0001]The present invention relates to a cold rolled steel sheet and galvanized cold-rolled steel sheet.
BACKGROUND
[0002]Recently, in view of greenhouse gas emissions regulations due to global warming, improved fuel economy of automobiles has been required. For weight reduction and crash safety of the vehicle body, the application of high-strength steel sheet is being increasingly enlarged. Recently, there is a growing need for more tensile strength 980MPa of ultra-high-strength steel sheet. The site where rust resistance among the vehicle body is required, ultra-high-strength galvanized steel sheet galvanized on the surface is obtained.
[0003]
　The steel sheet subjected to automobile parts, strength as well as press formability and weldability, etc., various workability required during component molding is required. Specifically, from the viewpoint of press formability, the steel sheet (total elongation in a tensile test: El) excellent stretch and stretch flangeability (bore expanding ratio: lambda) is often required.
[0004]
　In general, as the strength of a steel sheet, a total elongation: El and hole expansion rate: the lambda, together but it becomes difficult to secure a high level, utilizing the transformation-induced plasticity of retained austenite, high strength TRIP steel sheet having both workability (Transformation Induced Plasticity) is known as.
[0005]
　On the other hand, when considering the application of high-strength steel sheets for automotive used in cold climates, high strength steel sheet, it is required not to brittle fracture in a low temperature environment. Especially, when considering the application to automobile parts, low temperature toughness after plastic strain introduced by press working is obtained. However, TRIP steel sheets are generally known to be inferior in low-temperature toughness.
[0006]
　Patent Documents 1 to 3, the organizational structure fraction is controlled to a predetermined range, it discloses a technique relating to high-strength TRIP steel sheet having improved elongation and hole expansion rate.
[0007]
　Patent Document 4 and Patent Document 5, a structure fraction of microstructure in terms of controlled to a predetermined range, by controlling the distribution of crystal grains of IQ (Image Qualty) values ​​obtained by EBSD method within a predetermined range, It discloses a technique relating to high-strength TRIP steel sheet having improved low temperature toughness.
[0008]
　Patent Document 6, the microstructure and structure of tempered martensite mainly containing retained austenite and MA, MA and residual austenite is in contact with the tempered martensite, or increasing the proportion present in the grains of tempered martensite Accordingly, technology has been disclosed regarding high-strength TRIP steels having improved hole expansion.
[0009]
　Patent Document 7 discloses a technique for improving the toughness of DP (Dual Phase) steel sheet has been disclosed. Patent Document 8 and Patent Document 9, the structure fraction of the microstructure after having controlled within a predetermined range, by controlling the stacking fault density of residual austenite in a predetermined range, relating to high-strength steel sheet with improved low temperature toughness techniques have been disclosed.
CITATION
Patent Document
[0010]
Patent Document 1: WO 2013/151238 Patent
Patent Document 2: JP 2006-104532 Patent Publication
Patent Document 3: JP 2007-262494 Patent Publication
Patent Document 4: JP 2015-086468 Patent Publication
Patent Document 5: JP 2015-200006 JP
Patent Document 6: JP 2014-34716 JP
Patent Document 7: JP 2011-132602 JP
Patent Document 8: JP 2015-025208 JP
Patent Document 9: JP 2014-133944 JP
Summary of the Invention
Problems that the Invention is to Solve
[0011]
　In the technique Patent Documents 1 to 3, the low temperature toughness is not considered. The technique of Patent Document 4, since the structural fraction of the ferrite is 50% or more, it is difficult to ensure 980MPa class or higher strength. The technique of Patent Document 5, is not considered low-temperature toughness after necessary processing as automotive steel sheets. In the Patent Document 6 art does not consider low-temperature toughness. Steel of Patent Document 7 does not include the residual austenite little ductility is insufficient. In the technique of Patent Document 8 and Patent Document 9, significant hole expansion as processing of high-strength steel sheets is not considered.
[0012]
　In view of the state of the prior art, and in the high strength cold rolled steel sheet and high-strength galvanized cold rolled steel sheet, workability and low-temperature toughness, especially, it is an object to improve the low temperature toughness after the introduction of plastic strain high strength cold rolled steel sheet and high-strength hot-dip galvanized cold-rolled steel sheet for solving this problem (hereinafter, the "cold-rolled steel sheet" is also referred to as "cold rolled steel".) it is an object to provide a.
Means for Solving the Problems
[0013]
　The present inventors, when considering the method for solving the above problems, in addition to high strength and intensive studies for processability and microstructure capable of ensuring low-temperature toughness.
[0014]
　As a result, the strength of the target, elongation, hole expansion rate, and, in order to ensure the low temperature toughness, microstructure, following the (i) ~ (v), have found that it is necessary to simultaneously satisfy .
　(I) Ferrite: 1-29 area%
　(ii) residual austenite: 5-20 area%
　(iii) Martensite: less than 10 area%
　(iv) perlite: less than 5 area%
　(v) bainite and / or martensite : the balance
[0015]
　Further, locating the ferrite most soft tissues within the microstructure, that is the interface between the martensite or residual austenite hardest tissue serving as a starting point for destruction, the length of the interface both organizations are in contact with predetermined value below, specifically, to satisfy the following the (vi), found that it is possible to further improve the low temperature toughness after processing.
[0016]
　(Vi) ferrite and, the sum length of the circle equivalent radius 1μm or more martensite or interface at which the residual austenite is in contact, 1000 .mu.m 2 per 100μm or less
[0017]
　1 shows, the steel sheet having various ShigumaMA, after applying 5% prestrain, perform Charpy impact test shows the result of measuring the vTrs. In the present specification, ferrite and, at the interface in contact and the circle-equivalent radius 1μm or more martensite or residual austenite the sum of the lengths, referred to as ShigumaMA.
[0018]
　As shown in FIG. 1, there is a tendency that the vTrs after pre-straining as σMA smaller 5% decrease, vTrs is greatly reduced particularly in the case of 100μm or less. Here, ShigumaMA is for impact mechanism for the low temperature toughness after machining is considered as follows. When subjected to machining in steel, and ferrite is the most soft tissues within the microstructure, and concentrated strain on the interface between the martensite or retained austenite which is the most hard tissue, occurs microscopic interfacial delamination or cracking . The interfacial peeling or cracking, the starting point of brittle fracture. Therefore, the more the interface is small, that is, as σMA small, it is considered that the excellent low-temperature toughness after machining.
[0019]
　The present invention has been made based on the above findings, its gist the following.
[0020]
　(1) Tensile strength is a cold-rolled steel sheet is more than 980 MPa,
　the chemical composition, in
mass%,
C: 0.10 ~ 0.30%, Si: 0.50 ~
2.50%, Mn :
1.50 ~ 3.50%, Al: 0.001 ~
1.00%, P: 0.05% or
less, S: 0.01% or
less, N: 0.01% or
less, O: 0.01 % or
less,
Cr:
0 ~ 1.00%,
Mo: 0 ~ 1.00%, Sn: 0 ~ 1.00%,
Cu: 0 ~ 1.00%, Ni: 0 ~
1.00%, B:
~
0.005%
0,
~ 0.50%, Nb: 0 ~ 0.10%,
W: 0 ~ 0.50%, Ca: 0 ~ 0.010
%,
Mg:
0 ~ 0.010%,
Sb: 0 ~ 0.200%, Zr: 0 ~ 0.010%,
Bi: 0 ~ 0.010%, REM: 0 ~ 0.100%,
The balance is Fe and impurities,
　microstructure, in area%,
ferrite: 1-29%,
retained austenite: 5% to 20%,
martensite: less than 10%,
perlite: less than 5%,
the balance being bainite and / or tempering is martensite,
　and ferrite, the sum of the equivalent circle radius 1μm or more martensite or length of the interface and residual austenite are in contact is, 1000 .mu.m 2 is per 100μm or less,
cold-rolled steel sheet.
[0021]
　(2) the thickness of the steel sheet is 0.5 ~ 3.2 mm, cold-rolled steel sheet or galvanized cold rolled steel sheet of the above (1).
[0022]
　(3) above (1) or on the surface of the cold-rolled steel sheet (2) comprises a hot-dip galvanizing layer, galvanized cold rolled steel sheet.
[0023]
　(4) above (1) or on the surface of the cold-rolled steel sheet (2) comprises a galvannealed layer, galvanized cold rolled steel sheet.
The invention's effect
[0024]
　According to the present invention, excellent workability and low-temperature toughness, it is possible inter alia to provide a high strength cold rolled steel sheets and high-strength galvanized cold-rolled steel sheet excellent in low temperature toughness after the introduction of plastic strain.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025]
[1] and vTrs after giving 5% of prestrain is a diagram showing the relationship between ShigumaMA.
It is a diagram showing the results of examining the relationship lvalue and σMA in [2] formula (1).
3 is a diagram showing an example of a slab heating pattern.
4 is a diagram showing a relationship between the tertiary cooling rate C concentration in the residual γ (Cγ).
DESCRIPTION OF THE INVENTION
[0026]
　Hereinafter, plated steel sheet according to the steel sheet and the present invention according to the present invention, as well as methods for their production, it will be sequentially described.
[0027]
　First, a description will be given reasons for limiting the chemical composition of the steel sheet according to the present invention. Hereinafter, according to the chemical composition "%" means "% by mass".
[0028]
　Chemical Composition
　C: 0.10 ~ 0.30%
　C is an essential element to secure strength of the steel sheet. In order to obtain a sufficiently high strength, the C content is 0.10% or more. Preferably 0.13% or more, 0.15% or more, 0.17% or more, or 0.18% or more. On the other hand, excessive content, so reducing the workability and weldability, the C content to 0.30% or less. In order to suppress a decrease in press formability and weldability 0.27% or less, 0.25% or less, preferably 0.23% or 0.21%.
[0029]
　Si: 0.50 ~
　2.50% Si suppresses the formation of iron carbide, which is an element contributing to the improvement of strength and formability. In order to obtain the effect, the amount of Si should be 0.50% or more. For suppressing precipitation of iron-based carbides, 0.65% or more 0.80% or 0.90% or more, 1.00% or more, preferably 1.10% or more, or 1.20% or more. On the other hand, excessive content may crack cast slab, since cause embrittlement of the steel sheet, the Si content should be not more than 2.50%. Further, Si is in the annealing step, an oxide formed on the surface of the steel sheet, because it may inhibit the chemical conversion treatability and coating adhesion, the content of Si 2.25% is less, 2.00% or less, 1.85% or less, preferably 1.70% or less, or 1.60% or less. 1.50% or less is more preferable.
[0030]
　Mn: 1.50 ~
　3.50% Mn enhances the hardenability of the steel sheet, an element which contributes to improvement in strength. In less than 1.50% content of Mn, insufficient hardenability of the steel sheet, ferrite is a large amount of precipitates in the cooling after annealing, it is difficult to ensure a required strength. Thus, the content of Mn is set to 1.50% or more. Preferably 1.80% or more, 2.00% or more, 2.20% or more, or 2.30% or more. On the other hand, excessive content may manifest the Mn segregation, as it reduces the workability and toughness, the Mn content is not more than 3.50%. From the viewpoint of securing the weldability, the content of Mn is preferably not more than 3.00%. 2.80% or less, 2.70% or less, more preferably 2.60% or less, or 2.50% or less.
[0031]
　Al: 0.001 ～ 1.00
　Pasento Al is a deoxidizing element. In order to obtain the effect, the Al content is 0.001% or more. Preferably 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, even if excessively is contained, in addition to the effect of addition is lowered economical saturated, transformation temperature of the steel increases, the load of the hot rolling is increased, the content of Al is 1.00% below that. Preferably 0.50% is less, 0.20% or less, 0.10% or less, 0.060% or less or less, or 0.040%.
[0032]
　P: 0.05% or less
　P is a solid solution strengthening, an element which contributes to improvement in strength. When the content of P exceeds 0.05%, the weldability and the toughness is lowered, the content of P is 0.05% or less. Preferably less than 0.02%, or 0.015%. P is not particularly necessary to limit the lower limit of the content of, the lower limit is 0%. However, to reduce the content of P less than 0.001%, because the manufacturing cost is significantly increased, or the lower limit of 0.001%.
[0033]
　S: 0.01% or less
　S is an impurity element, an element that inhibits workability and weldability by forming MnS. Therefore, the S content is not more than 0.01%. Preferably 0.005% or less, or 0.003% or less, more preferably 0.002% or less. There is no need to particularly limit the lower limit of the content of S, the lower limit is 0%. Reducing the content of S to less than 0.0005%, the manufacturing cost is significantly increased, or the lower limit of 0.0005%.
[0034]
N: 0.01% or less
　N is an impurity element, an element that inhibits workability and toughness by forming a coarse nitrides. Therefore, the N content is 0.01% or less. Preferably 0.007% or below, it is 0.005% or less, or 0.004% or less. N is not particularly necessary to limit the lower limit of the content of, the lower limit is 0%. Reducing the content of N to less than 0.0005%, the manufacturing cost is significantly increased, or the lower limit of 0.0005%.
[0035]
　O: 0.01% or less
　O is an impurity element, an element that inhibits bendability and hole expansion to form coarse oxides. Therefore, the content of O is 0.01% or less. Preferably less than 0.005%, or 0.003%. O is not particularly necessary to limit the lower limit of the content of, the lower limit is 0%. If O of the content is reduced to less than 0.0001%, the manufacturing cost is significantly increased, or the lower limit of 0.0001%.
[0036]
　The steel sheet according to the present invention, if necessary, may include the elements described below.
[0037]

Cr:
0
~ 1.00% Mo: 0 ~ 1.00%
Sn: 0 ~ 1.00% Cu: 0 ~ 1.00%
Ni: 0 ~ 1.00% B: 0 ~
　0.005% Cr, Mo, Sn, Cu, Ni and B are both because it is an element contributing to the improvement of the strength of the steel sheet, may contain one or more kinds of these elements. However, even if excessively by containing these elements, the addition effect is saturated, so is uneconomical, Cr, Mo, Sn, the upper limit of the content of Cu and Ni are both set to 1.00%, the upper limit of the content of B is 0.0050%. A more preferred upper limit, Cr, Mo, Ni, Sn, Cu and Ni are both 0.60% 0.40% 0.20%, 0.10% or 0.050% B is it is 0.0020% or 0.0030% or. To obtain the above effect sufficiently is, Cr, Mo, Sn, the lower limit of the content of Cu and Ni, 0.001%, the lower limit of the content of B may be 0.0001%. A more preferred lower limit is, Cr, Mo, Sn, Cu and Ni is 0.010% or 0.020% or any, B is 0.0005% or 0.0010%. Possible to obtain the effect of the above is not essential. Therefore, Cr, Mo, Sn, there is no need to particularly limit the lower limit of the content of Cu and Ni, their lower limit is 0%.
[0038]

Ti:
0 ~ 0.30% V: 0 ~
0.50% Nb: 0 ~ 0.10% W: 0
　~ 0.50% Ti, V, Nb and W, a carbide is formed, the improvement of the strength of the steel sheet since it is an element contributing to, it may contain one or more kinds of these elements. However, even if excessively by containing these elements, the addition effect is saturated, so is uneconomical, 0.30% upper limit of the content of Ti, the upper limit of the V content is 0.50%, Nb upper limit 0.10% of the content of, the upper limit of the content of W is set to 0.50%. More preferable upper limit of Ti is 0.15% or 0.05%. More preferable upper limit of V is 0.30% or 0.08%. More preferable upper limit of Nb is 0.05% or 0.02%. More preferable upper limit of W is 0.25% or 0.05%. To obtain the above effect sufficiently, the Ti, V, lower limit of the content of Nb and W are both preferably set to 0.001% or 0.005%. A more preferred lower limit is also 0.010% each element. Possible to obtain the effect of the above is not essential. Therefore, Ti, V, need not be specifically limited to the lower limit of the content of Nb and W, their lower limit is 0%.
[0039]

Ca:
0 ~
0.010%, Mg: 0 ~ 0.010%, Sb: 0 ~
0.200%, Zr: 0 ~ 0.010%, Bi: 0 ~
0.010%, REM: 0 ~ 0
　.100%, Ca, Mg, Sb, Zr and REM are inclusions were finely dispersed, is an element contributing to the improvement of workability, Bi is reduce Mn, the micro-segregation of substitutional alloying elements such as Si and an element which contributes to the improvement of workability. Therefore, it may contain one or more kinds of these elements. However, if the content of these elements is excessive, since the ductility is lowered, Ca and Mg upper limit 0.010% of the content of, the upper limit of the content of Sb is 0.200%, Zr and Bi content upper limit 0.010%, the upper limit of the content of REM is 0.100%. A more preferred upper limit, Ca and Mg 0.005% or 0.003%, Sb is 0.150% or 0.05%, Zr and Bi is 0.005% or 0.002%, REM is 0.050 % or 0.004%. The content of the in order to obtain the effect sufficiently is, Ca and Mg content lower limit 0.0001%, the lower limit of the content of Sb and Zr 0.001%, or 0.005%, Bi and REM the lower limit preferably set to 0.0001% or 0.005%. A more preferred lower limit is, Ca and Mg 0.0010%, Sb and Zr 0.008%, Bi and REM are 0.0008%. Possible to obtain the effect of the above is not essential. Therefore, Ca, Mg, Sb, there is no need to particularly limit the lower limit of the content of Zr and REM, their lower limit is 0%. Incidentally, REM is, Sc, is a generic name for a total of 17 elements Y and lanthanoid, and the content of REM means the total amount of the element.
[0040]
　Chemical composition of the steel sheet according to the present invention, in addition to the above elements, but the balance is Fe and impurities, unavoidably contaminating elements from the steel raw materials and / or steel process, the characteristics of the steel sheet according to the present invention it may include a range that does not impair.
[0041]
　Next, a description will be given reasons for limitation of the microstructure of the steel sheet according to the present invention. Hereinafter, according to the microstructure "%" means "% by area".
[0042]
　Microstructure
　Ferrite: 1-29%
　retained austenite: 5% to 20%
　martensite: less than 10%
　Perlite: less than 5%
　balance: bainite and / or tempered martensite
　in the steel sheet according to the present invention, by forming the microstructure to ensure the required mechanical properties.
[0043]
　Ferrite, since it is sufficient elongation effective to ensure a tissue, amount of ferrite is 1% or more. Preferable lower limit is 3%, 5%, 7% or 9%. A more preferred lower limit is 10%, 11%, 12% or 13%. On the other hand, since the securing of sufficient strength is difficult when ferrite content is excessive, the amount of ferrite is not more than 29%. A preferred upper limit is 27%, 25%, 22% or 20%. More preferred upper limit is 19% or 18%.
[0044]
　Residual austenite, because it is effective tissue to ensure sufficient elongation, the amount of retained austenite is 5% or more. Preferred lower limit is 7%, 8% or 9%. A more preferred lower limit is 10% or 11%. On the other hand, since the amount of retained austenite to secure a sufficient strength it is difficult, if excessive, amount of retained austenite is 20% or less. A preferred upper limit is 17%, 16%, 15% or 14%.
[0045]
　Martensite and pearlite, if the respective amounts is excessive, sufficient hole expansion and can not ensure the low temperature toughness, the martensite amount is less than 10%, pearlite content is less than 5%. The preferable upper limit of the amount of martensite is 8%, 6%, is 5% or 4%, preferably the upper limit of the pearlite amounts, 3%, 2% or 1%. A more preferred upper limit is less than 1%. The lower limit of these quantities, it is not necessary to determine in particular, is 0%. However, in the steel sheet according to the present invention, often there is some martensite, optionally, 1% the lower limit of the amount of martensite, 2%, may be 3% or 4%. Pearlite amount is preferably 0%, but may the lower limit of 0.5% or 1%.
[0046]
　Remainder of the microstructure is bainite and / or martensite. 94% is the upper limit of the remaining structure, the lower limit is 36 percent. The lower limit, 40%, 50%, 55%, 60%, may be a 65% or 70%, the upper limit of 90%, 86%, 82%, may be 78% or 74%. In particular, tempered martensite amount is preferably 65% ​​or less or 60% or less, tempered martensite amount is preferably 30% or more or 40% or more.
[0047]
　Here, a method of calculating the area% of the steel sheet microstructure of the present invention. Cut rolling direction cross-section of the steel sheet, the current out of the microstructure corroded by nital solution, 1/4 of the thickness of the position tissue a scanning electron microscope (magnification: 5000 times, 5 fields) captured by the resulting micro from the tissue photographs, it calculates the area ratio (area%) by a point counting method.
[0048]
　Infrastructure does not emerge, and the area of ​​low luminance as ferrite, also without revealing the underlying tissue, and calculating the area ratio of high luminance region as martensite or residual austenite. The area where infrastructure is revealing to calculate the area ratio as a tempered martensite or bainite.
[0049]
　Area ratio of residual austenite, subjected to X-ray diffraction of the surface of the 1/4 thickness of the steel plate as the observation surface, the value calculated from the peak area ratio of the bcc and fcc and area ratio. The area ratio of martensite, the area ratio was calculated as martensite or retained austenite is obtained by subtracting the area ratio of retained austenite was obtained by X-ray diffraction.
[0050]
　Tissue fraction obtained by X-ray diffraction is essentially the volume ratio (vol%). However, the area ratio of the microstructure (area%) is equal approximately the volume rate (vol%), the ratio of the measured residual austenite by X-ray diffraction as described above, is as it is the area ratio of residual austenite.
[0051]
　The bainite and tempered martensite can be distinguished by observing the position and variants of cementite contained within the tissue. Tempered martensite is composed of a martensite lath, and cementite generated inside lath. In this case, the crystal orientation relationship martensite lath and cementite is present two or more, cementite constituting the tempered martensite has a plurality of variants.
[0052]
　Bainite is classified into upper bainite and lower bainite. Upper bainite, since the lath-shaped bainitic ferrite are composed of cementite generated in lath boundaries, can be easily distinguished from the tempered martensite. Lower bainite is composed of a lath-like bainitic ferrite, the cementite generated inside lath. At this time, bainitic ferrite and crystal orientation relationship cementite is one different from the tempered martensite, cementite constituting the lower bainite have the same variant. Accordingly, the lower bainite and tempered martensite, can be distinguished on the basis of the cementite variants.
[0053]
　Ferrite and a circle equivalent radius 1μm or more martensite or residual austenite are in contact with the interface of the length of the total: 1000 .mu.m 2 per 100μm or less
　martensite or residual austenite, inhibition when the circle equivalent radius is large, the processability and toughness to. In particular, when the circle equivalent radius of martensite or retained austenite than 1μm is in contact with the ferrite is soft tissue, which may degrade the workability and toughness. Therefore, it is necessary to manage ferrite and, at the interface in contact and the circle-equivalent radius 1μm or more martensite or residual austenite the sum of the lengths.
[0054]
　Sum of the lengths of the interface is obtained as follows.
　First, the captured microstructure photograph (1) ferrite, (2) martensite or residual austenite, and distinguishes the three areas (3) other organizations. The "(3) Other tissue", as described above, a region where infrastructure is revealing the microstructure photographs, corresponding to bainite and / or martensite.
[0055]
　Next, using a commercially available image analysis application, determine the area of martensite or residual austenite, respectively, it is converted into an equivalent circle radius. For the circle equivalent radius 1μm or more of all the martensite or residual austenite, to trace the boundary of ferrite to calculate the length. Then, the total sum of the length, 1000 ([mu] m 2 ) / measured field area ([mu] m 2 multiplying).
[0056]
　Image analysis application used in this case, as long as the above operation can be, but not particularly specified, for example, there are image-pro plus ver.6.1 (Media Cybernetics, Inc.).
[0057]
　To ensure the required processability and toughness, and ferrite, the sum of the equivalent circle radius 1μm or more martensite or length of the interface and residual austenite are in contact, 1000 .mu.m 2 to per 100μm or less. In that further improve the toughness, the sum of the length of the interface 80μm or less, preferably 70μm or less, or 60μm or less. More preferably 50μm or less, or 40μm or less.
[0058]
　　Next, a description will be given favorable mechanical properties of the steel sheet according to the present invention.
[0059]
　　Tensile strength: 980 MPa or more
　　total elongation: 10% or more
　　hole expansion rate: 30% or more
　　5% prestrain after seeing vTrs: -10 ° C. or less
　for securing the strength of an automotive steel sheet, the tensile strength of the steel sheet according to the present invention more than 980MPa is preferable. We need not determine the upper limit of the tensile strength in particular, 1250 MPa, may be 1200MPa or 1150 MPa. As automotive steel sheets, in order to ensure the workability can be molded into various shapes by press working or the like, the total elongation of 10% or more, the hole expanding ratio is preferably 30% or more. In order to secure the low temperature toughness as automotive steel sheets for cold climates, vTrs after seeing 5% prestrain is preferably -10 ° C. or less. Preferably at -30 ℃ or less.
[0060]
　The thickness of the steel sheet according to the present invention, there is also 0.5mm, or less than 3.2mm greater become casing, predominantly 0.5 ~ 3.2mm.
[0061]
　Plated steel sheet according to the present invention, cold-rolled steel sheet having a galvanized layer on the surface of the steel sheet according to the present invention, or a cold rolled steel sheet having a galvannealed layer. The presence of galvanized layer on the surface of the steel sheet, the corrosion resistance is further improved. The surface of the steel sheet, that Fe is present galvannealed layer taken into galvanized layer in an alloying treatment, it is possible to ensure excellent weldability and paintability.
[0062]
　In the plated steel sheet according to the present invention, on the galvanized layer or a galvannealed layer, in order to improve the paintability and weldability, it may be subjected to the upper layer plating. In the steel sheet according to the present invention, on the galvanized layer or a galvannealed layer, various processes, for example, chromate treatment, phosphate treatment, lubricity-improving treatment, the weldability improving treatment such as it may be subjected.
[0063]
　Next, description will be given of a manufacturing method suitable for the steel sheet according to the present invention.
[0064]
　When manufacturing the steel sheet according to the present invention comprises the steps described below for processing the slab of chemical composition of the steel sheet according to the present invention (A) ~ (C) is important. It, studies so far, when the following condition is satisfied, it has been confirmed that it is possible to obtain the microstructure, etc. of the present invention.
[0065]
　(A) (A1) ~ condition hot-rolling process according to the (A4)
　hot rolling step is carried out according to the following criteria.
[0066]
　(A1) slab heating satisfying formula (1)
[0067]
[Number 1]

[0068]
　The left side of the equation (1) is an expression representing the degree of heterogeneity of the Mn concentration that occurs at the time of slab heating. Molecule of formula (1) left-hand side, stay in alpha + gamma two-phase region during slab heating, a term which represents the amount of Mn to be dispensed from the alpha to gamma, the more this value is increased, Mn concentration distribution in the slab not homogenized. On the other hand, equation (1) the left side of the denominator, stay in the single phase region gamma during slab heating, a term corresponding to the distance of Mn atoms diffuse in gamma, Mn concentration higher this number is, in the slab distribution is homogenized. That, alpha + gamma two phase region (Ac 1 or more, Ac 3 below) the longer the residence time of the slab in, Mn amount distributed from alpha to gamma increases. On the other hand, single-phase temperature region gamma (Ac 3 the longer slab residence time above), Mn concentration distribution homogenized.
[0069]
　The larger lvalue of formula (1), is locally Mn concentrations in the steel higher Mn concentrated region is formed. Also, around the Mn concentrated region Mn lean region. These hot rolling, through cold rolling, is inherited to a final annealing process. Since Mn lean region has a low hardenability, in the final annealing step, easily transformed into preferentially ferrite. On the other hand, Mn concentrated region which lies adjacent the Mn lean region has high hardenability, ferrite transformation and bainite transformation difficult to occur in the final annealing step, easily transformed to martensite. Therefore, the Mn concentration is insufficient homogeneity, since the ferrite and martensite is easily formed adjacent the interface in contact and the ferrite and martensite or retained austenite is the sum of the length σMA increases.
[0070]
　Figure 2 is a graph showing the results of examining the relationship lvalue and σMA of formula (1). More ShigumaMA is increasing is larger lvalue of formula (1), ShigumaMA rapidly increases in particular where the left-hand side value of the formula (1) exceeds 1.0. From the above, to sufficiently homogenize the Mn concentration distribution in the steel, so the left side value of the formula (1) is 1.0 or less, it is necessary to select a slab heating conditions. Incidentally, Ac 1 and Ac 3 is calculated based on the empirical formula below. Element symbol denotes an element amount (mass%).
[0071]
　Ac 1 = 723-10.7Mn-16.9Ni +
　29.1Si + 16.9Cr Ac 3 = 910-203 · √C-15.2Ni + 44.7Si + 104V + 31.5Mo-30 mN-11Cr-20Cu + 700P + 400Al + 400Ti
　Each element symbol in the above formula, It means the respective contents (mass%).
[0072]
　Here, FIG. 3 shows an example of a slab heating pattern. In FIG. 3, (a), in Table 2 (infra), No.1 (invention examples, the left side value of the formula (1) is 0.52 <1.0) shows the slab heating pattern, (b) is in Table 2 (infra), No.2 (Comparative example, the left side value of the formula (1) is 1.25> 1.0) show the slab heating pattern. Slab heating pattern (a) a slab heating pattern (b) is found to differ significantly. Incidentally, the slab heating temperature, 1200 ° C. or higher, preferably 1300 ° C. or less.
[0073]
　(A2) 1050 ° C. or higher, the total rolling reduction at 1150 ° C. or less: 60% or more
　rough rolling, 1050 ° C. or higher, 1150 ° C. or less, the total rolling reduction: performing at least 60%. 1050 ° C. or more, when the total rolling reduction at 1150 ° C. or less is less than 60%, insufficient recrystallization during rolling, since the hot-rolled sheet tissues may become insufficient homogeneity, the total rolling reduction is to 60% or more.
[0074]
　(A3) 1050 ° C. or less to finish rolling of the final pass (finishing final pass) total rolling reduction before: 70 to 95%
　finish final pass reduction ratio of: 10-25%
　finish final pass temperature: 880 ~ 970 ° C.
1050 If ° C. the total rolling reduction to below-finishing final pass before is less than 70%, if reduction ratio of finishing final pass is less than 10%, or, when the temperature of finishing final pass exceeds 970 ° C., the hot rolled leaf tissue are coarsened, tissue coarsens workability of the final product sheet deteriorates. Therefore, the total rolling reduction of up to 1050 ° C. or less to finish the final pass before is 70% or more, reduction ratio of finishing final pass is 10% or more, the temperature of the finishing final pass (inlet side temperature) to 970 ° C. or less .
[0075]
　On the other hand, if the total rolling reduction of up to 1050 ° C. or less to finish the final pass before exceeds 95%, if the reduction ratio of finishing final pass exceeds 25% or when the temperature of finishing final pass is less than 880 ° C. the texture of the hot-rolled steel sheet is developed, the anisotropy in the final product plate is obvious. Thus, 1050 ° C. or less to finish the final pass to 95% or less total rolling reduction before, reduction ratio of finishing final pass of 25% or less, the temperature of the finishing final pass (inlet side temperature) to 880 ° C. or higher.
[0076]
　(A4) coiling temperature: 430 ~ 650 ° C.
　When the coiling temperature is below 430 ° C., the strength of the hot rolled steel sheet becomes excessively large, since the cold function rollability is impaired, the coiling temperature is set to 430 ° C. or higher. On the other hand, when the coiling temperature exceeds 650 ° C., Mn is concentrated in the cementite in the hot rolled steel sheet, Mn concentration distribution becomes heterogeneous, also, since the pickling property is reduced, coiling temperature and 650 ° C. or less to.
[0077]
　Incidentally, pickling the hot-rolled steel sheet may be carried out according to a conventional method. It may also be carried out skin pass rolling for shape correction and pickling improvement of hot-rolled steel sheet.
[0078]
　(B) reduction ratio: 30% or more, 80% or less of the cold rolling step
　in the final annealing step, it is necessary to refine the austenite grain size, the reduction ratio of 30% or more. On the other hand, when the reduction ratio exceeds 80%, the rolling load is becomes excessive, the load of the rolling mill is increased, the reduction ratio be 80% or less.
[0079]
　(C) (C1) ~ (C5) continuous annealing step through the process of
　(C1) a heating temperature: Ac 3 -30 ° C. or higher, 900 ° C. or less
　heating time (retention time): 30 seconds or more, or less 450 seconds
　heating temperature Ac 3 is less than -30 ° C., since sufficient austenitization does not proceed, the heating temperature is Ac 3 to -30 ° C. or higher. On the other hand, if it exceeds 900 ° C. heating temperature, coarsened austenite grain size, reduces the toughness and chemical conversion treatability, also, since the possibility that the annealing equipment is damaged occurs, the heating temperature is set to 900 ° C. or less.
[0080]
　When the heating time is less than 30 seconds, since austenitization does not proceed sufficiently, the heating time is 30 seconds or more. On the other hand, if it exceeds 500 seconds heating time, the productivity is lowered, the heating time is less 450 seconds.
[0081]
　(C2) primary cooling
　cooling rate: 5.0 ° C. / sec or less, the primary cooling finish temperature: 620 ~ 720 ° C.
　to control the ferrite fraction and pearlite fraction the required range, after the heating, the primary cooling, followed by performs secondary cooling (described later). If the cooling rate in the primary cooling exceeds 5.0 ° C. / sec, or, if the primary cooling end temperature exceeds 720 ° C., since the required ferrite fraction can not be obtained, the cooling rate is 5.0 ° C. / and sec, the primary cooling finish temperature is set to 720 ° C. or less. On the other hand, the primary cooling finish temperature is less than 620 ° C., since the required ferrite fraction is not obtained, the primary cooling finish temperature is set to 620 ° C. or higher.
[0082]
　(C3) the secondary cooling
　cooling rate: 20 ° C. / sec or more
　secondary cooling end temperature: 280 ~ 350 ° C.
　The secondary cooling conditions after primary cooling shall be as described above. If the secondary cooling rate is less than 20 ° C. / sec, the required fraction of ferrite and pearlite fraction is not obtained. If the secondary cooling end temperature is below 280 ° C., since the austenite fraction of untransformed is significantly reduced, residual austenite fraction is below the required value. If the secondary cooling end temperature is above 350 ° C., in a subsequent tertiary cooling step, since the bainite transformation does not proceed sufficiently, secondary cooling end temperature to 350 ° C. or less. Incidentally, the secondary cooling start temperature is the same as the primary cooling finish temperature.
[0083]
(C4) a low temperature heating
　(low temperature) heating temperature: 390 ~ 430 ° C.
　(low temperature) heating time (retention time): 10 seconds or less
　secondary cooling immediately after, performs the low-temperature heating. Heating temperature of less than 390 ° C., or when the heating temperature exceeds 430 ° C., subsequent bainite transformation during the tertiary cooling does not proceed sufficiently, stability of austenite is deteriorated. Heating rate need not be particularly limited, it is preferable to heat at 1 ° C. / sec or more from the viewpoint of production efficiency. Low temperature heating time is 10 seconds or less.
[0084]
(C5) tertiary cooling
　tertiary cooling end temperature: 280 ~ 350 ° C.
　Cooling rate: 0.15 ~ 1.5 ℃ / sec
　for the stabilization of austenite (austempering), carrying out the immediately tertiary cooling after low-temperature heating. Usually, austempering is held at a constant temperature, than to the slow cooling instead of isothermal holding, it is possible to further increase the stability of austenite. Tertiary cooling end temperature, and 280 ~ 330 ° C.. Incidentally, tertiary cooling start temperature is the same as the heating temperature during low temperature heating temperature.
[0085]
　Although not clear detailed mechanism of improving stability towards slow cooling austenite than isothermal holding in the case of isothermal holding, T C concentration in the untransformed austenite isothermal holding temperature 0 composition (austenite phase (FCC structure ) and ferrite phase (free energy of BCC structure) are equal, when the driving force of bainite transformation is 0, bainite transformation when it reaches the C concentration) in the austenite is stopped. On the other hand, in the case of slow cooling, T 0 because the composition is larger every moment as the temperature decreases due to gradual cooling, C concentration of the untransformed austenite is enhanced than in the case of isothermal holding. As a result, it is believed that the stability of the untransformed austenite is enhanced.
[0086]
　Figure 4 shows the relationship between C concentration in γ residual and tertiary cooling rate (C gamma). As shown in FIG. 4, the range of the tertiary cooling rate 0.15 ~ 1.5 ℃ / s, it can be seen that Cγ is maximized.
[0087]
　After the continuous annealing, a flat straightening of the steel sheet may be subjected to temper rolling to adjust the surface roughness. In this case, in order to avoid the deterioration of ductility, elongation rate is preferably 2% or less.
[0088]
　Next, a method for manufacturing a coated steel sheet according to the present invention.
[0089]
　Plated steel sheet according to the present invention, after the above (A) ~ in step (C) includes a step of following (D) or (E).
　(D) above (A) ~ plating step of forming a galvanized layer on the surface of the steel sheet according to the present invention process was prepared via the (C)
　produced through the step of (E) above (A) ~ (C) after forming the hot-dip galvanizing layer on the surface of the steel sheet according to the present invention it was, subjected to alloying treatment, an alloying step of forming a galvannealed layer
[0090]
　Hereinafter, the respective steps will be described.
[0091]
　(D) plating process
　the steel sheet according to the present invention is immersed in molten zinc plating bath to form a galvanized layer on the surface of the steel sheet. The forming of a molten zinc plated layer may be performed continuously after the above continuous annealing. Galvanizing bath is a plating bath composed mainly of zinc, or a plating bath composed mainly of zinc alloy. The temperature of the plating bath is preferably 450 ~ 470 ℃.
[0092]
　(E) alloying step
　is subjected to alloying treatment in galvanized layer formed on the steel sheet surface to form a galvannealed layer. Conditions of alloying treatment is particularly, but not limited to specific conditions, heated to 480 ~ 600 ° C., it is preferably maintained from 2 to 100 seconds at this temperature.
Example
[0093]
　Next, a description will be given of an embodiment of the present invention, conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is, in this single condition example the present invention is not limited. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0094]
　(Example)
　casting a slab having the chemical compositions shown in Table 1 were hot-rolled steel sheet subjected to hot rolling under the conditions shown in Table 2 and Table 3. Subjected to pickling the hot-rolled steel sheet was cold-rolled steel sheet subjected to cold rolling at a reduction ratio shown in Table 2 and Table 3. This cold-rolled steel sheet was subjected to heat treatment under the conditions shown in Table 2 and Table 3.
[0095]
[Table 1]

[0096]
[Table 2]

However, the meaning of each symbol in the tables are as follows.
SRT: slab heating temperature
R1: 1050 ~ total rolling rate at 1150 ℃
R2: 1050 ℃ or less to finish the final pass total reduction rate of up to before
R3: finish rolling reduction in the final pass
FT: the inlet side temperature of finishing the final pass
CT : coiling temperature
[0097]
[Table 3]

meanings, however, each symbol in the tables are as follows.
　T1: heating temperature
　t1: heating time
　CR1: the primary cooling rate
　T2: primary cooling finish temperature (secondary cooling start temperature)
　CR2: secondary cooling rate
　T3: secondary cooling end temperature
　HR: heating rate
　T4: low heating temperature
　t2 : low temperature heating time
　CR3: tertiary cooling rate
　T5: tertiary cooling end temperature
　CR: cold-rolled steel sheet,
　GI: galvanized steel sheet,
　GA: alloyed hot-dip galvanized steel sheet
[0098]
　From cold-rolled steel sheet after heat treatment, No. 5 tensile test piece JIS Z2241 was taken from a direction perpendicular to the rolling direction, subjected to a tensile test, tensile strength (TS), yield strength (YS), total elongation (EL) It was measured. Also performs hole expanding test according to JIS Z2256, it was measured hole expanding ratio (lambda).
[0099]
　Then, after applying a strain to the steel sheet (prestrain processing) by cold rolling the cold-rolled steel sheet elongation of 5% after the heat treatment, to prepare a Charpy specimen, brittle - to determine the ductile transition temperature (vTrs) Accordingly, to evaluate the low-temperature toughness after machining. Charpy specimens signed a steel plate plural superposed bolt, after confirming that there is no gap between the steel sheet, to produce a V-notched test pieces of depth 2 mm. Number of steel plates superimposed, the specimen thickness after lamination was set to closest to 10 mm. For example, if the plate thickness of 1.2mm was laminated eight, specimen thickness was 9.6 mm. Laminated Charpy test pieces were taken sheet width direction as the longitudinal. Incidentally, not laminated specimen, but is better to perform Charpy impact test at one of the test piece is simple, since the person laminated becomes more severe test conditions, was laminated specimens.
[0100]
　The test temperature, the -120 ℃ ~ + 20 ℃, measured at 20 ° C. intervals, brittle fracture rate was the temperature at which 50% and transition temperature (vTrs). Conditions other than the above, in accordance with JIS Z 2242. For reference, prestrain low temperature toughness of the prior application of (vTrs) was also evaluated.
[0101]
　The results are shown in Table 4.
[0102]
[Table 4]

However, the meaning of each symbol in the tables are as follows.
V.alpha: the area ratio of the ferrite
VP: pearlite area ratio
VM: the area fraction of martensite
V.gamma: residual austenite area ratio
remainder area ratio of bainite and / or martensite
ShigumaMA: ferrite and a circle equivalent radius 1μm or more Martens site or the sum of the length of the interface and residual austenite are in contact with ([mu] m / 1000 .mu.m 2 )
YS: yield strength
TS: tensile strength
El: total elongation
lambda: hole expansion ratio
vTrs: transition temperature
[0103]
　In the example of the range the chemical composition and manufacturing conditions of the present invention, since the structure fraction is within the scope of the present invention, tensile strength of at least 980 MPa, 10% or more of elongation, 30% or more of the hole expansion ratio, 5% vTrs after pre-strain is in the -10 ° C. or less. On the other hand, in the range of examples of chemical composition and either or both of the production conditions invention, tensile strength, elongation, hole expansion ratio, one of vTrs after strain 5% pre, not reach the required value Absent.
Industrial Applicability
[0104]
　As described above, according to the present invention, excellent workability and low-temperature toughness, it is possible inter alia to provide a high strength cold rolled steel sheets and high-strength galvanized cold-rolled steel sheet excellent in low temperature toughness after the introduction of plastic strain . Accordingly, the present invention has high applicability in the steel sheet manufacturing industry and the steel sheet use industries.

WE CLAIM

Tensile strength of a cold-rolled steel sheet is more than 980 MPa,
　the chemical composition, in
mass%,
C: 0.10 ~ 0.30%, Si: 0.50 ~
2.50%, Mn: 1.
~ 3.50% 50, Al: 0.001 ~
1.00%, P: 0.05% or
less, S: 0.01% or
less, N: 0.01% or
less, O: 0.01% or less, cr:
0 ~ 1.00%, Mo: 0 ~ 1.00%, Sn: 0 ~ 1.00%, Cu: 0 ~ 1.00%, Ni: 0 ~ 1.00%, B: 0 ~ 0 % .005, Ti: 0 ~ 0.30%, V: 0 ~ 0.50%, Nb: 0 ~ 0.10%, W: 0 ~ 0.50%, Ca: 0 ~ 0.010%, Mg : 0 ~ 0.010%, Sb: 0 ~ 0.200%, Zr: 0 ~ 0.010%, Bi: 0 ~ 0.010%, REM: 0 ~ 0.100%,

The balance is Fe and impurities,
　microstructure, in area%,
ferrite: 1-29%,
retained austenite: 5% to 20%,
martensite: less than 10%,
perlite: less than 5%,
the balance being bainite and / or tempering is martensite,
　and ferrite, the sum of the equivalent circle radius 1μm or more martensite or length of the interface and residual austenite are in contact is, 1000 .mu.m 2 is per 100μm or less,
cold-rolled steel sheet.
[Requested item 2]
The plate thickness of the steel sheet is 0.5 ~ 3.2 mm,
cold-rolled steel sheet or galvanized cold rolled steel sheet according to claim 1.
[Requested item 3]
　On the surface of the cold-rolled steel sheet according to claim 1 or claim 2 comprising a galvanized layer,
galvanized cold rolled steel sheet.
[Requested item 4]
　On the surface of the cold-rolled steel sheet according to claim 1 or claim 2 comprising a galvannealed layer,
galvanized cold rolled steel sheet.