JP2000045029A - Manufacture of high carbon steel sheet with high ductility and high hardenability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of surely manufacturable a high carbon steel sheet capable of meeting the recent demand of users for simplification of a working process and lowering in temperature and shortening in time of heat treatment, having high ductility and high hardenability, and having a composition specified by a carbon steel for machine structural use, a carbon tool steel stock, or a cold-rolled steel strip for spring. SOLUTION: At the time 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 plate where hot rolling is finished by performing heating treatment at a temperature of >=Ar3 after hot roughing and spheroidal comentite rate is regulated to >=80% is cold rolled at a draft of >=30% and then annealed in the temperature range between 600 deg.C and the Ac1 transformation point to form cementite grains of <=1.5 aspect ratio and ferrite grains of <=1.3 aspect ratio.

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 having high ductility and high hardenability, which does not contain a special alloy element such as Mo specified in 802).

[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 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 inventors of the present invention 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 processing steps and the reduction of the heat treatment time and time for the user. Steel sheets having such high ductility and high hardenability may not 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.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and a carbon steel for machine structural use having high ductility and high hardenability capable of responding to a simplification of a working process and a reduction in heat treatment time and time for a user. (JIS G 405
1) It is an object of the present invention to provide a method capable of 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 a carbon steel for machine structure, a carbon tool steel, or a cold-rolled steel strip for a spring. After the rough rolling, a step of performing a heat treatment at a temperature of Ar ₃ or more to terminate hot rolling, and, in the hot-rolled steel sheet, a steel sheet having a spheroidizing ratio of cementite of 80% or more is used.
%, And a step of annealing the cold-rolled steel sheet in a temperature range from 600 ° C. to the Ac ₁ transformation point, wherein the aspect ratio is 1.5 or less. This is a method for producing a high carbon steel sheet having high ductility and high hardenability, which forms ferrite grains having a ratio of 1.3 or less.

Here, the spheroidization ratio of cementite and the aspect ratio of cementite and ferrite grains are measured as follows. a. Spheroidization rate of cementite: A cross section formed in the rolling direction and the thickness direction was observed at 1500 times with an electron microscope,
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.

B. Aspect ratio of cementite: 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 about 500 pieces of cementite have a long axis and a short axis (perpendicular to the long axis). Determine the length ratio and average. c. Aspect ratio of ferrite grains: JIS G052
2 is determined in the same manner as in the elongation.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION JIS G 4051 and JIS G 440 allow the present inventors to simplify the processing steps on the user side and to cope with the shortening of the heat treatment at a low temperature and a short time.
1. When examining the high ductility and high quenchability of a high carbon steel sheet having a component system specified by JZS G4802,
A step of performing a heat treatment at a temperature of Ar ₃ or more after hot rough rolling and a step of cold rolling-annealing are required. To increase ductility, the spheroidization rate of cementite before cold rolling and cold rolling-
It became clear that the aspect ratio of the ferrite grains after annealing and the aspect ratio of the cementite after cold rolling-annealing had to be optimized for high hardenability.

[0011] Based on this finding, the present inventors have established JIS
G 4051, JIS G 4401, JIS G
By controlling the heating conditions of the coarse bar after hot rough rolling and the annealing conditions before cold rolling of the high-carbon steel sheet defined by 4802, the spheroidization rate of cementite is increased, and the cold rolling and final annealing conditions are further increased. By controlling the aspect ratio of ferrite grains and cementite within a certain range, high carbon steel sheet with high ductility and high hardenability that can respond to the simplification of the processing process and the low temperature and short time of heat treatment by the user. (JIS G 4051, JIS G 4401, J
The present inventors have found a method that can surely produce ISG 4802 (component definition), and have completed the present invention.

An embodiment of the present invention will be described below. Using a steel having a component system of S35C according to JIS G 4051, and using a bar heater heating after hot rough rolling,
After performing a heat treatment at 1050 ° C. for 15 seconds, changing a finishing temperature and a winding temperature, and performing a heat treatment at a temperature and time to change the cementitious spheroidization rate of a 2 mm-thick steel plate, the spheroidization rate of cementite was changed. %, And cold-rolled to a thickness of 1 mm at a rolling reduction of%, and annealed at different temperatures and times to produce samples. 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. In addition, along the rolling direction JI
An S5 test piece was cut out and subjected to a tensile test at a tensile speed of 10 mm / min, and the total elongation was determined to evaluate ductility. Further, a test piece cut out to a size of 50 × 100 mm was 8
After short-time heating at 20 ° C for 10 seconds, quenching in oil at 20 ° C, Rockwell C scale hardness (HR
C) was measured and the hardenability was evaluated.

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 terms of HRC. Therefore, the conditions for obtaining a total elongation of not less than 42% and a hardness after quenching of not less than HRC 52 are defined 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. When the spheroidization ratio of cementite before cold rolling is 80% or more and the aspect ratio of ferrite grains after annealing is 1.3 or less, the total elongation is 42% 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 is 52 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 a steel sheet having 0% or more is not particularly limited, but a conventional method, that is, a molten steel whose composition has been adjusted is converted into a slab by continuous casting or ingot / bulking rolling, and is heated directly or through a heating furnace. Hot rolling can be performed by subjecting a hot-rolled steel sheet that has been subjected to hot rough rolling to a heat treatment at a temperature of Ar ₃ or higher and then hot-rolled, and then subjected to spheroidizing annealing after cold rolling. Here, the reason for performing the heat treatment at a temperature of Ar ₃ or higher is to make the γ particle size uniform, as described later in Examples. Uniform γ grain size of steel sheet during hot rolling and uniform pearlite after transformation to reduce variation in cementite grain size and ferrite grain size after final annealing, improving ductility and hardenability Let it.

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. Hereinafter, examples of the present invention will be described to demonstrate the effects of the present invention.

[0019]

EXAMPLES (Example 1) S70 of JIS G 4802
Component system equivalent to C-CSP (C: 0.71% by weight, S
i: 0.19%, Mn: 0.75%, P: 0.01%,
S: 0.003%, Al: 0.01%, N: 0.004
0%) steel is manufactured by continuous casting.
After heating to 270 ° C. and before finish rolling, a bar heater is used to perform a heat treatment at 950 to 1100 ° C. for 3 to 30 seconds, and hot-roll at a finishing temperature of 800 to 900 ° C.
Take up at 700 ° C, pickle and wash at 560-720 ° C for 20-1
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 heat treatment reduces the variation in the cementite grain size and the ferrite grain size after the final annealing by making uniform the γ grain size of the steel sheet during hot rolling and forming uniform pearlite after transformation. Improves ductility and hardenability. 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. Note that the heat treatment includes temperature increase, temperature decrease, and temperature maintenance.

The results are shown in Table 1 (Nos. 1-4: Examples of the present invention, Nos. 5-11: Comparative examples). In all of the samples (Examples Nos. 1 to 4) produced by the method of the present invention, 37 samples were used.
% And a hardness after quenching of 62 or more by HRC, and an average total elongation of about 30% and hardness after quenching when a high-carbon steel sheet having a similar composition and thickness is manufactured by a conventional method. HRC shows higher ductility and hardenability compared to around 50.

On the other hand, comparative samples (Comparative Examples Nos. 5 to 11) produced by a method outside the scope of the present invention can obtain only ductility and hardenability equal to or less than those produced by the conventional method.

[0024]

[Table 1]

(Example 2) S4 of JIS G 4051
Component system equivalent to 5C (C: 0.44% by weight, Si:
0.19%, Mn: 0.76%, P: 0.01%, S:
(0.008%, Al: 0.01%, N: 0.003%)
A slab made of steel of
℃, before finishing rolling, 9
Perform heat treatment at 50 to 1100 ° C for 3 to 30 seconds, hot-roll at a finishing temperature of 820 to 900 ° C, and 500 to 700 ° C.
After pickling, box annealing was performed at 560 to 720 ° C. for 20 to 120 hours after pickling to produce steel sheets having different spheroidization rates of cementite. Next, this steel sheet is cold-rolled at a rolling reduction of 20 to 70%, and box-annealed at 580 to 720 ° C. for 4 to 40 hours to produce samples having different cementite aspect ratios and ferrite grain aspect ratios. did. 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. The results are shown in Table 2 (Nos. 12 to 15: Examples of the present invention, Nos. 16 to 22: Comparative examples). In all of the samples prepared by the method of the present invention (Examples Nos. 12 to 15 of the present invention), a total elongation of 42% or more, a hardness after quenching of 52 or more by HRC, and a similar component system and plate thickness were obtained. It shows higher ductility and hardenability than an average total elongation of about 35% and a hardness after quenching HRC of about 40 when a high carbon steel sheet is manufactured by a conventional method.

On the other hand, comparative samples (Comparative Examples Nos. 16 to 22) produced by a method out of the scope of the present invention can obtain only ductility and hardenability equal to or less than those produced by the conventional method.

[0028]

[Table 2]

[0029]

As described above, the present invention has a high ductility and a high hardenability that can cope with simplification of the working process and low-temperature and short-time heat treatment by specifying the steel structure and the manufacturing conditions. Carbon steel for machine structure (JI
SG4051), carbon tool steel (JIS G4
401), cold-rolled steel strip for spring (JIS G 480)
It is possible to provide a method capable of reliably producing a high-carbon steel sheet whose composition is specified in 2).

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the total elongation, the spheroidization ratio of cementite before cold rolling, and the aspect ratio of ferrite grains after annealing according to an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between hardness after quenching and aspect ratio of cementite after annealing according to the embodiment of the present invention.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasuyuki Takada 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Katsutoshi Ito 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Sun (72) Inventor Yoshihide Ishii 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Japan Nippon Steel Pipe Co., Ltd. (72) Inventor Noboru Shioya 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan 4K037 EA01 EA06 EA07 EA15 EA18 EA23 EA25 EA27 FA03 FB04 FC03 FC04 FC07 FE01 FE02 FE03 FF01 FF02 FG01 FH03 FJ04 JA06

Claims (1)

[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: A step of performing a heat treatment at a temperature of ₃ or more to finish hot rolling; and, in a hot-rolled steel sheet, cold rolling a steel sheet having a spheroidizing ratio of cementite of 80% or more at a rolling reduction of 30% or more. And a step of annealing the cold-rolled steel sheet in a temperature range from 600 ° C. to an Ac ₁ transformation point, wherein cementite having an aspect ratio of 1.5 or less and ferrite grains having an aspect ratio of 1.3 or less are formed. A method for producing a high carbon steel sheet having high ductility and high hardenability.