JP3049147B2 - Method for producing high-tensile cold-rolled steel sheet with excellent chemical conversion property and deep drawability - Google Patents

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-tensile cold-rolled steel sheet excellent in deep drawability and useful for use in steel sheets for automobiles and the like, and particularly for producing a steel sheet excellent in chemical conversion property. It is intended to propose a method.

[0002]

2. Description of the Related Art 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 as described above, as the mechanical properties of the steel sheet, a high r value (Rankford value) and a good ductility (El.
) Is necessary.

On the other hand, in recent years, for the purpose of reducing the weight of a vehicle body and improving safety, a tensile strength of 35 to 60 kgf / mm ^{2 has been used.}
Moment to use steel sheets with higher tensile strength has rapidly increased. Even with such a high-tensile steel sheet,
It is needless to say that during press forming, it is required to show excellent deep drawability, and therefore, a steel plate with higher tensile strength and a high r value equal to or higher than conventional steel and excellent ductility R & D is progressing.

[0004] In order to manufacture such a high-strength cold-rolled steel sheet for deep drawing, low-carbon steel containing Si, Mn, P or the like as a reinforcing component is used.
Generally, Al-killed steel is subjected to ordinary hot rolling, then to cold rolling, and subsequently to recrystallization annealing. However, in order to obtain a high tensile strength, a large amount of the above-described reinforcing component must be contained, and therefore, a texture that is not favorable for deep drawability is formed, and only a steel sheet having a low r-value has been obtained. Furthermore, since a steel sheet for automobiles is subjected to a chemical conversion treatment as a pretreatment for baking coating, it is necessary that the chemical conversion treatment property is good. However,
When Si, Mn, P or the like described above is contained, there is also a problem that these components are concentrated on the surface of the steel sheet, thereby deteriorating the chemical conversion treatment property.

[0005] In addition to the above, high tensile strength cold-rolled steel sheets for deep drawing are disclosed in JP-B-60-47328.
It has been shown that a cold rolled steel sheet having excellent deep drawability and bake hardenability can be produced by adding a composite of steel. However, even with such a steel sheet, the chemical conversion property was still not satisfactory.

[0006]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems. By regulating the steel composition and the production conditions, the present invention provides a chemical conversion treatment more excellent than the conventional one and an excellent deep drawing. It is an object of the present invention to propose a method capable of producing a high-tensile cold-rolled steel sheet having a property.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to improve the deep drawability, and as a result, by limiting the steel composition and the production conditions as described below, it is possible to obtain a good chemical conversion treatment and an excellent chemical conversion treatment. It has been found that a high-tensile cold-rolled steel sheet having good 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 wt%
Over 0.2 wt% or less, B: 0.0001-0.0030 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,
The respective contents of S and N, C, Ti, Nb, S and N are represented by the following formulas.

[Equation 2] 0.5 (N / 14 + S / 32) ≦ Ti / 48 ≦ 1.5 (
N / 14 + S / 32) (wt%) The basic component composition satisfies 0.7 (C / 12) ≦ Nb / 93 ≦ 1.5 (C / 12) (wt%), and the balance consists of Fe and inevitable impurities. After hot rolling of steel material, rolling reduction 60-90%
(1st invention). A method for producing a high-tensile cold-rolled steel sheet excellent in chemical conversion property and deep drawability, characterized by cold-rolling and subsequently performing recrystallization annealing at 750 to 980 ° C.

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

Hereinafter, the results of the research on which the present invention was developed will be described. 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.%, C and Nb, C: 0.001-0.
028 wt%, Nb: contained variously changed in the range of 0.01 to 0.22 wt% (note that (Nb / 93) / (C / 12) 1.0), and the balance is substantially a Fe bar. After hot rolling, cold rolling was performed at a rolling reduction of 78% to a sheet thickness of 0.7 mm, and recrystallization annealing was performed at 850 ° C. for 20 seconds to obtain a test steel.

The test steel thus obtained was examined for mechanical properties and chemical conversion treatment properties. The chemical conversion treatment was performed by degreasing the test steel after cold rolling and annealing, washing with water, and adjusting the phosphate treatment 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. This was performed by spraying with a spray for 120 seconds, and evaluated by the pinhole area ratio and the number of crystal precipitation nuclei when the pinhole test described below was performed.

In the pinhole test, a filter paper impregnated with a reagent (ferrooxyl solution) which reacts with iron ions and develops a color is brought into close contact with the test surface, and a portion of the steel sheet surface where phosphoric acid crystals are not adhered is detected. Is image-analyzed and quantified as a pinhole area ratio. When the pinhole area ratio is less than 2%,
It was evaluated as “△” for ９9%, and as “x” for exceeding 9%. The number of crystal nuclei was determined by SEM observation, and the number of crystal nuclei (× 10 ⁴ / mm
² ) The evaluation was as follows. The results of the pinhole test and the number of crystal precipitation nuclei were converted into chemical conversion indices from 1 to 5 based on the following Table 1.

[0013]

[Table 1]

The results thus obtained are graphed in terms of the effect of the amount of C on the r value, TS and chemical conversion property, and are shown in FIG. From the figure, it can be seen that the r-value after cold-rolling-annealing, the TS and the chemical conversion property strongly influence the amount of C, and 0.006 wt% ≦ C ≦ 0.021 wt%
, The TS increases without deterioration of the r value,
And it turns out that chemical conversion processability improves.

Next, in order to examine the effects of Ti and Nb in the steel composition on the r value and elongation, C: 0.01 wt%, Si: 0.
2 wt%, Mn: 0.3 wt%, P: 0.06 wt%, S: 0.01 wt%,
Al: 0.05% by weight, N: 0.002% by weight, B: 0.001% by weight, and Ti: 0.004 to 0.038% by weight, Nb: 0.02 to 0.14% by weight, with the balance being Fe After hot rolling, cold rolling is performed at a rolling reduction of 78% to a sheet thickness of 0.7 mm, and recrystallization annealing is performed.
The test was performed at 850 ° C for 20 s to obtain a test steel. FIG. 2 shows the effect of the Ti and Nb contents on the r value and elongation of the test steel thus obtained. From FIG.
The value depends on the contents of Ti and Nb, and 0.5 ≦ (Ti / 48) / (N / 14
+ S / 32) ≦ 1.5 and 0.7 (C / 12) ≦ Nb / 93
It can be seen that a high r value and high ductility are obtained by setting the range of ≦ 1.5 (C / 12).

[0016]

As described above, in the present invention, the steel component is important, and unless the above component composition range is satisfied, excellent deep drawability cannot be ensured. 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 strength and improving chemical conversion treatment. 0.006 C content
By setting the content to wt% or more, a large number of NbCs are formed due to the relationship with Nb to be described later. As a result, not only does the strength increase due to precipitation strengthening, but this NbC becomes a precipitation nucleus of phosphate crystals. Also, the chemical conversion property is improved. However, when the C content exceeds 0.02%, NbC becomes {111}.
Since the formation of a recrystallized texture is inhibited, the r value is degraded. Therefore, the C content is limited to 0.006 to 0.020 wt%.

Si: 1.5 wt% or less Si has an effect of strengthening steel and contains a necessary amount according to a desired strength. The content is 1.5 wt%.
Exceeding the range will adversely affect deep drawability and surface properties.
It was limited to 1.5 wt% or less. Mn: 0.1 to 3.0 wt% Mn has the effect of strengthening steel and contains a necessary amount according to the desired strength, but the content is 0.1 wt%.
If it is less than 3.0 wt%, the strength will be insufficient. On the other hand, if it exceeds 3.0 wt%, the deep drawability will be adversely affected, so it was limited to 0.1-3.0 wt%.

Ti: 0.005 to 0.2 wt% Ti is an important component in the present invention, and precipitates and fixes N and S in the steel as Ti (N, S), and is a {111} set which is advantageous for deep drawability. It has the effect of forming an organization preferentially. Solid solution if its content is less than 0.005% (N, S)
Has a disadvantage that a large amount of Si remains and a {111} texture is not formed. On the other hand, if it exceeds 0.2 wt%, a large amount of solute Ti is present, which causes inconveniences such as reduced ductility and deteriorated chemical conversion treatment. It was limited to the range of 0.2 wt%. Note that Ti
Is also limited in relation to the N and S contents as described later.

Nb: more than 0.03% by weight and not more than 0.2% by weight Nb is an important component in the present invention, and the solid solution C in the steel is precipitated and fixed as NbC, which is advantageous for deep drawability.
効果 It has the effect of forming a texture preferentially. Further, by forming a large number of NbC, NbC becomes a precipitation nucleus of phosphate crystals, and has an effect of improving chemical conversion treatment. If the content is 0.03% or less, a large amount of solid solution C remains, and {111}
There is a disadvantage that a texture is not formed. On the other hand, if it exceeds 0.2 wt%, a large amount of solute Nb is present, which causes disadvantages such as a decrease in ductility and a deterioration in chemical conversion treatment.
Limited to the 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 to improve the resistance to secondary working brittleness. If the content is less than 0.0001wt%, there is no effect,
On the other hand, if the content exceeds 0.003 wt%, the deep drawability deteriorates, so the content is limited to 0.0001 to 0.002 wt%. Al: 0.01 to 0.20 wt% Al is used to deoxidize and contain the necessary amount to improve the yield of carbonitride-forming components. If the content is less than 0.01 wt%, the effect is reduced. But not 0.20wt
%, A 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 effectively act for increasing the strength, while if it exceeds 0.20 wt%, the deep drawability is adversely affected, so it was limited to 0.04 to 0.20 wt%.

S: 0.05 wt% or less The smaller the amount of S, the better the deep drawability.
It is preferable to suppress the content as much as possible, but if the content is 0.05 wt% or less, there is no significant adverse effect.
It was limited to 0.05 wt% or less.

N: N: 0.006 wt% or less N becomes smaller, so that the deep drawability is improved.
It is preferable to suppress the content as much as possible, but if the content is 0.006 wt% or less, there is no significant effect, 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, the Ti content [Ti] is changed to the N and S content [N].
And [S], it is necessary to satisfy the above expression. As described above, Ti replaces N and S in steel with Ti (N,
S) is precipitated and fixed, and has an effect of preferentially forming a {111} texture advantageous for deep drawability. Ti content is 0.
5 (N / 14 + S / 32)> Ti / 48 deteriorates deep drawability because a large amount of solute N remains in the steel. On the other hand Ti / 48
> 1.5 (N / 14 + S / 32) at 0.5 (N / 14 + S / 32), since excess Ti is present, which leads to deterioration of ductility.
32) ≦ Ti / 48 ≦ 1.5 (N / 14 + S / 32) (wt%)

[0026]

0.7 (C / 12) ≦ Nb / 93 ≦ 1.5 (C / 12) (wt%) In the present invention, the above equation is expressed by the relationship between the Nb content [Nb] and the C content [C]. It is necessary to be satisfied. Nb has the effect of precipitating the {111} texture advantageous for deep drawability by precipitating and fixing solid solution C in steel as NbC as described above. Also, by forming a large number of NbC,
NbC becomes a precipitation nucleus of phosphate crystals, and has an effect of improving chemical conversion treatment. When this 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 is present, which leads to deterioration of ductility.
≦ Nb / 93 ≦ 1.5 (C / 12) (wt%).

Mo: 0.01 to 1.0 wt% Mo has an effect of strengthening steel, and is contained in the second invention according to a desired strength.
If the amount is less than wt%, there is no effect. On the other hand, if it exceeds 1.0 wt%, the deep drawability is adversely affected, so the amount is limited to 0.01 to 1.0 wt%.

Next, the reason why the manufacturing process is limited in the present invention will be described. Hot Rolling Step The hot rolling step is not particularly limited in the present invention, and may be produced 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 less 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 to the Ar ₃ transformation point or the temperature is maintained is also suitable for the present invention. The hot rolling finish temperature is preferably higher than the Ar ₃ transformation point for improving the material, but rolling from a slightly lower Ar ₃ transformation point is also possible from the viewpoint of energy saving. A suitable hot rolling temperature is 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 been subjected to cold rolling must be subjected to recrystallization annealing by a continuous annealing method. This recrystallization annealing has an annealing temperature of 75
The range is 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 is randomized due to α-γ transformation, and the deep drawability deteriorates.
Limited to 0 ° C. Needless to say, the steel strip after annealing may be subjected to a 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 an original sheet of a surface-treated steel sheet for processing. Surface treatments include zinc plating (including alloys),
There are tin plating and enamel.

[0031]

EXAMPLES Steel slabs were prepared by continuously casting molten steel having various component compositions shown in Table 2.

[0032]

[Table 2]

The slab was heated at a slab heating temperature of 1150
℃, finishing temperature: 900 ℃, winding temperature: 550 ℃ after hot rolling, after pickling, cold rolling at a rolling reduction shown in Table 3, the sheet thickness: 0.7 mm After forming the cold rolled steel strip, recrystallization annealing was performed in the continuous annealing equipment under the conditions shown in Table 3.
Table 3 also shows the results of an investigation on the material properties and chemical conversion treatment properties of the thus obtained cold rolled steel sheet. In Tables 2 and 3, the values out of the range of the present invention are underlined.

[0034]

[Table 3]

The tensile properties were measured using a JIS No. 5 tensile test piece, and the r-value was measured by a three-point method after applying a 15% tensile prestrain, and was measured in the L direction (rolling direction). Direction), the average value in the D direction (45 degrees from the rolling direction) and the average value in the C direction (90 degrees from the rolling direction) were determined from the equation: r = (r _L + 2r _D + r _C ) / 4. Further, as to the evaluation of the secondary working brittle resistance, a sample processed at a limit drawing ratio of 2.8 was cooled to -50 ° C, and then a crush test was performed to evaluate whether brittle cracks were generated.

Further, the chemical conversion property was evaluated by ×× according to the test method described above.

As is clear from Table 3, all of the conforming examples according to the present invention have excellent deep drawing properties and chemical conversion treatment properties as compared with the comparative examples.

[0038]

According to the present invention, by limiting the steel composition and the production conditions, it becomes possible to produce a high-tensile cold-rolled steel sheet having excellent chemical conversion properties and deep drawability.

[Brief description of the drawings]

FIG. 1 shows the effect of C on r-value, TS and chemical conversion properties.
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.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Nobuhiko Uesugi 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. Kawasaki Steel Corporation Mizushima Works (56) References JP-A-2-149624 (JP, A JP-A-61-41722 (JP, A) JP-A-60-149729 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/48, 8/04 C22C 38 / 00-38/58

Claims (2)

(57) [Claims] 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: more than 0.03 wt% and 0.2 wt% or less, 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, S and N Each content C
, Ti, Nb, S and N are given by the following equation: 0.5 (N / 14 + S / 32) ≦ Ti / 48 ≦ 1.5 (
N / 14 + S / 32) (wt%) The basic component composition satisfies 0.7 (C / 12) ≦ Nb / 93 ≦ 1.5 (C / 12) (wt%), and the balance consists of Fe and inevitable impurities. After hot rolling of steel material, rolling reduction 60-90%
A method for producing a high-tensile cold-rolled steel sheet having excellent chemical conversion properties and deep drawability, characterized by cold-rolling, followed by recrystallization annealing at 750 to 980 ° C. 2. The method for producing a high-tensile cold-rolled steel sheet according to claim 1, which further comprises Mo: 0.01 to 1.0 wt% in addition to the basic component composition.