JP2004277789A - High-strength and high-ductility cold-rolled steel sheet superior in secondary adhesiveness for hot salt water, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength and high-ductility cold-rolled steel sheet which has superior mechanical characteristics such as high ductility and high formability for extension flange, and further superior secondary adhesiveness for hot salt water after being electrodeposition coated. <P>SOLUTION: This steel sheet has a composition comprising, by mass%, 0.05-0.35% C, 0.5-3.0% Si, 0.5-3.0% Mn, 0.05% or less P, 0.005% or less S, less than 0.10% Al, 0.0020-0.0100% N, 0.001-0.20% Ti, 0.0010-0.010% Ca and/or REM, and the balance Fe with unavoidable impurities; and further has a composite structure comprising ferrite, retained austenite and a low temperature transformation phase, in which ferrite and retained austenite respectively constitute 30% or more and 2% or more by a volume ratio, and besides, a mean crystal grain size of ferrite and a concentration ratio of Si and C are respectively limited to 15 μm or smaller and into a predetermined range. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００５３】
【表２】

【００５４】
【表３】

【００５５】
表２および３に示したとおり、本発明の要件を満足する発明例はいずれも、強度−伸びバランス（ＴＳ×Ｅｌ）が２００００ＭＰａ・％以上と極めて良好な値を示し、また強度−穴拡げバランス（ＴＳ×λ）が４００００ＭＰａ・％以上と伸びフランジ性にも優れていた。さらに、スラブ製造時に表面割れの発生がなく、また電着塗装後の耐塩温水２次密着性も良好であった。
また、本発明の鋼板は、表面粗度形態を制御しているので、プレス成形時の摺動性および耐型かじり性に優れていることも確認されている。
【００５６】
実施例２
表４に示す成分組成になる溶鋼を、転炉にて溶製し、連続鋳造によりスラブとした。得られたスラブを、枚厚：３．０ｍｍまで熱間圧延し、ついで酸洗後、冷間圧延により枚厚：１．４ｍｍの冷延鋼板とした。
ついで、これらの冷延鋼板を、連続焼鈍ラインにて、表５に示す条件で加熱保持したのち、冷却し、圧下率：０．５％の調質圧延を施して製品とした。
得られた鋼板のミクロ組織および鋼板表面形状について調べた結果を、表５に併記する。
また、スラブ製造時における表面割れの発生状況、製品板の機械的特性および耐塩温水２次密着性について調査した結果を、表６に示す。
【００５７】
【表４】

【００５８】
【表５】

【００５９】
【表６】

【００６０】
表５および６に示したとおり、本発明の要件を満足する発明例はいずれも、強度−伸びバランス（ＴＳ×Ｅｌ）が２００００ＭＰａ・％以上と極めて良好な値を示し、また強度−穴拡げバランス（ＴＳ×λ）が４００００ＭＰａ・％以上と伸びフランジ性にも優れていた。さらに、スラブ製造時に表面割れの発生がなく、また電着塗装後の耐塩温水２次密着性も良好であった。
また、本発明の鋼板は、表面粗度形態を制御しているので、プレス成形時の摺動性および耐型かじり性に優れていることも確認されている。
【００６１】
【発明の効果】
かくして、本発明によれば、高延性でかつ、伸びフランジ性を有し、電着塗装後の耐塩温水２次密着性が良好な高強度冷延鋼板を安価にしかも安定的して製造することができる。
従って、本発明の冷延鋼板を自動車部品用素材として適用することにより、自動車の軽量化、低燃費化が可能になり、ひいては地球環境の改善に大きく貢献する。[0001]
[Industrial applications]
The present invention relates to a high-strength, high-ductility, cold-rolled steel sheet having excellent salt-water-resistant secondary adhesion after electrodeposition coating, and a method for producing the same.
[0002]
[Prior art]
2. Description of the Related Art In recent years, from the viewpoint of preserving the global environment, there has been a demand for improved fuel efficiency of automobiles. In addition, in order to protect occupants in the event of a collision, there is a demand for improved safety of the vehicle body. For this reason, the weight reduction of the vehicle body and the strengthening of the vehicle body have been actively promoted.
It is said that increasing the strength of component materials is an effective way to satisfy both weight reduction and strengthening of automobile bodies at the same time. Recently, high-strength steel sheets have been actively used for automobile parts. Have been.
[0003]
Further, since many automotive parts made of steel sheets are formed by press working, steel sheets for automotive parts are also required to have excellent press formability. In order to realize excellent moldability, it is essential to secure high ductility in the first place. Further, in press molding of automobile parts, stretch flange deformation is often used. In particular, in the case of a member constituting a skeleton member for ensuring the strength of an automobile body, a component constituting a reinforcement, or the like, a component molding using stretch flange deformation is often performed.
For this reason, a steel sheet for automobile parts is strongly required to have high ductility and excellent stretch flangeability.
[0004]
As a high-strength steel sheet having excellent ductility, a dual-phase steel sheet having a composite structure of ferrite and martensite is typical.
In recent years, for example, as described in Non-Patent Document 1, a high-ductility steel sheet using a transformation induced plasticity (TRIP) by adding an appropriate amount of Si and Mn to 0.11 mass% C and using transformation induced plasticity (TRIP) caused by retained austenite. Has also reached the stage of practical application.
However, as described in Non-Patent Document 2, such a medium carbon steel has a peritectic reaction (δ + L → γ) and an A4 transformation point near the solidus temperature on the Fe—C equilibrium phase diagram. δ → γ), solidification tends to be non-uniform, and there is a problem that surface cracks such as vertical cracks and horizontal cracks are likely to occur in the mold of the continuous casting machine.
[0005]
In the secondary cooling zone after casting, as described in Non-Patent Document 3, carbides and nitrides generated by the addition of alloy elements such as Al, Nb, V, and Cu cause grain boundary brittleness. Slabs are likely to be cracked and surface cracks are likely to occur. Therefore, at present, slabs are produced while taking measures such as low-speed casting and slow cooling.
Under such manufacturing conditions, the productivity of the steelmaking sector, which is an upstream process of the steel industry, is significantly reduced, and ultimately hinders the productivity of the entire steelworks.
[0006]
Further, a serious problem of this type of steel sheet is that since a large amount of Si is added, the oxide of Si is concentrated on the steel sheet surface during annealing, and the steel sheet after electrodeposition coating is inferior to salt hot water. When exposed to an unfavorable environment, the coating film is more likely to be peeled off than a normal steel plate.
[0007]
[Non-patent document 1]
Japan Iron and Steel Association: “Materials and Processes Vol. 4 (1991) -P. 1942”
[Non-patent document 2]
(I) Japan Iron and Steel Association: "Iron and Steel, Vol. 8 (1995) No. 9-P. 26"
[Non-Patent Document 3]
Japan Iron and Steel Association: "Third Edition, Iron and Steel Handbook II, Iron and Steelmaking, P.649"
[0008]
[Problems to be solved by the invention]
The present invention has been developed in view of the above circumstances, and solves the above-mentioned problems in the steelmaking stage by regulating the steel composition to a specific range, and also adjusts the steel structure in accordance with the regulation of the steel composition described above. By controlling, high mechanical strength such as high ductility and high elongation flangeability, which is suitable as a material for automobile parts, and high strength and high ductility cold, which is excellent in secondary adhesion of salt hot water after electrodeposition coating. It is an object of the present invention to provide a rolled steel sheet together with its advantageous production method.
[0009]
[Means for Solving the Problems]
By the way, the present inventors have conducted intensive studies on the component composition, microstructure and surface properties of a steel sheet to prevent the surface crack of the slab and optimize the ductility and stretch flangeability of the steel sheet in order to achieve the above object. Was piled up.
as a result,
(1) By adding appropriate amounts of Ti and N, it is possible to prevent AIN, Nb-based carbide, and V-based carbide that precipitate at low temperatures from preferentially precipitating at crystal grain boundaries.
(2) The structure of the high-strength cold-rolled steel sheet obtained after continuous annealing is a composite structure composed of ferrite, retained austenite, and a low-temperature transformation phase, and the volume ratio of each phase is set to a predetermined ratio, so that the steel sheet has excellent ductility. Can be expressed,
(3) By making the crystal grain size of ferrite, which is the main component of the above composite structure, fine, excellent stretch flangeability in addition to high ductility can be obtained.
I got the knowledge. Also,
(4) By suppressing the amount of concentration of Si and C on the surface of the steel sheet after annealing, the secondary adhesion with salt and hot water is significantly improved.
(5) Electrolytic pickling, or electrolytic pickling and brushing are required to suppress the amount of Si and C enrichment on the steel sheet surface after annealing. Electrolytic pickling or electrolytic pickling By suppressing the form of oxide on the surface of the steel sheet immediately before the treatment and the brush treatment, the salt-water-resistant secondary adhesion is greatly improved.
I also found this out.
The present invention is based on the above findings.
[0010]
That is, the gist configuration of the present invention is as follows.
1. In mass%,
C: 0.05 to 0.35%,
Si: 0.5 to 3.0%,
Mn: 0.5-3.0%,
P: 0.05% or less,
S: 0.005% or less,
Al: less than 0.10%,
N: 0.0020 to 0.0100% and
Ti: 0.001 to 0.20%
And
Ca and / or REM: 0.0010-0.010%
, The balance being Fe and unavoidable impurities, and a composite structure comprising ferrite, retained austenite and a low-temperature transformation phase, wherein the ferrite has a volume fraction of 30% or more and the residual austenite has a volume fraction of It has a steel structure containing at least 2%, has an average crystal grain size of the ferrite of 15 μm or less, and has a concentration ratio of Si and C represented by the following formula:
P (Si)_0-10/ P (Si)₁₀₀≦ 3.0
P (C)_0-10/ P (C)₁₀₀≦ 3.0
here,
P (Si)_0-10, P (C)_0-10: Peak values of Si and C concentrations from the surface of steel sheet to a depth of 10 nm in the thickness direction
P (Si)₁₀₀, P (C)₁₀₀: Si and C concentrations at a depth of 100 nm from the surface of the steel sheet in the thickness direction
A high-strength and high-ductility cold-rolled steel sheet having excellent secondary resistance to salt and hot water, characterized by satisfying the following conditions.
[0011]
2. In the above item 1, the steel sheet further contains, by mass%,
One or two selected from Nb and V: 0.001 to 0.30%
A high-strength, high-ductility, cold-rolled steel sheet having excellent secondary resistance to salt and hot water, characterized by having a composition containing:
[0012]
3. In the above 1 or 2, the steel sheet further contains, by mass%,
Cr: 1.5% or less
Mo: 0.5% or less and
B: 0.010% or less
A high-strength, high-ductility, cold-rolled steel sheet having excellent secondary adhesion to salt and hot water, wherein the steel sheet has a composition containing one or more selected from the group consisting of:
[0013]
4. In mass%,
C: 0.05 to 0.35%,
Si: 0.5 to 3.0%,
Mn: 0.5-3.0%,
P: 0.05% or less,
S: 0.005% or less,
Al: less than 0.10%,
N: 0.0020 to 0.0100% and
Ti: 0.001 to 0.20%
And
Ca and / or REM: 0.0010-0.010%
, A steel slab having a balance of Fe and inevitable impurities is hot-rolled, then cold-rolled, and then heated to a temperature equal to or higher than the Ac1 transformation point by continuous annealing. After cooling to a temperature range of 300 to 500 ° C. at a cooling rate of 60 to 500 seconds or more and staying in this temperature range for 60 seconds or more, during or after cooling, electrolytic pickling or electrolytic pickling and brushing are performed. Concentration ratio of Si and C represented by the following formula
P (Si)_0-10/ P (Si)₁₀₀≦ 3.0
P (C)_0-10/ P (C)₁₀₀≦ 3.0
here,
P (Si)_0-10, P (C)_0-10: Peak values of Si and C concentrations from the surface of the steel sheet to a depth of 10 nm in the thickness direction
P (Si)₁₀₀, P (C)₁₀₀: Si and C concentrations at a depth of 100 nm from the surface of the steel sheet in the thickness direction
A method for producing a high-strength, high-ductility, cold-rolled steel sheet having excellent secondary resistance to salt and hot water, characterized in that:
[0014]
5. In the above item 4, the SiO 2 of the steel sheet surface layer immediately before the electrolytic pickling treatment₂And Mn₂SiO₄Abundance ratio of SiO₂/ Mn₂SiO₄The method for producing a high-strength, high-ductility, cold-rolled steel sheet having excellent secondary adhesion to salt and hot water, wherein
[0015]
6. In the above 4 or 5, the steel slab further comprises, by mass%,
One or two selected from Nb and V: 0.001 to 0.30%
A method for producing a high-strength and high-ductility cold-rolled steel sheet having excellent secondary resistance to salt and hot water, comprising:
[0016]
7. In the above 4, 5 or 6, the steel slab further comprises:
Cr: 1.5% or less,
Mo: 0.5% or less and
B: 0.010% or less
A method for producing a high-strength, high-ductility, cold-rolled steel sheet having excellent secondary adhesion to salt and hot water, characterized in that the composition comprises one or more selected from the group consisting of:
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described specifically.
First, the reason why the composition of the steel sheet is limited to the above range in the present invention will be described.
In the present invention, “%” in the composition means “% by mass” unless otherwise specified.
C: 0.05-0.35%
C is an element indispensable for increasing the strength of steel, and is also an effective element for generating retained austenite and a low-temperature transformation phase. However, if the C content is less than 0.05%, a desired high strength cannot be obtained, while if it exceeds 0.35%, the weldability deteriorates. For this reason, C is limited to the range of 0.05 to 0.35%. More preferably, it is 0.10 to 0.25%.
[0018]
Si: 0.5 to 3.0%
Si is an element that not only contributes to strengthening of the steel by solid solution strengthening, but also stabilizes austenite and promotes generation of a retained austenite phase. However, if the content is less than 0.5%, the effect of the addition is poor, while if the content exceeds 3.0%, the ductility is deteriorated. For this reason, Si was limited to the range of 0.5 to 3.0%. More preferably, it is 1.0 to 2.0%.
[0019]
Mn: 0.5-3.0%
Mn not only strengthens the steel by solidification, but also has the effect of improving the hardenability of the steel and promoting the formation of retained austenite and low-temperature transformation phases. Such an effect is recognized when the amount of Mn is 0.5% or more. On the other hand, even if the content exceeds 3.0%, the effect reaches saturation, an effect corresponding to the content cannot be expected, and the cost is increased. For this reason, Mn was limited to the range of 0.5 to 3.0%. More preferably, it is 1.0 to 2.0%.
[0020]
P: 0.050% or less
P is a solid solution strengthening element and is usually an effective element for obtaining a high-strength steel sheet. However, the content of more than 0.05% causes a decrease in spot weldability. % As the upper limit. More preferably, it is 0.020% or less.
[0021]
S: 0.005% or less
S is a harmful element that forms MnS in the steel and reduces the stretch flangeability of the steel sheet. For this reason, it is preferable to minimize the incorporation of S, but it is acceptable if the content is 0.005% or less. More preferably, it is 0.003% or less.
[0022]
Al: less than 0.10%,
Al is an element that effectively contributes as a deoxidizing agent in the steelmaking stage and is necessary for separating nonmetallic inclusions that reduce hole expandability into slag. However, since the content of 0.10% or more increases the alloy cost, in the present invention, the content is set to less than 0.10%. The preferred range is 0.02 to 0.09%.
[0023]
N: 0.0020 to 0.0100%,
N is an impurity element that usually causes strain aging, but in a structure-strengthened steel sheet as in the present invention, strain aging does not occur, but rather, a surface crack of a slab is prevented by a precipitate such as TiN. It is a useful element. In order to prevent such surface cracking of the slab, the content of 0.0020% or more is required. However, since the content of N of more than 0.010% is difficult in steel making and increases the cost, N was limited to the range of 0.0020 to 0.0100%. More preferably, it is 0.0025 to 0.0080%.
[0024]
Ti: 0.001 to 0.20%
Ti is an element useful for suppressing the growth of the ferrite phase in the heating stage during continuous annealing, refining the steel structure, and significantly improving the hole expandability. Further, Ti precipitates Ti-based carbonitrides and sulfides at a high temperature during slab cooling, and AlN generated at a relatively low temperature, and Nb and V added for the purpose of crystal grain refinement, cause grain boundaries at Ti. It is an effective element in suppressing the precipitation of the generated Nb-based or V-based carbides and preventing slab surface cracks. In order to exhibit such an effect, the content needs to be at least 0.001%. On the other hand, a content exceeding 0.20% not only causes an increase in the alloy cost, but also increases the precipitation amount of TiC, reduces the retained austenite for exhibiting the TRIP effect, and also has a poor precipitation strengthening ability. It becomes too large and disadvantageous in that high ductility cannot be obtained. Therefore, in the present invention, the content of Ti is limited to the range of 0.001 to 0.20%. Preferably, it is 0.010 to 0.10%.
[0025]
Ca and / or REM: 0.0010-0.010%,
Ca and REM have an action of controlling the form of the sulfide-based inclusions, thereby effectively contributing to the improvement of the stretch flangeability of the steel sheet. However, if the total amount of one or two selected from Ca and REM is less than 0.0010%, the effect of the addition is poor, while if it exceeds 0.010%, the effect reaches saturation. For this reason, Ca and REM are contained in the range of 0.0010 to 0.010% in either case of single addition or composite addition. A more preferred range is 0.0010 to 0.005%.
[0026]
As described above, the essential components have been described. However, in the present invention, other elements described below can be appropriately contained.
One or two selected from Nb and V: 0.001 to 0.30%
Both Nb and V, like Ti, are useful for precipitating NbC and VC, suppressing the growth of the ferrite phase in the heating stage during continuous annealing, refining the steel structure, and significantly improving the hole expandability. Element. In order to exhibit such an effect, it is necessary to add at least 0.001%. On the other hand, if the content exceeds 0.30%, YS increases due to precipitation strengthening, and not only does workability decrease, but also disadvantages arise in that residual austenite for exhibiting the TRIP effect is reduced. In addition, Nb and V generate carbides at a lower temperature than Ti, and if this precipitates preferentially at crystal grain boundaries, it causes a surface crack of the slab. Therefore, in the present invention, Nb and V are contained in the range of 0.001 to 0.30% in either case of single addition or composite addition. A more preferred range is 0.001 to 0.10%.
[0027]
One or more selected from Cr: 1.5% or less, Mo: 0.5% or less and B: 0.010% or less
Cr, Mo and B are all useful elements that have the effect of improving the hardenability of steel and promoting the formation of a low-temperature transformation phase. However, the effect described above is saturated when the content of Cr is more than 1.5%, Mo is more than 0.5%, and B is more than 0.010%, and the effect corresponding to the content cannot be expected. Rather, it creates an economic disadvantage. Therefore, the content of Cr is 1.5% or less, the content of Mo is 0.5% or less, and the content of B is 0.010% or less. A more preferred range is 0.0005 to 1.0% in total of one or more selected from Cr, Mo, and B.
[0028]
Although the preferred component composition of the present invention has been described above, in the present invention, the steel structure is also important, and it is important to form a composite structure including ferrite, retained austenite, and a low-temperature transformation phase.
Ferrite
Ferrite is a soft phase that does not contain iron carbide, has high deformability, and improves the ductility of a steel sheet. In the steel sheet of the present invention, it is necessary to contain such ferrite in a volume ratio of 30% or more. This is because if the amount of ferrite is less than 30%, a remarkable ductility improving effect cannot be expected. A more preferred amount of ferrite is 50% or more.
[0029]
Retained austenite
The retained austenite transforms into martensite at the time of working, disperses locally applied working strain widely, and has an effect of improving the ductility of the steel sheet. In the steel sheet of the present invention, such retained austenite is contained at a volume ratio of 2% or more. If the amount of retained austenite is less than 2%, remarkable improvement in ductility cannot be expected. A more preferred amount of retained austenite is 5% or more.
[0030]
Low temperature transformation phase
The low-temperature transformation phase in the present invention refers to martensite or bainite.
Martensite and bainite are both hard phases and have the effect of increasing the strength of the steel sheet by strengthening the structure. Further, since the generation of transformation involves the generation of movable dislocations, it also has the effect of lowering the yield ratio of the steel sheet. In order to sufficiently obtain the above effects, it is preferable that the low-temperature transformation phase is martensite. The amount of the low-temperature transformation phase is not particularly limited, and may be appropriately distributed according to the strength of the steel sheet.
[0031]
In the present invention, the crystal grain size of the ferrite is also important.
That is, the refinement of the crystal grains contributes to the improvement of the stretch flangeability of the steel sheet.
Therefore, in the present invention, the average crystal grain size of ferrite in the composite structure is limited to 15 μm or less. The reason is that when the average crystal diameter of ferrite exceeds 15 μm, a remarkable action of improving stretch flangeability cannot be expected. More preferably, it is 10 μm or less.
[0032]
Furthermore, in the present invention, the surface properties of the steel sheet are also important, and it is necessary to regulate the steel and C enrichment ratios represented by the following formula.
Record
P (Si)_0-10/ P (Si)₁₀₀≦ 3.0
P (C)_0-10/ P (C)₁₀₀≦ 3.0
here,
P (Si)_0-10, P (C)_0-10: Peak values of Si and C concentrations from the surface of the steel sheet to a depth of 10 nm in the thickness direction
P (Si)₁₀₀, P (C)₁₀₀: Si and C concentrations at a depth of 100 nm from the surface of the steel sheet in the thickness direction
[0033]
That is, P (Si)_0-10/ P (Si)₁₀₀> 3.0 and / or P (C)_0-10/ P (C)₁₀₀In the case of> 3.0, due to Si oxide and C (graphite) on the surface of the steel sheet, the secondary adhesion of the coating film is remarkable when exposed to a poor environment such as salt water after the electrodeposition coating. It is because it deteriorates.
The deterioration of the secondary adhesion is caused by the fact that Si oxide and C (graphite) on the steel sheet garment surface are in the early stage of the formation process of the chemical conversion coating using zinc phosphate or the like, which is performed as a base treatment for electrodeposition coating. This is for inhibiting the etching. To improve the salt water secondary adhesion, P (Si)_0-10/ P (Si)₁₀₀≤3.0 and P (C)_0-10/ P (C)₁₀₀≤3.0. Preferably, P (Si)_0-10/ P (Si)₁₀₀≤2.0 and P (C)_0-10/ P (C)₁₀₀≦ 2.0.
[0034]
Here, since the concentration of Si and C at a depth exceeding 10 nm does not affect the secondary adhesion of the salt-resistant hot water at all, in the present invention, the peak value of the concentration from the steel sheet surface to a depth of 10 nm is considered. did. The concentrations of these elements can be evaluated, for example, by an intensity ratio calculated from an element intensity distribution curve in the thickness direction by GDS (glow discharge emission spectroscopy) analysis.
[0035]
Next, a method for manufacturing a steel sheet according to the present invention will be described.
The molten steel adjusted to the above preferable component composition is cast by a known method, hot-rolled according to a conventional method, then cold-rolled, then heated by continuous annealing, cooled, and held at a predetermined temperature during the heating. After cooling, the product is cooled.
[0036]
In the present invention, in the above manufacturing process, Ac₁Heating to a temperature above the transformation point, quenching at a rate of 10 ° C./s or more during cooling, ending the quenching temperature to 350 to 500 ° C., staying in this temperature range for 60 seconds or more, and cooling By performing electrolytic pickling, or electrolytic pickling and brushing during or after cooling, P (Si)_0-10/ P (Si)₁₀₀≤3.0 and P (C)_0-10/ P (C)₁₀₀It is important that ≦ 3.0.
In addition, the SiO 2 on the steel sheet surface immediately before the electrolytic pickling treatment₂And Mn₂Si₄Abundance ratio of SiO₂/ Mn₂SiO₄Is preferably 1.5 or less.
[0037]
Hereinafter, the reason why the manufacturing conditions are limited to the above range will be described.
Heating temperature: Ac₁Above the transformation point
The heating temperature in continuous annealing is Ac₁Must be above the transformation point. The reason is that if heating is not performed to the (α + γ) two-phase region, retained austenite cannot be obtained, and the TRIP effect does not appear. Therefore, in the present invention, the lower limit of the heating temperature is Ac₁The transformation point. When the heating temperature is higher than 850 ° C., the ferrite grain size increases, and the stretch flangeability may be reduced. Therefore, the upper limit of the heating temperature is preferably set to about 850 ° C.
[0038]
Cooling rate: 10 ° C / s or more
The reason why the cooling rate is set to 10 ° C./s or more is that if the cooling rate is less than 10 ° C./s, the austenite phase is transformed into pearlite or bainite, and the residual austenite disappears, so that the TRIP effect cannot be obtained. It is. More preferably, the rate is 20 ° C./s or more.
[0039]
Rapid cooling end temperature: 300-500 ° C
The reason for setting the quenching end temperature to 300 to 500 ° C. is that if the temperature is rapidly cooled to a temperature lower than 300 ° C., all the austenite phase is transformed into martensite. This is because they transform into pearlite or bainite, and the TRIP effect cannot be expected. A more preferred temperature range is 350 to 450 ° C.
[0040]
Retention time: 60 seconds or more
After quenching, to stay in the temperature range of 300 to 500 ° C for 60 seconds or more, when the temperature range of less than 300 ° C is reached in a short time of less than 60 seconds, most of the retained austenite is transformed into martensite, This is because the TRIP effect does not appear during press working. On the other hand, if the time is too long, the bainite transformation is completed, so that the retained austenite decreases, which is not preferable. A particularly preferred holding time is 60 to 1200 seconds.
[0041]
Electrolytic pickling during or after cooling, or electrolytic pickling and brushing
In order to improve the secondary adhesion of salted hot water after coating, it is necessary to increase the concentration of Si oxide, particularly SiO₂And C are removed, and the concentration ratio of Si and C, P (Si)_0-10/ P (Si)₁₀₀≤3.0 and P (C)_0-10/ P (C)₁₀₀It is essential to satisfy ≦ 3.0. For this reason, it is necessary to perform electrolytic pickling, or electrolytic pickling and brushing during or after the cooling.
[0042]
Here, Si oxide concentrated on the surface of the steel sheet, particularly SiO 2₂And C are removed, and the concentration ratio of Si and C is changed to P (Si)_0-10/ P (Si)₁₀₀≤3.0 and P (C)_0-10/ P (C)₁₀₀To simultaneously satisfy ≦ 3.0, electrolytic pickling is effective. Examples of the acid solution include hydrochloric acid, sulfuric acid, and nitric acid-hydrochloric acid, but are not limited thereto. For example, in a mixed solution of (1% hydrochloric acid + 10% nitric acid) at 50 ° C., 30 A / dm²Electrolytic pickling at a current density of 5 seconds or more is recommended. However, if the treatment is performed for a long time, the C concentration on the steel sheet surface increases, and the secondary adhesion of the salt-resistant water is deteriorated. Therefore, it is necessary to set the pickling treatment time so that the C concentration does not increase.
[0043]
Furthermore, by using a brush process together with the electrolytic pickling process, the concentration ratio of Si and C, P (Si)_0-10/ P (Si)₁₀₀≤3.0 and P (C)_0-10/ P (C)₁₀₀≦ 3.0 can be more easily achieved.
For example, a pressure: 29.4 N / cm while rotating a 300 mmφ brush roll having a core material in which SiC of # 240 is embedded after electrolytic pickling at a rotation speed of at least 1 rpm.²It is preferable to press against the steel sheet with the above pressure and rub the steel sheet surface.
[0044]
The Si oxide generated on the steel sheet surface is SiO 2₂And Mn₂SiO₄Of which are Mn₂SiO₄Can be easily removed by electrolytic pickling. Therefore, the SiO 2 on the steel sheet surface immediately before the electrolytic pickling treatment₂And Mn₂SiO₄Abundance ratio of SiO₂/ Mn₂SiO₄Is set to 1.5 or less, the concentration ratio of Si and C, P (Si) can be effectively increased by a short electrolytic pickling treatment._0-10/ P (Si)₁₀₀≤3.0 and P (C)_0-10/ P (C)₁₀₀.Ltoreq.3.0 can be satisfied at the same time.
[0045]
SiO on steel sheet surface just before electrolytic pickling₂And Mn₂SiO₄Abundance ratio of SiO₂/ Mn₂SiO₄Can be achieved by, for example, setting the dew point of the atmosphere in the furnace to −30 ° C. or more in the above continuous annealing step.
Where SiO₂And Mn₂SiO₄Can be determined by, for example, using an FTIR (Fourier transform infrared spectroscopy) apparatus to detect SiO2 around 1250 / cm.₂And Mn appearing near 1000 / cm₂SiO₄Can be evaluated by the peak intensity ratio.
[0046]
Further, after the above-described continuous annealing, it is advantageous to perform temper rolling for the purpose of shape correction and roughness adjustment. However, even if such temper rolling is not performed, there is no particular problem with the material.
[0047]
【Example】
Example 1
Molten steel having the component composition shown in Table 1 was smelted in a converter and made into a slab by a continuous casting method. The obtained slab was hot-rolled to a thickness of 3.0 mm, then pickled, and then cold-rolled into a cold-rolled steel sheet having a thickness of 1.6 mm.
Next, these cold-rolled steel sheets were heated and held in the continuous annealing line under the conditions shown in Table 2, then cooled, and cooled, and then cooled to 53 ° C. (0.6% HCl + 26% HNO).₃) 30 A / dm in the mixture²Electrolytic brushing treatment at a current density of 6 to 20 seconds, or a brush roll (300 mmφ) having a core material embedded with # 240 SiC after the electrolytic pickling treatment, pressure: 29.4 N / cm.²The brush was pressed at a rotation speed of 2 rpm, washed with water, dried, and then subjected to temper rolling at a rolling reduction of 0.5% to obtain a product.
Table 2 also shows the results obtained by examining the microstructure and the surface shape of the obtained steel sheet.
Table 3 shows the results of investigations on the occurrence of surface cracks during the production of the slab, the mechanical properties of the product plate, and the secondary adhesion of the salt-resistant hot water.
[0048]
The cracks in the slab were visually determined and evaluated based on the presence or absence of cracks.
Further, a sample for a high-temperature tensile test was cut out from the slab, processed into a 6 mmφ round bar tensile test piece, subjected to a solution treatment at 1200 ° C. for 10 minutes, and cooled to 800 ° C. at a cooling rate of 100 ° C./min. After holding for a minute, a tensile test is performed. From the original sectional diameter (Do) and the fractured surface diameter after tension (D), the following equation is obtained.
Aperture ratio (%) = (Do−D) / Do × 100
Thus, the aperture ratio was determined.
[0049]
The microstructure of the steel sheet was investigated by observing a cross section in the rolling direction of the steel sheet with an optical microscope or a scanning electron microscope. That is, using a photograph of a cross-sectional structure at a magnification of 1000 times, the occupied area ratio of the corresponding phase existing in a 100 mm square area arbitrarily set by image analysis was determined and defined as the volume ratio of the corresponding phase.
The amount of retained austenite was determined by polishing a steel sheet to the center plane in the thickness direction and measuring the diffraction X-ray intensity on the center plane in the thickness direction. MoKα rays were used as incident X-rays, and {111}, {200}, and {220} of the residual austenite phase with respect to the diffraction X-ray intensity of each of the {110}, {200}, and {211} planes of the ferrite phase. , {311}, the diffraction X-ray intensity ratio of each surface was determined, and the average value thereof was defined as the volume fraction of retained austenite.
The ferrite grain size was determined by measuring the crystal grain size in accordance with the method specified in JIS Z 0552 and converting it to an average crystal grain size.
[0050]
The mechanical properties of the steel sheet were measured by the following tensile test and hole expansion test. Table 3 shows the measurement results.
The tensile properties were determined by using a No. 5 test piece specified in JIS Z 2204, which was taken from a steel sheet in a direction perpendicular to the rolling direction, in accordance with the method specified in JIS Z 2241, in accordance with the method specified in JIS Z 2241, tensile strength (TS), tensile strength (TS), Breaking elongation (El) and yield elongation (YEl) were measured.
The stretch flangeability was evaluated by measuring the hole expansion ratio (λ) in accordance with the method specified in JFS T1001 and evaluating this value.
[0051]
The salt-warm water secondary adhesion can be determined by subjecting a 150 mm x 75 mm test piece to a chemical conversion treatment with zinc phosphate using a commercially available solution, applying a cationic electrodeposition coating to a thickness of 25 µm, and then using a cutter knife. After cutting the test piece and immersing it in a solution of 5% NaCl at 50 ° C. for 240 hours, the adhesive tape was stuck on the cut and peeled off. It was measured and evaluated. Here, if the maximum peeling full width is 5.0 mm or less, it can be said that the salt-water-resistant secondary adhesion is good.
[0052]
[Table 1]

[0053]
[Table 2]

[0054]
[Table 3]

[0055]
As shown in Tables 2 and 3, the invention examples satisfying the requirements of the present invention all show extremely good values of strength-elongation balance (TS × El) of 20,000 MPa ·% or more, and strength-hole expansion balance. (TS × λ) was 40000 MPa ·% or more, which was excellent in stretch flangeability. Furthermore, there was no surface cracking during the slab production, and the secondary adhesion with salt-and-warm water after electrodeposition coating was also good.
Further, since the steel sheet of the present invention controls the surface roughness form, it has been confirmed that the steel sheet is excellent in slidability and press galling resistance during press forming.
[0056]
Example 2
Molten steel having the composition shown in Table 4 was melted in a converter and slab was formed by continuous casting. The obtained slab was hot-rolled to a sheet thickness of 3.0 mm, then pickled, and then cold-rolled into a cold-rolled steel sheet having a sheet thickness of 1.4 mm.
Next, these cold-rolled steel sheets were heated and held in the continuous annealing line under the conditions shown in Table 5, then cooled, and subjected to temper rolling at a rolling reduction of 0.5% to obtain products.
Table 5 also shows the results obtained by examining the microstructure and the steel sheet surface shape of the obtained steel sheet.
Table 6 shows the results of investigations on the occurrence of surface cracks during the production of the slab, the mechanical properties of the product sheet, and the secondary adhesion of salt and hot water.
[0057]
[Table 4]

[0058]
[Table 5]

[0059]
[Table 6]

[0060]
As shown in Tables 5 and 6, the invention examples satisfying the requirements of the present invention all show extremely good values of strength-elongation balance (TS × El) of 20,000 MPa ·% or more, and strength-hole expansion balance. (TS × λ) was 40000 MPa ·% or more, which was excellent in stretch flangeability. Furthermore, there was no surface cracking during the production of the slab, and the secondary adhesion with salt and hot water after electrodeposition coating was good.
Further, since the steel sheet of the present invention controls the surface roughness form, it has been confirmed that the steel sheet is excellent in slidability and press galling resistance during press forming.
[0061]
【The invention's effect】
Thus, according to the present invention, it is possible to manufacture a high-strength cold-rolled steel sheet having high ductility, stretch flangeability, and good salt-hot water secondary adhesion after electrodeposition coating inexpensively and stably. Can be.
Therefore, by applying the cold-rolled steel sheet of the present invention as a material for an automobile part, it is possible to reduce the weight and fuel consumption of the automobile, thereby contributing greatly to the improvement of the global environment.

Claims (7)

In mass%,
C: 0.05 to 0.35%,
Si: 0.5 to 3.0%,
Mn: 0.5-3.0%,
P: 0.05% or less,
S: 0.005% or less,
Al: less than 0.10%,
N: 0.0020 to 0.0100% and Ti: 0.001 to 0.20%
And Ca and / or REM: 0.0010 to 0.010%
, The balance being Fe and unavoidable impurities, and a composite structure comprising ferrite, retained austenite and a low-temperature transformation phase, wherein the ferrite has a volume fraction of 30% or more and the residual austenite has a volume fraction of It has a steel structure containing 2% or more, and has an average crystal grain size of the ferrite of 15 μm or less, and a concentration ratio P (Si) _0-10 / P (Si) _{100 of} Si and C represented by the following formula. ≦ 3.0
P (C) _0-10 / P (C) ₁₀₀ ≦ 3.0
here,
P (Si) _0-10 , P (C) _0-10 : Peak values of Si and C concentrations from the surface of the steel sheet to a depth of 10 nm in the thickness direction P (Si) ₁₀₀ , P (C) ₁₀₀ : From the surface of the steel sheet to the plate A high-strength, high-ductility, cold-rolled steel sheet having excellent secondary resistance to salt and hot water, which satisfies Si and C concentrations at a depth of 100 nm in the thickness direction. The steel sheet according to claim 1, wherein the steel sheet further comprises:
One or two selected from Nb and V: 0.001 to 0.30%
A high-strength, high-ductility cold-rolled steel sheet having excellent secondary adhesion to salt and hot water, characterized by having a composition containing: The steel sheet according to claim 1 or 2, further comprising:
Cr: 1.5% or less Mo: 0.5% or less and B: 0.010% or less The composition contains one or more selected from the group consisting of: High-strength, high-ductility cold-rolled steel sheet with excellent heat resistance. In mass%,
C: 0.05 to 0.35%,
Si: 0.5 to 3.0%,
Mn: 0.5-3.0%,
P: 0.05% or less,
S: 0.005% or less,
Al: less than 0.10%,
N: 0.0020 to 0.0100% and Ti: 0.001 to 0.20%
And Ca and / or REM: 0.0010 to 0.010%
, A steel slab having a balance of Fe and inevitable impurities is hot-rolled, then cold-rolled, and then heated to a temperature equal to or higher than the Ac1 transformation point by continuous annealing. After cooling to a temperature range of 300 to 500 ° C. at a cooling rate of 60 seconds or more and staying in this temperature range for 60 seconds or more, during or after cooling, electrolytic pickling or electrolytic pickling and brushing are performed. Enrichment ratio of Si and C represented by the following formula: P (Si) _0-10 / P (Si) ₁₀₀ ≦ 3.0
P (C) _0-10 / P (C) ₁₀₀ ≦ 3.0
here,
P (Si) _0-10 , P (C) _0-10 : Peak values of Si and C concentrations from the surface of the steel sheet to a depth of 10 nm in the thickness direction P (Si) ₁₀₀ , P (C) ₁₀₀ : From the surface of the steel sheet to the plate A method for producing a high-strength, high-ductility cold-rolled steel sheet having excellent secondary resistance to salt and hot water, characterized by adjusting the Si and C concentrations at a depth of 100 nm in the thickness direction. In claim 4, the abundance ratio _SiO 2 _/ Mn 2 SiO ₄ of SiO ₂ and _Mn 2 SiO ₄ of the steel plate surface layer of the electrolytic pickling process most recently, salt hot water secondary adhesiveness, characterized in that more than 1.5 For producing high-strength and high-ductility cold-rolled steel sheets with excellent heat resistance. The steel slab according to claim 4 or 5, further comprising:
One or two selected from Nb and V: 0.001 to 0.30%
A method for producing a high-strength and high-ductility cold-rolled steel sheet having excellent secondary resistance to salt and hot water, comprising: The steel slab according to claim 4, 5 or 6, further comprising:
Cr: 1.5% or less,
Mo: 0.5% or less and B: 0.010% or less. High strength and high ductility excellent in secondary adhesion to salt hot water, characterized in that the composition contains one or two or more kinds selected from 0.010% or less. Manufacturing method of cold rolled steel sheet.