JP2995526B2 - Manufacturing method of cold rolled steel sheet which has excellent formability, has paint bake hardenability, and has little fluctuation in paint bake hardenability in the width direction - 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 cold-rolled steel sheet having a high degree of formability, workability and paint bake hardenability suitable for automobile panels and the like, and having little variation in paint bake hardenability in the width direction. About.

[0002]

2. Description of the Related Art Starting from global environmental problems, weight reduction of automobiles has become a major issue again. Automotive panels are no exception to weight reduction, and the trend toward thinner technology development is increasing. However, on the other hand, the cold rolled steel sheet for automobiles is increasingly required to have a higher degree of freedom in forming in order to cope with the progress of CAD and CAM in die design and to respond to the preference of customers for the shape. That is, there is an increasing demand for materials that can withstand advanced molding processes. Further, as a demand for a panel or the like, there is a dramatic improvement in panel surface quality.
The technical meaning lies in both the surface shape and the degree of plastic deformation resistance of the panel, that is, the dent resistance.

[0003] Further, in order to reduce the manufacturing cost of automobiles, integrated molding is aimed at, and a wide-width cold-rolled steel sheet is required.
In terms of material, it is required to minimize variations in the width direction. First, r value (Rankford value), elongation value or n value is a representative index for the formability, but its level is increasing more and more.

[0004] Further, surface distortion resistance and dent resistance are important for panel surface quality. The former is related to the shape freezing property and requires a low yield point strength. Dent resistance, on the other hand, is the strength of the product, i.e. after forming, assembling, assembling and painting. Of these, paint baking is usually a heat treatment at 170 ° C. for about 20 minutes, and paint bake curability (usually called BH property), which is a property of being cured by this heat treatment, is required. The paint bake hardenability utilizes strain aging due to solid solution C and N in steel, which can sufficiently diffuse even at a low temperature of about 170 ° C. (In this case, the strain is caused by skin pass rolling in the final steel plate manufacturing process and by an automobile factory. Is the sum of the strains in the forming process at.

An object of the present invention is to provide a method for producing a cold-rolled steel sheet having a high degree of workability and this BH property and having less variation in the BH property in the width direction. Ultra-low carbon steel is typically used for such applications. The present invention is also consistent with this ultra-low carbon steel. As a solute element involved in imparting BH property, solid solution C and N are used for steel as described above. On the other hand, BH property is a kind of aging property and causes deterioration in formability at room temperature. It is a problem if it is too large. That is, normal temperature delayed aging or non-aging
It is necessary to be compatible with accelerated aging at a temperature of about ° C. The temperature dependence on aging, ie the activation energy of aging is different between C and N, C is larger,
The effect of C on aging is characterized by slow aging at normal temperature and faster at higher temperatures. Therefore, solid solution C is generally used as a BH property imparting technique.

[0006] The main methods for producing extremely low carbon BH steel sheets are described in JP-A-59-31827, JP-A-59-38337, JP-A-63-128149 and JP-A-2-19754.
No. 9 is described in each gazette. In each case, Nb is added in a range not more than the stoichiometric equivalent to C. In addition, Japanese Unexamined Patent Publication
JP-A-194126 discloses that Ti is completely replaced by TiC.
A technique for adding as far as it is not fixed is described.

[0007] Further, in terms of components, a carbide-forming element is added in excess of carbon, but this carbide is dissolved by recrystallization annealing during steel sheet production to secure solid-solution carbon. For example, JP-A-63-24
No. 1122 describes this technique. But,
In these methods, there is no suggestion about a technique for reducing the fluctuation of paint baking hardenability in the width direction of the sheet and producing a steel sheet having a good shape.

That is, it is difficult with the prior art to produce a cold-rolled steel sheet with reduced dispersion of baking hardening in the width direction as intended by the present invention at low cost. In order to produce a cold rolled steel sheet having a good shape at a low cost with less variation in paint bake hardening property, a cooling method in a continuous annealing line is an important point. The BH property and the shape of the steel sheet are sensitive to the cooling rate and pattern after annealing.

Cooling methods in the continuous annealing line include steam cooling, GAS cooling, roll cooling, and water cooling. The present inventors used steels having the component systems and production conditions shown in Tables 1 and 2, and obtained a cooling rate distribution and B
The H distribution was investigated. The results are shown in FIG. As a result, it has been clarified that the steam-water cooling method has a small variation in the cooling rate distribution in the width direction of the sheet, and as a result, a cold-rolled steel sheet having a good shape can be obtained at a low cost with little variation in paint baking hardening property. .

[0010]

[Table 1]

[0011]

[Table 2]

[0012]

[Table 3]

In the present invention, the workability level is r ≧
2.0, El ≧ 48% (plate thickness 0.8 mm), n ≧ 0.2
3 (However, the r value is an in-plane average value and is 0 with respect to the rolling direction.
Characteristic values in the directions of °, 45 ° and 90 °
If expressed as 0, X45, X90, (X0 + 2X4
5 + X90) (defined as 4).

Here, the r value is an index for deep drawability, and is defined by [logarithmic strain in width direction 幅 logarithmic strain in plate thickness] with respect to the tensile direction. El is the elongation at break in the tensile test.
The n value is a work hardening index, which indicates the inflow of a material, and is also a typical workability index.

[0015]

The problem to be solved by the present invention is to provide, as described above, a mold workability capable of withstanding a high degree of processing, a surface distortion resistance and a dent resistance, and a baking in the width direction. It is an object of the present invention to provide a method for manufacturing a cold-rolled steel sheet with less variation in hardening properties. If this problem is specifically shown, r value ≧ 2.0 and El ≧ 48 for workability.
% (El is a case where the plate thickness is 0.8 mm in both cases. El
Depends on the sheet thickness), n value ≧ 0.23, but the BH property is unloaded after giving 2% pre-strain in the tensile test, 170
A heat treatment at 20 ° C. for 20 minutes was applied, the film was pulled again, and the yield point strength was evaluated by subtracting the yield stress from the flow stress at 2% prestrain. That is, the yield rise in the strain aging test at 2% pre-strain and 170 ° C. for 20 minutes.

[0016]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention combines specific trace element control, specific solid solution strengthening element addition, and specific conditions from hot rolling to continuous annealing line. The summary is as follows.

(1) C: 0.0010 to 0.0030
%, N: 0.0030% or less, Si: 0.2% or less, M
n: 0.02 to 0.3%, P: 0.03% or less, S:
0.01% or less, acid-soluble Al: 0.005 to 0.07
%, Nb: 0.03% or less and Nb / C (atomic weight ratio)
0.7 to 1.3, Ti: 24 / 14N (%) to 7
A steel containing 2 / 14N (%), the balance consisting of Fe and unavoidable impurities is hot-rolled at a finishing temperature not lower than the Ar ₃ transformation point, and quenched within 2 seconds after hot-rolling to 650 to 770 ° C.
And then cold-rolled at a cold rolling rate of 72 to 92%, followed by annealing at 830 to 880 ° C. for 20 seconds or more in annealing at a continuous annealing equipment having steam-water cooling equipment.
Cooling to more than 70 ° C at a cooling rate of 3 to 15 ° C / sec.
70 ° C or less is cooled at a cooling rate of 30 ° C / second or more, and then skin pass is performed at an elongation of 0.8 to 1.5%. A method for producing a cold-rolled steel sheet having little variation in baking hardenability in the width direction.

(2) C: 0.0010 to 0.0030
%, N: 0.0030% or less, Si: 0.2% or less, M
n: 0.02 to 0.3%, P: 0.03% or less, S:
0.01% or less, acid-soluble Al: 0.005 to 0.07
%, Nb: 0.03% or less and Nb / C (atomic weight ratio)
0.7 to 1.3, Ti: 24 / 14N (%) to 7
2 / 14N (%), and B: 0.0001 to
A steel containing 0.0020%, Cr: 1.5% or less, and the balance consisting of Fe and unavoidable impurities is hot-rolled at a finishing temperature not lower than the Ar ₃ transformation point, and after hot rolling, 2 After quenching within seconds, winding at 650 to 770 ° C., followed by cold rolling at a cold rolling rate of 72 to 92%, followed by annealing in a continuous annealing facility having a steam-water cooling facility, 83
After annealing at 0 to 880 ° C. for 20 seconds or more, cooling to a temperature exceeding 670 ° C. is performed at a cooling rate of 3 to 15 ° C./sec.
Cooling at a cooling rate of 0 ° C./second or more, followed by skin pass at an elongation of 0.8 to 1.5%. A method for producing cold rolled steel sheets with little change in paint bake hardenability.

[0019]

Next, the operation of each component and the reason for limiting the numerical values will be described. C: C is an immersion-type solid-solution element that is detrimental to the workability of the cold-rolled steel sheet, that is, harmful to the texture composition and the growth of sufficiently large grains, while BH is the solid-solution carbon in the final product sheet. It depends on the amount, so a minimum amount is required. For these reasons, the lower and upper limits of C are 0.0010%, respectively.
Must be 0.0030%. The upper limit is desirably 0.0025%.

N: N is also an immersion type solid solution element and is harmful. Further, since it is easy to diffuse at room temperature, it is difficult to achieve both the BH property and the aging resistance at room temperature, so that it is disadvantageous to use it for BH property. Therefore, it is necessary to be 0.0030% or less. Si: Si strengthens the steel by solid solution strengthening, but on the other hand, impairs workability and chemical conversion treatment, so the upper limit is made 0.2% or less.

Mn: Mn also strengthens steel by solid solution strengthening. In particular, it is a preferable strengthening element with less ductility deterioration of the material for its strength. However, too much addition reduces the ductility of the material and degrades workability. In addition, S in steel is changed to MnS
0.02% or more is necessary to fix the temperature. Therefore, Mn is added in an amount of 0.02 to 0.3%.

P: P is also a solid solution strengthening element and is effective in increasing the strength. However, on the other hand, P causes deterioration of workability and brittle fracture. S: Since S forms inclusions with impurities and reduces the workability of steel, the lower the S content, the better. When the amount of S is large, not only TiS is formed but also combined with C to form TS.
Produces compounds such as i ₄ C ₂ S ₂ . This compound is stable, and it is extremely difficult to decompose during the annealing to form a solid solution of C, which causes a reduction in BH property. That is, in order to stably impart high BH properties, it is desirable that carbides other than NbC are not generated as much as possible. For these reasons, S is set to 0.01% or less. Preferably 0.
Should be less than 004%.

Al: Al is used for deoxidation. It is also used to assist in fixing N, which is an intrusion type impurity. Therefore, 0.005% is necessary as acid-soluble Al. On the other hand, 0.
Addition of more than 07% deteriorates the workability of steel. Nb: Nb is a very important element in the present invention, and the upper limit is 0.03%. Further, in the present invention, it is necessary to satisfy the following relationship with respect to C and Nb.

0.70 ≦ Nb / C (atomic ratio) ≦ 1.30 When the value is less than the lower limit of this relational expression, the amount of solid solution C in the steel sheet is too large, and the workability such as r value and elongation is reduced. If the value exceeds the upper limit of the relational expression, the melting temperature of NbC increases, and Nb
C cannot be decomposed, so that solid solution C required for BH property cannot be obtained, and sufficient BH property cannot be obtained.

Ti: Ti is added for fixing N. However, if too much is added, fine TiC is formed in the hot rolling stage, and a good recrystallization texture cannot be obtained. Therefore, the stoichiometric equivalent of N (48/14 × Ti (%)) is 0.5 to 1.5.
Add within the range. N may be a little excessive,
In that case, the remaining N in the specific hot rolling of the present invention is fixed as AlN, and no solid solution N remains before cold rolling.

In the present invention, in the final product, solute carbon remains for imparting BH property, so that carbon is segregated also at the crystal grain boundaries and is good for embrittlement in secondary processing, but is more severe for secondary embrittlement. When work embrittlement is required, B is added. When the amount of B added is less than 0.0001%, the effect is not obtained.
The addition exceeding 020% deteriorates the workability of the steel. More preferably, the addition should be 0.0008% or less.

When the strength is to be further supplemented, Cr is added at 1.5%.
Add below. Cr is a preferable element that has a small solid solution strengthening ability but improves work hardening characteristics and minimizes deterioration of the n-value for high strength. Additions above 1.5% are not economical. The lower limit is not particularly required, but if it is less than 0.02%, the effectiveness is not recognized. Preferably, 0.1 to
1.0%.

Hot rolling conditions: Hot rolling is completed at a temperature not lower than the Ar ₃ transformation point. Hot rolling in the α phase region has an adverse effect on r-value formation. The cooling conditions after hot rolling are important. The crystal grain boundary of the hot-rolled sheet is a nucleation position having a crystal orientation that is preferable for the r value during recrystallization annealing. The finer the grain structure, the more active the nucleation and the better the r value. Therefore, it is necessary to rapidly cool within 2 seconds after the end of rolling. If the time exceeds 2 seconds, a coarse hot-rolled structure results, and a good r value cannot be obtained. Preferably, it should be quenched within 0.8 seconds. The quenching rate may be about 30 ° C./sec, which is the usual cooling rate for spraying, etc.
Cool at 0 ° C / sec or more and about 100 ° C or more.

The winding temperature must be 650-770 ° C. As a result, solid solution impurities such as C remaining in the hot rolling step are sufficiently scavenged. If the temperature is lower than 650 ° C., the diffusion is not sufficient and there is no scavenging effect. On the other hand, 77
When the temperature exceeds 0 ° C., crystal grain growth occurs, and the effect of performing specific hot rolling is lost. More preferably, the winding temperature is from 700 to
770 ° C.

Cold rolling / annealing conditions: The cold rolling ratio must be as high as 72 to 92% in order to obtain a high r value. It is preferably at least 77%. A cold rolling rate exceeding 95% is not realistic considering the current facilities and the like. After cold rolling, recrystallization annealing is performed in a continuous annealing line. At that time, the heating temperature was 83
It is necessary to be 0 to 880 ° C. Heating, that is, annealing, to obtain a {111} orientation uniform and sufficiently large recrystallized texture, and to partially dissolve NbC in Nb and C to secure solid solution carbon and impart BH property, 830
C is required. On the other hand, if the annealing temperature exceeds 880 ° C., the crystal grains become too large, which leads to a defect such as rough surface during press molding. The high-temperature holding time is also important for ensuring the workability and the BH property, and it is necessary to hold for 20 seconds or more.

The cooling rate after annealing is important in the present invention. If it exceeds 670 ° C., it is 3 to 15 ° C./sec. Below the lower limit, solid solution C precipitates as NbC, and sufficient BH properties cannot be obtained. If the upper limit is exceeded, the shape of the steel sheet deteriorates. 67
At 0 ° C or lower, the temperature is 30 ° C / second or higher. If the cooling rate is lower than this, the productivity decreases. In addition, solid solution C precipitates as NbC, and the BH property decreases.

The elongation percentage of the skin pass is 0.8 to 1.5%. If the value is less than the lower limit, the yield elongation remains in the product sheet, and stretcher strain occurs when the panel is pressed. If it exceeds the upper limit, work hardening occurs, the yield point strength increases, and the pressability decreases. The melting of the starting steel of the present invention is performed in a converter. After converter refining, it is decarbonized by vacuum degassing. Then, it is hot-rolled after being made into a slab by ingot making / bulking or continuous casting.

The hot rolling conditions may be conditions usually employed except for the finishing temperature, the cooling condition after finish rolling, and the winding temperature. However, in order to further improve the workability, the heating temperature is set to 115.
It is desirable that the temperature be 0 ° C. or lower. Annealing is performed in a continuous annealing line having steam-water cooling equipment. After annealing, the strip is air-water cooled, followed by a skin pass.

[0034]

EXAMPLE Next, an example will be described. Steel having the chemical components shown in Table 4 was tapped in a converter and melted. In each case, ultra-low carbon is obtained by RH vacuum degassing. Among these steels, steels with steel symbols A to J and M, N, O are in accordance with the present invention, but otherwise differ from the present invention in underlined items.

These steels were converted into slabs by continuous casting and then hot-rolled. After pickling, it was cold rolled, and subsequently passed through a continuous annealing line to obtain a product. Table 4 shows the hot rolling and annealing conditions. In addition, the hot rolling heating temperature was 1110-1150 degreeC. The resulting mechanical test values are also shown in Table 4. For the mechanical test, use the No. 5 test piece described in JISZ2201 and Z224
The yield point strength YP, tensile strength TS, and elongation at break El were measured according to the method described in 1. In addition, 10-20%
The n value was calculated from the strain.

The evaluation of paint baking curability was based on B
Indicated by H. In addition, to evaluate the aging resistance at room temperature, 40
The recovery of the yield point elongation after 30 days at
Indicated by l. YP-El is an amount corresponding to a stretcher strain defect, and this defect occurs unless it is within 0.2%.

[0037]

[Table 4]

[0038]

[Table 5]

[0039]

[Table 6]

[0040]

[Table 7]

[0041]

[Table 8]

[0042]

[Table 9]

As is clear from Tables 4 to 9, the steel sheets (A to G) according to the present invention have almost no fluctuation in the width direction.
It has a BH property of 0 MPa or more, has sufficiently low YP (surface distortion resistance), has good elongation, r value and n value (high formability),
As for the aging property, there is almost no restoration of YP-El at the normal temperature aging, and the non-aging property at normal temperature or the delayed aging property is exhibited. In contrast, none of the comparative steel sheets satisfy all these characteristics.

In the case of the steel H, since the roll cooling was used during the continuous annealing, the cooling speed at the time of cooling varied, and as a result, the BH
The variation in the properties was large, the shape of the steel sheet deteriorated, and ear waves were generated. In the case of steel I, since the GAS cooling was used during the continuous annealing, the production cost was higher than that of steam cooling. In the case of steel J, water cooling was used during continuous annealing, so that the shape of the steel sheet was deteriorated, and ear waves and warpage occurred.

In the case of steel K, since Nb exceeded the upper limit,
The dissolution temperature of NbC increased, and NbC could hardly be dissolved during continuous annealing, and required solid solution C could not be obtained. As a result, the BH property was reduced. In steel L, since C exceeded the upper limit, the amount of solute C was too large, and the r value and elongation decreased. Further, the yield point elongation after aging exceeded 0.2%, and the aging property was deteriorated.

In the case of the steel M, since the cooling rate exceeding 670 ° C. exceeded the upper limit, the shape of the steel sheet was deteriorated, and ear waves were generated.
In the case of N steel, the cooling rate at 670 ° C. or less was lower than the lower limit, so that the BH property was reduced. In addition, productivity has decreased. In the case of O steel, since the cooling rate exceeding 670 ° C. was less than the lower limit, solid solution C was precipitated as NbC, and the BH property was reduced.

[0047]

As described above, an automobile is related to an environmental problem, and is trying to reduce its body weight to reduce fuel consumption. Panels are no exception, and their weight as a percentage of vehicle weight is rather important. On the other hand, panels are the most prominent part of automobile quality, and their design is becoming more important. This leads to increasingly demanding complex shapes. From such a viewpoint, it is extremely important to have both excellent workability and paint bake hardenability as the object of the present invention. Furthermore, by reducing the variation in baking hardenability in the width direction, it has become possible to integrally mold panels and the like.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cooling rate distribution and a BH property distribution for each cooling method in a continuous annealing line.

Continuing from the front page (72) Inventor Koji Sakuma 1 Kimitsu, Kimitsu City, Chiba Prefecture Inside Kimitsu Works, Nippon Steel Corporation (56) References JP-A-7-278654 (JP, A) JP-A-5-278 171286 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21D 9/46-9/48 C21D 8/02-8/04 C22C 38/00-38/14

Claims (2)

(57) [Claims] 1. C: 0.0010 to 0.0030%,
N: 0.0030% or less, Si: 0.2% or less, Mn:
0.02-0.3%, P: 0.03% or less, S: 0.0
1% or less, acid-soluble Al: 0.005 to 0.07%, N
b: 0.03% or less and the value of Nb / C (atomic weight ratio) 0.7 to 1.3, Ti: 24 / 14N (%) to 72/1
A steel containing 4N (%), the balance consisting of Fe and unavoidable impurities is hot-rolled at a finishing temperature not lower than the Ar ₃ transformation point,
Rapid cooling within 2 seconds after hot rolling, winding at 650 to 770 ° C, and subsequent cold rolling at a cold rolling rate of 72 to 92%, followed by annealing in a continuous annealing facility having steam-water cooling facilities. After annealing at 830 to 880 ° C. for 20 seconds or more, 670 ° C.
Cooling at a cooling rate of 3 to 15 ° C./sec.
The following are cooled at a cooling rate of 30 ° C./second or more, and then the skin pass is performed at an elongation of 0.8 to 1.5%. Method of manufacturing cold rolled steel sheet with less variation in paint bake hardenability in different directions. 2. C: 0.0010 to 0.0030%,
N: 0.0030% or less, Si: 0.2% or less, Mn:
0.02-0.3%, P: 0.03% or less, S: 0.0
1% or less, acid-soluble Al: 0.005 to 0.07%, N
b: 0.03% or less and the value of Nb / C (atomic weight ratio) 0.7 to 1.3, Ti: 24 / 14N (%) to 72/1
4N (%), and B: 0.0001 to 0.0
020%, Cr: 1.5% or less of one or two kinds, and the balance consisting of Fe and unavoidable impurities is Ar ₃
Hot-rolled at the finishing temperature above the transformation point, quenched within 2 seconds after hot-rolling, wound up at 650-770 ° C, then 72-
After performing cold rolling at a cold rolling rate of 92%, when performing annealing in a continuous annealing equipment having steam-water cooling equipment, 830 to 8
After annealing at 80 ° C for 20 seconds or more, 3 to 1
Cool at a cooling rate of 5 ° C./sec.
Cooling at a cooling rate of at least 2 seconds, and then performing a skin pass at an elongation of 0.8 to 1.5%. Excellent in moldability, paint bake hardening, and paint bake in the width direction. Method for manufacturing cold-rolled steel sheets with little fluctuation in properties.