JP4010131B2 - Composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability and manufacturing method thereof - Google Patents

Description

【００５６】
【表２】

【００５７】
表２に示す結果から、本発明例は、いずれも、低い降伏応力（YS）、高い伸び（El）および低い降伏比（YR）を有し、さらに高いｒ値を示して、深絞り成形性に優れるとともに、引張り強さが（TS）が440MPa以上の高張力を有している。これに対し、本発明の範囲を外れる比較例では、降伏応力（YS）が高いか、伸び（El）が低いか、あるいはｒ値が低くなっている。
【００５８】
【発明の効果】
本発明によれば、優れた深絞り成形性を有する高張力の冷延鋼板を安定して製造することができるという産業上格段の効果を奏する。本発明の冷延鋼板を自動車部品に適用した場合、プレス成形が容易で、自動車車体の軽量化に十分に寄与できるという効果もある。
【図面の簡単な説明】
【図１】 鋼スラブ中のＶおよびＣ含有量がr値と降伏比（＝降伏応力（YS）/引張り強さ（TS）×100（％））に及ぼす影響を示すための図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability, which is useful for use in an automotive steel sheet or the like, in which the tensile strength of the steel sheet is 440 MPa or more, and a method for producing the same.
[0002]
[Prior art]
In recent years, reducing the weight of automobile bodies has become an extremely important issue in connection with exhaust gas regulations due to global environmental conservation issues. Recently, the strength of automobile steel sheets has been increased to reduce the weight of automobile bodies. Therefore, it has been studied to reduce the plate thickness.
[0003]
Since many automotive body parts made of steel sheets are formed by press working, the cold-rolled steel sheets used are required to have excellent press formability. In order to improve the press formability, as the mechanical properties of the steel sheet, high Rankford value (r value), high ductility (El), and low yield stress (YS) are required.
However, generally, when the strength of a steel plate is increased, the r value and ductility are lowered, the press formability is deteriorated, the yield stress is increased, the shape freezing property is deteriorated, and the problem of springback is likely to occur.
[0004]
As a typical example of a high-tensile steel plate having good press formability, a composite steel plate composed of a composite structure of soft ferrite and hard martensite can be mentioned, and in particular, a composite steel plate manufactured by gas jet cooling after continuous annealing, It has a low yield stress and combines high ductility and excellent bake hardenability. Although the above-mentioned composite structure steel plate is generally good in workability, it has a disadvantage that workability under severe conditions, particularly, r value is low and deep drawability is inferior.
[0005]
For this reason, attempts have been made to improve the deep drawability by increasing the r value of the composite structure steel plate. For example, in Japanese Patent Publication 55-10650 and JP after cold rolling, subjected to box annealing at a temperature of recrystallization temperature to A _c3 transformation point, then, after heating to 700 to 800 ° C. for a composite structure, the quenching and tempering A technique for performing continuous annealing is disclosed. However, in this method, since quenching and tempering is performed during continuous annealing, the yield stress is high and a low yield ratio cannot be obtained. This high yield stress steel sheet is not suitable for press forming and has the disadvantages that the shape freezing property of the pressed part is poor.
[0006]
Furthermore, a method for improving the high yield stress is disclosed in JP-A-55-100934. In this method, box annealing is first performed in order to obtain a high r value. The temperature during box annealing is set to a two-phase region of ferrite (α) -austenite (γ), and Mn is changed from α phase to γ phase during soaking. Thicken. This Mn-concentrated phase preferentially becomes a γ phase during continuous annealing, and a mixed structure can be obtained even at a cooling rate comparable to that of a gas jet, and the yield stress is low. However, this method requires a relatively high temperature and long-time box annealing in the α-γ two-phase region for Mn concentration. Therefore, frequent adhesion between steel plates, generation of temper color, and furnace body inner cover There are a number of problems in the manufacturing process, such as a reduction in the service life. Conventionally, it has been difficult to industrially stably produce a high-tensile steel plate having such a high r value and a low yield stress.
[0007]
In addition, in Japanese Patent Publication No. 1-35900, after cold rolling a steel having a composition of 0.012 mass% C-0.32 mass% Si-0.53 mass% Mn-0.03 mass% P-0.051 mass% Ti, After heating to 870 ° C, which is a two-phase region, and cooling at an average cooling rate of 100 ° C / s, a composite with a very high r value and low yield stress of r = 1.61, YS = 224 MPa, TS = 482 MPa A technique that enables the production of a textured cold-rolled steel sheet is disclosed. However, since a high cooling rate of 100 ° C./s cannot be achieved by ordinary gas jet cooling, water quenching equipment is required, and water-quenched cold-rolled steel sheet has a problem of surface treatment, There are problems with manufacturing equipment and materials.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to propose a composite structure type high-tensile cold-rolled steel sheet having a high r value and excellent deep drawability, and a method for producing the same, which have advantageously solved the above problems.
[0009]
[Means for Solving the Problems]
In order to achieve the above-mentioned problems, the present inventors have conducted extensive research on the influence of alloying elements on the microstructure and recrystallization texture of cold-rolled steel sheets. As a result, C in the steel slab is limited to a low content. In addition, by optimizing the V content in relation to the C content, before recrystallization annealing, C in the steel is precipitated as V-based carbides to reduce the solid solution C as much as possible. } A high r value can be obtained by developing a recrystallized texture, and by subsequently heating to a two-phase region of α-γ, V-type carbides are dissolved and C is concentrated in austenite. As a result, it was found that martensite is easily generated in the subsequent cooling process, and as a result, a high-strength cold rolled steel sheet having a high r value and excellent deep drawability can be stably produced.
[0010]
First, basic experimental results performed by the present inventors will be described.
Based on mass%, C: 0.03%, Si: 0.02%, Mn: 1.7%, P: 0.01%, S: 0.005%, Al: 0.04%, N: 0.002%, Mo: 0.15% By adding V in the range of 0.03 to 0.55 mass%, various sheet bars having different V contents are heated to 1250 ° C, soaked, and then subjected to 3-pass rolling so that the finish rolling finish temperature is 900 ° C. To obtain a plate thickness of 4.0 mm. In addition, after finishing rolling, the heat | fever equivalent process of 650 degreeC x 1h was performed as coil winding-up process. Subsequently, cold rolling with a rolling reduction of 70% was performed to a sheet thickness of 1.2 mm. Subsequently, these cold-rolled sheets were subjected to recrystallization annealing at 850 ° C. for 60 s and then cooled at a cooling rate of 30 ° C./s.
[0011]
About the obtained cold-rolled steel plate, the tensile test was implemented and the tensile characteristic was investigated. The tensile test was performed using a JIS No. 5 tensile test piece. The r value is the average r value in the rolling direction (r _L ), 45 ° direction (r _D ) in the rolling direction and 90 ° direction (r _c ) in the rolling direction {= (r _L + r _c + 2 × r _D ) / 4}.
[0012]
Figure 1 shows the effect of V content in steel slabs on r-value and yield ratio (= yield stress (YS) / tensile strength (TS) x 100 (%)). Is the atomic ratio of V content to C content ((V / 51) / (C / 12)), and the vertical axis shows the r value and the yield ratio divided up and down.
From FIG. 1, by limiting the V content in the steel slab to an atomic ratio with C in the range of 0.5 to 3.0, a high r value and a low yield ratio can be obtained, and a composite structure type excellent in deep drawability. It became clear that high-tensile cold-rolled steel sheets can be manufactured.
[0013]
In the cold-rolled steel sheet of the present invention, a high r value is obtained because the solid solution C and N are small before the recrystallization annealing and the {111} recrystallization texture develops strongly. On the other hand, by annealing in the α-γ two-phase region, V carbide dissolves and solute C concentrates abundantly in the austenite phase, so that austenite is easily transformed into martensite in the subsequent cooling process. It was clarified that a composite structure of ferrite and martensite can be obtained.
[0014]
Here, conventionally, Ti and Nb have been mainly used as carbide-forming elements. However, in order to obtain solid solution C effectively by annealing in a high temperature region, the present inventors have a solubility of carbides more than that of Ti and Nb. We also focused on high V. That is, V carbide is more easily dissolved during high-temperature annealing than Ti carbide and Nb carbide. As a result, annealing in the two-phase region of α-γ is sufficient to dissolve austenite into martensite. I found out that In addition, it has also been clarified that this phenomenon can be obtained by adding Nb and Ti in a similar manner, although the V component is most prominent.
[0015]
The present invention has been completed by further study based on the above-described findings, and the gist of the present invention is as follows.
(1) C: 0.01 to 0.08% by mass, Si: 2.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.005-0.20%, N: 0.004 % or less and V: 0.01 to 0.5%, and V and C are
0.5 × C / 12 ≦ V / 51 ≦ 3 × C / 12
The balance has a composition consisting of Fe and inevitable impurities, and the structure is composed of a ferrite phase as a first phase and a second phase containing a martensite phase with an area ratio of 1% or more with respect to the entire structure. A composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability.
[0016]
(2) By mass%, C: 0.01 to 0.08%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.005-0.20%, N: 0.004 % or less and V: 0.01 to 0.5%, Nb: 0.001 to 0.3% and Ti: 0.001 to 0.3%, or a total of 0.3% or less, and V, Nb, Ti and C And
0.5 × C / 12 ≦ (V / 51 + 2 × Nb / 93 + 2 × Ti / 48) ≦ 3 × C / 12
The balance has a composition consisting of Fe and inevitable impurities, and the structure is composed of a ferrite phase as a first phase and a second phase containing a martensite phase with an area ratio of 1% or more with respect to the entire structure. A composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability.
[0017]
(3) In addition to the above composition, the depth described in (1) or (2) above, further comprising, in mass%, one group or two groups of the following groups A and B: Composite structure type high-tensile cold-rolled steel sheet with excellent drawability.
Group A: 2.0% by mass or less of Cr Group B: 2.0% by mass or less of one or two of Cu and Ni in total
(4) By mass%: C: 0.01 to 0.08%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.005 to 0.20%, N: 0.004 % or less and V: 0.01 to 0.5%, and V and C are
0.5 × C / 12 ≦ V / 51 ≦ 3 × C / 12
The steel slab with the composition of Fe and the inevitable impurities in the balance is hot-rolled, subsequently pickled, then cold-rolled, and then continuously annealed in the temperature range of Ac1 to Ac3 transformation points. A method for producing a composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability.
[0019]
(5) C: 0.01 to 0.08% by mass, Si: 2.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.005-0.20%, N: 0.004 % or less and V: 0.01 to 0.5%, Nb: 0.001 to 0.3% and Ti: 0.001 to 0.3%, or a total of 0.3% or less, and V, Nb, Ti and C And
0.5 × C / 12 ≦ (V / 51 + 2 × Nb / 93 + 2 × Ti / 48) ≦ 3 × C / 12
The steel slab with the composition of Fe and the inevitable impurities in the balance is hot-rolled, subsequently pickled, then cold-rolled, and then continuously annealed in the temperature range of Ac1 to Ac3 transformation points. A method for producing a composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability.
[0020]
(6) Steel slabs, in addition to the above composition, by mass%, the (4), characterized in that it contains 1 group or 2 group of the group A and group B shown below, or (5) The manufacturing method of the composite structure type | mold high-tensile cold-rolled steel plate excellent in the deep drawability as described in 2.
Group A: 2.0% by mass or less of Cr Group B: 2.0% by mass or less of one or two of Cu and Ni in total
DETAILED DESCRIPTION OF THE INVENTION
The cold-rolled steel sheet of the present invention is a composite structure type high-tensile cold-rolled steel sheet having a tensile strength (TS) of 440 MPa or more and excellent deep drawability.
First, the reason for limiting the composition of the cold-rolled steel sheet of the present invention will be described. The mass% is simply written as%.
[0022]
C: 0.01-0.08%
C is an element that increases the strength of the steel sheet and further promotes the formation of a composite structure of ferrite and martensite. In the present invention, it is necessary to contain 0.01% or more, more preferably 0.015% or more from the viewpoint of forming the composite structure. is there. Note that C is also preferably set to 0.015% or more when aiming at increasing the strength of TS: 540 MPa or more. On the other hand, the content exceeding 0.08% inhibits the development of {111} recrystallized texture and lowers the deep drawability. For this reason, in this invention, C content was limited to 0.01 to 0.08%. From the viewpoint of deep drawability, it is preferably 0.05% or less.
[0023]
Si: 2.0% or less
Si is a useful strengthening element that can increase the strength of the steel sheet without significantly reducing the ductility of the steel sheet, but when its content exceeds 2.0%, it causes deterioration of deep drawability, The surface properties deteriorate. For this reason, Si was limited to 2.0% or less.
[0024]
Mn: 3.0% or less
Mn has the effect of strengthening steel, and further has the effect of reducing the critical cooling rate at which a composite structure of ferrite and martensite is obtained, and promoting the formation of a composite structure of ferrite and martensite. It is preferable to contain the quantity according to the cooling rate after crystal annealing. Further, since Mn is also an effective element for preventing aging cracks due to S, it is preferable to contain an appropriate amount according to the amount of S contained. However, if the Mn content exceeds 3.0%, the deep drawability and weldability deteriorate. For this reason, in the present invention, the Mn content is limited to 3.0% or less. The Mn content is preferably 0.5% or more in order to significantly exhibit the above effects, and more preferably 1.0% or more.
[0025]
P: 0.10% or less P has an effect of strengthening steel and can be contained in a necessary amount depending on a desired strength. However, if the P content exceeds 0.10%, press formability deteriorates. For this reason, P content was limited to 0.10% or less. In addition, when more excellent press formability is required, the P content is preferably 0.08% or less.
[0026]
S: 0.02% or less S is an element present in the steel sheet as an inclusion, and is an element that causes deterioration of the ductility and formability of the steel sheet, particularly stretch flangeability. Therefore, it is preferable to reduce it as much as possible, and reduce it to 0.02% or less. Then, since it does not have much adverse effects, the upper limit of the S content is 0.02% in the present invention. When more excellent stretch flange formability is required, the S content is preferably 0.01% or less, more preferably 0.005% or less.
[0027]
Al: 0.005-0.20%
Al is added as a deoxidizing element for steel, and is an element useful for improving the cleanliness of steel. However, if it is less than 0.005%, there is no effect of addition, while it is more than 0.20%. A further deoxidation effect cannot be obtained, and conversely the deep drawability deteriorates. For this reason, Al was limited to 0.005 to 0.20%. In the present invention, it does not exclude a melting method by a deoxidation method other than Al deoxidation. For example, Ti deoxidation or Si deoxidation may be performed. Included in the range. At that time, even if Ca, REM, etc. are added to the molten steel, the characteristics of the steel sheet of the present invention are not inhibited at all, and it is a matter of course that a steel sheet containing Ca, REM, etc. is also included in the scope of the present invention.
[0028]
N: 0.004 % or less N is an element that increases the strength of the steel sheet by solid solution strengthening or strain age hardening, but if it exceeds 0.004 %, nitride increases in the steel sheet, thereby deep drawing the steel sheet. Remarkably deteriorates. For this reason, N was limited to 0.004 % or less.
[0029]
V: 0.01 to 0.5% and 0.5 × C / 12 ≦ V / 51 ≦ 3 × C / 12
V is the most important element in the present invention. By precipitating and fixing solute C as V carbide before recrystallization, {111} recrystallized texture can be developed to obtain a high r value. Furthermore, during the two-phase annealing of α-γ, V carbides are dissolved to concentrate a large amount of solid solution C into the austenite phase, and are easily martensitic transformed in the subsequent cooling process, so that ferrite and martensite A composite steel sheet having a composite structure can be obtained. Such an effect is effective when the V content is 0.01% or more, more preferably 0.02% or more and 0.5 × C / 12 ≦ V / 51 in relation to the C content. On the other hand, when V content exceeds 0.5% or V / 51> 3 × C / 12 in relation to C content, dissolution of V carbide in the α-γ two-phase region is difficult to occur. It becomes difficult to obtain a composite structure of ferrite and martensite. Therefore, the V content was 0.01 to 0.5% and was limited to 0.5 × C / 12 ≦ V / 51 ≦ 3 × C / 12. V / 51 ≦ 2 × C / 12 is preferable for obtaining a composite structure of ferrite and martensite.
[0030]
Further, in the present invention, in addition to the above-described composition, in terms of mass%, one or two of Nb: 0.001 to 0.3% and Ti: 0.001 to 0.3% are contained in total of 0.3% or less, and V Instead of satisfying 0.5 × C / 12 ≦ V / 51 ≦ 3 × C / 12, the content and C content, each content of V, Nb, Ti and the content of C,
It is preferable to satisfy the relationship of 0.5 × C / 12 ≦ (V / 51 + 2 × Nb / 93 + 2 × Ti / 48) ≦ 3 × C / 12.
[0031]
One or two of Nb: 0.001 to 0.3% and Ti: 0.001 to 0.3% are contained in a total of 0.3% or less, and V, Nb, Ti and C are 0.5 × C / 12 ≦ (V / 51 + 2 × Nb / 93 + 2 × Ti / 48) ≦ 3 × C / 12
Nb and Ti are carbide-forming elements like V and have the same action as V described above. That is, by precipitating and fixing solute C as Nb and Ti carbide before recrystallization, {111} recrystallized texture can be developed and high r value can be obtained. During annealing, the Nb and Ti carbides are dissolved to concentrate a large amount of solid solution C into the austenite phase, and the martensite transformation is performed in the subsequent cooling process, thereby producing a composite steel sheet having a composite structure of ferrite and martensite. Obtainable. However, since the above-described effects of Nb and Ti are considerably smaller than V, the deep drawability, which is the effect of the present invention, can be achieved by adding only Nb and Ti without adding V to the steel slab. It cannot be raised sufficiently.
[0032]
Specifically, the Nb and Ti contents are 0.001% or more and 0.5 × C / 12 ≦ (V / 51 + 2 × Nb / 93 + 2 × Ti / 48) in relation to the C and V contents. It is preferable for exhibiting the effect. On the other hand, the total addition of Nb and Ti exceeds 0.3%, or the relationship between C and V content (V / 51 + 2 × Nb / 93 + 2 × Ti / 48)> 3 × C / 12 In this case, since it is difficult for the carbide to dissolve in the α-γ two-phase region, it becomes difficult to obtain a composite structure of ferrite and martensite. Therefore, when only one of Nb and Ti is added, both are in the range of 0.001 to 0.3%, and when Nb and Ti are added in combination, the total is 0.3% or less, and V and In relation to C, the range is limited to 0.5 × C / 12 ≦ (V / 51 + 2 × Nb / 93 + 2 × Ti / 48) ≦ 3 × C / 12.
[0033]
In addition, in the present invention, in addition to the composition of the steel slab described above, the following group A and group B, that is,
Group A: Cr is 2.0% or less Group B: One or two of Cu and Ni are preferably contained in one or two of 2.0% or less in total.
[0034]
Group A: Cr 2.0% or less Group A: Cr, like Mn, the critical cooling rate composite structure of ferrite and martensite is obtained by reducing the effect of promoting formation of a composite structure of ferrite and martensite It can be contained if necessary. The lower limit of the content of the preferred Cr for obtaining the above effects, Cr: 0.05%. However, when the content exceeds Cr 2.0%, deep drawability is deteriorated. For this reason, it is preferable to limit A group: Cr to 2.0% or less.
[0035]
Group B: 2.0% or less of one or two of Cu and Ni in total B group: Cu and Ni have an action of strengthening steel and can be contained in a necessary amount according to desired strength. When Cu and Ni are added alone or in total, the combined addition exceeds 2.0%, the deep drawability tends to deteriorate. For this reason, Cu or Ni is preferably 2.0% or less in total of one or two of them. In addition, the minimum value of preferable Cu and Ni content for acquiring the said effect is Cu: 0.05%, Ni: 0.05%.
[0036]
In the present invention, the components other than those described above are not particularly limited, but there is no problem even if B, Ca, Zr, REM, etc. are contained within the range of the normal steel composition.
[0037]
The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include, for example, Sb, Sn, Zn, Co, etc. The allowable ranges of these contents are Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: 0.1 % Or less.
[0038]
Next, the structure of the steel sheet of the present invention will be described. The cold-rolled steel sheet of the present invention has a structure composed of a ferrite phase that is a first phase and a second phase that includes a martensite phase of 1% or more in terms of the area ratio relative to the entire structure.
[0039]
In order to obtain a cold-rolled steel sheet having low yield stress (YS) and high ductility (El) and having excellent deep drawability, in the present invention, the structure of the steel sheet is composed of a ferrite phase as a first phase and martensite. It is necessary to have a composite structure with the second phase including the phase. The ferrite phase as the first phase is preferably 80% or more in terms of area ratio. This is because if the ferrite phase is less than 80% in area ratio, it is difficult to ensure high ductility and the press formability tends to decrease. Further, when better ductility is required, the ferrite phase is preferably 85% or more by area ratio. In order to take advantage of the composite structure, the ferrite phase needs to be 99% or less.
[0040]
Moreover, as a 2nd phase, in this invention, it is necessary to contain a martensite phase 1% or more by the area ratio with respect to the whole structure | tissue. If the martensite is less than 1% in area ratio, low yield stress (YS) and high ductility (El) cannot be satisfied at the same time. More preferably, the martensite phase is 3% or more by area ratio. The second phase may be composed of a martensite phase with an area ratio of 1% or more alone, or a martensite phase with an area ratio of 1% or more and other pearlite, bainite, and retained austenite phases as subphases. It may be mixed with either, and is not particularly limited. However, these pearlite phase, bainite phase, and residual austenite phase make the martensite phase more effective, so the total of these phases is 50% or less in terms of area ratio with respect to the structure of the second phase. Is preferred.
The cold-rolled steel sheet having the above-described structure is a steel sheet excellent in deep drawability having low yield stress and high ductility.
[0041]
Next, the manufacturing method of the cold rolled steel sheet of this invention is demonstrated.
Since the composition of the steel slab used in the production method of the present invention is the same as that of the above-described cold-rolled steel sheet, description of the reason for limiting the steel slab is omitted.
The cold-rolled steel sheet of the present invention comprises a steel slab having a composition within the above range as a raw material, hot-rolling the hot-rolled sheet by subjecting the raw material to hot rolling, and pickling the hot-rolled sheet. By sequentially performing a process, a cold rolling process in which the hot-rolled sheet is cold-rolled to form a cold-rolled sheet, and a recrystallization annealing process in which the cold-rolled sheet is subjected to recrystallization annealing to form a cold-rolled annealed sheet Manufactured.
[0042]
The steel slab to be used is preferably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot casting method or a thin slab casting method. After manufacturing the steel slab, in addition to the conventional method of cooling to room temperature and then heating it again, without cooling, the method of inserting it into a heating furnace as it is, or after performing a slight heat retention Energy-saving processes such as direct feed rolling and direct rolling, which are immediately rolled, can be applied without problems.
[0043]
The above-mentioned raw material (steel slab) is heated and subjected to a hot rolling step of hot rolling to obtain a hot rolled sheet. The hot rolling process only needs to be a condition that enables the production of a hot rolled sheet having a desired thickness, and there is no particular problem even if normal rolling conditions are used. For reference, suitable hot rolling conditions are shown below.
[0044]
Slab heating temperature: 900 ° C. or higher The slab heating temperature is preferably low because the precipitates are coarsened to develop {111} recrystallized texture and improve deep drawability. However, if the heating temperature is less than 900 ° C., the rolling load increases and the risk of trouble occurring during hot rolling increases. For this reason, it is preferable that slab heating temperature shall be 900 degreeC or more. In addition, the upper limit of the slab heating temperature is more preferably 1300 ° C. due to a decrease in yield due to an increase in scale loss accompanying an increase in oxidized weight. Needless to say, using a so-called sheet bar heater that heats the sheet bar from the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling is of course effective.
[0045]
Finish rolling end temperature: 700 ° C or higher Finish rolling end temperature (FDT) should be 700 ° C or higher to obtain a uniform hot-rolled base metal structure that provides excellent deep drawability after cold rolling and recrystallization annealing. Is preferred. In other words, if the finish rolling finish temperature is less than 700 ° C, the hot rolled base metal structure becomes non-uniform, the rolling load during hot rolling increases, and the risk of trouble occurring during hot rolling increases. is there.
[0046]
Winding temperature: 800 ° C. or lower The winding temperature is preferably 800 ° C. or lower. That is, when the coiling temperature exceeds 800 ° C., the scale increases and the yield tends to decrease due to scale loss. When the coiling temperature is less than 200 ° C., the shape of the steel sheet is significantly disturbed and the risk of causing problems in actual use increases. Therefore, the lower limit of the coiling temperature is more preferably 200 ° C.
[0047]
Thus, in the hot rolling process of the present invention, after the steel slab is heated to 900 ° C. or higher, the finish rolling finish temperature is hot rolled to 700 ° C. or higher and wound at a winding temperature of 800 ° C. or lower. Is preferred.
In the hot rolling process of the present invention, in order to reduce the rolling load during hot rolling, lubrication rolling may be performed between some or all passes of finish rolling. In addition, performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.10-0.25.
[0048]
Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.
[0049]
Next, the hot-rolled sheet is pickled to remove scale. The pickling step may be performed in accordance with a conventional method, and it is preferable to use, for example, hydrochloric acid or sulfuric acid-based processing solution as the pickling solution. Further, the hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet. The cold rolling condition is not particularly limited as long as it can be a cold-rolled sheet having a desired dimension and shape, but the rolling reduction during cold rolling is preferably 40% or more. This is because if the rolling reduction is less than 40%, the {111} recrystallized texture does not develop and an excellent deep drawability cannot be obtained.
[0050]
Thereafter, in the recrystallization annealing step, the cold rolled sheet is subjected to recrystallization annealing to obtain a cold rolled annealed sheet. Recrystallization annealing is performed in a continuous annealing line. The annealing temperature for recrystallization annealing needs to be performed in a two-phase region of (α + γ) in the temperature range of the A _{c1 to} A _c3 transformation points. This is because if the annealing temperature is lower than the A _c1 transformation point, a ferrite single-phase structure is formed and martensite cannot be generated. On the other hand, if the annealing temperature is higher than the A _c3 transformation point, the crystal grains become coarse. At the same time, it becomes an austenite single-phase region, and the {111} recrystallization texture does not develop and the deep drawability deteriorates significantly.
[0051]
The cooling during the recrystallization annealing is preferably performed at a cooling rate of 5 ° C./s or more in order to generate martensite and to obtain a composite structure of ferrite and martensite.
[0052]
In addition, after the recrystallization annealing step, temper rolling with an elongation of 10% or less may be added for adjustment of shape correction, surface roughness, and the like.
The cold-rolled steel sheet of the present invention can be applied not only as a cold-rolled steel sheet for processing but also as an original sheet of a surface-treated steel sheet for processing. Examples of the surface-treated steel sheet for processing include galvanized steel sheets (including alloy systems), tin-plated steel sheets, enamels and the like. Further, the cold-rolled steel sheet of the present invention may be subjected to a special treatment for improving chemical conversion treatment properties, weldability, press formability, corrosion resistance, and the like after galvanization.
[0053]
【Example】
Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. Next, after these steel slabs are heated to 1150 ° C, a hot-rolled steel strip with a thickness of 4.0 mm (hot) is applied by hot rolling at a finish rolling finish temperature of 900 ° C and a winding temperature of 650 ° C. Sheet). Subsequently, these hot-rolled steel strips (hot-rolled plates) were pickled and cold-rolled by cold rolling at a reduction ratio of 70% to obtain cold-rolled steel strips (cold-rolled plates) having a thickness of 1.2 mm. Then, these cold-rolled steel strips (cold-rolled sheets) were subjected to recrystallization annealing at the annealing temperatures shown in Table 2 in a continuous annealing line. The obtained steel strip (cold rolled sheet) was further subjected to temper rolling with an elongation of 0.8%.
[0054]
A specimen is collected from the obtained steel strip, and a microscopic structure is imaged using an optical microscope or a scanning electron microscope with respect to a cross section (C cross section) orthogonal to the rolling direction, and the first phase is used using an image analyzer. The structure fraction of ferrite and the type and structure fraction of the second phase were determined. In addition, JIS No. 5 tensile test specimens were collected from the obtained steel strip and subjected to a tensile test in accordance with the provisions of JIS Z 2241. Yield stress (YS), tensile strength (TS), elongation (El), Yield ratio (YR) and Rankford value (r value) were determined. These results are shown in Table 2.
[0055]
[Table 1]

[0056]
[Table 2]

[0057]
From the results shown in Table 2, each of the inventive examples has low yield stress (YS), high elongation (El), and low yield ratio (YR), and exhibits a higher r value, deep drawing formability. In addition, the tensile strength (TS) has a high tension of 440 MPa or more. On the other hand, in the comparative example outside the scope of the present invention, the yield stress (YS) is high, the elongation (El) is low, or the r value is low.
[0058]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, there exists an industrial remarkable effect that the high tension cold-rolled steel plate which has the outstanding deep drawing formability can be manufactured stably. When the cold-rolled steel sheet of the present invention is applied to automobile parts, press forming is easy, and there is an effect that it can sufficiently contribute to weight reduction of the automobile body.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph showing the influence of the contents of V and C in a steel slab on the r value and the yield ratio (= yield stress (YS) / tensile strength (TS) × 100 (%)).

Claims (6)

C: 0.01 to 0.08% by mass, Si: 2.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.005-0.20%, N: 0.004 % or less, and V: 0.01 ~ 0.5% and V and C are
0.5 × C / 12 ≦ V / 51 ≦ 3 × C / 12
The balance has a composition consisting of Fe and inevitable impurities, and the structure is composed of a ferrite phase as a first phase and a second phase containing a martensite phase with an area ratio of 1% or more with respect to the entire structure. A composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability. C: 0.01 to 0.08% by mass, Si: 2.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.005-0.20%, N: 0.004 % or less, and V: 0.01 And 0.5% or less, and Nb: 0.001 to 0.3% and Ti: 0.001 to 0.3%, or a total of 0.3% or less, and V, Nb, Ti, and C are contained.
0.5 × C / 12 ≦ (V / 51 + 2 × Nb / 93 + 2 × Ti / 48) ≦ 3 × C / 12
And the balance has a composition consisting of Fe and inevitable impurities,
A composite structure type high tension excellent in deep drawability, characterized by having a structure composed of a ferrite phase as a first phase and a second phase containing a martensite phase with an area ratio of 1% or more with respect to the entire structure. Cold rolled steel sheet. The composite having excellent deep drawability according to claim 1 or 2, further comprising one group or two groups of group A and group B shown below in mass% in addition to the above composition: Structure type high-tensile cold-rolled steel sheet.
Group A: Cr is 2.0% by mass or less Group B: One or two of Cu and Ni are 2.0% by mass or less in total C: 0.01 to 0.08% by mass, Si: 2.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.005-0.20%, N: 0.004 % or less, and V: 0.01 ~ 0.5% and V and C are
0.5 × C / 12 ≦ V / 51 ≦ 3 × C / 12
A steel slab having the composition of Fe and the inevitable impurities in the balance is hot-rolled, subsequently pickled, and then cold-rolled, and thereafter in the temperature range of the A _{c1 to} A _c3 transformation points. A method for producing a composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability, characterized by performing continuous annealing. C: 0.01 to 0.08% by mass, Si: 2.0% or less, Mn: 3.0% or less, P: 0.10% or less, S: 0.02% or less, Al: 0.005-0.20%, N: 0.004 % or less, and V: 0.01 And 0.5% or less, and Nb: 0.001 to 0.3% and Ti: 0.001 to 0.3%, or a total of 0.3% or less, and V, Nb, Ti, and C are contained.
0.5 × C / 12 ≦ (V / 51 + 2 × Nb / 93 + 2 × Ti / 48) ≦ 3 × C / 12
A steel slab having the composition of Fe and the inevitable impurities in the balance is hot-rolled, subsequently pickled, and then cold-rolled, and thereafter in the temperature range of the A _{c1 to} A _c3 transformation points. A method for producing a composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability, characterized by performing continuous annealing. The steel slab further contains, in addition to the above composition, 1% or 2 groups of the following groups A and B in mass%, and the deep drawability according to claim 4 or 5 For producing a high-strength cold-rolled steel sheet with excellent structure.
Group A: Cr is 2.0% by mass or less Group B: One or two of Cu and Ni are 2.0% by mass or less in total

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