JP2000303144A - High strength thin steel sheet excellent in secondary working embrittlement resistance and formability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength tin steel sheet excellent in secondary working embrittlement resistance, deep drawability and surface properties in press working, and to provide a method for producing the same. SOLUTION: This high strength thin steel sheet excellent in secondary working embrittlement resistance and formability is the one in which, as to steel having components contg., by weight, 0.0040 to 0.01% C, <=0.07% Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, <=0.02% S, <=0.004% N, <=0.15% Nb also so as to satisfy the inequality of (12/93)×Nb*/C>=1.2 (where Nb*=Nb-(93/14)×N), and the balance substantial Fe with inevitable impurities, after the end of hot finish rolling at >= Ar3, it is coiled at 500 to 700 deg.C and is subjected to cold rolling, annealing and galvanizing, and (n) value in a 2 point method of nominal distortion 1% and 10% by an uniaxial tensile test is >=0.21.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a high-strength cold-rolled steel sheet, a high-strength galvanized steel sheet which does not generate secondary working brittleness and stretcher strain in press working, and is excellent in deep drawability and surface properties. It relates to the method of manufacturing them, including automobile steel plates, household electrical appliances,
Can be widely used for building materials.

[0002]

2. Description of the Related Art A high-strength galvanized steel sheet formed by press working such as a steel sheet for automobiles has not only a high level of surface quality that can be used as an outer plate material but also a deep drawability and a non-stretching method for suppressing the occurrence of stretcher strain. Expiration is required. Until now, in order to enhance the deep drawability and the non-aging property, the amount of C has been reduced as much as possible, and at the same time, Ti and N
b added harmful solute C as carbide
High strength steel sheets based on steel have been developed.

[0003] However, in the IF steel, since the grain boundaries are clean and brittle, the part that has been subjected to high-strength compression processing during shrinkage flange deformation at the time of deep drawing has a secondary working brittleness that causes brittle fracture in subsequent processing. There is a problem of high sensitivity. On the other hand, various proposals have been made to increase the resistance to secondary working brittleness while maintaining the characteristics of IF steel.

[0004] First, a technique for partially leaving solid solution C has been proposed. For example, Japanese Patent Publication No. 61-32375 discloses a technique in which the equivalent ratio of Ti and C to N or less is reduced. By adding the following Nb, it is possible to prevent a decrease in r value, an increase in yield strength, and a decrease in elongation,
A technique has been disclosed in which a part of the solute C is left at the grain boundary to enhance the resistance to secondary working embrittlement (hereinafter, Conventional Technique 1).

Japanese Patent Application Laid-Open No. HEI 5-112845 proposes a technique for limiting the lower limit of the amount of C and actively adding Mn and Cr to increase the amount of solute C to increase the brittleness resistance in secondary working. (Hereinafter referred to as Conventional Technique 2). JP-A-5-7
No. 0836 discloses that the lower limit of the amount of C fixed by Ti and Nb is limited to secure the amount of carbide that suppresses the growth of crystal grains, and to obtain an excellent strength-ductility balance by refining crystal grains. At the same time, the upper limit of the amount of Si and P and B
A technique for increasing the resistance to secondary working embrittlement by addition has been proposed (hereinafter, Conventional Technique 3).

JP-A-2-175837 discloses that the amount of C is reduced as much as possible and the r value is increased by fixing with Ti, and fine NbC is precipitated by adding Nb to make the crystal grain boundaries in a sawtooth shape. A technique for improving secondary working brittleness has been disclosed (hereinafter, Conventional Technique 4).

[0007] However, the prior arts 1 and 2 both cause solid solution C to remain and increase the resistance to secondary working embrittlement, so that the problem of aging occurs when the temperature is kept for a long time in a relatively high temperature environment such as summer. Is concerned. In the prior art 1, the aging resistance is evaluated by an accelerated test at 100 ° C. for 1 hour. However, when evaluated by a long-term aging test for several months at room temperature, the amount of Ti or Nb added to C or N When the ratio is not more than the equivalent ratio, the yield elongation causing the stretcher strain is often observed even if it is determined that there is no problem in the above-mentioned acceleration test.

Further, the prior art 3 is a technique for enhancing the resistance to secondary working embrittlement by adding B. However, B segregates at the grain boundaries, suppresses crystal rotation during cold working and obtains a high r value. In this case, the development of a preferred texture is inhibited, so that the deep drawability deteriorates. The prior art 4 is a technique for increasing the brittle resistance in secondary processing by making the grain boundaries into a saw-like shape by adding Nb. However, the binding force of the grain boundaries is increased by the saw-like grain boundaries, and the deformation is concentrated in the crystal grains. And the ductility decreases. This tendency is particularly remarkable in local ductility, which causes a decrease in stretch flangeability.

Further, in this prior art, in order to increase the r value, Ti is added to C, N, S at an equivalent ratio or more, and in the embodiment, the addition amount exceeding substantially 0.03% is added. In need of. For this reason, surface unevenness called a striped Ti mark occurs in hot-dip galvanizing, and it cannot be used for applications requiring surface quality such as automobile outer panels. As described above, in the conventional technique, it is difficult to obtain a galvanized steel sheet that is non-ageable, has high deep drawability, and does not cause secondary working embrittlement.

[0010]

SUMMARY OF THE INVENTION The present invention relates to a high-strength thin steel sheet having high surface quality, non-aging property, high deep drawability, and excellent secondary work brittleness, which can be applied to automobile outer panels. ,
In particular, it is an object of the present invention to provide a high-strength thin steel sheet having a tensile strength of 340 MPa or more and a method for producing the same.

[0011]

Means for Solving the Problems The inventors of the present invention have proposed a residual solid solution C which is an obstacle to non-aging, addition of B which limits the improvement of r value, and a grain boundary shape by NbC which deteriorates stretch flangeability. Without using the control, the technology to improve the resistance to secondary working brittleness has been earnestly studied, and as a result, by controlling the amount of C, the amount of Nb and the Nb / C within a specific range,
The present inventors have found that a high-strength galvanized steel sheet having non-aging properties, deep drawability, and excellent secondary work brittleness resistance can be obtained, and completed the present invention. In the present invention, a thin steel sheet is a cold-rolled steel sheet,
Zinc-based galvanized steel sheets are collectively referred to.

That is, the present invention provides: % By mass, C: 0.0040 to 0.01%, Si:
0.05% or less, Mn: 0.1 to 1.0%, P: 0.0
1 to 0.05%, S: 0.02% or less, Al: 0.01 to 0.1
%, N: 0.004% or less, Nb: 0.15% or less, and the balance that satisfies the expression (1) has a component substantially consisting of Fe and unavoidable impurities, and is nominally strained by a uniaxial tensile test. A high-strength thin steel sheet excellent in secondary working brittleness resistance and formability, characterized in that the n value of the two-point method of 1% and 10% is 0.21 or more.

(12/93) × Nb ^{* /} C ≧ 1.2 (1) where Nb ^* = Nb− (93/14) × N, C (% by mass), Nb (% by mass), N (% by mass) %) 2. 2. The high-strength thin steel sheet excellent in secondary working brittleness resistance and formability according to 1, further containing 0.05% by mass or less of Ti by mass%.

3. B in 0.002% by mass%
3. A high-strength thin steel sheet excellent in secondary work brittleness resistance and formability according to 1 or 2, characterized by containing:

[0015] 4.1 A step of hot rolling a steel slab having the component described in any of 1 to 3 at a finishing temperature not lower than the Ar3 transformation point, and a step of winding the steel sheet after hot rolling at 500 to 700 ° C. And a step of subjecting the wound steel sheet to cold rolling and annealing. A method for producing a high-strength cold-rolled steel sheet having excellent secondary work brittleness resistance.

[0016] Steps of hot rolling a steel slab having the composition described in any one of the above items 1 to 3 at a finishing temperature not lower than the Ar3 transformation point, and a step of winding the steel sheet after hot rolling at 500 to 700 ° C. And a step of subjecting the wound steel sheet to cold rolling, annealing, and zinc-based plating. A method for producing a high-strength galvanized steel sheet having excellent secondary work brittleness resistance.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The component composition range of the present invention will be described below.
The manufacturing conditions will be described.

1. Component composition range C: 0.0040 to 0.01% C is added to secure strength. In order to ensure a tensile strength of 340 MPa or more, 0.0040% or more is added. However, if it exceeds 0.01%, precipitation of carbides will be recognized at the grain boundaries, and the secondary working brittleness will be deteriorated.
0.0040 to 0.01%. In order to properly control the morphology and dispersion state of the precipitates, further improve the resistance to secondary working brittleness, and derive more favorable overall performance, the amount of C added is preferably 0.0050 to 0.0080%, and more preferably,
It is preferable that the content be regulated in the range of 0.0050 to 0.0074%.

Si: not more than 0.05% Since excessive addition of Si deteriorates the galvanizing adhesion, it is set to not more than 0.05%.

Mn: 0.1 to 0.7% Mn precipitates S in the steel as MnS and prevents hot cracking of the slab. Further, it is added because the strength can be increased without deteriorating the galvanizing adhesion.

The content of S is set to 0.1% or more, which is necessary for the precipitation and fixation. However, if added excessively, the ductility decreases with an increase in strength. Therefore, the upper limit is set to 0.7%.

P: 0.01 to 0.05% P is an element effective for strengthening steel, and is added in an amount of 0.01% or more, but if added in excess of 0.05%, the zinc plating adhesion is deteriorated. Therefore, the content is set to 0.05% or less.

S: 0.02% or less S reduces the hot workability and increases the hot cracking susceptibility of the slab. Further, since the workability is deteriorated due to the precipitation of fine MnS, the upper limit is made 0.02%.

Al: 0.01-0.1% N in steel is precipitated as AlN, and is added in order to prevent solid solution N from remaining as much as possible. If it is less than 0.01%, such effects are not sufficient, and if it exceeds 0.1%, the remaining solid solution Al
, The ductility is reduced, so the upper limit is defined as 0.1%.

N: 0.004% or less Precipitates and renders harmless as AlN, but the content is made 0.004% or less so that the lower limit amount of Al is rendered as harmless as possible.

Nb: 0.15% or less Nb is added in order to fix solid solution C and to improve secondary work brittleness resistance and formability. However, since excessive addition lowers the ductility, the upper limit of Nb is set to 0.15%.
In order to properly control the morphology and dispersion state of the precipitates and further improve the resistance to secondary working brittleness, the amount of Nb added should be Nb> 0.
035%. In order to further improve secondary work brittleness resistance and further improve overall performance, Nb ≧ 0.08
It is desirable to set it to 0%. However, in consideration of cost, the upper limit is desirably set to Nb ≦ 0.140%.

(12/93) × Nb ^{* /} C ≧ 1.2 where Nb ^* = Nb− (93/14) × N In the present invention, the steel sheet is provided with excellent sensitivity to secondary working brittleness and r-value. And to make it non-aging, N
The amount b is defined so as to satisfy the above expression. In FIG. 1, C:
0.0040 to 0.01%, Nb: 0.01 to 0.15
%, Si: 0.01-0.05%, Mn: 0.1-1.
0%, P: 0.01-0.05%, S: 0.002-
A slab having a composition of 0.02% was prepared, and after hot rolling,
Pickling, cold rolling, continuous annealing at 800 ° C., and finally temper rolling of 0.5% are performed, and the results of evaluating the susceptibility to secondary working brittleness are shown.

The secondary work brittleness resistance is 105 m in diameter from the steel plate.
m, punched into a cup, deep-drawn into a cup, and the resulting cup sample was stored in a variety of refrigerants (such as ethyl alcohol). After that, the end of the cup was expanded with a conical punch to prevent ductile fracture. The temperature at which the transition to brittle fracture was measured was taken as the secondary working embrittlement temperature.

The secondary working embrittlement temperature is between 1% nominal strain and 1%.
The n value of the two-point method of 0% is 0.21 or more, and (12/9
3) × Nb ^{* /} C ≧ 1.2 where Nb ^* is Nb−
The effective Nb amount defined by (93/14) × N is remarkably reduced, and excellent secondary work brittle resistance is obtained. Details of such an effect are unknown, but are considered to be due to the following three combined effects.

1) By increasing the n value in the low strain region of 1% to 10%, the amount of strain at the punch bottom contact portion at the time of drawing is increased, and the amount of inflow in the deep drawing is reduced, resulting in shrinkage. The effect of reducing the degree of compression processing in flange deformation.

2) (12/93) × Nb ^* / C is 1.2
By the above, the size and dispersion form of carbides are optimized, and even under compression processing in shrinkage flange deformation during deep drawing, microscopic strain is uniformly dispersed, so dislocation to a specific grain boundary And no grain boundary embrittlement occurs.

3) Due to the precipitation of NbC, the grain size of the annealed sheet is made finer than that of the conventional steel, so that the toughness is improved.

In the present invention, in order to further improve the resistance to secondary working brittleness, the n value of the two-point method with a nominal strain of 1% and 10% is desirably 0.214 or more for the reason 1) above. .

FIGS. 2 and 3 show that C is 0.0040 to 0.01.
% Of the steel sheet was subjected to a tensile test after aging at r value and 30 ° C. for 3 months, and the yield elongation was measured, and (12/93) × Nb ^* / C was 1.2 or more. In this case, solid solution C can be completely fixed, and an excellent r value and non-aging property can be obtained.

In the present invention, (12/93) × Nb ^* / C must be set at 1.12 in order to control the morphology and dispersion state of the precipitates, and to secure more excellent secondary work brittleness resistance and formability.
It is desirable to regulate within the range of 3 to 2.2.

From FIGS. 1 to 3, the steel composition was changed to (12/93) ×
In the case of a steel having Nb ^* / C of 1.2 or more and a two-point method with a nominal strain of 1% and 10% in a uniaxial tensile test having an n-value of 0.21 or more, excellent susceptibility to secondary working brittleness , Moldability and non-aging properties.

The effects of the present invention can be achieved by the above-mentioned rules. In order to further improve the quality and the resistance to secondary working brittleness, Ti and B are each set to Ti ≦ 0.05%, B
It can be added in the range of ≦ 0.002%.

Ti: Formability is improved by forming carbonitrides and making the structure of the hot-rolled sheet finer. However, if it is added in excess of 0.05%, the precipitates become coarse and sufficient effects cannot be obtained. More preferably, the upper limit is 0.02%, particularly from the viewpoint of the surface properties of the hot-dip galvanized.
, And the lower limit is set to 0.
005% is desirable.

B: added to strengthen grain boundaries and improve secondary work brittleness resistance, but if added in excess of 0.002%, formability is significantly reduced, so the upper limit is set to 0. 0
02%. In the steel of the present invention, the crystal grains are refined and exhibit extremely excellent secondary work brittleness resistance. Therefore, in order to minimize the decrease in formability, the amount of B added is preferably 0.0%.
It is desirable to regulate it in the range of 001 to 0.001%.

Further, in the high-strength thin steel sheet of the present invention, since solid solution C and N are completely fixed according to the above formula (1),
H (baking hardenability) is less than 19.6 MPa, and aging does not pose a problem even when the temperature is maintained for a long time in an environment where the temperature is relatively high such as in summer. Furthermore, it has excellent workability of the welded part, and can respond to new technologies such as tailored blanks.

2. Manufacturing conditions Hot rolling is performed after or without heating a continuous cast slab of steel having the above components.

Hot rolling finish temperature: Ar3 point or higher If the finish temperature is lower than Ar3 point, the n value in the low strain region of 1 to 10% is reduced, and the secondary working brittleness is deteriorated. And

Winding temperature: 500 to 700 ° C. or less The winding temperature is 500 ° C. in order to sufficiently precipitate NbC.
As described above, the temperature is set to 700 ° C. or less in order to prevent indentation flaws due to scale peeling of the steel sheet surface.

Although the present invention is mainly directed to a hot-dip galvanized steel sheet, an electro-galvanized steel sheet can also achieve the intended effects. Further, even a cold-rolled steel sheet which is not plated can be used for various uses including an automobile steel sheet due to its excellent secondary work brittleness resistance, non-aging property, and workability. In the present invention, the cold rolling, annealing and galvanizing are performed by a conventional method, and the intended effects can be obtained. Further, after plating, an organic film treatment may be performed.

[0045]

EXAMPLE A steel having the chemical composition shown in Table 1 was melted,
After heating a continuous cast slab having a thickness of m to 1200 ° C., a finishing temperature of 890 to 940 ° C. and a winding temperature of 600 to 6 ° C.
Hot-rolled at 50 ° C. to form a hot-rolled steel sheet having a thickness of 2.8 mm,
After cold rolling to a thickness of 0.7 mm after pickling, an annealing temperature of 800 to 860 ° C. in a continuous galvanizing line,
Galvannealing was performed at a plating bath temperature of 460 ° C. and an alloying treatment temperature of 500 ° C.

Then, after temper rolling of 0.7% was performed,
Tensile test specimens were taken and 1% and 10%
Values, TS and r values were determined, and the susceptibility to secondary working embrittlement was investigated by the same test method using cup drawing as described above. The tensile test was performed using a JIS No. 5 tensile test piece taken from the L direction. After aging at 30 ° C. for 3 months, a tensile test was performed to measure the yield elongation to evaluate non-aging properties.

As is evident from Table 2, the steels of the present invention of steel Nos. 1 to 15 show excellent formability, and all have extremely excellent secondary work embrittlement temperatures of -85 ° C. or less. It has secondary working embrittlement, has no surface property problems, and is non-ageing. In addition, it was confirmed that the steel of the present invention was excellent in workability and fatigue properties of a welded part in addition to the above-mentioned properties.

On the other hand, for example, in Comparative Example No. 1
In Comparative Examples Nos. 6 and 21, since the amounts of C and P were out of the range of the present invention, the strength was insufficient. In Nos. 19 and 20, the surface properties are remarkably inferior because the amounts of Si and P are inappropriate. Comparative steel No. 17
In Comparative Example No. 2, the finish rolling temperature was not more than Ar3 point. Since the Nb ^* / C values of Nos. 18 and 22 are not appropriate, the moldability and the secondary work embrittlement resistance are not sufficient.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

As described above, according to the present invention,
It is possible to obtain galvanized steel sheet that is non-ageable and has high formability and excellent secondary work brittleness resistance without impairing the surface properties, and can be applied to various fields such as steel sheets for automobiles, Is extremely significant.

[Brief description of the drawings]

FIG. 1 is a diagram showing the influence of Nb and C on secondary work brittleness resistance.

FIG. 2 is a view showing the influence of Nb and C on deep drawability and non-aging property.

FIG. 3 is a diagram showing the effect of Nb and C on non-aging properties.

Continuing from the front page (72) Inventor Takeshi Fujita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Soto Kitano 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Inside (72) Inventor Masaya Morita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Yuji Yamazaki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Toshiaki Urabe 1-2-1, Marunouchi, Chiyoda-ku, Tokyo F-term (reference) 4K037 EA01 EA02 EA04 EA15 EA18 EA19 EA23 EA25 EA27 EA31 EB03 EB06 EB08 FC04 FC07 FE01 FE02 FE03 GA05 HA02 HA05

Claims (5)

[Claims] 1. C: 0.0040 to 0.01 by mass%
%, Si: 0.05% or less, Mn: 0.1 to 1.0%,
P: 0.01 to 0.05%, S: 0.02% or less, Al:
0.01-0.1%, N: 0.004% or less, Nb: 0.15%
Hereinafter, the balance satisfying the expression (1) has a component substantially consisting of Fe and unavoidable impurities, and the n value of a two-point method with a nominal strain of 1% and 10% by a uniaxial tensile test is 0.2.
A high-strength thin steel sheet excellent in secondary work brittleness resistance and formability, characterized by being at least one. (12/93) × Nb ^{* /} C ≧ 1.2 (1) where Nb ^* = Nb− (93/14) × N, C (% by mass), Nb (% by mass), N (% by mass) 2. The high-strength thin steel sheet according to claim 1, further comprising 0.05% by mass or less of Ti in mass%. 3. The high-strength steel sheet according to claim 1, further comprising 0.002% by mass or less of B in mass%. . 4. A step of hot rolling a steel slab having the composition according to claim 1 at a finishing temperature not lower than the Ar3 transformation point, and rolling the steel sheet after hot rolling at 500 to 700 ° C. A method for producing a high-strength cold-rolled steel sheet having excellent secondary work brittleness resistance, comprising a step of taking and a step of performing cold rolling and annealing on the wound steel sheet. 5. A step of hot-rolling a steel slab having the composition according to claim 1 at a finishing temperature not lower than the Ar3 transformation point, and rolling the steel sheet after hot rolling at 500 to 700 ° C. A method for producing a high-strength galvanized steel sheet having excellent secondary work brittleness resistance, comprising a step of taking and a step of subjecting the wound steel sheet to cold rolling, annealing and zinc-based plating.