JP3446003B2 - Method for producing high carbon steel sheet excellent in formability and hardenability - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to JIS G 40
51 (carbon steel for machine structure), JIS G 4401 (carbon tool steel material), JIS G 4802 (cold rolled steel strip for springs) contains special alloy elements such as Ni, Cr, Mo, etc. The present invention relates to a method of manufacturing a high carbon steel sheet.

[0002]

2. Description of the Related Art Automotive parts such as tools, blades, gears and seat belt fittings are made of JIS G 4 as a material.
051, JIS G 4401, JIS G 4802
The high carbon steel sheet whose composition is specified in 1. is used, and it is manufactured by subjecting it to a predetermined shape and then subjecting it to 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 come to consider simplification of the molding process and reduction of heat treatment time at a low temperature for cost reduction. As a high carbon steel sheet as described above, excellent formability capable of forming a complicated shape in a small number of steps and excellent hardenability capable of obtaining a desired hardness even by heat treatment at a low temperature for a short time are strongly demanded.

Therefore, various studies have been conducted to improve the formability and hardenability of high carbon steel sheets. For example, in Japanese Unexamined Patent Publication (Kokai) No. 5-9588, a steel strip after hot rolling is cooled at a cooling rate of 10 ° C./sec or more for 20 to 50.
After cooling to a temperature range of 0 ° C., reheating to a temperature range of 500 ° C. to (Ac ₁ transformation point + 30 ° C.) and winding at that temperature, or 650 ° C. after cold rolling ((Ac ₁ transformation point +
It is disclosed that a heat treatment is carried out in a temperature range of 30 ° C. for 1 hour or more to promote spheroidization of cementite, thereby making it soft and highly ductile to improve moldability. In addition, JP-A-6
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, when the inventors of the present invention examined the method described in Japanese Patent Laid-Open No. 5-9588, it was possible to cope with the simplification of the molding process by the user and the shortening of the heat treatment at low temperature. In some cases, a high carbon steel sheet having excellent formability and hardenability cannot always be obtained. Further, in the method of graphitizing carbon in steel described in JP-A-64-25946 and JP-A-8-246051, since the dissolution rate of graphite is slow, a quenching treatment at a low temperature for a short time is sufficient. There is a problem that it cannot be hardened.

The present invention has been made to solve such a problem, and JIS G 4051, JIS which is excellent in moldability and hardenability capable of responding to the simplification of the molding process by the user and the shortening of heat treatment at low temperature. G 440
It is an object of the present invention to provide a method for stably producing a high carbon steel sheet whose composition is defined by JIS G 4802.

[0007]

[Means for Solving the Problems]
4051 (carbon steel for machine structure) , JIS G4401
(Carbon tool steel) , JIS G 4802 (cooling for springs
Upon manufacturing a high carbon steel sheet having a component system defined between rolled steel strip), it was hot rolled at Ar ₃ transformation point or more of the temperature, the (b) the steel sheet after hot rolling (a) slab 5 ℃ / se
After cooling with water spray at a cooling rate of c or higher, and after a short heat treatment of stopping water spraying at a temperature in the temperature range of 560 to 650 ° C. and holding for 2 to 10 seconds, again cooling with water spray at a cooling rate of 7 ° C./sec or higher. (C) The cooled steel sheet is wound at a temperature of 600 ° C. or lower, (D) The rolled steel sheet is cold-rolled at a reduction ratio of 30% or more, and (E) the cold-rolled steel sheet is 580.
Annealed at a temperature of ℃ ~ Ac ₁ transformation point, the average grain size is 1.1
This is solved by a method for producing a high carbon steel sheet having excellent formability and hardenability, which forms cementite having an average aspect ratio of 1.5 or less and a ferrite grain having an average particle size of 2 μm or more at μm or less.

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 grain size of cementite: The cross section formed in the rolling direction and the thickness direction is 1500 by an electron microscope.
Observed at double magnification, the long axis of about 300 cementite,
The lengths in the minor axis (perpendicular to the major axis) and those 45 ° are determined and averaged. b) Average aspect ratio of cementite: Cross sections formed in the rolling direction, the thickness direction, and the width direction and the thickness direction are observed with an electron microscope at a magnification of 1,500, and about 500 cementites have a long axis and a short axis (perpendicular to the long axis. Direction) length ratio and average. c) Average grain size of ferrite grains: A cross section formed in the rolling direction and the thickness direction is observed with an optical microscope at a magnification of 500, and about 300 ferrite grains have a long axis, a short axis (a direction perpendicular to the long axis), and the like. And the length in the direction of 45 ° is obtained and averaged.

In order to enable the present inventors to cope with the simplification of the molding process on the user side and the shortening of the heat treatment at low temperature, JIS G 4051, JIS G 4401, JIS
Upon studying the improvement of formability and hardenability of high carbon steel sheet having the composition system specified in G4802, the cooling conditions after hot rolling and the morphology of cementite and ferrite grains after cold rolling-annealing play an important role. It became clear that he was playing. The details will be described below.

1) Hot rolling In hot rolling, when α-phase rolling is performed below the Ar ₃ transformation point, coarsening of ferrite grains occurs and a nonuniform structure of ferrite and pearlite, which is unfavorable for hardenability, is formed. Therefore, it is necessary to carry out at the Ar ₃ transformation point or higher. In addition, hot rolling, even if the slab produced by continuous casting or ingot casting / slabbing of molten steel with adjusted composition is directly rolled,
Moreover, you may roll 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 to be doing. For that purpose, the nose for pearlite transformation may be brought into the temperature range during cooling after hot rolling, and the steel plate may be kept in the temperature range near this nose to complete 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 by 2 is in the temperature range of 560 to 650 ° C, it is cooled at 5 ° C / sec or more after hot rolling, and 2 to the temperature range of 560 to 650 ° C. After holding for 10 seconds, again 7 ℃ / sec
It may be cooled at the above cooling rate and wound.

The cooling rate after hot rolling is less than 5 ° C./sec, the cooling rate after short-time holding in the nose is less than 7 ° C./sec, and the holding time in the temperature range of 560 to 650 ° C. If it exceeds 10 seconds, coarsening of pearlite is caused. Further, if the holding time in the temperature range of 560 to 650 ° C. is less than 2 sec, 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 holding in the temperature range of 560 to 650 ° C. is preferably 30 ° C./sec or less so as not to deteriorate the coil shape during winding. Moreover, holding in the temperature range of 560 to 650 ° C. does not necessarily have to be performed at a constant temperature in this temperature range, and sprinkling
Even if there is a temperature gradient to the extent that the cooling is stopped and natural cooling is performed, the temperature in this temperature range may be maintained for 2 to 10 seconds.
In order to actually maintain the temperature range for a short time, for example, cooling by water spray after hot rolling is stopped for a short time in this temperature range, that is, water spray is stopped and natural cooling is performed.

3) Winding temperature Since the winding temperature exceeds 600 ° C., pearlite is coarsened, the winding temperature must be 600 ° C. or lower. In addition, in order not to deteriorate the coil shape at the time of winding,
It is preferable to wind at a temperature of not less than ° C.

4) Cold Rolling-Average Diameter of Cementite after Annealing, Average Aspect Ratio and Average Diameter of Ferrite Grains A slab of a component system corresponding to S65C-CSP of JIS G 4802 is hot-worked at a temperature of 820 ° C. After rolling, 7 ℃ / sec
After cooling to a temperature of 550 to 660 ° C. at a cooling rate of 1 to 12 sec and maintaining the temperature range for 1 to 12 sec, 10 ° C./se
It was cooled at a cooling rate of c and wound at a temperature of 550 ° C. After that, cold rolling is performed at a reduction rate of 20 to 60% to obtain 550 to 72.
Annealing was performed for 20 hours at a temperature of 0 ° C. 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 grains were measured by the above-described methods. Also, 0 with respect to the rolling direction
JIS No. 5 test pieces were cut out along the °, 45 °, and 90 ° directions, and a tensile test was performed at a tensile speed of 10 mm / min.
Formability was evaluated by obtaining the total elongation averaged in each direction. Furthermore, a test piece cut into a size of 50 × 100 mm was heated at 760 ° C. for 10 seconds for a short time and then quenched in oil at 20 ° C. to obtain Rockwell C scale hardness (HR) on the steel plate surface.
C) It measured and evaluated hardenability.

According to Japanese Unexamined Patent Publication (Kokai) No. 5-9588, a plate having a composition similar to that of S65C-CSP and having a plate thickness of 1.
The average of the total elongation of the 2 mm spheroidized annealed material is at most about 31%, and the average hardness after quenching is 55 at the HRC.
Therefore, the present invention is defined as a condition that a total elongation of 33% or more and a hardness after quenching of HRC of 60 or more can be obtained.

FIG. 1 shows the relationship between the total elongation, the average aspect ratio of cementite, and the average grain size of ferrite grains.

The average aspect ratio of cementite is 1.5
It will be seen below that when the average grain size of the ferrite grains is 2 μm or more, the total elongation is 33% or more, and excellent formability is 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 the thickness is 1 μm or less, the hardness after quenching becomes 60 or more in HRC, and it is understood that stable quenching property can be obtained even by heating for a short time.

At this time, the rolling reduction during cold rolling is 30%.
If it is less than this, pearlite is not sufficiently destroyed and fine cementite having an average grain size of 1.1 μm or less and an average aspect ratio of 1.5 or less cannot be obtained after annealing, so it is necessary to set it to 30% or more. Although the upper limit is not specified, it is preferably 70% or less so as not to increase the load on the rolling mill.

If the annealing temperature after cold rolling is less than 580 ° C., unrecrystallized structure may remain, resulting in hard and low ductility. Further, if annealing is performed above the Ac ₁ transformation point, pearlite is generated, and the formability and hardenability are significantly impaired, so it is necessary to make the temperature below the Ac ₁ transformation point.

When the surface of the steel sheet thus manufactured is galvanized and then treated with a phosphate, even if gears, which are automobile parts, are molded by a method such as cold forging or ironing, which involves high surface pressure reduction, the galling of the die 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.

[0024]

[Example] (Example 1) S35 of JIS G 4051
Component system equivalent to C (wt: C: 0.36, Si: 0.1
8, Mn: 0.74, P: 0.008, S: 0.00
7, Al: 0.01) slab is manufactured by continuous casting, heated to 1250 ° C., and then hot rolled under the conditions shown in Table 1-primary cooling-short time heat treatment-secondary cooling-winding-cold rolling. -Annealing was sequentially performed to prepare 21 types of samples having a plate thickness of 1.2 mm. Here, the primary cooling rate and the secondary cooling rate in Table 1 were changed by changing the conditions of water spraying. The short-time heat treatment was performed by stopping the water spraying for the time shown in the table when the temperature shown in the table was reached.

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, and the hardness after quenching (heating temperature:
820 ° C.) was measured.

The results are shown in Table 1. The samples produced by the method of the present invention all have a total elongation of 35% or more and a hardness after quenching of 50 or more by HRC, and when a high carbon steel sheet having a similar composition and plate thickness is produced by a conventional method. In comparison with an average total elongation of about 30% and a hardness HRC after quenching of about 40, the moldability and hardenability are superior.

On the other hand, the comparative sample produced by the method other than the present invention can obtain the moldability and hardenability comparable to or less than those produced by the conventional method.

[0028]

[Table 1]

(Example 2) S6 of JIS G 4802
5C-CSP equivalent component system (wt% C: 0.65, S
i: 0.21, Mn: 0.76, P: 0.007, S:
A slab of 0.007, Al: 0.01) is manufactured by continuous casting, heated to 1280 ° C., and then hot-rolled-primary cooling-short time heat treatment-secondary cooling-winding-cooling under the conditions shown in Table 2. Hot rolling and annealing were sequentially performed to prepare 21 kinds of samples having a plate thickness of 1.2 mm. Here, the conditions for the primary cooling, the short-time heat treatment, and the secondary cooling were set in the same manner as in Example 1.

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, and the hardness after quenching (heating temperature:
750 ° C.) was measured.

The results are shown in Table 2. In the samples produced by the method of the present invention, a total elongation of 33% or more and a hardness after quenching of 60 or more by HRC were obtained, and when a high carbon steel sheet having a similar composition and plate thickness was produced by a conventional method. In comparison with an average total elongation of about 30% and a hardness HRC after quenching of about 50, the moldability and hardenability are more excellent.

On the other hand, the comparative sample manufactured by the method other than the present invention can obtain the moldability and hardenability comparable to or less than those manufactured by the conventional method.

[0033]

[Table 2]

(Example 3) No. 1 in Table 1 Use sample 1
, The surface of which is galvanized (Zn: 21 g / m ²)
After that, phosphate treatment (P coating: 1.95 mg / m²)
Then, a friction sliding test was conducted.

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

The test was conducted under the test conditions shown in the figure.
FIG. 4 shows the relationship between the pressing force and the friction coefficient. It can be seen that galvanization occurs when the surface is electrogalvanized and then treated with phosphate to cause a sharp increase in the friction coefficient with a higher pressing force than with a sample (bare material) not subjected to such surface treatment. . From this result, it can be said that, if the surface is electrogalvanized and then treated with phosphate, mold galling and cracks are less likely to occur even if the surface is formed by a method such as cold forging or ironing, which involves high surface pressure reduction.

[0037]

EFFECTS OF THE INVENTION Since the present invention is constituted as described above, it is possible to stabilize a high carbon steel sheet having excellent formability and hardenability which can cope with simplification of the forming process by the user and reduction of heat treatment time at low temperature. It is possible to provide a manufacturing method.

Further, if the surface is galvanized and then treated with a phosphate, the formability under high surface pressure such as cold forging and ironing can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between total elongation, average aspect ratio of cementite, and average grain size of ferrite grains.

FIG. 2 is a diagram showing a relationship between hardness after quenching and average particle size of cementite.

FIG. 3 is a diagram showing a friction sliding device used in a test.

FIG. 4 is a diagram showing a relationship between a pressing force and a friction coefficient.

[Explanation of symbols]

1 indenter 2 test pieces 3 sample table 4 Laura Leveler 5 Load cell for pressing force measurement 6 hydraulic cylinder 7 Load cell for pulling force measurement

Continuation of the front page (56) Reference JP-A-4-116137 (JP, A) JP-A-59-28527 (JP, A) JP-A-59-205417 (JP, A) JP-A-55-85626 (JP , A) JP-B-63-14045 (JP, B1) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04 C22C 38/00 -38/60

Claims (2)

(57) [Claims] 1. JIS G 4051 (carbon steel for machine structure), JIS G4401 (carbon tool steel material), JIS
In producing a high carbon steel sheet having a composition system defined by G 4802 (cold rolled steel strip for springs), (a) a slab is hot rolled at a temperature of Ar ₃ transformation point or higher, and (b) hot rolled. The rolled steel sheet is water- cooled at a cooling rate of 5 ° C./sec or more, subjected to a short-time heat treatment of stopping water spraying at a temperature in the range of 560 to 650 ° C. and holding for 2 to 10 seconds, and then again at 7 ° C./sec water spray cooling in a cooling rate higher than (c) coiling at a temperature steel plate of 600 ° C. or less after cooling, and cold-rolled at (d) winding reduction ratio of 30% or more of the steel sheet after removal, (e) Cold-rolled steel sheet is annealed at a temperature of 580 ° C. to Ac ₁ transformation point to obtain cementite having an average grain size of 1.1 μm or less and an average aspect ratio of 1.5 or less and ferrite grains having an average grain size of 2 μm or more. Of high carbon steel sheet with excellent formability and hardenability Law. 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.