CN101171355A - Cold-rolled steel sheet with high yield ratio and low anisotropy and manufacturing method thereof - Google Patents

Abstract

A Nb-Ti composite IF steel is disclosed, in which fine precipitates, such as CuS precipitates, with a particle size of less than or equal to 2 μm are distributed. The distribution of these fine precipitates in the Nb-Ti composite IF steel can improve yield strength and reduce the planar anisotropy index. The nanoscale precipitates can form fine grains. As a result, the amount of dissolved carbon present at the grain boundaries is greater than that within the grains, a characteristic that is beneficial for room temperature non-aging properties and bake hardening properties.

Description

Cold-rolled steel sheet with high yield ratio and low anisotropy and manufacturing method thereof

technical field

The present invention relates to a niobium (Nb)-based interstitial-free (IF) cold-rolled steel sheet that can be used as a material for automobiles, home electronic appliances, and the like. More specifically, the present invention relates to a high yield ratio IF cold-rolled steel sheet whose in-plane anisotropy is reduced due to the distribution of fine precipitates, and to a method for producing such a steel sheet.

Background technique

In general, cold-rolled steel sheets for automobiles and household electronic appliances are required to have excellent room temperature aging resistance and bake hardening properties, as well as high strength and excellent formability.

Aging is a strain aging phenomenon that results from hardening caused by the fixation of dissolved elements such as C and N in faults. Since aging causes defects called "tensile strain", it is important to ensure good resistance to room temperature aging.

Bake hardenability refers to the increase in strength due to the presence of dissolved carbon, which is the trace amount of carbon that remains in solid solution after press forming followed by painting and drying. A steel sheet having excellent bake hardenability can overcome difficulties in press formability due to high strength.

Aluminum (Al)-deoxidized steel can be imparted with room temperature aging resistance and bake hardenability by performing batch annealing. However, prolonging the batch annealing time will reduce the productivity of Al-deoxidized steel and cause severe changes in different parts of the steel. In addition, the bake hardening (BH) value (difference in yield strength before and after painting) of Al-deoxidized steel was 10–20 MPa, indicating a limited increase in yield strength.

In this case, interstitial-free (IF) steel with excellent room-temperature aging resistance and bake-hardenability can be formed by adding carbide and nitride-forming elements such as Ti and Nb, followed by continuous annealing.

For example, Japanese Patent Application Laid-Open Sho 57-041349 describes Ti-based IF steels whose strength is enhanced by adding 0.4-0.8% of manganese (Mn) and 0.04-0.12% of phosphorus (P). However, in very low carbon IF steels, P causes secondary processing embrittlement problems due to segregation at grain boundaries.

Japanese Patent Application Laid-Open (Hei) 5-078784 describes a method of increasing strength by adding Mn in an amount of more than 0.9% but not more than 3.0% as a solid solution strengthening element.

Korean Patent Application Laid-Open No. 2003-0052248 describes a method of improving secondary work embrittlement resistance as well as strength and workability by adding 0.5-2.0% of Mn, and replacing P with aluminum (Al) and boron (B).

Japanese Patent Application Publication Hei (Hei) 10-158783 describes a way to increase strength by reducing the P content and using Mn and Si as solid solution strengthening elements. According to the disclosure, Mn may be used in an amount up to 0.5%, Al as a deoxidizer is used in an amount of 0.1%, and nitrogen (N) as an impurity is limited to less than or equal to 0.01%. If the Mn content is increased, the plating characteristics will be deteriorated.

Japanese Patent Application Laid-Open (Hei) 6-057336 discloses a method for increasing the strength of IF steel, that is, adding 0.5-2.5% copper (Cu) to form ε-Cu precipitates. High-strength IF steels are obtained because of the presence of ε-Cu precipitates, but the machinability of IF steels decreases.

Japanese Patent Application Laid-Open Hei (Hei) 9-27951 and Hei 10-265900 propose techniques for improving machinability or improving surface defects caused by carbon due to the use of Cu as nuclei for precipitated carbides. According to the previous patent application publication, 0.005-0.1% Cu was added during tempering of IF steel to precipitate CuS, and the CuS precipitate was used as a crystal nucleus to form a Cu-Ti-C-S precipitate during hot rolling. In addition, the former patent application publication indicates that during recrystallization, the number of crystal nuclei forming {111} planes parallel to the steel sheet surface increases near Cu-Ti-C-S precipitates, which can improve workability. According to the publication of the latter patent application, 0.01-0.05% Cu is added to IF steel to obtain CuS precipitate, and then this CuS precipitate is used as the crystal nucleus of precipitated carbide to reduce the amount of dissolved carbon (C), thereby Improve surface defects. According to the prior art, since CuS coarse precipitates are used in the manufacture of cold-rolled steel sheets, carbides remain in the manufactured product. In addition, since the added amount of emulsion-forming elements such as Ti and Zr is larger than that of sulfur (S) in atomic weight ratio, most of sulfur (S) reacts with Ti or Zr instead of Cu.

On the other hand, Japanese Patent Application Laid-Open Hei (Hei) 6-240365 and Hei (Hei) 7-216340 describe adding a combination of Cu and P to improve the corrosion resistance of bake-hardening type IF steel. According to these patent application publications, the addition of Cu in an amount of 0.05-1.0% ensures improved corrosion resistance. However, in reality, the amount of Cu added is greater than or equal to 0.2%.

Japanese Patent Publication Hei (Hei) 10-280048 Hei (Hei) 10-287954 proposes to dissolve carbon sulfide (Ti-C-S base) in carbide during reheating and annealing to obtain a solid solution at the grain boundary, thus A bake hardening (BH) value (difference in yield strength before and after baking) of 30 MPa or more can be obtained.

According to the aforementioned disclosure, the strength can be improved by strengthening the 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 crystal nucleus of precipitated carbides. But there is no mention in these publications of increasing the high yield ratio (ie, yield strength/tensile strength) and reducing the in-plane anisotropy index. If the ratio of tensile strength to yield strength (ie yield ratio) of an IF steel is high, the thickness of the IF steel plate can be reduced, which can effectively reduce the weight. Furthermore, if an IF steel plate has a lower planar anisotropy index, less wrinkling and earing would occur during and after processing, respectively.

Contents of the invention

technical problem

An object of some embodiments of the present invention is to provide a Nb-based IF cold-rolled steel sheet and a method of manufacturing the same, which can achieve a high yield ratio and a low planar anisotropy index.

Another object of some embodiments of the present invention is to provide a method of manufacturing such a steel plate.

Technical solutions

According to one aspect of the present invention, there is provided a cold-rolled steel sheet having a high yield ratio and a low planar anisotropy index, the cold-rolled steel sheet has a composition comprising the following components: in terms of weight %, less than or equal to 0.01% of C, 0.01-0.2% of Cu, 0.005-0.08% of S, less than or equal to 0.1% of Al, less than or equal to 0.004% of N, less than or equal to 0.2% of P, 0.001-0.002% of B, 0.002-0.04% of Nb, and the balance of Fe and other unavoidable impurities, the composition satisfies the following relationship: 1≤(Cu/63.5)/(S/32)≤30, this steel plate contains CuS with an average particle size less than or equal to 0.2μm Precipitate.

In an embodiment of the present invention, the cold-rolled steel sheet has a composition comprising the following components: in terms of weight percent, less than or equal to 0.01% of C, 0.01-0.2% of Cu, 0.005-0.08% of S, less than or equal to 0.1% % of Al, less than or equal to 0.004% of N, less than or equal to 0.2% of P, 0.001-0.002% of B, 0.002-0.04% of Nb, and the balance of Fe and other unavoidable impurities, wherein, the composition Satisfying the following relationship: 1≤(Mn/55+Cu/63.5)/(S/32)≤30, the steel plate contains (Mn, Cu)S precipitates with an average particle size less than or equal to 0.2 μm.

In another embodiment of the present invention, the cold-rolled steel sheet has a composition comprising the following components: in terms of weight percent, less than or equal to 0.01% of C, 0.01-0.2% of Cu, 0.005-0.08% of S, less than or equal to Al equal to 0.1%, N less than or equal to 0.004%, P less than or equal to 0.2%, B 0.001-0.002%, Nb 0.002-0.04%, and the balance of Fe and other unavoidable impurities, wherein, The composition satisfies the following relational formula: 1≤(Cu/63.5)/(S/32)≤30, 1≤(Al/27)/(N/14)≤10, the steel plate contains (Mn, Cu)S precipitates.

In yet another embodiment of the present invention, the cold-rolled steel sheet has a composition comprising the following components: in terms of weight percent, less than or equal to 0.01% of C, 0.01-0.2% of Cu, 0.005-0.08% of S, less than or equal to 0.1% of Al, less than or equal to 0.004% of N, less than or equal to 0.2% of P, 0.001-0.002% of B, 0.002-0.04% of Nb, and the balance of Fe and other unavoidable impurities, wherein, the The composition satisfies the following relationship: 1≤(Mn/55+Cu/63.5)/(S/32)≤30, 1≤(Al/27)/(N/14)≤10, the steel plate contains an average grain size less than or equal to Precipitation of (Mn, Cu)S and AlN at 0.2 μm.

According to another aspect of the present invention, there is provided a cold-rolled steel sheet having a high yield ratio and a low in-plane anisotropy index, the cold-rolled steel sheet having a composition comprising: in terms of weight %, less than or equal to 0.01% C, less than or equal to 0.08% of S, less than or equal to 0.1% of Al, less than or equal to 0.004% of N, 0.2% of P, 0.0001-0.002% of B, 0.002-0.04% of Nb, selected from the following At least one substance: 0.01-0.2% of Cu, 0.01-0.3% of Mn and 0.004-0.2% of N, and the balance of Fe and other unavoidable impurities, wherein the composition satisfies the following relationship: 1≤( Mn/55+Cu/63.5)/(S/32)≤30, 1≤(Al/27)/(N/14)≤10, the N content is greater than or equal to 0.004%, and the steel plate contains an average grain size of less than or equal to 0.2 μm of at least one substance selected from (Mn, Cu)S precipitation and AlN precipitation.

In order to achieve the non-aging property at room temperature, the weight content of C and Nb satisfies the following relationship: 0.8≤(Nb/93)/(C/12)≤5.0. Also, for bake hardenability, solute carbon (Cs) is 5-30, where Cs=(C-Nb×12/93)×10,000.

Depending on the design of the composition, the cold-rolled steel sheet of the present invention is characterized as a soft cold-rolled steel sheet with a strength on the order of 280 MPa and a high-strength cold-rolled steel sheet with a strength on the order of 340 MPa or greater.

When the P content of the composition of the present invention is less than or equal to 0.015%, soft cold-rolled steel sheets of the order of 280 MPa can be produced. When the soft cold-rolled steel sheet also contains at least one solid solution strengthening element selected from Si and Cr, or when the P content is in the range of 0.015-0.2%, a high strength greater than or equal to 340 MPa can be achieved. The content of P in the high-strength steel containing only P is preferably 0.03-0.2%. The Si content of the high strength steel is preferably in the range of 0.1-0.8%. The Cr content of the high strength steel is preferably 0.2-1.2%. When the cold-rolled steel sheet of the present invention contains at least one element selected from Si and Cr, the P content can be freely limited to be less than or equal to 0.2%.

In order to achieve better workability, the cold-rolled steel sheet of the present invention may also contain 0.01-0.2% by weight of Mo.

According to yet another aspect of the present invention, there is provided a method of manufacturing a cold-rolled steel sheet, the method comprising the steps of: reheating a slab satisfying one of the compositions described above to a temperature higher than or equal to 1,100° C.; Hot-rolling the reheated slab at the finish rolling temperature of the specified point to provide hot-rolled steel plate; cooling the hot-rolled steel plate at a rate of 300°C/min; coiling the cooled steel plate at a temperature lower than or equal to 700°C; Cold-rolling the coiled steel sheet; and continuous annealing of the cold-rolled steel sheet.

best way

The present invention is described in detail below.

Fine precipitates with a particle size of less than or equal to 0.2 μm are distributed in the cold-rolled steel sheet of the present invention. Examples of such precipitates include MnS precipitates, CuS precipitates, and composite precipitates of MnS and CuS. These precipitates are abbreviated as "(Mn,Cu)S".

The present inventors have found that when fine precipitates are distributed in Nb-based IF steel, the yield strength of the IF steel increases and the plane anisotropy index decreases, thus resulting in improved machinability. The present invention has been accomplished based on these findings. The precipitates used in the present invention have received little attention in conventional IF steels. In particular, such precipitates are not actively used with regard to yield strength and planar anisotropy index.

The composition in Nb-based IF steels needs to be adjusted to obtain (Mn, Cu)S precipitates and/or AlN precipitates. If the IF steel contains Ti, Nb, Zr and other elements, S preferably reacts with Ti and Zr. Since the cold-rolled steel sheet of the present invention is Nb-added IF steel, S of (Mn, Cu)S is precipitated by adjusting the contents of Cu and Mn. N is precipitated as AlN by adjusting the content of Al and N.

The fine precipitate thus obtained is capable of forming fine grains. The fine grain size of the grains relatively increases the proportion of grain boundaries. Therefore, the amount of dissolved carbon present at the grain boundaries is greater than that within the grains, thus achieving excellent room temperature non-aging properties. Since the dissolved carbon present in the grains can migrate more freely, bind to mobile faults, thus affecting the room temperature aging properties. Conversely, dissolved carbon segregated at stable locations (eg, near grain boundaries and precipitates) is activated at higher temperatures (eg, those at which painting/bake treatments are performed), thus affecting bake hardenability.

The fine precipitates distributed in the steel sheet of the present invention have a positive effect on the increased yield strength due to the reinforcement of the precipitates, improved strength-ductility balance, planar anisotropy index and plastic anisotropy. Therefore, fine (Mn, Cu)S precipitates and AlN precipitates must be uniformly distributed. According to the cold-rolled steel sheet of the present invention, the content of the components affecting the deposits, the composition between the components, the preparation conditions and the specific cooling rate after hot rolling all have a great influence on the distribution of the fine deposits.

The composition components of the cold-rolled steel sheet according to the present invention will be explained below.

The carbon (C) content is preferably limited to less than or equal to 0.01%. Carbon (C) affects the room temperature aging resistance and bake hardenability of the cold-rolled steel sheet. When the carbon content exceeds 0.01%, it is necessary to add expensive reagents Nb and Ti to remove the residual carbon, which is economically unfavorable and also undesirable in terms of formability. When only the room temperature aging resistance is intended, it is preferable to keep the carbon content low, which can reduce the addition of expensive reagents Nb and Ti. When it is intended to ensure the required bake hardenability, the amount of carbon added is preferably greater than or equal to 0.001%, more preferably 0.005-0.01%. When the carbon content is less than 0.005%, it is not necessary to increase the amount of Nb and Ti to ensure room temperature resistance Aging.

The copper (Cu) content is preferably in the range of 0.01-0.2%.

The role of copper is to form fine CuS precipitates, which can make the crystal grains fine. Copper reduces the planar anisotropy index of cold-rolled steel sheets and increases the yield strength of cold-rolled steel sheets by promoting precipitation. In order to form fine precipitates, Cu precipitates must be greater than or equal to 0.01%. When the Cu content is greater than 0.2%, a coarse precipitate is obtained. The Cu content is more preferably in the range of 0.03-0.2%.

The manganese (Mn) content is preferably in the range of 0.01-0.3%.

The role of manganese is to convert the sulfur precipitates in the solid solution state in the steel into MnS precipitates, thereby preventing hot embrittlement caused by dissolved sulfur (or called solid solution reinforcing elements). From these technical viewpoints, a large amount of manganese is generally added. The inventors have found that when manganese content is reduced and sulfur content is optimized, very fine MnS precipitates are obtained. Based on this finding, the manganese content was limited to less than or equal to 0.3%. In order to ensure this characteristic, the manganese content must be greater than or equal to 0.01%. When the manganese content is less than 0.01%, that is, when the residual sulfur content in solid solution is high, hot brittleness may occur. When the manganese content is greater than 0.3%, coarse MnS precipitates are formed, so it is difficult to achieve the required strength. A more preferred Mn content is 0.01-0.12%.

The sulfur (S) content 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 increases. An increase in dissolved sulfur significantly deteriorates the ductility and formability of the steel sheet and increases the risk of hot embrittlement. In order to obtain as many CuS and/or MnS precipitates as possible, the sulfur content is preferably greater than or equal to 0.005%.

The aluminum (Al) content is preferably limited to 0.1% or less.

Aluminum reacts with nitrogen (N) to form fine AlN precipitates, thereby completely preventing aging caused by dissolved nitrogen. When the nitrogen content is greater than or equal to 0.004%, AlN precipitates are sufficiently formed. The fine AlN precipitates are distributed in the steel plate, which can form fine grains and increase the yield strength of the steel plate by strengthening the precipitation. The Al content is more preferably in the range of 0.01-0.1%.

The nitrogen (N) content is preferably limited to less than or equal to 0.02%.

When it is intended to use AlN precipitates, nitrogen can be added in an amount of up to 0.02%. In addition, the nitrogen content is controlled to be less than or equal to 0.004%. When the nitrogen content is less than 0.004%, the amount of AlN precipitates is small, so the small effect of grains and the effect of precipitation enhancement are negligible. On the contrary, when the nitrogen content is more than 0.02%, it is also difficult to secure aging properties by using dissolved nitrogen.

The phosphorus (P) content is preferably limited to 0.2% or less.

Phosphorus is an element that has an excellent solid solution strengthening effect and can slightly reduce the r-value. Phosphorus ensures the high strength of the steel plate of the present invention and controls precipitation in the steel plate. The ideal phosphorus content in steel requiring a strength on the order of 280 MPa is limited to less than or equal to 0.015%. The ideal phosphorus content for high strength steels with strengths on the order of 340 MPa is limited to greater than 0.015% but not greater than 0.2%. A phosphorus content exceeding 0.2% may result in a decrease in the ductility of the steel sheet. Therefore, 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 properly controlled to be less than or equal to 0.2%, so as to achieve the required strength.

The boron (B) content is preferably in the range of 0.0001-0.002%.

Boron is added to prevent secondary processing embrittlement. Therefore, it is preferable that the boron content is greater than or equal to 0.0001%. When the boron content exceeds 0.002%, the deep drawability of the steel sheet is obviously deteriorated.

The niobium (Nb) content is preferably in the range of 0.002-0.04%.

The purpose of adding Nb is to secure the non-aging properties and improve the formability of the steel sheet. Nb is a potential carbide-forming element, and niobium is added to steel to form NbC precipitates in the steel. In addition, NbC precipitates make the steel sheet well-textured during annealing, thus significantly improving the deep-drawability of the steel sheet. When the added Nb content was not more than 0.002%, a very small amount of NbC precipitate was formed. Therefore, the steel sheet cannot be textured well, and thus, the deep drawability of the steel sheet is hardly improved. In contrast, when the Nb content is greater than 0.04%, a very large amount of NbC precipitates are formed. Therefore, the deep drawability and elongation of the steel sheet are lowered, so that the formability of the steel sheet is remarkably deteriorated.

In order to form (Mn,Cu)S and AlN precipitates, the contents of Mn, Cu, S, Al and N are adjusted within the range defined by the following relationship. Each component indicated in the following relational formula is expressed in % by weight.

1≤(Cu/63.5)/(S/32)≤30 (1)

Relation 1 is related to the formation of (Mn,Cu)S precipitates. In order to obtain fine CuS precipitates, the value of relational expression 1 is preferably greater than or equal to 1. If the value of this relational expression 1 is greater than 30, coarse CuS precipitates are distributed, which is not desirable. In order to stably obtain CuS precipitates with a particle size less than or equal to 0.2 μm, the value of relational expression 1 is preferably in the range of 1-9, more preferably 1-6. The reason for this limitation is for the formation of fine (Mn,Cu)S precipitates.

1≤(Mn/55+Cu/63.5)/(S/32)≤30 (2)

Relation 2 is related to the formation of (Mn,Cu)S precipitates, which is obtained by adding the Mn content to Relation 1. For efficient formation of (Mn,Cu)S precipitates, the value of relation 2 must be greater than or equal to 1. When the value of relational expression 2 is greater than 30, coarse (Mn,Cu)S precipitates are formed. In order to stably obtain CuS precipitates with a particle size less than or equal to 0.2 μm, the value of relational expression 2 is preferably in the range of 1-9, more preferably 1-6.

1≤(Al/27)/(N/14)≤10 (3)

Relation 3 is related to the formation of AlN precipitates. When the value of relational expression 3 is less than 1, aging may occur due to dissolved N. When the value of Relational Expression 3 is greater than 10, coarse AlN precipitates are formed and thus sufficient strength cannot be obtained. The value of relational expression 3 is preferably in the range of 1-5.

The components of the cold-rolled steel sheet of the present invention may be combined in various ways depending on the kind of precipitates to be obtained. For example, the present invention provides a cold-rolled steel sheet having a high yield ratio and a low planar anisotropy index, the cold-rolled steel sheet having a composition comprising: in terms of weight percent, less than or equal to 0.01% of C, less than or equal to 0.08% of S, less than or equal to 0.1% of Al, less than or equal to 0.004% of N, 0.2% of P, 0.0001-0.002% of B, 0.002-0.04% of Nb, at least one selected from the following: 0.01 -0.2% of Cu, 0.01-0.3% of Mn and 0.004-0.2% of N, and the balance of Fe and other unavoidable impurities, wherein the composition satisfies the following relationship: 1≤(Mn/55+Cu/63.5 )/(S/32)≤30, 1≤(Al/27)/(N/14)≤10, and the N content is greater than or equal to 0.004%. Then, the steel plate contains at least one precipitate selected from the group consisting of: Nn[Mn? ]S precipitates, CuS precipitates, composite precipitates of MnS and CuS, and AlN precipitates, the average particle size of which is less than or equal to 0.2 μm. That is, one or more substances selected from 0.01-0.2% of Cu, 0.01-0.3% of Mn, and 0.004-0.2% of N produce each of (Mn, Cu)S and AlN precipitates with a particle size of not more than 0.2 μm. kind of combination.

In the steel sheet of the present invention, the carbon precipitates are in the form of NbC and TiC. Therefore, the room temperature aging resistance and bake hardenability of the steel sheet are affected by the state of dissolved carbon in which no NbC and TiC precipitates are formed. In consideration of these requirements, it is most preferable that the contents of Nb, Ti and C satisfy the following relationship.

0.8≤(Nb/93)/(C/12)≤5.0 (4)

Relation 4 is related to the formation of NbC precipitates to remove solid solution carbon, thereby achieving room temperature non-aging properties. When the value of relational expression 4 is less than 0.8, it is difficult to guarantee the room temperature non-aging property. On the contrary, when the value of relational expression 4 is greater than 5, the amount of Nb and Ti remaining in the steel in the solid solution state is relatively large, which will deteriorate the ductility of the steel. When it is intended to obtain room temperature aging resistance without ensuring bake hardenability, it is preferable to limit the carbon content to 0.005% or less. Although the carbon content is more than 0.005%, the room temperature non-aging property can be obtained when the relationship 4 is satisfied, but the amount of NbC precipitates increases, thus deteriorating the workability of the steel sheet.

Cs (solute carbon): 5-30, here

Cs＝(C-Nb×12/93)×10,000 (5)

Relation 5 is related to achieving bake hardenability. Cs represents the content of dissolved carbon in ppm. In order to obtain a high bake hardening value, the Cs value must be greater than or equal to 5 ppm. If the Cs value exceeds 30 ppm, the content of dissolved carbon increases, making it difficult to obtain room temperature non-aging properties.

It is advantageous that the fine precipitates are evenly distributed in the composition of the invention. The average particle size of the precipitate is preferably less than or equal to 0.2 μm. According to studies conducted by the present inventors, when the average particle size of the precipitates is larger than 0.2 μm, the strength of the steel sheet is poor and the planar anisotropy index is low. In addition, a large number of precipitates with a particle size smaller than or equal to 0.2 μm are distributed in the composition of the present invention. Although there is no particular limitation on the amount of precipitates distributed, it is more advantageous to have a larger amount of precipitates. The number of distributed precipitates is preferably greater than or equal to 1×10 ⁵ /mm ² , more preferably greater than or equal to 1×10 ⁶ /mm ² , most preferably greater than or equal to 1×10 ⁷ /mm ² . By increasing the amount of precipitates, the plasticity-anisotropy index was increased and the planar anisotropy index was decreased, resulting in significantly improved processability. There is a known limit to the increase in processability, since the planar anisotropy index increases with plasticity-anisotropy index. It is worth noting that when the amount of precipitates distributed in the steel plate of the present invention increases, the plasticity-anisotropy index of the steel plate increases, while the planar anisotropy index of the steel plate decreases. The steel plate of the present invention in which fine precipitates are formed satisfies the requirement for a yield strength ratio (yield strength/tensile strength) of 0.58 or more.

When the steel plate of the present invention is applied to a high-strength steel plate with a strength on the order of 340 MPa, the steel plate may also contain at least one solid solution reinforcing element selected from P, Si and Cr. The additional functions of P have been explained above, so their explanations are omitted.

The silicon (Si) content is preferably in the range of 0.1-0.8%.

Si is an element with a solid solution strengthening effect, which slightly reduces the elongation. Si ensures the high strength of the steel sheet of the present invention and controls precipitation in the steel sheet. Only when the Si content is greater than or equal to 0.1%, high strength can be guaranteed. However, when the Si content is greater than 0.8%, the ductility of the steel sheet deteriorates.

Chromium (Cr) content is preferably in the range of 0.2-1.2%.

Cr is an element with a solid solution strengthening effect. Cr reduces the secondary processing embrittlement temperature and reduces the aging index due to the formation of Cr carbides. Cr ensures the high strength of the steel plate of the present invention, controls the precipitation in the steel plate, and has the effect of reducing the planar anisotropy index of the steel plate. Only when the Cr content is greater than or equal to 0.2%, high strength can be guaranteed. However, when the Cr content exceeds 1.2%, the ductility of the steel sheet deteriorates.

The cold-rolled steel sheet of the present invention further contains molybdenum (Mo).

The content of molybdenum (Mo) in the cold-rolled steel sheet of the present invention is preferably in the range of 0.01-0.2%.

Adding Mo as an element can improve the plasticity-anisotropy index of the steel plate. Only when the molybdenum content is not less than 0.01%, the plasticity-anisotropy index of the steel plate can be improved. However, when the molybdenum content exceeds 0.2%, the plasticity-anisotropy index does not increase any more, and there is a danger of hot embrittlement.

Manufacture of cold-rolled steel sheets

Next, a method of manufacturing the cold-rolled steel sheet of the present invention will be described with reference to the following preferred embodiments. Various modifications can be made to the embodiment of the present invention, and these modifications are within the scope of the present invention.

The method of the present invention is characterized in that the steel meeting one of the steel compositions defined above can be processed by performing hot rolling and cold rolling to form precipitates with an average particle size less than or equal to 0.2 μm in the cold-rolled steel sheet. The average particle size of precipitates in cold-rolled steel sheets is affected by steel composition design and processing conditions, such as reheating temperature and coiling temperature. In particular, the cooling rate after hot rolling directly affects the average particle size of the precipitate.

Hot rolling condition

In the present invention, a steel material satisfying one of the compositions defined above is reheated and then hot rolled. The reheating temperature is preferably higher than or equal to 1,100°C. When the steel is reheated to a temperature below 1,100° C., the coarse precipitate formed during continuous casting cannot be completely dissolved and remains. Coarse precipitates remain even after hot rolling.

Hot rolling is preferably performed at a finish rolling temperature not lower than the Ar ₃ transformation point. When the finish rolling temperature is lower than the Ar ₃ transformation point, rolled grains are generated, which deteriorate workability and lower strength.

Cooling is preferably performed at a rate greater than or equal to 300° C./min before coiling but after hot rolling. Although the composition of the components was controlled to form fine precipitates, the average particle size of the precipitates formed at a cooling rate of less than 300° C./min was greater than 0.2 μm. That is, many crystal nuclei are generated as the cooling rate increases, so the grain size of the precipitate becomes finer and finer. Since the particle size of the precipitate decreases as the cooling rate increases, it is not necessary to limit the upper limit of the cooling rate. However, when the cooling rate is greater than 1,000° C./minute, there is no significantly enhanced effect of reducing the particle size of the precipitate. Therefore, the cooling rate is preferably 300-1000°C/min.

Winding conditions

After hot rolling, coiling is carried out at a temperature not exceeding 700°C. When the winding temperature is higher than 700°C, the precipitate formed is too coarse, so it is difficult to ensure high strength.

Cold rolling condition

The steel is cold rolled at a reduction rate of 50-90%. Because when the reduction ratio of cold rolling is less than 50%, a small number of crystal nuclei are produced after annealing and recrystallization, and the grains are excessively grown by annealing, making the grains recrystallized by annealing thicker, resulting in a decrease in strength and formability. A cold rolling reduction of more than 90% results in improved formability while producing too many crystal nuclei, and therefore, crystal grains recrystallized by annealing become too fine, which deteriorates the ductility of the steel.

continuous annealing

The continuous annealing temperature plays an important role in determining the mechanical properties of the final product. According to the invention, the continuous annealing is preferably carried out at a temperature of 700-900°C. When the continuous annealing is carried out at a temperature below 700°C, the recrystallization is not complete, so the required ductility cannot be guaranteed. On the contrary, when continuous annealing is carried out at a temperature higher than 900°C, the recrystallized grains become coarser, and thus the strength of the steel material deteriorates. Continue annealing until the steel has completed recrystallization. The recrystallization of steel is completed within 10 seconds or more. Continuous annealing is preferably performed for 10 seconds to 30 minutes.

Modes of Carrying Out the Invention

The present invention is illustrated in more detail with reference to the following examples.

The mechanical properties of the steel sheets produced in the following examples were evaluated according to the ASTM E-8 standard test method. Specifically, each steel plate was machined to prepare a standard sample. Yield strength, tensile strength, elongation, plasticity-anisotropy index (rm value) and planar anisotropy index (Δr value) and aging index were measured using a tensile strength testing machine (obtained from INSTRON Company as model 6025). The plasticity-anisotropy index rm and the planar anisotropy index (Δr value) are calculated using the following equations, respectively: r _m = (r ₀ +2r ₄₅ +r ₉₀ )/4 and Δr = (r ₀ -2r ₄₅ +r ₉₀ )/2.

The aging index of the steel sheet was defined as the elongation at yield point measured by annealing each sample, followed by 1.0% skin pass rolling and heat treatment at 100° C. for 2 hours. The bake hardening (BH) values of the standard samples were determined by applying a 2% strain to each sample and annealing the strained samples at 170°C for 20 minutes. The yield strength of the samples after annealing was determined. The BH value is calculated by subtracting the yield strength value measured before annealing from the yield strength value measured after annealing.

Example 1

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 1

No. Chemical composition (weight%) C Cu S al N P B Nb other A11 0.0008 0.15 0.008 0.029 0.0014 0.048 0.0005 0.009 - A12 0.0012 0.17 0.007 0.024 0.0014 0.052 0.0002 0.011 Si: 0.02 A13 0.0021 0.09 0.018 0.044 0.0019 0.083 0.0007 0.027 Si: 0.15 A14 0.0031 0.12 0.011 0.028 0.0024 0.118 0.0011 0.024 Si: 0.25 A15 0.0029 0.08 0.009 0.038 0.0018 0.085 0.0008 0.021 Si: 0.17Mo: 0.08 A16 0.0023 0.11 0.011 0.039 0.0029 0.088 0.001 0.032 Si: 0.18Cr: 0.19 A17 0.0024 0.11 0.01 0.035 0.0018 0.053 0 0 A18 0.0044 0 0.008 0.024 0.0021 0.122 0.0007 0.077

Table 2

No. (Cu/63.5)/(S/32) (Nb/93)/(C/12) Average particle size of CuS precipitate (□) Number of CuS precipitates (/mm ² ) A11 9.45 1.45 0.05 2.2×10 ⁷ A12 12.2 1.18 0.06 1.2×10 ⁷ A13 2.52 1.66 0.05 3.2×10 ⁷ A14 5.5 1 0.05 3.2×10 ⁷ A15 4.48 0.93 0.05 4.1×10 ⁷ A16 5.04 1.8 0.05 5.3×10 ⁷ A17 5.54 0 0.06 5.5×10 ⁶ A18 0 2.26 0.05 5.4×10 ³

table 3

No. mechanical properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI(%) SWE(DBTT-℃) A11 216 347 46 2.01 0.25 0 -70 IS A12 224 354 44 1.97 0.24 0 -70 IS A13 262 409 38 1.78 0.22 0 -60 IS A14 327 464 35 1.61 0.21 0 -50 IS A15 321 457 34 1.72 0.23 0 -50 IS A16 335 462 34 1.69 0.21 0 -60 IS A17 239 348 42 1.18 0.29 0.62 -70 CS

A18 325 465 25 1.49 0.48 0 -50 CS

*Note:

YS=yield strength, TS=tensile strength, El=elongation, r _m =plasticity-anisotropy index, Δr=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

Example 2

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 4

No. Chemical composition (weight%) C mn Cu S al N P B Nb other A21 0.0015 0.07 0.09 0.011 0.042 0.0024 0.041 0.001 0.032 A22 0.0016 0.05 0.05 0.015 0.04 0.0018 0.04 0.0007 0.011 Si: 0.04 A23 0.0028 0.08 0.08 0.015 0.03 0.0023 0.086 0.0007 0.031 Si: 0.24 A24 0.0018 0.07 0.12 0.007 0.05 0.0019 0.125 0.005 0.035 Si: 0.35 A25 0.0019 0.09 0.09 0.011 0.048 0.0023 0.08 0.0009 0.027 Si: 0.23Mo: 0.09 A26 0.0029 0.11 0.1 0.009 0.039 0.0031 0.075 0.001 0.031 Si: 0.31Cr: 0.24 A27 0.0038 0.42 0 0.0083 0.038 0.0024 0.052 0.005 0.051 A28 0.0015 0.07 0.08 0.012 0.032 0.0021 0.118 0 0 Si: 0.1

table 5

No. Mn+Cu (Mn/55+Cu/63.5)/(S/32) (Nb/93)/(C/12) Average particle size of CuS precipitates (□) Amount of CuS precipitate (/mm ² ) A21 0.16 7.27 2.77 0.05 6.2×10 ⁷ A22 0.1 3.62 0.89 0.04 5.5×10 ⁷ A23 0.16 5.79 1.43 0.04 6.0×10 ⁷ A24 0.19 14.5 2.51 0.03 7.2×10 ⁷ A25 0.18 8.88 1.83 0.04 6.5×10 ⁷ A26 0.21 12.7 1.38 0.04 6.9×10 ⁷ A27 0.42 29.4 1.73 0.25 1.5×10 ⁴ A28 0.15 6.75 0 0.06 5.3×10 ⁶

Table 6

No. mechanical properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI (%) SWE(DBTT-℃) A1 226 362 44 2.03 0.22 0 -70 IS A2 225 348 45 2.12 0.19 0 -70 IS A3 282 402 39 1.87 0.21 0 -60 IS A4 338 451 34 1.68 0.16 0 -50 IS A5 329 449 34 1.88 0.22 0 -50 IS A6 383 452 35 1.64 0.19 0 -50 IS A7 188 342 42 1.77 0.39 0 -60 CS A8 378 463 30 1.25 0.28 0.42 -50 CS

*Note:

YS=yield strength, TS=tensile strength, El=elongation, r _m =plasticity-anisotropy index, Δr=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

Example 3

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 7

No. Chemical composition (weight%) C Cu S al N P B Nb other A31 0.0019 0.17 0.009 0.024 0.0054 0.052 0.0007 0.028 A32 0.0015 0.06 0.009 0.044 0.0077 0.045 0.0004 0.031 Si: 0.03 A33 0.0024 0.11 0.01 0.068 0.008 0.085 0.0006 0.037 Si: 0.11 A34 0.0024 0.17 0.011 0.058 0.014 0.126 0.0004 0.025 Si: 0.18 A35 0.0028 0.12 0.009 0.043 0.0093 0.044 0.0005 0.032 Si: 0.07Mo: 0.06 A36 0.0025 0.09 0.012 0.033 0.012 0.039 0.0009 0.021 Cr: 0.22 A37 0.0036 0.35 0.01 0.034 0.0012 0.042 0.0005 0.074 A38 0.0014 0.42 0.009 0.055 0.0067 0.12 0.0005 0 Si: 0.13

Table 8

No. (Cu/63.5)/(S/32) (Nb/93)/(C/12) (Al/27)/(N/14) Average particle size of CuS precipitates (□) Amount of CuS precipitate (/mm ² ) A31 9.44 1.92 2.31 0.04 6.4×10 ⁷ A32 3.36 2.67 2.96 0.04 7.5×10 ⁷ A33 5.54 1.99 4.41 0.04 7.0×10 ⁷ A34 7.79 1.34 2.15 0.03 6.2×10 ⁷ A35 6.72 1.47 2.4 0.04 7.5×10 ⁷ A36 3.78 1.08 1.43 0.04 7.9×10 ⁷ A37 17.6 2.65 14.7 0.25 4.5×10 ⁴ A38 23.5 0 4.26 0.06 4.3×10 ⁵

Table 9

No. mechanical properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI(%) SWE(DBTT-℃) A31 227 353 44 1.92 0.19 0 -70 A32 209 352 42 2.07 0.24 0 -40 IS A33 261 402 37 1.88 0.27 0 -40 IS A34 341 452 33 1.88 0.22 0 -40 IS A35 231 392 37 1.94 0.23 0 -50 IS A36 226 372 38 1.74 0.21 0 -40 IS A37 242 369 36 1.62 0.42 0.62 -60 CS A38 373 465 33 1.21 0.57 0 -40 CS

*Note:

YS=yield strength, TS=tensile strength, El=elongation, r _m =plasticity-anisotropy index, Δr=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

Example 4

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 10

No. Chemical composition (weight%) C mn Cu S al N P B Nb other A41 0.0018 0.12 0.08 0.012 0.044 0.0069 0.036 0.0007 0.015 Si: 0.2 A42 0.0018 0.07 0.06 0.015 0.05 0.0063 0.04 0.0007 0.01 Si: 0.05 A43 0.0022 0.09 0.09 0.013 0.05 0.0082 0.083 0.0007 0.031 Si: 0.11 A44 0.0025 0.07 0.12 0.009 0.04 0.0078 0.129 0.005 0.028 Si: 0.15 A45 0.0026 0.11 0.1 1 0.011 0.041 0.012 0.029 0.0008 0.032 Si: 0.16Mo: 0.07 A46 0.0032 0.09 0.09 0.012 0.036 0.0093 0.031 0.0011 0.028 Si: 0.15Cr: 0.25 A47 0.0034 0.45 0 0.0083 0.038 0.0015 0.048 0.005 0.063 A48 0.0038 0.07 0.08 0.012 0.035 0.0024 0.13 0.005 0

Table 11

No. Mn+Cu (Mn/55+Cu/63.5)/(S/32) (Nb/93)/(C/12) (Al/27)/(N/14) Average particle size of CuS precipitates (□) The number of CuS precipitates (/□) A41 0.2 8.33 1.08 3.32 0.04 6.9×10 ⁷ A42 0.13 4.73 0.72 4.12 0.04 8.4×10 ⁷ A43 0.18 7.52 1.82 3.16 0.04 9.0×10 ⁷ A44 0.19 11.2 1.45 2.66 0.04 8.2×10 ⁷ A45 0.22 10.9 1.59 1.77 0.04 6.9×10 ⁷ A46 0.18 8.14 1.13 2.01 0.03 9.4×10 ⁷ A47 0.45 31.5 2.39 13.1 0.25 1.5×10 ⁴ A48 0.15 6.75 0 7.56 0.04 3.5×10 ⁵

Table 12

No. mechanical properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI(%) SWE(DBTT-℃) A41 238 368 44 2.18 0.22 0 -70 IS A42 220 345 44 2.25 0.19 0 -70 IS A43 268 403 38 1.82 0.17 0 -60 IS A44 325 461 34 1.79 0.19 0 -60 IS A45 234 359 42 2.32 0.23 0 -60 IS A46 228 355 43 2.25 0.25 0 -50 IS A47 202 355 38 1.59 0.39 0 -60 CS A48 338 458 twenty four 1.31 0.58 0.55 -70 CS

*Note:

YS=yield strength, TS=tensile strength, El=elongation, r _m =plasticity-anisotropy index, Δr=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

Example 5

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 13

No. Chemical composition (weight%) C mn P S al Nb B N other A51 0.0015 0.05 0.05 0.012 0.04 0.021 0.0007 0.0018 A52 0.0028 0.08 0.082 0.015 0.041 0.019 0.0007 0.0027 Si: 0.15 A53 0.0038 0.12 0.12 0.006 0.028 0.035 0.0011 0.0018 A54 0.0019 0.07 0.09 0.011 0.032 0.021 0.0009 0.0021 Mo: 0.06 A55 0.0027 0.05 0.09 0.009 0.041 0.033 0.001 0.0029 Cr: 0.13 A56 0.0024 0.07 0.053 0.01 0.035 0 0 0.0012 A57 0.0024 0.32 0.11 0.008 0.024 0.035 0.007 0.0013

Table 14

No. (Mn/55)/(S/32) (Nb/93)/(C/12) Average particle size of CuS precipitates (□) Number of CuS precipitates (/□) A51 2.42 1.81 0.06 4.2×10 ⁵ A52 3.1 0.88 0.05 5.2×10 ⁵ A53 11.6 1.19 0.05 3.2×10 ⁶ A54 3.7 1.43 0.05 2.9×10 ⁶ A55 3.23 1.58 0.04 3.2×10 ⁶ A56 4.07 0 0.06 4.5×10 ⁴ A57 23.3 1.88 0.22 2.3×10 ³

Table 15

No. mechanical properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI (%) SWE(DBTT-℃) A51 215 358 45 1.92 0.25 0 -70 IS A52 254 403 39 1.73 0.22 0 -60 IS A53 315 458 35 1.59 0.19 0 -50 IS A54 288 442 36 1.78 0.24 0 -40 IS A55 309 452 35 1.52 0.21 0 -50 IS A56 248 355 41 1.33 0.29 0 -40 CS A57 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=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

Example 6

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 16

No. Chemical composition (weight%) C S al N P B Nb other A61 0.0014 0.006 0.053 0.0072 0.051 0.0005 0.015 A62 0.0023 0.01 0.062 0.0082 0.082 0.0004 0.021 Si: 0.12 A63 0.0018 0.011 0.055 0.0127 0.118 0.0004 0.037 Si: 0.25 A64 0.0024 0.008 0.049 0.0074 0.029 0.0007 0.028 Si: 0.2Mo: 0.06 A65 0.0032 0.013 0.053 0.0088 0.109 0.0009 0.035 Si: 0.18Cr: 0.13 A66 0.0018 0.013 0.052 0.0018 0.052 0.005 0 A67 0.0035 0.009 0.008 0.023 0.125 0.0005 0.062

Table 17

No. (Al/27)/(N/14) (Nb/93)/(C/14) Average particle size of CuS precipitates (□) Number of CuS precipitates (/□) A61 3.82 1.38 0.06 5.3×10 ⁵ A62 3.92 1.18 0.05 5.6×10 ⁵ A63 2.25 2.65 0.05 6.8×10 ⁶ A64 3.43 1.51 0.05 5.5×10 ⁶ A65 3.12 1.41 0.04 6.3×10 ⁶ A66 15 0 0.11 4.5×10 ⁴ A67 0.18 2.29 0.08 8.4×10 ⁴

Table 18

No. mechanical properties mark YS(MP) TS(MPa) EL(%) r _m Δr SWE(DBTT-℃) 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=planar anisotropy index, SWE=secondary processing embrittlement, AI=aging index, IS=this Invention steel, CS=comparative steel

Example 7

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 19

No. Chemical composition (weight%) C Mn P S Al Nb B N other A71 0.0013 0.11 0.041 0.006 0.054 0.014 0.0005 0.0072 A72 0.0026 0.1 0.075 0.01 0.072 0.018 0.0007 0.0082 Si: 0.11 A73 0.0018 0.09 0.105 0.011 0.055 0.037 0.0005 0.0127 Si: 0.1 A74 0.0031 0.07 0.035 0.009 0.043 0.032 0.0005 0.0079 Si: 0.12Mo: 0.06 A75 0.0021 0.14 0.036 0.01 0.052 0.019 0.0007 0.0089 Cr: 0.13 A76 0.0018 0.68 0.045 0.009 0.048 0.022 0.0004 0.0021 A77 0.0037 0.1 0.114 0.01 0.008 0.01 0.0011 0.0067 Si: 0.04

Table 20

No. (Mn/55)/(S/32) (Al/27)/(N/14) (Nb/93)/(C/12) Average particle size of CuS precipitates (□) Amount of CuS precipitate (/mm ² ) A71 10.7 3.89 1.39 0.06 5.2×10 ⁵ A72 5.82 4.55 0.89 0.05 6.4×10 ⁵ A73 4.76 2.25 2.65 0.05 7.5×10 ⁶ A74 4.53 2.82 1.33 0.05 6.8×10 ⁶ A75 8.15 3.03 1.17 0.05 7.3×10 ⁶ A76 44 11.9 1.58 0.24 1.8×10 ³ A77 5.82 0.62 0.35 0.06 4.5×10 ⁴

Table 21

No. mechanical properties mark YS(MPa) TS(MPa) EL(%) r _m Δr SWE(DBTT-℃) AI(%) A71 205 357 45 1.99 0.25 -60 0 IS A72 275 398 38 1.82 0.29 -50 0 IS A73 345 453 34 1.83 0.27 -60 0 IS A74 232 363 42 2.12 0.24 -50 0 IS A75 229 362 44 1.89 0.22 -50 0 IS A76 185 348 42 1.92 0.42 -40 0 CS A77 378 461 27 1.12 0.34 -60 0.49 CS

*Note:

YS=yield strength, TS=tensile strength, El=elongation, r _m =plasticity-anisotropy index, Δr=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

Example 8

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 22

No. Chemical composition (weight%) C P S Al Cu Nb B N other B81 0.0028 0.008 0.008 0.029 0.04 0.009 0.0005 0.0014 B82 0.0032 0.077 0.01 0.035 0.09 0.004 0.0005 0.0019 B83 0.003 0.048 0.009 0.034 0.12 0.006 0.0005 0.0017 Si: 0.03 B84 0.0019 0.081 0.015 0.04 0.11 0.0004 0.0007 0.0022 Si: 0.14 B85 0.0044 0.112 0.013 0.023 0.09 0.012 0.0012 0.0013 Si: 0.26 B86 0.0028 0.082 0.011 0.033 0.16 0.006 0.0006 0.0018 Si: 0.15Mo: 0.084 B87 0.0037 0.085 0.01 0.025 0.12 0.006 0.001 0.0022 Si: 0.15Cr: 0.15 B88 0.0028 0.05 0.013 0.038 0.13 0.055 0 0.0014 - B89 0.0038 0.119 0.012 0.029 0 0 0.0005 0.0026 -

Table 23

No.   (Cu/63.5)/(S/32 ) Cs Average particle size of CuS precipitates (□) Amount of CuS precipitate (/mm ² ) B81 2.52 16.387 0.05 4.7×10 ⁶ B82 4.54 26.839 0.05 6.6×10 ⁶ B83 6.72 22.258 0.06 6.8×10 ⁶ B84 3.7 18.484 0.05 9.12×10 ⁶ B85 3.49 28.516 0.05 3.4×10 ⁷ B86 7.33 20.258 0.05 1.1×10 ⁷ B87 6.05 29.258 0.05 2.1×10 ⁷ B88 5.04 -42.97 0.05 2.7×10 ⁷ B89 0 38 0.07 5.4×10 ⁶ Cs=(C-Nb×12/93)×10000

Table 24

No. mechanical properties mark YS(Mpa) TS(MPa) EL(%) r _m Δr AI (%) BH value (MPa) SWE(DBTT-℃) B81 193 302 49 1.93 0.23 0 44 -50 B82 241 372 42 1.71 0.28 0 59 -50 B83 234 356 44 1.66 0.25 0 42 -60 IS B84 271 404 39 1.52 0.32 0 53 -70 IS B85 331 467 34 1.31 0.18 0 62 -60 IS B86 329 452 35 1.73 0.24 0 44 -50 IS B87 343 460 34 1.67 0.21 0 57 -60 IS B88 209 345 40 1.88 0.29 0 0 -10 CS B89 328 469 29 1.19 0.19 2.8 95 -70 CS

*Note:

YS=yield strength, TS=tensile strength, El=elongation, r _m =plasticity-anisotropy index, Δr=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

Example 9

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 25

No. Chemical composition (weight%) C Mn P S Al Cu Nb B N other B91 0.0018 0.1 0.008 0.009 0.035 0.082 0.003 0.0007 0.0013 - B92 0.0022 0.12 0.026 0.011 0.042 0.1 0.004 0.0005 0.0022 - B93 0.0018 0.07 0.044 0.012 0.029 0.068 0.003 0.0008 0.0027 Si: 0.05 B94 0.0028 0.11 0.082 0.013 0.043 0.084 0.006 0.0006 0.0023 Si: 0.26 B95 0.0039 0.07 0.12 0.011 0.035 0.11 0.008 0.0008 0.0023 Si: 0.39 B96 0.0028 0.09 0.083 0.012 0.033 0.15 0.006 0.0005 0.0021 Si: 0.27Mo: 0.085 B97 0.0037 0.08 0.073 0.01 0.052 0.13 0.008 0.0009 0.0015 Si: 0.33Cr: 0.28 B98 0.0025 0.42 0.055 0.009 0.043 0 0.038 0.005 0.0024 - B9 0.0039 0.07 0.115 0.011 0.036 0.11 0.003 0 0.0027 Si: 0.1

Table 26 No. Cu+Mn (Mn/55+Cu/63.5)/(S/32) Cs Average particle size of CuS precipitates (□) Amount of CuS precipitate (/mm ² ) B91 0.18 1 1.1 14.129 0.06 1.1×10 ⁷ B92 0.22 10.9 16.839 0.06 1.8×10 ⁷ B93 0.14 6.25 14.129 0.06 1.5×10 ⁷ B94 0.19 8.18 20.258 0.05 2.5×10 ⁷ B95 0.18 8.74 28.677 0.05 3.2×10 ⁷ B96 0.24 10.7 20.258 0.05 2.5×10 ⁷ B97 0.21 1 1.2 26.677 0.04 4.4×10 ⁷ B98 0.42 27.2 -24.03 0.25 6.5×10 ⁴ B99 0.18 8.74 35.129 0.06 5.3×10 ⁶ Cs=(C-Nb×12/93)×10000

Table 27

No. mechanical properties Mark YS(MPa) TS(MPa) EL(%) r _m Δr AI (%) BH value (MPa) SWE(DBTT-℃) B91 190 308 48 1.91 0.31 0 35 -50 IS B92 209 329 46 1.85 0.27 0 41 -50 IS B93 227 347 46 1.82 0.22 0 36 -70 IS B94 295 396 39 1.73 0.21 0 45 -60 IS B95 341 463 33 1.53 0.14 0 58 -50 IS B96 331 446 35 1.71 0.22 0 51 -50 IS B97 355 457 34 1.59 0.18 0 46 -50 IS B98 196 350 40 1.85 0.29 0 0 -50 CS B99 369 451 32 1.29 0.198 2.5 95 30 CS

*Note:

YS=yield strength, TS=tensile strength, El=elongation, r _m =plasticity-anisotropy index, Δr=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

Example 10

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 28 No. Chemical composition (weight%) C P S al Cu Nb B N other B01 0.0021 0.009 0.01 0.042 0.08 0.005 0.0007 0.0072 B02 0.0022 0.027 0.009 0.039 0.12 0.004 0.0005 0.0092 B03 0.0018 0.046 0.012 0.046 0.07 0.003 0.0004 0.0086 Si: 0.11 B04 0.0039 0.082 0.009 0.052 0.12 0.008 0.0009 0.0092 Si: 0.08 B05 0.004 0.122 0.011 0.039 0.15 0.009 0.0006 0.0121 Si: 0.21 B06 0.0028 0.045 0.011 0.049 0.09 0.004 0.0005 0.0085 Si: 0.07Mo: 0.064 B07 0.0039 0.041 0.011 0.042 0.12 0.008 0.0009 0.011 Cr: 0.22 B08 0.0028 0.043 0.012 0.036 0.37 0.053 0.0009 0.0022 B09 0.0042 0.116 0.011 0.052 0.35 0 0.0005 0.0021 Si: 0.13

Table 29

No. (Cu/63.5)/(S/32) (Al/27)/(N/14) Cs Average particle size of CuS precipitates (□) Amount of CuS precipitate (/mm ² ) B01 4.03 3.02 14.548 0.04 5.8×10 ⁷ B02 6.72 2.2 16.839 0.04 6.9×10 ⁷ B03 2.94 2.77 14.129 0.04 6.5×10 ⁷ B04 6.72 2.93 28.677 0.04 7.8×10 ⁷ B05 6.87 1.67 28.387 0.04 6.8×10 ⁷ B06 4.12 2.99 22.839 0.04 5.5×10 ⁷ B07 5.5 1.98 28.677 0.04 6.9×10 ⁷ B08 15.5 8.48 -40.39 0.25 6.5×10 ⁴ B09 16 12.8 42 0.06 5.5×10 ⁵ Cs＝(C-Nb×12/93)×10000

Table 30

No. mechanical properties Mark YA(MPa) Ts(MPa) EL(%) r _m Δr AI(%) BH value (MPa) SWE(DBTT-℃) B01 203 318 47 1.92 0.28 0 36 -50 IS B02 218 345 44 1.82 0.27 0 42 -40 IS B03 209 352 42 1.85 0.24 0 42 -40 IS B04 261 402 37 1.68 0.27 0 53 -50 IS B05 337 455 33 1.53 0.22 0 56 -40 IS B06 237 390 38 1.64 0.23 0 63 -50 IS B07 228 378 40 1.71 0.21 0 55 -40 IS B08 238 366 38 1.72 0.38 0 0 -60 CS B09 373 465 33 1.21 0.57 3.8 95 -40 CS

*Note:

YS=yield strength, TS=tensile strength, El=elongation, r _m =plasticity-anisotropy index, Δr=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

Example 11

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 31

No. Chemical composition (weight%) C Mn P S Al Cu Nb B N other B11 0.0019 0.08 0.008 0.009 0.042 0.11 0.003 0.0006 0.0074 B12 0.0022 0.1 0.027 0.009 0.039 0.09 0.005 0.0005 0.0092 B13 0.0016 0.11 0.042 0.013 0.052 0.08 0.003 0.0005 0.0073 Si: 0.09 B14 0.0027 0.09 0.085 0.011 0.045 0.11 0.006 0.0007 0.0082 Si: 0.1 B15 0.0039 0.12 0.12 0.012 0.024 0.09 0.008 0.0005 0.0084 Si: 0.12 B16 0.0032 0.09 0.033 0.009 0.047 0.11 0.005 0.0008 0.011 Si: 0.15Mo: 0.081 B17 0.0038 0.09 0.035 0.01 0.033 0.13 0.008 0.0011 0.00105 Si: 0.17Cr: 0.27 B18 0.0033 0.47 0.045 0.0053 0.039 0 0.033 0.0006 0.0015 B19 0.0045 0.18 0.128 0.008 0.045 0.12 0.006 0.0008 0.0018 Si: 0.11

Table 32 No. Cu+Mn (Mn/55+Cu/63.5)/(S/32) (Al/27)/(N/14) Cs Average particle size of CuS precipitates (□) Amount of CuS precipitate (/mm ² ) B11 0.19 11.3 2.94 15.129 0.05 2.2×10 ⁷ B12 0.19 11.5 2.2 15.548 0.05 2.7×10 ⁷ B13 0.19 8.02 3.69 12.129 0.05 2.1×10 ⁷ B14 0.2 9.8 2.85 19.258 0.05 3.1×10 ⁷ B15 0.21 9.6 1.48 28.677 0.04 4.5×10 ⁷ B16 0.2 12 2.22 25.548 0.05 3.4×10 ⁷ B17 0.22 11.8 16.3 27.677 0.05 2.7×10 ⁷ B18 0.47 51.6 13.5 -9.581 0.25 2.9×10 ⁴ B19 0.3 20.6 13 37.258 0.06 5.5×10 ⁵ Cs=(C-Nb×12/93)×10000

Table 33

No. mechanical properties Mark YS(MPa) TS(MPa) EL(%) r _m Δr AI(%) BH value (MPa) SWE(DBTT-℃) B11 196 322 49 1.92 0.31 0 37 -50 IS B12 208 342 46 1.87 0.29 0 42 -50 IS B13 227 352 43 1.85 0.28 0 34 -60 IS B14 263 397 39 1.7 0.25 0 48 -50 IS B15 336 449 33 1.58 0.22 0 62 -40 IS B16 240 355 43 1.88 0.33 0 50 -60 IS B17 232 361 42 1.75 0.31 0 66 -50 IS B18 209 348 39 1.95 0.36 0 0 -50 CS B19 342 461 29 1.33 0.21 3.5 106 -60 CS

*Note:

YS=yield strength, TS=tensile strength, El=elongation, r _m =plasticity-anisotropy index, Δr=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

Example 12

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 34

No. Chemical composition (weight%) C mn P S al Nb B N other B21 0.0021 0.12 0.008 0.009 0.034 0.004 0.0008 0.0023 B22 0.0018 0.07 0.045 0.013 0.047 0.003 0.0005 0.0021 Si: 0.12 B23 0.0031 0.11 0.085 0.012 0.039 0.006 0.0008 0.0025 B24 0.004 0.09 0.123 0.01 0.034 0.008 0.0009 0.0011 B25 0.004 0.09 0.092 0.009 0.041 0.009 0.0005 0.0027 Mo: 0.065 B26 0.0033 0.05 0.094 0.011 0.039 0.007 0.0008 0.0018 Cr: 0.16 B27 0.0027 0.48 0.051 0.009 0.028 0.032 0.0007 0.0022 B28 0.0042 0.09 0.11 0.011 0.028 0 0.007 0.0016

Table 35

No. (Mn/55)/(S/32) Cs Average particle size of CuS precipitates (□) Amount of CuS precipitate (/mm ² ) B21 7.76 15.839 0.06 1.9×10 ⁵ B22 3.13 14.129 0.06 2.9×10 ⁵ B23 5.33 23.258 0.05 3.7×10 ⁵ B24 5.24 29.677 0.05 4.4×10 ⁶ B25 5.82 28.387 0.05 3.2×10 ⁶ B26 2.64 23.968 0.05 2.9×10 ⁶ B27 31 -14.29 0.06 3.2×10 ⁴ B28 4.76 42 0.22 2.3×10 ⁵ Cs＝(C-Nb×12/93)×10000

Table 36

No. mechanical properties Mark YS(MPa) TS(MPa) EL(%) r _m Δr AI(%) BH value (MPa) SWE(DBTT-℃) B21 188 302 51 1.98 0.36 0 39 -40 IS B22 210 347 46 1.81 0.31 0 38 -50 IS B23 259 409 38 1.63 0.24 0 53 -60 IS B24 325 451 35 1.55 0.19 0 66 -50 IS B25 293 449 36 1.51 0.2 0 58 -40 IS B26 302 452 36 1.45 0.18 0 57 -50 IS B27 245 352 40 1.83 0.29 0 0 -40 CS B28 254 454 25 1.56 0.28 3.8 92 -60 CS

*Note:

YS=yield strength, TS=tensile strength, El=elongation, r _m =plasticity-anisotropy index, Δr=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

Example 13

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 37

No. Chemical composition (weight%) C P S Al Nb B N Other B31 0.0025 0.009 0.01 0.028 0.005 0.0008 0.0071 B32 0.0019 0.011 0.009 0.034 0.004 0.0007 0.0093 Si: 0.24 B33 0.0018 0.054 0.011 0.032 0.002 0.0005 0.0072 Si: 0.07 B34 0.0035 0.084 0.009 0.045 0.008 0.0008 0.0066 Si: 0.11 B35 0.004 0.11 0.012 0.062 0.009 0.0004 0.0091 Si: 0.22 B36 0.0039 0.033 0.009 0.042 0.007 0.0007 0.0071 Si: 0.2Mo: 0.062 B37 0.0035 0.11 0.009 0.044 0.008 0.0009 0.012 Si: 0.19Cr: 0.15 B38 0.0025 0.05 0.008 0.052 0.03 0.0005 0.0018 B39 0.0036 0.12 0.011 0.042 0 0.0005 0.013

Table 38

No. (Al/27)/(N/14) Cs   Average particle size of CuS precipitates (□) Amount of CuS precipitate (/mm ² ) B31 2.04 18.548 0.06 2.1×10 ⁵ B32 1.9 13.839 0.06 2.3×10 ⁵ B33 2.3 15.419 0.06 2.5×10 ⁵ B34 3.54 24.677 0.05 3.2×10 ⁵ B35 3.53 28.387 0.05 4.2×10 ⁶ B36 3.07 29.968 0.05 3.1×10 ⁵ B37 1.9 24.677 0.05 4.5×10 ⁵ B38 15 -13.71 0.32 1.6×10 ⁴ B39 1.68 36 0.06 2.8×10 ⁵ Cs＝(C-Nb×12/93)×10000

Table 39

No. mechanical properties mark YS(MPa) TS(MPa) EL(%) r _m Δr AI(%) BH value (MPa) SWE(DBTT-℃) B31 197 315 49 1.93 0.37 0 43 -40 IS B32 208 342 45 1.92 0.36 0 39 -40 IS B33 209 349 44 1.85 0.31 0 33 -40 IS B34 275 390 37 1.7 0.24 0 62 -50 IS B35 335 452 33 1.52 0.22 0 72 -40 IS B36 224 362 40 1.72 0.29 0 53 -60 IS B37 371 452 33 1.55 0.18 0 54 -60 IS B38 197 332 45 1.93 0.43 0 0 -40 CS B39 355 457 31 1.42 0.24 0 -40 CS

*Note:

YS=yield strength, TS=tensile strength, El=elongation, r _m =plasticity-anisotropy index, Δr=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

Example 14

First, steel slabs were prepared with compositions shown in the table below. Steel slabs are reheated and finish hot-rolled to provide hot-rolled steel sheets. The hot-rolled steel sheet was cooled at a rate of 400°C/min, coiled at 650°C, cold-rolled at a reduction rate of 75%, and then continuously annealed to obtain a cold-rolled steel sheet. At this time, finish hot rolling is carried out at 910°C, which is higher than the Ar ₃ transformation point, and the final cold steel sheet is produced by heating the hot-rolled steel sheet to 830°C at a rate of 10°C/sec and maintaining it for 40 seconds for continuous annealing. Rolled steel.

Table 40

No. Chemical composition (weight%) C Mn P S Al Nb B N Other B41 0.0028 0.08 0.009 0.009 0.039 0.006 0.0005 0.0092 B42 0.0022 0.1 0.026 0.01 0.048 0.005 0.0007 0.0072 B43 0.0019 0.09 0.043 0.011 0.042 0.004 0.0005 0.0082 Si: 0.04 B44 0.0033 0.12 0.078 0.009 0.067 0.005 0.0008 0.0078 Si: 0.11 B45 0.0039 0.09 0.097 0.015 0.037 0.008 0.001 0.0105 Si: 0.08 B46 0.0035 0.11 0.032 0.01 0.028 0.008 0.0007 0.0059 Si: 0.11Mo: 0.058 B47 0.0029 0.07 0.041 0.008 0.033 0.006 0.0007 0.008 Si: 0.05Cr: 0.33 B48 0.0027 0.68 0.041 0.008 0.035 0.028 0.0008 0.002 Si: 0.08 B49 0.0042 0.08 0.114 0.01 0.008 0 0.0011 0.0067 Si: 0.05

Table 41

No. (Mn/55)/(S/32) (Al/27)/(N/14) Cs Average particle size of CuS precipitates (□) Amount of CuS precipitate (/mm ² ) B41 5.17 2.2 20.258 0.05 4.2×10 ⁵ B42 5.82 3.46 15.548 0.05 3.5×10 ⁵ B43 4.76 2.66 13.839 0.05 3.8×10 ⁵ B44 7.76 4.45 26.548 0.05 5.8×10 ⁵ B45 3.49 1.83 28.677 0.05 5.4×10 ⁶ B46 6.4 2.46 24.677 0.05 4.5×10 ⁶ B47 5.09 2.14 21.258 0.05 3.7×10 ⁶ B48 49.5 9.07 -9.129 0.32 1.2×10 ⁴ B49 4.65 0.62 42 0.06 2.8×10 ⁵ Cs＝(C-Nb×12/93)×10000

Table 42

No. mechanical properties Mark YS(MPa) TS(MPa) EL(%) r _m Δr AI(%) BH value (MPa) SWE(DBTT-℃) B41 197 320 49 1.93 0.32 0 59 -40 IS B42 209 342 46 1.84 0.29 0 44 -50 IS B43 214 355 43 1.82 0.29 0 35 -50 IS B44 275 398 38 1.71 0.25 0 69 -50 IS B45 348 452 32 1.55 0.18 0 74 -60 IS B47 225 360 42 1.75 0.22 0 48 -40 IS B46 242 359 40 1.79 0.24 0 59 -50 IS B48 197 359 40 1.92 0.37 0 0 -50 CS B49 378 461 27 1.12 0.34 5.2 105 -60 CS

*Note:

YS=yield strength, TS=tensile strength, El=elongation, r _m =plasticity-anisotropy index, Δr=planar anisotropy index, AI=aging index, SWE=secondary processing embrittlement, IS=this Invention steel, CS=comparative steel

The preferred embodiments described in the present invention are not intended to limit the present invention, but are for illustrative purposes only. Any embodiment having substantially the same constitution and substantially the same operational effect as the technical spirit of the present invention defined by the claims is included in the technical scope of the present invention.

As can be understood from the above description, according to the cold-rolled steel sheet of the present invention, the distribution of fine precipitates in the Nb-based IF steel enables the formation of fine grains, and as a result, the planar anisotropy index is reduced and the yield strength is improved by enhancing the precipitation .

Claims (40)

1. A cold-rolled steel sheet having a high yield ratio and a low planar anisotropy index, said cold-rolled steel sheet having a composition comprising: by weight, less than or equal to 0.01% of C, 0.01- 0.2% of Cu, 0.005-0.08% of S, less than or equal to 0.1% of Al, less than or equal to 0.004% of N, less than or equal to 0.2% of P, 0.001-0.002% of B, 0.002-0.04% of Nb and The balance of Fe and other unavoidable impurities, Wherein, the composition satisfies the following relationship: 1≤(Cu/63.5)/(S/32)≤30, The steel plate contains CuS precipitates with an average particle size of less than or equal to 0.2 μm. 2. The cold-rolled steel sheet according to claim 1, characterized in that said composition also includes 0.01-0.3% Mn, and satisfies the following relationship: 1≤(Mn/55+Cu/63.5)/(S/32 )≤30, the steel plate contains (Mn, Cu)S precipitates with an average particle size less than or equal to 0.2 μm. 3. The cold-rolled steel sheet according to claim 1, characterized in that the N content is 0.004-0.02%, and the composition satisfies the following relational formula: 1≤(Al/27)/(N/14)≤10, the The steel sheet contains AlN precipitates with an average particle 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 relationship: 1≤(Mn/55+Cu/ 63.5)/(S/32)≤30, 1≤(Al/27)/(N/14)≤10, the steel plate contains (Mn, Cu)S precipitates and AlN precipitates with an average particle size less than or equal to 0.2 μm things. 5. A cold-rolled steel sheet having a high yield ratio and a low planar anisotropy index, said cold-rolled steel sheet having a composition comprising: by weight, C of less than or equal to 0.01%, less than or equal to S equal to 0.08%, Al less than or equal to 0.1%, N less than or equal to 0.004%, P less than or equal to 0.2%, B 0.0001-0.002%, Nb 0.002-0.04%, selected from 0.01-0.2% At least one of Cu, 0.01-0.3% of Mn and 0.004-0.2% of N, and the balance of Fe and other unavoidable impurities, Wherein, the composition satisfies the following relationship: 1≤(Mn/55+Cu/63.5)/(S/32)≤30, 1≤(Al/27)/(N/14)≤10, and the N content is greater than or equal to 0.004%, The steel sheet contains at least one substance selected from (Mn, Cu)S precipitates and AlN precipitates having an average particle size of 0.2 μm or less. 6. The cold-rolled steel sheet according to claim 1 or 5, characterized in that the C content and the Nb content satisfy the following relationship, by weight: 0.8≤(Nb/93)/(C/12)≤5.0. 7. The cold-rolled steel sheet according to claim 6, characterized in that the C content is less than or equal to 0.005%. 8. The cold-rolled steel sheet according to claim 1 or 5, characterized in that solute carbon (Cs) is 5-30, wherein Cs=(C-Nb×12/93)×10,000. 9. The cold-rolled steel sheet according to claim 8, characterized in that the C content is 0.001-0.01%. 10. The cold-rolled steel sheet according to any one of claims 1 to 5, characterized in that the cold-rolled steel sheet satisfies a yield strength ratio (yield strength/tensile strength) greater than or equal to 0.58. 11. The cold-rolled steel sheet according to any one of claims 1 to 5, characterized in that the amount of the precipitate is greater than or equal to 1×10 ⁶ /mm ² . 12. The cold-rolled steel sheet according to claim 1 or 5, characterized in that the P content is less than or equal to 0.015%. 13. The cold-rolled steel sheet according to claim 1 or 5, characterized in that the P content is 0.03-0.2%. 14. The cold-rolled steel sheet according to claim 1 or 5, wherein the composition further comprises at least one of 0.1-0.8% of Si and 0.2-1.2% of Cr. 15. The cold-rolled steel sheet according to claim 1 or 5, characterized in that said composition further comprises 0.01-0.2% Mo. 16. The cold-rolled steel sheet according to claim 14, wherein said composition further comprises 0.01-0.2% of Mo. 17. The cold-rolled steel sheet according to claim 2, 4 or 5, characterized in that the total amount of Mn and Cu is 0.08-0.4%. 18. The cold-rolled steel sheet according to claim 2, 4 or 5, characterized in that the Mn content is 0.01-0.12%. 19. The cold-rolled steel sheet according to claim 2, 4 or 5, characterized in that the value of (Mn/55+Cu/63.5)/(S/32) is 1-9. 20. The cold-rolled steel sheet according to claim 3, 4 or 5, characterized in that the value of (Al/27)/(N/14) is 1-5. 21. A method of manufacturing a cold-rolled steel sheet having a high yield ratio and a low planar anisotropy index, the method comprising the steps of: Reheating the slab to a temperature greater than or equal to 1100°C, the slab having a composition comprising, by weight, less than or equal to 0.01% C, 0.01-0.2% Cu, 0.005-0.08% S, Al less than or equal to 0.1%, N less than or equal to 0.004%, P less than or equal to 0.2%, B 0.001-0.002%, Nb 0.002-0.04%, and the balance of Fe and other unavoidable Impurities, the composition of which satisfies the following relationship: 1≤(Cu/63.5)/(S/32)≤30; Hot rolling the reheated slab at a finish rolling temperature higher than or equal to the _Ar3 transformation point to obtain a hot-rolled steel plate; cooling the hot-rolled steel sheet at a rate greater than or equal to 300°C/min; coiling said cooled steel sheet at a temperature lower than or equal to 700°C; Cold rolling of coiled steel sheets; Continuous annealing was performed on a cold-rolled steel sheet containing CuS precipitates with an average particle size of 0.2 μm or less. 22. The method according to claim 21, characterized in that said composition further comprises 0.01-0.3% of Mn, and satisfies the following relationship: 1≤(Mn/55+Cu/63.5)/(S/32)≤ 30. The steel plate contains (Mn, Cu)S precipitates with an average particle size less than or equal to 0.2 μm. 23. The method according to claim 21, characterized in that the N content is 0.004-0.02%, the composition satisfies the following relationship: 1≤(Al/27)/(N/14)≤10, and the steel plate contains AlN precipitates with an average particle size less than or equal to 0.2 μm. 24. The method according to claim 21, characterized in that the composition further comprises 0.01-0.3% of Mn and 0.004-0.02% of N, and satisfies the following relationship: 1≤(Mn/55+Cu/63.5) /(S/32)≤30, 1≤(Al/27)/(N/14)≤10, the steel plate contains (Mn, Cu)S precipitates and AlN precipitates with an average particle size less than or equal to 0.2 μm. 25. A method of manufacturing a cold-rolled steel sheet having a high yield ratio and a low planar anisotropy index, the method comprising the steps of: reheating the slab to a temperature greater than or equal to 1100° C., the slab having a composition comprising, by weight, 0.01% or less of C, less than or equal to 0.08% of S, less than or equal to 0.1 % of Al, less than or equal to 0.004% of N, less than or equal to 0.2% of P, 0.001-0.002% of B, 0.002-0.04% of Nb, at least one component selected from the following: 0.01-0.2% of Cu , 0.01-0.3% of Mn and 0.004-0.2% of N, and the balance of Fe and other unavoidable impurities, the composition satisfies the following relationship: 1≤(Mn/55+Cu/63.5)/(S/32 )≤30, 1≤(Al/27)(N/14)≤10, wherein the N content is greater than or equal to 0.004%; Hot rolling the reheated slab at a finish rolling temperature higher than or equal to the _Ar3 transformation point to obtain a hot-rolled steel plate; cooling the hot-rolled steel sheet at a rate greater than or equal to 300°C/min; coiling said cooled steel sheet at a temperature lower than or equal to 700°C; cold rolling the coiled steel sheet; Continuous annealing is performed on the cold-rolled steel sheet, the steel sheet including at least one selected from (Mn, Cu)S and AlN precipitates having an average particle size of 0.2 μm or less. 26. The method according to claim 21 or 25, characterized in that the content of C and Nb satisfies the following relationship, by weight: 0.8≤(Nb/93)/(C/12)≤5.0. 27. The method of claim 26, wherein the C content is less than or equal to 0.005%. 28. The method according to claim 21 or 25, characterized in that the solute carbon (Cs) is 5-30, wherein Cs=(C-Nb×12/93)×10,000. 29. The method according to claim 28, characterized in that the C content is 0.001-0.01%. 30. The method according to any one of claims 21-25, characterized in that the cold-rolled steel sheet satisfies a yield strength ratio (yield strength/tensile strength) greater than or equal to 0.58. 31. The method according to any one of claims 21-25, characterized in that the amount of precipitate is greater than or equal to 1 x ¹⁰⁶ / ^mm2 . 32. The method according to claim 21 or 25, characterized in that the P content is less than or equal to 0.015%. 33. The method according to claim 21 or 25, characterized in that the P content is 0.03-0.2%. 34. The method of claim 21 or 25, wherein the composition further comprises at least one of 0.1-0.8% Si and 0.2-1.2% Cr. 35. The method according to claim 21 or 25, characterized in that the composition further comprises 0.01-0.2% Mo. 36. The method of claim 34, wherein said composition further comprises 0.01-0.2% Mo. 37. A method as claimed in claim 22, 24 or 25, characterized in that the total amount of Mn and Cu is 0.08-0.4%. 38. A method as claimed in claim 22, 24 or 25, characterized in that the Mn content is 0.01-0.12%. 39. The method according to claim 22, 24 or 25, characterized in that the value of (Mn/55+Cu/63.5)/(S/32) is 1-9. 40. The method according to claim 23, 24 or 25, characterized in that the value of (Al/27)/(N/14) is 1-5.