CN101171356A - Cold-rolled steel sheet having excellent formability and high yield ratio and manufacturing method thereof - Google Patents

Abstract

Disclosed herein is a Nb-Ti composite IF steel in which fine precipitates, such as CuS precipitates, having a size of 0.2 mum or less are distributed. The distribution of fine precipitates in the Nb-Ti composite 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 handenability.

Description

Cold-rolled steel sheet having excellent formability and high yield ratio and manufacturing method thereof

technical field

The present invention relates to an interstitial-free (IF) cold-rolled steel sheet to which niobium (Nb) and titanium (Ti) are added, which can be used as materials for automobiles, home electronic appliances, and the like. More specifically, the present invention relates to a high formability IF cold-rolled steel sheet whose yield strength is increased due to the distribution of fine precipitates, and to a method of manufacturing such an IF cold-rolled 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, the protracted time of batch annealing 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 Publication Hei (Hei) 9-227951 and Hei (Hei) 10-265900 propose techniques for improving machinability or improving surface defects caused by carbides 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 Application Publication Hei (Hei) 10-280048 Hei (Hei) 10-287954 proposes to dissolve carbosulfides (Ti-C-S-based) in carbides during reheating and annealing to obtain solid solutions at grain boundaries, 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 has a lower in-plane anisotropy index, fewer wrinkles and ears occur during and after processing, respectively.

Contents of the invention

technical problem

An object of some embodiments of the present invention is to provide Nb and Ti added IF cold-rolled steel sheet capable of achieving high yield ratio and low in-plane anisotropy index.

Another object of some embodiments of the present invention is to provide a method of manufacturing such an IF cold-rolled steel sheet.

Technical solutions

According to the present invention, there is provided a cold-rolled steel sheet having a composition comprising the following components: represented 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, 0.0001-0.002% of B, 0.002-0.04% of Nb, 0.005-0.15% of Ti, and the balance of Fe and other unavoidable impurities, the composition satisfies the following relationship: 1≤(Cu/63.5)/(S ^* /32)≤30, S ^* =S-0.8×(Ti-0.8×(48/14)×N )×(32/48), this steel plate contains CuS precipitates with an average particle size less than or equal to 0.2 μm.

According to the present invention, a cold-rolled steel sheet is provided. The cold-rolled steel sheet has a composition comprising the following components: represented by weight %, less than or equal to 0.01% of C, 0.01-0.2% of Cu, 0.01-0.3% of Mn, 0.005 -0.08% S, less than or equal to 0.1% Al, less than or equal to 0.004% N, less than or equal to 0.2% P, 0.0001-0.002% B, 0.002-0.04% Nb, 0.005-0.15% Ti , 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, and S ^* =S-0.8 ×(Ti-0.8×(48/14)×N)×(32/48), the steel plate contains (Mn, Cu)S precipitates with an average particle size less than or equal to 0.2 μm.

According to the present invention, there is provided a cold-rolled steel sheet having a composition comprising the following components: represented 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, 0.004-0.02% of N, less than or equal to 0.2% of P, 0.0001-0.002% of B, 0.002-0.04% of Nb, 0.005-0.15% of Ti, and the balance of Fe and Other unavoidable impurities, wherein the composition satisfies the following relationship: 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), the steel plate contains CuS and AlN precipitates with an average particle size less than or equal to 0.2 μm.

According to the present invention, there is provided a cold-rolled steel sheet having a composition comprising the following components: represented by weight %, less than or equal to 0.01% of C, 0.01-0.2% of Cu, 0.01-0.3% of Mn, 0.005-0.08% S, less than or equal to 0.1% Al, 0.004-0.02% N, less than or equal to 0.2% P, 0.0001-0.002% B, 0.002-0.04% Nb, 0.005-0.15% Ti , 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, 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 plate contains precipitates of (Mn, Cu)S and AlN with an average particle size less than or equal to 0.2 μm.

According to the present invention, there is provided a cold-rolled steel sheet having a composition comprising the following components: in terms of weight percent, C of less than or equal to 0.01%, S of less than or equal to 0.08%, and S of less than or equal to 0.1% Al, 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%, Ti 0.005-0.15%, 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, wherein the composition satisfies the following relationship: 1≤(Mn/55+Cu/ 63.5)/(S ^* /32)≤30, 1≤(Al/27)/(N ^* /14)≤10 (provided that the N content is greater than or equal to 0.004%), 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 plate contains the average At least one of (Mn, Cu)S and AlN precipitates having a particle size of 0.2 μm or less.

When the cold-rolled steel sheet of the present invention satisfies the following relationship between the contents of C, Ti, Nb, N and S: 0.8≤(Ti ^* /48+Nb/93)/(C/12)≤5.0 and Ti ^* =Ti-0.8×((48/14)×N+(48/32)×S), this cold-rolled steel sheet exhibits room temperature non-aging properties. In addition, when the solute carbon (Cs) determined by the content of C and Ti [Cs=(C-Nb×12/93-Ti ^* ×12/48)×10000, wherein, Ti ^* =Ti-0.8×((48/ 14)×N+(48/32)×S), provided that when Ti ^* is less than 0, Ti ^* is defined as 0)) when the value is 5-30, the cold-rolled steel sheet of the present invention has bake hardenability.

Depending on the design of the composition, the characteristics of the cold-rolled steel sheet of the present invention are a soft cold-rolled steel sheet on the order of 280 MPa and a high-strength cold-rolled steel sheet on the order of 340 MPa or more.

When the P content of the composition of the present invention is less than or equal to 0.015%, soft cold-rolled steel sheets on 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 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 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 to provide a 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; The coiled steel sheet is cold rolled; and the cold rolled steel sheet is continuously annealed.

best way

The present invention is described in detail below.

Fine precipitates with particle size 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 and Ti-added IF steel (also simply referred to as "Nb-Ti composite IF steel"), the yield strength of the IF steel increases and the in-plane anisotropy index decreases, Improved processability is thus achieved. 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 in-plane anisotropy index.

The composition in Nb-Ti composite IF steel needs to be adjusted to obtain (Mn, Cu)S precipitates and/or AlN precipitates. If the IF steel contains Ti, Zr and other elements, S and N preferably react with Ti and Zr. Since the cold-rolled steel sheet of the present invention is a Nb-Ti composite IF steel, Ti reacts with C, N and S. Therefore, the composition needs to be adjusted so that S and N can be precipitated as (Mn,Cu)S and AlN forms, respectively.

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, so that excellent room temperature non-aging properties can be achieved. Because the dissolved carbon present in the grains can migrate more freely and 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 invention have a positive effect on the increase of yield strength due to precipitation enhancement, improvement of strength-extensibility balance, in-plane 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 remaining carbon, which is economically unfavorable and also undesirable from the aspect 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 make the crystal grains finer. Copper reduces the in-plane anisotropy index of the cold-rolled steel sheet and increases the yield strength of the cold-rolled steel sheet by promoting precipitation. In order to form fine precipitates, the Cu content 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 function of manganese is to precipitate the sulfur 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 this technical point of view, 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 will significantly deteriorate the ductility and formability of the steel sheet and increase 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 improve the yield strength of the steel plate through precipitation strengthening. 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 effect of grain fineness and precipitation strengthening effect can be neglected. 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 is 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.

The titanium (Ti) content is preferably in the range of 0.005-0.15%.

The purpose of adding titanium is to ensure the non-aging properties and improve the formability of the steel sheet. Ti is a potential carbide-forming element, and titanium is added to steel to form TiC precipitates in the steel. In addition, the TiC precipitate precipitates dissolved carbon to ensure non-aging properties. When the added Ti content is less than 0.005%, very little TiC precipitates are formed. Therefore, the steel sheet cannot be textured well, and thus, the deep drawability of the steel sheet is hardly improved. On the contrary, when Ti is added in an amount greater than 0.15%, a large amount of TiC precipitates are formed. Therefore, the miniaturization effect of crystal grains is reduced, resulting in a high in-plane anisotropy index, lowered yield strength, and markedly deteriorated plating characteristics.

To form (Mn,Cu)S and AlN precipitates, the contents of Mn, Cu, S, Nb, Ti, Al, N, and C 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)

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

In Relational Formula 1, S* is determined by Relational Formula 2, and represents the sulfur content that does not react with Ti and then reacts with Cu. In order to obtain fine CuS precipitates, it is preferable that the value of (Cu/63.5)/(S*/32) is greater than or equal to 1. If the value of (Cu/63.5)/(S*/32) is greater than 30, coarse CuS precipitates are distributed, which is undesirable. To stably form CuS precipitates with a particle size less than or equal to 0.2 μm, the value of (Cu/63.5)/(S*/32) is preferably in the range of 1-20, more preferably 1-9, most preferably 1-6.

1≤(Mn/55+Cu/63.5)/(S*/32)≤30 (3)

Relation 3 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 (Mn/55+Cu/63.5)/(S*/32) must be greater than or equal to 1. When the value of relational expression 3 is greater than 30, coarse (Mn,Cu)S precipitates are formed. In order to stably form (Mn, Cu)S precipitates with a particle size less than or equal to 0.2 μm, the value of (Cu/63.5)/(S*/32) is more preferably in the range of 1-20, more preferably 1-9, most preferably Preferably 1-6. When Mn and Cu are added together, the total amount of Mn and Cu is preferably 0.05-0.4%. The reason for limiting the total amount 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)

Relational formula 4 is related to the formation of fine (Mn, Cu)S precipitates. In relational formula 4, N* is determined by relational formula 5, which represents the N content that does not react with Ti and then reacts with Al. In order to obtain fine AlN precipitates, the value of (Al/27)/(N ^* /14) is in the range of 1-10. In order to obtain efficient AlN precipitation, the value of (Al/27)/(N ^* /14) must be greater than or equal to 1. If the value of (Al/27)/(N ^* /14) is greater than 10, coarse AlN precipitates are formed. , thus resulting in poor processability and low yield strength. The value of (Al/27)/(N ^* /14) is preferably in the range of 1-6.

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, 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, less than or equal to 0.08% of S, less than or equal to 0.1% Al, less than or equal to 0.004% of N, less than or equal to 0.2% of P, 0.0001-0.002% of B, 0.002-0.04% of Nb, 0.005-0.15% of Ti, 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, wherein the composition satisfies the following relationship: 1≤(Mn/55+Cu/ 63.5)/(S*/32)≤30, 1≤(Al/27)/(N*/14)≤10 (provided that the N content is greater than or equal to 0.004%), 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 plate contains at least one Precipitates selected from the group consisting of precipitates of MnS, CuS, MnS and AlN, the average particle size of said precipitates being less than or equal to 0.2 μm. That is, one or more components selected from 0.01-0.2% of Cu, 0.01-0.3% of Mn, and 0.004-0.2% of N produce (Mn, Cu)S and AlN precipitates with a particle size of not more than 0.2 μm Various combinations.

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≤(Ti*/48+Nb/93)/(C/12)≤5.0 (6)

Ti*＝Ti-0.8×((48/14)×N+(48/32)×S) (7)

Relation 6 is related to the formation of NbC and TiC precipitates to remove solid solution carbon, thus achieving room temperature non-aging properties. In Relational Expression 6, Ti* is determined by Relational Expression 7, and represents the content of titanium that reacts with N and S and then reacts with C.

When the value of (Ti*/48+Nb/93)/(C/12) is less than 0.8, it is difficult to guarantee the room temperature non-aging property. On the contrary, when the value of (Ti*/48+Nb/93)/(C/12) is greater than 5, the amount of Nb and Ti remaining in the steel in solid solution state is large, which will make the ductility of the steel worse. 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 6 is satisfied, but the amount of NbC and TiC precipitates increases, thus deteriorating the workability of the steel sheet.

Cs＝(C-Nb×12/93-Ti*×12/48)×10000 (8)

(The condition is that Ti* is defined as 0 when Ti* is less than 0.)

Relation 8 is related to achieving bake hardenability. Cs is expressed in ppm in relation 8 and represents the content of dissolved carbon that has not precipitated in the form of NbC and TiC. 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.

The fine precipitate is preferably uniformly distributed in the composition of the present 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 low and the in-plane anisotropy index is low. In addition, a large number of precipitates with an average particle size of 0.2 μm or less 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 in-plane anisotropy index was decreased, resulting in significantly improved processability. There is a known limit to the increase in processability, since the in-plane anisotropy index increases with increasing 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 in-plane 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, the steel plate may further 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 will slightly reduce 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 in-plane 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, and forming precipitates with an average particle size less than or equal to 0.2 μm in the steel plate after cold rolling. 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 lower than 1,100° C., coarse precipitates formed during continuous casting cannot be completely dissolved and remain. 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 a fine precipitate, when cooled at less than 300° C./min, the average particle size of the formed precipitate was greater than 0.2 μm. That is, many crystal nuclei are generated as the cooling rate increases, so 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 was greater than 1,000°C/min, the effect of reducing the particle size of the precipitate was no longer significantly improved. 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 generated 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. Cooling reduction greater 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, so the strength of the steel material deteriorates. Continue annealing until the steel has completed recrystallization. The recrystallization of the 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 (r _m value) and in-plane anisotropy index (Δr value) and aging were determined using a tensile strength testing machine (obtained from INSTRON Company as model 6025) index. The plasticity-anisotropy index r _m and the in-plane 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-rolled steel plate.

Table 1

Sample serial number Chemical composition (weight%) C Cu S A1 N P B Nb Ti other A11 0.0006 0.14 0.008 0.032 0.0012 0.048 0.0003 0.014 0.008 A12 0.0017 0.12 0.012 0.043 0.0026 0.082 0.0006 0.02 0.022 Si: 0.17 A13 0.0031 0.09 0.012 0.028 0.0016 0.106 0.0012 0.028 0.02 Si: 0.28 A14 0.0012 0.118 0.02 0.042 0.0015 0.078 0.0011 0.033 0.033 Si: 0.15Mo: 0.09 A15 0.0018 0.1 0.018 0.036 0.0019 0.085 0.0009 0.04 0.018 Si: 0.15Cr: 0.15 A16 0.0022 0.11 0.01 0.038 0.0015 0.059 0 0 0 A17 0.0012 0 0.011 0.034 0.0027 0.12 0.0008 0.03 0.16

Table 2

Sample serial number S ^* (Cu/63.5)/(S ^* /32) (Ti ^* /48+Nb/93)/(C/12) Average particle size of CuS precipitate (μm) Amount of CuS precipitate (mm ^-2 ) A11 0.0055 12.854 0.97 0.04 1.5×10 ⁷ A12 0.0041 14.858 1.59 0.05 2.5×10 ⁷ A13 0.0037 12.345 1.26 0.05 3.8×10 ⁷ A14 0.0046 12.943 4.57 0.05 4.1×10 ⁷ A15 0.0112 4.5077 1.64 0.04 5.2×10 ⁷ A16 0.0122 4.5458 -1.8 0.08 4.5×10 ⁶ A17 -0.07 0 32.3 0.08 6.7×10 ⁴ S ^* =S-0.8×Ti-0.8×(48/14)×N)×(32/48)Ti ^* =Ti-0.8×((48/14)×N+(48/32)×S)

table 3

Sample serial number Mechanical Properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI (%) SWE(DBTT-℃) A11 208 345 46 2.32 0.14 0 -70 IS A12 263 402 39 1.88 0.18 0 -60 IS A13 332 448 36 1.73 0.12 0 -50 IS A14 329 452 36 1.84 0.18 0 -50 IS A15 334 450 37 1.74 0.13 0 -60 IS A16 232 348 43 1.12 0.29 0.62 -70 CS A17 270 445 28 1.82 0.48 0 -50 CS

^* Note:

YS = yield strength, TS = tensile strength, El = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, AI = aging index, SWE = secondary processing embrittlement, IS = Steel product of the present invention, CS=comparative example steel product

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-rolled steel plate.

Table 4

Sample serial number Chemical composition (weight %) C Mn Cu S Al N P B Nb Ti other A21 0.0007 0.06 0.08 0.007 0.038 0.0011 0.05 0.0008 0.01 0.009 A22 0.0014 0.15 0.15 0.013 0.027 0.0018 0.082 0.0009 0.02 0.019 Si: 0.22 A23 0.0029 0.18 0.12 0.02 0.041 0.0025 0.12 0.0011 0.029 0.028 Si: 0.33 A24 0.0015 0.08 0.11 0.018 0.028 0.0026 0.085 0.0009 0.039 0.015 Si: 0.22Mo: 0.11 A25 0.0012 0.13 0.15 0.022 0.032 0.0011 0.073 0.0009 0.005 0.032 Si: 0.3Cr: 0.24 A26 0.0036 0.45 0.14 0.009 0.033 0.0024 0.048 0.005 0 0 Si: 0.05 A27 0.0015 0.13 0 0.008 0.038 0.0021 0.118 0 0.04 0.02 Si: 0.35

table 5

Sample serial number Cu+Mn S ^* (Mn/55+Cu/63.5)/(S ^* /32) (Ti ^* /48+Nb/93)/(C/12) Average particle size of (Mn, Cu)S precipitates (µm) Amount of (Mn, Cu)S precipitates (mm ^-2 ) A21 0.14 0.0038 19.748 0.98 0.04 3.3×10 ⁷ A22 0.3 0.0055 29.613 1.57 0.04 4.2×10 ⁷ A23 0.3 0.0087 18.937 1.04 0.03 5.0×10 ⁷ A24 0.19 0.0138 7.3879 1.07 0.04 4.5×10 ⁷ A25 0.28 0.0065 23.115 1.08 0.04 4.9×10 ⁷ A26 0.59 0.0125 26.566 -1.2 0.25 5.5×10 ⁶ A27 0.13 0.0004 186.6 4.21 0.16 4.3×10 ⁴ S ^* =S-0.8×(Ti-0.8×(48/14)×N)×(32/48)Ti ^* =Ti-0.8×((48/14)×N+(48/32)×S)

Table 6

Sample serial number Mechanical Properties mark YS(MPa) TS(MPa) El(%) r _m Δr SWE(DBTT-℃) AI(%) A21 218 348 45 2.1 0.19 -40 0 IS A22 262 410 36 1.94 0.17 -40 0 IS A23 328 455 33 1.89 0.17 -40 0 IS A24 247 401 35 1.89 0.19 -50 0 IS A25 229 392 38 1.79 0.17 -40 0 IS A26 233 359 37 1.11 0.62 -60 1.56 CS A27 283 425 33 1.81 0.57 -40 0 CS

^* Note:

YS = yield strength, TS = tensile strength, El = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, SWE = secondary processing embrittlement, AI = aging index, IS = Steel product of the present invention, CS=comparative example steel product

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-rolled steel plate

Table 7

Sample serial number Chemical composition (weight %) C Cu S Al N P B Nb Ti Other A31 0.0005 0.08 0.007 0.029 0.0139 0.044 0.0008 0.025 0.038 A32 0.0012 0.13 0.016 0.026 0.011 0.08 0.0008 0.05 0.028 Si: 0.11 A33 0.0025 0.14 0.011 0.04 0.0148 0.116 0.0009 0.032 0.051 Si: 0.26 A34 0.0013 0.17 0.012 0.031 0.0088 0.047 0.0011 0.043 0.029 Si: 0.09Mo: 0.12 A35 0.0005 0.15 0.015 0.03 0.0089 0.043 0.0009 0.009 0.04 Si: 0.11Cr: 0.22 A36 0.0038 0.09 0.013 0.032 0.0012 0.042 0.0005 0 0 A37 0.0014 0 0.009 0.055 0.012 0.12 0.0005 0 0.14 Si: 0.13

Table 8

Sample serial number S ^* (Cu/63.5)/(S ^* /32) (Ti ^* /48+Nb/93)/(C/12) N ^* (Al/27)/(N ^* /14) Average particle size of sediment (µm) Amount of sediment (mm ^-2 ) A31 0.0071 5.7046 2.19 0.007 2.15 0.04 3.3×10 ⁷ A32 0.0172 3.8181 0.92 0.0089 1.51 0.04 4.2×10 ⁷ A33 0.0055 12.944 1.37 0.006 3.47 0.03 5.0×10 ⁷ A34 0.0094 9.1075 2.43 0.0054 2.98 0.04 4.5×10 ⁷ A35 0.0067 11.306 1.12 0.0038 4.13 0.04 4.9×10 ⁷ A36 0.0148 3.0737 0 0.0048 3.43 0.25 5.5×10 ⁶ A37 0 0 17.2 0 -1.6 0.16 4.3×10 ⁴ 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

Sample serial number mechanical properties mark YS(MPa) Ts(MPa) El(%) r _m Δr SWE(DBTT-℃) AI(%) A31 218 348 45 2.1 0.19 -40 0 IS A32 262 410 36 1.94 0.17 -40 0 IS A33 328 455 33 1.89 0.17 -40 0 IS A34 247 401 35 1.89 0.19 -50 0 IS A35 229 392 38 1.79 0.17 -40 0 IS A36 233 359 37 1.11 0.62 -60 1.56 CS A37 283 425 33 1.81 0.57 -40 0 CS

^* Note:

YS = yield strength, TS = tensile strength, El = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, SWE = secondary processing embrittlement, AI = aging index, IS = Steel product of the present invention, CS=comparative example steel product

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-rolled steel plate

Table 10

Sample serial number Chemical composition (weight %) C Mn Cu S Al N P B Nb Ti other A41 0.0008 0.07 0.15 0.009 0.025 0.0089 0.045 0.0009 0.018 0.03 Si: 0.03 A42 0.0015 0.15 0.12 0.014 0.034 0.011 0.082 0.001 0.039 0.039 Si: 0.12 A43 0.0028 0.12 0.16 0.011 0.029 0.0109 0.118 0.0007 0.03 0.038 Si: 0.09 A44 0.0012 0.15 0.1 0.02 0.03 0.013 0.035 0.0011 0.012 0.063 Si: 0.12Mo: 0.09 A45 0.0019 0.13 0.14 0.017 0.053 0.0132 0.034 0.0008 0.045 0.05 Si: 0.09Cr: 0.22 A46 0.0034 0.45 0.1 0.0083 0.038 0.0015 0.048 0.0005 0 0 A47 0.0038 0.07 0 0.012 0.035 0.0024 0.13 0.0005 0 0.17 Si: 0.08

Table 11

Sample serial number Cu+Mn S ^* (Mn/55+Cu/63.5)/(S ^* /32) (Ti ^* /48+Nb/93)/(C/12) N ^* (Al/27)/(N ^* /14) Average particle size of sediment (µm) Amount of sediment (mm ^-2 ) A41 0.22 0.006 19.324 1.27 0.0044 2.93 0.04 9.4×10 ⁷ A42 0.27 0.0093 15.901 2.03 0.0058 3.03 0.03 9.0×10 ⁷ A43 0.28 0.0067 22.527 0.93 0.0051 2.94 0.04 8.2×10 ⁷ A44 0.25 0.0054 25.413 1.99 0.0039 3.99 0.04 7.9×10 ⁷ A45 0.27 0.0096 15.16 2.19 0.0063 4.37 0.03 9.6×10 ⁷ A46 0.55 0.0105 29.751 -1 0.0038 5.15 0.25 1.5×10 ⁴ A47 0.07 0 0 9.8 0 0 0.04 3.5×10 ⁵ 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

Sample serial number mechanical properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI(%) SWE(DBTT-℃) A41 222 357 43 2.22 0.09 0 -70 IS A42 260 409 35 1.93 0.06 0 -60 IS A43 332 453 34 1.73 0.06 0 -60 IS A44 229 367 40 2.18 0.08 0 -60 IS A45 231 359 45 1.89 0.07 0 -50 IS A46 202 355 38 1.59 0.39 0 -60 CS A47 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 = in-plane anisotropy index, AI = aging index, SWE = secondary processing embrittlement, IS = Steel product of the present invention, CS=comparative example steel product

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-rolled steel plate

Table 13

Sample serial number Chemical composition (weight%) C Mn P S Al Ti Nb B N other A51 0.0009 0.08 0.008 0.006 0.042 0.008 0.017 0.0005 0.0018 A52 0.0016 0.12 0.059 0.012 0.033 0.016 0.025 0.0008 0.0012 Si: 0.11 A53 0.0026 0.12 0.094 0.021 0.043 0.026 0.039 0.0011 0.0033 Si: 0.31 A54 0.0012 0.11 0.129 0.013 0.033 0.016 0.038 0.0009 0.0012 Si: 0.26Mo: 0.14 A55 0.0015 0.13 0.053 0.021 0.039 0.032 0.011 0.0009 0.0011 Si: 0.33Cr: 0.24 A56 0.0028 0.48 0.052 0.009 0.033 0.022 0.021 0.0005 0.0024 Si: 0.05 A57 0.0015 0.13 0.118 0.008 0.038 0 0 0 0.0021 Si: 0.35

Table 14

Sample serial number S ^* (Mn/55)/(S ^* /32) (Ti ^* /48+Nb/93)/(C/12) Average particle size of sediment (μm) Amount of sediment (mm ^-2 ) A51 0.0044 10.66 1.29 0.06 3.8×10 ⁶ A52 0.0052 13.37 1.75 0.06 4.6×10 ⁶ A53 0.012 5.8373 1.14 0.05 5.2×10 ⁶ A54 0.0062 10.286 3.48 0.06 4.1×10 ⁶ A55 0.0055 13.647 1.58 0.06 3.9×10 ⁶ A56 0.0008 359.18 1.38 0.28 1.2×10 ⁴ A57 0.0111 6.8313 -2.6 0.18 6.3×10 ⁵ S ^* =S-0.8×(Ti-0.8×(48/14)×N)×(32/48)Ti ^* =Ti-0.8×((48/14)×N+(48/32)×S)

Table 15

Sample serial number Mechanical Properties mark YS(MPa) TS(MPa) El(%) r _m (Δr) AI(%) SWE(DBTT-℃) A51 188 302 49 2.09 0.25 0 -60 IS A52 221 352 42 1.93 0.22 0 -50 IS A53 256 409 38 1.73 0.19 0 -40 IS A54 270 444 34 1.69 0.21 0 -40 IS A55 231 362 43 1.87 0.21 0 -50 IS A56 202 356 41 1.85 0.29 0 -40 CS A57 254 401 37 1.28 0.54 2.33 -40 CS

^* Note:

YS = yield strength, TS = tensile strength, El = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, AI = aging index, SWE = secondary processing embrittlement, IS = Steel product of the present invention, CS=comparative example steel product

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-rolled steel plate

Table 16

Sample serial number Chemical composition (weight%) C P S Al Ti Nb B N other A61 0.0007 0.01 0.009 0.042 0.039 0.03 0.0008 0.011 A62 0.0015 0.037 0.017 0.053 0.029 0.042 0.0009 0.0074 Si: 0.11 A63 0.0028 0.073 0.012 0.049 0.048 0.039 0.0009 0.0123 Si: 0.22 A64 0.0014 0.119 0.011 0.038 0.029 0.044 0.0011 0.0085 Si: 0.12Mo: 0.12 A65 0.0007 0.035 0.016 0.037 0.04 0.022 0.0009 0.0089 Si: 0.11Cr: 0.27 A66 0.0025 0.074 0.011 0.039 0.025 0.022 0.0005 0.0018 Si: 0.05 A67 0.0015 0.053 0.012 0.038 0 0 0 0.0026 Si: 0.35

Table 17

Sample serial number (Ti ^* /48+Nb/93)/(C/12) N ^* (Al/27)/(N ^* /14) Average particle size of sediment (μm) Amount of sediment (mm ^-2 ) A61 4.83 0.0044 4.93 0.05 6.8×10 ⁵ A62 1.66 0.0054 5.1 0.05 5.3×10 ⁵ A63 1.78 0.0045 5.7 0.05 7.2×10 ⁵ A64 2.71 0.0048 4.09 0.05 5.5×10 ⁵ A65 2.77 0.004 4.74 0.05 6.3×10 ⁵ A66 1.82 -0.001 -twenty one 0.05 2.8×10 ⁴ A67 0 0.006 3.31 0.05 3.3×10 ⁴ Ti ^* =Ti-0.8×((48/14)×N+(48/32)×S)N ^* =N-0.8×(Ti-0.8×(48/32)×S)×(14/48)

Table 18

Sample serial number mechanical properties Mark YS(MPa) TS(MPa) E1(%) r _m Δr SWE(DBTT-℃) AI(%) A61 218 352 43 2.03 0.25 -60 0 IS A62 228 369 42 1.82 0.24 -50 0 IS A63 269 417 36 1.73 0.27 -50 0 IS A64 289 452 33 1.71 0.29 -50 0 IS A65 222 358 40 1.83 0.21 -60 0 IS A66 202 356 40 1.92 0.34 -40 0 CS A67 254 401 37 1.28 0.54 -40 2.33 CS

^* Note:

YS = yield strength, TS = tensile strength, El = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, SWE = secondary processing embrittlement, AI = aging index, IS = Steel of the present invention, CS=comparative example 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-rolled steel plate

Table 19

Sample serial number Chemical composition (weight%) C Mn P S Al Ti Nb B N other A71 0.0007 0.09 0.009 0.012 0.039 0.028 0.025 0.0006 0.0084 A72 0.0017 0.14 0.038 0.015 0.039 0.041 0.047 0.0005 0.0109 Si: 0.14 A73 0.0025 0.16 0.078 0.012 0.047 0.039 0.029 0.0007 0.011 A74 0.0014 0.09 0.12 0.021 0.043 0.063 0.013 0.0011 0.0128 Si: 0.12Mo: 0.11 A75 0.0015 0.13 0.042 0.016 0.059 0.052 0.048 0.0009 0.013 Si: 0.1Cr: 0.29 A76 0.0028 0.48 0.052 0.009 0.033 0.022 0.021 0.0005 0.0024 Si: 0.05 A77 0.0015 0.13 0.118 0.008 0.038 0 0 0 0.0021 Si: 0.35

Table 20

Sample serial number S ^* (Mn/55)/(S ^* /32) (Ti ^* /48+Nb/93)/(C/12) V ^* (Al/27)/(N ^* /14) Average particle size of sediment (μm) Amount of sediment (mm ^-2 ) A71 0.0094 5.5976 1.24 0.0052 3.87 0.05 2.9×10 ⁷ A72 0.0091 8.9723 2.55 0.0055 3.65 0.05 3.5×10 ⁷ A73 0.0073 12.767 0.94 0.0053 4.63 0.05 3.3×10 ⁷ A74 0.0061 8.5498 1.68 0.004 5.6 0.04 4.5×10 ⁷ A75 0.0073 10.384 3.65 0.0053 5.72 0.04 4.2×10 ⁷ A76 0.0008 359.18 1.38 -2E-04 -80 0.28 1.2×10 ⁴ A77 0.0111 6.8313 0 0.0043 4.54 0.18 6.3×10 ⁵ 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

Sample serial number mechanical properties mark YS(MPa) TS(MPa) E1(%) r _m Δr SWE(DBTT-℃) AI (%) A71 215 347 46 2.1 0.25 -60 0 IS A72 254 404 38 1.91 0.27 -40 0 IS A73 265 411 36 1.61 0.22 -50 0 IS A74 292 450 32 1.65 0.24 -50 0 IS A75 246 395 37 1.67 0.22 -50 0 IS A76 202 356 41 1.85 0.29 -40 0 CS A77 254 401 37 1.28 0.54 -40 2.33 CS

^* Note:

YS = yield strength, TS = tensile strength, E1 = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, AI = aging index, SWE = secondary processing embrittlement, IS = Steel product of the present invention, CS=comparative example steel product

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-rolled steel plate

Table 22

sample Chemical composition (weight%) serial number C P S Al Cu Ti Nb B N Other B81 0.0017 0.009 0.009 0.039 0.11 0.005 0.005 0.0005 0.0012 B82 0.0016 0.032 0.007 0.042 0.09 0.004 0.004 0.0007 0.0021 B83 0.0018 0.048 0.01 0.034 0.09 0.005 0.004 0.0004 0.0005 Si: 0.05 B84 0.0026 0.083 0.011 0.038 0.09 0.012 0.005 0.0008 0.0021 Si: 0.15 B85 0.0028 0.11 0.012 0.042 0.14 0.02 0.006 0.001 0.0022 Si: 0.26 B86 0.0025 0.086 0.008 0.042 0.1 0.01 0.005 0.0007 0.0016 Si: 0.19Mo: 0.071 B87 0.0025 0.084 0.01 0.033 0.15 0.009 0.004 0.0006 0.0016 Si: 0.21Cr: 0.21 B88 0.0017 0.065 0.012 0.035 0.11 0.033 0.02 0.0009 0.0012 B89 0.0039 0.123 0.011 0.035 0 0 0 0.0008 0.0025

Table 23

Sample serial number (Cu/63.5)/(S ^* /32) Cs Average particle size of sediment (μm) Amount of sediment (mm ^-2 ) B81 6.85 17 0.06 3.3×10 ⁶ B82 5.71 16 0.06 3.5×10 ⁶ B83 5.62 18 0.06 3.1×10 ⁶ B84 5.91 26 0.05 4.5×10 ⁶ B85 15.5 21.3 0.05 4.8×10 ⁶ B86 10.1 25 0.05 5.2×10 ⁶ B87 10 25 0.05 4.1×10 ⁶ B88 -14 -36 0.08 2.5×10 ⁶ B89 0 39 0.08 6.2×10 ⁴ S ^* =S-0.8×(Ti-0.8×(48/14)×N)×(32/48)Cs=(C-Nb×12/93-Ti ^* ×12/48)×10000, Ti ^* = Ti-0.8×((48/14)×N+(48/32)×S)

Table 24

Sample serial number Mechanical Properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI(%) BH value (MPa) SWE(DBTT-℃) B81 189 308 49 2.04 0.31 0 42 -40 IS B82 193 320 45 2.01 0.34 0 44 -50 IS B83 209 352 43 1.93 0.28 0 37 -40 IS B84 276 406 39 1.78 0.25 0 58 -50 IS B85 335 450 35 1.62 0.19 0 55 -60 IS B86 329 452 36 1.55 0.21 0 49 -50 IS B87 333 449 34 1.66 0.24 0 45 -50 IS B88 210 346 42 1.98 0.22 0 0 -50 CS B89 285 463 29 1.22 0.28 3.9 89 -70 CS

^* Note:

YS = yield strength, TS = tensile strength, El = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, AI = aging index, SWE = secondary processing embrittlement, IS = Steel product of the present invention, CS=comparative example steel product

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-rolled steel plate

Table 25

samples Chemical composition (weight%) serial number C Mn P S A1 Cu Ti Nb B N other B91 0.0018 0.11 0.009 0.008 0.038 0.06 0.007 0.004 0.0005 0.0019 B92 0.0016 0.1 0.023 0.01 0.042 0.11 0.009 0.005 0.0008 0.0022 B93 0.0015 0.09 0.042 0.011 0.028 0.08 0.006 0.005 0.0006 0.0005 Si: 0.1 B94 0.0021 0.11 0.08 0.009 0.041 0.11 0.011 0.012 0.0008 0.0012 Si: 0.22 B95 0.0028 0.12 0.1 0.008 0.031 0.16 0.011 0.009 0.0005 0.002 Si: 0.31 B96 0.0019 0.09 0.081 0.011 0.042 0.11 0.005 0.008 0.0011 0.0029 Si: 0.25Mo: 0.15 B97 0.0023 0.1 0.078 0.008 0.035 0.13 0.007 0.005 0.0008 0.002 Si: 0.3Cr: 0.27 B98 0.0025 0.55 0.05 0.009 0.037 0 0.022 0.018 0.0009 0.0028 B99 0.0041 0.11 0.116 0.017 0.038 0.08 0 0 0.009 0.0021 Si: 0.33

Table 26

Sample serial number Cu+Mn (Mn/55+Cu/63.5)/(S ^* /32) Cs Average particle size of sediment (μm) Amount of sediment (mm ^-2 ) B91 0.17 13.4 18 0.05 3.5×10 ⁶ B92 0.21 13.5 16 0.05 3.7×10 ⁶ B93 0.17 10.9 15 0.05 3.3×10 ⁶ B94 0.22 24.4 13.2 0.05 5.2×10 ⁶ B95 0.28 29.7 26.6 0.05 5.5×10 ⁶ B96 0.2 8.57 19 0.05 4.3×10 ⁶ B97 0.23 17.2 twenty three 0.04 5.9×10 ⁶ B98 0.55 235 4.52 0.29 2.5×10 ⁴ B99 0.19 5.2 41 0.06 2.7×10 ⁵ S ^* =S-0.8×(Ti-0.8×(48/14)×N)×(32/48)Cs=(C-Nb×12/93-Ti ^* ×12/48)×10000, Ti ^* = Ti-0.8×((48/14)×N+(48/32)×S)

Table 27

Sample serial number Mechanical Properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI BH value (MPa) SWE(DBTT-℃) B91 197 308 47 1.95 0.31 0 42 -40 IS B92 210 332 47 1.92 0.29 0 35 -50 IS B93 222 350 45 1.92 0.27 0 35 -50 IS B94 292 405 39 1.71 0.22 0 44 -60 IS B95 341 456 35 1.65 0.2 0 61 -50 IS B96 338 452 34 1.62 0.23 0 42 -60 IS B97 333 457 35 1.61 0.22 0 46 -50 IS B98 193 347 41 1.99 0.35 0 0 -50 CS B99 327 446 32 1.17 0.2 5.3 93 -60 CS

^* Note:

YS = yield strength, TS = tensile strength, El = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, AI = aging index, SWE = secondary processing embrittlement, IS = Steel product of the present invention, CS=comparative example steel product

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-rolled steel plate

Table 28

Sample serial number Chemical composition (weight%) C P S Al Cu Ti Nb B N Other B01 0.0015 0.008 0.008 0.052 0.09 0.009 0.004 0.0006 0.0073 B02 0.0017 0.022 0.01 0.038 0.11 0.01 0.004 0.0009 0.011 B03 0.0018 0.045 0.008 0.032 0.12 0.005 0.003 0.0005 0.0075 Si: 0.07 B04 0.0023 0.081 0.011 0.052 0.13 0.011 0.004 0.001 0.0103 Si: 0.14 B05 0.0026 0.118 0.011 0.028 0.16 0.021 0.005 0.0009 0.012 Si: 0.2 B06 0.0021 0.046 0.021 0.052 0.09 0.038 0.004 0.0009 0.0118 Mo: 0.082 B07 0.0015 0.045 0.008 0.067 0.12 0.011 0.003 0.0007 0.0071 Cr: 0.23 B08 0.0022 0.044 0.01 0.028 0 0.021 0.022 0.0009 0.0015 B09 0.0042 0.12 0.009 0.052 0.13 0 0 0.0008 0.0073 Si: 0.15

Table 29

Sample serial number (Cu/63.5)/(S ^* /32) (Al/27)/(N ^* /14) Cs Average particle size of sediment (μm) Amount of sediment (mm ^-2 ) B01 3.27 3.624 15 0.06 2.7×10 ⁶ B02 2.67 1.718 17 0.06 3.9×10 ⁶ B03 3.71 1.935 18 0.06 3.6×10 ⁶ B04 3.24 2.493 twenty three 0.05 6.4×10 ⁶ B05 4.65 1.426 26 0.05 8.3×10 ⁶ B06 2.52 3.059 twenty one 0.05 8.5×10 ⁶ B07 4.83 5.129 15 0.04 7.9×10 ⁶ B08 0 -24.2 -8.5 0.2 3.7×10 ⁴ B09 3.33 2.746 42 0.05 5.1×10 ⁶ S ^* =S-0.8×(Ti-0.8×(48/14)×N)×(32/48)Cs=(C-Nb×12/93-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

Sample serial number Mechanical Properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI BH value (MPa) SWE(DBTT-℃) B01 201 315 48 2.05 0.29 0 38 -40 IS B02 213 347 46 1.96 0.27 0 42 -50 IS B03 212 353 42 1.93 0.27 0 38 -40 IS B04 294 418 35 1.79 0.24 0 53 -50 IS B05 323 451 34 1.69 0.21 0 48 -40 IS B06 254 394 38 1.79 0.28 0 55 -50 IS B07 231 387 37 1.71 0.27 0 35 -40 IS B08 205 348 40 2.03 0.46 0 0 -50 CS B09 299 452 31 1.21 0.17 4.4 84 -40 CS

^* Note:

YS = yield strength, TS = tensile strength, El = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, AI = aging index, SWE = secondary processing embrittlement, IS = Steel product of the present invention, CS=comparative example steel product

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-rolled steel plate

Table 31

Sample serial number Chemical composition (weight %) C Mn P S Al Cu Ti Nb B N Other B11 0.0014 0.11 0.008 0.008 0.042 0.08 0.009 0.004 0.0007 0.0072 B12 0.0019 0.08 0.0028 0.008 0.036 0.11 0.011 0.004 0.0005 0.011 B13 0.0015 0.09 0.043 0.007 0.034 0.09 0.01 0.003 0.0009 0.011 Si: 0.09 B14 0.0024 0.11 0.082 0.009 0.042 0.13 0.01 0.004 0.0011 0.012 Si: 0.12 B15 0.0027 0.08 0.11 0.008 0.067 0.12 0.025 0.006 0.0009 0.0087 Si: 0.1 B16 0.0025 0.15 0.037 0.012 0.073 0.14 0.02 0.005 0.0009 0.0072 Si: 0.11Mo: 0.087 B17 0.0022 0.1 0.037 0.012 0.041 0.13 0.009 0.004 0.0007 0.014 Si: 0.13Cr: 0.31 B18 0.0013 0.55 0.044 0.007 0.03 0 0.03 0.012 0.0005 0.0027 B19 0.0045 0.08 0.121 0.013 0.04 0.15 0 0 0.0008 0.0018

Table 32

Sample serial number Cu+Mn (Mn/55+Cu/63.5)/(S ^* /32) (Al/27)/(N ^* /14) Cs Average particle size of sediment (μm) Amount of sediment (mm ^-2 ) B11 0.19 7.6 2.967 14 0.06 2.3×10 ⁷ B12 0.19 5.6 1.749 19 0.06 2.9×10 ⁷ B13 0.18 5.5 1.659 15 0.06 2.5×10 ⁷ B14 0.24 6.1 1.787 twenty four 0.05 4.2×10 ⁷ B15 0.2 14.5 6.803 27 0.05 2.9×10 ⁷ B16 0.29 13.3 6.423 25 0.05 3.1×10 ⁷ B17 0.23 4.47 1.393 twenty two 0.04 3.4×10 ⁷ B18 0.55 -63 -6.65 -28 0.27 1.2×10 ⁴ B19 0.23 7.81 3.813 45 0.06 9.5×10 ⁵ S ^* =S-0.8×(Ti-0.8×(48/14)×N)×(32/48)Cs=(C-Nb×12/93-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

Sample serial number Mechanical Properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI BH value (MPa) SWE(DBTT-℃) B11 192 320 48 2.06 0.31 0 37 -40 IS B12 211 349 46 1.98 0.29 0 48 -60 IS B13 221 359 42 1.93 0.27 0 33 -60 IS B14 252 403 37 1.78 0.27 0 45 -50 IS B15 321 457 34 1.62 0.31 0 58 -60 IS B16 234 355 41 1.88 0.27 0 51 -60 IS B17 222 351 42 1.87 0.3 0 51 -50 IS B18 189 359 42 1.95 0.38 0 0 -50 CS B19 336 461 27 1.27 0.21 3.5 96 -60 CS

^* Note:

YS = yield strength, TS = tensile strength, El = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, AI = aging index, SWE = secondary processing embrittlement, IS = Steel product of the present invention, CS=comparative example steel product

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-rolled steel plate

Table 34

Sample serial number Chemical composition (weight%) C Mn P S Al Ti Nb B N other B21 0.0018 0.09 0.011 0.009 0.036 0.011 0.004 0.0005 0.0019 B22 0.0015 0.07 0.054 0.012 0.042 0.012 0.003 0.0007 0.0016 B23 0.0023 0.1 0.064 0.009 0.023 0.01 0.004 0.0008 0.0021 Si: 0.15 B24 0.0025 0.07 0.11 0.009 0.037 0.008 0.005 0.0005 0.003 B25 0.0028 0.12 0.09 0.01 0.024 0.015 0.006 0.0011 0.0015 Mo: 0.1 B26 0.0024 0.12 0.095 0.008 0.031 0.014 0.004 0.001 0.0016 Cr: 0.19 B27 0.0019 0.47 0.042 0.011 0.03 0.028 0.016 0.0007 0.002 B28 0.0042 0.32 0.12 0.01 0.024 0 0 0.0014 0.0013

Table 35

Sample serial number (Mn/55)/(S ^* /32) Cs Average particle size of sediment (μm) Amount of sediment (mm ^-2 ) B21 8.86 18 0.06 1.9×10 ⁵ B22 5.13 15 0.06 1.8×10 ⁵ B23 8.63 twenty three 0.05 2.7×10 ⁵ B24 4.46 25 0.05 3.3×10 ⁶ B25 16.6 twenty three 0.05 2.9×10 ⁶ B26 24.3 18.8 0.04 4.1×10 ⁶ B27 -271 -13 0.28 1.1×10 ⁴ B28 15.6 42 0.22 7.4×10 ³ S ^* =S-0.8×(Ti-0.8×(48/14)×N)×(32/48)Cs=(C-Nb×12/93-Ti ^* ×12/48)×10000, Ti ^* = Ti-0.8×((48/14)×N+(48/32)×S)

Table 36

Sample serial number mechanical properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI (%) BH value (MPa) SWE(DBTT-℃) B21 188 305 49 2.18 0.34 0 48 -40 IS B22 221 350 43 2.09 0.3 0 33 -50 IS B23 245 397 37 1.88 0.29 0 43 -50 IS B24 316 444 33 1.62 0.26 0 54 -50 IS B25 275 452 33 1.55 0.24 0 55 -40 IS B26 319 446 31 1.5 0.21 0 49 -50 IS B27 219 362 38 2.09 0.37 0 0 -50 CS B28 251 466 26 1.2 0.19 4.2 87 -60 CS

^* Note:

YS = yield strength, TS = tensile strength, El = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, AI = aging index, SWE = secondary processing embrittlement, IS = Steel of the present invention, CS=comparative example 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-rolled steel plate

Table 37

Sample serial number Chemical composition (weight%) C P S Al Ti Nb B N Other B31 0.0019 0.009 0.01 0.047 0.008 0.004 0.0005 0.0094 B32 0.0017 0.047 0.01 0.059 0.009 0.003 0.0008 0.0072 Si: 0.03 B33 0.0024 0.086 0.008 0.067 0.016 0.003 0.001 0.0068 Si: 0.11 B34 0.0026 0.118 0.012 0.047 0.027 0.005 0.0009 0.0125 Si: 0.25 B35 0.0024 0.037 0.01 0.051 0.036 0.003 0.0007 0.011 Si: 0.26Mo: 0.074 B36 0.0026 0.115 0.009 0.039 0.01 0.005 0.0011 0.01 Si: 0.22Cr: 0.23 B37 0.0022 0.057 0.011 0.035 0.02 0.024 0.0007 0.0011 B38 0.0045 0.125 0.015 0.042 0 0 0.0008 0.012

Table 38

Sample serial number (Al/27)/(N ^* /14) Cs Average particle size of sediment (μm) Amount of sediment (mm ^-2 ) B31 2.358 19 0.06 5.1×10 ⁶ B32 3.872 17 0.06 4.3×10 ⁶ B33 6.547 twenty four 0.05 4.4×10 ⁶ B34 2.549 26 0.05 6.3×10 ⁶ B35 4.897 twenty four 0.05 5.2×10 ⁶ B36 1.985 26 0.04 7.4×10 ⁶ B37 19.87 -83 0.29 1.1×10 ⁴ B38 1.344 45 0.06 2.8×10 ⁶ Cs=(C-Nb×12/93-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

Sample serial number Mechanical Properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI(%) BH value (MPa) SWE(DBTT-℃) B31 221 325 47 2.02 0.31 0 43 -40 IS B32 232 352 44 1.87 0.27 0 35 -50 IS B33 263 409 37 1.76 0.26 0 58 -50 IS B34 325 450 31 1.7 0.28 0 58 -50 IS B35 232 358 42 1.81 0.29 0 49 -50 IS B36 334 463 31 1.55 0.28 0 58 -50 IS B37 205 369 38 2.11 0.33 0 0 -40 CS B38 343 461 29 1.19 0.22 4.3 109 -40 CS

^* Note:

YS = yield strength, TS = tensile strength, El = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, AI = aging index, SWE = secondary processing embrittlement, IS = Steel of the present invention, CS=comparative example 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-rolled steel plate

Table 40

Sample serial number Chemical composition (weight %) C Mn P S Al Ti Nb B N Other B41 0.0012 0.11 0.009 0.008 0.052 0.009 0.004 0.0005 0.0072 B42 0.0017 0.09 0.024 0.011 0.039 0.011 0.003 0.0009 0.013 B43 0.0014 0.07 0.046 0.006 0.067 0.019 0.003 0.0007 0.0069 Si: 0.04 B44 0.0022 0.12 0.073 0.01 0.039 0.013 0.005 0.0005 0.0103 Si: 0.1 B45 0.0022 0.08 0.113 0.01 0.05 0.029 0.004 0.001 0.012 Si: 0.11 B46 0.0027 0.09 0.038 0.012 0.048 0.035 0.005 0.0008 0.0093 Si: 0.21Mo: 0.083 B47 0.0025 0.13 0.04 0.011 0.048 0.018 0.003 0.0011 0.011 Cr: 0.26 B48 0.0028 0.68 0.043 0.013 0.038 0.03 0.02 0.0005 0.0021 B49 0.0044 0.08 0.12 0.009 0.025 0 0 0.0011 0.0067 Si: 0.05

Table 41

Sample serial number (Mn/55)/(S ^* /32) (Al/27)/(N ^* /14) Cs Average particle size of sediment (μm) Amount of sediment (mm ^-2 ) B41 4.66 3.673 12 0.05 5.2×10 ⁶ B42 2.17 1.496 17 0.05 7.5×10 ⁶ B43 6.83 8.378 14 0.05 6.7×10 ⁶ B44 3.85 2.009 twenty two 0.05 6.9×10 ⁶ B45 3.85 3.227 twenty two 0.04 9.6×10 ⁶ B46 7.55 5.539 27 0.05 5.9×10 ⁶ B47 4.32 2.519 25 0.05 7.8×10 ⁶ B48 5495 -15.6 -6.1 0.21 1.2×10 ⁴ B49 2.48 1.406 44 0.06 8.7×10 ⁵ S ^* =S-0.8×(Ti-0.8×(48/14)×N)×(32/48)Cs=(C-Nb×12/93-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

Sample serial number Mechanical Properties mark YS(MPa) TS(MPa) El(%) r _m Δr AI (%) BH value (MPa) SWE(DBTT-℃) B41 196 308 48 2.03 0.3 0 37 -40 IS B42 211 349 47 1.92 0.29 0 47 -50 IS B43 220 362 43 1.87 0.31 0 38 -50 IS B44 263 390 37 1.7 0.28 0 44 -40 IS B45 320 457 32 1.62 0.21 0 51 -60 IS B46 231 364 43 1.73 0.31 0 57 -50 IS B47 218 360 44 1.61 0.28 0 53 -50 IS B48 209 359 39 1.92 0.37 0 0 -40 CS B49 356 471 28 1.25 0.18 5.6 93 -60 CS

^* Note:

YS = yield strength, TS = tensile strength, El = elongation, r _m = plasticity-anisotropy index, Δr = in-plane anisotropy index, AI = aging index, SWE = secondary processing embrittlement, IS = Steel of the present invention, CS=comparative example 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.

industrial application

As can be understood from the above description, according to the cold-rolled steel sheet of the present invention, the fine precipitates are distributed in the Nb-Ti composite IF steel, so that fine grains are formed, and as a result, the in-plane anisotropy index is reduced and the yield strength.

Claims (40)

1. cold-rolled steel sheet with less anisotropy and high-yield-ratio, described cold-rolled steel sheet has the composition that comprises following component, by weight: be less than or equal to Cu, the 0.005-0.08% of 0.01% C, 0.01-0.2% S, be less than or equal to 0.1% Al, be less than or equal to 0.004% N, be less than or equal to Nb, the Ti of 0.005-0.15% of B, the 0.002-0.04% of 0.2% P, 0.0001-0.002%, and the Fe of surplus and other unavoidable impurities

Wherein, described composition satisfies following relational expression:

1≤(Cu/63.5)/(S ^*/32)≤30，

S ^*＝S-0.8×(Ti-0.8×(48/14)×N)×(32/48)，

Described steel plate comprises the CuS throw out that mean particle size is less than or equal to 0.2 μ m.

2. cold-rolled steel sheet as claimed in claim 1 is characterized in that described composition also comprises the Mn of 0.01-0.3%, and satisfies following relational expression: 1≤(Mn/55+Cu/63.5)/(S ^*/ 32)≤30, described steel plate comprises (Mn, Cu) the S throw out that mean particle size is less than or equal to 0.2 μ m.

3. cold-rolled steel sheet as claimed in claim 1 is characterized in that N content is 0.004-0.02%, and described composition satisfies following relational expression:

1≤(Al/27)/(N ^*/14)≤10，

N ^*＝N-0.8×(Ti-0.8×(48/32)×S)×(14/48)，

Described steel plate comprises the AlN throw out that mean particle size is less than or equal to 0.2 μ m.

4. cold-rolled steel sheet as claimed in claim 1 is characterized in that described composition also comprises the Mn of 0.01-0.3% and the N of 0.004-0.02%, and satisfies following relational expression:

1≤(Mn/55+Cu/63.5)/(S ^*/32)≤30，

1≤(Al/27)/(N ^*/14)≤10，

N ^*＝N-0.8×(Ti-0.8×(48/32)×S)×(14/48)，

Described steel plate comprises (Mn, Cu) S throw out and the AlN throw out that mean particle size is less than or equal to 0.2 μ m.

5. cold-rolled steel sheet with less anisotropy and high-yield-ratio, described cold-rolled steel sheet has the composition that comprises following component, by weight: be less than or equal to 0.01% C, be less than or equal to 0.08% S, be less than or equal to 0.1% Al, be less than or equal to 0.004% N, be less than or equal to 0.2% P, the B of 0.0001-0.002%, the Nb of 0.002-0.04%, the Ti of 0.005-0.15%, at least a Cu that is selected from following component: 0.01-0.2%, the Mn of 0.01-0.3% and the N of 0.004-0.2%, and the Fe of surplus and other unavoidable impurities

Described composition satisfies following relational expression:

1≤(Mn/55+Cu/63.5)/(S ^*/32)≤30，

1≤(Al/27)/(N ^*/ 14)≤10, wherein N content is more than or equal to 0.004%,

S ^*＝S-0.8×(Ti-0.8×(48/14)×N)×(32/48)，

N ^*＝N-0.8×(Ti-0.8×(48/32)×S)×(14/48)，

Described steel plate comprises and is selected from (Mn, Cu) at least a throw out in S throw out and the AlN throw out that mean particle size is less than or equal to 0.2 μ m.

6. as claim 1 or 5 described cold-rolled steel sheets, it is characterized in that the content of C, Ti, Nb, N and S satisfies following relational expression:

0.8≤(Ti ^*/48+Nb/93)/(C/12)≤5.0，

Ti ^*＝Ti-0.8×((48/14×N+(48/32)×S)。

7. cold-rolled steel sheet as claimed in claim 6 is characterized in that C content is less than or equal to 0.005%.

8. as claim 1 or 5 described cold-rolled steel sheets, it is characterized in that the solute carbon of being determined by C and Ti content (Cs) is 5-30, Cs=(C-Nb * 12/93-Ti ^** 12/48) * 10000, Ti wherein ^*=Ti-0.8 * ((48/14) * N+ (48/32) * S), condition is to work as Ti ^*Less than 0 o'clock, Ti ^*Be defined as 0.

9. cold-rolled steel sheet as claimed in claim 8 is characterized in that C content is 0.001-0.01%.

10. as each described cold-rolled steel sheet in the claim 1 to 5, it is characterized in that described cold-rolled steel sheet satisfies the yield tensile ratio (yield strength/tensile strength) more than or equal to 0.58.

11., it is characterized in that sedimentary quantity is more than or equal to 1 * 10 as each described cold-rolled steel sheet in the claim 1 to 5 ⁶/ mm ²

12., it is characterized in that P content is for being less than or equal to 0.015% as claim 1 or 5 described cold-rolled steel sheets.

13., it is characterized in that P content is 0.03-0.2% as claim 1 or 5 described cold-rolled steel sheets.

14., it is characterized in that described composition also comprises one or both components among the Cr of the Si that is selected from 0.1-0.8% and 0.2-1.2% as claim 1 or 5 described cold-rolled steel sheets.

15., it is characterized in that described composition also comprises the Mo of 0.01-0.2% as claim 1 or 5 described cold-rolled steel sheets.

16. cold-rolled steel sheet as claimed in claim 14 is characterized in that described composition also comprises the Mo of 0.01-0.2%.

17. as claim 2,4 or 5 described cold-rolled steel sheets, the total amount that it is characterized in that Mn and Cu is 0.05-0.4%.

18., it is characterized in that Mn content is 0.01-0.12% as claim 2,4 or 5 described cold-rolled steel sheets.

19., it is characterized in that (Mn/55+Cu/63.5)/(S as claim 2,4 or 5 described cold-rolled steel sheets ^*/ 32) value is 1-9.

20., it is characterized in that (Al/27)/(N as any one described cold-rolled steel sheet in the claim 3 to 5 ^*/ 14) value is 1-6.

21. a manufacturing has the method for the cold-rolled steel sheet of less anisotropy and high-yield-ratio, this method may further comprise the steps:

With the plate slab reheat to the temperature that is greater than or equal to 1100 ℃, described plate slab has the composition that comprises following component, by weight: be less than or equal to Cu, the 0.005-0.08% of 0.01% C, 0.01-0.2% S, be less than or equal to 0.1% Al, be less than or equal to 0.004% N, be less than or equal to Nb, the Ti of 0.005-0.15% of B, the 0.002-0.04% of 0.2% P, 0.0001-0.002%, and the Fe of surplus and other unavoidable impurities, this composition satisfies following relational expression: 1≤(Cu/63.5)/(S ^*/ 32)≤30, S ^*=S-0.8 * (Ti-0.8 * (48/14) * N) * (32/48);

Be greater than or equal to Ar ₃Plate slab to described reheat under the final rolling temperature of transformation temperature carries out hot rolling, obtains the hot rolled steel plate;

To cool off described hot-rolled steel sheet more than or equal to 300 ℃/minute speed;

Described cooling metal sheets is reeled being less than or equal under 700 ℃ the temperature;

Carry out cold rolling to described coiling steel plate;

Described cold-rolled steel sheet is carried out continuous annealing, and described cold-rolled steel sheet comprises the CuS throw out that mean particle size is less than or equal to 0.2 μ m.

22. method as claimed in claim 21 is characterized in that described composition also comprises the Mn of 0.01-0.3%, and satisfies following relational expression: 1≤(Mn/55+Cu/63.5)/(S ^*/ 32)≤30, described steel plate comprises (Mn, Cu) the S throw out that mean particle size is less than or equal to 0.2 μ m.

23. method as claimed in claim 21 is characterized in that N content is 0.004-0.02%, described composition satisfies following relational expression:

1≤(Al/27)/(N ^*/14)≤10，

N ^*＝N-0.8×(Ti-0.8×(48/32)×S)×(14/48)，

Described steel plate comprises the AlN throw out that mean particle size is less than or equal to 0.2 μ m.

24. method as claimed in claim 21 is characterized in that described composition also comprises the Mn of 0.01-0.3%, N content is 0.004-0.02%, and described composition satisfies following relational expression:

1≤(Mn/55+Cu/63.5)/(S ^*/32)≤30，

1≤(Al/27)/(N ^*/14)≤10，

N ^*＝N-0.8×(Ti-0.8×(48/32)×S)×(14/48)，

Described steel plate comprises (Mn, Cu) S throw out and the AlN throw out that mean particle size is less than or equal to 0.2 μ m.

25. a manufacturing has the method for the cold-rolled steel sheet of less anisotropy and high-yield-ratio, this method may further comprise the steps:

With the plate slab reheat to the temperature that is greater than or equal to 1100 ℃, described plate slab has the composition that comprises following component, by weight: be less than or equal to 0.01% C, be less than or equal to 0.08% S, be less than or equal to 0.1% Al, be less than or equal to 0.004% N, be less than or equal to 0.2% P, the B of 0.0001-0.002%, the Nb of 0.002-0.04%, the Ti of 0.005-0.15%, at least a Cu that is selected from following component: 0.01-0.2%, the Mn of 0.01-0.3% and the N of 0.004-0.2%, and the Fe of surplus and other unavoidable impurities, this composition satisfies following relational expression:

1≤(Mn/55+Cu/63.5)/(S ^*/32)≤30，

1≤(Al/27) (N ^*/ 14)≤10, wherein N content is more than or equal to 0.004%,

S ^*＝S-0.8×(Ti-0.8×(48/14)×N)×(32/48)，

N ^*＝N-0.8×(Ti-0.8×(48/32)×S)×(14/48)；

Be greater than or equal to Ar ₃Under the final rolling temperature of transformation temperature described reheat plate slab is carried out hot rolling, obtain the hot rolled steel plate;

To cool off described hot-rolled steel sheet more than or equal to 300 ℃/minute speed;

Described cooling metal sheets is reeled being less than or equal under 700 ℃ the temperature;

Carry out cold rolling to described coiling steel plate;

Described cold-rolled steel sheet is carried out continuous annealing, and described cold-rolled steel sheet comprises and is selected from (Mn, Cu) the sedimentary at least a throw out of S and AlN that mean particle size is less than or equal to 0.2 μ m.

26., it is characterized in that the content of C, Ti, Nb, N and S satisfies following relational expression as claim 21 or 25 described methods:

0.8≤(Ti ^*/48+(Nb/93)/(C/12)≤5.0，

Ti ^*＝Ti-0.8×((48/14)×N+(48/32)×S)。

27. method as claimed in claim 26 is characterized in that C content is for being less than or equal to 0.005%.

28., it is characterized in that the solute carbon of being determined by C and Ti content (Cs) is 5-30, Cs=(C-Nb * 12/93-Ti as claim 21 or 25 described methods ^** 12/48) * 10000, Ti wherein ^*=Ti-0.8 * ((48/14) * N+ (48/32) * S), condition is to work as Ti ^*Less than 0 o'clock, Ti ^*Be defined as 0.

29. method as claimed in claim 28 is characterized in that C content is 0.001-0.01%.

30., it is characterized in that cold-rolled steel sheet satisfies the yield tensile ratio (yield strength/tensile strength) more than or equal to 0.58 as each described method in the claim 21 to 25.

31., it is characterized in that sedimentary quantity is more than or equal to 1 * 10 as each described method in the claim 21 to 25 ⁶/ mm ²

32., it is characterized in that P content is for being less than or equal to 0.015% as claim 21 or 25 described methods.

33., it is characterized in that P content is 0.03-0.2% as claim 21 or 25 described methods.

34., it is characterized in that described composition also comprises one or both components among the Cr of the Si that is selected from 0.1-0.8% and 0.2-1.2% as claim 21 or 25 described methods.

35., it is characterized in that described composition also comprises the Mo of 0.01-0.2% as claim 21 or 25 described methods.

36. method as claimed in claim 34 is characterized in that described composition also comprises the Mo of 0.01-0.2%.

37., it is characterized in that Mn and Cu total amount are 0.08-0.4% as claim 22,24 or 25 described methods.

38., it is characterized in that Mn content is 0.01-0.12% as claim 22,24 or 25 described methods.

39., it is characterized in that (Mn/55+Cu/63.5)/(S as claim 22,24 or 25 described methods ^*/ 32) value is 1-9.

40., it is characterized in that (Al/27)/(N as claim 23,24 or 25 described methods ^*/ 14) value is 1-6.