Cold Rolled Steel Sheet Having Superior Formability, Process for Producing the Same

- POSCO

Disclosed herein is a Ti-based IF steel in which fine precipitates, such as CuS precipitates, having a size of 0.2 μm or less are distributed. The distribution of fine precipitates in the Ti-based IF steel enhances the yield strength and lowers the in-plane anisotropy index. The nanometer-sized precipitates allow the formation of minute crystal grains. As a result, dissolved carbon is present in a larger amount in the crystal grain boundaries than within the crystal grains, which is advantageous in terms of room-temperature non-aging properties and bake hardenability.

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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.

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 E-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—Ti—C—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, 0.004% 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×(Ti−0.8×(48/14)×N)×(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, 0.004% 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×(Ti−0.8×(48/14)×N)×(32/48), and the steel sheet comprises (Mn,Cu)S 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, 0.005-0.08% of S, 0.1% or less of Al, 0.004-0.02% 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, 1≦(Al/27)/(N*/14)≦10, S*=S−0.8×(Ti−0.8×(48/14)×N)×(32/48) and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises CuS and AlN 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, 0.004-0.02% 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, 1≦(Al/27)/(N*/14)≦10, S*=S−0.8×(Ti−0.8×(48/14)×N)×(32/48) and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises (Mn,Cu)S and AlN 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, 0.08% or less of S, 0.1% or less of Al, 0.004% 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 and 0.004-0.2% of N, 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, where the N content is 0.004% or more, S=S−0.8×(Ti−0.8×(48/14)×N)×(32/48) and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), the steel sheet comprises at least one kind selected from (Mn,Cu)S and AlN precipitates having an average size of 0.2 μm or less.

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×((48/14)×N+(48/32)×S), they show room-temperature non-aging properties. In addition, when solute carbon (Cs) [Cs=(C−Ti*×12/48)×10000 in which Ti*=Ti−0.8×((48/14)×N+(48/32)×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 preferably 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 preferably 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 preferably 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 preferably 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 preferably 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 preferably 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 preferably 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 preferably 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.


1≦(Cu/63.5)/(S*/32)≦30  (1)


S*=S−0.8×(Ti−0.8×(48/14)×N)×(32/48)  (2)

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 preferred 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 um 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.


1≦(Mn/55+Cu/63.5)/(S*/32)≦30  (3)

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.


1≦(Al/27)/(N*/14)≦10  (4)


N=N−0.8×(Ti−0.8×(48/32)×S))×(14/48)  (5)

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 preferred 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, 0.004% 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 and 0.004-0.2% of N, 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 (with the proviso that the N content is 0.004% or more), S*=S−0.8×(Ti 0.8×(48/14)×N)×(32/48) and N*=N−0.8×(Ti−0.8×(48/32)×S))×(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 0.004-0.2% 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.


0.8≦(Ti*/48)/(C/12)≦5.0  (6)


Ti*=Ti−0.8×((48/14)×N+(48/32)×S)  (7)

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.


Cs=(C−Ti*×12/48)×10000  (8)

(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 1×105/mm2 or more, more preferably 1×106/mm2 or more, and most preferably 1×107/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 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.

It is preferred that the hot rolling is performed at 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 preferably 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 (rm 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 rm and in plane anisotropy index (Δr value) were calculated by the following equations: rm=(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 Chemical Components (wt %) No. 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 Average size of Number of (Mn/55 + CuS CuS Sample Cu/63.5)/ (Ti★/48)/ precipitates precipitates No. S★ (S★/32) (C/12) (μm) (mm−2) A11 0.0059 14.443 2.01 0.06 3.2 × 106 A12 0.0102 4.4402 0.97 0.06 4.1 × 106 A13 0.0108 5.5975 1.02 0.06 4.5 × 106 A14 0.0071 5.6665 1.83 0.05 5.1 × 106 A15 0.0139 3.9764 1.12 0.05 4.3 × 106 A16 0.0122 4.5458 0 0.08 4.5 × 106 A17 0 0 7.58 0.08 6.7 × 104 S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48), Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S)

TABLE 3 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%) rm Δr AI (%) (DBTT-° C.) Remarks 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 Chemical Components (wt % ) No. 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 Average size of Number of (Mn/55 + (Mn, Cu)S (Mn, Cu)S Sample Cu/63.5)/ (Ti★/48)/ precipitates precipitates No. Cu + Mn S★ (S★/32) (C/12) (μm) (mm−2) A21 0.2 0.0057 19.173 1 0.04 4.5 × 106 A22 0.2 0.0089 11.972 1.01 0.04 5.2 × 106 A23 0.27 0.0096 14.994 0.86 0.03 6.3 × 106 A24 0.2 0.008 13.535 1.67 0.04 7.3 × 106 A25 0.19 0.0147 7.0611 1.04 0.04 8.9 × 106 A26 0.59 0.0125 26.566 −1.2 0.25 1.5 × 104 A27 0.16 −0.065 −1.398 10.5 0.16 4.3 × 104 S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48) Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S)

TABLE 6 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%) rm Δr AI (%) (DBTT-° C.) Remarks 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, AI = Aging Index, SWE = Secondary Working Embrittlement, 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 Chemical Components (wt %) No. 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 (Mn/55 + Average size of Number of Sample Cu/63.5)/ (Ti★/48)/ (Al/27)/ precipitates precipitates No. S★ (S★/32) (C/12) N★ (N★/14) (μm) (mm−2) A31 0.0072 5.5772 0.99 0.0031 5.78 0.04 3.9 × 106 A32 0.0059 8.5273 0.91 0.0034 6.41 0.04 5.5 × 106 A33 0.0077 10.539 0.83 0.0033 6.4 0.03 6.2 × 106 A34 0.007 6.47 0.85 0.0032 4.01 0.04 5.3 × 106 A35 0.0071 9.2382 1.11 0.0034 5.58 0.04 5.9 × 106 A36 0.0148 3.0737 0 0.0048 3.43 0.25 5.5 × 106 A37 0 0 17.2 0 −1.6 0.16 4.3 × 104 S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48), Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S) N★ = N − 0.8 × (Ti − 0.8 × (48/32) × S)) × (14/48)

TABLE 9 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%) rm Δr (DBTT-° C.) AI (%) Remarks 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, AI = Aging Index, SWE = Secondary Working Embrittlement, 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 Chemical Components (wt %) No. 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 0.0083 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 (Mn/55 + Average size of Number of Sample Cu/63.5)/ (Ti★/48)/ (Al/27)/ precipitates precipitates No. Cu + Mn S★ (S★/32) (C/12) N★ (N★14) (μm) (mm−2) A1 0.17 0.0042 22.453 1.5 0.0032 8.03 0.06 4.4 × 107 A2 0.21 0.007 16.123 1.06 0.004 4.93 0.05 7.0 × 107 A3 0.21 0.0069 16.435 1.03 0.0032 3.89 0.06 6.2 × 107 A4 0.16 0.0048 18.039 1.49 0.0032 7.97 0.06 5.9 × 107 A5 0.22 0.0102 11.7 0.95 0.0033 7.29 0.06 6.4 × 107 A6 0.55 0.0105 29.751 0 0.0038 5.15 0.25 1.5 × 104 A7 0.07 0 −0.542 9.8 0 0 0.04 3.5 × 105 S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48) Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S) N★ = N − 0.8 × (Ti − 0.8 × (48/32) × S)) × (14/48)

TABLE 12 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%) rm Δr AI(%) (DBTT-° C.) Remarks 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 Chemical Components (wt %) No. 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 Average/ size of Number of (Mn/55 + precipi- precipi- Sample Cu/63.5)/ (Ti★/48)/ tates tates No. S★ (S★/32) (C/12) (μm) (mm−2) A51 0.0045 14.211 1.78 0.06 3.3 × 105 A52 0.0079 5.8631 1.16 0.06 3.6 × 105 A53 0.01 6.3706 1.02 0.05 3.8 × 106 A54 0.01 5.255 0.93 0.05 3.6 × 106 A55 0.0135 4.3217 1.54 0.05 3.8 × 106 A56 0.0125 20.927 −1.2 0.26 2.6 × 103 A57 −0.065 −1.165 10.5 0.06 4.5 × 105 S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48) Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S)

TABLE 15 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%) rm Δr AI(%) (DBTT-° C.) Remarks 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 Chemical Components (wt %) No. 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 Average/size of Number of Sample (Ti★/48)/ (Al/27)/ precipitates precipitates No. (C/12) N★ (N★/14) (μm) (mm−2) A61 0.67 0.003 7.32 0.05 6.3 × 105 A62 1.03 0.0032 7.06 0.05 6.3 × 105 A63 1.31 0.0024 8.59 0.05 8.4 × 106 A64 0.81 0.0033 3.77 0.05 7.3 × 106 A65 1.12 0.0034 5.78 0.05 6.2 × 106 A66 −1.2 0.0048 3.43 0.05 4.5 × 105 A67 17.2 −0.018 −1.6 0.28 3.5 × 103 Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S) N★ = N − 0.8 × (Ti − 0.8 × (48/32) × S)) × (14/48),

TABLE 18 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%) rm Δr (DBTT-° C.) AI(%) Remarks 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, AI = Aging Index, SWE = Secondary Working Embrittlement, 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 Chemical Components (wt %) No. 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 (Mn/55 + Average size of Number of Sample Cu/63.5)/ (Ti★/48)/ (Al/27)/ precipitates precipitates No. S★ (S★/32) (C/12) N★ (N★/14) (μm) (mm−2) A71 0.0035 18.419 1.38 0.0031 8.79 0.05 6.3 × 105 A72 0.007 7.512 0.93 0.0042 4.64 0.05 6.3 × 105 A73 0.006 10.703 3.46 0.0031 2.5 0.05 8.4 × 106 A74 0.0045 12.864 1.61 0.003 8.51 0.05 7.3 × 106 A75 0.0102 6.2698 0.95 0.0033 7.29 0.05 6.2 × 106 A76 −0.018 −21.59 5.62 −0.009 −2.9 0.05 4.5 × 105 A77 0.0198 2.9383 −2.1 0.0095 0.44 0.28 3.5 × 103 S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48) Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S) N★ = N − 0.8 × (Ti − 0.8 × (48/32) × S)) × (14/48)

TABLE 21 Mechanical Properties Sample YS TS El SWE No. (MPa) (MPa) (%) rm Δr (DBTT-° C.) AI(%) Remarks 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 Chemical components (wt %) No. 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 Number of Sample Average size of precipitates No. (Cu/63.5)/(S★/32) Cs precipitates (μm) (mm−2) B81 12.8 19.043 0.06 1.8 × 106 B82 24 10.957 0.06 2.1 × 106 B83 8.52 18 0.06 2.5 × 106 B84 23.3 23.286 0.05 3.2 × 106 B85 24.9 13.843 0.06 4.1 × 106 B86 26.5 11.529 0.06 3.2 × 106 B87 27.4 15.471 0.05 4.1 × 106 B88 −2.8 −75 0.08 4.5 × 105 B89 0 33 0.08 6.2 × 104 S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48), Cs = (C − Ti★ × 12/48) × 10000, Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S)

TABLE 24 Mechanical Properties Sample YS TS El BH value SWE No. (MPa) (MPa) (%) rm Δr AI(%) (MPa) (DBTT-° C.) Remarks 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 Chemical Components (wt %) No. 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 (Mn/55 + Average size of Number of Sample Cu + Cu/63.5)/ precipitates precipitates No. Mn (S★/32) Cs (μm) (mm−2) B91 0.23 29.1 19 0.05 2.8 × 106 B92 0.28 17 18 0.05 2.5 × 106 B93 0.19 20.8 15 0.05 2.8 × 106 B94 0.21 29.6 27 0.05 2.9 × 106 B95 0.26 25.9 22 0.05 3.9 × 106 B96 0.23 20.1 19 0.04 2.5 × 106 B97 0.2 19.9 25 0.04 3.9 × 106 B98 0.47 −23 −39 0.25 1.7 × 104 B99 0.16 5.45 37 0.08 6.3 × 104 S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48), Cs = (C − Ti★ × 12/48) × 10000, Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S)

TABLE 27 Mechanical Properties Sample YS TS El BH value SWE No. (MPa) (MPa) (%) rm Δr AI(%) (MPa) (DBTT-° C.) Remarks 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 Chemical Components (wt %) No. 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 Average size of Number of Sample (Cu/63.5)/ (Al/27)/ precipitates precipitates No. (S★/32) (N★/14) Cs (μm) (mm−2) B01 2.57 2.1 19 0.06 2.8 × 106 B02 4.2 2.6 17 0.06 3.7 × 106 B03 3.04 1.81 18 0.06 3.5 × 106 B04 2.43 1.75 16 0.05 4.7 × 106 B05 5.63 3.81 26 0.04 5.5 × 106 B06 5.75 7.44 25 0.05 4.3 × 106 B07 7.41 2.97 22 0.04 5.2 × 106 B08 0 −2.8 −77 0.2 2.5 × 104 B09 1.67 2.03 54 0.05 4.4 × 106 S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48), Cs = (C − Ti★ × 12/48) × 10000, Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S) N★ = N − 0.8 × (Ti − 0.8 × (48/32) × S)) × (14/48)

TABLE 30 Mechanical Properties Sample YS TS El BH value SWE No. (MPa) (MPa) (%) rm Δr AI (MPa) (DBTT-° C.) Remarks 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 Chemical Components (wt %) No. 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 Average size of Number of (Mn/55 + precipi- precipi- Sample Cu + Cu/63.5)/ (Al/27)/ tates tates No. Mn (S★/32) (N★/14) Cs (μm) (mm−2)) B11 0.19 6.11 2.28 14 0.06 1.1 × 107 B12 0.21 6.91 3.23 16 0.06 9.5 × 106 B13 0.17 5.62 2.86 17 0.06 1.7 × 107 B14 0.21 6.66 1.7 12 0.05 1.9 × 107 B15 0.29 24.3 5.68 24 0.05 3.2 × 107 B16 0.2 4.42 1.27 25 0.05 3.8 × 107 B17 0.21 14.1 3.1 22 0.04 4.5 × 107 B18 0.52 −15 −2 −79 0.25 1.8 × 104 B19 0.2 8.66 4.85 42 0.06 8.3 × 105 S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48), Cs = (C − Ti★ × 12/48) × 10000, Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S) N★ = N − 0.8 × (Ti − 0.8 × (48/32) × S)) × (14/48)

TABLE 33 Mechanical Properties Sample YS TS El BH value SWE No. (MPa) (MPa) (%) rm Δr AI (MPa) (DBTT-° C.) Remarks 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 Chemical Components (wt %) No. 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 Number Sample Average size of of precipitates No. (Mn/55)/(S★/32) Cs precipitates (μm) (mm−2) B21 7.37 18 0.06 1.2 × 105 B22 4.11 15 0.06 1.2 × 105 B23 22.7 23.657 0.05 1.8 × 105 B24 5.76 25 0.05 2.2 × 106 B25 8.83 13.3 0.05 3.1 × 106 B26 8.65 26 0.04 3.7 × 106 B27 −14 −72 0.06 3.4 × 104 B28 18.8 24 0.22 2.3 × 103 S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48), Cs = (C − Ti★ × 12/48) × 10000, Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S)

TABLE 36 Mechanical Properties Sample YS TS El BH value SWE No. (MPa) (MPa) (%) rm Δr AI(%) (MPa) (DBTT-° C.) Remarks 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 Chemical Components (wt %) No. 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 Number Sample Average size of of precipitates No. (Al/27)/(N★/14) Cs precipitates (μm) (mm−2) B31 1.96 11 0.06 3.5 × 106 B32 3.74 14 0.06 3.2 × 106 B33 6.06 26 0.05 4.1 × 106 B34 6.65 17 0.05 5.3 × 106 B35 2.95 26 0.05 4.4 × 106 B36 3.18 28 0.04 5.9 × 106 B37 −3.6 −63 0.21 1.8 × 104 B38 1.79 38 0.07 2.2104 Cs = (C − Ti★ × 12/48) × 10000, Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S) N★ = N − 0.8 × (Ti − 0.8 × (48/32) × S)) × (14/48)

TABLE 39 Mechanical Properties Sample YS TS El BH value SWE No. (MPa) (MPa) (%) rm Δr AI(%) (MPa) (DBTT-° C.) Remarks 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 Chemical Components (wt %) No. C Mn P S Al Ti B N Others B41 0.0015 0.12 0.009 0.007 0.039 0.01 0.0008 0.0073 B42 0.0018 0.08 0.024 0.009 0.042 0.008 0.0005 0.0094 B43 0.0012 0.09 0.044 0.008 0.043 0.009 0.0007 0.0079 Si: 0.06 B44 0.0026 0.11 0.077 0.012 0.054 0.022 0.0008 0.011 Si: 0.12 B45 0.0018 0.11 0.11 0.016 0.052 0.051 0.0011 0.0125 Si: 0.11 B46 0.0021 0.1 0.041 0.013 0.067 0.033 0.0009 0.0083 Si: 0.09 Mo: 0.056 B47 0.0019 0.11 0.041 0.008 0.042 0.019 0.0006 0.0095 Cr: 0.33 B48 0.0016 0.68 0.045 0.009 0.048 0.052 0.0004 0.0021 B49 0.0037 0.1 0.114 0.01 0.008 0 0.0011 0.0067 Si: 0.05

TABLE 41 Average size of Number of Sample (Mn/55)/ (Al/27)/ precipitates precipitates No. (S★/32) (N★/14) Cs (μm) (mm−2) B41 5.66 2.92 15 0.06 5.1 × 106 B42 2.52 2.17 18 0.06 4.9 × 106 B43 3.55 2.77 12 0.06 5.8 × 106 B44 3.91 3.03 26 0.05 6.9 × 106 B45 9.03 5.31 18 0.05 8.1 × 106 B46 7.71 8.19 21 0.05 6.8 × 106 B47 5.44 2.98 19 0.04 8.8 × 106 B48 −25 −3.3 −62 0.21 1.8 × 104 B49 2.94 0.44 37 0.07 8.3 × 105 S★ = S − 0.8 × (Ti − 0.8 × (48/14) × N) × (32/48), Cs = (C − Ti★ × 12/48) × 10000, Ti★ = Ti − 0.8 × ((48/14) × N + (48/32) × S) N★ = N − 0.8 × (Ti − 0.8 × (48/32) × S)) × (14/48)

TABLE 42 Mechanical Properties Sample YS TS El BH value SWE No. (MPa) (MPa) (%) rm Δr AI(%) (MPa) (DBTT-° C.) Remarks 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

The preferred embodiments illustrated in the present invention do not serve to limit the present invention, but are set forth for illustrative purposes. Any embodiment having substantially the same constitution and the same operational effects thereof as the technical spirit of the present invention as defined in the appended claims is encompassed within the technical scope of the present invention.

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

1. A cold rolled steel sheet with superior formability, the cold rolled 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 Al, 0.004% 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×(Ti−0.8×(48/14)×N)×(32/48), and
wherein the steel sheet comprises CuS precipitates having an average size of 0.2 μm or less.

2. The cold rolled steel sheet according to claim 1, wherein the composition further comprises 0.01-0.3% of Mn, and 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.

3. The cold rolled steel sheet according to claim 1, wherein the N content is 0.004-0.02%, and the composition satisfies the following relationships: 1≦(Al/27)/(N*/14)≦10 and N=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises AlN precipitates having an average size of 0.2 μm or less.

4. The cold rolled steel sheet according to claim 1, 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≦(Al/27)/(N*/14)≦10 and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises (Mn,Cu)S precipitates and AlN precipitates having an average size of 0.2 μm or less.

5. A cold rolled steel sheet with superior formability, the cold rolled steel sheet having a composition comprising 0.01% or less of C, 0.08% or less of S, 0.1% or less of Al, 0.004% 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 and 0.004-0.2% of N, 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, where the N content is 0.004% or more, and S*=S−0.8×(Ti−0.8×(48/14)×N)×(32/48) and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and
wherein the steel sheet comprises at least one kind selected from (Mn,Cu)S and AlN precipitates having an average size of 0.2 μm or less.

6. The cold rolled steel sheet according to claim 1 or 5, wherein the C, Ti, N and S contents satisfy the following relationships: 0.8≦(Ti*/48)/(C/12)≦5.0 and Ti=Ti−0.8×((48/14)×N+(48/32)×S).

7. The cold rolled steel sheet according to claim 6, wherein the C content is 0.005% or less.

8. The cold rolled steel sheet according to claim 1 or 5, wherein solute carbon (Cs) [Cs=(C−Ti*×12/48)×10000 in which Ti*=Ti−0.8×((48/14)×N+(48/32)×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.

9. The cold rolled steel sheet according to claim 8, wherein the C content is from 0.001 to 0.01%.

10. The cold rolled steel sheet according to any one of claims 1 to 5, wherein the cold rolled steel sheet satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher.

11. The cold rolled steel sheet according to any one of claims 1 to 5, wherein the number of the precipitates is 1×106/mm2 or more.

12. The cold rolled steel sheet according to claim 1 or 5, wherein the P content is 0.015% or less.

13. The cold rolled steel sheet according to claim 1 or 5, wherein the P content is from 0.03% to 0.2%.

14. The cold rolled steel sheet according to claim 1 or 5, wherein the composition further comprises at least on kind of 0.1 to 0.8% of Si and 0.2 to 1.2% of Cr.

15. The cold rolled steel sheet according to claim 1 or 5, wherein the composition further comprises 0.01 to 0.2% of Mo.

16. The cold rolled steel sheet according to claim 14, wherein the composition further comprises 0.01 to 0.2% of Mo.

17. The cold rolled steel sheet according to any one of claim 2, 4 or 5, wherein the sum of Mn and Cu is from 0.05% to 0.4%.

18. The cold rolled steel sheet according to any one of claim 2, 4 or 5, wherein the Mn content is from 0.01 to 0.12%.

19. The cold rolled steel sheet according to claim 2, 4 or 5, wherein the value of (Mn/55+Cu/63.5)/(S*/32) is from 1 to 9.

20. The cold rolled steel sheet according to claim 3, 4 or 5, wherein the value of (Al/27)/(N*/14) is from 1 to 6.

21. 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 Al, 0.004% 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, the composition satisfying the following relationships: 1≦(Cu/63.5)/(S*/32)≦30 and S*=S−0.8×(Ti−0.8×(48/14)×N)×(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, the cold rolled steel sheet comprises CuS precipitates having an average size of 0.2 μm or less.

22. The method according to claim 21, wherein the composition further comprises 0.01 to 0.3% of Mn, and satisfies the following relationships: 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.

23. The method according to claim 21, wherein the N content is 0.004-0.02%, and the composition satisfies the following relationships: 1≦(Al/27)/(N*/14)≦10 and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises AlN precipitates having an average size of 0.2 μm or less.

24. The method according to claim 21, wherein the composition further comprises 0.01 to 0.3% of Mn and 0.004 to 0.02% of N, and satisfies the following relationships: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10 and N*=N−0.8×(Ti−0.8×(48/32)×S))×(14/48), and the steel sheet comprises (Mn,Cu)S precipitates and AlN precipitates having an average size of 0.2 μm or less.

25. 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 Al, 0.004% 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 and 0.004-0.2% of N, by weight, and the balance of Fe and other unavoidable impurities, the composition satisfying a relationship: 1≦(Mn/55+Cu/63.5)/(S*/32)≦30, 1≦(Al/27)/(N*/14)≦10, where the N content is 0.004% or more, S*=S−0.8×(Ti 0.8×(48/14)×N)×(32/48) and N*=N−0.8×(Ti−0.8×(48/32)×S))×(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, the cold rolled steel sheet comprises at least one kind selected from (Mn,Cu)S and AlN precipitates having an average size of 0.2 μm or less.

26. The method according to claim 21 or 25, wherein the C, Ti, N and S contents satisfy the following relationships: 0.8≦(Ti*/48)/(C/12)≦5.0 and Ti=Ti−0.8×((48/14)×N+(48/32)×S).

27. The method according to claim 26, wherein the C content is 0.005% or less.

28. The cold rolled steel sheet according to claim 21 or 25, wherein solute carbon (Cs) [Cs=(C−Ti*×12/48)×10000 in which Ti*=Ti−0.8×((48/14)×N+(48/32)×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.

29. The method according to claim 28, wherein the C content is from 0.001 to 0.01%.

30. The method according to any one of claims 21 to 25, wherein the cold rolled steel sheet satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher.

31. The method according to any one of claims 21 to 25, wherein the number of the precipitates is 1×106/mm2 or more.

32. The method according to claim 21 or 25, wherein the P content is 0.015% or less.

33. The method according to claim 21 or 25, wherein the P content is from 0.03% to 0.2%.

34. The method according to claim 21 or 25, wherein the composition further comprises at least one kind or 0.1 to 0.8% of Si and 0.2 to 1.2% of Cr.

35. The method according to claim 21 or 25, wherein the composition further comprises 0.01 to 0.2% of Mo.

36. The method according to claim 34, wherein the composition further comprises 0.01 to 0.2% of Mo.

37. The method according to claim 22, 24 or 25, wherein the sum of Mn and Cu is from 0.08% to 0.4%.

38. The method according to claim 22, 24 or 25, wherein the Mn content is from 0.01 to 0.12%.

39. The method according to claim 22, 24 or 25, wherein the value of (Mn/55+Cu/63.5)/(S*/32) is from 1 to 9.

40. The method according to claim 23, 24 or 25, wherein the value of (Al/27)/(N*/14) is from 1 to 6.

Patent History
Publication number: 20080149230
Type: Application
Filed: May 3, 2006
Publication Date: Jun 26, 2008
Applicant: POSCO (Nam-ku, Pohang)
Inventors: Jeong-Bong Yoon (Kyungsangbook-do), Sang-Ho Han (Kyungsangbook-do), Sung-Il Kim (Kyungsangbook-do), Man-Young Park (Kyungsangbook-do), Kwang-Geun Chin (Kyungsangbook-do), Ho-Seok Kim (Kyungsangbook-do), Jin-Hee Chung (Kyungsangbook-do)
Application Number: 11/913,176
Classifications
Current U.S. Class: With Working At Or Below 120c Or Unspecified Cold Working (148/603); Age Or Precipitation Hardened Or Strengthed (148/328)
International Classification: C21D 8/02 (20060101); C22C 38/16 (20060101);