High carbon steel sheet

TECHNICAL FIELD

The present invention relates to a high carbon steel sheet having chemical composition specified by JIS G 4051 (Carbon steels for machine structural use), JIS G 4401 (Carbon tool steels) or JIS G 4802 (Cold-rolled steel strips for springs), and in particular to a high carbon steel sheet having excellent hardenability and toughness, and workability with a high dimensional precision, and a method of producing the same.

BACKGROUND ART

High carbon steel sheets having chemical compositions specified by JIS G 4051, JIS G 4401 or JIS G 4802 have conventionally much often been applied to parts for machine structural use such as washers, chains or the like. Such high carbon steel sheets have accordingly been demanded to have good hardenability, and recently not only the good hardenability after quenching treatment but also low temperature—short time of quenching treatment for cost down and high toughness after quenching treatment for safety during services. In addition, since the high carbon steel sheets have large planar anisotropy of mechanical properties caused by production process such as hot rolling, annealing and cold rolling, it has been difficult to apply the high carbon steel sheets to parts as gears which are conventionally produced by casting or forging, and demanded to have workability with a high dimensional precision.

Therefore, for improving the hardenability and the toughness of the high carbon steel sheets, and reducing their planar anisotropy of mechanical properties, the following methods have been proposed.

(1) JP-A-5-9588, (the term “JP-A” referred to herein signifies “Unexamined Japanese Patent Publication”) (Prior Art 1): hot rolling, cooling down to 20 to 500° C. at a rate of 10° C./sec or higher, reheating for a short time, and coiling so as to accelerate spheroidization of carbides for improving the hardenability.

(2) JP-A-5-98388 (Prior Art 2): adding Nb and Ti to high carbon steels containing 0.30 to 0.70% of C so as to form carbonitrides for restraining austenite grain growth and improving the toughness.

(3) “Material and Process”, vol.1 (1988), p.1729 (Prior Art 3): hot rolling a high carbon steel containing 0.65% of C, cold rolling at a reduction rate of 50%, batch annealing at 650° C. for 24 hr, subjecting to secondary cold rolling at a reduction rate of 65%, and secondary batch annealing at 680° C. for 24 hr for improving the workability; otherwise adjusting the chemical composition of a high carbon steel containing 0.65% of C, repeating the rolling and the annealing as above mentioned so as to graphitize cementites for improving the workability and reducing the planar anisotropy of r-value.

(4) JP-A-10-152757 (Prior Art 4): adjusting contents of C, Si, Mn, P, Cr, Ni, Mo, V, Ti and Al, decreasing S content below 0.002 wt %, so that 6 μm or less is the average length of sulfide based non metallic inclusions narrowly elongated in the rolling direction, and 80% or more of all the inclusions are the inclusions whose length in the rolling direction is 4 μm or less, whereby the planar anisotropy of toughness and ductility is made small.

(5) JP-A-6-271935 (Prior Art 5): hot rolling, at Ar3 transformation point or higher, a steel whose contents of C, Si, Mn, Cr, Mo, Ni, B and Al were adjusted, cooling at a rate of 30° C./sec or higher, coiling at 550 to 700° C., descaling, primarily annealing at 600 to 680° C., cold rolling at a reduction rate of 40% or more, secondarily annealing at 600 to 680° C., and temper rolling so as to reduce the planar shape anisotropy caused by quenching treatment.

However, there are following problems in the above mentioned prior arts.

Prior Art 1: Although reheating for a short time, followed by coiling, a treating time for spheroidizing carbides is very short, and the spheroidization of carbides is insufficient so that the good hardenability might not be probably sometimes provided. Further, for reheating for a short time until coiling after cooling, a rapidly heating apparatus such as an electrically conductive heater is needed, resulting in an increase of production cost.

Prior Art 2: Because of adding expensive Nb and Ti, the production cost is increased.

Prior Art 3: Δr=(r0+r90−2×r45)/4 is −0.47, which is a parameter of planar anisotropy of r-value (r0, r45, and r90 shows a r-value of the directions of 0° (L), 45° (S) and 90° (C) with respect to the rolling direction respectively). Δmax of r-value being a difference between the maximum value and the minimum value among r0, r45, and r90 is 1.17. Since the Δr and the Δmax of r-value are high, it is difficult to carry out a forming with a high dimensional precision.

Besides, by graphitizing the cementites, the Δr decreases to 0.34 and the Δmax of r-value decreases to 0.85, but the forming could not be carried out with a high dimensional precision. In case graphitizing, since a dissolving speed of graphites into austenite phase is slow, the hardenability is remarkably degraded.

Prior Art 4: The planar anisotropy caused by inclusions is decreased, but the forming could not be always carried out with a high dimensional precision.

Prior Art 5: Poor shaping caused by quenching treatment could be improved, but the forming could not be always carried out with a high dimensional precision.

DISCLOSURE OF THE INVENTION

The present invention has been realized to solve above these problems, and it is an object of the invention to provide a high carbon steel sheet having excellent hardenability and toughness, and workability with a high dimensional precision, and a method of producing the same.

The present object could be accomplished by a high carbon steel sheet having chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, in which the ratio of number of carbides having a diameter of 0.6 μm or less with respect to all the carbides is 80% or more, more than 50 carbides having a diameter of 1.5 μm or larger exist in 2500 μm

2

of observation field area of electron microscope, and the Δr being a parameter of planar anisotropy of r-value is more than −0.15 to less than 0.15.

The above mentioned high carbon steel sheet can be produced by a method comprising the steps of: hot rolling a steel having chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, coiling the hot rolled steel sheet at 520 to 600° C., descaling the coiled steel sheet, primarily annealing the descaled steel sheet at 640 to 690° C. for 20 hr or longer, cold rolling the annealed steel sheet at a reduction rate of 50% or more, and secondarily annealing the cold rolled steel sheet at 620 to 680° C.

The JIS G standards JIS G 4051 (1979), JIS G 4401:2000 and JIS G 4802:1999 and particularly the section of each disclosing the chemical composition, are hereby incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

shows the relationship between maximum diameter Dmax of carbide when 80% or more is the ratio of number of carbides having diameters ≦Dmax with respect to all the carbides and hardness after quenching treatment;

FIG. 2

shows the relationship between number of carbides having a diameter of 1.5 μm or larger which exist in 2500 μm

2

of observation field area of electron microscope and austenite grain size;

FIG. 3

shows the relationship between primary annealing temperature, secondary annealing temperature and Δmax of r-value; and

FIG. 4

shows the another relationship between primary annealing temperature, secondary annealing temperature and A max of r-value.

EMBODIMENTS OF THE INVENTION

As to the high carbon steel sheet containing chemical composition specified by JIS G 4051, JIS G 4401 or JIS G4802, we investigated the hardenability, the toughness and the dimensional precision when forming, and found that the existing condition of carbides precipitated in steel was a governing factor over the hardenability and the toughness, while the planar anisotropy of r-value was so over the dimensional precision when forming, and in particular for providing an enough dimensional precision when forming, the planar anisotropy of r-value should be made smaller than that of the prior art. The details will be explained as follows.

(i) Hardenability and Toughness

By making a steel having, by wt %, C: 0.36%, Si: 0.20%, Mn: 0.75%, P: 0.011%, S: 0.002% and Al: 0.020%, hot rolling at a finishing temperature of 850° C., coiling at a coiling temperature of 560° C., pickling, primarily annealing at 640 to 690° C. for 40 hr, cold rolling at a reduction rate of 60%, and secondarily annealing at 610 to 690° C. for 40 hr, steel sheets were produced. Cutting out samples of 50×100 mm from the produced steel sheets, and heating at 820° C. for 10 sec, followed by quenching into oil at around 20° C., the hardness was measured and carbides were observed by an electron microscope.

The hardness was averaged over 10 measurements by Rockwell C Scale (HRc). If the average HRc is 50 or more, it may be judged that the good hardenability is provided.

The carbides were observed using a scanning electron microscope at 1500 to 5000 magnifications after polishing the cross section in a thickness direction of the steel sheet and etching it with a picral. Further, measurements were made on the size and the number of carbides in an observation field area of 2500 μm

2

. The reason for preparing the observing field area of 2500 μm

2

was that if an observing field area was smaller than this, the number of observable carbides was small, and the size and the number of carbides could not be measured precisely.

FIG. 1

shows the relationship between maximum diameter Dmax of carbide when 80% or more is the ratio of number of carbides having diameters ≦Dmax with respect to all the carbides and hardness after quenching treatment.

If the ratio of number of carbides having a diameter of 0.6 μm or less with respect to all the carbides is 80% or more, the HRc exceeds 50 and the good hardenability may be obtained. This is considered to be because fine carbides below 0.6 μm in diameter are rapidly dissolved into austenite phase when quenching.

But, if the diameter of all the carbides are below 0.6 μm, all the carbides are dissolved into the austenite phase when quenching, so that the austenite grains are remarkably coarsened and the toughness might be deteriorated. For avoiding it, as shown in

FIG. 2

, more than 50 carbides having a diameter of 1.5 μm or larger should exist in 2500 μm

2

of observation field area of electron microscope.

(ii) Dimensional Precision when Forming

For improving the dimensional precision when forming, it is necessary that the Δr is made small as described above. But it is not known how small the Δr should be made to obtain an equivalent dimensional precision in gear parts conventionally produced by casting or forging. So, the relationship between Δr and dimensional precision when forming was studied. As a result, it was found that if the Δr was more than −0.15 to less than 0.15, the equivalent dimensional precision in gear parts produced by casting or forging could be provided.

If the Δmax of r-value instead of the Δr is made less than 0.2, the forming can be conducted with a higher dimensional precision.

The high carbon steel sheet under the existing condition of carbides as mentioned in (i) and having a Δr of more than −0.15 to less than 0.15 as mentioned in (ii), can be produced by a method comprising the steps of: hot rolling a steel having chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, coiling the hot rolled steel sheet at 520 to 600° C., descaling the coiled steel sheet, primarily annealing the descaled steel sheet at 640 to 690° C. for 20 hr or longer, cold rolling the annealed steel sheet at a reduction rate of 50% or more, and secondarily annealing the cold rolled steel sheet at 620 to 680° C. Detailed explanation will be made therefor as follows.

(1) Coiling Temperature

Since the coiling temperature lower than 520° C. makes pearlite structure very fine, carbides after the primary annealing are considerably fine, so that carbides having a diameter of 1.5 μm or larger cannot be produced after the secondary annealing. In contrast, exceeding 600° C., coarse pearlite structure is generated, so that carbides having a diameter of 0.6 μm or less cannot be produced after the secondary annealing. Accordingly, the coiling temperature is defined to be 520 to 600° C.

(2) Primary Annealing

If the primary annealing temperature is higher than 690° C., carbides are too much spheroidized, so that carbides having a diameter of 0.6 μm or less cannot be produced after the secondary annealing. On the other hand, being lower than 640° C., the spheroidization of carbides is difficult, so that carbides having a diameter of 1.5 μm or larger cannot be produced after the secondary annealing. Accordingly, the primary annealing temperature is defined to be 640 to 690° C. The annealing time should be 20 hr or longer for uniformly spheroidizing.

(3) Cold Reduction Rate

In general, the higher the cold reduction rate, the smaller the Δr, and for making Δr more than −0.15 to less than 0.15, the cold reduction rate of at least 50% is necessary.

(4) Secondary Annealing

If the secondary annealing temperature exceeds 680° C., carbides are greatly coarsened, the grain grows markedly, and the Δr increases. On the other hand, being lower than 620° C., carbides become fine, and recrystallization and grain growth are not sufficient, so that the workability decreases. Thus, the secondary annealing temperature is defined to be 620 to 680° C. For the secondary annealing, either a continuous annealing or a box annealing will do.

For producing the high carbon steel sheet under the existing condition of carbides as mentioned in (i) and having a Δmax of r-value of less than 0.2 as mentioned in (ii), the primary annealing temperature T1 and the secondary annealing temperature T2 in the above method should satisfy the following formula (1).

1024−0.6

×T

1

≦T

2≦1202−0.80×

T

1 (1)

Detailed explanation will be made therefore as follows.

By making a slab of, by wt %, C: 0.36%, Si: 0.20%, Mn: 0.75%, P: 0.011%, S: 0.002% and Al: 0.020%, hot rolling at a finishing temperature of 850° C. and coiling at a coiling temperature of 560° C., pickling, primarily annealing at 640 to 690° C. for 40 hr, cold rolling at a reduction rate of 60%, and secondarily annealing at 610 to 690° C. for 40 hr, steel sheets were produced, and the Δmax of r-value was measured.

As seen in

FIG. 3

, if the primary annealing temperature T1 is 640 to 690° C. and the secondary annealing temperature T2 is in response to the primary annealing temperature T1 to satisfy the above formula (1), the Δmax of r-value is less than 0.2.

At this time, if the secondary annealing temperature is higher than 680° C., carbides are coarsened, and carbides having a diameter of 0.6 μm or less cannot be obtained. In contrast, being lower than 620° C., carbides having a diameter of 1.5 μm or larger cannot be obtained. Therefore, the secondary annealing temperature is defined to be 620 to 680° C. For the secondary annealing, either a continuous annealing or a box annealing will do.

The Δmax of r-value can be made smaller, if the high carbon steel sheet is produced by such a method comprising the steps of: continuously casting into slab a steel having chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, rough rolling the slab to sheet bar without reheating the slab or after reheating the slab cooled to a certain temperature, finish rolling the sheet bar (rough rolled slab) after reheating the sheet bar to Ar3 transformation point or higher, coiling the finish rolled steel sheet at 500 to 650° C., descaling the coiled steel sheet, primarily annealing the descaled steel sheet at a temperature T1 of 630 to 700° C. for 20 hr or longer, cold rolling the annealed steel sheet at a reduction rate of 50% or higher, and secondarily annealing the cold rolled steel sheet at a temperature T2 of 620 to 680° C., wherein the temperature T1 and the temperature T2 satisfy the following formula (2).

1010−0.59

×T

1≦

T

2≦1210−0.80

×T

1 (2)

At this time, instead of finish rolling the sheet bar after reheating the sheet bar to Ar3 transformation point or higher, by finish rolling the sheet bar during reheating the rolled sheet bar to Ar3 transformation point or higher the similar effect is available. Detailed explanation will be made therefor as follows.

(5) Reheating the Sheet Bar

By finish rolling the sheet bar after reheating the sheet bar to Ar3 transformation point or higher or during reheating the rolled sheet bar to Ar3 transformation point or higher, crystal grains are uniformed in a thickness direction of steel sheet during rolling, the dispersion of carbides after the secondary annealing is small, and the planar anisotropy of r-value becomes smaller. Accordingly, more excellent hardenability and toughness, and higher dimensional precision when forming are obtained. The reheating time should be at least 3 seconds. As the reheating time is short like this, an induction heating is preferably applied.

(6) Coiling Temperature and Primary Annealing Temperature

If the sheet bar is reheated as above mentioned, the ranges of the coiling temperature and the primary annealing temperature are respectively enlarged to 500 to 650° C. and 630 to 700° C. as compared with the case where the sheet bar is not reheated.

(7) Relationship Between Primary Annealing Temperature T1 and Secondary Annealing Temperature T2

By making a slab of, by wt %, C: 0.36%, Si: 0.20%, Mn: 0.75%, P: 0.011%, S: 0.002% and Al: 0.020%, rough rolling, reheating the sheet bar at 1010° C. for 15 sec by an induction heater, finish rolling at 850° C., coiling at 560° C., pickling, primarily annealing at 640 to 700° C. for 40 hr, cold rolling at a reduction rate of 60%, and secondarily annealing at 610 to 690° C. for 40 hr, steel sheets were produced. Measurements were made on the (222) integrated reflective intensity in the thickness directions (surface, ¼ thickness and ½ thickness) by X-ray diffraction method.

As shown in Table 1, by reheating the sheet bar, the Δmax of (222) intensity being a difference between the maximum value and the minimum value of (222) integrated reflective intensity in the thickness direction becomes small, and therefore the structure is more uniformed in the thickness direction.

As seen in

FIG. 4

, within the range satisfying the above formula (2), the Δmax of r-value less than 0.15 is obtained. The range satisfying the above formula (2) is wider than that of the formula (1).

TABLE 1

Se-

Reheating

Primary

condary

of

an-

an-

sheet bar

nealing

nealing

Integrated reflective intensity (222)

(° C. ×

(° C. ×

(° C. ×

1/4

1/2

sec)

hr)

hr)

Surface

thickness

thickness

Δ max

1010 ×

640 ×

610 ×

2.81

2.95

2.89

0.14

15

40

40

1010 ×

640 ×

650 ×

2.82

2.88

2.95

0.13

15

40

40

1010 ×

640 ×

690 ×

2.90

2.91

3.02

0.12

15

40

40

1010 ×

680 ×

610 ×

2.37

2.35

2.46

0.11

15

40

40

1010 ×

680 ×

650 ×

2.40

2.36

2.47

0.11

15

40

40

1010 ×

680 ×

690 ×

2.29

2.34

2.39

0.10

15

40

40

—

640 ×

610 ×

2.70

3.01

2.90

0.31

40

40

—

640 ×

650 ×

2.75

2.87

2.99

0.24

40

40

—

640 ×

690 ×

2.81

2.90

3.05

0.24

40

40

—

680 ×

610 ×

2.34

2.27

2.50

0.23

40

40

—

680 ×

650 ×

2.39

2.23

2.51

0.28

40

40

—

680 ×

690 ×

2.25

2.37

2.45

0.20

40

40

For improving sliding property, the high carbon steel sheet of the present invention may be galvanized through an electro-galvanizing process or a hot dip Zn plating process, followed by a phosphating treatment.

To produce the high carbon steel sheet of the present invention, a continuous hot rolling process using a coil box may be applicable. In this case, the sheet bar may be reheated through rough rolling mills, before or after the coil box, or before and after a welding machine.

EXAMPLE 1

By making a slab containing the chemical composition specified by S35C of JIS G 4051 (by wt %, C: 0.35%, Si: 0.20%, Mn: 0.76%, P: 0.016%, S: 0.003% and Al: 0.026%) through a continuous casting process, reheating to 1100° C., hot rolling, coiling, primarily annealing, cold rolling, secondarily annealing, under the conditions shown in Table 2, and temper rolling at a reduction rate of 1.5%, the steel sheets A-H of 1.0 mm thickness were produced. Herein, the steel sheet H is a conventional high carbon steel sheet. The existing condition of carbides and the hardenability were investigated by the above mentioned methods. Further, mechanical properties and austenite grain size were measured as follows.

(a) Mechanical Properties

JIS No.5 test pieces were sampled from the directions of 0° (L), 45° (S) and 90° (C) with respect to the rolling direction, and subjected to the tensile test at a tension speed of 10 mm/min so as to measure the mechanical properties in each direction. The Δmax of each mechanical property, that is, a difference between the maximum value and the minimum value of each mechanical property, and the Δr were calculated.

(b) Austenite Grain Size

The cross section in a thickness direction of the quenched test piece for investigating the hardenability was polished, etched, and observed by an optical microscope. The austenite grain size number was measured following JIS G 0551.

The results are shown in Tables 2 and 3.

As to the inventive steel sheets A-C, the existing condition of carbides is within the range of the present invention, and therefore the HRc after quenching is above 50 and the good hardenability is obtained. The austenite grain size of these steel sheets is small, and therefore the excellent toughness is obtained. In addition, the Δr is more than −0.15 to less than 0.15, that is, the planar anisotropy is very small, and accordingly the forming is carried out with a high dimensional precision. At the same time, the Δmax of yield strength and tensile strength is 10 MPa or lower, the Δmax of the total elongation is 1.5% or lower, and thus each planar anisotropy is very small.

In contrast, the comparative steel sheets D-H have large Δmax of the mechanical properties and Δr. The steel sheet D has coarse austenite grain size. In the steel sheets E, G, and H, the HRc is less than 50.

TABLE 2

Coiling

Primary

Cold

Secondary

Steel

temperature

annealing

reduction

annealing

Number of carbides

Ratio of carbides

sheet

(° C.)

(° C. × hr)

rate (%)

(° C. × hr)

larger than 1.5 μm

smaller than 0.6 μm (%)

Remark

A

580

650 × 40

70

680 × 40

89

84

Present

invention

B

560

640 × 20

60

660 × 40

84

87

Present

invention

C

540

660 × 20

65

640 × 40

81

93

Present

invention

D

500

640 × 40

60

660 × 40

64

96

Comparative

example

E

560

710 × 40

65

660 × 40

103

58

Comparative

example

F

540

660 × 20

40

680 × 40

86

84

Comparative

example

G

550

640 × 20

60

720 × 40

98

61

Comparative

example

H

620

—

50

690 × 40

74

70

Comparative

example

TABLE 3

Mechanical properties before quenching

Yield strength (MPa)

Tensile strength (MPa)

Steel sheet

L

S

C

Δ max

L

S

C

Δ max

A

395

391

393

4

506

502

507

5

B

405

404

411

7

504

498

507

9

C

409

406

414

8

509

505

513

8

D

369

362

370

8

499

496

503

9

E

370

379

375

9

480

484

481

4

F

374

377

385

11

474

480

488

14

G

372

376

379

7

496

493

498

5

H

317

334

320

17

501

516

510

15

Mechanical properties before quenching

Total elongation (%)

r-value

Steel sheet

L

S

C

Δ max

L

S

C

Δr

A

35.7

36.4

35.9

0.7

1.06

0.97

1.04

0.04

B

35.8

36.8

36.2

1.0

1.12

0.98

1.23

0.10

C

35.2

36.4

35.3

1.2

0.98

1.19

1.05

−0.09

D

30.1

29.3

31.0

1.7

1.16

0.92

1.33

0.16

E

36.9

36.0

36.4

0.9

1.15

0.96

1.47

0.18

F

35.7

34.6

36.3

1.7

1.25

0.96

1.46

0.20

G

38.0

37.7

37.7

0.3

1.14

0.94

1.64

0.23

H

36.5

34.6

35.5

1.9

1.12

0.92

1.35

0.16

Hardness after

Austetine

quenching

Grain size

Steel sheet

(HRc)

size No.)

Remark

A

52

11.6

Present

invention

B

54

11.3

Present

invention

C

56

10.7

Present

invention

D

57

8.6

Comparative

example

E

44

12.2

Comparative

example

F

53

11.2

Comparative

example

G

40

12.1

Comparative

example

H

49

11.6

Comparative

example

EXAMPLE 2

By making a slab containing the chemical composition specified by S35C of JIS G 4051 (by wt %, C: 0.36%, Si: 0.20%, Mn: 0.75%, P: 0.011%, S: 0.002% and Al: 0.020%) through a continuous casting process, reheating to 1100° C., hot rolling, coiling, primarily annealing, cold rolling, secondarily annealing, under the conditions shown in Table 4, and temper rolling at a reduction rate of 1.5%, the steel sheets 1-19 of 2.5 mm thickness were produced. Herein, the steel sheet 19 is a conventional high carbon steel sheet. The same measurements as in Example 1 were conducted. The Δ max of r-value was calculated in stead of Δr.

The results are shown in Tables 4 and 5.

As to the inventive steel sheets 1-7, the existing condition of carbides is within the range of the present invention, and therefore the HRc after quenching is above 50 and the good hardenability is obtained. The austenite grain size of these steel sheets is small, and therefore the excellent toughness is obtained. In addition, the Δmax of r-value is below 0.2, that is, the planar anisotropy is extremely small, and accordingly the forming is carried out with a high dimensional precision. At the same time, the Δmax of yield strength and tensile strength is 10 MPa or lower, the Δmax of the total elongation is 1.5% or lower, and thus each planar anisotropy is very small.

In contrast, the comparative steel sheets 8-19 have large Δmax of the mechanical properties. The steel sheets 8, 10, 17 and 18 have coarse austenite grain size. In the steel sheets 9, 11, 15, 16 and 19, the HRc is less than 50.

TABLE 4

Coiling

Primary

Cold

Secondary

Number of

Ratio of carbides

Steel

temperature

annealing

reduction

annealing

Secondary annealing

carbides larger

smaller than

sheet

(° C.)

(° C. × hr)

rate (%)

(° C. × hr)

range by the formula (1)

than 1.5 μm

0.6 μm (%)

Remark

1

580

640 × 40

70

680 × 40

640-680

56

85

Present

invention

2

530

640 × 20

60

680 × 40

640-680

52

87

Present

invention

3

595

640 × 40

60

680 × 20

640-680

64

81

Present

invention

4

580

660 × 40

60

660 × 40

628-674

61

83

Present

invention

5

580

680 × 20

60

640 × 40

620-658

63

82

Present

invention

6

580

640 × 40

50

660 × 40

640-680

56

85

Present

invention

7

580

640 × 40

70

640 × 40

640-680

54

86

Present

invention

8

510

640 × 20

60

680 × 40

640-680

30

92

Comparative

example

9

610

640 × 20

60

680 × 20

640-680

68

61

Comparative

example

10

580

620 × 40

60

680 × 40

—

32

90

Comparative

example

11

580

720 × 40

60

680 × 40

—

68

65

Comparative

example

12

580

640 × 15

70

680 × 40

640-680

54

86

Comparative

example

13

580

640 × 40

30

680 × 40

640-680

58

84

Comparative

example

14

580

660 × 20

60

620 × 40

628-674

60

84

Comparative

example

15

580

640 × 20

60

700 × 40

640-680

66

73

Comparative

example

16

580

640 × 40

60

690 × 40

640-680

67

70

Comparative

example

17

580

690 × 40

60

615 × 40

620-650

33

88

Comparative

example

18

520

640 × 20

60

640 × 20

640-680

45

88

Comparative

example

19

620

—

50

690 × 40

—

51

67

Comparative

example

TABLE 5

Mechanical properties before quenching

Hardness after

Austetine

Yield strength (MPa)

Tensile strength (MPa)

Total elongation (%)

r-value

quenching

grain size

Steel sheet

L

S

C

Δ max

L

S

C

Δ max

L

S

C

Δ max

L

S

C

Δ max

(HRc)

(size No.)

Remark

1

398

394

402

8

506

508

513

5

36.2

37.4

37.0

1.2

1.07

0.99

1.00

0.08

54

11.1

Present

invention

2

410

407

412

5

513

512

516

4

36.8

38.0

36.8

1.2

1.02

1.01

1.11

0.10

56

10.9

Present

invention

3

350

348

351

3

470

474

472

2

36.3

36.8

36.2

0.6

1.01

1.01

1.09

0.08

51

11.6

Present

invention

4

395

398

404

9

507

506

509

3

36.6

37.5

37.3

0.9

1.09

0.99

1.01

0.10

52

11.5

Present

invention

5

392

397

400

8

502

503

501

2

37.9

38.2

38.0

0.3

0.95

1.13

1.00

0.18

51

11.5

Present

invention

6

401

398

407

9

509

509

512

3

37.5

37.9

38.5

1.0

0.94

1.07

1.02

0.13

53

11.3

Present

invention

7

404

401

410

9

510

509

512

3

35.3

36.7

36.6

1.4

1.03

1.18

1.01

0.17

55

11.0

Present

invention

8

374

367

374

7

507

505

508

3

29.9

28.4

31.3

2.9

1.17

1.01

1.43

0.42

58

8.3

Comparative

example

9

371

386

380

15

482

491

485

9

27.1

25.0

26.7

2.1

1.14

0.93

1.31

0.38

40

12.0

Comparative

example

10

395

396

399

4

512

512

515

3

27.0

25.4

28.2

2.8

1.27

0.98

1.28

0.30

58

8.9

Comparative

example

11

372

384

380

12

484

489

485

5

37.7

36.9

37.3

0.8

1.24

1.00

1.34

0.34

42

12.0

Comparative

example

12

390

384

377

13

490

500

498

10

29.0

24.9

29.4

4.5

1.19

0.94

1.29

0.35

56

10.9

Comparative

example

13

372

383

390

18

480

486

493

13

35.5

33.7

36.5

2.8

1.02

0.96

1.48

0.52

53

11.3

Comparative

example

14

404

401

410

9

510

508

513

5

35.1

37.0

36.7

1.9

1.01

1.28

0.94

0.34

52

11.4

Comparative

example

15

385

386

376

10

503

501

506

5

37.5

36.8

36.4

1.1

1.28

1.00

1.31

0.31

45

11.8

Comparative

example

16

388

389

378

11

504

501

507

6

37.3

36.5

36.0

1.3

1.18

0.98

1.36

0.38

43

11.9

Comparative

example

17

410

406

417

11

513

510

515

5

35.3

36.7

36.5

1.4

1.02

1.26

0.92

0.34

56

9.9

Comparative

example

18

412

406

415

9

514

511

519

8

35.1

36.5

36.3

1.4

0.97

1.22

0.88

0.34

57

9.4

Comparative

example

19

322

335

322

13

510

519

514

9

36.1

34.1

35.9

2.0

1.12

0.93

1.36

0.43

43

12.0

Comparative

example

EXAMPLE 3

By making a slab containing the chemical composition specified by S65C-CSP of JIS G 4802 (by wt %, C: 0.65%, Si: 0.19%, Mn: 0.73%, P: 0.011%, S: 0.002% and Al: 0.020%) through a continuous casting process, reheating to 1100° C., hot rolling, coiling, primarily annealing, cold rolling, secondarily annealing, under the conditions shown in Table 6, and temper rolling at a reduction rate of 1.5%, the steel sheets 20-38 of 2.5 mm thickness were produced. Herein, the steel sheet 38 is a conventional high carbon steel sheet. The same measurements as in Example 2 were conducted.

The results are shown in Tables 6 and 7.

As to the inventive steel sheets 20-26, the existing condition of carbides is within the range of the present invention, and therefore the HRc after quenching is above 50 and the good hardenability is obtained. The austenite grain size of these steel sheets is small, and therefore the excellent toughness is obtained. In addition, the Δmax of r-value is below 0.2, that is, the planar anisotropy is extremely small, and accordingly the forming is carried out with a high dimensional precision. At the same time, the Δmax of yield strength and tensile strength is 15 MPa or lower, the Δmax of the total elongation is 1.5% or lower, and thus each planar anisotropy is very small.

In contrast, the comparative steel sheets 27-38 have large Δmax of the mechanical properties. The steel sheets 27, 29 and 36 have coarse austenite grain size. In the steel sheets 28 and 38, the HRc is less than 50.

TABLE 6

Coiling

Primary

Cold

Secondary

Number of

Ratio of carbides

Steel

temperature

annealing

reduction

annealing

Secondary annealing

carbides larger

smaller than

sheet

(° C.)

(° C. × hr)

rate (%)

(° C. × hr)

range by the formula (1)

than 1.5 μm

0.6 μ m (%)

Remark

20

560

640 × 40

70

680 × 40

640-680

86

86

Present

invention

21

530

640 × 20

60

680 × 40

640-680

82

88

Present

invention

22

595

640 × 40

60

680 × 20

640-680

94

82

Present

invention

23

560

660 × 40

60

660 × 40

628-674

90

83

Present

invention

24

560

680 × 20

60

640 × 40

620-658

92

83

Present

invention

25

560

640 × 40

50

660 × 40

640-680

87

85

Present

invention

26

560

640 × 40

70

640 × 40

640-680

83

86

Present

invention

27

510

640 × 20

60

680 × 40

640-680

44

93

Comparative

example

28

610

640 × 20

60

680 × 20

640-680

101

62

Comparative

example

29

560

620 × 40

60

680 × 40

—

47

91

Comparative

example

30

560

720 × 40

60

680 × 40

—

100

64

Comparative

example

31

560

640 × 15

70

680 × 40

640-680

83

87

Comparative

example

32

560

640 × 40

30

680 × 40

640-680

88

85

Comparative

example

33

560

660 × 20

60

620 × 40

630-674

89

84

Comparative

example

34

560

640 × 20

60

700 × 40

640-680

98

72

Comparative

example

35

560

640 × 40

60

690 × 40

640-680

99

70

Comparative

example

36

560

690 × 40

60

615 × 40

620-650

49

89

Comparative

example

37

600

690 × 40

50

650 × 40

620-650

96

77

Comparative

example

38

620

—

50

690 × 40

—

100

65

Comparative

example

TABLE 7

Mechanical properties before quenching

Hardness after

Austetine

Yield strength (MPa)

Tensile strength (MPa)

Total elongation (%)

r-value

quenching

Grain size

Steel sheet

L

S

C

Δ max

L

S

C

Δ max

L

S

C

Δ max

L

S

C

Δ max

(HRc)

(size No.)

Remark

20

412

406

413

7

515

518

523

8

34.2

35.7

35.2

1.5

1.04

0.96

0.97

0.08

63

11.2

Present

invention

21

422

419

427

8

524

521

526

5

35.1

36.0

34.6

1.4

0.98

1.00

1.06

0.08

64

11.0

Present

invention

22

365

360

363

5

480

483

480

3

34.5

35.0

34.1

0.9

0.97

0.98

1.07

0.10

60

11.7

Present

invention

23

409

409

416

7

518

514

519

5

34.7

35.7

34.2

1.5

1.02

0.97

0.93

0.09

61

11.6

Present

invention

24

405

410

415

10

511

512

512

1

35.8

36.1

36.2

0.4

0.89

1.11

0.94

0.19

60

11.6

Present

invention

25

416

412

423

11

519

517

523

6

35.4

36.0

36.7

1.3

0.92

1.03

0.95

0.14

62

11.4

Present

invention

26

417

414

424

10

521

515

524

9

33.4

34.9

34.7

1.5

1.00

1.15

0.98

0.17

63

11.1

Present

invention

27

385

380

388

8

518

515

518

3

28.2

24.8

28.2

3.4

1.22

0.96

1.28

0.32

66

8.4

Comparative

example

28

385

400

395

15

489

500

493

11

25.7

23.2

25.2

2.5

1.15

0.89

1.22

0.33

48

12.2

Comparative

example

29

406

410

413

7

519

523

526

7

25.5

24.0

26.7

2.7

1.21

0.97

1.36

0.39

66

9.0

Comparative

example

30

384

397

394

13

492

500

496

8

35.8

34.6

35.6

1.2

1.20

0.90

1.18

0.30

50

12.1

Comparative

example

31

405

398

389

16

500

510

511

11

27.1

22.4

27.4

5.0

0.94

1.25

0.97

0.31

64

11.1

Comparative

example

32

386

396

406

20

486

497

503

17

33.7

31.9

34.8

2.9

0.81

1.17

0.94

0.36

62

11.4

Comparative

example

33

416

412

425

13

521

516

523

7

33.2

35.1

34.8

1.9

1.04

1.32

1.01

0.31

61

11.5

Comparative

example

34

402

391

388

14

512

510

515

5

35.7

34.8

34.3

1.4

1.22

0.97

1.34

0.37

53

11.9

Comparative

example

35

405

395

394

11

514

511

517

6

35.5

34.8

34.1

1.4

1.17

0.88

1.18

0.30

51

12.0

Comparative

example

36

420

417

431

14

523

519

525

6

33.3

34.8

34.5

1.5

1.00

1.26

0.93

0.33

65

10.0

Comparative

example

37

375

363

370

12

482

490

485

8

34.3

35.2

34.0

1.2

1.21

0.93

1.24

0.31

56

11.8

Comparative

example

38

336

350

331

19

517

528

526

11

34.5

32.4

33.8

2.1

1.10

0.83

1.29

0.44

46

12.4

Comparative

example

EXAMPLE 4

By making a slab containing the chemical composition specified by S35C of JIS G 4051 (by wt %, C: 0.36%, Si: 0.20%, Mn: 0.75%, P: 0.011%, S: 0.002% and Al: 0.020%) through a continuous casting process, reheating to 1100° C., hot rolling, coiling, primarily annealing, cold rolling, secondarily annealing, under the conditions shown in Tables 8 and 9, and temper rolling at a reduction rate of 1.5%, the steel sheets 39-64 of 2.5 mm thickness were produced. In this example, the reheating of sheet bar was conducted for some steel sheets. Herein, the steel sheet 64 is a conventional high carbon steel sheet. The same measurements as in Example 2 were conducted. The Δmax of (222) intensity as above mentioned was also measured.

The results are shown in Tables 8-12.

As to the inventive steel sheets 39-52, the existing condition of carbides is within the range of the present invention, and therefore the HRc after quenching is above 50 and the good hardenability is obtained. The austenite grain size of these steel sheets is small, and therefore the excellent toughness is obtained. In addition, the Δmax of r-value is below 0.2, that is, the planar anisotropy is extremely small, and accordingly the forming is carried out with a high dimensional precision. At the same time, the Δmax of yield strength and tensile strength is 10 MPa or lower, the Δmax of the total elongation is 1.5% or lower, and thus each planar anisotropy is very small. In particular, the steel sheets 39-45 of which the sheet bar was reheated have small Δmax of (222) intensity in the thickness direction, and therefore more uniformed structure in the thickness direction.

In contrast, the comparative steel sheets 53-64 have large Δmax of the mechanical properties. The steel sheets 53, 55, 62 and 63 have coarse austenite grain size. In the steel sheets 54, 56, 60, 61 and 64, the HRc is less than 50.

TABLE 8

Secondary

Reheating of

Coiling

Primary

Cold

Secondary

annealing range

Number of

Ratio of carbides

Steel

sheet bar

temperature

annealing

reduction

annealing

by the formula

carbides larger

smaller than

sheet

(° C. × sec)

(° C.)

(° C. × hr)

rate (%)

(° C. × hr)

(1)

than 1.5 μm

0.6 μm (%)

Remark

39

1050 × 15

580

640 × 40

70

680 × 40

632-680

55

86

Present

invention

40

1100 × 3

530

640 × 20

60

680 × 40

632-680

52

87

Present

invention

41

950 × 3

595

640 × 40

60

680 × 20

632-680

64

81

Present

invention

42

1050 × 15

580

660 × 40

60

660 × 40

620-680

60

84

Present

invention

43

1050 × 15

580

680 × 20

60

640 × 40

620-666

62

82

Present

invention

44

1050 × 15

580

640 × 40

50

660 × 40

632-680

56

85

Present

invention

45

1050 × 15

580

640 × 40

70

640 × 40

632-680

54

86

Present

invention

46

—

580

640 × 40

70

680 × 40

632-680

56

85

Present

invention

47

—

530

640 × 20

60

680 × 40

632-680

53

86

Present

invention

48

—

595

640 × 40

60

680 × 20

632-680

64

81

Present

invention

49

—

580

660 × 40

60

660 × 40

620-680

61

83

Present

invention

50

—

580

680 × 20

60

640 × 40

620-666

63

82

Present

invention

51

—

580

640 × 40

50

660 × 40

632-680

56

85

Present

invention

TABLE 9

Secondary

Reheating of

Coiling

Primary

Cold

Secondary

annealing range

Number of

Ratio of carbides

Steel

sheet bar

temperature

annealing

reduction

annealing

by the formula

carbides larger

smaller than

sheet

(° C. × sec)

(° C.)

(° C. × hr)

rate (%)

(° C. × hr)

(1)

than 1.5 μm

0.6 μm (%)

Remark

52

—

580

640 × 40

70

640 × 40

632-680

55

85

Present

invention

53

1050 × 15

510

640 × 20

60

680 × 40

632-680

30

92

Comparative

example

54

1100 × 3

610

640 × 20

60

680 × 20

632-680

67

61

Comparative

example

55

950 × 3

580

620 × 40

60

680 × 40

—

32

89

Comparative

example

56

1050 × 15

580

720 × 40

60

680 × 40

—

68

65

Comparative

example

57

1050 × 15

580

640 × 15

70

680 × 40

632-680

55

86

Comparative

example

58

1050 × 15

580

640 × 40

30

680 × 40

632-680

58

84

Comparative

example

59

1050 × 15

580

660 × 20

60

610 × 40

620-680

60

84

Comparative

example

60

1050 × 15

580

640 × 20

60

700 × 40

632-680

66

74

Comparative

example

61

1050 × 15

580

640 × 40

60

690 × 40

632-680

66

70

Comparative

example

62

1050 × 15

580

690 × 40

60

615 × 40

620-658

33

88

Comparative

example

63

1050 × 15

520

640 × 20

60

640 × 20

632-680

45

88

Comparative

example

64

1050 × 15

620

—

50

690 × 40

—

33

87

Comparative

example

TABLE 10

Mechanical properties before

Hardness after

Austetine

Yield strength (MPa)

Tensile strength (MPa)

Total elongation (%)

r-value

quenching

grain size

Steel sheet

L

S

C

Δ max

L

S

C

Δ max

L

S

C

Δ max

L

S

C

Δ max

(HRc)

(size No.)

Remark

39

398

394

398

4

506

508

512

6

36.5

37.4

37.0

0.9

1.07

0.99

1.02

0.08

55

11.0

Present

invention

40

410

407

410

3

514

512

516

4

36.8

37.7

36.8

0.9

1.04

1.01

1.11

0.10

56

10.9

Present

invention

41

351

348

350

3

470

474

473

4

36.4

36.8

36.2

0.6

1.03

1.01

1.09

0.08

51

11.6

Present

invention

42

395

398

400

5

508

506

509

3

36.8

37.5

37.3

0.7

1.09

0.99

1.02

0.10

53

11.4

Present

invention

43

395

397

400

5

501

503

501

2

37.9

38.2

38.1

0.3

0.95

1.09

1.00

0.14

52

11.4

Present

invention

44

401

399

404

5

509

510

512

3

37.7

37.9

38.5

0.8

0.94

1.07

1.04

0.13

53

11.3

Present

invention

45

404

401

405

4

511

509

512

3

35.7

36.7

36.6

1.0

1.03

1.15

1.01

0.14

55

11.0

Present

invention

46

397

394

402

8

506

508

513

7

36.2

37.4

37.1

1.2

1.14

0.99

1.00

0.15

54

11.1

Present

invention

47

409

407

412

5

514

512

516

4

36.8

38.0

36.9

1.2

1.02

1.01

1.14

0.16

55

11.0

Present

invention

48

351

348

351

3

470

474

469

5

36.4

36.8

36.2

0.6

1.01

0.98

1.13

0.15

51

11.6

Present

invention

49

395

397

404

9

507

505

509

4

36.6

37.5

37.2

0.9

1.13

0.96

1.01

0.17

52

11.5

Present

invention

50

392

396

400

8

502

505

501

4

37.2

38.2

38.0

1.0

0.95

1.14

1.00

0.19

51

11.5

Present

invention

51

403

398

407

9

509

505

512

3

37.5

37.7

38.5

1.0

0.94

1.12

1.02

0.18

53

11.3

Present

invention

TABLE 11

Hard-

Auste-

ness

tine

after

grain

Mechanical properties before quenching

quench-

size

Steel

Yield strength (MPa)

Tensile strength (MPa)

Total elongation (%)

r-value

ing

(size

Re-

Sheet

L

S

C

Δmax

L

S

C

Δmax

L

S

C

Δmax

L

S

C

Δmax

(HRc)

No.)

mark

52

405

401

410

9

510

507

512

5

35.3

36.7

36.4

1.4

1.03

1.19

1.00

0.19

54

11.1

Pre-

sent

in-

ven-

tion

53

372

364

374

10

507

503

508

5

29.8

28.4

31.3

2.9

1.26

1.02

1.37

0.35

58

8.3

Com-

para-

tive

ex-

am-

ple

54

371

386

379

15

482

491

484

9

27.1

25.0

26.3

2.1

1.27

0.98

1.27

0.29

41

12.0

Com-

para-

tive

ex-

am-

ple

55

392

396

399

7

512

509

515

6

27.2

25.4

28.2

2.8

1.33

1.04

1.36

0.32

58

9.0

Com-

para-

tive

ex-

am-

ple

56

372

385

380

13

484

489

486

5

37.7

36.6

37.3

1.1

1.23

0.95

1.25

0.30

42

12.0

Com-

para-

tive

ex-

am-

ple

57

390

384

378

12

490

500

497

10

28.8

24.9

29.4

4.5

1.16

0.89

1.20

0.31

55

10.9

Com-

para-

tive

ex-

am-

ple

58

372

385

390

18

480

487

493

13

35.4

33.7

36.5

2.8

0.88

1.19

0.91

0.31

53

11.3

Com-

para-

tive

ex-

am-

ple

59

405

401

410

9

510

506

513

7

35.1

37.0

36.6

1.9

1.01

1.27

0.94

0.33

52

11.4

Com-

para-

tive

ex-

am-

ple

60

383

386

376

10

504

501

506

5

37.5

36.9

36.4

1.1

1.18

0.94

1.29

0.35

45

11.7

Com-

para-

tive

ex-

am-

ple

61

387

389

378

11

503

501

507

6

37.3

36.6

36.0

1.3

1.16

1.00

1.45

0.45

44

11.9

Com-

para-

tive

ex-

am-

ple

62

410

404

417

13

513

507

515

8

35.3

36.7

36.1

1.4

0.87

1.17

0.88

0.29

56

9.9

Com-

para-

tive

ex-

am-

ple

63

411

406

415

9

515

511

515

8

35.1

36.5

36.0

1.4

1.02

1.32

1.00

0.32

57

9.4

Com-

para-

tive

ex-

am-

ple

64

323

335

322

13

510

519

513

9

36.1

34.1

35.5

2.0

1.10

0.93

1.35

0.40

43

12.0

Com-

para-

tive

ex-

am-

ple

TABLE 12

Integrated reflective intensity (222)

Steel

¼

½

sheet

Surface

thickness

thickness

Δ max

Remark

39

2.80

2.79

2.90

0.11

Present invention

40

2.85

2.92

3.00

0.15

Present invention

41

2.87

2.93

3.00

0.13

Present invention

42

2.72

2.80

2.84

0.12

Present invention

43

2.54

2.60

2.66

0.12

Present invention

44

2.85

2.93

2.99

0.14

Present invention

45

2.88

3.01

2.95

0.13

Present invention

46

2.75

2.90

3.03

0.28

Present invention

47

2.77

3.06

2.98

0.29

Present invention

48

2.79

2.74

3.02

0.28

Present invention

49

2.65

2.77

2.90

0.25

Present invention

50

2.48

2.58

2.75

0.27

Present invention

51

2.80

3.02

2.97

0.22

Present invention

52

2.83

2.80

3.04

0.24

Present invention

53

2.81

2.88

2.96

0.15

Comparative example

54

2.84

2.87

2.98

0.14

Comparative example

55

2.90

3.04

2.99

0.14

Comparative example

56

2.20

2.28

2.32

0.12

Comparative example

57

2.82

2.93

2.91

0.11

Comparative example

58

2.83

2.90

2.98

0.15

Comparative example

59

2.73

2.79

2.86

0.13

Comparative example

60

2.85

2.92

3.00

0.15

Comparative example

61

2.82

2.96

2.93

0.14

Comparative example

62

2.38

2.42

2.53

0.15

Comparative example

63

2.83

2.88

2.96

0.13

Comparative example

64

2.33

2.39

2.48

0.15

Comparative example

EXAMPLE 5

By making a slab containing the chemical composition specified by S65C-CSP of JIS G 4802 (by wt %, C: 0.65%, Si: 0.19%, Mn: 0.73%, P: 0.011%, S: 0.002% and Al: 0.020%) through a continuous casting process, reheating to 1100° C., hot rolling, coiling, primarily annealing, cold rolling, secondarily annealing, under the conditions shown in Tables 13 and 14, and temper rolling at a reduction rate of 1.5%, the steel sheets 65-90 of 2.5 mm thickness were produced. In this example, the reheating of sheet bar was conducted for some steel sheets. Herein, the steel sheet 90 is a conventional high carbon steel sheet. The same measurements as in Example 4 were conducted.

The results are shown in Tables 13-17.

As to the inventive steel sheets 65-78, the existing condition of carbides is within the range of the present invention, and therefore the HRc after quenching is above 50 and the good hardenability is obtained. The austenite grain size of these steel sheets is small, and therefore the excellent toughness is obtained. In addition, the Δmax of r-value is below 0.2, that is, the planar anisotropy is extremely small, and accordingly the forming is carried out with a high dimensional precision. At the same time, the Δmax of yield strength and tensile strength is 15 MPa or lower, the Δmax of the total elongation is 1.5% or lower, and thus each planar anisotropy is very small. In particular, the steel sheets 65-71 of which the sheet bar was reheated have small Δmax of (222) intensity in the thickness direction, and therefore more uniformed structure in the thickness direction.

In contrast, the comparative steel sheets 79-90 have large Δmax of the mechanical properties. The steel sheets 79, 81 and 88 have coarse austenite grain size. In the steel sheet 80, the HRc is less than 50.

TABLE 13

Secondary

Reheating of

Coiling

Primary

Cold

Secondary

annealing range

Number of

Ratio of carbides

Steel

sheet bar

temperature

annealing

reduction

annealing

by the formula

carbides larger

smaller than 0.6

Sheet

(° C. × sec)

(° C.)

(° C. × hr)

rate (%)

(° C. × hr)

(1)

than 1.5 μm

μm (%)

Remarks

65

1050 × 15

560

640 × 40

70

680 × 40

632-680

85

87

Present

invention

66

1100 × 3

530

640 × 20

60

680 × 40

632-680

82

88

Present

invention

67

950 × 3

595

640 × 40

60

680 × 20

632-680

94

82

Present

invention

68

1050 × 15

560

660 × 40

60

660 × 40

620-680

89

84

Present

invention

69

1050 × 15

560

680 × 20

60

640 × 40

620-666

91

83

Present

invention

70

1050 × 15

560

640 × 40

50

660 × 40

632-680

87

85

Present

invention

71

1050 × 15

560

640 × 40

70

640 × 40

632-680

83

86

Present

invention

72

—

560

640 × 40

70

680 × 40

632-680

86

86

Present

invention

73

—

530

640 × 20

60

680 × 40

632-680

83

87

Present

invention

74

—

595

640 × 40

60

680 × 20

632-680

94

82

Present

invention

75

—

560

660 × 40

60

660 × 40

620-680

90

83

Present

invention

76

—

560

680 × 20

60

640 × 40

620-666

92

83

Present

invention

77

—

560

640 × 40

50

660 × 40

632-680

87

85

Present

invention

TABLE 14

Secondary

Reheating of

Coiling

Primary

Cold

Secondary

annealing range

Number of

Ratio of carbides

Steel

sheet bar

temperature

annealing

reduction

annealing

by the formula

carbides larger

smaller than 0.6

Sheet

(° C. × sec)

(° C.)

(° C. × hr)

rate (%)

(° C. × hr)

(1)

than 1.5 μm

μm (%)

Remarks

78

—

560

640 × 40

70

640 × 40

632-680

84

85

Present

invention

79

1050 × 15

510

640 × 20

60

680 × 40

632-680

44

93

Comparative

example

80

1100 × 3

610

640 × 20

60

680 × 20

632-680

100

62

Comparative

example

81

950 × 3

560

620 × 40

60

680 × 40

—

47

90

Comparative

example

82

1050 × 15

560

720 × 40

60

680 × 40

—

100

64

Comparative

example

83

1050 × 15

560

640 × 15

70

680 × 40

632- 680

84

87

Comparative

example

84

1050 × 15

560

640 × 40

30

680 X 40

632- 680

88

85

Comparative

example

85

1050 × 15

560

660 × 20

60

610 × 40

620-680

89

84

Comparative

example

86

1050 × 15

560

640 × 20

60

700 × 40

632-680

98

73

Comparative

example

87

1050 × 15

560

640 × 40

60

690 × 40

632-680

98

70

Comparative

example

88

1050 × 15

560

690 × 40

60

615 × 40

620-680

49

89

Comparative

example

89

1050 × 15

600

690 × 20

50

650 × 40

632-680

96

77

Comparative

example

90

1050 × 15

610

—

50

690 × 40

—

99

71

Comparative

example

TABLE 15

Hard-

Auste-

ness

tine

after

grain

Mechanical properties before quenching

quench-

size

Steel

Yield strength (MPa)

Tensile strength (MPa)

Total elongation (%)

r-value

ing

(size

Re-

Sheet

L

S

C

Δmax

L

S

C

Δmax

L

S

C

Δmax

L

S

C

Δmax

(HRc)

No.)

mark

65

412

406

412

6

515

518

521

6

34.7

35.7

35.2

1.0

1.04

0.96

0.98

0.08

64

11.1

Pre-

sent

in-

ven-

tion

66

422

419

424

5

523

521

526

5

35.1

36.0

35.1

0.9

0.98

1.02

1.06

0.08

64

11.0

Pre-

sent

in-

ven-

tion

67

364

360

363

4

480

483

481

3

34.5

35.0

34.3

0.7

0.97

0.99

1.07

0.10

60

11.7

Pre-

sent

in-

ven-

tion

68

409

409

415

6

517

514

519

5

34.7

35.7

34.7

1.0

1.02

0.96

0.93

0.09

62

11.5

Pre-

sent

in-

ven-

tion

69

405

410

412

7

511

511

512

1

35.8

36.0

36.2

0.4

0.92

1.06

0.94

0.14

61

11.5

Pre-

sent

in-

ven-

tion

70

416

412

421

9

520

517

523

6

35.9

36.0

36.7

0.8

0.89

1.03

0.96

0.14

62

11.4

Pre-

sent

in-

ven-

tion

71

417

414

421

7

521

515

521

6

33.9

34.9

34.7

1.0

1.00

1.12

0.98

0.14

63

11.1

Pre-

sent

in-

ven-

tion

72

411

406

413

7

515

519

523

8

34.2

35.7

35.3

1.5

1.08

0.93

0.97

0.15

63

11.2

Pre-

sent

in-

ven-

tion

73

423

419

427

8

523

521

526

5

35.3

36.0

34.6

1.4

0.94

1.00

1.10

0.16

63

11.1

Pre-

sent

in-

ven-

tion

74

365

360

362

5

479

483

480

4

34.6

35.0

34.1

0.9

0.95

0.98

1.12

0.17

60

11.7

Pre-

sent

in-

ven-

tion

75

410

409

416

7

517

514

519

5

34.6

35.7

34.2

1.5

1.07

0.97

0.91

0.16

61

11.6

Pre-

sent

in-

ven-

tion

76

405

408

415

10

511

512

514

3

35.4

36.1

36.6

1.2

0.92

1.11

0.95

0.19

60

11.6

Pre-

sent

in-

ven-

tion

77

417

412

423

11

518

517

523

6

35.4

36.1

36.7

1.3

0.89

1.07

0.95

0.18

62

11.4

Pre-

sent

in-

ven-

tion

TABLE 16

Hard-

Auste-

ness

tine

after

grain

Mechanical properties before quenching

quench-

size

Steel

Yield strength (MPa)

Tensile strength (MPa)

Total elongation (%)

r-value

ing

(size

Re-

Sheet

L

S

C

Δmax

L

S

C

Δmax

L

S

C

Δmax

L

S

C

Δmax

(HRc)

No.)

mark

78

418

414

424

10

520

515

524

9

33.4

34.9

34.5

1.5

1.00

1.17

0.98

0.19

62

11.2

Pre-

sent

in-

ven-

tion

79

385

380

390

10

518

515

520

5

28.0

24.8

28.2

3.4

1.18

0.92

1.25

0.33

66

8.4

Com-

para-

tive

ex-

am-

ple

80

385

400

394

15

489

500

494

11

25.7

23.2

25.0

2.5

1.12

0.88

1.22

0.34

49

12.2

Com-

para-

tive

ex-

am-

ple

81

406

410

415

9

519

522

526

7

25.3

24.0

26.7

2.7

1.18

1.01

1.42

0.41

66

9.1

Com-

para-

tive

ex-

am-

ple

82

384

397

392

13

492

500

497

8

35.8

34.3

35.6

1.5

1.18

0.93

1.32

0.39

50

12.1

Com-

para-

tive

ex-

am-

ple

83

405

397

389

16

500

509

511

11

27.0

22.4

27.4

5.0

1.24

0.90

1.27

0.37

63

11.1

Com-

para-

tive

ex-

am-

ple

84

386

398

406

20

486

496

503

17

33.4

31.9

34.8

2.9

0.81

1.16

0.93

0.35

62

11.4

Com-

para-

tive

ex-

am-

ple

85

418

412

425

13

521

516

524

8

33.2

35.1

34.5

1.9

1.02

1.23

0.86

0.37

61

11.5

Com-

para-

tive

ex-

am-

ple

86

402

393

388

14

512

509

515

6

35.7

34.9

34.3

1.4

1.24

0.95

1.25

0.30

53

11.8

Com-

para-

tive

ex-

am-

ple

87

406

395

394

12

514

510

517

7

35.5

34.7

34.1

1.4

1.11

0.86

1.19

0.33

52

12.0

Com-

para-

tive

ex-

am-

ple

88

421

417

431

14

523

518

525

7

33.3

34.8

34.3

1.5

1.00

1.26

0.92

0.34

65

10.0

Com-

para-

tive

ex-

am-

ple

89

375

363

369

12

482

490

486

8

34.3

35.4

34.0

1.4

1.17

0.99

1.40

0.41

56

11.8

Com-

para-

tive

ex-

am-

ple

90

338

350

331

19

517

528

524

11

34.5

32.4

33.6

2.1

1.13

0.83

1.29

0.42

54

11.9

Com-

para-

tive

ex-

am-

ple

TABLE 17

Integrated reflective intensity (222)

Steel

¼

½

sheet

Surface

thickness

thickness

Δ max

Remark

65

2.87

2.82

2.97

0.15

Present invention

66

2.83

2.86

2.94

0.11

Present invention

67

2.85

2.90

2.97

0.12

Present invention

68

2.75

2.81

2.86

0.11

Present invention

69

2.58

2.64

2.71

0.13

Present invention

70

2.84

2.91

2.96

0.12

Present invention

71

2.85

2.99

2.95

0.14

Present invention

72

2.73

2.85

3.02

0.29

Present invention

73

2.76

3.03

2.97

0.27

Present invention

74

2.78

2.92

3.04

0.26

Present invention

75

2.69

2.82

2.96

0.27

Present invention

76

2.50

2.64

2.75

0.25

Present invention

77

2.81

3.03

2.99

0.22

Present invention

78

2.79

2.87

3.03

0.24

Present invention

79

2.83

2.87

2.96

0.13

Comparative example

80

2.84

2.88

2.99

0.15

Comparative example

81

2.92

3.03

2.95

0.11

Comparative example

82

2.22

2.26

2.34

0.12

Comparative example

83

2.85

2.97

2.92

0.12

Comparative example

84

2.88

2.94

3.02

0.14

Comparative example

85

2.73

2.75

2.87

0.14

Comparative example

86

2.84

2.87

2.99

0.15

Comparative example

87

2.86

3.01

2.92

0.15

Comparative example

88

2.40

2.42

2.54

0.14

Comparative example

89

2.89

2.98

3.04

0.15

Comparative example

90

2.37

2.40

2.50

0.13

Comparative example

	Number	Date	Country
Parent	PCT/JP01/00404	Jan 2001	US
Child	09/961843		US

High carbon steel sheet

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (1)

Parent Case Info

Foreign Referenced Citations (6)

Non-Patent Literature Citations (7)

Continuations (1)

Number	Date	Country
5-9588	Jan 1993	JP
5-98388	Apr 1993	JP
6-271935	Sep 1994	JP
52-47512	Apr 1997	JP
10-152757	Jun 1998	JP
2000-328172	Nov 2000	JP

Entry
Fukui et al, “Formable High Carbon Cold Rolled Steel Sheet Utilizing Graphitization of Cementite”, Report of the ISIJ Meeting of Current Advances in Materials and Processes, vol. 1 (1988), p. 1729, Published by The Iron and Steel Institute of Japan (with English language translation).
JIS G 4051 (1979).
JIS G 4401 (1983).
JIS G 4802 (1983).
JIS G 4802:1999 (1999).
JIS G 4401:2000 (2000).
English language version only.