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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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, r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
AI = Aging Index,
SWE = Secondary Working Embrittlement,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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,
r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
SWE = Secondary Working Embrittlement,
AI = Aging Index,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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,
r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
SWE = Secondary Working Embrittlement,
AI = Aging Index,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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,
r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
AI = Aging Index,
SWE = Secondary Working Embrittlement,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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,
r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
AI = Aging Index,
SWE = Secondary Working Embrittlement,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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,
r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
SWE = Secondary Working Embrittlement,
AI = Aging Index,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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,
r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
AI = Aging Index,
SWE = Secondary Working Embrittlement,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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,
r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
AI = Aging Index,
SWE = Secondary Working Embrittlement,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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,
r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
AI = Aging Index,
SWE = Secondary Working Embrittlement,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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,
r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
AI = Aging Index,
SWE = Secondary Working Embrittlement,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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, r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
AI = Aging Index,
SWE = Secondary Working Embrittlement,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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,
r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
AI = Aging Index,
SWE = Secondary Working Embrittlement,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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, r_m = Plasticity-anisotropy index,
Δr = In-plane anisotropy index,
AI = Aging Index,
SWE = Secondary Working Embrittlement,
IS = Inventive Steel,
CS = Comparative steel

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. 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 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) Sample No. Mechanical Properties Remarks YS (MPa) TS (MPa) El (%) r_m 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, r_m = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index, SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel

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.