JPH06102816B2 - Cold rolled steel sheet with a composite structure having excellent workability, non-aging at room temperature, and bake hardenability, and a method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention provides a cold-rolled steel sheet which has excellent deep drawability such as an outer panel for automobiles and which is required to have high plastic deformation resistance for finished products. In particular, the present invention relates to a cold-rolled steel sheet having excellent formability during processing, aging resistance at room temperature storage and the like and high bake hardenability (BH property), and a manufacturing method thereof.

(Prior Art) In a press-molded product used for an outer plate of an automobile or the like, external damage due to physical force is a great enemy, and the manufacturing industry of such a product desires to use a steel plate that is less likely to cause external damage, if possible. In such an outer plate, the resistance to dent damage due to collision is called dent resistance, and such a property can be generally obtained by increasing the yield stress of the steel plate.

On the other hand, however, in press working, it is desired to reduce the yield stress of the steel sheet from the viewpoint of energy required for working and accuracy of the shape.

As a steel plate that meets these conflicting requirements,
Some BH (Bake Hardening) steel sheets have an increased yield stress when subjected to a coating treatment including a high temperature holding step of about 0 to 200 ° C. after processing. In this steel sheet, solid solution C or solid solution N, particularly solid solution C, is present in the steel, and the solid solution C or the like applied to the movable dislocations generated by the working due to the high temperature during coating baking diffuses to cause the dislocations of the movable dislocations. It cures by taking advantage of the reduced mobility.

The problem with the above-mentioned hardening mechanism is that some dislocations are already fixed by the solid solution component before processing, and during processing, wavy surface defects called stretcher strains due to elongation at the yield point occur. At present, by using ultra-light reduction cold rolling for shape correction called temper rolling after annealing, mobile dislocations are separated from solid solution components, and strain concentrated portions are generated on the surface, and this strain concentrated portion is generated. Such an inconvenience is prevented by trapping solid solution C or the like in and dispersing to solid dislocations.

Although such means is effective in the short term, it does not completely suppress the progress of the age hardening phenomenon itself, and is particularly remarkable in the case of a high BH steel sheet having a BH amount of 3 kgf / mm ² or more at the yield yield point. ,
Even before processing, solid solution C or the like easily diffuses again to mobile dislocations by long-term room temperature storage or high-temperature processing at a maximum of about 550 ° C. in a surface treatment line or the like. Therefore, at present, the usage conditions of these steel sheets are limited.

As means for suppressing aging at room temperature and obtaining BH property,
Although a quenching treatment in annealing (Japanese Patent Publication No. 61-10014), partial fixation of C by carbonitride forming components (Japanese Patent Publication No. 61-9368, Japanese Patent Publication No. 61-281852), etc. have been proposed. However, in both cases, it is difficult to obtain a high BH amount in order to control the solid solution C with a delicate balance, and it is easy to lose compatibility between non-aging property and BH property due to changes in components and process conditions. was there.

On the other hand, applicants including the inventors have already published Japanese Patent Publication No. 61-1200.
No. 8 publication proposes a completely new type of non-aging, high BH type steel sheet. That is, when an ultra-low carbon steel sheet in which Nb and B are added together is heated to the α phase-γ phase coexisting temperature range of the Ac ₁ transformation point or higher and rapidly cooled, a two-phase structure of acicular ferrite + ferrite is formed. Although this structure has a high BH property containing solid solution C, most of the solid solution C is trapped in acicular ferrite having a large strain, that is, dislocations densely packed, and therefore, there is almost no yield point elongation even after annealing. No, and it does not easily age at room temperature.

However, in this type of non-aging and high BH type steel sheet, the workability, especially the elongation value, was inferior to the conventional non-aging and non-BH type soft steel sheet. Moreover, according to the investigation by the inventors, there was found an example in which even if the r-values were apparently the same, they could not be machined as much as a non-BH type steel plate in actual deep drawing. Further, the workability tends to deteriorate as the annealing temperature increases, and it is difficult to achieve both the best workability and the best non-aging property.

(Problems to be Solved by the Invention) An object of the present invention is to obtain a high bake hardenability (BH property) in a cold-rolled steel sheet used as an outer panel for an automobile,
Preserving at room temperature for a long period of time and aging resistance to temperature rise below the recrystallization temperature that does not include processing (hereinafter referred to as normal temperature non-aging in this specification), also non-aging and non-BH in processability To propose a cold-rolled steel sheet and a method for producing the same, which can be compared with a conventional deep deep-drawing steel sheet.

(Means for Solving the Problems) The present invention is C: 0.01 wt% (hereinafter simply referred to as%), S
i: 0.1% or less, Al: 0.5% or less and N: 0.02% or less,
And one or two of Mn: 0.01 to 2.0% and Cr: 0.005 to 5% is contained under the condition that Mn + Cr × 2 ≧ 0.2 (%) is satisfied, and Nb: 0.005 to 0.1% and
B: 0.0005 to 0.01% of 1 type or 2 types is contained under the condition that Nb + B × 10 ≧ 0.02 (%) is satisfied, and the balance is composed of Fe and unavoidable impurities. A cold-rolled steel sheet with a composite structure, which has an acicular ferrite structure with a volume ratio of 5% or less and a yield point elongation of 0.5% or less, and which is excellent in workability, non-aging at room temperature, and bake hardenability. Invention), and C: 0.01% or less, Si: 0.1% or less, Al: 0.5% or less, and N: 0.02.
% Or less, and Mn: 0.01 to 2.0% and Cr: 0.005 to 5%
1 or 2 of the above is contained under the condition that Mn + Cr × 2 ≧ 0.2 (%) is satisfied, and Nb: 0.005 to 0.1% and
B: 0.0005 to 0.01% of 1 or 2 is contained under the condition that Nb + B × 10 ≧ 0.02 (%) is satisfied, and the balance is steel material consisting of Fe and unavoidable impurities, hot rolled, then cold rolled After rolling, it is continuously annealed at a temperature higher than Ac ₁ transformation point −50 ° C. and lower than Ac ₁ transformation point, and subsequently cooled at a cooling rate of 10 ° C./s or more, and then temper rolling with a reduction rate of 0.8% or more. A method for producing a cold-rolled steel sheet having a composite structure excellent in workability, non-aging at room temperature and bake hardenability (second invention), and C: 0.01% or less, Si: 0.1% or less, Ti : 0.005-0.05%, Al:
0.5% or less, N: 0.02% or less and S: 0.05% or less are included under the condition that C- (12 / 48Ti-12 / 32S-12 / 14N) ≥ 0.0005 (%) is satisfied, and Mn: 0.01 to 2.0 % And Cr: 0.0
05 to 5% of 1 or 2 is contained under the condition that Mn + Cr x 2 ≥ 0.2 (%) is satisfied, and Nb: 0.005 to 0.1% and
B: 0.0005 to 0.01% of 1 type or 2 types is contained under the condition that Nb + B × 10 ≧ 0.02 (%) is satisfied, and the balance is composed of Fe and unavoidable impurities. A cold-rolled steel sheet with a complex structure, which has an acicular ferrite structure with a volume ratio of 5% or less and a yield point elongation of 0.5% or less and is excellent in workability, normal temperature non-aging property and bake hardenability (3rd Invention) and C: 0.01% or less, Si: 0.1% or less, Ti: 0.005 to 0.05%, Al:
0.5% or less, N: 0.02% or less and S: 0.05% or less are included under the condition that C- (12 / 48Ti-12 / 32S-12 / 14N) ≥ 0.0005 (%) is satisfied, and Mn: 0.01 to 2.0 % And Cr: 0.0
05 to 5% of 1 or 2 is contained under the condition that Mn + Cr x 2 ≥ 0.2 (%) is satisfied, and Nb: 0.005 to 0.1% and
B: 0.0005 to 0.01% of 1 or 2 is contained under the condition that Nb + B × 10 ≧ 0.02 (%) is satisfied, and the balance is steel material consisting of Fe and unavoidable impurities, hot rolled, then cold rolled After rolling, it is continuously annealed at a temperature higher than Ac ₁ transformation point −50 ° C. and lower than Ac ₁ transformation point, and subsequently cooled at a cooling rate of 10 ° C./s or more, and then temper rolling with a reduction rate of 0.8% or more. A method of manufacturing a cold rolled steel sheet with a composite structure having excellent workability, non-aging at room temperature and bake hardenability (4th invention), and C: 0.01% or less, Si: 0.1% or less, P : 0.03-0.15%, Al: 0.5
% Or less and N: 0.02% or less, and one or two of Mn: 0.01 to 2.0% and Cr: 0.005 to 5% under the condition of satisfying Mn + Cr × 2 ≧ 0.2 (%), and Nb: 0.005-0.1% and
B: 0.0005 to 0.01% of 1 type or 2 types is contained under the condition that Nb + B × 10 ≧ 0.02 (%) is satisfied, and the balance is composed of Fe and unavoidable impurities. A cold-rolled steel sheet with a complex structure, which has an acicular ferrite structure with a volume ratio of 5% or less and a yield point elongation of 0.5% or less, and which has excellent workability, normal temperature non-aging property, and bake hardenability. Invention) and C: 0.01% or less, Si: 0.1% or less, P: 0.03 to 0.15%, Al: 0.5
% Or less and N: 0.02% or less, and one or two of Mn: 0.01 to 2.0% and Cr: 0.005 to 5% under the condition of satisfying Mn + Cr × 2 ≧ 0.2 (%), and Nb: 0.005-0.1% and
B: 0.0005 to 0.01% of 1 or 2 is contained under the condition that Nb + B × 10 ≧ 0.02 (%) is satisfied, and the balance is steel material consisting of Fe and unavoidable impurities, hot rolled, then cold rolled After rolling, it is continuously annealed at a temperature higher than Ac ₁ transformation point −50 ° C. and lower than Ac ₁ transformation point, and subsequently cooled at a cooling rate of 10 ° C./s or more, and then temper rolling with a reduction rate of 0.8% or more. A method for producing a cold-rolled steel sheet having a composite structure excellent in workability, non-aging at room temperature and bake hardenability (the sixth invention), and C: 0.01% or less, Si: 0.1% or less, Ti : 0.005-0.05%, P: 0.
03-0.15%, Al: 0.5% or less, N: 0.02% or less and S: 0.05%
The following are included under the condition that C- (12 / 48Ti-12 / 32S-12 / 14N) ≧ 0.0005 (%) is satisfied, and Mn: 0.01 to 2.0% and Cr: 0.0
05 to 5% of 1 or 2 is contained under the condition that Mn + Cr x 2 ≥ 0.2 (%) is satisfied, and Nb: 0.005 to 0.1% and
B: 0.0005 to 0.01% of 1 type or 2 types is contained under the condition that Nb + B × 10 ≧ 0.02 (%) is satisfied, and the balance is composed of Fe and unavoidable impurities. A cold-rolled steel sheet with a composite structure, which has an acicular ferrite structure with a volume ratio of 5% or less and a yield point elongation of 0.5% or less, and which is excellent in workability, non-aging at room temperature, and bake hardenability. Invention) and C: 0.01% or less, Si: 0.1% or less, Ti: 0.005 to 0.05%, P: 0.
03-0.15%, Al: 0.5% or less, N: 0.02% or less and S: 0.05%
The following are included under the condition that C- (12 / 48Ti-12 / 32S-12 / 14N) ≧ 0.0005 (%) is satisfied, and Mn: 0.01 to 2.0% and Cr: 0.0
05 to 5% of 1 or 2 is contained under the condition that Mn + Cr x 2 ≥ 0.2 (%) is satisfied, and Nb: 0.005 to 0.1% and
B: 0.0005 to 0.01% of 1 or 2 is contained under the condition that Nb + B × 10 ≧ 0.02 (%) is satisfied, and the balance is steel material consisting of Fe and unavoidable impurities, hot rolled, then cold rolled After rolling, it is continuously annealed at a temperature higher than Ac ₁ transformation point −50 ° C. and lower than Ac ₁ transformation point, and subsequently cooled at a cooling rate of 10 ° C./s or more, and then temper rolling with a reduction rate of 0.8% or more. Is a method of manufacturing a cold rolled steel sheet having a composite structure excellent in workability, non-aging at room temperature and bake hardenability (eighth invention).

(Action) According to the conventional wisdom, suppression of aging due to strain concentrated in the second phase grains and / or grain boundaries is such that the second phase is at least 10% of the total.
If it did not reach a certain level, it would not work effectively.
This also applies to a steel sheet having acicular ferrite as the second phase.

However, the present inventors have found that, in a steel sheet having acicular ferrite as the second phase, when the yield point elongation is 0.5% or less by externally reinforcing strain concentration, the fraction of the second phase is It was discovered that it has an aging suppressing effect even if it is lower.

Hereinafter, the experiment that was the basis of the present invention will be described.

C: 0.0030%, Si: 0.01%, Mn: 0.61%, P: 0.015%, S: 0.0
07%, Al: 0.059%, N: 0.0025%, Nb: 0.012% and B: 0.00
Steel having a composition of 13% was subjected to the steps of continuous casting-hot rolling-cold rolling-continuous annealing while changing the annealing temperature to obtain the sheet thickness.
It was a 0.8 mm cold rolled steel sheet. Here the slab heating temperature is 1250
℃, Ar ₃ transformation point 850 ℃, hot rolling finish temperature 880 ℃, hot rolled sheet thickness was 3.2mm, hot rolling coiling temperature was 620 ℃. Cold rolling reduced to 0.8 mm. The Ac ₁ transformation point measured by the volume expansion coefficient measurement (temperature rising rate 1 ° C./s) after cold rolling was 910 ° C. The annealing temperature was various temperatures of 810 to 950 ° C (soaking for 5 seconds), and the subsequent cooling rate was 25 ° C / s.

The BH content of the steel sheet thus obtained was 5.0 to 5.5 kgf / mm ² . Here, as for the amount of BH, the difference between the nominal stress at 2% prestrain of nominal strain and the yield stress (nominal) after aging treatment at 170 ° C. for 20 minutes after prestrain was taken.

1.2% temper rolling is applied to each of the above cold rolled steel sheets, and the second phase volume ratio, elongation value, room temperature non-aging (yield point elongation) of each steel sheet before and after temper rolling Change first
Shown in the figure.

In the figure, in order to obtain sufficient non-aging at room temperature without temper rolling, the volume fraction of the second phase should be 10% or more, but if temper rolling (1.2%) is applied, the second phase Accelerated aging treatment (100 ℃, 10 hours aging; 30 ℃,
Equivalent to carbon aging for 6 months), non-aging at room temperature is also performed. In this region, the elongation value is good and the aging does not deteriorate.

Thus, the heat treatment (annealing) temperature at which the second phase volume ratio is 5% or less is lower than the Ac ₁ transformation point measured by thermal expansion. In this temperature range, acicular ferrite is only observed as fine grains at ferrite grain boundaries, especially at triple points. Therefore, it is considered that Mn was segregated in the grain boundary portion and only this portion was transformed into the γ phase at a temperature lower than the transformation temperature of the ferrite matrix phase during annealing.

There is a condition that the normal temperature non-aging property deteriorates just above the transformation point of the parent phase, but this is thought to be because the grain size suddenly became coarse due to the initiation of transformation of the parent phase, which prevented strain concentration due to temper rolling. .

It has been found that the steel sheet containing fine acicular ferrite in a total volume ratio of 5% or less in ferrite as described above is also excellent in workability. As shown in FIG.
Elongation values are better than when annealed above the ferrite phase transformation point. The same applies to the r value, but the actual workability with respect to the r value is also good.

Fig. 2 shows three types of cold-rolled steel sheets, that is, ferrite + fine grain acicular ferrite type steel sheet (C: 0.00
32-0.0036%, Si: 0.01%, Mn: 0.40-0.42%, Cr: 0.08
%, P: 0.020%, S: 0.010%, Al: 0.06%, N: 0.0026 to 0.0
027%, Nb: 0.015 to 0.025%, B: 0.0012%, slab heating temperature: 1200 ° C, Ar ₃ transformation point: 830 ° C, hot rolling end temperature: 850 ° C, hot rolled sheet thickness: 3.5 mm, hot rolled sheet Winding temperature: 600 ℃, cold rolled sheet thickness:
0.8mm, Ac ₁ transformation point: 890 ℃, annealing temperature: 870 ℃, soaking time: 1
0 seconds, cooling rate: 20 ° C / s, temper rolling reduction: 1.2%; non-aging, high BH type), conventional ferrite + coarse grain acicular ferrite type steel sheet (C: 0.0030 to 0.0033%, Si: 0.01) %, Mn: 0.
40%, P: 0.018%, S: 0.010%, Al: 0.05%, N: 0.0024〜
0.0028%, Nb: 0.015 to 0.025%, B: 0.0013%, slab heating temperature: 1200 ° C, Ar ₃ transformation point: 860 ° C, hot rolling end temperature: 890
℃, hot-rolled sheet thickness: 3.5mm, hot-rolled sheet winding temperature: 500 ℃, cold-rolled sheet thickness: 0.8mm, Ac ₁ transformation point: 910 ℃, annealing temperature: 940 ℃, soaking time: 5 seconds , Cooling rate: 20 ℃ / s, temper rolling reduction: 0.5%;
Non-aged / high BH type) and normal ferrite single-phase type steel sheet (C:
0.0022-0.0026%, Si: 0.01-0.03%, Mn: 0.08-0.12
%, P: 0.008% to 0.012%, S: 0.005 to 0.015%, Al: 0.03
% -0.05%, N: 0.0020-0.0028%, Nb: 0.015-0.020
%, Slab heating temperature: 1200-1250 ℃, Ar ₃ transformation point: 870 ℃,
Hot rolling end temperature: 880-900 ° C, hot-rolled sheet thickness: 3.5 mm, hot-rolled sheet winding temperature: 580-660 ° C, cold-rolled sheet thickness: 0.8 mm, Ac ₁ transformation point: 9
30 ° C, annealing temperature: 830-880 ° C, soaking time: 0-20 seconds, cooling rate: 15-30 ° C / s; non-aging / non-BH type) r value and LDR (limit drawing ratio) test (Wrinkle holding force: 700 kgf) The results of the investigation for the values are shown below. Here, the TS (tensile strength) level of each steel plate is set to 29 to 31 kgf / mm ² .

In FIG. 2, the conventional ferrite + coarse grain acicular ferrite type steel sheet has a slightly lower drawing ratio when the drawing is actually applied even if the r value is the same as that of the ferrite single-phase type steel sheet. It is considered that this is because when the hard working is applied, the hard second phase restrains the deformation of the ferrite grains and reduces the anisotropy of the plastic deformation due to the crystal orientation. However, if the second phase is a fine grain and its volume ratio is low, it is processed without restraining the anisotropy of plastic deformation of the ferrite grain, so that a drawing ratio similar to that of a normal ferrite single-phase steel sheet is obtained. Has been.

The inventors conducted further research based on the above findings,
The conditions necessary to obtain a high BH steel sheet having excellent workability and non-aging at room temperature have been found.

First, the particle size of the second phase (acicular ferrite) is 30μ
It must be m or less. If the particle size exceeds this, the workability is adversely affected. The volume ratio is
As described above, 5% or less is required to secure workability and non-aging at room temperature. In the experiments by the inventors, when the second phase was observed even at all, it had room temperature non-aging property, but the minimum value of the second phase volume ratio confirmed at this time was 0.1%.
Met.

Further, in order to obtain the grain size distribution as described above, the presence of Mn is important due to the mechanism of formation of the fine second phase, but the present inventors also have similar grain boundary segregation / γ phase formation promoting effect in Cr. I found that. Specifically, it is necessary to contain Mn and / or Cr in a content of Mn + Cr × 2 ≧ 0.2% in order to generate a fine γ phase during annealing and heating. Further, in order to convert the γ phase into an acicular ferrite during cooling, it is necessary to contain Nb and / or B in a content of Nb + B × 10 ≧ 0.02% to delay the γ → α transformation to the low temperature side.

Also in the steel sheet having the above-mentioned second phase distribution, when the initial yield point elongation exceeds 0.5%, a slight increase in the yield point elongation due to accelerated aging is recognized. It is necessary to keep the yield point elongation at 0.5% or less.

The reasons for limiting the composition range of each component will be described below.

C: C is an important component for imparting BH property, but 0.0
If it exceeds 1%, it becomes difficult to maintain non-aging at room temperature even with the method of the present invention. Further, the lower the amount of C, the more advantageous the material is, and if it exceeds 0.01%, good workability cannot be obtained. Therefore, the C content is 0.01% or less. Still higher BH
In order to obtain the property, the C content is preferably 0.0005% or more, and particularly when Ti, which is a strong carbide-forming component, is added as described later, the solid solution C content is 0.0005% or more is essential.

Si: Si does not hinder the inclusion of a little for increasing the strength of the steel sheet, but if it is present in excess of 0.1%, the elongation and drawability of the steel sheet deteriorate, so it is made 0.1% or less.

Al: Al is mainly added to steel for deoxidation during steelmaking.
Further, Al has the effect of fixing N as AlN and preventing the decrease in B yield due to BN formation, so it is desirable to contain 0.005% or more. However, the addition of Al in excess of 0.5% adversely affects the surface properties, so it must be kept below 0.5%. It is preferably 0.1% or less.

N: N deteriorates the deep drawability and, if not fixed with Al, bonds with B and significantly reduces the effect of adding B. Therefore, the larger the amount is, the more the required amount of Al is uneconomical. Further, since solid solution N originally has a high aging property at room temperature, it is not used as a baking effect component in the present invention. Therefore, it is desirable to keep the amount of N as low as possible.
0.02% or less. Preferably, the N content is 0.005% or less.

Mn, Cr: For the reasons already mentioned, it is necessary to add Mn + Cr × 2 ≧ 0.2%. Both Mn and Cr are components that can lower the transformation temperature of the entire steel sheet without deteriorating the deep drawability, and are also effective for increasing the strength of the steel sheet. However
Addition of less than 0.01% of Mn or less than 0.005% of Cr makes little contribution to the reduction of transformation temperature and the increase of steel plate strength. On the other hand, addition of Mn in excess of 2.0% or excess addition of Cr in excess of 5% deteriorates the balance between elongation and drawability of the steel sheet and strength. Further, addition of Mn in excess of 2% increases the endothermic reaction in the molten steel, and there is a possibility that vacuum degassing treatment may become impossible due to a decrease in molten steel temperature. Moreover, excessive addition of Cr exceeding 5.0% deteriorates the chemical conversion treatment property of the steel sheet surface. Therefore Mn: 0.01-2.0
%, Cr: 0.005 to 5%, one or two, Mn + Cr × 2 ≧ 0.
It is necessary to satisfy the content of 2%.

As described above, Nb, B: Nb and B each delay the γ → α transformation during cooling, either alone or in the presence of each other, to the low temperature side, thereby promoting the acicular ferrite + ferrite two-phase organization. Further, in the texture as well, remarkable accumulation of (111) orientation is caused, which has an effect of improving workability (especially r value). If Nb + B × 10 is less than 0.02%, the two-phase organization and texture improvement are insufficient, while the addition effect is saturated when the Nb content exceeds 0.1% or the B content exceeds 0.01%. Not only that, it causes remarkable deterioration of elongation and lowers the balance between strength and workability. Further, the addition of less than 0.0005% of the amount of B does not contribute to the effect of the joint addition, so there is no significance in adding B. The same applies to the addition of less than 0.005% of Nb. Therefore Nb: 0.005-0.1% and
B: 0.0005 to 0.01% of 1 type or 2 types, Nb + B × 10 ≧ 0.
It is necessary to satisfy the content of 02%.

Ti, S: In the present invention, it is not necessary to specify S as a general component in steel, but it is desirable to reduce S within a range commensurate with the cost as a steel plate for working, and 0.05%
The following is preferable. Also effective when Ti is added
The S addition amount has an important meaning in considering the Ti amount.

In the present invention, Ti may be added in order to improve workability and B yield. 0 to get the effect of Ti.
It is necessary to add 005% or more, but addition of more than 0.05% is not only uneconomical in terms of the effect of addition, but also causes an increase in production cost due to an increase in the transformation point. Further, in order to secure BH property, it is necessary to secure a solid solution C amount of 0.0005% or more, and regarding effective Ti (TiN excluding TiN and TiS forming components),
C- (12 / 48Ti-12 / 32S-12 / 14N) ≧ 0.0005 (%) must be satisfied. Further, when Ti is added, S is set to 0.05% or less in order to prevent a decrease in Ti yield. Therefore
The conditions for adding Ti are as follows: Ti: 0.005 to 0.05%, C- (12 / 48Ti-12 / 32S-12 / 14N) ≧ 0.0005 (%), and S: 0.05% or less.

P: P as a general component in the steel need not be specified, but it is preferably 0.05% or less for applications requiring softness. The addition of 0.15% or less as a steel plate reinforcing component does not impair the effects of the present invention.
On the other hand, since the effect of P strengthening the steel plate hardly appears unless 0.03% or more is added, the P amount must be 0.03% to 0.15% when P is added for the purpose of strengthening the steel plate.

Next, the reasons for limiting the steel sheet manufacturing conditions of the present invention will be described below.

First, steelmaking may be carried out according to a conventional method. In particular, the present invention does not require the limitation of the conditions, but it is preferable to use the continuous casting method in terms of cost and quality.

Hot rolling may be carried out by a conventional method, but fine γ during annealing.
In order to sufficiently concentrate Mn and / or Cr in the grain boundaries to generate grains, the coiling temperature after hot rolling is preferably set to 550 ° C or higher.

Cold rolling may be performed according to a conventional method, but a cold rolling reduction of 60% or more is desirable for obtaining workability by recrystallization.

As for the annealing after cold rolling, the box annealing method is not sufficient for forming a two-phase structure, so the continuous annealing method is adopted. Further, the annealing temperature is set to a temperature lower than the Ac ₁ transformation point of the ferrite phase which is the first phase, as is clear from FIG. 1, in order to obtain the desired fine second phase structure in the present invention. The lower limit of the annealing temperature may be a temperature at which the γ phase (austenite phase), which is the origin of the second phase, appears, but it is stable below the Ac ₁ transformation point of the ferrite phase −50 ° C. Two phases can be obtained. Since it is an annealing at a higher temperature than normal annealing, ferrite grains are likely to grow, and if the temperature is soaked for more than 15 seconds,
At room temperature, non-aging may be deteriorated due to a decrease in grain boundary area and a decrease in strain concentration. Therefore, the annealing soaking time is preferably 15 seconds or less.

In the cooling subsequent to the annealing, at least a part of the γ phase is converted to acicular ferrite during cooling, so that the cooling rate needs to be 10 ° C./s or more. On the other hand, from the viewpoint of workability, the cooling rate is preferably 80 ° C / s or less.

In the fine acicular ferrite + ferrite dual-phase steel sheet thus obtained, it is necessary to strengthen the strain concentration inside the steel sheet as described above, but temper rolling is most suitable as this method. In this case, the reduction ratio of temper rolling is in order to strengthen strain concentration and obtain excellent non-aging property.
0.8% or more is necessary, and 1% or more is preferable. 5
If the rolling reduction is more than%, the deterioration of the material becomes large, so it is desirable to avoid it.

The cold-rolled steel sheet of the present invention can be applied to a plated steel sheet, and is particularly suitable as a mother plate of a hot-dip galvanized steel sheet which is likely to be aged in the plating process.

(Example) Example 1 Steels having various compositional compositions shown in Table 1 were prepared.

These test steels are manufactured by continuous casting and rough rolling (reduction ratio
88%), finish rolling (88% reduction) to obtain a hot coil with a plate thickness of 3.5 mm, and then cold rolling to 0.8 mm. After that, it was pickled and continuously annealed and temper-rolled. Second by the main conditions of each process and the light microscopic examination of the obtained steel sheet
Table 2 shows the maximum particle size and volume fraction of the phase (acicular ferrite phase).

Table 3 shows the workability, BH property and aging property of the cold-rolled steel sheet thus obtained. The BH amount was the falling yield point value shown in FIG.

As is clear from Table 3, all of the examples of the present invention showed not only a high BH value and excellent non-aging at room temperature, but also excellent elongation / strength balance (shown by T.S + E1 in Table 3) and r value. It can be seen that it has workability.

On the other hand, Comparative Examples NO. 10 to 13 whose components are not compatible with this invention and Comparative Examples NO. 1B, 1C, 1D, 1 whose process conditions are not compatible with this invention.
Both G and 1H are inferior in BH property or normal temperature non-aging property as compared with the examples of the present invention because neither good second phase distribution is obtained or no second phase is obtained. Many are inferior.

Further, Comparative Examples 1E and 1F have a high yield point elongation before accelerated aging treatment, which is out of the problem.

Example 2 Inventive Examples 1A and 3 of Example 1 were passed through a hot dip galvanizing line. The soaking cycle of the plating line is 550 ℃, 20
It was seconds.

The material of plated steel sheet 1A is YS: 16.5kgf / mm ² , TS: 3
0.2kgf / mm ² , elongation value 56.2%, r value 2.50, BH amount 5.0kgf / m
m ² and yield point elongation were 0.0%. Steel plate 3 also has YS: 19.8
kgf / mm ² , TS: 35.4kgf / mm ² , elongation value 52.0%, r value 2.4
1. The BH content was 5.0 kgf / mm ² and the yield point elongation was 0.0%. In both cases, the workability is almost the same as before plating,
BH property and non-aging property at room temperature were also unchanged.

(Effects of the Invention) According to the present invention, it is possible to industrially safely produce a steel sheet having excellent workability comparable to that of a steel sheet for soft working, high BH property, and non-aging at room temperature. Such a steel sheet is particularly suitable as a steel sheet for outer plate processing, and is used under conditions where conventional BH steel sheets for processing cannot be used due to aging, for example, for long-term storage necessary for always securing inventory, long-term storage. It will open the way for applications such as exports requiring sea travel, and for alloy-plated original plates that pass high temperatures of around 500 ° C.

[Brief description of drawings]

FIG. 1 shows the second effect on workability and normal temperature non-aging of steel sheet.
2 is a graph showing the influence of the phase acicular ferrite content and the annealing temperature, FIG. 2 is a graph showing the relationship between the r value and the limiting drawing ratio of the steel of the present invention and the conventional one, and FIG. 3 is the bake hardenability (BH property). ) Is a diagram showing a measuring method of FIG.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-61-199054 (JP, A) JP-A-59-38337 (JP, A)

Claims (8)

[Claims] 1. C: 0.01 wt% or less, Si: 0.1 wt% or less, Al: 0.5 wt% or less and N: 0.02 wt% or less, and Mn: 0.01 to 2.0 wt% and Cr: 0.005 to 5 wt% 1 or 2 of Nb + Bx under the condition that Mn + Cr × 2 ≧ 0.2 (wt%) is satisfied, and 1 or 2 of Nb: 0.005-0.1wt% and B: 0.0005-0.01wt% is added to Nb + B. It is contained under the condition that × 10 ≧ 0.02 (wt%) is satisfied, and the balance is composed of Fe and unavoidable impurities, and has a structure of ferrite and acicular ferrite with a grain size of 30 μm or less and a total volume ratio of 5% or less. A cold-rolled steel sheet having a composite structure, which is excellent in workability, non-aging at room temperature, and bake hardenability, characterized in that the yield point elongation is 0.5% or less. 2. C: 0.01 wt% or less, Si: 0.1 wt% or less, Al: 0.5 wt% or less and N: 0.02 wt% or less, and Mn: 0.01 to 2.0 wt% and Cr: 0.005 to 5 wt% 1 or 2 of Nb + Bx under the condition that Mn + Cr × 2 ≧ 0.2 (wt%) is satisfied, and 1 or 2 of Nb: 0.005-0.1wt% and B: 0.0005-0.01wt% is added to Nb + B. × contained under conditions satisfying the 10 ≧ 0.02 (wt%), the balance in the steel material consisting of Fe and unavoidable impurities, hot rolling, and then subjected to cold rolling, from Ac ₁ transformation point -50 ° C. High workability characterized by continuous annealing at a temperature below the Ac ₁ transformation point, subsequent cooling at a cooling rate of 10 ° C / s or more, and temper rolling with a rolling reduction of 0.8% or more. Method for producing a cold rolled steel sheet having a composite structure, which is excellent in heat resistance and bake hardenability. 3. C: 0.01 wt% or less, Si: 0.1 wt% or less, Ti: 0.005 to 0.05 wt%, Al: 0.5 wt% or less, N: 0.02 wt% or less and S: 0.05 wt% or less -(12 / 48Ti-12 / 32S-12 / 14N) ≥ 0.0005 (wt%) is included, and one or two of Mn: 0.01 to 2.0wt% and Cr: 0.005 to 5wt% is included. It is contained under the condition that Mn + Cr × 2 ≧ 0.2 (wt%) is satisfied, and one or two of Nb: 0.005-0.1wt% and B: 0.0005-0.01wt% is added, Nb + B × 10 ≧ 0.02 (wt%) The balance is Fe and the composition of unavoidable impurities, and the composition is ferrite and an acicular flight structure with a grain size of 30 μm or less and a total volume ratio of 5% or less. The yield point elongation is 0.5%.
A cold-rolled steel sheet having a composite structure, which is excellent in workability, non-aging at room temperature, and bake hardenability, characterized in that: 4. C: 0.01 wt% or less, Si: 0.1 wt% or less, Ti: 0.005-0.05 wt%, Al: 0.5 wt% or less, N: 0.02 wt% or less, and S: 0.05 wt% or less -(12 / 48Ti-12 / 32S-12 / 14N) ≥ 0.0005 (wt%) is included, and one or two of Mn: 0.01 to 2.0wt% and Cr: 0.005 to 5wt% is included. It is contained under the condition that Mn + Cr × 2 ≧ 0.2 (wt%) is satisfied, and one or two of Nb: 0.005-0.1wt% and B: 0.0005-0.01wt% is added, Nb + B × 10 ≧ 0.02 (wt%) The steel material containing Fe and unavoidable impurities in the balance is hot-rolled and then cold-rolled, and then the Ac ₁ transformation point is higher than −50 ° C. and less than the Ac ₁ transformation point. A composite with excellent workability, non-aging at room temperature, and bake hardenability, characterized by continuous annealing at a temperature and subsequent cooling at a cooling rate of 10 ° C / s or more, followed by temper rolling with a rolling reduction of 0.8% or more. A method for manufacturing a structure cold-rolled steel sheet. 5. C: 0.01 wt% or less, Si: 0.1 wt% or less, P: 0.03 to 0.15 wt%, Al: 0.5 wt% or less and N: 0.02 wt% or less, and Mn: 0.01 to 2.0 wt% % And Cr: 0.005 to 5 wt% of 1 or 2 are contained under the condition that Mn + Cr x 2 ≥ 0.2 (wt%) is satisfied, and Nb: 0.005 to 0.1 wt% and B: 0.0005 to 0.01 wt% are contained. One or two kinds are contained under the condition that Nb + B × 10 ≧ 0.02 (wt%) is satisfied, and the balance becomes the structure of Fe and unavoidable impurities. Ferrite and grain size of 30 μm or less and total volume ratio of 5% or less A cold-rolled steel sheet with a complex structure, which has an acicular ferrite structure and a yield point elongation of 0.5% or less, and is excellent in workability, non-aging at room temperature, and bake hardenability. 6. C: 0.01 wt% or less, Si: 0.1 wt% or less, P: 0.03 to 0.15 wt%, Al: 0.5 wt% or less and N: 0.02 wt% or less, and Mn: 0.01 to 2.0 wt% % And Cr: 0.005 to 5 wt% of 1 or 2 are contained under the condition that Mn + Cr x 2 ≥ 0.2 (wt%) is satisfied, and Nb: 0.005 to 0.1 wt% and B: 0.0005 to 0.01 wt% are contained. 1 or 2 is contained under the condition that Nb + B × 10 ≧ 0.02 (wt%) is satisfied, and the balance is steel material consisting of Fe and unavoidable impurities, hot-rolled and then cold-rolled, It is characterized in that continuous annealing is performed at a temperature higher than the Ac ₁ transformation point −50 ° C. and lower than the Ac ₁ transformation point, followed by cooling at a cooling rate of 10 ° C./s or more, and then temper rolling with a reduction rate of 0.8% or more. A method for producing a cold rolled steel sheet having a composite structure, which is excellent in workability, non-aging at room temperature and bake hardenability. 7. C: 0.01 wt% or less, Si: 0.1 wt% or less, Ti: 0.005-0.05 wt%, P: 0.03-0.15 wt%, Al: 0.5 wt% or less, N: 0.02 wt% or less and S : 0.05Wt% or less is included under the condition that C- (12 / 48Ti-12 / 32S-12 / 14N) ≧ 0.0005 (wt%) is satisfied, and Mn: 0.01-2.0wt% and Cr: 0.005-5wt% 1 or 2 of Nb + Bx under the condition that Mn + Cr × 2 ≧ 0.2 (wt%) is satisfied, and 1 or 2 of Nb: 0.005-0.1wt% and B: 0.0005-0.01wt% is added to Nb + B. It is contained under the condition that × 10 ≧ 0.02 (wt%) is satisfied, and the balance is composed of Fe and unavoidable impurities, and has a structure of ferrite and acicular ferrite with a grain size of 30 μm or less and a total volume ratio of 5% or less. A cold-rolled steel sheet having a composite structure, which is excellent in workability, non-aging at room temperature, and bake hardenability, characterized in that the yield point elongation is 0.5% or less. 8. C: 0.01 wt% or less, Si: 0.1 wt% or less, Ti: 0.005-0.05 wt%, P: 0.03-0.15 wt%, Al: 0.5 wt% or less, N: 0.02 wt% or less and S : 0.05wt% or less under the condition that C- (12 / 48Ti-12 / 32S-12 / 14N) ≧ 0.0005 (wt%) is satisfied, and Mn: 0.01-2.0wt% and Cr: 0.005-5wt% 1) or 2) under the condition that the following formula Mn + Cr × 2 ≧ 0.2 (wt%) is satisfied, and further, 1 or 2 of Nb: 0.005-0.1wt% and B: 0.0005-0.01wt% is added. contains the conditions satisfying the following formula Nb + B × 10 ≧ 0.02 ( wt%), the balance in the steel material consisting of Fe and unavoidable impurities, hot rolling and then after performing cold rolling, Ac ₁ transformation point It is characterized by being continuously annealed at a temperature higher than −50 ° C and lower than the Ac ₁ transformation point, subsequently cooled at a cooling rate of 10 ° C / s or more, and then temper-rolled with a rolling reduction of 0.8% or more. , Cold rolling of composite structure with excellent non-aging at room temperature and bake hardenability Method of manufacturing the plate.