JP4114521B2 - Ultra-high strength cold-rolled steel sheet having excellent formability and method for producing the same - Google Patents

Description

【００７０】
【表２】

【００７１】
【表３】

【００７２】
表３に示したとおり、発明例はいずれも、引張強さ（ＴＳ）≧ 980 MPa、強度−伸びバランス（ＴＳ×Ｅｌ）≧ 17000 MPa・％、強度−伸びフランジ性バランス（ＴＳ×λ）≧ 65000 MPa・％という３つの目標値を全て満足しており、高強度と良好な加工性を兼備していることが分かる。
また、特に鋼板表面と鋼板内部のSi濃度比を 1.5以下に低減したものは、塗装後の耐食性にも優れていることが分かる。
【００７３】
【発明の効果】
かくして、本発明によれば、ＴＳ≧980 MPa という超高強度で、かつＴＳ×Ｅｌ≧ 17000 MPa・％、ＴＳ×λ≧ 65000 MPa・％という優れたプレス成形性を有し、またさらには塗装後の耐食性にも優れた冷延鋼板を、安定して得ることができる。
【図面の簡単な説明】
【図１】 冷延鋼板の強度−伸びバランス（ＴＳ×Ｅｌ）および強度−伸びフランジ性バランス（ＴＳ×λ）に及ぼすベイナイト相の体積分率およびベイナイト相とフェライト相の硬度比の影響を示した図である。
【図２】 冷延鋼板の強度−伸びバランス（ＴＳ×Ｅｌ）および強度−伸びフランジ性バランス（ＴＳ×λ）に及ぼす焼鈍温度の影響を示した図である。
【図３】 冷延鋼板の強度−伸びバランス（ＴＳ×Ｅｌ）および強度−伸びフランジ性バランス（ＴＳ×λ）に及ぼす冷却停止温度の影響を示した図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is an ultra-high strength with a tensile strength TS of 980 MPa or more suitable for use mainly in automobile parts that are press-molded, and has excellent workability such as ductility and stretch flangeability. The present invention relates to a cold-rolled steel sheet having excellent corrosion resistance and a method for producing the same.
[0002]
[Prior art]
By various strengthening methods, the material strength can achieve the target strength, but the workability decreases as the strength increases. In particular, in conventional high-strength steel sheets, due to structural inhomogeneities and local mixing of the hard and soft phases, there are many locations that become the origin of cracks during the hole expansion test for evaluating stretch flangeability. Thus, it is said that this causes stretch flangeability. In addition, such workability generally decreases greatly as the strength of the steel plate increases.
For this reason, it has been extremely difficult to achieve both high strength and workability such as ductility, bendability and stretch flangeability with conventional steel sheet manufacturing techniques.
[0003]
Various types of high-strength cold-rolled steel sheets that are excellent in the hole expansion ratio (λ), which is one of the indices of stretch flangeability, have been proposed in the past (see, for example, Patent Document 1 and Patent Document 2).
However, the above-mentioned Patent Document 1 only discloses a relatively high level steel plate having a tensile strength TS of 690 MPa or less, and it is disclosed in an ultra-high strength material as intended in the present invention. There is no knowledge that both stretch flangeability and ductility can be achieved together with strengthening. In addition, strength-elongation balance (tensile strength TS x elongation El) and strength-stretch flangeability balance (tensile strength TS x hole expansion ratio λ). ) Was also difficult to say.
Also, Patent Document 2 discloses only about the increase in λ, and the TS level of the target material is only 690 MPa, which is a steel material of a considerably lower level than the ultra-high strength material targeted by the present invention. There is only disclosure about. Moreover, the TS × El level is low, and there is no suggestion of coexistence with the TS-λ balance.
[0004]
Furthermore, a TS: 980 MPa grade cold-rolled steel sheet having a fine bainite structure has been proposed (see, for example, Patent Document 3).
However, since this patent document 3 is intended for processing by roll forming or bending, it only mentions the yield ratio (YP), TS, and elastic limit, and is merely a technique for a thin steel plate for light processing applications. Absent. Accordingly, there is no description of El and λ which are extremely important for obtaining a complicated pressed part shape, and nothing is mentioned about compatibility between these workability and high strength. In addition, referring to the examples, the TS: 980 MPa class example components are low-carbon steels, which are rapidly cooled to 300 ° C. or less in the production process, so the bainite phase produced has a high dislocation density. Therefore, it is generally expected that the ductility is bad. Furthermore, the effect of uniform fineness is only mentioned that the difference between YS and the elastic limit is reduced, and the TS-El balance and TS-λ balance assuming press forming are ensured for ultra-high strength steel sheets. It's not technology.
[0005]
Patent Document 4 proposes a high-strength hot-dip galvanized steel sheet and a high-strength steel sheet having a bainitic ferrite structure and excellent hole expansibility and ductility, and methods for producing them.
However, this technique is a technique related to hot-rolled steel sheets and hot-dip galvanized steel sheets obtained by hot-dip galvanizing on hot-rolled steel sheets, and can only deal with products having a large thickness. For example, the product plate thickness disclosed in Patent Document 4 is 2.6 mm. When the product plate is thick, it can be applied to an automobile foot part and the like, but it is difficult to apply it to a structural part that requires more formability. Further, from the viewpoint of reducing the weight of the vehicle body, it has been desired to develop a technology related to a cold-rolled steel plate that can easily cope with a thin plate thickness.
[0006]
In addition, in order to manufacture parts required by press molding at a tensile strength (TS) strength level of 980 MPa or more, it is required to have bending characteristics in addition to elongation and stretch flangeability. Since the steel sheet disclosed in No. 4 is mainly composed of hard bainitic ferrite and has a crystal grain size of 20 μm or less, it allows a non-uniform structure in which coarse grains and fine grains can be mixed, so that bending workability is low. There was a problem.
[0007]
Furthermore, cold-rolled steel sheets for processing applied to automobile parts are usually used after electrodeposition coating, but it is necessary to apply various platings to the steel sheet surface in order to improve the corrosion resistance after painting. In many cases, the cost is increased and the price is high. In addition, for ultra-high strength steel sheets with a TS of 980 MPa or more, the increase in additive elements can cause problems such as the occurrence of non-plating and variations in the amount of adhesion, which can lead to problems such as low plating quality. The steel plate excellent in is desired.
[0008]
As a technology developed from such a viewpoint, for example, a technology for improving the corrosion resistance by containing a large amount of P, Cu, Cr, Mo, Ni as disclosed in Patent Document 5 has been proposed.
However, this method has a problem that not only the cost increases due to the addition of these elements, but also the hole expandability and the elongation are lowered, and it is difficult to obtain good moldability.
[0009]
[Patent Document 1]
JP-A-9-263838 (Tables 4 and 5)
[Patent Document 2]
Japanese Patent Laid-Open No. 10-60593 (Table 3, Table 6, Table 7)
[Patent Document 3]
JP 2000-273576 A (Claim 1, Table 3)
[Patent Document 4]
JP 2001-355043 A (Claims)
[Patent Document 5]
Japanese Patent Laid-Open No. 5-140654 (Claims)
[0010]
[Problems to be solved by the invention]
As described above, it is common for strength and workability to show contradictory trends, and at present there is no known ultra-high strength cold-rolled steel sheet that combines good workability and a tensile strength of 980 MPa or more. .
An object of the present invention is to solve the above-described problem advantageously, and an object of the present invention is to propose an ultra-high-strength cold-rolled steel sheet excellent in formability together with its advantageous manufacturing method.
Another object of the present invention is to propose an ultra-high-strength cold-rolled steel sheet that has not only excellent formability but also excellent corrosion resistance after coating, together with its advantageous manufacturing method.
[0011]
Here, the target values of the strength and workability of the cold-rolled steel sheet having a thickness of about 0.8 to 2.5 mm in the present invention are as follows.
・ Tensile strength (TS) ≥ 980 MPa
・ Strength-elongation balance (TS × El) ≧ 17000 MPa ・%
・ Strength-Stretch Flange Balance (TS × λ) ≧ 65000 MPa ・%
[0012]
[Means for Solving the Problems]
Now, in order to achieve the above-mentioned object, the inventors conducted intensive experiments from the viewpoints of steel components, production conditions, metal structures, and the like, and repeated studies.
As a result, it is composed of a certain amount of ferrite phase and a certain amount of bainite phase in which the crystal grain size is controlled by controlling the heating temperature and cooling stop temperature especially during the manufacturing process after controlling the component composition within an appropriate range. By controlling the hardness of the ferrite phase and the bainite phase, it becomes possible to achieve uniform deformation macroscopically while eliminating the difference in local deformability at the same time. Thus, under the very high strength level, an unprecedented excellent strength-elongation balance and strength-stretch flangeability balance were obtained, and as a result, it was found that excellent press formability was obtained.
In addition, the decrease in corrosion resistance after painting is due to the oxide containing Si present on the surface of the steel sheet. I also found that it was the cause.
The present invention is based on the above findings.
[0013]
That is, the gist configuration of the present invention is as follows.
1. C: 0.12-0.18 mass%
Si: 0.2 to 0.8 mass%
Mn: 2.2-3.0 mass%
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
N: 0.0050 mass% or less and
Ti: 0.001 to 0.030 mass%
In the range satisfying the following formula (1), the balance is the composition of Fe and inevitable impurities, and the tensile strength (TS), elongation (El), and hole expansion rate (λ) are respectively Relational expression
TS ≧ 980 MPa,
TS × El ≧ 17000 MPa ・%
TS × λ ≧ 65000 MPa ・%
An ultra-high-strength cold-rolled steel sheet with excellent formability characterized by satisfying
Record
−100 [C] + 15 ≦ [Mn] ≦ −100 [C] +20 --- (1)
Here, [C] and [Mn] are the contents of C and Mn (mass%), respectively.
[0014]
2. C: 0.12-0.18 mass%
Si: 0.2 to 0.8 mass%
Mn: 2.2-3.0 mass%
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
N: 0.0050 mass% or less and
Ti: 0.001 to 0.030 mass%
In the range satisfying the following formula (1), the balance is the composition of Fe and inevitable impurities, the volume fraction of the ferrite phase is 10-50 vol%, the average grain size of the ferrite phase is 4.0 μm In the following, the volume fraction of the bainite phase is 50 to 80 vol%, and the ratio of the Vickers hardness (Hv (B)) of the bainite phase to the Vickers hardness (Hv (F)) of the ferrite phase (Hv (B) / An ultra-high strength cold-rolled steel sheet having excellent formability, characterized by having a steel structure with Hv (F)) of 1.6 or less.
Record
−100 [C] + 15 ≦ [Mn] ≦ −100 [C] +20 --- (1)
Here, [C] and [Mn] are the contents of C and Mn (mass%), respectively.
[0015]
3. C: 0.12-0.18 mass%
Si: 0.2 to 0.8 mass%
Mn: 2.2-3.0 mass%
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
N: 0.0050 mass% or less and
Ti: 0.001 to 0.030 mass%
In the range satisfying the following formulas (1) and (2), the balance is the composition of Fe and inevitable impurities, and the tensile strength (TS), elongation (El) and hole expansion ratio (λ) are Each of the following relational expressions
TS ≧ 980 MPa,
TS × El ≧ 17000 MPa ・%
TS × λ ≧ 65000 MPa ・%
An ultra-high-strength cold-rolled steel sheet with excellent formability, characterized in that the ratio of the Si concentration between the steel sheet surface and the steel sheet inside is 1.5 or less.
Record
−100 [C] + 15 ≦ [Mn] ≦ −100 [C] +20 --- (1)
[Ti] ≧ 3.43 [N] + 1.5 [S] − 0.006 --- (2)
Here, [C], [Mn], [Ti], [N], and [S] are C, Mn, Ti, N, and S, respectively.
Content (mass%)
[0016]
4). C: 0.12-0.18 mass%
Si: 0.2 to 0.8 mass%
Mn: 2.2-3.0 mass%
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
N: 0.0050 mass% or less and
Ti: 0.001 to 0.030 mass%
In the range satisfying the following formulas (1) and (2), the balance is the composition of Fe and inevitable impurities, the volume fraction of the ferrite phase is 10 to 50 vol%, the average grain size of the ferrite phase The diameter is 4.0 μm or less, the volume fraction of the bainite phase is 50-80 vol%, and the ratio of the Vickers hardness (Hv (B)) of the bainite phase to the Vickers hardness (Hv (F)) of the ferrite phase (Hv (B) / Hv (F)) has a steel structure of 1.6 or less, and the ratio of the Si concentration between the steel sheet surface and the steel sheet is 1.5 or less. .
Record
−100 [C] + 15 ≦ [Mn] ≦ −100 [C] +20 --- (1)
[Ti] ≧ 3.43 [N] + 1.5 [S] − 0.006 --- (2)
Here, [C], [Mn], [Ti], [N], and [S] are the contents of C, Mn, Ti, N, and S, respectively (mass%)
[0017]
5. In any one of the above 1 to 4, the steel plate is further
Cu: 0.01-0.50mass%,
Ni: 0.01-0.50mass%,
Mo: 0.01-0.50mass% and
Cr: 0.01 ~ 0.50mass%
An ultra-high strength cold-rolled steel sheet excellent in formability, characterized by having a composition containing one or more selected from among the above.
[0018]
6). In any one of the above 1 to 5, the steel plate is further
Nb: 0.001 to 0.050 mass%
A super-high-strength cold-rolled steel sheet excellent in formability, characterized by having a composition containing
[0019]
7). In any one of the above 1 to 6, the steel plate is further
V: 0.001 to 0.300 mass% and
Zr: 0.001 to 0.300 mass%
An ultra-high-strength cold-rolled steel sheet having excellent formability, characterized by having a composition containing at least one selected from the above.
[0020]
8). In any one of 1 to 7 above, the steel plate is further
B: 0.0001 to 0.0050 mass%
A super-high-strength cold-rolled steel sheet excellent in formability, characterized by having a composition containing
[0021]
9. In any one of 1 to 8 above, the steel plate is further
Ca: 0.0001 to 0.0050 mass% and
REM: 0.0001 ~ 0.0050mass%
An ultra-high-strength cold-rolled steel sheet having excellent formability, characterized by having a composition containing at least one selected from the above.
[0022]
10. C: 0.12-0.18 mass%,
Si: 0.2 to 0.8 mass%
Mn: 2.2-3.0 mass%
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
N: 0.0050 mass% or less and
Ti: 0.001 to 0.030 mass%
In a range satisfying the following formula (1), the balance being Fe and the inevitable impurities composition of steel slab immediately after casting or once cooled to 1050-1300 ℃, after finishing rolling, finish rolling Temperature: Hot-rolled at 850-950 ° C, rolled up at 450-650 ° C after rolling, cold-rolled, and then subjected to continuous annealing, within the temperature range represented by the following formula (3) After heating and annealing, it is cooled to the temperature range indicated by the following formula (4), and after cooling is completed, the steel plate temperature is not increased (cooling stop temperature to cooling stop temperature−100 ° C.) for 60 seconds. A method for producing an ultra-high-strength cold-rolled steel sheet having excellent formability, characterized by maintaining the temperature as described above.
Record
−100 [C] + 15 ≦ [Mn] ≦ −100 [C] +20 --- (1)
T ₁ ± 50 ° C (however, T ₁ = 950-150 [C] ^1/2 +50 [Si] -30 [Mn]) --- (3)
T ₂ ± 50 ° C (however, T ₂ = 500−450 [C] −30 [Mn]) --- (4)
Here, [C], [Mn], and [Si] are the contents of C, Mn, and Si, respectively (mass%)
[0023]
11. C: 0.12-0.18 mass%,
Si: 0.2 to 0.8 mass%
Mn: 2.2-3.0 mass%
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
N: 0.0050 mass% or less and
Ti: 0.001 to 0.030 mass%
In a range satisfying the following formulas (1) and (2), and the remainder of the steel slab having a composition of Fe and inevitable impurities is heated to 1050-1300 ° C. immediately after casting or once after cooling. Finishing rolling finish temperature: Hot rolling at 850 to 950 ° C, winding at 450 to 650 ° C after rolling, cold rolling and then performing continuous annealing at least during heating during annealing After heating and annealing to a temperature range represented by the following formula (3) under an atmosphere dew point (DP) of −30 ° C. or higher, cooling to the temperature range represented by the following formula (4), A method for producing an ultra-high-strength cold-rolled steel sheet having excellent formability, wherein the steel sheet is kept warm for 60 seconds or more in a temperature range (cooling stop temperature to cooling stop temperature −100 ° C.) without increasing the steel sheet temperature.
Record
−100 [C] + 15 ≦ [Mn] ≦ −100 [C] +20 --- (1)
[Ti] ≧ 3.43 [N] + 1.5 [S] − 0.006 --- (2)
T ₁ ± 50 ° C (however, T ₁ = 950-150 [C] ^1/2 +50 [Si] -30 [Mn]) --- (3)
T ₂ ± 50 ° C (however, T ₂ = 500−450 [C] −30 [Mn]) --- (4)
Here, [C], [Mn], [Ti], [N], and [S] are the contents of C, Mn, Ti, N, and S, respectively (mass%)
[0024]
12. In the above 10 or 11, the steel slab is further
Cu: 0.01-0.50mass%,
Ni: 0.01-0.50mass%,
Mo: 0.01-0.50mass% and
Cr: 0.01 ~ 0.50mass%
The manufacturing method of the ultra-high-strength cold-rolled steel plate excellent in the formability characterized by becoming a composition containing 1 type, or 2 or more types selected from among these.
[0025]
13. In any one of 10 to 12, the steel slab is further
Nb: 0.001 to 0.050 mass%
A method for producing an ultra-high-strength cold-rolled steel sheet having excellent formability, characterized by comprising a composition containing
[0026]
14. In any one of 10 to 13, the steel slab is further
V: 0.001 to 0.300 mass% and
Zr: 0.001 to 0.300 mass%
A method for producing an ultra-high-strength cold-rolled steel sheet having excellent formability, wherein the composition contains at least one selected from the above.
[0027]
15. In any one of the above 10 to 14, the steel slab is further
B: 0.0001 to 0.0050 mass%
A method for producing an ultra-high-strength cold-rolled steel sheet having excellent formability, characterized by comprising a composition containing
[0028]
16. In any of the above 10 to 15, the steel slab is further
Ca: 0.0001 to 0.0050 mass% and
REM: 0.0001 ~ 0.0050mass%
A method for producing an ultra-high-strength cold-rolled steel sheet having excellent formability, wherein the composition contains at least one selected from the above.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be specifically described below.
First, the reason why the component composition of steel is limited to the above range in the present invention will be described.
C: 0.12-0.18mass%
C is an essential element for strengthening steel using the low-temperature transformation phase, and it is necessary to contain at least 0.12 mass% to obtain a tensile strength of 980 MPa or more, but it exceeds 0.18 mass%. If it is contained, the weldability is remarkably deteriorated, the C concentration in the austenite becomes too high, the martensite transformation point (Ms) is lowered, and a hard retained austenite phase is present even after the heating, cooling, and heat retaining steps. Since stretch flangeability falls, C amount was limited to the range of 0.12-0.18 mass%.
[0030]
Si: 0.2 to 0.8 mass%
Si is a useful element that contributes to strength improvement. However, if the content is less than 0.2 mass%, the effect of addition is poor. On the other hand, if it exceeds 0.8 mass%, ferrite transformation is promoted and the low-temperature transformation phase. Strengthening due to is insufficient. Si is an element that has a large effect of promoting C concentration in austenite, and a phase other than ferrite (also referred to as a second phase) itself hardens, and a hard retained austenite in the steel sheet finally obtained. There is a detrimental effect on stretch flangeability due to the presence of a phase.
Furthermore, when Si is included in excess of 0.8 mass%, Si is concentrated on the surface significantly during continuous annealing, and reacts with oxygen present in the atmosphere to form a Si-based oxide on the surface. The chemical conversion treatment performed as a pretreatment for coating is reduced, and the adhesion to the coating is significantly reduced. Therefore, the Si content is limited to the range of 0.2 to 0.8 mass%. More preferably, it is the range of 0.2-0.65 mass%.
[0031]
Mn: 2.2 to 3.0 mass%
Mn is an element that plays an important role in suppressing ferrite transformation and obtaining a bainite structure. Ar _Three It contributes to refinement of crystal grains through the action of lowering the transformation point, and has the action of increasing the strength-elongation balance. In order to obtain a stable low-temperature transformation phase from the viewpoint of securing tensile strength, an amount of Mn of 2.2 mass% or more is necessary, but if it exceeds 3.0 mass%, the formation of a soft ferrite phase is excessively suppressed. TS-E1 balance decreases. Therefore, the amount of Mn was limited to the range of 2.2 to 3.0 mass%.
[0032]
P: 0.02 mass% or less
P is an element that has a high solid solution strengthening ability and is effective in achieving an improvement in strength. Therefore, P is preferably contained in an amount of 0.005 mass% or more. However, if excessively contained, P causes structural heterogeneity, Solidification segregation during casting becomes prominent, leading to internal cracking and deterioration of workability. Therefore, the P content is limited to 0.02 mass% or less. More preferably, it is 0.018 mass% or less.
[0033]
S: 0.0030 mass% or less
Since S exists as non-metallic inclusions in steel and becomes a stress concentration source during stretch flange forming, it is desirable to reduce the content thereof as much as possible. However, in the range where the S amount is 0.0030 mass% or less, even if the strength is high, the stretch flangeability is not so badly affected, so 0.0030 mass% was set as the allowable upper limit. More preferably, it is less than 0.0010 mass%.
[0034]
Al: 0.05 mass% or less
Al is an element effective for improving the yield of deoxidation and carbide forming elements, and is preferably contained in an amount of 0.01 mass% or more, but not only when the content exceeds 0.05 mass%, the effect is saturated. In addition, the amount of Al is limited to 0.05 mass% or less because it causes deterioration of workability and surface properties.
[0035]
N: 0.0050 mass% or less
N is present in the steel as AlN and solute N, and if contained in a large amount, the ductility of ferrite is reduced, so its content is limited to 0.0050 mass% or less. More preferably, it is 0.0030 mass% or less.
[0036]
Ti: 0.001 to 0.030 mass%
Ti exists as TiC in the slab heating stage, and not only suppresses the growth of austenite grains during heating and heating, but also induces dynamic recrystallization in the subsequent hot rolling process, making the structure fine and uniform. This is an element effective for improving the elongation and hole expansibility, and for this purpose it needs to contain at least 0.001 mass%. Further, when Ti is used in combination with Nb described later, the critical cooling rate at which ferrite transformation does not occur is reduced, and the effect of improving hardenability is brought about. On the other hand, when Ti exceeding 0.030 mass% is contained, a hard carbide or the like is formed and stretch flangeability is deteriorated, so the Ti amount is limited to a range of 0.001 to 0.030 mass%. More preferably, it is 0.005 to 0.015 mass%.
[0037]
Furthermore, in the present invention, it is important to satisfy the range of the following formula (1) for the C content and the Mn content.
−100 [C] + 15 ≦ [Mn] ≦ −100 [C] +20 --- (1)
Here, [C] and [Mn] are the contents of C and Mn (mass%), respectively.
This is because C and Mn have an extremely large influence on the strength, so that it is necessary to contain them in a balanced manner. Here, if the amount of Mn is less than −100 [C] +15, it is difficult to ensure sufficient strength, and the elongation (El) increases due to the formation of ferrite, but the hole expansion rate (λ) decreases. On the other hand, if the content exceeds -100 [C] +20, the formation of ferrite is suppressed, the elongation (El) decreases, and the strength level becomes too high.
[0038]
Furthermore, in order to improve the corrosion resistance after painting, it is important to satisfy the range of the following formula (2) with respect to the Ti amount, N amount, and S amount.
[Ti] ≧ 3.43 [N] + 1.5 [S] − 0.006 --- (2)
Here, [C], [Mn], [Ti], [N], and [S] are the contents of C, Mn, Ti, N, and S, respectively (mass%)
Ti bonds not only with C but also with N and S to form precipitates. Of these, TiC is not only effective for the uniformity of the structure, but also improves the hardenability, further improves the chemical conversion treatment, and effectively contributes to the improvement of the subsequent coating corrosion resistance. Ti also works to improve corrosion resistance after coating by fixing S as a precipitate. Here, if the amount of Ti is less than (3.43 [N] + 1.5 [S]-0.006), it becomes difficult to obtain the above-mentioned effects, particularly the corrosion resistance improvement effect after coating. Accordingly, Ti is preferably contained in a range that satisfies the above formula (2).
[0039]
The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
Cu: 0.01 to 0.50 mass%, Ni: 0.01 to 0.50 mass%, Mo: 0.01 to 0.50 mass%, and Cr: 0.01 to 0.50 mass%
Cu, Ni, Mo and Cr are all effective elements for improving the strength without greatly reducing the elongation, but less than 0.01 mass%, the effect is poor, while more than 0.50 mass%. Since there is no further effect even if it is contained in the composition, it is rather disadvantageous economically, so that these are contained in the range of 0.01 to 0.50 mass% in either case of single addition or combined addition. In addition, it is more preferable to contain these in the range of 0.01 to 0.20 mass% in either case of single addition or composite addition.
[0040]
Nb: 0.001 to 0.050 mass%
Nb is an element that affects the existence form of precipitates such as NbC and the recrystallization temperature. In particular, in the present invention, Nb effectively acts to make the structure fine and uniform, suppresses the formation of ferrite-pearlite, and has a high strength despite its high strength by forming a bainite-based structure that is a low-temperature transformation phase. It has the effect of elongating and providing a hole expansion rate. Such an effect is manifested by containing Nb in an amount of 0.001 mass% or more, but if it is contained in excess of 0.050 mass%, a large amount of hard precipitates are formed in the steel, and the stretch flangeability is reduced. The amount of Nb was limited to the range of 0.001 to 0.050 mass%. More preferably, it is 0.005 to 0.020 mass%.
[0041]
V: 0.001 to 0.300 mass% and / or Zr: 0.001 to 0.300 mass%
V and Zr are effective elements for increasing the strength of the steel sheet through the effect of suppressing the coarsening of the crystal grain size due to the formation of carbides. However, when the content is less than 0.001 mass%, the effect is poor, while 0.300 mass Even if it is contained in a large amount exceeding 5%, there is no further effect, rather it is economically disadvantageous. Therefore, V and Zr are each contained in the range of 0.001 to 0.300 mass%. In addition, even if these contain alone or in combination, they exhibit the same behavior.
[0042]
B: 0.0001 to 0.0050 mass%
B is also an element effective for increasing the strength. By containing B, the critical cooling rate at which ferrite is generated becomes slow, so that it is easy to form a low-temperature transformation phase by suppressing the formation of a soft ferrite phase during continuous cooling in the annealing process after cold rolling. Become. In order to acquire such an effect, it is necessary to contain 0.0001 mass% or more, but even if it contains more than 0.0050 mass%, a further effect is not acquired, Therefore B amount is 0.0001-0.0050 mass% Limited to range.
[0043]
Ca: 0.0001 to 0.0050 mass% and / or REM: 0.0001 to 0.0050 mass%
Both Ca and REM have the effect of suppressing the reduction in stretch flangeability by reducing the concentration of stress by reducing the stress concentration by spheroidizing precipitates such as sulfides, such as MnS. Yes. However, if the content is less than 0.0001 mass%, the effect is small. On the other hand, even if the content exceeds 0.0050 mass%, the effect is saturated and the cost is increased. Therefore, Ca and REM are contained in the range of 0.0001 to 0.0050 mass%, respectively.
[0044]
Next, the reason why the steel structure is limited to the above range will be described.
Volume fraction of ferrite phase: 10-50 vol%
If the volume fraction of the ferrite phase is less than 10 vol%, the absolute amount of the soft phase is too small and the ductility is reduced. On the other hand, if it exceeds 50 vol%, the soft phase becomes too much and the second phase Although it depends on the hardness, it is difficult to ensure the tensile strength TS. Accordingly, the ferrite phase is present in a volume fraction of 10 to 50 vol%. More preferably, it is the range of 15-35 vol%.
[0045]
Average grain size of ferrite phase: 4.0 μm or less
By making the average grain size of the ferrite phase as fine as 4.0 μm or less, a soft ferrite phase exists in the range of 10 to 50 vol%, and even when the second phase, that is, the hard low-temperature transformation phase is relatively small, it is high. Strengthening is possible, and as a result, a large amount of ferrite phase, which is a soft phase, is included, and as a result, ductility is improved. Furthermore, since the structure of the steel sheet is made uniform by refining the crystal grains, the stretch flangeability is also improved. Therefore, the average crystal grain size of the ferrite phase is set to 4.0 μm or less.
[0046]
Volume fraction of bainite phase: 50-80 vol%
When the volume fraction of the bainite phase exceeds 80 vol%, the strength is increased, and the bainite phase becomes close to the single-phase structure and becomes a uniform structure. Therefore, the stretch flangeability tends to be improved, but the ductility is decreased. On the other hand, if it is less than 50 vol%, it is difficult to ensure the tensile strength (TS). Moreover, in order to secure TS, it is necessary to strengthen the bainite phase itself, and the hardness difference from the ferrite phase is inevitably opened, so that the stretch flangeability is deteriorated. Therefore, the bainite phase was limited to a volume fraction of 50 to 80 vol%. More preferably, it is the range of 60-75 vol%.
Furthermore, in order to advantageously exhibit the properties of the ferrite phase and the bainite phase, it is preferable that the sum of both is 90 vol% or more in terms of the volume fraction with respect to the entire structure.
[0047]
In addition to the ferrite and bainite phases described above, cementite that precipitates along the grain boundaries, martensite and residual austenite that are slightly produced without transformation 100% when transformed from austenite to bainite during isothermal holding, etc. However, there is no problem as long as the ferrite phase and the bainite phase satisfy the above-mentioned range.
The bainite phase here refers to Fe that precipitates in the form of ferrite and needles that are formed by cooling from austenite to a predetermined temperature and then maintaining the temperature. _Three A low-temperature transformation phase composed of C, for example, austenite that is rapidly cooled, such as water cooling, and is also produced in the subsequent reheating process, including ferrite and Fe. _Three Although it is composed of C, it has a rod-like or elliptical Fe _Three Tempered martensite phase composed of C and ferrite, Fe _Three It is clearly distinguished from bainitic ferrite that does not contain C precipitation.
[0048]
Hardness of bainite phase (Hv (B)) / Hardness of ferrite phase (Hv (F)) ≦ 1.6
If Hv (B) / Hv (F) is greater than 1.6, a hard part is locally present in the soft phase instead of a uniform structure, and deformability is different at the time of strain introduction, resulting in nonuniform deformation. Therefore, moldability is reduced. That is, ferrite is deformed by the introduction of strain, but the deformation amount of bainite is small, so voids are easily generated at the interface of the second phase, and cracks are easily developed at the interface during the test. However, the hole expansion rate is reduced. Therefore, the ratio Hv (B) / Hv (F) between the hardness of the bainite phase and the hardness of the ferrite phase is set to 1.6 or less.
[0049]
C: 0.145 mass%, Si: 0.65 mass%, Mn: 2.58 mass%, P: 0.017 mass%, S: 0.0008 mass%, Al: 0.027 mass%, N: 0.0025 mass% and Ti: 0.011 mass% The balance is steel slab (T) with a composition of Fe and inevitable impurities. ₁ = 848 ℃, T ₂ = 357 ° C), slab heating temperature: 1100-1300 ° C, finish rolling temperature: 850-1000 ° C, winding temperature: 400-650 ° C, cold rolling reduction: 50%, annealing temperature: 750-920 ° C, annealing Subsequent cooling rate: 10 to 200 ° C./s, cooling stop temperature: 200 to 450 ° C., heat treatment temperature: 200 to 450 ° C.
The strength (TS), elongation (El) and hole expansion ratio (λ) of the cold-rolled steel sheet thus obtained were measured, and the strength-elongation balance (TS × El) and the strength-stretch flangeability balance (TS × λ). The obtained results are shown in FIG. 1 in relation to the volume fraction of the bainite phase.
In addition, the volume fraction of the ferrite phase of the obtained cold-rolled steel sheet was 5 to 70 vol%.
[0050]
As shown in the figure, high TS × El and TS × λ are obtained only when the volume fraction of the bainite phase is 50 to 80 vol% and the hardness of the bainite phase / the hardness of the ferrite phase ≦ 1.6. I was able to.
As described above, by adjusting the steel composition and hardness after adjusting the steel composition, TS ≧ 980 MPa, TS × E1 ≧ 17000 MPa ·%, TS × λ ≧ 65000 for the first time.
It was possible to obtain a cold-rolled steel sheet that achieved MPa ·%.
[0051]
Ratio of Si concentration between steel sheet surface and steel sheet: 1.5 or less
In steel containing Si like the present invention steel, Si is concentrated on the surface during hot rolling, and Si oxide is easily formed. Such Si oxide may remain without being completely removed in the pickling process after hot rolling. Further, during the continuous annealing after cold rolling, depending on the dew point and temperature in the furnace, Si is concentrated on the surface and Si oxide is formed. When Si oxide remains on the surface, the Si concentration on the surface increases. These Si oxides inhibit the dissolution of the base iron during the subsequent chemical conversion treatment of the steel sheet, adversely affect the formation of appropriate chemical conversion crystals, and further deteriorate the corrosion resistance after painting.
The present invention obtains excellent post-coating corrosion resistance by suppressing excessive Si concentration on the steel plate surface and preventing the Si oxide from remaining on the steel plate surface to such an extent that the post-coating corrosion resistance is degraded. It is what I did. The degree of concentration of Si on the steel sheet surface can be evaluated by measuring the Si concentration ratio between the steel sheet surface and the steel sheet interior by the method described below. Corrosion resistance after coating can be obtained.
[0052]
The Si concentration in the steel sheet surface and in the steel sheet was ground 0.5 mm from the surface of the steel sheet before and after annealing, and each surface after grinding was counted for Si intensity in the same area by fluorescent X-ray analysis. Can be measured. Here, Si concentration calculated | required about the steel plate before annealing represents Si concentration inside a steel plate. Accordingly, the Si concentration ratio between the steel plate surface and the steel plate is calculated by (Si concentration of steel plate grinding surface after annealing) / (Si concentration of steel plate grinding surface before annealing).
[0053]
Next, the reason why the manufacturing conditions are limited to the above range in the manufacturing method of the present invention will be described.
In the present invention, the steel slab adjusted to the above-mentioned preferred component composition is cast, immediately or once cooled, then reheated to a slab heating temperature described later, hot rolled, wound, Rolled and then subjected to continuous annealing.
[0054]
Slab heating temperature (SRT) during reheating: 1050-1300 ° C
For uniform refinement of crystal grains, the slab heating temperature is preferably as low as possible at 1300 ° C. or lower. This is because the lower the slab heating temperature, the smaller the initial austenite grain size during heating, which is effective in obtaining high stretch flangeability by refinement of the final product grain size. This is because, due to the effect of increasing the strength, it is possible to reduce the hard second phase fraction and improve the ductility. However, if the slab heating temperature is lower than 1050 ° C., it is difficult to ensure the finish rolling temperature, so the slab heating temperature needs to be 1050 ° C. or higher. Therefore, the slab heating temperature was limited to the range of 1050 to 1300 ° C. More preferably, it is the range of 1100-1200 degreeC.
[0055]
Finishing rolling finish temperature: 850-950 ° C
If the finish rolling finish temperature at the time of hot rolling is less than 850 ° C, the deformation resistance at the time of rolling is large and the hot rolling property is lowered. Further, the structure becomes non-uniform, and the ferrite and the second phase become a layered structure, that is, a band-shaped structure, and the non-uniform structure remains after cold rolling annealing, and both stretch flangeability and ductility are reduced. On the other hand, at a temperature higher than 950 ° C., austenite becomes coarse, a uniform fine structure cannot be obtained, and stretch flangeability deteriorates. Therefore, the finish rolling end temperature is set to a range of 850 to 950 ° C.
[0056]
Winding temperature: 450-650 ° C
When the coiling temperature is lower than 450 ° C, not only the hard martensite phase is generated and stretch flangeability deteriorates, but also the rolling load during cold rolling increases, and further the strength of the steel sheet in the width direction varies. The cold rolling property after hot rolling is reduced. On the other hand, when the temperature exceeds 650 ° C., TiC becomes coarse, a uniform structure cannot be obtained, and ductility after cold rolling annealing becomes insufficient. Moreover, it becomes a layered structure mainly composed of ferrite and pearlite, the level of C concentration distribution in the steel sheet is increased, the structure after cold rolling and annealing is not uniform, and both the hole expansion ratio (λ) and ductility (El) are adversely affected. Therefore, the coiling temperature was in the range of 450 to 650 ° C.
In addition, what is necessary is just to perform the cold rolling performed after coiling on the conditions as usual, and it is preferable to make the reduction rate in this cold rolling especially about 30 to 70%.
[0057]
Annealing temperature: T ₁ ± 50 ° C (however, T ₁ = 950-150 [C] ^1/2 +50 [Si] -30 [Mn])
Where T ₁ A _Three This is a temperature that is a measure of the transformation point.
Now, the annealing temperature during continuous annealing is T ₁ If it is lower than −50 ° C., it is difficult to completely eliminate the influence of the structure during cold rolling, so that a two-phase band structure, that is, a non-uniform structure is formed, and the hole expansion rate (λ) is lowered. On the other hand, the annealing temperature is T ₁ When the temperature is higher than + 50 ° C., the austenite grain size is abruptly coarsened in the temperature raising step, and the carbides are also coarsely localized, so that a fine and uniform structure cannot be obtained and λ is lowered. Moreover, ferrite transformation is delayed and ductility is also reduced.
Therefore, in order to balance the ductility and the hole expansion rate, the annealing temperature is T ₁ It is necessary to control within the range of ± 50 ℃.
[0058]
FIG. 2 shows the effect of annealing temperature on elongation (El) and hole expansion ratio (λ), and consequently on strength-elongation balance (TS × El) and strength-stretch flangeability balance (TS × λ). .
C: 0.154 mass%, Si: 0.55 mass%, Mn: 2.56 mass%, P: 0.017 mass%, S: 0.0008 mass%, Al: 0.027 mass%, N: 0.0022 mass% and Ti: 0.014 mass% The balance is steel slab (T) with a composition of Fe and inevitable impurities. ₁ = 842 ℃, T ₂ = 354 ° C), slab heating temperature: 1185 ° C, finish rolling temperature: 883 ° C, winding temperature: 455 ° C, cold rolling reduction: 45%, annealing temperature: T ₁ The cold-rolled steel sheet was processed under the conditions of ± 95 ° C., cooling rate: 22 ° C./s, cooling stop temperature: 360 ° C., heat retention temperature: 350 ° C., heat retention time: 145 s.
The elongation (El) and the hole expansion ratio (λ) of the cold-rolled steel sheet thus obtained were measured, and the results obtained for the strength-elongation balance (TS × El) and the strength-stretch flangeability balance (TS × λ) FIG. 2 shows the relationship with the annealing temperature.
[0059]
As is apparent from the figure, the annealing temperature is T ₁ By controlling in a range of ± 50 ° C., a ferrite phase and a bainite phase having a predetermined volume fraction can be obtained, and the hardness ratio of these phases also satisfies a predetermined range, and high TS × El and TS × λ are high. Has been obtained.
[0060]
In order to emphasize the corrosion resistance after coating and to improve it, as described above, it is necessary to suppress the concentration of Si on the steel sheet surface and make the Si concentration ratio 1.5 or less. It is preferable that the atmospheric dew point (DP) is at least −30 ° C. or higher during heating during annealing, that is, during heating up to the annealing temperature and during annealing.
Here, the reason why the above atmospheric dew point (DP) is set to −30 ° C. or more is that Si has a lower equilibrium oxygen partial pressure at which oxidation proceeds than Fe and Mn, and therefore has a dew point lower than −30 ° C. This is because when annealed, Si tends to be concentrated on the steel sheet surface, the Si concentration ratio between the steel sheet surface and the steel sheet exceeds 1.5, and the chemical conversion treatment performance decreases. A preferred dew point range is from -30 to -15 ° C.
[0061]
Cooling stop temperature: T ₂ ± 50 ° C (however, T ₂ = 500−450 [C] −30 [Mn])
Where T ₂ Is a temperature that is a measure of the Ms point.
In cooling after annealing, the cooling stop temperature is T ₂ If the temperature is below -50 ° C, a hard martensite phase is formed and the formation of bainite phase is reduced or the bainite phase is excessively hardened, but the tensile strength is increased, and the elongation and hole expansion rate are increased. Decreases. On the other hand, the cooling stop temperature is T ₂ When the temperature exceeds + 50 ° C., a hard retained austenite phase having an extremely high C concentration is generated, the hardness difference from the ferrite phase increases, and the hole expansion rate decreases. In addition, the strength level is lowered because the bainai soot phase is formed at a high temperature, and it is difficult to ensure TS ≧ 980 MPa. Therefore, the cooling stop temperature is T ₂ The temperature was controlled within the range of ± 50 ° C.
Incidentally, the cooling rate from the annealing temperature to the cooling stop temperature is defined by (annealing temperature (cooling start temperature) −temperature at the time of cooling stop) / cooling time (° C./s). If the cooling rate is too slow, a large amount of ferrite is produced, so that it is difficult to ensure the tensile strength. On the other hand, if it is too fast, the formation of ferrite is suppressed and problems such as a decrease in ductility occur. Therefore, unlike the case of water cooling, mist cooling, etc., the cooling rate may be about 5 to 80 ° C./s as an average cooling rate. Preferably it is the range of 10-50 degreeC / s.
[0062]
Insulation temperature: Cooling stop temperature to cooling stop temperature –100 ℃
In order to sufficiently perform the transformation from austenite to bainite, it is important to retain in a temperature range of (cooling stop temperature to cooling stop temperature−100 ° C.) for an appropriate time. If the temperature is kept below (cooling stop temperature−100 ° C.), the tensile strength increases due to the hardening of the second phase, and the elongation decreases. In addition, the hardness difference from ferrite is increased, cracks are easily generated and propagated, and the hole expansion rate is reduced.
In this respect, if the heat retention temperature is equal to or lower than the steel plate temperature at the end of cooling, a bainite phase, which is a low-temperature transformation phase having a sufficiently small hardness difference from ferrite, is generated, so that a sufficient hole expansion rate can be obtained. Moreover, it becomes a mixed structure of a bainite phase and a ferrite phase, which is not harder than martensite, and can be compatible with ductility. Therefore, the heat retention temperature is limited to the range of (cooling stop temperature to cooling stop temperature−100 ° C.), and after the cooling is finished, the temperature is kept within the above range without increasing the steel plate temperature.
[0063]
Insulation time: 60 seconds or more
Similar to the above-mentioned heat retention temperature, it is important for sufficient transformation from austenite to bainite, and if it is less than 60 seconds, hard martensite is generated and the hole expansion rate decreases, so the heat retention time is 60 seconds. It is necessary to do it above. In addition, even if the temperature is maintained for more than 240 seconds, the effect is saturated and the production efficiency only deteriorates. Therefore, the heat retention time is preferably 240 seconds or less. In addition, this heat retention treatment can be performed in an overaging zone or the like of a continuous annealing furnace.
[0064]
FIG. 3 shows the effect of the cooling stop temperature on the hole expansion ratio (λ) and elongation (El), and hence the strength-elongation balance (TS × El) and the strength-stretch flangeability balance (TS × λ). is there.
C: 0.149 mass%, Si: 0.44 mass%, Mn: 2.55 mass%, P: 0.017 mass%, S: 0.0007 mass%, Al: 0.035 mass%, N: 0.0028 mass% and Ti: 0.015 mass% The balance is steel slab (T) with a composition of Fe and inevitable impurities. ₁ = 838 ° C, T ₂ = 356 ° C), slab heating temperature: 1170 ° C, finish rolling temperature: 905 ° C, winding temperature: 520 ° C, cold rolling reduction: 50%, annealing temperature: 850 ° C, cooling rate: 25 ° C / s, cooling Stop temperature: (T ₂ -70 ° C) to (T ₂ + 75 ° C), heat retention temperature: cooling stop temperature -10 ° C, heat retention time: 200 seconds.
The elongation (El) and the hole expansion ratio (λ) of the cold-rolled steel sheet thus obtained were measured, and the results obtained for the strength-elongation balance (TS × El) and the strength-stretch flangeability balance (TS × λ) FIG. 3 shows the relationship with the cooling stop temperature.
[0065]
As is apparent from the figure, the cooling stop temperature is T ₂ By controlling the temperature within a range of ± 50 ° C. and performing a heat treatment under appropriate conditions, a ferrite phase and a bainite phase having a predetermined volume fraction can be obtained, and the hardness ratio of these phases is also appropriate, and high TS × El and It can be seen that TS × λ is obtained.
[0066]
In addition, after said heat retention process, it is preferable to cool to about 200 degreeC by standing_to_cool or a cooling rate of about 10-60 degreeC / min. Moreover, about subsequent cooling, there is no restriction | limiting regarding a cooling method or a cooling rate, such as water cooling, mist cooling, and natural cooling.
As described above, the method of the present invention does not require a process such as rapid cooling from the annealing temperature or reheating of the steel sheet after the end of cooling, so it has high productivity and low fuel consumption, and at a low level of total energy cost. Industrial production is possible.
[0067]
【Example】
Steel slabs having various component compositions shown in Table 1 were processed under the conditions shown in Table 2 to produce cold-rolled steel sheets having a plate thickness of 1.0 to 1.8 mm. The rolling reduction during cold rolling was 40 to 60%. The cooling rate to 200 ° C. after the heat treatment was 30 to 50 ° C./min.
Table 3 shows the results of examining the steel structure and various mechanical properties of the cold-rolled steel sheet thus obtained.
[0068]
In addition, the evaluation method of each characteristic and the measurement method of a structure | tissue are as follows.
-Tensile properties: Evaluation was performed by performing a tensile test based on JIS Z 2241 using a JIS Z 2201 No. 5 test piece having a longitudinal direction (tensile direction) perpendicular to the rolling direction. -Hole expansion rate λ: Implemented based on the Japan Iron and Steel Federation Standard JFSTl001. That is, the initial diameter d _o ＝ After punching a 10mm hole, when the hole was widened by raising the 60 ° circular punch, the punch was stopped when the crack penetrated the plate thickness, and the punched hole diameter d after the crack was measured Hole expansion ratio λ = [(d−d _o ) / D _o ) × 100 (%).
-Bending characteristics: Using a test piece of 40 mm width x 200 mm length with the rolling direction as the longitudinal direction, an adhesion bending test by a push bending method based on JIS Z 2248 was performed and evaluated.
・ Grain size of ferrite phase
The measurement position was derived from the area of the ferrite phase and the number of ferrite phases by image analysis based on a 3000 times SEM image near the 1 / 4th of the plate thickness, and n = 3 simple average calculated by the quadrature method. Value.
-Ferrite phase volume fraction and bainite phase volume fraction: Based on a 5000 times SEM image in the vicinity of the 1 / 4th plane of the plate thickness, the area ratio was obtained by performing two gradations by image analysis, and a simple average was obtained at n = 5 Value. This area ratio was used as the volume fraction.
-Hardness of bainite phase and ferrite phase: The measurement position is in the vicinity of the 1/4 thickness plane, using a micro Vickers hardness tester, load: n = 3 simple average of 3 g test value.
・ Si concentration ratio between steel plate surface and steel plate interior
After cold rolling and pre-annealing steel plate and annealed steel plate, after grinding them 0.5 mm from the surface, each surface after grinding was counted for the same area by fluorescent X-ray analysis. Was the Si concentration. Here, the Si concentration obtained for the steel plate before annealing, that is, the Si strength count, represents the Si concentration inside the steel plate. Therefore, the Si concentration ratio is calculated by (Si concentration of steel plate grinding surface after annealing (Si strength count)) / (Si concentration of steel plate grinding surface before annealing (Si strength count)).
・ Corrosion resistance after painting
A 150mm x 75mm test piece is phosphated, electrodeposited to a thickness of 25μm, 45mm with a cutter knife, 3 pieces / notch of the test piece, 5% NaCl, 55 ° C salt After immersing in warm water for 240 hours, cellotape (registered trademark) was cut and pasted on top, and the peel width after peeling was measured. When the maximum one-side peel width was 2.5 mm (total width: 5 mm) or less, it was determined to be acceptable (◯). In addition, when it peeled in parts other than a notch, it was set as the disqualification (x).
[0069]
[Table 1]

[0070]
[Table 2]

[0071]
[Table 3]

[0072]
As shown in Table 3, all of the inventive examples have tensile strength (TS) ≧ 980 MPa, strength-elongation balance (TS × El) ≧ 17000 MPa ·%, strength-stretch flangeability balance (TS × λ) ≧ All three target values of 65000 MPa ·% are satisfied, indicating that both high strength and good workability are achieved.
It can also be seen that those with a reduced Si concentration ratio between the steel sheet surface and inside the steel sheet to 1.5 or less are excellent in corrosion resistance after painting.
[0073]
【The invention's effect】
Thus, according to the present invention, the super high strength of TS ≧ 980 MPa, the excellent press formability of TS × El ≧ 17000 MPa ·%, TS × λ ≧ 65000 MPa ·%, and further coating A cold-rolled steel sheet having excellent later corrosion resistance can be obtained stably.
[Brief description of the drawings]
FIG. 1 shows the effects of volume fraction of bainite phase and hardness ratio of bainite phase and ferrite phase on strength-elongation balance (TS × E1) and strength-stretch flangeability balance (TS × λ) of cold-rolled steel sheets. It is a figure.
FIG. 2 is a diagram showing the influence of annealing temperature on the strength-elongation balance (TS × El) and the strength-stretch flangeability balance (TS × λ) of a cold-rolled steel sheet.
FIG. 3 is a graph showing the influence of the cooling stop temperature on the strength-elongation balance (TS × El) and the strength-stretch flangeability balance (TS × λ) of a cold-rolled steel sheet.

Claims (16)

C: 0.12-0.18 mass%
Si: 0.2 to 0.8 mass%
Mn: 2.2-3.0 mass%
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
N: 0.0050 mass% or less and
Ti: 0.001 to 0.030 mass%
In the range satisfying the following formula (1), the balance is the composition of Fe and inevitable impurities, and the tensile strength (TS), elongation (El), and hole expansion rate (λ) are respectively Relational formula TS ≧ 980 MPa,
TS × El ≧ 17000 MPa ・%
TS × λ ≧ 65000 MPa ・%
An ultra-high-strength cold-rolled steel sheet with excellent formability characterized by satisfying
-100 [C] + 15 ≤ [Mn] ≤ -100 [C] + 20 --- (1)
Here, [C] and [Mn] are the contents of C and Mn (mass%), respectively. C: 0.12-0.18 mass%
Si: 0.2 to 0.8 mass%
Mn: 2.2-3.0 mass%
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
N: 0.0050 mass% or less and
Ti: 0.001 to 0.030 mass%
In the range satisfying the following formula (1), the balance is the composition of Fe and inevitable impurities, the volume fraction of the ferrite phase is 10-50 vol%, the average grain size of the ferrite phase is 4.0 μm In the following, the volume fraction of the bainite phase is 50 to 80 vol%, and the ratio of the Vickers hardness (Hv (B)) of the bainite phase to the Vickers hardness (Hv (F)) of the ferrite phase (Hv (B) / An ultra-high strength cold-rolled steel sheet having excellent formability, characterized by having a steel structure with Hv (F)) of 1.6 or less.
-100 [C] + 15 ≤ [Mn] ≤ -100 [C] + 20 --- (1)
Here, [C] and [Mn] are the contents of C and Mn (mass%), respectively. C: 0.12-0.18 mass%
Si: 0.2 to 0.8 mass%
Mn: 2.2-3.0 mass%
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
N: 0.0050 mass% or less and
Ti: 0.001 to 0.030 mass%
In the range satisfying the following formulas (1) and (2), the balance is the composition of Fe and inevitable impurities, and the tensile strength (TS), elongation (El) and hole expansion ratio (λ) are The following relational expression TS ≧ 980 MPa,
TS × El ≧ 17000 MPa ・%
TS × λ ≧ 65000 MPa ・%
An ultra-high-strength cold-rolled steel sheet with excellent formability, characterized in that the ratio of the Si concentration between the steel sheet surface and the steel sheet inside is 1.5 or less.
-100 [C] + 15 ≤ [Mn] ≤ -100 [C] + 20 --- (1)
[Ti] ≧ 3.43 [N] + 1.5 [S] − 0.006 --- (2)
Here, [C], [Mn], [Ti], [N], and [S] are the contents of C, Mn, Ti, N, and S, respectively (mass%) C: 0.12-0.18 mass%
Si: 0.2 to 0.8 mass%
Mn: 2.2-3.0 mass%
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
N: 0.0050 mass% or less and
Ti: 0.001 to 0.030 mass%
In the range satisfying the following formulas (1) and (2), the balance is the composition of Fe and inevitable impurities, the volume fraction of the ferrite phase is 10 to 50 vol%, the average grain size of the ferrite phase The diameter is 4.0 μm or less, the volume fraction of the bainite phase is 50-80 vol%, and the ratio of the Vickers hardness (Hv (B)) of the bainite phase to the Vickers hardness (Hv (F)) of the ferrite phase (Hv (B) / Hv (F)) has a steel structure of 1.6 or less, and the ratio of the Si concentration between the steel sheet surface and the steel sheet is 1.5 or less. .
-100 [C] + 15 ≤ [Mn] ≤ -100 [C] + 20 --- (1)
[Ti] ≧ 3.43 [N] + 1.5 [S] − 0.006 --- (2)
Here, [C], [Mn], [Ti], [N], and [S] are the contents of C, Mn, Ti, N, and S, respectively (mass%) In any one of Claims 1-4, a steel plate is further
Cu: 0.01-0.50mass%,
Ni: 0.01-0.50mass%,
Mo: 0.01-0.50mass% and
Cr: 0.01 ~ 0.50mass%
An ultra-high strength cold-rolled steel sheet excellent in formability, characterized by having a composition containing one or more selected from among the above. In any one of Claims 1-5, a steel plate is further
Nb: 0.001 to 0.050 mass%
A super-high-strength cold-rolled steel sheet excellent in formability, characterized by having a composition containing In any one of Claims 1-6, a steel plate is further V: 0.001-0.300 mass% and
Zr: 0.001 to 0.300 mass%
An ultra-high-strength cold-rolled steel sheet having excellent formability, characterized by having a composition containing at least one selected from the above. In any one of Claims 1-7, a steel plate is further B: 0.0001-0.0050mass%.
A super-high-strength cold-rolled steel sheet excellent in formability, characterized by having a composition containing In any one of Claims 1-8, a steel plate is further
Ca: 0.0001 to 0.0050 mass% and
REM: 0.0001 ~ 0.0050mass%
An ultra-high-strength cold-rolled steel sheet having excellent formability, characterized by having a composition containing at least one selected from the above. C: 0.12-0.18 mass%
Si: 0.2 to 0.8 mass%
Mn: 2.2-3.0 mass%
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
N: 0.0050 mass% or less and
Ti: 0.001 to 0.030 mass%
In a range satisfying the following formula (1), the balance being Fe and the inevitable impurities composition of steel slab immediately after casting or once cooled to 1050-1300 ℃, after finishing rolling, finish rolling Temperature: Hot-rolled at 850-950 ° C, rolled up at 450-650 ° C after rolling, cold-rolled, and then subjected to continuous annealing, within the temperature range represented by the following formula (3) After heating and annealing, it is cooled to the temperature range indicated by the following formula (4), and after cooling is completed, the steel plate temperature is not increased (cooling stop temperature to cooling stop temperature−100 ° C.) for 60 seconds. A method for producing an ultra-high-strength cold-rolled steel sheet having excellent formability, characterized by maintaining the temperature as described above.
-100 [C] + 15 ≤ [Mn] ≤ -100 [C] + 20 --- (1)
T ₁ ± 50 ° C (However, T ₁ = 950-150 [C] ^1/2 +50 [Si] -30 [Mn]) --- (3)
T ₂ ± 50 ° C (However, T ₂ = 500−450 [C] −30 [Mn]) --- (4)
Here, [C], [Mn], and [Si] are the contents of C, Mn, and Si, respectively (mass%) C: 0.12-0.18 mass%
Si: 0.2 to 0.8 mass%
Mn: 2.2-3.0 mass%
P: 0.02 mass% or less,
S: 0.0030 mass% or less,
Al: 0.05 mass% or less,
N: 0.0050 mass% or less and
Ti: 0.001 to 0.030 mass%
In a range satisfying the following formulas (1) and (2), and the remainder of the steel slab having a composition of Fe and inevitable impurities is heated to 1050-1300 ° C. immediately after casting or once after cooling. Finishing rolling finish temperature: Hot rolling at 850 to 950 ° C, winding at 450 to 650 ° C after rolling, cold rolling and then performing continuous annealing at least during heating during annealing After heating and annealing to a temperature range represented by the following formula (3) under an atmosphere dew point (DP) of −30 ° C. or higher, cooling to the temperature range represented by the following formula (4), A method for producing an ultra-high-strength cold-rolled steel sheet having excellent formability, wherein the steel sheet is kept warm for 60 seconds or more in a temperature range (cooling stop temperature to cooling stop temperature −100 ° C.) without increasing the steel sheet temperature.
-100 [C] + 15 ≤ [Mn] ≤ -100 [C] + 20 --- (1)
[Ti] ≧ 3.43 [N] + 1.5 [S] − 0.006 --- (2)
T ₁ ± 50 ° C (However, T ₁ = 950-150 [C] ^1/2 +50 [Si] -30 [Mn]) --- (3)
T ₂ ± 50 ° C (However, T ₂ = 500−450 [C] −30 [Mn]) --- (4)
Here, [C], [Mn], [Ti], [N], and [S] are the contents of C, Mn, Ti, N, and S, respectively (mass%) The steel slab according to claim 10 or 11, further comprising:
Cu: 0.01-0.50mass%,
Ni: 0.01-0.50mass%,
Mo: 0.01-0.50mass% and
Cr: 0.01 ~ 0.50mass%
The manufacturing method of the ultra-high-strength cold-rolled steel plate excellent in the formability characterized by becoming a composition containing 1 type, or 2 or more types selected from among these. The steel slab according to any one of claims 10 to 12,
Nb: 0.001 to 0.050 mass%
A method for producing an ultra-high-strength cold-rolled steel sheet having excellent formability, characterized by comprising a composition containing The steel slab according to any one of claims 10 to 13, further comprising V: 0.001 to 0.300 mass% and
Zr: 0.001 to 0.300 mass%
A method for producing an ultra-high-strength cold-rolled steel sheet having excellent formability, wherein the composition contains at least one selected from the above. The steel slab according to any one of claims 10 to 14, further comprising B: 0.0001 to 0.0050 mass%.
A method for producing an ultra-high-strength cold-rolled steel sheet having excellent formability, characterized by comprising a composition containing The steel slab according to any one of claims 10 to 15, further
Ca: 0.0001 to 0.0050 mass% and
REM: 0.0001 ~ 0.0050mass%
A method for producing an ultra-high-strength cold-rolled steel sheet having excellent formability, wherein the composition contains at least one selected from the above.

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