WO2015141097A1 - 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

Provided is a high strength cold rolled steel sheet which has a tensile strength of 980 MPa or greater and which exhibits excellent ductility and bendability. The high strength cold rolled steel sheet satisfies a specified component composition, and the structure of the steel sheet at a depth corresponding to a quarter of the sheet thickness satisfies all of (1) to (5) below when measured using specified methods. (1) The areal ratio of ferrite relative to the overall structure is not less than 5% but less than 50 %, with the remainder being hard phases. (2) The areal ratio of a mixed structure of fresh martensite and residual austenite relative to the overall structure is greater than 0% but not greater than 30%. (3) A region in which the concentration of Mn is enriched to at least 1.2 times the concentration of Mn in the steel sheet is present at a proportion of not less than 5% by area. (4) When the fraction of a region in which the concentration of Mn is enriched to at least 1.2 times the concentration of Mn in the steel sheet is measured for 100 sections each measuring 2 μm square, the standard deviation is not lower than 4.0%. (5) The concentration of Mn in a ferrite phase is not higher than 0.9 times the concentration of Mn in the steel sheet.

Description

High strength cold rolled steel sheet and high strength galvanized steel sheet excellent in ductility and bendability, and method for manufacturing them

The present invention relates to a high strength cold rolled steel sheet and a high strength galvanized steel sheet excellent in ductility and bendability, and a method of manufacturing them. Specifically, a high strength cold rolled steel sheet excellent in ductility and bendability in a region having a tensile strength of 980 MPa or more, a high strength electrogalvanized steel sheet, a high strength galvanized steel sheet and a high strength alloyed galvanized steel sheet, The present invention relates to a manufacturing method capable of efficiently manufacturing these steel plates.

In order to realize low fuel consumption of automobiles and transport aircraft, it is desirable to reduce the weight of the automobile and transport aircraft. For example, it is effective to use a high strength steel plate and reduce the thickness to reduce the weight. However, when the steel sheet is made high in strength, the ductility is deteriorated, so that the workability is deteriorated. Therefore, high strength steel sheets are required to have excellent ductility and bendability required for press forming. In addition, from the viewpoint of corrosion resistance, galvanizing (EG, Electro-Galvanizing), galvanizing (GI, Galvanized Iron), galvanizing such as alloying galvanization (GA, Galva-Annealed) are applied to steel parts for automobiles. In many cases, a steel plate that has been subjected to There are cases where the electrogalvanized steel sheet, the galvanized steel sheet, and the alloyed galvanized steel sheet are hereinafter represented by "galvanized steel sheet". Also in these galvanized steel sheets, characteristics similar to those of high strength steel sheets are required.

As techniques for improving the processability of high strength steel plates, for example, Patent Documents 1 to 4 propose ultra-high strength cold rolled steel plates excellent in bendability.

JP, 2011-179030, A Patent No. 5299591 JP, 2011-225976, A International Publication No. 2012/036269

However, a steel plate excellent in ductility and bendability has not been provided yet in a region where the tensile strength is 980 MPa or more.

The present invention has been made focusing on the above circumstances, and the object thereof is a high strength cold rolled steel sheet having a tensile strength of 980 MPa or more and excellent in ductility and bendability, and high strength galvanizing. Steel sheets and methods for manufacturing them with high productivity, specifically, high strength cold rolled steel sheets excellent in ductility and bendability in a region of tensile strength of 980 MPa or more, high strength electrogalvanized steel sheets, high strength galvanized steel It is an object of the present invention to provide a steel sheet, a high strength alloyed galvanized steel sheet, and a manufacturing method capable of efficiently manufacturing these steel sheets.

In the present invention which has solved the above problems, the component composition of the steel plate is, in mass%, C: 0.10% or more and 0.30% or less, Si: 1.2% or more and 3% or less, Mn: 0. 5% or more and 3.0% or less, P: more than 0% and 0.1% or less, S: more than 0% and 0.05% or less, Al: 0.005% or more and 0.2% or less, N: more than 0% 0 .01% or less and O: more than 0% to 0.01% or less, the balance being composed of iron and unavoidable impurities, and the structure at the 1/4 thickness position of the steel plate has the following (1) to (5) It has a gist to satisfy all of. Hereinafter, "%" means "mass%" about the component composition of a steel plate.
(1) When observed with a scanning electron microscope, the area ratio of ferrite to the entire 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 entire structure is more than 0% and 30% or less when subjected to repeller corrosion and observed with an optical microscope.
(3) When analyzed with an electron beam microprobe analyzer, there are 5 area% or more of a region in which the Mn concentration is 1.2 times or more the Mn concentration in the steel plate, and (4) □ 2 μm section The fraction of the region where the Mn concentration is 1.2 times or more the Mn concentration in the steel sheet is measured, and the standard deviation when measured in 100 sections is 4.0% or more.
(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 a preferred embodiment that the volume fraction of retained austenite to the entire structure is 5% or more, as measured by the X-ray diffraction method.

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

In the practice of the present invention, at least one other element selected from the group consisting of (A) Cr: more than 0% and 1% or less and Mo: more than 0% and 1% or less as another element; (B) Ti: 0 %, At least one selected from the group consisting of more than 0.15%, Nb: more than 0%, and less than 0.15%, and V: more than 0% and less than 0.15%; (C) Cu: more than 0% And at least one selected from the group consisting of Ni: more than 0% and 1% or less; (D) B: more than 0% and 0.005% or less; (E) Ca: more than 0% and 0.01% or less, Mg It is also a preferred embodiment to contain at least one selected from the group consisting of: more than 0% and 0.01% or less, and REM (Rare Earth Metal): 0% or more and 0.01% or less. .

In the present invention, furthermore, a high strength electrogalvanized steel sheet in which an electrogalvanized layer is formed on the surface of the high strength cold rolled steel sheet; a hot dip galvanized layer is formed on the surface of the high strength cold rolled steel sheet Also included is a high strength galvanized steel sheet; and a high strength alloyed galvanized steel sheet having an alloyed galvanized layer 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 a process of hot rolling a steel sheet having the above-described composition, and wound at 500 ° C. or more and 800 ° C. or less. After holding at 800 ° C. or less for 3 hours or more, cool to room temperature, cold-roll, hold soaking in a temperature range of (Ac ₁ point + 20 ° C.) to Ac ₃ point, and then average cooling rate up to 500 ° C. Cool at 10 ° C / sec or more and 500 ° C or less at an average cooling rate of 10 ° C / sec or more to a temperature range of 500 ° C or less, then reheat to a temperature range of 250 ° C or more and 500 ° C or less and hold for 30 seconds or more Then, it has a gist on cooling to room temperature.

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

The method for producing a high strength galvanized steel sheet according to the present invention, which has solved the above problems, is a process of hot rolling a steel sheet having the above-mentioned composition, and wound at 500 ° C. or more and 800 ° C. or less. After holding at 3 ° C or more and 800 ° C or less for 3 hours or more, cool to room temperature, cold-roll, hold soaking in a temperature range of (Ac ₁ point + 20 ° C) or more and Ac ₃ point, and then average cooling to 500 ° C Cool at a rate of 10 ° C / sec or more and 500 ° C or less at an average cooling rate of 10 ° C / sec or more to a temperature range of 500 ° C or less, then reheat to a temperature range of 250 ° C or more and 500 ° C or less, for 30 seconds or more It has a gist to hold | maintain and to carry out hot dip galvanization within this holding time, and to cool to room temperature.

In the present invention, it is also preferable to perform alloying in a temperature range of 450 ° C. or more and 550 ° C. or less after applying the above-described hot-dip galvanization.

According to the present invention, high strength cold rolled steel sheet and high strength galvanized steel sheet excellent in ductility and bendability even at 980 MPa or more, in detail, high strength cold rolled steel sheet excellent in the above characteristics, high strength An electrogalvanized steel sheet, a high strength galvanized steel sheet, and a high strength alloyed galvanized steel sheet can be provided. Moreover, according to the manufacturing method of this invention, these steel plates can be manufactured efficiently. Therefore, the high-strength cold-rolled steel plate or the like of the present invention is extremely useful particularly in the industrial field such as automobiles.

FIG. 1 is a schematic explanatory view showing an example of a heat treatment pattern in the manufacturing method of the present invention.

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

As a result, on the premise that the component composition is appropriately controlled, the ferrite phase and the hard phase in the metal structure of the steel sheet are optimized, and if segregation of Mn is appropriately controlled, high strength of 980 MPa or more is ensured. It has been found that the ductility and bendability can be improved, leading to the present invention.

Hereinafter, the reason for defining the metal structure in the present invention will be described in detail. In addition, the fraction measured by microscopic observation means the ratio to 100% of the whole structure | tissue of a steel plate. The metal structure which comprises this invention differs in the measuring method by metal structure. Therefore, when all the metal structures specified in the present invention are totaled, it may exceed 100%, but this is only by measuring residual γ which constitutes a mixed structure of fresh martensite and retained austenite by optical microscope observation. It is because it is also measured by X-ray diffraction in an overlapping manner. Hereinafter, retained austenite is referred to as "remaining γ", and a mixed structure of fresh martensite and retained austenite is sometimes 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 bendability of a steel sheet. In the present invention, by increasing the area fraction of ferrite, it is possible to improve the ductility and bendability in a high strength region having a tensile strength of 980 MPa or more. 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 amount of ferrite is excessive, the strength of the steel sheet is reduced, which makes it difficult to secure high strength of 980 MPa or more. Therefore, the area ratio of ferrite is less than 50%, preferably 45% or less, more preferably 40% or less. The area ratio of ferrite is a value measured by observing a 1/4 thickness position of a steel plate by a scanning electron microscope (SEM: Scanning Electron Microscope).

Hard Phase The hard phase is the texture necessary to improve tensile strength. In the present invention, by increasing the area fraction of the hard phase, a high strength of 980 MPa or more can be achieved while soft ferrite is present within the range of the above 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 phase harder than ferrite, and is, for example, at least one selected from the group consisting of bainitic ferrite, bainite, tempered martensite, and MA structure, and in the present invention, it is shown below As at least including the MA organization. Among the above hard phases, bainitic ferrite, bainite, and tempered martensite are values measured by SEM observation of a 1/4 thickness position of a steel plate. The residual γ is present in the ravine of bainitic ferrite or in the MA structure.

Area ratio of MA structure to whole tissue: more than 0% and 30% or less The presence of an 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, more preferably 4% or more. On the other hand, when the area ratio of the MA structure becomes too large, the bendability deteriorates. Therefore, in the present invention, the area ratio of the MA structure is 30% or less, preferably 20% or less, and more preferably 15% or less.

In addition, the fresh martensite which comprises MA structure means the thing of the state which untransformed austenite transformed to martensite in the process which cools a steel plate from heating temperature to room temperature, and distinguishes it from the tempered martensite after heat processing. . In the present invention, the area which was whitened when repeller corrosion was observed with an optical microscope was taken as an MA structure. In addition, since it is difficult to distinguish fresh martensite and residual γ by optical microscope observation, the composite structure of fresh martensite and residual γ is measured as an MA structure. The MA structure is a value measured by optical microscope observation at a 1/4 thickness position of a steel plate.

A region where the Mn concentration is concentrated 1.2 times or more of the Mn concentration in the steel plate: 5 area% or more, and a region where the Mn concentration is concentrated 1.2 times or more of the Mn concentration in the steel plate in 2 μm sections Standard deviation of area fraction: 4.0% or more In the present invention, the area where concentration of Mn is concentrated is a cross section of a steel plate with a beam diameter of 1 μm or less and a range of 20 μm × 20 μm with an electron probe microprobe analyzer (Electron Probe) It is defined using the Mn concentration obtained by analysis using Microanalyzer, EPMA). Moreover, "the Mn concentration in a steel plate" is a Mn concentration obtained by chemically analyzing a base steel plate by inductively coupled plasma emission spectroscopy. Therefore, in the region where the Mn concentration is concentrated by 1.2 times or more of the Mn concentration in the steel plate, the measured value of the Mn concentration obtained by EPMA analysis is 1.2 times or more higher than the Mn concentration in the base steel plate. It is an area and is measured in the range of 20 μm × 20 μm.

Further, “□ 2 μm section” is a 2 μm square section, and in the present invention, an EPMA measuring range of 20 μm × 20 μm is divided into 100 sections of 2 μm square section obtained by drawing lines at intervals of 2 μm each. The area fraction of the region where the Mn concentration is 1.2 times or more higher in each section is measured, and the standard deviation is statistically determined in 100 sections.

In the present invention, in the Mn concentration distribution, a region concentrated by 1.2 times or more of the Mn concentration in the steel plate is 5 area% or more, and Mn is concentrated 1.2 times or more in a section of 2 μm It was found that the bendability is greatly improved if the standard deviation when measuring the area fraction is 4.0% or more. Hereinafter, a region concentrated 1.2 times or more of the Mn concentration in the steel plate is referred to as “a region of 1.2 times or more of the Mn concentration”, and Mn is concentrated 1.2 times or more in a section of 2 μm □ The standard deviation when the area fraction is measured may be referred to as "the standard deviation of the area in which the Mn concentration is 1.2 times or more concentrated" or simply "the standard deviation".

That is, the region with an Mn concentration of 1.2 times or more is mainly a hard phase. The larger the area ratio of the region having a Mn concentration of 1.2 or more, the relatively lower the Mn concentration in the ferrite phase, the lower the hardness of the ferrite phase, and the bendability can be improved. Although the standard deviation increases as the segregation of Mn increases, the Mn concentration in the ferrite phase contributing to the improvement of the bendability decreases, and the hardness of the ferrite phase can be reduced.

In order to obtain such an effect, the area having a Mn concentration of 1.2 times or more is 5.0 area% or more, preferably 5.2 area% or more, more preferably 5.5 area% or more. On the other hand, if the proportion occupied by the region having a Mn concentration of 1.2 times or more is too high, the Ms point of austenite may decrease and the MA structure may increase. Therefore, preferably 20 areas of a region having a Mn concentration of 1.2 times or more % Or less, more preferably 15 area% or less.

In addition, the standard deviation of a region in which 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. If the standard deviation is less than 4.0%, the distribution of Mn is insufficient and uniformly distributed, so the reduction in 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.

The spheroidized hard phase can be dispersed in the ferrite phase by setting the area with an Mn concentration of 1.2 times or more to 5 area% or more and the standard deviation to be 4.0% or more, and the strength of the steel material It is possible to combine the improvement effect with the bendability improvement effect by ferrite. Hereinafter, the spheroidized hard phase is referred to as "spherical hard phase". Here, the spherical hard phase is a part of the hard phase, and is composed of bainitic ferrite, bainite, tempered martensite, an MA structure, and the like as the above-mentioned hard phase. It has been considered that if the hardness difference between the hard phase and the ferrite phase is large, an interface crack is generated during bending and the bendability is deteriorated, but it is 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 should be as small as possible, and preferably has an aspect ratio of 3 or less, more preferably 2.5 or less, still more preferably 2 or less. Preferably it is 2 micrometers or less, More preferably, it is 1.8 micrometers or less, More preferably, it is 1.5 micrometers or less. Further, in order to exert the above effect, the spherical hard phase is preferably 0.70% by volume or more, more preferably 0.75% by volume or more, still more preferably 0.80% by volume or more with respect to the above hard phase. Do.

Since the Mn concentration in the ferrite is 0.90 times or less of 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 is deteriorated. The Mn concentration of Mn 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 may decrease and the strength may be insufficient. Therefore, the Mn concentration in the ferrite is preferably 0.3 times or more of the Mn concentration in the steel sheet, Preferably, it is 0.4 times or more. The Mn concentration in the ferrite phase can be measured by EPMA.

The volume ratio of residual γ to the entire structure: 5% or more The residual γ is deformed by strain when processing a steel sheet, and can transform to martensite to ensure good ductility and harden the deformed portion during processing This is a structure necessary to improve the strength-ductility balance of the steel sheet because it has the effect of promoting the strain concentration and suppressing the concentration of strain. In order to exert such an effect effectively, the volume ratio of residual γ is preferably 5% or more, more preferably 6% or more, and still more preferably 7% or more. The upper limit of the volume fraction of residual γ is not particularly limited, but is at most 20% or less within the range of the component composition and production conditions of the present invention. The residual γ is a value measured by X-ray diffractometry at a quarter thickness of the steel plate.

The residual γ is contained between the lass of bainitic ferrite or in the MA structure. The effect of the residual γ is exerted regardless of the present form, so in the present invention, the residual γ which can be confirmed when measured is taken as the residual γ regardless of the present form.

Next, the component composition of the high strength steel plate of the present invention will be described.

C: 0.10% or more and 0.30% or less C is an element necessary for securing the strength and enhancing the stability of the residual γ. 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, more preferably 0.15% or more. However, if the C content is excessive, the strength after hot rolling will increase, cracks will occur during cold rolling, and the weldability of the final product will decrease, so the C content is preferably 0.30% or less, preferably It is 0.26% or less, more preferably 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 formation of carbides, effectively acting on the formation of residual γ, and securing an excellent TS × EL balance. In order to exert such an effect effectively, the Si content is 1.2% or more, preferably 1.35% or more, more preferably 1.5% or more. However, when the Si content is excessive, a marked scale is formed during hot rolling, scale marks may be formed on the surface of the steel sheet, and the surface properties may be deteriorated. It also degrades the acid washability. Therefore, the Si content is 3% or less, preferably 2.8% or less, and more preferably 2.6% or less.

Mn: 0.5% or more and 3.0% or less Mn is an element that improves the hardenability and contributes to the strengthening of the steel sheet. It is also an element that effectively acts to stabilize γ and generate residual γ. In order to exert such an effect effectively, the Mn content is 0.5% or more, preferably 0.6% or more, more preferably 1.0% or more, further preferably 1.5% or more, further Preferably, it is 2.0% or more. However, when the Mn content is excessive, the strength after hot rolling is increased, which causes cracking during cold rolling or deterioration of the weldability of the final product. Moreover, addition of excessive Mn causes Mn to segregate to deteriorate the workability. Therefore, the Mn content is 3.0% or less, preferably 2.8% or less, more preferably 2.6% or less.

P: more than 0% and 0.1% or less P is an element which is contained unavoidable and is an element which degrades the weldability of a steel plate. Therefore, the P content is 0.1% or less, preferably 0.08% or less, and more preferably 0.05% or less. The lower limit is not particularly limited because the P content is preferably as small as possible, but the industrially lower limit is 0.0005%.

S: more than 0% and 0.05% or less S, like P, is an element that is inevitably contained and is an element that degrades the weldability of a steel plate. Moreover, S forms a sulfide type inclusion in a steel plate, and becomes a cause of reducing the workability of a steel plate. Therefore, the S content is 0.05% or less, preferably 0.01% or less, and more preferably 0.005% or less. The lower limit is not particularly limited because the S content is preferably as small as possible, but the industrially lower limit is made 0.0001%.

Al: 0.005% or more and 0.2% or less Al is an element acting as a deoxidizer. In order to exert such an effect effectively, the Al content is made 0.005% or more, more preferably 0.01% or more. However, if the Al content is excessive, the weldability of the steel sheet is significantly degraded, so the Al content is 0.2% or less, preferably 0.15% or less, and more preferably 0.10% or less.

N: more than 0% and 0.01% or less N is an element which is contained unavoidably, but is an element which precipitates a nitride in a steel plate and contributes to high strengthening of the steel plate. 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 to cause elongation, which causes deterioration of stretch flangeability λ, bendability and the like. Therefore, the N content is 0.01% or less, preferably 0.008% or less, and more preferably 0.005% or less.

O: more than 0% and 0.01% or less O is an element contained unavoidably, and when it is contained in excess, it is an element which causes a decrease in ductility and bendability at the time of processing. Therefore, the O content is 0.01% or less, preferably 0.005% or less, and more preferably 0.003% or less. The lower limit is not particularly limited because the O content is preferably as small as possible, but the industrially lower limit is 0.0001%.

Other Components The steel sheet of the present invention satisfies the above-mentioned composition, and the balance is iron and unavoidable impurities. The unavoidable impurities include, for example, the above-mentioned P, S, N, O, and tramp elements such as Pb, Bi, Sb, Sn, etc., which may be brought into steel depending on the conditions of raw materials, materials, manufacturing facilities, etc. Sometimes. In addition, the following elements can be positively contained as other elements within the range not adversely affecting the action of the present invention.

The steel sheet of the present invention may further contain, as another element,
(A) Cr: at least one selected from the group consisting of more than 0% and less than 1% and Mo: more than 0% and less than 1%,
(B) at least one selected from the group consisting of Ti: more than 0% and 0.15% or less, Nb: more than 0% and 0.15% or less, and V: 0% and more than 0.15% or less,
(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% or less,
(D) B: more than 0% and less than 0.005%,
(E) Ca: at least one selected from the group consisting of more than 0% and 0.01% or less, Mg: more than 0% and 0.01% or less, and REM: more than 0% and 0.01% or less May be These elements (A) to (E) can be contained alone or in combination. The reason for defining such a range is as follows.

(A) At least one selected from the group consisting of Cr: more than 0% and 1% or less and Mo: more than 0% and 1% or less Both Cr and Mo are effective in enhancing the hardenability and improving the strength of the steel sheet Elements, which can be used alone or in combination. In order to exert such an effect effectively, the content of each of Cr and Mo is preferably 0.1% or more, more preferably 0.3% or more. However, if it is contained in excess, the processability is lowered and the cost becomes high. Therefore, when each of Cr and Mo is contained alone, it is preferably 1% or less, more preferably 0.8% or less. More preferably, it is 0.5% or less. When Cr and Mo are used in combination, each of them is independently within the above upper limit, and preferably the total amount is 1.5% or less.

(B) at least one of Ti, Nb, and at least one selected from the group consisting of Ti: more than 0% and 0.15% or less, Nb: more than 0% and 0.15% or less, and V: more than 0% and 0.15% or less V is an element having a function of forming precipitates of carbides and nitrides in the steel plate to improve the strength of the steel plate and refining the old γ grains, and can be used alone or in combination. . In order to exert such an effect effectively, the content of Ti, Nb and V is preferably 0.005% or more, more preferably 0.010% or more. However, if it is contained in excess, carbides precipitate at grain boundaries, and the stretch flangeability and bendability of the steel sheet deteriorate. Therefore, the content of each of Ti, Nb and V is preferably 0.15% or less, more preferably 0.12% or less, and still more preferably 0.10% or less.

(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% or less Cu and Ni are elements effectively acting on formation and stabilization of retained austenite, Furthermore, it is an element also having the effect of improving the corrosion resistance, and can be used alone or in combination. In order to exert such effects, the contents of Cu and Ni are preferably 0.05% or more, more preferably 0.10% or more. However, if the Cu content is excessive, the Cu content is preferably 1% or less, more preferably 0.8% or less, still more preferably 0.5, because the hot workability deteriorates if the Cu content is excessive. % Or less. Since an excessive content of Ni results in high cost, the Ni content is preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. When Cu and Ni are used in combination, the above-mentioned action is easily exhibited, and when Ni is contained, the deterioration of hot workability due to the addition of Cu is suppressed. Therefore, when Cu and Ni are used in combination, the total amount is preferably 1 .5% or less, more preferably 1.0% or less.

(D) B: more than 0% and 0.005% or less B is an element improving the hardenability, and is an element effective for stably causing austenite to reach room temperature. In order to exert such effects effectively, the B content is preferably 0.0005% or more, more preferably 0.0010% or more, and still more preferably 0.0015% or more. However, when it is contained in excess, the B content is preferably 0.005% or less, more preferably 0.004% or less, and still more preferably 0.0035% or less, because boride is generated to deteriorate ductility. Do.

(E) Ca: at least one selected from the group consisting of Ca: more than 0% and 0.01% or less, Mg: more than 0% and 0.01% or less, and REM: more than 0% and 0.01% or less REM is an element having an effect of finely dispersing inclusions in a steel sheet, and may be contained singly or in combination of two or more selected arbitrarily. In order to exert such an effect effectively, the content of each of Ca, Mg and REM is preferably made 0.0005% or more, more preferably 0.0010% or more, alone. However, when it is contained in excess, it causes deterioration of castability, hot workability and the like. Therefore, the content of each 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.

In the present invention, REM is an abbreviation of a rare earth element and is a meaning including lanthanoid elements, that is, 15 elements from La to Lu, and scandium and yttrium.

Next, a method of manufacturing the high strength cold rolled steel sheet, high strength electrogalvanized steel sheet, high strength galvanized steel sheet and high strength alloyed galvanized steel sheet of the present invention, particularly high strength cold rolled steel sheet and high strength molten zinc The manufacturing method of a plated steel plate is demonstrated. The method for producing a high strength cold rolled steel sheet and the method for producing a high strength galvanized steel sheet according to the present invention are as follows: “The hot rolling process of the steel sheet having the above-mentioned composition has a winding temperature of 500 ° C. or more and 800 ° C. or less. Hold at 500 ° C or more and 800 ° C or less for 3 hours or more, cool to room temperature, cold-roll, hold soaking in a temperature range of (Ac ₁ point + 20 ° C) or more and Ac ₃ point, and then average up to 500 ° C Since the steps up to “cooling to a temperature range of 500 ° C. or less at a cooling rate of 10 ° C./sec or more and 500 ° C. or less at an average cooling rate of 10 ° C./sec or more are the same, the steps will be described together, The reheating process after “cooling to a temperature range of 500 ° C. or less” is different between the two, so the process will be described separately for the high strength cold rolled steel sheet and the high strength galvanized steel sheet.

In the method for producing a high strength cold rolled steel sheet and a high strength galvanized steel sheet according to the present invention, annealing described later is performed on a steel sheet obtained by performing hot rolling and cold rolling on a steel satisfying the above-mentioned component composition. Do. In the method of manufacturing a high strength cold rolled steel sheet, reheating is performed after the annealing. Furthermore, a high-strength electrogalvanized steel sheet can be obtained by performing the galvanizing treatment appropriately in combination as needed. In the method for producing a high strength galvanized steel sheet, after annealing, reheating and hot dip galvanizing are performed. Furthermore, a high-strength alloyed hot-dip galvanized steel sheet can be obtained by performing the alloying treatment in combination as appropriate. In the present invention, by appropriately controlling the production conditions, it is possible to obtain a high strength cold rolled steel sheet, a high strength galvanized steel sheet, etc. having a desired structure.

For example, as shown in FIG. 1, hot rolling is performed according to a conventional method using a steel having the above-described component composition. In hot rolling, for example, after hot rolling so that the finish rolling temperature is Ac ₃ or more, winding is performed at a winding temperature of 500 ° C. or more and 800 ° C. or less. Then, after holding at 500 degreeC or more and 800 degrees C or less for 3 hours or more, it cools to room temperature and cold-rolls. The cooling after finish rolling is about 500 ° C./sec at the upper limit of operation.

After cold rolling, as an annealing process, soaking is maintained in a two-phase temperature range of Ac ₁ point + 20 ° C. or more and less than Ac ₃ point, and then cooled to 500 ° C. at an average cooling rate of 10 ° C./sec or more It cools to a temperature range of 500 degrees C or less at 500 degrees C or less at an average cooling rate of 10 degrees C / sec or more. Next, the method of manufacturing the high strength cold rolled steel sheet includes a step of reheating to a temperature range of 250 ° C. or more and 500 ° C. or less, holding the temperature range for 30 seconds or more, and cooling to room temperature. Further, in the method for producing a high strength galvanized steel sheet, after cooling to the temperature range of 500 ° C. or less, it is then reheated to a temperature range of 250 ° C. or more and 500 ° C. or less and held for 30 seconds or more in the temperature range. Galvanize in a holding time and then cool to room temperature.

Hereinafter, the reason which specified each said conditions is explained in full detail.

Take up at winding temperature 500 ° C. or more and 800 ° C. or less, then hold at 500 ° C. or more and 800 ° C. or less for 3 hours or more, then cool to room temperature After hot rolling, take up at winding temperature 500 ° C. or more and 800 ° C. or less Then, by maintaining at 500 ° C. or more and 800 ° C. or less for 3 hours or more, the above-mentioned predetermined Mn concentration distribution is generated to precipitate carbides containing Mn, and spheroidized in ferrite phase by annealing after cold rolling It becomes a hard phase. In order to obtain such an effect, the winding temperature is set to 500 ° C. or more, preferably 550 ° C. or more, more preferably 600 ° C. or more. However, if the coiling temperature is too high, a large amount of scale, grain boundary oxidation and the like occur in the steel sheet and the acid washability deteriorates, so that the coiling temperature is 800 ° C. or less, preferably 750 ° C. or less, more preferably 700 ° C. It is assumed that Further, the temperature range to be maintained after winding is 500 ° C. or more, preferably 510 ° C. or more, more preferably 520 ° C. or more, further preferably 550 ° C. or more, still more preferably 580 ° C. or more. On the other hand, if the holding temperature is too high, as in the case where the coiling temperature is too high, a large amount of scale, intergranular oxidation, etc. may occur on the steel sheet and the acidity may deteriorate, so the holding temperature is preferably 800 ° C. or less Is set to 780 ° C. or less, more preferably 750 ° C. or less, and still more preferably 700 ° C. or less. The holding time in the temperature range is 3 hours or more, preferably 4 hours or more, more preferably 5 hours or more, still more preferably 7 hours or more, still more preferably 10 hours or more. On the other hand, if the holding time is too long, a large amount of scale, intergranular oxidation, etc. may occur on the steel plate as in the case where the coiling temperature is too high, and the acid washability may be deteriorated. The following time is more preferably 60 hours or less.

In the present invention, holding at a predetermined temperature does not necessarily mean holding the same temperature at the same temperature, and it may be fluctuated as long as it is within a predetermined temperature range. For example, the temperature may be maintained within the range of the above-mentioned holding temperature, or within this range, it is intended to include changes such as temperature decrease due to temperature decrease or heating, temperature increase due to recuperation accompanying transformation.

In the present invention, after holding for a predetermined time in the above temperature range, cooling is performed to room temperature, but the cooling rate at that time is not particularly limited, and may be, for example, air cooling.

After pickling, cold rolling and hot rolling, pickling is performed if necessary, and cold rolling is performed at a cold rolling ratio of about 30 to 80%.

Annealing As an annealing process after cold rolling, soaking is maintained in a two-phase zone of Ac ₁ point + 20 ° C or more and Ac ₃ point, and then the temperature range up to 500 ° C is cooled at an average cooling rate of 10 ° C / sec or more Then, the temperature range of 500 ° C. or less is cooled at an average cooling rate of 10 ° C./sec or more, and cooled to a temperature range of 500 ° C. or less.

By maintaining soaking holding temperature in a two-phase area of Ac ₁ point + 20 ° C. or more and less than Ac ₃ point, it is possible to secure the desired amount of ferrite while maintaining the Mn concentration distribution of the present invention. If the soaking holding temperature is lower than Ac ₁ point + 20 ° C., the amount of ferrite in the metallographic structure of the finally obtained steel sheet is too large to secure sufficient strength. Therefore, the soaking holding temperature is set to Ac ₁ point + 20 ° C. or higher, preferably Ac ₁ point + 25 ° C. or higher, more preferably Ac ₁ point + 50 ° C. or higher, and still more preferably Ac ₁ point + 80 ° C. or higher. On the other hand, if the Ac ₃ point or more, the ferrite can not be sufficiently formed and grown during soaking holding, the ductility is lowered, the Mn concentration distribution becomes uniform, and the spherical hard phase formed in the ferrite phase is Decrease. Therefore, the soaking holding temperature is set to a temperature less than Ac ₃ point, preferably Ac ₃ point −5 ° C. or less, more preferably Ac ₃ point −10 ° C. or less, still more preferably Ac ₃ point −20 ° C. or less.

The average temperature rising rate at the time of raising the temperature to the above-mentioned soaking holding temperature range is not particularly limited, and can be appropriately selected. For example, an average temperature rising rate of about 0.5 to 50 ° C./second may be used. .

In the present invention, the holding time in the soaking holding temperature range is not particularly limited. However, if the holding time is too short, the machined structure may remain and the ductility of the steel may decrease, so the holding time is preferably 40 seconds or more, 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 proceeds and the Ms point may decrease to increase the MA structure. Therefore, the holding time is preferably 3600 seconds or less, more preferably 3000 seconds or less Do.

Further, as described above, in the present invention, holding at a predetermined temperature does not have to be always maintained at the same temperature, and within the predetermined temperature range, it may be fluctuated. For example, in the case of holding at the above-mentioned soaking 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 above Ac ₁ point and Ac ₃ point can be calculated from the following formulas (a) and (b) described in “Leslie Iron and Steel Material Chemistry”, Maruzen Co., Ltd., May 31, 1985, page 273. In the formula, [] indicates 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%.
Ac ₁ (° C.) = 723-10.7 × [Mn] −16.9 × [Ni] + 29.1 × [Si] + 16.9 × [Cr] + 290 × [As] + 6.38 × [W] · (A)
Ac ₃ (° C.) = 910-203 × √ [C] −15.2 × [Ni] + 44.7 × [Si] +10 ⁴ × [V] + 31.5 × [Mo] + 13.1 × [W] − (30 x [Mn] + 11 x [Cr] + 20 x [Cu]-700 x [P]-400 x [Al]-120 x [As]-400 x [Ti]) (b)

After holding the temperature uniform, the temperature range up to 500 ° C. is cooled at an average cooling rate of 10 ° C./sec or more. By controlling the cooling rate from the above-mentioned soaking holding temperature, it is possible to suppress the formation of ferrite having a high Mn concentration and to suppress the generation amount of ferrite. When the average cooling rate is low, ferrite having a high Mn concentration may be formed during cooling, which may deteriorate the bendability or reduce the strength. Therefore, the average cooling rate is 10 ° C./sec or more, preferably 15 ° C./sec or more, and more preferably 20 ° C./sec or more. The upper limit of the average cooling rate is not particularly limited, and may be water cooling or oil cooling.

After cooling the temperature range up to 500 ° C. at the above average cooling rate, cooling is performed at 500 ° C. or less at an average cooling rate of 10 ° C./sec or more. By setting the average cooling rate of 500 ° C. or less to 10 ° C./sec or more, it is possible to suppress the formation of soft high-temperature bainite and to suppress the self-tempering of martensite to improve the strength. In order to obtain such an effect, the average cooling rate at 500 ° C. or less is 10 ° C./sec or more, preferably 15 ° C./sec or more, more preferably 20 ° C./sec or more. The upper limit of the average cooling rate is not particularly limited, and may be water cooling or oil cooling.

The average cooling rate up to 500 ° C. and the average cooling rate below 500 ° C. may be the same or different, and may be appropriately adjusted within the above range.

The cooling stop temperature in the case of cooling at 500 ° C. or less at an average cooling rate of 10 ° C./sec 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, and the strength can not be secured, and the MA structure is increased to deteriorate the bendability. Therefore, the cooling stop temperature is set to 500 ° C. or less, preferably 400 ° C. or less, more preferably 350 ° C. or less, and still more preferably 300 ° C. or less. The lower limit of the cooling stop temperature is not particularly limited, but in operation, it is up to room temperature.

Hereinafter, the reheating process is divided into a method of manufacturing a cold-rolled steel sheet and a method of manufacturing a hot-dip galvanized steel sheet.

Reheating in the method for producing a cold rolled steel sheet After stopping cooling at the above-mentioned temperature range of 500 ° C. or less, then reheating to a temperature range of 250 ° C. or more and 500 ° C. or less, holding for 30 seconds or more and cooling to room temperature Do.

After cooling is stopped, reheating is performed to a temperature range of 250 ° C. or more and 500 ° C. or less, and by holding for 30 seconds or more, hard phases such as martensite can be tempered and untransformed austenite can be transformed. When reheating is not performed or the holding temperature is too low, tempering of the hard phase does not proceed, high-density dislocation may occur, or a large amount of MA structure may be left, which may deteriorate the bendability. Therefore, the reheating temperature is set to 250 ° C. or more, preferably 300 ° C. or more, more preferably 350 ° C. or more. On the other hand, when the holding temperature is too high, the strength is reduced. Therefore, the reheating temperature is 500 ° C. or less, preferably 470 ° C. or less, more preferably 450 ° C. or less. In the present invention, this "reheating" means heating from the cooling stop temperature to 500 ° C. or less, that is, temperature rise, as it is written. Therefore, the reheating temperature is a temperature higher than the cooling stop temperature, and even in the temperature range of 250 ° C. or more and 500 ° C. or less, isothermal holding and cooling stop with the same cooling stop temperature and reheating temperature The cooling process from temperature to lower temperatures is not included in this reheating.

If the holding time in the reheating temperature range is too short, the hard phase can not be sufficiently tempered, and untransformed austenite can not be transformed. Therefore, the holding time is set to 30 seconds or more, preferably 50 seconds or more, more preferably 100 seconds or more, and further preferably 200 seconds or more. On the other hand, the upper limit of the holding time is not particularly limited, but if it is held for a long time, the productivity is lowered and the strength is lowered, so it is preferably 1500 seconds or less, more preferably 1000 seconds or less.

After holding for a predetermined time in the above 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 ° C./sec or more, more preferably 0.4 ° C./sec or more, preferably 200 ° C./sec or less, more preferably 150 ° C./sec or less It suffices to cool at an average cooling rate.

In the present invention, an electrogalvanized layer may be formed on the surface of the steel sheet obtained above.

The formation method of said electrogalvanized layer is not specifically limited, The electric galvanization processing method of a conventional method is employable. For example, in the case of producing an electrogalvanized steel sheet, a method of performing an electrogalvanizing treatment by immersing in a 55 ° C. zinc solution and conducting electricity is mentioned. Further, the amount of plating adhesion per one side is also not particularly limited, and, for example, in the case of an electrogalvanized steel plate, it may be about 10 to 100 g / m ² .

Reheating in the manufacturing method of hot-dip galvanized steel sheet After cooling is stopped in the above-mentioned temperature range of 500 ° C. or less, next, it is re-heated to a temperature range of 250 ° C. or more and 500 ° C. or less and held for 30 seconds or more Galvanize in time and then cool to room temperature.

After cooling is stopped, reheating is performed to a temperature range of 250 ° C. or more and 500 ° C. or less, and by holding for 30 seconds or more, hard phases such as martensite can be tempered and untransformed austenite can be transformed. When reheating is not performed or the holding temperature is too low, tempering of the hard phase does not proceed, high-density dislocation may occur, or a large amount of MA structure may be left, which may deteriorate the bendability. Therefore, the reheating temperature is set to 250 ° C. or more, preferably 300 ° C. or more, more preferably 350 ° C. or more. On the other hand, when the holding temperature is too high, the strength is reduced. Therefore, the reheating temperature is 500 ° C. or less, preferably 470 ° C. or less, more preferably 450 ° C. or less. In the present invention, this "reheating" means heating from the cooling stop temperature to 500 ° C. or less, that is, temperature rise, as it is written. Therefore, the reheating temperature is a temperature higher than the cooling stop temperature, and even in the temperature range of 250 ° C. or more and 500 ° C. or less, isothermal holding and cooling stop with the same cooling stop temperature and reheating temperature The cooling process from temperature to lower temperatures is not included in this reheating.

If the holding time in the reheating temperature range is too short, the hard phase can not be sufficiently tempered, and untransformed austenite can not be transformed. Therefore, the holding time is set to 30 seconds or more, preferably 50 seconds or more, more preferably 100 seconds or more, and further preferably 200 seconds or more. On the other hand, the upper limit of the holding time is not particularly limited, but if it is held for a long time, the productivity is lowered and the strength is lowered, so it is preferably 1500 seconds or less, more preferably 1000 seconds or less.

In the present invention, the hot dip galvanizing treatment is performed within the holding time of 30 seconds or more in the reheating temperature range to form a hot dip galvanized layer on the steel sheet surface. In the present invention, it is performed by combining hot-dip galvanizing and holding in the reheating temperature range. That is, in order to perform appropriate management such as metal structure and strength by reheating, it is necessary to perform hot dip galvanization in the above-mentioned holding time of the reheating temperature range. The method for forming the hot-dip galvanized layer is not particularly limited, and a conventional hot-dip galvanizing method can be adopted. For example, the steel sheet may be dipped in a plating bath whose temperature is adjusted to the above reheating temperature range to carry out the hot dip galvanization treatment. The plating time should just satisfy the above-mentioned holding time, and should just be adjusted suitably so that a desired amount of plating can be secured. The plating time is preferably, for example, 1 to 10 seconds.

There are the following various patterns as a combination of hot-dip galvanizing treatment and reheating without heating treatment only;
(I) After performing only heating, hot dip galvanizing is performed.
(Ii) After the hot dip galvanizing treatment, only heating is performed.
(Iii) Only heating, galvanizing, and heating are performed in this order.

When the reheating temperature in the case of only the heating and the hot dip galvanizing temperature, that is, the temperature of the plating bath are different, the case of heating or cooling from one temperature to the other temperature may be included. As a method of the said heating, furnace heating, induction heating, etc. are mentioned.

In the case of forming an alloyed hot dip galvanized layer on the surface of a steel sheet, alloying may be performed after the above-described hot dip galvanization. The alloying temperature is not particularly limited, but is preferably 450 ° C. or more, more preferably 460 ° C. or more, and still more preferably 480 ° C. or more because alloying does not proceed sufficiently if the alloying temperature is too low. On the other hand, if the alloying temperature is too high, the alloying proceeds too much to increase the Fe concentration in the plating layer and the plating adhesion deteriorates, so it is preferably 550 ° C. or less, more preferably 540 ° C. or less, still more preferably It is 530 ° C. or less. Further, the time of alloying treatment is not particularly limited, and may be adjusted to obtain desired alloying. The alloying treatment time is preferably 10 seconds to 60 seconds. Since the alloying treatment is performed after being held 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 holding for a predetermined time in the above 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 ° C./sec or more, more preferably 0.4 ° C./sec or more, preferably 200 ° C./sec or less, more preferably 150 ° C./sec or less It suffices to cool at an average cooling rate.

The technique of the present invention can be suitably adopted particularly for thin steel plates having a thickness of 6 mm or less.

The high strength cold rolled steel sheet and high strength galvanized steel sheet of the present invention are steel sheets having a tensile strength of 980 MPa or more, preferably 1,000 MPa or more, more preferably 1,010 MPa or more. The ductility is represented by a balance of strength and ductility, that is, “tensile strength in unit MPa × ductility in unit%”, preferably 15,000 MPa ·% or more, more preferably 15,100 MPa ·% or more, further preferably The pressure shall be 15,200 MPa ·% or more. Flexibility is represented by the balance of strength and VDA (Verband der Automobilindustrie) bending angle determined by the method described in Examples described later, that is, "VDA bending angle indicated by tensile strength in unit MPa x unit degree", Preferably, it is 100,000 MPa · ° or more, more preferably 100,500 MPa · ° or more, and further preferably 101,000 MPa · ° or more.

The present application is related to Japanese Patent Application No. 2014-053399 filed on March 17, 2014, Japanese Patent Application No. 2014-053400 filed on March 17, 2014, and September 22, 2014 Claiming the benefit of priority based on Japanese Patent Application No. 2014-192757 filed in The entire content of the specification of Japanese Patent Application No. 2014-053399 filed on March 17, 2014, the entire specification of Japanese Patent Application No. 2014-053400 filed on March 17, 2014 The contents and the entire contents of the specification of Japanese Patent Application No. 2014-192757 filed on September 22, 2014 are incorporated for reference of the present application.

EXAMPLES Hereinafter, the present invention will be more specifically described by way of examples. However, the present invention is of course not limited by the following examples, and appropriate modifications may be made as long as the present invention can be applied to the purpose. Of course, implementation is also possible, and all of them are included in the technical scope of the present invention.

Example 1
A steel having the component composition shown in Table 1 below was melted and subjected to hot rolling → cold rolling → continuous annealing under the following conditions to produce a cold rolled steel sheet. The steels of the component compositions shown in Table 1 have the balance of iron and unavoidable impurities, and the blanks indicate that no element is added.

Hot rolling The slab was heated to 1250 ° C. and hot rolled to a plate thickness of 2.3 mm so that the rolling reduction was 90% and the finish rolling temperature was 920 ° C. Thereafter, the film is cooled from this temperature to the “rolling temperature (° C.)” shown in Table 2 or Table 3 at an average cooling rate of 30 ° C./s and then taken up, and then “holding temperature 1 (° C.)” shown in Table 2; And “hold time (hour)” or “hold start temperature (° C.)”, “hold end temperature (° C.)”, and “hold time (hour)” shown in Table 3. Then, it air-cooled to room temperature and manufactured the hot rolled sheet steel.

Cold rolling The obtained hot rolled steel sheet was pickled to remove surface scale, and then cold rolling was performed to produce 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 soaking holding → cooling → reheating under the conditions shown in Table 2 or Table 3 to produce a test steel. In addition, Table 2 No. 32 is a comparative example which does not reheat, and it shows in the reheat column that it hold | maintained for 300 seconds at the said cooling stop temperature from 480 degreeC to 350 degreeC instead of reheat after cooling at this temperature .

In the table, the temperature at which the soaking is maintained is "soaking temperature (° C)", the average cooling rate to 500 ° C after soaking is "average cooling rate 1 (° C / sec)", and the cooling rate at 500 ° C or less is " Average cooling rate 2 (° C / sec), cooling stop temperature is "cooling stop temperature (° C)", holding temperature at reheating after cooling stop is "reheating holding temperature (° C)", holding time at the holding temperature Was described as "reheat holding time (seconds)", respectively. In the present embodiment, the holding time at the soaking holding temperature is set to 100 seconds to 600 seconds. After the above-mentioned "reheating holding temperature (° C)" and after "reheating holding time (seconds)", it was allowed to cool to room temperature to obtain a test steel. In addition, about the cold-rolled steel plate which did not perform postscript galvanization, "CR" was entered in the "kind" column in the table.

Production of electrogalvanized steel plate A portion of the above test steel is immersed in a galvanizing bath at 55 ° C., electroplated under conditions of a current density of 30 to 50 A / dm ² , washed with water and dried to obtain electricity A galvanized steel sheet was obtained. The adhesion amount of zinc plating per one side was 10 to 100 g / m 2. Further, in the above plating treatment, washing treatment such as alkaline aqueous solution immersion degreasing, water washing, and acid washing was appropriately performed to obtain a test steel having an electrogalvanized layer on the surface. The electrogalvanized steel plate entered "EG" in the "variety" column in the table.

The metal structure, the Mn concentration, and various mechanical properties were evaluated for each of the test steels, as described in detail below, and are shown in Table 4 or Table 5.

Measurement of Metallographic Structure The area ratio of ferrite, the area ratio of hard phase, the area ratio of MA structure, the volume ratio of residual γ, and the ratio of spherical hard phase were measured as follows. That is, the cross section of the test steel was polished and corroded as described below, and then a quarter position of the plate thickness was observed using an optical microscope or a scanning electron microscope. Then, a metallographic photograph taken with an optical microscope or SEM was subjected to image analysis to measure the proportion of each tissue. Details are shown below.

Area ratio of ferrite Corroded by nital after the above polishing, 1 field size: 100 μm × 100 μm at a magnification of 1000 times by SEM, a total of 3 fields of view are observed, and ferrite with a grid spacing of 5 μm and a grid point of 20 × 20 The area ratio of was measured, and the average value of 3 fields of view was calculated. The results are shown in "ferrite (area%)" in the table. In addition, the hard phase in a ferrite phase is remove | excluded to the area ratio of a ferrite.

Area Ratio of Hard Phase The structure other than the above ferrite was used as a hard phase, and the value obtained by removing the area ratio of ferrite from 100% of the observation field of view was defined as the area ratio of the hard phase. The results are described in "Hard phase (area%)" in the table. The structure of the hard phase was also observed, and it was confirmed that the hard phase is at least one selected from the group consisting of bainitic ferrite, bainite, tempered martensite, residual γ, and MA structure.

Area ratio of MA structure After the above polishing, corrode with repeller, and observe 1 field size: 100 μm × 100 μm with a total of 3 fields of view with a magnification of 1000 × with an optical microscope. The area ratio of the MA tissue was measured, and the average value of three fields of view was calculated. The results are described in "MA (area%)" in the table. In addition, the location whitened by the said repeller corrosion was observed as MA structure | tissue.

Volume rate of residual γ After polishing using # 1000 to # 1500 sandpaper to 1/4 plate thickness position, the surface is electropolished to a depth of 10 to 20 μm, and then X-ray diffractometer, RIGAKU Co., Ltd. It measured using manufactured RINT1500. Specifically, a Co target is used, 40 kV-200 mA is output, and a range of 40 ° to 130 ° is measured at 2θ, and the diffraction peaks (110), (200), (b A quantitative measurement of residual γ was performed from diffraction peaks (111), (200), (220) and (311) of 211) and fcc (γ). The results are described in “residual γ (volume%)” in the table.

Spherical hard phase in ferrite phase After the above polishing, it is corroded by nital and spherical hard phase with an aspect ratio 1 to 3 with an equivalent circle diameter of 2 μm or less in the ferrite phase is 1000 × magnification in SEM. : 100 μm × 100 μm were observed for a total of three fields of view, and image analysis was performed to determine the ratio of the spherical hard phase to the above hard phase. The results are described in "Spherical hard phase (area%)" in the table.

The percentage of the area where the Mn concentration is 1.2 times or more the concentration of the Mn concentration in the steel sheet The Mn concentration cuts the test steel in the cross section, embeds it in the resin, and polishes EPMA within the range of 20 μm × 20 μm. The beam diameter was measured under the condition of 1 μm or less. The obtained Mn concentration was divided by the Mn concentration of the steel plate subjected to chemical analysis by inductively coupled plasma emission spectroscopy to determine the ratio of the region concentrated 1.2 times or more of the Mn concentration to the Mn concentration in the steel plate . Thereafter, the region of 1.2 times or more of the Mn concentration and the region of less than 1.2 times were color-coded to obtain the area% of the region having the Mn concentration of 1.2 times or more. The results are shown in "area ratio (%) of 1.2 times of Mn concentration" in the table.

Standard deviation of the region where the Mn concentration is concentrated 1.2 times or more of the Mn concentration in the steel plate The image color-coded according to the Mn concentration in the steel plate is divided into 100 divisions of 2 μm division, and Mn in each division The fraction of the area in which the concentration is 1.2 times or more concentrated was measured, and the standard deviation of 100 sections was determined. The results are shown in "standard deviation (area%) of 1.2 times area of Mn concentration" in the table.

The ratio of the Mn concentration in the ferrite phase to the Mn concentration in the steel plate: The above Mn concentration was measured by EPMA analysis, and the same visual field of 20 μm × 20 μm was observed by SEM. Each ferrite grain and its Mn concentration distribution were identified by comparing the EPMA analysis result and the SEM image. The point at which the major axis and minor axis of the ferrite grain intersect is the center position of the ferrite grain, and the Mn concentration at the center position is the Mn concentration of the ferrite grain. The Mn concentration at the ferrite grain center position in the 20 μm × 20 μm range is identified by the above method, and the Mn concentration of the highest Mn concentration ferrite grain in each 20 μm × 20 μm range is divided by the Mn concentration of the steel plate to obtain a ferrite phase. The ratio of the Mn concentration in the steel sheet to the Mn concentration of the steel plate was determined. In the present invention, 20 μm × 20 μm is set as one field of view, and the average value of the three fields of view is the ratio of the Mn concentration in the ferrite phase to the Mn concentration of the steel plate. The results are shown in the “proportion of Mn concentration in ferrite phase” in the table.

Evaluation of mechanical properties The mechanical properties of the test steel were subjected to a tensile test using a No. 5 test piece specified in JIS Z2201, and tensile strength and ductility were measured. The test piece was cut out of 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 strength-ductility balance is “TS × EL (MPa ·%)”.

In the present invention, when the tensile strength was 980 MPa or more, it was high strength and was accepted, and when it was less than 980 MPa, the strength was insufficient and was evaluated as rejection.

The ductility is evaluated by the strength-ductility balance, and when TS × EL is 15,000 MPa ·% or more, the ductility is considered to be excellent, and when less than 15,000 MPa ·%, the ductility is evaluated as disqualified. did.

Evaluation of bendability The bendability was evaluated under the following measurement conditions based on the VDA standard "VDA 238-100" defined by the German Automobile Manufacturers Association. In the present invention, the displacement at the maximum load obtained in the bending test is converted into an angle on the basis of VDA, and the bending angle is determined. The results are described in “VDA bending angle (°)” in the table. Also, the bendability was evaluated from the tensile strength and the bending angle. The results are described in “TS × VDA (MPa · °)” in the table. When TS × VDA was 100,000 MPa · ° or more, it was regarded as passability as excellent in bendability, and when less than 100,000 MPa · °, it was evaluated as failure as bendability insufficient.
Measurement conditions Test method: Roll support, punch press Roll diameter: φ30 mm
Punch shape: Tip R = 0.4 mm
Distance between rolls: 2.9 mm
Pushing speed: 20 mm / min
Specimen size: 60 mm × 60 mm
Bending direction: Rolled right angle direction Testing machine: SIMAZU AUTOGRAPH 20kN

The following can be understood from Tables 1 to 5. Experiment No. 1 manufactured under the annealing conditions specified in the present invention using steel types A to T and W to AC satisfying the component composition of the present invention. The steel plates of 1 to 18, 20, 24 to 26, 28, 29, and 33 to 43 were excellent in ductility and bendability in the region of a tensile strength of 980 MPa or more.

On the other hand, steel plates other than the above did not satisfy the component composition and manufacturing conditions specified in the present invention as described in detail below, and desired properties were not obtained.

The steel type U in Table 1 has a C content, and the steel type V has an Mn content exceeding the upper limit of the present invention, and fracture occurred during cold rolling, so that a test steel could not be produced.

Experiment No. No. 19 is an example which did not reheat after cooling to 500 degrees C or less, MA structure | tissue increased and bending property deteriorated.

Experiment No. No. 21 was an example which did not hold | maintain at predetermined | prescribed temperature after winding-up, the standard deviation of the 1.2 density | concentration area | region of Mn concentration was low, and the bendability was bad.

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

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

Experiment No. No. 27 was an example in which the soaking temperature was high, and ferrite was not generated, and the ductility was poor because the area ratio of Mn concentration 1.2 times and the standard deviation of the 1.2 concentration area of Mn concentration were low.

Experiment No. No. 30 was an example in which the cooling stop temperature was high, and the ferrite and MA structures were many, the strength was low, and the ductility and the bendability were also poor.

Experiment No. No. 31 is an example where the cooling rate to 500 ° C. is slow, and the bendability is deteriorated because the Mn concentration in the ferrite phase is too high.

Experiment No. No. 32 is an example which did not reheat after cooling to 500 degrees C or less, and MA structure | tissue increased and bending property deteriorated.

Example 2
A steel having the component composition shown in Table 6 below was melted and subjected to hot rolling → cold rolling → continuous annealing under the following conditions to produce a cold rolled steel sheet. The steels of the component compositions shown in Table 6 have the balance of iron and unavoidable impurities, and the blanks indicate that no element is added.

Hot rolling The slab was heated to 1250 ° C. and hot rolled to a plate thickness of 2.3 mm so that the rolling reduction was 90% and the finish rolling temperature was 920 ° C. Thereafter, the film is cooled from this temperature to the “rolling temperature (° C.)” shown in Table 7 or Table 8 at an average cooling rate of 30 ° C./sec, and then “holding temperature 1 (° C.)” shown in Table 7; And “hold time (hour)” or “hold start temperature (° C.)”, “hold end temperature (° C.)”, and “hold time (hour)” shown in Table 8; Then, it air-cooled to room temperature and manufactured the hot rolled sheet steel.

Cold rolling The obtained hot rolled steel sheet was pickled to remove surface scale, and then cold rolling was performed to produce a cold rolled steel sheet having a thickness of 1.2 mm.

Annealing of cold rolled steel sheet, manufacture of galvanized steel sheet and alloyed galvanized steel sheet The obtained cold rolled steel sheet is subjected to soaking holding → cooling → reheating → plating treatment under the conditions shown in Table 9 or Table 10. The test steel was manufactured. In addition, Table 7 No. 29 is a comparative example which does not reheat, and it shows in the reheat column that it hold | maintained at the said temperature for 45 seconds after cooling to the cooling stop temperature of 470 degreeC to 400 degreeC instead of reheating. .

In the table, the temperature at which the soaking is maintained is "soaking temperature (° C)", the average cooling rate to 500 ° C after soaking is "average cooling rate 1 (° C / sec)", and the cooling rate at 500 ° C or less is " Average cooling rate 2 (° C / sec), cooling stop temperature is "cooling stop temperature (° C)", holding temperature at reheating after cooling stop is "reheating holding temperature (° C)", holding time at the holding temperature “Reheating holding time (seconds)”, plating bath temperature “plating bath temperature (° C.)” and plating time “hot dip galvanizing time (seconds)” respectively. In addition, "reheating holding time (second)" is a total time including "hot-dip galvanization processing time (second)".

The steel sheet is immersed in the galvanizing bath after being held substantially ("Reheat holding time (second)"-"hot dip galvanizing treatment time (second)") at the above "reheat holding temperature (° C)" The galvanizing layer was formed in "galvanization processing time (second)". Experiment No. In 31 and 33, immediately before immersion in the zinc plating bath, heating was performed from “reheat holding temperature (° C.)” to “plating bath temperature (° C.)” in Table 8 and then the sheet was immersed in the zinc plating bath. In addition, after carrying out the hot dip galvanization process to the steel plate in part, the alloying process was performed. In the table, the alloying temperature at this time is expressed as “alloying temperature (° C.)”, and the holding time at the alloying temperature is expressed as “alloying treatment time (seconds)”. After holding for a predetermined time, it was allowed to cool to room temperature to obtain a test steel. In addition, the steel plate which performed only the hot dip galvanization process described with "GI" and the steel plate which performed the alloying process as "GA" to the "kind" column in a table | surface.

For each test steel, as described in detail below, the metal structure, the Mn concentration, and the evaluation of various mechanical properties were performed in the same manner as in Example 1, and are shown in Table 9 or Table 10.

The following can be understood from Tables 6 to 10. Experiment No. 1 manufactured under the annealing conditions specified in the present invention using steel types A to N and Q to U satisfying the component composition of the present invention. The steel plates of 1, 2, 4, 5, 10, 11, 13-16, 18-23, 25-28, and 30-35 were excellent in ductility and bendability in the region of tensile strength of 980 MPa or more.

On the other hand, steel plates other than the above did not satisfy the component composition and manufacturing conditions specified in the present invention as described in detail below, and desired properties were not obtained.

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

Experiment No. No. 3 is an example in which the holding time at the reheating holding temperature after cooling to 500 ° C. or less was short, and the MA structure increased and the bendability deteriorated.

Experiment No. 6 is an example which was not kept at a predetermined temperature after winding, and the standard deviation of the region of 1.2 times the Mn concentration was low, and the bendability was bad.

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

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

Experiment No. 9 is an example in which reheating after cooling to 500 ° C. or lower was low, and the MA structure increased and the bendability deteriorated.

Experiment No. No. 12 is an example in which the soaking temperature was high, and the ductility was poor because ferrite was not sufficiently generated and the Mn concentration in the ferrite phase was too high.

Experiment No. No. 17 was an example in which the cooling stop temperature was high, and the ferrite and MA structures were many, the strength was low, and the bendability was also poor.

Experiment No. No. 24 is an example where the cooling rate up to 500 ° C. is slow, and the bendability is deteriorated because the Mn concentration in the ferrite phase becomes too high.

Experiment No. No. 29 is an example which did not reheat after cooling to 500 degrees C or less, MA structure | tissue increased and bending property deteriorated.

Claims (11)

1. The composition of the steel plate is, in mass%,
   C: 0.10% or more and 0.30% or less,
   Si: 1.2% or more and 3% or less,
   Mn: 0.5% or more and 3.0% or less,
   P: more than 0% and less than 0.1%,
   S: more than 0% and less than 0.05%,
   Al: 0.005% or more and 0.2% or less,
   And more than N: 0% and 0.01% or less, and O: more than 0% and 0.01% or less, and the balance consists of iron and unavoidable impurities, and
   A high strength cold rolled steel sheet having a tensile strength of 980 MPa or more and excellent in ductility and bendability characterized in that the structure at a 1/4 thickness 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 entire 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 entire structure is more than 0% and 30% or less when subjected to repeller corrosion and observed with an optical microscope.
   (3) When analyzed with an electron beam microprobe analyzer, there are 5 area% or more of a region in which the Mn concentration is 1.2 times or more the Mn concentration in the steel plate, and (4) □ 2 μm section The fraction of the region where the Mn concentration is 1.2 times or more the Mn concentration in the steel sheet is measured, and the standard deviation when measured in 100 sections is 4.0% or more.
   (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.
2. The high strength cold rolled steel sheet according to claim 1, wherein the volume ratio of retained austenite to the entire structure is 5% or more when measured by the X-ray diffraction method.
3. The high strength according to claim 1, wherein the hard phase comprises a mixed structure of the fresh martensite and retained austenite; and at least one structure selected from the group consisting of bainitic ferrite, bainite, and tempered martensite. Cold rolled steel plate.
4. The high-strength cold-rolled steel sheet according to claim 1, wherein the component composition further includes one or more of the following (A) to (E) in mass% as other elements.
   (A) Cr: at least one selected from the group consisting of more than 0% and less than 1%, and Mo: more than 0% and less than 1%;
   (B) Ti: more than 0% and less than 0.15%,
   Nb: at least one selected from the group consisting of more than 0% and less than 0.15%, and V: more than 0% and less than 0.15%;
   (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% or less;
   (D) B: more than 0% and less than 0.005% (E) Ca: more than 0% and less than 0.01%, Mg: more than 0% and less than 0.01%, and REM: more than 0% and less than 0.01% At least one selected from the group consisting of
5. 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.
6. A high strength 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.
7. A high strength alloyed galvanized steel sheet characterized in that an alloyed galvanized steel layer is formed on the surface of the high strength cold rolled steel sheet according to any one of claims 1 to 4.
8. A method for producing the high strength cold rolled steel sheet according to any one of claims 1 to 4, comprising:
   In the hot rolling process of a steel plate having the above composition,
   Take up at a winding temperature of 500 ° C. or more and 800 ° C. or less, hold at 500 ° C. or more and 800 ° C. or less for 3 hours or more, and then cool to room temperature, cold rolled,
   Soaking is maintained in a temperature range of (Ac ₁ point + 20 ° C.) or more and Ac ₃ point, and then an average cooling rate of 10 ° C./sec or more and 500 ° C. or less is an average cooling rate of 10 ° C./sec or more up to 500 ° C. Cool to a temperature range of 500 ° C or less,
   Then, the method of manufacturing a high strength cold rolled steel sheet having a tensile strength of 980 MPa or more excellent in ductility and bendability by reheating to a temperature range of 250 ° C. to 500 ° C., holding for 30 seconds or more and cooling to room temperature.
9. A method for producing a high strength electrogalvanized steel sheet, which further comprises applying galvanizing to the high strength cold rolled steel sheet obtained by the method according to claim 8.
10. A method for producing the high strength galvanized steel sheet according to claim 6, wherein
    In the hot rolling process of a steel plate having the above composition,
    Take up at a winding temperature of 500 ° C. or more and 800 ° C. or less, hold at 500 ° C. or more and 800 ° C. or less for 3 hours or more, and then cool to room temperature, cold rolled,
    Soaking is maintained in a temperature range of (Ac ₁ point + 20 ° C.) or more and Ac ₃ point, and then an average cooling rate of 10 ° C./sec or more and 500 ° C. or less is an average cooling rate of 10 ° C./sec or more up to 500 ° C. Cool to a temperature range of 500 ° C or less,
    Next, re-heating to a temperature range of 250 ° C. to 500 ° C., holding for 30 seconds or more, hot-dip galvanizing within the holding time, and cooling to room temperature, tensile strength excellent in ductility and bendability Manufacturing method of high strength galvanized steel sheet of 980 MPa or more.
11. The method for producing a high-strength galvanized steel sheet according to claim 10, wherein after hot-dip galvanizing, alloying is performed in a temperature range of 450 ° C to 550 ° C.

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