JP2002080935A - High strength thin steel sheet having excellent workability and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength thin steel sheet having excellent workability (a balance of strength-stretch flanging properties and a balance strength-ductility) and impact resistance and to provide its production method. SOLUTION: Steel having a composition containing, by weight, 0.04 to 0.1% C, <=0.5% Si, 0.5 to 2% Mn, <=0.05 P, <=0.005% O and <=0.005% S and, if required, containing one or more kinds selected from Ti, Nb, V, Mo and Cr by 0.01 to O.3% in total is subjected to continuous casting into a slab, is subjected to hot rolling in which rolling finishing temperature is controlled to Ar3 or higher, is immediately cooled to 600 to 750 deg.C at a cooling rate of 100 to 2,000 deg.C/s, is subsequently cooled to 450 to 650 deg.C and is coiled to form a bandlike second phase structure in which the average ferrite grain size is <=10 μm, and the forming frequency (A) is <=20 mm/mm2; wherein, the forming frequency (A): the total length of the bandlike second phase structure observed per mm in the cross section of the sheet thickness in the rolling direction of the thin steel sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength thin steel sheet and a method for producing the same, and more particularly to a sheet having excellent strength-stretch flangeability balance, strength-ductility balance, and impact resistance, and a method for producing the same.

[0002]

2. Description of the Related Art Since hot-rolled steel sheets and cold-rolled steel sheets are mainly used as members formed by press working in the fields of automobiles, home electric appliances and industrial machines, various workability is required according to the shape of the members. Is required.

[0003] In particular, in recent years, automobile manufacturers and the like have a strong need for weight reduction, and the use ratio of high-tensile materials is high. However, high-tensile materials are inferior in workability to 270 MPa-grade soft materials, and therefore have high pressability. Cracking during processing and variations in quality are often problems, and improvement of workability is required as a basic quality.

For example, high tensile strength materials of 340 MPa class or higher are required to have high stretch flangeability at the time of burring. For collision safety, which has become particularly important in automotive applications in recent years, there is a problem. It has been required to have high impact absorption energy and excellent impact resistance.

As a prior art relating to improvement of workability of high-tensile steel, Japanese Patent No. 25555536, Japanese Patent Publication No. 7-56053
And JP-A-4-88125. Japanese Patent No. 25555536 discloses that a Ti-based precipitation strengthened steel is cooled at a cooling rate of 30 to 150 ° C./s and a winding temperature of 250 after finish rolling.
~ 540 ℃ to make ferrite + bainite structure,
The purpose is to improve the stretch flangeability of a 50-60K class high tensile material.

Japanese Patent Publication No. 7-56053 discloses that a cooling rate after hot-rolling a ferrite + pearlite steel (45-50K class high-tensile steel) is 10 ° C./s or more (in the example, 95%).
(° C / s is the highest) to improve the stretch flangeability of the hot-dip galvanized steel sheet.

JP-A-4-88125 discloses that 0.0005 to 0.0050 of Ca is added to ferrite + pearlite steel.
%, And after finishing hot rolling at a high temperature of Ar3 + 60 to 950 ° C., a cooling rate of 50 ° C./s or more within 3 seconds, provided that
Preferably cooled at 150 ° C / s or less, stopped at 410 to 620 ° C depending on the steel composition-air-cooled, and then wound at 350 to 500 ° C, for the purpose of improving the stretch flangeability of 50 to 70K class high-tensile steel. And

[0008]

However, the stretch flangeability obtained by the invention steel of Japanese Patent No. 25555536 does not greatly exceed the level of the conventional steel, and the winding temperature is as low as 250 to 540 ° C. Therefore, elongation at break was low, and there was also a problem with the shape of the coil.

In the case of Japanese Patent Publication No. 7-56053, the cooling rate after hot-rolling finish is at most 95 ° C./s, and no substantial improvement in stretch flangeability has been obtained. JP-A-4-881
In the case of Japanese Patent No. 25, since a small amount of Ca is added, an RH degassing step is indispensable in the steel making stage, and the steel making cost increases.

[0010] Further, the stretch flangeability is not remarkably improved by the cooling after the hot rolling, which is a characteristic feature, and the winding temperature is as low as 350 to 500 ° C.
The breaking elongation was low, and there was also a problem with the shape of the coil.

Further, none of these prior arts considers the impact resistance.

The present invention has been made in view of the above points, and an object of the present invention is to provide a high-strength thin steel sheet excellent in stretch flangeability, workability of elongation at break, and impact resistance, and a method for producing the same. Aim.

The high-strength thin steel sheet of the present invention includes a galvanized steel sheet (a hot-dip galvanized sheet, a galvannealed steel sheet, an electroplated steel sheet and a steel sheet further subjected to a surface treatment).

[0014]

The present inventors have studied in detail the relationship between the steel structure, the stretch flangeability, and the elongation at break, and in order to drastically improve these properties,
It is effective to increase the yield strength of the material within a range that does not impair the workability in order to eliminate the band structure existing in the entire thickness direction due to the concentration of C, Mn, etc. and to improve the impact resistance. I found something.

The present invention has been further studied based on these findings, that is, the present invention provides: Characterized by having the following structure,
C: 0.04 to 0.1%, Si: 0.5% or less, Mn:
0.5-2%, P: 0.05% or less, O: 0.005%
Hereinafter, S: a high-strength thin steel sheet containing 0.005% or less and having excellent workability.

(1) Average ferrite grain size: 10 μm or less (2) Band-shaped second phase structure with a formation frequency (A) of 20 mm / mm ² or less, where formation frequency (A): rolling direction of thin steel sheet-sheet 1. Total length of band-like second phase structure observed per 1 mm ^{2 in} thick section. As steel components, Ti, Nb, V, Mo, Cr
2. A high-strength thin steel sheet excellent in workability according to 1, wherein one or more kinds of the above are contained in a total of 0.01 to 0.3%.

3. A method for producing a high-strength thin steel sheet having excellent workability, comprising the following steps.

(1) A step in which a steel having the composition described in 1 or 2 is formed into a slab by continuous casting, and hot rolling is performed at a rolling end temperature of Ar3 or higher, directly or through a reheating step.

(2) Immediately after hot rolling, 100 to 20
600-750 within 2 seconds at a cooling rate of 00 ° C / s
Cooling to ℃.

(3) Then, a step of cooling to 450 to 650 ° C. and winding in the temperature range.

4. 4. The method for producing a high-strength thin steel sheet excellent in workability according to 3, further comprising the following step (1) or (2).

(1) Step of annealing after pickling.

(2) Step of annealing after pickling and cold rolling.

5. 5. The method for producing a high-strength thin steel sheet excellent in workability according to 3 or 4, wherein a segregation reduction treatment is performed during continuous casting.

6. After hot rolling, 100-2000 ° C /
6. The method for producing a high-strength thin steel sheet according to any one of 3 to 5, wherein a temperature fluctuation after cooling for sec is within 60 ° C. in a width direction and a longitudinal direction of the hot-rolled steel strip.

7. 7. The method for producing a high-strength thin steel sheet according to 6, wherein the hot-rolled steel strip is cooled at a heat transfer coefficient of 2000 Kcal / m ² hr ° C. or higher.

8. Fluctuations in tensile strength in the width and longitudinal directions of the steel strip are ±± of the average value of tensile strength in the coil.
3. The high-strength steel sheet according to 1 or 2, wherein the content is within 8%.

9. A high-strength thin steel sheet manufactured by the manufacturing method according to 6, wherein a variation in tensile strength in a width direction and a longitudinal direction is within ± 8% of an average value of tensile strength in the coil.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the composition of steel, the steel structure, and the manufacturing conditions are specified.

1. Component composition C C is added to secure the strength of the steel sheet. 0.04%
If less than 340 MPa strength is not obtained,
%, The workability deteriorates.
% (0.04% or more, 0.1% or less).

Si Si is a solid solution strengthening element and is added to secure strength. If it exceeds 0.5%, the surface properties are deteriorated.
5% or less.

Mn Mn is added to improve the toughness of the steel sheet and strengthen the solid solution. If it is less than 0.5%, these effects cannot be obtained, and if it exceeds 2%, the workability is significantly deteriorated.
Is added.

PP strengthens the solid solution of the steel sheet. However, if it exceeds 0.05%, the workability is deteriorated due to segregation.

The content of O 2 O is specified in order to suppress the occurrence of cracks on the slab surface or slab surface layer during continuous casting. 0.
If it exceeds 005%, slab cracks are remarkable and workability deteriorates.

If S exceeds 0.005%, the amount of sulfide increases and the workability deteriorates. Therefore, the content of S is set to 0.005% or less. If the strength-stretch flangeability balance is particularly good, 0.0
It is preferably at most 03%.

The steel of the present invention contains the above elements as the basic component composition, and when the strength needs to be adjusted, Ti, Nb,
One, two or more of V, Mo, Zr, and Cr are contained as a selective element.

One or more of Ti, Nb, V, Mo, Zr, and Cr in a total amount of 0.01 to 0.3% These elements form a carbonitride and improve the strength.
If the total amount of one type or two or more types is less than 0.01%, the effect cannot be obtained, and if it exceeds 0.3%, the stretch flangeability is deteriorated.

In the present invention, other elements are not particularly defined. For example, Cu: 2% or less, Sn: 0.01
% Or less.

2. Steel structure Average ferrite grain size: 10 μm or less Ferrite grains ensure good strength-ductility balance in hot-rolled material, hot-rolled-alloyed hot-dip galvanized material, hot-rolled-cold rolled-alloyed hot-dip galvanized material. Therefore, in order to finely disperse the second phase structure (carbite, pearlite, bainite, martensite, austenite, etc.) as fine as possible, the average particle diameter is set to 10 μm or less.

Band-shaped second phase structure having a generation frequency (A) of 20 mm / mm ² or less When the second phase structure is formed in a band shape, it has a great effect on strength-stretch flangeability. Generation frequency (A) is 2
It exceeds 0 mm / mm ^2, the strength over stretch flangeability balance to deteriorate greatly, and 20 mm / mm ² or less.

FIG. 1 shows the strength-stretch flangeability balance (T
S × λ) and the effect of the ferrite grain size and the second phase structure on the strength-ductility balance (TS × El), wherein the average ferrite particle size is 10 μm or less and the frequency of formation of a band-like second phase structure Is not more than 20 mm / mm ² , excellent strength-stretch flangeability balance (TS × λ) and strength-ductility balance (TS × El) are obtained.

In the present invention, the smaller the band-like structure, the better, and the generation frequency is 0, that is, steel having a structure in which the average ferrite grain size is 10 μm or less and no band-like structure is observed is also included in the scope of the present invention. .

3. Manufacturing Conditions Preferred manufacturing conditions in the present invention include the following steps. A slab is formed by continuous casting, and a step of performing hot rolling at a rolling end temperature of Ar3 or higher through a direct or reheating step. In the present invention, the slab is manufactured by continuous casting to make the components uniform, and the temperature uniformity of the slab is obtained. Hot rolling is performed directly or through a reheating process in order to secure the mechanical properties in the width direction of the coil. When reheating the slab,
It is preferable to heat to 1250 ° C. or lower without cooling to room temperature.

In order to more effectively suppress the formation of a band-like structure, a segregation reduction treatment is performed during continuous casting, and M
Segregation during casting of elements such as n and C is suppressed. As the segregation reduction treatment, it is preferable to carry out electromagnetic stirring, light pressure casting, increase of the cooling rate of a slab or the like singly or in combination.

In the hot rolling, the finish rolling finish temperature is set to Ar3 or more in order to reduce the ferrite crystal grain size after transformation and the pearlite.

Immediately after hot rolling, 100 to 2000 ° C.
Step of Cooling to 600 to 750 ° C. within 2 Seconds at a Cooling Rate of / s After hot rolling, primary cooling to 600 to 750 ° C. reduces the ferrite crystal grain size after transformation and the second phase structure. In order to improve the stretch flangeability by suppressing the band structure existing in the central part of the sheet thickness, the cooling rate is set to 100 to 2
000 ° C./s.

The reason why the band-like structure is suppressed by the rapid cooling is that the band-like second phase structure corresponding to the C, Mn concentrated portion in the solidification stage promotes ferrite transformation at a cooling rate of 100 ° C./s or more. It is considered that the elements are homogenized.

When the cooling rate is less than 100 ° C./s, the band portion undergoes austenite-ferrite transformation at a low temperature and generates a large amount of pearlite. The cooling rate is better as soon as possible, and is set to 2000 ° C./s, which is the upper limit that can be realized industrially at the present time.

The cooling rate is preferably at least 200 ° C./s from the viewpoint of ferrite crystal grain size and pearlite miniaturization, and 400 ° C./s to further improve workability.
It is preferable to make the above.

If the cooling stop temperature exceeds 750 ° C., the ferrite is not refined and the dispersion of the second phase becomes non-uniform.
λ balance (TS × λ) decreases. On the other hand, when the temperature is lower than 600 ° C., the second phase becomes a hard low-temperature transformation phase, and the TS-El balance (TS × El) decreases.

FIG. 2 shows the strength-stretch flangeability balance (T
S × λ) and the effect of the primary cooling stop temperature on the strength-ductility balance (TS × El). Cooling stop temperature is 600
In the case of ~ 750 ° C, good workability is obtained.

The primary cooling is started within 2 seconds after the finish rolling, in order to refine the ferrite crystal grain size after transformation and the second phase structure. However, from the viewpoint of suppressing the band-shaped second phase structure, the time is preferably longer than 0.5 seconds in order to reduce the material fluctuation by homogenizing the austenite structure before transformation.

In the present invention, when the primary cooling end temperature is made more uniform, an induction heating device is provided on the entrance side of the continuous hot finishing rolling mill or between the stands of the continuous hot finishing rolling mill, and the width direction of the coarse bar is set. Heating the edge and adjusting the temperature can be done without any problem.

In the continuous hot rolling process using a coil box, the heating of the rough rolling bar is performed in addition to the above.
It can take place before or after the coil box, during or after rough rolling, or even before and after the welding machine behind the coil box.

After cooling to 450 to 650 ° C., winding in the above temperature range The secondary cooling is performed at a cooling rate of 50 ° C./sec to appropriately transform the austenite structure into the second phase structure and obtain good workability. It is preferred to be less than s. If the winding temperature exceeds 650 ° C., coarse pearlite, which is harmful to ductility, is formed, and the ductility is significantly reduced.

On the other hand, when the temperature is lower than 450 ° C., a structure mainly composed of a low-temperature transformation phase is formed, and the workability of a hot-rolled material is deteriorated.

In order to make the mechanical properties more uniform, it is preferable that the difference in the winding temperature in the coil be within 50 ° C.

As described above, in the present invention, by reducing the temperature fluctuation in the coil, the fluctuation (the maximum value and the minimum value) of the tensile strength in the width direction and the longitudinal direction of the hot-rolled steel strip can be reduced. A thin steel plate characterized by being within ± 8% of the average value of the tensile strength in the coil can be obtained.

Such a steel sheet having a small material variation (variation in material) has a small variation in a press-workable coil such as a springback at the time of bending. The yield and shape accuracy can be improved, and the performance as a material is excellent.

In order to reduce the material variation of the hot-rolled steel strip to a more preferable level, the above-mentioned quenching stop temperature is set within the range of the present invention, and the temperature in the coil width direction and the longitudinal direction after quenching is reduced. It is desirable that the fluctuation range (maximum value-minimum value) be within 60 ° C.

More preferably, the variation in tensile strength is ± 4.
%, The performance at the customer can be significantly improved. In this case, by setting the temperature fluctuation width of the coil after the rapid cooling within 40 ° C., the fluctuation of the material can be narrowed in this way. Furthermore, the variation in tensile strength is ± 2
%, The temperature fluctuation range of the coil after the rapid cooling is set to 20 ° C. or less.

As a quenching capacity, a heat transfer coefficient of 2000 Kc
By performing the cooling at al / m ² hr ° C., the fluctuation in the temperature after the rapid cooling can be reduced. To reduce temperature fluctuations, the preferred heat transfer coefficient is 5000 Kcal
/ M ² hr ° C. or higher, more preferably 8000 Kcal
/ M ² hr ° C or more.

In the present invention, the temperature in the coil width direction is set to a temperature excluding 30 mm from both edges of the coil width in consideration of the measuring method of the sensor. The fluctuation range of the temperature is determined by the maximum value minus the minimum value.

The tensile characteristics were investigated using samples taken from positions excluding 30 mm from both edges in the coil width direction and 5 m from both ends in the coil longitudinal direction, and the average value of all values was calculated as the average value in the coil. And

The steel of the present invention is subjected to pickling-annealing, or pickling-cold rolling-annealing after hot rolling, to pickling material, hot-dip galvanized material, cold-rolled material, It can be a hot-dip galvanized material or the like for a base material. When the workability is further improved, the annealing temperature is preferably 650 to 850 ° C.

[0066]

[Example 1] A steel having a chemical composition shown in Table 1 was melted, and then a hot-rolled sheet (sheet thickness 2.3 mm) N was prepared under the conditions shown in Table 2.
o. 1 to 6. Material No. The hot-rolled sheet of No. 3 was pickled, cold-rolled, and then subjected to hot-dip galvanizing. The hot-rolled sheet of No. 4 was subjected to hot-dip galvanizing after pickling, and then examined for mechanical properties. The evaluation of the stretch flangeability was based on the hole expansion ratio (λ). The heat transfer coefficient in the primary cooling in the example of the present invention is 35.
00 to 4000 Kcal / m ² hr ° C.

Table 2 shows the results of the evaluation test. Material No. as an example of the present invention. Nos. 1 to 4 and 6 have excellent workability (strength-stretch flangeability balance, strength-ductility balance), higher yield strength than the comparative material, and excellent impact resistance. In particular, N
o. Nos. 1 to 4 are cases where the segregation reduction treatment was performed, and No. 6 is superior in hole expansion rate.

On the other hand, the material No. 5 indicates that the primary cooling rate is 3
At 0 ° C./s, it is out of the scope of the invention according to any one of claims 3 to 5, and both the workability and the impact resistance are inferior. FIG. 1, 2, and 5 microstructures are shown. Material No. of the present invention In Nos. 1 and 2, no band-like structure was observed.

[0069]

[Table 1]

[0070]

[Table 2]

[0071]

[Table 3]

Example 2 After smelting steel having the chemical components shown in Table 3, a hot-rolled sheet (sheet thickness 2.8 mm) was manufactured under the conditions shown in Table 4, annealed at 800 ° C., and alloyed. Hot-dip galvanizing was performed and the mechanical properties were examined. Table 4 shows the results together with the manufacturing conditions.
Shown in

Material No. Nos. 7 and 8 are examples of the present invention satisfying the chemical components and production conditions within the range of the present invention.
No. 9 is a comparative example in which the primary cooling rate is out of the range of the present invention. Material No. 7 and 8 are comparative material Nos. Compared with No. 9, the workability (strength-stretch flangeability balance, strength-ductility balance) is superior, the yield strength is higher than that of the comparative material, and the impact resistance is clearly superior.

[0074]

[Table 4]

[0075]

[Table 5]

Example 3 After smelting steel having the chemical composition shown in Table 3, a hot-rolled sheet (sheet thickness 2.8 mm) was manufactured under the conditions shown in Table 5, and annealed at 800 ° C., followed by alloying. Hot-dip galvanizing was performed, and changes in mechanical properties in the width direction and the longitudinal direction of the coils were examined.

Table 5 shows the results together with the production conditions. The heat transfer coefficient in the primary cooling in the example of the present invention is 12000 Kcal.
/ M ² hr ° C. The heat transfer coefficient in the comparative example is 1000K
cal / m ² hr ° C.

In the example of the present invention, the fluctuation of the primary cooling stop temperature in the coil is smaller than that of the comparative example by the conventional laminar cooling, and the fluctuation of the mechanical properties is reduced to a more preferable level.

Although not shown in the table, the average value of TS of Example No. 10 of the present invention is 604 MPa, and the average value of El is 3
4%, the average TS value of Comparative Example No. 11 was 625 MPa,
The average El value was 30%.

[0080]

[Table 6]

[0081]

According to the present invention, a ferrite having an average crystal grain size of not more than 10 μm and a structure in which a band-like structure in which C, Mn, etc. are concentrated is eliminated, and stretch flangeability and breaking elongation are processed. High-strength thin steel sheet excellent in heat resistance and impact resistance and its manufacturing method can be obtained.Furthermore, rapid cooling can reduce the variation in cooling stop temperature, thereby making the material in the coil more uniform. Very useful in industry.

[Brief description of the drawings]

FIG. 1 Strength-stretch flangeability balance (TS × λ),
The figure which shows the influence of the ferrite average particle diameter and the 2nd phase structure on the strength-ductility balance (TS * El).

FIG. 2: Strength-stretch flangeability balance (TS × λ),
The figure which shows the influence of the primary cooling stop temperature on a strength-ductility balance (TS * El).

FIG. 3 is a microstructure photograph in the thickness direction showing the presence or absence of a band-like structure.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/04 C22C 38/04 38/38 38/38 (72) Inventor Takayuki Otake One Marunouchi, Chiyoda-ku, Tokyo 1-2-2 Nihon Kokan Co., Ltd. (72) Inventor Hiroyasu Kikuchi 1-2-1 Marunouchi Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. F term (reference) 4K037 EA05 EA11 EA15 EA17 EA19 EA22 EA23 EA25 EA27 EA31 EA32 EB05 EB06 EB08 EB11 FA03 FA05 FC07 FD04 FE01 FE02 FG00 FH00 HA03 JA06 JA07

Claims (9)

[Claims] 1. The method according to claim 1, which has the following organization:
In mass%, C: 0.04 to 0.1%, Si: 0.5% or less, Mn: 0.5 to 2%, P: 0.05% or less, O:
High-strength steel sheet excellent in workability containing 0.005% or less and S: 0.005% or less. (1) Average ferrite grain size: 10 μm or less (2) Band-shaped second phase structure with a formation frequency (A) of 20 mm / mm ² or less, where formation frequency (A): Rolling direction of thin steel sheet—cross section of thickness Total length of band-like second phase structure observed per 1 mm ^{2 of} 2. The steel composition further includes Ti, Nb, V,
One or more of Mo and Cr are used in a total amount of 0.01 to 0.1.
The high-strength thin steel sheet having excellent workability according to claim 1, wherein the steel sheet contains 3%. 3. A method for producing a high-strength thin steel sheet excellent in workability, comprising the following steps. (1) A step of forming a steel having the composition according to claim 1 or 2 into a slab by continuous casting, and performing hot rolling at a rolling end temperature of Ar3 or higher directly or through a reheating step. (2) A step of immediately cooling to 600 to 750 ° C. within 2 seconds at a cooling rate of 100 to 2000 ° C./s after hot rolling. (3) Then, a step of cooling to 450 to 650 ° C. and winding in the temperature range. 4. The method for producing a high-strength steel sheet excellent in workability according to claim 3, further comprising the following step (1) or (2). (1) Step of annealing after pickling. (2) Step of annealing after pickling and cold rolling. 5. The method for producing a high-strength thin steel sheet excellent in workability according to claim 3, wherein a segregation reduction treatment is performed during continuous casting. 6. After hot rolling, 100 to 2000 ° C./sec.
The method for producing a high-strength thin steel sheet according to any one of claims 3 to 5, wherein the temperature fluctuation after cooling in the step (c) is set within 60 ° C in the width direction and the longitudinal direction of the hot-rolled steel strip. 7. A method for cooling a hot-rolled steel strip, wherein the heat transfer coefficient is 20.
The method for producing a high-strength thin steel sheet according to claim 6, wherein the cooling is performed at a temperature of at least 00 Kcal / m ² hr ° C. 8. The variation of the tensile strength in the width direction and the longitudinal direction of the steel strip is ± 8% of the average value of the tensile strength in the coil.
The high-strength thin steel sheet according to claim 1 or 2, wherein: 9. A method of manufacturing according to claim 6, wherein the variation in the tensile strength in the width direction and the longitudinal direction is within ± 8% of the average value of the tensile strength in the coil. High strength steel sheet.