JP2000265239A - High carbon steel sheet excellent in blankability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve blankability by providing a specific composition consisting of C, Si, Mn, P, total Al, and the balance essentially Fe, also providing a structure in which carbides of a specific average grain size and a specific spheroidizing rate are dispersed through a ferritic matrix of specific grain size, and further providing a specific notch tensile elongation. SOLUTION: The steel sheet has a composition consisting of, by weight, 0.70-1.20% C, <=0.40% Si, <=1.0% Mn, <=0.03% P, <=0.10% total Al, and the balance essentially Fe and also has a structure in which carbides of 0.3-1.2 μm average grain size and >=80% spheroidizing rate are dispersed through a ferritic matrix of >=5 μm ferrite grain size. Further, when tensile test is performed by using a test piece which is prepared by providing a V-notch of 45 deg. opening angle and 2 mm depth to width-directional both sides in the central position, in a longitudinal direction, of the parallel part of a JIS No.5 tensile test piece, the notch tensile elongation represented as an elongation percentage after breaking based on 10 mm gauge length in the central part in the longitudinal direction of the parallel part is regulated to >=10%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention has excellent punching workability,
The present invention relates to a high carbon steel sheet used for various machine parts and the like.

[0002]

2. Description of the Related Art Gears and other mechanical parts that have a complicated shape and require high dimensional accuracy and wear resistance are made of high carbon steel, and are formed and finished by cutting or the like and then require quenching and tempering. It has been manufactured by heat treatment. Since the manufacturing cost is high in the cutting process, it has been studied to replace the cutting process with a punching process. In the case of punching, in particular, in the portion that comes into contact with parts such as gear teeth, cracks due to punching are suppressed or eliminated,
It is required that the shear surface be large. In addition, in order to obtain a product having a complicated shape and high dimensional accuracy, a high carbon steel sheet exhibiting excellent punching properties is required as a raw material, in addition to an improvement in punching technology.

When controlling the hot-rolling condition of a steel strip to suppress the formation of bainite, it is known that a steel sheet which does not crack by punching even if the hot-rolled material is obtained can be obtained.
No. 9329. However, the target steel type is a steel sheet having a C content of 0.60% by weight or less, and cannot be applied to a high carbon steel sheet having a C content of 0.70% by weight or more.
Regarding the punching property of high carbon steel, reduction of burr height and prolongation of mold life can be achieved by making carbide grain size finer and controlling ferrite grain size (Japanese Patent Application Laid-Open No. 9-316595), special elements The height of the burrs and the life of the mold can be reduced by the addition of (Japanese Patent Application Laid-Open Nos. 57-110622, 3-44447 and 4-2352).
No. 52), graphitization of carbide is effective for reducing noise during punching and extending the life of a mold (Japanese Patent Application Laid-Open No. 56-119).
No. 758) is known.

[0004]

Any of the conventional measures for improving the punching performance are merely aimed at reducing the burr height and extending the life of the mold. In addition, special elements used for improving the punching property cannot be applied to general high carbon steel, and cause an increase in manufacturing cost. As described above, despite the growing needs for high carbon steel sheets with particularly excellent punching surface properties among the punching properties, general high carbon steels have small cracks and large shear surfaces. An effective method to improve the properties has not been established. The present invention has been devised in order to solve such a problem.
By controlling the grain size of carbide in a high carbon steel sheet containing 0 to 1.20 wt% C, the notch tensile elongation closely related to the punching property is improved, and a high carbon steel sheet excellent in the punching property is provided. The purpose is to do.

[0005]

The high carbon steel sheet of the present invention comprises:
In order to achieve the object, C: 0.70 to 1.20% by weight, Si: 0.40% by weight or less, Mn: 1.0% by weight or less, P: 0.03% by weight or less, total Al: 0% .10% by weight
Including the following, the balance substantially has a composition of Fe, and has a structure in which carbide having an average particle diameter of 0.3 to 1.2 μm and a carbide spheroidization ratio of 80% or more is dispersed in a ferrite matrix having a ferrite particle diameter of 5 μm or more. A tensile test was performed using a test piece having a V-notch with an opening angle of 45 degrees and a depth of 2 mm on both sides in the width direction at the center in the longitudinal direction of the parallel portion of the JIS No. 5 tensile test piece, Point-to-point distance 10m
The notch tensile elongation expressed as the elongation percentage after breaking with respect to m is 10% or more.

[0006] The high carbon steel sheet further contains Cr: 1.6 wt% or less, Mo: 0.3 wt% or less, Cu: 0.3 wt% or less, Ni: 2.0 wt% or less, and Ca: 0.01. It may contain one or more kinds by weight of not more than one. S contained as an impurity is preferably regulated to 0.01% by weight or less. When observing the metallographic structure of the steel plate cross section, the carbide spheroidization ratio is determined by observing the region where the total number of carbides is 300 or more in the observation visual field and determining the maximum length p. The number of carbides having a ratio p / q of less than 3 (hereinafter referred to as spheroidized carbides) is expressed as a ratio (%) to the total number of carbides in the observation visual field. In addition, the carbide average particle diameter is represented by a value obtained by averaging the equivalent circle diameters measured for individual carbides in the observation field of the same carbide total number of 300 or more for all the measured carbides.

[0007]

The present inventors have studied various methods for improving the punching properties of general high carbon steel sheets, and found that the notch tensile elongation, which is one of the indexes of local ductility, and the shear surface ratio at the time of punching were used. It was found that the sex had a good correlation. It was also found that the form of the carbide dispersed in the steel sheet had a great effect on the notch tensile elongation, and the notch tensile elongation was improved by spheroidizing the carbide and increasing the average grain size. Cracks and cracks are thought to be the result of very local defects generated when the steel sheet was deformed during punching, and propagated inside the material as the deformation progressed. In a high carbon steel sheet, the generation and growth of microvoids originating from carbides (cementite), MnS-based inclusions, and the like can be cited as defects.

[0008] Under such a premise, it can be said that the adjustment of the metal structure and the reduction of inclusions that can suppress the generation and growth of microvoids as much as possible during working deformation are effective in improving the punching performance. Suppressing the generation and growth of microvoids also improves local ductility. Observation of the microvoids of the test pieces actually subjected to the notch tensile test showed that the formation and growth of microvoids were greatly affected by the morphology of the metallographic structure, and were very similar to the formation and growth of microvoids during punching. This suggests that there is a close relationship between the punching property and the notch tensile elongation.

[Components / Composition] In the present invention, C: 0.70
It is intended for high carbon steels containing up to 1.20% by weight. C
Is the most basic alloy component in high carbon steel,
Workability, quenching hardness, carbide amount, etc. greatly vary depending on the content. If the C content is less than 0.70% by weight, a sufficient amount of undissolved carbide does not remain after quenching, and desired wear resistance cannot be obtained. Conversely, if the C content exceeds 1.20% by weight, the productivity and handleability of the steel strip deteriorate due to a decrease in toughness after hot rolling, and sufficient ductility cannot be obtained even after annealing. The desired excellent punching property cannot be expected. Si is an alloy component that has a significant effect on local ductility. When an excessive amount of Si is added, the ferrite is hardened by a solid solution strengthening action, which causes cracks during molding. Excessive Si addition promotes the generation of scale flaws on the steel sheet surface during the manufacturing process, and also causes the surface quality to deteriorate. Then, S
It is preferable that the upper limit of the i content is set to 0.40% by weight, and particularly for applications requiring workability, the upper limit is set to 0.20% by weight or less.

Mn is an alloy component effective for improving wear resistance. However, when a large amount of Mn exceeding 1.0% by weight is contained, ferrite is hardened and workability is deteriorated. Since P is a component that adversely affects ductility and toughness, the upper limit is defined as 0.03% by weight. Al is a component added as a deoxidizing agent for molten steel. However, if the total Al content in the steel exceeds 0.1% by weight, the cleanliness of the steel material is impaired, and the surface of the steel sheet is liable to have flaws.

In order to further improve the heat treatment property, Cr, M
One or more of o, Cu, Ni, and Ca are added as needed. Cr is an alloy component effective for improving hardenability, and exhibits an effect of increasing tempering softening resistance.
However, when contains a large amount of Cr exceeding 1.6% by weight, hardly softened even annealed accompanied by prolonged annealing and A ₁ transformation point or more heating below the A ₁ transformation point, rather punching properties Decrease. Therefore, when adding Cr, the upper limit of the Cr content is set to 1.6% by weight. Mo
Has the effect of improving the hardenability and the tempering softening resistance in the same manner as Cr when added in a small amount. However, when contains a large amount of Mo exceeding 0.3% by weight, hardly softened even annealed accompanied by prolonged annealing and A ₁ transformation point or more heating below the A ₁ transformation point, rather punching properties Decrease. Therefore, when adding Mo, the upper limit of the Mo content is set to 0.1.
Set to 3% by weight. Cu has an effect of improving the releasability of the oxide scale generated during hot rolling and improving the surface quality of the steel sheet. However, when a large amount of Cu exceeding 0.3% by weight is included, fine cracks are easily generated on the steel sheet surface due to the embrittlement of the molten metal. When adding Cu, the range of 0.10 to 0.15% by weight is preferable.

Ni is an alloy component effective for improving hardenability and improving low-temperature toughness. In addition, it also has the effect of counteracting the adverse effect of embrittlement of molten metal due to the addition of Cu. To prevent the embrittlement of molten metal, 0.2% by weight or more of Cu
It is effective to add Ni in an amount equivalent to the amount of Cu added. However, if adding a large amount of Ni in excess of 2.0 wt%, it is difficult to softening even annealed accompanied by prolonged annealing and A ₁ transformation point or more heating below the A ₁ transformation point, rather punching property descend.

[0013] The punchability is also improved by regulating the S content and adding Ca. S is a component that generates MnS-based inclusions. Since the local ductility is deteriorated when the amount of the MnS-based inclusions increases, the S content in the steel is preferably reduced as much as possible. However, as long as the carbide morphology defined in the present invention can be obtained, an extremely low S is required. Without this, the effect of improving the punchability can be obtained even for general commercial steel. However, in order to stably secure high local ductility even when the C content is increased to nearly 1.20% by weight, it is necessary to use steel in which the S content is reduced to 0.01% by weight or less. preferable. The morphology of the MnS-based inclusions is effectively controlled by adding Ca. Ordinary MnS-based inclusions have an elongated shape and tend to be the starting point of microvoid formation during punching. On the other hand, Mn, S, Ca
And inclusions are spheroidized to suppress generation of microvoids. However, when an excessive amount of Ca exceeding 0.01% by weight is added, a problem caused by coarsening of inclusions appears. Therefore, when adding Ca, the upper limit of the Ca content is set to 0.01% by weight.

[Spheroidization rate of carbide]
The ratio of “spheroidized carbide” to the total carbide is shown.
In the present specification, the ratio p / q of the maximum length p and the maximum length q in the direction orthogonal to the maximum length p in the metallographic observation field of view of the steel sheet cross section is 3
Less than the carbides were treated as "spheroidized carbides". For example, the carbide in the recycled pearlite is almost the carbide of p / q ≧ 3. On the other hand, in a carbide grown from the undissolved carbide remaining after heating at the Ac ₁ transformation point or higher, the ratio p / q is less than 3. It is difficult to accurately determine the shape of the carbide by three-dimensionally grasping it, and it is also troublesome to determine the suitability of the product steel sheet. On the other hand, it is easy to observe the planar metal structure of the steel plate cross section. The present inventors can appropriately evaluate the influence of the carbide shape on the local ductility of the steel sheet when capturing the degree of spheroidization using the ratio p / q for the carbide shape observed in the metal structure of the steel sheet cross section. It was confirmed. According to various experimental results, when the number of “spheroidized carbides” having a ratio p / q of less than 3 accounts for 80% or more of the total number of carbides, and further when adjusting the average carbide particle size to a specific range, Showed high punching properties.

It is supposed that the reason why the punching property is improved when the carbide spheroidization ratio is increased is that the carbide having a high spheroidization ratio is unlikely to be a starting point of microvoid formation during processing. In a steel sheet having a low carbide spheroidization rate, among the dispersed carbides, a carbide having insufficient spheroidization, such as carbide of recycled pearlite, has a different deformability from surrounding ferrite grains. For this reason, it is considered that the insufficiently spheroidized carbide serves as a starting point for the generation of microvoids, and promotes the generation and connection of microvoids, leading to the generation of cracks. Therefore, in order to improve the punchability, it is effective to adjust the carbide spheroidization ratio of the steel sheet to 80% or more in combination with the adjustment of the average carbide particle size.

[Average Particle Size of Carbide] Punching properties and local ductility are also remarkably improved by increasing the average particle size of carbide. An increase in the average grain size means a decrease in the total number of carbides because the amount of carbon in the steel is constant. It is presumed that the reduction in the total number of carbides suppresses the connection of microvoids generated from individual carbides, and consequently contributes to remarkable improvement in punchability and local ductility. The average carbide particle size is represented by a value obtained by averaging the equivalent circle diameters measured for individual carbides in the observation field of view of all the measured carbides when observing the metal structure of the cross section of the steel sheet. Specifically, the area of each carbide is measured, and the equivalent circle diameter is calculated from the obtained area. The area of the carbide can be easily measured using an image processing device. The sum of the circle equivalent diameters of all the measured carbides is determined, and the value obtained by dividing the sum by the total number of the measured carbides is defined as the average carbide particle size. In order to increase the reliability of the numerical values, it is preferable to select an observation visual field in which the total number of measured carbides is 300 or more. As a result of detailed punching experiments performed by the present inventors, it has been found that when the carbide spheroidization ratio is 80% or more and the average carbide particle size is 0.3 μm or more, a steel sheet exhibiting excellent punchability and local ductility can be obtained. Was. However, 1.2
Even if the average carbide particle size is coarsened to at least μm, the effect of improving the local ductility corresponding to the increase in the carbide particle size is small, and long-term annealing needs to be performed, so that the economical disadvantage is increased. Therefore, in the present invention, the average carbide particle size in the steel sheet is specified in the range of 0.3 to 1.2 μm.

[Ferrite grain size] Ferrite grain after annealing
Diameter is also a factor that affects the improvement of punchability and local ductility.
is there. When the ferrite grain size is less than 5 μm,
There is a tendency for the ductility to decrease. In this regard, carbide dispersion
In order to maximize the effect of shape optimization,
The grain size (average grain size) of the particles must be 5 μm or more.
preferable. Furthermore, mixed grains with irregular ferrite grain size
The organization has a negative effect on workability, so
It is preferable that the grain size is uniform.
In order to obtain a sized structure, the ferrite grain size (average grain size)
Diameter) is preferably adjusted in the range of 5 to 35 μm,
Mixed ferrite grains with an average grain size exceeding 35 μm
Easy to become an organization. The steel sheet having the above characteristics is
₁ Although it can be obtained by long-time annealing below the transformation point,
It can be obtained in a relatively short time annealing by devising the method
You. For example, Ac₁ Specific temperature range just below and just above the transformation point
Annealing that appropriately combines heating in the enclosure
You. Specifically, (A_C1-50 ° C) to (A_C1Less than warm
Temperature) in hot rolled steel sheet or cold rolled steel sheet for 0.5 hours or more
First stage heating, A _C1~ (A_C1+ 100 ° C) temperature
The second stage of heating for 0.5 to 20 hours in the temperature range, then
(A_r1-80 ° C) to A_r1Temperature for 2 to 60 hours
The third stage heating is continued, and the third stage
The cooling rate to the eye holding temperature is 5 to 30 ° C./hour 3
Carbide dispersion form was properly controlled by step annealing
A steel plate is manufactured.

[0018]

EXAMPLES Various steels having the compositions shown in Table 1 were melted, and slabs obtained by continuous casting were hot-rolled. At this time, the structure of the hot-rolled steel strip was changed by adjusting the winding temperature. After pickling the hot-rolled steel strip, the steel sheet was annealed under various conditions so that the carbide spheroidization rate and the average carbide grain size of the steel sheet were different. Test No. 1 in Table 2
For 3 to 9, 11 to 14, the winding temperature is 450 to 600 ° C
In after hot rolled, and held for 4 hours at a temperature lower than Ac ₁ transformation point, and held for 4 hours at a constant temperature above Ac ₁ transformation point of the seven hundred thirty to seven hundred and seventy ° C., then cooled at a cooling rate of 10 ° C. / time, Ac ₁ It was kept at 690 ° C. below the transformation point for 4 to 40 hours, and then cooled to 760 ° C. at a cooling rate of 10 ° C./hour, followed by air-cooling heat treatment. In Test Nos. 2 and 10, after pickling the hot-rolled steel strip obtained at a winding temperature of 550 ° C., 30 to 6
Cold-rolled at 0% and heated to 700 ° C lower than the Ac ₁ transformation point.
Heat treatment was performed for 30 hours.

[0019]

Test pieces for a tensile test, a notch tensile test and a punching property evaluation test were cut out from a 2.0 mm thick steel strip. The carbide spheroidization ratio was determined by observing a predetermined region of the steel plate cross section using a scanning electron microscope and selecting a portion where a total of 300 to 1000 carbides were precipitated as an observation region. The ratio p between the maximum length p of the carbide and the maximum length q in the direction perpendicular thereto.
Those in which / q was less than 3 were counted as “spheroidized carbides”, and the ratio of the number of “spheroidized carbides” to the total number of measured carbides was calculated as the carbide spheroidization rate. The average carbide particle diameter was determined by performing image processing on the observation visual field in which the carbide spheroidization ratio was measured, calculating the circle equivalent diameter of each carbide, and averaging the calculated values with all the measured carbides. The ferrite grain size was determined by measuring the number of ferrite crystal grains cut along two orthogonal lines according to the cutting method defined in JIS G0522, and averaging the results of 10 visual field measurements.

For the tensile test, a JIS No. 5 test piece was used, and the distance between the reference points of the parallel portions was set to 50 mm. In the notch tensile test, a test piece having a V-notch with an opening angle of 45 degrees and a depth of 2 mm at both sides in the width direction at the center in the longitudinal direction of the parallel portion of the JIS No. 5 tensile test piece was used. Then, the elongation percentage with respect to the gauge length of 10 mm at the central part in the longitudinal direction of the parallel part was measured after breaking, and the obtained elongation percentage was determined by the notch tensile elongation El _V.
And The El _V value is an index indicating the local ductility, and can more quantitatively evaluate the local ductility more appropriately than the local elongation obtained as (total elongation) − (uniform elongation) in a normal tensile test. In the punching property evaluation test, a steel sheet was punched out using a punch having a diameter of 10 mm and the clearance was set to 5% of the sheet thickness, and the average of the shear surface ratio and the crack depth of 100 test pieces were obtained. The shear ratio is calculated as [shear length in plate thickness direction] / [plate thickness] ×
It was calculated as 100 (%). The crack depth was determined by observing a cross section in the rolling direction near the punched surface of each test piece with a microscope, and measuring the depth of the deepest crack at the fractured portion by the length from the shear plane.

As can be seen from the investigation results in Table 2, in Test No. 14, since the steel F was used as a material having a C content exceeding 1.20% by weight, the carbide grain size, the carbide spheroidization rate, and the ferrite grain size were used. Is within the range specified in the present invention,
7. Notch tensile elongation El _V is low, and punching property is also shear surface ratio.
8% and a crack depth of 0.26 mm were inferior. Test number 2
Indicates that the carbide particle size has not reached 0.3 μm,
In Test No. 10, the ferrite grain size was less than 5 μm, the notch tensile elongation El _V did not reach 10%, and the punching property was 1%.
Less than 0% and crack depth of 0.25 mm or more were inferior. On the other hand, in Test Nos. 1, 3 to 7, 9, 11 to 13 in which the carbide average particle diameter, the carbide spheroidization ratio and the ferrite particle diameter were adjusted to the ranges specified in the present invention, a high shear surface ratio of 15% or more was obtained. , The crack depth was as small as 0.15 mm or less, indicating good punching properties.

[0023]

[0024]

As described above, the high carbon steel sheet of the present invention has improved punchability and local ductility by setting the carbide spheroidization ratio, the average carbide particle size and the ferrite particle size in appropriate ranges. ing. This high-carbon steel sheet has a markedly improved punching property as compared with conventional high-carbon steel sheets, and is therefore used in a wide range of fields as materials for automobile parts having various shapes and various mechanical parts. In addition, since it is softened, it is effective in extending the life of the press die.

Claims (3)

[Claims] 1. C: 0.70 to 1.20% by weight, Si:
0.40% by weight or less, Mn: 1.0% by weight or less, P:
0.03% by weight or less, total Al: 0.10% by weight or less, the balance being substantially Fe, with an average particle size of 0.3
It has a structure in which carbide having a spheroidization ratio of 80% or more is dispersed in a ferrite matrix having a ferrite particle size of 5 μm or more. Degree, depth 2
A tensile test using a test piece having a V notch of 10 mm, the notch tensile elongation expressed as the elongation after fracture with respect to the distance of 10 mm between the gauges at the center in the longitudinal direction of the parallel portion is 10% or more. Excellent high carbon steel sheet. 2. C: 0.70 to 1.20% by weight, Si:
0.40% by weight or less, Mn: 1.0% by weight or less, P:
0.03% by weight or less, total Al: 0.10% by weight or less, Cr: 1.6% by weight or less, Mo: 0.3% by weight
In the following, one or more of Cu: 0.3% by weight or less and Ni: 2.0% by weight or less are contained, and the balance substantially has a composition of Fe, and the average particle diameter is 0.3 to 1.2 μm. Has a structure in which carbides having a carbide spheroidization ratio of 80% or more are dispersed in a ferrite matrix having a ferrite particle size of 5 μm or more.
A tensile test was conducted using a test piece having a 45 degree opening angle and a 2 mm deep V-notch on both sides in the width direction at the center in the longitudinal direction of the parallel portion of the tensile test piece. A high-carbon steel sheet excellent in punchability, having a notch tensile elongation of 10% or more expressed as a percentage of elongation after fracture with respect to a distance of 10 mm. 3. The high carbon steel sheet according to claim 1, further comprising 0.01% by weight or less of Ca.