JP4176403B2 - Thin steel sheet for processing with excellent low-temperature bake hardenability and aging resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin steel sheet for working which has excellent low temperature seizure hardenability and aging resistance, and also has satisfactory workability. <P>SOLUTION: The steel sheet has a composition comprising, by mass, 0.0020 to 0.010% C, &le;1.0% Si, 0.05 to 1.5% Mn, &le;0.05% P, &le;0.02% S, &le;0.005% N, Al: 15&times;N(%) to 0.10% Al and Nb: 0.5&times;(93/12)&times;C(%) to (93/12)&times;(C(%)-0.001), and the balance Fe with inevitable impurities. The mean crystal grain diameter d of the steel sheet is controlled to 2 to 12 &mu;m, and also the low temperature seizure hardening index A expressed by the following formula (1) is controlled to &le;10: the formula (1) is shown by A=(12/93)&times;(Nb(%)/C(%))&times;d(&mu;m)-10&times;C*(%); wherein C*(%)=C(%)-(12/93)&times;Nb(%). <P>COPYRIGHT: (C)2004,JPO

Description

【００４５】
【表２】

【００４６】
【表３】

【００４７】
【表４】

【００４８】
表４から明かなように、本発明に従い得られた発明例はいずれも、平均結晶粒径が５〜12μm 、低温焼付硬化指数Ａが10以下で、しかも 100℃−ＢＨで示される低温焼付硬化量が30 MPa以上でかつ常温６ヶ月相当時効後のY-Elが 0.6以下と良好であった。また、ｒ値で代表される加工性にも優れていた。
これに対し、本発明の適正範囲を逸脱する比較例はいずれも、低温焼付硬化性が十分ではないか、耐時効性が十分ではなかった。
【００４９】
【発明の効果】
かくして、本発明によれば、 100℃−ＢＨが30 MPa以上、常温６ヶ月時効後のY-ELが 0.6％以下、ｒ値が 1.6以上という、優れた低温焼付硬化性と耐時効性を有し、かつ良好な加工性を兼ね備える加工用薄鋼板を得ることができる。
従って、本発明によれば、自動車鋼板として、その軽量化および安全性の向上に大きく貢献する。
【図面の簡単な説明】
【図１】 低温焼付硬化指数Ａと焼付硬化量（ＢＨ量）との関係を示した図である。[0001]
[Industrial application fields]
The present invention relates to a thin steel sheet for processing that is excellent in low-temperature bake hardenability and aging resistance, which is suitable mainly for automobile bodies.
That is, the present invention not only exhibits good characteristics in bending, press molding, drawing, and the like, but also exhibits high bake hardenability, particularly in low-temperature paint baking treatments, and is excellent in aging resistance. It is a thin steel sheet for processing, and is advantageously adapted to applications such as surface-treated steel sheets.
[0002]
[Prior art]
Among steel plates for automobiles, steel plates having a relatively thin plate thickness are used for outer plate parts such as doors and fenders, so that dent resistance and stiffness are required. Therefore, a bake hardened steel plate (BH steel plate) whose yield strength increases after press forming and paint baking is frequently used.
Currently, the baking temperature of paint is normally 170 ° C, but from the viewpoint of reducing energy costs and protecting the environment, lowering of baking temperature is being studied, and low-temperature baking type (about 160 ° C) paint has also been developed. Has been. In the future, it is expected that the temperature will be further lowered. Therefore, it is necessary to develop a low-temperature bake-hardening type steel sheet corresponding to that.
[0003]
In order to impart low temperature bake hardenability, it is already known that increasing the amount of BH, that is, increasing the amount of C is effective, but simply increasing the amount of C deteriorates room temperature aging resistance. For this reason, wrinkles due to stretcher strain are likely to occur during press molding, and the appearance is impaired.
[0004]
From this viewpoint, conventionally, various methods have been proposed in order to combine the bake hardenability and aging resistance of a steel sheet.
For example, JP-A-7-75803, JP-A-2001-140038, and JP-A-2001-200337 disclose a method for increasing the elongation in temper rolling to suppress normal temperature aging deterioration. Japanese Patent No. 2000-336431 proposes a method of suppressing deterioration of normal temperature aging by introducing strain into a steel sheet surface layer by laser irradiation or the like.
[0005]
On the other hand, as a technique for improving low-temperature bake hardenability, for example, in Japanese Patent No. 2560168 and Japanese Patent Laid-Open No. 6-73498, there is a method for controlling the precipitate distribution of iron carbide, and Japanese Patent Laid-Open No. 6-299289. Have proposed a method for controlling the precipitation of NbC in the hot-rolled sheet and a method for optimizing the amounts of B and O added to Japanese Patent No. 2876966, both of which have excellent low-temperature bake hardenability. It is disclosed that it is obtained.
[0006]
[Problems to be solved by the invention]
However, any of the cold-rolled steel sheets manufactured by the known techniques disclosed in the above-mentioned JP-A-7-75803, JP-A-2001-140038, JP-A-2001-33737 and JP-A-2000-336431 is used. Since the aging resistance is improved by introducing strain (movable dislocation) into the steel plate, the required amount of strain increases as the BH increases.
In order to increase the amount of strain, it is necessary to increase the elongation in temper rolling, but there is a limit to the elongation that can be imparted with a high-strength steel sheet, and production in a continuous line becomes practically difficult.
Therefore, there has been a problem that a steel sheet satisfying both high bake hardenability and excellent aging resistance cannot be obtained.
[0007]
On the other hand, with respect to low temperature bake hardenability, the techniques disclosed in Japanese Patent No. 2560168 and Japanese Patent Laid-Open No. 6-73498 are techniques that harden using precipitation strengthening at a low temperature, and are required for the solid solution required for dent resistance. The technical content is different from the improvement in yield strength obtained by fixing C, N and dislocations. In JP-A-6-299289, a large amount of solid solution C remains in the crystal grains, and there is a concern that the room temperature aging resistance is lowered. Further, Japanese Patent No. 2876966 attempts to obtain excellent low-temperature bake hardenability by reducing the solid solution C existing at the grain boundary by adding B. Since C is increased, the room temperature aging resistance is deteriorated.
[0008]
The present invention is an advantageous solution of the above problems, and proposes a thin steel sheet for processing having excellent low-temperature bake hardenability, good aging resistance, and excellent workability. With the goal.
[0009]
[Means for Solving the Problems]
Now, as a result of earnestly researching the solution under the above-mentioned problem recognition, the inventors have adjusted the component composition of steel to an appropriate range including the interrelationship between each component, The inventors have found that the above problems can be advantageously solved by controlling the cold rolling conditions, the annealing conditions of the cold-rolled sheet, and the cooling conditions after the annealing, and have completed the present invention.
[0010]
That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.0020 to 0.010%,
Si: 1.0% or less,
Mn: 0.05 to 0.52 %,
P: 0.05% or less,
S: 0.02% or less,
N: 0.005% or less,
Al: 15 × N (%) to 0.10% and
Nb: 0.5 × (93/12) × C (%) to (93/12) × [C (%) − 0.001]
The balance is Fe and inevitable impurities, the steel sheet has an average grain size d of 2 to 12 μm, and a low-temperature bake hardening index A expressed by the following formula (1) is 10 or less. A thin steel sheet for processing having excellent low temperature bake hardenability and aging resistance.
A = (12/93) × (Nb (%) / C (%)) × d (μm) −10 × C ^* (%) --- (1)
However, C ^* (%) = C (%)-(12/93) x Nb (%)
[0011]
2. % By mass
C: 0.0020 to 0.010%,
Si: 1.0% or less,
Mn: 0.05 to 0.52 %,
P: 0.05% or less,
S: 0.02% or less,
N: 0.005% or less,
Al: 0.01-0.06%,
Nb: 0.5 × (93/12) × C (%) to (93/12) × [C (%) − 0.001] and
Ti: (48/14) × N (%) to (48/12) × [C (%) − (12/93) × Nb (%) −0.001]
+ (48/14) x N (%) + (48/32) x S (%)
The balance is Fe and inevitable impurities, the steel sheet has an average grain size d of 2 to 12 μm, and a low-temperature bake hardening index A expressed by the following formula (1) is 10 or less. A thin steel sheet for processing having excellent low temperature bake hardenability and aging resistance.
A = (12/93) × (Nb (%) / C (%)) × d (μm) −10 × C ^* (%) --- (1)
However, C ^* (%) = C (%)-(12/93) x Nb (%)-(12/48) x [Ti (%)-(48/14)
× N (%) − (48/32) × S (%)]
[0012]
3. A thin steel sheet for processing having excellent low-temperature bake hardenability and aging resistance, wherein the steel sheet further comprises a composition containing B: 0.0030% or less by mass%.
[0013]
4). In any one of the above 1-3, the steel sheet is further in mass%.
Cr: 2.0% or less,
Cu: 2.0% or less,
Ni: 2.0% or less and
Mo: A thin steel sheet for processing excellent in low-temperature bake hardenability and aging resistance, characterized in that the composition contains one or more selected from 1.0% or less.
[0014]
The present invention will be specifically described below.
First, the experiment that led to the present invention will be described.
Five types of steel materials having the composition shown in Table 1 were manufactured by changing the continuous annealing heat pattern (cooling conditions after annealing) in various ways. The relationship between the baking hardening amount (BH amount) at 100 ° C., 140 ° C. and 170 ° C. was investigated. The above cold-rolled steel sheet is heated and soaked at 1250 ° C after the sheet bar having the composition shown in Table 1 is hot-rolled at a finishing temperature not lower than the Ar ₃ transformation point and wound at 600 ° C. , Followed by pickling, cold rolling with a cold rolling reduction: 75-80%, followed by continuous annealing at a temperature of 850 ° C, followed by cooling at a cooling rate of 20-25 ° C / s Then, after holding at a temperature of 480 to 430 ° C. for 200 to 250 seconds, cooling at a rate of 10 to 20 ° C./s, temper rolling at 0.8 ± 0.1% was performed.
[0015]
Among the above characteristic values, the bake hardening amount (BH amount) is JIS No. 5 tensile test piece, and after applying 2.0% pre-strain with a tensile tester, the temperatures are 100 ° C, 140 ° C and 170 ° C, respectively. It shows the amount of increase in deformation stress when heat treatment is performed for 20 minutes. Originally, the amount of stress increase due to heat treatment at 170 ° C. for 20 minutes is called the BH amount, but for convenience in this experiment, it will be called 100 ° C.-BH, 140 ° C.-BH, 170 ° C.-BH. The larger the amount of BH at a lower temperature, the better the low-temperature bake hardenability. Moreover, if the amount of BH is 30 MPa or more, it can be said that it has good bake hardenability.
In addition, the tensile direction of the tensile specimen used for obtaining these characteristic values was the direction perpendicular to the rolling direction (C direction).
Further, the crystal grain structure of the cross section in the rolling direction was photographed at 400 times with an optical microscope, and the average crystal grain diameter d (μm) was calculated from the photograph by a cutting method.
From these values, the low-temperature bake hardening index A was calculated by the following equation (1).
A = (12/93) × (Nb (%) / C (%)) × d (μm) −10 × C ^* (%) --- (1)
However, C ^* (%) = C (%)-(12/93) x Nb (%)
[0016]
FIG. 1 shows the obtained results.
As is clear from the figure, it can be seen that the low-temperature bake hardening index A decreases and the BH amount tends to increase at any baking temperature.
[0017]
Furthermore, by repeating the same experiment, in order to satisfy 30 MPa or more at 100 ° C.-BH, it is necessary to set the low-temperature bake hardening index A to 10 or less, and particularly the Nb content in the steel components. It was found that it was necessary to adjust to the range of 0.5 × (93/12) × C (%) to (93/12) × [C (%) − 0.001].
It has also been found that the maximum heating temperature during annealing is preferably in the temperature range of 820 to Ar ₃ transformation point.
[0018]
In addition to the above components, a steel sheet added with an appropriate amount of Ti further improves workability, and a steel sheet added with an appropriate amount of B further improves secondary work brittleness resistance, and Cr, Ni, It has been found that steel sheets to which an appropriate amount of one or more selected from Mo and Cu are added can improve the strength while suppressing deterioration of workability.
[0019]
As described above, the reason why the low-temperature bake hardenability is improved by controlling the component composition, recrystallization annealing conditions, and the like is considered as follows.
That is, by annealing a cold-rolled steel sheet to which Nb is added in a stoichiometric equivalent with respect to C under appropriate conditions, C precipitated and fixed as Nb carbide during hot rolling becomes solid solution C by decomposition of Nb carbide. Generate as By refining crystal grains due to the precipitation of NbC and the presence of solid solution C, dislocation and interaction (hardening) within the grains can be achieved even when the C diffusion distance from the grain boundary is relatively short (low baking temperature). It is considered that the desired bake hardenability can be obtained even in low temperature baking.
In addition to the dispersion of the NbC precipitates described above, a method of controlling the transformation point can be used as the crystal grain refinement method, and is not particularly limited.
[0020]
In addition, in the manufacturing process, P exists at the crystal grain boundary in a high temperature range (about 600 ° C. or higher), so that normally solid solution C tends to exist in the crystal grain. It is considered that P segregation on the surface decreases, and instead C easily segregates at the grain boundaries. That is, C and P site competition occurs at the grain boundary, and the distribution ratio to the grain boundary increases.
Thereby, it is estimated that it also has favorable normal temperature aging resistance.
[0021]
Next, the reason why the component composition of steel is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.0020 to 0.010%
When the content of C increases, the workability, in particular, the r-value and the elongation deteriorate, and the effect becomes significant when the content exceeds 0.010%. Therefore, the upper limit of C is set to 0.010%. However, if it is less than 0.0020%, a sufficient low-temperature bake hardening amount cannot be obtained, so the lower limit of the C amount is set to 0.0020%.
[0022]
Si: 1.0% or less
Si has the effect of strengthening steel, and is added in the required amount depending on the desired strength. However, if the content exceeds 1.0%, the deep drawability and corrosion resistance deteriorate, so it should be contained at 1.0% or less. did. A preferable addition range is 0.50% or less in consideration of plating properties and chemical conversion treatment properties.
[0023]
Mn: 0.05-1.5%
Mn effectively contributes to prevention of hot brittleness caused by S and strengthening of steel. This hot brittleness prevention effect is manifested at 0.05% or more, but if it exceeds 1.5%, the deep drawability deteriorates, so the Mn content was limited to the range of 0.05 to 1.5%. From the viewpoint of plating properties, 1.0% or less is preferable. Further, from the viewpoint of preventing hot brittleness, it is preferable that Mn (%) / S (%) ≧ 10.
[0024]
P: 0.05% or less P has an effect of strengthening steel without significantly degrading the deep drawability, and is added in a necessary amount depending on the desired strength. However, if the content exceeds 0.05%, not only the aging resistance at normal temperature deteriorates, but also there is a risk of segregation at the grain boundaries to cause embrittlement. Therefore, the P content is limited to a range of 0.05% or less.
[0025]
S: 0.02% or less S is more preferable as it is less because it causes hot brittleness and also adversely affects deep drawability. Since these adverse effects become prominent when the content exceeds 0.02%, S is suppressed to 0.02% or less. In particular, from the viewpoint of press formability, it is preferably 0.005% or less.
[0026]
Al: 15 × N (%) -0.10% or 0.01-0.06%
Al is added for deoxidation and precipitation fixation of N in steel in Ti-free steel. At this time, if the added amount of Al is less than 15 × N (%), sufficient workability cannot be obtained. On the other hand, if it exceeds 0.10%, not only the workability is deteriorated but also the surface properties are deteriorated. Therefore, the Al content is limited to the range of 15 × N (%) to 0.10%. Preferably, N is 0.003% or less, and Al: 20 × N (%) to 0.08%.
Further, in Ti-added steel, N in the steel is precipitated and fixed by adding Ti, so Al is necessary only for deoxidation. In this case, the Al content is preferably in the range of 0.01 to 0.06%.
[0027]
N: 0.005% or less N not only adversely affects deep drawability, but a large amount of N requires a large amount of Al and deteriorates the surface properties, so the smaller the content, the better. In particular, when the amount of N exceeds 0.005%, the adverse effect becomes remarkable. Therefore, N must be 0.005% or less, preferably 0.003% or less.
[0028]
Nb: 0.5 × (93/12) × C (%) to (93/12) × [C (%) − 0.001]
Since Nb improves workability by reducing the solid solution C before annealing, it is necessary to add at least 0.5 × (93/12) × C (%). On the other hand, in order to obtain bake hardenability, a necessary amount of solid solution C must be present in the steel sheet. For that purpose, it is necessary to satisfy the following equation (2). When this equation is modified, the upper limit of the Nb amount is (93/12) × [C (%) − 0.001].
C ^* (%) = C (%)-(12/93) x Nb (%) ≥ 0.001 --- (2)
[0029]
Ti: (48/14) × N (%) to (48/12) × [C (%) − (12/93) × Nb (%) − 0.001] + (48/14) × N (%) + (48/32) x S (%)
Ti is added for the precipitation fixation of N and S, but since the precipitation fixation force of N is particularly stronger than that of Al, there is an effect of further improving the workability. In order to obtain this effect, the addition amount of Ti must be at least about the stoichiometric amount of N. Therefore, when it is added, it is set to (48/14) × N (%) or more. On the other hand, when added excessively, solid solution C cannot be ensured similarly to Nb. In order to obtain bake hardenability, the required amount of solute C must be present in the steel sheet. For this purpose, the following formula (3) must be satisfied. Is (48/12) x [C (%)-(12/93) x Nb (%) -0.001] + (48/14) x N (%) + (48/32) x S (%) .
C ^* (%) = C (%)-(12/93) x Nb (%)-(12/48) x [Ti (%)-(48/14) x N (%)-(48/32) × S (%)] ≧ 0.001 --- (3)
[0030]
The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
B: 0.0030% or less B may be added to further improve the secondary work brittleness resistance. However, if added over 0.0030%, the workability, particularly the r value, deteriorates, so the upper limit of the B amount is set to 0.0030%. In order to improve the secondary work brittleness resistance, the B content is preferably 0.0002% or more. More preferably, it is 0.0003 to 0.0020% of range.
[0031]
One or more selected from Cr: 2.0% or less, Cu: 2.0% or less, Ni: 2.0% or less, and Mo: 1.0% or less
Cr, Cu, Ni and Mo are all effective for strengthening the steel sheet, and the necessary amount is added depending on the strength. However, if added excessively, the workability will be reduced, so each may be contained in the above range. Good. In order to obtain the effect of improving the strength, the range is preferably Cr: 0.03-1.0%, Ni: 0.03-1.0%, Mo: 0.03-0.50%, Cu: 0.03-1.0%. Also, the total amount of these is desirably 2.0% or less, more preferably 1.5% or less.
[0032]
Although the preferred component composition range of the present invention has been described above, it is not sufficient for the present invention to limit the component composition to the above range, and it is also important to control the crystal grain size within a predetermined range.
Average crystal grain size d: 2 to 12 μm
The average crystal grain size is an important factor in the present invention. That is, the diffusion distance of C is shortened by lowering the baking temperature, the presence of a large amount of C in the grain boundary from the viewpoint of normal temperature aging resistance, and the interaction between dislocations and solid solution C in the crystal grain is bake hardening. It is important to make the crystal grains finer in view of contributing to the above.
Here, if the average crystal grain size d of the steel sheet exceeds 12 μm, the bake hardening amount at a low temperature becomes insufficient. On the other hand, if it is less than 2 μm, the ductility and the r value decrease, so the workability decreases. Therefore, the average crystal grain size d of the steel sheet is limited to the range of 2 to 12 μm.
[0033]
In the present invention, it is also important to control the low-temperature bake hardening index A represented by the following formula within an appropriate range.
Low temperature bake hardening index A = (12/93) × (Nb (%) / C (%)) × d−10 × C ^* (%) ≦ 10
This low-temperature bake hardening index A is effective as an index of bake hardenability at low temperatures. When this index A exceeds 10, a bake hardening amount (BH amount) that is sufficiently satisfactory as shown in FIG. 1 is obtained. I can't.
Therefore, in the present invention, the low-temperature bake hardening index A shown by the above formula is limited to 10 or less.
[0034]
Next, the preferable manufacturing conditions of the present invention will be described together with the reasons for limiting the main requirements.
First, although it is not necessary to specifically limit hot rolling, it is preferable to use the following manufacturing method for the purpose of improving workability.
That is, the slab heating temperature is preferably in the temperature range of 1150 to 1250 ° C. The hot rolling finishing temperature is preferably just above the Ar ₃ transformation point, that is, about Ar ₃ to (Ar ₃ +20) ° C. from the viewpoint of workability. Moreover, you may perform a rapid cooling process immediately after finish rolling. Furthermore, the coil winding temperature is preferably 600 ° C. or higher. From the viewpoint of energy saving, the continuous casting slab may be subjected to rough rolling immediately or after heat treatment without reheating or lowering the temperature to the Ar ₃ transformation point or less after continuous casting.
[0035]
Cold rolling reduction: 60-90%
If the rolling reduction in cold rolling is less than 60%, sufficient workability will not be obtained. On the other hand, if it exceeds 90%, a favorable texture for workability will not be sufficiently developed, resulting in deterioration of workability. The rolling reduction is desirably 60 to 90%, preferably 75 to 85%.
[0036]
Annealing temperature: An annealing process in which the maximum heating temperature is 820 to Ac ₃ transformation point is an important process in the present invention, and a {111} recrystallized texture is developed to increase the r value and impart bake hardenability. It plays a big role. That is, when the maximum heating temperature of this annealing is less than 820 ° C., decomposition of Nb carbide precipitated during hot rolling becomes insufficient, and sufficient bake hardenability cannot be obtained. On the other hand, when the maximum heating temperature exceeds the Ac ₃ transformation point, a large amount of austenite is formed during heating, transformation from austenite to ferrite occurs in the cooling process, and the recrystallized texture is randomized, so only a low r value is obtained. I can't. Furthermore, sufficient low-temperature bake hardenability cannot be obtained due to the coarsening of crystal grains.
Therefore, it is advantageous that the annealing temperature is in the temperature range where the maximum heating temperature is 820 to Ac ₃ transformation point.
[0037]
Cooling after annealing: Cooling rate of 20 ° C / s or more The cooling step after annealing is also an important step in the present invention, and in order not to reprecipitate the Nb carbide decomposed by the above annealing, the cooling rate after annealing is small. Is 20 ° C./s or higher, preferably 30 ° C./s or higher. The equipment used for the annealing is not particularly required, but a continuous annealing line or a hot dip galvanizing line is desirable in terms of productivity and cost.
[0038]
Holding for 60 to 400 s in a temperature range of 500 to 150 ° C. This step is the most important step in the present invention.
That is, by maintaining the above temperature range for an appropriate time, the diffusion of the solid solution C to the grain boundary is promoted, and by increasing the grain boundary C, an excellent effect of improving room temperature aging resistance can be obtained. Can do.
More preferably, the treatment is performed in a temperature range of 500 to 400 ° C. for 100 to 300 seconds. In the above holding treatment, it is advantageous to avoid as much as possible the range of 200 to 400 ° C., which is the precipitation temperature range of Fe ₃ C.
[0039]
Temper rolling reduction ratio: 0.3-1.5%
Temper rolling is performed on the cold-rolled steel sheet after annealing and cooling produced by the method described above. Here, if the rolling reduction is less than 0.3%, the yield elongation preventing effect cannot be obtained. On the other hand, if it exceeds 1.5%, workability deterioration such as a reduction in elongation is caused, so the rolling reduction in temper rolling is 0.3 to 1.5%. Is desirable. Preferably it is 0.6 to 1.0% of range.
[0040]
Even if the cold-rolled steel sheet thus obtained is passed through an electroplating line, the material properties thereof do not change, and therefore various types of electric plating may be applied after annealing. Furthermore, special treatments may be applied to improve chemical conversion properties, weldability, press formability, corrosion resistance, and the like.
[0041]
In addition, since the material properties of the thin steel plate of the present invention do not substantially change even when applied to the manufacture of a hot dip galvanized steel plate, hot dip galvanizing and alloying hot dip galvanizing may be performed after annealing. For this production, it is most efficient to use the aforementioned hot dip galvanizing line. Furthermore, this surface-treated steel sheet may be subjected to a special treatment for improving chemical conversion properties, weldability, press formability, corrosion resistance, and the like.
[0042]
【Example】
A steel slab having the composition shown in Table 2 was heated and soaked to 1200 ° C, hot rough rolled and then hot finished rolled, and then wound on a coil at 630 ° C. Subsequently, the obtained hot-rolled sheet was pickled and then cold-rolled at the cold rolling reduction shown in Table 3 to obtain each sheet thickness, and then recrystallized and annealed under the conditions shown in Table 3. Then, the holding | maintenance process and temper rolling in the temperature range of 400-500 degreeC were performed on the conditions shown in Table 3.
Table 4 shows the results of examining the tensile properties, BH amount, average crystal grain size, low-temperature bake hardening index A, and aging resistance of the thin steel sheet thus obtained.
[0043]
Here, the tensile properties were measured using a JIS No. 5 tensile test piece, and the Rankford value (r value) was measured by a three-point method after giving a tensile pre-strain of 15%. The r 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 C direction (90 ° direction with respect to the rolling direction).
Moreover, about 170 degreeC-BH and 100 degreeC-BH, an average crystal grain diameter, and the low-temperature bake hardening index A, it measured and computed according to the same method as the experiment mentioned above.
Furthermore, in order to judge whether the aging resistance was good or not, after the accelerated aging treatment at 40 ° C. for 20 days (corresponding to normal temperature of 6 months), the same tensile test as described above was performed to measure Y-El.
[0044]
[Table 1]

[0045]
[Table 2]

[0046]
[Table 3]

[0047]
[Table 4]

[0048]
As is clear from Table 4, all of the inventive examples obtained according to the present invention have an average crystal grain size of 5 to 12 μm, a low-temperature bake hardening index A of 10 or less, and low-temperature bake hardening represented by 100 ° C.-BH. The amount was 30 MPa or more and the Y-El after aging corresponding to room temperature for 6 months was as good as 0.6 or less. Moreover, the processability represented by the r value was also excellent.
On the other hand, any of the comparative examples that deviate from the appropriate range of the present invention has insufficient low-temperature bake hardenability or insufficient aging resistance.
[0049]
【The invention's effect】
Thus, according to the present invention, 100 ° C.-BH is 30 MPa or more, Y-EL after aging at room temperature for 6 months is 0.6% or less, and r value is 1.6 or more. In addition, a thin steel sheet for processing having good workability can be obtained.
Therefore, according to this invention, it contributes greatly to the weight reduction and safety | security improvement as an automotive steel plate.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between a low-temperature bake hardening index A and a bake hardening amount (BH amount).

Claims (4)

% By mass
C: 0.0020 to 0.010%,
Si: 1.0% or less,
Mn: 0.05 to 0.52 %,
P: 0.05% or less,
S: 0.02% or less,
N: 0.005% or less,
Al: 15 × N (%) to 0.10% and
Nb: 0.5 × (93/12) × C (%) to (93/12) × [C (%) − 0.001]
The balance is Fe and inevitable impurities, the steel sheet has an average grain size d of 2 to 12 μm, and a low-temperature bake hardening index A expressed by the following formula (1) is 10 or less. A thin steel sheet for processing having excellent low temperature bake hardenability and aging resistance.
A = (12/93) × (Nb (%) / C (%)) × d (μm) −10 × C ^* (%) --- (1)
However, C ^* (%) = C (%)-(12/93) x Nb (%) % By mass
C: 0.0020 to 0.010%,
Si: 1.0% or less,
Mn: 0.05 to 0.52 %,
P: 0.05% or less,
S: 0.02% or less,
N: 0.005% or less,
Al: 0.01-0.06%,
Nb: 0.5 × (93/12) × C (%) to (93/12) × [C (%) − 0.001] and
Ti: (48/14) × N (%) to (48/12) × [C (%) − (12/93) × Nb (%) −0.001]
+ (48/14) x N (%) + (48/32) x S (%)
The balance is Fe and inevitable impurities, the steel sheet has an average grain size d of 2 to 12 μm, and a low-temperature bake hardening index A expressed by the following formula (1) is 10 or less. A thin steel sheet for processing having excellent low temperature bake hardenability and aging resistance.
A = (12/93) × (Nb (%) / C (%)) × d (μm) −10 × C ^* (%) --- (1)
However, C ^* (%) = C (%)-(12/93) x Nb (%)-(12/48) x [Ti (%)-(48/14)
× N (%) − (48/32) × S (%)] The thin steel sheet for processing according to claim 1 or 2, wherein the steel sheet further has a composition containing B: 0.0030% or less by mass%, and having excellent low-temperature bake hardenability and aging resistance. In any one of Claims 1-3, a steel plate is further in the mass%.
Cr: 2.0% or less,
Cu: 2.0% or less,
Ni: 2.0% or less and
Mo: A thin steel sheet for processing excellent in low-temperature bake hardenability and aging resistance, characterized in that the composition contains one or more selected from 1.0% or less.

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