KR20180061395A - High strength cold rolled steel sheet and high strength galvanized steel sheet having excellent ductility and bendability, and methods for producing same - Google Patents

Abstract

A high strength cold rolled steel sheet having a tensile strength of 980 MPa or more and excellent ductility and bending property. The high-strength cold-rolled steel sheet satisfies all of the following (1) to (5) when the specified component composition is satisfied and the structure at the 1/4 plate thickness of the steel sheet is measured by a prescribed method. (1) the area ratio of the mixed structure of fresh martensitite and retained austenite to the entire structure is more than 0%, (2) the area ratio of ferrite to the whole structure is 5% or more and less than 50% (3) a concentration of Mn of not less than 1.2% of the concentration of Mn in the steel sheet is not less than 5% by area, and (4) a concentration of Mn of not less than 1.2 times (5) the Mn concentration in the ferrite phase is 0.9 times or less the Mn concentration in the steel sheet.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength cold-rolled steel sheet and a high-strength hot-dip galvanized steel sheet excellent in ductility and bendability, and a method of manufacturing the same. BACKGROUND ART [0002]

The present invention relates to a high-strength cold-rolled steel sheet and a high-strength hot-dip galvanized steel sheet excellent in ductility and bendability, and a method of manufacturing the same. Specifically, in a region having a tensile strength of 980 MPa or more, a high strength cold rolled steel sheet, a high strength galvanized steel sheet, a high strength hot dip galvanized steel sheet and a high strength alloyed hot dip galvanized steel sheet excellent in ductility and bendability, To a method of manufacturing the same.

In order to realize low fuel consumption of automobiles and transportation vehicles, it is desired to reduce the weight of automobiles and transportation vehicles. For example, it is effective to use a high-strength steel plate to reduce the thickness of the plate. However, if the strength of the steel sheet is increased, the ductility is deteriorated and the workability is deteriorated. Therefore, a high strength steel sheet is required to have excellent ductility and bendability required for press forming. In addition, from the viewpoint of corrosion resistance, steel parts for automobiles have been galvanized such as electro galvanizing (EG), galvanized iron (GI), and galvanized steel (GA) Steel plates are often used. Galvanized steel sheets, hot-dip galvanized steel sheets, and galvannealed galvanized steel sheets are hereinafter sometimes referred to as " zinc-coated steel sheets ". These galvanized steel sheets also require the same characteristics as those of high-strength steel sheets.

As a technique for improving the workability of high-strength steel sheets, for example, Patent Documents 1 to 4 propose ultra-high strength cold-rolled steel sheets excellent in bendability.

Japanese Patent Application Laid-Open No. 11-179030 Japanese Patent No. 5299591 Japanese Patent Application Laid-Open No. 2011-225976 International Publication No. 2012/036269

However, a steel sheet excellent in ductility and bending property has not yet been provided in a region having a tensile strength of 980 MPa or more.

The present invention has been made in view of the above circumstances, and its object is to provide a high-strength cold-rolled steel sheet and a high-strength hot-dip galvanized steel sheet excellent in ductility and bending property with a tensile strength of 980 MPa or more, Specifically, in a region having a tensile strength of 980 MPa or more, a high strength cold rolled steel sheet, a high strength galvanized steel sheet, a high strength hot dip galvanized steel sheet and a high strength alloyed hot dip galvanized steel sheet excellent in ductility and bendability, And a method for producing the same.

The present invention, which has solved the above problems, relates to a steel sheet comprising a steel sheet having a composition of C: 0.10 to 0.30%, Si: 1.2 to 3%, Mn: 0.5 to 3.0% More than 0% and not more than 0.01%, S: not less than 0% and not more than 0.05%, Al: not less than 0.005% and not more than 0.2% (1) to (5) of the steel plate at the 1/4 sheet thickness of the steel sheet. Hereinafter, "% " means " mass% " with respect to the composition of the steel sheet.

(1) When observed with a scanning electron microscope, the area ratio of ferrite to the whole structure is 5% or more and less than 50%, and the balance is a hard phase.

(2) The area ratio of the mixed structure of fresh martensite and retained austenite with respect to the whole structure is more than 0% and 30% or less when Lepera corrosion is performed and observation is made with an optical microscope.

(3) When analyzed by an electron beam microprobe analyzer, the area where the Mn concentration is 1.2 times or more the concentration of Mn in the steel sheet is present in an area of 5% by area or more, and

(4) A standard deviation of 4.0% or more when measured in 100 sections by measuring the fraction of the area in which the concentration of Mn is 1.2 times or more the concentration of Mn in the steel sheet in the 2μm section.

(5) When analyzed by an electron beam microprobe analyzer, the Mn concentration in the ferrite phase is 0.90 times or less of the Mn concentration in the steel sheet.

In the present invention, it is also preferable that the volume percentage of retained austenite with respect to the whole structure is 5% or more as measured by X-ray diffractometry.

Further, the hard phase comprises a mixed structure of the fresh martensite and the residual austenite; At least one structure selected from the group consisting of bainitic ferrite, bainite, and tempering martensite.

(A) at least one member selected from the group consisting of Cr: more than 0% and not more than 1%, and Mo: more than 0% and not more than 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% to 0.15% (C) at least one member selected from the group consisting of Cu: more than 0% and 1% or less, and Ni: more than 0% and 1% or less; (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 (Rare Earth Metal): more than 0% to 0.01% Is further contained as a preferred embodiment.

According to the present invention, there is provided a high-strength electro-galvanized steel sheet in which an electro-galvanized layer is formed on the surface of the high-strength cold-rolled steel sheet; A high-strength hot-dip galvanized steel sheet on which a hot-dip galvanized layer is formed on a surface of the high-strength cold-rolled steel sheet; And a high-strength alloyed hot-dip galvanized steel sheet on which an alloyed hot-dip galvanized layer is formed on the surface of the high-strength cold-rolled steel sheet.

The method for producing a high-strength cold-rolled steel sheet according to the present invention, which has solved the above problems, is characterized in that in the hot rolling process of the steel sheet having the above-mentioned composition, the coiling is carried out at a coiling temperature of 500 ° C to 800 ° C, (Ac ₁ point + 20 ° C) or more and kept at a temperature lower than the Ac ₃ point, and then cooled to 500 ° C at an average cooling rate of 10 ° C / sec Or more is cooled to a temperature range of 500 ° C or lower at an average cooling rate of 10 ° C / sec or higher, and then reheated to a temperature range of 250 ° C or higher and 500 ° C or lower, and the temperature is maintained for 30 seconds or longer and then cooled to room temperature Have a gist of things.

In the present invention, it is also preferable that the steel sheet obtained by the above production method is further subjected to electro-galvanizing.

A method of manufacturing a high strength hot-dip galvanized steel sheet according to the present invention which can solve the above problems is characterized in that in the hot rolling process of a steel sheet having the above composition, the steel sheet is rolled at a coiling temperature of 500 ° C to 800 ° C, ° C. or lower for 3 hours or more and then cooled to room temperature. After the cold rolling, the steel sheet was kept at a temperature range of (Ac ₁ point + 20 ° C.) and lower than the Ac ₃ point and then maintained at 500 ° C. at an average cooling rate of 10 ° C. / Sec or more and 500 ° C or less is cooled to a temperature range of 500 ° C or less at an average cooling rate of 10 ° C / sec or more, then reheated to a temperature range of 250 ° C to 500 ° C, maintained for 30 seconds or more, Hot dip galvanizing is carried out within a holding time, and then cooling is carried out to room temperature.

In the present invention, it is also preferable to carry out alloying at a temperature of 450 ° C or more and 550 ° C or less after the above hot-dip galvanizing.

According to the present invention, a high-strength cold-rolled steel sheet and a high-strength hot-dip galvanized steel sheet excellent in ductility and bendability even at a temperature of 980 MPa or higher, specifically, a high strength cold rolled steel sheet, a high strength galvanized steel sheet, An alloyed hot-dip galvanized steel sheet can be provided. Further, according to the production method of the present invention, these steel sheets can be efficiently produced. Therefore, the high-strength cold-rolled steel sheet of the present invention is extremely useful particularly in industrial fields such as automobiles.

1 is a schematic explanatory diagram showing an example of a heat treatment pattern in the production method of the present invention.

The inventors of the present invention have conducted intensive investigations to improve the ductility and bendability of particularly high strength cold-rolled steel sheets and high-strength hot-dip galvanized steel sheets having a tensile strength of 980 MPa or more.

As a result, on the premise that the composition of the components is appropriately controlled, the ferrite phase and the hard phase in the metal structure of the steel sheet are optimized and the segregation of Mn is suitably controlled, thereby ensuring high strength of 980 MPa or more, And the present invention has been accomplished.

Hereinafter, the reason why the metal structure is defined in the present invention will be described in detail. On the other hand, the fraction measured by microscopic observation means the percentage of the steel sheet occupying 100% of the whole tissue. The metal structure constituting the present invention is different from the measurement method depending on the metal structure. Therefore, when the total amount of the metal structures defined in the present invention is all, it may exceed 100%. This is because the residual? Constituting the mixed structure of fresh martensite and retained austenite is measured by optical microscope observation In addition, it is measured by overlapping even by X-ray diffraction. Hereinafter, the residual austenite is referred to as "residual γ", and the mixed structure of fresh martensite and retained austenite may be referred to as "MA (Martensite-Austenite Constituent) structure".

Area ratio of ferrite: 5% or more and less than 50%

Ferrite is a structure having an effect of improving ductility and bending property of a steel sheet. In the present invention, by increasing the area fraction of ferrite, ductility and bendability in a high strength region having a tensile strength of 980 MPa or more can be improved. In order to exhibit such an effect, the area ratio of ferrite is set to 5% or more, preferably 7% or more, and more preferably 10% or more. However, if the ferrite is excessive, the strength of the steel sheet is lowered, and it becomes difficult to secure a high strength of 980 MPa or more. Therefore, the area ratio of the ferrite is less than 50%, preferably 45% or less, and more preferably 40% or less. The area ratio of the ferrite is a value measured by a scanning electron microscope (SEM) observation of the 1/4 plate thickness of the steel sheet.

Hard phase

The hard phase is the tissue required to improve the tensile strength. In the present invention, by increasing the area fraction of the hard phase, high strength of 980 MPa or more can be achieved while soft ferrite is present within the area ratio. In order to exhibit such an effect, the remaining metal structure other than ferrite needs to be a hard phase. In the present invention, the hard phase is a harder phase than ferrite, and at least one species selected from the group consisting of bainitic ferrite, bainite, tempering martensite, and MA structure is used. In the present invention, As such, it includes at least MA tissue. Among the above hard phases, bainitic ferrite, bainite, and tempering martensite are measured values by SEM observation of the 1/4 sheet thickness of the steel sheet. On the other hand, the residual? Exists in the ras liver or MA structure of bainitic ferrite.

Area ratio of MA tissue to total tissue: more than 0% and less than 30%

The presence of MA structure can improve strength and ductility. Therefore, from the viewpoint of improving the strength-ductility balance, the area ratio of the MA structure is preferably 3% or more, and more preferably 4% or more. On the other hand, if the area ratio of the MA structure becomes excessively large, the bending property deteriorates. Therefore, in the present invention, the area ratio of the MA structure is set to 30% or less, preferably 20% or less, more preferably 15% or less.

On the other hand, the unstructured austenite is in a martensitic state in the process of cooling the fresh martensititan or steel sheet constituting the MA structure from the heating temperature to the room temperature, and distinguishes it from the tempering martensitic after heat treatment . In the present invention, whitened portions were observed as an MA structure when observed under an optical microscope after refraining. On the other hand, since the fresh martensite and the residual? Are difficult to be distinguished by optical microscopic observation, the composite structure of fresh martensite and residual? Is measured as MA structure. MA is the value measured by optical microscope at 1/4 plate thickness of the steel sheet.

A standard deviation of the area fraction of the area where the Mn concentration is 1.2 times or more of the Mn concentration in the steel sheet: 5 area% or more and the Mn concentration is 1.2 times or more the Mn concentration in the steel sheet in the 2 占 퐉 section: 4.0% More than

In the present invention, the region where the concentration of Mn is concentrated is obtained by measuring the cross section of the steel sheet in the range of the beam diameter of 1 mu m or less to 20 mu m x 20 mu m by the Mn concentration obtained by the analysis using an electron probe microanalyzer (EPMA) . The " Mn concentration in the steel sheet " is the Mn concentration obtained by chemical analysis of the base steel sheet by inductively coupled plasma emission spectroscopy. Therefore, a region where the Mn concentration is 1.2 times or more the concentration of Mn in the steel sheet means a region where the measurement value of the Mn concentration obtained by EPMA analysis is 1.2 times or more higher than the Mn concentration in the base steel sheet and is measured in a range of 20 占 20 占 퐉 have.

In the present invention, the EPMA measurement range of 20 占 퐉 占 20 占 퐉 is divided into 100 segments of 1 占 2 占 square obtained by dividing the line of 2 占 퐉 intervals in the vertical and horizontal directions, and Mn The area fraction of the region with a concentration higher than 1.2 times is measured and statistically standard deviation is obtained from 100 compartments.

In the present invention, the fraction of a region in which a concentration of 1.2 times or more of the concentration of Mn in the steel sheet is greater than or equal to 5% by area and a concentration of Mn is 1.2 times or more in a segment of 2 탆 is measured When the standard deviation is 4.0% or more, the bendability is remarkably improved. Hereinafter, a region in which the concentration of Mn in the steel sheet is 1.2 times or more of the concentration of Mn is referred to as a " region with a Mn concentration of 1.2 times or more ", and a standard deviation Quot; standard deviation of the region where the Mn concentration is 1.2 times or more concentrated " or simply " standard deviation ".

That is, the region having a Mn concentration of 1.2 times or more is mainly a hard phase. The larger the area ratio of the region having the Mn concentration of 1.2 times or more, the lower the Mn concentration in the ferrite phase and the lower the hardness of the ferrite phase, and the bendability can be improved. Further, the larger the segregation of Mn, the larger the standard deviation, but the Mn concentration in the ferrite phase, which contributes to the improvement in the bendability, is lowered, and the hardness of the ferrite phase can be lowered.

In order to obtain such an effect, the area of 1.2 times or more the Mn concentration is 5.0 area% or more, preferably 5.2 area% or more, and more preferably 5.5 area% or more. On the other hand, if the proportion occupying an area of 1.2 times or more the Mn concentration is excessively high, the Ms point of the austenite is lowered and the MA structure may be increased. Therefore, the area of 1.2 times or more the Mn concentration is preferably 20 percent by area or less, 15 Area% or less.

The standard deviation of the region where the Mn concentration is 1.2 times or more concentrated is 4.0% or more, preferably 4.5% or more, and more preferably 5.0% or more. When the standard deviation is smaller than 4.0%, since the distribution of Mn is insufficient and uniformly distributed, the lowering of the hardness of the ferrite phase contributing to the improvement of the bendability is insufficient. On the other hand, the upper limit of the standard deviation is not particularly limited, and is preferably 10% or less.

By making the area of the Mn concentration 1.2 times or more greater than or equal to 5% by area and the standard deviation to 4.0% or more, the spheroidized hard phase can be dispersed in the ferrite phase and the effect of improving the strength of the steel material and the bending property . Hereinafter, the spheroidized hard phase is referred to as " spherical hard phase ". Here, the spherical hard phase is a part of a hard phase, and is composed of bainitic ferrite, bainite, tempering martensite, MA structure and the like as the hard phase. It has been conventionally believed that if the hardness difference between the hard phase and the ferrite phase is large, interface cracks occur at the time of bending, and the bendability is deteriorated. However, it has been found that the interface crack can be suppressed by the spherical hard phase in the ferrite phase. In order to obtain such an effect, the spherical hard phase in the ferrite phase is preferably small, and the aspect ratio is preferably 3 or less, more preferably 2.5 or less, still more preferably 2 or less, and preferably 2 탆 or less , More preferably not more than 1.8 mu m, and further preferably not more than 1.5 mu m. In order to exhibit the above effect, the spherical hard phase is preferably 0.70% by volume or more, more preferably 0.75% by volume or more, and still more preferably 0.80% by volume or more with respect to the hard phase.

The Mn concentration in the ferrite is 0.90 times or less the Mn concentration in the steel sheet

If the Mn concentration in the ferrite phase is too high, the hardness of the ferrite phase can not be sufficiently reduced and the bendability deteriorates. Therefore, the Mn concentration in the ferrite phase needs to be lower than the Mn concentration in the steel sheet. Therefore, the Mn concentration in the ferrite phase is 0.90 times or less, preferably 0.85 times or less, more preferably 0.80 times or less of the Mn concentration in the steel sheet. On the other hand, if the Mn concentration in the ferrite is too low, the hardness of the ferrite decreases and the strength may be insufficient. Therefore, the Mn concentration in the ferrite is preferably at least 0.3 times, more preferably at least 0.4 times . On the other hand, the Mn concentration in the ferrite phase can be measured by EPMA.

Volume ratio of residual γ to total tissue: 5% or more

The residual? Is distorted and deformed when the steel sheet is processed, transformed into martensite, so that good ductility can be ensured and at the same time, the residual? Promotes the hardening of the deformed portion at the time of processing and suppresses the concentration of distortion Therefore, it is a structure necessary for improving the strength-ductility balance of the steel sheet. In order to effectively exhibit such effects, the volume ratio of residual? Is preferably 5% or more, more preferably 6% or more, and still more preferably 7% or more. On the other hand, the upper limit of the volume ratio of the residual? Is not particularly limited, but is at most 20% within the range of the composition of the present invention and the production conditions. The residual? Is a value measured by X-ray diffractometry at a plate thickness 1/4 position of the steel sheet.

On the other hand, the residual? Exists in the ras liver or MA structure of the bainitic ferrite. Since the effect of the residual gamma is exerted irrespective of the existence form, in the present invention, the residual gamma that can be confirmed when the measurement is performed is made to be residual gamma irrespective of the existence form.

Next, the composition of the high-strength steel sheet of the present invention will be described.

C: not less than 0.10% and not more than 0.30%

C is an element necessary for securing strength and further improving the stability of residual?. In order to secure a tensile strength of 980 MPa or more, the C content is 0.10% or more, preferably 0.12% or more, and more preferably 0.15% or more. However, if the C content is excessive, the strength after hot rolling increases to cause cracking during cold rolling or the weldability of the final product decreases. Therefore, the C content is preferably 0.30% or less, preferably 0.26% , It should be 0.23% or less.

Si: 1.2% or more and 3% or less

Si is an element that contributes to the strengthening of steel as a solid solution strengthening element. In addition, it is an element effective in suppressing the generation of carbide and effectively acting to generate residual?, Thereby ensuring an excellent TS 占 EL balance. In order to effectively exhibit such an effect, the Si content is 1.2% or more, preferably 1.35% or more, and more preferably 1.5% or more. However, when the Si content is excessive, a remarkable scale is formed at the time of hot rolling, scratch marks on the surface of the steel sheet are scratched, and the surface properties are sometimes deteriorated. Further, the acidity is deteriorated. Therefore, the Si content is 3% or less, preferably 2.8% or less, and more preferably 2.6% or less.

Mn: not less than 0.5% and not more than 3.0%

Mn is an element contributing to the strengthening of the steel sheet by improving the hardenability. Further, it is an element which effectively works to stabilize? To generate residual?. In order to effectively exhibit such an effect, the Mn content is 0.5% or more, preferably 0.6% or more, more preferably 1.0% or more, still more preferably 1.5% or more, still more preferably 2.0% or more . However, if the Mn content is excessive, the strength after hot rolling increases, causing cracks during cold rolling or deteriorating the weldability of the final product. Further, the addition of excess Mn causes segregation of Mn, which causes deterioration of workability. Therefore, the Mn content is set to 3.0% or less, preferably 2.8% or less, and more preferably 2.6% or less.

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

P is an element that inevitably contains, and is an element that deteriorates the weldability of the steel sheet. Therefore, the P content is 0.1% or less, preferably 0.08% or less, and more preferably 0.05% or less. On the other hand, since the P content is preferably as small as possible, the lower limit is not particularly limited, but the lower limit is industrially 0.0005%.

S: more than 0% and less than 0.05%

S, like P, is an element that inevitably contains and is an element that deteriorates the weldability of a steel sheet. In addition, S forms a sulfide inclusion in the steel sheet, which causes the workability of the steel sheet to deteriorate. Therefore, the S content is 0.05% or less, preferably 0.01% or less, and more preferably 0.005% or less. Since the S content is preferably as small as possible, the lower limit is not particularly limited, but the lower limit is set to 0.0001% industrially.

Al: 0.005% or more and 0.2% or less

Al is an element acting as a deoxidizer. In order to exhibit such an effect effectively, the Al content should be 0.005% or more, and more preferably 0.01% or more. However, if the Al content is excessive, the weldability of the steel sheet is remarkably deteriorated. Therefore, the Al content should be 0.2% or less, preferably 0.15% or less, more preferably 0.10% or less.

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

N is an element that inevitably contains, but contributes to the enhancement of the strength of the steel sheet by precipitating nitride in the steel sheet. From this viewpoint, the N content is preferably 0.001% or more. However, when the N content is excessive, a large amount of nitride is precipitated, which causes deterioration such as elongation, elongation flange lambda, bending property and the like. Therefore, the N content is set to 0.01% or less, preferably 0.008% or less, more preferably 0.005% or less.

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

O is an element inevitably included, and if it is contained in excess, it is an element which causes deterioration of ductility and bending property at the time of processing. Therefore, the content of O is set to 0.01% or less, preferably 0.005% or less, and more preferably 0.003% or less. On the other hand, since the O content is preferably as small as possible, the lower limit is not particularly limited, but the lower limit is industrially 0.0001%.

Other ingredients

The steel sheet of the present invention satisfies the above composition and the balance is iron and inevitable impurities. Examples of the inevitable impurities include the P, S, N, O, Pb, Bi, Sb, and Sn tram elements, which may enter the steel according to the conditions of raw materials, materials, There is a case. It is also possible to positively further contain the following elements as other elements within a range not adversely affecting the operation of the present invention.

The steel sheet of the present invention, as another element,

(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:

(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% The 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% and Mo: more than 0% to 1%

Both Cr and Mo are effective elements for enhancing the hardenability of the steel sheet and can be used singly or in combination. In order to effectively exhibit such an effect, the content of Cr and Mo is preferably 0.1% or more, more preferably 0.3% or more. However, if it is contained excessively, the workability is lowered and the cost becomes higher. Therefore, when the content of Cr and Mo is individually contained, the content is preferably 1% or less, more preferably 0.8% or less, Is 0.5% or less. When Cr and Mo are used in combination, they are individually contained within the upper limit and preferably the total amount is 1.5% or less.

(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 a function of improving the strength of the steel sheet and of refining the spherical γ-grains by forming precipitates of carbides or nitrides in the steel sheet, and they can be used alone or in combination . In order to effectively exhibit such an effect, the content of Ti, Nb and V is preferably 0.005% or more, and more preferably 0.010% or more. However, if it is contained in excess, carbide precipitates on the grain boundary, and the stretch flangeability and bendability of the steel sheet deteriorate. Therefore, the content of Ti, Nb and V is preferably 0.15% or less, more preferably 0.12% or less, and further preferably 0.10% or less.

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

Cu and Ni are elements which act effectively for generation and stabilization of retained austenite and further have an effect of improving corrosion resistance, and they can be used alone or in combination. In order to exhibit such action, the content of Cu and Ni is preferably 0.05% or more, and more preferably 0.10% or more. However, when Cu is excessively contained, the hot workability is deteriorated. Therefore, when Cu is added alone, the Cu content is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. The Ni content is preferably 1% or less, more preferably 0.8% or less, and more preferably 0.5% or less, because it is expensive to contain Ni in excess. When Cu and Ni are used together, the above-mentioned action is easy to be manifested. Further, by containing Ni, deterioration of hot workability due to Cu addition is suppressed. Therefore, when Cu and Ni are used together, the total amount is preferably 1.5% , More preferably 1.0% or less.

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

B is an element for improving the hardenability and is an effective element for stably maintaining the austenite at room temperature. In order to effectively exhibit such a function, the B content 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 in an excessive amount, boron is produced to deteriorate ductility. Therefore, the B content is preferably 0.005% or less, 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 a function of finely dispersing inclusions in the steel sheet, and may be contained singly or two or more kinds may be arbitrarily selected. In order to effectively exhibit such an effect, the content of Ca, Mg, and REM is preferably 0.0005% or more, and more preferably 0.0010% or more. However, if it is contained in excess, it causes deterioration in casting and hot workability. Therefore, the content of Ca, Mg, and REM is preferably 0.01% or less, more preferably 0.008% or less, and still more preferably 0.007% or less.

On the other hand, in the present invention, REM is abbreviation of rare earth element and includes lanthanoid element, that is, 15 elements from La to Lu, and scandium and yttrium.

Next, a method of manufacturing a high-strength cold-rolled steel sheet, a high-strength galvanized steel sheet, a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet according to the present invention will be described. On the other hand, the production method of a high-strength cold-rolled steel sheet and the method of manufacturing a high-strength hot-dip galvanized steel sheet refer to " a steel sheet having the composition described above is rolled at a coiling temperature of 500 ° C to 800 ° C, (Ac ₁ point + 20 ° C) or higher and a crack was maintained at a temperature range lower than the Ac ₃ point. Thereafter, the steel sheet was cooled to 500 ° C at an average cooling rate of 10 ° C / And cooling down to a temperature range of 500 ° C or lower and an average cooling rate of 10 ° C / sec or higher to 500 ° C or lower is described below. Since the reheating process after cooling to the station is different in both cases, the process will be described separately for the high-strength cold-rolled steel sheet and the high-strength hot-dip galvanized steel sheet.

In the method for manufacturing a high-strength cold-rolled steel sheet and a high-strength hot-dip galvanized steel sheet according to the present invention, a steel sheet obtained by performing hot rolling and cold rolling on steel satisfying the above-mentioned composition is subjected to annealing described later. In the method for producing a high-strength cold-rolled steel sheet, reheating is performed after the annealing. Further, if necessary, electro galvanizing treatment is suitably combined to obtain a high strength galvanized steel sheet. In the method of manufacturing a high strength hot-dip galvanized steel sheet, after the annealing is performed, the hot-dip galvanizing treatment is performed together with the reheating. Further, if necessary, alloying treatment is appropriately combined to obtain a high strength alloyed hot-dip galvanized steel sheet. In the present invention, a high-strength cold-rolled steel sheet having a desired structure, a high-strength hot-dip galvanized steel sheet or the like can be obtained by suitably controlling the production conditions.

For example, as shown in Fig. 1, hot rolling is performed on the basis of a conventional method using a steel having the above composition. In the hot rolling, hot rolling is performed, for example, so that the finish rolling temperature is equal to or higher than Ac ₃ point, and then the coiling is performed at a coiling temperature of 500 ° C or more and 800 ° C or less. Thereafter, it is maintained at 500 ° C or more and 800 ° C or less for 3 hours or more, and then cooled to room temperature to perform cold rolling. On the other hand, the cooling after finishing rolling is about 500 deg. C / sec as the upper limit of the operating temperature.

After the cold rolling, as the annealing step, the cracks were maintained at a Ac ₂ point + 20 ° C or higher and a 2-phase temperature lower than Ac ₃ point, and then cooled to 500 ° C at an average cooling rate of 10 ° C / The temperature is lowered to 500 DEG C or lower and the temperature is lowered to 500 DEG C or lower at an average cooling rate of 10 DEG C / second or higher. In the method of manufacturing a high strength cold rolled steel sheet, a step of reheating the steel sheet to a temperature range of not less than 250 ° C and not more than 500 ° C, cooling the steel sheet to room temperature after holding for 30 seconds or longer in the temperature range. Further, in the method of manufacturing a high-strength hot-dip galvanized steel sheet, the steel sheet is cooled to a temperature range of 500 ° C or lower, then reheated to a temperature range of 250 ° C or higher and 500 ° C or lower and maintained at that temperature for 30 seconds or more, And then performing a hot-dip galvanizing process and then cooling to room temperature.

Hereinafter, the reason for defining each condition will be described in detail.

Rolled at a coiling temperature of 500 ° C or higher and 800 ° C or lower and then held at 500 ° C or higher and 800 ° C or lower for 3 hours or longer,

Rolled at a coiling temperature of 500 ° C or higher and 800 ° C or lower and then maintained at 500 ° C or higher and 800 ° C or lower for 3 hours or longer to generate the predetermined Mn concentration distribution to precipitate carbides containing Mn In addition, it becomes a hard phase spheronized in a ferrite phase by annealing after cold rolling. In order to obtain such an effect, the coiling temperature is set to 500 deg. C or higher, preferably 550 deg. C or higher, and more preferably 600 deg. However, if the coiling temperature is too high, a large amount of scale or oxidation of the grain boundary will occur on the steel sheet and the pickling property will deteriorate. Therefore, the coiling temperature is set to 800 캜 or less, preferably 750 캜 or less, more preferably 700 캜 or less. Further, the temperature range to be maintained after winding is set to 500 deg. C or higher, preferably 510 deg. C or higher, more preferably 520 deg. C or higher, more preferably 550 deg. C or higher, and even more preferably 580 deg. On the other hand, if the holding temperature is too high, a large amount of scale or grain boundary oxidation may occur on the steel sheet as in the case where the coiling temperature is excessively high, and the pickling property may be deteriorated. Therefore, the holding temperature is preferably 800 占 폚 or less, ° C. or less, more preferably 750 ° C. or less, further preferably 700 ° C. or less. The holding time at the temperature range is at least 3 hours, preferably at least 4 hours, more preferably at least 5 hours, more preferably at least 7 hours, even more preferably at least 10 hours. On the other hand, if the holding time is too long, a large amount of scale, grain boundary oxidation or the like may be caused on the steel sheet in the same way as the winding temperature is excessively high, and the acidity may deteriorate. Therefore, the holding time is preferably 72 hours or less, Shall be 60 hours or less.

In the present invention, holding at a predetermined temperature means that it is not necessarily maintained at the same temperature, and it may fluctuate within a predetermined temperature range. For example, the temperature may be maintained within the range of the holding temperature, and the change may be within this range, that is, the temperature increase due to the temperature decrease or heating, and the temperature increase due to the double heat accompanying the transformation.

In the present invention, the temperature is maintained at the above-mentioned temperature for a predetermined time, and then cooled to room temperature. The cooling rate at that time is not particularly limited and may be, for example, air cooling.

Pickling, cold rolling

After hot rolling, pickling is carried out if necessary, and cold rolling at a cold rolling rate of about 30 to 80% is carried out.

Annealing

As the annealing step after cold rolling, cracks were maintained at the Ac ₁ point + 20 ° C or higher and less than Ac ₃ point, and then the temperature range up to 500 ° C was cooled to an average cooling rate of 10 ° C / sec or higher, The temperature range of 500 DEG C or less is cooled to an average cooling rate of 10 DEG C / second or more and cooled to a temperature range of 500 DEG C or less.

The desired amount of ferrite can be ensured while maintaining the Mn concentration distribution of the present invention by maintaining the crack holding temperature at the Ac ₁ point + 20 ° C or higher and less than the Ac ₃ point. If the crack holding temperature is lower than Ac ₁ point + 20 占 폚, the amount of ferrite in the finally obtained metal structure of the steel sheet becomes excessively large, and sufficient strength can not be ensured. Therefore, the crack holding temperature is preferably at least _one of Ac ₁ point + 20 ° C, preferably Ac ₁ point + 25 ° C or more, more preferably Ac ₁ point + 50 ° C or more, more preferably Ac ₁ point + do. On the other hand, when the Ac ₃ point or more is reached, the ferrite can not be sufficiently generated and grown during the crack holding, the ductility is lowered, the Mn concentration distribution becomes uniform, and the spherical hard phase produced in the ferrite phase is decreased. Therefore, the crack holding temperature is set to be lower than Ac ₃ point, preferably Ac ₃ point -5 ° C or lower, more preferably Ac ₃ point -10 ° C or lower, and more preferably Ac ₃ point -20 ° C or lower .

On the other hand, the average temperature raising rate at the time of raising the temperature to the temperature of the crack holding temperature is not particularly limited, and can be appropriately selected. For example, the average raising rate may be about 0.5 to 50 占 폚 / sec.

In the present invention, the holding time at the crack holding temperature is not particularly limited. However, if the holding time is too short, the processed structure may remain and the ductility of the steel may deteriorate. Therefore, the holding time is preferably 40 seconds or more, and more preferably 60 seconds or more. On the other hand, if the holding time is too long, the concentration of Mn in the austenite phase is advanced, and the Ms point is lowered to increase the MA structure. Therefore, the holding time is preferably 3600 seconds or less, more preferably 3000 seconds or less .

In addition, as described above, holding at a predetermined temperature in the present invention means that it is not necessarily maintained at the same temperature, and it may fluctuate within a predetermined temperature range. For example, in the case of holding at the above-mentioned crack holding temperature, the temperature may be kept constant within the range of Ac ₁ point + 20 ° C or more and less than Ac ₃ point, or may be changed within this range.

The Ac ₁ point and Ac ₃ point can be calculated from the following formulas (a) and (b) described in "Leslie Steel Material Chemistry", Maruzen Co., Ltd., published on May 31, 1985, . In the formula, [] represents the content of each element in mass%, and the content of the element not contained in the steel sheet may be calculated as 0 mass%.

As a result, the following relationship can be obtained: Ac ₁ (° C) = 723-10.7 × [Mn] -16.9 × [Ni] + 29.1 × [Si] + 16.9 × [Cr] + 290 × [As] + 6.38 × [W]

_{Ac 3 (℃) = 910-203 ×} √ [C] -15.2 × [Ni] + 44.7 × [Si] +10 4 × [V] + 31.5 × [Mo] + 13.1 × [W] - (30 × [ Mn] + 11 x [Cr] + 20 x [Cu] -700 x [P] -400 x [Al] -120 x [As] -400 x [Ti]

After the cracks are retained, the temperature range up to 500 DEG C is cooled to an average cooling rate of 10 DEG C / second or more. By controlling the cooling rate from the crack holding temperature, generation of ferrite with high Mn concentration can be suppressed and the amount of ferrite produced can be suppressed. When the average cooling rate is low, ferrite having a high Mn concentration is generated during cooling, which may deteriorate the bendability or lower the strength. Therefore, the average cooling rate is 10 ° C / second or more, preferably 15 ° C / second or more, and more preferably 20 ° C / second or more. The upper limit of the average cooling rate is not particularly limited, and may be water cooling or oil cooling.

The temperature range up to 500 DEG C is cooled to the average cooling rate, and then cooled to 500 DEG C or less at an average cooling rate of 10 DEG C / second or more. By setting the average cooling rate at 500 占 폚 or lower to 10 占 폚 / sec or more, it is possible to suppress the generation of soft high-temperature bainite and to suppress the magnetic tempering of the martensite and improve the strength. In order to obtain such an effect, the average cooling rate at 500 캜 or lower is 10 캜 / second or higher, preferably 15 캜 / second or higher, and more preferably 20 캜 / second or higher. The upper limit of the average cooling rate is not particularly limited, and may be water cooling or oil cooling.

On the other hand, the average cooling rate up to 500 占 폚 and the average cooling rate below 500 占 폚 may be the same or may be appropriately adjusted within the above range.

The cooling stop temperature in the case of cooling the temperature below 500 ° C at an average cooling rate of 10 ° C / second or more is a temperature range of 500 ° C or less. When the cooling-stop temperature is higher than 500 ° C, the hard phase is reduced, the strength is not ensured, and the MA structure is increased to deteriorate the bending property. Therefore, the cooling stop temperature is set to 500 ° C or less, preferably 400 ° C or less, more preferably 350 ° C or less, and further preferably 300 ° C or less. The lower limit of the cooling stop temperature is not particularly limited, but is, up to room temperature, for the purpose of operation.

Hereinafter, the reheating step will be described separately for the cold-rolled steel sheet producing method and the hot-dip galvanized steel sheet producing method.

Reheating in cold rolled steel sheet manufacturing method

After the cooling is stopped in the temperature range of 500 DEG C or lower, reheating is continued from 250 DEG C to 500 DEG C, and the temperature is maintained for 30 seconds or longer, and then the temperature is cooled to room temperature.

After the cooling is stopped, reheating is carried out up to a temperature of not lower than 250 ° C and not higher than 500 ° C and maintained for not less than 30 seconds, whereby the hard phase such as martensite can be tempered and the untransformed austenite can be transformed. When the reheating is not performed or the holding temperature is too low, hard tempering may not proceed and a high density potential may be generated, or MA structure may remain in a large amount, resulting in poor bendability. Therefore, the reheating temperature is set to 250 deg. C or higher, preferably 300 deg. C or higher, and more preferably 350 deg. On the other hand, if the holding temperature is excessively high, the strength is lowered. Therefore, the reheating temperature is set to 500 ° C or lower, preferably 470 ° C or lower, more preferably 450 ° C or lower. On the other hand, in the present invention, this " reheating " means heating from the cooling stop temperature to 500 DEG C or lower, that is, the temperature rise. Therefore, even if the reheating temperature is higher than the cooling stop temperature and the temperature is in the range of 250 DEG C or higher and 500 DEG C or lower, the cooling process from the cooling stop temperature to the lower temperature or the isothermal holding with the cooling stop temperature and the reheating temperature being the same , This is not included in reheating.

If the holding time at the reheating temperature is too short, the hard phase can not be sufficiently tempered and the untransformed austenite can not be transformed. Therefore, the holding time is at least 30 seconds, preferably at least 50 seconds, more preferably at least 100 seconds, more preferably at least 200 seconds. On the other hand, the upper limit of the holding time is not particularly limited. However, if the holding time is excessively long, the productivity is lowered, and the strength is lowered. Therefore, the holding time is preferably 1,500 seconds or less, more preferably 1,000 seconds or less.

After being held for a predetermined time in the reheating temperature range, it is cooled to room temperature. The average cooling rate at this time is not particularly limited and is preferably 0.1 占 폚 / sec or more, more preferably 0.4 占 폚 / sec or more, preferably 200 占 폚 / sec or less, more preferably 150 占 폚 / It is only necessary to cool down at a cooling rate.

In the present invention, an electro-galvanized layer may be formed on the surface of the obtained steel sheet.

The method of forming the above-described electroplated zinc plating layer is not particularly limited, and an electroplated zinc plating treatment of a conventional method may be employed. For example, in the case of producing an electrogalvanized steel sheet, there is a method in which electrodeposition is carried out while immersing in a zinc solution at 55 ° C to conduct an electroplating process. The amount of plating on one surface is not particularly limited, and for example, it is about 10 to 100 g / m ^{2 in} the case of an electrogalvanized steel sheet.

The reheating in the production method of the hot-dip galvanized steel sheet

After the cooling is stopped at a temperature range of 500 ° C or lower, reheating is continued from 250 ° C to 500 ° C and maintained for 30 seconds or longer, Cool.

After the cooling is stopped, reheating is carried out up to a temperature of not lower than 250 ° C and not higher than 500 ° C and maintained for not less than 30 seconds, whereby the hard phase such as martensite can be tempered and the untransformed austenite can be transformed. When the reheating is not performed or the holding temperature is too low, hard tempering may not proceed and a high density potential may be generated, or MA structure may remain in a large amount, resulting in poor bendability. Therefore, the reheating temperature is set to 250 deg. C or higher, preferably 300 deg. C or higher, and more preferably 350 deg. On the other hand, if the holding temperature is excessively high, the strength is lowered. Therefore, the reheating temperature is set to 500 ° C or lower, preferably 470 ° C or lower, more preferably 450 ° C or lower. On the other hand, in the present invention, this " reheating " means heating from the cooling stop temperature to 500 DEG C or lower, that is, the temperature rise. Therefore, even if the reheating temperature is higher than the cooling stop temperature and the temperature is in the range of 250 DEG C or more and 500 DEG C or less, the isothermal holding temperature and the reheating temperature are kept at the same temperature or the cooling process from the cooling stop temperature to the lower temperature , This is not included in reheating.

If the holding time at the reheating temperature is too short, the hard phase can not be sufficiently tempered and the untransformed austenite can not be transformed. Therefore, the holding time is at least 30 seconds, preferably at least 50 seconds, more preferably at least 100 seconds, more preferably at least 200 seconds. On the other hand, the upper limit of the holding time is not particularly limited. However, if the holding time is excessively long, the productivity is lowered, and the strength is lowered. Therefore, the holding time is preferably 1,500 seconds or less, more preferably 1,000 seconds or less.

In the present invention, a hot-dip galvanizing treatment is performed within a holding time of 30 seconds or longer at the reheating temperature to form a hot-dip galvanized layer on the surface of the steel sheet. In the present invention, the hot dip galvanizing and the holding at the reheating temperature are also performed. That is, in order to properly manage the metal structure and strength by reheating, it is necessary to perform hot dip galvanizing at the holding time in the reheating temperature range. A method of forming the hot-dip galvanized layer is not particularly limited, and a hot-dip galvanizing method of a conventional method can be adopted. For example, the steel sheet may be immersed in a plating bath whose temperature has been regulated to the reheating temperature, to perform a hot-dip galvanizing treatment. The plating time is only required to satisfy the above-mentioned holding time, and may be appropriately adjusted so as to secure a desired plating amount. The plating time is preferably set to, for example, 1 to 10 seconds.

A hot dip galvanizing treatment in reheating; There are various patterns as a combination of: no plating treatment by heating alone.

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

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

(iii) heating only, hot dip galvanizing, heating only.

And the case where the reheating temperature in the case of only heating is different from the temperature of the hot dip galvanizing bath, that is, the plating bath, from one temperature to the other. Examples of the heating method include furnace heating and induction heating.

In the case of forming a galvannealing hot-dip galvanized layer on the surface of a steel sheet, the hot-dip galvanizing may be performed after alloying. Although the alloying temperature is not particularly limited, it is preferably 450 占 폚 or higher, more preferably 460 占 폚 or higher, and still more preferably 480 占 폚 or higher because the alloying can not sufficiently proceed if the alloying temperature is too low. On the other hand, if the alloying temperature is too high, the alloying becomes excessively advanced to increase the Fe concentration in the plating layer and deteriorate the plating adhesion, so that it is preferably 550 占 폚 or lower, more preferably 540 占 폚 or lower, still more preferably 530 占 폚 or lower. The time of the alloying treatment is not particularly limited and may be adjusted so as to obtain desired alloying. The alloying treatment time is preferably 10 seconds or more and 60 seconds or less. On the other hand, since the alloying treatment is carried out after holding for a predetermined time in the reheating temperature range, the alloying treatment time is not included in the holding time in the reheating temperature range.

After being held for a predetermined time in the reheating temperature range, it is cooled to room temperature. The average cooling rate at this time is not particularly limited and is preferably 0.1 占 폚 / sec or more, more preferably 0.4 占 폚 / sec or more, preferably 200 占 폚 / sec or less, more preferably 150 占 폚 / It is only necessary to cool down at a cooling rate.

The technique of the present invention can be suitably applied particularly to thin steel sheets having a thickness of 6 mm or less.

The high-strength cold-rolled steel sheet and the high-strength hot-dip galvanized steel sheet of the present invention are intended to have a tensile strength of 980 MPa or more, preferably 1,000 MPa or more, and more preferably 1,010 MPa or more. The ductility is expressed by a balance between strength and ductility, that is, " tensile strength at unit MPa x ductility at unit% ", preferably at least 15,000 MPa ·%, more preferably at least 15,100 MPa ·% Is not less than 15,200 MPa ·%. The bending property is represented by the balance and the balance of the VDA (Verband der Automobilindustrie) bending angle obtained by the method described in Examples described later, that is, the " VDA bending angle expressed by tensile strength in unit MPa unit x deg. Is not less than 100,000 MPa ·, more preferably not less than 100,500 MPa ·, more preferably not less than 101,000 MPa ·.

This application is a continuation-in-part of Japanese Patent Application No. 2014-053399 filed on March 17, 2014, Japanese Patent Application No. 2014-053400 filed on March 17, 2014, and Japanese Patent Application No. It claims the benefit of priority based on application no. 2014-192757. The entire contents of the specification of Japanese Patent Application No. 2014-053399 filed on March 17, 2014, the entire contents of the specification of Japanese Patent Application No. 2014-053400 filed on March 17, 2014, and September 22, 2014 The entire contents of the specification of Japanese Patent Application No. 2014-192757 filed on the same date are incorporated herein by reference.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is of course not limited by the following Examples, and it is of course possible to carry out the present invention by appropriately modifying it within the range suitable for the purposes of the preceding and latter parts All of which are included in the technical scope of the present invention.

Example 1

A steel having the composition shown in the following Table 1 was melted and subjected to hot rolling, cold rolling and continuous annealing under the following conditions to produce a cold rolled steel sheet. The steels having the constituent compositions shown in Table 1 are those in which the remainder is iron and inevitable impurities, which means that the elements are not added.

Hot rolling

The slab was heated to 1250 占 폚 and hot-rolled to a plate thickness of 2.3 mm such that the reduction rate was 90% and the finish rolling temperature was 920 占 폚. Thereafter, from this temperature, the steel sheet was cooled to an "coiling temperature (占 폚)" shown in Table 2 or Table 3 at an average cooling rate of 30 占 폚 / sec and wound up. (Time) ", or maintained under the conditions of" holding start temperature (° C.) "," holding end temperature (° C.) ", and" holding time (time) "shown in Table 3. Subsequently, the steel sheet was air-cooled to room temperature to prepare 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.

Annealing of cold-rolled steel sheet

The obtained cold-rolled steel sheet was subjected to crack holding, cooling, and reheating under the conditions shown in Table 2 or Table 3 to produce a steel sheet. On the other hand, 32 is a comparative example in which reheating is not carried out. In place of reheating, the reheating column shows that cooling from the cooling stop temperature of 480 占 폚 to 350 占 폚 and holding at that temperature for 300 seconds is shown.

In the table, the "cracking temperature (° C)", the average cooling rate up to 500 ° C after cracking are "average cooling rate 1 (° C./sec)", the cooling rate at 500 ° C. or lower is " The holding temperature at reheating time (° C) is 2 (° C / sec), the cooling stop temperature is "cooling stop temperature (Second) "respectively. On the other hand, in this embodiment, the holding time at the crack holding temperature is set to 100 seconds to 600 seconds. After the "reheating holding time (sec)" at the "reheating holding temperature (캜)", the room temperature was cooled to room temperature to obtain a steel. On the other hand, for cold-rolled steel sheets which were not subjected to later galvanizing, "CR" was entered in the column "Variety" in the table.

Manufacture of galvanized steel sheet

A part of the above-mentioned steel was immersed in a zinc plating bath at 55 ° C, electroplated at a current density of 30 to 50 A / dm ² , washed with water, and dried to obtain an electrogalvanized steel sheet. On the other hand, the adhesion amount of zinc plating per side was 10 to 100 g / m ² . Further, in the plating treatment, a cleaning treatment such as dipping, washing, pickling and the like was appropriately carried out to obtain a coated steel having an electroplated zinc plating layer on its surface. For the galvanized steel sheet, "EG" is entered in the "Variety" column in the table.

As to each of the disclosed steel, the metal structure, the Mn concentration, and various mechanical properties were evaluated as described below, and the results are shown in Table 4 or Table 5.

Measurement of metal structure

The area ratio of the ferrite, the area ratio of the hard phase, the area ratio of the MA structure, the volume ratio of the residual?, And the ratio of the spherical hard phase were measured as follows. That is, the cross section of the disclosed steel was polished, and after corrosion as shown below, a position of 1/4 of the plate thickness was observed using an optical microscope or a scanning electron microscope. Then, the photographs of metal tissues photographed with an optical microscope or SEM were subjected to image analysis and the ratios of the respective tissues were measured. Details are given below.

Area ratio of ferrite

After the above polishing, the area ratio of the ferrite was measured by a gradual method with a lattice spacing of 5 탆 and a lattice point number of 20 횞 20 by observing a total of three viewing angles at a magnification of 1000 times and a field of view size of 100 탆 x 100 탆 with SEM , And the average value of the three visual fields was calculated. The results are shown in "Ferrite (area%)" in the table. On the other hand, the area ratio of the ferrite excludes the hard phase in the ferrite phase.

Area ratio of hard phase

A structure other than the ferrite was taken as a hard phase, and a value obtained by subtracting the ferrite area ratio from the observation field of 100 area% was taken as the hard coat area ratio. The results are shown in " hard phase (area%) " On the other hand, the hard phase structure was also observed, and it was confirmed that the hard phase was at least one selected from the group consisting of bainitic ferrite, bainite, tempering martensite, residual?, And MA structure.

Area ratio of MA tissue

After the above polishing, the substrate was corroded with referee and observed with an optical microscope at a magnification of 1000 times, a field of view size of 100 mu m x 100 mu m in total of 3 fields, and the area ratio of the MA structure And an average value of the three fields of view was calculated. The results are shown in "MA (area%)" in the table. On the other hand, the portion whitened by the above-mentioned Repera corrosion was observed as MA structure.

Volume ratio of residual γ

Polishing was carried out using sandpaper # 1000 to # 1500 up to a plate thickness of 1/4, and further, the surface was electrolytically polished to a depth of 10 to 20 μm and then measured using an X-ray diffractometer, RINT 1500 manufactured by Rigaku Corporation. More specifically, the Co target was used to measure the range of 40 to 130 degrees in 2? By outputting 40 kV to 200 mA, and the diffraction peaks 110, 200, 211, and quantitative measurement of the residual? was performed from the diffraction peaks 111, 200, 220, and 311 of fcc (?). The results are described in the column "Residual γ (vol%)" in the table.

The spherical hard phase in the ferrite phase

After the above polishing, the spherical hard phase having an aspect ratio of 1 to 3 in terms of the circle equivalent diameter present on the ferrite phase was corroded with the abrasion, and the spherical hard phase having an aspect ratio of 1 to 3 was sieved by SEM at a magnification of 1000 times, And a visual image was observed and an image was analyzed to determine the proportion of the spherical hard phase occupying the hard phase. The results are described in " spherical hard phase (area%) "

The ratio of the area where the concentration of Mn is 1.2 times or more the concentration of Mn in the steel sheet

The Mn concentration was measured in the range of 20 mu m x 20 mu m in the condition of the beam diameter of 1 mu m or less by using EPMA after cutting the disclosed steel into a cross section and buried in the resin. The obtained Mn concentration was divided by the Mn concentration of the steel sheet chemically analyzed by inductively coupled plasma emission spectroscopy, and the ratio of the concentration of the Mn concentration 1.2 times or more to the Mn concentration in the steel sheet was determined. Thereafter, the area% of the area having the Mn concentration of 1.2 times or more was determined by sorting the area where the Mn concentration is 1.2 times or more and the area where the Mn concentration is 1.2 times or less. The results are shown in the table "Area ratio (%) of Mn concentration 1.2 times".

The standard deviation of the region where the Mn concentration is 1.2 times or more the concentration of Mn in the steel sheet

An image distinguished according to the Mn concentration in the steel sheet was divided into 100 sections in a 2 占 퐉 section and a fraction of areas in which the Mn concentration in each section was 1.2 times or more enriched to obtain a standard deviation of 100 sections. The results are shown in the table as " standard deviation (area%) in the region of 1.2 times the Mn concentration ".

The ratio of the Mn concentration in the ferrite phase to the Mn concentration in the steel sheet

The Mn concentration was observed by SEM observation in the same field as 20 탆 x 20 탆 measured by EPMA analysis. The results of EPMA analysis and SEM image were compared, and the distribution of Mn concentration of each ferrite lid was identified. The point at which the major axes and minor axes of the ferrite lips intersect is taken as the center position of the ferrite lips and the Mn concentration at the corresponding center position is taken as the Mn concentration of the ferrite lips. The Mn concentration at the center of the ferrite grains in the range of 20 mu m x 20 mu m was identified by the above technique and the Mn concentration of the ferrite grains having the highest Mn concentration in each 20 mu m x 20 mu m range was divided by the Mn concentration of the steel sheet, The ratio of the Mn concentration in the ferrite phase to the Mn concentration of the steel sheet was obtained. In the present invention, the average value of the three field-of-view is set as a ratio of the Mn concentration in the ferrite phase to the Mn concentration of the steel sheet, with 20 mu m x 20 mu m being one field of view. The results are described in " ratio of Mn concentration in ferrite phase "

Evaluation of mechanical properties

The tensile strength and ductility of the steel specimens were measured by a tensile test using the No. 5 test specimen specified in JIS Z2201. The test piece was cut out from the test steel so that the direction perpendicular to the rolling direction was the longitudinal direction. The strength-ductility balance was calculated from the obtained tensile strength and ductility. In the table, the tensile strength is "TS (MPa)", the ductility is "EL (%)" and the balance of strength and ductility is "TS × EL (MPa ·%)".

In the present invention, when the tensile strength is 980 MPa or more, it is determined that the high strength is acceptable, and when the tensile strength is less than 980 MPa, the strength is inadequate.

The ductility was evaluated by the strength-ductility balance. When the TS × EL was 15,000 MPa ·% or more, it was judged that the ductility was excellent and the ductility was less than 15,000 MPa ·%.

Evaluation of Bendability

The bendability was evaluated based on the VDA standard "VDA238-100" specified by the German Automobile Manufacturers Association under the following measurement conditions. In the present invention, the displacement at the maximum load obtained by the bending test is converted into an angle based on the VDA, and the bending angle is obtained. The results are shown in "VDA bending angle (°)" in the table. The bending property was also evaluated from the tensile strength and the bending angle. The results are shown in "TS × VDA (MPa · °)" in the table. When TS x VDA was 100,000 MPa 占 or more, it was judged that the bending property was excellent, and when it was less than 100,000 MPa 占, it was evaluated that the bending property was insufficient.

Measuring conditions

Test method: roll support, punch push

Roll diameter: φ30mm

Punch shape: tip R = 0.4 mm

Distance between rolls: 2.9 mm

Pushing speed: 20 mm / min

Dimensions of specimen: 60mm × 60mm

Bending direction: direction perpendicular to the rolling direction

Tester: SIMAZU AUTOGRAPH 20kN

The following can be seen from Tables 1 to 5. Experiments Nos. A to T and W to AC, which were produced under the annealing conditions specified in the present invention, were used for the steel types A to T and W to AC satisfying the component composition of the present invention. The steel sheets 1 to 18, 20, 24 to 26, 28, 29, and 33 to 43 were excellent in ductility and bendability in the region having a tensile strength of 980 MPa or more.

On the other hand, the steel sheet other than the above does not satisfy the composition and manufacturing conditions specified in the present invention, as described below, and desired characteristics are not obtained.

The steel grade U in Table 1 had a C content, the steel grade V had a Mn content exceeding the upper limit of the present invention, and fracture occurred during cold rolling.

Experiment No. 19 was an example in which it was not reheated after cooling to 500 ° C or lower, and the bending property deteriorated due to an increase in MA structure.

Experiment No. 21 was an example in which the steel sheet was not held at a predetermined temperature after winding, and the standard deviation of the area of 1.2 times the Mn concentration was low and the bendability was bad.

Experiment No. 22 shows an example in which the coiling temperature and the holding temperature were low, the area ratio of 1.2 times the Mn concentration and the standard deviation of the area of 1.2 times the Mn concentration were low and the bendability was bad.

Experiment No. 23 is an example in which the holding time at a predetermined temperature was short after winding, and the standard deviation of the range of 1.2 times the Mn concentration was low and the bendability was bad.

Experiment No. 27 was an example in which the cracking temperature was high and ductility was bad because no ferrite was produced and the area ratio of the Mn concentration was 1.2 times and the standard deviation of the Mn concentration was 1.2 times lower.

Experiment No. 30 is an example in which the cooling stop temperature is high, and ferrite and MA structure are increased, the strength is low, ductility and bendability are also bad.

Experiment No. 31 is an example in which the cooling rate to 500 ° C is slow, and the Mn concentration in the ferrite phase is excessively high, so that the bendability is deteriorated.

Experiment No. 32 is an example in which reheating is not performed after cooling to 500 ° C or lower, and the bending property is deteriorated due to an increase in MA structure.

Example 2

A cold-rolled steel sheet was produced by subjecting a steel having the constituent composition shown in Table 6 below to a hot-rolling, cold-rolling and continuous annealing under the following conditions. The steels having the constituent compositions shown in Table 6 are iron and unavoidable impurities, and the blank means that no element is added.

Hot rolling

The slab was heated to 1250 占 폚 and hot-rolled to a plate thickness of 2.3 mm such that the reduction rate was 90% and the finish rolling temperature was 920 占 폚. Then, after cooling from this temperature to an "coiling temperature (占 폚)" shown in Table 7 or Table 8 at an average cooling rate of 30 占 폚 / sec and wound up, Hours (hours) ", or maintained under the conditions of" holding start temperature (° C.) "," holding end temperature (° C.) ", and" holding time (time) "shown in Table 8. Subsequently, the steel sheet was air-cooled to room temperature to prepare 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.

Annealing of cold-rolled steel sheets, manufacture of hot-dip galvanized steel sheets or galvannealed galvanized steel sheets

The cold-rolled steel sheet thus obtained was subjected to the conditions shown in Table 9 or Table 10 to maintain cracking, cooling, reheating, and plating to produce a steel sheet. On the other hand, 29 is a comparative example in which reheating is not carried out. In place of reheating, the reheating column shows that cooling is stopped at a cooling stop temperature of 470 캜 to 400 캜 and then maintained at that temperature for 45 seconds.

In the table, the "cracking temperature (° C)", the average cooling rate up to 500 ° C after cracking are "average cooling rate 1 (° C./sec)", the cooling rate at 500 ° C. or lower is " The holding temperature at reheating time (° C) is 2 (° C / sec), the cooling stop temperature is "cooling stop temperature (Second) ", the temperature of the plating bath is referred to as" plating bath temperature (° C.) ", and the plating treatment time is referred to as" hot dip galvanizing treatment time (second) ". On the other hand, the "reheating holding time (second)" is the total time including the "hot dip galvanizing treatment time (second)".

(The "reheating holding time (sec)" - "hot dip galvanizing treatment time (sec)") at the "reheating holding temperature (° C.)", the steel sheet is immersed in the zinc plating bath, Treatment time (sec) ". Experiment No. 31 and 33, immediately before immersion in a zinc plating bath, heating was performed from "reheating holding temperature (° C)" to "plating bath temperature (° C)" in Table 8, and then immersed in a zinc plating bath. On the other hand, some steel sheets were subjected to a hot-dip galvanizing treatment and then to an alloying treatment. In the table, the alloying temperature at this time is referred to as "alloying temperature (° C.)", and the holding time at the alloying temperature is referred to as "alloying treatment time (second)". After holding for a predetermined time, it was allowed to cool to room temperature to obtain a disclosed steel. On the other hand, "GI" was used for the steel plate subjected to only the hot dip galvanizing treatment in the column "Variety" in the table, and "GA" was used for the steel sheet subjected to the alloying treatment.

The evaluation of the metal structure, the Mn concentration, and various mechanical properties was carried out in the same manner as in Example 1, as shown in Table 9 or Table 10, as described below.

The following can be seen from Tables 6 to 10. Experiments Nos. A to N and Q to U, which satisfy the composition of the present invention, were used for annealing conditions specified in the present invention. The steel sheets of 1, 2, 4, 5, 10, 11, 13 to 16, 18 to 23, 25 to 28 and 30 to 35 were excellent in ductility and bending property in the region of tensile strength of 980 MPa or more.

On the other hand, the steel sheet other than the above does not satisfy the composition and manufacturing conditions specified in the present invention, as described below, and desired characteristics are not obtained.

The steel grade O in Table 6 had a C content, the steel grade P had a Mn content exceeding the upper limit of the present invention, and fracture occurred during cold rolling, so that a steel sheet could not be produced.

Experiment No. 3 was an example in which the holding time at the reheating holding temperature after cooling to 500 ° C or lower was short, and the bending property deteriorated due to an increase in the MA structure.

Experiment No. 6 was an example in which the steel sheet was not held at a predetermined temperature after winding, and the standard deviation of the area of 1.2 times the Mn concentration was low and the bendability was poor.

Experiment No. 7 was an example in which the coiling temperature and the holding temperature were low, the area ratio of 1.2 times the Mn concentration and the standard deviation of the Mn concentration 1.2 times were low and the bendability was poor.

Experiment No. 8 was an example in which the holding time at a predetermined temperature was short after winding, and the standard deviation of the area of 1.2 times the Mn concentration was low and the bendability was bad.

Experiment No. 9 was an example in which reheating after cooling to 500 ° C or lower was low, and bending properties deteriorated due to an increase in MA structure.

Experiment No. 12 is an example in which the cracking temperature is high and ductility is bad because ferrite is not sufficiently generated and the Mn concentration in the ferrite phase is too high.

Experiment No. 17 is an example in which the cooling stop temperature is high, and ferrite and MA structure are increased, the strength is low, and the bending property is bad.

Experiment No. 24 is an example in which the cooling rate to 500 ° C is slow, and the Mn concentration in the ferrite phase is excessively high, so that the bendability is deteriorated.

Experiment No. 29 was an example in which the steel sheet was not reheated after cooling to 500 ° C or lower, and the bending property was deteriorated due to an increase in MA structure.

Claims (11)

The composition of the steel sheet in mass%
C: not less than 0.10% and not more than 0.30%
Si: 1.2% or more and 3% or less,
Mn: not less than 0.5% and not more than 3.0%
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 0.2% or less,
N: not less than 0% and not more than 0.01%, and
O: more than 0% and not more than 0.01%
, The balance being iron and inevitable impurities, and
A high strength cold rolled steel sheet having a tensile strength of 980 MPa or more excellent in ductility and bending property, characterized in that the structure at the plate thickness 1/4 position of the steel sheet satisfies all of the following (1) to (5).
(1) When observed with a scanning electron microscope, the area ratio of ferrite to the whole structure is 5% or more and less than 50%, and the balance is a hard phase.
(2) The area ratio of the mixed structure of fresh martensite and retained austenite with respect to the whole structure is more than 0% and 30% or less when Lepera corrosion is performed and observation is made with an optical microscope.
(3) When analyzed by an electron beam microprobe analyzer, the area where the Mn concentration is 1.2 times or more the concentration of Mn in the steel sheet is present in an area of 5% by area or more, and
(4) A standard deviation of 4.0% or more when measured in 100 sections by measuring the fraction of the area in which the concentration of Mn is 1.2 times or more the concentration of Mn in the steel sheet in the 2μm section.
(5) When analyzed by an electron beam microprobe analyzer, the Mn concentration in the ferrite phase is 0.90 times or less of the Mn concentration in the steel sheet. The method according to claim 1,
A high strength cold rolled steel sheet having a volume percentage of retained austenite of 5% or more as measured by X-ray diffractometry. The method according to claim 1,
The hard phase comprises a mixed structure of the fresh martensite and the residual austenite; At least one structure selected from the group consisting of bainitic ferrite, bainite, and tempering martensite. The method according to claim 1,
The high-strength cold-rolled steel sheet according to any one of claims 1 to 5, further comprising one or more of the following (A) to (E) as the other constituents in terms of mass%.
(A) at least one member selected from the group consisting of Cr: more than 0% and 1% or less, and Mo: more than 0% and 1% or less;
(B) Ti: more than 0% and not more than 0.15%
Nb: more than 0% and not more than 0.15%, and
V: at least one member selected from the group consisting of 0% to 0.15%;
(C) at least one member selected from the group consisting of Cu: more than 0% and 1% or less, and Ni: more than 0% and 1% or less;
(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: A high strength electrogalvanized steel sheet characterized in that an electrogalvanized layer is formed on the surface of the high-strength cold-rolled steel sheet according to any one of claims 1 to 4. A high strength hot-dip galvanized steel sheet characterized in that a hot-dip galvanized layer is formed on the surface of the high-strength cold-rolled steel sheet according to any one of claims 1 to 4. A high strength alloyed hot-dip galvanized steel sheet characterized in that a galvannealed layer is formed on the surface of the high-strength cold-rolled steel sheet according to any one of claims 1 to 4. A method for producing the high-strength cold-rolled steel sheet according to any one of claims 1 to 4,
In the hot rolling process of the steel sheet having the above composition,
Rolled at a coiling temperature of 500 ° C or higher and 800 ° C or lower, and then held at 500 ° C or higher and 800 ° C or lower for 3 hours or longer, cooled to room temperature,
(Ac ₁ point + 20 ° C) or more and less than the Ac ₃ point, and thereafter cooling to 500 ° C at an average cooling rate of 10 ° C / sec or more and 500 ° C or less at an average cooling rate of 10 ° C / Cooled to a temperature range of 500 DEG C or lower,
Followed by reheating to a temperature of not less than 250 ° C and not more than 500 ° C, holding it for 30 seconds or longer, and then cooling it to room temperature, and having a tensile strength of 980 MPa or more excellent in ductility and bendability. A process for producing a high-strength electrogalvanized steel sheet, characterized in that the high-strength cold-rolled steel sheet obtained by the manufacturing method according to claim 8 is further subjected to electro-galvanizing. A method for producing the high-strength hot-dip galvanized steel sheet according to claim 6,
In the hot rolling process of the steel sheet having the above composition,
Rolled at a coiling temperature of 500 ° C or higher and 800 ° C or lower, and then held at 500 ° C or higher and 800 ° C or lower for 3 hours or longer, cooled to room temperature,
(Ac ₁ point + 20 ° C) or more and less than the Ac ₃ point, and thereafter cooling to 500 ° C at an average cooling rate of 10 ° C / sec or more and 500 ° C or less at an average cooling rate of 10 ° C / Cooled to a temperature range of 500 DEG C or lower,
Followed by reheating to a temperature of not less than 250 ° C and not more than 500 ° C to maintain the temperature for not less than 30 seconds and to perform hot dip galvanization within the holding time and then cooling to room temperature to obtain a high tensile strength having a ductility and bending property of not less than 980 MPa A method of manufacturing a hot dip galvanized steel sheet. 11. The method of claim 10,
And performing alloying at a temperature in a range of 450 ° C to 550 ° C after the hot dip galvanizing is performed.

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