JPH05230540A - Production of high tensile strength cold rolled steel sheet excellent in chemical conversion treating property and deep drawability - Google Patents

Abstract

PURPOSE:To produce a high tensile strength cold rolled steel sheet excellent in chemical conversion treating property and deep drawability. CONSTITUTION:A steel stock having a composition which consists of, by weight, 0.006-0.020% C, <=1.5% Si, 0.1-3.0% Mn, 0.005-0.2% Ti, 0.03-0.2% Nb, 0.0001-0.0030% B, 0.01-0.20% Al, 0.04-0.20% P, <=0.05% S, <=0.006% N, and the balance Fe with inevitable impurities and in which C, Ti, Nb, and S satisfy inequalities 0.5(N/14+S/32)=Ti/48 <=1.5(N/14+S/32)(wt.%) and 0.7(C/12)<=Nb93<=1.5(C/12)(wt.%), where the C, Ti, Nb, S, and N represent respective contents of the above-mentioned C, Ti, Nb, S, and N, is used. This steel stock is hot- rolled, cold-rolled at 60-90% draft, and successively subjected to recrystallization annealing at 750-980 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength cold-rolled steel sheet having excellent deep drawability, which is useful for use in automobile steel sheets and the like, and particularly to produce a steel sheet having excellent chemical conversion treatability. It is intended to propose a method.

[0002]

BACKGROUND OF THE INVENTION Cold-rolled steel sheets used for automobile panels and the like are required to have excellent deep drawability. In order for the steel sheet to exhibit excellent deep drawability, the mechanical properties of the steel sheet are high r value (Rankford value) and good ductility (El.
) Is required.

On the other hand, in recent years, the tensile strength is 35 to 60 kgf / mm ² for the purpose of weight reduction and safety improvement of automobile bodies.
There has been a rapid increase in the motivation to use higher tensile strength steel sheets. Even with such a high-tensile steel plate,
Needless to say, it is required to exhibit excellent deep drawability during press forming. Therefore, a steel sheet having higher tensile strength and a high r value equal to or higher than that of conventional steel and excellent ductility. Research and development is underway.

For the production of such a high-strength cold-rolled steel sheet for deep drawing, low carbon containing Si, Mn, P, etc. as a strengthening component is used.
In general, Al-killed steel is generally hot-rolled, then cold-rolled, and then recrystallized and annealed. However, in order to obtain a high tensile strength, a large amount of the above-mentioned reinforcing components must be contained, so that a texture which is not preferable for deep drawability is formed, and only a steel sheet having a low r value was obtained. Further, since a steel sheet for automobiles is subjected to a chemical conversion treatment as a pretreatment for baking coating, it is necessary that this chemical conversion treatability is good. However,
When Si, Mn, P, or the like described above is contained, these components are concentrated on the surface of the steel sheet, so that there is a problem that the chemical conversion treatment property is deteriorated.

In addition to the above, as a high-strength cold-rolled steel sheet for deep drawing, Japanese Examined Patent Publication No. 60-47328 discloses ultra low carbon steel such as Ti, Nb and B.
It has been shown that a cold-rolled steel sheet having excellent deep drawability and bake hardenability can be produced by the combined addition of. However, even such a steel sheet was still unsatisfactory in chemical conversion treatability.

[0006]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above problems, and by controlling the steel composition and manufacturing conditions, it has better chemical conversion treatability and superior deep drawing than before. An object of the present invention is to propose a method capable of producing a high-strength cold-rolled steel sheet having properties.

[0007]

[Means for Solving the Problems] As a result of intensive studies to improve the deep drawability, the inventors have found that good chemical conversion treatability and excellent chemical conversion treatability can be achieved by limiting the steel composition and manufacturing conditions as follows. It has been found that a high-strength cold-rolled steel sheet having deep drawability can be obtained.

The gist of the present invention based on the above findings is as follows. (1) C: 0.006 to 0.020 wt%, Si: 1.5 wt% or less, Mn:
0.1 to 3.0 wt%, Ti: 0.005 to 0.2 wt%, Nb: 0.03 to 0.
2 wt%, B: 0.0001 to 0.0030 wt%, Al: 0.01 to 0.20 wt
%, P: 0.04 to 0.20 wt%, S: 0.05 wt% or less and N: 0.
006 wt% or less, and the content of each of C, Ti, Nb, S and N is expressed by the following formula:

[Formula 2] 0.5 (N / 14 + S / 32) ≤ Ti / 48 ≤ 1.5 (N / 14 + S / 32) (wt%) 0.7 (C / 12) ≤ Nb / 93 ≤ 1.5 (C / 12) (wt %), And the balance is 60-90% after hot rolling a steel material consisting of Fe and unavoidable impurities.
Cold rolling, followed by recrystallization annealing at 750 to 980 ° C., a method for producing a high-strength cold-rolled steel sheet excellent in chemical conversion treatment and deep drawability (first invention).

(2) In addition to the steel composition of the first invention, Mo: 0.01-
A method for producing a high-strength cold-rolled steel sheet containing 1.0 wt% and having excellent deep drawability (second invention).

The following describes the results of the research on which the invention was developed. Si: 0.2 wt%, Mn: 0.3 wt%, P: 0.06 wt%, S: 0.01
wt%, Al: 0.05 wt%, N: 0.002 wt%, Ti: 0.02 wt% and B: 0.001 wt%, and C and Nb contained C: 0.001 to 0.
028 wt%, Nb: variously contained in the range of 0.01 to 0.22 wt% (note that (Nb / 93) / (C / 12) ≈1.0), and the balance is essentially Fe sheet bar After hot rolling, cold rolling was applied at a reduction rate of 78% to a plate thickness of 0.7 mm, and recrystallization annealing was performed at 850 ° C for 20 s to obtain a sample steel.

The test steel thus obtained was examined for mechanical properties and chemical conversion treatability. The chemical conversion treatability was adjusted by degreasing the test steel after cold rolling-annealing, washing it with water, and treating it with phosphate to a total acidity of 14.3 and a free acidity of 0.5 at 55 ° C using BT3122 manufactured by Nippon Parkerizing Co., Ltd. The test was performed by spraying for 120 seconds, and the pinhole area ratio and the number of crystal precipitation nuclei in the pinhole test described below were evaluated.

In the pinhole test, a filter paper impregnated with a reagent (ferrooxyl solution) that reacts with iron ions to develop a color is brought into close contact with the test surface, and the portion where the phosphoric acid crystals are not attached remains on the surface of the steel sheet. Image analysis was performed and digitized as a pinhole area ratio. If the pinhole area ratio is less than 2%, ○ 2
It was evaluated as Δ in the case of ˜9% and × in the case of exceeding 9%. The number of crystal nuclei is determined by SEM observation, and the number of crystal nuclei (× 10 ⁴ / mm
² ) was evaluated as ◯ when 5 exceeded, Δ when 5 to 3 and x when less than 3. The results of the pinhole test and the number of crystal precipitation nuclei were indexed into 1 to 5 as the chemical conversion treatment index based on the following Table 1.

[0013]

[Table 1]

The results thus obtained are graphed by the effect of the C value on the r value, TS and chemical conversion treatability, and are shown in FIG. From the figure, the r value, TS and chemical conversion treatability after cold rolling-annealing strongly influence the C content, and 0.006 wt% ≤ C ≤ 0.021 wt%
As a result, TS increases without deterioration of r value,
Moreover, it can be seen that the chemical conversion treatability is improved.

Next, in order to investigate the influence of Ti and Nb in the steel components on the r value and elongation, C: 0.01 wt% and Si: 0.
2 wt%, Mn: 0.3 wt%, P: 0.06 wt%, S: 0.01 wt%,
Al: 0.05 wt%, N: 0.002 wt%, B: 0.001 wt%, Ti: 0.004 to 0.038 wt%, Nb: 0.02 to 0.14 wt% with various changes, and the balance Fe. And a sheet bar consisting of unavoidable impurities is hot-rolled, and then cold-rolled at a reduction rate of 78% to a plate thickness of 0.7 mm, followed by recrystallization annealing.
The test steel was made under the conditions of 850 ° C and 20s. The effects of the Ti and Nb contents on the r-value and elongation of the test steel thus obtained are summarized in FIG. From the figure, r
The value depends on the Ti and Nb contents, and is 0.5 ≦ (Ti / 48) / (N / 14
+ S / 32) ≤ 1.5 and 0.7 (C / 12) ≤ Nb / 93
It can be seen that the high r value and the high ductility are obtained by setting the range of ≦ 1.5 (C / 12).

[0016]

As described above, in the present invention, the steel composition is important, and if the above composition range is not satisfied, excellent deep drawability cannot be secured. Hereinafter, the reason for limiting the range of each component will be described.

C: 0.006 to 0.020 wt% C is an important component in the present invention, and is necessary for increasing the strength and improving the chemical conversion treatability. C amount is 0.006
When the content is more than wt%, a large amount of NbC is formed due to the relationship with Nb which will be described later. As a result, not only the strength is increased by precipitation strengthening, but also this NbC becomes a precipitation nucleus of a phosphate crystal. The chemical conversion treatability is also improved. However, when the C content exceeds 0.02%, NbC is {111}
Since the formation of recrystallized texture is hindered, the r value deteriorates. Therefore, the amount of C is limited to 0.006 to 0.020 wt%.

Si: 1.5 wt% or less Si has a function of strengthening steel, and a necessary amount is contained according to a desired strength, but the content is 1.5 wt%.
If it exceeds the range, the deep drawability and surface properties will be adversely affected.
Limited to 1.5 wt% or less. In order to exert the above-mentioned action, it is preferable to contain about wt% or more. Mn: 0.1-3.0 wt% Mn has the effect of strengthening steel and contains the required amount according to the desired strength, but the content is 0.1 wt%.
If the content is less than 1.0, the strength will be insufficient, while if it exceeds 3.0 wt%, the deep drawability will be adversely affected, so the content was limited to 0.1-3.0 wt%.

Ti: 0.005 to 0.2 wt% Ti is an important component in the present invention. N and S in steel are precipitated and fixed as Ti (N, S), and {111} aggregates advantageous for deep drawability. It has the effect of preferentially forming an organization. If the content is less than 0.005%, it forms a solid solution (N, S)
Has a disadvantage that {111} texture is not formed. On the other hand, if it exceeds 0.2 wt%, a large amount of solid solution Ti is present, which causes disadvantages such as reduction of ductility and deterioration of chemical conversion treatability. It was limited to the range of 0.2 wt%. Note that Ti
The content of is also limited by the relationship with the N and S contents as described later.

Nb: 0.03 to 0.2 wt% Nb is an important component in the present invention, and solid solution C in steel is precipitated and fixed as NbC, which is advantageous for deep drawability {111
} There is an effect of preferentially forming a texture. Further, by forming a large number of NbC, NbC serves as a precipitation nucleus of phosphate crystals, which has an effect of improving chemical conversion treatability. If the content is less than 0.03%, a large amount of solute C remains, {11
1} There is a disadvantage that a texture is not formed, while 0.2 wt%
If it exceeds 1.05, a large amount of solid solution Nb is present, which causes disadvantages such as reduced ductility and deterioration of chemical conversion treatability.
It was limited to the wt% range. The Nb content is also limited by the relationship with the C content, as described later.

B: 0.0001 to 0.0030 wt% B is contained in order to improve the secondary processing brittleness resistance. If the content is less than 0.0001 wt%, there is no effect,
On the other hand, if the content exceeds 0.003 wt%, the deep drawability deteriorates, so it was limited to 0.0001 to 0.002 wt%. Al: 0.01 to 0.20 wt% Al is contained in a necessary amount for deoxidizing and improving the yield of carbonitride forming components. If the content is less than 0.01 wt%, the effect is None, meanwhile 0.20wt
%, The further deoxidizing effect cannot be obtained, so the content was limited to 0.01 to 0.20 wt%.

P: 0.04 to 0.20 wt% P is an important component in the present invention and is necessary for strengthening the steel. If the content is less than 0.04 wt%, it does not work effectively for strengthening, while if it exceeds 0.20 wt%, it adversely affects the deep drawability, so it was limited to 0.04 to 0.20 wt%.

S: 0.05 wt% or less Since S is less, the deep drawability is improved.
It is preferable to suppress the content as much as possible, but if the content is 0.05 wt% or less, it does not have a bad effect so much.
Limited to 0.05 wt% or less.

N: N: 0.006 wt% or less Since the smaller N is, the deep drawability is improved.
It is preferable to suppress the content as much as possible. However, if the content is 0.006 wt% or less, it does not exert a bad influence so much, so the content is limited to 0.006 wt% or less.

0.5 (N / 14 + S / 32) ≦ Ti / 48 ≦ 1.5 (N / 14 + S / 32) (wt%) In the present invention, Ti content [Ti] is N and S content [N]
And the relationship with [S], it is necessary to satisfy the above equation. As mentioned above, Ti is the same as Ti (N,
It has the effect of precipitating and fixing as S) and preferentially forming a {111} texture that is advantageous for deep drawability. Ti content is 0.
When 5 (N / 14 + S / 32)> Ti / 48, a large amount of solid solution N remains in the steel, and the deep drawability deteriorates. Meanwhile Ti / 48
> 1.5 (N / 14 + S / 32), since excess Ti exists, it leads to deterioration of ductility, so 0.5 (N / 14 + S / 32)
32) ≤ Ti / 48 ≤ 1.5 (N / 14 + S / 32) (wt%).

[0026]

## EQU00003 ## 0.7 (C / 12) ≤Nb / 93≤1.5 (C / 12) (wt%) In the present invention, the Nb content [Nb] is related to the C content [C] by It is necessary to be satisfied. As described above, Nb has the effect of precipitating and fixing the solid solution C in the steel as NbC and preferentially forming the {111} texture, which is advantageous for deep drawability. Also, by forming a large number of NbC,
NbC serves as a precipitation nucleus for phosphate crystals, and has the effect of improving chemical conversion treatability. When the content is 0.7 (C / 12)> Nb / 93, a large amount of solute C remains in the steel, so that the deep drawability deteriorates. On the other hand, when Nb / 93> 1.5 (C / 12), excess Nb exists, which leads to deterioration of ductility, so 0.7 (C / 12)
It was limited to ≦ Nb / 93 ≦ 1.5 (C / 12) (wt%).

Mo: 0.01 to 1.0 wt% Mo has the effect of strengthening steel, and is contained according to the desired strength in the second invention, but its content is 0.01.
If it is less than wt%, it has no effect, while if it exceeds 1.0 wt%, it adversely affects the deep drawability, so it was limited to 0.01-1.0 wt%.

Next, the reason why the manufacturing process is limited in the present invention will be described. Hot Rolling Process The hot rolling process is not particularly limited in the present invention, and may be manufactured according to an ordinary method. For example, the slab heating temperature may be about 950 to 1300 ° C, and low temperature heating of 1200 ° C or lower is effective for improving the material and reducing the energy cost. Further, a direct-feed rolling process in which the temperature is maintained immediately after the continuous casting without lowering the temperature at the transformation point of Ar ₃ or is kept warm is also suitable for the present invention. The hot rolling finishing temperature is preferably Ar ₃ transformation point or higher for material improvement, but from the viewpoint of energy saving, rolling at a temperature slightly lower than Ar ₃ transformation point is also possible. The hot rolling temperature is preferably 350 to 800 ° C.

Cold rolling step This step is indispensable for obtaining a high r value, and it is essential that the cold rolling reduction is 60 to 90%. If the cold rolling reduction is less than 60% or more than 90%, excellent deep drawability cannot be obtained.

Annealing Step The cold-rolled steel strip that has undergone cold rolling needs to be subjected to recrystallization annealing by a continuous annealing method. This recrystallization annealing has an annealing temperature of 75.
It shall be in the range of 0 to 980 ° C. In the low temperature range where the annealing temperature is less than 750 ° C, excellent deep drawability cannot be obtained. On the other hand, if the annealing temperature exceeds 980 ° C, the crystal orientation becomes random due to α-γ transformation and the deep drawability deteriorates.
Limited to 0 ° C. Needless to say, the annealed steel strip may be subjected to temper rolling of 10% or less, which is usually performed, in order to adjust the surface roughness and the like. Further, the cold-rolled steel sheet obtained by the present invention can be applied to the original plate of the surface-treated steel sheet for processing. As surface treatment, zinc plating (including alloy type),
There are tin plating, enamel, etc.

[0031]

EXAMPLES Molten steels having various compositional compositions shown in Table 2 were continuously cast to prepare steel slabs.

[0032]

[Table 2]

Slab heating temperature of these slabs: 1150
℃, finishing temperature: 900 ℃, coiling temperature: 550 ℃ After hot rolling, after pickling, cold rolling at the reduction ratio shown in Table 3 to obtain a plate thickness of 0.7 mm. After forming the cold-rolled steel strip, recrystallization annealing was performed in a continuous annealing facility under the conditions shown in Table 3.
Table 3 also shows the results of an examination of the material properties and chemical conversion treatability of the cold-rolled steel sheet thus obtained. In Tables 2 and 3, the numerical values outside the range of the present invention are underlined.

[0034]

[Table 3]

The tensile properties were measured using JIS No. 5 tensile test pieces, and the r-value was measured by the 3-point method after applying a 15% tensile prestrain, and the L direction (rolling Direction), the D direction (45 ° direction from the rolling direction) and the C direction (90 ° direction from the rolling direction) were averaged by the formula r = (r _L + 2r _D + r _C ) / 4. Further, as to the evaluation of the secondary processing brittleness resistance, the sample processed with the limiting drawing ratio of 2.8 was cooled to -50 ° C, and then the crushing test was conducted to evaluate the presence or absence of brittle cracking.

Further, the chemical conversion treatability was evaluated by ◯ by the test method described above.

As is apparent from Table 3, all the conforming examples according to the present invention have excellent deep drawability and chemical conversion treatability as compared with the comparative examples.

[0038]

According to the present invention, it is possible to produce a high-strength cold-rolled steel sheet excellent in chemical conversion treatment and deep drawability by limiting the steel composition and production conditions.

[Brief description of drawings]

FIG. 1 shows the effect of C on r-value, TS and chemical conversion treatability.
It is a graph which shows the influence of a component.

FIG. 2 is a graph showing the effect of Ti and Nb contents on r value and elongation.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chiba-shi, Chiba Within the Technical Research Division, Kawasaki Steel Co., Ltd. ) Kawasaki Steel Co., Ltd. Mizushima Steel Works

Claims (2)

[Claims] 1. C: 0.006-0.020 wt%, C: 0.006-0.020 wt%, Si: 1.5 wt% or less, Mn: 0.1-3.0 wt%, Ti: 0.005-0.2 wt%, Nb: 0.03-0.2 wt% %, B: 0.0001 to 0.0030 wt%, Al: 0.01 to 0.20 wt%, P: 0.04 to 0.20 wt%, S: 0.05 wt% or less and N: 0.006 wt% or less, and the above C, Ti, Nb, Each content of S and N C
, Ti, Nb, S and N are the following equations: 0.5 (N / 14 + S / 32) ≤ Ti / 48 ≤ 1.5 (N / 14 + S / 32) (wt%) 0.7 (C / 12) ≤ Nb / 93 ≤ 1.5 (C / 12) (wt%), the basic composition is satisfied, and the balance is steel material consisting of Fe and unavoidable impurities. After hot rolling, the rolling reduction is 60 to 90%.
The method for producing a high-strength cold-rolled steel sheet excellent in chemical conversion treatability and deep drawability, which comprises subjecting the steel sheet to cold rolling and then recrystallization annealing at 750 to 980 ° C. 2. The method for producing a high-strength cold-rolled steel sheet excellent in chemical conversion treatability and deep drawability according to claim 1, which contains Mo: 0.01 to 1.0 wt% in addition to the basic composition.