JPH07278654A - Manufacturing method of high-strength cold-rolled steel sheet for automobiles, which has excellent formability, has paint bake hardenability, and has little fluctuation in paint bake hardenability in the width direction - Google Patents

Abstract

(57) [Summary] [Object] To provide a method for producing a high-strength cold-rolled steel sheet having excellent formability and high bake hardenability and having little variation in the bake hardenability in the width direction. [Structure] To extremely low C and N steels, specific Nb is added to C in an atomic weight ratio, and Ti is added in an approximately equal amount to N.
The steel to which i, Mn, and P are added under specific conditions is processed in a continuous annealing line having specific hot rolling conditions, cold rolling conditions, and specific equipment. B and / or Cr may be added as appropriate. [Effect] Cold-rolled steel sheet, which has workability and dent resistance suitable for automobile panels and has little fluctuation in coating bake hardenability in the width direction, can be manufactured at low cost, thus forming an integral panel. This enabled the reduction of automobile manufacturing costs.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a high strength cold-rolled steel sheet having high formability, workability and paint bake hardenability suitable for automobile panels and the like, and having little variation in the paint bake hardenability in the width direction. The present invention relates to a manufacturing method.

[0002]

2. Description of the Related Art The weight reduction of automobiles has become a major issue again due to global environmental problems. Automotive panels are no exception to weight reduction, and the trend towards technological thinning is increasing. On the other hand, however, cold-rolled steel sheets for automobiles are increasingly required to have flexibility in forming in order to respond to the progress of CAD and CAM in die design and customer's preference for shape. That is, there is an increasing demand for materials that can withstand a high degree of molding processing. In addition, as a requirement for a panel or the like, a dramatic improvement in panel surface quality can be given. Its technical meaning lies in both the surface shape and the plastic deformation resistance of the panel, that is, the dent resistance. In addition, in order to reduce the manufacturing cost of automobiles, integral forming is aimed, and a wide cold-rolled steel sheet is required. As for the material, it is required to minimize the variation in the width direction.

First, r-value (Rankford value), elongation value or n-value is a representative index for moldability, but its level is increasing. Further, surface strain resistance and dent resistance are important for the surface quality of the panel. The former is related to shape fixability and requires low yield strength. On the other hand, dent resistance is the product's
Assembly The strength after assembly, mounting and paint baking. Of these, baking is usually 170 ° C for 20 min.
It is a heat treatment of a certain degree, and a coating bake hardenability (normally called BH property), which is a characteristic of hardening by this heat treatment, is required. The paint bakeability utilizes the strain aging due to solid solution C and N in the steel that can sufficiently diffuse even at a low temperature of about 170 ° C (in this case, the strain is strain due to skin pass rolling which is the final steel plate manufacturing process and at the automobile factory). Is the sum of strain in the molding process).

It is an object of the present invention to provide a method for producing a cold rolled steel sheet which has a high workability and this BH property and has a small variation in the BH property in the width direction. Very 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, solute C and N are used for steel as described above, but on the other hand, BH property is a kind of aging property and causes deterioration of moldability at room temperature. It becomes a problem if it is too large. That is, at room temperature delayed aging or non-aging 170
It is necessary to be compatible with accelerated aging at a temperature of about ° C. The temperature dependence of aging, that is, the activation energy of aging is different between C and N, and C is larger,
The effect of C on aging is that the aging at room temperature is slow, and the higher the temperature, the faster. Therefore, solid solution C is usually used as the BH property imparting technique.

The main methods for producing ultra low carbon BH steel sheets are as follows: JP-A-59-31827, JP-A-59-38337, JP-A-63-128149, and JP-A-2-1975.
It is described in each publication of No. 49. In each case, Nb is added in the range of not more than the stoichiometric equivalent to C. Further, in Japanese Patent Application Laid-Open No. 2-194126, Ti, C is completely TiC.
The technique of adding in the range that is not fixed is described. Further, in terms of composition, a carbide forming element is excessively added to carbon, but this carbide is melted by recrystallization annealing during steel sheet production to secure solid solution carbon, and as an example of hot dip galvanized steel sheet, This technique is described in JP-A-63-241122. However, these methods do not suggest any technique for reducing the variation in the paint bake hardenability in the plate width direction and producing a steel plate having a good shape. That is, it is difficult with the conventional technique to reduce the variation in the paint baking property in the width direction, which is the object of the present invention, and to manufacture at low cost.

[0006]

SUMMARY OF THE INVENTION The problems to be solved by the present invention are, as described above, the molding workability capable of withstanding a high degree of working, the surface strain resistance and the dent resistance, and further, the coating baking in the width direction. This is to realize a method for manufacturing a cold-rolled steel sheet with little variation in properties. If this problem is shown concretely, for workability, r value ≧ 2.0, El ≧ 48%
(If El is 0.8 mm in plate thickness, El depends on the plate thickness), n value ≧ 0.23, but the BH property is 2% pre-strained in the tensile test before unloading. , 170 ℃, 2
It is evaluated by a value obtained by subtracting the yield strength at the 2% prestrain from the flow stress at a pre-strain of 2% by applying a heat treatment of 0 min. That is, 2% prestrain, 170 ° C, 20min
It is the increase in yield point in the strain aging test.

That is, in the present invention, as workability levels, r ≧ 1.8, El ≧ 40% (plate thickness 0.8 mm), n
≧ 0.21, (where the r value is an in-plane average value and the characteristic values in the directions of 0 °, 45 ° and 90 ° with respect to the rolling direction are represented by X0, X45 and X90, respectively, (X0
+ 2X45 + X90) / 4) is defined as the target level. Here, the r value is an index for the deep drawability, and it is [logarithmic strain in the width direction ÷
Thickness logarithmic strain]. El is the elongation at break in the tensile test. Further, n value is a work hardening index,
It represents the inflow of materials and is also a representative index of workability.

[0008]

In order to overcome the above problems, the present invention combines specific trace element control with the addition of specific solid solution strengthening elements and specific conditions leading to hot rolling to continuous annealing lines. The gist of the present invention is (1) C: 0.0010 to 0.0030%, N: 0.0
030% or less, Si: 0.5% or less, Mn: 0.3 to
1.5%, P: 0.03 to 0.08%, S: 0.03%
Hereinafter, acid-soluble Al: 0.005 to 0.07%, Nb:
0.03% or less and a value of Nb / C (atomic weight ratio) of 0.
7 to 1.3, Ti: 24 / 14N (%) to 72 / 14N
(%) And the balance unavoidable impurities in steel
Hot rolling at a finishing temperature of ₃ transformation points or higher, quenching within 2 seconds after hot rolling, and winding at 650 to 770 ° C, followed by 72
After performing cold rolling at a cold rolling rate of up to 92%, and then performing annealing in a continuous annealing facility having a steam cooling facility, 830 to
After annealing for 20 s or more at 880 ° C, 3 to more than 670 ° C
Cool at a cooling rate of 15 ° C / s, and maintain 670 ° C or lower at 30 ° C
/ S or more, followed by skin pass with an elongation rate of 0.8 to 1.5%, which has excellent moldability, has paint bake hardenability, and has a paint bake in the width direction. A method for producing a high-strength cold-rolled steel sheet for automobiles with little fluctuation in hardenability.

(2) C: 0.0010 to 0.0030
%, N: 0.0030% or less, Si: 0.5% or less, M
n: 0.3 to 1.5%, P: 0.03 to 0.08%,
S: 0.03% or less, acid-soluble Al: 0.005-0.0
7%, Nb: 0.03% or less and Nb / C (atomic weight ratio) value of 0.7 to 1.3, Ti: 24/14 N (%)
˜72 / 14 N (%), and further B: 0.000
1 to 0.0020%, Cr: 1.5% or less of one or two kinds of steel containing the unavoidable impurities as the balance, and Ar ₃
Hot-rolling is performed at a finishing temperature equal to or higher than the transformation point, rapid cooling is performed within 2 seconds after hot-rolling, and the material is wound at 650 to 770 ° C., and subsequently 72-
After performing cold rolling at a cold rolling rate of 92%, and performing annealing in a continuous annealing facility having a steam cooling facility, 830 to 8
After annealing at 80 ° C for 20 s or more, 3 to 1 up to 670 ° C
Cool at a cooling rate of 5 ° C / s, and maintain 670 ° C or less at 30 ° C / s.
Cooling at a cooling rate of s or more, followed by skin pass at an elongation rate of 0.8 to 1.5%, excellent moldability, coating bake hardenability, and widthwise coating bakeability. It is a method for producing a high-strength cold-rolled steel sheet for automobiles with less fluctuation.

The present invention will be described in detail below. In the present invention, a cooling method in a continuous annealing line is an important point in order to produce a steel sheet having a good shape with less variation in paint bakeability at low cost. Since the BH property and the shape of the steel sheet are sensitive to the cooling rate and pattern after annealing, it is necessary to determine the cooling conditions. The cooling method in the continuous annealing line includes steam cooling, GAS cooling, roll cooling, and water cooling. The present inventors investigated the cooling rate distribution and BH property distribution in the plate width direction for each of the above cooling methods using steels having the component systems and manufacturing conditions shown in Tables 1 and 2. The results are shown in FIG. 1 and Table 3. That is, FIG.
FIG. 1A is a diagram showing a relationship between a part in the plate width direction and a cooling rate, and FIG. 1B is a diagram showing a relationship between a part in the plate width direction and a BH property. From this, it was clarified that the steam cooling method has little variation in the cooling rate distribution in the plate width direction, and as a result, the variation in paint bakeability is small and a steel sheet having a good shape can be obtained at low cost.

[0011]

[Table 1]

[0012]

[Table 2]

[0013]

[Table 3]

[0014]

Next, the function of each constituent element and the reason for limiting the numerical value will be described. C: C is an interstitial solid solution element, which is harmful to the workability of cold-rolled steel sheets, that is, is harmful to texture formation and sufficiently large crystal grain growth, and decreases as much as possible, while BH is solid solution carbon in the final product plate. It depends on the amount and therefore the minimum amount is needed. For these reasons, the lower and upper limits of C are 0.0010%,
It is necessary to set it to 0.0030%. The upper limit is preferably 0.0025%.

N: N is also an interstitial solid solution element and harmful. Further, since it easily diffuses at room temperature, it is difficult to satisfy both BH property and room temperature aging resistance, and therefore it is disadvantageous to use it for BH property. Therefore, it is necessary to set it to 0.0030% or less. Si: Si strengthens steel by solid solution strengthening, but on the other hand, it hinders workability and chemical conversion treatability, so the upper limit is made 0.5% or less. Mn: Mn also strengthens steel by solid solution strengthening. In particular, it is a preferable strengthening element because the deterioration of the ductility of the material is small for the strengthening. However, too much addition reduces the ductility of the material and deteriorates the workability. Therefore, Mn is 0.3% to 1.5%
Is added.

P: P is also a solid solution element and is effective for increasing strength, but on the other hand, it causes deterioration of workability and brittle fracture, so 0.03 to 0.08% is added. S: S forms inclusions due to impurities and reduces the workability of steel, so it is necessary to set it to 0.03% or less. It should preferably be less than 0.004%. Al: Al is used for deoxidation. It is also used to help fix N, which is an interstitial impurity. Therefore, 0.005% is required as acid-soluble Al. On the other hand, addition of more than 0.07% deteriorates the workability of steel.

Nb: Nb is an extremely important element in the present invention, and its upper limit is 0.03%. Furthermore, in the present invention, the following relations regarding C and Nb must be satisfied. 0.70 ≦ Nb / C (atomic ratio) ≦ 1.30 Below 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 deteriorates. If the upper limit of the relational expression is exceeded, the melting temperature of NbC will increase, and NbC will increase during annealing.
C cannot be decomposed, solid solution C required for BH property cannot be obtained, and sufficient BH property cannot be obtained.

Ti: Ti is added to fix 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 amount of N (48/14 × Ti (%)) 0.5 to 1.5
Add in the range of. A small amount of N may be excessive, but in that case, the N remaining in the specific hot rolling of the present invention is fixed as AlN, and the solid solution N does not remain before cold rolling.

In the present invention, solid solution carbon remains in the final product for imparting BH property, so that carbon is segregated in the grain boundaries and good for secondary work embrittlement, but more severe secondary resistance. When work embrittlement is required, B is added. If the addition amount of B is less than 0.0001%, the effect is not obtained, and 0.0
Addition of more than 020% deteriorates the workability of steel. More preferably, the amount added should be 0.0008% or less.

When the strength is further supplemented, Cr is 1.0%.
Add below. Although Cr has a small solid solution strengthening ability, it is a preferable element that improves work hardening characteristics and minimizes deterioration of n value for higher strength. Addition of more than 1.0% is not economical. No lower limit is required, but 0.0
If it is less than 2%, it is not effective. Preferably 0.1
~ 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. Cooling conditions after hot rolling are important. The crystal grain boundaries of the hot-rolled sheet are the nucleation positions of the crystal orientation that are preferable for the r value during recrystallization annealing, and the finer grain structure is more active in nucleation, and a better r value is obtained. Therefore, it is necessary to cool rapidly within 2 seconds after the completion of rolling. If it exceeds 2 s, a coarse hot-rolled structure is formed, and a good r value cannot be obtained. It should preferably be cooled within 0.8 s. The quenching rate may be about 30 ° C./s or more, which is a cooling rate of a spray or the like which is usually used, but preferably 50 ° C./s or more, and about 100 ° C. or more.

The coiling temperature must be 650 to 770 ° C. As a result, the solid solution impurities such as C remaining in the hot rolling stage are sufficiently scavenged. If the temperature is lower than 650 ° C, the diffusion is insufficient and the scavenging effect is not obtained. On the other hand, 77
If the temperature exceeds 0 ° C., grain growth occurs and the effect of performing the specific hot rolling is lost. More preferably, the winding temperature is 700 to
770 ° C.

Cold rolling / annealing condition: The cold rolling rate needs to be as high as 72 to 92% in order to obtain a high r value. It is preferably 77% or more. A cold rolling rate of over 95% is not realistic considering the current equipment. After cold rolling, recrystallization annealing is performed in a continuous annealing line. At that time, the heating temperature is 8
It is necessary to set the temperature to 30 to 880 ° C. Heating, that is, annealing, is to obtain a recrystallized texture in which the {111} orientation is uniform and large, and to dissolve NbC in a part of 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 of rough skin during press molding. The high temperature holding time is also important in order to secure workability and BH property, and it is necessary to hold for 20 s or more.

The cooling rate after annealing is important in the present invention. If it exceeds 670 ° C, it is set to 3 ° C / s to 15 ° C / s. If it is less than the lower limit, solid solution C is precipitated as NbC, and sufficient BH property cannot be obtained. If the upper limit is exceeded, the shape of the steel sheet will deteriorate. At 670 ° C. or lower, it is 30 ° C./s or higher. If the cooling rate is less than this, the productivity will decrease. In addition, solid solution C is Nb
C precipitates and BH property deteriorates. The elongation rate of the skin pass is 0.8 to 1.5%. If it is less than the lower limit, the yield elongation remains on the product sheet, and stretcher strain occurs when the panel is pressed. If it exceeds the upper limit, work hardening occurs, yield strength increases, and pressability deteriorates.

The melting of the steel is carried out in a converter. After the converter refining, it is decarburized by vacuum degassing. Then, it is hot-rolled after being made into a slab by ingot making, lump making, or continuous casting. The hot rolling conditions may be those that are normally taken except for the finishing temperature, the cooling conditions after finish rolling, and the winding temperature. However, in order to further improve the workability, it is desirable that the heating temperature be 1150 ° C or lower. Annealing is performed in a continuous annealing line having a steam cooling system. After annealing, the strip is water cooled, followed by a skin pass.

[0026]

EXAMPLES Next, examples will be described. Steel having the chemical composition shown in Table 4 was tapped in a converter and melted. All of them are made to have extremely low carbon by RH vacuum degassing. Among these steels, the steels with steel symbols A to J and M, N, O are in accordance with the present invention, but other than that are different from the present invention in the underlined items. These steels were continuously cast into slabs and then hot rolled. After pickling, the product was cold-rolled, and then a continuous annealing line was passed through it to obtain a product. Table 5 shows hot rolling and annealing conditions.
Shown in. The hot rolling heating temperature was 1110-1150 ° C. The resulting mechanical test values are shown in Table 6. Mechanical test is J
Using the No. 5 test piece described in ISZ2201, the same Z2241
The yield point strength YP, the tensile strength TS, and the breaking elongation El were measured according to the method described. Also, 10% to 20%
The n value was calculated from the strain. The evaluation of the paint bakeability was shown by the BH property described above. In addition, in order to evaluate the room temperature aging resistance, the recovery amount of the yield point elongation after standing at 40 ° C. for 30 days is shown by YP-El. YP-El is the amount corresponding to the stretcher strain defect, and this defect occurs unless it is within 0.2%.

[0027]

[Table 4]

[0028]

[Table 5]

[0029]

[Table 6]

As is clear from Tables 4 to 6, the steel sheets (A to G) according to the present invention have a BH property of 30 MPa or more with almost no fluctuation in the width direction and a sufficiently low YP (surface strain resistance). Property, good elongation, r value, and n value (high moldability)
With respect to aging, YP-El was hardly restored at room temperature aging, and showed no room temperature or delayed aging. In contrast, none of the steel sheets for comparison satisfy all of these characteristics. H
In the steel No. 2, since the roll cooling was used at the time of continuous annealing, the cooling rate at the time of cooling was varied, and as a result, the BH property was greatly changed, the shape of the steel sheet was deteriorated, and a seismic wave was generated. In steel I,
Since GAS cooling was used at the time of continuous annealing, the manufacturing cost increased as compared with steam cooling.

In the steel of J, since water cooling was used during continuous annealing, the shape of the steel sheet deteriorated, causing seismic waves and warpage. K
In the steel No. 3, since Nb exceeded the upper limit value, the melting temperature of NbC increased, and NbC could hardly be melted during continuous annealing, and required solid solution C could not be obtained. Therefore, the BH property was lowered. In the steel of L, since C exceeded the upper limit value, the amount of solute C was too large, and the r value and the elongation decreased. Also, the yield point elongation after aging exceeds 0.2%, and the aging property is poor. In the steel of M, the cooling rate above 670 ° C. exceeded the upper limit value, so the shape of the steel sheet deteriorated and a seismic wave was generated. 670 ℃ for N steel
Since the cooling rate below was less than the lower limit value, the BH property deteriorated.
In addition, productivity was reduced. In the case of the O steel, the cooling rate above 670 ° C. was less than the lower limit value, so the solid solution C was precipitated as NbC and the BH property deteriorated.

[0032]

EFFECTS OF THE INVENTION Since automobiles are related to environmental problems, they are trying to reduce the weight of their vehicles in order to reduce fuel consumption. Panels are no exception, and their importance to the weight of automobiles is rather important. On the other hand, panels are the most prominent part of automobile quality, and the importance of their design is increasing. This leads to increasingly complex shapes being required. From this point of view, 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 paint bake hardenability in the width direction, it became possible to integrally form panels and the like.

[Brief description of 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.

Claims (2)

[Claims] 1. C: 0.0010 to 0.0030%,
N: 0.0030% or less, Si: 0.5% or less, Mn:
0.3-1.5%, P: 0.03-0.08%, S:
0.03% or less, acid-soluble Al: 0.005-0.07
%, Nb: 0.03% or less and Nb / C (atomic weight ratio)
Value of 0.7 to 1.3, Ti: 24/14 N (%) to 7
Steel containing 2/14 N (%) and the balance unavoidable impurities is hot-rolled at a finishing end temperature not lower than the Ar ₃ transformation point, rapidly cooled within 2 s after hot rolling, and wound at 650 to 770 ° C.
Then, after cold rolling at a cold rolling rate of 72 to 92%, in performing annealing in a continuous annealing facility having a steam cooling facility, after annealing at 830 to 880 ° C. for 20 s or more, 670 ° C.
670 ℃, cooling up to over at a cooling rate of 3 to 15 ℃ / s
The following are cooled at a cooling rate of 30 ° C./s or more, and then skin pass is performed at an elongation rate of 0.8 to 1.5%, which is excellent in moldability, has a coating bake hardenability, and has a width. Method for producing high-strength cold-rolled steel sheet for automobiles with little fluctuation in paint baking property in the horizontal direction. 2. C: 0.0010 to 0.0030%,
N: 0.0030% or less, Si: 0.5% or less, Mn:
0.3-1.5%, P: 0.03-0.08%, S:
0.03% or less, acid-soluble Al: 0.005-0.07
%, Nb: 0.03% or less and Nb / C (atomic weight ratio)
Value of 0.7 to 1.3, Ti: 24/14 N (%) to 7
2 / 14N (%) is contained, and further B: 0.0001-
Steel containing 0.0020%, Cr: 1.5% or less of 1 type or 2 types and the balance of unavoidable impurities is hot-rolled at a finishing finish temperature of Ar ₃ transformation point or higher, and within 2 seconds after hot rolling. Quench and wind at 650-770 ° C, then 72-92
After cold-rolling at a cold rolling rate of%, 830 to 880 are used for annealing in a continuous annealing facility having a steam cooling facility.
After annealing at ℃ for 20s or more, 670 ℃ over 3 ℃
/ S at a cooling rate of 670 ° C. or lower, and at a cooling rate of 30 ° C./s or higher at a temperature of 670 ° C. or lower.
Excellent formability, which is characterized by 8 to 1.5%,
A method for producing a high-strength cold-rolled steel sheet for automobiles, which has paint bake hardenability and has little fluctuation in paint bakeability in the width direction.