JP4276504B2 - High carbon hot-rolled steel sheet with excellent stretch flangeability - Google Patents

Description

  The present invention relates to a high carbon hot-rolled steel sheet having excellent stretch flangeability used for machine structural parts such as an automobile drive system.

  High-carbon steel sheets are usually formed by cutting, punching, bending, drawing, etc. to a predetermined size and shape, and then subjected to heat treatment such as quenching and tempering, and after tempering to the required strength, Often used for applications. In addition, a method of integrally forming a part that has been manufactured by joining a plurality of parts by welding or the like by rolling or FCF processing is becoming widespread.

  Some of these parts require stretch flangeability. High carbon steel sheets temper necessary properties such as hardness by heat treatment such as quenching and tempering, so that the amount of C added is inevitably high. For high carbon steel, a method of improving workability by spheroidizing carbide is generally used, and the size of carbide, average distance of carbide, and the like are controlled. However, since spheroidizing annealing does not change the presence of many hard carbides in steel, it is not suitable for stretch flange processing.

Patent Documents 1 to 8 listed below disclose techniques for improving stretch flange workability of a high carbon hot rolled steel sheet.
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 2000-178678 A JP 2001-73033 A JP 2001-214234 A JP 2001-220463 A

  The steel sheet disclosed in Patent Document 1 is a steel sheet having a spheroidization rate of carbide of 90% or more, an average carbide distance of 0.8 μm or more, and a ferrite crystal grain size adjusted to 20 μm or more as necessary. Both requirements are within the range of the prior art, and cannot be applied to products that are subjected to severe stretch flange processing or further processing such as increasing the thickness of stretch flanged portions.

The technique disclosed in Patent Document 2 improves stretch flangeability by adopting a special annealing cycle. Although this technique has an effect of slightly improving the stretch flange, it cannot be applied to a product that is subjected to severe stretch flange processing or further processing such as increasing the thickness of the stretch flange processed portion.
The technique disclosed in Patent Document 3 is the same technique as the technique disclosed in Patent Document 1 that defines the composition of steel.

The technology disclosed in Patent Document 4 is to disperse carbides in ferrite so that the spheroidization rate of carbide is 90% or more and the average carbide particle size is 0.4 to 1.0 μm. The ferrite crystal grain size is adjusted to 20 μm or more. This technique is a technique in which the range of the carbide average particle size disclosed in Patent Document 1 is changed, and cannot be applied to products that are subjected to severe stretch flange processing or further processing such as increasing the thickness of stretch flange processed portions. .
The technique disclosed in Patent Document 5 is a technique related to a steel sheet having a large C content of 0.7 to 1.5%, but the technical idea is the same as the technique disclosed in Patent Document 3.

  The technology disclosed in Patent Document 8 specifies the degree of dispersion of carbide and the average particle size of carbide, and its technical idea is to increase the particle size of the carbide, disperse it uniformly, and to create a macro resulting from the carbide during processing. It is intended to reduce cracks, and is in the range of the prior art, and cannot be applied to products that are subjected to severe stretch flange processing or further processing such as increasing the thickness of stretch flange processed portions.

The technique disclosed in Patent Document 7 is a technique that defines the average particle size of carbides and the volume fraction of ferrite grains that do not contain carbides. This technique is the same technical idea as the technique disclosed in Patent Document 8, in which the dispersion of carbide is replaced by the volume fraction of ferrite grains not containing carbide, and is technically the same.
The technique disclosed in Patent Document 6 is obtained by changing the special annealing technique disclosed in Patent Document 2 into a metallurgical rule, and the technical idea is the same.

  Thus, although the conventional disclosed technique is a technique for controlling the size distribution of carbide and improving the stretch flangeability, it is not a technique for drastically improving the stretch flangeability of a high carbon steel sheet, so that It cannot be applied to products that undergo further processing such as flange processing or stretch flange processing, and development of new technologies is desired.

  The problem to be solved by the present invention is to provide a high carbon hot-rolled steel sheet that is excellent in stretch flangeability before quenching and that has a predetermined hardness and toughness after quenching.

  As a result of intensive studies on the stretch flangeability and the toughness after quenching of the high carbon steel sheet, the inventors of the present invention have excellent stretch flangeability by controlling the steel composition, ferrite grain size, and shape of inclusions, and It was found that a high carbon hot rolled steel sheet having excellent toughness after quenching can be obtained.

The present invention has been completed based on this finding, and the gist thereof is as follows.
(1) In mass%,
C: 0.20-0.60%, Si: 0.5% or less,
Mn: 2.0% or less, P: 0.03% or less,
Al: 0.08% or less, N: 0.01% or less,
Cr: 0.08 to 0.75% , S: 0.005% or less ,
Stretch flange characterized by the composition of the balance Fe and unavoidable impurities , ferrite average particle diameter of 1 to 10 μm , standard deviation of ferrite particle diameter of 3.0 μm or less, and inclusion shape ratio of 2.0 or less High carbon hot rolled steel sheet with excellent properties.
(2) By mass%
C: 0.20-0.60%, Si: 0.5% or less,
Mn: 2.0% or less, P: 0.03% or less,
Al: 0.08% or less, N: 0.01% or less,
Cr: 0.75% or less, S: 0.005% or less,
Ti: 0.010-0.050%, B: 0.0003-0.0050% ,
Stretch flange characterized by the composition of the balance Fe and unavoidable impurities , ferrite average particle diameter of 1 to 10 μm , standard deviation of ferrite particle diameter of 3.0 μm or less, and inclusion shape ratio of 2.0 or less High carbon hot rolled steel sheet with excellent properties.

  According to the present invention, in obtaining a high carbon hot-rolled steel sheet with excellent stretch flangeability, the average grain size of ferrite grains and the standard deviation thereof are controlled, and the shape ratio of inclusions is specified together to expand the hole. Generation of cracks during processing can be suppressed. It is possible to provide a high-carbon hot-rolled steel sheet that does not crack even when the upsetting process is performed after the hole expanding process, has excellent stretch flangeability, and is excellent in impact resistance after quenching and induction hardenability. By using such a high carbon hot-rolled steel sheet, it is possible to increase the degree of processing in processing of transmission parts and the like, and as a result, it is possible to manufacture parts and the like at low cost by omitting the manufacturing process. Become.

First, the fact that it has been found that good stretch flangeability can be obtained when the ferrite average crystal grain size, which is an important constituent factor of the present invention, is 10 μm or less will be described.
In a vacuum melting furnace, a steel having a composition of C: 0.35%, Si: 0.05%, Mn: 0.40%, Cr: 0.15%, S: 0.003% is melted and is 90 mm thick. Build a steel ingot, heat to 1250 ° C, change hot rolling rolling schedule, hot rolling finishing temperature, cooling rate after hot rolling, cooling pattern, cooling end point temperature in a holding furnace at the same temperature as the cooling end point temperature. After holding for 1 hour, it was air-cooled.
After this steel plate was pickled, it was annealed at 670 ° C. for 18 hours, and a hole expansion test and a microstructure were investigated.

In the hole expansion test, a 20φ hole was punched with a clearance of 12%, and this hole was expanded with a 60 ° conical punch, and the difference between the hole diameter at the time when a crack penetrating the plate thickness occurred around the hole and the initial hole diameter was calculated. Divided by the initial hole diameter and expressed as a percentage.
The ferrite grain size was determined by using the ferrite grain size No. defined by JIS, assuming that the ferrite grains were spheres, and determining the diameter. The standard deviation of the ferrite grain size is to determine the apparent area of each ferrite grain on the same observation surface where the ferrite grain diameter was measured, and the diameter when the apparent ferrite grain is assumed to be a circle is multiplied by 4 / π. Standard deviation was determined. The number of ferrite grains was measured in the range of 500 to 1000.

  The relationship between the average ferrite grain size and the hole expansion rate is shown in FIG. As can be seen from FIG. 1, it can be seen that the hole expansion rate is improved by setting the average ferrite crystal grain size to 10 μm or less. From this fact, the average grain size of ferrite grains was specified to be 10 μm or less. The lower limit need not be particularly limited. However, the lower limit of the average grain size that can be produced by the current industrial technology is about 1 μm.

  FIG. 2 shows the relationship between the standard deviation of the ferrite particle size and the hole expansion rate for the sample having a ferrite particle size of 10 μm or less. As can be seen from FIG. 2, it can be seen that when the standard deviation of the ferrite grain size is 3.0 μm or less, good hole expanding property can be obtained stably. From this fact, a standard deviation of ferrite grain size of 3.0 μm or less was specified. A preferable range is 2.0 μm or less for the same reason.

Hereinafter, the steel component which comprises this invention is demonstrated.
C is an element that deteriorates workability. However, when it is used after quenching and tempering, it needs to be 0.20% or more. On the other hand, if the amount of C exceeds 0.60%, it becomes hard and hard carbides increase, and sufficient stretch flangeability cannot be obtained.

Cr is an element that hardens the steel sheet and degrades workability, but it is necessary to refine the ferrite grain size of the hot-rolled steel sheet, which is an important factor for stretch flangeability, and to reduce the standard deviation of the ferrite grain size. Element. Further, it is an element that improves the toughness after quenching and tempering. Therefore, it is added at 0.75% or less. Since there are other techniques for controlling ferrite grains, the lower limit of the Cr amount does not need to be particularly limited, but a preferable range is 0.08% or more .

  Since S becomes an inclusion such as MnS and deteriorates stretch flangeability, it is necessary to make it 0.005% or less.

  B is known to be an element that enhances hardenability. Also in the present invention, it is desirable to add in the range of 0.0003 to 0.0050% when steel with a low amount of C, or particularly when hardenability is required. If it is less than 0.0003%, there is almost no effect of improving hardenability, and if it exceeds 0.0050%, it causes defects such as internal defects during the production of steel, so this range is desirable. More preferably, it is 0.0005 to 0.0030% for the same reason.

  Ti is added in the range of 0.010 to 0.050% in order to exert the effect of B when B is added.

  Inclusions are factors that affect stretch flangeability as well as the structure of the steel sheet. The influence of the inclusions on the stretch flangeability is the projected length, and the greater this length, the worse the stretch flangeability. Therefore, even with the same inclusion size, the stretch flangeability deteriorates as the degree of extension increases. For this reason, it is desirable that the shape ratio represented by the ratio of the short axis to the long axis of the inclusion is 2.0 or less.

The reason why the stretch flangeability is improved by controlling the ferrite crystal grain size and the standard deviation of the ferrite grain size is not clear, but the present inventors consider as follows.
The punched surface before stretch flange processing is hardened by shear deformation, and microcracks are already generated before stretch flange processing. When this is stretch flanged, a large crack is generated starting from microcracks, which is the stretch flange processing limit. Therefore, firstly, the stretch flangeability depends on the degree of macro cracks on the shear surface. Second, if there is no macro crack on the shear plane, the stretch flange depends on how uniformly the processing strain is shared, and if the strain distribution is uneven, the stretch flangeability deteriorates and the strain is evenly distributed. Then, stretch flangeability becomes favorable.

  The macro crack on the punched surface is generated from a portion having a large deformation amount and a portion having a poor deformability. The deformation unit is a ferrite grain size. If the ferrite grain size is not uniform, the deformation concentrates on large ferrite grains that are deformed by a small stress, and if the deformability is exceeded, cracks occur. The same behavior is exhibited when the ferrite grain size is large. In a uniform structure with small crystal grains, each crystal grain evenly distributes strain during shearing and hole expansion, thereby suppressing the occurrence of cracks.

  Steel used in the present invention is: C: 0.20-0.60%, Cr: 0.75% or less, S: 0.005% or less, and if necessary, Ti: 0.010-0.050%, B: In addition to 0.0003 to 0.0050%, if the ferrite particle size (10 μm or less) and its dispersion, standard deviation (3.0 μm or less), inclusion shape ratio (2.0 or less) The other chemical components Si, Mn, P, Al, and N may be added in a normal range and need not be specified. The addition amount of these elements may be determined in relation to other characteristics, and the feature of the present invention is not impaired by the addition amount.

However, the following is preferable.
Since Si hardens the steel sheet when the addition amount is large, it is preferably 0.5% or less.
Since Mn impairs ductility when added in excess, it is preferably added at 2.0% or less.
P not only impairs the workability but also deteriorates the toughness after quenching and tempering. Therefore, P is preferably 0.03% or less.
If Al is added excessively, the hardenability deteriorates, so 0.08% or less is preferable.
When N is added excessively, not only the ductility deteriorates but also the toughness after quenching and tempering deteriorates. Therefore, N is preferably made 0.01% or less.

  Furthermore, elements such as Cu, Ni, Nb, V, Zr, Ca, and Mg may be added in a range that is usually added according to the purpose. Even if other impurities are mixed in the manufacturing process, the characteristics of the present invention are not impaired.

  Reducing the ferrite grain size and its standard deviation is, in other words, controlling a uniform structure. The components of the steel as described above are adjusted using a converter or an electric furnace, and if necessary, vacuum degassing. The high carbon steel whose components have been adjusted in this way is made into a slab by ingot-bundling rolling or continuous casting. If the component segregation is large in the slab, even if it is manufactured with great care, the structure becomes uneven and it is difficult to obtain a steel sheet that satisfies the requirements of the present invention. For this reason, when manufacturing a slab, it is preferable to employ a method of reducing segregation such as electromagnetic stirring and unsolidified region pressure.

  This slab is subjected to hot rolling. In the hot rolling, the ferrite grain size and its dispersion cannot be reduced even if the normal hot rolling is performed. The heating temperature is preferably 1250 ° C. or lower in order to prevent surface defects due to scale generation.

In finishing hot rolling, it is preferable to increase the cumulative reduction ratio of the temperature immediately above the Ar3 point temperature as much as possible. Specifically, it is preferable that the cumulative rolling reduction in the temperature range from the finishing temperature to the Ar3 point temperature + 20 ° C. is 40% or more.
Also, lubrication hot rolling may be performed in order to reduce the difference in structure between the surface layer and the inside of the steel sheet due to shear deformation of hot rolling. The finishing temperature is preferably between Ar3 point temperature and Ar3 point temperature + 40 ° C. Even if rolling is finished at a temperature higher than this temperature range or rolling is finished at a lower temperature, the structure tends to be a mixed grain structure.

  As for cooling after finish hot rolling, it is important to cool the ferrite transformation temperature range and the pearlite transformation temperature range after finish hot rolling. It is important to cool the ferrite transformation temperature range by increasing the cooling rate and lowering the ferrite transformation temperature. The pearlite transformation temperature range is cooled so that the heat generated by pearlite transformation and heat removal by water injection are balanced, and the pearlite transformation is cooled so that it progresses isothermally to form a uniform pearlite structure, which is wound after the pearlite transformation. Is preferred. The winding temperature may be any number of times as long as the pearlite transformation is completed in the cooling zone, but in the present invention, the winding temperature is in the range of 520 to 600 ° C.

  The hot rolled coil thus manufactured is subjected to spheroidizing annealing of carbide after descaling. Spheroidizing annealing is performed at a temperature below the Ac1 point. As the annealing temperature increases, the spheroidized carbides increase and cracks are likely to occur during processing, so it is desirable to anneal at as low a temperature as possible.

As described above, by strictly controlling the slab manufacturing, hot rolling, pearlite transformation temperature control, and annealing conditions within a predetermined range, it is possible to manufacture a steel sheet having a uniform structure for the first time.
The steel sheet manufactured in this way is subjected to temper rolling as needed and provided to the product.

In mass%, C: 0.35%, Si: 0.15%, Mn: 0.73%, Cr: 0.28%, P: 0.015%, S: 0.003%, Al: 0.00. Steel of 0025%, N: 0.0035% was melted in a converter and a slab was made by continuous casting. A hot rolled coil was manufactured by changing the distribution of the slab under hot rolling and changing the cooling conditions. The hot rolling finishing temperature was changed to 780 to 880 ° C., and the winding temperature was changed in the range of 470 to 670 ° C. The hot rolled coil was pickled and then annealed at 650 to 710 ° C. for 14 to 48 hours to produce a hot rolled coil having a thickness of 4.0 mm.
Samples were taken from these steel plates, and the ferrite average grain size, standard deviation of ferrite grain size, inclusion shape ratio, and hardness were measured. At the same time, a hole expansion test and an upsetting test were performed after the hole expansion test.

  The ferrite grain size is obtained by polishing a cross section parallel to the rolling direction of the steel sheet, corroding with 5% nital, measuring the crystal grain size number by the method of JIS, and assuming that the ferrite grain is a sphere from the ferrite grain size number. The diameter was calculated. The standard deviation of the ferrite grain size is obtained by calculating the apparent area of each ferrite grain on the same observation surface where the ferrite grain diameter is measured, and multiplying the standard deviation by 4 / π to the diameter assuming that the apparent ferrite grain is a circle. Asked. The number of ferrite grains measured was in the range of 800 to 1000. For the shape ratio of inclusions, the long side and short side of 100 inclusions were measured, and the average value of the ratio of long side to short side was determined.

The hole expansion test is performed by punching a 10mmφ (do) 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 the cracks penetrating the hole were measured, and the hole expansion ratio λ (%) defined by the following equation was obtained.
λ = (d−do) / do × 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 cracks were observed was rated as ◯, a case where cracks were slightly observed was evaluated as Δ, and a case where cracks were present was rated as ×. These measurement results are shown in Table 1.

Nos. 1 to 5 in Table 1 are examples of the present invention having an average ferrite particle size of 10 μm or less, a standard deviation of ferrite particle size of 3.0 μm or less, and an inclusion shape ratio of 2.0 or less.
Steel Nos. 6 to 9 are comparative examples, and Steel Nos. 6 and 7 have an average ferrite crystal grain size of 10 μm or more and a standard deviation of ferrite grain size of 3.0 μm or more. Steel No. 8 has an average ferrite grain size and inclusion shape ratio within the scope of the present invention, but the standard deviation of the ferrite grain size exceeds 3.0 μm and is outside the scope of the present invention. Steel No. 9 has the ferrite grain size and the standard deviation of the ferrite grain size within the scope of the present invention, but the inclusion shape ratio is outside the scope of the present invention of 2.0 or more.

From Table 1, it can be seen that Nos. 1 to 5 of the examples of the present invention have a hole expansion ratio of 66 to 78%, excellent stretch flangeability and upsetting workability after hole expansion.
On the other hand, steel Nos. 6 and 7, whose standard deviation between the ferrite grain average grain size and the ferrite grain size is outside the scope of the present invention, have a low hole expansion rate despite being low in hardness and soft. Cracks are observed after upsetting and it can be seen that stretch flangeability is inferior to the examples within the scope of the present invention.
Steel No. 8 is a comparative example in which the standard deviation of the ferrite grain size is outside the scope of the present invention, but it can be seen that the stretch flangeability is also inferior. Steel No. 9 is a comparative example in which the average grain size and standard deviation of ferrite grains are within the scope of the present invention, but the inclusion shape ratio is outside the scope of the present invention. It can be seen that this steel also has poor stretch flangeability.

Although there are various methods for producing the steel sheet of the present invention, the production conditions for steel No. 4 will be described as an example.
This steel was cast at a solidification rate 1.5 times faster than the normal casting conditions during continuous casting to produce a slab. When this slab is hot-rolled by a six-stand hot rolling mill, the first two stands of the finishing rolling stock are rolled at a temperature of 1000 ° C. or more and cooled between each of the two to four stands. The temperature was 850 ° C., lubrication was performed at a reduction rate at which the rolling strain of each stand in the latter half of the rolling mill row was equal, and the finishing temperature was 830 ° C.

Immediately after finishing hot rolling, the cooling rate of the upper and lower surfaces is equal and the cooling rate is such that the cooling rate does not cause ferrite transformation. Cooling to 600 ° C is performed, and then the pearlite transformation temperature range is within 20 ° C. The amount of water was controlled and cooled, and a hot rolled coil was wound at 540 ° C. The coil was manufactured by pickling and annealing at 660 ° C. for 14 hours.
Thus, not only the steel composition but also the control of continuous casting conditions, the control of hot rolling conditions, and the control of cooling after hot rolling enable the production of the steel sheet of the present invention for the first time.

In mass%, C: 0.35%, Si: 0.20%, Mn: 0.55%, S: 0.003%, P: 0.012%, Al: 0.022%, Ti: 0.00. Steel of 024%, B: 0.0012%, N: 0.0045% was melted in a converter and a slab was made by continuous casting. The slab was heated to 1250 ° C., and hot rolled coils were manufactured by changing the distribution under hot rolling and changing the cooling conditions. The hot rolling finishing temperature was changed to 790 to 880 ° C, and the coiling temperature was changed in the range of 485 to 670 ° C. This hot rolled coil was pickled and then annealed at 650 to 710 ° C. for 16 to 48 hours to produce a hot rolled coil having a plate thickness of 4.0 mm.
Samples were collected from these steel plates, and the ferrite average grain size, standard deviation of ferrite grain size, inclusion shape ratio, and surface hardness were measured. The upsetting test was performed after the hole expansion test and the hole expansion test of the steel sheet. The test method and the like are the same as in Example 1. The measurement results are shown in Table 2.

  Steel Nos. 10 to 11 are examples of S35C in which B is added within the scope of the present invention in terms of average ferrite grain size, standard deviation of ferrite grain size, and inclusion shape ratio. Those within the scope of the present invention have a high hole expansion ratio, no cracks are observed in the upsetting test, and all have excellent stretch flangeability.

Steel No. 13 is a comparative example in which the average grain size of ferrite grains and the standard deviation of ferrite grains are outside the scope of the present invention. This steel sheet had a hole expansion rate worse than that of the steel of the examples within the scope of the present invention, and cracks occurred in the upsetting test.
Steel No. 14 is a comparative example in which the average grain size of ferrite is within the range of the present invention, but the variation in ferrite grain size is large. It can be seen that when the variation in the ferrite grain size is large, the hole expansion ratio is lower than that of the example of the present invention, and excellent stretch flangeability cannot be obtained.

C: 0.50%, Si: 0.20%, Mn: 0.78%, S: 0.003%, P: 0.015%, Cr: 0.12%, Al: 0.025%, N : 0.0043% steel was melted in a converter and a slab was made by continuous casting. A hot rolled coil was manufactured by changing the distribution under the hot rolling and cooling conditions of this slab. Among the hot rolling conditions, the hot rolling finishing temperature was changed to 770 to 880 ° C, and the winding temperature was changed in the range of 505 to 670 ° C. The hot rolled coil was pickled and then annealed at 650 to 710 ° C. for 16 to 48 hours to produce a hot rolled coil having a thickness of 2.8 mm.
Samples were collected from these steel plates, and the ferrite average grain size, standard deviation of ferrite grain size, inclusion shape ratio, and surface hardness were measured. At the same time, the upsetting test was conducted after the hole expansion test and the hole expansion test. These test methods are the same as in Example 1. Table 3 shows the measurement results.

  Steel Nos. 15 and 16 are examples of the S50C class within the scope of the present invention in terms of average ferrite grain size, standard deviation of ferrite grain size, and inclusion shape ratio. Those within the scope of the present invention have a high hole expansion ratio, no cracks are observed in the upsetting test, and it can be seen that even steel equivalent to S50C has excellent stretch flangeability.

Steel No. 17 is a comparative example in which the average grain size of ferrite grains and the standard deviation of the ferrite grain size are outside the scope of the present invention. It can be seen that the hole expansion rate is inferior compared to the examples within the scope of the present invention, the cracking is observed in the upsetting test, and the stretch flangeability is inferior.
Steel No. 18 is a comparative example in which the average grain size of ferrite grains is within the range of the present invention, but the variation in ferrite grain size is large. It can be seen that the stretch flangeability is poor.

  As described in detail in the above examples, by specifying both the average grain size and standard deviation of ferrite grains and the shape ratio of inclusions, a high carbon hot rolled steel sheet having excellent stretch flangeability can be obtained. I understand. It can be seen that if any one of the above factors falls outside the scope of the present invention, a high-carbon hot-rolled steel sheet having good stretch flangeability cannot be obtained.

  Steel with the composition shown in Table 4 is hot rolled into a hot rolled steel sheet having a thickness of 4.0 mm, annealed at 680 ° C. for 24 hours, ferrite average crystal grain size, standard deviation of ferrite crystal grain size, inclusion shape The ratio was measured. The steel plate was 880 ° C. × 50 minutes heated and then quenched in oil at 60 ° C., and tempered under conditions to obtain hardness according to each steel type, JIS No. 4 impact test pieces were made, and the impact test was conducted at 20 ° C. Further, in order to investigate the induction hardenability, the sample was heated at 900 ° C. for 3 seconds at high frequency, then cooled with water, and the hardness was measured. These values are shown in Table 5.

  As can be seen from Tables 4 and 5, the steel sheet that satisfies the requirements of the present invention has an impact value after quenching and tempering superior to that of the comparative material. In addition, it can be seen that the hardness after induction hardening is higher in the examples of the present invention than in the comparative examples and is excellent in induction hardenability.

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

Claims (2)

% By mass
C: 0.20-0.60%,
Si: 0.5% or less,
Mn: 2.0% or less,
P: 0.03% or less,
Al: 0.08% or less,
N: 0.01% or less,
Cr: 0.08~ 0.75%,
S: 0.005% or less ,
Stretch flange characterized by the composition of the balance Fe and unavoidable impurities , ferrite average particle diameter of 1 to 10 μm , standard deviation of ferrite particle diameter of 3.0 μm or less, and inclusion shape ratio of 2.0 or less High carbon hot rolled steel sheet with excellent properties. % By mass
C: 0.20-0.60%,
Si: 0.5% or less,
Mn: 2.0% or less,
P: 0.03% or less,
Al: 0.08% or less,
N: 0.01% or less,
Cr: 0.75% or less,
S: 0.005% or less,
Ti: 0.010 to 0.050%,
B: 0.0003 to 0.0050% ,
Stretch flange characterized by the composition of the balance Fe and unavoidable impurities , ferrite average particle diameter of 1 to 10 μm , standard deviation of ferrite particle diameter of 3.0 μm or less, and inclusion shape ratio of 2.0 or less High carbon hot rolled steel sheet with excellent properties.

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