JP4616568B2 - Thin steel plate excellent in slow aging at room temperature and bake hardenability and method for producing the same - Google Patents

Description

  The present invention relates to a thin steel plate excellent in normal temperature slow aging and bake hardenability.

  In order to reduce the weight of automobile bodies, it is desired to reduce the thickness of the steel sheet used, and the strengthening of automobile steel sheets has been studied. However, increasing the strength of the steel sheet tends to deteriorate the press formability of the steel sheet, and a high-tensile steel sheet having excellent press formability has been desired.

  As a steel sheet that achieves both press formability and high strength, a paint bake-hardening type automobile steel sheet has been developed. This steel plate is a steel plate in which the yield stress is increased by performing a coating baking process including holding at a high temperature of usually 150 to 200 ° C. after press forming.

  By having solute C or solute N present in the steel, the high temperature heating during the coating baking process causes C or N to adhere to the dislocations introduced during press forming, thereby preventing the movement of dislocations, resulting in yield stress. To rise. The increase in yield stress is the bake hardening amount (BH amount), and the BH amount is generally increased by increasing the solute C amount or the solute N amount.

  The problem of such a curing mechanism is as follows.

  If the amount of solute C or solute N is increased to increase the amount of BH, some dislocations are already fixed by C or N before forming (normal temperature aging), and a stretcher due to yield point elongation during press forming. Wavy surface defects called strains are produced, and the product characteristics are significantly deteriorated.

  It has been a long-standing problem to solve the problem of normal temperature aging and to realize a thin steel sheet having high bake hardenability and excellent aging resistance.

  In response to this problem, Patent Document 1 and Patent Document 2 disclose methods for controlling the addition amount of Nb and Al to achieve bake hardenability and aging resistance.

  This method is a method for obtaining aging resistance by appropriately adjusting the amount of solute N and the amount of solute C, but aging deterioration occurs when the amount of solute C is increased in order to increase the amount of BH. Therefore, it is not possible to produce steel having high bake hardening characteristics.

  Further, Patent Document 3 discloses a method for simultaneously obtaining normal temperature aging and bake hardenability by adding specified amounts of Mo, Cr, W, and the like. However, in this method, since the mechanism is unknown, there is no sufficient material design guideline, and it is not possible to sufficiently cope with the problems of higher bake hardenability and delayed aging. In addition, such a method of adding a specific element has a problem in terms of cost.

  Furthermore, Patent Document 4 discloses a method for producing a paint bake hardened steel sheet having excellent aging resistance by specifying components and production conditions and increasing the amount of grain boundary segregation C.

  However, the amount of C that segregates at the grain boundary is actually small compared to the amount of C in the grain, and the diffusion distance from the grain boundary is also smaller than the grain size, so that all the dislocations in the grain are fixed. It is not possible to obtain a high BH amount.

Japanese Patent Laid-Open No. 5-331553 JP-A-7-300623 JP-A-5-25549 Japanese Patent Laid-Open No. 11-229085

  This invention makes it a subject to provide the thin steel plate excellent in normal temperature slow aging property and bake hardenability in view of such the present condition.

  As a result of intensive studies to solve the above problems, the present inventors have obtained the following new knowledge.

  When dislocations are introduced and aged in a thin steel sheet, interstitial C or N diffuses, forms a Cottrell atmosphere around the introduced dislocations, fixes the dislocations, and hardens the steel.

  Furthermore, when new dislocations were introduced into this steel by temper rolling, it was found that C or N forming a Cottrell atmosphere hardly diffused into the new dislocations, so that aging deterioration at room temperature was suppressed.

  And the present inventors also introduced a larger amount of dislocations in the steel by further processing, and when the temperature of the steel sheet was increased by paint baking, a part trapped in the weak dislocation strain field It has been found that C or N of ## STR2 ## desorbs from the strain field and diffuses into dislocations introduced by processing.

  The diffused C or N forms a Cottrell atmosphere at new dislocations and fixes the dislocations. Furthermore, old dislocations are also fixed, and the steel is strengthened.

  Therefore, dislocations in steel can be used as trap sites that suppress C or N diffusion at room temperature, and by using this mechanism, aging resistance at room temperature and bake hardenability at high temperature can be realized at the same time. Can do.

  That is, as described above, the present invention utilizes dislocations in steel as C or N trap sites based on a new idea completely different from the conventional method, and the gist thereof is as follows.

(1) By mass%, C: 0.0001 to 0.2%, N: 0.0001 to 0.2%, C + N: 0.002 to 0.3%, Mn: 0.01 to 3%, Si : 0.001-2%, P: 0.001-0.1%, S: 0.05% or less, Al: 0.0001-0.1%, and either Nb or Ti A dislocation having a density of 1 × 10 ⁹ to 2 × 10 ¹⁰ cm ⁻² is introduced into a thin steel plate containing 0.001 to 0.02% in total of 1 type or 2 types and the balance being Fe and inevitable impurities. After that, the thin steel sheet was aged at 100 ° C. or more and less than 300 ° C. for 1 minute or more and 2 hours or less. Subsequently, the thin steel sheet was newly added with a density of 1 × 10 ⁹ to 2 × 10 ¹⁰ cm ⁻² . dislocations introduced, the total amount of solid solution C amount and amount of dissolved N to room temperature slow aging property and bake hardenability, characterized in that a 0.001 to 0.005 percent Thin steel sheet manufacturing method of that was.

(2) By mass%, C: 0.0001 to 0.2%, N: 0.0001 to 0.2%, C + N: 0.002 to 0.3%, Mn: 0.01 to 3%, Si : 0.001-2%, P: 0.001-0.1%, S: 0.05% or less, Al: 0.0001-0.1%, and either Nb or Ti After temper rolling with a strain amount of 0.2-2.0% on a thin steel plate containing 0.001% to 0.02% in total of 1 type or 2 types, the balance being Fe and inevitable impurities , less than 300 ° C. 100 ° C. or more thin steel sheet, and aging under conditions of less than 2 hours or more for 1 minute, followed by have rows temper rolling strain amount 0.2 to 2.0% to the thin steel sheet, solid A method for producing a thin steel sheet excellent in room temperature slow aging and bake hardenability, characterized in that the total amount of dissolved C and dissolved N is 0.001 to 0.005% .

(3) the 100 ° C. to 300 below ° C., instead of the aging performed at 1 minute or more 2 hours following conditions, and characterized by performing the aging at 50 ° C. or higher 100 ° C. less than the following conditions 10 days or more 1 hour The manufacturing method of the thin steel plate excellent in normal temperature slow aging property and bake hardenability as described in said (1) or (2).

(4) the 100 ° C. to 300 below ° C., instead of the aging performed at 1 minute or more 2 hours following conditions, 10 ° C. or higher 50 ° C. less than, and characterized by performing the aging under the following conditions over 10 days 180 days The manufacturing method of the thin steel plate excellent in normal temperature slow aging property and bake hardenability as described in said (1) or (2).

(5) By mass%, C: 0.0001 to 0.2%, N: 0.0001 to 0.2%, C + N: 0.002 to 0.3%, Mn: 0.01 to 3%, Si : 0.001-2%, P: 0.001-0.1%, S: 0.05% or less, Al: 0.0001-0.1%, and either Nb or Ti 1 × 10 ⁹ to 2 × 10 ¹⁰ cm ^−, which is a thin steel plate containing 0.001 to 0.02% in total of 1 type or 2 types, the balance being Fe and inevitable impurities, and fixed by C or N and dislocations of the ^second density, have a density dislocations of the fixed which do not ^{1 × 10 9 ~2 × 10 10} cm -2, the total amount of solid solution C amount and solute N amount is 0.001 to 0. excellent steel sheet to room temperature slow aging property and bake hardenability, wherein 005% der Rukoto.

  According to the present invention, it is possible to provide a thin steel plate excellent in normal temperature slow aging and bake hardenability.

  The thin steel plate according to the present invention may be either a hot rolled steel plate or a cold rolled steel plate. Furthermore, the hot rolling process and the cold rolling process are not particularly limited. In the JIS definition, a steel plate having a thickness of less than 3 mm is referred to as a thin steel plate.

  The reasons for limiting the components contained in the thin steel sheet according to the present invention are shown below. In addition,% of content represents the mass%.

  C and N are elements that are important in developing bake hardenability, and in order to express paint bake hardenability, it is essential to contain 0.002% or more as the amount of C + N. However, if the amount of C + N is too large, the amount of solid solution increases and it becomes difficult to ensure normal temperature aging, so the upper limit was made 0.3%.

  The reason why the lower limit values of C and N are set to 0.0001%, respectively, is that reduction to less than these lower limit values not only greatly increases the cost in steelmaking, but also cannot provide high bake hardenability. Furthermore, it is because it becomes impossible to make ultrafine precipitates made of carbon and nitride at high density.

  On the other hand, the reason why the upper limit values of C and N are each 0.2% is that when these upper limit values are exceeded, the strength becomes too high and the workability is impaired.

  The thin steel sheet according to the present invention contains Mn, Si, P, S, and Al in addition to the above elements.

  Mn, Si, and P are essential components for obtaining the strength required for a thin steel plate. The lower limit values of Mn, Si, and P are 0.01% for Mn, 0.001% for Si, and 0.001% for P. If the lower limit values are less than these lower limit values, the strength of the thin steel sheet is insufficient.

  The upper limit values of Mn, Si, and P are 3% for Mn, 2% for Si, and 0.1% for P. When these upper limit values are exceeded, the strength of the thin steel sheet becomes too high and the workability is impaired.

  If S is contained in excess of 0.05%, it may cause red-hot embrittlement during hot rolling and crack at the surface, so-called hot embrittlement. Therefore, S needs to be 0.05% or less.

  Al is an element effective as a deoxidizer, and 0.0001% or more is necessary to function as a deoxidizer. The reason why the upper limit value is set to 0.1% is that if the content exceeds the upper limit, the strength of the thin steel sheet increases and the workability is impaired.

  Furthermore, the thin steel plate according to the present invention contains Ti and Nb as necessary in addition to the above elements.

  Nb and Ti are elements that are effective for improving workability, increasing strength, and further miniaturizing and homogenizing the structure. Therefore, any one or two of them may be added in a total amount of 0.001 to 0 as necessary. It may be added in the range of 1%.

  However, when the addition amount is less than 0.001%, the effect of addition does not appear. On the other hand, when the addition amount exceeds 0.1%, it precipitates as carbonitride and makes it difficult to secure solute C and solute N. Or the recrystallization temperature rises and causes deterioration of the material. A more preferable range is 0.005 to 0.02%.

  Next, the dislocation density of the thin steel sheet of the present invention will be described.

  As described above, the thin steel sheet of the present invention exhibits excellent room temperature slow aging and bake hardenability by utilizing dislocations in steel as C or N trap sites. In the thin steel sheet of the present invention, the dislocation density is important because the dislocations in the steel are used as C or N trap sites.

  The thin steel sheet of the present invention is one in which temper rolling is performed once after annealing and dislocation is introduced, then the steel is aged, and then the second temper rolling is performed again to introduce dislocation.

  In general, the reason why dislocations are introduced by temper rolling after annealing is to improve workability by dispersing movable dislocations in steel, and usually temper rolling is performed only once.

  Moreover, as a method of temper rolling, skin pass rolling is generally used, but methods such as shot projection and laser irradiation may be used.

The density of dislocations before the aging of the thin steel sheet of the present invention (dislocations that trap C or N) is preferably 1 × 10 ⁹ to 2 × 10 ¹⁰ cm ⁻² . When the dislocation density before aging is less than 1 × 10 ⁹ cm ⁻² , C or N trap sites are insufficient, and aging deterioration is caused by untrapped C and N dissolved in the steel. Will be.

When the dislocation density before aging exceeds 2 × 10 ¹⁰ cm ⁻² , all C or N in the steel is strongly trapped in the dislocations and cannot be disengaged from the dislocations, and the thin steel sheet has excellent aging resistance. Bake hardenability cannot be ensured. Furthermore, the reduction in ductility and the decrease in the r value become prominent, causing deterioration of workability.

The dislocations introduced into the thin steel sheet of the present invention after aging are movable dislocations for improving workability, and the dislocation density is preferably 1 × 10 ⁹ to 2 × 10 ¹⁰ cm ⁻² .

If the dislocation density after aging is less than 1 × 10 ⁹ cm ⁻² , the movable dislocation density is insufficient, and stretcher strain may occur during processing.

When the dislocation density after aging exceeds 2 × 10 ¹⁰ cm ⁻² , sufficient workability may not be obtained due to work hardening, or ductility may be reduced.

  FIG. 1 shows the relationship between the amount of strain and dislocation density due to skin pass rolling. A method for calculating the dislocation density will be described later. The amount of strain and dislocation density increased almost uniformly, and up to about 4% were distributed almost uniformly in the steel. At higher strains, the formation of dislocation cells was observed.

As shown in this figure, in order to introduce a dislocation with a dislocation density of 1 × 10 ⁹ cm ^−2, a temper rolling amount of 0.2% is necessary, and a dislocation with a dislocation density of 2 × 10 ¹⁰ cm ⁻² is required. It can be seen that a temper rolling amount of 2% is necessary to introduce slag.

Therefore, in order to introduce dislocations having a dislocation density of 1 × 10 ⁹ to 2 × 10 ¹⁰ cm ⁻² , a temper rolling amount of 0.2 to 2% is required.

  Next, the manufacturing method of the thin steel plate of this invention is described.

  As described above, the thin steel sheet of the present invention is subjected to temper rolling once to introduce dislocations, then aged steel, and then to temper rolling for the second time to introduce dislocations.

  In the present invention, it is necessary to introduce the dislocation once into the thin steel sheet and then age the steel to trap C and N in the dislocation.

  In general, the higher the aging temperature, the greater the diffusion rate of C and N, so the aging time may be shortened. However, when aging is performed at a temperature exceeding 300 ° C., unnecessary carbides or nitrides are generated in the steel and do not contribute to bake hardenability.

  Further, when the aging temperature is as low as less than 10 ° C., the diffusion rate of C and N is remarkably lowered, so that a very long time is required for sufficient aging, which hinders productivity.

  When the aging time is insufficient, a sufficient amount of C and N for dislocation is not trapped, resulting in poor aging resistance.

  Therefore, the aging conditions are most preferably from 100 ° C. to less than 300 ° C. for 1 minute to 2 hours from the viewpoint of productivity, and from 50 ° C. to less than 100 ° C. for 60 minutes to 10 days, and then preferably 10 ° C. Next, it is preferably 10 days or more and 180 days or less at less than 50 ° C.

  Aging of 100 ° C. or more and less than 300 ° C. for more than 2 hours, aging of 50 ° C. or more and less than 100 ° C. for more than 10 days, and aging of 10 ° C. or more and less than 50 ° C. for more than 180 days may result in precipitation of carbonitride, etc. This is not preferable because the decrease in bake curability and room temperature slow aging increases.

  Although the optimum value of the aging time varies depending on the composition of the steel sheet and the aging temperature, it is generally preferable to use the aging time before complete aging. Moreover, although the aging method of a thin steel plate is not limited, for example, box-type annealing (BAF) may be used.

  The method for producing a thin steel sheet of the present invention limits the dislocation density and the presence of C and N in the thin steel sheet, and does not limit hot rolling, winding, cold rolling conditions, etc. Since the amount of solute C or the amount of solute N contributes to bake hardenability, it is necessary to leave an appropriate amount on the steel sheet before temper rolling before aging.

  The total amount of the solute C amount and the solute N amount is preferably 0.001 to 0.005%. If the total amount is less than 0.001%, sufficient bake hardenability cannot be obtained, and if it exceeds 0.005%, sufficient aging resistance cannot be obtained. More preferably, it is 0.0015 to 0.0035%.

  As such production conditions, for example, the preferred hot rolling heating temperature is 1100 to 1200 ° C, the finishing temperature is 850 to 950 ° C, and the winding temperature is 400 to 700 ° C. Moreover, the cold rolling rate is 30 to 90%.

Further, the annealing conditions vary with the components in the steel, such as, after holding 0.2-3 minutes at 780 ° C. or higher Ac ₃ temperatures below at a cooling rate of 10 to 100 ° C. / s. If necessary, an overaging treatment may be performed at 200 to 400 ° C. for 1 to 10 minutes. Further, hot dip galvanizing or other plating may be applied.

  Next, a method for evaluating the aging resistance and bake hardenability of the thin steel sheet of the present invention and a method for measuring the dislocation density in the steel will be described.

  Normal temperature aging can be examined by holding a thin steel sheet in an atmosphere of 40 ° C. for 70 days, conducting a tensile test, and measuring the yield point elongation (YP-El) at this time.

  In the method of the present invention, aging resistance is evaluated instead by an artificial acceleration test at 100 ° C. × 1 hour, and a YP-El value of 0.4% or less is considered good.

  In addition, the bake hardenability is measured by measuring the upper yield stress (YP) after holding a thin steel plate at 2% tension and holding at 170 ° C. for 20 minutes, and measuring the difference in strength when the 2% tensile test is performed first. Evaluated as BH amount. A BH amount of 70 MPa or more is considered good.

  An electron microscope is used to measure the dislocation density in steel. There are several methods for estimating the dislocation density. In the present invention, the following method is used.

  The thickness of the sample is measured in advance, and the number of dislocations actually present is derived by measuring the number of dislocations that can be observed under specific diffraction conditions, and the average length of dislocations is estimated. Using these values, the dislocation density is obtained based on the following equation (1).

In order to suppress variation in data depending on the observation location, 10 points were measured for a plurality of crystal grains, and an average value was obtained.
(Dislocation density) = (Dislocation number) × (Dislocation average length) / (Volume of observation region) Formula (1)
It is difficult to judge dislocations fixed by C or N and dislocations not fixed from direct observation with an electron microscope or the like.

  Therefore, the density of fixed dislocations is measured as the density of dislocations contained in the steel before complete aging.After temper rolling performed after complete aging, the total dislocation density is newly measured, and the aging is determined from this value. The value obtained by subtracting the previous dislocation density was defined as the dislocation density not fixed.

  Next, the working effects of the present invention will be described more specifically by way of examples. However, they are merely illustrative and the present invention is not unduly limited thereby.

Example 1
Test materials having chemical compositions a to d described in Table 1 were melted. In addition,% of a chemical component represents the mass%. a is the chemical composition of the present invention, but b to d are chemical compositions outside the scope of the present invention.

  The heating temperature for hot rolling was 1100 ° C., the finishing temperature was 900 ° C., and the winding temperature was 650 ° C. The cold rolling rate was 80%, and a 0.8 mm thin plate was used. Annealing was held at 800 ° C. for 1 minute and then cooled at a cooling rate of 50 ° C./s. A thin plate subjected to an overaging treatment at 350 ° C. for 3 minutes was used.

  Subsequently, under conditions A to I described in Table 2, pre-aging skin pass rolling, aging treatment, and post-aging skin pass rolling were performed to produce a paint bake-hardening type steel sheet, which was used as a sample. A to D are production conditions of the present invention, while E to I are production conditions outside the scope of the present invention.

  E is a condition for performing skin pass rolling once without performing an aging treatment, and F is a condition for performing skin pass rolling again without performing an aging treatment after skin pass rolling, as in the conventional manufacturing conditions.

  G and H are conditions that make the amount of skin pass before and after aging out of the scope of the present invention.

  I is a condition in which the aging time is outside the scope of the present invention.

  Next, each test piece was subjected to a tensile test by the above-described method to evaluate the BH characteristics and the yield point elongation.

  Table 3 shows the results of the mechanical test. The amount of BH was 70 in the table as good and indicated by ◯ in the table, and the value of less than 70 MPa was indicated by x in the table. The yield point elongation (YP-El) value after an artificial acceleration test at 100 ° C. for 1 hour is shown as ◯ in the table as good when 0.4% or less, and as poor as more than 0.4% as x in the table. Indicated.

In the steel sheet having the chemical composition a , good results were obtained in both bake hardenability and aging resistance under the production conditions of A, B, C, and D.

  On the other hand, both the bake hardenability and the aging resistance were inferior under the production conditions of E and F. Under these manufacturing conditions, it is considered that a large number of movable dislocations were fixed within 100 ° C. × 1 hour because there was no solute C or solute N trap site shown in the present invention.

  Under the production conditions of G and H, the aging resistance was good, but the bake hardenability was inferior. It is considered that the amount of BH was small because the pre-aging skin pass strain amount was too large under the G condition and the post-aging skin pass strain amount was too large under the H condition.

  Under the production conditions of I, aging resistance and bake hardenability were inferior. Since the aging time is too long under the condition of I, it is considered that precipitation of carbonitrides or the like occurs in the dislocation, and the bake hardenability and aging resistance are reduced.

  A steel plate having a chemical composition d was used and subjected to skin pass rolling and aging under conditions A to C. Although the aging resistance was good, the bake hardenability was inferior.

  In the steel sheet d, since the amount of C + N added is less than the specified amount of the present invention, these trap deeply in the dislocations introduced before aging, cannot diffuse even after aging, and cannot fix the dislocations introduced after aging. It is thought that it was because of.

Although not described here, a test material obtained by subjecting steel sheets having chemical compositions a to c to skin pass rolling and aging under production conditions A to D has a dislocation density of 1 × 10 ⁹ by skin pass rolling performed before aging. a ~2 × ^{10 10} cm ^-2, further dislocation density after skin pass rolling was performed after aging had become ^{2 × 10 9 ~4 × 10 10} cm -2.

  This increase is the dislocation introduced by skin pass rolling performed after aging.

Therefore, the density of dislocations fixed by C or N is 1 × 10 ⁹ to 2 × 10 ¹⁰ cm ⁻² , and the density of dislocations not fixed is 1 × 10 ⁹ to 2 × 10 ¹⁰ cm ^−2. It was.

(Example 2)
Using a 0.8 mm thin plate manufactured under the same chemical composition and manufacturing conditions as steel type a in Example 1, the bake hardenability and aging resistance were examined by changing the amount of skin pass rolling before and after aging. The aging conditions were 2 hours at 120 ° C.

  The results are shown in FIG. In the figure, ◯ indicates a condition in which both the bake hardenability and the aging resistance are good, and a black triangle indicates a condition in which both or one of the characteristics is bad. When the skin pass amount was within the range of 0.2 to 2.0% both before and after aging, both the bake hardenability and aging resistance were good.

A 0.2% strain steel has a dislocation density of 1 × 10 ⁹ cm ⁻² , and a 2.0% strain applied steel has a dislocation density of about 2 × 10 ¹⁰ cm ⁻² . The dislocation density of the steel to which 2 to 2.0% strain was applied was 1 × 10 ⁹ to 2 × 10 ¹⁰ cm ⁻² .

  As described above, according to the present invention, it is possible to provide a thin steel plate excellent in normal temperature slow aging and bake hardenability. Therefore, it can be said that the industrial value of the present invention is extremely high.

It is a figure which shows the correlation of the distortion amount by temper rolling, and the dislocation density in steel. It is a figure which shows the relationship between the amount of distortion before and behind aging, bake hardenability, and aging resistance.

Claims (5)

% By mass
C: 0.0001 to 0.2%,
N: 0.0001 to 0.2%
C + N: 0.002 to 0.3%,
Mn: 0.01 to 3%
Si: 0.001-2%,
P: 0.001 to 0.1%,
S: 0.05% or less,
Al: 0.0001 to 0.1%,
1 × 10 ⁹ to 2 in a thin steel sheet containing 0.001 to 0.02% in total of any one or two of Nb and Ti, with the balance being Fe and unavoidable impurities. After introducing dislocations with a density of × 10 ¹⁰ cm ⁻² , the thin steel plate was aged at 100 ° C. or higher and lower than 300 ° C. for 1 minute or longer and 2 hours or shorter. Room temperature slow aging, characterized by introducing dislocations with a density of 10 ^{9 to} 2 × 10 ¹⁰ cm ⁻² and making the total amount of solute C and solute N 0.001 to 0.005% And manufacturing method of thin steel plate with excellent bake hardenability % By mass
C: 0.0001 to 0.2%,
N: 0.0001 to 0.2%
C + N: 0.002 to 0.3%,
Mn: 0.01 to 3%
Si: 0.001-2%,
P: 0.001 to 0.1%,
S: 0.05% or less,
Al: 0.0001 to 0.1%,
Further, any one or two of Nb and Ti are contained in a total amount of 0.001 to 0.02%, and the balance is Fe and inevitable impurities. After temper rolling at 2.0%, the thin steel plate is aged at 100 ° C. or higher and lower than 300 ° C. for 1 minute or longer and 2 hours or shorter, and subsequently the strain amount of 0.2 to 2 is applied to the thin steel plate. There .0% rows temper rolling was excellent on the total amount of solid solution C amount and amount of dissolved N to room temperature slow aging property and bake hardenability, characterized in that a 0.001 to 0.005% Manufacturing method of thin steel sheet.   2. The aging is performed under the conditions of 50 ° C. or more and less than 100 ° C. for 1 hour or more and 10 days or less, instead of aging performed under the conditions of 100 ° C. or more and less than 300 ° C. for 1 minute or more and 2 hours or less. Or 2. A method for producing a thin steel sheet having excellent room temperature slow aging and bake hardenability according to 2.   The aging is performed under the conditions of 10 ° C or more and less than 50 ° C, 10 days or more and 180 days or less, instead of aging performed under the conditions of 100 ° C or more and less than 300 ° C and 1 minute or more and 2 hours or less. Or 2. A method for producing a thin steel sheet having excellent room temperature slow aging and bake hardenability according to 2. % By mass
C: 0.0001 to 0.2%,
N: 0.0001 to 0.2%
C + N: 0.002 to 0.3%,
Mn: 0.01 to 3%
Si: 0.001-2%,
P: 0.001 to 0.1%,
S: 0.05% or less,
Al: 0.0001 to 0.1%,
In addition, it is a thin steel plate containing 0.001 to 0.02% in total of any one or two of Nb and Ti, with the balance being Fe and inevitable impurities, and is fixed by C or N. was 1 × 10 ⁹ possess a dislocation density of ~2 × ^{10 10 cm -2,} the dislocation density of ^{1 × 10 9 ~2 × 10 10} cm -2 that are not fixed, solid solution and amount of solute C thin steel sheet excellent in cold slow aging property and bake hardenability of the total amount of N content and wherein from 0.001 to 0.005% der Rukoto.

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