EP1002884B1

EP1002884B1 - Cold rolled steel plate of excellent moldability, panel shape characteristics and denting resistance, molten zinc plated steel plate, and method of manufacturing these steel plates

Info

Publication number: EP1002884B1
Application number: EP98944222A
Authority: EP
Inventors: Fusato Kitano; Masaya Morita; Yoshihiro Hosoya; Takeshi Fujita; Tadashi Inoue; Masahiro Iwabuchi; Takeo Ishii
Original assignee: NKK Corp; Nippon Kokan Ltd
Current assignee: JFE Engineering Corp
Priority date: 1998-04-27
Filing date: 1998-09-24
Publication date: 2009-02-25
Anticipated expiration: 2018-09-24
Also published as: CN1261408A; JP4177478B2; CN1138016C; EP2172575A1; EP1002884A1; KR20010014238A; EP1002884A4; WO1999055927A1; DE69840595D1; CN1405352A; JPH11310849A; KR100345012B1; BR9810485A; CN1084797C

Description

Technical Field

The present invention relates to a cold-rolled steel sheet and a galvanized steel sheet, which are excellent in formability, panel shapeability, and dent-resistance required for an outer panel of a motor car, and a method of manufacturing the same.

Background Art

An excellent formability, a satisfactory shape after a panel formation and a high dent-resistance (resistance to local depression) are required for a steel sheet for an outer panel of a motor car. The panel formability is evaluated by indexes such as yield strength, elongation, and an n-value (work-hardening index) of the steel sheet. Also, the panel shapeability and the dent-resistance are evaluated in many cases by yield strength and the yield strength after the working and the coating-baking treatment. If the yield strength of the steel sheet is weakened, the press formability can be improved. However, the dent-resistance after the panel formation is rendered unsatisfactory. On the other hand, if the yield strength of the steel sheet is increased, the dent-resistance is improved. However, problems are generated in terms of the press formability such as occurrence of wrinkles or cracks. Such being the situation, vigorous researches are being made in an attempt to obtain a steel sheet having a low yield point in the press forming and a high yield strength after the forming and baking as an outer panel for a motor car. As a cold-rolled steel sheet meeting these two contradictory requirements in terms of the yield strength, a bake-hardenable steel sheet, hereinafter referred to as a "BH steel sheet", utilizing a strain aging phenomenon of the carbon atoms within the steel has been developed.
Particularly, known is a method of manufacturing a BH steel sheet having a excellent deep drawability, which is a cold-rolled steel sheet prepared by adding elements capable of forming carbonitrides such as Nb and Ti to a steel having a very low carbon content of about 50 ppm, the addition amount of such an element being not larger than 1 in terms of the atomic ratio of carbon. For example, Japanese Patent Publication (Kokoku) No. 60-46166 teaches that a Nb or Ti added low-carbon steel is annealed at a high temperature close to 900°C for manufacturing the particular BH steel sheet. Also, Japanese Patent Disclosure (Kokai) No. 61-276928 teaches that an extra low carbon BH steel sheet is manufactured by annealing under a temperature region of about 700 to 850°C.
The technology disclosed in JP '166 is certainly advantageous in that the BH properties and an r-value can be improved. However, since the annealing is performed at a high temperature, the rough surface derived from enlargement of the ferrite grains is worried about. In addition, since the steel sheet itself is softened, the yield strength after the press forming and the baking steps is not acceptably high, though high BH properties may be obtained. On the other hand, in the technology disclosed in JP '928, the annealing temperature is relatively low, compared with that employed in JP '166, and, thus, is desirable in the required surface properties and the yield strength. However, it is substantially impossible to improve as desired the BH properties and the r-value. It should also be noted that these prior arts are mainly intended to improve the BH properties of a steel sheet in order to allow the steel sheet to exhibit an improved dent-resistance. Therefore, deterioration in the resistance to natural aging, i.e., occurrence of stretcher strain in the press forming, which is derived from generation of a yield point elongation during storage under room temperature, is worried about. Under the circumstances, the BH amount is suppressed at 60 MPa or less in view of the practical use of the steel sheet.
As described above, the cold-rolled sheet manufactured by the conventional method is not sufficiently satisfactory in the surface properties, the resistance to natural aging, and the dent-resistance, which are required for the steel sheet used for an outer panel of a motor car.
An object of the present invention is to provide a cold-rolled steel sheet and a galvanized steel sheet, which are satisfactory in any of the surface properties, the resistance to natural aging, and the dent-resistance, which are required for the steel sheet used for an outer panel of a motor car, and a method of manufacturing the same.

Disclosure of Invention

(1) The present invention provides a cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance, comprising 0.005 to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.65 to 1.5% by weight of Mn, 0.02 to 0.07% by weight of P, 0.006 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, not higher than 0.004% by weight of N, not higher than 0.003% by weight of O, 0.04 to 0.23% annealing temperature is relatively low, compared with that employed in JP '166, and, thus, is desirable in the required surface properties and the yield strength. However, it is substantially impossible to improve as desired the BH properties and the r-value. It should also be noted that these prior arts are mainly intended to improve the BH properties of a steel sheet in order to allow the steel sheet to exhibit an improved dent-resistance. Therefore, deterioration in the resistance to natural aging, i.e., occurrence of stretcher strain in the press forming, which is derived from generation of a yield point elongation during storage under room temperature, is worried about. Under the circumstances, the BH amount is suppressed at 60 MPa or less in view of the practical use of the steel sheet.

As described above, the cold-rolled sheet manufactured by the conventional method is not sufficiently satisfactory in the surface properties, the resistance to natural aging, and the dent-resistance, which are required for the steel sheet used for an outer panel of a motor car.
Another prior art technology is taught by U.S. patent 3 876 390 (Elias et al. ) which discloses an invention related to a cold-rolled steel sheet suitable for deep drawing and hot dip galvanizing.
However, as it will be detailed in the description which follows, a cold-rolled steel sheet according to the invention must satisfy several conditions which are not met by Elias et al. As a result, the composition of the obtained cold-rolled steel sheet is enterly different that the one of the invention. For example, claim 3 of Elias et al. recites steel having yield strength of about 620 MPa. It concerns a technology relating to Al coated steel sheet which is full hard material. Such a sheet is not formable enough to be used for an outer panel of motorcar, for example.
Furthermore, it is also possible to point out important differences between the present invention as detailed in the description which follows and the teaching of Elias et al., In particular, the cold-rolled steel sheet according to Elias et al. is manufactured by batch annealing, not by continuous annealing.
An object of the present invention is to provide a cold-rolled steel sheet and a galvanized steel sheet, which are satisfactory in any of the surface properties, the resistance to natural aging, and the dent-resistance, which are required for the steel sheet used for an outer panel of a motor car, and a method of manufacturing the same.

Disclosure of Invention

(1) The present invention provides a cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance, comprising 0.005 to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.45 to 1.5% by weight of Mn, 0.01 to to 0.07% by weight of P, 0.006 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, not higher than 0.004% by weight of N, not higher than 0.003% by weight of O, 0.04 to 0.23% by weight of Nb, the amounts of Nb and C meeting the relationship given in formula (1), and a balance of Fe and unavoidable impurities, the cold-rolled steel sheet meeting the relationship given in formula (2): $1.0 ≦ (Nb % \times 12) / (C % \times 93) ≦ 3.0$
$\exp (ε) \times (5.29 \times \exp (ε) - 4.19) ≦ σ / σ_{0.2} ≦ \exp (ε) \times (5.64 \times \exp (ε) - 4.49)$

where 0.002 < ε ≦ 0.096, ε represents a true strain, σ_0.2 represents a 0.2% proof stress, and σ represents a true stress relative to ε;
and said steel having an average r-value meeting the relationship: 1.8 ≤ r-value ≤ 2.2; and a tensile strength of 340 MPa or more.
(2) The present invention provides the cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance defined in item (1) above, further comprising 0.0001 to 0.002% by weight of B.
(3) The present invention provides a galvanized steel sheet excellent in formability, panel shapeability and dent-resistance, which is obtained by applying a galvanizing to the cold-rolled steel sheet defined in item (1) or item (2) above.
(4) The present invention provides a method of manufacturing a cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance defined in item (1) or item (2) above, comprising the steps of:
- preparing a molten steel and continuously casting the steel;
- applying a hot-rolling process such that a finish rolling is performed at (Ar₃-100)°C or more and the rolled steel sheet is coiled at 500 to 700°C; and
continuously applying a cold-rolling treatment to the hot-rolled steel sheet and an annealing treatment to the cold-rolled steel band, the annealing treatment being carried out within a recrystallization temperature region of the ferrite phase.
(5) The present invention provides a method of manufacturing a galvanized steel sheet, the steel sheet being excellent in formability, panel shapeability and dent-resistance, defined in item (3) above, comprising the steps of:
- preparing a molten steel and continuously casting the steel;
- applying a hot-rolling process such that a finish rolling is performed at (Ar₃-100)°C or more and the rolled steel sheet is coiled at 500 to 700°C; and
continuously applying a cold-rolling treatment to the hot-rolled steel sheet and an annealing treatment and galvanizing treatment to the cold-rolled steel band, the annealing treatment being carried out within a recrystallization temperature region of the ferrite phase.

Brief Description of Drawings

FIGS. 1A and 1B show the relationships between the elongation El and (Nb × 12)/(C × 93) and between the r-value and (Nb × 12)/(C × 93) according to a first embodiment of the present invention;
FIG. 2 shows a method of evaluating the dent-resistance and the shapeability according to the first embodiment of the present invention;
FIG. 3 is a graph showing how P0.1 (dent-resistance load of a panel imparted with strains of 2%, 4% and 8%) and δ (spring back amount of 2% panel) are affected by σ/σ _0.2, exp(ε), and components of the steel composition according to the first embodiment of the present invention;
FIG. 4 is a graph showing how P0.1 (dent-resistance load of a panel imparted with strains of 2%, 4% and 8%) and δ (spring back amount of 2% panel) are affected by σ/σ_0.2, exp(ε), and components of the steel composition according to the first embodiment of the present invention;
FIG. 5 is a graph showing how P0.1 (dent-resistance load of a panel imparted with strains of 2%, 4% and 8%) and δ (spring back amount of 2% panel) are affected by σ/σ_0.2, exp ( ε ), and components of the steel composition according to the first embodiment of the present invention;
FIG. 6 is a graph showing how the finishing temperature and the coiling temperature have an influence on P0.1 (dent-resistance load of a panel imparted with strains of 2%), δ, and Wca (Arithmetic Average Waviness Height) according to the first embodiment of the present invention;

Best Mode of Carrying Out the Invention

The present inventors have conducted an extensive research in an attempt to obtain a cold-rolled steel sheet and a galvanized steel sheet, which are excellent in the surface properties, the resistance to natural aging and the dent-resistance required for the steel used for an outer panel of a motor car, and a method of manufacturing the same.
As a result, it has been found that the dent-resistance of a panel can be improved by an alloy design with an emphasis placed on the work-hardening behavior in a low strain region in the panel forming step, unlike the prior art in which the dent-resistance required for an outer panel of a motor car is improved by increasing the BH value. It has also been found that good surface properties and resistance to natural aging can be imparted to the steel sheet by positively suppressing the BH value. These findings have enabled the present inventors to develop a technology for stably manufacturing a cold-rolled steel sheet and a galvanized steel sheet, being excellent in the panel surface shapeability and the dent-resistance and exhibiting such a high tensile strength as at least 340 MPa.
Some embodiments of the present invention will now be described.

(First Embodiment)

Described in the following are the reasons for using the additives, the reasons for limiting the amounts of the additives, the reasons for limiting the tensile characteristics, and the reasons for limiting the manufacturing conditions according to the first embodiment of the present invention. In the following description, "%" represents "% by weight".

(1) Amounts of additives

C: 0.005 to 0.015%
A carbide formed together with Nb affects the work-hardening in a low strain region in panel forming step and contributes to an improvement of the dent-resistance. The particular effect cannot be obtained, if the C amount is less than 0.005%. Also, if the C amount exceeds 0.015%, the dent-resistance of the panel is certainly improved. However, the shape of the panel is impaired. It follows that the C amount should fall within a range of between 0.005 and 0.015%.
Si: 0.01 to 0.2%
Silicon is effective for strengthening the steel. However, if the Si amount is smaller than 0.01%, it is impossible to obtain a capability of the solid solution strengthening. On the other hand, if the Si amount is larger than 0.2%, the surface properties of the steel sheet are impaired. In addition, striped surface defects are generated after galvanizing. Therefore, the Si amount should fall within a range of between 0.01 and 0.2%.
Mn: 0.2 to 1.5%
Manganese serves to precipitate sulfide and to suppress deterioration of the hot ductility.
Also, Mn is effective for strengthening the steel. If the Mn amount is less than 0.2%, hot brittleness of the steel sheet is brought about, leading to a low yield. In addition, a high mechanical strength characterizing the steel sheet of the present invention cannot be obtained. Further, Mn, which relates to an improvement in the workability of the steel sheet, is necessary for controlling the morphology of the MnS in the hot rolling step. It should be noted that fine MnS particles are formed by the process of resolution and re-precipitation in the hot rolling step. These MnS particles impair the grain growth of the steel. However, if Mn is added in an amount not smaller than 0.2%, it is possible to eliminate the above-noted adverse effect produced by the presence of the MnS particles. In order to control effectively the morphology of the MnS particles in the hot rolling step, it is more desirable to add Mn in an amount of at least 0.45%. However, if the Mn amount exceeds 1.5%, the steel sheet is hardened and the panel shapeability of the steel sheet are deteriorated. It follows that Mn amount should fall within a range of between 0.2% and 1.5%.
P: 0.01 to 0.07%
Phosphorus is most effective for the solid solution strengthening of steel. If the P amount is smaller than 0.01%, however, P fails to exhibit a sufficient strengthening capability. On the other hand, if the P amount exceeds 0.07%, the ductility of the steel sheet is deteriorated. Also, a defective coating is brought about in the step of the alloying treatment during the continuous galvanizing process. It follows that the P amount should fall within a range of between 0.01 and 0.07%.
S: 0.006 to 0.015%
Sulfur, if added in an amount exceeding 0.015%, brings about hot brittleness of the steel. If the S amount is smaller than 0.006%, however, the peeling capability of the scale is impaired in the hot rolling step, and surface defects tend to be generated markedly. It follows that the S amount should fall within a range of between 0.006 and 0.015%.
Sol. Al: 0.01 to 0.08%
Aluminum serves to deoxidize the steel and fix N as nitride. If the Al amount is smaller than 0.01%, however, the deoxidation and the fixation of N cannot be achieved sufficiently. On the other hand, if the Al amount is larger than 0.08%, the surface properties of the steel sheet are deteriorated. Therefore, the Al amount should fall within a range of between 0.01 and 0.08%.
N ≦ 0.004%
Nitrogen is fixed in the form of AlN. If the N amount exceeds 0.004%, however, it is impossible to obtain a desired formability of the steel sheet. Naturally, the N amount should not exceed 0.004%.
O ≦ 0.003%
Oxygen forms inclusions involving oxides so as to adversely affect the grain growth of the steel. If the O amount exceeds 0.003%, the grain growth is impaired in the annealing step, resulting in failure to obtain satisfactory formability and panel shapeability. Naturally, the O amount should not exceed 0.003%. In order to suppress the O amount at 0.003% or less in the steel of the composition specified in the present invention, it is necessary to employ optimum manufacturing conditions. For example, the sol. Al should be controlled at a suitable level, and O should be controlled up in the process steps after the secondary refining process.
Nb: 0.04 to 0.23%
Niobium is bonded to C to form fine carbide particles. These fine carbide particles affect the work-hardening behavior in the panel forming step so as to contribute to an improvement in the dent-resistance of the panel. If the Nb amount is smaller than 0.04%, however, it is impossible to obtain the particular effect. On the other hand, if the Nb amount exceeds 0.23%, the panel shapeability such as the spring back and the surface deflection is deteriorated, though the dent-resistance is certainly improved. Naturally, the Nb amount should fall within a range of between 0.04 and 0.23%. $(Nb \times 12) / (C \times 93) : 1.0 to 3.0$
In the present invention, it is absolutely necessary to control (Nb × 12)/(C × 93) in order to improve the formability of the steel sheet. If the value of (Nb × 12)/(C × 93) is less than 1.0, C cannot be fixed sufficiently, resulting in failure to obtain a high r-value and a high ductility aimed at in the present invention. If the value exceeds 3.0, however, the amount of Nb forming a solid solution is rendered excessively high, leading to a low ductility. In this case, it is impossible to obtain a formability aimed at in the present invention. It follows that the value of (Nb × 12)/(C × 93) should fall within a range of between 1.0 and 3.0. FIGS. 1A and 1B show the relationships between the elongation El and (Nb × 12)/(C × 93) and between the r-value and (Nb × 12)/(C × 93).
In order to improve the dent-resistance as desired, it is desirable to add B in an amount given below in addition to the additives described above.
B: 0.0001 to 0.002%
If B is added, the grain boundary is strengthened so as to improve the resistance to the secondary working brittleness. Also, the ferrite grains are diminished so as to ensure an absolute value of the yield strength and, thus, to improve the dent-resistance. However, these effects cannot be obtained if the B amount is smaller than 0.0001%. On the other hand, if the B amount exceeds 0.002%, the yield point is increased and, thus, the panel shapeability is impaired. It follows that the B amount should fall within a range of between 0.0001 and 0.002%.

(2) Tensile Characteristics

exp ( ε ) × (5.29 × exp( ε ) - 4.19) ≦ σ/σ_0.2 ≦ exp( ε ) × (5.64 × exp( ε ) - 4.49), where 0.002 < ε ≦ 0.096, ε represents a true strain, σ_0.2 represents a 0.2% proof stress, and σ represents a true stress relative to ε.
In the steel sheet of the present invention comprising the additives described in item (1) above, Fe and unavoidable impurities, a ratio of flow stress σ obtained by a tensile test under the condition that a true strain ε is larger than 0.002 and not larger than 0.096, i.e., 0.002 < ε ≦ 0.096, to a 0.2% proof stress σ_0.2, i.e., σ/σ_0.2, should fall within a range of between exp( ε ) × (5.29 × exp ( ε ) - 4.19) and exp( ε ) × (5.64 × exp( ε ) - 4.49).
If the ratio σ/σ_0.2 is lower than the lower limit noted above, the dent-resistance load under the conditions of 2%P0.1, 4%P0.1, 8%P0.1 is as high as 160 to 190N as shown in FIGS. 3 to 5. For measuring the dent-resistance load, a steel sheet is formed to a model panel shown in FIG. 2 with strain of 2%, 4% or 8% imparted to the steel sheet, followed by applying a heat treatment at 170°C for 20 minutes. Then, measured is a load required for imparting a residual displacement of 0.1 mm to the model panel. However, the spring back δ (measured for a panel having a strain of 2%) is as large as 7 to 10% so as to impair the panel shapeability, if the ratio σ/σ_0.2 is lower than the lower limit noted above. On the other hand, if the ratio σ/σ_0.2 is higher than the upper limit noted above, the spring back δ is as small as 2 to 5% to improve the panel shapeability. However, the dent-resistance is as low as 140 to 175N. In other words, the dent-resistance cannot be improved. Under the circumstances, the ratio σ/σ_0.2 should fall within a range of between exp( ε ) × (5.29 × exp ( ε ) - 4.19) and exp ( ε ) × (5.64 × exp( ε ) - 4.49).
A cold-rolled steel sheet and a galvanized steel sheet excellent in the panel surface properties and the dent-resistance required for the steel used for an outer panel of a motor car can be obtained by controlling the additive components as described in item (1) above and the tensile characteristics as described in item (2) above.
The steel sheet exhibiting the particular properties can be manufactured as follows.

(3) Steel Sheet Manufacturing Process

In the first step, steel of the composition given in item (1) above is melted. A converter method is generally employed for melting the steel composition, or an electric furnace method can also be employed. After the molten steel is continuously cast to obtain a slab, the slab is heated immediately after the casting, or after the slab is once cooled, for applying a hot rolling. The hot rolling is performed under the conditions that the finishing temperature is set at temperature not less than (Ar₃-100)°C and that the coiling temperature is set at 500°C to 700°C. If the finishing temperature is lower than (Ar₃-100)°C, 2%P0.1, i.e., the dent-resistance load of the panel imparted with 2% of strain) is as low as 140 to 150N, as shown in FIG. 6. In other words, the dent-resistance of the panel cannot be improved. Also, where the coiling temperature is lower than 500°C, the value of 2%P0.1 is high, i.e., 155 to 165N. However, the value of δ, i.e., the spring back amount of the panel imparted with 2% of strain, is as large as 8% to 10%, leading to a poor shapeability. On the other hand, where the coiling temperature exceeds 700°C, the value of Wca (i.e., Arithmetic Average Waviness Height; measuring length of 25 mm; average of the values measured at 10 optional points around the apex of the panel) is large, which falls within a range of between a value exceeding 0.4 µm and 0.6 µm, leading to a poor panel shapeability. It follows that the finishing temperature should be not lower than (Ar₃-100)°C and that the coiling temperature should fall within a range of between 500°C and 700°C.
In the next step, the hot-rolled steel band is subjected to pickling, cold-rolling and, then, a continuous annealing. Alternatively, galvanizing is applied after the continuous annealing. The cold-rolling reduction should desirably be at least 70% in order to improve the deep drawability (r-value) of the steel sheet. The annealing should desirably be carried out within a recrystallization temperature region of the ferrite phase. Further, the coating employed in the present invention is not limited to continuous galvanizing. Specifically, even if a surface treatment such as coating with zinc phosphate or an electrolytic galvanizing is applied to the steel sheet obtained by the continuous annealing, no problem is brought about in the characteristics of the resultant steel sheet.
Some Examples of the present invention will now be described to demonstrate the prominent effects produced by the present invention.

Examples:

(Example 1)

Molten steel of the composition shown in Table 1 were prepared in a laboratory, followed by continuously casting the steel to prepare a slab having a thickness of 60 mm. Samples Nos. 1 to 7 shown in Table 1 represent the steel of the composition specified in the present invention, with samples Nos. 8 to 15 denoting the steel for Comparative Examples. The slab was treated by a blooming mill to reduce the thickness of the steel sheet to 30 mm, followed by heating the steel sheet at 1050°C for 1.5 hours under the atmosphere for the hot rolling treatment (by roughing mill). After the rough rolling, a finish rolling was applied at 900°C, followed by applying a coiling simulation at 630°C so as to obtain a hot rolled sheet having a thickness of 3 mm. Then, the hot rolled steel sheet was pickled, followed by applying a cold rolling to reduce the thickness of the steel sheet to 0.8 mm and subsequently applying a continuous annealing at 840°C for 90 seconds. Alternatively, after the continuous annealing at 840°C for 90 seconds, a galvanizing was applied at 460°C, followed by applying an alloying treatment at 530°C. Further, 1.0% of temper rolling was applied to the annealed steel sheet or the galvanized steel sheet so as to prepare samples for the experiments. These samples were used for the tensile test (test piece of JIS No. 5; tested in accordance with the method specified in JIS Z 2241) and for measuring the r-value, 2% BH amount (measured in accordance with the method specified in JIS G 3135), and ΔYPel (restoring amount of yield point elongation of the sample stored at 25°C for 6 months after the temper rolling). Also, the sample was formed into the model panel shown in FIG. 2 (formed at three levels of the forming strain of 2, 4 and 8%). After a heat treatment was applied at 170°C for 20 minutes, the dent-resistance of the panel and the shapeability of the panel were examined. The dent-resistance was evaluated under a load of P0.1, in which 0.1 mm of residual displacement was imparted to the panel (in the following description, expressions of 2%P0.1, 4%P0.1 and 8%P0.1 are used for denoting the panel imparted with strain of 2, 4 and 8%, respectively). On the other hand, the panel shapeability was evaluated by the spring back amount δ and Wca: Arithmetic Average Waviness Height (JIS B 0610). The spring back amount δ was defined by using a curvature radius R' of the panel imparted with 2% of strain and a curvature radius R of the press mold, i.e., δ was defined by (R'/R-1) × 100. Where δ was not larger than 6%, i.e., δ ≦ 6%, the evaluation was marked by ○. Where δ was 7 to 10%, i.e., δ = 7 to 10%, the evaluation was marked by Δ. Further, where δ was larger than 10%, i.e., δ > 10%, the evaluation was marked by x. On the other hand, the surface waviness height each having a length of 25 mm were measured at optional 10 points in the vicinity of the apex of the panel, and the average measured value is denoted by Wca. Where Wca was not larger than 0.2 µm, i.e., Wca ≦ 0.2 µm, the evaluation was marked by ○. Where Wca was larger than 0.2 µm but not larger than 0.4 µm, i.e., 0.2 µm < Wca ≦ 0.4 µm, the evaluation was marked by Δ. Further, where Wca was larger than 0.4 µm and not larger than 0.6 µm, i.e., 0.4 µm < Wca ≦ 0.6 µm, the evaluation was marked by x.
Table 2 shows the results of measurements and evaluations. In samples Nos. 1 to 7 each having a composition falling within the range specified in the present invention, the value of the elongation El was as large as 41.6% to 45.0%. The average r-value, i.e., (r0 + 2r45 + r90)/4, was as large as 1.80 to 2.20. The value of ΔYPel was 0% in any of the samples of the present invention. On the other hand, the spring back amount δ and the Waviness Height Wca were small, i.e., 3% to 5% and 0.09 µm to 0.17 µm, respectively, supporting a good panel shapeability. Further, the dent-resistance P0.1 of the panel imparted with strains of 2%, 4% and 8% was as high as 158N to 193N.
On the other hand, the steel samples Nos. 8 to 15, each having a composition failing to fall within the range specified in the present invention, did not satisfy simultaneously the formability, the shapeability, and the dent-resistance. Specifically, each of Comparative Samples Nos. 8 and 9 exhibited a 2% BH as high as 33 MPa to 42 MPa and a ΔYPel of 0.9% to 2.2%, indicating that these samples were not satisfactory in the resistance to natural aging. Also, the dent-resistance P0.1 under strains of 2% to 8% was found to be 165N to 193N, supporting a high dent-resistance. However, each of these Comparative samples was low in each of the elongation El and the r-value and large in each of the spring back amount δ and the value of Wca, supporting that these Comparative samples were not satisfactory in formability and shapeability. Comparative steel sample No. 10 was high in the elongation El and the r-value, and low in δ and Wca, supporting that this sample was satisfactory in each of formability and shapeability. However, the dent-resistance load P0.1 under strains of 2% to 8% was as low as 148 to 172N. Comparative steel sample No. 11 was high in σ_0.2, which was 265 MPa to 270 MPa, supporting that this sample was satisfactory in dent-resistance. However, the steel sample was high in each of δ and Wca, supporting a poor panel shape. Further, this steel sample was low in the elongation El and the r-value. Each of Comparative steel samples Nos. 12 and 13 was high in the r-value, which was 2.02 to 2.20, but low in El, which was 35.8% to 36.8%. Also, these steel samples were somewhat high in σ_0.2, which was 240 MPa to 250 MPa, supporting a satisfactory dent-resistance. However, since the values of δ and Wca were large, the panel shape of each of these Comparative steel samples was not satisfactory. Further, each of Comparative steel samples Nos. 14 and 15 was low in El, which was 37.0 to 38.5%, and in the r-value, which was 1.51 to 1.69, supporting a poor shapeability.

(Example 2)

A molten steel having a composition of steel sample No. 2 of the present invention shown in Table 1 was prepared by melting and casting in a laboratory, followed by casting the molten steel to prepare a slab having a thickness of 50 mm. The slab was treated by a blooming mill to reduce the thickness of the steel sheet to 25 mm, followed by heating the steel sheet at 1250°C for 1 hour under the atmosphere and subsequently applying a hot rolling treatment to reduce the thickness of the steel sheet to 2.8 mm. The finishing temperature and the coiling temperature in the hot rolling treatment were changed within ranges of 770°C to 930°C and 450°C to 750°C, respectively. Then, the hot rolled steel sheet was pickled, followed by applying a cold rolling to reduce the thickness of the steel sheet to 0.75 mm and subsequently applying a soaking treatment at 825°C for 90 seconds. Further, a temper rolling was applied at an elongation of 1.2%. The mechanical characteristics and the panel characteristics of the thin steel sheet thus prepared were examined as in Example 1. Table 3 shows the results. The finishing temperature for each of steel samples Nos. 1 to 3 of the present invention was lower than (Ar₃-100)°C. Also, each of these steel samples exhibited a low P0.1 under strains of 2% to 8%, i.e., 139N to 159N, and a high Wca, i.e., 0.35 µm to 0.40 µm, indicating that these steel samples were poor in the dent-resistance and in the shapeability. Further, the r-value for these steel samples was as low as 1.69 to 1.77. The coiling temperature for each of steel samples Nos. 7 and 12 was lower than 500°C. Also, each of these steel samples exhibited a high σ_0.2 value, i.e., 243 MPa and 248 MPa, respectively, supporting a good dent-resistance. However, the δ value was as high as 8% and the Wca value was as high as 0.30 µm, indicating that these steel samples were poor in the panel shape. The coiling temperature for each of steel samples Nos. 11, 15 and 18 was higher than 700°C. Also, each of these steel samples exhibited a low σ_0.2 value, i.e., 210 MPa to 216 MPa, and such a low δ value of 2%. However, the Wca value was as high as 0.42 µm to 0.43 µm. Also, the dent-resistance load was low in each of these steel samples. On the other hand, each of steel samples Nos. 4-6, 8-10, 13, 14, 16 and 17, which fell within the scopes specified in the present invention in respect of the finishing temperature and the coiling temperature, was found to be satisfactory in each of the formability, the dent-resistance and the shapeability.
As described above, the present invention makes it possible to manufacture stably a cold-rolled steel sheet and a galvanized steel sheet satisfying the dent-resistance of a panel, the surface shapeability and resistance to natural aging and having a tensile strength of 340 MPa or more, which are required for steels used for an outer panel of a motor car, by specifying the steel composition, the tensile characteristics and the manufacturing conditions. It follows that the present invention is highly valuable in the steel industries and in the motor car industries.

Claims

A cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance, comprising 0.005 to 0.015% by weight of C, 0.01 to 0.2% by weight of Si, 0.45 to 1.5% by weight of Mn, 0.01 to 0.07% by weight of P, 0.006 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, not higher than 0.004% by weight of N, not higher than 0.003% by weight of O, 0.04 to 0.23% by weight of Nb, the amounts of Nb and C meeting the relationship given in formula (1), and a balance of Fe and unavoidable impurities, said cold-rolled steel sheet meeting the relationship given in formula (2): $1.0 ≦ (Nb % \times 12) / (C % \times 93) ≦ 3.0$
$\exp (ϵ) \times (5.29 \times \exp (ϵ) - 4.19) ≦ σ / σ_{0.2} ≦ \exp (ϵ) \times (5.64 \times \exp (ϵ) - 4.49)$

where 0.002 < ε ≦ 0.096, ε represents a true strain, σ_0.2 represents a 0.2% proof stress, and σ represents a true stress relative to ε;
and said steel having an average r-value meeting the relationship: 1.8 ≤ r-value ≤ 2.2; and a tensile strength of 340 MPa or more.
The cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance according to claim 1, further comprising 0.0001 to 0.002% by weight of B.
A steel sheet coated with a molten zinc excellent in formability, panel shapeability and dent-resistance, which is obtained by applying a galvanizing to the cold-rolled steel sheet defined in claim 1 or 2.
A method of manufacturing a cold-rolled steel sheet excellent in formability, panel shapeability and dent-resistance defined in claim 1 or 2, comprising the steps of:
preparing a molten steel and continuously casting said steel;

applying a hot-rolling treatment such that a finish rolling is performed at (Ar₃-100)°C or more to form a hot-rolled steel band and the rolled steel band is coiled at 500 to 700°C; and

continuously applying a cold-rolling treatment to the hot-rolled steel sheet and an annealing treatment to the cold-rolled steel band, the annealing treatment being carried out within a recrystallization temperature region of the ferrite phase.
A method of manufacturing a galvanized steel sheet, said steel sheet being excellent in formability, panel shapeability and dent-resistance, defined in claim 3, comprising the steps of:
preparing a molten steel and continuously casting said steel;

applying a hot-rolling treatment such that a finish rolling is performed at (Ar₃-100) °C or more to form a hot-rolled steel band and the rolled steel band is coiled at 500 to 700°C; and
continuously applying a cold-rolling treatment to the hot-rolled steel sheet and an annealing treatment and galvanizing treatment to the cold-rolled steel band, the annealing treatment being carried out within a recrystallization temperature region of the ferrite phase.