JP2003064447A - High tensile strength cold rolled steel sheet having excellent heat treatability to increase its strength after forming and deep drawability, and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet which has excellent heat treatability to increase its strength after forming, and has excellent deep drawability, and to provide a production method therefor. SOLUTION: A steel stock having a composition containing <0.0050% C, 0.005 to 0.03% Al, 0.005 to 0.040% N, 0.005 to 0.050% Nb, 0.0003 to 0.0015% B and 0.01 to 0.5% V, and also in the conditions wherein the contents (mass%) of C, Al, N, Nb and V satisfy 0<= V/51+Nb/93-C/12} and 0.5<= V/51+Nb/93-C/12+Al/27}/(N/14)<=3.0 is subjected to hot rolling at a heating temperature, and a finishing temperature in finish rolling of >=800 deg.C, and is coiled. The steel is thereafter subjected to cold rolling at a draft of 60 to 95%, and is next subjected to recrystallization annealing at >=700 deg.C. Thus, the cold rolled steel sheet having heat treatability to increase its strength after forming satisfying >=500 MPa ΔTS, and having deep drawability satisfying an r value of >=1.2 is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength 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 having both excellent deep drawability and heat treatment capability for increasing strength after forming. .

[0002]

2. Description of the Related Art In recent years, reduction of the vehicle body weight of automobiles has become an extremely important issue in connection with the regulation of exhaust gas from the problem of preservation of the global environment. Furthermore, ensuring safety such as passenger protection in a collision accident is also an important issue. Under these circumstances, recently, in order to reduce the weight of the vehicle body and ensure collision safety, it has been considered to use a high-strength thin steel sheet having a high strength as well as a thin steel sheet for an automobile body.

Since many automobile body parts made of steel sheet are formed by press working, the steel sheet used is required to have excellent press formability.
In order to improve press formability, the mechanical properties of the steel sheet include high Rankford value (r value) and high ductility (elongation E).
l) is required. But in general,
When the strength of the steel sheet is increased, the r value and the ductility are lowered, and the press formability tends to be deteriorated.

When the steel sheet is pressed into a press-formed body such as a vehicle body part of an automobile, it is desired that the steel sheet is soft in order to facilitate the press working. On the other hand, a product (part) is required to have high strength after being subjected to paint baking treatment on the press-formed product. A BH steel sheet has been developed as a steel sheet capable of simultaneously satisfying the properties before and after the press working. This BH steel sheet is a steel sheet that is soft before press forming and has the property of being hardened by a paint baking treatment after press forming and increasing in strength.

As a method for improving the characteristic of hardening by coating baking treatment after press forming, that is, so-called baking hardening property, for example, Japanese Patent Laid-Open Publication No. 55-141526 discloses the contents of solid solution C, N and Al in steel. Nb is added according to
By limiting (solid solution C + solid solution N) within a specific range and controlling the cooling rate after annealing, the solid solution C in the steel is dissolved.
The method for adjusting the amount and the amount of solute N is described. Also,
Japanese Examined Patent Publication No. 61-45689 proposes a method of improving the bake hardenability by adding Ti and Nb in combination.

However, in the steel sheet manufactured by the above-mentioned technique, the strength of the material steel sheet is set low in order to make it a material having excellent deep drawability, and there is a problem that the strength is not always sufficient as a structural material. It was Further, in the steel sheet manufactured by the above-mentioned technique, the strength after forming-paint baking is increased due to the action of a small amount of solid solution C and solid solution N in the steel sheet. Only increases the yield strength, not the tensile strength. Therefore, the steel sheet manufactured by the above-described technique has an effect of increasing the deformation start stress of the component, but has a sufficient effect of increasing the stress required for the deformation over the entire deformation range from the deformation start to the deformation end (tensile strength after forming). I couldn't say that.

On the other hand, as a cold-rolled steel sheet whose tensile strength increases after forming, for example, JP-A-10-310847 discloses 200-4.
Disclosed is an alloyed hot-dip galvanized steel sheet whose tensile strength increases by 60 MPa or more in a heat treatment temperature range of 50 ° C. This steel sheet has a weight percentage of C: 0.01 to 0.08% and Mn: 0.01 to 3.0.
%, And one or more of W, Cr, and Mo in a total amount of 0.05 to 3.0%, and Ti: 0.005 if necessary.
Is a steel sheet having a microstructure mainly composed of ferrite or ferrite and having a composition containing one or more of 0.1 to 0.1%, Nb: 0.005 to 0.1%, and V: 0.005 to 0.1%.

[0008] However, since this technique forms fine carbides in the steel sheet by heat treatment after forming and effectively propagates dislocations with respect to the strain applied during pressing, the amount of strain is increased. It is necessary to perform heat treatment in the temperature range of 220 to 370 ° C, and there is a drawback that the heat treatment temperature is higher than the general coating baking temperature. Further, as a hot rolled steel sheet whose tensile strength increases after forming, for example, Japanese Patent Publication No. 8
Japanese Patent Laid-Open No. 23048 discloses a method for producing a hot-rolled steel sheet which is soft at the time of working and can significantly increase the tensile strength effective for improving fatigue properties by baking treatment after working.

In this technique, the C content is 0.02% to 0.13 mass%.
And adding N in a large amount of 0.0080 to 0.0250 mass%, and controlling the finishing rolling temperature and the winding temperature to leave a large amount of solute N in the steel, and the metallic structure mainly composed of ferrite and martensite. It is said that the increase in tensile strength of 100 MPa or more can be achieved at the post-forming heat treatment temperature of 170 ° C. by using such a composite structure.

Further, in Japanese Patent Laid-Open No. 10-183301, among the steel components, C and N are particularly C: 0.01 to 0.12 mass% and N:
It is possible to secure a high BH amount of 80 MPa or more by limiting the average crystal grain size to 8 μm or less while limiting the content to 0.0001 to 0.01 mass% and to control the AI value to 45 MPa or less. A hot rolled steel sheet having excellent room temperature aging resistance is disclosed.

[0011]

However, since the steel sheets described in Japanese Patent Publication No. 8-23048 and Japanese Patent Laid-Open No. 10-183301 are all hot-rolled steel sheets, the austenite / ferrite transformation after finish rolling is performed. As a result, the texture of ferrite becomes random, so the r value is at most about 0.8, which is required for steel sheets for automobiles, for example, 1.2.
It is difficult to obtain the above-mentioned high r value, and it cannot be said to have sufficient deep drawability.

Moreover, even if cold rolling and recrystallization annealing are performed using the hot-rolled steel sheet obtained by these techniques as a starting material, the tensile strength increase after the forming-heat treatment is not necessarily the same as that of the hot-rolled steel sheet. It is not always possible. This is because the steel structure has a microstructure different from that during hot rolling due to cold rolling and recrystallization annealing. In addition, since there is no idea that solid solution N and solid solution C are fixed and fixed (IF) before cold rolling,
A large amount of solid solution C and solid solution N remain during cold rolling, and a steel sheet having a sufficiently high r value cannot be obtained.

The present invention advantageously solves the above-mentioned problems of the prior art, and has a high-strength cold-rolled steel sheet having an excellent post-forming strength increasing heat treatment ability and an excellent deep drawability, and a post-forming strength increasing heat treatment ability and deep drawing. An object of the present invention is to propose a method for producing a high-strength cold-rolled steel sheet, which can stably produce a cold-rolled steel sheet having excellent properties. Incidentally, "excellent in strength increasing heat treatment after forming" referred to in the present invention means that the steel sheet is
Heat treatment (aging treatment) at a temperature of 0 ° C for 20 minutes
When the increase in tensile strength before and after this heat treatment (aging treatment) (described as △ TS; △ TS = (tensile strength after pre-deformation heat treatment)-(tensile strength before pre-deformation)) was 50 MPa or more. It means something. The term "excellent in deep drawability" as used in the present invention means that the r value is 1.2 or more.

[0014]

[Means for Solving the Problems] In order to achieve the above-mentioned objects, the present inventors have conducted extensive studies on the influence of alloying elements on the microstructure and recrystallization texture of cold-rolled steel sheets. As a result, Ti has traditionally been used as a carbonitride forming element.
Although Nb and Nb have been mainly used, the present inventors have focused on V, which is more likely to dissolve a nitride at a lower temperature than Ti nitride or Nb nitride. Then, by setting the C content to an extremely low carbon region and containing N and V in appropriate ranges, it is possible to reduce the solid solution C and the solid solution N as much as possible before the recrystallization annealing. 1 1 1} Recrystallized texture develops strongly and 1.2
The above-mentioned high r value is obtained, and the V-based nitride is melted by performing annealing, and a large amount of solute N can be contained in the steel sheet after annealing, and the heat treatment ability for increasing strength after forming is improved. The inventors have found that it is possible to manufacture a high-strength cold-rolled steel sheet that is capable of satisfying both excellent post-forming strength increasing heat treatment ability and excellent deep drawability at the same time. Furthermore, the present inventors have found that, by incorporating Nb and B in addition to V, the structure can be further refined.

First, the basic experimental results conducted by the present inventors will be described. % By mass, C: 0.002%, Si: 0.
02%, Mn: 1.2%, P: 0.04%, S: 0.004%, Al: 0.0
Each sheet bar containing 11%, N: 0.014%, Nb: 0.015%, B: 0.0014% and having a composition changed from V: 0.03 to 0.35% was heated to 1250 ° C and soaked, and then finish rolling was completed. Plate thickness by 3-pass hot rolling so that the temperature reaches 910 ℃
It was a 4.0 mm hot rolled sheet. After finishing rolling, these hot-rolled sheets were processed at 700 ° C x 1h for coil winding.
Was subjected to heat retention treatment. Then, these hot-rolled sheets were cold-rolled at a rolling reduction of 70% to obtain cold-rolled sheets having a plate thickness of 1.2 mm. Then, these cold-rolled sheets 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 sheets.

Test pieces were taken from each of the obtained cold-rolled and annealed sheets, and the tensile properties, heat treatment capacity for increasing strength after forming and r value were determined. The tensile properties were determined by collecting JIS No. 5 tensile test pieces from each cold rolled annealed sheet and performing a tensile test. Tensile strain of 10% was applied to JIS No. 5 tensile test pieces taken from each cold rolled annealed sheet.
After pre-deformation, heat treatment was performed under the condition of holding at 170 ° C. for 20 minutes, and then a tensile test was performed to determine the tensile strength after the heat treatment. The amount of increase in tensile strength before and after heat treatment (denoted as ΔTS; ΔTS = (tensile strength after heat treatment) − (tensile strength before pre-deformation)) was taken as the strength-increasing heat treatment ability after molding.

Further, JIS cold test pieces of each cold rolled annealed sheet were sampled from the rolling direction (L direction), the rolling direction of 45 degrees (D direction), and the direction perpendicular to the rolling direction (C direction). The width strain and the plate thickness strain of each test sample when a uniaxial tensile prestrain of% was applied to the test sample were obtained, and the ratio of the width strain and the plate thickness strain was r = ln (w / w ₀ ) / ln (t / t ₀ ), where w ₀ and t ₀ are the width and plate thickness of the test piece before the test, and w and t are the width and plate thickness of the test piece after the test) to the r value (r _L , r _C , and r _D ) were respectively calculated, and the average r value r _mean was calculated by the following formula r _mean = (r _L + r _C + 2 × r _D ) / 4. In the present invention, the r value means the average r value unless otherwise specified.

FIG. 1 shows the effect of V, C, Al and N contents on the r value and the heat treatment capacity for increasing strength (ΔTS) after forming. The horizontal axis is the parameter (V / 51 + Nb / 93-C
/ 12 + Al / 27) / (N / 14). Where V, C, A
l, Nb, and N are the contents (mass%) of each element. From Fig. 1, (V / 51 + Nb / 93-C / 12 + Al / 27) / (N / 14)
It can be seen that when r satisfies 0.5 or more and 3.0 or less, a high r value of 1.2 or more and a high heat treatment ability for increasing strength after forming of 50 MPa or more at ΔTS are simultaneously satisfied for the first time. That is, (V / 51 + Nb / 93 -C / 12 + Al / 27) / (N / 14)
It was found that it is possible to manufacture a high-strength cold-rolled steel sheet having excellent deep drawability and excellent post-forming strength increasing heat treatment ability by adjusting the components so that the value is 0.5 or more and 3.0 or less.

The present invention has been completed by further studies 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:
0.005-0.050%, B: 0.0003-0.0015%, V: 0.01-
0.5% is included, and C, Al, N, Nb, and V are expressed by the following equation (1) and the following equation (2) 0 ≦ {V / 51 + Nb / 93−C / 12} (1) 0.5 ≦ { V / 51 + Nb / 93 -C / 12 + Al / 27} / (N / 14) ≤ 3.0 (2) (where C, Al, N, Nb, V: content of each element (mass%) ) Is satisfied under the condition that
A high-strength cold-rolled steel sheet excellent in heat treatment ability for strength increase after forming and deep drawability, characterized in that it has a composition containing 0.0015% or more of Al and the balance Fe and unavoidable impurities. (2) In (1), in addition to the above composition, further, in mass%, one or two selected from Cu, Ni, Cr, and Mo.
A high-strength cold-rolled steel sheet containing 2.0% or less in total of at least one kind. (3) 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:
0.005-0.050%, B: 0.0003-0.0015%, V: 0.01-
0.5%, and C, Al, N, Nb, and V are expressed by the following equation (1) and the following equation (2) 0 ≦ {V / 51 + Nb / 93−C / 12} (1) 0.5 ≦ { V / 51 + Nb / 93 -C / 12 + Al / 27} / (N / 14) ≤ 3.0 (2) (where C, Al, N, Nb, V: content of each element (mass%) ) Is satisfied under the condition that the steel material has a composition consisting of the balance Fe and unavoidable impurities, and the heating temperature: 9
After hot rolling at a temperature of at least 00 ° C and a finish rolling finish temperature of at least 800 ° C to form a hot-rolled sheet, the hot-rolled sheet is wound at a temperature of 400-
After winding at 800 ° C, cold rolling the hot rolled sheet at a rolling reduction of 60 to 95% to obtain a cold rolled sheet, and then subjecting the cold rolled sheet to recrystallization annealing at a temperature of 700 ° C or higher. A method for producing a high-strength cold-rolled steel sheet which is excellent in heat treatment ability for increasing strength after forming and deep drawability. (4) In (3), in addition to the above composition, further, in mass%, one or two selected from Cu, Ni, Cr and Mo.
A method for producing a high-strength cold-rolled steel sheet, which comprises 2.0% or less in total of at least one kind.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The cold-rolled steel sheet of the present invention is a high-strength cold-rolled steel sheet having an excellent heat-treating ability to increase strength after forming and an excellent deep drawability. First, the reasons for limiting the composition of the cold-rolled steel sheet of the present invention will be described. In addition, mass% is simply described as%.

C: less than 0.0050% C increases the strength of the steel but reduces the ductility and deteriorates the press formability and deep drawability. Therefore, in the present invention, it is preferable to reduce as much as possible.
From the viewpoint of improving the deep drawability, it is preferable that C has an extremely low carbon level, and specifically, it is less than 0.0050%. The content is preferably 0.0030% or less. Further, the lower limit of the amount of C that can be reached without an extreme increase in cost is considered to be about 0.0005% in the current manufacturing technology.

Si: 0.005 to 1.0% Si is a useful element for suppressing the decrease in elongation and improving the strength, but such an effect is observed at a content of 0.005% or more, while 1.0 If the content exceeds%, the surface quality is deteriorated and ductility is deteriorated. Therefore, Si is 0.00
It was limited to the range of 5 to 1.0%. In addition, preferably 0.005
~ 0.75%.

Mn: 0.01 to 1.5% Mn is effective as a strengthening component of steel and also has the effect of forming MnS and suppressing embrittlement due to S. Such an effect is recognized when the content is 0.01% or more, but the content exceeding 1.5% causes deterioration of surface properties and ductility. Therefore, Mn is limited to the range of 0.01 to 1.5%. Incidentally, it is preferably 0.01 to 0.75%.

P: 0.1% or less P is an element that effectively contributes to the strengthening of steel by solid solution strengthening, and can be contained in a predetermined amount according to the required strength level, but if it is contained in excess of 0.1%, deep drawability is obtained. Is reduced. Therefore, P is limited to 0.1% or less. S: 0.01% or less S is mainly present as inclusions (sulfides) in steel and causes a decrease in ductility. Therefore, it is desirable to reduce S as much as possible, but 0.01% is acceptable.

Al: 0.005 to 0.030% Al is contained as a deoxidizing agent and for improving the yield of carbonitride forming elements, but if the content is less than 0.005%, there is no sufficient effect, while 0.030% If the content exceeds N, the amount of N that should be contained in the steel is increased, and slab defects are likely to occur during steelmaking.

N: 0.005 to 0.040% N is an important element in the present invention which plays a role of imparting a heat treatment ability for increasing strength after forming to a steel sheet. But 0.005%
If the content is less than the above, it is difficult to secure a sufficient amount of solute N, and sufficient heat treatment ability for increasing strength after molding cannot be obtained. On the other hand, if the content exceeds 0.040%, the press formability deteriorates. Therefore, N is limited to the range of 0.005 to 0.040%. The content is preferably 0.008 to 0.015%.

Nb: 0.005 to 0.050% Nb is contained together with B, contributes to the refinement of the hot-rolled structure and the cold-rolled recrystallization annealed structure, and combines with the solid solution C to precipitate as NbC and fix the solid solution C. It is an element that has the effect of Further, Nb has a function of binding to solid solution N and fixing it as NbN. When the Nb content is less than 0.005%, not only is it difficult to precipitate and fix the solid solution C, but also the refinement of the hot rolled structure and the cold rolled recrystallization annealed structure becomes insufficient. On the other hand, if the content exceeds 0.050%, ductility is lowered.
Therefore, Nb is limited to the range of 0.005 to 0.050%. The content is preferably 0.010 to 0.030%.

B: 0.0003 to 0.0015% B, by containing it in combination with Nb, contributes to the refinement of the hot rolled structure and the cold rolled recrystallized structure and improves the secondary work embrittlement resistance. It is an element that has. B content is 0.
If it is less than 0003%, a sufficient miniaturization effect cannot be obtained. on the other hand,
If it is contained in excess of 0.0015%, the amount of BN precipitation increases and the solution treatment in the slab heating stage is hindered. Therefore, B is limited to the range of 0.0003 to 0.0015%. In addition, it is preferably 0.0007 to 0.0012%.

V: 0.01 to 0.5% V is the most important element in the present invention. Before recrystallization annealing, it precipitates as V nitride and fixes N to reduce the solid solution N, and recrystallizes. It has a function of strongly developing a {111} recrystallization texture during annealing and obtaining a high r value.
Further, the precipitated V nitride is re-melted during recrystallization annealing, and has an action of increasing solid solution N again and improving the heat treatment ability for increasing strength after forming. Such an effect is recognized when the content is 0.01% or more, but when it exceeds 0.5%,
Remelting of the V-nitride is less likely to occur during annealing, and excellent post-forming strength increasing heat treatment ability cannot be obtained. For this reason,
V is limited to the range of 0.01 to 0.5%.

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 as follows.
It is contained under the condition that the formula (1) and the following formula (2) are simultaneously satisfied. 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%)) When the relationship between the C content, the Nb content and the V content does not satisfy the formula (1), A small amount of carbide is generated, the amount of solid solution C is increased, the desired recrystallization texture is weakly developed during recrystallization, and a high r value cannot be obtained.

Further, the relationship among the contents of C, Al, N, V and Nb is
(2) is not satisfied, that is, (V / 51 + Nb / 93 −
C / 12 + Al / 27) / (N / 14) is less than 0.5, or
If it exceeds 3.0, the amount of solute N is too large or too small, and as shown in FIG. 1, one of the r value and the post-molding strength increasing heat treatment ability (ΔTS) decreases, which is excellent. The deep drawability and the excellent heat treatment ability to increase strength after molding cannot be satisfied at the same time.

Solid solution N: 0.0015% or more In the cold-rolled steel sheet of the present invention, in order to secure sufficient strength and to obtain an excellent heat treatment capability for increasing strength after forming, N (solid solution state) (Also referred to as N) must be present in an amount (concentration) of 0.0015% or more. Here, the amount of solid solution N is obtained by subtracting the amount of precipitated N from the total amount of N in steel. As a method of analyzing the amount of precipitated N, it is effective to obtain it by an electrolytic extraction analysis method using a potentiostatic electrolysis method, according to the results of comparative examination of various analysis methods by the present inventors. In addition, as a method of dissolving the ground iron used for extraction analysis, there are an acid decomposition method, a halogen method, an electrolytic method, and the like.
Among these, the electrolysis method can stably dissolve only the base iron without decomposing unstable deposits such as charcoal and nitride. Acetylacetone is used as the electrolytic solution, and electrolysis is performed at a constant potential. In the present invention, the result of measuring the amount of precipitated N using the potentiostatic electrolysis method showed the best correspondence with the actual strength of the component.

From the above, in the present invention, the residue extracted by the potentiostatic electrolysis method is chemically analyzed to determine the N content in the residue, which is used as the precipitated N content. In addition, higher △ TS
In order to obtain the above, the amount of solute N is preferably 0.0030% or more, more preferably 0.0045% or more to obtain a higher value, and 0.0060% or more to obtain a further higher value. Further, in the present invention, in addition to the above composition, further Cu, Ni, Cr,
It is preferable to contain one or two or more of Mo in a total amount of 2.0% or less.

Cu, Ni, Cr and Mo all have the effect of strengthening steel, and can be selected and contained as required. The contents of these elements can be appropriately determined according to the desired strength, but in order to obtain the above effects, Cu: 0.05%
As described above, it is desirable to contain Ni: 0.05% or more, Cr: 0.05% or more, Mo: 0.05% or more, but if they are excessively contained, deep drawability deteriorates. For this reason, it is preferable that the total content of Cu, Ni, Cr, and Mo is one or more, and 2.0% or less.

As a component other than the above components, C
There is no problem even if it contains a, Zr, REM, etc. The balance other than the above components is Fe and inevitable impurities.
Inevitable impurities include Sb: 0.01% or less, Sn: 0.1%
Below, Zn: 0.01% or less and Co: 0.1% or less are acceptable.

Next, a method for manufacturing the cold rolled steel sheet according to the present invention will be described. Steel having the above composition is melted by a commonly known melting method such as a converter, and a steel material (slab) is manufactured by using a commonly known continuous casting method, thin slab casting method, or ingot casting method.
It is preferable that From the viewpoint of preventing macrosegregation of the components, it is more preferable to manufacture by the continuous casting method.

Next, a steel material (slab) having the above composition
Is heated to a heating temperature of 900 ° C. or higher, and then hot-rolled to a finish rolling end temperature of 800 ° C. or higher to form a hot-rolled sheet, which is wound at a winding temperature of 400 to 800 ° C. In the present invention, the steel material (slab) is cooled to room temperature once and then heated again to 900 ° C or higher as in the conventional case.
Alternatively, without cooling to room temperature, insert it into the heating furnace as it is and heat it to 900 ℃ or more, or if the temperature of the steel material (slab) is 900 ℃ or more, after slightly keeping heat Energy-saving processes such as immediate rolling, direct feed rolling and direct rolling can be applied without any problems.

Heating temperature: 900 ° C. or higher The heating temperature of the steel material (slab) is fixed to solid solution C and solid solution N in order to improve the deep drawability, and coarsely precipitate as a precipitate {111 } It is advantageous for the development of recrystallized texture, and for that purpose, it is desirable to keep the temperature as low as possible. However, if the heating temperature is lower than 900 ° C, the rolling load during hot rolling increases and the risk of rolling trouble increases. Therefore, the heating temperature of the steel material is preferably 900 ° C or higher. Note that the slab heating temperature is preferably 1300 ° C. or lower due to an increase in scale loss accompanying an increase in oxidized weight.

From the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling, it goes without saying that utilizing a so-called sheet bar heater for heating the sheet bar is an effective method. Yes. Finish rolling end temperature: 800 ° C. or higher In the present invention, the finish rolling end temperature FDT is preferably 800 ° C. or higher. By setting the FDT to 800 ° C or higher, a uniform hot rolled mother board structure can be obtained,
Excellent deep drawability is obtained after recrystallization annealing. On the other hand, FD
If T is less than 800 ° C, the structure of the hot-rolled base plate becomes non-uniform, and the rolling load during hot rolling increases, and the risk of troubles during hot rolling increases. Therefore, the FDT of hot rolling is preferably 800 ° C or higher.

Winding temperature: 400 ° C. or higher and 800 ° C. or lower If the winding temperature CT exceeds 800 ° C., the scale increases and the yield tends to decrease due to scale loss, and the hot-rolled sheet crystal grains become coarse and deep. The drawability deteriorates. CT
Is less than 400 ° C., precipitation of VN in the hot-rolled sheet is insufficient and the deep drawability is deteriorated, and the steel sheet shape is significantly disturbed, increasing the risk of causing problems in actual use. Therefore, the CT is preferably 400 ° C. or higher and 800 ° C. or lower.

In the present invention, part or all of the finish rolling may be lubrication rolling in order to reduce the rolling load during hot rolling. Performing the lubrication rolling is effective from the viewpoint of uniformizing the shape of the steel sheet and the material. The coefficient of friction during lubrication rolling is preferably in the range of 0.25-0.10. Further, it is desirable from the viewpoint of the operational stability of hot rolling that a continuous rolling process in which adjacent sheet bars are joined together and continuously finish rolled.

Then, the hot-rolled sheet is cold-rolled at a rolling reduction of 60 to 95% to obtain a cold-rolled sheet. The hot rolled sheet is preferably subjected to pickling treatment before cold rolling to remove the hot rolled scale. Cold rolling reduction: 60 to 95% In cold rolling, the rolling reduction is preferably 60 to 95%.
When the rolling reduction is less than 60%, the {111} recrystallized texture does not develop, the r value is low, and significant improvement in deep drawability cannot be expected. Further, when the rolling reduction exceeds 95%, the r value rather decreases.

Then, the cold-rolled sheet is subjected to recrystallization annealing at a temperature of 700 ° C. or higher to obtain a cold-rolled sheet. Annealing temperature for recrystallization annealing: 700 ° C. or higher The annealing temperature for recrystallization annealing is preferably 700 ° C. or higher in order to remelt V nitride and secure solid solution N. If the annealing temperature of the recrystallization annealing is less than 700 ° C, the recrystallization is not sufficiently completed, and the remelting of VN is insufficient, so that a high r value and a high post-forming strength increasing heat treatment ability cannot be obtained. The temperature is preferably 800 ° C or higher and 900 ° C or lower. Further, 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 may be insufficiently redissolved. Therefore, the annealing temperature is preferably 5 s or more. Further, the cooling rate after soaking in the recrystallization annealing is VN.
In order to suppress re-precipitation, it is preferable to set the cooling rate in the temperature range from the annealing temperature to 500 ° C or higher to 30 ° C / s or higher.
In addition, it is more preferably 50 ° C./s or more.

Needless to say, after the recrystallization annealing step, temper rolling with an elongation of 10% or less may be added in order to correct the shape and adjust the surface roughness. 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 plate of a surface-treated steel sheet for processing. The surface treatment includes zinc plating (including alloy system), tin plating, enamel and the like.

Further, the cold-rolled steel sheet of the present invention is subjected to a special treatment for improving chemical treatment property, weldability, press formability and corrosion resistance after galvanization, for example, application of a solid lubricant,
Fe-P electroplating may be applied.

[0046]

Example A 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. Then, after heating these steel materials (slabs) to various temperatures shown in Table 2,
Hot rolling was performed under the conditions shown in Table 2 to obtain a hot-rolled steel strip (hot-rolled sheet) having a sheet thickness of 4.0 mm. Subsequently, these hot-rolled steel strips (cold-rolled strips) were subjected to pickling treatment and cold-rolled under the conditions shown in Table 2 to obtain cold-rolled steel strips (cold-rolled strips) having a plate thickness of 1.2 mm. Then,
These cold-rolled steel strips (cold-rolled sheets) were continuously annealed in Table 2
Recrystallization annealing was performed at the annealing temperature and time shown in to obtain a cold rolled annealed steel strip (cold rolled annealed sheet). After annealing, it was cooled from the annealing temperature to 500 ° C under the conditions shown in Table 2. Further, these cold-rolled and annealed steel strips (cold-rolled and annealed sheets) were further temper-rolled at an elongation of 0.8%.

A test piece was taken from the obtained cold rolled steel strip,
Tensile properties, r-values, and post-molding strength increasing heat treatment ability were measured by the test methods shown below. The amount of dissolved N in the obtained cold rolled steel strip was measured by the method described above. (1) Tensile properties JIS 5 tensile test pieces were taken from each cold-rolled steel strip obtained,
A tensile test was carried out in accordance with the regulations of JIS Z 2241, and the yield stress YS, tensile strength TS, and elongation EI were obtained. (2) r value JIS 5 sampled from each of the obtained cold-rolled steel strips in the rolling direction (L direction), in the rolling direction 45 ° (D direction), and in the rolling direction 90 ° (C direction) Width strain and plate thickness strain of each test piece when 15% uniaxial tensile prestrain was applied to the No. test piece, the ratio of width strain and plate thickness strain, r = 1n (w / w _o ) / From 1 n (t / t _o ) (where w _o and t _o are the width and plate thickness of the test piece before the test, and w and t are the width and plate thickness of the test piece after the test). The r value in the direction was calculated, and the average r value r _mean was calculated by the following formula r _mean = (r _L + 2r _D + r _C ) / 4, and this was defined as the r value. Here, 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), and r _C is the rolling direction (L direction). ) Is the r value in the 90 ° direction (C direction). (3) Strength increasing heat treatment after forming From each cold-rolled steel strip obtained, JIS No. 5 test piece was sampled in the rolling direction, 10% tensile prestrain was given, and then 170 ℃ × 20m
After in-heat treatment (aging treatment), a tensile test was carried out to obtain the tensile strength TS _HT after pre-deformation-heat treatment (aging treatment), and ΔTS = TS _HT −TS was calculated. In addition, TS is the tensile strength of the cold-rolled steel strip before pre-deformation, that is, obtained.

The results are shown in Table 3.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

Each of the examples of the present invention has a high elongation El of 35% or more El and a high heat treatment for increasing strength after forming of ΔTS: 50 MPa or more, and a high r value of r value: 1.2 or more. As a result, it is a cold-rolled steel sheet that has excellent strength-increasing heat treatment after forming and deep drawability. On the other hand, in the comparative examples outside the scope of the present invention, the steel sheet has a low ΔTS or a low r value.

[0053]

According to the present invention, it is possible to stably manufacture a cold-rolled steel sheet having an excellent deep drawability of r value: 1.2 or more and a high post-forming strength increasing heat treatment ability of ΔTS: 50 MPa or more. And, it has a great effect on the industry. Further, 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 an automobile body.

[Brief description of drawings]

FIG. 1 is a graph showing the influence of V, Nb, C, Al, and N contents on the r value and the heat treatment capacity for increasing strength (ΔTS) after forming.

Continued front page    (72) Inventor Kei Sakata             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Technical Research Institute of Iron Co., Ltd. F-term (reference) 4K037 EA01 EA02 EA04 EA11 EA13                       EA15 EA17 EA18 EA19 EA20                       EA23 EA25 EA27 EA32 FA02                       FA03 FC03 FC04 FC05 FE01                       FE02 FE03 FG00 FJ05 FJ06                       FJ07

Claims (4)

[Claims] 1. Mass% C: less than 0.0050%, Si: 0.005-1.0%, Mn: 0.01-1.5%, P: 0.1% or less, S: 0.01% or less, Al: 0.005-0.03%, 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 are expressed by the following formula (1) and formula (2). It is contained under a satisfactory condition, further contains 0.0015% or more of N in a solid solution state, and has a composition consisting of balance Fe and unavoidable impurities. High tensile cold rolled steel sheet. Note 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. In addition to the above composition, C in% by mass is further included.
A total of 2.0% or less of one or more selected from u, Ni, Cr and Mo is contained.
High-strength cold-rolled steel sheet according to. 3. 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 are expressed by the following formula (1) and formula (2). A steel material containing under the satisfying conditions and having a composition consisting of balance Fe and unavoidable impurities was hot-rolled at a heating temperature of 900 ° C or higher and a finish rolling end temperature of 800 ° C or higher to obtain a hot-rolled sheet. Then, the coiling temperature of the hot rolled sheet:
Winding at 400-800 ℃, then rolling ratio on the hot-rolled sheet: 60-
After cold rolling at 95% to make a cold-rolled sheet, 70%
A method for producing a high-strength cold-rolled steel sheet excellent in post-forming strength increasing heat treatment ability and deep drawability, which comprises performing recrystallization annealing at a temperature of 0 ° C or higher. Note 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. In addition to the composition, in addition, in% by mass, C
4. A total of 2.0% or less of one or more selected from u, Ni, Cr and Mo.
The method for producing a high-strength cold-rolled steel sheet according to.