JP3350096B2 - Cold-rolled steel sheet or hot-dip galvanized cold-rolled steel sheet having excellent bake hardenability and formability, and methods for producing them - Google Patents

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 a hot-dip galvanized cold-rolled steel sheet excellent in bake hardenability and formability, and a method for producing the same. The cold-rolled steel sheet according to the present invention is used by press-forming automobiles, home appliances, buildings, and the like. Further, it includes both cold-rolled steel sheets in a narrow sense without surface treatment and cold-rolled steel sheets subjected to surface treatment such as Zn plating or alloyed 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 thickness of the conventional steel sheet in use, that is, to reduce the weight.
Therefore, it is considered that it can contribute to global environmental 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 carbon steel. At present, the demand for ultra-low carbon steel sheets having good workability is increasing.
Among them, for example, the ultra-low carbon steel sheet to which Ti and Nb are added in combination as disclosed in JP-A-59-31827 and JP-A-59-38337 has extremely good workability. Combines paint bake hardening (BH) properties,
As it has excellent hot-dip galvanizing properties, it is occupying an important position. However, the amount of BH is normal BH
It does not exceed the level of a steel sheet, and has the drawback that non-aging at room temperature cannot be ensured if an additional amount of BH is to be added.

On the other hand, many attempts have been made in the past to increase the strength while ensuring workability. In particular,
When the tensile strength related to the present invention is 35 to 50 kgf / mm ² , P, Si and the like have been added to steel, and the strength has been increased by utilizing these solid solution strengthening mechanisms. For example, JP-A-59-31827 and JP-A-59-3833.
No. 7 discloses a method of manufacturing a high-strength cold-rolled steel sheet having a tensile strength of up to 45 kgf / mm ² class by adding Si and P mainly to an ultra-low carbon steel sheet to which Ti and Nb are added. Japanese Patent Publication No. 57-57945 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.

[0004] As described above, conventionally, P,
Next, Si is frequently used. This is because it has been considered that P or Si has a very high solid solution strengthening ability and can be increased in strength by addition of a small amount, and does not decrease ductility and deep drawability so much, and does not increase addition cost so much. However, in fact, when trying to achieve an increase in strength with only these elements, not only the strength but also the yield strength increases significantly at the same time, resulting in poor surface shape, which may restrict the use of automotive panels. is there. Further, in the case of hot-dip galvanizing, there is a problem that Si causes plating failure and that P and Si significantly lower the alloying speed, thereby lowering productivity.

On the other hand, it is 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.
Is added to a Ti-containing ultra-low carbon steel sheet.
Japanese Patent No. 42742 and the above-mentioned Japanese Patent Publication No. 57-57945 disclose a technique of adding Mn and Cr to a Ti-added ultra-low carbon steel. The elements P and Si only play an auxiliary role, and thus the obtained cold-rolled steel sheet also has a high yield strength for strength, and (ii) for purposes other than the above (i),
For example, (a) is added not only to make the structure after annealing characteristic of the present invention into a mixed structure, but also (b) improving the work hardening rate, and (c) BH
Imparting properties, (d) improving secondary workability, (e)
Neither is it positively added for the purpose of improving the plating properties of hot-dip galvanizing.

Further, Japanese Patent Application Laid-Open No. 2-111841 discloses that a very low carbon steel with Ti added is 1.5% or more and 3.5% or less.
Disclosed are good workability cold rolled steel sheets and hot-dip galvanized steel sheets having bake hardenability with less than Mn added. By adding a large amount of Mn, the aim is to improve the operation stability of hot rolling and the uniformity of the metal structure by lowering the Ar ₃ transformation point. Further, for the purpose of further improving ductility, 0.2 to 1.
Additions of up to 0% are also disclosed. However, it is not based on the idea of improving the mechanical properties, particularly the balance between strength and ductility, by adding a large amount of Mn or Cr. Furthermore, the BH amount does not deviate from the usual level here, and it has not been possible to achieve both higher BH and non-aging at room temperature.

[0007] A steel sheet having a composite structure is known in addition to a steel sheet having a ferrite single-phase structure as described above. 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, a so-called Dual Phase steel in which a ferrite phase and a martensite phase are mixed together ( D
What is called P steel) is a typical example. Such DP steel has a very low yield ratio (Y
P) and is known to have a high BH due to non-aging at room temperature. However, there is a disadvantage that the average r value is as low as about 1.0 and the deep drawability is poor. Incidentally, the production method of such a cold-rolled steel sheet is described in JP-B-53-3.
No. 9368, JP-A-50-75113 and JP-A-51-39524.

[0008] Japanese Patent Publication No. 3-2224 and Japanese Patent Publication No. 3-21611 disclose a composite structure steel sheet made of an ultra-low carbon steel as a composite steel sheet made of a low carbon aluminum killed steel. Have been. These are composed of a very low carbon steel with a large amount of Nb and B, and also with the addition of Ti, to make the structure after annealing a composite structure of a ferrite phase and a low-temperature transformation forming phase, with a high average r value, high BH, high ductility, and normal temperature. It is intended to obtain a cold-rolled steel sheet having non-aging properties.

However, as a result of intensive studies made by the present inventors, it is apparent that the following problems arise when a composite structure is formed by adding Nb, B, and in some cases, Ti. It became. 1) In steels containing a large amount of Nb, B, and Ti, the α → γ transformation point does not necessarily decrease, but annealing at an extremely high temperature is indispensable to obtain a composite structure. It may cause troubles such as sheet breakage during annealing 2)
Since the temperature range of α + γ is extremely narrow, the structure changes in the sheet width direction, and as a result, the material may vary greatly, or the composite structure may or may not become a composite structure due to a change in the annealing temperature of several degrees Celsius, and the production is extremely unstable. Becomes Further, a large amount of B causes not only 3) deterioration of ductility but also 4) poor plating and the like, and is unsuitable as a hot-dip galvanized steel sheet. 5) Not only is it difficult to provide BH of 5 kgf / mm ² or more, but also the amount of BH is 5 kgf / mm ^2.
If it exceeds 50%, the YP-El after artificial aging exceeds 0.2%, so that non-aging at room temperature cannot be ensured.

Japanese Unexamined Patent Publication (Kokai) No. 3-277774 discloses that a steel obtained by adding Nb, B, Ti, Mn, and Cr to a very low carbon steel is annealed at a temperature from Ac ₁ -50 ° C. to less than the Ac ₁ transformation point. Discloses a technique for providing a steel sheet having both BH property, non-aging property at room temperature, and workability by making the structure a composite structure composed of acicular ferrite and ferrite having a volume ratio of 5% or less. I have. However, as a result of a detailed study by the present inventors, it has been found that there are the following problems. That is, it is difficult to provide BH of the conventional structure or higher, that is, 5 kgf / mm ² or more, with a composite structure steel sheet having a volume fraction of the second phase of 5% or less, and the BH amount exceeds 5 kgf / mm ² . YP-El after artificial aging may exceed 0.2%, indicating that it is extremely difficult to ensure non-aging at room temperature. As an example, 0.004% C-0.01Si-1.5Mn-
1.0Cr-0.05P-0.025Nb-0.04A
By changing the annealing temperature of steel having a composition of 1-0.0025N-0.01S, the volume fraction of the second phase is reduced to 0 to 0.
20%, BH content and YP-El after artificial aging
FIG. 1 shows the results of an examination of the relationship between the two. As is clear from this, when the volume fraction of the second phase is 5% or less, non-aging at room temperature is difficult to secure. It is considered that this is because the volume ratio of the second phase is small, and thus the mobile dislocation density introduced into the ferrite is not sufficient.

As described above, several proposals have been made for composite structure steel sheets in ultra-low carbon steel.
The H content did not deviate from the conventional level at all, and the room temperature non-aging property was slightly higher than the conventional level.

[0012]

A steel sheet used for a panel of an automobile or the like is strictly required to have a good surface shape which does not cause a springback or a surface distortion after pressing. By the way, it is well known that the lower the yield strength, the better the surface shape is. However, the high strength of steel sheets
As described in the prior art, it generally involves a significant increase in yield strength. Therefore, when increasing the strength, it is necessary to suppress the increase in the yield strength as much as possible. Furthermore, 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 vehicle. In the case where the plate thickness is constant, the higher the deformation stress after the press working and the coating baking, the better the dent resistance property becomes. Therefore, when considering steel sheets having the same yield strength, the paint bake hardening ability is high, and the higher the work hardening ability, the better the dent resistance is.

As described above, a desirable steel sheet used for a panel of an automobile is a steel sheet which has not so high a yield strength, is extremely work-hardened, and has a high paint bake hardenability. Of course, it is necessary to have excellent workability such as an average r value (deep drawing property) and elongation (extension property), and it is necessary to be substantially non-aging at room temperature. The present invention satisfies the above-mentioned demands, and in particular, can provide a high BH amount of up to about 10 kgf / mm ^{2 with} respect to the coating baking hardening ability according to the purpose, and can perform non-aging at room temperature. It is an object of the present invention to provide an unprecedented cold-rolled steel sheet having both properties.

[0014]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have intensively studied and obtained the following unprecedented knowledge. That is, Nb, T
Focusing on the structure and tensile properties of B, Mn, and Cr after cold rolling, annealing, and temper rolling, and particularly on the α → γ transformation behavior during annealing, based on the ultra-low carbon steel to which i is added alone or in combination. And examined it in detail. As a result, a composite structure composed of ferrite and a low-temperature transformation product could be obtained by adding B, but 1) annealing at a temperature much higher than usual is essential to form a composite structure. 2) In addition, since the temperature range for forming a composite structure has only a very narrow range, the variation in the material at the time of manufacturing is extremely large. 3) Furthermore, in such steel, BH is 5 kgf / mm ^2.
It is not only difficult to give above, but BH is 5kgf /
When it exceeds ² mm, the yield point elongation after artificial aging (YP-E
l) exceeds 0.2%, and the non-aging property at room temperature cannot be ensured. 4) It is extremely sensitive to cooling conditions after annealing, which also makes material properties such as BH amount and average r value extremely unstable. These facts show the same tendency in any of the cases of the composite addition of Nb and B, the composite addition of Ti and B, and the composite addition of Nb, Ti and B.

On the other hand, in steels to which Mn and / or Cr are added, 1) since these elements are γ-forming elements, they are extremely low carbon steels but have a low α → γ transformation point, so that annealing is so high. It does not require a temperature and 2) has an extremely wide α + γ2 phase region, so that material variations during manufacture are extremely small. Further, 3) BH property of 5 kgf / mm ² or more can be easily provided.
Even at about kgf / mm ² , YP-El after artificial aging did not exceed 0.2%, indicating that both excellent non-aging property at normal temperature and BH property were achieved. Although the cause is not necessarily clear, in a steel having a mixed structure using Mn or Cr, the mobile dislocation density in the generated low-temperature transformation product and in the ferrite introduced around the product is such that Nb, Ti, and B have a dislocation density of Nb, Ti, and B. This is probably due to the fact that it is much higher than that of the composite tissue obtained by the composite addition. Also,
4) In the mixed-structure steel sheet to which Mn and Cr are added, mechanical properties such as an average r value and BH are good irrespective of cooling conditions after annealing, and it is also a major feature that manufacturing is easy.
One. Further, although the reason is not always clear, these properties cannot be achieved even in a steel to which Mn or Cr is added if B is added too much at the same time.

Next, as a result of examining how each of Mn, Cr, P, and Si, which are considered as strengthening elements in increasing the strength, has an effect on the mechanical properties, the following new findings were obtained. . That is, Si and P, which have been frequently used as solid solution strengthening elements, a) firstly increase the yield strength remarkably by adding a small amount thereof, and b) as a result, the work hardening rate in a low strain region is significantly reduced. It has been found. On the other hand, when Mn or Cr, which has not been widely used as a solid solution strengthening element, is added, a) the yield strength hardly increases and the tensile strength increases. B) As a result, the work hardening rate in the low strain region is rather increased. A very important new finding was obtained, which was increased by these additions. In addition to the mixed structure of Mn and Cr, this also seems to be the reason that the steel of the present invention exhibits a low yield ratio. Also, like this,
The reduction of P and Si is significant in lowering the Ac ₁ point.

Further, the present inventors have found that the steel of the present invention has advantages as a hot-dip galvanized cold-rolled steel sheet. That is, it is known that in steels to which Si or P is added in a large amount, the plating property during hot-dip galvanizing and furthermore, the subsequent alloying reaction is delayed.
It has been found that in the steel to which n or Cr is added, even if a large amount of Si or P is contained at the same time, the galvanizing characteristics are not impaired. In addition, the effect of B was also examined, and it became clear that a large amount of B had an adverse effect on the plating properties and alloying reaction characteristics in hot-dip galvanizing. The present invention has been constructed based on such ideas and new findings, and the gist thereof is as follows. (1) In weight%, C: 0.0005 to 0.0070%, Si: 0.001 to 0.8%, Mn: 1.15 to 4.0%, P: 0.005 to 0.15% S: 0.0010 to 0.015%, Al: 0.005 to 0.1%, N: 0.0003 to 0.0060%, B: less than 0.0005%, and Ti: 0.003 to 0.1% and Nb: 0.003 to
0.1% of at least one of the alloys, the composition comprising the balance of Fe and unavoidable impurities, and having a mixed structure composed of a low-temperature transformation generation phase and ferrite having a total volume of more than 5%. Cold rolled steel sheet or hot-dip galvanized cold rolled steel sheet with excellent curability and formability. (2) The cold-rolled steel sheet or hot-dip galvanized cold-rolled steel sheet further containing Cr: 0.01 to 3.0% and having excellent bake hardenability and formability as described in 1 above. (3) In hot rolling a slab containing the component of the above item 1 or 2, hot-rolling is performed at a temperature of (Ar ₃ -100) ° C. or more, and wound at a temperature of room temperature to 750 ° C., and 60%
_{3. The} method for producing a cold-rolled steel sheet according to the above item 1 or 2, wherein the cold rolling is performed at the above-mentioned rolling ratio, and the annealing temperature in the continuous annealing is set to the α-γ transformation point or more and the Ac ₃ transformation point or less. (4) In hot rolling a slab containing the component of the above item 1 or 2, hot-rolling is performed at a temperature of (Ar ₃ -100) ° C. or more, and wound at a temperature from room temperature to 750 ° C., and 60%
_3. The hot-dip galvanizing method according to the above 1 or 2, wherein cold rolling is performed at the above rolling ratio, and in-line annealing type hot-dip galvanizing in which the annealing temperature is equal to or higher than the α → γ transformation point and equal to or lower than the Ac ₃ transformation point. Manufacturing method of plated cold rolled steel sheet.

[0018]

The reason why the steel composition and the manufacturing conditions are limited as described above in the present invention will be further described. C: C is a very important element that determines the material properties of the product. The present invention is based on ultra-low carbon steel subjected to vacuum degassing, but if C is less than 0.0005%, the grain boundary strength decreases, the secondary workability deteriorates, and the production cost increases significantly. Therefore, the lower limit is set to 0.0005%. On the other hand, if the C content exceeds 0.0070%, the moldability is deteriorated and the non-aging property at room temperature is not ensured, so the upper limit is made 0.0070%.

Si: Si is known as an element that increases strength at low cost, and its addition amount changes according to the intended strength level, but when the addition amount exceeds 0.8%, the yield strength increases. Surface distortion occurs during press molding due to excessive strain. Further, the α → γ transformation point rises, and the annealing temperature for obtaining a mixed structure becomes extremely high. Further, problems such as a decrease in chemical conversion treatment, a decrease in adhesion to hot-dip galvanizing, and a decrease in productivity due to a delay in alloying reaction occur. The lower limit is 0.001% from the viewpoint of steelmaking technology and cost.

Mn, Cr: Mn and Cr are one of the most important elements in the present invention. That is, Mn, C
r is lower than the α → γ transformation point, so that a high temperature is not required to obtain a mixed structure, and since the α + γ2 phase region is expanded, the volume fraction of the mixed structure can be easily controlled, There is little variation, which leads to an improvement in productivity. Moreover, in a mixed structure steel sheet obtained by utilizing Mn and Cr, it cannot be obtained usually.
BH amount of kgf / mm ² or more can be easily given,
Even when it has a BH property of not less than kgf / mm ² , it shows extremely excellent non-aging property at room temperature. This property is peculiar to a mixed-structure steel sheet obtained by utilizing Mn or Cr and cannot be obtained by a ferrite single-phase structure steel sheet or a composite structure steel sheet obtained by combining several kinds of Nb, B, and Ti. . Further, Mn, C
r is an effective solid solution strengthening element that increases the strength without significantly increasing the yield strength, and also has the effect of improving chemical conversion treatment properties and improving hot-dip galvanizing properties. If the amount of Mn is less than 1.15 %, the above-mentioned effects are not significantly exhibited , so the lower limit is set to 1.15 %. On the other hand, if it exceeds 4.0%, a good mixed structure cannot be obtained, so the upper limit is made 4.0%. Cr is 0.01%.
%, The lower effect is not exhibited.
If the content exceeds 3.0%, a good mixed structure cannot be obtained, so the upper limit is set to 3.0%.

P: As with Si, P is known as an element which increases the strength at low cost, and the amount of P varies depending on the intended strength level. When the addition amount exceeds 0.15%, the annealing temperature for obtaining a mixed structure becomes extremely high,
Further, the yield strength is excessively increased, which causes a poor surface shape at the time of pressing. Furthermore, the alloying reaction during continuous hot-dip galvanizing becomes extremely slow, and the productivity is reduced. Further, the secondary workability also deteriorates. Therefore, the upper limit is 0.15%
And The lower limit is set to 0.005% from the viewpoint of steelmaking technology and cost.

S: The S content is preferably as low as possible, but 0.00
If it is less than 1%, the production cost increases, so this is set as the lower limit. On the other hand, if the content exceeds 0.015%, a large amount of MnS precipitates and the workability deteriorates. Al: Al is used for deoxidation preparation and for fixing N when no Ti is added, but when less than 0.005%, Ti or N
The yield of b decreases. On the other hand, if it exceeds 0.1%, the cost is increased, so the upper limit is made 0.1%.

Ti, Nb: Ti, Nb has the role of securing the workability and non-aging properties of ultra-low carbon steel by fixing all or part of N, C, and S. Furthermore, the crystal grains of the hot-rolled sheet are refined, and the workability of the product sheet is improved. T
If i and Nb are less than 0.003%, the effect of addition is not exhibited, so this is set as the lower limit. On the other hand, if it exceeds 0.1%, a significant increase in alloy cost is caused, so the upper limit is made 0.1%.

N: N is preferably low. However, 0.
Reducing it to less than 0003% results in a significant increase in cost.
On the other hand, if the amount is too large, a large amount of Ti, Nb, Al is required, or the workability is deteriorated. Therefore, the upper limit is made 0.0060%. B: B may be added because it is effective in preventing embrittlement in secondary processing. However, when the content of BH is 0.0005% or more, when the BH amount exceeds 5 kgf / mm ² , non-aging at room temperature cannot be ensured. Also, the upper limit is set to 0.
Less than 0005%.

Next, the reasons for limiting the manufacturing conditions will be described. The hot rolling finish temperature needs to be Ar ₃ -100 ° C. or higher from the viewpoint of ensuring the workability of the product sheet. The winding temperature is from room temperature to 750 ° C. The present invention is characterized in that the material of the product is not so affected by the hot rolling temperature. One reason for this is considered to be that Mn, Cr and the like are considerably added and the structure of the hot-rolled sheet is extremely fine and uniform. The upper limit of the winding temperature is 750 ° C
Is determined from the viewpoint of preventing a decrease in yield due to deterioration of the material at both ends of the coil. The cold rolling may be performed under ordinary conditions, and the rolling reduction is set to 60% or more for the purpose of ensuring the deep drawability after annealing. The annealing temperature of the continuous galvanizing equipment of the continuous annealing or the in-line annealing method is set to the α → γ transformation point or more and the Ac ₃ transformation point or less. When the annealing temperature is lower than the α → γ transformation point, a mixed structure having a second phase volume ratio of more than 5%, which is a feature of the present invention, cannot be obtained.
Further, if annealing is performed at a temperature exceeding the Ac ₃ transformation point, the workability is significantly deteriorated. Therefore, the upper limit of the annealing temperature is defined as the Ac ₃ transformation point.

Thus, according to the present invention, the yield strength is low, the work hardens remarkably, the paint baking hardenability is high, and the workability such as the average r value (deep drawing property) and elongation (extension property) is improved. Excellent steel plate can be obtained. In particular, with respect to the paint bake hardening ability, it is possible to provide a high BH amount of about 10 kgf / mm ² as required, and to provide a cold-rolled steel sheet having non-aging properties at room temperature. Next, the present invention will be described with reference to examples.

[0027]

【Example】

Example 1 A steel having a composition shown in Table 1 was melted and hot-rolled at a slab heating temperature of 1180 ° C., a finishing temperature of 910 ° C., and a winding temperature of 600 ° C. to obtain a steel strip having a thickness of 4.0 mm. After pickling, cold rolling was performed at a rolling reduction of 80% to form a 0.8 mm-thick cold rolled sheet. Then, the heating rate was 10 ° C./s, and the soaking was 810 to 92%.
Continuous annealing was performed at 0 ° C. × 50 s and at an average cooling rate of 60 ° C./s. In addition, temper rolling at a reduction rate of 0.5% is performed, and JIS
A No. 5 tensile test piece was sampled and subjected to a tensile test. Table 2 summarizes the results of the tensile test.

Here, the WH amount is the amount of work hardening when 2% tensile strain is applied in the rolling direction, and is the amount obtained by subtracting the yield stress (YP) from the 2% deformation stress. Also, BH
The amount of increase in stress when a 2% prestrained material is subjected to a heat treatment equivalent to paint baking at 170 ° C. for 20 minutes and then subjected to a tensile test again (2% deformation stress from the drop yield stress at the time of the retensile test) (Subtracted value). The secondary working embrittlement transition temperature is
This is a ductile-brittle transition temperature when a blank having a diameter of 50 mm is punched from a temper-rolled steel sheet, then cup-formed with a punch having a diameter of 33 mm, and subjected to a drop test at various temperatures.

[0029]

[Table 1]

[0030]

[Table 2]

As is clear from Table 2, the steel of the present invention has an unprecedented high BH property and is very excellent at room temperature compared with a conventional steel sheet having the same level of tensile strength. It turns out that it has aging effect. This means that Mn
B and N in steel sheets with a mixed structure using Cr and Cr
It seems that the main cause is that it has a preferable dislocation density as compared with a steel sheet having a composite structure using b. Further, the steel of the present invention has low yield strength, excellent surface shape, and WH
The amount and the average r value are also high. Therefore, it is a suitable material for the outer and inner panel of an automobile, for example.

Example 2 Steels 3-2 and 3-4 in Table 1
The effect of soaking temperature in continuous annealing was investigated using The conditions of the hot rolling and the cold rolling are the same as in the first embodiment. Then, it heats at 10 degreeC / s, and 860-920 degreeC
At 50 ° C / s for an average cooling rate of 60 ° C / s.
Was continuously annealed. Further, temper rolling was performed at a rolling reduction of 0.5%, and a JIS No. 5 tensile test piece was sampled and subjected to a tensile test. Table 3 summarizes the results of the tensile tests.

As is evident from Table 3, it can be seen that the steel of the present invention can obtain excellent material properties stably even when the soaking temperature changes. On the other hand, in the comparative steel 3-4, the strength changes remarkably with only a slight change in the soaking temperature, and the BH amount,
The average r value also varied greatly.

[0034]

[Table 3]

Example 3 Steels 3-2 and 3-4 in Table 1
The effect of cooling conditions after soaking in continuous annealing was investigated using JIS. The conditions of the hot rolling and the cold rolling are the same as in the first embodiment. After cold rolling, heating at 10 ° C./s, holding at 50 ° C. at 880 ° C. and 900 ° C., respectively, the average cooling rate to 750 ° C. is 3 to 60 ° C./s.
And then cooled to room temperature at 750-60 ° C / s. In addition, temper rolling at a rolling reduction of 0.5%
An S5 tensile test piece was sampled and subjected to a tensile test. Table 4 summarizes the results of the tensile test.

As is evident from Table 4, the steel of the present invention can obtain excellent material properties extremely stably even when the cooling rate after soaking changes. On the other hand, the comparative steel 3-4 has a remarkable change in strength with only a slight change in the cooling rate,
Further, the BH amount and the average r value also varied greatly.

[0037]

[Table 4]

Example 4 Hot rolling was performed on steels 3-1 to 3-5 and 4-1 to 4-4 shown in Table 1 under the conditions of a slab heating temperature of 1220 ° C., a finishing temperature of 900 ° C., and a winding temperature of 500 ° C. And a steel plate having a thickness of 3.8 mm. After pickling, cold-rolled to a 7.5 mm thick cold-rolled sheet, then heated to a maximum heating temperature of 890 ° C at a heating temperature of 15 ° C / s, cooled at about 70 ° C / s, and conventionally used at 460 ° C. (Al concentration in the bath is 0.11%), and further heated at 520 ° C.
After the alloying treatment for 0 seconds, it was cooled to room temperature at about 20 ° C./s.
The plating appearance of the obtained alloyed galvanized steel sheet,
The powdering property and the Fe concentration in the plating film were measured. These results are summarized in Table 5.

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

Here, the powdering property was determined by performing a 180 ° close contact bending to determine the peeling state of the zinc film from the amount of peeling plating adhered to the tape after the cellophane tape was adhered to the bent portion. The evaluation was based on the following five levels. 1: Large peeling 2: Peeling off 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.

[0041]

[Table 5]

As is clear from Table 5, the steel of the present invention has a better plating appearance and powdering property than the conventional steel, and the δ ₁ phase which is considered to be a desirable phase in which the Fe concentration in the alloy layer is also desirable. Is equivalent to that of This is presumably because in the present invention, P, B, and Si, which degrade plating adhesion and reduce the alloying reaction rate, are reduced, and Mn and Cr are added. Further, it is understood that when Mn or Cr is added, even if a certain amount of P or Si is contained, the plating characteristics are not impaired.

[0043]

As is apparent from the above description, according to the present invention, it is possible to obtain a cold-rolled steel sheet having both unprecedented BH property and non-aging property at room temperature. Further, the steel of the present invention has extremely good press-formability, and is also excellent in hot-dip galvanizing properties, so that it can also exhibit a rust prevention function. As a result, when the steel of the present invention is used for a body or a frame of an automobile, the thickness of the body can be reduced, that is, the weight of the vehicle body can be reduced. Thus, the industrial significance of the present invention is extremely large.

[Brief description of the drawings]

FIG. 1: Volume fraction of second phase and BH and YP after artificial aging
It is a figure showing the relationship with -El.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI C23C 2/06 C23C 2/06 (72) Inventor Kunio Nishimura 1-1-1, Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation (56) References JP-A-3-277741 (JP, A) JP-A-5-112858 (JP, A) JP-A-5-331612 (JP, A) JP-A-5-78783 ( JP, A) JP-B 61-12008 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 301 C21D 8/02 C21D 9/46 C22C 38/14 C22C 2 / 06

Claims (4)

(57) [Claims] C: 0.0005 to 0.0070%, Si: 0.001 to 0.8%, Mn: 1.15 to 4.0%, P: 0.005 to 0.4% by weight. 15%; S: 0.0010 to 0.015%; Al: 0.005 to 0.1%; N: 0.0003 to 0.0060%; B: less than 0.0005%; 003-0.1% and Nb: at least one of 0.003-0.1%, and the composition is composed of the balance of Fe and unavoidable impurities. A cold-rolled steel sheet or hot-dip galvanized cold-rolled steel sheet having an excellent bake hardenability and formability, characterized by having a mixed structure comprising: 2. The cold-rolled steel sheet or hot-dip galvanized cold-rolled steel sheet according to claim 1, which contains 0.01 to 3.0% of Cr. 3. The method according to claim 1, wherein (Ar ₃ -1)
00) Finish hot rolling at a temperature of at least
The winding at a temperature of 0 ° C., cold rolling at a rolling rate of 60% or more, and the annealing temperature in continuous annealing is set to the α → γ transformation point or more and the Ac ₃ transformation point or less. 2. The method for producing a cold-rolled steel sheet according to 2. 4. The method according to claim 1, wherein (Ar ₃ -1)
00) Finish hot rolling at a temperature of at least
Winding at a temperature of 0 ° C, cold rolling at a rolling rate of 60% or more, and performing in-line annealing type hot-dip galvanizing with an annealing temperature of α → γ transformation point or more and Ac ₃ transformation point or less. The method for producing a hot-dip galvanized cold-rolled steel sheet according to claim 1 or 2.