JP3508657B2 - High strength cold rolled steel sheet excellent in ductility and stretch flangeability and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength cold-rolled steel sheet having excellent ductility and stretch flangeability, which is suitable for use mainly as a steel sheet for automobiles, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of conservation of the global environment,
There is a demand for improved fuel efficiency of automobiles. Further, in order to ensure the safety of passengers in the event of a collision, it is required to improve the safety of the vehicle body. Under such circumstances, the weight reduction of the automobile body and the reinforcement of the automobile body are being actively promoted. In order to achieve both weight reduction and strengthening of an automobile body at the same time, it is effective to increase the strength of component materials. Therefore, recently, high strength steel plates are actively used for automobile parts.

By the way, most of automobile parts made of steel sheet are formed by press working, and therefore excellent formability is required as a steel sheet for automobile parts. Here, in order to realize excellent moldability, it is essential to secure high ductility in the first place. Further, in press molding of automobile parts, stretch flange deformation is often used. In particular, in parts that constitute members such as skeleton members and reinforcements to secure the strength of the automobile body,
In many cases, parts are formed by making extensive use of stretch flange deformation. For this reason, it is strongly demanded for steel sheets for automobile parts to have both excellent ductility and stretch flangeability.

As a high-strength steel sheet having excellent ductility, a two-phase steel sheet having a composite structure of ferrite and martensite is typical. In recent years, the transformation-induced plasticity (Transformation Induced Plas-
High ductility steel sheets using sticity (TRIP) have reached the stage of practical application. However, such a structure-strengthened steel sheet has a problem that the local elongation is low because the hard martensite is the main strengthening factor, and therefore the stretch flangeability is poor.

On the other hand, as a method for producing a high-strength steel sheet excellent in both ductility and stretch-flangeability, Japanese Patent No. 2821481 proposes a method for producing a high-strength steel sheet mainly composed of tempered martensite containing retained austenite. There is. However, the ductility of the high-strength steel sheet obtained by this method cannot be said to be sufficient yet, and it has not been sufficiently satisfactory as a high-strength steel sheet widely used as a steel sheet for automobiles.

[0006]

SUMMARY OF THE INVENTION The present invention has been developed in view of the above situation, and provides a high-strength cold-rolled steel sheet having excellent ductility and stretch flangeability, which is suitable as a material for automobile parts. It aims at proposing it with various manufacturing methods.

[0007]

[Means for Solving the Problems] In order to achieve the above-mentioned object, the inventors have studied the influence of the composition and microstructure of the steel sheet on the ductility and stretch flangeability of the steel sheet.
We have earnestly studied. As a result, the structure of the high-strength cold-rolled steel sheet obtained after annealing is a ferrite, a composite structure consisting of retained austenite and a low temperature transformation phase, and by controlling the volume ratio of each phase to a predetermined ratio, excellent ductility is obtained. We have found that it can be expressed. It was also found that by making the crystal grain size of ferrite, which is the main constituent of the composite structure, finer, not only high ductility but also excellent stretch flangeability can be obtained. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. C: 0.05 to 0.2% in mass percentage, Si: 0.
5 to 3.0%, Mn: 0.6 to 3.0%, P: less than 0.015%,
S: 0.005% or less, Al: less than 0.10%, N: 0.005% or less, Ti: Ti ^* of 0.10% or less, where Ti ^* = Total Ti-
[(48/32) S + (48/14) N] and contains Ca and / or REM: 0.0010 to 0.0100%, the balance is Fe and inevitable impurities, and the steel structure is ferrite, It has a composite structure consisting of retained austenite and a low temperature transformation phase, contains 30% or more by volume of the above ferrite, contains 2% or more by volume of the above retained austenite, and further has an average crystal grain size of the ferrite of 15 μm or less, Ti as TiC /
A high-strength cold-rolled steel sheet excellent in ductility and stretch-flangeability, characterized by having a Ti ^* ratio of 0.05 or more.

2. The high-strength cold-rolled steel sheet having excellent ductility and stretch-flangeability, characterized in that the steel material according to the above-mentioned item 1 further has a composition containing Nb and / or V: 0.001 to 0.10% by mass.

3. In 1 or 2 above, the steel material further has a composition containing one or two or more selected from a mass percentage of Cr: 1.5% or less, Mo: 0.5% or less and B: 0.01% or less. High strength cold rolled steel sheet with excellent ductility and stretch flangeability.

4. C: 0.05 to 0.2% in mass percentage, Si:
0.5-3.0%, Mn: 0.6-3.0%, P: less than 0.015%,
S: 0.005% or less, Al: less than 0.10%, N: 0.005% or less, Ti: Ti ^* of 0.10% or less, where Ti ^* = Total Ti-
A steel slab containing [(48/32) S + (48/14) N] and having a composition containing Ca and / or REM: 0.0010 to 0.0100% is hot-rolled according to a conventional method, and further cold-rolled. After rolling and heating to a temperature above the Ac ₁ transformation point, 10
High-strength cooling with excellent ductility and stretch flangeability, characterized by cooling to a temperature range of 300 to 500 ° C at a cooling rate of ℃ / s or more, holding this temperature range for 60 seconds or more, and then cooling. Manufacturing method of rolled steel sheet.

In the present invention, the target characteristic values for ductility and stretch-flangeability are total elongation: El ≧
35%, hole expansion ratio: λ ≧ 90%.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. First, the reason why the composition of the steel sheet is limited to the above range in the present invention will be described. In this specification,% used to indicate the content of each component is a mass percentage. C: 0.05 to 0.20% C is an element which is not only indispensable for increasing the strength of steel, but is also effective for generating retained austenite and a low temperature transformation phase. However, if the content is less than 0.05%, the desired high strength cannot be achieved, while if it exceeds 0.20%, the weldability deteriorates.
C was limited to the range of 0.05 to 0.20%.

Si: 0.5-3.0% Si has the functions of strengthening the steel by solid solution strengthening, stabilizing austenite, and promoting the formation of a retained austenite phase. This kind of action has a Si content of 0.5%.
The above is recognized. However, if the content of Si exceeds 3.0%, the chemical conversion treatability deteriorates significantly, so Si is 0.5 to 3.
It was limited to the range of 0%.

Mn: 0.6-3.0% Mn has the effects of strengthening the steel by solid solution strengthening, improving the hardenability of the steel, and promoting the formation of retained austenite and the low temperature transformation phase. This action has the effect that the Mn content is
Although it is recognized at 0.6% or more, if it exceeds 3.0%, the effect reaches saturation and rather the cost rises.
It was limited to the range of 0.6 to 3.0%.

P: less than 0.015% P is a solid solution strengthening element and is usually an element effective for obtaining a high strength steel sheet, but if the content is 0.015% or more, the spot weldability deteriorates. In this invention 0.015
Limited to less than%. The lower P is, the better.

S: 0.005% or less S is an impurity element that forms MnS in steel and reduces the stretch flangeability of the steel sheet. Therefore, it is desirable to reduce the content of S as much as possible, but 0.005% or less is acceptable. It is more preferably 0.003% or less.

Al: less than 0.10% Al is an element added as a deoxidizing agent in the steelmaking stage,
It is effective for fixing N as AlN. However, if the content is 0.10% or more, the alloy cost increases, so in the present invention, Al is limited to less than 0.10%.

N: 0.005% or less N is an element that usually induces strain aging and deteriorates characteristics such as El, so it is necessary to reduce it as much as possible. In the present invention, N is changed to TiN, by adding Ti or Al.
Since it is fixed as AlN, there is no risk of aging deterioration, but if N is contained in a large amount exceeding 0.005%, a large amount of Ti and Al must be added to fix N. Was set to 0.005%.

Ti: Ti ^* is 0.10% or less where Ti ^* = To
tal Ti − [(48/32) S + (48/14) N] Ti precipitates TiC and effectively suppresses the growth of the ferrite phase during the heating stage during annealing, making the ferrite grains finer and forming holes. It is an element useful for significantly improving spreadability. Ti also has the effect of preventing key crack defects in the slab. In order to bring out these effects, it is necessary to add Ti in an amount of an effective Ti amount (Ti ^* = Total Ti-[(48/32) S + (48/14) N] exceeding 0. When the Ti amount exceeds 0.10%, YS increases due to precipitation strengthening, which not only causes deterioration of workability but also decreases retained austenite for exhibiting the TRIP effect, which is not preferable.
Therefore, in this invention, the addition amount of Ti is the effective Ti amount (Ti ^* )
Therefore, the range is 0 <Ti ^* ≦ 0.10%. Here, the TRIP effect is a phenomenon in which retained austenite becomes martensite by plastic working such as press working, and that part becomes hard and the deformation stops, but the deformation of a softer neighboring part progresses. It is a phenomenon that continues until the whole is deformed.

Ca and / or REM: 0.0010 to 0.0100% Ca and REM both have the function of controlling the morphology of sulfide inclusions, which has the effect of improving the stretch flangeability of the steel sheet. The effect is such that the content is 0.0010%
It appears above and becomes saturated when it reaches 0.01%. Therefore, Ca,
The content of REM was limited to the range of 0.0010 to 0.0100% in both cases of single addition and complex addition. A more preferable range is 0.0010 to 0.0050%.

Although the essential components have been described above, the following elements can be appropriately contained in the present invention, if necessary. Nb and / or V: 0.001 to 0.10% Nb and V both precipitate NbC and VC, like Ti, suppress the growth of ferrite phase in the heating stage during annealing, refine ferrite grains, and It is an element effective for significantly improving the expandability. And, in order to realize this effect, it is necessary to add at least 0.001%.
However, if the content exceeds 0.10%, YS increases due to precipitation strengthening, not only the workability is deteriorated, but also retained austenite for expressing the TRIP effect is decreased, which is not preferable. Therefore, in the present invention, Nb, V
The content of is limited to the range of 0.001 to 0.10% in either case of single addition or complex addition.

Cr: 1.5% or less, Mo: 0.5% or less, B: 0.
01% or less Cr, Mo, and B are all useful elements that improve the hardenability of steel and promote the formation of low-temperature transformation phase. However, if the content exceeds the above range, the effect of addition reaches saturation. , The effect corresponding to the content cannot be expected, which is economically disadvantageous. Therefore, Cr, Mo and B should be contained in the above ranges. A more preferable range of addition is Cr, Mo,
One or more selected from B, 0.0005 in total
It is 1.0%.

Next, in the present invention, the reason why the steel structure is a composite structure consisting of ferrite, retained austenite and low temperature transformation phase having the above ratio will be explained.
The tempered martensite in the present invention refers to a phase formed when lath martensite is heated. Ferrite: 30% or more Ferrite is a soft phase, has a high deformability, and has the effect of improving the ductility of the steel sheet. If the ratio of the above-mentioned ferrite is less than 30%, the remarkable effect of improving the ductility cannot be expected. Therefore, in the present invention, such ferrite is contained in a volume ratio of 30% or more.

Retained austenite: 2% or more Retained austenite undergoes strain-induced transformation into martensite during processing and widely disperses locally applied processing strain,
It has the effect of improving the ductility of the steel sheet. If the ratio of such retained austenite is less than 2%, the above-mentioned remarkable improvement in ductility cannot be expected. Therefore, in the present invention,
Such a retained austenite is contained in a volume ratio of 2% or more. Incidentally, it is preferably 5% or more,
The higher the amount of retained austenite, the better, but in practice
It is 10% or less.

Low Temperature Transformation Phase The low temperature transformation phase in the present invention refers to martensite or bainite that has not been tempered. Both martensite and bainite are hard phases and have the effect of increasing the strength of the steel sheet by strengthening the structure. In addition, since a mobile dislocation is generated when the transformation is generated, it also has an action of lowering the yield ratio of the steel sheet. Here, in order to sufficiently obtain the above-mentioned action, it is preferable that the low temperature transformation phase is martensite. In the present invention, the amount of the low temperature transformation phase is not particularly limited and may be appropriately distributed according to the strength of the steel sheet. The volume ratio is preferably 5 to 20%.

Further, in the present invention, it is important to make the crystal grain size of ferrite fine and to deposit an appropriate amount of TiC, as shown below. Ferrite crystal grain size: 15 μm or less Fine graining of ferrite crystal grains is useful for improving stretch flangeability of the steel sheet. If the average crystal grain size of ferrite exceeds 15 μm, a remarkable effect of improving stretch flangeability cannot be expected. Therefore, in the present invention, the average crystal grain size of ferrite in the composite structure is limited to 15 μm or less.

Precipitating an appropriate amount of Ti as TiC / Ti ^* ≧ 0.05 TiC is extremely useful for refining ferrite grains. Here, Ti as TiC / Ti
^Since the above effect cannot be sufficiently exhibited with a precipitation amount of which the value indicated by ^* ratio is less than 0.05, the TiC precipitation amount in the present invention is 0.05 in terms of Ti as TiC / Ti ^* ratio.
The above range is set.

In the composite structure of the steel sheet of the present invention, the ferrite is the main phase, and the balance is the retained austenite and the low temperature transformation phase. Such retained austenite and the low temperature transformation phase are dispersed and present between the grains of the main phase ferrite. Therefore, the average grain size of the retained austenite and the low temperature transformation phase is smaller than the average grain size of the ferrite which is the main phase, and is not particularly limited in the present invention.

Next, the manufacturing conditions of the present invention will be described. First, a steel having the above composition is melted, cast into a slab by a usual method, and then hot-rolled and further cold-rolled according to a conventional method to obtain a steel sheet. Also,
If necessary, steps such as pickling or annealing can be added. Then, in the annealing step, preferably in a continuous annealing line, the above steel sheet is heat-treated, then rapidly cooled, and held or gradually cooled during the cooling to generate an appropriate amount of retained austenite and a low temperature transformation phase. Less than,
Specific processing conditions of each step will be described.

Heating temperature: Ac ₁ transformation point or higher This heating treatment is preferably carried out by continuous annealing, in which case the heating temperature needs to be the Ac ₁ transformation point or higher. This is because retained austenite cannot be obtained without heating to the (α + γ) 2 phase region, and the TRIP effect will not be exhibited. However, if the heating temperature exceeds 850 ° C, the ferrite grain size becomes large and the stretch flangeability deteriorates. Therefore, it is preferable to set the heating upper limit to about 850 ° C.

Cooling rate: 10 ° C./s or more After the above heat treatment, a rapid cooling process is performed. The cooling rate in the rapid cooling process must be 10 ° C./s or more. The reason is that at a cooling rate of less than 10 ° C / s, the γ phase transforms into pearlite or bainite, the residual austenite disappears, and the TRIP effect cannot be obtained. More preferably, it is 20 ° C / s or more.

Quenching end temperature: 300 to 500 ° C. The reason for setting the quenching ending temperature to 300 to 500 ° C. is that when the temperature is rapidly cooled to a temperature below 300 ° C., all γ phases are transformed into martensite, while temperatures above 500 ° C. Then, most of the γ phase is transformed into pearlite or bainite, and TR
This is because the IP effect cannot be expected. A more preferable temperature range is 370 to 450 ° C.

Holding time: 60 seconds or more The time required for holding or gradual cooling after quenching is set to 60 seconds or more, because most of retained austenite is generated when a temperature range of less than 300 ° C. is reached in a short time of less than 60 seconds. This is because it transforms into martensite and the TRIP effect does not appear during press working. However, if the holding time exceeds 900 seconds, the nose of bainite transformation is generated and the retained austenite decreases, which is not preferable. More preferably
It is about 120 to 600 seconds.

After the above-mentioned series of treatments, with or without temper rolling for the purpose of straightening the shape and adjusting the roughness, the present invention does not cause a large change in the mechanical properties. No particular restrictions are placed on this point.

[0036]

EXAMPLE Steels having various compositional compositions shown in Table 1 were melted in a converter and made into slabs by a continuous casting method. The obtained steel slab was hot-rolled to a plate thickness of 3.0 mm, then pickled and cold-rolled to obtain a cold-rolled plate having a plate thickness of 1.4 mm. Then, these cold-rolled sheets were subjected to a heat treatment in a continuous annealing line under the conditions shown in Table 2, followed by a rapid cooling treatment and a holding / slow cooling treatment in the middle thereof, followed by a rolling reduction of 0.5.
% Temper rolling was performed to obtain a product plate. Table 3 shows the results of the microstructure investigation and the mechanical property investigation of the product plate thus obtained.

The microstructure of the steel sheet was investigated by observing the section in the rolling direction with an optical microscope or a scanning electron microscope. In addition, using a cross-sectional micrograph with a magnification of 1000 times, the occupied area ratio of the relevant phase existing in a 100 mm square square castle arbitrarily set by image analysis was determined and defined as the volume ratio of the relevant phase. Further, the amount of retained austenite was obtained by polishing the steel plate to the center plane in the plate thickness direction and measuring the diffraction X-ray intensity at the plate thickness center plane. MoK for incident X-rays
Using (α) rays, the retained austenite phase of {11
1}, {200}, {220}, and {311} diffracted X-ray intensity ratios of the respective surfaces were obtained, and the average value thereof was taken as the volume ratio of the retained austenite. The ferrite grain size was converted into the average grain size by measuring the grain size according to the method specified in JIS Z 0552. The mechanical properties of the steel sheet were measured by a tensile test and a hole expansion test. Tensile properties are measured according to JIS Z 2204 using JIS No. 5 test piece taken from steel plate in the direction perpendicular to the rolling direction.
The yield strength (YS), tensile strength (TS), and elongation at break (El) were measured by the methods specified in Z 2241. For stretch flangeability, the hole expansion ratio (λ) was measured by the method specified in JFS T 1001. For spot weldability, a tensile shear test prescribed in JIS Z 3136 was conducted, and the standard tensile shear load of 11062 N or more, which is the standard tensile shear load when the plate thickness is 1.4 mm, is "excellent" and less than that is "inferior." Was evaluated as

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

As shown in Table 3, each of the conforming examples obtained according to the present invention has a tensile strength (T.
S.) and strength-elongation balance (TS x El) of 20
It had excellent ductility of 000 MPa ·% or more, and strength-hole expansion ratio balance (TS × λ) of 54000 MPa ·% or more, and excellent stretch flangeability. On the other hand, none of the comparative examples in which the component composition and the steel structure deviate from the scope of the present invention show high values for both strength-elongation balance and strength-hole expansion ratio balance, and ductility and stretch-flangeability No two were better at the same time.

[0042]

As described above, according to the present invention, a high-strength cold-rolled steel sheet having extremely excellent ductility and stretch-flangeability can be obtained inexpensively and stably, and is industrially very useful. Further, by applying the steel sheet manufactured according to the present invention, it becomes possible to reduce the weight and fuel consumption of the automobile, and thus contribute greatly to the improvement of the global environment.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryusuke Yamaguchi 1-chome, Mizushima Kawasaki-dori, Kurashiki City, Okayama Prefecture (no street number) Kawasaki Steel Co., Ltd. Mizushima Steel Works (58) Fields investigated (Int.Cl. ⁷ , DB name) ) C22C 38/00-38/60 C21D 9/46

Claims (4)

(57) [Claims] 1. A mass percentage of C: 0.05 to 0.2%, Si: 0.5 to 3.0%, Mn: 0.6 to 3.0%, P: less than 0.015%, S: 0.005% or less, Al: less than 0.10%, N: 0.005. % Or less, Ti: Ti ^* is 0.10% or less, provided that Ti ^* = Total Ti − [(48/32) S + (48/14) N] is included and Ca and / or REM: 0.0010 to 0.0100% is included. , The balance is composed of Fe and unavoidable impurities, the steel structure has a composite structure consisting of ferrite, retained austenite and a low temperature transformation phase, and the above ferrite has a volume ratio of 30% or more, and the above retained austenite has a volume ratio. 2% or more, the average crystal grain size of the ferrite is 15 μm or less, and the Ti as TiC / Ti ^* ratio is 0.05 or more. A high-strength cold-rolled steel sheet excellent in ductility and stretch flangeability. 2. The high strength excellent in ductility and stretch flangeability according to claim 1, characterized in that the steel material further has a composition containing Nb and / or V: 0.001 to 0.10% in mass percentage. Cold rolled steel sheet. 3. The steel material according to claim 1 or 2, further comprising one or more selected from a mass percentage of Cr: 1.5% or less, Mo: 0.5% or less and B: 0.01% or less. A high-strength cold-rolled steel sheet having excellent ductility and stretch-flangeability. 4. A mass percentage of C: 0.05 to 0.2%, Si: 0.5 to 3.0%, Mn: 0.6 to 3.0%, P: less than 0.015%, S: 0.005% or less, Al: less than 0.10%, N: 0.005. % Or less, Ti: Ti ^* is 0.10% or less, provided that Ti ^* = Total Ti − [(48/32) S + (48/14) N] is included and Ca and / or REM: 0.0010 to 0.0100% is included. The steel slab having the composition is hot-rolled and further cold-rolled according to a conventional method, and then heated to a temperature of Ac ₁ transformation point or higher, and then cooled at a cooling rate of 10 ° C / s or higher to 300 ° C.
A method for producing a high-strength cold-rolled steel sheet excellent in ductility and stretch-flangeability, which comprises cooling to a temperature range of up to 500 ° C, maintaining this temperature range for 60 seconds or more, and further cooling.