JPH11323476A - Cold rolled steel sheet small in plane anisotropy and excellent in deep drawability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the nucleation and granular growth in the orientation making the plane anisotropy of (r) value at the time of recrystallization remarkable and to optimize the texture by adding Mg to extra-low carbon steel, specifying the componental compsn. therein and specifying the size and distribution of Mg and Ti oxides and multiple oxides in the steel sheet. SOLUTION: This steel sheet has a compsn. contg., by mass, <=0.005% C, 0.005 to 0.1% Si, 0.01 to 0.5% Mn, <=0.05% P, <=0.02% S, <=0.004% Al, 0.0001 to 0.01% Mg, 0.001 to 0.01% O and 0.001 to 0.01% N, also satisfying the inequality of 0.05>Ti>3.42N+1.5S and the inequality of (Ti-3.42N-1.5S)/48-C/12>0, and the balance Fe. The steel sheet contg. Mg and Ti oxides and/or multiple oxides by 0.005 to 0.1 μm average grain size and by 40 to 1000 pieces/mm<2> surface density is obtd. In this way, the steel sheet sufficiently high in the average (r) value-rm and in which the absolute value of Δr which is the value of the plane plastic isotropy which hardly causing problems as for the height of earing generated after forming is <=0.2 can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a cold-rolled steel sheet having a small in-plane anisotropy and excellent in deep drawability.

[0002]

2. Description of the Related Art In recent years, the material properties required for steel sheets for automobiles have been increasing year by year. Particularly for automobile fenders and oil pans, since extremely severe press molding is performed, further improvement in deep drawability and ductility has been expected. Since then, with the development of vacuum degassing technologies such as RH and DH, ultra-low carbon steel with reduced solid solution elements in steel and improved deep drawability has been developed. Used instead of steel. More recently, as a steel in which deep-drawing properties have been dramatically improved by scavenging solid solution C and N in the steel with Ti, Nb, etc.
No. 25727, Japanese Patent Application Laid-Open No. 2-34722, and other interstitial atoms.
Freesteel (hereinafter IF steel) has been widely used.

On the other hand, in recent years, the needs of ordinary consumers for automobiles have been diversified, and their silhouettes have become more important in design than in function. As a result, the shapes of the components of the automobile have become more complicated, and the number of components that are difficult to form with conventional steel plates has been increasing. Therefore, there is a strong demand for further improvement in deep drawability. However, from the viewpoint of uniformity of molded parts, prevention of cracking during pressing, and the like, the average Rankford value (hereinafter, rm: r0, r45
And r90 are 0 ° direction with respect to the rolling direction, 4
Rankford values in 5 ° and 90 ° directions,
Not only rm = (r0 + 2 × r45 + r90) / 4) but also the value of in-plane plastic anisotropy (hereinafter Δr: Δr =
It is very important to improve (r0−2 × r45 + r90) / 2). If the in-plane anisotropy of the r value is notable and the press forming including the element of deep drawing is performed,
Since the r-value in the specific direction is low, the material does not flow sufficiently and a direction called an ear appears, and a crack is generated at that portion.

As a measure for improving the in-plane anisotropy of the r value, there is the following known technique. Japanese Patent Publication No. Sho 62-161919 discloses a hot-rolled steel sheet having a low anisotropy in which a suitable AlN is precipitated by lowering the slab heating temperature by hot rolling and raising the winding temperature to 85-93%. A technique for reducing anisotropy by cold rolling at a high cold rolling rate and recrystallization annealing has been reported. However, the above technique requires a high cold rolling rate of 85% or more to improve anisotropy, and has a problem that the load of cold rolling increases.

[0005] JP-A-5-117758 discloses that T
By using a very low carbon steel slab containing appropriate amounts of i, Nb, B and Al as a raw material, by appropriately optimizing hot rolling and annealing conditions in the manufacturing process, a depth with less in-plane anisotropy of r value is obtained. A technique for manufacturing a cold-rolled thin steel sheet for drawing is disclosed. However, methods such as the above-mentioned known art,
Since the composite precipitate of (Ti, Nb) C and (Ti, Al) N must be precipitated at an appropriate timing during winding and annealing, it is difficult to stably produce a material.

Japanese Patent Publication No. 7-110977 discloses that after hot rolling, cold rolling and annealing are performed twice, and the respective cold rolling rates and annealing temperatures are optimally controlled to thereby provide an in-plane anisotropy of the r value. An invention for reducing the performance has been proposed. . However, the above-mentioned method has a problem in that cold rolling and annealing are performed twice, and production is troublesome and therefore costly.

[0007] Japanese Patent Application Laid-Open No. 7-188854 discloses T
There is disclosed a technique of providing a cold-rolled steel sheet having a small in-plane anisotropy of an r value by adjusting the amounts of C, Mn, and B of an ultra-low carbon nitrogen steel containing no i or Nb. However, the above-mentioned technology has a problem that since the elements fixing C, such as Ti and Nb, are not added, solute C is present in the steel sheet and the aging resistance is poor.

[0008]

The present invention relates to a cold-rolled steel sheet suitable for deep drawing, in which the size and distribution of fine dispersed particles are optimized in order to improve the in-plane anisotropy of the r value. Nucleation in an orientation that makes the in-plane anisotropy of the r-value remarkable during recrystallization annealing, and a cold-structure optimized to suppress grain growth and improve the in-plane anisotropy of the r-value It is intended to provide a rolled steel sheet.

[0009]

The gist of the present invention is as follows: (1) In terms of mass%, C ≦ 0.005%, Si = 0.
5 to 0.1%, Mn = 0.01 to 0.5%, P ≦ 0.0
5%, S ≦ 0.02%, Al ≦ 0.004%, Mg =
0.0001-0.01%, O = 0.001-0.01
%, N = 0.001 to 0.01%, and 0.05>Ti> 3.42N + 1.5S and (Ti-3.42N-1.5S) / 48-C / 12> 0. A cold-rolled steel sheet having a small in-plane anisotropy and excellent in deep drawability, characterized by being satisfied and the balance being Fe and unavoidable impurities. (2) The in-plane surface according to (1), further satisfying 0.05>Nb> 93 {C / 12- (Ti-3.42N-1.5S) / 48} by mass%. Cold rolled steel sheet with low anisotropy and excellent deep drawability. (3) Further, in mass%, B = 0.0003 to 0.0
The cold-rolled steel sheet having a small in-plane anisotropy and excellent in deep drawability according to the above (1) or (2), wherein the cold-rolled steel sheet contains 0.2%. (4) The steel according to any of (1) to (3), wherein the steel contains an oxide and / or a composite oxide of Mg and Ti having an average particle diameter of 0.005 μm or more and 0.1 μm or less. 2. A cold-rolled steel sheet having a small in-plane anisotropy according to item 1 and excellent in deep drawability. (5) The in-plane anisotropy according to (4), wherein the areal density of the oxides and / or composite oxides of Mg and Ti contained in the steel is 40 to 1000 / mm 2 or more. A cold rolled steel sheet that is small and has excellent deep drawability. It is in.

Here, oxides and composite oxides of Mg and Ti are Mg alone such as MgO and TiO, Ti2
This refers to not only oxide particles of Ti alone such as O3 and TiO2 and composite oxide particles thereof, but also composite particles containing other oxides and precipitates such as TiC, TiN and MnS.

The direction and height at which the ear is generated are closely related to the in-plane anisotropy of the r value, and the absolute value of Δr (hereinafter: |
The ear height decreases as Δr |) decreases. r90 ≧
If the V-shape distribution is r0> r45, ears are generated in the 0 ° and 90 ° directions from the rolling direction, and if the inverse V-shape distribution is r45> r90 ≧ r0, the ears are generated in the 45 ° direction.

The in-plane anisotropy of the r value is due to the texture. For example, "Iron and Steel", 57 (1971) 113
According to pages 4 to 1154, when the {110} <100> orientation is strong, the anisotropy of the V-shaped distribution of r90 ≧ r0> r45 is shown, and when the {100} <110> orientation is strong, r45 > R90 ≧ r0. Therefore, in order to improve the in-plane anisotropy of the r value, the {110} <100> orientation and {100} <110>
The nucleation of the orientation or the grain growth may be suppressed.

The present inventors have attempted to improve the in-plane anisotropy of the r-value of a cold-rolled steel sheet with a view to production by an industrial-scale continuous annealing equipment or a continuous hot-dip galvanizing equipment which is currently generally employed. As a result of intensive studies, it has been newly found that | Δr | decreases when Mg is added even if the crystal grain sizes after annealing are the same. Further, as a result of further detailed investigation, it was found that there was a strong correlation between the size of Mg and Ti oxides and / or composite oxides in the steel sheet and | Δr |, and Mg and Ti oxides and / or By optimizing the size and distribution of the composite oxide, it is possible to produce a steel sheet having a sufficiently high rm and | Δr | ≦ 0.2 in which the height of the ears generated after forming is practically negligible. I found out. The present invention described above is also applicable to hot-dip galvanized steel sheets.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, experimental results on which the present invention is based will be described. First, the relationship between the grain size of the annealed steel sheet and | Δr | was compared between the case where Mg was added and the case where Mg was not added. 0.002% C-0.02% Si-
0.2% Mn-0.01% P-0.003% S-0.0
50 kg of Mg-containing steel with pure Mg metal (99% or more) wrapped in iron foil and Mg-free steel with no Mg added, based on steel with a composition of 03% Al-0.05% Ti
In the laboratory. The obtained ingot is 1
After heating at 230 ° C for 1 hour, finishing temperature (FT) 900 to 9
It was hot rolled at 30C and wound up at 600-700C. After the pickling, cold rolling is performed at a rolling ratio of 80%, and 780 in an annealing furnace.
After annealing at ８860 ° C. for 1 minute, cooling was performed at a cooling rate of 20 ° C./S, and 1% skin pass rolling was performed.

The relationship between the crystal grain size of the steel sheet thus obtained and | Δr | was investigated. However, the average grain size of the obtained steel sheet was measured in a cross section parallel to the rolling direction,
It was arranged by the particle size number by the cutting method described in SG0522. Further, the r value is defined as a value obtained by dividing the change in the thickness of the test piece before and after the 15% tensile change by the change in the width of the test piece on the No. 5 test piece described in JIS Z 2201, and r0, r45 and r90. Were measured to obtain | Δr |. FIG.
Shows the relationship between the particle size (particle size number) and | Δr |. Thus, it was found that when Mg was contained, | Δr | could be significantly reduced as compared with the case where Mg was not contained.

Next, oxides of Mg and Ti in the steel sheet and / or
Alternatively, the relationship between the size of the composite oxide and rm and | Δr | was investigated. 0.002% C-0.02% Si-0.2
% Mn-0.01% P-0.003% S-0.003%
Al-0.05% Ti-0.0005% Mg based steel, deoxidizers Al, Si, Ti and M
The slab obtained by changing the order of addition of g, the addition interval, etc. in the steelmaking process in various ways is heated at 1230 ° C. for 1 hour, then hot-rolled at a finishing temperature (FT) of 920 ° C., and wound at 650 ° C. Was. After pickling, cold rolling was performed at a rolling reduction of 80%, and after annealing in an annealing furnace at 810 ° C. for 1 minute, cooling was performed at a cooling rate of 20 ° C./S, and further 1% skin pass rolling was performed.

The mechanical properties of the steel sheet thus obtained and the relationship between the particle size d of Mg and Ti oxides and / or composite oxides and | Δr | were investigated. However,
The mechanical properties were measured on a No. 5 test piece described in JIS Z 2201 by a tensile test method described in JIS Z 2241. | Δr | was obtained by the same method as described above.

On the other hand, the state of the oxides and the composite oxides of Mg and Ti was determined by taking a transmission electron microscope sample from a quarter of the thickness of the test steel and performing energy dispersive X-ray spectroscopy (Energy Dispersive X-ray spectroscopy). ray
Spectroscope (EDS) or electron energy loss spectroscopy (Electron Energy Loss)
It was observed by a transmission electron microscope equipped with a field emission type electron gun (Field Emission Gun: FEG) with an accelerating voltage of 200 kV and a composition analysis function of Spectroscope (EELS). The composition of the observed particles was determined by the above EDS and EELS.
And Ti oxides and composite oxides. Further, the average particle diameter of the oxides and composite oxides of Mg and Ti specified in the present application is observed at a magnification of 10,000 to 500,000 times over an area of at least 1000 μm 2, and each target particle is measured. It is the average value at.

FIG. 2 shows the measurement results of rm and | Δr | and the particle size d of Mg and Ti oxides and composite oxides. As described above, the particle size of Mg and Ti oxides and composite oxides having both excellent rm and | Δr |
It was found to be 5 μm or more and 0.1 μm or less.

Next, the components of the present invention will be described. First, the reasons for limiting the chemical components of the present invention will be described. C: If C exists in the steel in a solid solution state during cold rolling,
Since an orientation unfavorable for deep drawability is generated during subsequent annealing, the content is preferably 0.005% or less, and more preferably 0.003% or less.

Si: Si is a solid solution strengthening element and is added as necessary. However, if it is less than 0.005%, the production cost at the steel making stage will increase, so it is made 0.005% or more. On the other hand, if added in an amount of 0.1% or more, solid solution hardening becomes remarkable and not only becomes unsuitable for working, but also deteriorates rm and deteriorates adhesion of hot-dip galvanized, so that 0.1% or more is added.
It was as follows. Preferably, it is 0.05% or less.

Mn: Mn is a solid solution strengthening element like Si, and is added as necessary. However, if the content is less than 0.01%, the production cost at the steel making stage is increased, so that the content is 0.01%.
% Or more. On the other hand, if 0.5% or more is added, rm is deteriorated and ductility is also reduced.

P: P is an impurity, and is an element which segregates at the grain boundary to cause grain boundary embrittlement and causes secondary working cracks, so that the smaller the content, the better. However, if it is 0.05% or less, it is within an allowable range. S: S is an impurity, and if it is too much, it not only causes cracks during hot rolling, but also causes deterioration of rm. Therefore, S should be reduced as much as possible. Al: Al is usually added for deoxidation of molten steel, but in the present invention, a smaller amount is more preferable because it hinders the effect of Mg addition. However, if it is 0.004% or less, it is within an allowable range.

Mg: Mg is one of the most important additive elements in the present invention. Mg combines with oxygen to form a fine oxide. Mg and Ti finely dispersed in steel
The oxides and composite oxides suppress the coarsening of crystal grains at the hot-rolled sheet stage by the pinning effect, and reduce | Δr |. Also, during recrystallization annealing, these particles have a size of # 11
It is considered that there is an effect of suppressing nucleation or grain growth in the 0｝ <100> orientation and the {100｝ <110> orientation. However, if the content is less than 0.0001%, the effect is insufficient. On the other hand, if added over 0.01%, not only does the effect of reducing | Δr | become saturated, but also it is extremely difficult in steelmaking technology. Therefore, the added amount of Mg is 0.0001%
Not less than 0.01%.

O: O is required to be 0.001% or more in order to obtain fine particles containing Mg and / or Ti which have an effect of reducing | Δr |. However, 0.01
If it exceeds%, the oxide becomes coarse and its effect is lost, and moreover, C-based inclusions that are not preferable for the moldability increase. Therefore, the content of O is set to 0.001% or more and 0.01% or less.

N: N is involved in the formation of fine particles containing Mg and Ti oxides and composite oxides that have an effect of reducing | Δr |, and promotes the effect of reducing | Δr |. In order to obtain this effect, N must be contained at 0.001% or more. However, the effect is saturated even if the content exceeds 0.01%, so that the content of N is 0.001% or more and 0.1% or more.
01% or less.

Ti: Ti is one of the most important additive elements in the present invention. Ti is necessary for forming oxides and composite oxides of Mg and Ti having an effect of reducing | Δr | and uniformly and finely dispersing them. Further, it is considered that these particles have an effect of suppressing nucleation or grain growth in the {110} <100> direction and the {100} <110> direction during recrystallization annealing. Also, Ti precipitates as TiN and TiS in the high temperature region of the γ phase, but when Ti ≦ 3.42N + 1.5S, the dissolved N is not sufficiently fixed as TiN and the remaining dissolved N only deteriorates the deep drawability. In addition, B which precipitates as BN and is effective for secondary workability cannot be secured. Therefore, Ti: Ti> 3.42N
+1.5 S is added to satisfy the condition. But T
When i is added in an amount of 0.03% or more, the adhesion of hot-dip galvanized coating deteriorates and powdering occurs during press molding.
0.05% or less, preferably less than 0.03%. Further, in order to suppress the deterioration of deep drawability due to the residual solid solution C,
(Ti-3.42N-1.5S) / 48-C / 12> 0
And

Nb is preferably added because it fixes C as a precipitate and further improves formability. However, ｛(Ti−3.42N−1.5S) / 48 + Nb /
When 93 ° -C / 12 ≦ 0, solid solution C remains, which is not preferable for deep drawability. Therefore, ｛(Ti-3.42N-
1.5S) / 48 + Nb / 93｝ -C / 12> 0. However, Nb is 0.03%
Addition of the above increases the recrystallization temperature, so that the recrystallization cannot be sufficiently completed during annealing and the rm deteriorates. Therefore, the content is preferably 0.05% or less, and preferably less than 0.03%.

B: B is preferably added to prevent secondary working cracks. However, if the addition is less than 0.0003%, the effect is not obtained, and if the addition exceeds 0.002%, the effect is saturated. Therefore, the amount of B added is 0.
0003% or more and 0.002% or less. In the present invention, the components other than the above are Fe, but the inclusion of unavoidable impurities mixed from the smelting raw material such as scrap is allowed.

Next, Mg and Ti contained in the steel sheet of the present invention are described.
The state of existence of the oxides and composite oxides will be described. These oxides have an average particle diameter of 0.005
If it is less than μm, grain growth during recrystallization annealing is hindered, and good rm cannot be obtained. On the other hand, if it exceeds 0.1 μm, it is not preferable for the in-plane anisotropy of the r value {110} <10
Nucleation and grain growth during recrystallization annealing in the 0> and {100} <110> directions become insufficient | Δ
r | ≦ 0.2 cannot be obtained. Therefore, rm and | Δr
In order to satisfy both | ≦ 0.2, the average particle diameter of Mg and Ti oxides and composite oxides must be 0.005 μm.
At least 0.1 μm.

If the areal density of the oxides and composite oxides of Mg and Ti is less than 40 particles / mm 2, the number of these particles is small, which is not preferable for the in-plane anisotropy of r value.
Sufficient suppression of nucleation and grain growth during recrystallization annealing of the 10｝ <100> orientation and the {100｝ <110> orientation becomes insufficient, and | Δr | ≦ 0.2 cannot be obtained. Therefore, Mg
And Ti oxide and composite oxide have an area density of 40 /
mm2 or more. On the other hand, if the areal density of the oxides and composite oxides of Mg and Ti is more than 1000 / mm 2, {110} <10 is not preferable for the in-plane anisotropy of the r value.
The effects of suppressing nucleation and grain growth during recrystallization annealing in the 0> orientation and {100} <110> orientation are not only saturated, but also degrade elongation. Therefore, the areal density of the oxides and composite oxides of Mg and Ti is set to 1000 / mm 2 or less.

[0032]

The present invention will be further described below with reference to examples. After continuously casting and smelting steels A to M having the chemical components shown in Tables 1 and 2 in a converter, the slabs were reheated at a heating temperature of 1150 ° C, and finished at 910 ° C to 930 ° C. After rolling to 6.0 mm in the rolling temperature range, 600 ° C to 70 ° C
Winded at 0 ° C. Cold-rolled to 0.8mm after pickling, continuous annealing line (maximum heating temperature 800 ~ 830 ℃,
(Skin pass rolling ratio: 0.8%). However, for steel F, after cold rolling, hot-dip galvanizing line (maximum heating temperature 810 ° C, hot-dip galvanizing 460 ° C, alloying treatment 520 ° C x 20 seconds, skin pass rolling ratio 0.8
%). The indications of the chemical compositions in Tables 1 and 2 are% by mass.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

The relationship between rm and | Δr | and the mechanical properties of the steel sheet thus obtained and the particle size of Mg and Ti oxides and / or composite oxides were investigated.
However, the mechanical properties are as described in JIS Z 2201
The tensile test method described in JISZ2241 was used for the test piece. | Δr | was obtained by the same method as described above.

On the other hand, the state of the oxides and the composite oxides of Mg and Ti was determined by taking a transmission electron microscope sample from a quarter of the thickness of the test steel and performing energy dispersive X-ray spectroscopy (Energy Dispersive X-ray spectroscopy). ray
Spectroscope (EDS) or electron energy loss spectroscopy (Electron Energy Loss)
It was observed by a transmission electron microscope equipped with a field emission type electron gun (Field Emission Gun: FEG) with an accelerating voltage of 200 kV and a composition analysis function of Spectroscope (EELS). The composition of the observed particles was determined by the above EDS and EELS.
And Ti oxides and composite oxides. Further, their particle diameters are 10,000 to 50,000.
This is the average value in one visual field of the target particles measured at a magnification of 0 over an area of at least 1000 μm 2 or more. On the other hand, the areal densities (the number per 1 mm 2) of the target size particles were also measured.

According to the present invention, steels A, C, D, E,
F, I, and L steels. For other steels, cold-rolled steel sheets with small in-plane anisotropy and excellent deep drawability have not been obtained for the following reasons. Steel B has a low areal density of oxides and composite oxides of Mg and Ti because the Al content is out of the range of the present invention, and | Δr | is out of the range of the present invention. Steel G has a low rm because the value of C * is outside the scope of the present invention. Since steel H does not contain Mg, the diameter of the oxide particles is large, and | Δr | is out of the range of the present invention. Steel J has a higher C content than is within the scope of the present invention and therefore a lower rm. Steel K has a larger O content than the range of the present invention, and thus has a large particle size of Mg and Ti oxides and composite oxides. Therefore, | Δr | is out of the range of the present invention. Since the steel M has a lower O content than the range of the present invention, the areal density of the particles of the oxides and composite oxides of Mg and Ti is low, and | Δr | is out of the range of the present invention.

[0039]

As described above, the present invention provides a cold rolled steel sheet having a small in-plane anisotropy and excellent in deep drawability. By using these steel sheets, the occurrence of ears after press forming can be reduced. It can be significantly reduced.

[Brief description of the drawings]

FIG. 1 shows the grain size numbers N and | of recrystallized grains after annealing in a cross section parallel to the rolling direction in an experiment on which the present invention is based.
FIG. 6 is a diagram illustrating a relationship of Δr |.

FIG. 2 is a diagram showing the range of the present invention in relation to the average particle diameter d of Mg and Ti oxides and composite oxides and | Δr | and rm.

Claims (5)

[Claims] 1. In mass%, C ≦ 0.005%, Si = 0.005-0.1%, Mn = 0.01-0.5%, P ≦ 0.05%, S ≦ 0.02 %, Al ≦ 0.004%, Mg = 0.001-0.01%, O = 0.001-0.01%, N = 0.001-0.01%, and further 0.05 >Ti> 3.42N + 1.5S and (Ti-3.42N-1.5S) / 48-C / 12> 0, with the balance being Fe and inevitable impurities. Cold rolled steel sheet with low anisotropy and excellent deep drawability. 2. The surface according to claim 1, further satisfying 0.05>Nb> 93 {C / 12- (Ti-3.42N-1.5S) / 48} by mass%. Cold rolled steel sheet with low internal anisotropy and excellent deep drawability. 3. The cold-rolled material according to claim 1, wherein the content of B is 0.0003 to 0.002% by mass%. steel sheet. 4. An average particle size of 0.005 μm or more and 0.1 or more.
4. The steel according to claim 1, wherein an oxide and / or a composite oxide of Mg and Ti having a particle size of not more than μm is contained in the steel.
The cold-rolled steel sheet having a small in-plane anisotropy according to any one of the above, and having excellent deep drawability. 5. The areal density of oxides and / or composite oxides of Mg and Ti contained in steel is 40 to 1000 / m.
5. The cold-rolled steel sheet according to claim 4, having a small in-plane anisotropy and an excellent deep drawability.