of ultra-low carbon Ti-added steel, and further, corrosion resistance of a steel sheet can be remarkably improved by the combined addition of Cu and P, and corrosion resistance can be improved. In addition to Cu and P, Si and Mn are used for higher strength and corrosion resistance.
By adding Ni, Mo, Cr, etc., a cold-rolled steel sheet for deep drawing having excellent bake hardenability and corrosion resistance has been obtained.

[0007]

In other words, the present invention is C: 0.0% by weight.
01-0.008, Si: 1.5 or less, Mn: 0.05
-1.8, P: 0.03-0.20, S: 0.015 or less, Cu: 0.05-1.5, so1A1: 0.005
~ 0.100, N: 0.005 or less, the balance is steel slab consisting of iron and unavoidable impurities, hot-rolled, pickled, cold-rolled, and then on a continuous annealing line. Provided is a method for producing a cold-rolled steel sheet for deep drawing, which is excellent in bake hardenability and corrosion resistance, which comprises performing annealing at 700 to 950 ° C.

The present invention also provides C: 0.001-wt%.
0.008, Si: 1.5 or less, Mn: 0.05 to 1.
8, P: 0.03 to 0.20, S: 0.015 or less, C
u: 0.05 to 1.5, solAl: 0.005 to 0.
100, N: 0.005 or less, and further, a steel slab composed of one or more of Ni up to 1%, Mo up to 3% and Cr up to 7%, and the balance iron and unavoidable impurities. Of cold-rolled steel sheet for deep drawing excellent in bake hardenability and corrosion resistance, which comprises hot rolling, pickling, cold rolling, and then annealing at 700 to 950 ° C. in a continuous annealing line. A manufacturing method is provided.

The present invention also provides C: 0.001-wt%.
0.008, Si: 1.5 or less, Mn: 0.05 to 1.
8, P: 0.03 to 0.20, S: 0.015 or less, C
u: 0.05 to 1.5, solAl: 0.005 to 0.
100, N: 0.005 or less, Ti: 0.003 to 0.
05 and [Ti] = Ti-48 / 14.N-48 /
Satisfies the relation that [Ti] according to 32 · S is 0 or less,
The balance is bake hardenability, which is obtained by hot rolling a steel slab consisting of iron and unavoidable impurities, pickling, cold rolling, and then annealing at 700 to 950 ° C. in a continuous annealing line. Provided is a method of manufacturing a cold-rolled steel sheet for deep drawing that has excellent corrosion resistance.

The present invention also provides C: 0.001-wt%.
0.008, Si: 1.5 or less, Mn: 0.05 to 1.
8, P: 0.03 to 0.20, S: 0.015 or less, C
u: 0.05 to 1.5, solAl: 0.005 to 0.
100, N: 0.005 or less, Ti: 0.003 to 0.
05 and [Ti] = Ti-48 / 14.N-48 /
Satisfies the relation that [Ti] according to 32 · S is 0 or less,
Further, a steel slab comprising one or more of Ni up to 1%, Mo up to 3% and Cr up to 7%, and the balance consisting of iron and inevitable impurities is hot-rolled, and after pickling,
Cold rolling is performed, and then 700 to 9 in a continuous annealing line.
Provided is a method for producing a cold-rolled steel sheet for deep drawing, which is excellent in bake hardenability and corrosion resistance, which comprises performing annealing at 50 ° C.

The present invention also provides that the steel has a B content of 0.0003 to.
There is provided a method for producing any of the above steel plates, which comprises including 0.003%.

First, the actions of various components of the method for manufacturing a steel sheet according to the present invention and the reasons for limiting the above range will be described. C is less preferable because it deteriorates ductility, but if C is less than 0.001%, sufficient bake hardenability cannot be obtained. On the other hand, if it exceeds 0.008%, the amount of solid solution C increases, and a very high bake hardenability is obtained, but aging at room temperature occurs and ductility rapidly deteriorates. Therefore, C is 0.00
It was set to 1 to 0.008%.

Si is a preferable element for improving the strength of steel without impairing the workability, but if Si exceeds 1.5%, it becomes hard and the ductility deteriorates, so the upper limit was made 1.5%.

Mn is effective in preventing hot brittleness due to S, and for that purpose, at least 0.05% or more is necessary. On the other hand, although it is a preferable element for improving strength, 1.8
%, The ductility and deep drawability deteriorate, so the lower limit was made 0.05% and the upper limit was made 1.80%.

P and Cu are characteristic elements in the present invention, and the corrosion resistance is remarkably improved by the combined addition of these elements. P is 0.03 to improve corrosion resistance
%, Cu is required to be 0.05% or more. On the other hand, when P exceeds 0.20% and Cu exceeds 1.5%, the improvement effect is saturated and the ductility deteriorates. Therefore, P has a lower limit of 0.03% and an upper limit of 0.20%. Cu has a lower limit of 0.05% and an upper limit of 1.5%.

S is an element which is essentially harmful to steel, and the smaller the content, the more desirable it is, but 0.015% is acceptable, so S was made 0.015% or less.

In order to function as a deoxidizer, Al is required to be 0.005% or more, but if it exceeds 0.10%, inclusions such as Al ₂ O ₃ increase, resulting in workability and surface finish. Since the quality is deteriorated, the lower limit is set to 0.005% and the upper limit is set to 0.10%.

N is an element that is essentially harmful to steel, and the smaller the amount, the more desirable it is, but 0.005% can be tolerated, so N was made 0.005% or less.

Ti is an element effective for ensuring deep drawability, but if it is less than 0.003%, its effect is small.
The C content specified in the present invention, namely 0.001
If Ti exceeds 0.05% at 0.008%, bake hardenability cannot be obtained. Therefore, Ti is 0.002-
It is contained in the range of 0.05%. However, Ti is C,
Since it acts to fix S and N as precipitates of TiC, TiS, TiN, etc.
If i is added, bake hardenability cannot be obtained. Therefore,
It is necessary to satisfy the relationship that [Ti] according to [Ti] = Ti-48 / 12 · N-48 / 32 · S is 0 or less. Under the condition that [Ti] is 0 or less, C is 0. 00
It is 1 to 0.08%.

Further, in the present invention, in order to increase the strength of the steel sheet and improve the corrosion resistance, Ni up to 1% and Mo up to 3% are used.
And up to 7% of one or more of Cr can be contained.

Ni effectively acts to prevent hot brittleness and improve pitting corrosion resistance due to Cu, but if it exceeds 1%,
The effect is saturated and the manufacturing cost is high. Therefore, the upper limit is set to 1%.

Mo effectively acts to improve the strength of the steel sheet and to improve the resistance to pitting corrosion, but if it exceeds 3%, the effect is saturated and the manufacturing cost becomes high, so the upper limit was made 3%. ..

Cr effectively acts to improve the resistance to pitting corrosion, but if it exceeds 7%, the manufacturing cost becomes very high, so the upper limit was made 7%.

B is effective in improving the secondary processing embrittlement,
For that purpose, 0.0003% or more must be added. On the other hand, even if added over 0.003%, the effect is saturated. Therefore, B is 0.0003 to 0.003%
And

In the present invention, the steel containing such components is subjected to a hot rolling step and a cold rolling step to form a cold rolled steel sheet. In this case, the finishing temperature in the hot rolling step is A
Below the r3 transformation point, deep drawability is poor. Winding temperature is 50
If it is less than 0 ° C, the deep drawability is poor and the plate shape is poor. If it exceeds 750 ° C, the pickling property is poor and the coil is deformed after winding. Therefore, the hot rolling finish temperature is A
The coiling temperature is preferably 500 ° C to 750 ° C above the r3 transformation point.

In the cold rolling step, a cold rolling rate of 50 to 95% is preferable in order to secure deep drawability. If the cold rolling rate is less than 50%, the deep drawability is poor, and if the cold rolling rate is more than 95%, the load on the cold rolling mill is large and the productivity is poor.

The lower limit of the annealing temperature in the continuous annealing line is 700 ° C. or higher in order to improve the workability above the recrystallization temperature, and the upper limit of 950 ° C. or lower improves the workability. This is because the surface defects are likely to occur in the continuous annealing line while being saturated.

[0028]

[Detailed Disclosure of the Invention]

Example 1 A steel sheet having a composition shown in Table 1 was used to form a hot-rolled sheet having a plate thickness of 3.2 mm by hot rolling under the conditions shown in Table 2, and after pickling, a hot rolled plate having a sheet thickness of 0.8 mm was prepared by cold rolling. Cold rolled steel sheet was annealed in a continuous annealing line. After that, skin pass rolling with an elongation of 0.8% was performed, and the mechanical properties and corrosion resistance of the obtained steel sheet were investigated, and the results are also shown in Table 2.

For mechanical properties, JIS Z2201 No. 5 test piece was used. For the corrosion resistance test, a 70 × 150 mm test piece was cut out and a complex corrosion test was performed. Complex corrosion test is J
According to the salt spray test of ISZ2371, the salt water concentration was 1.
The maximum erosion depth due to corrosion was measured by using the salt spray test changed to 0% for 5 hours → 60 ° C. hot air drying for 4 hours → wetting test for 14 hours → blast drying for 1 hour for 24 hours as one cycle.

As can be seen from the results in Table 2, C is high and P is high.
And Cu whose amount is less than the amount specified in the present invention. The cold-rolled steel sheet produced by using the comparative steel of No. 1 has a high yield point (YS) and a low elongation (El) although the tensile strength (TS) is low. Further, since the bake hardenability (BH) is too high, room temperature aging occurs. And the corrosion resistance is poor. No. 3 containing less P and containing P. The cold-rolled steel sheet produced using Comparative Steel No. 6 has relatively good tensile strength (TS), elongation (El) and low bake hardenability (BH), but also poor corrosion resistance. On the other hand, No. 3 having a component composition within the range specified in the present invention. 3-5 and No. The cold-rolled steel sheet manufactured using 7 to 10 steel has a high tensile strength (TS) but a yield point (Y
It has a low S), good elongation (El), good bake hardenability (BH), and excellent corrosion resistance.

Example 2 A steel having a composition shown in Table 3 was hot-rolled under the conditions shown in Table 4 to obtain a hot-rolled sheet having a thickness of 3.2 mm, which was pickled and then cold-rolled. A cold-rolled steel sheet having a thickness of 0.8 mm was annealed in a continuous annealing line. After that, skin pass rolling with an elongation of 0.8% was performed. The properties of the obtained steel sheet were evaluated in the same manner as in Example 1. The results are also shown in Table 4.

As can be seen from the results of Table 4, No. 1 having higher C and lower P and Cu than those specified in the present invention. The cold-rolled steel sheet produced by using No. 11 steel has a high yield point (TS) and a low elongation (El) despite its low tensile strength (TS). Also,
Since the amount of Ti added is too large, the bake hardenability (BH) is poor. And the corrosion resistance is poor. No. containing less P and less P The cold-rolled steel sheets manufactured using the 17 comparative steels show relatively good tensile strength (TS) and elongation (El), but Ti
Is too large, the bake hardenability (BH) is low and the corrosion resistance is also poor. On the other hand, the cold-rolled steel sheets manufactured using the No. 12 to 16 steels and the No. 18 to 20 steels having the compositional composition in the range specified in the present invention have a high tensile strength (TS) but a low yield point (YS). , Elongation (El), plastic strain ratio (r
Value), good bake hardenability (BH), and excellent corrosion resistance.

[0033]

As described above, the present invention clarifies a method for producing a cold-rolled steel sheet which is excellent in bake hardenability and corrosion resistance while maintaining workability such as deep drawability and ductility. INDUSTRIAL APPLICABILITY The present invention greatly contributes to weight reduction of automobiles, and its industrial significance and profit are extremely large.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiro Yamada 11-1 Showa-cho, Kure-shi, Hiroshima Nisshin Steel Co., Ltd.

Claims (5)

[Claims] 1. C: 0.001-0.008 by weight%,
Si: 1.5 or less, Mn: 0.05 to 1.8, P: 0.
03-0.20, S: 0.015 or less, Cu: 0.05
~ 1.5, so1A1: 0.005 to 0.100, N:
A steel slab with a balance of 0.005 or less and the balance of iron and unavoidable impurities is hot-rolled, pickled and cold-rolled, and then 700 to 950 in a continuous annealing line.
A method for producing a cold-rolled steel sheet for deep drawing which is excellent in bake hardenability and corrosion resistance, which comprises performing annealing at ℃. 2. C: 0.001-0.008 by weight%,
Si: 1.5 or less, Mn: 0.05 to 1.8, P: 0.
03-0.20, S: 0.015 or less, Cu: 0.05
~ 1.5, solAl: 0.005 to 0.100, N:
Ni less than 0.005 and up to 1% Ni, 3
% Mo and 1% or more of Cr up to 7%, the balance is steel slab consisting of iron and inevitable impurities, hot-rolled, pickled, cold-rolled, and then
A method for producing a cold-rolled steel sheet for deep drawing, which is excellent in bake hardenability and corrosion resistance, which comprises performing annealing at 700 to 950 ° C in a continuous annealing line. 3. C: 0.001-0.008 by weight%,
Si: 1.5 or less, Mn: 0.05 to 1.8, P: 0.
03-0.20, S: 0.015 or less, Cu: 0.05
~ 1.5, solAl: 0.005 to 0.100, N:
0.005 or less, Ti: 0.003 to 0.05 and satisfying the relation that [Ti] = Ti-48 / 14 · N-48 / 32 · S [Ti] is 0 or less, and the balance is A steel slab consisting of iron and unavoidable impurities is hot-rolled, pickled, cold-rolled, and then annealed at 700 to 950 ° C. in a continuous annealing line to improve bake hardenability and corrosion resistance. A method for producing an excellent cold-rolled steel sheet for deep drawing. 4. C: 0.001-0.008 by weight%,
Si: 1.5 or less, Mn: 0.05 to 1.8, P: 0.
03-0.20, S: 0.015 or less, Cu: 0.05
~ 1.5, solAl: 0.005 to 0.100, N:
Satisfies the relation that 0.005 or less, Ti: 0.003 to 0.05, and [Ti] according to [Ti] = Ti-48 / 14 · N-48 / 32 · S is 0 or less, and 1%
To Ni, up to 3% Mo and up to 7% Cr, one or more, and the balance iron and unavoidable impurities. A steel slab is hot-rolled, pickled, and then cold-rolled. Then, a method for producing a cold-rolled steel sheet for deep drawing which is excellent in bake hardenability and corrosion resistance, and which is then performed at 700 to 950 ° C. in a continuous annealing line. 5. The method according to claim 3 or 4, wherein the steel contains B in an amount of 0.0003 to 0.003%.