JPH09235652A - Cold rolled steel sheet and galvannealed steel sheet, excellent in press workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet and a galvannealed steel sheet, having excellent press formability, chemical conversion treating property, and powdering resistance. SOLUTION: This cold rolled steel sheet has a composition consisting of, by mass, <=0.003% C, <=0.3% Si, 0.05-0.5% Mn, <=0.02% P, <=0.02% S, 0.01-0.1% Al, (3.43N+1.5S) to 0.040% Ti, 0.008-0.015% Nb, 0.002-0.015% Zr, <=0.005% N, and the balance Fe with inevitable impurities. It is preferable to regulate the upper limit of Ti so that Ti+Zr<=0.040% is satisfied. Further, in the case of a galvannealed steel sheet, it is preferable to regulate the upper limit of Ti content in the composition to 0.035%. Both steel sheets are characterized by respectively prescribed ranges of contents of Nb and Zr in particular.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a cold-rolled steel sheet and an alloyed hot-dip galvanized steel sheet which are steel sheets suitable for press working of automobile steel sheets and the like and have excellent press workability.

[0002]

2. Description of the Related Art Steel sheets, such as automobile bodies, which require high press formability are required to have Ti or Ni dissolved therein in order to precipitate and fix solute carbon and solute nitrogen which deteriorate ductility, deep drawability, aging resistance and the like. Ultra-low carbon steel to which a carbonitride forming element such as Nb is added, so-called IF steel is used.

Further, as disclosed in Japanese Patent Publication No. 61-32375, the addition of a relatively small amount of carbonitride-forming element improves the chemical conversion treatment property and plating adhesion, and has an excellent deep drawability. Ti-Nb compound addition IF
Steel plates are used as cold rolled steel plates for press working and hot-dip galvanized steel plates.

[0004]

In recent years, the integral molding of a plurality of parts for the purpose of reducing the weight of an automobile body and the complexity of the body design have been advanced, and further press workability has been required. However, despite the use of the Ti-Nb composite-added IF steel sheet, it cannot be said that it has sufficient press workability.

In order to improve the press workability, Ti-
In the production of an Nb composite-added IF steel sheet, a method of quenching after hot rolling and performing rapid heating during annealing after cold rolling (Japanese Patent Application Laid-Open No.
No. 1-276927), and a method of lowering the slab heating temperature and controlling the rough rolling temperature lower than before has been proposed (Japanese Patent Application Laid-Open No. 2-259023). In addition, since special manufacturing conditions are required, the manufacturing process becomes complicated and its management is difficult.

The present invention has been made in view of the above problems, and can be manufactured without requiring special process control, and has excellent press formability, excellent chemical conversion treatment property, and plating adhesion. Provided are cold rolled steel sheets and galvannealed steel sheets.

[0007]

[Means for Solving the Problems] As a result of various studies on the ultra low carbon cold-rolled steel sheet in order to solve the above problems, the inventors have reduced the amount of C added in the Ti-Nb added steel. However, by strictly controlling the Nb amount within the optimum range and adding a small amount of Zr accordingly, the elongation and r value that influence the press workability without adversely affecting the chemical conversion treatment property and the plating adhesion property. The inventors have found that the above can be significantly improved and have completed the present invention.

That is, the cold-rolled steel sheet according to the first aspect of the present invention has a mass% of C: 0.003% or less, S:
i: 0.3% or less, Mn: 0.05 to 0.5%, P:
0.02% or less, S: 0.02% or less, Al: 0.0
1 to 0.1%, Ti: 3.43N + 1.5S ≦ Ti ≦
0.040%, Nb: 0.008 to 0.015%, Z
r: 0.002 to 0.015%, N: 0.005% or less, and the balance Fe and unavoidable impurities.
In addition, the element symbol in the formula showing the Ti content indicates the content of the element.

In the above components, as described in claim 2, the upper limit of the Ti content is Ti + Zr in relation to the content of Zr, which is an element that easily forms an oxide like Ti.
By defining ≦ 0.040%, not only chemical conversion treatability but also plating adhesion during electroplating or hot metal plating can be improved.

Further, as described in claim 3, claim 1
In the steel sheet components, the P content is 0.008 to 0.02.
%, And the upper limit of Ti is set to 0.035%, it is possible to effectively prevent the powdery peeling phenomenon (powdering) of the plating when the hot-dip galvanized layer is alloyed. As a result, it is possible to prevent surface defects caused by the deposition of plating strips on the press die. Of course, the cold-rolled steel sheet according to claim 1 or 2 can be used as an original plate of a galvanized steel sheet such as hot-dip galvanizing.

Here, the reason for limiting the steel components in the present invention will be explained.

C: 0.003% or less If a large amount of C is added, the workability deteriorates. In the present invention, since the mild steel sheet of the highest processing grade is targeted, the upper limit is made 0.003%.

Si: 0.3% or less Since the workability deteriorates when Si is added in a large amount, the upper limit is made 0.3%.

Mn: 0.05 to 0.5% Mn has an effect of preventing hot brittleness. If the content is less than 0.05%, the effect is too small. On the other hand, if the content exceeds 0.5%, the workability deteriorates.
The upper limit is 0.5%.

P: 0.02% or less If P is added in a large amount, workability deteriorates and secondary work embrittlement occurs. Therefore, P is limited to 0.02% or less. In the case of alloyed hot-dip galvanized steel sheet, since P has an action of suppressing excessive alloying of Zn plating when Ti is contained,
From the viewpoint of suppressing powdering, the content of 0.008% or more is preferable.

S: 0.2% or less Since S is an impurity element, it is desirable to reduce it as much as possible. However, if 0.02% or less, the influence on the material is small, so the upper limit is made 0.02%. Desirably 0.01%
It is better to stop below.

Al: 0.01 to 0.1% Al is an element useful as a deoxidizing material during molten steel refining,
It is necessary to add 0.01% or more. However, addition of a large amount leads to deterioration of workability and high refining cost, so the upper limit is made 0.1%.

Ti: 3.43N + 1.5S≤Ti≤0.
040% Ti is (3.43N) for the purpose of fixing and fixing N and S.
+ 1.5S)% or more. If it is less than this value,
Fixation of N and S becomes insufficient, and the deep drawability deteriorates. On the other hand, when it exceeds 0.040%, the chemical conversion treatment property and the plating adhesion are deteriorated. As described later, in the present invention, Zr is added as an essential element for improving the elongation (El), but since both Ti and Zr are elements that easily form an oxide, chemical conversion treatability and plating adhesion In order to further improve the above, the total amount of Ti and Zr should be restricted to 0.040% or less. On the other hand, in the case of performing galvannealing, if it exceeds 0.035%, a hard Fe-zinc alloy layer is likely to develop at the steel plate-plating interface and the powdering resistance is deteriorated. It is better to set it to 035%.

Nb: 0.008 to 0.015% Nb is contained in order to simultaneously achieve high El and r values. If it is less than 0.008%, a high r value cannot be obtained. On the other hand, if it exceeds 0.015%, El deteriorates. Therefore, the lower limit is 0.008% and the upper limit is 0.01
5%.

Zr: 0.002 to 0.015% Zr has an effect of improving El without affecting other properties. If it is less than 0.002%, its action is too small to obtain a high El, while if it is added over 0.015%, the effect is saturated, which only causes an increase in cost. The amount is 0.002% and the upper limit is 0.015%. Regarding the upper limit, in order to secure a higher El, preferably 0.010%, more preferably 0.00
8% is recommended.

N: 0.005% or less If the amount of N is too large, Ti necessary for fixing and precipitating it
Is added, and the workability and chemical conversion treatability are deteriorated accordingly. Therefore, the content is limited to 0.005% or less.

Next, a method for producing the steel sheet of the present invention will be described. A steel having the above components is melted and cast by a usual method, and the obtained steel slab is subsequently hot-rolled, pickled, cold-rolled and then annealed. Further, plating is performed as necessary. In the case of a hot-dip galvanized steel sheet, the cold-rolled steel sheet may be plated by a conventional method, but if necessary, an alloying treatment of reheating to about 600 ° C. may be performed. In the case of galvannealed steel sheet, the lower limit of P is set to 0.
As described above, it is preferable to define the upper limits of 008% and Ti to 0.035%.

The production conditions may be in accordance with the ordinary production conditions for an ultra-low carbon cold rolled steel sheet, but the following conditions are preferred.

First, in hot rolling, the hot rolling finishing temperature is preferably set to Ar ₃ point to (Ar ₃ point + 150) ° C. Below Ar ₃ points, a texture, which is disadvantageous to deep drawability after annealing, develops (Ar ₃ points + 150).
If the temperature exceeds ℃, grain growth in the austenite region becomes remarkable, the grain size after γ → α transformation becomes large, and the deep drawability after annealing is adversely affected.

The winding temperature is preferably set to 500 to 750 ° C. When the winding temperature is less than 500 ° C., the carbonitride does not sufficiently precipitate, so that the ductility and deep drawability deteriorate. On the other hand, if it is higher than 750 ° C,
The material variation in the longitudinal direction of the coil increases, and the descaling property also deteriorates.

The rolled hot-rolled steel sheet is pickled by a usual method and then cold-rolled. Cold rolling rate is 60-9
5% is preferred. If it is less than 60%, a texture favorable for deep drawability does not sufficiently develop after annealing, and if it exceeds 95%, the in-plane anisotropy increases. A more preferred range is 75
~ 90%.

The coil after cold rolling is subjected to recrystallization annealing. Regarding the annealing method, either box annealing or continuous annealing is possible, but in order to ensure ductility and deep drawability, it is preferable to perform annealing at a temperature not lower than the recrystallization temperature and not higher than _one Ac point. Further, the cold-rolled steel sheet according to the present invention is suitable not only for chemical conversion treatment but also for surface reactivity, and thus is suitable as an original plate such as an electrogalvanized steel sheet and a hot-dip galvanized steel sheet.

[0028]

【Example】

Example A As for the Nb content, those outside the scope of the present invention were prepared.
The other components were 3.2 mm at a finishing temperature of 910 ° C and a winding temperature of 700 ° C.
Hot-rolled to the plate thickness of 0.7 mm, pickled, and then 0.7 mm (cold rolling rate 78
%) And subjected to recrystallization annealing at 820 ° C.

Steel composition (unit mass%, balance substantially Fe) C: 0.0015 to 0.0023%, Si: 0.01 to 0.02%, M
n: 0.15 to 0.17%, Al: 0.024 to 0.035%, P
: 0.008 to 0.010%, S: 0.005%, N: 0.0018 to
0.0029%, Ti: 0.028 to 0.030%, Zr: 0.005 to 0.
007%, Nb: Tr to 0.016%

N on the El and r values of the obtained cold rolled steel sheet
b The effect of the added amount (content) was examined. The result is shown in FIG. From the figure, the r value rises as the Nb amount increases,
Reach the maximum. On the other hand, El does not change so much up to 0.015%, but then drops sharply. It can be seen that both properties are excellent in balance within the range of the present invention.

Although the reason why the El and r values change depending on the Nb content is not clear, increasing the Nb decreases the solute C, thereby increasing the r value, while the Nb system which adversely affects El is adversely affected. It is considered that El rapidly decreases because the carbide increases from the point where Nb exceeds 0.015%.

The chemical conversion treatability of the above cold-rolled steel sheets was investigated, and good results were obtained for all the steel sheets. The chemical conversion treatability was evaluated by treating the steel sheet after cold rolling annealing with a phosphate treatment liquid (trade name: S
D5000, manufactured by Nippon Paint Co., Ltd.) for 2 minutes, and then the number of urate crystal nuclei per unit area was measured by a scanning electron microscope. The crystal nucleus of silicate in any steel sheet exceeds 10 × 10 ⁶ / cm ² ,
The chemical conversion treatability was good.

Further, the powdering property was examined using a steel sheet which had been alloyed and hot-dip galvanized using the cold rolled steel sheet as a base sheet, but there was no particular problem because the Ti content was low. The powdering property was evaluated by visually observing the state of plating peeling on the inside of the bent portion of the 60 ° V bending test piece on a tape.

Example B Regarding the Zr content, those having the Zr content within the range of the present invention were prepared,
A cold-rolled steel sheet was manufactured under the same conditions as in Example A using a steel slab having the following components within the scope of the present invention.

Steel composition (unit mass%, balance substantially Fe) C: 0.0016 to 0.0019%, Si: 0.01 to 0.02%, M
n: 0.13 to 0.15%, Al: 0.022 to 0.030%, P
: 0.010 to 0.015%, S: 0.006%, N: 0.0016 to
0.0022%, Ti: 0.028 to 0.032%, Zr: Tr to 0.00
7%, Nb: 0.011 to 0.013%

Z on the El and r values of the obtained cold rolled steel sheet
The effect of the added amount (content) of r was investigated. The result is shown in FIG. As can be seen from the figure, the r value hardly changes with the increase of the Zr amount, but the El value increases sharply with the addition of Zr.
Although the detailed reason for such Zr action is not clear, it is considered that addition of a small amount of Zr affects precipitation of carbonitrides in Ti—Nb-added steel, thereby improving El.

The chemical conversion treatability was examined by the above method.
All were good. Further, the powdering property was examined using an alloyed hot-dip galvanized steel sheet using the above cold-rolled steel sheet as a base sheet, but there was no problem in this example because the Ti content was low.

From the above results, in the steel in which the amount of C is reduced to the limit, the amount of Nb added is 0.008 to 0.015%.
By controlling to a narrow range and adding a trace amount of Zr, El is improved by 2% or more and r value is improved by 0.2 or more as compared with the conventional Zr-free Ti-Nb-added ultra-low carbon steel. It was confirmed that This improvement is equivalent to an improvement of one rank or more in terms of mild steel processing grade. By using the steel sheet of the present invention, forming of difficult-to-machine parts, such as for integral forming, which could not be formed by a conventional steel sheet. Can be possible. It is particularly important that the integrally molded part has a high El because it is difficult for the material to flow during pressing due to its shape.

Example C Steels having the components shown in Table 1 below were melted in a converter and made into a slab by a continuous casting method. Finish temperature of this slab is 910
Hot rolling to a thickness of 3.2mm at 700 ° C and a winding temperature of 700 ° C, cold rolling to 0.7mm (78% cold rolling reduction) after pickling,
Recrystallization annealing was performed at 820 ° C.

[0040]

[Table 1]

The mechanical properties and chemical conversion treatability of a predetermined sample of the obtained cold rolled steel sheet were examined. Table 2 shows the results. The chemical conversion treatability was evaluated in the same manner as in Example A. The case where the phosphate crystal nuclei exceeded 10 × 10 ⁶ / cm ² was marked with “⊚” and 10 × 10 ⁶ / cm ² or smaller. 10x
"○", 10 x 10 if it exceeds 10 ⁵ / cm ^2.
Those having a density of ⁵ / cm ² or less are indicated by “x”.

[0042]

[Table 2]

From Table 2, sample No. 1 has a Ti content and T
Since the i + Zr content is high, the mechanical properties are good, but the chemical conversion treatability is poor. No. 2 has good mechanical properties, but its Ti + Zr content is a little high, and therefore the chemical conversion treatability is slightly inferior to the examples in which the Ti + Zr content is 0.040% or less. No. 3 has a high Ti + Zr content and no Nb added, so the r value is low and the chemical conversion treatability is somewhat poor. No. 4
Has a Ti + Zr content of 0.040% or less and good chemical conversion treatability, but has a low r value due to a low Nb content. That is, it is understood that the addition of an appropriate amount of Nb is indispensable in order to achieve both the chemical conversion treatability and the excellent press workability. No. 5 is an example with a high Nb content, No. 6 is an example with a low Ti content, No. 7 is an example with a high C content, and No. 8 is an example with a low Zr content. Or, and the r value is low. On the other hand, sample Nos. 9 to 14 are the examples of the present invention according to claim 1, and are good in all of El, r value and chemical treatment embedding property.

Further, a predetermined one of the cold rolled steel sheets was subjected to recrystallization annealing, and then subjected to hot dip galvanizing treatment and alloying treatment. The mechanical properties and powdering property of the alloyed hot-dip galvanized steel sheet sample thus obtained were examined. Table 3 shows the results. The powdering property was evaluated in the same manner as in Example A.

[0045]

[Table 3]

From Table 3, since the sample No. 21 has a large Ti content, the mechanical properties are good, but the powdering property is poor. Since Sample Nos. 22 to 26 have components outside the scope of the invention of claim 2, they have low El or r value. On the other hand, Sample Nos. 27 to 32 corresponding to the embodiment according to claim 3 are
Good results were obtained in terms of El, r value, and powdering property.

[0047]

As described above, according to the present invention, C
Since the Nb content was strictly controlled within a predetermined range and a small amount of Zr was added while suppressing the content to a very small amount, the cold-rolled steel sheet of the present invention according to claim 1 has excellent press workability. The cold-rolled steel sheet according to claim 2 has excellent chemical conversion treatability and plating adhesion, and the galvannealed steel sheet according to claim 3 also has excellent powdering resistance. Therefore, it is possible to sufficiently deal with complicated processing such as integral molding. Moreover, it can be easily manufactured without requiring a special manufacturing process.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between Nb addition amount and El and r values.

FIG. 2 is a bluff diagram showing the relationship between Zr addition amount and El and r values.

Claims (3)

[Claims] 1. C .: 0.003% or less by mass%, S
i: 0.3% or less, Mn: 0.05 to 0.5%, P:
0.02% or less, S: 0.02% or less, Al: 0.0
1 to 0.1%, Ti: 3.43N + 1.5S ≦ Ti ≦
0.040%, Nb: 0.008 to 0.015%, Z
r: 0.002 to 0.015%, N: 0.005% or less, balance Fe and unavoidable impurities, and a cold-rolled steel sheet excellent in press workability. 2. The cold rolled steel sheet excellent in press workability according to claim 1, wherein Ti: 3.43N + 1.5S ≦ Ti and Ti + Zr ≦ 0.040%. 3. The component according to claim 1, wherein P:
0.008-0.02%, Ti: 3.43N + 1.5S
An alloyed hot-dip galvanized steel sheet excellent in press workability, characterized in that a cold-rolled steel sheet defined as ≦ Ti ≦ 0.035% is subjected to galvannealing.