JP2013064169A - High-strength steel sheet and plated steel sheet excellent in bake-hardenability and formability, and method for production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength steel sheet excellent in bake-hardenability and formability.SOLUTION: The high-strength steel sheet contains, by mass, 0.0010-0.0040% C, ≤0.05% Si, 0.1-1.0% Mn, ≤0.10% P, ≤0.03% S, 0.01-0.10% Al, ≤0.0050% N, 0.005-0.025% Nb, and the balance Fe with inevitable impurities, and further satisfies [%Nb]/[%C]≤10 and [%Mn]/[%C]≥100. The steel sheet has the tensile strength (TS) of ≥340 MPa, bake-hardening amount (BH) of ≥30 MPa, uniform elongation of ≥18%, and yield elongation (YP-EL) after progressive aging of ≤1.0%.

Description

The present invention is a high strength thin film excellent in bake hardenability and formability that is optimal for panel parts such as automobile doors or hoods, and parts to be baked, such as vending machines, desks, home appliances / OA equipment, and building materials. The present invention relates to steel plates and plated thin steel plates, and methods for producing them.
In the present invention, the thin steel plate means a cold-rolled steel plate having a thickness of 2.0 mm or less.

In recent years, in response to growing interest in the global environment, there has been an increasing demand to reduce the use of steel sheets with large CO ₂ emissions during manufacturing. In the automobile field and the like, there is an increasing need for improving fuel efficiency by reducing the body weight and reducing exhaust gas.
Therefore, it is considered effective to increase the strength and thickness of the steel sheet. However, when the strength of the steel sheet is increased, there is a problem that cracking occurs due to a shape defect due to springback during pressing or strain concentration caused by insufficient uniform elongation.

  Conventionally, many stamped parts of thin steel sheets are baked and painted after press working. For such parts, heat generated during baking is used for high strength steel sheets that can be further strengthened after press working. The needs were very large.

  Here, as a steel plate excellent in bake hardenability, for example, in Patent Document 1, aging is improved by fixing N as B / N = 0.5 to 1.6 in steel of C ≦ 0.01% by weight%. , Nb / C = 0.5-4, and a technique for imparting bake hardenability by allowing solid solution C to remain is disclosed. In Patent Document 2, in steel of weight% C = 0.001 to 0.0035% and Ti ≧ 0.005%, (Ti / 48) / (S / 32 + N / 14) ≦ 1.0, S and N are fixed with Ti. In addition, a technique for imparting bake hardenability by controlling the added C to be the total amount of solute C is disclosed.

However, the technique of Patent Document 1 has a problem that it is difficult to increase the strength of the steel sheet.
Moreover, although the technique of patent document 2 can obtain a certain amount of strength, there is a problem that sufficient uniform elongation cannot be secured.

JP 58-84929 A Japanese Patent Laid-Open No. 2-197549

As described above, it has been difficult to provide a high-strength thin steel sheet excellent in bake hardenability and formability with the conventional technology.
The present invention advantageously solves the above-mentioned problems of the prior art, and aims to provide a high-strength thin steel sheet excellent in bake hardenability and formability, and a plated thin steel sheet, together with an advantageous manufacturing method thereof. And

  Inventors repeated earnest examination in order to solve the subject mentioned above. As a result, for a steel material having a specific composition, after hot rolling, cooling and winding the coil at a coiling temperature of 550 ° C. or higher, then pickling, cold rolling, and performing annealing, In order to optimize the heating rate, soaking temperature, and soaking time, and to perform temper rolling, by optimizing the sheet thickness reduction rate, the desired bake hardenability and forming of the manufactured thin steel sheet It has been found that properties and strength can be obtained.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
(1) By mass%, C: 0.0010 to 0.0040%, Si: 0.05% or less, Mn: 0.1 to 1.0%, P: 0.10% or less, S: 0.03% or less, Al: 0.01 to 0.10%, N: 0.0050% And Nb: 0.005 to 0.025%, and satisfy [% Nb] / [% C] ≦ 10 and [% Mn] / [% C] ≧ 100, with the balance being the composition of Fe and inevitable impurities Become
Bake hardening characterized by tensile strength (TS) of 340 MPa or more, bake hardening (BH) of 30 MPa or more, uniform elongation of 18% or more, and yield elongation after accelerated aging (YP-EL) of 1.0% or less. High-strength thin steel sheet with excellent properties and formability.
However, [% M] indicates the content (mass%) of the M element in the steel.

(2) The high-strength thin steel sheet excellent in bake hardenability and formability as described in (1) above, further containing, by mass%, B: 0.0005 to 0.0030%.

(3) The high-strength thin steel sheet having excellent bake hardenability and formability as described in (1) or (2) above, further containing, by mass%, Ti: 0.003 to 0.050%.

(4) Any one of the above (1) to (3), further containing 0.005 to 0.050% of one or more selected from V, Ta, W and Mo in mass% A high strength thin steel sheet having excellent bake hardenability and formability as described in 1.

(5) The composition according to any one of (1) to (4) above, further containing 0.01 to 0.10% of one or more selected from Cr, Ni and Cu, respectively, by mass% High strength thin steel plate with excellent bake hardenability and formability.

(6) The high strength thin steel sheet excellent in bake hardenability and formability according to any one of the above (1) to (5), further comprising Sb: 0.005 to 0.050% by mass.

(7) The bake hardening according to any one of (1) to (6) above, further containing 0.0005 to 0.01% of one or two selected from Ca and REM by mass%. High-strength thin steel sheet with excellent properties and formability

(8) A plated thin steel sheet further comprising a plating layer on the surface of the steel sheet according to any one of (1) to (7).

(9) The steel material according to any one of the above (1) to (7) is hot-rolled, cooled and wound into a coil, then pickled, cold-rolled, and then annealed. In producing thin steel sheets by temper rolling,
The coiling temperature is 550 ° C. or higher, and the annealing is performed at a heating rate from 500 ° C. to a soaking temperature range: 0.1 × ([% Nb] / [% C]) ° C./s or more, soaking temperature: (650 + 10 × [% Nb] / [% C]) to 900 ° C., soaking time: 10 to 1000 s, and the thickness reduction rate of the temper rolling is 0.8 × [% Mn] to (2 + [% Mn] The method for producing a high-strength thin steel sheet having excellent bake hardenability and formability as described in any one of (1) to (8) above.
However, [% M] indicates the content (mass%) of the M element in the steel.

(10) A method for producing a plated thin steel sheet, comprising subjecting the surface of the thin steel sheet obtained by the production method according to (9) to a plating treatment after annealing.

(11) The method for producing a plated thin steel sheet according to (10), wherein after the plating treatment, the plating layer is further subjected to an alloying treatment.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the high strength thin steel plate excellent in bake hardenability and formability, and a plated thin steel plate with the advantageous manufacturing method.

For each specimen, the relationship between [% Nb] / [% C] and the bake hardening amount (BH) is shown. The relationship between [% Mn] / [% C] and yield elongation (YP-EL) is shown for each specimen. For each specimen, the relationship between the heating rate and the uniform elongation is shown. The relationship between the yield elongation (YP-EL) and the heating rate is shown for each specimen. The relationship between soaking temperature and uniform elongation is shown for each specimen. The relationship between soaking temperature and bake hardening amount (BH) is shown for each specimen. For each specimen, the relationship between the thickness reduction rate of temper rolling and uniform elongation is shown.

Hereinafter, the present invention will be specifically described.
First, the reason why the composition of the thin steel plate is limited to the above range in the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.

C: 0.0010 to 0.0040%
C combines with Nb to form fine carbides, contributes to increasing the strength of the steel sheet, and improves the work hardening rate. In addition, the presence of solute C improves the bake hardenability. Therefore, the C content needs to be 0.0010% or more. However, a large amount of C not only lowers the uniform elongation due to an increase in carbides or solid solution C, but when a large amount of solid solution C exists, the yield elongation (YP-EL) after accelerated aging increases. Therefore, the C amount needs to be 0.0040% or less, preferably 0.0030% or less, more preferably 0.0025% or less, and still more preferably 0.0020% or less.

Si: 0.05% or less
If Si is added in a large amount, the workability deteriorates due to hardening, and the plating property is hindered due to the formation of Si oxide during annealing. Therefore, Si needs to be 0.05% or less, preferably 0.03% or less, more preferably 0.02% or less, and still more preferably 0.01% or less.

Mn: 0.1-1.0%
Mn not only contributes to strengthening by solid solution strengthening, but also suppresses the increase in yield elongation (YP-EL) after accelerated aging caused by solid solution C by interacting with solid solution C. Can do. In addition, by suppressing recovery during heating during annealing, uniform recrystallized grains can be formed during soaking, and uniform elongation can be improved. Furthermore, it has the effect of detoxifying harmful S in steel as MnS. In order to obtain such an effect, Mn needs to be 0.1% or more. On the other hand, a large amount of Mn not only lowers the uniform elongation due to hardening, but also inhibits plating properties due to the formation of Mn oxide during annealing. Therefore, the amount of Mn needs to be 1.0% or less.

P: 0.10% or less
P segregates at the grain boundary and deteriorates ductility and toughness, so it is necessary to keep it at 0.10% or less. The lower limit is not particularly set, but it is preferably 0.03% or more, more preferably 0.05% or more, because it effectively acts to increase the strength.

S: 0.03% or less S causes a hot cracking by remarkably reducing hot ductility and remarkably deteriorates surface properties. Further, S hardly contributes to the strength, but also reduces the ductility by forming coarse MnS as an impurity element. These problems become significant when the S content exceeds 0.03%, and it is desirable to reduce them as much as possible. Therefore, the S amount needs to be 0.03% or less, preferably 0.02% or less, more preferably 0.01% or less.

Al: 0.01-0.10%
Al can suppress aging deterioration due to solute N by fixing N as a nitride. In order to obtain such an effect, Al needs to be 0.01% or more, preferably 0.03% or more. On the other hand, a large amount of Al needs to be 0.1% or less because it causes an increase in the aluminum oxide in the steel and lowers the ductility.

N: 0.0050% or less N combines with Ti to form TiN, or combines with Al to form AlN. However, when the N content exceeds 0.01%, these nitrides are dispersed in the ferrite grains and the work hardening rate is lowered. For this reason, N amount needs to be 0.01% or less, preferably 0.0030% or less, more preferably 0.0020% or less.

  In the present invention, it is necessary to contain at least one element selected from Ti and Nb.

Nb: 0.005-0.025%
Nb can contribute to hardening by forming fine carbides with C. Furthermore, the fine carbide of Nb suppresses recovery during heating during annealing, so that uniform recrystallized grains can be formed during soaking, and uniform elongation can be improved. Therefore, Nb needs to be 0.005% or more, preferably 0.010% or more. On the other hand, the addition of a large amount of Nb not only reduces the bake hardenability by reducing the solid solution C, but also increases the hot deformation resistance value and makes rolling difficult. Therefore, Nb needs to be 0.025% or less, preferably 0.020% or less.

[% Nb] / [% C] ≦ 10
Further, if the ratio of the Nb content to the C content is large, carbides are easily formed, and it becomes difficult to leave the solid solution C. Therefore, [% Nb] / [% C] needs to be 10 or less, preferably 7.7 or less, more preferably 6.5 or less. In addition, [% M] shows content (mass%) of M element in steel.

[% Mn] / [% C] ≧ 100
By increasing the ratio of Mn to C, an increase in yield elongation (YP-EL) after accelerated aging due to interaction with solute C can be suppressed. In order to obtain such an effect, [% Mn] / [% C] needs to be 100 or more, preferably 150 or more, more preferably 200 or more. In addition, [% M] shows content (mass%) of M element in steel.

  The balance of the present invention is Fe and inevitable impurities. This means that the contents of the present invention include unavoidable impurities and other trace elements unless the effects and effects of the present invention are impaired.

  Furthermore, the following elements can be added for the purpose of improving strength, bake hardenability, ductility, and yield elongation after aging.

B: 0.0005-0.0030%
B can suppress aging deterioration due to solute N by fixing N as a nitride. In addition, segregation at the grain boundary can improve the secondary work brittleness resistance. In order to obtain such an effect, 0.0005% or more of B is preferably added. On the other hand, a large amount of B increases the hot deformation resistance and makes rolling difficult. Therefore, when B is added, the content is preferably 0.0030% or less, more preferably 0.0020% or less.

Ti: 0.003-0.050%
Ti can suppress aging deterioration due to solute N by fixing N as nitride. In order to obtain such an effect, it is preferable to add 0.003% or more of Ti. On the other hand, when a large amount of Ti is added, it becomes difficult to form C and carbides and leave solid solution C. Therefore, when adding Ti, it is preferable to set it as 0.050% or less, More preferably, it shall be 0.030% or less.

0.005 to 0.050% of one or more selected from V, Ta, W and Mo
V, Ta, W and Mo can contribute to high strength by forming fine precipitates. In order to acquire such an effect, when adding 1 type, or 2 or more types selected from V, Ta, W, and Mo, it is preferable to add 0.005% or more, respectively. On the other hand, when these elements are added in a large amount, the ductility is greatly lowered. Therefore, the content of these elements is preferably 0.050% or less.

0.01 to 0.10% of one or more selected from Cr, Ni and Cu, respectively
Cr, Ni, and Cu can contribute to high strength by refining the structure. In order to acquire such an effect, when adding 1 type, or 2 or more types selected from Cr, Ni, and Cu, it is preferable to add each element 0.01% or more respectively. On the other hand, when these elements are added in a large amount, the ductility is greatly lowered. Therefore, the content of these elements is preferably 0.10% or less.

Sb: 0.005 to 0.050%
Sb can suppress deterioration of aging due to N by preventing segregation on the surface and nitriding of the slab in a heating furnace during hot rolling. In order to obtain such an effect, when Sb is added, 0.005% or more is preferably added. On the other hand, when a large amount of Sb is added, the production cost increases. Therefore, when Sb is added, the amount is preferably 0.050% or less.

0.0005 to 0.01% of one or two selected from Ca and REM
Ca and REM can improve ductility by controlling the form of sulfide. In order to obtain such an effect, it is preferable to add one or two of Ca and REM in an amount of 0.0005% or more. On the other hand, the addition of a large amount increases the production cost, so the content of these elements is preferably 0.01% or less.

  It should be noted that there is no problem in characteristics even if the impurities contain Sn, Mg, Co, As, Pb, Zn, O, etc. in a total of 0.5% or less.

Tensile strength (TS): 340 MPa or more The high-strength thin steel sheet of the present invention is characterized by a tensile strength (TS) of 340 MPa or more. This is because by setting TS to 340 MPa or more, the steel sheet can be thinned with respect to a member that requires strength. Here, the TS can be measured by a JIS5 tensile test piece cut out from the direction perpendicular to the rolling direction and a tensile test based on JIS Z 2241.

Bake hardening amount (BH): 30 MPa or more The high strength thin steel sheet of the present invention has a bake hardening amount (BH) of 30 MPa or more. This is because by setting BH to 30 MPa or more, the load at the time of press molding can be reduced and the strength after press molding can be increased. Here, the BH can be measured by a paint bake hardening amount test method in accordance with JIS G3135 by cutting out a JIS5 tensile test piece from the direction perpendicular to the rolling.

Uniform elongation: 18% or less The high-strength thin steel sheet of the present invention is characterized by a uniform elongation of 18% or more. This is because by setting the uniform elongation to 18% or more, it is possible to suppress the concentration of strain during press molding and to suppress the occurrence of cracks.

Yield elongation after accelerated aging (YP-EL): 1.0% or less The high strength thin steel sheet of the present invention is characterized in that the yield elongation after accelerated aging (YP-EL) is 1.0% or less. This is because the generation of wrinkles during press molding can be suppressed by setting the yield elongation after accelerated aging to 1.0% or less. Here, YP-El after accelerated aging can be measured as the elongation at yield when a tensile test is performed after cutting a JIS5 tensile test piece from the direction perpendicular to the rolling and holding at 100 ° C. for 6 hours.

  The steel sheet of the present invention may have a plating film on the surface. By forming a plating film on the surface of the steel plate, the corrosion resistance of the thin steel plate is improved. In addition, as a plating film, electrogalvanization, for example, Zn-Ni electroalloy plating, etc. other than a hot dip galvanization film and an alloying hot dip galvanization film, etc. are mentioned, for example.

Next, the manufacturing method of the thin steel plate of this invention is demonstrated.
In the present invention, the slab obtained by continuous casting is preferably a steel material, hot rolled, cooled and wound into a coil, then pickled, cold rolled, annealed and temper rolled. To manufacture thin steel sheets.
The coiling temperature is set to 550 ° C. or more, and the annealing is performed at a heating rate from 500 ° C. to a soaking temperature range: 0.1 × ([% Nb] / [% C]) ° C./s or more, soaking temperature: ( 650 + 10 × [% Nb] / [% C]) to 900 ° C., soaking time: 10 to 1000 s, and the thickness reduction rate of the temper rolling is 0.8 × [% Mn] to (2 + [% Mn ])%.

Winding temperature after hot rolling: 550 ° C or higher If the coiling temperature after hot rolling is low, precipitation of AlN is suppressed and N remains in a solid solution state, so aging deterioration due to N occurs. . Moreover, the precipitation of NbC is also suppressed, and if solute C remains at the stage of the hot rolled steel sheet, a large amount of shear strain is introduced during cold rolling, resulting in a significant reduction in uniform elongation. Furthermore, the generation of the acicular ferrite hardens the steel sheet, and the load during subsequent cold rolling also increases, resulting in operational difficulties. Therefore, the winding temperature needs to be 550 ° C. or higher, preferably 600 ° C. or higher. The upper limit of the coiling temperature is not particularly set, but if the coiling temperature is high, not only the scale generation is promoted and the yield of the steel sheet is lowered, but also surface defects caused by the remaining scale during pickling occur. It is preferable that the temperature is not higher than 700C, more preferably not higher than 700 ° C, still more preferably not higher than 650 ° C.

Heating rate from 500 ° C to soaking temperature range in annealing: 0.1 x ([% Nb] / [% C]) ° C / s or more If the heating rate during annealing is small, recovery is promoted during heating, soaking Sometimes coarse recovery grains remain as they are, and uniform recrystallization is suppressed, so that uniform elongation is lowered. In addition, the decrease in the work dislocations stabilizes the precipitates and suppresses the re-solution of NbC during the subsequent soaking, resulting in a decrease in the solid solution C, resulting in a decrease in bake hardenability. . The recovery during heating becomes remarkable at 500 ° C. or higher, and such an effect becomes more apparent as the ratio of Nb content to C content ([% Nb] / [% C]) increases. It is necessary to heat at an average of 0.1 × ([% Nb] / [% C]) ° C./s or more, and preferably 0.2 × ([% Nb] / [% C]) ° C./s or more. More preferably, it is 0.3 × ([% Nb] / [% C]) ° C./s or more, and further preferably 0.5 × ([% Nb] / [% C]) ° C./s or more. There is no particular upper limit on the heating rate, and heating may be performed at 100 ° C./s or higher using IH or the like, but 30 ° C./s or lower is sufficient when no special heating device is used. In addition, [% M] shows content (mass%) of M element in steel.

Soaking temperature in annealing: (650 + 10 × [% Nb] / [% C]) to 900 ° C
When the soaking temperature is low, not only recrystallization is not completed, but also re-solution of NbC is suppressed, so that the solid solution C decreases, and the bake hardenability decreases. Since such an effect becomes more apparent as the ratio of Nb content to C content [% Nb] / [% C] increases, the soaking temperature is (650 + 10 × [% Nb] / [% C]) ° C. It is necessary to set it above, preferably (650 + 15 × [% Nb] / [% C]) ° C. or higher, more preferably (650 + 20 × [% Nb] / [% C]) ° C. or higher. On the other hand, when the soaking temperature increases, the ferrite grains become coarse and the strength decreases, and the re-solution of NbC is promoted and the amount of solid solution C increases too much. As a result, the YP-El after the decrease in uniform elongation and accelerated aging Will lead to an increase. Therefore, the soaking temperature needs to be 900 ° C. or lower, preferably 860 ° C. or lower, more preferably 840 ° C. or lower. In addition, [% M] shows content (mass%) of M element in steel.

Soaking time in annealing: 10-1000s
If the soaking time is short, the recrystallization is not completed and the uniform elongation is remarkably lowered. Therefore, the soaking time needs to be 10 s or more, preferably 30 s or more, more preferably 100 s or more. On the other hand, if the soaking time is long, the ferrite grains become coarse and the strength decreases, so the soaking time needs to be 1000 s or less, preferably 500 s or less, more preferably 300 s or less, more preferably 200s or less.

Sheet thickness reduction rate in temper rolling: 0.8 x [% Mn] to (2+ [% Mn])%
By performing temper rolling after annealing, it is possible to reduce YP-EL and suppress wrinkling during press molding. Furthermore, in steel with increased intragranular strength by adding Mn, the strain introduced during temper rolling can be concentrated near the grain boundary to promote intragranular deformation during processing and improve uniform elongation. be able to. In order to obtain such an effect, the greater the amount of Mn, the more strain is required. Therefore, the sheet thickness reduction rate in temper rolling must be 0.8 × [% Mn]% or more. On the other hand, when the plate thickness reduction rate by temper rolling increases, the uniform elongation due to processing strain decreases. However, the smaller the Mn, the more the decrease in uniform elongation becomes more pronounced with less strain, so the plate thickness reduction rate is (2+ [% Mn])% or less is required. In temper rolling, rolling by a rolling roll may be applied, or processing by tension with a tension applied to a steel sheet may be added. Further, it may be a combination of rolling and tension.

In carrying out the invention, a melting method can be appropriately applied, such as a normal converter method, an electric furnace method, or the like. The molten steel is hot-rolled as it is after being cast into a slab, or by reheating a hot piece or cold piece slab. What is necessary is just to heat at about 1100-1250 degreeC when heating by hot rolling.
In finish rolling after rough rolling, it is preferable to finish rolling in the austenite region.
The cooling rate from finish rolling to winding is also not particularly specified, but a cooling rate higher than air cooling is sufficient, but even if quenching at 20 ° C / s or higher or ultra-rapid cooling at 100 ° C / s or higher is performed. Good.

  Then, when performing cold rolling after normal pickling, rolling may be performed at a cold pressure rate of about 50 to 80%. In the annealing, the heating rate in the heating process up to 500 ° C. is arbitrary, but if it is slow, the working efficiency is lowered, so annealing may be performed at a heating rate of 3 ° C./s or more. Cooling after soaking is optional, but if it is slow, work efficiency will decrease, so it may be cooled at a cooling rate of 5 ° C / s or more. There is no particular problem even if so-called overaging treatment is performed during cooling, which is performed at a temperature of 300 to 450 ° C. for 30 to 600 seconds.

Moreover, hot dip galvanization can also be performed by immersing in the galvanization bath of 420-500 degreeC as needed in the middle of cooling.
Furthermore, after soaking in the plating bath, reheating can be performed to a temperature of about 460 to 570 ° C., and so-called alloying treatment can be performed in which zinc and iron are alloyed by holding for 1 s or more, more preferably 5 s or more.
In plating, in addition to zinc, Al plating or composite plating of zinc and Al can also be performed. In addition, when plating is not performed during annealing, electrogalvanization or Ni plating may be performed, and a film can be formed on a cold-rolled steel plate or a plated steel plate by chemical conversion treatment or the like. .

Examples will be described below.
Table 1 shows the chemical composition of the specimen, and Table 2 shows the production conditions of the steel sheet. The molten steel having the composition shown in Table 1 was continuously cast into a slab (steel material). The obtained slab is hot-rolled according to the conditions in Table 2, cooled and wound into a coil, then pickled, cold-rolled and then annealed, and then subjected to temper rolling to produce a thin steel sheet did.

  Regarding the plating treatment, GA is alloyed hot dip galvanizing, GI is hot dip galvanizing, EG is electrogalvanizing, GA and GI are during cooling during annealing, and EG is plated after annealing. .

Moreover, the tensile test for measuring tensile strength (HS) cut out the tensile test piece from the rolling right angle direction, and performed the tensile test based on JIS Z 2241.
The uniform elongation was defined as the total elongation at the maximum test force described in JIS Z 2241.
The bake hardening amount (BH) was measured by giving a pre-strain of 2% to the specimen, holding it at 170 ° C. for 20 minutes, and then increasing the yield point (YP) after work hardening by pre-strain.
Yield elongation after accelerated aging (YP-EL) was measured by simulating a state of holding at 100 ° C. for 6 hours and aging at 25 ° C. for 6 months.
The measured and calculated results are shown in Table 3.

FIG. 1 shows the influence of [% Nb] / [% C] on the bake hardening amount (BH) for specimens Nos. 1 to 22, 35, and 36. By setting [% Nb] / [% C] ≦ 10, BH ≧ 30 MPa can be realized.
FIG. 2 shows the influence of [% Mn] / [% C] on the yield elongation (YP-EL) for specimen Nos. 1 to 22, 30, and 37. By setting [% Mn] / [% C] ≧ 100, YP-EL ≦ 1.0% can be realized.
FIG. 3 shows the influence of the heating rate on the uniform elongation for specimens Nos. 1-24. By setting the heating rate ≧ 0.1 × ([% Nb] / [% C]) ° C./s, uniform elongation ≧ 18% can be realized. FIG. 4 shows the influence of the heating rate on the yield elongation (YP-EL) (%) for specimens Nos. 1 to 24. By setting the heating rate ≧ 0.1 × ([% Nb] / [% C]) ° C./s, YP-EL ≦ 1.0% can be realized. 3 and 4, the horizontal axis represents the value obtained by dividing the heating rate by ([% Nb] / [% C]).
FIG. 5 shows the effect of soaking temperature on uniform elongation for specimens Nos. 1 to 22, 26, and 27 having a soaking temperature of 900 ° C. or less. Uniform elongation ≧ 18% can be realized by setting soaking temperature ≧ 650 + 10 × ([% Nb] / [% C]) ° C. FIG. 6 shows the influence of the soaking temperature on the bake hardening amount (BH) for specimens Nos. 1 to 22, 26, and 27 having a soaking temperature of 900 ° C. or less. By setting soaking temperature ≧ 650 + 10 × ([% Nb] / [% C]) ° C., BH ≧ 30 MPa can be realized. 5 and 6 represent values obtained by dividing the soaking temperature by (650 + 10 × ([% Nb] / [% C])).
FIG. 7 shows the influence of the sheet thickness reduction rate of the temper rolling on the uniform elongation for specimens Nos. 1 to 22, 40, and 41. Uniform elongation ≧ 18% can be realized by adjusting the pressure regulation rate to 0.8 × [% Mn] to (2 + [% Mn])%. The horizontal axis in FIG. 7 represents the value obtained by dividing (thickness reduction rate−0.8 × [% Mn]) by {(2 + [% Mn]) − 0.8 × [% Mn]}. When the reduction rate is (0.8 × [% Mn])% (lower limit), it is 0, and when the plate thickness reduction rate is (2 + [% Mn])% (upper limit), it is 1.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the high-strength thin steel plate excellent in bake hardenability and a moldability, and a plated thin steel plate with the advantageous manufacturing method, and there exists a remarkable industrial effect.

Claims (11)

In mass%, C: 0.0010 to 0.0040%, Si: 0.05% or less, Mn: 0.1 to 1.0%, P: 0.10% or less, S: 0.03% or less, Al: 0.01 to 0.10%, N: 0.0050% or less, and Nb : 0.005 to 0.025%, and [% Nb] / [% C] ≦ 10 and [% Mn] / [% C] ≧ 100 are satisfied, and the balance is composed of Fe and inevitable impurities,
Bake hardening characterized by tensile strength (TS) of 340 MPa or more, bake hardening (BH) of 30 MPa or more, uniform elongation of 18% or more, and yield elongation after accelerated aging (YP-EL) of 1.0% or less. High-strength thin steel sheet with excellent properties and formability.
However, [% M] indicates the content (mass%) of the M element in the steel.   The high-strength thin steel sheet having excellent bake hardenability and formability according to claim 1, further comprising B: 0.0005 to 0.0030% in mass%.   The high strength thin steel sheet having excellent bake hardenability and formability according to claim 1 or 2, further comprising Ti: 0.003 to 0.050% by mass.   The bake hardenability according to any one of claims 1 to 3, further comprising 0.005 to 0.050% of one or more selected from V, Ta, W and Mo in mass%. And high strength thin steel sheet with excellent formability.   Furthermore, the bake hardenability and shaping | molding in any one of Claims 1-4 which contain 0.01-0.10% of the 1 type (s) or 2 or more types selected from Cr, Ni, and Cu by the mass%, respectively. High strength thin steel sheet with excellent properties.   The high strength thin steel sheet excellent in bake hardenability and formability according to any one of claims 1 to 5, further comprising Sb: 0.005 to 0.050% in mass%.   Furthermore, it is excellent in the bake hardenability and the moldability according to any one of claims 1 to 6, characterized by containing 0.0005 to 0.01% of one or two selected from Ca and REM, respectively. High strength thin steel sheet.   A plated thin steel sheet, further comprising a plating layer on the surface of the steel sheet according to claim 1. The steel material having the composition according to any one of claims 1 to 7, after hot rolling, cooled and wound into a coil, then pickled, cold-rolled and then annealed, and then tempered In producing thin steel sheets by rolling,
The coiling temperature after the hot rolling is set to 550 ° C. or higher, and the annealing is performed at a heating rate from 500 ° C. to a soaking temperature range: 0.1 × ([% Nb] / [% C]) ° C./s or higher, Heat temperature: (650 + 10 × [% Nb] / [% C]) to 900 ° C., soaking time: 10 to 1000 s, and the thickness reduction rate of the temper rolling is 0.8 × [% Mn] to ( The method for producing a high-strength thin steel sheet excellent in bake hardenability and formability according to any one of claims 1 to 8, characterized in that 2 + [% Mn])%.
However, [% M] indicates the content (mass%) of the M element in the steel.   A method for producing a plated thin steel sheet, comprising subjecting the surface of the thin steel sheet obtained by the production method according to claim 9 to a plating treatment after annealing.  The method for producing a plated thin steel sheet according to claim 10, wherein after the plating process, the plating layer is further subjected to an alloying process.

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