[1]The present invention relates to a high strength steel sheet, and relates to a method of manufacturing the same, and more particularly, automobile outer drawing which can be preferably applied to the material of the plate material, such as resistance and bake curing has excellent high strength steel sheet and a method of manufacturing the same.
[2]
BACKGROUND
[3]
Steel is applied as the material of the inside of a car, the outer plate member (door, hood, fenders, floors, etc.), as well as high strength with excellent formability is required. This is to promote the safety of passengers from an accident, and to improve fuel economy through kkoehagi body weight.
[4]
However, the increase in the steel sheet strength than the high-performance, such as the two factors (strength, moldability) very difficult and is to satisfy both at the same time, in particular the door inner plate (Door Inner) and the rear floor (Rear Floor), as they result in a deterioration moldability the components required to form a molding defect such as a crack during processing occurs frequently caused by the application of high strength steel for these components is still minor situation.
[5]
[6]
To the excellent strength and the moldability developed so far known in the steel sheet has a so-called (Interstitial Free Steel) IF steel. This is a strong carbonitride forming elements titanium (Ti) and / or niobium (Nb), such as by the addition of carbon (C), nitrogen (N), sulfur (S) ensuring the strength and moldability by removing the employed elements, such as at the same time It is written by and is representatively described in Patent documents 1 to 4. However, the IF steel has average plastic anisotropy factor (Lankford value, r value) refers to the 1.5 ~ 1.8, it was very insufficient level hagieneun parts existing soft cold-rolled steel sheet of DDQ (Deep Drawing Quality) class that used alternative.
[7]
[8]
[Patent Document 1] Japanese Laid-Open Patent Publication No. 1992-280943 No.
[9]
(Patent Document 2) Japanese Unexamined Patent Publication No. 1993-070836 No.
[10]
(Patent Document 3) Japanese Unexamined Patent Publication No. 1993-263184 No.
[11]
[Patent Document 4] Japanese Laid-Open Patent Publication No. 1998-096051 No.
[12]
Detailed Description of the Invention
SUMMARY
[13]
One of the objects of the present invention is to provide a process for producing drawability and bake curing has excellent high strength steel sheet with them.
[14]
Problem solving means
[15]
One aspect of the present invention is, in weight%, C: 0.0005 ~ 0.003%, Si: 0.5% or less (excluding 0%), Mn: less than 1.2% (excluding 0%), P: 0.005 ~ 0.12%, S: 0.008 % or less, N: 0.005% or less, acid soluble Al: 0.1% or less (excluding 0%), Ti: 0.01 ~ 0.04%, the balance Fe and including unavoidable impurities, to a more than 80% P defined by equation (1) and, the drawability and bake curing has a 4MPa or more baking hardening amount (BH) provides excellent high strength steel sheet.
[16]
[수학 식 1] P (%) = {N / (N + N GB )} × 100
[17]
(However, N in is the number of carbides having a size of 20nm or less present in the crystal grains, N gb is being the number of carbides having a size of 20nm or less present in the grain boundaries)
[18]
[19]
In addition, the other aspect of the invention, in weight%, C: 0.0005 ~ 0.003%, Si: 0.5% or less (excluding 0%), Mn: less than 1.2% (excluding 0%), P: 0.005 ~ 0.12%, S: 0.008% or less, N: 0.005% or less, acid soluble Al: 0.1% or less (excluding 0%), Ti: 0.01 ~ 0.04%, the balance Fe and the inevitable rolling a steel slab containing the impurity hot to obtain a hot rolled steel sheet step, step for winding the hot-rolled steel sheet at a temperature of 450 ~ 750 ℃, 75 ~ 85% of the take-up the hot-rolled steel sheet reduction ratio, the pressure Harvey condition of 5 to 15% of the final calender roll to obtain a cold-rolled steel sheet to cold rolling step, after the cold-rolled steel sheet was heated at a rate of 7 ℃ / sec or less (excluding 0 ℃ / sec) to 830 ~ 880 ℃, the step of continuous annealing by keeping it for 30 ~ 80sec, and wherein the annealed cold-rolled steel sheet 650 ℃ the production method of 2 ~ 10 ℃ / average drawings comprises cooling at a cooling rate of sec properties and baking curability excellent in high strength steel sheet is started up.
[20]
[21]
In addition, solving means of the above-described problems is, it is not enumerate all features of the present invention. The various features and advantages and effects thereof of the present invention will be understood in more detail with reference to the specific embodiments described below.
[22]
Effects of the Invention
[23]
As one of the many effects of the present invention, high strength steel sheet according to one embodiment of the present invention can be preferably applied to the material, such as drawability and bake hardening is very good to the automobile outer plate.
[24]
Brief Description of the Drawings
[25]
Figure 1 is a photomicrograph showing the observed cross section of 1 / 4t point of steel sheet to the replica by using the TEM.
[26]
Best Mode for Carrying Out the Invention
[27]
The present inventors have combined the addition of singly or a titanium (Ti) and niobium (Nb) of titanium (Ti) results, strong carbonitride forming elements the steel in depth studies to solve the problems of the aforementioned prior art, the carbon (C), nitrogen (N), sulfur (S), but remove the employment elements such as, by Kane properly control the carbides such as those generated as a result of employment element removal location distribution and the proper level remaining the re-dissolve during the annealing employ carbon, drawability It confirmed that the curing and baking to prepare a high strength steel sheet remarkably improved, and accomplished the present invention.
[28]
[29]
Will now be described in detail with respect to one side of drawability and bake curing has excellent high strength steel sheet of the present invention. First, the present invention will be described in detail with respect to the alloy composition of the high strength steel sheet.
[30]
[31]
Carbon (C): 0.0005 ~ 0.003 wt.%
[32]
C will effect a significant effect on the texture of the steel sheet in the cold-forming and annealing processes employed by the interstitial elements. In particular, when the steel increases the amount employed carbon, the growth of crystal grains having an advantageous {111} texture in the drawing is suppressed, the growth of crystal grains with {110} and {100} texture is promoted the drawability of the annealed sheet It is deteriorated. Further, if the content of C over, not only disadvantageous from an economic point of view becomes the amount of Ti required over to precipitate it as a carbide, since the problem of the fine TiC precipitates in large quantity distribution in a steel drastically deteriorate the drawability , the upper limit of the content of C is preferably in the managing 0.003% by weight, and more preferably manages to 0.0028 wt%, it is more desirable than the managing to 0.0025% by weight. On the other hand, it is a problem that the C The content of the low one glass has improved drawability of the content is too low, a case, a sharp decrease baking curability of the annealed sheet, the content of the lower limit of the C is preferably manages to 0.0005 wt% and, and more preferably it manages to 0.0008 wt%, it is more desirable than the managing to 0.0010% by weight.
[33]
[34]
Silicon (Si): 0.5% or less (excluding 0% by weight)
[35]
Si contributes to improvement in strength of a steel sheet by solid solution strengthening. However, if the Si content is excessive, to cause surface defects, so the scale is a problem that the coated surface property deterioration, the present invention manages the upper limit of the Si content to 0.5% by weight.
[36]
[37]
Manganese (Mn): 1.2 wt% or less (excluding 0% by weight)
[38]
Mn serves to not only contribute to improvement in strength by solid solution strengthening element for steel to precipitate S as MnS. However, if the content is excessive, the excess of Mn contained in solid solution so that the problem of deterioration of the drawability, the present invention manages the upper limit of the Mn content to 1.2% by weight.
[39]
[40]
Of (P): 0.005 ~ 0.12% by weight
[41]
P is the most effective element to hire the best and effective, without significantly harming the drawing Province, securing the strength of the steel. To indicate this effect in the present invention manages a preferred less than 0.005% by weight, and more preferably 0.008 to managing weight% or higher, it is more desirable than managing more than 0.01% by weight. However, if the content is excessive, so that the problem that excessive P to precipitate a FeTiP the drawability of disadvantage, the upper limit of the P content is preferable to managing 0.12% by weight.
[42]
[43]
Sulfur (S): 0.008 (excluding 0% by weight)% or less, nitrogen (N): (excluding 0% by weight) 0.005% by weight
[44]
S and N are added there is unavoidably present as an impurity in the steel, in order to secure an excellent welding characteristic is preferably controlled as low as possible for the content. In the present invention, the content of S is less than 0.008, and preferably manages wt%, and the content of N is preferably 0.005% or less is managing.
[45]
[46]
Acid soluble aluminum (Al): 0.1% or less (excluding 0% by weight)
[47]
Acid soluble Al is to precipitate AlN contribute to teaching drawing properties and improved ductility. In the present invention, the lower limit of the acid soluble Al content may be one not particularly limited, and preferably 0.01% by weight, and may be more preferably from 0.02% by weight. However, if the content is excessive, since the problem that the steel sheet and internal defects caused by steel-making operation when Al inclusions hyperplasia occurs, the content of the upper limit of the acid soluble Al is in a preferred, and 0.08 wt% manages to 0.1% by weight managing is more preferable, and more preferably is managing to 0.05% by weight.
[48]
[49]
Titanium (Ti): 0.01 ~ 0.04% by weight
[50]
Ti is an element which contributes to improvement of the drawability of the steel sheet by precipitation of Ti-based carbonitride react with the employment of carbon and nitrogen in the hot rolling employed. To indicate this effect in the present invention is preferably contained more than 0.01% by weight, more preferably contained at least 0.012% by weight, it is more preferable that contain at least 0.015% by weight. On the other hand, if the content is excessive, employment and by carbon and employing nitrogen as the reaction and the rest of Ti is formed excessive FeTiP precipitates in combination with P, and is a fear be the drawability disadvantage, TiC or TiN precipitates a large amount distribution in the steel employed there is a fear that the C content be less curable baking is inferior. The upper limit of the content of Ti is that the 0.04% by weight, and more preferably 0.03% by weight.
[51]
[52]
In addition it comprises the balance Fe and unavoidable impurities. However, in the conventional manufacturing process it is not intended from the raw material or the environment, because impurities may be unavoidably incorporated, it is impossible to exclude them. These impurities do not specifically mentioned in this specification for all that information because it can be seen by anyone by those skilled in the art. Not necessarily to be added to the exclusion of the effective components in addition to the above compositions, it may further comprise the following components shown below in order in particular to improve the mechanical properties of the steel sheet.
[53]
[54]
Niobium (Nb): 0.005 ~ 0.04% by weight
[55]
Nb is hot, by precipitating the rolling of the employment of carbon in the form of C complex carbide (Ti, Nb), it serves to facilitate the formation of the texture of the annealing, and further if the appropriate amount of Nb was added, direction by plastic anisotropy ( 0 °, 45 °, 90 °) to plastic deformation anisotropy (r-value) plane anisotropy as a result the material to the increase in the got the improvement effect of 90 ° compared to 0 °, and 45 ° direction (△ r, planar anisotropy) is Zero (0-level) is reached at a value represented the characteristics of the r-value uniformly distributed in the plate surface it has the advantage that the ear (earing properties) form the molding defect prevention of the molding material. To indicate this effect in the present invention preferably contain at least 0.005% by weight. However, if the content is excessive, the steel employed carbon is mostly precipitated as fine NbC solute C is almost mothayeo be redissolved baking curability deteriorates even after annealing, as well as, the fine (Ti, Nb) C less the amount of the complex carbide precipitation drawing as well as the property deterioration, there is a disadvantage to get the material deterioration due to the recrystallization temperature. The upper limit of the Nb content is 0.04% by weight, and that the preferred, more preferably from 0.03% by weight, and is more preferably not more than 0.025% by weight.
[56]
[57]
Boron (B): 0.002% by weight (excluding 0% by weight)
[58]
B serves to suppress the secondary work embrittlement of the steel P. In the present invention, the lower limit of the B content is one not particularly limited, and preferably may be 0.0005% by weight or more, and may be more preferably not less than 0.0007% by weight. However, if the content is excessive, the ductility of the steel sheet involves decreased, so the upper limit of the content of B is preferably a 0.002% by weight, more preferably from 0.0015% by weight.
[59]
[60]
Hereinafter, the invention will now be described with respect to the precipitate, and the organization of the drawability and bake curing has excellent high strength steel sheet.
[61]
[62]
The high strength steel sheet of this invention is preferably a P defined by equation (1) to less than 80%, more preferably not less than 82%. If the ratio (P) is less than 80%, that is, when a large amount of carbides precipitate on the grain boundaries, it becomes higher significant cracking likely occurs during processing, thereby there is a possibility that the ductility and drawability deteriorates. The higher the proportion (P), so the ductility and drawability of the glass increase, the present invention is not particularly limited to the upper limit of the ratio (P). On the other hand, where carbides means a C composite carbide TiC alone carbide, NbC alone or carbide (Ti, Nb).
[63]
Equation 1
[64]
P (%) = {N in / (N in + N GB )} × 100
[65]
(However, N in is the number of carbides having a size of 20nm or less present in the crystal grains, N gb is being the number of carbides having a size of 20nm or less present in the grain boundaries)
[66]
[67]
According to one embodiment of the invention, the invention is a high strength steel sheet FeTiP precipitate unit area (㎛ 2 and to include more than 0.2 per) preferably 0.1 pieces is more preferred to include less. FeTiP the precipitate has a problem which is mainly precipitated as a needle-like decrease the development of {111} orientation during annealing. The precipitate is FeTiP 0.2 pieces / ㎛ 2 if formed in excess of the, there is a fear that the drawability deteriorates. On the other hand, since the less the number of precipitates per unit area FeTiP glass to improve drawability, in the present invention is not particularly limited in the lower limit of the number FeTiP precipitate.
[68]
[69]
According to one embodiment of this invention, high strength steel sheet is gamma (γ) - preferably has a fiber texture. The array having a predetermined surface orientation and the generated internal to the crystal as the texture (texture), and to one aspect development target (band) is the texture thereof in a predetermined direction is referred to as fiber (Fiber) texture. Texture is gamma side has a close relationship and drawings, (111) of which the texture is formed perpendicular to the rolling plane (γ) - has the higher the surface strength of the fiber texture value known to the drawing workability improvement .
[70]
On the other hand, the inventors have found that the gamma (γ) as described above - in order to form a fiber texture in the plate thickness direction t / 4 from the surface of the steel sheet: at (t thickness of the steel sheet) position (111) <011> - {111} < 112> found that the developmental level is important for the National Guard. More specifically, the plate thickness direction from the surface of the steel sheet t / 4: one in the area of ​​up to (t thickness of the steel sheet), the <110> direction parallel to the rolling direction (α) - fiber texture (RD // <110> ) for gamma (γ) in-fiber texture (ND // <111>) a {111} <011> - {111} <112> average random diffraction intensity ratio of the IDF indicating the degree of development to control the five or more of If, average plastic anisotropy factor (Lankford value, r value), it was confirmed that there can be secured the excellent drawability of more than 2.2. On the other hand, the {111} <011> - {111} The higher average random diffraction intensity of the IDF in the <112> ratio gamma (γ) - it is formed with a fiber texture fine glass gender drawing, the upper limit does not particularly limited. For reference, a random diffraction intensity Sembilan divided by the X-ray intensity material disclosed obtained by measuring under the conditions that do not have an integrated standard sample and the disclosed material, such as an X-ray intensity of a specific orientation by X-ray diffraction by an X-ray intensity of the standard sample is the value.
[71]
[72]
On the other hand, average plastic anisotropy factor (Lankford value, r value) obtained from plastic anisotropy coefficient as measured by direction to the rolling direction as a typical material property value representing the drawability, the value calculated from the following expression (1).
[73]
r-value = (r0 + r90 + 2r45) / 4 (Equation 1)
[74]
(Where, ri represents the plastic anisotropy coefficient measured on specimens collected at the i ° direction from the rolling direction).
[75]
The greater the r value, wherein it is possible to increase the depth of the forming cup during drawing process can be determined that a good drawability. Thin steel sheet according to one embodiment of the present invention exhibit excellent drawability obtain the r-value of 2.2 or more.
[76]
[77]
In addition, obtain the steel sheet is 4MPa or more, more preferably 10MPa or more, more preferably 15MPa or more baking hardening amount than the (BH) according to one embodiment of the present invention, exhibits excellent burn-curable.
[78]
[79]
Drawability and bake curing has excellent high strength steel sheet of the present invention described above can be prepared in a variety of ways, a production method is not particularly limited. However, as one preferred example, it may be prepared by the method described below.
[80]
[81]
Will now be described in detail with respect to the production method of the present invention, another aspect of drawability and bake curing has excellent high strength steel sheet of the.
[82]
[83]
Production process of the other one side surface of high strength steel sheet of the invention, step a steel slab having the above-described alloy composition to obtain a hot rolled steel sheet by hot rolling; Step for winding the hot-rolled steel sheet; To afford a cold rolled steel sheet by cold-rolling the wound hot rolled steel sheet; And a step of continuous annealing the cold-rolled steel sheet; And a step of cooling the annealed the cold rolled steel sheet to 650 ℃; includes.
[84]
[85]
During the winding, the coiling temperature is the winding temperature is preferably in the 450 ~ 750 ℃, more preferably 500 ~ 700 ℃. If, when the coiling temperature of 450 ℃ below, the FeTiP precipitate a large amount precipitates a drawability is degraded, if there are a possibility that the plate-like distortion caused, on the other hand, exceeding 750 ℃, with precipitate coarsening, during annealing employ carbon there is a possibility that the re-dissolving hard baking hardening amount (BH) is deteriorated.
[86]
[87]
When the cold rolling, cold rolling reduction is preferably 75 ~ 85%. If the rolling reduction during the cold rolling is too low, the gamma (γ) - the fiber texture does not sufficiently grow a problem that the drawability of disadvantage. Therefore, the lower limit of the cold rolling reduction during rolling is preferably 75%. On the other hand, if the cold rolling reduction during rolling is too high, there is a problem that the rolling roll when the load is deep shape of the steel sheet poor. Therefore, the upper limit of the cold rolling reduction during rolling is preferably 85%.
[88]
[89]
When the cold rolling, a final roll (Roll) pressure Harvey (final roll (Roll) reduction rate / total rolling reduction) is more preferably from 5 to 15%. If the final roll reduction ratio is too low, and a large amount of carbides precipitate on the grain boundaries, gamma (γ) - the nucleation and development of fiber texture not enough, there is a problem that the drawability deteriorated. Therefore, the lower limit of the final roll pressure Harvey is preferably 5%. On the other hand, when the final roll reduction ratio is too high, there is a problem due to the rolling load increased the possibility determining fracture occurs. Therefore, the upper limit of the final roll pressure Harvey is preferably 15%.
[90]
[91]
Rate of temperature increase up to the time of continuous annealing, the annealing temperature is preferably less than 7 ℃ / sec (excluding 0 ℃ / sec). If, when the temperature elevation rate is more than 7 ℃ / sec {111} because the nucleation and development of the tissue enough there is a fear the drawability becomes inferior.
[92]
[93]
When the continuous annealing, the annealing temperature is preferably 830 ~ 880 ℃, more preferably 850 ~ 870 ℃. If, when the annealing temperature is lower than 830 ℃, favorable gamma (γ) in workability - it is apprehended be fiber texture drawing failed to sufficiently growth potential is inferior, of annealing precipitates do not re-dissolve baking hardening amount (BH) is there is likely to be deteriorated. On the other hand, if the annealing temperature exceeds 880 ℃, workability there may cause problems with the equipment to the heating of glass, one, the steel sheet becomes poor shape because of the grain size variation annealing.
[94]
[95]
The holding time at the time of the continuous annealing, the annealing temperature is 30 ~ 80sec is the preferred, more preferably 40 ~ 70sec. Gamma (γ) - if sufficiently secured after sufficient development of fiber texture, keeping the annealing time, when some carbonaceous there is re-dissolved in solid solution C, the cooling in a state in which such a solid solution C is present, in the final steel sheet with solid solution C is left at an appropriate level exhibit a superior baking hardening amount (BH). If the annealing retention time due to insufficient 30sec remelting of the lack of time that the final steel sheet within the solid solution C does not remain less than, appears to baking hardening amount (BH) The disadvantage, on the other hand, the holding of excessive if it exceeds 80sec time this is because the crystal grains coarsened, and the shape deterioration due to grain deviation occurs, it is due to a too low conveyance speed disadvantageous in an economic point of view.
[96]
[97]
Wherein during cooling, the average cooling rate up to 650 ℃ is preferably in the range of 2 ~ 10 ℃ / sec, more preferably 3 ~ 8 ℃ / sec. If the average cooling rate is 2 ℃ / sec is less than the employed carbon-remelting of annealing the deposited material to a carbonized material, and the possibility that the baking curing decreases, the re-precipitated carbide is diffused to step I, and the possibility that the drawability decreases , there may cause a twisting a plate-like if the average cooling rate exceeds 10 ℃ / sec. On the other hand, as the temperature is 650 ℃ the precipitation and diffusion of the carbide is the most complete, no particular limitation on the cooling conditions later.
[98]
[99]
Conditions are not particularly limited as long as the above-described other conditions, can be carried out in accordance with the production conditions of conventional steel sheet. Additionally, further comprising the step of performing hot-dip galvanizing on the surface of the steel sheet after annealing, or may further comprise the step of the alloying heat treatment after conducting hot-dip galvanizing. As one preferred embodiment, it will be described with respect to the process conditions in each step as follows.
[100]
[101]
Hot finish rolling temperature: Ar3 or more
[102]
If the finish rolling, fundamentally it is advantageous to carried out in the austenite single-phase reverse. This is because it can be added basically uniform deformation than in tissue consisting of a single-phase crystal grains by performing the finish rolling in the austenitic single-phase inverse increase in the uniformity of tissue. In case that a hot finish rolling temperature is lower than Ar3, there are two possibilities sangyeok rolling are concerned the material increases the non-uniformity is caused. For reference, Ar3 can be calculated from the following expression (2).
[103]
Ar3 (℃) = 910 - 310 [C] - 80 [Mn] - 20 [Cu] - 15 [Cr] - 55 [Ni] - 80 [Mo]
[104]
(Where, [C], [Mn], [Cu], [Cr]. [No]. [Mo] Each of the means the weight% of the element)
[105]
[106]
Alloying heat treatment temperature: 450 ~ 600 ℃
[107]
As described above, further comprising the step of performing hot-dip galvanizing on a surface of the after annealed steel sheet, or may further include the step of alloying heat treatment after conducting hot-dip galvanizing, at this time, the alloying heat treatment temperature is 450 ~ 600 ℃ of it is preferred. If alloying is not sufficient if the alloying heat treatment temperature is lower than 450 ℃, is likely to cause a decrease in the degradation or the coating adhesion of the victim way function, on the other hand, if it exceeds 600 ℃, it proceeds to the alloying is excessive powdering drop of It can cause. On the other hand, Fe is the concentration of the alloying heat treatment after the coating layer is preferably 8-12% by weight.
[108]
Mode for the Invention
[109]
It will be described in more detail the present invention through the following examples. However, the following examples are only one example for explaining the present invention in more detail, but the embodiment is not limited to the scope of the present invention.
[110]
[111]
Heating the steel slab having a thickness of 220mm with an alloy composition shown in Table 1 to 1200 ℃, and the hot rolling was wound under the conditions described in, the following Table 2 were prepared by the hot-rolled steel sheet having a thickness of 3.2mm. At this time, the finish rolling temperature was equal to about 930 ℃ immediately above the Ar3. Then, after pickling the hot-rolled steel sheet, to cold rolling, annealing and cooling under the conditions shown in Table 2, the steel sheet was prepared.
[112]
[113]
Then, the precipitates were a number with respect to the steel sheet is manufactured, respectively, and distribution, such as the texture observation and measurement, the results are shown in Table 3 below. More specifically, one defense and FeTiP precipitate the number of carbides, after observing the precipitate in replica by using the TEM, was 5 places count calculated by the average value to precipitate the number per unit length (μm), the texture is of steel plate 1 / the R (Rolling), T (Transverse), the intensity of each orientation with the EBSD based on the ND direction determination bangwido in terms of N (Vertical) ratio (ODF used) at the point 4t were calculated and analyzed.
[114]
[115]
Then, the r value and the amount of bake hardening (BH) with respect to the steel sheet produced each was measured. JIS was collected for the test piece according to No. 5 standard, r value was measured using the ASTM STD specimen, baking hardening quantity is maintained at 170 ℃ the specimen and the yield strength values ​​after a 2% pre-strain again for 20 minutes It was evaluated as a difference between the yield strength values ​​after.
[116]
[117]
TABLE 1
[118]
TABLE 2
[119]
TABLE 3
[120]
As can be seen from Table 1-3, in the case of the present invention, the alloy composition and Examples 1 to 7 satisfying the production conditions to offer to include the carbide having a size of 20nm or less present in the FeTiP deposit number, ferrite crystal grains per unit area and at a ratio of 1/4 thickness position (111) <011> - {111}, and satisfy all the range <112> average random diffraction intensity ratio of the IDF is to control the present invention, basically, the yield strength above 170MPa, more 4MPa the BH property and r value of 2.2 or more can be obtained and understood that.
[121]
[122]
However, in the case of Comparative Examples 1 to 7, the alloy composition is one satisfying the range proposed in the present invention, at least any one of the production conditions do not satisfy the range proposed in the present invention, the drawability and bake hardening showed the disadvantage . In the case of Comparative Examples 8 to 11 do not satisfy the range of the alloy composition proposed by the present invention it showed that the drawability and bake curing inferior.
[123]
[124]
On the other hand, Figure 1 is a photograph showing by using the TEM observation of cross-section of 1 / 4t point of the steel sheet as a replica, and Fig. 1 (a) is a thin steel sheet according to Inventive Example 1, Fig. 1 (b) is a comparative example a steel sheet according to the first.

Claims

[Claim 1]By weight%, C: 0.0005 ~ 0.003% , Si: 0.5% or less (excluding 0%), Mn: 1.2% or less (excluding 0%), P: 0.005 ~ 0.12%, S: 0.008% or less, N: 0.005% or less, acid soluble Al: 0.1% or less (excluding 0%), Ti: 0.01 ~ 0.04%, the balance Fe and below, and including unavoidable impurities, and P is more than 80% as defined by equation 1, 4MPa amount greater than the bake hardening (BH) for having drawability and bake curing has excellent high strength steel sheet. [equation 1] P (%) = {N in / (N in + N gb )} × 100 (However, N in is present in the crystal grain is the number of carbides having a size of 20nm or less, N gb is being the number of carbides having a size of 20nm or less present in the grain boundaries)
[Claim 2]
According to claim 1, in weight%, Nb: drawing further comprises 0.005 ~ 0.04% property and bake curing has excellent high strength steel sheet.
[Claim 3]
According to claim 1, in weight%, B: 0.002% or less (excluding 0%), further comprising the drawability and bake curing it has excellent high strength steel sheet.
[Claim 4]
The method of claim 1, wherein the steel sheet is 0.2 pieces / ㎛ 2 drawing containing the precipitate of less than FeTiP properties and baking curability excellent in high strength steel sheet.
[Claim 5]
According to claim 1, wherein the steel sheet in the thickness direction from the surface of t / 4: one is a <110> direction to the rolling direction in parallel in the area of ​​up to (t thickness of the steel sheet) (α) - to the fiber texture { 111} <011> - {111} <112> average random intensity ratio of 5 or higher drawability and bake curing has excellent high strength steel sheet of the National Guard.
[Claim 6]
The method of claim 1, wherein 2.2 or more r value (Lankford value) having the drawability and bake curing has excellent high strength steel sheet.
[Claim 7]
The method of claim 1, further comprising: drawing a hot-dip galvanizing layer formed on a surface of the steel sheet properties and baking curability excellent in high strength steel sheet.
[Claim 8]
The method of claim 1, further comprising drawing the alloyed hot-dip galvanizing layer formed on a surface of the steel sheet properties and baking curability excellent in high strength steel sheet.
[Claim 9]
By weight%, C: 0.0005 ~ 0.003%, Si: 0.5% or less (excluding 0%), Mn: 1.2% or less (excluding 0%), P: 0.005 ~ 0.12%, S: 0.008% or less, N: 0.005% or less, acid soluble Al: 0.1% or less (excluding 0%), Ti: to afford a 0.01 ~ 0.04%, the balance Fe and unavoidable steel slab to hot-rolled steel sheet containing the impurities by hot rolling; Step for winding the hot-rolled steel sheet at a temperature of 450 ~ 750 ℃; to afford a cold rolled steel sheet by cold rolling the said take-up the hot-rolled steel sheet with pressure Harvey condition of 5 to 15% of a 75-85% reduction ratio, the last calender roll; after the cold-rolled steel sheet was heated at a rate of 7 ℃ / sec or less (excluding 0 ℃ / sec) to 830 ~ 880 ℃, the step of continuous annealing by keeping it for 30 ~ 80sec; And a step of cooling the annealed cold-rolled steel sheet in the 2 ~ 10 ℃ / sec average cooling rate to 650 ℃; The method of drawability and bake curing has excellent high strength steel sheet comprising a.
[Claim 10]
The method of claim 9 wherein said steel slab has a wt%, Nb: 0.005 ~ 0.04% The method of the drawability and bake curing has excellent high strength steel sheet further comprises.
[Claim 11]
10. The method of claim 9, wherein the steel slab B: 0.002% or less The method of drawability and bake curing has excellent high strength steel sheet further comprises a (excluding 0%).
[Claim 12]
10. The method of claim 9, wherein the production of the hot rolling when, the finish rolling temperature is less than Ar3 drawability and bake curing has excellent high strength steel sheet.
[Claim 13]
10. The method of claim 9, wherein after the production of the continuous annealing, drawing and further comprising conducting hot-dip galvanizing on a surface of the steel sheet properties and baking curability excellent in high strength steel sheet.
[Claim 14]
10. The method of claim 9, wherein after the continuous annealing, the hot-dip galvanized steel sheet on the surface of the embodiment; And a method of producing a 450 ~ 600 ℃ alloying heat treatment step the drawing further comprises properties and baking curability superior to high-strength steel sheet at a temperature of.