JP2003268490A - Thin steel sheet for working excellent in bake hardenability and aging resistance and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin steel sheet for working excellent in bake hardenability, aging resistance, and workability and its production method. <P>SOLUTION: A hot rolled steel sheet containing 0.0020-0.010 mass% C, 0.07 mass% or lower P, 0.005 mass% or lower N, Al in a content of N(mass%)×15-0.10 mass%, and Nb in a content of C(mass%)×3-C(mass%)×8+0.02 mass%, Nb (at%)/C (at%) being 0.3-0.6 and C (at%)/(P (at%) being 0.4 or higher, is cold rolled under a draft of 60-90%, then annealed at a highest heating temperature of 820°C-(Ac<SB>3</SB>transformation point +20)°C, cooled at 20°C or higher, and subjected to temper rolling of 0.3-1.5%, thus giving a steel sheet having a BH of 75 MPa or higher, a ratio (BH/AI) of BH(MPa) to AI(MPa) of 1.50 or higher, and a YEl after an aging treatment of 0.6% or lower. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin steel sheet mainly used for automobile bodies and the like, and is particularly excellent in properties such as bending workability, press workability and deep drawability, and high baking during coating baking treatment. This is a proposal regarding thin steel sheets such as cold-rolled steel sheets and surface-treated steel sheets, which have excellent aging resistance despite exhibiting hardenability, and a manufacturing method thereof.

[0002]

2. Description of the Related Art Recently, workability has been improved from the viewpoint of reducing the weight of a vehicle body for the purpose of improving fuel efficiency with the tightening of exhaust gas regulations for automobiles, and the strength of the vehicle body for improving safety. The range of use of excellent high-strength steel sheets is expanding. However, the use of high-strength steel sheets has made it possible to reduce the plate thickness, but for automobile outer panel parts, especially parts with a relatively thin plate such as doors and fenders, the reduction in plate There is concern that the dent properties and the tensile rigidity may be reduced. Therefore, a steel plate having a bake hardenability (BH steel plate) is conventionally used for such an application.

The above-mentioned bake hardenability is a phenomenon in which the yield point, the deformation strength and the tensile strength of a steel sheet increase due to strain aging in the coating baking treatment performed after press forming. If this phenomenon is used, a soft material can be used during molding, so the load on processing machines such as presses can be reduced, while higher strength can be achieved by subsequent painting / baking processing. Comparing steel sheets having the same steel sheet strength afterwards, there is an advantage that deterioration of workability due to higher strength is smaller than that of a material that is not age hardened.

However, by increasing the BH of the steel plate,
Aging at room temperature deteriorates, and yield elongation easily occurs even at room temperature. Therefore, during press forming, there is a problem that wrinkles due to stretcher strains occur and the appearance is impaired, which is an obstacle to expanding the application of BH steel sheets to outer panel parts. On the other hand, in a region where dent resistance and tensile rigidity are problems, a steel sheet having higher BH property while maintaining formability is required in order to achieve higher strength.

From these viewpoints, while maintaining moldability,
A manufacturing technique for a high-strength steel sheet having both bake hardenability and aging resistance superior to those of conventional steel sheets has been proposed. For example, in JP-A-7-75803, JP-A-2001-140038, JP-A-2001-200337, etc., there is a method of increasing elongation at temper rolling to suppress aging deterioration at room temperature. Open 2000-3
Japanese Patent No. 36431 discloses a method of suppressing aging deterioration at room temperature by introducing strain into the surface layer of a steel sheet by laser irradiation or the like.

[0006]

However, since the cold-rolled steel sheets produced by the above-mentioned conventional techniques are intended to improve the aging resistance by introducing strain (movable dislocation) into the steel sheet, the BH content is increased. The larger the value of, the larger the amount of strain to be introduced. Therefore, in order to manufacture a steel sheet having a higher BH property, it is necessary to further increase the elongation percentage of temper rolling. However, in the case of high-tensile steel sheets, there is a limit to the increase in elongation rate, so it is practically difficult to manufacture with conventional lines. Therefore, there is a problem that it is difficult to obtain a high-strength steel sheet that simultaneously satisfies high bake hardenability and excellent aging resistance.

An object of the present invention is to propose a thin steel plate for working which has high bake hardenability and excellent aging resistance, and also has excellent workability, and a manufacturing method thereof.
Specifically, the bake hardening index (BH) is 75 MPa or more, the aging index (AI) is 50 MPa or less, and the ratio of these (BH / A
The object of the present invention is to propose a thin steel plate for working satisfying the characteristics of I) of 1.50 or more and a manufacturing method thereof.

[0008]

[Means for Solving the Problems] Under the recognition of the above-mentioned problems, the inventors diligently studied for solving the problems. As a result, the compositional composition of steel is controlled within an appropriate range, and the interrelationship between each composition, in particular, the relationship between Nb / C and C / P is controlled within an appropriate range. In addition, cold rolling conditions and annealing of cold-rolled sheet are performed. The inventors have found that the above problems can be solved by controlling the conditions and the cooling conditions after annealing, and have completed the present invention.

That is, according to the present invention, C: 0.0020 to 0.010 m
ass%, Si: 1.0 mass% or less, Mn: 0.05 to 1.5 mass%,
P: 0.07 mass% or less, S: 0.02 mass% or less, N: 0.005 m
ass% or less, Al: N (mass%) x 15 to 0.10 mass%, Nb: C
(mass%) x 3 to C (mass%) x 8 + 0.02 mass% is included, and
Nb and P are Nb (at%) / C (at
%): 0.3 to 0.6, C (at%) / P (at%): 0.4 or more, the content is satisfied, and the balance has a composition of Fe and inevitable impurities, and BH: 75 MPa or more, and BH (MPa) to AI (MPa) ratio (BH / AI) is 1.50 or more, and YEL after aging treatment at 40 ° C x 20 days is 0.6% or less, bake hardenability and aging resistance. A thin steel plate for processing with excellent properties.

Further, according to the present invention, C: 0.0020 to 0.010 mass
%, Si: 1.0 mass% or less, Mn: 0.05 to 1.5 mass%, P: 0.
07mass% or less, S: 0.02mass% or less, N: 0.005mass%
Below, Al: 0.01 to 0.10 mass%, Nb: C (mass%) x 3 to C
(mass%) x 8 + 0.02 mass%, Ti: N (mass%) x 3.43 ~ N
(mass%) × 3.43 + S (mass%) × 1.5 + 0.01mass%, and Nb and P in relation to C are Nb (at%) /
C (at%): 0.3 to 0.6, C (at%) / P (at%): 0.4 or more are contained, and the balance has a composition of Fe and inevitable impurities, and BH: 75 MPa. Above, and the ratio of BH (MPa) and AI (MPa) (BH / AI) is 1.50.
Above, YEL after aging treatment at 40 ℃ × 20 days is 0.6
% Or less, which is a thin steel sheet for working excellent in bake hardenability and aging resistance.

The steel sheet of the present invention further contains 0.0002 to 0.0030 mass% of B in addition to the above component composition, or further contains Cr: 0.03 to 2.0 mass% and Cu: 0.03 to 2.0 mass%.
%, Ni: 0.03 to 2.0 mass% and Mo: 0.03 to 1.0 mass%, and a total amount of one or more selected from 2.0 mass.
It is preferable to contain s% or less.

Further, according to the present invention, C: 0.0020 to 0.010 mass
%, Si: 1.0 mass% or less, Mn: 0.05 to 1.5 mass%, P: 0.
07mass% or less, S: 0.02mass% or less, N: 0.005mass%
Hereinafter, Al: N (mass%) × 15 to 0.10 mass%, Nb: C (mass
%) × 3 to C (mass%) × 8 + 0.02mass%, and Nb and P in relation to C are Nb (at%) / C (at%): 0.
3 to 0.6, C (at%) / P (at%): A hot rolled steel sheet that contains 0.4 or more of the conditions and the balance is Fe and unavoidable impurities, and cold-rolls with a reduction rate of 60 to 90%. After annealing, annealing is performed at the maximum heating temperature of 820 ℃ ~ (Ac ₃ transformation point + 20) ℃, and then
We propose a method for manufacturing a thin steel sheet for working with excellent bake hardenability and aging resistance, which is characterized by performing 0.3 to 1.5% temper rolling after cooling at 20 ° C / sec or more.

Further, according to the present invention, C: 0.0020 to 0.010 mass
%, Si: 1.0 mass% or less, Mn: 0.05 to 1.5 mass%, P: 0.
07mass% or less, S: 0.02mass% or less, N: 0.005mass%
Below, Al: 0.01 to 0.10 mass%, Nb: C (mass%) x 3 to C
(mass%) x 8 + 0.02 mass%, Ti: N (mass%) x 3.43 ~ N
(mass%) × 3.43 + S (mass%) × 1.5 + 0.01mass%, and Nb and P in relation to C are Nb (at%) /
C (at%): 0.3 to 0.6, C (at%) / P (at%): 0.4 or more are contained, and the balance is Fe and inevitable impurities. was subjected to rolling of 90% cold, annealed for maximum heating temperature 820 ℃ ~ (Ac ₃ transformation point +20) ° C., then cooled at 20 ° C. / sec or more, a temper rolling of 0.3 to 1.5% The present invention proposes a method for producing a thin steel sheet for working which is characterized by being applied and which is excellent in bake hardenability and aging resistance.

In addition to the above component composition, the production method of the present invention further contains 0.0002 to 0.0030 mass% B, or further contains Cr: 0.03 to 2.0 mass% and Cu: 0.03 to 2.0 ma.
ss%, Ni: 0.03 to 2.0 mass% and Mo: 0.03 to 1.0 mass%
2.0m in total of one or more selected from
It is preferable to contain as% or less.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION First, an experiment conducted to complete the present invention will be described. In this experiment, C: 0.
0033 to 0.0045 mass%, Si: 0.01 mass%, Mn: 0.40 to 1.00
mass%, P: 0.01 to 0.02 mass%, S: 0.005 to 0.009 mass
%, Al: 0.03 to 0.04 mass%, N: 0.0012 to 0.0030 mass%
And Nb: 0.006 to 0.015 mass%, Nb (at%) / C
(at%): 0.40 to 0.59, C (at%) / P (at%): 0.17 to 0.74
For a cold-rolled steel sheet having a thickness of 0.8 mm and manufactured by changing the continuous annealing heat pattern, the bake hardening index (BH) and aging index (AI) and their ratio (BH /
(AI) and 40 ° C. × 20 days (equivalent to room temperature for 6 months), the yield elongation (YEl) after aging treatment was investigated. The cold-rolled steel sheet, after heating and soaking the sheet bar having the above-mentioned composition to 1250 ° C, hot rolling at a finishing temperature of Ar ₃ transformation point or higher, and then winding at 600 ° C, followed by Pickled, rolling reduction 75-80
% Cold rolling, followed by recrystallization annealing at a temperature of 830 to 880 ° C and continuous annealing at a cooling rate of 20 to 25 ° C / sec, followed by 0.8 ± 0.1% temper rolling. It was done.

Among the above-mentioned characteristic values, AI is an aging when JIS No. 5 tensile test piece is used, and a 7.5% prestrain is applied by a tensile tester, and then an aging heat treatment is applied at 100 ° C. for 30 minutes. This is the amount of increase in deformation stress after aging compared to before, and the smaller this value, the better the aging resistance. In addition, BH uses JIS No. 5 tensile test pieces, and after applying 2.0% prestrain with a tensile tester, heat treatment at 170 ° C x 20 minutes simulating the paint baking process before heat treatment This is the amount of increase in deformation stress after heat treatment, and the larger this value, the better the bake hardenability. In addition, the tensile direction of the tensile test piece used to obtain these characteristic values is the direction perpendicular to the rolling direction.
(C direction).

The cold-rolled steel sheets obtained as described above all had a high BH property with a BH amount of 75 MPa or more. FIG. 1 shows the relationship between BH / AI of these steel sheets and YEl after aging. As is clear from FIG. 1, BH
It can be seen that the increase in / AI tends to decrease the YEl after aging, that is, the YEl after aging at room temperature for 6 months. At the same time, an index that does not cause stretcher strain (yield elongation after aging treatment at 40 ° C for 20 days (YE
It is also understood that BH / AI needs to be 1.5 or more to satisfy (l): 0.6% or less).

As a result of further experimentation, in order to obtain a large amount of BH, the composition of the steel is such that the atomic ratio of Nb and C (Nb
(at%) / C (at%)) is 0.3 to 0.6, and the maximum continuous annealing temperature must be in the temperature range of 820 ° C to (Ac ₃ transformation point + 20) ° C. all right.

Further, in order to control BH / AI, C
It is also effective to control the atomic ratio of C and P (C (at%) / P (at%)), specifically C (at%) / P (at%) of 0.40 or more. found.

Further, in addition to the above-mentioned components, the steel sheet to which Ti is added in an appropriate amount has further improved workability, and the steel sheet to which B has been added in an appropriate amount has further improved secondary work embrittlement resistance.
It has also been found that a steel sheet to which an appropriate amount of one or more selected from Mo and Cu is added can increase strength while suppressing deterioration of workability.

As described above, the reason why high bake hardenability as well as aging resistance can be obtained by controlling the component composition, recrystallization annealing conditions, etc. is considered as follows. That is, a cold-rolled sheet in which the Nb content in the steel sheet was added in an approximately equal amount to the C content in the atomic ratio was 820 ° C to (Ac ₃ transformation point + 20) ° C
Nb which was precipitated during hot rolling by annealing at the temperature
The carbide is decomposed and solid solution C is generated. By further quenching after annealing, the solid solution C remains as solid solution C in the crystal grains without reprecipitation. Usually, this solid solution C tends to segregate at the grain boundaries. However, in the high temperature region (about 600 ° C. or higher), P is also an element that has a strong tendency to segregate at the crystal grain boundaries, so in a material with a large amount of P, segregation of C at grain boundaries is difficult to proceed. Therefore, increasing C / P (increasing C, decreasing P) reduces the segregation of P at the crystal grain boundaries, and instead makes it easier for C to segregate at the grain boundaries.
When is increased, site competition of C and P occurs at the crystal grain boundary, and the ratio of C distribution at the grain boundary increases. Fig. 2 shows the results of measuring the amount of C present in the grain boundary / intragrain of high P steel (0.046 mass%) and low P steel (0.013 mass%) by Auger analysis. It is clear that the amount of C present at the grain boundaries is increasing.

That is, by increasing C / P, the grain boundary C increases and the intragranular C, which is a factor of deterioration of the aging resistance, decreases. As a result, the intragranular dislocation fixing phenomenon is suppressed and the aging resistance is improved. It is thought that it is a mechanism to do. On the other hand, the solid solution C segregated at the grain boundaries is the coating / baking condition (170 ° C x
At a temperature of (20 minutes), it is considered that it diffuses and moves into the crystal grains and the solid solution C in the grains increases, which contributes to ensuring a high bake hardenability. Here, the abundance of C was evaluated using the parameter C / Fe obtained by Auger analysis. That is, a sample held at extremely low temperature with liquid nitrogen was brittlely fractured with a hammer, and each of the intergranular fracture surface and the intergranular fracture surface at this time was subjected to Auger analysis at 5 points to measure the C and Fe counts. To do. Then, the ratio of the count numbers of C and Fe at this time is obtained, and the average of each 5 points is calculated,
This value was set as C / Fe and used as a parameter indicating the amount of C present.

Next, in the present invention, the reason why the chemical composition of steel is limited to the above-mentioned constitution will be explained. C: 0.0020 to 0.010 mass% C deteriorates workability, especially r value and elongation. The effect becomes remarkable when it exceeds 0.010 mass%, so the upper limit of C is made 0.010 mass%. However, 0.0020mass
If less than 100%, a sufficient bake hardening amount cannot be obtained, so 0.0020
It is necessary to secure mass% or more.

Si: 1.0 mass% or less Si has a function of strengthening steel and is added in a required amount according to desired strength. If the added amount exceeds 1.0 mass%, deep drawability and corrosion resistance are improved. As it deteriorates, 1.0mass
% Or less. The preferred range of addition is
It is 0.50 mass% or less in consideration of plating property and 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. To 1.0 mass% or less. From the viewpoint of preventing hot brittleness, Mn (mass%) /
It is preferable that S (mass%) ≧ 10.

P: 0.07 mass% or less, C (at%) / P (at
%): 0.4 or more P has the effect of strengthening the steel without significantly deteriorating the deep drawability, and is added in the required amount according to the desired strength. However, if the addition amount exceeds 0.07 mass%, the deep drawability deteriorates, and a large amount of segregates at the grain boundaries, causing embrittlement.
0.07 mass% or less, preferably 0.05 ma from the viewpoint of plating property
Add in the range of ss% or less. Furthermore, C (at%) / P (at
%) Is 0.4 or more, when less than 0.4, P to the grain boundary
This is because the amount of segregation increases, so that segregation of C to the grain boundaries is hindered, and as a result, the effect of increasing grain boundaries C effective for improving the aging resistance cannot be sufficiently obtained. Therefore, C (at%) / P (at%) is 0.
It is 4 or more, preferably 0.5 or more.

S: 0.02 mass% or less S not only causes hot brittleness but also adversely affects deep drawability, so the smaller the content, the better. These effects are 0.02 mass
%, It becomes remarkable, so 0.02 mass% or less. Particularly, from the viewpoint of press formability, it is preferably 0.005 mass% or less.

Al: N (mass%) × 15 to 0.10 mass% or 0.
01 to 0.06 mass% Al is added for deoxidizing and precipitating and fixing N in steel in Ti-free steel. If the amount of Al added is less than N (mass%) × 15 mass%, the workability is insufficient. On the other hand, if the amount added exceeds 0.10 mass%, not only the workability is deteriorated but also the surface properties are deteriorated. Therefore, the amount of Al added is N (mass
%) × 15 to 0.10 mass%, preferably N (mass%) × 20 to 0.0
8 mass% Further, in Ti-added steel, N in the steel is precipitated and fixed by Ti, so the addition of Al is necessary only for deoxidation, and the addition amount may be 0.01 to 0.06 mass%.

N: 0.005 mass% or less N has a bad influence on deep drawability, and a large amount of N is Al.
It is necessary to add a large amount of
The smaller the content, the better. N content is 0.005 mass%
If it exceeds, the effect becomes remarkable, so 0.005 mass% or less, preferably 0.003 mass% or less.

Nb: C (mass%) × 3 to C (mass%) × 8 + 0.02
mass%, Nb (at%) / C (at%): 0.3 to 0.6 Nb has the effect of fixing C as Nb carbide to reduce the solid solution C before annealing and improving the workability, so at least It is necessary to add C (mass%) × 3. On the other hand, in order to obtain bake hardenability, the required amount of solid solution C in the steel sheet after annealing is required.
The solid solution C is obtained mainly by the decomposition of Nb carbide during high temperature annealing. To do so, the amount of Nb added should be C (mass%) x 8 considering the solubility product of C and Nb.
It is necessary to limit it to +0.02 mass% or less. Further, it is necessary to control the atomic ratio of Nb and C (Nb (at%) / C (at%)) to 0.3 to 0.6. When Nb (at%) / C (at%) is less than 0.3, the r value is significantly reduced, while when it exceeds 0.6, the target BH
The amount cannot be obtained, so it is necessary to set it to 0.3-0.6.

Ti: N (mass%) × 3.43 to N (mass%) × 3.43
+ S (mass%) × 1.5 + 0.01mass% Ti is added in order to precipitate and fix N or N and S. Especially, since the precipitation fixing force of N is stronger than Al, the workability is further improved. . In order to obtain this effect, it is necessary to add Ti in an amount equal to the atomic equivalent of N. on the other hand,
Excessive addition of Ti deteriorates workability, especially ductility, so the amount is set to about the atomic equivalent of N + S. Therefore, the amount of Ti added is N (mass%) × 3.43 to N (mass%) × 3.43 + S (mas
s%) × 1.5 + 0.01 mass% is added.

In the present invention, the following components are preferably added in addition to the above essential components. B: 0.0002 to 0.0030 mass% B is an element that improves secondary work embrittlement resistance, and can be added depending on the application. The amount of B added is 0.0002mass
If it is less than%, the effect cannot be obtained. On the other hand, 0.0030mass%
If it is added in excess of 1.0, the workability, especially the r value, is deteriorated, so the addition amount is 0.0002 to 0.0030 mass%, more preferably 0.
It is desirable to set it in the range of 0003 to 0.0020 mass%.

Cr: 0.03 to 2.0 mass%, Ni: 0.03 to 2.0 mass
%, Mo: 0.03 to 1.0 mass% and Cu: 0.03 to 2.0 mass%, and a total amount of one or more selected from 2.0 mass
s% or less Any of these elements is effective in increasing the strength without impairing the workability, and a necessary amount can be added depending on the strength. However, if added excessively, the workability is deteriorated, so 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%
2.0m in total of one or more selected from
ass% or less, more preferably Cr: 0.03 to 1.0 mass%, Ni:
0.03-1.0mass%, Mo: 0.03-0.50mass%, Cu: 0.03-
It is desirable to add one or two or more selected from 1.0 mass% in a total amount of 1.5 mass% or less.

Next, the reasons for limiting the main requirements of the manufacturing method of the present invention will be described. Hot rolling Hot rolling is preferably performed under the following conditions from the viewpoint of improving workability. Slab heating temperature is 1050-1300 ℃
The temperature range of 1050 to 1200 ° C. is preferable in order to improve the ductility by coarsening the precipitates. The hot rolling finishing temperature is preferably in the range of (Ar ₃ −10) to (Ar ₃ +50) ° C., and from the viewpoint of workability, it is directly above the Ar ₃ transformation point, that is, Ar ₃ to (Ar ₃ +3).
It is more preferable that the temperature is 0) ° C. Further, the quenching treatment may be performed immediately after the finish rolling. The coil winding temperature is
The temperature is preferably 600 ° C or higher. From the viewpoint of energy saving, the continuous casting slab is directly fed and rolled, that is, without reheating or without lowering the temperature to a temperature below the Ar ₃ transformation point after continuous casting, immediately after continuous casting or subjected to heat retention treatment. Subsequent rough rolling does not affect the characteristics of the present invention and may be carried out.

Cold rolling If the rolling reduction is less than 60%, a texture favorable for workability does not grow and sufficient workability cannot be obtained. On the other hand, if it exceeds 90%, the workability deteriorates because the texture preferable for workability does not develop sufficiently, so the rolling reduction is in the range of 60 to 90%, preferably in the range of 75 to 85%. is necessary.

Annealing Temperature (Maximum Heating Temperature) The annealing step is a particularly important step in the present invention,
It develops a {111} recrystallized texture to increase the r value and plays a major role in imparting bake hardenability. If the maximum heating temperature of this annealing is less than 820 ° C, the N precipitated during hot rolling
b Carbide is not decomposed sufficiently, solute C is not secured, and a large 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, and the recrystallization texture is randomized, resulting in a low r value. However, the workability is deteriorated.
Therefore, the annealing temperature is 820～ maximum temperature (Ac ₃ transformation point + 20)
Must be done in the range of ° C.

Cooling after annealing The cooling step after annealing is also an important step in the present invention. In the above annealing step, at least the cooling rate after annealing needs to be 20 ° C./sec or more, preferably 30 ° C./sec or more in order to prevent re-precipitation of the solid solution C generated by the decomposition of Nb carbide. Regarding the equipment used for the above-mentioned annealing, it is desirable to use a continuous annealing line or hot dip galvanizing line equipment from the viewpoint of productivity, cost and the like.

Temper rolling The temper rolling is carried out in a rolling reduction range of 0.3 to 1.5% in order to improve the shape correction, the roughening and the mechanical properties of the cold rolled steel sheet after annealing. If the rolling reduction is less than 0.3%, the yield elongation cannot be completely eliminated, while if it exceeds 1.5%, the workability is deteriorated such as the yield point is increased and the elongation is decreased. It is preferably 0.6 to 1.0%.

Since the material characteristics of this cold rolled steel sheet do not change even when it is passed through the electroplating line, various electroplating may be performed after annealing. Further, even if a special surface treatment is applied to improve chemical conversion treatability, weldability, press formability and corrosion resistance, the characteristics of the present invention are not adversely affected.

Further, even when the present invention is applied to a hot-dip galvanized steel sheet, the mechanical properties of the steel sheet are substantially the same as those of the continuous annealed material. May be given. For production, it is most efficient to use the equipment of the hot dip galvanizing line described above.
Further, the plated steel sheet may be subjected to a special surface treatment in order to improve the chemical conversion treatment property, weldability, press formability, corrosion resistance and the like.

[0041]

EXAMPLE A steel slab having the composition shown in Table 1 was
After heating and soaking at 00 ° C, hot rolling was performed and the coil was wound at 630 ° C. This hot-rolled sheet was pickled, cold-rolled at a reduction rate shown in Table 2 to a sheet thickness of 0.8 mm, and then recrystallized and annealed under the conditions shown in Table 2. Then, temper rolling was further performed under the conditions shown in Table 2. The obtained thin steel sheet was subjected to measurement of tensile properties, mechanical properties such as r value, bake hardenability, and aging property. Here, the tensile properties were measured using a JIS No. 5 tensile test piece, and the Lankford value (r value) was measured by a 3-point method after applying a tensile prestrain of 15%. Rolling direction),
The average value in the D direction (45 ° to the rolling direction) and the C direction (90 ° to the rolling direction) was used. Also, the aging index (AI) and the bake hardening index (BH) were measured according to the same method as the above-mentioned experiment. Also, the aging resistance at room temperature is 40
After the accelerated aging treatment at 20 ° C. for 20 days (corresponding to 6 months at room temperature), the above-mentioned tensile test was performed to measure and evaluate YEl.

[0042]

[Table 1]

The results of the above experiment are shown in Table 2. As is clear from Table 2, the thin steel sheet manufactured according to the manufacturing method of the present invention has a BH of 75 MPa or more, an AI of 50 MPa or less, and a BH /
The AI had a value of 1.5 or more, and the YEl after aging for 6 months at room temperature was 0.6 or less. Moreover, it is understood that the workability represented by the r value is also excellent. On the contrary,
In Comparative Examples that deviate from the conditions of the present invention, it can be seen that at least one of BH / AI and YEl after aging is inferior and bake hardenability is insufficient or aging resistance is insufficient.

[0044]

[Table 2]

[0045]

As described above, according to the present invention,
In addition to having excellent bake hardenability and aging resistance, it becomes possible to manufacture thin steel sheets for working that combine good workability. Further, according to the present invention, BH is 75 MPa or more, AI is 50 MPa or less, r
It is possible to manufacture a thin steel plate for processing having excellent properties with a value of 1.6 or more. Therefore, according to the present invention, it is possible to provide a high workability cold rolled steel sheet having high BH and good aging resistance,
It greatly contributes to the weight reduction and safety improvement of automobiles.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between BH / AI and YEl after aging.

FIG. 2 is a graph showing the influence of C / P on the location of solid solution C.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kei Sakata             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Technical Research Institute of Iron Co., Ltd. F-term (reference) 4K037 EA01 EA04 EA11 EA13 EA15                       EA17 EA18 EA19 EA20 EA23                       EA25 EA27 EA31 EB02 EB03                       EB06 EB07 EB08 EB09 FB00                       FG00 FH00 FJ05 FJ06 FK03                       FM02 JA01

Claims (8)

[Claims] 1. C: 0.0020 to 0.010 mass%, Si: 1.0 mass%
Below, Mn: 0.05-1.5mass%, P: 0.07mass% or less, S: 0.02mass% or less, N: 0.005mass% or less, Al: N (mass%) × 15-0.10mass%, Nb: C (mass %) × 3 to C (mass%) × 8 + 0.02mass%, and Nb and P are Nb (at
%) / C (at%): 0.3 to 0.6, C (at%) / P (at%): 0.4 or more are contained, and the balance is a component composition consisting of Fe and inevitable impurities. BH: 75MPa
Above, and the ratio (BH / AI) of BH (MPa) and AI (MPa) is 1.
It is 50 or more, and YEl after aging treatment at 40 ° C for 20 days is 0.
A thin steel sheet for processing with excellent bake hardenability and aging resistance, which is 6% or less. 2. C: 0.0020 to 0.010 mass%, Si: 1.0 mass%
Below, Mn: 0.05 to 1.5 mass%, P: 0.07 mass% or less,
S: 0.02 mass% or less, N: 0.005 mass% or less, Al: 0.01 to 0.10 mass%, Nb: C (mass%) × 3 to C (mass%) × 8 + 0.02 mass%, Ti: N (mass%) × 3.43〜N (mass%) × 3.43 + S (mass%)
X1.5 + 0.01mass%, and Nb and P are related to C by Nb (at
%) / C (at%): 0.3 to 0.6, C (at%) / P (at%): 0.4 or more are contained, and the balance is a component composition consisting of Fe and inevitable impurities. BH: 75MPa
Above, and the ratio (BH / AI) of BH (MPa) and AI (MPa) is 1.
It is 50 or more, and YEl after aging treatment at 40 ° C for 20 days is 0.
A thin steel sheet for processing with excellent bake hardenability and aging resistance, which is 6% or less. 3. In addition to the above component composition, 0.0002 of B is further added.
~ 0.0030mass% is included, Claim 1 or 2 characterized by the above-mentioned.
Thin steel plate for processing according to. 4. In addition to the above component composition, Cr: 0.03 to
2.0mass%, Cu: 0.03-2.0mass%, Ni: 0.03-2.0mass
% And Mo: any one selected from 0.03 to 1.0 mass%
4. The thin steel plate for working according to claim 1, wherein the total amount of the alloys is two or more, and 2.0 mass% or less is contained. 5. C: 0.0020 to 0.010 mass%, Si: 1.0 mass%
Below, Mn: 0.05 to 1.5 mass%, P: 0.07 mass% or less,
S: 0.02mass% or less, N: 0.005mass% or less, Al: N (mass%) × 15 to 0.10mass%, Nb: C (mass%) × 3 to C (mass%) × 8 + 0.02mass% , And Nb and P are Nb (at
%) / C (at%): 0.3 to 0.6, C (at%) / P (at%): 0.4 or more are contained, and the balance is hot rolled steel sheet containing Fe and inevitable impurities. After performing cold rolling at a rate of 60 to 90%, annealing at a maximum heating temperature of 820 ° C to (Ac ₃ transformation point + 20) ° C, and then cooling at 20 ° C / sec or more, then 0.3 to 1.5
% Temper rolling, which is a method for manufacturing a thin steel sheet for working having excellent bake hardenability and aging resistance. 6. C: 0.0020 to 0.010 mass%, Si: 1.0 mass%
Below, Mn: 0.05 to 1.5 mass%, P: 0.07 mass% or less,
S: 0.02 mass% or less, N: 0.005 mass% or less, Al: 0.01 to 0.10 mass%, Nb: C (mass%) × 3 to C (mass%) × 8 + 0.02 mass%, Ti: N (mass%) × 3.43〜N (mass%) × 3.43 + S (mass%)
X1.5 + 0.01mass%, and Nb and P are related to C by Nb (at
%) / C (at%): 0.3 to 0.6, C (at%) / P (at%): 0.4 or more are contained, and the balance is hot rolled steel sheet containing Fe and inevitable impurities. After performing cold rolling at a rate of 60 to 90%, annealing at a maximum heating temperature of 820 ℃ ~ (Ac ₃ transformation point + 20) ℃, and then cooling at 20 ℃ / sec or more, 0.3 ~ 1.5
% Temper rolling, which is a method for manufacturing a thin steel sheet for working having excellent bake hardenability and aging resistance. 7. In addition to the above component composition, B is 0.0002.
~ 0.0030mass% is included, Claim 5 or 6 characterized by the above-mentioned.
The method for manufacturing a thin steel plate for processing according to. 8. In addition to the above component composition, Cr: 0.03 to
2.0mass%, Cu: 0.03-2.0mass%, Ni: 0.03-2.0mass
% And Mo: any one selected from 0.03 to 1.0 mass%
The method for producing a thin steel plate for working according to any one of claims 5 to 7, characterized in that the total amount of the two kinds or more is 2.0 mass% or less.