JP5765411B2 - Cold rolled steel sheet manufacturing method - Google Patents

Description

The present invention relates to a method of manufacturing a cold rolled steel plate. More particularly, the present invention is suitable as a material for automobiles or industrial equipment, small in-plane anisotropy excellent in deep drawability, to method of manufacturing a cold-rolled steel sheet excellent in fatigue properties.

A steel sheet to be deep-drawn is required not only to have a high elongation and plastic strain ratio (hereinafter also referred to as “r value”) but also to have a small in-plane anisotropy of the r value in order to reduce earrings. . Furthermore, since durability is also required for steel sheets used for automobile parts, it is important to have excellent fatigue characteristics.

As an index of the in-plane anisotropy of the r value, when the r value in each direction of 0 °, 45 °, and 90 ° with respect to the rolling direction is r ₀ , r ₄₅ , r ₉₀ , Δr = (r Δr defined by ₀ −2 × r ₄₅ + r ₉₀ ) / 2 is known. The smaller the absolute value of Δr, the smaller the in-plane anisotropy.

As a technique for improving the deep drawability of a cold rolled steel sheet, for example, JP 2007-2915 A
14 (Patent Document 1) includes, in mass%, C: 0.01 to 0.05%, Si, Mn,
The steel adjusted for Al, P, S, N, and O is 3 passes or more in the temperature range below the Ar ₃ transformation point, the cumulative rolling reduction in the temperature range is 20 to 90%, the rolling end temperature is 700 to 850 ° C., Winding temperature is 600
A cold-rolled steel sheet having a small in-plane anisotropy of YS and r values obtained by cold-rolling a hot-rolled base material at ˜800 ° C. has been proposed. However, in hot rolling in the two-phase region, the austenite phase and the ferrite phase coexist, so the rolling load fluctuates greatly, and it is difficult to carry out the sheeting stably. There is a problem that the shape cannot be obtained.

In Japanese Patent Application Laid-Open No. 2004-76060 (Patent Document 2), C: 0.1% or less, Si: 0.5
%: Mn: 1.0% or less, P: 0.05% or less, S: 0.03% or less, Al: 0.01 to
Steel having a content of 0.1% or less is hot-rolled at an Ar ₃ transformation point or more, and cooling is started within 2 seconds after rolling, and the cooling is performed at a cooling rate of 70 ° C./s or more at 100 ° C. or more. A cold-rolled steel sheet having a small in-plane anisotropy of less than | Δr | <0.15 has been proposed by performing over a temperature range, cold rolling and annealing. However, fatigue properties have not been studied.

JP 2007-291514 A Japanese Patent Laid-Open No. 2004-76060

The present invention is provided with in-plane anisotropy of r value is superior to the less deep drawability, excellent in fatigue characteristics, a method of manufacturing a cold rolled steel plate also suitable for motor vehicles not only for industrial equipment The purpose is to do.

For the purpose of solving the above problems, the present inventors have conducted various examinations and experiments on the fatigue characteristics and in-plane anisotropy of cold- rolled steel sheets, and have found that the following cold- rolled steel sheets can solve the problems.

That is, in one aspect, by mass%, C: 0.046% to 0.096% ; Si: 0.01% to 0.04% ; Mn: 0.29% to 1.02% ; P : 0.021% or less ; S: 0.0040% or less ; sol. Al: 0.016% or more and 0.023% or less ; N: 0.0029% or more and 0.0048% or less ; and O: 0.0040% or less , and having a chemical composition consisting of the balance Fe and impurities, TS _ave defined by the following formula (1) is 300 MPa or more, YR _ave defined by the following formula (2) is 0.67 or more, r _ave and Δr defined by the following formulas (3) and (4) are A cold-rolled steel sheet having mechanical properties satisfying the following formulas (5) and (6).

TS _ave = (TS ₀ + 2 × TS ₄₅ + TS ₉₀ ) / 4 (1)
YR _ave = (YR ₀ + 2 × YR ₄₅ + YR ₉₀ ) / 4 (2)
r _ave = (r ₀ + 2 × r ₄₅ + r ₉₀ ) / 4 (3)
Δr = (r ₀ −2 × r ₄₅ + r ₉₀ ) / 2 (4)
| Δr | ≦ 0.20 (5)
r _ave /|Δr|≧4.7 (6)
In the above formula,
TS ₀ : Tensile strength in rolling direction, TS ₄₅ : Tensile strength in 45 ° direction with respect to rolling direction, TS _9
0 : Tensile strength in 90 ° direction with respect to rolling direction; YR ₀ : Yield ratio in rolling direction, YR ₄₅ : Yield ratio in 45 ° direction with respect to rolling direction, YR ₉₀ : 90 ° direction with respect to rolling direction Yield ratio, r ₀ :
The plastic strain ratio in the rolling direction, r ₄₅ : the plastic strain ratio in the 45 ° direction with respect to the rolling direction, and r ₉₀ : the plastic strain ratio in the 90 ° direction with respect to the rolling direction.

The chemical composition is Ti, Nb, V, Mo, B, Cr, Ca, Mg instead of part of Fe.
And one or more of REM may be contained in the amounts described below.
The mechanical properties preferably further satisfy the following formula (7):
YR _ave × r _ave /|Δr|≧4.7 (7).

The cold-rolled steel sheet can be easily manufactured by the method according to the present invention including the following steps (A) to (C):
(A) A hot rolling process in which a steel material having the above chemical composition is subjected to multi-pass hot rolling to obtain a hot rolled steel sheet having a ferrite grain size of 2.0 μm or more and 4.3 μm or less , provided that hot rolling starts The temperature is more than 1000 ° C. and not more than 1350 ° C., and the total reduction amount in hot rolling is 86% or more in terms of sheet thickness reduction rate in order to promote the refinement of ferrite, the reduction rate before the final two passes, and the final 1 The rolling reduction before the final pass and the final rolling reduction are 35% to 50%, 30% to 40%, and 25% to 50%, respectively. The interval time is 0.28 seconds or more and 0.68 seconds or less , the completion temperature of the final rolling pass is 850 ° C. or more and 910 ° C. or less , the cooling stop temperature is 687 ° C. or more and 715 ° C. or less, and the cooling rate is 833 ° C. / Second to 1120 ° C / second , And the Ri 0.21 seconds der less cooling time than 0.17 seconds to 720 ° C. after the rolling completion is the residence time at from 600 to 720 ° C. is more than 2 seconds, 650 coiling temperature 400 ° C. or higher ℃ Ru der below;
(B) A cold rolling step in which the hot-rolled steel sheet is subjected to cold rolling at a rolling reduction of 40% or more and 90% or less to form a cold-rolled steel sheet; and (C) Ac ₁ point or more on the cold-rolled steel sheet, Ac _An annealing process for annealing in a temperature range of ₃ points or less.

  A structural member can be manufactured by processing the cold-rolled steel sheet according to the present invention.

According to the present invention, it is possible to provide a cold-rolled steel sheet having excellent deep drawability, a small amount of earrings, and excellent fatigue characteristics through hot rolling in an austenite region that can be carried out stably and easily. It becomes. This cold-rolled steel sheet is particularly suitable as a steel sheet for automobiles, but it can of course be used for other applications such as industrial equipment parts that are subjected to deep drawing.

Hereinafter, the present invention will be described in more detail. In the following description, “%” relating to the chemical composition of steel means “mass%”.
(A) Chemical composition C: 0.015% or more and 0.15% or less C is an element having an action of increasing the strength of steel. C also has the effect of lowering the transformation temperature from austenite to ferrite, so that it is possible to lower the rolling completion temperature of hot rolling and is useful for promoting the refinement of ferrite crystal grains. C content is 0
If it is less than 0.015%, it is difficult to obtain the effect by the above action. Therefore, the C content is set to 0.015% or more. Preferably it is 0.02% or more, More preferably, it is 0.025% or more. On the other hand, if the C content exceeds 0.15%, the r value and the ductility are remarkably reduced. Therefore, the C content is 0.15% or less. Preferably it is 0.135% or less, More preferably, it is 0.12% or less.

Si: 2.0% or less Si is an element contained as an impurity in steel, but is also an element that contributes to strengthening ferrite and improving ductility. Therefore, Si may be positively included. However, Si
If the content exceeds 2.0%, the problem of surface oxidation during hot rolling becomes obvious. Therefore,
The Si content is 2.0% or less. Preferably it is 1.0% or less, More preferably, it is 0.5% or less, Most preferably, it is 0.3% or less.

Mn: 0.1% or more, 3.0% or less Since Mn has an action of lowering the transformation temperature from austenite to ferrite,
It is an element that makes it possible to lower the rolling completion temperature in hot rolling and to promote the refinement of ferrite crystal grains. If the Mn content is less than 0.1%, it is difficult to obtain the effect by the above action. Therefore, the Mn content is 0.1% or more. On the other hand, if the Mn content exceeds 3.0%, the formability is lowered due to segregation of Mn and the formability is lowered due to a decrease in ferrite volume fraction. Therefore, the Mn content is 3.0% or less. Preferably it is 2.5% or less, More preferably, it is 2.0% or less.

P: 0.05% or less P is an element contained as an impurity in steel, but has the effect of increasing strength.
P may be positively included. However, when the P content exceeds 0.05%, embrittlement due to grain boundary segregation becomes significant. Therefore, the P content is 0.05% or less. Preferably 0.03%
Hereinafter, it is more preferably 0.02% or less.

S: 0.05% or less S is an element contained as an impurity in steel, and has the effect of reducing the workability by forming sulfide inclusions in the steel. If the S content is more than 0.05%, the workability deteriorates remarkably. Therefore, the S content is set to 0.05% or less. When it is desired to further improve the workability, the S content is preferably 0.008% or less, and more preferably 0.003% or less.

sol. Al: not less than 0.001% and not more than 0.1% Al has an action of deoxidizing steel and is an element effective for making steel healthy. sol. Al
If the content is less than 0.001%, it is difficult to obtain the effect by the above action. Therefore, sol. Al content shall be 0.001% or more. Preferably it is 0.010% or more, More preferably, it is 0.015% or more. On the other hand, sol. If the Al content exceeds 0.1%, the transformation temperature from austenite to ferrite increases remarkably, and the rolling completion temperature of hot rolling must be increased, making it difficult to refine the ferrite crystal grains. . In addition, when the continuous casting method is applied, stable operation becomes difficult. Therefore, sol. Al content is 0
.1% or less Preferably it is 0.080% or less, More preferably, it is 0.060% or less.

N: 0.001% or more and 0.01% or less N is an element contained as an impurity in the steel and has an effect of reducing ductility and deep drawability. If the N content exceeds 0.01%, the ductility and deep drawability are significantly reduced. Therefore,
N content shall be 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.0.
0.7% or less. On the other hand, when Ti, Nb, or the like is contained, N precipitates as nitrides or carbonitrides, thereby refining the hot-rolled steel sheet, which is a base material for cold rolling, and as a result, cold-rolled steel sheet Contributes to the improvement of mechanical properties. Therefore, the N content is 0.001% or more.
Preferably it is 0.0015% or more, more preferably 0.002% or more.

O: 0.01% or less O (oxygen) is an element contained as an impurity in the steel, and reduces the cleanliness of the steel.
Degrading its mechanical properties. When the O content exceeds 0.01%, the mechanical properties are significantly deteriorated. Therefore, the O content is set to 0.01% or less. Preferably it is 0.005% or less.

One or two selected from the group consisting of Ti: 0.1% or less, Nb: 0.1% or less, V: 0.5% or less, Mo: 0.5% or less and B: 0.005% or less More than seeds Ti, Nb, V, Mo and B precipitate as carbides, nitrides or carbonitrides, and contribute to refinement of the steel structure and YS improvement. Accordingly, one or more of these elements may optionally be contained in the steel. However, Ti content exceeds 0.1%, Nb content is 0.1%
If the V content exceeds 0.5%, the Mo content exceeds 0.5%, or the B content exceeds 0.005%, a large amount of carbide, nitride or carbonitride precipitates in the steel. As a result, the in-plane anisotropy increases and the deep drawability decreases. Therefore, the content of each element is Ti: 0.1
% Or less, Nb: 0.1% or less, V: 0.5% or less, Mo: 0.5% or less, and B: 0.005
% Or less. The contents of Ti and Nb are each preferably 0.05% or less, and more preferably 0.03% or less. The contents of V and Mo are preferably 0.3% or less, and more preferably 0.1% or less. B content is 0.00
It is preferably 3% or less, and more preferably 0.001% or less. In addition,
In order to more reliably obtain the effect of the above action, any one of Ti, Nb, Mo and V is set to 0.0.
It is preferable to contain 01% or more or B to contain 0.0001% or more.

Cr: 2.0% or less Since Cr has the effect of further increasing the strength of the steel material by solid solution strengthening, it may be contained in the steel in some cases. However, if the Cr content exceeds 2.0%, the workability may be significantly deteriorated. Therefore, the Cr content is 2.0% or less. Preferably it is 1.5% or less, More preferably, it is 1.0% or less, More preferably, it is 0.5% or less. Note that the Cr content is preferably 0.01% or more in order to more reliably obtain the effect of the above action.

One or more selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less Ca, Mg, and REM (rare earth elements) are solidified during solidification. It has the effect | action which refines | miniaturizes the oxide and nitride which precipitate and improves the soundness of a steel ingot or a steel piece. Accordingly, one or more of these elements may optionally be contained in the steel. However, both elements are 0.0
Even if it contains exceeding 1%, the effect by the said effect | action will be saturated, and a cost rise will be caused. Therefore, the content of each element is set to 0.01% or less. In addition, in order to acquire the effect by the said action | operation more reliably, it is preferable that content of any element shall be 0.0002% or more. Here, REM is a combination of 15 elements of lanthanoid and Y and Sc.
Means an element.

(B) Mechanical properties TS _ave : 300 MPa or more In order to obtain a steel plate with high fatigue resistance, the higher the tensile strength (TS), the better. And
In order to obtain high fatigue resistance against repeated stresses in various directions, it is preferable that the tensile strength in each direction in the plane of the steel sheet is generally high. Therefore, the TS _ave defined by the following formula (1) in the present invention be at least 300 MPa. The higher the TS _{ave, the} better the fatigue resistance.
S _ave preferably 340MPa or more, and more preferably not less than 390 MPa.

TS _ave = (TS ₀ + 2 × TS ₄₅ + TS ₉₀ ) / 4 (1)
Here, TS _{0 is} the tensile strength in the rolling direction, TS _{45 is} the tensile strength in the 45 ° direction with respect to the rolling direction, and TS ₉₀ is the tensile strength in the 90 ° direction with respect to the rolling direction.

YR _ave : 0.67 or more The TS of the steel sheet is determined according to the required characteristics of the parts in which the steel plate is used, but even if the TS is the same, the higher the YS, the better the fatigue characteristics. Therefore, in the present invention, the YR _ave defined by the following formula (2) 0.67 or more. YR _ave is preferably 0.69 or more, more preferably 0.8.
71 or more, particularly preferably 0.73 or more.

YR _ave = (YR ₀ + 2 × YR ₄₅ + YR ₉₀ ) / 4 (2)
Where YR ₀ : Yield ratio in rolling direction, YR ₄₅ : Yield ratio in 45 ° direction with respect to rolling direction,
YR ₉₀ : Yield ratio in the 90 ° direction with respect to the rolling direction.

| Δr |: 0.20 or less Earring when a steel sheet is deep-drawn by reducing the absolute value | Δr | of ΔR defined by Δr = (r ₀ −2 × r ₄₅ + r ₉₀ ) / 2 Is reduced. Therefore, in the present invention, | Δr | is 0.20 or less. | Δr | is preferably 0.15 or less.

r _ave / | Δr |: 4.7 or more Generally, as the average r value [r _ave = (r ₀ + 2 × r ₄₅ + r ₉₀ ) / 4] increases, the deep drawing forming limit of the steel sheet increases, and | Δr | The larger the size, the larger the amount of earrings. The r value of the steel sheet is strongly influenced by the texture, and with the development of the plane orientation that improves the r value, the average r value increases and the deep drawability improves, but | Δr | The directionality may decrease. Therefore, in order to achieve both good deep drawability and small in-plane anisotropy, it is necessary to limit the balance. In the present invention, the r _ave | Δr | ratio (r _ave / | Δr |) 4
.7 or more. This ratio is preferably 5.4 or more, more preferably 6.1 or more.

YR _ave × r _ave / | Δr |: 4.7 or more As described above, the higher YR is preferable from the viewpoint of fatigue resistance, and the higher r _ave / | Δr | is preferable from the viewpoint of deep drawability. When the content of an element that enhances the hardenability, such as Mn, is increased to increase the strength of the steel sheet, the effect of improving fatigue resistance is small because YR decreases. On the other hand, when a precipitation strengthening element such as Ti or Nb is included, YR is improved, but r _ave and | Δ
r | decreases. Therefore, the higher the balance between YR and r _ave / | Δr |, the higher the fatigue characteristics and deep drawability. As an index, YR _ave × r _ave / | Δr
| Can be used, and is preferably 4.7 or more. This value is more preferably 5.0 or more.

(C) Manufacturing method
(1) Hot rolling process Hot rolling is performed by multipass rolling in the austenite region using a lever mill or a tandem mill. From the viewpoint of industrial productivity, at least the final several stages are preferably rolled using a tandem mill.

The steel material to be subjected to hot rolling may be a steel ingot obtained by continuous casting, a steel piece obtained by casting and partial rolling, or a steel plate obtained by strip casting. If necessary, those obtained by applying hot or cold processing to them in advance can also be used. If the steel material to be subjected to rolling is in a high temperature state as in the case of direct feed rolling, it may be subjected to hot rolling directly or without being heated without being heated. If the steel material to be rolled is a cold piece, it may be heated and subjected to hot rolling.

When the hot rolling start temperature is 1000 ° C. or less, the rolling load increases, and it becomes difficult to perform rolling at a sufficient reduction rate, or it becomes difficult to complete rolling at a temperature of Ar ₃ or higher. As a result, desired mechanical properties may not be obtained. Therefore, it is preferable that the temperature of the steel material used for hot rolling is higher than 1000 ° C. More preferably 1025 ° C. or higher,
Especially preferably, it is 1050 degreeC or more. The upper limit of the temperature of the steel material to be subjected to hot rolling need not be specified in particular, but in order to suppress austenite grain coarsening and to suppress equipment costs and heating fuel costs, it is preferably 1350 ° C. or lower. More preferably, it is 1250 ° C. or lower.

It is the final product by appropriately adjusting the time between rolling passes (the time between final rolling passes) between the final rolling pass immediately before the final rolling pass in the hot rolling process (the rolling pass immediately before the final rolling pass) and the final rolling pass. It has been found that it is possible to improve the r value and the anisotropy of the cold rolled steel sheet.
The cause of this is not clear, but the texture of the hot-rolled steel sheet, which is the cold-rolled base metal, changes due to the recovery and recrystallization of some processed austenite between the rolling pass immediately before the final rolling pass and the final rolling pass. As a result, it is assumed that the texture of the cold-rolled steel sheet changes. In order to obtain the above effect, it is preferable to set the time between the final rolling passes, which is the time between the rolling passes immediately before the final rolling pass and the final rolling pass, to 0.3 seconds or more. On the other hand, if the time between the final rolling passes is excessively long, the grain growth of recrystallized austenite becomes remarkable and a fine structure cannot be obtained. For this reason, the time between final rolling passes shall be 4.0 seconds or less. The time between final rolling passes is preferably 0.4 seconds or more and 3.0 seconds or less.

The rolling completion temperature (final rolling pass completion temperature) is set to a temperature range of Ar ₃ or higher and 780 ° C. or higher in order to transform the structure from austenite to ferrite after the completion of rolling to refine the structure. When the rolling completion temperature is lower than the Ar ₃ point, ferrite is generated during rolling. In addition, when the rolling completion temperature is less than 780 ° C., the rolling load is remarkably increased and it becomes difficult to apply sufficient reduction, or ferrite transformation may occur in the surface layer portion of the steel sheet during rolling. The rolling completion temperature is preferably Ar ₃ point or higher and 800 ° C. or higher. The rolling completion temperature is preferably as low as possible if it is in the temperature range of Ar ₃ or higher and 800 ° C. or higher.
This is because, when the rolling completion temperature is lower, the effect of accumulating processing strain introduced into austenite by rolling becomes larger, and the refinement of crystal grains is promoted. The Ar ₃ point of the steel type used in the present invention is approximately 780 ° C to 900 ° C.

The total rolling reduction in hot rolling is 86% in thickness reduction rate in order to promote the refinement of ferrite.
% Or more is preferable. The plate thickness reduction rate is more preferably 90% or more, and particularly preferably 94% or more. Moreover, it is preferable that the plate | board thickness reduction | decrease rate in the temperature range more than rolling completion temperature-(rolling completion temperature +100 degreeC) or less shall be 40% or more. Above rolling completion temperature ~ (
More preferably, the sheet thickness reduction rate in the temperature range of (rolling completion temperature + 80 ° C.) or less is 60% or more. At least the final rolling pass and the final rolling pass are continuous multi-pass rolling. The reduction amount per pass is preferably 15 to 60%.

After completion of hot rolling, cooling is performed so that the cooling time to 720 ° C. is within 0.4 seconds.
This is by cooling to a temperature range where ferrite transformation becomes remarkable while suppressing the release of processing strain introduced into austenite as much as possible, and by transforming from austenite to ferrite as a driving force at once, the processing strain introduced into austenite is transformed, This is to produce a fine ferrite crystal grain structure. The cooling time to 720 ° C. after the completion of rolling is preferably 0.2 seconds or less. It is desirable to use water cooling for the cooling, and the cooling rate is preferably an average cooling rate during the forced cooling time excluding the air cooling time, and is preferably 400 ° C./second or more.

Here, the reason for prescribing the cooling conditions up to 720 ° C. after the completion of rolling is that the processing introduced into the austenite by the processing before the fine ferrite is formed when the cooling is stopped or slowed down at a temperature exceeding 720 ° C. This is because the strain is released or the existence form of the processing strain is changed and becomes ineffective for nucleation of ferrite, and the ferrite crystal grains are remarkably coarsened.

When the temperature reaches 720 ° C. or lower, it enters a transformation temperature range in which ferrite transformation is activated. The ferrite transformation temperature range where the above ferrite structure is obtained is a temperature range between this temperature and 600 ° C. Therefore, when the coiling temperature is lower than 600 ° C., after reaching 720 ° C. or lower, it is possible to temporarily stop the cooling or slow down the speed and hold it in this temperature range for 2 seconds or more. Is preferable for ensuring the formation of a stable ferrite crystal grain structure. When the coiling temperature is higher than 600 ° C., this holding time is generally satisfied naturally.

The winding is preferably performed after the steel sheet temperature is lowered to 700 ° C. or less by water cooling or air cooling. This is because if the winding temperature exceeds 700 ° C., the iron-phosphorus compound may precipitate during the slow cooling after winding, and the deep drawability may deteriorate. When emphasizing deep drawability, the coiling temperature is 600
It is preferable to set it to at least 700 ° C. On the other hand, when emphasizing fatigue characteristics, the winding temperature is preferably less than 600 ° C.

In the present invention, the equipment for performing the above cooling is not limited. Industrially, it is preferable to use a water spray device having a high water density. For example, it is possible to cool by disposing high-pressure water having a sufficient water density from the top and bottom of the plate by disposing a water spray header between the rolled plate conveyance rollers.

The hot-rolled steel sheet obtained in this way typically has a fine grain structure in which the average grain size of ferrite at the 1/4 depth position of the sheet thickness from the steel sheet surface satisfies the following formula (8). When the average ferrite grain size of the hot-rolled steel sheet does not satisfy the following formula (8), the cold-rolled steel sheet having the mechanical properties defined in the present invention even after cold rolling and annealing under the conditions defined in the present invention Is often not obtained.

D ≦ 3.1 + 5000 / (5 + 350 × C + 40 × Mn) ² (8)
In the above formula, D is the average grain diameter of ferrite (μ) at the 1/4 depth position of the plate thickness from the steel plate surface.
m), and C and Mn mean the content (% by mass) of each element in the steel.

(2) Cold rolling process The hot rolled steel sheet obtained in the hot rolling process is cold rolled to obtain a cold rolled steel sheet. If the rolling reduction in the cold rolling is too small, the crystal grains after the cold rolling and annealing are coarsened, and desired mechanical properties cannot be obtained. Therefore, the lower limit of the rolling reduction in cold rolling is set to 40%. on the other hand,
If this rolling reduction is too large, the load on the cold rolling equipment becomes excessive, making operation difficult. Therefore, the upper limit of the rolling reduction in cold rolling is set to 90%.

(3) Annealing process When a cold-rolled steel sheet obtained by cold rolling is annealed according to a conventional method , a cold-rolled steel sheet is obtained. When the annealing temperature is less than ₁ Ac, the production efficiency decreases because recrystallization of ferrite takes a long time. On the other hand, if annealing is performed at a temperature exceeding Ac ₃ points, the structure at the time of annealing becomes an austenite single phase, so that it is difficult to obtain the effect of refining the cold-rolled steel sheet. Therefore, the annealing temperature is a so-called two-phase temperature of Ac ₁ point or more and Ac ₃ point or less. The annealing time is not particularly limited as long as the time required for recrystallization of the ferrite can be ensured, but is preferably 5 seconds or more in order to make the recrystallization of ferrite more reliable. On the other hand, from the viewpoint of suppressing the grain growth of ferrite, it is preferably set to 300 seconds or less. The cooling conditions after annealing are not particularly limited.

The cold-rolled steel sheet after annealing can be subjected to a skin pass according to a conventional method, if necessary.
Further, using a continuous hot dipping line, hot dipping may be performed following the high-temperature cold-rolled steel sheet after annealing. Examples of the 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 and the like. . Moreover, electroplating can also be performed to the cold-rolled steel plate after annealing. Examples of electroplating include electrogalvanizing and electro-Zn-Ni alloy plating. These plated steel sheets can be subjected to chemical conversion treatment to improve corrosion resistance as necessary.
(D) Structural member
Cold-rolled steel sheet is suitable for producing structural members by known processing methods such as press molding or stamping of such deep drawing and hole expansion processing, as the material of the structural member, in particular deep drawing is performed Is preferred. As the structural member, a steel plate member for automobiles is typical.

Hereinafter, the present invention will be described in more detail by way of examples.
Steels of steel types A to H having the chemical composition shown in Table 1 were melted and made into steel pieces having a thickness of 30 mm by hot forging. The steel slab was reheated to 1050 ° C. or higher, and then hot-rolled with a small test tandem mill to finish a thickness of 2 mm to 3.5 mm. In all rolling, multi-pass rolling of 3 or more passes was performed within a temperature range from hot rolling completion temperature to [hot rolling completion temperature + 100 ° C.]. Cooling to 720 ° C. after completion of rolling was performed by water cooling or air cooling.

Table 2 shows the hot rolling conditions and the ferrite grain size of the obtained hot-rolled steel sheet.
The ferrite grain size is determined by observing a cross section of the steel plate thickness using a scanning electron microscope and analyzing the crystal orientation using the EBSP (Electron Back Scattering Pattern) method at a depth of 1/4 of the plate thickness from the plate surface. Asked.

The obtained hot-rolled steel sheet was cold-rolled and annealed under the conditions shown in Table 3 to obtain a cold-rolled steel sheet. The holding time at the annealing temperature is 60 seconds, and after the annealing, it is cooled at a cooling rate of 50 ° C./s,
After holding at 400 ° C. for 150 seconds, it was cooled to room temperature.

The tensile properties of the cold-rolled steel sheet thus obtained were 10 at room temperature using a JIS No. 5 tensile test piece.
A tensile test was performed at a test speed of mm / min or less for evaluation. The test results are summarized in Table 4. In addition, the underlined part in the table indicates that it is out of the requirements defined in the present invention.

Test numbers 2, 6, 8, 9, 11, 14, and 18 which are comparative examples show that YR _ave is small or Δ
r is large. On the other hand, all of the inventive examples satisfy all the requirements of the mechanical properties defined in the present invention, and are excellent in in-plane anisotropy, and hence deep drawability and fatigue properties.

Claims (4)

A method for producing a cold-rolled steel sheet comprising the following steps (A) to (C):
(A) In mass%, C: 0.046% to 0.096% ; Si: 0.01% to 0.04% ; Mn: 0.29% to 1.02% ; P: 0.0. 021% or less ; S: 0.0040% or less ; sol. A steel material containing Al: 0.016% or more and 0.023% or less ; N: 0.0029% or more and 0.0048% or less ; and O: 0.0040% or less , and having a chemical composition composed of the remainder Fe and impurities. A hot rolling process in which a multipass hot rolling is performed to obtain a hot rolled steel sheet having a ferrite grain size of 2.0 μm or more and 4.3 μm or less , provided that the hot rolling start temperature is more than 1000 ° C. and 1350 ° C. or less. The total rolling reduction in hot rolling is 86% or more in terms of sheet thickness reduction rate in order to promote the refinement of ferrite, and the reduction rate before the final 2 pass, the reduction rate before the final 1 pass, and the reduction rate of the final pass are , 35% or more and 50% or less, 30% or more and 40% or less, and 25% or more and 50% or less, respectively, and the time between rolling passes of the rolling pass immediately before the final rolling pass and the final rolling pass is 0.28 seconds or more and 0.68 seconds. Of the final rolling pass Completion temperature is 850 ° C. or higher and 910 ° C. or lower , cooling stop temperature is 687 ° C. or higher and 715 ° C. or lower, cooling rate is 833 ° C./second or higher and 1120 ° C./second or lower, and cooling time is 720 ° C. after completion of rolling. There Ri der below 0.21 seconds 0.17 seconds, and the residence time at 600 to 720 ° C. is more than 2 seconds, the coiling temperature is Ru der 400 ° C. or higher 650 ° C. or less;
(B) a cold rolling step in which the hot rolled steel sheet is cold rolled at a rolling reduction of 40% or more and 90% or less to form a cold rolled steel sheet; and (C) the cold rolled steel sheet has Ac ₁ point or more, Ac _An annealing process for annealing in a temperature range of ₃ points or less. The chemical composition is replaced by a part of the Fe, in terms of mass%, Ti: 0.019 % or less, Nb: 0.021 % or less, V: 0.05 % or less, Mo: 0.02 % or less and B: The method for producing a cold-rolled steel sheet according to claim 1, comprising one or more selected from the group consisting of 0.0004 % or less. The method for producing a cold-rolled steel sheet according to claim 1 or 2, wherein the chemical composition contains Cr: 0.05 % or less in mass% instead of a part of the Fe. Claims containing one or two kinds selected from the group consisting of 0.0005% or less: the chemical composition, instead of a part of the Fe, by mass%, Ca: 0.0010% or less contact and REM The manufacturing method of the cold-rolled steel plate in any one of Claims 1-3.