EP1888799B1 - Cold rolled steel sheet having superior formability, process for producing the same - Google Patents

Description

[Technical Field]
• The present invention relates to titanium (Ti) based interstitial free (IF) cold rolled steel sheets that are used as materials for automobiles, household electronic appliances, etc. More specifically, the present invention relates to highly formable Ti based IF cold rolled steel sheets whose yield strength is enhanced due to the distribution of fine precipitates, and a process for producing the Ti-based IF cold rolled steel sheets.
[Background Art]
• In general, cold rolled steel sheets for use in automobiles and household electronic appliances are required to have excellent room-temperature aging resistance and bake hardenability, together with high strength and superior formability.
• Aging is a strain aging phenomenon that arises from hardening caused by dissolved elements, such as C and N, fixed to dislocations. Since aging causes defect, called "stretcher strain", it is important to secure excellent room-temperature aging resistance.
• Bake hardenability means increase in strength due to the presence of dissolved carbon after press formation, followed by painting and drying, by leaving a slight small amount of carbon in a solid solution state. Steel sheets with excellent bake hardenability can overcome the difficulties of press formability resulting from high strength.
• Room-temperature aging resistance and bake hardenability can be imparted to aluminum (Al)-killed steels by batch annealing of the Al-killed steels. However, extended time of the batch annealing causes low productivity of the Al-killed steels and severe variation in steel materials at different sites. In addition, Al-killed steels have a bake hardening (BH) value (a difference in yield strength before and after painting) of 10-20 MPa, which demonstrates that an increase in yield strength is low.
• Under such circumstances, interstitial free (IF) steels with excellent room-temperature aging resistance and bake hardenability have been developed by adding carbide and nitride-forming elements, such as Ti and Nb, followed by continuous annealing.
• EP-A-1136575 discloses a method of producing cold-rolled IF steel sheet having good press forming and deep forming properties.
• For example, Japanese Unexamined Patent Publication No. Sho 57-041349 describes an enhancement in the strength of a Ti-based IF steel by adding 0.4-0.8% of manganese (Mn) and 0.04-0.12% of phosphorus (P). In very low carbon IF steels, however, P causes the problem of secondary working embrittlement due to segregation in grain boundaries.
• Japanese Unexamined Patent Publication No. Hei 5-078784 describes an enhancement in strength by the addition of Mn as a solid solution strengthening element in an amount exceeding 0.9% and not exceeding 3.0%.
• Korean Patent Laid-open No. 2003-0052248 describes an improvement in secondary working embrittlement resistance as well as strength and workability by the addition of 0.5-2.0% of Mn instead of P, together with aluminum (Al) and boron (B).
• Japanese Unexamined Patent Publication No. Hei 10-158783 describes an enhancement in strength by reducing the content of P and using Mn and Si as solid solution strengthening elements. According to this publication, Mn is used in an amount of up to 0.5%, Al as a deoxidizing agent is used in an amount of 0.1%, and nitrogen (N) as an impurity is limited to 0.01% or less. If the Mn content is increased, the plating characteristics are worsened.
• Japanese Unexamined Patent Publication No. Hei 6-057336 discloses an enhancement in the strength of an IF steel by adding 0.5-2.5% of copper (Cu) to form ε-Cu precipitates. High strength of the IF steel is achieved due to the presence of the ε-Cu precipitates, but the workability of the IF steel is worsened.
• Japanese Unexamined Patent Publication Nos. Hei 9-227951 and Hei 10-265900 suggest technologies associated with improvement in workability or surface defects due to carbides by the use of Cu as a nucleus for precipitation of the carbides. According to the former publication, 0.005-0.1% of Cu is added to precipitate CuS during temper rolling of an IF steel, and the CuS precipitates are used as nuclei to form Cu-Ti-C-S precipitates during hot rolling. In addition, the former publication states that the number of nuclei forming a {111} plane parallel to the surface of a plate increases in the vicinity of the Cu-TiC-S precipitates during recrystallization, which contributes to an improvement in workability. According to the latter publication, 0.01-0.05% of Cu is added to an IF steel to obtain CuS precipitates and then the CuS precipitates are used as nuclei for precipitation of carbides to reduce the amount of dissolved carbon (C), leading to an improvement in surface defects. According to the prior art, since coarse CuS precipitates are used during production of cold rolled steel sheets, carbides remain in the final products. Further, since emulsion-forming elements, such as Ti and Zr, are added in an amount greater than the amount of sulfur (S) in an atomic weight ratio, a main portion of the sulfur (S) reacts with Ti or Zr rather than Cu.
• On the other hand, Japanese Unexamined Patent Publication Nos. Hei 6-240365 and Hei 7-216340 describe the addition of a combination of Cu and P to improve the corrosion resistance of baking hardening type IF steels. According to these publications, Cu is added in an amount of 0.05-1.0% to ensure improved corrosion resistance. However, in actuality, Cu is added in an excessively large amount of 0.2% or more.
• Japanese Unexamined Patent Publication Nos. Hei 10-280048 and Hei 10-287954 suggest the dissolution of carbosulfide (Ti-C-S based) in a carbide at the time of reheating and annealing to obtain a solid solution in crystal grain boundaries, thereby achieving a bake hardening (BH) value (a difference in yield strength before and after baking) of 30 MPa or more.
• According to the aforementioned publications, strength is enhanced by strengthening solid solution or using ε-Cu precipitates. Cu is used to form ε-Cu precipitates and improve corrosion resistance. In addition, Cu is used as a nucleus for precipitation of carbides. No mention is made in these publications about an increase in high yield ratio (i.e. yield strength/tensile strength) and a reduction in in-plane anisotropy index. If the tensile strength-to-yield strength ratio (i.e. yield ratio) of an IF steel sheet is high, the thickness of the IF steel sheet can be reduced, which is effective in weight reduction. In addition, if the in-plane anisotropy index of an IF steel sheet is low, fewer wrinkles and ears occur during processing and after processing, respectively.
[Disclosure] [Technical Problem]
• It is one object of certain embodiments of the present invention to provide Ti based IF cold rolled steel sheets that are capable of achieving a high yield ratio and a low in-plane anisotropy index.
• It is another object of certain embodiments of the present invention to provide a process for producing the IF cold rolled steel sheets.
[Technical Solution]
• According to the present invention, there is provided a cold rolled steel sheet which has a composition comprising 0.01% or less of C, 0.01-0.2% of Cu, 0.005-0.08% of S, 0.1% or less of Al, 0.02% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, by weight, and the balance of Fe and other unavoidable impurities, wherein the composition satisfies the following relationships: 1 ≤ (Cu/63.5)/(S*/32) ≤ 30 and S* = S - 0.8 x (Ti - 0.8 x (48/14) x N) x (32/48) and the steel sheet comprises CuS precipitates having an average size of 0.2 µm or less.
• According to the present invention, there is provided a cold rolled steel sheet which has a composition comprising 0.01% or less of C, 0.01-0.2% of Cu, 0.01-0.3% of Mn, 0.005-0.08% of S, 0.1% or less of Al, 0.02% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, by weight, and the balance of Fe and other unavoidable impurities, wherein the composition satisfies the following relationships: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32) ≤ 30 and S* = S - 0.8 x (Ti - 0.8 x (48/14) x N) x (32/48), and the steel sheet comprises (Mn,Cu)S precipitates having an average size of 0.2 µm or less.
• Preferred embodiments are given in the dependent claims.
• When the cold rolled steel sheets of the present invention satisfy the following relationships between the C, Ti, N and S contents: 0.8 ≤ (Ti*/48)/(C/12) ≤ 5.0 and Ti* = Ti - 0.8 x ((48/14) x N + (48/32) x S), they show room-temperature non-aging properties. In addition, when solute carbon (Cs) [Cs = (C - Ti* x 12/48) x 10000 in which Ti* = Ti - 0.8 x ((48/14) x N + (48/32) x S), provided that when Ti* is less than 0, Ti* is defined as 0], which is determined by the C and Ti contents, is from 5 to 30, the cold rolled steel sheets of the present invention show bake hardenability.
• Depending on the design of the compositions, the cold rolled steel sheets of the present invention have characteristics of soft cold rolled steel sheets of the order of 280 MPa and high-strength cold rolled steel sheets of the order of 340 MPa or more.
• When the content of P in the compositions of the present invention is 0.015% or less, soft cold rolled steel sheets of the order of 280 MPa are produced. When the soft cold rolled steel sheets further contain at least one solid solution strengthening element selected from Si and Cr, or the P content is in the range of 0.015-0.2%, a high strength of 340 MPa or more is attained. The P content in the high-strength steels containing P alone is preferably in the range of 0.03% to 0.2%. The Si content in the high-strength steels is preferably in the range of 0.1 to 0.8%. The Cr content in the high-strength steels is preferably in the range of 0.2 to 1.2. In the case where the cold rolled steel sheets of the present invention contain at least one element selected from Si and Cr, the P content may be freely designed in an amount of 0.2% or less.
• For better workability, the cold rolled steel sheets of the present invention may further contain 0.01-0.2 wt% of Mo.
• According to the present invention, there is provided a process for producing the cold rolled steel sheets, the process comprising reheating a slab satisfying one of the compositions to a temperature of 1,100°C or higher, hot rolling the reheated slab at a finish rolling temperature of the Ar₃ transformation point or higher to provide a hot rolled steel sheet, cooling the hot rolled steel sheet at a rate of 300 °C/min., winding the cooled steel sheet at 700°C or lower, cold rolling the wound steel sheet, and continuously annealing the cold rolled steel sheet.
[Best Mode]
• The present invention will be described in detail below.
• Fine precipitates having a size of 0.2 µm or less are distributed in the cold rolled steel sheets of the present invention. Examples of such precipitates include MnS precipitates, CuS precipitates, and composite precipitates of MnS and CuS. These precipitates are referred to simply as "(Mn,Cu)S".
• The present inventors have found that when fine precipitates are distributed in Ti-based IF steels, the yield strength of the IF steels is enhanced and the in-plane anisotropy index of the IF steels is lowered, thus leading to an improvement in workability. The present invention has been achieved based on this finding. The precipitates used in the present invention have drawn little attention in conventional IF steels. Particularly, the precipitates have not been actively used from the viewpoint of yield strength and in-plane anisotropy index.
• Regulation of the components in the Ti-based IF steels is required to obtain (Mn,Cu)S precipitates and/or AlN precipitates. If the IF steels contain Ti, Zr and other elements, S and N preferentially react with Ti and Zr. Since the cold rolled steel sheets of the present invention are Ti added IF steels, Ti reacts with C, N and S. Accordingly, it is necessary to regulate the components so that S and N are precipitated into (Mn,Cu)S and AlN forms, respectively.
• The fine precipitates thus obtained allow the formation of minute crystal grains. Minuteness in the size of crystal grains relatively increases the proportion of crystal grain boundaries. Accordingly, the dissolved carbon is present in a larger amount in the crystal grain boundaries than within the crystal grains, thus achieving excellent room-temperature non-aging properties. Since the dissolved carbon present within the crystal grains can more freely migrate, it binds to movable dislocations, thus affecting the room-temperature aging properties. In contrast, the dissolved carbon segregated in stable positions, such as in the crystal grain boundaries and in the vicinity of the precipitates, is activated at a high temperature, for example, a temperature for painting/baking treatment, thus affecting the bake hardenability.
• The fine precipitates distributed in the steel sheets of the present invention have a positive influence on the increase of yield strength arising from precipitation enhancement, improvement in strength-ductility balance, in-plane anisotropy index, and plasticity anisotropy. To this end, the fine (Mn,Cu)S precipitates and AlN precipitates must be uniformly distributed. According to the cold rolled steel sheets of the present invention, contents of components affecting the precipitation, composition between the components, production conditions, and particularly cooling rate after hot rolling, have a great influence on the distribution of the fine precipitates.
• The constituent components of the cold rolled steel sheets according to the present invention will be explained.
• The content of carbon (C) is limited to 0.01% or less.
• Carbon (C) affects the room-temperature aging resistance and bake hardenability of the cold rolled steel sheets. When the carbon content exceeds 0.01%, the addition of the expensive agents Ti is required to remove the remaining carbon, which is economically disadvantageous and is undesirable in terms of formability. When it is intended to achieve room-temperature aging resistance only, it is preferred to maintain the carbon content at a low level, which enables the reduction of the amount of the expensive agents Ti added. When it is intended to ensure desired bake hardenability, the carbon is preferably added in an amount of 0.001% or more, and more preferably 0.005% to 0.01%. When the carbon content is less than 0.005%, room-temperature aging resistance can be ensured without increasing the amounts of Ti.
• The content of copper (Cu) is in the range of 0.01-0.2%.
• Copper serves to form fine CuS precipitates, which make the crystal grains fine. Copper lowers the in-plane anisotropy index of the cold rolled steel sheets and enhances the yield strength of the cold rolled steel sheets by precipitation promotion. In order to form fine precipitates, the Cu content must be 0.01% or more. When the Cu content is more than 0.2%, coarse precipitates are obtained. The Cu content is more preferably in the range of 0.03 to 0.2%.
• The content of manganese (Mn) is preferably in the range of 0.01-0.3%.
• Manganese serves to precipitate sulfur in a solid solution state in the steels as MnS precipitates, thereby preventing occurrence of hot shortness caused by the dissolved sulfur, or is known as a solid solution strengthening element. From such a technical standpoint, manganese is generally added in a large amount. The present inventors have found that when the manganese content is reduced and the sulfur content is optimized, very fine MnS precipitates are obtained. Based on this finding, the manganese content is limited to 0.3% or less. In order to ensure this characteristic, the manganese content must be 0.01% or more. When the manganese content is less than 0.01%, i.e. the sulfur content remaining in a solid solution state is high, hot shortness may occur. When the manganese content is greater than 0.3%, coarse MnS precipitates are formed, thus making it difficult to achieve desired strength. A more preferable Mn content is within the range of 0.01 to 0.12%.
• The content of sulfur (S) is limited to 0.08% or less.
• Sulfur (S) reacts with Cu and/or Mn to form CuS and MnS precipitates, respectively. When the sulfur content is greater than 0.08%, the proportion of dissolved sulfur is increased. This increase of dissolved sulfur greatly deteriorates the ductility and formability of the steel sheets and increases the risk of hot shortness. In order to obtain as many CuS and/or MnS precipitates as possible, a sulfur content of 0.005% or more is preferred.
• The content of aluminum (Al) is limited to 0.1% or less.
• Aluminum reacts with nitrogen (N) to form fine AlN precipitates, thereby completely preventing aging by dissolved nitrogen. When the nitrogen content is 0.004% or more, AlN precipitates are sufficiently formed. The distribution of the fine AlN precipitates in the steel sheets allows the formation of minute crystal grains and enhances the yield strength of the steel sheets by precipitation enhancement. A more preferable Al content is in the range of 0.01 to 0.1%.
• The content of nitrogen (N) is limited to 0.02% or less.
• When it is intended to use AlN precipitates, nitrogen is added in an amount of up to 0.02%. Otherwise, the nitrogen content is controlled to 0.004% or less. When the nitrogen content is less than 0.004%, the number of the AlN precipitates is small, and therefore, the minuteness effects of crystal grains and the precipitation enhancement effects are negligible. In contrast, when the nitrogen content is greater than 0.02%, it is difficult to guarantee aging properties by use of dissolved nitrogen.
• The content of phosphorus (P) is limited to 0.2% or less.
• Phosphorus is an element that has excellent solid solution strengthening effects while allowing a slight reduction in r-value. Phosphorus guarantees high strength of the steel sheets of the present invention in which the precipitates are controlled. It is desirable that the phosphorus content in steels requiring a strength of the order of 280 MPa be defined to 0.015% or less. It is desirable that the phosphorus content in high-strength steels of the order of 340 MPa be limited to a range exceeding 0.015% and not exceeding 0.2%. A phosphorus content exceeding 0.2% can lead to a reduction in ductility of the steel sheets. Accordingly, the phosphorus content is limited to a maximum of 0.2%. When Si and Cr are added in the present invention, the phosphorus content can be appropriately controlled to be 0.2% or less to achieve the desired strength.
• The content of boron (B) is preferably in the range of 0.0001 to 0.002%.
• Boron is added to prevent occurrence of secondary working embrittlement. To this end, a preferable boron content is 0.0001% or more. When the boron content exceeds 0.002%, the deep drawability of the steel sheets may be markedly deteriorated.
• The content of titanium (Ti) is in the range of 0.005 to 0.15%.
• Titanium is added for the purpose of ensuring the non-aging properties and improving the formability of the steel sheets. Ti, which is a potent carbide-forming element, is added to steels to form TiC precipitates in the steels. The TiC precipitates allow the precipitation of dissolved carbon to ensure non-aging properties. When the content of Ti added is less than 0.005%, the TiC precipitates are obtained in very small amounts. Accordingly, the steel sheets are not well textured and thus there is little improvement in the deep drawability of the steel sheets. In contrast, when the titanium is added in an amount exceeding 0.15%, very large TiC precipitates are formed. Accordingly, minuteness effects of crystal grains are reduced, resulting in high in-plane anisotropy index, reduction of yield strength and marked worsening of plating characteristics.
• To obtain (Mn,Cu)S and AlN precipitates, the Mn, Cu, S, Ti, Al, N and C contents are adjusted within the ranges defined by the following relationships. The respective components indicated in the following relationships are expressed as percentages by weight. $$\mathrm{1}\le \left(\mathrm{Cu}/\mathrm{63.5}\right)/\left(\mathrm{S}*/\mathrm{32}\right)\le \mathrm{30}$$

  

  $$\mathrm{S}*=\mathrm{S}-\mathrm{0.8}\times \left(\mathrm{Ti}-\mathrm{0.8}\times \left(\mathrm{48}/\mathrm{14}\right)\times \mathrm{N}\right)\times \left(\mathrm{32}/\mathrm{48}\right)$$

  
• In Relationship 1, S*, which is determined by Relationship 2, represents the content of sulfur that does not react with Ti and thereafter reacts with Cu. To obtain fine CuS precipitates, it is required that the value of (Cu/63.5)/(S*/32) be equal to or greater than 1. If the value of (Cu/63.5)/(S*/32) is greater than 30, coarse CuS precipitates are distributed, which is undesirable. To stably obtain CuS precipitates having a size of 0.2 µm or less, the value of (Cu/63.5)/(S*/32) is preferably in the range of 1 to 20, more preferably 1 to 9, and most preferably 1 to 6. $$\mathrm{1}\le \left(\mathrm{Mn}/\mathrm{55}+\mathrm{Cu}/\mathrm{63.5}\right)/\left(\mathrm{S}*/\mathrm{30}\right)\le \mathrm{30}$$

  
• Relationship 3 is associated with the formation of (Mn,Cu)S precipitates, and is obtained by adding a Mn, content to Relationship 1. To obtain effective (Mn,Cu)S precipitates, the value of (Mn/55 + Cu/63.5)/(S*/32) must be 1 or greater. When the value of Relationship 3 is greater than 30, coarse (Mn,Cu)S precipitates are obtained. To stably obtain (Mn,Cu)S precipitates having a size of 0.2 µm or less, a more preferable value of (Cu/63.5)/(S*/32) is preferably in the range of 1 to 20, more preferably 1 to 9, and most preferably 1 to 6. When Mn and Cu are added together, the sum of Mn and Cu is more preferably 0.05-0.4%. The reason for this limitation to the sum of Mn and Cu is to obtain fine (Mn,Cu)S precipitates. $$\mathrm{1}\le \left(\mathrm{Al}/\mathrm{27}\right)/\left({\mathrm{N}}^{*}/\mathrm{14}\right)\le \mathrm{10}$$

  

  $${\mathrm{N}}^{*}=\mathrm{N}-\mathrm{0.8}\times \left(\mathrm{Ti}-\mathrm{0.8}\times \left(\mathrm{48}/\mathrm{32}\right)\times \mathrm{S}\right)\times \left(\mathrm{14}/\mathrm{48}\right)$$

  
• Relationship 4 is associated with the formation of fine (Mn,Cu)S precipitates. In Relationship 4, N*, which is determined by Relationship 5, represents the content of nitrogen that does not react with Ti and thereafter reacts with Al. To obtain fine AlN precipitates, it is required that the value of (Al/27)/(N*/14) be in the range of 1-10. To obtain effective AlN precipitates, the value of (Al/27)/(N*/14) must be 1 or greater. If the value of (Al/27)/(N*/14) is greater than 10, coarse AlN precipitates are obtained and thus poor workability and low yield strength are caused. It is preferred that the value of (Al/27)/(N*/14) be in the range of 1 to 6.
• The components of the cold rolled steel sheets according to the present invention may be combined in various ways according to the kind of precipitates to be obtained. For example, the present invention provides a cold rolled steel sheet which has a composition comprising 0-01% or less of C, 0.08% or less of S, 0.1% or less of Al, 0.02% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, at least one kind selected from 0.01-0.2% of Cu, 0.01-0.3% of Mn , by weight, and the balance of Fe and other unavoidable impurities wherein the composition satisfies the following relationships: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32) ≤ 30, 1 ≤ (Al/27)/(N*/14) ≤ 10, S* = S - 0.8 x (Ti - 0.8 x (48/14) x N) x (32/48) and N* = N - 0.8 x (Ti - 0.8 x (48/32) x S)) x (14/48), and the steel sheet comprises at least one kind selected from MnS, CuS, MnS and AlN precipitates having an average size of 0.2 µm or less. That is, one or more kinds selected from the group consisting of 0.01-0.2% of Cu, 0.01-0.3% of Mn and 0-0.2% of N lead to various combinations of (Mn,Cu)S and AlN precipitates having a size not greater than 0.2 µm.
• In the steel sheets of the present invention, carbon is precipitated into NbC and TiC forms. Accordingly, the room-temperature aging resistance and bake hardenability of the steel sheets are affected depending on the conditions of dissolved carbon under which NbC and TiC precipitates are not obtained. Taking into account these requirements, it is most preferred that the Ti and C contents satisfy the following relationships. $$\mathrm{0.8}\le \left({\mathrm{Ti}}^{*}/\mathrm{48}\right)/\left(\mathrm{C}/\mathrm{12}\right)\le \mathrm{5.0}$$

  

  $${\mathrm{Ti}}^{*}=\mathrm{Ti}-\mathrm{0.8}\times \left(\left(\mathrm{48}/\mathrm{14}\right)\times \mathrm{N}+\left(\mathrm{48}/\mathrm{32}\right)\times \mathrm{S}\right)$$

  
• Relationship 6 is associated with the formation of TiC precipitates to remove the carbon in a solid solution state, thereby achieving room-temperature non-aging properties. In Relationship 6, Ti*, which is determined by Relationship 7, represents the content of titanium that reacts with N and S and thereafter reacts with C.
• When the value of (Ti*/48)/(C/12) is less than 0.8, it is difficult to ensure room-temperature non-aging properties. In contrast, when the value of (Ti*/48)/(C/12) is greater than 5, the amounts of Ti remaining in a solid solution state in the steels are large, which deteriorates the ductility of the steels. When it is intended to achieve room-temperature non-aging properties without securing bake hardenability, it is preferred to limit the carbon content to 0.005% or less. Although the carbon content is more than 0.005%, room-temperature non-aging properties can be achieved when Relationship 6 is satisfied but the amounts of TiC precipitates are increased, thus deteriorating the workability of the steel sheets. $$\mathrm{Cs}=\left(\mathrm{C}-{\mathrm{Ti}}^{*}\times \mathrm{12}/\mathrm{48}\right)\times \mathrm{10000}$$

  

  (provided that when Ti* is less than 0, Ti* is defined as 0.)
• Relationship 8 is associated with the achievement of bake hardenability. Cs, which is expressed in ppm by Relationship 8, represents the content of dissolved carbon that is not precipitated into TiC forms. In order to achieve a high bake hardening value, the Cs value must be 5 ppm or more. If the Cs value exceeds 30 ppm, the content of dissolved carbon is increased, making it difficult to attain room-temperature non-aging properties.
• It is advantageous that the fine precipitates are uniformly distributed in the compositions of the present invention. It is preferable that the precipitates have an average size of 0.2 µm or less. According to a study conducted by the present inventors, when the precipitates have an average size greater than 0.2 µm, the steel sheets have poor strength and low in-plane anisotropy index. Further, large amounts of precipitates having a size of 0.2 µm or less are distributed in the compositions of the present invention. While the number of the distributed precipitates is not particularly limited, it is more advantageous with higher number of the precipitates. The number of the distributed precipitates is preferably 1 x 10⁵/mm² or more, more preferably 1 x 10⁶/mm² or more, and most preferably 1 x 10⁷/mm² or more. The plasticity-anisotropy index is increased and the in-plane anisotropy index is lowered with increasing number of the precipitates, and as a result, the workability is greatly improved. It is commonly known that there is a limitation in increasing the workability because the in-plane anisotropy index is increased with increasing plasticity-anisotropy index. It is worth noting that as the number of the precipitates distributed in the steel sheets of the present invention increases, the plasticity-anisotropy index of the steel sheets is increased and the in-plane anisotropy index of the steel sheets is lowered. The steel sheets of the present invention in which the fine precipitates are formed satisfy a yield ratio (yield strength/tensile strength) of 0.58 or higher.
• When the steel sheets of the present invention are applied to high-strength steel sheets, they may further contain at least one solid solution strengthening element selected from P, Si and Cr. The addition effects of P have been previously described, and thus their explanation is omitted.
• The content of silicon (Si) is preferably in the range of 0.1 to 0.8%.
• Si is an element that has solid solution strengthening effects and shows a slight reduction in elongation. Si guarantees high strength of the steel sheets of the present invention in which the precipitates are controlled. Only when the Si content is 0.1% or more, high strength can be ensured. However, when the Si content is more than 0.8%, the ductility of the steel sheets is deteriorated.
• The content of chromium (Cr) is preferably in the range of 0.2 to 1.2%.
• Cr is an element that has solid solution strengthening effects, lowers the secondary working embrittlement temperature, and lowers the aging index due to the formation of Cr carbides. Cr guarantees high strength of the steel sheets of the present invention in which the precipitates are controlled and serves to lower the in-plane anisotropy index of the steel sheets. Only when the Cr content is 0.2% or more, high strength can be ensured. However, when the Cr content exceeds 1.2%, the ductility of the steel sheets is deteriorated.
• The cold rolled steel sheets of the present invention may further contain molybdenum (Mo).
• The content of molybdenum (Mo) in the cold rolled steel sheets of the present invention is preferably in the range of 0.01 to 0.2%.
• Mo is added as an element that increases the plasticity-anisotropy index of the steel sheets. Only when the molybdenum content is not lower than 0.01%, the plasticity-anisotropy index of the steel sheets is increased. However, when the molybdenum content exceeds 0.2%, the plasticity-anisotropy index is not further increased and there is a danger of hot shortness.
Production of cold rolled steel sheets
• Hereinafter, a process for producing the cold rolled steel sheets of the present invention will be explained with reference to the preferred embodiments that follow. Various modifications of the embodiments of the present invention can be made, and such modifications are within the scope of the present invention.
• The process of the present invention is given in claims 23 to 44 and characterized in that a steel satisfying one of the steel compositions defined above is processed through hot rolling and cold rolling to form precipitates having an average size of 0.2 µm or less in a cold rolled sheet. The average size of the precipitates in the cold rolled plate is affected by the design of the steel composition and the processing conditions, such as reheating temperature and winding temperature. Particularly, cooling rate after hot rolling has a direct influence on the average size of the precipitates.
Hot rolling conditions
• In the present invention, a steel satisfying one of the compositions defined above is reheated, and is then subjected to hot rolling. The reheating temperature is preferably 1,100°C or higher. When the steel is reheated to a temperature .lower than 1,100°C, coarse precipitates formed during continuous casting are not completely dissolved and remain. The coarse precipitates still remain even after hot rolling.
• The hot rolling is performed at a rate of 300
  a finish rolling temperature not lower than the Ar₃ transformation point. When the finish rolling temperature is lower than the Ar₃ transformation point, rolled grains are created, which deteriorates the workability and causes poor strength.
• The cooling is performed at a rate of 300 °C/min or higher before winding and after hot rolling. Although the composition of the components is controlled to obtain fine precipitates, the precipitates may have an average size greater than 0.2 µm at a cooling rate of less than 300 °C/min. That is, as the cooling rate is increased, many nuclei are created and thus the size of the precipitates becomes finer and finer. Since the size of the precipitates is decreased with increasing cooling rate, it is not necessary to define the upper limit of the cooling rate. When the cooling rate is higher than 1,000 °C/min., however, a significant improvement in the size reduction effects of the precipitates is not further shown. Therefore, the cooling rate is preferably in the range of 300-1000 °C/min.
Winding conditions
• After the hot rolling, winding is performed at a temperature not higher than 700°C. When the winding temperature is higher than 700°C, the precipitates are grown too coarsely, thus making it difficult to ensure high strength.
Cold rolling conditions
• The steel is cold rolled at a reduction rate of 50-90%. Since a cold reduction rate lower than 50 % leads to creation of a small amount of nuclei upon annealing recrystallization, the crystal grains are grown excessively upon annealing, thereby coarsening of the crystal grains recrystallized through annealing, which results in reduction of the strength and formability. A cold reduction rate higher than 90 % leads to enhanced formability, while creating an excessively large amount of nuclei, so that the crystal grains recrystallized through annealing become too fine, thus deteriorating the ductility of the steel.
Continuous annealing
• Continuous annealing temperature plays an important role in determining the mechanical properties of the final product. According to the present invention, the continuous annealing is preferably performed at a temperature of 700 to 900°C. When the continuous annealing is performed at a temperature lower than 700°C, the recrystallization is not completed and thus a desired ductility cannot be ensured. In contrast, when the continuous annealing is performed at a temperature higher than 900°C, the recrystallized grains become coarse and thus the strength of the steel is deteriorated. The continuous annealing is maintained until the steel is completely recrystallized. The recrystallization of the steel can be completed for about 10 seconds or more. The continuous annealing is preferably performed for 10 seconds to 30 minutes.
[Mode for Invention]
• The present invention will now be described in more detail with reference to the following examples.
• The mechanical properties of steel sheets produced in the following examples were evaluated according to the ASTM E-8 standard test methods. Specifically, each of the steel sheets was machined to obtain standard samples. The yield strength, tensile strength, elongation, plasticity-anisotropy index (r_m value) and in-plane anisotropy index (Δr value), and the aging index were measured using a tensile strength tester (available from INSTRON Company, Model 6025). The plasticity-anisotropy index r_m and in-plane anisotropy index (Δr value) were calculated by the following equations: r_m = (r₀ + 2r₄₅ + r₉₀)/4 and Δr = (r₀ - 2r₄₅ + r₉₀)/2, respectively.
• The aging index of the steel sheets is defined as a yield point elongation measured by annealing each of the samples, followed by 1.0% skin pass rolling and thermally processing at 100°C for 2 hours. The bake hardening (BH) value of the standard samples was measured by the following procedure. After a 2% strain was applied to each of the samples, the strained sample was annealed at 170°C for 20 minutes. The yield strength of the annealed sample was measured. The BH value was calculated by subtracting the yield strength measured before annealing from the yield strength value measured after annealing.
Example 1
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 1

  Sample No.	Chemical Components (wt%)
  	C	Cu	S	Al	N	P	B	Ti	Others
  A11	0.0008	0.17	0.026	0.027	0.0005	0.05	0.0004	0.039	Si:0.02
  A12	0.0015	0.09	0.037	0.042	0.0032	0.082	0.0007	0.059	Si:0.15
  A13	0.0028	0.12	0.047	0.023	0.0026	0.117	0.0012	0.075	Si:0.25
  A14	0.0015	0.08	0.036	0.035	0.0014	0.083	0.0007	0.058	Si:0.17
  									Mo:0.07
  A15	0.0017	0.11	0.05	0.034	0.0016	0.082	0.0009	0.072	Si:0.18
  									Cr:0.17
  A16	0.0022	0.11	0.01	0.038	0.0015	0.059	0	0
  A17	0.0046	0	0.011	0.029	0.0027	0.125	0.0008	0.16

  TABLE 2

  Sample No.	S★	(Cu/63.5)/ (S★/32)	(Ti★/48)/(C/1 2)	Average size of CuS precipitates (µm)	Number of CuS precipitates (mm-2)
  A11	0.0059	14.443	2.01	0.06	3.2X106
  A12	0.0102	4.4402	0.97	0.06	4.1X106
  A13	0.0108	5.5975	1.02	0.06	4.5X106
  A14	0.0071	5.6665	1.83	0.05	5.1X106
  A15	0.0139	3.9764	1.12	0.05	4.3X106
  A16	0.0122	4.5458	0	0.08	4.5X106
  A17	0	0	7.58	0.08	6.7X104
  S★=S-0.8x(Ti-0.8x(48/14)xN)x(32/48)
  Ti★=Ti-0.8x((48/14)xN+(48/32)xS)

  TABLE 3

  Sample No.	Mechanical Properties							Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	AI (%)	SWE (DBTT-°C)
  A11	219	348	46	2.22	0.34	0	-70	IS
  A12	260	398	40	1.93	0.32	0	-60	IS
  A13	325	451	37	1.85	0.36	0	-50	IS
  A14	321	457	34	1.82	0.31	0	-50	IS
  A15	337	455	35	1.79	0.31	0	-60	IS
  A16	232	348	43	1.12	0.29	0.62	-70	CS
  A17	275	448	28	1.82	0.48	0	-50	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

Example 2
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 4

  Sample No.	Chemical Components (wt%)
  	C	Mn	Cu	S	Al	N	P	B	Ti	Others
  A21	0.0007	0.11	0.09	0.02	0.035	0.0008	0.043	0.0007	0.029	Si:0.08
  A22	0.0012	0.08	0.12	0.032	0.039	0.0021	0.08	0.0009	0.049	Si:0.17
  A23	0.0028	0.11	0.16	0.041	0.025	0.0019	0.11	0.0005	0.064	Si:0.3
  A24	0.0013	0.09	0.11	0.035	0.043	0.0023	0.082	0.0011	0.057	Si:0.26
  										Mo:0.1
  A25	0.0015	0.1	0.09	0.05	0.025	0.001	0.075	0.0012	0.069	Si:0.32
  										Cr:0.21
  A26	0.0035	0.45	0.14	0.009	0.033	0.0024	0.048	0.005	0
  A27	0.0031	0.13	0.03	0.012	0.038	0.0021	0.118	0	0.15	Si:0.33

  TABLE 5

  Sample No.	Cu+Mn	S★	(Mn/55+Cu/63.5) /(S★/32)	(Ti★/48) /(C/12)	Average size of (Mn,Cu)S precipitates (µm)	Number of (Mn,Cu)S precipitate (mm-2)
  A21	0.2	0.0057	19.173	1	0.04	4.5X106
  A22	0.2	0.0089	11.972	1.01	0.04	5.2X106
  A23	0.27	0.0096	14.994	0.86	0.03	6.3X106
  A24	0.2	0.008	13.535	1. 67	0.04	7.3X106
  A25	0.19	0.0147	7.0611	1.04	0.04	8.9X106
  A26	0.59	0.0125	26.566	-1.2	0.25	1.5X104
  A27	0.16	-0.065	-1.398	10.5	0.16	4.3X104
  S★=S-0.8x(Ti-0.8x(48/14)xN)x(32/48)
  Ti★=Ti-0.8x((48/14)xN+(48/32)xS)

  TABLE 6

  Sample No.	Mechanical Properties							Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	AI (%)	SWE (DBTT-°C)
  A21	222	352	46	2.04	0.39	0	-70	IS
  A22	288	402	39	1.87	0.32	0	-60	IS
  A23	338	454	35	1.68	0.29	0	-50	IS
  A24	329	449	34	1.88	0.28	0	-50	IS
  A25	383	452	35	1.64	0.29	0	-50	IS
  A26	238	342	43	1.21	0.59	1.73	-60	CS
  A27	302	433	30	1.65	0.48	0	-50	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, SWE = Secondary Working Embrittlement, AI = Aging Index, IS = Inventive Steel, CS = Comparative steel

Example 3
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 7

  Sample No.	Chemical Components (wt%)
  	C	Cu	S	Al	N	P	B	Ti	Others
  A31	0.0005	0.08	0.023	0.035	0.01	0.044	0.0007	0.057	Si:0.06
  A32	0.0016	0.1	0.025	0.042	0.0132	0.084	0.001	0.072	Si:0.16
  A33	0.0026	0.16	0.034	0.041	0.0148	0.121	0.0009	0.09	Si:0.21
  A34	0.0011	0.09	0.025	0.025	0.0114	0.044	0.0007	0.065	Si:0.09
  									Si:0.09
  									Mo:0.08
  A35	0.0005	0.13	0.023	0.037	0.011	0.046	0.0008	0.06	Cr:0.22
  A36	0.0038	0.09	0.013	0.032	0.0012	0.042	0.0005	0
  A37	0.0014	0	0.009	0.055	0.012	0.12	0.0005	0.14	Si:0.13

  TABLE 8

  Sample No.	S★	(Cu/63.5)/ (S★/32)	(Ti★/48)/ (C/12)	N★	(Al/27)/ (N★/14)	Average size of precipita tes (µm)	Number of precipitates (mm-2)
  A31	0.0072	5.5772	0.99	0.0031	5.78	0.04	3.9X106
  A32	0.0059	8.5273	0.91	0.0034	6.41	0.04	5.5X106
  A33	0.0077	10.539	0.83	0.0033	6.4	0.03	6.2X106
  A34	0.007	6.47	0.85	0.0032	4.01	0.04	5.3X106
  A35	0.0071	9.2382	1.11	0.0034	5.58	0.04	5.9X106
  A36	0.0148	3.0737	0	0.0048	3.43	0.25	5.5X106
  A37	0	0	17.2	0	-1.6	0.16	4.3X104
  S★=S-0.8x(Ti-0.8x(48/14)xN)x(32/48)
  Ti★=Ti-0.8x((48/14)xN+(48/32)xS)
  N★=N-0.8x(Ti-0.8x(48/32)xS))x(14/48)

  TABLE 9

  Sample No.	Mechanical Properties							Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	SWE (DBTT-°C)	AI (%)
  A1	211	352	44	2.11	0.34	-40	0	IS
  A2	269	408	37	1.98	0.37	-40	0	IS
  A3	331	452	34	1.81	0.33	-40	0	IS
  A4	241	392	36	1.89	0.41	-50	0	IS
  A5	224	384	39	1.81	0.37	-40	0	IS
  A6	233	359	37	1.11	0.62	-60	1.56	CS
  A7	283	425	33	1.81	0.57	-40	0	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, SWE = Secondary Working Embrittlement, AI = Aging Index, IS = Inventive Steel, CS = Comparative steel

Example 4
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 10

  Sample No.	Chemical Components (wt%)
  	C	Mn	Cu	S	Al	N	P	B	Ti	Others
  A1	0.0006	0.11	0.06	0.017	0.05	0.0113	0.042	0.0009	0.055	Si:0.05
  A2	0.0012	0.09	0.12	0.027	0.038	0.0141	0.08	0.001	0.077	Si:0.11
  A3	0.0026	0.1	0.11	0.035	0.024	0.0158	0.12	0.0008	0.096	Si:0.09
  A4	0.0012	0.08	0.08	0.024	0.049	0.0135	0.032	0.0009	0.073	Si:0.12
  										Mo:0.075
  A5	0.0026	0.11	0.11	0.043	0.046	0.0155	0.03	0.0011	0.104	Si:0.09
  										Cr:0.22
  A6	0.0034	0.45	0.1	0.008 3	0.038	0.0015	0.048	0.005	0
  A7	0.0038	0.07	0	0.012	0.035	0.0024	0.13	0.005	0.17	Si:0.08

  TABLE 11

  Sample No.	Cu+Mn	S*	(Mn/55+Cu /63.5)/(S */32)	(Ti*/48)/ (C/12)	N*	(Al/27) /(N*14)	Average size of precipitates (µm)	Number of precipitates (mm-2)
  A1	0.17	0.0042	22.453	1.5	0.0032	8.03	0.06	4.4 x 107
  A2	0.21	0.007	16.123	1.06	0.004	4.93	0.05	7.0 x 107
  A3	0.21	0.0069	16.435	1.03	0.0032	3.89	0.06	6.2 x 107
  A4	0.16	0.0048	18.039	1.49	0.0032	7.97	0.06	5.9 x 107
  A5	0.22	0.0102	11.7	0.95	0.0033	7.29	0.06	6.4 x 107
  A6	0.55	0.0105	29.751	0	0.0038	5.15	0.25	1.5 x 104
  A7	0.07	0	-0.542	9.8	0	0	0.04	3.5 x 105
  S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48)
  Ti*=Ti-0.8x((48/14)xN+(48/32)xS)
  N*=N-0.8x(Ti-0.8x(48/32)xS))x(14/48)

  TABLE 12

  Sample No.	Mechanical Properties							Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	AI (%)	SWE (DBTT-°C)
  A1	218	355	44	2.14	0.39	0	-70	IS
  A2	265	402	38	1.85	0.35	0	-60	IS
  A3	328	455	35	1.68	0.4	0	-60	IS
  A4	234	363	41	2.11	0.37	0	-60	IS
  A5	219	350	44	2.06	0.35	0	-50	IS
  A6	202	355	38	1.59	0.39	0	-60	CS
  A7	338	458	24	1.31	0.58	0.55	-70	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

Example 5
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 13

  Sample No.	Chemical Components (wt%)
  	C	Mn	P	S	Al	Ti	B	N	Others
  A51	0.0009	0.11	0.008	0.022	0.039	0.035	0.0007	0.0008
  A52	0.0013	0.08	0.032	0.031	0.043	0.049	0.0009	0.0021	Si:0.15
  A53	0.0025	0.11	0.058	0.043	0.028	0.067	0.0005	0.0019	Si:0.33
  A54	0.0017	0.09	0.082	0.037	0.047	0.057	0.0011	0.0023	Si:0.24
  									Mo:0.082
  A55	0.0016	0.1	0.118	0.052	0.022	0.075	0.0012	0.001	Si:0.31
  									Cr:0.13
  A56	0.0035	0.45	0.048	0.009	0.033	0	0.005	0.0024
  A57	0.0031	0.13	0.118	0.012	0.038	0.15	0	0.0021	Si:0.33

  TABLE 14

  Sample No.	S★	(Mn/55)/ (S★/32)	(Ti★/48)/ (C/12)	Average size of precipitates (µm)	Number of precipitates (mm-2)
  A51	0.0045	14.211	1.78	0.06	3.3X105
  A52	0.0079	5.8631	1.16	0.06	3.6X105
  A53	0.01	6.3706	1.02	0.05	3.8X106
  A54	0.01	5.255	0.93	0.05	3.6X106
  A55	0.0135	4.3217	1.54	0.05	3.8X106
  A56	0.0125	20.927	-1.2	0.26	2.6X103
  A57	-0.065	-1.165	10.5	0.06	4.5X105
  S★=S-0.8x(Ti-0.8x(48/14)xN)x(32/48)
  Ti★=Ti-0.8x((48/14)xN+(48/32)xS)

  TABLE 15

  Sample No.	Mechanical Properties							Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	AI (%)	SWE (DBTT-°C)
  A51	189	295	49	2.21	0.35	0	-50	IS
  A52	209	332	45	1.93	0.28	0	-50	IS
  A53	315	362	41	1.96	0.22	0	-50	IS
  A54	234	380	36	1.75	0.24	0	-40	IS
  A55	238	407	38	1.63	0.21	0	-50	IS
  A56	243	339	44	1.38	0.42	3.6	-40	CS
  A57	225	404	38	1.79	0.43	0	-40	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

Example 6
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 16

  Sample No.	Chemical Components (wt%)
  	C	P	S	Al	Ti	B	N	Others
  A61	0.0008	0.008	0.023	0.042	0.059	0.0007	0.0103
  A62	0.0017	0.035	0.025	0.044	0.074	0.001	0.0135	Si:0.13
  A63	0.0025	0.061	0.034	0.039	0.095	0.0009	0.015	Si:0.24
  A64	0.0012	0.085	0.025	0.024	0.066	0.0007	0.0117	Si:.11
  								Mo:0.06
  A65	0.0006	0.12	0.023	0.038	0.061	0.0008	0.0112	Cr:0.13
  A66	0.0038	0.042	0.013	0.032	0	0.0005	0.0012
  A67	0.0014	0.12	0.009	0.055	0.14	0.0005	0.012	Si:0.13

  TABLE 17

  Sample No.	(Ti★/48) /(C/12)	N★	(Al/27)/ (N★/14)	Average size of precipitates (µm)	Number of precipitates (mm-2)
  A61	0.67	0.003	7.32	0.05	6.3X105
  A62	1.03	0.0032	7.06	0.05	6.3X105
  A63	1.31	0.0024	8.59	0.05	8.4X106
  A64	0.81	0.0033	3.77	0.05	7.3X106
  A65	1.12	0.0034	5.78	0.05	6.2X106
  A66	-1.2	0.0048	3.43	0.05	4.5X105
  A67	17.2	-0.018	-1.6	0.28	3.5X103
  Ti★=Ti-0.8x((48/14)xN+(48/32)xS)
  N★=N-0.8x(Ti-0.8x(48/32)xS))x(14/48)

  TABLE 18

  Sample No.	Mechanical Properties							Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	SWE (DBTT-°C)	AI(%)
  A61	209	349	44	2.03	0.25	-60	0	IS
  A62	282	399	37	1.72	0.24	-50	0	IS
  A63	339	457	34	1.73	0.27	-50	0	IS
  A64	219	360	42	2.21	0.29	-50	0	IS
  A65	354	449	33	1.73	0.21	-60	0	IS
  A66	189	348	45	1.32	0.43	-40	0.94	CS
  A67	335	457	26	1.53	0.24	-40	0	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, SWE = Secondary Working Embrittlement, AI = Aging Index, IS = Inventive Steel, CS = Comparative steel

Example 7
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 19

  Sample No.	Chemical Components (wt%)
  	C	Si	Mn	P	S	Al	Ti	B	N	Others
  A71	0.0009	0	0.11	0.038	0.017	0.053	0.058	0.0005	0.0119
  A72	0.0012	0.11	0.09	0.053	0.026	0.038	0.076	0.001	0.0147	Si:0.11
  A73	0.0008	0.1	0.11	0.109	0.033	0.015	0.094	0.0008	0.0158	Si:0.1
  A74	0.0012	0.12	0.1	0.032	0.024	0.049	0.073	0.0009	0.0133	Si:0.12
  										Mo:0.05
  A75	0.0026	0.09	0.11	0.03	0.043	0.046	0.104	0.0011	0.0155	Si:0.09
  										Cr:0.28
  A76	0.0018	0	0.68	0.045	0.009	0.048	0.057	0.0004	0.0021
  A77	0.0037	0.05	0.1	0.114	0.01	0.008	0	0.0011	0.0067	Si:0.05

  TABLE 20

  Sample No.	S*	(Mn/55) /(S*/32)	(Ti*/48) /(C/12)	N*	(Al/27)/(N*/14)	Average size of precipitates (µm)	Number of precipitates (mm-2)
  A71	0.0035	18.419	1.38	0.0031	8.79	0.05	6.3 x 105
  A72	0.007	7.512	0.93	0.0042	4.64	0.05	6.3 x 105
  A73	0.006	10.703	3.46	0.0031	2.5	0.05	8.4 x 106
  A74	0.0045	12.864	1.61	0.003	8.51	0.05	7.3 x 106
  A75	0.0102	6.2698	0.95	0.0033	7.29	0.05	6.2 x 106
  A76	-0.018	-21.59	5.62	-0.009	-2.9	0.05	4.5 x 105
  A77	0.0198	2.9383	-2.1	0.0095	0.44	0.28	3.5 x 103
  S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48)
  Ti*=Ti-0.8x((48/14)xN+(48/32)xS)
  N*=N-0.8x(Ti-0.8x(48/32)xS))x(14/48)

  TABLE 21

  Sample No.	Mechanical Properties							Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	SWE (DBTT-°C)	AI (%)
  A71	215	357	46	2.04	0.39	-40	0	IS
  A72	243	382	41	1.89	0.35	-50	0	IS
  A73	271	425	34	1.75	0.27	-50	0	IS
  A74	232	371	42	1.84	0.24	-50	0	IS
  A75	226	364	41	1.89	0.22	-60	0	IS
  A76	189	347	42	1.92	0.42	-40	0	CS
  A77	293	418	36	1.32	0.34	-60	3.51	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

Example 8
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 22

  Sample No.	Chemical components (wt%)
  	C	P	S	Al	Cu	Ti	B	N	Others
  B81	0.0021	0.009	0.011	0.037	0.09	0.017	0.0005	0.0011
  B82	0.0017	0.026	0.01	0.026	0.11	0.021	0.0009	0.0024
  B83	0.0018	0.05	0.012	0.027	0.08	0.015	0.0004	0.0005	Si:0.02
  B84	0.0028	0.082	0.01	0.032	0.12	0.018	0.0007	0.0015	Si:0.18
  B85	0.0021	0.113	0.011	0.034	0.12	0.021	0.001	0.0018	Si:0.24
  B86	0.0017	0.082	0.008	0.033	0.09	0.017	0.0007	0.0019	Si:0.18
  									Mo:0.074
  B87	0.0022	0.082	0.01	0.029	0.12	0.019	0.0006	0.0016	Si:0.18
  									Cr:0.21
  B88	0.0022	0.063	0.008	0.029	0.11	0.055	0.0005	0.0012
  B89	0.0033	0.12	0.009	0.037	0	0	0.0008	0.0027

  TABLE 23

  Sample No.	(Cu/63.5)/(S★/32)	Cs	Average size of precipitates (µm)	Number of precipitates (mm-2)
  B81	12.8	19.043	0.06	1.8X106
  B82	24	10.957	0.06	2.1X106
  B83	8.52	18	0.06	2.5X106
  B84	23.3	23.286	0.05	3.2X106
  B85	24.9	13.843	0.06	4.1X106
  B86	26.5	11.529	0.06	3.2X106
  B87	27.4	15.471	0.05	4.1X106
  B88	-2.8	-75	0.08	4.5X105
  B89	0	33	0.08	6.2X104
  S★=S-0.8x(Ti-0.8x(48/14)xN)x(32/48),
  Cs=(C-Ti★12/48)X10000,
  Ti★=Ti-0.8x((48/14)xN+(48/32)xS)

  TABLE 24

  Sample No.	Mechanical Properties								Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	AI (%)	BH value (MPa)	SWE (DBTT-°C)
  B81	183	305	49	1.93	0.32	0	37	-40	IS
  B82	193	332	48	1.88	0.32	0	41	-50	IS
  B83	204	349	44	1.88	0.29	0	47	-50	IS
  B84	267	402	39	1.75	0.27	0	67	-60	IS
  B85	329	450	36	1.65	0.19	0	37	-50	IS
  B86	325	455	35	1.61	0.31	0	41	-50	IS
  B87	333	449	34	1.66	0.24	0	45	-50	IS
  B88	232	348	43	1.92	0.29	0	0	-50	CS
  B89	279	453	29	1.22	0.48	3.8	92	-70	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

Example 9
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 25

  Sample No.	Chemical Components (wt%)
  	C	Mn	P	S	Al	Cu	Ti	B	N	Others
  B91	0.0019	0.11	0.008	0.008	0.038	0.12	0.01	0.0008	0.0011
  B92	0.0018	0.14	0.024	0.011	0.042	0.14	0.008	0.0007	0.0015
  B93	0.0015	0.09	0.041	0.009	0.034	0.1	0.009	0.0005	0.0005	Si:0.08
  B94	0.0027	0.1	0.083	0.011	0.046	0.11	0.017	0.0008	0.0013	Si:0.18
  B95	0.0022	0.11	0.1	0.011	0.039	0.15	0.016	0.0005	0.002	Si:0.28
  B96	0.0019	0.1	0.083	0.010	0.033	0.13	0.013	0.0009	0.0021	Si:0.27
  										Mo:0.11
  B97	0.0025	0.09	0.076	0.013	0.033	0.11	0.02	0.0011	0.0021	Si:0.31
  										Cr:0.24
  B98	0.0022	0.47	0.051	0.008	0.031	0	0.042	0.0007	0.0016
  B99	0.0037	0.13	0.12	0.013	0.034	0.03	0	0.005	0.0025	Si:0.32

  TABLE 26

  Sample No.	Cu+Mn	(Mn/55+Cu/63.5)/(S★/32)	Cs	Average size of precipitates (um)	Number of precipitates (mm-2)
  B91	0.23	29.1	19	0.05	2.8X106
  B92	0.28	17	18	0.05	2.5X106
  B93	0.19	20.8	15	0.05	2.8X106
  B94	0.21	29.6	27	0.05	2.9X106
  B95	0.26	25.9	22	0.05	3.9X106
  B96	0.23	20.1	19	0.04	2.5X106
  B97	0.2	19.9	25	0.04	3.9X106
  B98	0.47	-23	-39	0.25	1.7X104
  B99	0.16	5.45	37	0.08	6.3X104
  S★=S-0.8x(Ti-0.8x(48/14)xN)x(32/48),
  Cs=(C-Ti★x12/48)X10000,
  Ti★=Ti-0.8x((48/14)xN+(48/32)xS)

  TABLE 27

  Sample No.	Mechanical Properties								Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	AI(%)	BH value (MPa)	SWE (DBTT-°C)
  B91	188	309	48	1.91	0.32	0	43	-50	IS
  B92	210	331	46	1.88	0.29	0	44	-40	IS
  B93	225	357	45	1.85	0.35	0	39	-50	IS
  B94	292	399	39	1.75	0.32	0	47	-60	IS
  B95	343	452	34	1.61	0.28	0	53	-50	IS
  B96	333	447	34	1.66	0.28	0	42	-50	IS
  B97	328	452	35	1.65	0.27	0	55	-60	IS
  B98	201	351	41	1.92	0.45	0	0	-50	CS
  B99	312	437	31	1.21	0.2	4.5	89	-50	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

Example 10
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 28

  Sample No.	Chemical Components (wt%)
  	C	P	S	Al	Cu	Ti	B	N	Others
  B01	0.0019	0.008	0.008	0.039	0.09	0.006	0.0005	0.0088
  B02	0.0017	0.027	0.01	0.042	0.14	0.007	0.0005	0.0072
  B03	0.0018	0.042	0.009	0.038	0.12	0.007	0.0007	0.01	Si:0.07
  B04	0.0016	0.086	0.011	0.04	0.1	0.016	0.001	0.0125	Si:0.14
  B05	0.0026	0.12	0.018	0.062	0.16	0.045	0.0009	0.0139	Si:0.2
  B06	0.0025	0.044	0.025	0.055	0.09	0.065	0.0006	0.012	Si:0.11
  									Mo:0.084
  B07	0.0022	0.043	0.009	0.033	0.12	0.029	0.0009	0.01	Cr:0.27
  B08	0.0025	0.041	0.012	0.054	0	0.063	0.0005	0.0012
  B09	0.0054	0.11	0.011	0.055	0.09	0	0.001	0.011	Si:0.15

  TABLE 29

  Sample No.	(Cu/63.5)/ (S★/32)	(Al/27)/ (N★/14)	Cs	Average size of precipitates (µm)	Number of precipitates (mm-2)
  B01	2.57	2.1	19	0.06	2.8X106
  B02	4.2	2.6	17	0.06	3.7X106
  B03	3.04	1.81	18	0.06	3.5X106
  B04	2.43	1.75	16	0.05	4.7X106
  B05	5.63	3.81	26	0.04	5.5X106
  B06	5.75	7.44	25	0.05	4.3X106
  B07	7.41	2.97	22	0.04	5.2X106
  B08	0	-2.8	-77	0.2	2.5X104
  B09	1.67	2.03	54	0.05	4.4X106
  S★=S-0.8x(Ti-0.8x(48/14)xN)x(32/48),
  Cs=(C-Ti★12/48)X10000,
  Ti★=Ti-0.8x((48/14)xN+(48/32)xS)
  N★=N-0.8x(Ti-0.8x(48/32)xS))x(14/48)

  TABLE 30

  Sample No.	Mechanical Properties								Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	AI	BH value (MPa)	SWE (DBTT-°C)
  B01	209	325	50	1.91	0.35	0	48	-50	IS
  B02	219	344	47	1.83	0.29	0	38	-40	IS
  B03	217	355	43	1.88	0.31	0	42	-50	IS
  B04	292	411	36	1.79	0.29	0	43	-50	IS
  B05	339	450	33	1.66	0.25	0	55	-40	IS
  B06	248	390	38	1.75	0.32	0	52	-50	IS
  B07	243	389	39	1.77	0.35	0	45	-40	IS
  B08	202	339	40	1.99	0.52	0	0	-50	CS
  B09	291	431	32	1.28	0.19	3.9	104	-40	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

Example 11
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 31

  Sample No.	Chemical Components (wt%)
  	C	Mn	P	S	Al	Cu	Ti	B	N	Others
  B11	0.0014	0.1	0.007	0.008	0.042	0.09	0.009	0.0005	0.0094
  B12	0.0016	0.13	0.023	0.011	0.052	0.08	0.01	0.0007	0.0076
  B13	0.0017	0.09	0.044	0.01	0.053	0.08	0.018	0.0009	0.011	Si:0.07
  B14	0.0012	0.1	0.084	0.009	0.035	0.11	0.02	0.0008	0.0128	Si:0.12
  B15	0.0024	0.13	0.117	0.015	0.061	0.16	0.055	0.0011	0.0142	Si:0.09
  B16	0.0025	0.11	0.035	0.026	0.028	0.09	0.038	0.0009	0.013	Si:0.11
  										Mo:0.072
  B17	0.0022	0.12	0.033	0.009	0.043	0.09	0.04	0.0009	0.014	Si:0.09
  										Cr:0.25
  B18	0.0018	0.52	0.045	0.009	0.035	0	0.06	0.006	0.0022
  B19	0.0042	0.11	0.127	0.01	0.043	0.09	0	0.005	0.0018	Si:0.08

  TABLE 32

  Sample No.	Cu+Mn	(Mn/55+Cu/63. 5)/(S★/32)	(Al/27)/ (N★/14)	Cs	Average size of precipitates (µm)	Number of precipitates (mm-2))
  B11	0.19	6.11	2.28	14	0.06	1.1X107
  B12	0.21	6.91	3.23	16	0.06	9.5X106
  B13	0.17	5.62	2.86	17	0.06	1.7X107
  B14	0.21	6.66	1.7	12	0.05	1.9X107
  B15	0.29	24.3	5.68	24	0.05	3.2X107
  B16	0.2	4.42	1.27	25	0.05	3.8X107
  B17	0.21	14.1	3.1	22	0.04	4.5X107
  B18	0.52	-15	-2	-79	0.25	1.8X104
  B19	0.2	8.66	4.85	42	0.06	8.3X105
  S★=S-0.8x(Ti-0.8x(48/14)xN)x(32/48),
  Cs=(C-Ti★x12/48)X10000,
  Ti★=Ti-0.8x((48/14)xN+(48/32)xS)
  N★=N-0.8x(Ti-0.8x(48/32)xS))x(14/48)

  TABLE 33

  Sample No.	Mechanical Properties								Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	AI	BH value (MPa)	SWE (DBTT-°C)
  B11	201	321	48	1.94	0.34	0	35	-40	IS
  B12	211	342	46	1.89	0.31	0	42	-50	IS
  B13	221	359	45	1.91	0.35	0	36	-60	IS
  B14	269	410	37	1.77	0.32	0	39	-60	IS
  B15	332	462	33	1.63	0.31	0	47	-60	IS
  B16	237	360	42	1.85	0.31	0	53	-60	IS
  B17	227	353	42	1.83	0.33	0	55	-50	IS
  B18	184	352	39	1.99	0.45	0	0	-50	CS
  B19	343	453	25	1.27	0.21	6.2	93	-60	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

Example 12
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 34

  Sample No.	Chemical Components (wt%)
  	C	Mn	P	S	Al	Ti	B	N	Others
  B21	0.0018	0.08	0.011	0.008	0.037	0.007	0.0004	0.0014
  B22	0.0015	0.05	0.052	0.009	0.044	0.008	0.0006	0.0016
  B23	0.0029	0.11	0.08	0.011	0.029	0.02	0.0009	0.0017
  B24	0.0025	0.09	0.108	0.011	0.032	0.011	0.0007	0.0027	Si:0.14
  B25	0.0017	0.07	0.089	0.015	0.038	0.031	0.0009	0.0042	Mo:0.077
  B26	0.0026	0.12	0.093	0.011	0.039	0.014	0.001	0.0031	Cr:0.14
  B27	0.0021	0.45	0.045	0.009	0.038	0.058	0.0007	0.0021
  B28	0.0024	0.32	0.11	0.008	0.024	0	0.007	0.0013

  TABLE 35

  Sample No.	(Mn/55)/(S★/32)	Cs	Average size of precipitates (µm)	Number of precipitates (mm-2)
  B21	7.37	18	0.06	1.2X105
  B22	4.11	15	0.06	1.2X105
  B23	22.7	23.657	0.05	1.8X105
  B24	5.76	25	0.05	2.2X106
  B25	8.83	13.3	0.05	3.1X106
  B26	8.65	26	0.04	3.7X106
  B27	-14	-72	0.06	3.4X104
  B28	18.8	24	0.22	2.3X103
  S★=S-0.8x(Ti-0.8x(48/14)xN)x(32/48),
  Cs=(C-Ti★x12/48)X10000,
  Ti★=Ti-0.8x((48/14)xN+(48/32)xS)

  TABLE 36

  Sample No.	Mechanical Properties								Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	AI (%)	BH value (MPa)	SWE (DBTT-°C)
  B21	189	301	51	2.02	0.35	0	43	-50	IS
  B22	227	356	44	1.97	0.32	0	39	-50	IS
  B23	259	409	38	1.81	0.27	0	59	-60	IS
  B24	321	459	34	1.58	0.21	0	54	-50	IS
  B25	280	447	32	1.59	0.24	0	35	-40	IS
  B26	313	457	32	1.49	0.21	0	53	-50	IS
  B27	211	354	40	1.96	0.33	0	0	-40	CS
  B28	254	454	25	1.56	0.28	0		-70	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

Example 13
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 37

  Sample No.	Chemical Components (wt%)
  	C	P	S	Al	Ti	B	N	Others
  B31	0.0011	0.009	0.011	0.039	0.005	0.0006	0.0084
  B32	0.0014	0.05	0.008	0.053	0.009	0.0008	0.0072
  B33	0.0026	0.084	0.013	0.062	0.031	0.0008	0.0089	Si:0.11
  B34	0.0017	0.11	0.01	0.05	0.051	0.001	0.013	Si:0.27
  B35	0.0026	0.033	0.012	0.033	0.041	0.0007	0.012	Si:0.23
  								Mo:0.055
  B36	0.0028	0.11	0.011	0.05	0.019	0.0011	0.0095	Si:0.18
  								Cr:0.12
  B37	0.0013	0.055	0.01	0.052	0.052	0.0007	0.0019
  B38	0.0038	0.12	0.012	0.022	0	0.0009	0.003

  TABLE 38

  Sample No.	(Al/27)/(N★/14)	Cs	Average size of precipitates (µm)	Number of precipitates (mm- 2)
  B31	1.96	11	0.06	3.5X106
  B32	3.74	14	0.06	3.2X106
  B33	6.06	26	0.05	4.1X106
  B34	6.65	17	0.05	5.3X106
  B35	2.95	26	0.05	4.4X106
  B36	3.18	28	0.04	5.9X106
  B37	-3.6	-63	0.21	1.8X104
  B38	1.79	38	0.07	2.2104
  Cs=(C-Ti★x12/48)X10000,
  Ti★=Ti-0.8x((48/14)xN+(48/32)xS)
  N★=N-0.8x(Ti-0.8x(48/32)xS))x(14/48)

  TABLE 39

  Sample No.	Mechanical Properties								Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	AI (%)	BH value (MPa)	SWE (DBTT-°C)
  B31	188	312	51	1.99	0.31	0	36	-40	IS
  B32	217	344	45	1.88	0.25	0	37	-50	IS
  B33	271	404	38	1.7	0.23	0	52	-50	IS
  B34	330	458	32	1.74	0.31	0	42	-50	IS
  B35	220	362	41	1.89	0.29	0	58	-50	IS
  B36	333	453	32	1.59	0.21	0	58	-60	IS
  B37	196	355	41	1.32	0.43	0	0	-40	CS
  B38	329	452	27	1.21	0.18	5.2	88	-40	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

Example 14
• First, steel slabs were prepared in accordance with the compositions shown in the following tables. The steel slabs were reheated and finish hot-rolled to provide hot rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min., wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing to produce cold rolled steel sheets. At this time, the finish hot rolling was performed at 910°C, which is above the Ar₃ transformation point, and the continuous annealing was performed by heating the hot rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the final cold rolled steel sheets. TABLE 40

  Sample No.	Chemical Components (wt%)								Others
  	C	Mn	P	S	Al	Ti	B	N
  B41	0.0015	0.12	0.00 9	0.007	0.039	0.01	0.0008	0.0073
  B42	0.0018	0.08	0.02 4	0.009	0.042	0.00 8	0.0005	0.0094
  B43	0.0012	0.09	0.04 4	0.008	0.043	0.00 9	0.0007	0.0079	Si:0.06
  B44	0.0026	0.11	0.07 7	0.012	0.054	0.02 2	0.0008	0.011	Si:0.12
  B45	0.0018	0.11	0.11	0.016	0.052	0.05 1	0.0011	0.0125	Si:0.11
  B46	0.0021	0.1	0.04 1	0.013	0.067	0.03 3	0.0009	0.0083	Si:0.09
  									Mo:0.056
  B47	0.0019	0.11	0.04 1	0.008	0.042	0.01 9	0.0006	0.0095	Cr:0.33
  B48	0.0016	0.68	0.04 5	0.009	0.048	0.05 2	0.0004	0.0021
  B49	0.0037	0.1	0.11 4	0.01	0.008	0	0.0011	0.0067	Si:0.05

  TABLE 41

  Sample No.	(Mn/55)/ (S★/32)	(Al/27)/ (N★/14)	Cs	Average size of precipitates (µm)	Number of precipitates (mm-2)
  B41	5.66	2.92	15	0.06	5.1X106
  B42	2.52	2.17	18	0.06	4.9X106
  B43	3.55	2.77	12	0.06	5.8X106
  B44	3.91	3.03	26	0.05	6.9X106
  B45	9.03	5.31	18	0.05	8.1X106
  B46	7.71	8.19	21	0.05	6.8X106
  B47	5.44	2.98	19	0.04	8.8X106
  B48	-25	-3.3	-62	0.21	1.8X104
  B49	2.94	0.44	37	0.07	8.3X105
  S★=S-0.8x(Ti-0.8x(48/14)xN)x(32/48),
  Cs=(C-Ti★x12/48)X10000,
  Ti★=Ti-0.8x((48/14)xN+(48/32)xS)
  N★=N-0.8x(Ti-0.8x(48/32)xS))x(14/48)

  TABLE 42

  Sample No.	Mechanical Properties								Remarks
  	YS (MPa)	TS (MPa)	El (%)	rm	Δr	AI (%)	BH value (MPa)	SWE (DBTT-°C)
  B41	194	311	49	1.98	0.41	0	38	-50	IS
  B42	209	325	47	1.82	0.37	0	45	-40	IS
  B43	219	355	43	1.79	0.39	0	38	-50	IS
  B44	267	395	39	1.71	0.29	0	48	-40	IS
  B45	322	459	33	1.51	0.25	0	39	-60	IS
  B46	239	360	41	1.61	0.26	0	44	-50	IS
  B47	233	368	42	1.57	0.28	0	41	-50	IS
  B48	185	348	42	1.92	0.42	0	0	-40	CS
  B49	378	461	27	1.12	0.34	4.1	96	-60	CS

  * Note: YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

[Industrial Applicability]
• As apparent from the above description, according to the cold rolled steel sheets of the present invention, the distribution of fine precipitates in Ti-based IF steels allows the formation of minute crystal grains, and as a result, the in-plane anisotropy index is lowered and the yield strength is enhanced by precipitation enhancement.

Claims (44)

1. A cold rolled steel sheet with superior formability, the steel sheet having a composition comprising 0.01% or less of C, 0.01-0.2% of Cu, 0.005-0.08% of S, 0.1% or less of A1, 0.02% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, by weight, and the balance of Fe and other unavoidable impurities,
   wherein the composition satisfies the following relationships: 1 ≤ (Cu/63.5)/(S*/32) ≤ 30 and S* = S-0.8 x (Ti - 0.8 x (48/14) x N) x (32/48), and
   wherein the steel sheet comprises CuS precipitates having an average size of 0.2 µm or less.
2. The steel sheet according to claim 1, wherein the composition comprises 0.004% or less of N.
3. The steel sheet according to claim 1, wherein the composition comprises 0.004-0.02% of N, the composition satisfies the following relationships: 1 ≤ (A1/27)/(N*/14) ≤ 10 and N* = N - 0.8 x (Ti - 0.8 x (48/32) x S) x (14/48), and the steel sheet comprises A1N precipitates having an average size of 0.2 µm or less.
4. The steel sheet according to claim 1, wherein the composition further comprises 0.01-0.3% of Mn, the composition satisfies the following relationship: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32) ≤ 30, and the steel sheet comprises (Mn,Cu)S precipitates having an average size of 0.2 µm or less.
5. The steel sheet according to claim 1, wherein the composition comprises 0.004-0.02% of N and further comprises 0.01-0.3% of Mn, the composition satisfies the following relationships: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32) ≤ 30, 1 ≤ (A1/27)/(N*/14) ≤ 10 and N* = N - 0.8 x (Ti - 0.8 x (48/32) x S) x (14/48), and the steel sheet comprises (Mn,Cu)S precipitates and A1N precipitates having an average size of 0.2 µm or less.
6. A cold rolled steel sheet with superior formability, the steel sheet having a composition comprising 0.01% or less of C, 0.08% or less of S, 0.1% or less of A1, 0.02% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, at least one of 0.01-0.2% of Cu and 0.01-0.3% of Mn, by weight, and the balance of Fe and other unavoidable impurities,
   wherein the composition satisfies the following relationships: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32) ≤ 30, 1 ≤ (A1/27)/(N*/14) ≤ 10, where the composition comprises 0.004% or more of N, S* = S - 0.8 x (Ti - 0.8 x (48/14) x N) x (32/48) and N* = N - 0.8 x (Ti - 0.8 x (48/32) x S) x (14/48), and
   wherein the steel sheet comprises at least one of (Mn,Cu)S and A1N precipitates having an average size of 0.2 µm or less.
7. The steel sheet according to claim 6, wherein the composition comprises 0.004% or less of N.
8. The steel sheet according to claim 1 or 6, wherein the composition satisfies the following relationships: 0.8 ≤ (Ti*/48)/(C/12) ≤ 5.0 and Ti* = Ti - 0.8 x ((48/14) x N + (48/32) x S).
9. The steel sheet according to claim 8, wherein the composition comprises 0.005% or less of C.
10. The steel sheet according to claim 1 or 6, wherein solute carbon (Cs) [Cs = (C - Ti* x 12/48) x 10000 in which Ti* = Ti - 0.8 x ((48/14) x N + (48/32) x S), provided that when Ti* is less than 0, Ti* is defined as 0] is from 5 to 30.
11. The steel sheet according to claim 10, wherein the composition comprises 0.001-0.01% of C.
12. The steel sheet according to claim 1 or 6, wherein the composition comprises 0.015% or less of P.
13. The steel sheet according to claim 1 or 6, wherein the composition comprises 0.03-0.2% of P.
14. The steel sheet according to claim 1 or 6, wherein the composition further comprises at least one of 0.1-0.8% of Si and 0.2-1.2% of Cr.
15. The steel sheet according to claim 1 or 6, wherein the composition further comprises 0.01-0.2% of Mo.
16. The steel sheet according to claim 1 or 6, wherein the composition further comprises 0.01-0.2% of Mo, and at least one of 0.1-0.8% of Si and 0.2-1.2% of Cr.
17. The steel sheet according to any of claims 4 to 6, wherein the composition comprises 0.05-0.4% of Mn and Cu.
18. The steel sheet according to any of claims 4 to 6, wherein the composition comprises 0.01-0.12% of Mn.
19. The steel sheet according to any of claims 4 to 6, wherein the value of (Mn/55 + Cu/63.5)/(S*/32) is from 1 to 9.
20. The steel sheet according to claim 3, 5 or 6, wherein the value of (A1/27)/(N*/14) is from 1 to 6.
21. The steel sheet according to any of claims 1 to 20, wherein the steel sheet satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher.
22. The steel sheet according to any of claims 1 to 21, wherein the number of the precipitates is 1 x 10⁶/mm² or more.
23. A method for producing a cold rolled steel sheet with superior formability, the method comprising the steps of:

    reheating a slab to a temperature of 1,100°C or higher, the slab having a composition comprising 0.01% or less of C, 0.01-0.2% of Cu, 0.005-0.08% of S, 0.1% or less of A1, 0.02% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, by weight, and the balance of Fe and other unavoidable impurities, and the composition satisfying the following

    relationships: 1 ≤ (Cu/63.5)/(S*/32) ≤ 30 and S* = S-0.8 x (Ti - 0.8 x (48/14) x N) x (32/48);

    hot rolling the reheated slab at a finish rolling temperature of the Ar₃ transformation point or higher to provide a hot rolled steel sheet;

    cooling the hot rolled steel sheet at a rate of 300 °C/min or higher;

    winding the cooled steel sheet at 700°C or lower;

    cold rolling the wound steel sheet; and

    continuously annealing the cold rolled steel sheet, wherein the cold rolled steel sheet comprises CuS precipitates having an average size of 0.2 µm or less.
24. The method according to claim 23, wherein the composition comprises 0.004% or less of N.
25. The method according to claim 23, wherein the N content is 0.004-0.02%, the composition satisfies the following relationships: 1 ≤ (A1/27)/(N*/14) ≤ 10 and N* = N - 0.8 x (Ti - 0.8 x (48/32) x S) x (14/48), and the steel sheet comprises A1N precipitates having an average size of 0.2 µm or less.
26. The method according to claim 23, wherein the composition further comprises 0.01-0.3% of Mn, the composition satisfies the following relationship: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32) ≤ 30, and the steel sheet comprises (Mn,Cu)S precipitates having an average size of 0.2 µm or less.
27. The method according to claim 23, wherein the composition further comprises 0.01-0.3% of Mn, and 0.004-0.02% of N, and satisfies the following relationships: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32) ≤ 30, 1 ≤ (A1/27)/(N*/14) ≤ 10 and N* = N - 0.8 x (Ti - 0.8 x (48/32) x S) x (14/48), and the steel sheet comprises (Mn,Cu)S precipitates and A1N precipitates having an average size of 0.2 µm or less.
28. A method for producing a cold rolled steel sheet with superior formability, the method comprising the steps of:

    reheating a slab to a temperature of 1,100°C or higher, the slab having a composition comprising 0.01% or less of C, 0.08% or less of S, 0.1% or less of A1, 0.02% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.005-0.15% of Ti, at least one of 0.01-0.2% of Cu and 0.01-0.3% of Mn, by weight, and the balance of Fe and other unavoidable impurities, and the composition satisfying the following relationships: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32) ≤ 30, 1 ≤ (A1/27)/(N*/14) ≤ 10, where the composition comprises 0.004% or more of N, S* = S-0.8 x (Ti - 0.8 x (48/14) x N) x (32/48) and N* = N-0.8 x (Ti - 0.8 x (48/32) x S) x (14/48);

    hot rolling the reheated slab at a finish rolling temperature of the Ar₃ transformation point or higher to provide a hot rolled steel sheet;

    cooling the hot rolled steel sheet at a rate of 300 °C/min or higher;

    winding the cooled steel sheet at 700°C or lower;

    cold rolling the wound steel sheet; and

    continuously annealing the cold rolled steel sheet, wherein the cold rolled steel sheet comprises at least one of (Mn,Cu)S and A1N precipitates having an average size of 0.2 µm or less.
29. The method according to claim 28, wherein the composition comprises 0.004% or less of N.
30. The method according to claim 23 or 28, wherein the composition satisfies the following relationships: 0.8 ≤ (Ti*/48)/(C/12) ≤ 5.0 and Ti* = Ti - 0.8 x ((48/14) x N + (48/32) x S).
31. The method according to claim 30, wherein the composition comprises 0.005% or less of C.
32. The method according to claim 23 or 28, wherein solute carbon (Cs) [Cs = (C - Ti* x 12/48) x 10000 in which Ti* = Ti - 0.8 x ((48/14) x N + (48/32) x S), provided that when Ti* is less than 0, Ti* is defined as 0] is from 5 to 30.
33. The method according to claim 32, wherein the composition comprises 0.001-0.01% of C.
34. The method according to claim 23 or 28, wherein the composition comprises 0.015% or less of P.
35. The method according to claim 23 or 28, wherein the composition comprises 0.03-0.2% of P.
36. The method according to claim 23 or 28, wherein the composition further comprises at least one of 0.1-0.8% of Si and 0.2-1.2% of Cr.
37. The method according to claim 23 or 28, wherein the composition futher comprises 0.01-0.2% of Mo.
38. The method according to claim 23 or 28, wherein the composition further comprises 0.01-0.2% of Mo, and at least one of 0.1-0.8% of Si and 0.2-1.2% of Cr.
39. The method according to any of claims 26 to 28, wherein the composition comprises 0.05-0.4% of Mn and Cu.
40. The method according to any of claims 26 to 28, wherein the composition comprises 0.01-0.12% of Mn.
41. The method according to any of claims 26 to 28, wherein the value of (Mn/55 + Cu/63.5)/(S*/32) is from 1 to 9.
42. The method according to claim 25, 27 or 28, wherein the value of (A1/27)/(N*/14) is from 1 to 6.
43. The method according to any of claims 23 to 42, wherein the cold rolled steel sheet satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher.
44. The method according to any of claims 23 to 43, wherein the number of the precipitates is 1 x 10⁶/mm² or more.