KR101970095B1 - High-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more excellent in workability and impact property and a method for manufacturing the same - Google Patents

Abstract

A high strength cold rolled steel sheet having a tensile strength of 980 MPa or higher, which has good processability evaluated by ductility and elongation flangeability, and which is excellent in collision characteristics, and a process for producing the same. A high strength cold rolled steel sheet in which the metal structure at the 1/4 position of the sheet thickness satisfies the following (1) to (4).
(1) the area ratio of the ferrite is more than 10% and not more than 65%, and the remainder is at least one selected from the group consisting of bainitic ferrite, bainite and tempering martensite, including quenched martensite and retained austenite Lt; / RTI >
(2) The volume ratio V _? Of the retained austenite is 5% or more and 30% or less.
(3) the area ratio V _MA of the MA structure in which quenched martensite and retained austenite are combined is 3% or more and 25% or less, and the average circle-equivalent diameter of the MA structure is 2.0 μm or less.
(4) The ratio V _MA / V _? Of the area ratio V _MA of the MA structure to the volume ratio V _? Of the retained austenite is 0.50 to 1.50.

Description

High-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more excellent in workability and impact property and a method for manufacturing the same

The present invention relates to a high strength cold rolled steel sheet having a tensile strength of 980 MPa or more, which is excellent in workability and impact property, and a method for producing the same. Specifically, the high-strength cold-rolled steel sheet, the high-strength galvanized steel sheet having an electro-galvanized layer formed on the surface of the high-strength cold-rolled steel sheet, the high-strength hot-dip galvanized steel sheet having a hot- To a high-strength alloyed hot-dip galvanized steel sheet on which a galvannealed hot-dip galvanized layer is formed on the surface of a steel sheet, and a method for producing the same.

In order to realize low fuel consumption of automobiles and transportation vehicles, it is desired to reduce the weight of automobiles and transport vehicles. In order to reduce the weight, it is effective to reduce the plate thickness by using, for example, a high-strength steel plate. However, if the steel sheet is made to have a high strength, ductility and stretch flangeability are deteriorated, so that workability in a product shape is deteriorated.

In addition, from the viewpoint of corrosion resistance, the surface of the steel part for automobiles may be coated with electro-galvanized steel (hereinafter sometimes referred to as EG), hot-dip galvanized steel (hereinafter sometimes referred to as GI), galvannealed (Hereinafter may be collectively referred to as a galvanized steel sheet) is often used in many cases. These galvanized steel sheets are required to have strength and workability as in the high strength steel sheet.

For example, Patent Document 1 discloses a method in which a ferrite has a metal structure in which martensite or retained austenite is mixed and has a high tensile strength TS of 490 to 880 MPa due to its composite structure, A steel sheet is disclosed.

In Patent Document 2, it is described that TS (Tensile Strength) is 590 MPa or more and moldability is excellent, specifically TS 占 EL (elongation, elongation) is 23000 MPa% or more, Resistant steel sheet excellent in corrosion resistance after coating even in a severe environment such as a steel sheet or a composite cycle corrosion test. The metal structure of this steel sheet is a structure including ferrite, retained austenite, bainite and / or martensite. It is described that the retained austenite has an effect of improving the ductility of the steel sheet, that is, a so-called TRIP effect.

[0004] However, steel parts for automobiles are also required to have excellent collision characteristics that efficiently absorb impact when a car is hit. As a technique for improving the collision characteristic, for example, Patent Document 3 is known. Patent Document 3 discloses a high strength galvanized steel sheet having a maximum tensile strength of 900 MPa or more excellent in collision absorbing energy and capable of achieving a maximum static strength of 900 MPa or more and a static friction ratio of 590 MPa in a steel sheet level, have. This manufacturing method is characterized in that after performing galvanization, cooling, and rolling using a roll having an illuminance (Ra) of 3.0 or less.

Japanese Patent No. 3527092 Japanese Patent No. 5076434 Japanese Patent No. 5487916

According to the techniques described in Patent Documents 1 and 2, the workability of the steel sheet can be improved. However, nothing is considered about the collision characteristic. On the other hand, according to the technique described in Patent Document 3, the collision characteristics of the steel sheet can be improved. However, nothing is taken into account for the processability which is evaluated as ductility and elongation flangeability.

Disclosure of the Invention The present invention has been made in view of the above circumstances and provides a high strength cold rolled steel sheet having a tensile strength of 980 MPa or more and a high strength cold rolled steel sheet having good workability evaluated by ductility and stretch flangeability, It is on. Another object of the present invention is to provide a high-strength galvanized steel sheet having a high-strength cold-rolled steel sheet having an electro-galvanized layer on the surface thereof, a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the high- A hot-dip galvanized steel sheet having a galvannealing layer on its surface. Another object of the present invention is to provide a method of manufacturing a high strength cold rolled steel sheet, a high strength hot dip galvanized steel sheet and a high strength alloyed hot dip galvanized steel sheet having the above characteristics.

A high strength cold rolled steel sheet having a tensile strength of 980 MPa or more according to the present invention is characterized by containing 0.10 to 0.5% of C, 1.0 to 3% of Si, 1.5 to 7% of Mn, P: not less than 0% to not more than 0.1%, S: not less than 0% to not more than 0.05%, Al: not less than 0.005% to not more than 1%, N: more than 0% to not more than 0.01% Added iron and inevitable impurities. Further, the metal structure at 1/4 of the plate thickness satisfies the following (1) to (4). On the other hand, MA is an abbreviation of Martensite-Austenite Constituent.

(1) When the metal structure is observed with a scanning electron microscope, the area ratio of the ferrite to the entire metal structure is more than 10% but not more than 65%, and the balance includes quenched martensite and retained austenite, Ferrite, bainite, and tempering martensite. The term " hard phase "

(2) When the metal structure is measured by X-ray diffractometry, the volume percentage V _? Of the retained austenite is 5% or more and 30% or less with respect to the entire metal structure.

(3) When the metal structure is observed with an optical microscope, the area ratio V _MA of the MA structure in which the quenched martensite and the retained austenite are combined is not less than 3% and not more than 25% The average circle equivalent diameter is 2.0 占 퐉 or less.

(4) The ratio V _MA / V _γ of the area ratio V _MA of the MA structure to the volume ratio V _γ of the retained austenite satisfies the following formula (i).

0.50? V _MA / V _?? 1.50 ... (i)

The steel sheet may further contain, as other elements, in mass%

(a) at least one member selected from the group consisting of Cr: more than 0% to 1% and Mo: more than 0% to 1%

(b) at least one selected from the group consisting of Ti: more than 0% to 0.15% or less, Nb: more than 0% to 0.15% or less, and V: more than 0%

(c) at least one selected from the group consisting of Cu: more than 0% to 1% and Ni: more than 0% to 1%

(d) B: more than 0% and not more than 0.005%

(e) at least one member selected from the group consisting of Ca: more than 0% to 0.01% or less, Mg: more than 0% to 0.01% or less, and REM:

And the like.

The present invention also provides a hot-dip galvanized steel sheet having a high-strength galvanized steel sheet having an electro-galvanized layer on the surface of the high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the high- A high strength alloyed hot-dip galvanized steel sheet having a plated layer is also included.

The high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more excellent in workability and collision characteristics according to the present invention is obtained by subjecting a steel satisfying the above-mentioned composition to a rolling finish of 5 to 25% in the final stand of finish rolling, _three or more hot rolling to less than 900 ℃, and wound by a take-up temperature less than 600 ℃, cooled to room temperature, cold rolling, and the average rate of temperature rise 10 ℃ / a second or more than 800 ℃ Ac ₃ point below the temperature of (Uniformly heated) for 50 seconds or longer at the corresponding temperature range, and is cooled to an optional cooling stop temperature T ° C in a temperature range of 50 ° C or higher and Ms point or lower at an average cooling rate of 10 ° C / Cooling it and heating it to maintain it at the above-mentioned cooling stop temperature T ° C or higher and 550 ° C or lower for at least 50 seconds, and then cooling it to room temperature.

The high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more excellent in workability and impact properties according to the present invention can be obtained by subjecting a steel satisfying the above-mentioned composition to a rolling finish in a final stand of finish rolling in an amount of 5 to 25% the Ar ₃ to point to or higher than 900 ℃ wound to below the hot rolling, 600 ℃ the coiling temperature, cooling to room temperature, cold rolling, and the average rate of temperature rise 10 ℃ / a second or more than 800 ℃ Ac less than ₃ points , Cooled at an average cooling rate of 10 DEG C / sec or more to an optional cooling stop temperature T DEG C in a temperature range of 50 DEG C or more and Ms point or less, , And the mixture is heated and maintained at the above-mentioned cooling stop temperature in a temperature range of more than T ° C to 550 ° C or more for 50 seconds or more, and further subjected to hot-dip galvanization in a holding time period and then cooled to room temperature It can be prepared by.

The high-strength alloyed hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more and excellent in workability and impact properties according to the present invention is characterized in that the steel satisfying the above-mentioned composition is rolled by 5 to 25% in the final stand of finish rolling, to a temperature in a range from 900 ℃ Ar ₃ point and hot-rolled, wound by the take-up temperature less than 600 ℃, then cooled to room temperature, cold rolling, and the average rate of temperature increase above 800 ℃ to 10 ℃ / s or Ac ₃ point And is cooled at an average cooling rate of 10 占 폚 / sec or more to an optional cooling stop temperature T 占 폚 in a temperature range of 50 占 폚 or more and Ms point or less. And maintained at a temperature in the range of more than T ° C and not more than 550 ° C for not less than 50 seconds during the cooling stoppage and is subjected to hot dip galvanization within the holding time, After Li may be prepared by cooling down to room temperature.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, a high-strength electrogalvanized steel sheet excellent in both workability evaluated by ductility and elongation flangeability, A galvanized steel sheet and a high strength alloyed hot-dip galvanized steel sheet can be provided. The high strength cold rolled steel sheet, the high strength galvanized steel sheet, the high strength hot dip galvanized steel sheet and the high strength galvannealed steel sheet according to the present invention are particularly excellent in ductility among the processability. Further, according to the present invention, it is possible to provide a method of manufacturing the high-strength cold-rolled steel sheet, the high-strength electro-galvanized steel sheet, the high-strength hot-dip galvanized steel sheet and the high-strength galvannealed steel sheet. The high-strength cold-rolled steel sheet, high-strength galvanized steel sheet, high-strength hot-dip galvanized steel sheet and high-strength galvannealed steel sheet according to the present invention are extremely useful particularly in industrial fields such as automobiles.

1 is a schematic explanatory view showing an example of a heat treatment pattern performed in the embodiment.

The inventors of the present invention have conducted intensive investigations to improve both ductility, stretch flangeability and collision characteristics of a high strength cold rolled steel sheet having a tensile strength of 980 MPa or more. As a result, in order to secure the tensile strength, in order to improve the ductility after setting the ferrite fraction in the metal structure within the predetermined range and the residual structure in the hard phase, a predetermined amount of ferrite is produced, to that suitably controls for the entire, hardened martensite and non-V _MA / V _γ of the residual austenite area ratio of the compound of MA organization V _MA and the retained austenite volume ratio of V _γ, the stretch flangeability In order to improve the MA structure, it has been found that, in order to improve the collision property, it is necessary to make the MA structure finer, the MA structure can be miniaturized and the non-V _MA / V _? .

First, the metal structure characterizing the present invention will be described.

The high strength cold rolled steel sheet according to the present invention is characterized in that the metal structure at the 1/4 position of the plate thickness satisfies the following (1) to (4).

(1) When the metal structure is observed with a scanning electron microscope, the area ratio of the ferrite to the entire metal structure is more than 10% but not more than 65%, and the balance includes quenched martensite and retained austenite, Ferrite, bainite, and tempering martensite. The term " hard phase "

(2) When the metal structure is measured by X-ray diffractometry, the volume percentage V _? Of the retained austenite is 5% or more and 30% or less with respect to the entire metal structure.

(3) When the metal structure is observed with an optical microscope, the area ratio V _MA of the MA structure in which the quenched martensite and the retained austenite are combined is not less than 3% and not more than 25% The average circle equivalent diameter is 2.0 占 퐉 or less.

(4) The volume ratio V _? Of the retained austenite and the area ratio V _MA of the MA structure satisfy the following formula (i).

0.50? V _MA / V _?? 1.50 ... (i)

The metal structure is observed at 1/4 of the plate thickness on the basis of the steel plate.

On the other hand, the fraction of the metal structure prescribed in the above (1) to (3) differs from the measurement method in some cases. That is, in the above (1), the metal structure is observed with a scanning electron microscope, and the measured area ratio is a ratio when the entire metal structure is taken as 100%. In the area ratio measured using a scanning electron microscope, quenched martensite and retained austenite are included as hard surface area ratios. On the other hand, in the above (2), the residual austenite fraction in the metal structure is calculated by the X-ray diffraction method, and in the above (3), the area ratio of the MA structure in which the quenched martensite and the residual austenite are combined is referred to as optical It is observed under a microscope. Therefore, the fraction of retained austenite and quenching martensite is measured in duplicate by a plurality of methods. Hereinafter, the residual austenite may be referred to as residual? In some cases. Therefore, when the respective fractions of the metal structure prescribed in the above (1) to (3) are summed up, they may exceed 100%. In addition, the structure in which the quenched martensite and the residual? Are combined is referred to as MA structure.

(1) In the present invention, when the metal structure is observed with a scanning electron microscope, the area ratio of the ferrite to the entire metal structure is set to be more than 10% but not more than 65%. Ferrite is a structure that improves ductility, especially ductility, of a steel sheet. In order to exhibit such an effect, in the present invention, the area ratio of the ferrite is set to exceed 10%. The area ratio of the ferrite is preferably 15% or more, and more preferably 20% or more. However, if the ferrite is excessive, the strength of the steel sheet is lowered, and a tensile strength of 980 MPa or more can not be secured. Therefore, in the present invention, the area ratio of ferrite is set to 65% or less. The area ratio of the ferrite is preferably 60% or less, more preferably 50% or less.

The remainder of the metal structure is a hard phase comprising at least one member selected from the group consisting of bainitic ferrite, bainite, and tempering martensite, including quench martensite and residual y as an essential structure. These hard phases are harder than ferrite, and the strength of the steel sheet can be increased to 980 MPa or more by forming a predetermined amount of ferrite and then making the remaining structure hard. The inclusion of quenched martensite and residual? As an essential structure is to produce a predetermined amount of MA structure in which quenched martensite and residual? Are combined, as described below.

The metal structure may include at least one selected from the group consisting of pearlite and cementite in addition to ferrite and hard phase. The total area ratio of pearlite and cementite is not particularly limited as long as the effect of the present invention is not impaired, but is preferably 20% or less, for example. The total area ratio is more preferably 15% or less, and more preferably 10% or less.

The area ratio of the metal structure may be calculated by observation with a scanning electron microscope after the ¼ position of the plate thickness is eroded and removed, and the observation magnification may be set to, for example, 1000 times.

(2) In the present invention, when the metal structure is measured by the X-ray diffraction method, the volume ratio V _{? Of the} residual _{? Is set} to 5% or more and 30% or less with respect to the entire metal structure. The residual? Is deformed by distorting when the steel sheet is processed, and is transformed into martensite, thereby promoting the hardening of the deformed portion at the time of processing and suppressing the concentration of distortion. Therefore, the strength-elongation balance of the steel sheet is improved, and the ductility can be improved. In order to exhibit such an effect, the volume ratio of residual? Needs to be 5% or more. The volume ratio of the residual? Is preferably at least 6%, more preferably at least 7%. However, if the volume ratio of the residual? Is excessively increased, the stretch flangeability deteriorates. Therefore, in the present invention, the volume ratio of the residual? Is set to 30% or less. The volume ratio of the residual? Is preferably 25% or less, more preferably 20% or less.

The volume ratio of the residual? May be obtained by measuring the 1/4 position of the plate thickness by the X-ray diffraction method. On the other hand, the residual? Exists between the laths of the bainitic ferrite or exists in the MA structure. Since the effect of the residual? Is exerted irrespective of the existence form, in the present invention, the volume ratios are obtained by adding up the amounts of all residual? Measured by the X-ray diffraction method, regardless of the existence form.

(3) In the present invention, when the metal structure is observed with an optical microscope, the area ratio V _MA of the MA structure is set to 3% or more and 25% or less with respect to the entire metal structure. The MA structure is a structure for improving the strength-elongation balance of a steel sheet, and can improve ductility. In order to exhibit such an effect, it is necessary to set the area ratio of the MA structure to 3% or more. The area ratio of the MA structure is preferably 4% or more, and more preferably 5% or more. However, if the area ratio of the MA structure excessively increases, the collision characteristics deteriorate. Therefore, in the present invention, the area ratio of the MA structure is set to 25% or less. The area ratio of the MA structure is preferably not more than 23%, more preferably not more than 20%.

In the present invention, the average circle-equivalent diameter of the MA structure is set to 2.0 탆 or less. By making the MA structure finer, stretch flangeability and collision characteristics can be improved. In order to exhibit such an effect, the average circle-equivalent diameter of the MA structure needs to be 2.0 占 퐉 or less. The average circle-equivalent diameter of the MA structure is preferably 1.8 占 퐉 or less, more preferably 1.5 占 퐉 or less. On the other hand, as the MA structure becomes finer, the stretch flangeability and collision characteristics become better, the lower limit of the average circle equivalent diameter of the MA structure is not particularly limited, but it is limited to about 0.1 μm industrially.

The MA structure refers to a structure in which quenched martensite and residual? Are combined, and quenched martensitron means a structure in which untransformed austenite is martensitic in a process of cooling the steel sheet from a heating temperature to room temperature . The quench martensite can be distinguished from the tempering martensite tempered by heat treatment by observing with an optical microscope. That is, the quenching martensite is observed as white when observed with an optical microscope after referee corrosion of the metal structure, whereas the tempering martensite is observed as gray.

On the other hand, since quenching martensite and residual? Are difficult to distinguish from observation by observation with an optical microscope, in the present invention, a structure in which quenched martensite and residual? Are combined is measured as MA structure.

The area ratio of the MA structure is a value measured at the 1/4 plate thickness of the steel sheet.

The average circle-equivalent diameter of the MA structure is a value obtained by calculating a circle-equivalent particle size based on the area of each MA structure with respect to all the MA tissues identified in the observation field, and averaging these.

(4) In the present invention, it is important that the ratio V _MA / V _γ of the area ratio V _MA of the MA structure to the volume ratio V _γ of the residual γ satisfies the following formula (i).

0.50? V _MA / V _?? 1.50 ... (i)

By controlling the value of the ratio V _MA / V _y to satisfy the above formula (i), both the ductility and the collision characteristics can be achieved. That is, as described above, in the present invention, the residual? Is positively generated in order to improve the strength-elongation balance as an index of ductility. As a result, MA structure is unavoidably formed in the steel sheet. When the predetermined amount of residual? Is generated, the area ratio V _MA of the MA structure may be controlled so that the value of the ratio V _MA / V _{? Is not} less than 0.50 . The value of the ratio V _MA / V _{? Is} preferably 0.55 or more, and more preferably 0.60 or more. However, if the value of the non-V _MA / V _y is excessively large, MA structure is excessively generated. Since quenching martensite present in MA structure is a very hard tissue, when MA structure is excessively produced, cracks tend to occur at the interface with other tissues in collision, and the collision characteristics are rather deteriorated. Therefore, in the present invention, the value of the non- _VMA / V _{? Is} set to 1.50 or less in order to reduce the area ratio of the quenching martensite in the MA structure to secure the collision characteristics. The value of the ratio V _MA / V _{? Is} preferably 1.40 or less, more preferably 1.30 or less.

As described above, the metal structure of the high-strength cold-rolled steel sheet that characterizes the present invention has been described.

Next, the composition of the high-strength cold-rolled steel sheet according to the present invention will be described. On the other hand, "%" as to the composition of the steel sheet means "% by mass".

[C: 0.10% or more and 0.5% or less]

C is an element necessary for securing a tensile strength of 980 MPa or more, increasing the stability of residual?, And securing a predetermined amount of residual?. In the present invention, the C content is 0.10% or more. The amount of C is preferably 0.12% or more, and more preferably 0.15% or more. However, when the amount of C is excessive, the strength after hot rolling increases, cracking occurs in cold rolling, and weldability of the final product is deteriorated. Therefore, the C content should be 0.5% or less. The amount of C is preferably 0.40% or less, more preferably 0.30% or less, and still more preferably 0.25% or less.

[Si: 1.0% or more and 3% or less]

Si acts as a solid solution strengthening element and contributes to the strengthening of steel. In addition, Si is an element necessary for suppressing the generation of carbide and effectively acting to generate ferrite and residual?, Thereby securing excellent strength-elongation balance. In the present invention, the amount of Si is 1.0% or more. The Si content is preferably 1.2% or more, more preferably 1.35% or more, and further preferably 1.5% or more. However, when the amount of Si is excessive, a remarkable scale is formed at the time of hot rolling, scratch marks are formed on the surface of the steel sheet, and the surface properties are deteriorated. Also, the acidity is deteriorated. Therefore, the amount of Si should be 3% or less. The amount of Si is preferably 2.8% or less, and more preferably 2.6% or less.

[Mn: 1.5% or more and 7% or less]

Mn is an element contributing to the strengthening of the steel sheet by improving the hardenability. Mn is an element necessary for stabilizing? To generate residual?. In the present invention, the amount of Mn is 1.5% or more. The amount of Mn is preferably 1.6% or more, more preferably 1.7% or more, further preferably 1.8% or more, and even more preferably 2.0% or more. However, if the amount of Mn is excessive, the strength after hot rolling increases, cracking occurs during cold rolling or weldability of the final product deteriorates. Further, when Mn is excessively added, Mn segregates and causes deterioration in ductility and stretch flangeability. Therefore, in the present invention, the amount of Mn is 7% or less. The amount of Mn is preferably 5.0% or less, more preferably 4.0% or less, and further preferably 3.0% or less.

[P: more than 0% and not more than 0.1%]

P is an impurity element inevitably contained, and if it is contained in excess, the weldability of the final product deteriorates. Therefore, in the present invention, the P content is 0.1% or less. The P content is preferably 0.08% or less, and more preferably 0.05% or less. The amount of P is preferably as small as possible, but it is industrially difficult to set the amount to 0%. The lower limit of the amount of P is industrially 0.0005%.

[S: more than 0% and not more than 0.05%]

S, like P, is an impurity element inevitably contained, and if it is contained in excess, the weldability of the final product deteriorates. Further, S forms a sulfide inclusion in the steel sheet, which causes the ductility and stretch flangeability of the steel sheet to deteriorate. Therefore, in the present invention, the amount of S is 0.05% or less. The amount of S is preferably 0.01% or less, more preferably 0.005% or less. The amount of S should be as small as possible, but it is industrially difficult to set the amount of S to 0%. The lower limit of the amount of S is industrially 0.0001%.

[Al: 0.005% or more and 1% or less]

Al is an element acting as a deoxidizing agent. In order to exhibit such action, in the present invention, the amount of Al is set to 0.005% or more. The amount of Al is more preferably 0.01% or more. However, if the amount of Al is excessive, the weldability of the final product is remarkably deteriorated. Therefore, in the present invention, the amount of Al is 1% or less. The amount of Al is preferably 0.8% or less, more preferably 0.6% or less.

[N: more than 0% and not more than 0.01%]

N is an impurity element which is inevitably included, and if it is contained excessively, a large amount of nitride precipitates, which deteriorates ductility, stretch flangeability and collision characteristics. Therefore, in the present invention, the N content is set to 0.01% or less. The N content is preferably 0.008% or less, and more preferably 0.005% or less. On the other hand, a small amount of nitride contributes to the enhancement of the strength of the steel sheet, so the N content may be 0.001% or more.

[O: more than 0% and not more than 0.01%]

O is an impurity element which is inevitably contained, and if it is contained in excess, it is an element that lowers ductility and collision characteristics. Therefore, in the present invention, the amount of O is set to 0.01% or less. The amount of O is preferably 0.005% or less, and more preferably 0.003% or less. The amount of O is preferably as small as possible, but it is industrially difficult to make 0%. The lower limit of the amount of O is industrially 0.0001%.

The cold-rolled steel sheet according to the present invention satisfies the above composition and the balance is iron and unavoidable impurities. Examples of the inevitable impurities include P, S, N, and O, and other tramp elements such as Pb, Bi, Sb, and Sn, which may be mixed in the steel depending on the conditions of raw materials, materials, May be included.

The cold-rolled steel sheet of the present invention may further contain, as other elements,

(a) at least one member selected from the group consisting of Cr: more than 0% to 1% and Mo: more than 0% to 1%

(b) at least one selected from the group consisting of Ti: more than 0% to 0.15% or less, Nb: more than 0% to 0.15% or less, and V: more than 0%

(c) at least one selected from the group consisting of Cu: more than 0% to 1% and Ni: more than 0% to 1%

(d) B: more than 0% and not more than 0.005%

(e) at least one member selected from the group consisting of Ca: more than 0% to 0.01% or less, Mg: more than 0% to 0.01% or less, and REM:

And the like.

These elements (a) to (e) may be contained singly or arbitrarily in combination. The reason for setting this range is as follows.

[(a) at least one selected from the group consisting of Cr: more than 0% to 1% or less and Mo: more than 0% to 1%

Both Cr and Mo are effective elements for enhancing the hardenability and improving the strength of the steel sheet. In order to effectively exhibit such an effect, Cr and Mo are each preferably 0.1% or more, and more preferably 0.3% or more. However, if it is contained in excess, the ductility and stretch flangeability deteriorate. In addition, excessive addition is expensive. Therefore, when Cr and Mo are added alone, the content is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. Cr and Mo can be used alone or in combination. When Cr and Mo are used in combination, it is preferable that the total content of Cr and Mo is 1.5% or less in the above range when contained alone.

(b) at least one selected from the group consisting of Ti: more than 0% to 0.15% or less, Nb: more than 0% to 0.15% or less, and V: more than 0% to 0.15%

Ti, Nb and V are elements having carbide and nitride formed in the steel sheet to improve the strength of the steel sheet and to refine the spherical γ-grains. In order to effectively exhibit such an effect, Ti, Nb and V are each preferably 0.005% or more, and more preferably 0.010% or more. However, if it is contained in an excess amount, the carbide precipitates at the grain boundary, and the stretch flangeability and collision characteristics of the steel sheet deteriorate. Therefore, in the present invention, Ti, Nb and V are each preferably not more than 0.15%, more preferably not more than 0.12%, further preferably not more than 0.10%. These elements may be used alone or in any combination thereof.

[(c) at least one selected from the group consisting of Cu: more than 0% and 1% or less and Ni: more than 0% and 1%

Cu and Ni are elements effective for generation and stabilization of residual?. Cu and Ni also have an effect of improving the corrosion resistance of the steel sheet. In order to effectively exhibit such an effect, Cu and Ni are each preferably 0.05% or more, and more preferably 0.10% or more. However, when Cu is excessively contained, the hot workability deteriorates. Therefore, when Cu is added singly, it is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. On the other hand, if Ni is contained excessively, the cost becomes high. Therefore, the amount of Ni is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. Cu and Ni may be used alone or in combination. When the combination of Cu and Ni is used in combination, the above-mentioned action is easy to be manifested, and by containing Ni, deterioration of hot workability due to addition of Cu is easily suppressed. When Cu and Ni are used in combination, the total amount is preferably 1.5% or less, and more preferably 1.0% or less.

[(d) B: more than 0% and not more than 0.005%

B is an element that improves the hardenability and is an element that acts to stably maintain the austenite at room temperature. In order to exhibit such an effect effectively, the amount of B is preferably 0.0005% or more, more preferably 0.0010% or more, and still more preferably 0.0015% or more. However, if it is contained excessively, boride may be generated to deteriorate the ductility. Therefore, the amount of B is preferably 0.005% or less. The amount of B is more preferably 0.004% or less, and still more preferably 0.0035% or less.

[(e) at least one member selected from the group consisting of Ca: more than 0% to 0.01% or less, Mg: more than 0% to 0.01% or less, and REM:

Ca, Mg and REM are elements having an effect of finely dispersing inclusions in the steel sheet. In order to effectively exhibit such an effect, the amounts of Ca, Mg, and REM are preferably 0.0005% or more, and more preferably 0.0010% or more. However, if it is added in excess, it causes deterioration of main constitution, hot workability and the like. Therefore, the amounts of Ca, Mg, and REM are preferably 0.01% or less, more preferably 0.008% or less, and still more preferably 0.007% or less. These elements may be used alone or in any combination thereof. In the present invention, REM is an abbreviation of Rare earth metal (rare earth element), and it means a lanthanoid element, that is, 15 elements from La to Lu, and Sc and Y.

The high strength cold rolled steel sheet according to the present invention has been described above.

The high-strength cold-rolled steel sheet may have an electro-galvanized layer, a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer on its surface. That is, in the present invention, a high-strength galvanized steel sheet (hereinafter sometimes referred to as an EG steel sheet) having an electro-galvanized layer on the surface of the high-strength cold-rolled steel sheet, a high- Galvanized steel sheet (hereinafter sometimes referred to as GI steel sheet), and a high-strength alloyed hot-dip galvanized steel sheet having a galvannealed galvanized layer on the surface of the high-strength cold-rolled steel sheet (hereinafter sometimes referred to as a GA steel sheet) .

Next, a method of manufacturing a high-strength cold-rolled steel sheet according to the present invention will be described.

The high-strength cold-rolled steel sheet is hot-rolled by setting the steel satisfying the above-described composition of the constituents to a final stand of finish rolling at a rolling rate of 5 to 25% and a finishing rolling finish temperature of Ar ₃ to 900 ° C., Rolled to a room temperature, cold-rolled, heated to a temperature range of 800 ° C or more and less than Ac ₃ point at an average heating rate of 10 ° C / sec or more, and maintained at the temperature range for at least 50 seconds And cooled to an arbitrary cooling stopping temperature T ° C in a temperature range of 50 ° C or higher and Ms point or lower and cooled to an average cooling rate of 10 ° C / And then cooling the mixture to room temperature.

Hereinafter, description will be made in order.

[Rolling rate in the final stand of finishing rolling: 5 to 25%]

First, the steel satisfying the above composition is heated according to a conventional method. The heating temperature is not particularly limited, but is preferably 1000 to 1300 占 폚, for example. If the heating temperature is less than 1000 占 폚, solidification of carbide becomes insufficient, and sufficient strength is hardly obtained. On the other hand, if the heating temperature exceeds 1300 DEG C, the structure of the hot-rolled steel sheet is coarsened, and the MA structure of the cold-rolled steel sheet is also likely to be coarsened. As a result, the collision characteristic tends to be lowered.

After heating, hot rolling is performed. In the present invention, it is important to set the reduction rate in the final stand of the finish rolling to 5 to 25%. If the rolling ratio is less than 5%, the austenite grain size after hot rolling becomes coarse, and the average circle equivalent diameter of the MA structure in the cold-rolled steel sheet after annealing becomes large. As a result, stretch flangeability is deteriorated. Therefore, in the present invention, it is necessary to set the rolling rate to 5% or more. The reduction rate is preferably 6% or more, more preferably 7% or more, and still more preferably 8% or more. However, even if the rolling rate exceeds 25%, the average circle equivalent diameter of the MA structure becomes large, and the stretch flangeability and collision characteristics are deteriorated. This mechanism is unclear, but it is thought that the structure after hot rolling is heterogeneous. In the present invention, the rolling rate should be 25% or less. The reduction rate is preferably not more than 23%, more preferably not more than 20%.

[Finish rolling finish temperature: Ar ₃ point or more and 900 占 폚 or less]

If the finish rolling finish temperature is lower than the temperature of Ar ₃ point, the steel sheet structure after hot rolling becomes inhomogeneous and the stretch flangeability is lowered. On the other hand, when the finish rolling finish temperature exceeds 900 캜, recrystallization of austenite occurs and crystal grains are coarsened, and the average circle equivalent diameter of the MA structure in the cold-rolled steel sheet becomes large. As a result, stretch flangeability is deteriorated. Therefore, in the present invention, the finishing rolling finishing temperature needs to be 900 캜 or lower. The finishing rolling finishing temperature is preferably 890 DEG C or lower, more preferably 880 DEG C or lower.

On the other hand, the temperature of the Ar ₃ point was calculated based on the following formula (ii). In the formula, [] represents the content (mass%) of each element, and the content of the element not contained in the steel sheet may be calculated as 0 mass%.

Ar ₃ point (℃) = 910-310 × [C ] -80 × [Mn] -20 × [Cu] -15 × [Cr] -55 × [Ni] -80 × [Mo] ... (ii)

[Coiling temperature: 600 占 폚 or less]

When the coiling temperature exceeds 600 DEG C, the crystal grains become coarse, and the average circle equivalent diameter of the MA structure in the cold-rolled steel sheet becomes large. As a result, stretch flangeability is deteriorated. Therefore, in the present invention, the coiling temperature is set to 600 占 폚 or less. The coiling temperature is preferably 580 占 폚 or lower, more preferably 570 占 폚 or lower, still more preferably 550 占 폚 or lower.

[Cold Rolling]

After the hot rolling, it is rolled up, cooled to room temperature, pickled by a conventional method as required, and then cold-rolled according to a conventional method. The cold rolling rate in the cold rolling may be, for example, 30 to 80%.

[Annealing]

After the cold rolling, annealing is performed by heating at a temperature raising rate of not less than 800 ° C and not more than Ac ₃ point at an average heating rate of 10 ° C / sec or more, and maintaining at least 50 sec. When the average temperature raising rate to the above temperature range after cold rolling is less than 10 캜 / sec, the austenite lips grow and coarsen during heating, so that the average circle equivalent diameter of the MA structure in the cold rolled steel sheet becomes large, . Therefore, in the present invention, the average temperature raising rate is set to 10 ° C / second or more. The average temperature raising rate is preferably 12 캜 / second or more, and more preferably 15 캜 / second or more. The upper limit of the average temperature raising rate is not particularly limited, but is usually about 100 캜 / second at the maximum.

By setting the cracking temperature at 800 ° C or higher and lower than the Ac ₃ point, a desired amount of ferrite can be secured. If the cracking temperature is lower than 800 占 폚, the reverse transformation to the austenite becomes insufficient and the processed structure remains on the cold-rolled steel sheet, and the workability is lowered. Therefore, in the present invention, the cracking temperature is set at 800 DEG C or higher. The cracking temperature is preferably 805 DEG C or higher, and more preferably 810 DEG C or higher. However, cracks when the temperature is above the temperature of the Ac ₃ point, can not secure a desired amount of ferrite, the ductility is deteriorated. Therefore, in the present invention, the soaking temperature is below the temperature of the Ac ₃ point. The cracking temperature is preferably Ac ₃ point -10 캜 or lower, and more preferably Ac ₃ point -20 캜 or lower.

If the cracking time is less than 50 seconds, the processed structure remains on the cold-rolled steel sheet, and the ductility deteriorates. Therefore, in the present invention, the cracking time is set to 50 seconds or more. The cracking time is more preferably 60 seconds or more. The upper limit of the cracking time is not particularly limited, but if the cracking time is too long, the concentration of Mn in the austenite phase progresses, and the Ms point is lowered to increase and coarsen the MA structure in some cases. Therefore, the cracking time is preferably 3600 seconds or less, and more preferably 3000 seconds or less.

The crack holding at the above-mentioned temperature range does not have to be kept constant at the same temperature but may be varied by heating, cooling, or the like within the temperature range.

The temperature of the Ac ₃ point can be calculated based on the following formula (iii) described in " Leslie steel material chemistry " (Maruzen Co., Ltd., published on May 31, 1983, page 273). In the formula, [] represents the content (mass%) of each element, and the content of the element not contained in the steel sheet may be calculated as 0 mass%.

W = 30? X? ₃ ? C = 910-203 x [C] ^1/2 -15.2 x [Ni] + 44.7 x [Si] + 104 x [V] + 31.5 x [Mo] + 13.1 x [W] [Mn] + 11 x [Cr] + 20 x [Cu] -700 x [P] -400 x [Al] -120 x [As] -400 x [Ti] (iii)

[Cooling]

After the crack is retained, it is cooled to an arbitrary cooling stop temperature T ° C in a temperature range of 50 ° C or more and Ms or less. By cooling to this temperature range, the untransformed austenite can be transformed into the martensite and the hard bainite phase, and the MA structure can be made finer. At this time, the martensite is present as a quench martensite immediately after the transformation, but is tempered while remaining reheated in the post-process, and remains as a tempering martensite. This tempering martensite does not adversely affect the ductility, stretch flangeability and impact properties of the steel sheet. However, when the cooling stop temperature T exceeds the Ms point, no martensite is produced, and the MA structure generated in the reheating holding process at a high temperature is coarsened, the local deformability is lowered, and the stretch flangeability can be improved none. Therefore, in the present invention, the cooling stop temperature T is set to the temperature of the Ms point or less. The cooling stop temperature T is preferably Ms point -20 ° C or lower, more preferably Ms point -50 ° C or lower. On the other hand, if the cooling stop temperature T is lower than 50 캜, residual γ and MA structure are hardly produced, and ductility can not be improved. Therefore, in the present invention, the lower limit of the cooling stop temperature T is set to 50 DEG C or higher. The cooling stop temperature T is preferably 60 DEG C or higher, more preferably 70 DEG C or higher.

The temperature of the Ms point can be calculated based on the following formula (iv). In the formula, [] represents the content (mass%) of each element, and the content of the element not contained in the steel sheet may be calculated as 0 mass%. In the equation, Vf represents the area ratio of ferrite to the entire metal structure.

Ms point (° C.) = 561-474 × [C] / (1-Vf / 100) -33 × [Mn] -17 × [Ni] -17 × [Cr] (iv)

It is also important to set the average cooling rate up to the cooling stop temperature T in the above temperature range to 10 DEG C / second or more after the crack is retained. By appropriately controlling the cooling rate up to the cooling stop temperature T after the crack is maintained, excessive production of ferrite can be suppressed. That is, when the average cooling rate is less than 10 캜 / second, excessive ferrite is generated during cooling, and the tensile strength is lowered. Therefore, in the present invention, the average cooling rate is 10 ° C / second or more. The average cooling rate is preferably 15 ° C / second or more, and more preferably 20 ° C / second or more. On the other hand, the upper limit of the average cooling rate is not particularly limited, and may be cooled by water cooling or oil cooling.

[Reheating Process]

After cooling to an arbitrary cooling stop temperature T ° C in the temperature range of 50 ° C or higher and Ms point or lower, it is reheated to the above-mentioned cooling stop temperature T ° C to 550 ° C or lower, It is important to keep. The hard phase of the martensite or the like can be tempered and the untransformed austenite can be transformed into bainitic ferrite or bainite by reheating the quenching temperature to a temperature in the range of T ° C to 550 ° C. When the reheating is not performed, the balance between the residual? And the MA formation amount is deteriorated and the ratio V _MA / V _? Of the area ratio V _MA of the MA structure to the volume ratio V _{? Of the} residual? Can not be controlled within an appropriate range . As a result, the collision characteristics can not be improved. Further, the hard phase can not be tempered, and a high-density electric potential is also generated. Therefore, in the present invention, after cooling to the cooling stop temperature T, the wafer is reheated to a temperature exceeding the cooling stop temperature T. The reheating temperature is preferably T + 20 占 폚 or higher, more preferably T + 30 占 폚 or higher, and still more preferably T + 50 占 폚 or higher. However, if the reheating temperature exceeds 550 캜, residual γ and MA structure are hardly produced, so that the tensile strength is lowered, the value of TS × λ is also reduced, and the stretch flangeability can not be improved. Therefore, in the present invention, the reheating temperature is set to 550 占 폚 or less. The reheating temperature is preferably 520 占 폚 or lower, more preferably 500 占 폚 or lower, even more preferably 450 占 폚 or lower.

On the other hand, in the present invention, "reheating" means heating, that is, raising the temperature from the cooling stop temperature T, in a word. Therefore, even if the reheating temperature is higher than the cooling stop temperature T and the reheating temperature is, for example, a temperature range of 50 ° C to 550 ° C, the cooling stop temperature T and the reheating temperature are the same, When the reheating temperature is lower than T, it does not correspond to the reheating of the present invention.

After reheating to a temperature in the range of above the cooling-stop temperature T ° C to 550 ° C or lower, it is held for at least 50 seconds in the temperature range. If the reheating holding time is less than 50 seconds, MA structure is excessively generated, and ductility can not be improved. Further, since the MA structure is coarse and the average circle equivalent diameter can not be properly controlled, stretch flangeability can not be improved. In addition, since the ratio V _MA / V _gamma of the area ratio V _MA of the MA structure to the volume ratio V _gamma of the residual gamma can not be properly controlled, the collision characteristics can not be also improved. In addition, the hard phase can not be sufficiently tempered, and the transformation from untransformed austenite to bainitic ferrite or bainite does not proceed sufficiently. Therefore, in the present invention, the reheating holding time is set to 50 seconds or more. The reheating holding time is preferably 80 seconds or more, more preferably 100 seconds or more, and still more preferably 200 seconds or more. The upper limit of the reheating holding time is not particularly limited, but if the holding time is prolonged, the productivity is lowered and the tensile strength tends to decrease. From this point of view, the reheating holding time is preferably 1500 seconds or less, and more preferably 1000 seconds or less.

After reheating, cool to room temperature. The average cooling rate at the time of cooling is not particularly limited. For example, it is preferably 0.1 占 폚 / second or more, and more preferably 0.4 占 폚 / second or more. Further, the average cooling rate is preferably, for example, 200 占 폚 / second or less, and more preferably 150 占 폚 / second or less.

[Plating treatment]

After the reheating is maintained, the high-strength cold-rolled steel sheet according to the present invention obtained by cooling to room temperature may be subjected to electro-galvanizing, hot-dip galvanizing or alloying hot-dip galvanizing according to a conventional method.

For example, electro-galvanizing may be conducted by immersing the above-mentioned high-strength cold-rolled steel sheet in a zinc solution at 50 to 60 캜 (particularly 55 캜) while conducting electric galvanizing treatment. The plating adhesion amount is not particularly limited and may be, for example, about 10 to 100 g / m ² per one side.

The hot-dip galvanizing may be performed, for example, by immersing the high-strength cold-rolled steel sheet in a hot-dip galvanizing bath at a temperature of 300 ° C to 550 ° C. The plating time may be appropriately adjusted so as to secure a desired coating adhesion amount, and is preferably set to, for example, 1 to 10 seconds.

The galvannealed hot-dip galvanizing may be performed after the hot-dip galvanizing process. The alloying treatment temperature is not particularly limited. However, if the alloying treatment temperature is too low, the alloying does not sufficiently proceed. Therefore, the temperature is preferably 450 캜 or higher, more preferably 460 캜 or higher, and still more preferably 480 캜 or higher. However, if the alloying treatment temperature is excessively high, the alloying proceeds excessively, the Fe concentration in the plating layer becomes high, and the plating adhesion is deteriorated. From this viewpoint, the alloying treatment temperature is preferably 550 占 폚 or lower, more preferably 540 占 폚 or lower, and still more preferably 530 占 폚 or lower. The alloying treatment time is not particularly limited and may be adjusted so that the hot-dip galvanizing is alloyed. The alloying treatment time is, for example, 10 to 60 seconds.

The high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more excellent in workability and impact properties according to the present invention can be obtained by subjecting a steel satisfying the above-mentioned composition to a rolling finish in a final stand of finish rolling in an amount of 5 to 25% the Ar ₃ to point to or higher than 900 ℃ wound to below the hot rolling, 600 ℃ the coiling temperature, cooling to room temperature, cold rolling, and the average rate of temperature rise 10 ℃ / a second or more than 800 ℃ Ac less than ₃ points , Cooled at an average cooling rate of 10 DEG C / sec or more to an optional cooling stop temperature T DEG C in a temperature range of 50 DEG C or more and Ms point or less, , And the mixture is heated and maintained at the above-mentioned cooling stop temperature in a temperature range of more than T ° C to 550 ° C or more for 50 seconds or more, and further subjected to hot-dip galvanization in a holding time period and then cooled to room temperature Even it can be prepared. That is, the steps from the cooling stop temperature to the temperature in the temperature range from T ° C to 550 ° C are the same as the above-described manufacturing method of the high strength cold rolled steel sheet according to the present invention, It may be carried out by holding for more than 50 seconds and performing hot-dip galvanizing.

The hot-dip galvanizing may be carried out within the holding time at the reheating temperature, that is, at the cooling stop temperature T ° C or higher and 550 ° C or lower, and the specific plating method may be a conventional method. For example, if the steel sheet heated to a temperature in the range of above the cooling-stop temperature T ° C to 550 ° C or less is immersed in a plating bath adjusted to a temperature in the range of the cooling-stop temperature T ° C to 550 ° C or less and subjected to the hot- do. The plating time may be appropriately adjusted so as to secure a desired amount of plating within the time of holding the reheat. The plating time is preferably set to, for example, 1 to 10 seconds.

There are the following patterns (I) to (III) as a combination of hot-dip galvanizing treatment and reheating only heating and no plating treatment.

(I) is heated only, and then subjected to a hot-dip galvanizing treatment.

(II) After the hot dip galvanizing treatment, only heating is performed.

(III) heating is performed, then hot dip galvanizing is performed, and heating is performed again in this order.

The reheating temperature in the case of only performing the heating may be different from the temperature of the plating bath in performing the hot dip galvanizing. In the present invention, the temperature may be either heated to the other temperature or cooled. Examples of the heating method include furnace heating and induction heating.

The high-strength alloyed hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more and excellent in workability and impact properties according to the present invention is characterized in that the steel satisfying the above-mentioned composition is rolled by 5 to 25% in the final stand of finish rolling, to a temperature in a range from 900 ℃ Ar ₃ point and hot-rolled, wound by the take-up temperature less than 600 ℃, then cooled to room temperature, cold rolling, and the average rate of temperature increase above 800 ℃ to 10 ℃ / s or Ac ₃ point And is cooled at an average cooling rate of 10 占 폚 / sec or more to an optional cooling stop temperature T 占 폚 in a temperature range of 50 占 폚 or more and Ms point or less. And maintained at a temperature in the range of more than T ° C and not more than 550 ° C for not less than 50 seconds during the cooling stoppage and is subjected to hot dip galvanization within the holding time, Lee and then can be manufactured also by cooling to room temperature. That is, the steps from the cooling stop temperature to the temperature in the temperature range from T ° C to 550 ° C are the same as the above-described manufacturing method of the high strength cold rolled steel sheet according to the present invention, The holding for more than 50 seconds and the hot-dip galvanizing may be performed in combination, and then the hot-dip galvanizing layer may be alloyed and then cooled to room temperature.

The alloying treatment temperature is not particularly limited. However, if the alloying temperature is too low, the alloying does not proceed sufficiently. Therefore, the temperature is preferably 450 ° C or higher, more preferably 460 ° C or higher, and still more preferably 480 ° C or higher. However, if the alloying treatment temperature is excessively high, the alloying proceeds excessively, the Fe concentration in the plating layer becomes high, and the plating adhesion is deteriorated. From this viewpoint, the alloying treatment temperature is preferably 550 占 폚 or lower, more preferably 540 占 폚 or lower, and still more preferably 530 占 폚 or lower.

The alloying treatment time is not particularly limited and may be adjusted so that the hot-dip galvanizing is alloyed. The alloying treatment time is, for example, 10 to 60 seconds. On the other hand, since the alloying treatment is performed after the hot-dip galvanizing treatment is performed for a predetermined time in the temperature range exceeding the above-mentioned cooling stop temperature T ° C and 550 ° C or lower, the time required for the alloying treatment exceeds the above- It is not included in the holding time within the temperature range of 550 DEG C or lower.

The hot-dip galvanizing may be carried out within the holding time at the cooling stop temperature T ° C or higher and 550 ° C or lower, and if necessary, the steel sheet may be cooled to room temperature. The average cooling rate at the time of cooling is not particularly limited. For example, it is preferably 0.1 占 폚 / second or more, and more preferably 0.4 占 폚 / second or more. Further, the average cooling rate is preferably, for example, 200 占 폚 / second or less, and more preferably 150 占 폚 / second or less.

The high strength cold rolled steel sheet according to the present invention has a tensile strength of 980 MPa or more. The tensile strength is preferably 1000 MPa or more, and more preferably 1010 MPa or more. The high-strength cold-rolled steel sheet has excellent workability evaluated by ductility and elongation flangeability, and is also excellent in collision characteristics.

The ductility can be evaluated by the strength-elongation balance. In the present invention, the product of the tensile strength TS (MPa) and elongation EL (%) is at least 17000 MPa ·%. The value of TS EL is preferably 17100 MPa ·% or more, more preferably 17200 MPa ·% or more.

The tensile strength TS (MPa) and the hole expanding ratio? (%) Multiplied by 2000 MPa ·% or more are acceptable in the present invention. The value of TS x? Is preferably 21000 MPa ·% or more, more preferably 22000 MPa ·% or more.

The impact characteristic can be evaluated by the strength-VDA bending angle balance. In the present invention, the product of the tensile strength TS (MPa) and the VDA bending angle (DEG) is 9000 MPa · or more. The value of TS 占 VDA bending angle is preferably 90500 MPa 占 or more, and more preferably 91000 MPa 占 or more.

The thickness of the high-strength cold-rolled steel sheet according to the present invention is not particularly limited, but it is preferably a thin steel sheet of 6 mm or less.

This application claims the benefit of priority based on Japanese Patent Application No. 2015-071438 filed on March 31, 2015, and Japanese Patent Application No. 2015-225507 filed on November 18, 2015. The entire contents of the above Japanese Patent Application No. 2015-071438 and Japanese Patent Application No. 2015-225507 are hereby incorporated by reference.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited by the following Examples, and it is of course possible to carry out the present invention by making changes within the scope of the present invention, , All of which are included in the technical scope of the present invention.

And the remainder was made of a steel consisting of iron and unavoidable impurities, followed by hot rolling, cold rolling and continuous annealing to produce a cold-rolled steel sheet. In Table 1, " - " means not containing an element. Table 1 below shows the temperature of the Ar ₃ point calculated based on the formula (ii) and the temperature of the Ac ₃ point calculated based on the formula (iii). FIG. 1 shows an example of a heat treatment pattern performed by continuous annealing. In Fig. 1, reference numeral 1 denotes a heating step, 2 denotes a cracking step, 3 denotes a cooling step, 4 denotes a reheating holding step, and 5 denotes a cooling stopping temperature.

[Hot Rolling]

The slab obtained by the melting was heated to 1250 캜 and the rolling reduction rate in the final stand of the finish rolling was set to the reduction ratio shown in Table 2-1 or Table 2-2, Hot rolling was performed at a temperature shown in Table 2-2 to a plate thickness of 2.3 mm. After hot rolling, the steel sheet was cooled to the coiling temperature shown in Table 2-1 or Table 2-2 at an average cooling rate of 30 占 폚 / sec and rolled up. After winding, the steel sheet was air-cooled to room temperature to produce a hot-rolled steel sheet.

[Cold Rolling]

The obtained hot-rolled steel sheet was pickled to remove the scale of the surface, and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.2 mm.

[Continuous Annealing]

The obtained cold-rolled steel sheet was continuously annealed based on the heat treatment pattern shown in Fig. That is, the obtained cold-rolled steel sheet was heated to the cracking temperature shown in Table 2-1 or Table 2-2 at the average heating rate shown in Table 2-1 or Table 2-2 as a heating step, Crack temperature was maintained. Table 2-1 and Table 2-2 below show crack times. The following Tables 2-1 and 2-2 show values calculated by subtracting the crack temperature from the Ac ₃ point temperature.

After the cracking, the cooling step was cooled to the cooling stopping temperature T ° C shown in Table 2-1 or Table 2-2 at the average cooling rate shown in the following Table 2-1 or Table 2-2.

After cooling, the material was heated up to the reheating temperature shown in Table 2-1 or Table 2-2, and the reheating was maintained at the reheating temperature and then cooled to room temperature to produce a sealing material. Table 2-1 and Table 2-2 show the reheat holding time. The following Tables 2-1 and 2-2 show the values calculated by subtracting the cooling stop temperature T from the reheating temperature.

On the other hand, in Table 2-1, 8, and 11 are examples in which the reheating holding step is not performed after the cooling is stopped at the cooling stop temperature T shown in Table 2-1 below. That is, No. 8 was cooled to a cooling stop temperature T of 480 캜, cooled to 350 캜 lower than this temperature, and maintained at 350 캜 for 300 seconds. In the following Table 2-1, for the sake of convenience, the reheating temperature is described as 350 ° C, and the reheating retention time as 300 seconds. No. 11 was cooled at a cooling stop temperature T of 330 占 폚, cooled to 300 占 폚 lower than this temperature, and maintained at 300 占 폚 for 300 seconds. In Table 2-1, for convenience, the reheating temperature is described as 300 ° C, and the reheating time as 300 seconds.

[Electrolytic zinc plating]

Table 2-1. 2 is an example in which the above-mentioned specimen is immersed in a zinc plating bath at 55 ° C, subjected to electro-galvanizing treatment, and then washed with water and dried to produce an electro-galvanized steel sheet. The electro-galvanizing treatment was carried out at a current density of 40 A / dm ² . The amount of zinc plating adhered per side was 40 g / m ² . On the other hand, in the above electrogalvanizing treatment, a cleaning material such as an alkali aqueous solution dipping, degreasing, washing, pickling or the like was suitably performed to prepare a sealing material having an electroplated zinc layer on the surface of the cold-rolled steel sheet. In Table 2-1, In column of division of 2, we mentioned "EG".

[Hot-dip galvanizing]

Table 2-2. 36 is an example in which a hot dip galvanized steel sheet of 460 占 폚 was immersed in the hot dip galvanized steel sheet, and a hot-dip galvanized steel sheet was produced. The amount of hot dip galvanized coating was 30 g / m ² per side. In Table 2-2, We list "GI" in column of division of 36.

[Alloying Hot-dip galvanizing]

Table 2-1. 18 is an example of preparing a galvannealed galvanized steel sheet by immersing the specimen in a hot-dip galvanizing bath at 460 ° C, performing hot-dip galvanizing treatment, and then heating it to 500 ° C for alloying treatment. The amount of galvannealed galvanized coating was 30 g / m ² per side. In Table 2-1, We list "GA" in column of division of 18.

On the other hand, for the non-galvanized steel sheet, the hot-dip galvanized steel sheet, or the galvanized steel sheet, "cold-rolled steel sheet" is listed in the column of Table 2-1 and Table 2-2.

With respect to the obtained sealing material, the metal structure was observed in the following procedure.

[Observation of metal structure]

(Area ratio of ferrite and hard phase)

The cross-section of the obtained specimen was polished, then detached and corroded, and a 1/4 position of the plate thickness was photographed with a scanning electron microscope at a magnification of 1000 times at 3 o'clock. The observation field size is 100 占 퐉 100 占 퐉 at 1 field of view. The area ratio of the ferrite was measured by an oblique method with a lattice spacing of 5 占 퐉 and a lattice score of 20 占 20 to calculate an average value Vf of the three fields. The calculation results are shown in Tables 3-1 and 3-2. On the other hand, the area ratio of the ferrite was calculated excluding the area ratio of the hard phase present in the ferrite phase.

The Ms point was calculated from the formula (iv) on the basis of the composition of the components shown in the following Table 1 and the average area ratio Vf of the ferrite shown in Tables 3-1 and 3-2 below, -1 and Table 2-2. In Tables 2-1 and 2-2, values obtained by subtracting the temperature of the Ms point from the cooling stop temperature T were also shown.

Similarly, the total area ratio of pearlite and cementite was measured by the point-of-gravity method, and the average value of the three fields of view was calculated. The calculation results are shown in Tables 3-1 and 3-2. On the other hand, the total area ratio of pearlite and cementite is shown as " other structure " in the following Tables 3-1 and 3-2.

In the present embodiment, the structure other than ferrite, pearlite and cementite calculated by the above-mentioned point-graphening method is set to be a hard phase. That is, a value obtained by subtracting the area ratio of ferrite and the total area ratio of pearlite and cementite from 100% was calculated as the area ratio of the hard phase, and the results are shown in Tables 3-1 and 3-2.

On the other hand, as a result of observing the specific structure constituting the hard phase, the hard phase was at least one selected from the group consisting of bainitic ferrite, bainite and tempering martensite, including quench martensite and residual y.

(Volume ratio V _{? Of} residual?)

The obtained specimens were polished to 1/4 of the plate thickness using # 1000 to # 1500 sandpaper, and the surface was further electrolytically polished to a depth of 10 to 20 μm. Thereafter, the volume ratio of residual gamma -gas V _{? Was} measured. Specifically, a range of 40 ° to 130 ° in 2θ was measured by outputting 40 kV-200 mA using a Co target using "RINT 1500" manufactured by Rigaku Corporation as an X-ray diffractometer. From the obtained diffraction peaks (111), (200), (220), and (311) of diffraction peaks 110, 200, 211 and fcc _gamma] . The results are shown in Tables 3-1 and 3-2.

(Area ratio of MA structure V _MA and average circle equivalent diameter)

The cross-section of the obtained specimen was polished, then subjected to Repera erosion, and a 1/4 position of the plate thickness was observed with an optical microscope at a magnification of 1000 times at 3 o'clock. The observation field size is 100 占 퐉 100 占 퐉 at 1 field of view. The area ratio of the MA structure was measured by using an MA structure and a lattice spacing of 5 占 퐉 and a lattice score of 20 占 20, and the average value of the three fields of view was calculated. The calculation results are shown in Tables 3-1 and 3-2.

Images photographed with the optical microscope were subjected to image analysis, and the circle-equivalent diameter d of each MA structure was calculated, and an average value was obtained. The results are shown in Tables 3-1 and 3-2.

(The ratio of the area ratio of the volume rate V _MA and V _γ MA tissue of the residual γ)

Based on the volume percentage of residual γ measured by the above-described procedure V _γ and the area ratio of the MA organization V _MA, a ratio V _MA / V _γ of the area ratio of V _MA of the MA organization for the volume ratio of retained γ V _γ Respectively. The calculation results are shown in Tables 3-1 and 3-2.

Next, the obtained specimens were evaluated for mechanical properties, ductility, stretch flangeability, and collision characteristics in the following procedure.

[Evaluation of mechanical properties and ductility]

No. 5 test specimen specified in JIS Z2201 was cut out in such a manner that the direction perpendicular to the rolling direction of the obtained specimen was the longitudinal direction, and the tensile strength TS and elongation EL were measured by using this specimen. The measurement results are shown in Tables 3-1 and 3-2.

In the present embodiment, when the tensile strength is 980 MPa or more, it is evaluated as a high strength and when it is less than 980 MPa, it is evaluated as a failure because of lack of strength.

Further, on the basis of the measured tensile strength TS and elongation EL, the value of tensile strength TS x elongation EL was calculated. The calculation results are shown in Tables 3-1 and 3-2. The value of TS 占 EL L represents the strength-elongation balance, and is an index for evaluating ductility.

In the present embodiment, when the value of TS 占 EL is 17000 MPa 占% or more, the ductility is evaluated to be excellent and the acceptance is evaluated. When the value is less than 17000 MPa 占%, the ductility is worse.

[Evaluation of stretch flangeability]

In order to evaluate the stretch flangeability of the seal material, a hole expansion test was conducted based on the Steel Federation Standard JFST 1001 to measure the hole expanding ratio?. The measurement results are shown in Tables 3-1 and 3-2.

Further, on the basis of the measured tensile strength TS and the value of the hole expanding ratio [lambda], the value of tensile strength TS x hole expanding factor [lambda] was calculated. The calculation results are shown in Tables 3-1 and 3-2. The value of TS x lambda represents the strength-hole expansion factor balance, and is an index for evaluating the stretch flangeability.

In the present embodiment, when the value of TS x? Is 20,000 MPa ·% or more, it is evaluated that the stretch flangeability is excellent and the passing is evaluated. When the value is less than 20,000 MPa ·%, the stretch flangeability is poor.

[Evaluation of collision characteristics]

It is described in the following document that the collision characteristic is correlated with the bending angle.

P. Larour, H. Pauli, T. Kurz, T. Hebesberger: "Influence of post-uniform tensile and bending properties on the crash behavior of AHSS and press-hardening steel grades", IDDRG 2010

Therefore, a bending test is conducted under the following conditions based on the VDA standard (VDA238-100) prescribed by the German Automobile Manufacturers Association, and the displacement at the maximum load measured in the bending test is converted into an angle from the VDA standard, I got it. The conversion results are shown in Tables 3-1 and 3-2.

(Measuring conditions)

Test method: roll support, punch press

Roll diameter: φ30mm

Punch shape: tip R = 0.4 mm

Distance between rolls: 2.9 mm

Punch press speed: 20 mm / min

Dimensions of specimen: 60mm × 60mm

Bending direction: perpendicular to the rolling direction

Tester: SIMAZU AUTOGRAPH 20kN

Further, the value of the tensile strength TS 占 VDA bending angle was calculated based on the values of the tensile strength TS and the VDA bending angle measured in the tensile test. The calculation results are shown in Tables 3-1 and 3-2.

In the present embodiment, when the value of TS 占 VDA is 90000 MPa 占 or more, it is evaluated that the impact property is excellent and the result is acceptable, and when the value is less than 90000 MPa 占, the collision property is worse.

Based on the above results, when the value of TS is 980 MPa or more, the value of TS 占 EL is 17000 MPa 占 · or more, the value of TS 占 is 20000 MPa 占 이상 or more, and the value of TS 占 VDA is 90000 MPa 占 or more And the acceptance is described in the column of the comprehensive evaluation in the following Tables 3-1 and 3-2. On the other hand, as a comparative example, any one of the values of TS, TS 占 EL, TS 占 or TS 占 VDA does not satisfy the acceptance criteria, We list failure in field of comprehensive evaluation of -2.

[Table 1]

[Table 2-1]

[Table 2-2]

[Table 3-1]

[Table 3-2]

From Table 1, Table 2-1, Table 2-2, Table 3-1, and Table 3-2, it can be considered as follows.

In Table 3-1 and Table 3-2, all of the examples described as "Pass" in the column of the comprehensive evaluation are steel sheets satisfying the requirements specified in the present invention, and TS × EL Value, the value of TS 占 and the value of TS 占 VDA all satisfy the acceptance criterion value. It can be seen that these steel sheets have good workability evaluated by ductility and elongation flangeability, particularly excellent ductility and excellent collision characteristics.

On the other hand, the example in which "Failed" is described in the total evaluation column is a steel sheet that does not satisfy any of the requirements specified in the present invention, and at least one of ductility, stretch flangeability, or collision characteristics could not be improved. The details are as follows.

No. 3 is an example in which the MA structure is coarsened because the finishing rolling finishing temperature is too high. As a result, the value of TS x lambda became small, and the stretch flangeability could not be improved.

No. 4 is an example in which the MA structure is coarsened because the reduction rate in the final stand during finish rolling exceeds the range specified by the present invention and is too high. As a result, the value of TS x lambda became small, and the stretch flangeability could not be improved. Further, the value of TS 占 VDA became smaller, and the collision characteristic could not be improved.

No. 5 is an example in which the MA structure is coarsened because the reduction rate in the final stand during finish rolling is below the range specified in the present invention and is too low. As a result, the value of TS x lambda became small, and the stretch flangeability could not be improved.

No. 7, since the crack at a temperature which exceeds the temperature range of above 800 ℃ less than Ac ₃ point, an example was not able to secure a ferrite amount in the range specified in the present invention. As a result, the value of TS 占 EL EL becomes smaller, and the ductility can not be improved.

No. 8 is an example in which the value of V _MA / V _gamma is excessively large because the post-crack cooling stop temperature T exceeds the temperature range of 50 DEG C or higher and Ms point or lower and is excessively high and the reheating heat is not maintained after cooling. As a result, the value of TS x VDA becomes smaller, and the collision characteristics can not be improved.

No. 10 is an example in which a predetermined amount of residual gamma and MA structure can not be secured because the post-cracking cooling stop temperature T is less than 50 DEG C, and the value of _VMA / V _{? Is} smaller than the specified range . As a result, the value of TS 占 EL EL becomes smaller, and the ductility can not be improved.

No. 11 shows that the reduction in the final stand during finish rolling exceeds the range defined by the present invention and is too high and since the reheating heat is not maintained after cooling, the MA structure is coarsened and _VMA / The value of V _? Is too large. As a result, the value of TS x VDA becomes smaller, and the collision characteristics can not be improved.

No. 14 is an example in which the MA structure is coarsened because the reheat holding time was too short. As a result, the value of TS x lambda became small, and the stretch flangeability could not be improved. In addition, MA tissue was excessively produced. As a result, the value of TS 占 EL EL becomes smaller, and the ductility can not be improved. In addition, the value of V _MA / V _? Is excessively large. As a result, the value of TS 占 VDA became smaller and the collision characteristic deteriorated.

No. 16 and 37 are examples in which the MA structure is coarsened because the average heating rate after winding is too small. As a result, the value of TS x lambda became small, and the stretch flangeability could not be improved.

No. 19 is an example in which the MA structure is coarsened because the cooling stop temperature T after cracking is too high exceeding the temperature range of 50 ° C or higher and Ms or lower. As a result, the value of TS x lambda became small, and the stretch flangeability could not be improved.

No. 25 is an example in which a reheating temperature after cooling was too high to cause decomposition of austenite and a certain amount of residual? And MA structure could not be secured. As a result, TS was lowered. Further, the value of TS x lambda was reduced, and the stretch flangeability could not be improved. Further, the value of TS 占 VDA became smaller, and the collision characteristic could not be improved.

No. 27 and 38 are examples in which ferrite is excessively produced because the average cooling rate after cracking is too small. As a result, TS was lowered. Further, the value of TS x lambda was reduced, and the stretch flangeability could not be improved.

No. 29 is an example in which the MA structure is coarsened because the coiling temperature was too high. As a result, the value of TS x lambda became small, and the stretch flangeability could not be improved.

No. 33 is an example in which the amount of C is too small and the residual amount of gamma in the range specified in the present invention can not be ensured and the value of V _MA / V _? Exceeds the range specified in the present invention. As a result, the value of TS 占 EL became smaller and the ductility deteriorated.

No. 34 is an example in which the amount of Si is too small, and the amount of ferrite within the range specified in the present invention can not be ensured. As a result, the value of TS 占 EL became smaller and the ductility deteriorated.

No. 35 is an example in which the amount of Mn is too small, the hardenability becomes insufficient, and the ferrite is excessively produced, so that the TS decreases. Further, the value of TS 占 becomes smaller, and the stretch flangeability deteriorates.

1: Heating process
2: Cracking process
3: Cooling process
4: Reheating process
5: Cooling stop temperature

Claims (8)

In terms of% by mass,
C: not less than 0.10% and not more than 0.5%
Si: not less than 1.0% and not more than 3%
Mn: 1.5% or more and 7% or less,
P: more than 0% and not more than 0.1%
S: not less than 0% and not more than 0.05%
Al: 0.005% or more and 1% or less,
N: not less than 0% and not more than 0.01%, and
O: more than 0% and not more than 0.01%
And the remainder being iron and unavoidable impurities,
A high strength cold rolled steel sheet having a tensile strength of 980 MPa or more and excellent in workability and impact properties, characterized by satisfying the following (1) to (4) in the metal structure at 1/4 of the plate thickness.
(1) When a metal structure was observed with a scanning electron microscope,
The area ratio of the ferrite to the entire metal structure is more than 10% and not more than 65%
The remainder is a hard phase comprising at least one member selected from the group consisting of bainitic ferrite, bainite and tempering martensite, including quenched martensite and retained austenite.
(2) When the metal structure is measured by X-ray diffractometry, the volume percentage V _? Of the retained austenite is 5% or more and 30% or less with respect to the entire metal structure.
(3) When the metal structure is observed with an optical microscope, the area ratio V _MA of the MA structure in which the quenched martensite and the retained austenite are combined is not less than 3% and not more than 25% The average circle equivalent diameter is 2.0 占 퐉 or less.
(4) The ratio V _MA / V _γ of the area ratio V _MA of the MA structure to the volume ratio V _γ of the retained austenite satisfies the following formula (i).
0.50? V _MA / V _?? 1.50 ... (i) The method according to claim 1,
The high-strength cold-rolled steel sheet according to claim 1, wherein the steel sheet further contains at least one of the following elements (a) to (e) in mass% as other elements.
(a) at least one member selected from the group consisting of Cr: more than 0% to 1% and Mo: more than 0% to 1%.
(b) at least one member selected from the group consisting of Ti: more than 0% to 0.15%, Nb: more than 0% to 0.15%, and V: more than 0% to 0.15%
(c) at least one selected from the group consisting of Cu: more than 0% to 1% and Ni: more than 0% to 1%.
(d) B: more than 0% and not more than 0.005%.
(e) at least one member selected from the group consisting of Ca: more than 0% to 0.01% or less, Mg: more than 0% to 0.01% or less, and REM: more than 0% to 0.01% A high-strength electro-galvanized steel sheet characterized by having an electro-galvanized layer on the surface of the high-strength cold-rolled steel sheet according to claim 1 or 2. A high strength hot-dip galvanized steel sheet characterized by having a hot-dip galvanized layer on the surface of the high-strength cold-rolled steel sheet according to any one of claims 1 to 3. A high-strength alloyed hot-dip galvanized steel sheet characterized by having a galvannealing layer on the surface of the high-strength cold-rolled steel sheet according to any one of claims 1 to 3. A steel satisfying the composition of claim 1 or 2,
Rolled at a final stand of finishing rolling at a rolling rate of 5 to 25% and finish rolling finish temperature of Ar ₃ point to 900 캜, rolled at a coiling temperature of 600 캜 or less, cooled to room temperature,
Cold-rolled,
Heated to a temperature of 800 ° C or higher and less than Ac ₃ point at an average heating rate of 10 ° C / sec or more,
Cooling to an arbitrary cooling stop temperature T ° C in a temperature range of 50 ° C or higher and Ms point or lower at an average cooling rate of 10 ° C /
Heating the steel sheet at a temperature in the range of more than T ° C to 550 ° C for at least 50 seconds and then cooling to room temperature. A steel satisfying the composition of claim 1 or 2,
Rolled at a final stand of finishing rolling at a rolling rate of 5 to 25% and finish rolling finish temperature of Ar ₃ point to 900 캜, rolled at a coiling temperature of 600 캜 or less, cooled to room temperature,
Cold-rolled,
Heated to a temperature of 800 ° C or higher and less than Ac ₃ point at an average heating rate of 10 ° C / sec or more,
Cooling to an arbitrary cooling stop temperature T ° C in a temperature range of 50 ° C or higher and Ms point or lower at an average cooling rate of 10 ° C /
Heating and holding the steel sheet at a cooling stop temperature in a temperature range of more than T ° C to 550 ° C or more for 50 seconds or longer and performing hot dip galvanization in a holding time and then cooling to room temperature. Or more of the hot-dip galvanized steel sheet. A steel satisfying the composition of claim 1 or 2,
Rolled at a final stand of finishing rolling at a rolling rate of 5 to 25% and finish rolling finish temperature of Ar ₃ point to 900 캜, rolled at a coiling temperature of 600 캜 or less, cooled to room temperature,
Cold-rolled,
Heated to a temperature of 800 ° C or higher and less than Ac ₃ point at an average heating rate of 10 ° C / sec or more,
Cooling to an arbitrary cooling stop temperature T ° C in a temperature range of 50 ° C or higher and Ms point or lower at an average cooling rate of 10 ° C /
Heating and maintaining at a temperature in the range of more than T ° C and not more than 550 ° C for not less than 50 seconds and then performing hot dip galvanizing within the holding time and then further alloying treatment and then cooling to room temperature. A method of producing a high strength alloyed hot-dip galvanized steel sheet having excellent characteristics and having a tensile strength of 980 MPa or more.

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