JP3446004B2 - Method for producing high carbon steel sheet having high ductility and high hardenability - Google Patents

Description

Description: TECHNICAL FIELD [0001] The present invention relates to JIS G40.
Includes special alloy elements such as Ni, Cr, and Mo specified in JIS G 4401 (carbon tool steel), JIS G 4802 (cold rolled steel strip for spring) No high carbon steel sheet manufacturing method. 2. Description of the Related Art Automobile parts such as tools, blades or gears, and seat belt fittings are used as raw materials according to 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 processing steps and shortening of heat treatment time and time in order to reduce costs. There is a strong demand for high carbon steel sheets having excellent workability for processing complex shapes in a small number of steps, in particular, high ductility and high hardenability to obtain a desired hardness even by heat treatment at low temperature for a short time. [0004] Therefore, various studies have hitherto been made to increase the ductility and the quenchability of the 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 for 1 hour or more in a temperature range of 30 ° C.) to promote spheroidization of cementite to achieve softness and high ductility. JP-A-64-25946 and JP-A-8-246051 also propose a method of graphitizing carbon in steel to achieve softness and high ductility. [0005] However, the present inventors have studied the method described in Japanese Patent Application Laid-Open No. Hei 5-9588. In some cases, a steel sheet having high ductility and high hardenability that can cope with the above problem cannot always be obtained. In addition, Japanese Patent Application Laid-Open Nos.
The method of graphitizing carbon in steel described in Japanese Patent No. 246051 has a problem that the rate of dissolution of graphite is low, so that it cannot be sufficiently hardened in a quenching treatment at a low temperature for a short time, resulting in poor hardenability. The present invention has been made to solve such a problem, and has high ductility and high hardenability JIS G 4051 and JIS which can respond to simplification of a processing step and low-temperature and short-time heat treatment by a user. G 44
01, It is an object of the present invention to provide a method capable of reliably producing a high-carbon steel sheet defined by JIS G4802. Means for Solving the Problems The above problems are solved by JIS G
4051 (Carbon steel for machine structure) , JIS G4401
(Carbon tool steel) , JIS G 4802 (Cold for spring )
In producing a high carbon steel sheet having a component system defined by a rolled steel strip) , a hot-rolled steel sheet is heated at 640 to 710 ° C. for 20 to 4 times.
0 h (10 to 30 h above 700 ° C)
The <br/> steel sheet and the spheroidization ratio of the cementite is 80% or more is subjected to blunt, after cold rolling at a reduction rate of more than 30%, 600 ° C. ~
Annealing in the temperature range of the Ac ₁ transformation point, cementite having an aspect ratio of 1.5 or less and an aspect ratio of 1.3.
The problem is solved by the following method for producing a high carbon steel sheet having high ductility and high hardenability, which forms ferrite grains. Here, the spheroidization ratio of cementite and the aspect ratio of cementite and ferrite grains are measured as follows. 1) Spheroidization ratio of cementite: The cross section formed in the rolling direction and the thickness direction is 1500 by an electron microscope.
Observation is performed at × ² , and the area percentage of spherical cementite and lamellar cementite in a visual field of 0.5 mm ² is determined by a linear analysis method. 2) Aspect ratio of cementite: The cross sections formed in the rolling direction and the thickness direction and in the width direction and the thickness direction are observed at 1500 times with an electron microscope, and the major axis and the minor axis (perpendicular to the major axis) of about 500 cementites are observed. ) Determine the length ratio and average. 3) Aspect ratio of ferrite grains: JIS G052
2 is determined in the same manner as in the elongation. [0009] JIS G 4051, JIS G 4401, JIS G 4051, etc., so that the present inventors can cope with the simplification of the processing steps on the user side and the shortening of the heat treatment at low temperature and short time.
Examination of the high-ductility and high-quenching properties of the high-carbon steel sheet having the component system defined by G4802 requires a cold rolling-annealing step. It became clear that the spheroidization ratio and the aspect ratio of ferrite grains after cold rolling and annealing, and the aspect ratio of cementite after cold rolling and annealing had to be optimized for high hardenability. . The details will be described below. [0010] A steel sheet having a thickness of 2 mm, which has been hot rolled by changing the finishing temperature and the winding temperature using a steel having a component system of S35C according to JIS G 4051, is subjected to heat treatment at different temperatures and times, and the spheroidization ratio of cementite is changed. Then, the sample was cold-rolled to a thickness of 1 mm at a rolling reduction of 50%, and annealed at different temperatures and times to produce a sample. Then, the spheroidization ratio of cementite before cold rolling, the aspect ratio of cementite after final annealing, and the aspect ratio of ferrite grains were measured by the above-described methods. Further, a JIS No. 5 test piece was cut out along the rolling direction, a tensile test was performed at a tensile speed of 10 mm / min, and the total elongation was determined to evaluate the ductility. In addition, 50 × 100mm
After heating at 820 ° C. for a short period of 10 seconds, the test piece cut into a size of was quenched in oil at 20 ° C., and the Rockwell C scale hardness (HRC) on the steel plate surface was measured to evaluate hardenability. Incidentally, Japanese Patent Application Laid-Open No. Hei 5-9588 discloses S
The total elongation of the spheroidized annealed material having a component system equivalent to 35C and a plate thickness of 1 mm is described as about 35%.
When a steel sheet of a component system equivalent to C is sufficiently heated and quenched, the hardness after quenching is about 50 in HRC. Therefore, the conditions for obtaining a total elongation of 40% or more and a hardness after quenching of 50 or more in HRC were set as the present invention. FIG. 1 shows the relationship between the total elongation and the spheroidization ratio of cementite before cold rolling and the aspect ratio of ferrite grains after annealing. The spheroidizing ratio of cementite before cold rolling is 8
0% or more, the aspect ratio of ferrite grains after annealing is 1.
When it is 3 or less, the total elongation is 40% or more, and it can be seen that high ductility can be reliably obtained. FIG. 2 shows the relationship between the hardness after quenching and the aspect ratio of cementite after annealing. When the aspect ratio of the cementite after annealing is 1.5 or less, the hardness after quenching becomes 50 or more by HRC, and it can be seen that high hardenability can be reliably obtained even by heating for a short time. The spheroidizing ratio of cementite before cold rolling is 8
The method for producing steel sheets of 0% or more is to form molten steel whose composition has been adjusted into a slab by continuous casting or ingot-forming / bulking rolling, and to spheroidize hot-rolled steel sheets that have been hot-rolled directly or through a heating furnace. Can be manufactured by annealing. If the rolling reduction during cold rolling is less than 30%, ferrite grains after annealing become coarse and ductility deteriorates.
It needs to be 30% or more. The upper limit is not particularly defined, but is preferably set to 80% or less so as not to increase the load on the rolling mill. If the annealing temperature after the cold rolling is lower than 600 ° C., the unrecrystallized structure may remain and the steel may become hard and have low ductility. Moreover, Ac ₁ exceeds the transformation point when annealed pearlite generates, since significantly inhibit ductility and hardenability, it is necessary to below Ac ₁ transformation point. (Embodiment 1) S70 of JIS G 4802
Component system equivalent to C-CSP (C: 0.71, wt.%, S
i: 0.19, Mn: 0.75, P: 0.01, S:
A slab made of steel of 0.003, Al: 0.01, N: 0.0040) is manufactured by continuous casting, heated to 1270 ° C., and then hot-rolled at a finishing temperature of 800 to 900 ° C.
Up to 700 ° C, pickling, 560 to 720 ° C and 20 to 1
By performing box annealing for 20 hours, steel sheets having different spheroidization rates of cementite were produced. Next, the steel sheet was rolled with a reduction ratio of 20 to
Cold rolling was performed at 60%, and box annealing was performed at 580 to 720 ° C for 4 to 40 hours to prepare samples having different aspect ratios of cementite and ferrite grains. The thickness of the sample was adjusted to 1.2 mm by adjusting the thickness after hot rolling and the rolling reduction in cold rolling. The spheroidization ratio of cementite before cold rolling, the aspect ratio of cementite after final annealing, the aspect ratio of ferrite grains, the total elongation, and the hardness after quenching were measured by the above-described methods. Since this sample had a high C content, the heating temperature in the quenchability test was set to 750 ° C. The results are shown in Table 1. All of the samples produced by the method of the present invention have a total elongation of 35% or more, a hardness after quenching of 60 or more by HRC, and a high-carbon steel sheet having the same composition and thickness as the conventional method. Shows higher ductility and hardenability compared to an average total elongation of around 30% and a hardness HRC after quenching of around 50. On the other hand, a comparative sample produced by a method other than the method of the present invention can obtain only ductility and hardenability equal to or less than those produced by a conventional method. [Table 1] (Embodiment 2) S4 of JIS G 4051
Component system equivalent to 5C (C: 0.44, Si: 0.
19, Mn: 0.76, P: 0.01, S: 0.00
8, Al: 0.01, N: 0.003) steel slab is manufactured by continuous casting, heated to 1180 ° C, hot-rolled at a finishing temperature of 820 to 900 ° C, and 500 to 700 ° C.
After winding and pickling, box annealing was performed at 560 to 720 ° C. for 20 to 120 hours to produce steel sheets having different spheroidizing rates of cementite. Next, this steel sheet was cold-rolled at a rolling reduction of 20 to 70%, and box-annealed at 580 to 720 ° C. for 4 to 40 hours to prepare samples having different aspect ratios of cementite and ferrite grains. The thickness of the sample is adjusted by adjusting the thickness after hot rolling and the rolling reduction of cold rolling.
Each was set to 2.3 mm. Then, the same measurement as in Example 1 was performed. Table 2 shows the results. All of the samples prepared by the method of the present invention have a total elongation of 40% or more, a hardness after quenching of 50 or more by HRC, and a high carbon steel sheet having the same composition and thickness as the conventional method. It shows higher ductility and hardenability as compared with an average total elongation of about 35% when manufactured and a hardness HRC after quenching of about 40. On the other hand, in the comparative sample produced by a method other than the method of the present invention, only the ductility and hardenability equal to or less than those produced by the conventional method can be obtained. [Table 2] Since the present invention is configured as described above, JIS G 4051 having high ductility and high hardenability that can respond to the simplification of the processing steps and the low-temperature and short-time heat treatment by the user. , JIS G 4401,
It is possible to provide a method capable of reliably producing a high-carbon steel sheet defined by JIS G4802.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relationship between the total elongation, the spheroidization ratio of cementite before cold rolling, and the aspect ratio of ferrite grains after annealing. FIG. 2 is a diagram showing a relationship between hardness after quenching and aspect ratio of cementite after annealing.

──────────────────────────────────────────────────続 き Continued from the front page (72) Inventor Katsutoshi Ito 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-59-28527 (JP, A) JP-A-4 -116137 (JP, A) JP-A-59-205417 (JP, A) JP-A-55-85626 (JP, A) JP-B-63-14045 (JP, B1) (58) Fields investigated (Int. . ^7, DB name) C21D 9/46 - 9/48 C21D 8/00 - 8/04 C22C 38/00 - 38/60

Claims (1)

(57) [Claims] [Claim 1] JIS G 4051 (Carbon steel for machine structure), JIS G4401 (Carbon tool steel), JIS
When manufacturing a high carbon steel sheet having a composition specified by G 4802 (cold rolled steel strip for spring), 6
20 to 40 hours at 40 to 710 ° C (however,
10-30 h) spheroidizing annealing is performed to form a steel sheet having a spheroidizing rate of cementite of 80% or more. After cold rolling at a rolling reduction of 30% or more , the steel sheet is annealed in a temperature range of 600 ° C. to the Ac ₁ transformation point. And a method for producing a high carbon steel sheet having high ductility and high quenchability to form cementite having an aspect ratio of 1.5 or less and ferrite grains having an aspect ratio of 1.3 or less.