JPH06122939A - Cold rolled steel sheet or galvanized cold rolled steel sheet excellent in baking hardenability and formability and production thereof - Google Patents

Abstract

PURPOSE:To impart baking hardenability and formability to a steel sheet and also to impart excellent cold nonaging properties thereto by rolling steel having a specified compsn. independently or compositely added with Nb and Ti and subjecting it to continuous annealing and galvanizing under specified conditions. CONSTITUTION:The compsn. of the steel is constituted of a one contg., by weight, 0.0005 to 0.0070% C, 0.001 to 0.8% Si, 0.01 to 4.0% Mn, 0.005 to 0.15% P, 0.0010 to 0.015% S, 0.005 to 0.1% Al and 0.0003 to 0.0060% N and furthermore contg. one or more kinds of 0.003 to 0.1% Ti and 0.003 to 0.1% Nb, and the balance Fe with inevitable impurities. This steel slab is subjected to finishing in hot rolling at the Ar3-100 deg.C or above, is coiled at a room temp. to 750 deg.C and is subjected to cold rolling at >=60% draft. Next, it is subjected to continuous annealing at the annealing temp. of the Ae3 transformation point or above. Moreover, it is applied with galvanizing. In this way, the cold rolled steel sheet or galvanized cold rolled steel sheet having a low temp. transformed product single phase structure and excellent in press formability can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet or hot-dip galvanized cold-rolled steel sheet excellent in bake hardenability and non-aging at room temperature, and a method for producing them. The cold-rolled steel sheet according to the present invention is used for automobiles, household electric appliances, buildings and the like. Further, it includes both a cold-rolled steel sheet in a narrow sense which is not surface-treated and a cold-rolled steel sheet which has been subjected to surface treatment such as Zn plating, alloyed Zn plating or electric Zn plating for rust prevention. Since the steel sheet according to the present invention is a steel sheet having both strength and workability, it is possible to reduce the plate thickness in use, that is, to reduce the weight, in use. Therefore, it is considered that it can contribute to global environment conservation.

[0002]

2. Description of the Related Art With the recent progress in vacuum degassing of molten steel, it has become easier to produce ultra-low oxygen steel, and the demand for ultra-low oxygen steel sheet having good workability is increasing more and more.
Among them, for example, the ultra-low carbon steel sheet containing the composite addition of Ti and Nb disclosed in JP-A-59-31827 and JP-A-59-38337 has extremely good workability, Since it has both paint bake hardening (BH) properties and excellent hot dip galvanizing properties, it is becoming an important position. However, the BH amount does not exceed the level of a normal BH steel plate, and further BH
It has a drawback in that the non-aging at room temperature cannot be ensured when an amount is given.

On the other hand, many attempts have heretofore been made to increase the strength while ensuring the workability. In particular,
When the tensile strength is 35 to 50 kgf / mm ² , P, Si, etc. are added to the steel and the strength is increased by utilizing the solid solution strengthening mechanism of these. For example, JP-A-59-3
In Japanese Patent No. 1827 and Japanese Patent Laid-Open No. 59-38337, S is mainly used for an extremely low oxygen steel sheet to which Ti and Nb are added.
Disclosed is a method for producing a high-strength cold-rolled steel sheet having a tensile strength of up to 45 kgf / mm ² by adding i and P. Japanese Patent Office Sho 5
7-57945 is a typical prior art relating to a method for producing a high-strength cold-rolled steel sheet by adding P to a Ti-added ultra-low carbon steel.

On the other hand, it is also known to use Mn or Cr as a solid solution strengthening element. JP-A-63-19014
No. 1 and JP-A-64-62440 disclose Mn.
Was added to the Ti-containing ultra-low carbon steel sheet, and
No. 42742 and Japanese Patent Publication No. 57-57945 described above disclose a technique of adding Mn and Cr to a Ti-added ultra-low carbon steel, but the structure after annealing is referred to as a low-temperature transformation product single-phase structure. It is not added to do so.

Further, Japanese Patent Laid-Open No. 2-111841 discloses that ultra-low carbon steel containing Ti has a content of 1.5% or more and 3.5% or more.
Disclosed are a good workability cold-rolled steel sheet and a hot-dip galvanized steel sheet having bake hardenability containing less than Mn. By adding a large amount of Mn, the purpose is to improve the operation stability of hot rolling and the homogeneity of the metal structure due to the reduction of the Ar ₃ transformation point. Further, for the purpose of further improving the ductility, 0.2 to 1.
Additions up to 0% are also disclosed. However, the addition of Mn and Cr is not for making the structure after annealing a low-temperature transformation product single-phase structure. Therefore, the amount of BH does not deviate from the normal level here as well, and the higher BH and the non-aging at room temperature have not been achieved at the same time.

In contrast to the steel sheet having the ferrite single phase structure as described above, a steel sheet having a composite structure is also known. By adding alloying elements such as Si, Mn, and Cr to low carbon aluminum killed steel and optimizing the continuous annealing temperature and the subsequent cooling rate, so-called Dual Phase steel (mixed with ferrite phase and martensite phase ( D
What is called P steel) is a typical example. Such DP steel has a high yield strength but a very low yield ratio (Y
It is known to have R) and have a high BH at room temperature and non-aging. However, it has a defect that the average r value is as low as about 1.0 and the deep drawability is inferior. By the way, the manufacturing method of such a cold rolled steel sheet is described in JP-B-53-3.
9368, JP-A-50-75113, JP-A-51-
39524, Japanese Patent Publication No. 62-56209, Japanese Patent Publication No. 62
It is disclosed in Japanese Patent Publication No.-40405.

In contrast to these composite steel sheets made of low carbon aluminum killed steel, Japanese Patent Publication Nos. 3-2224 and 3-21611 disclose composite steel sheets made of ultra-low carbon steel. Has been done. These are ultra-low carbon steels in which a large amount of Nb and B, and further Ti, are added in combination to make the structure after annealing into a composite structure of a ferrite phase and a low-temperature transformation forming phase. It is intended to obtain a cold rolled steel sheet having both aging properties. However, as a result of diligent studies by the present inventors, Nb,
It has become clear that the following problems occur when the composite structure is formed by adding B, or Ti in some cases.

1) Since the temperature range of α + γ is extremely narrow, the structure changes in the plate thickness, the plate width, and the longitudinal direction of the plate.
As a result, there are cases in which the materials vary widely, and there are cases where a composite structure does not form due to changes in the annealing temperature of several degrees Celsius, and manufacturing becomes extremely unstable. 2) Further, it is difficult to apply BH of 5 kgf / mm ² or more, and if the BH amount exceeds 5 kgf / mm ² , YP-El after artificial aging exceeds 0.2%, and the room temperature is Non-prescription is not secured. JP-A-3-27
No. 7741 discloses that a steel obtained by adding Nb, B, Ti, Mn, and Cr to an ultra-low carbon steel is Ac ₁ -50 ° C. or higher Ac ₁
By annealing at a temperature below the transformation point, the structure is reduced to 5
There is disclosed a technique of providing a steel sheet having both BH property, non-aging at room temperature, and workability by forming a composite structure composed of acicular-ferrite and ferrite having a volume ratio of not more than%.

However, as a result of detailed investigations by the present inventors, it became clear that there were the following problems. That is, it is difficult to provide BH of the conventional level or more, that is, 5 kgf / mm ² or more, and the BH amount of 5k is 5k or less in the steel sheet having a composite structure with the volume fraction of the second phase of 5% or less.
When it exceeds gf / mm ² , YP-El after artificial aging is 0.
Since it exceeds 2%, it has been found that it is extremely difficult to ensure non-aging at room temperature. It is considered that this is because the volume fraction of the second phase is small, and therefore the density of mobile dislocations introduced into ferrite is not sufficient. As described above, some proposals have been made for the steel sheet having a microstructure in the ultra-low carbon steel, but the BH amount does not deviate from the conventional level at all, and the room temperature non-aging property slightly exceeds the conventional level. It stayed around.

[0010]

Steel sheets used for automobile panels and the like are strictly required to have good surface formability without causing springback or surface distortion after pressing. By the way, it is well known that the lower the yield strength is, the more preferable the surface shape property is. Further, the steel sheet after press forming is required to have dent resistance. The dent resistance means resistance to permanent dent deformation of a steel plate when a stone or the like hits the assembled automobile. When the plate thickness is constant, the dent resistance property becomes better as the deformation stress after press working and paint baking becomes higher. Therefore, when a steel plate having the same yield strength is considered, the higher the paint bake hardenability, the better the dent resistance.

From the above, a desirable steel sheet used for an automobile panel or the like is a steel sheet which does not have a high yield strength and has a high paint bake hardening ability and non-aging at room temperature. Of course, it is also necessary to excel in processability such as average r value (deep drawing property) and elongation (protrusion property). The present invention satisfies the above demands, and particularly with respect to paint bake hardening ability, it is higher than the conventional level, that is, 5 kg.
It is an object of the present invention to provide an unprecedented cold-rolled steel sheet capable of imparting a high BH amount of f / mm ² or more according to the purpose and also having room-temperature non-aging properties.

[0012]

[Means for Solving the Problems] The present inventors have earnestly conducted research in order to achieve the above-mentioned goals, and obtained the following unprecedented findings. That is, Nb, T
Regarding the microstructure and tensile properties after cold rolling, annealing, and temper rolling with the addition of B, Mn, and Cr based on the ultra-low carbon steel with i added alone or in combination, especially when annealed in the α + γ2 phase region The difference from the case of annealing in the γ single phase region was investigated. When annealed in the α + γ2 phase region, a composite structure composed of ferrite and a low temperature transformation product could be obtained.
1) The temperature range for forming a composite structure is extremely narrow, and therefore the variation in the material during manufacturing is extremely large. 2) Further, in such steel, BH is 5 k
It is not only difficult to give gf / mm ² or more,
When BH is 5 kgf / mm ² or more, the yield point elongation (YP-El) after artificial aging exceeds 0.2%, and normal temperature non-aging cannot be secured.

On the other hand, when annealed in the γ single-phase region, 1) annealing is performed in the γ single-phase region, so that the structure after annealing can be a low-temperature transformation product single-phase structure. The material variation over time is extremely small. The low-temperature transformation product as used herein includes all structures other than so-called polygonal ferrite obtained when annealed in the ferrite single-phase temperature range. 2) In the γ region, since the carbides such as TiC and NbC formed during hot rolling or winding are redissolved, it is possible to easily give a BH property of 5 kgf / mm ² or more. 3) Even if BH Even when the amount was about 10 kgf / mm ² , YP-El after artificial aging did not exceed 0.2%, and it was found that both excellent non-aging at room temperature and BH property were excellent. The cause of this is not necessarily clear, but it is considered that the reason is that the mobile dislocation density introduced into the formed low temperature transformation product is considerably high.

The present invention is an unprecedented new steel plate constructed on the basis of such an idea and new knowledge, and its gist is as follows. (1) C: 0.0005 to 0.0070% by weight,
Si: 0.001-0.8%, Mn: 0.01-4.0
%, P: 0.005-0.15%, S: 0.0010
0.015%, Al: 0.005-0.1%, N: 0.
0003 to 0.0060%, Ti: 0.003
.About.0.1% and Nb: 0.003 to 0.1%, at least one of which is composed of the balance Fe and unavoidable impurities, and has a low-temperature transformation product single-phase structure. Cold-rolled steel sheet or hot-dip galvanized cold-rolled steel sheet excellent in bake hardenability and formability. (2) B: The cold-rolled steel sheet or hot-dip galvanized cold-rolled steel sheet according to (1), containing less than 0.0030%. (3) The cold-rolled steel sheet or hot-dip galvanized cold-rolled steel sheet according to (1) or (2), which contains Cr: 0.01 to 3.0%.

(4) The slab is hot-rolled at a temperature of (Ar ₃ -100) ° C. or higher, wound at room temperature to 750 ° C., and cold-rolled at a rolling ratio of 60% or higher,
The method for producing a cold rolled steel sheet according to any one of (1) to (3), characterized in that the annealing temperature in continuous annealing is set to Ae ₃ transformation point or higher. (5) The slab is finished by hot rolling at a temperature of (Ar ₃ −100) ° C. or higher, wound at a temperature of room temperature to 750 ° C., cold rolled at a rolling ratio of 60% or higher, and the annealing temperature is changed. The method for producing a hot-dip galvanized cold-rolled steel sheet according to any one of (1) to (3), characterized by performing in-line annealing hot-dip galvanizing with an Ae ₃ transformation point or higher.

[0016]

The reason why the steel composition and manufacturing conditions are limited as described above in the present invention will be further described. C: C is an extremely important element that determines the material properties of the product. The present invention is premised on a vacuum degassed ultra low carbon steel, but if C is less than 0.0005%, the grain boundary strength decreases, the secondary workability deteriorates, and the manufacturing cost significantly increases. Therefore, the lower limit is made 0.0005%. On the other hand, if the amount of C exceeds 0.0070%, the formability is deteriorated and normal temperature non-aging cannot be ensured, so the upper limit is made 0.0070%. In order to exhibit sufficient paint bake hardening performance and non-aging property, it is preferable that the content of C: 0.0020 or more and less than 0.0050%.

Si: Si is known as an element that inexpensively increases the strength, and the addition amount thereof changes depending on the target strength level, but if the addition amount exceeds 0.8%, the yield strength increases. If too much, surface distortion occurs during press molding. Further, the Ae ₃ transformation point rises, and the annealing temperature for obtaining the low temperature transformation product single-phase structure becomes extremely high. Further, problems such as deterioration of chemical conversion treatment property, deterioration of adhesion of hot dip galvanizing, and deterioration of productivity due to delay of alloying reaction occur. The lower limit is 0.001% from the viewpoint of steelmaking technology and cost.

Mn, Cr: Mn and Cr are the most important elements in the present invention. That is, Mn and Cr are
It does not require too high a temperature to obtain the low temperature transformation product single phase structure because it lowers the Ae ₃ transformation point. Moreover, the low-temperature transformation product single-phase structure steel sheet obtained by utilizing Mn and Cr cannot obtain 5 kg normally.
A BH amount of f / mm ² or more can be easily applied,
Even when it has a BH property of 5 kgf / mm ² or more, it exhibits extremely excellent non-aging at room temperature. This property is peculiar to the low temperature transformation product single phase steel sheet, and is a characteristic that cannot be obtained with a ferrite single phase steel sheet or a composite steel sheet obtained by combining several kinds of Nb, B and Ti. Further, it is important to note that in ordinary steel, the r value is significantly deteriorated when annealed in the α + γ2 phase region or the γ single phase region, but in the steel in which Mn and Cr are positively added, That is, r is hardly deteriorated even if annealed in the γ single phase region.

Further, Mn and Cr are effective solid solution strengthening elements that increase the strength without increasing the yield strength, and also have the effect of improving the chemical conversion treatment property and the hot dip galvanizing property. The lower limit of Mn is set to 0.01% from the viewpoint of steelmaking technology, but in order to obtain the above-mentioned effects, it is preferable to add 0.3% or more. On the other hand, if it exceeds 4.0%, a preferable low temperature transformation product single phase structure cannot be obtained, so the upper limit is made 4.0%.
Also, when Cr is added, the above effect is not exhibited if it is less than 0.01%, so the lower limit is made 0.01%,
If it exceeds 3.0%, a good structure cannot be obtained, so the upper limit is made 3.0%. When Cr is added, it is desirable to add 0.1% or more.

P: P is known as an element that can increase the strength at a low cost like Si, and the amount of addition changes depending on the target strength level. If the amount added exceeds 0.15%, the annealing temperature for obtaining the low-temperature transformation product single-phase structure will be remarkably high, and the yield strength will increase too much, causing a defective surface shape during pressing. Further, during continuous hot dip galvanizing, the alloying reaction becomes extremely slow, which lowers productivity. Further, the secondary workability also deteriorates. Therefore, the upper limit value is set to 0.15%. The lower limit is 0.005% from the viewpoint of steelmaking technology and cost.

S: The lower the S content, the better, but 0.00
If it is less than 1%, the manufacturing cost is remarkably increased, so this is made the lower limit. On the other hand, if it exceeds 0.015%, MnS
Is deposited in large numbers and the workability deteriorates, so this is made the upper limit. Al: Al is used for deoxidation and for fixing N when Ti is not added, but if less than 0.005%, Ti and N are used.
The yield of b decreases. On the other hand, if it exceeds 0.1%, the cost increases, so the upper limit is made 0.1%.

Ti, Nb: Ti, Nb has the role of securing the workability and non-aging property of the ultra-low carbon steel by fixing all or part of N, C, S. Further, the crystal grains of the hot-rolled sheet are refined to improve the workability of the product sheet. T
If i and Nb are less than 0.003%, the effect of addition does not appear, so this is made the lower limit. On the other hand, if it exceeds 0.1%, the alloy cost will be significantly increased, so the upper limit is made 0.2%.

N: N is preferably low. However, 0.
If it is less than 0003%, a significant cost increase is incurred.
On the other hand, if it is too much, a large amount of Ti, Nb, Al is required or the workability is deteriorated, so 0.0060% is made the upper limit. B: B is effective in preventing secondary work embrittlement, and is also effective in obtaining a low-temperature transformation product single-phase structure.
Add less than 30%. However, if it exceeds 0.0030%, the workability is deteriorated, so the upper limit is 0.0030.
Less than%. The strength of the steel sheet according to the present invention is 2
Although all those having a pressure of 5 kgf / mm ² or more are targeted, it is desirable that the strength be 35 kgf / mm ^{2 in} order to obtain a good r value even as a low temperature transformation product single phase structure.

Next, the reasons for limiting the manufacturing conditions will be described. The finishing temperature for hot rolling needs to be Ar ₃ -100 ° C. or higher from the viewpoint of ensuring workability of the product sheet. The winding temperature is from room temperature to 750 ° C. The present invention is characterized in that the product material is not so much affected by the hot rolling coiling temperature. The upper limit of the coiling temperature of 750 ° C. is determined from the viewpoint of preventing a decrease in yield due to material deterioration at both ends of the coil. Cold rolling may be performed under normal conditions, and the rolling rate is set to 60% or more for the purpose of ensuring deep drawability after annealing.

The annealing temperature of the continuous hot-dip galvanizing equipment of the continuous annealing or in-line annealing system is set to the Ae ₃ transformation point or higher. If the annealing temperature is lower than the Ae ₃ transformation point, the low-temperature transformation product single-phase structure, which is a feature of the present invention, cannot be obtained.
The cooling conditions after soaking during annealing are not particularly specified, but when a high r value is required, the annealing temperature and 600 ° C. to 700 ° C.
It is desirable to cool at an average cooling rate of 30 ° C./s or less. When extremely high BH property is required, it is desirable to cool the temperature range between the annealing temperature and 600 ° C to 700 ° C at an average cooling rate of 30 ° C / s or more. However, neither is an essential condition.

Thus, according to the present invention, a steel sheet having a high paint bake hardening ability and room-temperature non-aging characteristics, and having excellent workability such as average r value (deep drawing characteristic) and elongation (protrusion characteristic) is obtained. Obtainable. In particular, regarding the coating bake hardenability, it is possible to provide a high BH amount of 5 kgf / mm ² or more as needed, and it is possible to provide a cold-rolled steel sheet having room-temperature non-aging resistance.

Next, the present invention will be described with reference to examples.

【Example】

Example 1 Steels having the compositions shown in Table 1 were melted and slab heating temperature 1
Hot rolling was performed at 180 ° C., finishing temperature 910 ° C., and winding temperature 600 ° C. to obtain a steel strip having a thickness of 4.0 mm. 80% after pickling
Cold rolling with a reduction ratio of 0.8 mm to form a cold rolled sheet with a thickness of 0.8 mm
Then heating rate 10 ° C / s, soaking 830-980 ° C x 5
Continuous annealing was performed at 0 s and an average cooling rate of 80 ° C./s. Further, temper rolling with a rolling reduction of 0.5% is performed, and JIS5
The No. 10 tensile test piece was sampled and subjected to a tensile test. The results of the tensile test are summarized in Table 2. Here, the BH amount is the amount of increase in stress when a tensile test is performed again after heat treatment equivalent to coating baking of 170 ° C. × 20 minutes for a 2% pre-strained material (from the lower yield stress during the re-tension test to 2 The value obtained by subtracting% deformation stress). The secondary processing embrittlement transition temperature was determined by punching a blank with a diameter of 50 mm from a temper-rolled steel plate,
It is a ductile-brittle transition temperature when a cup is formed with a 3 mm punch and subjected to a drop weight test at various temperatures.

[0028]

[Table 1]

As is clear from Table 2, the examples of the present invention in which the structure has a single phase of a low temperature transformation product have a high BH property which has never been obtained, and also have a very good non-aging property at room temperature. I understand. This seems to be because the steel sheet having a low-temperature transformation product single-phase structure has a preferable dislocation density as compared with other steel sheets. Moreover, according to this invention, it turns out that it is also excellent in r value. Therefore, for example, it is a suitable material for outer and inner panel of automobiles.

[0030]

[Table 2]

Example 2 Using Steel 3-2 shown in Table 1, the effect of soaking temperature in continuous annealing was examined. The conditions of hot rolling and cold rolling are the same as in Example 1. Then, heat at 10 ° C / s for 8
After holding at 40 to 930 ° C. for 50 s, continuous annealing was performed at an average cooling rate of 60 ° C./s. Further, temper rolling was performed at a rolling reduction of 0.5%, and JIS No. 5 tensile test pieces were sampled and subjected to a tensile test. The results of the tensile test are summarized in Table 3. As is clear from Table 3, when a low temperature transformation product single phase structure is obtained by annealing in the γ single phase region as in the present invention, a stable and excellent material is obtained even if the soaking temperature changes. You can see that you get the characteristics. On the other hand, when annealed in the α + γ2 phase temperature range, the soaking temperature changes only slightly, the BH content varies widely, and the YP after artificial aging changes.
-El exceeded 0.2%, and the room-temperature non-aging property was not secured.

[0032]

[Table 3]

Example 3 Steels 3-1 to 3-4 and 4-1 to 4-4 shown in Table 1 were hot-rolled under the conditions of a slab heating temperature of 1220 ° C., a finishing temperature of 900 ° C. and a winding temperature of 500 ° C. A steel plate having a thickness of 3.8 mm was used. After pickling, cold rolling is carried out to obtain a cold-rolled sheet having a thickness of 0.75 mm, and then a maximum heating temperature of 840 to 840 at a heating temperature of 15 ° C / s.
After heating at 980 ℃, cooling at about 70 ℃ / s, 4
Conventional hot dip galvanizing was performed at 60 ° C. (Al concentration in the bath was 0.11%), further heating and alloying treatment at 520 ° C. for 20 s, and cooling to room temperature at about 20 ° C./s. With respect to the obtained galvannealed steel sheet, the plating appearance, powdering property and Fe concentration in the plating film were measured. These results are summarized in Table 4.

[0034]

[Table 4]

Here, the appearance of the plating property was evaluated according to the following criteria. ◎: A state where 100% of the area ratio is attached ○: A state where 90% or more of the area ratio is attached △: A state where 60 to 90% of the area ratio is attached ×: 30 to 60% of the area ratio Adhered state XX: State in which plating is adhered only at an area ratio of 30% or less

The adhesion of the plating was determined by determining the peeling condition of the zinc coating after the cellophane tape was adhered to the bent portion after peeling the zinc coating by 180 °. The evaluation was made into the following 5 grades. 1: Large peeling 2: During peeling 3: Small peeling 4: Small amount of peeling
5: No peeling at all The Fe concentration in the plating layer was determined by X-ray diffraction. As is apparent from Table 4, in the present invention, the plating appearance and the powdering property are good, and the Fe concentration in the alloy layer is an amount corresponding to that of the δ ₁ phase which is considered to be a desirable phase.

[0037]

As is apparent from the above description, according to the present invention, it is possible to obtain a cold-rolled steel sheet having both BH properties and non-aging at room temperature which have not been obtained in the past. Further, the steel of the present invention has good press formability and also has excellent hot-dip galvanizing properties, and therefore can exhibit a rust preventive function. as a result,
When the steel of the present invention is used in the body or frame of an automobile, the thickness of the plate can be reduced, that is, the weight of the vehicle body can be reduced. Therefore, the present invention can greatly contribute to the conservation of the global environment, which has been receiving attention recently. Thus, the industrial significance of the present invention is extremely great.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI technical display location C23C 2/02 2/06 (72) Inventor Yoshikazu Matsumura 20-1 Shintomi, Futtsu-shi, Chiba Made in New Japan (72) Inventor, Kunio Nishimura, 1-1 Hibatacho, Tobata-ku, Kitakyushu, Fukuoka Prefecture, Nippon Steel Co., Ltd., Yawata Works

Claims (5)

[Claims] 1. C: 0.0005-0.00, by weight.
70%, Si: 0.001-0.8%, Mn: 0.01
~ 4.0%, P: 0.005-0.15%, S: 0.0
010 to 0.015%, Al: 0.005 to 0.1%,
N: 0.0003 to 0.0060%, Ti:
0.003-0.1% and Nb: 0.003-0.1
%, A cold-rolled steel sheet excellent in bake hardenability and formability, characterized in that it has a composition comprising a balance Fe and unavoidable impurities and has a low-temperature transformation product single-phase structure, or Hot-dip galvanized cold rolled steel sheet. 2. The cold-rolled steel sheet or hot-dip galvanized cold-rolled steel sheet according to claim 1, containing B: less than 0.0030%. 3. The cold-rolled steel sheet or hot-dip galvanized cold-rolled steel sheet according to claim 1, which contains Cr: 0.01 to 3.0%. 4. A slab is hot-rolled at a temperature of (Ar ₃ -100) ° C. or higher, wound at a temperature of room temperature to 750 ° C., cold-rolled at a rolling ratio of 60% or more, and continuously. The method for manufacturing a cold-rolled steel sheet according to claim 1, wherein the annealing temperature in the annealing is set to the Ae ₃ transformation point or higher. 5. The slab is finished by hot rolling at a temperature of (Ar ₃ -100) ° C. or higher, and is rolled at a temperature of room temperature to 750 ° C. and cold-rolled at a rolling rate of 60% or more, and then an annealing temperature is set. The method for producing a hot-dip galvanized cold-rolled steel sheet according to any one of claims 1 to 3, wherein in-line annealing type hot-dip galvanizing is performed at a temperature of Ae ₃ or higher.