JP2016074967A - Method for manufacturing cold rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength cold rolled steel sheet having excellent ductility, process hardening property and stretch flanging property.SOLUTION: A slub having a predetermined chemical composition is heated to the temperature of 1100 to 1280°C, is subjected to hot rolling at a finishing temperature of an Arpoint or more and 850 to 950°C, is started to cool within 0.5 second, is cooled to the temperature of (a hot rolling finishing temperature - 50) to 750°C at an average cooling rate of 200°C/s or more, is let to cool for 0.3 to 2.0 seconds, is cooled to 700°C or less at an average cooling rate of 30°C/s or more, is thereafter rolled up at less than 600°C. The hot rolled steel sheet is subjected to cool rolling and the resultant cool rolled steel sheet is heated to the temperature of (Acpoint - 40) to 900°C according to a condition in which an average temperature-rising rate from 700°C to the completion of heating is set to be 1°C/s or more, is held at the aforementioned temperature, is cooled to the temperature of 200 to 450°C according to a condition in which an average cooling rate from (Acpoint - 60) to the completion of cooling is set to be 30°C/s or more, and is thereafter held in the temperature range of 300 to 450°C.SELECTED DRAWING: None

Description

  The present invention relates to a method for manufacturing a cold-rolled steel sheet.

  Now that the industrial technology field is highly fragmented, special and advanced performance is required for materials used in each technical field. For example, steel sheets used by press forming are also required to have better formability with the diversification of press shapes. In addition, high strength is required, and application of high-tensile steel sheets is being studied. In particular, regarding automotive steel sheets, in consideration of the global environment, the demand for thin-walled, high-formability, high-tensile steel sheets has been remarkably increasing in order to reduce vehicle weight and improve fuel efficiency. In press molding, since the thinner the steel sheet used, the easier it is to generate cracks and wrinkles, a steel sheet with better ductility and stretch flangeability is required. However, these press formability and high strength of the steel sheet are contradictory characteristics, and it is difficult to satisfy these characteristics at the same time.

Until now, as a method for improving the press formability of a high-tensile cold-rolled steel sheet, many techniques relating to micronization of the microstructure have been proposed. For example, Patent Document 1 discloses a method for producing an ultrafine-grained high-strength hot-rolled steel sheet that performs rolling with a total sheet thickness reduction rate of 80% or more in a temperature range near the Ar ₃ point in a hot rolling process. Patent Document 2 discloses a method for producing ultrafine-grained ferritic steel in which rolling with a sheet thickness reduction rate of 40% or more is continuously performed in a hot rolling process.

  Patent Document 3 discloses a method for producing a hot-rolled steel sheet having ultrafine grains, in which a reduction in a dynamic recrystallization region is performed by a reduction pass of 5 stands or more in a hot rolling process.

  Regarding cold-rolled steel sheets having a microstructure, in Patent Document 4, residual austenite having an average crystal grain size of 5 μm or less is dispersed in ferrite having an average crystal grain size of 10 μm or less. An excellent high strength cold rolled steel sheet for automobiles is disclosed.

  Patent Document 5 discloses a high-strength steel sheet excellent in elongation and stretch flangeability in which a second phase composed of retained austenite and / or martensite is finely dispersed in crystal grains.

  Patent Document 6 discloses a high-tensile melt excellent in ductility, stretch flangeability, and fatigue resistance, in which retained austenite and low-temperature transformation product phase are dispersed in ferrite and tempered martensite having an average crystal grain size of 10 μm or less. A galvanized steel sheet is disclosed. Tempered martensite is an effective phase for improving stretch flangeability and fatigue resistance, and it is said that these properties will be further improved if tempered martensite is refined.

  In Patent Document 7, immediately after hot rolling, it is rapidly cooled to 720 ° C. or lower and held in a temperature range of 600 to 720 ° C. for 2 seconds or more, and the obtained hot-rolled steel sheet is subjected to cold rolling and annealing in fine ferrite. A method for producing a cold-rolled steel sheet in which retained austenite is dispersed is disclosed.

JP 58-123823 A JP 59-229413 A Japanese Patent Laid-Open No. 11-152544 JP-A-11-61326 JP 2005-179703 A JP 2001-192768 A International Publication No. 2007/15541 Pamphlet

The techniques 1 and 2 of Patent Documents improve the balance between strength and ductility in hot-rolled steel sheets. However, there is no description in the above-mentioned Patent Documents about a method of making a cold-rolled steel sheet finer and improving press formability. According to the study by the present inventors, when cold rolling and annealing are performed using a fine-grained hot-rolled steel sheet obtained by rolling under large rolling as a base material, the crystal grains are likely to be coarsened and have excellent press formability. It is difficult to get. In particular, in the manufacture of a cold-rolled steel sheet having a microstructure including a low-temperature transformation generation phase and residual austenite that requires annealing in a high temperature range of Ac ₁ point or higher, coarsening of crystal grains during annealing is remarkable. In addition, the advantage of the cold-rolled steel sheet having excellent ductility cannot be enjoyed.

  In the technique of Patent Document 3, it is necessary to extremely reduce the temperature drop during hot rolling, and it is difficult to carry out with normal hot rolling equipment. Moreover, although the example which performed cold rolling and annealing after hot rolling is shown, the balance of tensile strength and hole expansibility is bad, and press formability is inadequate.

  As described in Patent Document 4, a steel sheet containing retained austenite in a metal structure exhibits a large elongation due to transformation-induced plasticity (TRIP) generated by austenite becoming martensite during processing. And the hole expandability is impaired. In the cold-rolled steel sheet disclosed in Patent Document 4, ductility and hole expandability are improved by refining ferrite and retained austenite, but the hole expansion ratio is 1.5 at most, and sufficient press It is hard to say that it has moldability. Further, in order to improve the work hardening index and improve the collision resistance safety, the main phase needs to be a soft ferrite phase, and it is difficult to obtain a high tensile strength.

  In the technique of Patent Document 5, in order to refine the second phase to a nano size and disperse it in the crystal grains, a large amount of expensive elements such as Cu and Ni are contained, and a solution treatment is performed at a high temperature for a long time. It is necessary to increase the manufacturing cost and the productivity.

  In order to obtain a metal structure containing tempered martensite and retained austenite as in the technique described in Patent Document 6, primary annealing for generating martensite, tempering martensite, and further obtaining retained austenite. Subsequent annealing is required, and productivity is greatly impaired.

  The technique of Patent Document 7 does not release the processing strain accumulated in austenite after hot rolling, but transforms the ferrite using the processing strain as a driving force, thereby forming a fine grain structure, resulting in workability and thermal stability. This is excellent in that a cold-rolled steel sheet with improved slab is obtained. However, in recent years, there has been a demand for cold-rolled steel sheets having high strength, good ductility, good work hardenability and good stretch flangeability at the same time due to the need for higher performance.

  The present invention has been made to meet such demands, and an object of the present invention is to provide a method for producing a high-tensile cold-rolled steel sheet having a tensile strength of 780 MPa or more and having excellent ductility, work-hardening properties and stretch flangeability. And

  The inventors of the present invention have made extensive studies to solve the above problems and have obtained the following knowledge.

  (A) A hot-rolled steel sheet manufactured through a so-called immediate quenching process in which water quenching is performed immediately after hot rolling, specifically, quenching to a temperature range of 720 ° C. or less within 0.40 seconds after completion of hot rolling. When the hot-rolled steel sheet manufactured in this manner is cold-rolled and annealed, the ductility and stretch flangeability of the cold-rolled steel sheet are improved as the annealing temperature rises. However, if the annealing temperature is too high, the austenite grains become coarse, and the ductility and stretch flangeability of the annealed steel sheet may deteriorate rapidly.

  (B) When the final reduction ratio of hot rolling is increased, it is possible to suppress the austenite grains from becoming coarse during the high-temperature annealing. The reason for this is not clear, but the larger the final rolling reduction, the more (a) the ferrite fraction increases in the metal structure of the hot-rolled steel sheet and the ferrite becomes finer, and (b) the metal structure of the hot-rolled steel sheet. (C) Since the ferrite grain boundary functions as a nucleation site in the transformation from ferrite to austenite during annealing, the nucleation frequency increases as the amount of fine ferrite increases. It is presumed that austenite is refined and (d) the coarse low-temperature transformation formation phase becomes coarse austenite grains during annealing.

  (C) In the winding process immediately after the rapid cooling process, when the winding temperature is increased, austenite grain coarsening that may occur during annealing at a high temperature after cold rolling is suppressed. The reason for this is not clear, but (a) immediately after the hot-rolled steel sheet is refined by a rapid cooling process, the precipitation amount of iron carbide in the hot-rolled steel sheet increases remarkably as the coiling temperature rises. (B) Since iron carbide functions as a nucleation site in the transformation from ferrite to austenite during annealing, the nucleation frequency increases as the precipitation amount of iron carbide increases, and austenite becomes finer (c ) It is presumed that the undissolved iron carbide suppresses the austenite grain growth and the austenite becomes finer.

  (D) As the Si content in the steel increases, the effect of preventing coarsening of austenite grains becomes stronger. The reason for this is not clear, but as the Si content increases, (a) the iron carbide becomes finer and its number density increases. (B) This increases the nucleation frequency in the transformation from ferrite to austenite. It is presumed to be caused by the further increase and (c) the increase in undissolved iron carbide further suppresses the grain growth of austenite and further refines the austenite.

  (E) When cooling is performed by soaking at a high temperature while suppressing coarsening of austenite grains, a metal structure in which a fine low-temperature transformation generation phase is the main phase and the second phase contains fine retained austenite is obtained.

  (F) In order to suppress the coarsening of austenite grains, in addition to the above conditions, it is important to increase the heating rate after cold rolling, that is, 1 ° C./second or more.

  This invention is made | formed based on said knowledge, and makes a summary the manufacturing method of the following cold-rolled steel plate.

[1] A method for producing a cold-rolled steel sheet comprising the following steps (A) to (C), wherein the main phase is a low-temperature transformation generation phase and has a metal structure containing residual austenite:
(A) The chemical composition is mass%,
C: more than 0.10% and less than 0.30%,
Si: more than 0.50% and 2.50% or less,
Mn: more than 1.00% and 3.50% or less,
P: 0.10% or less,
S: 0.010% or less,
sol. Al: 1.50% or less and N: 0.010% or less,
At least one selected from Ti: less than 0.001 to less than 0.100% and Nb: less than 0.001 to less than 0.050%;
V: 0 to 0.50%
Cr: 0 to 1.0%,
Mo: 0 to 0.50%,
B: 0 to 0.010%,
Ca: 0 to 0.010%,
Mg: 0 to 0.010%,
REM: 0 to 0.050% and Bi: 0 to 0.050%,
Remaining part: A slab composed of Fe and impurities is heated to 1100 to 1280 ° C., and hot-rolled under a condition where the finishing temperature is Ar ₃ or higher and a temperature range of 850 to 950 ° C.,
Cooling is started within 0.5 seconds after completion of hot rolling, and is cooled to a temperature range of (hot rolling finishing temperature −50) to 750 ° C. under the condition that the average cooling rate is 200 ° C./second or more.
After cooling for 0.3 to 2.0 seconds and cooling to 700 ° C. or lower under the condition that the average cooling rate is 30 ° C./second or higher,
Winding in a temperature range of less than 600 ° C. to obtain a hot-rolled steel sheet;
(B) A step of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet, and (C) a condition that an average temperature increase rate from 700 ° C. to heating stop is 1 ° C./second or more. in heating to a temperature range of (Ac ₃ point -40) to 900 ° C., and held at that temperature _range, under the condition that the _(Ar 3 -60) average cooling rate from ° C. until the cooling stops 30 ° C. / sec or more The process of hold | maintaining in the 300-450 degreeC temperature range, after cooling to the temperature range of 200-450 degreeC.

[2] In the step (A), the time t (second) between the start of rolling in the final rolling pass and the completion of rolling in the pass immediately before the final rolling pass is the rolling completion temperature T in the pass immediately before the final rolling pass. The method for producing a cold-rolled steel sheet according to the above [1], which satisfies the following formula (1) in relation to (° C.).
0.0015 / exp {−6080 / (T + 273)} ≦ t ≦ 2.0 (1)

[3] The chemical composition is mass%,
V: contains 0.010 to 0.50%,
The method for producing a cold-rolled steel sheet according to [1] or [2].

[4] The chemical composition is mass%,
Cr: 0.20 to 1.0%
Containing one or more selected from Mo: 0.05 to 0.50% and B: 0.0010 to 0.010,
The manufacturing method of the cold rolled steel sheet in any one of said [1]-[3].

[5] The chemical composition is mass%,
Ca: 0.0005 to 0.010%,
Mg: 0.0005-0.010%,
Containing one or more selected from REM: 0.0005 to 0.050% and Bi: 0.0010 to 0.050%,
The manufacturing method of the cold rolled steel plate in any one of said [1]-[4].

  According to the present invention, it is possible to produce a high-tensile cold-rolled steel sheet having a tensile strength of 780 MPa or more and having excellent ductility, work-hardening properties, and stretch flangeability.

  The metal structure and chemical composition of the high-tensile cold-rolled steel sheet produced by the method according to the present invention and the rolling and annealing conditions for producing such a steel sheet efficiently, stably and economically will be described in detail below. .

1. Metal Structure The cold-rolled steel sheet of the present invention has a metal structure in which the main phase is a low-temperature transformation generation phase and contains retained austenite. Such a metal structure is suitable for improving ductility, work hardenability and stretch flangeability while maintaining tensile strength. If the main phase is polygonal ferrite that is not a low-temperature transformation generation phase, it is difficult to ensure tensile strength and stretch flangeability.

  The main phase means a phase or structure having the maximum volume fraction, and a phase and structure other than the main phase are called a second phase. The low temperature transformation generation phase refers to a phase and a structure generated by low temperature transformation, such as martensite, tempered martensite, bainite, bainitic ferrite and the like. Bainitic ferrite is distinguished from polygonal ferrite in that it has a lath-like (plate-like) or granular-like (bulk-like) form and a high dislocation density, and there is no iron carbide inside and at the interface of the grains. Distinguished from bainite. This low temperature transformation product phase may contain two or more phases or structures, for example, martensite and bainitic ferrite. When the low temperature transformation product phase includes two or more phases and structures, the sum of the volume fractions of these phases and tissues is defined as the volume fraction of the low temperature transformation product phase.

  In order to improve the ductility, the volume ratio of the retained austenite with respect to the entire structure is preferably more than 5.0%. This volume fraction is more preferably more than 6.0%, particularly preferably more than 8.0% and most preferably more than 10.0%. On the other hand, if the volume ratio of retained austenite is excessive, stretch flangeability deteriorates. Accordingly, the volume ratio of retained austenite is preferably less than 25.0%. More preferably it is less than 18.0%, particularly preferably less than 16.0%, and most preferably less than 14.0%.

  In order to further improve the balance between ductility and stretch flangeability, the average carbon concentration of retained austenite is preferably 0.80% or more. More preferably, it is 0.84% or more. On the other hand, if the average carbon concentration of retained austenite is excessive, stretch flangeability deteriorates. Therefore, the average carbon concentration of retained austenite is preferably less than 1.7%. More preferably, it is less than 1.6%, more preferably less than 1.4%, and particularly preferably less than 1.2%.

The martensite, retained austenite, or a mixed structure thereof (collectively referred to as “MA (MA Constituents)”) preferably has a circle equivalent particle size of 1.2 μm or more and a volume ratio of 20% or less. . This is because stretch flangeability may be impaired. More preferably, it is 15% or less, More preferably, it is 10% or less, Most preferably, it is 8% or less. From the viewpoint of ensuring both strength and elongation, it is preferably 1% or more, more preferably 2% or more, still more preferably 3% or more, and particularly preferably 4% or more. Moreover, when there are many coarse grains having a particle size of MA of 2 μm or more, work hardening and stretch flangeability may be impaired. Therefore, the number density of MA having a circle-equivalent particle diameter of 2 μm or more is preferably 20 × 10 ⁻⁴ pieces / μm ² or less. More preferably, it is 15 × 10 ⁻⁴ pieces / μm ² or less, and particularly preferably 10 × 10 ⁻⁴ pieces / μm ² or less.

  In order to further improve the ductility and work hardenability, the second phase may contain polygonal ferrite in addition to retained austenite. The volume ratio of the polygonal ferrite to the entire structure is preferably 0 to 30%. More preferably, it is 25% or less, more preferably 20% or less, particularly preferably 15% or less, and most preferably 10% or less.

  The metal structure of the cold rolled steel sheet according to the present invention is measured as follows. That is, the volume ratio of the low-temperature transformation generation phase and polygonal ferrite is obtained by taking a test piece from a steel plate, polishing a longitudinal section parallel to the rolling direction, and subjecting it to a corrosion treatment with nital. The metal structure is observed using the SEM at the depth position, and the area ratios of the low-temperature transformation generation phase and the polygonal ferrite are measured by image processing, and the respective volume ratios are obtained assuming that the area ratio is equal to the volume ratio.

  The volume ratio of MA was observed with an optical microscope using a sample subjected to repeller corrosion, and the total area ratio of MA having a circle-equivalent particle size of 1.2 μm or more was measured by image analysis. The area ratio was determined as being equal to the volume ratio. The reason why the thickness is 1.2 μm or more is that it is a relatively coarse material that affects the stretch flangeability. In addition, the number density of MA having a circle-equivalent particle diameter of 2 μm or more was also determined by image analysis through structural observation with an optical microscope.

  The volume ratio of retained austenite is obtained by collecting a specimen for XRD measurement from a steel plate, chemically polishing the rolled surface from the steel plate surface to a 1/4 depth position of the plate thickness, performing an X-ray diffraction test, and measuring the volume of retained austenite. The fraction was measured. Specifically, RINT 2500 made by Rigaku is used for the X-ray diffractometer, Co-Kα rays are incident, and α phase (110), (200), (211) diffraction peaks and γ phase (111), (200) The integrated intensity of the (220) diffraction peak was measured to determine the volume fraction of retained austenite.

The lattice constant dγ (Å) was determined from the diffraction angles of the γ phase (111), (200), and (220) diffraction peaks, and the average carbon concentration Cγ (mass%) of retained austenite was determined by the following conversion formula.
Cγ = (dγ−3.572 + 0.000015 × Si−0.0012 × Mn) /0.033

  In the present invention, in the case of a cold-rolled steel sheet, the above-described metal structure is defined at a position at a quarter depth of the sheet thickness from the steel sheet surface. This is because the metal structure at the 1/4 depth position of the plate thickness represents the metal structure in the entire region in the depth direction of the plate thickness.

  As a mechanical property that can be realized based on the above-mentioned features on the metal structure, the steel sheet of the present invention has a tensile strength (TS of 750 MPa, preferably 780 MPa or more in the direction orthogonal to the rolling direction in order to ensure shock absorption. ), Preferably 950 MPa, more preferably 980 MPa or more. Furthermore, TS × u. It is preferable that the value of El is 12000 MPa% or more, the value of TS × El is 20000 MPa% or more, and the value of the hole expansion ratio λ is 40% or more. Where u. El is uniform elongation.

2. Chemical composition of steel C: more than 0.10% and less than 0.30% When the C content is 0.10% or less, it is difficult to obtain a metal structure mainly composed of a low-temperature phase. Therefore, the C content is more than 0.10%. Preferably it is more than 0.12%, more preferably more than 0.14%, particularly preferably more than 0.16%. On the other hand, if the C content is 0.30% or more, not only the stretch flangeability of the steel sheet is impaired, but also the weldability deteriorates. Therefore, the C content is less than 0.30%. Preferably it is 0.25% or less, more preferably 0.22% or less, particularly preferably 0.20% or less.

Si: more than 0.50% and not more than 2.50% Si has an effect of improving ductility, work hardenability and stretch flangeability. Moreover, it is an element which has the effect | action which improves the stability of austenite and is effective in obtaining said metal structure. When the Si content is 0.50% or less, it is difficult to obtain the effect by the above action. Therefore, the Si content is more than 0.50%. Preferably it is more than 0.70%, more preferably more than 0.90%, particularly preferably more than 1.20%. On the other hand, when the Si content exceeds 2.50%, the surface properties of the steel sheet deteriorate. Furthermore, the plating property is significantly deteriorated. Therefore, the Si content is 2.50% or less. Preferably it is less than 2.2%, More preferably, it is less than 2.0%, Most preferably, it is less than 1.8%. When Al is contained, Si and sol. The total content of Al is preferably 0.60% or more, more preferably 0.90% or more, and particularly preferably 1.20% or more.

Mn: more than 1.00% and not more than 3.50% Mn has an effect of improving the hardenability of steel and is an effective element for obtaining the above metal structure. If the Mn content is 1.00% or less, it is difficult to obtain the above metal structure. Therefore, the Mn content is more than 1.00%. Preferably it is more than 1.20%, more preferably more than 1.40%, particularly preferably more than 1.50%. If the Mn content is excessive, a coarse low-temperature transformation phase that extends in the rolling direction occurs in the metal structure of the hot-rolled steel sheet, and coarse residual austenite grains increase in the metal structure after cold rolling and annealing. , Work hardenability and stretch flangeability deteriorate. Therefore, the Mn content is 3.50% or less. Preferably it is less than 3.20%, more preferably less than 3.0%, particularly preferably less than 2.80%.

P: 0.10% or less P is an element contained in the steel as an impurity, and segregates at the grain boundaries to embrittle the steel. For this reason, the smaller the P content, the better. Therefore, the P content is 0.10% or less. Preferably it is less than 0.050%, more preferably less than 0.020%, particularly preferably less than 0.015%.

S: 0.010% or less S is an element contained in steel as an impurity, and forms sulfide inclusions to deteriorate stretch flangeability. For this reason, the smaller the S content, the better. Therefore, the S content is set to 0.010% or less. Preferably it is less than 0.005%, more preferably less than 0.003%, particularly preferably less than 0.002%.

sol. Al: 1.50% or less Al has an action of deoxidizing molten steel. In the present invention, since Si having a deoxidizing action is contained in the same manner as Al, Al is not necessarily contained. That is, it may be as close to 0% as possible. Al, like Si, has the effect of increasing the stability of austenite and is an effective element for obtaining the above-described metal structure. Therefore, Al can be contained for this purpose. However, sol. If the Al content is too high, not only surface flaws are likely to occur due to alumina, but the transformation point is greatly increased, making it difficult to obtain a metal structure having a low-temperature transformation generation phase as a main phase. Therefore, sol. The Al content is 1.50% or less. sol. Al is preferably contained in an amount of 0.0050% or more, more preferably more than 0.020%, more preferably 0.10% or more, still more preferably 0.12% or more, and particularly preferably 0.14% or more. . On the other hand, sol. Al is preferably contained in an amount of less than 1.0%, more preferably less than 0.80%, even more preferably less than 0.50%, and particularly preferably less than 0.30%. When it is included for the purpose of deoxidation only, the sol. Al is preferably less than 0.10%, more preferably less than 0.080%, and still more preferably less than 0.060%.

N: 0.010% or less N is an element contained in steel as an impurity, and deteriorates ductility. For this reason, the smaller the N content, the better. Therefore, the N content is set to 0.010% or less. Preferably it is 0.006% or less, More preferably, it is 0.005% or less.

Ti: 0.001 to 0.100%
Nb: 0.001 to less than 0.050% Ti and Nb have the effect of increasing the work strain by suppressing recrystallization in the hot rolling process and refining the metal structure of the hot-rolled steel sheet. Moreover, it precipitates as a carbide | carbonized_material or nitride, and has the effect | action which suppresses the coarsening of the austenite during annealing. Therefore, at least one of these elements is contained. To obtain these effects, Ti is contained by 0.005% or more, and Nb is contained by 0.005% or more. However, even if it contains excessively, the effect by the said effect | action will be saturated and it will become uneconomical. In addition, the recrystallization temperature during annealing increases, the metal structure after annealing becomes non-uniform, and stretch flangeability is also impaired. Furthermore, the precipitation amount of carbide or nitride increases, the yield ratio increases, and the shape freezing property also deteriorates. Therefore, the Ti content is less than 0.100% and the Nb content is less than 0.050%. The Ti content is preferably less than 0.080%, more preferably less than 0.060%, more preferably less than 0.050%, even more preferably less than 0.040%, particularly preferably less than 0.030%. is there. The Nb content is preferably less than 0.040%, more preferably less than 0.030%, more preferably less than 0.02%. The lower limit of the Ti content is preferably 0.010% or more, and the lower limit of the Nb content is preferably 0.010% or more.

V: 0 to 0.50%
V, like Ti and Nb, has the action of increasing the work strain by suppressing recrystallization in the hot rolling process and refining the metal structure of the hot-rolled steel sheet. Moreover, it precipitates as a carbide | carbonized_material or nitride, and has the effect | action which suppresses the coarsening of the austenite during annealing. For this reason, you may contain V. However, even if it contains excessively, the effect by the said effect | action will be saturated and it will become uneconomical. In addition, the recrystallization temperature during annealing increases, the metal structure after annealing becomes non-uniform, and stretch flangeability is also impaired. Moreover, the precipitation amount of carbide or nitride increases, the yield ratio increases, and the shape freezing property also deteriorates. Therefore, when V is contained, the content is set to 0.50% or less. The V content is preferably 0.30% or less, and more preferably less than 0.050%. In order to sufficiently obtain the above effects, the V content is preferably 0.010% or more, and more preferably 0.020% or more.

Cr: 0 to 1.0%
Mo: 0 to 0.50%
B: 0 to 0.010%
Cr, Mo, and B have an effect of improving the hardenability of steel and are effective elements for obtaining the above-described metal structure. Therefore, you may contain 1 or more types of these elements. However, even if these elements are contained excessively, the above effects are saturated and uneconomical. Therefore, when these elements are contained, the C content is 1.0% or less, the Mo content is 0.50% or less, and the B content is 0.010% or less. The Cr content is preferably 0.50% or less, the Mo content is preferably 0.10% or less, and the B content is preferably 0.0003% or less. In order to sufficiently obtain the above effects, it is preferable that Cr is contained in an amount of 0.20% or more, Mo is 0.05% or more, and B is contained in 0.0010% or more.

Ca: 0 to 0.010%
Mg: 0 to 0.010%
REM: 0 to 0.050%
Bi: 0 to 0.050%
Ca, Mg, and REM have the effect of improving the stretch flangeability by adjusting the shape of inclusions, and Bi by refining the solidified structure. Therefore, you may contain 1 or more types of these elements. However, even if these elements are contained excessively, the above effects are saturated and uneconomical. Therefore, the Ca content is 0.010% or less, the Mg content is 0.010% or less, the REM content is 0.050% or less, and the Bi content is 0.050% or less. Preferably, the Ca content is 0.0001% or less, the Mg content is 0.000020% or less, the REM content is 0.000020% or less, and the Bi content is 0.010% or less. In order to sufficiently obtain the above effect, it is preferable to contain 0.0005% or more of Ca, 0.0005% or more of Mg, 0.0005% or more of REM, and 0.0010% or more of Bi. Note that REM means a rare earth element and is a generic name for a total of 17 elements of Sc, Y and lanthanoid, and the REM content is the total content of these elements.

  The chemical composition of the cold-rolled steel sheet obtained by the method of the present invention includes each of the above elements in a prescribed range, and the balance is composed of Fe and impurities. An impurity means the component mixed by raw materials and other factors, such as an ore and a scrap, when manufacturing steel materials industrially.

3. Manufacturing Conditions (1) Hot Rolling Step First, a steel ingot or steel slab having the above chemical composition (hereinafter referred to as “slab”) is heated at 1100 to 1280 ° C. If the heating temperature is less than 1100 ° C., it is generally difficult to ensure a finishing temperature of Ar 3 or higher, and if it exceeds 1280 ° C., the heating cost increases and the yield decreases due to scale loss. Therefore, the heating temperature of the slab is set to 1100 to 1280 ° C.
The equipment for performing hot rolling may be either a reverse mill or a tandem mill. From the viewpoint of industrial productivity, it is preferable to use a tandem mill for at least the last several stages.

Subsequently, the slab heated to the above temperature is hot-rolled under conditions where the finishing temperature is not less than Ar ₃ and is in the temperature range of 850 to 950 ° C. In the present invention, strain is accumulated in austenite crystal grains by hot rolling, release of the accumulated strain is suppressed by cooling after hot rolling, and this strain is used as a driving force when it reaches a predetermined low temperature range. The crystal grains are refined by rapidly promoting the transformation from austenite to ferrite. Therefore, since it is necessary to perform hot rolling in the austenite region, the finishing temperature is Ar ₃ or more and 850 to 950 ° C. In addition, the said temperature is the surface temperature of a steel plate, and can be measured with a radiation thermometer etc.

  The total rolling reduction in hot rolling is preferably 90% or more in terms of sheet thickness reduction rate in order to promote the refinement of ferrite. Furthermore, it is preferably 92%, particularly preferably 94% or more. From the same point of view, the sheet thickness reduction rate in the temperature range above the rolling completion temperature (rolling completion temperature + 100 ° C.) or less is preferably 40% or more, and the temperature range above the rolling completion temperature (rolling completion temperature + 80 ° C.) or less. More preferably, the plate thickness reduction rate is 60% or more.

  The plate thickness reduction rate in the temperature range of 1150 to 1050 ° C. in hot rolling is preferably 40% or more. This is to suppress the precipitation of Nb or Ti and promote the refinement of γ grains during cold rolling annealing. More preferably, it is 45% or more, more preferably 50% or more, still more preferably 55% or more, particularly preferably 60% or more, and most preferably 70% or more.

  When hot rolling is performed by multi-pass rolling, by making the sheet thickness reduction rate per pass 15% or more, strain to austenite can be efficiently accumulated, and after hot rolling up to room temperature By making the transformation from austenite to ferrite in the cooling process, it becomes easy to refine the structure. Therefore, the plate thickness reduction rate per pass is preferably 15% or more. Moreover, since the excessive increase in rolling load is suppressed by setting the sheet thickness reduction rate per pass to 60% or less, not only can the enlargement of the rolling equipment be avoided, but also the plate shape Control is also easy. Therefore, the plate thickness reduction rate per pass is preferably 60% or less. According to the present invention, fine ferrite crystal grains can be obtained even by rolling in a plurality of passes with a plate thickness reduction rate per pass of 50% or less. Therefore, particularly when it is desired to easily control the plate shape, the plate thickness reduction rate of the final two passes is preferably set to 45% / pass or less.

A time t (second) between the rolling start in the final rolling pass and the rolling completion in the pass immediately before the final rolling pass is expressed by the following formula (1) in relation to the rolling completion temperature T (° C.) in the pass immediately before the final rolling pass. ) Is preferable.
0.0015 / exp {−6080 / (T + 273)} ≦ t ≦ 2.0 (1)

  By setting the time t to 0.0015 / exp {−6080 / (T + 273)} or more, recrystallization of austenite can be promoted. On the other hand, when the time t is set to 2.0 or less, the grain growth of austenite is suppressed, and the hot-rolled steel sheet that dulls the effect of improving the deep drawability of the cold-rolled steel sheet by refining the crystal grains of the hot-rolled steel sheet. Crystal grain refinement can be achieved while suppressing the texture development and enhancing the recrystallization promotion effect. From the viewpoint of promoting the recrystallization of austenite and improving the deep drawability of the cold-rolled steel sheet by refining the crystal grains of the hot-rolled steel sheet, particularly improving the anisotropy, the time t is 0.0018 / exp {- 6080 / (T + 273)} or more is preferable, and 0.002 / exp {−6080 / (T + 273)} or more is more preferable. The time t is preferably 1.5 seconds or less, and more preferably 1.0 seconds or less.

  The hot-rolled steel sheet obtained by the above method starts cooling within 0.5 seconds after completion of hot rolling, and the average cooling rate is 200 ° C./second or more (hot rolling finishing temperature −50). It is cooled to a temperature range of ˜750 ° C. (hereinafter referred to as “first cooling”).

  The distribution of strain applied by hot rolling in the thickness direction increases from the center of the plate thickness toward the steel plate surface, but in extremely low carbon steel with high grain growth, strain in the region near the steel plate surface is extremely easy. Will be released. However, after the hot rolling is completed, if it is rapidly cooled (average cooling rate: 200 ° C./second or more) to a temperature range of (hot rolling finishing temperature −50) to 750 ° C., it is introduced into austenite by hot rolling. Since the release of the processed strain is suppressed, after the secondary cooling to be described later is stopped, the strain can be transformed from austenite to ferrite using the strain as a driving force, and as a result, the structure can be refined.

  Therefore, the time from the completion of hot rolling to the start of cooling should be within 0.5 seconds. The time from the completion of hot rolling to the start of cooling is preferably within 0.4 seconds, and more preferably within 0.3 seconds. In particular, the time from the completion of hot rolling to the start of cooling is less than 0.1 seconds from the viewpoint of suppressing the release of work strain of austenite, further promoting ferrite nucleation, and promoting the refinement of the structure. It is good. More preferably, it is 0.05 second or less, More preferably, it is less than 0.02 second.

  If the average cooling rate of the first cooling is too slow, the strain in the vicinity of the steel sheet surface is released and the structure cannot be refined, so the average cooling rate is set to 200 ° C./second or more. On the other hand, when the cooling stop temperature of the first cooling exceeds (hot rolling finishing temperature−50) ° C., the strain in the vicinity of the steel sheet surface is released and the structure cannot be refined. On the other hand, when the temperature is lower than 750 ° C., the ferrite transformation proceeds in a high temperature range, so that the structure cannot be refined. Therefore, it is important to rapidly cool to a temperature range of (hot rolling finish temperature −50) to 750 ° C.

  After stopping the cooling of the first cooling, it is allowed to cool for 0.3 to 2.0 seconds, and is cooled to 700 ° C. or lower under the condition that the average cooling rate is 30 ° C./second or higher (hereinafter referred to as “second cooling”). ).

  By setting the time from the first cooling stop to the second cooling start to be 0.3 seconds or more, the temperature variation of the cooling can be reduced, and the uniformity of the material characteristics is improved. Further, if this time is used to measure the plate thickness, plate shape, plate temperature, etc., accelerated rolling can be performed, and productivity is dramatically increased. During this time, air cooling or air cooling may be used. The time from the first cooling stop to the second cooling start is preferably 0.4 seconds or more. On the other hand, if the time from the first cooling stop to the second cooling start exceeds 2.0 seconds, the processing strain introduced into the austenite by hot rolling is released, making it difficult to refine the structure. Become. Therefore, the water cooling stop time is set to 2.0 seconds or less. Preferably it is less than 1.5 seconds.

  In the secondary cooling, when the average cooling rate is 30 ° C./second or more and 700 ° C. or less, strain can be transformed from austenite to ferrite at a stretch using the driving force, and as a result, the structure can be refined. When the average cooling rate at the time of cooling to 700 ° C. or lower is less than 30 ° C./second, the processing strain introduced into the austenite by hot rolling is released, and it becomes difficult to refine the structure. Therefore, in the secondary cooling, the cooling is started within 0.3 to 2.0 seconds after the first cooling stop, and the cooling is performed to 700 ° C. or less under the condition that the average cooling rate is 30 ° C./second or more. . The average cooling rate is preferably 50 ° C./second or more, and more preferably 100 ° C./second or more. The cooling stop temperature is preferably 680 ° C. or lower.

  Winding is performed in a temperature range below 600 ° C. When the coiling temperature is 600 ° C. or higher, not only Ti or Nb nitrides or carbides are coarsely precipitated during slow cooling after winding, but also the crystal grains are coarsened and coarse iron carbides are precipitated. In the cold-rolled steel sheet after cold rolling and annealing, a desired structure cannot be obtained. As a result, ductility and hole expandability are reduced. In addition, problems such as generation of surface defects and a decrease in yield due to scale loss may occur. Therefore, the coiling temperature needs to be less than 600 ° C. Preferably it is 550 degrees C or less.

  In addition, there is no restriction | limiting in the heat history until winding after secondary cooling. However, 600 to 700 ° C. is a ferrite transformation temperature range where the ferrite transformation is activated, and it is preferable to appropriately manage the steel plate temperature during this period. That is, in the secondary cooling, it is preferable that the cooling is stopped at an arbitrary temperature of 700 ° C. or lower, or the cooling rate is slowed to 20 ° C./second or lower, and the mixture is allowed to cool for 1 second or longer in the ferrite transformation temperature range. This facilitates the formation of a thermally stable ferrite crystal grain structure. Then, by cooling to the coiling temperature at an average cooling rate of 20 ° C./second or more, the above thermally stable ferrite crystal grain structure can be obtained more reliably. In particular, it is preferable to cool in a temperature range of 620 to 700 ° C. for 2 seconds or more. Moreover, after standing to cool, it is good to cool to coiling temperature with the average cooling rate of 30 degrees C / sec or more.

A cold-rolled steel sheet obtained by subjecting the hot-rolled steel sheet to cold rolling is subjected to a treatment such as degreasing according to a known method, if necessary, followed by a predetermined heat treatment. In this heat treatment, the cold-rolled steel sheet is heated to a temperature range of (Ac ₃ points −40) to 900 ° C. under the condition that the average rate of temperature increase from 700 ° C. to the heating stop is 1 ° C./second or more. It is to hold.

The reason why the holding temperature (soaking temperature) in the heat treatment is set to (Ac ₃ points−40) or more is that the main structure is a low-temperature transformation generation phase and contains retained austenite. In order to increase the volume ratio of the low-temperature transformation generation phase and improve stretch flangeability, the above holding temperature is preferably set to a temperature exceeding (Ac ₃ point−20 ° C.), and the temperature exceeding Ac ₃ point More preferably. However, if the above holding temperature becomes too high, austenite becomes excessively coarse, and the bainite transformation is delayed, so that ductility, work hardenability and stretch flangeability are liable to deteriorate. For this reason, the upper limit of soaking temperature shall be 900 degrees C or less. In order to promote the formation of fine polygonal ferrite and improve ductility and work hardenability, the temperature is preferably 880 ° C. or less, and more preferably 860 ° C. or less.

  Although there is no restriction | limiting in particular in said holding time (soaking time), In order to acquire the stable mechanical characteristic, it is preferable to set it as time exceeding 15 seconds, and it is more preferable to set it as time exceeding 60 seconds. On the other hand, if the holding time is too long, the austenite becomes excessively coarse, and ductility, work hardenability and stretch flangeability tend to deteriorate. For this reason, the holding time is preferably less than 150 seconds, and more preferably less than 120 seconds.

In order to promote recrystallization and make the metal structure after annealing uniform and improve stretch flangeability, it is necessary to set the average temperature rising rate to 1.0 ° C./second or more. Here, the average heating rate means an average heating rate from 700 ° C. to the heating stop temperature when the cold-rolled steel sheet is heated and held in a temperature range of (Ac ₃ point−40) to 900 ° C. When this average temperature rise rate is less than 1 ° C./second, austenite coarsening proceeds and the bainite transformation is delayed. As a result, the generation of retained austenite becomes insufficient and the ductility is lowered, and the generation of coarse MA becomes remarkable, and the hole expandability deteriorates. If this average temperature rising rate is too high, the heating cost is unnecessarily large and the productivity is hindered, so that the temperature is preferably less than 10.0 ° C./second. More preferred is less than 8.0 ° C / second, and even more preferred is less than 5.0 ° C / second.

After the above heat treatment, in order to obtain a metal structure as the main phase of the low-temperature transformation product phase, under conditions such that the (Ar 3 _-60) Average cooling rate from ° C. until the cooling stops 30 ° C. / sec or more from 200 to 450 It is necessary to cool to a temperature range of ° C. This is because if the cooling stop temperature exceeds 450 ° C., the volume ratio of the low-temperature transformation phase decreases, and predetermined mechanical properties cannot be obtained, and if it is less than 200 ° C., residual austenite cannot be obtained in the subsequent steps. . The higher the average cooling rate, the higher the volume ratio of the low temperature transformation product phase. Therefore, the average cooling rate is preferably more than 35 ° C./second, more preferably more than 45 ° C./second. On the other hand, if the average cooling rate is too high, the shape of the steel sheet is impaired, so it is preferable to set it to 200 ° C./second or less. More preferably, it is less than 150 ° C./second, more preferably less than 130 ° C./second. The lower limit temperature for stopping cooling is preferably 230 ° C. or higher, more preferably 280 ° C. or higher, further preferably 330 ° C. or higher, particularly preferably 360 ° C., and most preferably 370 ° C. The upper limit of the holding temperature is preferably 440 ° C, and more preferably 430 ° C.

  After cooling is stopped, it is necessary to keep the temperature in the range of 300 to 450 ° C. This is to obtain retained austenite. In order to improve the stability of retained austenite and improve ductility, work hardenability and stretch flangeability, the lower limit of the holding temperature is preferably 360 ° C, more preferably 370 ° C, and the upper limit of the holding temperature. Is preferably 440 ° C., more preferably 430 ° C. In addition, the process hold | maintained in the 300-450 degreeC temperature range may be performed as it is after cooling stops, and may be performed after once heating.

  The holding time in the above temperature range is not particularly limited, but is preferably 30 seconds or more. The longer the holding time, the higher the stability of retained austenite. Therefore, the holding time is more preferably 60 seconds or more. More preferably, it is 120 seconds or more, and particularly preferably more than 300 seconds.

  The heat treatment (annealing) may be a continuous type or a batch type. Moreover, after performing a heat treatment using a continuous hot dip galvanizing line, hot dip galvanizing or alloying hot dip galvanizing may be performed. The steel plate after the heat treatment may be subjected to electroplating, for example, zinc-based plating (including zinc alloy plating such as pure zinc plating and Zn—Ni alloy plating). Any of these plating may be performed according to a conventional method.

  In the case of producing an electroplated steel sheet, the cold-rolled steel sheet produced by the above-described method is subjected to a known pretreatment for surface cleaning and adjustment as necessary, and then electroplated according to a conventional method. The chemical composition and adhesion amount of the plating film are not limited. Examples of types of electroplating include electrogalvanizing and electro-Zn-Ni alloy plating.

  When manufacturing a hot-dip galvanized steel sheet, the annealing process is performed by the above-described method, and the steel sheet is heated as necessary, and then immersed in a plating bath to perform hot dip plating. The alloying treatment may be performed by heating after hot dipping. The chemical composition and the amount of adhesion of the plating film are not limited. Examples of hot dip plating include hot dip galvanizing, alloyed hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, etc. The

  In order to further improve the corrosion resistance, the plated steel sheet may be subjected to an appropriate chemical conversion treatment after plating. The chemical conversion treatment is preferably carried out using a non-chromium chemical conversion treatment solution (for example, silicate-based, phosphate-based, etc.) instead of the conventional chromate treatment.

  The cold-rolled steel sheet thus obtained (including a steel sheet having a surface plated) may be subjected to temper rolling according to a conventional method. However, when the elongation rate of temper rolling is high, ductility is deteriorated, and therefore, the elongation rate of temper rolling is preferably 1.0% or less. A more preferable elongation is 0.5% or less.

  Using a laboratory vacuum melting furnace, steel having the chemical composition shown in Table 1 was melted and cast to obtain a steel ingot. These steel ingots were made into steel pieces having a thickness of 40 mm by hot forging. These steel slabs were heated to 1200 ° C. using an electric heating furnace, held for 60 minutes, and then hot rolled under the conditions shown in Table 2 using a laboratory hot rolling mill. The thermal history of the winding was charged in an electric heating furnace maintained at the winding temperature and held for 30 minutes, then cooled to room temperature at an average cooling rate of 20 ° C./h, and gradually cooled after winding. Was simulated.

  The obtained hot-rolled steel sheet was pickled and used as a cold-rolled base material, and cold-rolled at a sheet thickness reduction rate of 50% to obtain a cold-rolled steel sheet having a thickness of 1.0 mm. The obtained cold-rolled steel sheet was annealed under the heat treatment conditions shown in Table 3 using a continuous annealing simulator. After performing a heat treatment equivalent to the overaging treatment, the steel sheet was cooled to room temperature to obtain an annealed steel sheet.

<Confirmation of metal structure>
A specimen for SEM observation was collected from the annealed steel sheet, and after polishing a longitudinal section parallel to the rolling direction, it was subjected to corrosion treatment with nital, and the metal structure at the 1/4 depth position of the plate thickness was observed from the steel sheet surface, The volume fraction of the low-temperature transformation generation phase and polygonal ferrite was measured by image processing. The total volume ratio of MA was determined by corroding a mirror-polished steel plate with a repeller corrosive solution and observing with an optical microscope and image analysis.

On the other hand, after specimens for XRD measurement were collected from the annealed steel sheet, the rolled surface was chemically polished from the steel sheet surface to a 1/4 depth position of the sheet thickness, and then subjected to an X-ray diffraction test to determine the volume fraction of retained austenite and Average carbon concentration was measured. Specifically, RINT 2500 made by Rigaku is used for the X-ray diffractometer, Co-Kα rays are incident, and α phase (110), (200), (211) diffraction peaks and γ phase (111), (200) The integrated intensity of the (220) diffraction peak was measured to determine the volume fraction of retained austenite. Further, the lattice constant dγ (Å) is obtained from the diffraction angle of the γ phase (111), (200), (220) diffraction peaks, and the average carbon concentration Cγ (mass%) of the retained austenite is obtained by the following conversion formula. It was.
Cγ = (dγ−3.572 + 0.000015 × Si−0.0012 × Mn) /0.033

  Further, a specimen for EBSP measurement was collected from the annealed steel sheet, and after electropolishing the longitudinal section parallel to the rolling direction, the metal structure was observed at a 1/4 depth position from the steel sheet surface, and image analysis was performed. The volume fraction of polygonal ferrite was determined.

  A JIS No. 5 tensile test piece was taken from the annealed steel sheet along a direction perpendicular to the rolling direction, and a tensile test was performed in accordance with JIS Z2241 (2011). Stretch flangeability was evaluated by conducting a hole expansion test specified in Japan Iron and Steel Federation Standard JFST1001 and measuring the hole expansion ratio (λ).

Table 4 shows the results of metal structure observation and performance evaluation of the cold-rolled steel sheet after annealing. Each performance was judged to be good when the following conditions were satisfied.
TS: 750 MPa or more YR: 0.60 or more λ: 40% or more TS × u. El: 12000 MPa% or more TS × El: 20000 MPa% or more

  As shown in Table 4, in the steel plates (test numbers 1 to 15) that satisfy all the conditions specified in the present invention, the uniform elongation (u.El), total elongation (El), and hole expansibility (λ) are as follows. High, excellent good ductility, stretch flangeability and a high yield ratio were obtained. On the other hand, in steel plates (test numbers 16 and 17) whose chemical composition is outside the range defined by the present invention, and steel plates (test numbers 18 to 20) whose manufacturing conditions are outside the range defined by the present invention, ductility and elongation Either or all of flangeability and yield ratio were inferior.

  According to the present invention, it is possible to produce a high-tensile cold-rolled steel sheet having a tensile strength of 780 MPa or more and having excellent ductility, work-hardening properties, and stretch flangeability.

Claims (5)

A method for producing a cold-rolled steel sheet comprising the following steps (A) to (C), wherein the main phase is a low-temperature transformation generation phase and has a metal structure containing residual austenite:
(A) The chemical composition is mass%,
C: more than 0.10% and less than 0.30%,
Si: more than 0.50% and 2.50% or less,
Mn: more than 1.00% and 3.50% or less,
P: 0.10% or less,
S: 0.010% or less,
sol. Al: 1.50% or less and N: 0.010% or less,
At least one selected from Ti: less than 0.001 to less than 0.100% and Nb: less than 0.001 to less than 0.050%;
V: 0 to 0.50%
Cr: 0 to 1.0%,
Mo: 0 to 0.50%,
B: 0 to 0.010%,
Ca: 0 to 0.010%,
Mg: 0 to 0.010%,
REM: 0 to 0.050% and Bi: 0 to 0.050%,
Remaining part: A slab composed of Fe and impurities is heated to 1100 to 1280 ° C., and hot-rolled under a condition where the finishing temperature is Ar ₃ or higher and a temperature range of 850 to 950 ° C.,
Cooling is started within 0.5 seconds after completion of hot rolling, and is cooled to a temperature range of (hot rolling finishing temperature −50) to 750 ° C. under the condition that the average cooling rate is 200 ° C./second or more.
After cooling for 0.3 to 2.0 seconds and cooling to 700 ° C. or lower under the condition that the average cooling rate is 30 ° C./second or higher,
Winding in a temperature range of less than 600 ° C. to obtain a hot-rolled steel sheet;
(B) A step of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet, and (C) a condition that an average temperature increase rate from 700 ° C. to heating stop is 1 ° C./second or more. in heating to a temperature range of (Ac ₃ point -40) to 900 ° C., and held at that temperature _range, under the condition that the _(Ar 3 -60) average cooling rate from ° C. until the cooling stops 30 ° C. / sec or more The process of hold | maintaining in the 300-450 degreeC temperature range, after cooling to the temperature range of 200-450 degreeC. In the step (A), the time t (second) between the rolling start in the final rolling pass and the rolling completion in the pass immediately before the final rolling pass is the rolling completion temperature T (° C.) in the pass immediately before the final rolling pass. The manufacturing method of the cold rolled steel sheet of Claim 1 which satisfies following formula (1) by relationship with these.
0.0015 / exp {−6080 / (T + 273)} ≦ t ≦ 2.0 (1) The chemical composition is mass%,
V: contains 0.010 to 0.50%,
The manufacturing method of the cold rolled steel sheet of Claim 1 or 2. The chemical composition is mass%,
Cr: 0.20 to 1.0%
Containing one or more selected from Mo: 0.05 to 0.50% and B: 0.0010 to 0.010,
The manufacturing method of the cold rolled steel plate in any one of Claim 1 to 3. The chemical composition is mass%,
Ca: 0.0005 to 0.010%,
Mg: 0.0005-0.010%,
Containing one or more selected from REM: 0.0005 to 0.050% and Bi: 0.0010 to 0.050%,
The manufacturing method of the cold rolled steel plate in any one of Claim 1 to 4.