JP4280202B2 - High carbon steel plate with excellent hardenability and stretch flangeability - Google Patents

Description

  The present invention relates to a high carbon steel sheet having excellent hardenability and stretch flangeability.

  High carbon steel sheets used for automobile parts (gears, missions) and the like are subjected to heat treatment such as quenching and tempering after punching and forming, and adjusted to a predetermined strength. In general, high carbon steel sheets may be stamped, bent, light drawn, and slightly stretched. In addition, when the part shape is complicated, it is often produced by welding several parts. However, in recent years, in order to reduce the manufacturing cost of parts, part processing steps have been omitted and integrated molding has been promoted. For this reason, the high carbon steel plate as a raw material is required to have excellent workability. There has also been attempted a method of performing secondary processing in a different processing form on the processed portion. For example, a thickening process or the like may be performed after the punching hole is expanded. In order to meet these requirements, it is impossible to withstand the processing simply by having a good stretch flangeability.

  The techniques for improving the stretch flangeability of the high carbon steel sheet include Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, Patent Document 5, Patent Document 6, Patent Document 7, Patent Document 8, and Patent Document. 9, Patent Literature 10, Patent Literature 11, Patent Literature 12, Patent Literature 13, Patent Literature 14, Patent Literature 15, and Patent Literature 16 are disclosed. Patent Documents 1 to 10 are characterized by steel components and hot rolling conditions, but the basic technique is that the temperature is maintained at Ac1 -50 ° C to Ac1 at the time of annealing and then held at a temperature of Ac1 to Ac1 + 100 ° C and then the cooling rate is 5 to 30 ° C. It is characterized by an annealing cycle that is held again in the temperature range of Ac1 to Ac1-50 ° C. during cooling at / hr. This method requires a long time for annealing, and the V-notch elongation shown in the specification disclosing the technology is good, but the actual stretch flangeability is not necessarily good. Patent Documents 12 and 13 are both technologies that employ a thermal cycle that cools at a cooling rate of 50 ° C./hr or less after heating in the γ region during annealing, which is basically the same as the previous technology and has sufficient elongation. Flangeability cannot be obtained.

Since Patent Document 11 is a ferrite + pearlite structure because it is a hot-rolled material, sufficient workability cannot be obtained. Patent Document 14 is a technique for setting the carbide particle size to 0.1 to 1.2 μm and the volume fraction of ferrite grains not containing carbide to 15% or less. This technique also stabilizes a high carbon steel sheet having an excellent stretch flange. Cannot be manufactured, and there are problems such as cracking during thickening after severe stretch flange processing. In the techniques of Patent Document 15 and Patent Document 16, the cooling rate after hot rolling is cooled at 120 ° C./S or more, the carbide particle size is 0.1 to 1.2 μm, and the ferrite grain volume ratio not containing carbide is 15%. Although it is the technique which manufactures the steel plate made below, since it cools with the cooling rate of 120 degrees C / S or more after hot rolling, the stable steel plate cannot be obtained. In addition, after severe stretch flange processing, there is a problem that cracking occurs in the thickening processing.
Japanese Patent Laid-Open No. 11-80884 Japanese Patent Laid-Open No. 11-80885 Japanese Patent Laid-Open No. 11-140544 Japanese Patent Laid-Open No. 11-256272 JP 11-256268 A JP-A-11-269552 JP-A-11-269553 JP 2000-178678 A JP 2000-178679 A JP 2000-273537 A Japanese Patent Laid-Open No. 2001-64751 JP 2001-73033 A JP 2001-220463 A JP 2001-214234 A JP 2003-13144 A JP 2003-13145 A

  In a steel sheet having a non-uniform structure, voids are generated on the punched end surface during the punching process of stretch flange processing. For example, even if it has excellent local ductility in the case of a smooth surface, if stretched flange processing is applied to the punched portion, voids are connected and cracks occur at the beginning of processing, so stretch flangeability Is insufficient. On the other hand, a steel sheet whose structure is made uniform by rapid cooling after hot rolling hardly causes voids in the punching process, but cannot withstand severe stretch flange processing because the ductility of the steel sheet is low. Therefore, an object of the present invention is to provide a steel sheet that is soft and excellent in ductility, is less likely to cause voids during punching, and has excellent hardenability.

  In the stretch flangeability, the punched surface is sheared and deformed at the time of punching, and the void is already generated before the stretch flange processing. When this is stretch flanged, a large crack occurs starting from the void, which becomes the stretch flange processing limit. On the other hand, if no voids occur during punching, it depends on how uniformly the strain of stretch flange processing is shared, and if the strain distribution becomes uneven, stretch flangeability deteriorates. The property becomes good. However, even if the strain is evenly distributed, if the steel sheet has poor ductility, it cannot withstand severe stretch flange processing. Therefore, it is difficult to produce voids during punching, and a steel plate having a uniform ductility and excellent ductility is required during stretch flange processing. The high carbon steel sheet has a mixed structure of ferrite and pearlite or a mixed structure of ferrite and cementite. Pearlite is a structure in which ferrite and cementite are laminated in a layered form and has poor ductility. Therefore, when workability is required, pearlite is annealed and used as a mixed structure of ferrite and spherical carbide (cementite). Since this spherical carbide is hard, there is almost no deformability, strain is discontinuous around the carbide, voids are generated around the carbide, and the voids are connected to cause cracks. Therefore, the smaller the carbide, the better the workability. However, the amount of carbide depends on the amount of C, and if the amount of carbide is small, hardness after quenching cannot be obtained, so a certain amount of carbide is necessary.

  Therefore, as a result of examining the influence of the existence form of carbide on stretch flangeability when the amount of carbide is constant, (1) voids are likely to occur around large carbides during processing, and stretch flangeability deteriorates. (2) If the carbide is small, the distortion that causes voids increases, but if voids occur, they are connected and cracks are likely to occur, and the stretch flangeability is still insufficient. (3) Even if the average carbide particle size is the same, if the variation in the carbide particle size is large, the stretch flangeability is inferior, and even if only the average particle size of the carbide is controlled, a steel sheet having excellent stretch flangeability cannot be obtained. An excellent stretch-flangeable steel sheet can be obtained by reducing the particle diameter and its variation. (4) In order to reduce the dispersion of carbides, it has been found that addition of an appropriate amount of Cr can be achieved together with solidification conditions for reducing segregation, hot rolling conditions for making the hot rolled structure uniform, and annealing conditions for preventing generation of large carbides. (5) If Ti and B are added, the hardness equivalent to that of steel with a high C content can be obtained under normal heat treatment conditions without degrading stretch flangeability, and excellent toughness can be obtained. can get.

Based on the above knowledge, the present invention has been completed, and it has become possible to provide a high carbon steel sheet having excellent stretch flangeability and hardenability. The gist is mass%, C: 0.22 to 0.45 % , Si: 1.0% or less, Mn: 1.5% or less, P: 0.03% or less, S: 0.03% or less. , Al: 0.08% or less, N: 0.0080% or less, Cr: 0.01~0.70%, Ti: 0.005~0.050%, B: 0.0003~0.0050%, It consists of the remainder Fe and inevitable impurities , the average particle size of the carbide is 0.1 to 1.0 μm, and the ratio of the standard deviation of the carbide particle size / the average particle size of the carbide is 0.5 or more and 1.0 or less. It is a high carbon steel sheet with excellent hardenability and stretch flangeability.

Hereinafter, the reasons for limitation of the present invention will be described.
C directly affects the hardness after quenching. If the amount of C is less than 0.20% by mass, sufficient strength as a machine structural component cannot be obtained after quenching. If the C content exceeds 0.45% by mass, the amount of carbide increases, voids are generated around the carbides during processing, and breakage progresses by a mechanism in which these voids are connected, so that stretch flangeability deteriorates. For this reason, the upper limit of C was specified as 0.45 mass%. A preferable range is 0.22 to 0.35 mass% for the same reason.
Since Cr serves to reduce the variation in the particle size of the spheroidized carbide, it is added within 0.70% by mass. Addition of Cr exceeding 0.70% requires a long annealing time for spheroidization, which not only increases the manufacturing cost but also makes the steel plate hard and deteriorates workability. Means for reducing the variation in the carbide particle diameter exist in addition to Cr addition, such as solidification conditions, hot rolling conditions, annealing conditions, etc., so the lower limit of the Cr amount is 0.01% by mass.
B is known as an element that enhances hardenability. Also in this invention, 0.0050 mass% is added at the upper limit for this purpose. When the amount of B exceeds 0.0050%, defects occur in the continuous casting slab, the surface flaws increase in the product, and the yield decreases. On the other hand, when the addition amount is less than 0.0003 mass%, the effect of improving the hardenability becomes insufficient. For this reason, the lower limit was specified as 0.0003 mass%.
Ti is added to manifest the effect of addition of B and to increase toughness after quenching. If the addition amount exceeds 0.05% by mass, the toughness may be deteriorated depending on the manufacturing conditions. Therefore, the upper limit is specified as 0.05% by mass. On the other hand, when the addition amount is less than 0.005% by mass, the above effect cannot be exhibited, so the lower limit was specified as 0.005% by mass.

  Carbide particle size greatly affects void formation in processability and hole expandability. When carbides become finer, the strain that creates voids increases, but the strain that causes cracks due to the connection of voids decreases and the workability deteriorates. For this reason, the lower limit of the average carbide particle size is specified to 0.1 μm. did. On the other hand, when the carbide particle size exceeds 1.0 μm, the breakage progresses starting from voids formed around the carbide and the stretch flangeability deteriorates. For this reason, the upper limit of the carbide average particle size was specified as 1.0 μm. For the same reason, a preferable range is a range in which the carbide average particle size is 0.3 to 0.8 μm.

  The variation in the carbide particle size has a great influence on the generation of voids during the hole expanding process, similarly to the carbide particle size. When the variation in carbides increases, even if the average carbide particle size is in the range of 0.1 to 1.0 μm, the ratio of large carbides and small carbides increases, and stretch flangeability deteriorates. If the standard deviation of the carbide particle size exceeds 1.0 in terms of the average carbide particle size ratio, the effect of worsening the stretch flangeability becomes significant. Therefore, the standard deviation of the carbide particle size is 1. Must be 0 or less. This is: C: 0.30%, Si: 0.12%, Mn: 0.67%, Cr: 0.30%, Ti: 0.016%, B: 0.0015% (all by mass) A steel slab is formed by continuous casting, heated to 1250 ° C., hot-rolled under various conditions, and spheroidized and annealed under various annealing conditions. A specimen for microstructural observation of the cross section was collected. The hole-expanded test piece was punched with a 10φ hole at the center of the plate with a clearance of 12%. The hole was pushed up with a conical punch of 60 degrees, and the test was stopped when a crack penetrating the punched surface and plate thickness occurred. The difference between the hole diameter and the initial hole diameter was divided by the initial hole diameter, and the ratio expressed as 100 minutes was defined as the hole expansion ratio. The carbide having a cross section parallel to the rolling direction was observed with a scanning microscope, and the average particle size of the carbide and the standard deviation of the carbide particle size were measured. Carbide observation sites were at least 10 fields of view. Relationship between the ratio of the standard deviation of carbide / average carbide particle size and the hole expansion ratio in the case where the measured average particle size of carbide is in the range of 0.3 to 0.4 μm and the average particle size of carbide is 0.7 to 0.75 μm. Is shown in FIG. Regardless of the average carbide particle size, the hole expansion rate decreases as the standard deviation of the carbide particle size increases. In particular, when the standard deviation is equal to or greater than the average carbide particle size, the tendency of the hole expansion rate to decrease becomes significant. Based on this fact, the ratio of the standard deviation of the carbide particle size to the average carbide particle size was specified to be 1.0 or less.

  In this invention, in order to achieve both the improvement of stretch flangeability and hardenability, not only simply controlling the particle size of carbide, but also adjusting the steel components appropriately and controlling the variation of carbide, It is possible to suppress the generation of voids on the end face of the metal and to delay the growth of cracks in the hole expanding process. As a result, it is possible to provide a high carbon steel sheet having extremely excellent stretch flangeability, excellent hardenability, and excellent toughness after quenching. By using this high-carbon steel sheet, it is possible to obtain a high degree of processing in processing of automobile drive system machine parts, etc., making it possible to manufacture parts etc. at low cost by omitting the manufacturing process. It is a very useful invention.

The steel of this invention has C: 0.22-0.45 mass%, Cr: 0.05-0.70 mass%, Ti: 0.010-0.050 mass%, B: 0.0003-0. Except for containing 0050% by mass, the average particle size and variation of the carbide may be specified in the above-mentioned range. Other chemical components are not particularly defined, and even if elements such as Si, Mn, P, S, Al, and N are contained in a normal range, the characteristics of the present invention are not impaired. However, the following is preferable.
Si hardens the steel sheet, impairs workability, and at the same time causes surface defects of the steel sheet.
When Mn is added in excess, it causes a decrease in ductility and at the same time tends to increase the variation in the particle size of the carbide. Therefore, Mn is preferably 1.5% by mass or less.
When P and S are added excessively, the ductility is lowered.
Since adding Al excessively tends to cause surface defects of the steel sheet, it is preferable to add Al in the range of 0.08% by mass or less.
When N is added in a large amount, it forms Al, Ti, B and nitride and deteriorates the hardenability. Therefore, N is preferably added in the range of 0.0080% by mass or less. Furthermore, elements such as Cu, Ni, Mo, Nb, V, Ca, and Mg may be added in a range that is usually added according to the purpose. These elements do not particularly affect the characteristics of the present invention. Further, elements and impurities inevitably mixed in the manufacturing process do not impair the characteristics of the present invention.

  The steel whose components have been adjusted as described above is made into a slab by continuous casting or ingot-bundling rolling. At this time, the component segregation of the steel ingot increases the variation in the carbide particle size, and therefore it is preferable to employ a method of reducing solidification segregation such as electromagnetic stirring under unsolidified region pressure. The slab is hot-rolled. At this time, the slab heating temperature is preferably set to 1280 ° C. or less in order to avoid deterioration of the surface condition due to scale generation. The finishing temperature of hot rolling is desirably Ar3 or higher from the viewpoint of workability. About coiling temperature, it is desirable to set it as 500-650 degreeC from a viewpoint of control of the size of carbide and its distribution. In addition, since cooling after finish rolling greatly affects the size distribution of carbides, it is desirable to cool by pouring water so that pearlite undergoes a constant temperature transformation. The hot-rolled steel strip produced in this manner is subjected to spheroidizing annealing after descaling, or cold rolling, annealing, or cold rolling and annealing after spheroidizing annealing, and supplied to the product. When annealing a hot-rolled steel strip after pickling, it is desirable to carry out at a temperature not higher than the Ac1 point temperature in order not to increase carbides having a large particle size when annealing after cold rolling. The cold rolling rate when the hot-rolled steel strip is cold-rolled after pickling or spheroidizing annealing is preferably 20% or more in order to control the carbide to a uniform size. On the other hand, when the cold rolling rate is high, the thickness of the hot-rolled steel strip is inevitably increased, and the dispersion of carbide tends to increase. Therefore, it is preferable to adopt a cold rolling rate of 70% or less. The steel strip manufactured in this way is subjected to temper rolling as needed to process machine structural parts and the like. The high-carbon steel sheet of the present invention may be a hot-rolled steel sheet or a cold-rolled steel sheet, and in any case, the effects of the features of the present invention can be obtained.

In mass%, C: 0.28%, Si: 0.15%, Mn: 0.58%, Cr 0.30%, S: 0.002%, Ti: 0.018%, B: 0.0015% N: 0.0038% of the composition, steel within the scope of the present invention was melted in a converter and a slab was made by continuous casting. At the time of continuous casting, at the time of a part of casting, the casting was performed in the unsolidified region at a reduction, electromagnetic stirring, and the casting speed 1.5 times faster than usual. This slab was heated to 1240 ° C. and hot rolled. The hot rolling finishing temperature was 790 to 880 ° C, and the winding temperature was 450 to 680 ° C. After this hot-rolled steel sheet was pickled, box annealing was performed at 615 to 705 ° C. for 14 to 96 hours to produce a steel sheet having a thickness of 4.0 mm. Samples were taken from these steel plates, and the average particle size of carbide, the standard deviation of carbide particle size, and the hardness were measured. At the same time, the upsetting test was conducted after the hole expansion test and the hole expansion test. The carbide particle size was obtained by polishing a cross section parallel to the rolling direction of the steel sheet, corroding with a picric acid solution, measuring each carbide area by observation with a scanning electron microscope, and determining the diameter assuming that this was a perfect circle. In the measurement of carbide, the number of fields of view was adjusted so that the number of carbides was 2000 or more. The standard deviation of the measured carbide particle size was determined. The hole expansion test was performed by punching out a 10mmφ (d0) hole at the center of a 150mm square steel plate with a clearance of 12%, and then pushing it up with a 60 ° conical punch. The hole diameter (d) at the time when a crack penetrating the thickness occurred was measured, and the hole expansion ratio λ (%) defined by the following equation was obtained.
λ = (d−d0) / d0 × 100
In the upsetting test, a hole of 15 mmφ was punched at a clearance of 12% at the center of 100 corners, and the hole diameter was pushed up with a flat bottom punch with a diameter of 20 mmφ until the hole diameter became the same as the punch diameter. The head of the hole expansion was compressed with a compression tester, and it was evaluated whether or not a crack occurred in the head when the height reached 50%. A case where no crack was observed was evaluated as “◯”, a case where a crack was slightly observed as “Δ”, and a case where a crack was present as “×”. The measurement results are shown in Table 1.

  Steel No. No. 1 employs electromagnetic stirring under conditions that minimize segregation during continuous casting, hot rolling finish temperature: 820 ° C., coiling temperature: 600 ° C., and cooling after finish rolling has a pearlite transformation temperature between 600 and 610 ° C. The water was poured to finish. In normal hot rolling, the transformation temperature range is relatively large. For example, the pearlite transformation of the comparative steel 4 is between 590 and 650 ° C. After hot rolling, it was pickled, annealed at 670 ° C. for 18 hours, and subjected to characteristic investigation. Carbide size distribution with an average carbide particle size of 0.76 μm, a standard deviation of carbide particle size of 0.45 μm, a standard deviation of carbide particle size / an average particle size of carbide of 0.592 is an example within the scope of the present invention. It has excellent stretch flangeability with a hole expansion ratio of 68%. Steel No. In No. 2, the electromagnetic stirring and casting speed during continuous casting was 1.5 times higher than the normal one. Hot rolling finishing temperature: 800 ° C., coiling temperature: 600 ° C. Water was poured in consideration of transformation heat generation so that the pearlite transformation was completed within 10 ° C. Carbide size distribution with an average particle size of carbide of this steel of 0.56 μm, standard deviation of carbide particle size of 0.32 μm, standard deviation of carbide particle size / average average particle size of carbide of 0.571 It is an example. It has excellent stretch flangeability with a hole expansion ratio of 75%. Steel No. No. 3 was subjected to 5% reduction in an unsolidified region during continuous casting, cast to make a slab, and subjected to hot rolling. Hot-rolling finishing temperature: 815 ° C., coiling temperature: 580 ° C., and pearlite temperature transformation temperature was poured and cooled so as to be 590-600 ° C. Carbide size distribution with an average particle size of carbide of 0.45 μm, standard deviation of carbide particle size of 0.28 μm, standard deviation of carbide particle size / carbide average particle size of 0.622 is an example within the scope of the present invention. . It has excellent stretch flangeability with a hole expansion ratio of 78%. Moreover, steel No. which is an Example within the scope of the present invention. Nos. 1, 2 and 3 show that cracks are not observed in the upsetting test after hole expansion, and all have excellent stretch flangeability.

  Steel No. No. 4 has an average particle size of carbide of 0.82 μm, a standard deviation of carbide particle size of 0.90 μm, a standard deviation of carbide particle size / an average particle size of carbide of 1.10, and the average particle size of carbide is within the scope of the present invention. However, this is a comparative example having a large variation in carbide particle size and deviating from the scope of the present invention. This steel has a hole expansion rate of 35%, which is lower than that of the example of the present invention, and it can be seen that cracks occur in the upsetting test after the hole expansion test and the stretch flangeability is inferior. Steel No. No. 5 has an average carbide particle size of 0.56 μm, a standard deviation of carbide particle size of 0.80 μm, a standard deviation of carbide particle size / an average particle size of carbide of 1.429, and the standard deviation of carbide particle size is out of the scope of the present invention. It is a comparative example. The hole expansion rate was as low as 41%, and cracks were observed in the upsetting test after the hole expansion. Steel No. No. 6 has a carbide average particle size of 1.25 μm, a carbide particle size standard deviation of 0.80 μm, a carbide particle size standard deviation / carbide average particle size of 0.64, and the carbide average particle size is outside the scope of the present invention. It is a comparative example. The hole expansion rate of this steel sheet is 42%, and it can be seen that the hole expansion rate is inferior compared to the examples within the scope of the present invention. Thus, even if the steel components are the same, it is not possible to provide a steel sheet having excellent stretch flangeability when either the carbide average particle diameter, the standard deviation of carbide particle diameter / the average particle diameter of carbide is outside the scope of the present invention. I understand that.

In mass%, C: 0.28%, Si: 0.15%, Mn: 0.58%, Cr 0.30%, S: 0.002%, Ti: 0.18%, B: 0.0015% , N: A steel having a composition of 0.0038% was melted in a converter and a slab was formed by continuous casting. At the time of continuous casting, casting was carried out by lowering the unsolidified zone pressure during part casting, electromagnetic stirring, and casting speed 1.5 times higher than usual. This slab was heated to 1240 ° C. and hot rolled. The hot rolling finishing temperature was 790 to 880 ° C, and the winding temperature was 450 to 680 ° C. After this hot-rolled steel sheet was pickled and cold-rolled at a cold rolling rate of 25 to 65%, box annealing was performed at 615 to 705 ° C. for 14 to 96 hours to produce a steel sheet having a thickness of 2.0 mm. Samples were taken from these steel plates, and the average particle size of carbide, the standard deviation of carbide particle size, and the hardness were measured. At the same time, the upsetting test was conducted after the hole expansion test and the hole expansion test. Carbide grain size was obtained by polishing a cross section parallel to the spreading direction of the steel sheet, corroding with a picric acid solution, and measuring each carbide area by observation with a scanning electron microscope, and calculating the diameter assuming that this was a perfect circle. . In the measurement of carbide, the number of fields of view was adjusted so that the number of carbides was 2000 or more. The standard deviation of the measured carbide particle size was determined. The hole expansion test was performed by punching out a 10mmφ (d0) hole at the center of a 150mm square steel plate with a clearance of 12%, and then pushing it up with a 60 ° conical punch. The hole diameter (d) at the time when a crack penetrating the thickness occurred was measured, and the hole expansion ratio λ (%) defined by the following equation was obtained.
λ = (d−d0) / d0 × 100
In the upsetting test, a hole of 15 mmφ was punched at a clearance of 12% at the center of 100 corners, and the hole diameter was pushed up with a flat bottom punch with a diameter of 20 mmφ until the hole diameter became the same as the punch diameter. The head of the hole expansion was compressed with a compression tester. When the height reached 50%, the crack was evaluated as to whether or not a crack would occur in the head. Those with cracks were rated as △, and those with cracks were rated as x. The measurement results are shown in Table 2.

  Steel No. No. 7 is hot rolling finishing temperature: 800 ° C., pearlite transformation is poured and cooled so as to be 600 to 610 ° C., wound at 590 ° C., pickled, cold rolling rate: 40% cold rolling, 660 It was manufactured by annealing with heating and holding for 24 hours. The carbide size distribution is 0.56 μm, the standard deviation of carbide particle size is 0.45 μm, the standard deviation of carbide particle size / carbide average particle size is 0.804, and both carbide size distributions are examples within the scope of the present invention. . It has excellent stretch flangeability with a hole expansion ratio of 83%. Steel No. In No. 8, a slab was formed by applying unsolidified zone reduction during continuous casting. In hot rolling, the hot rolling zone is lubricated and rolled, the cooling after hot rolling is controlled so that the pearlite transformation proceeds in the range of 560 to 570 ° C. at a finishing temperature of 815 ° C., and the winding temperature is 550 ° C. there were. Annealing was performed at 650 ° C. for 24 hours. The average particle size of carbide is 0.35 μm, the standard deviation of carbide particle size is 0.25 μm, the standard deviation of carbide particle size / carbide average particle size is 0.714, and both carbide size distributions are examples within the scope of the present invention. . The hole expansion ratio has an excellent value of 79%. In addition, in the upsetting test after the hole expansion, it is understood that no cracks are observed and that the film has excellent stretch flangeability. Steel No. No. 9 raised the pearlite transformation temperature range during cooling after hot rolling, and performed hot rolling controlled at 620 to 630 ° C. Other conditions are No. Same as 8. The average particle size of the carbide is 0.78 μm, the standard deviation of the carbide particle size is 0.45 μm, the standard deviation of the carbide particle size / the average particle size of the carbide is 0.577, and both the carbide size distributions are examples within the scope of the present invention. . The hole expansion rate has an excellent value of 77%. In addition, in the upsetting test after the hole expansion, it is understood that no cracks are observed and that the film has excellent stretch flangeability.

  Steel No. No. 10 has an average carbide particle size of 0.78 μm, a standard deviation of carbide particle size of 0.91 μm, a standard deviation of carbide particle size / an average particle size of carbide of 1.167, and an average carbide particle size of Steel No. Same as 9, but with large variations in carbides. The hole expansion rate of this steel plate is the same as that of steel No. Compared to 9, it is greatly deteriorated. Further, in the upsetting test after the hole expansion, a slight crack is observed, and it can be seen that the stretch flangeability is inferior as compared with the steel within the range of the present invention. Steel No. 11 is a comparative example in which the carbide average particle size is 1.30 μm, the standard deviation of carbide / carbide average particle size is 0.754, and the carbide average particle size is out of the scope of the present invention. Although the hardness of this steel plate is HRB: 75, which is softer than other steels, the hole expansion rate is as low as 43%, cracks are observed in the upsetting test after hole expansion, and the stretch flangeability is the present invention. It is inferior compared with an Example. Steel No. No. 12 is a comparative example in which the carbide average particle size is 1.20 μm, the standard deviation of carbide particle size / carbide average particle size is 1.083, and the carbide size and distribution are both out of the scope of the present invention. The steel sheet has a hole expansion ratio of 35%, which is inferior in stretch flangeability compared to the embodiment of the present invention. It can be seen that even a cold-rolled high carbon steel sheet can provide a steel sheet having excellent stretch flangeability for the first time by controlling both the average grain size of carbide and its variation within the scope of the present invention.

Steels having the compositions shown in Table 3 were melted in a converter and slabs were made by continuous casting. During continuous casting, casting was carried out with electromagnetic stirring and casting speed increased to 1.5 times that of a normal one during partial casting. This slab was heated to 1240 ° C. and hot rolled. The hot rolling finishing temperature was 790 to 880 ° C, and the winding temperature was 450 to 680 ° C. This hot-rolled steel sheet was pickled and then subjected to box annealing at 615 to 710 ° C. for 14 to 96 hours to produce a 4.0 mm steel sheet. The pickled steel sheet was subjected to box annealing at 615 to 705 ° C. for 14 to 96 hours after cold rolling with a cold rolling rate of 50%. A steel plate having a thickness of 2.0 mm was manufactured. Samples were taken from these steel plates, and the average particle size of carbide, the standard deviation of carbide particle size, and the hardness were measured. At the same time, the upsetting test was conducted after the hole expansion test and the hole expansion test. The carbide particle size was obtained by polishing a cross section parallel to the rolling direction of the steel sheet, corroding with a picric acid solution, measuring each carbide area by observation with a scanning electron microscope, and determining the diameter assuming that this was a perfect circle. In the measurement of carbide, the number of fields of view was adjusted so that the number of carbides was 2000 or more. The standard deviation of the measured carbide particle size was determined. The hole expansion test was performed by punching out a 10mmφ (d0) hole at the center of a 150mm square steel plate with a clearance of 12%, and then pushing it up with a 60 ° conical punch. The hole diameter (d) at the time when a crack penetrating the thickness occurred was measured, and the hole expansion ratio λ (%) defined by the following equation was obtained.
λ = (d−d0) / d0 × 100
In addition, after heating these steel plates to 850 ° C., holding them for 10 minutes, quenching them in oil at 60 ° C., measuring the quenching Vickers hardness, and quenching hardness of Hv: 450 or more by tempering, The test piece was adjusted to Hv: 450, a JIS No. 4 impact subsize (thickness of 4 mm), and the impact value at 20 ° C. was measured. Table 4 shows the properties of the steel plates annealed with the pickled plates, and Table 5 shows the measurement results of the cold-rolled and annealed steel plates.

  The characteristics of the steel sheet obtained by annealing the hot-rolled sheet after pickling will be described. Steel A-1 is an example in which the carbide average particle diameter and the ratio of carbide particle diameter standard deviation / carbide average particle diameter are both within the scope of the present invention. This steel plate is made with a magnetic slab under conditions that reduce segregation during continuous casting, and a slab is made after the hot rolling finish rolling stand, and the rolling reduction is increased by 30% compared to normal rolling, and the finishing temperature is 800. Performed at ° C. Cooling after hot rolling was performed at a winding temperature of 580 ° C. by pouring water so that the pearlite transformation proceeded between 600 to 610 ° C. Annealing is 670 ° C. × 36 hours. The hole expansion ratio is as high as 78% and has excellent stretch flangeability. Moreover, it can be seen that the quenching hardness is as high as Hv: 600, the impact value after tempering is high, and not only has excellent stretch flangeability but also excellent quenchability and toughness. Steel B-1 is a comparative example in which the amount of B deviates from the scope of the present invention. Although the stretch flangeability is good, the quenching hardness is insufficient as Hv: 330, and it is impossible to achieve both quenchability and stretch flangeability. Steel C-1 is an example in which the amount of C deviates from the lower limit of the range of the present invention. This steel also has a quenching hardness of Hv: 400, which is insufficient in hardenability. D-1 is a comparative example in which the Ti content deviates from the scope of the present invention. This steel also has a quenching hardness as low as Hv: 330, and the quenchability is insufficient. Steel E-1 is a comparative example having a C content of 0.55% and deviating from the scope of the present invention. Although the quenching hardness is high and the quenchability is sufficient, the toughness after tempering is inferior to that of the examples within the scope of the present invention, and the stretch flangeability is also poor.

  A-2 to E-2 shown in Table 5 show the properties of the cold-rolled steel sheet. A-2 is a steel sheet produced by cold rolling A-1 at a cold rolling rate of 50%, annealing at 650 ° C. for 18 hours, and temper rolling at 0.5%. This steel is an example within the scope of the present invention in terms of components, carbide particle size, standard deviation of carbide particle size / carbide average particle size, and has an excellent hole expansion ratio and quenching hardness. It can be seen that B-2 deviating from the scope of the present invention, C-2 deviating from the amount of C, and D-2 deviating from the amount of Ti have insufficient quenching hardness and cannot be applied to machine structural parts. On the other hand, even when the carbide particle size and the standard deviation of carbide particle size / carbide average particle size are within the range of the present invention, it can be seen that E-2 having too much C content has insufficient stretch flangeability.

  As described in detail in the above examples, only by controlling not only the average particle diameter of steel components and carbides but also the variation thereof, the stretch flangeability is excellent, and the hardenability and toughness after heat treatment are excellent. An excellent high carbon steel sheet can be provided.

  In this invention, in order to achieve both the improvement of stretch flangeability and hardenability, not only simply controlling the particle size of carbide, but also adjusting the steel components appropriately and controlling the variation of carbide, It is possible to suppress the generation of voids on the end face of the metal and to delay the growth of cracks in the hole expanding process. As a result, it is possible to provide a high carbon steel sheet having extremely excellent stretch flangeability, excellent hardenability, and excellent toughness after quenching. By using this high-carbon steel sheet, it is possible to obtain a high degree of processing in processing of automobile drive system machine parts, etc., making it possible to manufacture parts etc. at low cost by omitting the manufacturing process. It is a very useful invention.

The figure which shows the relationship between the standard deviation of carbide particle diameter / carbide average particle diameter, and a hole expansion rate.

Claims (1)

% By mass
C: 0.22-0.45 % ,
Si: 1.0% or less,
Mn: 1.5% or less,
P: 0.03% or less,
S: 0.03% or less,
Al: 0.08% or less,
N: 0.0080% or less,
Cr: 0.01-0.70 %
Ti: 0.005 to 0.050 % ,
B: 0.0003 to 0.0050 %,
The balance Fe and inevitable impurities ,
The average particle size of the carbide is 0.1 to 1.0 μm,
A high carbon steel sheet having excellent hardenability and stretch flangeability, wherein a ratio of standard deviation of carbide particle diameter / average particle diameter of carbide is 0.5 or more and 1.0 or less.

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