JP4556362B2 - High-tensile cold-rolled steel sheet excellent in heat-treating ability and strength of deep drawing after forming and manufacturing method thereof - Google Patents

Description

【００５０】
【表２】

【００５１】
【表３】

【００５２】
本発明例は、いずれも、El：35％以上の高い伸びElとΔＴＳ：50MPa 以上の高い成形後強度上昇熱処理能を有し、さらにｒ値：1.2 以上の高いｒ値を有して、成形後強度上昇熱処理能と深絞り性に優れた冷延鋼板となっている。これに対し、本発明の範囲を外れる比較例では、△ＴＳが低いか、あるいはｒ値が低下した鋼板となっている。
【００５３】
【発明の効果】
本発明によれば、ｒ値：1.2 以上という優れた深絞り性とΔＴＳ：50MPa 以上という高い成形後強度上昇熱処理能を有する冷延鋼板を、安定して製造することが可能となり、産業上格段の効果を奏する。さらに、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】ｒ値と成形後強度上昇熱処理能（△ＴＳ）とにおよぼすＶ、Nb、Ｃ、Al、Ｎ含有量の影響を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-tensile cold-rolled steel sheet suitable for use in automobile steel sheets and the like, and more particularly to a high-tensile cold-rolled steel sheet excellent in both deep drawability and post-forming strength increasing heat treatment ability.
[0002]
[Prior art]
In recent years, in connection with exhaust gas regulations due to global environmental conservation issues, the reduction of vehicle body weight has become a very important issue. In addition, ensuring safety such as occupant protection in a collision accident is also an important issue. For these reasons, recently, in order to reduce the weight of the vehicle body and ensure collision safety, it has been studied to use a high-strength thin steel plate having a reduced strength while simultaneously reducing the thickness of the steel plate.
[0003]
Since many automotive body parts made of steel sheets are formed by press working, the steel sheets used are required to have excellent press formability. In order to improve press formability, it is necessary that the steel sheet has a high Rankford value (r value) and a high ductility (elongation El) as mechanical properties of the steel sheet. However, generally, when the strength of a steel plate is increased, the r value and ductility are lowered, and the press formability tends to deteriorate.
[0004]
Further, when forming a steel sheet into a press-molded body such as a car body part of an automobile by pressing, it is desired that the steel sheet is soft in order to facilitate the pressing. On the other hand, the product is required to have high strength after the press-molded body is subjected to a paint baking process to obtain a product (part). A BH steel sheet has been developed as a steel sheet that can simultaneously satisfy such characteristics before pressing and characteristics after processing. This BH steel sheet is a steel sheet that has a characteristic that it is soft before press molding and is hardened by a coating baking process after press molding to increase the strength.
[0005]
As a method for improving the so-called bake hardenability, which is a characteristic that is hardened by a coating baking process after press molding, for example, in Japanese Patent Application Laid-Open No. 55-141526, depending on the content of solid solution C, N, and Al in steel By adding Nb and limiting Nb / (Solution C + Solution N) to a specific range at at% and controlling the cooling rate after annealing, the amount of solute C and the amount of solute N in steel The method of adjusting is described. Japanese Patent Publication No. 61-45689 proposes a method for improving the bake hardenability by adding Ti and Nb in combination.
[0006]
However, the steel plate manufactured by the above-described technology has a problem that the strength of the material steel plate is set low in order to make the material excellent in deep drawability, and the strength is not necessarily sufficient as a structural material. Moreover, in the steel plate manufactured by the above-described technology, the strength after forming-paint baking is increased by the action of a small amount of solid solution C and solid solution N in the steel plate. Only increases the yield strength and does not increase the tensile strength. Therefore, the steel plate manufactured by the above-described technique has the effect of increasing the stress at the start of deformation of the part, but the effect of increasing the stress required for deformation over the entire deformation area from the start of deformation to the end of deformation (post-forming tensile strength) is sufficient. I couldn't say that.
[0007]
On the other hand, as a cold-rolled steel sheet whose tensile strength increases after forming, for example, JP-A-10-310847 discloses an alloyed hot-dip galvanized steel sheet whose tensile strength increases by 60 MPa or more in a heat treatment temperature range of 200 to 450 ° C. It is disclosed. This steel sheet contains C: 0.01 to 0.08%, Mn: 0.01 to 3.0% by weight percentage, and contains 0.05 to 3.0% of one or more of W, Cr and Mo in total, and is necessary. A steel sheet having a composition containing one or more of Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, and V: 0.005 to 0.1%, and having a microstructure mainly composed of ferrite or ferrite. is there.
[0008]
However, this technique forms fine carbides in the steel sheet by heat treatment after forming, effectively increases the amount of strain by increasing the amount of strain by effectively increasing dislocations against the strain applied during pressing. Heat treatment needs to be performed in a temperature range of ° C., and there is a problem that the heat treatment temperature is higher than a general paint baking temperature.
Also, as a hot-rolled steel sheet whose tensile strength increases after forming, for example, in Japanese Patent Publication No. 8-23048, the tensile strength that is soft at the time of processing and effective in improving the fatigue characteristics by baking processing after processing is greatly increased. A method of manufacturing a hot-rolled steel sheet that can be raised is disclosed.
[0009]
In this technology, the amount of C is set to 0.02% to 0.13 mass%, N is added in a large amount of 0.0080 to 0.0250 mass%, and the finish rolling temperature and the winding temperature are controlled to add a large amount of solute N into the steel. It is said that an increase in tensile strength of 100 MPa or more is achieved at a post-molding heat treatment temperature of 170 ° C. by making the metal structure a composite structure mainly composed of ferrite and martensite.
[0010]
JP-A-10-183301 discloses that, among steel components, C and N are particularly limited to C: 0.01 to 0.12 mass% and N: 0.0001 to 0.01 mass%, and the average crystal grain size is 8 μm or less. A hot-rolled steel sheet having excellent bake hardenability and room temperature aging resistance capable of ensuring a high BH amount of 80 MPa or more and controlling the AI value to 45 MPa or less by controlling is disclosed.
[0011]
[Problems to be solved by the invention]
However, since the steel sheets described in Japanese Patent Publication No. 8-23048 and JP-A-10-183301 are both hot-rolled steel sheets, the ferrite texture is randomized by the austenite / ferrite transformation after finish rolling. Therefore, the r value is at most about 0.8, and it is difficult to obtain a high r value of 1.2 or more, which is required as a steel sheet for automobiles, and it is difficult to say that it has sufficient deep drawability.
[0012]
Moreover, even if cold rolling and recrystallization annealing are performed using the hot-rolled steel sheet obtained by these techniques as a starting material, an increase in the tensile strength after forming-heat treatment equivalent to that of the hot-rolled steel sheet is not necessarily obtained. Not exclusively. This is because the steel structure becomes a microstructure different from that during hot rolling due to cold rolling and recrystallization annealing. In addition, since there is no idea that the solid solution N and solid solution C are precipitated and fixed (IF) before cold rolling, a large amount of solid solution C and solid solution N remain at the time of cold rolling and have a sufficiently high r value. A steel plate cannot be obtained.
[0013]
The present invention advantageously solves the above-mentioned problems of the prior art, and has a high strength cold-rolled steel sheet having excellent post-forming strength increasing heat-treating ability and excellent deep drawing ability, and both post-forming strength increasing heat-treating ability and deep drawing ability. It aims at proposing the manufacturing method of the high tension cold-rolled steel plate which can manufacture the excellent cold-rolled steel plate stably. In the present invention, “excellent heat-strength heat treatment after forming” means that when a steel plate is pre-deformed with a tensile strain of 10% and then heat-treated (aging treatment) at a temperature of 170 ° C. for 20 minutes. (Aging treatment) Amount of increase in tensile strength before and after (denoted as △ TS; △ TS = (tensile strength after pre-deformation heat treatment)-(tensile strength before pre-deformation)) is 50 MPa or more And In the present invention, “excellent deep drawability” refers to a case where the r value is 1.2 or more.
[0014]
[Means for Solving the Problems]
In order to achieve the above-mentioned problems, the present inventors have conducted intensive studies on the influence of alloy elements on the microstructure and recrystallization texture of cold-rolled steel sheets. As a result, Ti and Nb have been mainly used as carbonitride-forming elements in the past, but the present inventors have focused on V in which nitride is easily dissolved at a lower temperature than Ti nitride and Nb nitride. And by making C content into an extremely low carbon region and containing N and V in an appropriate range, solid solution C and solid solution N can be reduced as much as possible before recrystallization annealing, and by recrystallization annealing, { 1 1 1} The recrystallized texture is strongly developed and a high r value of 1.2 or more can be obtained, and the V-based nitride is dissolved by annealing, and a large amount of solute N is dissolved in the steel sheet after annealing. It is possible to produce a high-tensile cold-rolled steel sheet that can simultaneously satisfy the excellent strength-increasing heat-treating ability after forming and the excellent deep drawability. Obtained knowledge. Furthermore, the present inventors have found that by adding Nb and B in addition to V, the structure can be further refined.
[0015]
First, basic experimental results performed by the present inventors will be described.
In mass%, C: 0.002%, Si: 0.02%, Mn: 1.2%, P: 0.04%, S: 0.004%, Al: 0.011%, N: 0.014%, Nb: 0.015%, B: including 0.0014% V: Each sheet bar having a composition changed from 0.03 to 0.35%, heated to 1250 ° C and soaked, and then hot rolled for 3 passes so that the finish rolling finish temperature is 910 ° C, and the sheet thickness is 4.0 mm. The hot rolled sheet was used. In addition, after finishing rolling, these hot-rolled sheets were subjected to a heat retention treatment of 700 ° C. × 1 h as a coil winding equivalent process. Subsequently, these hot-rolled sheets were cold-rolled at a reduction ratio of 70% to obtain cold-rolled sheets having a thickness of 1.2 mm. Then, these cold-rolled plates were subjected to recrystallization annealing at 840 ° C. for 40 s and then cooled at a cooling rate of 30 ° C./s to obtain cold-rolled annealed plates.
[0016]
Test pieces were sampled from the obtained cold-rolled annealed plates, and the tensile properties, post-molding strength increasing heat treatment ability and r value were determined.
Tensile properties were determined by collecting JIS No. 5 tensile test pieces from each cold-rolled annealed plate and conducting a tensile test. In addition, JIS No. 5 tensile test specimens collected from each cold-rolled annealed plate were heat-treated after pre-deformation with a tensile strain of 10% and maintained at 170 ° C for 20 minutes, and then a tensile test was conducted. Tensile strength was determined. The amount of increase in tensile strength before and after the heat treatment (denoted as ΔTS; ΔTS = (tensile strength after heat treatment) − (tensile strength before pre-deformation)) was defined as the post-molding strength increasing heat treatment ability.
[0017]
In addition, JIS test specimens were collected from each cold-rolled annealed sheet in the rolling direction (L direction), 45 degrees direction (D direction) in the rolling direction, and perpendicular direction (C direction) to the rolling direction. % Width and thickness strain of each specimen when% uniaxial tensile prestrain is applied, and the ratio of width strain to thickness strain
r = ln (w / w ₀ ) / Ln (t / t ₀ )
Where w ₀ , T ₀ Is the width and thickness of the specimen before the test, and w and t are the width and thickness of the specimen after the test)
R value in each direction (r _L , R _C , R _D ) For each
r _mean = (R _L + R _C + 2 × r _D ) / 4
The average r value r _mean Asked. In the present invention, the r value means an average r value unless otherwise specified.
[0018]
FIG. 1 shows the influence of the contents of V, C, Al, and N on the r value and the post-molding strength increasing heat treatment ability (ΔTS). The horizontal axis is parameter (V / 51 + Nb / 93-C / 12 + Al / 27) / (N / 14). Here, V, C, Al, Nb, and N are the contents (mass%) of each element.
From Fig. 1, when (V / 51 + Nb / 93-C / 12 + Al / 27) / (N / 14) satisfies 0.5 or more and 3.0 or less, it is the first time that a high r value of 1.2 or more and ΔTS of 50 MPa or more. It can be seen that a high post-molding strength-increasing heat treatment ability is simultaneously satisfied. That is, by adjusting the components so that (V / 51 + Nb / 93-C / 12 + Al / 27) / (N / 14) is 0.5 or more and 3.0 or less, excellent deep drawability and excellent strength after molding. It was found that a high-tensile cold-rolled steel sheet having ascending heat treatment ability can be produced.
[0019]
The present invention has been completed by further study based on the above findings.
That is, the gist of the present invention is as follows.
(1) By mass%, C: less than 0.0050%, Si: 0.005 to 1.0%, Mn: 0.01 to 1.5%, P: 0.1% or less, S: 0.01% or less, Al: 0.005 to 0.03%, N: 0.005 to 0.040%, Nb: 0.005 to 0.050%, B: 0.0003 to 0.0015%, V: 0.01 to 0.5%, and C, Al, N, Nb, and V in the following formulas (1) and (2)
0 ≦ {V / 51 + Nb / 93-C / 12} (1)
0.5 ≦ {V / 51 + Nb / 93 −C / 12 + Al / 27} / (N / 14) ≦ 3.0 (2)
(Here, C, Al, N, Nb, V: content of each element (mass%))
Excellent in heat-treating strength after forming and deep drawability, characterized by having a composition comprising 0.0015% or more of solid solution N and the balance Fe and inevitable impurities. High tensile cold-rolled steel sheet.
(2) In (1), in addition to the above composition, the composition further comprises one or two or more selected from Cu, Ni, Cr, and Mo in a mass% of 2.0% or less in total. High tensile cold-rolled steel sheet.
(3) By mass%, C: less than 0.0050%, Si: 0.005 to 1.0%, Mn: 0.01 to 1.5%, P: 0.1% or less, S: 0.01% or less, Al: 0.005 to 0.03%, N: 0.005 to 0.040%, Nb: 0.005 to 0.050%, B: 0.0003 to 0.0015%, V: 0.01 to 0.5%, and C, Al, N, Nb, and V in the following formulas (1) and (2)
0 ≦ {V / 51 + Nb / 93-C / 12} (1)
0.5 ≦ {V / 51 + Nb / 93 −C / 12 + Al / 27} / (N / 14) ≦ 3.0 (2)
(Here, C, Al, N, Nb, V: content of each element (mass%))
A steel material having a composition consisting of the remaining Fe and inevitable impurities, hot-rolled to a heating temperature of 900 ° C or higher and a finish rolling finish temperature of 800 ° C or higher. After that, the hot-rolled sheet is wound at a coiling temperature of 400 to 800 ° C., and then the hot-rolled sheet is subjected to cold rolling at a reduction ratio of 60 to 95% to obtain a cold-rolled sheet. A method for producing a high-tensile cold-rolled steel sheet excellent in post-forming strength-increasing heat treatment ability and deep drawability, characterized by subjecting to recrystallization annealing at a temperature of 700 ° C.
(4) In (3), in addition to the above composition, the composition further comprises one or two or more selected from Cu, Ni, Cr, and Mo in a mass% of 2.0% or less in total. A method for producing a high-tensile cold-rolled steel sheet.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
The cold-rolled steel sheet of the present invention is a high-tensile cold-rolled steel sheet having excellent post-forming strength increasing heat treatment ability 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%.
[0021]
C: Less than 0.0050%
C increases the strength of the steel but lowers the ductility and degrades press formability and deep drawability. For this reason, although it is preferable to reduce as much as possible in the present invention, C is preferably set to an extremely low carbon level from the viewpoint of improving press formability and deep drawability, specifically, less than 0.0050%. In addition, Preferably, it is 0.0030% or less. In addition, the lower limit value of the amount of C that can be reached without extreme cost increase is considered to be about 0.0005% in the current manufacturing technology.
[0022]
Si: 0.005 to 1.0%
Si is a useful element that suppresses the decrease in elongation and improves the strength, but such an effect is recognized with a content of 0.005% or more, while content exceeding 1.0% deteriorates the surface properties. Cause a drop in ductility. For this reason, Si was limited to the range of 0.005 to 1.0%. In addition, Preferably it is 0.005-0.75%.
[0023]
Mn: 0.01-1.5%
Mn is effective as a strengthening component of steel and has the effect of suppressing the embrittlement due to S by forming MnS. Such an effect is recognized when the content is 0.01% or more, but when the content exceeds 1.5%, surface properties are deteriorated and ductility is lowered. For this reason, Mn was limited to the range of 0.01 to 1.5%. In addition, Preferably it is 0.01 to 0.75%.
[0024]
P: 0.1% or less
P is an element that contributes effectively to strengthening steel by solid solution strengthening, and can be contained in a predetermined amount according to the required strength level, but if it exceeds 0.1%, deep drawability decreases. For this reason, P was limited to 0.1% or less.
S: 0.01% or less
S is present mainly as inclusions (sulfides) in steel and causes a reduction in ductility. Therefore, it is desirable to reduce S as much as possible, but it is allowed up to 0.01%.
[0025]
Al: 0.005 to 0.030%
Al is included as a deoxidizer and to improve the yield of carbonitride-forming elements, but if the content is less than 0.005%, there is no sufficient effect, while the content exceeding 0.030% is contained in the steel. The amount of N to be increased is increased, and slab defects are easily generated during steelmaking.
[0026]
N: 0.005 to 0.040%
In the present invention, N is an important element that plays a role of imparting post-forming strength increasing heat treatment ability to the steel sheet. However, if the content is less than 0.005%, it is difficult to secure a sufficient amount of solute N, and sufficient post-molding strength increasing heat treatment ability cannot be obtained. On the other hand, if it exceeds 0.040%, the press formability is lowered. For this reason, N was limited to the range of 0.005 to 0.040%. In addition, Preferably it is 0.008 to 0.015%.
[0027]
Nb: 0.005-0.050%
Nb is an element that is contained together with B, contributes to refinement of the hot-rolled structure and the cold-rolled recrystallization annealed structure, and has an action of binding to the solid solution C to precipitate and fix the solid solution C as NbC. Nb also has an action of fixing solute N as NbN by combining with solute N. When the Nb content is less than 0.005%, not only is it difficult to precipitate and fix the solid solution C, but the refinement of the hot-rolled structure and the cold-rolled recrystallization annealed structure becomes insufficient. On the other hand, if it exceeds 0.050%, ductility is lowered. For this reason, Nb is limited to the range of 0.005 to 0.050%. In addition, Preferably, it is 0.010 to 0.030%.
[0028]
B: 0.0003-0.0015%,
B is an element that contributes to refinement of the hot-rolled structure and the cold-rolled recrystallized structure and has the effect of improving the secondary work brittleness resistance by being contained in combination with Nb. If the B content is less than 0.0003%, a sufficient fine effect cannot be obtained. On the other hand, when it contains exceeding 0.0015%, the amount of BN precipitation will increase and it will interfere with the solution formation in the slab heating stage. For this reason, B was limited to the range of 0.0003 to 0.0015%. In addition, Preferably it is 0.0007 to 0.0012%.
[0029]
V: 0.01 to 0.5%
V is the most important element in the present invention. Before recrystallization annealing, V precipitates as V nitride and fixes N to reduce solute N, and {111} recrystallization texture during recrystallization annealing. Has the effect of obtaining a high r value. Further, the precipitated V nitride is re-dissolved during recrystallization annealing, and has the effect of increasing the solid solution N again and improving the strength-increasing heat treatment ability after forming. Such an effect is recognized when the content is 0.01% or more. However, when the content exceeds 0.5%, re-melting of the V nitride is difficult to occur during annealing, and excellent post-molding strength increasing heat treatment ability cannot be obtained. For this reason, V was limited to the range of 0.01 to 0.5%.
[0030]
The cold-rolled steel sheet of the present invention has a composition within the above range, and further contains C, Al, N, V, and Nb under the conditions that satisfy the following formulas (1) and (2) at the same time. To do.
0 ≦ {V / 51 + Nb / 93-C / 12} (1)
0.5 ≦ {V / 51 + Nb / 93 −C / 12 + Al / 27} / (N / 14) ≦ 3.0 (2)
(Here, C, Al, N, Nb, V: content of each element (mass%))
If the relationship between the C content, Nb content and V content does not satisfy the formula (1), the amount of carbide generated is small, the amount of solid solution C increases, and the desired recrystallization texture during recrystallization The development of this becomes weak, and a high r value cannot be obtained.
[0031]
Further, the relationship among the contents of C, Al, N, V, and Nb does not satisfy the formula (2), that is, (V / 51 + Nb / 93-C / 12 + Al / 27) / (N / 14) is 0.5. If it is less than or more than 3.0, the amount of dissolved N is too much or too little, and as shown in FIG. 1, either the r value or the heat treatment ability for increasing strength (ΔTS) decreases. However, excellent deep drawability and excellent post-molding strength increasing heat treatment ability cannot be satisfied at the same time.
[0032]
Solid solution N: 0.0015% or more
In the cold-rolled steel sheet of the present invention, in order to ensure sufficient strength and to obtain an excellent heat-treating ability for increasing strength after forming, the amount of N in solid solution (also referred to as solid solution N) is 0.0015% or more (concentration). ) Must be present.
Here, the solute N amount is obtained by subtracting the precipitated N amount from the total N amount in the steel. It should be noted that, as a method for analyzing the amount of precipitated N, it is effective to obtain by an electrolytic extraction analysis method using a constant potential electrolysis method according to the results of comparisons of various analysis methods by the present inventors. In addition, there are an acid decomposition method, a halogen method, an electrolysis method and the like as a method for dissolving the base iron used for the extraction analysis. Among these, the electrolytic method can stably dissolve only the ground iron without decomposing unstable precipitates such as charcoal and nitride. Electrolysis is performed at a constant potential using an acetylacetone system as the electrolytic solution. In the present invention, the result of measuring the amount of precipitated N using a constant potential electrolysis method showed the best correspondence with the actual component strength.
[0033]
For this reason, in the present invention, the residue extracted by the constant potential electrolysis method is chemically analyzed to determine the amount of N in the residue, which is used as the amount of precipitated N. In order to obtain a higher ΔTS, the amount of dissolved N is preferably 0.0030% or more, 0.0045% or more for obtaining a higher value, and 0.0060% or more for obtaining a higher value. .
Further, in the present invention, in addition to the above-described composition, it is preferable that one or more of Cu, Ni, Cr and Mo are further contained in a total of 2.0% or less.
[0034]
Cu, Ni, Cr, and Mo all have an action of strengthening steel, and can be selected and contained as necessary. The content of these elements can be appropriately determined according to the desired strength, but in order to obtain the above-described effects, Cu: 0.05% or more, Ni: 0.05% or more, Cr: 0.05% or more, Mo : 0.05% or more is desirable, but if it is excessively contained, deep drawability deteriorates. For this reason, Cu, Ni, Cr, and Mo are preferably one or two or more in total to 2.0% or less.
[0035]
It should be noted that there is no problem if Ca, Zr, REM or the like is contained as a component other than the components described above.
The balance other than the above components is Fe and inevitable impurities. As unavoidable impurities, Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: 0.1% or less are acceptable.
[0036]
Below, the manufacturing method of the cold-rolled steel plate of this invention is demonstrated.
The steel having the above composition is melted by a generally known melting method such as a converter, and a steel material (slab) is obtained by using a generally known continuous casting method, thin slab casting method or ingot forming method. preferable. In addition, it is more preferable to manufacture by a continuous casting method from a viewpoint of preventing the macrosegregation of a component.
[0037]
Next, the steel material (slab) having the above composition is heated to a heating temperature of 900 ° C. or higher, and then subjected to hot rolling at a finish rolling finish temperature of 800 ° C. or more to obtain a hot rolled sheet. Wind as 400-800 ° C.
In the present invention, the steel material (slab) is once cooled to room temperature and then heated again to 900 ° C. or higher, or it is not cooled to room temperature, and it remains in the heating furnace. If the steel material (slab) temperature is 900 ° C or higher, it will be rolled immediately after a little heat retention. Applicable without any problem.
[0038]
Heating temperature: 900 ℃ or more
The heating temperature of the steel material (slab) is advantageous for the development of {111} recrystallization texture by fixing solid solution C and solid solution N and precipitating coarsely as precipitates in order to improve deep drawability. Therefore, it is desirable to make the temperature as low as possible. However, if the heating temperature is less than 900 ° C, the rolling load during hot rolling increases and the risk of occurrence of rolling trouble increases. For this reason, it is preferable that the heating temperature of a steel material shall be 900 degreeC or more. Note that the slab heating temperature is desirably 1300 ° C. or less because of an increase in scale loss accompanying an increase in oxidized weight.
[0039]
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.
Finishing rolling finish temperature: 800 ℃ or more
In the present invention, the finish rolling finish temperature FDT is preferably 800 ° C. or higher. By setting the FDT to 800 ° C. or more, a uniform hot-rolled mother board structure can be obtained, and excellent deep drawability can be obtained after cold rolling and recrystallization annealing. On the other hand, if the FDT is less than 800 ° 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. For this reason, the hot rolling FDT is preferably 800 ° C. or higher.
[0040]
Winding temperature: 400 ℃ or more and 800 ℃ or less
When the coiling temperature CT exceeds 800 ° C., the scale increases and the yield tends to decrease due to the scale loss, and the hot rolled plate crystal grains become coarse and the deep drawability decreases. If the CT is less than 400 ° C., the precipitation of VN in the hot-rolled sheet is insufficient and the deep drawability is deteriorated, and the shape of the steel sheet is significantly disturbed, which increases the risk of causing problems in actual use. For this reason, CT is preferably set to 400 ° C. or higher and 800 ° C. or lower.
[0041]
In the present invention, part or all of finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling. 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 0.25-0.10. Moreover, it is desirable from the viewpoint of the operational stability of hot rolling that a continuous rolling process is performed in which successive sheet bars are joined and finish-rolled continuously.
[0042]
Next, the hot-rolled sheet is subjected to cold rolling at a reduction ratio of 60 to 95% to obtain a cold-rolled sheet. In addition, it is preferable that a hot-rolled sheet is pickled before cold rolling, and a hot-rolled scale is removed.
Cold reduction rate: 60-95%
Cold rolling is preferably performed at a reduction ratio of 60 to 95%. If the rolling reduction is less than 60%, the {111} recrystallization texture does not develop, and the r value is low and a significant improvement in deep drawability cannot be expected. On the other hand, when the rolling reduction exceeds 95%, the r value decreases.
[0043]
Next, the cold-rolled sheet is subjected to recrystallization annealing at a temperature of 700 ° C. or higher to be a cold-rolled annealed sheet.
Recrystallization annealing temperature: 700 ℃ or more
The annealing temperature for recrystallization annealing is preferably set to 700 ° C. or higher in order to re-dissolve the V nitride and secure solid solution N. When the annealing temperature for recrystallization annealing is less than 700 ° C., recrystallization is not sufficiently completed, and remelting of VN is insufficient, and a high r value and high post-molding strength increasing heat treatment ability cannot be obtained. It is preferably 800 ° C. or higher and 900 ° C. or lower. The soaking time (annealing time) at the annealing temperature is preferably 5 s or more. If the annealing time is less than 5 s, recrystallization may be insufficient, or VN remelting may be insufficient. For this reason, it is preferable that an annealing temperature shall be 5 s or more.
Moreover, in order to suppress the reprecipitation of VN, the cooling rate after soaking in the recrystallization annealing is preferably set to a cooling rate of 30 ° C./s or more in the temperature range from the annealing temperature to 500 ° C. More preferably, it is 50 ° C./s or more.
[0044]
Needless to say, after the recrystallization annealing step, temper rolling with an elongation of 10% or less may be added to adjust the 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 treatment include galvanization (including alloy system), tin plating, enamel and the like.
[0045]
Also, the cold-rolled steel sheet of the present invention is specially treated for improving chemical processability, weldability, press formability, corrosion resistance, etc. after galvanization, such as application of solid lubricant, Fe-P electroplating, etc. May be applied.
[0046]
【Example】
Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) by a continuous casting method. Subsequently, these steel materials (slabs) were heated to various temperatures shown in Table 2, and then hot-rolled under the conditions shown in Table 2 to obtain hot-rolled steel strips (hot-rolled plates) having a thickness of 4.0 mm. Subsequently, these hot-rolled steel strips (cold-rolled sheets) were pickled and cold-rolled under the conditions shown in Table 2 to obtain cold-rolled steel strips (cold-rolled sheets) having a thickness of 1.2 mm. Then, these cold-rolled steel strips (cold-rolled sheets) were subjected to recrystallization annealing at the annealing temperature and time shown in Table 2 in a continuous annealing line to obtain cold-rolled annealed steel bands (cold-rolled annealed plates). In addition, it cooled from annealing temperature to 500 degreeC on the conditions shown in Table 2 after annealing. Furthermore, these cold-rolled annealed steel strips (cold-rolled annealed sheets) were further subjected to temper rolling with an elongation of 0.8%.
[0047]
Test pieces were collected from the obtained cold-rolled steel strips and measured for tensile properties, r value, and post-strength strength increasing heat treatment ability by the following test methods. In addition, the amount of solute N of the obtained cold-rolled steel strip was measured by the method described above.
(1) Tensile properties
A JIS No. 5 tensile test piece was collected from each of the obtained cold-rolled steel strips, and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to obtain yield stress YS, tensile strength TS, and elongation EI.
(2) r value
To each JIS 5 test piece taken from the rolling direction (L direction) of each cold-rolled steel strip, 45 ° direction (D direction) with respect to the rolling direction, and 90 ° direction (C direction) with respect to the rolling direction, Obtain the width strain and plate thickness strain of each specimen when 15% uniaxial tensile pre-strain was applied, and the ratio of the width strain to the plate thickness strain,
r = 1n (w / w _o ) / 1n (t / t _o )
(Where _o , T _o Is the width and thickness of the test piece before the test, and w and t are the width and thickness of the test piece after the test. )
The r value in each direction is obtained from
r _mean = (R _L + 2r _D + R _C ) / 4
The average r value r _mean This was determined as the r value. Where r _D Is the r value in the rolling direction (L direction), r _D Is the r value in the 45 ° direction (D direction) with respect to the rolling direction (L direction). _C Is the r value in the 90 ° direction (C direction) with respect to the rolling direction (L direction).
(3) Strength increasing heat treatment ability after molding
From each cold-rolled steel strip obtained, a JIS No. 5 specimen was taken in the rolling direction, given a pre-strain of 10%, and then subjected to heat treatment (aging treatment) at 170 ° C for 20 minutes, followed by a tensile test. And tensile strength TS after pre-deformation-heat treatment (aging treatment) _HT △ TS = TS _HT -TS was calculated. TS is the tensile strength of the cold-rolled steel strip before pre-deformation, that is, obtained.
[0048]
These results are shown in Table 3.
[0049]
[Table 1]

[0050]
[Table 2]

[0051]
[Table 3]

[0052]
Each of the examples of the present invention has a high elongation El of 35% or higher and a high heat treatment ability for increasing strength after molding of ΔTS: 50 MPa or more, and further has a high r value of 1.2 or more. It is a cold-rolled steel sheet with excellent post-strength heat treatment ability and deep drawability. On the other hand, in the comparative example outside the scope of the present invention, the steel sheet has a low ΔTS or a low r value.
[0053]
【The invention's effect】
According to the present invention, it is possible to stably produce a cold-rolled steel sheet having excellent deep drawability of r value: 1.2 or more and high post-forming strength increasing heat treatment ability of ΔTS: 50 MPa or more. Has the effect of. Furthermore, when the cold-rolled steel sheet of the present invention is applied to automobile parts, there is an effect that press forming is easy and it can sufficiently contribute to weight reduction of the automobile body.
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of the contents of V, Nb, C, Al, and N on r value and heat-treating ability to increase strength after forming (ΔTS).

Claims (4)

C: less than 0.0050% by mass%, Si: 0.005-1.0%,
Mn: 0.01 to 1.5%, P: 0.1% or less,
S: 0.01% or less, Al: 0.005 to 0.03%,
N: 0.005-0.040%, Nb: 0.005-0.050%,
B: 0.0003-0.0015%, V: 0.01-0.5%
And containing C, Al, N, Nb, and V under the conditions satisfying the following formula (1) and the following formula (2), further containing 0.0015% or more of N in a solid solution state, the balance Fe and inevitable A high-tensile cold-rolled steel sheet excellent in post-forming strength-increasing heat-treating ability and deep drawability, characterized by having a composition comprising mechanical impurities.
0 ≦ {V / 51 + Nb / 93-C / 12} (1)
0.5 ≦ {V / 51 + Nb / 93 −C / 12 + Al / 27} / (N / 14) ≦ 3.0 (2)
Here, C, Al, N, Nb, V: Content of each element (mass%) 2. The high content according to claim 1, further comprising, in addition to the composition, 2.0% or less in total of one or more selected from Cu, Ni, Cr, and Mo by mass%. Tensile cold-rolled steel sheet. C: less than 0.0050% by mass%, Si: 0.005-1.0%,
Mn: 0.01 to 1.5%, P: 0.1% or less,
S: 0.01% or less, Al: 0.005 to 0.03%,
N: 0.005-0.040%, Nb: 0.005-0.050%,
B: 0.0003-0.0015%, V: 0.01-0.5%
A steel material containing C, Al, N, Nb, and V under the conditions satisfying the following formula (1) and the following formula (2), and having a composition comprising the balance Fe and inevitable impurities: Temperature: 900 ° C. or higher, finish rolling finish temperature: 800 ° C. or higher, hot rolled into a hot rolled sheet, then rolled up at a coiling temperature of 400 to 800 ° C., then the hot rolled sheet After forming into a cold-rolled sheet by subjecting it to a cold rolling of 60 to 95%, the cold-rolled sheet is subjected to recrystallization annealing at a temperature of 700 ° C. or higher. A method for producing a high-tensile cold-rolled steel sheet with excellent drawability.
0 ≦ {V / 51 + Nb / 93-C / 12} (1)
0.5 ≦ {V / 51 + Nb / 93 −C / 12 + Al / 27} / (N / 14) ≦ 3.0 (2)
Here, C, Al, N, Nb, V: Content of each element (mass%) 4. The high content according to claim 3, further comprising, in addition to the composition, 2.0% or less in total of one or more selected from Cu, Ni, Cr, and Mo by mass%. A method for producing a tension cold-rolled steel sheet.

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