JP2000045031A - Manufacture of high carbon steel sheet excellent in formability and hardenability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of surely manufacturing a high carbon steel sheet capable of meeting the recent demand of users for simplification of a forming process and lowering in heat treatment temperature and shortening in time of heat treatment and excellent in formability and hardenability. SOLUTION: In the method of manufacturing a high carbon steel sheet having a component system specified by a carbon steel for machine structural use or a carbon tool steel stock or a cold rolled steel strip for spring, a steel is hot-rough-rolled and subjected to heating treatment at a temperature of >=Ar3, and the resultant slab is hot-rolled at a temp. of >=Ar3. The resultant hot-rolled steel plate is cooled at a cooling rate of >=5 deg.C/sec, held at a temp. of 560 to 650 deg.C for 2 to 10 sec to undergo short-time heat treatment, and cooled again at a cooling rate of >=7 deg.C/sec. Then, the steel plate after cooling is coiled at a temp. of <=600 deg.C, cold-rolled at a draft of >=30%, and annealed at a temperature between 580 deg.C and the Ac1 transformation point, thereby, cementite grains of <=1.1 μm average grain size and <=1.5 average aspect ratio and ferrite grains of >=2 μm average grain size can be formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a carbon steel for machine structure (JIS G 4051) and a carbon tool steel (JIS).
G 4401), cold-rolled steel strip for springs (JIS G 4
The present invention relates to a method for producing a high carbon steel sheet which does not contain a special alloy element such as Mo specified in 802) and has excellent formability and hardenability.

[0002]

2. Description of the Related Art Automobile parts such as tools, cutting tools or gears, and seat belt fittings are made of JIS G4.
051, JIS G 4401, JIS G 4802
A high-carbon steel sheet whose composition is specified in (1) is used, which is processed into a predetermined shape and then subjected to a heat treatment such as quenching and tempering.

In recent years, manufacturers of such tools and parts, that is, users of high-carbon steel sheets, have been studying simplification of the forming process and shortening of the heat treatment time and time in order to reduce costs. There is a strong demand for a high carbon steel sheet having excellent formability that can form a complicated shape in a small number of steps and excellent hardenability that can obtain a desired hardness even by heat treatment at a low temperature for a short time.

[0004] Therefore, various studies have hitherto been made to improve the formability and hardenability of a high carbon steel sheet. For example, JP-A-5-9588 discloses that a steel strip after hot rolling is cooled at a cooling rate of 10 ° C./sec or more for 20 to 50 seconds.
It is cooled to a temperature range of 0 ° C. and then reheated to a temperature range of 500 ° C. to (Ac ₁ transformation point + 30 ° C.) and wound at that temperature, and further cold-rolled to 650 ° C. to (Ac ₁ transformation point +
A method is disclosed in which heat treatment is performed in a temperature range of 30 ° C. for 1 hour or more to promote spheroidization of cementite, and to improve softness and ductility to improve moldability. In addition, Japanese Unexamined Patent Publication
JP-A-4-25946 and JP-A-8-246051 also propose a method of graphitizing carbon in steel to make it soft and highly ductile to improve formability.

[0005]

However, the present inventors have studied the method described in Japanese Patent Application Laid-Open No. Hei 5-9588, and found that it is possible to respond to the simplification of the molding process and the shortening of the heat treatment time and temperature by the user. A high carbon steel sheet excellent in such formability and hardenability may not always be obtained. In addition, the method of graphitizing carbon in steel described in JP-A-64-25946 and JP-A-8-246051 is not sufficient in a quenching treatment at a low temperature and a short time because the dissolution rate of graphite is low. There is a problem that it cannot be hardened.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and a carbon steel for machine structural use excellent in formability and hardenability which can respond to simplification of a forming process by a user and shortening of a heat treatment time to a short time. (JIS G 405
1) It is an object of the present invention to provide a method for reliably producing a high carbon steel sheet whose composition is defined by a carbon tool steel material (JIS G 4401) and a cold-rolled steel strip for a spring (JIS G 4802).

[0007]

In order to solve the above problems and achieve the object, the present invention uses the following means. (1) The production method of the present invention is a method for producing a high carbon steel sheet having a component system defined by carbon steel or carbon tool steel for machine structure or cold rolled steel strip for spring. the after hot rough rolling, and performing heat treatment in Ar ₃ or higher, (b) a step of hot rolling the heated slab at Ar ₃ transformation point or more of the temperature, (c) hot rolling The steel sheet after cooling is cooled at a cooling rate of 5 ° C./sec or more,
Performing a short-time heat treatment at a temperature of 0 ° C. for 2 to 10 seconds, and then cooling again at a cooling rate of 7 ° C./sec or more; and (d) winding the cooled steel sheet at a temperature of 600 ° C. or less. (E) cold rolling the rolled steel sheet at a rolling reduction of 30% or more, and (f) cold rolling the steel sheet to 58%.
Annealing at a temperature of 0 ° C. to the Ac ₁ transformation point to form cementite having an average particle size of 1.1 μm or less and an average aspect ratio of 1.5 or less and ferrite particles having an average particle size of 2 μm or more, This is a method for producing a high carbon steel sheet having excellent formability and hardenability.

(2) The production method of the present invention is to produce a high carbon steel sheet excellent in formability and hardenability as described in (1) above, wherein the surface of the steel sheet after annealing is subjected to a galvanizing treatment and then a phosphate treatment. Is the way.

Here, the average particle size of cementite, the average aspect ratio of cementite, and the average particle size of ferrite particles are measured as follows. a) Average particle size of cementite: A cross section formed in the rolling direction and the thickness direction was observed at 1500 times with an electron microscope,
About 300 cementites, the major axis, the minor axis (perpendicular to the major axis), and the lengths thereof in the direction of 45 ° are determined and averaged.

B) Average aspect ratio of cementite: A cross section formed in the rolling direction and the thickness direction and in the width direction and the thickness direction is observed at 1500 times with an electron microscope, and is observed at about 500 times.
The ratio between the lengths of the major axis and the minor axis (in the direction perpendicular to the major axis) of the cementite is determined and averaged.

C) Average grain size of ferrite grains: A cross section formed in the rolling direction and the thickness direction is observed at 500 times by an optical microscope, and the major axis and the minor axis (perpendicular to the major axis) of about 300 ferrite grains are observed. ) And their lengths in the 45 ° direction are averaged.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION JIS G 4051 and JIS G 440 allow the present inventors to simplify the molding process on the user side and to cope with shortening of the heat treatment time and temperature.
1. When the improvement of formability and hardenability of a high carbon steel sheet having a component system specified by JIS G4802 was examined, a step of performing a heat treatment at a temperature of Ar ₃ or more after hot rough rolling and a step of It was found that the cooling conditions and the morphology of cementite and ferrite grains after cold rolling and annealing play important roles.

[0013] Based on this finding, the present inventors have established JIS
G 4051, JIS G 4401, JIS G
The average grain size and aspect ratio of cementite are controlled by controlling the heat treatment of the coarse bar after hot rough rolling of the high carbon steel sheet defined by the component 4802, the cold rolling after hot rolling, and the annealing conditions.
By controlling the average grain size of ferrite grains within a certain range, a high-carbon steel sheet (JIS G 4051) having excellent formability and hardenability capable of responding to simplification of the forming process and shortening of the heat treatment time and time by the user. , JIS G
4401, JIS G 4802), and found a method capable of reliably producing the same, thereby completing the present invention.

An embodiment of the present invention will be described below. The heat treatment aims to make the γ grain size of the steel sheet uniform during hot rolling, and to make the pearlite uniform after transformation, thereby reducing the variation in the cementite grain size and ferrite grain size after final annealing, ductility and quenching. Improve the performance. Actually, the heating is performed at least once after the rough rolling, before the finish rolling, or during the finish rolling, and the heating temperature is set to Ar ₃ or more to uniform the γ grain size. The heating time is desirably at least 3 seconds or more. In addition, the heat treatment raises the temperature,
Includes cooling and maintaining temperature.

1) Hot Rolling In the hot rolling, when the α-phase rolling is performed at a temperature lower than the Ar ₃ transformation point, ferrite grains are coarsened and a non-uniform structure of ferrite and pearlite which is not preferable for hardenability is formed. Therefore, it is necessary to perform the process at the Ar ₃ transformation point or higher.

In the hot rolling, a slab produced by continuous casting or ingot-bulking rolling of molten steel whose composition has been adjusted may be directly rolled, or may be rolled through a heating furnace. . 2) Cooling after hot rolling The steel sheet after hot rolling has a structure in which fine pearlite is uniformly distributed in order to form a structure preferable for hardenability in the subsequent spheroidizing annealing. It is necessary. For this purpose, the pearlite transformation nose may be in the temperature range during cooling after hot rolling, and the steel sheet may be maintained in a temperature range near the nose to start the pearlite transformation in a short time. Specifically, JIS G
4051, JIS G 4401, JIS G 480
Since the pearlite nose of the high carbon steel having the component system defined in 2 is in the temperature range of 560 to 650 ° C, it is cooled at 5 ° C / sec after hot rolling, and is cooled to 2 to 10 in the temperature range of 560 to 650 ° C. After holding for 2 seconds, the film may be cooled again at a cooling rate of 7 ° C./second or more and wound.

The cooling rate after hot rolling is less than 5 ° C./sec, or the cooling rate after holding for a short time in the nose is 7 ° C.
When the holding time in the temperature range of 560 to 650 ° C. exceeds 10 seconds, the pearlite becomes coarse. If the holding time in the temperature range of 560 to 650 ° C. is less than 2 seconds, the pearlite transformation does not end in this temperature range, and a structure in which fine pearlite is uniformly distributed cannot be obtained.

The cooling rate after maintaining the temperature in the range of 560 to 650 ° C. is desirably 30 ° C./sec or less so as not to deteriorate the coil shape at the time of winding. Also, 5
The holding in the temperature range of 60 to 650 ° C. does not necessarily have to be performed at a constant temperature in this temperature range, and it is sufficient that the holding in this temperature range is 2 to 10 seconds even if there is a temperature gradient. Actually, the temperature can be maintained for a short time in this temperature range by, for example, suspending the cooling of the hot rolling squirt by watering for a short time in this temperature range, that is, by stopping the watering and cooling naturally.

3) Regarding the winding temperature When the winding temperature exceeds 600 ° C., the pearlite becomes coarse.

Preferably, the coil is wound at 480 ° C. or higher so as not to deteriorate the shape of the coil during winding. 4) Cold Rolling-Average Grain Size, Average Aspect Ratio and Average Ferrite Grain Size of Cementite after Annealing A slab of a component system equivalent to S65C-CSP according to JIS G 4802 is heated to 10 by a bar heater before finish rolling.
After performing a heat treatment at 10 ° C. for 15 seconds and hot rolling at a temperature of 820 ° C., the sample was cooled to a temperature of 550 to 660 ° C. at a cooling rate of 7 ° C./second, and kept at that temperature range for 1 to 12 seconds. Thereafter, it was cooled at a cooling rate of 10 ° C./sec and wound at a temperature of 550 ° C. Thereafter, cold rolling is performed at a rolling reduction of 20 to 60%,
Annealing was performed at a temperature of 550 to 720 ° C. for 20 hours to produce a high carbon steel sheet having a thickness of 1.2 mm. Then, the average particle size of cementite, the average aspect ratio, and the average particle size of ferrite particles were measured by the above methods. In addition, JIS No. 5 test pieces were cut out along the 0 °, 45 °, and 90 ° directions with respect to the rolling direction, a tensile test was performed at a tensile speed of 10 mm / min, and the total elongation in each direction was obtained to evaluate formability. did. Further, a test piece cut into a size of 50 × 100 mm was heated at 760 ° C. for 10 seconds and then quenched in oil at 20 ° C., and the Rockwell C scale hardness (HRC) on the steel plate surface was measured to evaluate hardenability. did.

According to JP-A-5-9588, a component system similar to that of S65C-CSP is used, and the plate thickness is 1.
The average of the total elongation of the 2 mm spheroidized annealed material is at most about 31%, and the average of the hardness after quenching is at most 55% by HRC.
Therefore, the condition for obtaining a total elongation of 35% or more and a hardness after quenching of 62 or more in HRC was determined as the present invention.

FIG. 1 shows the relationship between the total elongation and the average aspect ratio of cementite and the average grain size of ferrite grains. When the average aspect ratio of cementite is 1.5 or less,
When the average particle size of the ferrite particles is 2 μm or more, the total elongation becomes 35% or more, and it can be seen that excellent moldability can be obtained.

FIG. 2 shows the relationship between the hardness after quenching and the average particle size of cementite. The average particle size of cementite is 1.
When it is 1 μm or less, the hardness after quenching is 62 or more by HRC, and it can be seen that excellent hardenability can be obtained stably even by heating for a short time.

At this time, the rolling reduction during cold rolling is 30%.
If it is less than 10, pearlite is not sufficiently broken, and fine cementite having an average particle size of 1.1 μm or less and an average aspect ratio of 1.5 or less cannot be obtained after annealing. Therefore, the content needs to be 30% or more. The upper limit is not particularly defined, but is preferably set to 70% or less so as not to increase the load on the rolling mill.

If the annealing temperature after cold rolling is lower than 580 ° C., an unrecrystallized structure may remain and the steel may become hard and low ductile. Moreover, Ac ₁ exceeds the transformation point to the pearlite generates annealing, since significantly inhibit moldability and hardenability, it is necessary to below Ac ₁ transformation point.

When the surface of the steel sheet manufactured in this way is subjected to a phosphate treatment after galvanizing, even if a gear such as an automobile part is formed by a high surface pressure reduction method such as cold forging or ironing, a mold galling occurs. And cracks are less likely to occur. In addition,
Galvanization can be performed by an electrogalvanizing method, a hot-dip galvanizing method, or the like.

[0027]

EXAMPLES (Example 1) S35 of JIS G 4051
Component system equivalent to C (C: 0.36% by weight, Si: 0.
18%, Mn: 0.74%, P: 0.008%, S:
(0.007%, Al: 0.01%) slab is produced by continuous casting, heated to 1250 ° C., and then heated under the conditions shown in Table 1—hot rolling—primary cooling—short time heat treatment—secondary cooling -Winding-Cold rolling-Annealing in order, thickness 1.2m
m samples of 21 types were prepared. Here, the primary cooling rate and the secondary cooling rate in Table 1 were changed by changing the sprinkling conditions. The short-time heat treatment was performed by stopping watering for the time shown in the table when the temperature reached the temperature shown in the table.

With respect to these samples, the average particle size and average aspect ratio of cementite, the average particle size of ferrite particles, the total elongation, the hardness after quenching (heating temperature:
820 ° C).

The results are shown in Table 1 (Nos. 1 to 11: Examples of the present invention, Nos. 12 to 22: Comparative examples). In the samples prepared by the method of the present invention (Examples Nos. 1 to 11 of the present invention), a total elongation of 37% or more and a hardness after quenching of 52 or more by HRC were obtained. In comparison with an average total elongation of around 30% and a hardness after quenching HRC of around 40 when the high carbon steel sheet of the present invention is manufactured by a conventional method, the steel sheet shows more excellent formability and hardenability.

On the other hand, comparative samples (Comparative Examples No. 12 to No. 22) produced by a method out of the range of the present invention can obtain only moldability and hardenability equal to or lower than those produced by the conventional method. Absent.

[0031]

[Table 1]

(Example 2) S6 of JIS G 4802
Component system corresponding to 5C-CSP (C: 0.65% by weight%,
Si: 0.21%, Mn: 0.76%, P: 0.007
%, S: 0.007%, Al: 0.01%) slab is manufactured by continuous casting, heated to 1280 ° C., and then heat-treated under hot rolling, primary rolling, and short-time heat treatment under the conditions shown in Table 2. -Secondary cooling-winding-cold rolling-annealing were sequentially performed to produce 21 types of 1.2 mm-thick samples. Here, the conditions for the primary cooling, the short-time heat treatment, and the secondary cooling were set in Example 1.
Performed in the same manner as

For these samples, the average particle size and average aspect ratio of cementite, the average particle size of ferrite particles, the total elongation, the hardness after quenching (heating temperature:
750 ° C.).

The results are shown in Table 2 (Nos. 23 to 33: Examples of the present invention, Nos. 34 to 44: Comparative examples). In the samples prepared by the method of the present invention (Examples Nos. 23 to 33 of the present invention), a total elongation of 35% or more and a hardness after quenching of 62 or more by HRC were obtained. In comparison with a high carbon steel sheet manufactured by the conventional method, an average total elongation of about 30% and a hardness HRC after quenching of about 50, more excellent formability and hardenability are exhibited.

On the other hand, comparative samples (Comparative Examples Nos. 34 to 44) produced by a method out of the range of the present invention can obtain only moldability and hardenability equal to or less than those produced by the conventional method.

[0036]

[Table 2]

(Example 3) 1 (Example of the present invention)
The surface of the sample was electrogalvanized (Zn: 21).
g / m ² ) and then phosphated (P coating: 1.95 mg /
m ² ) to perform a friction sliding test.

FIG. 3 shows the friction sliding device used for the test. After the test piece 2 is set on the sample table 3 which can move horizontally on the roller leveler 4, the roller leveler 4 is lifted upward by the hydraulic cylinder 6, and the test piece 2 is pressed by the pressing force of the indenter 1, and the sample table 3 is pressed. Move horizontally. At this time, indenter 1
Since the horizontal force applied between the test piece 2 and the surface of the test piece 2 is measured by the pull-out force measuring load cell 7 and the vertical force applied by the pressing force measuring load cell 5, the friction coefficient can be measured.

The test was performed under the test conditions shown in the figure.
FIG. 4 shows the relationship between the pressing force and the friction coefficient. If the surface is electrogalvanized and then phosphated, the friction coefficient increases sharply with a higher pressing force and galling occurs, compared to a sample without such surface treatment (bare material). . From these results, it can be said that when a phosphate treatment is performed after electrogalvanizing the surface, mold galling and cracking hardly occur even when the surface is formed by a method under high surface pressure such as cold forging or ironing.

[0040]

As described above, according to the present invention, by specifying the steel structure and the manufacturing conditions, excellent formability and hardenability that can respond to the simplification of the forming process and the reduction of the heat treatment time and time can be achieved. A method for stably producing a high carbon steel sheet can be provided. If the surface is subjected to a phosphate treatment after galvanizing, the formability under a high surface pressure such as cold forging or ironing can be improved.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between the total elongation, the average aspect ratio of cementite, and the average grain size of ferrite grains according to an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between hardness after quenching and average particle size of cementite according to the embodiment of the present invention.

FIG. 3 is a view showing a friction sliding device used in a friction sliding test according to the embodiment of the present invention.

FIG. 4 is a diagram showing a relationship between a pressing force and a friction coefficient according to the embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Indenter, 2 ... Test piece, 3 ... Sample stand, 4 ... Roller leveler, 5 ... Load cell for measuring pressing force, 6 ... Hydraulic cylinder, 7 ... Load cell for measuring pulling force.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Katsutoshi Ito 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Yoshihide Ishii 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Sun Inside the Honko Co., Ltd. (72) Inventor Noboru Shioya 1-2-1, Marunouchi, Chiyoda-ku, Tokyo F-term in the Nippon Kokan Co., Ltd. 4K037 EA01 EA06 EA15 EA23 EA25 EA27 EC01 FA03 FC03 FC04 FC07 FD02 FD03 FD08 FE01 FE02 FE06 FG01 FH03 FJ04 FJ05 GA02 GA05 JA06 4K044 AA02 AB02 BA10 BA17 BB03 BB09 BC01 BC06 CA18 CA64

Claims (2)

[Claims] 1. A method for producing a high carbon steel sheet having a component system defined by a carbon steel for machine structural use, a carbon tool steel or a cold rolled steel strip for a spring, comprising the steps of: Later, a step of performing a heat treatment at a temperature of Ar ₃ or higher; (b) a step of hot rolling the heat-treated slab at a temperature of the Ar ₃ transformation point or higher; Cooling at a cooling rate of not less than 7 ° C./sec, performing a short-time heat treatment at a temperature of 560 to 650 ° C. for 2 to 10 seconds, and cooling again at a cooling rate of 7 ° C./sec or more; ) A step of winding the steel sheet after cooling at a temperature of 600 ° C. or lower; (e) a step of cold rolling the steel sheet after winding at a rolling reduction of 30% or more; and a step of annealing at a temperature of 580 ° C. to Ac ₁ transformation point, the average particle size of the average a below 1.1μm And aspect ratio of 1.5 or less of cementite and an average particle size to form a more ferrite grains 2 [mu] m,
Method for producing high carbon steel sheet with excellent formability and hardenability. 2. After galvanizing the steel sheet surface after annealing,
The method for producing a high-carbon steel sheet having excellent formability and hardenability according to claim 1, which is subjected to a phosphate treatment.