# Patent

## Title
High carbon steel and production method thereof

## Abstract

To provide a high carbon steel sheet having excellent hardenability and toughness, and low planar anisotropy of tensile properties affecting workability, and a method of producing the same.
A high car 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), wherein more than 50 carbides having a diameter of 1.5 μm or larger exist in 2500 μm2, 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, and the Δr is more than −0.15 to less than 0.15, herein Δr=(r0+r90−2×r45)/4, and 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.

## Background
This application is a divisional application of application Ser. No. 09/961,843 filed Sep. 24, 2001 now U.S. Pat. No. 6,652,671, which is a continuation application of International Application PCT/JP01/00404 filed Jan. 23, 2001.


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-AP-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 directions 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+rπ−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 μm2 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 μm2 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 Δmax 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 G 4802, 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 μm2. The reason for preparing the observing field area of 2500 μm2 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 μm2 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 therefore 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×T1≦T2≦1202−0.80×T1 . . .   (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×T1≦T2≦1210−0.80×T1 . . .   (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, 1⁄4 thickness and 1⁄2 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









Integrated reflective intensity (222)













Reheating of
Primary
Secondary

1⁄4




sheet bar
annealing
annealing

thick-
1⁄2


(° C. × sec)
(° C. × hr)
(° C. × hr)
Surface
ness
thickness
Δmax
















1010 × 15
640 × 40
610 × 40
2.81
2.95
2.89
0.14


1010 × 15
640 × 40
650 × 40
2.82
2.88
2.95
0.13


1010 × 15
640 × 40
690 × 40
2.90
2.91
3.02
0.12


1010 × 15
680 × 40
610 × 40
2.37
2.35
2.46
0.11


1010 × 15
680 × 40
650 × 40
2.40
2.36
2.47
0.11


1010 × 15
680 × 40
690 × 40
2.29
2.34
2.39
0.10


—
640 × 40
610 × 40
2.70
3.01
2.90
0.31


—
640 × 40
650 × 40
2.75
2.87
2.99
0.24


—
640 × 40
690 × 40
2.81
2.90
3.05
0.24


—
680 × 40
610 × 40
2.34
2.27
2.50
0.23


—
680 × 40
650 × 40
2.39
2.23
2.51
0.28


—
680 × 40
690 × 40
2.25
2.37
2.45
0.20









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
Remark


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










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



























sheet
L
S
C
Δmax
L
S
C
Δmax
L
S
C
Δmax
L
S
C
Δr
(HRc)
No.)
Remark





























A
395
391
393
4
506
502
507
5
35.7
36.4
35.9
0.7
1.06
0.97
1.04
0.04
52
11.6
Present





















inven-





















tion


B
405
404
411
7
504
498
507
9
35.8
36.8
36.2
1.0
1.12
0.98
1.23
0.10
54
11.3
Present





















inven-





















tion


C
409
406
414
8
509
505
513
8
35.2
36.4
35.3
1.2
0.98
1.19
1.05
−0.09
56
10.7
Present





















inven-





















tion


D
369
362
370
8
499
496
503
9
30.1
29.3
31.0
1.7
1.16
0.92
1.33
0.16
57
8.6
Compa-





















rative





















example


E
370
379
375
9
480
484
481
4
36.9
36.0
36.4
0.9
1.15
0.96
1.47
0.18
44
12.2
Compa-





















rative





















example


F
374
377
385
11
474
480
488
14
35.7
34.6
36.3
1.7
1.25
0.96
1.46
0.20
53
11.2
Compa-





















rative





















example


G
372
376
379
7
496
493
498
5
38.0
37.7
37.7
0.3
1.14
0.94
1.64
0.23
40
12.1
Compa-





















rative





















example


H
317
334
320
17
501
516
510
15
36.5
34.6
35.5
1.9
1.12
0.92
1.35
0.16
49
11.6
Compa-





















rative





















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 Δmax 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 0.6 μm


sheet
(° C.)
(° C. × hr)
rate (%)
(° C. × hr)
range by the formula (1)
than 1.5 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
500
640 × 20
60
640 × 20
640–690
45
88
Comparative










example


19
620
—
50
690 × 40
—
51
67
Comparative










example





















TABLE 5










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



























sheet
L
S
C
Δmax
L
S
C
Δmax
L
S
C
Δmax
L
S
C
Δmax
(HRc)
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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven





















tion


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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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 0.6 μm


sheet
(° C.)
(° C. × hr)
rate (%)
(° C. × hr)
range by the formula (1)
than 1.5 μm
(%)
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
610
690 × 40
50
650 × 40
610–650
96
77
Comparative










example


38
620
—
50
690 × 40
—
100
65
Comparative










example





















TABLE 7










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



























sheet
L
S
C
Δmax
L
S
C
Δmax
L
S
C
Δmax
L
S
C
Δmax
(HRc)
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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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
Compa-





















rative





















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







Coiling

Cold

Secondary

Ratio of carbides




Reheating of
tempe-
Primary
reduc-
Secondary
annealing range
Number of
smaller than


Steel
sheet bar
rature
annealing
tion
annealing
by the formula
carbides larger
0.6 μm


sheet
(° C. × sec)
(° C.)
(° C. × hr)
rate (%)
(° C. × hr)
(1)
than 1.5 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


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 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 0.6


sheet
(° C. × sec)
(° C.)
(° C. × hr)
rate (%)
(° C. × hr)
(l)
than 1.5 μm
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










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



























sheet
L
S
C
Δmax
L
S
C 
Δmax
L
S
C 
Δmax
L
S
C
Δmax
(HRc)
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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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
11.8
Present





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion


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





















inven-





















tion





















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



























sheet
L
S
C
Δmax
L
S
C 
Δmax
L
S
C 
Δmax
L
S
C
Δmax
(HRc)
No.)
Remark





























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
Present





















inven-





















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-





















parative





















example


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-





















parative





















example


55
392
396
399
7
512
509
515
6
27.2
25.4
28.2
2.8
1.33
1.04
1.38
0.32
58
9.0
Com-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


63
411
406
415
9
515
511
515
8
35.1
38.5
36.0
1.4
1.02
1.32
1.00
0.32
57
9.4
Com-





















parative





















example


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-





















parative





















example



















TABLE 12









Integrated reflective intensity (222)













Steel

1⁄4
1⁄2




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)
(l)
than 1.5 μm
μm (%)
Remark







65
1050 × 15
580
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
620–686
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)
(l)
than 1.5 μm
μm (%)
Remark
























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 × 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



























sheet
L
S
C
Δmax
L
S
C 
Δmax
L
S
C 
Δmax
L
S
C
Δmax
(HRc)
No.)
Remark





























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
Present





















inven-





















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
Present





















inven-





















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
Present





















inven-





















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
Present





















inven-





















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
Present





















inven-





















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
Present





















inven-





















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
Present





















inven-





















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
Present





















inven-





















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
Present





















inven-





















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
Present





















inven-





















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
Present





















inven-





















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
Present





















inven-





















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
Present





















inven-





















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



























sheet
L
S
C
Δmax
L
S
C 
Δmax
L
S
C 
Δmax
L
S
C
Δmax
(HRc)
No.)
Remark





























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
Present





















inven-





















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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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-





















parative





















example


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
58
11.8
Com-





















parative





















example


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-





















parative





















example



















TABLE 17









Integrated reflective intensity (222)













Steel

1⁄4
1⁄2




sheet
Surface
thickness
thickness
Δ max
Remark





65
2.87
2.82
2.97
0.15
Present invention


68
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

## Claims

1. A method of producing a high carbon steel sheet, 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,
annealing the descaled steel sheet at 640 to 690° C. for 20 hr or longer (primary annealing),
cold rolling the annealed steel sheet at a reduction rate of 50% or more, and
annealing the cold rolled steel sheet at 620 to 680° C. (secondary annealing), and
wherein the temperature T1 of the primary annealing and the temperature T2 of the secondary annealing satisfy the following formula (1),


1024−0.6×T1≦T2≦1202−0.80×T1 . . .   (1).

2. A method of producing a high carbon steel sheet, 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 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,
annealing the descaled steel sheet at a temperature T1 of 630 to 700° C. for 20 hr or longer (primary annealing),
cold rolling the annealed steel sheet at a reduction rate of 50% or higher, and
annealing the cold rolled steel sheet at a temperature T2 of 620 to 680° C. (secondary annealing),
wherein the temperature T1 and the temperature T2 satisfy the following formula (2),


1010−0.59×T1≦T2≦1210−0.80×T1 . . .   (2).

3. A method of producing a high carbon steel sheet, 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 during reheating the rolled sheet bar to Ar3 transformation point or higher,
coiling the finish roiled steel sheet at 500 to 650° C.,
descaling the coiled steel sheet,
annealing the descaled steel sheet at a temperature T1 of 630 to 700° C. for 20 hr or longer (primary annealing),
cold rolling the annealed steel sheet at a reduction rate of 50% or higher, and
annealing the cold rolled steel sheet at a temperature T2 of 620 to 680° C. (secondary annealing),
wherein the temperature T1 and the temperature T2 satisfy the following formula (2)


1010−0.59×T1≦T2≦1210−0.80×T1 . . .   (2).