JP3814720B2 - High strength and high ductility cold-rolled steel sheet excellent in salt hot water secondary adhesion and method for producing the same - Google Patents

Description

【００５１】
【表２】

【００５２】
【表３】

【００５３】
表２，３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−延びバランス（TS×El）が 20000 MPa・％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 40000 MPa・％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摩擦係数が非常に小さく、プレス成形性に優れていることも確認されている。
【００５４】
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、板厚：3.0 mmまで熱間圧延し、ついで酸洗後、冷間圧延により板厚：1.4 mm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表面性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。
【００５５】
【表４】

【００５６】
【表５】

【００５７】
【表６】

【００５８】
表５，６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−延びバランス（TS×El）が 20000 MPa・％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 40000 MPa・％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摩擦係数が非常に小さく、プレス成形性に優れていることも確認されている。
【００５９】
【発明の効果】
かくして、本発明によれば, 高延性でかつ、優れた伸びフランジ性を有する高強度冷延鋼板を、安価にしかも安定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]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength, high-ductility cold-rolled steel sheet having excellent salt hot water secondary adhesion after electrodeposition coating and a method for producing the same.
[0002]
[Prior art]
In recent years, there has been a demand for improvement in fuel efficiency of automobiles from the viewpoint of conservation of the global environment. In addition, from the viewpoint of occupant protection in the event of a collision, it is also required to improve the safety of the automobile body. Therefore, the weight reduction and reinforcement of the automobile body are being actively promoted.
It is said that increasing the strength of component materials is effective in achieving both weight reduction and strengthening of automobile bodies. Recently, high-strength steel sheets have been actively used for automotive parts. Yes.
[0003]
In addition, since many automobile parts made of steel plates are formed by press working, steel sheets for automobile parts are also required to have excellent press formability. In order to achieve excellent press formability, first of all, it is important to ensure high ductility. In press molding of automobile parts, stretch flange deformation is also frequently used. In particular, parts forming a member, reinforcement or the like, which is a skeleton member for securing the strength of an automobile body, are often molded using parts that are subjected to stretch flange deformation.
For this reason, the steel sheet for automobile parts is strongly required to have excellent ductility as well as high ductility.
[0004]
A typical example of a high-strength steel sheet having excellent ductility is a dual-phase steel sheet having a composite structure of ferrite and martensite.
In recent years, for example, as described in “Materials and Processes vol.4 (1991) P.1942” edited by the Japan Iron and Steel Institute, appropriate amounts of Si and Mn are added to 0.11 mass% C to obtain retained austenite. Highly ductile steel sheets using transformation induced plasticity (TRIP) due to the phenomenon have been put to practical use.
However, as described in “Iron and Steel vol.8 (1995) No.9, P.26” edited by the Japan Iron and Steel Association, such medium carbon steel is an Fe-C equilibrium diagram. Since the peritectic reaction (δ + L → γ) and the A ₄ transformation (δ → γ) occur near the solidus temperature above, solidification tends to be non-uniform. There was a problem that surface cracks such as transverse cracks were likely to occur.
[0005]
In addition, in the secondary cooling zone after casting, Al, Nb, V, Cu, etc., as described in “3rd edition Steel Handbook II Steelmaking and Steelmaking P.649” edited by Japan Iron and Steel Association Grain boundary embrittlement occurs due to carbides and nitrides generated by the addition of alloying elements, and surface cracking of slabs is likely to occur, so it is currently produced while taking measures such as low-speed casting and slow cooling It is.
Such manufacturing conditions greatly reduce the productivity in the steelmaking sector, which is the upstream process of the steel industry, and ultimately become a cause of hindering the productivity of the entire steelworks.
[0006]
Furthermore, a serious problem with this type of steel sheet is that it contains a large amount of Si, so that the oxide of Si is concentrated on the surface of the steel sheet during annealing, and the steel sheet after electrodeposition coating is inferior like salt warm water. When exposed to a different environment, the coating film is more easily peeled off than a normal steel plate.
[0007]
[Problems to be solved by the invention]
The present invention was developed in view of the above-mentioned actual situation, and by restricting the steel composition to a specific range, it solves the above-mentioned problems in the steel making stage, and in addition to the steel composition regulation described above, by controlling, suitable, high ductility to ensure excellent mechanical properties of high stretch flangeability, a high strength and high more excellent in salt tolerance heated secondary adhesion after electrodeposition coating as a material for automobile parts An object is to provide a ductile cold-rolled steel sheet together with its advantageous production method.
[0008]
[Means for Solving the Problems]
Now, in order to achieve the above-mentioned object, the inventors have conducted intensive research on the component composition, microstructure and surface properties of a steel plate which prevents surface cracking of the slab and optimizes the ductility and stretch flangeability of the steel plate. Piled up.
as a result,
(1) By adding appropriate amounts of Ti and N, it is possible to prevent preferential precipitation of AlN, Nb-based carbides and V-based carbides precipitated at low temperatures at the 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 low-temperature transformation phase, and the volume ratio of each phase is set to a predetermined ratio, resulting in excellent ductility in the steel sheet. Can be expressed,
(3) The inventors have obtained knowledge that excellent stretch flangeability can be obtained in addition to high ductility by refining the crystal grain size of ferrite, which is the main component of the composite structure. Also,
(4) It has also been found that the secondary adhesion of salt-warm water is greatly improved by controlling the oxide concentration on the steel sheet surface after annealing.
The present invention is based on the above findings.
[0009]
That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.05 to 0.35%
Si: 0.5-3.0%
Mn: 0.5-3.0%
P: 0.05% or less,
S: 0.005% or less,
Al: less than 0.10%,
N: 0.0020-0.0100% and
Ti: 0.001 to 0.20%
And including
Ca and / or REM: 0.0010 to 0.010%
And the balance is composed of Fe and inevitable impurities, and is a composite structure composed of ferrite, residual austenite and a low-temperature transformation phase, wherein the ferrite is 30% or more by volume and the residual austenite is by volume It has a steel structure containing 2% or more, and the average grain size of the ferrite is 15 μm or less, and the Si / Fe strength ratio Si / Fe is 6.0 or less by GDS analysis from the outermost surface layer of the steel sheet to a depth of 10 nm. A high-strength, high-ductility cold-rolled steel sheet having excellent salt-resistant warm water secondary adhesion.
[0010]
2. % By mass
C: 0.05 to 0.35%
Si: 0.5-3.0%
Mn: 0.5-3.0%
P: 0.05% or less,
S: 0.005% or less,
Al: less than 0.10%,
N: 0.0020-0.0100% and
Ti: 0.001 to 0.20%
And including
Ca and / or REM: 0.0010 to 0.010%
And the balance is composed of Fe and inevitable impurities, and is a composite structure composed of ferrite, residual austenite and a low-temperature transformation phase, wherein the ferrite is 30% or more by volume and the residual austenite is by volume It has a steel structure containing 2% or more, the average grain size of the ferrite is 15 μm or less, and the strength ratio of SiO ₂ and Mn ₂ SiO ₄ by FT-IR analysis in the surface layer of the steel sheet SiO ₂ / Mn ₂ SiO A high-strength, high-ductility cold-rolled steel sheet having excellent salt hot water secondary adhesion, wherein ₄ is 1.5 or less.
[0011]
3. In the above 1 or 2, the steel sheet is further in mass%,
One or two types selected from Nb and V: 0.001 to 0.30%
A high-strength, high-ductility cold-rolled steel sheet excellent in salt hot water secondary adhesion, characterized in that it contains a composition containing
[0012]
4). In the above 1, 2 or 3, the steel sheet is further in 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 excellent in salt hot water secondary adhesion, characterized by containing one or more kinds .
[0013]
5). % By mass
C: 0.05 to 0.35%
Si: 0.5-3.0%
Mn: 0.5-3.0%
P: 0.05% or less,
S: 0.005% or less,
Al: less than 0.10%,
N: 0.0020-0.0100% and
Ti: 0.001 to 0.20%
And including
Ca and / or REM: 0.0010 to 0.010%
A steel slab with a composition of Fe and the inevitable impurities is hot-rolled, cold-rolled, and then heated to a temperature equal to or higher than the Ac ₁ transformation point by continuous annealing, and then 10 ° C./s After cooling to the temperature range of 300-500 ℃ at the above cooling rate and staying in this temperature range for 60 seconds or more, pickling treatment and / or brushing treatment is performed during or after cooling, from the outermost layer of the steel plate A method for producing a high-strength, high-ductility cold-rolled steel sheet excellent in salt hot water secondary adhesion, characterized in that the Si / Fe strength ratio Si / Fe by GDS analysis up to 10 nm depth is 6.0 or less.
[0014]
6). % By mass
C: 0.05 to 0.35%
Si: 0.5-3.0%
Mn: 0.5-3.0%
P: 0.05% or less,
S: 0.005% or less,
Al: less than 0.10%,
N: 0.0020-0.0100% and
Ti: 0.001 to 0.20%
And including
Ca and / or REM: 0.0010 to 0.010%
A steel slab having a composition of Fe and inevitable impurities is hot-rolled and then cold-rolled, and then continuously annealed to obtain Ac under a condition where the atmospheric dew point in the furnace is −30 ° C. or higher. _When heated to a temperature above ₁ transformation point, SiO ₂ and Mn ₂ SiO ₄ by FT-IR analysis on the surface layer of the steel sheet Strength ratio of SiO ₂ / Mn ₂ SiO ₄ The temperature is reduced to 1.5 or less, then cooled to a temperature range of 300 to 500 ° C at a cooling rate of 10 ° C / s or more, and then retained in this temperature range for 60 seconds or more, followed by cooling. A method for producing a high-strength, high-ductility cold-rolled steel sheet having excellent adhesion.
[0015]
7). In the above 5 or 6, the steel slab is further in mass%,
One or two types selected from Nb and V: 0.001 to 0.30%
The manufacturing method of the high intensity | strength high ductility cold-rolled steel plate excellent in the salt hot water secondary adhesiveness characterized by the above-mentioned.
[0016]
8). In the above-mentioned 5, 6 or 7, the steel slab is further in 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 excellent in salt hot water secondary adhesion, characterized by containing one or more kinds Manufacturing method.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be specifically described below.
First, the reason why the component composition of the steel sheet is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.05-0.35%
C is an element indispensable for increasing the strength of steel, and is also an element effective for the formation of retained austenite and a low-temperature transformation phase. However, if the C content is less than 0.05%, the desired high strength cannot be obtained, while if it exceeds 0.35%, weldability is deteriorated. For this reason, C was limited to the range of 0.05 to 0.35%. More preferably, it is 0.10 to 0.25%.
[0018]
Si: 0.5-3.0%
Si not only contributes to strengthening steel by solid solution strengthening, but also stabilizes austenite and promotes the formation of residual austenite phase. However, if the content is less than 0.5%, the effect of addition is poor, while if it exceeds 3.0%, 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 solid solution strengthening, but also improves the hardenability of the steel and promotes the formation of retained austenite and low-temperature transformation phase. Such an effect is observed when the Mn content is 0.5% or more. On the other hand, if the content exceeds 3.0%, the effect reaches saturation, and an effect corresponding to the content cannot be expected, but rather the cost increases. 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: 0.05% or less P is a solid solution strengthening element, and is usually an element useful for obtaining a high-strength steel sheet. However, if containing over 0.05% causes a decrease in spot weldability, in the present invention, The upper limit was 0.05%. More preferably, it is 0.020% or less.
[0021]
S: 0.005% or less S is a harmful element that forms MnS in steel and lowers the stretch flangeability of the steel sheet. For this reason, mixing of S is preferably reduced as much as possible, but 0.005% or less is acceptable. More preferably, it is 0.003% or less.
[0022]
Al: Less than 0.10%
Al is an element necessary for separating nonmetallic inclusions in the slag, which contributes effectively as a deoxidizer in the steelmaking stage and lowers the hole expandability. However, the content of 0.10% or more increases the alloy cost, so in the present invention, the content is less than 0.10%. A preferred range is 0.02 to 0.09%.
[0023]
N: 0.0020-0.0100%
N is usually an impurity element that causes strain aging. However, in a structure-strengthened steel sheet as in the present invention, strain aging does not occur, but rather it is useful to prevent surface cracking of the slab by precipitates such as TiN. It is an element. In order to prevent such surface cracking of the slab, it is necessary to contain 0.0020% or more. However, it is difficult to make the steel contain more than 0.0100%, and the cost is high, so N is 0.0020. Limited to a range of ~ 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 remarkably improving the hole expandability. Ti also precipitates Ti-based carbonitrides and sulfides at high temperatures during slab cooling, and is produced at relatively low temperatures, and Nb and V added for the purpose of grain refinement are used to reach grain boundaries. It is an element that is also effective in suppressing the formation of Nb-based and V-based carbide precipitates and preventing slab surface cracks. In order to exhibit such an effect, the content of at least 0.001% is required. On the other hand, the content exceeding 0.20% not only increases the alloy cost, but also increases the precipitation amount of TiC, reduces the retained austenite for expressing the TRIP effect, and increases the precipitation strengthening ability. Therefore, there is a disadvantage that high ductility cannot be obtained. Therefore, in the present invention, the Ti content 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 to 0.010%
Ca and REM have the effect of controlling the form of sulfide inclusions, and thereby contribute effectively to the improvement of 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 addition is poor, whereas 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 preferable 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 types selected from Nb and V: 0.001 to 0.30%
Both Nb and V, like Ti, precipitate NbC and VC, suppress the growth of the ferrite phase in the heating stage during continuous annealing, refine the steel structure, and remarkably improve the hole expandability. It is a useful element. In order to exhibit such an effect, addition of at least 0.001% is required. On the other hand, if the content exceeds 0.30%, YS increases due to precipitation strengthening, and not only the workability decreases, but also the disadvantage that the retained austenite for expressing the TRIP effect decreases. Nb and V generate carbides at a lower temperature than Ti, and when this precipitates preferentially at the grain boundaries, it causes surface cracks in the slab. Therefore, in the present invention, Nb and V are contained in a range of 0.001 to 0.30% in both cases of single addition and composite addition. A more preferable 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 above effects are saturated when Cr is more than 1.5%, Mo is more than 0.5%, and B is more than 0.010%. Cause disadvantages. Therefore, Cr is 1.5% or less, Mo is 0.5% or less, and B is 0.010% or less. A more preferable range is 0.0005 to 1.0% in total of one or two selected from Cr, Mo, and B.
[0028]
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 have a composite structure composed of ferrite, retained austenite, and a low-temperature transformation phase.
Ferrite Ferrite is a soft phase containing no iron carbide, has high deformability, and improves the ductility of the 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 ferrite content is less than 30%, a significant ductility improvement effect cannot be expected. A more preferable amount of ferrite is 50% or more.
[0029]
Residual austenite Residual austenite has the effect of strain-induced transformation into martensite during processing, widely dispersing locally applied processing strain, and improving the ductility of the steel sheet. In the steel sheet of the present invention, such retained austenite is contained in a volume ratio of 2% or more. This is because if the amount of retained austenite is less than 2%, a significant improvement in ductility cannot be expected. A more preferable 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. In addition, since the generation of movable dislocation is accompanied at the time of transformation generation, it also has the effect of reducing the yield ratio of the steel sheet. In order to sufficiently obtain the above action, it is more 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 plate.
[0031]
In the present invention, the crystal grain size of the ferrite is also important.
That is, the refinement of crystal grains effectively contributes to the improvement of 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 if the average crystal grain size of ferrite exceeds 15 μm, a significant improvement in 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 the Si / Fe strength ratio Si / Fe is 6.0 or less by GDS analysis from the steel sheet outermost layer to a depth of 10 nm, or the FT-IR in the surface layer of the steel sheet. the SiO ₂ and Mn ₂ intensity ratio SiO ₂ / Mn ₂ SiO ₄ of SiO ₄ analysis is required to be 1.5 or less.
This is because when the strength ratio (Si / Fe) exceeds 6.0, the Si oxide on the steel sheet surface causes the electrodeposition coating to be exposed to a poor environment such as salt warm water. This is because the secondary adhesion of the coating film is significantly deteriorated. A more preferable range is Si / Fe ≦ 3.0.
[0033]
Si oxides generated on the steel sheet surface are mainly SiO ₂ and Mn ₂ SiO _4. Of these, Mn ₂ SiO ₄ has no adverse effect on chemical conversion treatment, and SiO ₂ is comparable to Mn ₂ SiO ₄ . When the amount is small, it has been found that good chemical conversion treatment is exhibited. When the generated oxide is analyzed by FT-IR, if the strength ratio SiO ₂ / Mn ₂ SiO ₄ between SiO ₂ and Mn ₂ SiO ₄ exceeds 1.5, the steel oxide film and the coating film are in a salt warm water environment. It was found that the oxidation-reduction reaction easily occurs and the secondary adhesion of the coating film is deteriorated.
Therefore, the strength ratio SiO ₂ / Mn ₂ SiO ₄ between SiO ₂ and Mn ₂ SiO ₄ when using FT-IR analysis is 1.5 or less, preferably 1.2 or less.
Here, SiO ₂ and Mn ₂ intensity ratio SiO ₂ / Mn ₂ SiO ₄ of the SiO ₄ are, Wave number The at FT-IR analysis: 1250 / cm appearing in vicinity SiO ₂ peak and Wave number: 1000 / cm vicinity The height ratio of the Mn ₂ SiO ₄ peak appearing in FIG.
[0034]
Incidentally, either the intensity ratio Si / Fe of Si and Fe by GDS analysis described above is 6.0 or less, or SiO ₂ and Mn ₂ intensity of SiO ₄ ratio SiO ₂ / Mn ₂ SiO ₄ by FT-IR analysis of 1.5 or less If one of the conditions is satisfied, the secondary adhesion of the coating can be secured, but the Si / Fe strength ratio Si / Fe is 6.0 or less and the SiO ₂ and Mn ₂ SiO ₄ strength ratio SiO ₂ / Mn ₂ SiO ₄ is 1.5. Needless to say, it is more preferable to satisfy both of the following conditions.
[0035]
In addition, since the depth region exceeding 10 nm from the outermost surface layer of the steel plate has no effect on the salt-resistant warm water secondary adhesion, in the present invention, the depth at which the strength ratio of Si and Fe should be measured by GDS analysis. The depth was 10 nm from the outermost layer of the steel plate. Note that the intensity ratio of SiO ₂ and Mn ₂ SiO ₄ by FT-IR analysis, measurement depth not dare matter.
[0036]
Next, the manufacturing method of the steel plate of this invention is demonstrated.
The molten steel prepared in the above preferred component composition is cast by a known method, hot-rolled and then cold-rolled according to a conventional method, then heated by continuous annealing, cooled, and during that time, a holding treatment is performed at a predetermined temperature. After that, cool it down to make a product.
[0037]
In the present invention, in the above manufacturing process, heating to a temperature of the Ac ₁ transformation point or higher during continuous annealing, quenching at a rate of 10 ° C./s or more during cooling, and a quenching end temperature of 300 to The strength ratio of Si and Fe by GDS analysis from the outermost surface of the steel sheet to the depth of 10 nm by applying a pickling treatment and / or a brush treatment during or after cooling for 500 seconds at a temperature of 500 ° C. By making Si / Fe 6.0 or less, or by setting the atmospheric dew point in the furnace to −30 ° C. or more in the above-described continuous annealing process, SiO ₂ and Mn by FT-IR analysis from the steel sheet outermost layer to a depth of 10 nm. It is important that the ₂ SiO ₄ strength ratio SiO ₂ / Mn ₂ SiO ₄ is 1.5 or less.
[0038]
Hereinafter, the reason why the manufacturing conditions are limited to the above range will be described.
Heating temperature: Ac ₁ transformation point or higher The heating temperature in continuous annealing must be higher than the Ac ₁ transformation point. The reason is that the retained austenite cannot be obtained without heating to the (α + γ) two-phase region, and the TRIP effect does not appear. Therefore, in the present invention, the lower limit of the heating temperature is set as the Ac ₁ transformation point. Note that when the heating temperature is higher than 850 ° C., the ferrite grain size increases and the stretch flangeability may be lowered. Therefore, the upper limit of the heating temperature is preferably about 850 ° C.
[0039]
Further, the dew point in the furnace in the Ac ₁ transformation point or more temperature region by a -30 ° C. or higher, the SiO ₂ / Mn ₂ SiO ₄ ratio mentioned above can be 1.5 or less. Ac is less than the furnace atmosphere dew point -30 ° C. in ₁ transformation point or more temperature range, the more the formation versus SiO ₂ of Mn ₂ SiO _4, the intensity ratio SiO ₂ / Mn ₂ SiO ₄ exceeds 1.5 . In the temperature range below the Ac ₁ transformation point, the amount of SiO ₂ produced is small, so there is no need to adjust the dew point of the furnace atmosphere.
When the Si / Fe strength ratio Si / Fe is set to 6.0 or less by pickling treatment or brush treatment, which will be described later, it is not always necessary to adjust the atmospheric dew point in the temperature range above the Ac ₁ transformation point.
[0040]
Cooling rate: 10 ° C / s or more The reason for setting the cooling rate to 10 ° C / s is that when the cooling rate is less than 10 ° C / s, the austenite phase transforms into pearlite or bainite and the residual austenite disappears. This is because the TRIP effect cannot be obtained. More preferably, the rate is 20 ° C./s or more.
[0041]
Rapid cooling end temperature: 300-500 ° C
The reason for setting the quenching end temperature to 300-500 ° C is that when it is rapidly cooled to a temperature below 300 ° C, the austenite phase is transformed into martensite, and at temperatures above 500 ° C, the austenite phase is mostly pearlite. Or it will be transformed into bainite and the TRIP effect cannot be expected. A more preferable temperature range is 350 to 450 ° C.
[0042]
Retention time: After quenching for 60 seconds or more, stay in the temperature range of 300 to 600 ° C for 60 seconds or more. Most of the retained austenite is martensite when it reaches the temperature range of less than 300 ° C in a short time of less than 60 seconds. This is because the TRIP effect does not appear during press working. On the other hand, an excessively long time is not preferable because a nose of bainite transformation is generated and retained austenite is reduced. A particularly preferable holding time is 60 to 1200 seconds.
[0043]
In order to improve the secondary adhesion of salt-resistant hot water during or after cooling, pickling treatment and / or brush treatment, Si oxide concentrated on the steel sheet surface is removed, and Si and Fe are analyzed by GDS analysis. It is essential that the strength ratio Si / Fe is 6.0 or less. For this purpose, pickling and / or brushing is performed during or after the cooling.
In order to reduce Si / Fe to 6.0 or less by pickling, for example, a method of pickling by immersing in a mixed solution of 50 ° C. (1% hydrochloric acid + 25% nitric acid) for 10 seconds or more can be used. Is not to be done.
In order to reduce Si / Fe to 6.0 or less by brushing, for example, while rotating a 300 mmφ brush roll having a core material embedded with # 240 SiC at a rotational speed of at least 1 rpm, a pressure of 29.4 N / It is preferable to press against the steel plate with a pressure of cm ² or more and rub the steel plate surface.
In addition, when the atmospheric dew point in the temperature range above the Ac ₁ transformation point is controlled to be −30 ° C. or higher during the above-described continuous annealing, such pickling treatment or brush treatment is not necessarily required.
[0044]
Further, after the continuous annealing as described above, it is advantageous to perform temper rolling for the purpose of shape correction or roughness adjustment. However, there is no problem in terms of material even without such temper rolling.
[0045]
【Example】
Example 1
Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab by continuous casting. The obtained slab was hot-rolled to a thickness of 3.0 mm, then pickled, and cold-rolled to obtain a cold-rolled steel plate having a thickness of 1.6 mm.
Next, these cold-rolled steel sheets were heated and held in a continuous annealing line under the conditions shown in Table 2 and then cooled, and after cooling, in a mixed liquid of 53 ° C. (0.6% HCl + 26% HNO ₃ ). Pickling treatment for 6-21 seconds or pickling treatment or brush treatment for 17 seconds in 5% HCl solution at 55 ° C, then water washing, drying, temper rolling at a rolling reduction of 0.5% The product.
The results of examining the microstructure and surface properties of the obtained steel sheet are also shown in Table 2.
Table 3 shows the results of investigations on the occurrence of surface cracks during slab production, the mechanical properties of the product plate, and the salt hot water secondary adhesion.
[0046]
About the crack of the slab, it judged visually and evaluated by the presence or absence of the crack.
Also, a sample for high-temperature tensile test is cut out from the slab, processed into a tensile test piece, solution treated at 1200 ° C for 10 minutes, cooled to 800 ° C at a cooling rate of 100 ° C / minute, and held for 20 minutes. After that, a tensile test was performed, and the following drawing ratio (%) = (D ₀ −D) / D ₀ × 100 based on the original section diameter (D ₀ ) and the fractured section diameter after tension (D).
Thus, the aperture ratio was obtained.
[0047]
The microstructure of the steel sheet was examined by observing a cross section in the rolling direction of the steel sheet with an optical microscope or a scanning electron microscope. Magnification: The area ratio of the ferrite phase present in a square area of 100 mm square arbitrarily set by image analysis was determined using a cross-sectional structure photograph of 1000 times, and the volume ratio of the corresponding phase was obtained.
The amount of retained austenite was obtained by polishing the steel plate to the center plane in the plate thickness direction and measuring the diffraction X-ray intensity at the plate thickness center plane. MoKα rays are used as incident X-rays, and the diffracted X-ray intensity ratios of the {111}, {200}, {220}, and {311} surfaces of the retained austenite phase are determined. Rate.
For the ferrite grain size, the crystal grain size was measured in accordance with the method specified in JIS Z 0552, and converted to an average crystal grain size.
[0048]
The mechanical properties of the steel sheet were measured by the following tensile test and hole expansion test.
Tensile properties were measured using JIS Z 2204 No. 5 test specimens taken in the direction perpendicular to the rolling direction from the steel sheet, in accordance with the methods specified in JIS Z 2241. Yield strength (YS), Tensile strength (TS), The elongation at break (El) and the yield elongation (YEl) were measured.
Stretch flangeability was evaluated by measuring the hole expansion rate (A) according to the method specified in JFS T 1001, and evaluating this value.
[0049]
For salt-resistant warm water secondary adhesion, after chemical conversion treatment was performed on a 150 mm x 75 mm test piece using a commercially available liquid, electrodeposition was applied to a thickness of 25 µm, and then with a cutter knife (length: 45mm, 3 pieces) / After cutting the test piece and immersing it in 5% NaCl, 55 ° C salt water for 240 hours, put a cellophane tape on the cut and measure the peel width after peeling. ,evaluated. Here, if the maximum peel width is 5.0 mm or less, it can be said that the salt hot water secondary adhesion is good.
[0050]
[Table 1]

[0051]
[Table 2]

[0052]
[Table 3]

[0053]
As shown in Tables 2 and 3, all of the invention examples that satisfy the requirements of the present invention showed a very good value of strength-elongation balance (TS × El) of 20000 MPa ·% or more, and strength-hole expansion. The balance (TS x λ) was 40000 MPa ·% or more, and it was excellent in stretch flangeability. Furthermore, there was no surface cracking during slab production, and the salt-resistant warm water secondary adhesion after electrodeposition coating was good.
Moreover, since the steel plate of this invention is controlling the surface roughness form, it has also been confirmed that a friction coefficient is very small and it is excellent in press formability.
[0054]
Example 2
Molten steel having the component composition shown in Table 4 was melted in a converter and made into a slab by continuous casting. The obtained slab was hot-rolled to a thickness of 3.0 mm, then pickled, and then cold-rolled to obtain a cold-rolled steel plate having a thickness of 1.4 mm.
Next, these cold-rolled steel sheets were heated and held in a continuous annealing line under the conditions shown in Table 5, then cooled, and subjected to temper rolling at a reduction ratio of 0.5% to obtain products.
The results of examining the microstructure and surface properties of the obtained steel sheet are also shown in Table 5.
Table 6 shows the results of investigations on the occurrence of surface cracks during slab production, the mechanical properties of the product plate, and the salt hot water secondary adhesion.
[0055]
[Table 4]

[0056]
[Table 5]

[0057]
[Table 6]

[0058]
As shown in Tables 5 and 6, all of the invention examples satisfying the requirements of the present invention showed a very good value of strength-elongation balance (TS × El) of 20000 MPa ·% or more, and strength-hole expansion. The balance (TS x λ) was 40000 MPa ·% or more, and it was excellent in stretch flangeability. Furthermore, there was no surface cracking during slab production, and the salt-resistant warm water secondary adhesion after electrodeposition coating was good.
Moreover, since the steel plate of this invention is controlling the surface roughness form, it has also been confirmed that a friction coefficient is very small and it is excellent in press formability.
[0059]
【The invention's effect】
Thus, according to the present invention, a high-strength cold-rolled steel sheet having high ductility and excellent stretch flangeability can be manufactured at low cost and stably.
Therefore, by applying the cold-rolled steel sheet of the present invention as a material for automobile parts, it becomes possible to reduce the weight and fuel consumption of the automobile, thereby contributing greatly to the improvement of the global environment.

Claims (8)

% By mass
C: 0.05 to 0.35%
Si: 0.5-3.0%
Mn: 0.5-3.0%
P: 0.05% or less,
S: 0.005% or less,
Al: less than 0.10%,
N: 0.0020-0.0100% and
Ti: 0.001 to 0.20%
And including
Ca and / or REM: 0.0010 to 0.010%
And the balance is composed of Fe and inevitable impurities, and is a composite structure composed of ferrite, residual austenite and a low-temperature transformation phase, wherein the ferrite is 30% or more by volume and the residual austenite is by volume It has a steel structure containing 2% or more, and the average grain size of the ferrite is 15 μm or less, and the Si / Fe strength ratio Si / Fe is 6.0 or less by GDS analysis from the outermost surface layer of the steel sheet to a depth of 10 nm. A high-strength, high-ductility cold-rolled steel sheet having excellent salt-resistant warm water secondary adhesion. % By mass
C: 0.05 to 0.35%
Si: 0.5-3.0%
Mn: 0.5-3.0%
P: 0.05% or less,
S: 0.005% or less,
Al: less than 0.10%,
N: 0.0020-0.0100% and
Ti: 0.001 to 0.20%
And including
Ca and / or REM: 0.0010 to 0.010%
And the balance is composed of Fe and inevitable impurities, and is a composite structure composed of ferrite, residual austenite and a low-temperature transformation phase, wherein the ferrite is 30% or more by volume and the residual austenite is by volume It has a steel structure containing 2% or more, the average grain size of the ferrite is 15 μm or less, and the strength ratio of SiO ₂ and Mn ₂ SiO ₄ by FT-IR analysis in the surface layer of the steel sheet SiO ₂ / Mn ₂ SiO A high-strength, high-ductility cold-rolled steel sheet having excellent salt hot water secondary adhesion, wherein ₄ is 1.5 or less. In Claim 1 or 2, a steel plate is further mass%,
One or two types selected from Nb and V: 0.001 to 0.30%
A high-strength, high-ductility cold-rolled steel sheet excellent in salt hot water secondary adhesion, characterized in that it contains a composition containing In Claim 1, 2 or 3, the steel sheet is further in 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 excellent in salt hot water secondary adhesion, characterized by containing one or more kinds . % By mass
C: 0.05 to 0.35%
Si: 0.5-3.0%
Mn: 0.5-3.0%
P: 0.05% or less,
S: 0.005% or less,
Al: less than 0.10%,
N: 0.0020-0.0100% and
Ti: 0.001 to 0.20%
And including
Ca and / or REM: 0.0010 to 0.010%
A steel slab with a composition of Fe and the inevitable impurities is hot-rolled, cold-rolled, and then heated to a temperature equal to or higher than the Ac ₁ transformation point by continuous annealing, and then 10 ° C./s After cooling to the temperature range of 300-500 ℃ at the above cooling rate and staying in this temperature range for 60 seconds or more, pickling treatment and / or brushing treatment is performed during or after cooling, from the outermost layer of the steel plate A method for producing a high-strength, high-ductility cold-rolled steel sheet excellent in salt hot water secondary adhesion, characterized in that the Si / Fe strength ratio Si / Fe by GDS analysis up to 10 nm depth is 6.0 or less. % By mass
C: 0.05 to 0.35%
Si: 0.5-3.0%
Mn: 0.5-3.0%
P: 0.05% or less,
S: 0.005% or less,
Al: less than 0.10%,
N: 0.0020-0.0100% and
Ti: 0.001 to 0.20%
And including
Ca and / or REM: 0.0010 to 0.010%
A steel slab having a composition of Fe and inevitable impurities is hot-rolled and then cold-rolled, and then continuously annealed to obtain Ac under a condition where the atmospheric dew point in the furnace is −30 ° C. or higher. _When heated to a temperature above ₁ transformation point, SiO ₂ and Mn ₂ SiO ₄ by FT-IR analysis on the surface layer of the steel sheet Strength ratio of SiO ₂ / Mn ₂ SiO ₄ The temperature is reduced to 1.5 or less, then cooled to a temperature range of 300 to 500 ° C at a cooling rate of 10 ° C / s or more, and then retained in this temperature range for 60 seconds or more, followed by cooling. A method for producing a high-strength, high-ductility cold-rolled steel sheet having excellent adhesion. In Claim 5 or 6, steel slab is further mass%,
One or two types selected from Nb and V: 0.001 to 0.30%
The manufacturing method of the high intensity | strength high ductility cold-rolled steel plate excellent in the salt hot water secondary adhesiveness characterized by the above-mentioned. In Claim 5, 6 or 7, steel slab is further 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 excellent in salt hot water secondary adhesion, characterized by containing one or more kinds Manufacturing method.