Art
[1]
The present invention relates to that, more specifically, the high strength excellent moldability which can be preferably applied as a material for automobile outer plate material for a steel sheet and a manufacturing method of the 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 method for manufacturing high strength steel sheet excellent in formability with them.
[14]
Problem solving means
[15]
One aspect of the present invention is, in weight%, C: 0.001 ~ 0.004%, Si: 0.5% or less (excluding 0%), Mn: less than 1.2% (excluding 0%), P: 0.005 ~ 0.12%, S: 0.01 % or less, N: 0.01% or less, acid soluble Al: 0.1% or less (excluding 0%), Ti: 0.01 ~ 0.04%, the balance Fe and including unavoidable impurities, wherein the Ti, the content of N and S is the following relation 1 satisfaction, and the sheet thickness direction by t / 4: from about (t thickness of the steel sheet), the point (001) [1-10] ~ 110 [1-10] mean random intensity ratio (a) of the National Guard ( 111) [1-10] - (111) [- 1-12] the ratio of the average random intensity ratio (b) of the National Guard (b / a) is more than 2.3, the amount of bake hardening (BH) is 4MPa or higher high strength steel sheet It provides.
[16]
[Expression 1] -0.02≤ [Ti] - (24/7) [N] - (3/2) [S] ≤0.025
[17]
(Where, [Ti], [N] and [S], each means a content (% by weight) of the element)
[18]
[19]
Another aspect of the invention is, in weight%, C: 0.001 ~ 0.004%, Si: 0.5% or less (excluding 0%), Mn: less than 1.2% (excluding 0%), P: 0.005 ~ 0.12%, S: 0.01% or less, N: 0.01% or less, acid soluble Al: 0.1% or less (excluding 0%), Ti: 0.01 ~ 0.04%, the balance Fe and unavoidable step the steel slab containing the impurities to obtain a hot rolled steel sheet by hot rolling, to afford a step for winding the hot-rolled steel sheet at a temperature of 450 ~ 750 ℃, cold-rolled steel sheet to the wound hot rolled steel sheet cold-rolling of more than 75% of rolling reduction, the cold-rolled steel sheet by raising the temperature to the annealing temperature of 830 ~ 880 ℃ then, the step of holding at the annealing temperature for the annealing time of 30 ~ 80sec continuous annealing comprising the steps of: cooling the successive annealing, the cold-rolled steel sheet at a rate of 2 ~ 10 ℃ / sec up to a temperature not higher than 650 ℃, the cooled cold-rolled comprising the step of temper rolling the steel sheet with a reduction ratio of 0.3 to 1.6%, and during the temperature increase of the cold rolled steel sheet, (recrystallization starting temperature +20) ℃ from annealing on The average cooling rate to provide a method for manufacturing a high strength steel sheet characterized in that not more than 5 ℃ / sec.
[20]
Effects of the Invention
[21]
As one of the many effects of the present invention, the steel sheet according to the invention may be in strength and excellent formability preferably applied as a material for automobile outer plate for.
[22]
Brief Description of the Drawings
[23]
Figure 1 is a graph that analyzes the developmental level of a set of honors to one organization.
[24]
Best Mode for Carrying Out the Invention
[25]
The inventors have by the addition of depth study results, the addition of the titanium (Ti) strong carbonitride-forming elements of the steel by itself, or titanium (Ti) and niobium (Nb) in order to solve the problems of the aforementioned prior art, a compound carbon (C), nitrogen (N), sulfur (S), but remove the employment elements such as, the strength and the drawability by appropriately controlling the position distribution of the carbides such as those generated as a result of employment element removal, and control the texture It can be remarkably improved, and, leading to the confirmation that the re-dissolution of the carbon employed annealing may remain significantly improve the bake hardenable by Kane appropriate level, and completed the present invention.
[26]
[27]
It will now be described in detail with respect to one aspect of moldability and excellent high strength steel sheet of the present invention.
[28]
[29]
First, a detailed description of the alloy composition and preferred content range of the high strength steel sheet. All the content of each component described below is, unless otherwise stated in advance that puts out a weight basis.
[30]
[31]
C: 0.001~0.004%
[32]
Carbon has a significant influence on the formation of a set of cold-rolled steel sheet structure and in the annealing process as the interstitial element employed. In particular, when the large amount employ carbon in steel, the growth of crystal grains having an advantageous {111} texture on the drawing workability is suppressed, and the {110} and {100} aggregation is promoted the growth of crystal grains with a tissue drawing of a steel sheet Castle this can be degraded. Further, if the carbon content is too excessive, the Ti content is required, as well as excessive becomes disadvantageous in an economic point of view to this, a carbide precipitation, there is a problem with a large amount of fine carbides TiC steel distribution to rapidly degrade the drawability. Therefore, the present invention controls the upper limit of the carbon content to 0.004%, and preferably controlled to 0.0035%. On the other hand, the lower the carbon content, but can be beneficial include improved drawability, it is a problem that the curability of the thin steel sheet is baked rapidly deteriorated if the content is too low. Therefore, the present invention controls the lower limit of the carbon content to 0.001%, and preferably controlled to 0.0012%.
[33]
[34]
Si: 0.5% or less (excluding 0%)
[35]
Silicon contributes to improvement in strength of a steel sheet by solid solution strengthening. However, if the content is too much to cause a surface defect scale, so the problem of deterioration of the plating surface properties, in the present invention, controlling the upper limit to 0.5%, preferably controlled to 0.05%. On the other hand, although in the present invention, not particularly limited to the lower limit of the silicon content, it may be preferably 0.001%, and may be more preferably 0.002%.
[36]
[37]
Mn: 1.2% or less (excluding 0%)
[38]
Mn serves to not only contribute to the improved strength of the steel as a solid solution strengthening element to precipitate S in steel as MnS to suppress the breaking plate and high temperature embrittlement caused by S during hot rolling. However, if the content is over the excess Mn is employed, there is a problem of deteriorating the drawability. In the present invention control the upper limit of the manganese content to less than 1.2% and, preferably is controlled to less than 1.0%, and more preferably controlled to less than 0.8%. On the other hand, in the present invention, though it not particularly limited to the lower limit of the manganese content, and preferably may be about 0.01%, and may be more preferably 0.1%.
[39]
[40]
P: 0.005~0.12%
[41]
Which is the most effective element for improving the strength of the steel book if employment effects are very good, and greatly impair the drawability. And controlling the amount of the lower limit in the present invention, 0.005%, and preferably controlled to 0.008%, and more preferably controlled to 0.010%. However, if the content is excessive, the excess P is precipitated in FeTiP there is a problem that the drawability deteriorates. And controlling the amount of the upper limit in the present invention as 0.12%, and preferably controlled to 0.10%, and more preferably controlled to 0.08%.
[42]
[43]
S: 0.01% or less, N: 0.01% or less
[44]
Sulfur and nitrogen, it is preferable to control the content thereof as low as possible in order to, obtain an excellent welding characteristic as the impurities inevitably present in steel. In the present invention, each administration to less than 0.01%, the upper limit of the content of sulfur and nitrogen in terms of ensuring appropriate welding characteristics.
[45]
[46]
Sol.Al: 0.1% or less (excluding 0%)
[47]
Acid soluble aluminum is to precipitate AlN and contributes to improvement of the ductility and drawability of steel sheet. However, if the content is excessive, the Al-based inclusions formed during steel-making operation is excessive, there is a problem that the steel sheet and internal defects occur. In the present invention control the upper limit of the acid soluble aluminum content of 0.1% and, preferably is controlled to 0.08%, and more preferably controlled to 0.05%. On the other hand, in the present invention, though not particularly limited to the lower limit of the acid soluble aluminum content, it may be preferably 0.01%, and may be more preferably 0.02%.
[48]
[49]
Ti: 0.01 ~ 0.04%
[50]
Titanium is an element which contributes to improvement of the drawability of steel sheet by precipitation of Ti-based carbonitride react with the employment of carbon and nitrogen in the hot rolling employed. In the present invention, it is controlling the lower limit of the titanium content of 0.01% or more, and preferably controlled to 0.012% or more, and is more preferably controlled to 0.015% or more. However, by which the risk that the content is to gyeoung, employment of carbon and employing nitrogen as the reaction and the remaining excess Ti will be coupled to the drawability deteriorated to form a redundant FeTiP precipitate and P, TiC or TiN precipitates a large amount distribution of the steel employed the amount of carbon is too low there is a fear that deterioration of the bake hardenable steel sheet. In the present invention it controls the upper limit of the titanium content of 0.04%, and preferably controlled to 0.03%.
[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. In addition, it is not excluded that the addition of the effective components in addition to the above compositions, particularly may further comprise the following components shown below in order to improve the mechanical properties of the steel sheet.
[53]
[54]
Nb: 0.005~0.04%
[55]
Niobium serves to facilitate the formation of a set of annealing by precipitates in the form of C complex carbide the employment of carbon hot-rolled (Ti, Nb) tissue. Further, when the appropriate amount of Nb to be added, direction by firing anisotropic plastic deformation anisotropy (r-value) of the (0 °, 45 °, 90 °) there is effect is improved, 90 ° direction than 0 ° and 45 ° direction increases and, as a result, the plane anisotropy of the material (Δr, planar anisotropy) is zero (0), and reaches the vicinity of, exhibits a characteristic that the r value is evenly distributed over the plate surface ear at the time of the molding compound (earing properties) shaped molding of there is an advantage that a defect is prevented. In order to obtain such an effect in the present invention, it is preferable to control the lower limit of the niobium content of 0.005% or more, and more preferably is also controlled to 0.008% or more. However, if the content is too excessive, the precipitation of most employ carbon in steel fine NbC employ carbon is almost mothayeo be redissolved baking curability deteriorates even after annealing, and further, the fine (Ti, Nb) C amount composite carbide precipitation as well as the relatively small deterioration of drawability (r-value), there is a problem to get the material deterioration due to the recrystallization temperature. A niobium content of 0.04% and the upper limit is not preferable, more preferably 0.03%, and further more preferably 0.025%.
[56]
[57]
B: 0.002% or less (excluding 0%)
[58]
Boron inhibits the secondary working embrittlement caused by P in steel. However, if the content is over, the bar can be accompanied by a decrease ductility of the steel sheet, the present invention controls the upper limit of the boron content to less than 0.002%, and preferably controlled to less than 0.0015%. On the other hand, in the present invention it may be, but not particularly limited to the lower limit of boron content, preferably 0.0003%, and may be more preferably from 0.0005%.
[59]
[60]
On the other hand, when the content of the alloy design of steel sheet having a composition range as described above, the Ti, N and S are preferably to satisfy the following relation: 1. If, [Ti] - (24/7) [N] - (3/2) [S] and the value is low as -0.02 for the Ti content to precipitate the steel C as TiC, if less than absolute processability evaluation index the value of r this significantly becomes low, while, if the value exceeds 0.025 TiC precipitates in addition to the advantageous FeTiP precipitate formed and the workability is significantly inhibited the development of the {111} orientation during annealing. And more preferably controlled to -0.01 to 0.01 for it.
[61]
[Expression 1] -0.02≤ [Ti] - (24/7) [N] - (3/2) [S] ≤0.025
[62]
(Where, [Ti], [N] and [S], each means a content (% by weight) of the element)
[63]
[64]
Tissue and the like will be described in detail below precipitate, high strength steel sheet.
[65]
[66]
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 . Normal alpha (α) - the fiber texture is defined as RD // <110> gamma (γ) - the fiber texture is defined as the ND // <111>.
[67]
[68]
On the other hand, the inventors have found that the gamma (γ) as described above - fibers set in order to form a tissue with a thickness direction from the surface of the steel sheet t / 4: - fiber texture ((001-alpha (α) from (t thickness of the steel sheet) position ) [1-10] ~ 110 [1-10] Guard average random intensity ratio (a) of a) and gamma (γ) - the fiber texture (111 [1-10] ~ 111 [1 -12] found that the average rate is critical in the random intensity ratio (b) of the National Guard). More specifically, the plate thickness direction from the surface of the steel sheet t / 4 (t: thickness of steel foil) at a point (001) [1-10] ~ 110 [1-10] mean random intensity ratio of the IDF (a 111 on) [1-10] - (111) [- 1-12] average plastic anisotropy factor when the ratio (b / a) of an average random intensity ratio (b) of the National Guard be secured more than 2.3 (Lankford value , r value), it was confirmed that it is possible to achieve good drawability can be obtained by more than 1.9. On the other hand, gamma (γ) - the fiber texture ((111) [1-10] - (111) [- 1-12] Guard) As the average random intensity ratio is higher as the relative glass gender drawing, the upper limit in the present invention It is not particularly limited.
[69]
[70]
In particular, the present invention should ensure good drawability by different direction than the specific direction at the time of molding automotive parts make this an integral part molded without cracking possible and gamma (γ) - analyzing the development of the degree of fiber texture 0 ~ 90 ° all when displaying the value found that express full formability. That is gamma (γ) - 0 ° ((111) [1-10]), 30 ° ((111) [1-21]), 60 ° ((111) [0-11]) of the fiber texture, 90 ° ((111) [- 1-12]) the development of the average random intensity ratio with respect to all directions of the glass is higher in general.
[71]
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).
[72]
r-value = (r0 + r90 + 2r45) / 4 (Equation 1)
[73]
(Where, ri represents the plastic anisotropy coefficient measured on specimens collected at the i ° direction from the rolling direction).
[74]
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 r value of 1.9 or more import.
[75]
[76]
In one example, the average grain size of the high strength steel sheet may be greater than 5μm, preferably may be at least Advantageously, 7μm. Here, it means the average grain size is the average circle equivalent diameter of crystal grains (equivalent circular diameter). It is advantageous in the present invention, to obtain a coarse crystal grains as possible, because the more advantageous in terms of formability dialog grain size tank. For this purpose, the lower the C content from the component control with very low carbon steel level of 40ppm or less by controlling the carbide precipitation as effectively as possible to promote the grain growth during annealing. As the grain size of coarse carbides precipitate easily in the grain boundary than in the grain, because it can significantly reduce the likelihood of cracking during processing. On the other hand, the larger the average grain size advantageous bar in moldability aspect, the present invention, though not particularly limited to the upper limit of the average grain size, as caused by over 860 ℃ high-temperature annealing, for grain growth annealing, there is a risk of my refractory damage considering the aspect there may be limited, the upper limit to 20μm.
[77]
[78]
In one example, high strength steel sheet of the present invention is P, defined by equation 1 below in which the number is more than 80%, preferably be greater than or equal to 82%. The ratio (P in the case) is less than 80%, that is, when a large amount of carbides precipitate at grain boundaries, cracks likely occur significantly during processing it becomes high, whereby there is a possibility that the ductility and drawability deteriorates. The ratio (P in higher) because the glass to improve the ductility and drawability, in the present invention, the ratio (P in is not particularly limited to the upper limit of). On the other hand, where carbides means a C composite carbide TiC alone carbide, NbC alone or carbide (Ti, Nb).
[79]
Equation 1
[80]
P in(%)={N in/(N in+N gb)} × 100
[81]
(However, N in is the number of carbides having a circle-equivalent diameter of 20nm or less present in the crystal grains, N gb is the number of carbides having a circle-equivalent diameter of 20nm or less present in the grain boundaries Im)
[82]
[83]
According to one embodiment, the present invention is a high strength steel sheet FeTiP precipitate unit area (㎛ 2 may comprise more than 0.2 per), may preferably include more than 0.1. FeTiP the precipitate thereby precipitated mainly in the bed decreases 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.
[84]
[85]
In one example, the invention high strength steel sheet is more than 4MPa, take more preferably 10MPa or more, more preferably 15MPa or more than the amount of bake hardening (BH), exhibits excellent burn-curable.
[86]
[87]
According to one embodiment, the present invention has a high strength steel sheet has a thickness of less than 0.8mm, the yield strength (YS, MPa) and obtain the value of multiplication more than 290MPa average plastic anisotropy factor (Lankford value, r-value), the external physical the dent (dent), which means the resistance to strength properties and formability is excellent, it can be preferably applied as a material for automobile outer panel.
[88]
[89]
The 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.
[90]
[91]
It will now be described in detail with respect to the production method of the present invention, another aspect of moldability and is excellent in high strength steel sheet.
[92]
[93]
First, to obtain a hot rolled steel sheet by hot rolling a steel slab having the above-mentioned component.
[94]
In one example, during hot rolling, the finish rolling may be carried out in the austenite single-phase inverse temperature (a temperature above Ar3 ℃). If, there is a fear that if the hot finish rolling temperature is below Ar3 ℃ 2 sangyeok high rolling possible material non-uniformity is to be caused. For reference, Ar3 ℃ can be calculated from the following expression (2).
[95]
[Formula 2]
[96]
Ar3 (℃) = 910 - 310 [C] - 80 [Mn] - 20 [Cu] - 15 [Cr] - 55 [80] - [I]
[97]
(Where, [C], [Mn], [Cu], [Cr]. [No]. [Mo] Each of the means the weight% of the element)
[98]
[99]
Next, the take-up the hot-rolled steel sheet.
[100]
At this time, the take-up temperature is preferably in the 450 ~ 750 ℃, more preferably 500 ~ 700 ℃. If, when the coiling temperature of 450 ℃ is less than FeTiP precipitates are much precipitation and the drawability deterioration, there is a possibility that the plate-like warpage occurs, whereas, if it exceeds 750 ℃, re-dissolution of the employment of carbon of annealing, with precipitate coarsening there is a fear that it is difficult to bake hardening amount (BH) is deteriorated.
[101]
[102]
According to one example, the average cooling rate until coiling temperature from the hot rolling finish temperature may be 10 ~ 200 ℃ / sec. If, when the average cooling rate is 10 ℃ / sec less, when growing the ferrite crystal grains uniform, and it is difficult to moldability ensure that purpose in the present invention is FeTiP precipitate is formed, exceeds the other hand, 200 ℃ / sec, excessive cooling as the temperature of the hot-rolled steel sheet can be the shape of the hot rolled steel sheet becomes nonuniform because of the bad.
[103]
[104]
Next, the cold-rolling the hot-rolled steel sheet rolled to obtain a cold-rolled steel sheet.
[105]
At this time, the cold rolling reduction is preferably not less than 75%. If, when the cold rolling reduction is less than 75% of the gamma (γ) - does not grow sufficiently fiber aggregate structure has a problem that the drawability of disadvantage. On the other hand, the higher the cold-rolling reduction of the gamma (γ) - In the present invention, so the glass to the fiber texture growth, but not particularly limited to the upper limit of the cold rolling reduction, when the cold rolling reduction is too high, the rolling when the roll load simhaejyeo steel when in the shape can become defective bar, take this into account it is possible to limit the upper limit of 85%.
[106]
[107]
Next, the continuous annealing the cold-rolled steel sheet.
[108]
At this time, annealing temperature (T) is preferably 830 ~ 880 ℃, more preferably 840 ~ 870 ℃. If the annealing temperature (T) the workability favorable gamma (γ) to less than 830 ℃ - it is apprehended be the drawability failed to grow enough fiber texture disadvantage, because the precipitate is not re-dissolved during annealing baking hardening amount (BH ) this may possibly be deteriorated. On the other hand, if the workability annealing temperature (T) exceeds 880 ℃ include, but can be beneficial, it becomes a plate-shaped defects due to grain size variation, there is a fear that a problem occurs in the plant to heat annealing.
[109]
[110]
On the other hand, the annealing time (t), that is maintained at the annealing temperature for the time should preferably be 30 ~ 80sec preferable, more preferably 40 ~ 70sec. Gamma (γ) - it was fully developed the fiber aggregate structure, if sufficient annealing time, some carbonaceous there is re-dissolved by employing carbon, when such employment carbon is present as a cooled in a state in which, employing carbon steel sheet It is to remain in the moderate exhibit a superior baking hardening amount (BH). If the annealing time (t) appears as is the case 30sec less than remelting not within the steel sheet due to the lack of time employed enough carbon does not remain baking hardening amount (BH) The disadvantage, on the other hand, the holding of excessive if it exceeds a 80sec time this is due to the coarse grain, it becomes a sheet-like defects in the grain size deviation, which is disadvantageous in terms of economy.
[111]
[112]
In one example, in continuous annealing, the annealing temperature (T, ℃) and annealing time (t, sec) may be satisfied with the following relation 2. If, 0.001 * T * when t value 30 is less than drawability and can bake curing may be deteriorated, while, 0.001 * T * t If the value exceeds 70 plate shape with grain coarsening and grain size variations occur poor It can be made.
[113]
[Expression 2] 30≤0.001 * T * t≤70
[114]
[115]
On the other hand, it is preferable that the average rate of temperature rise is 5 ℃ / sec or less to continuous annealing, recrystallization starting temperature + annealing from 20 ℃ temperature, more preferably 4.5 ℃ / sec or less, more preferably 3.8 ℃ / sec or less. Here, the recrystallization starting temperature is defined as the temperature at which the new recrystallized grains begin to form in the course of the rolling tissue stretched and hold by the cold rolling annealing, specifically, the 50% area fraction of new recrystallized grains of the total grain than This is defined as the temperature of the occupied time. In the early stages of re-crystallization is started there is to be accompanied by nucleation and growth of new grains, is to lower the temperature raising rate at this stage is low increases the nucleation of advantageous {111} texture on workability, resulting in a high r-value It can be secured. If the workability as required in the present invention that the heating rate in the temperature range not sufficient even nucleation of the (111) texture during recrystallization When exceeding 5 ℃ / sec, the fine grain size required by the present invention is not sufficient there is not a concern. On the other hand, in the nucleation and nuclei growth, the present invention because the glass of the temperature range favorable (111) texture in the temperature increase the more the speed is slow working property in not particularly limited to the lower limit value.
[116]
[117]
Next, the continuous annealing the cold-rolled steel sheet is cooled to a temperature not higher than 650 ℃.
[118]
At this time, the average cooling rate is preferably 2 ~ 10 ℃ / sec, and more preferably 3 ~ 8 ℃ / sec. If a twisting a plate-like occur when if the average cooling rate of 2 ℃ / sec is less than the re-dissolution employed carbon is precipitated material with the carbide of the annealing and the possibility that the baking curable degradation, on the other hand, exceeding 10 ℃ / sec there is a concern. On the other hand, as the temperature is 650 ℃ the precipitation and diffusion of the carbide is the most complete, are not particularly limited in terms of the cooling later.
[119]
[120]
Next, the skin-pass rolling the cooled cold-rolled steel sheet to obtain a high strength steel sheet.
[121]
In this case, the temper rolling reduction is preferably 0.3 to 1.6%. Temper rolling increases the yield strength of the river and increasing the endoscopic Hyosung by a large amount of mobile dislocations introduced during rolling, thereby increasing the bake-curing by interaction with the employment of carbon and potential. If the temper rolling reduction of 0.3% is not only disadvantageous to the plate-like control, mobile dislocations are high this can not be secured sufficiently likely to stretch strain fault occurs, whereas, if it exceeds 1.6% customers parts cracking in molding is less than as well as it increases the likelihood of, and displayed a tendency to decrease the formability index of r value.
[122]
[123]
Next, if necessary, subjected to hot-dip galvanizing on a surface of the high strength steel sheet or obtain a hot-dip galvanized steel sheet, it is possible to obtain a hot dip zinc plating after a galvannealed steel sheet to alloying heat treatment. At this time, the alloying heat treatment temperature is preferably 450 ~ 600 ℃. In case that the alloying heat treatment temperature is lower than 450 ℃ has no alloying is insufficient, which can lead to degradation or coating adhesion degradation of the victim way function, on the other hand, if it exceeds 600 ℃ has been conducted 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.
[124]
Mode for the Invention
[125]
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.
[126]
[127]
Table 1 below with the heating of the alloy composition having steel slab (220mm thick) to 1200 ℃, and hot-rolled to prepare a hot-rolled steel sheet (thickness 3.2mm). At this time, the finish rolling temperature was equal to about 930 ℃ immediately above the Ar3. Then, taking up a hot-rolled steel sheet under the conditions of Table 2, the cold rolling, continuous annealing, temper rolling and cooling the steel sheet was prepared.
[128]
[129]
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. Meanwhile, FIG. 1 is a graph that analyzes the developmental level of a set of organizations to honor one showed any tendency to fame are all similar to Example 1.
[130]
[131]
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.
[132]
[133]
TABLE 1
[134]
TABLE 2
[135]
TABLE 3
[136]
[137]
When referring to Table 3, the alloy composition and Examples 1 to satisfying the production conditions to offer to the invention 7 ratio and the average of the carbides having a size of less than 20nm for the case of unit FeTiP precipitate the number per unit area, present in the ferrite grain random intensity ratio (b / a) both satisfies the range in which the control of this invention, by default, r-value is obtained at least 1.9 from, (yield strength * r value) value as well as possible to secure at least 290MPa BH Chengdu 4 MPa of it can be seen that over and secured.
[138]
[139]
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.

Claims
[Claim 1]
By weight%, C: 0.001 ~ 0.004%, Si: 0.5% or less (excluding 0%), Mn: 1.2% or less (excluding 0%), P: 0.005 ~ 0.12%, S: 0.01% or less, N: 0.01% or less, acid soluble Al: 0.1% or less (excluding 0%), Ti: 0.01 ~ 0.04%, the balance Fe and unavoidable comprising impurities, and satisfies the above Ti, N and the content of S is preferably the following relation 1, in the sheet thickness direction by t / 4: in (t thickness of the steel sheet), the point (001) [1-10] ~ 110 [1-10] 111 to the average random intensity ratio (a) of the National Guard [1-10] - (111) [- 1-12] ratio (b / a) is 2.3 or more, the amount of bake hardening (BH) is 4MPa or higher high strength steel sheet having an average random intensity ratio (b) of the National Guard. [Expression 1] -0.02≤ [Ti] - (24/7) [N] - (3/2) [S] ≤0.025 (where, [Ti], [N] and [S], each content of the element means the weight percent)
[Claim 2]
According to claim 1, in weight%, Nb: 0.005 ~ 0.04%, and B: 0.002% or less The high strength steel sheet further comprises at least one selected from the group consisting of (excluding 0%).
[Claim 3]
According to claim 1, wherein the following equation is defined as P 1 in the in the 80% or more The high strength steel sheet. [Equation 1] P in (%) = {N in / (N in + N gb )} × 100 (However, N in is the number of carbides having the equivalent circle of 20nm or less present in the crystal grain diameter, N gb Im is the number of carbides having a circle-equivalent diameter of 20nm or less present in the grain boundaries)
[Claim 4]
The method of claim 1 wherein one 0.2 / μm 2 The high strength steel sheet containing FeTiP precipitate below.
[Claim 5]
The method of claim 1, wherein the yield strength (Yield Strength, YS) and the average plastic anisotropy factor (Lankford value, r-value) multiplication 270MPa or more of the high strength steel sheet.
[Claim 6]
By weight%, C: 0.001 ~ 0.004%, Si: 0.5% or less (excluding 0%), Mn: 1.2% or less (excluding 0%), P: 0.005 ~ 0.12%, S: 0.01% or less, N: 0.01% 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 obtain a cold-rolled steel sheet to the take-up the hot-rolled steel sheet cold-rolled to a reduction ratio of 75% or more steps; After the cold-rolled steel sheet annealing temperature was raised to a temperature of 830 ~ 880 ℃, the step of continuous annealing and maintained at the annealing temperature for the annealing time of 30 ~ 80sec; Step of cooling the continuous annealing the cold-rolled steel sheet at a rate of 2 ~ 10 ℃ / sec up to a temperature not higher than 650 ℃; Wherein the cooled cold-rolled steel sheet to temper rolling step with a reduction ratio of 0.3 to 1.6%; upon including, the temperature increase of the cold rolled steel sheet, the average cooling rate of up (recrystallization starting temperature + 20) from the annealing temperature ℃ 5 ℃ / method for manufacturing a high strength steel sheet characterized in that not more than sec.
[Claim 7]
7. The method of claim 6, in weight%, Nb: 0.005 ~ 0.04%, and B: 0.002% or less of the method for manufacturing high strength steel sheet further comprises at least one member selected from the group consisting of (excluding 0%).
[Claim 8]
7. The method of claim 6, wherein the production of the hot rolling when, the finish rolling temperature is high strength steel sheet more than Ar3 ℃.
[Claim 9]
The method of claim 8 wherein the method of manufacturing an average cooling rate of 10 to high strength, characterized in that 200 ℃ / sec steel from the finishing rolling temperature to the coiling temperature.
[Claim 10]
7. The method of claim 6 wherein the continuous annealing when annealing temperature (T, ℃) and annealing time (t, sec) is a method for manufacturing a high strength steel sheet characterized in that it satisfies the following relation 2. [Expression 2] 30≤0.001 * T * t≤60
[Claim 11]
The method of claim 6 wherein the method for manufacturing a high strength steel sheet further comprises the step of hot-dip zinc plating the surface of the skin-pass-rolling the cold-rolled steel sheet.
[Claim 12]
12. The process of claim 11, wherein the production of the hot-dip galvanizing after the high strength steel sheet further comprises the step of alloying heat treatment at 450 ~ 600 ℃.