JPH07300623A - Production of sheet steel for working excellent in baking hardenability and aging resistance - Google Patents

Abstract

PURPOSE:To produce a sheet steel for working, excellent in baking hardenability, aging resistance, and workability, by applying specific annealing and cooling to a cold rolled steel sheet of specific composition consisting of C, Si, Mn, P, S, N, Al, Nb, and Fe. CONSTITUTION:A hot rolled steel plate, consisting of, by mass, 0.0010-0.0025% C, <=1.0% Si, 0.05-1.5% Mn, <=0.10% P, <=0.02% S, <=0.004% N, 15N to 0.10% Al, 3C to (8C+0.02%) Nb, and the balance Fe with inevitable impurities and further containing, if necessary, prescribed amounts of Ti, B, Cr, Ni, Mo, and Cu, is cold-rolled at 60-90% draft. The resulting cold rolled steel sheet is annealed under the conditions of 820 to (Ac3 point +20 deg.C) maximum heating temp. and >=10sec holding time at 780 to (Ac3 point +20 deg.C). Subsequently, this steel sheet is subjected to cooling consisting of primary cooling in which average cooling rate between 780 and 600 deg.C is regulated to >=25 deg.C/sec and secondary cooling in which average cooling rate between 400 and 200 deg.C is regulated to <=15 deg.C/sec.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has good properties for use in bending, press molding, drawing, etc., mainly for automobile bodies, and has a high bake hardening especially in paint baking. The present invention relates to a method for producing a thin steel sheet that exhibits excellent aging resistance while exhibiting properties, that is, a cold rolled steel sheet and a surface-treated steel sheet that have these characteristics.

[0002]

2. Description of the Related Art Recently, a high-strength steel sheet having excellent workability has been demanded from the viewpoint of weight reduction and safety improvement of a vehicle body for the purpose of improving fuel efficiency in accordance with exhaust gas regulations of automobiles. As a method of increasing the tensile strength, a method of increasing the strength of the steel sheet itself without degrading the workability has been studied so far, while the yielding in each part of the press-formed product due to the difference in the amount of plastic strain after working Methods have been studied for alleviating differences in strength and imparting bake hardenability that improves tensile rigidity. Bake-hardening is a phenomenon in which the yield point of a steel sheet rises due to strain aging and the deformation strength rises, as a result of heating by paint baking treatment performed after press forming. According to this method, high strength can be achieved while the load of the processing machine such as a press is small, so that the deterioration of the workability due to the higher strength is small when compared with steel sheets with the same strength after processing and baking coating. There are advantages such as.

From these viewpoints, various methods have heretofore been proposed to combine the workability and bake hardenability of a steel sheet. For example, Japanese Examined Patent Publication No. 60-17004 and Japanese Examined Patent Publication No. 61
-9365 gazette and Japanese Patent Publication No. 61-54089 gazette show that solid solution C and N hardly exist before annealing.
b) Addition of bake hardenability and workability is achieved by re-solidifying the carbide formed during hot rolling by high temperature annealing in ultra-low carbon steel by continuous annealing and subsequent re-precipitation by rapid cooling. A method of manufacturing a cold rolled steel sheet or a surface treated steel sheet is disclosed.

[0004]

However, in the cold-rolled steel sheet manufactured by the above-mentioned known method, when the C content is small, sufficient bake hardenability cannot be obtained, and on the other hand, sufficient bake hardenability is not obtained. When bake hardenability is obtained, there is a problem that it is not possible to obtain a product that satisfies both high bake hardenability and excellent aging resistance, such as poor aging resistance.

Therefore, a main object of the present invention is to provide a thin steel plate for working which has bake hardenability, excellent aging resistance even when high bake hardenability is imparted, and excellent workability as well. Proposing a manufacturing method. And
Another object of the present invention is to have a bake hardening index (BH) of 45 MPa.
As mentioned above, it is to propose a method of manufacturing a thin steel sheet for working satisfying the characteristics that the aging index (AI) is 30 MPa or less, the r value is 1.6 or more, and TS × El is 14000 (MPa ·%) or more.

[0006]

Under the recognition of such problems, the inventors of the present invention have conducted diligent research to solve the problems, and as a result, the content range of the composition of the steel, the mutual relation between the respective components,
The inventors have found that the problems described above can be solved by controlling the cold rolling conditions, the annealing conditions for cold rolled sheets, and the cooling conditions after annealing, and have completed the present invention. That is, the gist of the present invention is as follows.

(1) C: 0.0010 to 0.0025 mass%, Si: 1.
0 mass% or less, Mn: 0.05 to 1.5 mass%, P: 0.10.
mass% or less, S: 0.02 mass% or less, N: 0.00
4 mass% or less, Al: N (mass%) × 15 to 0.10 mass%, N
b: C (mass%) × 3 to C (mass%) × 8 + 0.02 mass
% Containing the balance in the hot-rolled steel sheet having a composition consisting of Fe and unavoidable impurities, was subjected to rolling reduction 60% to 90% cold rolling, the maximum heating temperature is 820 ~ (Ac ₃ transformation point + 20 ° C.) ℃, and
780 to the retention time in the temperature range of (Ac ₃ transformation point + 20 ° C.) is annealed at least 10 sec, 1 primary cooling and is then 780-600 average cooling rate between ° C. is 25 ° C. / sec or higher, 40
The average cooling rate between 0 and 200 ℃ is 15 ℃ / sec or less 2
A method for producing a thin steel sheet for working, which is excellent in bake hardenability and aging resistance, characterized by performing cooling including the following cooling.

(2) C: 0.0010 to 0.0025 mass%, Si: 1.
0 mass% or less, Mn: 0.05 to 1.5 mass%, P: 0.10.
mass% or less, S: 0.02 mass% or less, Al: 0.01
~ 0.06 mass%, N: 0.004 mass% or less, Nb: C (mass
%) × 3 to C (mass%) × 8 + 0.02 mass% Ti: N (mass%) × 3.43 to N (mass%) × 3.43 + S (mas
s%) x 1.5 + 0.01 mass%, the balance of which is Fe and unavoidable impurities in a hot-rolled steel sheet with a rolling reduction of 60 to
After 90% cold rolling, the maximum heating temperature is 820 ~ (Ac ₃
Transformation point + 20 ℃) ℃, and 780 ~ (Ac ₃ holding time in the temperature range of transformation point + 20 ° C.) is annealed at least 10 sec, then 780 to 600 average cooling rate is 25 ° C. / sec between ° C.
For processing with excellent bake hardenability and aging resistance, which is characterized by performing primary cooling as described above and secondary cooling having an average cooling rate between 400 and 200 ° C of 15 ° C / sec or less. Manufacturing method of thin steel sheet.

(3) The hot rolled steel sheet according to (1) or (2),
In addition to the above component composition, B: 0.0002 to 0.0030ma
It contains ss% and the rolling reduction is 60
After 90% cold rolling, the maximum heating temperature is 820 ~ (A
c ₃ transformation point + 20 ° C) and 780 to (Ac ₃ transformation point + 20 ° C) holding time is 10 seconds or more, then annealing is performed, then the average cooling rate between 780 and 600 ° C is 25 ° C / sec
For processing with excellent bake hardenability and aging resistance, which is characterized by performing primary cooling as described above and secondary cooling having an average cooling rate between 400 and 200 ° C of 15 ° C / sec or less. Manufacturing method of thin steel sheet.

(4) The hot-rolled steel sheet according to any one of (1) to (3) further comprises Cr: 0.03 to
2.0 mass%, Ni: 0.03 to 2.0 mass%, Mo: 0.03 to 1.0 ma
ss% and Cu: 0.03 to 2.0 mass% of any one kind or two or more kinds are contained, and after cold rolling with a rolling reduction of 60 to 90%, Annealing with a maximum heating temperature of 820 to (Ac ₃ transformation point + 20 ° C) ℃ and a holding time of 10 seconds or more in the temperature range of 780 to (Ac ₃ transformation point + 20 ° C), and then between 780 to 600 ℃ Bake hardenability, characterized by performing primary cooling with an average cooling rate of 25 ° C / sec or more and secondary cooling with an average cooling rate of 400 ° C to 200 ° C of 15 ° C / sec or less And a method for manufacturing a thin steel sheet for processing having excellent aging resistance. However, regarding Cr, Ni, Mo and Cu that are selectively added, it is recommended to add the following combinations. 0.03 to 2.0 mass% Cr- (any one or more selected from Ni, Mo and Cu 0.03 to 2.0 mass% Ni- (any one or two selected from Mo and Cu 0.03 to 1.0 mass% Mo- ( Cu) 0.03 to 2.0 mass% Cu

(5) The hot dip galvanizing treatment is performed between the primary cooling and the secondary cooling after annealing (1) to (4)
A method for manufacturing a thin steel plate for processing according to any one of 1.

[0012]

First, the experimental results which are the basis of the present invention will be described. C: 0.001 to 0.0020 mass%, Nb: 0.008 to 0.016 mass
%, Nb (mass%) / C (mass%): 5 to 8, S
i: 0.01 mass%, Mn: 0.15 mass%, P: 0.01 mass%,
S: 0.004 mass%, Al: 0.05 mass% and N: 0.002 mass%
The relationship between the aging index (AI) and the bake hardening index (BH) was investigated for cold-rolled steel sheets having a thickness of 0.7 mm and having different s% component compositions and different continuous annealing heat patterns (cooling conditions after annealing). These steel sheets were first heated and soaked to a sheet bar of the above composition at 1200 ° C, then hot-rolled at a finishing temperature of 880 ° C and wound at 630 ° C, followed by pickling and cold rolling reduction of 75 ~ 80% after cold rolling to ~ 80%
It is manufactured by continuous annealing at a temperature of ~ 900 ° C. The continuous annealing heat pattern at this time is shown in FIG. The heat pattern (a) is 780 to 100 during the cooling process during continuous annealing.
Cooling rate between ℃ is 30 ℃ / sec or more, heat pattern
(b) Cools between 780 and 600 ° C at the same cooling rate of 30 ° C / sec as in heat pattern (a) and between 10 and 50 at 600 to 400 ° C.
℃ / sec, 10 ℃ / sec or less between 400 and 100 ℃. Also,
Among the above characteristic values, AI is JIS No. 5 tensile test piece, and after applying 7.5% pre-strain by tensile tester, at 100 ℃
It is the amount of increase in deformation stress when subjected to heat treatment for 30 minutes,
Steel sheets with this value exceeding 30 MPa are said to have problems with aging resistance. BH uses JIS No. 5 tensile test piece,
This is the amount of increase in deformation stress when heat treatment is performed for 20 minutes at 170 ° C simulating the baking coating process after applying 2.0% prestrain with a tensile tester. The larger this value, the better the bake hardenability. . Further, the tensile direction of the tensile test piece used for obtaining these characteristic values was the rolling direction.

The results are shown in FIG. As is clear from Fig. 1, the relationship between AI and BH largely depends on the heat pattern, and as compared with the case of quenching from the annealing temperature to near room temperature, it cools rapidly at high temperatures, but cools in the temperature range near 300 ° C. If the speed is slowed, even if the AI is the same, B
It was found that H was remarkably improved. As a result of a more detailed experiment, if the average cooling rate between at least 780 and 600 ° C is set to 25 ° C / sec or more and the average cooling rate between 400 and 200 ° C is set to 15 ° C / sec or less, The same relationship between AI and BH as the heat pattern (b) of
It was found that the cooling condition between 〜400 ℃ was not so dependent. Since this cooling condition can be realized in a hot dip galvanizing line whose heat pattern is different from that of the continuous annealing line, it was examined that the relationship between AI and BH obtained in the continuous annealing heat pattern was the same in the hot dip galvanizing heat pattern. It turned out to be obtained.

By further experimenting, a large B
To obtain the H content, the steel composition should be Nb (mass%) / C (mass
%) Is 3 to 8, and the maximum heating temperature for annealing is 820 to
It was also found that the holding time in the temperature range of (Ac ₃ transformation point + 20 ° C) and 780 to (Ac ₃ transformation point + 20 ° C) is required to be 10 seconds or more.

Further, in addition to the above components, the steel sheet to which Ti is added in the proper range amount has further improved workability, and the steel sheet to which B has been added in the proper range amount has further improved secondary work embrittlement resistance,
It has also been found that the steel sheet to which one or more selected from Cr, Ni, Mo and Cu is added in an appropriate range amount has little deterioration in workability with an increase in strength.

As described above, the details of the reason why high bake hardenability as well as aging resistance can be obtained by controlling the component composition, cooling conditions after cold rolled sheet annealing, etc. are not necessarily clear, but are as follows. It is believed that there is. That is, by annealing the cold-rolled steel sheet in which the amount of Nb in the steel sheet is approximately equal to the amount of C in the atomic ratio, by annealing in the above condition range,
C precipitated and fixed as Nb carbide during hot rolling produces solid solution C by decomposition of Nb carbide. Intermediate steel plate in this state
The grain boundary / intragranular equilibrium of the solid solution C when rapidly cooled to the temperature (600 ° C) is advantageously changed by gradually cooling between 400 and 200 ° C, and the grain boundary solids effective for bake hardenability are changed. It is considered that the amount of solute C increased and the amount of solute C in the grains, which adversely affects the aging resistance, decreased.

Next, in the present invention, the reason why the composition of the steel and the manufacturing conditions are limited to the above-mentioned constitution will be explained. C: 0.0010 to 0.0025 mass%; C deteriorates workability, especially r value and elongation. The effect becomes remarkable when it exceeds 0.0025 mass%, so the upper limit of C is made 0.0025 mass%, preferably 0.0020 mass%. However, 0.0010mass%
If less than 0.0010mass, a sufficient bake hardening amount cannot be obtained.
% Or more must be added.

Si: 1.0 mass% or less; Si has the effect of strengthening steel, and is added in a required amount according to the desired strength.
If the addition amount exceeds 1.0 mass%, deep drawability and corrosion resistance are deteriorated, so the addition amount is set to 1.0 mass% or less. The preferable addition range is 0.30 mass% or less in consideration of the plating property and the chemical conversion treatment property.

Mn: 0.05 to 1.5 mass%; Mn has an action of preventing hot brittleness caused by S and strengthening of steel. The effect of preventing hot brittleness appears at 0.05 mass% or more, but if added in excess of 1.5 mass%, deep drawability deteriorates, so the addition amount is 0.05 to 1.5 mass%, preferably from the viewpoint of plating property. It should be 1.0 mass% or less. From the viewpoint of preventing hot brittleness, Mn (mass%) / S (mass
%) ≧ 10 is preferable.

P: 0.10 mass% or less; P has a function of strengthening steel, and is added in a required amount according to desired strength.
However, if the added amount exceeds 0.10 mass%, the deep drawability is deteriorated, and a large amount is segregated at the grain boundaries to cause embrittlement, so 0.10 mass% or less, preferably from the viewpoint of plating property.
Add within the range of 0.08 mass% or less.

S: 0.02 mass% or less; S is a cause of hot brittleness and adversely affects deep drawability. These effects become remarkable when the content exceeds 0.02 mass%, so the content should be 0.02 mass% or less. Particularly, from the viewpoint of press moldability, it is preferably 0.005 mass% or less.

Al: N (mass%) × 15 to 0.10 mass%, 0.01
~ 0.06mass%; Al is deoxidized in Ti-free steel and N in steel
It is added for precipitation fixing of. The amount of Al added is N (mass
%) × less than 15 mass%, the workability is insufficient, while
Addition of more than 0.10 mass% not only deteriorates workability but also deteriorates surface properties. Therefore, the addition amount of Al is N (mass%) × 15 to 0.10 mass%, preferably N
(mass%) x 20 to 0.08 mass%. Further, in Ti-added steel, N in the steel is precipitated and fixed by Ti, so addition of Al is necessary only for deoxidation, and the amount is 0.01 to 0.06.
mass%.

N: 0.004 mass% or less; N adversely affects the deep drawability, and a large amount of N requires a large amount of Al and deteriorates the surface properties. Therefore, the smaller the content, the better.
When the N content exceeds 0.004 mass%, the effect becomes remarkable, so 0.004 mass% or less, preferably 0.002 mass%
Must be:

Nb: C (mass%) × 3 to C (mass%) × 8
+0.02 mass%; Nb reduces the solid solution C before annealing to improve the workability, so that it is necessary to add at least about C atom equivalent, that is, C (mass%) × 3. On the other hand, in order to obtain bake hardenability, the required amount of solid solution C
Must be present in the steel sheet, which is obtained mainly from the decomposition of Nb carbide during high temperature annealing. To do so, the amount of Nb added should be C (mass%) × considering the solubility product of C and Nb.
It is necessary to limit it to 8 + 0.02 mass% or less.

Ti: N (mass%) × 3.43 to N (mass%) ×
3.43 + S (mass%) x 1.5 + 0.01 mass%; Ti is
N or N and S are added for precipitation fixing, but in particular,
Since the precipitation fixing force of N is stronger than that of Al, the workability is further improved. In order to obtain this effect, it is necessary to add Ti in an atomic equivalent amount of N or N + S.
Excessive addition deteriorates workability, especially ductility. Therefore, Ti is N (mass%) × 3.43 to N (mass%) × 3.43 +
S (mass%) × 1.5 + 0.01 mass% is added.

B: 0.0002 to 0.0030 mass%; B is added to further improve the secondary work embrittlement resistance, but 0.0002
If less than mass%, the effect cannot be obtained, while 0.0030mass
%, The workability, especially the r-value, is deteriorated, so the addition amount is 0.0002 to 0.0030 mass%, preferably 0.0003.
It should be in the range of ~ 0.0020mass%.

Cr: 0.03 to 2.0 mass%, Ni: 0.03 to 2.0 ma
ss%, Mo: 0.03 to 1.0 mass% and Cu: 0.03 to 2.0 mass
Any one or two or more selected from the total amount
2.0 mass% or less; all of these elements are effective in strengthening the steel sheet, and the necessary amount is added according to the strength, but if added in excess, the workability is reduced, so C
r: 0.03 to 2.0 mass%, Ni: 0.03 to 2.0 mass%, Mo: 0.0
The total amount of any one or more selected from 3 to 1.0 mass% and Cu: 0.03 to 2.0 mass% is 2.0 mass% or less, preferably Cr: 0.03 to 1.0 mass%, Ni: 0.03 to 1.0.
mass%, Mo: 0.03 to 0.50 mass%, Cu: 0.03 to 1.0 mass
%, And the total amount is 1.5 mass% or less.

Next, the manufacturing method of the present invention will be described together with the reasons for limiting the main requirements thereof. First, the hot rolling is not particularly limited, but the following manufacturing method is preferable for the purpose of improving workability. The slab heating temperature may be in the temperature range of 1050 to 1300 ° C, and is preferably in the range of 1050 to 1200 ° C in order to improve the ductility by coarsening the precipitates. The hot rolling finishing temperature is preferably (Ar ₃ -10) to (Ar ₃ + 50 ° C), and from the standpoint of workability, it is just above the Ar ₃ transformation point, that is, Ar ₃ 〜.
(Ar ₃ + 30 ° C) is preferred. Further, it is also good to improve the workability by subjecting it to a quenching treatment immediately after finish rolling. Further, the coil winding temperature is preferably 600 ° C. or higher. From the viewpoint of energy saving, even if the continuous casting slab is not reheated or cooled down to a temperature below the Ar ₃ transformation point after continuous casting, or immediately after heat retention treatment and rough rolling is performed, the feature of the present invention is obtained. Since it has no effect, direct rolling may be performed.

Cold rolling reduction: Cold rolling of 60 to 90%;
If the rolling reduction of cold rolling is less than 60%, sufficient workability cannot be obtained, and if it exceeds 90%, the workability deteriorates. Therefore, the rolling reduction is in the range of 60 to 90%, preferably 75 to It is necessary to set the range to 85%.

Annealing temperature / time: Maximum heating temperature is 820-
Holding time in the temperature range of (Ac ₃ transformation point + 20 ° C), 780 to (Ac ₃ transformation point + 20 ° C) is 10 seconds or more; the annealing step is a particularly important step in the present invention, and the {111} recrystallization texture is formed. It develops and increases the r value, and plays a major role in imparting bake hardenability. That is, the maximum heating temperature of this annealing is less than 820 ℃, or 780〜 (Ac ₃ transformation point +20
Even if the holding time in the temperature range of (° C.) Is less than 10 seconds, the Nb carbide precipitated during hot rolling is insufficiently decomposed and bake hardenability cannot be obtained. On the other hand, when the maximum heating temperature exceeds (Ac ₃ transformation point + 20 ° C), a large amount of austenite is formed during heating, transformation occurs from austenite to ferrite during the cooling process, the recrystallization texture is randomized, and a low r value is obtained. I can only get it. Thus, annealing the maximum heating temperature is 820 ~ (Ac ₃ transformation point + 20 ℃), 780 ~ ( A
Hold time in the temperature range of c ₃ transformation point + 20 ° C) is 10 seconds
You need to do this. Where 780 ~ (Ac ₃ transformation point +20
The upper limit of the holding time in the temperature range of (° C.) is not particularly specified, but it is preferably 90 seconds or less from the viewpoint of ductility deterioration due to crystal grain coarsening.

Cooling after annealing: Average cooling rate between 780 and 600 ° C. is 25 ° C./sec or more, average cooling rate between 400 and 200 ° C. is 15 ° C./sec or less; It is a particularly important step in the invention, and was decomposed by the above-mentioned annealing.
To prevent re-precipitation of Nb carbide, at least 780 ~
Average cooling rate between 600 ℃ is 25 ℃ / sec or more, preferably 30
℃ / sec or more, and in order to secure aging resistance, the average cooling rate between 400 and 200 ℃ must be 15 ℃ / sec or less. The equipment used for the above-mentioned annealing need not be specified in particular, but from the viewpoint of productivity, cost, etc., equipment of a continuous annealing line or a hot dip galvanizing line is desirable.

The annealed cold-rolled steel sheet produced by the above-mentioned method may be subjected to a reduction of (sheet thickness (mm) +0.5)% or less as necessary for the purpose of preventing yield elongation, sheet shape correction, surface adjustment, and the like. The temper rolling may be performed at a rate. Further, this cold-rolled steel sheet may be subjected to various electroplating after annealing because its material properties do not change even if it is passed through an electroplating line. Further, the cold-rolled steel sheet obtained by the present invention, after annealing or plating, the effect of the invention will not be impaired unless a heat cycle in which the sheet temperature is raised to 200 ° C. or more and then rapidly cooled, chemical conversion treatment A special treatment may be performed to improve the workability, weldability, press formability, corrosion resistance and the like.

When the present invention is applied to the production of hot dip galvanized steel sheet, it is most efficient to use the equipment of the hot dip galvanizing line described above. In this case,
After the primary cooling in the cooling step of annealing, the plating treatment may be performed, and then the secondary cooling in the above step may be performed.

[0034]

EXAMPLE A steel slab having the composition shown in Table 1 (values outside the range of the present invention are underlined) was
Rough rolling and then finish rolling were performed without heating and soaking at ℃ or reheating after continuous casting. The coil winding temperature was 630 ° C. This hot-rolled sheet was pickled, cold-rolled at a cold rolling reduction rate shown in Table 2 to obtain each sheet thickness, and then re-rolled in a continuous annealing line or a hot dip galvanizing line under the conditions shown in Table 2. Crystal annealing was performed. After that, further plate thickness (mm)%
Was temper-rolled. Further, some of the cold rolled steel sheets were electroplated. The tensile properties of the thus obtained thin steel sheets were determined. Here, the tensile properties were measured using a JIS No. 5 tensile test piece,
The Rankford value (r value) was measured by a three-point method after applying a tensile prestrain of 15%. Each characteristic value was expressed as an average value in the L direction (rolling direction), the D direction (45 ° direction with respect to the rolling direction) and the T direction (90 ° direction with respect to the rolling direction). That is, the average property value and X, X each direction characteristic values respectively _L, when the X _D and X _T, was determined by _{X = (X L + 2X D} + X T) / 4. Further, the aging index (AI) and the bake hardening index (BH) were measured according to the same method as the experiment described above.

[0035]

[Table 1]

[0036]

[Table 2]

The obtained evaluation results are shown in Table 2. As shown in Table 2, the thin steel sheet manufactured according to the method of the present invention has a bake hardenability index of 45 MPa or more and an aging index of 30 MPa or less, and further, in the strength-ductility balance and r value shown by TS × El. It can be seen that the typical workability is also excellent. On the other hand, it is understood that the comparative examples that deviate from the conditions of the method of the present invention are inferior in at least one of the bake hardening index, the aging index and the workability.

[0038]

As described above, according to the present invention,
In addition to having excellent bake hardenability and aging resistance index, it becomes possible to manufacture a thin steel sheet for working having good workability and a good balance of strength and ductility. Further, according to the present invention, the bake hardening index is
45MPa or more, aging index 30MPa or less, r value 1.6 or more,
It is possible to manufacture thin steel sheets for processing that have excellent characteristics with TS x El of 14000 (MPa ·%) or more. Therefore, according to the present invention, it is possible to provide a cold-rolled steel sheet having both high strength and high workability, which greatly contributes to weight reduction and safety improvement of an automobile.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a bake hardening index and an aging index.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location C22C 38/50

Claims (5)

[Claims] 1. C: 0.0010 to 0.0025 mass%, Si: 1.0 ma
ss% or less, Mn: 0.05 to 1.5 mass%, P: 0.10 mass% or less, S: 0.02 mass% or less, N: 0.004 mass% or less, Al: N (mass%) x 15 to 0.10 mass%, Nb: C (mass%) x 3 to C (mass%) x 8 + 0.02 mass
% Containing the balance in the hot-rolled steel sheet component composition consisting of Fe and unavoidable impurities, was subjected to rolling reduction 60% to 90% cold rolling, the maximum heating temperature is 820 ~ (Ac ₃ transformation point + 20 ° C. ) ,And
780 to the retention time in the temperature range of (Ac ₃ transformation point + 20 ° C.) is annealed at least 10 sec, 1 primary cooling and is then 780-600 average cooling rate between ° C. is 25 ° C. / sec or higher, 40
The average cooling rate between 0 and 200 ℃ is 15 ℃ / sec or less 2
A method for producing a thin steel sheet for working, which is excellent in bake hardenability and aging resistance, characterized by performing cooling including the following cooling. 2. C: 0.0010 to 0.0025 mass%, Si: 1.0 ma
ss% or less, Mn: 0.05 to 1.5 mass%, P: 0.10 mass% or less, S: 0.02 mass% or less, Al: 0.01 to 0.06 mass%, N: 0.004 mass% or less, Nb: C (mass%) x 3 ~ C (mass%) x 8 + 0.02 mass
% Ti: N (mass%) × 3.43 to N (mass%) × 3.43 + S (mas
s%) x 1.5 + 0.01 mass%, the balance being a hot-rolled steel sheet with a composition that consists of Fe and unavoidable impurities.
After cold rolling 60 ~ 90%, the maximum heating temperature is 820 ~
Annealing is performed for 10 seconds or more in the temperature range of (Ac ₃ transformation point + 20 ° C) and 780 to (Ac ₃ transformation point + 20 ° C), and then the average cooling rate between 780 and 600 ° C is 25 ° C / sec
For processing with excellent bake hardenability and aging resistance, which is characterized by performing primary cooling as described above and secondary cooling having an average cooling rate between 400 and 200 ° C of 15 ° C / sec or less. Manufacturing method of thin steel sheet. 3. The hot rolled steel sheet according to claim 1 or 2, in addition to the above component composition, B: 0.0002 to 0.0030 mass
%, The rolling reduction is 60 to
After 90% cold rolling, the maximum heating temperature is 820 ~ (Ac ₃
Annealing with a holding time of 10 seconds or more in the temperature range of 780 to (Ac ₃ transformation point + 20 ° C)
Then, the average cooling rate between 780 and 600 ℃ is 25 ℃ / sec or more, and the average cooling rate between 400 and 200 ℃ is
A method for producing a thin steel sheet for working, which is excellent in bake hardenability and aging resistance, characterized by performing a secondary cooling at 15 ° C / sec or less. 4. The hot-rolled steel sheet according to claim 1, further comprising Cr: 0.03 to 2.
0 mass%, Ni: 0.03 to 2.0 mass%, Mo: 0.03 to 1.0 mass
% And Cu: any one selected from 0.03 to 2.0 mass% 1
Will contain more species or two or, with respect to the hot-rolled steel sheet, after being subjected to rolling reduction 60% to 90% cold rolling, - the maximum heating temperature is 820 (Ac ₃ transformation point + 20 ° C.), and Annealing with a holding time of 10 seconds or more in the temperature range of 780 to (Ac ₃ transformation point + 20 ° C) is performed, and then primary cooling with an average cooling rate of 25 ° C / sec or more between 780 and 600 ° C is performed. A method for producing a thin steel sheet for working, which is excellent in bake hardenability and aging resistance, characterized by performing cooling comprising secondary cooling having an average cooling rate of from 15 to 200 ° C of 15 ° C / sec or less. 5. The method for manufacturing a thin steel sheet for working according to claim 1, wherein hot dip galvanizing is performed between the primary cooling and the secondary cooling.