JP3238211B2 - Manufacturing method of cold rolled steel sheet or hot-dip galvanized cold rolled steel sheet with excellent bake hardenability and non-aging property - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel sheet or hot-dip galvanized cold-rolled steel sheet having excellent bake hardenability and non-aging property. 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.

[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, JP-A-59-31827,
And the ultra-low carbon steel sheet to which Ti and Nb are added in combination as disclosed in JP-A-59-38337 and the like have extremely good workability, and have both paint bake hardening (BH) properties and molten zinc. It is occupying an important position because of its excellent plating characteristics. However, the BH amount does not exceed the level of a normal BH steel sheet,
There is a drawback that non-aging at room temperature cannot be ensured if an amount 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 according to the present invention is 30 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 limits the use of automotive panels. There is. When hot-dip galvanizing is performed, poor plating may be caused by Si or P, Si
However, there is a problem that the alloying speed is remarkably reduced and the productivity is lowered.

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 Publication 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.

(I) Addition of Mn or Cr only plays an auxiliary role of P or Si which is a main additive element.
The obtained cold-rolled steel sheet also has a high yield strength instead of strength, and (ii) for the purpose other than the above (i), for example, (a) to make the structure after annealing, which is a feature of the present invention, a mixed structure. It is of course not added, (b) improving the work hardening rate, (c) imparting BH properties,
Neither is it positively added for the purpose of (d) improving the secondary workability, or (e) improving the galvanizing property of hot-dip galvanizing.

Further, Japanese Patent Application Laid-Open No. 2-111841 discloses that ultra-low carbon steel with Ti added is 1.5% to 3.5%.
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 that the addition of a large amount of Mn or Cr improves the mechanical properties, especially the balance between strength and ductility. 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.

A steel sheet having a composite structure is also 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.

[0010] Such DP steel is known to have a very low yield ratio (YR) despite its high strength, and 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, a method for producing such a cold-rolled steel sheet is disclosed in JP-B-53-39368 and JP-A-50-751.
No. 13 and JP-A-51-39524.

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 alloys contain a large amount of Nb, B, and Ti in an extremely low-carbon steel to form a structure after annealing to form a composite structure of a ferrite phase and a low-temperature transformation generation phase, with a high r value, a high BH, a high ductility, and a non-normal temperature. It is intended to obtain a cold-rolled steel sheet having aging properties.

However, as a result of intensive studies made by the present inventors, it is clear that the following problems arise when a composite structure is formed by adding Nb, B, and in some cases, Ti. It became. 1) When a composite structure is formed by utilizing such a large amount of Nb, B and Ti, the Ac ₁ transformation point does not decrease, but annealing at an extremely high temperature is essential to obtain a composite structure. 2) Since the temperature range of α + γ is extremely narrow, the structure changes in the width direction of the sheet, and as a result, the material greatly varies or the annealing temperature of several degrees Celsius The change may or may not result in a composite structure, making the manufacture extremely unstable.

Further, a large amount of B not only causes 3) deterioration of ductility but also causes 4) poor plating.
It is not suitable as a galvanized steel sheet. 5) Also,
Not only is it difficult to apply BH of 5 kgf / mm ² or more, but if the BH amount exceeds 5 kgf / mm ² , the YP
-El exceeds 0.2% and 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 investigation 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. 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 on 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.

[0017]

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, increasing the strength of a steel sheet generally involves a significant increase in yield strength as described in the prior art. Therefore, when increasing the strength, it is necessary to suppress the increase in the yield strength as much as possible.

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 vehicle. When the plate thickness is constant, the dent resistance becomes better as the deformation stress after press working and coating baking is higher. 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.

From the above, it is desirable that the steel sheet used for automobile panels and the like be a steel sheet which has not so high a yield strength, is extremely work-hardened, and has 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 particularly, with respect to the paint baking hardening ability, 5 kgf / mm
^An object of the present invention is to provide an unprecedented cold-rolled steel sheet that can provide a high BH amount of ² or more according to the purpose and also has non-aging property at room temperature.

[0021]

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
Microstructure and tensile properties after cold rolling, annealing, and temper rolling with addition of B, Mn, and Cr based on ultra-low carbon steel to which i is added alone or in combination, especially when annealing 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. However, 1) Since the temperature range for forming the composite structure exists only in a very narrow range, the production was carried out. Sometimes, the variation of the material is extremely large. 2) Furthermore, BH is 5
It is not only difficult to apply more than kgf / mm ²
Is 5 kgf / mm ² or more, the yield point elongation after artificial aging (Y
P-El) may exceed 0.2%, making it difficult to ensure normal-temperature non-aging properties.

On the other hand, when annealing is performed 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 variation of the material at the time is extremely small. Low-temperature transformation products are obtained when annealed in ferrite single phase temperature range,
Includes all structures other than the so-called polygonal ferrite.

2) In the γ region, carbides such as TiC and NbC formed during hot rolling or winding are redissolved, so that a BH property of 5 kgf / mm ² or more can be easily provided. Also, even if the BH content is about 10 kgf / mm ² , the YP-El after artificial aging does not exceed 0.2%, and both the excellent non-aging property at normal temperature and the BH property are compatible. I understood that. The reason for this is not necessarily clear, but is thought to be due to the fact that the mobile dislocation density introduced into the formed low-temperature transformation product is considerably high.

FIG. 2 shows the result of studying the cooling rate after annealing on the relationship between the BH amount and the yield point elongation after artificial aging (YP-El). The chemical composition of the material used is 0.004% C-0.01% Si-1.5% Mn-
0.07% P-0.005% S-0.02% Nb-0.
At 04% Al-0.0015% N, the heat cycle is as shown in FIG.

As is clear from this, in order to achieve both high BH and non-aging property (YP-El <0.2%), it is necessary to set the cooling rate after annealing to 30 ° C./s or more. I understood that. Although the cause is not necessarily clear, it is considered that the cooling at a rate of 30 ° C./s or more provides a sufficient mobile dislocation density to secure non-aging properties.

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 mechanical properties, the following new findings were obtained. . In other words, 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)
It was found that the work hardening rate in the low strain region was significantly reduced.

On the other hand, when Mn or Cr, which has not been used as a solid solution strengthening element in the past, is added, a) the yield strength hardly increases and the tensile strength increases. B) As a result,
A very important new finding was that the work hardening rate in the low strain range was rather increased by these additions. In addition to having a low-temperature transformation product single-phase structure with Mn and Cr, this also seems to be the reason that the steel of the present invention exhibits a low yield ratio.
Further, such reduction of P and Si is also significant in lowering the Ae ₃ point.

Furthermore, 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.

The present invention is an unprecedented and completely new steel plate constructed on the basis of such ideas and new findings, and its gist is as follows. (1) By weight%, C: 0.0005 to 0.0070%,
Si: 0.001 to 0.8%, Mn: 0.6 to 4.0
%, P: 0.003 to 0.15%, S: 0.0010
0.015%, Al: 0.005 to 0.1%, N: 0.
0003-0.0060%, and Ti: 0.003
-0.1% and Nb: 0.003-0.1%
A slab containing at least one kind and having a composition consisting of the balance of Fe and unavoidable impurities is subjected to hot rolling at a temperature of (Ar ₃ -100) ° C. or more, and is wound at a temperature of room temperature to 750 ° C. Cold rolling at a rolling rate of at least
The annealing temperature in the continuous annealing is set to the Ae ₃ transformation point or higher, and the temperature range from the annealing temperature to (Ar ₁ -50 ° C.) to (Ar ₁ + 50 ° C.) is cooled at an average cooling rate of 30 ° C./s or more, and low-temperature transformation is generated. A method for producing a cold-rolled steel sheet having excellent bake hardenability and formability, characterized by having a single-phase structure.

(2) B: The method for producing a cold-rolled steel sheet excellent in bake hardenability and non-aging property according to (1), wherein a slab containing less than 0.0030% is used.

(3) The method for producing a cold-rolled steel sheet according to (1) or (2), wherein the slab contains Cr: 0.01 to 3.0% and has excellent bake hardenability and non-aging properties.

(4) The slab having the chemical composition described in (1), (2) or (3) is subjected to hot rolling at a temperature of (Ar ₃ -100) ° C. or more, and is heated from room temperature to 750 ° C.
And cold rolling at a rolling rate of 60% or more. In the in-line annealing type hot-dip galvanizing line, the annealing temperature is set to the Ae ₃ transformation point or higher, and the annealing temperature is (Ar ₁ -50 ° C.) to (Ar ₁ + 50 ° C.) The average cooling rate is 30 ° C./s or more, and the hot-dip galvanizing cold is excellent in baking hardenability and non-aging characteristics, characterized by having a low-temperature transformation product single phase structure. Manufacturing method of rolled steel sheet.

[0034]

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. Also, the Ae ₃ transformation point rises, and the annealing temperature for obtaining a low-temperature transformation product single-phase 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 the most important elements in the present invention. That is, Mn and Cr are
Not so high temperatures are required to obtain a low temperature transformation product single phase structure to lower the Ae ₃ transformation point.

In addition, a single-phase structure steel sheet obtained by utilizing Mn and Cr cannot be usually obtained.
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 single-phase steel sheet obtained by utilizing Mn or Cr, and is a ferrite single-phase steel sheet, a composite steel sheet obtained by combining several kinds of Nb, B, and Ti, or a low-temperature transformation product. This is a characteristic that cannot be obtained with a single-phase structure steel sheet.

Further, it is important to note that, in ordinary steels, it is known that the r-value is significantly deteriorated when annealed in the α + γ2 phase region or the γ single phase region, but Mn and Cr were positively added. In steel, even if annealed in the γ single phase region, r is hardly deteriorated.

Further, Mn and Cr are effective solid solution strengthening elements for increasing the strength without significantly increasing the yield strength, and also have the effect of improving the chemical conversion treatment property and the galvanizing property. In the present invention, Mn is essential, and Cr is added as needed.

That is, from the viewpoint of lowering the Ae ₃ transformation point, Mn is more effective than Cr,
Take advantage of n. Cr exerts an excellent effect from the viewpoint of improving the BH property and enhancing the work hardening ability, and is added when it is desired to further enhance these properties. With respect to Mn, if the addition is less than 0.6%, the above-mentioned effects are not significantly exhibited, so the lower limit is set to 0.6%. on the other hand,
If it exceeds 4.0%, a desirable low-temperature transformation product single phase structure cannot be obtained, so the upper limit is made 4.0%.

If the content of Cr is less than 0.01%, the above effect is not exhibited, so the lower limit is made 0.01% and 3.0% is set.
%, The good structure cannot be obtained, so the upper limit is set to 3.0%.

P: As with Si, P is known as an element that increases strength at low cost, and the amount of P added varies according to the intended strength level. When the addition amount exceeds 0.15%, the annealing temperature for obtaining a single-phase structure of the low-temperature transformation product becomes remarkably high, and the yield strength is excessively increased to cause poor surface shape at the time of pressing.

Further, 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 In addition, the lower limit is set to 0.003% 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 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 Ti is not added, but if it is less than 0.005%, the yield of Ti and Nb 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 a role of securing workability and non-aging property of the ultra-low carbon steel by fixing all or a part of N, C, 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 0.10%, a significant increase in alloy cost is caused.
%.

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 is effective in preventing the embrittlement of secondary processing and is effective in obtaining a single-phase structure of a low-temperature transformation product, so B can be added in an amount of less than 0.0030%. However, when the content is 0.0030% or more, the workability is deteriorated. Therefore, the upper limit is set to less than 0.0030%.

Next, the reasons for limiting the manufacturing conditions will be described. The material for hot rolling is not particularly limited, and may be a continuously cast slab, a thin cast strip cast by a strip caster or the like, or another slab.

The finishing temperature for hot rolling must 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. This is because Mn and Cr
It is considered that one of the causes is that the structure of the hot-rolled sheet is extremely fine and uniform because of the considerable addition of such materials. The fact that the upper limit of the winding temperature is 750 ° C. is determined from the viewpoint of preventing a decrease in yield due to material deterioration 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 Ae ₃ transformation point or higher. When the annealing temperature is lower than the Ae ₃ transformation point, a low-temperature transformation product single-phase structure characteristic of the present invention cannot be obtained.

The cooling condition after soaking during annealing is such that the average cooling rate is 30 ° C./s or more in the temperature range from the annealing temperature to (Ar ₁ -50 ° C.) to (Ar ₁ + 50 ° C.). As a result, a sufficient amount of solid solution C for ensuring high BH is secured, and a high mobile dislocation density required for obtaining excellent non-aging properties can be obtained.

Thus, according to the present invention, the yield strength is low, the work hardens remarkably, it has both high baking hardenability and non-aging property, and the average r value (deep drawing property), elongation (extension property), etc. It is possible to obtain a steel sheet having excellent workability.
In particular, with regard to paint bake hardening ability, a high BH amount of about 10 kgf / mm ² can be provided as necessary, and a cold-rolled steel sheet having non-aging properties at room temperature can be provided.

[0054]

【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 700 ° C. to obtain a 4.0 mm thick steel strip. After pickling, cold rolling was performed at a rolling reduction of 80% to form a cold-rolled sheet having a thickness of 0.8 mm, and then a heating rate of 10 ° C./s and a soaking temperature of 820 to 99.
The continuous annealing was performed at 0 ° C. × 50 s from the annealing temperature to 600 ° C. at an average cooling rate of 80 ° C./s and from 600 ° C. to room temperature at 70 ° C./s. Further, temper rolling was performed at a rolling reduction of 0.8%, and a JIS 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 a work hardening amount when a tensile strain of 2% is applied in the rolling direction, and is an 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.

[0056]

[Table 1]

[0057]

[Table 2]

As is clear from the annealing temperatures in Table 1, it is understood that in the steel of the present invention, the annealing temperature for obtaining the low-temperature transformation product single phase structure is considerably lower than that of the comparative steel. Therefore, it can be manufactured without imposing an unreasonable burden on the continuous annealing equipment.

Further, as is clear from Table 2, the steel of the present invention has an unprecedentedly high BH property and is extremely excellent as compared with the steel sheet having the same level of tensile strength as the conventional steel. It turns out that it has non-aging property at room temperature. This is considered to be mainly due to the fact that a steel sheet having a low-temperature transformation product single-phase structure using Mn, Cr or B has a preferable dislocation density as compared with other steel sheets.

Another characteristic is that, according to the present invention, the r value hardly deteriorates despite annealing in the γ single phase temperature range. Further, the steel of the present invention has low yield strength, excellent surface shape, and a high WH amount. Therefore, it is a suitable material for the outer and inner panel of an automobile, for example.

Example 2 The effect of soaking temperature in continuous annealing was examined using steel 3-2 in Table 1. 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 it is 5 at 830-920 degreeC.
After holding for 0 s, the average cooling rate from the annealing temperature to 600 ° C was 80 ° C / s, and the average cooling rate from 600 ° C to 350 ° C was 70 ° C.
/ S, kept at 350 ° C. for 3 minutes, and continuously annealed from 350 ° C. to room temperature at 70 ° C./s.

Further, temper rolling at a rolling reduction of 0.8% is performed.
JIS No. 5 tensile test pieces were collected and subjected to a tensile test. Table 3 summarizes the results of the tensile tests.

As is evident from Table 3, when the single-phase structure of the low-temperature transformation product is obtained by annealing in the γ single-phase region as in the present invention, even if the soaking temperature changes, it is stable. It can be seen that excellent material properties are obtained. On the other hand, α + γ2
In the case of annealing in the phase temperature range, B
The amount of H is sensitive, and the amount is smaller than that during annealing in the γ region.

[0064]

[Table 3]

Example 3 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, it was cold-rolled into a cold-rolled sheet having a thickness of 7.5 mm, and then heated at a heating temperature of 15 ° C / s to a maximum heating temperature of 830 to 990 ° C, and then heated for about 7 hours.
Cool at 0 ° C / s, perform conventional hot-dip galvanizing at 460 ° C (Al concentration in the bath is 0.11%), and further heat to 5%.
After the alloying treatment at 20 ° C. for 20 seconds, it was cooled to room temperature at about 20 ° C./s. About the obtained alloyed galvanized steel sheet, the plating appearance, the powdering property, and the Fe concentration in the plating film were measured. These results are summarized in Table 4.

[0066]

[Table 4]

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 plating adhesion was determined by performing 180 ° contact bending to determine the peeling state of the zinc film from the amount of peel 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.

As is evident from Table 4, in the present invention, the plating appearance and the powdering property are good and the F content in the alloy layer is good.
The e concentration is also equivalent to that of the δ ₁ phase, which is considered a desirable phase. This is because, in the present invention, P, B,
This is probably because Si was reduced and Mn and Cr were 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.

[0070]

According to the present invention, it is possible to obtain a cold-rolled steel sheet having both the unprecedented BH property and the normal temperature non-aging property. 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 is a heat cycle for examining the effect of cooling rate after annealing on BH and YP-El after artificial aging.

FIG. 2 is a table showing a relationship between a cooling rate after annealing and BH, YP-El.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Kunio Nishimura 1-1-1, Hibata-cho, Tobata-ku, Kitakyushu-shi Nippon Steel Corporation Yawata Works (56) References JP-A-4-136123 (JP, A) JP-A-6-122939 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/02-8/04

Claims (4)

(57) [Claims] C .: 0.0005 to 0.0070%, Si: 0.001 to 0.8%, Mn: 0.6 to 4.0%, P: 0.003 to 0. 15%, S: 0.0010 to 0.015%, Al: 0.005 to 0.1%, N: 0.0003 to 0.0060%, and Ti: 0.003 to 0.1% and Nb : 0.003 ~
Of the 0.1%, one or more slabs having a composition consisting of the balance of Fe and unavoidable impurities are subjected to hot rolling at a temperature of (Ar ₃ -100) ° C. or more, and at a temperature of room temperature to 750 ° C. Winding, 60%
Cold rolling is performed at the above rolling ratio, the annealing temperature in continuous annealing is set to the Ae ₃ transformation point or higher, and (Ar ₁ −
50 ℃) ~ (Ar ₁ + a 50 ° C.) the temperature range of up to an average cooling rate of 30 ° C. / s or higher, baking hardenability and moldability, characterized by chromatic <br/> the low-temperature transformation product single phase structure And excellent cold rolled steel sheet manufacturing method. 2. The method for producing a cold-rolled steel sheet according to claim 1, wherein a slab containing B: less than 0.0030% is used. 3. The method for producing a cold-rolled steel sheet having excellent bake hardenability and non-aging property according to claim 1, wherein a slab containing Cr: 0.01 to 3.0% is used. 4. A slab having the chemical composition according to claim 1, 2 or 3, is subjected to hot rolling at a temperature of (Ar ₃ -100) ° C. or higher, and is wound at a temperature from room temperature to 750 ° C. Cold rolling is performed at a rolling rate of 60% or more, and in the in-line annealing type hot-dip galvanizing line, the annealing temperature is set to the Ae ₃ transformation point or higher, and the annealing temperature is changed to (Ar ₁ −50).
(C) to (Ar ₁ + 50 ° C.), with an average cooling rate of 30 ° C./s or more, and having a low-temperature transformation product single-phase structure and excellent in bake hardenability and non-aging properties. Manufacturing method of galvanized cold rolled steel sheet.