JP2002173737A - High strength and high ductility cold rolled steel sheet having excellent press formability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength and high ductility cold rolled steel sheet which has no generation of surface defects in a slab, and has excellent ductility, spot weldability and press formability as well. SOLUTION: The steel sheet has a composition containing 0.05 to 0.40% C, 0.5 to 3.0% Si, 0.5 to 3.0% Mn, <=0.05% P, <=0.005% S, <0.10% Al, <=0.0100% N and <=0.20% Ti, and further containing one or two kinds of Ca and rare earth metals, and has a structure containing, by volume, >=30% ferrite and retained γ of >=3% by the average in the total sheet thickness direction, and in which the content of residual γ in the region from the steel sheet surface to a position of 1/16 in the sheet thickness is <=50% to the content of residual γin the central part in the sheet thickness direction. Ti/N>=2.5 is preferably satisfied. The steel sheet may contain one or two kinds of Nb and V, and one or more kinds selected from Cr, Mo and B.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength cold-rolled steel sheet, and in particular, can prevent the occurrence of defects such as vertical cracks and horizontal cracks in a slab in a manufacturing process, and is suitable for molded parts represented by automobile parts. The present invention relates to a high-strength and high-ductility cold-rolled steel sheet having excellent spot weldability and excellent press formability.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of preserving the global environment,
There is a demand for improved fuel efficiency of automobiles. In addition, in order to protect occupants in the event of a collision, there is a demand for improved safety of the vehicle body. For these reasons, recently, the weight reduction of automobile bodies and the strengthening of automobile bodies have been actively promoted. Particularly, in order to reduce the weight of an automobile body, it has been considered to increase the strength of a steel sheet for automobile parts and reduce the thickness of the steel sheet.

[0003] Further, since many automotive parts made of steel plates are formed by press working, steel sheets for automotive parts are required to have excellent press formability. In order to realize excellent press formability, it is essential to secure high ductility in the first place. In press molding of automobile parts, stretch flange molding is often used. In particular, in the case of a member constituting a skeleton member for ensuring the strength of an automobile body, a component constituting a reinforcement, or the like, a variety of component forming such as stretch flange forming is often performed. Therefore, there is a strong demand for steel sheets for automobile parts to have high strength and excellent ductility and stretch flangeability.

[0004] As a high-strength steel sheet having excellent ductility, a dual-phase steel sheet having a composite structure of ferrite and martensite is typical. In recent years, for example, the Iron and Steel Institute of Japan, Materials and Processes, vol.4 (1991), p.1942
As described in the above, high-ductility steel sheets utilizing the transformation-induced plasticity resulting from retained austenite (γ) by adding appropriate amounts of Si and Mn to 0.11% C have also reached the stage of practical use.

Japanese Patent Application Laid-Open No. 10-130776 discloses that C:
0.06-0.25%, Si: 2.5% or less, Mn: 0.5-3.0%,
P: 0.1% or less, S: 0.03% or less, Al: 0.1 to 2.5%,
Ti: 0.003 to 0.08%, N: 0.01% or less, and Ti
Satisfies (48/14) N ≦ Ti ≦ [(48/14) N + (48/32) S + 0.01], and the structure after cold rolling and recrystallization annealing contains 5% or more of residual γ Which is a high-ductility type high-tensile cold-rolled steel sheet. In the technology described in JP-A-10-130776, while using Al as a stabilizing element for suppressing carbide formation and residual γ, by containing Ti, to prevent surface flaws generated during continuous casting, It is said to be a steel sheet with excellent surface properties and shock absorption.

[0006] Japanese Patent Application Laid-Open No. 10-273752 discloses that C:
0.05 to 0.20%, Mn: 0.5 to 2.0%, the total amount of one or two of Si and Al is 0.5 to 4.0%, or N
b: 0.2% or less, Ti: 0.2% or less, B: One or more of 0.02% or less, with the balance being Fe as a main component and the microstructure of a steel sheet after 5% forming, A high-strength steel sheet for automobiles having an occupation ratio of residual γ of 3% or more and a work hardening index of 0.130 or more and excellent in impact resistance and formability is disclosed. Also, JP-A-10-273752 discloses that
In order to obtain the above-mentioned microstructure, continuous annealing is performed from Ac ₁ to
And held at a temperature of Ac ₃ 10 seconds or more, performs an average cooling rate is not less than 10 ° C. / sec cooling 700 to 500 ° C., 500 ° C. at a temperature
It is disclosed that the condition is maintained for 30 seconds or more. However, in the steel sheet described in JP-A-10-273752,
During production, surface defects such as vertical cracks and horizontal cracks of the slab occur, and there is a problem that stable production is difficult.

[0007]

SUMMARY OF THE INVENTION
No. 10-130776, high-strength steel sheets described in JP-A-10-273752 and the like, `` Iron and steel, vol.63 (1982) No.9, p.11
As shown in "0", there is a problem that the dimensional defect due to springback after press molding becomes remarkable as the material strength increases. Dimensional failure due to springback is a major problem when using high-strength steel sheets,
It is required to have excellent shape freezing properties after press molding.

The present invention advantageously solves the above-mentioned problems of the prior art, prevents the occurrence of defects such as vertical cracks and horizontal cracks in a slab in a continuous casting process, and is suitable for molded parts represented by automobile parts. It is an object of the present invention to propose a high-strength, high-ductility cold-rolled steel sheet having excellent ductility, excellent spot weldability, and excellent press formability such as stretch flangeability, slidability, shape freezing property, and the like. The high-strength and high-ductility cold-rolled steel sheet in the present invention is desirably manufactured using a continuous annealing line.

[0009]

Means for Solving the Problems To achieve the above-mentioned object, the present inventors have developed a slab surface crack, a ductility of steel sheet,
Intensive research was conducted on the influence of the steel composition and the microstructure of the steel sheet on the press formability such as stretch flangeability, slidability and shape freezing property. As a result, (1) C, Si, Mn, etc. can be prevented by (1) containing Ti, further reducing the Al content, or further adjusting Ti in relation to the N content. While adjusting the alloy element content of the alloy within a predetermined range, the continuous annealing conditions were adjusted, and the steel sheet structure after annealing was a composite structure composed of ferrite, retained austenite, and a low-temperature transformation phase, and the abundance ratio of each phase was set to a predetermined value. By setting the ratio, the ductility of the cold-rolled annealed sheet can be improved. (3) By adjusting the amount of Ti and making the crystal grain size of ferrite, which is the main component of the composite structure, fine, it is excellent in addition to high ductility. (4) By adjusting the amount of strain applied to the surface of the steel sheet, the structure immediately below the surface of the steel sheet is compared with the structure at the center of the sheet thickness to obtain a residual γ amount. A composite structure with less Sliding property during molding, it can be an excellent cold-rolled annealed sheets in shape fixability, to obtain a finding that.

The present invention has been made by further study based on the above findings. That is, in the present invention, C: 0.05 to 0.40%, Si: 0.5 to 3.0% by mass%,
Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.005% or less, Al: less than 0.10%, N: 0.0100% or less, Ti: 0.20% or less, and one or more of Ca and REM It contains 0.0010-0.0500% of seeds in total, has a composition consisting of the balance Fe and unavoidable impurities, and a composite structure consisting of ferrite, residual austenite, and a low-temperature transformation phase.
The ferrite content is 30% or more, the retained austenite is 3% or more in average in the entire thickness direction, and the amount of retained austenite in the region from the steel sheet surface to the 1 / 16th sheet thickness position is the retained austenite in the central part in the thickness direction. It is a high-strength and high-ductility cold-rolled steel sheet excellent in press formability characterized by being 50% or less with respect to the amount, and in the present invention, the composition may satisfy Ti / N: 2.5 or more. Preferably, in the present invention, one or two of Nb and V are added in a total amount of 0.001% by mass% in addition to the above-mentioned respective compositions.
Preferably, it is contained in an amount of up to 0.30%.

Further, in the present invention, in addition to the above-mentioned respective compositions, Cr: 1.5% or less, Mo: 0.5% or less, B:
It is preferable to contain one or more selected from 0.01% or less. Further, the present invention provides, in mass%,
C: 0.05 to 0.40%, Si: 0.5 to 3.0%, Mn: 0.5 to 3.0
%, P: 0.05% or less, S: 0.005% or less, Al: less than 0.10%, N: 0.0100% or less, Ti: 0.20% or less.
One or two of Ca and REM are 0.0010 to 0.05 in total
A steel material containing 00% is hot-rolled into a hot-rolled sheet, the hot-rolled sheet is pickled, cold-rolled into a cold-rolled sheet, and then the cold-rolled sheet is subjected to a continuous annealing line. , And heat treatment to heat the Ac ₁ transformation point or higher to 850 ° C or lower, and rapidly cool to a temperature range of 300 to 500 ° C at a cooling rate of 1 ° C / s or higher.
This is a high-strength, high-ductility, cold-rolled steel sheet with excellent press formability, which is subjected to continuous annealing at a temperature in the range of 300 to 500 ° C for at least 10 s, followed by bending back or temper rolling. .

[0012]

First, the reasons for limiting the composition of the steel sheet of the present invention will be described. Hereinafter,% in the composition means mass%. C: 0.05 to 0.40% C is an element indispensable for increasing the strength of steel, has an effect of promoting the formation of retained austenite and a low-temperature transformation phase, and is an essential element in the present invention. In order to secure the desired high strength, the content of 0.05% or more is required in the present invention. On the other hand, a content exceeding 0.40% causes deterioration of weldability. For this reason, C was limited to the range of 0.05 to 0.40%. More preferably, it is 0.10 to 0.30%.

Si: 0.5-3.0% Si is an element that strengthens steel by solid solution strengthening, stabilizes austenite, and promotes the formation of a residual austenite phase. Such an effect is 0.5
%. On the other hand, if the content exceeds 3.0%, the ductility deteriorates. Therefore, Si is 0.5-3.0%
Limited to the range. More preferably, it is 1.0 to 2.5%.

Mn: 0.5 to 3.0% Mn is an element that strengthens the steel by solid solution strengthening, improves the hardenability of the steel, and promotes the formation of retained austenite and a low-temperature transformation phase. Such an effect
It is recognized at a content of 0.5% or more. On the other hand, if the content exceeds 3.0%, the effect saturates, and an effect commensurate with the content cannot be expected, resulting in an increase in cost and an economic disadvantage.
For this reason, Mn is limited to the range of 0.5 to 3.0%. More preferably, it is 1.0 to 2.0%.

P: 0.050% or less P is an element that strengthens steel by solid solution strengthening.
Effective element for obtaining high-strength steel sheet, 0.050%
If the content exceeds 3, the spot weldability is reduced. Therefore, in the present invention, the upper limit of the P content is set to 0.05%. It is more preferably at most 0.020%.

S: not more than 0.005% S is an element that forms MnS in the steel and lowers the stretch flangeability of the steel sheet. In the present invention, it is desirable to reduce as much as possible. However, since an excessive reduction leads to an increase in refining costs, the upper limit of the S content is limited to 0.005% in the present invention. The content is more preferably 0.003% or less.

Al: less than 0.10% Al acts as a deoxidizing agent and has the effect of separating non-metallic inclusions, which lower the hole expandability, as slag. However, when the content is 0.10% or more, the amount of inclusions increases, the stretch flangeability decreases, and the alloy cost increases, which is economically disadvantageous. For this reason, in the present invention, Al content is less than 0.10%. In addition, it is preferably in the range of 0.02 to 0.09%.

N: 0.0100% or less N is a harmful element that causes strain aging, and it is desirable to reduce it as much as possible. However, in the present invention, since Ti is contained, Ti
The risk of aging degradation can be avoided by the precipitation of N. However, when the content exceeds 0.0100%, Ti corresponding to the content must be contained, which increases the cost. Therefore, in the present invention, N is limited to 0.0100% or less. Note that the content is more preferably 0.0050% or less. Ti: 0.20% or less Ti is an element effective for suppressing the growth of the ferrite phase in the heating stage during continuous annealing, refining ferrite grains, and remarkably improving hole expandability. Such an effect is 0.01
Although the content becomes remarkable at a content of 0% or more, a content exceeding 0.20% causes an increase in alloy cost, increases the amount of TiC precipitated, and reduces the retained austenite for exhibiting the transformation-induced plasticity effect. Further, as the Ti content further increases, the strength increase due to precipitation strengthening increases, and high ductility cannot be obtained. For these reasons, Ti was limited to 0.20% or less. Preferably, it is at most 0.10%.

[0019] In addition, during the cooling of the slab, Ti precipitates Ti-based carbonitride and Ti-based sulfide at a high temperature, and forms AlN at a relatively low temperature.
Also, it is an effective element that suppresses precipitation of Nb-based carbide and V-based carbide generated at the grain boundary by Nb and V added for the purpose of crystal grain refinement, thereby preventing slab surface cracking. In order to remarkably exhibit such a slab surface crack preventing effect, the Ti content preferably satisfies Ti / N: 2.5 or more in relation to the N content. It is more preferable that Ti: 0.10% or less and Ti / N: 3.0 or more be satisfied.

One or two selected from Ca and REM
Species: Total: 0.0010-0.0500% Ca, REM has the effect of controlling the form of sulfide inclusions, thereby improving the stretch flangeability of the steel sheet. Such an effect is saturated when the content of one or two selected from Ca and REM exceeds 0.0500% in total. Therefore, the content of one or two of Ca and REM is preferably limited to 0.0500% or less in total. A more preferred range is 0.001 to 0.03%.

One or two selected from Nb and V
Species: Total: 0.001 to 0.30% Nb, V forms carbide: NbC, VC like Ti, suppresses the growth of ferrite phase in the heating stage during continuous annealing, refines crystal grains, and expands holes It is an element effective for significantly improving the properties, and can be contained as necessary. These effects are
It becomes remarkable when the total content of Nb and V is 0.001% or more.
When the total content of Nb and V exceeds 0.30%, the yield strength (YS) is increased by precipitation strengthening, the workability is reduced, and the retained austenite for exhibiting the transformation-induced plasticity effect is reduced.

Further, if the generated carbides of Nb and V preferentially precipitate at the crystal grain boundaries, they may cause surface cracks of the slab. Therefore, in the present invention, the content of Nb and V is set to 0.001 to 0.1%.
Preferably, it is 30%. In addition, more preferably, 0.00
1 to 0.10%. One or more selected from the group consisting of Cr: 1.5% or less, Mo: 0.5% or less, and B: 0.01% or less Cr, Mo, and B improve the hardenability of steel and form a low-temperature transformation phase. Is an element that has the effect of promoting and can be contained as necessary. Such an effect is as follows: Cr: 0.2% or more, Mo: 0.
It becomes remarkable when the content is 05% or more and B: 0.0005% or more. On the other hand, if the content exceeds 1.5% for Cr, 0.5% for Mo, and 0.01% for B, the effect saturates, an effect corresponding to the content cannot be expected, and it is economically disadvantageous. Therefore, Cr:
It is desirable to contain one or more of 1.5% or less, Mo: 0.5% or less, and B: 0.01% or less. More preferably, one or more of Cr, Mo, and B are 0.0005 to 1.0% in total.

Next, the structure of the steel sheet of the present invention will be described. The steel sheet of the present invention is a steel sheet having a composite structure composed of ferrite, retained austenite, and a low-temperature transformation phase. Ferrite phase: 30% or more by volume ratio Ferrite is a soft phase that does not contain iron carbide, has high deformability, and improves ductility of a steel sheet. In the present invention, such a ferrite phase is contained in a volume ratio of 30% or more. If the proportion of the ferrite phase is less than 30%, remarkable improvement in ductility cannot be expected. For this reason, the amount of ferrite in the composite structure is limited to 30% or more. In addition, more preferably
50% or more.

Average grain size of ferrite: 15 μm or less Refining the grain size has the effect of improving the stretch flangeability of the steel sheet. In the steel sheet of the present invention, the average grain size of ferrite in the composite structure is preferably set to 15 μm or less. If the average crystal grain size of ferrite exceeds 15 μm, a remarkable improvement in stretch flangeability cannot be expected. In addition,
More preferably, it is 10 μm or less.

Retained austenite phase: 3% or more in volume ratio on average in the entire thickness direction. Retained austenite undergoes strain-induced transformation into martensite during processing, widely dispersing locally applied processing strain,
It has the effect of improving the ductility of the steel sheet. Therefore, in the present invention, retained austenite is contained at 3% (volume ratio) or more. Retained austenite content is 3% (vol%)
If it is less than 1, a remarkable improvement in ductility cannot be expected. For this reason, the amount of retained austenite was limited to 3% or more. In addition, it is preferably at least 5%. The content of retained austenite here refers to the average (average value) in the entire sheet thickness direction (sheet thickness cross section).

The average amount of retained austenite in the entire sheet thickness direction (sheet thickness section) was determined by the X-ray diffraction method to be right below the surface of the sheet thickness section, 1/16, 1/8, 1 / 4, 1 /
At each position of 2, the diffraction X-ray intensity is measured, and the diffraction X-ray intensity ratio between the austenite phase and the ferrite phase is calculated to be the average value in the entire thickness direction. (Amount of retained austenite in the region from the steel sheet surface to 1 / 16th sheet thickness) / (Amount of retained austenite at the center of the steel sheet thickness): 50% or less In the steel sheet of the present invention, the thickness of the steel sheet is 1 The average amount of retained austenite in the region up to the / 16 position is set to 50% or less of the amount of retained austenite at the center of the thickness of the steel sheet. The central part of the thickness of the steel sheet is defined as 1/4 to 1 of the thickness.
/ 2 area.

Further, the amount of retained austenite in the region from the steel plate surface to the 1 / 16th of the plate thickness is determined by polishing the cross section of the plate thickness of the steel plate, and measuring the diffraction X The line intensity is measured, the diffraction X-ray intensity ratio between the austenite phase and the ferrite phase is calculated, and the ratio is determined from the average value. The amount of retained austenite at the center of the thickness of the steel sheet was determined by measuring the diffraction X-ray intensity at each position of １ ／ of the thickness and の of the thickness of the steel sheet. The X-ray intensity ratio is calculated and determined from the average value of these.

By making the average retained austenite amount in the region from the steel sheet surface to the sheet thickness 1/16 position less than or equal to 50% of the residual austenite amount at the center of the steel sheet thickness, the surface layer area of the steel sheet is reduced. As a result, the precipitation strengthening of TiC or the like is increased, the yield point is increased, the work hardening coefficient (n value) is reduced, and the shape freezing property of the steel sheet is improved. here,
The "shape freezing property" is determined based on the amount of change in the shape after the press working into a shape (hat-shaped cross section: broken line) as shown in FIG.

Further, the structure of the surface layer of the steel sheet is reduced to 50% or less of the ratio of the retained austenite phase as compared with the central part of the sheet thickness to have a structure having a high yield point, so that the sliding at the time of press forming is improved. The performance is improved. On the other hand, from the surface,
If the amount of retained austenite in the region up to 16 positions exceeds 50% with respect to the central portion, the shape freezing property after press working is reduced and the slidability during press forming is reduced.

When the range in which the amount of retained austenite is 50% or less with respect to the amount of retained austenite at the center of the sheet thickness exceeds 1/16 position of the sheet thickness from the surface, it is difficult to obtain a highly ductile steel sheet. Become. For this reason, in the present invention, the amount of retained austenite in the region from the surface to the 1/16 position of the plate thickness is set to 50% or less of that in the center of the plate thickness. Preferably, it is at most 40%. Further, the region where the amount of retained austenite is 50% or less of the amount of retained austenite at the center of the sheet thickness is 1/1
It is preferred that the area be up to 20 positions.

In the structure of the steel sheet of the present invention, the remainder other than the ferrite phase and the retained austenite phase is a low-temperature transformation phase. The low-temperature transformation phase in the present invention refers to martensite or bainite. Both martensite and bainite are hard phases and have the effect of increasing the strength of the steel sheet by strengthening the structure. Further, since transformation is accompanied by the occurrence of movable dislocations, it also has the effect of lowering the yield ratio of the steel sheet. In addition,
In order to sufficiently obtain such an effect, it is preferable that the low-temperature transformation phase be martensite. In the present invention, the amount of the low-temperature transformation phase is not particularly limited. What is necessary is just to distribute suitably according to the intensity | strength of a steel plate.

Next, a method for producing a high-strength and high-ductility cold-rolled steel sheet according to the present invention will be described. First, a steel having the above-described composition is melted by a generally known method, cast by a generally known method to obtain a steel material, and then hot-rolled by a generally known method to obtain a hot-rolled sheet. The obtained hot-rolled sheet is subjected to cold rolling by a generally known method to obtain a cold-rolled sheet. Then, on these cold rolled sheets,
Anneal.

The annealing is preferably performed in a continuous annealing line. Annealing is performed by performing a heat treatment of heating to a temperature not lower than the Ac ₁ transformation point and not higher than 850 ° C., and then rapidly cooling from the heating temperature to a temperature range of 300 to 500 ° C. at a cooling rate of 1 ° C./s or more, and then 300 ° C.
Preferably, the treatment is to stay in the temperature range of up to 500 ° C. for 10 seconds or more. If the heating temperature is lower than the Ac ₁ transformation point, no residual austenite is obtained without heating to the α + γ two-phase region, so that no transformation-induced plasticity effect is exhibited, and improvement in ductility cannot be expected. On the other hand, when the heating temperature exceeds 850 ° C., the grain size of the ferrite phase increases, and the stretch flangeability decreases. Therefore, the heating temperature is 850 ℃ above the Ac ₁ transformation point.
It is preferable to set the following. , Cooling rate after heat treatment,
At less than 1 ° C./s, austenite generated by heating transforms to pearlite or bainite during cooling, no residual austenite remains, no transformation-induced plasticity effect is exhibited, and improvement in ductility cannot be expected. For this reason, the cooling rate is preferably set to 1 ° C./s or more. Note that the temperature is more preferably 10 ° C./s or more.

When the cooling stop temperature is lower than 300 ° C., almost all austenite is transformed into martensite,
No retained austenite remains, no transformation induced plasticity effect is exhibited, and improvement in ductility cannot be expected. On the other hand, if the cooling stop temperature exceeds 500 ° C., austenite is transformed into pearlite or bainite, no retained austenite remains, no transformation-induced plasticity effect is exhibited, and improvement in ductility cannot be expected. For this reason, the cooling stop temperature is 300 to 500
The temperature is preferably in the range of ° C. In addition, more preferably,
370-450 ° C.

In a short time of less than 10 s after the quenching stop in the temperature range of 300 to 500 ° C., most of the remaining austenite is transformed into martensite, and the desired amount of retained austenite can be secured. As a result, the transformation-induced plasticity effect is not exhibited during press working. On the other hand, 600
If the holding time is longer than s, bainite transformation occurs, and the amount of retained austenite decreases. For this reason, it is preferable that the residence time in the temperature range of 300 to 500 ° C. is 10 s or more, more preferably 60 s or more and 600 s or less.

By subjecting the cold-rolled sheet to an annealing treatment under the above conditions, a composite structure comprising a fine ferrite phase, a predetermined amount or more of a retained austenite phase and a low-temperature transformation phase can be obtained. After annealing, it is preferable to further perform temper rolling to obtain a product (a cold-rolled annealed plate). In order to make the structure just below the surface of the steel sheet, that is, the region from the surface to the position of 1/16 of the sheet thickness, a desired structure, in the final stage of continuous annealing, that is, in the final cooling process at a steel sheet temperature of 300 ° C. or less It is preferable that the steel sheet is subjected to a bending-back process of applying a bending-back by a small-diameter roll, or a temper rolling without applying tension.

By this bending back, retained austenite becomes martensite in a region immediately below the steel sheet surface,
Finally, the structure in this area is
The structure has a small amount of retained austenite. In order to obtain such a structure, it is preferable that the amount of the bending back is 1% or more and the number of times of the bending back is 1 or more. In addition, it is preferable to use a small-diameter roll having a diameter of 50 to 500 mmφ from the viewpoint of adding distortion.

Further, the temper rolling without applying tension causes the retained austenite to become martensite in a region immediately below the surface of the steel sheet, and finally, the structure in this region has a smaller residual austenite amount than the structure in the center of the sheet thickness. Become a small organization. In order to obtain such a structure, it is preferable that the rolling reduction of the temper rolling is in the range of 0.2 to 2.0%. If the elongation of the temper rolling is less than 0.2%, a desired structure cannot be obtained.
On the other hand, if it exceeds 2.0%, there is a problem that ductility (elongation) deteriorates. Needless to say, the steel sheet of the present invention may be applied as a steel sheet for electroplating. Even if electroplating is performed, there is no change in the mechanical properties of the steel sheet of the present invention.

[0039]

Next, the present invention will be described in detail with reference to examples. Steel having the composition shown in Table 1 was melted in a converter and cast into slabs by a continuous casting method. Next, the obtained slab was hot-rolled into a hot-rolled sheet having a thickness of 3.0 mm. Then, after pickling these hot-rolled sheets, the sheet thickness was 1.
A 4 mm cold rolled sheet was used.

These cold-rolled sheets were subjected to heat treatment, rapid quenching and 300-300 ° C. under the conditions shown in Table 2 in a continuous annealing line.
Continuous annealing consisting of staying (slow cooling) in a temperature range of 500 ° C. was performed to obtain a cold-rolled annealed sheet. In addition, the temper rolling without tension | tensile_strength was performed with the temper rolling mill which is the last stage equipment of continuous annealing. About the obtained cold rolled annealed sheet, the microstructure of the steel sheet,
Tensile properties, stretch flangeability, spot weldability, slidability,
The shape freezing property was investigated. The survey method is as follows. (1) Microstructure A test specimen is sampled from each cold-rolled annealed sheet, embedded in resin so that the cross-section of the steel sheet becomes the observation surface, and used as an observation test specimen, and the microstructure is observed with an optical microscope or a scanning electron microscope. did. The test specimen for observation was etched at room temperature in a liquid obtained by mixing “an aqueous solution obtained by adding 1 g of sodium pyrosulfite per 100 ml of pure water” and “a liquid obtained by adding 4 g of picric acid to 100 ml of ethanol”. For 120 seconds. By this etching, under an optical microscope, the ferrite phase is observed as a black part, and the other second phases are observed as white parts. After etching, magnify the tissue: × 1000
And the area ratio of ferrite was determined using an image analyzer and converted to a volume ratio. The ferrite particle size is
The crystal grain size was measured according to JIS Z 0552 and converted to an average crystal grain size.

The amount of retained austenite in each cold-rolled annealed sheet was determined by taking a test piece from each cold-rolled annealed sheet and performing X-ray diffraction using MoK α-rays as incident X-rays. The diffraction X-ray intensity was determined by measurement. Austenitic phases ｛111｝, ｛200｝, ｛220｝, ｛311
｛, And ｛200｝, ｛110｝, ｛211 in ferrite phase
｝ Determine the diffraction X-ray intensity of the surface and calculate the following equation: Vγ = {γ / (α + γ)} × 100% γ = [I (111) γ / 0.7387 + I (200) γ / 0.3547 + I
(220) γ / 0.2083 + I (311) γ / 0.2186] / 4α = [I (110) α / 1.0000 + I (200) α / 0.1587 + I (211) α
/0.2923]/3, the residual austenite amount (volume ratio) (Vγ
(%)) Was calculated.

The average amount of retained austenite in the entire thickness direction was determined by measuring the measurement surface at the surface, 1/16, 1/8, 1 /
4 and 1/2, the amount of retained austenite at each position was determined by the above method and averaged. In addition, from the steel sheet surface,
The amount of retained austenite in the region up to the / 16 position is
With the measurement surface being 1/16 of the plate thickness, the amount of retained austenite at each position was determined in the same manner as described above and averaged. The amount of retained austenite at the center in the thickness direction is
The amount of retained austenite at each position was determined in the same manner as described above, with the measurement surface being １ ／ and １ ／ of the plate thickness, and averaged. (2) Tensile properties JIS Z 2204 sampled from each cold-rolled annealed sheet in the direction perpendicular to the rolling direction
Using a JIS No. 5 test piece conforming to the regulations, a strain rate of 7
A tensile test was performed under the conditions of × 10 ⁻³ / s, and the proof stress (YS), tensile strength (TS), elongation at break (El), and yield elongation (YEl) were measured. (3) Stretch flangeability A test piece was sampled from each cold-rolled annealed plate, and the hole expansion ratio (λ) was measured in accordance with the provisions of JFS T1001. (4) Spot weldability For each cold-rolled annealed plate, using a dome-shaped electrode with a tip diameter of 6φ, electrode force: 4.3 kN, welding current: 8 kA,
squeeze time: 25cyc, setup time: 3cyc, welding tim
e: spot welding was performed under the welding conditions of 13 cyc and holding time of 1 cyc to produce a spot welded joint sample. For each of the obtained welded joint samples, a tensile load (TSS) was determined by a tensile shear test in accordance with the provisions of JIS Z3136. For each of the obtained welded joints, a tensile load (CTS) was determined by a cross-shaped tensile test in accordance with the provisions of JIS Z 3137.
I asked. If the obtained tensile load (TSS) is 11062N or more, which is the standard tensile shear load for a sheet thickness of 1.4 mm, and the ductility ratio (CTS / TSS) is 0.25 or more, it is regarded as “excellent”. Those that were not satisfied were evaluated as “poor”. (5) Sliding properties Test specimens (tmm × width 10mm × length 300mm) from each cold-rolled annealed plate
Was collected and placed between a pair of flat tools (plane length 10 mm) as shown in FIG. Then, apply 1.5 g / m ² of commercially available rust-proof oil to one side of the test piece, and use a flat tool (contact length 10
mm) with a surface pressure of 1 kgf / mm ² (9.8 kN / mm ² ), and the pulling force when the test piece was pulled out at a friction speed of 20 mm / s was determined.
The coefficient of friction was calculated. The friction coefficient μ is μ = (pull-out force) /
Calculated from (2 × surface pressure). (6) Shape Freezing A test piece having a thickness of tmm, a width of 100 mm and a length of 300 mm was collected from each cold-rolled annealed sheet. These test pieces were press-formed with a hat bending die with a plate holder to form a hat-shaped cross-sectional shaped part (dotted line) having an opening length W (= 100 mm) shown in FIG.
After the press molding, after the mold was removed from the mold, the amount of change ΔW in the length of the opening was measured. The ratio of ΔW to the tensile strength of the steel sheet, ΔW / TS, was used as an index for shape freezing.

With respect to the slab, the surface condition was observed over the entire length of the slab, and the presence or absence of surface cracks was examined. In addition, II-6 according to JIS G 0567
Shaped test pieces were collected, subjected to high-temperature tension at a test temperature of 900 ° C., and the aperture value was determined. Table 3 shows the obtained results.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

In each of the examples of the present invention, the drawing value at high temperature is high, there is no surface crack of the slab, the elongation El of 30% or more,
Excellent strength-elongation balance (TS × El) of GPa% or more,
Further, it shows an excellent strength-hole expansion ratio balance (TS × λ) of 40 GPa% or more, and has excellent stretch flangeability. In addition, the examples of the present invention have excellent friction properties with a low friction coefficient of 0.12 or less, low ΔW / TS, low springback after press molding, excellent shape freezing properties, and further excellent spot weldability. have.

On the other hand, in Comparative Examples outside the scope of the present invention, one or both of the strength-elongation balance (TS × El) and the strength-hole expansion ratio balance (TS × λ) are reduced, and Has either or both of ductility and stretch flangeability reduced, and also has reduced slidability and shape freezing property.

[0049]

As described above, according to the present invention,
In addition to preventing horizontal and vertical slab cracks during continuous casting,
A high-strength cold-rolled steel sheet that has excellent ductility, excellent stretch flangeability, slidability, and shape freezing property, and is excellent in press formability and is really suitable as a molded product material represented by automotive parts. Can be manufactured stably at low cost, and it has a remarkable industrial effect.

Further, the steel sheet of the present invention has a good strength-elongation balance (TS × El) and a good strength-hole expansion ratio balance (TS × λ), and contributes to the weight reduction and low fuel consumption of automobiles for automobile parts. It can greatly contribute to the improvement of the global environment.

[Brief description of the drawings]

FIG. 1 is an explanatory view schematically showing a pressed shape of a test piece for evaluating shape freezing properties and a shape after being pressed and removed from a mold.

FIG. 2 is an explanatory view schematically showing a slidability test.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Eiji Iizuka 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. Chome (without address) Inside the Mizushima Works of Kawasaki Steel Corporation

Claims (5)

[Claims] 1. Mass%, C: 0.05 to 0.40%, Si: 0.5 to 3.0%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.005% or less, Al: less than 0.10%, N: 0.0100% or less, Ti: 0.20% or less, further contains one or two of Ca and REM in a total amount of 0.0010 to 0.0500%, and the composition consisting of the balance of Fe and unavoidable impurities, ferrite, residual austenite and It has a composite structure consisting of a low-temperature transformation phase, contains at least 30% by volume of the ferrite and 3% or more of the retained austenite on the average in the entire thickness direction, and has a volume fraction of 1/16 from the steel sheet surface to the sheet thickness 1/16 position. A high-strength, high-ductility cold-rolled steel sheet excellent in press formability, characterized in that the amount of retained austenite in the region is 50% or less of the amount of retained austenite in the central part in the thickness direction. 2. The high-strength and high-ductility cold-rolled steel sheet according to claim 1, wherein the composition satisfies Ti / N ≧ 2.5. 3. The composition according to claim 1, further comprising:
b, one or two of V are 0.001 to 0.30% in total
The high-strength and high-ductility cold-rolled steel sheet according to claim 1 or 2, wherein the cold-rolled steel sheet has excellent press formability. 4. In addition to the above composition, C
The high strength according to any one of claims 1 to 3, wherein one or two or more selected from among r: 1.5% or less, Mo: 0.5% or less, and B: 0.01% or less are contained. High ductility cold rolled steel sheet. 5. The ferrite having an average crystal grain size of 15 μm.
5. The high-strength and high-ductility cold-rolled steel sheet according to claim 1, wherein the thickness is not more than m.